Switch in photocatalytic reaction selectivity: The effect of oxygen partial pressure on carbon-carbon bond dissociation over hydroxylated TiO₂(1 1 0) surfaces

Photocatalytic oxidation of ethanol over rutile TiO₂(1 1 0) in the presence of O₂ have been studied with scanning tunneling microscopy and on-line mass spectrometry to elucidate the reaction mechanisms. The O₂ partial pressure has a direct impact on C-C bond cleavage, resulting in a shift of selectivity in gas phase products from acetaldehyde (dehydrogenation) to methyl radicals (C-C bond dissociation) with increasing pressure. This differs from the behavior of anatase TiO₂(1 0 1) single crystal, where at all investigated pressures negligible C-C bond dissociation occurs. The prevalence of the methyl radical species at high oxygen pressures is correlated with an increase in the surface population of an adsorbed species bound to Ti_5c after the reaction, which are identified as formate moieties. Parallel XPS C1s, Ti2p and O1s further confirmed the assignment of surface population, by STM, to ethoxides at 300 K, in dark conditions (C1s at 286.7 and 285.4 eV attributed to -CH₂O- and -CH₃ groups respectively). After photoreaction, a large fraction of the surface was covered by formates (XPS C1 at 289.7 eV). This also correlated with the STM assignment where species spaced by 6 Å along the [0 0 1] direction and with a height of ca. 1.1 Å attributed to formates. Moreover the profile for CH₃ radical desorption in the gas phase as a function O₂ partial pressures correlated with the increasing surface population of formates. Analysis of the rate of methyl radical formation reveals fast and slow regimes, with photoreaction cross-sections between 10⁻¹⁷ cm² and 10⁻¹⁹ cm². The parallel channel of acetaldehyde production has a non-varying cross-section of ca. 2 × 10⁻¹⁹ cm². A schematic description of the two different reaction channels (dehydrogenation and C-C bond dissociation) is given and discussed.