Magnetic susceptibility measurements applied to the study of the reversible hydrogen adsorption on high surface area CeO2-ZrO2 mixed oxides in presence of precious metals

Article. Published in Physical Chemistry Chemical Physics (PCCP). — 2003. — №5 — P. 3557-3564Magnetic susceptibility measurements were used to investigate the interactions of hydrogen with Ce_xZr_1-xO₂ mixed oxide supported noble metal catalysts (NM = Pd, Rh and Pt). The protocol included the reduction by H2 at 298–973 K followed by desorption cycles up to at least 773 K. This allowed the quantification of the cerium ions involved in the irreversible reduction corresponding to the formation of anionic vacancies in the support and the reversible reduction which is due to hydrogen spill-over. The irreversible reduction was found to occur to some extent already at room temperature. It increases upon insertion of zirconium in the ceria lattice, in relation with the increased mobility of oxygen of the mixed oxides. In agreement with literature, the addition of a NM to the mixed oxides improves the efficiency of the reduction process the effect being far more obvious at lower temperatures. At T > 773 K, the metal does not greatly assist the oxygen removal from the surface. Concerning the reversibility, the composition of the mixed oxide and the nature of the noble metal have no significant influence on the formation of the hydroxyls layer. For low reduction temperature, the number of hydroxyls per unit of surface area is almost constant (4.6 mmol H₂ m^-2) in a wide temperature range (373–623 K) whatever the cerium content, although the later varies by a factor 2. This coefficient was found to increase to 6.6 with a low specific surface area support. It seems that for ceria–zirconia the reversible reduction process involves the formation of hydroxyls both on the surface and in the subsurface. When the reduction is carried out at T > 650 K, the number of hydroxyl groups remains very great at the surface, even if the reversibility slightly decreases. One can think that at these high temperatures, the oxygen mobility in the oxide is high enough to have a continuous flow of oxygen ions coming to the surface, thus continuously creating potential sites to adsorb hydrogen.