Oxygenation of vanadyl(IV). Effect of coordinated surface hydroxyl groups and hydroxide ion

The oxygenation kinetics of vanadyl in solution and in the presence of δ-Al2O3 and TiO2 (anatase) was investigated as a function of pH, pO2, temperature, and surface area concentrations. In the presence of the hydrous oxides the oxidation reaction is preceded by specific adsorption, characterized and measured in terms of formation of surface complexes: 2(=MOH) + VO2+ ⇄ VO(OM=)2 + 2H+. The rate laws can be interpreted in terms of a rate dependence on the concentrations of dissolved oxygen and the concentrations of VO(OH)+ in the homogeneous solution and on that of V(IV) bound to the hydrous oxide surface, VO(OM=)2 in the heterogeneous system, respectively. The kinetic interpretation given is in line with an inner-sphere nature of the surface complex. The rate constants (25 °C, M-1 s-1) are 1.07, 0.051, and 0.028, respectively, for the homogeneous system, in the presence of TiO2 and Al2O3. These results are compared with published data on the oxygenation of Fe(II) and Mn(II) to illustrate more generally that complex formation with surface hydroxyl groups produces similar kinetic effects on the oxygenation as complex formation with OH- in solution.