Characterization of the acid-base properties of the TiO2(110) surface by adsorption of amines

The adsorption of ammonia, dimethylamine, and ethylamine on stoichiometric and slightly defective TiO2-(110) surfaces was studied by means of TPRS and XPS. The slightly defective surfaces were prepared by exposing a clean stoichiometric surface to electron irradiation, resulting in the creation of point defects (oxygen vacancies) while minimizing the structural damage inflicted to the surface. Adsorption is proposed to take place on this surface by the binding of the nitrogen atom in the amine to a Till cation. On the stoichiometric surface, the amines adsorbed and desorbed primarily intact, with a small fraction (5-8%) nonselectively decomposing into CO, N-2, and H-2 in the cases of dimethylamine and ethylamine. On the defective surfaces, the adsorption behavior of the amines was not significantly altered, as they still adsorbed and desorbed mainly intact. The observed coverage on the defective surfaces was smaller (3-19%) than that on the stoichiometric surface, suggesting that adsorption on the point defects blocks adsorption at more than one neighboring Ti4+ cation. The activation energies for desorption of the amines on the TiO2(110) surface were shown to correlate with the gas-phase basicities of these species, reflecting the Lewis acid character of the Till cations.