PROCESS FOR PURIFYING EXHAUST GASES FROM INTERNAL COMBUSTION ENGINES

1392528 Treating exhaust gas KALICHEMIE AG 2 Aug 1972 [14 Aug 1971 27 Nov 1971] 36050/72 Heading F1B A process for purifying I.C. engine exhaust gases in a catalytic two-bed reactor comprises the steps of heating the first reactor bed by initially adding secondary air to the exhaust gases so that oxidizing conditions prevail in the first reactor bed and then diverting the secondary air to the second reactor bed so that reducing conditions prevail in the first bed and oxidizing conditions in the second bed, the first catalyst bed comprising a support which has been calcined at 1000 to 1500‹C and which contains, by weight, either 0.02 to 0.10% rhodium and 0.05 to 0.2% palladium or 0.005 to 0.05% of each of rhodium and iridium. The ratio of rhodium to iridium can vary between 1:10 and 10:1 and the ratio of rhodium to palladium can vary between 1:0.5 and 1:10. Catalyst support Moulded aluminium oxides optionally containing silica, or aluminosilicates. Honeycomb or corrugated ceramics e.g. cordierite, #-spodumene, mullite, α-alumina or combinations thereof, optionally coated with aluminium oxide. The support is calcined for 10-120 minutes at 1000-1500‹C and then impregnated with solutions of salts of rhodium and iridium or palladium, separately or mixed. The first bed catalyst is dried at 100-200‹C, calcined at 700-900‹C and then reduced in a hydrogen atmosphere at 400-600‹C or in an aqueous hydrazine hydrochloride solution. Artificial ageing in alternate oxidizing and reducing atmospheres for 200 hours at 800‹C showed that the catalysts did not lose their reduction properties.