State and localization of cobalt, nickel and copper ions adsorbed on titania (anatase)

Abstract

Spectroscopic methods and chemical analysis are used to investigate Co²⁺/TiO₂, Ni²⁺/TiO₂ and Cu²⁺/TiO₂ samples prepared by adsorption of the corresponding ions from alkali solutions on anatase. Titania is characterized by the presence of two kinds of surface hydroxyl groups and both strong and weak Lewis acid sites. The adsorption of copper ions results in blocking of both types of Lewis acid sites whereas after adsorption of cobalt and nickel ions the strong sites only are blocked. In all three cases the hydroxyl groups of anatase do not participate in the process.

The three cations are stabilized in a divalent state on the surface. Cobalt, nickel and part of the copper ions are found to have octahedral surroundings. Introduction of CO to the activated Co²⁺/TiO₂ and Ni²⁺/TiO₂ samples (450 °C, in vacuo) causes the formation of carbonyls of the types Ti⁴⁺—CO, Co²⁺—CO and Ni²⁺—CO. In the case of sample Cu²⁺/TiO₂, Cu²⁺ is reduced to Cu⁺ and carboxylates are observed together with two kinds of Cu⁺—CO carbonyls. The structures of the supported phases and the possible sites of cation adsorption are discussed.