# Progress of Metal Oxide (Sulfide)-Based Photocatalytic Materials for Reducing Nitrogen to Ammonia

Authors: Jianjun Yang
Journal: Journal of Chemistry
 (2018)

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## Abstract

The Haber–Bosch process has been an important approach to produce ammonia for meeting the food need of increasing population and the worldwide need of nitrogenous fertilizers since 1913. However, the traditional ammonia production process is a high energy-consumption process, which usually produces 1 metric ton ammonia with releasing around 1.9 metric tons CO2. Photocatalytic ammonia synthesis under solar light as energy source, an attractive and promising alternative approach, is a very challenging target of reducing fossil energy consumption and environmental pollution. Therefore, photocatalytic ammonia production process would emerge huge opportunities by directly providing nitrogenous fertilizers in a distributed manner as needed in the agricultural fields. In this article, different metal oxide (sulfide)-based photocatalytic materials for reducing nitrogen to ammonia under ambient conditions are reviewed. This review provides insights into the most recent advancements in understanding the photocatalyst materials which are of fundamental significance to photocatalytic nitrogen reduction, including the state-of-the-art, challenges, and prospects in this research field.


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## Body

## 1. Introduction

As the important chemicals to our planet, nitrogen(N2) compounds, such as ammonia (NH3), nitrates, and urea, have played an essential role in meeting the growing demand for food and the worldwide need of nitrogenous fertilizer since 1913 [1]. It is essential for all living organisms to being provided the N2 compounds needed for growing tissue [2]. Moreover, the global N2 cycle, whose small fraction is related to the atmospheric ionization, mainly depends on the biogeochemical cycles [3]. In addition, N2 photoreduction to nitrogenous compounds (e.g., NH3) in soils and sands as catalysts also is an important part of global N2 cycle. Through continuous exploration of nitrogen fixation, the Haber–Bosch process, a thermo-chemical catalytic conversion technology, becomes a primary choice of synthesizing N2 fixation compounds which were produced from the reaction:(1)N2+3H2⟶2NH3In the past over decades [4], the process has fed the world’s growing population and has significantly contributed to the human beings development. The Haber–Bosch process reacted with the pure feed gases at high temperatures and pressures, requiring almost 2% of global energy consumption [5]. On the contrary, there are some negative impacts of the Haber–Bosch process in terms of the overuse of nitrogenous fertilizer that affects the ecosystem balance, human health, climate change, etc. [6]. Then, reducing the fossil energy consumption and environmental pollution would be a very challenging target for this process.Ambient reduction of N2 to NH3 has been a significant research subject of efficiently fixing N2 under mild conditions. Energy coming from sustainable source as solar, an alternative, sustainable NH3 synthesis process based on the biological N2 fixation would be more energy efficient than the Haber–Bosch process [7]. Additionally, the distribution of nitrogenous fertilizers produced by the Haber–Bosch process requires efficient transportation which may be least easy in the nonindustrialized countries than in the industrialized countries. Then, it would be possible to produce nitrogenous fertilizers close to the farm field as needed and reduce greenhouse gas emission and control the global N2 cycle. Obviously, due to its energy saving and friendly environmental, photocatalytic reduction of N2 to NH3 could be an excellent alternative to the Haber–Bosch process. That is to say, developing efficient photocatalysts for synthesizing NH3 would emerge huge opportunity to directly provide nitrogenous fertilizers in agricultural fields as needed in a distributed manner. Chemists should discover how to activate the N≡N bond (941 kJ·mol−1) to synthesize ammonia in the presence of novel photocatalysts with less fossil energy consumption and more solar energy [8].In initial works, there was a popular perspective that the photocatalytic reduction of N2 to NH3 should take place over abundant soil minerals and sand in nature [9]. Since 1977, the TiO2-based synthetic materials as photocatalysts were firstly applied for N2 fixation under UV light, and a series of studies [10] have gradually been conducted on photocatalytic N2 reduction to NH3 with water and air under light driving force. Although there have been many new photocatalysts and approaches to solve the problems of the Haber–Bosch process, till now, no viable and efficient catalysts could match all requirements of an active, selective, scalable, long-lived catalyst for the sustainable photocatalytic reduction of N2 to NH3 [11]. This review tries to make them possible to ascertain a comprehensive understanding of the current knowledge regarding to photocatalytic materials for producing nitrogenous fertilizers from N2 and to give a solid research perspectives for the next step work in the research field.


## 2. Photocatalytic Reduction Process of N2 to NH3

The photocatalytic reduction of N2 to NH3 under ambient conditions is a sustainable NH3 production process without fossil energy consumption and environmental pollutions. Photocatalysis on semiconductors is a promising method for synthesizing NH3 from N2 with water (H2O) as a reducing reagent. The photocatalytic NH3 production process mainly consists of the photocatalytic oxidation of H2O to protons and the photocatalytic reduction of N2 to NH3.(2)H2O+2h+⟶0.5O2+2H+(3)N2+6H++6e−⟶2NH3(4)2N2+3H2O⟶photocatalystlight2NH3+1.5O2The reactions mentioned above imply that the photocatalysts created on the surface of robust semiconductors capable of oxidizing H2O (Equation (2)) may reduce N2 by the photoformed conduction band (CB) electrons (Equation (3)). As a result of these behaviours, N2 is reduced to NH3 by the CB electrons (Equation (2)). That is to say, NH3 could be produced from N2 and H2O (Equation (4)) on a metal-free photocatalyst under ambient conditions in these processes. The valence band (VB) should be lower than the oxygen evolution potential, while the position of the CB should be higher than the reduction potential of N2 hydrogenation. With synthesizing ammonia, the metal oxide-based material as photocatalyst is dynamically converted between its oxidized and reduced states in the process.Solar light as energy source is a good choice for photocatalytic reduction of N2 to NH3 under ambient conditions because it is difficult for the existing power plants to meet the large energy need of producing NH3 (Figure 1). The CB position of the semiconductor should be more negative than the reduction potential of the N2 hydrogenation, as well as the VB should be more positive than the oxygen evolution potential (Table 1). Therefore, something must be taken into account that both reduction potential of the adsorbate and position of the energy band are important for the photocatalytic redox reaction occurrence when making a decision on the choice of semiconductor photocatalyst materials.Figure 1
Schematic of metal oxide-based photocatalysts for reducing N2 to NH3. MeOx(MeOx−n): metal oxide-based materials as photocatalysts and Me denotes metals.Table 1
Redox potentials of reducing N2 to NH3 [10–12]].
No.
Reaction
E
0 (V)
1
H
2
O
+
2
h
+
⟶
0.5
O
2
+
2
H
+
1.33b
2
H
+
+
e
−
⟶
0.5
H
2
−0.42a
3
N
2
+
e
−
⟶
N
2
−
−4.16b
4
N
2
+
H
+
+
e
−
⟶
N
2
H
−3.2b
5
N
2
+
6
H
+
+
6
e
−
⟶
2
NH
3
0.55b
a
E
0 vs. NHE at pH 7; bE0 vs. NHE at pH 0.Normally, the photocatalytic reduction process of N2 to NH3 can be divided into several reaction steps [13, 14]. Firstly, the photogenerated electrons (e−) are promoted to leave from vacant holes in the VB to the CB. Secondly, some electrons and holes recombine each other in order to improve the solar conversion efficiency and apparent quantum yield of the reduction. Protons could be generated in the endothermic process of H2O oxidation when solar energy is enough to be absorbed and converted into the chemical energy which can meet the large free energy gain of N2 to NH3 (⊿G0 = 339 kJ·mol−1) [15]. Thirdly, the others transfer onto the surface of the photocatalysts for the redox reaction (Equation (4)) which the photoformed VB holes (h+) oxidize H2O to protons (H+) and O2 under visible light. In the process of photocatalytic ammonia synthesis, the photogenerated holes must also be consumed to satisfy the charge neutrality (CB e− + VB h+ = 0) (Equation (2) in Table 1).By synthesizing NH3 from atmospheric N2 and H2O in the presence of photocatalysts under visible light (Equation (3)) [16], photons as the driving force actually can promote to reduce N2 to NH3 after a series of multistep injections of photogenerated electrons and H2O-derived protons. Therefore, the first electron transfer (−4.16 V vs. NHE) and proton-coupled electron transfer (−3.2 V vs. NHE) (Table 1) must be overcome to reduce N2 to NH3. Then, the recombination of charge carriers and a small band gap are important for satisfying semiconductor photocatalysts preferably in the visible light. And it is also necessary for ideal semiconductor materials to have the characteristics of the charge carrier recombination and a small band gap in the visible light region.


## 3. Photocatalytic Materials for Reducing N2 to NH3

As we all know, photocatalytic NH3 synthesis from N2 also needs to overcome the N≡N band energy barrier with the high-energy UV light. Necessarily, the large thermodynamic driving force is required for the reduction of N2 to NH3 at atmospheric conditions [17]. To solve the problem, developing the efficient photocatalytic materials for the N2 reduction process maybe are indispensable parts for increasing NH3 yields significantly.It is first time that Dhar and his coworkers [18] discovered this photocatalytic reaction in the 1940s. Schrauzer and Guth [13] demonstrated that the photocatalytic NH3 synthesis on a variety of sterile desert sands until late 1970s. Following this work, some independent results were achieved as gaining synthetic materials for speeding the photocatalytic N2 reduction rate [19]. However, there are still some differences among these results some of which are in conflict with each other or confused, and no consensus was reached in regard to the photocatalytic materials challenges for the reaction of N2 and H2O to NH3 and oxygen. In this section, the metal oxide-based photocatalytic materials for the reduction of N2 to NH3 are classified (Tables 2 and 3) according to their different elemental compositions and the state-of-the-art research advancements are described, respectively.Table 2
Summary of metal oxide-based materials for photocatalytic reduction of N2 to NH3.
Catalytic materials
Catalysts
T (K)
Light source
Organic scavenger
Average NH3 production rate
Year
Ref.
Titanium oxide-based materials
TiO2@C/g-C3N4
ACs
Visible light
CH3OH
250.6μmol g−1 h−1
2018
[20]
Pb/Fe2O3-TiO2
298
Xenon light
Distilled water
1150μM·h−1
2017
[21]
Fe-doped TiO2
—
UV
Ethanol
400μM·h−1
2014
[22]
RuCl3-TiO2
RT
UV-Vis
Humic acid
4μM·h−1·cm−2
2008
[23]
P3MeT/TiO2
293
UV
None
8.3μmol·h−1·m−2
2001
[24]
Fe-doped TiO2
313
UV-Vis
None
10μmol·g−1·h−1
1997
[13]
Ru-doped TiO2
—
UV-Vis
None
22.7μmol·g−1·h−1
1996
[19]
Fe3+-doped TiO2
353
UV
None
6μmol·g−1·h−1
1991
[25]
TiO2
RT
UV
Wet nitrogen flux
0.83μmol g−1 h−1
1988
[26]
BaTiO3
313
UV
None
0.87μmol·g−1·h−1
1983
[27]
Other metal oxide-based materials
g-C3N4/Mg1.1Al0.3Fe0.2O1.7
303
Visible light
Ethanol
166.8μM·h−1
2017
[28]
BiO
ACs
Solar light
Deionized water
50.5μM·h−1
2017
[29]
Fe2O3
298
UV-Vis
Ethanol
1362.5μM·h−1
2017
[21]
H-Bi2MoO6
RT
Solar light
None
1300μmol g−1 h−1
2016
[30]
BiOBr
RT
Visible light
None
1042μM·h−1
2015
[31]
ß-Ga2O3
298
UV
Tert-butanol
36.1%
2015
[32]
Pt-loaded ZnO
—
UV
Na-EDTA
860μmol·g−1·h−1
2010
[33]
Cu2O.xH2O.CuCl
ACs
UV
None
70μM·h−1
1989
[34]
Fe2O3 + Fe3O4
303
Xenon light
None
10μmol·g−1·h−1
1987
[35]
RT denotes room temperature. ACs denote ambient conditions.Table 3
Summary of metal sulfide-based materials for photocatalytic reduction of N2 to NH3.
Catalytic materials
Catalysts
T (K)
Light source
Organic scavenger
Average NH3 production rate
Year
Ref.
Metals sulfide-based materials
MoS2
298
Xenon light
None
325μmol·g−1·h−1
2017
[12]
[Mo2Fe6S8(SPh)3]-[Fe4S4]
ACs
Xenon light
None
18.82μM·h−1
2016
[36]
Mo0.1Ni0.1Cd0.8S
303
Visible light
Ethanol
71.2μM·h−1
2016
[37]
Zn0.1Sn0.1Cd0.8S
303
Visible light
Ethanol
105.2μM·h−1
2016
[38]
[Mo2Fe6S8(SPh)3]3+ and [Sn2S6]4−
RT
Solar light
—
18.82μM·h−1
2015
[39]
Pt/CdS·Ag2S/RuO2
303
Visible light
K[Ru(EDTA-H)Cl]2H2O
1260μM·h−1
1991
[40]
CdS/Pt/RuO2
303
Visible light
None
620μM·h−1
1988
[41]
CdS/Pt
311
UV
None
3.26μmol·g−1·h−1
1980
[42]
RT denotes room temperature. ACs denotes ambient conditions.


### 3.1. Titanium Oxide-Based Materials

Photocatalytic N2 fixation has received more and more attentions since synthesizing NH3 from N2 was available in the presence of titanium-based catalysts. Dhar and Chowdhry [43] found that titanium oxide (TiO2) as photocatalytic material could provide the larger NH3 yields from reducing N2 than ferric oxide and zinc oxide as photocatalysts. Schrauzer and Guth [13] also discovered this phenomenon of N2 photoreduction. UV irradiation of TiO2 with a large number of surface Ti3+ species could produce NH3 from N2 and H2O under ambient conditions, which shows the species are actually the active sites for the reduction of N2 to NH3 [14].


#### 3.1.1. Iron-Doped Materials

As key factors underlying photocatalytic performance, the influence of the crystalline phase and iron dopants on the catalytic activity of TiO2 photocatalytic material for the N2 reduction was focused. Under UV light, NH3 yields reached a maximum value when over 0.2% Fe2O3 was doped into TiO2 material. However, Hirakawa et al. [14] demonstrated that Ru, Pt, or Pd particles loaded in the TiO2 materials were unhelpful to increase the NH3 yields due to their particles covering the catalytic active site Ti3+ of TiO2 photocatalytic material and changing surface defects. Interestingly, Augugliaro et al. [44] supported Fe2O3-hybridized TiO2 on Al2O3 as photocatalyst which was applied in gas-solid fluidized bed reactors to improve NH3 yields because of the influence of iron ions on the TiO2 crystalline [45]. Lashgari and Zeinalkhani [21] photosynthesized NH3 in a H2O photosplitting setup in the presence of some synthetic Fe2O3 and TiO2-based uniform nanoparticles and obtained the maximum NH3 yield. Bourgeois et al. [26] suggested that thermal pretreatments could generate surface defects or impurity states on the surface of unmodified TiO2 that showed the photocatalytic activity of reducing N2 after annealing in air. As a highly efficient method, high pretreatment temperatures (∼1000°C) inducing surface defects in TiO2 material and the small amounts (<1%) of iron impurities are necessary to enhance the NH3 yields [46].Observing no reduction of N2 to NH3 in the presence of pure TiO2 material, Augugliaro et al. [47] put forward a hypothesis that Fe3+ of photoassisted organ iron compound could temporarily trap photons and promote the separation of charge carriers, which has played an important role in the NH3 production processes. Exposing the facets of Fe-doped TiO2 with ethanol as the scavenger enhanced the photocatalytic reduction of N2 to NH3 [22]. No nitrite was formed in the ethanol solution that could prevent the oxidation of NH3. Keeping a low doping concentration on the surface of TiO2 material was very important to inhibit charge recombination for forming Fe2+ and Fe4+ and to transfer electrons and holes to Ti4+ and OH− for generating Ti3+ and OH [18]. Similarly, Ranjit and Viswanathan [48] observed that the FeTi2O5 phase in Fe-doped titanium materials could enhance the separation of charge carriers and also demonstrated this hypothesis. However, Soria et al. [25] suggested that excess Fe dopants might lead TiO2 materials to lose themselves all photocatalytic activities. Therefore, the low bulk Fe3+ concentrations significantly assist in charge separation. TiO2 nanoparticles codoped with N2 and Fe3+ were prepared using the homogeneous precipitation-hydrothermal method. The cooperation of N2 and Fe3+ leads to narrow the band gap, improve the photocatalytic activity in the visible light region, and promote the separation of the photogenerated electrons and holes to accelerate the transmission of the photocurrent carrier [49].Generally, titanium oxide-based photocatalytic materials for the N2 reduction have made much progress in several aspects, such as the importance of iron dopants. However, some atomic-scale behaviours of TiO2 photocatalytic N2 reduction to NH3 are chaos, such as the N≡N dissociation transition-states and active sites of TiO2 photocatalytic materials. Future research works should mainly discover the secrets on how to reduce N2 to NH3 in the presence of titanium-based materials from a molecular-level viewpoint.


#### 3.1.2. Transition Metal-Doped Materials

Transition metals as dopants in photocatalytic materials have some advantages for the higher NH3 yields, such as less charge carrier recombination, stronger photocatalyst absorbance, and lower overpotential for the N2 reduction. In the process of preparing titanium-based photocatalytic materials for NH3 synthesis, preprocessing titanium-based materials with high temperatures (≈1000°C) and adding transition metal impurities (<1%) were necessary and benefit to obtain efficient titanium dioxide-based photocatalytic materials [46]. Since then, CuCl, WO3, and FeOx as photocatalysts for NH3 synthesis from N2 have been investigated [34, 50]. Those surface oxygen defects of titanium metal could play an important role in the atmospheric N2 photoreduction.Some transition metals taking place of iron as dopants in TiO2 materials were investigated [13], such as Co, Mo, Ni, Pd, V, Cr, Cu, Pt, Ag, Au, and Pb. Only did Co, Mo, and Ni dopants among these metals enhance the NH3 yields, and the other metals not. Differently, Palmisano found that chromium-doped TiO2 material was effective in the photocatalytic N2 reduction to NH3 [51]. Besides their dopant role, transition metals on the surface of TiO2 materials could be effective in NH3 synthesis as cofactors of photocatalysts. For example, Ru-doped TiO2 could photocatalyze the reduction of N2 to NH3 [42]. As cocatalysts of TiO2 materials, the photocatalytic activities of transition metals are associated with the strength of the M–H bonds (M: transition metal), such as Ru > Rh  > Pd > Pt [19].Due to lower Fermi level than the others, transition-metal dopants as electron sinks in titanium photocatalysts could minimize the probability of carrier recombination to promote greater NH3 yields, which is a critical factor for designing photocatalytic systems. To increase N2 reduction driving force to NH3 synthesis, Co, Mo, Ni, Ru, and Pt dopants have been added into the titanium-based materials, respectively, which resulted in increasing the NH3 yields [9, 19]. Anatase TiO2 was prepared by the sol-gel method through the hydrolysis of titanium tetrachloride and doped with transition metal ions like V5+ and Zn2+. Although both doped samples showed similar red shift in the band gap, Zn2+ (0.06 at.%) doped TiO2 materials showed enhanced activity which was attributed to smaller crystallite size and larger surface area for accelerating the interfacial charge transfer [52]. Under ambient temperature and atmosphere, the photocatalytic activity of the Ag-TiO2 multiphase nanocrystal composite thin films prepared by the liquid phase deposition method exceeded that of pure TiO2 thin films when the AgNO3 concentration was kept in the range of 0.03–0.05 [53]. The presence of dopants in the band gap could effectively improve photocatalytic performance through inducing defect states which assist in charge separation of photogenerated electrons and holes [26]. However, all transition metal dopants did not show more perfect photocatalytic activities than iron dopant in the TiO2 photocatalytic NH3 synthesis process [54]. Due to their lower Fermi level, transition metals act as electron sinks which creates a Schottky barrier, trapping photogenerated electrons and minimizing the probability for carrier recombination [20]. Therefore, reducing carrier recombination is a significant issue to design photocatalytic materials for the NH3 synthesis from N2.


#### 3.1.3. Other Metal Oxide-Based Materials

In addition to titanium oxide-based materials, other metal oxide-based materials as photocatalyst have also obtained more and more attentions on their behaviours of reducing N2 to NH3 in the past several decades. All of these metal oxide-based materials reported have different degrees of photocatalytic activities for NH3 synthesis from N2 under ambient conditions.Iron was early used as catalysts in the catalytic ammonia production processes by Dhar and Chowdhry [43]. As we know, iron plays an important role in the Haber–Bosch process, but it is ferric oxide (Fe2O3) not iron as the alternative to titanium that could photocatalytic the reduction of N2 to NH3. However, pure Fe2O3 is not capable of photocatalytic NH3 synthesis from N2 unless partially reduced [20] and hydrous [23] Fe2O3. To solve the problem, Khader et al. [35] partially reduced a-Fe2O3 to Fe3O4 for photoactivating N2, which resulted in detecting NH3 in aqueous slurry of the catalyst. Furthermore, the photoactivity of mesoporous β-Ga2O3 nanorods in N2 photoreduction was ameliorated in the presence of different alcohols, such as ethanol and tert-butanol [32]. Besides ZnO prepared by means of wet etching or precipitation methods, it was reported that the NH3 yield of unmodified ZnO was higher than that of Pt-loaded ZnO materials [33].As-synthesized bismuth monoxide (BiO) materials were applied in the photocatalytic reduction of N2 to NH3 under solar light. Research results [28] showed that the NH3 synthesis rate is up to 1226 mmol·g−1·h−1 which is about 1000 times higher than that of the Fe-TiO2 photocatalyst (Table 2). Obviously, deactivation of this photocatalyst did not occur even after being used more than 120 hours. Bismuth oxyhalides, BiOX (X = Cl, Br, and I), have recently become popular due to their excellent applications in photocatalytic NH3 synthesis from N2. BiOBr has also been focused on their material defects. Li et al. [29] employed BiOBr nanosheets with oxygen vacancies (OVs) to reduce N2 under visible light, and the N2 reduction rate was estimated to be 104.2 mmol·h−1. The structure of BiOBr provides enough space to polarize atoms, and the efficient separation and transformation of charge carriers must depend on the as-formed internal electric field [30]. Moreover, the large number of OVs on the surface of BiOBr hampered the recombination of electron-hole pairs.Interestingly, except the N-type semiconductors above, there are some P-type semiconductors which should be also suitable for the photocatalytic N2 reductions, such as CrO, MnO, FeO, NiO, Cu2O, VO2, Cr2O3, and Ag2O. In addition, being added into some impurity atoms such as boron atoms, aluminum atoms, indium atoms, and gallium atoms, the others would turn into P-type semiconductors whose conductivity (i.e., charge carriers) mainly depends on positive vacant holes.So far, hydrated P-type semiconductors as photocatalysts have been mainly used to reduce N2 to NH3. Early in 1987, hydrous FeO photocatalysts were discovered by Tennakone et al. [55] and were superior to TiO2 in the case of reducing N2 to NH3 under visible light [56]. Hydrous Cu2O after impregnation with CuCl was able to photoreduce N2 to NH3. Presumably, the high NH3 yield resulted from the reduction sites Cu2O·xH2O [34]. In 1992, vanadium-doped hydrous FeO could enhance reducing N2 to NH3 and the average NH3 production rate was 200 mM·h−1 or so, which benefited from the V impurities increasing vacant holes [57] whose concentration is much larger than the free electrons concentration. Therefore, the more the impurity is added, the higher the concentration of multivacant holes, the stronger the conductivity of the semiconductor.


#### 3.1.4. Metal Sulfide-Based Materials

Similar to the metal oxide-based photocatalysts, metal sulfides have recently become a hot research topic in the field of photocatalytic NH3 synthesis due to their strong absorption of visible light (Table 3). The NH3 synthesis rate is up to 325 μmol·g−1·h−1 in the presence of ultrathin MoS2 as photocatalysts, but without charged excitons, the bulk MoS2 did not have the ability of reduction of N2 under the same conditions [32]. Miyama et al. [42] suggested that NH3 yield of CdS/Pt binary photocatalysts was drastically higher than that of pristine CdS under UV irradiation. CdS/Pt/RuO2 [33] and Pt/CdS-Ag2S/RuO2 [31] photocatalysts were successively applied to reduce N2 to NH3 under visible light. The holes in the valence band trapped the electrons that released from RuO2. To keep the high photocatalytic activity of CdS for a longer time, maybe some measures must be taken to stop the degradation of CdS to S and Cd2+.Multicomponent metal sulfides with sulphur vacancies, such as Zn0.1Sn0.1Cd0.8S [34] and Mo0.1Ni0.1Cd0.8S [27], could reduce N2 as photocatalysts under visible light, and the concentration of sulphur vacancies trapping electrons were linear related to the NH3 yields. In addition, G-C3N4/ZnSnCdS [42] and g-C3N4/ZnMoCdS [35] were, respectively, employed for the fixation of N2. A tight junction coupling between g-C3N4 and ZnMoCdS was the key for efficient charge transfer.Only nitrogenases can catalyze reduction of N2 to NH3 at room temperature and atmospheric pressure [55]. The nitrogenase complex consists of two proteins: Fe-protein which is responsible for the supply of electrons and MoFe-protein which uses the provided electrons to reduce N2 to NH3 [57]. Mimicking the catalytic activity sites FeMo-cofactor of MoFe-protein, a synthetic complex of Fe, Mo, and S should be taken into account, and organic-sulfide catalysts have also been designed for enhanced N2 fixation activity. Banergee et al. [39] demonstrated that synthetic FeMoS inorganic clusters could reduce N2 to NH3 in aqueous media under light, which showed that structural analogues of nitrogenase can be functional to photocatalytic N2 fixation. Katherine et al. [31] reported that cadmium sulfide (CdS) nanocrystals can be used to drive the enzymatic reduction of N2 to NH3 by photosensitizing the MoFe-protein of nitrogenase not ATP hydrolysis. Under optimal conditions, the turnover rate was 75 per minute, 63% of the ATP-coupled reaction rate for the nitrogenase complex. Both [Mo2Fe6S8(SPh)3] and [Fe4S4] clusters comprised in nitrogenase could do so at ambient temperature and pressure, and results suggest [58] that the nitrogenase could keep its photocatalytic activity when the [Fe4S4] clusters were replaced with other inert ions such as Sb3+, Sn4+, and Zn2+. In this process, Fe is necessary but Mo is not; however it does not mean that Mo is not playing a role in N2 binding. In addition, the biohybrids of CdS and MoFe-protein provide a photochemical model for achieving the photocatalytic NH3 production process. Therefore, redox-active iron sulfide containing clusters with high-energy photoexcited states could photocatalyze the reduction of N2 to NH3.


## 3.1. Titanium Oxide-Based Materials

Photocatalytic N2 fixation has received more and more attentions since synthesizing NH3 from N2 was available in the presence of titanium-based catalysts. Dhar and Chowdhry [43] found that titanium oxide (TiO2) as photocatalytic material could provide the larger NH3 yields from reducing N2 than ferric oxide and zinc oxide as photocatalysts. Schrauzer and Guth [13] also discovered this phenomenon of N2 photoreduction. UV irradiation of TiO2 with a large number of surface Ti3+ species could produce NH3 from N2 and H2O under ambient conditions, which shows the species are actually the active sites for the reduction of N2 to NH3 [14].


### 3.1.1. Iron-Doped Materials

As key factors underlying photocatalytic performance, the influence of the crystalline phase and iron dopants on the catalytic activity of TiO2 photocatalytic material for the N2 reduction was focused. Under UV light, NH3 yields reached a maximum value when over 0.2% Fe2O3 was doped into TiO2 material. However, Hirakawa et al. [14] demonstrated that Ru, Pt, or Pd particles loaded in the TiO2 materials were unhelpful to increase the NH3 yields due to their particles covering the catalytic active site Ti3+ of TiO2 photocatalytic material and changing surface defects. Interestingly, Augugliaro et al. [44] supported Fe2O3-hybridized TiO2 on Al2O3 as photocatalyst which was applied in gas-solid fluidized bed reactors to improve NH3 yields because of the influence of iron ions on the TiO2 crystalline [45]. Lashgari and Zeinalkhani [21] photosynthesized NH3 in a H2O photosplitting setup in the presence of some synthetic Fe2O3 and TiO2-based uniform nanoparticles and obtained the maximum NH3 yield. Bourgeois et al. [26] suggested that thermal pretreatments could generate surface defects or impurity states on the surface of unmodified TiO2 that showed the photocatalytic activity of reducing N2 after annealing in air. As a highly efficient method, high pretreatment temperatures (∼1000°C) inducing surface defects in TiO2 material and the small amounts (<1%) of iron impurities are necessary to enhance the NH3 yields [46].Observing no reduction of N2 to NH3 in the presence of pure TiO2 material, Augugliaro et al. [47] put forward a hypothesis that Fe3+ of photoassisted organ iron compound could temporarily trap photons and promote the separation of charge carriers, which has played an important role in the NH3 production processes. Exposing the facets of Fe-doped TiO2 with ethanol as the scavenger enhanced the photocatalytic reduction of N2 to NH3 [22]. No nitrite was formed in the ethanol solution that could prevent the oxidation of NH3. Keeping a low doping concentration on the surface of TiO2 material was very important to inhibit charge recombination for forming Fe2+ and Fe4+ and to transfer electrons and holes to Ti4+ and OH− for generating Ti3+ and OH [18]. Similarly, Ranjit and Viswanathan [48] observed that the FeTi2O5 phase in Fe-doped titanium materials could enhance the separation of charge carriers and also demonstrated this hypothesis. However, Soria et al. [25] suggested that excess Fe dopants might lead TiO2 materials to lose themselves all photocatalytic activities. Therefore, the low bulk Fe3+ concentrations significantly assist in charge separation. TiO2 nanoparticles codoped with N2 and Fe3+ were prepared using the homogeneous precipitation-hydrothermal method. The cooperation of N2 and Fe3+ leads to narrow the band gap, improve the photocatalytic activity in the visible light region, and promote the separation of the photogenerated electrons and holes to accelerate the transmission of the photocurrent carrier [49].Generally, titanium oxide-based photocatalytic materials for the N2 reduction have made much progress in several aspects, such as the importance of iron dopants. However, some atomic-scale behaviours of TiO2 photocatalytic N2 reduction to NH3 are chaos, such as the N≡N dissociation transition-states and active sites of TiO2 photocatalytic materials. Future research works should mainly discover the secrets on how to reduce N2 to NH3 in the presence of titanium-based materials from a molecular-level viewpoint.


### 3.1.2. Transition Metal-Doped Materials

Transition metals as dopants in photocatalytic materials have some advantages for the higher NH3 yields, such as less charge carrier recombination, stronger photocatalyst absorbance, and lower overpotential for the N2 reduction. In the process of preparing titanium-based photocatalytic materials for NH3 synthesis, preprocessing titanium-based materials with high temperatures (≈1000°C) and adding transition metal impurities (<1%) were necessary and benefit to obtain efficient titanium dioxide-based photocatalytic materials [46]. Since then, CuCl, WO3, and FeOx as photocatalysts for NH3 synthesis from N2 have been investigated [34, 50]. Those surface oxygen defects of titanium metal could play an important role in the atmospheric N2 photoreduction.Some transition metals taking place of iron as dopants in TiO2 materials were investigated [13], such as Co, Mo, Ni, Pd, V, Cr, Cu, Pt, Ag, Au, and Pb. Only did Co, Mo, and Ni dopants among these metals enhance the NH3 yields, and the other metals not. Differently, Palmisano found that chromium-doped TiO2 material was effective in the photocatalytic N2 reduction to NH3 [51]. Besides their dopant role, transition metals on the surface of TiO2 materials could be effective in NH3 synthesis as cofactors of photocatalysts. For example, Ru-doped TiO2 could photocatalyze the reduction of N2 to NH3 [42]. As cocatalysts of TiO2 materials, the photocatalytic activities of transition metals are associated with the strength of the M–H bonds (M: transition metal), such as Ru > Rh  > Pd > Pt [19].Due to lower Fermi level than the others, transition-metal dopants as electron sinks in titanium photocatalysts could minimize the probability of carrier recombination to promote greater NH3 yields, which is a critical factor for designing photocatalytic systems. To increase N2 reduction driving force to NH3 synthesis, Co, Mo, Ni, Ru, and Pt dopants have been added into the titanium-based materials, respectively, which resulted in increasing the NH3 yields [9, 19]. Anatase TiO2 was prepared by the sol-gel method through the hydrolysis of titanium tetrachloride and doped with transition metal ions like V5+ and Zn2+. Although both doped samples showed similar red shift in the band gap, Zn2+ (0.06 at.%) doped TiO2 materials showed enhanced activity which was attributed to smaller crystallite size and larger surface area for accelerating the interfacial charge transfer [52]. Under ambient temperature and atmosphere, the photocatalytic activity of the Ag-TiO2 multiphase nanocrystal composite thin films prepared by the liquid phase deposition method exceeded that of pure TiO2 thin films when the AgNO3 concentration was kept in the range of 0.03–0.05 [53]. The presence of dopants in the band gap could effectively improve photocatalytic performance through inducing defect states which assist in charge separation of photogenerated electrons and holes [26]. However, all transition metal dopants did not show more perfect photocatalytic activities than iron dopant in the TiO2 photocatalytic NH3 synthesis process [54]. Due to their lower Fermi level, transition metals act as electron sinks which creates a Schottky barrier, trapping photogenerated electrons and minimizing the probability for carrier recombination [20]. Therefore, reducing carrier recombination is a significant issue to design photocatalytic materials for the NH3 synthesis from N2.


### 3.1.3. Other Metal Oxide-Based Materials

In addition to titanium oxide-based materials, other metal oxide-based materials as photocatalyst have also obtained more and more attentions on their behaviours of reducing N2 to NH3 in the past several decades. All of these metal oxide-based materials reported have different degrees of photocatalytic activities for NH3 synthesis from N2 under ambient conditions.Iron was early used as catalysts in the catalytic ammonia production processes by Dhar and Chowdhry [43]. As we know, iron plays an important role in the Haber–Bosch process, but it is ferric oxide (Fe2O3) not iron as the alternative to titanium that could photocatalytic the reduction of N2 to NH3. However, pure Fe2O3 is not capable of photocatalytic NH3 synthesis from N2 unless partially reduced [20] and hydrous [23] Fe2O3. To solve the problem, Khader et al. [35] partially reduced a-Fe2O3 to Fe3O4 for photoactivating N2, which resulted in detecting NH3 in aqueous slurry of the catalyst. Furthermore, the photoactivity of mesoporous β-Ga2O3 nanorods in N2 photoreduction was ameliorated in the presence of different alcohols, such as ethanol and tert-butanol [32]. Besides ZnO prepared by means of wet etching or precipitation methods, it was reported that the NH3 yield of unmodified ZnO was higher than that of Pt-loaded ZnO materials [33].As-synthesized bismuth monoxide (BiO) materials were applied in the photocatalytic reduction of N2 to NH3 under solar light. Research results [28] showed that the NH3 synthesis rate is up to 1226 mmol·g−1·h−1 which is about 1000 times higher than that of the Fe-TiO2 photocatalyst (Table 2). Obviously, deactivation of this photocatalyst did not occur even after being used more than 120 hours. Bismuth oxyhalides, BiOX (X = Cl, Br, and I), have recently become popular due to their excellent applications in photocatalytic NH3 synthesis from N2. BiOBr has also been focused on their material defects. Li et al. [29] employed BiOBr nanosheets with oxygen vacancies (OVs) to reduce N2 under visible light, and the N2 reduction rate was estimated to be 104.2 mmol·h−1. The structure of BiOBr provides enough space to polarize atoms, and the efficient separation and transformation of charge carriers must depend on the as-formed internal electric field [30]. Moreover, the large number of OVs on the surface of BiOBr hampered the recombination of electron-hole pairs.Interestingly, except the N-type semiconductors above, there are some P-type semiconductors which should be also suitable for the photocatalytic N2 reductions, such as CrO, MnO, FeO, NiO, Cu2O, VO2, Cr2O3, and Ag2O. In addition, being added into some impurity atoms such as boron atoms, aluminum atoms, indium atoms, and gallium atoms, the others would turn into P-type semiconductors whose conductivity (i.e., charge carriers) mainly depends on positive vacant holes.So far, hydrated P-type semiconductors as photocatalysts have been mainly used to reduce N2 to NH3. Early in 1987, hydrous FeO photocatalysts were discovered by Tennakone et al. [55] and were superior to TiO2 in the case of reducing N2 to NH3 under visible light [56]. Hydrous Cu2O after impregnation with CuCl was able to photoreduce N2 to NH3. Presumably, the high NH3 yield resulted from the reduction sites Cu2O·xH2O [34]. In 1992, vanadium-doped hydrous FeO could enhance reducing N2 to NH3 and the average NH3 production rate was 200 mM·h−1 or so, which benefited from the V impurities increasing vacant holes [57] whose concentration is much larger than the free electrons concentration. Therefore, the more the impurity is added, the higher the concentration of multivacant holes, the stronger the conductivity of the semiconductor.


### 3.1.4. Metal Sulfide-Based Materials

Similar to the metal oxide-based photocatalysts, metal sulfides have recently become a hot research topic in the field of photocatalytic NH3 synthesis due to their strong absorption of visible light (Table 3). The NH3 synthesis rate is up to 325 μmol·g−1·h−1 in the presence of ultrathin MoS2 as photocatalysts, but without charged excitons, the bulk MoS2 did not have the ability of reduction of N2 under the same conditions [32]. Miyama et al. [42] suggested that NH3 yield of CdS/Pt binary photocatalysts was drastically higher than that of pristine CdS under UV irradiation. CdS/Pt/RuO2 [33] and Pt/CdS-Ag2S/RuO2 [31] photocatalysts were successively applied to reduce N2 to NH3 under visible light. The holes in the valence band trapped the electrons that released from RuO2. To keep the high photocatalytic activity of CdS for a longer time, maybe some measures must be taken to stop the degradation of CdS to S and Cd2+.Multicomponent metal sulfides with sulphur vacancies, such as Zn0.1Sn0.1Cd0.8S [34] and Mo0.1Ni0.1Cd0.8S [27], could reduce N2 as photocatalysts under visible light, and the concentration of sulphur vacancies trapping electrons were linear related to the NH3 yields. In addition, G-C3N4/ZnSnCdS [42] and g-C3N4/ZnMoCdS [35] were, respectively, employed for the fixation of N2. A tight junction coupling between g-C3N4 and ZnMoCdS was the key for efficient charge transfer.Only nitrogenases can catalyze reduction of N2 to NH3 at room temperature and atmospheric pressure [55]. The nitrogenase complex consists of two proteins: Fe-protein which is responsible for the supply of electrons and MoFe-protein which uses the provided electrons to reduce N2 to NH3 [57]. Mimicking the catalytic activity sites FeMo-cofactor of MoFe-protein, a synthetic complex of Fe, Mo, and S should be taken into account, and organic-sulfide catalysts have also been designed for enhanced N2 fixation activity. Banergee et al. [39] demonstrated that synthetic FeMoS inorganic clusters could reduce N2 to NH3 in aqueous media under light, which showed that structural analogues of nitrogenase can be functional to photocatalytic N2 fixation. Katherine et al. [31] reported that cadmium sulfide (CdS) nanocrystals can be used to drive the enzymatic reduction of N2 to NH3 by photosensitizing the MoFe-protein of nitrogenase not ATP hydrolysis. Under optimal conditions, the turnover rate was 75 per minute, 63% of the ATP-coupled reaction rate for the nitrogenase complex. Both [Mo2Fe6S8(SPh)3] and [Fe4S4] clusters comprised in nitrogenase could do so at ambient temperature and pressure, and results suggest [58] that the nitrogenase could keep its photocatalytic activity when the [Fe4S4] clusters were replaced with other inert ions such as Sb3+, Sn4+, and Zn2+. In this process, Fe is necessary but Mo is not; however it does not mean that Mo is not playing a role in N2 binding. In addition, the biohybrids of CdS and MoFe-protein provide a photochemical model for achieving the photocatalytic NH3 production process. Therefore, redox-active iron sulfide containing clusters with high-energy photoexcited states could photocatalyze the reduction of N2 to NH3.


## 3.1.1. Iron-Doped Materials

As key factors underlying photocatalytic performance, the influence of the crystalline phase and iron dopants on the catalytic activity of TiO2 photocatalytic material for the N2 reduction was focused. Under UV light, NH3 yields reached a maximum value when over 0.2% Fe2O3 was doped into TiO2 material. However, Hirakawa et al. [14] demonstrated that Ru, Pt, or Pd particles loaded in the TiO2 materials were unhelpful to increase the NH3 yields due to their particles covering the catalytic active site Ti3+ of TiO2 photocatalytic material and changing surface defects. Interestingly, Augugliaro et al. [44] supported Fe2O3-hybridized TiO2 on Al2O3 as photocatalyst which was applied in gas-solid fluidized bed reactors to improve NH3 yields because of the influence of iron ions on the TiO2 crystalline [45]. Lashgari and Zeinalkhani [21] photosynthesized NH3 in a H2O photosplitting setup in the presence of some synthetic Fe2O3 and TiO2-based uniform nanoparticles and obtained the maximum NH3 yield. Bourgeois et al. [26] suggested that thermal pretreatments could generate surface defects or impurity states on the surface of unmodified TiO2 that showed the photocatalytic activity of reducing N2 after annealing in air. As a highly efficient method, high pretreatment temperatures (∼1000°C) inducing surface defects in TiO2 material and the small amounts (<1%) of iron impurities are necessary to enhance the NH3 yields [46].Observing no reduction of N2 to NH3 in the presence of pure TiO2 material, Augugliaro et al. [47] put forward a hypothesis that Fe3+ of photoassisted organ iron compound could temporarily trap photons and promote the separation of charge carriers, which has played an important role in the NH3 production processes. Exposing the facets of Fe-doped TiO2 with ethanol as the scavenger enhanced the photocatalytic reduction of N2 to NH3 [22]. No nitrite was formed in the ethanol solution that could prevent the oxidation of NH3. Keeping a low doping concentration on the surface of TiO2 material was very important to inhibit charge recombination for forming Fe2+ and Fe4+ and to transfer electrons and holes to Ti4+ and OH− for generating Ti3+ and OH [18]. Similarly, Ranjit and Viswanathan [48] observed that the FeTi2O5 phase in Fe-doped titanium materials could enhance the separation of charge carriers and also demonstrated this hypothesis. However, Soria et al. [25] suggested that excess Fe dopants might lead TiO2 materials to lose themselves all photocatalytic activities. Therefore, the low bulk Fe3+ concentrations significantly assist in charge separation. TiO2 nanoparticles codoped with N2 and Fe3+ were prepared using the homogeneous precipitation-hydrothermal method. The cooperation of N2 and Fe3+ leads to narrow the band gap, improve the photocatalytic activity in the visible light region, and promote the separation of the photogenerated electrons and holes to accelerate the transmission of the photocurrent carrier [49].Generally, titanium oxide-based photocatalytic materials for the N2 reduction have made much progress in several aspects, such as the importance of iron dopants. However, some atomic-scale behaviours of TiO2 photocatalytic N2 reduction to NH3 are chaos, such as the N≡N dissociation transition-states and active sites of TiO2 photocatalytic materials. Future research works should mainly discover the secrets on how to reduce N2 to NH3 in the presence of titanium-based materials from a molecular-level viewpoint.


## 3.1.2. Transition Metal-Doped Materials

Transition metals as dopants in photocatalytic materials have some advantages for the higher NH3 yields, such as less charge carrier recombination, stronger photocatalyst absorbance, and lower overpotential for the N2 reduction. In the process of preparing titanium-based photocatalytic materials for NH3 synthesis, preprocessing titanium-based materials with high temperatures (≈1000°C) and adding transition metal impurities (<1%) were necessary and benefit to obtain efficient titanium dioxide-based photocatalytic materials [46]. Since then, CuCl, WO3, and FeOx as photocatalysts for NH3 synthesis from N2 have been investigated [34, 50]. Those surface oxygen defects of titanium metal could play an important role in the atmospheric N2 photoreduction.Some transition metals taking place of iron as dopants in TiO2 materials were investigated [13], such as Co, Mo, Ni, Pd, V, Cr, Cu, Pt, Ag, Au, and Pb. Only did Co, Mo, and Ni dopants among these metals enhance the NH3 yields, and the other metals not. Differently, Palmisano found that chromium-doped TiO2 material was effective in the photocatalytic N2 reduction to NH3 [51]. Besides their dopant role, transition metals on the surface of TiO2 materials could be effective in NH3 synthesis as cofactors of photocatalysts. For example, Ru-doped TiO2 could photocatalyze the reduction of N2 to NH3 [42]. As cocatalysts of TiO2 materials, the photocatalytic activities of transition metals are associated with the strength of the M–H bonds (M: transition metal), such as Ru > Rh  > Pd > Pt [19].Due to lower Fermi level than the others, transition-metal dopants as electron sinks in titanium photocatalysts could minimize the probability of carrier recombination to promote greater NH3 yields, which is a critical factor for designing photocatalytic systems. To increase N2 reduction driving force to NH3 synthesis, Co, Mo, Ni, Ru, and Pt dopants have been added into the titanium-based materials, respectively, which resulted in increasing the NH3 yields [9, 19]. Anatase TiO2 was prepared by the sol-gel method through the hydrolysis of titanium tetrachloride and doped with transition metal ions like V5+ and Zn2+. Although both doped samples showed similar red shift in the band gap, Zn2+ (0.06 at.%) doped TiO2 materials showed enhanced activity which was attributed to smaller crystallite size and larger surface area for accelerating the interfacial charge transfer [52]. Under ambient temperature and atmosphere, the photocatalytic activity of the Ag-TiO2 multiphase nanocrystal composite thin films prepared by the liquid phase deposition method exceeded that of pure TiO2 thin films when the AgNO3 concentration was kept in the range of 0.03–0.05 [53]. The presence of dopants in the band gap could effectively improve photocatalytic performance through inducing defect states which assist in charge separation of photogenerated electrons and holes [26]. However, all transition metal dopants did not show more perfect photocatalytic activities than iron dopant in the TiO2 photocatalytic NH3 synthesis process [54]. Due to their lower Fermi level, transition metals act as electron sinks which creates a Schottky barrier, trapping photogenerated electrons and minimizing the probability for carrier recombination [20]. Therefore, reducing carrier recombination is a significant issue to design photocatalytic materials for the NH3 synthesis from N2.


## 3.1.3. Other Metal Oxide-Based Materials

In addition to titanium oxide-based materials, other metal oxide-based materials as photocatalyst have also obtained more and more attentions on their behaviours of reducing N2 to NH3 in the past several decades. All of these metal oxide-based materials reported have different degrees of photocatalytic activities for NH3 synthesis from N2 under ambient conditions.Iron was early used as catalysts in the catalytic ammonia production processes by Dhar and Chowdhry [43]. As we know, iron plays an important role in the Haber–Bosch process, but it is ferric oxide (Fe2O3) not iron as the alternative to titanium that could photocatalytic the reduction of N2 to NH3. However, pure Fe2O3 is not capable of photocatalytic NH3 synthesis from N2 unless partially reduced [20] and hydrous [23] Fe2O3. To solve the problem, Khader et al. [35] partially reduced a-Fe2O3 to Fe3O4 for photoactivating N2, which resulted in detecting NH3 in aqueous slurry of the catalyst. Furthermore, the photoactivity of mesoporous β-Ga2O3 nanorods in N2 photoreduction was ameliorated in the presence of different alcohols, such as ethanol and tert-butanol [32]. Besides ZnO prepared by means of wet etching or precipitation methods, it was reported that the NH3 yield of unmodified ZnO was higher than that of Pt-loaded ZnO materials [33].As-synthesized bismuth monoxide (BiO) materials were applied in the photocatalytic reduction of N2 to NH3 under solar light. Research results [28] showed that the NH3 synthesis rate is up to 1226 mmol·g−1·h−1 which is about 1000 times higher than that of the Fe-TiO2 photocatalyst (Table 2). Obviously, deactivation of this photocatalyst did not occur even after being used more than 120 hours. Bismuth oxyhalides, BiOX (X = Cl, Br, and I), have recently become popular due to their excellent applications in photocatalytic NH3 synthesis from N2. BiOBr has also been focused on their material defects. Li et al. [29] employed BiOBr nanosheets with oxygen vacancies (OVs) to reduce N2 under visible light, and the N2 reduction rate was estimated to be 104.2 mmol·h−1. The structure of BiOBr provides enough space to polarize atoms, and the efficient separation and transformation of charge carriers must depend on the as-formed internal electric field [30]. Moreover, the large number of OVs on the surface of BiOBr hampered the recombination of electron-hole pairs.Interestingly, except the N-type semiconductors above, there are some P-type semiconductors which should be also suitable for the photocatalytic N2 reductions, such as CrO, MnO, FeO, NiO, Cu2O, VO2, Cr2O3, and Ag2O. In addition, being added into some impurity atoms such as boron atoms, aluminum atoms, indium atoms, and gallium atoms, the others would turn into P-type semiconductors whose conductivity (i.e., charge carriers) mainly depends on positive vacant holes.So far, hydrated P-type semiconductors as photocatalysts have been mainly used to reduce N2 to NH3. Early in 1987, hydrous FeO photocatalysts were discovered by Tennakone et al. [55] and were superior to TiO2 in the case of reducing N2 to NH3 under visible light [56]. Hydrous Cu2O after impregnation with CuCl was able to photoreduce N2 to NH3. Presumably, the high NH3 yield resulted from the reduction sites Cu2O·xH2O [34]. In 1992, vanadium-doped hydrous FeO could enhance reducing N2 to NH3 and the average NH3 production rate was 200 mM·h−1 or so, which benefited from the V impurities increasing vacant holes [57] whose concentration is much larger than the free electrons concentration. Therefore, the more the impurity is added, the higher the concentration of multivacant holes, the stronger the conductivity of the semiconductor.


## 3.1.4. Metal Sulfide-Based Materials

Similar to the metal oxide-based photocatalysts, metal sulfides have recently become a hot research topic in the field of photocatalytic NH3 synthesis due to their strong absorption of visible light (Table 3). The NH3 synthesis rate is up to 325 μmol·g−1·h−1 in the presence of ultrathin MoS2 as photocatalysts, but without charged excitons, the bulk MoS2 did not have the ability of reduction of N2 under the same conditions [32]. Miyama et al. [42] suggested that NH3 yield of CdS/Pt binary photocatalysts was drastically higher than that of pristine CdS under UV irradiation. CdS/Pt/RuO2 [33] and Pt/CdS-Ag2S/RuO2 [31] photocatalysts were successively applied to reduce N2 to NH3 under visible light. The holes in the valence band trapped the electrons that released from RuO2. To keep the high photocatalytic activity of CdS for a longer time, maybe some measures must be taken to stop the degradation of CdS to S and Cd2+.Multicomponent metal sulfides with sulphur vacancies, such as Zn0.1Sn0.1Cd0.8S [34] and Mo0.1Ni0.1Cd0.8S [27], could reduce N2 as photocatalysts under visible light, and the concentration of sulphur vacancies trapping electrons were linear related to the NH3 yields. In addition, G-C3N4/ZnSnCdS [42] and g-C3N4/ZnMoCdS [35] were, respectively, employed for the fixation of N2. A tight junction coupling between g-C3N4 and ZnMoCdS was the key for efficient charge transfer.Only nitrogenases can catalyze reduction of N2 to NH3 at room temperature and atmospheric pressure [55]. The nitrogenase complex consists of two proteins: Fe-protein which is responsible for the supply of electrons and MoFe-protein which uses the provided electrons to reduce N2 to NH3 [57]. Mimicking the catalytic activity sites FeMo-cofactor of MoFe-protein, a synthetic complex of Fe, Mo, and S should be taken into account, and organic-sulfide catalysts have also been designed for enhanced N2 fixation activity. Banergee et al. [39] demonstrated that synthetic FeMoS inorganic clusters could reduce N2 to NH3 in aqueous media under light, which showed that structural analogues of nitrogenase can be functional to photocatalytic N2 fixation. Katherine et al. [31] reported that cadmium sulfide (CdS) nanocrystals can be used to drive the enzymatic reduction of N2 to NH3 by photosensitizing the MoFe-protein of nitrogenase not ATP hydrolysis. Under optimal conditions, the turnover rate was 75 per minute, 63% of the ATP-coupled reaction rate for the nitrogenase complex. Both [Mo2Fe6S8(SPh)3] and [Fe4S4] clusters comprised in nitrogenase could do so at ambient temperature and pressure, and results suggest [58] that the nitrogenase could keep its photocatalytic activity when the [Fe4S4] clusters were replaced with other inert ions such as Sb3+, Sn4+, and Zn2+. In this process, Fe is necessary but Mo is not; however it does not mean that Mo is not playing a role in N2 binding. In addition, the biohybrids of CdS and MoFe-protein provide a photochemical model for achieving the photocatalytic NH3 production process. Therefore, redox-active iron sulfide containing clusters with high-energy photoexcited states could photocatalyze the reduction of N2 to NH3.


## 4. Conclusions

Although the Haber–Bosch process is an important chemical industrial approach to the worldwide population, this traditional NH3 production process is a high energy-consumption and environmental pollution process. Therefore, there have been more and more research interests in photocatalytic reduction of N2 to NH3 in the past decades. Difficultly, photocatalytic NH3 synthesis from N2 also needs to overcome the energy barrier of N≡N triple band under the spin-polarized plane-wave pseudopotential method. The Density-functional theory has been used to calculate the electronic band structures and the optical absorption spectra of nitrogen-doped and oxygen-deficient anatase TiO2. The calculated results are in good agreement with our experimental measurements. These calculations reveal that the optical absorption of nitrogen-doped TiO2 in the visible light region is primarily located between 400 and 500 nm, while that of oxygen-deficient TiO2 is mainly above 500 nm. These results have important implications for the understanding and further development of photocatalytic materials that are active under visible light. However, synthesis of photocatalytic NH3 is still kept in a laboratory-scale level.This work would like to assist in understanding state-of-the-art in the photocatalytic NH3 synthesis field for promoting the research field of N2 fixation. Past reports on photocatalytic NH3 synthesis focused primarily on titanium oxide-based materials. However, recent studies demonstrated that the N2 reduction could occur in different photocatalytic reaction systems composed of transition metal-doped materials, metal sulfide-based materials, and other metal oxide-based materials (e.g., P-type semiconductors) as photocatalysts, respectively. Unfortunately, up to our knowledge, no viable and efficient photocatalysts for sustainable NH3 synthesis could meet all requirements of an active, selective, scalable, long-lived catalyst. With the application of modern computational and experimental techniques, developing efficient photocatalysts should be encouraged to discover the mechanism on how N2 reduction happens with the photocatalysts in the molecular level.



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*Source: 3286782-2018-11-22.xml*