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Mechanism of CO2-formation promotion by Au in plasma-catalytic oxidation of CH4 over Au/γ-Al2O3 at room temperature

Yao, Shuiliang, Chen, Zhizong, Weng, Shan, Mao, Linai, Zhang, Xuming, Han, Jingyi, Wu, Zuliang, Lu, Hao, Tang, Xiujuan, Jiang, Boqiong, Nozaki, Tomohiro
Journal of hazardous materials 2019 v.373 pp. 698-704
ambient temperature, carbon dioxide, carbonates, catalysts, desorption, gold, greenhouse gas emissions, methane, methane production, oxidation, oxygen
The plasma-catalytic oxidation of methane (CH4) is a potential reaction for controlling CH4 emissions at low temperatures. However, the mechanism of the CH4 plasma-catalytic oxidation is still unknown, which inhibits the further optimization of the oxidation process. Herein, a CH4 oxidation mechanism over an Au/γ-Al2O3 catalyst was proposed based on our experimental findings. CH4 is first decomposed to CH3 and H by the discharge, and a fraction of the CH3 is adsorbed on γ-Al2O3 surface for deep oxidation. The oxygen atoms produced by the discharge react with H2O to yield surface reactive OH groups that contribute to the CH3 oxidation. Oxygen atoms also promote the release of H2O from the surfaces of the γ-Al2O3 and Au/γ-Al2O3 and especially promote CO2 desorption from the surface of the Au/γ-Al2O3. When γ-Al2O3 was used as the catalyst, the CO2 selectivity was only 15 vol.%, and the CH4 conversion decreased after 7 h of plasma-catalytic oxidation. In contrast, when Au/γ-Al2O3 was used, the CO2 selectivity was 80 vol.%, long-term CH4 conversion was obtained. Experimental results revealed that Au was beneficial for the decomposition of surface carbonate species into gaseous CO2, whereas the carbonate species accumulated on γ-Al2O3 when Au was absent.