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

Abstract

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.

TiO2 Photocatalysis for the Transformation of Aromatic Water Pollutants into Fuels

“…The photocatalytic oxidation of organic pollutants proceeds either by the direct attack of the photogenerated holes or via the attack of the highly reactive hydroxyl radicals generated at the surface of the photocatalyst [110]. The photocatalytically generated OH • radicals can abstract hydrogen atoms from the organic molecules, causing a chain of reactions toward lower molecular in- By far, heterogeneous photocatalysis has gained the most attention as one of the most realistic and viable solutions due to its ability to clean-up a wide range of environmental pollutants besides the use of low-cost and chemical stable photocatalysts [101][102][103][104][105][106][107][108]. This promising approach relies on the excitation of a semiconductor with suitable light, e.g., the sunlight, to drive different redox reactions.…”

“…Another major field in photocatalysis includes light-driven water splitting into H 2 and O 2 [128]. H 2 is regarded as the most recommended replacement for fossil fuels since its energy cycle is free of pollutants and greenhouse gases [105,129,130]. Achieving dualfunctional photocatalysis, i.e., the photocatalytic degradation of organic pollutants and the simultaneous production of hydrogen gas is an added value of this technique [131].…”

“…Such a technique has a great advantage as it can benefit from solar light to treat wastewater, meanwhile, the evolved CO 2 can be consumed by natural photosynthesis [157]. In the photocatalytic reforming process, the photogenerated holes in the valence band can oxidize adsorbed organic substrates (electron donors or sacrificial reagents), whereas the photogenerated electrons in the conduction band can reduce the protons (electron acceptor) to molecular hydro-gen [104,105,130,158,159]. Such an adsorbed organic substrate can react irreversibly with the photogenerated holes, minimizing the undesired electron/hole recombination [150].…”

“…Catalysts 2021, 11, x FOR PEER REVIEW 13 of 45 [104,105,130,158,159]. Such an adsorbed organic substrate can react irreversibly with the photogenerated holes, minimizing the undesired electron/hole recombination [150].…”

“…As O 2 is not produced in these systems, the back reaction to produce water is suppressed, avoiding a subsequent gas separation stage [164]. A wide range of organic compounds such as alcohols, organic acids, and hydrocarbons had proven activity as electron donors for photocatalytic H 2 production [101,104,105,131,150,[165][166][167]. The evolution of H 2 and its kinetic reactions pathway is dependent on the concentration and the nature of the organic substrate [168,169].…”