EFFECT OF INCORPORATION OF COPPER OR NICKEL ON HYDROGEN STORAGE IN CERIA - MECHANISM OF REDUCTION

Interaction of hydrogen with a series of mixed oxides CeM(x) (M = Cu, Ni; 0 < x less than or equal to 5) precursors of catalysts for hydroge nation 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 d ifferent temperatures under flowing H-2 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 cer ia-based lattice which has been attributed to the high amount of hydro gen stored in the host oxide and the reduction of Ce4+ to Ce3+. The M( 2+) reduction is slowed down, depending on the content of M(2+) in the ceria lattice: redox processes between Ce4+, Ce3+, M and M(+) or M(2) have been clearly demonstrated. Finally, a reduction mechanism, part ly based on heterolytic dissociation of H-2, is proposed. This may be extended to bulk mixed oxides with other transition metals, and to a c ertain extent to metals of Group VIII (Rh, Pd, Ir, Pt) deposited on ce ria.