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

of Developments in their Application in Industrywww.platinummetalsreview.com

VOL. 48 JULY 2004 NO. 3

Contents

Platinum Metals Review: E-Journal 90Editorial by N. A. P. Carson; A. I. Wood; Susan V. Ashton

Derivatives of Magnus� Green Salt 91By Walter Caseri

Applications of Coordination Complexes 101A book review by J. M. Fisher, R. J. Potter and C. F. J. Barnard

Sonochemical Asymmetric Hydrogenation with Palladium 104

Ethanol Reactions over the Surfaces of Noble Metal/Cerium Oxide 105CatalystsBy H. Idriss

The Chemistry of Selenium and Tellurium 116A conference review by V. K. Jain and H. B. Singh

Piezochromism and Related Phenomena Exhibited by Palladium 117Complexes

By Hideo D. Takagi, Kyoko Noda, Sumitaka Itoh and Satoshi Iwatsuki

Treatment of Platinum Flotation Products 125By A. V. Tatarnikov, I. Sokolskaya, Ya. M. Shneerson, A. Yu. Lapin and P. M. Goncharov

Platinum/Carbon Nanotubes in PEMFCs 132

Optical Hydrogen Sensors Using Palladium-Silicon 132

�Platinum 2004� 133

The Minting of Platinum Roubles: Part III 134By David B. Willey and Allin S. Pratt

Production of Fine Iridium Fibre 138

IOM3 Materials Congress 2004 139A review by A. Bridle

Abstracts 140

New Patents 143

Final Analysis: Thermocouples � Minimising Drift 145By R. Wilkinson

Communications should be addressed to: The Editor, Susan V. Ashton, Platinum Metals Review, [email protected] Johnson Matthey Public Limited Company, Hatton Garden, London EC1N 8EE

Platinum Metals Rev., 2004, 48, (3), 90 90

Platinum Metals Review is now a fully-fledgedE-journal, supporting the science and technolo-gy of the platinum group metals (pgms).Platinum Metals Review forms the centre of thisnew free website that aims to help the work ofthe scientists and technologists who are the cre-ators of the exceptional wealth of research andapplications involving the pgms.

In 1956, it was decided to launch a free quar-terly technical journal to disseminate knowledgeof the science and technology of the pgms to aworldwide readership, to support the platinumindustry and encourage research. A year later,with support from the Directors of JohnsonMatthey and Rustenburg Platinum Mines (theworld�s largest producer of platinum), PlatinumMetals Review was founded.

Since 1957, Platinum Metals Review has fol-lowed the growth of pgm technologies as they

moved from laboratory to industrial scale. It hasreported technologies, such as platinum anti-cancer drugs, catalysts for vehicle emissioncontrol, and a host of chemical, catalytic, elec-trical and electronic, medical and metallurgicaluses as they developed; besides more latterlyfuel cells, sensors and nanotechnology applica-tions. Indeed, Platinum Metals Review has alwaysencouraged scientists to describe their research;and for the last 48 years the Journal has been asource of free information on the unique prop-erties and industrial applications of pgms.

We hope readers will enjoy this new freeE-journal, and we welcome everyone to the newwebsite. N. A. P. CARSON

Neil Carson became the Chief Executive of Johnson Matthey inJuly 2004. He joined Johnson Matthey in 1980, becomingManaging Director, Catalysts & Chemicals in 1999. In 2002 heassumed board level responsibility for the Precious MetalsDivision.

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Anglo Platinum and Johnson Matthey have had a commercial link since 1931 when RustenburgPlatinum (Anglo Platinum) was formed to work platinum deposits in the Bushveld IgneousComplex (the Merensky Reef) in the Transvaal. Johnson Matthey became the refiners and distribu-tors of the pgms produced. In the mid-1950s, Rustenburg Platinum (now as then the largestplatinum producer in the world) encouraged Johnson Matthey to found Platinum Metals Review toreport on scientific developments in the pgm industry. As the output from the platinum minesincreased so did the range and scope of the work that Platinum Metals Review was able to describe �often moving in unexpected directions. Publishing this information has contributed to the devel-opments of the commercial markets for pgms, and has thus supported Anglo Platinum�s objectives.

We hope that Platinum Metals Review and its website will continue to be a valuable tool forplatinum group metals scientists everywhere. A. I. WOOD

Sandy Wood is the Executive Director: Commercial at Anglo Platinum Corporation Limited, in Johannesburg, South Africa.

Platinum Metals Review: E-Journal

DOI: 10.1595/147106704X1522

Johannesburg, South Africa, July 2004

Trafalgar Square, London, July 2004

With good wishes from Trafalgar Square and Johannesburg, welcome to our new website.Besides the E-journal, the website carries features in the �PGM Science Mine� section to providecontacts, advice, data and information, and more will be added later.

We are always happy to hear from our readers and we hope you will enjoy our new website. Onbehalf of my colleagues, welcome to www.platinummetalsreview. SUSAN V. ASHTON, Editor

Hatton Garden, London, July 2004

Platinum Metals Rev., 2004, 48, (3), 91�100 91

Magnus� green salt, [Pt(NH3)4][PtCl4], was firstprepared by Gustav Magnus around 1830 by thedissolution of platinum(II) chloride in hydrochlo-ric acid followed by addition of ammonia (1, 2).The green crystals which slowly precipitatedattracted considerable attention as characterisedammonia complexes were rare at that time. Sincethen several procedures for synthesising Magnus�green salt have been developed, for example, withthe starting materials PtCl2 and ammonia (3�5);K2[PtCl4] and ammonia (6, 7, 8); PtCl2 and[Pt(NH3)4]Cl2 (9); K2[PtCl4] and [Pt(NH3)4]Cl2(10�13); or (NH3)2[PtCl4] and [Pt(NH3)4]Cl2 (14).

Although the elemental composition ofMagnus� green salt was correctly analysed byMagnus himself (1, 2), a detailed insight into thestructure of Magnus� green salt developed onlyslowly over a very long period. Initially, confusionarose due to the early discovery of cis-[Pt(NH3)2Cl2](Peyrone�s chloride (9, 15)) and trans-[Pt(NH3)2Cl2](Reiset�s second chloride (5, 16�18)) which possessthe same stoichiometric composition as Magnus�green salt. While some authors attributed Magnus�

green salt to a stoichiometric mixture of[Pt(NH3)4]Cl2 and PtCl2 (9, 15), other authors,around the year 1850, tried to explain the nature ofplatinum(II)-ammonia complexes on the basis of acondensed state of two ammonia molecules(N2H6) and two modifications of atomic platinumstates (19�23). One form of the platinum stateswas called �platinicum�, and given the symbol pt.Platinicum, was believed to occur only as dimers(pt2). The other form was regarded as a condensedstate of pt2 and was designed as �platinosum� andconnected with the symbol Pt.

It was suggested that hydrogen atoms in ammo-nia or in the condensed state of ammonia could besubstituted by the respective equivalents of �pla-tinicum� or �platinosum�, resulting in moieties suchas NH2Pt (platosamine), N2H5Pt (diplatosamine),NHpt2 (platinamine), and N2H4pt2 (diplatinamine).

In 1886, it was realised that the proposed con-versions of N-H into N-Pt bonds could notdescribe the nature of platinum-ammonia complexesbecause pyridine does not contain N-H bonds butnonetheless forms complexes analogous to those

Derivatives of Magnus� Green SaltFROM INTRACTABLE MATERIALS TO SOLUTION-PROCESSED TRANSISTORS

By Walter CaseriDepartment of Materials, ETH Zentrum, CH-8092 Zürich, Switzerland; E-mail: [email protected]

Magnus� green salt is a quasi-one-dimensional compound of composition [Pt(NH3)4][PtCl4]

comprising linear arrays of platinum(II) ions. It is essentially insoluble in water and organic

solvents and therefore difficult to process, which limits its use. Recently, soluble and thus

processible derivatives of Magnus� green salt have been synthesised by substituting the ammonia

by linear and branched aminoalkanes. The Pt-Pt distances and the properties of these Magnus�

salt derivatives depend on the detailed structure of the aminoalkane. In particular, in compounds

with branched aminoalkanes weak but noteworthy interactions arise between adjacent platinum

atoms, as is evident from their colour, their electrical conductivity, and their UV and IR spectra.

Compounds with optically active branched aminoalkanes exhibit circular dichroism with a

bisignate Cotton effect and unusually high absolute values for the chiral anisotropy factors.

The complex [Pt(NH2dmoc)4][PtCl4] with dmoc designating (S)-3,7-dimethyloctyl is of particular

importance since its colour and electrical conductivity strongly resemble those of Magnus�

green salt. Films of [Pt(NH2dmoc)4][PtCl4] can function as an active semiconducting layer

in field effect transistors. Remarkably, such devices have superior stability in air and water

to unprotected field effect transistors fabricated with typical organic polymers. Hence, Magnus�

salt derivatives might find use in components of mass-produced �plastic electronics�.

DOI: 10.1595/147106704X1504

Platinum Metals Rev., 2004, 48, (3) 92

of ammonia, for example, [Pt(pyridine)2Cl2] and[Pt(pyridine)4][PtCl4] (24). As an alternative, theisomers were interpreted with the help of pentava-lent nitrogen atoms, which allow the formation of,for instance, Pt(-NH3-Cl)2 or Cl-Pt-NH3-NH3-Clentities (24). By 1906 it was known from experi-ments that Magnus� green salt was composed ofPtCl4 and Pt(NH3)4 moieties, but the structure ofMagnus� green salt was still explained using pen-tavalent nitrogen atoms. This resulted in thefollowing possibilities (25): Pt(-Cl-Cl-NH3-NH3-Pt-NH3-NH3-Cl-Cl-)2Pt and Pt(-NH3-NH3-Cl-Cl-)2Pt.

It is noteworthy that as late as 1930 discussionsabout the general structure of platinum complexeshad not ended (26); indeed, the detailed structureof Magnus� green salt was finally elucidated only in1957 by X-ray diffraction, revealing alternatelystacked [Pt(NH3)4)]2+ and [PtCl4]2� moieties com-prising linear arrays of platinum atoms separatedby 3.25 Å, see Figure 1 (27). This distance is sig-nificantly larger than the typical Pt-Pt bond lengthsof 2.6�2.8 Å (28�35). Hence, it was suggested thatit was mainly the electrostatic interactions betweenthe oppositely charged coordination units that leadto the stacking of the coordination units ofMagnus� green salt into a quasi-one-dimensionalstructure, rather than bonds between the platinumatoms (36, 37). Nonetheless, Pt-Pt interactions

involving the 6pz atomic orbitals, although weak,also exist (36) and, in fact, are the origin of impor-tant materials� properties � as will become evident.

Magnus� Pink SaltA pink or red isomer of Magnus� green salt,

usually designated as Magnus� pink salt, has alsobeen described. Some authors (38, 39) attributedthe first synthesis of Magnus� pink salt toVauquelin in 1817 (40, 41), but we failed to findevidence for such a compound in the relatedreports (it appears that the corresponding red sub-stances concern platinum(IV) complexes). In 1906,the discovery of Magnus� pink salt was ascribedto a student named Bjerrum, and it was deducedthat Magnus� pink salt had the composition[Pt(NH3)4][PtCl4] (25). Remarkably, the detailedstructure of Magnus� pink salt is still unknown,since attempts to get crystals suited for single-crys-tal X-ray diffraction have failed (27, 42). It couldbe concluded from powder X-ray diffraction pat-terns, however, that the platinum atoms inMagnus� pink salt are separated by more than 5 Å(27). The formation of Magnus� pink salt seems tobe kinetically favoured, but it readily converts toMagnus� green salt during synthesis if it is not pre-pared at low temperatures and rapidly dried (43).Once isolated, however, it can be stored formonths or years under ambient atmosphere.

Derivatives of Magnus� Green SaltMotivated by the existence of Magnus� green

salt, numerous compounds of the type[PtL4]2+[PtL¢4]2� (L and L¢ are ligands or coordinat-ing parts of multidentate ligands) have beenproduced, and an overview has been presented inthe literature (44). Indeed, complexes with struc-tures investigated by X-ray analysis reveal a linearbackbone of platinum atoms separated by dis-tances of 3.1�4.0 Å, depending on the ligands (44,45). A systematic dependence of the interplatinumdistance on the nature of the ligands has not yetbeen found and, remarkably, compounds with twodifferent Pt-Pt spacings also exist (44).

This paper will focus on compounds of the type[Pt(NH2R)4][PtCl4] with R denoting an alkyl group,see Figure 1. Such complexes have been typically

Fig. 1 Schematicrepresentation of thequasi-one-dimensional structureof compounds of composition [Pt(NH2R)4][PtCl4]R = H (Magnus�green salt) orR = alkyl group

prepared in aqueous solution from K2[PtCl4] and[Pt(NH2R)4]Cl2 (46); K2[PtCl4] and the 1-aminoalka-ne (7, 24, 43); or PtCl2 and the 1-aminoalkane (47).

Derivatives with Linear 1-AminoalkanesOf the compounds with linear 1-aminoalkanes

reported so far, only the aminomethane complexhas been noted to be green (25, 37, 46�50). Its Pt-Pt spacings (3.25�3.29 Å (7, 49, 51)) are similar tothose of Magnus� green salt (see above).Compounds with linear 1-aminoalkanes rangingfrom aminoethane to 1-aminotetradecane are,however, pink (or reddish) (37, 43, 46, 49). For R= ethyl, butyl, propyl and 2-methylpropyl (47, 50,52), brown or greenish-grey modifications werealso described but there is evidence that theseproducts were not (pure) Magnus� salt derivatives(43). However, single crystals of any of the pinkcompounds suitable for X-ray analysis have so faronly been obtained for the aminoethane deriva-tive, revealing linearly arranged platinum atomsseparated by 3.62 Å (49), well below the Pt-Pt dis-tance in Magnus� pink salt (see above). Remarkably,the plane of the platinum-nitrogen coordinationsquare of the tetrakis(aminoethane)platinum(II)coordination unit is inclined by 29° to that of thetetrachloroplatinate(II) plane (49), indicating thatnot only electrostatic attractions, but also crystalpacking effects determine the Pt-Pt distance.

Derivatives with Branched 1-AminoalkanesInterestingly, the colours of the Magnus� salt

derivatives with branched 1-aminoalkanes pre-pared so far (45, 53) differ from the colour ofrelated complexes with linear 1-aminoalkanes ofsimilar chain lengths, although it was claimed ear-lier (46) that aminoalkanes with two or morecarbon atoms only give rise to pink Magnus� saltderivatives due to an increase in the Pt-Pt dis-tance by the packing of the alkyl groups. Thecomplex [Pt(NH2dmoc)4][PtCl4] (dmoc is (S)-3,7-dimethyloctyl) is crystalline, exhibits the colour ofMagnus� green salt and is characterised by a Pt-Ptdistance of 3.1 Å. The 1-amino-2-ethylhexyl (NH2

eh) (eh is ethylhexyl) Magnus� salt derivative isdark violet (or greyish: probably depending on thesurface roughness of the solids or differences in

light reflection), apparently similar in colour to[Pt(en)2][PtCl4] (en = 1,2-diaminoethane) (51) whichexhibits an interplatinum distance of 3.41 Å (7).Since the colour of [Pt(NH2R)4][PtCl4] complexesappears to be related to the interplatinum spacings,the Pt-Pt distance in [Pt(NH2eh)4][PtCl4] is proba-bly smaller than that of the pink compounds butsimilar to that in [Pt(en)2][PtCl4]. However, uponcooling, [Pt(NH2eh)4][PtCl4] undergoes a reversiblecolour change to green at ca. �55°C, which isprobably connected with a diminishing Pt-Pt dis-tance (the pink derivatives with linear 1-amino-alkanes stay pink to at least �196°C). Differentialscanning calorimetric (DSC) measurements didnot reveal a pronounced energy change in the tem-perature region of the colour change at �55°C.Besides the colour transition, [Pt(NH2eh)4][PtCl4]differs from the other 1-aminoalkane derivativesby its essentially amorphous state � evident frompolarisation microscopy and X-ray diffraction.

Colour, IR and UV-Vis SpectraAs already evident from the above section, the

colour of [Pt(NH2R)4][PtCl4] complexes dependson the substituent. In the case of methyl and (S)-3,7-dimethyloctyl groups it is green; for complexeswith linear alkyl groups comprising more than onecarbon atom it is pink, and it is dark violet when Ris 2-ethylhexyl. Indeed, it appears that the greencolour is restricted to compounds with relativelyshort Pt-Pt distances; this is also reflected in theUV and IR spectra.

The Pt-Cl stretching vibration of the pink com-pounds emerges around 320 cm�1, which is closeto the related frequency of K2[PtCl4], but signifi-cantly different from those of Magnus� green salt(311 cm�1), [Pt(NH2 eh)4][PtCl4] (306 cm�1) and[Pt(NH2dmoc)4][PtCl4] (303 cm�1). The lowabsorption frequencies of the latter three com-pounds appear to indicate proximity betweenadjacent coordination units (10, 54). The UV-visspectra of Magnus� green salt and its derivativeswith significant Pt-Pt interactions, are of complexnature and have been discussed extensively in theliterature (7, 8, 37, 46, 51, 55). The UV-vis spectraof Magnus� green salt and its alkyl derivatives men-tioned above are dominated in the wavelength



range of 200�800 nm by a band in the UV region.Smaller peaks in the visible wavelength rangeappear to be due to d�d transitions, which mayshift towards lower energies in compounds of theMagnus� salt type with metal-metal spacings in theregion of 3.25�3.6 Å (7). Magnus� green salt and itsaminomethane derivative (55) show an absorptionmaximum (lmax) at 290 nm in the solid state; solid[Pt(NH2dmoc)4][PtCl4] at 310 nm; [Pt(NH2eh)4][PtCl4] in the solid state at 292 nm and in tetrahy-drofuran at 263 nm with a shoulder at ~ 290 nm;and the solid aminoethane (55) and the 1-aminooc-tane derivative in tetrahydrofuran at 251 and 255nm, respectively.

Thus, it seems that a band at ~ 300 nm arises inthe modifications that are not pink and is indica-tive of significant interactions between adjacentplatinum atoms. The related band has been attrib-uted to a transition of the dz2 orbital of the [PtCl4]2�

to the pz orbital of the [Pt(NH2R)4]2+ unit (7, 54). Itshould be noted that the UV spectra of the pinkcomplexes are not simply composed of a superpo-sition of the spectra of the respective cationic andanionic coordination units. This indicates that veryweak Pt-Pt interactions are also present in therelated pink complexes in spite of the presumablyrelatively large interplatinum distance in them.

Circular DichroismThe 5d z2 � 6p z charge transfer transition in

Magnus� salt derivatives can be involved in circulardichroism (CD) induced by coordinated optically

active aminoalkanes (56). Generally, induction ofCD can proceed via association of optically activemolecules in the second coordination sphere(outer-sphere association, for example, via disper-sion interactions (57, 59)), where the opticallyactive moieties do not show a preferential orienta-tion with respect to the coordination sphere (59).Induced CD emerges in Magnus� salt derivatives oftype [Pt(NH2R)4][PtCl4] where R denotes (S)-3,7-dimethyloctyl or (R)-2-ethylhexyl via hydrogenbonds (57, 58) or via dispersion interactions,respectively, (56). In the region of the 5dz2 � 6pz

transition, the CD spectra of solid[Pt(NH2dmoc)4][PtCl4] and [Pt(NH2 eh)4][PtCl4]reveal strong signals (see Figure 2 for examplewhich also includes the related UV spectrum).

Surprisingly, a bisignate Cotton effect arises forboth of the compounds, with a negative sign atlower energy (first Cotton effect at 314 nm or 291nm, respectively) and a positive sign at higher ener-gy (second Cotton effect at 298 nm or 276 nm,respectively), indicating strong exciton couplingbetween the chromophores. The negative sign ofthe first Cotton effect and the positive sign of thesecond Cotton effect imply a left-handed screw-ness of the electric transition dipole moments ofthe neighbouring chromophores (negative excitonchirality) in both complexes. In general, in organicstructures, the screwness is commonly associatedwith a helical arrangement of the chromophores.Since the electric 5dz2�6pz transition is z-polarised(7, 55), the bisignate Cotton effect would � in

Fig. 2 Circular dichroism (CD) shown bythe solid line, and the UV spectrum shown by the dotted line, for[Pt(NH2dmoc)4][PtCl4]

those terms � imply a left-handed helical arrange-ment of the platinum backbone. However, CDspectra of metal complexes should be regarded asa consequence of the chirality of the metal-ligandsystem as a whole, rather than of the individualchromophores (60). Hence, the helical feature inthe Magnus� salt derivatives is not necessarily pre-sent in the platinum backbone itself.

The chiral anisotropy factors, gabs, for both theNH2 dmoc and the NH2 eh derivatives are0.10�0.12, and are larger than typical valuesreported for other transition metal complexes,including those found for induced CD in d�d tran-sitions in Magnus� salt derivatives (59, 61) (the 5dz2�6pz transitions were not mentioned in 59 and61). Rather high chiral anisotropy factors � up to0.06 � have been observed for cobalt(III) com-plexes with bidentate cyclohexane derivatives (62,63) and for [Pt(NH3)2(dmbn)] with dmbn = (S)-3,3-dimethyl-1,2-butanediamine (64); but thesevalues fall below those of [Pt(NH2dmoc)4][PtCl4].

Solubility and ProcessibilityBesides the chemical and structural aspects of

Magnus� salts attention has also been paid to theirmaterial properties, for instance:� the dichroism observed with linearly polarisedlight of Magnus� green salt (7�10, 37, 55) and itsaminomethane (24, 46), aminoethane (46) and 1-aminobutane (24) derivatives, or � the electrical semiconductivity (65�73) ofMagnus� green salt, which appears to arise as aconsequence of the above mentioned Pt-Pt inter-actions (electrical conductivity is higher in thedirection of the platinum arrays than perpendicu-lar to them, see also below), or� the thermal and chemical stability, which hadalready been recognised by 1847, when it wasdescribed that Magnus� green salt is notably stabletowards heat, acid and alkaline solutions (74),although it is decomposed at elevated tempera-tures by nitric acid (3).

Hence, Magnus� green salt and its derivativesare of interest to materials scientists, all the moreso as their structures, see Figure 1, resemble thoseof rigid-rod polymers with a metal element mainchain; this is a class of polymers which has

received relatively little attention. However, mostMagnus� salt type complexes (including Magnus�green salt) are largely insoluble in water and organ-ic solvents. This unfavourable property is typicalof rigid-rod polymers and often severely restrictsor even prevents potential applications. Onlyrecently have soluble and therefore processibleMagnus� salts been disclosed, in particular deriva-tives with linear 1-aminoalkanes in the range ofheptyl to tetradecyl (the butyl-substituted com-pound is still insoluble in organic solvents andwater), and the branched aminoalkanes mentionedabove (43, 45, 53).

It was shown that the [Pt(NH2eh)4]2+ and the[PtCl4]2� units assemble into supramolecular struc-tures not only in the solid state but also in toluene,in the studied concentration range of 0.1�1.0%w/w and temperature 37°C, as evident from inves-tigations with membrane osmometry. Themeasurements yielded a number average molecularweight (Mn) of 4 ´ 105 g mol�1, which correspondsto a supramolecular assembly containing ca. 750platinum atoms. The existence of high molecularweight structures composed of [Pt(NH2eh)4]2+ and[PtCl4]2� in toluene at 37°C was qualitatively con-firmed by viscosimetric measurements and itsextremely sluggish dissolution behaviour (a prop-erty characteristic of high molecular weightsubstances).

The Mn of [Pt(NH2dmoc)4][PtCl4], determinedby vapour phase osmometry in toluene in the con-centration range 0.1�1.15% w/w at 70ºC, resultedin a value of 9 ´ 103 g mol�1, which corresponds toabout 16 platinum atoms in an assembled forma-tion. The existence of supramolecular structures insolution implies that electrostatic attraction betweenthe oppositely charged coordination units indeedplays an important role in the formation of thequasi-one-dimensional structures of Magnus� salts.

The complexes with long linear 1-aminoalkanesdissolved preferentially at elevated temperature.Hot solutions of these complexes converted, uponcooling to room temperature, into pink gels abovea concentration of ca. 0.1�0.5% w/w. These gelsare stable for months. The gelation process is ther-mally reversible and, hence, appears to beassociated with an ordering or crystallisation



process. When observed in the optical microscope,the gels reveal birefringent, highly ordered, fibrillarstructures. The birefringence is lost when the gelsare heated to adopt the fluid state again. TEMs ofdried gels show fibrillar structures of typical widthbetween 20 nm and 3 µm, and are indicative of acollapsed network. Importantly, the dried gels canagain be dissolved at elevated temperatures, andthe resulting solutions again form gels upon cool-ing to room temperature. In contrast to theMagnus� salt derivatives with linear 1-aminoalka-nes, solutions of [Pt(NH2eh)4][PtCl4] did not yieldgels when cooled from elevated temperature toroom temperature, while [Pt(NH2dmoc)4][PtCl4] inthe concentration range of 2�5% w/w in tolueneseparated into a solvent and a thermo-reversiblegel-like phase upon cooling from 80°C to roomtemperature.

Fibres, Films and Electronic DevicesDue to their outstanding solubility/processibil-

ity, fibres and films have been readily obtained forcompounds of type [Pt(NH2R)4][PtCl4] where R is1-aminooctane, 1-amino-2-ethylhexane and (S)-1-amino-3,7-dimethyloctane (43, 75, 76). Fibres, seeFigure 3, were manufactured by electrostatic spin-ning. Polarisation microscopy disclosed that theplatinum complexes were oriented in the fibres aswell as in film. Films comprising oriented struc-tures of the 1-aminooctane derivative could be

prepared by manual stretching of related gels (seeabove). The growth of highly oriented films of[Pt(NH2dmoc)4][PtCl4] and [Pt(NH2eh)4][PtCl4]proceeded well by deposition of the compoundsfrom super-saturated solutions onto glass slidescovered by a thin layer of highly oriented poly-(tetrafluoroethylene) (PTFE) that had been frictiondeposited onto the slides at elevated temperature(77). Parallel strips of very long, needle-like[Pt(NH2dmoc)4][PtCl4] crystals, which were orient-ed along the PTFE molecules, were readilydetected in the films in TEM images.

Outstanding orientation in the arrays of thecoordination planes in the [Pt(NH2dmoc)4][PtCl4]units was evident from atomic force microscopy(AFM), see Figure 4, and also from electron diffra-ction patterns. The orientation of [Pt(NH2dmoc)4]-[PtCl4] in the films was also evident from the UV-vis absorption spectra recorded with polarisedlight with the incident light polarised parallel andperpendicular to the platinum arrays. In the paral-lel case, the absorption maximum at 310 nm wassubstantially more pronounced than at perpendic-ular orientation. An analogous phenomenon was observed in the UV-vis absorption spectra of

Fig. 3 Polarised optical microscope image of a fibre of[Pt(NH2R)4][PtCl4], R = 2-ethylhexyl; prepared by electrostatic spinning. The Pt complexes were oriented inthe fibres Scale 100 mm

Fig. 4 Atomic force microscopy (AFM) image of an oriented [Pt(NH2dmoc)4][PtCl4] film showing parallelstripes, each built up of linearly assembled coordinationunits Image size ~ 18 × 20 nm


oriented [Pt(NH2eh)4][PtCl4] films.The bulk electrical conductivity of pressed sam-

ples of [Pt(NH2dmoc)4][PtCl4] was 1.6 ´ 10�7 Scm�1 at room temperature, which is in the range ofthat found for pressed samples of Magnus� greensalt measured under similar conditions (5 ´ 10�6 Scm�1). The pressed samples typically are pellets ofca. 1 cm diameter that are pressed under a load ofa few tons. In oriented films, the conductivity par-allel to the platinum arrays (10�4 to 10�3 S cm�1) wasseveral orders of magnitude higher than the val-ues for the perpendicular orientation (ca. 10�8 Scm�1). These anisotropic conductivities are consis-tent with a mobile charge path via the platinum

arrays. The temperature dependence of the con-ductivity of pressed samples followed theexpression:

Ea

s = s0 e kBT

where s is electrical conductivity, s0 is an arbitraryconstant, Ea is the activation energy, kB isBoltzmann�s constant, and T the temperature (inK). This temperature dependence is characteristicfor semiconductors with a single thermally activat-ed conduction process in the observedtemperature interval (70), and, in agreement withthe above equation, a logarithmic representationof the conductivity versus the inverse temperatureresulted in a straight line, see Figure 5. From itsslope an activation energy of 0.24 eV was calculat-ed, which is in the range of the values reported forMagnus� green salt (0.1�0.4 eV) (67�70). Hence,[Pt(NH2dmoc)4][PtCl4] may indeed be regarded asa soluble equivalent of Magnus� green salt.

The intrinsic mobility of charge carriers in[Pt(NH2dmoc)4][PtCl4] was determined with thepulse-radiolysis time-resolved microwave conduc-tivity (PR-TRMC) technique, resulting in amobility along the Pt-chains of 0.06 cm2 Vs�1 (76).This value compares favourably with those foundfor p-stacked discotic materials and p-bond conju-gated polymers (78, 79). One should be aware thatthe PR-TRMC technique yields mobilities of thetrap-free state, that is, mobilities expected to be

Fig. 5 Dependence of the logarithm of the conductivity(s) of a pressed sample of [Pt(NH2dmoc)4][PtCl4] on theinverse temperature (1/T), with s0 = 1 S cm�1

Selected Data for Magnus’ Salts and Magnus Salt Derivatives of Type [Pt(NH2R)4][PtCl4]d(Pt-Pt) denotes the interplatinum distance; s the electrical conductivity at room temperature;lmax the absorption maximum wavelength in the UV-vis region; andn(Pt-Cl) the position of the IR absorption of the Pt-Cl stretching vibration

Compound Colour d (Pt-Pt), s, lmax, n(Pt-Cl),Å S cm–1 nm cm–1

Magnus’ green salt green 3.24(b) 5·10–6 290 311Magnus’ pink salt pink > 5 n.a. n.a. 321R = methyl green 3.27(c) n.a. 290 n.a.R = ethyl pink 3.62 n.a. 251 n.a.R = octyl pink n.a. < 10–10 n.a. 319R = 2-ethylhexyl violet(a) n.a. 7·10–10 292 306R = 3,7-dimethyloctyl green 3.1 2·10–7 310 303

(a) or greyish, depending on the observation angle; at ca. �55°C reversible colour transition to green(b) reported values ranging from 3.23�3.25 Å (c) reported values ranging from 3.25�3.29 Å n.a. = not applicable

close to the optimum value that could be achievedin a DC-device structure for a well-organised layerof a semiconductor between the electrodes. Well-aligned, defect-free layers of [Pt(NH2 dmoc)4]-[PtCl4] would therefore approach mobilities of ~0.1 cm2 Vs�1. It appears that Magnus� salt deriva-tives can be capable of sustaining attractive currentdensities and switching times.

Field effect transistors (FETs) that have[Pt(NH2dmoc)4][PtCl4] as the active semiconduc-tor layer were produced under ambient conditionsin air, both with highly oriented films grown ontoPTFE orientated layers, see Figure 6, and withisotropic, spin-coated films of the Magnus� saltderivative (76). Devices in which the platinumarrays were aligned parallel to the current transportdirection exhibited (p-type) transistor action withfield effect mobilities of the order of 10�3�10�4 cm2

Vs�1. In contrast to the more microscopic PR-TRMC measurements, the mobility observed inFET devices is still most likely to be limited bytransport in disordered regions of the film, pre-sumably in the grain boundaries. In as-prepareddevices, the relatively high film-conductivity of theorder of 10�7 S cm�1 limited the ON-OFF currentratio of the transistors to less than 10. Whileunprotected FETs comprising an organic polymeror oligomer, under current scrutiny for FETs,appear typically to suffer from degradation uponexposure to oxygen and water, the transistors with

[Pt(NH2 dmoc)4][PtCl4] did not significantlydegrade, even after immersion in water at a tem-perature of 90°C for a period of 12 hours. Chargecarrier mobilities in devices with the channel per-pendicular to the oriented [Pt(NH2dmoc)4][PtCl4]molecules or in devices containing an isotropic[Pt(NH2 dmoc)4][PtCl4] layer produced by spin-coating were two or three orders of magnitudelower than the above values in the oriented state,respectively. It is thus essential to control thestructural order in the active semiconductor layer.This is easily achieved for [Pt(NH2dmoc)4][PtCl4].

The simple synthesis of Magnus� salt deriva-tives, convenient processibility of solublecompounds of the type [Pt(NH2R)4][PtCl4], andoutstanding resistance to relatively harsh environ-mental conditions could render Magnus� saltderivatives suited for mass-produced electronicproducts. Note that in spite of the presence ofplatinum, the cost of the principal starting materi-al for Magnus� salt derivatives has been estimatedto be about only one-fifth of that of substitutedpoly(phenylene vinylenes) and pentacene whichare among those species being considered for thepreparation of �plastic electronics�.

ConclusionsSoluble and thus processible derivatives of

Magnus� green salt can be obtained by substitutionof ammonia by linear and branched 1-aminoalka-


234

175 mm

5

234

175 mm

5

1 Evaporated gold electrodes2 Magnus� salt derivative3 Oriented PTFE layer4 SiO2 layer (200 nm)5 n++-doped silicon wafer

1

Fig. 6 Optical micrograph (A) and schematic representation (B) of a field effect transistor with an aligned [Pt(NH2dmoc)4][PtCl4] film

A B

2

Alignmentdirection

nes. The quasi-one-dimensional structures of thecorresponding Magnus� salts are believed to arisemainly as a consequence of electrostatic attractionbetween the oppositely charged coordinationunits, although packing effects of the aminoalka-nes also appear to be of importance. In addition,weak but noteworthy interactions between adja-cent platinum atoms can also appear. These arereflected in the colour of the various complexes, aswell as in their electrical conductivity and in theirUV and IR spectra. The compounds with compa-rably short Pt-Pt spacings are green, semi-conducting, and display a UV band around 300nm, which is due to a dz2 �pz transition betweenplatinum atoms of adjacent coordination units,and a Pt-Cl stretching vibration well below 320cm�1. A comparably long spacing between adjacentplatinum atoms is accompanied by a pink colourof the related complexes and a Pt-Cl stretchingvibration close to 320 cm�1.

The Pt-Pt distances in the complexes withbranched aminoalkanes, [Pt(NH2eh)4][PtCl4] and[Pt(NH2dmoc)4][PtCl4], appear to be considerablyshorter than those in corresponding compoundswith linear alkyl groups. The 1-amino-2-ethylhexylderivative is amorphous, cryochromic and in solu-tion forms supramolecular structures whichcontain hundreds of platinum atoms. The complex[Pt(NH2dmoc)4][PtCl4] is green with a particularlyshort Pt-Pt distance of 3.1 Å and besides thecolour also shows the semiconducting propertiesof Magnus� green salt. It displays electrical (semi-conductor) conductivity of 1.6 ´ 10�7 S cm�1 atroom temperature and a thermal activation energyof the conduction process of 0.24 eV.

The derivatives with the optically active dmocas well as with (R)-2-ethylhexyl exhibit circulardichroism in the region of the d z2�p z transition.Interestingly, a bisignate Cotton effect arises, sug-gesting [Pt(NH2R)4][PtCl4] has a helical structure.The absolute values of the chiral anisotropy fac-tors are of the order of 0.1, which is among thehighest values reported for metal complexes.

Oriented fibres could be prepared for all solu-ble Magnus� salt derivatives. Related films of[Pt(NH2dmoc)4][PtCl4] can function as the activesemiconducting layer in, for instance, FETs.

Remarkably, such devices show a stability towardsair and water which is superior to that of unpro-tected FETs with typical organic polymers. Hence,Magnus� salt derivatives may pave the way formass-produced �plastic electronics�.

AcknowledgementThe author is particularly indebted to, and has written this

review also on behalf of: J. Bremi, M. Fontana, P. Smith, H.Chanzy, M. G. Debije, M. P. de Haas, I. P. Dolbnya, K.Feldman, R. H. Friend, J. G. P. Goossens, E. W. Meijer, A. P.H. J. Schenning, H. Sirringhaus, N. Stutzmann, T. Tervoort, A.M. van de Craats, and J. M. Warman, for many years of fruitfuland collegial collaboration and their seminal contribution to thisproject.

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The AuthorWalter Caseri has been active as a Senior Scientist in the Institutfür Polymere at the ETH Zentrum, Zürich in Switzerland since 1996.He is involved in research and teaching. His interests are inpolymers containing both organic and inorganic components(polymeric structures with inorganic backbones, nanocompositesand polymers at inorganic interfaces).



This 9th volume in the set of books:�Comprehensive Coordination Chemistry II�, cov-ers applications of coordination chemistry and hasan ambitious remit: to survey the developments inthe applications of coordination chemistry since1982. It follows on from the review of Volume 6,published in our April issue (1).

Coordination Complexes as CatalystsThe first half of the book focuses on the use of

coordination complexes as catalysts for a diverserange of organic reactions. Chapters on supportedmetal complexes and combinatorial catalysis arealso included in this half of the book.

Chapter 1, covering catalysts for polymerisa-tion, is not an area where platinum group metal(pgm) complexes would be expected to dominate.There is, however, an interesting section on the re-emergence of Ru initiators for polymerisation. Theuse of Ru complexes (some now commerciallyavailable) for ring opening metathesis polymerisa-tion (ROMP) of cyclic alkenes is discussed.

Chapter 2 by C. Pettinari, F. Marchetti and D.Martini covers metal complexes as hydrogenationcatalysts with, naturally, a heavy emphasis on enan-tioselective reactions. I liked this chapter with itsfocus on mechanism and careful choice of exam-ple to demonstrate each point. Work which hasbecome of industrial importance is covered togeth-er with proper discussion of Noyori�s groundbreaking design and synthesis of RuCl2(diphos-phine)(1,2-diamine) catalysts. The second half ofthe chapter concentrates on new developmentsand covers hydrogenation and enantioselectivehydrogenation in aqueous systems and in super-critical CO2, and biphasic catalysis amongst othertopics. Additionally hydrogenation by Os and Rucluster complexes is also discussed.

Metal complexes as catalysts for the addition ofcarbon monoxide is the focus of Chapter 3. This is

obviously a huge area and the authors (P. W. N. M.van Leeuwen and C. Claver) have concentrated onwork which explicitly uses coordination complexesrather than materials prepared in situ. Additionallythey cite a large number of review articles. Thechapter begins by discussing the BP CativaTM

process (2) where Ir has found advantage over Rhfor the carbonylation of methanol. A large sectionon hydroformylation covers PtIISnCl2/phosphinesas well as the extensive Rh chemistry dominated byphosphine or phosphite ligand systems. Drent�swork with Pd is also included. Asymmetric hydro-formylation with these types of systems is dealtwith in separate sections. The development ofthermoplastics has required the copolymerisationof alkenes and carbon monoxide; and the use ofPd complexes with chelating bidentate phosphineligands, amongst others, is surveyed. The chapterconcludes with sections on reductive carbonyla-tion of nitro groups and the use of Pd complexesfor hydroxy- and methoxycarbonylation.

In Chapter 4, T. Katsuki discusses metal com-plexes as catalysts for oxygen-, nitrogen- andcarbon-atom transfer to alkenes. Ru metallopor-phyrin and metallosalene complexes feature ascatalysts for chiral epoxidation and mention ismade of work where molecular oxygen is the oxi-dant. RuII salen complexes have been found to beefficient catalysts for aziridination. Os chemistrydominates the section on dihydroxylation. In par-ticular complexes with diamine and bis-cinchonaligands have excellent enantioselectivity. TheSharpless and Carey mechanistic models are thor-oughly covered. There is a large section on the useof Rh and Ru catalysts for inter- and intramolecu-lar cyclopropanation.

The addition of H-X (X = B, CN, Si, N and P)to carbon-carbon multiple bonds is reviewed by M.K. Whittlesey in Chapter 5. The comprehensivesection on hydroboration covers a lot of work with

Applications of Coordination ComplexesCOMPREHENSIVE COORDINATION CHEMISTRY II. FROM BIOLOGY TO NANOTECHNOLOGY

Volume 9 APPLICATIONS OF COORDINATION CHEMISTRYEDITED BY M. D. WARD; EDITORS-IN-CHIEF, JON A. McCLEVERTY AND THOMAS J. MEYER, Elsevier, Amsterdam, 2003, 876 pages,ISBN 0-08-0443311 (Volume 9); ISBN 0-08-0437486 (Set), U.S.$ 5975, E 6274 per Set

DOI: 10.1595/147106704X1621

Rh catalysts � largely with phosphine ligands. Pdcatalysts find favour for the addition of HB toalkynes and enyes. The section on hydrosilylationcovers both Rh and Pt with a variety of chiral lig-ands and Pd with ferrocenyl phosphine ligands forthe hydrosilylation of styrene. Rh and Ir are large-ly used for hydroamination, with some Pdphosphine systems suitable for activated alkenes.

Chapter 6 (metal complexes as catalysts for C-C cross-coupling reactions) by I. P. Beletskaya andA. V. Cheprakov, is possibly the best of all; a goodintroduction describes the various methods ofcross-coupling. It is clearly laid out with discussionon leaving groups, activation of reagents and cata-lysts. There is a pleasing emphasis on the rationalebehind the design of catalysts. Most of the workfocuses on Pd although Ni is not neglected.

In Chapter 7, J. F. Hartwig describes metalcomplexes as catalysts for carbon heteroatomcross-coupling reactions, and not surprisinglyreveals a huge emphasis on Pd in every type ofreaction. The majority of the reactions discussedare aminations, but C-O, C-S, C-P and C-Se reac-tions are also included. The discussion of reactionintermediates at the end of the chapter is particu-larly interesting and it was a shame that moredetailed discussions on individual reactions isbeyond the scope of the chapter.

S. Kobayashi, Y. Mori and Y. Yamashita inChapter 8 cover the use of metal complexes asLewis acid catalysts in organic synthesis. In gener-al, pgm coordination complexes do not play animportant role in this area. However, it was inter-esting to learn that a Ag+BINAP complex has beenfound to activate aldehydes and imines effectivelyallowing asymmetric allylations, aldol reactions andMannich-type reactions to proceed in high yieldwith high selectivity. Additionally both Ag(I) andAu (I) ferrocenylphosphine complexes promoteasymmetric aldol reactions of a-isocyanocarboxy-lates to form chiral oxazolines.

Although some of the preceding chaptersbriefly mention immobilisation, Chapter 9 by F.Quignard and A. Choplin reviews supported metalcomplexes. Most of the supports covered are con-ventional but new advances including dendrimersand meso-structured materials are mentioned. I

felt that the newer approaches could have beencovered in a little more detail but I liked the con-clusion and perspective section at the end of thechapter.

In Chapter 10, entitled �ElectrochemicalReactions Catalyzed by Transition MetalComplexes�, A. Deronzier and J.-C. Moutet review20 years of literature of mainly electroreductionreactions with target molecules such as CO2,organic halides and oxides of nitrogen.Surprisingly, given the importance of electroreduc-tion to, for example, fuel cells, the description ofthe catalysis of oxygen reduction using coordina-tion compounds is comparatively brief. Rhcomplexes, especially with P-containing ligand sys-tems, are popular choices for reduction reactions,including hydride transfer in, for example, NADHcatalysis. Ru complexes appear to be better suitedfor heavy-duty oxidation work such as oxygen evo-lution.

Chapter 11 by M. T. Reetz, discusses combina-torial methods in catalysis by metal complexes.Methods of catalyst screening are discussed withthe calorimetric assay developed to screen thehydroamination of 1,3-dienes by various phos-phines with Pd, Rh, Ir, Ni and Ru precursors beingone of the examples. Capillary electrophoresis isalso mentioned with the optimisation of the Pd-catalysed annulation reaction of an indolederivative as a prime example. The evaluation ofPd 1,2-diimine complexes for ethylene polymerisa-tion by a combinatorial approach is also included.The bulk of this chapter focuses on the use ofcombinatorial methods for enantioselectiveprocesses and reviews new methods for highthroughput ee-assays as well as the modular syn-thesis of chiral ligands. This is an excellent résuméof an exciting field of work.

Optical Properties of CoordinationCompounds

Five chapters (Chapters 12 to 16) deal withoptical properties of coordination compounds as acentral theme, and the predominance of Ru-N lig-and species as systems for academic study willcome as no surprise. The sweep of the field is vast,both in terms of the science and the history, and


the reader does get a comprehensive review of the lit-erature. However, in Chapter 12, by P. Gregory,on metal complexes as speciality dyes and pig-ments, pgms feature very little: most of the metalcomplexes described are from the early transitionelements. Nevertheless, the history of dyes andpigments is well accounted for. Key advancesthrough the ages include the discovery of mor-danting (typically using alum) and the synthesis ofartificial dyes, which paved the way for the mod-ern colours and coatings industry. Increasingly,coordination compounds are being used to pro-vide functionality, as well as decorative effect incoatings and surface finishes. The role of metalcomplexes in photography, electrophotograpy andink-jet printing are concisely described.

The pgms reappear in Chapter 13 (R. J.Mortimer and N. M. Rowley) entitled �Metal com-plexes as dyes for optical data storage andelectrochromic materials�. In contrast to the tech-nology described in the previous chapter,electrochromic devices have yet to achieve wide-spread application, mainly due to poor durabilityand slow response times. The lack of stability isinitially surprising given that the ubiquitousphthalocyanine, porphyrin and polypyridyl com-plexes of transition metals, which confer suchstability in other applications, fail to do so here,until one considers the stresses and strains inducedin thin films by rapid ion movement. Optical datastorage on the other hand, has flourished despitemany practical constraints. Here the phthalocya-nines have reigned supreme although azo-typedyes are presently the system of choice for DVDs.

In Chapter 14 entitled �Nonlinear optical prop-erties of metal complexes� by B. J. Coe, theinvestigation of molecular complexes exhibitingboth quadratic and cubic (non-linear) optical (NLO)responses is described. The list of candidate ligandsystems again includes phthalocyanines, por-phyrins, and polypyridyl complexes, as well as adiverse range of less-common conjugated ligands.The importance of inter-system-crossing (ISC) ofspin-states in NLO means that complexes withheavy metal ions such as Pt, Os, Ir and Ru, arepopular choices for many of these studies.

Many of the precursors of inorganic phosphors

are coordination compounds, as described by J.Silver in Chapter 15, �Metal compounds as phos-phors�. Phosphor materials usually rely heavily onGroup 10 and the rare earths for the key proper-ties required. However, the ISC properties ofcomplexes with the pgms are usefully applied inseveral types of organo-electroluminescent devicesdescribed in this Chapter.

In Chapter 16, by Md. K. Nazeeruddin and M.Grätzel, the conversion and storage of solar ener-gy using dye-sensitised nanocrystalline TiO2 cells isdiscussed. Since the pioneering work of Grätzelsome 15 years ago, Ru-based polypyridyl ligandshave remained the first choice due in part to goodabsorption characteristics and remarkably efficientcharge injection (into the inorganic semiconduc-tor). Although other metal-based dye systems arealso active, the stability of the Ru (and also Os)complexes is a major advantage in their use. Morerecently, advantage has been taken of the axialcoordination sites on the Ru ion to fine-tune themolecular environment of the charge injection sitefor both solar cell and photoelectrocatalysis appli-cations.

Hydrometallurgy and ExtractionChapter 17 relates to �Metal complexes in

hydrometallurgy and extraction�, especially, mineralprocessing, leaching, separation and concentration.This covers both base metals and precious metals:Pt(IV), Pt(II), Pd(II), Au(I), Au(III), and Ag(I).

Coordination Compounds forMedicine and Biology

The interaction of transition metal ions withbiological molecules provides one of the most fas-cinating areas of coordination chemistry. Theapplication of this field to biomedical uses is dealtwith in 5 chapters. Chapter 18 by N. Farrell, dealswith the use of metal complexes as drugs andchemotherapeutic agents. Farrell concentrates onPt anticancer drugs and, in particular, the differinginteractions of mono-, di- and trinuclear complex-es with DNA and the differing antitumour effectsthis may produce. Chapter 19, by É. Tóth, L.Helm and A. E. Merbach, describes the applica-tion of transition metal ions and in particular



Gd(III) as MRI contrast agents. The spectaculargrowth in use of MRI as a diagnostic tool wouldnot have occurred without the use of contrastagents. Radioactive nuclei, for example, Rh,described in Chapter 20, by S. Z. Lever, J. D.Lydon, C. S. Cutler and S. S. Jurisson, have been avital part of medicine for a much longer period butdevelopments are still being made through coordi-nation chemistry in modifying the distribution ofthe elements. Chapter 21, by S. Faulkner and J. L.Matthews, describes the use of fluorescent com-pounds in diagnosis. A different method of cancertreatment is discussed in Chapter 22 on Ru, Pdand Pt complexes for photodynamic therapy. Thisarea is dominated by porphyrin complexes.

ConclusionsIn summary this book certainly is a compre-

hensive overview of the applications of coord-ination chemistry, and in particular the use ofpgms in catalysis, in dyes for optical applications,

and in medicinal and biomedical applications. It isan informative read and the individual chaptersoffer good introductions to the various areas.

J. M. FISHER, R. J. POTTER AND C. F. J. BARNARD

References1 A. K. Keep, Platinum Metals Rev., 2004, 48, (2), 642 J. H. Jones, Platinum Metals Rev., 2000, 44, (3), 94

The Reviewers

Janet Fisher, who reviewed Chapters 1–9 and 11, is a PrincipalScientist at the Johnson Matthey Technology Centre, SonningCommon, U.K. Her primary interests are in catalyst preparationand characterisation.

Rob Potter, who reviewed Chapters 10 and 12–16, is a SeniorPrincipal Scientist at the Johnson Matthey Technology Centre. Hismain interests include energy conversion technology,electrochemistry, and meta materials.

Chris Barnard, who reviewed Chapters 18–22, is a ScientificConsultant in the Liquid Phase Catalysis Group at the JohnsonMatthey Technology Centre, with interests in homogeneouscatalysis employing the platinum group metals. He is alsointerested in the application of platinum compounds as cancertherapy.

Enantioselective hydrogenation is one of themost versatile methods of asymmetric synthesis,with heterogeneous catalysis, using chiral modi-fiers, rapidly becoming an alternative to the�traditional� homogeneous methods. The role ofmodifiers in asymmetric hydrogenations is toenhance catalysis, with the bonding mode andgeometry of adsorption being important, as well asthe modifier concentration and the type and posi-tion of the substituent groups in the aromatic ring.

Ultrasonic irradiation (sonication) is known to bebeneficial in catalytic asymmetric hydrogenations.Sonication removes catalyst surface impurities, andgives enhanced adsorption to the chiral modifiers.

Now a team from Michigan TechnologicalUniversity, Houghton, U.S.A. (S. C. Mhadgut, I.Bucsi, M. Török and B. Török, Chem. Commun.,2004, (8), 984-985; DOI: 10.1039/b315244h) hasrevisited the Pd-catalysed, proline-modified, asym-metric hydrogenation of isophorone (3,3,5-trimethyl-2-cyclohexen-1-one (with a C=C bond)).They examined the catalyst, the modifier and theeffects of sonication.

Pd/Al2O3 was found to give a better, thoughlow, enantiomeric excess (ee) than Pd/C. Proline

and its derivatives (isomeric hydroxyl-prolines,prolinols and proline esters) were tested as chiralmodifiers for Pd/Al2O3. Proline was the best mod-ifier, and both enantiomers gave ee £ 35%.

Presonication was found to enhance the enan-tioselectivity when both the Pd/Al2O3 catalyst andthe proline modifier were present. �Modifier-free�presonication and the presence of substrate duringpretreatment decreased the enantioselectivity.

The reaction was performed at 50 bar pressureand 25ºC. Presonication for 20 minutes gave thehighest optical yields, and increased optical yieldsacross all the H2 pressure range. Maximum eeoccurred at a 1 :2 isophorone:proline ratio, andwith optimised conditions and presonication, theee for the Pd/Al2O3-(S)-proline catalytic systemwas £ 85%.

Ultrasonic cleaning of the catalyst enhancedboth the adsorption of the modifier and the mod-ifier-induced surface restructuring of the Pd. Thehigh ee was due to proline adsorption on the Pdsurface. New catalysts that can strongly adsorbproline could thus become important in heteroge-neous catalysis for C=C double bond hydro-genation of a,b-unsaturated carbonyl compounds.

Sonochemical Asymmetric Hydrogenation with Palladium

DOI: 10.1595/147106704X1892

Man-made chemicals affect our health andenvironment through the air, water and soil. Henceseeking suitable, safer alternative chemicals andparticularly fuels to replace conventional ones is anongoing objective. There are several candidates foralternative fuels that can be derived from non-crude oil resources. These include natural gas,liquid petroleum gas (LPG), propane, methanol,ethanol, and hydrogen. Table I characterises vari-ous current and alternative fuels (1(a)).

Ethanol as a Source of HydrogenEthanol is an interesting alternative for replac-

ing gasoline as a motor fuel. Ethanol productionfrom sugar- or starch-containing crops is an indus-trially well-established technique that has beenused for many years. Ethanol could indirectlyreduce net carbon dioxide emissions from vehicleswhen used as a 95% blend with gasoline for light-

duty vehicles. This is due to the consumption ofcarbon dioxide by crops used as feedstock for theproduction of ethanol fuel. Sulfur oxide emissionswould also be lowered (by 60 to 80%). Volatileorganic compound emissions would be 13 to 15%lower than those of reformulated gasoline (1(b)).Although ethanol is considered to be an attractivereplacement fuel, with quite low emissions of com-plex hydrocarbons, its partial oxidation toacetaldehyde (a potential carcinogen) poses athreat. Hence, the problem of effectively control-ling the emissions caused by ethanol oxidationalong with the desired conversion requires betterunderstanding.

There are four basic methods for hydrogen pro-duction: water electrolysis, a gasification reactionusing coal or coke as the feedstock, a partial oxida-tion reaction using heavy or residual oil as thefeedstock, and steam reforming using various


Ethanol Reactions over the Surfaces ofNoble Metal/Cerium Oxide CatalystsBy H. IdrissThe University of Auckland, Department of Chemistry, Auckland, New Zealand; E-mail: [email protected]

This review focuses on the reactions of ethanol on the surfaces of platinum, palladium, rhodium

and gold supported on ceria (of size 10�20 nm). The bimetallic compounds: Pt-Rh, Rh-Au,

Rh-Pd, and Pt-Pd were also investigated. Initially this work was aimed at understanding the

roles of the different components of automobile catalytic converters on the reactions of ethanol,

which is used as a fuel additive. Some of the catalysts that showed high activity for ethanol

oxidation were also investigated for hydrogen production. The addition of any of the above

metals to CeO2 was found to suppress the oxidation of ethanol to acetates at room temperature,

as there are fewer surface oxygen atoms available to oxidise the ethanol (the remaining oxygen

atoms did not produce efficient oxidation). Ethanol dehydrogenation to acetaldehyde was

facilitated by the presence of Pt or Pd; at higher temperatures the acetaldehyde condensed

to other organic compounds, such as crotonaldehyde. By contrast, in the presence of Rh

only traces of acetaldehyde or other organic compounds were seen on the surface, and detectable

amounts of CO were found upon ethanol adsorption at room temperature. This indicates the

powerful nature of Rh in breaking the carbon-carbon bond in ethanol. The effects of prior

reduction were also investigated and clear differences were seen: for example, a shift in reaction

selectivity is observed for the bimetallic Rh-containing catalysts. Methane was the dominant

hydrocarbon on the reduced catalysts while acetaldehyde was the main product for the non-

reduced ones. Hydrogen formation was monitored during steady state ethanol oxidation and

Pt-Rh and Rh-Au were found to be the most active catalysts.

DOI: 10.1595/147106704X1603


kinds of hydrocarbons as the feedstock. Currentlysteam reforming is the most important process forhydrogen manufacture; steam reforming of natur-al gas accounts for almost 50% of the worldfeedstock for hydrogen (1(c)):

Water electrolysis: 2H2O ® 2H2 + O2 (i)

Coal gasification: CH0.8 + 0.6H2O + 0.7O2 ®CO2 + H2 (ii)

Partial oxidation: CH1.8 + H2O + 0.5O2 ®CO2 + 1.9H2 (iii)

Steam reforming: CH4 + 2H2O ® CO2 + 4H2 (iv)

Much work has been devoted to producing H2

from ethanol, either by steam reforming or partialoxidation. Fundamental studies of ethanol reac-tions on metals and oxides, examining therelationships between surface properties (struc-ture, surface defects, etc.) and the reaction yields,have helped to further our understanding of thevarious processes.

Ethanol Reactions on SurfacesThe ethanol molecule contains two carbon

atoms. It can be split at several bonds, see SchemeI. Depending on the type of interaction with the

surface of a solid material, the nature of the solid,and the diverse reaction conditions, one may beable to orient the reaction into desired product(s).

The different reactions occurring with ethanolon metal and metal oxide surfaces so far observedare summarised in Scheme II. The reaction can beshifted from CO to CO2 and from H2 to H2O,depending on the ethanol :oxygen ratio. Thus, thebalance is not simple and the role of the catalyst iscrucial. As the main objective is to find ways toobtain H2 and CO2, the catalyst must be active forthe water gas shift reaction and for the direct oxidation of CO to CO2, Equation (xiii) andFigure 1.

The Interaction of Ethanol withCeO2 and M/CeO2

The desired catalyst should not be poisoned byCO at the reaction temperature, should allow for afast O transfer (from the bulk to surface), shouldallow for fast rejuvenation of the surface oxygen

Table I

Characteristics of Various Fuels

Fuel LHV, A/FBTU/lbm

Gasoline 18,341 14.6Light diesel 18,574 14.5Methane 21,459 17.2Natural gas 19,257 16.2LPG 19,757 15.8Propane 19,879 15.7Methanol 8,538 6.5Ethanol 11,546 9.0Hydrogen 51,352 34.3

LHV (Lower heating value) is used when product-containingwater is in its vapour form. The unit of LHV is British ThermalUnit (BTU) per pound mass (lbm).A/F (Air to Fuel ratio) represents the amount of air required forcomplete combustion of fuel. Heating value (HV) is defined as the amount of energy releasedwhen a fuel is burned completely in a steady flow process (Adapted from 1(a))

Scheme IApproximate bond energies for ethanol (2)

Fig. 1 Schematic of two activated processes: CO oxidation to CO2 and H2 oxidation to H2O. The catalyst chosen would depend on the process temperature. Here, H2 oxidation has low activation energy while CO to CO2 has high activation energy. In these conditions increasing the reaction temperaturewould favour CO to CO2 and not H2O formation

CH3CH2OH + 3/2O2 ® 2CO2 + 3H2


CH3CH2OH (g) ® CH3CH2O(a) + H(a) (v) on most oxide surfaces and some metals,

such as Pd (3), Ni (4), and Rh (5)

CH3CH2O(a) ® CH3CHO(g) + H(a) (vi(a)) DHr ((v) + (vi(a)) = 111.5 kJ mol�1

on large numbers of oxides (6) and some metals (3)

CH3CH2O(a) ® CH2CH2(g) + OH(a) (vi(b)) DHr ((v) + (vi(b)) = 88.4 kJ mol�1

on some oxides, usually acidic (7) or in

sub-stoichiometric form (8, 9)

CH3CH2O(a) ® CH2CH2O(a) + H(a) (vi(c)) on Rh (10, 11)

CH3CHO(g) + O(s) ® CH3COO(a) + H(a) (vii) mainly on basic oxides, such as CeO2 (12)

and ZnO (13)

2CH3COO(a) ® CH3C(O)CH3 + CO2 + O(a) (viii) DHr (acetic acid(g) to acetone, CO2 and water)

= 13 kJ mol�1; on stoichiometric oxides such as

TiO2 (14), CeO2 (12) and Fe2O3 (15)

CH3CHO(g) ® CH4(g) + CO(g) (ix) DHr = �18.7 kJ mol�1; mainly on metals (16)

2CH3CHO(g) ® CH3CH=CHCHO(g) + H2O(g) (x) DHr = �10.3 kJ mol�1; on stoichiometric oxides

such as TiO2 (14), CeO2 (17), and MgO (18)

2CH3CHO(g) + 2Vo ® CH3CH=CHCH3(g) + 2O(a) (xi) DHr (acetaldehyde to trans-butene and O2) =

321 kJ mol�1; Vo is oxygen vacancy;

on TiO2�x (14) and UO2 (19)

3CH3CHO(g) ® C6H6(g) + 3H2O(g) (xii) DHr = �144.2 kJ mol�1; on Pt/CeO2 (20) and

UO2�x (21)

CH3CH2OH +1.5O2 ® 2CO2 + 3H2 (xiii) DHr = �551 kJ mol�1

The elementary steps of Reaction (xiii) can be written as follows (xiii(a)�xiii(d):

CH3CH2OH + 0.5O2 ® CH3CHO + H2O (xii(a)) DHr = �172.9 kJ mol�1

CH3CHO ® CH4 + CO (xiii(b)) DHr = �18.7 kJ mol�1

CH4 + H2O ® CO + 3H2 (xiii(c)) DHr = 205.7 kJ mol�1

2CO + O2 ® 2CO2 (xiii(d)) DHr = �566 kJ mol�1

Methane reforming (xiii(c)) and CO oxidation (xiii(d)) may also be interchanged with methane oxidation

(xiii(c' )) and the water gas-shift reaction (xiii(d' )):

(CH4 + O2 ® 2H2 + CO2) (xiii(c' )) DHr = �319.1 kJ mol�1)

(CO + H2O ® H2 + CO2) (xiii(d' )) DHr = �41.2 kJ mol�1)

Scheme IISummary of reactions occurring with ethanol on the surfaces of metals and metal oxides

(a) = adsorbed; (g) = gas phase; adsorption and desorption reactions are omitted for simplicity

defects (Mars van Krevelen mechanism (22)) andshould contain metals that are capable of oxida-tion/reduction cycles with minimum deactivation.

Catalysis by noble metals is usually achieved via thehigh dispersion of low loading metal(s) (less thanone atomic per cent) on an appropriate support.The supports are usually of relatively inexpensiveoxide, such as alumina, silica or titania.

Indeed, automobile catalytic converters havenow been used for almost three decades and areformed of highly dispersed metals on ceria (CeO2).Ceria has the fluorite structure (a relatively openstructure allowing for a facile oxygen diffusion). Itis an ionic oxide of basic character and can bemade with high surface area and of nano-dimen-sion particulates. These catalysts (or a variant of


Scheme IIIInteraction of an alcohol molecule with the surface of

CeO2 leading to dissociation of the O-H bond

Fig. 2 IR spectra collectedupon adsorption of ethanolat 300 K on CeO2,Pd/CeO2, Pt/CeO2 andRh/CeO2. Note the presence of acetatespecies (na = 1572 cm�1) onCeO2 and their absence onall the other catalysts. Note also the presence ofacetaldehyde on Rh/CeO2

(n (CO) at 1709 cm�1)


them), as will be shown here, can be used forhydrogen production.

The interaction of a polar molecule such asethanol with the CeO2 surface would be favouredthrough the dipole-dipole type as shown inScheme III. This type of interaction is moreknown as an acid-base interaction, where the Hatom of the acid (in this case ethanol) interactswith one surface O2� (the base). Simultaneously,the Ce4+ site (acting as Lewis acid) interacts withthe O (2p) orbitals of the oxygen of the adsorbedethanol molecule. As a result, ethoxide and surfacehydroxyl species are formed. Figure 2 shows IRspectra following adsorption of ethanol on theCeO2 and M/CeO2 (M = Rh, Pt and Pd) (20, 23,24). The formation of ethoxides can be seen (twomodes of these species are most likely present onCeO2: monodentate at 1107 cm�1, and bidentate at1057 cm�1).

CeO2 is made by precipitation fromCe(NO3)3.6H2O solution by addition of ammoniawith stirring at 373 K (pH = 8�8.5). After filtra-tion, washing and drying (at 373 K) calcinationwas conducted at 773 K for 4 hours under air-flow. Metals were deposited on CeO2 byimpregnation from their salts (RhCl3.3H2O, PtCl4,PdCl2 and HAuCl4) as detailed in (24) and (29).

Three main differences occur in the presenceof Pt, Pd or Rh metals (the �as prepared� metals areactually in oxide form (see (20, 22, 23, 28) formore details on catalyst characterisation):[1] The two bands (at 1107 and 1057 cm�1) asso-ciated to two modes of adsorption for ethoxideshave shifted to lower wavenumbers and in addi-tion the first band has become far more

pronounced. This is not only due to reduction ofCeO2 because of the presence of the metal (seebelow) but might also occur due to a direct inter-action with the metal particles, see Figure 3.[2] Acetate species are not observed; as shownbelow this is due to a partial reduction of the CeO2

surface.[3] Bands due to adsorbed acetaldehyde species inh1(O) configuration are seen. Adsorption ofethanol on H2-reduced CeO2 has also been studiedand Figure 10 in (23) shows the absence of acetatespecies due to depletion of surface oxygen.

Extent of CeO2 Reduction Due to Presenceof Noble Metal and CO2/CO Values

The extent of the reduction of CeO2 is not con-stant. It depends on the amount of metal presentand also on its nature. One can obtain informationfrom XPS analyses of the O(1s) to Ce(3d) ratios ofthe �as prepared� metal containing catalysts. Acomplementary way of seeing the effect of thereduction is to analyse the CO2 : CO ratio duringethanol temperature programmed desorption(TPD). This is because any CO2 formed during theethanol decomposition would contain one oxygenatom from the lattice.

Figure 4 shows a plot of CO2/CO as a functionof the extent of reduction due to the presence ofthe metal(s). The extent of the reduction is definedas: DO/at.% M; where DO = 2 � x ; 2 is takenbecause the ratio O to Ce is ideally = 2 (althoughusually slightly more than 2 on the surface), seeTable II for more details. Thus, it is a gauge of anydeviation from stoichiometry normalised to theamount of metal. In all metals x is positive

Fig. 3 Schematic representation of the modes of ethoxide species on CeO2 and in the presence of Pt ions. The presence of stepped surfaces of CeO2 (such as the 310) makes a bidentate configuration of ethoxide species possible


(decreasing DO) with the exception of Au. Thereare two main features that can be seen in Figure 4:· First, there is a linear relationship betweenCO2/CO and the extent of reduction of CeO2 fora series containing Rh, Pt, and Pd, as well as for Pt-Pd and Pt-Rh.· Second, there is a deviation by Au-containingcatalysts from this linearity. Thus, based on these�stoichiometric reaction� results, if one wants todecompose ethanol into CO2 with high selectivityAu is a far better choice.

CO Adsorption at 90 K, a Molecular ProbeCO adsorption at low temperatures has often

been used as a way of characterising specificadsorption sites on metals and metal oxides. It isimportant to emphasise that the metals being dis-cussed here, are actually in an oxidised form � asindicated before. Figure 5 shows the irreversibleadsorption of CO at 90 K on CeO2 alone and

Table II

A: Effect of Metal Addition on the Reduction of CeO2, as Determined from the Surface (plus nearsurface) O to Ce Atomic Ratios

Catalyst at.% M O(1s)/Ce(3d) = x DO = 2 – x DO/at.% M

Pd/CeO2 0.25 1.76 0.24 0.96Pt/CeO2 0.26 1.59 0.41 1.58Rh/CeO2 0.17 1.91 0.09 0.53Au/CeO2 0.19 2.32 –0.32 –1.68Pt-Pd/CeO2 Pt = 0.38; Pd = 0.45 1.51 0.49 0.59Au-Rh/CeO2 Au = 0.27; Rh = 0.57 1.49 0.51 0.61Pt-Rh/CeO2 Pt = 0.21; Rh = 0.21 1.66 0.34 0.81

B: State of Metal Ions on CeO2 in the ‘As Prepared’ Catalysts

Catalyst Binding Energy, eV, XPS Assignment

Pd/CeO2 Pd(3d5/2) = 336.5 PdOPt/CeO2 Pt(4f7/2) = 74.0 PtORh/CeO2 Rh(3d5/2) = 309.5 RhO2

Au/CeO2 Au(4f7/2) = 85.6 Au2O3

Pt-Pd/CeO2 Pd(3d5/2) = 337.5;Pt(4f7/2) = 73.0

Au-Rh/CeO2 Rh(3d5/2) = 308.7;Au(4f7/2) = 85.5

Pt-Rh/CeO2 Rh(3d5/2) = 309.5;Pt(4f7/2) = 73.0

Fig. 4 Plot of CO2 to CO peak area ratios computedfrom TPD of ethanol, and the extent of surface reduction(defined in the text). Au/CeO2 and Rh-Au/CeO2 catalystsdeviate from the linearity observed for the other catalysts.DO = 2 � x; x = O(1s)/Ce(3d)


when impregnated with metals, and the followingpoints should be noted:[a] The presence of both Ce4+ and Ce3+ on thesurface of CeO2 are clearly seen by CO adsorptionas the probe. CO is adsorbed via an electrostaticinteraction that disappears by 150 K.[b] In the presence of Rh ions, bands character-istic of linear CO are seen (note the relativestability of these bands compared to those of COon CeO2 alone).

[c] The presence of both Rh and Au ionstogether appear masking the CO electrostaticadsorption on Ce4+ ions.

Oxidation/Reduction ReactionsIn a system where oxidation reactions occur

(such as ethanol to CO2) the state of the metal mayoscillate between oxidation and reduction. It isthus worth studying the effect of prior reductionof the metal on the outcome of the ethanol

Fig. 5 Irreversible CO adsorption after saturation at 90 K over CeO2, Au/CeO2, Rh/CeO2 and Rh-Au/CeO2. The catalysts were heated at the indicated temperatures after CO adsorption at 90 K, then quenched back to 90 K for further data collection


decomposition. There is a clear shift in the reactionselectivity as shown by representative data below.The �as prepared� Pd/CeO2, Pt/CeO2, Au/CeO2

and Rh/CeO2 show the formation of acetaldehyde.Acetaldehyde made by dehydrogenation (in thepresence as well as the absence of oxygen) ofethanol has been seen by IR spectroscopy (as anadsorbed species), during TPD and in steady state

catalytic conditions. The main point is that priorreduction shifts the Rh/CeO2 catalysts fromCH3CHO directly to CO formation (23).

In the absence of a good hydrogenating catalystmost of the CO desorbs. The addition of a secondmetal (Pt or Pd) is dramatic. Figure 6 shows theselectivity shifts from acetaldehyde, on the unre-duced bimetallic Rh-containing catalysts, tomethane on the reduced catalysts. In the absenceof Rh, acetaldehyde is only marginally affected bythe reduction process. This can be interpreted asfollows: Rh is an efficient catalyst for carbon-car-bon bond dissociation and this is shown by thepresence of adsorbed CO, even at low tempera-ture, upon dosing the rhodium surface withethanol. In the presence of a good hydrogenationcatalyst (such as Pd) and upon heating CO is con-verted to methane.

Comparing TPD to Catalytic ReactionsGood similarity between TPD and catalytic

reactions can be seen in Figure 7. The turnovernumber (TON) determined from the rate ofethanol reaction in steady state catalytic reactionstracks the desorption temperature of acetaldehyde(during ethanol-TPD) for Rh/CeO2, Pd/CeO2 andPt/CeO2.

Acetaldehyde is weakly adsorbed on the surfaceand its desorption is reaction limited (as soon as

Fig. 6 Switch in reaction selectivitybetween reduced andnon-reduced M/CeO2

catalysts prior toethanol oxidation. The presence of Rhshifts the reactionproducts fromacetaldehyde tomethane when the catalyst has beenreduced with H2

Fig. 7 Correlation between TPD peak desorption temperature and the TON for ethanol oxidation. TheTON is defined as the number of molecules of ethanolconverted per surface M atom (as computed from thecorrected XPS signal) per second. The higher the TONthe lower is the desorption temperature of acetaldehyde


the H of the CH2 is abstracted from the ethoxidespecies acetaldehyde is formed and desorbs). Thismeans that the desorption temperature duringTPD contains in large part the activation of the C-H bond of the CH2 of ethoxides. The higher thedesorption temperature of acetaldehyde duringTPD the lower is the TON and vice versa.

Hydrogen ProductionFrom the above studies it appears that bimetal-

lic catalysts containing Rh are better suited forethanol decomposition. We have conducteddetailed studies of Pt-Rh/CeO2 and Rh-Au/CeO2

by IR spectroscopy and TPD and in steady statereaction conditions. Both catalysts are very activefor ethanol decomposition. Table III (A to C)summarises most of the steady state reaction data.The reactions ethanol undergoes first proceed viaa dehydrogenation of ethanol to acetaldehyde. Thefaster acetaldehyde is decomposed, the more effi-cient is the total decomposition (as in a typicalconsecutive reaction). The addition of Rh dramat-ically destabilises acetaldehyde and this is probablydue not only to further conversion of acetaldehydeto CO and CH4 but also to a specific direct inter-action between Rh and ethoxide species (as an

Table III

A: Product Distribution (in mol%) during Ethanol Reaction [Ethanol]/[O2] = 2/3; [Catalyst] = 50 mg;F = 200 ml min–1 over Au/CeO2 at the Indicated Temperatures

473 K 573 K 673 K 773 K 873 K 973 K 1073 K

CH3CHO 56.44 25.52 9.88 6.46 5.01 0.1 0.07CH4 7.53 5.67 18.50 7.26 8.81 24.77 24.65CH3C(O)CH3 – 3.18 6.69 36.98 26.82 0.75 –CO – 8.25 10.23 16.80 22.02 31.12 31.34CO2 31.91 55.54 52.32 27.85 32.52 34.14 35.01CO/CO2 0 0.15 0.20 0.60 0.68 0.91 0.90H2 4.12 1.86 2.37 4.65 4.81 9.11 8.93

B: Product Distribution (in mol%) during Ethanol Reaction [Ethanol]/[O2] = 2/3; [Catalyst] = 50 mg;F = 200 ml min–1 over Rh-Au/CeO2 at the Indicated Temperatures

473 K 573 K 673 K 773 K 873 K 973 K 1073 K

CH3CHO 66.60 0.74 0.04 0.03 0.04 0.03 0.01CH4 17.99 19.04 16.95 14.39 11.24 10.97 12.63CH3C(O)CH3 – 1.49 0.12 0.23 – – –CO – 24.34 13.98 15.65 19.70 21.62 23.58CO2 – 49.44 56.16 54.62 53.68 52.75 49.00CO/CO2 0.49 0.25 0.29 0.37 0.41 0.48H2 15.42 4.95 12.74 15.09 15.34 14.63 14.77

C: Product Distribution (in mol%) during Ethanol Reaction [Ethanol]/[O2] = 2/3; [Catalyst] = 50 mg;F = 200 ml min–1 over Pt-Rh/CeO2 at the Indicated Temperatures

473 K 573 K 673 K 773 K 873 K 973 K 1073 K

CH3CHO 48.72 0.97 2.72 0.66 – – –CH4 10.84 17.71 17.10 15.78 11.98 9.07 9.53CH3C(O)CH3 – – 1.18 2.28 – – –CO – 15.25 13.33 12.12 12.14 13.06 16.26CO2 32.62 61.67 58.31 57.46 58.82 60.26 59.86CO/CO2 0.25 0.23 0.21 0.21 0.22 0.27 0.25H2 7.82 4.39 7.36 11.69 17.07 17.61 14.35

oxametallacycle) (10, 11). The decomposition ofthe latter would also give CO and CH4. Most COis oxidised to CO2 at low temperatures. Acetoneresults as a secondary reaction of acetaldehyde (viaacetate species), which explains its disappearancein the presence of Rh. At low temperaturesAu/CeO2 showed the lowest CO to CO2 ratios butthe effect of Au seems to disappear at higher tem-peratures. Figure 8 shows H2 production as afunction of reaction temperature in steady stateconditions. Although Rh-Au/CeO2 and Pt-Rh/CeO2 are of comparable activity towards H2

formation at low temperatures, the latter catalyst is

far better above 700 K.Scheme IV has been constructed from spectro-

scopic and kinetic data. The addition of Rh metallargely suppressing the dehydrogenation route ismost likely due to the formation of a bicoordinat-ed species (via the O and the C ends: oxametalla-cycle). This five-member oxametallacycle specieshas not been seen on these catalysts but has beencomputed (and proposed) by other workers as themost probable species resulting from iodo-ethanoland ethylene oxide molecules on Ag and Rh sur-faces (10, 24, 25, 26). Further decomposition ofthis species will yield CO and CH4 and/or CHx

depending on the nature of the metal. The hightemperature production of H2 is most likely associ-ated with the oxidative dehydrogenation as well asreforming of CH4 and CO. There is a great dealstill to be learned in this generalised reactionscheme and particularly the role of these metalsboth in reforming and in the water gas shiftreaction (see for example (27�36)).

ConclusionsThe complexity of the ethanol reactions on the

surfaces of noble metals/cerium oxide catalysts isoutlined. Hydrogen production from ethanol isdirectly related to two main steps. The firstinvolves breaking the carbon-carbon bond, and Rhappears the most suitable compound for this reac-tion at reasonable operating temperatures. Thesecond involves CO oxidation to CO2. While fine-ly dispersed Au is a very active catalyst for COoxidation during TPD, the high temperaturerequirement for good hydrogen yield in steady stateconditions may favour other metals, such as Pt.


Fig. 8 H2 production during the catalytic reaction ofethanol on Au/CeO2, Rh-Au/CeO2 and Pt-Rh/CeO2.[Ethanol]/[O2] = 2/3; [Catalyst] = 50 mg; F(ethanol flow) = 200 ml min�1; [Ethanol]o = 2.5 ´ 10 �6 mol ml �1 for Au/CeO2 and 3.2 ´ 10�6 mol ml�1 for both Rh-Au/CeO2 and Pt-Rh/CeO2.All measurements taken under steady state conditions

Scheme IVThe difference in the reaction pathways between Rh and Pd and Pt catalysts;(a) for adsorption; [O] for surface oxygen and M for a metal or a Ce ion site


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References

The AuthorHicham Idriss is an Associate Professor in the Department of Chemistry at the University of Auckland,New Zealand. His research interests include the fundamental and applied study of surface-gasinteractions on oxide and metal/oxide surfaces; the design, synthesis and study of new oxide materials ofrelevance to environmental, petrochemical and electronic applications. Current work involves the totaldecomposition of ethanol to hydrogen with high yield and low CO production. This includes fundamentalstudies of Au and Rh (in their metallic and oxidised states) on titania single crystals.He began his career as a lecturer in the School of Chemistry, University of Strasbourg, France, followedby postdoctoral/research associate work (Department of Chemical Engineering), University of Delaware,U.S.A. After this he was Group Leader, Catalysis and Technology, SABIC, Saudi Arabia; then a ResearchAssociate (Department of Chemical Engineering), University of Illinois, Urbana, IL, U.S.A.Since 1995 he has been in New Zealand at the University of Auckland.

Over 160 delegates from 15 countries attendedthe IX International Conference on the Chemistryof Selenium and Tellurium (ICCST-9) held at theIndian Institute of Technology Bombay, from23rd to 27th February, 2004. This is a triennialinternational conference, the first one being held inNew York in 1971. ICCST-9 covered variousaspects of selenium (Se) and tellurium (Te) chem-istry (including biochemistry, materials andenvironmental chemistry). There were 12 plenary,19 session and 23 invited lectures, and about 60poster presentations. Only lectures related to theplatinum group metals are reported here.

S. Uemura (Kyoto University, Japan) delivereda plenary lecture entitled �Retrospect of my Se andTe chemistry�. Among many other reactions of Seand Te compounds, he described oxidative addi-tion of (MeY)2 (Y = S, Se, Te) to [Cp*Ru(m3-Cl)]4

affording organochalcogenolato bridged dirutheni-um complexes, [Cp*RuCl(m-YMe)]2. The use ofthe latter molecules as catalyst for propargylic sub-stitution reactions has been examined.

Versatile synthetic routes to Pd(II) and Pt(II)diselenolene complexes containing tertiary phos-phines and Pt selenaketocarbene derivatives weredescribed by C. P. Morley (University of WalesSwansea, U.K.). Significant differences between Pdand Pt chemistry have been reported. The newlysynthesised complexes were characterised by X-raycrystallography, NMR and mass spectrometry.

The synthesis of several hybrid telluroethersand telluroether containing Schiff bases was pre-sented by A. K. Singh (Indian Institute ofTechnology Delhi, New Delhi, India). These mol-ecules were shown to be versatile ligands. Theircoordination chemistry with Pd, Pt and Ru wasdescribed.

V. K. Jain (Bhabha Atomic Research Centre) (1)presented the chemistry of Pd and Pt complexesderived from N,N-dimethylaminoalkyl chalco-genolates, a family of ligands designed and devel-oped at BARC. Complexes with diverse nuclearity(mono-, bi-, tri- and hexa-) were described. The

nuclearity is greatly influenced by the nature of thechalcogen atom (S, Se, Te) and by the number ofcarbon atoms separating the nitrogen and chalco-gen centres. These complexes have beencharacterised by IR, NMR (1H, 13C, 31P, 77Se, 125Te,195Pt) and UV-vis spectroscopies, FAB mass spec-trometry and X-ray crystallography. A weak abs-orption in the UV-vis spectra of [MCl(ECH2CH2-NMe2)(PR3)] (E = S, Se, Te; M = Pd or Pt) hasbeen attributed to metal mediated ligand-to-ligandcharge transfer. This absorption is red-shifted onmoving from S ® Se ® Te. The use of some ofthese complexes as molecular precursors for thesynthesis of Pd chalcogenides has been demon-strated.

Five of the poster presentations showed thechemistry of Pd, Pt and Ru complexes containingSe and Te ligands.

In conclusion the ICCST-9 provided a mediumfor groups to exchange views on current andemerging themes in Se and Te chemistry. Theinternational advisory board of ICCST has pro-posed broadening the scope of the conference toalso cover the chemistry of S and Po. A specialissue of Phosphorus, Sulfur, and Silicon and the RelatedElements will contain the presentations at ICCST-9.

The tenth conference in the series is planned totake place in June 2007 in Lodz, Poland, under thechairmanship of Professor Marian Mikolajczyk,E-mail: [email protected]

V. K. JAIN AND H. B. SINGH

Reference1 S. Dey and V. K. Jain, Platinum Metals Rev., 2004, 48,

(1), 16

The AuthorsVimal K. Jain is Head, Synthesis & Pure Materials Section, NovelMaterials & Structural Chemistry Division, Bhabha AtomicResearch Centre, Mumbai 400 085, India. His interests lie in thedesign and development of molecular precursors for advancedinorganic materials. He was co-chairman of ICCST-9.E-mail: [email protected]

Harkesh B. Singh is a Professor of Chemistry, at the Indian Instituteof Technology Bombay, Powai, Mumbai 400 076. His interests arein selenium and tellurium chemistry. He was chairman of ICCST-9.

The Chemistry of Selenium and TelluriumPLATINUM METALS ASPECTS OF THE IX INTERNATIONAL CONFERENCE HELD IN MUMBAI, INDIA

DOI: 10.1595/147106704X1595

116Platinum Metals Rev., 2004, 48, (3), 116


Piezochromic phenomena � colour changes insolid specimens or solution samples induced byexternal pressures � are explained by pressure per-turbation to the HOMO and/or LUMO (highestoccupied molecular orbital and/or lowest unoccu-pied molecular orbital) energy levels of the relatedelectronic transition. The piezochromism of solidinorganic and organic materials has been investi-gated by examining phase transition phenomena.Specific electronic properties of the solids,acquired by tuning the external pressure, may beused as electronic devices and as pressure sensors.

Changes in the absorption and emission spectraof metal complexes in solution are related tochanges in solvent polarity at each pressure: a grad-ual increase of the dielectric constant of thesolvent with pressure affects the energies of theHOMO and LUMO levels involved in the elec-tronic transitions within the metal complexes, anda corresponding colour change may be observed.However, such a pressure perturbation to thedielectric constant of solvents is usually small (1)and the piezochromic effect of samples in solutionis rather ambiguous � partly because of the narrowrange of applied pressures (< 5000 bar).

It is known that the compressibility of solids is

much smaller (< 0.001%) than that of liquids. Thissmall compressibility is explained by the difficultyof intermolecular and/or interionic compressionin the crystals that comprise the solid and by thedifficulty of compression along the bond axis inthe molecules or complex ions. For example, later-al compression between the chains takes place foralkylsilicon and alkylgermanium polymers at rela-tively low pressures (ca. 10,000 bar), followed bycompression along the Si�Si and Ge�Ge axes athigher pressures (> 20,000 bar) (2�7). Moreover,deformation of compounds by the external pres-sure does not take place in a free way: there is aquantum mechanical restriction � �symmetryrules� (8, 9) � that governs the direction of defor-mation. Specific interactions, such as hydrogenbonding and ion-pair interactions, also perturb thestructures at elevated pressures (10). In addition toknowing the �symmetry rules,� it is essential tocomprehend the �theories of electronic transi-tions� and �molecular symmetry and vibrations�for a proper understanding of piezochromiceffects (11).

In this short review the effects of pressure per-turbations on the absorption and emission spectrathat are exhibited by solid palladium complexes are

Piezochromism and Related PhenomenaExhibited by Palladium ComplexesBy Hideo D. Takagi*, Kyoko Noda and Sumitaka ItohResearch Center for Materials Science, Nagoya University, Furocho, Chikusa, Nagoya 464-8602, Japan

*E-mail: [email protected]

and Satoshi IwatsukiDepartment of Chemistry, Waseda University, Okubo, Shinjuku, Tokyo 169-8555, Japan

Piezochromic phenomena are explained by pressure perturbation to the HOMO and/or LUMO

energy levels of the related electronic transition. The piezochromism of solid inorganic and

organic materials has been investigated by examination of the phase transition phenomena.

Specific electronic properties of the solids, acquired by tuning the external pressure, may be

used as electronic devices and as pressure sensors. The effects of pressure perturbations on

the absorption and emission spectra exhibited by solid palladium complexes are reviewed

here. Related phenomena exhibited by platinum complexes and other metal complexes are

included for comparison.

DOI: 10.1595/147106704X1630

summarised. Related phenomena exhibited byplatinum complexes and other metal complexesare included for comparison.

Square-Planar Pd(II) ComplexesProbably the most classical studies of the

piezochromism of metal complexes concernNi(II), Pd(II) and Pt(II) complexes with dimethyl-glyoximate, L1: dmg�. (Ligands appearing in thisreview and other related ligands are summarised inthe Scheme.) These complexes all have square-pla-nar, D4h, symmetry, and the absorption spectra foreach complex in solution and solid form are verydifferent. The crystal structure of [Ni(dmg)2] wasreported by Godychi and Rundle (12). The planar[Ni(dmg)2] units are stacked one upon anotherwith a rotation of 90º for each alternate layer. TheNi(II) ions line up with an average Ni---Ni distanceof 3.233 Å. The Pd(II) and Pt(II) complexes exhib-it similar structures with the Pd---Pd and Pt---Ptdistances being 3.253 and 3.25 Å, respectively (13,14). As the M(II)---M(II) distances in these com-plexes are quite similar, the factor that governs thedistances is attributed to the organic groups in theligand. The complexes show sharp absorptionbands near 19,000 cm�1 in the solid, but theserather strong absorption bands are not observed insolution. The bands originate from M(II)---M(II)interactions. Furthermore, the absorption bandsare dichroic with the perpendicular componenthaving high intensity.

Zahner and Drickamer examined the pressuredependence of these complexes as solids (15).They used diluted salt pellets for the measure-ments. For all the complexes, the absorption bandnear 19,000 cm�1 shifted towards lower energy withpressure. The observed pressure effects may besummarised as follows:[a] A very large red shift was observed at first(6100 cm�1 and 9800 cm�1 for the Ni(II) and Pd(II)complexes, respectively) at 100 kbar, see Figure 1.[b] The red shift started to level off at ca. 120�150kbar for the Ni(II) and Pd(II) complexes.[c] For the Pt(II) complex, a reversal in shift wasobserved after a red shift of 8700 cm�1 at 63 kbar;the blue shift at higher pressures was very large.[d] The external pressure induced a broadening ofthis absorption band in the order of Ni(II) <Pd(II) < Pt(II). As this band is related to the M(II)---M(II) interactions, the observations wereexplained by a decrease in the M(II)---M(II) dis-tance with pressure, see Figure 2. In the originalD4h complexes d-d transitions are Laporte forbid-den, while the d-p transitions are allowed.However, in complexes with D4h symmetry the


SchemeLigands appearing in this review

Fig. 1 Frequency shift for [Ni(dmg)2], [Pd(dmg)2] and[Pt(dmg)2] vs. pressure. From J. C. Zahner and H. G. Drickamer, J. Chem. Phys., 1960, 33, 1625 Copyright American Institute of Physics (2004)

energy of d-p transitions is very large and onlyweak absorption bands corresponding to the for-bidden d-d transitions are observed in the visibleregion. This situation explains the absorptionspectra in solution.

In the solid, the distance between the M(II)ions is very small. The positive charges located onthe z axis of the planar complex cause an energydecrease in the vacant pz and s orbitals, althoughthe energy levels of the px and py orbitals are littleinfluenced by the adjacent M(II). Therefore, thespecific absorption band observed in the solid isassigned to the allowed (z- and x,y-polarised) d-pz

transitions; the d-s transition is Laporte forbidden.When external pressure is applied to the solid

specimen, the effect of the positive charge on thevacant pz and s orbitals increases: as the distancebetween two M(II) ions decreases, the energy levelof the vacant pz orbital continuously decreasesuntil a point where the vacant pz orbital and the

occupied dz2 orbital on the adjacent M(II) ionsstart to overlap. Further compression of the solidthen leads to an increase of the pz energy level andinduces a blue shift of the absorption band. As theprincipal quantum number increases, it is expectedthat the corresponding s, p and d orbitals gradual-ly expand. However, such a tendency does notapply to the 6s orbital of the third-row transitionmetals.

The larger relativistic effect for the third-rowtransition elements causes a contraction of the 6sorbital and the 5d and 6p orbitals become relative-ly higher in energy than is expected for theseelements. Therefore, it seems that for M = Pt, theoverlap of the vacant 6pz orbital and the occupied5dz2 orbital on adjacent M(II) ions takes place at arather low pressure. The pressure broadening ofthe absorption bands was explained by delocalisa-tion of the electron density from A1g and Eg in theone-dimensional metallic lattice. A similar pressure


Fig. 2 Energies of the nd, (n + 1)s and (n + 1)p orbitals on [M II(dmg)2] complexes. Allowed transitions from Eg toEu, from A1g to Eu, and from B1g to Eu are z-; x,y-; and x,y-polarised, respectively. The d-d transitions as well as the d-s transition are Laporte forbidden. The turnover point was observed at lower pressure for Pt(II), mainly because ofthe relativistic effect described in the text

dependence of the absorption spectra wasobserved for the Ni(II) and Pd(II) complexes with1,2-cyclohexanedione dioximate (L2: niox� ).

A series of studies, including measurements ofthe electrical conductivity at very high pressures(up to 14 GPa), was carried out for the complexdimethylglyoximatoplatinum(II) (16, 17). PowderX-ray diffraction at elevated pressures revealedthat the Pt---Pt distance continuously decreased byca. 17% up to 14 GPa. A levelling-off in resistivitywith pressure was observed at ca. 6.5 GPa. Thiswas attributed to the phase transition from theone-dimensional �metal� to the �semiconductor.�Most interesting results were reported for the shiftof the UV-vis bands at very high pressures: theMLCT (metal to ligand charge transfer ) band thatappeared at the 320 nm region at ambient pressurewas observed at ca. 500 nm at 9 GPa. It seems thatthe large red shift of the MLCT band is related tothe �metal� to �semiconductor� phase transition.

An extension of these studies appeared in thisJournal in 1987 (18). The dz2(a1g) ® pz(a2u) absorp-tion in the PdII(niox)2 complex exhibited a largepressure dependence. Accordingly, the colour ofthis complex changed from yellow orange (0.8�1.4GPa) to red (1.4�2.3), to purple (2�3), to blue(2.7�5.5), to yellow green (5.5�6.5), and to pale yel-low (above 7 GPa), see Figure 3 (18).

It has been suggested that the use of thispiezochromism would enable us to monitor pres-sure in situ, although further investigationsconcerning the relationship between the thickness

of the specimen and the observedcolours under each pressure may have tobe carried out.

Mixed-Valent Pd(II)/Pd(IV)Complexes

Pressure effects on the conductivityof crystals and powders of various plat-inum compounds have been investigatedin relation to the electronic interactionsbetween two metal sites through bridg-ing ligands. Pt and Pd complexes with d8

electronic configuration normally prefersquare-planar coordination geometry,while metal ions with d7 and d6 electron-

ic configuration prefer octahedral 6-coordination.Although early studies of 1-electron oxidised M(II)complexes, MAX3 (M = Pt or Pd; A = (NH3)2; X= Cl�, Br�, or I� ) suggested the existence of M(III)species (19), later structural analyses revealed thatthese complexes are mixtures of square-planarM(II) and octahedral M(IV) complexes withchains: [---M(II)�X�M(IV)�X---M(II)---] in thecrystals (20, 21). These complexes are diamagneticand have been classified as �Class II compounds�,according to the criteria of Robin and Day (22).The observed absorption bands for these com-pounds are therefore superpositions of theindividual absorption bands for M(II) and M(IV)complexes with a M(II) to M(IV) inter-valencetransfer (IT) band in the visible region. The inten-sity of the latter IT bands is high and thesecompounds are highly coloured.

Interrante and colleagues investigated the possi-bility of a spectral shift and a change in cond-uctivity upon compression of the crystals along the ---M(II)---X�M(IV)�X--- axis for variousMAX3 complexes:� ([MII(NH3)2X2][MIV(NH3)2X4] for Pd and Ptwith X = Cl�, Br�;� [MII(en)X2][MIV(en)X4] for M = Pt, X = Cl�

and Br� (en = ethylenediamine); and� [MII(C2H5NH2)4][MIV(C2H5NH2)4X2]X4�4H2Ofor M = Pt, X = Cl� and Br� ) (23). No irreversiblephase change with pressure was observed, exceptin the case of [Pt(C2H5NH2)4]Br3�2H2O. The X-raypowder diffraction pattern for each complex was


Fig. 3 The absorption spectra of [Pd(niox)2] at various pressures (18)

examined at ambient pressure and at ca. 60 kbar.For Pd(NH3)2Cl3, isotropic compression in all

three directions of the orthorhombic unit cell wasobserved, including compression along the intra-chain Pd(II)---Cl�Pd(IV) distance. Observation ofthe absorption spectra revealed that the IT bandwas very strong for most of the complexes, evenat ambient pressure, and measurement becameimpossible at elevated pressures.

An exceptional result was obtained for com-plex Pt(C2H5NH2)4Cl3�2H2O where an absorptionchange with pressure in the UV-vis region wasobserved. The absorption maximum at 16,000cm�1 at ambient pressure for a single crystal (con-sistent with the previously reported absorptionband corresponding to the M(II) to M(IV) inter-valence transition) shifted towards longerwavelengths at ca. 30 kbar. This shift in the ITband indicates that the compression along theM(II)---X�M(IV)�X--- axis is due to the appliedpressure.

The conductivities of these complexes at ambi-ent temperature reversibly increased with pressure.The plot of the logarithmic value of the conduc-tivity (ohm�1 cm�1) against the reciprocaltemperature at each pressure was linear. The acti-vation barrier for the change in conductance wasplotted against pressure, see Figure 4. The activa-tion barrier was initially found to decrease withpressure up to ca. 106 kbar and then to begin toincrease with pressure. However, no phase transi-tion to the metallic state was observed, even at 140kbar. The inhibition of this phase transition maybe caused by the increase in the activation barrierat pressures higher than 106 kbar. Therefore, itwas not possible to achieve the Class III state(where a complete electronic delocalisation occurs,according to the Robin and Day definition (22))for this complex even at very high pressures. ThePd(II)---Cl and Pd(IV)�Cl distances have beenreported as 322 pm and 199 pm, respectively, atambient pressure. As compression of thePd(IV)�Cl distance is difficult, compression of thePd(II)---Pd(IV) distance by ca. 35 pm at 60 kbarmay be attributed to the decrease in the Pd(II)---Cldistance. However, such a change in the Pd(II)---Cl distance is still too small for the transition to the

Class III state, where the Cl atom is expected to sitbetween a Pd(II) and a Pd(IV) ion.

A more recent analysis of the complex[Pd(chxn)2][Pd(chxn)2Br2]Br4 (L3: chxn = 1R,2R-cyclohexanediamine) indicates that Pd(II) andPd(IV) in the one-dimensional chain are energeti-cally close and produce 10�3 of paramagneticPd(III) (24). Moreover, this mixed-valence palladi-um complex, which is dark brown at ambientpressure, turns light green upon exposure to Br2:an uptake of two Br2 molecules to each[Pd(chxn)2][Pd(chxn)2Br2] (25). This processinduced the oxidation of Pd(III) to Pd(IV).

Electronic conduction in solid samples general-ly takes place through: [1] the direct interaction of the dz2 orbitals viabridged ligands as seen in MACl3 and/or through[2] the interaction of p orbitals as found in tetra-benzoporphyrin (26).

In the former case, the application of externalpressures may strengthen the interactions betweendz2 orbitals by axial compression: the phase transition to achieve the ultimate electronic delo-calisation may be expected to occur. In the lattercase, the ultimate state/condition for this type ofinteraction is as �organic metal�, such as the


Fig. 4 DE as a function of pressure for [Pd(NH3)2Br3].From L. V. Interrante, K. W. Browall and F. P. Bundy,Inorg. Chem., 1974, 13, 1158Copyright American Chemical Society (2004)

TCNQ-TTF (tetracyanoquinodimethane-tetrathia-fulvalene).

A mixed-valence palladium complex with aconjugated planar macrocyclic ligand, tetrabenzo-[b,f,j,n]-1,5,9,13-tetraazacyclohexadecine (L4: TAAB),was examined at elevated pressures (27). Themixed-valence complex was prepared by the partialoxidation of the corresponding Pd(II) complex byI2. Powder and crystalline samples of the resulting[Pd(TAAB)]2.7+(I3

� )2.7 was examined by IR andRaman spectroscopic methods. It was revealedthat the I3

� anion exists as a linear unit in the crys-tal, based on the selection rules. Although X-rayanalysis was not carried out for this mixed-valencecomplex, it was concluded that the I3

� ion is locat-ed in space with little perturbation from theneighbouring group. Therefore, this mixed-valencecomplex does not have a one-dimensional chainstructure. The mixed-valence complex is dark redand this colour may relate to the ML/LMCT band.The conductivity of the complex increased from10�8 ohm�1 cm�1 at ambient pressure to 10�5 ohm�1

cm�1 at 10 kbar. The conductivity of solid[PdII(TAAB)]2+ was 10�8 ohm�1 cm�1 at ambientpressure and was independent of pressure.Moreover, the spectra of the mixed-valence com-plex hardly changed with pressure. Therefore, thesmall pressure-dependent conductivity of thismixed-valence complex was attributed to the crys-tal packing effect.

Pressure Effects on Emission Spectra,Emission Intensities and Lifetimesof Excited Pd(II) Complexes

One of the most intensively studied Pd(II)complexes in the last decade is [PdIIL4]2+ (L =SCN� or SeCN� ). The XCN� ligands are ambiden-tate and may coordinate through X or N,depending on the polarisability of the central metalion. Crystal structures of the [Pd(XCN)4]2� frag-ments have D4h symmetry; the average Pd�S andPd�Se distances are 2.33 and 2.44 Å, respectively,and the M�X�C angles were 109º and 105�107ºfor the tetra-n-butylammonium salt of the SCN�

and SeCN� complexes, respectively (28).Rhode and coworkers (28) also reported normal

coordinate analyses for these complexes. The force

constants for Pd�S and Pd�Se were ca. 1.17 mdynÅ�1, which is significantly smaller than those forthe corresponding Pt(II) complexes: Pt�S is 1.44mdyn Å�1 and Pt�Se is 1.42 mdyn Å�1. The a1g

totally symmetric vibrations of the Pd�S andPd�Se bonds were 274�303 cm�1 and 180�195cm�1, respectively, while b1g non-totally symmetricstretching and b2g non-totally symmetric bendingmodes were observed at 260�290 and 170�187cm�1 for Pd�SCN, and at 140�150 and 100�110cm�1 for Pd�SeCN complexes, respectively. Theground-state electronic configuration for thesecomplexes is: a1g(dz2)2b2g(dxy)2eg(dxz,dyz)4b1g(dx2 �y2)0

(29) and one-electron excitation from the eg orbitalto the s antibonding b1g orbital creates degenerate3Eg and 1Eg excited states. The electron occupancyof the s antibonding b1g orbital induces totallysymmetric (a1g) elongation of all Pd�X bonds,while the doubly degenerate electronic excitedstates cause Jahn-Teller distortion � that is, elonga-tion/contraction along one diagonal X�Pd�X axistakes place.

The luminescence bands for these complexesare broad, indicating that the potential energy min-imum of the lowest excited 3Eg state is coupledstrongly with the Pd�X vibration modes. At ambi-ent temperature, the emission centred at ca. 12,500cm�1 is almost unobservable, however the intensi-ties as well as the emission lifetime increase withdecreasing temperature. Therefore, effective par-ticipation of the non-radiative decay mechanismexists at relatively high temperatures. Furthermore,the low temperature emission spectra of thesecomplexes exhibit well-resolved vibronic struc-tures (< 50 K).

Reber and coworkers examined the pressureeffects on the d-d luminescence spectra of thesePd(II)�XCN and Pt(II)�XCN complexes (X = Sand Se) up to 40 kbar (30). The intensity of theluminescence increased with pressure for all thecomplexes. The peak position steadily blue-shiftedwith pressure: 24, 12, 29 and 25 cm�1 kbar�1 forPt�SCN, Pd�SCN, Pt�SeCN and Pd�SeCN,respectively. However, such a blue-shift may notbe explained by the axial metal---metal interactionsthat [Pt(CN)4]2� exhibits. Indeed, the metal---metaldistance in the crystals of these complexes (ca.


13 Å) are much longer than in the tetracyanoplati-nate complex (ca. 3.1�3.7 Å) (28).

In [Pd(XCN)4]2� and [Pt(XCN)4]2� the pressuredependence of the luminescence intensities andlifetimes was attributed to the loss of the inversioncentres caused by the applied pressure: the proba-bility of the d-d transition increases with thedecreasing centrosymmetry of the complex (11). Itwas shown that the participation of the slow b2g

bending motion coupled with the intermolecularforces induced by the external pressure also con-tributes to the enhanced intensities of the emissionspectra. Such a distortion of the excited stateexplains the spectral shifts: the bending motioninherent in these complexes may reduce the pp-dpinteractions between the HOMO (eg orbitals) andligand np orbital and cause a blue shift of the lumi-nescence bands with pressure. This distortion alsoenhances the luminescence lifetimes by contribut-ing to the loss of centrosymmetry in thechromophore. No enhancement of the lumines-cence intensity was observed for K 2[PtBr4] inwhich pressure-induced coupling of the b2g bend-ing mode is not expected.

The luminescence lifetimes of the [Pd(XCN)4]2�

complexes were significantly enhanced with pres-sure: 62 ms and 48 ms at ca. 30 kbar, and 4 ms and541 ns at 7 kbar, for X = Se and S, respectively, seeFigure 5. By contrast, the lifetimes for the corre-

sponding Pt complexes were not enhanced much:6 ms and 13 ms at ca. 30 kbar, and 2 ms and 750 nsat 5 kbar, for X = Se and S, respectively. The radi-ation lifetime is expected to decrease when the d-dtransition becomes more allowed by the loss ofcentrosymmetry with pressure. Therefore, theenhanced luminescence lifetimes observed forthese complexes indicate the significant decreasein the rate of the non-radiative process with pres-sure. The validity of this was verified by the use ofEnglman-Jortner�s radiationless decay theory (31).

ConclusionsIn this short review topics concerning

piezochromism and related phenomena in solidpalladium complexes have been summarised. Thedata may help towards finding use for these metalcomplexes, such as for in situ pressure sensors oras conductors in extreme environments.

AcknowledgementThe authors wish to pay their respects to the late Professor

Harry G. Drickamer, the pioneer of this research field.

References1 N. S. Isaacs, �Liquid Phase High Pressure

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The Authors

Hideo D. Takagi (right) has been an Associate Professor of Inorganic Chemistry at the ResearchCenter for Materials Science, Nagoya University since 2002. His research areas are: synthetic andphysical inorganic chemistry, structure/reactivity relations of metal complexes, and inorganic reactionsin solution at elevated pressures. He graduated from Tokyo Institute of Technology in 1978 andreceived his Ph.D. (Nuclear Engineering) from there in 1983. From 1981 to 1982 he was an ExchangeStudent at the Department of Chemistry, University of East Anglia, U.K., under the supervision ofDr R. D. Cannon. From 1983 to 1985 he was an Instructor at the College of Industrial Technology,Nihon University, and from 1985 to 1992 he was a Post Doctoral Fellow and Research Associate inthe Department of Chemistry at the University of Calgary in Canada with Prof. T. W. Swaddle. From1992 to 2002 he was Associate Professor of Chemistry, Department of Chemistry, Nagoya University.

Satoshi Iwatsuki is a Post Doctoral Fellow in the Department of Chemistry, Waseda University, (21COE “PracticalNano-Chemistry” from MEXT). His research involves the catalytic reactions of binuclear Pt(III) complexes atelevated pressures. He graduated from the Department of Chemistry, Nagoya University (1997) and obtained hisPh.D. in chemistry in 2003 from the Graduate School of Science, Nagoya University.

Kyoko Noda is a Ph.D. student in theGraduate School of Science, NagoyaUniversity. Her research includessyntheses, structural analyses andreactions of metal complexes withnovel hybrid ligands havingphosphorus as the donating atoms.

Sumitaka Itoh is a Ph.D. student inthe Graduate School of Science,Nagoya University. His researchinvolves gated electron transferreactions at elevated pressures.


Most of the primary platinum group metals(pgms) comes from low-sulfide platinum ores(South Africa and the U.S.A) and from sulfide cop-per-nickel ores (Russian and Canadian deposits).Recent exploration in Russia and elsewhere hasresulted in the discovery of some new deposits ofplatinum-bearing low-sulfide ores. Many existingtechnologies for treating pgm ore are based onmechanical beneficiation, high-temperature smelt-ing and converting operations, and hydro-metallurgical processing. Therefore, the develop-ment of complex hydrometallurgical technology torecover pgms and non-ferrous metals from low-sulfide pgm-bearing ores has considerablysimplified the treatment, decreased the operatingcosts and improved the environmental conditions.

Our investigations involved flotation concen-trates of low-sulfide platinum-bearing chrome oresfrom South Africa. The ore had the compositionsshown in the Table. An X-ray microscopy study ofthe phase composition of the flotation concentrate

showed that nickel is present as pentlandite andcopper as chalcopyrite. Iron is present in theseminerals and is also present as pyrrhotite and inrock-forming minerals (pyroxene, spinel and talc).Platinum group metals are found as their own sul-fides and in mixed sulfide minerals, which areeither individual, or associated with non-ferrousmetals sulfide minerals and pyrrhotite, see Figure1. Minor amounts of gold and ferroplatinum inmetallic forms have also been discovered.

Based on results of the investigation, the pre-ferred treatment of the flotation concentrate is thatshown in the flowsheet, see Figure 2.

Autoclave oxidative leaching (AOL) at a tem-perature of 150ºC and O2 partial pressure of 1 MPaenables non-ferrous metals to pass into solutionselectively. This does not involve precious metalminerals as they are resistant to inorganic acids (1).The effect of sulfuric acid consumption and theprocess time on the AOL performance were stud-ied. The AOL values were found to be dependent

Treatment of Platinum Flotation ProductsBy A. V. Tatarnikov* and I. SokolskayaAll-Russian R & D Institute of Chemical Technology, 33 Kashirskoye Road, 115409 Moscow, Russia*E-mail: [email protected]

and Ya. M. Shneerson*, A. Yu. Lapin and P. M. GoncharovGipronickel Institute JS, 11 Grazhdansky Prospekt, 195220 St. Petersburg, Russia*E-mail: [email protected]

A flowsheet has been developed for the production of rich concentrates of precious and non-

ferrous metals by a complex treatment of the flotation products from South African platinum-

containing chrome ores. The procedure involves: autoclave leaching, roasting, hydrochlorination

and precious metal recovery by sorption. Autoclave oxidative leaching of the initial material

allows the non-ferrous metals to pass into solution from where they are recovered as a rich

sulfide concentrate (> 30% nickel and copper). Recovering the precious metals into solution

combines two operations: sinter roasting and hydrochlorination. Roasting destroys the precious

metal acid-proof mineral forms. The precious metals are recovered from solution by ion

exchange using anionites that are finally burned. The ash from the burning is a concentrate

of precious metals ( > 75% in total) which are recovered in three forms: ammonium

chloroplatinate with purity > 98%, palladium dichlorodiamine with purity > 96%, and a

mixture of rhodium, ruthenium and iridium hydrates. The flowsheet uses full water rotation

and minimum consumption of reagents, and gives a good recovery of metals to commodity

concentrates (nickel > 98%, copper > 80% and precious metals (in total) > 95%).

DOI: 10.1595/147106704X1667

on the sulfur content in the initial material. Forexample, during dissolution of concentrate con-taining a relatively high sulfide content (about 7%S in a sulfide form) the AOL process is carried outin an automatic mode without sulfuric acid addi-tion to the initial slurry. Nickel and copper rec-overies are 96 and 80%, respectively, into solution.

Leaching of feed with relatively low sulfide con-tent (< 2% S in a sulfide form) is possible underthe same conditions, but with acidic additions tothe initial slurry (30% of the solids� weight). In thiscase 94% Ni and 62.5% Cu are recovered intosolution. Platinum group metals recoveries intosolution are not high for these types of concentrateand are not more than 0.5% for platinum and pal-ladium, and not more than 1.2% for rhodium andruthenium. The AOL process is considered in (2).

Platinum group metals are present in the insol-uble leach residues in natural chalcogenide mineralforms; they do not undergo chemical transforma-tion at the AOL stage. Platinum metals may berecovered by a combination of oxidative roastingand hydrochlorination. At the roasting stage pgmminerals are broken down to produce metallicforms which are then dissolved during hydrochlo-rination to form pgm complex compounds.

The effect of the roasting temperature and timeon the Pt and Pd recovered into the chlorinationsolution is shown in Figures 3(a) and 3(b). Theoptimum conditions for oxidative roasting are:temperature 1000ºC, time 1 hour, and rate of heat-ing the material 6.5�7.5ºC min�1. In order todecrease sulfur dioxide production, a method tochange sulfur to sulfate during roasting by addingcalcium oxide to the initial feed was tested.

The effects of parameters, such as the type ofoxidising agent, HCl concentration, slurry density,and time and sequence of the process on the finalhydrochlorination values were studied so that max-imum amounts of pgm could be recovered fromthe sinter. The effects on the pgm recoveries intosolution of preliminary grinding before sinteringand reduction by hydrazine were estimated.Chlorine gas was the preferred oxidising agent atthe sinter hydrochlorination stage rather thanhydrogen peroxide or potassium permanganate.The optimal acidity of the slurry for maximumpgm recovery into solution corresponds to an HClconcentration of 220 g l�1, see Figure 4, consistentwith industrial practice (3). The platinum metalsassociates and rhodium, in particular, are sensitiveto variations in the HCl concentration. The pgmsrecovered at the hydrochlorination stage decreaseas the initial slurry density grows, see Figure 5. The


Fig. 1 Optical microscopy shows platinum group element minerals associated with pentlandite and chalcopyrite in reflected electrons (Cp is chalcopyrite)

Chemical Composition of the Initial Concentrates

Component Content

Concentrate 1, % Concentrate 2, %

Cu 0.62 2.07Ni 1.04 3.11Fe 7.9 12.4S 1.72 7.09Mg 9.3 9.45Al 3.1 3.19Cr 2.8 1.22SiO2 42.24 38.5

Concentrate 1, Concentrate 2,g t–1 g t–1

Pt 156.0 450Pd 74.7 310Rh 26.3 106Ru 52.7 151Ir 0.59 37Os 1.2 26Au 1.91 4.2Ag not analysed 17.4

Fig. 2 Flowsheet showing the treatment of flotation concentrates of low-sulfide platinum-containing ores


maximum drop in recovery is seen with a L:S (liq-uid : solid) ratio change from 3 to 2. Variations inthe slurry density in the range L : S = 3�5 havepractically no effect on the platinum metals recov-

eries. When the chlorination time was increasedfrom 2 to 4 hours, there was an increase in thepgms recoveries into solution. In general, platinummetals recoveries increase when the two-stageleaching process is used in a countercurrent mode.During a two-stage leaching process in solutionscontaining 170 g l�1 HCl, platinum extractionexceeded 99%, while extraction of palladium andplatinum metals associates was over 90%.

The platinum metals recoveries as a function ofthe time and sequence of the chlorination processare given in Figure 6. Sinter grinding and prelimi-nary reduction by hydrazine allow the platinummetals recovered into solution to increase by5�10% on average, see Figure 7.

Particle size analysis of the final chlorinationcakes showed that the size distribution of the plat-inum metals is in proportion to the size yields. The


Fig. 4 Platinum metals recoveries into the chlorinationsolution as a function of HCl concentration (L:S ratio = 10, chlorination time = 2 hours)

Fig. 3(a) The effect of roasting temperature on platinumand palladium recovery into chlorination solution (roasting time = 2 hours)

Fig. 6 Effect of the chlorination time on platinum groupmetals recoveries into solution (120 g l �1 HCl, L:S ratio = 2)

Fig. 5 Platinum metals recoveries into chlorination solution as a function of initial slurry density (170 g l �1 HCl, chlorination time = 2 hours)

Fig. 3(b) Effect of roasting time on platinum and palladium recovery into chlorination solution (roasting temperature = 1000ºC)


maximum pgms content occurs at a size of ~ 10mm while the minimum content occurs in therange > 44 mm but < 74 mm. The minor concentra-tion of rare platinum metals (Rh, Ru, Ir) in the ~10 mm fraction is noted. Mineralogical analysis ofthe hydrochlorination cakes, see Figure 8, showedthat all the platinum metals are present in acid-sol-uble forms (intermetallic compounds, dioxides).The incomplete recovery of platinum metals intothe chlorination solution may be explained by theirisolation by the rock minerals. An additional flota-tion recovery of platinum metals from the finalhydrochlorination cakes enables the production ofsufficiently rich concentrates (~ 500 g t�1) with themass of the initial material decreasing by up tothree times. The concentrates may then be

returned to the roasting stage of the flowsheet.Platinum metals recovery from hydrochlorina-

tion solutions was carried out by ion exchange(sorption). Anionite Rossion-11, a porous sorbentbased on styrene and divinylbenzene with func-tional groups of primary, secondary and tertiaryamines (-NH2, =NH, ºN) groups, characterisedby high selectivity towards the pgms, was used asthe sorbent. It was found that for maximumamounts of pgms to pass to the sorbent, the HClconcentration in solution should not be higherthan 125 g l�1, while the ferric iron concentrationshould not be more than 15 g l�1. The solution oxi-dation-reduction potential should be < 800 mV;this is also important to avoid sorbent destruction.

During the ion exchange process, with the rateof solution passing being: 2 volumes of solution to1 volume of resin per hour, the following residualpgm content in the sorbate was (in mg l�1): plat-inum < 1; palladium 1�1.5; rhodium < 1;ruthenium 3�4; iridium 1�2. The residual contentscorrespond to a total pgms recovery of 95 to 98%.However, if the sorbate, containing residualamounts of pgms, is used to prepare hydrochlori-nation solutions � thus returning pgms to thechlorine leaching process � the pgm recoveries atthe ion exchange stage will reach 99.5%.

During the investigations it was found that theoptimum ion exchange modes developed usingsimulated solutions are reproduced in existingtechnological solutions for all the pgms, except

Fig. 7 Effect of sinter grinding on platinum group metals recoveries into solution (220 g l�1 HCl, L:S ratio = 2, chlorination time = 2 hours)

Fig. 8 Platinum group metals and their compounds in the final hydrochlorination cakes

rhodium. The reasons for the incomplete recoveryof rhodium into the sorbent during ion exchangehave been determined. Rhodium recovery intosolution decreases as the sinter hydrochlorinationtime reduces due to the production of poorlysorbed forms of rhodium, such as [RhCl6]2�,[RhCl6�x(H2O)x]

x�3, [RhCl6�x(H2O)x]x�2, etc. Based

on the chlorination process time of 2 hours thesorption recovery figure is 70.3%. An additionaltwo-hour treatment of the solution by chlorine at90ºC helps to increase rhodium recovery to 76%,while a six-hour treatment increases it to 93.8%. Itis also noted that if the six-hour solution is held at90ºC (while mixing), then the free chlorine in solu-tion facilites rhodium transformation to the mostsorbed form, ([RhCl6]3�), and increases rhodiumrecovery into the sorbent up to 97%, see Figure 9.

The resin produced from the sorption containspgms and gold in the amount 90 kg of preciousmetal per tonne of air-dry resin. These studiesshowed there was the potential for pgm recoveryfrom resin by both combined and selective pgmdesorption and sorbent burning.

Burning the preliminary dried resin for 4 hoursat 1000ºC results in pgm-rich concentrates. Thepgm and gold content in the concentrates is80�85%. The ash yield (pgm concentrate) is 3�6%of the weight of the sorbent feed for burning.Gases, such as CO, CO2, CH4, are produced fromthe burning resin, while nitrogen oxides areformed when the resin is burned in an oxidisingatmosphere. These gases require attention before

being vented to the atmosphere.It was found earlier, that anionites based on

vinylpyridine sorb platinum and palladium wellfrom nitrate and chloride solutions. Palladium iseasily desorbed from anionite with dilute ammoniasolution (5% on NH3). Technology based on thishas been used to separate platinum and palladiumfrom silver nitrate electrolyte at an affinage workssince 1994. Electrolytes containing as much as 600g l�1 silver and 80 g l�1 copper, have undergonepurification. The platinum content of the elec-trolytes is in the range 30�500 mg l�1, and thepalladium content is in the range 200�2500 mg l�1.The exchange resin capacity for platinum and pal-ladium together is up to 45 g l�1. Palladium isdesorbed from the resin by ammonia solution, pre-cipitating palladium dichlordiamine salt of puritymore than 99% (~ 99.9%) from the palladiumstrippant. The resin is returned to the sorption.The platinum accumulated in the resin afterammonia flushing needs to be desorbed withthiourea, to precipitate an acid-soluble platinic-pal-ladium concentrate from the strippant (5).

Similar technology has been used since 2000 toprocess solutions from leached pgm materials. Atwo-stage pgm sorption recovery from hydrochlo-rination solutions was developed (Figure 10).

In the first stage Pt and Pd are recovered.Anionite containing pyridine groups has been syn-thesised for this. The distinctive feature of theanionite is that ammonia can desorb platinum andpalladium from it. Quadrivalent platinum isreduced on anionite to bivalent platinum and as aresult can be transferred to ammonia solution bydesorption. The optimum composition of the resinfunctional groups and the structure have beendetermined. After oxidation, ammonia chloroplati-nate and palladium dichlorodiamine are precipit-ated from the ammonia strippant. The anionitecapacity is ~ 35 g l�1 for platinum and palladiumtogether.

In the second stage Rh, Ru and Ir are recoveredby Rossion-11 anionite. Platinum group metals aredesorbed from anionite with hot (60�80ºC)thiourea solution. A mixture of Rh, Ru and Irhydrates is precipitated from the thiourea solution.The iron and non-ferrous metals content of the

Fig. 9 The degree of rhodium sorption recovery fromsolutions (ERh) as a function of the total chlorinationtime (sinter hydrochlorination and additional treatmentof solution)


concentrate produced does not exceed 4% of thetotal (Rh, Ru and Ir).

At the oxidative roasting stage of autoclaveleaching the insoluble residue, and during burningthe saturated resin, sublimation of volatile osmiumand, partly, ruthenium oxides to the gas phasetakes place. It is therefore necessary to collect,recover and then separate the products. Methodsof collecting osmium tetraoxide from roaster gasesare employed at existing plants in Russia (4).

ConclusionsA flowsheet for treating flotation concentrates

produced from low-sulfide platinum-containingores has been developed. It provides low con-

sumption of cheap reagents, complete water circu-lation and gives a high recovery of platinum groupmetals and non-ferrous metals into rich concen-trates. These are then suitable for refining.

References1 C. H. McLaren and J. P. De Villiers, �The platinum

group chemistry and mineralogy at the UG-2chromite layers of the Bushveld complex�, Econ.Geol., 1982, 77, (6), 1348

2 Ya. M. Shneerson, P. A. Goncharov, A. Yu. Lapinand V. M. Shpaer, �Selective recovery of non-ferrousmetals from complex feed�, Proc. of GipronickelInstitute JS, St. Petersburg, 2000

3 M. A. Meretukov and A. M. Orlov, �Metallurgy ofprecious metals (foreign experience)�, Moscow,Metallurgy, 1990, 416 pp.

PGM solution

I SORPTION

1 anionite sorbate

I DESORPTION

NH4Cl

1 anionite

1 desorbate

II SORPTION

sorbate 2 anionite

I DESORPTION

thiourea

2 desorbate 2 anioniteNaOHHCl

PRECIPITATION(NH3)2PdCl2

PRECIPITATIONRh, Ru, Ir hydrates

(NH3)2PdCl2 solution H2O2

PRECIPITATION(NH4)2PtCl6

(NH4)2PtCl6 solution

thiourea Rh, Ru, Ir hydrates


Fig. 10 Two-stage pgm sorption recovery from hydrochlorination solutions

4 T. N. Greiver, E. L. Kassatsier, T. V. Vergizova, V.M. Khudyakov et al., Russian Patent 2,044,084, C22B11/00, published in Bulletin 26, 1995

5 A. Tatarnikov, I. Sokolskaya, Ya. Shneerson, A.Lapin and P. Goncharov, �Complex treatment ofplatinum flotation concentrates�, Metallurgy, refrac-tories and environment; Proc. Vth Int. Conf.Metallurgy, Refractories and Environment, StaraLesna, High Tatras, Slovakia, 2002

6 Ya. M. Shneerson, A. Yu. Lapin, P. A. Goncharov,V. M. Shpayer, T. N. Greiver, G. V. Petrov and A.V. Tatarnikov, �Hydrometallurgical processing tech-nology for low-sulfide platinum-containingconcentrates�, Tsvetnye Metally, 2001, (3), 26

The Authors

A. V. Tatarnikov is a Senior Staff Scientist at the All-Russian R & DInstitute of Chemical Technology, Moscow, Russia. He is involved in the recovery of noble and rare metals from ores andbyproducts. His speciality is sorption concentrating of platinumgroup metals from solutions and slurries.

I. Sokolskaya is a Research Scientist at the All-Russian R & DInstitute of Chemical Technology, Moscow, Russia. Her interestsare in the synthesis of sorbents with given characteristics forapplication to the recovery of platinum group metals from solutions.

Ya. M. Shneerson is Head of the Hydrometallurgical Laboratory atthe Gipronickel Institute JS, St. Petersburg, Russia. His interestsare in the hydrometallurgical treatment of ores and concentratescontaining non-ferrous and noble metals.

A. Yu. Lapin is a Leading Staff Scientist at the Gipronickel InstituteJS, St. Petersburg, Russia. He is interested in non-ferrous andplatinum group metals recovery from concentrates by means ofhydrometallurgical autoclave treatment.

P. M. Goncharov is a Senior Staff Scientist at the GipronickelInstitute JS, St. Petersburg, Russia. He is involved in the treatmentof ores and byproducts containing noble metals by means ofhydrometallurgical autoclave treatment.


Hydrogen (H2) can be detected in several waysbut the technique chosen must take into accountthe conditions of its use, other impurities likely tobe present and the physical demands upon it. Themethods include semiconductor metal oxides,electrochemical methods, pellistors, palladium andoptical means. Response time and the thresholdlimit are important factors. As H2 becomes morewidely used, reliable detectors able to detect hydro-gen before it gets to explosive amounts in air ( >4.65 vol.% H2) are increasingly important.

Now scientists at the University of California,San Diego, U.S.A., have used optical interferome-try to detect H2 using Pd-coated porous Si (H. Lin,T. Gao, J. Fantini and M. J. Sailor, Langmuir, 2004,20, (12), 5104�5108; DOI: 10.1021/1a04974lu).Thin porous Si films were immersion plated withPd. On exposure to H2, the wavelength and inten-

sity of their Fabry-Pérot fringes, obtained from theinterferometric reflectance spectrum were simulta-neously measured. The intensity of the fringesdepends on the reflectivity of the Pd/porous Sicomposite and their wavelength depends on therefractive index of the Pd film. H2 expands the Pdlattice and this shifts the optical fringes anddecreases the intensity of the reflected light.

The set-up used by the researchers gave a detec-tion limit of H2 at room temperature of ~ 0.2% (byvolume) in nitrogen, with the lowest concentrationdetected being ~ 0.17%. The response time was afew seconds. This sensor design reliably detects H2

concentrations well below the explosive limit. Thesensor is safe, sensitive, selective, reproducible andcan operate at room temperature. However, as COimpedes the adsorption and desorption of H2, theresponse time is longer, if CO is present.

Optical Hydrogen Sensors Using Palladium-Silicon

Platinum/Carbon Nanotubes in PEMFCsProton exchange membrane fuel cells (PEMFCs)

generate electric power efficiently without produc-ing exhaust gases, so are very desirable for use inLEVs or ZEVs (low or zero emission vehicles) andas power sources for small portable electronics.However, achieving minimised metal content withthe costly platinum group metals catalysts is one ofthe challenges to their commercialisation.

Now, researchers at the University ofCalifornia, Riverside, U.S.A., have investigatedincreasing Pt utilisation in PEMFCs by using car-bon multiwalled nanotubes (MWNTs) as the Ptsupport (C. Wang, M. Waje, X. Wang, J. M. Tang,R. C. Haddon and Y. Yan, Nano Lett., 2004, 4, (2),345�348; DOI: 10.1021/nl034952p).

MWNTs were grown directly onto C paper bychemical vapour deposition. The Pt catalyst wasthen electrodeposited onto the MWNTs. The Ptparticles had an average diameter of 25 nm (com-mercial Pt/C catalysts are 2�3 nm). There wasgood electrical contact between the MWNTs andthe C paper and excellent adhesion. The surfacearea of the MWNT-C paper composite was ~80�140 m2 g�1 (< 2 m2 g�1 for the C paper alone).

A membrane electrode assembly was preparedwith two of the composite electrodes and tested ina fuel cell station. Its performance was lower thanthat of a conventional PEMFC, but its robustnesswas confirmed. It is suggested that reducing the Ptparticle size to ~ 2.5 nm, improving MWNT yieldand reducing tube diameter will give C nanotube-based fuel cells of superior performance.

DOI: 10.1595/147106704X1874

DOI: 10.1595/147106704X1883


In May, Johnson Matthey published�Platinum 2004�, its annual survey of the sup-ply and demand of the platinum group metals(pgms) for 2003, including a discussion of theshort-term outlook for the pgms markets.

Demand for platinum (Pt) in 2003,increased by ~ 1 per cent to 6.52 million oz.Purchases of Pt by the autocatalyst industryincreased by 23 per cent to 3.19 million oz. InNorth America, the car industry bought moremetal than the previous year, when inventoriesof Pt were used to supplement purchases.European demand climbed to a record highdue to further growth in diesel car sales, whilein Japan autocatalyst demand for Pt was alsoboosted by new heavy-duty diesel emissionsregulations for Tokyo. Indeed, tighteningemission standards worldwide have helped tosupport Pt use.

Jewellery demand for Pt dropped sharplyby 13 per cent in 2003 to 2.44 million ozresulting from the sharp rise in the price of Pt(see later). This affected sales of Pt jewellery inboth China and Japan.

Industrial demand for Pt weakened to 1.52million oz. The use of the metal in electronicgoods containing hard disks increased, butglass manufacturers reduced their metal hold-ings and less new capacity was added in Asia.

Supplies of Pt grew by 4.5 per cent to 6.24million oz. South African output expandedsteadily and Russian sales increased, butNorth American production dropped.Overall, the Pt market remained in deficit forthe fifth year in succession.

In 2003, the Pt price increased by morethan 35 per cent from the opening fixing of theyear of $600 to the final fixing of $814. Thespeculative buying of Pt had a major influenceon the price, and was part of a substantial flowof funds into commodities as a whole.

Demand for palladium (Pd) recovered by 9per cent but, at 5.26 million oz, this was stillthe third lowest total for a decade. Purchasesof Pd for autocatalysts were up by 410,000 ozto 3.46 million oz as U.S. car companies usedless metal from inventories. The electronicsindustry used 18 per cent more Pd but the useof Pd in dental alloys dropped by 8 per cent.Supplies of Pd increased by 23 per cent to 6.45million oz. Russian production was fully soldand South African output expanded. Withsupplies rising faster than demand, the Pdprice weakened, ending the year at $193, some$41 below the opening fixing in January.

Net purchases of rhodium (Rh) rose by 6per cent to 627,000 oz. The autocatalyst mar-ket grew strongly as average Rh levelsincreased ahead of tighter emissions legisla-tion. Demands from the glass industry fell.

Purchase of ruthenium increased by 18 percent to 496,000 oz due to a strong demand forchemical catalyst applications and for elec-tronics. Demand for iridium (Ir) alsorecovered, rising by 30 per cent to 103,000 oz.The growth was driven by increased use of Ircatalysts for the chemicals industry and byincreased orders for Ir crucibles from the elec-tronics sector.

�Platinum 2004� also contains two specialreports on: �PGM Mining in Russia� and �30Years in the Development of Autocatalysts�.

The Interim Review will be published inNovember and there will be two further elec-tronic updates: in August 2004 and inFebruary 2005. For an E-mail alert, please reg-ister online at: www.platinum.matthey.com.A copy of �Platinum 2004� can be obtainedfrom: Johnson Matthey PLC, 40�42 HattonGarden, London EC1N 8EE, U.K.; E-mail:[email protected]; Fax: +44 (0)20 72698389.

Platinum 2004

DOI:10.1595/147106704X1612

The Innovation Group based at the JohnsonMatthey Technology Centre was approached toexamine and characterise four Russian platinumroubles belonging to Johnson Matthey. The coinsare:

1828 3 rouble 1830 6 rouble1834 3 rouble 1835 3 rouble

Forged Russian roubles have been identified asbeing of pure platinum metal, while the genuine

coins contain iron impurities of up to 4 wt.% (3, 4).The coins were analysed by four methods:[i] Magnetic, namely permeameter measurements,[ii] Density measurements, see Table I[iii] Scanning electron microscopy (SEM), seeTable II, and[iv] X-ray diffraction (XRD), see Table III.

In measuring the magnetic characteristics of thecoins, a rare earth-transition metal type magnet


The Minting of Platinum RoublesPART III: THE PLATINUM ROUBLES OF JOHNSON MATTHEY

By David B. Willey and Allin S. Pratt*

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

It is not known for certain how four platinum roubles came to be in Johnson Matthey�s possession.

There is rumour that, at the end of World War I, A. B. Coussmaker of Johnson Matthey,

negotiated with the White Russians to smuggle out of Russia a hoard of coins which had

been withdrawn by the government years before. The hoard was reputed to be on a train to

the West when the Reds caught up with it. Rather than stop the transaction, they thought it a

good idea as it would raise capital for them � at that time, the refining capacity of the young

U.S.S.R. had been disrupted. So they took over the deal and let the consignment continue its

journey to Johnson Matthey where it was refined and the platinum sold on their behalf. However,

this is speculation (1). Eye witnesses state that two roubles were definitely in the company�s

possession in 1956, and that two more came from the desk of Dr Leslie B. Hunt, the founder

of this Journal (1). The roubles have thus been in Johnson Matthey�s possession for almost 50

years and probably for longer. More likely to be true is a brief note in a typewritten statement

in the possession of Johnson Matthey, stating no more than �the specimens formed part of a

consignment sent to Johnson Matthey for refining about 1870� (2). As there is always interest

in platinum coins and particularly in Russian roubles which were the first platinum coins to

be minted, it was decided to investigate the metal content of the Johnson Matthey roubles to

find if they conformed to recognised Russian roubles � or were forgeries.

DOI: 10.1595/147106704X1649

Table I

Densities of the Roubles (Measured in Ethanol) at Room Temperature

Year 1828 1830 1834 1835

Mass in air, g 10.3549 20.7039 10.1612 10.3120Mass + wire in liquid, g 10.3727 20.3061 10.1845 10.3136Mass of wire in liquid, g 0.4017 0.4017 0.4017 0.4017Real mass in liquid, g 9.9710 19.9044 9.7828 9.9119Difference, g 0.3839 0.7995 0.3784 0.4001Volume, cm3 0.4868 1.0138 0.4798 0.5074

Density, g cm–3 21.27084 20.42163 21.17633 20.32503

Density of ethanol 0.7886 g cm�3


Platinum 3 rouble coin dated 1828 Platinum 6 rouble coin dated 1830

Platinum 3 rouble coin dated 1834 Platinum 3 rouble coin dated 1835


Micrographs and EDX spectra of the 6 rouble coin dated 1830; EDX: left 139 counts/s; right 15 counts/s



was used a reference. However, the remanencesand coercivities of the coins were too small fordetection.

Density measurements were carried out (TableI), with the coins suspended by wire in ethanol. Thetheoretical density of pure platinum is 21.45 g cm�3,and any substantial decrease from this value wouldindicate the presence of other foreign elements, thatis, a genuine coin. The 1830 and 1835 coins wereobserved to have lower densities than the othertwo. From this measurement, and within experi-mental error, the Johnson Matthey archive thusappears to hold two genuine and two forged coins.

SEM was performed on the materials, usingenergy dispersive X-rays (EDX) to identify the ele-ments present (Table II). Trace amounts of ironwere found in three of the coins. The 1828 coinappeared to be ~ 100% pure platinum.

Finally, XRD was performed on the 1834 and1835 coins to find if the 1834 coin was pure plat-inum. The two coins were both indexed to pureplatinum. It was observed that the 1834 coin hasan exact match to these parameters, while the 1835coin has a definite shift towards a platinum/ironphase that is indexed. It is likely that the 1834 coinis in fact pure platinum and thus a forgery. TableIII details the lattice parameters and crystallite sizesof the coins. Pure platinum has a lattice parameterof 0.3925 nm which is very close to the valueobtained for the 1834 coin. The 1835 coin has aslightly lower value, indicating unit cell volumedepression caused by the iron.

ConclusionsFrom these measurements we conclude that the

1828 coin is a forged rouble. It is more than likelythat the 1834 is also a forgery as its platinum con-

tent is too high. The 1830 and 1835 coins are gen-uine roubles as they contain other elements, mostnotably iron.

AcknowledgementsWe would like to thank Alan Stubbs for the SEM work and

Hoi Wong for the XRD analysis.

References1 A. Austin, private E-mail communication, 28th July,

19992 Johnson Matthey, London, internal manuscript3 C. J. Raub, Platinum Metals Rev., 2004, 48, (2), 66; and

references therein4 D. F. Lupton, op. cit., (Ref. 3), 72; and references

therein

The AuthorsAllin Pratt is a Principal Scientist within the Johnson MattheyInnovation Group. His main interests are the application of metallurgy and materials science to new areas of research as wellas conventional applications in materials, catalysis, biomedicalapplications, and renewable energy systems including batteriesand hydrogen storage.

David Willey specialised in the interactions of materials and hydrogen with respect to battery materials, diffusion systems andmetallurgical processes while at the Johnson Matthey TechnologyCentre. He also had experience in fuel cell technology. David iscurrently a consultant at Buchanan Communications, London, andis involved in strategic financial communications for a range ofcompanies including Renewable Energy, E & P and Chemicals.

Table II

Elemental Analysis of the Roubles

Year Coin Iron, % Platinum, %

1828 3 rouble Trace 1001830 6 rouble 1.6 98.41834 3 rouble 0.7 99.31835 3 rouble 4.8 95.2

Table III

Lattice Parameters and Crystallite Sizes ofthe 3 Rouble Coins

Year Crystallite size, nm Lattice parameters, nm

1834 34.29 0.3921835 32.29 0.391

Production of Fine Iridium FibreIn the last issue, K. Mori of Tanaka Kikinzoku KogyoKK described the production of flocculate platinumfibre and non-woven fabric, which are used as electri-cally conductive fillers for porcelain enamel (PlatinumMetals Rev., 2004, 48, (2), 56). Now, Furuya KinzokuKK of Japan have produced fine iridium (Ir) and Iroxide fibre from linear Ir compounds with Ir�Irbonds as the main chains in a fibre-like shape (JapaneseAppl. 2004-027,399). The Ir compounds are thermal-ly treated either in H2 or O2, to form fine Ir or Iroxide microfilament, respectively. The fibre size is0.1�5 mm by £ 20 mm, with surface area > 1 m2 g�1.The Ir fibre displays a high melting point, chemicalstability, and has excellent characteristics as a catalyst.

DOI:10.1595/147106704X1937


The recent Institute of Materials, Minerals andMining (IOM3) Materials Congress 2004, held inLondon from 30th March to 1st April, 2004,included a symposium entitled �Platinum � fromCradle to Grave�. Chaired by Professor Peter R.Simpson (Imperial College London and Admini-strative Secretary to the U.K. Parliamentary andScientific Committee) and introduced by ProfessorJane Plant (Imperial College London and ChiefScientist of the British Geological Survey, U.K.),the symposium featured a range of papers span-ning geological, economic and environmentalissues relating to the platinum group metal (pgm)industry. This selective review focuses on geologi-cal aspects, in particular ones relating to SouthAfrica.

Between 1999 and 2003 demand for platinum(Pt) from the jewellery, automotive and industrialsectors has consistently outweighed supply of newmetal from mines. In the same period the Pt pricehas risen from around $350/troy oz to over $800/troy oz, and in 2004 has moved to over $900/troyoz (1). Alongside the growing demand for theother pgms, these factors have encouraged anincrease in pgm exploration projects around theworld, while established major producers seek toexpand operations to satisfy metal demand. Paperspresented during the morning session providedan insight into some of the economic and socialchallenges faced by producers in South Africa asthey look to increase production levels, and theissues surrounding exploration of platinum groupelements (PGEs) including one exploration pro-gramme in the U.K.

In 2002, the South African Government intro-duced the Black Economic Empowerment (BEE)policy. This requires South African mining compa-nies to transfer 26 per cent of their mining assetsto Historically Disadvantaged South Africans(HDSA) within ten years, and has placed a newresponsibility on all the Bushveld producers.Professor Dennis Buchanan (Imperial CollegeLondon) presented a financial evaluation modelfor a South African PGE mine. This highlighted

the economic effects of the BEE policy, and a newroyalties policy.

J. F. W. Bowles (Mineral Science Ltd., France)gave an interesting presentation outlining some ofthe issues faced by PGE exploration programmes.He highlighted the difficulties of identifying PGE-bearing minerals in the field, particularly workingamong the highly weathered and poorly exposedformations of certain African countries such asthose in Sierra Leone and Madagascar.

S. J. Thompson (Agricola Resources, U.K.)introduced a potential PGE exploration pro-gramme within the U.K. The company has recentlysecured an agreement (via Beowulf Gold PLC)with a landowner for acquiring exclusive explo-ration rights for PGEs on property on the Isle ofUnst, one of the Shetland Isles. Unst is the mostnortherly inhabited island in the British Isles, andis the only part of the U.K. currently being investi-gated for PGEs. The exploration area covers 8 km2

of an ophiolite complex, that was mined forchromite between 1824 and 1945. The presence ofPGEs associated with the chromite deposits hasbeen known since the nineteenth century and in1985 a British Geological Survey reconnaissanceprogramme discovered medium to high levels ofPGEs at several locations. If Agricola exercises itsoption, it intends to carry out a systematic geolog-ical investigation, including soil and rockgeochemical analysis. If results are favourable, adiamond drilling programme will be undertaken.

Other papers covered commercial fuel cellsdevelopments and the environmental effects ofautocatalysts. It is hoped that many of the paperspresented will be published in a future issue of theIOM3 journal Applied Earth Science, see:www.ingenta.com/journals/browse/maney/aes

A. BRIDLE

Reference1 �Platinum 2004�, Johnson Matthey, London, 2004

Asa Bridle is a Marketing Executive at Johnson Matthey PreciousMetals Marketing in London. With a background in geology, he hasa particular interest in the mining and exploration of the platinumgroup elements.

IOM3 Materials Congress 2004

DOI: 10.1595/147106704X1658

PROPERTIESTheoretical Investigation of Isomer Stability inPlatinum–Palladium Nanoalloy ClustersL. D. LLOYD, R. L. JOHNSTON, S. SALHI and N. T. WILSON,J. Mater. Chem., 2004, 14, (11), 1691�1704

The interatomic interactions of Pt-Pd nanoalloyclusters (1) were modelled by the Gupta many-bodypotential. For 18�20 atom clusters, a general algo-rithm was used to establish the lowest energystructures for each size and for all possible composi-tions. In the lowest energy isotopes (homotops), thePd atoms are mainly at the surface, with the Pt atomspreferentially occupying interior sites.

Dendrimer-Templated Ag–Pd BimetallicNanoparticlesY.-M. CHUNG and H.-K. RHEE, J. Colloid Interface Sci., 2004,271, (1), 131�135

Ultrafine amine-terminated fourth-generation star-burst poly(amidoamine) dendrimer-templated Ag�Pdbimetallic nanoparticles (1) were prepared fromsilver(I)-bis(oxalato)palladate(II) (2). The use of (2),in which two metal ions exist in one complex, pre-vented Ag halide formation. The particle sizedistributions of (1) were all in the narrow range of 2�5nm, but were dependent upon the Ag/Pd ratio.

CHEMICAL COMPOUNDSThe Construction of P-O/P-N Ligands on Platinumand PalladiumP. BERGAMINI, V. BERTOLASI and F. MILANI, Eur. J. Inorg.Chem., 2004, (6), 1277�1284

PtII and PdII diphenyldiphosphinites were preparedby adding diols to a solution containing cis-[MCl2(PPh2Cl2] (M = Pt or Pd) preformed in situ fromcis-[MCl2(1,5-COD)] and PPh2Cl. As for the P-Obond, the P-N bond can be formed via nucleophilicattack of an amine group on coordinated PPh2Cl inthe presence of base to give aminophosphine-phos-phinite and N,N¢-bis(antipyryl-4-methyl)- piperazinecomplexes of PtII and PdII.

C–H and N–H Activation by Pt(0) in N- andO-Heteroaromatic CompoundsJ. T. CHANTSON and S. LOTZ, J. Organomet. Chem., 2004, 689,(7), 1315�1324

[Pt(PEt3)4] reacts with azoles to give Pt(II) hydridecomplexes, trans-[PtH(1-azolyl)(PEt3)2] (azolyl =indolyl, imidazolyl, benzimidazolyl, pyrazolyl, inda-zolyl), by oxidative insertion of the Pt centre into theN�H bonds of the respective azoles. Pyrrole wasmuch less reactive. The trans-[PtH(R)(PEt3)2] com-plexes (R = 2-furyl, 2-benzoxazolyl, 2-benzothiazolyl)were prepared via C�H bond activation.

ELECTROCHEMISTRYThe Electrochemical Formation and Properties ofBilayers Composed of Polypyrrole and C60Pd FilmsM. WYSOCKA, K. WINKLER and A. L. BALCH, J. Mater. Chem.,2004, 14, (6), 1036�1042

Bilayers of polypyrrole and C60Pd were prepared bysequential electropolymerisation of the parentmonomers and investigated by cyclic voltammetry.For the electrode/polypyrrole/C60Pd bilayer, the highpermeability of the C60Pd film for the supportingelectrolyte ions allowed the oxidation of the polypyr-role inner layer. For the electrode/C60Pd/polypyrrolebilayer, the outer polypyrrole layer inhibited thereduction of the inner C60Pd layer.

PHOTOCONVERSIONEffects of Chlorine Gas Exposure on the OpticalProperties of Rhodium Phthalocyanine FilmsL. GAFFO, O. D. D. COUTO, R. GIRO, M. J. S. P. BRASIL,D. S. GALVÃO, F. CERDEIRA, O. N. DE OLIVEIRA andK. WOHNRATH, Solid State Commun., 2004, 131, (1), 53�56

Rh phthalocyanine films (1) were deposited on glassby the Langmuir-Blodgett technique. On exposureto Cl2 (1) changed from blue to transparent.Incorporation of Cl2 caused quenching of the charac-teristic triplet centred around the Q-absorption bandat 662 nm. Leaving prior Cl2 exposed (1) in air for sev-eral hours resulted in a slow partial recovery of theoptical spectra.

A Synthesis and Luminescence Study of Ir(ppz)3

for Organic Light-Emitting DevicesE. J. NAM, J. H. KIM, B.-O. KIM, S. M. KIM, N. G. PARK, Y. S. KIM,Y. K. KIM and Y. HA, Bull. Chem. Soc. Jpn., 2004, 77, (4),751�755

OLEDs were fabricated with doped films of tris(1-phenyl-kC 1-pyrazolato-kN 2 )iridium (1) in severalhosts. The electroluminescence peak occurred at 450nm. The luminance of the OLEDs was pure blue, butluminous efficiencies were low since the LUMO of(1) was higher than those of the hosts.

Photobleaching and Single Molecule Detection ofa Phosphorescent Organometallic Iridium(III)ComplexM. VACHA, Y. KOIDE, M. KOTANI and H. SATO, J. Luminesc.,2004, 107, (1�4), 51�56

The photostability of bulk samples of Ir(ppy)3 (1)(ppy = 2-phenylpyridine) in polymer films was exam-ined by measuring the photobleaching kinetics underintense laser irradiation. On a single molecule level,(1) was characterised by phosphorescence time traces,distribution of phosphorescence intensity levels andby polarisation modulated excitation.


ABSTRACTSof current literature on the platinum metals and their alloys

DOI: 10.1595/147106704X1900

Synthesis and Characterization of Naphthyridineand Acridinedione Ligands CoordinatedRuthenium(II) Complexes and Their Applicationsin Dye-Sensitized Solar CellsS. ANANDAN, J. MADHAVAN, P. MARUTHAMUTHU, V.RAGHUKUMAR and V. T. RAMAKRISHNAN, Sol. Energy Mater.Sol. Cells, 2004, 81, (4), 419�428

The title complexes were synthesised and charac-terised, and then used in dye-sensitised solar cells.From the I�V curves, the short-circuit photocurrent(ISC) and the open-circuit photovolatage (VOC) weremeasured. A maximum current conversion efficiencyof ~ 7.7% was achieved by the 5-amino-4-phenyl-2-(4-methylphenyl)-7-(pyrrolidin-1-yl)-1,6-naphthyridine-8-carbonitrile coordinated Ru(II) complex.

ELECTRODEPOSITION AND SURFACECOATINGSMesoporous Microspheres Composed of PtRu AlloyJ. JIANG and A. KUCERNAK, Chem. Mater., 2004, 16, (7),1362�1367

Electrochemical co-reduction of H2PtCl6 and RuCl3dissolved in the aqueous domains of the liquid crys-talline phase of an oligoethylene oxide surfactant(C16EO8) gave the title microspheres (1) (0.5�1 µm).The ordered mesoporous internal structure of (1)involves periodic pores of ~ 2.4 nm in diameter sep-arated by walls of ~ 2.4 nm thick. (1) have highspecific surface area.

Synthesis of PtNx Films by Reactive Laser AblationG. SOTO, Mater. Lett., 2004, 58, (16), 2178�2180

Thin films of Pt (1) with ~ 14 at.% N were pre-pared by reactive laser ablation in molecular N2

ambient. AES, XPS and electron energy loss spec-troscopy were used to characterise (1). The existenceof chemisorbed N was supported by the N1s bindingenergy of 398.4 eV. The +0.2 eV shift of the Pt4fpeak position indicated charge transfer from Pt to N.The Pt formed an incipient nitride phase with com-position near to Pt6N.

APPARATUS AND TECHNIQUECalorimetric Hydrocarbon Sensor for AutomotiveExhaust ApplicationsM.-C. WU and A. L. MICHELI, Sens. Actuators B: Chem., 2004,100, (3), 291�297

The title sensor (1) is a thermoelectric device sup-ported on a planar Al2O3 substrate. A non-selectivePt catalyst was used in (1) to detect hydrocarbonswith high selectivity. For CO detection (1) uses a COoxidation catalyst of Pb-modified Pt, which exhibitsexcellent CO selectivity at 200�400ºC. (1) gave a lin-ear output of 0�2.75 mV over 0�1000 ppm ofpropylene (at 350ºC). Engine dynamometer evalua-tion showed that the response of (1) paralleled thechange in concentration of the CO and hydrocarbonswhen the engine air:fuel ratio was varied.

Chemiresistor Coatings from Pt- andAu-Nanoparticle/Nonanedithiol Films: Sensitivityto Gases and Solvent VaporsY. JOSEPH, B. GUSE, A. YASUDA and T. VOSSMEYER, Sens.Actuators B: Chem., 2004, 98, (2�3), 188�195

Layer-by-layer self-assembly using 1,9-nonanedi-thiol and dodecylamine-stabilised nanoparticles of Ptor Au gave films (1) of thickness, 66 ± 2 and 31 ± 1nm, respectively. The sensitivity of (1) was investigat-ed by dosing them with NH3, CO and vapours ofH2O and toluene (300 ppb�5000 ppm). (1) have ahigh signal:noise ratio. A detection limit for NH3 <100 ppb was achieved. NH3 and CO bind to vacantsites on the metal nanoparticle cores.

HETEROGENEOUS CATALYSISPalladium(II) Chloride Catalyzed SelectiveAcetylation of Alcohols with Vinyl AcetateJ. W. J. BOSCO and A. K. SAIKIA, Chem. Commun., 2004, (9),1116�1117

PdCl2 with CuCl2 can be used for the catalytic acety-lation of primary and secondary alcohols with vinylacetate. The reaction is carried out in dry toluene atroom temperature. The catalyst system can be recov-ered by filtration. The acetaldehyde byproduct can beremoved by evaporation along with the toluene sol-vent. This mild reaction proceeds more rapidly withprimary alcohols.

Assembled Catalyst of Palladium and Non-Cross-Linked Amphiphilic Polymer Ligand for theEfficient Heterogeneous Heck ReactionY. M. A. YAMADA, K. TAKEDA, H. TAKAHASHI and S. IKEGAMI,Tetrahedron, 2004, 60, (18), 4097�4105

An insoluble catalyst (1) was prepared from self-assembly of (NH4)2PdCl4 and a non-cross-linkedamphiphilic phosphine polymer. (1) in only 5.0 × 10�5

mol equiv. concentration was effective for the het-erogeneous Heck reaction of aryl iodides withacrylates, styrenes and acrylic acid. (1) could be recy-cled five times. (1) showed good stability in tolueneand H2O and so efficiently catalysed the Heck reac-tion in these media. (1) can be used for the synthesisof resveratrol, a cyclooxygenase-II inhibitor.

Effect of Activated Carbon and Its SurfaceProperty on the Activity of Ru/AC CatalystW. HAN, B. ZHAO, C. HUO and H. LIU, Chin. J. Catal., 2004, 25,(3), 194�198

Activated C was pretreated by washing with HNO3

and then calcined before the preparation of Ru/acti-vated C catalysts (1) for NH3 synthesis. The HNO3

treatment increased the catalytic activity from 17.4 to18.4%, due to the reduction of S and ash. Gas phaseoxidation of the C caused the Ru to be more dis-persed in (1) by changing the texture and surfacegroups of the support. Overall, the Ru dispersion andcatalytic activity were improved from 43 to 62% and17.4 to 19%, respectively.



HOMOGENEOUS CATALYSISThe First Platinum-Catalyzed Hydroamination ofEthyleneJ.-J. BRUNET, M. CADENA, N. C. CHU, O. DIALLO, K. JACOB andE. MOTHES, Organometallics, 2004, 23, (6), 1264�1268

The hydroamination of ethylene with aniline, usingPtBr2 as a catalyst precursor in n-Bu4PBr under 25 barof ethylene pressure, gave N-ethylaniline with 80turnovers after 10 h at 150ºC. 2-Methylquinoline wassimultaneously produced in ~ 10 cycles. Additions ofP(OMe)3 (2 equiv./PtBr2) or of a proton source (3equiv./PtBr2) were beneficial. A TON of 145 after 10h at 150ºC was achieved with a biphasic system (n-Bu4PBr/decane) in the presence of C6H5NH3

+ (3equiv./PtBr2). The lower the basicity of the aryl-amine, the higher the TON.

Important Consequences for Gas ChromatographicAnalysis of the Sonogashira Cross-Coupling ReactionE. H. NIEMELÄ, A. F. LEE and I. J. S. FAIRLAMB, Tetrahedron Lett.,2004, 45, (18), 3593�3595

Typical quenching procedures for GC analysis ofthe Sonogashira reaction for 4-bromo-6-methyl-2-pyrone with phenylacetylene (catalysed by(Ph3P)2PdCl2) cannot be used. Turnover continues tooccur in sample vials even after quenching by com-monly used SiO2 adsorption and product elution withCH2Cl2. Trace amounts of Pd are carried through theSiO2 plug. Addition of 1,2-bis(diphenylphosphino)-ethane to the sample inhibited any further reaction.

Role of Base in Palladium-Catalyzed Arylation ofCarbanionsA. V. MITIN, A. N. KASHIN and I. P. BELETSKAYA, J. Organomet.Chem., 2004, 689, (6), 1085�1090

The arylation of carbanions, derived from varioussulfones, cyanoacetic ester and malononitrile, witharyl bromides using Pd2dba3/3L, L = PPh3, P

tBu3, aswell as the reaction of the carbanions with 1 equiv. of4-CF3C6H4 Pd(PPh3)2Br were carried out. A basestronger than the initial carbanion was required. Thereaction mechanism includes the acceleration of thereductive elimination due to the deprotonation of theintermediate ArPdL2CHXY.

New Chiral Bis(oxazoline) Rh(I)-, Ir(I)- andRu(II)-Complexes for Asymmetric TransferHydrogenations of KetonesN. DEBONO, M. BESSON, C. PINEL and L. DJAKOVITCH,Tetrahedron Lett., 2004, 45, (10), 2235�2238

The transfer hydrogenation of acetophenone in 2-propanol was used to examine the title complexes.The Ru(II)-based catalyst exhibited good activity with50% conversion and high enantioselectivity (89%),whereas the Rh(I)- and Ir(I)-complexes gave lowconversions (~ 20%) and poor enantioselectivities(16�20%). A free hydroxyl group on the ligand was aprerequisite for high enantioselectivity. The strongerthe base present, the higher the conversion.

FUEL CELLSEffect of Preparation Conditions of Pt/C Catalystson Oxygen Electrode Performance in ProtonExchange Membrane Fuel CellsJ. H. TIAN, F. B. WANG, ZH. Q. SHAN, R. J. WANG and J. Y. ZHANG,J. Appl. Electrochem., 2004, 34, (5), 461�467

Pt/C catalysts with 3.2 nm Pt crystallites were pre-pared by the impregnation-reduction method,varying conditions such as the reaction temperature,the concentration of H2PtCl6 and different reducingagents. Heat treatment in N2 of the C black supportimproved Pt dispersion and increased the relativecontent of Pt (111) orientation. This benefited theacceleration of the oxygen reduction reaction in thePEMFC.

Preparation and Performance of Pt-Co/C Catalystfor PEMFCY. ZHANG, X. LI, X. WU, M. XU and L. DENG, Precious Met. (Chin.),2004, 25, (1), 19�23

Pt-Co/C electrocatalysts (1) were prepared by theliquid deposition and high temperature alloyingmethod. The Pt and Pt-Co have f.c.c. structure, withsmall particle size and high dispersity. The Co addi-tion shortened the length of the Pt�Pt bond. (1)showed better performance than Pt/C in PEMFCs.The Co addition exhibited high catalytic activity.

ELECTRICAL AND ELECTRONICENGINEERINGMicrostructure and Magnetic Properties ofBamboo-Like CoPt/Pt Multilayered NanowireArraysY.-K. SU, D.-H. QIN, H.-L. ZHANG, H. LI and H.-L. LI, Chem. Phys.Lett., 2004, 388, (4�6), 406�410

Double-pulse electrodeposition into the pores of aporous anodic Al oxide template gave highly orderedCoPt/Pt multilayered nanowire arrays (1). Thenanowires had a bamboo-like structure. The sectionlengths could be adjusted by varying the pulse widthand pulse intensity. (1) exhibited in-plane anisotropy.High coercivity (Hc = 1.8 kOe) and squareness (Mr/Ms) ~ 0.35 were obtained in (1) when the field wasapplied perpendicular to the wire axis of (1). This isattributed to the disordered f.c.c. CoPt formation.

Effects of Post-Annealing on the DielectricProperties of Au/BaTiO3/Pt Thin Film CapacitorsE. J. H. LEE, F. M. PONTES, E. R. LEITE, E. LONGO, R. MAGNANI,P. S. PIZANI and J. A. VARELA, Mater. Lett., 2004, 58, (11),1715�1721

BaTiO3 thin films (1) were prepared by the poly-meric precursor method and deposited onto Pt/Ti/SiO2/Si. The BaTiO3 perovskite phase formationwas investigated. (1) were post-annealed in O2 and N2

at 300ºC for 2 h. Post-annealing in O2 increased thedielectric relaxation phenomenon, whereas in N2 aslight dielectric relaxation was produced.

143Platinum Metals Rev., 2004, 48, (3), 143�144

NEW PATENTSPHOTOCONVERSIONElectrochromic MirrorTOKAI RIKA CO LTD European Appl. 1,400,839

A reflecting mirror suitable for use as an interiormirror, or as a rear-view mirror in a vehicle, compris-es a H storing-metal or alloy of Pd, Rh and Pt in anelectrically conductive reflecting film (1). (1) also con-tains an ion conductive dielectric film of Ta2O5, SiO2

or MgF2. A predetermined voltage is applied between(1) and a transparent ITO electrode (2) to release Hions. The H ions move through the ion conductingfilm to bond with a W oxide colouring film posi-tioned between (2) and (1) making it bluish in colour.

Organo-Electroluminescence ElementSANYO ELECTRIC CO LTD European Appl. 1,418,217

An organic electroluminescence device has a holeinjection electrode formed on a glass substrate; a holetransport layer, a light emitting layer (1) and a holeblocking layer are formed sequentially on the holeinjection electrode. (1) includes an organic Pt groupmetal compound (2) composed of a phenanthridinederivative and a Pt group metal element. (2) can emitred-orange light via a triplet excited state.

Rh and/or Ir Doped SrTiO3 for Water DecompositionJAPAN SCI. TECHNOL. CORP Japanese Appl. 2004-008,963

A visible light active catalyst (1) is claimed, whichcomprises Rh and/or Ir doped SrTiO3 and with, inparticular, a Pt metal catalyst. (1) is used for H2Odecomposition to generate H2 from an aqueousMeOH solution under visible light irradiation.

APPARATUS AND TECHNIQUEGas SensorNGK SPARK PLUG CO LTD European Appl. 1,418,421

A gas sensor has a detecting element with an elec-trode containing a Pt metal on the surface of a solidelectrolyte. It is manufactured by: (a) applying nucleiof a Pt metal by sputtering (to catalyse a gas to bemeasured); and (b) growing the nuclei by electrolessplating from an aqueous solution of platinic ammineor platinous ammine, and a hydrazine reducer.

Hydrogen Contamination MonitorBOEING CO U.S. Patent 6,734,975

A H detection system includes a H sensor (1)formed of at least Pd which can detect contaminationwithin a Pd reaction member and can generate a Hcontamination signal. A surface spectroscopic system(2), having an output device configured to respond toa resonant frequency of a H�Pd bond, operates inconjunction with (1) to determine the H contamina-tion and to generate a sensor contamination signal. Acontroller is electrically coupled to (1) and (2) andcompares both signals; the controller can thus gener-ate a corrected H contamination signal.

HETEROGENEOUS CATALYSISC7+ Paraffin Isomerisation ProcessHALDOR TOPSOE A/S European Appl. 1,402,947

High-octane gasoline is produced by a selective iso-merisation of C4+ hydrocarbons containing ³ 20 wt.%of C7+ hydrocarbons in the presence of 0.01�5% of Ptand/or Pd catalysts supported on Al2O3-ZrO2 modi-fied with a W oxyanion. The reaction proceeds in H2

with a H2:hydrocarbon ratio of 0.1�5, at 150�300ºC,a total pressure of 1�40 bar and a LHSV of 0.1�30h�1. The feed may optionally also include shorterparaffins, aromatics or cycloparaffins.

Stabilised Alumina Supports for Partial OxidationCONOCOPHILLIPS CO World Appl. 2004/043,852

Stabilised supports (1) stable at > 800ºC are pre-pared by adding a rare earth metal to an Al-containingprecursor prior to calcining. The stabilised Al2O3

catalyst support comprises a rare earth aluminate witha molar ratio of Al:rare earth metal of ³ 5:1. Catalystscomprising Rh, Ru and/or Ir or their combinations,loaded onto (1) are used for synthesis gas productionvia the partial oxidation of light hydrocarbons.

Catalytic Partial Oxidation of HydrocarbonsUNIV. MINNESOTA World Appl. 2004/044,095

C6�C30 hydrocarbons are produced by using a filmof a fuel source that includes at least one organiccompound, on a wall of a reactor. The fuel source iscontacted with a source of oxygen; the mixture of fueland oxygen are then vaporised and contacted with asupported catalyst containing Rh and/or Pt, underautothermal conditions for £ 25 ms. Preferred prod-ucts include a-olefins and synthesis gas.

Exhaust Gas Purification CatalystTOYOTA JIDOSHA KK Japanese Appl. 2004-008,932

The activity of supported Pt metal particles in anexhaust gas purification catalyst is improved by con-trolling the oxidation of the Pt oxide surfaces by usingPt oxide with lower Pt valence than that of stoichio-metric PtOx (0 < x < 2 and x ¹ 1). Electrons areinvolved in the gas adsorption, so unstable gas is eas-ily adsorbed, and its reactivity increases. The cycle ofadsorption and desorption of the gas is shortened andthe amount of adsorbed gas per unit time is increased.

HOMOGENEOUS CATALYSISChiral Organometallic CompoundsAVECIA LTD European Appl. 1,417,030

Chiral organometallic compounds, used in asym-metric synthesis, comprise a non-symmetricallysubstituted cyclopentadiene complexed to Pt, Pd, Rh,Ru, Ir, Co, Fe or Mn, etc. The cyclopentadiene has asecond coordinating group which also complexes theabove metal and is attached to the cyclopentadiene bya chiral connecting chain.

DOI: 10.1595/147106704X1919

144Platinum Metals Rev., 2004, 48, (3)

Alkylidene Ruthenium ComplexesW. A. HERRMANN et al. U.S. Appl. 2004/095,792

Ru alkylidene complexes (1) containing N-hetero-cyclic carbene ligands are claimed and used as highlyactive, selective catalysts for olefin metathesis.Acyclic olefins with ³ 2C atoms or/and cyclic olefinshaving ³ 4C atoms can be made from acyclic olefinswith ³ 2C atoms or/and from cyclic olefins with ³4C atoms by olefin metathesis in the presence of (1)with addition of HCl or HBr, and BF3 or AlCl3, etc.

Carbonylation of Conjugated Dienes SHELL OIL CO U.S. Patent 6,737,542

Carbonylation (1) of conjugated dienes proceeds byreacting a conjugated diene with CO and a hydroxylgroup-containing compound in the presence of a cat-alyst system comprising: a source of Pd cations; aP-containing ligand X1-R-X2; and a source of anionswherein X1 and X2 contain substituted or non-substi-tuted cyclic group of ³ 5 ring atoms, one being a Patom. R is a bivalent organic bridging group and con-nects both P atoms. (1) can be performed batchwise,semi-continuously or continuously.

FUEL CELLSReduction of Ammonia during Fuel ReformingNUVERA FUEL CELLS INC World Appl. 2004/043,851

The formation of NH3 in a fuel reforming process,such as an autothermal reforming process, is reducedby reacting a fuel with air and H2O in a reformingunit containing a Pt group metal catalyst bed, forexample, Pt, Pd, Rh, Ru and/or Ir, to produce a H-containing reformate stream substantially free ofNH3. The operating temperature and pressure for thereforming unit can be controlled, and the amount ofcatalyst should be sufficient to minimise the forma-tion of NH3 in the reformate stream to < 50 ppm.

High Surface Area Material Films and Membranes HEWLETT-PACKARD DEV. CO U.S. Appl. 2004/048,466

Patterns of spikes, bristles, dimples, pores, etc., areproduced on wafers and transferred to film of Ru,Rh, Pd, Os, Pt, Co, Fe, etc., or conductive polymerfilm, such as Nafion, or to biological material film,such as of lipid, protein, enzyme, etc.; by repetitiveprocesses, such as electroplating and embossing. Thisgives low cost, high surface area film of ~ 10 µm inthickness and spikes < 1 µm. High surface area mem-brane is extremely valuable in fuel cells; the patternsmay be used to cast inexpensive fuel cell electrodes.

Catalyst for Oxidising Reformed GasTANAKA KIKINZOKU KOGYO KK U.S. Patent 6,726,890

A catalyst (1) for oxidising reformed gas containingH2 can selectively oxidise CO into CO2 with high per-formance. The CO contained in the H2 fuel for aSPFC, acts as a catalyst poison in the fuel cell. (1)contains a zeolite support, such as M-type mordenite,and a bimetallic alloy of Pt and 20�50% of Ru, Rh,Fe, Co, Mo, Ni and Mn. (1) can convert ³ 60% CO.

ELECTRICAL AND ELECTRONICENGINEERINGHigh-Density Readable Only Optical Disk SAMSUNG ELECTRON. CO LTD European Appl. 1,403,860

A high-density readable only optical disk (1), withlarge storage capacity, includes a substrate with pits;and mask layer(s), with a super resolution near fieldstructure, made from dielectric material of metaloxide, nitride, sulfide, fluoride or their mixture, suchas ZnS-SiO2, and metal particles of Pt, Rh, Pd, Au ortheir mixture. (1) can be obtained without decreasingthe wavelength of a laser diode or increasing thenumerical aperture of an objective lens.

Magnetoelectronics Information DeviceMOTOROLA INC U.S. Patent 6,714,446

A magnetoelectronics information device includesmultilayer structures with spacer layers interposedbetween them. The first and second spacer layers arepartially formed of one of Ru, Os, Rh, Cr, Re and Cu.A pinned magnetic region comprises an antiferro-magnetic layer, formed of IrMn, FeMn, RhMn, PtMnand PtPdMn, and a ferromagnetic layer of Ni, Fe, Mnand Co. Spacer layers interposed between the twomagnetic sublayers provide antiferromagnetic exchangecoupling quantified by a saturation field.

Enhancing Adhesion of a Ruthenium Layer MICRON TECHNOL. INC U.S. Patent 6,737,313

A Ru metal layer is formed on a dielectric layer of aSiO2 layer that has been prior treated with a Sihydride gas, such as silane, disilane or methylatedsilanes. The Si-containing gas treatment enhancesadhesion between the dielectric and the Ru withoutrequiring the addition of a separate adhesion layerbetween the dielectric layer and the Ru metal layer.

Nitride Semiconductor Element NICHIA CHEM. IND. LTD Japanese Appl. 2004-006,991

A nitride semiconductor element (1) has excellentexternal quantum efficiency. (1) is made from a p-type nitride semiconductor layer (2). An electrode (3)containing Rh and Ir having high reflection coeffi-cient is formed on (2). Ohmic contact is achievedbetween (2) and (3). The external quantum efficiencyof (1) is good because the electrode has a high reflec-tion coefficient, therefore reduces the absorption oflight.

Semiconductor DeviceMATSUSHITA ELECTRIC IND. CO LTD

Japanese Appl. 2004-014,716A GaN-based compound semiconductor has a

Schottky electrode (1) of Cu alloy, such as Pd-Cu with£ 20% Cu, or Cu-Pt, Cu-Au, etc. A buffer layer, anundoped GaN layer and a n-type GaN active layer areformed on a sapphire substrate. Ohmic electrodes assource and drain electrodes, and a (1) as gate electrodeare formed on the n-type GaN active layer. (1) has anexcellent Schottky characteristic and high adhesiveness.

To achieve consistent and reliable readingsfrom a thermocouple over its service life (1), it isnecessary to minimise the drift that steadilyreduces the output. Drift is generally caused bycontamination of the Pt limb. Contamination canbe built in, acquired in service or originate fromthe thermocouple�s RhPt alloy limb.

The user must ensure that a thermocouple isnot contaminated before or during service.Cleanliness is essential. It is critical not to build incontamination such as metal swarf derived fromend caps, etc. Burning-out ceramics and wipingwires with industrial methylated spirits are bothgood practices to follow.

The upper operating temperatures of Pt ther-mocouples are sufficiently high to destabiliselower grade ceramics, releasing metallic elementsfrom their oxides. Thus, the preferred insulationfor both twin bore and outer sheaths is high puri-ty recrystallised alumina.

Another source of contamination comes fromthe furnace load; metallic vapours in vacuum braz-ing furnaces can condense on the thermocoupleand cause damage. It is important that the designof the installation protects the thermocouple withsuitable physical barrier layers of metallic andceramic closed end tubes. However, while addi-tional layers prolong the life of the couple, they dothis to the detriment of the response rate andaccuracy. Thick metal cladding, which conductsheat to the furnace wall, can act as a �heat sink�,attracting and condensing contamination.

Rhodium DriftThis is the transfer of Rh from the alloy limb to

the Pt limb. The metal transfer is possible becauseRh oxide is volatile above 1200ºC and the gas dif-fuses or convects to cooler areas where itcondenses. It is not very stable and, once dissoci-ated, the Rh metal contaminates the Pt limb. Rhoxide can often be seen as a black layer coveringthe alloy limb. It indicates the in-service tempera-ture contour from 800 to 1200ºC. (The blackening

can be removed by resistance heating the alloylimb wire to 1450°C in air.)

Reducing convection and diffusion using phys-ical barriers reduces the Rh drift, for example, byusing one-piece ceramics. An alternative approachis to limit the formation of Rh oxide by reducingthe available oxygen. However, very low oxygenpressure can destabilise other harmless oxides,reducing them to harmful contaminants, for exam-ple, silicon.

The rate of Rh drift is dependent on the physi-cal and thermal geometry and the peaktemperature of an installation. It is recommendedthat at first users should recalibrate more fre-quently in order to find the rate of drift and hencethe thermocouple�s life expectancy.

The various forms of contamination or Rh lossresult in wire that is no longer homogeneous alongits length. The lack of homogeneity can cause aproblem when the thermocouple is recalibrated, asthe temperature profile in the calibration furnace isunlikely to be the same as the temperature profilein the user�s furnace. In this case, different lengthsof the wire will be responsible for producing thevoltages during both calibration and service. Thelack of homogeneity can also explain the differ-ences seen when comparing results from differentcalibration methods or from different laboratories.

There are also open circuit faults and compen-sating circuits that use compensating leads. Thesewill be discussed later. For accuracy, users shouldensure that: · the environment is clean· the thermocouples are not stressed· they are annealed before use, and· high quality ceramics are used and mechanicaldamage is avoided. R. WILKINSON


FINAL ANALYSIS

Thermocouples � Minimising Drift

DOI: 10.1595/147106704X3467

Reference1 R. Wilkinson, Platinum Metals Rev., 2004, 48, (2), 88

Roger Wilkinson is a Senior Materials Scientist at JohnsonMatthey Noble Metals in Royston, U.K. He has worked withplatinum thermocouples since 1987 in manufacturing, calibrationand customer technical support.

platinum metals review support from the directors of johnson matthey and rustenburg platinum mines...

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