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 was studied by comparing two Pt/CeO2-Al2O3 catalysts prepared from hexachloroplatinic acid and platinum acetylacetonate. In absence of chlorine, a dispersion value higher than 100% has been measured by irreversible hydrogen chemisorption. To interpret this result, the interaction of H-2 with the support and the two catalysts were studied by magnetic measurements. The variations of the Ce3+ content have given evidence that different redox processes may occur. The hydrogen spillover is clearly observed in presence of platinum with the phenomenon being more pronounced for the ex-acetylacetonate catalyst. The limited amount of hydrogen irreversibly adsorbed at 294 K on the 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 mobility of hydroxyl species responsible for the hydrogen spillover. Moreover, independently to the presence of chlorine, a reduction at 573 K seems to stabilise a significant fraction of Ce3+ ions against reoxidation under air at room temperature. This was not the case for bulk ceria and probably originates from ceria/alumina interactions occurring after reduction. (C) 1999 Elsevier Science B.V. All rights reserved.