Hydrogen formation in the reaction of steam with Rh/CeO2 catalysts:: a tool for characterising reduced centres of ceria

A 3.7% Rh/CeO2 catalyst was prepared by wet impregnation of a low-surface-area ceria (23 m(2) g(-1)) with an aqueous solution of rhodium nitrate. This catalyst was reduced in a flow of H-2 at temperatures T-R ranging from 200 to 1000 degreesC. The number of reduced centres of ceria in Rh/CeO2 catalysts was measured by in situ reoxidation of these centres at T-OX by pulses of water according to the following reaction: Reduced centre + H2O --> Oxidised centre + H-2. The amount of hydrogen, Q(H), evolved in the reaction allows us to calculate the number of reduced centres of the support since the noble metal would not be oxidised. Q(H) remains relatively small for T-R not exceeding 500 degreesC, then increases rapidly to reach a pseudo-plateau at about 850 degreesC. At 900-1000 degreesC, the oxide composition would be close to CeO1.75. Contrasting with the effect of T-R, relatively small variations are recorded with T-OX for a catalyst reduced at 850 degreesC: a maximum value of Q(H) is reached for T-OX close to 850 degreesC, but 74% of this maximum yield in H-2 is already obtained when the reaction with steam is carried out at 100 degreesC. Oxidation by steam was also carried out on a catalyst prereduced in deuterium. It was shown that the hydrogen formed comes from the decomposition of water (H-2) and not from the storage of deuterium in the support during the reduction stage. In situ characterisation by XRD showed that there is a deep perturbation of the cubic structure at 850 degreesC, followed by a reduction of CeO2 into Ce2O3 (hexagonal) at higher temperatures (900-1000 degreesC). A treatment in water restores the cubic structure of CeO2. The support having been previously reduced at 900 degreesC, there is virtually no sintering of the ceria crystallites upon these high-temperature treatments in hydrogen and steam. (C) 2002 Elsevier Science (USA). All rights reserved.