Copper–palladium catalyst shows high efficiency in green ammonia production

A team of functional materials researchers in China developed a copper–palladium catalyst that has been shown to improve catalytic activity and selectivity in the electrochemical nitrate reduction reaction (NO₃RR), leading to improved ammonia yields. Scaling this process could significantly reduce the energy and environmental burden of the ammonia industry as a whole.

At present, the large-scale production of NH₃ relies on the Haber–Bosch process, which requires high-temperature and high-pressure conditions while consuming significant quantities of fossil fuels and emitting large amounts of greenhouse gases. Given the importance of ammonia as a chemical feedstock in fertilisers, pharmaceuticals, textiles and other industries, there is significant incentive to improve efficiency and address externalities in the production of ammonia. It is thanks to advances in electrochemical technologies that the reduction reaction has emerged as a viable alternative.

NO₃RR is an environmentally friendly method of reducing nitrates to ammonia, and holds major potential for sustainable management of the nitrogen cycle and the green production of ammonia. It can be driven by renewable electricity, and utilises nitrate byproducts from other human activities. If it can be scaled up, the process could make a significant contribution to addressing nitrate pollution.

Identifying potential catalysts that could provide high Faradaic efficiency (FE) and ammonia yield has proven to be an obstacle to the large-scale application of NO₃RR. Recent studies have favoured transition metals such as copper, cobalt and nickel, as well as noble metals including ruthenium, silver and palladium. Copper has shown strong NO₃− activation capacity, though in isolation it is characterised by excessive NO₂− production and relatively low FE and yield. Palladium exhibits good adsorption–desorption characteristics, and is regarded as an efficient electrocatalyst.

In the recent study, the team prepared a self-supported nanoporous bimetallic copper and palladium-based catalyst (Cu–Pd@ZrCuAl) through dealloying and galvanic replacement. This structure provides a large surface area and unique electronic properties, essential for high catalytic activity. It was shown that the addition of just a tiny amount of palladium enhanced H₂O dissociation, providing sufficient H atoms for the hydrogenation of nitrate to produce NH₃, while suppressing the competing hydrogen evolution reaction and reducing NO₂ accumulation. An outstanding FE of 95% was achieved, with an impressive NH₃ yield rate approx. 10% higher in comparison to other bimetallic catalysts.

The authors theorised that the Cu–Pd heterostructure modifies the electronic structure of the copper such that the adsorption behaviour of the intermediates is optimised and the energy barrier of the rate-determining step (RDS) – the ammonia desorption – is reduced. In practice this means that the Cu–Pd@ZrCuAl electrocatalyst is highly effective in the reduction of nitrate to ammonia, and is therefore able to provide rapid ammonia generation. The catalyst also shows outstanding long-term stability.

Looking ahead, ammonia is also viewed as a promising potential energy carrier, and its emissions-free production could support efforts to advance the energy transition and adopt a circular economy.

The authors of the study are all representatives of the State Key Laboratory of Precious Metal Functional Materials, the Tianjin Key Laboratory of Composite and Functional Materials and the School of Materials Science and Engineering, all of which are based in Tianjin, China. Support for the project was provided by the National Natural Science Foundation of China.

Source: https://pubs.acs.org/doi/10.1021/acsami.5c02269