Kinetic and Spectroscopic Study of¹O₂Generation from H₂O₂Catalyzed by LDH–MoO₄²⁻(LDH=Layered Double Hydroxide)

Abstract

Layered double hydroxides (LDHs), exchanged with molybdate, decompose H2O2 to form one molecule of singlet-state dioxygen (1O2) 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 (X-ray diffraction (XRD), scanning electron microscopy (SEM), IR spectroscopy, N2 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 1O2 liberation. Time-resolved diffuse reflectance and Raman observations of the various MoO4(2-)-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 1O2. 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 per Mo atom are at least twice as high for the heterogeneous LDH catalyst as for the homogeneous systems.

Molybdate-exchanged layered double hydroxides for the catalytic disproportionation of hydrogen peroxide into singlet oxygen: Evaluation and improvements of 1O2 generation by combined chemiluminescence and trapping experiments

“…H 2 O 2 and alkyl hydroperoxide solutions are strongly oxidizing and should be handled with appropriate precautions. )-[Mg,Al]LDH support with a Mg to Al molar ratio of 2 to 1 was prepared by co-precipitation of Mg and Al nitrate salts [9,10]. In a 1 L three-neck round-bottom flask, 100 mL of deionized and boiled water was brought to pH 10 with NaOH (1 M).…”

“…The reaction mixture was stirred at 20 8C for 30 min. Since the Mo-LDH shows optimum activity in less concentrated H 2 O 2 solutions, the total amount of H 2 O 2 was added in portions [10]. Each portion contained about 50 equivalents of H 2 O 2 with respect to Mo.…”

“…On the other hand, the rate of 1 O 2 generation by the Mo-LDH catalysts is much higher compared to the rate for soluble molybdate. The higher activity of the Mo-LDHs in the disproportionation of H 2 O 2 might be due to the electrostatic field exerted by the highly charged LDH support, resulting in enhanced polarization of the Mo-O 2 and O-O bonds in the immobilized peroxomolybdate complexes compared to the corresponding dissolved species [10]. This might also explain the increase in activity at low Mg/Al ratios (high Al content) since the polarity of the LDHs is determined by the Mg/Al ratio.…”

“…Recently, some of us reported the use of layered double hydroxides (LDHs) as inorganic anion exchangers for the immobilization of the catalytically active MoO 4 2À species [8][9][10][11]. The resulting heterogeneous molybdate-exchanged layered double hydroxide (Mo-LDH) catalysts show several advantages over homogeneous molybdate salts.…”

“…Moreover, in contrast to sodium molybdate, Mo-LDHs can be used in aprotic organic solvents. Also, it was found that the activity per Mo atom is significantly higher at the LDH surface than in solution [10]. One drawback of the Mo-LDH catalysts is the less efficient use of H 2 O 2 compared to Na 2 MoO 4 .…”