Mechanistic Investigation of Methanol Aqueous-Phase Reforming via Water Dissociation over Pt and Pt–Mn Catalysts: A Density Functional Theory Insight

Catalytic conversion of oxygenated hydrocarbons via aqueous-phase reforming (APR) is a promising catalytic platform for hydrogen production and water remediation. Herein, we apply periodic density functional theory (DFT) calculations to extensively investigate the energetics of methanol (CH3OH) APR on Pt and Pt-Mn catalysts modeled as single-crystal facets, fcc(111), of extended surfaces and understand their catalytic behavior. First, on pure Pt, we find that the preferable path goes through successive hydrogen abstractions from adsorbed CHxOH species to generate hydrogen (H) atoms, which are strongly bound to the Pt(111) surface, resulting in moderate hydrogen poisoning of the surface. The reaction mechanism of CH3OH-APR under high coverage of hydrogen is then investigated under the proper resting-state conditions (at variance with previous theoretical studies that typically investigated low coverage). We determine that H2O dissociation to OH + H is the rate-determining step for CH3OH-APR on high-hydrogen-coverage Pt(111), with an energy barrier of 0.86 eV (and an overall energy barrier of 0.96 eV). We then find that Mn in Pt-Mn alloys at Pt2Mn composition promotes H2O dissociation by reducing the activation barrier to 0.64 eV. Additionally, Mn atoms alleviate hydrogen poisoning issues by decreasing the interaction between hydrogen and Pt surface atoms and promoting hydrogen evolution, in agreement with experimental observations. Finally, to understand the experimental results after Mn leaching out from the catalysts under APR reaction conditions showing that, surprisingly, the reused catalysts still exhibit good catalytic performance, we model systems with very low subsurface Mn content. We find that mitigation of hydrogen poisoning occurs even when the content of Mn doping is low, thus offering a possible rationalization of the experimental puzzle.