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

Abstract

Large number of low-concentration gas (\begin{document}${C_{{\rm{C}}{{\rm{H}}_{\rm{4}}}}} $\end{document} < 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.

Keywords

• low-concentration cmm
• methanol
• visible-light catalysis
• z-type heterojunction
• composite catalysts