Small pore molecular sieve supported copper catalysts durable against lean/rich aging for the reduction of nitrogen oxides

A method of using a catalyst comprising exposing a catalyst to at least one reactant comprising nitrogen oxides in a chemical process comprising exhaust gas treatment, wherein the catalyst comprises copper and a small pore molecular sieve framework having a maximum ring size of eight tetrahedral atoms selected from the group of Framework Type Codes consisting of CHA, LEV, ERI and DDR, the chemical process undergoes at least one period of exposure to a reducing atmosphere, the catalyst has an initial activity, and the catalyst has a final activity after the at least one period of exposure to the reducing atmosphere, wherein the final activity is within 10% of the initial activity at a temperature between 250 and 350° C.