# Ammonia cracking by microwave plasma under reduced pressure

M. Awaji, L. Pentecoste-Cuynet, C. Noël, T. Gries, M. Belmahi, T. Belmonte

► To cite this version:

M. Awaji, L. Pentecoste-Cuynet, C. Noël, T. Gries, M. Belmahi, et al. Ammonia cracking by microwave plasma under reduced pressure. International Journal of Hydrogen Energy, 2025, 119, pp.377-385. 10.1016/j.ijhydene.2025.03.118. hal-05140670

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# Ammonia cracking by microwave plasma under reduced pressure

M. Awaji¹,², L. Pentecoste-Cuynet¹, C. Noël¹, T. Gries¹, M. Belmahi¹, and T. Belmonte¹*

¹Université de Lorraine, CNRS, IJL, F-54000 Nancy, France

²Department of Physical Science, Physics Division, College of Science, Jazan University, P.O. Box 114, Jazan 45142, Kingdom of Saudi Arabia

* Corresponding author.

E-mail address: [thierry.belmonte@univ-lorraine.fr](mailto:thierry.belmonte@univ-lorraine.fr)

## Keywords

Microwave plasma; ammonia cracking, hydrogen production

## Disclose of the use of generative AI

During the preparation of this work the authors used ChatGPT in order to improve the English language of the text. After using this tool/service, the authors reviewed and edited the content as needed and take full responsibility for the content of the publication

## Author contributions

M. Awaji and L. Pentecoste-Cuynet made investigation on the process, acquired and cured data, and developed the experimental methodology.

C. Noël supervised the measurements by optical emission spectroscopy and developed the softwares used to determine the rotational temperature.

T. Gries supervised the measurements by microwave interferometry and gas chromatography.

M. Belmahi participated in the experimental investigations, the methodology and data analysis.

T. Belmonte conceptualized the work, wrote the original draft of the paper and acquired the fundings.

## Graphical Abstract
<figure><img src="image_2.png" /></figure>

**Highlights**

* Microwave discharges are showed to be efficient sources for ammonia dissociation

* The overall energy cost is only about a factor of three with respect to thermal cracking

* Microwave plasmas are among the best plasma sources to crack ammonia

* The role of surface processes at moderate pressure is not negligible

* The management of gradients within the plasma volume is certainly key in the overall efficiency of the process

*No need for color in any figures in print*

**Abstract**

The cracking of pure ammonia by microwave discharges under reduced pressure (typically 50 to 500 mbar) has been studied as a hydrogen formation process. Under the best experimental conditions obtained, the lowest energy cost associated with the production of one kilogram of hydrogen is 33 kWh (i.e. 13% energy efficiency for a dissociation ratio of 28%). With the same process, NH₃ can be fully dissociated at a greater energy expense (50 kWh / kgH₂). The plasmas produced have electron densities of no more than 5×10¹¹ cm⁻³ at electron temperatures close to 0.5 eV. The gas is heated to around 2,500 K, enabling thermal dissociation of NH₃. We show that the molecule is also dissociated by electron collisions. Some of the waste heat lost in this process can be recovered in post-discharge by activating surface reactions. With flow rates exceeding 10 liters per minute, the maximum used in the present study, it is plausible that the efficiency of the process could be further improved.
# 1. Introduction

Ammonia serves as a carbon-free energy carrier, leveraging existing global infrastructure, which renders it an appealing option for facilitating hydrogen trade. Ammonia cracking is achieved through thermal catalytic processes with minimal energy expenditure. The enthalpy of dissociation of NH₃ requires 4.3 kWh per kg of H₂. Catalytic cracking plants producing up to 100 Nm³ / h display energy consumptions around 16 kWh / kgH₂ [Kapsom2024].

The use of plasmas presents an alternative to this high-throughput process in cases where local production conditions prohibit continuous exploitation, such as when intermittent energy sources are required. Several studies have explored the feasibility of cracking ammonia using dielectric barrier discharges [Bang2023, Andersen2023, Hayakawa2020, Navascués2020, Gorky2021, Akiyama2018] or arc discharges [Lin2021, Mlotek2021]. As Peng et al. [Peng2023] claim it legitimately, plasma-based decomposition must achieve energy efficiencies around 10 kWh / kgH₂ or lower to become suitable for exploitation.

Zhang and Cha [Zhang2024] recently investigated the dissociation of highly diluted ammonia in argon (0.5% v/v) using microwave discharge. They demonstrated, through extrapolation of their data, the potential for achieving complete ammonia conversion at an energy cost of 530 kWh per kg of H₂. Previous research by Truscott et al. [Truscott2015] also proposed kinetic pathways for the decomposition of ammonia diluted in H₂ (1% v/v) using microwave discharge. One significant aspect of microwave discharges is the typically high temperatures they can generate. However, there would be no point in unnecessarily heating the gas if ammonia cracking were to be essentially thermal. Consequently, the temperature must be reduced as much as possible to take advantage of the non-equilibrium effect of the plasma.

However, the behavior of a pure ammonia discharge, essential for achieving high rates of hydrogen production, differs strongly from media where NH₃ is highly diluted. The primary mechanisms responsible for energy absorption by the molecule are distinct from those of argon, hydrogen and even nitrogen gases. The complexity of vibrational energy storage by NHₓ molecules and the limited information available on energy transfer processes between NH₃ and its fragments render any attempt to describe pure ammonia discharges less credible.

This study focuses on investigating the conditions required for dissociating pure NH₃ using microwave discharge. The impact of key process parameters is assessed and correlated with dissociation yield and process energy efficiency. The interest of resorting to such a process is finally discussed.

# 2. Experimental setup

The reactor consists of a fused silica tube with an inner diameter of 26 mm, supplied with pure ammonia (99.999%, AlphaGaz1, Air Liquide) via a Bronkhorst flowmeter (0-20 slm, Fig. 1a). Ammonia is injected tangentially at two points using a specialized injector to create a swirling flow (see **Supplemental Material 1**). This injection method ensures a longer residence time for the gas compared to straight injection, in a ratio that depends on conditions. The ammonia is drawn through a primary pump, achieving a minimum vacuum of approximately 4 Pa. The pressure is maintained between 50 and 500 kPa. Operating the discharge below atmospheric pressure minimizes the risk of leaks, which is crucial in this set of experiments to prevent user exposure to ammonia. Additional safety measures, such as
enclosing the entire reactor in a ventilated box, using ammonia sensors, and treating exhaust gases, were mandatory for this study.

For microwave injection (Fig. 1a), a WR-340 waveguide connects the resonant cavity to a SAIREM generator (1200 W, 2.45 GHz) that is protected by a water load to prevent damage from reflected waves. A window above the cavity provides optical access. The window is a rectangular parallelepiped measuring 4.318 cm in width and height, and 9.6 cm in length.

Gas chromatography (GC) is used to analyze the plasma effluent (Fig. 1b). The chromatograph, a PerkinElmer CLARUS 580, is equipped with two columns: RT-U-bond (polar) and MolSieve (for permanent gases). Since GC operates at atmospheric pressure, a sampling unit is required to connect the low-pressure afterglow to the GC column. Gas injection is performed using a device specially designed by ANTELIA to handle gases sampled at sub-atmospheric pressures (above 5 kPa). A micrometer valve connects a tube from the reactor outlet to the chromatograph. This sampling system involves filling a balloon (shown in the bottle in Fig. 1c) and then emptying it into the analysis columns. The system must be calibrated to determine the concentrations of the products formed. For this purpose, two mixtures were used: NH₃-N₂ and NH₃-H₂, with accurately known flow rates for each component.

<figure><img src="image_3.png" /><figcaption>**Figure 1:** a) Overview of the microwave discharge system used to dissociate ammonia. b) Chromatograph used to determine the dissociation yield of NH₃. c) Sampling system used to manage the differential pressure between the discharge and the chromatograph. The bottle contains a balloon that inflates when the processed gas mixture is sampled and deflates to inject gases into the column when the measurement begins.</figcaption></figure>
Optical emission spectroscopy was performed with a 550 mm focal length monochromator (Jobin-Yvon TRIAX 550) equipped with a 1200 gr. mm⁻¹ grating blazed at 500 nm for overall spectra in the visible range (250–900 nm). It was coupled with a Spectrum ONE MTE CCD 2000 detector. Each measurement is averaged over 50 spectra recorded with an exposure time of 10 ms.

Microwave interferometry (Miwitron MWI 2650-A @ 26.5 GHz) was used along with image analysis of the discharges to determine the average electron density. Image analysis was necessary to determine the absorption length of the microwave signal, assuming this length is equal to the plasma emission length along the 26.5 GHz microwave trajectory.

To accurately estimate the energy balance of the process, it is essential to know the actual power consumption of the entire system. Pumps, gauges, and flowmeters consume constant power, while the microwave generator delivers variable power depending on the selected conditions. Therefore, it is important to precisely define the different power levels mentioned in this work: the *total power*, which includes losses due to the DC-microwave transformation; the *nominal power* set at the microwave power generator, which does not include these losses; and the *injected power* in the plasma, which is the difference between the nominal power and the reflected power. As shown in **Supplemental Material 2**, the efficiency of the generator used in this study is only 30% for a nominal power of 100 W, but increases to 60% for a nominal power of 1000 W.

# 3. Results and discussion

## 3.1. Optical emission spectroscopy

The analysis of OES spectra is challenging due to the complexity of the emissive systems observed (see **Supplemental Material 3**). The following emission systems have been identified:

- the N₂ (C³Πu-B³Πg) second positive system,

- the NH (A³Π-X³Σ⁻) Ångström system,

- the Hα, Hβ, sometimes Hγ, and rarely Hδ Balmer lines,

- a large continuum composed of many lines and arbitrarily attributed to NH₂*.

The origin of this continuum remains unclear, likely involving emissions from several excited states, which adds to the complexity of the spectrum. Modeling the corresponding set of transitions was not feasible within the scope of this study.

The rotational temperatures of N₂* and NH* can be determined by modeling their combined rotational spectra. An example of this approach is provided in **Supplemental Material 4, Fig. 2** shows the variation of these temperatures with pressure at different power levels. The rotational temperature for NH ranges from 2800-3300 K, while for N₂ it ranges from 2000-2700 K, with an accuracy of ±150 K. These temperatures increase slightly with pressure. These high temperatures are assumed to be close to the maximum gas temperature in the reactor, as they align closely with the fusion temperature of quartz (approximately 1900 K), because of the thermal gradient existing between the discharge center and the reactor wall. Notably, when the injected power exceeds 700 W, the fused silica tube begins to melt.

The difference in rotational temperatures between NH* and N₂* likely arises from the different regions in the plasma where these species emit. This study confirms that the rotational temperatures of NH and N₂ overlap and are probably close to the reactor's
maximum temperature. The temperature gradient is around 1000 K over a distance of 1.3 cm, which is plausible due to the swirling gas flow that cools the reactor walls more effectively than a parabolic flow would. It is also important to note that the temperature variation is relatively narrow overall.

These temperatures are far higher than those required for the thermal dissociation of NH₃. However, since this high temperature is confined to a small volume, it does not imply that ammonia dissociation is predominantly thermal. The relative contributions of thermally-activated and plasma-assisted dissociation processes will be discussed later.

<figure><img src="image_4.png" /><figcaption>**Figure 2:** Evolution of the rotational temperatures of the N₂* and NH* probes as a function of the pressure at different injected powers (color scales).</figcaption></figure>

## 3.2. Evolution of the dissociation ratio

The dissociation ratio of ammonia is defined as $\alpha = \frac{1-x_{\text{NH}_3}^{\text{out}}}{1+x_{\text{NH}_3}^{\text{out}}}$ where $x_{\text{NH}_3}^{\text{out}}$ is the volume fraction of NH₃ at the reactor outlet, measured by gas chromatography (Supplemental Material 5). The influence of the main process parameters –flowrate, pressure, injected power, and temperature– on $\alpha$ is depicted in **Fig. 3**

Increasing the flow rate by an order of magnitude significantly reduces precursor dissociation (Fig. 3a) due to the shorter transit time of the molecules in the discharge. As pressure increases, this trend shifts slightly toward higher dissociation values. The influence of pressure and power (Figs. 3b and 3c) at various flow rates is less pronounced. As these parameters increase, dissociation rises gradually and almost linearly within the studied range, with shifts occurring as the flow rate changes.

The effect of rotational temperature (Fig. 3d), which is assumed to represent the gas temperature, appears to be negligible for both probes. No clear dependence is observed, with data points spread between $\alpha \approx 0.1$ to $\alpha \approx 0.9$ across a relatively narrow temperature range. This suggests that thermal dissociation of NH₃ is not the primary mechanism driving molecular cracking in this process.

Notably, nearly 100% dissociation (97% precisely) can be achieved using this process at a pressure of 400 mbar (approximately half of atmospheric pressure), 640 W of power, and a flow rate of 2 slm, for example (Fig. 3b). In this case, the energy efficiency is about 8.4%, as showed later.
<figure><img src="image_5.png" /><figcaption>**Figure 3:** Evolution of the dissociation ratio as a function of a) the ammonia flowrate at different pressures (injected powers around 600 W are mentioned), b) the pressure at different flowrates (injected powers around 600 W are also mentioned), c) the injected power at different flowrates for a pressure set at 100 mbar, and d) the rotational temperatures of N₂* and NH* at different injected powers.</figcaption></figure>

These data can be consolidated into a single graph showing the evolution of the dissociation ratio as a function of the residence time of the species in the reactor (Fig. 4). To determine the dependence of $\alpha$ on the residence time, it is necessary to describe how the volume fractions of NH₃ and H₂ (or N₂) vary with this parameter. Assuming a quasi first-order reaction:

$$ \frac{d[\text{NH}_3]}{dt} = -\frac{1}{\tau}[\text{NH}_3] \qquad (\text{Eq.1}) $$

we observe that the time evolutions of these three parameters are satisfactorily reproduced. $\tau$ is a characteristic time decay constant.
<figure><img src="image_6.png" /><figcaption>**Figure 4:** a) Evolution of the dissociation ratio as a function of the residence time of species in the reactor at different injected power levels. b) Volume fraction of NH₃ as a function of the residence time of species in the reactor at different injected power levels. c) Volume fraction of H₂ produced by ammonia cracking as a function of the residence time of species in the reactor at different injected power levels.</figcaption></figure>

This process is not solely or directly the result of the thermal decomposition of the molecule, as the dissociation ratio does not depend on temperature. Therefore, we still have to examine the role of electrons in the dissociation of ammonia.

### 3.3. Evolution of the electron density

To clarify the role of electron collisions in the dissociation process, their density was determined using microwave interferometry. The results are presented in **Fig. 5**. Within the uncertainty of the measurements, it is difficult to ascertain whether the electron density decreases smoothly or experiences a local maximum near 4 kPa. These variations are influenced by the absorption length of the microwaves, which is directly related to the plasma diameter. This parameter fluctuates slightly and was measured through photographs to correct the raw data and refine the measurements. However, the uncertainty in this parameter cannot be substantial, as the plasma often fills the tube. If the plasma were slightly hollow, the error in the diameter could typically not exceed 30%, as estimated from picture analysis.

The key point here is the average decrease in electron density ($n_e$) by a factor of ~5 when the pressure increases from 1 to 30 kPa. It is more common to observe in non-equilibrium low-pressure plasmas that increasing the pressure increases electron density and reduces electron temperature. However, we will see that the electron temperature remains almost constant within this range, close to 0.5 eV. At this energy, the vibrational excitation of NH₃ could explain this behavior, especially considering Chen's cross sections [Chen2023], where the excitation of the v = 4 level reaches half of Itikawa's momentum cross section (13 vs 26×10⁻¹⁶ cm²) [Itikawa2017], which is substantial. This excitation could serve as a stepwise process for ionization, thereby enhancing the generation of electrons.

### 3.4. Possible origin of the first-order decay

Let's consider the various mechanisms studied for the plasma-assisted decomposition of pure ammonia. If we assume that NH₃ dissociation is electronically driven through an overall reaction such as:

$$NH_3 + e \xrightarrow{k_1} \frac{1}{2}N_2 + \frac{3}{2}H_2 + e \quad (R1)$$

then the dissociation rate is given by:
$$ \frac{d[\text{NH}_3]}{dt} = -k_1(T_e)[\text{NH}_3][e] \qquad (\text{Eq.2}) $$

<figure><img src="image_7.png" /><figcaption>**Figure 5:** Evolution of the electron density as a function of the pressure in the reactor a) at constant injected power for different flowrates and b) at constant flowrate for different injected powers.</figcaption></figure>

This reaction could be considered a first-order process if $k_1(T_e) \times [e]$ remains constant over the investigated pressure range. As $n_e$ decreases by a factor of ~5, $k_1(T_e)$ should typically increase from ~6×10⁻¹¹ to ~3×10⁻¹⁰ cm³ s⁻¹.

An approximate electron temperature was determined using the Boltzmann plot method from the first transitions of the Balmer series (Hα, Hβ and Hγ when available). Regardless of the conditions (pressure between 50 and 200 mbar, flow rates from 1.375 to 11.0 slm, and powers from 300 to 1000 W), the electron temperature consistently remains around 0.5 ± 0.1 eV. Therefore, direct dissociation of NH₃ by electron impact seems unlikely under these present conditions.

D'Agostino et al. [D'Agostino1981], using a radiofrequency plasma between 6 and 50 mbar, proposed a mechanism relying on three principal reactions:

$$ \mathrm{NH_3} \xrightarrow{k_2} \mathrm{NH_2} + \mathrm{H} \qquad (\mathrm{R2}) $$

$$ \mathrm{H_2} \xrightarrow{k_3} \mathrm{H} + \mathrm{H} \qquad (\mathrm{R3}) $$

$$ \mathrm{NH_3} + \mathrm{H} \xrightarrow{k_4} \mathrm{NH_2} + \mathrm{H_2} \qquad (\mathrm{R4}) $$

This approach drew inspiration from CH₄ and other hydrocarbon decompositions, without assigning a specific role to electrons. According to this mechanism, the authors demonstrated that the dissociation process appears to follow zero-order kinetics, which is not applicable to the current situation.

Reaction 2, considered a second-order process [Baulch1994, Hanson1984], is extremely challenging to achieve thermally (with a rate constant as low as 7×10⁻¹⁵ cm³ s⁻¹ at 3,000 K, for instance).

NH₃ is known to decompose at lower temperature through surface-assisted reactions [Takahashi2016, Zhang2005, Hellman2006], even on fused silica walls [Cooper1988]. The latter mechanism is most likely applicable in our situation. The surface dissociation of NH₃:
$$NH_3 + s \xrightarrow{k_s} \frac{1}{2}N_2 + \frac{3}{2}H_2 + s \quad (R5)$$

follows a first-order rate given by:

$$\frac{d[\text{NH}_3]}{dt} = -\frac{\gamma v_{th}}{2R} [\text{NH}_3] \quad (\text{Eq.3})$$

where R is the tube radius, $v_{th}$ is the thermal velocity of the gas, and $\gamma$ the loss probability of NH$_3$ on the walls. The value of $k_s$ given by Cooper and Ljungstroem [Cooper1988] ($k_s = 5.12 \times 10^6 \exp(-\frac{147,000}{RT_w})$) matches the present experimental value, i.e. $1/0.069 \text{ s}^{-1}$, when the temperature of the wall $T_w$ is close to 1400 K, as estimated. Assuming a mean gas temperature of 2500 K, then, $\gamma$ is approximately $2.6 \times 10^{-4}$, which aligns with typical values found for surface processes on ceramic walls [Lefèvre2000].

Gicquel et al. [Gicquel1986, Gicquel1989] argue that the decomposition of NH$_3$ in plasma is governed by an endothermic step involving the vibrationally excited states of NH$_3$:

$$\text{NH}_3(v) + e \xrightarrow{k_v} \text{NH}_x(J) + \text{H}_{3-x} + e \quad (\text{R6})$$

where $x = 1, 2$ or $3$. Similar to reaction (R1), this process is not first-order. However, it could enhance surface decomposition by accelerating surface mechanisms; vibrationally excited states of ammonia are easier to decompose than the ground state. With such a contribution, ammonia decomposition would not be solely driven by thermally activated surface processes but also by plasma-induced processes involving vibrational states.

To test this assumption, experiments were conducted using a larger surface area of fused silica. A set of 13 tubes (6 mm outer diameter, 4 mm inner diameter, 63 mm in length) was inserted into the reactor tube, as shown in Fig. 6.

The tubes were positioned 2-3 millimeters downstream of the plasma zone. At 50 mbar, 320 W, and 1 slm NH$_3$, the dissociation yield increased from 44.2% without the tubes to 53.1% with the tubes. At 100 mbar, 480 W, and 2 slm NH$_3$, the dissociation yield rose from 48.1% without the tubes to 59.2% with them.

<figure><img src="image_8.png" /><figcaption>**Figure 6:** Picture of the reactor with the set of 13 small tubes inserted into the reactor tube.</figcaption></figure>

These results confirm the role of fused silica in enhancing dissociation. The approximate 10% increase in dissociation yield is significant. However, this improvement is likely due to the
small portion of the tubes located in the post-discharge region, where the temperature decreases. In this setup, we utilize the thermal energy generated by the plasma, but only over a limited distance, beyond which the medium becomes too cool for further significant reactions. Thus, we are able to recover some waste heat from the process, though only to a certain extent.

Nevertheless, increasing the exposed surface area (~550 cm², with the active portion estimated to be about one-third of this) is insufficient to achieve complete dissociation. This indicates that while surface processes may contribute to NH₃ cracking, they are not the primary contributors to hydrogen production.

### 3.5. Process efficiency

To assess process efficiency, we applied the calculations from Andersen et al. [Andersen2022] to our results. Therefore, to evaluate the energy efficiency of ammonia dissociation η<sub>NH₃</sub> (in %), we used the following formula:

$$ \eta_{\text{NH}_3}(\%) = \frac{\alpha \cdot \Delta_r H}{\frac{P_{mw}}{F_{\text{NH}_3}^{\text{in}}} \cdot \frac{M_{\text{NH}_3}}{\rho_{\text{NH}_3}}} \quad (\text{Eq.4}) $$

where Δ<sub>r</sub>H is the enthalpy of NH₃ decomposition (46.2 kJ. mol<sup>-1</sup>), P<sub>mw</sub> is the total microwave power delivered to the process, F<sub>NH₃</sub><sup>in</sup> is the molar flowrate of NH₃ into the reactor, and M<sub>NH₃</sub> and ρ<sub>NH₃</sub> the molar mass and density of NH₃, respectively.

Results are presented in Fig. 7. As expected, the energy efficiency varies linearly with α/P<sub>mw</sub> and α·F<sub>NH₃</sub><sup>in</sup>. Under our conditions, η<sub>NH₃</sub> reaches a maximum of ~13.25% at 10 slm, 200 mbar (i.e. a dissociation ratio of 28% for an energy consumption of 57 kWh / kg<sub>H₂</sub>). Interestingly, efficiency increases with flow rate (Fig. 7a), despite previous observations that NH₃ dissociation ratio decreases with increasing the flow rate. This discrepancy is explained by considering the quantity of material transformed and the power consumed to achieve this transformation. Thus, even if 100% of ammonia introduced into the reactor at 2 slm is transformed, the same quantity of ammonia can be transformed with only 20% conversion at 10 slm, given the same power input.

However, this calculation does not include the additional costs of post-treatment required to separate pure hydrogen from N₂ and residual NH₃, which would incur significant additional costs. In Fig. 7b, it can be observed that, logically, at constant conversion rate and flow rate, reducing the injected power increases energy efficiency.

We also estimated the yield of H₂ normalized by the energy input, considering the total microwave power P<sub>mw</sub>. It was calculated using the following equation:

$$ R_{\text{H}_2}(\text{g} \cdot \text{kWh}^{-1}) = \frac{3}{2} \alpha \frac{F_{\text{NH}_3}^{\text{in}} M_{\text{H}_2}}{P_{\text{mw}}} \quad (\text{Eq.5}) $$

The energy required to produce 1 kg of hydrogen is simply the reciprocal of R<sub>H₂</sub>, with appropriate units. These results are summarized in Table 1 and compared with other findings in the literature on various NH₃ cracking processes for hydrogen production.
<figure><img src="image_9.png" /></figure>

<figure><img src="image_10.png" /><figcaption>**Figure 7:** Evolution of the energy efficiency η<sub>NH₃</sub> expressed in percent: a) as a function of the α/P<sub>mw</sub> ratio at different flow rates and b) as a function of the α · F<sup>in</sup><sub>NH₃</sub> at different injected powers.</figcaption></figure>

The energy per kg of H₂ is about 8 times higher than the energy needed for bond dissociation and is comparable to the findings of Wang *et al.* [Wang2019] in a heated DBD reactor with catalyst. It is however 3 times (2 times without catalyst) higher than Lin *et al.*'s results [Lin2021] where the flow rate is up to 3 times higher. In Lin *et al.*'s process, the average gas temperature varies from 700 to 1000 K when the power increases from 200 to 700 W.

Although the arc discharge operates in a non-equilibrium state, its efficiency compared to those of thermal treatments suggests that a significant portion of energy is inefficiently utilized. In thermal treatment, maintaining a constant temperature can ensure uniform efficiency throughout, neglecting changes in gas composition. Conversely, in continuous discharge, sections of gas passing through may not reach sufficient temperatures for complete NH₃ decomposition, while other areas may become overheated beyond necessity, yet limited to an efficiency that cannot exceed full dissociation. Electronic processes are expected to compensate for this inefficiency caused by thermal gradients, aiming to prevent wasted energy in pathways where molecules might not fully decompose and could recombine or lose internal energy through relaxation. Thus, competing with thermal processes poses significant challenges.

Zhang *et al.* [Zhang2024] demonstrated the predominant influence of gas temperature in microwave plasma jet-induced NH₃ cracking. They found that electron-induced reactions played a minor role in their study. However, these authors did not consider surface processes within a fused silica reactor tube (with an inner radius of only 2 mm), due to incomplete filling of the tube volume by the discharge.

According to Lin *et al.* [Lin2021], in comparison to other cold plasma reactors, their reactor benefits from high gas temperatures and a high electron density, around 10¹² cm⁻³ compared to 10¹⁰-10¹¹ cm⁻³ in the present experiment. The use of swirl flow was also shown to enhance energy exchange between ammonia molecules and plasma, thereby promoting ammonia decomposition.
The best result from this study is still 2.4 times less efficient than the findings of Lin *et al*. We believe the key difference lies in how the temperature is distributed along the flow. In their setup, the process begins with an overheated electrode, where thermal decomposition is highly effective, and waste heat is utilized downstream to further enhance ammonia cracking. Flow rates of up to 30 slm likely contribute to the high performance of the non-thermal plasma torch used in their work, as inferred from **Fig. 7a**. However, the inefficiency of the microwave power supply—about 40% of the input power being lost—remains an unavoidable limitation with this type of plasma source.

## 4. Conclusion

Microwave discharges in ammonia are characterized by relatively low electron temperatures and electron densities, ranging from 2.5×10¹¹ cm⁻³ at 1 kPa to 5.0×10¹⁰ cm⁻³ at 20 kPa at 500 W.

When ammonia is cracked in a microwave discharge, three primary mechanisms are responsible for decomposition:

* Thermal cracking: The discharge reaches temperatures high enough to decompose ammonia thermally.

* Non-thermal dissociation: This occurs within the plasma volume through electron collisions and exchanges with excited species.

* Surface processes: Although these are limited at high pressures, they remain non-negligible in this context.

The waste heat generated by the discharge can be harnessed in the afterglow, over a distance where the temperature exceeds the threshold needed for NHₓ cracking. This improves the dissociation ratio, which could be further enhanced by using catalysts instead of fused silica.

Increasing both the flow rate and absorbed power could yield results comparable to those of Lin *et al*. [Lin2021]. However, this would require redesigning the reactor to limit excessive heating around the plasma zone and prevent the fused silica tube from melting. New designs will be explored in future work.

## Acknowledgement

We gratefully acknowledge Sakowin, an eco-conscious deeptech company founded in 2017, which produces decarbonized hydrogen through methane plasmalysis, for their financial support of this study and fruitful discussions.

The microwave interferometer used in this study is a shared tool of the French cold plasma network from the MITI-CNRS. We also warmly thank this network for lending us the equipment.
<table><caption>**Table 1:** Main characteristics of processes for ammonia cracking. Data in the first line correspond to conditions with the highest energy efficiency obtained with the present process, whereas data in parenthesis correspond to conditions where the conversion was almost 100%. Water electrolysis is presented for comparison.</caption><thead><tr><th>Process</th><th>Gases</th><th>Catalyst</th><th>Conversion α (%)</th><th>R<sub>H₂</sub> (gH₂ / kWh)</th><th>kWh / kgH₂</th><th>η<sub>NH₃</sub> (%)</th><th>Ref.</th></tr></thead><tbody><tr><td colspan="8"><strong>PLASMA PROCESSES</strong></td></tr><tr><td>MW plasma cavity</td><td>NH₃</td><td>[-]</td><td>28 (97)</td><td>30 (20)</td><td>33 (50)</td><td>13 (8)</td><td>This study</td></tr><tr><td>MW plasma jet</td><td>NH₃/Ar</td><td>[-]</td><td>76</td><td>1.89</td><td>529</td><td></td><td>[Zhang2024]</td></tr><tr><td>DBD</td><td>NH₃</td><td>[-]</td><td>13</td><td>1.32</td><td>758</td><td></td><td>[Hayakawa2020]</td></tr><tr><td>DBD</td><td>NH₃</td><td>[-]</td><td>20</td><td>1.44</td><td>694</td><td></td><td>[Nevascues2020]</td></tr><tr><td>DBD</td><td>NH₃/N₂</td><td>[-]</td><td>100</td><td>0.79</td><td>1266</td><td></td><td>[Akiyama2018]</td></tr><tr><td>DBD+catalyst</td><td>NH₃/N₂</td><td>MgAl₂O₄</td><td>82</td><td>0.45</td><td>2222</td><td></td><td>[Andersen2022]</td></tr><tr><td>DBD+catalyst</td><td>NH₃</td><td>Perovskite</td><td>44</td><td>0.67</td><td>1493</td><td></td><td>[Gorky2021]</td></tr><tr><td>DBD (650 K)+catalyst</td><td>NH₃</td><td>Co/fumed SiO₂</td><td>98</td><td>32</td><td>31.25</td><td>30.16</td><td>[Wang2019]</td></tr><tr><td>DBD (600 K)</td><td>NH₃/N₂</td><td>[-]</td><td>17</td><td>0.69</td><td>1449</td><td></td><td>[Bang2023]</td></tr><tr><td>Nonthermal arc+catal.</td><td>NH₃</td><td>NiO/Al₂O₃</td><td>28</td><td>97.2</td><td>10</td><td></td><td>[Lin2021]</td></tr><tr><td>Gliding arc</td><td>NH₃/N₂</td><td>[-]</td><td>45</td><td>10.61</td><td>94</td><td></td><td>[Mlotek2021]</td></tr><tr><td colspan="8"><strong>OTHER PROCESSES</strong></td></tr><tr><td>Theory<br>(ΔH<sub>r</sub> = 46.2 kJ mol<sup>-1</sup>)</td><td>NH₃</td><td></td><td>100</td><td>235</td><td>4.27</td><td>100</td><td></td></tr><tr><td>Thermal + catalyst</td><td>NH₃</td><td></td><td>99.7</td><td>62.5</td><td>16</td><td></td><td>[Kapsom2024]</td></tr><tr><td>Water electrolysis</td><td></td><td></td><td>100</td><td>18-23</td><td>30-90</td><td></td><td>[ElShafie2023]</td></tr></tbody></table>
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SUPPLEMENTAL MATERIAL 1

<figure><img src="image_11.png" /></figure>

**Supplemental Material 1:** Schematic representation of the stainless steel gas injector for orthogonal (lane 1) or tangential (lane 2; used in this work) injection. The width and height of the part are 7 cm. Internal hole diameter is 8 mm.
SUPPLEMENTAL MATERIAL 2

<figure><img src="image_12.png" /></figure>

**Supplemental Material 2:** a) Device for measuring the power consumed by the microwave generator at the electricity meter. b) Graph showing the ratio, expressed in percent, of the power injected into the plasma to the total power consumed by the generator together with the total power consumed by the generator as a function of the injected power.
SUPPLEMENTAL MATERIAL 3

In Fig. S3.1, a typical spectrum of the light emitted by the discharge is depicted. The following emission systems are identified:

- The N₂ (C³Πu-B³Πg) second positive system

- The NH (A³Π-X³Σ⁻) Ångström system

- The Hα Balmer line at 656 nm

- A large continuum composed of many lines and arbitrarily attributed to NH₂*.

A detailed analysis of the last transition showed that this part of the spectrum includes unidentified transitions. As they do not correspond to the usually observed NH₂(Ã²A¹ – X̃²B¹) transition and NH₃ Schuster's bands at 563.5 and 567 nm, we simply assigned the part of the spectrum to higher emissive states of NH₂, denoted here NH₂*.

<figure><img src="image_13.png" /></figure>

**Supplemental Material 3.1:** Typical spectrum of the light emitted by the microwave discharge beyond 50 mbar in the wavelength range spanning from 250 to 900 nm.

The continuum extending from 450 to 900 nm and named NH₂* is not due to the Â²A₁(0, v'₂, 0) → X̃²B₁(0,0,0) transition, as showed in Fig. S3.2. It might be due to higher excited states and/or to specific NH₃ transitions that could not be identified in this work.
<figure><img src="image_14.png" /><figcaption>**Supplemental Material 3.2:** Detail of the sub-bands of the NH₂ spectrum corresponding to the $\tilde{A}^2A_1(0, v'_2, 0) \rightarrow \tilde{X}^2B_1(0,0,0)$ transition. Wavelength of lines for the considered transition are given by Dressler *et al.* in vacuum and corrected for air [Dressler1959]. For the vacuum to air conversion, $\lambda_{\text{air}} = \lambda_{\text{vac}} / n$, $n$ being the refraction index given by the formula given in [Morton2000]: $n = 1 + 8.34254 \times 10^{-5} + 2.406147 \times 10^{-2} / (130 - s^2) + 1.5998 \times 10^{-4} / (38.9 - s^2)$, where $s = 10^4 / \lambda_{\text{vac}}$ and $\lambda_{\text{vac}}$ is in Å.</figcaption></figure>

Increasing the power from 300 to 1000 W increases the intensity of all the transitions in the recorded spectra (Fig. S3.3)

<figure><img src="image_15.png" /></figure>
**Supplemental Material 3.3:** Evolution with the injected power of the main transitions observed in Fig. S3.2.
SUPPLEMENTAL MATERIAL 4

<figure><img src="image_16.png" /></figure>

**Supplemental Material 4:** Experimental and modelled rotational spectra of N₂ (C³Πu-B³Πg) second positive system and NH (A³Π-X³Σ⁻) Ångström system emissions around 336-337 nm with the evolution of the calculated residual vs the wavelength (experimental conditions: 2 slm, 200 mbar, 540 W).
SUPPLEMENTAL MATERIAL 5

The total gas flux varies in the reactor because of the decomposition of one NH₃ molecule into 2 molecules: 1/2 N₂ and 3/2 H₂. This quantity reads:

$$F_{Tot}^{out} = F_{NH_3}^{out} + F_{N_2}^{out} + F_{H_2}^{out}$$

with $F_{NH_3}^{out} = (1 - \alpha) \times F_{NH_3}^{in}$; $F_{N_2}^{out} = \frac{1}{2}\alpha \times F_{N_2}^{in}$; $F_{H_2}^{out} = \frac{3}{2}\alpha \times F_{H_2}^{in}$ as $F_{N_2}^{in} = F_{H_2}^{in} = 0$.

The dissociation ratio of ammonia is defined as $\alpha = \frac{1-x_{NH_3}^{out}}{1+x_{NH_3}^{out}}$ where $x_{NH_3}^{out}$ is the volume fraction of NH₃ at the reactor outlet, measured by gas chromatography

Then,

$$F_{Tot}^{out} = (1 + \alpha) \times F_{NH_3}^{in}$$

and

$$x(NH_3) = \frac{F_{NH_3}^{out}}{F_{Tot}^{out}} = \frac{1 - \alpha}{1 + \alpha}$$

$$x(N_2) = \frac{F_{N_2}^{out}}{F_{Tot}^{out}} = \frac{1}{2} \left( \frac{\alpha}{1 + \alpha} \right)$$

$$x(H_2) = \frac{F_{H_2}^{out}}{F_{Tot}^{out}} = \frac{3}{2} \left( \frac{\alpha}{1 + \alpha} \right)$$

Thus:

$$x(NH_3) = 1 - \frac{4}{3}x(H_2)$$