Increase of reducibility of NiO by H2, due to pretreatment with salt solutions

Abstract The impregnation of a high temperature preparation of pure NiO with various salt solutions, followed by drying and extraction with water, affects the reducibility of NiO differently according to the nature of the cation. Whereas no positive effect is observed with Li, Cs, Zn, Cd, Hg, Al, La, Fe, or Co, the rate of reduction in dry hydrogen becomes about twice as much that of the starting NiO for Na, K, Rb, Mg, Sr, Ba, Au, and Ni. This is ascribed to the formation of some disordered superficial areas on NiO. More pronounced effects occur with Cu, Pt, Pd, Ru, Rh, Os, Ir, and are related to a cationic exchange with surface Ni 2+ during impregnation. The effect of most Group VIII a elements changes with the percentage of d character of the metallic bond as does the specific catalytic activity in some particular reactions. A large difference of the thermal stability of artificial nucleation areas formed during the considered treatments is evidenced. From this point of view Pt is much more favorable than Os. Finally, the fraction of platinum which is extractable by water after impregnation of NiO with H 2 PtCl 6 or Pt(NH 3 ) 2 (NO 2 ) 2 gives rise to a metallic phase which increases the rate of reduction only slightly.