A tuned Lewis acidic catalyst guided by hard–soft acid–base theory to promote N2 electroreduction

Abstract

The electrocatalytic N₂ reduction reaction (NRR) to ammonia (NH₃) driven by intermittent renewable electricity under ambient conditions offers an alternative to the energy-intensive Haber–Bosch process. However, as a distinct core of the process, the design strategy of the electrocatalyst for enhancing the N₂ activation ability is still in a trial-and-error stage due to the absence of theoretical guidance. As a result, the corresponding NH₃ yield rate and selectivity are much lower than that required for implementation at scale. In this work, on the basis of the hard–soft acid–base theory, we report a paradigm for the design of an electrocatalyst with tuned Lewis acidity to efficiently activate and reduce N₂ to NH₃. As a proof of concept, it is revealed that enhancing the Lewis acidity of the molybdenum sulfide (MoS_x) model catalyst supported on carbon nanotubes can greatly improve its activation ability toward the N₂ molecule. Accordingly, a high faradaic efficiency of 21.60 ± 2.35% and NH₃ yield rate of 40.4 ± 3.6 μg h⁻¹ mg_cat.⁻¹ are obtained over the modified MoS_x, which are ∼2 times enhanced in comparison with the original MoS_x, respectively. Density functional theory calculations verify that the electron transfer from the occupied σ orbitals of N₂ to the empty d orbitals of Mo sites within MoS_x can be greatly accelerated by tuning the Lewis acidity of MoS_x to match with the basicity of N₂, thereby enhancing the N₂ activation process via the σ → d donation mechanism.