Identification of oxy-radicals on an irradiated molybdena catalyst using electron paramagnetic resonance spectroscopy

Abstract

An electron paramagnetic resonance study is reported of radicals formed on irradiation of a silica-supported molybdena catalyst in the presence of H₂ and O₂ at low temperatures, at λ= 340 nm. In H₂ alone photoreduction of the catalyst occurs. In O₂ alone no paramagnetic species are formed. In H₂+O₂ mixtures irradiation at 77 K produces O^–₂ and HO^˙₂ radicals. On warming above 77 K, HO^˙₂ is removed and O^–₂ is converted to a second O^–₂ species which may be hydrogen bonded. Traces of a fourth radical identified as O^– are also detected. None of the oxy-radicals produced are stable at room temperature. Complete g- and hyperfine-(¹⁷O, ²H and ⁹⁵Mo) tensor data are provided for the three major species observed. The photochemistry of MoO₃–SiO₂ is compared with that of TiO₂.