# Tri-reforming of methane over a hydroxyapatite supported nickel catalyst

Thanh Son Phan, Doan Pham Minh

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## To cite this version:

Thanh Son Phan, Doan Pham Minh. Tri-reforming of methane over a hydroxyapatite supported nickel catalyst. ChemCatChem, 2024, 16 (16), pp.e202400192. 10.1002/cctc.202400192. hal-04537513

HAL Id: hal-04537513

https://imt-mines-albi.hal.science/hal-04537513v1

Submitted on 8 Jul 2024

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# Tri-reforming of methane over a hydroxyapatite supported nickel catalyst

Thanh Son Phan[a, b], Doan Pham Minh[a]

[a] Dr. Thanh Son Phan, Prof. Dr. Doan Pham Minh
CNRS UMR 5302, Centre RAPSODEE
Université de Toulouse, IMT Mines Albi
Campus Jarlard, F-81013 Albi Cedex 09, France
E-mail: doan.phamminh@mines-albi.fr

[b] Dr. Thanh Son Phan
Faculty of Chemical Engineering
The University of Danang – University of Science and Technology
Danang 550000, Vietnam

**Abstract:** For the first time, a catalyst containing 5wt.% Ni supported on a hydroxyapatite support (HAP) has been synthesized and evaluated in tri-reforming of methane (TRM) for synthetic gas (syngas) production. Nickel nanoparticles could be easily formed on HAP surface by using the conventional incipient wetness impregnation technique. In TRM, under unfavorable reaction conditions from the thermodynamic point of view (e.g. medium reforming temperature of 700-800 °C, low steam-to-carbon ratio, etc.), this catalyst showed high methane conversion (up to 90%), however, some deactivation took place. The latter could be explained by both the thermal sintering of nickel nanoparticles and the solid carbon formation. The impact of the main operational parameters has been studied. Increasing the reaction temperature, the molar ratio of oxygen-to-carbon and steam-to-carbon ratio are favorable for the methane conversion and mostly for the stability of the catalyst. High methane conversion of ca. 90% with a perfect catalytic stability during 300 hours-on-stream could be achieved at 800 °C and 1.4 bar, using a mixture containing low ratio of oxidant-to-carbon (molar ratio of CH₄/CO₂/H₂O/O₂/N₂ = 1.0/0.67/0.9/0.1/0). These results offer the opportunity to further design an optimal Ni/HAP catalyst by improving metal-support interaction and downsizing nickel nanoparticles.

## Introduction

Important part of collected household waste is treated by landfill [1]. The latter takes place in a landfill site, where household waste is slowly degraded under anaerobic conditions [1] The main valuable product of a given landfill site is called landfill gas, which mainly contains methane (35-65 vol.%) and carbon dioxide (15-40 vol.%) [2]. However, other compounds such as nitrogen (0-40 vol.%), oxygen (0-5 vol.%), hydrogen (0-3 vol.%), carbon monoxide (0-3 vol.%), water vapor (1-5 vol.%) could also be present, together with pollutants such as hydrosulfide (hundreds ppmv), halogenated compounds (hundreds ppmv) siloxanes (0-50 mgSi/Nm³), light hydrocarbons (0-4500 mg/Nm³), etc. [3],[4], [5], [6] Today, landfill gas is principally upgraded into biomethane for injection to gas grids [7]. As example, to-date, there are 634 biomethane injection points in metropolitan France for a total

production capacity of 11341 GWh/year [8]. However, the reforming of landfill gas is a promising valorization way which allows obtaining synthetic gas (syngas) as a versatile platform gas mixture. According to our recent work [9], the reforming of landfill gas can be performed via different reforming processes including steam reforming of methane (SRM), dry reforming of methane (DRM), partial oxidation of methane (POM), or tri-reforming of methane (TRM). Among them, TRM, which combines all the three individual methane reforming reactions, has been found to be the most efficient. In fact, TRM converts the main components of a typical landfill gas into syngas according to the Eq. 1. This allows not only valorizing all the main components of landfill gas into syngas, but also offering a possibility to reach the desired molar ratio of H₂/CO via the control of the feeding composition.

$$x\text{CH}_4 + y\text{CO}_2 + z\text{H}_2\text{O} + t\text{O}_2 \rightarrow u\text{CO} + v\text{H}_2 \qquad \text{Eq. 1}$$

TRM reaction is generally catalyzed by a solid catalyst. The development of catalysts for TRM process has been recently reviewed by Pham et al. [10]. Thus, nickel nanoparticles have commonly been considered as the most interesting active phase in TRM, which can be dispersed on the surface of various catalyst supports (e.g. Al₂O₃, ZrO₂, CeO₂, etc.) by different preparation methods [10]. Highly-dispersed nickel nanoparticles or sub-nanoparticles are required to reach not only high specific catalytic activity, but also to limit solid carbon formation [11]. Consequently, the selection of an adequate catalyst support plays an important role in the design of a performing TRM catalyst [10]. Mixed oxides such as MgAl₂O₄, CeO₂-ZrO₂-Al₂O₃, CaO-ZrO₂ etc. have been found to be the most efficient supports because they offer the possibilities to tailor and optimize the physico-chemical properties, which are favorable for TRM reaction [10]. Particularly, new potential catalyst supports for TRM reaction have also been proposed, including hydroxyapatite (HAP: Ca₁₀(PO₄)₆(OH)₂) [10]. The latter has been studied in a large variety of chemical reactions [12]. To-date, HAP-based catalysts have been studied in DRM showing high catalytic performances [13], [14], [15], but, to the best of our knowledge, they have not been investigated in TRM yet.

Thus, this work aims to develop highly performing Ni/HAP catalyst for TRM reaction, in view of valorizing a typical landfill gas into
syngas. The main parameters impacting the catalyst performance have also been investigated. The stability of the catalyst has also been investigated for a long reaction time of 300 h (12.5 days consecutively) showing excellent catalytic performance.

# Results and Discussion

## Fresh catalyst characterization

ICP-AES results allow determining the bulk Ca/P molar ratio of the calcined 5Ni/HAP equal to 1.73, which is slightly higher than the targeted stoichiometric value (1.67). In fact, the Ca/P molar ratio in HAP depends on various synthesis factors such as the precursor of calcium, the initial and final pH, the ageing time, etc. [17]. In the case of our HAP support, the value of the Ca/P molar ratio of 1.73 would lead to high surface basicity, thus enhancing CO₂ adsorption on the catalyst surface, which in turn favors methane reforming reaction [18], [19].

The Ni content of the 5Ni/HAP catalyst determined by ICP-AES reached 4.0 wt.%, which is slightly smaller than the theoretical value of 5.0 wt%. The loss could be due to the deposition of nickel precursor on the wall of the recipient during the incipient wetness impregnation (IWI) step.

<figure><img src="image_3.png" /><figcaption>Figure 1. (a) FTIR spectrum of the HAP support and the calcined 5Ni/HAP catalyst; and (b) TPR profile of the calcined 5Ni/HAP catalyst.</figcaption></figure>

Figure 1a presents the FTIR spectra of the dried powder HAP (support) and 5Ni/HAP (catalyst). HAP support shows the main characteristic vibration peaks of phosphate (565, 603, 960, 1037 and 1091 cm⁻¹), hydroxyl groups (636 and 3570 cm⁻¹), and

carbonate groups (873, 1414, and 1454 cm⁻¹) in the apatitic structure [20], [21]. The presence of CO₃²⁻ groups could be explained by the insertion of CO₂ into the apatitic structure during the support synthesis (dissolution of atmospheric CO₂ in the synthesis solution) and/or during contact of the supports with the air [22]. The deposition of nickel nanoparticles on the HAP surface did not make any notable modification to the surface functional groups in FTIR of the initial HAP support.

Figure 1b shows the TPR profile of the calcined 5Ni/HAP catalyst. As expected for a Ni-based catalysts, three reduction peaks could be observed: the first one centered around 395°C indicating a weak metal-support interaction, the second one around 445 °C with the highest intensity showing strong metal-support interaction, and the last one around 525 °C proving the presence of nickel species incorporated in the support structure [13].

Figure 2 displays the XRD patterns of the prepared catalysts calcined at 500 and 700°C under the air atmosphere. Characteristic peaks corresponding to the crystalline phase of the apatitic structure of the HAP support were observed at the 2θ angle of 25.9, 31.8, 32.9, 46.8, and 49.7 degrees [16], [23]. The sample calcined at 700 °C exhibits the same characteristic peaks in comparison with those of the sample calcined at 500 °C, but with higher crystallinity. The characteristic peaks of NiO were also observed at 2θ angle of 37.32, 43.36, and 63.52, since the thermal treatment was performed under the air. From XRD results and by applying the Scherrer equation (Eq. 2), the average size of the NiO crystallites could be determined to be equal to 33.8 nm.

<figure><img src="image_4.png" /><figcaption>Figure 2. XRD patterns of 5Ni/HAP catalyst calcined at different temperatures under the air.</figcaption></figure>

SEM images of the fresh calcined 5Ni/HAP catalyst are presented in Figure 3 (a) and (b). As expected, Figure 3 (a) confirms the obtention of HAP particles in the range of 125-315 µm according to the experimental section. Figure 3 (b) illustrates the similarity in the size and the good distribution of NiO particles on the HAP surface. Figure 3 (c) illustrates NiO particle size obtained by TEM, which could be confirmed by EDX analysis (Figure 3 (d)). From a large number of TEM images, NiO particle size of the fresh calcined catalyst could be determined and further presented in
<figure><img src="image_5.png" /><figcaption>Figure 3. (a) and (b): SEM images of the calcined 5Ni/HAP catalyst; (c) and (d): TEM image and EDX mapping of the calcined 5Ni/HAP catalyst.</figcaption></figure>

The nitrogen adsorption-desorption isotherms are depicted in Figure 4a. The hysteresis loops indicate that both HAP and 5Ni/HAP catalysts are mesoporous materials. Both the HAP support and the 5Ni/HAP catalyst exhibit similar isotherm and hysteresis loops, which could be assigned to the type IV isotherm and H1 hysteresis, respectively, according to the IUPAC classification. This explains the narrow distribution of pore sizes (Figure 4b) [24]. The deposition of Ni on HAP led to a decrease of the specific surface area from 86 m²/g to 65 m²/g.

<figure><img src="image_6.png" /><figcaption>Figure 4. Nitrogen adsorption-desorption isotherms (a) and pore size distribution (b).</figcaption></figure>

Performance of the catalyst in TRM

<figure><img src="image_7.png" /><figcaption>Figure 5. Impact of the O/C ratio on the catalytic performance of 5Ni/HAP in TRM: CH₄ conversion (a), CO₂ conversion (b), and H₂/CO molar ratio (c). Reaction conditions: reaction temperature 700 °C; reaction total pressure 1.4 bar; feeding molar ratio of CH₄/CO₂/H₂O/O₂/N₂ = 1.0/1.1/0.6/0/0.5 (for O/C = 0), 1.0/1.1/0.6/0.05/0.5 (for O/C = 0.05), and 1.0/1.1/0.6/0.1/0.5 (for O/C = 0.1); inlet CH₄ flowrate = 45 NmL/min; catalyst mass: 340 mg.</figcaption></figure>

The content of oxygen in the landfill gas depends on its production and recovery, and can reach 0-5 vol.% as presented in the Introduction section. Figure 5 compares the catalytic performance of the 5Ni/HAP catalyst at different O/C ratios in the range of 0.0-0.1. For these tests, only the inlet flowrate of O₂ changed, while all other parameter, e.g. catalyst mass, other inlet gas flowrate, reaction temperature and pressure were unchanged (see Table 1 in the Experimental section). Figure 5a shows that increasing the O/C ratio led to an increase of the initial CH₄ conversion. The latter
reached 63%, 68%, and 70% at the O/C ratio of 0.00, 0.05, and 0.10 respectively. These values of CH₄ conversion were below the theoretical values calculated at the thermodynamic equilibrium (Table 1). Similar findings were observed in the study conducted by Song *et al.* over nickel supported on different metal oxides [25]. This could be explained by the high reactivity of oxygen in the partial oxidation of methane (POM reaction). For each studied O/C ratio, the CH₄ conversion decreased with the reaction time, suggesting a catalyst deactivation, which did not finish even after 50 hours on stream. The deactivation rate (slope of the CH₄ conversion curves) decreased with the increase of the O/C ratio, which could be attributed to the partial removal of carbon deposits on the catalyst surface facilitated by the presence of oxygen.

In Figure 5b, a significant decrease in CO₂ conversion is observed as the O/C ratio increases from 0 to 0.05. This could be explained by the competition between CO₂ and O₂ in the reforming of methane, where O₂ exhibits higher reactivity than CO₂. In fact, in all TRM tests implying the presence of O₂ in the feed, no trace of O₂ could be detected at the outlet of the reactor, suggesting its complete consumption in TRM reaction. However, this effect was almost negligible when the O/C ratio increased from 0.05 to 0.1. As observed in Figure 5a for methane conversion, the CO₂ conversion decreased with the reaction time, which could also be explained by the catalyst deactivation.

Finally, the presence of O₂ in the feed allowed increasing the H₂/CO ratio from ca. 1 (for O/C = 0) to ca. 1.2-1.4 (for O/C = 0.05-0.1) (Figure 5c). Similar results have also been observed in the work of Majewski *et al.* [26].

<figure><img src="image_8.png" /><figcaption>Figure 6. (a) TG and DTG curves, (b) and SEM image of 5Ni/HAP catalyst recovered after 50 h on stream in TRM. Reaction conditions: 700 °C; 1,4 bar; feed molar ratio of CH₄/CO₂/H₂O/O₂/N₂ = 1.0/1.1/0.6/0.1/0.5; inlet CH₄ flowrate = 45 NmL/min; catalyst mass: 340 mg. Arrows: positions of nickel particles.</figcaption></figure>

Figure 6a shows TG and DTG curves under the air atmosphere of the catalyst recovered after the test with the O/C ratio of 0.1. A small gain of mass (< 0.2 wt.%) took place around 430 °C, probably due to the oxidation of metallic nickel inside the nickel nanoparticles. Then, a clear mass loss happened around 630 °C, which corresponds to the complete oxidation of solid carbons formed on the surface of the catalyst during the TRM reaction. In fact, Figure 6b shows an example of TEM image of this used catalyst, which clearly highlighted the formation of solid carbon, mostly under the form of carbon nanofibers and nanotubes. Since the solid carbon formation is still favoured at 700 °C from the thermodynamic point of view [27], the presence of large oxygen content in the feed (e.g. 3.2 vol.% of O₂ in the feed in the case of the O/C ratio = 0.1) was not enough to prevent coke formation.

## Effect of the S/C ratio

In a given landfill gas, the content of CH₄, CO₂ and O₂ can be found in the range of 35-65 vol.%, 15-50 vol.% and 0-5 vol.%, respectively [27]. To perform TRM with a landfill gas, it is convenient to add an appropriate fraction of steam. In this study, the impact of the S/C ratio in the range of 0.6-0.9 was studied (Table 1), which is much smaller than the S/C employed in the industrial steam reforming of methane (S/C = 3-4) [9].

As expected, Figure 7a shows the positive impact of the S/C ratio in TRM reaction, where CH₄ conversion was always higher at S/C = 0.9 than at S/C = 0.6. During the first 5 hours on stream, the CH₄ conversion decreased from 64% to 61% for S/C = 0.9, and from 60% to 52% for S/C = 0.6. These values of CH₄ conversion were below the theoretical values calculated at the thermodynamic equilibrium (Table 1). After this first deactivation, CH₄ conversion remained mostly stable up to the end of the reaction at S/C = 0.9, while it continuously decreased at S/C = 0.6, highlighting the importance of steam to preserve the performance of the catalyst against the deactivation by solid carbon deposition. Similar result was also found by Majewski and Wood [26]. As shown in Figure 7c (Supporting Information), solid carbon deposition is less important at S/C = 0.9 that at S/C = 0.6. However, TG analysis of the catalyst recovered after the TRM at S/C = 0.9 and 700 °C shows that the solid carbon formation was still important (Figure 8e below).

In Figure 7b, CO₂ conversion was higher at the S/C ratio = 0.6 in comparison with that at the S/C = 0.9. This is explained by the competition of H₂O and CO₂ in the reforming of methane, where steam exhibits higher reactivity. The decrease of CO₂ conversion with the reaction time could be explained not only by the catalyst deactivation (in the case of the ratio S/C = 0.6), but also the presence of side reaction such as the reserve water-gas-shift (rWGS), which implies CO₂. The decrease of the CO₂ conversion (Figure 7b) and the increase of the H₂/CO ratio (Figure 7c) suggests that the activity of the catalyst in rWGS decreased with the reaction time.

The H₂/CO ratio was higher at S/C = 0.9 that at S/C = 0.6 (Figure 7c). Increasing the S/C ratio favored the steam reforming of methane, leading to higher H₂/CO ratio, since SRM theoretically leads to higher H₂/CO ratio than DRM. Similar results were also observed by Song *et al.* with Ni/Al₂O₃, Ni/MgO/CeZrO, and
Ni/MgO catalysts [25]. Moreover, these results are consistent with the trend predicted by the thermodynamic analysis of TRM [27].

<figure><img src="image_9.png" /><figcaption>Figure 7. Impact of the S/C ratio on the catalytic performance of 5Ni/HAP catalyst in TRM reaction at 700 °C and 1.4 bar. Inlet gas composition: CH₄/CO₂/H₂O/O₂/N₂ = 1.0/0.67/0.6/0.1/0 and 1.0/0.67/0.9/0.1/0; inlet CH₄ flowrate = 45 NmL/min; catalyst mass: 340 mg. (a): CH₄ conversion; (b): CO₂ conversion; (c): H₂/CO molar ratio; and (d): Ni particle size distribution of the fresh and used catalysts.</figcaption></figure>

In addition to solid carbon deposition as a reason of the catalyst deactivation, the thermal sintering of nickel nanoparticles also causes the catalyst deactivation in methane reforming. Figure 7d compares the nickel nanoparticle size distribution of the fresh 5Ni/HAP catalyst and the used catalyst recovered after TRM reaction at the S/C of 0.6 and 0.9 (at 700 °C). The fresh catalyst exhibited nickel particles ranging from 10 nm to 90 nm. On the other hand, after 50 hours on stream in TRM reaction, larger nickel particles up to 170-180 nm appeared, for both S/C ratios of 0.6 and 0.9. The sintering of nickel particles could be favored by the presence of water vapor at high temperature since metal atoms become more mobile [28] [29].

## Effect of reaction temperature

Thermodynamic study of the TRM reaction has shown that increasing the reaction temperature can limit solid carbon formation [27], but this can also increase the risk of thermal sintering. A compromise should be found for each catalyst system. In the present study, all the TRM tests presented in the previous sections were performed at 700 °C, where both the solid carbon formation and the thermal sintering of nickel nanoparticles took place, as revealed by the characterization of the used catalysts. Thus, TRM reaction was also conducted at 800 °C in view of limiting the formation of solid carbon. This temperature is still much lower than that used in the industrial steam reforming of methane (e.g. 900-950 °C) [9]. Figure 8 compares the catalytic results at 700 °C and 800 °C. As expected, increasing the reaction temperature from 700 °C to 800 °C favored both the CH₄ and CO₂ conversions (Figure 8a and Figure 8b). This is explained by the fact that both the steam reforming and dry reforming of methane are endothermic and are favored by increasing the reaction temperature. At both reaction temperatures, methane conversions were smaller in comparison with the values calculated at the thermodynamic equilibrium (Table 1). A slight deactivation still took place at both 700 and 800 °C during the first hours on stream (Figure 8a, methane conversion). Then, the catalyst became stable. This initial deactivation could be due to the thermal sintering of nickel particles (Figure 8d), and the solid carbon formation (Figure 8e and Figure 8f). In fact, Figure 8d evidences the increase of the size of Ni nanoparticles after TRM reaction at both reaction temperatures and this was more pronounced at 800 °C than at 700 °C, while Figure 8e and Figure 8f indicate the presence of solid carbon in the used catalyst. However, while the mass loss due to solid carbon oxidation was important after TRM reaction at 700 °C (Figure 8e), it was mostly negligible after TRM at 800 °C (Figure 8f). This result indicates that the developed 5Ni/HAP catalyst can be considered as an efficient anti-coking catalytic material for TRM reaction at 800 °C. About the H₂/CO ratio, it reached approximately 1.9-2.0, and was mostly stable along the reaction at both the reaction temperatures (Figure 8c). Globally, the reaction temperature at 800 °C appeared as the good temperature for the 5Ni/HAP catalyst in TRM reaction in terms of both the catalytic activity and stability.
<figure><img src="image_10.png" /></figure>

<figure><img src="image_11.png" /><figcaption>Figure 8. Impact of the reaction temperature on the catalytic performance of 5Ni/HAP catalyst in TRM reaction: (a) CH₄ conversion, (b) CO₂ conversion, (c) H₂/CO molar ratio, (d) Ni particle size distribution, (e) TG and DTG curves of the used catalyst at 700 °C, (f) TG and DTG curves of the used catalyst at 800 °C. Other reaction conditions: total pressure = 1.4 bar; feed gas composition: CH₄/CO₂/H₂O/O₂/N₂ = 1.0/0.67/0.9/0.1/0; inlet CH₄ flowrate: 45 NmL/min; catalyst mass: 340 mg.</figcaption></figure>

## Catalyst stability

To confirm the catalytic stability of the 5Ni/HAP catalyst in TRM at 800 °C, another test was performed for 300 hours on stream. The results were displayed in Figure 9. Along the reaction time, the conversion of methane was kept stable at ca. 90%, showing a perfect catalytic activity and stability. It is worth noticing that the reaction conditions employed in this test were much softer than those used for steam reforming of methane [9]. On the other hand, the conversion of CO₂ fluctuated between 35 and 50%. This behavior in CO₂ conversion has been observed before but is less visible at shorter reaction times (50 hours). This fluctuation might be related to the operation of the HPLC pump, which injected liquid water pulses to the reactor and thus impacted CO₂ conversion due to the competitiveness of steam and CO₂ in methane reforming. Overall, the CO₂ conversion was relatively stable. The fluctuation of CO₂ conversion slightly impacted the H₂/CO ratio. The latter closely varied in the range of 1.7-1.9, which is suitable for downstream processes such as Fisher-Tropsch synthesis. The selectivity into H₂ and CO was excellent (Figure 9b). The selectivity into H₂ reached closely 95-100%, while the selectivity into CO was closely 90-95%.

Despite the very good catalytic stability of the catalyst along the reaction time, TEM analysis of the fresh and used catalyst (Figure 9c) showed that the thermal sintering clearly took place, confirmed the observation in Figure 8d. We suppose that this thermal sintering quickly happened at the first minutes of reaction, and did not further evolve, which allowed keeping the catalyst stable for the rest of reaction time. TG analysis confirmed again that the solid carbon was not notably formed (Figure 9d) at 800 °C under the reaction conditions used.
<figure><img src="image_12.png" /><figcaption>Figure 9. Catalytic stability of the 5Ni/HAP catalyst in TRM at 800 °C and 1.4 bar: (a) CH₄ and CO₂ conversions, (b) selectivity into H₂ and CO, and H₂/CO molar ratio, (c) Ni particle size distribution, (d) TG and DTG curves of the used catalyst. Other reaction conditions: feed gas composition: CH₄/CO₂/H₂O/O₂/N₂ = 1.0/0.67/0.9/0.1/0; inlet CH₄ flowrate: 45 NmL/min; catalyst mass: 340 mg.</figcaption></figure>

Figure 10 shows the mass balance in the TRM reaction at 800 °C. Atomic flux of hydrogen, oxygen and carbon were calculated from the flowrate and the content of each gas (e.g. CH₄, CO₂, CO, H₂, H₂O) at the inlet and outlet of the reactor, while solid carbon is considered as negligible (see Figure 9d). In all cases, the outlet

atomic flux of each element was close to the corresponding inlet atomic flux, leading to good mass balances along the reaction.

<figure><img src="image_13.png" /><figcaption>Figure 10. Mass balance in the TRM reaction over the 5Ni/HAP catalyst at 800 °C, 1.4 bar. Other conditions: CH₄/CO₂/H₂O/O₂/N₂ = 1.0/0.67/0.9/0.1/0; inlet CH₄ flowrate: 45 NmL/min; catalyst mass: 340 mg.</figcaption></figure>

The performance of the 5Ni/HAP catalyst developed in this work is compared to those of nickel-based catalysts reported in the literature under the similar conditions of temperature and pressure of TRM (Table S1.1 in the Supporting Information). It is worth noticing that, Table S1.1 only shows a relative comparison, since the works reported in this table were not performed under the same conditions (e.g. nickel loading, method for catalyst preparation, catalyst pretreatment conditions, contact time and feeding composition in TRM, etc.). At 700 °C, the methane conversion over 5Ni/HAP is much higher than that over a 11wt.%Ni/SiO₂ catalyst, and is only slightly smaller than those obtained at 750 °C over 9.1wt%Ni/MgAl₂O₄ or 9.2wt.%Ni/CeZrO₂/MgAl₂O₄, the latter being prepared on a referenced support (MgAl₂O₄) in methane reforming. At 800 °C, 5Ni/HAP is situated among the most active and stable catalysts, together with the counterparts prepared on ZrO₂ or CeO₂ supports.
This highlights the potential of HAP-containing nickel catalysts in TRM reaction.

## Conclusion

In this work, the 5Ni/HAP catalyst prepared by IWI method has been investigated in TRM reaction, in view of the valorization of a landfill gas into syngas. As expected, TRM reaction was favored by increasing the molar ratio of O/C and S/C, and by increasing the reaction temperature. Particularly, this catalyst has been found extremely performing under relatively “soft” reforming conditions. At 800 °C, 1.4 bar and for the feeding gas composition of CH₄/CO₂/H₂O/O₂/N₂ = 1.0/0.67/0.9/0.1/0 (with the inlet CH₄ flowrate of 45 NmL/min), the methane conversion reached ca. 90%, the H₂/CO ratio reached 1.7-1.9, with a perfect catalytic stability during a long reaction time of 300 hours.

Future work would be focused on the optimization of this catalyst by downsizing the nickel nanoparticles, and mostly to enhance the metal-support interaction to limit the thermal sintering under TRM conditions.

## Material and methods

### Synthesis of the catalyst

HAP support was prepared by the coprecipitation method using Ca(NO₃)₂·4H₂O (99%, CAS number: 13477-34-4) and NH₄H₂PO₄ (98%, CAS number: 7722-76-1) as calcium and phosphate precursors. The coprecipitation was performed in an aqueous solution at 40 °C under magnetic stirring (500 rpm) for 48 h, followed by filtration and washing several times with distilled water, and finally by drying in an oven at 105 °C overnight. The obtained solid was crushed and sieved, and only grain fraction of 125-315 µm was used for the preparation of the catalyst. The specific surface area of this support, determined by the nitrogen adsorption-desorption method, reached 86 m²/g, with a mean pore diameter of 22.4 nm.

The deposition of nickel nanoparticles on the HAP support synthesized above was performed by the conventional incipient wetness impregnation (IWI), using Ni(NO₃)₂·6H₂O (98%, CAS number: 13478-00-7) as nickel precursor. The concentration of the aqueous solution of nickel nitrate was calculated to reach the theoretical Ni content of 5.0 wt.%. After the impregnation step, the mixture was dried at 105 °C overnight, then calcined under the air atmosphere at 500 °C or 700 °C for 3 h to convert nickel precursor into nickel oxide particles. The resulting solid was named 5Ni/HAP.

### Catalyst characterization and evaluation in TRM reaction

Fresh and used catalysts were characterized by different techniques. Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES) was performed on a HORIBA Jobin Yvon Ultima 2 for elemental analysis. Solid samples were dissolved in a mixture of concentrated HCl and HNO₃ prior to ICP-AES analysis. Fourier-transform infrared spectroscopy (FTIR) spectroscopy was realized with a SHIMADZU - 8400S spectrometer within 4000 à 500 cm⁻¹ wavelength range.

Thermogravimetry analysis (TGA) was done with a SDT Q600 (TA Instrument). Solid samples were heated (5°C/min heating rate) under a flux of air (100 NmL/min) from 30 to 950 °C. X-ray diffraction (XRD) was performed with a X'pert Pro MPD (PANalytical) within the 2Θ angle range of 10-80°. The identification of the crystallin phases was done using the International Centre for Diffraction Data (ICDD) and Crystallography Open Database (COD) database. From the XRD results, the size of nickel oxide crystallites could be calculated according to the Scherrer equation (Eq. 2).

$$d = \frac{k \cdot \lambda}{FWHM \cdot \cos\theta} \qquad \text{Eq. 2}$$

where d is the crystallite size of nickel oxide (nm), k is a dimensionless shape factor which has usually the value of 0.9, λ is the X-ray wavelength (nm), Θ is the Bragg diffraction angle (°), FWHM is the full-width at half-maximum of the considered peak (rad).

Scanning electronic microscopy (SEM) and transmission electronic microscopy (TEM) coupled with Energy-dispersive X-ray spectroscopy (EDX) analyses were performed with a Philips XL30 ESEM FEG and a FEG JEOL JEM 2100F, respectively. The nitrogen adsorption-desorption isotherm was recorded at 77 K with a Micromeritics Tristar II 3020. Before the analysis, solid samples were degassed at 105 °C and 50-100 mbar for 24 h. Then, the specific surface area was obtained by the BET (Brunauer – Emmet – Teller) model, while the pore size distribution was obtained by the BJH (Barrett-Joyner-Halenda) model. Temperature-programed reduction (TPR) was performed with the catalyst previously calcined at 700 °C using a Micromeritics Autochem 2920 machine. The sample was heated from 30 to 700 °C (10 °C/min heating rate) under H₂ flux (4%vol) diluted in He (100 NmL/min total flowrate). The reduction was detected by a Thermal Conductivity Detector (TCD).

### Catalytic reactor used for TRM tests

TRM reaction was carried out in a tubular fixed-bed reactor (8 mm inner diameter, 250 mm length), heated by an electric oven (Figure SI. 2, Supporting Information). The tube of the reactor was made of chemically inert refractory ceramic. For each TRM test, the reactor was filled with a layer of inert alumina power, which was already sintered at 1000 °C for 5 h (specific surface area < 2 m²/g), then, with a layer of 5Ni/HAP catalyst (340 mg) diluted twice in the inert alumina powder, and finally with another layer of the same inert alumina. The quantity of each layer was calculated to set the catalyst bed at the center of the tubular reactor. A thermocouple was placed at the center of the catalyst bed in order to control and keep constant the temperature of the reaction. Prior to the reaction, the catalyst was in-situ reduced at 700 °C by a flow of 5 vol.%H₂/N₂ for 1 h. Then, the temperature of the reactor was adjusted to the desired value (700 or 800 °C). Finally, the TRM reaction started by feeding the reactor with the reaction gas mixture. The latter was prepared from different gas bottles (purity > 99.95%) using mass flow meters, and distilled water using a precision HPLC pump. Water and the different gases were preheated to 120 °C before entering to the reactor.
At the outlet of the reactor, the mixture was cooled down to 5 °C, which allowed condensing steam. Then, the gas mixture passed through a silica gel tube to remove all eventual traces of water vapor. The composition of the dried gas mixture was analyzed by a µ-GC. From the µ-GC results, the conversion, the selectivity, and the mass balance of the reaction were calculated according to Eq. 3 to Eq. 10:

$$ \text{CH}_4 \text{ conversion: } X_{\text{CH}_4}(\%) = \frac{Q_{\text{CH}_4}^{\text{inlet}} - Q_{\text{CH}_4}^{\text{outlet}}}{Q_{\text{CH}_4}^{\text{inlet}}} \times 100 \qquad \text{Eq. 3} $$

$$ \text{CO}_2 \text{ conversion: } X_{\text{CO}_2}(\%) = \frac{Q_{\text{CO}_2}^{\text{inlet}} - Q_{\text{CO}_2}^{\text{outlet}}}{Q_{\text{CO}_2}^{\text{inlet}}} \times 100 \qquad \text{Eq. 4} $$

$$ \text{Selectivity into } H_2: S_{H_2}(\%) = \frac{Q_{H_2\text{formed}}}{2x(Q_{\text{CH}_4}^{\text{inlet}} - Q_{\text{CH}_4}^{\text{outlet}}) + Q_{H_2O}^{\text{inlet}} - Q_{H_2O}^{\text{outlet}}} \times 100 \qquad \text{Eq. 5} $$

$$ \text{Selectivity into CO: } S_{\text{CO}}(\%) = \frac{Q_{\text{CO\text{formed}}}}{Q_{\text{CH}_4}^{\text{inlet}} - Q_{\text{CH}_4}^{\text{outlet}} + Q_{\text{CO}_2}^{\text{inlet}} - Q_{\text{CO}_2}^{\text{outlet}}} \times 100 \qquad \text{Eq. 6} $$

$$ \text{Selectivity into solid carbon } S_c(\%) = \frac{Q_c}{Q_{\text{CH}_4}^{\text{inlet}} - Q_{\text{CH}_4}^{\text{outlet}} + Q_{\text{CO}_2}^{\text{inlet}} - Q_{\text{CO}_2}^{\text{outlet}}} \times 100 \qquad \text{Eq. 7} $$

$$ H_{\text{balance}} = \frac{H_{\text{out}}}{H_{\text{in}}} \qquad \text{Eq. 8} $$

$$ O_{\text{balance}} = \frac{O_{\text{out}}}{O_{\text{in}}} \qquad \text{Eq. 9} $$

$$ C_{\text{balance}} = \frac{C_{\text{out}}}{C_{\text{in}}} \qquad \text{Eq. 10} $$

with $Q_{CH_4}^{\text{outlet}}$, $Q_{CO_2}^{\text{outlet}}$ and $Q_{H_2O}^{\text{outlet}}$: molar flowrate of methane, carbon dioxide and water at the reactor outlet, respectively (mol/min), $Q_{CH_4}^{\text{inlet}}$, $Q_{CO_2}^{\text{inlet}}$ and $Q_{H_2O}^{\text{inlet}}$: molar flowrate of methane, carbon dioxide and water at the reactor inlet, respectively (mol/min). $Q_c$: estimated value of the solid carbon formed during the reaction by difference; $H_{in}$, $O_{in}$, and $C_{in}$: Calculated atomic flux (mol/min) of hydrogen, oxygen and carbon in the feed of the reactor, respectively. $H_{out}$, $O_{out}$, and $C_{out}$: Calculated atomic flux (mol/min) of hydrogen, oxygen and carbon at the outlet of the reactor, respectively.

The main operation parameters investigated in this work include the oxygen-to-carbon ratio (O/C, feeding molar ratio of oxygen to methane), the steam-to-carbon ratio (S/C, feeding molar ratio of steam to methane), and the reaction temperature, as summarized in Table 1. The feeding composition has been selected to adapt to the average composition of the landfill gas produced on an industrial site in the South of France [9]. The molar ratio of S/C and O/C have been selected according to our previous study on the thermodynamic aspect of the TRM [27]. The total pressure inside the reactor reached 1.4 bar due to pressure drop of the catalyst bed. From the reaction conditions used, methane conversion at the thermodynamic equilibrium could be calculated as detailed elsewhere [9] and are displayed in Table 1.

<table><caption>Table 1. Experimental conditions for TRM tests. The total pressure was 1.4 bar and the catalyst mass was 340 mg for all the tests. $X_{CH_4}^{\text{thermo}}$: CH$_4$ conversion (%) calculated at the thermodynamic equilibrium.</caption><thead><tr><th></th><th>Inlet gas flowrate of CH<sub>4</sub>/CO<sub>2</sub>/H<sub>2</sub>O/O<sub>2</sub>/N<sub>2</sub> (NmL/min)</th><th>Molar ratio of the gas mixture in the feed: CH<sub>4</sub>/CO<sub>2</sub>/H<sub>2</sub>O/O<sub>2</sub>/N<sub>2</sub></th><th>T (°C)</th><th>X<sub>CH<sub>4</sub></sub><sup>thermo</sup></th></tr></thead><tbody><tr><td>O/C=0</td><td>45.0/49.5/17.8/0/25.0</td><td>1.0/1.1/0.6/0.0/0.5</td><td>700</td><td>82.5</td></tr><tr><td>O/C=0.05</td><td>45.0/49.5/17.8/2.3/22.5</td><td>1.0/1.1/0.6/0.05/0.5</td><td>700</td><td>84.6</td></tr><tr><td>O/C=0.1</td><td>45.0/49.5/17.8/4.5/20.5</td><td>1.0/1.1/0.6/0.1/0.5</td><td>700</td><td>86.4</td></tr><tr><td>S/C=0.6</td><td>45.0/30.1/17.8/4.5/0</td><td>1.0/0.67/0.6/0.1/0</td><td>700</td><td>82.4</td></tr><tr><td>S/C=0.9</td><td>45.0/30.1/26.7/4.5/0</td><td>1.0/0.67/0.9/0.1/0</td><td>700</td><td>82.8</td></tr><tr><td>S/C=0.9</td><td>45.0/30.1/26.7/4.5/0</td><td>1.0/0.67/0.9/0.1/0</td><td>800</td><td>97.7</td></tr></tbody></table>

## Acknowledgment

The authors gratefully thank ADEME (Agence de l'environnement et de la maîtrise de l'énergie, France) for the financial support of the VABHYOGAZ3 project. The authors also thank our colleagues at the RAPSODEE center (CNRS UMR 5302, France) for technical help.

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## Entry for the Table of Contents

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## Text for Table of Contents

For the first time, hydroxyapatite supported nickel catalyst has been investigated in tri-reforming of methane for syngas production. Excellent activity, selectivity and stability could be achieved under unfavorable reaction conditions of methane reforming.