Methane (CH4), contributing 70-90% to the composition of natural gas, has been recognized as an indispensable feedstock in the manufacture of fuels and chemicals. Its large-scale transformation to liquid fuels and chemicals is performed with traditional syngas and Fischer-Tropsch technology via an indirect process under high operation temperature (800 oC - 900 oC).
Photocatalytic CH4 oxidation can generate many types of value-added chemicals at room temperature using sustainable radiation with light instead of thermal activation. Amongst, methanol (CH3OH) and formaldehyde (HCHO) are two major targets because of their application as basic and widely used chemicals or building blocks.
However, due to the complicated and energy-downhill process of CH4 oxidation: CH4 → CH3OOH → CH3OH → HCHO → CO2, there is a trade-off between catalytic productivity and selectivity to the intermediates of CH3OH and HCHO, respectively.
Figure. Gibbs free energy corresponding to formation of CH3OOH, CH3OH, HCHO and CO2 from CH4 oxidation at 298 K (left), and proposed reaction mechanism for photocatalytic CH4 oxidation by q-BiVO4 in presences of H2O and O2 (right).
Herein, scientists reported a highly selective aerobic conversion of CH4 to CH3OH and HCHO using quantum-sized BiVO4 nanoparticles (q-BiVO4) as photocatalyst at room temperature. A hard-template method was developed for preparation of the q-BiVO4 nanoparticles，whose size (~4.5 nm) is close to the Bohr radius (2 nm), indicating a strong quantum confinement effect.
Benefiting from the quantum size, q-BiVO4 is characteristic of high kinetic energy and large specific surface enabling effective conversion of CH4 to CH3OH and HCHO. The selectivity to CH3OH and HCHO can be tuned by altering the amount of oxygen and solvent, reaction time, irradiation wavelength and intensity.
Since HCHO is the oxidation product of CH3OH, increasing the oxidation capacity is an efficient strategy for elevating its selectivity. Thus, ultraviolet irradiation (300-400 nm, 170 mW cm-2) was used to promote the conversion of CH4 and accelerate the oxidation of CH3OH to HCHO to be 86.7% selectivity. Conversely, diminished oxidation can enhance the selectivity for CH3OH. Using visible light (400-780 nm), a remarkable CH3OH selectivity of 96.6% was achieved.
According to the conducted isotopic tests, the capture of intermediates, and the identification of the rate-determining step, a feasible radical mechanism for the oxidation of CH4 on q-BiVO4 is proposed. Under light irradiation, q-BiVO4 is excited to produce hydroxyl radicals (HO·) for C-H bond cleavage. The generated H3C· combines with O2, a proton and one electron to form CH3OOH, which will be reduced by electrons or decomposed under UV irradiation to CH3OH. Upon oxidation, the as-formed CH3OH is further activated to produce HCHO.
Hopefully, this research will provide a more accurate and detailed understanding of the photocatalytic methane oxidation process and lay the foundation for future methane oxidation work.
This work was sponsored by National Natural Science Foundation of China, “Strategic Priority Research Program” of Chinese Academy of Sciences and Frontier Science Key Project of Chinese Academy of Sciences.
National Center for Nanoscience and Technology