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This report deals with the catalytic reduction of nitrogen oxides in exhaust gases, over mixed NiFe2_xCrx04 oxides in a bulk state or deposited on a pelletized «-alumina carrier. Initial activity tests (laboratory or motor-bench tests) indicate a better activity and selectivity for the ternary Fe203-Cr203-Ni0 oxide, when compared with the three binary combinations (FeCr, FeNi, and CrNi oxides). The effects of composition variations on catalytic properties of the ternary mixed oxide were studied. The mechanism of aging was investigated for the NiFe2/3Cr3/404 composition by TGA, X-ray diffraction, and activity tests after aging; it was concluded that the aging is probably due to chemical reactions between the «-alumina carrier and the nickel contained in the ternary oxide. This reaction is enhanced when the engine exhaust gases contain the most residual oxygen and can be prevented by adding precious metals to the base oxide formula.

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