Photoreactions of Organic Compounds with TiO2 Single Crystal Surfaces

In this work the photoreactions of two organic molecules: acetic acid and ethanol, over rutile TiO2(110), TiO2(011) and the (011) reconstructed surface of TiO2(001) single crystals are presented. Comparison with the corresponding dark reaction is also given. The steady state reaction at 320 K of acetic acid over the {011}-faceted surface of TiO2(001) under UV irradiation indicated the formation of methane, ethane and CO2. These are the expected products of the so-called “Photo-Kolbe” reaction of acetic acid. While the formation of methane was not sensitive to the addition of molecular oxygen and could be carried out in UHV conditions catalytically that of ethane required the continual addition of gas phase molecular oxygen for its photo-catalytic production to occur. The quantum yield of the reaction was computed equal to 0.05, while the depletion layer width and barrier height were found equal to 18 nm and 0.18 eV, respectively. The photoreaction of ethanol conducted over the rutile TiO2(110) surface indicated that, oxidation of ethoxides to acetate species occurs under molecular oxygen within the investigated pressure range (10−6–10−9 torr). In the absence of molecular oxygen (base pressure 2 × 10−10 torr) or in presence of molecular hydrogen with a pressure up to 10−6 torr negligible photo-reaction occurred. Fitting the observed decay of ethoxides upon irradiation with a Langmuir-type analysis, as a function of gas phase molecular oxygen, the binding constant of the later was found equal to ca. 2 × 107 torr−1. This value, together with the initial surface coverage of ethoxide of 0.5 with respect to surface Ti atoms, may indicate that the number of adsorbed molecular oxygen needed for maximum reaction rate is similar to that of the free surface Ti atoms.