2.2 Catalytic tests
The experimental setup is shown in SI, section 2, Scheme S2. Calibrated mass flow controllers were used to control the flow rate of CH4 and O2 (both 99.999 % purity), which were mixed well before flowing into the DBD reactor. The DBD reactor, featuring two layers of quartz glass, comprised an inner tube with a 10 mm outer diameter filled with 1.25 g catalyst granules (20-40 mesh). The outer tube had a 30 mm outer diameter, and there was circulated water between both tubes, functioning both as a temperature control for the discharge area and as the grounding electrode. A 2 mm stainless steel rod placed inside the inner tube, was used as the high voltage electrode.
The effective length of the discharge area was fixed at 50 mm. Throughout plasma-assisted DOMtM, the discharge frequency was set at 9.2 kHz. The discharge voltage, current and input power were monitored using a digital oscilloscope (DPO 3012, Tektronix, USA). The gas flow rates before and after reaction were measured by a mass flow controller to account for volume compression or expansion due to the chemical reaction, when determining the CH4 conversion and product yield/selectivity. Gas products were analyzed using a gas chromatograph (GC-7900, Tianmei, China) with a thermal conductivity detector (TDX-01 column) and a flame ionization detector (alumina-filled column). Liquid products were condensed in a cold trap (a mixture of isopropanol and liquid nitrogen) and subsequently analyzed using a gas chromatograph (GC-2014C, Shimadzu, Japan), GC-MS (5975C, Agilent, USA), and 1H-NMR (AVANCE Ⅲ 500, Bruker, Switzerland). Further details on the qualitative and quantitative analysis of products in CH4/O2 NTP are provided in SI (section 3), including the formulas of the standard calibrated concentration curves (presented in Table S1). Each catalyst underwent three tests to establish experimental error bars.
To evaluate the reaction performance of the catalysts, the selectivity of reaction products and the CH4 conversion were calculated using the following equations. Carbon-based selectivity is defined here, excluding H2O and H2 from these equations.
The CH4 conversion was calculated by:
\(\mathrm{X}_{\mathrm{\text{CH}}_{\mathrm{4}}}\mathrm{\ (\%)\ =\ }\frac{\mathrm{\text{moles\ }}\mathrm{\text{of\ }}\mathrm{\text{CH}}_{\mathrm{4}}\mathrm{\text{\ converted}}}{\mathrm{\text{moles\ of\ initial\ }}\mathrm{\text{CH}}_{\mathrm{4}}}\mathrm{\ }\mathrm{\times\ 100\ \%}\)(1)
In the gaseous products, only CO and CO2 were collected, and no hydrocarbons (C2 or higher) were detected. The selectivity of the gaseous products was calculated as:
\(\mathrm{S}_{\mathrm{\text{CO\ }}}\mathrm{\ (\%)\ =\ }\frac{\mathrm{\text{moles\ }}\mathrm{\text{of\ CO\ produced}}}{\mathrm{\text{moles\ }}\mathrm{\text{of\ }}\mathrm{\text{CH}}_{\mathrm{4}}\mathrm{\text{\ converted}}}\mathrm{\ }\mathrm{\times\ 100\ \%}\)(2)
\(\mathrm{S}_{\mathrm{\text{CO}}_{\mathrm{2}}}\mathrm{\ (\%)\ =\ }\frac{\mathrm{\text{moles\ }}\mathrm{\text{of\ }}\mathrm{\text{CO}}_{\mathrm{2}}\mathrm{\text{\ produced}}}{\mathrm{\text{moles\ }}\mathrm{\text{of\ }}\mathrm{\text{CH}}_{\mathrm{4}}\mathrm{\text{\ converted}}}\mathrm{\ }\mathrm{\times\ 100\ \%}\)(3)
The selectivity of the liquid products was calculated as follows:
\(\mathrm{Total\ selectivity\ of\ liquid\ products\ (\%)\ =\ 100\ \%\ -}\mathrm{\ }\mathrm{(S}_{\mathrm{\text{CO}}}\mathrm{+\ }\mathrm{S}_{\mathrm{\text{CO}}_{\mathrm{2}}}\mathrm{\ )}\)(4)
The selectivity of every single oxygenates, CxHyOz, can be calculated as:
\(\mathrm{S}_{\mathrm{\text{\ C}}_{\mathrm{x}}\mathrm{H}_{\mathrm{y}}\mathrm{O}_{\mathrm{z}}}\mathrm{\ (\%)\ =\ \ }\frac{X\ \times\ \mathrm{N}_{\mathrm{\text{\ C}}_{\mathrm{x}}\mathrm{H}_{\mathrm{y}}\mathrm{O}_{\mathrm{z}}}\ }{\sum X_{i}N_{i}}\mathrm{\ \times\ total\ selectivity\ of\ liquid\ }\)products (5)
Where\(N_{\mathrm{\text{\ C}}_{\mathrm{x}}\mathrm{H}_{\mathrm{y}}\mathrm{O}_{\mathrm{z}}}\)represents the number of moles of various oxygenates in the liquid fraction.
Finally, we defined the energy consumption for CH3OH formation (kJ/mmol) as:
\(\mathrm{\text{Energy\ }}\mathrm{\text{consumption}}\mathrm{\ \ =\ }\frac{\mathrm{\text{discharge\ power}}\mathrm{\ }\mathrm{(}\mathrm{J/s}\mathrm{)}}{\mathrm{\text{rate}}\mathrm{\ }\mathrm{\text{of}}\mathrm{\ }\mathrm{C}H_{3}\mathrm{\text{OH}}\mathrm{\ }\mathrm{\text{produced}}\mathrm{\ (mol/s)}}\times 10^{-6}\)(6)