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CO adsorption on both clean and hydroxylated TiO 2 -B (100) surfaces with terminal and bridging hydroxyl groups is investigated via first-principles density functional theory calculations. The adsorption mechanisms of CO molecules on both clean and hydroxylated surfaces are discussed. CO molecules preferentially adsorb at five coordinated Ti sites of TiO 2 through C atoms. The calculated adsorption energies range from 13.11 to 43.03 kJ/mol. Moreover, lower concentrations of CO gas can strongly bind to the surface. From structure point of view, CO molecules interact with the surface mainly via its 2π* state. The adsorption is accompanied by electron transfers (0.02À0.08 e) between the CO molecule and the surface. Both the terminal and bridging hydroxyl groups can slightly facilitate CO adsorption, however, in different levels. When CO molecules adsorb near the bridging hydroxyl groups (E ads = 43.56 kJ/mol), it can increase more CO adsorption than the terminal hydroxyl groups (E ads = 33.87 kJ/mol). Furthermore, our calculations indicates that the surface donates electrons to the CO molecule when the latter is adsorbed near the bridging hydroxyl groups, which is different from observations made for adsorption onto the clean surface and near the terminal hydroxyl groups. This unique mechanism provides a possible explanation for the larger increment in CO adsorption near the bridging hydroxyl groups.

A previous study showed that TiO(2)-B (100) surface is very unique. It is characterised by high activity and a loose structure. In this study, we studied the adsorption of ammonia on TiO(2)-B (100) surface at coverages ranging from 1/6 ML to 1 ML using ab initio density functional calculations. We also investigated the adsorption of an isolated ammonia molecule on TiO(2)-B (001) surface to compare the different activities of TiO(2)-B (100) and (001) surfaces towards NH(3). The results showed that the TiO(2)-B (100) surface is more reactive towards NH(3) molecule than TiO(2)-B (001) surface, and the Lewis acid site on TiO(2)-B (100) surface is more acidic. The decrease rate of the average molecular adsorption energy of NH(3) with coverage on TiO(2)-B (100) surface is substantially lower than that on a rutile (011) surface above 1/2 ML coverage due to the open structure of TiO(2)-B (100) surface. The average molecular adsorption energy shows a linear dependence on the coverage of y = 111.0 - 36.3x on TiO(2)-B (100) surface. The possibility of NH(3) molecule onto the Ti(5c) site is nearly equal to forming a dimer with adsorbed NH(3) on TiO(2)-B (100) surface at 5/6 ML coverage.

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