Electrocatalytic Nitrate Reduction

Humans contribute over 10⁸ tonnes of reactive nitrogen to the environment each year, largely from fertilizer runoff and industrial processes.¹ A major consequence is that aqueous nitrate (NO₃^–) is one of the most widespread water pollutants.^2,3 Consuming nitrate has been linked to infant methemoglobinemia (“blue baby syndrome”), childbirth complications, and ovarian cancer.^2,4 High nitrate levels in lakes and oceans also cause mass death of aquatic life through eutrophication.⁵ Finding an effective strategy to balance the nitrogen cycle is a National of Academy of Engineering grand challenge.⁶ Concentrated nitrate also exists in low-level nuclear waste,⁷ and could facilitate spent fuel recovery if removed from the waste stream.⁸

The electrocatalytic nitrate reduction reaction (NO₃RR) is a promising strategy for aqueous denitrification. It converts nitrates to benign or valuable products, such as nitrogen gas (N₂) or ammonia (NH₃). Unlike thermocatalytic, physical, or biological denitrification, the NO₃RR does not require additional chemicals or generate a secondary waste stream.⁹ This makes NO₃RR approaches desirable for modular, decentralized applications. A technoeconomic analysis of low-level nuclear waste found that using the NO₃RR to convert nitrate to NH₃ and N₂ gas could be cost-competitive if a sufficiently stable, active, and selective catalyst exists.¹⁰ However, researchers have yet to identify such an electrocatalyst.

My research focuses on discovering active, selective, and stable electrocatalysts to promote NO₃RR. I am currently studying bimetallic alloys of earth-abundant metals as well as metal sulfides for this reaction, in collaboration with experimental researchers in the Nirala Singh lab at University of Michigan. Recent projects include an examination of how PtRu alloy composition affects catalyst activity as well as whether metal sulfides are resistant to halide poisoning.

PtRu Alloys

The electrocatalytic nitrate reduction reaction (NO₃RR) converts nitrate (NO₃^–) ions in water to benign or valuable products, such as nitrogen (N₂) gas, ammonia (NH₃), hydroxylamine (NH₂OH), or ammonium nitrate (NH₄NO₃).¹¹ The rate of conversion and selectivity towards each product depends on a complex reaction mechanism and operating conditions such as solution pH and applied cell voltage. On transition metal surfaces at moderately low (< 1 M) nitrate concentrations and, NO₃RR follows a reaction mechanism involving dissociation of surface-bound NO_3– to other surface-bound nitrogen intermediates, as well as surface reaction between these intermediates and surface-bound H.¹² This mechanism appears below:¹³

A major challenge in commercializing aqueous-phase electrocatalytic denitrification is the discovery of a sufficiently active, selective, and stable electrocatalyst. Such an electrocatalyst must also attain high activity and selectivity at relatively low (< 1 V) overpotentials.

Of the pure transition metals, Rh is the most active towards NO₃RR, as the proposed rate-limiting step (dissociation of NO₃* to NO₂* and O*) requires a high coverage of NO₃, and Rh is able to bind NO₃ to its surface more strongly than other metals do. However, Rh is very expensive (~$8,300/oz) and rare, so it is not a practical choice for widespread denitrification applications.

Previous and recent studies have investigated the performance of bimetallic alloys for nitrate reduction and have demonstrated that alloy catalysts can often achieve performance that is better than either pure metal alone. This is accomplished through ligand, strain, and ensemble effects.¹⁴ For example, alloying Pt and Sn helps increase the rate of the NO₃RR rate-limiting step (nitrate dissociation) and also makes the reaction more selective toward producing hydroxylamine.¹⁵ Additionally, a Cu₅₀Ni₅₀ alloy was shown to be six times as active as pure Cu at 0 V vs. RHE.¹⁶ It was also shown that the NO₃RR activity on the CuNi alloy is a function of the alloy composition, and that there exists an optimal composition that maximizes activity.

Our previous computational work¹³ predicted that Pt₃Ru is a highly active alloy for NO₃RR. Our research around PtRu alloys explores whether this is true, and whether NO₃RR activity depends on PtRu alloy composition, as it does for CuNi alloys. Preliminary experimental and computational studies suggest that NO₃RR activity on Pt_xRu_y alloys is indeed a function of alloy composition, and is maximized at an approximate composition of Pt₇₈Ru₂₂. We are also interested in investigating

• how PtRu alloy composition impacts the selectivity of the reaction towards one or more products,
• which step in the reaction is rate-determining at a variety of alloy compositions and potentials,
• the stability of PtRu alloys in acidic media during long periods of operation, and
• the denitrification cost per gram of catalyst or per mole of nitrate consumed.

The answers to these questions will inform catalyst design rules and reveal insight about which denitrification applications are most appropriate for PtRu alloy electrocatalysts. Our findings are also relevant to other aqueous-phase reactions beyond nitrate reduction.

Metal sulfides

One understudied aspect of heterogeneous catalyst discovery is poison resistance. Poisoning occurs when substances other than reactants compete for and/or block active sites on the catalyst surface, preventing the catalyst from carrying out the desired reaction, as shown in this figure:¹⁷

In the context of electrocatalytic nitrate reduction (NO₃RR), poisoning can be addressed through upstream preprocessing. However, many catalysts can be poisoned with even trace amounts of contaminants in the reactor feed,¹⁸ and upstream purification methods may not remove enough poison. Finding a catalyst that intrinsically resists poisoning is a more robust solution.

My research focuses on halide poisoning, which has been extensively studied in the experimental electrochemical literature as well as in electrochemistry applications (such as PEM fuel cells).¹⁸ Halide poisoning is observed experimentally and predicted computationally for many pure metals. A DFT ab initio thermodynamics study found that Cl^– was likely to achieve surface coverages of at least 1/3 monolayer in the potential range for nitrate reduction to NH₃ and N₂.¹⁹ Additionally, voltammetry experiments on Pt electrodes show that even trace amounts of Cl^– ions (e.g., less than 100 μM) drastically reduce current.²⁰

Metal sulfides (M_xS_y) are a class of chalcogen compounds which have been shown to resist halide poisoning in some reactions. A mixed-metal sulfide (Co_0.4Ru_0.6S₂) exhibited hydrogen evolution reaction (HER) activity similar to that of Pt in the presence of Br^–, yet maintained a stable reaction current while Pt was continuously deactivated.²¹ Rh sulfides (Rh_xS_y) were also tested against Pt electrodes in a electrolytic cell, where Rh_xS_y was found to maintain moderate HER current long after Pt was completely deactivated.²² Metal sulfides also show promise for enhancing the figures of merit for NO₃RR. An oxo-MoS₂ catalyst was shown to reduce NO₂^– to with a selectivity of 13.5% Faradaic efficiency under neutral pH conditions, which is the highest N₂ selectivity reported for a metal sulfide electrocatalyst.²³ This combination of known halide poison resistance and possible selectivity preference, along with the fact that Rh is the most active transition metal for NO₃RR, motivates my current study of Rh_xS_y electrocatalysts.

I am interested in computationally investigating the rate and selectivity of NO₃RR on a Rh_xS_y catalyst surface models, focusing on Cl^– as a model poison. As Rh_xS_y catalysts have received very little study about halide poison resistance for NO₃RR in acidic media, a primary goal is to quantify the degree to which Cl^– decreases the reaction rate or changes the selectivity of desirable products such as NH₃ and N₂. Another important goal is to elucidate the active site for NO₃RR on Rh_xS_y surface models through selective poisoning and defect experiments. These insights will help determine whether there exists a selective, active, and stable NO₃RR electrocatalyst that is also poison-resistant to one of the most common wastewater poisons. It will also inform design guidelines about how to maximize poison resistance for metal sulfide catalysts in aqueous-phase reactions beyond nitrate reduction.