On the reactivity of plasma-treated photo-catalytic TiO2 surfaces for oxidation of C_2H_2 and CO

摘要

The objective of this study is to understand fundamental aspects of interactions of plasmas with catalytic surfaces. Based on this approach the reactivity of plasma treated and stimulated catalytic surfaces of TiO _2 is studied by analysing the oxidation (i) of C2H _2 to CO and CO_2 and (ii) of CO to CO_2. The inner surface of a Pyrex discharge tube is coated with TiO_2 films impregnated with TiO_2 nanoparticles, which provides a surface area of about 4 m~2. In addition to the exposure of the TiO_2 surface by low-pressure radio-frequency plasmas using O_2, Ar or N_2 (f = 13.56 MHz, p = 0.53 mbar, P = 17 W) the surfaces are stimulated by heating and UV radiation treatment. The temporal development of the concentrations of the precursor gases C_2H_2 or CO and of the reaction products is monitored using quantum cascade laser absorption spectroscopy, which provides multi-component detection in the mid-infrared spectral range. The C_2H_2 concentration was found to be nearly constant over time after a pre-treatment with Ar or N_2 discharges using an initial gas mixture of 1% C_2H_2 in Ar. However, a strong decay of the concentration of C_2H_2 is observed for pure O_2 plasma pre-treatment. In general, the decay is found to be nearly exponential with time constant in the order of about 10 min. The reactive adsorption of C_2H_2 molecules on the inner surface of the tube reactor showed a density of about 7.5 × 10 12 C_2H_2 molecules cm~(-2). This behaviour demonstrates that the reaction produces some adsorbed intermediates, which can be thermally or photo-catalytically oxidized to CO_2. In contrast, when 1% CO in Ar is used as an initial gas mixture no adsorption processes on the TiO_2 surface could be detected. An effective destruction of CO took part via photo-catalytic oxidation.