2 . 3 . 9 . Oxo - Anion Modified Oxides

In the interaction of two or more catalyst components, two extremes can be found: The formation of a compound or a solid solution constitutes the most intimate and homogeneous type of interaction, while the formation of a non-wetting surface species on an inert support can be seen as the minimal type of interaction. For supported catalysts, the goal is usually the dispersion of the active species; the structural integrity of the support remains unchanged although strong interactions may occur at the interface of the support and the dispersed phase (see Chapter 3.2.5). In this sense, anion-modified oxides such as sulfated or tungstated zirconia or sulfated titania could simply be understood as supported systems, specifically as mounted acids. In a typical preparation route, however, the second component— sulfate, tungstate—is added to the precursor and is already present when the crystalline oxide is formed by thermal treatment. As a consequence, the textural and structural properties—particularly zirconia features a vivid phase chemistry—of the matrix oxide are severely influenced by the additive. If further components such as promoters are added, the situation will become more complex. Although the product might be considered a surface-functionalized oxide, the systems are characterized by a strong interaction of the functionalizing agent and the matrix, leading to a mutual directing of their structures. The final product ideally features only the matrix oxide as crystalline phase. In that, these systems are also distinguished from coprecipitated catalysts such as Ni/Al2O3 and Cu/ZnO, which also feature precursors with intimate mixing of the components, but the final product obtained after a reduction step consists of separate phases, which are individually detectable by diffraction. Oxo-anion modified oxides can thus be viewed as a separate class of catalysts. 2.3.9.1.2 Motivation for Selection of System