Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds: Part 3. Electron paramagnetic resonance (EPR) studies of plasma-treated porous alumina

Abstract

The effect of plasma processes inside the intra-particle volume of porous materials (especially Al2O3) was studied in order to evaluate the potential of the combination of non-thermal plasma (NTP) and in situ heterogeneous catalysis (plasma catalysis), for the improvement of efficiency and selectivity towards total oxidation of organic pollutants in gas cleaning applications. Electron paramagnetic resonance (EPR) spectroscopy was applied as an appropriate method to detect both the formation of radical species by the NTP as well as the initiation of structural changes to the catalyst.The presence of paramagnetic oxygen or hydroxyl species (O−, O2− or OH) could not be detected by EPR spectroscopy. The observed signal was not significantly influenced either by the type of atmosphere present during NTP treatment or by applying reducing agents to the sample after plasma treatment.However, by using non-porous and porous alumina (α- and γ-Al2O3) as model catalysts, the effect of NTP modifying the surface structure in the interior of a porous material could be clearly demonstrated. A paramagnetic species probably related to an AlOO aluminium peroxyl group was formed by NTP processes independently of the oxygen content of the gas atmosphere. It was not formed when the alumina sample was positioned in the off-gas flow of a plasma reactor, i.e. used in the post-treatment mode.The structure of the paramagnetic site was investigated by employing several spectroscopic tools (X- and Q-band EPR, electron spin echo envelope modulation [ESEEM] and EPR measurements after pre-deuteration).