Despite the proven properties of the anatase phase of TiO2 related to photocatalysis, detailed mechanistic information regarding a photo-oxidation reaction has not yet been derived from single crystal studies. In this work, we have studied the photo-oxidation of ethanol (as a prototype hole scavenger organic molecule) adsorbed on an anatase TiO2 (101) surface by STM and on-line mass spectrometry to determine the adsorbate species in the dark and UV illumination in the presence of O2 and to extract kinetic reaction parameters under photo-excitation. The reaction rate for the photo-oxidation of ethanol to acetaldehyde is found to depend on the O2 partial pressure and surface coverage, where the order of the reaction with respect to O2 is close to 0.15. Carbon-carbon bond dissociation leading to CH3 radicals in the gas phase was found to be a minor pathway, which is contrary to the case of rutile TiO2 (110) single crystal. Our STM images distinguished two types of surface adsorbates upon ethanol exposure that can be attributed to its molecular and dissociative modes. A mixed adsorption is also supported by our DFT calculations in which we determine similar Energies of adsorption (Eads) for molecular (1.11 eV) and dissociative modes (0.93 eV). Upon UV exposure at (and above) 3×10 −8 mbar O2, a third species is identified on the surface as a reaction product, which can be tentatively attributed to acetate/formate species on the basis of C1s XPS results. The kinetics of the initial oxidation steps are evaluated using the STM and mass spectrometry data.
Unlike thermally driven catalytic reactions by metal nanoparticles, reaction rates in photocatalysis do not scale with either the density of nanoparticles or their size. Because of the complexity of multi-component photo-catalysts in powder form, this lack of correlation, routinely observed for decades, is still not yet understood. In order to explore this phenomenon, H2 production from ethanol over Au clusters with different coverages deposited on single crystal rutile TiO2(110) were studied by scanning tunneling microscopy and online mass spectrometry. There is a non-linear increase of the H2 production with increasing gold coverage. The key determining factor appears to be the Au inter-cluster distance. Increasing this distance resulted in an increase in the normalized production. These results are explained in terms of competition between clusters for excited electrons to reduce H + (of surface OH groups) to H2. It was possible to determine the proportionality factor between the hydrogen production and the number of absorbed photons. A slope close to one is found, which is in line with the "current doubling effect" in electrocatalysis. Moreover, pump probe transient absorption spectroscopy measurements were conducted. Results show that excited electrons transfer from the conduction band of TiO2 to Au particles within the first picoseconds after UV excitation. The fact that Au metal inter-cluster distances directly affect the reaction rate indicates that there is an optimum arrangement between the metal and the semiconductor that could potentially be achieved by nano-structuring.
Photocatalytic oxidation of ethanol over rutile TiO2(110) 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(101) 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 Ti5c 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 300K, 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 6Å along the [001] direction and with a height of ca. 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 cm 2 and 10 -19 cm 2 . The parallel channel of acetaldehyde production has a nonvarying cross-section of ca. 2×10 -19 cm 2 . A schematic description of the two different reaction channels (dehydrogenation and C-C bond dissociation) is given and discussed.
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