Hydrogen evolution from water using Mo–oxide clusters in the gas phase: DFT modeling of a complete catalytic cycle using a Mo2O4−/Mo2O5− cluster couple

Abstract

Density functional theory (DFT) calculations using a small metal cluster couple, Mo₂O₄⁻/Mo₂O₅⁻, are used to model a complete catalytic cycle for H₂ production from water. While Mo₂O₄⁻ is known to readily react with water to form Mo₂O₅⁻ and release H₂, the principal challenge is in reducing Mo₂O₅⁻ to Mo₂O₄⁻ to complete the cycle. We investigate the role of several potential sacrificial reagents (ethylene, propylene, CO and acetylene) that can reduce Mo₂O₅⁻ after the initial oxidation. DFT calculations of the free energy reaction pathways demonstrate the presence of overall kinetically accessible barriers that are below the entrance channel (separated reactants) in the Mo₂O₄⁻ + H₂O reaction (step I) followed by the Mo₂O₅⁻ + sacrificial reagent reactions (step II). Though the overall reaction is thermodynamically favorable, the first step is highly exothermic while the second step is endothermic. The deepest part of the potential energy surface is a complex of Mo₂O₅⁻ with the sacrificial reagent. If the energy gained in the first reaction and the succeeding complex formation is not lost due to collisions, the subsequent barriers can be overcome, leading to possible catalytic applications of the Mo₂O₄⁻/Mo₂O₅⁻ cluster couple in H₂ production reactions.