Patent ID: 7175822

Claim:
A method for catalytic reduction of nitrogen oxides in exhaust gases which comprises supplying and subjecting fuel to combustion with periodic rich/lean excursions wherein rich conditions have an air/fuel weight ratio of 10-14 and lean conditions have an air/fuel weight ratio of 20-40 and mainly under the lean conditions and contacting the resulting exhaust gases with a catalyst which comprises: (A) an outer catalyst layer comprising at least 80% by weight of an outer layer catalyst component wherein the outer layer catalyst component consists essentially of a first catalyst component consisting essentially of a mixture or a composite oxide (except mixtures or composite oxides of cerium oxides and zirconium oxides) of (a) ceria and (b) at least one oxide of at least one element selected from zirconium, gadolinium, terbium, samarium and lanthanum, and 0.05–3% by weight of a second catalyst component consisting essentially of (c) at least one catalyst component selected from rhodium and its oxides, and (B) an inner catalyst layer comprising at least 50% by weight of an inner layer catalyst component wherein the inner layer catalyst component comprises (a) 0.05–5% by weight in terms of metals of at least one catalyst component selected from platinum, rhodium and their oxides, and (b) a carrier material.