Reactions between small group 6 transition metal suboxide clusters, M xOy- (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 Oy- and W2 Oy- 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 Oy- 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 xOy-)> 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 Oy- than for Wx Oy -. No evidence of cluster reduction by CO is observed.
ASJC Scopus subject areas
- Physics and Astronomy(all)
- Physical and Theoretical Chemistry