# NOx decomposition through seaweed biocarbon as biosourced catalyst: Experimental and density functional theory approaches

Ronan Pelé, María González Martínez, Ange Nzihou

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Ronan Pelé, María González Martínez, Ange Nzihou. NOx decomposition through seaweed biocarbon as biosourced catalyst: Experimental and density functional theory approaches. Journal of Environmental Management, 2026, 401, pp.128992. 10.1016/j.jenvman.2026.128992. hal-05514352

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Research article

# NOx decomposition through seaweed biocarbon as biosourced catalyst: Experimental and density functional theory approaches

Ronan Pelé<sup>a</sup>, María González Martínez<sup>a,b</sup>, Ange Nzihou<sup>a,b,*</sup>

## ARTICLE INFO

Keywords:

Biocarbon

Seaweed

NOx abatement

Biosourced catalyst

DFT simulation

## ABSTRACT

Understanding the NOx abatement and deactivation mechanisms is essential to improve the performance and regeneration of bio-based catalysts. In this study, biocarbon obtained from marine algae (SW), rich in nitrogen and minerals, was used as a biosourced catalyst for the decomposition of a flux of 1021 ppm of NO in argon (deNOx). Several parameters were studied, including pyrolysis temperature (650 and 800 °C) and deNOx operating temperature (200, 350, and 500 °C). Additionally, a theoretical study using density functional theory (DFT) calculations investigated the role of nitrogen and catalytic (Fe) or promoter (K) elements and functional groups (O-groups) in NO adsorption. The results indicated that the bio-based catalyst produced at 800 °C (SW800) achieved the highest conversion rate of 22.5 ± 1% at 350 °C compared to the catalyst produced at 650 °C (SW650), which achieved a conversion rate of 15.6 ± 1%. This result is attributed to the higher mineral content (52.99 wt%, db) of the SW800 catalyst rather than its specific surface area, which was not identified as a discriminating parameter under the studied conditions. Regarding the reaction mechanism, nitrogen and oxygen atomic balances confirm the multistep decomposition hypothesis. Furthermore, the slight increase in lactone and carbonyl groups after the deNOx process suggests that oxygen remains on the biocarbon. This contributes to reduce active sites and to deactivate the biocarbon catalyst, as observed. Coke deposition on active sites and metal sintering can also contribute to deactivation. Based on the calculations on representative nitrogen-rich biocarbon structures, the adsorption energies (E<sub>ads</sub>) on pyridinic N (–71.10 kJ mol<sup>-1</sup>) and pyrrolic N (–69.30 kJ mol<sup>-1</sup>) sites are high, demonstrating their roles in NO abatement. However, the presence of an oxygen group close to a nitrogen site inhibits NO adsorption due to its high electronegativity. This information is in agreement with the deactivation of nitrogen sites by residual oxygen from NO decomposition, as observed experimentally.

## 1. Introduction

Nitrogen oxides (NOx), mainly produced by high-temperature combustion processes, are responsible for serious environmental and health problems such as acid rain, ozone depletion, and respiratory diseases (Cesar et al., 2015). Thermal NOx production can occur for both fossil and biosourced fuels; thus, exhaust gas treatment for NOx reduction (deNOx) is mandatory. The direct decomposition of NO is thermodynamically feasible (ΔrG = –87.6 kJ mol<sup>-1</sup>) and the reaction is exothermic (ΔrH = –91.3 kJ mol<sup>-1</sup>). A kinetic mechanism developed by Konnov and De Ruyck for the NO decomposition in the gas phase, consisting of 27 reactions and 11 species, highlighted the extremely slow kinetics of the decomposition (Konnov and De Ruyck, 1999). This

mechanism is composed of two ignition reactions:

$$NO + NO \leftrightarrow N_2 + O_2 \qquad \text{Eq. 1}$$

$$NO + M \leftrightarrow N + O + M \qquad \text{Eq. 2}$$

These reactions showed high activation energies, 272 and 620 kJ mol<sup>-1</sup>, respectively, which are an obstacle to NO decomposition. In another work, the energy barrier of NO decomposition is estimated to be 335 kJ mol<sup>-1</sup> (Sun et al., 2018). Therefore, a catalyst is required to lower this energy barrier and promote the reaction. In this regard, selective catalytic reduction (SCR) is one of the most efficient and widely used technologies. However, it uses noble metal catalysts such as vanadium and platinum, which have high cost, toxicity, and environmental impact

<footnote><sup>a</sup> Université de Toulouse, IMT Mines Albi, RAPSODEE CNRS UMR, 5302, Campus Jarlard, F., Albi Cedex 09, 81013, France</footnote>

<footnote><sup>b</sup> Andlinger Center for Energy and the Environment, Princeton University, Princeton, NJ, 08544, United States</footnote>

<footnote>* Corresponding author. Université de Toulouse, IMT Mines Albi, RAPSODEE CNRS UMR, 5302, Campus Jarlard, F., Albi Cedex 09, 81013, France.
E-mail address: ange.nzihou@mines-albi.fr (A. Nzihou).</footnote>
(Han et al., 2019). In a previous study, the use of raw fern or willow biocarbons, which are naturally rich in metals or impregnated with nickel and iron, was a promising alternative route (Graul et al., 2024). However, the use of bioresources with high availability and naturally rich in less toxic metals would avoid artificial impregnation and thus metal extraction, by favoring a circular economy approach (Bird et al., 2011).

Biocarbon is a biosourced carbon-based material obtained as the solid fraction of bioresource pyrolysis (500-800 °C, N₂). Its physico-chemical properties, close to those of fossil fuels, make it a suitable alternative to reduce greenhouse gas emissions in industry (Meng et al., 2024). Biocarbon is also efficient for pollution remediation, soil fertility improvement, and carbon sequestration (Tomczyk et al., 2020). The high metal content in some biocarbon can be an advantage in enhancing catalytic activity. This may include hydrogen or biodiesel production (Mishra et al., 2024), when a high content of alkaline and alkaline-earth metals, AAEM (Ca, K, Mg), is finely dispersed within the porous structure of biocarbon (Park et al., 2024). Metal enrichment with Ni can improve the catalytic performance of the biocarbon in hydrogen-rich syngas production (Fang et al., 2024). Furthermore, the porous structure of biocarbon, close to that of activated carbon, allows taking advantage of physical and chemical properties with a suitable metallic composition to promote the use of biocarbon as a catalyst for the decomposition of nitrogen oxides (NOx) (Lee et al., 2017). Indeed, oak chips biocarbon produced at 550 °C and activated at 900 °C under CO₂ showed a drastic reduction of NO (100 % removal) at ambient temperature from the air corresponding to an urban environment (5 ppm of inlet NO) (Díaz-Maroto et al., 2023a), (Díaz-Maroto et al., 2023b). However, a higher temperature of 800 °C is necessary to remove 90% of 500 ppm of NO from the inlet flow using elm biocarbon (Gong et al., 2021). This NO conversion rate is comparable to standard SCR catalysts (vanadium, manganese, titanium, and cerium), which have 80-90% NOx removal efficiency, but perform between 150 and 550 °C (Sunil Kumar et al., 2023). The high temperatures used for biocarbon catalysts directly imply biocarbon decomposition, which calls into question the use of this material as a catalyst without destruction or modification. Impregnating the biocarbon with a metal catalyst can reduce the conversion temperature while maintaining NO removal efficiency. For example, Ni-impregnated biocarbon demonstrated efficient performance (30%) at 500 °C with less than 10% mass loss. However, higher temperatures (>500 °C) result in excessive mass loss (Graul et al., 2024). Therefore, it is important to choose a resource with an intrinsic presence of a catalytic metal, thus avoiding impregnation, to work at low temperatures (>500 °C) and ensure carbon matrix stability.

Several mechanisms explain the catalytic pathways for NO decomposition using various catalysts. They all assume that NO molecules are adsorbed onto the active sites and are dissociated into N₂ and O₂. However, the adsorption sequence of NO molecules, the reaction intermediates, and the rate-determining step (RDS) remain controversial (Xie et al., 2021). The Langmuir-Hinshelwood model identified the formation of the intermediate N₂O as the RDS, while the Eley-Rideal mechanism emphasized the production of N₂O₂ as the RDS. Reactions over metals were reported to be favored for zerovalent metals. NO molecules are adsorbed, and the metal is oxidized by N₂ desorption. The resulting metal oxides are then reduced by carbon, desorbing CO or CO₂ to form zerovalent metals again. The abovementioned mechanisms assume that the active sites are regenerated after the desorption. However, while on stream, the adsorption sites are degraded, and the main deactivation was initially caused by the rapid sintering of metallic active sites (Wang et al., 2023a). Deactivation may also be explained by pore closing and active site coverage by coke deposition (Wang et al., 2023a).

The presence of nitrogen (N) in biocarbon structure was reported to provide better performance in deNOx applications (Guo et al., 2016). N has three possible structures in biocarbon: pyridinic, pyrrole, and graphitic (Guo et al., 2016). Overall, the pyrrole structure is predominant (54 %), followed by the pyridinic structure (26 %), and the

graphitic structure (20 %) (Yang et al., 2022a). Doping elm biocarbon with up to 3.01 % N improved NO removal performance by up to 25 % (Yang et al., 2022a). Porous carbon doped with 14% of nitrogen significantly enhanced the direct decomposition of NO, with an efficiency up to 93% at 500 °C (Wang et al., 2018a). Due to the pyridinic structures, nitrogen-doped graphene can decompose NO efficiently by providing an extra electron in the outer orbit to boost catalysis (Wang et al., 2018b). Furthermore, a DFT (Density Function Theory) study showed that a biocarbon doped with N has more attractive sites (lower adsorption energy, Eads) (Yang et al., 2022b). The presence of N in the adsorption sites decreased Eads for the CO₂, CH₄, and N₂O and acted as an electron donor. Nevertheless, the positive effect of a graphitic N adsorption site on NO seemed to be less important with the presence of some O-functional groups such as -CHO and -COOH (Liu et al., 2024). On the contrary, the interaction was reported as stronger for the -OH group and the pyrrolic N adsorption site with -OH, -CHO, and -COOH. Algal biochars required attention due to their content of up to 31% and 56% of nitrogen and ash, which favors potential environmental applications (Wang et al., 2024a).

No studies have explored the use in catalytic processes of biocarbon naturally rich both in nitrogen and minerals. This study aims to elucidate NOx abatement mechanisms (deNOx) within a low temperature range (200-500 °C) using seaweed biocarbon, which is naturally rich in nitrogen and minerals. Unlike previous studies, no pretreatment (activation or impregnation) was carried out on the biocarbon, to explore the role of biocarbon surface functional groups and inorganic composition on deNOx. The influence of the feedstock composition, biocarbon production temperature, and deNOx operating conditions will be discussed and compared to the literature. A special focus is put on the interaction between NO and surface metals and oxygen-functional groups, by considering the presence of nitrogen in seaweed biocarbon. Experimental results were completed by a modelling approach based on DFT calculations to accurately represent NO adsorption and catalytic conversion mechanisms through biocarbon surface chemistry determined from seaweed biocarbon characterization.

## 2. Materials and methods

### 2.1. Biocarbon catalysts preparation

Seaweed (Fucus ssp.) from CEVA (Centre d'Etude et de Valorisation des Algues), Pleubian, France, was selected as the feedstock due to its high ash and nitrogen contents. The biocarbon production was carried out in a pilot-scale fixed-bed reactor (Fig. S1, Sup. Mat.) (Bertus, 2025). 1.4 kg of feedstock was placed in the furnace and then heated under N₂ flow from room temperature up to the final temperature (650 or 800 °C) at 20 °C.min⁻¹. During pyrolysis, clean dry permanent gases were monitored online thanks to a micro-GC connected to the exhaust gas using a tar protocol (1st bottle at ambient temperature filled with isopropanol, 2nd and 3rd bottle at -15 °C filled with isopropanol, and 4th bottle empty at -15 °C). Pyrolysis was stopped by switching off the heating process in the oven after 4h or when there was no more CO, CO₂ production. 550 g of biocarbon from seaweed were obtained at 650 °C (named SW650) and 500 g at 800 °C (SW800), used as biosourced catalysts for the deNOx experiment. The resulting biocarbons have similar particle size ranges from 3 mm to 0.1 mm, with to a porosity of 0.46.

### 2.2. DeNOx experiments

The experimental setup (Fig. S2, Sup. Mat.) consisted of a fixed-bed reactor, a cold trap system, an online µGC (Agilent 990), and an online NOx analyzer (ThermoScientific, 42i-HL, with a span value of 1.017) (Graul et al., 2024). The fixed bed reactor was a quartz tube placed vertically in a Heraeus furnace (D-6450 Hanau) capable of operating up to 1100 °C. A porous disk placed in the isothermal zone of the furnace allowed handling the crucible with the sample. A tar protocol composed
of three impinger bottles allowed trapping tars and moisture before permanent gas analysis. The first cold trap with isopropanol and cooled at 5 °C trapped soot and tars. The second trap, filled with silica gel, and the last one, empty, both placed at room temperature, trapped residual moisture. The gas composition (N₂, O₂, CO₂, CO, and H₂) was analyzed thanks to an online micro-GC with an interval of 2.5 min. The NO and NO₂ were measured with the online analyzer based on chemiluminescence at 1-min intervals.

For each test (SW650 and SW800), 4 g (i.e., a bed height of approximately 2 cm) of biocarbon was used. The temperature profile started with a first step of heating up to 140 °C under argon (400 mL min⁻¹, 1 bar), at 10 °C.min⁻¹, which was held for 15 min to remove moisture. Then, the temperature was raised to the test temperature (200 °C, 350 °C, 500 °C) at 10 °C.min⁻¹. Ar was maintained for 30 min to remove CO, CO₂, H₂, and CH₄ released from biocarbon thermal decomposition. The inlet flow was then switched to a NO flow (1021 ppm of NO in Ar, 400 mL min⁻¹, 1 bar, set as t = 0 min). The Gas hour space velocity (GHSV) is fixed at 5287 h⁻¹. This concentration was chosen to simulate the NOx concentration in a real flue gas (Bertus, 2025), (Najafi et al., 2009). One hour after that, the steady state was reached, so the gas was switched to Ar, and the reactor was cooled down. The experiments were repeated twice. No significant increase in NO₂ was measured in any of the tests.

The global NO decomposition reaction is proposed as follows (Eq. (3)).

$$NO \leftrightarrow \frac{1}{2} N_2 + \frac{1}{2} O_2 \qquad \text{Eq. 3}$$

The conversion rate of NO (RNO) was defined in the steady state (between 40 and 60 min) according to Eq. (4).

$$R_{NO} = \frac{NO_{inlet} - NO_{outlet}}{NO_{inlet}} \qquad \text{Eq. 4}$$

The proportions of nitrogen/oxygen converted from NO to N₂ ($\Phi_{N_2}$, Eq. (5)) and to O₂ ($\Phi_{O_2}$, Eq. (6)), respectively, normalized to the stoichiometric ratio were calculated. They allow the quantification of the conversion by the global reaction following the nitrogen and oxygen balance.

$$\Phi_{N_2} = 2 \cdot \frac{[N_2]_{Output}}{[NO]_{Converted}} \qquad \text{Eq. 5}$$

$$\Phi_{O_2} = 2 \cdot \frac{[O_2]_{Output}}{[NO]_{Converted}} \qquad \text{Eq. 6}$$

As n the reaction order of NO concentration lower than 1, (Garin, 2001), the apparent activation energy Ea can be calculated as follows (Gong et al., 2021):

$$\ln\{T[1 - (1 - R_{NO})^{1-n}]\} = \ln\left(k_0 \frac{(1 - n)e^{\beta_{char}}}{c_{NO}^{1-n}}\right) - \frac{E_a}{RT} \qquad \text{Eq. 7}$$

where T is the reaction temperature, k₀ is the reaction rate constant, R is the universal gas constant, and β is the reaction order of the biochar concentration (=1, (Gong et al., 2021)).

The uncertainty on the activation energy can be expressed as:

$$\Delta E_a = \sqrt{(\Delta E_a)_{\text{Reaction order}}^2 + (\Delta E_a)_{\text{Statistical}}^2} \qquad \text{Eq. 8}$$

with reaction order uncertainty:

$$(\Delta E_a)_{\text{Reaction order}} = \left| \frac{\partial E_a}{\partial n} \right| \Delta n \qquad \text{Eq. 9}$$

The uncertainty from the reaction order is 0.75 kJ mol⁻¹, while the statistical uncertainties are 6.12 and 5.66 kJ mol⁻¹, respectively, for SW650 and SW800 at these temperatures. The deep analysis is shown in the supplementary material, Fig. S3.

The uncertainty of the present setup, ΔRNO is represented by three terms, representing experimental hardware, measurement of NO outlet, and statistical errors. The experimental hardware error, which depends on the temperature, is evaluated as:

$$(\Delta R_{NO})_{\text{Hardware}} = \left| \frac{\partial R_{NO}}{\partial T} \right| \Delta T \qquad \text{Eq. 10}$$

The gradient term and ΔT are 0.0002 %.K⁻¹ and 1 K, respectively, meaning the hardware error is 0.0002 %. The error in the NO outlet measurement can be defined as follows:

$$(\Delta R_{NO})_{\text{Measurement}} = \left| \frac{\partial R_{NO}}{\partial NO_{Outlet}} \right| \Delta NO \qquad \text{Eq. 11}$$

The gradient term is calculated using Eq. (4), equaling 0.1 %.K⁻¹ and 0.1 ppm for ΔNO. The error is 0.01 %. The statistical error (standard deviation) varies between 1 % and 2 % across all conditions. The total error is defined as follows:

$$\Delta R_{NO} = \sqrt{(\Delta R_{NO})_{\text{Hardware}}^2 + (\Delta R_{NO})_{\text{Measurement}}^2 + (\Delta R_{NO})_{\text{Statistical}}^2} \\ \approx (\Delta R_{NO})_{\text{Statistical}} \qquad \text{Eq. 12}$$

Due to negligible hardware and measurement errors, the total error can be estimated using the statistical error.

## 2.3. Characterization of biocarbon catalysts

Biosourced catalysts were characterized before and after deNOx experiments. The organic element composition (carbon, hydrogen, nitrogen, sulfur) was determined by a Flash 2000 elemental analyzer. Ash content was determined according to ISO 21656 at 815 °C. Inorganic element content was determined by Ultima 2 induced-coupled plasma atomic emission spectroscopy (ICP-AES) after alkaline fusion (Claisse LeNeo). The thermal stability of biocarbons was determined by thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC, Seteram Labsys Evo 1600). Specific surface area was characterized by N₂ adsorption using Brunauer-Emmett-Teller calculations (BET, Micromeritics Tristar II 3020). The chemical structure of the biocarbon samples was analyzed by Fourier transform infrared spectroscopy (Thermo Fisher Nicolet IS10 mid-FTIR spectrometer) with a resolution of 2 cm⁻¹, by using disks of 2 wt% of biocarbon in KBr. Helium Temperature-Programmed Desorption (He-TPD) coupled to gas chromatography (microGC, Micromeritics Autochem 2920) was used to analyze the oxygenated surface groups. Their quantities per surface were determined by spectrum deconvolution (Matlab program) and the integration over the characteristic temperature ranges of the surface species, and the uncertainty is up to 5% (Fig. S4, Sup. Mat.). XPS spectra were acquired using a ThermoFisher Scientific K-ALPHA spectrometer with a 400 µm X-ray spot size and a monochromatised Al-Kα source (hv = 1486.6 eV). The powders were pressed onto indium foils. Full spectra (0–1150 eV) were obtained with a constant pass energy of 200 eV, and high-resolution spectra were recorded with a constant pass energy of 40 eV. Charge neutralisation was applied during the analysis. High-resolution spectra (i.e. C 1s, O 1s and N 1s) were quantified using Avantage software (provided by ThermoFisher Scientific) by applying Scofield sensitivity factors. Attention was primarily focused on the N 1s spectra to determine the chemical environment after fitting.

## 2.4. Adsorption energy calculation

Adsorption energy was calculated using DFT and the Generalized Gradient-Approximation (GGA) of Perdew-Burke-Ernzerhof (PBE). A pseudopotential method with Kresse-Joubert (PAW) pseudopotentials was used (files: H.pbe-n-kjpw_psl.1.0.0UPF, C.pbe-n-kjpw_psl.1.0.0UPF, N.pbe-n-kjpw_psl.1.0.0UPF and, O.pbe-n-kjpw_psl.1.0.0UPF, Fe.pbe-spn-kjpw_psl.1.0.0UPF, and K.pbe-spn-kjpw_psl.1.0.0UPF)
(Garijo et al., 2003).

All calculations were performed with the PWscf code from the Quantum ESPRESSO distribution with the spin polarization turned on, van der Waals corrections (DFT-D3), square box size of 40 bohr, and the gamma point. Kinetic energy and charge density cutoffs were set to 47 Ry and 326 Ry, respectively. Convergence requirements for structural relaxation included an energy convergence criterion of $10^{-5}$ Ry, a force convergence criterion of $10^{-5}$ Ry. Bohr$^{-1}$, and an SCF convergence criterion of $10^{-8}$ Ry. The adsorption energy was calculated by considering the adsorption energy of biocarbon and NO, subtracting from this value the adsorption energies of biocarbon and NO individually (Yang et al., 2022b), (Quantum Espresso) (Eq. (12)).

$$E_{ads} = E_{biochar+NO} - E_{biochar} - E_{NO} \qquad \text{Eq. 13}$$

where $E_{biochar+NO}$, $E_{biochar}$, and $E_{NO}$ are the total energies of the system biocarbon with NO adsorbed, the isolated biocarbon, and the isolated NO molecule. If $E_{ads}$ is less than -50 kJ mol$^{-1}$, adsorption falls into the chemical process, while if $E_{ads}$ is between -30 and -10 kJ mol$^{-1}$, it falls into the physical adsorption range (Quantum Espresso).

Bader charge calculations were carried out to estimate the surface electron changes after NO adsorption.

# 3. Results

## 3.1. Biocarbon characterization

Seaweed biocarbons (SW650, SW800) were characterized by elemental composition (Table 1). As expected, C, H, N, and O content decreased with the increase in the pyrolysis temperature due to devolatilization. N content was high (2.33 and 2.40 wt%, db) and close to values reported in the literature for seaweed (from 1.9 to 4.0 wt%) (Bird et al., 2011) and for doped biocarbon for NO reduction (from 2.75 to 5.69 wt%) (Yang et al., 2022a). The ash content was relatively high, between 43.79 and 52.99 wt%, db, the same order of magnitude as other seaweed biocarbons from the literature (around 41 wt%) (Bird et al., 2011).

Overall, the biocarbon produced at 800 °C presented a higher metal content than that produced at 650 °C (Table 2). This can be explained by a higher decomposition of the carbon matrix at 800 °C compared to 650 °C, which is also reflected in the elemental composition. Metals found in both SW samples were alkali metals (up to 60 %) and alkaline earth metals (31 %, Table 2), such as biocarbons produced from various resources (plant residues, manure, and sludge) (Su et al., 2024). The high amount of AAEM promotes NO decomposition. Several studies have shown that Na (Xu et al., 2017), K (Chen et al., 2023), Ca (Xu et al., 2023), and Mg (Zhang et al., 2020) are catalytic promoters. Biocarbons used for deNOx and catalytic applications in general also present a higher metal content, such as fern, willow (Graul et al., 2024), and oak (Díaz-Maroto et al., 2023b). In these cases, biocarbons present high K (up to 180.8 mg·g$_{biocarbon}^{-1}$), Na (5.9 mg·g$_{biocarbon}^{-1}$), and Ca (103.0 mg·g$_{biocarbon}^{-1}$). In the case of SW biocarbon, the large content of Na and AAEM is directly related to the marine nature of the algae. Compared to biocarbons produced from common resources, the AAEM contents of SW biocarbons are in the upper end of the range, such as K (~70.0 this study vs. 1.4 to 91.4 mg·g$_{biocarbon}^{-1}$ (Su et al., 2024)), Ca (~43.0 in this study vs.

<table><caption>**Table 2**
Inorganic composition of seaweed biocarbon.</caption><thead><tr><th rowspan="2">Metallic Group</th><th>SW650</th><th>SW800</th><th rowspan="2">Metallic element</th><th>Raw SW</th><th>SW650</th><th>SW800</th></tr><tr><th colspan="2">mg·g<sub>biocarbon</sub><sup>-1</sup></th><th colspan="2">mg·g<sub>biocarbon</sub><sup>-1</sup></th></tr></thead><tbody><tr><td>Alkali metals</td><td>118.94</td><td>125.46</td><td>K</td><td>33.34</td><td>69.82</td><td>73.85</td></tr><tr><td></td><td></td><td></td><td>Na</td><td>20.09</td><td>49.12</td><td>51.61</td></tr><tr><td>Alkaline earth metals</td><td>60.49</td><td>64.93</td><td>Ca</td><td>12.70</td><td>42.28</td><td>43.91</td></tr><tr><td>Nonmetals</td><td>8.74</td><td>13.29</td><td>Mg</td><td>6.17</td><td>18.21</td><td>21.02</td></tr><tr><td></td><td></td><td></td><td>S</td><td>5.81</td><td>5.80</td><td>9.19</td></tr><tr><td></td><td></td><td></td><td>P</td><td>1.99</td><td>2.94</td><td>4.10</td></tr><tr><td>Metalloids</td><td>6.56</td><td>8.63</td><td>Si</td><td>1.92</td><td>6.56</td><td>8.63</td></tr><tr><td>Transition metals</td><td>3.21</td><td>4.13</td><td>Fe</td><td>0.56</td><td>1.50</td><td>2.41</td></tr><tr><td></td><td></td><td></td><td>Mn</td><td>0.16</td><td>0.63</td><td>0.63</td></tr><tr><td></td><td></td><td></td><td>Cr</td><td>0.08</td><td>0.48</td><td>0.50</td></tr><tr><td></td><td></td><td></td><td>Cu</td><td>0</td><td>0</td><td>0</td></tr><tr><td></td><td></td><td></td><td>Ni</td><td>0.10</td><td>0.30</td><td>0.28</td></tr><tr><td></td><td></td><td></td><td>Co</td><td>0.08</td><td>0.30</td><td>0.31</td></tr><tr><td>Post-transition metals</td><td>0.25</td><td>0.24</td><td>Al</td><td>0</td><td>0</td><td>0</td></tr><tr><td></td><td></td><td></td><td>Cd</td><td>0</td><td>0.25</td><td>0.24</td></tr><tr><td></td><td></td><td></td><td>Pb</td><td>0</td><td>0</td><td>0</td></tr><tr><td></td><td></td><td></td><td>Zn</td><td>0</td><td>0</td><td>0</td></tr></tbody></table>

1.3 to 94.0 mg·g$_{biocarbon}^{-1}$ (Su et al., 2024)), and Mg (~20.0 in this study vs. 0.6 to 28 mg·g$_{biocarbon}^{-1}$ (Su et al., 2024)). +6.

Transition metals such as Fe have a well-known catalytic performance in general but also in NO reduction (Basile et al., 2006), (Zhou et al., 2023). Similar or higher contents of catalytic metals (Fe, Ni) were found between SW biocarbons (2.0 mg·g$_{biocarbon}^{-1}$) and biocarbons from fern, willow (Graul et al., 2024), and oak (Díaz-Maroto et al., 2023b) used for deNOx (0.1 to 5.3 mg·g$_{biocarbon}^{-1}$). Even if Fe and Ni are in the minority compared to the content of AAEM in the biocarbon, their catalytic effect will be enhanced by AAEM as promoters. Sulfur has a negative effect due to its reaction with Ca, resulting in calcium sulfate, which has no catalytic properties (Zhao et al., 2024).

SW800 presented a much higher specific surface area than SW650 (Table 1), due to the difference in pyrolysis temperature. The higher pyrolysis temperature supplies the activation energy used to reach almost total devolatilization of biomass constituents, developing microporosity, and promoting the evolution of pore structure (Zevenhoven and Hupa, 1998). These values remain in the range of biocarbon used as biosourced catalysts (8.8 to 42.4 m$^2$·g$^{-1}$) (Graul et al., 2024) but are lower than those typically observed for biocarbon exclusively dedicated to adsorption processes (around 1000 m$^2$·g$_{biocarbon}^{-1}$) (Leng et al., 2021a). BET isotherms suggested a mesoporous structure of the samples (Fig. S5, Sup. Mat.), which agrees with typical structures for biocarbon obtained under similar conditions in the literature (Sadegh et al., 2024). The pore volume of SW is in the lower range of biocarbons (Poo et al., 2018). SW800 has a higher pore volume than SW650 without changing the pore size (Table 1). The higher pyrolysis temperature can explain this, although the specific surface area changes slightly above 500 °C (Poo et al., 2018). This difference could be mainly attributed to the different K and Na content in biocarbons (Table 2), which act as self-activators for pore development during pyrolysis (Leng et al., 2021b).

Thermal stability was determined by TGA (Fig. S6, Sup. Mat.). Less than 5 % and 10 % of mass loss were observed for operating

<table><caption>**Table 1**
Elemental composition of seaweed biocarbons.</caption><thead><tr><th rowspan="2">Samples</th><th>C</th><th>H</th><th>N</th><th>S</th><th>O</th><th>Ash</th><th>Specific surface area</th><th>Pore volume</th><th>Pore size</th></tr><tr><th>wt.%, db</th><th></th><th></th><th></th><th></th><th></th><th>m$^2$·g$_{biocarbon}^{-1}$</th><th>cm$^3$·g$_{biocarbon}^{-1}$</th><th>nm</th></tr></thead><tbody><tr><td>Raw SW</td><td>34.96 (±1.50)</td><td>4.10 (±0.22)</td><td>2.20 (±0.50)</td><td>&lt;0.1</td><td>29.86</td><td>28.54 (±1.25)</td><td>-</td><td>-</td><td>-</td></tr><tr><td>SW650</td><td>39.40 (±2.75)</td><td>0.83 (±0.43)</td><td>2.40 (±0.83)</td><td>&lt;0.1</td><td>13.57</td><td>43.79 (±2.22)</td><td>17.7 (±2.3)</td><td>0.020 (±0.001)</td><td>14.2 (±0.9)</td></tr><tr><td>SW800</td><td>37.90 (±5.31)</td><td>0.50 (±0.17)</td><td>2.33 (±0.23)</td><td>&lt;0.1</td><td>6.27</td><td>52.99 (±4.31)</td><td>100.6 (±2.8)</td><td>0.026 (±0.003)</td><td>14.1 (±1.1)</td></tr></tbody></table>

<footnote>* wt.%, db: mass percentage, dry basis; O was calculated by difference.</footnote>
temperatures below 350 °C and 500 °C, respectively. These results are in agreement with the mass loss reported for fern and willow catalysts at 500 °C (Graul et al., 2024). Higher temperatures were reported to enhance biocarbon devolatilization and depolymerization, contributing to biocarbon carbonaceous structure degradation (Raymundo-Piñero et al., 2009). However, the stabilization of the biocarbon carbon matrix for its use at higher temperatures could be achieved by graphitization (Collard and Blin, 2014).

The main oxygenated surface groups were identified thanks to He-TPD coupled to microGC analysis (Fig. 1). Overall, the biocarbon surface remained unchanged after 1h of the deNOx process. However, a slight decrease in CO₂ emission was observed before and after deNOx experiments for the temperature range between 100 °C and 500 °C. This decrease is mainly attributed to carbonyl groups (Fig. 1d), which may decompose during the heating step before reaching the deNOx temperature, corresponding with the mass loss determined by TGA.

SW800 has fewer oxygenated surface groups than SW650 by a factor of 7 (Fig. 1c). The lower quantity of oxygenated surface groups with high-energy bonds, such as lactone, carbonyl + quinone, and pyrone, for SW800 compared to SW650 is explained by the higher pyrolysis temperature, which kinetically favors the decomposition of high-energy bonds (Raymundo-Piñero et al., 2009). Nevertheless, the presence of O-functional groups such as -OH, -CHO, and -COOH near to adsorption sites (N-site or metal site) reduces their NO adsorption ability (Liu et al., 2024).

Nitrogen structures in biocarbon catalysts were analyzed in a first step using IRTF spectra (Fig. 2), and the bands were assigned according to the literature (Romero Millan et al., 2023), (Leng et al., 2020a). Depending on the environment, the band of a chemical bond can shift, as observed for SW650 and SW800. The peak corresponding to the red band is slightly shifted to the right, and the peak corresponding to the blue band is shifted to the left. This can be explained by the high inorganic content, which may affect the vibration of the chemical bond. Two nitrogen bands (graphitic nitrogen: 1106-1121 cm⁻¹ and ring structures of nitrogen: 1288-1388 cm⁻¹ (Romero Millan et al., 2023), (Leng et al., 2020a)) seem to indicate that graphitic, pyridinic, and pyrrolic nitrogen structures are present in the biocarbon. To confirm the nitrogen structures, additional biocarbons without nitrogen (from wood and rice husk) are compared (Fig. S7, Sup. Mat.), and none of them have these two nitrogen bands. It can be concluded that the two bands observed for SW650 and SW800 are due to nitrogen structures (graphitic, pyridinic, and pyrrolic).

In a second step, nitrogen structures in biocarbon catalysts were quantitatively analyzed by XPS (Table 3). Deconvolution results are shown (Fig. S8, Sup. Mat.). The main peaks corresponding to pyridinic, pyrrole, and graphitic nitrogen were identified according to the literature. Total nitrogen atomic percentage on the surface was higher for SW650 (2.40%) compared to SW800 (0.97%) due to its lower pyrolysis

<figure><img src="image_3.png" /><figcaption>**Fig. 1.** CO₂ and CO He-TPD spectra of the two biosourced catalysts (a) SW650, (b) SW800, (c) quantity per surface comparison before deNOx and (d) after deNOx.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 2. FTIR spectra of seaweed biocarbon produced at 650 °C (SW650) and 800 °C (SW800, bands (Leng et al., 2020a)).</figcaption></figure>

production temperature. Due to this same reason, carbon atomic ratio diminished between SW650 and 800 (53.00 to 33.37%), while oxygen atomic ratio increased (18.85 to 24.15%). Total nitrogen diminished for SW650 after deNOx (1.51%), while it increased for SW800 (1.83% for deNOx at 500 °C). SW650 nitrogen structures were revealed to be mainly pyridinic (42%), pyrrolic (40%) and amine (10%). SW800 main structures detected were amines (39%), metal nitrides (24%) and pyridinic (21%). No change in N-structure type was detected between biocarbon catalysts before and after deNOx, both at 200 and 500 °C. In terms of proportions for SW650, pyridinic structures slightly changed between before and after deNOx(42 vs. 44%, respectively). Amine structures were drastically reduced (–86%), while graphitic nitrogen (–43%), oxidized nitrogen (–75%) and pyrrolic structures increased (40% vs. 47%, before and after deNOx respectively). For SW800, pyridinic structures increased (21% vs. 27% for deNOx at 200 °C and vs. 30% for deNOx at 500 °C) as pyrrolic structures (15% vs. 23% for both deNOx at 200 °C and at 500 °C).

The XRD results showed differences in the inorganic phases between SW650 and SW800 (Fig. 3). Higher pyrolysis temperatures convert calcite to calcium oxide. This thermal degradation occurs around 750 °C (Karunadasa et al., 2019). Silicon oxide is not detected for SW800, which may be due to the reaction with some CaCO₃, CaO, and Ca(OH)₂ to form CaSiO₃ (Chen et al., 2025) observed at high-temperature pyrolysis, 800 °C. Potassium chloride has been observed to have catalyst

properties for carbon tube production (Lv et al., 2022) which can favor the NO decomposition. The higher KCl quantity found for SW800 should favor the NO decomposition, but it can be balanced by the higher calcium sulfate (CaS), which has no catalytic properties (Zhao et al., 2024). Magnesium Oxide (MgO) seems to be important on the surface of biochars and has been highlighted as a good catalyst (Elkhalifa and Friedrich, 2018). After the process, the XDR patterns of biochars are slightly changed.

## 3.2. NO decomposition

The concentration of N₂ and converted NO in the exhaust gas after NOx abatement through SW catalysts was investigated (Fig. 4). The amount of NO converted was calculated by subtracting the inlet concentration from the outlet concentration. A delay of 40 min is observed before flow rates stabilize, which is the time required to fill the setup with the inlet mixture (NO + Ar). The N₂ flow rate was measured at the outlet during NO conversion as an indicator of NO decomposition through catalytic reactions over the SW biocarbon.

The results showed similar NO conversions for both catalysts, SW650 and SW800. At 200 °C and 500 °C, they presented similar minimum and maximum NO conversions (15.6 and 20.5 %, respectively, Fig. 5), which were reached in about 40 min. The large difference in specific surface area between the two catalysts does not appear to affect the deNOx performance under the conditions tested in this study, which is consistent with the literature. The pyrolysis temperature of semi-coke char (900 °C to 1300 °C) was reported to have no apparent effect on the NO reduction, even when the specific surface area was increased (from 9.61 to 52.02 m² g⁻¹) (Wang et al., 2023b). This may be due to the low specific surface area (<100.6 m² g⁻¹ biocarbon, Table 1), whereas carbon with a specific surface area of 1000 m² g⁻¹ biocarbon is used for bio-absorbent (Leng et al., 2021a). However, the results showed that the conversion rate at 350 °C was dependent on the biocarbon, while the performance of SW650 was similar at 350 °C and 200 °C (15.6 %), the performance of SW800 was closer for 350 °C and 500 °C tests (22.5 %, Fig. 5). The higher pyrolysis temperature for biocarbon production allowed higher RNO and N₂ flow (Fig. 4b) at a lower deNOx temperature (350 °C), which means higher catalytic conversion. This may be related to the higher metal content in SW800 (Table 2). The catalytic reaction rates increase with temperature and the metal content, which is consistent with the results. In the literature, the effect of the operating temperature from 600 °C to 850 °C with semi-coke char showed a total conversion at 850 °C (Wang et al., 2023b). At 600 °C, the reported NO conversion was about 5%, much lower than in this study at 500 °C. This can be explained by the difference in ash content (9.61 (Wang et al., 2023b) vs. 43.79-52.99 %, this study), which indicates the crucial role of

<caption>**Table 3**</caption>

Nitrogen structures in seaweed biocarbon produced at 650 °C (SW650) and 800 °C (SW800), before and after deNOx at 200 °C (SW800) and 500 °C (SW650 and SW800).

<table><thead><tr><th rowspan="2">Samples</th><th rowspan="2"></th><th>C 1s</th><th>O 1s<br>(total)</th><th colspan="2">N 1s</th><th colspan="2">Pyrrolic (Yang and Jiang, 2014)</th><th colspan="2">Graphitic (Maliutina et al., 2018)</th><th colspan="2">Oxidized (Mahaninia et al., 2015)</th><th rowspan="2">Total</th></tr><tr><th></th><th></th><th>Metal nitrides (Greczynski et al., 2017)</th><th>Pyridinic (Yuan et al., 2018)</th><th>Amine (Yang and Jiang, 2014)</th><th></th><th></th><th></th><th></th><th></th></tr></thead><tbody><tr><td colspan="13" style="text-align: left; font-weight: bold;">Binding Energy peak (eV)</td></tr><tr><td colspan="13" style="text-align: left;">284.55 - ~397.6</td></tr><tr><td colspan="13" style="text-align: left; font-weight: bold;">Atomic percentage (%)</td></tr><tr><td rowspan="2">Raw</td><td>SW650</td><td>53.00</td><td>18.85</td><td>-</td><td>1.01</td><td>0.22</td><td>0.95</td><td>0.14</td><td>0.08</td><td>2.40</td><td></td></tr><tr><td>SW800</td><td>33.37</td><td>24.15</td><td>0.24</td><td>0.20</td><td>0.38</td><td>0.15</td><td>-</td><td>-</td><td>0.97</td><td></td></tr><tr><td rowspan="3">Spent</td><td>SW650<br>deNOx500</td><td>43.81</td><td>28.77</td><td>-</td><td>0.67</td><td>0.03</td><td>0.71</td><td>0.08</td><td>0.02</td><td>1.51</td><td></td></tr><tr><td>SW800<br>deNOx200</td><td>43.45</td><td>28.82</td><td>0.32</td><td>0.45</td><td>0.50</td><td>0.39</td><td>-</td><td>-</td><td>1.66</td><td></td></tr><tr><td>SW800<br>deNOx500</td><td>42.82</td><td>28.37</td><td>0.25</td><td>0.54</td><td>0.63</td><td>0.42</td><td>-</td><td>-</td><td>1.83</td><td></td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 3. XRD patterns of seaweed biocarbon produced at 650 °C and 800 °C before and after deNOx.</figcaption></figure>

ash catalytic and promoter metals in NO conversion. Nevertheless, the decrease in NO conversion from 350 to 500 °C with SW800 may be due to the deactivation mechanism, such as the sintering of metal, which reduces the number of active sites, thereby decreasing the reactivity.

The comparison with the literature (Fig. 5, literature data from Table S1, Supplementary Material) shows that carbon catalysts reach a high conversion rate at high temperature (>500 °C) and are inefficient at low temperature (200 °C). However, SW biocarbons were shown to be efficient from low temperatures (20-500 °C), as the sludge biocarbon without any treatment (Chen et al., 2020), in contrast to the activated carbon (Díaz-Maroto et al., 2023a). Moreover, the performances of SW800 are similar to biocarbon of willow impregnated with Fe, showing that seaweeds are the most suitable resource among those explored up to now for deNOx at low temperature.

No significant CO₂, H₂, CH₄, or CO were measured during the deNOx experiments, except for CO at 500 °C (Fig. 6). Before switching to NO + Ar, CO peaks corresponded to biocarbon thermal decomposition under Ar. When NO + Ar flow started (t = 0 min), an increase in CO was observed, resulting from NO decomposition. According to the proposed mechanism (Wang et al., 2024b):

<figure><img src="image_6.png" /><figcaption>Fig. 4. NO conversions and profiles for the two biosourced catalysts (a) SW650 and (b) SW800.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 5. Conversion rate RNO as a function of the operating temperature for carbon catalysts (Fir BC (Gong et al., 2021), Char and Gasified char (Leng et al., 2020b), Willow and Willow -Fe BC (Graul et al., 2024), Activated char (Díaz-Maroto et al., 2023a), Sludge BC (Chen et al., 2020), Semi-coke (Wang et al., 2023b) and, SW650 and SW800 from this work).</figcaption></figure>

$$2C^* + NO \rightarrow C(N) + C(O) \qquad \text{Eq. 14}$$

$$2C(N) \rightarrow N_2 + 2C^* \qquad \text{Eq. 15}$$

$$C(N) + NO \rightarrow N_2 + C(O) \qquad \text{Eq. 16}$$

$$C(O) \rightarrow CO + C^* \qquad \text{Eq. 17}$$

$$C(O) + NO \rightarrow CO_2 + C(N) \qquad \text{Eq. 18}$$

$$2C(O) \rightarrow CO_2 + C^* \qquad \text{Eq. 19}$$

$$C(O) + CO \rightarrow CO_2 + C^* \qquad \text{Eq. 20}$$

CO could be formed by Eq. (17) directly from the degradation of the carbon adsorption site. However, this CO could not come from oxygenated surface groups such as -COOH, -CO, quinone, lactone, and pyrone, as they were not destroyed, as shown by TPD results (Fig. 1). This suggests that oxygenated surface groups are not direct adsorption sites for NO, unlike metals (Table 2), but they can modify the attractiveness of the adsorption site of metals (Liu et al., 2024). CO desorption may result from metal reduction by carbon. The CO emission and partial destruction of the carbon structure have also been reported in a previous DFT study (Quantum Espresso).
<figure><img src="image_8.png" /><figcaption>Fig. 6. Evolution of CO during deNOx experiment at 500 °C with SW650 and SW800.</figcaption></figure>

The proportions of nitrogen/oxygen converted from NO to N₂ ($\Phi_{N_2}$) and to O₂ ($\Phi_{O_2}$), normalized to the stoichiometric ratio, were calculated (Fig. S9, Sup. Mat.) for the two catalysts at the three deNOx operating temperatures. A flux diagram (Fig. 7) of nitrogen and oxygen is proposed to summarize the process (from Fig. 4 and Fig. S9, Sup. Mat.). The first step represents the adsorption and quantifies the ratio of NO adsorbed on the surface. The second step represents catalysis and quantifies the ratio $\Phi_{N_2}$, and $\Phi_{O_2}$. In each test, just over 50 % of the nitrogen from the adsorbed NO was converted to N₂, and less than 50 % of the oxygen from the absorbed NO was converted to O₂. In the literature, the carbonaceous particles from char studied in a fluidized bed reactor have also provided a higher nitrogen conversion than oxygen conversion (about 100 % and 90 %, respectively) (Chan et al., 1983). In the present study, the unconverted nitrogen may remain on the biocarbon as C(N) or C (N₂), since N₂ desorption is the RDS of the NO conversion on metals. For oxygen, the CO release may explain the non-total conversion to O₂ at high temperature (500 °C, Fig. 6) and the oxygen remaining at the surface as C(O). TPD analysis (Fig. 1) showed a small increase in lactone and carbonyl groups, indicating the residual oxygen in the carbon structure. The non-total conversion of N₂ and O₂ (≠100 %) indicates that NO decomposition occurs via the major intrinsic catalytic metals of the biocarbon (K, Na, Ca, Mg, Table 2) through multiple reactions rather than direct decomposition (Eq. (3)). The N₂ formation is globally constant with the temperature for SW650 while O₂ formation increases with the temperature, favoring the O₂ catalytic formation and desorption. For SW800, a peak of N₂ formation at 350 °C was observed as NO adsorption. The lower efficiency and N₂ formation at 500 °C can be explained

<figure><img src="image_9.png" /><figcaption>Fig. 8. Specific surface areas after deNOx process for SW800.</figcaption></figure>

by some deactivation as metal sintering, due to the higher inorganic content in SW800 (Table 2).

To explore the deactivation mechanism, the specific surface area after the deNOx process was investigated only for SW800 (variations for SW650 were too small and within the uncertainty range). At 200 °C, a weight loss of 2.5 % (Fig. S6, Sup. Mat.) was observed due to the decomposition of the carbon matrix, as shown in the TDP-He spectra (Fig. 1) with the CO₂ release favoring the surface development (Fig. 8). However, at 350 and 500 °C, the mass losses are 4 and 6.5 %, respectively, with higher carbon decomposition, while the specific surface area has decreased, possibly due to pore blockage by the remaining ash (Fig. 8). At medium and high temperatures, coke deposits can also reduce the specific surface area (Arayedh et al., 2025). In addition, the reaction of deNOx over the biocarbon can saturate active sites with nitrogen and oxygen as C(O) and C(N) according to the mechanism (Wang et al., 2024b) which is consistent with the observations of TDP-He spectra (Fig. 1) showing a slight increase in carbonyl and quinone structures for biocarbon used at high temperatures.

In the present study, the addition of gas such as CO₂, CO, or O₂ in the feed stream to simulate real treatment conditions was not investigated to avoid the carbonaceous structure of biocarbon. Rather, the study focuses on understanding the NO interaction with SW biocarbon. However, it has been reported in the literature that CO addition improved NO decomposition in contrast to O₂ addition (Wang et al., 2023b), (Gong

<figure><img src="image_10.png" /><figcaption>Fig. 7. Atomic flux rates for SW650 (left) and SW800 (right) at the three deNOx operating temperatures (200, 350, and 500 °C).</figcaption></figure>
et al., 2021). It can be concluded that NO decomposition with biocarbon as a post-combustion treatment is easier under rich conditions with CO than under lean conditions with O₂ in the flue gas. Future work should explore the effect of the gas atmosphere on NO removal through SW biocarbons.

The apparent activation energy of the global reaction was calculated at a low temperature range (20-500 °C, Fig. 9). The SW and sludge biocarbons have the lower apparent activation energies (>9 kJ mol⁻¹), demonstrating their low-temperature efficiency for NO decomposition compared to the other carbon catalysts. The apparent activation energy was globally inversely proportional to ash content. Nonetheless, some variations were observed and depend on the ash composition. For example, Fern BC and Willow-Fe BC have similar ash content (14.4 vs. 16.7 wt%) while their activation energies differ by a factor 2 (19.13 vs. 9.94). This is due to the large content of Fe in Willow-Fe BC (0.02 vs. 4.6 wt%), which has catalytic performance, and its low amount of Si, which is an inhibitor (1.7 vs. 0.2 wt%) compared to Fern BC (Graul et al., 2024).

No direct relation exists with the specific surface area at low operating temperatures, demonstrating that the DeNOx process is not only a physical process (adsorption). The adsorbed molecules react through chemical paths (Langmuir-Hinshelwood and Eley-Rideal mechanisms) favored by the catalytic metals. These results are in agreement with the experimental part of this study.

### 3.3. Adsorption energy

Two conditions must be met for a site to be favorable for NO decomposition: the reaction must be favored (lower activation energy), and the adsorption energy must be strongly negative. This study focuses on the adsorption of NO onto active sites, the first step of the catalytic process. For this purpose, DFT simulations are used to obtain the adsorption energy of NO (Eq. (13)) on biocarbon. This information confirms whether an active site can be efficient for NO adsorption.

#### 3.3.1. Effect of nitrogen

DFT simulations highlighted the critical role of nitrogen in the biocarbon structure for NO adsorption. The XPS results have shown that for seaweed biocarbons the major nitrogen structures are pyridinic and pyrrolic (42% and 40% respectively) while the graphitic structure is

minor (6%). The molecular optimization of NO adsorption on different adsorption sites was simulated by DFT calculations (Fig. 10) and based on experimental results. The FTIR spectra suggested nitrogen structures as graphitic, pyridinic, and pyrrolic. The E<sub>ads</sub> of graphitic-C was weak –11.9 kJ mol<sup>-1</sup>, (Table 4), which means less favorable for NO adsorption than the graphitic-N –26.8 kJ mol<sup>-1</sup> (Table 4), where the value is close to the literature, 33.1 kJ mol<sup>-1</sup> (Xu et al., 2017). The presence of three N atoms decreased the E<sub>ads</sub> by at least a factor of 11, which significantly improved the NO adsorption. The structures of the simulated biocarbon surfaces with pyridinic and pyrrolic structures are deformed, which is consistent with the simulations of CO<sub>2</sub>, CH<sub>4</sub>, and N<sub>2</sub>O adsorptions on biocarbon-doped nitrogen (Yang et al., 2022b). According to their NO attraction (Table 4), the adsorption sites can be classified as pyrrolic-N, pyridinic-N, graphitic-N, and graphitic-C.

The adsorption of NO on nitrogen structure can be classified into two classes in function of the biocarbon/N-NO distance and adsorption energy. The NO adsorption over the graphitic-N site fell into the physical adsorption range (–30 < E<sub>ads</sub> < –10 kJ mol<sup>-1</sup>), while over pyridinic-N and pyrrolic-N, it fell into the chemical process (E<sub>ads</sub> < –50 kJ mol<sup>-1</sup>). The shorter distances for pyridinic-N and pyrrolic-N (Table 4, 2.78 and 2.71) indicate the formation of a chemical bond, which reflects a higher absolute value of E<sub>ads</sub>. This is confirmed by the Bader charge difference (electron difference), which highlights the transfer of one electron from oxygen to nitrogen inside NO molecule. The nitrogen sites favor NO attraction, and one of the two conditions for catalytic activity is confirmed. For the second condition, the reaction pathways of NO decomposition on N-graphitic (Xu et al., 2017) and N-pyrrolic (Liu et al., 2024) structures have been investigated, demonstrating NO removal potential. However, they did not evaluate the Langmuir-Hinshelwood model and the Eley-Rideal mechanism, classical kinetic models. Future work must determine the Langmuir-Hinshelwood and the Eley-Rideal mechanisms over the three nitrogen sites (graphitic, pyridinic, and pyrrolic sites).

#### 3.3.2. Effect of catalysts (Fe), promoters (K), and functional groups (O-groups)

The effect of the nitrogen groups can be influenced by other elements present in biochar, such as inorganic components. According to the ultimate analysis (Table 2), K is the main element for SW650 and SW800, known as a catalytic promoter, and Fe is the major transition metal,

<figure><img src="image_11.png" /><figcaption>**Fig. 9.** Apparent activation energy at low decomposition temperature (<500 °C) as a function of the carbon catalysts (Bamboo, Fir, and Elm BC (Gong et al., 2021), Char and Gasified chars (Leng et al., 2020b), Willow/Fern-/Fe-Ni BC (Graul et al., 2024), Activated char (Díaz-Maroto et al., 2023a), Sludge BC (Chen et al., 2020) and SW650 and, SW800 from this work) compared to the ash content and the specific surface area.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 10.** Top and side views of NO adsorption with the Bader charge difference on NO atoms on biocarbon with a (a) graphitic-C, (b) graphitic-N, (c) pyridinic-N, and (d) pyrrolic-N adsorption site.</figcaption></figure>

<table><caption>**Table 4**

Properties of NO adsorption in biocarbon with and without nitrogen.</caption><thead><tr><th>Adsorption site</th><th>Graphitic-C</th><th>Graphitic-N</th><th>Pyridinic-N</th><th>Pyrrolic-N</th></tr></thead><tbody><tr><td>Distance biocarbon/N-NO [bohr]</td><td>5.91</td><td>5.73</td><td>2.78</td><td>2.71</td></tr><tr><td>E<sub>ads</sub> [kJ.mol<sup>-1</sup>]</td><td>-11.9</td><td>-26.8</td><td>-130.6</td><td>-150.0</td></tr></tbody></table>

known for its catalytic potential. To investigate the role of oxygen groups, a carbonyl group was added according to the experimental results (Fig. 1). K, Fe, and O were added at three opposite sites on nitrogen-free and pyridinic biocarbon (a suitable site with few surface

modifications). Each calculation focused on the NO adsorption, and on one active site at a time, K, Fe, O, C-graphitic, and N-pyridinic, respectively. The NO positions resulting from each calculation were plotted (Fig. 10).

<figure><img src="image_13.png" /><figcaption>**Fig. 11.** Top views of NO adsorption associated with E<sub>ads</sub> [kJ.mol<sup>-1</sup>] on different active sites of biocarbon with a structure of graphitic-C and pyridinic-N. The dark zones correspond to major absolute Bader charge differences (absolute value higher than 0.1 electron).</figcaption></figure>
The adsorption energy results highlighted the strong attraction of Fe to NO in both configurations (Fig. 11), with a similar $E_{ads}$ for Fe (from -303.0 to -307.2 kJ mol⁻¹) than those previously reported in the literature (-166.9 kJ mol⁻¹) (Basile et al., 2006). This difference is due to the different position of Fe between the configurations: Fe is placed on the top of the graphitic structure as an impregnation (surface deposit) (Basile et al., 2006), and the bond between Fe and C is weak. However, in this study, Fe is considered as an element of the structure with a strong bond corresponding to the inorganic content naturally present in the biocarbon. The Bader charge difference indicated a gain of 1.5 electrons on nitrogen of NO explicated the strong attraction.

On the contrary, $E_{ads}$ on potassium is enhanced by the presence of pyridinic structure, from -51.6 kJ mol⁻¹ to -77.1 kJ mol⁻¹ (Fig. 11). The major absolute Bader charge difference zone has increased due to the nitrogen presence in the biocarbon, making electron transfer to form a chemical bond. The Bader charge difference on nitrogen of NO is up to 1.3 electrons in the presence of a pyridinic structure, while 0.5 electrons on the nitrogen-free structure. The nitrogen in biocarbon favors the electron charge repartition on the structure. The adsorption on the carbon site was slightly improved (from -11.9 to -29.8 kJ mol⁻¹) in the presence of functional groups, but remained weak compared to the

others. Conversely, the adsorption on the pyridinic site was drastically degraded (from -130.6 to -36.2 kJ mol⁻¹) by the presence of the functional groups. NO adsorption on the carbonyl group was shown to be efficient without the nitrogen structure around it. The pyridinic structure reduced the adsorption energy on the carbonyl by 97 %, and the carbonyl group reduced the adsorption energy on the pyridinic structure by 86 %.

The simultaneous presence of nitrogen and oxygenated functional groups inhibited NO adsorption. To confirm this argument, an additional calculation was performed to replace the carbonyl with a hydrogen atom (Fig. 12). The decrease in adsorption energies in the presence of nitrogen and oxygen in the biocarbon was demonstrated, confirming the inhibitory effect of one functional group on the other. This result was also found in the literature with the functional group -COOH (Liu et al., 2024), and can be explained by the strong atomic electronegativity of O and N (3.44 and 3.04, respectively). In the absence of O or N, respectively, the functional group acts as an electron donor, as shown by the Bader charge difference (-0.59 and -0.86 for O and N, respectively) favors the adsorption. However, in the presence of both elements, the electron transfer during the NO adsorption is limited due to the carbons bonded to nitrogen or oxygen. Their electron charges

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Top views of NO adsorption associated with $E_{ads}$ [kJ.mol⁻¹] and the Bader charge difference (<0.1 electron) on each atom for different active sites of biocarbon with a structure of (a) graphitic-C, (b-c) pyridinic-N, and (d) pyridinic-N without O.</figcaption></figure>
were unchanged due to the high electronegativity of oxygen or nitrogen close to them, which retained the electrons and blocked the charge repartition on the surface. This reduced electron attraction decreased the potential for NO adsorption. A correlation was established between the biocarbon surface and the E<sub>ads</sub> of a molecule (Yang et al., 2022c). The adsorption energy depended on the surface properties as the maximum of the electrostatic potential, the LUMO (lowest unoccupied molecular orbital), and the energetic gap between the LUMO and HOMO (highest occupied molecular orbital). This result was consistent with the change in electronegativity of the biocarbon due to the presence of nitrogen and oxygen.

### 3.4. Catalyst regeneration/recyclability strategy

According to the experimental and DFT results, the main deactivation mechanism of this work was shown to be the oxidation of active sites (metals and N-structures) due to NO decomposition. However, this mechanism is expected to be slow, as only small performance losses were observed for 1 h of deNOx tests. The literature describes coke deposition and metal sintering as important deactivation mechanisms. Future work should involve long experimental tests to observe catalyst deactivation, identify the main deactivation mechanisms, and develop a regeneration strategy. Nonetheless, extended deNOx experiments at low temperature (4h at 500 °C) (Graul et al., 2024) showed no significant deactivation of catalysts.

Different reactivation strategies are proposed in the literature, such as regeneration under CO₂ and air to remove carbon deposits (Rego de Vasconcelos et al., 2018). However, air regeneration would probably destroy the biocarbon matrix, so only partial oxidation should be applied; 0.4% O₂ was sufficient to reduce the coke deposit on the biochar catalyst (Sun et al., 2022). However, the main disadvantage is the highly exothermic nature of the reaction, which makes temperature control difficult.

To remove oxygen from the active sites, H₂ or CO reduction can be employed, as is typically done for metal reduction (Ma et al., 2025).

Metal sintering is an irreversible process that operates at a high temperature, dependent on the metal (e.g. Fe sintering takes place at 1000–1200 °C). Redispersing the metal is difficult, so it is better to prevent metal sintering by operating the catalyst at a low temperature, as executed in this study.

Upon the conclusion of the functional lifespan of the SW catalysts, they can be subjected to reuse to enhance soil amendment. The SW catalysts employed for deNOx are devoid of any toxic pollutants or metals (pesticides or arsenic). The chemical and physical characteristics of the samples (CHNS, ash, specific surface area) are comparable to those of sludge char, which has been demonstrated to provide the most significant increases in phytomass (Zhang et al., 2025). The beneficial effects on plant growth are likely to be derived from the relatively high nitrogen (N) content of the sludge char. The nitrogen from the NO decomposition on the catalyst could thus promote plant development.

Industrial-scale use is challenging due to variations in production from batch to batch, which makes it extremely difficult to scale up biochar production at this time (Zomorodbakhsh et al., 2025).

## 4. Conclusion

This study investigated the performance of biosourced catalysts rich in nitrogen and minerals. Seaweed biocarbons (SW650 and SW800) were prepared at two pyrolysis temperatures (650 and 800 °C, N₂), characterized, and tested for NO removal. The produced biocarbons showed a high N (2.3 to 2.4 wt%), Fe (0.15 to 0.24 wt%), and AAEM (17.94 to 19.04 wt%), which can act as promoters for the catalytic effect. The specific surface area was not a parameter affecting the NO removal for the experimental conditions investigated, as confirmed by the literature data. In terms of NO removal, both catalysts gave good performances, up to 22.5 % (SW800) and 20.5 % (SW650) for 1 h, equivalent

or better than literature results at these temperatures. The higher operating temperature favored the NO decomposition even more on SW800 due to its higher metal content. The non-N₂ and O₂ stoichio-metric ratio may suggest catalytic decomposition in several steps rather than direct NO decomposition. In addition, it is suggested that some nitrogen from NO remains on the biocarbon. It was also observed that CO emission during the deNOx process at 500 °C may be due to a partial biocarbon decomposition, confirming the non-direct NO decomposition. The CO released during the deNOx process cannot be attributed to the oxygenated surface groups such as -COOH, -CO, quinone, lactone, and pyrone, because they are still present after the deNOx process. However, CO desorption could result from metal reduction by carbon.

The deactivation of the biosourced catalyst may be due to coke deposit and to atomic nitrogen or oxygen remaining on the surface. This was confirmed by the decrease in the specific surface area and by the slight increase in carbonyl and quinone structures.

The role of N in the biocarbon structure on the NO adsorption was evaluated by DFT pyridinic-N, pyrrolic-N, graphitic-N, and graphitic-C adsorption sites. The adsorption energy for NO decreased drastically in N adsorption sites (from -19.36 to -71.10 kJ mol⁻¹) compared to graphitic carbon sites (-2.57 kJ mol⁻¹). Furthermore, E<sub>ads</sub> depends on the N-structures: the pyridinic structure was the most attractive for NO adsorption. This strong interaction suggested a possible catalytic role in NO decomposition. Nonetheless, oxygen functional groups near the nitrogen site inhibited NO adsorption due to their high electronegativities. This information highlights the potential deactivation of the nitrogen sites by the oxygen remaining from the NO decomposition, as experimentally observed. In terms of minerals, iron presents a high efficiency in adsorbing NO, and potassium shows a similar performance as pyridinic structure (-54.5 Vs. -71.1 kJ mol⁻¹ respectively), highlighting their crucial role in NO adsorption.

Future work will explore catalyst cycling and regeneration strategies (CO₂/H₂), as well as the detailed elucidation of the kinetic parameters of the chemical reactions involved in the mechanism of NOx decomposition through seaweed biocarbon catalysts.

### CRediT authorship contribution statement

**Ronan Pelé:** Writing – original draft, Visualization, Methodology, Investigation, Formal analysis. **María González Martínez:** Writing – review & editing, Validation, Supervision, Resources. **Ange Nzihou:** Writing – review & editing, Validation, Supervision, Resources, Project administration, Funding acquisition, Conceptualization.

### Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

The authors would like to specially thank technicians and research engineers who have helped on biochar characterizations (Céline Madrières, Sylvie Del Confetto, Véronique Nallet and Jean-Marie Sabatier, from RAPSODEE), XPS interpretation results (Christine Labrugère-Sarroste, from CNRS) and set-up technical support (Mickaël Ribeiro and Pierre Bertorelle from RAPSODEE).

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.jenvman.2026.128992.
# Data availability

Data will be made available on request.

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