Chemistry - a European Journal vol:7 issue:12 pages:2547-2556
Layered double hydroxides (LDHs), exchanged with molybdate, decompose H2O2 to form one molecule of singlet-state dioxygen (O-1(2)) from two molecules of H2O2. The dependence of the kinetics of H2O2 decomposition on Mo and H2O2 concentrations and on temperature has been related to structural characteristics of the material (Xray diffraction (XRD), scanning electron microscopy (SEM), IR spectroscopy, N-2 adsorption, thermogravimetry) and to molybdate speciation as revealed by in-situ studies in the presence of peroxide: (FT Raman, diffuse reflectance UV/visible spectroscopy). The H2O2 decomposition rate is linearly correlated with the amount of LDH-exchanged molybdate, except when a considerable fraction of the molybdate occupies less accessible interlayer positions. A maximum in the H2O2 decomposition rate as the H2O2 concentration is increased is due to the successive formation of mono-, di-, tri-, and tetraperoxomolybdates. This behavior was modeled successfully by using the equilibrium constants for formation of the Mo-peroxo complexes, and the rate constants for decay of the peroxomolybdates with O-1(2) liberation. Time-resolved diffuse reflectance and Raman observations of the various MoO42--peroxide adducts are in line with the proposed kinetic scheme, Of all the Mo-peroxo species on the LDH, the triperoxomolybdate has the highest rate for decay to O-1(2), Comparison with the kinetics of dissolved molybdate shows that the monomolecular decay of all peroxomolybdate species proceeds much faster at the LDH surface than in solution, Consequently, maximal rates pet Mo atom are at least twice as high for the heterogeneous LDH catalyst as for the homogeneous systems.