Air cold plasmas as a new tool for nitrogen fixation in agriculture: underlying mechanisms and current experimental insights

Nitrogen fixation is crucial for plant growth and global agriculture, especially with the projected population growth requiring a significant increase in food production. Traditional nitrogen fixation relies on the Haber-Bosch (H-B) process, which is energy-intensive and environmentally harmful due to greenhouse gas emissions. Emerging technologies, such as cold plasma, offer promising alternatives with lower energy consumption. Cold plasma facilitates reactive nitrogen species generation under ambient conditions, potentially improving the production efficiency of nitrogen oxides (NO_x). However, optimizing cold plasma nitrogen fixation requires a synergy between experimental and theoretical approaches. Accurate input data are essential for refining theoretical models, which can then guide the design of more efficient processes. This integrated approach can leverage renewable energy, operate on smaller scales, and minimize environmental impacts, making cold plasma a sustainable solution for future nitrogen fixation needs.

1 Introduction

Nitrogen (N) is essential for plant development, as it is needed for synthesizing proteins and nucleic acids. Nitrogen fixation is a crucial process for providing these compounds. This is a big issue considering an expected population of nine billion people by 2050 that would require an expansion of global agricultural output by 70%–100% [1].

The established synthesis technology for N fertilizers relies strongly on the Haber-Bosch (H-B) process, to convert N₂ gas into biologically available ammonia (NH₃). The current industrial production of reactive N (RNS) is 120 teragrams per year (Tg yr⁻¹), twice the amount from all-natural land processes (63 Tg yr⁻¹ [2]). Fertilizer production consumes about 80% of this RNS [3], and is essential for sustaining half of the global human population [4]. However, the efficiency of RNS utilization is poor, with 50%–70% lost to the environment [5, 6]. The accumulation of excess RNS, such as NH₃, nitrous oxide (N₂O), and nitrate (NO₃⁻), in the atmosphere and ecosystems leads to serious environmental and climate issues [2, 3, 7, 8]. These issues encompass alterations in ecosystem productivity and biodiversity [9, 10], the eutrophication and nitrate pollution of freshwater [10, 11], the deterioration of ozone and air quality [11, 12], and the exacerbation of climate change due to greenhouse gases [3, 7].

At the beginning of the 20^th century, the fertilizers produced by H-B substituted the non-renewable mineral ones. Nowadays, the H–B process drains more than 1% of the globally produced energy, releasing more than 3 × 10¹¹ kg year⁻¹ of CO₂, and requiring about 2% of the global natural gas production, to synthesize the needed hydrogen for NH₃ synthesis [13–15]. Therefore, while H-B processes should be made more sustainable [16], greener strategies in nitrogen fixation should also be pursued. In Figure 1 the energy consumption in terms of MJ consumed per N mole of various processes is represented together with the calculated theoretical limits. Among the explored processes for nitrogen fixation, the most efficient is still the H-B, surpassed only by biological nitrogen fixation. Other industrial-scale processes adopted in the past like the thermal plasma-based Birkeland-Eyde, or the Frank-Caro process leading to calcium cyanamide are far more energy-consuming. Metallo-complexes fixation currently has poor efficiency and a theoretical limit far from the H-B.

Figure 1

Figure 1. Comparison of the energy consumption for nitrogen fixation with different methods (Reprinted from [15] with permission of Elsevier B.V.).

Since the 1970s, plasma technologies have been pointed to as a possible alternative for large-scale NH₃ production, in the presence of a catalyst or not. Different plasma sources have been proposed for ammonia synthesis, such as glow discharge, gliding arc, microwave, radio frequency, pulsed discharges, and dielectric barrier discharges. The current reported energy yield for the plasma and catalytic materials process is ranging from 1.71–58.8 MJ/mol N [17–33]. Best performances are achieved by pulsed discharge systems and dielectric barrier discharge reactors at atmospheric pressure that facilitates the integration of catalytic materials.

Novel approaches based on Cold Plasma are currently being extensively investigated and promise versatile applications. These innovative methods entail subjecting agricultural systems to non-thermal plasma, generating reactive species such as reactive oxygen species (ROS) and reactive nitrogen species (RNS) [34, 35]. One crucial allure of these technologies is the achievement of lower energy consumption (EC) than the H-B process, according to theoretical calculations [36]. It is our opinion, that the main advantage of cold plasma technology is related to their intrinsic non-thermal nature and the energy transfer processes involved in reactive species productions. We will try to give our opinion on the elementary processes leading to nitrogen oxide production and their potential impact on the agrifood industry of this technology reporting briefly on the current status of the ongoing research.

2 Plasma nitrogen fixation in dry air

Direct, efficient production of NO_x from air, at variance with current indirect production by NH₃ synthesis, using H-B process, would open plenty of possibilities in recycling. NO_x allows the direct synthesis of nitric acid (HNO₃) [37], which can be used for chemical fertilizers production, but also the fast maturation of natural fertilizers from livestock slurry, an environment-friendly process, which is currently being developed [38].

2.1 Plasma production of NOx from air: the mechanism

Plasma production of NO_x from air, such as in lightning, by a thermal plasma was industrially realized by the Birkeland-Eyde process. The NO_x production efficiency of a few percent limits its application to a handful of special uses thus making the process not competitive with current industrial processes.

An energy analysis [15, 36] on the use of cold plasma can however envisage a completely different development, with a theoretical maximum limit of 35% efficiency [36], exceeding the current total efficiency of industrial NO_x production [37].

Cold plasma behavior is complex and influenced by numerous parameters and conditions. For instance, electron temperatures might reach 10000 K while gas remains at 400 K and molecular vibrational temperature at 3000 K [39]. To properly study the kinetics of a cold plasma, therefore we need to know vibrational state-to-state processes involving, at least, electron-molecule (e-M) and molecule-molecule collisions. e-M processes transfer electron energy from the discharge to the internal energy, mostly vibrational, of air molecular species. Then, by molecular collisions and vibration-to-vibration (VV) exchange processes, as in N₂ (v₁)+N₂ (v₂)→N₂ (v₁-1)+N₂ (v₂+1), a significant fraction of molecules increases its vibrational energy [36, 40], provided low-lying levels are continuously replenished by e-M processes. When vibrational energy reaches oxygen dissociation energy (≈5eV), O atoms can initiate the Zel’dovich chain (Equations 1, 2), while N₂ remains mostly undissociated.

$\mathrm{O}+{\mathrm{N}}_2\left(\mathrm{v}\right)\to \text{NO}\left(\mathrm{v}’\right)+\mathrm{N}{1}^{\text{st}}{\text{Zel}}^{\prime }\text{dovich\hspace{0.17em}reaction}(1)$

$\mathrm{N}+{\mathrm{O}}_2\left(\mathrm{v}\right)\to \text{NO}\left(\mathrm{v}’\right)+\mathrm{O}{2}^{\text{nd}}{\text{Zel}}^{\prime }\text{dovich\hspace{0.17em}reaction}(2)$

This is favorable for energy efficiency because it is more efficient to make N₂ react rather than obtaining N atoms from dissociation. For the same reasons processes involving excited electronic states of N₂ and O₂ should be avoided. The ideal Zel’dovich reaction path of Equations 1, 2 is thought to be the most efficient one [36, 40]. However, many other concurrent collision processes take place, such as, for example,: vibration-to-translation (VT) deactivation processes (as in N + O₂(v)→N + O₂ (v-1)); the reverse of the Zel’dovich (Equation 1) reaction; plasma–walls vibrational deactivation collision; NO dissociation/reaction by collisions with other air species.

2.2 Cold plasma production of NOx from air: coupling modeling with experiment issue

The non-equilibrium nature of cold plasmas requires a guiding model to design experiments to investigate the possible reaction pathways occurring in the discharge systems. Therefore, chemical kinetics modelling is crucial in this context, provided the data used as input for this modelling is accurate and detailed. Up to now, no one has been able to replicate the predictions reported in Ref. [36] (except for [41]) by detailed modelling. Indeed, the main problem for modelling is the extremely approximate data taken as input. For example, the first Zel’dovich reaction Equation 1, i.e., the core of the NO production mechanism from air, has been known for many years only by empirical models. The model of Gordiets [42] considered the possibility of reaction once the reaction threshold has been reached, which occurs, at room temperature, when N₂ is at least in v = 12 state. More recent studies [43–46] contemplating the adiabatic path to reaction on the triplet potential energy surfaces (PES), showed a much more complex situation. In [45] a complete compilation of vibrationally detailed data (i.e., considering the whole vibrational ladder of reagents and products) concerning the adiabatic path has been calculated, with an excellent agreement with the experimental thermal rate. The differences in results obtained from the simple models adopted in the past and still used in the literature are huge (orders of magnitude, see [45, 47]), particularly in the energy region between vibrational levels v = 12–30 where most of the reaction occurs. Another aspect of great impact is the determination of the final NO vibration in reaction (Equation 1). In [48] a comparison of a kinetic scheme in air is presented using the same set of rate coefficients calculated in [45, 46, 49, 50], but considering state-to-state or only initially state-selected rate coefficients in (1). The results of N and NO production are qualitatively different, stressing the importance of accurate and detailed molecular dynamics calculations as input of the kinetics.

With a higher probability than reaction (Equation 1), the VT deactivation:

$\mathrm{O}+{\mathrm{N}}_2\left(v\right)\to \mathrm{O}+{\mathrm{N}}_2\left(v^{\prime }<v\right)\text{\hspace{0.17em}VT\hspace{0.17em}deactivation}(3)$

can take place starting from the same collision. This is a big issue for nitrogen fixation because it depletes the N₂ vibrational population essential for good efficiency in Equation 1. In fact, in cold plasmas, the strongly endothermic reaction (Equation 1) should be preferentially activated by vibrational pumping of molecular species involved rather than by heating, in order to keep the rate of VT processes like (Equation 3) low. However, the accurate rate value of the process (Equation 3) for v > 1 has remained unknown for decades, with only the v = 1 to v' = 0 rate coefficient known experimentally. The only known result, however, was also dramatically different (even 20 orders of magnitude at 300 K) from theoretical results obtained by quasi-classical (QCT), semiclassical (SC), and quantum methods [45, 51, 52]. Only in very recent years the problem has been solved [52, 53], by recognizing the fundamental role of vibronic transitions of the collisional system and calculating other transitions from v ≠ 1. Indeed, process (Equation 3) shows even other issues when studied theoretically. The methods used for studying molecular collisions of heavy particles can be QCT, SC, or quantum methods on a scale of increasing accuracy and computational cost. In studies on air species aiming to create vibrationally detailed databases, using accurate quantum methods is simply unfeasible. While these methods are essential for establishing benchmarks (as demonstrated in [44]), they are unsuitable for the comprehensive calculations necessary for kinetic modeling. Semiclassical and, first, QCT must be used instead, requiring huge computational resources. However, some processes are difficult to treat even with these simpler methods, due to their specific limitations. In particular, the process in Equation 3 cannot be reproduced theoretically by QCT at low total energy [54]. However, at collision energy values comparable to or higher than the reaction (Equation 1) threshold, QCT becomes suitable for accurate results for the inelastic process (Equation 3). Deactivation processes are surely present in all collision processes of interest in air plasmas. Most of the VT data used in current kinetic models of air plasmas have been calculated by methods based on a forced harmonic oscillator (FHO) semiclassical model [55] or by QCT [56, 57]. However, FHO is designed to treat non-reactive cases at not very high total energy, while QCT tends to be more accurate at higher energy, especially over the threshold for reactions originating from the collisions considered [46]. Merging of different semi- and quasi-classical methods for cold plasma modeling input data calculation is therefore desirable. This merging of methods could be successfully applied potentially to all collisional systems involving air species of interest in this context, such as N₂-O₂, from which O atoms production depends, and research is active in this sense [39, 46, 54].

Another important collisional system is of course the second Zel’dovich reaction. Even in this case, recent results in [49] are quite complete, accurate, and more suited for nitrogen fixation studies than older studies on this topic [58].

2.3 Current experimental approaches for nitrogen fixation

In most air plasmas, the Zel’dovich reactions (Equations 1, 2) leading to NO productions can further proceed towards NO₂, resulting in a gas-phase mixture of nitrogen oxides referred to as NO_x. These reactions involve ozone or oxygen radicals oxidizing NO (Equations 4–6):

${\mathrm{O}}^{*}+\text{NO\hspace{0.17em}}\to {\text{NO}}_2(4)$

${\mathrm{O}}^{*}+{\mathrm{O}}_2\to {\mathrm{O}}_3(5)$

${\mathrm{O}}_3+\text{NO\hspace{0.17em}}\to {\text{NO}}_2+{\mathrm{O}}_2(6)$

Following earlier studies on thermal plasma (i.e., electric arc exploiting the Birkeland-Eyde reaction), other plasma types and reactors have been examined for NO_x production. This has led to an overabundance of possible sources, making any comparison between different methodologies quite complicated, as no standard reactors or even measurements techniques for different parameters are widely accepted. Energy consumption, for example, is dependent on power and NO_x concentration. NO_x concentrations can be determined via a plethora of methods: Fourier-transform infrared spectroscopy, mass spectroscopy, chemiluminescence, ion chromatography, as well as a nondispersive infra-red sensor with an ultra-violet sensor through a gas analyzer, in situ laser induced fluorescence, laser Raman spectroscopy, optical absorption. Power can be calculated by the Lissajous method [59, 60], numerically integrating the product of the voltage and current and multiplying by the frequency, or by calorimetric methods by multiplying air density, gas temperature, heat capacity of the air, and volume [61]. Moreover, for a real industrial application, the total power should include all the energy consumption in the production pipeline, such as gas flow system and storage, power sources absorption, etc., that are not always considered.

A great number of plasma sources have been investigated in the context of nitrogen fixation into NO_x. Gliding arc (GA) reactors are promising for gas conversion, achieving efficient NO_x production with reduced energy. GA plasmas feature a reduced electric field of less than 100 Td and electron energies around 1 eV, ideal for the vibrational excitation of gas molecules. Reactor optimizations, including controlling pressure, specific energy input, and reactor designs, enhance NO_x yield and energy efficiency across various setups (EC 0.67–4.8 MJ/mol N) [62–69].

NO_x generation in transient spark discharge, which involves non-thermal and thermal plasma phases, has also been achieved. Although a limited volume, which means that only a portion of the N₂ gas is exposed to the plasma, innovations like floating electrodes could improve NO_x yield and efficiency, reducing energy needs (EC 1.9–40 MJ/mol N) [62, 70–74].

Dielectric Barrier Discharge (DBD) systems could be of great interest for the great versatility they offer in reactor design and for the possibility to operate them at atmospheric pressure. For example, other than simple cylindrical, plug-flow-like reactors, it is possible to incorporate water (for nitrogen fixation into NO₃) or other materials in the interelectronic gap. Packed-bed DBD reactors with various catalysts improve NO_x yield, resulting in higher energy costs. Promising catalytic materials include γ-Al₂O₃ and Al₂O₃-supported metallic nanoparticles (EC 17–33 MJ/mol N) [62, 75–77].

Low-pressure microwave plasmas achieved the best energy consumption and NO yield. However, past claims (EC = 0.28 MJ/mol N [41]) are unconfirmed. Energy use excludes reactor cooling and vacuum needs, that eventually add up and make total consumption higher (EC 0.84–3.76 MJ/mol N) [78, 79].

NO_x formation by plasma jets in air or N₂ reacting with water results in NO₂ and NO₃ due to oxygen presence. Key factors affecting NO_x production include gas composition, flow rate, and temperature. Studies show reducing the flow rate and increasing oxygen content boosts NO_x concentration, while lower gas temperature and electric field enhance production efficiency (EC 0.42–3.6 MJ/mol N) [80–89].

From the existing literature, it can be concluded that efficient NO_x production in plasma systems relies on optimizing reduced electric fields to favor vibrational excitation (enhancing Zel’dovich reactions), controlling gas temperature to prevent NO reconversion, and utilizing appropriate catalysts. Key parameters include voltage, electric field strength, gas composition and flow rates, and reactor design.

All the reported results show that the current technology is still far from theoretical limits, but is closing the gap between H-B processes.

3 Future perspective

In countries where agriculture is significant, environmentally sustainable HNO₃ production could serve as an important power-to-chemicals channel, accumulating excess renewable power into valuable chemicals using cold plasma reactors operating at room temperature and atmospheric pressure, thus requiring lower investments than an H-B plant [37]: this would be vital to boosting agriculture in low-income countries. Moreover, a diffused grid energy/fertilizer production usage would be more resilient to natural disasters and armed conflicts than centralized power production and fertilizer import, avoiding direct and indirect related import, transportation, and distribution costs. In our opinion the most viable possible solutions are currently two: production of hybrid fertilizer from slurry and plasma-activated water.

3.1 Hybrid fertilizers

Efficient air plasma production of NO_x offers significant advantages for the fertilizer industry, allowing direct synthesis of nitric acid (HNO₃) [37], for chemical and hybrid fertilizers. Acidification of livestock slurry process has been used in agriculture [90] since 2003, but the novelty is the easy-to-manage direct fertilizer production by air plasma [38]. The latter process facilitates easy production of fertilizers from slurry and air, with benefits including recycling livestock wastewater, eliminating inorganic feedstock, preserving natural manure components, shorter organic fertilizer production times, avoiding stocking dangerous acids, reducing greenhouse gases (i.e., N₂O and CH₄), and preventing ammonia emissions. It supports precision agriculture [91], minimizes environmental losses, and can also be applied in principle to human wastewater. Preliminary experiments in the fields show that this “hybrid” (artificial/natural) manure can be extremely effective [92].

3.2 Plasma-activated water

Concurrently, alongside advancements in low-temperature plasma applications, plasma-activated water (PAW) has surfaced as a cutting-edge tool in modern agriculture, with the potential to enhance crop productivity and mitigate industry challenges [93–98]. PAW production involves subjecting water to non-thermal plasma, generating reactive species such as ROS and RNS. The physicochemical alterations induced by plasma treatment on water can be tailored for diverse applications [99]. PAW reactive species elicit beneficial effects on plants, inducing physiological and biochemical changes conducive to growth and stress tolerance [97, 98, 100]. PAW offers a sustainable and eco-friendly alternative to conventional chemical fertilizers and pesticides, applicable in watering crops, seed soaking, and foliage spraying.

4 Conclusion

Concerning future developments in nitrogen fixation, cold plasma seems to be the only promising and industrially scalable method because its use could further lower the EC of the process, based on thermodynamic calculations. The highest energy efficiency is expected when vibrationally excited nitrogen molecules are formed in sufficient quantities, the temperature of the resulting gases is not high enough to decompose the reaction products, and the reaction is channeled into a particular reaction pathway. Atmospheric pressure cold plasma nitrogen fixation has many environmental advantages as it requires only electricity, air, and, depending on the application, wastewater (hybrid fertilizers) or water (PAW) as feedstock. Contrary to large-scale high-pressure ammonia synthesis plants, plasma nitrogen fixation may be performed on a much smaller scale, locally producing the fertilizers and eliminating the problems associated with fertilizers’ transportation and storage. Moreover, such plants can be ideally powered by renewable energy sources, further reducing the environmental impact of fertilizer production.

However, detailed modeling tools are needed to maximize cold plasma nitrogen fixation efficiency. An essential step is establishing an efficient dialogue between practical and theoretical experiments: accurate data sets should be fed to models so that they could shed light on the mechanism pathways that should be preferred to optimize the reaction.

Data availability statement

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding author.

Author contributions

DA: Data curation, Writing - review and editing, Writing - original draft, Validation, Conceptualization. PA: Funding acquisition, Data curation, Writing - review and editing, Writing - original draft, Validation, Supervision, Conceptualization. FE: Writing - review and editing, Writing - original draft, Validation, Supervision, Conceptualization.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. European Cooperation in Science and Technology, CA19110; Regione Puglia, Riparti - POC PUGLIA FESRT-FSE 2014/2020.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

The author(s) declared that they were an editorial board member of Frontiers, at the time of submission. This had no impact on the peer review process and the final decision.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Abbreviations

DBD, Dielectric Barrier Discharge; EC, Energy Consumption; e-M, electron-Molecules collisions; FHO, forced harmonic oscillator; GA, Gliding Arc; H-B, Haber Bosch; MJ/mol N, MegaJoules consumed per mole of Nitrogen (fixed) produced; N, nitrogen atoms; NOx, Nitric Oxides; PAW, Plasma Activated Water; QCT, quasi-classical modeling methods; RNS, Reactive Nitrogen Species; ROS, Reactive Oxygen Species; SC, semiclassical modeling methods; Tg yr⁻¹, Tera grams per year; VT, Vibration-to-Translation energy transfer; VV, Vibration-to-Vibration energy transfer.

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