Performance study on visible-light catalytic conversion of low concentration coalmine gas to methanol with Z-type heterojunction Ag₂S/AgVO₃

Large number of low-concentration gas (C_\rmC\rmH_\rm4 < 30%) is discharged directly due to the lack of reasonable and effective utilization pathways. Research and development of high-value conversion and utilization methods and technologies for low-concentration gas is of great significance to ensure the safe production in coal seams and mitigate the greenhouse effect, and it is also an indispensable part of realizing China’s “double carbon” strategic goal. The production of liquid fuel methanol from low-concentration gas as a carbon source is considered to be one of the ideal utilization pathways. Photocatalytic technology driven by clean solar energy can activate and convert methane at room temperature and atmospheric pressure, providing a novel approach for low-carbon utilization of low-concentration gas. AgVO₃ was firstly prepared by hydrothermal method. Ag₂S/AgVO₃ heterojunction was constructed by in-situ decoration of Ag₂S on the surface of AgVO₃ via ion exchange strategy using thioacetamide as a sulfur source. Molar ratio of Ag₂S could be regulated by varying the amount of thioacetamide. The microstructure of composite catalysts was characterized using XRD, SEM, TEM and UV-Vis diffuse reflectance spectroscopy. A methane-air mixture with a volume ratio of 1∶12 was used as the simulated low-concentration gas, and the effects of Ag₂S compound ratio, oxidant concentration and light intensity on methane conversion, methanol productivity and selectivity were systematically investigated. The intrinsic mechanism of Z-type heterojunction Ag₂S/AgVO₃ for enhancing the conversion performance of simulated gas was explored by means of transient photocurrent response spectroscopy and electron paramagnetic resonance (EPR) spectroscopy. The results indicated that, the prepared AgVO₃ shown a fibrous morphology with monoclinic crystalline phase structure. The in-situ composite Ag₂S was in the form of nanoparticles with an average particle size of 60 nm, and the Ag₂S particles were uniformly distributed on the surface of AgVO₃ fibers. Composite Ag₂S/AgVO₃ exhibited enhanced light absorption compared to single AgVO₃ and Ag₂S. The bandgap energy, valence band and conduction band potentials of AgVO₃ were 2.08 eV, 2.21 V and 0.13 V, and those of Ag₂S were 0.91 eV, 0.34 V and −0.57 V, respectively. The composite catalyst Ag₂S/AgVO₃ shown significantly enhanced gas conversion performance compared to AgVO₃. The methane conversion and methanol production of the optimal catalyst 20% Ag₂S/AgVO₃ irradiated with visible light for 1 h was 3.10 mmol/g and 2.45 mmol/g, which was 1.72 times and 2.63 times higher than that of the single AgVO₃, respectively, and the methanol selectivity was up to 78.9%. The result of 6 cyclic tests shown the excellent catalytic stability of Ag₂S/AgVO₃. The results of energy band structure analysis and EPR test shown that, the Ag₂S/AgVO₃ heterojunction followed the Z-type charge transfer mechanism, which not only enhanced the spatial separation of photogenerated charges, but also maintained strong oxidation-reduction ability, which significantly improved the catalytic performance in the directional conversion of low concentration gas to methanol, and provided a novel idea for the high-efficiency utilization of low-concentration gas in a low-carbon way.