Effect of incorporation of copper or nickel on hydrogen storage in ceria. Mechanism of reduction

Abstract

Interaction of hydrogen with a series of mixed oxides CeM_x(M = Cu, Ni; 0 < x⩽ 5) precursors of catalysts for hydrogenation reactions has been studied in the 300–1073 K temperature range. In situ XRD, XPS, EPR and thermogravimetric techniques have been used to characterize the processes occurring during the hydrogen treatment, and the amounts of hydrogen occluded in the solids when treated at different temperatures under flowing H₂ have also been determined. The most interesting phenomena have been found to occur in the activation temperature range. They involve a non-negligible expansion of the ceria-based lattice which has been attributed to the high amount of hydrogen stored in the host oxide and the reduction of Ce⁴⁺ to Ce³⁺. The M²⁺ reduction is slowed down, depending on the content of M²⁺ in the ceria lattice: redox processes between Ce⁴⁺, Ce³⁺, M and M⁺ or M²⁺ have been clearly demonstrated. Finally, a reduction mechanism, partly based on heterolytic dissociation of H₂, is proposed. This may be extended to bulk mixed oxides with other transition metals, and to a certain extent to metals of Group VIII (Rh, Pd, Ir, Pt) deposited on ceria.