Origin of coverage dependence in photoreactivity of carboxylate on TiO$_{\mathrm{2}}$(110) surface.

Employing scanning tunneling microscopy (STM) and ultraviolet photoelectron spectroscopy (UPS), we have observed a strong nonlinear decay of the reaction rate constant with coverage for the photolysis of trimethyl acetate on TiO$_{\mathrm{2}}$(110). This effect was not linked to intermolecular interactions of TMA but to the accumulation \quad of the coadsorbed bridging hydroxyls (HO$_{\mathrm{b}})$ deposited during (thermal) dissociative adsorption of the parent, trimethylacetic acid (TMAA). Confirmation of the hindering influence of HO$_{\mathrm{b}}$ groups was obtained by the observation that HO$_{\mathrm{b}}$ species originated from H$_{\mathrm{2}}$O dissociation at O-vacancy sites have a similar hindering effect on TMA photochemistry. Though HO$_{\mathrm{b}}$'s are photoinactive on TiO$_{\mathrm{2}}$(110) under ultrahigh vacuum conditions, UPS results show that these sites trap photoexcited electrons, which in turn likely (electrostatically) attract and neutralize photoexcited holes, thus suppressing the hole-mediated photoreactivity of TMA. This negative influence of surface hydroxyls on hole-mediated photochemistry is likely a major factor in other anaerobic photochemical processes on reducible oxide surfaces.