Direct observation of ozone formation on SiO₂ surfaces in O ₂ discharges

Ozone production is studied in a pulsed O₂ discharge at pressures in the range 1.3-6.7mbar. Time-resolved absolute concentrations of O₃ and O are measured in the post-discharge using UV absorption spectroscopy and two-photon absorption laser-induced fluorescence. In a bare silica discharge tube ozone is formed mainly by three-body gas-phase recombination. When the tube surface is covered by a high specific surface silica catalyst heterogeneous formation becomes the main source of ozone. The efficiency of this surface process increases with O₂ pressure and is favoured by the presence of OH groups and adsorbed H₂O on the surface. At p=6.7mbar ozone production accounts for up to 25% of the atomic oxygen losses on the surface.