(708a) Effect of Water On Photocatalytic Decomposition of Formic Acid and Tetrahydrofuran On TiO2
Water can either promote or inhibit photocatalytic reactions. For example, during benzene photocatalytic decomposition (PCD) on TiO2, water at low humidity increased the decomposition rate, but the reaction was inhibited at high humidity. Competitive adsorption between benzene and water for active TiO2 sites appears to cause the rate decrease, but the reasons for rate increase are less clear. Formic acid adsorption on TiO2 was studied to probe the effects and mechanisms by which water accelerates PCD of a volatile organic. Water dramatically increases the PCD rate of formic acid. Similar effects have also been observed for tetrahydrofuran (THF). These effects were studied using infrared spectroscopy and temperature-programmed desorption (TPD). Density functional theory (DFT) calculations were also employed to elucidate the preferred adsorption structure for formic acid on anatase (101) surface.
Density functional theory calculations indicate that water co-adsorbed with formic acid promotes O-H bond dissociation to form formate ions. Coverage of formic acid influences the amount of water required for dissociation. For ¼ ML formic acid coverage, a 1:1 ratio of water to formic acid is sufficient. At one ML formic acid coverage a 2:1 ratio of water to formic acid is required for OH dissociation because additional stabilization through hydrogen bonding is necessary on the more crowded surface. Infrared spectroscopy confirms that water induces dissociation of formic acid to monodentate formate. Furthermore, comparison of infrared spectra with DFT frequency calculations indicates that water converts bidentate formate to monodentate, and preferentially displaces formic acid from the surface.