CO₂ reduction by group 6 transition metal suboxide cluster anions

Reactions between small group 6 transition metal suboxide clusters, M _xO_y^- (M= M 98 o or W 186; x=1-4; y3x) and both CO₂ and CO were studied in gas phase using mass spectrometric analysis of high-pressure, fast flow reaction products. Both Mo₂ O_y^- and W₂ O^y- show evidence of sequential oxidation by CO₂ of the form, M₂ O ^Oy^- + CO₂ → M₂ O _y+1^- +CO for the more reduced species. Similar evidence is observed for the trimetallic clusters, although Mo₃ O ₆^- appears uniquely unreactive. Lower mass resolution in the M₄ O_y^- range precludes definitive product mass assignments, but intensity patterns suggest the continued trend of sequential oxidation of the more reduced end of the M₄ O _y^- oxide series. Based on thermodynamic arguments, cluster oxidation by CO₂ is possible if D₀ (O-Mo _xO_y^-)> 5.45 eV. Although simple bond energy analysis suggests that tungsten oxides may be more reactive toward CO ₂ compared to molybdenum oxides, this is not born out experimentally, suggesting that the activation barrier for the reduction of CO₂ by tungsten suboxide clusters is very high compared to analogous molybdenum suboxide clusters. In reactions with CO, suboxides of both metal-based oxides show CO addition, with the product distribution being more diverse for Mo _x O_y^- than for W^x O_y ^-. No evidence of cluster reduction by CO is observed.