INFLUENCE OF RHODIUM-PROMOTION AND CERIA-PROMOTION OF AUTOMOTIVE PALLADIUM CATALYST ON ITS CATALYTIC BEHAVIOR UNDER STEADY-STATE AND DYNAMIC OPERATION

The dynamic behaviour of honeycomb type, unpromoted, and rhodium (Rh) and ceria (CeO2) promoted palladium/alumina (Pd/Al2O3) automotive cata lysts has been tested under constant air/fuel ratios as well as under symmetric and asymmetric cycling of simulated exhaust feed gas. Combin ed use of FTIR spectroscopy and mass spectrometry allowed simultaneous monitoring of the exhaust components. Light-off tests carried out in the range 150-500 degrees C indicated drastic differences in the conve rsion of the main target species NOx, CO and hydrocarbons during warm up, depending on the presence of ceria and/or Rh. Best performance wit h regard to NO conversion under steady feed conditions was observed wi th the Rh promoted Pd, whereas under cycling, addition of ceria result ed in a further improvement of NOx conversion and lowering of undesira ble NH3 formation. CO conversion was substantially enhanced by ceria a ddition as well as cycling operation. As concerns the behaviour in hyd rocarbon conversion, Rh had a much more pronounced influence than ceri a, significantly enhancing the average conversion during light-off. Th e benefits of lambda-cycling were generally lower light-off temperatur es for NO, CO and C3H8 conversion and an improved catalytic behaviour of the ceria-containing catalysts, especially for higher amplitudes an d frequencies. (C) 1998 Elsevier Science B.V.