Beyond Fossil-Fuel-Driven Nitrogen Transformations

Abstract

Nitrogen is fundamental to all of life and to many industrial processes. Nitrogen in its various oxidation states comprises the global nitrogen cycle, with the change between forms being redox reactions involving electrons and protons. The interchange of nitrogen oxidation states constitutes some of the most important industrial processes, with the energy for these processes being provided largely by fossil fuel. A key goal of research in the field of nitrogen chemistry is to minimize the use of fossil fuels by developing more efficient heterogeneous, homogeneous, or biological catalysts, or by inventing new energy-efficient processes that rely on catalysts. These approaches, as well as the challenges involved, are discussed in this review.

One-Sentence Summary

This review article reports on the current state of the field of nitrogen activation chemistry and discusses future directions.

Reduced forms of nitrogen, particularly ammonia (NH₃), are vital to life. Prior to the early 1900’s, all reduced N came from biological nitrogen fixation occurring in microbes. This landscape changed in the early 1900’s with the invention of an industrial process to reduce N₂ to NH₃ (N₂ + 3H₂ ⇌ 2NH₃) by Haber and Bosch (H-B). The H-B process, in turn, has led to fundamental changes in the way food is produced, and its impact is underscored by the fact that about 50% of the N atoms in humans today originate from the H-B process (1). For these reasons and others, the H-B process has been called the most impactful invention of the 20th century (2). The H-B process is, however, energy intensive, consuming 1–2% of the world’s annual energy output (3). The main energy requirements arise from the high reaction temperatures (~700 K) and pressures (~100 atm), and, most importantly, from the need for large quantities of H₂. Nevertheless, at scale H-B is surprisingly energy efficient. In addition to the high energy demand, however, there is a second problem with H-B that is potentially more serious. Specifically, the source of H₂ is usually natural gas, (4) and consequently H-B generates about 1.9 metric tons of CO₂ per metric ton of NH₃ produced (3 CO₂ per 8 NH₃) (5).

Nitrogen in its more oxidized forms is also industrially, biologically, and environmentally important. Oxidation of N₂ to nitric oxide (NO) and nitrogen dioxide (NO₂), which are collectively referred to as NO_x (the representation of neutral forms of oxidized nitrogen), occurs naturally in lightning and also during combustion of fuels in air (6). In fact, plasma-driven oxidation of N₂ to NO (the Birkeland-Eyde, or B-E, process) preceded H-B as the first commercial nitrogen fixation process (7). However, B-E is not economically competitive with the H-B process, and as a result the demand for oxidized nitrogen, primarily in the form of nitric acid (HNO₃), is satisfied by catalytic oxidation of H-B-generated NH₃ at elevated temperatures via the Ostwald process (8).

Nitrogen oxides (NO_y, which represents both neutral and anionic forms of oxidized nitrogen) are also produced on a large scale by bacteria. These processes include oxidation of NH₃ to nitrate (NO₃^-) by nitrifying bacteria and reduction of NO₃^- to nitrite (NO₂^-), NO, nitrous oxide (N₂O), and ultimately to N₂ by denitrifying bacteria. The conversion of NH₃ and NO₃^- to N₂ by the action of these bacteria is used in waste-water treatment and is the primary pathway for loss of applied fertilizers in agriculture. Nitrogen oxides are common environmental pollutants, and thus catalysts to reduce these oxides to N₂ are also of considerable practical importance. For example, NO_x generated during combustion is a primary component of photochemical smog (9). The development of active catalysts for reducing NO_x to N₂ (the TWC, or three-way catalytic converter) led to remarkable improvements in air quality in the late 1970’s (10), and there are now TWCs operating on hundreds of millions of vehicles worldwide. However, increasingly stringent emissions regulations coupled with the low exhaust temperatures from high efficiency engines present challenges for future combustion emissions control (11). The soluble forms of the nitrogen oxides, primarily NO₂^- and NO₃^- 2 3 , are similarly hazardous to human health and contribute to the eutrophication of waterways (12). Sustainable approaches for catalytically converting these pollutants to N₂ remain an unmet challenge (13).

In the present article, we report on the results of a U. S. Department of Energy workshop held in October, 2016 that focused on the challenges and opportunities associated with fundamental aspects of nitrogen chemistry. We begin by discussing the thermochemistry of nitrogen transformations, and then focus on the reduced forms of nitrogen, primarily NH₃, followed by the oxidized forms.

Thermochemistry of nitrogen fixation

The energy expenditures for various reductive and oxidative N₂ fixation pathways are compared in Figure 1 (14–16). Hydrocarbons, including biomass and fossil fuels (particularly methane), and water are the only two sources of hydrogen atoms that can sustain long-term, large-scale NH₃ production. The use of water as the hydrogen source for NH₃ production requires substantially more energy than methane (by 613.4 kJ/mol N₂), but it is also more environmentally benign, does not contribute to the accumulation of greenhouse gases, and does not compete for valuable and limited hydrocarbon resources. However, these advantages can be fully realized only if the energy comes from a clean, renewable source such as the sun or wind.

Figure 1.

Atom and energy economy of nitrogen fixation. The numerical values represent standard (for gaseous reactants and products and liquid H₂O, as defined in ref. 14) Gibbs free energies in kJ/mol of fixed N₂ in the direction of the arrows. All thermochemical data are from ref. 15, except for H₂NOH (ref. 16).

When water is used as the feedstock, there is only a small (32.9 kJ/mol N₂) thermodynamic advantage for the direct synthesis of NH₃ compared to a two-stage synthesis involving water splitting followed by H-B. The overall energy saving may, however, become more significant if the H-B process is avoided altogether and direct reductive N₂ fixation is conducted under milder conditions.

The two most important N-containing commodity chemicals are NH₃ and HNO₃. In 2017, their worldwide production was estimated to be 150 (17) and 50 (18) million metric tons , respectively. Presently, nearly all HNO₃ is manufactured using a three-stage process: steam reforming of methane, H-B, and then the Ostwald process (Figure 1), but this approach is energetically wasteful compared to direct oxidative N₂ fixation.

An attractive alternative route to HNO₃, which avoids the NH₃ intermediate, is direct N₂ oxidation to aqueous HNO₃ (eq 1).

$${\text{N}}_{2(\text{g})}+{2.50}_{2(\text{g})}+{\text{H}}_2{\text{O}}_{(\text{liq})}\rightleftarrows {\text{2H}}^{\text{+}}{}_{(\text{aq})}+2{\text{NO}}_{{}^{3^{-}(\text{aq})}}$$ (1)

The equilibrium constant for this reaction is ~2.7×10⁻³ M⁴/bar^3.5 (ΔG⁰ = 14.6 kJ/mol N₂) (14–16, 19), which is large enough to drive spontaneous formation of ~0.1 M HNO₃ in any pool of water on the surface of the earth. Moreover, if equilibrium were to be achieved between the earth’s atmosphere and the world’s oceans, the oceans would contain ~0.02 M HNO₃ and ~90% of atmospheric O₂ would be consumed. Fortunately for life on Earth, under ordinary conditions and in the absence of a catalyst, eq 1 is immeasurably slow in both directions.

Reduction of N₂ to NH₃

Overview.

As discussed in the previous two sections, the most important N₂ reduction reaction is NH₃ synthesis, specifically N₂ + 3H₂ ⇌ 2NH₃. Given that this reaction is slightly thermodynamically favorable under ambient conditions, major effort has been devoted to developing alternative and environmentally friendly processes that would allow NH₃ to be produced at distributed sources under more benign conditions rather than the large-scale centralized H-B process. This goal is particularly important in developing countries where the population, and hence the need for food, is increasing rapidly. In these countries, access to fertilizer is hampered by poor transportation infrastructure and insufficient capital to build large chemical plants. For example, there are no large-scale NH₃ production facilities in all of East Africa. To address this need, there has been substantial effort expended to understand N₂ reduction using a variety of catalysts; including: heterogeneous, enzymes, and homogeneous catalysts, and electro- and photocatalysts.

Heterogeneous catalysis.

The commercial H-B process is carried out using a heterogeneous catalyst based on iron and promoted with Al₂O₃ and potassium. Typically, 2500 tons of NH₃ per day are produced in a single plant where natural gas and water are available in large quantities. It is now well accepted that the relative activity of metallic catalysts can be correlated to their binding energies with N-containing species in terms of a volcano-shaped relationship. Metals that bind nitrogen too strongly or too weakly are on either side of the volcano (20). For those having low binding energies, N₂ dissociation is rate-limiting. For those metals exhibiting high binding energies, N₂ dissociation occurs but desorption of the resulting atomic N (and other N-containing intermediates) is slow, limiting the number of available binding sites and thereby slowing the catalytic rate (21).

Recent calculations suggest that the energies of all intermediates and transition states involved in a conventional NH₃ synthesis reaction network scale with the N binding energy. As shown in Figure 2, these correlations limit the rate of NH₃ synthesis on transition metals to much lower values than would be possible on a material with both a low N₂ dissociation barrier and more moderate intermediate binding energies (22). Clearly, an important challenge is to break this type of scaling relationship, which should lead to the development of catalysts yielding reaction rates that are potentially orders of magnitude higher than the current state of the art.

Figure 2.

Linear scaling between nitrogen binding energy and activation barrier for N₂ dissociation. TOF = turnover frequency (from ref. 22).

Advances in the synthesis, characterization, and modeling of non-traditional heterogeneous catalysts, such as intermetallics, alloys containing a single heteroelement, and shape-controlled materials, whose discovery might be driven by computational models, offer the promise for more active catalysts. These combined efforts should lead to catalysts that could potentially operate at lower temperatures and thus reduce the operating pressures of the H-B process. Another opportunity to reduce the CO₂ footprint of the H-B process is to replace the methane reformer with a water-electrolyzer to provide CO₂-free H₂, but this would require cost-effective, large-scale electrolyzers.

Enzyme catalysis.

A range of microbes (bacteria and archaea) living in all major ecosystems are able to reduce N₂ to NH₃ using the enzyme nitrogenase. Nitrogen fixation by these microbes contributes to global supply of NH₃, but it is insufficient to support modern intensive agriculture food production. Exciting frontier areas for the use of microbial N₂ fixation include engineering the nitrogen fixation genes into eukaryotes (23), such as plants, and using nitrogen-fixing microbes in microbial electrocatalysis cells that permit sustained N₂ reduction to NH₃ (24). Such systems offer the promise of providing localized solutions to N₂ reduction, but are not likely to displace the need for large-scale N₂ reduction in the near term.

One important lesson that can be learned from biological nitrogen fixation is understanding how the enzyme nitrogenase accomplishes reduction of N₂ to NH₃ at low temperatures and pressures and without H₂. The first nitrogenase to be purified (and the most widely distributed) is Mo-dependent and, as shown in Figure 3, has a metal-cluster active site containing Mo, Fe, S, C, and homocitrate (FeMo-cofactor).

Figure 3.

FeMo-cofactor with Fe in rust, S in yellow, C in gray, Mo in magenta, and O in red. Made from PDB: 1M1N

An important step toward mechanistic understanding of nitrogenase emerged from trapping an intermediate with four H⁺ and four e^- accumulated immediately prior to binding N₂. Characterization of this intermediate (termed the E₄ state) has revealed the presence of bridging hydrides (Fe-H-Fe). The E₄ state undergoes reductive elimination of the two hydrides with release of H_2, with concomitant activation of the metal core by two e^- for binding and reduction of N₂ to a metal-bound diazenido species (HN=N-Fe). Although these studies have provided insights into the central catalytic step, which is the cleavage of the N₂ triple bond, many challenges remain before the entire mechanism can be elucidated. These include a molecular level understanding of both the early and late stages of the N₂ reduction pathways, an understanding of the roles of ATP hydrolysis in driving electron transfer, and then reconciliation of the resulting empirical mechanism with computational studies.

These foregoing challenges primarily involve substrate moieties bound as catalytic intermediates. In parallel, important questions remain about the FeMo-cofactor active site of Mo-dependent nitrogenase. For example, what changes in redox and spin states of the seven Fe centers and the Mo center accompany the stepwise transformations of substrates? Additionally, the FeMo-cofactor contains the first example of a carbide (C^4–) in biology. The significance and function of this carbide with respect to mechanism are unknown. Understanding the structure of FeMo-cofactor intermediates and synthetic analogues could provide important clues to guide the design of new homogeneous and heterogeneous catalysts for N₂ reduction (25). Biomimetic approaches could take advantage of not only the coordination chemistry in the FeMo-cofactor, but also the cooperative interactions with nearby acidic amino acid residues in the nitrogenase protein.

Electrochemical techniques offer a powerful alternative method of delivering electrons to nitrogenase without a requirement for ATP hydrolysis, and they also offer new avenues for addressing aspects of the nitrogenase mechanism. Electron transfer from an electrode, previously demonstrated for a number of redox enzymes, has been applied to nitrogenase but only via mediators (i.e., no direct electron transfer) (26–28). As illustrated in a recent study, this approach, in combination with computations (29), provides insights into catalytic nitrogen transformations.

A related frontier area is using nanomaterials to deliver electrons to nitrogenase. For example, it has been recently demonstrated that a CdS nanorod-nitrogenase MoFe protein hybrid achieves light-dependent N₂ reduction to NH₃ using a sacrificial electron donor (30). Such systems offer a means to use light as the exclusive energy source to drive N₂ reduction.

Homogeneous catalysis.

Synthetic homogeneous catalysts designed for N₂ reduction provide well-defined molecular pre-catalysts and intermediates, which can be thoroughly characterized by diverse spectroscopic techniques and thereby provide excellent opportunities for determining mechanistic information about N₂ reduction to NH₃. The reactivity of synthetic inorganic complexes provides insights into molecular reactivity and individual bond-making processes that occur in nitrogenases. The discovery of nitrogenase, along with a transition metal compound that contains N₂ bound to ruthenium (31), initiated a race to prepare NH₃ catalytically under mild conditions via a transition metal complex. Indeed, entire institutes were created in England and Russia in the late 1960s for this purpose. The stoichiometric N₂-to-NH₃ conversion using H₂SO₄ was demonstrated using low-valent Mo-N₂ and W-N₂ complexes in the 1970s (32); typically the electrons required came from the metal complex. In 1985, the synthesis of NH₃ was accomplished by Pickett and Talarmin using an electrochemical route starting from the tungsten complex [W(N₂)₂(PMe₂Ph)₄], with the electrons supplied by the electrode and protons from an added acid (33). The catalytic reduction of N₂ to NH₃ by H⁺/e^- was reported in subsequent studies, using, for example, a well-defined Mo-containing catalyst (34). Up to eight equivalents of NH₃ per Mo were formed, along with H₂. Both experiments and calculations support a mechanism involving the addition of 6H⁺ and 6e^- to the N₂ initially bound to a Mo(III) center. Altogether, eight of these proposed intermediates were prepared and characterized.

Additional Mo pre-catalysts for reduction of N₂ using chemical reductants and an acid, a dimolybdenum–dinitrogen complex (35) and a molybdenum nitride (36), were subsequently reported in 2011 and 2015, respectively. In 2013, Fe complexes were reported that catalyze reduction of N₂ to NH₃ using a strong reducing agent (KC₈) and a strong acid at 200 K (37); improvements to the Fe system recently produced as many as 84 equivalents of NH₃ per Fe (38). These Fe complexes were designed to reflect the trigonal symmetry of the Fe in the catalytic face of FeMo-cofactor (Figure 3) and to allow variation of an axial ligand, so as to explore tuning of N₂ binding/reduction by the unusual carbide at the center of the FeMo-cofactor.

In all these cases it appears that catalytic N₂ reduction is a 6-electron process, involving many intermediate chemical species [M-N _xH_yⁿ⁺], but unlike nitrogenase without a mechanistic requirement for production of H₂. Nonetheless, in all cases N₂ reduction is accompanied by production of H₂, and at least with the Fe complexes, this has been shown to involve a catalytic process, with H⁺ reduction competing with N₂ reduction.

Much work remains to move this field forward. For example, rational design of potential catalysts will be guided by accurate determination of the bond dissociation free energies (BDFEs) of N-H bonds, including likely intermediates such as M-N=NH, M=N-NH₂, and M=N-NH₃. Initial experimental and theoretical estimates have shown that N-H BDFEs of coordinated NH₃ can vary significantly and demonstrate the importance of metal identity and its oxidation state as well as the possibility of ligand control to tune the stability of potential intermediates. In addition, determination of the energies of these bonds will allow evaluation of energetic requirements of the individual chemical steps that ultimately lead to NH₃. It is also important to identify features that tune the reduction potentials of relevant complexes to ensure that the multiple reductions required for catalysis are energetically feasible. Thus, research should focus on exploring complexes designed to simultaneously promote N₂ binding and multiple reductions at the metal site.

Electrocatalysis and photocatalysis.

The potential-pH predominance (Pourbaix) diagram for the N₂-H₂O system shown in Figure 4 reveals that reduction of N₂ to NH₄⁺/NH₃ (eq 2 and the N₂ reduction line in Figure 4) is thermodynamically possible under moderately reducing conditions.

$${\text{N}}_{\text{2(g)}}{\text{ + 8H}}^{\text{+}}{}_{\text{(aq)}}{\text{(6H}}^{\text{+}}{}_{\text{(aq)}}\text{) + 6e-\hspace{0.17em}\hspace{0.17em}}\rightleftarrows {\text{2NH}}_{\text{4}}{{}^{\text{+}}}_{\text{(aq)}}{\text{(2NH}}_{\text{3}}{}_{\text{(aq)}}\text{)}$$ (2)

Moreover, with a sufficiently active electrocatalyst, the N₂ reduction reaction can, in principle, proceed in a narrow range of negative potentials without interference from water reduction (line a in Figure 4) over the entire pH range. However, at potentials below line a, an extremely selective electrocatalyst would be required to suppress parasitic water reduction. Note that the nitrogen reduction reaction could be paired with a water oxidation half-reaction to supply the H⁺ and e^- required for a complete electrolysis cell. Such a cell would essentially combine water splitting with NH₃ formation as depicted by the direct reductive fixation route in Figure 1. Despite the inviting prospect of a scalable NH₃ electrolysis cell running on renewable electricity at or near room temperature, the difficulty of selectively catalyzing the nitrogen reduction reaction has prevented development of an efficient electrochemical system.

Figure 4.

Partial Pourbaix diagram for the N₂-H₂O system showing lines corresponding to N₂ reduction to NH₄⁺/NH₃ and N₂ oxidation to NO₃⁻ . Lines a and b straddle the region of water stability (reduction to H₂ and oxidation to O₂, respectively). See ref. 14 for details and refs. 15 and 19 for primary data.

At present, only a few examples of pairing the nitrogen reduction reaction with water oxidation at ambient temperature and pressure have been reported, and in all cases, due to competing H₂ evolution, the faradaic efficiencies for NH₃ are so low that even its reliable detection is challenging (39, 40). For example, an efficiency of 0.83% and a production rate of 9.4 × 10⁻¹⁰ mol-NH₃ cm⁻² s⁻¹ were measured at 353 K at an applied cell voltage of 1.2 V using Pt electrodes and a Nafion electrolyte (41). Another electrochemical approach involved an electrochemical cycling process for producing NH₃ (42). The cycling included separate steps of LiOH electrolysis, Li nitridation, and Li₃N hydrolysis, thereby circumventing the parasitic H₂ evolution reaction and leading to higher faradaic efficiencies for NH₃.

Proton-conducting ceramic oxides have been explored as electrodes for nitrogen reduction, and when they operate at higher temperatures both the rate and faradaic efficiency increase (43). For example, efficiencies in excess of 50% and rates in the range of 10⁻⁹-10⁻⁸ mol-NH₃ cm⁻² s⁻¹ have been attained above 673 K using oxide or molten chloride electrolytes and a variety of metal or metal oxide cathodes (43, 44). Note, however, that higher temperatures introduce an additional energy burden and hence compromise the advantage of this approach compared to H-B.

Photochemical reduction of N₂ to NH₃ has also been reported, and there may be guidance in these studies relevant to electrochemistry (45, 46). Photocatalytic routes to NH₃ have been summarized recently (47). One of the biggest challenges for photocatalytic NH₃ synthesis is the discovery of active and stable photoelectrode materials responsive to visible light. Synthetic nitrogenase mimics in the form of chalcogels, composed of Mo and Fe-containing biomimetic clusters, can potentially accomplish photocatalytic N₂ fixation and conversion to NH₃ at ambient temperature and pressure. Both Fe–S and Mo-Fe–S chalcogels have displayed promising activity toward N₂ reduction (48).

Plasma-driven transformations.

The feasibility of using plasmas consisting of ions, electrons, and excited molecular species to drive chemical reactions that are thermally inaccessible has been known for many years (7). For example, NH₃ was synthesized in laboratory-scale, non-thermal N₂ + H₂ plasmas at temperatures and pressures lower than those used in H-B as early as the 1970s, and more recently the synthesis has been demonstrated under much milder conditions (1 atm and 140°C) (49). The plasma synthesis of NH₃ has been carried out both homogeneously and in combination with heterogeneous catalysts (7, 49, 50).

Several mechanisms have been proposed to account for enhancements in catalytic activity in the presence of non-thermal plasmas (51–53). However, the fundamental science and practical engineering of plasma-driven NH₃ syntheses lag behind thermal and electrochemical routes and are therefore ripe for additional research. Further progress in this field will draw on the insights gained from traditional heterogeneous and electrochemical reductions.

Chemical looping.

Chemical looping is perhaps most familiar from combustion where two connected fluidized beds are used to cycle solid particles between an oxidized and a reduced state to combust a carbon-containing fuel with oxygen (54). Chemical looping could also be performed with two fixed beds instead of fluidized beds. Nitrogen can be used instead of oxygen for a similar looping approach in NH₃ synthesis (55). First, N₂ is contacted with a suitable solid-state transition metal to yield a nitride (activation). In the second step, NH₃ is harvested by contacting the nitride with steam or H₂ (56). Recently, lithium has been employed as the parent metal for the nitride, but this approach requires liquid-phase electrolysis to recycle lithium and this step is usually not present in chemical looping (42).

Potential advantages of chemical looping include the ability to independently control process conditions for N₂ activation and product harvest. In this sense chemical looping enables breaking the scaling relationship alluded to in Figure 2. Operation at atmospheric pressure is another major advantage of chemical looping for nitrogen activation (57). Based on results from a pilot-scale plant, chemical looping for combustion, including CO₂ capture, has been compared to conventional combustion and appears to be economically attractive (58). This may be an indication of the possible economic feasibility of chemical looping for nitrogen activation.

Oxidation of N₂

Overview.

The dominance of the H-B process in commercial N₂ fixation is arguably a direct consequence of the ready availability of H₂ from fossil hydrocarbons. As shown in Figure 1, oxidative fixation of N₂ can, in principle, be achieved with a lower energy input than reductive fixation if N₂, H₂O, and O₂ are used as reactants. Other nitrogen oxides are accessible at even lower free energies, especially when coupled with water to form the corresponding oxoacids.

Although H-B and natural processes have motivated substantial research into N₂ reductive fixation, direct N₂ oxidation remains largely unexplored despite its great practical value as a replacement for the Ostwald process. The fact that living organisms have not evolved to consume N₂ and O₂ and produce aqueous HNO₃ suggests fundamental chemical challenges that merit investigation. As a result, there is very little literature on the homogeneous oxidative chemistry of N₂.

Direct oxidation of N₂ with O₂.

NO_x is unavoidably generated at the high temperatures that prevail during combustion in air. NO is the primary combustion-generated component of NO_x, and its formation can be viewed in the context of its equilibrium with N₂ and O₂ (eq 3).

$${\text{N}}_{2(\text{g})}+{\text{O}}_{2(\text{g})}\rightleftarrows 2{\text{NO}}_{(\text{g})}$$ (3)

The standard enthalpy of this reaction is 90.3 kJ/mol NO and the entropy is 12.4 J/(mol NO K) (14, 15), indicating that equilibrium is unfavorable for NO production under ambient conditions. Due to the positive entropy, however, the equilibrium NO concentration rises rapidly with temperature, exceeding 100 ppm above 2000 K (59).

The direct bimolecular reaction of N₂ and O₂ to produce NO is symmetry-forbidden and has a negligible rate even at combustion temperatures. Rather, NO is produced during combustion via eqs 4 and 5, where O (and similarly OH) radicals attack N₂, as originally proposed by Zeldovich (60).

$$\text{O\hspace{0.17em}}+{\text{N}}_2\to \text{\hspace{0.17em}NO + N}$$ (4)

$$\text{N\hspace{0.17em}}+{\text{O}}_2\to \text{\hspace{0.17em}NO + O}$$ (5)

Analogous reactions are at play in high-temperature thermal plasmas, as in the original B-E arc process. Thermal plasmas are unlikely to be energy-competitive with the H-B process, even if the latter uses a renewable H₂ source (61), because the overall energy efficiencies of N₂ activation are low and rapid thermal quenching is required to suppress NO decomposition back to N₂ and O₂.

The aforementioned fundamental limitations can be mitigated using low-temperature, non-thermal plasmas generated by either a dielectric barrier discharge or microwave absorption at or below atmospheric pressure, potentially coupled with appropriate catalysts (7). According to current estimates, theoretical energy consumption for N₂ fixation via eq 3 in a non-thermal plasma (~400 kJ/mol N₂) is more than 2.5 times lower than that for the H-B process with methane-derived H₂ (61–63), and the energy efficiencies already attained in the laboratory (600–1200 kJ/mol N₂, assuming 100% efficient plasma generation (62, 63)) are better than the H-B process using H₂ from water electrolysis (~3000 kJ/mol N₂ (61)), 40% of which would go to electrolysis (64). Experiments and kinetic models show that the primary effect of the nonthermal plasma is to accelerate Zeldovich-like reactions involving athermal, vibrationally excited N₂. Practical implementation of non-thermal plasmas is hampered by their relatively low power per unit reactant mass and thus low throughput.

A thermal, heterogeneous N₂ oxidation catalyst would have to activate N₂ and O₂ and be stable at the extreme temperatures at which NO production becomes thermodynamically favored. Because of these stringent demands, non-thermal electro-, photo-, or plasma-assisted oxidation routes likely offer the greatest opportunities for progress. The development of catalytic systems that can effectively couple with these external energy sources will be key to practical advances. Preliminary results with non-thermal plasmas, especially coupled to catalysts, are promising. There is a pressing need to uncover the fundamental mechanisms that couple plasma chemistry, surface chemistry, catalysts, and theory to guide optimization of plasma and rational development of appropriate catalytic materials.

Electrochemical oxidation of N₂ to HNO₃.

The oxidation line in Figure 4 shows that N₂ is electrochemically unstable toward NO₃^- under moderately oxidizing conditions (eq 6).

$${\text{N}}_{\text{2(g)}}{\text{ + 6H}}_2{\text{O}}_{(\text{liq})}\rightleftarrows 2{\text{NO}}_{3^{-}(\text{aq})}+12{\text{H}}^{+}{}_{(\text{aq})}+10\text{e-}$$ (6)

At pH > 1.3, this ten-electron reaction is more thermodynamically favorable than the parasitic four-electron water oxidation to O₂. Thus, it is possible for NO₃^- to be the only product of an anodic process, particularly in neutral and alkaline solutions, if a sufficiently active and selective electrocatalyst for eq 6 can be discovered. The corresponding cell-completing cathodic reaction can be either water reduction to H₂ or O₂ reduction to water. No progress has yet been made in developing such catalysts, but there is no reason to believe that they cannot eventually be discovered given the necessary resources.

By coupling cathodic N2 reduction (eq 2) with anodic N2 oxidation (eq 6), an electrochemical cell consuming only atmospheric N₂ and water as feedstock to yield aqueous NH₄NO₃ (eq 7) can be envisioned.

$${\text{N}}_{2\left(\text{g}\right)}+3{\text{H}}_2{\text{O}}_{\left(\text{liq}\right)}\rightleftarrows {\text{NH}}_4{{}^{+}}_{\left(\text{aq}\right)}+{\text{ NO}}_3{{}^{-}}_{\left(\text{aq}\right)}+{\text{ H}}_2{}_{\left(\text{g}\right)}$$ (7)

The additional H₂ production is necessary for matching cathodic and anodic currents and maintaining the electrolyte pH constant. Such a cell would require a minimum of 1.08 V to operate in neutral or alkaline electrolyte, which is slightly lower than the minimum water electrolysis voltage of 1.23 V. Advances relating to electrochemical N₂ oxidation will require development of catalysts that are sufficiently active (capable of good current densities at low overpotentials), selective (with respect to water oxidation), and stable (toward deactivation under harsh redox and pH operating conditions).

NO_y Reduction and NH₃ Oxidation

Overview.

The redox chemistry of forms of nitrogen other than N₂ is immense and rich, and it is extensively incorporated into biological systems. Both NO_y and NH₃ are environmentally hazardous pollutants generated by industrial and transportation activity, and thus extensive research has gone into developing and applying NO_y reduction and NH₃ oxidation catalysts. As is often the case, the understanding of these catalysts lags their applications

Ammonia oxidation and NO_y reduction are critical to environmental protection and are much less well understood or optimized than the synthesis of NH₃. There are appealing opportunities to apply the experimental and computational tools of modern heterogeneous catalysis to these reactions, both to develop a fundamental understanding of catalytic processes and to identify superior materials and catalytic processes; for example, catalysts that can selectively reduce NO_y to N₂ using hydrocarbon reductants. Environmental applications generally demand high longevity, durability, and tolerance to poisons. Key gaps in understanding include how activity and selectivity can be achieved in relatively cool gas streams, how catalyst structure evolves over long periods of time, how sulfur and other common poisons interfere with reactions, and especially how these effects can be mitigated.

NO_y reduction.

NO decomposition, the reverse of eq 3, is thermodynamically downhill but difficult to catalyze under the oxygen-rich conditions of interest for environmental protection (65). Nevertheless, NO decomposition has been demonstrated using both homogeneous and heterogeneous catalysts. Decomposition appears to involve a two-electron reduction of two NO molecules to a hyponitrite (eq 8).

$$2\text{N}=\text{O }+{\text{ e}}^{-}\to {\text{N}}_2{\text{O}}_2{}^{-}$$ (8a)

$${\text{N}}_2{\text{O}}_2{}^{-}+{\text{e}}^{-}\to {\text{N}}_2{\text{O}}_2{}^{2-}$$ (8b)

$${\text{N}}_2{\text{O}}_2{}^{2–}+2{\text{H}}^{+}\to {\text{N}}_2\text{O\hspace{0.17em}}+{\text{H}}_2\text{O}$$ (8c)

Coordination complexes of hyponitrites are not uncommon, and a tri-Cu hyponitrite coordination complex has been isolated as an intermediate in the catalytic decomposition of NO to N₂ (66). A Cu dimer is similarly implicated as the active site in a Cu-exchanged zeolite catalyst for NO decomposition (67).

NO_x can be catalytically reduced to N₂ via several routes. Precious metals are active for NO_x reduction by CO and hydrocarbons, but only under conditions in which O₂ concentrations are low (10). Vanadia/titania and metal-exchanged zeolites catalyze selective reduction of NO_x by NH₃ (68). Nitrosamine appears to be the key N₂-forming intermediate in the catalytic pathway (eq 9).

$${\text{NH}}_3+\text{\hspace{0.17em}NO\hspace{0.17em}}\to {\text{H}}_2\text{NNO\hspace{0.17em}}+{\text{H}}^{+}+{\text{e}}^{-}$$ (9a)

$${\text{H}}_2\text{NNO\hspace{0.17em}}\to {\text{N}}_2+{\text{H}}_2\text{O}$$ (9b)

This N₂-forming reaction is exothermic, implying significant, but perhaps not insurmountable, kinetic challenges to reversing the chemistry.

NO_x can also be catalytically reduced with H₂ over Pd and Pt catalysts (13), and N₂ is typically the desired product (65). By suitable selection of promoters and control of the NO/H₂ ratio, however, NH₃ can be produced over Pt catalysts (69). Likewise, NH₃ competes with N₂ as the product of the catalytic reduction of NO₂^- and NO₃^- by H₂ over Pd catalysts (13).

Bacteria analogously reduce NO₃^- and NO₂^- through a series of gaseous intermediates leading ultimately to N₂ in a process called denitrification (eq 10). In these bacteria, NO_y acts as a terminal electron acceptor in an anaerobic respiration process. Electrons flowing through membrane-bound proteins produces a proton gradient that in turn is used to make ATP. Each step in the denitrification process is catalyzed by well-studied metalloenzymes that use some rather unusual metal clusters to achieve their chemistry (70, 71). From a global nitrogen perspective, denitrification by these bacteria functions to convert fixed forms of N (NO_y) into N₂. In some cases, this process is beneficial, such as during waste-water treatment to remove nitrates. In other cases, such as in agriculture, denitrification results in loss of N that was applied to crops as fertilizer.

$$2{\text{NO}}_3{}^{-}\text{\hspace{0.17em}+\hspace{0.17em}}10{\text{e}}^{-}+12{\text{ H}}^{+}\to {\text{N}}_2+6{\text{H}}_2\text{O}$$ (10)

NH₃ oxidation.

Heterogeneous NH₃ oxidation with O₂ is at the heart of the Ostwald process and is practiced in a variety of environmental protection applications. Precious metals remain the workhorse catalysts, and therefore opportunities exist to develop lower-cost materials having equivalent or better activity and selectivity. The preferred NH₃ oxidation products shift from N₂ at low temperatures to NO at higher temperatures, with N₂O being a minor but highly undesirable product. At present, a full mechanistic understanding of this process and of the kinetic origins of selectivity is not available.

NH₃ and/or NO₂^– are fuel for bacteria and archaea that employ multi-electron oxidative reactions to generate reducing equivalents for cellular respiration and to establish a proton gradient for ATP synthesis (72, 73). Ammonia-oxidizing bacteria (AOB) and archaea (AOA) stoichiometrically convert NH₃ to NO₂^–. AOA and AOB first carry out the selective conversion of NH₃ to hydroxylamine (NH₂OH) using an integral membrane Cu monooxygenase called ammonia monooxygenase (AMO) (eq 11). Active AMO has never been purified, and thus this challenging transformation awaits mechanistic elucidation.

$${\text{NH}}_3+\raisebox{1ex}{$1$}\!\left/ \!\raisebox{-1ex}{$2$}\right.{\text{O}}_2\to {\text{NH}}_2\text{OH}$$ (11)

The NH₂OH generated by AMO is oxidized to provide a net electron flow for cellular respiration. How AOA processes NH₂OH is entirely unknown (74). In AOB, this process is carried out by hydroxylamine oxidoreductase (HAO), which contains a rare, covalently modified c-heme, heme P460, to which NH₂OH binds at Fe and undergoes proton-coupled oxidation (72). Recent experiments strongly support NO as the enzymatic product of HAO reactivity, contrary to decades of dogma asserting NO₂^– as the enzymatic product (75). Assuming a 2-electron turnover of AMO, the 3-electron oxidation of NH₂OH by HAO furnishes one net electron for cellular respiration. Stoichiometric production of NO₂^– from NH₃ by AOB likely involves an additional, but as-yet unidentified, third enzyme in the pathway. Cytotoxicity of both NO and NH₂OH may be managed by cytochrome P460, a monoheme enzyme that selectively produces N₂O from the reaction of NH₂OH with Fe-bound NO (76). NO₂^– is itself cellular fuel for nitrite-oxidizing bacteria (NOB) that effect the 2-electron oxidation of NO₂^– to NO₃^– and for recently discovered “complete ammonia oxidation” (COMAMMOX) bacteria that effect the 8-electron oxidation of ${\text{NH}}_3\text{\hspace{0.17em}to\hspace{0.17em}}{\text{NO}}_3{}^{-}$ (77).

Opportunity abounds to extract broadly applicable insights through the study of the oxidative enzymes operative in nitrification. Establishing the mechanism of NH₃ oxidation by AMO remains a grand challenge whose difficulty is exacerbated by the integral membrane nature of the enzyme and the sluggish growth of its host organisms. The end goal is tantalizing technology: selective hydroxylation of relatively inert bonds generalizable beyond N-species, especially considering that enzymes related to AMO, such as particulate methane monooxygenase, effect similar transformations of unactivated alkanes (78).

Mechanistic understanding of NH₂OH and NO₂^- oxidation in turn offers insights into proton-coupled, multi-electron transformations that are crucial to redox catalysis. NH₂OH is an energetic, cytotoxic metabolite that AOB and AOA harness as a source of reducing equivalents. The absence of requisite c-heme biosynthesis pathways in AOA necessitates substitution of HAO for either a non-heme or Cu-based enzyme for NH₂OH oxidation (79). Thus, opportunities exist to fill a “missing link” in the biogeochemical nitrogen cycle while expanding understanding of how transition metals can transduce energy from redox-active small molecules.

Concluding outlook

Since the beginning of the 20^th century, both the ability to produce and the need to remediate nitrogen-containing compounds have been coupled to fossil fuels. Demands for greater energy efficiency, smaller and more flexible distributed processes, and environmental protection provide growing impetus to expand the scope of practically viable oxidative and reductive transformations of nitrogen that are not driven by fossil fuels. Opportunities exist to achieve radically improved, new, and different pathways, but progress in this regard will require a molecular-level understanding of nitrogen transformation reactions, as well as the translation of these core insights to the discovery of new catalytic systems and alternative means of delivering the energy needed to drive those reactions. These understandings will emerge through the collective knowledge and insights to be gained from fundamental research that integrates experiments and theory in hetero- and homogeneous catalysis science, in photon- and electron-driven processes, and in biochemistry and biology.