H 2 production from reactions between water and small molybdenum suboxide cluster anions

Abstract

Reactions between molybdenum suboxide cluster anions, Mo(x)O(y)(-) (x=1-4; y < or = 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(2)O(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(3)O(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)O(y+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.