*4.3. Plasmacatalysis*

The plasmacatalysis process was described as a possible alternative to many chemicals' high-temperature and pressure synthesis systems. In addition to positive and negative ions, plasma often contains a large number of neutral particles, such as atoms, molecules, radicals and excited particles, resulting in highly reactive physical and chemical reactions when used in chemical synthesis. The plasma's ionised and excited species concentration is considerably higher than the traditional thermally heated gas phases. These properties can, therefore, benefit from attaining further effective interaction even without a catalyst [145]. In the interaction of plasma and catalyst, plasma creates a more active spot yielding to higher catalytic activity. When the beneficial effect of plasma and catalyst is combined effectively, it is likely to produce a much higher yield [146].

Depending on the thermal equilibrium or not, plasma could be classified into thermal and non-thermal plasma (NTP). The temperature of plasma, like that of any other gas, is determined by the average energies of the plasma particles (neutral and charged) and their degrees of freedom (translational, rotational, vibrational and those related to electronic excitation) [147,148]. Plasmas can thus exhibit multiple temperatures as a multi-component system. In common electrical discharge for plasma generation, energy is transferred to heavy articles by collision with electron. In thermal plasma, electron and heavy particles achieved thermal equilibrium due to joule heating. Joule heating or ohmic heating define the process in which the energy of an electric current is converted into heat as it flows through a resistance. The temperature of the gas in thermal plasma is extremely high, typically ranging from 4000 K to 20,000 K. On the other hand, non-thermal plasma is characterised by multiple different temperatures related to different plasma particles and different degrees of freedom. In non-thermal plasma, thermal equilibrium between electron and heavy particles is not achieved, and the temperature of the NTP may be as low as room temperature, although the electron, the excited and the ionised species have a high temperature (Te >> T0) [147].

The temperature of the gas in thermal plasma is extremely high, typically ranging from 4000 K to 20,000 K and is equivalent to that of the electron, which has achieved a thermodynamic equilibrium between the electron and other species. On the other hand, the temperature of the NTP may be as low as room temperature, although the electron, the excited and the ionised species have a high temperature. Since NTP offers less power input, this plasma is a more attractive option for chemical synthesis.

Many researchers have explored the mechanism of plasmacatalytic synthesis of ammonia since the 1900s. The first attempt to utilise plasma to synthesis ammonia can be traced back to 1929 when Brewer et al. [149] successfully synthesised ammonia using glow discharge plasma and achieved an energy yield of 3.03 g-NH3/kWh. The system used was complex in that high voltage, vacuum and magnetic fields were applied. The magnetic field was reported to have no significant impact on the yield of ammonia. Since these experiments demonstrated the principle of using plasma to produce ammonia, researchers have begun to conduct detailed investigations into the process of plasma-assisted ammonia synthesis using other types of plasma. Table 9 summarises previous research on plasmacatalytic ammonia synthesis.


**Table 9.** Summary of previous research on plasma-chemical for ammonia synthesis.


**Table 9.** *Cont.*

Note: GD: glow discharge, RF: radiofrequency, MW: microwave; DBD: dielectric barrier discharge.

Following the initial study, Eremin et al. [151] revealed that ammonia is formed by surface reactions. Afterwards, Venugopalan et al. [152] achieved high productivity of ammonia on Ag coated quartz. Uyama et al. [156,180] found the formation of nitride in addition to hydrazine and ammonia in their study. Nakajima and Sekiguchi [161] found that when plasma is generated by H2/N2 gas mixture, the nitrogen gas activation in the plasma has been depressed by hydrogen while hydrogen injection into the afterglow area increased the production of ammonia. In addition to the plasma mentioned above, dielectric barrier discharge was also widely explored. In 2000, researchers [164] tested MgO as a catalyst in combination with DBD. The result shows that the catalyst could increase the ammonia yield by up to 75% more than a plasma-assisted reaction. The group also studied the synthesis of ammonia from methane and nitrogen without any catalyst and achieved an energy yield of as much as 0.69 g-NH3/kWh [168]. The use of porous materials for ammonia absorption was also studied by Peng et al. [171]. They found that by using a porous material for ammonia absorption, the rate of ammonia synthesis increases due to the lower gas phase of ammonia. The group also examined some metal catalysts as a promoter for improving Ru loaded on magnesium-oxide particles. Caesium (Cs) was reported as the best promoter and capable of achieving energy yield as high as 2.41 g-NH3/kWh [171]. Akay and Zhang [181] performed research where barium titanate enhanced by Ni/SiO2 was used as the catalyst. The overall energy yield of 1.9 g-NH3/kWh was achieved by this configuration. More recently, in a pulsed DBD plasma reactor, Peng et al. [179] used MgCl2 as a catalyst and ammonia absorber. The study discovered that MgCl2 was effective to store generated ammonia for later extraction. The configuration also achieved a very high energy yield with 20.5 g-NH3/kWh.

More recently, an attempt was made to synthesis NH3 in the catalyst-free plasma– water interfaces system based on the batch reactor process. Breakthroughs have recently occurred in which plasmas in contact with water surfaces have achieved significant results, putting the interfaces between plasma and water as an NH3 reaction locus by using a combination of electrochemical and plasma. In these studies, the metal anode was replaced with N2-plasma gas and successfully produced up to 0.44 mg/hour of NH3 on 1 mm<sup>2</sup> plasma–liquid interfaces [182].

#### **5. Ammonia as a Renewable Fuel**
