Photocatalytic H₂ Evolution from Oxalic Acid: Effect of Cocatalysts and Carbon Dioxide Radical Anion on the Surface Charge Transfer Mechanisms

Photocatalytic reforming of carboxylic acids on cocatalyst-loaded semiconductors is an attractive process for H₂ generation that has been studied for years. Experimental support for the surface reaction mechanisms, nevertheless, is still insufficient. Herein, the mechanism of the photocatalytic conversion of oxalic acid in anaerobic conditions together with the total yields have been deeply investigated by employing self-prepared TiO₂ photocatalysts loaded with different noble metals (Pt and/or Au). While the photocatalytic H₂ evolution remarkably occurs over bare TiO₂, the loading with a cocatalyst significantly boosts the activity. Pt/TiO₂ shows higher photonic efficiencies than Au/TiO₂, whereas Au–Pt/TiO₂ has no additional advantage. The turnover numbers (TONs) of complete degradation have been calculated as 4.86 and 12.14 over bare TiO₂ and noble-metals/TiO₂, respectively, confirming true photocatalytic processes. The degradation of oxalic acid has been experimentally confirmed to proceed via the photo-Kolbe reaction, forming ^•CO₂^– radicals. The contribution of the current-doubling mechanism and the effect of the disproportionation reaction of radicals on the total yield is discussed, showing a loss of efficiency due to secondary reactions. A remarkable diversion of H₂ evolution was recorded in all cases with Pt/TiO₂ showing an ∼30% decrease in the evolved amounts of H₂ with respect to the theoretically expected amount. This diversion can be attributed to (i) the increase in charge carrier recombination due to oxalic acid consumption, (ii) the incomplete scavenging of the photogenerated electrons by Pt nanoparticles as proved by solid-phase EPR spectroscopy, (iii) the formation of byproducts depending on the nature of the cocatalyst, and (iv) the disproportionation of ^•CO₂^– radicals, which reduces the contribution of the current doubling. Formate and formaldehyde have been experimentally detected, and EPR spin-trap experiments confirm a surface charge transfer mechanism through the TiO₂/oxalic acid interface. This work helps in closing the gap of knowledge between the theoretical and experimental aspects.