Role of anionic impurities in the formation of the active state of catalysts based on transition metals

Abstract

Experimental data concerning the structure of transition metal-based catalysts are analyzed. Anionic impurities in the oxide precursor markedly modify both its structure and the structure of the catalyst resulting from its reduction. The modifying anionic impurities exert a significant effect on the local environment of the transition metal cations (particularly on that of Jahn-Teller cations) and even on the very possibility of existence of a mixed oxide precursor. In some cases, the changes in the local environment of the cations under the action of modifying ions show themselves as radical changes in the catalyst reduction kinetics. The formation of epitaxial bonding between the particles of the active metal and the surface of the oxide support, as well as the decoration of the particles with an amorphous oxyhydroxide layer, can be favorable for the stabilization of the active metal particles in the reduced state. Presumably, the activation of hydrogen molecules and the substrate being hydrogenated (e.g., CO) occurs on the surface of metal particles completely covered by a thin layer of the amorphous oxyhydroxide. The experimentally observed high activity of these catalysts is unlikely to be solely due to the increase in the specific catalytic activity of the remaining uncovered surface of the metal. It should also be attributed to the high inherent catalytic activity of the metal particles decorated with the oxyhydroxide layer.