Toward a Comprehensive Understanding of Photocatalysis: What Systematic Studies and Alcohol Surface Chemistry on TiO₂(110) Have to Offer for Future Developments

Abstract

Heterogeneous photocatalytic systems are usually described based on electrochemistry, which the vast majority of interpretations and strategies for optimizing photocatalysts rely on. Charge carrier dynamics are usually in the spotlight, whereas the surface chemistry of the photocatalyst is neglected. This is unjustified, because studies on alcohol photoreforming on metal-decorated rutile single crystals revealed that the electrochemical reaction model is not generally applicable. Hence, many photocatalytic reactions may proceed in a different manner and the thermal chemistry needs to be accounted for. The new mechanism is particularly relevant for reactions in gaseous environments in the absence of solvated ionic species. Here, we compare both mechanisms and highlight their differences and consequences for photocatalysis. Based on alcohol photochemistry, we demonstrate the importance of thermal reactions in photocatalytic mechanisms and the relevance of systematic studies in different environments for a holistic understanding of photocatalysis.

Mechanistic understanding of the thermal-assisted photocatalytic oxidation of methanol-to-formaldehyde with water vapor over Pt/SrTiO3

“…The strict separation of half reactions, often made in photocatalysis, is thus not applicable in that case. 19,58,59 However, we observed a strong dependence of the reaction kinetics on the presence of water being the H + transport medium, and for the Pt loading an optimum Pt amount for every photocatalytic system was derived. 9 Additionally, EPR measurements indicated that Pt NPs capture photogenerated electrons supporting the commonly accepted mechanism of H + reduction at the surface of Pt NPs after surface migration via hydroxyl groups being favored in the presence of water.…”

The Influence of the Reaction Conditions on the Photocatalytic Gas‐Phase Conversion of Methanol with Water Vapor over Pt/SrTiO3 in a Continuously Operated Flow Reactor

“…The depicted reaction pathways are strongly simplified and still under debate. [2,31] increased, whereas the selectivity toward the valuable products formaldehyde and methyl formate decreased. In comparison to earlier reports on the synergistic effect of temperature and light in thermally assisted photocatalytic reactions using Ru/TiO 2 , [8] Cu/TiO 2 , [9] or Pt/TiO 2 [2] photocatalysts, the influence of light irradiation is significant for the conditions used here.…”

“…Thermocatalytic and photocatalytic reactions are marked in red and green, respectively. The depicted reaction pathways are strongly simplified and still under debate [2,31].…”