CATALYSIS WITH PALLADIUM DEPOSITED ON RARE-EARTH-OXIDES - INFLUENCE OF THE SUPPORT ON REFORMING AND SYNGAS ACTIVITY AND SELECTIVITY

The influence of the support has been tested on the reactivity of Pd/rare earth oxides catalysts (La2O3, CeO2, Pr6O11, Nd2O3, Tb4O7). According to BET surface area, chemisorption, temperature-programmed reduction (TPR) and oxidation (TPO), X-ray diffraction (XRD) and X-ray photoemission (XPS) characterizations, these catalysts have been classified into three classes according to their ability to create anion vacancies: (i) oxides of the type Re2O3 which are unreducible, (ii) CeO2 where anion vacancies can be created extrinsically by the reduction process, and (iii) Pr6O11 and Tb4O7 where anion vacancies exist due to the nonstoichiometric nature of these oxides. We emphasize also the role of chlorine, coming from the palladium precursor salt, which reacts with the support to form a stable oxychloride phase surrounding the metallic particle and interacting with it. concerning the catalytic activity, (i) the active site is purely metallic in methycyclopentane hydrogenolysis, with small selectivity changes on fluorite oxides as compared to Pd/Al2O3 catalysts due to some electronic interaction with the support, but (ii) the mechanism is found to be partly bifunctional in 3-methylhexane aromatization with a large increase in aromatization on Pr6O11 and Tb4O7 supports, and (iii) in syngas conversion, production of high alcohols occurs at the metal-support interface and is favored by the presence of intrinsic anion vacancies on Pr6O11 and Tb4O7 supports. A correlation is found between the density of anion vacancies on these supports and the chain growth probability deduced from the Anderson-Schulz-Flory plot.