Methylene bromide chemistry and photochemistry on rutile TiO 2(110)

Abstract

The chemistry and photochemistry of methylene bromide (CD 2Br 2) on the rutile TiO 2(110) surface was probed using temperature programmed desorption (TPD). CD 2Br 2 desorbed in three desorption states at 145, 160 and 250 K tentatively assigned to desorption from the multilayer, from an η 1-CD 2Br 2 species and a bridging η 2-CD 2Br 2 species, respectively. The latter two TPD states presumably involve binding of CD 2Br 2 molecules to the surface through Br coordination at five-coordinate Ti 4+ 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 Ti 4+ site density (5.2 × 10 14 cm − 2 ). No thermal decomposition of CD 2Br 2 was observed on either the clean surface or with preadsorbed O 2. UV irradiation of CD 2Br 2 on TiO 2(110) resulted in predominately photodesorption, with trace amounts of photodecomposition evidenced in TPD. The rate of CD 2Br 2 photodesorption from TiO 2(110) occurred with a low cross section (~ 2 × 10 − 21 cm 2) similar to that expected from direct optical excitation of CD 2Br 2. This observation suggests that charge carriers generated in TiO 2(110) were no more effective in activating adsorbed CD 2Br 2 molecules than would be expected through direct molecular excitation. These findings suggest that photocatalytic destruction of halocarbons such as CD 2Br 2 on TiO 2 may preferentially occur though indirect processes (such as OH radical attack) as opposed to direct electron transfer processes involving charge carriers generated in TiO 2 by bandgap excitation.