Influence of rhodium- and ceria-promotion of automotive palladium catalyst on its catalytic behaviour under steady-state and dynamic operation

Abstract

The dynamic behaviour of honeycomb type, unpromoted, and rhodium (Rh) and ceria (CeO 2) promoted palladium/alumina ( PdAl 2O 3 ) automotive catalysts has been tested under constant air/fuel ratios as well as under symmetric and asymmetric cycling of simulated exhaust feed gas. Combined use of FTIR spectroscopy and mass spectrometry allowed simultaneous monitoring of the exhaust components. Light-off tests carried out in the range 150–500°C indicated drastic differences in the conversion of the main target species NO x , CO and hydrocarbons during warm up, depending on the presence of ceria and/or Rh. Best performance with regard to NO conversion under steady feed conditions was observed with the Rh promoted Pd, whereas under cycling, addition of ceria resulted in a further improvement of NO x conversion and lowering of undesirable NH 3 formation. CO conversion was substantially enhanced by ceria addition as well as cycling operation. As concerns the behaviour in hydrocarbon conversion, Rh had a much more pronounced influence than ceria, significantly enhancing the average conversion during light-off. The benefits of λ-cycling were generally lower light-off temperatures for NO, CO and C 3H 8 conversion and an improved catalytic behaviour of the ceria-containing catalysts, especially for higher amplitudes and frequencies.