Selectivity changes during organic photooxidation on TiO 2: Role of O 2 pressure and organic coverage

Abstract

The selectivity of trimethyl acetate (TMA) photodecomposition on TiO 2(110) as a function of O 2 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 TiO 2(110) is sensitive to the initial TMA coverage and O 2 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 TiO 2 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 O C O bond angle and formation of CO 2 and a t-butyl radical by cleavage of the C C bond between these groups. The CO 2 product desorbs from the surface at RT, but the t-butyl radical has several options for thermal chemistry. In ultrahigh vacuum (UHV), where the O 2 partial pressure is < 1 × 10 −10 Torr , the TMA photodecomposition results in a near 1:1 yield of isobutene ( i-C 4H 8) and isobutane ( i-C 4H 10) from surface chemistry of the t-butyl radicals. STM results show that the reaction occurs fairly homogeneously across the TiO 2(110) surface. In the presence of O 2, the photodecomposition selectivity switches from initially i-C 4H 8 to a mixture of i-C 4H 8 and i-C 4H 10 and then back to predominately i-C 4H 8. 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 TiO 2 and also how factors such as O 2 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 TiO 2.