# A promising CO2 methanation catalyst system based on modified halloysites as supports

Ahmed Gamal, Khouloud Jlassi, Khulood Shafi, Mohamed M Chehimi,

Aboubakr M Abdullah

► To cite this version:

Ahmed Gamal, Khouloud Jlassi, Khulood Shafi, Mohamed M Chehimi, Aboubakr M Abdullah. A promising CO2 methanation catalyst system based on modified halloysites as supports. Emergent Materials, 2025, 8, pp.2737-2747. 10.1007/s42247-024-00929-1. hal-05375877

HAL Id: hal-05375877

https://hal.science/hal-05375877v1

Submitted on 21 Nov 2025

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# A promising CO₂ methanation catalyst system based on modified halloysites as supports

Ahmed Gamal<fnref n="1" /> · Khouloud Jlassi<fnref n="1" /> · Khulood Shafi<fnref n="1" /> · Mohamed M. Chehimi<fnref n="2" /> · Aboubakr M. Abdullah<fnref n="1" />

Received: 9 July 2024 / Accepted: 5 November 2024

© The Author(s) 2025

## Abstract

Earth's climate is warming due to anthropogenic emissions of greenhouse gases, especially carbon dioxide (CO₂). Different reactions are allocated to mitigate the CO₂ in the atmosphere. However, CO₂ methanation is a pivotal research hotspot due to its ability to produce methane at low operating temperatures (200–400 °C). Halloysite nanotubes (HNTs)-based catalysts have attracted significant attention in various catalytic applications. However, Halloysite is rarely reported for thermal CO₂ methanation. The selected halloysite clay was modified first using the 3-Aminopropyl triethoxy silane (NH₂) as coupling agent, the resulting materials (HNTs), and then doped with Ni at different weight concentrations (5%, 10%, 20%, 40%, 50%). materials can offer high surface area and porous structure, which can improve metal dispersion. The prepared Ni/HNTs catalysts were characterized using various techniques, such as XRD, XPS, SEM, and TEM, which confirmed the existence of nanotubes and porous structures. The propensity of the prepared Ni/HNTs were evaluated to catalyse the CO₂ methanation reactions at a temperature range of 250 to 500 °C. The catalyst containing 20 wt.% of Ni (20Ni/HNTs) showed the highest CO₂ conversion at all reaction temperatures and the highest selectivity of methane at 450 °C (82%). This study paves the way for the large utilization of the HNTs as a strong support for different metals used in thermal catalytic reactions, not limited to the CO₂ methanation.

**Keywords** Halloysite nanotubes · Nanocatalysts · Methanation · CO₂ utilization

## 1 Introduction

Excessive usage of fossil fuels as main energy resources resulted in detrimental carbon dioxide (CO₂) emissions, which are the main reason for global warming. Therefore, the frontlines in science and engineering are persuaded to solve this issue using different strategies such as CO₂ capture and utilization [1–4]. One of the most promising reactions

of CO₂ utilization is CO₂ hydrogenation to methane (Eq. 1) or methanol (Eq. 2) [5–9]. In these equations, enthalpy and free energy are calculated at standard state. However, the path of CO₂ hydrogenation to methanol necessitates harsh parameters, for example, high reaction temperatures and high applied pressures [10–12]. Oppositely, CO₂ hydrogenation to methane does not require high temperatures or high pressure [13]. Thus, CO₂ methanation has attracted researchers in the last decade to explore it intensively, which is evidenced by the significant increase in the number of publications on CO₂ methanation in the decade last decade (over 2700 research articles, source).

Kouloud Jlassi
khouloud_jlassi@qu.edu.qa

Mohamed M. Chehimi
mohamed.chehimi@cnrs.fr

Aboubakr M. Abdullah
bakr@qu.edu.qa

$$ 
\begin{aligned}
CO_2 + 4H_2 \rightleftharpoons CH_4 + 2H_2O \quad \Delta H^{\circ}_{298K} \\
= -165kJ/mol \quad \Delta G^{\circ}_{298K} \\
= -141.9kJ/mol
\end{aligned} 
\quad (1) $$

$$ 
\begin{aligned}
CO_2 + 3H_2 \rightleftharpoons CH_3OH + H_2O \quad \Delta H^{\circ}_{298K} \\
= -49.5kJ/mol \quad \Delta G^{\circ}_{298K} \\
= -3.5kJ/mol
\end{aligned} 
\quad (2) $$

<footnote n="1">¹ Center for Advanced Materials, Qatar University, 2713 Doha, Qatar</footnote>

<footnote n="2">² UMR 7086, Université Paris Cité, CNRS, ITODYS, 75013 Paris, France</footnote>
Both noble and transition metals, such as Ru [14] and Ni [8], were reported with various supports, i.e. biochar, [15–17] SiO₂, and Al₂O₃ [18]. Of relevance to this work, aluminosilicates such as zeolites, dealuminated zeolites [19, 20], and natural clays [21–24] have also been considered as supports for CO₂ methanation nanocatalysts mostly based on nickel, its alloys and mixtures with oxides, and its mixtures. Many promising achievements in the metal-catalyzed CO₂ hydrogenation to methane were reported, such as the Ni-promoted Ce catalyst, which is supported on multiwall carbon nanotubes (Ni-Ce/MWCNT) [25] that offered an interesting performance, achieving 100% methane selectivity and 83.8% CO₂ conversion at 350 °C. Ni-promoted Fe, supported on CNTs (Ni-Fe/CNTs) [26] presented 100% carbon dioxide conversion and 88% methane yield at 350 °C. Moreover, the Ni-promoted Ce catalyst, which is supported on reduced graphene oxide (Ni-Ce/RGO) [27] showed 83% CO₂ conversion with 99.9 methane selectivity at 350 °C, competing with the catalysts Ni-Ce/MWCNT [25] and Ni-Ce/CNTs [26]. Furthermore, Ru-based catalysts, such as Ru/ABC [28], achieved 90% CO₂ conversion and 99% methane selectivity at 400 °C, showing outstanding performances. However, using the expensive Ru is a significant disadvantage precluding the scaling up of the process.

The high surface area, adsorption ability, physicochemical properties, low cost, and special morphology to induce excellent gas permeability are the main reasons for the halloysite nanotubes being used in the catalytic gas phase reactions [29]. The preparation of the composite catalysts could be cumbersome or utilize microwave assistance [19, 30]. However, halloysite are rarely reported as a support for the CO₂ methanation catalysts. Halloysite is a natural clay mineral under the umbrella of the orthosilicate group, with the chemical formula Al₂-Si₂O₅(OH)₄ [31]. It is derived from the dehydration of kaolinite, which causes folding of the kaolinite layers, creating multilayer nanotubes, a distinctive feature of the halloysite structure. The formed halloysite nanotubes (HNTs) are composed of alternating aluminum-oxygen and silicon-oxygen layers linked by oxygen bridges [31]. That unique structure provides exceptional adsorption capacity, which can be further improved through surface modifications to be used as mesoporous supports. To enhance attachment of nickel ions to the underlying support, we propose to silanize the HNTs as aminosilane enhance nickel and other metal ions adsorption [32, 33]. To fill in the gap, we propose a simple wet impregnation method for the immobilization of nickel nitrate onto silanized HNTs, followed by pyrolysis in order to prepare nickel catalyst-loaded HNTs. The HNTs-based composite catalysts were tested in the CO₂ methanation reaction, under two important parameters (mass loading of catalyst and temperature effect). Halloysite is a less common platform for CO₂

methanation catalysts and is worth the investigation as other supports such as biochar [15].

The challenge of the carbon dioxide methanation process is to obtain high methane selectivity at lower temperatures. Thus, in this work, we prepared HNTs-supported catalysts loaded by Ni as active metal in the CO₂ methanation process with different concentrations starting from 5 wt.% until 50 wt.% of Ni. The synthesized catalysts were characterized by different techniques such as XRD, FTIR, XPS, SEM, and TEM. The catalysts were tested in a fixed bed reactor under atmospheric pressure, using a hydrogen and carbon dioxide mixture in the 250–500 °C temperature range. The catalytic performance showed that Ni-supported catalysts on the HNTs were successful and active catalysts for the CO₂ methanation process. The catalyst that included 20 wt.% of Ni offered high carbon dioxide conversion (58% at 500 °C) and high methane selectivity (82% at 450 °C). This study will open the way for the wider use of HNTs as supports for different metals to be used in the carbon dioxide methanation process.

# 2 Methodology

## 2.1 Apparatus

The catalytic examinations were performed out in a fixed bed reactor under atmospheric pressure, with operational temperatures ranging from 250 to 500 °C, depicted in Fig. 1, illustrating the experimental setup. A quartz tube, containing 50 mg of the catalyst under test, was flanked by two quartz wool pieces. The outlet gas pipe was directly linked to an online Infrared Gas Analyzer (IR400–Yokogawa). The reaction's feed-in gases comprised 40 ml/min of 40% H₂ and 10% CO₂ balanced in argon (WHSV=48,000 mL/g.h). Initially, the catalysts underwent activation through reduction using 40 mL/min of 10 vol.% H₂/Ar for one hour at 500 °C before initiating the reaction. Subsequently, the system was cooled to room temperature, and the feed gas was introduced to the catalyst as the reaction temperature increased from 250 to 500 °C. Ultimately, the CO₂ conversion is computed using Eq. 3, methane selectivity is determined by Eq. 4, methane yield is calculated with Eq. 5, and the reaction rate (r) is derived from Eq. 6.

$$CO_2 \text{ Conversion} = [CO_{2\text{In}} - CO_{2\text{Out}}/CO_{2\text{In}}] \times 100 \quad (3)$$

$$CH_4 \text{ Selectivity} = CH_{4\text{Out}}/CO_2 \text{ converted} \quad (4)$$

$$CH_4 \text{ Yield} = CH_{4\text{Out}}/CO_{2\text{In}} \quad (5)$$

$$r = n(CO_{2\text{In}} - nCO_{2\text{Out}})/g_{\text{cat.}}T_{\text{min.}} \quad (6)$$
<figure><img src="image_3.png" /><figcaption>Fig. 1 A schematic diagram of the experimental setup</figcaption></figure>

<table><caption>**Table 1** Synthesis compositions and conditions</caption><thead><tr><th>Catalyst</th><th>HNT (g)</th><th>Ni-nitrate-hexahydrate (g)</th><th>Ni (wt.%)</th></tr></thead><tbody><tr><td>5Ni/HNTs</td><td>0.95</td><td>0.25</td><td>5</td></tr><tr><td>10Ni/HNTs</td><td>0.90</td><td>0.50</td><td>10</td></tr><tr><td>20Ni/HNTs</td><td>0.80</td><td>1</td><td>20</td></tr><tr><td>40Ni/HNTs</td><td>0.60</td><td>2</td><td>40</td></tr><tr><td>50Ni/HNTs</td><td>0.50</td><td>2.5</td><td>50</td></tr></tbody></table>

## 2.2 Catalysts synthesis

The nickel nitrate hexahydrate was purchased from VWR Company, UK. 3-Aminopropyltriethoxysilane (NH₂, 96%), HNT, and analytical-grade organic solvents were purchased from Sigma-Aldrich. 3-Aminopropyltriethoxysilane (NH₂, 96%) was purified by passing through basic alumina. 2 g of halloysite nanotubes were dispersed in 100 mL of distilled water under sonication for 5 min. A silane solution was prepared by diluting 114.7 mg 3-aminopropyltriethoxysilane in 25 mL of ethanol/acetic acid mixture (95/5 v/v). This silane solution was added dropwise to the halloysite suspension under vigorous stirring. The silanization process was allowed to proceed for 72 h at room temperature [34].

The resulting silanized halloysite clay, called hereafter (HNTs) was washed by four cycles of centrifugation (8000 rpm for 15 min) and then dispersed in ethanol. The HNTs was finally centrifuged and dried in the oven at 60 °C for 24 h. The silanized HNTs sample was loaded with different weight percent of Ni to observe the effect they would have on the CO₂ methanation reaction.

A solution of Ni(NO₃)₂·6H₂O was used as a nickel precursor and mixed with a suspension of the aforementioned modified HNTs. The amount of Ni(NO₃)₂·6H₂O to HNTs was calculated according to the different wt% of Ni/HNTs catalysts: 5, 10, 20, 40, and 50 wt% Ni, as reported in Table 1. Then the samples were annealed at 500 °C in N₂

atmosphere for one hour with a heating rate of 5 K/min, and then cooled down to the room temperature and the collected powder denoted as 5Ni/HNTs, 10Ni/HNTs, 20Ni/HNTs, 40Ni/HNTs, and 50Ni/HNTs, respectively.

## 2.3 Characterization

Nova Nano SEM machine was utilized to obtain imaging for the synthesized catalysts. Transmission Electron Microscopy (TEM) was employed for topographical and morphological images using FEI-TECNAI-G2-TEM and TF20. XRD-PANalytical-EMPYREAN was employed to characterize the bulk phase and structure of the catalysts. Surface chemical analysis was performed using XPS, Thermo, UK, for the pristine and nanocatalyst-coated aminated clay samples. The apparatus is fitted with mono-Al Kα source (1486.6 eV) and a flood gun for static charge compensation. The surface composition was computed using the manufacturer's sensitivity factors. Thermo Fisher Scientific DXR Raman Microscope and TGA Pyris-6 were also used for thermal gravimetric analysis.

# 3 Results and discussion

## 3.1 Characterization

Figure 2a shows the XRD patterns of the bare and Ni-loaded HNTs samples. The XRD spectrum of HNTs is aligned with findings in prior literature on halloysite [35]. The identifiable basal reflection at approximately 2θ=12.1° corresponds to a (001) plane, confirming the presence of multiwall tubular structures within HNTs. Furthermore, the detection of a (002) diffraction peak around 2θ=24.54° confirms the dehydrated state of HNTs. Additionally, the basal reflection observed at 2θ=62.41°, assigned to the (006) plane, further supports the characterization of HNTs
<figure><img src="image_4.png" /></figure>

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /></figure>

<figure><img src="image_7.png" /></figure>

<figure><img src="image_8.png" /><figcaption>**Fig. 2** (a) XRD, (b) FTIR, and (c) TGA of the prepared catalysts. (d) TGA before and after treatment and (e) TPR of 20Ni/HNTs Vs metal-free HNTs</figcaption></figure>

as a dioctahedral mineral. XRD results of 5Ni/HNTs, 10Ni/HNTs, 20Ni/HNTs, 40Ni/HNTs, and 50Ni/HNTs approved the presence of nickel in the HNTs (Fig. 2a). The XRD peak appeared ~44° is corresponding to the Ni [200] phase

[36]. The decrease in the intensity of the HNTs peaks with increasing metal content is likely attributed to the surface coverage of the HNTs and the loss of crystallinity of the halloysite, induced by calcination at 500 °C [37].
The FTIR analysis showed the main two bands around 912 and 1080 cm⁻¹, which are ascribed to the O–H deformation of inner hydroxyl groups and the in-plane Si–O stretching mode, respectively, (Fig. 2b) [38]. In addition, the bands in the range of 3350 to 3510 cm⁻¹ are attributed to the stretching vibration mode of the amine group (NH₂) [39].

TGA plots are displayed in Fig. 2c. Because the thermal analysis was performed in air, the organic functional group from the silane are unstable and decomposed as noted from the mass loss up to ~450 °C. However, the silane is intended to withstand the CO₂ methanation process in the temperature range 250–500 °C, but to complex the nickel ions on the HNTs. This helps to obtain better nanoparticle dispersion. Figure 2c shows that the main weight loss of the HNTs occurs in the 450–550 °C temperature range, which is attributed to the dehydroxylation of the structural aluminum groups of the HNTs (Fig. 2c) [40, 41]. The remaining weight percentage of HNTs after increasing the temperature to 800 °C was 82%, indicating the robust thermal behavior of HNTs. The gradual addition of Ni to the HNTs resulted in an increased residual mass of the nanocomposite (Fig. 3c), which reflects the control over the preparation of the HNTs-immobilized Ni systems. Furthermore, as shown in Fig. 2d, the TGA analyses of HNTs before and after treatment revealed a calculated weight loss difference of approximately 0.8%, which can be attributed to the loading of amine functional groups on the HNTs.

The H₂-TPR profile of the catalyst 20Ni/HNTs showed a broad reduction peak starting from 300 to 550 °C with a maximum hydrogen consumption around 400 °C, as seen in Fig. 2e. That is corresponded to the conversion of the nickel oxides to the metallic phase, which matches with other reported results [42]. The area below 400 °C is attributed to the reduction of the large crystallites of NiO interacting weakly with the support, while the higher temperature reduction area around 400–550 °C is ascribed to the enhanced interaction between the NiO species and the support [42]. On the other hand, the metal-free sample (HNTs) did not show any reduction peaks, as it does not contain Ni.

The SEM images of all samples confirmed the tubular structures of HNTs. Figures 3a-b shows the SEM images of the catalyst 20Ni/HNTs that exhibited tubular porous structures with open ends, as seen in the circled spots of Fig. 3b. Thus, the tubular porous structures are expected to offer high surface area. The EDX analysis of 20Ni/HNTs approved the preparation of nickel with the HNTs, as shown in Fig. 3c with roughly 41, 21, 20, 18 wt.% of O, Al, Si, and Ni, respectively. In addition, the TEM images of the metal-free HNTs confirmed the successful existence of the nanotubes with different diameters varying from 20 to 100 nm, as displayed in Fig. 3d-e. Moreover, Fig. 3f shows the TEM image of the 20Ni/HNTs catalyst that approved the perfect existence of nickel on the HNTs as nanoparticles, without agglomeration. The Ni nanoparticles are well-dispersed on the surface of the HNTs, which results in a larger surface

<figure><img src="image_9.png" /><figcaption>**Fig. 3** SEM images of 20Ni/HNTs (a-b), EDX of 20Ni/HNTs (c), TEM images of metal-free HNTs (d-e), and TEM image of 20Ni/HNTs (f)</figcaption></figure>
area to be exposed to the reactants. This increased surface area enhances the availability of active sites, and it can enhance the ability of the catalyst to prevent agglomeration, which finally improves the catalytic efficiency.

In addition, the mean particle size of the Ni nanoparticles was 4.7 nm (see histogram inserted in Fig. 3f), which is smaller than that of the catalyst 40Ni/HNTs that presented an average particle size of 5.6 nm.

In contrast to the images displayed in Fig. 3, SEM images of the high-metal loaded catalyst 40Ni/HNTs clearly indicate a metal agglomeration on the HNTs, as seen in the circled spots in Fig. 4a. Such an agglomeration was confirmed by the TEM images of the 40Ni/HNTs, compared to the particles of 20Ni/HNTs (Fig. 4b), hence higher particle size of the catalyst 40Ni/HNTs. In addition, The SEM images of the spent catalyst 20Ni/HNTs did not show any significant or indicating differences other than the fresh catalyst, as shown in Fig. 4C, which is the main reason for keeping the catalyst active in the second cycle and stable for long time during the time on stream reaction, carried out for ten hours.

Figure 5 displays the XPS surface analysis results of the HNTs at indicated, targeted nickel loading. HNTs is characterized by their main peak Al2p (~74 eV), Si2p (102 eV), and O1s (~532 eV), as well as a low-intensity N1s peak centered at 400 eV (Fig. 5a). Nickel is characterized by its Ni2p region (850–890 eV). The maximum loading induced a higher relative intensity for the Ni2p doublet. Figure 5b displays the high-resolution Ni2p region. Interestingly, the Ni2p<sub>3/2</sub> portion of the spectrum (850–870 eV) shows two main signals centred at 854.7 eV due to NiO, and at 856.2 eV due to surface effects [43]. The spectra feature NiO, but at low nickel loading, one could distinguish a low-intensity peak at 852.5 eV, ascribed to metallic nickel. NiO is confirmed by the appearance of an O1s peak component at 529.6 eV [44] in the case of 50% Ni-HNTs, compared to the catalyst-free HNTs (Fig. 5c). It is to note that XPS is a surface specific analysis technique compared to XRD, hence the detection of any oxidation that could occur at the surface. Figure 5d displays the Ni/(Al+Si) atomic ratio versus

the targeted nickel loading; stepwise loading nickel leads to increased atomic ratio until a plateau value is reached for 20% loading and above. It is to note that this XPS surface characterization is not done in operand and cannot give the composition during catalysis, but it shows that the composition is correlated with the catalytic performances as will be shown below.

## 3.2 Catalytic tests

First, all samples were pretreated in an H₂ atmosphere at 500 °C for one hour, as an activation step reassuring that the nickel oxides are reduced to Ni<sup>0</sup> which is reported to be the active phase in reactions of methanation of carbon dioxide [45, 46]. The metal-free HNTs was not active at any temperature. In contrast, the Ni/HNTs samples showed high catalytic activity, which jumped after raising the nickel amounts up to 20 Wt. %, and then inhibited with raising the nickel loading, due to the agglomeration that occurred in the higher-loaded catalysts. Accordingly, 20Ni/HNTs presented the best activity among the tested samples, as it achieved the biggest CO<sub>2</sub> conversion hitting 58% at 500 °C, as depicted in Fig. 6a. It also reached the best selectivity for methane (82%), and the lowest CO selectivity (27%) at 450 °C, as depicted in Fig. 6b-c. 5Ni/HNTs, 10Ni/HNTs, 40Ni/HNTs, and 50Ni/HNTs showed less catalytic activity than 20Ni/HNTs with less CO<sub>2</sub> conversion and less selectivity to the formation of CH<sub>4</sub> in addition to higher CO selectivity at all reaction temperatures. Besides, the CH<sub>4</sub> yield increased with raising the reaction temperatures to 450 °C, and then, decreased with increasing the heating, because of increasing the chances of the reverse water gas shift reaction (RWGS) to start (Fig. 6c). 20Ni/HNTs reached 43% methane yield at 450 °C, the best obtained yield.

It is worthy to be mentioned that the selectivity and yield of methane was raising with increasing the heating up to 450 °C. The selectivity of CH<sub>4</sub> production then inhibited with increasing the heating higher than 450 °C, which is also attributed to the RWGS [30, 47], as depicted in Eq. 7.

<figure><img src="image_10.png" /><figcaption>Fig. 4 (a) SEM image of (a) 40Ni/HNTs, (b) TEM image of 40Ni/HNTs, and SEM image of the spent 20Ni/HNTs (c)</figcaption></figure>
<figure><img src="image_11.png" /><figcaption>Fig. 5 XPS surface analysis of HNTs samples: survey regions (a), high-resolution Ni2p spectra (b), O1s (c), and Ni/(Al+Si) effective atomic ratio versus the targeted nickel mass loading (d)</figcaption></figure>

That RWGS starts at high reaction temperatures switching the reaction to produce CO instead of CH$_{4}$, which then reacts with H$_{2}$O to produce CH$_{3}$OH [30, 47]. Thus, increasing the reaction temperature of 20Ni/HNTs catalyst over 450 °C resulted in higher CO selectivity, as shown in Fig. 6d.

$$CO_2 + H_2 \rightleftharpoons CO + H_2O \quad \Delta H_{298K} = 41.2 kJ/mol \quad (7)$$

Figure 6e compares the activity of all HNT-based catalysts, operating at 450 °C, the optimal temperature for the selectivity to methane. The 20Ni/HNTs catalyst thus appears as the highest CO$_{2}$ conversion and the lowest CO selectivity at 450 °C. This is ascribed to the small size of the Ni nanoparticles and the excellent Ni dispersion, confirmed by the TEM. 20Ni/HNTs showed the highest reaction rate (r) among all tested catalysts as testified by Fig. 6f. The highest yield coincides with the onset of nanocatalyst loading saturation noted in XPS (see Fig. 5d).

## 3.3 Stability and recycling

The stability of the 20Ni/HNTs catalyst was tested for twelve hours at 450 °C with WHSV 48000 mL/g.h. 20Ni/HNTs showed a stable behavior during the whole course of the reaction without any deactivation, as it could keep the conversion of CO$_{2}$ around 51%, the CH$_{4}$ selectivity at the range of 82, and the yield about 43%, as seen in Fig. 7a. A recycling test was performed testing the spent catalyst 20Ni/HNTs, as the best-performing catalyst, under the same operational conditions. The used 20Ni/HNTs presented a strong activity, showing similar results to the fresh one with a negligible difference, as seen in Fig. 7b-d. It approves that the catalyst is not influenced by harsh operation conditions, such as heating. In addition, the high catalytic performance of the spent catalyst might be ascribed to the reduction atmosphere, caused by the hydrogen that sustains the metallic phase of nickel.

Furthermore, the catalyst 20Ni/HNTs could be considered as one of the active monometallic catalysts, as shown in Table 2. For example, 20Ni/HNTs showed 51% CO$_{2}$ conversion and 82% CH$_{4}$ selectivity at 450°C, which is better than the nickel catalyst supported on wheat straw biochar [48], which presented 42% CO$_{2}$ conversion and 65% CH$_{4}$ selectivity at the same temperature. Some other catalysts such as Ni/ZrO$_{2}$ [49] presented higher CO$_{2}$ conversion (79%) than 20Ni/HNTs. However, it could not achieve the high methane selectivity of 76% achieved with 20Ni/HNTs. On the other hand, some other promoted supports such as zirconia presented higher catalytic activity than 20Ni/HNTs. The catalyst Ni/CeO$_{2}$-ZrO$_{2}$, prepared by wet impregnation, at 350 °C, could achieve 51% CO$_{2}$ conversion and 99% methane selectivity, and when it is prepared by ammonia
<figure><img src="image_12.png" /><figcaption>Fig. 6 (a) CO₂ conversion, (b) CH₄ selectivity, (c) CH₄ yield, of all Ni/HNTs catalysts, and (d) the CO selectivity of 20 Ni/HNTs. (e) a comparison among the catalysts at 450 °C. (f) A comparison among the reaction rates of all catalysts</figcaption></figure>

evaporation method it achieved 54% CO₂ conversion and 99.5% CH₄ selectivity [50]. That high catalytic performance is attributed to the effect of the added ceria as a promoter into the zirconia support.

## 4 Conclusion

In this work, silanized Halloysite nanotubes (HNTs) hybrids were decorated with Ni salt, via wet impregnation, and calcined at 500 °C. The Ni loading was tuned with the initial concentration of nickel nitrate, and the targeted loading was 5, 10, 20, 40, and 50 wt.% Ni into the HNTs. All Ni-HNTs were tested as catalysts of CO₂ methanation and found to be effective in this conversion process. This is ascribed to the unique structure offered by the HNTs, as the HNTs have a high surface area and pore structure, providing more sites for the catalytic reactions. This allows for a greater number of reactant molecules to interact with the catalyst, improving the overall reaction rate. It also allows better dispersion

of catalyst nanoparticles, which maximizes the availability of active sites and prevents agglomeration.

The catalyst 20Ni/HNTs presented the best activity, as it showed the highest conversion of CO₂ and the best selectivity for CH₄ at 450 °C (82%). That high catalytic performance lies in the amount of nanocatalysts and particularly its excellent dispersion on the HNTs, as confirmed by the TEM. The catalysts with higher Ni amounts, i.e. 40Ni/HNTs and 50Ni/HNTs, showed less CO₂ conversion than 20Ni/HNTs and less CH₄ selectivity, due to the agglomeration caused by the higher concentrations of nickel particles, as judged from TEM imaging. In addition, it is noteworthy that CH₄ selectivity improved with heating until 450 °C, and then it decreased due to the RWGS. In a short, the HNTs can be used successfully as a promising support for nickel in the CO₂ methanation reactions achieving high catalytic performance obtained for a simple, and scalable user-friendly protocol of materials design.
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<figure><img src="image_15.png" /></figure>

<figure><img src="image_16.png" /><figcaption>**Fig. 7 A)** Stability test of the 20Ni/HNTs at 450 °C for 12 h, (b) CO₂ conversion, (c) CH₄ selectivity, and (d) CO selectivity of the fresh and spent catalyst 20Ni/HNTs</figcaption></figure>

<table><caption>**Table 2** A prepared comparison of nickel-based catalysts</caption><thead><tr><th>Sample</th><th>Synthesis method</th><th>Reaction Temperature (°C)</th><th>CO<sub>2</sub> conversion (%)</th><th>CH<sub>4</sub> selectivity (%)</th><th>Ref</th></tr></thead><tbody><tr><td>20Ni/HNTs</td><td>Wet Impregnation</td><td>450°C</td><td>51</td><td>82</td><td>This paper</td></tr><tr><td>Ni-ZrO<sub>2</sub></td><td>Plasma Decomposition</td><td>350°C</td><td>79</td><td>76</td><td>[49]</td></tr><tr><td>Ni@MOF</td><td>Wet impregnation</td><td>350°C</td><td>35</td><td>81</td><td>[51]</td></tr><tr><td>Ni@MOF</td><td>Solvothermal</td><td>350°C</td><td>15</td><td>45</td><td>[51]</td></tr><tr><td>Meso/Rh</td><td>Chemical Reduction</td><td>550°C</td><td>98</td><td>81</td><td>[52]</td></tr><tr><td>NP/Rh</td><td>Chemical Reduction</td><td>550°C</td><td>43</td><td>17</td><td>[52]</td></tr><tr><td>Ni/Biochar</td><td>Pyrolysis</td><td>450°C</td><td>42</td><td>65</td><td>[48]</td></tr><tr><td>Ni/Cellulose Carbon</td><td>Pyrolysis</td><td>400°C</td><td>52</td><td>67</td><td>[53]</td></tr><tr><td>Ni-CeO<sub>2</sub>-ZrO<sub>2</sub></td><td>Wet impregnation</td><td>350°C</td><td>51</td><td>99</td><td>[50]</td></tr><tr><td>Ni-CeO<sub>2</sub>-ZrO<sub>2</sub></td><td>Ammonia evaporation</td><td>350°C</td><td>54</td><td>99</td><td>[50]</td></tr></tbody></table>
**Acknowledgements** This work was supported by (i) the Qatar National Research Fund (QRDI, a member of the Qatar Foundation) through the National Priority Research Program Grant (ARG01-0530-230416) and (ii) Qatar University through an International Research Collaboration Co-Fund grant, IRCC-157. The authors are indebted to Philippe Decorse (Université Paris Cité) for the assistance with XPS analyses.

**Funding** Open Access funding provided by the Qatar National Library.

## Declarations

**Conflict of interest** The authors declare no conflict of interest that could have appeared to influence the presented work.

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