New Advances in Gold Catalysis Part 1*

David Thompson

Consultant to Vf0rld Cold Council and Technical Editor; Cold Bulletin

'Newlends', The Village, Whitchurch Hill, Reading, RC8 7PN, UK

As for many aspects of gold science and technology, gold catalysis has unique features. Until recently, goldcatalysis has had an uncertain history, but research work in a number of laboratories has now shown that ifcare is taken in the preparation of well defined supported gold systems then these systems can be unusuallyactive and/or selective for a number of reactions of commercial importance. These include both selective andcomplete oxidation, hydrochlorination and hydrogenation reactions. In fact gold has been demonstrated tobe the element of choice for some catalytic reactions. As a result, both chemical processing andenvironmental applications are foreseen for supported gold catalyst systems. In this, the first of two articles,the current knowledge on the hydrochlorination of ethyne and the low temperature oxidation of carbonmonoxide using supported gold catalysts is reviewed. In the second part, some further examples ofheterogeneous catalysis will be given and the importance of some recently reported early examples ofhomogeneous gold catalysis, in solution, assessed.

Since the beginning of 1996 Gold Bulletin has reviewedthe current status of gold science and technology inmost of its principal areas of application, includingmetallurgy and jewellery; pharmaceuticals anddentistry; chemistry and catalysis; analytical methods;and electronics and electrochemistry, plating andelectrocatalysis. We should also include the use of goldfor decoration of ceramic tableware and decorativeplates. A striking outcome of all these recent articles isthe emergence of a common theme, ie the uniquenature of the properties of gold and its derivatives.A summary of the properties of gold in relation to itsapplications is given in Figure 1 (1). The uniqueness ofgold can be rationalized in terms of its relativisticcontraction - Figure 2 demonstrates the 'relativisticcontraction' for the 6s orbitals of the heavy elements asa function of the atomic number Z. The element gold(Z=79) is at a pronounced local minimum. Electronsin atoms with high atomic numbers are influenced bythe increased nuclear point charge and reach speedswhich are approaching the velocity of light, andtherefore must be treated according to Einstein's theoryof relativity (1, 2). This relativistic effect consists ofthree components, ie s-orbital and, to a smaller extent,p-orbital contraction, spin orbit coupling, and f-orbitalexpansion. The unique yellow colour of gold derivesfrom the relativistic effect. The Van der Waalsinteractions between gold atoms are correlation effects.

(fiiJ' ColdBulletinI998,31(4)

Pyykko had predicted gold - rare gas chemical bondsbefore a compound containing Au-Xe bonds had beenprepared (3). The Au+ ion is not noble in a classicalsense - it is reactive, especially in the gas phase.

Professor Peter Schwerdtfeger has indicated themany apparent anomalies in the physical properties ofgold compared with those of the other elements ofGroup 11 (IB), ie silver and copper, including colour,specific resistivity, electrical heat capacity, meltingpoint and boiling point. Whereas the predominantoxidation states for copper are Cu(I) and Cu(II), andAg(I) predominates for silver, Au(I) and Au(III) are themost stable for gold. There is good agreement betweenthe various methods for calculating the relativisticbond contraction in AU2' Relativistic effects lead toincreased Van der Waals interaction and increasedphysisorption energies on gold surfaces. In a recentpaper (4), the structures of AuX 3 and AU2X6 have beenrationalized.

In addition, it is noticeable that most areas of goldscience are at an earlier stage of evolution than thatalready attained for neighbouring elements in thePeriodic Table, and although some of the potential

* The material for this article was the basis for the first part of a talk

presented at the IPMI Annual Meeting in Toronto, 17 June 1998 and will

be published as part of the Precious Metals 1998 'Proceedings of the

Conference held in Toronto, Ontario, Canada'.

III

Uniqu e Nature of Gold Sc ienc e and Techno logy

Figure 1 Examples illustrating the applications ofgold and its

derivatives in relation to its properties

Field Characteristic s Applications

METALLURGY Incorrupt ibl e JewelleryElectron icsDent istry

CHEMISTRY Au(I), AU(III) dom inant liquid Crysta lsMOCVDLuminescence

MEDICINE

ELECTROCHEMISTRY

CATALYSIS

Gold -prot ein inter action Al1hr itis treatmentChemoth erapy

Oxide depo sits on Active state for go ldsu rface of metal elet rocataly sls

PlatingElectroform ing

Spec ial Topic of th is Repol1

application as fuel cells and some forms of vehiclepollution control can be foreseen as possibilities, aswell as in masks for removing toxic gases such ascarbon monoxide.

Initial results in both homogeneous and supportedhomogeneous catalysis indicate that the field of goldcatalysis as a whole is beginning to progress and isworthy of closer examination. Increased funding forresearch into gold catalysis would be likely to producenew applications. Parallels can be drawn with thegrowing numbers of applications for platinum metalscatalysis achieved in the recent past, and it is reasonableto speculate that a significant proportion of gold salescould in due course be for catalyst manufacture.

1.0

.§~ 0.95

8.~ 0.90;;;~

0.85

Figure 2 'Relativistic contraction' (rre/rnonre) for the 6s orbitals

ofthe heavy elements as a function of the atomic

number Z Cold (Z = 79) representsa pronounced

local minimum (based on reference 1)

applications are at an early stage of development, manysignificant developments can be anticipated in the future.

Whereas in earlier studies gold had been regardedas a useful promoter in some catalytic reactions, it hasnow been demonstrated that in some circumstancessupported gold catalyst systems themselves can be themost active, and this has been rationalized via

theoretical concepts. That gold systems are best forsome reactions has been demonstrated both in thesynthesis of vinyl chloride from acetylene and in thelow temperature oxidation of carbon monoxide.

The methods used to prepare gold catalysts arecritically important and for example co-precipitation issuperior to impregnation, the method widely used forpreparing platinum group metal catalysts. Recentresults indicate that further investigation could lead tonew applications for gold catalyst systems in both thechemical processing and environmental fields.Although much more research is required in order toidentify and realize the full potential of the catalystsstudied to date, use in such important areas of

112

PLATINUM USAGE COMPAREDWITH THAT FOR GOLD

In Figure 3 the marketing position for platinum isindicated in terms of annual demand in 1975 and1997. It can be seen that there have been significantchanges in the spectrum of uses.

With the exception of the traditional area ofmetallurgy, most of the other areas of gold science andtechnology are very underdeveloped. When more isknown about the technical capabilities of goldderivatives the spectrum of uses for this metal couldalso change. A knowledge of gold metallurgy has beenwell advanced for many centuries, as for example isevident for the historical articles we publish in Gold

Bulletin. This knowledge has led to the current wideuse of gold and its alloys in jewellery. It is notsurprising therefore that the predominant industrialuse for gold is in the manufacture of jewellery and thiscurrently dwarfs all other applications as is indicated in

Platinum Usage -1 975 and 1997 (' 000 oz)

DEMAND BYAPPLICATION 1975 1997

Autoc atalyst: gross 360 1,840recovery 0 (370)

Chemical 345 235

Electrical 225 305

Glass 65 275

Investment: small 0 180large 0 110

Jewellery 1.210 2,160

Petro leum 165 170

Other 210 295

TOTAL DEMAND 2,580 5,200

Figure 3 A comparison ofplatinum usagebetween 1975and

1995 Source: 'Platinum 1998; johnson Mattheyplc,

May 1998, andprivate communication

(@' Cold Bulletin 1998,31 (4)

Gold Usage 1997 (tonnes)

Figure 4 Cold usagein its most important applications - from

Cold Fields Mineral Survey Ltd data

HETEROGENEOUS CATALYSISUSING GOLD

Two early reviews of gold catalysis rationalized what wasthought to be the comparatively low catalytic activity ofgold in many applications (5). In 1972 Bond (6) stated:"Although the catalytic properties of gold are surpassedby those of the Group VIII metals, especially palladiumand platinum, possible applications of gold in catalyticprocesses have been widely studied". In 1985, Schwank(7) commented: "In spite of its low intrinsic activity,

HC==CH + HCl---.,. CH2=CHCl

gold can influence the activity and selectivity of GroupVIII metals". Indeed, for many years, gold was known tohave poor catalytic activity in many applications, andgold systems were investigated more in the hope ofboosting or modifying the known catalytic activity ofother metals, including other precious metals (platinumgroup metals). Gold catalysts have been shown to beactive for oxidation reactions, egoxidation of sulfides (8)and dithioalkanes (9), dehydrogenation reactions, egbenzene formation from cyclohexene (10) isomerization,dehydrocyclization and hydrogenolysis reactions, egpentane (11), and oxidative dehydrogenation reactions(6). However, in all these applications, gold systems didnot exhibit any significant catalytic advantages overother metal catalyst systems. Many of these studiesinvolved the introduction of gold to another, moreactive, metal to influence the selectivity of the reactionat the expense of activity.

Recently, however, there has been a re-awakening ofinterest in catalysis by gold species (12), and the reactionsdiscussed here are those where gold has beendemonstrated to have potential advantages, especiallyincreased activity or selectivity. This is prompted by recentdiscoveries that have shown that the observation of lowcatalytic activity with gold is not a universal rule, and thatgold can be the catalytic metal of choice when itdemonstrates the highest catalytic activity and!orselectivity. This has been demonstrated to be the case, forexample in the hydrochlorination of ethyne usingsupported metal chlorides, and in the oxidation of carbonmonoxide to carbon dioxide at ambient temperature.There are also other possibilities and, for example, in theproduction of hydrogen peroxide, where work to date hasbeen predominantly with Pd-based catalysts, supportedon activated carbon, it has been predicted that gold couldbe more active than Pd, Pt, and Ag (13). From aconsideration of the energetics of the reactions involved, itwas concluded that Au and Ag should produce thehighest yields of hydrogen peroxide, and since Au has thehighest activation energy for oxygen dissociation it mayprove to be the best overall.

HydrochJorination ofEthyne

The heterogeneously catalysed addition of hydrogenchloride to ethyne (acetylene) has been used for theindustrial manufacture of chloroethene (vinyl chloride)(5):

Supported metal chlorides have been used for thisreaction, including mercuric chloride supported on

3,328

237

99

226

3,890337

28

4,255

Bar Hoard ing

Gild Loans

Jewellery

Electro nics

Offi cal Coins (Sales)

Other

Total Fabrication:

TOTAL DEMAND

DEMAND BY APPLICATION

Fabricatio n:

Figure 4. Will the same be true in 20 years time?Certainly a wider spread of uses for gold would help toincrease its price stability. Gold jewellery is thepredominant commercial outlet for sales of gold. Mostof the other uses for gold and its derivatives have beendeveloped much more recently. Gold and its alloyshave significant applications in dentistry and golddrugs are used for the treatment of arthritis and havepotential for chemotherapy. The stability of goldmeans that it has found use in electrical circuitry andthe recent developments in gold nanotechnology signalexciting new developments in electronics. Thetechnologies associated with the electroforming of andelectroplating with gold are well known but continueto be improved and developed, for example by usinggold-matrix composite coatings.

What is addressed in the present two articles is thepromise of gold catalysis, and this is one factor whichmay change the balance of gold demand over the nexttwenty years, ie a question we are evaluating is "Willgold catalysis make significant new demands for gold?"We will be considering future prospects rather thanexisting major applications, but I think you will agreethat it is an area with significant promise for increaseduse of gold.

(00' Cold Bulletin 1998, 31(4) 113

Hydrochlorination ofEthyne using GoldCatalyst SystemsActive gold-containing catalysts were prepared byimpregnation by chloroauric acid solutions ontoactivated carbon and the best results were obtained ifthe chloroauric acid was dissolved in aqua regia. Oneunfortunate observation for all metal chloride catalystsfor this reaction is that they deactivate with time onstream when used in a standard fixed bed reactor.However, gold catalyst systems have a lower rate ofdeactivation than would be expected (Figure 7) .Furthermore, the rate of deactivation is significantlydecreased for higher gold loadings, although the lowestgold loadings give the highest initial reaction rate.Surprisingly, the deactivation rate is very temperaturedependent and high rates of deactivation are observedat both high and low temperatures. The lowtemperature deactivation is due to coke deposition,probably as a result of surface polymerization reactionsof chloroethene and ethyne. The highest reaction rates

including: Cu, Ag, Au, Na, K, Rb, Zn, Cd, Hg, Pd,Os, Ce, AI, Mg, Ca, Sr, Ba and are consideredpossible for Pt, Ru, Rh, Ir. The stability of theseacetylides can be expected to be an importantparameter in controlling the activity of these metalsand their salts. From a study of a range of metalchlorides, the activity could be correlated with theelectron affinity of the cation. However, one of themost extensive studies of metal chloride catalysts wascarried out by Shinoda (14) and with more data thesimple correlation with electron affinity was nolonger valid. Shinoda considered that a correlationexisted with the electron affinity divided by the metalvalence (oxidation state).

Hutchings proposed (15) that the standardelectrode potential should be a more suitableparameter for the correlation of catalytic activity andthe correlation is shown in Figure 5. The data fall ona smooth curve and can be satisfactorily representedby a single function. The importance of this, is thatthis correlation can be used predictively, since anymetal cation with a higher electrode potential thanHg2+ would be expected to give enhanced catalyticactivity. On this basis gold cations would be predictedto be the most active catalysts for this reaction.Subsequent research (16) confirmed this prediction(Figure 6) and on an initial rate basis gold catalystsare about three times more active than mercuricchloride catalysts. This was the first example of areaction for which a gold catalyst was found toexhibit the highest activity.

2

MCln + HC==CH + HCl ---.,. MCln.HC==CH.HCl

Figure 6 Correlation ofinitial hydrochlorination activity (mol

HCl mol mctel! h-!) ofmetal chlorides supported on

carbon (453 K, CHSV 1140hi) with standard

electrodepotential. Catalysts contained 5 x 10-4 mol

metal /1OOg catalyst (based on reference 18)

400o:~g 300

]'c 200

~ 75

§.;;; 50

~o 25u

500

100 .-----or------..,.-----.---.....,....,............,...---.

Figure 5 Correlation ofhydrochlorination activity ofmetal

chlorides supported on carbon (473 K, CHSV

150H!) with standard electrodepotential (based on

reference 15)

Active catalysts should therefore be able to formsurface metal-ethyne and metal HCl complexes.Metastable acetylides are known for many metals

carbon which has been used industrially. Mercuricchloride catalysts deactivate quite rapidly underreaction conditions and, in view of the toxicity of thesecatalysts, there have been a number of studies on theuse of alternative catalysts.

Prediction ofthe Enhanced Activity ofGoldCatalystsIn the previous studies, carbon was found to be thepreferred support since when silica was used somepolymerization products were observed. Further studiesindicated that the rate-determining step involves theadditibn of HCl to a surface ethyne complex (5):

114 (00' Cold Bulletin 1998, 31(4)

Carbon Monoxide Oxidation Using GoldCatalystsHaruta et a1 (20, 21) were the first to show that goldcatalysts could be effective at ambient temperature. Henoted that gold catalysts prepared using a conventionalimpregnation procedure were considerably less activethan platinum group metals catalysts prepared in asimilar way. It is probably for this reason that gold wasthen overlooked by many researchers as an oxidationcatalyst (5). Haruta showed that the method of catalystpreparation is very important and that if a co­precipitation procedure is used, high activity catalystsare obtained. The catalysts were prepared fromaddition of an aqueous solution of chloroauric acidand a metal nitrate to an aqueous sodium carbonatesolution. The precipitate was then washed, vacuumdried and calcined at 673 K. In addition, Haruta et a1

(22) have also shown that catalysts can be preparedusing deposition precipitation. Haruta evaluated arange of metal oxide supports (Figure 8) and observedthat the best results were obtained with a-Fez03' witha gold loading of 5%.

Haruta observed that this catalyst was eveneffective at sub-ambient temperatures and activity was

chloroethene selectivity (19). In this way the goldcatalysts can be used for extended reaction periods.This represents a very interesting example of on-linereactivation and may be applicable to other goldcatalysis applications.

Low Temperatare Oxidation ofCarbonMonoxideThe oxidation of carbon monoxide at ambienttemperature is a very important reaction for airpurification systems and breathing apparatus. Theindustrial catalyst used at present is hopcalite (5) which isa mixed oxide of copper and manganese, CuMnZ03, butthis catalyst deactivates quite rapidly and is unsuitable forlong term use. Certain applications require the catalyst tosurvive and remain effective for many weeks, eg in thepurification of air in long duration space travel. There isanother reason for the intense interest in low temperaturecarbon monoxide oxidation and this concerns the lowtemperature performance of car exhaust catalysts. Onstart-up car exhaust catalysts can take a little time tobecome effective, and this is mainly due to the timerequired to heat the catalyst using the exothermicity ofthe combustion reactions. If a catalyst component couldbe incorporated that is effective for combustion at lowertemperatures, then this initial warm-up period could beshortened.

14

Pt

OL-_ J.-_ ....L..._ ....L..._ ....I-_ --'-_ --L_ --Jo

100

~ 200e

400o:~ 300u"

500.-- ,-- ...,-- ...,-- -.-- --.--- .....,..- --.

Figure 7 Correlation between initial catalyst activityand

conversion decay rate (averagelossofconversion (%)for the initial 3 h reaction). Catalysts contained

5x10 4 molmetaJl1 OOg catalyst (based on reference 18)

are observed at 453 K, and the cause of the deactivationat this temperature was elucidated using 197AuMossbauer spectroscopy, which gave the relativeconcentrations of Au (0), Au (I) and Au (Ill) present inthe catalysts. These studies confirmed that deactivationat the higher temperature was due to reduction ofAu(Ill) to metallic gold (5). In addition, Au(I) was alsoobserved to be present in these catalysts. Studies usingcatalysts containing only Au (0), Au (I) and both Au (Ill)and Au(O) (it was not possible to prepare catalystscontaining only Au(Ill)) showed that the rate ofreaction decreased in the order Au (Ill) > Au (I) > Au (0) .

Deactivation is a common operational problem forheterogeneous catalysts and therefore it is important todevise a strategy for reactivation so that the catalyst canbe continued to be used. In the studies with mercuricchloride catalysts it was found that some activity couldbe restored with an off-line treatment of the catalystwith hydrogen chloride at >453 K (5). However, aprinciple cause for activity loss was found to be loss ofmercuric chloride from the catalyst during use (17).This is not the case for the gold catalyst since underthe reaction conditions employed no gold is lost anddeactivation is mainly due to reduction of both Au (Ill)and Au (I) to Au (0) (18). For gold catalysts, off-linehydrogen chloride treatment of the catalyst at >453 Kdid restore some activity but, as such a treatment couldnot oxidize the elemental gold, full activity was notrestored. Treatments of the deactivated catalyst off-linewith chlorine, nitric oxide and nitrous oxide at elevatedtemperatures (>453 K) were effective and the bestresults were obtained using nitric oxide. However, themost significant observation was that nitric oxide couldbe fed together with the reactants, and deactivationwas virtually eliminated, and there was no effect on

(00' Cold Bulletin 1998, 31(4) 115

when reaction temperatures of >328 K are used. Ironoxide is not a commonly used catalyst support forother precious metals and the optimum crystallite sizeis larger (ca 2 - 5 nm) for gold than for many otherprecious metal systems. The reasons for thesedifferences should be further investigated in order tothrow more light on how the gold systems work - thiscould lead to optimization of their use, ie improvingthe activities so far obtained.

Studies by Gardner et a1 (25) confirmed the highactivity of gold catalysts and, in particular, contrastedthe high activity gold catalysts with a supportedplatinum catalyst. They showed that the gold catalystswere more active than the platinum catalyst by a factorof five (Figure 9).

One of the main problems for supported goldcatalysts for this reaction is deactivation. All Au/metaloxide catalysts deactivate during carbon monoxideoxidation, and particularly when carbon dioxide ispresent in the feed. This is of course a major problemfor breathing air applications, since significant carbondioxide concentrations are present (26).

Hutchings et a1 (5) have studied the problem ofdeactivation using their knowledge of the effect ofageing of catalyst precipitation precursors in solutionon the eventual catalyst performance for copper-basedcatalysts. Au/CuO/ZnO, Au/ZnO and the Au/CuOcatalysts were prepared using a co-precipitationmethod in which the initial precipitate is stirred, ieaged, in the solution for 180 min prior to recovery andwashing. These catalysts were found to give sustainedactivity without deactivation during the 800 minexperimental test (Figure 10). The non- Au-containingCuO/ZnO catalyst prepared using the same co­precipitation procedure displays a much lower activityand a general decline in carbon monoxide conversiontogether with oscillations in catalytic activity. Suchphenomena had been noted previously for non-Au­containing catalysts for this reaction (27). Examinationof the catalysts by electron microscopy showed that nomorphological differences were observed between anyparticular used catalyst and its unused counterpart. Acommon characteristic of all the catalysts is thepresence of discrete gold particles which frequentlyexhibit multiple twinning. There is, however, a distinctdifference in the sizes and dispersion of the goldparticles in these different catalysts. The bestperformance was obtained with the Au/ZnO catalystin which much smaller gold particles (mean particlesize is ca 2 - 5 nm) are supported on feathery zinc­containing crystallites.

The method of co-precipitation used is important

42 3T ime (days)

273 373 473 573Tem perature (K)

O~;;==miftft~o

100

~ 75c0

.~

50OJ:>§

25u

Figure 9 The carbon monoxide oxidation activity found for 10

at%AulMnOx (0,_),19.5 wt%PtlSnOx (0,_),and 2 wt% PtlSnOx (h.,.Ii..) at 328 K without

pretreatment (open symbols) andpretreatment in 5vol% carbon monoxide/helium at 323 K (solid

symbols). Rates are expressedin mol {g cat}! s! x10- 6. Under the conditions ofthese experiments a

rate of1.5 x 10- 6 corresponds to 100% conversion

(based on reference25)

Figure 8 Oxidation eff1ciencies for carbon monoxide oxidation

for Aulcx-Fez03 in relation to catalyst temperature in

comparison with other catalysts. 1 Aulcx-Fez03 (AulFe

=1:19), prepared by eo-precipitation, 2 0.5 wt%Pdly-A1z03,prepared by impregnation; 3 gold f1ne

powder; 4 C030 4 ex-carbonate;5 MO; 6 cx-Fez03;

75wt% Aulcx-Fez03' prepared by impregnation;8 5wt% AuIy-A1z03,prepared by impregnation(basedon reference 21)

observed at temperatures as low as 197 K. However,except for a few reports of activity being observed atthese low temperatures, recent research papers haveconcentrated on the use of ambient temperatures ortemperatures up to 353 K. The detailed work ofHaruta has led to an understanding of the improvedcatalytic activity of the eo-precipitated catalysts. He hasfound that this method results in a very uniformdispersion of small metallic gold particles on thesupport. The best results are obtained when theseparticles are ca 5 nm or less in diameter (23, 24) and

116 (00' Cold Bulletin 1998, 31(4)

+§

70 /\ ++ tt + t~'E V+l \A + \1'> 60 .* •• v. •". V<\/ V§u + + +

50 +

40 +0 200 400 600 800

Time T ime 0 11 line (min)

Figure 10 Catalytic performance for carbon monoxide

oxidation at 293 K (0. 45% CD, CHSV = 33,000

hi) for Au/ZnD (D); Au/CuD (0); Au/CuD/ZnD

(b.); and CuD/ZnD (+) (aged 300 min] (based on

reference 5)

100

<f.75 B

§50-~

'">t:0

25u

or..-:~ ""- ...I

273 323 275 323Tem perat ure (K)

Figure 11 Conversion eff1ciencyfor catalystsderived trom

Auy4gZr14 (4)andAusF'eZr14 (B) measured for

ascending {11Ued circles} and descending (open circles)

temperature. Each point representscarbon monoxide

conversion after steady state was achieved (basedon

reference30)

for the attainment of the sustained high activitycatalysts. In the earlier work by Haruta et a1 (21, 28),the catalysts were prepared using a precipitationmethod in which the precipitate was not aged in thesolution, ie the precipitate was recovered immediatelyafter the precipitation step rather than allowing it toremain in contact with the precipitating solution for anextended time period. Earlier studies had shown thatthe ageing of the CuO/ZnO precipitate could becrucial with respect to the formation of an activecatalyst for the hydrogenation of carbon monoxide tomethanol and in general the preferred catalysts areproduced from a precursor that has been aged for atleast 180 min (29). When gold is present in the co­precipitation mixture the initial precipitate that formsis amorphous. During the ageing process theprecipitate gradually crystallizes. The sustained high

(00' Cold Bulletin 1998, 31(4)

activity of these catalysts was evident in that the carbonmonoxide conversion for the 2% Au/ZnO catalyst wastypically >90% over the test period and the rate ofcarbon monoxide oxidation was typically 6 mol/kgcatalyst/h. Although the previously cited gold catalysts,prepared by co-precipitation without an ageing step,generally gave initial carbon monoxide conversionsthat were equivalent, they were known to deactivaterapidly at 293 K and half lives of only 100-200 minwere observed. These results (5) were not optimizedbut they were indicative of the improved catalyticperformance that can be achieved using a method ofpreparation that is a combination of the co­precipitation procedure together with controlledageing of the precipitate.

A more traditional approach to gold alloy catalystpreparation (30) gave systems which are highly activefor carbon monoxide oxidation at around ambienttemperatures (Figure 11). Amorphous metal alloyswith the compositions (at%) AuSFeZfj4 andAu SAgZfj4 were used as precursors for the preparationof gold/zirconia/iron oxide and gold/silver/zirconiacatalysts for low temperature oxidation of carbonmonoxide. The catalysts were prepared by in situ

activation (oxidation) of the glassy metal alloys undercarbon monoxide oxidation conditions at 553 K.Kinetic tests performed in a continuous tubularmicroreactor in the temperature range from 253 to323 K showed that both catalysts are highly active forthe oxidation of carbon monoxide at low temperatures.Carbon monoxide oxidation rates increasedconsiderably if the CO:OZ ratio in the reactant feedwas changed from stoichiometry (2:1) to conditions ofexcess oxygen (1:2).

CONCLUSIONS

We can conclude from the work described in thisarticle that supported gold catalysts are the most activefor the hydrochlorination of acetylene (ethyne), andthat Au/Fez03 is the best catalyst found to date for theoxidation of carbon monoxide at low temperatures.The control of gold particle size to between about twoand four to five nanometres is important (31) and thiscan be achieved using deposition precipitation orco-precipitation techniques.

In Part II of this pair of articles we will considerother heterogeneous reactions catalysed by goldincluding catalytic combustion of hydrocarbons,selective oxidation, hydrogenation of carbon oxides,the reduction of nitrogen oxides with propene, carbon

117

monoxide or hydrogen, and the oxidative decompositionof halogen compounds. We will also consider reactionscatalysed by homogeneous and supported homogeneousgold systems.

Present indications are that catalysis by gold isdistinctly different from catalysis using other preciousmetals, ie some aspects are unique. Although these areearly days, there are a number of areas where gold catalysishas industrial potential. In-depth study of these areas islikely to lead to a better understanding of the effects so farrecorded and to provide the basis for new applications inchemical processing and pollution control. An increasedspectrum of uses for gold would help to stabilize its price.On the basis of what has happened to platinum sales overthe last 20 years it is reasonable to speculate that asignificant proportion of gold sales could soon begin toincrease outside traditional areas.

ACKNOWLEDGEMENTS

The author is grateful for supply of information by DrMasatake Haruta (Osaka National Research Institute,Japan) and for helpful discussions with ProfessorsGeoffrey Bond (BruneI University, Uxbridge, UK) andGraham Hutchings (Cardiff University, UK); parts ofthis article rely heavily on the review by ProfessorHutchings entitled 'Catalysis: A Golden Future' andpublished previously in Gold Bulletin (1998, 29, 123).Figures 2 and 5-11 were re-drawn from the originalliterature during preparation for a review article entitled'Catalysis by Gold' by G.C. Bond and D.T.Thompsonand recently accepted for publication in CatalysisReviews - Science and Engineering. Thanks are also due toDr Lucy Thompson for literature searches and to theIMPI for permission to publish the whole of theToronto talk (see footnote to p. 111).

ABOUT THE AUTHOR

David Thompson has participated in and managedupstream, basic and applied research activities inuniversities (Imperial College, London and Universityof California, Los Angeles), ICI, and johnson Matthey.His published papers are concerned with catalysis andmaterials topics, particularly those involved withprecious metals, as well as with organic and inorganicchemistry. He is Technical Editor of Gold Bulletin andConsultant to World Gold Council. In his freelancework as a chemical consultant he advises on catalyst

118

system design for chemical processing, pollutioncontrol and gas detection. Some of this work is carriedout in collaboration with universities, and he helpsindustrial organizations to optimize their contacts withuniversity research. He has contributed to and assistedin the editing of a new book to be published in 1999entitled 'The Company of the Future'.

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(00' Cold Bulletin 1998, 31(4)