SELECTIVE CATALYTIC OXIDATION OF CO IN PRESENCE OF H2

Abstract: A catalyst which comprises gold, a transition metal oxide to which the gold is complexed and tin oxide captured on an oxide support which comprises a mixture of cerium oxide and titanium dioxide. The tin oxide is present in an amount of less than 0,4 % by weight of the catalyst. The catalyst has particular application in selectively oxidising carbon monoxide in a carbon monoxide/hydrogen mixture.

Description: SELECTIVE CATALYTIC OXIDATION OF CO IN PRESENCE OF H2 BACKGROUND OF THE INVENTION This invention relates to catalytic oxidation. Fuel cells are electrochemical devices for continuously converting chemical energy into direct-current electricity. The cell consists of two electronic- conductor electrodes separated by an ionic conducting electrolyte with provision for the continuous flow of fuel, oxidant and reaction product into and out of the cell. The fuel may be gaseous or a liquid; the electrolyte liquid or solid; and the oxidant is gaseous. The electrodes are solid, but may be porous and contain a catalyst. Fuel cells differ from batteries in that electricity is produced from chemical fuels fed to them as needed. One of the fuels which is used in a fuel cell is hydrogen. A contaminant in many sources of hydrogen is carbon monoxide which tends to poison the electrodes. Carbon dioxide, on the other hand, does not poison the electrodes of a fuel cell and it is therefore desirable to oxidise as much of the carbon monoxide to carbon dioxide before such hydrogen is fed to the fuel cell. One of the problems encountered in oxidising carbon monoxide to carbon dioxide is that there is simultaneous oxidation of the hydrogen. Selective oxidation of the carbon monoxide is difficult to achieve. SUMMARY OF THE INVENTION According to the invention, a method of selectively oxidising carbon monoxide in a carbon monoxide/hydrogen mixture comprises contacting the mixture with a suitable oxidant, generally oxygen, in the presence of a catalyst which comprises gold, a transition metal oxide, preferably cobalt oxide, to which the gold is complexed, and tin oxide captured on an oxide support comprising a mixture of cerium oxide and titanium dioxide, the tin oxide being present in an amount of less than 0,4% by weight of the catalyst According to another aspect of the invention, there is provided a catalyst which comprises gold, a transition metal oxide, preferably cobalt oxide, to which the gold is complexed, and tin oxide captured on an oxide support comprising a mixture of cerium oxide and titanium dioxide, the tin oxide being present in an amount of less than 0,4% by weight of the catalyst The complexing of the gold and the transition metal oxide includes both chemical and physical bonding Some such complexing of the tin oxide and the gold may also be present BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a graph showing CO oxidation of a gas containing CO, H 2 and O 2 , and Figure 2 is a graph showing H 2 oxidation of a gas containing CO, H 2 and O 2 DESCRIPTION OF EMBODIMENTS It has been found that the gold-based catalyst described above is particularly effective in the selective oxidation of carbon monoxide in a mixture of carbon monoxide and hydrogen "Selective" means that the oxidation of the carbon monoxide takes place in preference to that of the hydrogen. Some oxidation of the hydrogen will occur. The oxidation of the mixture will take place in the presence of an oxidant such as oxygen and will typically take place at a temperature in the range 80 to 130°C. The ratio of cerium oxide to titanium dioxide in the oxide support will generally be in the range of 4:1 to 5:1 on a weight basis. The catalyst of the invention is further characterised by containing tin oxide. It is important that this oxide is present in a relatively low amount, i.e. 0,4 percent by weight of the catalyst or less. Preferably, such oxide is present in an amount of 0,03 to 0,3 percent by weight of the catalyst. The oxide support may be made by providing a particulate mixture of the cerium oxide and titanium dioxide and, drying the mixture. The mixture will be in finely particulate form and will typically have a surface area of approximately 80 to 200 m 2 tg. The catalyst of the invention may be made by methods described in the art. Preferably, however, the catalyst is made by a method which includes the steps of providing a mixture of cerium oxide and titanium oxide, impregnating the mixture with a transition metal solution and optionally a tin solution, drying the impregnated oxide mixture, heating the impregnated oxide mixture at a temperature of at least 250°C in the presence of oxygen, contacting the thus treated oxide mixture with a gold solution at a pH above 12, decreasing the pH of the gold solution to a pH in the range 7 to 8,5, causing gold to precipitate on to the oxide mixture, optionally drying the thus treated oxide mixture and heating the treated oxide mixture at a temperature of at least 250°C in the presence of oxygen. An example of a preferred catalyst of the invention has the following composition (all percentages by weight) 83 % Ce0 2 15 % TιO 2 0,3% SnO 2 1 % Au 1 % Co, as an oxide The catalyst may be made as follows 42g of CeO 2 and 8g TιO 2 are mixed for 10 minutes in an automatic mixer The mixed oxides are dried at 120°C for 30 minutes and impregnated with 22,5ml of 0,4 molar Co(NO 3 ) 2 and 10,5ml of 0,1 molar SnCι 2 solutions at 50°C The so formed sludge is dried at 120°C for 12 hours and impregnated with 21 l of 0,13 molar HAuC/ The material is washed with water and dried at 120°C for 3 hours and calcined at 400°C for 30 minutes The catalyst is preferably made as follows Stage One Powdered cerium oxide and titanium oxide are accurately measured into a 100ml sample bottle and mixed in a mixer mill for 10 minutes This mixture is heated at 120°C for 30 minutes Solutions of cobalt nitrate and tin nitrate are introduced into a vessel and heated to 80°C The heated mixture of cerium and titanium oxides, the support for the catalyst, is then added under constant stirring The pressure in the mixing vessel is reduced and water is distilled off at a constant temperature of 65°C over a 4 hour period The oxide support impregnated with cobalt and tin nitrates is dried at 120°C for 2 hours The dried oxide support is sieved through a 35 micron mesh sieve and then heated under air at 300°C for 6 hours This has the effect of converting the cobalt and tin nitrates to their oxides Stage Two Hydrogen tetrachloroaurate solution is added to 500ml potassium hydroxide solution at pH 13 The combined solution has a pH of just below 13 The temperature of the solution is increased to 60°C and the preheated oxide support impregnated with the cobalt and tin oxides is added to form a slurry The pH of the slurry is reduced to 8 by addition of 0,1 M HNO 3 Magnesium citrate solution (six times the mole equivalent of the hydrogen tetrachloroaurate) is added to the slurry at a constant rate over a 30 minute period The slurry is maintained at pH 8 and 60°C over a 60 minute period The slurry is filtered producing a filtered catalyst The filtered catalyst is re- slurned in de-ionised water and stirred for a further 30 minutes before being filtered and washed with de-ionised water The catalyst is then dried at 120°C under air for 2 hours The dried catalyst is sieved to -35μm and calcined at 450°C under 5% 0 2 for 3 hours The catalyst is stored prior to use The selective oxidation of carbon monoxide in a mixture containing 0,5 percent CO, 2 percent H 2 , 0,8 percent O 2 , the balance being nitrogen, was evaluated using a catalyst of the type described above, designated FC6, and a similar catalyst, except that the tin oxide was omitted, designated FC2. The oxidation of CO is shown in Figure 1 and the oxidation of H 2 is shown in Figure 2. It will be noted from the graphs that the carbon monoxide oxidation of the catalyst without the tin oxide achieved excellent carbon monoxide oxidation, but also significant hydrogen oxidation, particularly at higher temperatures. In contrast, at higher temperatures, the oxidation of the carbon monoxide in the presence of the catalyst of the invention approached that of the other catalyst but with significantly lower hydrogen oxidation.

Claims: CLAIMS 1. A method of selectively oxidising carbon monoxide in a carbon monoxide/hydrogen mixture comprises contacting the mixture with a suitable oxidant in the presence of a catalyst which comprises gold, a transition metal oxide, to which the gold is complexed, and tin oxide captured on an oxide support comprising a mixture of cerium oxide and titanium dioxide, the tin oxide being present in an amount of less than 0,4% by weight of the catalyst. 2. A method according to claim 1 wherein the transition metal oxide is cobalt oxide. 3. A method according to claim 1 or claim 2, wherein the ratio of cerium oxide to titanium dioxide is in the range 4:1 to 5:1 on a weight basis. 4. A method according to any one of claims 1 to 3, wherein the tin oxide is present in an amount of 0,03 to 0,3 percent by weight of the catalyst. 5. A method according to claim 1 , wherein the catalyst has the composition (all percentages being by weight): 83 % CeO 2 15 % TiO 2 0,3% SnO 2 1 % Au 1 % Co, as an oxide. 6. A method according to any one of the preceding claims wherein the oxidant is oxygen. A method according to any one of the preceding claims wherein the oxidation takes place at a temperature in the range 80 to 130°C A catalyst comprising gold a transition metal oxide to which the gold is complexed. and tin oxide captured on an oxide support comprising a mixture of cerium oxide and titanium oxide, the tin oxide being present in an amount of less than 0,4% by weight of the catalyst A catalyst according to claim 8, wherein the transition metal oxide is cobalt oxide A catalyst according to claim 8 or claim 9, wherein the ratio of cerium oxide to titanium dioxide is in the range 4 1 to 5 1 on a weight basis A catalyst according to any one of claims 8 to 10, wherein the tin oxide is present in an amount of 0,03 to 0,3 percent by weight of the catalyst A catalyst according to claim 8, which has the composition (all percentages by weight) 83 % CeO 2 15 % TιO 2 0,3% SnO 2 1% Au 1 % Co, as an oxide A method according to claim 1 , substantially as herein described A catalyst according to claim 8 substantially as herein described

Inventors: GREYLING FOURIE, GRIGOROVA BOJIDARA, MELLOR JOHN, PALAZOV ATANAS, GREYLING, FOURIE, GRIGOROVA, BOJIDARA, MELLOR, JOHN, PALAZOV, ATANAS
Applicants: ANGLO AMERICAN RES LAB PTY LTD, ANGLO AMERICAN RESEARCH LABORATORIES (PROPRIETARY) LIMITED
Application year: 2000
Main classification: B01J23
IPC codes: B01J23, C01B3, C10K3, H01M8