H_2 production from reactions between water and small molybdenum suboxide cluster anions

摘要

Reactions between molybdenum suboxide cluster anions, Mo_x_Oy- (x=1-4;y≤3x), and water (H_2 O and D_2 O) have been studied using mass spectrometric analysis of products formed in a high-pressure, fast-flow reactor. Product distributions vary with the number of metal atoms in the cluster. Within the MoO_y~- oxide series, product masses correspond to the addition of one water molecule, as well as a H/D exchange with MoO_4~(H-). Within the Mo_2 O _y~- oxide series, product evolution and distribution suggest sequential oxidation via Mo_2 O_y~- + H_2 O/ D_2 O→ Mo_2O_(y+1)~- + H_2 / D_2 reactions for y<5, while for Mo_2 O_5~-, Mo_2 O~6 H_2 / D _2~- is produced. Mo_2 O_6~- does not appear to be reactive toward water. For the Mo_3O _y~- oxide series, sequential oxidation similarly is suggested for y<5, while Mo_3 O_5~- reactions result in Mo_3 O~6 H_2 / D_2~- formation. Mo_3 O_6~- appears uniquely unreactive. Mo_3 O_7~- and Mo_3 O ~8- react to form Mo_3 O~8 H_2 / D _2~- and Mo_3 O~9 H_2 / D _2~-, respectively. Lower mass resolution in the Mo _4 O_y~- mass range prevents unambiguous mass analysis, but intensity changes in the mass spectra do suggest that sequential oxidation with H_2 / D_2 evolution occurs for y<6, while Mo_4 Oy+1 H_2 / D_2~- addition products are formed in Mo_4 O_6~- and Mo_4 O _7~- reactions with water. The relative rate constants for sequential oxidation and H_2 O/ D_2 O addition for the x=2 series were determined. There is no evidence of a kinetic isotope effect when comparing reaction rates of H_2 O with D_2 O, suggesting that the H_2 and D_2 losses from the lower-oxide/hydroxide intermediates are very fast relative to initial reaction complex formation with H_2 O or D_2 O. The rate constants determined here are two times higher than those determined in identical reactions between W_2 O_y~- + H_2 O/ D_2 O.