Theoretical design and experimental implementation of Ag/Au electrodes for the electrochemical reduction of nitrate

Abstract

The current imbalance in the biogeochemical cycle of nitrogen is as serious as that of carbon. One way to mitigate this problem is through the electrochemical reduction of nitrates under mild conditions, which is an appealing though not fully understood process. Therefore, deeper insight into the electrocatalytic reaction mechanism is needed to optimize this process. Here we thoroughly analyse the adsorption energy of nitrate with DFT calculations on various surface facets of pure Au, Ag, and their near-surface and surface alloys, as the adsorption and subsequent reduction of nitrate are thought to be rate limiting in the electrocatalytic reaction. The observed systematic trends allow prediction of the surface with highest electrocatalytic activity for the reduction of nitrate. This prediction was verified experimentally by depositing sub-monolayer amounts of Ag on polycrystalline Au electrodes. We observe a well-defined volcano curve which correlates the amount of Ag deposited on the surface with the current density at a fixed potential, with the peak activity around 2/3 ML Ag surface coverage. The electrocatalytic activity and stability of the bimetallic Ag-Au systems, found through the interplay of theoretical modelling and empirical observations, serve as a clear example for the rational design of novel catalytic materials and confirm the key role that the adsorption of nitrate plays in the overall nitrate reduction rate.

Electrocatalytic Reduction of Nitrate on a Pt Electrode Modified by p‐Block Metal Adatoms in Acid Solution

“…Nitrate species can bind to the surface in a monodentate, bidentate, or tridentate fashion to the surface atoms. The bidentate configuration was found to be most stable on all of the SAs and surfaces with adatoms considered in this study, in agreement with previous studies 35. In the case of the SAs, the most stable configurations are found when ✶NO 3 binds with an oxygen atom to a Pt atom and another oxygen atom to a Sn/In atom.…”

“…To improve our understanding of the role of promoters for nitrate reduction, we performed DFT calculations of pure and modified Pt(1 1 1) with Sn and In adatoms, the two best promoters from the experimental study, to gain some insight on the role of these promoters in the reduction of nitrate. Our DFT calculations followed a strategy similar to a recent combined experimental–computational study of nitrate reduction on Au–Ag bimetallic electrodes35 and were based on the fact that the rate‐limiting step in this electrocatalytic process was known to be the conversion of nitrate into nitrite. Thus, the adsorption energies of ✶NO 3 might be used as descriptor for the overall reaction rate.…”

“…We noted that the total energy of gas‐phase nitric acid was not correctly estimated within standard DFT, as was discussed in detail in ref. 35. A simple verification of this statement is given by the standard formation energy of this compound, based on Equation (5).…”

Square Wave Voltammetry Measurements of Low Concentrations of Nitrate Using Au/AgNPs Electrode in Chloride Solutions

“…Other authors also managed to highlight the formation of co‐products depending on the experimental conditions (pH, electrolyte, nitrate concentration) used . According to the bibliography, nitrate reduction requires first the adsorption of NO 3 − on the electrode surface (reaction 1) . The step determining the rate of the reaction on most electrodes is the reduction of NO 3 − (ads) to NO 2 − (ads) (reaction 2) .…”

“…Adsorption is also promoted on small nanoparticles as the effective surface is bigger and so makes the reduction on this electrode easier. This is why pure bare gold electrode is inactive toward nitrate reduction (weak adsorption of the reactants) . The maximum catalytic activity should be for a surface partially covered with AgNPs which is the case for all the charges tested as the characteristic reduction of O 2 in two steps on gold is always visible on Figure .…”