Elucidation of Pathways for NO Electroreduction on Pt(111) from First Principles

Abstract

The mechanism of nitric oxide electroreduction on Pt(111) is investigated using a combination of first principles calculations and electrokinetic rate theories. Barriers for chemical cleavage of N-O bonds on Pt(111) are found to be inaccessibly high at room temperature, implying that explicit electrochemical steps, along with the aqueous environment, play important roles in the experimentally observed formation of ammonia. Use of explicit water models, and associated determination of potential-dependent barriers based on Bulter-Volmer kinetics, demonstrate that ammonia is produced through a series of water-assisted protonation and bond dissociation steps at modest voltages (<0.3 V). In addition, the analysis sheds light on the poorly understood formation mechanism of nitrous oxide (N2 O) at higher potentials, which suggests that N2 O is not produced through a Langmuir-Hinshelwood mechanism; rather, its formation is facilitated through an Eley-Rideal-type process.

Atomistic Insights into Nitrogen-Cycle Electrochemistry: A Combined DFT and Kinetic Monte Carlo Analysis of NO Electrochemical Reduction on Pt(100)

“…30 Even at the saturated coverages, however, we found that formation of the trans-(NO) 2 dimer on Pt(100) requires a 1.32 eV kinetic barrier, indicating that this reaction will not occur at room temperature.…”

“…52 Corrections to surface adsorbate energies from hydrogen bonding effects are applied, as discussed in a previous study. 30 Reaction barriers for N-O bond dissociation and protonation of intermediates are modeled within a 3 × 3 unit cell and determined using the climbing image nudged elastic band method (CI-NEB) implemented by Henkelman and co- workers. 53 A hydrogen shuttling model is employed for protonation barrier calculations.…”

“…0.4 V SHE , with exclusively NH 4 + production from NO stripping using single crystalline Pt, and NH 4 + and N 2 O from continuous reduction of NO using polycrystalline Pt. It has also been observed that the Pt(111) surface, which has been analyzed by both experiments and theory, 9,23,27,30 shows high activity for the conversion of NO to ammonia in acid electrolytes. Recently, our group has studied the NO reduction mechanism in acid electrolyte on the single crystal Pt(111) surface using periodic, self-consistent density functional theory calculations (DFT), 30 wherein we illustrated that both NH 4 + and N 2 O could be produced on Pt(111) through a series of water-assisted protonation steps at different potentials.…”

“…9,30 This result is intriguing for two reasons. First, it is often the case that bond formation, in this case N-N bonds, can occur with lower overbarriers on the square lattice of the (100) surface as compared to the hexagonal (111) surface.…”

“…In our previous study on the Pt(111) surface, we illustrated how both the thermodynamics and potentialdependent protonation, through a series of water-assisted hydrogen shuttling steps, play a key role in NO electroreduction. 30 These effects are also expected to play an important role on Pt(100), but in addition, coverage-dependent interactions could have a substantial impact on the (100) results, given the strong binding of NO to this surface. To describe all of these interlocking effects, a simulation that takes into account coverage-dependent interactions, activation barriers, and potential-dependent energetics in real time would be highly desirable.…”