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

The electrocatalytic N-2 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 N-2 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 N-2 to NH3. As a proof of concept, it is revealed that enhancing the Lewis acidity of the molybdenum sulfide (MoSx) model catalyst supported on carbon nanotubes can greatly improve its activation ability toward the N-2 molecule. Accordingly, a high faradaic efficiency of 21.60 +/- 2.35% and NH3 yield rate of 40.4 +/- 3.6 mu g h(-1) mg(cat.)(-1) are obtained over the modified MoSx, which are similar to 2 times enhanced in comparison with the original MoSx, respectively. Density functional theory calculations verify that the electron transfer from the occupied sigma orbitals of N-2 to the empty d orbitals of Mo sites within MoSx can be greatly accelerated by tuning the Lewis acidity of MoSx to match with the basicity of N-2, thereby enhancing the N-2 activation process via the sigma -> d donation mechanism.

Automated summary

The study presents a paradigm for designing an electrocatalyst with tuned Lewis acidity to efficiently activate and reduce N-2 to NH3 based on the hard-soft acid-base theory. Increasing the Lewis acidity of the molybdenum sulfide (MoSx) model catalyst supported on carbon nanotubes greatly improves its ability to activate the N-2 molecule, resulting in significantly enhanced NH3 yield rate and selectivity. Density functional theory calculations confirm that tuning the Lewis acidity of MoSx to match the basicity of N-2 can accelerate the N-2 activation process via the sigma -> d donation mechanism.