Hybrid plasma catalysis-thermal system for non-oxidative coupling of methane to ethylene and hydrogen

Abstract

We report a two-stage hybrid plasma catalysis-thermal system for non-oxidative coupling of methane (NOCM) to ethylene (C2H4) and hydrogen (H2), achieving 66 % C2H4 selectivity and 60 % H2 selectivity with 28 % CH4 conversion. This corresponds to a C2H4 yield of 18 %, which is one of the highest reported in literature. The system consists of the first plasma catalysis stage (stage 1) and the second thermal cracking stage (stage 2). Comprehensive analyses using in-situ mass spectrometry (MS) and gas chromatography (GC) reveal that the combination of plasma and Pt/ZrO2 catalyst in stage 1 predominantly facilitates the conversion of methane to ethane. Characterizations employing X-ray photoelectron spectroscopy (XPS), high-resolution transmission electron microscopy-scanning transmission electron microscopy-energy dispersive X-ray (HRTEM-STEM-EDX) mapping, and hydrogen temperature-programmed reduction (H2-TPR), indicate that the coordinatively unsaturated site of Pt could be the active sites for C–H bond cracking to generate abundant CH3 radicals (CH3·), enhancing the C2 selectivity by inhibiting the non-selective formation of coke and higher hydrocarbons. Subsequently, the products from stage 1 (especially C2H6) undergo pyrolysis reactions at 880 °C in stage 2, which further boosts the C2H4 selectivity and yield. Based on the reaction performance, catalyst characterization, optical emission spectroscopy (OES), and in-situ Fourier transform infrared spectroscopy (FTIR) results, we reveal the reaction mechanism within the hybrid two-stage plasma catalysis-thermal system for converting CH4 to C2H4 and H2.