A Mini Review of Catalytic Reducing Nitrogen to Ammonia under Ambient Conditions

Ammonia(NH 3 ) has played an essential role in meeting the increasing demand for food and the worldwide need of nitrogenous fertilizer since 1913. Unfortunately, the traditional Haber-Bosch process for producing NH 3 from nitrogen(N 2 ) is a high energy-consumption process. Under ambient conditions catalytic reducing N 2 to NH 3 is an attractive and promising alternative approach which would emerge huge opportunity to directly provide nitrogenous fertilizers in agricultural fields as need in a distributed manner. In this review, some research findings showed alterna -tive, available, sustainable NH 3 production processes from N 2 in the presence of electro-catalysts and photo-catalysts under ambient conditions.


Introduction
As one of the most important chemicals to our planet, NH 3 has played an essential role in meeting the growing demand for food and the worldwide need of nitrogenous fertilizer since 1913 [1]. The total worldwide NH 3 production exceeded 140 million tons, and demand for NH 3 continues to grow in 2014 [2], and about 80% of the NH 3 product usually is used as nitrogenous fertilizer. Fritz Haber discovered that NH 3 could be directly synthesized by reacting atmospheric N 2 with hydrogen in the temperature range of 400-500℃ and at pressures of 130-170bar [3], and Carl Bosch subsequently developed it on an industrial scale [4]. As the most important invention of the 20th century, the Haber-Bosch process, a thermo-chemical catalytic conversion technology, is a primary choice of industrial production of NH 3 for human beings until now. According to the Haber-Bosch process, NH 3 is produced from the reaction: The Haber-Bosch process reacts the pure feed gases at high temperatures and pressures, requiring energy input of around 485 kJ mol-1of N 2 and almost 2% of global energy consumption [5]. The high dissociation energy of triply bonded N 2 molecule (911 kJ mol -1 ) presents a significant activation energy barrier, however, the negative entropy (∆H300= -46.35 kJ mol -1 ) of the reaction dictates that N 2 could be converted to NH 3 at lower temperatures [6]. There are several advantages for NH 3 synthesis at low temperatures [7,8]. Firstly, the reaction of producing NH 3 from N 2 with dihydrogen is spontaneous. Secondly, the proton conductivity of low-temperature electrolytes has more excellent behaves than the other temperature ones. Thirdly, the reaction kinetics of NH 3 production processes is extremely slow when the operating temperatures are below 100℃. It is Kordali et. al. [9] who firstly reported that NH 3 was produced from N 2 and water using a Nafion electrolyte at low temperature in 2000. In additional, it would be a key joint to build foundational principles of designing new efficient catalysts for sustainable NH 3 synthesis production process. The new methods of catalyst design need us to understand the catalytic mechanisms by integrating theory and experiment of discovering the active, scalable, selective, long-lived efficient catalysts for sustainable NH 3 synthesis.
Therefore, how to activate the N≡N bond to produce NH 3 with less fossil energy consumption is great opportunity and challenges for chemists. Although researchers tried to develop the artificial catalysts to facilitate the reaction at more ambient conditions and there are many new approaches, the nowadays used industrial catalysts are extremely similar to the original one discovered by Mittasch [10] in 1910s. In this review, some research findings showed alternative, available, sustainable NH 3 production processes from N 2 in the presence of electrocatalysts, photo-catalysts and analogous catalysts of nitrogenases metalloclusters under ambient conditions.

Organic and Medicinal Chemistry International Journal Discussion
Recently, electro-catalysis depending on renewable electricity produced from renewable energy (such as wind or solar) has played an increasingly role in the NH 3 synthesis at ambient temperature(T<100℃) [2,7,8,11]. Some novel functional materials as catalysts used in typical N 2 reductions were explored by researchers [12].

Electro-Driven Catalytic Reducing N 2 to NH 3
Under ambient conditions, Liu et al. [13,14] developed Sm1.5Sr0.5MO 4 (M=Ni, Co, Fe) and SmFe0.7Cu0.3-xNix-O3(x=0-0.3) as electro-catalysts for NH 3 synthesis with Nafion used as proton solid electrolyte. Such systems, while impressive in current efficiency, require careful control experiments due to low overall yields and observed ambient NH 3 absorption by polymer electrolyte membranes. Further, researchers have taken their efforts to develop ideal electro-catalysts for improving N 2 reduction rates and the NH 3 yield. Impressively, SmFe0. 7Cu0.1Ni0.2O3 showed the record-high NH 3 production rate of 1.1×10 -8 mol·s -1 ·cm -2 at 80℃. The NH 3 yield was up to 8.7×10 -9 mol·s -1 ·cm -2 in the present of electro-catalyst SmBaCuMO5+i (M=Fe, Co,Ni) at 80℃ and 2.5V [15]. However, there are still big spaces to improve the NH 3 yield which currently are not enough to the future industry application.
An alternative strategy would like to consider using metal nitrides as electro-catalyst cathodes for N 2 reduction except pure metals. Respectively at potentials of -0.76V and -0.51V, producing NH 3 from N 2 took place on the surface of Zr N and VN as electro-catalyst cathodes which were not covered by adsorbed H-atoms [16], the same to Nb N and CrN [17]. The theoretical analysis of Abghoui et al. [18][19][20] predicted stable operation and Faraday Efficiency(FE) higher than 75% for V, Cr, Nb and Zr mono-nitrides at applied bias between 0.5 and 0.76V. Unfortunately, there would not be such nitride materials which were tested experimentally like above mentioned for NH 3 synthesis [21]. Beside mono-nitrides, binary nitrides such as Co 3 Mo 3 N are amongst the most active electro-catalysts for NH 3 synthesis [22]. A thorough investigation of their electro-catalytic behaves should be performed in real NH 3 synthesis situations.
These reports show that there was a universal phenomenon that these electro-catalysts often contain big proportions of oxides on their surfaces, which is harmful to keep their normal conductivity and could significantly reduce their catalytic efficiency. Actually, an ideal electro-catalyst should have the capacities of high catalytic activity and excellent electronic conductivity at ambient conditions and could suppress the hydrogen evolution reaction. The electro-catalytic NH 3 synthesis is still under experiment stage, huge challenges are still faced the lack of the solid oxide electrolyte materials [23]. Future research should focus on theory-guided screening and discovery of new NH 3 catalysts, as well as developing novel methods for NH 3 synthesis at low temperatures and pressures. One important issue on the successful demonstration of NH 3 being from reducing N 2 and Overcoming N 2 and NH 3 contamination should be done with 15 N 2 dinitrogen, FTIR analysis and choice of system materials.

Photo-Driven Catalytic Reducing N 2 to NH 3
With the global population increasing, the photon-driven N 2 fixation technologies will become very critical to capture solar energy and produce nitrogenous fertilizers [24]. In the late 1970s, Schrauzer and Guth discovered that TiO 2 powders doped with Fe 2 O 3 could catalyze the reduction of N 2 to NH 3 under ambient conditions [25,26]. Since then, TiO 2 became the workhorse photo-catalyst for subsequent studies, and CuCl, WO3 and FeOx as possible photo-catalysts for NH 3 synthesis have been investigated [27,28]. Maybe, surface oxygen defects of titanium metal caused by high temperatures could play an important role in the photo-driven catalytic reduction of N 2 to NH 3 . Although there is not an insight regarding the atomic scale phenomena on photo-catalytic N 2 reduction, the TiO 2 crystal polymorph and its iron impurity concentration are identified to be the key factors to underlay photo-catalytic performance.
Co-catalysts, such as Co, Mo, Ni, Ru and Pt dopants, have been added to the titanium-based materials for increasing the NH 3 yield [24,29]. Transition metal dopants as electron sinks in titanium photo-catalysts could minimize the probability for carrier recombination to promote greater NH 3 yields. Dopants inducing defect states could assist in charge separation of photogenerated electrons and holes but are not a part of the active site for N 2 dissociation. Therefore, the conduction band position of photo-catalysts ( Figure 1) should be more negative than the reduction potential of the N 2 hydrogenation, as well as the valance band should be more positive than the oxygen evolution potential [30]. The partial current densities for photo-catalytic producing NH 3 are lower 3 to 5 orders of magnitude than the other electrodriven NH 3 production processes. To solve the problem, maybe the efficient photo-catalysts attached to electrodes would show us an amazing result of the higher NH 3 yields. In additional, like electrochemical NH 3 synthesis, photo-catalytic approaches must employ proper methods and controls.

Conclusion
Under ambient conditions, some seminal literatures have been reviewed on describing the electro-catalytic NH 3 synthesis from commonly available feedstock such as purified H 2 and N 2 . The electro-catalytic NH 3 synthesis is still under experiment stage due to the lack of the suitable electrolyte materials as electro-catalysts with high catalytic activity, excellent electronic conductivity, and hydrogen evolution reaction. Transition metals which can achieve selective N 2 reduction maybe are good choice for further researches. Additionally, it is with the high-energy UV light that photo-catalytic NH 3 synthesis from N 2 could overcome the N≡N band energy barrier under ambient conditions. Currently, titanium oxide with transition metal dopants is a not bad choice in practice. Future studies should encourage investigating on how N 2 reduction happen on the photo-catalysts with the application of modern computational and experimental techniques from molecular level viewpoint.