NOx Adsorber catalyst

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A lean NOx trap catalyst for diesel engine comprising (i) a first layer consisting essentially of a mixture or alloy of rhodium and platinum, a cerium-containing support material and an inorganic oxide, (ii) a second layer underneath the first one and (iii) a substrate. The weight ratio of rhodium to platinum may range from 1 : 10 to 10 : 1 and is preferably about 1 : 1 and the total loading of Pt and Rh may range from 17.7 to 1766 g/m3. The cerium-containing support material may be selected from ceria, a ceria-zirconia mixed oxide and an alumina-ceria-zirconia mixed oxide and is preferably cerium oxide. The inorganic oxide may be alumina, ceria, magnesia, silica, titania, zirconia, niobia, tantalum oxide, molybdenum oxide, tungsten oxide and mixed oxides or composite oxides thereof, preferably alumina. The first layer is preferably free of palladium, alkali and alkaline earth metals and rare earth metals other than cerium. Additional layers may be present and any of the layers may form a flow-through or filter substrate. A method of treating exhaust gas from a lean burn diesel internal combustion engine is also claimed.