Fate of methanol under one-pot artificial photosynthesis condition with metal-loaded TiO2 as photocatalysts

• Photocatalytic reaction of methanol and water vapors on M–TiO2 was studied.
• Fifteen different reactions simultaneously take place during this reaction.
• One-pot photoreaction of CO2 and H2O on M–TiO2 has been studied for long time.
• Explains why CH4 and CO are the major products from the above one-pot reaction.
• Explains why CH3OH, HCOH, and HCOOH cannot be intermediates in the above reaction.

The experimental artificial photosynthesis (AP) systems, which have attempted AP by photo-irradiation of single-chamber reactors containing CO2, H2O, and metal-loaded TiO2, have received great attention during the last three decades. Such one-pot AP systems cannot be efficient because the catalysts have water oxidation sites which can oxidize the carbon-containing organic fuels more readily than water. Despite this, methanol has been the most desired product from such AP systems due to its many merits. However, CH4 and CO have been produced as major products under normal conditions (1 bar of CO2, 1 sun, neutral condition, and irradiation wavelength > 350 nm). From a systematic study aimed to elucidate the fate of methanol in such one-pot AP systems with novel metal nanoparticle loaded TiO2 (Mn–TiO2, M = Pd, Pt, Cu and Au) as the catalyst we found that methanol and its related products formaldehyde and formic acid are not produced from such one-pot AP systems, indicating that the gaseous products should be produced from the pathways which do not involve methanol and the less-reduced products as intermediates or side products. We also elucidated that when methanol is added into the AP system, as many as fifteen different reactions take place as shown in Scheme 1. The reaction is initiated by photoinduced excitation of the charge-transfer (CT) band from methanol to TiO2 surface, which appears in the UV region, by the UV part of the solar light. These reactions bear the potential to be used for production of various compounds.

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