Room-temperature photooxidation of low-concentration coalbed methane to methanol over CuOx/D-WO3 combined with gas-solid-liquid interface: From pollutant to clean fuel

Abstract

Direct transformation of CH4 in low-concentration coalbed methane (LC-CBM) to clean fuel methanol is a more sustainable route as industrial CH4 transformation is usually energy-intensive. Solar-driven photocatalysis with near-zero carbon emissions can activate and transform CH4 at ambient temperatures, whereas the reported methanol productivities of traditional solid-liquid photocatalysis are still far from satisfactory. Herein, amorphous CuOx decorated defective WO3 nanocomposites (CuOx/D-WO3) were first synthesized and used as full-spectrum responsive photocatalysts for selective oxidation of LC-CBM to methanol. Introducing oxygen vacancies endows CuOx/D-WO3 with near-infrared (NIR) response. Loading CuOx boosts the separation of photogenerated charge carriers and the generation of ·OH via H2O2 reduction by photoelectrons on CuOx sites, confirmed by photoelectrochemical characterizations and in-situ XPS. Under simulated solar light irradiation, CuOx/D-WO3 exhibited significantly promoted activity of CH4 oxidation compared with the counterpart D-WO3. Even under NIR illumination, the optimal 3.0 % CuOx/D-WO3 showed a methanol production of 1.57 mmol·gcat−1 with selectivity of 79.8 %. Furthermore, a gas-solid-liquid triphase interface was constructed by immobilizing hydrophilic 3.0 % CuOx/D-WO3 on superhydrophobic carbon fiber paper (CFP) to promote methanol generation through strengthening CH4 mass transfer. This triphase photocatalysis exhibited a methanol production of 25.42 mmol·gcat−1, which was ∼4 times higher than solid-liquid diphase catalysis. Methanol selectivity of CuOx/D-WO3/CFP triphase photocatalysis was up to 95.3 %. It was mainly due to enhanced mass transfer of CH4 from gas phase to photocatalysts layer through hydrophobic-hydrophilic wettability abrupt interface of CuOx/D-WO3/CFP, which could be evidenced by significantly increased ·CH3 radicals from in-situ EPR spectra. This work broadens the avenue toward cleaner and efficient utilization of LC-CBM in a sustainable way.