The interaction of H2O2 with exchanged titanium oxide systems (TS-1, TiO2, [Ti]-APO-5, Ti-ZSM-5)

Abstract

The interaction of hydrogen peroxide with a range of polycrystalline titanium oxide systems, including TiO₂, TS-1, [Ti]-APO-5 and ion-exchanged Ti-ZSM-5, has been investigated using c.w. X-band EPR spectroscopy. Decomposition of the peroxide occurs immediately upon contact with all of the metal oxides investigated, resulting in the formation of a series of paramagnetic oxygen centred radicals. In the TiO₂/H₂O₂ system, intense signals assigned to adsorbed O₂⁻ anions were observed. Traces of physisorbed water was found to retard the motional characteritics of the adsorbed O₂⁻ anion, as manifested in the variable-temperature EPR spectra. In addition, covalently bound superoxo species were also identified on the TiO₂/H₂O₂ surface. In the TS-1/H₂O₂ system, two different bound peroxo radicals were observed and identified firstly as (1) Ti(OSi)₄/O₂⁻/H₂O or Ti(OSi)₄/OO˙/H₂O and secondly (2) as HOTi(OSi)₃/OO˙/H₂O. In both cases, the adsorbed radical interacts with an adjacent water molecule, while the two models differ by the presence or absence of a titanol function. An EPR signal of the weakly adsorbed hyperperoxy radicals (HO₂˙) was also observed in H₂O₂-treated TS-1. In the [Ti]-APO-5/H₂O₂ and Ti-ZSM5/H₂O₂ systems bound peroxo radicals are observed in both cases, but broader EPR resonances are observed in the latter case over ZSM5 due to the heterogeneity of extraframework Ti⁴⁺ adsorption sites for stabilisation of the radical anions.