Fishbone derived-hydroxyapatite supported Ni-Zr nanocatalyst for CO₂ methanation: Synergistic effects of support and zirconia

2.1 Fabrication of catalyst

The raw salmon bone was collected from seafood manufacturers in Vietnam. All the chemicals used in the experiment were purchased from Merck and used without further purification. The preparation of HA was taken place as described in our previous research (Tri et al., 2020). 1.0 g of the dried salmon bone was introduced to 50 mL of distilled water, then 14 mL H₃PO₄ 1 wt% was added dropwise into the above mixture. The mixture was adjusted to pH = 10 by NH₄OH 5 wt% solution and stirred vigorously for 2 h. After stirring, the mixture was transferred into Teflon-line auto-clave and heated to 120 °C for 7 h. The precipitate after the hydrolysis was isolated by centrifuging, washed with distilled water and ethanol, then dried at 80 °C for 12 h. The HA support was obtained after calcining the dry precipitate in the air at 800 °C for 1 h.

Ni-ZrO₂/HA was prepared by the impregnation method. Calculated amounts of Ni(NO₃)₂·6H₂O, ZrOCl₂·8H₂O, and HA support were mixed with a decent amount of distilled water. After mixing, the mixture was dried at 3 levels of temperature (80, 100, and 120 °C) for 2 h for each temperature. The calcination was carried out at 600 °C for 4 h (a heating rate of 10 °C/min) to obtain the NiO-ZrO₂/HA. Before the catalytic activity evaluation, a pre-treatment of NiO-ZrO₂/HA in a pure H₂ flow (3 L/h) was taken place and the Ni-ZrO₂/HA catalyst was officially obtained. The Ni content was fixed at 10 wt%. The catalysts were assigned 10NixZr/HA, in which × value is the wt.% of ZrO₂ and ×  = 2, 4, and 6.

2.2 Characterization

The physical–chemical characteristics of the obtained samples were studied by several methods (Van et al., 2020), including X-ray diffraction on a Bruker D2 Phaser Xray diffractometer with Cu Kα radiation and recorded in 2θ = 10 − 80° with a step size of 0.03°; EDS spectrum on a JEOL JST-IT 200 instrument; ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) on a V-770 UV–visible/NIR spectrophotometer; nitrogen adsorption − desorption isotherms at − 197 °C on a Nova 2200e instrument with the sample pretreated in a 1.8 L/h N₂ flow at 300 °C for 2 h; field-emission scanning electron microscopy on Hitachi S4800 instrument; high-resolution transmission electron microscopy (HR-TEM) on JEOL JEM 1400 instrument; hydrogen temperature-programmed reduction (H₂-TPR) carried out on a microreactor in a gas mixture of 10% H₂/N₂ at a flow rate of 1.8 L/h using a gas chromatograph GOW-MAC 69–––350 with a TCD detector and the used sample of 50 mg; and CO₂-programmed temperature desorption (CO₂-TPD) also carried out on a microreactor in He at a flow rate of 1.8 L/h from 50 to 800 °C at a ramping rate of 10 °C/min with 100 mg of catalyst adopted in a quartz reactor and reduced at 450 °C for 1 h in pure H₂ at a flow rate of 1.8 L/h prior to being cooled down to 50 °C.

2.3 Catalytic activity

For catalytic activity testing, a quartz tube placed in a furnace was used as the reactor system. The catalyst bed is a mixture of 0.5 g quartz and 0.5 g catalyst in the same size of 0.25–––0.50 mm. The reduction process was carried out under a 3 L/h H₂ flow at 400 °C for 2 h with a heating rate of 10 °C/min. After the pre-treatment, the catalytic activity of all samples was tested at the temperature range of 250–––400 °C with a total flow rate of 6 L/h consisting of a CO₂/H₂/N₂ ratio of 3/12/85 mol%. To determine the CO₂ conversion and CH₄ selectivity in product flow, the reaction mixture was analyzed on a gas chromatograph (Agilent 5890 Series II) using a thermal conductivity detector with a capillary column (TG-BON Q, 30 m length, 12 μm inter diameter, 0.32 μm thickness) and a flame ionization detector with a capillary column (DB624, 30 m length, 0.32 mm outer diameter, 0.25 μm thickness). The gas mixtures of CH₄ 6 mol.% in N₂ and CO 6 mol.% in N₂ were used as a reference to monitor CH₄ and CO produced in the output. CO₂ conversion ( $X_{{\mathit{CO}}_2}$ ) and CH₄ selectivity ( $S_{C{H}_4}$ ) were calculated according to Eq. (1) and Eq.(2):

3.1 Physico-chemical characteristics

The XRD patterns of catalysts with different Zr contents (Fig. 1a) shows that the presence of HA hexagonal phase in all samples is indicated by the diffraction peaks at 2θ = 10.9, 16.9, 21.6, 22.8, 26.1, 27.8, 28.7, 31.8, 32.0, 32.9, 34.0, 39.9, 46.8, 49.5, 50.5, 51.3, 52.0, 53.0, 60.2, 61.7, 64.0, 65.1, and 77.0° (JCPDS card No.09–432). Besides, the β-TCP Ca₃(PO₄)₂ phase with low concentration was also detected with low intensive diffraction peaks at 2θ = 13.7, 31.2, 34.7, 35.6, 37.6, 57.1, 58.0, and 78.3° (JCPDS card No.09–169). The presence of β-TCP Ca₃(PO₄)₂ relates to pH and sintering temperature (Le et al., 2012; Boukha et al., 2019). The same phenomenon was also observed in Taimai’s study (Tamai et al., 2007), in which a ratio of Ca/P < 1.67 induced the decomposition of HA, resulting in the formation of β-TCP Ca₃(PO₄)₂. The characteristic diffraction peaks of Ni appear at 2θ = 44.7, 51.9, and 76.8° (JCPDS No.04–0850). According to Standard Card mp-20194, the characteristic peaks for ZrO₂ appear at 2θ = 24.3, 29.7, 33.7, 34.8, 42.1, 49.3, 50,1, 57.9, 59.3, 61.7, 70.9, and 73.5°. However, these ZrO₂ peaks are hardly observed. This phenomenon could account for the relatively low content of ZrO₂, or maybe some of the Ni’s peaks have coincided with the HA peaks (2θ = 33.7, 49.3, 50.1, and 61.7°). The texture properties of HA support and undoped nickel as well as zirconia-doped nickel catalysts are given in the Table 1. The addition of zirconia to the 10Ni/HA catalyst shows the effect of decreasing the Ni crystalline size (15.3 nm down to 5.5 nm), which agrees with several pieces of literature investigating the impact of ZrO₂ promoter in the Ni-based catalysts (Roh et al., 2004; Cai et al., 2011; Chong et al., 2019). The improved dispersal of active sites could result in the amelioration of the catalyst’s overall activity. The Ni crystalline size calculated by the Debye-Scherrer equation is 5.5 nm for the reduced 10Ni4Zr/HA sample, three times smaller than that of the pure 10Ni/HA sample.

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Fig. 1

XRD patterns (a), H₂-TPR (b) and CO₂-TPD (c) profiles, the hydrogen consumption (H) from H₂-TPR results and the desorbed CO₂ amount ( $({\mathrm{m}}_{{\mathrm{C}\mathrm{O}}_2})$ $m_{C{O}_2}$ ) from CO₂-TPD results (d), N₂ adsorption/desorption isotherms (e), BJH pore diameter distribution (f), and and of 10NixZr/HA catalysts.

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The H₂-TPR results (Fig. 1b and Table 1) were obtained to assess the reducibility of the supported Ni species (Boukha et al., 2022). Fig. 1b illustrates that the H₂-TPR results of zirconia-doped catalysts exhibit typical peaks corresponding to the same temperature areas as the profiles of unmodified catalysts. The 10Ni/HA sample’s TPR profile exhibited four reduction peaks below 450 °C, which can respectively be classified as 4 types of NiO species (α, β₁, β₂, and γ). The first characteristic is ascribed to the reduction of surface oxygen species (α species), which induces the oxygen vacancies on the catalyst’s surface. The second and third reduction peaks can be attributed to the reduction of NiO that exhibits a weak and medium interaction with HA (β species), which further classified into β₁ and β₂ (Boukha et al., 2019). The last reduction peak at higher temperature, which assigned to the γ- NiO species, might be induced from the bulk NiO particles that strongly attached to the HA support surface. Intriguingly, the addition of Zr alters the peaks toward lower temperatures, demonstrating the promoter effect of zirconium on the reduction of Ni species. More specifically, the reduction temperatures of these catalysts are lower than those of the unmodified catalysts, suggesting improved metal phase dispersion since ZrO₂ minimizes the weakening of the NiO–support contact and enhances the capacity to scatter NiO on the HA surface (Boukha et al., 2022). In addition, the Fig. 1b demonstrates that the zirconia-doped catalysts exhibit faint reduction signals at 600 °C; these reduction signals pertain to the reduction process of the NiO–ZrO₂ interaction phases or surface ZrO₂ in the doped catalysts. The ZrO₂ doping level is modest (2–6 wt%); hence the decrease signal is not strong. Furthermore, regardless of the Zr-modified sample, the amounts of consumed H₂ are considerably greater than those required for the reduction of a stoichiometric NiO phase. The observed tendency can be attributed to the presence of highly reducible Ni sites, most likely those located close to the NiO-ZrO₂ interface. This is consistent with the observed increase in the density of defective metallic Ni crystallites with increasing Zr loading, as demonstrated by XRD data for reduced samples. Notably, our results show that Zr promotes the reducibility of 10NixZr/HA catalysts.

To evaluate the CO₂ adsorption capacity of the ZrO₂ modified catalyst for the methanation process, the CO₂-TPD desorption method was performed and the results are shown in Fig. 1c. All catalysts show a wide and high intensity CO₂ desorption region in the temperature range of 80–––300 °C, indicating that the catalyst surface contains a large number of medium and weak base centers. Low CO₂ desorption peaks at temperatures from 500 °C onwards, especially at above 700 °C are possibly accounted for strong base sites. It was found that CO₂ adsorbed on medium-intensity basic sites could be readily desorbed from the catalyst surface (Pan et al., 2014) to promote CO₂ methanation. From Table 1, the CO₂ desorption capacity of the 10NixZr/HA catalysts is higher than that of the 10Ni/HA catalyst. This could be associated to the more uniform Ni nanoparticles dispersing on the ZrO₂-HA surface. According to researches (Diallo-Garcia et al., 2014; Mihet and Lazar, 2018), CO₂ is primarily adsorbed on surface hydroxyl groups and metal nanoparticles affect CO₂ adsorption capacity. The addition of ZrO₂ also provided new active sites that could absorb CO₂, since ZrO₂ is a quantitative oxide and the hydroxyl provided by it can also increase the number of reactive sites and enhance CO₂ adsorption. ZrO₂ can inhibit carbon deposition and Ni sintering due to its strong oxygen storage capacity (Yang et al., 2020). The enhanced Ni dispersion and the slight decrease of primary sites could suggest a more harmonious more harmony combined adsorption of H₂ and CO₂, proposing a better catalytic activity in the hydrogenation of CO₂ to methane.

The hydrogen consumption (in H₂-TPR analysis) and CO₂ desorption capacity (in CO₂-TPD analysis) are illustrated in Fig. 1d. As being mentioned above, the addition of ZrO₂ promotes the reduction of NiO species, corresponding to the increased amount of consumed hydrogen in H₂-TPR analysis. Although the strong metal-support interaction can enhance the catalytic stability through preventing the Ni sites from agglomerating, weakening those interactions by adding ZrO₂ may also improve the catalytic activity with more presenting Ni metal sites. The maximum hydrogen consumption is observed in the 10Ni4Zr/HA’s profile, indicating more ZrO₂ loading might cover Ni sites and slightly decreases the Ni site density on the surface. The dispersion of Ni onto the HA surface didn’t alter the basic property of the catalyst, implied by the approximately same CO₂ desorption capacity as seen in Fig. 1d. Adding ZrO₂ produces more CO₂ adsorption sites, and the CO₂ adsorption capacity reaches its peak in the 4 wt% ZrO₂ modification. The excess ZrO₂ (higher than 4 wt%) does not induce more basic sites.

Fig. 1e indicates that the isotherm N₂ adsorption–desorption diagrams of all the examined samples are of type III with hysteresis loops corresponding to the H3 class of the IUPAC classification (Li et al., 2023). The presence of a medium pore structure, as indicated by isotherms of type III, is advantageous for CO₂ adsorption as well as the transport of reactants and intermediates during the reaction (Wang et al., 2019). The absence of capillary condensation in the isotherms of the catalyst sample at low pressures indicates that HA does not have a porous capillary structure (El Shafei et al., 2004). Fig. 1f reveals that the pore size distribution for all samples ranges between 10–––60 Å, which is typical for materials with medium pore sizes. The 10Ni4Zr/HA catalyst has a maximum mean pore diameter of 26 Å, with a slightly higher pore volume compared to other analyzed samples. This textural property is suitable for use for application in catalytic CO₂ methanation (the kinetic diameters of CO₂ and H₂ correspond to 0.33 nm and 0.29 nm) (Li et al., 2006). The specific surface area of the modified catalyst is moderately more considerable than that of the unmodified one, which could be induced by the smaller nanoparticles, having a larger area/volume ratio compared to their bulk material.

The FESEM images (Fig. 2) show that all catalysts have rough and porous surfaces, and the particles have a spherical shape. Two particle sizes were observed on the catalyst surface: large HA particles and smaller particles of the active metal phase. The unmodified catalyst has a surface of highly concentrated and dense nanoparticles, stacked on top of each other, explaining why this catalyst has the lowest specific surface area (Fig. 2a). Meanwhile, on the surface of ZrO₂ modified Ni/HA catalysts (Fig. 2b-d), it is observed that small spherical particles, with a size of approximate 10 nm, are sparsely distributed and relatively more evenly spread. The FESEM result is consistent with the increase in specific surface area of the modified catalyst compared with the unmodified one in BET result.

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Fig. 2

SEM images of samples; a) 10Ni/HA, b) 10Ni2Zr/HA, c) 10Ni4Zr/HA, and d) 10Ni6Zr/HA.

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The SAED and HR-TEM results (Fig. 3a-c) show the microstructural properties of the 10Ni4Zr/HA sample. The SAED image (Fig. 3a) shows that the nanoparticles are uniformly crystalline with several observed diffraction rings of HA (d = 0.34 and 0.28 nm assigned to (0 0 2) and (2 1 1) planes), and Ni (d = 0.20 and 0.17 nm associated to (1 1 1) and (2 0 0) plane). In addition, a faded ring with a respective d-spacing of 0.24 nm could be attributed to the (1 1 1) plane of NiO, indicating a part of the catalyst is not reduced into Ni⁰. The HRTEM images (Fig. 3b and c) show that the nanoparticles exist with a size range of 30–––40 nm, which is consistent with the captured SEM images and the crystal size calculated from the XRD result. The particles tend to clump together. The HRTEM image also shows bright patches corresponding to ZrO₂ and Ni centers corresponding to dark patches. ZrO₂ particles tend to agglomerate to form large masses with capillary pores and Ni evenly distributed on the surface and in the developed pores.

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Fig. 3

SAED (a), HRTEM (b and c) images, EDS spectrum and component (d) and mapping (e) of elements, and the UV–Vis DRS spectrum (f) of 10Ni4Zr/HA sample.

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Based on the EDS results (Fig. 3d), it was found that the mass composition of the elements on the catalyst surface is relatively close to the theoretical calculation, which are O (35.81%), P (15.93%), Ca (34.26%), Ni (10%), and Zr (4%). Slight deviations between experimental and theoretical values exist, possibly due to the impregnation method, since adequate contact cannot be guaranteed in all cases between the metal salt solution and the support during sample drying (Daza et al., 2009). The Ca/P molar ratio < 1.67 agrees with the appeared diffraction peaks of β-TCP Ca₃(PO₄)₂ analyzed in the XRD spectrum. Moreover, the colourful dots on the EDS mapping results (Fig. 3e) demonstrated the uniform distribution of components on the material surface and the high dispersion of Ni on the support. The UV–Vis analysis results of the typical catalyst, 10Ni4Zr/HA (Fig. 3f), reveal three distinct absorbance regions. The first region lies within the wavelength range of 320 nm, characteristic of ZrO₂ absorption (Soni et al., 2022). The second region falls within the wavelength range of 380 nm, indicative of Ni or NiO absorption (Srihasam et al., 2020). Finally, the third region is observed at a wavelength of 585 nm, representing HA absorption (Pal et al., 2020). Based on these findings, it can be deduced that the material 10Ni4Zr/HA has been successfully synthesized.

3.2 Activity of catalysts

The catalytic CO₂ conversion of catalysts is given in Fig. 4a as temperature functions. Under the same reaction condition, in the temperature range of 250–––400 °C, the CO₂ conversion of the ZrO₂-modified catalyst samples was significantly higher than that of the unmodified catalyst sample. At low temperature, the CO₂ conversion increased slowly, but from about 300 °C, the CO₂ conversion increased rapidly and reached a maximum value at 400 °C for all modified samples. ZrO₂ is a weak hydrophilic (Li and Chen, 2019), beneficial for water desorption and thermodynamically promoting CH₄ formation. ZrO₂ plays many roles in different catalysts, a reasonable load of ZrO₂ can enhance the active metal's dispersion, reducing the crystal size of the catalyst, enlarging the surface area and increasing the Ni⁰ active site (from H₂-TPR result), improving H₂ adsorption and CO₂ conversion.

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Fig. 4

CO₂ conversion (a) and CH₄ selectivity (b) of 10NixZr/HA catalysts in CO₂ methanation.

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The effect of ZrO₂ can be explained by the mechanism by which ZrO₂ adsorbs CO₂, while Ni adsorbs and dissociates H₂. The adsorbed CO₂ will interact with bridging OH groups on the surface of ZrO₂ to form bicarbonate, an essential intermediate for methane synthesis. The role of ZrO₂ is also involved in creating vacancy oxygen sites. This enhancement is due to many oxygen vacancies on the surface of m-ZrO₂, which enhances the adsorption capacity. t-ZrO₂ with abundant emptiness also affects the catalytic performance (Li and Chen, 2019).

As shown in SEM images, the compactness of the catalyst particles in NiZr/HA samples was less than that of Ni/HA. It is demonstrated that zirconia can limit agglomeration on the support, promoting Ni particles' dispersion and stabilization. Combined with the results of BET and XRD, 10Ni4Zr/HA has a specific surface area of 19.7 m²/g and the best Ni dispersion on the HA support, which could be one of the main reasons for the highest catalytic activity. At 400 °C, the catalyst achieved 92.9% CO₂ conversion, 8.3% higher than the unmodified one. However, when the denaturation content increased from 4 to 6 wt%, the CO₂ conversion decreased, although the ZrO₂ addition reduced the crystal size. This could be due to decreases in the Ni⁰ active site (calculated by the hydrogen consumption (H) in Fig. 1d) as well as the dispersion of catalyst shown in the SEM images (Fig. 2) when increasing the ZrO₂ content up to 6 wt%, suggesting that the ZrO₂ content has an optimal range in affecting the catalyst performance. When the ZrO₂ content exceeds the required value, the activity of the catalyst decreases due to the influence of the specific surface area of the catalyst, the dispersion of Ni decreases, or the size of NiO crystals increases too much higher than the value desired. In the case of the catalyst containing higher ZrO₂ content, the lower amount of Ni can prevent the conversion of CO₂ to CO via RWGS, and the right amount of ZrO₂ will benefit methane formation. This fact proved that 10Ni4Zr/HA is one of the catalysts having high activity for CO₂ methanation.

The enhanced interaction between Ni and Zr not only promotes the overflow of H spilling from Ni to the support, which is beneficial for CO₂ hydrogenation and intermediates but also alleviates the clogging of the pore structure by restricting the average size of the Ni particles on the used catalysts to a size smaller than the average pore size. In addition, the increasing Ni particle size improved the low-temperature CO₂ conversion by enhancing the CO₂ adsorption of the catalysts to OCO*. The addition of Zr inhibited carbon deposition by reducing the number of active metal sites and increasing the surface oxygen, slightly improving the CH₄ selectivity. However, adding too much Zr will reduce the catalyst's reactivity because the irregular pore structure increases the Ni particle size, leading to the ability to adsorb OCO* too strongly and reducing the activity of the catalyst. Another property leading to the distinction in product selectivity in the temperature range of 250–––300 °C (Fig. 4b) could be the CO₂ adsorption capacity, relating to the catalysts’ CO₂-TPD profiles. While suitable amount of ZrO₂ with moderate basic sites could promote the formation of CH₄ in the low temperatures, the excess loading of ZrO₂ inducing a lower Ni density on the surface could favor the formation of nickel carbonyl, which later convert into CO in the product flow. The slow kinetic nature of the reaction in the low temperature combining with more CO₂ adsorbed on the catalyst’s surface than H₂ might take responsibility for the generation of the metallic carbonyl species (Aziz et al., 2015; Wen et al., 2021). Therefore, adding an appropriate amount of Zr can improve the reactivity of the catalysts. Within the scope of this work, the 10Ni4Zr/HA catalyst showed optimal CO₂ conversion. A schematic of CO₂ methanation on ZrO₂-doped nickel nanocatalyst supported on hydroxyapatite from Salmon bone could be represented in Fig. 5.

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Fig. 5

Schematic representation of CO₂ methanation on NiZr/HA catalyst.

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Reaction mechanism investigations were carried out to determine how methane is generated on the Ni/HA catalyst surface. In recent years, the CO₂ methanation reaction mechanism has been intensively investigated in order to design a more active and selective catalyst for the synthesis of methane (Wai et al., 2020). Pan et al. proposed a reaction mechanism in which formate species is the primary reaction intermediate while carbonates and hydrogen carbonates are formed in CO₂ methanation (Jia et al., 2020). As shown in [steps (1)–(11)], a reaction mechanism for CO₂ methanation over Ni/HA catalyst is postulated on the basis of these previous studies. The H₂ molecule splits into two hydrogen atoms (step 1) on the Ni surface (*). In steps (2)-(5), gaseous CO₂ chemisorbs and reacts with surface hydroxyl (OH*) to form hydrogen-carbonates (HCO₃*). The hydrogen-carbonate species becomes formate (HCOO*) by reacting with H atoms on the surface (step (6)). In step 4, surface H atoms react with surface O atoms to replenish the surface hydroxyl groups. Through additional hydrogenation (steps (7)-(8)), surface formate also transforms into OCH₂OH* species and then into methoxy species (OCH₃*). The OCH₃* species is then hydrogenated (step (9)) to form CH₄. Additionally, methoxy formation is also associated with the presence of Lewis acidic sites (Kim et al., 2019). Since HA catalytic support is utilized in this work for CO₂ methanation, the Ca (II) sites in the HA unit cell structure are Lewis acidic (Yook et al., 2022), indicating that the HA support plays a role in stabilizing methoxy species generation and influencing methane synthesis.

To provide insight into the suitability of the 10Ni4Zr/HA catalyst, we compare its efficacy to that of various Ni catalysts at the same of reaction temperature and the CO₂: H₂ molar ratio reported in the literature in Table 2. Numerous supports, including SBA-15 (Van et al., 2020); MCM-41 (Yang et al., 2020), CNTs (Romero-Sáez et al., 2018); SiO₂ (Guo and Lu, 2014), Al₂O₃ (Mutz et al., 2017), Pr₂O₃–CeO₄, Sm₂O₃–CeO₄ (Siakavelas et al., 2021), etc have been investigated. In the majority of cases, Ni content higher than 10% wt were supported on them. Despite their relatively modest Ni content, we may conclude that they can compete favourably with these reference formulations. Because HA can self-exchange cations and anions, this can be explained by the characteristic properties of HA. The active metal centers for the reaction can be introduced into the HA structure via ion exchange, and Ca²⁺ ions give the HA surface a weak base nature with the ability to dissociate CO₂ and the OH group to stabilize the active metal, which promotes CO₂ hydrogenation to CH₄. In addition to the active metal's dispersion, the catalyst's fundamental properties and the amount of surface oxygen in the catalyst significantly influence its reactivity. Adding alkaline earth metals (Ca present in HA) can increase the number of active metal sites on the catalyst that influence CO₂ adsorption and activation. Moreover, it is evident that the positive effect of Zr on the activity of the Ni catalyst is more pronounced in our HA-based materials, indicating that our green support is suitable for the CO₂ methanation reaction. It should be emphasized that the enhanced structural, chemical, and catalytic properties of Ni-based catalysts supported on hydroxyapatite-derived Salmon bone waste represent an advantageous alternative to conventional basic catalysts.

3.3 The stability of 10Ni4Zr/HA catalyst

The stability test, performed at 350 ◦C, reveals a robust performance of the optimized 10Ni4Zr/HA catalyst very notable stability during long time on stream (Fig. 6a). After an activation period of 50 h of reaction, reaching a CO₂ conversion close to 76% and a high selectivity towards methane production (>95%). In addition, compared with the characterizations of the initial fresh 10Ni4Zr/HA catalyst (Fig. 2c and 3b), the characterizations of the spent one (Fig. 6b-d) revealed insignificant differences in their properties and the distribution of active phases on the HA surface based on the impregnation technique employed.

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Fig. 6

The activity stability of the best catalyst (10Ni4Zr/HA) in CO₂ methanation at 350 °C during 50 h on stream (a), XRD pattern (d), and SEM (b) and TEM (c) images of spent catalyst.

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