Study of the promoting influence of transition metals on the reduction of cupric oxide by temperature programmed reduction

Abstract

Temperature programmed reduction (t.p.r.) has been used to study the hydrogen reduction of cupric oxide and the influence on the reduction of transition metals added to the bulk oxide. X.r.d. and X.p.s. were used to characterise the samples.

For cupric oxide containing 2 mol% metal additive, three types of reduction behaviour were indicated. (i)For Cr, Mn, Fe, Co and Ni, a single reduction process some 30 K below that observed for pure CuO. (ii) For Pd and Ru, two reduction processes some 100 K below that observed for pure CuO. (iii) For Pt, Rh and Ir, complex reduction behaviour with the bulk of the reduction occurring some 100 K below that for CuO. For Ag and Au, similar complex behaviour with the main reduction some 30 K below that for CuO.

A detailed study was made of the reduction of CuO containing Pd (0.05–4.0 mol%) and Pt (0.05–10 mol%).

Measured activation energies were essentially independent of the metal additive indicating that rate limiting steps for the reductions involve the growth of metal nuclei.

The promoting effects of metal additives are discussed in terms of their ability (i) to remain within the CuO lattice, (ii) to produce extra nucleation sites and (iii) to increase the concentration of surface hydrogen. Thus the first row transition metals produce extra copper nucleation sites by non-specific distortion of the CuO lattice. For Group VIII and Ib additives, the nucleation sites are provided by the preferential reduction of the additive ion. The Group VIII metals further promote the reduction of CuO by mechanisms involving hydrogen adsorption and spillover.