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Methane (CHâ‚„), the colorless and odorless gas that makes up most natural gas on Earth, has so far been converted into useful fuels and chemicals via energy-intensive processes that need to be carried out at high temperatures. Some energy researchers, however, have been exploring the possibility of transforming this gas into useful hydrocarbons and chemicals via photocatalysis.

Photocatalysis is a process through which the energy contained in light, typically solar energy, activates a material known as a "catalyst," driving desired chemical reactions. Converting CHâ‚„ into specific fuels or chemicals via photocatalysis instead of conventional methods that rely on the burning of fossil fuels could be highly advantageous, as it could contribute to the reduction of greenhouse gas emissions.

Researchers at Hebei University and other institutes in China recently introduced a new photocatalysis-driven approach to convert CH₄ into propane (C₃H₈), a hydrocarbon that is easier to use in real-world settings, as it becomes liquid at specific pressures, which facilitates its storage and transport.

Their proposed method, outlined in a paper in Nature Energy, enables the efficient photocatalytic oxidative coupling of methane (POCM), a process through which methane molecules exposed to oxygen can combine and form larger hydrocarbons, utilizing titanium dioxide (TiOâ‚‚) as a catalyst.

"POCM enables the production of value-added fuels and chemicals using renewable solar energy," wrote Wenfeng Nie, Liwei Chen and their colleagues in their paper. "Unfortunately, despite recent advances in the production of C₂ chemicals (for example, ethane), POCM systems that selectively produce industrially useful and transportable C₃+ hydrocarbons remain elusive."

The photocatalytic process through which this team of researchers converted CH₄ into C₃H₈ is driven by an enhanced version of the photocatalyst TiO₂. This is a widely used semiconducting material, yet the team improved its photocatalytic capabilities by embedding gold (Au) nanoparticles in its pores (i.e., confined spaces in its structure).

"We report that Au-embedded porous TiO₂, activated by steam during the POCM process, enables efficient and selective flow synthesis of propane with a productivity of 1.4 mmol h^−1," wrote Nie, Chen and their colleagues. "At this productivity, we achieve a high propane selectivity of 91.3% and an apparent quantum efficiency of 39.7% at a wavelength of 365 nm."

In initial tests, the enhanced TiO₂-based catalyst introduced by the researchers was found to enable the synthesis of propane with remarkable productivity, quantum efficiency and selectivity. The results so far are highly promising, hinting at the possibility of realizing the efficient solar energy-powered photocatalytic conversion of methane into more valuable fuels on a large scale.

"Mechanistic studies reveal that the tensile-strained Au and the nanopore-confined catalytic microenvironment jointly stabilize key ethane intermediates, boosting deeper C₂–C₁ coupling to form propane," wrote the authors. "Meanwhile, the steam-activated surface lattice oxygen on TiO₂ accelerates hydrogen species transfer, thus enhancing POCM kinetics. This approach is economically feasible for practical application under concentrated solar light."

This recent study could soon open new possibilities for the clean upgrading of methane into hydrocarbons and chemicals that are still widely used both for transportation and in industrial settings. In the future, other research teams could build on the findings gathered by Nie, Chen and their colleagues to design other highly performing catalysts and POCM approaches.

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