Electron-induced decomposition of methanol on the vacuum-annealed surface of TiO2(110)

The 100 eV electron-induced decomposition (EID) of methanol adsorbed on the vacuum-annealed surface of TiO2(110) at 135 K was examined with temperature-programmed desorption (TPD) and electron-stimulated desorption (ESD). By annealing at 850 K, a TiO2(110) surface was reproducibly prepared with an oxygen vacancy coverage of about 0.08 ML (where 1 ML = 5.2 x 10(14) sires cm(-2)). In the absence of electron irradiation. CH3OH adsorbed on the vacuum-annealed surface in three main TPD states: a molecular state at 295 K and two dissociative states at 350 and 480 K. The 480 K state was assigned to methoxyls at oxygen vacancy sites, and the 350 K state was due to methoxyls at non-vacancy sites. The surface coverages in these states for the saturated monolayer were 0.40 ML (295 K), 0.15 ML (350 K) and 0.08 ML (480 K). Although CH3OH dissociated on the surface, no irreversible decomposition was observed, and CH3OH was the only desorption product in TPD. By heating a multilayer CH3OH exposure to 197, 310 and 410 K, followed by recooling to 135 K, methanol adlayers could be prepared containing only the saturated monolayer, only both types of methoxyl and only the methoxyls at vacancies, respectively. Given these preparation conditions, the 100 eV EID of each methanol-related species was examined. Using CD3OD, the major positive ESD ions detected from multilayer methanol were D+, O+/CD2+ and OD+/CD3+ (the latter were mostly O+ and OD+ based on results with CH3OH) with weaker signals from C+, CD+, CO+, DCO+ and CD2OD+. However, the monolayer gave D+ and O+, with weak signals from OD+ and CD+. The EID cross-section for molecularly adsorbed CH3OH (1.7 x 10(-16) cm(2)) was only a factor of three less than the literature values for the total dissociative cross-section in the gas phase suggesting that the TiO2(110) surace had little or no influence on the dissociative ionization process. No carbon-containing surface products were detected in post-irradiation TPD associated with EID of molecular CH3OH, including no additional methoxyl formation. The initial EID cross-sections for the two types of methoxyls were approximately equivalent regardless of the surface condition, but were a factor of 5 greater in the presence of CH3OH (3.0-3.4 x 10(-15) cm(2)) than in its absence (5.8-6.2 x 10(-16) cm(2)). EID of both vacancy and non-vacancy methoxyl resulted in H2CO products bound at the same sites, but vacancy-bound H2CO was resistant to further EID, whereas non-vacancy H2CO was decomposed with further electron exposure. Total D+ ESD cross-sections were several orders of magnitude lower than those measured by post-irradiation TPD, suggesting that the major EID channels involved ejection of neutral species. These results demonstrated the ability of low-energy electrons to active organics adsorbed on oxide surfaces with high cross-sections, and suggest that the EID cross-sections and products for surface organics depend on the coverage, adsorption state and adsorption site as in the case of methanol in TiO2(110). Based on these conclusions, low-energy electrons produced from adsorption of ionizing radiation may play a significant role in the radiocatalytic destruction of organics over oxide catalysis. (C) 1998 Elsevier Science B.V. All rights reserved.