(750b) Rhodium Sulfide (RhxSy) as a Halide-Resistant Nitrate Reduction Electrocatalyst for Wastewater Remediation

Electrocatalytic nitrate reduction (NO₃RR), where nitrate is reduced in aqueous solution on electrode surfaces, is a promising method for sustainable remediation of nitrate to value-added chemicals such as NH₃. A challenge for NO₃RR is that nitrate adsorbs weakly to many catalyst surfaces and must compete with hydrogen and reaction intermediates for active sites. This problem is exacerbated in real waste streams, where halides such as chloride are often present (e.g., introduced via resin recovery in ion-exchange membranes) and can compete for active sites.

In this talk, we will discuss the competitive adsorption of nitrate and hydrogen and the reaction mechanism of NO₃RR. By using adsorption energies of nitrate and hydrogen as descriptors, we qualitatively understand many of the observed trends in NO₃RR activity on metal surfaces through a Langmuir-Hinshelwood reaction mechanism[1]. We show the voltage dependence of NO₃RR on platinum group metals, where competitive adsorption of hydrogen and nitrate or nitrate intermediates causes a maximum in NO₃RR activity with potential. Using cyclic voltammetry on Pt and Rh, we observe that the chloride adsorption voltage window overlaps with the maximum activity for NO₃RR, due to the related adsorption energies of nitrate and chloride. We show that even 1 mM chloride lowers NO₃RR activity by 30-60%, resulting from blocked sites on Pt and Rh. Using DFT, we compute the chloride and nitrate adsorption energies on a series of metals and observe linear scaling relations, such that it is unlikely any transition metal binds chloride weakly while adsorbing nitrate strongly. To address this chloride poisoning, we examine rhodium sulfide (Rh_xS­_y), which is an electrocatalyst with notable halide resistance. We will discuss the activity of Rh_xS­_y for NO₃RR in acidic media with and without chloride, and DFT modeling of plausible active sites.

[1]Liu, Richards, Singh, Goldsmith. ACS Catalysis. 9 (2019). 7052-7064.