Electrochemical Reduction of Nitric Oxide on Pt from a combined DFT and kinetic Monte Carlo calculations

COMP Seminar (Otakaari 1). Speaker: Prof. Karoliina Honkala (University of Jyväskylä).

Studying the low temperature electrocatalytic NO reduction is a first step towards understanding the fundamentals of nitrate and nitrite electrochemistry and developing improved electrochemical denitrification technologies to remove NO_x species from groundwater. Furthermore in many electrocatalytic processes, adsorbed NO is considered to be a selectivity-determining species, which influences the overall rate of reaction. A lack of atomic-level understanding of the molecular pathways that control the overpotential and product distribution have limited the advancement of NO chemistry to the same level of well-understood  electrocatalytic processes like oxygen reduction reaction.

We have employed a combined density functional theory (DFT) and kinetic Monte Carlo approach to gain atomic level insight into the reaction pathways for NO reduction on the Pt(100) surface under experimental reaction conditions.  Periodic, self-consistent DFT calculations were used to illustrate the thermodynamics and kinetics of a series of intermediates for the electrochemical conversion of NO to possible product species: ammonium (NH₄⁺), nitrogen gas (N₂), and nitrous oxide (N₂O) at both low and saturated NO coverages.  Our DFT results suggest that at low coverage a HNO intermediate dominates, while at experimentally observed NO coverages there is a significant thermodynamic and kinetic competition between two pathways proceeding either via NOH or HNO intermediates.  Kinetic Monte Carlo simulations enable modeling experimental conditions (i.e., voltage sweep over time) and shed light on the reaction mechanism.  This allows showing a direct one-to-one comparison to voltammetric sweep data obtained in previous experimental reports and identifying the essential intermediates and mechanism for NO reduction at experimentally observed potentials. Furthermore, our approach solves the mystery of why, though thermodynamically preferred, N₂ (g) (as well as N₂O (g)) is not produced in the electrochemical reduction of NO on the Pt(100) surface.