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# NOₓ decomposition using Ni- and Fe-loaded biocarbon catalysts

Théodore Graul<sup>a</sup>, María González Martínez<sup>a</sup>, Yi Qiu<sup>a,b</sup>, Floriane Fourgeaud<sup>a</sup>, Kuo Zeng<sup>b</sup>, Ange Nzihou<sup>a,*</sup>

## ARTICLE INFO

### Keywords:

Biocarbon

Catalyst

NO decomposition

Nickel

Iron

## ABSTRACT

Fern and willow (W) impregnated with heavy metals (Ni/Fe) were pyrolyzed (800 °C, N₂) to produce heavy metal-loaded biocarbon to catalyze NO decomposition (deNOₓ). The effects of reaction temperature (200, 350 and 500 °C), biomass type and impregnated metals on deNOₓ performance were investigated. WFe and WNi achieved the highest deNOₓ performance at 200 °C (16.5%) and 500 °C (30.6%) respectively. The biocarbon composition and structure were crucial for NO adsorption and metal dispersion, which induced higher CO₂ adsorption (TPO), high specific surface area (419.1 m²/g for WNi), and highly dispersed small Ni particles (SEM). The main routes for deNOₓ evidenced by on-line monitoring were direct decomposition of NO into N₂ and O₂, and NO reduction to N₂, CO and CO₂ by biocarbon sites. Dispersed catalytic metals, as well as NO adsorption and reactivity by biocarbon functional groups, reflect the cost-effective and eco-friendly deNOₓ potential of bio-carbon catalysts.

## 1. Introduction

As one of the major air pollutants, nitrogen oxides (NOₓ) are responsible to the formation of acid rain, photochemical smog, the depletion of ozone and respiratory diseases, seriously affecting human health and the environment [1–3]. NOₓ and mainly nitrogen oxide (NO, > 95% emissions) are mostly emitted from fossil fuel combustion and fundamental industrial processes [4,5]. Currently, selective catalytic reduction (SCR) and three-way catalysts are widely used to decompose NOₓ in fixed and mobile sources, respectively [6–8].

The homogeneous decomposition of NO (Eq.1) is thermodynamically feasible with ΔG = -86.6 kJ/mol at 100 °C [2]. However, the strong binding energy between N and O atom (~ 630.6 kJ/mol) results in a high activation energy (~335 kJ/mol) for this reaction [9].

$$NO \rightarrow \frac{1}{2} N_2 + \frac{1}{2} O_2 \quad (1)$$

The N—O binding energy complicates activation on catalytic surfaces [10,11]. Therefore, the catalytic pathways for NO direct decomposition remain controversial with various mechanisms being proposed [2], including the Langmuir–Hinshelwood model (Eq.2–5, * for vacant active sites) where reactive species react on the catalyst surface [12,13].

Another is the Eley-Rideal (Eq.6–10) model where reactive species adsorbed on the surface react with others in the medium surrounding the catalyst [13,14].

$$2[NO + ^{*} \leftrightarrow NO^{*}] \quad (2)$$

$$2NO^{*} \rightarrow N_{2}O^{*} + O^{*} \quad (Rate Determining Step, RDS) \quad (3)$$

$$N_{2}O^{*} \rightarrow N_{2} + O^{*} \quad (4)$$

$$2O^{*} \leftrightarrow O_{2} + 2^{*} \quad (5)$$

$$NO + ^{*} \leftrightarrow NO^{*} \quad (6)$$

$$NO + NO^{*} \rightarrow N_{2}O_{2}^{*} \quad (RDS) \quad (7)$$

$$N_{2}O_{2}^{*} + ^{*} \rightarrow N_{2}O^{*} + O^{*} \quad (8)$$

$$N_{2}O^{*} \rightarrow N_{2} + O^{*} \quad (9)$$

$$2O^{*} \leftrightarrow O_{2} + 2^{*} \quad (10)$$

Noble metals [15–18], simple metal oxides [19–22], rare-earth metal

<footnote><sup>a</sup> Université de Toulouse, Mines Albi, UMR CNRS 5302, Centre RAPSODEE, Campus Jarlard, Route de Teillet, 81000 Albi, France</footnote>

<footnote><sup>b</sup> State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, 1037 Luoyu Road, Wuhan, Hubei 430074, China</footnote>

<footnote>* Corresponding author at: Université de Toulouse, Mines Albi, UMR CNRS 5302, Centre RAPSODEE, Campus Jarlard, Route de Teillet, 81000 Albi, France.
E-mail addresses: theodore.graul@mines-albi.fr (T. Graul), maria.gonzalez_martinez@mines-albi.fr (M. González Martínez), yi.qiu@mines-albi.fr (Y. Qiu), Floriane.fourgeaud@mines-albi.fr (F. Fourgeaud), zengkuo666@hust.edu.cn (K. Zeng), ange.nzihou@mines-albi.fr (A. Nzihou).</footnote>
oxides [13,23,24], complex metal oxides [25-27] to zeolites [28-30] and graphene catalysts [13,31] can bring down the NO activation energy. These catalysts however suffer high prices from solvent and energy-intensive processes, high reaction temperatures and low oxygen-related activity that could impede their use and the environment [2,32]. Therefore, there is an urgent need to develop catalysts that decompose NOx pollutants effectively, economically and sustainably. Transition metals generally have strong redox ability because of their unique valence electron structure [8,33,34]. Among them, iron (Fe) and nickel (Ni) are widely available, cheap and not toxic (Fe). Above 200 °C, the addition of both Ni and Fe on carbon surface significantly enhanced deNOx, with Ni generally outperforming Fe [35]. The combined effect of both metals has only been recently studied, with improved NOx adsorption and reaction by the complementary behavior of the two active sites [36].

Biocarbon (or biochar) is an eco-friendly carbon-rich material produced through biomass thermal conversion, which has been given diverse applications including catalytic supports and catalysts [37-40]. Biocarbon has been found effective in deNOx without the use of additional reducing agents such as ammonia [41,42]. This has been linked to intrinsic heteroatoms (e.g. N, O, S, P) and biocarbon inherent or impregnated metallic compounds [18,37]. Impregnation incorporates active metallic species into the biocarbon structure and imitates plants that enrich themselves with heavy metals during their growth [43-45]. Active interfaces and binding sites are created during impregnation, resulting in higher active surface area and total pore volume [46]. NO will preferentially react with zerovalent Ni and Fe impregnated on biocarbon, and form N2 or N2O. The resulting metal oxides are then reduced by carbon, forming zerovalent metals again (Eq.11-13, with M = Ni or Fe, and x = 1 or 2).

$$M + ^{+} \rightarrow M^{+} \quad (11)$$

$$2M^{+} + 2NO \rightarrow 2MO^{+} + N_{2} \quad (12)$$

$$xMO^{+} + C \rightarrow xM^{+} + CO_{x} \quad (13)$$

In this study, fern and willow impregnated with Ni and Fe before pyrolysis were used to mimic an easily available hyperaccumulative bioresource with short growth cycle [45,47,48]. Biocarbon catalysts were characterized before and after deNOx experiments in terms of elemental and inorganic composition, specific surface area, surface functional groups, porosity, and thermal stability. The effects of reaction temperature, nature of biomass and impregnated metals, linked to the biocarbon catalyst properties, on deNOx performance were investigated.

## 2. Materials and methods

### 2.1. Preparation of biocarbon catalysts

Fern (F) and willow (W) were selected as raw materials for this study. As short rotation coppices, they present a fast growth and short cycle, which justified their interest in phytoremediation and rapid metal accumulation. For the large-scale production of biocarbon catalysts, availability and abundance of the bioresource in the territory needs to be considered for its selection. To simulate heavy metal content, impregnation of raw biomass with Ni and Fe nitrates (Ni(NO3)2, Fe(NO3)3) was carried out.

Wetness impregnation (WI) was applied to raw biomass. This method consists of the active metallic species incorporation into biocarbon structures via (in-situ) mixing of feedstock with metal precursors, forming active interfaces and binding sites [46,49]. For each round of impregnation, 20 g of biomass was submerged in 1 L aqueous solutions containing Fe or Ni nitrates, stirred for 3 days to reach optimal dispersion of metallic species [50]. The amount of Fe and Ni nitrates used are calculated based on the final goal of reaching a same quality of metal

load in biocarbon (3 wt.% in biocarbon). Three different Fe and Ni impregnation concentrations were used for fern (FFe*, FNi* and FNiFe were only tested at 500 °C). Following the impregnation, the biomass was filtered and dried for 24 h at 60 °C [50]. The impregnated biomass was pyrolyzed under 1 L/min N2 from 25 to 800 °C, at 2 °C/min, followed by a 1 h-isothermal step at 800 °C. For each pyrolysis experiment, 9-10 g of biomass was placed into the crucible and the biocarbon generated was 2-3 g depending on the sample. This would give a proximate solid yield of 25 wt.%. Unimpregnated fern biocarbon (RF) and willow biocarbon (RW) were prepared under the same pyrolysis conditions for comparison.

### 2.2. DeNOx experiments

The performance of prepared biocarbon catalysts in deNOx was tested at laboratory scale. The experimental set-up (Fig. 1) consisted in a fixed-bed reactor followed by a cold trap system, an on-line µGC (Aglent 990) and an on-line NOx analyzer (ThermoScientific, 42i-HL). The fixed-bed reactor consisted of a quartz tube with a porous disc in the isothermal area of the reactor, so the sample could be placed in a quartz crucible on it. The fixed-bed reactor was arranged vertically in a Heraeus furnace (D-6450 Hanau), able to work up to 1100 °C. Cold traps were used to trap impurities and moisture that may end up in the circuit. The first cold trap, filled with isopropanol and cooled by a thermostatic bath, was used to trap soot and tars that can form during treatments at high temperatures (500 °C). The other two traps, at room temperature, respectively filled with silica gel and empty, were used to trap residual moisture to prevent damage to the analytical devices. On-line µGC monitored the outlet gas composition (N2, O2, CO2, CO and H2) with an interval of 2.5 min. On-line NOx analyzer monitored outlet NO and NO2 concentration with an averaging time of 1 min.

4 g (i.e. a bed height of around 2 cm) of biocarbon catalyst were placed in a quartz crucible inside the quartz reactor [51]. The temperature of the oven was then raised to 140 °C under constant argon (Ar) flow (300 mL/min, 1 atm) and kept for 15 min to eliminate traces of moisture and undesirable compounds [52-54]. After that, the temperature was raised to the experiment temperature (200, 350 or 500 °C) with a heating rate of 10 °C/min. The beginning of the experiment was considered when the isothermal step reached the experiment temperature (set as t = 0 min, Fig. 2). During the first 30 min, CO, CO2 and H2 were released from the biocarbon. The production was related to the thermal decomposition of the catalysts and was measured by the online µGC under Ar flow. At 30 min, the inlet gas was switched to a constant flow of 1021 ppm NO in Ar (300 mL/min, 1 atm). This concentration was chosen to simulate the high concentration of nitric oxide (600-1200 ppm) in exhaust gas [10,42]. The deNOx performance was tested for 1 h. Finally, the gas was switched again to Ar and kept for 30 min to observe the desorption phase. At the end of the experiment, the reactor was cooled down and the spent biocarbon catalyst was stored for characterization. Extended experiments were carried out for selected samples to evaluate the degradation of catalysts, with a longer exposure under NO flow (4 h).

To compare the deNOx performance, the deNOx ratio (XNO) was calculated as the percentage of NO being removed after contact with biocarbon catalysts (Eq.14).

$$X_{NO} = \frac{C_{NO,in} - C_{NO,out}}{C_{NO,in}} \quad (14)$$

Where XNO is the deNOx ratio, CNO,in is the inlet NO concentration, CNO,out is the outlet NO concentration, which is the measured value on the NOx analyzer. For CNO,in, it is calculated with the average NO concentration of respective blank test.

Repetitions of experiments were conducted and resulted in a relative standard deviation (RSD) of XNO (Eq.14) inferior to 2%.
<figure><img src="image_3.png" /><figcaption>**Fig. 1.** Experiment set-up for testing deNOₓ performance of biocarbon catalysts.</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** Temperature program of the oven during one standard experiment.</figcaption></figure>

## 2.3. Characterization of biocarbon catalysts

The organic element content (carbon, hydrogen, nitrogen, sulfur, CHNS) and inorganic element content were determined using the elemental analyzer Flash 2000 and induced-coupled plasmas atomic emission spectroscopy (ICP-AES) Ultima 2, before and after deNOₓ experiments. The dispersion of metals on the carbonaceous matrix were investigated with scanning electron microscopy (SEM). The biocarbon thermal stability and ash content were determined with thermogravimetric analysis coupled with differential scanning calorimetry (TGA-DSC, Seteram Labsys Evo 1600). The surface area and surface chemical groups were characterized by N₂ adsorption using Brunauer-Emmett-Teller calculations (BET, Micromeritics Tristar II 3020) and temperature programmed desorption, reduction and oxidation (TPD, TPR, TPO, Micromeritics Autochem 2920), respectively.

# 3. Results and discussion

## 3.1. Biocarbon characterization

### 3.1.1. Elemental composition

The biocarbon catalysts produced were characterized in terms of elemental composition before and after deNOₓ experiments at 500 °C (Table 1). The results showed higher C content for willow-based than for fern-based catalysts, which was coherent with their biomass type (woody vs. herbaceous) [47,55]. Impregnation of both Ni and Fe

<caption>**Table 1**</caption>

CHNS elemental composition of biocarbon catalysts characterized before and after 500 °C deNOₓ experiments.

<table><thead><tr><th rowspan="2">Sample</th><th rowspan="2">Biomass</th><th rowspan="2">Metal</th><th rowspan="2">deNO<sub>x</sub></th><th>C</th><th>H</th><th>N</th><th>S</th></tr><tr><th colspan="4">(wt.%, dry basis/db)</th></tr></thead><tbody><tr><td rowspan="2">RF</td><td rowspan="2">Fern</td><td rowspan="2">None</td><td>Fresh</td><td>72.78</td><td>0.93</td><td>1.47</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>70.92</td><td>1.00</td><td>1.52</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">FNi</td><td rowspan="2">Fern</td><td rowspan="2">Ni</td><td>Fresh</td><td>71.23</td><td>0.73</td><td>1.86</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>72.78</td><td>0.83</td><td>1.57</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">FNi*</td><td rowspan="2">Fern</td><td rowspan="2">Ni</td><td>Fresh</td><td>54.68</td><td>0.86</td><td>1.28</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>57.66</td><td>0.52</td><td>1.22</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">FFe</td><td rowspan="2">Fern</td><td rowspan="2">Fe</td><td>Fresh</td><td>71.01</td><td>0.49</td><td>1.07</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>72.05</td><td>0.51</td><td>0.94</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">FFe*</td><td rowspan="2">Fern</td><td rowspan="2">Fe</td><td>Fresh</td><td>52.43</td><td>0.47</td><td>0.74</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>53.36</td><td>0.38</td><td>0.71</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">FNiFe</td><td rowspan="2">Fern</td><td rowspan="2">Ni,Fe</td><td>Fresh</td><td>66.99</td><td>0.84</td><td>1.15</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>72.28</td><td>0.52</td><td>1.16</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">RW</td><td rowspan="2">Willow</td><td rowspan="2">None</td><td>Fresh</td><td>80.07</td><td>1.32</td><td>0.81</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>83.22</td><td>1.24</td><td>0.89</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">WNi</td><td rowspan="2">Willow</td><td rowspan="2">Ni</td><td>Fresh</td><td>80.34</td><td>0.73</td><td>0.71</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>87.14</td><td>0.51</td><td>0.85</td><td>&lt; 0.01</td></tr><tr><td rowspan="2">WFe</td><td rowspan="2">Willow</td><td rowspan="2">Fe</td><td>Fresh</td><td>82.55</td><td>0.74</td><td>0.62</td><td>&lt; 0.01</td></tr><tr><td>Spent</td><td>86.12</td><td>0.69</td><td>0.67</td><td>&lt; 0.01</td></tr></tbody></table>

resulted in a decrease in the organic composition of biocarbon, of at most 20 wt.% C when heavily impregnated. A slight increase of C content was generally observed after deNOₓ experiments due to thermal decomposition, the total observed mass loss being around 5 to 10%.

The inorganic composition of biocarbon catalysts were based on ICP-AES analysis (Table 2). Raw biocarbon (RF and RW) had negligible content of Fe and Ni, while the impregnation brought the value above the thresholds of hyperaccumulation (> 0.3 wt.% biocarbon) [45,56]. Both RF and RW biocarbon had high alkaline and alkali-earth (AAEM) content: RF was especially rich in K and had high content of Na, Mg and Ca; while RW was especially rich in Ca and had high content of K. Impregnation with Ni and Fe salts resulted in the leaching of inherent AAEM, which may impact catalytic performance. AAEM ions have been proved to be effective in promoting the deNOₓ activity for many simple metal oxides [2]. They play multiple roles including increasing the surface area, creating basic sites to promote NO adsorption, facilitating the desorption of O₂ and forming active sites, leading to enhanced activity [2]. Therefore, AAEM can be seen as inherent metals that may
<table><caption>**Table 2**
Inorganic composition of biocarbon catalysts before deNOₓ experiments.</caption><thead><tr><th rowspan="2">Sample</th><th colspan="7">(wt.% in biocarbon)</th></tr><tr><th>Fe</th><th>Ni</th><th>K</th><th>Na</th><th>Mg</th><th>Ca</th><th>Si</th></tr></thead><tbody><tr><td>RF</td><td>&lt; 0.1</td><td>0.1</td><td>1.9</td><td>0.4</td><td>0.3</td><td>0.9</td><td>1.7</td><td>14.4</td></tr><tr><td>FNi</td><td>&lt; 0.1</td><td>8.6</td><td>&lt;</td><td>&lt;</td><td>&lt;</td><td>0.4</td><td>2.3</td><td>20.4</td></tr><tr><td>FNi*</td><td>&lt; 0.1</td><td>38.0</td><td>1.2</td><td>0.2</td><td>0.2</td><td>0.8</td><td>2.2</td><td>46.6<sup>1</sup></td></tr><tr><td>FFe</td><td>13.2</td><td>&lt;</td><td>0.4</td><td>0.1</td><td>&lt;</td><td>0.3</td><td>4.4</td><td>28.1</td></tr><tr><td>FFe*</td><td>64.2</td><td>&lt;</td><td>0.9</td><td>0.2</td><td>0.2</td><td>0.8</td><td>3.6</td><td>76.2<sup>1</sup></td></tr><tr><td>FNiFe</td><td>10.3</td><td>8.1</td><td>0.4</td><td>0.1</td><td>0.1</td><td>0.5</td><td>4.6</td><td>26.2<sup>1</sup></td></tr><tr><td>RW</td><td>&lt; 0.1</td><td>&lt;</td><td>0.7</td><td>&lt;</td><td>0.2</td><td>2.3</td><td>0.1</td><td>6.7</td></tr><tr><td>WNi</td><td>&lt; 0.1</td><td>2.4</td><td>0.2</td><td>&lt;</td><td>&lt;</td><td>0.7</td><td>&lt;</td><td>10.1</td></tr><tr><td>WFe</td><td>4.6</td><td>&lt;</td><td>0.1</td><td>&lt;</td><td>&lt;</td><td>0.1</td><td>&lt;</td><td>16.7</td></tr></tbody></table>

1. Estimated ash content based on sum of inorganic composition (average moisture content: 8.5 wt.%).

present a catalytic activity for deNOₓ. Si content is very high in fern biocarbon, compared to the relatively small content in willow biocarbon, which is caused by the nature of biomass. SiO₂ is an inhibitor for deNOₓ. It can deactivate the inherent K by forming K-silicate in pores and encapsulates active sites, which reduces the accessibility to active sites, thus reducing catalytic activity [57]. Additionally, the increase in Ni and Fe content helps catalyze pyrolysis and dilutes the organic content by increasing inorganic mass [58]. Both could explain the decrease in organic content (Table 1) and the decrease seems more important as inorganic content is high (FFe* and FNi* compared to RF).

### 3.1.2. Surface characterization

The biocarbon catalysts were characterized in terms of specific surface area and porosity type through BET analysis under N₂. Furthermore, the surface chemical groups were analyzed through temperature programmed desorption (TPD, NH₃), oxidation (TPO, CO₂) and reduction (TPR, H₂) (Table 3, spectra in Supporting Information, Figs. A.1–3).

The quantity of chemical surface groups and specific surface area are lower for RF biocarbon than for RW biocarbon. Lower specific surface of RF biocarbon (8.8 m²/g) could be due to lower volatile matter content in reported herbaceous type biomass resulting in less porosity generated during pyrolysis and therefore less specific surface area [55,59,60]. Impregnation impacts little the chemical adsorption, as the total adsorption of three gases (NH₃, CO₂ and H₂) are similar as long as the biocarbon composition remains the same. However, biocarbon impregnated with both Ni and Fe showed higher specific surface area (> 150 m²/g) than raw biocarbon (< 50 m²/g), which was also observed in previous studies [61]. This increase could be attributed to the intensifying volatile release during pyrolysis, resulting in the formation of

internal porous structure, while the presence of Ni and Fe (Table 2) catalyzes this process [62,63]. Introduction of metals onto the biocarbon could lead to a blocking of pores that reduces access to surface area and adsorption sites for gas (Table 3, FFe*) [64]. Indeed, a loss of pores induces a decrease in surface area. If increased, then porosity is developed and can reflect better access to adsorption sites and could result in higher adsorbed gases. This increase due to impregnation is however not as noticeable compared to the difference in adsorption between fern and willow biocarbon. This means that the surface groups that the biocarbon develops during pyrolysis could have a higher impact on the adsorption of gases than the presence of metals. Therefore, the combination of highly dispersed active sites that perform deNOₓ and the omnipresence of NO adsorption sites provided by the surrounding biocarbon could possibly result in high performance. Additionally, metal impregnation (FNi*) could also promote the aromatization of the biocarbon which when coupled with the formation of reduced metallic Ni, agglomerated or nanoparticle, during biomass pyrolysis results in less O-containing sites and may explain a reduction in H₂ adsorption (Table 3) [65–67].

Imagery of the catalysts was also performed through SEM (Fig. 3). Dispersion of metals was observable for willow biocarbon but not for fern biocarbon due to the higher metal content and their lack of discernible feature (dots of Ni). This lack of distinctive structure could be indicative of agglomerated metals and could explain lesser activity and gas adsorption capabilities on behalf of the fern biocarbon (Table 3). However, fern biocarbon possesses higher inorganic content (Table 2) which could be beneficial to deNOₓ. It was also possible to see some exposed inherent metals from the structure of willow and is associated with wood bark [61,68]. It was also noted that some metals may be covered by a sheet of carbon that could result in loss of active sites (Fig. 4). This sheet could have been formed during pyrolysis and deposited on the reactive metals, and could induce loss in reactive sites and gas adsorption capabilities without losing specific surface area [69]. This could also be a possible source of C for selective reduction of NO or a reduction source for oxidized metals (Eq. 13) [70].

## 3.2. deNOₓ experiments

### 3.2.1. deNOₓ performance: NO analysis

The concentration of NO and NO₂ was analyzed on-line every minute (in ppm). No significant production of NO₂ was observed (Fig. A.4, Supporting Information) during the experiments conducted and it may be due to its formation mechanism being hindered by NO saturation of the catalyst active sites [71]. Therefore, only XNO was calculated and averaged in 15 min (Fig. 5), which indicated the overall deNOₓ performance of different biocarbon catalysts. For all the samples, XNO did not significantly change after 50 min and became almost stable after 75 min. Therefore, the averaging XNO from 75 to 90 min was defined as the steady state XNO (indicated at the curves ends in the corresponding figures).

The values obtained for XNO reaching at most 30.6% (500 °C, gas hourly space velocity of 8235 h⁻¹, direct NO decomposition) could seem insignificant. However, literature on biocarbon for NOₓ reduction is

<table><caption>**Table 3**
Surface chemical groups and specific surface area of biocarbon catalysts before deNOₓ.</caption><thead><tr><th rowspan="2">Sample</th><th colspan="2">TPD-NH<sub>3</sub></th><th colspan="2">TPD-CO<sub>2</sub></th><th colspan="2">TPD-H<sub>2</sub></th><th rowspan="2">Specific surface area (m<sup>2</sup>/g)</th></tr><tr><th>Total adsorption (mmol/g)</th><th>T<sub>max</sub> (°C)</th><th>Total adsorption (mmol/g)</th><th>T<sub>max</sub> (°C)</th><th>Total adsorption (mmol/g)</th><th>T<sub>max</sub> (°C)</th></tr></thead><tbody><tr><td>RF</td><td>0.779</td><td>913</td><td>12.003</td><td>915</td><td>2.545</td><td>993</td><td>8.8</td></tr><tr><td>FNi</td><td>1.480</td><td>912</td><td>16.207</td><td>920</td><td>2.921</td><td>986</td><td>151.6</td></tr><tr><td>FNi*</td><td>0.645</td><td>939</td><td>8.975</td><td>934</td><td>1.684</td><td>999</td><td>187.5</td></tr><tr><td>FFe</td><td>0.553</td><td>951</td><td>8.258</td><td>905</td><td>0.158</td><td>981</td><td>309.6</td></tr><tr><td>FFe*</td><td>0.485</td><td>956</td><td>6.360</td><td>940</td><td>1.657</td><td>990</td><td>259.4</td></tr><tr><td>FNiFe</td><td>1.088</td><td>923</td><td>15.980</td><td>924</td><td>0.994</td><td>1000</td><td>367.9</td></tr><tr><td>RW</td><td>1.460</td><td>889</td><td>23.288</td><td>907</td><td>1.501</td><td>999</td><td>42.4</td></tr><tr><td>WNi</td><td>1.968</td><td>949</td><td>25.458</td><td>914</td><td>0.282</td><td>988</td><td>419.1</td></tr><tr><td>WFe</td><td>1.249</td><td>960</td><td>27.773</td><td>935</td><td>2.108</td><td>985</td><td>384.2</td></tr></tbody></table>
<figure><img src="image_5.png" /><figcaption>Fig. 3. SEM imagery of different biocarbon catalysts (RW, FNi and WNi biocarbon).</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>Fig. 4. SEM imagery of FNi biocarbon showing encapsulation of metals.</figcaption></figure>

recent [8,72–74]. Also, most studies concern selective reduction using NH₃, H₂ or CO as a reductant (coupled with O₂) [11,13,22,52]. Indeed, in optimized conditions (200 °C, 16000 h⁻¹, same concentration NO/NH₃ with 5% O₂), NH₃ can improve NOₓ reduction capabilities from nearly 0% (no NH₃) to 40% (with NH₃) with biocarbon alone or 90% with N-activated biocarbon-supported perovskite oxide [13]. NH₃ was not used here as it is a health and environmental hazard [75,76].

### 3.2.1.1. Effects of temperature.
Higher temperature has shown promoting effects on the deNOₓ performance for all the samples (Fig. 5). The raw biocarbon (RF and RW, Fig. 5a and d) were not fully activated at 200 and 350 °C and had very limited deNOₓ effects. Rising temperature to 500 °C significantly enhanced their catalytic activity, as the activation energy needed for the inherent metals was achieved. However, impregnated biocarbon presented a higher XNO with increasing temperature, which should be related to the presence of Fe and Ni (Table 2). This was also shown for WNi (Fig. 5e), except that its performance at 500 °C was the highest (XNO = 30.6%). For FNi (Fig. 5b), the performance was close between 350 and 500 °C, both significantly higher than that at 200 °C. This indicated a lower activation temperature for this

catalyst for a similar efficiency. This could be related to the high CO₂ adsorption capabilities of willow biocarbon (Table 3) that are related to the capacity to adsorb NO. This adsorption by the surface of the biocarbon means more NO is in contact of active sites and this higher quantity should result in more frequent reaction with NO and therefore higher XNO. For WFe (Fig. 5f), XNO was already high at 200 °C and steadily increased, almost linearly, with temperature, which indicated a wider range of temperature for the use of this catalyst. For FFe (Fig. 5c), similar XNO was achieved at both 200 and 350 °C, both remarkably lower than that at 500 °C. This indicated a higher activation temperature for FFe. Therefore, the high presence of other metals such as AAEM in fern biocarbon did not contribute to diminishing energy barriers and could even be inhibiting at higher temperatures. Then, the gas adsorbing capabilities of willow biocarbon could contribute to its higher performance. Nevertheless, the results show that temperature did increase the deNOₓ performance of all biocarbon catalysts.

### 3.2.1.2. Effects of biomass.
The nature of the biomass was an important factor that could influence the effectiveness of the biocarbon catalyst. Indeed, the composition of the initial biomass determined the
<figure><img src="image_7.png" /><figcaption>Fig. 5. NO removal ratio X<sub>NO</sub> of different biocarbon catalysts (RF, FNi, FFe, RW, WNi, WFe) under 1021 ppm NO/Ar.</figcaption></figure>

composition and structure of the biocarbon formed, favorable or not to
the NO adsorption and deNOx. In addition, the effectiveness of
impregnation may also depend on biomass characteristics. At both 200
and 350 °C, fern biocarbon achieved obviously higher deNOx perfor-
mance than their respective willow biocarbon (the only exception is FFe
vs. WFe). This was directly relevant to the difference in the inherent
metal content of fern and willow, especially K, Na and Mg (Table 2). RF,
for example, was significantly better than RW from 200 to 500 °C, which
best reflected the influence of biomass and its inherent metals as there
was no interference from the impregnated metals. Furthermore, the
higher presence of carbon from willow biocarbon (Table 1) could
interfere with the small amount of potential active sites (Fig. 4). When
impregnated however, the capacity to adsorb higher quantities of NO by
the willow-based biocarbon catalysts (Table 3) is highly beneficial to
their deNOx performance.

### 3.2.1.3. Effects of impregnated metals.
The effects of impregnated metals were also important. First of all, impregnation successfully enhanced the performance of the biocarbon catalysts at least at both 200 and 350 °C, as a result of more catalysts available for the adsorption and decomposition. Remarkable examples were FFe at 200 °C (Fig. 5c), and WFe at 200 and 350 °C (Fig. 5f), which at least doubled X<sub>NO</sub> compared with respective raw biocarbon. According to literature, the deNO<sub>x</sub> efficiency of Ni-impregnated and Fe-impregnated active carbon were similar at 300 °C and both significantly higher than that of raw active carbon [77–79]. This also agrees with the fact that metal-loaded carbons show high activity due to the dissociative NO chemisorption [79,80]. In this

study, the performance was not always positively correlated to the metal
amount because during pyrolysis the impregnated metals could be
coated with carbon or aggregated and were therefore not accessible by
NO. For example, FNi and FFe (Fig. 5b and c) were both high in their
loaded metal content (Table 2), but their performance at 500 °C was
worse than the raw biocarbon (RF, Fig. 5a). This was the result of poor
availability of metals after pyrolysis at high temperature. For WNi and
WFe (Fig. 5e and f), though the loaded metal content was not so high,
their performance was better.

To investigate the synergistic effects of Ni and Fe, a bimetallic
impregnation was carried out to produce a fern-based biocarbon catalyst
containing both Ni and Fe (FNiFe). The deNOx performance of this
sample was compared to that of fern biocarbon (FNi and FFe, Fig. 5b and
c). None of the samples achieved better performance than raw biocarbon
(RF) at 500 °C. FNi and FFe were close in steady stage XNO (23.4% vs.
23.6%), which were both better than the bimetallic sample, FNiFe
(Supporting information, Fig. A.5). Therefore, a synergistic effect of Ni
and Fe was not found.

### 3.2.1.4. Effects of metal content.
It was revealed above that not only the kind of impregnated metals but also the loaded metals content on biocarbon has significant influence on the deNOx performance of biocarbon catalysts. Therefore, the behavior of fern-based catalysts with different Ni and Fe content were tested in deNOx experiments at 200 °C (FFe*, Fig. 6, left) and 500 °C (FNi* and FFe*, Fig. 6).

For FFe, the effects of additional metal content were different at 200 and 500 °C (Fig. 6, left). At 200 °C, FFe* (with 64.17 wt.% Fe) performed
<figure><img src="image_8.png" /><figcaption>**Fig. 6.** X<sub>NO</sub> of fern-based biocarbon catalyst impregnated with higher Fe content and used in deNO<sub>x</sub> at 200 (left, full) and 500 °C (left, dotted), and impregnated with higher Ni content and used in deNO<sub>x</sub> at 500 °C (right).</figcaption></figure>

slightly better (17.0% > 13.9%) than FFe (with 13.17 wt.% Fe), both significantly better than RF (7.1%). Therefore, higher Fe content (≤ 64 wt.%) could result in better deNO<sub>x</sub> performance at 200 °C. At 500 °C, however, highest values of metal content drastically reduced the performance of biocarbon. This indicated the metal blocking of biocarbon pores (Table 3) happened mainly at higher temperature. Therefore, the consideration of optimal metal content to improve deNO<sub>x</sub> capabilities might be different depending on the temperature of the emission sources.

In the case of Ni-based catalysts, the steady state X<sub>NO</sub> of FNi was enhanced from 23.4% to 27.0% at 500 °C when the Ni content was increased from 8.62 wt.% (FNi) to 37.96 wt.% (FNi*, Fig. 6, right). This made its performance slightly better than that of RF. In an industrial application, it needs to be evaluated if the improvement achieved justifies the higher metal cost.

### 3.2.1.5. Catalyst deactivation.
Extended deNO<sub>x</sub> experiments (4 h versus 1 h) were conducted to evaluate the catalyst deactivation (Fig. 7), as well as their thermal stability (Table 4).

Within the experimental conditions considered, no significant deactivation of catalysts was observed even in extended deNO<sub>x</sub> experiments (Fig. 7), as the NO concentration remained stable after initial transitional stage (30 min), indicating constant deNO<sub>x</sub>. Therefore, stability could be expected for the biocarbon catalysts in the conditions of the experiment. In terms of thermal stability (Table 4), the impregnated biocarbon seemed to lose <5 wt.% after 1 h deNO<sub>x</sub> experiments. The raw biocarbon, however, had a mass loss of 5 to 10 wt.%. This mass loss could be due to reaction with NO, and result in biocarbon surface functional group loss. It could also be linked to CO and CO<sub>2</sub> release during the heating stage [57,81–84]. This could be possibly related to catalyzed pyrolysis that, in addition to reducing organic content (more O

<figure><img src="image_9.png" /><figcaption>**Fig. 7.** NO concentration curves of selected biocarbon catalysts (WNi, WFe, FNi, RF and blank test) during 500 °C deNO<sub>x</sub> experiments for extended time (4 h), under 1021 ppm NO/Ar.</figcaption></figure>

<table><caption>**Table 4**
Mass loss of biocarbon catalysts after standard deNO<sub>x</sub> experiments (1 h).</caption><thead><tr><th rowspan="2">T (°C)</th><th colspan="6">Mass loss (wt.%, dry basis)</th></tr><tr><th>RF</th><th>FNi</th><th>FFe</th><th>RW</th><th>WNi</th><th>WFe</th></tr></thead><tbody><tr><td>200</td><td>8.6</td><td>3.6</td><td>3.2</td><td>5.9</td><td>5.0</td><td>3.4</td></tr><tr><td>350</td><td>6.3</td><td>2.8</td><td>3.4</td><td>7.7</td><td>7.6</td><td>4.7</td></tr><tr><td>500</td><td>9.9</td><td>4.6</td><td>2.1</td><td>7.2</td><td>4.8</td><td>7.2</td></tr></tbody></table>

and H, Table 1), could stabilize the carbon matrix [85].

### 3.2.2. deNO<sub>x</sub> performance through N<sub>2</sub>, CO, CO<sub>2</sub> and H<sub>2</sub> fate analysis

#### 3.2.2.1. N<sub>2</sub> fate analysis.
Due to the input and replacement of reactor containing biocarbon catalysts at the beginning of each deNO<sub>x</sub> experiment, air background is inevitable throughout the experiment. The time needed for gas replacement from gas bottles to µGC analyzer was estimated as 15 min considering the total volume of gas channel. Therefore, the background N<sub>2</sub> concentration (C<sub>N<sub>2,BG</sub></sub>) was calculated by averaging the measured N<sub>2</sub> concentration from t = 30 to 45 min (before the inlet NO gas completely replaced Ar). Then the N<sub>2</sub> concentrations (C<sub>N<sub>2</sub></sub>) were normalized (C<sub>N<sub>2,nor</sub></sub>) according to this standard to represent the N<sub>2</sub> produced during the test (Eq. 15).

$$C_{N_2,nor} = C_{N_2} - C_{N_2,BG} \qquad (15)$$

The normalized N<sub>2</sub> concentration C<sub>N<sub>2,nor</sub></sub> can be seen as the indicator of direct decomposition reaction of NO (Fig. 8). In most cases, C<sub>N<sub>2,nor</sub></sub> was positive during NO phase. Even for those with initial negative values, the curves started to rise after. These indicated that NO is decomposed into N<sub>2</sub> either through direct decomposition or through catalytic reduction (Eq. 10–12). For RF, the catalyzing performance was enhanced with the increase of temperature, resulting in its higher X<sub>NO</sub> at higher temperature (Fig. 5). Also, RF performed better than RW, which was related to their difference in inherent metal contents (Table 2). FNi and FFe achieved highest decomposition performance at 200 °C, in contrary to WNi and FFe. This caused their higher X<sub>NO</sub>, while the NO removed by WNi and WFe was mostly through adsorption at this temperature. With the increase of temperature, both FNi and FFe showed a trend of catalyst deactivation, which should be due to the inhibiting effects of their higher Si content (Table 2), while these did not happen to the respective willow-based biocarbon catalysts (WNi and WFe).

#### 3.2.2.2. CO and CO<sub>2</sub> fate analysis.
Selective reduction with biocarbon will lead to the formation of N<sub>2</sub>, CO and CO<sub>2</sub> [35], which was also observed in the experiments. This is a side reaction that can cause the degradation of biocarbon.

From the results, the CO<sub>2</sub> evolution followed similar trends among samples (Fig. 9). CO<sub>2</sub> concentration was very high at the beginning and decreased sharply up to 30 min (under Ar). This may be explained by the oxygen groups on biocarbon surface releasing large amounts of CO<sub>2</sub> (Fig. 3). It is fair to assume that the peak of CO<sub>2</sub> should be during the
<figure><img src="image_10.png" /><figcaption>Fig. 8. Normalized N₂ concentration of different biocarbon catalysts (RF, FNi, FFe, RW, WNi, WFe), under 1021 ppm NO/Ar.</figcaption></figure>

temperature rising stage (t < 0). In addition, this process was significantly influenced by temperature, as biocarbon released significantly more CO₂ from 350 °C than at 200 °C, which indicated a threshold between 200 and 350 °C. When Ar is switched to NO, CO₂ concentration kept at a low value and steadily decreased. The reasons behind this should be a combination of residue oxygen groups releasing CO₂ and the reduction of NO by biocarbon producing CO₂. This showed that raw biocarbon released more CO₂ than impregnated biocarbon, which corresponded to their higher mass loss (Table 4). This may be an indicator of the supposedly better regenerability of impregnated biocarbon.

Regarding to the CO concentration, there was no CO release at 200 °C and negligible release at 350 °C, therefore only the evolution of CO concentration at 500 °C is presented (Fig. 10). Compared to CO₂, the quantity of CO released was much smaller, indicating CO₂ as the major product of selective reduction reaction. Unlike for CO₂, the impregnated biocarbon released more CO than the raw biocarbon.

### 3.2.2.3. H₂ fate analysis.
H₂ release was not observed at 200 °C, so the evolution of H₂ concentration at 350 and 500 °C was presented (Fig. 11). H₂ release was not so significant at 350 °C and was mostly during the Ar phase. At 500 °C, willow biocarbon released more H₂ than respective fern biocarbon. At both temperatures, almost no H₂ release was found for raw biocarbon.

## 3.3. Proposed mechanisms

Based on results discussed in this work, a graphical illustration of

reported mechanisms for deNOₓ was represented (Fig. 12). Additionally, the change in mechanism brought by the presence of metal was also represented. In this work, the metal-based mechanism is likely, with C provided by the biocarbon directly (Eq. 16–18), but further studies would be needed to confirm this. Other studies mention that, in addition to metallic active sites, O vacancies present on the biocarbon surface or formed during deNOₓ through the reduction of O containing functional groups by NO and CO forming NO₂ and CO₂, can serve as adsorption and reaction sites [11,13]. NO₂ can quickly react with CO to form CO₂ and N₂ and may attest to the little production in this study (Fig. A.1, Supporting Information) [11]. H₂ may also contribute to NO reduction [22]. Furthermore, only specific functional groups may take part in deNOₓ and result in others being spectators or even inhibitors by adsorbing and isolating co-reactants such as CO or NH₃ [11,76]. To increase NO uptake, new research focuses on impregnating biocarbon with metals that present high oxygen storage capacity such as ceria (Ce) or manganese (Mn) [8,72,73].

deNOₓ reaction over metal:

$$M + ^{\cdot} \rightarrow M^{\cdot} \qquad (16)$$

$$2M^{\cdot} + 2NO \rightarrow 2MO^{\cdot} + N_{2} \qquad (17)$$

$$xMO^{\cdot} + C \rightarrow xM^{\cdot} + CO_{x} \qquad (18)$$

## 4. Conclusions

Lignocellulosic fern and willow impregnated with heavy metals (Ni/
<figure><img src="image_11.png" /><figcaption>Fig. 9. CO₂ concentration of different biocarbon catalysts (RF, FNi, FFe, RW, WNi, WFe); blank area: Ar, shadow area: 1021 ppm NO/Ar.</figcaption></figure>

<figure><img src="image_12.png" /><figcaption>Fig. 10. CO concentration of different biocarbon catalysts during 500 °C deNOₓ experiments (a: fern biocarbon; b: willow biocarbon); blank area: Ar, shadow area: 1021 ppm NO/Ar.</figcaption></figure>

Fe) were pyrolyzed (800 °C, N₂) to produce biocarbon catalysts, which showed promising performances in NO decomposition (deNOₓ) at 200, 350 and 500 °C. Generally, higher temperatures showed better performances but a strict linear correlation was not observed. At 200 and 350 °C, adsorption of NO on biocarbon surface was facilitated by the high specific surface area of impregnated biocarbon (151.6 to 419.1 m²/g), compared to that of raw biocarbon (< 50 m²/g). This resulted in a remarkable enhancement of deNOₓ performance at this temperature range. At 500 °C, adsorption and decomposition were competing in the deNOₓ mechanism, and the inherent metals from raw biocarbon seemed to play an important part. As a result, the effects of impregnated metals

were less positive or even negative at this temperature, except for WNi, which achieved a remarkably high deNOₓ ratio of 30.6% (10.5% higher than RW). Fern biocarbon achieved higher deNOₓ performance than the respective willow biocarbon at both 200 and 350 °C, mostly due to their higher content of inherent metals (K, Na) that could facilitate activity from loaded metals (Ni or Fe). At 500 °C, willow biocarbon could outperform their fern counterparts as the loaded metals could be activated and benefit from the high NO adsorption of the biocarbon. Increasing the metal content generally slightly enhanced their performance in deNOₓ at 200 °C, but not significantly at 500 °C, and could justify use of a metal-poor biocarbon. No significant synergistic effects
<figure><img src="image_13.png" /><figcaption>**Fig. 11.** H₂ concentration of different biocarbon catalysts during 350 (top) and 500 °C (bottom) deNOₓ experiments (left: fern biocarbon; right: willow biocarbon); blank area: Ar, shadow area: 1021 ppm NO/Ar.</figcaption></figure>

<figure><img src="image_14.png" /><figcaption>**Fig. 12.** Graphical representation of deNOₓ mechanism reported in literature.</figcaption></figure>

were found with fern biocarbon impregnated with both Ni and Fe.

A production of N₂ was observed during deNOₓ experiments under NO flow, suggesting that the direct decomposition of NO was the main reaction, facilitated by higher temperature for most catalysts. The release of CO₂ observed under both Ar and NO flow was mostly from the surface chemical groups on biocarbon surface and peaked at the temperature rising stage (under Ar). The selective reduction of NO with biocarbon also contributed slightly to this release and was a side reaction. CO and H₂ was detected in smaller quantities. The impregnated biocarbon released less gas (CO₂, CO and H₂) than raw biocarbon, resulting in lower mass loss after deNOₓ, and suggested better stability and regenerability.

Further work will be carried out to study the influence of activating

the biocarbon catalysts and use of a real gaseous effluent containing nitrogen oxides. Furthermore, potential regeneration routes, in addition to longer experimental duration, will be explored to facilitate the upscaling in the use of these biocarbon catalysts as opposed to metal catalysts.

CRediT authorship contribution statement

**Théodore Graul:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Methodology. **María González Martínez:** Writing – review & editing, Visualization, Validation, Supervision, Resources, Project administration, Funding acquisition. **Yi Qiu:** Writing – original draft, Visualization, Methodology, Investigation, Formal
analysis. Floriane Fourgeaud: Writing – review & editing, Visualization, Methodology, Investigation, Formal analysis. Kuo Zeng: Writing – review & editing, Validation, Supervision. 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.

## Data availability

Data will be made available upon reasonable request to the corresponding author.

## Acknowledgments

The authors sincerely acknowledge the support provided by Eizhy, which provided shrubland biomass used in this study, as well as the European Union's Horizon 2020 research and innovation program under grant agreement No 637020–MOBILE FLIP for the willow biomass used in this study.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.apcato.2024.206982.

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