Plasma-assisted ammonia synthesis under mild conditions for hydrogen and electricity storage: Mechanisms, pathways, and application prospects

Abstract

Ammonia, with its high hydrogen storage density of 17.7 wt.% (mass fraction), cleanliness, efficiency, and renewability, presents itself as a promising zero-carbon fuel. However, the traditional Haber−Bosch (H−B) process for ammonia synthesis necessitates high temperature and pressure, resulting in over 420 million tons of carbon dioxide emissions annually, and relies on fossil fuel consumption. In contrast, dielectric barrier discharge (DBD) plasma-assisted ammonia synthesis operates at low temperatures and atmospheric pressures, utilizing nitrogen and hydrogen radicals excited by energetic electrons, offering a potential alternative to the H−B process. This method can be effectively coupled with renewable energy sources (such as solar and wind) for environmentally friendly, distributed, and efficient ammonia production. This review delves into a comprehensive analysis of the low-temperature DBD plasma-assisted ammonia synthesis technology at atmospheric pressure, covering the reaction pathway, mechanism, and catalyst system involved in plasma nitrogen fixation. Drawing from current research, it evaluates the economic feasibility of the DBD plasma-assisted ammonia synthesis technology, analyzes existing dilemmas and challenges, and provides insights and recommendations for the future of nonthermal plasma ammonia processes.

Graphical abstract

Keywords

plasma catalysis / nitrogen fixation / ammonia synthesis / hydrogen storage / catalyst / carbon neutralization

1 Introduction

The synthesis of ammonia has significantly impacted human society. Ammonia plays a crucial role in the production of fertilizers, which contributes over 40% to food production, thus playing a key role in addressing challenges related to population growth and food security [1–4]. As the second largest chemical globally, ammonia is essential for the growth of the world economy [5–7]. It serves as a fundamental raw material in various industries, such as pharmaceuticals for antibiotic production, metal processing for iron and steel treatment [8,9], and food manufacturing for alkalizing agents [10–13]. Moreover, in the textile industry, ammonia is utilized not only in the production of materials like nylon and synthetic fibers, but also in the dyeing and washing processes of cotton and silk [14,15]. Furthermore, ammonia finds diverse applications in the electronics industry, metallurgy, refrigerants, commercial explosives production, and other sectors [16,17].

Ammonia is not only an essential industrial raw material, but also a crucial hydrogen storage material and zero-carbon fuel, particularly in the current push for “carbon neutrality” [18,19]. It offers several advantages: ① cleanliness and environmental friendliness: controlled ammonia combustion results in only water and nitrogen, without the release of harmful or greenhouse gases [20,21], ② renewability: ammonia can be produced through the reaction of hydrogen and nitrogen, with the nitrogen being recyclable, creating a sustainable energy loop [22–24], ③ high energy density: with 3.0 kWh/kg, ammonia provides a significant calorific value when burned [25,26], ④ ease of transportation: liquid ammonia is more convenient and cost-effective to store and transport compared to liquid hydrogen, ⑤ superior hydrogen storage capacity: ammonia contains more hydrogen than liquid hydrogen, allowing for over 60% more hydrogen storage in the same volume [27], and ⑥ enhanced safety: NH₃ has a narrower explosion limit (16%–25%) compared to H₂ (4%–76%), and its distinctive smell aids in easy detection [28]. As a result, many countries worldwide are embracing green ammonia initiatives, with China leading the way by establishing comprehensive industrial chains, industry standards, and safety regulations for NH₃ production, storage, transportation, and utilization.

The Haber−Bosch (H−B) process, developed in the early 20th century, is widely used for ammonia synthesis in industry [29–31]. It requires high temperatures (up to 500 °C) and high pressures (15–30 MPa) [32–34]. The H−B process and its accompanying methane reforming industry, among others, consume 2% of global energy and 2% of global natural gas production. Large, centralized H−B plants also require a significant amount of energy for product transportation [35–37]. Efforts have been made to reduce energy consumption and carbon emissions, but the high-temperature and high-pressure conditions limit optimization possibilities [32,38,39]. Despite decades of development, the H−B process is approaching its maximum efficiency, making it challenging to enhance effectiveness through catalyst and structural improvements.

In recent years, the transition from H−B ammonia industry to distributed renewable energy ammonia industry has become increasingly obvious [40–42]. The H−B process, currently relying on large, centrally located plants for ammonia synthesis, does not fully utilize renewable energy sources, such as wind and solar energy, which are often far from population centers. Additionally, the transportation of ammonia from centralized H−B plants is energy-intensive. The development of novel technologies for green ammonia synthesis, in combination with the electrolytic water technology, enables on-site production of ammonia without emissions. This technology is particularly beneficial for industries or processes heavily reliant on ammonia products, such as agricultural fertilizer production in regions where wind and solar energy are not fully utilized [43]. Researchers are actively exploring various mild ammonia synthesis technologies, including electrochemical synthesis [44–52], biological nitrogen fixation [53–55], photocatalysis [56–58], and chemical looping synthesis [59–61]. However, the high-energy barriers for N₂ dissociation limit their efficiencies [39,62]. Plasma synthesis of ammonia, on the other hand, can overcome these barriers by breaking N≡N through the excitation of high-energy electrons, potentially surpassing the efficiency of the H−B process. This emerging clean and mild technology can be integrated with distributed energy sources and hydrogen production from electrolyzed water, offering a promising alternative to the traditional process [63–65]. This review aims to summarize the progress in plasma-assisted ammonia synthesis at atmospheric-pressure and low-temperature dielectric barrier discharge (DBD), covering catalytic pathways, catalyst systems, and mechanisms. In addition, it assesses the competitiveness of plasma-assisted ammonia synthesis in terms of environmental friendliness and economy. Moreover, it envisions the future development and application prospects of atmospheric-pressure and non-thermal DBD plasma-assisted ammonia synthesis.

2 Plasma nitrogen fixation

Plasma ammonia synthesis is an emerging technology that offers a clean and mild alternative to the traditional H−B method [66–68]. It has the potential to utilize distributed energy sources and achieve a synthesis rate of ammonia higher than that of the H−B method. One key distinction between plasma ammonia synthesis and the conventional method is the generation of high-energy electrons in plasma, which can stimulate gas-phase molecules to enhance surface reactions on catalyst [69,70]. Plasma-assisted catalytic reactions involve a combination of homogeneous plasma-phase reactions, where free radicals are excited in space without catalysts, and non-homogeneous surface reactions that take place on the catalyst surface [71,72]. In contrast to thermally catalyzed reactions that rely on adsorption−dissociation reactions for N≡N bond breaking, plasma excites high-energy electrons to break N≡N bonds.

In conventional thermocatalytic ammonia synthesis, it is difficult to regulate the energy of these steps independently because the activation and adsorption energy are correlated on metal catalysts, i.e., the optimization of these catalysts is limited by linear scaling relations [73]. In contrast, plasma reactions do not rely on dissociation reactions at the catalyst surface for breaking N≡N bonds, but rather on plasma discharge reactions. Mehta et al. [73] suggested that plasma could facilitate the entry of N₂ into less stable vibrational or electronically excited states, potentially lowering the activation energy for N₂ dissociation without impacting subsequent reaction steps. Liu et al. [74] proposed that above certain critical radical concentrations-N radicals might become the primary source of N at the binding site. As a result, plasma-assisted ammonia synthesis is being explored as a strategy to disrupt the traditional linear scaling relationship.

Different plasma generation forms include DBD, microwave discharges, and jet discharge. In 1971, Eremin et al. [63] successfully synthesized ammonia using the DBD technique, marking an early example of ammonia synthesis with DBD. DBD involves inserting gas into an insulating dielectric within the discharge region, leading to gas breakdown and plasma formation. The setup is illustrated in Fig. 1(a) [75]. This method is widely used owing to its efficient utilization of gas in the reaction region and high energy efficiency at atmospheric pressure. Jet discharge initially dissociates the gas into plasma ejected from the device, facilitating interatomic reactions (Fig. 1(b), N₂ plasma injection into water) [76]. While this method offers a wide excitation bandwidth and quick response time, its rate and energy efficiency are relatively low. Microwave discharges convert microwave energy into internal gas energy for creating plasma (Fig. 1(c), consisting of a microwave generator, rectangular waveguide, microwave resonance cavity, reactor, and impingers for collection of the ammonia produced) [77]. This process boasts a high energy efficiency and high conversion rates but is limited to smaller scales.

The mechanism of the plasma nitrogen fixation reaction is illustrated in Fig. 1(d) [78]. N₂ and H₂ undergo dissociation into radicals under the influence of high-energy electrons, leading to hydrogenation for ammonia production at the metal active sites of the catalyst. This reaction is intricate, representing a fusion of the L−H reaction and the E−R reaction. Furthermore, the behavior of the catalyst may impact the plasma and vice versa, with the plasma also influencing the catalyst and catalytic mechanism. Therefore, delving into the analysis of the catalytic mechanism, the catalyst is crucial for enhancing the understanding of the plasma nitrogen fixation reaction.

3 Catalysts

The coaxial medium-blocked plasma reaction involves a high-energy voltage supplied by a power supply and a regulator to excite gas molecules into a plasma state. This reaction typically combines a homogeneous plasma-phase reaction, such as ammonia synthesis through oscillating excited radicals in space without a catalyst, with a non-homogeneous surface reaction that takes place on the surface of the catalyst [78,79]. While plasma-assisted ammonia synthesis can occur without a catalyst, the rate is low. However, the addition of a catalyst can increase the reaction rate by at least 50% or more, as the reactions on the catalyst surface enhance the hydrogenation efficiency for ammonia production [80,81]. Therefore, enhancing the effect of catalysts in DBD plasma reactions is crucial for maximizing the value of plasma applications. Presently, mainstream catalytic systems include both Mono-catalysts [82,83] and metal-carrier catalysts [84–86].

3.1 Mono-catalysts

Mono-catalysts are catalysts composed of a single type of material, in contrast to metal-carrier catalysts. These catalysts do not contain carriers, such as pure metal catalysts [83], zeolites [86,87], and metal-organic frameworks [82,88]. Mono-catalysts offer the benefits of straightforward structures and easy accessibility, but require no complex preparation processes. Iwamoto et al. [83] studied the impact of non-thermal DBD plasma-assisted ammonia synthesis using various metals as catalysts. Their findings indicated that Au exhibited the highest catalytic activity in non-thermal DBD plasma synthesis of ammonia, with nitrogen and hydrogen reacting more efficiently due to the appropriate instability of its surface nitride (Fig. 2(a)). The initial ammonia yield from H₂ and N₂ was 2.5% without any heating. Additionally, zeolite imidazolium frameworks (ZIFs) ZIF-8 and ZIF-67 utilized in non-thermal plasma-assisted ammonia synthesis demonstrated a notable ammonia synthesis rate of 28.52 μmol NH₃/(min·g_cat) (Fig. 2(b)), attributed to the relatively low ammonia storage capacity caused by dipole-dipole interactions between polar ammonia molecules and the polar walls of ZIFs [86]. Shah et al. [82] developed a Ni-MOF-74 for DBD plasma-assisted ammonia synthesis, enhancing the catalytic efficiency through improved pore size, open Ni metal sites, and reduced hydrogen complexation surfaces, resulting in an energy cost of 265 MJ/mol [82]. In summary, while single catalytic systems offer simple preparation and diverse structures, they may not fully leverage synergistic interactions between the active component and the carrier, limiting their maximum reaction rate due to their singular structure.

3.2 Metal-carrier catalysts

Metal-carrier catalysts offer a significant advantage over mono-catalysts by combining the high mass transfer efficiency of porous carriers with the catalytic effect of metal active sites, resulting in a notable enhancement in the rate of ammonia synthesis. The performance of these catalysts is influenced by the choice of carriers and active metals, with current research predominantly focusing on optimizing these two aspects [67,68,72].

3.2.1 Carriers

Mesoporous (medium pore) materials enhance the plasma nitrogen fixation reaction by graded ordered pore size, which enhances the pore size effect and facilitates the diffusion of free radicals [85]. Macroporous materials, do not significantly impact gas mass transfer and active site distribution. The inability to generate plasma discharge reactions in the pores of most microporous materials, due to the limitation of the Debye length, reduces their efficiency. The main carriers currently used include porous alumina [71,89], silica [70,90], activated carbon (AC) [91], and mesoporous molecular sieves [92,93].

γ-Al₂O₃ is a commonly used mesoporous material due to its widespread availability, affordability, and successful commercialization, making it an ideal carrier for DBD plasma nitrogen fixation reactions. Zhu et al. [71] conducted a comprehensive study on the impact of commercial γ-Al₂O₃ on the efficiency of plasma synthesis of ammonia. Their findings revealed that the addition of γ-Al₂O₃ increased the efficiency of plasma-assisted synthesis of NH₃ by nearly 30%, and the highest energy efficiency reached 6.58 {\text{g}}_{\text{N}{\text{H}}_{\text{3}}}/kWh (Figs. 1(c) and 1(d)). In a study by Zhao et al. [94], the DBD reaction rate with γ-Al₂O₃ fillers doubled the concentration observed in an empty tube plasma reactor, attributed to the improved mass transfer and discharge effects facilitated by the mesoporous structure of γ-Al₂O₃.

Mesoporous silicon dioxide, characterized by a regular nanoscale pore structure ranging from 2 to 50 nm, exhibits mechanical stability and heat resistance, making it suitable for use in strong electric field environments like DBD plasma synthesis of ammonia. Chen et al. [70] demonstrated that porous SiO₂ outperformed non-porous glassy materials in plasma nitrogen fixation due to the plasma reaction in the catalyst mesopores. The large specific surface area of mesoporous materials facilitates these surface adsorption reactions. Li et al. [95] utilized mesoporous silicon dioxide (SBA-15) as a carrier for plasma-assisted ammonia synthesis, resulting in a catalyst with an average particle size of 5.1 ± 1.1 nm [95]. The unique pore structure of SBA-15 significantly enhanced the plasma effect. Gorky et al. [69] compared the performance of silica with three structures: nonporous silica, fumed silica, and mesoporous SBA-15 silica. Mesoporous SBA-15 exhibited a threefold higher ammonia synthesis rate per gram of catalyst when supplied with hydrogen-rich gas compared to nonporous and fumed silica. The uniform and dynamic mixing of discharge diffusion into the pores with the oxide-induced plasma in mesoporous SBA-15 led to high ammonia synthesis rates and efficient energy utilization.

AC has unique properties such as a large surface area and a strong pore system that have been found to be beneficial for the thermally catalyzed synthesis of NH₃ [96]. Previous research has indicated that catalysts loaded with carbon exhibit higher activity levels compared to those with other carriers [91]. Hu et al. [91] investigated the plasma-enhanced synthesis of NH₃ using a variety of M/AC catalysts (M represents Ru, Co, Ni, and Fe). AC improved the reaction performance when used as a catalyst compared to not adding any filler. By loading the AC with different active metals, the reaction rate was increased up to nearly 40% (Fig. 3(a)). The highest energy efficiency of 0.72 g/kWh was obtained for Ru-loaded AC at SIE of 8.0 kJ/L.

Mesoporous molecular sieve materials with highly ordered pores are considered optimal carriers in plasma ammonia catalysts. These materials exhibit a low conductivity, which helps provide a stable microdischarge environment. Peng et al. [92] described a method for loading metallic Ru onto mesoporous Si-MCM-41 and investigated the plasma synthesis of ammonia. The highest energy consumption ratio recorded was 1.7 g/kWh at 5000 V and 26000 Hz. During the plasma reaction, the transfer of electrons from the promoter to the Ru catalyst facilitated the synthesis of NH₃ by providing dissociation energy to the inlet gas and forming precursors (Fig. 3(b)). The high surface area, low conductivity, and abundant pore structure of mesoporous molecular sieve materials enhanced the performance of ammonia synthesis when compared to metal oxide carriers. Table 1 displays the plasma ammonia synthesis rates for various mono-catalysts and metal-carrier catalysts. The ammonia synthesis rates of the metal-carrier catalysts are notably higher than those of the mono-catalysts in approximate power, indicating the need for further investigation in this field.

Mesoporous materials demonstrate superior performance in plasma synthesis of ammonia when used as carriers. The enhanced gas mass transfer and microporous discharge effect originating from their highly ordered pore structure contribute to this improved performance. However, it has been suggested that due to the limitation of the Debye length, the discharge reaction no longer occurs in the nanoscale pore structure. Wang et al. [97] proposed that the absence of plasma discharge in MCM-41 mesopores provides a “shielding protection”, allowing desorbed NH₃ to diffuse into the ordered mesopores and shield the decomposition process, ultimately enhancing the rate of ammonia synthesis indirectly. To support this hypothesis, Ni was deposited at different locations on the mesoporous molecular sieves, including the inside of the mesoporous skeleton (Ni/MCM in), the outer surface (Ni/MCM out), or the entire skeleton (Ni/MCM both). When comparing different catalysts, H₂ plasma-treated Ni/MCM out showed the highest total H₂ consumption, followed by Ni/MCM both and Ni/MCM in (Fig. 3(c)). Meanwhile, the ammonia synthesis rate of Ni/MCM-41 with three different loading positions showed a positive proportional relationship with the number of its Ni sites (Fig. 3(d)). This suggests that the plasma reaction intensity aligns with this order, indicating that the discharge reaction is limited in the mesopores of MCM-41 due to plasma discharge length constraints (Fig. 3(e)). Despite the unclear impact of mesoporous material pore structure on the discharge reaction or the “shielding protection” effect, its undeniable enhancement of plasma nitrogen fixation warrants further investigation into the specific enhancement mechanism and internal reactions.

3.2.2 Active metals

In plasma nitrogen fixation reactions, understanding the interaction of plasma species with active metals on the catalyst surface and the subsequent hydrogenation process for ammonia production are crucial factors influencing NH₃ synthesis. Liu et al. [74] conducted a systematic investigation on the surface reaction energy barriers and reaction pathways of various metals using the density functional theory. These pathways include the collision of N·(H·) radicals with N*(H*) to produce N₂(g)[H₂(g)], the generation of NH_Y, and the reactions leading to NNH_Y and H_XNNH_Y.

In all metals, the nitrogen complex (r1, ΔE_rxn from –7.89 eV (Ag) to –3.37 eV (Fe)

\begin{array}{}\text{(r1)}& \mathrm{N}\cdot +{\mathrm{N}}^{\ast }\to {\mathrm{N}}_2(\mathrm{g})\end{array}

Significantly better than hydrogen composite (r2, ΔE_rxn from –2.45 eV (Sn) to –1.37 eV (Fe))

\begin{array}{}\text{(r2)}& \mathrm{H}\cdot +{\mathrm{H}}^{\ast }\to {\mathrm{H}}_2(\mathrm{g})\end{array}

Reactions competing with r1 and r2 lead to the formation of NH^*. At this point, the E−R reaction can only proceed in the following order

\begin{array}{}\text{(r3)}& \mathrm{H}\cdot +\mathrm{N}{\mathrm{H}}^{\ast }\to {\mathrm{N}{\mathrm{H}}_2}^{\ast }\end{array}

\begin{array}{}\text{(r4)}& \mathrm{H}\cdot +{\mathrm{N}{\mathrm{H}}_2}^{\ast }\to {\mathrm{N}{\mathrm{H}}_3}^{\ast }\end{array}

However, r4 must compete with r5 (which forms NNH₂^*)

\begin{array}{}\text{(r5)}& \mathrm{N}\cdot +{\mathrm{N}{\mathrm{H}}_2}^{\ast }\to {\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\end{array}

However, r6 and r13 are thermodynamically easier (i.e., more exothermic) than r11 and r12. Once NNH^* or NNH₂^* is formed, the E−R reaction follows a different reaction order as follows

\begin{array}{}\text{(r6)}& \mathrm{H}\cdot +\mathrm{N}\mathrm{N}{\mathrm{H}}^{\ast }\to {\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\end{array}

\begin{array}{}\text{(r7)}& \mathrm{H}\cdot +\mathrm{N}\mathrm{N}{\mathrm{H}}^{\ast }\to \mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}^{\ast }\end{array}

\begin{array}{}\text{(r8)}& \mathrm{H}\cdot +{\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\to {\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\end{array}

\begin{array}{}\text{(r9)}& \mathrm{H}\cdot +\mathrm{N}\mathrm{N}{\mathrm{H}}^{\ast }\to {\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\end{array}

\begin{array}{}\text{(r10)}& \mathrm{H}\cdot +\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}^{\ast }\to {\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\end{array}

\begin{array}{}\text{(r11)}& \mathrm{H}\cdot +{\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\to {\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\ast \end{array}

\begin{array}{}\text{(r12)}& \mathrm{H}\cdot +{\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\to {\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\end{array}

\begin{array}{}\text{(r13)}& \mathrm{H}\cdot +{\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\to {\mathrm{H}}_2{\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\end{array}

\begin{array}{}\text{(r14)}& \mathrm{H}\cdot +{\mathrm{H}\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\to {\mathrm{H}}_2{\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\end{array}

\begin{array}{}\text{(r15)}& \mathrm{H}\cdot +{\mathrm{H}}_2{\mathrm{N}\mathrm{N}{\mathrm{H}}_2}^{\ast }\to {\mathrm{H}}_2{\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\end{array}

\begin{array}{}\text{(r16)}& \mathrm{H}\cdot +{\mathrm{H}}_2{\mathrm{N}\mathrm{N}{\mathrm{H}}_3}^{\ast }\to {2\mathrm{N}{\mathrm{H}}_3}^{\ast }\end{array}

Based on the thermodynamically easiest sequence of hydrogenation for ammonia production, following the formation of NNH* via r6, Fe, Sn, Au, and Ag will proceed through the hydrogenation steps r7 (HNNH*) → r10 (HN{\text{NH}}_2^{\ast }) → r13 (H₂N{\text{NH}}_2^{\ast }) → r15 (H₂N{\text{NH}}_3^{\ast }) → r16 (2{\text{NH}}_3^{\ast }). Conversely, Ni, Co, Pd, Ga, and Cu will undergo the sequence r6 (NN{\text{NH}}_2^{\ast }) → r9 (HN{\text{NH}}_2^{\ast }) → r13 (H₂N{\text{NH}}_2^{\ast }) → r15 (H₂N{\text{NH}}_3^{\ast }) → r16 (2{\text{NH}}_3^{\ast }). Following the formation of NN {\text{NH}}_2^{\ast } through r5, all metals will then follow the hydrogenation steps r9 (HN {\text{NH}}_2^{\ast }) → r13 (H₂N{\text{NH}}_2^{\ast }) → r15 (H₂N{\text{NH}}_3^{\ast }) → r16 (2{\text{NH}}_3^{\ast }) [74]. The lower the nitrophilicity of the metal, the more favorable the hydrogenation step and the higher theoretical ammonia synthesis rate. This relationship is further supported by Winter et al. [98] through in situ characterization. They found that Ni/Al₂O₃ promoted plasma NH₃ production and favored surface-adsorbed NH_x species, while Fe/Al₂O₃ exhibited the presence of N₂H_y and a lower total concentration of nitrogenous adsorbates.

Mehta et al. [73] constructed a microscopic model using density-functional theory to forecast the surface processes involved in plasma-assisted ammonia synthesis reactions. Their comparison of the rates of plasma-assisted ammonia synthesis reactions at 473 K and 101.325 kPa with thermocatalytic rates, shown in Figs. 4(a) and 4(b), highlighted a significant difference. The peak plasma reaction rate, depicted as the peak of the volcano curve, was several orders of magnitude higher than that of thermal catalysis for both types of reactions. Notably, for the step position (Fig. 4(b)), the volcano curve of plasma reactions exhibited considerably higher theoretical reaction rates for metals with a lower nitrophilicity such as Co and Ni. When comparing the atmospheric pressure non-thermal plasma-volcano curves with the high temperature and high-pressure H−B catalytic curves, it was found that the optimal plasma rate on the Ru catalyst was already comparable to the rate on the optimal thermal catalyst at this juncture.

Barboun et al. [89] conducted experiments to evaluate the plasma nitrogen fixation efficiency of three metals (Ni, Co, and Ru) supported on porous Al₂O₃ with varying particle sizes, as illustrated in Fig. 4(c). The catalytic performance of Co and Ni was found to be significantly higher compared to Ru, contrary to theoretical predictions based on volcano plots. This discrepancy may be attributed to suboptimal reaction temperatures during the experiments. In a separate study, Wang et al. [99] observed a notably higher rate of ammonia synthesis reaction with less nitrophilic metals like Ni and Cu in comparison to more nitrophilic metals such as Fe, when used as active metals (Fig. 4(d)). It is important to note that Liu et al. [100] demonstrated that the presence of a bimetal reduced both the total number and intensity of acidic sites on the catalyst surface, leading to improved NH₃ desorption. Additionally, the bimetal presence enhanced the discharge effect of plasma, resulting in an increased efficiency of ammonia synthesis. However, further research is needed to fully understand the surface reaction mechanism and synergistic catalytic effects of the bimetals.

Current studies have consistently shown that the L−R reaction on nitrophilic metals like Fe cannot compete with the plasma-assisted ammonia synthesis reaction. This results in the loss of the primary advantage of the nitrophilic metal, which is promoting the dissociation of N₂ on the surface. As a result, the enhancement of plasma-assisted ammonia synthesis performance is not significant. On the other hand, nitrogen-phobic metals have a weak adsorption of both N₂ and NH₃, leading to a poor performance in the traditional H−B process due to higher surface N₂ dissociation energy barriers. However, in plasma-assisted ammonia synthesis, N₂ is mainly dissociated through the excitation of high-energy electrons. The exceptional NH₃ desorption of nitrogen-phobic metals has emerged as a critical factor in enhancing their performance in ammonia synthesis, which breaks the famous “linear scaling relationship” of ammonia synthesis.

Researchers have extensively investigated the relationship between relevant carriers and active sites, as well as developed catalysts with high activity, stability, and recyclability. However, many catalyst designs are primarily based on trial and error. Therefore, future research should prioritize a deeper exploration of the catalytic mechanism inherent to the catalyst itself, and a comprehensive understanding of the hydrogenation and ammonia production pathways in plasma nitrogen fixation reactions through in situ characterization and other methodologies. This approach will provide valuable insights into inform catalyst design, such as elucidating the pore structure in the plasma environment and the impact of plasma reactions on the catalyst surface.

4 Reaction pathways

4.1 N₂ and H₂

N and H are the direct constituents of NH₃, and the synthesis of ammonia through the plasma reaction of N₂ and H₂ can effectively prevent competing reactions and achieve the highest theoretical rate of nitrogen fixation. Rouwenhorst et al. [101] categorized the plasma nitrogen fixation reaction into four pathways. The first pathway involves the generation of ammonia through random combinations of free radicals in the atmosphere, known as the plasma phase reaction (Fig. 4(e)). The second pathway occurs when plasma species react on the catalyst surface, leading to hydrogenation reactions and ammonia production on the catalyst surface (Fig. 4(f)). Both pathways are diffusion-limited and non-selective processes. The third pathway, semi-catalytic ammonia synthesis (Fig. 4(g)), involves N and H reacting differently, with N forming radicals before adsorbing on the surface and H₂ dissociating directly on the catalyst. The surface sites of the catalyst are occupied by intermediates and H, hindering the dissociation of N₂. The fourth pathway, direct plasma-enhanced synthesis of ammonia (Fig. 4(h)), includes adsorption of both H₂ and N₂ on the catalyst surface before dissociation. Vibrational excitation of N occurs first in an atmospheric plasma environment, followed by the final breaking of the bond between the nitrogen atoms and the dissociation of H₂. The subsequent hydrogenation process for ammonia production and desorption of NH₃ occurs at the active site, which is not affected by the plasma. The plasma reaction for ammonia synthesis, as currently recognized by most, is a combination of the first pathway and the fourth pathway [102–104]. Barboun et al. [89] distinguished between the contributions of the plasma-phase reaction, the thermo-catalytic reaction, and the plasma-enhanced catalytic ammonia synthesis. They computed reaction rate formulas for all three, which were generally consistent with the experimental results. The theoretical calculations of Mehta et al. [73] using the fourth pathway also showed a good agreement with experimental data.

While both H₂ and N₂ dissociate at the catalyst surface, their reaction rates differ. Theoretically, the highest rate of ammonia synthesis occurs at the N₂ to H₂ ratio of 1:3, reflecting the molar ratio of N and H in NH₃. However, Rouwenhorst et al. [85] observed peak nitrogen fixation rates at H₂ and N₂ ratios of 1:1 and 1:2, with optimal reaction ratios falling between 1:1 and 1:2. Similarly, Peng et al. [105] reported that ammonia synthesis rates were highest at the N₂ share of 0.625, with the maximum energy efficiency at 0.5 [105]. This is different from what is expected because the N₂-rich environment requires a higher breakdown voltage than that in the H₂-rich environment. However, a higher H₂ content results in a lower breakdown voltage, and at the same time a large number of H radicals are produced, far more than the amount of produced N radicals. This imbalance results in the limitation of the reaction rate by the number of N radicals. On the other hand, in environments with elevated N₂ occupancy, while the breakdown reaction becomes more challenging and the number of H radicals decreases rapidly, the number of N radicals increases. This results in the ratio of N radicals to H radicals reaching an optimal ratio of 3:1 in the NH₃ molecule at higher N₂ occupancy, since much more energy is required for the dissociation of N₂ than that for the dissociation of H₂.

4.2 Air as a nitrogen source

Plasma can efficiently convert stabilized N₂ molecules into more reactive nitrogen oxides like {\text{NO}}_3^{-} and {\text{NO}}_2^{-}. Nitrogen oxides can undergo reduction to NH₃ through two distinct pathways: electron transfer reduction and atomic hydrogen reduction [106]. The electrochemical nitrate reduction reaction via electron transfer initiates with nitrate adsorption on the electrode surface, followed by its reduction to nitrite. Nitrite, highly reactive on the electrode surface, generates the adsorbed NO. This adsorbed NO can then be further reduced to {\text{NH}}_4^{+} as the final product [107]. Alternatively, nitrate reduction can be facilitated by atomic hydrogen (H(ads)), which is generated through water reduction in the Volmer process [106,108]. Atomic hydrogen, a potent reducing agent (E⁰(H/H) = −2.31 V vs. standard hydrogen electrode (SHE)), is capable of reducing intermediate products of {\text{NO}}_2^{-} and NO. Attributed to the kinetic favorability of atomic hydrogen (adsorption)-mediated N−H bond formation over N−N bond formation, the primary end product of this process is ammonia [109]. These NO_x species can easily produce NH₃ through electrochemical methods compared to the direct electrocatalytic nitrogen reduction reaction (eNRR) process, which has a higher reaction rate and selectivity. The higher standard reduction potential of NO₃^–RR (E⁰, 0.69 V) in comparison to eNRR (0.093 V) makes e{\text{NO}}_3^{-}RR thermodynamically more favorable [110]. Additionally, the larger standard reduction potential difference between e{\text{NO}}_3^{-}RR and the hydrogenolysis reaction (HER) (0.69 V) allows e{\text{NO}}_3^{-}RR to operate over a wider range of reduction potentials without interference from competing HER side reactions, enhancing selectivity. By generating NO_x directly from the air in a single step, the conventional N₂ and H₂ preparation processes, which typically require more energy, are bypassed. Consequently, the combination of converting air to NO_x through plasma catalysis followed by electrochemical reduction to NH₃ has a great promise for large-scale environmentally friendly ammonia production.

Ren et al. [110] conducted a study on an integrated plasma-electrochemical process system to investigate the activation and recombination processes of N₂ and O₂ molecules (Fig. 5(a)). The concentration of NO_x produced in the plasma system was found to be dependent on the discharge length and the volume ratio of N₂ and O₂ feed gases. The system demonstrated a high NH₃ yield of approximately 40 nmol/(s·cm²) and a Faraday efficiency of nearly 90%, surpassing other electrochemical methods in the literature (Fig. 5(b)). Li et al. [111] implemented a similar system utilizing a core-shell nickel boride electrocatalyst with a boron-enriched surface to enhance NH₃ yield by improving NO_x adsorption, inhibiting hydrogen precipitation, and enhancing surface Ni oxidation for increased activity and selectivity. This system achieved an ammonia yield of up to 198.3 μmol/(h·cm²) and a Faraday efficiency close to 100%. Sun et al. [112] were able to convert NO_x intermediates to ammonia at a rate of 23.2 mg/h (42.1 nmol/(s·cm²)) using a homemade scalable reaction tank. Liu et al. [113] proposed a non-thermal plasma-assisted nitrogen fixation (NTPNF) atmospheric liquid contact process based on a water-fall film medium blocking discharge, in which air and H₂O were directly used as feedstock, achieving a high synthesis rate (198.3 μmol/min) and a low energy consumption (39.6 MJ/mol). In brief, the coupled plasma and electrochemical system allowing direct access to industrially required NH₃ from air is unrivaled and attractive.

4.3 N₂ and H₂O

When using it as the hydrogen source for plasma nitrogen fixation reactions, H₂O can effectively reduce energy and raw material consumption compared to traditional methods like electrolysis of water or methane steam reforming. This approach allows for a higher reaction rate without being limited by the hydrogen production process. However, a drawback is the inability to directly synthesize NH₃ in a single step, resulting in the formation of most NO_x products. According to Zhou et al. [115], the decomposition of H₂O generates H and OH radicals, with OH radicals playing a crucial role in the ammonia synthesis reaction. H₂O serves as a hydrogen source in two main ways: direct participation in the plasma process by mixing H₂O vapor with N₂ gas for discharge, leading to H atom dissociation and subsequent ammonia synthesis [116]; or involvement in the ammonia synthesis reaction post-plasma, such as in the case of nitrogen plasma jet reacting with water [117]. In this scenario, only a small amount of H radicals is produced at the plasma-liquid interface. For instance, Sakakura et al. [114] explored nitrogen immobilization and its conversion between reduction and oxidation in plasma/liquid interface reactions, successfully generating nitrogen oxides through plasma-excited reaction of N₂ with liquid H₂O. The balance between oxidizing and reducing reactions can be controlled by manipulating the reactive nitrogen species (Fig. 5(c)). Haruyama et al. [118] constructed a system for one-step noncatalytic synthesis of ammonia from atmosphere (a gas mixture containing 78% N₂ + 21% O₂ + traces of other gases) and water (H₂O) to produce 11.7 μmol of ammonia in 10 min.

The principles and advantages of the three plasma nitrogen fixation pathways are shown in Fig. 6. Considering that both the latter two methods can only convert N₂ into NO_x and require additional reactions to yield NH₃, the primary pathway involving the direct generation of NH₃ through the reaction of N₂ and H₂ plasma remains the most widely used approach in current research. This method offers advantages such as high purity of the product, rapid reaction rates, and a high conversion efficiency. It has achieved an energy efficiency that is more than 20 times higher than that achieved without a catalyst, making it a promising option for environmentally friendly ammonia production on an industrial scale. However, the actual implementation of this method still relies on further advancements in the efficiency of plasma nitrogen fixation and improvements in the hydrogen production process.

5 Evaluation of economization application

Although the H−B process technology is well-established and numerous new catalysts are being developed, it still faces challenges in overcoming the kinetic limitations of high temperature and pressure conditions. Industrial ammonia synthesis remains energy-intensive, heavily reliant on fossil fuels. Current research is increasingly focused on alternative ammonia synthesis processes that can easily integrate with renewable energy sources. Plasma-assisted ammonia synthesis, a novel clean-energy technology, shows promise due to its ability to utilize distributed energy sources and water electrolysis for hydrogen production, potentially surpassing the efficiency of the traditional H−B process. Unlike the H−B process, plasma-assisted ammonia synthesis leverages high-energy electrons to enhance catalyst surface reactions without the need for large-scale equipment or extreme reaction conditions. This process can be initiated with a small plasma generator and seamlessly integrated with renewable energy sources like wind power, reducing energy consumption during product transportation. However, the high energy consumption of plasma nitrogen fixation poses a significant challenge, requiring further evaluation of its energy efficiency, CO₂ emissions, and ammonia pricing competitiveness.

Winter et al. [119] conducted a comparative analysis of various processes in terms of energy consumption, CO₂ emissions, and price per kg of NH₃. The study utilized the most recent experimental efficiency data available (28.0 \mathrm{k}\mathrm{W}\mathrm{h}/\mathrm{k}{\mathrm{g}}_{\mathrm{N}{\mathrm{H}}_3}) [64]. The results illustrated in Fig. 7(a) indicate that the energy efficiency of the plasma process consistently lags behind that of SMR + HB, unless the efficiency of PL is enhanced significantly. This suggests that, with current plasma efficiency levels, it remains less competitive than the traditional H−B process utilizing methane vapor reforming. Moreover, as depicted in Fig. 7(b), when considering CO₂ emissions and pricing with both natural gas and renewable energy sources, plasma nitrogen fixation emits lower CO₂ levels compared to SMR + HB at existing efficiencies, regardless of whether hydrogen is produced from water electrolysis or methane steam reforming. However, if electricity is generated from natural gas, plasma nitrogen fixation using methane steam reforming will emit more CO₂ than H−B process unless the efficiency is boosted by a factor of 28.5. Additionally, natural gas-driven EC + PL results in higher CO₂ emissions compared to other scenarios examined [119].

When comparing prices based on energy costs (Fig. 7(c)), it is evident that plasma-based ammonia synthesis utilizing renewable energy electrolyzed water for hydrogen production would need a 6.4-fold improvement in efficiency to economically rival the conventional H−B process. Similarly, methane-to-hydrogen reforming with renewable energy-driven plasma would require a 17.6-fold enhancement, and a natural gas-powered plasma nitrogen fixation process would necessitate a 25.5-fold improvement. Given the current technological landscape and simplified economic considerations, it appears unlikely that plasma-based technologies could compete with traditional ammonia synthesis methods. Nonetheless, achieving a less than 10-fold efficiency enhancement could render renewable energy-driven EC + PL economically competitive with the H−B process while reducing CO₂ emissions significantly. Furthermore, a 20-fold improvement could position SMR + PL as a viable competitor to the H−B process while emitting slightly lower levels of CO₂ [119].

As shown in Fig. 7(d), at the highest efficiencies reported to date, both processes using plasmas powered by any fossil electricity emit more CO₂ than the H−B process until the electricity is primarily derived from CO₂-free renewable energy sources (< 0.06 \mathrm{k}{\mathrm{g}}_{\mathrm{C}{\mathrm{O}}_2}/kWh). A plasma powered by grid-supplied electricity from methane steam reforming to hydrogen would require a 20- to 30-fold improvement in energy efficiency to match or reduce the CO₂ emissions of the H−B process. Unless CO₂-free energy sources supply most of the electricity, electricity-driven EC + PL will continue to emit more CO₂ than the H−B process. At the same time even under the maximum carbon pricing scenario, the H−B process is currently still much more economical than any of the alternatives considered (Fig. 7(e)). However, assuming a 10-fold increase in plasma efficiency, renewables-driven EC + PL would be cheaper than the H−B process even without a carbon price. Ammonia produced by coupling methane steam reforming with renewable energy-driven plasma is less expensive than ammonia synthesized by the H−B process at a carbon price of at least 0.072 $/\mathrm{k}{\mathrm{g}}_{\mathrm{C}{\mathrm{O}}_2} [119].

Rouwenhorst & Lefferts [120] also compared the process costs of plasma nitrogen fixation and the H−B process. Even with the most cost-effective equipment available, a plasma nitrogen fixation circuit costs more than 10 times as much as a conventional H−B synthesis circuit. This is due to the need for large compressors to perform multiple cycles. In addition, expensive ammonia separation equipment is required due to the low NH₃ concentration in the off-gas. Plasma synthesis circuits can be operated at low pressures and therefore do not require a feed compressor, but current plasma-assisted ammonia synthesis technology has a low unidirectional conversion of about 1%, and therefore requires more expensive recirculation compressors.

The plasma-assisted ammonia synthesis process currently lags behind the H−B process in terms of efficiency and conversion rates. However, as advancements in technology lead to improved efficiencies and lower operating costs, the appeal of plasma-assisted ammonia synthesis is expected to grow. This is particularly relevant in the ongoing shift from fossil fuels to renewable energy sources, where a zero-carbon process like plasma-assisted ammonia synthesis, powered by green energy, will likely gain more traction.

6 Conclusions and outlook

After years of rapid advancement, the mechanisms involved in the current plasma-assisted ammonia synthesis process have been increasingly understood, leading to enhanced reaction rates and energy efficiency. With the growing emphasis on carbon neutrality, the technology of green ammonia synthesis using entirely renewable energy sources is gaining significant attention. Nevertheless, the plasma nitrogen fixation technology still grapples with various challenges in both fundamental scientific comprehension and practical engineering applications. Key areas needing urgent investigation include:

1) The microscopic discharge mechanism at the nanoscale pore level is unclear. According to the Debye length theory, the range of influence of the electric field on any charge in a plasma is limited, and interaction forces between two charged particles in a plasma occur only when their distance is less than the Debye length. Whether the discharge reaction can occur in the pores of mesoporous materials at the micrometre or even nanometre scale deserves further exploration.

2) The interaction between plasma species and catalysts remains ambiguous. Reactants produced in the plasma phase impact the catalyst surface, influencing its electronic and chemical properties like adsorption probability, oxidation state, and work function. Moreover, the presence of the catalyst in the plasma region can alter the electric field and discharge type, subsequently affecting the physical and chemical properties of the plasma. This intricate interaction poses challenges to quantitative analysis.

3) The energy efficiency and conversion rates of catalysts remain low, despite a nearly 10-fold improvement in the past decade. Significant enhancements in catalyst efficiency are required, around 10 to 20 times higher than current levels, to compete with the H−B process on a large scale. Achieving this goal depends on continuous advancements in reaction mechanisms and material properties.

Ammonia synthesis using non-thermal medium-blocked discharge plasma at atmospheric pressure, as a zero-carbon technology that does not rely on fossil energy, holds great promise for future development. Advancements in understanding and breakthroughs in technology will enhance the industrialization of plasma-based ammonia synthesis and support the achievement of the “dual-carbon” goal.

References

[1] Liu L, Xu W, Wen Z. . Modeling global oceanic nitrogen deposition from food systems and its mitigation potential by reducing overuse of fertilizers.Proceedings of the National Academy of Sciences of the United States of America, 2023, 120(17): 2221459120

[2] Palys M J, Daoutidis P. Optimizing renewable ammonia production for a sustainable fertilizer supply chain transition.ChemSusChem, 2023, 16(22): 202300563

[3] Mushtaq M A, Kumar A, Liu W. . A metal coordination number determined catalytic performance in manganese borides for ambient electrolysis of nitrogen to ammonia.Advanced Materials, 2024, 36(21): 2313086

[4] Yang P, Gong F, Liu C. . Mechanistic insights into the stepwise lithium-mediated electrochemical nitrogen reduction for enhanced ammonia synthesis.Chemical Engineering Journal, 2024, 488: 151098

[5] Joseph Sekhar S, Said Ahmed Al-Shahri A, Glivin G. . A critical review of the state-of-the-art green ammonia production technologies—Mechanism, advancement, challenges, and future potential.Fuel, 2024, 358: 130307

[6] Fu E, Gong F, Wang S. . Ni-Mn-N derived composite nitrogen carriers for enhanced chemical looping ammonia production.Fuel Processing Technology, 2023, 252: 107971

[7] Liu X, Liu C, He X. . Fe-doped Co₃O₄ nanowire strutted 3D pinewood-derived carbon: A highly selective electrocatalyst for ammonia production via nitrate reduction.Nano Research, 2023, 17(4): 2276–2284

[8] Duong P A, Ryu B R, Song M K. . Safety assessment of the ammonia bunkering process in the maritime sector: A review.Energies, 2023, 16(10): 4019

[9] Wang S, Gong F, Zhou Q. . Transition metal enhanced chromium nitride as composite nitrogen carrier for sustainable chemical looping ammonia synthesis.Applied Catalysis B: Environmental, 2023, 339: 123134

[10] Aziz M, Juangsa F B, Irhamna A R. . Ammonia utilization technology for thermal power generation: A review.Journal of the Energy Institute, 2023, 111: 101365

[11] Adeniyi A, Bello I, Mukaila T. . Trends in biological ammonia production.BioTech, 2023, 12(2): 41

[12] Liang J, Li Z, Zhang L. . Advances in ammonia electrosynthesis from ambient nitrate/nitrite reduction.Chem, 2023, 9(7): 1768–1827

[13] Liu Q, Xu T, Luo Y. . Recent advances in strategies for highly selective electrocatalytic N₂ reduction toward ambient NH₃ synthesis.Current Opinion in Electrochemistry, 2021, 29: 100766

[14] Kafle K, Greeson K, Lee C. . Cellulose polymorphs and physical properties of cotton fabrics processed with commercial textile mills for mercerization and liquid ammonia treatments.Sage Journals, 2014, 86(16): 1692–9

[15] Liu H, Ji X, Guo Z. . A high-current hydrogel generator with engineered mechanoionic asymmetry.Nature Communications, 2024, 15(1): 1494

[16] Qi Y, Liu W, Liu S. . A review on ammonia-hydrogen fueled internal combustion engines.eTransportation, 2023, 18: 100288

[17] Tian J, Wang L, Xiong Y. . Enhancing combustion efficiency and reducing nitrogen oxide emissions from ammonia combustion: A comprehensive review.Process Safety and Environmental Protection, 2024, 183: 514–543

[18] Macfarlane D R, Cherepanov P V, Choi J. . A roadmap to the ammonia economy.Joule, 2020, 4(6): 1186–1205

[19] Yu S, Xiang T, Alharbi N S. . Recent development of catalytic strategies for sustainable ammonia production.Chinese Journal of Chemical Engineering, 2023, 62: 65–113

[20] Li T, Duan Y, Wang Y. . Research progress of ammonia combustion toward low carbon energy.Fuel Processing Technology, 2023, 248: 107821

[21] Kim D, Surendran S, Janani G. . Nitrogen-impregnated carbon-coated TiO₂ nanoparticles for N₂ reduction to ammonia under ambient conditions.Materials Letters, 2022, 314: 131808

[22] Akay G, Zhang K. Process intensification in ammonia synthesis using novel coassembled supported microporous catalysts promoted by nonthermal plasma.Industrial & Engineering Chemistry Research, 2017, 56(2): 457–468

[23] Yu Y, Geng M, Wei D. . Effect of potassium on the performance of a CuSO₄/TiO₂ catalyst used in the selective catalytic reduction of NO_x by NH₃.Acta Physico-Chimica Sinica, 2022, 39(4): 2206034

[24] Kim D, Surendran S, Lim Y. . Spinel-type Ni₂GeO₄ electrocatalyst for electrochemical ammonia synthesis via nitrogen reduction reaction under ambient conditions.International Journal of Energy Research, 2022, 46(4): 4119–4129

[25] Murakami K, Manabe R, Nakatsubo H. . Elucidation of the role of electric field on low temperature ammonia synthesis using isotopes.Catalysis Today, 2018, 303: 271–275

[26] Ogo S, Sekine Y. Catalytic reaction assisted by plasma or electric field.Chemical Record, 2017, 17(8): 726–738

[27] Aziz M, Wijayanta A T, Nandiyanto A B D. Ammonia as effective hydrogen storage: A review on production, storage and utilization.Energies, 2020, 13(12): 3062

[28] Ge Y, Yang Z, He H. . Risk evaluation of ammonia leakage based on modified probability calculation formulas.Journal of Thermal Science, 2023, 32(2): 854–865

[29] Aihara K, Akiyama M, Deguchi T. . Remarkable catalysis of a wool-like copper electrode for NH₃ synthesis from N₂ and H₂ in non-thermal atmospheric plasma.Chemical Communications, 2016, 52(93): 13560–13563

[30] Liu N, Sun Z, Zhang H. . Emerging high-ammonia-nitrogen wastewater remediation by biological treatment and photocatalysis techniques.Science of the Total Environment, 2023, 875: 162603

[31] Patil B S, Wang Q, Hessel V. . Plasma N₂-fixation: 1900–2014.Catalysis Today, 2015, 256: 49–66

[32] Kandemir T, Schuster M E, Senyshyn A. . The Haber–Bosch process revisited: On the real structure and stability of “ammonia iron” under working conditions.Angewandte Chemie International Edition, 2013, 52(48): 12723–12726

[33] Lee K, Liu X, Vyawahare P. . Techno-economic performances and life cycle greenhouse gas emissions of various ammonia production pathways including conventional, carbon-capturing, nuclear-powered, and renewable production.Green Chemistry, 2022, 24(12): 4830–4844

[34] Zhou Q, Gong F, Xie Y. . 1+1>2: Learning from the interfacial modulation on single-atom electrocatalysts to design dual-atom electrocatalysts for dinitrogen reduction.Green Energy & Environment, 2023, 8(6): 1753–1763

[35] Liu X, Elgowainy A, Wang M. Life cycle energy use and greenhouse gas emissions of ammonia production from renewable resources and industrial by-products.Green Chemistry, 2020, 22(17): 5751–5761

[36] Ahmed M I, Assafiri A, Hibbert D B. . Li-mediated electrochemical nitrogen fixation: Key advances and future perspectives.Small, 2023, 19(52): 2305616

[37] M. Nguyen H, Omidkar A, Song H.Technical challenges and prospects in sustainable plasma catalytic ammonia production from methane and nitrogen. ChemPlusChem, 2023, 88(7): 202300129

[38] Bogaerts A, Neyts E C. Plasma technology: An emerging technology for energy storage.ACS Energy Letters, 2018, 3(4): 1013–1027

[39] Li Z, Zhou Q, Liang J. . Defective TiO_2–x for high-performance electrocatalytic NO reduction toward ambient NH₃ production.Small, 2023, 19(24): 2300291

[40] Li K, Chen S, Li M. . Plasma-catalyzed ammonia synthesis over La(OH)₃ catalyst: Effects of basic sites, oxygen vacancies, and H₂ plasma treatment.International Journal of Hydrogen Energy, 2024, 59: 1287–1296

[41] Zhou G, Zhao H, Wang X. . Plasma-catalytic ammonia synthesis on Ni catalysts supported on Al₂O₃, Si-MCM-41 and SiO₂.International Journal of Hydrogen Energy, 2024, 60: 802–813

[42] Kamarinopoulou N S W, Wittreich G R, Vlachos D G. Direct HCN synthesis via plasma-assisted conversion of methane and nitrogen.Science Advances, 2024, 10(13): eadl4246

[43] Jing Y, Gong F, Wang S. . Activating the synergistic effect in Ni-Co bimetallic MOF for enhanced plasma-assisted ammonia synthesis.Fuel, 2024, 368: 131686

[44] Giddey S, Badwal S P S, Kulkarni A. Review of electrochemical ammonia production technologies and materials.International Journal of Hydrogen Energy, 2013, 38(34): 14576–14594

[45] Yang B, Ding W, Zhang H. . Recent progress in electrochemical synthesis of ammonia from nitrogen: Strategies to improve the catalytic activity and selectivity.Energy & Environmental Science, 2021, 14(2): 672–687

[46] Zhou Q, Gong F, Xie Y. . A general strategy for designing metal-free catalysts for highly-efficient nitric oxide reduction to ammonia.Fuel, 2022, 310: 122442

[47] Liang J, Zhou Q, Mou T. . FeP nanorod array: A high-efficiency catalyst for electroreduction of NO to NH₃ under ambient conditions.Nano Research, 2022, 15(5): 4008–4013

[48] He X, Li Z, Yao J. . High-efficiency electrocatalytic nitrite reduction toward ammonia synthesis on CoP@TiO₂ nanoribbon array.iScience, 2023, 26(7): 107100

[49] Cai Z, Zhao D, Fan X. . Rational construction of heterostructured Cu₃P@TiO₂ nanoarray for high-efficiency electrochemical nitrite reduction to ammonia.Small, 2023, 19(30): 2300620

[50] Fan X, Zhao D, Deng Z. . Constructing Co@TiO₂ nanoarray heterostructure with Schottky contact for selective electrocatalytic nitrate reduction to ammonia.Small, 2023, 19(17): 2208036

[51] Yu M S, Jesudass S C, Surendran S. . Synergistic interaction of MoS₂ nanoflakes on La₂Zr₂O₇ nanofibers for improving photoelectrochemical nitrogen reduction.ACS Applied Materials & Interfaces, 2022, 14(28): 31889–31899

[52] An T Y, Surendran S, Jesudass S C. . Promoting electrochemical ammonia synthesis by synergized performances of Mo₂C-Mo₂N heterostructure.Frontiers in Chemistry, 2023, 11: 1122150

[53] Mahmud K, Makaju S, Ibrahim R. . Current progress in nitrogen fixing plants and microbiome research.Plants, 2020, 9(1): 97

[54] Mus F, Crook M B, Garcia K. . Symbiotic nitrogen fixation and the challenges to its extension to nonlegumes.Applied and Environmental Microbiology, 2016, 82(13): 3698–3710

[55] Wang W L, Moore J K, Martiny A C. . Convergent estimates of marine nitrogen fixation.Nature, 2019, 566(7743): 205–211

[56] Bo Y, Wang H, Lin Y. . Altering hydrogenation pathways in photocatalytic nitrogen fixation by tuning local electronic structure of oxygen vacancy with dopant.Angewandte Chemie International Edition, 2021, 60(29): 16085–16092

[57] Dong G, Ho W, Wang C. Selective photocatalytic N₂ fixation dependent on g-C₃N₄ induced by nitrogen vacancies.Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2015, 3(46): 23435–23441

[58] Li P, Zhou Z, Wang Q. . Visible-light-driven nitrogen fixation catalyzed by Bi₅O₇Br nanostructures: Enhanced performance by oxygen vacancies.Journal of the American Chemical Society, 2020, 142(28): 12430–12439

[59] Feng S, Gao W, Wang Q. . A multi-functional composite nitrogen carrier for ammonia production via a chemical looping route.Journal of Materials Chemistry. A, Materials for Energy and Sustainability, 2021, 9(2): 1039–1047

[60] Chen J G, Crooks R M, Seefeldt L C. . Beyond fossil fuel-driven nitrogen transformations.Science, 2018, 360(6391): eaar6611

[61] Gao W, Guo J, Wang P. . Production of ammonia via a chemical looping process based on metal imides as nitrogen carriers.Nature Energy, 2018, 3(12): 1067–1075

[62] Fu E, Gong F, Wang S. . Chemical looping technology in mild-condition ammonia production: A comprehensive review and analysis.Small, 2024, 20(1): 2305095

[63] Zhang J, Li X, Zheng J. . Non-thermal plasma-assisted ammonia production: A review.Energy Conversion and Management, 2023, 293: 117482

[64] Kim H H, Teramoto Y, Ogata A. . Atmospheric-pressure nonthermal plasma synthesis of ammonia over ruthenium catalysts.Plasma Processes and Polymers, 2017, 14(6): 1600157

[65] Shao K, Mesbah A. A study on the role of electric field in low-temperature plasma catalytic ammonia synthesis via integrated density functional theory and microkinetic modeling.JACS Au, 2024, 4(2): 525–544

[66] Neyts E C, Ostrikov K K, Sunkara M K. . Plasma matalysis: Synergistic effects at the nanoscale.Chemical Reviews, 2015, 115(24): 13408–13446

[67] Zhao L, Wang W, Zhou W. . Experimental investigation of DBD parameters and Ru/γ-Al₂O₃ catalyst on plasma-assisted ammonia synthesis.IEEE Transactions on Plasma Science, 2023, 51(12): 3538–3545

[68] Nguyen H M, Gorky F, Guthrie S. . Sustainable ammonia synthesis from nitrogen wet with sea water by single-step plasma catalysis.Catalysis Today, 2023, 418: 114141

[69] Gorky F, Guthrie S R, Smoljan C S. . Plasma ammonia synthesis over mesoporous silica SBA-15.Journal of Physics. D, Applied Physics, 2021, 54(26): 264003

[70] Liu J, Zhu X, Hu X. . Plasma-assisted ammonia synthesis in a packed-bed dielectric barrier discharge reactor: Roles of dielectric constant and thermal conductivity of packing materials.Plasma Science & Technology, 2022, 24(2): 025503

[71] Zhu X, Hu X, Wu X. . Ammonia synthesis over γ-Al₂O₃ pellets in a packed-bed dielectric barrier discharge reactor.Journal of Physics. D, Applied Physics, 2020, 53(16): 164002

[72] Wu H, Yang L, Wen J. . Plasma-driven nitrogen fixation on sodium hydride.Advanced Energy Materials, 2023, 13(27): 2300722

[73] Mehta P, Barboun P, Herrera F A. . Overcoming ammonia synthesis scaling relations with plasma-enabled catalysis.Nature Catalysis, 2018, 1(4): 269–275

[74] Liu T W, Gorky F, Carreon M L. . Energetics of reaction pathways enabled by N and H radicals during catalytic, plasma-assisted NH₃ synthesis.ACS Sustainable Chemistry & Engineering, 2022, 10(6): 2034–2051

[75] Gorky F, Best A, Jasinski J. . Plasma catalytic ammonia synthesis on Ni nanoparticles: The size effect.Journal of Catalysis, 2021, 393: 369–380

[76] Lamichhane P, Paneru R, Nguyen L N. . Plasma-assisted nitrogen fixation in water with various metals.Reaction Chemistry & Engineering, 2020, 5(11): 2053–2057

[77] Nakajima J, Sekiguchi H. Synthesis of ammonia using microwave discharge at atmospheric pressure.Thin Solid Films, 2008, 516(13): 4446–4451

[78] Bogaerts A, Tu X, Whitehead J C. . The 2020 plasma catalysis roadmap.Journal of Physics. D, Applied Physics, 2020, 53(44): 443001

[79] Wang X, Du X, Chen K. . Predicting the ammonia synthesis performance of plasma catalysis using an artificial neural network model.ACS Sustainable Chemistry & Engineering, 2023, 11(12): 4543–4554

[80] Qu Z, Zhou R, Sun J. . Plasma-assisted sustainable nitrogen-to-ammonia fixation: Mixed-phase, synergistic processes and mechanisms.ChemSusChem, 2023, 17(6): e202300783

[81] Peng P, Li Y, Cheng Y. . Atmospheric pressure ammonia synthesis using non-thermal plasma assisted catalysis.Plasma Chemistry and Plasma Processing, 2016, 36(5): 1201–1210

[82] Shah J, Wu T, Lucero J. . Nonthermal plasma synthesis of ammonia over Ni-MOF-74.ACS Sustainable Chemistry & Engineering, 2019, 7(1): 377–383

[83] Iwamoto M, Akiyama M, Aihara K. . Ammonia synthesis on wool-like Au, Pt, Pd, Ag, or Cu electrode catalysts in nonthermal atmospheric-pressure plasma of N₂ and H₂.ACS Catalysis, 2017, 7(10): 6924–6929

[84] Hosseini H. Dielectric barrier discharge plasma catalysis as an alternative approach for the synthesis of ammonia: A review.RSC Advances, 2023, 13(40): 28211–28223

[85] Rouwenhorst K H R, Mani S, Lefferts L. Improving the energy yield of plasma-based ammonia synthesis with in situ adsorption.ACS Sustainable Chemistry & Engineering, 2022, 10(6): 1994–2000

[86] Gorky F, Lucero J M, Crawford J M. . Plasma-induced catalytic conversion of nitrogen and hydrogen to ammonia over zeolitic imidazolate frameworks ZIF-8 and ZIF-67.ACS Applied Materials & Interfaces, 2021, 13(18): 21338–21348

[87] GorkyFNamboACarreonM A, . Plasma catalytic conversion of nitrogen and hydrogen to ammonia over silico alumino phosphate (SAPO) zeolites. Plasma Chemistry and Plasma Processing, 2023, early access, https://doi.org/10.1007/s11090-023-10397-w

[88] Guo H, Wang M, Liu J. . Facile synthesis of nanoscale high porosity IR-MOFs for low-k dielectrics thin films.Microporous and Mesoporous Materials, 2016, 221: 40–47

[89] Barboun P, Mehta P, Herrera F A. . Distinguishing plasma contributions to catalyst performance in plasma-assisted ammonia synthesis.ACS Sustainable Chemistry & Engineering, 2019, 7(9): 8621–8630

[90] Chen Z, Koel B E, Sundaresan S. Plasma-assisted catalysis for ammonia synthesis in a dielectric barrier discharge reactor: Key surface reaction steps and potential causes of low energy yield.Journal of Physics. D, Applied Physics, 2022, 55(5): 055202

[91] Hu X, Zhu X, Wu X. . Plasma-enhanced NH₃ synthesis over activated carbon-based catalysts: Effect of active metal phase.Plasma Processes and Polymers, 2020, 17(12): 2000072

[92] Peng P, Cheng Y, Hatzenbeller R. . Ru-based multifunctional mesoporous catalyst for low-pressure and non-thermal plasma synthesis of ammonia.International Journal of Hydrogen Energy, 2017, 42(30): 19056–19066

[93] Zhao Y, Li K, Du J. . Binary heterogroup-templated scaffolds of polyoxopalladates as precatalysts for plasma-assisted ammonia synthesis.ACS Applied Materials & Interfaces, 2023, 15(37): 43899–43908

[94] Zhao H, Song G, Chen Z. . In situ identification of NNH and N₂H₂ by using molecular-beam mass spectrometry in plasma-assisted catalysis for NH₃ synthesis.ACS Energy Letters, 2021, 7(1): 53–58

[95] Li S, Shao Y, Chen H. . Nonthermal plasma catalytic ammonia synthesis over a Ni catalyst supported on MgO/SBA-15.Industrial & Engineering Chemistry Research, 2022, 61(9): 3292–3302

[96] Changhai L, Zhaobin W, Qin X. . Ammonia-treated activated carbon as support of Ru-Ba catalyst for ammonia synthesis.Reaction Kinetics and Catalysis Letters, 2004, 83: 39–45

[97] Wang Y, Yang W, Xu S. . Shielding protection by mesoporous catalysts for improving plasma-catalytic ambient ammonia synthesis.Journal of the American Chemical Society, 2022, 144(27): 12020–12031

[98] Winter L R, Ashford B, Hong J. . Identifying surface reaction intermediates in plasma catalytic ammonia synthesis.ACS Catalysis, 2020, 10(24): 14763–14774

[99] Wang Y, Craven M, Yu X. . Plasma-enhanced catalytic synthesis of ammonia over a Ni/Al₂O₃ catalyst at near-room temperature: Insights into the importance of the catalyst surface on the reaction mechanism.ACS Catalysis, 2019, 9(12): 10780–10793

[100] Liu Y, Wang C W, Xu X F. . Synergistic effect of Co–Ni bimetal on plasma catalytic ammonia synthesis.Plasma Chemistry and Plasma Processing, 2022, 42(2): 267–282

[101] Rouwenhorst K H R, Kim H H. Lefferts vibrationally excited activation of N₂ in plasma-enhanced catalytic ammonia synthesis: A kinetic analysis.ACS Sustainable Chemistry & Engineering, 2019, 7(20): 17515–17522

[102] Shao K, Mesbah A. A study on the role of electric field in low-temperature plasma catalytic ammonia synthesis via integrated density functional theory and microkinetic modeling.JACS Au, 2024, 4(2): 525–544

[103] Brown S, Ahmat Ibrahim S, Robinson B R. . Ambient carbon-neutral ammonia generation via a cyclic microwave plasma process.ACS Applied Materials & Interfaces, 2023, 15(19): 23255–23264

[104] Andersen J A, Holm M C, van ’t Veer K. . Plasma-catalytic ammonia synthesis in a dielectric barrier discharge reactor: A combined experimental study and kinetic modeling.Chemical Engineering Journal, 2023, 457: 141294

[105] Peng P, Chen P, Addy M. . Atmospheric plasma-assisted ammonia synthesis enhanced via synergistic catalytic absorption.ACS Sustainable Chemistry & Engineering, 2019, 7(1): 100–104

[106] Zhang X, Wang Y, Liu C. . Recent advances in non-noble metal electrocatalysts for nitrate reduction.Chemical Engineering Journal, 2021, 403: 126269

[107] Long J, Chen S, Zhang Y. . Direct electrochemical ammonia synthesis from nitric oxide.Angewandte Chemie International Edition, 2020, 59(24): 9711–9718

[108] Gao J, Jiang B, Ni C. . Enhanced reduction of nitrate by noble metal-free electrocatalysis on P doped three-dimensional Co₃O₄ cathode: Mechanism exploration from both experimental and DFT studies.Chemical Engineering Journal, 2020, 382: 123034

[109] Shin H, Jung S, Bae S. . Nitrite reduction mechanism on a Pd surface.Environmental Science & Technology, 2014, 48: 12475–13022

[110] Ren Y, Yu C, Wang L. . Microscopic-level insights into the mechanism of enhanced NH₃ synthesis in plasma-enabled cascade N₂ oxidation–electroreduction system.Journal of the American Chemical Society, 2022, 144(23): 10193–10200

[111] Li L, Tang C, Cui X. . Efficient nitrogen fixation to ammonia through integration of plasma oxidation with electrocatalytic reduction.Angewandte Chemie International Edition, 2021, 60(25): 14131–14137

[112] Sun J, Alam D, Daiyan R. . A hybrid plasma electrocatalytic process for sustainable ammonia production.Energy & Environmental Science, 2021, 14: 865–872

[113] Liu Z, Tian Y, Niu G. . Direct oxidative nitrogen fixation from air and H₂O by a water falling film dielectric barrier discharge reactor at ambient pressure and temperature.ChemSusChem, 2021, 14(6): 1507–1511

[114] Sakakura T, Murakami N, Takatsuji Y. . Nitrogen fixation in a plasma/liquid interfacial reaction and its switching between reduction and oxidation.Journal of Physical Chemistry C, 2020, 124(17): 9401–9408

[115] Zhou D, Zhou R, Zhou R. . Sustainable ammonia production by non-thermal plasmas: Status, mechanisms, and opportunities.Chemical Engineering Journal, 2021, 421: 129544

[116] Takahashi J, Sasaki K. Production rates and destruction frequencies of ammonia in inductively coupled H₂O/N₂ and H₂/N₂ plasmas.Contributions to Plasma Physics, 2023, 64(3): e202300167

[117] RamoyMShiraiNSasakiK. Catalyst-free ammonia synthesis using DC-driven atmospheric-pressure plasma in contact with liquid. Journal of Physics D: Applied Physics, 2024, 57(1)

[118] Haruyama T, Namise T, Shimoshimizu N. . Non-catalyzed one-step synthesis of ammonia from atmospheric air and water.Green Chemistry, 2016, 18(16): 4536–4541

[119] Winter L R, Chen J G. N₂ fixation by plasma-activated processes.Joule, 2021, 5(2): 300–315

[120] Rouwenhorst K H R, Lefferts L. Feasibility study of plasma-catalytic ammonia synthesis for energy storage applications.Catalysts, 2020, 10(9): 999

RIGHTS & PERMISSIONS

Higher Education Press 2024