Recent advances in photocatalytic nitrogen fixation and beyond

Yunxia Wei ^a, Wenjun Jiang ^b, Yang Liu ^c, Xiaojuan Bai ^d, Derek Hao *^e and Bing-Jie Ni *^e
aCollege of Chemistry and Chemical Engineering, Lanzhou City University, Lanzhou, Gansu 730070, China
^bQian Xuesen Laboratory of Space Technology, China Academy of Space Technology, Beijing 100094, China
^cSchool of Materials Science and Engineering, Henan Normal University, Xinxiang, Henan 453007, China
^dBeijing Engineering Research Center of Sustainable Urban Sewage System Construction and Risk Control, Beijing University of Civil Engineering and Architecture, Beijing 100044, China
^eCentre for Technology in Water and Wastewater (CTWW), School of Civil and Environmental Engineering, University of Technology Sydney (UTS), Ultimo, NSW 2007, Australia. E-mail: d.hao@griffith.edu.au; bingjieni@gmail.com

First published on 24th January 2022

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1. Introduction

Nitrogen is an irreplaceable element for the growth and development of plants because it is the source for the generation of chlorophyll and amino acids.¹ Plants cannot utilize atmospheric nitrogen until it is converted from its free state to a compound state, which is called nitrogen fixation.² In the natural environment, the nitrogen fixation process is mainly achieved by lightning and nitrogen-fixing microorganisms. However, these are not enough to support human demand. To achieve a sustainable agricultural industry for the increasing population, more and more nitrogen-based fertilizers are required, thus artificial nitrogen fixation was invented. Currently, commercial artificial nitrogen fixation is achieved through the Haber process, which was a milestone for human development.³ However, a huge amount of energy is required to finalize this process. As a result, about 2% of global energy is consumed by the Haber process each year and it is responsible for 1.6% of global CO₂ emissions.⁴ To reduce energy consumption and CO₂ emissions, researchers are seeking new approaches to replace the Haber process.

Recently, photocatalytic nitrogen fixation has become a hot research frontier, and has broad potential to be utilized on a large scale.^5–8 Compared with the traditional Haber process, photocatalytic nitrogen fixation has some distinctive advantages (Table 1). In detail, the hydrogen source of photocatalytic nitrogen fixation is water, rather than natural gas, which means that a high temperature is not required to convert the natural gas to hydrogen. The photocatalytic nitrogen fixation reaction can be completed under ambient conditions, without the requirement of high temperature and pressure.^9,10 More importantly, the photocatalytic nitrogen fixation reaction is driven by solar energy, rather than fossil fuel, indicating that the consumption of traditional energy and the emission of CO₂ can be significantly decreased. However, there is still a huge gap for photocatalytic nitrogen fixation to be applied in mass production, due to the low ammonia yield rate. Therefore, it is urgent to develop novel materials to improve photocatalytic nitrogen fixation efficiency.

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In this review, we introduce different mechanisms for nitrogen fixation and then summarize the recent advances in nanomaterial photocatalysts. After that, we propose several approaches that can efficiently boost the photochemical nitrogen fixation performance. This review may provide some new ideas and inspiration for the development of artificial ammonia synthesis under ambient conditions.

2. Mechanisms of photocatalytic ammonia synthesis

A detailed understanding of fundamental nitrogen fixation mechanisms is important, because the more we know about the reaction process, the more we can work to improve nitrogen fixation efficiency with pertinence. Currently, there are three kinds of well-recognized mechanisms including four pathways for catalytic nitrogen fixation, which include the dissociative, distal associative, alternative associative and enzymatic pathways.¹¹ In the dissociative pathway (Fig. 1a), strong energy is used to completely break the triple bonds of the nitrogen molecules, after which, hydrogenation will happen, and ammonia can be formed.¹² In the Haber process, the dissociative pathway is the approach by which ammonia is generated. In the two kinds of associative pathways, hydrogenation happens with the step-by-step cleavage of the NN, but the hydrogenation occurs differently. In the distal associative pathway (Fig. 1b), hydrogenation first occurs at the nitrogen atom furthest away from the catalyst. Then hydrogenation will occur at the second nitrogen atom until the first ammonia molecule is released. In the alternative associative pathway (Fig. 1c), hydrogenation happens to the two nitrogen atoms simultaneously. In the enzymatic pathway (Fig. 1d), hydrogenation usually occurs in the reactions carried out by nitrogenases and other catalysts. Different from the previously mentioned pathways, herein, the nitrogen molecule is adsorbed on the nitrogenases or catalysts on the edge of each atom, rather than one side of only one atom. In this pathway, hydrogenation also happens to each nitrogen atom simultaneously.

In 2019, Ling et al. proposed a new nitrogen reduction mechanism called the surface hydrogenation mechanism that can happen on the surface of the noble-metal-based metal catalysts (Fig. 2).¹⁴ The first step of this mechanism is converting H⁺ to *H using little energy and it is regarded as a potential rate determining step. After that, nitrogen molecules will directly react with *H to form *N₂H₂ with higher energy. Then *N₂H₂ will react with H⁺ and an e⁻ to form ammonia. Since alkali–metal cations work similarly to H⁺ in an N₂ activation, increasing the concentration of alkali–metal cations may be a potentially effective approach for accelerating the nitrogen fixation reaction.

3. Recent materials advances in photocatalytic ammonia synthesis

Different materials have different properties. In recent years, various kinds of materials have been developed to boost the efficiency of artificial photochemical nitrogen fixation. These methods and strategies may provide some inspiration for future development in this topic. In this review, some recent materials advances in photocatalytic ammonia synthesis are summarized and discussed.

3.1 Bismuth-based photocatalysts for ammonia synthesis

Bismuth-based photocatalysts are emerging materials with a lot of advancements including facile preparation, non-toxicity, controllable band structure, high solar-energy utilization, etc. More importantly, bismuth is a less active hydrogen evolution reaction (HER) material, making the competing HER obstructed, which can significantly boost nitrogen fixation.⁴ Using the surface plasmon resonance (SPR) effect of bismuth is regarded as an efficient way to improve the activity of photocatalytic nitrogen reduction. For example, Wang et al. reported a Bi/InVO₄ photocatalyst with a 5.2-fold enhanced photocatalytic ammonia yield rate compared to the pure InVO_4.¹⁵ The photogenerated electrons in the conduction band (CB) of InVO₄ are transferred to Bi because of the thermodynamic potential gap, as a result, the charge transfer is boosted and the nitrogen fixation is significantly increased (Fig. 3a). These strategies can also be used to modify other semiconductors to boost their photochemical nitrogen fixation efficiencies.^16–18 Elemental doping is another way to improve the photocatalytic activity of bismuth-based photocatalysts. For instance, Meng et al. prepared an Fe-doped Bi₂MoO₆ for an efficient photocatalytic N₂ fixation. The photocatalytic activity of the best sample can reach 106.5 μmol g⁻¹ h⁻¹ and it is 3.7-times that of the pure Bi₂MoO₆. Fe-doping can not only increase the surface area of the catalysts, but also increase the light absorption (Fig. 3b and c). Fe-doping can likewise further develop the charge assortment through an Fe³⁺/Fe²⁺ redox pathway, which fills in as active sites for nitrogen reduction.¹⁹ It has been confirmed that many kinds of bismuth-based photocatalysts can be used for ammonia synthesis like Bi₃FeMo₂O₁₂,²⁰ bismuth oxyhalides,²¹ Bi₂O₂CO₃,²² Bi₂O₃,²³etc. Using defect engineering is a good strategy to boost catalytic efficiency as it can work as an active site. Feng et al. prepared a series of Bi₂O₂CO₃ with abundant oxygen vacancies via the addition of glyoxal during a hydrothermal reaction.²² The oxygen vacancies made the photocatalytic ammonia yield rate increase 10-fold, because the vacancies can boost the chemisorption and activation of the nitrogen molecules. Bismuth-based semiconductors are good photocatalysts for ammonia synthesis. More importantly, metallic bismuth has an SPR effect and it can be used to activate nitrogen molecules.^24,25 Currently, the photocatalytic activity of bismuth-based materials for nitrogen fixation still needs improvement. Besides, the cycling stability of bismuth-based materials is usually not good as bismuth is beam sensitive (Fig. 3).²⁶

3.2 Noble metal-modified photocatalysts for ammonia synthesis

Noble metal-modified photocatalysts are generally divided into noble metal-based photocatalysts and noble metal-doped photocatalysts. Noble metals with surface plasmon resonance, such as gold, silver, etc., can inject active high-energy hot electrons into the anti-bonding orbital of nitrogen molecules, thereby reducing the chemical bond energy of the nitrogen molecules, making it easier to complete the first step of hydrogenation.^27,28 Plasma-enhanced noble metal-based photocatalysts can bypass the linear constraints of conventional catalytic processes and are a very promising direction in the research of nitrogen reduction. For example, Hu et al. designed an “AuRu core-antenna nanostructure”, where gold nanocrystals were selected as plasmonic nanostructures to obtain a wide range of light absorption (Fig 4a and b). This mixed structure with Au as the core and Ru as the antenna promoted the activation process of nitrogen molecules on Ru and achieved an ammonia yield of 101.4 μmol g⁻¹ h⁻¹.²⁹ Chen et al. developed a MOF film encapsulating gold nanoparticles for the direct immobilization of photocatalytic nitrogen.³⁰ The “gas-membrane–solution interface” composed of the film can achieve effective gas diffusion. Each Au nanoparticle is dispersed in the MOF matrix and simultaneously acts as a photosensitizer (acquiring light and generating electrons) and an auxiliary catalyst (catalytic reduction of nitrogen). In the design of the reaction interface, a porous MOF substrate (such as UiO-66) not only serves as a stable substrate, but also ensures the contact of these AuNPs with N₂ molecules and protons, and provides an interconnected nanoreactor with an ultra-high surface area (Fig 4d–g). Zheng et al. designed a hydrophobic PTFE porous framework on a silicon substrate as a gas diffusion layer, with gold nanoparticles covering the PTFE porous framework as photocatalytic active sites (Fig 4c).³¹ Such a hydrophilic–hydrophobic structure helps to improve nitrogen fixation. The solubility of the catalytic site and effective inhibition of the hydrogen evolution reaction have achieved a maximum Faraday efficiency of 37.8%. The development of noble metal photocatalysts with different shapes and array structures based on the plasma enhancement effect still has great potential. Metal–semiconductor heterostructures composed of precious metals and semiconductor materials have also been developed. Recently, Yang et al. reported a strategy of immobilizing gold nanoparticles on titanium dioxide nanosheets for photocatalytic nitrogen fixation.³² The oxygen vacancies on the surface of titanium dioxide can pre-adsorb and activate nitrogen molecules, and then the nitrogen molecules will be reduced by the hot electrons generated by the plasma excited gold nanoparticles to form a “working-in-tandem” pathway, which achieves a quantum efficiency of 0.82% under 550 nm light. Wang et al. reported a heterojunction photocatalyst composed of Ru, RuO₂ and g-C₃N₄.³³ Under light irradiation, electrons in g-C₃N₄ transition from the valence band to the conduction band and can be easily captured by Ru. Finally, the electrons are transferred to the π* anti-bond orbital of the nitrogen molecule to promote the cleavage and activation of the nitrogen–nitrogen triple bond. The holes are transferred to RuO₂, which reacts with methanol in the solution and is consumed, finally achieving an ammonia yield of 13.3 μmol g⁻¹ h⁻¹. Oshikiri et al. reported a photoelectrocatalytic cathode nitrogen reduction catalyst with hemispherical gold nanoparticles supported on strontium titanate, which achieved an ammonia yield of 10 nmol h⁻¹ cm⁻².³⁴ The author also studied the effect of solution pH and bias voltage on the final product. A noble metal with good nitrogen fixation activity can not only serve as a catalytically active site after being combined with a semiconductor, but also promote the movement of electrons in the semiconductor to the surface, inhibit the recombination of holes and electrons, and improve light utilization. This is a high-efficiency catalyst design strategy with more research value. Li et al. reported a photocatalyst with monoatomic platinum anchored at the –N₃ site in covalent triazine framework nanosheets.³⁵ Theoretical calculations showed that the alternating mechanism is more energy-efficient than the remote mechanism, and the single-atom dispersed platinum atoms help the electron–hole separation. The transfer of electrons in the conduction band to platinum atoms can promote the activation and reduction of adsorbed nitrogen molecules. Without sacrificial agents, an ammonia yield of 171.40 μmol g⁻¹ h⁻¹ was achieved. Although the SPR of noble metals can significantly improve the activation of nitrogen molecules, they are not cost-effective enough to achieve mass production. Strategies should be developed to find a balanced amount of utilization of noble metals to get the best photocatalytic activity (Fig. 4).

3.3 Metal-free semiconductor photocatalysts for ammonia synthesis

g-C₃N₄ is one of the most popular metal-free semiconductor photocatalysts, and is widely used for photocatalytic ammonia synthesis. However, pure g-C₃N₄ shows poor photocatalytic ammonia synthesis performance on account of the severe recombination of photogenerated carriers, and the limited adsorption and activation capacity of nitrogen. Great efforts have been made to solve these problems. Nonmetallic doping (B, P, S, and I) is an effective way to promote the photocatalytic ammonia synthesis activity of g-C₃N₄. Lv et al. reported a single B atom anchored on holey g-C₃N₄ (B@g-CN) for photocatalytic ammonia synthesis.³⁶ As shown in Fig. 5a, strong covalent B–N bonds were formed between the B atoms and sp²-bonded N atoms. As a result, one empty sp² orbital and one half-occupied sp² orbital of the B atom could promote N₂ activation (Fig. 5b). Three possible mechanisms (distal, alternating, and enzymatic) for N₂ reduction are shown in Fig. 5c. DFT calculations proved that the enzymatic pathway is the most rational mechanism, and the accelerated activity comes from the “σ donation–π* back-donation” mechanism.

Metallic doping (Cu, K, Co, Fe, and Ag) has also been widely adopted to accelerate photocatalytic ammonia photosynthesis. Huang et al. reported a single Cu atom modified g-C₃N₄ (Cu-CN) for ammonia synthesis with a quantum efficiency of 1.01% at 420 nm.³⁷ X-ray absorption fine structure (XAFS) showed that a threefold coordination structure of Cu atoms was formed (Fig. 5d). In situ FTIR spectra proved that the N–H bonds (1553 and 1685 cm⁻¹) were formed under light irradiation thanks to the decoration of a single Cu atom (Fig. 5e). DFT calculations proved that the valence-electron was isolated from the conjugated π electron cloud (Fig. 5f), which is beneficial for the adsorption and activation of N₂ over the positively charged metal ions.

Defect strategies provide an efficient way of adjusting the properties of the photocatalysts since the defect sites could act as active centres for ammonia synthesis and further increase intrinsic activity. Wang et al. prepared g-C₃N₄ with N vacancies (V_N-P-GCN) for N₂ fixation by replacing the corner-site C with P (Fig. 5g).³⁸ The chemical environment was changed on account of the synergy of the P doping and N vacancies. Both the CB and VB of V_N-P-GCN exhibited an upshift compared to GCN (Fig. 5h). In addition, the DFT calculations proved that the introduction of P could boost the activation ability of the N vacancies to the adsorbed N₂, which is beneficial for the N₂ photofixation performance.

Coupling with other semiconductors is also a beneficial strategy to promote the separation efficiency of carriers. Qiu reported a black phosphorus nanosheet-modified g-C₃N₄ (BPCNS) for photocatalytic ammonia synthesis.³⁹ Electron paramagnetic resonance showed that the peak intensity of BPCNS under visible light was much higher than that of CNS and demonstrated that the electrons of BPCNS are much easier to be excited thanks to the change of the π-conjugated system induced by the C–P bonds (Fig. 6a). The fluorescence lifetime of BPCNS was longer than that of CNS, demonstrating the effective electron transfer from the CNS to BP (Fig. 6b and c).

The covalent triazine framework is also a potential candidate for ammonia synthesis thanks to its high visible light adsorption capability and adjustable π-conjugated units. A single Pt atom modified covalent triazine framework was synthesized by Li et al.³⁵ The isotopically labeled experiment demonstrated that the generated NH₄⁺ indeed came from the ammonia synthesis (Fig. 2d). Both the CB and VB of the Pt-SACs/CTF exhibited an upshift compared to CTF-PDDA-TPDH (Fig. 2e), which is thermodynamically beneficial for the reduction of N₂. The DFT calculations proved that the Pt–N₃ sites were the active sites (Fig. 2f) and that the alternating pathway was the main reaction path other than the distal pathway.

Although a large number of articles concerning g-C₃N₄ have been published, there are still several key issues that need to be addressed. First, ammonia production should be measured with caution since it is possible for g-C₃N₄ to break down to produce nitrogen-containing groups during the reaction. These nitrogen-containing groups may have an impact on the test results. Second, the ethanol and heavy metal ions used in the synthesis of g-C₃N₄ may also affect the accuracy of Nessler's reagent method.⁴⁰ In addition, ethanol is widely used as the sacrificial agent of holes, which is an interference factor that affects the detection results. In terms of accuracy, it is necessary to detect ammonium ions by NMR or ion chromatography.^41–43 Third, most of the current research about metal-free semiconductor photocatalysts focuses on g-C₃N₄. The exploitation of new metal-free semiconductor photocatalysts beyond g-C₃N₄ is urgently needed.⁴⁴

3.4 Other semiconductor photocatalysts

Currently, the research on photocatalysts for nitrogen fixation is mainly focused on bismuth-based materials, g-C₃N₄-based materials and metallic materials with SPR. However, some other kinds of semiconductors also showed broad potential in this application. Titanium dioxide (TiO₂) is a well-studied photocatalyst, and it has been confirmed to be a good photocatalyst for photocatalytic nitrogen fixation. Meanwhile, some recent achievements have been accomplished to improve the photocatalytic nitrogen reduction activity. For example, Zhang et al. synthesized a tunable defective TiO₂via heating TiO₂ with NaBH₄ under high temperatures (310–360 °C) in an Ar atmosphere_.⁴⁵ As shown in Fig. 7, the color of the TiO₂ samples became darker as the temperature increased. At the same time, the absorption edge showed a redshift. Among all the samples, the one obtained at 340 °C reached the highest ammonia yield of 324.86 μmol h⁻¹ g⁻¹, which is 3.85-fold better than pristine TiO₂. The oxygen vacancies can increase the adsorption capacity of nitrogen, as well as boost the charge separation, leading to significantly increased photocatalytic activity. The construction of the heterojunction can also improve the photochemical nitrogen fixation performance of TiO₂. For instance, Rong et al. reported a Z-scheme TiO₂/ZnFe₂O₄ heterojunction photocatalyst, which showed much higher photocatalytic activity than the single TiO₂ or ZnFe₂O₄.⁴⁶ The Z-scheme heterojunction can boost the transfer and separation of electron–hole pairs, leading to good photocatalytic activity.

With proper modification, some other kinds of novel photocatalysts for nitrogen fixation have been developed and they also have some potential for further use. For example, Luo et al. reported an Fe doped SrMoO₄ and it showed much better photocatalytic activity for nitrogen fixation_.⁸ The reason why the catalytic activity can be improved is because of the narrower bandgap, expanded light absorption, and newly formed Fe–Mo active centres. Constructing heterojunctions is a highly efficient approach to boost the separation and transfer of photogenerated electron–hole pairs. A CeO₂/FeS₂ heterojunction and CeCO₃OH/g-C₃N₄/CeO₂ heterojunction have been reported as good photocatalysts for nitrogen fixation.^47,48 Defect engineering is the most widely used approach for the modification of catalysts to achieve highly efficient nitrogen fixation. Sun et al. prepared sulfur vacancy-rich oxygen-doped MoS₂ modified CdS nanorods, which exhibited a photochemical ammonia synthesis rate of 8.2 mmol L⁻¹ g⁻¹ h⁻¹.⁴⁹ They found that the sulfur vacancy and incorporation of oxygen can improve the conductivity as well as reduce the energy barrier for nitrogen reduction, and that is a key factor for excellent photocatalytic activity. Recently, Zhang et al. found that layered double hydroxides (LDH) can be used as photocatalysts for nitrogen reduction.⁵⁰ They used oxygen vacancies and Cu^δ+ to modify ZnAl-LDH and got an ammonia yield rate of 110 μmol g⁻¹ h⁻¹.

4. Beyond nitrogen fixation

4.1 Alternative nitrogen sources for ammonia synthesis

Due to the high bond energy, high stability and poor solubility of nitrogen molecules, the current photocatalytic nitrogen fixation rate is too low to meet mass production needs. Alternatively, there are many other kinds of nitrogen-based compounds that can be used as nitrogen sources, such as nitrate, nitrite, nitrogen oxides, etc. In this way, we do not need to worry about the cleavage of nitrogen molecules, more importantly, the potential environmental hazards can be converted into valuable chemicals. These alternatives will be easier and more impactful than nitrogen fixation for sustainable development and a green planet.

4.2 Generation of other valuable chemicals

As an important raw material, ammonia is widely used for the synthesis of a lot of cleaning and pharmaceutical products. Except converting nitrogen to ammonia, coupling CO₂ reductions with nitrogen reductions, a lot of other high-value chemicals can be synthesized, which will be more valuable than nitrogen fixation (Fig. 8).⁵¹ In 2020, Chen et al. reported the direct electrochemical synthesis of urea by coupling N₂ with CO₂ in water under ambient conditions.⁵² The reactions were completed utilizing an electrocatalyst comprised of PdCu alloy nanoparticles on TiO₂ nanosheets. It opened a new path for the catalytic synthesis of high-value chemicals. Recently, there have been some significant achievements in photocatalytic urea synthesis. Maimaiti et al. achieved the photochemical synthesis of urea using N₂, CO₂ and H₂O, with TiO₂ loaded on an Fe-carbon nanotube.⁵³ The urea yield can reach 710.1 μmol (L g)⁻¹ in four hours.

4.3 Lithium-mediated nitrogen fixation

In electrochemical ammonia synthesis, lithium-mediated nitrogen fixation has attracted broad interest because Li is an excellent material for N₂ activation with small energy barriers.⁵⁴ Recently, some important findings on this topic have been published. For instance, Kim et al. developed a biphasic system of aqueous 1 M LiClO₄ and 1 M LiClO₄/propylene carbonate reinforced with poly(methyl methacrylate), which is membrane-free and can reach a high faradaic efficiency of 57.2% of nitrogen reduction.⁵⁵ Suryanto et al. used a phosphonium salt as a proton shuttle to boost the lithium-mediated nitrogen fixation, and a high faradaic efficiency of 69 ± 1% was achieved.⁵⁶ In the lithium-mediated nitrogen fixation process, Li⁺ first reacts with electrons to form Li⁰. Then the Li⁰ will react with nitrogen to generate Li₃N, which can further react with H⁺ to obtain ammonia and Li⁺. Although some great achievements have been made in electrochemical lithium-mediated nitrogen fixation, it has not been applied in photochemical ammonia synthesis. The key reason is that the reduction potential of Li⁺ to Li₀ is −3.04 V (vs. SHE).⁵⁵ This step is difficult to achieve by most photocatalysts. However, photoelectrocatalysis can overcome this problem and lithium-mediated photoelectrocatalytic nitrogen fixation may be a good topic worth investigating.

5. Summary and outlook

To date, many kinds of novel photocatalysts have been developed to improve photocatalytic nitrogen fixation performance. A lot of photocatalysts including TiO₂, g-C₃N₄, Bi-based semiconductors, etc. have shown broad prospects in ammonia synthesis. However, the current ammonia yield is still too low to meet the needs of mass production. Due to the strong bond energy of NN, it is challenging to achieve mass nitrogen fixation under ambient conditions. Nevertheless, photoelectrocatalytic lithium-mediated nitrogen fixation might be a good way to overcome this problem. Besides, using another nitrogen source like nitrate, nitrite or nitrogen oxides is more promising. Meanwhile, coupling N-reduction with CO₂ reduction deserves more investigation as it will create more economic value and be more sustainable for the planet.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work is supported by an Australian Research Council (ARC) Future Fellowship (FT160100195). Prof. Yunxia Wei acknowledges the support of the Key Talent Project of the Gansu Province.

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