PNNL

Abstract

The chemistry and photochemistry of methylene bromide (CD2Br2) on the rutile TiO2(110) surface was probed using temperature programmed desorption (TPD). CD2Br2 desorbed in three desorption states at 145, 160 and 250 K assigned to desorption from the multilayer, from an ?1-CD2Br2 species and a bridging ?2-CD2Br2 species, respectively, the latter two bound to five-coordinate Ti4+ surface sites. The 160 and 250 K TPD states saturated at coverages of 1.0 and 0.33 ML, respectively, where 1 ML is equivalent to the surface Ti4+ site density (5.2x1014 cm-2). No thermal decomposition of CD2Br2 was observed on either the clean surface or with preadsorbed O2. UV irradiation of CD2Br2 on TiO2(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidence, for example, by DBr in TPD. The mechanism of CD2Br2 photodesorption from TiO2(110) occurred with a low cross section (~2x10-21 cm2) similar that expected for direct optical excitation of CD2Br2 suggesting that charge carriers generated in TiO2(110) had little or no impact on adsorbed CD2Br2. These results suggest that photocatalytic destruction of halocarbons such as CD2Br2 on TiO2 likely occurs via indirect processes (such as OH radical attack) as opposed to direct mechanisms involving charge carriers generated in TiO2 by bandgap excitations.