Two-stage catalysis of engine exhaust

A method of removing nitric oxide from a hydrocarbon burning engine exhaust stream, the engine operating under varying load conditions from a high load to a low load wherein the exhaust stream temperature varies from a high of about 800.degree. C. to a low of about 200.degree. C., comprising:

• contacting a combusted gas stream comprising nitric oxide and oxygen with ammonia, the contacting over a first catalyst bed having a catalyst comprising a zeolite which catalyzes a reductive reaction of ammonia with nitric oxide, the amount of ammonia from about 0.5 to about 2 times the stoichiometric amount for the reductive reaction of ammonia with nitric oxide, the contacting at a temperature between about 200.degree. C. to about 800.degree. C., the first catalyst bed retaining ammonia when the temperature is above about 315.degree. C. and desorbing a portion of the ammonia at a temperature from about 200.degree. C. to about 315.degree. C., to form an effluent gas stream having oxygen, a reduced amount of nitric oxide content, and having the desorbed portion of ammonia when said contacting is at a temperature from about 200.degree. C. to about 315.degree. C.;
• and
• contacting said effluent gas stream with a second catalyst bed comprising a porous solid carrier having a catalytically active oxidation catalyst metal disposed thereon which is sufficiently active to catalyze the oxidation of the desorbed portion of ammonia to form a substantially nitric oxide free and ammonia free exhaust stream.