Preparation of alumina-supported ceria .2. Measurement of ceria surface area after impregnation with platinum or rhodium

The surface area of cerium oxide was tentatively determined in model three-way catalysts using (i) a methodology based on the exploitation of the hydrogen temperature-programmed reduction (TPR) profiles and (ii) the adsorption of CO2 on the hydroxyl groups of alumina followed by FTIR spectroscopy. These two methods were performed on a ceria-alumina support after impregnation with RhCl3 and H2PtCl6 and calcination under nitrogen at 773 K. Whereas the two methods gave agreeing results on the initial support, the results were not so straightforward for catalysts. In this case, the TPR profiles are deeply modified with respect to the support. Due to the presence of platinum and rhodium, the reduction peak corresponding to the ceria surface reduction is shifted toward lower temperatures. It also includes the quantity of hydrogen necessary for the reduction of the precious metal oxide. After examining the metal mean oxidation degree, it has been possible to calculate the ceria surface area in the catalyst. A 20-30% decrease was found compared to the initial support. On the contrary, with the CO2-FTIR method, the calculated ceria surface area increases after impregnation. It can be attributed to the fact that the density of OH species responsible for CO2 adsorption is modified by the presence of chlorine introduced during the preparation. These results are discussed and compared to those for the support alone. A reexamination of the TPR curves has permitted the extent of ceria surface not in contact with precious metal to be taken into account. It appears that 10 to 30% of the initial ceria surface would remain uncovered by the precious metals after impregnation. Finally, the TPR method can be considered as the most reliable and potentially rich source of informations on the catalyst surface state. (C) 1997 Academic Press.