Hollow AMn₂O_4-δ (A = Co, Zn, Ni) nanotube for direct photo-oxidation of methane to C1 and C2 alcohols at atmospheric pressure and room temperature

Methane (CH₄) conversion is brought into research hotspot in the field of environment, and direct photo-oxidation of methane into C1 and C2 alcohols under moderate conditions remains a huge challenge. Herein, we report a series of hollow AMn₂O_4-δ (A = Co, Zn, Ni) spinel nanotubes (HNTs) for the direct photo-oxidation of methane (DPOM) reaction. The results of catalytic performance demonstrated that the CH₄ conversion and the yields of high value-added liquid products arrange in order of ZnMn₂O_4-δ HNTs > CoMn₂O_4-δ HNTs > NiMn₂O_4-δ HNTs. The ZnMn₂O_4-δ HNTs exhibited a superior CH₄ conversion of 53.9 %, a liquid product yield of 241.7 μmol g⁻¹h⁻¹ (CH₃OH 40.1 μmol g⁻¹h⁻¹, CH₃CH₂OH 200.9 μmol g⁻¹h⁻¹) and a selectivity of 100 % under light irradiation at atmospheric pressure and room temperature conditions. Xray photoelectron spectroscopy (XPS) and low-temperature electron paramagnetic resonance (EPR) analysis showed that the substitution of Zn ion at the A-site of manganate spinel significantly improved surface chemisorbed oxygen (O_α) and oxygen vacancy activity, which were beneficial to the formation of •CH₃. Multi-technology photocatalytic activity characterizations results showed that ZnMn₂O_4-δ HNTs possessed a narrower band gap and greatly favored the separation of photogenerated carriers. Moreover, the DPOM reaction mechanism involving the formation and dehydrogenation of alkyl and alkoxy intermediates was proposed over AMn₂O_4-δ (A = Zn, Co, Ni) HNTs, which was investigated through temperature-programmed desorption of CH₄ (CH₄-TPD), in situ EPR, in situ Fourier transform infrared spectroscopy (FT-IR) and density functional theoretical (DFT) calculations. It emphasized that CH₃OH was formed via the combination of •CH₃ and •OH. And it was more inclined to generate CH₃CH₂OH through the intermediates *CH₂CH₃/*CH₃O. This study could broaden the avenue toward the application of manganese-based spinel in the direct photo-oxidation of methane into alcohols and offer a disparate perspective on the role of the substitution of metal ion at the A-site in enhancing the photocatalytic performance.