Nitrate (NO3-) produced from industrial and agricultural processes has imbalanced the global nitrogen cycle. Electrocatalytic reduction is a promising route to remediate nitrate while generating products such as NH3 or N2. However, catalysts with improved stability, activity, and selectivity are needed to enable practical electrocatalytic nitrate reduction (NO3RR). To guide catalyst design, we have studied NO3RR on metals, alloys, and metal sulfides. Using density functional theory (DFT) calculations and microkinetic modeling, we predicted activity and selectivity trends of transition metals for NO3RR.[1] The adsorption strengths of O and N atoms act as descriptors for the overall activity and selectivity of electrocatalysts. Theoretical volcano plots, selectivity maps, and surface coverages were predicted for metal electrocatalysts. The microkinetic model rationalizes several experimental observations including the activity trends of pure metals and in situ X-ray absorption spectroscopy measurements of competitive adsorption between hydrogen and nitrate on Pt/C. Based on the volcano plots, we predicted that PtxRuy alloys are promising catalysts for NO3RR. To verify these predictions, the activity of platinum-ruthenium (PtxRuy/C) catalysts of different compositions for NO3RR were measured. The PtxRuy/C are more active than Pt/C, with Pt78Ru22/C six times more active than Pt/C at 0.1 V vs. RHE. DFT calculations rationalize the maximum activity observed experimentally through tuning of the nitrate and hydrogen adsorption energies with alloy composition. By experimentally confirming the use of N and O thermodynamic descriptors and the volcano plot to find active alloys, these findings provide a blueprint to select alloy compositions for NO3RR.[2] Besides active and selective catalysts, it is critical to develop catalysts that can withstand the conditions present in many nitrate-laden waste streams. In particular, chloride poisoning is a serious problem for NO3RR but is not well understood. Catalytic inhibition of NO3RR by chloride has been reported for Pt, Rh, Fe, and Cu electrodes. To address this issue, we demonstrate rhodium sulfide (RhxSy) as an active and chloride-resistant electrocatalyst. We discuss the role of sulfur vacancies in RhxSy as plausible active sites for NO3RR.

Electrocatalytic nitrate reduction on metals, alloys, and metal sulfides

ACS Fall 2021

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