New mechanism revealed for ambient nitrogen fixation
A palladium catalyst enables efficient electrochemical ammonia synthesis at low overpotential under ambient conditions, which is attributed to a unique hydride transfer mechanism for nitrogen hydrogenation.
The paper in Nature Communications is here: https://go.nature.com/2k4UZXy
Nitrogen fixation is a research topic that has received continuous interest, due to the vast need of ammonia for producing fertilizers to feed the world’s ever-growing population. Current industrial ammonia production via the Haber-Bosch process is energy-intensive (costing 1−2% of the global annual energy supply), which relies on fossil fuels and releases large amount of CO2. A sustainable process for ammonia production is urgently needed to mitigate the global energy and environment crisis.
Among the numerous efforts to advance sustainable ammonia synthesis, electrochemical reduction of N2 at ambient conditions has recently received rising interest in the academia, as it is compatible with intermittent energy supply from renewable sources such as solar and wind. Nevertheless, there are major challenges for electrochemical ammonia synthesis, due to the sluggish kinetics and the competition from hydrogen evolution reaction (HER). Previous studies indicated that the rate-limiting step is adding the first hydrogen to a nitrogen molecule through a proton-coupled electron transfer process. Inspired by a recent work of palladium-catalyzed electrohydrogenation of CO2 to formate (J. Am. Chem. Soc. 2015, 137, 4701), we hypothesized that palladium may also be a good catalyst for N2 hydrogenation to NH3 due to the formed palladium hydride.
As palladium is a highly active catalyst for HER, we first found an electrolyte that can effectively suppress the undesired HER, that is, a neutral phosphate buffer solution. In such an electrolyte, we observed interesting catalytic performance for palladium-catalyzed nitrogen reduction, showing a high activity and selectivity at a potential near the equilibrium potential (Note: it is different from the standard potential as our experimental conditions are non-standard). Our computational studies revealed a palladium-catalyzed hydride transfer process for nitrogen fixation with a lower energy barrier. Basically, palladium first absorb hydrogen in it forming a palladium hydride, and then the hydrogen atoms in palladium can hop and add to nitrogen molecules. Such a mechanism involves an electrochemical formation of palladium hydride and a chemical transfer of hydride to nitrogen. The reaction rate showed a weak dependence on the potential, indicating that the rate-limiting step should be the chemical hydride transfer.
While interesting, there is still much to do to understand and improve it. We will further investigate the catalytic process using advanced techniques including synchrotron-based X-ray spectroscopy, which can be used to reveal catalyst structure and chemical state during electrochemical reactions. The mechanism revealed in our research may be an electrochemical promotion of catalysis (EPOC) effect, which has rarely been reported in room-temperature aqueous systems. Similar principles may also be utilized to address other challenging reactions for renewable energy conversion.