HREM STUDY OF THE BEHAVIOR OF A RH/CEO2 CATALYST UNDER HIGH-TEMPERATURE REDUCING AND OXIDIZING CONDITIONS

This work reports on the HREM study of a series of Rh/CeO2 catalysts reduced at temperatures ranging from 623 to 1173 K. Computer simulation and digital processing techniques were also used to arrive at a finer interpretation of the experimental micrographs. Throughout the whole range of reduction temperatures, images in both profile and planar views show the existence of a well-defined structural relationship between metal and support. The profile imaging technique has allowed us to study the rhodium/ceria interface, It consists of Rh(111) in epitaxial relationship with (111) ceria surface planes. For the whole metal particle two different crystallographic orientations with respect to the support, both compatible with a Rh(111)/CeO2(111) interface, were observed. Metal decoration phenomena have also been found. They could only be observed on catalysts reduced at either 973 or 1173 K, the covering effects being much heavier in the latter case, The metal particle size distributions corresponding to the reduced catalysts show that metal sintering is slight up to 973 K, with mean sizes ranging from 3.5 to 3.7 nm, and much stronger when the catalysts are reduced at 1173 K, mean size: 6.7 nm. Reoxidation studies at 373, 523, 773 and 1173 K, have revealed that upon O-2 treatment at 773 K the metal particles thoroughly transform into Rh2O3, the precise structural nature of which could not be unequivocally established. These studies have also shown that for oxygen treatments up to 773 K, metal redispersion is rather modest, being necessary to heat to 1173 K to induce the spread of the oxidized rhodium phase onto the support and the subsequent redispersion. For the catalyst reduced at 1173 K, the reoxidation at 773 K or below does not lead to the recovery of the metal decoration effects. The ensemble of results presented here are discussed in relation to the nature of the metal/support interaction phenomena occurring in Rh/CeO2 catalysts. The influence of both the reduction and reoxidation treatments on the metal deactivation/regeneration of these catalysts is also discussed.