Kinetic and spectroscopic study of O-1(2) generation from H2O2 catalyzed by LDH-MoO42- (LDH = layered double hydroxide)

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 H2O 2 concentrations and on temperature has been related to structural characte ristics of the material (Xray diffraction (XRD), scanning electron microsco py (SEM), IR spectroscopy, N-2 adsorption, thermogravimetry) and to molybda te speciation as revealed by in-situ studies in the presence of peroxide: ( FT Raman, diffuse reflectance UV/visible spectroscopy). The H2O2 decomposit ion rate is linearly correlated with the amount of LDH-exchanged molybdate, except when a considerable fraction of the molybdate occupies less accessi ble interlayer positions. A maximum in the H2O2 decomposition rate as the H 2O2 concentration is increased is due to the successive formation of mono-, di-, tri-, and tetraperoxomolybdates. This behavior was modeled successful ly by using the equilibrium constants for formation of the Mo-peroxo comple xes, and the rate constants for decay of the peroxomolybdates with O-1(2) l iberation. Time-resolved diffuse reflectance and Raman observations of the various MoO42--peroxide adducts are in line with the proposed kinetic schem e, 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 specie s proceeds much faster at the LDH surface than in solution, Consequently, m aximal rates pet Mo atom are at least twice as high for the heterogeneous L DH catalyst as for the homogeneous systems.