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.