Selective Partial Oxidation of Methane by Photocatalysis under Ambient Conditions

Abstract

Photocatalytic partial oxidation of methane to high-value chemicals as an environment-friendly, cost-efficient and energy-efficient technology has a great potential to substitute the current energy-intensive multistage thermal catalytic pathway to activate methane for the chemical industry. The expected products include selective oxidised compounds (e.g. methanol, formic acid), long-chain products (e.g. ethylene, ethanol) and even cyclic compound (e.g. benzene). In the project, the feasibility of photocatalytic activation of methane was firstly investigated by TiO2 photocatalyst. Highly dispersed iron species on titanium dioxide was optimised to present a high selectivity for the transformation of methane in the presence of H2O2. It showed a methane conversion of 15% with an alcohol selectivity of over 97% (methanol selectivity over 90%) and a yield of 18 moles of alcohol per mole of iron active sites in just 3 hours, which was far better than the just reported benchmark results. Advanced characterisation data confirmed the function of the major iron-containing species—namely, FeOOH and Fe2O3, which enhanced charge transfer and separation, decreased the overpotential of the reduction reaction and improved the selectivity towards methanol over carbon dioxide production. However, the light absorption capability of TiO2 was limited by the relative wide bandgap of 3.2 eV and band positions for such metal oxides were hard to be manipulated by modification. Therefore, one group of polymer-based photocatalysts, covalent triazine-based framework, with tuneable band structure was then studied. The photocatalytic activity and band structure were first investigated by a similar but relatively easy process of water splitting half-reactions. By tuning the degree of conjugation and carbonisation, the capacity of charge transfer and separation was enhanced, resulting in an unprecedented activity for both oxygen and hydrogen evolution under visible light irradiation. The apparent quantum efficiencies (AQE) of 3.8% for O2 evolution and over 6% for H2 evolutional were achieved at 420 nm. The activity was near 20 times higher than the benchmark polymer photocatalyst g-C3N4 for oxygen evolution and 50 times higher for hydrogen evolution under visible light irradiation. Furthermore, the optimised covalent triazine-based framework (CTF)-1 was investigated for photocatalytic methane transformation. By using water-saturated air instead of expensive H2O2 as an oxidant, such approach could continuously convert methane to ethanol instead of methanol in a fluidic reactor with an extremely high selectively of ~ 80 % at one atmospheric pressure and room temperature, realising conversion and C-C coupling in one step. The reaction mechanism was proposed based on spectroscopic measurements and structural analysis. Isotopic labelling confirmed that water molecules enhanced the C-H cleavage of methane and calorimetry experiments suggested energetically favourable desorption of ethanol comparing to methanol on the polymer photocatalysts, making ethanol as the major product. Such catalysts presented no activity decay during 12-hour stability tests, resulting in ca. 250 μmol g-1 h-1 ethanol generation and ca. 7% apparent quantum efficiency at the applied single wavelength which was generated by the most economical LED light source