NO Electrochemical Reduction on Pt Electrocatalysts: A DFT Approach

Electrochemical denitrification is a promising technology for the removal of toxic nitrate and nitrite species from groundwater due to the process’s environmental compatibility, energy efficiency, safety, and product selectivity. The adsorbed NO is generally considered to be a selectivity-determining species during this electrochemical potential dependent reaction. Atomic-scale studies using density functional theory (DFT), in turn, can provide powerful molecular-level information about the elementary mechanisms of reductive NO electrocatalysis. These basic building blocks can further serve as a starting point for future trends-based analysis on different transition metal surfaces.  

In this work, we employ periodic, self-consistent DFT calculations to clarify the adsorption structures and thermochemistry of NO and its related reaction intermediate species on different Pt single crystal surfaces ((111) and (100)). Kinetics are further determined by calculating the activation barriers for N-O dissociation, protonation, and N-N bond formation. We begin by describing results for the reaction at low NO coverage (ca. 0.11 ML). After describing the thermodynamics (the adsorption) and kinetics (N-O bond breaking and protonation barrier) for the dentirification reaction scheme, a plausible mechanistic reaction pathway is proposed. Water-assisted protonation barriers are generally shown to be lower than barriers for non-electrochemical reactions, such as surface hydrogenation or N-O bond breaking. Consistent with available experimental evidence, from the most probable reaction pathway, we conclude that ammonium ion would be the most favorable product that would evolve. There is no evidence for the formation of hydroxylamine (H₂NOH) at low coverage. At higher NO coverages (ca. 0.45 ML), the relative energy states of the intermediates show a similar trend for the energetics when it is compared with the study with lower NO coverage.

Finally, N₂O formation is examined. We find that trans NO dimer species could be a precursor state for the N­₂O formation due to its lower kinetic barrier than barriers that are found for cis-type NO dimers. The simulation demonstrates that N₂O could be formed at higher potentials through two protonations of trans NO species, and the reaction is thermodynamically more favorable at higher NO coverage.  This barrier could be surmountable at room temperature, and these mechanistic conclusions hold on both the (111) and (100) surfaces.    

Based on our study, we find that reduction of NO proceeds through the water-assisted protonations which will finally lead to ammonia. The primary pathway for N2O formation occurs through trans-type NO dimer species. The proposed pathways occur with different applied electrochemical potentials and NO coverages.