Role of weakly bound complexes in temperature-dependence and relative rates of MxOy− + H2O (M = Mo, W) reactions

Results of a systematic comparison of the MoxOy− + H2O and WxOy− + H2O reaction rate coefficients are reported and compared to previous experimental and computational studies on these reactions. WxOy− clusters undergo more direct oxidation by water to yield WxOy+1− + H2, while for MoxOy− clusters, production of MoxOyH2− (trapped intermediates in the oxidation reaction) is comparatively more prevalent. However, MoxOy− clusters generally have higher rate coefficients than analogous WxOy− clusters if MoxOy+1H2− formation is included. Results of calculations on the M2Oy− + H2O (M = Mo, W; y = 4, 5) reaction entrance channel are reported. They include charge-dipole complexes formed from long-range interactions, and the requisite conversion to a Lewis acid-base complex that leads to MxOy+1H2− formation. The results predict that the Lewis acid-base complex is more strongly bound for MoxOy− clusters than for WxOy− clusters. The calculated free energies along this portion of the reaction path are also consistent with the modest anti-Arrhenius temperature dependence measured for most MoxOy− + H2O reactions, and the WxOy− + H2O reaction rate coefficients generally being constant over the temperature range sampled in this study. For clusters that exhibit evidence of both water addition and oxidation reactions, increasing the temperature increases the branching ratio toward oxidation for both species. A more direct reaction path to H2 production may therefore become accessible at modest temperatures for certain cluster stoichiometries and structures.