(639c) Trends in the Electrocatalytic Nitrate Reduction Reaction across Transition Metals

Electrocatalytic reduction is a promising approach to remediate nitrate (NO₃^−), one of the world's most widespread water pollutants. In this talk, we will discuss our efforts to find activity and selectivity trends of transition metals for electrocatalytic nitrate reduction to benign or value-added products such as N₂ and NH₃. Using density functional theory (DFT) calculations, we find that the adsorption strengths of oxygen and nitrogen atoms act as descriptors for the overall activity and selectivity of nitrate reduction electrocatalysts. Volcano plots, surface species coverages, and the degree of rate control were predicted for transition metal electrocatalysts as a function of applied potential using DFT-based microkinetic modeling. Our microkinetic model rationalizes a number of experimental observations for nitrate reduction, including the activity trends of transition metals and the dependence of activity on applied potential. The rate-determining step and rates for nitrate reduction were predicted to be potential dependent due to significant changes in surface coverages of adsorbed species, particularly the influence of potential on adsorbed hydrogen and nitrate. We show that in situ XANES and EXAFS can be used to directly probe the changes in adsorption with potential in the presence of nitrate and explain the observed reactivity dependence on Pt, consistent with previous hypotheses and our microkinetic model. The highest predicted electrocatalytic activities for single metals are on Rh, followed by Cu, Pt and Pd at positive potentials (0 – 0.4 V) because of the moderate nitrate and hydrogen adsorption energy of these four metals. Although Fe is predicted to be the most selective for N₂ production at negative potentials, it is unstable in acids (as a single metal) unless cathodically protected. Our theoretical volcano plots predict Fe₃Ru, Fe₃Ni, Fe₃Cu and Pt₃Ru are potential catalysts for N₂ formation at ≤ 0 V, which may be stable in acids. We also predict Rh₃Sn and Pt₃Ni alloys will be active for NO formation at > 0 V vs RHE. Some of the identified alloys match previously reported experimental work of alloys that have synergistic activity, i.e., Rh-Sn alloys are more active than Rh or Sn individually. Future work will consist of experimentally synthesizing and testing the new predicted alloys for activity and selectivity. Ultimately, this work gives deeper insight into nitrate reduction on transition metal surfaces and can guide the design of improved electrocatalysts for nitrate remediation.