Experimental study on catalytic oxidation of low concentration 14CH4 by Non-thermal Plasma

Background: Carbon-14 (14C) has become the largest contributor of annual effective dose to the surrounding public in radioactive effluent during normal operation of nuclear power plants. However, an effective treatment method for 14C in the form of hydrocarbons has not yet been established. Purpose: This study aims to investigate the treatment performance of a Non-thermal plasma/catalytic coupling system for low concentrations of 14CH4, the largest and most chemically stable of the 14C hydrocarbons. Methods: Considering the optimization of radiation protection, 12CH4, which possesses the same physicochemical properties as 14CH4, was selected as the experimental subject. The Pd/g-Al2O3 catalyst were prepared by segmental heat treatment and wet impregnation. The discharge behaviors of the plasma were analyzed by the Lissajous figure. The CH4 treatment performance of the Non-thermal plasma before and after the introduction of the catalyst under different discharge voltages, gas flow rates and temperatures were analyzed by the constructed experimental system. The microstructural changes of the catalyst were analyzed by N2 adsorption-desorption isotherm, X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Results: The introduction of Pd/Al2O3 catalyst can significantly improve CH4 treatment performance in Non-thermal plasma, and the CH4 treatment efficiency and CO2 selectivity can reach 100% and 83.7% by increasing the discharge voltage, lowering the reaction temperature and reasonably adjusting the gas flow. Conclusion: The Non-thermal plasma and the Pd/Al₂O₃ catalyst exhibit a synergistic effect. The Pd/Al₂O₃ catalyst can reduce the reaction barriers, optimise the reaction paths and significantly improve the CH₄ treatment performance of the plasma. Furthermore, the plasma-excited reactive species are conducive to the formation of PdO, which is the key reactive phase in the catalyst.