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PlatinumMetalsReview

www.platinummetalsreview.comE-ISSN 1471–0676

VOLUME 52 NUMBER 2 APRIL 2008

E-ISSN 1471–0676

PLATINUM METALS REVIEWA Quarterly Survey of Research on the Platinum Metals and

of Developments in their Application in Industrywww.platinummetalsreview.com

VOL. 52 APRIL 2008 NO. 2

ContentsSafer, Faster and Cleaner Reactions Using Encapsulated 64

Metal Catalysts and Microwave HeatingBy M. R. Pitts

Practical New Strategies for Immobilising Ruthenium 71Alkylidene Complexes: Part IBy Ileana Dragutan and Valerian Dragutan

“Green Chemistry and Catalysis” 83A book review by Duncan Macquarrie

A Disordered Copper-Palladium Alloy Used as a Cathode Material 84By Philippe Poizot, Lydia Laffont-Dantras and Jacques Simonet

Dalton Discussion 10: Applications of Metals 96in Medicine and Healthcare

A conference review by Christian G. Hartinger

Platinum as a Reference Electrode in 100Electrochemical Measurements

By Kasem K. Kasem and Stephanie Jones

Faraday Discussion 138: Nanoalloys – From Theory to Applications 107A conference review by Geoffrey C. Bond

Challenges in Catalysis for Pharmaceuticals and Fine Chemicals 110A conference review by Chris Barnard

The Periodic Table and the Platinum Group Metals 114By W. P. Griffith

Frederick A. Lewis 120An appreciation by Ted B. Flanagan

“Fuel Cell Today Industry Review 2008” 123Abstracts 124

New Patents 127Final Analysis: Crystallite Size Analysis of 129

Supported Platinum Catalysts by XRDBy Tim Hyde

Communications should be addressed to: The Editor, Barry W. Copping, Platinum Metals Review, [email protected]; Johnson Matthey Public Limited Company, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K.

Platinum Metals Rev., 2008, 52, (2), 64–70 64

Microwave heating has developed as an impor-tant tool for research chemists, enabling reactionsto be carried out and optimised more quickly thanusing traditional heating methods (1–3). Directirradiation of the reaction mixture produces amore uniform and homogeneous heating profilethan does, for example, an oil bath. In most casesthe observed increase in rate can be explained bythe extremely efficient energy transfer and homo-geneous heating effect. This can lead tosuperheating of the reaction mixture (4): indeed,even microwave heating of an open vessel canachieve temperatures several degrees higher thanthe boiling point of the solvent (5).

In certain cases the presence of elements thatstrongly absorb microwave energy and release itefficiently as heat can cause localised ‘hotspots’tens of degrees higher than the bulk temperature,generating significant rate enhancements (6–8).This effect can be exploited to heat materials oflow microwave absorbance by the use of ‘passiveheating elements’ (9). Non-polar and poorlyabsorbing solvents can also be superheated byadding small amounts of a strongly absorbingcosolvent such as an ionic liquid (10–13). Theapplication of this selective heating can be particu-larly striking when the element is a heterogeneous

catalyst (14–16). A localised increase in tempera-ture at a catalyst surface over the bulk temperature,or a selective absorption of microwave energy bycatalytic species or organometallic intermediateson a reaction pathway, can lead to increased selec-tivity for the catalytic process while unwanted(thermally driven) side reactions are minimised bya relatively low bulk temperature (17). A synergis-tic advantage between microwave heating andplatinum group metal catalysis can therefore bedemonstrated (18).

The use of commercially available focused(monomode) microwave units (19–21) enhancesthe safety and reproducibility of reactions. Thestandard integration of monomode units intomany laboratory environments has expanded thearmoury of techniques available to chemists, allow-ing ready access to previously difficult-to-achievechemistries. These include high-temperature reac-tions such as Ullmann couplings (22); someheterocycle preparations previously requiringmetal baths (23, 24); the use of near-critical wateras solvent (25–29); and shortening the reactiontime on slow processes such as cycloadditions (30)to practically useful timescales, including replacingthe need for autoclaves (31); and automated pep-tide synthesis (32, 33).

Safer, Faster and Cleaner Reactions UsingEncapsulated Metal Catalysts andMicrowave HeatingPERFORMANCE ENHANCEMENT OF PALLADIUM, PLATINUM AND OSMIUM CATALYSTS

By M. R. Pitts*Reaxa Ltd., Hexagon Tower, Blackley, Manchester M9 8ZS, U.K.; E-mail: [email protected]

The combination of focused microwave heating and encapsulated metal promoters (EnCatTM)offers a safer, cleaner and more cost-effective solution to a wide range of catalyst-mediatedreactions, some of which are not widely accessible to the bench chemist due to high hazardratings. These include the palladium-catalysed Sonogashira cross-coupling, palladium-catalysed transfer hydrogenation, platinum-mediated hydrogenation and osmium tetroxide-catalysed dihydroxylation.

*Present address: Chemistry Innovation KTN, The Heath, Runcorn WA7 4QZ, U.K.; E-mail: [email protected]

DOI: 10.1595/147106708X292526

For the reasons discussed, metal-catalysedreactions work particularly well under microwave irra-diation; however safety and isolation issues still arisefrom their use. Elemental metal can deposit fromreaction mixtures onto the side of the glass tube,causing localised superheating of the glass and explo-sive rupture of the vessel (34). This can occur withboth homogeneous and heterogeneous catalysts. Itcan also be difficult to remove metal species selective-ly from the product on completion of the reaction.

The EnCatTM range of encapsulated metal cata-lysts were designed to address these issues ofpurification and reuse. Unlike other immobilisedhomogeneous catalysts such as FibreCatTM, wherephosphine ligands are attached to polyethylene fibres(35), the homogenous catalyst in EnCat is containedwithin a resin microcapsule. The use of such support-ed or ‘heterogenised’ catalysts industrially is beingdriven by regulatory pressures towards lower residuallevels of metal catalysts within active pharmaceuticalintermediates (APIs) (36, 37).

EnCats are prepared by an interfacial micropoly-merisation of an organic solution containing thehomogeneous metal catalyst, monomers (function-alised isocyanates) and additives, dispersed as asuspension in an aqueous phase. Reactive groupsgenerated at the interface combine to form polymerwalls and, as the surrounding matrix forms, the cata-lyst is entrapped to give spherical microcapsules (38).The individual catalytic species gain additional stabil-isation through interaction with the amidefunctionality of the polyurea matrix, resulting in verylow levels of metal leaching. Consequently the cata-lyst can be recovered efficiently by simple filtrationand reused.

Examples of catalysts already encapsulated thisway include palladium(II) acetate (39, 40) with andwithout various phosphine ligands (41), palladium(0)nanoparticles (42), platinum(0) (43) and osmiumtetroxide (44). Here we describe how EnCats providea homogeneous catalyst in a more effective form foruse with microwave heating.

EnCats in Microwave HeatedReactions

EnCats have been shown to be highly compatiblewith microwave heating (45, 46). Following the excel-

lent work by Ley and coworkers in demonstratingmicrowave-enhanced palladium EnCat-catalysedSuzuki couplings in both batch and flow modes (47),we were keen to understand the role of EnCat inheating bulk solution. Ley found that cooling reac-tions while providing a fixed microwave powerequivalent to that required for good conversion in thenon-cooled method resulted in cleaner products atsimilar or better conversions. The lower bulk temper-ature in the case of cooling may explain the reductionin side reactions, with the temperature ‘inside’ theEnCat beads potentially much higher. It is knownthat Pd/C preferentially absorbs microwave energywhen suspended in a virtually microwave-transparentsolvent, and ‘passively’ heats the surroundings (48).To investigate whether EnCat acts in the same way, a5 cm3 sample of anhydrous toluene, with variousadditives, was irradiated at a constant power of 200W for 5 minutes and the temperature recorded(Figure 1). Adding 250 mg of Pd EnCat had a negli-gible effect on the heating profile, as did the additionof ‘blank’ EnCat beads containing no metal. Additionof an equivalent amount of homogeneous palladiumacetate (27 mg) also had no effect on the heatingbehaviour, whereas 50 mg of palladium (5%) on car-bon caused a significantly increased rate of heating.

These results suggest Pd EnCat does not causesuperheating of the bulk solution, and behaves morelike homogeneous palladium acetate than palladiumon carbon.

Palladium(II) for Cross-CouplingReactions

Considerable effort has been focused on the useof Pd EnCat to facilitate cross-coupling reactions(41). The extremely low leaching of metal species andease of handling of EnCat beads greatly simplifypurification of these reactions. Many examples havebeen published regarding the use of EnCats withmicrowave heating for the acceleration of specificreactions (49–52). An important advantage, often notconsidered, is improved safety when using EnCats ina microwave reactor. Deposition of a film of elemen-tal metal on the glass walls of microwave tubes byprecipitation from solution is a common problemwith conventional metal catalysts. This has beenshown not to occur with Pd EnCat (53). Where a

Platinum Metals Rev., 2008, 52, (2) 65

film is deposited, it absorbs microwave energystrongly, and hotspots can form, resulting in vesselfailure. With modern microwave reactor designssuch ruptures are well contained; however therelease of vapours and subsequent decontamina-tion pose serious issues. These can lead torestrictions on the use of particularly hazardousreagents.

By way of example, a useful palladium-mediatedmicrowave process is the carbonylation of arylhalides with solid sources of carbon monoxide(54). Molybdenum hexacarbonyl has been shownto be an effective carbon monoxide releasing agent(55, 56), however it is a very toxic substance withrelatively high volatility (57). The risk of vessel rup-ture in such procedures can be greatly reduced bysubstituting Pd EnCat for the traditional palladiumcatalyst. The reaction proceeds with quantitativeconversion as shown in Scheme I.

EnCats have been applied in flow chemistrywith the beads packed in simple columns and

reagents passed over them. The initial work inthis area is extremely promising for the process-intensification of homogeneous catalytic reactions(47, 58–60).

A low degree of leaching of the catalytic speciesis vital in a continuous process, in order to avoidrapid deactivation and resulting contamination ofthe product flow stream. Certain substrates areknown to induce leaching of palladium fromEnCat resins, with aryl iodides and alkynes show-ing a high propensity. Indeed, running themicrowave-assisted Sonogashira reaction inScheme II with Pd EnCat 30 resulted in productwith a palladium content of 83 ppm. The triph-enylphosphine-entrapped Pd EnCat (polyTPP30)resin demonstrates an extremely high retention ofboth the palladium and phosphorus ligand, and hasbeen used to great effect in the same reaction(Scheme II). Using Pd EnCat polyTPP30 as thecatalyst, the residual palladium concentration in theproduct was only 14 ppm.


0

20

40

60

80

100

120

140

160

180

0 62000 124000 186000 248000 310000 372000 434000

time (data points)

tem

pera

ture

(C)

tolueneblank beads0.48 mmol/g Pd EnCatpalladium acetate5% Pd/C

Time, 1000 data points

0 62 124 186 248 310 372 434

Tem

pera

ture

, ºC 5%Pd/C

PdEnCat(0.48mmol Pd g–1 )BlankbeadsToluenePalladiumacetate

Fig. 1 Rate of heating of toluene containing various dopants under microwave irradiation

Pd EnCat 30Mo(CO)6

DBU, THFMicrowave

120ºC30 min

Me

I

Ph+ H2N

Yield 98%

PhN

H

NH

Me

O

DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene

Scheme I


Palladium(0) for HydrogenationReactions

The nanoparticulate palladium(0) EnCatcatalyst has been demonstrated as a highlychemoselective hydrogenation and transferhydrogenation catalyst (61, 62). In additionto the improved selectivity shown by PdEnCat NP30, a superior safety profile andease of handling make it a powerful alterna-tive to palladium on charcoal.

Transfer hydrogenation with Pd EnCatNP30 is easily performed in the microwave,allowing reactions in minutes rather thanhours. A recent paper by Quai and coworkersdemonstrated the efficiency of microwave-assisted transfer hydrogenation for O-benzyldeprotection (Scheme III) (63). The use ofEnCat was recommended to improve thesafety of the process and reduce palladiumcontamination of the products.

Scheme IV shows a representative exam-ple of an aromatic nitro reduction. Thesereactions are conventionally carried out atambient temperature overnight (64). How-ever, the microwave transfer hydrogenationprocedure gave a quantitative conversion tothe final product in only 5 minutes.

Platinum(0) for Hydrogenation andReduction Reactions

To complement the palladium(0) EnCatrange, a platinum(0) EnCat has recently beendeveloped, offering the same benefits over itscarbon-supported equivalents as the palladiumversion: improved safety profile, ease of handlingand low metal leaching. Pt(0) EnCat 40 performssimilarly to Pt/C in hydrogenation reactions, andis particularly useful in selective reductions in thepresence of aryl chlorides. The reaction shown inScheme V gave 3-chloroaniline with > 98%selectivity at room temperature under an atmos-phere of hydrogen after one hour (65).Microwave-assisted hydrogenations have recent-ly been investigated (66), and equipment to runthem in the laboratory is becoming commerciallyavailable (67, 68). With microwave reactorsdesigned to meter pressures up to 15 bar and runat them, such technology offers the benchchemist simple, safe access to hydrogenation.

The microwave-assisted hydrogenation of 3-chloronitrobenzene shown in Scheme V was runusing a standard microwave vial. A hydrogenatmosphere (at slight positive pressure) wasintroduced via a needle and manifold cycledbetween vacuum and hydrogen from a lecture

Pd EnCat 30or polyTPP30

CuI, Et3N, THFMicrowave

140ºC20 min Yield 99%

Ph

Me

O

Ph

Me

O

I

+ Scheme II

Pd(0) EnCat NP30HCOONH4, DMF

Microwave (cooled)80ºC

10 minOPh

R

R = NH2, NHMe, COOH, CN, COR, heterocycle, etc.

HO

R

Scheme III

Yield > 99%

Pd(0) EnCat NP30HCOONH4, EtOH

Microwave80ºC5 min

HO HO

NH2NO2

Scheme IV


bottle. Following irradiation at a constant power(30 W) for 13 minutes all the starting materialwas consumed, giving 3-chloroaniline in 85%yield. With equipment designed to charge gas to agiven pressure and monitor the pressure drop, it isto be expected that this reaction could be opti-mised to higher selectivities.

Encapsulated Osmium Tetroxidefor Dihydroxylation Reactions

The osmium tetroxide-catalysed dihydroxyla-tion reaction is Nobel Prize-winning chemistry(69); however the routine use of osmium in thelaboratory is avoided where possible due to its tox-icity, the likelihood of contact due to its volatilityand its propensity to cause burns (70). Os EnCat40 is an encapsulated osmium tetroxide that issafer to handle because no osmium tetroxidevapour can escape the polymer matrix (44). TheEnCat acts as a reservoir of osmium tetroxide,releasing catalytic amounts under oxidation reac-tion conditions, but retaining sufficient activity forrecycling (71). Following the reaction only verylow levels of residual osmium are detectable in thereaction media. Os EnCat 40 has been successful-ly applied to asymmetric dihydroxylation reactions(72). To demonstrate the application of Os EnCat40 under microwave conditions, the simple dihy-droxylation in Scheme VI was carried out at 80ºCand was complete in 20 minutes. The correspond-ing reaction at ambient temperature, when allowedto proceed overnight, gave the product in 86%yield (73). With the reaction performed in a sealedmicrowave tube, the contents could be removed

via syringe with a fine filter fitting, minimising con-tact and potential hazards, and allowing routine,safe use of such chemistry.

ConclusionsMicrowave heating has expanded the arsenal of

synthetic methods available to the bench chemist.The use of encapsulated platinum group metal cat-alysts coupled with the inherently safe design ofmodern microwave apparatus enables safe accessto an even greater range of useful transformations.Such a synergistic combination of technologiesenables reactions to be performed that furnishclean products with very low levels of residualmetal, thus simplifying the preparation of complexmolecules.

Pt(0) EnCat 40H2, EtOH

Microwave 30 W13 min

Yield 85%

Cl NO2 Cl NH2

Scheme V

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Ph

Os EnCatNMO

H2O/acetone

Microwave80ºC

20 min

Ph

OH

Ph

OH

PhScheme VI

Yield 91%NMO = N-methylmorpholine N-oxide


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The AuthorMike Pitts obtained his first degree at LoughboroughUniversity, U.K., in 1997. Zeneca sponsored a project ondioxirane chemistry in his final year, following a successfulindustrial placement year as part of the degree. He then movedto the University of Exeter, U.K., to obtain a Ph.D. withProfessor Chris Moody on ‘Selective Reductions with IndiumMetal’. A postdoctoral stay with Professor Johann Mulzer at theUniversity of Vienna, Austria, followed, where he completed aformal total synthesis of laulimalide as part of a European

Network focused on antitumour natural products. Mike returned to the U.K. in 2002to work for StylaCats Ltd., a start-up company from the University of Liverpool,where he initiated and developed a microwave research platform. In September 2005he moved to Reaxa Ltd. in Manchester, a technology spin-out from Avecia, to developmicrowave processes with their proprietary catalysts. In August 2007 he took up hiscurrent position managing Sustainable Technologies at the Chemistry InnovationKnowledge Transfer Network.

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65 Results from Reaxa laboratories are available in

‘Pt(0) EnCatTM 40 User Guide’, Reaxa Ltd., March2007:http://www.reaxa.com/images/stories/Reaxa%20Pt(0)%20EnCatT%20User%20Guide_mar_07.pdf

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73 Results from Reaxa laboratories are available in‘User Guide – Catalytic Oxidations with OsEnCatTM Microencapsulated Osmium TetroxideCatalysts’, Reaxa Ltd.:http://www.reaxa.com/images/stories/reaxaosencatuserguide.pdf

1. IntroductionCompelling environmental and health-and-safety

demands are presently driving fundamental changein the design of chemical processes, especially thoseinvolving catalytic and/or highly hazardous reactions.The last few years have seen substantial progressin designing and implementing novel, clean andsustainable technologies, but considerable challengesremain for future academic and industrial research.

In this regard, the immobilisation of welldefined homogeneous catalytic complexes hasproved a beneficial strategy, combining the advan-tages of homogeneous and heterogeneous catalyticsystems (1–7). This technique offers multiple ben-efits for organic synthesis, such as simplification ofthe reaction scheme, greater control of processselectivity, better removal of the catalyst from thereaction products, the recycling of expensive cata-lysts, the possibility of designing continuous-flowprocesses on a large scale and, in polymer synthe-sis, the precise control of polymer morphology andbulk density in high polymers (8–14). However,immobilisation shares with heterogeneous catalysisthe major drawback of a diminished catalytic per-formance as compared with that of thehomogeneous counterpart. This effect is oftenattributed to non-uniform local concentration ofthe catalyst, limited access of reactants to the activesites and, in certain cases, to opposing groups on

the heterogeneous support or to steric effects ofthe latter.

The commonly applied methodology to trans-form a homogeneous catalytic reaction into aheterogeneous process involves anchoring theactive catalyst on a solid support possessing a largesurface area (15, 16). This procedure should notunduly affect the intrinsic catalytic properties ofthe complex, and the system should benefit effec-tively from the characteristics of both thedeposited catalyst and the solid support.

Recently, the coordination and organometallicchemistry of ruthenium complexes has seenunprecedented development, due to the emer-gence of the increasing potential of this class asefficient promoters of versatile catalytic processes(17–23). Most of these complexes possess anappropriate balance between the electronic andsteric properties within the ligand environmentand, as a result, exhibit attractive catalytic proper-ties; in particular enhanced activity, chemo-selectivity and stability in targeted chemical trans-formations (24–29).

Olefin metathesis, a most efficient transitionmetal mediated reaction for forming C–C bonds,has proved to be a powerful synthetic strategy forobtaining fine chemicals, pharmaceuticals and bio-logically active compounds, structurally complexassemblies, novel materials and functionalised


Practical New Strategies for ImmobilisingRuthenium Alkylidene Complexes: Part IIMMOBILISATION VIA PHOSPHANE, ALKYLIDENE AND N-HETEROCYCLIC CARBENE LIGANDS

By Ileana Dragutan* and Valerian Dragutan**Institute of Organic Chemistry “Costin D. Nenitescu”, Romanian Academy, 202B Spl. Independentei, PO Box 35-108,

060023 Bucharest, Romania; E-mail: *[email protected]; **[email protected]

The paper critically presents various routes for immobilising ruthenium alkylidene complexesthrough their ligands. This part (Part I) describes immobilisation via coordinating/actorligands (phosphane/alkylidene), and established ancillary ligands such as N-heterocycliccarbenes. Other ligands commonly encountered in immobilisation protocols, such as Schiffbases, arenes, anionic ligands and specifically tagged (ionic liquid tag, fluoro tag) substituentswill be the topic of Part II. Selected applications of some of these ruthenium complexes inolefin metathesis reactions are highlighted where they are particularly advantageous.

DOI: 10.1595/147106708X297477

polymers tailored for specific uses. Examples ofapplications for the latter include sensors, semi-conductors and microelectronic devices (30–36).Procedures such as ring-closing metathesis (RCM),ring-opening metathesis (ROM), cross-metathesis(CM), enyne metathesis and ring-opening metathe-sis polymerisation (ROMP), are sometimescombined in tandem with non-metatheticalprocesses. This has resulted in broad diversifica-tion towards progressive technologies and newperspectives for industrial applications (37–42).Advances have mainly been due to the discoveryof a wide range of functional group-tolerant ruthe-nium alkylidene complexes, resistant to air andmoisture, bearing appropriate ancillary ligandssuch as phosphanes (1 and 2, R = phenyl (Ph) orcyclohexyl (Cy)), N-heterocyclic carbenes (3 and4), Schiff bases (5 and 6) or arene groups (7)(Scheme I).

Although some of these complexes exhibit agood selectivity profile and activity in the freestate, immobilising them on organic or inorganicsupports has emerged as an improvement in theircapability for ‘green’ metathesis chemistry, enhanc-ing their potential as clean, recyclable and highlyefficient catalysts (43–47). Most frequently, theruthenium complexes 1–7 have been immobilisedby binding one of their stable ligands to the sup-port (48–51). Both anionic and neutral ligands

have so far been employed. Table I summarisescurrently well developed methods for immobilisingruthenium metathesis catalysts.

2. Immobilisation via the PhosphaneLigand

Since the first well defined and widely appliedhomogeneous ruthenium metathesis catalystsincorporated phosphines as ligands, it was not sur-prising that immobilisation through the phosphanewas tried first. It was obvious that while perfor-mance of the resulting catalyst depends on releaseof the active species into solution, its recyclabilityis strongly affected by the poor ability of thebound phosphine to recapture the ruthenium.Consequently, disadvantages associated with thismode of immobilisation were to be expected.

An early report on the immobilisation of ametathesis catalyst was by Nguyen and Grubbs(52), who anchored the homogeneous Ru vinyl-carbene complex 1 (R = Ph or Cy) on a polystyrenesupport through both its phosphane ligands,obtaining the well defined immobilised complexes8–10 (Scheme II).

Despite the apparent practical advantages ofapplications of precatalysts 8–10 in metathesis ofcis-2-pentene and polymerisation of norbornene,the activity of these precatalysts was found to be atleast two orders of magnitude less than that of the


Ru

PR3

PR3

Cl

Cl

Ph

HRu

PR3

PR3

Cl

Cl

Ph

Ph

NNMes Mes

ClCl

Ru

O

Ru

PCy3

Cl

Cl Ph

NNMesMes

4321

RuCl Ph

PhPCy3

PF6Ru

Cl

O

N

Br

RuPh

PCy3

ClO

N

Br

5 6 7

Ru RuRu

RuPh

PhPF6

Ph

PhPh

Ph

PR3

PR3 PR3

PR3

PCy3

Cl

Cl

ClClCl

Cl

Cl

Cl

H

N NNN

MesMesMesMes

PCy3 O

ClRu

Scheme I Homogeneous metathesis ruthenium complexes suitable for immobilisation on solid supports

homogeneous complex 1. This result was ratio-nalised in terms of the detrimental effect of thetwo chelated phosphane ligands on the dissocia-tive reaction pathway, and the need for thesubstrate to diffuse into the polymer cavities.Subsequently, immobilisation of complex 2,through only one of its phosphane ligands, to givecomplexes 11 and 12, was reported by Verpoort etal. (53). A phosphinated mesoporous aluminosili-cate matrix (P-MCM-41) was used as the solidsupport (Scheme III).

Gratifyingly, the immobilised catalysts 11 and12 displayed good activity in norbornene poly-merisation (yield up to 70%) and very high activityin RCM of diallylamine and diethyl diallylmalonate(yield up to 100%). Moreover, catalyst 12 wasactive even in an aqueous environment. Since, bycontrast with complexes 8–10, in catalysts 11 and

12 the Ru-alkylidene entity is grafted onto the sup-port through only one phosphane ligand, thedissociative mechanism of the metathesis reactionis favoured in this case.

3. Immobilisation via the AlkylideneLigand

A remarkable innovation came with the designof the so-called ‘boomerang’ catalyst 13 (54), inwhich the ruthenium complex is anchored ontothe vinyl polystyrene support (vinyl-PS resin)through its alkylidene ligand (Scheme IV).Polymer-supported catalyst 13 was readilyobtained by CM of vinyl polystyrene with theruthenium complex 2, and was isolated as anorange-brown solid, after filtration and washing.Catalyst 13 was found to be effective in RCM, itsactivity being comparable with that of the homo-

Ru

Cl

ClPh

PhPPh2

PPh2n

PCy2

PCy2

RuCl

ClPh

Ph

n


CH2

CH2

PCy2

PCy2

RuCl

ClPh

Ph

n10

98n n

n

PPh2

PPh2

PCy2

PCy2

PCy2

PCy2

CH2

CH2

Scheme IIImmobilised vinyl-carbene rutheniumcomplexes 8–10

Table I

Approaches for Immobilisation of Ruthenium Metathesis Catalysts

Mode of immobilisation Section† References

Immobilisation via the phosphane ligand I/2 52, 53 (Part I)

Immobilisation via the alkylidene ligand I/3 54–63 (Part I)

Immobilisation via the N-heterocyclic carbene (NHC) ligand I/4 64–84 (Part I)

Immobilisation via the Schiff base ligand II/1 Part II

Immobilisation via the arene ligand II/2 Part II

Immobilisation via anionic ligands II/3 Part II

Tagged ruthenium alkylidene complexes II/4 Part II

†I = Part I (this paper); II = Part II, to be published in a future issue of Platinum Metals Review

Ph

Ph

Ph

Ph

Ph

Ph

geneous catalyst 2. It was suggested that during theinitial reaction step with the diene substrate theactive catalytic species becomes detached from thevinyl polystyrene support, acts then as a homoge-neous RCM catalyst in solution and, after all of thediene has been consumed, reattaches itself to thevinyl polystyrene support. Under these conditions,the inhibiting necessity for reactants to diffuse tothe active sites of the immobilised complex is fullyeliminated, and the advantages of a homogeneouscatalytic system are enjoyed. Catalyst 13 could berecycled several times by simple filtration, and theresidual ruthenium in the product mixture wasconsiderably reduced, as compared with the caseof the homogeneous catalyst 2 (55). Improvedimmobilised ruthenium alkylidene complexes havesubsequently been reported by Nolan (56–58) andBarrett (50).

The increased strength of the coordinativeRu–O bond in catalyst 4 (of the Hoveyda type)could render such catalysts even more suitable forimmobilisation. Indeed, a highly efficient polymer-bound, recyclable catalyst 14 has been prepared byBlechert et al. (59) via ROMP of the norbornene

derivative 15 in the presence of complex 3(Scheme V). The procedure has been furtherextended to the synthesis of the supported catalyst16, where an oxanorbornene benzoate co-monomer was employed in conjunction with 15and the ruthenium complex 3 (59) (Scheme VI).

Excellent conversions have been obtained inRCM of a variety of diene substrates, leadingreadily to five-, six-, seven- and higher-ring carbo-and heterocyclic compounds. It is important tonote that the recyclability of catalysts such as 16 inmetathesis reactions is remarkable. Catalyst 16affords high conversions of diallyl tosyl amide to1-tosylpyrroline (> 98%), even after seven reac-tion cycles, and complete recovery of the catalystwas possible (59). The synthesis and olefinmetathesis activity in protic solvents of a new,phosphine-free ruthenium alkylidene 17, bound toa hydrophilic PEGA resin support (PEGA =polyethylene glycol amine), has been reported byConnon and Blechert (60) (Scheme VII). Thisheterogeneous catalyst promotes relatively effi-cient RCM and CM reactions in both methanoland water.


OH

OH

OH

Si (CH2)x PR2

EtO

EtO

EtO

O

O

O

Si (CH2)x PR2

O

O

O

Si (CH2)x PR2 Ru PR3

Cl Cl

Ph

R3P Ru PR3

Cl Cl

PhO

O

O

Si (CH2)x PR2

11 (x = 2)12 (x = 3)

x x

x x

Scheme III Synthesis ofzeolite-supported rutheniumcomplexes 11 and 12

Ru

PCy3

PCy3Cl

Cl

PhRu

PCy3

PCy3Cl

Cl

1 - 2 hCH2Cl2,

Vinyl-PS Resin 13

1–2 hVinyl-PS resin

Scheme IV Synthesis of theimmobilised ruthenium ‘boomerang’complex 13

+

On using an appropriate linker (generated byCM from the styryl ether 18, and allyl-dimethylchlorosilane), Hoveyda and coworkers(61) bound the resulting isopropoxy benzylideneRu complex 19 on a monolithic sol-gel, thuspreparing in an advantageous ‘one-pot’ procedurea series of highly active and recyclable supportedRu complexes 20–22 (Scheme VIII and SchemeIX). Practically, these supported catalysts providedproducts in RCM and tandem ROM/CM that are

of excellent purity, even before silica gel chro-matography or distillation. They are readilyemployed in combinatorial synthesis in air andwith reagent-grade commercial solvents.

An interesting soluble polymer-bound rutheni-um alkylidene catalyst 23 was prepared by Lamatyet al. (62) through exchange of the benzylidene unitfrom the commercially available Grubbs catalyst 3with the supported ligand 24 (PEG = polyethyleneglycol) (Scheme X). This catalyst was fully charac-


O O

O i P r R u P C y 3

N N M e s M e s

P h C l C l

C H2 C l 2 , C u C l

O O

O

O i P r

O O i P r R u C l

C l

N N M e s

M e s

x y n Ru

PhPCy3Cl

Clnx

Mes MesN N

CH2Cl2, CuCln(x + y)

OiPr

OO

Mes

Mes

NN

RuClCl

OiPr O O

O

OiPr

O

y

n15 14 (x:y = 1:99)

x

Scheme V Synthesis of immobilised NHC ruthenium complex 14

n

CH2Cl2, CuCl

Ru

PCy3

NNMes Mes

PhCl

Cl

OO

OiPr O

O2CPh

+ y + z

15

OiPr

16 (x:y:z = 1:9:30)

OO

O

O2CPh

nz

RuCl

ClPCy3

Ph

Mes N N Mes

n

CH2Cl2, CuCl

Mes

Mes

Ru

ClCl

N

N

OiPr

OiPr

O O OO

OOO

x y z

n

n(x + y)

Scheme VI Synthesis of immobilised NHC ruthenium complex 16

terised by solution nuclear magnetic resonance(NMR) spectroscopy and matrix-assisted laser des-orption/ionisation (MALDI) mass spectrometry,and tested in RCM reactions. It proved to be par-ticularly active and could be used in the parallel

synthesis of cyclic amino esters. Most significantly,catalyst 23 could be recovered and recycled; 1HNMR analysis provided key information concern-ing the recovery of the catalyst at the end of thereaction.


O

OHO

O

NO

RuCl

Cl

N NMesMes

O

NO

Cat 3

PEGA Resin

Catalyst 3

17 PEGA resin

Scheme VII Synthesis of immobilised NHC ruthenium complex 17

O O

O R u P C y 3

N N M e s M e s

P h C l C l

S i M e 2 C lC H 2 C l 2 , 2 2 ° C ,

1 h O O

R u O

N N M e s M e s C l C l

S i M e 2 C l CH2Cl2, 22ºC, 1 h

18 19

RuClCl

NN MesMes

Mes MesNN

ClCl

RuO

O

O O

O

O

PCy3

SiMe2Cl

SiMe2Cl

O O

R u O

C l C l

S i M e 2 O S i

O O

S iO O

O R S i O R

O

M e s M e s N N

C H 2 C l 2 4 0 ° C , 5 d

O H S i

O O

S i O O

O H S i O H

O

SiMe2

Si Si Si

SiSiSi

O

OO

O OO

O

O

OO

OO

OO

OROR

OHOH OH Mes

RuCl

N

Cl

Mes N

CH2Cl2, 40ºC, 5 d

20

Ph

Scheme VIII Synthesis of the supported NHC ruthenium complex 20

The synthesis of a highly efficient, fluorine-con-taining, immobilised metathesis catalyst 25, derivedfrom the Grubbs second-generation rutheniumalkylidene complex 3, has been described by Yao(63) (Scheme XI). The air-stable polymer-boundruthenium alkylidene complex 25 showed highreactivity in RCM of a broad spectrum of dieneand enyne substrates, leading to the formation of

di-, tri-, and tetrasubstituted cyclic olefins in “min-imally fluorinated solvent systems”(PhCF3/CH2Cl2, 1:9–1:49 vol./vol.). The catalystcould readily be separated from the reaction mix-ture by extraction with FC-72 (perfluoro-n-hexane)and repeatedly reused. The practical advantage ofrecyclability offered by this fluorinated catalyst hasbeen demonstrated by its sequential use in up tofive different metathesis reactions (63).

4. Immobilisation via the NHCLigand

Immobilisation via the NHC ligand capitaliseson the NHC’s characteristic of generally formingstrong σ-bonds with the metal (64–70); conse-quently, these ligands have been successfullyemployed as suitable linkers for anchoring metalcomplexes onto solid supports. This propensity


Me2Si

O

O Ru

NArArN

Cl

ClO

SiO OO

SiSiO O

OROR

H

H

SiO OO

SiSiO O

OROR

OClCl

ArN

NAr

Ru

ArN

NAr

ClCl

O

Ru

O

O

H

H

OMe2Si

RuRu

Ru

ClCl

Cl

Cl

Cl

Cl

SiSiSiSi

SiSi

Me2Si Me2Si

O

O

O

OOO

O

OO

O

OOO

O

O

O

O

O

OR OROROR

NAr

ArN

ArN

NArArNNAr

H

HH

H

21 22

Scheme IX Supported NHC ruthenium complexes 21 and 22 (Ar = 2,4,6-trimethylphenyl)

PEG ORu

PCy3

Cl

Cl Ph

NN

RuCl

Cl

NN

O

PEG

Ph

CH2Cl2

(CuCl)RuO

O

PEG

RuCl

Cl Cl

Cl

NNNN

PhPhPCy3

CH2Cl2

(CuCl)

24 3 23

+ +PEG

Scheme X Synthesis of soluble polymer-bound NHC ruthenium complex 23

Ru

NNMes MesCl

ClO O

O FluorousPolyacrylate

25

Fluorouspolyacrylate

Scheme XI Fluorine-containing polymer-boundruthenium alkylidene complex 25

has been exploited by Blechert (71) to prepare apermanently immobilised and highly active NHCruthenium benzylidene complex 26, by attaching 2to a polymeric support through an NHC ligand.The approach consisted in synthesising first a suit-ably immobilised precursor 27, starting from thediamine A (Scheme XII). Compound A, preparedfrom 2,3-dibromo-1-propanol and 2,4,6-trimethyl-aniline, was attached by an ether linkage, afterdeprotonation of the hydroxyl group, to Merrifieldresin (polystyrene crosslinked with 1% divinyl ben-zene (DVB)), yielding quantitatively theimmobilised diamine B; this diamine was cyclisedunder acidic conditions and, after anion exchange,gave the support-bound 1,3-dimesityl-4,5-dihy-

droimidazolium salt 27. Precursor 27 was convert-ed into the protected carbene 28 (2-tert-butoxy-4,5-dihydroimidazoline), which through insitu deprotection in the presence of the diphos-phane ruthenium benzylidene complex 2 (with R =Ph) yielded the support-bound NHC rutheniumcomplex 26.

Immobilised complex 26 proved to be an excel-lent precatalyst for various metathesis reactions. Itcleanly cyclised diallyl or dihomoallyl derivatives tothe respective carbocycles and heterocycles, inhigh yields (90 to 100%). Macrocyclic and dicyclicarchitectures were also accessible in considerableyields (80 to 100%), starting from the correspond-ing α,ω-dienes (Scheme XIII). It is remarkable that


Cl

PS-DVBO

N N

H

OH

NH

NH

O

NH

NH

PS-DVB

b.PS-DVBa.KOtBu

DMF

c.HC(OMe)3,HCO2H Toluene

d.HCl/THF

O

N N

H

O

N N

O t B u H

C l a . T M S O T f , C H 2 C l 2 R T , 3 0 m i n

b . K O t B u , T H F R T , 6 0 m i n

O

N N

R u P h P C y 3

C l C l

O

N N

O t B u H

P h R u

C l C l

P C y 3

P C y 3

T o l u e n e , 7 0 - 8 0 ° C

OH

NH (a) KOtBu(b) PS-DVB

DMFNH

NH

NH

NN N N

N N

N NNN

OO

OO

O O

(c) HC(OMe)3, HCO2HToluene

(d) HCl/THF

PS-DVBPS-DVB

H

PS-DVB

HH

H

Cl

Cl

Cl

ClOtBu

OtBu

RuPh

PCy3

PCy3

RuPhPCy3

Toluene, 70–80ºC

(a) TMSOTf, CH2Cl2RT, 30 min

(b) KOtBu, THFRT, 60 min

A B 27

27 28

28 26

PS-DVB PS-DVB

PS-DVB

Scheme XII Synthesis of the immobilised NHC ruthenium complex 26 (TMSOTf = trimethylsilyltrifluoromethanesulfonate)

Cl

Cl

enantiomerically pure α,ω-dienes could rearrangequantitatively in the presence of 26 and ethyleneinto new compounds of high enantiomeric purity(Scheme XIV).

In addition to ring closing, some demandingenyne cross-metatheses have readily been per-formed, to produce functionalised 1,3-dienes inhigh yield by a simple and efficient atom econom-ical procedure (71) (Scheme XV).

In the context of experimental endeavours in‘green’ chemistry, novel water-soluble ruthenium-based olefin metathesis catalysts (29 and 30),supported via poly(ethylene glycol)-NHC ligands,have recently been introduced by Grubbs andcoworkers (72, 73) (Scheme XVI). These soluble

catalysts display greater activity in aqueous RCMand ROMP than do other previously reported(74–77) water-soluble metathesis catalysts.Significantly, RCM and ROMP with 29, in proticsolvents (such as methanol), proceeded compara-bly to reactions with the earlier water-solublecatalysts. It is impressive that catalyst 30 provedhighly active in RCM of α,ω-heterodiene salts inwater, giving substantial yields (95%) of the corre-sponding heterocyclic structures (Scheme XVII).Related water-soluble, immobilised rutheniumalkylidene complexes have been devised by Yao(78), Bowden (79) and Gnanou (80) and success-fully applied in RCM of dienes and ROMP ofnorbornene.


Scheme XIII Synthesisof macrocyclic anddicyclic compoundsvia RCM usingcatalyst 26(15 = 15-memberedring; Ns = p-nitrobenzenesulfonyl)

O

O7 3

O

O

15

E / Z = 1.4

NN

Ns Ns

N N

Ns Ns

80%

100%

CH2Cl2,

CH2Cl2,

26 (5 mol%)

26 (5 mol%)

45 oC

45 oC

Yield 100%

45ºC

Yield 80%

45ºC

E/Z

100%

CH2Cl2,

26 (5 mol%)

45 oCTs

OTBS

H

Ts

HOTBS

C2H4

45ºC

Yield 100%

Scheme XIV Synthesis of enantiomericallypure compounds using catalyst 26 (Ts =tosyl; TBS = (tert-butyl)dimethylsilyl)

RO SiMe3

SiMe3

RO SiMe3

SiMe3

CH2Cl2,

CH2Cl2,

26 (5 mol%)

26 (5 mol%)

100%

80%

45 oC

45 oC

E/Z = 1.2

E/Z = 1.6

(3 equiv.)

(3 equiv.)

3 345ºC

45ºC

E/Z

E/Z

Yield 100%

Yield 80%

Scheme XVSynthesis offunctionalised1,3-dienes bycross-metathesisusing catalyst26

+

+

+

+

Immobilisation of a ruthenium complexthrough its NHC ligand, as in 31, has beenachieved by Buchmeiser et al. (81) by an interestingapproach using a monolithic support; the latterwas modified by ROMP of norbornene, or itsfunctionalised derivatives, in order to be suitablefor anchoring the homogeneous complex (Scheme XVIII).

Another well designed strategy, introduced bythe same group, employs a silica-based support tocreate immobilised NHC Ru complexes (82).Various polymer monolithic materials have alsobeen ingeniously applied to heterogenise welldefined Ru complexes (83, 84).

ConclusionOverall, this first part of the survey convinc-

ingly illustrates that ruthenium alkylidenecomplexes can be effectively immobilised ontosolid and soluble polymers by various routes.These capitalise on beneficial attributes of boththe catalysts’ actor/spectator ligands and theirsupports. This strategy has emerged as animprovement in the catalysts’ capability for‘green’ metathesis chemistry, enhancing theirpotential as clean, recyclable and highly efficientcatalysts and paving the way for scaling up toindustrial applications.

The concluding paper of this series, Part II, willbe published in a future issue of Platinum MetalsReview; see Table I for the projected topics in Part II.

Note added in proof : When certain types ofimmobilised catalyst are used for olefin metathesis,ruthenium byproducts may be removed from theproducts by simple aqueous extraction (85).


Scheme XVI Water-solubleruthenium-based olefinmetathesis catalysts 29 and30

H2NH2N

ClCl

30

H2O> 95%

(5 mol%)

Yield > 95%

Scheme XVII RCM of α,ω-heterodiene salts in waterwith immobilised ruthenium complex 30

OO

N N

Ru

PCy3

Ad Ad

PhClCl

Monolithicsupport

n mn m

Monolithicsupport

31

Scheme XVIII NHC ruthenium complex immobilised onmonolithic support 31 (Ad = adamantyl)

N N

RuPhCl PCy3

Cl

N

OMeO

OO

Hn

29

N N

RuClCl

O30

n

nO O OMe

Cl

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31 “Metathesis Chemistry: From NanostructureDesign to Synthesis of Advanced Materials”, eds. Y.Imamoglu and V. Dragutan, NATO Science SeriesII: Mathematics, Physics and Chemistry, Vol. 243,Springer Verlag, Berlin, Heidelberg, 2007

32 K. J. Ivin and J. C. Mol, “Olefin Metathesis andMetathesis Polymerization”, 2nd Edn., AcademicPress, London, 1997

33 V. Dragutan, M. Dimonie and A. T. Balaban,“Olefin Metathesis and Ring-OpeningPolymerization of Cycloolefins”, John Wiley &Sons, Chichester, New York, 1985




37 “Handbook of Metathesis”, ed. R. H. Grubbs, in 3vols., Wiley-VCH, Weinheim, Germany, 2003, Vols.II–III

38 V. Dragutan and I. Dragutan, J. Organomet. Chem.,2006, 691, (24–25), 5129

39 D. E. Fogg and E. N. dos Santos, Coord. Chem. Rev.,2004, 248, (21–24), 2365

40 K. C. Nicolaou, P. C. Bulger and D. Sarlach, Angew.Chem. Int. Ed., 2005, 44, (29), 4490

41 D. E. Fogg and H. M. Foucault, in “ComprehensiveOrganometallic Chemistry III”, in 13 vols., eds. R.Crabtree and M. Mingos, Elsevier, Amsterdam,2006, Vol. 11, pp. 623–652

42 V. Dragutan and R. Streck, “CatalyticPolymerization of Cycloolefins – Ionic, Ziegler-Natta and Ring-Opening MetathesisPolymerization”, Studies in Surface Science andCatalysis, Vol. 131, Elsevier, Amsterdam, 2000


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(10), 1955


The AuthorsIleana Dragutan is a Senior Researcher at theInstitute of Organic Chemistry “Costin D.Nenitescu” of the Romanian Academy. Herinterests lie in the synthesis of stable organicradicals, EPR spin probe applications inorganised systems and biologicalenvironments, late transition metal complexeswith radical ligands, ruthenium catalysis inorganic and polymer chemistry, iminocyclitolsand prostaglandin-related prodrugs.

Valerian Dragutan is a Senior Researcher at theInstitute of Organic Chemistry “Costin D.Nenitescu” of the Romanian Academy. Hisresearch interests are homogeneous catalysisby transition metals and Lewis acids; olefinmetathesis and ROMP of cycloolefins;bioactive organometallic compounds; andmechanisms and stereochemistry of reactionsin organic and polymer chemistry. He is amember of several national and internationalchemical societies, and has contributedsignificant books, book chapters, patents andpapers to the scientific literature.

Catalysis is behind virtually every chemical weuse today, and is involved in some way in virtual-ly every consumer product on the market. Thishas been true for a century or more, and will con-tinue to be true for the foreseeable future. Thelast century saw catalyst scientists and technol-ogists master the conversion of crude oil into anenormous array of chemicals, using solid-phaseacids such as zeolites to convert the original oilinto a range of more useful products. These couldthen be elaborated using systems such as redoxcatalysts, metal-centred coupling catalysts orenzymes, all of which provide excellent controlover reactivity and selectivity.

As a consequence of the uncertainties overcrude oil supply, this century is likely to see afocus on converting renewables, for examplebiopolymers such as starch and cellulose, intomore sophisticated molecules. The C:O ratio ofthese molecules is typically 1 (compare with crudeoil, where it is > 100), and that of the desiredproducts is typically between 3 and 10. Thismeans that the processes required to convertthese feedstocks will involve different blends ofchemistries, with reduction perhaps becomingmore central than oxidation as a conversion tech-nology. Reduction catalysis will be expected toplay a very prominent role, with the outlook forpalladium, platinum and related reduction cata-lysts in deoxygenation type chemistry being veryinteresting. Catalysis will certainly retain its cen-tral importance to chemistry.

Additionally, the role of chemistry in ‘green-ing’ existing processes will drive the developmentof more efficient and selective catalysts, and theirmore effective use. Improved processing, separa-tion and recovery are key concepts, and reducedenergy costs will also be vital.

For these reasons, this book is very valuable,

as it pulls together all the main catalytic technol-ogies, with a focus on green chemistry andprocessing. The book is structured to cover thekey areas of catalysis, with chapters on solid acidsand bases, oxidation (including ruthenium cata-lysts), reduction, C–C bond forming (includingpalladium and ruthenium catalysts) and hydro-lysis. Other important aspects are also covered;for example, the chapter on alternative reactionmedia covers developments in solvent choice. Inrecent years, many new solvent types have beendeveloped in order to improve the environmentalimpact of processes, since solvents are typicallyby far the largest components of a reaction mix-ture and, given their volatility, are one of thehardest parts to control. The newer solventsinclude ‘typical’ organic solvents with lower toxi-city, plus novel systems such as fluorous solvents,supercritical media and ionic liquids. Biphasiccatalysis is also covered here. These newer sol-vents are the focus of intense research activityand are likely to find uses in industry; a fewprocesses already use them.

Biocatalysis is also an emerging theme, andmay find itself more suited to transformations ofbiomolecules (which are given a chapter) thanpetrochemicals. Process design and integration isalso given a chapter, as it can produce significantimprovements by intelligent combination of twocatalytic steps.

The book deals with chemical catalysis andbiocatalysis as two parts of the same whole,something which is pleasing to see. The twoparts are often seen as competing, whereas theycan be combined to great effect. The coverage ofthe area is well rounded and the chapters are upto date, well written and referenced. Overall, thisis an impressive book, and a valuable additionto the field.

Platinum Metals Rev., 2008, 52, (2), 83 83

“Green Chemistry and Catalysis”BY ROGER A. SHELDON, ISABEL ARENDS and ULF HANEFELD (Delft University of Technology, The Netherlands), Wiley-VCH,

Weinheim, Germany, 2007, 448 pages, ISBN 978-3-527-30715-9, £95.00, €142.50, U.S.$190.00

Reviewed by Duncan MacquarrieDepartment of Chemistry, University of York, Heslington, York YO10 5DD, U.K.; E-mail: [email protected]

DOI: 10.1595/147106708X299646

84

For achieving novel reactions in organic elec-trochemistry the design and construction of anideal, multi-purpose electrode remains a perennialgoal (1). Within the cathodic range, the use of mer-cury is now banned for environmental reasons (2).Platinum, owing to both its cost and its weakhydrogen over-voltage, is difficult to use over awide cathodic domain. Various carbon interfaces(such as graphite, glassy carbon and carbon felts)may provide useful working electrodes but they arenot always inactive. Their neutrality towards elec-trochemical insertion of ions as well as theirtendency to graft free radicals have been noted (3).Electrodes modified with functionalised conduc-tive polymers have been claimed to be useful ininterfacial synthesis since they can mimic some ofthe mechanisms of organic chemistry on solid sup-ports (4). Consequently, some new insights inelectrochemical synthesis may be linked to the

development of solid electrodes with very specificproperties. Following this approach, pure silver (5)as well as other solid metal electrodes modified byadatoms (6) could offer interesting prospects whentailored to specific reactions.

Two-electron cathodic cleavage reactionsinvolving halides, sulfones, sulfonamides or tosy-lates are of importance in organic synthesis, sincethey can be applied to deprotection processes, seefor example (7). However, most of these reactionshave been reported as slow electrochemicalprocesses, depending on the electrochemicalpotential necessary to cleave the carbon–hetero-atom bonds. Such cleavage reactions have beenshown to occur at quite negative potentials – basi-cally lower than –2 V vs. SCE. Thus the use ofaqueous or water-wet organic solvents is inappro-priate when considering most solid metals for useas cathodes.

Platinum Metals Rev., 2008, 52, (2), 84–95

A Disordered Copper-Palladium AlloyUsed as a Cathode MaterialTHE ONE-ELECTRON CLEAVAGE OF CARBON–HALOGEN BONDS

By Philippe Poizot and Lydia Laffont-DantrasLRCS, UMR 6007, Université de Picardie Jules Verne, 33 rue Saint-Leu, 80039 Amiens Cedex, France

and Jacques SimonetLaboratoire MaSCE, UMR 6226, Université de Rennes 1, Campus de Beaulieu, 35042 Rennes Cedex, France;

E-mail: [email protected]

A novel method of forming a palladised copper (Cu/Pd) interface of well defined structure isdescribed. The CuPd alloy is straightforwardly obtained by immersing a copper substratein acidic solutions of palladium salts. Depending on the composition of the salt/acid solution,the copper surface is virtually instantly covered with a CuPd deposit. With nitric and sulfuricacid solutions and the corresponding Pd(II)-based salt, the deposit is composed of nanoparticlesof disordered CuPd alloy dispersed at the copper interface. The alloy-modified surface wassuccessfully used as an efficient promoter of bond cleavage reactions, especially those ofcarbon–iodide and carbon–bromide bonds in alkyl halides. The catalytic activity is specificallycharacterised by a very large shift in potential as between the use of a regular glassy carbonsurface and the palladised copper interface. With alkyl halides (RBr and RI), the shift towardless cathodic potentials is so large that it enables the one-electron cleavage of C–I andC–Br bonds. This method should enable the heterogeneous generation of free alkyl radicalsas transients in electrochemical reactions. These novel cathodic materials could also be ofconsiderable interest for the disposal of halogenated waste.

DOI: 10.1595/147106708X292517

The tailoring of surfaces by means of a specif-ic deposit of catalyst is an alternative approach tonovel electrode design. Here, the activationpotential may become so large as to transformthe nature of the cathodic reactions. Largepotential shifts can be observed, which in princi-ple enable the overall reaction process to befundamentally changed. Following thisapproach, we have developed a very convenientmethod to produce a modified electrode basedon a copper-palladium alloy (8). Preliminaryresults have shown that the as-obtained Cu/Pdinterface appears particularly efficient in acceler-ating the cleavage of carbon–halogen bonds.These cleavage reactions have often been notedas possessing a high activation energy to achievethe first electron transfer (9–14).

In the present paper, we intend to fully definethe characteristics of the copper-palladium layerproduced onto copper substrates by displace-ment reactions from several Pd(II)-basedprecursors, and to investigate the efficiency ofthe as-produced surface towards the electro-chemical cleavage of several alkyl halides RX(with X = Cl, Br and I). At regular solid metallicelectrodes, these cleavages are commonly report-ed to occur with very large activation energies. Aseries of organic halides were therefore used asprobes to compare their cleavage modes as con-ventionally observed at glassy carbon electrodes(GCEs) with those at palladised surfaces. Thepalladised surfaces were prepared by electro-chemical deposition (onto platinum or glassycarbon substrates under the same experimentalconditions). The advantages of this Cu-Pd sur-face, such as its great catalytic activity, itsstability, and its simplicity of synthesis will bepresented. The use and specificity of this newinterface are discussed in terms of potential shiftvalues related to its catalytic efficiency. It mustbe borne in mind that these interfaces specifical-ly promote unexpected one-electron processes,which involve the transient formation of freealkyl radicals. With aryl halides, however, reduc-tion processes retain a two-electron mechanism.This is in agreement with the observed high reac-tivity of aryl radicals (15, 16).

ExperimentalFormation of the Palladised CopperInterface

The copper/palladium interface was simplyprepared by dipping into a fresh acidic solution ofPd(II) for 15 s a copper substrate (grid or sheet)previously cleaned with acetone. Three differentsolutions were prepared by dissolving a palladiumsalt Pd(Yn–)2/n (with Y = SO4

2–, NO3– and Cl–) into

the corresponding acid. For example, 1 g of palla-dium(II) sulfate dihydrate (Pd(SO4)·2H2O) (AlfaAesar) was dissolved in 100 cm3 of 0.1 N H2SO4

solution. The dipping procedure produced a virtu-ally instant deposit onto the copper surface, due tothe displacement of copper by palladium cations,together with the unexpected formation of a pal-ladised copper interface. The shiny layer appearedto be quite stable, and sonication had no visibleeffect on its adhesion to the copper substrate. Toprevent any residue of acidic impurities more orless strongly adsorbed onto the surface, a prelimi-nary cleaning step is recommended; the modifiedelectrode is dipped into a dilute aqueous solutionof tetramethylammonium hydroxide, followed byrinsing with water, alcohol and finally acetonebefore drying with a hot air flow (at about 60ºC).Such electrodes were easily reused, giving coherentdata, since they were rinsed regularly following theabove procedure.

Texture and Structure AnalysisModified surface samples were first examined

by X-ray diffraction (XRD) measurements atroom temperature. However, the nanometric scaleof the deposits made it difficult to identify the as-produced material correctly. To preciselydetermine the structure/texture of thecopper/palladium interface, investigations weremade using high-resolution transmission electronmicroscopy (HRTEM). Commercially availablecopper grids for electron microscopy were used asthe substrate to study the Cu/Pd interface. Threesamples were prepared for TEM investigation bydipping a copper grid into a fresh acidic solutionof Pd(Yn–)2/n. Electron-transparent specimenswere obtained. The TEM and HRTEM imagingwere performed using a FEI Tecnai F20 S-TWIN


microscope. The elemental composition was alsodetermined by energy dispersive spectroscopy(EDS) at nanometre resolution. The diffractionpatterns were obtained using the selected area elec-tron diffraction (SAED) mode or by Fouriertransform of the HRTEM imaging.

Electrochemical Procedures: Salts andSolvents

In all the electrochemical experiments, tetra-n-butylammonium tetrafluoroborate (TBABF4) wasused as the supporting salt at a fixed concentrationof 0.1 M. Its purity (at least 98%, Aldrich) wasconsidered suitable for the experiments; there wasno further purification. The dimethylformamide(DMF) solvent (SDS, France) was typicallyemployed without drying. However, if ultra-drysolutions were required, DMF stored over activat-ed alumina was used. Alumina activation was byheating at 340ºC under vacuum overnight.Alumina could be added into the electrode cell ifnecessary, and this in situ drying technique gave amoisture level well below 100 ppm. It is worthmentioning that the procedures given below donot require extremely dry solutions. If one wishesto reach potentials as low as –2 V vs. SCE, thesolution could be dried more efficiently to avoidhydrogen evolution via the reduction of residualwater, thereby increasing the electrical yield of theoverall organic cleavage. The organic halides (RX)used in the present work were purchased fromAldrich (minimum purity 95%) and used as sup-plied.

All electrochemical experiments were per-formed under an inert atmosphere (dry argon)using a three-electrode cell with a glass separator,as described elsewhere (8). Potential values givenin this study are quoted versus SCE. The electrodesused here had an apparent surface area ofS = 0.8 mm2, except for those using copper as asubstrate (S = 1.6 or 3.2 mm2). Glassy carbon, purepalladium disc and copper electrodes were alwayscarefully polished with silicon carbide paper(Struer) or with Norton polishing paper (grades 02and 03). Before use, the conventional workingelectrodes were rinsed twice with water, then alco-hol and finally acetone before drying with a hot air

flow. Palladised electrodes (including those usedfor comparison purposes) were prepared by a gal-vanostatic deposit of Pd from a palladium chloridesolution onto several types of metallic substrates(platinum, gold or palladium). The plating bathcontained 10 g l–1 of PdCl2 (Alfa Aesar) in aqueous0.1 N HCl. In the present experiments, the chargedensity for galvanostatic deposition was4 mC mm–2 throughout, with current densities ofthe order of a few hundreds of μA cm–2.

Coulometry and ElectrolysesCoulometric experiments and electrolyses of

organic chlorides, bromides and iodides were car-ried out using three-electrode cells allowing a totalcatholyte volume of about 5 to 10 cm3. The anod-ic compartment was separated by a fritted glass ofweak porosity. Substrate volume was about0.1 mM. In order to avoid disturbance resultingfrom the possible presence of copper oxide,which could depend on the history of the coppersubstrate, the solution was always pre-electrolysedbefore adding the RX compound to the cell.Owing to the high reactivity of the Cu/Pd inter-face towards impurities (in particular dioxygen),there was efficient argon bubbling in all cases,ensuring a good reproducibility of results,especially in voltammetry.

ResultsCharacterisation of the Palladised CopperInterface

A complete TEM study was performed on allthe samples in order to determine the texture, thestructure and the precise composition of the palla-dium-based layer. The bright field image (Figure1(a)) shows a dendritic-like growth of the layer(particle size < 50 nm) obtained with the palladiumsulfate solution. The HRTEM image of one part ofthe bright field image represents one of the 12 nmnanoparticles of which the dendrite was com-posed. The morphology and dimensions (around10–15 nm) of these particles are homogeneous,and each is well crystallised. For the two othersamples, prepared with palladium chloride and pal-ladium nitrate solutions, the growth of thePd-based layer is not dendritic (Figures 1(b) and


1(c), respectively). However, the layer is alwayscomposed by the juxtaposition of nanoparticles,the size of which is closely dependent on the pre-cursors and varies from 3 to 5 nm (Figure 1(b))and from 5 to 8 nm (Figure 1(c)). All thesenanoparticles are well crystallised. The SAED pat-terns obtained from these three samples areidentical, as shown in Figure 1(d). They are com-posed of diffraction circles due to randomlyoriented nanoparticles. Thanks to the‘ProcessDiffraction’ software, a line profile of theelectron diffraction pattern may be plotted, simi-larly to the X-ray diffraction pattern (Figure 1(d)).Hence it was determined that the layer can berelated to the CuPd disordered alloy (ICDD cardNo. 48-1551, space group: Fm3m). The nature andcrystallographic properties of this layer are pre-sented and developed in the Discussion section ofthis article. The EDS analysis of these layers (notgiven here) systematically indicates a Cu:Pd ratio

near to unity, corroborating the CuPd alloy forma-tion. It was concluded that, for all samples, thePd-based modified electrodes consisted of a thinlayer of the stoichiometric CuPd alloy.Additionally, as mentioned in a previous paper(17), it is worth noting that, when using palladiumchloride as the salt in solution, the sparingly solu-ble compound copper(I) chloride (CuCl) may alsobe incorporated into the surface electrode layer, inwhich case Cu(I) is stabilised by the excess chlo-ride as a transient in the redox process.

Primary Alkyl IodidesWhen using Cu-Pd electrodes, and in particular

those obtained with palladium sulfate and nitratesolutions, the results regarding the electrochemicalreduction of primary alkyl halides differ from thosealready described for conventional solid electrodes.Several alkyl iodides such as 1-iodobutane,1-iodohexane, 1-iodooctane and 1-iodohexadecane


20 40 60 80 100 120 140 1602θ (Cu Kα)

160140120100

80604020

Inte

nsity

, a.u

.CuPd

(ICDD No. 48–1551)*

*

***

*

***

(d)(c)

(a) (b)

50 nm

5 nm

50 nm100 nm

5 nm

5 nm

0

Fig. 1 Bright field images of the palladium-based layer obtained with the solution of: (a) palladium sulfate; (b) palladium chloride; and (c) palladium nitrate, (insets: HRTEM images of CuPd nanoparticles composing theselayers); (d) common SAED pattern of these three layers combined with a graph similar to an X-ray diffraction patternwhich enables characterisation of the disordered CuPd alloy

were tested (see Figure 2). 1-Iodobutane wasfound to exhibit a strong activation phenomenonat any Cu-Pd electrode. This activation is quantifi-able as a positive shift in potential with respect toresults obtained at a GCE, which is supposed to bethe ideal type of electrode with zero activitytowards alkyl halide molecules. Thus under stan-dard conditions, the half-peak potential ofiodobutane (Ep/2 = –1.93 V vs. SCE) has beenunambiguously assigned to the classical two-elec-tron reaction step, which is regarded as unchangedby the nature of the interface. By contrast, using aCu-Pd electrode prepared from a PdSO4 solutionyields a cathodic step that is strongly shifted inpotential, and of much smaller limiting current(Ep/2 = –1.44 V). This electrochemical process isdiffusion-controlled, but strongly irreversible. Thetransfer coefficient estimated from half-peak widthmeasurements was found to be smaller than 0.2.The catalytic efficiency of the Cu/Pd interface wasalso compared with those of palladised interfacessuch as smooth platinum and polished palladium.

As already reported (18) palladised surfaces such asthose of Pt/Pd electrodes led to a higher potentialshift (Ep/2 = –1.36 V) as compared with that of aGCE. As a general trend, all types of palladisedsurfaces exhibit highly significant potential shiftswith alkyl iodides. Peak currents are halved whenusing Cu-Pd electrodes, suggesting that the overallelectrochemical process has become a one-elec-tron reaction. Coulometric measurements verifiedthis proposal satisfactorily with all the primaryalkyl iodides tested. It is worth noting that smoothcopper also exhibits a one-electron step with butyliodide, but located at more negative potentials, typ-ically –1.82 V vs. SCE. Therefore the soleparticipation of the copper substrate in the overallactivation process is vanishingly unlikely.

Whatever the formation mode, the one-elec-tron reduction process at Cu-Pd surfaces could beverified by microcoulometric measurements onmillimolar amounts of reactant. At low reductionpotentials (Er < –1.2 V vs. SCE) the measuredcharge values are consistently very close to 1F mol–1. Since nitrones (classically used as spinmarkers) do not disturb electrochemical reductionreactions of alkyl iodides, complementary investigations were performed with N-tert-butyl-α-phenylnitrone (PBN). In all cases, a distinctone-electron process was observed, whereas theformation of paramagnetic nitroxides was demon-strated. Under these conditions, 1-iodobutaneproduces a six-line ESR signal with the couplingconstants aH = 3.16 G and aN = 14.9 G. Thisremains in good agreement with previous resultsobtained at regular palladised surfaces (18).

Alkyl BromidesA large suite of long-chain primary alkyl bro-

mides were reduced at different solid electrodesand their voltammetric data were compared.Almost all of this range exhibited a two-electronirreversible step at a GCE at quite strongly reduc-ing potentials (i.e. Ep/2 < –2.5 V vs. SCE). Smoothpalladium electrodes also yielded a main reductionstep that occurs at very negative potentials (withina comparable potential range < –2.4 V) since theelectrolyte was thoroughly dried by addingactivated alumina in situ. The reduction of


i, μA

E, V/SCE

–3.0 –2.0 –1.0

10

2

1

A

B

C

Fig. 2 Voltammetry of 1-iodohexane (concentration:9 mM) in 0.1 M TBABF4 using DMF as solvent, recordedat different microelectrodes. Scan rate: 100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a palladised platinum electrode (S = 0.8mm2)(C) Response at a Cu-Pd modified electrode preparedfrom a palladium sulfate solution (S = 1.6 mm2)

short-chain alkyl bromides at palladised electrodesmay be effective at much less negative potentialsthan –2 V, but currents are generally small.However, these voltammetric steps exhibited akinetically controlled character, and appeared tovanish completely upon repeated scans withincreasing alkyl chain length. At a smooth copperinterface, a reduction step was generally observedbeyond –1.8 V, together with the possible occur-rence of an adsorption-like step attributable to thereduction of copper oxide at moderate potentials.By contrast, Cu-Pd modified electrodes yieldedsurprising results: much larger reduction steps foralkyl bromides were consistently observable atmuch higher potentials than –2.0 V (see Figure 3for the case of 1-bromodecane). The stepobtained from the second scan is generally S-shaped, with the overall current indicating aprocess close to a one-electron transfer. Thenature of the step strongly suggests a kind of self-inhibition, probably due to the adsorption at theelectrode surface of the free radical produced.

Voltammetric experiments have shown that theshape of the reduction step is strongly sensitive toimpurities in the solution. Thus, with traces ofdioxygen, there is no pre-peak and the main step isclearly shifted. The second scan of an ‘impurity-free’ solution also exhibits such a potential shift,

probably underlining that the catalysis is sloweddown by the decay of the free active surface. Theactivated surface can only be regenerated by rins-ing the electrode according to the proceduredescribed above. However, a pre-peak of variableheight is obtained with a freshly produced micro-electrode, depending on the nature and theconcentration of the alkyl bromides (see Figure 4in the case of 1-bromohexadecane). The totalheight of the overall cathodic step is a linear func-tion of alkyl bromide concentration. With R =n-propyl, n-butyl, n-pentyl, n-hexyl, n-octyl and n-decyl, the total current of the reduction step(found to be diffusion-controlled throughout) isroughly half of the current observed at a GCE.This observation argues in favour of a one-elec-tron reduction. Within the scanned potential range(i.e. –2.5 ≤ E ≤ –0.5 V vs. SCE), there is no appear-ance of a second step attributable to the reductionof the free radical release by the alkyl halide reduc-tion. Moreover, it has been verified that there is noevidence of a partial reduction of the alkyl halideonto a pure copper cathode at potentials above–2 V.

In order to produce cheap and strongly acti-vated electrodes, we attempted to build aCu/Pd interface onto a GCE. Copper was


10

–3.0 –2.0 –1.0

1021

A

B

E, V/SCE

i, μA

Fig. 3 Voltammetry of 1-bromodecane (concentration:9 mM) in 0.1 M TBABF4 using DMF as solvent, recordedat different microelectrodes. Scan rate: 100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a Cu-Pd modified electrode preparedfrom a palladium chloride solution (S = 1.6 mm2). Firsttwo sweeps

25

–3.0 –2.0 –1.0

2 1A

B

E, V/SCE

i, μA

Fig. 4 Voltammetry of 1-bromohexadecane(concentration: 9 mM) in 0.1 M TBABF4 using DMF assolvent, recorded at different microelectrodes. Scan rate:100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a Cu-Pd modified electrode preparedfrom a palladium sulfate solution (S = 1.6 mm2). Firsttwo sweeps

galvanostatically deposited from a copper nitratesolution prepared by dissolving 0.1 g of the salt in100 cm3 of 0.1 N HNO3. The charge density waslimited to 5 × 10–3 C mm–2 and the current wasfixed at 0.5 mA. After obtaining the copperdeposit, (estimated average thickness ≈ 0.2 μm),the electrode was briefly dipped into a palladiumsulfate solution. The glassy carbon surface emergedshiny. Results from use of the interface as avoltammetric electrode were interesting, since thedegree of activation appeared extremelyfavourable. The ‘pre-peak’ turned out to be themain peak (see Figure 5, curve (C)). If the mainreduction step decays during repetitive sweeps, abrief pause at 0 V may regenerate most of the orig-inal current. It is likely that a finely divided depositof the alloy Cu-Pd can be superimposed on thethin copper deposit, producing quite a large activat-ed surface. This procedure has so far only beenachieved with a glassy carbon support.

Our observations suggest that using Cu-Pd elec-trodes at much less negative potentials than those

already reported with conventional electrodematerials leads, at least with alkyl bromides, toone-electron processes. To verify this hypothesis,an extended series of coulometric experimentswas carried out on a large suite of primary alkylhalides. Cu-Pd electrodes (visible as a brightmetallic deposit) formed from palladium sulfate ornitrate solutions could be reused for a large num-ber of experiments without any apparentdeterioration in efficiency. This was not the casewith electrodes produced from palladium chlo-ride, which turned blue over time, probably due tothe oxidation of residual cuprous ions inside thelayer. It was found for all alkyl bromides in theseries that the Cu-Pd electrode then consistentlyproduced a one-electron process. Finally, theanalysis by gas chromatography/mass spectrome-try (GC/MS) of the R–Br electrolysis productsshowed that R–R dimers and/or mixtures ofR(H)/R(–H) in equal amounts were obtained withR = C8, C10 and C12.

The formation of free alkyl radicals in the cleav-age of primary alkyl bromides at Cu-Pd cathodes isstrongly corroborated by the spin marker tech-nique. 10–20 mg of the alkyl halide, dissolved in5 cm3 of DMF, was reduced in the presence of athreefold excess of N-tert-butyl-α-phenylnitrone(PBN) (electrolysis current = 10–15 mA). By wayof example, the reduction current for 1-bromohep-tane at –1.5 V on a Cu-Pd electrode vanishedcompletely at 1 F mol–1. ESR analysis of the elec-trolyte in the absence of dioxygen disclosed astrong paramagnetic signal, fully attributable to thetrapping of the n-heptyl radical. The nitroxide rad-ical obtained (see Structure 1) displayed a six-rayspectrum with coupling constants aN =14.379 Gand aH = 2.614 G.

6-Bromo-1-hexene, which is known to afford acyclisable free radical, usable as a ‘radical clock’,gives very similar results. Thus the reduction at aGCE shows Ep/2 = –2.34 V, whereas the use of a


25

–3.0 –2.0 –1.0

21

A

C

E, V/SCE

i, μA

B

2

1

5

Fig. 5 Voltammetry of 1-bromodecane (concentration:9 mM) in 0.1 M TBABF4 using DMF as solvent, recordedat different microelectrodes. Scan rate: 100 mV s–1

(A) Response at a GCE (S = 0.8 mm2)(B) Response at a freshly made CuPd cathode preparedfrom a palladium chloride solution (S = 3.2 mm2). Firsttwo sweeps(C) Response at a GCE first covered by a galvanostaticdeposit of copper (S = 0.8 mm2) and then treated bypalladium sulfate solution. First two steps

N

H

Ph

CH3(CH2)6

tBu

O

C

1

Cu-Pd working electrode (still prepared withPdSO4) produces a spectacular shift toEp/2 = –1.40 V. As shown in Figure 6(a), the pres-ence of nitrone at the reduction led to twoparamagnetic transients, with the formation of twoparent nitroxides. It is presently premature toassign these two nitroxides to the trapping of theuncyclised and cyclised n-hexenyl radical.

Finally, fixed potential electrolyses on alkyl bro-mides (all exhibiting one-electron processes) led tomixtures of R–R, RH and R(–H). The ratioRH:R(–H) was equal to 1, as shown by GC/MSexperiments with C8, C10 and C12 bromides.

n-Alkyl ChloridesIt was found possible to reduce 1-chloroalkanes

at Cu-Pd cathodes. An appreciable potential shiftwas also observed. However, in all cases, half-peakpotentials were still located at very negative

potentials (E < –2.5 V vs. SCE). This precludesobtaining one-electron reduction processes similarto those observed with alkyl bromides and iodides.

DiscussionIn order to characterise the electrochemical effi-

ciency of our as-prepared electrodes, whatever thepalladium precursor used, the primary objectivewas to unambiguously identify the structure of thedeposited layer. Electron diffraction was an appro-priate technique here, since the nanometric scale ofthe metallic particles made valid identification diffi-cult when using a conventional XRD analysis. Forall deposits, the electron diffraction line profileenabled identification of the deposited layer as adisordered CuPd alloy, thanks to a perfect matchwith the diffraction data given by Nekrasov (19)and more specifically by Zhu et al. (20). The latterperformed a thorough study of clusters of


Inte

nsity

, a.u

.

3450 3480 3510Magnetic field, Gauss

40,000

0

–40,000 g = 2.00641aN = 14.161 GaH = 2.397 G

Inte

nsity

, a.u

.

100,000

0

–100,000

g = 2.01aN = 14.692 GaH = 2.564 G

3420 3440 3460 3480 3500 3520 3540Magnetic field, Gauss

(a)

(b)

Fig. 6 ESR signals obtained from:(a) 6-bromo-1-hexene and (b) phenyliodide when reduced in 0.1 MTBABF4 using DMF and withdissolved TBPN (threefold excess). Inboth cases, reductions werecompleted after a total consumptionof 1 F mol–1 based on the halideamount

disordered CuPd (i.e. nanoparticles) via a theoret-ical approach using the bond order simulation(BOS) model for metals and the corrected effectmedium (CEM) theory. The simulation model ofZhu et al. predicts diffraction patterns and rela-tive peak intensities, which are in goodagreement with the reported experimental data.

Having demonstrated the formation of thedisordered CuPd phase via TEM investigations,data regarding the Cu-Pd system must be consid-ered, since the disordered structure is notexpected at room temperature. Thermo-dynamically speaking, below the solidus, the Cu-Pd system is first characterised by a continuoussolid solution showing a face-centred cubic(f.c.c.) structure (21) with a lattice spacing rang-ing from 3.615 Å (pure copper) to 3.892 Å (purepalladium) (22). At the 50:50 atomic composi-tion, the disordered CuPd A1-type alloy (solidsolution) has a cell constant close to 3.77 Å (22),and is formed of copper and palladium that ran-domly occupy, with 50% probability, each site ofthe f.c.c. structure (Table I). As the temperaturedecreases (T < 600ºC), ordering of Cu and Pdatoms is energetically favoured (23–25) and thecubic CuPd alloy adopts the b.c.c.-based struc-ture (CsCl-type). This phase, which is alsoreferred to as B2 or CuPd (β), shows an alterna-tion of (001) planes of Cu and Pd (Table I). It isworth noting that the f.c.c.-based L10 orderedsuperstructure (CuAu-type) with alternating

(001) planes of Cu atoms and (001) planes of Pdatoms does not exist (Table I). The competitionbetween the B2 and L10 ordered phases of CuPdresolves in favour of the former, thanks to a sub-stantially lower energy of formation (for moredetails see (25)). Moreover, the high stability ofthe B2-type structure is substantiated empiricallyby the recent discovery of the correspondingmineral (skaergaardite) (26). Consequently, underour experimental conditions, the layer growthmust be kinetically controlled, since it leads tothe A1-type alloy, a metastable phase at roomtemperature.

A noteworthy result of this study is that wesucceeded in synthesising very straightforwardlynanoparticles of the disordered CuPd alloy byimmersing a copper substrate (grid, sheet, Cuelectrodeposit) into a fresh acidic solution of apalladium salt. Other methods known to date arevery much more complicated, usually involvingpolyvinylpyrrolidone (PVP) as stabiliser to obtainnanoparticles. Esumi’s method (27) (or adapta-tions) yields this alloy at nanometric scale bythermal decomposition of mixtures of copperand palladium precursors in high-boiling organicsolvents (20, 27–29) or by condensing Pd and Cuatoms at 350ºC under ultra-high vacuum (30, 31).Interestingly, CuPd alloys also show potential asgas-phase catalysts in enhancing the selectivity ofhydrogenation of dienes (32) and the reductionof NO by CO (30, 33, 34).


Phase formula CuPd “CuPd(α)”* CuPd(β)

Pearson symbol cF4 tP4 cP2

Space group Fm3m P4/mmm Pm3m

Strukturbericht A1 L10 B2designation

Crystal structure

Pd

Cu

Cu or Pd

Table I

Crystal Structure Information for the f.c.c. and b.c.c. Copper-Palladium Alloys

*Note the L10 structure is given only for comparison since “CuPd(α)” phase does not exist for energetic reasons

ConclusionIt may be concluded that a simple redox dis-

placement reaction between Cu0 and Pd2+,operating in acidic solution at room temperatureoffers a route to a thin and very stable layer, charac-terised as a well crystallised nanometric CuPd alloy.The displacement is simply achieved in the presenceof Pd(II)-based salts such as sulfate, nitrate andchloride. However, a pure CuPd alloy is formedonly with palladium sulfate and nitrate. Electro-chemical data obtained from the reduction of alarge series of organic halides (mainly iodides andbromides) showed that the use of such alloys ascathode materials very strongly activates the cleav-age of the carbon–halide bond, sometimesdisplaying a +1 V shift in potential. There were nostrong passivating phenomena during the electroly-ses, even though a moderate decay of the cathodiccurrent could be observed after a few minutes. Thedeposit was shown to act as a porous material, andits structure may change dramatically with time; thiscorroborates the assumption that palladium reactswith alkyl halides (Figure 7 depicts the modificationin morphology of the Cu-Pd layer during the reduc-tion of alkyl bromides).

We have already mentioned (18) the use of pal-ladium deposits as modifier of the cathode surface(for example deposits onto platinum or glassy car-bon). The mode of action of palladium probablystems from the finely divided nature of the deposits(nanosized particles). Hitherto it was believed thatelectrolytic deposition (from Pd2+ in acidic solu-tions) was a prerequisite for electrocatalytic activity(here quantified mainly in terms of a shift of themain voltammetric step toward much less negativepotentials).

The mode of catalysis is not yet fully determined,but it is conceivable as the insertion of palladiuminto the carbon–halide bond, giving a stronglyadsorbed chain species such as C–Pd–X. Such aninsertion may corroborate the catalytic hypothesis,given the constant regeneration of the copper-palla-dium alloy (see Scheme I), promoted by the strongelectronic interaction between Pd and Cu uponalloying with a specific feature (33, 34).

In the process proposed here, the rates ofadsorption and insertion of palladium into the

C–halogen bond would be rapid compared with dif-fusion of the electroactive species. As stressedabove, catalysis by the Cu-Pd surface is of very greatinterest for C–Br bond cleavage reactions. The C–Icleavage reaction is also facilitated, but results arequite similar to those already observed with pal-ladised surfaces. Very often (but not invariably), thepotential shift is so large that the cathodic reactionis fundamentally changed, and turns out to bemono-electronic. The method may therefore beseen as an efficient source of free radicals (with apossible coupling reaction outside the cathodiclayer) more or less strongly adsorbed at the inter-face. These results are in full agreement withprevious estimates by Lund et al. (35–37) concern-ing the standard potentials corresponding to thereduction of a large number of free alkyl radicals inDMF between –1.39 and –1.72 V vs. SCE, undervery similar experimental conditions.


200 nm

200 nm

(a)

(b)

Fig. 7 SEM images of the Cu-Pd layer before and afterreduction of 1-bromodecane (concentration 2 × 10–2 M).The metal layer (shown in (a)) has been obtained after adipping of a copper sheet into PdSO4 (see Experimentalsection) for 2 minutes. The structural change (b) of thelayer (consecutive to the catalytic reduction of the RBrcompound) was obtained by electrolysis at –1.9 V vs. SCEafter 2 C cm–2 have passed through the cell. Averagecurrent density: 0.5 mA cm–2


1 D. G. Peters, in “Organic Electrochemistry”, 4thEdn., eds. H. Lund and O. Hammerich, MarcelDekker, New York, Basel, 2001, Chapter 8, p. 341

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3 E. Coulon, J. Pinson, J.-D. Bourzat, A. Commerçonand J. P. Pulicani, Langmuir, 2001, 17, (22), 7102

4 E. Steckhan, in “Organic Electrochemistry”, eds. H.Lund and O. Hammerich, Marcel Dekker, NewYork, Basel, 2001, Chapter 27, p. 1103

5 S. B. Rondinini, P. R. Mussini, F. Crippa and G.Sello, Electrochem. Commun., 2000, 2, (7), 491

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Su, J. Am. Chem. Soc., 1986, 108, (4), 63811 J. M. Savéant, J. Am. Chem. Soc., 1992, 114, (26),

1059512 J. Grimshaw, J. R. Langan and G. A. Salmon, J.

Chem. Soc., Faraday Trans., 1994, 90, (1), 7513 C. P. Andrieux, I. Gallardo and J. M. Savéant, J. Am.

Chem. Soc., 1989, 111, (5), 162014 J. M. Savéant, in “Advances in Physical Organic

Chemistry”, ed. T. T. Tidwell, Academic Press, NewYork, 2000, Vol. 35, p. 117 and references therein

15 P. Hapiot, V. V. Konavalov and J. M. Savéant, J.Am. Chem. Soc., 1995, 117, (4), 1428

16 C. P. Andrieux and J. Pinson, J. Am. Chem. Soc.,2003, 125, (48), 14801

17 J. Simonet, Electrochem. Commun., 2005, 7, (6), 619

18 J. Simonet, J. Electroanal. Chem., 2005, 583, (1), 3419 I. Nekrasov and V. Ustinov, Dokl. Acad. Sci. USSR,

Earth Sci. Sect. (Engl. Transl.), 1993, 328, 12820 L. Zhu, K. S. Liang, B. Zhang, J. S. Bradley and A.

E. DePristo, J. Catal., 1997, 167, (2), 41221 “Binary Alloy Phase Diagrams”, 2nd Edn., eds. T.

B. Massalski, H. Okamoto, P. R. Subramanian andL. Kacprzak, in 3 vols., ASM International, Ohio,U.S.A., 1990, Vol. 2, p. 1454

22 W. B. Pearson, “A Handbook of Lattice Spacingsand Structure of Metals and Alloys”, PergamonPress, New York, 1967

23 S. Takizawa, S. Blügel, L. Terakura and T. Oguchi,Phys. Rev. B, 1991, 43, (1), 947

24 Z. W. Lu, S.-H. Wei, A. Zunger, S. Frota-Pessoa andL. G. Ferreira, Phys. Rev. B, 1991, 44, (2), 512

25 G. Bozzolo, J. E. Garcés, R. D. Noebe, P. Abel andH. O. Mosca, Prog. Surf. Sci., 2003, 73, (4–8), 79

26 N. S. Rudashevsky, A. M. McDonald, L. J. Cabri, T.F. D. Nielsen, C. J. Stanley, Yu. L. Kretzer and V. N.Rudashevsky, Mineral. Mag., 2004, 68, (4), 615

27 K. Esumi, T. Tano, K. Torigoe and K. Meguro,Chem. Mater., 1990, 2, (5), 564

28 J. S. Bradley, E. W. Hill, C. Klein, B. Chaudret andA. Duteil, Chem. Mater., 1993, 5, (3), 254

29 N. Toshima and Y. Wang, Langmuir, 1994, 10, (12),4574

30 S. Giorgio and C. Henry, Microsc. Microanal.Microstruct., 1997, 8, (6), 379

31 S. Giorgio, H. Graoui, C. Chapon and C. Henry, in“Metal Clusters in Chemistry”, eds. P. Braunstein, L.A. Oro and P. R. Raithby, in 3 vols., Wiley-VCH,Weinheim, Germany, 1999, Chapter 2, p. 1194

32 J. Philips, A. Auroux, G. Bergeret, J. Massardier andA. Renouprez, J. Phys. Chem., 1993, 97, (14), 3565

33 Y. Debauge, M. Abon, J. C. Bertolini, J. Massardierand A. Rochefort, Appl. Surf. Sci., 1995, 90, (1), 15

Scheme Iads = adsorbed; sol = solution; ≡ represents that an interaction existsbetween Cu and Pd atoms in the solidstate

RBrCu-Pdinterface

[RBr]ads

Pd

Cu[R ≡ Br]ads

e–

– Br– [R•]ads + Cu-Pd R•sol

R•sol

R–R (coupling)

R(H) + R(–H) (disproportionation)

References

AcknowledgementsThe authors are grateful to Professor Viatcheslav Jouikov (Laboratoire MaSCE) for the ESR measure-ments and to Michèle Nelson (LRCS) for helpful assistance.


Jacques Simonet is Directeur deRecherche Emérite in theElectrochemistry Group, Université deRennes 1 (UMR 6226), France. Hisprincipal interests are organicelectrochemistry, the activation oforganic reactions by electron transfer,electro-polymerisation and the formation

of redox polymers. He also researches on the reversible cathodiccharging of precious metals (platinum and palladium) in super-dry conditions, in contact with polar organic solvents containingelectrolytes, mimicking Zintl phases for transition metals.

Philippe Poizot is presently AssistantProfessor at the Department ofChemistry (LRCS, UMR 6007) of theUniversité de Picardie Jules Verne(Amiens, France) where he studiedChemistry, and completed his Ph.D. inMaterials Science in 2001. His researchtopics are mainly focused on the

lithium-ion battery and the synthesis of nanostructuredelectrode materials using soft chemistry routes such aselectrodeposition.

Lydia Laffont-Dantras is Assistant Professorat the Department of Chemistry (LRCS,UMR 6007) of the Université de PicardieJules Verne (Amiens, France). Her principalinterest is the study of organic and inor-ganic compounds by transmission electronmicroscopy (TEM) and electron energy lossspectroscopy (EELS). Her research work is

currently focused on the characterisation (morphology andnanostructure) of electrochemical devices such as electrochromicthin films or lithium-ion batteries by TEM and EELS.

The Authors

and references therein34 A. Rochefort, M. Abon, P. Delichère and J. C.

Bertolini, Surf. Sci., 1993, 294, (1–2), 4335 D. Occhialini, S. U. Pedersen and H. Lund, Acta

Chem. Scand., 1990, 44, (7), 715

36 D. Occhialini, J. S. Kristensen, K. Daasbjerg and H.Lund, Acta Chem. Scand., 1992, 46, (5), 474

37 D. Occhialini, K. Daasbjerg and H. Lund, ActaChem. Scand., 1993, 47, (11), 1100

96

Dalton Discussion 10 on the topic“Applications of Metals in Medicine andHealthcare” was held at the University of Durham,U.K., from 3rd to 5th September 2007 (1). DaltonDiscussions represent a conference concept quitedifferent from the norm with a clear focus, as thename implies, on the discussion. Therefore themajority of the presentations were short fiveminute talks on papers submitted for a DaltonTransactions special issue (2), distributed in advanceto all the participants, followed by a discussion ofabout twenty-five minutes. Additionally, fiveKeynote lectures, given by experts in the field, andapproximately sixty poster presentations wereincluded. The conference was perfectly suited toinitiate collaborations, develop ideas or simply dis-cuss. The follow-up meeting “Dalton Discussion11: The Renaissance of Main Group Chemistry”was announced by Professor Robin Perutz, thePresident of the Dalton Division Council of theRoyal Society of Chemistry, and will take place in2008 at the University of California, Berkeley,U.S.A. (3).

About 100 participants from both academia andindustry, including chemists, biologists and clini-cians, discussed recent results obtained formetal-based therapeutics and diagnostics.Currently, metal compounds are not the drugs offirst choice in clinical application or for companiesto develop. As several experts pointed out, there isa need to initiate long-term discussion and interdis-ciplinary research, and to convince both cliniciansand society of the benefit of metal-based drugs.The example of the vanadium compound bis(mal-tolato)oxovanadium(IV) (BMOV), which hasinsulin mimetic properties (developed by Chris

Orvig and colleagues), which re-entered clinical tri-als in 2007 after several years with a lack of interestfrom drug development companies, was used tounderline the fact that patience is required, andthat nobody in the field can expect to developdrugs overnight. This is a long term process withan average duration of about ten years, there aremany failures and costs are high.

Antitumour and Anti-HIVApplications of PGMs

With regard to platinum group metals (pgms),an overwhelming number of presentations atDalton Discussion 10 focused on their applicationas antineoplastic agents. Platinum complexes areapplied in half of all chemotherapeutic schemesagainst a wide range of tumours, although they areeffective against only a handful of tumourigenicdiseases. Keynote presenter Chi-Ming Che(University of Hong Kong, China) et al. reportedon their recent developments of platinum(II),ruthenium(II), ruthenium(III) and ruthenium(IV)complexes alongside non-pgm compounds (gold,iron and vanadium) as anticancer and anti-HIVagents (4). Che presented Pt complexes which bindnon-covalently to biomolecules, and are capable ofbinding in an electrostatic or hydrophobic manneras well as via intercalation. Some of the complexeswere found to be up to 100 times more potent invitro than cisplatin. Furthermore, amino-alcohol-platinum complexes were proposed asprotein-staining reagents in sodium dodecyl sulfate(SDS)-polyacrylamide gels, due to their high bind-ing affinities to proteins, and protein interaction isalso accompanied by an enhancement of the emis-sion. In addition, Ru complexes with quinone-


Dalton Discussion 10: Applications ofMetals in Medicine and HealthcareAPPLICATIONS OF PLATINUM GROUP METALS IN CANCER AND HIV TREATMENT

Reviewed by Christian G. HartingerInstitut des Sciences et Ingénierie Chimiques, École Polytechnique Fédérale de Lausanne (EPFL), CH-1015 Lausanne, Switzerland;

and University of Vienna, Institute of Inorganic Chemistry, Währinger Str. 42, A-1090 Vienna, Austria;

E-mail: [email protected]

DOI: 10.1595/147106708X298296


diimine as auxiliary ligands were shown to interca-late into DNA, but were found to exhibit mildcytotoxicities of about 200 μM against epidermalKB-3-1 and KB-V-1 carcinoma cell lines. A ruthe-nium-oxo oxalato cluster was presented whichexhibited promising anti-HIV properties, beingabout ten times more active than the commonHIV-1 RT inhibitor 3'-azido-3'-deoxythymidine-5'-phosphate.

Nicholas P. Farrell (Virginia CommonwealthUniversity, U.S.A.) and coworkers, who developedthe trinuclear Pt compound BBR3464 up to clinicaltrials, reported on BBR3464 analogues which arenot capable of binding covalently to bio-molecules (5). They observed by mass spectrome-try, circular dichroism and fluorescence studies thepre-association of these compounds with humanserum albumin (HSA) at an initial stage. It isthought that non-covalent interaction of these Ptcomplexes with HSA might circumvent the deacti-vation of Pt drugs by binding to serum proteins, andthis suggests a new mode of action for this com-pound class.

The contribution from Peter J. Sadler’s group,presented by Abraha Habtemariam (University ofWarwick, U.K.), was the 106Ru-radiolabelled putativeantitumour organometallic compound [(η6-fluo-rene)RuCl(en)]PF6 (6). Synthesised with thepurpose of following and locating the Ru species invivo, the compound was administered to a non-tumour bearing male albino rat. The compound wasfound to be distributed over the whole animal, withthe highest level in liver and kidneys, which illus-trates the difficulty in finding drugs that do notaccumulate in these organs.

The Keynote talk of Simon P. Fricker(AnorMED Inc., Canada), entitled ‘Metal baseddrugs: from serendipity to design’, was focused onestablished Pt anticancer agents but also reportedon new developments in the field, including com-pounds such as picoplatin, iproplatin and the orallyadministerable satraplatin, as well as non-Pt com-plexes (7). The advantages of second and thirdgeneration compounds in comparison to cisplatinwere highlighted: carboplatin has lower toxicity,satraplatin is orally bioavailable and picoplatin over-comes resistance. Notably, an interesting road-map

from the presenter’s point of view was given (Figure 1).

Fricker pointed out that the advantages of metal-based drugs are thought to derive from a precise 3Dconfiguration, leading to precise target/drug interac-tion; a capacity to coordinate to biomolecules, whichis also tuneable by modification of the ligand sphere;and capacities to participate in biological redoxprocesses and to undergo ligand exchange reactions.The speaker reviewed progress in the field of non-Ptanticancer drug candidates, notably Ru anticanceragents including the two Ru compounds in clinicaltrials, indazolium trans-[tetrachlorobis(1H-indazole)ruthenate(III)] (KP1019) and imidazoliumtrans-[tetrachloro(imidazole)(dimethyl sulfoxide)-ruthenate(III)] (NAMI-A) (8).

In his Keynote lecture, Trevor W. Hambley(University of Sydney, Australia) described recentdevelopments in metal-based pharmaceuticals (9)and classified metallodrugs into seven classes (seeFigure 2). Several pgm compounds are representedin classes (i) and (iii). In particular, the Ru-basedglycogen synthase kinase 3β (GSK3β) inhibitorDW1/2, developed in Eric Meggers’s group(Philipps-Universität Marburg, Germany) (10), wasmentioned as an example of a class (i) metallodrug.Since most of the known anticancer Pt complexesare believed to exhibit their activity in other thantheir administered forms, cisplatin and carboplatin aswell as Ru(III), Ru–arene and other Pt(II) and Pt(IV)complexes can be considered as representativesof class (iii).

Fig. 1 A road-map for the development of drugs (ADME = adsorption, distribution, metabolism andexcretion)


Janice R. Aldrich-Wright (University ofWestern Sydney, Australia) et al. reported on atargeted approach exploiting molecular hosts asdrug delivery vehicles (11). Notably, some Ptcomplexes containing the (1S,2S)-cyclohexane-diamine (chxn) moiety were found to be moreactive in vitro than oxaliplatin analogues with the(1R,2R)-chxn ligand. Loading such Pt(II) com-plexes onto cucurbit[n]urils resulted in onlysmall modifications of the cytotoxicity of thecomplexes, indicating only minor influence ofthe drug delivery system on the activity of thecomplexes. However, the application of thesecompounds is limited by their low solubility,therefore other molecular hosts such ascyclodextrins, calix[n]arenes and dendrimers arebeing evaluated.

Finally, the contribution from Paul J. Dyson’s(École Polytechnique Fédérale de Lausanne(EPFL), Switzerland) group dealt with the designof Ru organometallics, the tuning of lipophilicity,which influences the cellular uptake, and alteringthe compounds’ reactivity towards DNA modelsand proteins (12). All these efforts were under-taken with a view to improving the efficacy of Rucompounds. Based on these studies, it was con-cluded that modifications which lead to increasedinteractions of a drug with DNA at the expenseof protein binding are more toxic towardshealthy cells, and therefore are likely to exhibitmore unwanted side effects in patients.

Other MetalsBesides the talks mentioned above, there were

numerous fascinating presentations and posters onapplications of other metal compounds for medici-nal purposes. These include gold, titanium, rheniumand iron complexes for cancer chemotherapy, CO-releasing molecules, radio-metallodrugs fordiagnosis and therapy (for example, copper com-plexes for positron emission tomography (PET)imaging and technetium binding to peptides),gadolinium complex-based contrast agents, zinc andcopper complexes as probes for in vitro fluorescenceimaging and α-emitters such as 213bismuth, 211asta-tine and 225actinium as therapeutics.

Concluding RemarksIn summary, a broad variety of applications in

metals in medicine and healthcare was presented,with Pt, Ru and other metal-based drugs demon-strating the potential to become the majortreatments for some common diseases. DaltonDiscussion 10 was a very interesting conference at apleasant venue in the old city of Durham, and hadan appropriate size to benefit from the special con-ference mode with its focus on discussion. All thecontributions can be read in the special issue ofDalton Transactions, published in autumn 2007 (2).

References1 Dalton Discussion 10: Applications of Metals in

Medicine and Healthcare, 3rd–5th September, 2007,Durham University, U.K.:http://www.rsc.org/DD10

2 Dalton Trans., 2007, (43), 4873–50923 Dalton Discussion 11: The Renaissance of Main

Group Chemistry, 23rd–25th June, 2008, Universityof California, Berkeley, U.S.A.:http://www.rsc.org/DD11

4 R. W.-Y. Sun, D.-L. Ma, E. L.-M. Wong and C.-M.Che, Dalton Trans., 2007, (43), 4884

5 E. I. Montero, B. T. Benedetti, J. B. Mangrum, M. J.Oehlsen, Y. Qu and N. P. Farrell, Dalton Trans.,2007, (43), 4938

6 J. D. Hoeschele, A. Habtemariam, J. Muir and P. J.Sadler, Dalton Trans., 2007, (43), 4974

7 S. P. Fricker, Dalton Trans., 2007, (43), 49038 C. G. Hartinger, S. Zorbas-Seifried, M. A. Jakupec,

B. Kynast, H. Zorbas and B. K. Keppler, J. Inorg.Biochem., 2006, 100, (5–6), 891

9 T. W. Hambley, Dalton Trans., 2007, (43), 4929

Fig. 2 Classification of metallodrugs


10 K. S. M. Smalley, R. Contractor, N. K. Haass, A. N.Kulp, G. E. Atilla-Gokcumen, D. S. Williams, H.Bregman, K. T. Flaherty, M. S. Soengas, E. Meggersand M. Herlyn, Cancer Res., 2007, 67, (1), 209

11 N. J. Wheate, R. I. Taleb, A. M. Krause-Heuer, R. L.

Cook, S. Wang, V. J. Higgins and J. R. Aldrich-Wright, Dalton Trans., 2007, (43), 5055

12 C. Scolaro, A. B. Chaplin, C. G. Hartinger, A.Bergamo, M. Cocchietto, B. K. Keppler, G. Savaand P. J. Dyson, Dalton Trans., 2007, (43), 5065

The ReviewerChristian G. Hartinger received his M.S. and Ph.D. in Chemistry in 1999 and2001, respectively, from the University of Vienna, Austria, under thesupervision of Bernhard K. Keppler. Up to 2006, he worked as a researchassistant at the same department. He has recently joined the working group ofPaul Dyson at the EPFL in Switzerland as a Schrödinger Fellow. His researchinterests include the development of mono- and multinuclear platinum groupmetal complexes as anticancer agents, and the elucidation of the transportmechanism and mode of action for such compounds using modern separationand mass spectrometric techniques.

100

Traditional reference electrodes used for elec-trochemical measurements, such as the calomeland silver/silver chloride electrodes, have a limitedrange of applicability. The liquid junction is prob-lematic with these electrodes, and they cannot beused either with wholly solid-state electrochemicalcells or for very high-temperature reactions such asthose in molten electrolytes. The use of solid plat-inum electrodes in molten salts has been reported(1–14), but there are problems associated with theuse in molten media of Pt electrodes. Understrongly alkaline conditions, they actually functionas oxidation electrodes (2). It has been demonstrat-ed that Pt foil cannot act as a reference electrode inmolten electrolytes, since it is neither stable nordepolarised (4). On the other hand, Pt wireimmersed in molten NaCl/KCl can maintain asteady electrode potential for more than 12 hours,and shows electrochemical irreversibility. It cantherefore can be used as a pseudo-reference elec-trode for the study of electrode reaction kinetics,with the advantages of simplicity, convenience andease of operation (7). The Pt reference electrodeperforms well in geothermal brine solutions at highpressure and temperature (~ 250ºC). Unlike con-ventional reference electrodes (even whenmodified for high temperature), the Pt referenceelectrode is applicable to measurements in com-plex polluted brines (8).

In certain electrolytes, modification of the Ptsurface is important for its stability. Anodised, non-porous Pt has demonstrated its usefulness as asolid-state reference electrode by virtue of its near-Nernstian behaviour, low hysteresis and rapidresponse (15). Modifications to Pt wire may extendits usefulness to more electrochemical systems. Theuse of polypyrrole (16), poly-1,3-phenylendiamine(15) or polyvinyl ferrocene (17) as a surface modifi-er can successfully suppress significant interferenceby any coupled redox systems or contaminants.The fact that a Pt electrode can be modified withnitrogen-based polymers or be incorporated as partof a biosensor assembly (18) indicates its resistanceto interference from these compounds.

Some of the problems associated with the refer-ence electrode can be solved outside theelectrochemical cell. A reference electrode isdefined as an ideal non-polarisable electrode; thusits potential does not vary with the current passing.In practice, no electrode follows this ideal behav-iour; consequently, the interfacial potential of thecounterelectrode in the two-electrode systemvaries with the flow of current passed through thecell. In order to overcome this problem, a three-electrode cell can be used. The functions of thecounterelectrode (in a three-electrode cell) aredivided between the reference and auxiliary elec-trodes. The passage of current between the


Platinum as a Reference Electrode inElectrochemical MeasurementsBy Kasem K. Kasem* and Stephanie JonesDepartment of Natural, Information, and Mathematical Science, Indiana University Kokomo, Kokomo, IN, 46904-9003, U.S.A.;

*E-mail: [email protected]

The usefulness of platinum as an electrochemical reference electrode was investigated. Wellknown redox systems with one-electron single or multiple redox waves, and two-electronmultiple redox waves were used as test regimes. The effects on electrode performance ofvariables such as the solvent, the physical state of the electrolyte and its temperature wereinvestigated. Cyclic voltammetry (CV) was used to derive kinetic parameters for comparisonwith corresponding measurements on traditional reference electrodes. The results indicatethat Pt can be used as a reference electrode under specific conditions in which traditionalreference electrodes cannot be used.

DOI: 10.1595/147106708X297855

working and auxiliary electrodes ensures that lesscurrent passes through the reference electrode.Furthermore, the three-electrode cell allows thepotential between the working and reference elec-trodes to be controlled. Most electrochemicaldevices include an operational amplifier of highinput impedance for the reference electrode input,to eliminate the possibility of any current passingthrough the reference electrode. Since no Faradaicprocess takes place at the reference electrode, itsarea relative to that of working electrode has noeffect on the electrochemical results.

The physical form of the Pt reference electrodemay contribute to its performance. Studies (19)indicate that the Pt mesh electrode yields veryreproducible results, and that it can be used as aconvenient reference electrode. On the otherhand, Pt sheet or wire has been used in all-solid-state electrochemical cells at room temperature(20–22), and in reactions in frozen agar or frozenaqueous electrolytes (23, 24). Most of these stud-ies involved only one-electron redox systems.

In this study, the usefulness of Pt as a referenceelectrode in electrochemical systems was investi-gated using CV techniques. Single ormulti-electron redox systems involving one- ortwo-electron redox waves were used in this study.To verify the suitability of Pt as a reference elec-trode, kinetic parameters were determined forcomparison with corresponding measurements ontraditional reference electrodes.

Experimental DetailsThe reagents FeCl3, KCl, K3[Fe(CN)6] and

K4[Fe(CN)6] were of analytical grade. A purifiedagar powder was obtained from Sigma ChemicalCo. All other reagents were of at least reagentgrade and were used without further purification.Analytical grade nitrogen gas was used to purgeoxygen from the electrolyte. Unless otherwise stat-ed, experiments were performed at 25ºC and 1 atmpressure.

To test the suitability of Pt as a reference elec-trode, a Pt wire reference electrode was coupledwith a standard Ag/AgCl/Cl– reference electrodein a beaker containing 0.5 M KCl with or withoutthe addition of 2 mM K3Fe(CN)6 (potassium hexa-

cyanoferrate (III)) electrolyte. The two electrodeswere connected briefly to the inputs of a Fluke 27multimeter that has input impedance 200 MΩ.The voltage reading was used to assess the qualityand suitability of the Pt reference electrode.

Electrochemical experiments were carried outusing a 10 cm3 cylindrical cell (Figure 1). The ref-erence electrode, unless otherwise stated, was Pt.CV was performed first using Pt as a referenceelectrode and then using Ag/AgCl/Cl– as a refer-ence electrode. The counter (auxiliary) electrodewas a Pt wire, and the working electrodes wereglassy carbon (0.07 cm2) or Pt (0.02 cm2) in disc ormicroelectrode (10 μm diameter or 7.85 × 10–7

cm2) configuration. Electrodes were positioned inthe cell in a similar way. The Pt wire referenceelectrode was coiled around the Teflon jacket ofthe working electrode; the counter electrode wasplaced at a distance from both the reference andworking electrodes. The working electrodes werecleaned by polishing with 1 μm α-alumina paste ordiamond paste, and rinsed with water and acetoneprior to use. A BAS 100B ElectrochemicalAnalyzer (Bioanalytical System, Inc.) was used to


Working electrode

Counter electrodeReference electrode

Electrodecross-section

Fig. 1 Schematic drawing for the electrochemical cellused in this study

perform the electrochemical studies. For frozenelectrolyte experiments, the electrolyte was firstfrozen at –20ºC and measurements were per-formed at –5ºC.

Suitability Tests on Platinum WireReference Electrodes

Prior to use as a reference electrode, Pt wirewas mechanically polished using 600 grade sandpa-pers, followed by 2 μm diamond paste, and rinsedwith deionised water. In an alternative method, themechanically cleaned Pt wire was heated to 1000ºCfor 10 minutes. The Pt wires with the differenttreatments were each coupled with a referenceelectrode of known potential as described in theExperimental Details section. The voltammetricreading was less than 0.2 mV in each case. Thisvalue is much lower than the standard maximumallowed for this test, which is 3 mV. These resultsindicate that Pt wire is an adequate reference elec-trode for routine laboratory use.

The suitability test showed that stirring or agita-tion has no effect on the performance of the Ptwire reference electrode. Several Pt wires of differ-ent lengths were subjected to the referenceelectrode suitability test. The results indicate thatthe surface area of the Pt wire has no effect on per-formance.

One of the most undesirable effects which areference electrode can cause is a change in poten-tial during the course of an experiment. In CVstudies, the quantity ΔEp (the difference betweenthe reduction peak potential Epc and the oxidationpeak potential Epa) is very important in the calcu-lation of the charge transfer rate constant kCT

when diffusion is the dominant process. The calcu-lation is made using Equation (i) (25):

(DO/DR)α kCT (i)

(NF π ν DO/RT)1/2

where: Ψ is a dimensionless rate parameter, thevalue of which decreases from 20 to 0.1 as ΔEp

increases from 0.061 V to 0.212 V (25); DO = dif-fusion coefficient of oxidation; DR = diffusioncoefficient of reduction; N = number of electrons;ν = scan rate (V s–1); F = Faraday constant;T = temperature; R = gas constant; α = transfer

coefficient. The validity of Pt as a reference elec-trode has been tested in each of the three systemsdescribed below.

One-Electron Redox SystemLiquid aqueous, agar gel and frozen systems

containing 5 mM of K3Fe(CN)6 and 100 mM KClas supporting electrolytes were chosen as modelsystems for one-electron redox reactions. Figure2(a) indicates that the use of Pt as a reference elec-trode shows a clear potential window in aredox-free electrolyte, whereas Figure 2(b) showsonly a parallel shift in both Epc and Epa withoutaffecting the value of ΔEp, even in agar medium.Typical characteristics of diffusional redox wavesare reported and displayed in Figures 2(c)–2(e).

The use of Pt as a reference electrode in afrozen electrolyte shows CV outcomes similar tothose generated when Ag/AgCl/frozen Agar (KClsaturated) was used as a reference electrode. Theresults are displayed in Figure 3. These results wereinterpreted using a high pressure effect model (24).

Multi One-Electron System1 mM of TCQN (7,7,8,8-tetracyanoquino-

dimethane) in acetonitrile containing 0.2 M LiClO4

(lithium perchlorate) was used as a medium toinvestigate a multi-one electron redox system.Figure 4 displays the CV of this system using a Ptdisc working electrode with a Pt wire electrode(solid trace), and Ag/AgCl/KCl (dashed trace) asreference electrodes. Two one-electron redoxwaves can be identified. It can be observed thatincreasing the scan rate from 5 mV s–1 (Figure 4(a))to 50 mV s–1 (Figure 4(b)) generated well definedredox waves. Measured ΔEp for redox waves after‘IR’ compensation (for the ohmic potential drop)indicates a typical one-electron wave character(amplitude 61 mV) which is very close to the theo-retical amplitude of 59 mV. The fact that theTCQN CV shows a consistent behaviour regard-less of the reference electrode used indicates thatthe results are attributable to the redox system andnot to the type of reference electrode. This conclu-sion demonstrates the usefulness of Pt as referenceelectrode in non-aqueous media for multi one-electron redox systems.

Ψ =


Multi Two-Electron Redox System5 mM H3PMo12O40 (a Keggin heteropolyacid)

immobilised in agar gel containing H2SO4/KClwas used as a model for a multi two-electron redoxsystem. Either a glassy carbon microelectrode (10μm diameter) or a disc electrode (0.07 cm2) wasused as a working electrode. The results are dis-played in Figure 5(a) (microelectrode) and Figure5(b) (disc electrode). Figure 5 clearly illustrates themulti two-electron redox waves typical of phos-phomolybdic acid. The measured ΔEp for each ofthe first two redox waves was less than 28 mV.The position of the formal potential of each ofthese waves was negatively shifted by approxi-mately 600 mV from that observed when aAg/AgCl/KCl reference electrode was used.

ConclusionsUnder certain conditions, such as high temper-

ature or in molten electrolytes, where the usualreference electrodes such as calomel orAg/AgCl/KCl electrodes cannot be used in elec-trochemical measurements, this study hasdemonstrated that Pt is the reference electrode ofchoice. Our study also shows that, under condi-tions where traditional reference electrodes areviable, Pt can replace them. Furthermore, in high-pressure electrochemical systems the change in theformal potential of the redox system followsEquation (ii):

ΔV° = –(δE°)TNF/δP (ii)

where δP is the partial derivative of the pressure.


= ipc= ipa

(d) (e)

= Epc= Epa

i p, μ

A

50

40

30

20

10

0 0.1 0.2 0.3 0.4 0.5V½

i p, μ

A

50

40

30

20

10

0 0.1 0.2V s–1

(c)

20 μA

+0.4 –0.4E, V

0

+1.0 +0.5 0 –0.4E, V

20 μA

(b)

–1.0 –2.0E, V

(a)

+1.0 0

Fig. 2 (a) CV at 0.20 V s–1 of: ⎯ GCE; - - - Pt reference electrode, in agar gel containing KCl; (b) CV at 100 V s–1 ofPt electrode in agar gel containing 5 mM K3[Fe(CN)6]/KCl: ⎯ vs. Ag/AgCl (agar saturated KCl) as referenceelectrode; - - - vs. Pt electrode; (c) CV of Pt electrode (0.02 cm2) in agar gel containing 5 mM K4[Fe(CN)6]/KCl at:0.020, 0.050, 0.10 and 0.150 V s–1 (inner to outer CV traces, respectively); (d) Plot of ip vs. V½ ( = ipc, = ipa);(e) Plot of ip vs. V sec–1 ( = Epc, = Epa)


In liquid electrolytes the change of the volume(ΔV°) originates in the outer sphere molecules inthe solvation layer. Traditional reference electrodecomponents contribute to this volume change. Thecapability of a Pt wire to act alone as a referenceelectrode without any associated solvated ions,eliminates the error in calculation of ΔV°.Consequently, the measured δE° relates morespecifically to the particular redox system. Ourstudies show a 3.5 mV change in the formal poten-tial (ΔE°) of [Fe(CN)6]–3/–4 when the aqueous liquidelectrolyte is frozen. ΔE° was 98 mV when aqueousgel electrolyte is frozen. It has been reported (26)that a change in the formal potential of the[Fe(CN)6]–3/–4 system in a liquid electrolyte of 3.93 ×10–5 V atm–1 took place when the system was sub-ject to pressure. This change in the formal potentialis equivalent to applying 89 atm and 2500 atm tofrozen aqueous and gel electrolytes of [Fe(CN)6]–3/–4

respectively. The volumes of both the reference andcounter electrode components will remain constant,because Pt wires were used as reference and counterelectrodes; the change in the formal potential is dueto the change in the volume of the redox ion. Thisis an additional advantage of using Pt as a referenceelectrode over the use of a traditional referenceelectrode under these conditions.

In conclusion, we have shown that where con-ventional reference electrodes are not suitable forsome electrochemical measurements, Pt wire is asatisfactory reference electrode in various electro-chemical systems such as aqueous, non-aqueous,gel or frozen electrolytes, and for measurementsunder high pressure. Single or multi one- or two-electron redox systems were studied, with peakseparation (ΔEp) indicating that Pt can be usedreliably as a reference electrode under a variety ofconditions.

+0.6 0 –0.6E, V

10 μA

(a)

0 –0.6E, V

(b)

6 μA

500 pA

(c)

+0.6 +0.3 0 –0.3E, V

+0.6

Fig. 3 CV at 100 mV s–1 for 5 mM [Fe(CN)6]3–/4–: (a) Pt disc in aqueous KCl; (b) Pt disc in frozen KCl electrolyte;(c) Pt microelectrode in frozen KCl electrolyte


1 D.-S. Kim, H.-J. Park, H.-M. Jung, J.-K. Shin, P.Choi, J.-H. Lee and G. Lim, Jpn. J. Appl. Phys., 2004,43, (6B), 3855

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6 Yu. K. Delimarskii, L. S. Berenblyum and I. N.Sheiko, Zh. Fiz. Khim., 1951, 25, 398

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Fig. 4 CV of GCE (0.07 cm2) in agar gel containing 1mM TCQN (7,7,8,8-tetracyanoquinodimethane) in 0.2 MLiClO4 in acetonitrile: ⎯ Pt reference electrode, - - -Ag/AgCl/Cl– reference electrode: (a) at 0.005 V s–1; (b) at0.50 V s–1

1 nA

10 μA

+1.0 0 –1.0E, V

(b)

0 –0.5 –1.1E, V

(a)

Fig. 5 CV at 0.5 V s–1 in 5 mM H3PMoO40 in agar gelcontaining H2SO4/KCl for: (a) carbon microelectrode;(b) GCE (0.07 cm2)

References

(a)

(b)

0 –0.2 –0.4 –0.6E, V

+0.2

+0.2 –0.6–0.4–0.20

E, V

4.0 μA

3.0 μA


19 E. P. Jacobs and F. Doerr, Ber. Bunsen-Ges., 1972, 76,(12), 1271

20 J. A. Cox, A. M. Wolkiewicz and P. J. Kulesza, J.Solid State Electrochem., 1998, 2, (4), 247

21 P. J. Kulesza and Z. Galus, J. Electroanal. Chem.,1992, 323, (1–2), 261

22 P. J. Kulesza, L. R. Faulkner, J. Chen and W. G.Klemperer, J. Am. Chem. Soc., 1991, 113, (1), 379

23 K. K. Kasem, J. New Mater. Electrochem. Syst., 2005, 8,(3), 189

24 L. S. Books, C. Harris and K. K. Kasem, Am. J.Undergrad. Res., 2007, 5, (4), 25

25 A. J. Bard and L. R. Faulkner, “ElectrochemicalMethods: Fundamentals and Applications”, 1stEdn., John Wiley & Sons, New York, 1980, p. 231

26 J. I. Sachinidis, R. D. Shalders and P. A. Tregloan,Inorg. Chem., 1994, 33, (26), 6180

The AuthorsDr Kasem K. Kasem is a Professor of Chemistry at IndianaUniversity Kokomo, U.S.A. He is interested indevelopments in the field of applied electrochemistry,especially in physical and analytical applications ofchemically-modified electrodes. He also has interests insemiconductor photoelectrochemistry, the electrochemicalbehaviour of polymeric thin films, activation andmetallisation of polymers, and the electrodeposition ofmetals and alloys.

Stephanie Jones participated in this work during her senioryear at Indiana University Kokomo, of which she is now analumna, having graduated with a bachelor’s degree inchemistry. She works with a chemical company inBloomington, Indiana, U.S.A.

A Faraday Discussion on “Nanoalloys”,organised by the Royal Society of Chemistry, andsponsored by Johnson Matthey and theCollaborative Research Network for Nanotech-nology (CRNNT) of the University ofBirmingham, was held in the Department ofChemistry, University of Birmingham, U.K., on3rd to 5th September 2007 (1). It was attended byabout sixty participants from the U.K., Europeand the U.S.A., mainly from academia. As usualwith Faraday Discussions, preprints were avail-able in advance, and authors had only fiveminutes to present the highlights of their work.There was therefore ample time for discussion,which never flagged and was at times vigorousand forthright.

The term ‘nanoalloy’ has been coined todescribe the assembly of a small number of atomsof metallic elements of two (or sometimes more)kinds. Although there is no consensus, the term‘cluster’ is used for an assembly formed in the gasphase, where the number of atoms is small (e.g. < 100) and countable by mass spectrometry,whereas the term ‘nanoparticle’ describes largerassemblies usually made by chemical routes (e.g.as colloids) and ranging in size from about 1 to 10nm. Both clusters and nanoparticles can bedeposited on supports, and their physical and cat-alytic properties examined in that state. Suchsmall assemblies are of great current interestbecause their properties often differ significantlyfrom those of the corresponding bulk materials,and nanoalloys are formed between pairs of ele-ments that do not form homogenous bulk alloys.

Theory and SimulationOf the twenty-five papers presented, seven-

teen concerned one or more of the platinum

group metals (pgms), the palladium-gold combi-nation being the most popular. Many of thepapers involved collaborations between severalinstitutions; the details of these are given in (2).The Discussion was divided into four parts: thefirst part dealt with ‘Theory and simulation ofnanoalloy structures and dynamics’, and need notdetain us, as in the main the work presentedappeared to be somewhat remote from practicalreality, and failed to produce insights into observ-able properties. In some of the papers, imagin-ative structures were invented and studied, irre-spective of whether they had been or could beprepared. Thus F. Calvo (Université ClaudeBernard Lyon 1, France) studied the interchangeof palladium and platinum atoms in icosahedralparticles containing alternating layers of the twokinds of atoms, and E. P. M. Leiva (UniversidadNacional de Córdoba, Argentina) et al. examinedby computer simulation how bimetallic nano-particles could arise by collision of two clusters,one of each kind. There were no computations ofthe number of angels that could dance on thepoint of a nanoalloy particle, but it would nothave been surprising if there had been.

Optoelectronic and Catalytic PropertiesThe Discussion returned to earth for the sec-

tion on ‘Electronic, optical and magneticproperties of nanoalloys’; the term ‘opto-electronic’ covers all three. Condensation con-trolled by laser vaporisation, reported by M. S.El-Shall (Virginia Commonwealth University,U.S.A.) et al. enabled nanoalloy particles to bemade of a number of binary combinations (PdAu,PtAu, PdFe, PtFe, PdNi and PtNi), those involv-ing iron and nickel being superparamagnetic. Themagnetism of CoRh nanoparticles was reported


Faraday Discussion 138: Nanoalloys –From Theory to ApplicationsPRE-EMINENT ROLE OF THE PLATINUM GROUP METALS

Reviewed by Geoffrey C. Bond59 Nightingale Road, Rickmansworth WD3 7BU, U.K.; E-mail: [email protected]

DOI: 10.1595/147106708X298278

by G. M. Pastor (Universität Kassel, Germany)and colleagues. The average spin moment peratom is larger than for macroscopic materials,and the likely structures have a rhodium coreand a cobalt-rich outer layer. The structure ofCoPt particles was investigated theoreticallyby C. Mottet (Centre de Recherche en MatièreCondensée et Nanosciences (CRMC-N),Marseille, France) et al.

The six papers in ‘Nanoalloys in Catalysis’ oncatalytic properties of nanoalloys having one ofthe pgms as a component stimulated a lively dis-cussion. G. J. Hutchings (Cardiff University,U.K.) et al. reported their latest results on hydro-gen peroxide synthesis by oxidation of hydrogenusing catalysts containing palladium and/orgold. With carbon as support, high activitieswere obtained, and binary compositions con-tained homogeneous alloy particles, but withtitania and alumina supports core-shell struc-tures occurred. P. A. Sermon (University ofSurrey, U.K.) et al. advocated using alkane con-versions (hydrogenolysis, isomerisation) as ameans of characterising the surface of PtAu andPtSn nanoparticles, illustrating their catalyticcapability by reference to the reactions ofn-hexane and methylcyclopentane. P. Wells(University of Southampton, U.K.) and col-leagues showed that the oxygen reductionactivity of the Pt3Cr nanoalloy phase was betterthan that of platinum alone in model fuel cells;this work had input from D. Thompsett of theJohnson Matthey Technology Centre, U.K.

Structure-performance relationships inPdRh/γ-Al2O3 catalysts for the NO-CO reactionwere described by M. Tromp (University ofSouthampton, U.K.) and coworkers. Inclusionof palladium prevented dissociative oxidation ofrhodium by NO, but did not stop its extensivedisruptive oxidation by CO. Sir John MeurigThomas (University of Cambridge, U.K.) andassociates showed that organometallic clustercompounds comprising ruthenium and tinatoms could be decomposed on a silica supportto give effective catalysts for the selective hydro-genation of cyclododecatriene to cyclododeceneand for other reactions.

Structural StudiesThe fourth section of the Discussion concen-

trated on ‘Structural studies of nanoalloys’, thepalladium-gold combination proving the mostpopular. The use of energy dispersive X-rayspectroscopy for investigating the structure ofsupported PdAu catalysts used for hydrogenperoxide synthesis was explained by C. J. Kiely(Lehigh University, Bethlehem, Pennsylvania,U.S.A.) and colleagues. The principal conclu-sions of this work were mentioned inconnection with the Cardiff University work (seeabove). Strongly size-controlled synthesis oficosahedral palladium-gold nanoparticles hasbeen accomplished in inert-gas condensation ina sputtering reactor by E. Pérez-Tijerina(Universidad Autónoma de Nuevo Léon,Monterrey, Mexico) and colleagues; particleswere homogeneous and did not show core-shellstructures. This combination was also studied byC. R. Henry (CRMC-N, Marseille, France) andhis associates; they prepared bimetallic particlesuniformly dispersed on nanostructured aluminafilm by sequential condensation of the twokinds of atoms.

Concluding RemarksIn conclusion, this Discussion clearly

demonstrated the potential for practical appli-cations of small bimetallic particles. What is ofparticular interest and importance is the factthat small homogeneous alloy particles can beformed from pairs of elements for which thebulk phase diagram shows a miscibility gap.This point did not receive emphasis in thisDiscussion, and it is unfortunate that theoreti-cians have not addressed the problem, thenature of which has recently been considered inthe context of the platinum-gold pair (3).

Discussion on the papers relating to cataly-sis focused on the utility of physicalcharacterisation of catalysts before (or occa-sionally after) use in understanding theircatalytic performance. It was noted that inmany papers much more time and effortappeared to have been spent on the character-ising than on the catalytic reaction, the



investigation of which was often brief and sim-ple. Failure to consider why a particularstructure or composition behaved as it didbetrays a lamentable lack of curiosity, andretards the development of basic theory. Thediscussions, together with the texts of thepapers, will be published in full by the RoyalSociety of Chemistry in 2008 (2); they shouldmake interesting reading.

References1 Faraday Discussion 138: Nanoalloys – From

Theory to Applications, 3rd–5th September, 2007,University of Birmingham, U.K.:http://www.rsc.org/FD138

2 “Nanoalloys from Theory to Applications”,Faraday Discussions No 138, RSC Publishing,Cambridge, U.K., 2008:http://www.rsc.org/shop/books/2008/9780854041190.asp

3 G. C. Bond, Platinum Metals Rev., 2007, 51, (2), 63

The AuthorGeoffrey Bond held academic postsat Leeds and Hull Universitiesbefore joining Johnson Matthey PLCin 1962 as Head of CatalysisResearch. In 1970 he was appointedProfessor in Brunel University’sChemistry Department, and is nowEmeritus Professor.


Around eighty participants with an interest inhomogeneous catalysis came together inLondon on 6th November 2007 to discuss“Challenges in Catalysis for Pharmaceuticalsand Fine Chemicals” at a symposium organisedby the Society of Chemical Industry FineChemicals Group and the Royal Society ofChemistry Applied Catalysis Group. Inresponse to increased focus on environmentalissues and sustainability for the preparation ofactive pharmaceutical ingredients and finechemicals, industry and academia are collaborat-ing to identify catalytic processes that willreduce waste, energy demand and safety hazardsby replacing the use of some stoichiometricreagents. This symposium highlighted some ofthe areas where progress is being made. A seriesof seven oral presentations was supported by aposter session where students presented theirwork on environmental improvements in chem-istry. Content involving platinum group metalsis described here.

Improvements in PharmaceuticalManufacturing

Peter Dunn (Pfizer, U.K.) reviewed the back-ground to this meeting and in particular the roleof the American Chemical Society GreenChemistry Institute Pharmaceutical Roundtable(1). This forum allows an exchange of informa-tion between the major pharmaceuticalcompanies with regard to improving the sustain-ability of manufacturing for active pharmaceuticalingredients. A survey of the reactions used insuch preparations (2) provided the backgroundinformation for identifying those areas whereimprovements were most needed. The results ofthese further discussions were also published (3).

The topics were divided between the categoriesof common reactions requiring better reagents,and more aspirational reactions where new tech-nology is required for industrial application. Apriority ranking was used to highlight five exam-ples in each category, for example, amideformation avoiding poor atom economy reagentswas the item with the highest priority in the for-mer category. The Roundtable has the capabilityto offer research grants to meet these objectives,with two programmes already operating (one inhydrogenation of amides and one in C–H activa-tion for biaryl coupling) and further grants are onoffer for 2008 and 2009.

Rhodium-catalysed asymmetric hydrogenationis one of the most commonly used homogeneouscatalysis processes in the pharmaceutical industry.Nevertheless, Johannes de Vries (DSMPharmaceutical Products and University ofGroningen, The Netherlands) noted that thereare relatively few examples due to a number ofobstacles: long process development time (requir-ing improved, automated screening); high cost(requiring cheaper metals, simpler ligands andgreater catalyst turnover numbers); limited toler-ance for substrate variations and lengthy orinefficient routes for substrate preparation.Improvements can be achieved through a modu-lar approach to ligand design, allowing ligandlibraries to be generated by automated equip-ment. Ligand design can thus become part of awider, automated screening process. The devel-opment of chiral monophosphoramidite ligands,1, (4), which have proved to give enantioselectiv-ities equivalent to or even better than those of thewell known chiral bisphosphine ligands in Rh-catalysed asymmetric hydrogenation, was used toillustrate this. Another elegant approach to the

Challenges in Catalysis forPharmaceuticals and Fine ChemicalsPLATINUM GROUP METALS IN CATALYTIC PROCESSES

Reviewed by Chris BarnardJohnson Matthey Technology Centre, Blounts Court, Sonning Common, Reading RG4 9NH, U.K.; E-mail: [email protected]

DOI: 10.1595/147106708X298287


design of bidentate ligands is the use ofsupramolecular chemistry to bring two moleculestogether so that the donor atoms are brought intothe correct positions for coordination.

While much attention has been given to devel-oping catalysis for particular substrates, ofteninsufficient attention has been paid to the impactof preparation of these substrates on the overallprocess efficiency. Thus the benefits of the cat-alytic step may be nullified if the synthesis andpurification of the substrate add extra steps. Forexample, the preparation of pure E- or Z-vinylcompounds is often difficult and the purificationof imines may present an issue. This issue meritsgreater attention to prevent subsequent effort onthe optimisation of the catalytic step beingwasted.

The lack of generality in application of cat-alytic methods can also be a limiting factor.For example, unfunctionalised olefins may givelow enantioselectivity, and molecules contain-ing sulfur donors may act as catalyst poisons.No effective catalyst is available for homo-geneous hydrogenation of substituted aromaticcompounds, and the more reactive hetero-aromatic compounds generally give poorenantioselectivity.

One area where synthesis of the substrate hasa major impact on catalytic chemistry is the use ofaryl boronic acid derivatives as coupling partnersin palladium-catalysed biaryl synthesis (Suzuki-Miyaura reaction). The process would be greatly

simplified if C–H activation of aromatic groupscould be achieved directly. This was discussed byRobin Bedford (University of Bristol, U.K.).Significant progress in this area has been madefor intramolecular systems, while, with the aid ofdirecting groups, some intermolecular reactionscan also be carried out effectively. The initialreport of Pd-catalysed cyclisation of 2-substitutedhalogenoarenes to form dibenzofurans and relat-ed heterocycles (intramolecular 5-membered ringformation) appeared in 1984 (5). Statistically it ismore difficult to form larger rings, but 7- and 8-membered ring formation has now beenreported. Bedford, himself, has shown how newrings can be formed by combining sequential Pd-catalysed aryl bromide and C–H activationreactions (6). The major challenges to the imple-mentation of chemistry based on C–H activationare selectivity (both with regard to single versusmultiple substitution and regioselectivity), catalystloading/activity, and the complexity and corre-sponding high cost of the catalysts. However,progress is being made on each of these issues.

Hydroformylation is a well-established chem-istry in the bulk preparation of aldehydes. Thedevelopment of the Rh-catalysed LP OxoSM

process was recently reviewed in this Journal(7, 8). However, the reaction is little used in thepharmaceutical industry. Graham Meek(DowpharmaSM, U.K.) described his company’sefforts to make hydroformylation more attractiveto the pharmaceutical industry. Having inheritedUnion Carbide’s expertise following thattakeover, Dowpharma have further developed arange of bisphosphite ligands, particularly oneknown as BIPHEPHOS, 2, (9), that gives satis-factory rates and high yields of linear aldehydesfor a wide variety of substrates. Reactions cantypically be carried out at 3 bar pressure and 80ºCwith a substrate:catalyst ratio of around 1000.The reactivity of the aldehyde product means thatunder certain circumstances tandem reactions canbe carried out. Thus, for example, hydroamino-methylation (the combination of hydro-formylation and reductive amination) may beused to synthesise secondary and tertiary aminesfrom olefins.

R1 = R2 = Me (MonoPhos)R1 = R2 = EtR1, R2 = –(CH2)5– (PipPhos)R1, R2 = –[(CH2)2]2O (MorfPhos)R1 = H, R2 = (R)-CHMePhR1 = H, R2 = iPr

1


For enantioselective hydroformylation, earlywork studied the reaction of styrene, withBINAPHOS a preferred ligand. More recently,chiral phosphites and diazaphospholanes havebeen developed by Dowpharma. The enantiose-lectivity of these reactions is often highly sensitiveto minor structural changes in the ligand, so theavailability of an extensive ligand library of thesecompounds is a distinct advantage.

The synthesis of amides with minimal waste isa highly desired transformation. Andy Whiting(University of Durham, U.K.) described a rangeof studies aimed at achieving this from carboxylicacid and amine substrates. Because of the difficul-ties of this direct approach, various other routesto amides are being developed. For example, thereaction of amines and alcohols to form amidesusing ruthenium-pincer catalysts has been report-ed by Milstein (10). So far, the reaction haslimited scope for substrates, being sensitive tosteric hindrance at the α-position of either part-ner and not giving the desired product withsecondary amines. Amides can also be formed byaminocarbonylation of aryl halides. Buchwald hasrecently reported (11) that earlier work on thisPd-catalysed reaction of aryl iodides and bro-mides can be extended to aryl chlorides in thepresence of sodium phenoxide. The reaction pro-ceeds via the initial formation of the phenyl ester,which then undergoes a phenoxide-catalysed acyltransfer reaction to form the amide.

The asymmetric addition of –NH and –OHfunctions to C=C bonds was reviewed by MimiHii (Imperial College London, U.K.). Alternativeroutes to the chiral amine and alcohol productsinclude hydroboration or hydrogenation, but con-ditions for more direct reaction are desirable. Themajority of the research has been carried out onthe hydroamination reaction. As usual, theintramolecular reaction was performed first withthe intermolecular enantioselective reaction beingreported by Togni in 1997 (12) using iridium(I)catalysts, but later work by Hartwig (13) and byHii (14) has shown that significantly improvedresults can be obtained with Pd catalysts.Thermodynamics provides a constraint for theseconversions, since there is little driving force forthe reaction (ΔG ~ 0). Only limited yields ofproducts are possible in some cases. The reactionof activated alkenes is also known as the aza-Michael reaction. In this case, reaction of primaryaromatic amines has been achieved under mildconditions (< 60ºC in toluene).

Compared to hydroamination, the addition of–OH has received little attention. Cyclisation byenantioselective –OH addition (an intramolecu-lar, asymmetric version of the Wacker reaction)was first reported in 1981 (15) with modest enan-tiomeric excess. More recent work by Hayashi(16) and others has improved these results, butexamples of this chemistry are still limited.

Poster PresentationsIn the poster presentations, Asma Qazi

(Queen Mary University, London, U.K.)described various forms of Pd(II) immobilised onsilica. While Pd coordination to sulfur donorsgave materials with low leaching, they showedlow activity in Suzuki coupling (catalyst loadingca. 5 mol%). Better activity was seen for an ethyl-phosphatrioxaadamantane Pd catalyst that waseffective at 0.1 mol%. Direct arylation for hetero-cycles is often more efficient than for arylcompounds due to the strong directing effect ofthe heteroatom. The C-5 arylation of thiazoleswas reported by Gemma Turner (University ofEdinburgh, U.K.). This reaction can be carriedout with Pd catalysis in only 12 hours at 60ºC

MeO

MeO

O

O

P

P

O

O

O

O

2

OO

O

O

O

O

P

P

MeO

MeO

BIPHEPHOS


using water as solvent.Nathan Owston (University of Bath, U.K.)

reported on studies of metal-catalysed amidepreparation from alcohols. Catalyst loadings canbe significantly reduced by changing from an Ircatalyst to a Ru one, and the methodology hasnow been adapted to allow formation of a varietyof acyl derivatives (e.g. esters). Xiaofeng Wu(University of Liverpool, U.K.) described amonotosylated ethylenediamine Ir(III) complexwhich can be used as catalyst for hydrogenationof aldehydes in water as solvent (using hydrogengas or by transfer hydrogenation).

Concluding RemarksIt is clear that improved catalytic chemistry will

be required to replace wasteful stoichiometricroutes for pharmaceuticals and fine chemicals inthe future. Improvements to catalytic processeswill involve adaptation to more readily availablesubstrates, simpler ligand preparation and theminimisation of metal costs through lower cata-lyst loadings. It is hoped that developmentstowards these goals will provide the focus for fur-ther meetings on this topic in the future, to beorganised by the Society of Chemical Industry andthe Royal Society of Chemistry.

1 American Chemical Society Green ChemistryInstitute Pharmaceutical Roundtable: http://portal.acs.org/portal/PublicWebSite/greenchemistry/industriainnovation/roundtable/index.htm

2 J. S. Carey, D. Laffan, C. Thomson and M. T.Williams, Org. Biomol. Chem., 2006, 4, (12), 2337

3 D. J. C. Constable, P. J. Dunn, J. D. Hayler, G. R.Humphrey, J. L. Leazer, Jr., R. J. Linderman, K.Lorenz, J. Manley, B. A. Pearlman, A. Wells, A.Zaks and T. Y. Zhang, Green Chem., 2007, 9, (5),411

4 D. J. Ager, A. H. M. de Vries and J. G. de Vries,Platinum Metals Rev., 2006, 50, (2), 54

5 D. E. Ames and A. Opalko, Tetrahedron, 1984, 40,(10), 1919

6 R. B. Bedford and M. Betham, J. Org. Chem., 2006,71, (25), 9403

7 R. Tudor and M. Ashley, Platinum Metals Rev., 2007,51, (3), 116

8 R. Tudor and M. Ashley, Platinum Metals Rev., 2007,51, (4), 164

9 C. J. Cobley, R. D. J. Froese, J. Klosin, C. Qin andG. T. Whiteker, Organometallics, 2007, 26, (12), 2986

10 C. Gunanathan, Y. Ben-David and D. Milstein,Science, 2007, 317, (5839), 790

11 J. R. Martinelli, T. P. Clark, D. A. Watson, R. H.Munday and S. L. Buchwald, Angew. Chem. Int. Ed.,2007, 46, (44), 8460

12 R. Dorta, P. Egli, F. Zürcher and A. Togni, J. Am.Chem. Soc., 1997, 119, (44), 10857

13 M. Kawatsura and J. F. Hartwig, J. Am. Chem. Soc.,2000, 122, (39), 9546

14 K. Li, P. N. Horton, M. B. Hursthouse and K. K.(M.) Hii, J. Organomet. Chem., 2003, 665, (1–2), 250

15 T. Hosokawa, T. Uno, S. Inui and S. Murahashi, J.Am. Chem. Soc., 1981, 103, (9), 2318

16 Y. Uozumi, K. Kato and T. Hayashi, J. Am. Chem.Soc., 1997, 119, (21), 5063

The ReviewerChris Barnard is a Scientific Consultantin the Liquid Phase Catalysis Group atthe Johnson Matthey TechnologyCentre, U.K., with interests inhomogeneous catalysis employing theplatinum group metals. He is alsointerested in the application of platinumcompounds as cancer therapy.

References


The Periodic Table and the PlatinumGroup MetalsBy W. P. GriffithDepartment of Chemistry, Imperial College, London SW7 2AZ, U.K.; E-mail: [email protected]

The year 2007 marked the centenary of the death of Dmitri Mendeleev (1834–1907). Thisarticle discusses how he and some of his predecessors accommodated the platinum groupmetals (pgms) in the Periodic Table, and it considers the placing of their three transuraniccongeners: hassium (108Hs), meitnerium (109Mt) and darmstadtium (110Ds). Over twenty-fiveyears ago McDonald and Hunt (1) wrote an excellent account of the pgms in their periodiccontext. This account is indebted to that work. The present article introduces new perspectivesand shows some of the relevant tables. There are good books on the history of the PeriodicTable, e.g. (2, 3) and other texts (4, 5) which provide a fuller picture than it is possible togive here.

DOI: 10.1595/147106708X297486

Discovery and Early Classificationof the Platinum Group Metals

Antoine-Laurent Lavoisier (1743–1794) in 1789defined the element as being “the last point thatanalysis can reach”, and it was largely this clear state-ment which brought about the discovery of 51 newelements in the nineteenth century alone. JohnDalton’s (1766–1844) recognition in 1803 of theatom as being the ultimate constituent of an element,with its own unique weight, was crucial. StanislaoCannizzaro (1826–1910), at the celebrated KarlsruheCongress (1860), published a paper recognising thetrue significance of Avogadro’s molecular hypothesisand thereby clarified the difference between atomicand molecular weights. From then, reasonably accu-rate atomic weights of known elements becamereadily available and greatly helped the constructionof useful Periodic Tables. Atomic (or elemental)weights were useful but were not a sine qua non fortable construction. A number of tables were pro-duced with incorrect values, or, as Mendeleev laternoted, inconsistencies in published atomic weightsbecame apparent from these tables. We have thebenefit of hindsight and know that atomic numbersare crucial factors for periodicity.

Platinum is a metal of antiquity, but the other fivepgms were isolated in the nineteenth century. Thebicentenaries of four were marked in this Journal:William Hyde Wollaston’s (1766–1828) discovery of

palladium and rhodium in 1802 and 1804 (6) andSmithson Tennant’s (1761–1815) isolation of iridi-um and osmium in 1804 (7, 8). Ruthenium was thelast to be isolated, by Karl Karlovich Klaus(1796–1864) in 1844 (9–11). Thus, five of the sixwere known by 1804, and the sixth by 1844, in goodtime for the development of the Periodic Table.

The pgms are now known to fall into two hori-zontal groups: Ru-Rh-Pd and Os-Ir-Pt, but webenefit from some 200 years of hindsight in thisobservation. Johann Döbereiner (1780–1849) notedsimilarities in the chemical behaviour of ‘triads’ ofelements, in which the equivalent weight of the mid-dle element lay roughly halfway between those of theother two. In 1829, when Professor of Chemistry atJena, he used his equivalent weights for these metals(based on oxygen = 100) to demonstrate that Pt-Ir-Os and Pd-‘pluran’-Rh ‘triads’ existed (12). ‘Pluran’had been reported together with two other ‘new’ ele-ments in 1827 by Gottfried Osann (1796–1866). Itmay possibly have contained some ruthenium, butBerzelius was unable to confirm the novelty of thesethree elements, and Osannn subsequently withdrewhis claim (13).

In 1853 John Hall Gladstone (1827–1902), thena chemist at St. Thomas’s Hospital, London, notedthat the Rh-Ru-Pd triad was related to that ofPt-Ir-Os, while the ‘atomic weights’ (sic) of the lattertriad were roughly twice those of the former (14). In


1857 William Odling (1829–1921), then teachingchemistry at Guy’s Hospital, London, noted thegreat similarity of Pd, Pt and Ru, that the ‘atomicweight’ (sic) of Pt (98.6) was about twice that of Pd(53.2), and that Pt, Ir and Os were chemically similar(15). The stage was now set for a periodic classifica-tion of these and indeed all the elements thenknown.

The Development of PeriodicClassifications

In 1862 Alexandre-Emile Béguyer deChancourtois (1820–1886), Professor at the Écoledes Mines, Paris, devised a ‘vis tellurique’ (telluricscrew) (16), a helix on a vertical cylinder on whichsymbols of the elements were placed at heights pro-portional to their atomic weights. Although somepgms appeared on it (Rh and Pd on one incline andIr and Pt on another), no relationships betweenthem are discernible.

Karl Karlovich Klaus, then professor of chem-istry at the University of Kazan (now in Tatarstan),had discovered Ru in 1844 (9–11) and knew moreabout the pgms than anyone else. In 1860he arranged the three most abundant ones in aPrincipal series (Haupt Reihe), and beneath themplaced a Secondary series (Neben Reihe), notingalso the chemical similarities of each vertical pair(17–19) (Figure 1 (18)).

Klaus’s table shows the correct vertical pairs, butnot in the now accepted sequence. The pgms werenot set in the context of other elements. In 1864 theanalytical chemist John Alexander Raina Newlands(1837–1898) proposed the first of his tables, arrang-ing the known 61 elements in order of ascendingatomic weights (20, 21). In his subsequent ‘law ofoctaves’ he noted that the chemical properties ofsome elements were repeated after each series ofseven, and assigned ordinal numbers to elements inthe sequence of their ascending atomic weights: an

early form of the atomic number (e.g. H = 1, Li = 2etc.) (22). Although the pgms featured in Newlands’stables they were often out of place. William Odling(born, like Newlands, in Southwark, London),whose pgm triads we have noted above (15), pro-duced in 1864 a table of 61 elements in which the sixpgms were grouped together (Ro is rhodium). Hewas the first to arrange them in a reasonably logicalway in a Periodic Table (Figure 2) (23).

The stage was now set for two giants of periodic-ity, Lothar Meyer and, above all, Dmitri Mendeleev.In 1868 Julius Lothar Meyer (1830–1895), Professorof Chemistry at Tübingen arranged 52 elements inan unpublished table with Ru & Pt, Rh & Ir, Pd &Os side-by-side. His slightly later table, published in1870 (24), places the pgms correctly, but a numberof other elements lie in a sequence different fromthat of modern tables:

Mn = 54.8 Ru = 103.5 Os = 198.6?Fe = 55.9 Rh = 104.1 Ir = 196.7Co = Ni = 58.6 Pd = 106.2 Pt = 196.7

On 6th March, 1869, Dmitri Mendeleev(1834–1907) produced his first table (25, 26).Mendeleev was born in Tobolsk, Siberia, the last offourteen children. His father became blind whenDmitri was sixteen, and his indomitable mother,determined that he should be well educated, hitch-hiked with him on the 1400 mile journey to theUniversity at Moscow. Here he was refused admit-tance because he was Siberian; they travelled afurther 400 miles to St. Petersburg. There in 1850Mendeleev got a job as a trainee teacher; his motherdied from exhaustion in the same year. In 1866, aftera spell of study in Germany (he had attended the1860 Karlsruhe Congress) and France, Mendeleevbecame Professor of Chemistry at the University ofSt. Petersburg.

Mendeleev’s interest in periodicity may well havedated from the Karlsruhe Congress and been

Fig. 1 Klaus’s arrangement of theplatinum group metals of 1864 (18)


cemented by a textbook on inorganic chemistry, partof which he finished in 1868. More than any of hispredecessors in the field of periodicity, he had aremarkable knowledge of the chemistry of the ele-ments. His first published version placed the pgmstogether but with unusual pairings (25, 26):

Rh 104.4 Pt 197.4Ru 104.4 Ir 198Pd 106.6 Os 199

The version normally regarded as Mendeleev’sdefinitive table appeared in 1871, first printed in aRussian journal (27) and then reprinted in Annalen inthe same year (Figure 3) (28). By then Mendeleevhad seen Lothar Meyer’s paper and almost certainlyknew of Newlands’s and Odling’s work, but his tablerepresents a major advance in classification of theelements, for the first time placing the pgms in theirmodern sequence and in context. The dashes underthe Ru-Rh-Pd-Ag listing under Group VIII misledsome later workers to think that missing elementswere being denoted (13). Acceptance of his table was

partly brought about by his astonishingly accuratepredictions of the properties of the then unknownscandium (shown as ‘–- = 44’ in Figure 3), gallium‘–- = 68’ and germanium ‘–- = 72’. Mendeleev’s pre-dictions also led to the subsequent discovery of otherelements including francium, radium, technetium,rhenium and polonium. Other factors such as thesuccessful accommodation or placement of the ele-ments were also important, a topic well discussed ina recent book (3).

It is apparent from Mendeleev’s tables that forhim (and others) the pgms, some of the transitionmetals, lanthanides and actinides then known poseda problem; here we concentrate on the pgms. Henoted their very similar properties and that therewere very small differences between the atomicweights of Ru-Rh-Pd and between those of Os-Ir-Pt (28). He knew that only Ru and Os demonstratedoctavalency in Group VIII (‘RO4’; R denotes an ele-ment), but includes Rh, Pd, Ir and Pt in GroupVIII. Mendeleev also placed iron, cobalt and nickel,and the coinage metals copper, silver and gold in

Fig. 2 William Odling’s table ofelements from 1864 (23)


Group VIII; he additionally accommodated thecoinage metals in Group I. His problems with all hisGroup VIII elements continued to trouble him: aslate as 1879 he published two papers in ChemicalNews which tried to address this difficulty (29, 30).In the first paper he split Groups I–VII into left-hand ‘even’ and right-hand ‘odd’ blocks, withGroup VIII in the centre, Cu, Ag and Au beingaccommodated in both VIII and the ‘odd’ I–VIIblock (29). In the second paper he ruefully refers toGroup VIII as ‘special’ and ‘independent’ (30).

Mendeleev published some thirty PeriodicTables and left another thirty unpublished (3), butthe 1871 one (Figure 3) (28) is his most successful:it is the definitive Periodic Table of the nineteenthcentury and the basis of all later ones. As late as1988, the leading inorganic textbook “AdvancedInorganic Chemistry”, by Cotton and Wilkinson(fifth edition) (31) shows Group VIII as containingthe nine elements Fe, Co, Ni and the pgms (Cu, Agand Au are designated as Group IB). It was only inthe sixth edition of 1999 that the modern form(Figure 4), in which the pgm vertical pairs are inGroups 8, 9 and 10, was used (32).

The Transuranic Congeners of thePlatinum Group Metals

The story now moves forward to the SecondWorld War, when there was discussion as towhether uranium, neptunium and plutonium were

appropriately placed in the fourth row of the tran-sition metals (using 6d orbitals), or were membersof a lanthanide-like series, the ‘actinides’, using 5forbitals. The latter view prevailed (33), and now allthe actinides (thorium to lawrencium inclusive) areknown. Indeed, elements up to and including 118are now established, with the exception of element117 (34). These elements are recognised by theInternational Union of Pure and AppliedChemistry (IUPAC), although only those up to 111have ‘official’ names (Figure 4) (35); see also (36).Mendeleev’s table (28) omits most of the lan-thanides and actinides and, of course, the noblegases which were not known when he made up histable. However, some 140 years earlier, his versionhad essentially contained the kernel of our modernPeriodic Tables.

Recent chemical work on a few very short-livedatoms of each element strongly suggests that ele-ments 104 to 111 are members of a fourthtransition metal series involving 6d orbitals. Thus104rutherfordium, 105dubnium, 106seaborgium and107bohrium have properties analogous to those ofhafnium (Group 4), tantalum (Group 5), tungsten(Group 6) and rhenium (Group 7) respectively.

The next three elements were all made in thelinear accelerator in the city of Darmstadt, Hessen,Germany. Hassium was first made in 1984, andnamed from the Latin ‘Hassias’ for the state ofHessen. Meitnerium was first made in 1982, and

Fig. 3 Mendeleev's Periodic Table of 1871 (28)


named after Lise Meitner (1878–1968), the dis-coverer of protactinium in 1917. Darmstadtiumwas first made in 1994, and named afterDarmstadt. For any meaningful chemistry to becarried out on a given element, at least four atomsare necessary, of half-life (t½) > 1 second, and aproduction rate of at least one atom per week isrequired. The nuclear reactions producing the ele-ments should give only single products. For thesethree elements the most useful nuclear reactionsare (Equations (i)–(iii)):

Of these, 269Hs and 270Hs have t½ = 14 and 23 srespectively; 266Mt has t½ = 6 × 10–3 s and 271Dt hast½ = 6 × 10–2 s, so at present chemistry can only becarried out on hassium. It is clearly a congener ofOs: using just seven atoms it was found to form avolatile tetroxide (37) which in alkaline NaOHgives a species which is probably cis-Na2[HsO4(OH)2] (38). For studies on meitneriumand darmstadtium to be made, longer-lived iso-topes are essential – they would also be much

more difficult to study chemically, since distinc-tive volatile Ir and Pt compounds are rare anddifficult to synthesise on a very small scale, unlikeHsO4, although the fluorides IrF6 and PtF6 arevolatile above 60ºC. It seems likely, however, thatthese three elements are congeners of Os, Ir andPt, particularly since it has recently been shownthat the unnamed (at the time of writing) element112 is itself volatile. This suggests that it is a con-gener of mercury (39), as would be expected ifelements 104–111 inclusive form a fourth transi-tion metal series.

ConclusionsThe story of the Periodic Table is convoluted,

and this article has concentrated on the pgms. Itis clear that they represented a challenge to themakers of the tables, but the problem was finallyresolved by Mendeleev some 140 years ago (28).The three man-made congeners of these ele-ments, hassium, meitnerium and darmstadtium,are likely to have chemistries similar to those ofosmium, iridium and platinum. At the time ofwriting it has been possible to demonstrate thisonly for hassium.

Fig. 4 The current Periodic Table (35) based on IUPAC recommendations

248Cm + 26Mg → 269, 270Hs + 5 or 4 1n (i)209Bi + 58Fe → 266Mt + 1n (ii)208Pb + 64Ni → 271Ds + 1n (iii)

96

83

82

12

26

28

108

109

110

0

0

0


AcknowledgementsI am grateful to Professor Christoph Düllmann

(Gesellschaft für Schwerionenforschung mbH,

Darmstadt, Germany) and Dr Simon Cotton(Uppingham School, Rutland, U.K.) for their adviceon aspects of transuranium chemistry.

1 D. McDonald and L. B. Hunt, “A History ofPlatinum and its Allied Metals”, Johnson Matthey,London, 1982, p. 333

2 J. W. van Spronsen, “The Periodic System ofChemical Elements: A History of the First HundredYears”, Elsevier, Amsterdam, 1969

3 E. R. Scerri, “The Periodic Table: Its Story and ItsSignificance”, Oxford University Press, New York,U.S.A., 2007

4 M. E. Weeks and H. M. Leicester, “Discovery of theElements”, 7th Edn., Journal of ChemicalEducation, Easton, Pennsylvania, U.S.A., 1968

5 W. H. Brock, “The Fontana History of Chemistry”,Fontana Press, London, 1992

6 W. P. Griffith, Platinum Metals Rev., 2003, 47, (4), 1757 W. P. Griffith, Platinum Metals Rev., 2004, 48, (4), 1828 M. Usselman, in “The 1702 Chair of Chemistry at

Cambridge”, eds. M. D. Archer and C. D. Haley,Cambridge University Press, Cambridge, U.K., 2005,Chapter 5, p.113

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18112 J. W. Döbereiner, Ann. Phys. Chem. (Poggendorff), 1829,

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Inorganic Chemistry: A Comprehensive Text”, 5thEdn., John Wiley & Sons, Chichester, U.K., 1988

32 F. A. Cotton, G. Wilkinson, C. A. Murillo and M.Bochmann, “Advanced Inorganic Chemistry”, 6thEdn., John Wiley & Sons, Chichester, U.K., 1999

33 G. T. Seaborg, Chem. Eng. News, 10th December,1945, 23, (23), 2190

34 S. Cotton, “Lanthanide and Actinide Chemistry”,John Wiley & Sons, Chichester, U.K., 2006

35 Periodic Table, World Wide Web version preparedby G. P. Moss, London, U.K., 2007:http://www.chem.qmul.ac.uk/iupac/AtWt/table.html

36 IUPAC Periodic Table of the Elements, 2007:http://www.iupac.org/reports/periodic_table/index.html

37 Ch. E. Düllmann, W. Brüchle, R. Dressler, K.Eberhardt, B. Eichler, R. Eichler, H. W. Gäggeler, T.N. Ginter, F. Glaus, K. E. Gregorich, D. C.Hoffman, E. Jäger, D. T. Jost, U. W. Kirbach, D. M.Lee, H. Nitsche, J. B. Patin, V. Pershina, D. Piguet,Z. Qin, M. Schädel, B. Schausten, E. Schimpf, H.-J.Schött, S. Soverna, R. Sudowe, P. Thörle, S. N.Timokhin, N. Trautmann, A. Türler, A. Vahle, G.Wirth, A. B. Yakushev and P. M. Zielinski, Nature,2002, 418, (6900), 859

38 A. von Zweidorf, R. Angert, W. Brüchle, S. Bürger,K. Eberhardt, R. Eichler, H. Hummrich, E. Jäger,H.-O. Kling, J. V. Kratz, B. Kuczewski, G.Langrock, M. Mendel, U. Rieth, M. Schädel, B.Schausten, E. Schimpf, P. Thörle, N. Trautmann, K.Tsukada, N. Wiehl and G. Wirth, Radiochim. Acta,2004, 92, (12), 855

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References

The AuthorBill Griffith is an Emeritus Professor of Chemistry at Imperial College, London. He has much experience with the platinumgroup metals, particularly ruthenium and osmium. He has published over 260 research papers, many describing complexesof these metals as catalysts for specific organic oxidations. He has written seven books on the platinum metals, and iscurrently writing another on oxidation catalysis by ruthenium complexes. He is the Secretary of the Historical Group of theRoyal Society of Chemistry.


Frederick Alastair Lewis died in Belfast,Northern Ireland, on 22nd May 2007 after along illness. He had been a frequent contributorto Platinum Metals Review. Born in Belfast in1926, Fred served in the Royal Navy during theSecond World War. After his naval service(1944–47), he began the study of Chemistry atQueen’s University, Belfast, which culminatedin his Ph.D. degree under the direction ofProfessor A. R. Ubbelohde. He subsequentlycarried out postdoctoral studies at ImperialCollege, London, again under the direction ofProfessor Ubbelohde. While at ImperialCollege, Fred and Ubbelhohde coauthored thebook, “Graphite and Its CrystallineCompounds” (1). In 1956 Fred returned toQueen’s University as a Lecturer and laterbecame a Reader in Inorganic Chemistry.

During his career at Queen’s, Fred carriedout research on metal-hydrogen systems andspecifically on palladium and its alloys. His 33contributions to Platinum Metals Review (2–34)included numerous review articles; his first,‘The Hydrides of Palladium and PalladiumAlloys’, was published in 1960 (2) and his last in2003 (33), ‘Uphill Effects on HydrogenDiffusion Coefficients in Pd0.77Ag0.23 AlloyMembranes’, coauthored with Tong, Cermákand Bell. His paper of 1982, ‘The Palladium-Hydrogen System’ (11) is particularly frequentlycited.

Fred especially enjoyed interacting with col-leagues from foreign countries, andcollaborating in research with them. During the“Iron Curtain” period, J. Cermák (Institute ofPhysics, Academy of Science, Prague, CzechRepublic), R. Bucur (Institute of StableIsotopes, Cluj, Romania) and B. Baranowski(Institute of Physical Chemistry, Academy ofScience, Warsaw, Poland) collaborated withFred, at Queen’s and at their home institutions.During that time, scientific exchanges withEastern European scientists provided valuablecontacts for both the Eastern and WesternEuropean scientists. Fred also collaborated withscientists from other countries, for example, F.Mazzolai (Department of Physics, University ofPerugia, Italy), Y. Sakamoto (Department ofMaterial Science and Engineering, NagasakiUniversity, Japan), X.-Q. Tong (Department ofMaterials Science, Tsinghua University, Beijing,China) and K. Kandasamy (Department ofPhysics, University of Jaffna, Sri Lanka).

Fred played a unique role by attending con-ferences which were often not well attended byother Western European scientists. Some exam-ples were the “First International Workshop onStress and Diffusion”, Balatonfüred, Hungary,

Frederick A. LewisAN APPRECIATION

DOI: 10.1595/147106708X298836

Frederick Alastair Lewis (Courtesy of Donetsk NationalTechnical University, Ukraine)


1995 (25), “Hypothesis III” symposium, in St.Petersburg, Russia, 1999 (31), “HydrogenTreatment of Materials”, HTM-2001, Donetsk-Mariupol, Ukraine, 2001 (32), and the “SecondInternational Symposium on Safety andEconomy of Hydrogen Transport”, IFSSEHT-2003, Sarov, Russia, 2003 (34). Useful reportson these conferences by Fred appeared inPlatinum Metals Review (25, 31, 32, 34).

Fred participated in the “HydrogenTreatment of Materials” conferences held inDonetsk, Ukraine, organised triannually since1995 by Professor V. A. Goltsov of theDonetsk State Technical University, Ukraine(26, 30, 32). Professor Goltsov is a recognisedauthority on H diffusion in Pd and other met-als. Professor Goltsov has referred to thephenomenon of uphill diffusion of H in Pd asthe ‘Lewis Effect’ because it was first observedand characterised by Fred Lewis and hiscoworkers (35). Fred first observed this phe-nomenon in 1983, he realised its importance,and, with international coworkers, X.-Q. Tong,R. Burcur, B. Baranowski, K. Kandasamy, Y.Sakamoto and others, actively pursued it untilhis retirement. An official award was given toFred in 2001 by the International Associationfor Hydrogen Energy (IAHE) and thePermanent Working International ScientificCommittee on Hydrogen Treatment ofMaterials (PWISC HTM) in recognition of theLewis Effect (36).

The book “The Palladium-HydrogenSystem” was written by F. A. Lewis and pub-lished in 1967 by Academic Press (37).Palladium alloys and isotopes of hydrogen werealso included in the book which has continued

as a valuable reference forty years after its pub-lication. Fred was very meticulous about citingreferences properly which makes this book andhis many review articles valuable for searches ofthe literature. In 2000, Fred with Kandasamyand Tong published a book-length review (295pages), entitled ‘Platinum- and Palladium-Hydrogen’, in “Hydrogen in Metal Systems II”,Solid State Phenomena, Scitec Publications,Zürich (38). This constituted a useful update ofhis earlier book. Fred and A. Aladjem edited theseries of reviews, Parts I and II, which includedhis article with Kandasamy and Tong. Reviewsby other scientists in this series covered mostmetals which absorb hydrogen.

In 1985 Fred organised an “InternationalSymposium on Hydrogen in Metals” held inBelfast (39). The Symposium was well attendedand considered to be very successful. The pro-ceedings were published in Zeitschrift fürPhysikalische Chemie Neue Folge (40), and then asa book (41). The editor of the journal at thattime was Ewald Wicke, Director of the Instituteof Physical Chemistry, University of Münster,Germany. Fred and Professor Wicke collaborat-ed in the editing of the symposium papers forthe published proceedings in the Zeitschrift. Along friendship developed between the two sci-entists as a result of this collaboration.

Fred will be missed by his many scientificfriends because he was an excellent scientistand a jovial, kind-hearted person.

TED B. FLANAGAN

References1 A. R. Ubbelohde and F. A. Lewis, “Graphite and

Its Crystalline Compounds”, Oxford UniversityPress, Oxford, U.K., 1960

2 F. A. Lewis, ‘The Hydrides of Palladium andPalladium Alloys’, Platinum Metals Rev., 1960, 4, (4),132

3 F. A. Lewis, ‘The Hydrides of Palladium andPalladium Alloys’, Platinum Metals Rev., 1961, 5, (1),21

4 F. A. Lewis, ‘The Hydrided Palladium Electrode’,Platinum Metals Rev., 1962, 6, (1), 22

5 F. A. Lewis, ‘Novel Method of Electrical Storage’,Platinum Metals Rev., 1963, 7, (3), 88

6 F. A. Lewis, ‘Hydrogen in Palladium and its Alloys’,Platinum Metals Rev., 1968, 12, (4), 140

Award of gold medaland diploma to FredLewis on behalf of theIAHE and PWISCHTM in 2001(Courtesy of DonetskNational TechnicalUniversity, Ukraine)


The AuthorTed Flanagan was a postdoctoral worker with Fred Lewis at Queen’s University, Belfast, from 1957 to 1959; he had an enjoyable andproductive time. The collaboration with Fred resulted in twelve papers in the area of H2 absorption by Pd and its alloys. After that,Flanagan spent two years at Brookhaven National Laboratory, Upton, New York, and then joined the University of Vermont,Burlington, Vermont, where he has remained. Flanagan’s main interests have been in the area of metal, alloy and intermetallic-Hsystems. Following in the footsteps of his collaboration with Fred Lewis, his special interests have been Pd- and Pd alloy-Hsystems. Flanagan received his B.S. from the University of California, Berkeley and his Ph.D. from the University of Washington,Seattle. He subsequently received a Ph.D. honoris causa from Uppsala University (1992) for his research on H in Pd-basedcompounds such as Pd6P. He is on the Editorial Advisory Board of the Journal of Alloys and Compounds and the Materials ScienceForum. Although he is now an Emeritus Professor, he has continued his research on metal-H systems.

7 F. A. Lewis, ‘Absorption of Hydrogen by PalladiumAlloys’, Platinum Metals Rev., 1970, 14, (4), 131

8 R. Burch and F. A. Lewis, ‘The Form of theInteraction between Palladium and Hydrogen’,Platinum Metals Rev., 1971, 15, (1), 21

9 F. A. Lewis, ‘Current Research on the Palladium-Hydrogen System’, Platinum Metals Rev., 1977, 21,(4), 134

10 F. A. Lewis, ‘Platinum Group Metal Hydrides’,Platinum Metals Rev., 1980, 24, (3), 102

11 F. A. Lewis, ‘The Palladium-Hydrogen System’,Platinum Metals Rev., 1982, 26, (1), 20



14 F. A. Lewis, ‘Hydrogen in Palladium and Its Alloys’,Platinum Metals Rev., 1984, 28, (1), 13

15 F. A. Lewis, K. Kandasamy and B. Baranowksi,‘The “Uphill” Diffusion of Hydrogen’, PlatinumMetals Rev., 1988, 32, (1), 22

16 F. A. Lewis, ‘Hydrogen in Amorphous PalladiumAlloys’, Platinum Metals Rev., 1988, 32, (2), 83

17 R.-A. McNichol and F. A. Lewis, ‘The HydridePhase Miscibility Gap in Palladium-Rare EarthAlloys’, Platinum Metals Rev., 1990, 34, (2), 81

18 F. A. Lewis, X. Q. Tong and R. V. Bucur,‘Permeation of Hydrogen through Palladium-SilverMembranes’, Platinum Metals Rev., 1991, 35, (3), 138

19 F. A. Lewis, ‘Metal-Hydrogen Systems and theHydrogen Economy’, Platinum Metals Rev., 1992, 36,(4), 196

20 F. A. Lewis, ‘Hydrogen Interstitial Structures inPalladium-Silver Membranes’, Platinum Metals Rev.,1993, 37, (4), 220

21 F. A. Lewis, ‘Hydrogen Material Science and MetalHydride Chemistry’, Platinum Metals Rev., 1994, 38,(1), 20

22 F. A. Lewis, ‘Palladium-Hydrogen System’, PlatinumMetals Rev., 1994, 38, (3), 112

23 F. A. Lewis, ‘International Conference on “Nobleand Rare Metals”’, Platinum Metals Rev., 1995, 39,(1), 29

24 F. A. Lewis, ‘Fundamentals and Applications ofMetal-Hydrogen Systems’, Platinum Metals Rev.,1995, 39, (2), 75

25 F. A. Lewis, ‘First International Workshop onDiffusion and Stresses’, Platinum Metals Rev., 1995,39, (3), 127

26 F. A. Lewis, ‘Hydrogen Treatment of Materials’,Platinum Metals Rev., 1996, 40, (1), 36

27 F. A. Lewis, ‘Problems of the Palladium-HydrogenSystem’, Platinum Metals Rev., 1996, 40, (4), 180

28 F. A. Lewis, ‘Rapid Hydrogen Permeation inPalladium and Palladium Alloys’, Platinum MetalsRev., 1997, 41, (1), 33

29 F. A. Lewis, ‘Platinum Metals Involvement in theHydrogen Economy’, Platinum Metals Rev., 1997, 41,(4), 163


31 F. A. Lewis, ‘Advancements in HydrogenTechnology’, Platinum Metals Rev., 1999, 43, (4), 166


33 X. Q. Tong, F. A. Lewis, S. E. J. Bell and J. Cermák,‘Uphill Effects on Hydrogen DiffusionCoefficients in Pd77Ag23 Alloy Membranes’,Platinum Metals Rev., 2003, 47, (1), 32

34 F. A. Lewis, ‘Hydrogen Economy Forum inRussia’, Platinum Metals Rev., 2003, 47, (4), 166

35 F. A. Lewis, J. P. Magennis, S. G. McKee and P. J.M. Ssebuwufu, Nature, 1983, 306, (5944), 673

36 V. A. Goltsov and T. N. Veziroglu, ‘A Step on theRoad to Hydrogen Civilization, The “HTM-2001”review’, in The Third International Conference“Hydrogen Treatment of Materials”, DonetskNational Technical University, Ukraine, 14th–18thMay, 2001:http://donntu.edu.ua/hydrogen/conf.html

37 F. A. Lewis, “The Palladium-Hydrogen System”,Academic Press, London, New York, 1967

38 F. A. Lewis, K. Kandasamy and X. Q. Tong,‘Platinum and Palladium-Hydrogen’, in “Hydrogenin Metal Systems II”, eds. F. A. Lewis and A.Aladjem, Solid State Phenomena, Vols. 73–75,Scitec Publications, Zürich, Switzerland, 2000,Chapter 6, pp. 207–501

39 P. A. Sermon, ‘Symposium on Hydrogen in Metals’,Platinum Metals Rev., 1985, 29, (3), 115

40 F. A. Lewis and E. Wicke (Eds.), Z. Phys. Chem.Neue Folge, 1985, 143, 1–254; 1985, 145, 1–278;1985, 146, 137–242; and 1986, 147, 1–290

41 “Hydrogen in Metals”, eds. F. A. Lewis and E.Wicke, R. Oldenbourg Verlag, Munchen, Germany,1986, Proceedings of the International Symposiumon Hydrogen in Metals, 26th–29th March, 1985,Queen’s University of Belfast, Northern Ireland

123Platinum Metals Rev., 2008, 52, (2), 123

DOI: 10.1595/147106708X292508

The commercialisation of fuel cells started in2007, according to Fuel Cell Today in its 2008Industry Review, released on 30th January. TheReview, titled “Fuel Cells: Commercialisation”,reports that in the last year the fuel cell industryhas seen a growth of 75 per cent in new unitsdelivered, with some 12,000 new fuel cell unitsshipped during 2007. Fuel Cell Today believesthat the current global manufacturing capabilityfor fuel cells is around 100,000 units per annum,with a quarter of this coming from companieswhose business activity is exclusively the develop-ment of hydrogen and fuel cell technologies.

The Fuel Cell Industry Review aims to pro-vide a concise and accurate summary ofworldwide fuel cell activity. Alongside informa-tion on legislation, finance, applications and keyfuel cell companies, the Review publishes, for thefirst time, the Fuel Cell Today analysts’ forecastsof fuel cell shipments for the next two years. Theforecasts include data by geographical region, fuelcell technology type and end use application.

According to the Review, the last three yearshave seen the commercialisation of a number offuel cell products at the luxury end of the market.Currently, fuel cells are relatively expensive and anumber of issues are still outstanding in terms ofresearch, development and demonstration(RD&D), codes and standards, and fuel infra-structure/distribution. However, price reduc-tions are expected as manufacturing costs fall andsubsidies for adoption become available.

The Review shows that worldwide govern-ment funding for RD&D topped £500 million(U.S.$1000 million) during 2007, with seven coun-tries making up £400 million (U.S.$800 million) ofthis. Government funding has helped to supportdevelopment of fuel cells for stationary and trans-port applications, while funding for portable fuelcells has come mainly from the private sector.

Fuel Cell Today believes that the current com-mercial opportunities for fuel cells favour thelow-temperature electrolytes, direct methanol fuelcells (DMFCs) and proton exchange membranes(PEMs), with over 98 per cent of manufacturingtoday being low-temperature units. The cost ofPEM products currently varies from £1500(U.S.$3000) per kW for a 5 kW unit up to £17,000(U.S.$34,000) per kW for a micro 100 W fuel cell.Annual cost reductions of between 10 and 20 percent are currently being reported.

Dr Kerry-Ann Adamson, Principal Analyst atFuel Cell Today, said: “Fuel cells are starting theprocess of becoming a mainstream market tech-nology and although this will not be completeduntil well after the period under scrutiny in thisreport, commercialisation has finally begun”.

The Fuel Cell Today Industry Review (ISBN:978-0-9557963-0-2; ISSN: 1756-3186), priced at£500 (U.S.$1000), is available to order fromhttp://www.fuelcelltoday.com/events/industry-review. For more information please contact DrKerry-Ann Adamson:[email protected].

Fuel Cell Today Industry Review 2008“FUEL CELLS: COMMERCIALISATION”

Horizon Fuel Cell Technologies’ H-racerand refuelling set. Horizon came to theattention of the wider public in 2006 withthe release of this toy-scale, six-inch longfuel cell car, which circumvents the need foran external refuelling infrastructure bycoming complete with a miniature hydrogenproduction plant powered by a solar cell(Courtesy of Horizon Fuel CellTechnologies)

CATALYSIS – APPLIED AND PHYSICAL ASPECTSFT-IR Investigation of NOx Storage Properties ofPt–Mg(Al)O and Pt/Cu–Mg(Al)O CatalystsObtained from Hydrotalcite CompoundsS. MORANDI, F. PRINETTO, G. GHIOTTI, M. LIVI and A. VACCARI,Microporous Mesoporous Mater., 2008, 107, (1–2), 31–38

The NOx storage capability upon admission of NOand NO2 with or without excess O2 at ≤ 623 K wasinvestigated by in situ FT-IR for the title catalysts. PureNO2 is adsorbed by a dismutation reaction with simul-taneous formation of nitrates and nitrites that evolveto nitrates (“dismutation route”); nitrite evolution ispromoted by the metal phase. When metal phase ispresent, the nitrites’ oxidation is further accelerated byO2. When O2 is present, NO is stored by: (a) oxidationto nitrites followed by their oxidation to nitrates; or (b)oxidation to NO2, followed by the “dismutationroute”. (a) and (b) are promoted by the metal phases.

Complete Oxidation of Methane over PalladiumSupported on Alumina Modified with Calcium,Lanthanum, and Cerium IonsB. STASINSKA, W. GAC, T. IOANNIDES and A. MACHOCKI,J. Nat. Gas Chem., 2007, 16, (4), 342–348

The supports for Pd/Al2O3 and Pd/(Al2O3 + MOx)(M = Ca, La, Ce) were prepared by a sol-gel method.They were dried either conventionally or with sc-CO2,and then impregnated with Pd nitrate solution. Theintroduction of Ca, La or Ce oxide caused a decreaseof the surface area, dependent on the support pre-cursor drying method. These modifiers decreased theactivity of the Pd catalysts for CH4 oxidation.Improvement of the Pd activity by La and Ce supportmodifier was observed only at low temperatures.

Carbon Microsphere Supported Pd Catalysts forthe Hydrogenation of EthyleneK. C. MONDAL, L. M. CELE, M. J. WITCOMB and N. J. COVILLE,Catal. Commun., 2008, 9, (4), 494–498

C microspheres were prepared from acetylene at800ºC. The microspheres were loaded with 2% Pd,both before and after H2SO4/HNO3 acid treatment.The acid-treated C microsphere-supported Pd cata-lyst exhibited better ethylene hydrogenation activity.

Selective Oxidation of Styrene to Acetophenoneover Supported Au–Pd Catalyst with HydrogenPeroxide in Supercritical Carbon DioxideX. WANG, N. S. VENKATARAMANAN, H. KAWANAMI and Y.IKUSHIMA, Green Chem., 2007, 9, (12), 1352–1355

Selective oxidation of styrene to acetophenone wascarried out over supported Pd-Au catalysts with H2O2

in sc-CO2. The Al2O3 support showed the best cat-alytic performance. The presence of the sc-CO2

improved the oxidation of styrene to acetophenoneand inhibited the formation of byproducts.

CATALYSIS – REACTIONSStille Cross-Coupling Reaction Using Pd/BaSO4 asCatalyst ReservoirA. V. COELHO, A. L. F. DE SOUZA, P. G. DE LIMA, J. L. WARDELLand O. A. C. ANTUNES, Appl. Organomet. Chem., 2008, 22, (1),39–42

Stille cross-couplings of iodobenzene and tributyl-phenyltin were achieved in EtOH/H2O using differentamounts of Pd/BaSO4 as catalyst reservoir in a ligand-free system. The catalyst was reused up to three timeswith some loss in activity. Filtration of the catalyst andproduct extraction gave a solution that kept its activity,indicating that Pd(0)/Pd(II) are the catalytic species.

α-Arylation of Ketones Using Highly Active, Air-Stable (DtBPF)PdX2 (X = Cl, Br) CatalystsG. A. GRASA and T. J. COLACOT, Org. Lett., 2007, 9, (26),5489–5492α-Arylation of ketones with aryl chlorides and bro-

mides using (DtBPF)PdX2 (X = Cl, Br) catalysts gave80–100% yield of the coupled products under rela-tively mild conditions at low catalyst loadings. TheX-ray structure determination of (DtBPF)PdCl2

revealed the largest P–Pd–P bite angle (104.2º) for aferrocenyl bisphosphine ligand. 31P NMR monitoringof the (DtBPF)PdCl2-catalysed reaction of 4-chloro-toluene with propiophenone indicated that theDtBPF remained coordinated in a bidentate mode.

C-C Coupling Reaction of Triphenylbismuth(V)Derivatives and Olefins in the Presence ofPalladium Nanoparticles Immobilized in SphericalPolyelectrolyte BrushesY. B. MALYSHEVA, A. V. GUSHCHIN, Y. MEI, Y. LU, M. BALLAUFF,S. PROCH and R. KEMPE, Eur. J. Inorg. Chem., 2008, (3),379–383

C–C couplings were carried out at 50ºC using Ph3BiX2

(X = O2CH, O2CMe, O2CEt, O2CnPr, O2CnBu, O2CtBu,O2CPh, O2CCH2Cl, O2CCCl3, O2CCF3) and a range ofolefins in the presence of Pd nanoparticles immobilisedin spherical polyelectrolyte brushes (Pd@SPB).Ph3Bi(O2CCF3)2 was the most active. This route allowsthe formation of Heck-type products without the addi-tion of base. A very low Pd loading was used.

Ruthenium-Based NHC-Arene Systems as Ring-Opening Metathesis Polymerisation CatalystsN. LEDOUX, B. ALLAERT and F. VERPOORT, Eur. J. Inorg. Chem.,2007, (35), 5578–5583

The coordination of the standard NHC ligand H2IMesto [(p-cymene)RuCl2]2 was established to be unattain-able, so bidentate NHC analogues were synthesisedinstead. These analogues are O-hydroxyaryl-substitutedNHCs, capable of binding with the metal centrethrough the O atom as well as through the carbene Catom. Their chelating properties improved the stabilityof the corresponding Ru arene complexes.


ABSTRACTSDOI: 10.1595/147106708X300535


FUEL CELLSSynergistic Effect of CeO2 Modified Pt/C Electrocatalystson the Performance of PEM Fuel CellsH. XU and X. HOU, Int. J. Hydrogen Energy, 2007, 32, (17),4397–4401

Pt/C electrocatalysts were modified by CeO2 withsol-gel and dipping processes. TEM and CV resultsshowed that some Pt active surfaces were covered byCeO2, the electrochemical surface area of modifiedPt/C was less than that of the unmodified one, andthe sol-gel process covered less electrocatalyst surfacearea than the dipping process. Using modified Pt/Cas a cathode electrocatalyst enhanced performance.

Synthesis and Characterization of ElectrodepositedNi–Pd Alloy Electrodes for Methanol OxidationK. S. KUMAR, P. HARIDOSS and S. K. SESHADRI, Surf. Coat.Technol., 2008, 202, (9), 1764–1770

A wide compositional range of Ni-Pd alloy electro-catalysts (1) were prepared by electrodeposition foruse as anode materials for DMFCs in alkaline condi-tions. As-plated (1) were nanocrystalline, single phase,f.c.c. materials, indicating the formation of a completesolid solution in the alloy. Compositional analysis ofthe alloys indicated that the Pd composition increasedwith decrease in current density. (1) were active forMeOH oxidation in alkaline medium.

Pd-Co Carbon-Nitride Electrocatalysts for PolymerElectrolyte Fuel CellsV. DI NOTO, E. NEGRO, S. LAVINA, S. GROSS and G. PACE,Electrochim. Acta, 2007, 53, (4), 1604–1617

Two groups of materials with the formulaKn[PdxCoyCzNlHm] were synthesised with: (a) molarratio y :x > 1; and (b) molar ratio y :x < 1. Vibrationalstudies revealed that (a) and (b) systems consisted oftwo polymorphs of α- and graphitic-like CN nano-materials. The electrochemical performance of thePd-Co-CNs of (a) obtained at tf ≥ 700ºC was higherthan that measured for a Pt-based commercial elec-trocatalyst in terms of both activity towards the Oreduction and H oxidation reactions; also the resis-tance towards poisoning by MeOH.

METALLURGY AND MATERIALSInfrared Spectroscopy of Physisorbed andChemisorbed N2 in the Pt(111)(3×3)N2 StructureK. GUSTAFSSON, G. S. KARLBERG and S. ANDERSSON, J. Chem.Phys., 2007, 127, (19), 194708 (6 pages)

The adsorption of N2, at 30 K, on Pt(111) andPt(111)(1×1)H surfaces was investigated using IRspectroscopy and LEED. The IR spectra revealedthat N2 exclusively physisorbed on the Pt(111)(1×1)Hsurface, whereas both physisorbed and chemisorbedN2 were detected on the Pt(111) surface. PhysisorbedN2 was the majority species in the latter case, and thetwo adsorption states showed an almost identicaluptake behaviour, which indicates that they are intrin-sic constituents of the growing (3×3) N2 islands.

Synthesis of Colloidal Particles of Poly(2-vinylpyridine)-Coated Palladium and Platinum inOrganic Solutions under the High Temperaturesand High PressuresM. HARADA, M. UEJI and Y. KIMURA, Colloids Surf. A:Physicochem. Eng. Aspects, 2008, 315, (1–3), 304–310

Colloidal dispersions of PVP-coated Pd and Pt parti-cles in toluene/1-propanol were synthesised by thedecomposition and reduction of Pd(acac)2 and Pt(acac)2

respectively under high-temperature and high-pressureconditions. At 473 K and 25 MPa, colloidal dispersions([Pd] = 7.5 mM) of Pd particles of average diameter 1.9nm with narrow particle size distributions, were synthe-sised within seconds. Pt particles (average diameter of2.1 nm) were also obtained.

Synthesis of Palladium Nanowire Arrays withControlled Diameter and LengthG. KARTOPU, S. HABOUTI and M. ES-SOUNI, Mater. Chem. Phys.,2008, 107, (2–3), 226–230

Pd nanowire (NW) arrays were synthesised usingporous alumina templates and direct current elec-trodeposition. The electrolyte was K2PdCl4 in H2SO4.Final pore sizes of the alumina templates were ~ 65and 35 nm. A high filling rate (> 90%) was obtainedusing 65 μm thick templates. The NWs synthesised in65 nm pores were polycrystalline and textured, butthose in 35 nm pores were single crystalline. The alu-mina template was dissolved away, leaving self-standingNWs supported on a conductive thin film.

Creep Deformation Mechanisms in Ru-Ni-AlTernary B2 AlloysF. CAO and T. M. POLLOCK, Metall. Mater. Trans. A, 2008, 39,(1), 39–49

The creep behaviour of five Ru-Ni-Al alloys withcompositions across the ternary RuAl-NiAl B2 phasefield was studied within the range 1223–1323 K. Thesealloys exhibited exceptional creep strength compared toother high melting temperature intermetallics. A con-tinuous increase of the melting temperature and creepresistance with increasing Ru:Ni ratio was observed.

APPARATUS AND TECHNIQUETemperature-Independent Ceria- and Pt-DopedNano-Size TiO2 Oxygen Lambda Sensor UsingPt/SiO2 Catalytic FilterF. HAGHIGHAT, A. KHODADADI and Y. MORTAZAVI, Sens.Actuators B: Chem., 2008, 129, (1), 47–52

The overlap of TiO2-based O2 sensor responses inthe rich and lean regions was eliminated by using a 1.0wt.% Pt/SiO2 catalytic filter (1) located in front of thesensors. Nanosized TiO2 was prepared by amicroemulsion method and then doped with 1.0wt.% Pt and 10.0 wt.% CeO2 by an impregnationmethod. The sensor was exposed to synthetic exhaustgases with λ values in the range 0.8–1.4. All of thesensors showed low-high transitions at about λ = 1.0.By using (1) only CO or O2 reaches the sensors.

Platinum Metals Rev., 2008, 52, (2)

Electrochemical DNA Biosensors Based onPalladium Nanoparticles Combined with CarbonNanotubesZ. CHANG, H. FAN, K. ZHAO, M. CHEN, P. HE and Y. FANG,Electroanalysis, 2008, 20, (2), 131–136

MWCNTs and Pd nanoparticles were dispersed inNafion, and used to modify a GCE. Oligonucleotideswith amino groups at the 5' end were covalently linkedto carboxylic groups of MWCNTs on the electrode.The hybridisation events were monitored by differen-tial pulse voltammetry using methylene blue as anindicator. The detection limit of the method for targetDNA was 1.2 × 10–13 M.

Hydrogen Permeation of Thin, Free-StandingPd/Ag23% Membranes Before and After HeatTreatment in AirA. L. MEJDELL, H. KLETTE, A. RAMACHANDRAN, A. BORG andR. BREDESEN, J. Membrane Sci., 2008, 307, (1), 96–104

The title membranes (thicknesses ~ 1.3–5.0 μm)were produced by magnetron sputtering. Thermaltreatment in air at 300ºC significantly enhanced theirH2 flux. The permeability values became fairly similarafter treatment, indicating bulk diffusion was the mainrate-limiting step for H2 flux. The effect on permeationwas found to depend on the membrane thickness, withless enhancement for the ~ 5.0 μm thick membranes.The treated samples had higher surface roughness,larger surface area and larger surface grains.

BIOMEDICAL AND DENTALOn the Hydrolysis Mechanism of the Second-Generation Anticancer Drug CarboplatinM. PAVELKA, M. F. A. LUCAS and N. RUSSO, Chem. Eur. J., 2007,13, (36), 10108–10116

The hydrolysis reaction mechanisms of carboplatinwere investigated by combining DFT with the con-ductor-like dielectric continuum model (CPCM)approach. The theoretical calculations on carboplatinwere used to obtain energy profiles and optimisedstructures for the rate-limiting process in its neutralhydrolysis. The results indicated that if carboplatinundergoes a hydration process, it should be doublyhydrated prior to reaction with DNA.

CHEMISTRYDouble Complex Salts of Pt and Pd Ammines withZn and Ni Oxalates – Promising Precursors ofNanosized AlloysA. V. ZADESENETS, E. YU. FILATOV, K. V. YUSENKO, YU. V.SHUBIN, S. V. KORENEV and I. A. BAIDINA, Inorg. Chim. Acta,2008, 361, (1), 199–207

[M(NH3)4][M'(Ox)2(H2O)2]·2H2O (M = Pd, Pt; M'= Ni, Zn) were synthesised from solutions containing[M(NH3)4]2+ and [M'(Ox)2(H2O)2]2–. Thermal decom-position of the prepared salts in He or H2 atmosphereat 200–400ºC resulted in formation of nanosizedbimetallic powders with crystallite sizes 50–250 Å.

PHOTOCONVERSIONAggregation-Induced Phosphorescent Emission(AIPE) of Iridium(III) ComplexesQ. ZHAO, L. LI, F. LI, M. YU, Z. LIU, T. YI and C. HUANG, Chem.Commun., 2008, (6), 685–687

An AIPE was observed for Ir(ppy)2(DBM) andIr(ppy)2(SB) (DBM = 1,3-diphenyl-1,3-propanedione,SB = 2-(naphthalen-1-yliminomethyl)phenol). TheseIr(III) complexes, in powder form, exhibited moder-ately intense emissions. Furthermore, addition ofnon-solvent H2O into dilute MeCN solutions canturn on their photoluminescent emission.

Ultrafast Luminescence in Ir(ppy)3

G. J. HEDLEY, A. RUSECKAS and I. D. W. SAMUEL, Chem. Phys.Lett., 2008, 450, (4–6), 292–296

For Ir(ppy)3, an emission with a lifetime componentof 230 fs in the spectral region 500–560 nm is assignedto the population equilibration between electronic sub-states of the lowest excited triplet state, with energydissipation by intramolecular vibrational redistribution.At shorter wavelengths a strong emission with a fasterdecay was observed, which is attributed to a state witha higher admixture of singlet character. A slower decayon a 3 ps timescale is attributed to vibrational cooling.

SURFACE COATINGSPlatinum OMCVD Processes and PrecursorChemistryC. THURIER and P. DOPPELT, Coord. Chem. Rev., 2008, 252,(1–2), 155–169

Organometallic chemical vapour deposition (OMCVD)allows the formation of Pt thin films as a fine disper-sion of Pt particles. Pt precursors having goodvolatility and a good thermal stability window areavailable. The best systems are MeCpPtMe3 andEtCpPtMe3, the latter being O2- and H2O-stable atambient temperature. (cod)Pt(Me)2 is less volatile butit is easily synthesised in high yield. These precursorsbenefit from facile decomposition under the CVDconditions. Decomposition is rapid in the presence ofO2(g) or H2(g). Films can be obtained with only tracesof impurities, C being the most common. (97 Refs.)

Electrochemical Formation of Ir Oxide/PolyanilineComposite FilmsH. ELZANOWSKA, E. MIASEK and V. I. BIRSS, Electrochim. Acta,2008, 53, (6), 2706–2715

IrOx/PANI composite films were made by formingan anodic IrOx film on bulk Ir and then depositingPANI into its pores. All of the PANI film that waselectrochemically active was in direct electrical con-tact with the Ir surface at the base of the IrOx filmpores. Thin films of Ir nanoparticles, subsequentlyconverted to IrOx, were also used as a template forPANI formation within the porous structure. Thesehybrid films exhibited an enhanced internal porosity,high charge densities, unusual electrochromic behav-iour, and very rapid charge transfer kinetics.

126

CATALYSIS – APPLIED AND PHYSICAL ASPECTSPlatinum-Ruthenium Catalyst for Methanol OxidationJPN. ADV. INST. SCI. TECHNOL. HOKURIKU

Japanese Appl. 2007-190,454A PtRu-based catalyst with increased uniformity of

Pt particle size is claimed, where coagulation of Ptparticles is prevented. Ru particles are dispersed ona carrier surface, followed by Pt particles with aver-age diameter 0.5–15 nm. Standard deviation of thePt particle diameter is 7–13. The supported particlesare heat treated at < 300ºC in a non-oxidisingatmosphere.

CATALYSIS – INDUSTRIAL PROCESSHydroformylation Process with Rhodium RecoveryDOW GLOBAL TECHNOL. INC World Appl. 2007/133,379

A non-aqueous hydroformylation process with liq-uid catalyst recycle includes a hydroformylation stepand one or more phase separation stages to recoverhigh molecular weight aldehyde product and Rh cat-alyst. Hydroformylation is carried out at 250–450psia (1724–3103 kPa). The product mix containsaldehydes, conjugated polyolefins, a Rh-organophos-phorus ligand complex, free organophosphorusligand and a polar organic solubilising agent. Phaseseparation stages use added H2O with CO(g), H2(g) ora mixture, and are carried out at 20–400 psia(138–2758 kPa). Sum of pressures in both steps is> 360 psia (2482 kPa).

Preparation of 3-Methylbut-1-eneOXENO OLEFINCHEMIE GmbH

World Appl. 2008/006,633The title compound is prepared from a hydrocar-

bon stream containing ≥ 70 wt.% isobutene withlinear butenes or olefins containing 3–5 C atoms, byhydroformylation in the presence of a Rh catalystwith organophosphorus ligands, followed by hydro-genation of the resulting aldehyde to an alcohol.Elimination of H2O gives the final product.

CATALYSIS – REACTIONSCross Metathesis of Cyclic OlefinsMATERIA INC World Appl. 2008/008,440

Ring-opening, ring insertion cross metathesis ofcyclic olefins with internal olefins such as seed oils iscarried out in the presence of a Ru alkylidene olefinmetathesis catalyst. Olefinic substrates may includean unsaturated fatty acid or alcohol or an esterifica-tion product of an unsaturated fatty acid with asaturated or unsaturated alcohol. The Ru catalystmay be a Grubbs-Hoveyda complex and may con-tain an N-heterocyclic carbene ligand associatedwith the Ru centre, and is present in < 1000 ppmconcentration relative to olefinic substrate.

Synthesis of 10-HydroxycamptothecinUNIV. FUDAN Chinese Appl. 1,054,381

The title compound is synthesised from20(S)-camptothecin by catalytic hydrogenation usingPt/C or Rh/C, in the presence of a mitigator containingorganic compound, followed by oxidation of theresulting tetrahydrocamptothecin to obtain the desiredproduct. Yield is 70–75% and product purity is > 98.5%.

EMISSIONS CONTROLRemoving Mercury from Gas StreamsJOHNSON MATTHEY PLC World Appl. 2007/141,577

Heavy metals such as Hg can be removed fromhigh-temperature gases such as coal-derived syngasstreams, using a sulfided Pd-containing absorbent,preferably Pd4S. Pd content is > 1.5 wt.%, preferably~ 2 wt.%, loaded on a support, preferably γ-alumina.Hg forms a PdHg phase on contact with absorbent.

Exhaust Gas Purifying CatalystMAZDA MOTOR CORP European Appl. 1,859,851

A catalyst for purifying exhaust gas containing HC,CO, NOx and H2O contains a catalyst layer on ahoneycomb support. A first catalyst powder containscomposite oxide RhZrCeNdO and a second containsRhZrXO, where X = a rare earth element other thanCe, and Rh is present on the surface. RhZrXO forms1–50% of the total catalyst powder.

Diesel Particulate FilterNISSAN MOTOR CO LTD Japanese Appl. 2007-239,522

A DPF which can be partially regenerated at rela-tively low temperatures is claimed. A Pt catalyst iscoated on the surface of a porous monolithic filter,with Pt concentration higher in the centre part toincrease the probability of contact between the Pt cat-alyst and exhaust particulate.

FUEL CELLSPalladium-Ruthenium ElectrocatalystJOHNSON MATTHEY PLC World Appl. 2008/012,572

An electrocatalyst for the anode of a DMFC is madefrom a PdRu alloy with a single crystalline phase, andcontains (in at.%): 5–95 Pd, 5–95 Ru and < 10 othermetals, but not 50 Pd and 50 Ru. Preferred composi-tions contain (in at.%): 5–49 Pd, 51–95 Ru and < 10other metals on a support of high surface area.

Water Management of PEMFC StackGM GLOBAL TECHNOL. OPER. INC

Japanese Appl. 2007-194,195A fuel cell system includes a means of humidifying

the cathode inlet airflow and the H2(g) to the anode. Asurface active agent such as EtOH is added to reducesurface tension and allow wicking of H2O to the flowfield channels. The catalyst layers may include Ru aswell as Pt to mitigate poisoning of Pt by CO formedby oxidation of EtOH on the cathode side.

127Platinum Metals Rev., 2008, 52, (2), 127–128

DOI: 10.1595/147106708X295127

NEW PATENTS

Platinum Catalyst on Carbon Nanotube SupportKOREA INST. ENERGY RES. Korean Patent 0,726,237

A highly dispersed C-nanotube supported Pt cata-lyst for a fuel cell is prepared by growing C nanotubeson graphite paper by CVD, pretreating to removeimpurities and modify surface structure, then treatingwith chloroplatinic acid in aqueous H2SO4 solution todeposit Pt by an electrochemical process.

METALLURGY AND MATERIALSPalladium-Iridium Hydrogen Storage AlloyKYUSHU UNIV. Japanese Appl. 2007-239,052

A H2 storage alloy is composed of PdIr nanoparti-cles which may form a core/shell structure with a coreof Pd and a shell of Ir, or may be a solid solution withsingle crystal lattice. The alloy contains 40–90 at.% Pdand 10–60 at.% Ir. H2 storage content at 303 K andH2 pressure 0.1 MPa is ≥ 0.4 mol% and is claimed toexceed that of PdPt nanoparticles or bulk Pd.

Platinum Alloy for JewellerySEKI KK Japanese Appl. 2007-239,089

A Pt alloy contains ≥ 99.7 wt.% Pt with 0.002–1.0wt.% P, S or Be, preferably 0.005–0.3 wt.%. The alloycan be hallmarked Pt 1000, and Pt content is controlledin the range 98.90–99.94 wt.%, preferably 99.70–99.94wt.%. Good wear and deformation resistance and lowsusceptibility to casting defects are claimed.

APPARATUS AND TECHNIQUETemperature Measuring SystemWEBRESULTS SRL European Appl. 1,860,414

A temperature measuring system includes a Pt resis-tance thermometer sensor, a managing circuit and acontrol circuit. The sensor incorporates at least tworheophores made from Pt or Ag (preferably99.9999% Pt), with interconnecting wires made fromthe same material, sealed inside a metallic sheath withinert gas or dry air.

Electrochemical Detection of DNAGENEOHM SCI. U.S. Appl. 2008/0,026,397

An assay for detecting a polynucleotide such asDNA includes the steps of immobilising the polynu-cleotide on an electrode, contacting with a Rucomplex having a reduction potential which does notcoincide with that of O2(g), such as Ru(III) pentaamine pyridine, and electrochemically detectingthe Ru complex as an indicator of the presence ofimmobilised target polynucleotide. The process canbe carried out in the presence of O2(g) and no deaera-tion step is required.

Iridium Spark Plug AlloyTANAKA KIKINZOKU KOGYO KK

World Appl. 2008/013,159A spark plug chip is made from Ir with (in wt.%):

0.2–6.0 Cr plus 2.0–12.0 Fe and/or Ni. The surfacemay be oxidised by heating at 300–900ºC in an oxi-dising atmosphere, to give an oxide of Cr-Fe, Cr-Nior Cr-Fe-Ni of thickness 5–100 μm.

BIOMEDICAL AND DENTALAnticancer Drug CombinationsBAYER PHARM. CORP World Appl. 2007/139,930

Drug combinations and pharmaceutical composi-tions are claimed for treating cancer such asnon-small cell lung carcinoma. The compositionscontain at least one substituted-diaryl urea, at leastone taxane and at least one Pt complex antineoplasticnucleic acid binding agent such as carboplatin,oxaplatin or cisplatin.

ELECTRICAL AND ELECTRONICSRechargeable Battery with UltracapacitorAPOGEE POWER INC European Appl. 1,876,669

A composite battery set for an electronic deviceincludes a Li-ion, Li-polymer or Ni metal hydride bat-tery and an ultracapacitor made from Pt, Au orpreferably a metal-ceramic Ru oxide. The set mayoptionally include a protective circuit module. Pulserise time provided to the electronic device is < 5 ms.

Palladium-Plated Lead Finishing StructureSHINKO ELECTR. IND. CO LTD U.S. Appl. 2007/0,272,441

A Pd-plated lead finishing structure for a semicon-ductor part includes Pd or Pd alloy plated at ≤ 0.3 μmthickness onto the surfaces of external connectionterminals made from Cu, Cu alloy, Fe or a Fe-Nialloy. No intermediate or underlying layer is required.Au or Au alloy may optionally be plated onto the Pdor Pd alloy to a thickness of ≤ 0.1 μm. Short circuitsbetween terminals due to whiskers are prevented.

Inkjet Printhead with Platinum AlloySAMSUNG ELECTR. CO LTD U.S. Appl. 2008/0,012,906

Thermal inkjet printheads include a heater to heatink by direct contact, formed from Pt-Ru or Pt-Ir-X,where X = Ta, W, Cr, Al or O. Thickness of theheater is 500–3000 Å and the area of the heat gener-ation part is ≤ 650 μm2. The Pt-Ru alloy contains20–80% Ru; the Pt-Ir-X alloy may contain a propor-tion of Pt ≈ Ir, with 0–30% Ta or 0–40% O.

SURFACE COATINGSPlatinum-Coated Refractory Oxide Ceramic PartJOHNSON MATTHEY PLC World Appl. 2007/148,104

A refractory metallic oxide ceramic part for use inmolten glass processing is surface treated to provide anarray of slots or closed-end holes, and may then bespray coated with a Pt group metal or alloy of thickness200–500 μm for erosion and corrosion protection.

Dental Mirror with Ruthenium CoatingI. A. McCABE U.S. Appl. 2007/0,268,603

A dental mirror includes a glass substrate coatedwith a Ru film on either the front or rear surface. Thecoating thickness is 250–650 Å, preferably 350–550Å. An adhesion enhancing layer may optionally beincluded between the glass substrate and the Ru coat-ing, and an optical layer such as a reflection enhancinglayer may be coated over the Ru.

128Platinum Metals Rev., 2008, 52, (2)


FINAL ANALYSIS

Crystallite Size Analysis of SupportedPlatinum Catalysts by XRD

X-Ray diffraction (XRD) is often used in com-bination with transmission electron microscopy(TEM) and, for fuel cell electrocatalysts, electro-chemical methods such as cyclic voltammetry, inthe characterisation of supported platinum cata-lysts. Crystal/particle size information obtainedfrom fresh or aged samples generally correlateswith catalytic activity.

Average Pt crystallite size is frequently calculat-ed from XRD peak broadening using the Scherrerequation (1). As a bulk technique, XRD has certainadvantages over TEM digital processing of parti-cles (2). Direct comparison of the resulting XRDvolume-weighted size of the crystallite with theTEM number-weighted size of the particle (oftenformed from several primary crystallite grains) ishowever often erroneously made in the literature.

In order to obtain a realistic determination ofPt crystallite size by XRD, the effects of Pt parti-cle shape, microstructure and very small, so-called‘XRD amorphous’ Pt must be considered duringX-ray analysis. The effect of interference fromsupports of high surface area and the presence ofpoorly crystalline oxidic Pt phases must also beconsidered. In addition, super-Lorentzian peakshapes are often encountered (Figure 1), which are

a result of either a broad lognormal or multimodalPt crystallite size distributions (3).

Issues outlined above have been successfullyaddressed for Pt/C catalysts by using the smallangle X-ray scattering (SAXS) technique (4), asSAXS is well suited to size determination of verysmall particles (< 3 nm diameter). Although SAXSmeasurements can be made either in the labora-tory or at a synchrotron facility, instrumentation isnot commonplace, and data analysis of supportedcatalysts is often not routine.

Alternative approaches more suited to thelaboratory utilise XRD whole-pattern fittingbased on the Rietveld method (5), using eitherFourier (6) or non-Fourier transform method-ology. One non-Fourier approach that allows forthe determination of metric and microstructuralparameters is given in the program FormFit (7).Similar to the Rietveld method, an analytical func-tion is used to describe the measured X-raypattern. Each line of the pattern is described by asplit pseudo-Voigt function in terms of line width,Lorentzian fraction and an asymmetry term. Afteraccounting for the instrument apparative intensitydistribution function, the microstructure of thespecimen can be determined.

DOI: 10.1595/147106708X299547

1200

1000

800

600

400

200

0

Inte

nsity

, cou

nts

per s

econ

d

30 40 50 60 70 80 90 100 110 120 1302-Theta-Scale

Aged 20 wt.% Pt/CPlatinum

Fig. 1 X-Ray powder diffractiondata for an aged 20 wt.%platinum/carbon electrocatalyst

It is usual to assume a lognormal distributiongLN of diameters D of spherical crystallites withthe median D0 and a variance parameter σ (seeEquation (i)):

gLN(D) =[D lnσ (2π)½]–1 exp[–(ln(D/D0))2/2(lnσ)2] (i)

Having an empirical relationship betweenthis variance and the parameter η describing theLorentzian fraction, FormFit can derive micro-structural parameters for Pt such as anisotropiccrystallite sizes and their distribution width,mean microstrain and stacking-fault densitiescalculated according to various models.

A comparison of normalised, volume-weighted FormFit XRD, TEM and SAXSapproaches is summarised in Figure 2 for anaged 20 wt.% Pt/C electrocatalyst sample. Allapproaches give on refinement a bimodal distri-bution of crystallites with a close match in XRDand SAXS distributions. Differences in themagnitude of this bimodal distribution are evi-dent on comparing to TEM. In this examplemany of the particles measured by TEM are

likely to contain several primary crystallitegrains leading to a larger TEM particle size thanthat measured by XRD or SAXS.

TIM HYDE

References1 P. Scherrer, Nachr. Ges. Wiss. Göttingen, 26 September,

1918, 98 2 D. Ozkaya, Platinum Metals Rev., 2008, 52, (1), 613 N. C. Popa and D. Balzar, J. Appl. Cryst., 2002, 35,

(3), 3384 D. A. Stevens, S. Zhang, Z. Chen and J. R. Dahn,

Carbon, 2003, 41, (14), 27695 H. M. Rietveld, J. Appl. Cryst., 1969, 2, (2), 656 P. Scardi and P. L. Antonucci, J. Mater. Res., 1993, 8,

(8), 18297 R. Haberkorn, ‘FormFit V5.5 – A Program for X-ray

Powder Pattern Deconvolution and Determinationof Microstructure’, Analytik und Datenverarbeitung,Dudweiler, Germany, 2005

Fig. 2 Comparison of normalised,volume-weighted size distributionsdetermined by XRD, TEM and SAXSfor an aged 20 wt.% platinum/carbonelectrocatalyst

0.25

0.20

0.15

0.10

0.05

0 10 20 30 40 50D, nm

Freq

uenc

y, a

.u.

XRDSAXS

TEM

130Platinum Metals Rev., 2008, 52, (2)

The Author

Dr Tim Hyde is a Principal Scientist in theAnalytical Department at the Johnson MattheyTechnology Centre, Sonning Common, U.K.Since joining Johnson Matthey in 1989 he hasspecialised in analytical characterisation ofcatalysts, primarily by laboratory X-ray powderdiffraction and more recently synchrotronradiation techniques.

Platinum Metals ReviewJohnson Matthey Plc, Precious Metals Marketing, Orchard Road, Royston, Hertfordshire SG8 5HE, U.K.

E-mail: [email protected]://www.platinummetalsreview.com/

platinum metals review - johnson matthey technology review

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