(212e) NO Electrochemical Reduction on Pt(100) from First Principles

Electrochemical denitrification is a promising technology for the removal of toxic nitrate and nitrite species from groundwater due to this processâ??s environmental compatibility, energy efficiency, safety, and product selectivity.^[1] The adsorbed NO is generally considered to be a selectivity-determining species for the reaction in acid electrolytes.^[2]However, molecular-level insights into the mechanism are challenging to obtain from experiments alone, and atomic-scale studies using density functional theory (DFT) can thus provide important information about general reaction scheme of NO electrocatalysis. These basic building blocks, determined on the most common electrocatalyst for this reaction, platinum, may, in turn, serve as a starting point for future trends-based analysis on different transition metal surfaces.

While Pt(111) terraces are the lowest in energy of any single crystal Pt surface, terraces of Pt(100) have shown unusual activity and selectivity patterns for the electroreduction of NO.^[3] Therefore, in this work, we use periodic, self-consistent DFT calculations to clarify the adsorption structures and energetics of NO and its reaction intermediate species on the Pt(100) surface. Solvation energies and protonation kinetics are calculated using water-incorporated systems in the DFT analysis.^[4]In addition, we implement a rigorous kinetic Monte Carlo (kMC) approach that takes into account coverage-dependent interactions and activation barriers and that as that produces potential-dependent values of current from multiple reaction pathways as a function of voltage.

In our simulation, the most favorable pathway is the ammonia formation. The mechanism consists of two consecutive proton-electron transfer steps after the adsorption of NO (denoted the EE mechanism), with favorable formation of NOH* as a first intermediate. The Tafel slope, which was obtained by plotting peak potentials at different scan rates in kMC simulations, supports the EE mechanism rather than the chemical dissociation steps. Also, due to the fast protonation, the probabilities of forming N₂ or N₂O are negligible. Finally, implications of these results for both NO and nitrate electroreduction on other transition metal surfaces will be discussed.

[1] M. Duca and M. T. M. Koper, Energ. Environ. Sci. 5 (2012) 9726.
[2] A. Cuesta, M. Escudero, Phys. Chem. Chem. Phys. 10 (2008) 3628.
[3] V. Rosca, M. T. M. Koper, J. Phys. Chem. B 109 (2005) 16750.
[4] A. Clayborne, H. J. Chun, R. B. Rankin, J. Greeley, Angew. Chem. Int. Ed. 54 (2015) 8255.