Infrared study of the effects of oxidation/reducation treatments on Pt dispersion in Pt/Al2O3 catalysts

Abstract

Chlorided Pt/Al₂O₃ catalysts have been subjected to a series of consecutive oxidation / reduction cycles and the quality of Pt dispersion after each reduction stage monitored by infrared study of adsorbed CO. Low-temperature (613 or 723 K) oxidation treatments failed to improve Pt dispersion after subsequent reduction. Platinum dispersion was enhanced to an increasing extent with the increasing oxidation temperatures 793, 843 and 893 K. However, this effect was reversed after several oxidation/reduction cycles during which chlorine was lost from the sample. High oxidation temperature (993 K) led to drastic sintering of dispersed platinum. The results are discussed in relation to the essential role of mobile Pt^IVO_xCl_y species on the support surface during the redispersion process. Enhancement in platinum dispersion is discussed in terms of not only increases in the number of exposed platinum atoms but also the redistribution of platinum to provide stronger interactions with the alumina support.