Room-Temperature Photooxidation of CH₄ to CH₃OH with Nearly 100% Selectivity over Hetero-ZnO/Fe₂O₃ Porous Nanosheets

Abstract

The huge challenge for CH 4 photooxidation into CH 3 OH lies in the activation of the inert C−H bond and the inhibition of CH 3 OH overoxidation. Herein, we design two-dimensional in-plane Z-scheme heterostructures composed of two different metal oxides, with efforts to polarize the symmetrical CH 4 molecules and strengthen the O−H bond in CH 3 OH. As a prototype, we first fabricate ZnO/Fe 2 O 3 porous nanosheets, where high-resolution transmission electron microscopy and in situ X-ray photoelectron spectroscopy affirm their in-plane Z-scheme heterostructure. In situ Fourier transform infrared spectra and in situ electron paramagnetic resonance spectra demonstrate their higher amount of •CH 3 radicals relative to the pristine ZnO porous nanosheets, in which density functional theory calculations validate that the high local charge accumulation on Fe sites lowers the CH 4 adsorption energy from 0.14 to 0.06 eV. Moreover, the charge-accumulated Fe sites strengthen the polarity of the O−H bond in CH 3 OH through transferring electrons to the O atoms, confirmed by the increased barrier from 0.30 to 2.63 eV for *CH 3 O formation, which inhibits the homolytic O−H bond cleavage and thus suppresses CH 3 OH overoxidation. Accordingly, the CH 3 OH selectivity over ZnO/Fe 2 O 3 porous nanosheets reaches up to nearly 100% with an activity of 178.3 μmol −1 g cat −1, outperforming previously reported photocatalysts without adding any oxidants under room temperature and ambient pressure.

Atomically Dispersed Nickel Anchored on a Nitrogen‐Doped Carbon/TiO2Composite for Efficient and Selective Photocatalytic CH4Oxidation to Oxygenates

“…The yield and selectivity of liquid oxygenates of Ni−NC/TiO 2 are higher than those of Ni−NPs/TiO 2 and TiO 2 , demonstrating the superiority of single atom Ni−NC cocatalysts for the photocatalytic CH 4 oxidation. The excellent photocatalytic performance observed over Ni−NC/TiO 2 is comparable to or even outperforms most reported photocatalysts decorated with either noble metal or non‐noble metal cocatalysts under similar experiment conditions (Table S2) [12, 13, 16, 17, 19, 20, 22–25, 38–40] …”

Elevating Photooxidation of Methane to Formaldehyde via TiO2 Crystal Phase Engineering

“…Distinct from the thermocatalytic systems, photocatalysis exploits clean solar energy to provide the Gibbs free energy for the reaction and simultaneously generates highly active free radical species, realizing the conversion of methane into value-added products (methanol, formaldehyde, formic acid, ethanol, etc .) under mild conditions. − Among those products, formaldehyde is highly desired since it is widely used as feedstocks in over 50 industrial processes . Actually, 40% of the industrial methanol nowadays is employed to synthesize formaldehyde. , Therefore, direct photocatalytic conversion of methane to formaldehyde is of great significance for sustainable development.…”

Targeted H2O activation to manipulate the selective photocatalytic reduction of CO2to CH3OH over carbon nitride-supported cobalt sulfide

“…11,12 Furthermore, the thermodynamically unstable CH 3 OH is easily over-oxidized during photocatalytic CO 2 reduction. [13][14][15][16] Therefore, it is a great challenge to achieve the selective photocatalytic reduction of CO 2 to CH 3 OH.…”