Rhodium Sulfides (Rh
x
S
y
) as a Halide-Resistant Nitrate Reduction Electrocatalyst for Wastewater Remediation
Authors: Richards Danielle , Young Samuel , Goldsmith Bryan R. , Singh Nirala 
Published: July 2022

Excess nitrate (NO3
–) accumulation in the environment due to human activities has adverse environmental and health effects and requires intervention. 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 [1]. A challenge for NO3RR is that nitrate adsorbs weakly to many catalyst surfaces and must compete with hydrogen and reaction intermediates for active sites [2, 3]. Metal catalysts which adsorb nitrate strongly at low overpotentials and are active for NO3RR, such as Rh, can be expensive. Better understanding the relationship between nitrate and hydrogen adsorption energies and nitrate reduction activity for known catalysts would allow faster identification of new, less expensive, active catalysts. Additionally, competition of nitrate for active sites is exacerbated in real waste streams, where other anions such as chloride are often present (e.g., introduced via resin recovery in ion-exchange membranes) and can compete for active sites, lowering the NO3RR activity. Determining chloride adsorption energies for nitrate reduction catalysts affected by the presence of chloride would provide useful information for understanding which materials would be affected by chloride and act as a guide for selecting chloride-resistant NO3RR catalysts.
We report 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 [3]. 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. Identifying these activity descriptors allows rapid computational screening to identify new promising catalysts. 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 [4]. Using steady state current densities, we show that Rh is more active than Pt for NO3RR in acidic conditions but the addition of even 1 mM chloride lowers NO3RR activity by 30-60%, with Rh more affected by chloride than Pt. The lowering of activity is attributed to competitive adsorption between chloride and nitrate for active sites. 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 chloride poisoning, we examine rhodium sulfide (Rh
x
S
y
), which is an electrocatalyst with notable halide resistance. We show that Rh
x
S
y
is more active for NO3RR in acidic media with and without chloride than Pt or Rh and discuss plausible active sites.
References:
[1] P.H. van Langevelde, I. Katsounaros, M.T.M. Koper, Electrocatalytic Nitrate Reduction for Sustainable Ammonia Production, Joule, 5 (2021) 1-5.
[2] Z. Wang, D. Richards, N. Singh, Recently Discoveries in the Reaction Mechanism of Heterogeneous Electrocatalytic Nitrate Reduction, Catal. Sci. & Tech., 11 (2021) 705-725.
[3] J.-X. Liu, D. Richards, N. Singh, B.R. Goldsmith, Activity and Selectivity Trends in Electrocatalytic Nitrate Reduction on Transition Metals, ACS Catal., 9 (2019) 7052-7064.
[4] D. Richards, S.D. Young, B.R. Goldsmith, N. Singh, Electrocatalytic Nitrate Reduction on Rhodium Sulfide Compared to Pt and Rh in the Presences of Chloride, Catal. Sci & Tech. 11 (2021) 7331-7346.

Figure 1