Catalytic purification of engine exhaust gas

摘要

A combined catalyst-hydrocarbon trap for the treatment of the exhaust gas of an engine to combat air pollution comprises (1) an underlayer comprising adsorber which adsorbs hydrocarbon from the exhaust gas at a lower temperature and desorbs it at a higher temperature, the adsorber comprising zeolite of silica to alumina weight ratio greater than 300, and (2) an overlayer, arranged over the underlayer, which overlayer comprises catalyst on a support which does not adsorb the hydrocarbon preferentially compared to water, the catalyst being catalytically active for the conversion of the hydrocarbon to carbon dioxide and water, the underlayer and overlayer being so arranged that exhaust gas passes through the overlayer to the underlayer for the underlayer to adsorb the hydrocarbon at the lower temperature, and that the hydrocarbon subsequently desorbed at the higher temperature passes back through the overlayer.