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Photon-driven Nitrogen Fixation: Current Progress, Thermodynamic Considerations, and Future Outlook Andrew J. Medford, and Marta C Hatzell ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b00439 • Publication Date (Web): 28 Feb 2017 Downloaded from http://pubs.acs.org on February 28, 2017

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Photon-driven Nitrogen Fixation: Current Progress, Thermodynamic Considerations, and Future Outlook Andrew J. Medford∗,† and Marta C. Hatzell∗,‡ †School of Chemical & Biomolecular Engineering, Atlanta, GA, USA ‡George W. Woodruff School of Mechanical Engineering, Atlanta, GA, USA E-mail: [email protected]; [email protected]

Abstract

78% of the atmosphere’s volume; yet reactive “fixed” nitrogen, the form necessary for living organisms, is available in much smaller concentrations. 1 Fixed nitrogen compounds are defined by their lack of N-N bonds, and can take many forms, including ammonia, nitrates, and urea. These compounds are essential to living organisms due to their integral role in basic biological building blocks such as amino acids and DNA/RNA nucleotides, providing the nitrogen content needed to grow tissue in all plants and animals. 2,3 The conversion to and from fixed nitrogen is the backbone of the global nitrogen cycle, one of the most important biogeochemical cycles on earth. For nearly 2 billion years, earth relied on natural processes to drive the production of fixed nitrogen (150-200 Tg yearly). 4 Most of this (∼90%) can be ascribed to biological fixation by the nitrogenase enzyme. A small fraction of the global fixed nitrogen comes from atmospheric ionization; 5 however, uncertainty exists in all biogeochemical nutrient fluxes, limiting our ability to fully define all possible nutrient sources and sinks. 6,7 In fact, abiotic photofixation of dinitrogen in soils and sands has been suggested to be a third significant source of natural nitrogen fixation. 8,9 Human influence on the nitrogen cycle is a new phenomenon on a geological time scale, but has been a prevalent practice in preindustrial and even ancient agroecosystems. The scientific understanding of fixed nitrogen on crop growth

Over the last century the industrialization of agriculture and the consumption of fossil fuels have resulted in a significant imbalance and redistribution in nitrogen containing resources. This has sparked an interest in developing more sustainable and resilient approaches for producing nitrogen-containing commodities such as fertilizers and fuels. One largely neglected but emerging approach is photocatalytic nitrogen fixation. There is significant evidence that this process occurs spontaneously in terrestrial settings, and it has been demonstrated in numerous engineered systems. Yet many questions still remain unanswered regarding the rates, mechanisms and impacts of photocatalytically producing fixed nitrogen “out of thin air”. This work reviews the fascinating history of the reaction and examines current progress toward understanding and improving photo-fixation of nitrogen. This is supplemented by a quantitative review of the thermodynamic considerations and limitations for various reaction mechanisms. Finally, future prospects and preliminary performance targets for photocatalytic nitrogen fixation are discussed.

1

Introduction

Nitrogen in its nonreactive form (dinitrogen, N2 ) is readily available in nature, making up

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approaches for producing fixed nitrogen and controlling the global nitrogen cycle. The transformational implications of a process for efficient nitrogen fixation at mild conditions has made ambient dinitrogen fixation the subject of significant research in chemistry and catalysis. Numerous strategies have been explored, including homogeneous catalysis and coordination chemistry, 23–25 biological and biomimetic approaches based on the nitrogenase enzyme, 26–32 electrochemical and electrocatalytic approaches, 33–35 and catalyst discovery for heterogeneous thermocatalytic processes. 36,37 These strategies have been wellstudied and reviews exist detailing the state of research. However, the prospect of photondriven nitrogen fixation has received relatively little attention despite the fact that it relies only on clean, readily-available feedstocks (water, air, and light) and has been reported to occur at ambient conditions. Although numerous studies have been conducted, the existing literature is rather disjointed, making it difficult to ascertain a comprehensive view of the current knowledge regarding the possibility of producing renewable fertilizers through solar energy. This work seeks to provide a thorough picture of the current state of research in the field of photon-driven nitrogen fixation and provide a solid foundation for future work in this direction.

2

Figure 2: Histogram of number of publications per year on nitrogen photofixation classified by type of catalyst. the photocatalytic production of ammonia over a variety of sterile desert sands. 9,38–41 This work was followed by several independent groups who were able to achieve similar results with titaniabased catalysts, particularly when doped with transition-metals such as iron. 42–47 However, other groups were unsuccessful in reproducing these results and pointed out the thermodynamic challenges with the proposed conversion of nitrogen and water (two very stable reactants) to ammonia and oxygen (two relatively reactive products). This led to a contentious debate in Angewandte, 48–51 but no consensus was reached regarding the source of the disagreement. Despite the initial promising results on this important process, the number of publications dwindled over the next decades. Recently, there has been a renewed interest in photocatalytic nitrogen fixation, and several papers have demonstrated ammonia 52–54 and nitrate 55 formation using a variety of semiconducting catalysts, plasmon-enhanced systems, and biomimetic systems. Yet, relatively little emphasis has been placed on understanding the fundamental phenomena governing the reaction, and many of the discrepancies within the literature remain unexplained. Thus, photocatalytic fixation of nitrogen on earth-abundant catalyst at ambient conditions remains enigmatic nearly 70 years after its initial discovery. This section presents a roughly chronological summary of these previous studies, beginning with the early results on natural materials. Subsequently we review the more controlled

Current Progress

At first glance, one might assume that the conversion of nitrogen to ammonia at atmospheric conditions is unrealistic due to the large thermodynamic driving force required. However, production of ammonia “out of thin air” has indeed been demonstrated in numerous experiments, and has a rather fascinating history (Figure 2). This reaction was first discovered in the 1940’s by an Indian soil scientist 8 but drifted into obscurity, perhaps due to war and the rise of industrial ammonia production. The hypothesis was revisited in the late 1970’s and early 1980’s with the pioneering work of Schrauzer and Guth who demonstrated


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studies on engineered synthetic semiconductor catalysts, followed by results indicating the potential of co-catalysts and plasmon enhancement, and finish with a brief overview of more unique strategies for harnessing photons to fix nitrogen. The focus will be on semiconductor and metal/semiconductor systems where the materials and device-level design strategies developed for photocatalytic water splitting 56 are more directly applicable, while more exotic approaches will be treated in less detail.

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more, the “theory” Dhar suggests to explain his findings is not consistent with the modern understanding of thermochemistry. Dhar speculated that nitrogen fixation occurs through a multi-step process driven by “chemical energy” liberated from the oxidation of carbonaceous species, 57,62 though the nature of this energy and its transfer are not described. One possible explanation is that the hydrocarbons are more easily oxidized/reduced than water, which could enhance the nitrogen reduction/oxidation rate if the reaction is limited by its converse half-reaction (i.e. the hydrocarbons act as sacrificial reagents). It is also plausible that the hydrocarbons may directly react with atmospheric nitrogen, leading to carbonaceous fixation products such as urea. 63 Alternatively, the effects of carbon content may simply indicate that the sterilization procedures used in this work did not effectively kill all nitrogen fixing bacteria, since sugars and fats are known to aid in cultivating microorganisms. It is also possible that photo-decomposition of residual amino acids from dead microorganisms may account for some of the measured ammonia. 64 Despite the fact that much of this early work may be unreliable, the results and main conclusions opened the discussion on the photofixation of dinitrogen by oxide catalysts. The first rigorous work on photocatalytic nitrogen fixation by natural materials was performed by Schrauzer and Guth who independently re-discovered the process in the early 1980’s. 9,41 Schrauzer and Guth studied global desert sands from a variety of locales (California, Kuwait, India, Egypt, China, Saudia Arabia), rather than fertile soils where biological fixation is known to occur. Deserts, with their vast surface area, intense solar flux, oxide-rich sands, and lack of microorganisms, were speculated to serve as an unaccounted sink for dinitrogen through photofixation, potentially responsible for fixing 106 tons of N2 per year. 9 This is comparable to the amount fixed by lightning. 5 The hypothesis was tested with sterilized desert sands where control experiments showed no nitrogen fixation, and isotopically labeled nitrogen was utilized to provide conclusive evidence that photon-driven nitrogen fixation oc-

Natural materials: Soils and sands

Initial studies of photocatalytic nitrogen fixation aimed to investigate the role of the naturally abundant photocatalyst (soils and desert sands), rather than synthesized metals and metal oxides. 8,9,57 The first work was led by N.R. Dhar, a prominent Indian soil scientist 58–61 who was motivated by the need for a nitrogen fixation industry in India. Dhar began investigating photofixation in the 1940’s, and obscure references in his work suggest that knowledge of this reaction may date as far back as 1910. 57 These studies included numerous unconventional experiments involving sterilized soil in the light and dark, at different temperatures, and in the presence of an extremely diverse range of hydrocarbons including glucose, cane sugar, and clarified butter. 8,57,62 The primary conclusion drawn by Dhar and colleagues was that “nitrogen fixation in sunlight is greater than in the diffused light or in the dark”. 8 Dhar further hypothesized that oxides (including TiO2 , ZnO, and Fe2 O3 ) in the soil were responsible for the excess nitrogen fixed in experiments with sterile (abiotic) soils, 62 and noted that fixation could occur through either an oxidative or reductive path. 57 The results of Dhar’s numerous experiments consistently indicate that illumination with sunlight enhances the rate of nitrogen fixation by sterilized soils; however, the experiments were not conducted with modern techniques and should be viewed with some skepticism since even control experiments resulted in non-zero levels of fixed nitrogen. 8,57 Further-


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2.2

Yield of Ammonia (nmol/year)

curs at atmospheric conditions. This, along with the consistency of the results across many types of sand lends credence to the findings and suggests that the phenomenon is rather robust. Despite the fact that these sands were not well-defined catalysts, some clear trends emerged. The crystallinity was concluded to have a significant effect on the activity of titania, with the activity increasing roughly linearly with the percent of rutile (Figure 3a). This finding is the opposite of most CO2 and H2 O photocatalytic studies, where anatase titania is typically more active. 65 Photoxidation products measured were also significantly lower than photoreductive products, indicating photoreduction may be the primary means for dinitrogen fixation. 9 These early studies laid the groundwork for many subsequent investigations based on more well-defined synthetic catalysts; however, the role of photon-driven nitrogen fixation in the environment has received relatively little attention. Photocatalytic dinitrogen conversion is not widely acknowledged as a significant source of natural nitrogen fixation, despite the fact that it may be a dominant source of fixed nitrogen in arid environments. More involved studies of this photogeochemical phenomenon may prove interesting within the environmental, geochemical, and soil science communities.

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%-wt Fe2O3 in TiO2

Figure 3: (a) Reproduced plot of ammonia yield against the rutile concentration for desert sands, 41 (b) ammonia concentration based on catalyst weight percent (Ti/Fe). 66 observed in control experiments, raising similar questions of contamination that plague his earlier work. Schrauzer’s work on titania photocatalysts, beginning with his seminal 1977 paper, 38 is far more rigorous. The phenomenon of nitrogen photofixation was re-discovered during the evaluation of the photoreductive path of acetylene to various hydrocarbons on titania catalysts. When quantifying the products formed with varying concentrations of argon and nitrogen, ammonia and hydrazine were detected. These findings led to more comprehensive studies focusing on the influence of crystalline phase and iron dopants on titania catalysts, 41 a filed patent on photoreduction of nitrogen 39 and demonstration of a prototype “nitrogen reducing solar cell”. 40 These early experiments, along with the previous work on sands, led to a number of findings regarding the nature of the titaniabased catalysts. Nitrogen photofixation activity increased with increasing rutile concentration (see Figure 3(a)), confirming the impor-

Titania-based materials

Titania-based catalysts were the first nitrogen fixation photocatalysts, and have received the most attention of any material for photocatalytic nitrogen fixation. Both Dhar and Schrauzer conducted extensive experiments on lab-synthesized metal oxides in addition to their work on soils and sands. 38–41,62,67 In Dhar’s later works, he evaluated the photofixation process on titania, zinc oxide, and ferric oxide. 62 Again he supplied carbon sources such as glucose and starch in sterile and non-sterile conditions. In both abiotic and biotic conditions titania was found to provide the largest yield of fixed nitrogen. However, this work also relied on the same “chemical energy” hypothesis and significant baseline nitrogen fixation was


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tance of rutile. An optimum ammonia yield >5 times greater than pure anatase was found with ∼25% rutile, though this decreased as rutile concentration increased further. The ammonia yield of pure rutile was found to be about twice that of anatase. 38,41 This decrease was attributed to a loss of rutile surface area, since photocatalysts with a larger percentage of rutile generally consist of larger titania particles. The optimal rutile particle size was determined to be 0.1-0.3 µm, though the particle size distributions were not well-controlled or wellcharacterized. 41,67 On the contrary, some other studies have indicated that the anatase phase of TiO2 is more active than rutile, 68,69 though it is noted that in both cases catalysts were classified as anatase or rutile based on bulk measurements, while more detailed experiments have indicated that even bulk anatase may contain rutile at the surface. 70 Despite the lack of consensus regarding the active phase, a common observation is that high pre-treatment temperatures (∼1000◦ C) are required, and that small amounts (Rh>Pd>Pt, and this was attributed to the metal-H bond strength. It was hypothesized that stabilizing the Hads species was necessary to minimize hydrogen evolution. 47 This is consistent with other work investigating Ru, Fe and Os co-catalysts, where it was observed that metal co-catalysts for ammonia production need to have a high overpotential for hydrogen evolution. With metals with high overpotentials for the hydrogen evolution reaction (Ru and Fe) having higher ammonia activity than metals with low hydrogen evolution reaction overpotential (Os). 46,47 However, we note that in this role the transition metals would not be acting as co-catalysts in a strict sense, as they would be reducing the rate of hydrogen evolution rather than increasing the rate of ammonia synthesis. This would necessitate that the metals effectively block the hydrogen evolution sites on the semiconductor catalyst, acting as a poison for hydrogen evolution rather than (or in addition to) being a catalyst for ammonia synthesis. Advances have also been made in understanding the trends in thermochemical ammonia synthesis over transition-metal catalysts. 37,110 The trend in improvement of photocatalytic ammonia synthesis for metal cocatalysts (Ru>Rh>Pd>Pt) is remarkably consistent with the trend for thermochemical ammonia synthesis catalysts, which suggests an

in promoting greater ammonia yields, though there have been relatively few systematic studies of the fundamental processes that lead to these improvements. One role of transiton-metal ions is as dopants. Metals and noble metals act as electron sinks because their Fermi level is lower than most semiconducting materials. This creates a Schottky barrier at the metal/semiconductor interface which traps photogenerated electrons, minimizing the probability for carrier recombination. 107 The timescales for charge carrier recombination are known to be on the order of 10-100 ns, 108 therefore designing photocatalytic systems to minimize recombination is a critical issue. This improved charge carrier separation is hypothesized to be the primary role of iron dopants in titania, as discussed in Section 2.2. Another suggested role of transitionmetal dopants is an indirect enhancement through mediation of the catalyst crystallinity. Schrauzer and Guth, in their initial work, investigated a range of metal dopants (Fe, Co, Mo, Ni, Pd, Pt, Ag, Au, V, Cr, Pb and Cu). They showed that only Fe, Co, Mo, and Ni enhanced ammonia formation, and all other dopants suppressed ammonia formation when compared with controls. This was linked to the percentage of rutile titania present in each sample. The synthesis approach used high temperatures to load the dopant into the titania photocatalyst, and was hypothesized to change the cystallinity of the photocatalyst. In catalysts with Fe, Co, Mo and Ni dopants the titania was primarily rutile (10-95%) while all other metal loaded titania structures showed lower (500 nm wavelengths. Interestingly, Au particles were hypothesized to act as reduction catalysts, although Au has not been reported

2.5

Biomimetic materials

Nitrogenase enzymes are biological catalysts that convert dinitrogen to ammonia at ambient conditions. 119 Prior to synthetic fertilizers, the majority of terrestrial fixed nitrogen was produced using nitrogenase enzymes. For nearly


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the nitrogenase enzyme. 124 However, the protein also catalyzes the hydrogen evolution reaction at a rate of approximately 10x greater than ammonia, leading to the requirement of nearly 100 photons (hν ∼3 eV) per ammonia molecule. Furthermore, performance degraded significantly after 5 hours of operation, 124 indicating that substantial progress is needed to make the approach practical. Nonetheless, this work highlights the possibility of interfacing a biocatalyst with a photon absorber to create a photocatalytic system. In an alternative biomimetic approach Bannerjee et al. have utilized porous chalcogenide aerogels as photocatalysts. 125,126 These catalyst utilize Mo2 Fe6 S8 (SPh)3 125 and Fe4 S4 126 clusters as analogs to the MoFe active site of the nitrogenase catalyst, and these clusters are linked by Sn2 S6 ligands to form amorphous chalcogel structures. While a range of these chalcogels exhibited nitrogen reduction activity, interestingly the most active catalysts did not contain Mo, suggesting that the active sites of these chalcogels may differ from the nitrogenase enzyme. These materials exhibit good aqueous stability and have small bandgaps below 1 eV, allowing them to harvest a large amount of the solar spectrum. The yields are higher than observed for most oxide-based catalysts 126 suggesting that these materials are promising for practical photocatalytic nitrogen fixation. However, a number of differences in experimental setup including the the use of sacrificial reagents such as pyridinium hydrochloride and sodium ascorbate make it difficult to directly compare the performance to that of oxide catalysts.

five decades, efforts have been made to discern the fundamentals of this biological fixation process 120–122 with the hope that understanding the details of this mechanism will enable the design of new bio-inspired and biomimetic catalysts for effective nitrogen fixation at benign conditions. The nitrogenase enzymes produce ammonia through a catalytic process driven by two proteins with different metal active centers (Fe and MoFe). The Fe based protein is termed the nitrogenase reductase, and the MoFe protein termed the nitrogenase. Electrons are repeatedly transferred from the nitrogenase reductase to the nitrogenase (MoFe) through binding and hydrolysis of ATP. The electrons transferred to the nitrogenase are then stored as a Fe-hydride, which react with dinitrogen to eventually produce ammonia and hydrogen. 123 While the biological process has the great advantage of operating at ambient conditions, the repeated hydrolysis of ATP, and transfer of 8 electrons makes the process fairly inefficient, leaving room for optimization through catalyst engineering. 121 The idea of biomimetic nitrogen fixation catalysts has, in general, received considerable attention due primarily to the benign conditions of the biological reaction. 28–32,121 Recently, several exciting experimental studies have demonstrated the potential of bio-hybrid and bioinspired catalysts to harness photons for nitrogen fixation. 124–126 Brown et al. 124 explored this idea by combining the MoFe protein with CdS nanocrystals to create a bio-hybrid system. CdS is a photocatalytic semiconductor with a bandgap of ∼2.4 eV, and a conduction band edge at -0.8 V (vs. NHE). This is low enough to reduce the MoFe protein, therefore the CdS catalyst acts as an electron source when exposed to visible light, and the MoFe protein acts as the electron scavenger. This stepwise process aids in controlling the rate at which electrons are supplied to the nitrogenase. Initial performance revealed that the shape and size of the CdS significantly altered the performance, with only rod like shaped particles producing measurable ammonia. Performance values reached a quantum yield for ammonia of 3.3%, corresponding to 63% of the activity of

2.6

Carbon materials

Photon-driven nitrogen fixation has also been reported to occur over carbon-based materials with no transition metal components. For example, Nishibayashi et al. reported that capped buckminsterfullerenes can photocatalyze the fixation of dinitrogen to ammonia. A neutral pH was found to be optimum, and the reaction produced relatively high yields (360 ppm) of ammonium under illumination with visible


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light. 127 It was hypothesized that this occurs due to the redox properties of C60 , although relatively few details were provided and to our knowledge this study has not been followed up. More recent work has demonstrated nitrogen fixation over graphitic carbon nitride, including isotopic labeling experiments to ensure that the measured ammonium originates from dinitrogen rather than the nitride material. Nitrogen vacancies were suggested to be active sites in the case of nitrogen photofixation. 100 Doped graphite and graphene materials have also recently been reported as efficient catalysts for the oxygen evolution/reduction reactions, 128 and nitrogen dopants have been identified as active sites. 129 These findings suggest that doped carbon materials are an interesting and versatile class of catalytic materials that may play an increasingly important role in nitrogen photofixation. A fundamentally different approach for using carbon materials to fix nitrogen has been to utilize diamond in order to generate “solvated electrons”. The solvated electrons mitigate the need for adsorption and desorption of dinitrogen to the catalyst. In this case solvated electrons are ejected into the solution from the conduction band of a hydrogen-terminated diamond photoabsorber with a very large 5.5 eV bandgap. These solvated electrons reduce protons to hydrogen radicals 130,131 with dinitrogen forming diazenyl radicals and eventually ammonia. In this case, ammonia is formed indirectly in the aqueous phase rather than on the surface. Degradation of the H-termination sites on the diamond surface caused the catalyst to deactivate. 130 In addition, practical challenges associated with diamonds large band gap (5.5 eV), allow only a small fraction of the UV spectrum to be captured (190-225 nm). A similar approach was employed by Lu et. al. 132 who utilized a 3D graphene as an absorber to generate “hot” electrons. The 3D graphene absorber was coupled to an iron catalyst where the hot electrons provided a driving force for sequential reduction of dinitrogen to diazene, hydrazine, and ammonia. These studies indicate that the principle of nitrogen fixation through solvated electrons or other high-energy interme-

diates/radicals provides a novel strategy for nitrogen photofixation.

2.7

Polymeric materials

An alternative strategy involves fixation of nitrogen through the formation of solid ammonium perchlorate. 133,134 This has been achieved by using catalyst/conducting-polymer composites. A composite made of ClO− 4 doped poly(3methylthiophene) (P3MeT) and titanium dioxide was prepared through electrochemical oxidation, and ammonium perchlorate crystals grew as the system was irradiated by white light. The mechanism by which the crystals grew was described as a combination of ClO− 4 dedoping from the polymer and photocatalytic dinitrogen reduction which produced NH3 , followed by an acid-base reaction. 133,134 While the process does yield fixed nitrogen, long term sustained operation would not be practical since crystals would need to be intermittently removed from the electrode surface and the conductive polymer would need to be redoped over time. Furthermore, the fixation of nitrogen is being catalyzed by the titania catalyst, making the formation of solid ammonium perchlorate more of a separation technique than an approach for producing fixed nitrogen. Nonetheless, these studies highlight the possibility of utilizing creative chemistries in order to improve the efficiency of photofixation of atmospheric nitrogen.

3

Thermodynamic Considerations

The literature on photocatalytic nitrogen fixation over titania-based catalysts suffers from a lack of consensus regarding the global reaction. The majority of the studies discussed in Section 2 demonstrate nitrogen fixation via reduction to ammonia; however, several other works indicate that nitrogen can also be oxidized to nitrates over titania catalysts. 55,84,85,89 The fact that ammonia oxidation to nitrates is thermodynamically favorable and known to occur photocatalytically over titania 136 is one ob-


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Table 1: Thermodynamic potentials of global reactions at standard temperature and pressure (298 K, 1 atm) taken from the NIST CCCBDB. 135 Reaction

∆ G [eV]

N2 (g) + H2 O (l) + 25 O2 (g) → HN O3 (g) 0.93 N2 (g) + 2O2 (g) → 2N O2 (g) 1.08 1.55 N2 (g) + H2 O (l) + 23 O2 (g) → HN O2 (g) N2 (g) + O2 (g) → 2N O (g) 1.81 N2 (g) + 3O2 (g) → 3N O3 (g) 2.46 N2 (g) + 56 H2 O (l) → 65 N O (g) + 54 N H3 (g) 3.90 N2 (g) + H2 O (l) + 21 O2 (g) → 2HN O2 (g) 4.94 7.03 N2 (g) + 3H2 O (l) → 2N H3 (g) + 23 O2 (g) N2 (g) + 3H2 O (l) → 2N H2 OH (g) + 21 O2 (g) N H3 (g) + 45 O2 (g) → N O (g) + 32 H2 O (l) N H3 (g) + O2 (g) → HN O (g) + H2 O (l) H2 O (l) → H2 (g) + 21 O2 (g) 2H2 O (l) → H2 (g) + H2 O2 (g) 2H2 O (l) → H2 (g) + 2OH ∗ (g)

vious explanation for this observation, and indeed in-situ oxidation of formed ammonia has been observed in the case of nitrogen photoreduction. 67,78,81 The work of Ileperuma et al. indicates that nitrates and ammonia may be produced simultaneously. The results show that the ammonia/nitrate ratio varies over time with catalysts first producing ammonia that is subsequently reduced. 85 However, there are also reports where nitrates are observed but ammonia is not detected. 55,84 For example, the relatively recent study of Yuan et al. 55 indicates that titania nanoparticles photocatalytically oxidize dinitrogen without the formation of ammonia. The lack of consensus regarding the products of photocatalytic nitrogen fixation makes it difficult to develop consistent hypotheses regarding the reaction. In order to provide perspective on photondriven nitrogen fixation it is useful to examine the thermodynamic driving forces which govern the process. The thermodynamic considerations can be divided into two types: the thermodynamics of the global- and half-reactions of the process (catalyst independent) and the thermodynamics of surface reactions and photoninduced processes (catalyst dependent). The

n

e

[#]

∆ V [V]

10 0.093 8 0.135 6 0.258 4 0.453 12 0.205 4 0.975 2 2.47 6 1.17 7.31 2 3.655 -2.61 5 -0.52 -1.05 4 -0.26 2.46 2 1.23 3.82 2 1.91 5.63 2 2.81

∆ eN [#]

+5 +4 +3 +2 +6 +2,-3 +1 -3 -1 (13) +5 (14) +4 (15) N/A (16) N/A (17) N/A (18)

(5) (6) (7) (8) (9) (10) (11) (12)

primary focus of this section is the former, as these represent fundamental limitations and thus provide a foundation for any hypotheses involving photo(electro)catalytic fixation of nitrogen. The influence of the catalyst redox potential, band edge energies, and band gap is also briefly discussed, with an emphasis on commonly used titania catalysts described in Section 2.2.

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Reaction Thermodynamics

Considering the thermodynamics of the global reactions and redox half-reactions is an important first step to formulating sound scientific hypotheses regarding the photo(electro)catalytic fixation of nitrogen. Global reaction thermodynamics are of particular importance for photocatalysis, where the lack of applied bias makes it difficult to systematically separate and study the redox half-reactions.Such photoelectrochemical applied bias studies for nitrogen conversion have been limited. 44,95,98,104 Without high-quality results from a 3-electrode photoelectrochemical setup the overall reaction and half-reactions can only be inferred or speculated based on


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observed products. As such, a comprehensive thermodynamic analysis of common nitrogencontaining compounds with well-determined enthalpies and entropies is presented in order to establish the thermodynamic feasibility of various nitrogen fixation routes. The N-N triple bond is one of the strongest in nature (9.44 eV @ SATP), and interconversion of mononitrogen compounds (e.g. ammonia to nitrates) is relatively facile compared to conversion from dinitrogen to mononitrogen compounds. 137,138 Nitrogen fixation can be broadly classified as reductive or oxidative. Thermochemically these are distinct processes, with nitrogen fixation occurring through reduction in the Haber-Bosch process 138 and subsequent oxidation to nitrates through the Ostwald process. 137 However, the case of photocatalytic nitrogen fixation is less clear, since both reductive (ammonia) and oxidative (nitrate) products have been observed from titania-based photocatalysts at ambient conditions. 41,55 The thermodynamics of both oxidative and reductive reactions at ambient conditions are given in Table 1 based on values tabulated by NIST. 139 The free energy values are given at ambient temperature (298K) and the pressure of each species is equal to 1 bar for convenience. We have assumed that the only reactants available at ambient conditions are nitrogen, oxygen, and liquid water, and all possible products with a ∆G < 8 eV are shown. The number of electrons transferred (ne ), voltage per electron (∆V), and change in nitrogen redox state (∆eN ) are also given. Unsurprisingly, all reactions are endothermic, with some exhibiting extremely large amounts of free energy per N2 molecule; however, the energy needed per electron (i.e. applied voltage) is relatively low (< 1V) in many cases. For comparison, the wellstudied water splitting reaction is also included, and we note that this reaction requires a larger voltage than most oxidative or reductive nitrogen fixation pathways, demonstrating that these reactions are indeed thermodynamically feasible in contrast to some previous opinons. 48 We note that the water splitting reaction (reaction 16) can be combined with oxidative reactions (e.g. reactions 6,8,9) in order to make

water the oxidizing agent; however, the significantly higher oxidative potential of O2 makes it a more thermodynamically favorable oxidizing agent. The thermodynamic data in Table 1 indicates that photo(electro)catalysis can theoretically fix nitrogen with a cell voltage lower than 0.1 V. Although this is a remarkably low voltage, it is worth noting that most processes with low theoretical cell voltages (e.g. direct formation of HNO3 ) require many (> 6) electron transfers, making them kinetically improbable. Furthermore, some reactions are likely to proceed sequentially since concerted multi-electron transfer reactions are improbable. 140 In particular, conversion of NO and NO2 to HNO2 and HNO3 is known to occur readily through reaction with water. 137 This is illustrated in Figure 6 which shows that the production of NO is the most thermodynamically challenging step, and indicates that the formation of further oxidized products will be spontaneous. Further, Figure 6 shows that in order to achieve nitric acid production at the theoretical voltage of 0.093 V it would be necessary for a catalyst to bind NO very strongly (>1.5 eV) but bind NO2 very weakly ( 2 V applied overpotential. However, in the case of oxidative nitrogen fixation the alkaline oxygen reduction half-reaction would only have ∼0.55 eV applied overpotential (see reaction 34), which is smaller than the overpotential needed to reduce oxygen over titania. 151,152 This indicates that the reductive half-reaction will likely be rate-limiting since the band alignment of TiO2 will deliver high-energy holes that can drive even kinetically challenging oxidative reactions. Furthermore, this simple analysis indicates that there is tremendous potential to optimize the performance of nitrogen photocatalysis through bandgap engineering and co-catalysts since TiO2 catalysts possess a bandgap >2 V larger than thermodynamically required for nitrogen fixa-

Surface and Band-gap Energetics

As outlined in the previous section, there are a range of dinitrogen reduction and oxidation reactions which may play a role in photofixation. The diversity of previously reported results indicates that the dominant reaction likely depends on the photocatalyst characteristics and reaction kinetics of individual elementary processes. The previous discussion has relied purely on established thermodynamic facts. The relevant thermodynamic free energies are well-known because they rely solely on gasand liquid-phase products and reactants with well-defined structures and properties. However, these analyses only provide bounds and limitations. A full understanding of photocatalytic nitrogen fixation will require a detailed understanding of the active-site structure, electronic properties, and energetics of adsorbed intermediates for a specific catalytic system. These quantities are generally not known for any catalyst in the context of nitrogen fixation. Nonetheless a few concepts and possibilities are reviewed here with a focus on what is currently known regarding the most well-studied titaniabased photocatalysts. The energetics of half-reactions discussed in the previous section assume that the catalyst does not play a direct role. However, there is also the possibility that a reducible oxide catalyst is directly involved in the redox cycle. For example, titania could catalyze nitrogen fixation through the following redox reactions: N2 (g) + 2T iO2 + 4e− → 2T iO + 2N O (35)


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Figure 8: Band-gap energies and conduction/valence band levels of common semiconductors relative to water splitting and N2 reduction/oxidation potentials. The width of conduction bands shows uncertainty or range of values where reported. Band edges for Fe2 Ti2 O7 are taken from Rusina et al. 82 and all others are taken from Tamirat et al. 149 . tion, and numerous other semiconductors with smaller bandgaps straddle the nitrogen fixation redox potentials (see Figure 8). These include several materials that have demonstrated preliminary nitrogen photofixation activity (GaP, Cu2 O, and CdS as discussed in Section 2.3), indicating promise for the development of improved photocatalytic nitrogen fixation catalysts. These opportunities are discussed further in Section 4.3.2.

4

tion, as well as possible strategies for reaching these targets. This discussion is not meant to be authoritative; rather, we hope that the simple analyses presented will inspire more detailed techno-economic studies to more clearly define targets and bottlenecks in the quest to develop practical schemes for photon-driven nitrogen fixation.

4.1

Future Outlook

Establishing Standards and Benchmarks

One significant challenge in considering the current state of photocatalytic nitrogen fixation is the lack of consistency in the reported experimental conditions and catalytic behavior of the vast variety of materials that have been tested. Many experiments utilize inconsistent setups. Light sources vary between mercury and xenon lamps, hole scavengers are often used, and parameters such as humidity, pH, catalyst concentration, and temperature vary considerably. Furthermore, the catalytic activity is not measured in a consistent way, with many reports simply reporting total yield, while others report weight-normalized rates, and a few include various measures of quantum efficiency. Originally it was our intention to compile a quantitative overview of reported photocatalytic activities for nitrogen fixation, but we discovered that creating a fair comparison between various

The global application of fixed nitrogen in agriculture increased from 3 Tg-N per year in the 1950s to over 100 Tg-N per year today. 153 This resulted in a fourfold increase in crop yield, while ammonia production increased by a factor of more than 30. 16 Maintaining this growth will remain critical as the global population continues to rise. In addition to fertilizers, ammonia is necessary for creating many chemical compounds, and has sparked interest as an energy rich alternative fuel and hydrogen storage medium. 154 Low-infrastructure based approaches for ammonia production, such as those discussed within this work, may aid in meeting rising fixed nitrogen demands. Here we discuss potential solar-to-ammonia targets needed to sustain fertilizer and alternative fuel produc-


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4.2

studies is not possible given the information reported. The general importance of benchmarking in catalysis has been acknowledged by the community, 155 and several recent studies have established a set of valuable standards and references for photoelectrocatalytic water splitting. 156,157 We recommend that future studies involving photoelectrocatalytic nitrogen fixation leverage these clearly defined procedures in order to produce easily comparable results. However, most photon-driven nitrogen fixation occurs on unsupported photocatalysts. Photocatalysis remains one of the more difficult areas to establish standardized testing because of the diversity of systems which can include both homogeneous and supported or unsupported heterogeneous catalysts as well as solid/gas and solid/liquid systems. One recent work by Kisch et. al. 158 establishes some best practices for measuring photocatalytic rates by ensuring that activity is measured at the optimal concentration, and by reporting quantum yields for solid/gas systems and rates for solid/liquid systems. Further refinement of these standards for the specific case of photocatalytic nitrogen fixation will be necessary. This includes a standard pH, temperature, and light source. It is our opinion that measuring the ammonia production rate at standard temperature and pressure at neutral pH without scavengers under illumination by an AM 1.5 solar simulator will provide a valuable measurement of the practical performance of a nitrogen fixation photocatalyst at environmental conditions. More rigorously defining the conditions of benchmark tests should be a community-driven effort and is beyond the scope of this review; however, thorough reporting of the experimental conditions (including humidity, catalyst surface area, illumination area, and catalyst concentration) and nitrogen fixation rates (normalized to both geometric illumination area and catalyst surface area) and including quantum yields will enable improved comparisons in the future.

4.2.1

Targets for Practical Relevance Ammonia for Fertilizer

The average global fixed nitrogen demand reported for 154 countries was 55 ± 11 kg-N ha−1 y−1 in 2010 (95% confidence). 159,160 Specific nitrogen loading ranges extensively from only 10 kg-N ha−1 y−1 in developing countries to over 300 kg-N ha−1 y−1 in East Asia. 160–162 This has created a global disparity in fixed nitrogen consumption, with higher-income countries consuming nearly 25 kg-N per capita and low-income countries only consuming 15 kg-N per capita. 3 Closing this nutrient gap could aid in meeting many health and sustainability related initiatives. 21,163 Due to socioeconomic disparities and geographic challenges uniform distribution of nitrogen-containing resources can not be attained unless low cost and distributed technological solutions can be realized. The average global nitrogen load is equivalent to a molar flux of 12.5 nmol-N m−2 s−1 . In order for photocatalytic nitrogen fixation to become a practical route to replace or supplement current thermochemical based approaches device production rates should begin to approach this base load. For simplicity we assume that two moles of ammonia are produced photocatalytically through a six electron reductive pathway (Table 1, reaction 12). One approach to estimate the required performance is from the perspective of energy efficiency. Using a solar constant of 1 kW/m2 and the fact that each mol of NH3 requires 3.52 eV (340 kJ/mol), and assuming 8 hours of daylight per day, leads to a required ”solar to ammonia” (STA) efficiency of approximately 1 ×10−3 %. However, this number implicitly assumes that the amount of land area used for solar harvesting is equal to the land area used for farming, which is likely unrealistic. We propose that this technology would become practical if less than 1% of land is necessary for solar harvesting, leading to a still-low efficiency of 0.1 % solar to ammonia, several orders of magnitude below the current solar to hydrogen efficiencies that can exceed 10%. 164,165


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ies have focused on reporting ammonia yield or mass-normalized rate rather than quantum efficiency, and have utilized a variety of light sources and experimental setups making it difficult to extract reliable estimates of the efficiencies. More recent studies have occasionally reported quantum efficiency values, which range from 10-5 to 0.1%. 52,53,113,130 This corresponds to roughly 30 nmol g−1 s−1 for the best reported quantum efficiencies 52 (illuminated with a xenon lamp). These values provide evidence that efficiencies capable of meeting fertilizer demands are realistic through rational optimization of the process and/or discovery of improved catalytic materials.

The previous analysis presents a lower limit of the necessary quantum efficiency because it assumes that all photons are absorbed. In reality, a photocatalyst will only absorb the photons with sufficient energy to excite the bandgap, and each photon will excite a single electron/hole pair. The probability that this electron/hole pair participates in a reaction is known as the quantum efficiency, which is a wavelength-dependent quantity. Here define the cumulative quantum efficiency (%) for ammonia production as: CQE = 100 × Nhν =

Z

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3NN H3 Nhν

∞

ǫQ (λ)dλ λg

where NNH3 is the ammonia flux (mol-NH3 m-2 s-1 ), Nhv is the absorbed photon flux (molphoton m-2 s-1 ), ǫQ is the wavelength-dependent photon flux (mol-photon m−2 s−1 nm−1 ), λ is the wavelength (nm) and λg is the wavelength of the bandgap. The wavelength-dependent photon flux was obtained using a standard air mass (AM) 1.5 global solar spectrum, 166 and the required cumulative quantum efficiencies are plotted as a function of bandgap in Figure 9. The thermodynamic driving force required for the reaction is 1.17 V (see reaction 12), which sets a lower bound on the energy of photons capable of driving the process. Assuming the same target of 1% of land used for solar capture, the required cumulative quantum efficiencies range from 0.3 - 3%, which is considerably lower than the quantum efficiencies of state-of-theart photocatalytic water splitting. With this in mind, achieving 100% of our fixed nitrogen needs through solar driven means is potentially feasible if systems with moderate efficiencies are developed. We propose that this technology has the largest potential in remote and impoverished regions where soil nutrient contents are low, traditional fertilizers are expensive to transport, the amount of sunlight may exceed 8 hours per day and land available for solar capture may well exceed 1%. In these situations the required efficiencies could decrease by an order of magnitude. Most photofixation stud-

Figure 9: Approximate quantum efficiency required to meet base nitrogen loads needed in agricultural practices based on 1 kW/m2 illumination for 8 hours per day and 1% of land dedicated to solar harvesting.

4.2.2

Ammonia for Fuel

In addition to being a coveted nutrient, ammonia is an energy rich fuel and an effective hydrogen storage medium (17.6% hydrogen by weight). Ammonia is an effective fuel source for rockets, fuel cells, and internal combustion engines due to its high energy density (4.3 kWh/L). In addition, with the growing interest in non-hydrogen solar fuels from abundantly available feedstocks (e.g. CO2 ) there is potential motivation for the use of dinitrogen as a


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Table 4: Potential solar-to-fuel reactions, Gibbs free energy (∆ G), number of electrons transferred (ne ), thermodynamic potential (∆ V), and lower heating value (LHV) of fuels produced from H2 O, CO2 , and N2 at ambient conditions #

Reaction

∆ G [eV]

ne [#]

∆ V [V]

LHV [kJ mol−1 ]

1 2 3 4 5

H2 O (l) → 12 O2 (g) + H2 (g) CO2 (g) → CO (g) + 21 O2 (g) N2 (g) + 3H2 O (g) → 2N H3 (g) + 23 O2 (g) CO2 (g) + 2H2 O (l) → CH 4 (g) + 2O2 (g) 2CO2 (g) + 3H2 O (l) → C2 H5 OH (l) + 3O2 (g)

2.46 2.66 7.03 8.46 13.74

2 2 6 8 12

1.23 1.33 1.17 1.06 1.14

241 282 316 889 1235

4.3

feedstock. The overall fuel energy requirements for photochemical fuel production from H2 O, N2 and CO2 are similar (Table 4), motivating evaluation of all solar fuels. 167 The solar-to-fuel targets set by the Department of Energy (DOE) for solar-to-hydrogen (STH) applications are 25% STH and 2-4 $ per kg H2 . 106 State-of-the-art STH devices currently achieve of 12-18% STH, and projected costs range from 1.6-10.4 $/kg. 164,165 It should be noted that most of these cost estimates consider hydrogen production facilities which have access to a pipeline, and therefore take into account only moderate compression of hydrogen (20 bar). If liquid phase transport of generated hydrogen were needed (e.g. 700 bar compression), costs would increase substantially. Regardless, in order for solar ammonia to be considered as an alternative hydrogen carrier, STA targets would need to approach to approach 25% in order to obtain the same energy storage content. We note that CO2 -based solar fuels technologies generally have lower efficiency (1-7%) and selectivity targets than STH. One major advantage for CO2 -based solar fuels is the potential environmental and economic impact associated with removing greenhouse gases from the atmosphere, an advantage not shared by solar ammonia. Based on this analysis we expect that STA efficiencies of >20% would be required for STA to compete with STH technologies. Given that this is more than 2 orders of magnitude greater efficiency than required for fertilizer applications we propose that the most practical path to capitalize on nitrogen photofixation is the development of solar fertilizer processes.

Strategies for Improving Performance

Solar-driven N2 fixation, if realized, would provide a means for distributed and clean fertilizer production. Yet, as described above, the currently reported yields and efficiencies are 12 orders of magnitude below the performance needed for practical relevance. There are many technical and scientific challenges which limit ammonia production from photon driven processes. Near term challenges will need to focus on device and materials selection, guided by an improved understanding of the reaction pathway and elementary steps. Many of the challenges associated with materials and device configuration have been the subject of numerous reviews. 56,111,168,169 In short, materials should have high absorptivity and catalytic activity, and devices should be designed to separate products and maximize energy efficiency. Here we do not aim to reiterate the many prospective configurations, catalyst and photoabsorbers for solar fuel applications, but rather we will highlight potential important challenges which are specific to photon driven nitrogen fixation. 4.3.1

Device Configuration

Due to the emerging nature of the dinitrogen photofixation field, little work has been completed to explore reactor or device designs. Furthermore, nearly all photofixation studies have focused on particle-based photocatalytic systems rather than photoelectrocatalytic (PEC) device configurations. Particle-based systems


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provide simplicity and reduce the overall system cost, but the PEC-based systems have the advantage of decoupling the photon absorption and catalytic processes. This enables the use of absorber material(s) with optimal band-gaps and co-catalyst with minimal overpotentials. This strategy has been applied to solar hydrogen production and CO2 reduction has been shown to improve the STF efficiency through reducing carrier recombination, increasing light absorption, and minimizing transport losses. Furthermore, a sensitivity analysis has confirmed that STF efficiency is the most important parameter to optimize for low cost operation at scale. 106 Most reported STH efficiency values for particle based systems achieve lower than 1%, whereas PEC devices have attained 18% STH. Panel PEC devices are effective because they are able to efficiently capture light, and their catalytic reactions are well known. Furthermore, well-known selective electrocatalyst can be easily integrated into the panel. This indicates that PEC-based systems have the potential to significantly improve STA effeciencies as well, and this section provides some preliminary analyses of the possible efficiences of PEC-based systems for photocatalytic ammonia production. The use of panels allows for a range of potential device configurations. Single band gap material are generally not feasible for solar-fuel systems, because few materials conduction and valance bands straddle both redox reactions (N2 /NH3 and H2 O/O2 , see Figure 8). This limitation is overcome by decoupling the adsorption and catalytic processes using a photovoltaic (PV) cell coupled with an electrolysis cell (PV/electrolysis). PV/electrolysis promotes higher yields, selectivity and quantum efficiency for solar-fuel applications, 106,170 and better utilization of both the catalyst and photoabsorber materials. However, PV/electrolysis systems are generally not ideal as system complexity and potential cost increases. In terms of dinitrogen fixation, it is unknown if this proposed route would be optimal, as selective electrocatalyst for nitrogen reduction do not exist. If nitrogen reduction can occur electrochemically with high faradaic efficiency, a

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PV/electrolysis configuration should perform similarly to other STF operations. However, if the catalytic reaction process undergoes intermediate reactions where the light absorption process is coupled to the catalytic mechanism then PV/electrolysis would not be a feasible path forward. There is a clear need to identify both the mechanisms responsible for electrochemical nitrogen reduction (NRR), and those responsible for photofixation of nitrogen. We will touch upon this issue in Section 4.3.2. Tandem dual-band-gap PECs configurations actively couple both the adsorption and catalytic processes in the same material system, minimizing system requirements. By combining two lower band gap semiconductors together, this configuration effectively captures a larger portion of the solar spectrum (when compared to single band gap materials), and produces greater photovoltages necessary for redox reactions. 168 There are a range of photoabsorbers which can be combined to create the ideal optical and electrochemical environments needed for photofixation of dinitrogen. To model these approaches, many researchers have expanded early photochemical models to evaluate tandem PECs through combining photo (scattering, light concentration, recombination) and electrochemical (activation, ohmic, mass transport losses) phenomena. 171,172 More recently, a web-based model developed by Seger et al. has provided an open access means to assess the STF efficiency of tandem PECs. 173 This approach builds off of established models that couple photochemistry and electrochemistry. For solar-to-ammonia (STA) the efficiency can be written as Jop εN H3 LHVN H3 Pout = PS nN H3 F PS where Pout is power generated by the fuel, and PS is solar power per area, Jop is operating current density, εN H3 is Faradaic efficiency, LHVN H3 is lower heating value per mole of ammonia, nN H3 is number of electrons transfered per mole of ammonia, and F is Faradays conC stant (96485 mol ). In an ideal solar-to-ammonia device, the Jop and εN H3 need to be optimized. Jop in theory is limited by the Shockley-Quisser ηST A =


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7.0 6.0

60

5.0 4.0

40

3.0 20

2.0

2.8

1.0

1.5

0.8

2.6 1.2

2.4 2.2

0.90

2.0 0.60

1.8 1.6

0.30

STA efficiency (%)

Faradaic Efficiency (%)

8.0 80

c

3.0

STA Efficiency (%)

b Bottom Absorber bandgap (eV)

a 100

STA Efficiency (%)

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0.6

0.4

0.2

1.4 0.0

200 300 400 500 600 700 800 900 1000

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2

Overpotential KOER+KNRR (mV)

Top absorber bandgap (eV)

0.0 1.2

1.6

2.0

2.4

2.8

3.2

Band Gap (eV)

Figure 10: Projected maximum STA efficiency for a tandem cell with variable reaction overpotentials and Faradaic efficiency (Jop =10 mA cm-2 ) (a). White line indicates operating conditions for (b and c). Projected STA efficiency for a tandem dual band gap (b) and single band gap (c) device operating with 10% NRR Faradaic efficiency and 800 mV reaction overpotential. limit, and in solar-fuel applications is significantly limited by the additional cell reaction overpotentials (η). For simplicity here we assume that a STA device would contain the nitrogen reduction reaction (NRR) and oxygen evolution reaction (OER), which have corresponding reaction overpotentials (ηN RR and ηOER ). The OER overpotential is well documented, and approaches 400 mV at an operating current density of 10 mA/cm2 for the best catalysts. 156 The NRR reaction overpotentials are not well known, but are anticipated to be as high as 1 V. Faradaic efficiency, with a perfectly selective electrocatalyst would approach 100%. In reality, Faradaic efficiency for electrochemical nitrogen reduction (εN H3 ) has only approached 10%, 174,175 and typically is ≤ 1%. 176 Utilizing this as a basis, it is expected that STA values will be low. With a tandem dual band gap configuration, and an optimistic operating current density of 10 mA/cm2 , the STA values will range from 1 to 8.5% (Figure 10 a). However, the upper end of this range is only achievable if perfectly selective catalyst can be designed (εN H3 =100%), which has yet to be demonstrated. Reducing the overpotential substantially will improve STA performance, but has less impact on STA than improving selectivity. Therefore significant efforts must be placed on designing selective electrocatalyst rather then necessarily improving electrocatalyst activity. This “selectivity challenge” has

already been identified, and is expected to be challenging based on theoretical analysis. 73 If a realistic Faradaic efficiency is assumed (10%) and moderate operating reaction overpotential (400 mV for OER, 400 mV for NRR), the ideal tandem cell cell configuration would combine a top photoabsorber with Eg =1.3 with a bottom photoabsorber which has Eg =2. In this configuration STA values would approach 1.5 % (Figure 10 b). This is a substantial improvement when compared with single band gap device, where 0.4% STA is the maximum value attainable with a photoabsorber with Eg =2.5 (Figure 10 c).These simple calculations corroborate previous conclusions that attaining necessary STA values to promote ammonia production for fuel purposes is unlikely (Section 4.2.2). However, 1% STA would be viable for meeting fertilizer demands previously outlined (Section 4.2.1) Another possibility is to employ a particle slurry photoreactor design. This approach has received less attention for water splitting primarily because of the challenge in separating the explosive H2 and O2 mixture produced. 177 Furthermore, taking advantage of tandem absorbers is not possible in particle slurries, lowering the maximum possible efficiencies. However, for the case of solar fertilizer production particle slurry reactors may be a more attractive design option due to their simplicity and low cost. The ammonia produced stays in the liquid solution as ammonium ions, and evolved


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oxygen can be released into the atmosphere, making it possible to design reactors that are open or partially open to the environent. Furthermore, the ammonium-rich aqueous effluent could potentially be used directly as a fertilizer, or as the base of a more complex fertilization mixture. This would remove the requirement of product separation and allow for extremely simple and low-cost operation. Reactor design for particle slurry photoreactors has been analyzed, and key factors include particle concentration, size/shape, and necessary flow rates. 177,178 However, optimizing the reactor design will require significantly more knowledge about the ideal operating conditions and catalyst morphology for photocatalytic nitrogen fixation. Nonetheless, the relatively low efficiencies required for fertilizer production indicate that this may be a viable route. 4.3.2

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DFT studies have highlighted the advantages of these catalysts based on evaluating the catalyst i) N binding strength and ii) surface structuring. The N binding strength is important, because if the strength is too large, NH3 may not desorb from the surface, and if the N binding strength is too small N2 may not adsorb. 146 Therefore, there is an optimum strength ideal catalyst must exhibit. The binding strength is also highly dependent on the catalyst surface structuring (flat, stepped, etc.). The activity has been hypothesized to be greater at the step location, due to stable intermediate ion configuration. Yet, significant experimental testing must be completed to conclude the potential selectivity and activity changes with various catalyst surface structuring. 72 Interestingly, the photocatalytic systems seem to overcome this selectivity challenge both for single-material 41 and co-catalyst systems 47 (see Figure 4). In the case of transition-metal co-catalysts this has been attributed to the suppression of the HER reaction, and a clear trend has emerged with respect to metal-H binding energy, as shown in Figure 11. We note that this trend is also roughly consistent with nitrogen binding energy, 37 suggesting that the co-catalysts may play a more direct role in NN bond scission. Furthermore, the fact that these materials are not selective for ammonia synthesis in an electrochemical setup suggests that there may be a non-trivial interaction between the catalyst and semiconducting support. Improved understanding of the mechanism of co-catalysis will help identify more appropriate descriptors and allow the discovery of more active co-catalysts for ammonia formation. In the case of single-material photocatalytic systems such as titania and other transition-metal compounds the difficulty of adsorbing relatively inert N2 molecules has been suggested to be the limiting factor, and the presence of defects has been shown to enhance N2 adsorption 52,105 (see Figure 5). This suggests that pursuing strategies to maximize the presence of surface defects or searching for semiconducting materials with strong N2 binding energies are also promising routes of improving photocatalytic nitrogen fixation rates.

Materials Design and Discovery

In addition to optimization of device design, it will be critical to obtain an enhanced understanding of how to optimize and discover novel absorber and catalytic materials. As discussed in Section 4.3.1 one critical open question is whether the absorption and catalytic processes can be decoupled or whether photon absorption is directly involved in the nitrogen fixation process. Some studies have shown evidence that transition-metal co-catalysts can significantly enhance the nitrogen fixation rates, which suggests that catalysis and photon absorption can be decoupled. On the other hand, it has proven extremely challenging to discover a catalytic material capable of selectively electrocatalytically reducing nitrogen into ammonia, which supports the hypothesis that photon absorption plays a critical role. One of the most significant limitations with photofixing dinitrogen via a reductive pathway is the competition between the hydrogen evolution reaction (HER, reaction 31) and the nitrogen reduction reaction (NRR, reaction 30) which have very similar redox potentials. A range of transition metals (Mo, Fe, Rh, Co) and metal oxides (Fe2 O3 ) have been utilized for catalytic and electrocatalytic NRR. 179–181 Recent


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