Mechanism of Ethanol Photooxidation on Single-Crystal Anatase TiO2(101)

Abstract

Despite the proven properties of the anatase phase of TiO2 related to photocatalysis, detailed mechanistic information regarding a photooxidation reaction has not yet been derived from single-crystal studies. In this work, we have studied the photooxidation of ethanol (as a prototype hole-scavenger organic molecule) adsorbed on the anatase TiO2(101) surface by STM and online mass spectrometry to determine the adsorbate species in the dark and under UV illumination in the presence of O2 and to extract kinetic reaction parameters under photoexcitation. The reaction rate for the photooxidation of ethanol to acetaldehyde was 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 the formation of CH3 radicals in the gas phase was found to be a minor pathway, which is contrary to the case of the 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 determined similar energies of adsorption (Eads) for the molecular (1.11 eV) and dissociative (0.93 eV) modes. Upon UV exposure at (and above) 3 × 10-8 mbar O2, a third species was identified on the surface as a reaction product that can be tentatively attributed to acetate/formate species on the basis of C 1s XPS results. The kinetics of the initial oxidation steps were evaluated using the STM and mass spectrometry data.