Spectroscopic investigation of species arising from CO chemisorption on titania-supported manganese

The nature of manganese species on the surface of TiO2 (anatase) has been investigated by means of XPS, visible absorption spectroscopy, and FTIR spectroscopy of adsorbed CO. The catalysts were prepared by ion-exchange from aqueous manganese(II) chloride solution and impregnation. The application of ion-exchange results in a loading of manganese oxide phase corresponding to a monolayer coverage. On the surface of the ion-exchanged sample two kinds of Mn3+ ions are stabilized, differing in their coordinative saturation and localization. The Mn3+ ions form two kinds of linear carbonyls characterized by absorption at 2194 and 2187 cm(-1), respectively The impregnated catalyst contains a mixture of Mn3+ and Mn2+ species. The latter ions produce a carbonyl band at 2114 cm(-1). The adsorption of CO at room temperature on the catalysts studied results in formation of formate, carbonate, and hydrogen carbonate structures as web. It is found that the formation of formate species is associated with the Mn3+ ions and the possible mechanism is discussed. The stabilization of the hydrogen carbonates is favored by Mn2+ ions. The reduction of the catalyst studied with hydrogen strongly suppresses the adsorption of CO and is indicative of the occurrence of a strong metal-support interaction. (C) 2000 Academic Press.