Critical impacts of interfacial water on C–H activation in photocatalytic methane conversion

Abstract

On-site and on-demand photocatalytic methane conversion under ambient conditions is one of the urgent global challenges for the sustainable use of ubiquitous methane resources. However, the lack of microscopic knowledge on its reaction mechanism prevents the development of engineering strategies for methane photocatalysis. Combining real-time mass spectrometry and operando infrared absorption spectroscopy with ab initio molecular dynamics simulations, here we report key molecular-level insights into photocatalytic green utilization of methane. Activation of the robust C–H bond of methane is hardly induced by the direct interaction with photogenerated holes trapped at the surface of photocatalyst; instead, the C–H activation is significantly promoted by the photoactivated interfacial water species. The interfacial water hydrates and properly stabilizes hydrocarbon radical intermediates, thereby suppressing their overstabilization. Owing to these water-assisted effects, the photocatalytic conversion rates of methane under wet conditions are dramatically improved by typically more than 30 times at ambient temperatures (~300 K) and pressures (~1 atm) in comparison to those under dry conditions. This study sheds new light on the role of interfacial water and provides a firm basis for design strategies for non-thermal heterogeneous catalysis of methane under ambient conditions.

Beyond Reduction Cocatalysts: Critical Role of Metal Cocatalysts in Photocatalytic Oxidation of Methane with Water**

“… The kinetic oxidation model of methane and methane‐derived surface intermediates on the catalyst surface covered with one layer of adsorbed water. The water oxidation process is omitted from the diagram because it is not rate‐determining in the photocatalytic oxidation of methane with water [7b] . a) Kinetic model and b) possible eight‐step reaction pathways and intermediates (X i, i=1–8 shown in Scheme 1a; e.g.…”

“…Based on the detailed kinetic analyses [7b] provided in Supporting Information Section 4, the observed P CH4 profiles of r CO2 and r C2H6 for the Pt/Ga 2 O 3 photocatalyst under steady‐state reaction conditions could be simply described as the first‐ and second‐order reactions of the initially adsorbed intermediate X 1 as r CO2 $${{\rm { \propto }}}$$ θ X1 and r C2H6 $${{\rm { \propto }}}$$ θ X1 2 , respectively, where θ X1 is the coverage of X 1 characterized by the Langmuir adsorption isotherm‐like P CH4 ‐dependence as follows: $$$\vcenter{\openup.5em\halign{$\displaystyle{#}$\cr {\theta }_{{X}_{1}}{\rm { =}}{{{{\rm { P}}}_{{{\rm { CH}}}_{{\rm { 4}}}}}\over{{{\rm { P}}}_{{{\rm { CH}}}_{{\rm { 4}}}}{\rm { \ +\ }}{{\rm { K}}}^{{\rm \ }-{\rm { 1}}}}}\hfill\cr}}$$$ …”

“…Although water itself is not involved in the equation of methane coupling (2CH 4 →C 2 H 6 +H 2 ), the presence of the adsorbed water layer enhanced ethane formation (Figure S6). The roles of the adsorbed water species in photocatalytic methane oxidation are investigated and discussed in detail in our previous study [7b] …”

High-density frustrated Lewis pairs based on Lamellar Nb2O5 for photocatalytic non-oxidative methane coupling

“…In recent years, widespread interest has been aroused in converting methane into high value-added chemicals through photocatalytic oxidative or non-oxidative way, which can not only promote the development of clean energy but also effectively achieve the purpose of environmental protection by consuming greenhouse gases 1 – 10 . Researchers have used various strategies to improve the efficiency of photocatalytic methane conversion, such as introducing other auxiliary oxidants 11 – 15 , heat-assisted photoreaction 16 , and improving the separation efficiency of photogenerated carriers 17 . Unfortunately, the photocatalytic methane conversion efficiency is still commonly low, especially for the non-oxidative methane coupling (NOCM) reaction due to the much easier photocatalyst deactivation in the absence of oxidants 18 , 19 , which however is more desirable considering the high carbon-atom efficiency and less CO 2 emission.…”

Metal-Loaded Semiconductor-Photocatalysis of Alcohols for Selective Organic Synthesis: A Personal Account

“…67 68 Sugimoto and co-workers recently reported that methyl radicals would be stabilized on an SP surface, which would also be affected by H 2 O molecules adsorbed on the surface, based on their theoretical calculations. 69 Thus, the water content of the reaction mixture should be carefully optimized 37 for efficient and selective β-scission.…”