Rare-earth metal-organic frameworks (REMOFs) based on polynuclear metal clusters are an emerging class of materials that have shown promise for CO2 capture and conversion. In this work, copper nanoparticles (CuNPs) were successfully installed on a cluster-based Y(III) MOF to yield a composite material, CuNP-Y-TBAP. The abundance of Cu binding sites on the Y(III) clusters allowed a remarkably high Cu loading to be achieved, and electron microscopy demonstrated that the MOF-supported CuNPs are exceptionally small and monodisperse. CuNP-Y-TBAP was found to be an active heterogeneous catalyst for electrochemical reduction of CO2, yielding CO and CH4 as the primary CO2 reduction products.

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

Chloride poisoning is a serious problem for the electrocatalytic reduction of aqueous nitrate (NO3−) and improved electrocatalysts are needed.

The importance of maintaining a balanced nitrogen cycle and rectifying the accumulation of nitrate in water streams creates a need for technologies that can convert nitrate species. Heterogeneous electrocatalytic nitrate reduction (NO3RR) is a promising technology because it can use renewable electricity to convert nitrate to nitrogen or ammonia without chemical reductants, hydrogen gas, or the production of biological waste. This review discusses the fundamental mechanism of nitrate reduction by exploring the rate-determining conversion of nitrate to nitrite and NO, and the selectivity pathways that lead to ammonia, nitrogen gas, and nitrogen oxides. Additionally, it explores several important techniques for evaluating nitrate reduction electrocatalysts, including methods to quantify the electrochemically active surface area and the product selectivity. This review highlights the activity and selectivity trends for electrocatalysts which include metals, alloys, and more recent work on sulfides, oxysulfides, oxides, phosphides, and N-doped materials. Factors that influence the reactivity and selectivity trends, such as adsorption energy of the intermediates, reaction conditions, and surface contaminants are also discussed. Overall, this review shows how mechanistic studies have led to a better understanding of NO3RR. Targeted and controlled NO3RR studies can lead to better synergy between experimental and computational results, predictive mechanistic understanding, and discovery of new NO3RR electrocatalysts.

Electrocatalytic reduction is a promising approach to remediate nitrate (NO3-), one of the world's most widespread water pollutants. In the present work, we elucidate activity and selectivity trends of transition metals for electrocatalytic nitrate reduction to benign or value-added products such as N-2 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. Nitrate reduction rates, 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 including the activity trends of pure metals and our in situ X-ray absorption spectroscopy measurements of competitive adsorption between hydrogen and nitrate on Pt/C. We also predict that Fe3Ru, Fe3Ni, Fe3Cu, and Pt3Ru are promising catalysts for nitrate electroreduction toward N-2 with relatively high activity and selectivity. Ultimately, this work gives insight into nitrate reduction on transition metal surfaces and can guide the design of improved electrocatalysts for nitrate remediation.

Renewable electricity generated from solar or wind is inherently intermittent, so finding routes to electrochemically store this energy in chemical bonds as fuels or chemicals is an important scientific challenge. For electrocatalytic reduction reactions such as bio-oil hydrogenation1,2 or nitrate conversion to ammonia,3 the rates and efficiency of converting electricity to energy stored in chemical bonds is related to the surface chemistry steps. A particular challenge in understanding and controlling these surface chemistry steps at a molecular level for electrocatalysis is that the complexity of the liquid phase makes it hard to characterize the catalyst surface and adsorbed species. A further challenge is that it is unclear when insight from first-principles calculations such as density functional theory will apply to electrocatalysis, because modeling solvation and the double layer accurately remains a matter of intense effort. In this talk we will discuss a few techniques to detect surface coverages and intermediates for aqueous electrocatalytic reduction reactions (i.e., phenol hydrogenation and nitrate reduction), and how these techniques have given insight into observed reaction rates. We will discuss how site counting of adsorbed hydrogen on platinum group metals using cyclic voltammetry can be used to determine reactant species coverages as a function of concentration (Figure 1). We will also compare these results to electrochemical impedance spectroscopy measurements of the change in the double layer with adsorption (i.e., decreases in capacitance due to adsorbed organic molecules). Also, we report results of in situ spectroscopy under reaction conditions, including near edge (XANES) and extended X-ray absorption fine structure (EXAFS) to determine catalyst oxidation state, structure, and adsorbate distances. Time permitting, we will discuss vibrational spectroscopy techniques to understand adsorbed species structure, such as surface enhanced Raman spectroscopy applied in the aqueous phase under electrochemical conditions,4 and our efforts to extend the work in this area to understand the effect of applied potential and catalyst surface for bio-oil hydrogenation. We compare our experimental results to first-principles calculations to show where trends allow us to better understand changes in reactivity with catalyst, and how a combined experimental and computational effort can give insight into the electrocatalyst/electrolyte interface in terms of reactant adsorption. References (1) Singh, N.; Song, Y.; Gutiérrez, O. Y.; Camaioni, D. M.; Campbell, C. T.; Lercher, J. A. ACS Catal. 2016, 6 (11). (2) Song, Y.; Gutiérrez, O. Y.; Herranz, J.; Lercher, J. A. Appl. Catal. B Environ. 2016, 182, 236. (3) Duca, M.; Koper, M. T. M. Energy Environ. Sci. 2012, 5, 9726. (4) Weaver, M. J. J. Raman Spectrosc. 2002, 33 (5), 309. Figure 1