Plasma synthesis of ammonia in a tangled wire dielectric barrier discharge reactor: Effect of electrode materials
Introduction
Recent years have witnessed a rising demand for ammonia due to its vital role in the chemical and agricultural sectors. The estimated global production of ammonia is over 160 million tonnes each year, and approximately 80% of the ammonia is consumed in fertiliser production [1]. The advent of synthetic nitrogen fertiliser has led to fundamental changes in food production and now feeds nearly half of the world's population [2]. Nowadays, ammonia is being considered as a carbon-free energy storage vector for the long-distance transport of hydrogen due to its large hydrogen mass density and the mild temperature required for liquefaction [3]. It is also promising for short-term electrochemical energy storage (i.e. batteries) to deliver on-demand energy in conjunction with fuel cells, which would enable better uptake of the irregular supply from sustainable electricity [4]. Nevertheless, conventional ammonia synthesis via the Haber-Bosch (H-B) process takes up 1–2% of the global energy consumption and releases more than 300 tonnes of CO2 on an annual basis [5,6]. Success in developing green, efficient, and commercially viable alternatives to the H-B process that accommodate renewable energy and CO2-free hydrogen will reinforce the importance of ammonia in traditional fertiliser production and pioneering energy storage.
Although the ammonia synthesis from N2 and H2 is exothermic (R1), elevated temperatures (∼700 K) are needed for the dissociation of the robust dinitrogen triple bond, which is the rate-limiting step in the H-B process [7]. In the context of high temperatures, high pressures around 100 bar are required to shift the equilibrium towards the formation of ammonia. The harsh conditions of the H-B process require large-scale centralised plants and continuous operation with an uninterrupted power supply. Thus, activating the inert dinitrogen at lower temperatures and lower pressures would be significant for addressing these challenges and improving the processes efficiency [8].
Non-thermal plasma (NTP) has been considered as an ideal candidate for sustainable N2 fixation under mild conditions [9]. In an NTP, highly energetic electrons with a mean electron energy of 1–10 eV are initially generated, inducing a cascade of chemically reactive species (e.g., excited molecules, ions, and atoms) that facilitates the kinetically limited dinitrogen activation at low bulk gas temperatures [[10], [11], [12], [13]]. This unique feature enables the plasma-driven ammonia synthesis to be performed under thermodynamically favourable conditions at atmospheric pressure. Due to the mild working conditions, the NTP technology is compact, flexible, and easily switched on and off; this is desirable for the accommodation of intermittent and highly decentralised renewable energy sources such as solar, wind, and tidal energy for the decentralised small-scale green ammonia production [14,15]. Moreover, the NTP process can readily integrate with water electrolysis using intermittent renewable electricity to achieve net-zero ammonia synthesis and alleviate the environmental impact [16].
In recent years, considerable efforts have been devoted to the plasma synthesis of ammonia using dielectric barrier discharges (DBDs) [[17], [18], [19], [20], [21], [22], [23]]. Bai et al. reported plasma synthesis of NH3 in a cylindrical DBD reactor at ambient conditions [20]. Wang et al. developed a unique water-cooled DBD reactor for the synthesis of ammonia at near room temperature and ambient pressure [21]. Aihara et al. employed a metallic wire electrode to synthesise ammonia in a DBD reactor, achieving an NH3 yield of 3.5% and an energy cost of 18.6 MJ mol−1. Their results proved that using a metal wire as the high-voltage inner electrode contributes to a more productive ammonia synthesis via the increased surface area [22]. Besides, Iwamoto et al. [23] and Mehta et al. [24] evaluated the activities of different metals via density functional theory (DFT) calculations and microkinetic modelling. The employment of proper metal electrodes in a DBD reactor can open a new route to tailor the formation of reactive species and facilitate ammonia production. However, reducing the energy cost and enhancing the ammonia yield remain challenges that require further innovation and improvement in plasma reactor design. Moreover, the individual effect of metallic electrodes such as electrode materials and electrode configurations on the electrical properties of the discharge is still underexplored in plasma NH3 synthesis [25,26]. Further efforts are required to develop a systematic understanding of these fundamental plasma properties in ammonia synthesis; this will reduce the energy consumption of NH3 production under ambient conditions, shedding light on the reaction mechanism of the plasma ammonia synthesis, and generating essential and invaluable knowledge for the future development of this disruptive and emerging technology.
In this work, the ammonia synthesis from nitrogen and hydrogen was carried out using a DBD reactor with tangled metal electrodes. The influence of process parameters including the N2/H2 molar ratio and total gas flow rate on the NH3 concentration and energy cost were investigated. The chemical performances and discharge characteristics were also examined using different tangled electrode materials. A possible reaction mechanism was discussed based on the comprehensive analysis of experimental findings with a view to achieving process optimisation of ammonia synthesis in a tangled wire DBD reactor.
Section snippets
Experimental setup
In this work, a coaxial DBD reactor was used to investigate the plasma-synthesis of ammonia under ambient conditions. Fig. 1(a) shows a schematic diagram of the experimental setup. N2 and H2 were controlled by mass flow controllers (Omega, FMA-2404) before being introduced into the DBD reactor. The configuration of the tangled wire DBD reactor is depicted in Fig. 1(b). A quartz tube with an outer diameter of 22 mm and a wall thickness of 1.5 mm was used as the dielectric layer. A
Effect of electrode materials on the plasma-driven NH3 synthesis
Fig. 3 shows the influence of different electrode materials on ammonia synthesis in terms of NH3 concentration and energy cost for ammonia production. Compared with the rod electrode, the NH3 production was more efficient using the tangled wire electrodes, this can be attributed to the larger surface area (63 cm2) of the tangled wire electrode compared to the rod electrode (45 cm2). Besides, it was reported that the surface of the tangled wire electrode became bumpy after the plasma treatment,
Discussion
In the Haber-Bosch process, initially the dissociative adsorption of N2 and H2 takes place on the catalyst surface, followed by the step-wise hydrogenation of adsorbed N atoms to form NH3 (R2-R6, ∗ denotes the adsorption site on the catalyst surface) [39].
In contrast, NTP enables the activation and dissociation of reactants in the gas phase, thus creating new reaction routes for ammonia production. It is well accepted that
Conclusions
The NH3 synthesis was carried out in a DBD plasma reactor under ambient conditions. A tangled wire electrode was used to improve NH3 production and decrease energy costs. Compared with the rod electrode, the tangled wire electrode substantially enhanced the conversion of N2 and H2, increased the NH3 concentration, and reduced the energy cost of the synthesis process. The material of the electrode significantly influenced the NH3 production. Among the tested materials, including NiFe, SS, Ti,
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
The support of this work by the UK EPSRC Impact Acceleration Account (IAA) is gratefully acknowledged. Y. Ma also acknowledges the PhD fellowship co-funded by the University of Liverpool and the Chinese Scholarship Council (CSC).
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