Oxide solid solutions as catalysts

This review traces the development of oxide solid solutions as catalysts from their first use in the 1960s to their current application in basic and applied research. Oxide solid solutions provide the means to control the properties of catalytically active ions in defined surface environments. When applied to transition metal (TM) ions, interaction with neighbors can be suppressed or progressively developed, depending on the concentration chosen for the active solute and the structure of the insulating matrix selected as solvent oxide. Simple examples are nickel, cobalt and chromium ions in MgO and MgAl2O4. The successful preparation of solid solutions demands a knowledge of the reactivity of solids and the behavior of crystal defects. This is exemplified in the methods described for preparing solid solutions of low and high specific surface area, respectively. Characterization receives detailed attention and the methods specific to oxide solid solutions are illustrated. Emphasis is placed on quantitative determination of surface composition for which X-ray photo- electron spectroscopy is the most widely applicable technique. The acidity and basicity of oxide solid solution surfaces is linked with coordinative unsaturation and this aspect of characterization involves adsorption calorimetry and infra-red spectroscopy. The account of oxide solid solutions as catalysts is divided into two parts. The first comprises studies where solid solutions have been used as model catalysts to identify and compare the catalytic properties of individual TM ions. For this purpose the catalysis of N2O decomposition, CO oxidation and H2-D2 equilibration have long served as prototypical test reactions. These simple reactions enable issues such as the distinctive behavior of isolated ions, pairs and chains to be addressed and matrix effects to be explored. The motivation here is detailed understanding of catalysis on highly characterized microcrystalline oxides. The second catalytic part is broader in scope and focuses to a greater extent on the application of oxide solid solutions as catalysts for reactions of industrial interest. Combustion of hydrocarbons is a high-temperature reaction for which perovskite-structured solid solution catalysts are especially attractive since they accommodate a wide range of TM and main group ions in solid solution. A second sector covered is selective oxidation of hydrocarbons. Oxide solid solutions containing TM ions made an early entry as catalysts for alkene conversion and remained when interest switched to alkanes. The solid solution approach featured strongly in the search for methane coupling catalysts and currently contributes in a new guise in titanium silicalite. The acidity developed at solute sites is the source of activity for carbenium ion catalysis. SAPOs and MeAPOs fall within the solid solution domain as covalently-bonded counterparts of ionic solid solutions. Finally, reduced solid solutions with phase-separated transition metal clusters are effective catalysts for reforming of alkanes.