BACKGROUND OF THE INVENTION

[0001] This invention relates to a catalyst which is suitable for use in the oxidation of carbon monoxide, and to the production and use thereof.

[0002] Precious metals are known to act as catalysts for the conversion of carbon monoxide to carbon dioxide. For example, US-4839327 discloses catalysts for oxidising carbon monoxide to carbon dioxide comprising an ultra-fine deposit of gold upon metal oxides including MnO2, Fe2O3, CuO, CuMnO2, AI2O3, SiO2, TiO2. In Chem. Commun., 1999, 1373-1374, Taylor et al disclose the use of a mixed oxide of copper and zinc for this purpose.

[0003] International patent publication WO 2004/002247 discloses several catalysts including zinc aluminate, and gold deposited upon a rare earth oxide such as cerium oxide. US-2003/0075193 discloses catalysts in the form of nanoparticles of metal oxides such as Fe2O3, CuO, TiO2, CeO2, Ce2O3, AI2O3, Y2O3 doped with Zr and MnO2 doped with Pd. Chinese patent specification CN-1464058 discloses a complex catalyst system which has three active components (Au, Pt and Pd) and at least 2 promoters (Fe, Cu and, optionally, Zn) on a support made of alumina, silica, bentonite or molecular sieve material.

[0004] Other documents which relate to gold catalysts for the oxidation of carbon monoxide to carbon dioxide include US patent 5,068,217, where the catalyst comprises gold on a porous ceramic carrier material which contains Fe2O3; US publication No. 2003/099586 which focuses on gold Fe2O3; and US patent 4,698,324 which describes the deposition of gold or a mixture of gold with a catalytic metal oxide (Cr2O3, MnO, NiO, Fe2O3, Co3O4, CuO) on a carrier of silica, alumina or magnesia.

[0005] Carbon monoxide is a colourless, odourless and highly toxic compound which is produced in many chemical industrial processes or as a product of incomplete combustion e.g. in furnaces or internal combustion engines, as well as in mines. [0006] A need exists for a catalyst which can be used for the oxidation of carbon monoxide to carbon dioxide at ambient temperature, often in conditions with high humidity, e g in CO2 lasers, submarines, fuel cells, sensors, breathing masks, respirators, motor vehicle ventilation and filters for air conditioners These potential applications are given merely as non-limiting examples of the use of the catalyst of the invention

SUMMARY OF THE INVENTION

[0007] The invention provides a catalyst formed from gold on a carrier which includes at least zinc oxide

[0008] The carrier may be formed from a mixed oxide of zinc and at least one metal

[0009] Preferably, the gold has a particle size less than 100 nanometres (e g below 70 nm), advantageously less than 50 nanometres (e g below 25nm) and desirably less than 10 nanometres (e g 5 nm or lower)

[0010] The amount of gold is preferably at least 0 01 wt-% of the catalyst, advantageously at least 0 05 wt %, desirably at least 0 5 wt % and conveniently 1 wt-% or more The catalysts tend to be more effective as the amount of gold on the carrier increases Preferably, however, for reasons of cost, the amount of gold on the carrier should not exceed 5 wt-% and is desirably no more than 4 wt-%

[0011] Gold is easier to deposit upon substrates that have iso-electric points in the pH range from 6 to 10 At least one other metal is therefore preferably selected so that the mixed oxide has an iso-electric point of from 6 to 10 Pure zinc oxide has an iso-electric point of about 9, which will alter when mixed with another oxide with a different iso-electπc point Suitable other metals for incorporation in the earner therefore include aluminium (which forms the oxide AI2O3 which has an iso-electric point from 8 to 9), cerium (which forms CeO2 with an iso-electric point of 7), titanium (which forms TiO2 with an iso-electric point of 6), zirconium (which forms ZrO2 with an iso-electric point of 6 7), iron (which forms Fe2O3 with an ιsorelectπς point of 6 5-6 9), and copper (which forms cupric oxide with an iso¬ electric point of about 6 5)

[0012] Effective catalysts are those in which the carrier is a mixed oxide of zinc and aluminium or a mixed oxide of copper and zinc

[0013] The mixed oxide may be in the form of a mixture of two oxides of metals with individual crystal structures, for example a mixture of zinc oxide and aluminium oxide

[0014] The relative quantities of zinc and the other metal may vary widely For example, the weight ratio of zinc to the other metal in the mixed metal oxide may vary from 100 1 to 1 100, but preferably the range is from 80 1 to 1 80 Typically the weight ratios of zinc to the other metal will be at least 5 and preferably at least 6 5 parts of zinc to 80 parts of the other metal Higher relative amounts of zinc can be used, e g 10, 20, 30, 40, 50, 60, 70, or 80 parts of zinc to 80 of the other metal Zinc may be present in greater weight proportions than the other metal, for example in ratios of 2, 5, 10, 20, 30, 40 50, 60, 70 or 80 parts to 1 of the other metal

[0015] Zinc to other metal weight ratios in the range of 80-20 1 are typically useful, preferably 25-65 1, desirably 35 45:1, especially weight ratios of about 40 1 These ranges are applicable particularly where the other metal is at least one of aluminium, graphite, silicon, titanium, zirconium, copper and iron

[0016] The aforementioned ranges also apply to possible molar ratios of zinc to the other metal For example the molar ratio of zinc to the other metal may be in the range of 8O- 1 to 1 80, and intermediate values, eg 2 1 to 1 2 Preferably the molar ratio of zinc to the other metal is greater than 1 1

[0017] In carriers composed of oxides of zinc and another metal, zinc is the more important metallic element Although it may be present in amounts as low as 2% by weight of the catalyst, it is preferably present in amounts of at least 5% by weight, preferably at least 10%, desirably at least 15% and advantageously at least 20%, 25 wt% or 30 wt-% The carrier may be composed predpminantly of zinc oxide, and may contain up to 80% by weight of zinc, for example 75 wt-% or preferably 70 wt-%, or conveniently 65 wt-%, or less, such as 50 wt-%, 45 wt-% or 40 wt-%.

[0018] The preferred catalysts may be prepared in any appropriate way e.g. by using co- precipitation, impregnation or inverse deposition-precipitation techniques, as described in the specification of South African patent application No 2003/8981.

[0019] Where co-precipitation is used, gold and the metal oxides, or precursors thereof, are simultaneously precipitated from solution.

[0020] The invention also extends to a method of producing a catalyst of the aforementioned kind wherein an aqueous solution containing at least one water soluble gold compound (for example tetrachloroauric acid, gold cyanide or sodium tetrachloroaurate and other such complexes or compounds) and water soluble salts (for example the nitrates) of zinc and at least one other metal is prepared, using quantities required to produce a desired ratio of gold, zinc and the other metal in the finished mixed metal oxide. The pH of the solution may be increased by the addition of a base such as ammonium or an alkali metal carbonate or hydroxide (e.g. NH4OH, NaOH, KOH, Na2CO3, K2CO3 or Li2CO3) or urea to a point at which gold is precipitated simultaneously with the hydroxides, carbonates, hydroxy- carbonates, oxides (or a combination thereof) of zinc and the at least one other metal. This will normally be in the pH range of 7 to 10, depending on the precursors used for the gold and the other metal of the carrier. The resulting gold-bearing catalyst material is separated from the aqueous liquor, washed and dried. Washing is particularly important where the gold is deposited from chloride-containing solutions.

[0021] In a different approach, which makes use of inverse deposition-precipitation or impregnation techniques, the gold catalyst is produced by depositing gold on a carrier material. The carrier may be prepared in a variety ways including, but not limited to, co- precipitation of metal precursors and physical mixing of metal oxides. Alternatively, any commercially available ?ιnc containing carrier can be used with no particular restriction thereon

[0022] The carrier material generally has a surface area of from 4 to 180 m2/g (as determined by the BETVN2 method (ASTM D3037)

[0023] Gold may be deposited upon the carrier by treating the carrier with an aqueous solution of a gold precursor compound The amount of gold deposited may be varied by varying the relative proportions of gold solution and carrier.

[0024] The gold precursor compound may be selected at least from, gold (I) cyanide [AuCN], sodium tetrachloroaurate (III) [NaAuCI4], potassium dicyanoaurate (I) [KAu(CN)2], gold (III) acethylacetonate [(CHa)2Au(CH3COCH2COCH3) and tetrachloroauric (III) acid [HAUCI4] The gold precursor may be dissolved to form a treatment solution with a gold content in the range of from 1x10"1 to 1x10"4, preferably in the range 1x10"2to 5x10'3M. The treatment solution may initially have a pH below 7, preferably in the range of from 1 to 3. The pH of the treatment solution may be adjusted to a desired value, e g. from 2 to 8.5, by adding a suitable alkali such as sodium carbonate solution. Upon altering the pH the gold species in solution may undergo chemical changes. For example, HAuCI4 produces gold species of the general formula [Au(OH)nCI4-n]" (where n = 1 - 4). The relative proportion of these species is dependent on the degree of hydrolysis that the HAuCI4 species undergoes. As such, the degree of hydrolysis is in part a function of solution pH, with higher pH's promoting further hydrolysis

[0025] The treatment solution and the carrier are preferably brought into contact at an elevated temperature, preferably in the range 20-90°C , ideally 40-800C, conveniently 65- 75°C

[0026] When the pH and temperature of the treatment solution are stabilised the solution is contacted with the carrier Gold is then precipitated from the solution, for example by increasing the pH of the solution to a desired value by adding an alkaline solution of, for example sodium, potassium or ammonium hydroxide or carbonate, or urea This will normally be in the pH range of 4 to 11. The form in which the gold is precipitated will depend upon the precursor used to form the precipitation mixture. For example, it may be precipitated as the hydroxide or deposited as [Au(OH)nCI4.,,]" (where n = 1-4). The pH at which precipitation will occur similarly depends on the nature of the precipitate. [Au(OH)nCI4.n]" (where n = 1 ,2) adsorbs/impregnates at lower pH values (typically in the range 4 to 6.5, preferably 4.5 to 6, e.g. about pH 5). Gold hydroxide however precipitates at higher pH values, (typically from 7 to 10, preferably 8 to 9.5, ideally about 8.5). This is advantageous because high pH values lead to well-dispersed gold with a low particle size in the nanometre range. In addition, the gold species prevalent at high pH values contain less chloride. The presence of chloride species is believed to increase the formation of larger gold particles, thereby decreasing catalytic activity, due to the sintering of the gold particles as a result of the formation of Cl-Au-Cl bridges. Residual chloride may also serve to poison the catalytic active site. The contacting time of the gold containing solution and the carrier material is performed for a suitable period, for example 0.5 to 3 hours, preferably about 1 hour. The solid material is then separated and washed. The washing is preferably carried out with a base, such as aqueous ammonia solution, NaOH or urea, which also assists in the removal of chloride ions. After washing, the catalyst material is dried, preferably at a temperature of from 20 to 1500C, e.g. at 1200C. Further washing with an alkaline solution can be performed on the dried catalyst material to remove excess chloride and other undesired ions, if required.

[0027] The activity of the catalyst may be modified by further treatment of the gold-bearing carrier. The catalytic mechanism of gold is not fully understood, but may depend upon the presence of both metallic gold (Au0) species and positively charged gold species (Au5+). Without wishing to be limited by any theory, the presence of these gold species can be affected by further treatment of the catalyst material. For example, catalyst material obtained by a co-precipitation, impregnation or inverse deposition-precipitation technique may be calcined before use, typically by heating the catalyst material to a temperature from 50 to 5000C (preferably from 100 to 3000C) for a period of from 1 to 5 hours. The effect of calcining is to reduce Au(OH)3 species present in the catalyst to metallic gold Alternatively the catalyst material may be treated in a reducing atmosphere, for example hydrogen or, preferably, carbon monoxide Such treatment also affects the nature of the gold species on the surface of the catalyst

5 DESCRIPTION OF PREFERRED EMBODIMENTS

[0028] The invention is further described with reference to the following examples

Examples 1 - 8 Au-ZnAIOx Catalysts

Carrier Preparation

[0029] A 1 34M aqueous solution of zinc nitrate [Zn(NO3)2 6H2O] and a 0 081 M aqueous I O solution of aluminium nitrate [AI(NO3)39H2O] were mixed together in proportions such that the weight ratio zinc aluminium was 40 1 The solution was heated to a temperature of approximately 800C, and the pH of the solution was adjusted from an initial value of 1 1 to a value of 9 1 by adding an aqueous solution of sodium carbonate while stirring, thereby producing a precipitate of the hydroxides of zinc and aluminium The pH and temperature of 15 the solution were maintained to allow the precipitate to age in contact with the solution for about 5 hours The precipitate was filtered from the solution, washed with high-purity water to remove unwanted ions, and dried in air at approximately 1200C for 16 hours The dried precipitate was then calcined by heating in air at 4000C for four hours The resulting earner material (A) was then ground into a powder Examination of the crystal structure of the 20 material indicated that it consisted of hexagonal zincite, ZnO and amorphous alumina, AI2O3 with traces of zinc aluminium carbonate hydroxy hydrate

[0030] The aforementioned procedure was repeated with different quantities of zinc and aluminium nitrates, and different reaction conditions, shown in Table 1 to produce catalyst carriers B and C with properties as specified in Table 1 For the purposes of comparison, 25 carriers D and E, consisting of zinc oxide and aluminium oxide alone, were prepared by a similar process Table 1: Varying Zv: Al ratio

Catalyst preparation

[0031] A volume of deionised water sufficient to support a sample of carrier A in

suspension was heated to 7O0C. A solution of tetrachloroauric [HAuCU] acid containing

sufficient gold was added to the water. The pH of the solution was adjusted from an initial

pH of 2 to 5.5 by adding a solution of Na2CO3. Having stabilised the temperature and pH,

the carrier was added, and the suspension agitated for a period of 1 hour. The solid material

was separated, washed with a ammonia solution of a desired concentration and dried in air

at 1200C.

[0032] This procedure was repeated using carriers A to E with different quantities of

tetrachloroauric acid, and wash solutions of different ammonium concentration, as tabulated

in Table 2 to produce the catalysts in Examples 1 to 8 in accordance with the invention, and

comparative catalysts in Examples I to V.

[0033] In order to test their catalytic activities, 0.25g samples of the catalysts of Examples

1 to 8, and of Examples I to V, having a particle size fraction of from 700 to 1400μm were

loaded into a continuous fixed-bed micro-reactor and tested under adiabatic conditions,

through which a test gas consisting of a 1% by volume mixture of CO in air with 100% relative humidity vyas passed at a flow rate of 495 ml/min, equivalent to a space velocity (SV)

of 118,800 ml.g'Vh"1 (millilitres of test gas per gram of catalyst per hour). The activity of the

catalyst was obtained by measuring the micro-mols of CO converted per second per gram of

gold on the catalyst (μmol CO.gAu"1.s"1). The resulting rates of conversion of CO to CO2 are

set forth in Table 2.

Table 2: Catalyst preparation ofi carrier materials

[0034] As can be seen from Table 2 gold, deposited upon alumina alone, does not

function as a catalyst for oxidation of CO under the conditions tested. Gold deposited upon

zinc oxide shows some catalytic function, but the best catalytic function is exhibited by gold

when deposited upon a carrier composed of the mixed oxides of zinc and aluminium,

especially when the ratio of zinc to aluminium in the carrier is over 20:1. Also the catalysts

with higher gold loading (2% by weight or more) perform more effectively.

Examples 9 - 14: Au/CuZnOx catalyst

Samples of catalyst material comprising gold on a carrier material consisting of the mixed

oxides of copper and zinc were prepared. Carrier Preparation

[0035] To prepare the carrier materials, solutions of Cu(NO3)2.3H2O and Zn(NO3)2.6H2O with various coppeπzinc molar ratios, as set forth in Table 3, were mixed and heated to 800C, and the pH was adjusted to a value of 8.5 with an aqueous Na2CO3 solution. The pH and temperature were maintained for 5 hours during precipitation. Thereafter, the resulting precipitates were filtered, washed several times with deionised water and dried overnight at 120°C. The powders were then calcined either at 200°C for 1 hour, or at 300°C for 5 hours. The calcined material was then pelletised, crushed and sieved to a particle size range of 500 to 1000 μm.

Catalyst Preparation

[0036] Gold catalysts were produced on the aforementioned carriers by means of inverse deposition precipitation using an aqueous solution of HAuCI4 of desired concentration. This solution was preheated to 700C and the pH adjusted to 8.5 by adding Na2CO3 solution. The carrier materials were then added to separate batches of the gold solution and each was aged for 1 hour at 70°C with pH control. Thereafter, the catalyst materials were filtered, washed several times with deionised water, dried in an oven at 120°C for 16 hours and calcined if necessary.

[0037] Activity tests were performed analogously to those of Examples 1 - 8, with the exception that catalyst samples of 0.5g were tested under both 0 and 100% humidity with a gas flowrate of 60 ml/min.

Effect of Cu:Zn ratios and calcination temperature

[0038] For the purposes of comparison, tests were also carried out on samples of the carrier material, with Cu:Zn ratios of 1:2, 1:1 and 2:1, that had not been treated with gold. Each sample had a very low catalytic activity under dry and humid conditions, less than 0.02 μmol CO.gCat"1.s"1. Table 3:. Effect of calcination temperature and CicZn ratio

[0039] All the samples carrying gold exhibited significant catalytic activity, especially those

that contained zinc in the form of zincite (ZnO) rather than rosasite or aurichalcite as a result

of calcination of the carrier material at higher temperatures. Also, the activity of the catalysts

is increased by 50% to 100% under humid conditions.

Examples 15 - 17: Au/ZnAIOx catalyst

Carrier and catalyst preparation

[0040] A batch of support material was prepared using the method described for Examples

1 to 8, with a Zn: Al ratio of 40:1. The resulting material was treated with gold as described in

Examples 9 - 14, with the exception that the solid-liquid contacting (ageing) time and

temperatures were varied as shown in Table 4.

Ageing

[0041] The effect of ageing time and temperature on catalytic activity is shown in Table 4.

The activity of the catalysts was tested in a CO/CO2 system, as described with reference to

Examples 1 - 8.

[0042] Satisfactory results were obtained for ageing at 700C for a duration of 1 hour. It was

found that the ageing conditions strongly influenced the activity of the catalysts. When the

ageing step was omitted almost no activity was observed. Catalyst preparation and ageing (1 hour) at room temperature produced an inactive catalyst. The gold loading of the catalysts

was also much lower than that of catalysts aged for 1 hour at 7O0C.

Table 4: Effect of ageing

Examples 18 - 22: Au/ZnAIOx catalyst

Carrier and catalyst preparation

[0043] Supports with zinc to aluminium ratios of 20:1, 40:1 , 80:1 as well as 100% ZnO and

100% AI2O3 were prepared according to the method highlighted in Examples 1 - 8. Gold was

deposited onto these carriers as described in Examples 9 - 14.

Effect of zinc to aluminium ratio on catalyst activity and gold loading

[0044] The catalysts were tested for CO oxidation in 100% relative humidity conditions as

described in Examples 1 - 8. Very high CO conversions were obtained when the catalysts

contained zinc and aluminium at a ratio of 40:1. A synergistic effect appears to exist when

the catalyst contains some aluminium. The investigation revealed the optimum zinc to

aluminium ratio to be approximately 40 to 1. Gold on AI2O3 showed poor activity, while gold

supported on zinc oxide performed well — see . Table 5. ' Table 5: Effect of varying Zn to Al ratios on catalyst activity

[0045] The gold adsorption is also affected by Zn to Al ratios and the highest gold loading

was obtained at Zn/AI ratios of 40:1. The alumina catalyst had very low gold loading. This

trend is shown in Table 5.

Carrier and catalyst preparation

[0046] Gold catalysts were prepared by the inverse deposition precipitation method at both

high (pH=8.5, Examples 9 - 14) and low (pH=5.5, Examples 1 - 8) pH values on three

carriers composed of commerciallly available zinc-aluminium oxide (supplied by Sud-Chemie

under product reference No G72D), zinc-oxide (supplied by Zinchem under product

reference No B. P. grade ZnO), and γ-AI2O3 (supplied by Degussa under product reference

No 2010). Various washing techniques were employed and are shown in Table 6. All

catalysts were tested as per the method shown in Examples 9 - 14.

Table 6: Effect of washing at high and low pH preparation on AwZnAlOx

[0047] As can be seen in Tables 6 to 8, catalysts prepared on G72D (Zn/AI 40:1) are more

active than the gold catalysts prepared on the individual metal oxide carriers alone. Gold

loading from solution onto the all the carriers is higher at low pH when compared to high pH

preparation, the inverse is true for CO oxidation activity.

[0048] Ammonia washing is needed to induce reasonable catalytic activity for catalysts

prepared at low pH.

Examples 29 - 37: Au/ZnO impregnated on AI7Qn [0049] In another example of the invention gold was deposited or impregnated onto an AI2O3 carrier impregnated with ZnO. A support was prepared comprising a ZnO layer on AI2O3 using one of the following techniques:

Deposition of Zn as [Zn(NH3)^I2+ on AI7O3

(a) 19.2 g Zn(NO3)2 were dissolved in 296 ml of 2M NH4OH to form the zinc amine complex [Zn(NH3)4]2+ before adding 30 g γ-alumina (particle size between 500 and 1000 μm). The solution was left standing for 24 hours. (pHi = 10.60, pHf = 11.30). After filtration the support was washed with 1 I high-purity water, dried at 12O0C overnight and calcined at 4000C for 4 hours to ensure complete conversion to ZnO.

(b) The procedure was repeated with 1x (see method (a)), 2x and 5x stoichiometric amounts of zinc nitrate. In addition to contacting the zinc complex/alumina by standing, an 'impregnation' method as follows was also used.

19.2 g Zn(NOs)2 were dissolved in 296 ml of 2M NH4OH to form the zinc amine complex [Zn(NH3)4]2+, before adding 30 g γ-alumina (particle size between 500 and 1000 μm). The slurry was contacted at 73 0C and an initial vacuum of 200 mbar (decreasing over time), using a Bϋchi rotary evaporator, until it was dry (approximately 2 hours). Washing, drying and calcining were carried out as described in method (a).

Deposition of Zn as Zn(NO3)?

(C) Zn(NO3)2 was dissolved in 200 ml high - purity water (1x (pHi = 3.78) and 2x (pHf = 3.42) stoichiometric amounts) before addition of AI2O3 (pH values of 4.74 and 4.53 respectively). The mixture was subjected to impregnation described in method (b). Washing, drying and calcining followed as described in method (a).

Deposition of Zn as Zn(OH)? (d) A stoichiometric amount of Zn(NO3)2 was dissolved in 200 ml high - purity water (pH

= 3.30). Alumina was added (pH = 4.56) followed by pH-adjustment to pH = 7 with

2 M NaOH (-55 ml) to precipitate Zn(OH)2. The mixture was contacted with AI2O3

for 24 hours, decanted, followed by washing, drying and calcining as described in

method (a).

(e) A stoichiometric amount of Zn(NO3)2 was dissolved in 200 ml high - purity water (pH

= 3.81); alumina was added (pH = 4.71) followed by pH-adjustment to pH = 7 with 2

M NaOH (50 ml) to precipitate Zn(OH)2. The mixture was impregnated for

approximately 2 hours at 74°C (vacuum 300mbar, decreasing to 200 mbar),

followed by washing, drying and calcination as described in method (a).

Preparation of gold onto support comprising of ZnO on AI2Oa

[0050] The gold catalyst was prepared by the inverse deposition precipitation method

highlighted in Examples 1 - 8.

Table 9: Activity and gold loading on Zn impregnated AhOs

[0051] The results indicate that impregnation with [Zn(NH3)4]2+ gave the highest catalytic activity. Higher catalytic activity was obtained with the presence of ZnO than for Au deposited onto AI2O3 alone.

Examples 38 - 40: Au/ZnAIOx catalyst

Gold Solution Recycling

[0052] Reuse/recycle of the gold containing solution is required, from a cost point of view, as only a portion of the gold is loaded from solution onto the carrier material. Several catalysts (Zn:AI=40:1 , mass ratio) were prepared by recycling the gold solution. The virgin solution consists of tetrachloroauric acid, with the subsequent catalysts synthesised using the spent solution. Prior to each catalyst synthesis the spent solution was adjusted with fresh tetrachloroauric acid, to maintain a constant solution volume and gold concentration (3.94x10"3M).

[0053] Catalysts were prepared by contacting the necessary amount of carrier material and gold containing solution to obtain a maximum gold loading of 1 wt%. The deposition- precipitation conditions employed are similar to those described in Examples 9 - 14. Catalytic activity was determined using 0.05 - 0.1 g of catalyst in a micro fixed-bed reactor under conditions identical to those described in Examples 9 - 14.

Table 10: Activity and loading of catalysts prepared with recycled gold solution

[0054] Catalysts prepared using the recycled gold solution compare well in terms of gold loading and catalytic activity to their freshly prepared counterpart. Recycling of the gold solution used in catalyst synthesis is possible.

Examples 41 - 43: Au/ZnAIOx catalyst

Effect of Gold Concentration

[0055] Catalysts (Zn:AI=40:1 , mass ratio) were prepared using various gold concentrations by an inverse-deposition-precipitation technique, and tested for CO oxidation activity, under conditions similar to those described in Examples 9-14. A commercially available zinc- aluminium oxide (supplied by Sϋd-Chemie under product reference No G72D) was used as the carrier material.

Table 11: Effect of gold cone, on activity and gold loading

[0056] It appears that an increase in gold concentration results not only in a higher gold loading but in an increase in the relative amount of gold loaded from solution onto the carrier material. Conversely an increase in gold concentration results in a decrease in activity on a per gram gold and per gram catalyst basis. Preparation at relatively low gold concentrations (1x10'2 and 5x10'3M) is therefore advantageous in producing highly active material, although the uptake of gold from solution is poor at these low concentrations.

Examples 44 - 49: Au/ZnAIOx and Au/CuZnOx Pre-treatment in reducing atmospheres

[0057] Gold catalysts were prepared by depositing gold onto two carriers composed of a commercially available zinc-aluminium oxide (supplied by Sϋd-Chemie under product reference No G72D) and a commercially available copper-zinc oxide (supplied by Sud- Chemie under product reference No G66B). In each case the carrier was crushed and sieved to a particle size in the range 500-1000μm. Gold was deposited onto the carrier by an inverse deposition precipitation technique as described in Example 9 - 14.

[0058] 5g samples of each catalyst were then contacted in a fluidised bed reactor with reducing atmospheres consisting of a dry 1.45% carbon monoxide-air mixture; a humidified 1.45% carbon monoxide-air mixture, a 10% hydrogen-argon mixture and, for comparison purposes, air, at flow rates of 10 litres/min for the periods and at the temperatures indicated in Table 12. The reductive process caused a change in colour of the catalyst from light yellow or white in the oxidised state, to purple in the reduced state, which can be attributed to the change in oxidation state of the deposited gold from the oxidised state to a mixture of species comprising metallic gold and oxidised species:

Au3+ → Au0 + Au5+

[0059] The activities of the resulting catalysts on the oxidation of CO were compared by using a piston to draw a specific volume of 1.45% CO in air mixture for a duration of two seconds from a pre-filled gas-tight bag over a bed of 200mg of the catalyst packed in a glass tube and retained therein by glass wool plugs. Gas samples were taken at one minute intervals. The emerging gas was analysed using a commercially available carbon monoxide infra-red analyser sold under the trade name Signal 7000GFG. The % conversions of CO to CO2 in the 1st and 8th gas sample are shown in Table 12.

Table 12: Effect of gas treatment

[0060] As can be seen from Table 12, whilst the activities of the untreated catalysts and

catalysts treated in air increase over the 8-gas sample test period, the activities of the

catalysts that were pre-treated in a reducing atmosphere are significantly enhanced in

comparison. This enhancement of activity correlates with an increase in Au(O) species

present in gold deposited upon the carrier.

Examples 50 - 52: Au/ZnAIOx and Au/CuZnOx

Pre-treatment by calcination

[0061] 5g samples of each of the catalysts prepared for Examples 70 - 72 were subject to

calcinations at 200 and 3000C for 2 hours in static air. Temperature programmed reduction

tests on the resulting catalysts confirmed that at 3000C, the gold species are almost

completely reduced to the Au(O) state whereas, after calcination at 2000C, approximately half

the original Au(III) remains.

[0062] The catalytic effects of the samples upon carbon monoxide were tested according to

the procedure used in Examples 70 - 72. The results are shown in Table 13.

Table 13: Effect of heat treatment

[0063] As can be seen from Table 13, heat treatment, especially at a higher temperature, enhances the activities of the catalysts and sustains their performances over the 8-gas sample cycle in comparison with the untreated samples, although low temperature reductive treatments still yield higher activity which is also consistent with the transformation of Au(III) species to Au(O) caused by calcination.