Some Recent Results on Metal/Support Interaction Effects in NM/CeO2 (NM Noble Metal) Catalysts

Abstract In this work, some recent chemical and nanostructural characterisation studies on ceria-supported noble metal (NM/CeO2; NM: Rh, Pd, Pt) catalysts are reviewed. According to the information available, a proposal is made about the nature of the metal/support interaction phenomena occurring in NM/CeO2 catalysts as the reduction temperature is increased. By combining the information provided by the HREM and the hydrogen chemisorption (volumetric adsorption, TPD-MS and H2/D2 isotopic exchange) studies, it is proposed that, for reduction temperatures up to 773 K, the observed partial deactivation effects are mainly due to electronic metal/support interactions. Metal decoration phenomena do also occur on the three NM/CeO2 catalysts. In contrast with that reported for NM/TiO2, however, they have only been observed upon reduction at 873 (Pd) or 973 K (Rh, Pt). At the highest reduction temperature, 1173 K, a crystallographically well-defined alloy phase could only be observed on Pt/CeO2 (CePt5). On Pd/CeO2 a solid solution of Ce in Pd may also be formed. The phases resulting from the reoxidation of the decorated and/or alloyed metal microcrystals have also been characterised by means of HREM. Significant differences with the NM/TiO2 catalysts are observed. The classic reoxidation treatment at 773 K does not allow the segregation of the NM and CeO2 phases, and therefore no complete recovery of the catalytic systems is achieved.