Some recent results on metal/support interaction effects in NM/CeO2 (NM : noble metal) catalysts

In this work, some recent chemical and nanostructural characterisation stud ies 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 infor mation provided by the HREM and the hydrogen chemisorption (volumetric adso rption, TPD-MS and H-2/D-2 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 deco ration 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 r eduction at 873 (Pd) or 973 K (Rh, Pt). At the highest reduction temperatur e, 1173 K, a crystallographically well-defined alloy phase could only be ob served 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 HRE M. Significant differences with the NM/TiO2 catalysts are observed. The cla ssic reoxidation treatment at 773 K does not allow the segregation of the N M and CeO2 phases, and therefore no complete recovery of the catalytic syst ems is achieved. (C) 1999 Elsevier Science B.V. All rights reserved.