Molybdenum Oxide Cluster Anion Reactions with C₂H₄ and H₂O: Cooperativity and Chemifragmentation

To probe the mechanism of sacrificial reagents in catalytic processes, product distributions from Mo_xO_y^- clusters reacting individually with C₂H₄ and H₂O are compared with those from reactions with a C₂H₄ + H₂O mixture, with the thermodynamics explored computationally. These molecules were chosen to model production of H₂ from H₂O via H₂O + C₂H₄ → H₂ + CH₃CHO, mediated by Mo_xO_y^- clusters. H₂O is known to sequentially oxidize Mo_xO_y^- suboxide clusters while producing H₂, resulting in less reactive clusters. Mo_xO_y^- (y ∼ x) clusters undergo chemi-fragmentation reactions with C₂H₄, with Mo_xO_yC₂H_z^- complexes forming as the cluster oxidation state increases. Unique species observed in reactions with the C₂H₄ + H₂O mixture, Mo₂O₅C₂H₂^- and MoO₃C₂H₄^-, suggest that the internal energy gained in new Mo-O bond formation from oxidation by H₂O opens additional reaction channels. C₂H₃O^- is observed uniquely in reactions with the C₂H₄ + H₂O mixture, giving indirect evidence of CH₃CHO formation via the cluster mediated H₂O + C₂H₄ → H₂ + CH₃CHO reaction; C₂H₃O^- can form via dissociative electron attachment to CH₃CHO. Calculations support mechanisms that invoke participation of two ethylene molecules on thermodynamically favorable pathways leading to experimentally observed products.