Direct and Selective Photocatalytic Oxidation of CH₄ to Oxygenates with O₂ on Cocatalysts/ZnO at Room Temperature in Water

Abstract

Direct conversion of methane into methanol and other liquid oxygenates still confronts considerable challenges in activating the first C−H bond of methane and inhibiting overoxidation. Here, we report that ZnO loaded with appropriate cocatalysts (Pt, Pd, Au, or Ag) enables direct oxidation of methane to methanol and formaldehyde in water using only molecular oxygen as the oxidant under mild light irradiation at room temperature. Up to 250 micromoles of liquid oxygenates with ∼95% selectivity is achieved for 2 h over 10 mg of ZnO loaded with 0.1 wt % of Au. Experiments with isotopically labeled oxygen and water reveal that molecular O 2 , rather than water, is the source of oxygen for direct CH 4 oxidation. We find that ZnO and cocatalyst could concertedly activate CH 4 and O 2 into methyl radical and mildly oxidative intermediate (hydroperoxyl radical) in water, which are two key precursor intermediates for generating oxygenated liquid products in direct CH 4 oxidation. Our study underlines two equally significant aspects for realizing direct and selective photooxidation of CH 4 to liquid oxygenates, i.e., efficient C−H bond activation of CH 4 and controllable activation of O 2 .

Electrocatalytic Synthesis of Hydrogen Peroxide over Au/TiO2 and Electrochemical Trace of OOH* Intermediate

“…There is no denying that the electron-rich state is favorable for OOH* radical adsorption. [19] Therefore, Au is an ideal catalyst for the two-electron process. After the TiO 2 is loaded with Au, Au is in an electron-rich state due to the presence of Ti 3 + defects and oxygen vacancies, as a catalytically active site, whose ability to adsorb OOH* radicals is enhanced, which is profitable for the two-electron process.…”

Photocatalytic selective oxidation of methane by quantum sized bismuth vanadate

“…Optimizing the yield of both CH 3 OH and HCHO is carried out through varying O 2 amount ( Fig. 3b) 12,23 .…”

“…Amongst the investigated liquid products, methanol (CH 3 OH) and formaldehyde (HCHO) are two major targets because of their roles as the basic and widely used chemicals 10,11,12 . Until now, a few photocatalytic CH 4 conversions with satis ed yield of CH 3 OH and HCHO have been reported; for instance, photocatalytic oxidation of CH 4 over 0.1 wt% Au/ZnO and 0.1 wt% Ag/ZnO achieved 41.2 μmol CH 3 OH (15.7% selectivity) and 112.1 μmol HCHO (42.6% selectivity) 12 , respectively, under moderate reaction condition.…”

Catalytic Oxidation of Methane to Oxygenated Products: Recent Advancements and Prospects for Electrocatalytic and Photocatalytic Conversion at Low Temperatures

“…demonstrated by Song et al [156] The photogenerated holes and electrons of the catalyst system activated CH 4 and molecular O 2 into •CH 3 and •OOH radicals, respectively, which subsequently converted to oxygenates. Notably, the mild reactive •OOH radicals stopped further oxidation of the oxygenates.…”