Parallel Pathways for Photocatalytic Decomposition of Acetic Acid on TiO₂

ISSN： 0021-9517

Source： Journal of Catalysis, Vol.187, Iss.1, 1999-10, pp. : 230-237

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Abstract

Acetic acid decomposes photocatalytically on TiO₂ at room temperature in an inert atmosphere through two parallel pathways. In one pathway, acetic acid decomposes to gas-phase CO₂ and apparently forms hydrogen and methyl groups, which combine on the surface to form CH₄. In the other pathway, acetic acid extracts oxygen from the TiO₂ lattice to form adsorbed H₂O and gas-phase CO₂ and C₂H₆. The extracted oxygen is replenished by diffusion from the bulk in an inert atmosphere or by gas-phase O₂. The formation of CH₄ and CO₂ in the first pathway does not consume lattice oxygen. The first step in photocatalytic decomposition (PCD) of acetic acid appears to be dissociation of the O–H bond, producing surface acetates. However, molecularly adsorbed acetic acid reacts at the same rate and with the same selectivity as surface acetates. Only the α-carbon forms CO₂ during PCD. When gas-phase O₂ is present, adsorbed methyl groups oxidize before they are hydrogenated to CH₄. The oxidizing agent during photocatalytic oxidation (PCO) is different from that during PCD and is most likely adsorbed oxygen. Adsorbed oxygen reacts with acetic acid in a different pathway from the two reactions observed for PCD, so a Mars Van Krevlen mechanism for PCO appears unlikely. The TiO₂ surface is not homogeneous and some surface sites are more active during both PCD and PCO. Co-adsorbed water increases the rate of CH₄ formation, apparently by reacting with CH_3(ads) to form CH₄, but in contrast to adsorbed O₂, water does not react with acetic acid in a separate pathway that is different from those observed for PCD without water.