Surface Chemistry of Organic Pollutants: Styrene, Ozone, and Water on TiO2(110)

The modification in reactivity of styrene by ozone and water was investigated on TiO2( 10) as a model for heterogeneous reactions of hazardous pollutants using temperature programmed desorption. Styrene desorbs from TiO2(110) at 180, 270, and 340 K, in peaks tentatively assigned here to multilayer sublimation and desorption from Ti4+ in the first layer and defects (including step edges), respectively. Water and styrene compete for sites when these species are coadsorbed, resulting in a shift of the 270 K styrene desorption peak to 200 K. Styrene, however, is still able to bind to defects. Ozone preadsorption suppresses styrene desorption associated with defects but results in a new styrene desorption feature at 535 K. Ozone also promotes styrene oxidation to formaldehyde (325 K), benzaidehyde (380 K), and styrene epoxide (545 K). Two pathways for the reaction of ozone with styrene were identified. First, ozonolysis of the terminal C=C bond of styrene occurs, yielding benzaidehyde and formaldehyde, analogous to the gas-phase reaction. Second, a surface-mediated reaction converts styrene to styrene epoxide. The saturation behavior and thermal stability of the corresponding oxidizing species suggests it is possibly the result of surface-mediated decomposition of ozone into adsorbed O adatoms. Our results suggest that the heterogeneous thermal chemistry of styrene, and possibly nonpolar organic molecules in general, is dependent on moisture and ozone levels in the air, and that volatile organic compound (VOC) concentrations can be significantly affected by anthropogenic interfaces such as titania-containing paints and coatings.