Infrared and optical spectroscopic study of Co-Mo-Al 2O 3 catalysts

Abstract

The oxide form of the CoMoAl 2O 3 system and its interaction with CO, H 2O, thiophene, and pyridine have been studied by ir and electronic spectroscopy. From changes induced in the spectrum of Co 2+ on adsorption of the above molecules, it is inferred that, even though a major fraction of cobalt is tetrahedrally coordinated to oxygen, a significant portion occurs on the surface with a trigonal (C 3 v ) coordination and adsorbs these molecules. The presence of molybdenum oxide is necessary for the formation of such trigonal Co 2+ ions since, in its absence, the optical spectrum does not reveal the presence of trigonal Co 2+. Infrared spectroscopy was used to study the nature of the hydroxyl groups on CoAl 2O 3 MoAl 2O 3, and CoMoAl 2O 3. Cobalt was found to exert a major control on the mode of interaction of molybdenum species with the surface hydroxyls of alumina. In the presence of cobalt, reaction of selective OH groups with molybdenum-containing species occurs, leading to the presence of molybdenum in specific sites (probably in parallel rows) on the alumina surface. From the ir spectrum of adsorbed pyridine, it is found that both Brønsted and Lewis acid sites occur on the oxide surface. On sulfidation, Brønsted sites disappear. At least some of the Lewis sites constitute coordinatively unsaturated Co 2+ and Al 3+ ions.