Magnetic study of the interaction of hydrogen with a Pt/CeO2-Al2O3 catalyst: influence of the presence of chlorine

The influence of a chlorinated precursor on the redox properties of ceria w as studied by comparing two Pt/CeO2-Al2O3 catalysts prepared from hexachlor oplatinic acid and platinum acetylacetonate. In absence of chlorine, a disp ersion value higher than 100% has been measured by irreversible hydrogen ch emisorption. To interpret this result, the interaction of H-2 with the supp ort and the two catalysts were studied by magnetic measurements. The variat ions of the Ce3+ content have given evidence that different redox processes may occur. The hydrogen spillover is clearly observed in presence of plati num with the phenomenon being more pronounced for the ex-acetylacetonate ca talyst. The limited amount of hydrogen irreversibly adsorbed at 294 K on th e support for this free chlorine reduced catalyst is sufficient to explain the excessive value obtained for the platinum dispersion. The results show a fixation of chlorine on the supported ceria phase which inhibits the mobi lity of hydroxyl species responsible for the hydrogen spillover. Moreover, independently to the presence of chlorine, a reduction at 573 K seems to st abilise a significant fraction of Ce3+ ions against reoxidation under air a t room temperature. This was not the case for bulk ceria and probably origi nates from ceria/alumina interactions occurring after reduction. (C) 1999 E lsevier Science B.V. All rights reserved.