Ti-Peroxo Species in the TS-1/H2O2/H2O System
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Abstract
This work is devoted to the study of the active species present on the TS-1-H2O2/H2O catalytic system and to the elucidation of the mechanisms of formation and interconversion of these species. In particular new experimental data and computational evidence are reported concerning (i) the acidic character of hydroperoxo species, (ii) the role of water in the hydroperoxo−peroxo interconversion, and (iii) the role of water as co-reactant in favoring the H2O2 adsorption at Ti sites. It is demonstrated that gentle drying of the TS-1-H2O2/H2O system leads to the transformation of the yellow peroxo groups into noncolored species, still located at Ti centers. The process is reversible and the colored species can be partially restored again by successive rehydration. As the hydroperoxo and peroxo species have different oxidizing ability, these results indicate that the presence of water molecules at hydrophilic Ti sites can correlate with the catalytic activity. As it is well-known, hydroperoxo and peroxo species are unstable and their concentration declines with aging. However, UV−vis spectroscopic results demonstrate that small fractions of peroxo species (characterized by a cream color) persist on the sample even after prolonged aging. On this basis and on EXAFS results, a comprehensive scheme of the formation and mutual conversion of oxidized species is concluded. Spectroscopic results, supported by acidity measurements and titration of active oxygen content, show the following: (i) the O−O species, responsible to the yellow color of the catalyst, is a side-on peroxo complex, probably generated by the reversible rupture of one Ti−O−Si bridge, with the formation of Ti(O2H) and H−O−Si groups; (ii) the stability of this peroxo complex is low in the absence of an excess of H2O; (iii) a strong enhancement of the acidity of the TS-1/H2O2/H2O system with respect to that of TS-1/H2O has been observed, demonstrating a synergic role between Ti(IV) centers and hydrogen peroxide; and (iv) a small fraction of oxidized species has a negligible oxidizing ability and probably does not play any role in the catalysis.
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