*3.3. Electrically-Driven Haber–Bosch Process*

Notwithstanding the technological reliability of the existing Haber–Bosch process, the question remains as to whether or not such a process can be CO2-free. To answer these questions, research and development are underway worldwide to substitute the centuriesold Haber–Bosch method for the production of ammonia, driven by renewable electricity. It includes switching H2 obtained from the steam-reformed CH4 to H2 obtained from the electrolysed H2O. Given the trend in renewable energy prices to compete with fossil fuels, the green Haber–Bosch process is no longer a vision. Figure 5 shows the electrically driven Haber–Bosch plant powered by renewable energy.

**Figure 5.** Schematic of renewable energy-driven Haber–Bosch plant [62].

Ammonia synthesis processes powered by renewable energy have been demonstrated in many countries. For example, in the USA, Schmuecker Pinehurst Farm LLC has built a solar ammonia plant and has been in operation for many years where H2 and N2 are generated from water and air by electrolysis and power swing system before pressurised and fed into the ammonia production facility [63]. In Zimbabwe, Africa, an ammonium nitrate plant was developed where ammonia is supplied from renewable energy-driven Haber– Bosch process. This production facility has been productive for years, with 240,000 tonnes of ammonium nitrate produced annually [64]. In Australia, several projects have been developed near Yara Pilbara, Western Australia [65] and Port Lincoln, South Australia [66] to evaluate the feasibility of renewable energy-driven ammonia plants. At the same time as optimisation of the renewable energy-driven Haber–Bosch process, the development of alternative methods to allow the N2 reduction reaction at atmospheric pressure and moderate ambient temperature, such as photocatalysis, electrocatalysis and plasmacatalysis has attracted widespread interest in ammonia synthesis today.

#### **4. Innovative Approaches for Ammonia Synthesis**

In the absence of high temperatures and pressures, nature converts molecular N2 to NH3. This natural process uses enzyme nitrogenises containing metal ions (iron and molybdenum) to induce ammonia reactions from atmospheric nitrogen, electrons and protons. This phenomenon has aroused the researcher's interest in imitating nature.

In recent times, significant progress in understanding the process for nitrogenises reactions and in creating a modern synthetic method has been achieved. Photocatalysis, plasmacatalysis and electrocatalysis have been studied as alternative green routes for ammonia production. These new techniques offer distinct advantages compared to the old Haber–Bosch method.

Among those, the plasma-enabled ammonia synthesis is both energy and cost-effective in theory. The potential energy consumption of non-thermal plasma (NTP) ammonia production has been reported to be around 0.2 MJ/mol, which is lower than the Haber– Bosch technique (0.48 MJ/mol) [67]. This section details the method and provides a discussion on emerging technology for facilitating the N2 reduction reaction to ammonia.

## *4.1. Photocatalysis*

Photocatalytic ammonia production from water and air at low temperature and pressure show enormous potential, attracting increased research interest from scientists. The process is relatively safe, inexpensive and accessible to a free energy source (light). In general, photons are used in the photocatalytic mechanism to drive N2 activation. The fixation process of N2 into NH3 by photocatalysts can be represented by the following equation:

$$\text{N}\_2 + 3\text{H}\_2\text{O} \overset{\text{light}}{\rightarrow} 2\text{NH}\_3 + \frac{3}{2}\text{O}\_2\tag{5}$$

The N2 fixation photocatalytic process involves several steps. In short, the electron generated by the photocatalyst effect is driven into the conduction band, creating an empty hole in the valance band. Some holes and electrons are then recombined together, while others transfer to the catalytic surface and take part in the redox reaction. H2O is then oxidised to O2 by holes, while N2 is reduced to NH3 by the reaction of waterderived protons and photo-generated electrons. Figure 6 illustrated the catalytic process of the photocatalyst.

**Figure 6.** Schematic of photocatalyst reaction for ammonia synthesis [68].

TiO2-based metal oxide photocatalysts were studied early in nitrogen fixation because of less expense and higher stability. Following the pioneering research in 1977 by Schrauzer and Guth [69], many semiconductors have been proposed for the process of photocatalysis viz. metal oxide, metal sulphide, oxyhalides and other graphitic nitride carbon materials.

In 1988, Bourgeois et al. [70] studied photocatalytic action of unmodified TiO2 after annealing under atmospheric air pressure. Thermal pre-treatment is believed to trigger surface defects that caused defects or impurities in the semiconductor bandgap. In the most recent study, the photocatalytic N2 fixation into NH3 on TiO2 surface oxygen vacancies was systematically investigated by Hirakawa et al. [71]. They found that the active sites for N2 reduction are the oxygen vacancies of the Ti3+ species. The superficial Ti3+ provided an abundance of active N2 fixing sites by acting as an electron donor, resulting in relatively easy dissociation of the N≡N bond. In other studies, many other metal species are used as doping in the TiO2 catalyst; however, the photocatalytic was unsatisfactory. The transition metal atoms loaded into TiO2 can function as co-catalysts while acting as a dopant in the TiO2 matrix. In addition, the Schottky junction formation on the semiconductor interface and the transition metal induces electrical fields and facilitates the separation of photoinduced electrons and holes.

In addition to metal doping, noble metals may also be embedded onto the surface of TiO2. Ranjit et al. [72], in 1996, compared four noble metals (Ru, Rh, Pd, Pt) as TiO2 co-catalysts. They found Ru > Rh > Pd > Pt to be the order of photoactivity between metals, which is closely related to the strength of the noble metals and the hydrogen bond. The experiment revealed that noble metals with higher barriers to H2 evolution exhibit surprisingly higher NH3 yields.

The development of metal-doped TiO2 in N2 fixation sparked interest in the development of binary metal oxide and ternary metal oxides as doping. The study by Lashgari and Zeinalkhani [73], on the synthesis of Fe2O3 by precipitation method, found that the ammonia is efficiently formed from N2 at Fe2O3 nanoparticles. It was also reported that partially reduced Fe2O3 may be used for the reduction of photocatalytic N2. More recently, bismuth monoxide (BiO) was employed for N2 photoreduction. It has been stated that BiO quantum dots will significantly decrease N2 at 1226 μmol gcat−1h−<sup>1</sup> NH3 with the simulated rate of sunlight [74].

In addition to metal oxides, the study of metal sulphides in the field of photocatalysis has also seen a significant increase in the scientific community. The metal sulphide band gap is conducive to intense visible light absorption, resulting in highly effective solar use. As a result of the growing multidisciplinary research in biology and material science, organic-sulphide has also been developed as a catalyst for NH3 synthesis [68]. Researchers questioned whether sulphur vacancies could effectively promote the photocatalytic event. Motivated by this idea, Hu et al. [75] studied the sulphur vacancies effect on the efficiency of ternary metal sulphide for N2 fixation. Sulphur vacancies have been found to introduce surface chemical adsorption sites, which have helped to activate N2 molecules by widening the bonding space from 1.164 Å to 1.213 Å. In the recent development, Cd0.5Zn0.5S solid solution loaded with Ni2P was effectively reduced N2 with the NH<sup>3</sup> yield of 253.8 μmol gcat−<sup>1</sup> h−<sup>1</sup> [76]. Other materials, such as Bismuth oxyhalides, are also found to be promising photocatalysts due to their layered crystal structure and suitable band gaps [68,77].

Other than the material mentioned above, graphitic nitride carbon photocatalyst for N2 fixation has been designed and developed in recent years. Dong et al. [78] have shown that introducing vacancies in nitrogen will dramatically boost the photocatalytic behaviour of g-C3N4. It was because the nitrogen vacancies, which had the size and forms as the nitrogen atoms, are advantageous for adsorbing and activating the chemically inert N2. In another study, honeycombed g-C3N4 doped with Fe3+ has shown enhanced photocatalytic performance [79]. After that, various forms of photocatalysts based on g-C3N4 were employed in photochemical ammonia synthesis. A summary of various photochemical catalysts by previous studies is presented in Table 7.


**Table 7.** Recent studies on photochemical catalysis systems for ammonia production.


**Table 7.** *Cont.*
