Direct Detection of OH Radicals and Indirect Detection of H2O2 Molecules in the Gas Phase near a TiO2 Photocatalyst Using LIF

Abstract

The formation of OH radicals and its diffusion into the gas phase during the excitation of TiO2 in the presence of H2O has been studied using the very sensitive and selective detection method of laser-induced fluorescence (LIF). The time-resolved evolution of the OH radical concentration has been observed at pressures between 4 and 600 Torr and at varying distances between the photocatalytic surface and the detection volume. OH radicals have been detected even at the highest pressures, opening the assumption that gas-phase reactions with OH radicals may very well be involved in the photocatalytic degradation of VOCs, even at atmospheric pressure. Interestingly, a second fluorescence signal peak has been observed at longer delays with respect to the excitation pulse. The use of different fluorescence laser energies leads us to interpret this second peak as H2O2, released from the surface into the gas phase and detected by a two-photon process; in fact, we use fluorescence laser fluencies high enough to photolyze H2O2 molecules at the fluorescence−excitation wavelength (282 nm) and excite the generated OH radicals within the same laser pulse.