EFFECTS OF PROMOTER OXIDES ON THE REDUCTION OF NICKEL-OXIDE

Bulk nickel oxide samples promoted with 2-3 wt.% CuO, Ag2O, Al2O3 and ZrO2 were tested for reducibility in hydrogen using three independent techniques: magnetization measurements, thermal gravimetric reduction and temperature-programmed reduction. All three showed the same trend. Reducibility, as measured by the extent of reduction at a given temperature, follows the order CuO almost-equal-to Ag2O > none > Al2O3 > ZrO2. X-ray diffraction indicated the absence of any promoter crystallites larger than 2.5 nm. This and the variation of derived lattice strain parameters are consistent with their successful incorporation into the NiO lattice. Lattice parameters and crystallite sizes were essentially the same. A model is proposed in which the initial stage of nickel oxide reduction is nucleation of nickel clusters, aided or hindered by the presence of more or less easily reduced cation-oxygen bonds in the surface. The conflict about the role of silver in promoting reduction of nickel oxide has been resolved. A steam reforming-type catalyst was tested for activity during self-reduction with and without promotion by Ag2O. Moderate improvement in reducibility occurred but not when the catalyst was precalcined at 400-degrees-C, showing that decomposition to silver metal destroys the effect.