Enabling Specific Photocatalytic Methane Oxidation by Controlling Free Radical Type

Abstract

Selective CH4 oxidation to CH3OH or HCHO with O2 in H2O under mild conditions provides a desired sustainable pathway for synthesis of commodity chemicals. However, manipulating reaction selectivity while maintaining high productivity remains a huge challenge due to the difficulty in the kinetic control of the formation of a desired oxygenate against its overoxidation. Here, we propose a highly efficient strategy, based on the precise control of the type of as-formed radicals by rational design on photocatalysts, to achieve both high selectivity and high productivity of CH3OH and HCHO in CH4 photooxidation for the first time. Through tuning the band structure and the size of active sites (i.e., single atoms or nanoparticles) in our Au/In2O3 catalyst, we show alternative formation of two important radicals, •OOH and •OH, which leads to distinctly different reaction paths to the formation of CH3OH and HCHO, respectively. This approach gives rise to a remarkable HCHO selectivity and yield of 97.62% and 6.09 mmol g–1 on In2O3-supported Au single atoms (Au1/In2O3) and an exceptional CH3OH selectivity and yield of 89.42% and 5.95 mmol g–1 on In2O3-supported Au nanoparticles (AuNPs/In2O3), respectively, upon photocatalytic CH4 oxidation for 3 h at room temperature. This work opens a new avenue toward efficient and selective CH4 oxidation by delicate design of composite photocatalysts.

Retrofitting Zr-Oxo Nodes of UiO-66 by Ru Single Atoms to Boost Methane Hydroxylation with Nearly Total Selectivity

“…48 In contrast, a higher energy barrier of 1.09 eV is required to activate CH 4 to S30), indicating the importance of • OOH/ • OH in methane hydroxylation. 49 These results indicate that the Ru 1 �O* species can efficiently activate CH 4 to produce the main product of CH 3 OOH, in agreement with the experimental observations (Figure 4c). On the other hand, the reaction at the Zr oxo − • OH* terminal was also investigated.…”

Methane Photooxidation with Nearly 100 % Selectivity Towards Oxygenates: Proton Rebound Ensures the Regeneration of Methanol

“…After that, the localized photo‐induced carriers will combine with the adsorbed O 2 and H 2 O to generate the reactive oxygen species, which is considered to play an important role in the selective dehydrogenation process [18b, 22] . For example, it is generally believed that the high concentration of hydroxyl radicals (⋅OH) will accelerate the dehydrogenation in the reaction process of CH 4 conversion [6d, e, 23] . Except for the influence induced by ⋅OH, previous studies reported that in the reaction system with the oxidant O 2 , molecular oxygen will convert to hydroperoxyl radicals (⋅OOH).…”

Fenton-like Chemistry by a Copper(I) Complex and H2O2 Relevant to Enzyme Peroxygenase C–H Hydroxylation

“…More broadly, it has been recently suggested that nature may control metal-ion active site oxidative chemistries by utilizing the Fenton reaction in a “constructive manner” . It should also be noted that the hydroxyl radical may be generated by photolysis of water at metal/alloy surfaces (or even at the water–gas surface of water microdroplet) and in a controlled manner be utilized for organic oxidations including conversion of methane to methanol, removal of contaminants in water purification, and chemistry applied to bleaching; it may even be applied to cancer therapies …”