Selectivity changes during organic photooxidation on TiO2: Role of O2 pressure and organic coverage

The selectivity of trimethyl acetate (TMA) photodecomposition on TiO2(110) as a function of O2 pressure and TMA coverage was probed at room temperature (RT) using isothermal mass spectrometry (ISOMS) and scanning tunneling microscopy (STM). The selectivity of TMA photodecomposition on TiO2(110) is sensitive to the initial TMA coverage and O2 pressure. TMA bridge-bonds to the surface via the carboxylate end of the molecule in a manner consistent with the binding of other carboxylate species (e.g., formate and acetate) on TiO2 surfaces. Under all conditions, photodecomposition of TMA was initiated via hole reaction with the electron in carboxylate's π system, resulting in opening of the OCO bond angle and formation of CO2 and a t-butyl radical by cleavage of the CC bond between these groups. The CO2 product desorbs from the surface at RT, but the t-butyl radical has several options for thermal chemistry. In ultrahigh vacuum (UHV), where the O2 partial pressure is <1×10−10 Torr<1×10−10 Torr, the TMA photodecomposition results in a near 1:1 yield of isobutene (i-C4H8) and isobutane (i-C4H10) from surface chemistry of the t-butyl radicals. STM results show that the reaction occurs fairly homogeneously across the TiO2(110) surface. In the presence of O2, the photodecomposition selectivity switches from initially i-C4H8 to a mixture of i-C4H8 and i-C4H10 and then back to predominately i-C4H8. The latter selectivity change occurs at the point at which void regions form and grow in the TMA overlayer. At this point, the photodecomposition rate accelerates and the reaction occurs preferentially at the interface between the TMA-rich and TMA-void regions on the surface. These results illustrate both the changing dynamics of a typical photooxidation reaction on TiO2 and also how factors such as O2 pressure and TMA coverage impact the photooxidation reaction selectivity. We also present results that suggest the rate of photodecomposition of monodentate carboxylates is greater than that of bidentate (bridging) carboxylates. This implies that the structural arrangement of Ti cation sites on the surface is an important issue that influences photocatalytic rates on TiO2.