Reduction of model steam reforming catalysts: Effect of oxide additives

This paper is part of a series on the reduction of NiO catalysts and reports results for 4-15 wt.-% NiO, supported on alpha-Al2O3 and containing small amounts of additive oxides, such as SrO, La2O3, CaO, and MgO. Rupture of Ni-O bonds by H-2 was measured by H-2 consumption, both isothermally and with temperature programmed reduction, and growth of Ni atoms into metallic crystallites with isothermal magnetization techniques, Chemical reduction follows a shrinking core model, whereas crystallite growth is best described with a nucleation model in which diffusion of NiO atoms from reduction sites to nucleation centers controls the rate. The rates of both these processes are impeded by adsorbed H2O. Hydrophilic additives retain surface H2O, even when low levels of these additives are present, thereby slowing reduction rates so that reduction must be carried out at significantly higher temperatures. A model is presented to explain these effects and to rationalize the order for reduction temperature: none < SrO < La2O3 < CaO < MgO.