(346h) Nitrogen Electroreduction: Elementary Kinetic Analysis and Surface Functionalization for Improved Catalysis

In a future with plentiful renewable electricity, electro-chemical systems to carry out chemical transformations can provide an efficient and sustainable alternative to fossil-fuel based energy consumption. Several important chemical reactions, such as CO₂ and N₂ reduction, have electro-chemical counterparts to form useful products through a series of proton-coupled electron transfer (PCET) reactions. The activity and selectivity of PCET reactions can significantly be impacted by the electrode-electrolyte interfacial properties, and tuning this near-surface environment can be used as a control towards developing active and selective electrochemical conversions. Surface-bound amino acid chains can be used to alter the electrocatalytic performance by altering the active site structure and local environment.

We first use a simple Density Functional Theory (DFT) based approach to estimate the electrochemical barriers [1] to elucidate the elementary kinetics of the possible associative mechanisms for N₂ electroreduction (NRR) on two low index surfaces of Fe [2]. Calculated activation barriers suggest significantly larger overpotentials for NRR than those inferred from consideration of only elementary reaction free energies. Key step barriers on low index surfaces of late transition metals resulted in a “kinetic volcano” for nitrogen reduction over (111) surfaces of FCC metals. This analysis suggests that transition metal catalysts will be ineffective for NRR, due to high elementary step barriers and low selectivity [3].

We further use DFT to study the surface binding of short amino acid chains on Fe₂O₃ surfaces to understand how they alter the interfacial environment through surface-ligand interactions. We examine the effects of small amino acid binding on the stability of different Fe₂O₃ terminations in the electrochemical environment. We consider how presence of continuum solvation affects the binding preferences of the amino acid. Coverage effects on binding energies are also evaluated.

Significance

This work demonstrates the extreme challenge in developing active and selective N₂ electroreduction catalyst to produce ammonia. Also, use of amino acid as co-catalyst to modify catalyst surface and environment provides a unique design variable to improve catalytic activity and selectivity.

References

[1] G. Rostamikia et al., J. Power Sources 196(22), 9228–9237 (2011).

[2] S. Maheshwari et al., J. Chem. Phys. 150, 041708 (2018)

[3] G. Rostamikia et al. Cat. Sci. & Tech, 9 (2019) 174-181.