Surface acidity relations of NiO-SiO 2

Abstract

Solids composed of varying ratios of NiO to SiO 2 were prepared by mechanically mixing hydrated Ni(OH) 2 and silica hydrogel. The acid strengths and concentrations were estimated by the amine titration method on the products of thermal treatment at 500 °C, 600 °C and 800 °C. Most of the products showed an acid strength H o of +2.0 that was independent of the chemical nature or the chemical composition of the solids. However, most of the acid sites are very weak, with characteristic H o values of + 3.3 or less. At about 45 mol.% NiO the minimum acidity is obtained at 500 °C, but at 600 °C and above this solid is characterized by a maximum in the weak acidity and the presence of a small number of strong acid sites ( H o ⩽ + 2.0). A sample prepared by coprecipitating the two components did not differ much from the corresponding mechanically mixed solid, apart from the presence of an abundance of weak acid centres. Owing to the large separation of the moderate acid sites ( H o = + 2.0), which are calculated to have a distribution density of less than 1 site per 100 Å 2, it isdoubtful whether NiO-SiO 2 would be able to catalyse any reactions.