Adsorption of CO, CO₂, H₂, and H₂O on titania surfaces with different oxidation states

The adsorptive properties of titania surfaces with different oxidation states were probed by temperature-programmed desorption (TPD) of CO, H₂, CO₂, and H₂O. Auger electron spectroscopy and X-ray photoelectron spectroscopy revealed that vacuum annealing an oxidized titanium foil at temperatures from 300 to 800 K was an effective means of systematically varying the average surface oxidation state from Ti⁴⁺ to Ti²⁺. Carbon monoxide weakly adsorbed (desorption energy of 44-49 kJ·mol^-1) in a carbonyl fashion on coordinatively unsaturated cation sites. Titania surfaces were inert with respect to H₂ adsorption and dissociation. Carbon dioxide adsorbed in a linear molecular fashion at exposed cation sites or as monodentate carbonates at surface oxygen anions (desorption energy of 45 kJ·mol^-1). A constant fraction (ca. 0.8) of the total adsorbed CO₂ dissociated upon heating, releasing CO in two distinct desorption peaks at 240 and 330 K. The atomic oxygen produced in this process remained on the surface. Activation energies for CO₂ dissociation were estimated to be 58 and 81 kJ·mol^-1 corresponding to the peak temperatures of the two reaction-limited desorption peaks. Water adsorbed both molecularly and dissociatively, with molecular hydrogen evolution becoming more extensive on surfaces which were initially more reduced. These results are discussed in terms of the role of titania oxidation state in CO hydrogenation over titania-supported metal catalysts.