Infrared spectroscopic study of the adsorption of hydrogen and carbon monoxide on highly dehydroxylated thoria

Abstract

Hydrogen adsorption on thoria activated at 973 K (ThO₂-973) leads to weak bands at 1115 and 860 cm^–1 and a shoulder at 910 cm^–1 due to partially reversibly adsorbed bridged species Th—H—Th. The uptake is very small. The adsorption of CO on ThO₂-973 turns the disc orange-yellow and gives rise to a large number of species, some of which are reversibly adsorbed. These species are characterized by a band at 2171 cm^–1 and are coordinated to Lewis-acid sites. In addition to the carbonate and formate species, at least four types of irreversibly adsorbed species are formed. Three of them are characterized by bands in the 1200–1000 cm^–1 frequency range and can be assigned to v(CO) vibrations of ^–O—CC—O^– species; another possibility is the formation of CO^2–₂ species because of the strong basicity of thoria. The irreversibly adsorbed species are very sensitive to O₂ and H₂O but are not reduced by H₂. Irreversibly adsorbed hydrogen species react with reversibly adsorbed CO species or CO gas, leading to species similar to those formed on adsorption of HCHO.