The chemistry of methanol on the surface:: the TiO2 (110) influence of vacancies and coadsorbed species

The chemistry of methanol was explored on the vacuum annealed TiO2(110) surface, with and without the presence of coadsorbed water and oxygen, using temperature programmed desorption (TPD), high resolution electron energy loss spectroscopy (HREELS), static secondary ion mass spectrometry (SSIMS) and low energy electron diffraction (LEED). The vacuum annealed TiO2(110) surface possessed about 8% oxygen vacancy sites, as determined with H2O TPD. Although evidence is presented for CH3OH dissociation to methoxy groups on the vacuum annealed TiO2(110) surface using SSIMS and HREELS, particularly at vacancy sites, the majority of the adlayer was molecularly adsorbed, evolving in TPD at 295 K. Although no evidence of irreversible decomposition was found in the TPD, dissociative CH3OH adsorption at 135 K on the vacuum annealed TiO2(110) surface led to recombinative desorption states at 350 and 480 K corresponding to methoxys adsorbed at non-vacancy and vacancy sites, respectively. Coadsorbed water had little or no influence on the chemistry of CH3OH on the vacuum annealed TiO2(110) surface, however new channels of chemistry were observed when CH3OH was adsorbed on the surface after O-2 adsorption at various temperatures. In particular, O-2 exposure at 300 K resulted in O adatoms (via dissociation at vacancies) that led to increased levels of CH3O-H bond cleavage. The higher surface coverage of methoxy then resulted in a disproportionation reaction to form CH3OH and H2CO above 600 K. In contrast, low temperature exposure of the vacuum annealed TiO2(110) surface to O-2 resulted in low temperature state of O-2 (presumably an O-2(-) species) that oxidized CH3OH to H2CO by C-H bond cleavage. These results provide incentive to consider alternative thermal and photochemical oxidation mechanisms that involve the interaction of organics and oxygen at surface defect sites.