Mechanistic insights into hydrogen evolution from methanol photoreforming on metal-loaded anatase: development of a microkinetic model

Abstract

A microkinetic model was developed to describe the kinetics of hydrogen production via methanol photoreforming under UV and visible light irradiations. The model integrates DFT calculations with experimental results to uncover mechanistic insights on the photoreforming process. The reaction mechanism and its governing rate coefficients were embedded into the design equation of a batch reactor to simulate the experimentally measured hydrogen evolution rates. Methanol exhibits a moderate reaction barrier to the methoxy anion (0.39 eV) and a low barrier to formaldehyde (0.16 eV), which serves as a key intermediate enabling multiple reaction pathways. In contrast, water shows moderately accessible formation of the hydroxyl radical (reaction barrier: 0.43 eV) while the formation of oxygen radicals is less accessible (reaction barrier: 0.66 eV). Radical coupling results in the exothermic formation of formic acid. Direct oxidation of formaldehyde yields carbon monoxide, whereas its hydroxylation leads to the formation of carbon dioxide. The microkinetic model incorporating DFT-derived energies showed good agreement with experimental methanol photoreforming data. The production of CH₂O closely follows that of H₂, emerging as the most kinetically accessible oxidation pathway. The production of O₂ is predicted to be negligible across all metals, and the production of CO and CO₂ is predicted to be minimal. These findings demonstrate the power of microkinetic modelling methodologies to accurately predict photocatalytic behaviour and guide the rational design of more efficient hydrogen evolution systems.

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