Impact of Solvent on Photocatalytic Mechanisms: Reactions of Photodesorption Products with Ice Overlayers on the TiO2(110) Surface

The effects of water and methanol ice overlayers on the photodecomposition of acetone on rutile TiO2(110) were evaluated in ultrahigh vacuum (UHV) using photon-stimulated desorption (PSD) and temperature-programmed desorption (TPD). In the absence of ice overlayers, acetone photodecomposed on TiO2(110) at 95 K by ejection of a methyl radical into the gas phase and formation of acetate on the surface. With ice overlayers, the methyl radicals are trapped at the interface between TiO2(110) and the ice. When water ice was present, the ejected methyl radicals reacted either with each other to form ethane or with other molecules in the ice (e.g., water or displaced acetone) to form methane (CH4), ethane (CH3CH3), and other products (e.g., methanol) with all of these products trapped in the ice. The new products were free to revisit the surface or to depart during desorption of the ice. Using isotopic labeling, we show that a significant portion (similar to 50%) of methane formed resulted from reactions of methyl radicals with water in the ice. Because the methane formation from reaction of methyl radical and water is highly endothermic, the ejected methyl radicals must be emitted hyperthermally with the reaction occurring during the initial collision of the radical with a neighboring water molecule. Formation of ethane (and other products) likely comes as a consequence of unfavorable methyl radical and water collisions in the ice. Similar results were obtained using methanol ice (instead of water) except that methane and ethane products slowly leaked through the methanol ice overlayers into vacuum at 95 K but not through the water ice overlayers. These results provide new insights into the product formation routes and solution-phase radical formation mechanisms that are important in heterogeneous photocatalysis.