New palladium-gold alloy catalyst boosts methane-to-ethylene conversion with solar power

Researchers just hit two benefits with one catalyst. They converted methane (CH₄), a potent greenhouse gas, into ethylene (C₂H₄), a highly demanded raw material in industrial sectors, using sunlight and a newly designed palladium–gold (PdAu) alloy cocatalyst. This process not only removed an unwanted gas from the atmosphere but also turned it into a valuable resource by harnessing solar power.

Methane is not usually what comes to mind when we think of greenhouse gases responsible for climate change. It is the second-highest contributor to global warming after carbon dioxide, with heat-trapping abilities significantly greater than those of the leading contender. Methane does not often take center stage in climate discussions because it has a short life span of about a decade, while carbon dioxide can remain in the atmosphere for hundreds of years.

However, over 100 years, methane’s impact can be about 28 to 36 times that of carbon dioxide. Reducing its atmospheric emissions can help lower global temperatures in the long run.

In this study published in the Journal of the American Chemical Society, the team explored how a range of palladium (Pd), gold (Au), and PdAu alloy-based catalysts performed at photocatalytic oxidative coupling of methane (POCM).

They found that coating PdAu cocatalysts onto the surface of a commonly used titanium dioxide (TiO₂) photocatalyst led to methane conversion rates of 13.73 mmol g^-1 h^-1 and ethylene production rates of 0.18 mmol g^-1 h^-1, with a quantum efficiency of 12% at 350 nm. These values achieved by PdAu/TiO₂ cocatalyst outperformed most POCM catalysts reported to date.

Making and breaking carbon bonds

POCM gained popularity as a sustainable method for converting methane into useful chemicals. The process involves a photocatalyst absorbing energy from sunlight to trigger chemical reactions that break the strong C–H bond in methane and form a new C–C bond, yielding multicarbon products such as ethane (C₂H₆) or ethylene (C₂H₄).

Most POCM catalysts available today have successfully produced ethane but have struggled with ethylene. Since ethylene is more valuable and widely used, scientists are searching for ways to improve the efficiency of converting methane into this product.

One crucial step toward achieving this goal is understanding exactly how carbon–carbon (C–C) bonds form during the reaction.

The researchers of this study first synthesized a range of PdAu alloy-based cocatalysts—including PdAu/TiO₂—and then applied advanced spectroscopic tools and theoretical calculations to uncover the POCM mechanism and investigate the methane conversion efficiency of the designed catalysts.

Analyzing the reaction intermediates and products revealed that the strong bonding between the metal cocatalyst and the hydrocarbon fragments (*CH₂ or *CH₃) was the driving force behind the quick and selective conversion of methane. They found that coating the TiO₂ photocatalyst with PdAu alloy made methane activation significantly easier.

The alloy lowered the energy needed for carbon–carbon coupling and helped generate *CH₂ intermediates, making it easier for two *CH₂ units to link. Together, these three factors steered the reaction toward producing more ethylene.

The researchers note that these results demonstrate how alloy engineering can be a powerful strategy for improving methane conversion and selectively producing economically important chemicals.

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