Vacancy-enabled N2 activation for ammonia synthesis on an Ni-loaded catalyst

Abstract

Ammonia (NH₃) is pivotal to the fertilizer industry and one of the most commonly produced chemicals¹. The direct use of atmospheric nitrogen (N₂) had been challenging, owing to its large bond energy (945 kilojoules per mole)^2,3, until the development of the Haber-Bosch process. Subsequently, many strategies have been explored to reduce the activation barrier of the N≡N bond and make the process more efficient. These include using alkali and alkaline earth metal oxides as promoters to boost the performance of traditional iron- and ruthenium-based catalysts^4-6 via electron transfer from the promoters to the antibonding bonds of N₂ through transition metals^7,8. An electride support further lowers the activation barrier because its low work function and high electron density enhance electron transfer to transition metals^9,10. This strategy has facilitated ammonia synthesis from N₂ dissociation¹¹ and enabled catalytic operation under mild conditions; however, it requires the use of ruthenium, which is expensive. Alternatively, it has been shown that nitrides containing surface nitrogen vacancies can activate N₂ (refs. ^12-15). Here we report that nickel-loaded lanthanum nitride (LaN) enables stable and highly efficient ammonia synthesis, owing to a dual-site mechanism that avoids commonly encountered scaling relations. Kinetic and isotope-labelling experiments, as well as density functional theory calculations, confirm that nitrogen vacancies are generated on LaN with low formation energy, and efficiently bind and activate N₂. In addition, the nickel metal loaded onto the nitride dissociates H₂. The use of distinct sites for activating the two reactants, and the synergy between them, results in the nickel-loaded LaN catalyst exhibiting an activity that far exceeds that of more conventional cobalt- and nickel-based catalysts, and that is comparable to that of ruthenium-based catalysts. Our results illustrate the potential of using vacancy sites in reaction cycles, and introduce a design concept for catalysts for ammonia synthesis, using naturally abundant elements.