Relationship between hydroxyl groups on oxide surfaces and the properties of supported metals. Progress report, June 1, 1992--January 31, 1994.

摘要

Supported metal catalysts are commonly prepared by depositing catalytic precursors from aqueous solutions of electrolytes onto high surface area oxides. A general conclusion of our previous studies was that the performance of the finished catalyst depends on the characteristic properties of the hydroxyl inventory on the surface of the oxide support, both in wet and in (pseudo)-dry conditions. Hydroxyl groups serve as adsorption or exchange sites during catalyst preparation. On the other hand, the configuration of hydroxyl groups still remaining on oxides after dehydration determines the acid-base characteristics of the catalyst, which is an important catalytic property. The purpose of the investigation is to characterize the relationship between the complex inventory of hydroxyl groups at oxide surfaces, the acid-base properties of oxides (both in aqueous solution and in the pseudo-dry state) and the resultant effects on the properties of catalytic materials formed by adsorption/impregnation onto these hydroxylated supports during catalyst preparation. We use a common crystallographic model to describe the local configuration of hydroxyl groups on both the pseudo-dry surface and -the oxide/aqueous solution interface. This allows us to extend the concept of structurally determined intrinsic heterogeneity of pseudo-dry surfaces (as already known from the IR spectra of isolated surface hydroxyls) to the oxide/solution interface. We examine the consequences of that heterogeneity upon the impregnation step during catalyst preparation.