| Oxygen-free conversion of methane to ethylene (as chemical feedstocks) and COx-free hydrogen (as clean energy) would be very important to ease oil crisis. Herein, oxygen-free conversion of methane to ethylene and hydrogen was achieved at low temperature using a plasma-catalytic process. The results are mainly as follows:1. Oxygen-free conversion of methane to acetylene by pure plasmaOxygen-free conversion of methane was carried out by pulsed corona discharges and pulsed spark discharges. Using pulsed corona discharges with energy density of 18.7 kJ/L, the conversion of methane was 36.8% and the selectivities of C2 and acetylene were 59.2% and 34.5%, respectively. By contrast, using pulsed spark discharges with energy density of 19.2 kJ/L, the conversion of methane was 60.2% and the selectivities of C2 and acetylene were 93.1% and 88.5%, respectively. According to the results of optical emission spectra, it can be inferred that the methane was highly dissociated into·CH,·C and·H radicals by electron impact, which may cause the high selectivity of acetylene.The methane conversion was carried out using AC spark discharges and pulsed spark discharges. Comparative experiments showed that AC spark discharges could convert methane more efficiently than pulsed spark discharges. Using pulsed spark discharges, 47.3% of methane conversion was obtained with energy density of 29.1 kJ/L and 14 eV of energy cost for converting per methane molecule. However, using AC spark discharges, 79.8% of methane conversion was obtained with energy density of 38.4 kJ/L and 11 eV of energy cost for converting per methane molecule.2. Selective hydrogenation of acetylene to ethylene over highly dispersed Pd-based catalystsHighly dispersed Pd-based catalyst (denoted as Pd/SiO2-N )was prepared using aminopropyl-containing silica support modified with formaldehyde. The result of TEM indicated that the particle size of Pd was 1-2 nm for Pd/SiO2-N. The novel Pd/SiO2-N catalyst exhibits unusual catalytic performance for selective hydrogenation of acetylene, that is, the selectivity of ethylene rises significantly with the increase of acetylene conversion through increasing reaction temperature or hydrogen partial pressure until 100% acetylene conversion. The in-situ DRIFTS spectra of CO adsorption show that the organic groups presented on the silica affect the electronic property of the very small Pd nanoparticles. Pd-Ag/SiO2-N catalyst was prepared by introducing silver onto Pd/SiO2-N catalyst through in-situ reduction of silver nitrate. Compared with Pd/SiO2-N catalyst, Pd-Ag/SiO2-N catalyst has higher ethylene selectivity.3. Oxygen-free conversion of methane to ethylene by plasma catalysisEthylene produced from methane conversion was performed by combining spark discharges and catalyst in the reactors of in-plasma catalysis (IPC) and post-plasma catalysis (PPC). The experimental results indicated that the stable methane conversion and high yield of ethylene could be obtained by combining spark discharges and catalyst in the PPC reactor under oxygen-free condition. However, the unstable discharge was observed by combining spark discharges and catalyst in the IPC reactor due to the coke deposition.Combining AC spark discharges and Pd-Ag/SiO2-N catalyst in the PPC reactor with energy density of 38.4 kJ/L at 423 K of reaction temperature, the single-pass yield of ethylene was 52.6% and the yield of hydrogen was 43.2% at 80.0% of methane conversion. The concentrations of ethylene and COx-free hydrogen were as high as 18.8% and 62.0% in the effluent gas, respectively. No acetylene was detected in the effluent gas.Additionally, the preparation of Au catalyst supported on aminopropyl-containing silica support by an in-situ reduction process was explored for selective hydrogenation of acetylene. Compared with Au catalyst supported on commercial silica prepared by deposition-precipitation, it is more stable for selective hydrogenation of acetylene and gives higher ethylene selectivity.
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