Tuning of Ni metallic sites over strontium hydroxyapatite nanorods for syngas methanation

Abstract

We investigate CO methanation over nickel-decorated strontium hydroxyapatite (NiSH) nanorods based catalysts by varying Ni loadings, prepared by the impregnation method. The catalytic experiments were conducted at light-off temperatures ranging from 225°C to 400 °C, under ambient pressure. The key properties of prepared catalysts were characterized by XRD, N₂ physisorption, TEM, SEM, TPR and H₂-TPD and H₂-chemisorption. The basicity of the catalysts was measured by CO₂-TPD analysis. A 10NiSH catalyst is optimal for CO methanation, with over 90% CH₄ selectivity at 300 °C and 0.1 MPa, with complete CO conversion. The synergistic interaction between highly dispersed Ni species and the basic Sr-modified hydroxyapatite support significantly enhances catalytic activity and CH₄ selectivity. Unlike conventional Ni-supported catalysts, the present 10NiSH system provides improved Ni dispersion, optimized reducibility, enhanced CO adsorption/activation, and superior hydrogenation ability, resulting in high yield of CH₄ at relatively low temperature. In-situ DRIFT spectroscopy evidenced reaction intermediates, and a reaction mechanism is proposed. The basicity of SH role is CO₂ adsorption in conjunction with Ni metal in hydrogenation activity are key roles. The optimal loading exhibited better properties in terms of Ni particle size, narrowly distributed histograms, and better reducibility. Based on the catalyst performance results, overall, the 10NiSH is the most active and selective catalyst due to its high Ni dispersion.

1 Introduction

Natural gas has historically been considered as the cleanest and most affordable fossil fuel source of energy (Cui et al., 2024). Due to limited resources, environmental concerns, increased fossil depletion, and rising in natural gas prices, the syngas (H₂/CO) era has grown in interest recently in producing synthetic natural gas (Stangeland et al., 2017; Hatta et al., 2022; Lv et al., 2018). Traditionally CO methanation is an important and a well-known process in producing synthetic natural gas. This process is a crucial step and plays a significant role in purifying the ammonia synthesis process and also purifying the H₂ rich gas stream for the proton-exchange membrane fuel cells (Pinzón et al., 2024; Richard et al., 2024). For CO methanation technology, it is essential to produce a stable, extremely efficient catalyst that operates at moderate to low temperatures is crucial. Many noble and transition metal-based catalysts are applied in CO/CO₂ methanation process (Mendoza-Merlano et al., 2024). Ni-based catalysts are so popular in selective CO methanation because of their optimal level of selectivity, high reactivity, and low cost (Frontera et al., 2017; Su et al., 2016). Especially, the support materials has been studied extensively with Ni based catalysts, e.g., Al₂O₃, CeO₂, SiO₂, mixed oxides and metal oxides with binary, tertiary oxides as the potentially useful support materials due to distinctive features such as high thermal stability, redox behavior, high CO adsorption capacity and surface functionality (Langer and Freund, 2024; Strucks et al., 2021; Italiano et al., 2020).

The methanation reaction can be represented as:

The improvement of Ni-based catalysts for CO methanation has been a major focus due to their ease of use, ability to dissociate CO, and ability to hydrogenate surface carbon species. Several studies indicate that catalyst performance is significantly influenced by metal-support interactions and support materials. It has been demonstrated that Ni supported reducible oxides, such as CeO₂, exhibits enhanced CO₂ activation compared to Al₂O₃ systems due to oxygen vacancies and better dispersion of Ni (Liu et al., 2024). Lee reported that CeO₂-supported Ni catalysts exhibited superior low-temperature methanation activity because of their enhanced reducibility, oxygen vacancy concentration, and strong metal–support interaction, which facilitate CO₂ adsorption and activation. In their study, Wang et al. found that Ni/CeO₂ catalysts could convert over 90% of CO₂ at temperatures between 300 °C and 350 °C, while achieving 98% selectivity for CH₄ (Wang et al., 2024). Compared to unpromoted Ni/Al₂O₃, Ru-Ni/CeO₂ showed twice the turnover frequency, though its CO conversion was higher at 300 °C–350 °C (Tommasi et al., 2024). Hassan et al. reported that incorporating Sr into the 5Ni/W−Zr samples led to an enhancement in surface basicity due to the development of robust basic sites. The 5Ni + 3Sr/W−Zr catalyst showed remarkable activity and stability, achieving a CO₂ conversion rate of 90% and a CH₄ yield of 82%, compared to the 62% and 57% achieved by the 5Ni/W−Zr catalyst, respectively (Al-fatesh et al., 2023). In these advancements, it is demonstrated that higher Ni dispersion, optimized metal-support interactions, addition of promoter and surface basicity are critical to achieving high CH₄ selectivity and lower operating temperatures.

Herein, the support material is still under research and development for Ni-based catalysts. Combination of basicity, high adsorption and surface reactivity is the key for the support material. The structure of hydroxyapatite, which is simply a mineral containing calcium hydroxyapatite (CaHAp) or calcium phosphate (CaP) (Ca₁₀(PO₄)₆(OH)₂), is being studied in catalysis due to its abundance and tunable acido-basicity properties (Van Nguyen et al., 2022). It is a well-known candidate used in many heterogeneous catalysed reactions such as hydrogenation, methanation, and other applications, and it also possesses varied acid-basicity properties that can be tuned based on the reaction characteristics (Stangeland et al., 2017; Pham et al., 2022). Salmon bone-derived HAp-supported Ni catalysts exhibited enhanced Ni dispersion, improved reducibility, and superior CO₂ adsorption capacity, leading to excellent methanation performance. Furthermore, the incorporation of CeO₂ promoter significantly enhanced catalytic activity, achieving CO₂ conversion of 92.5% with nearly 100% CH₄ selectivity at 325 °C (Van Nguyen et al., 2022). These findings indicate that modified hydroxyapatite supports can effectively stabilize active Ni species and improve methanation efficiency through synergistic effects between Ni and the support structure. However, Ca can be replaced by other ions such as divalent or trivalent cations for enhanced properties. As reported previously by Yasukawa et al., 2012, substitution of Ca by other divalent cations such as rare metal divalent and trivalent ions can significantly change the phase, surface re-structing, adsorption capacity and activity (Yasukawa et al., 2012). Some of the divalent cations such as Sr²⁺ are somewhat similar chemistry with that of Ca²⁺. Varying the Ca to P ratios can tune the total acidity and basicity of the CaP. Similarly, the ratios of Sr to P can exhibit distinctive properties, such as amount and strength of acidity and basicity.

Strontium hydroxyapatite (SH) is an amphoteric material with excellent ion exchange properties, adsorption efficiency, and thermal resistance. The chemical formula for SH is Sr₁₀(PO₄)₆(OH)₂), which is typically used in biomedical fields owing to its osteointegration properties and biocompatibility (Iriarte-Velasco et al., 2018). The versatility of hydroxyapatite’s characteristic features makes it suitable for various applications, particularly in heterogeneous catalysis. Only a few scientific studies have been published on the use of SH in catalysis so far. In a recent study, Unai et al. investigated the water-gas shift (WGS) reaction catalyst using Ni-supported CaHAp (Iriarte-Velasco et al., 2018). The results demonstrated that the Ni/CaHAp promoted the impacts of its pervasive species, decreased basicity on the surface, and had relatively strong interactions with the Ni active phase.

Strontium-doped mixed oxide influenced the acid-basicity and the textural properties of the catalyst. Over 1.9 wt% Sr modified Mg-AlO catalyst exhibited the highest selectivity of ethylene and butanol due to combination of strong basicity and mild acid sites during the conversion of ethanol. The product selectivity depends on the Sr content, and ethanol dehydrogenation is more pronounced with increased Sr content (Seekhiaw et al., 2023). In Ogo et al., 2017, Sr-based catalysts Co/Sr-P were shown to be highly active and stable due to high coke resistance in ethanol steam reforming (Ogo et al., 2017). Recently, CO methanation from our group was published, i.e., Ni/SrHAp in selective methanation, and this work is an extension of the previously reported study (Seelam et al., 2023). In that work, we studied the effect of size nanorod on the CO methanation activity. Here, we examined CO methanation activity with the impact of Ni loadings over one particular size of SH nanorod and also conducted DRIFT studies to explore reaction intermediates and reaction mechanism. It was discovered that crystallite size and metal loading significantly affect the efficiency of selective CO methanation catalysis. The metallic surface area was investigated by adjusting the metal loading. Additionally, the DRIFT analysis method was used to investigate the process of CO methanation on the surface of the NiSH catalyst methodically.

2 Materials and methods

2.1 Materials

Ammonia solution [NH₄OH] (99.9% purity) was acquired from Kermel Reagents Co., Ltd., Sinopharm Chemical Reagent Co., Ltd. Supplied chemicals like di ammonium hydrogen phosphate [(NH₄)₂HPO₄] (99% purity), strontium nitrate [Sr(NO₃)₂] (99 9% purity)., and nickel nitrate hexahydrate [Ni (NO₃)₂·6H₂O] (98% purity).

2.2 Preparation of support material

In 40 mL of deionized water, 1.58 g of (NH₄)₂HPO₄ were typically dissolved. To achieve a final pH of 11, the NH₄OH solution was added drop by drop with rapid agitation. Sr(NO₃)₂ weighed 4.23 g in a 40 mL aqueous solution in another beaker. At room temperature, drops of this solution were gradually mixed with the previously mentioned solution. After adjusting the pH to 11 with NH₄OH solution, the mixture was stirred for 0.5 h. Subsequently, it was placed inside a 100 mL Teflon-lined autoclave. The autoclave was maintained at 160 °C in an electric oven for 12 h. Following filtration, the resulting products were removed, treated with DI water, and allowed to dry at 90 °C for 24 h. Further, the final sample was called SH after being calcined for 6 hours at 550 °C.

2.3 Catalyst preparation

A wet impregnation technique was employed to prepare Ni/SrHAp catalysts from an aqueous Ni (NO₃)₂·6H₂O solution with varying Ni nominal/calculated loadings of 5, 10, and 15% weight percent. The obtained products were dried for 12 h at 120 °C and calcined for 4 h at 500 °C. The final samples are XNiSH, where X is the nominal weight percentage of Ni loading.

2.4 Catalyst characterization

XRD: Rigaku D/Max2500 V/PC powder diffractometer recorded the X-ray diffraction (XRD) patterns of the synthesized samples. It has a Cu Kα radiation source running at 40 kV and 100 mA current. Plasma-Spec-II spectrometer and inductively coupled plasma atomic emission spectroscopy equipment (ICPS-8100, Shimadzu) were used to determine the metal loading.

N₂Isotherms: Nitrogen sorption measurements were performed at −196 °C using the Autosorb-1 analyzer (Quanta chrome). Prior to the analysis, vaccum degassing were performed at 250 °C for 6 h. The specific surface area and pore size distribution were determined using the Brunauer-Emmett-Teller (BET) and Barrett-Joyner-Halenda (BJH) methods, respectively.

ICP: Analysis were conducted for the metal loading using Inductively coupled plasma atomic emission spectroscopy (ICPS-8100 equipment, Shimadzu) and a Plasma-Spec-II spectrometer.

TEM: A Hitachi HT7700 transmission electron microscope was used to capture the TEM images. The samples were evenly distributed in ethanol, and a few drops of the diluted samples were placed on a copper grid reinforced with carbon. The samples were then allowed to dry outside under a lamp. With an FEI Tecnai G2 F30 Spirit microscope (300 kV), high-resolution TEM (HR-TEM) images were obtained.

SEM: The microscopic images were recorded by the Philips Fei Quanta 200F scanning electron microscopy (SEM) instrument, which operates at 20 kV. The sample was placed on carbon conductive tape, fixed to an aluminum sample holder, before being coated with gold.

TPR: The AutoChem 2,920 (Micromeritics) equipment was utilized for temperature programmed reduction (TPR) analysis. A quartz sample tube shaped like a U was filled with 0.05 g of sample for the TPR tests. The catalyst is held in position with a quartz wool stopper. Before the TPR investigations, the sample was exposed to N₂ gas at 200 °C with 30 mL/min flow for 1 h. The collected material was set aside to cool to ambient temperature (25 °C) after pretreatment step. At 10 °C/min, the sample was heated to 700 °C for the TPR study, which was carried out in a 5 vol.% H₂-Ar with 30 mL/min gas flow.

2.5 Catalytic screening

CO methanation was conducted in an 8-mm diameter fixed-bed quartz reactor at ambient pressure in a conventional downflow configuration. The reactor was filled with around 0.1 g of catalyst between quartz wools. The reactor employed temperature controllers and K-type thermocouples to regulate the furnace’s temperature and monitor the temperature in the catalyst bed. The catalyst samples underwent treatment with 5 vol.% H₂/Ar gas, applying a ramp rate of 2 °C/min for a duration of 2 h to facilitate in situ reduction at 450 °C. Once it cooled to the reaction temperature, a combination of 20 vol% CO - 80 vol% H₂ gas was injected under atmospheric pressure at a 25 mL/min flow rate. A flame ionization detector and a DB-WAX column (60 m length, 0.25 mm diameter, 0.250 m film thickness) were fitted to the Agilent 7890A gas chromatograph, which was used to analyze the effluent online. Moreover, the selectivity and conversion of CO were computed using the following Equations 1–3.

3 Results and discussion

The crystal structure details were analyzed by XRD analysis over the 2θ range of 10°–80° (5^o/min). The X-ray diffraction patterns of the pure SrHAp (SH) and Ni-supported SrHAp catalysts are presented in Figures 1a,b. Figure 1a depicts the XRD profile of the pure SH, which is in good agreement with reported Sr apatite XRD patterns (Mi et al., 2020). All the samples shown similar diffraction patterns, which are well-matched with the apatite crystal structure in the reference data (i.e., hexagonal crystal system of group-P63/m,176 and cell constants such as a = 9.7670, b = 9.7670 and c = 7.265), but with different peak intensities and sharpness (Xu et al., 2018). Lower Ni loadings resulted in a more noticeable XRD peak intensity and sharpness, which tend to decline with Ni loading gradually. Using the Scherrer equation, the average size of the SH particles was estimated to be approximately 40.4 nm. After the Ni loading on SH, its structure did not alter; moreover, increasing in Ni loadings were associated with a notable drop in crystallite size from 41.8 to 34.8 nm. This could be explained by a decrease in the distribution of pore sizes, as seen in Table 1. A small reduction in SH lattice characteristics was also observed after Ni loading. Further, the diffraction peaks with low intensity of the (Figure 1b) centered at 37.5^o appeared due to supported Ni catalysts (Balla et al., 2021). This can be attributed to the NiO phase, which enhances the intensity by increasing Ni loadings confirmed by the particle size of NiO (Supplementary Table S1).

FIGURE 1

As indicated in Table 1, N2 physisorption isotherms at −196 °C were used to examine the textural characteristics of the catalysts. According to the IUPAC classification, all isotherms shown in Figure 1c can be categorized as type II isotherms. The BET surface area decreased progressively with increasing Ni loading, indicating that the incorporation of Ni affected the textural characteristics of the SH support. Compared with pure SH, all Ni-supported catalysts exhibited lower surface area, pore size, and pore volume values. Figure 1d presents the pore size distribution profiles of the synthesized catalysts. All samples exhibit a narrow pore size distribution, with pore diameters primarily centered between 3 and 5 nm. The pure SH support showed an average pore size of 20.3 nm and a pore volume of 0.085 cc g^-1, whereas these values decreased further after Ni incorporation. The reduction in surface area and pore characteristics is likely due to partial pore blockage by Ni particles. In addition, the increase in Ni particle size at higher metal loadings may have further contributed to these observed changes. From ICP, it is used to determine the actual Ni loadings of catalysts, and the results are summarised in Table 1. The ICP results revealed that the Ni content was slightly lower than those with nominal Ni loading.

SH surface morphology and uniformity were analysed by scanning and transmission electron microscopies (SEM, TEM). As shown in Figures 2a,b, the SH sample showed rod-shaped morphology measuring between 50-200 nm in width and 1.2–2 µm in length. The structure of the SH is distributed evenly with rod-shaped needle-like particles.

FIGURE 2

During the TPR studies, the reducibility of NiO species and their interactions with SH were investigated. Figure 3a shows the H₂-TPR of all the samples under the 100–700 C temperature range. The TPR profile of the bare SH catalyst showed one low-intensity reduction peak, which can be attributed to the absorption of surface oxygen-based anions above 650 C (Boukha et al., 2018). After Ni incorporation, the reduction profile of 5NiSH resulted in two peaks, a sharp peak between 280 °C and 510 °C, and a broad peak located at 510 °C–720 °C. Besides SH reduction peak, additional H₂ uptake peaks were noticed due to surface and bulk NiO reduction. Moreover, it is noteworthy that the existence of Ni ions increased the intensity of the SH peak and allowed it to migrate to lower temperatures. A notable variation in the reduction peaks was noticed with varying nickel loadings and the decrease of bulk and surface nickel oxides at different temperatures. In the case of the 10NiSH catalyst, three peaks were seen at 326, 359 °C and 587 °C. NiO particles that are highly dispersed were found to be effectively reduced at low temperatures (Pedroarena et al., 2023). In addition, two or more peaks were associated at high-temperature regions corresponding to bulk NiO species; these peaks were observed in 10NiSH and 15NiSH samples. Generally, the lower temperature region peak is associated with highly dispersed small NiO particles (Kuna et al., 2022). However, the rise in the higher temperature zone may have resulted from the bulk NiO species interacting with the support at a very low level. Also, with the increased Ni loadings, the temperature maxima of peaks shifted towards higher values.

FIGURE 3

Transmission Electron Microscopic (TEM) images of various reduced Ni-supported SH samples were captured. Figure 4 presents the results of TEM imaging and the associated histograms of the particle size distribution, including statistical analysis of mean, maximum, and minimum particle sizes. The TEM image of 5NiSH (Figure 4a) revealed that the isolated Ni particles were perfectly anchored on the SH nanorods with an average size of ∼4.3 ± 1.6 nm. When increasing the Ni loading from 10 to 15 wt.%, the average size of the particle is increased from 9.2 ± 2.2 to 12.2 ± 3.3 nm. The 15NiSH sample exhibits a broader particle size distribution.

FIGURE 4

H₂ chemisorption measurements were conducted to determine the average particle size and active Ni surface area of the NiSH catalysts, presented in Table 2. Interestingly, the 10NiSH sample exhibited the slightly highest active surface area (6.86 m²/g_cat) with 9.5 nm average-sized particles. Moreover, the addition of 15 wt.% Ni resulted in the deposition of larger particle sizes (∼16.6 nm) corresponding to Ni active surface area of 6.17 m²/g_cat. In this instance, the measured active Ni surface area on studied catalysts exhibits the following pattern: 10NiSH > 15NiSH > 5NiSH. The obtained H₂ chemisorption results are in line with TEM results.

The CO₂-TPD technique operates between 40 °C and 650 °C to measure the basicity of all the samples using CO₂-balanced He gas. The desorption curves shown in Figure 3b were obtained after 0.5 h of CO₂ adsorption at 30 °C followed by He flush at 110 °C and desorbed values are shown in Table 2. In general, the desorption temperatures determine three categories of basic sites: Weak (50 °C–200 °C), medium (200 °C–500 °C), and strong (500 °C–650 °C) (Abbasizadeh and Karimzadeh, 2019). Three desorption peaks, with temperatures ranging from 40 °C to 150 °C to 200 °C–400 °C and 400 °C–650 °C, were observed in the CO₂-TPD pattern of 5NiSH, indicating the presence of several adsorption sites with varying strengths of basicity. The surface hydroxyl (-OH) groups typically bind to the weak basic sites, metal-oxygen bonds to the medium basic sites, and low-coordination oxygen anions attach to the strong basic sites. As the Ni loading increases from 5 to 15 wt%, the surface coverage of the NiO phase causes the two peaks (between 200 °C to 400 °C and 400 °C–650 °C) intensities to decline progressively, consistent with previous reports (Kuna et al., 2022). Preferably weak and moderate basic sites are those critical in methanation reactions over Sr-O-P and Sr-OH centers.

XPS measurements were performed to study the reduced 10NiSH catalyst surface by evaluating the chemical state and elemental composition (Figure 5). According to survey spectra, the 10NiSH sample comprises Ni, O, P, and Sr elements. The high-resolution XPS spectrum of Ni2p provides evidence of the existence of the Ni element. It mainly possesses Ni 2p3/2 and Ni 2p1/2 peaks at 852.9 and 870.5 eV, respectively, hence confirming the successful reduction of NiO to metallic nickel species. In addition to that, two satellite peaks were noticed at 861.6 and 876.1 eV (Grosvenor et al., 2006), which are distinctive characteristics of NiO-like environments resulting from charge-transfer interactions between Ni 3d and O 2p orbitals. The concurrent existence of Ni⁰ and Ni²⁺ species indicates that while the majority of NiO was converted to metallic Ni, a minor portion of oxidized nickel persisted on the surface, potentially due to the minimal surface re-oxidation upon exposure to air during the XPS analysis. The O1s spectrum is associated with the O₂⁻ state, obtained from the metal-oxygen bonding in the PO₄²⁻ molecule. The overlapped spectra of P 2p and Sr 3d levels were noticed, which are well matched with previous reports (López et al., 2022; Kimball et al., 2008). It confirmed the successful incorporation of phosphate and Sr species into the hydroxyapatite lattice.

FIGURE 5

3.1 Catalytic activity

Table 3 presents the catalytic performance of the pure and Ni-modified SH catalysts. All samples exhibited similar trends and the conversion rates increased with reaction temperature. The underlying kinetic limitations of the CO methanation reaction within the low-temperature range are studied. Moreover, CO methanation is preferred to operate at moderate temperatures above 200 °C to achieve high conversions. For the evaluated catalyst series, the CO conversion rates were observed according to the increasing trend: 5NiSH< 15NiSH < 10NiSH. Under the evaluated reaction conditions, all catalysts showed exceptional selectivity toward CH₄ in terms of reaction selectivity. The catalytic performance of the 10NiSH catalyst strongly depends on Ni particle size, surface area, and catalyst basicity (Supplementary Figures S1–S3). The 10NiSH catalyst exhibited the highest reaction rate and methane yield due to its optimal Ni particle size and higher exposed metallic Ni surface area. Although 5NiSH showed higher CO adsorption capacity, its methanation activity was lower because of insufficient active Ni sites. In contrast, excessive Ni loading in 15NiSH slightly reduced performance due to particle agglomeration and lower Ni dispersion. To assess the relationship between structure and performance, the CH₄ yield was plotted against the accessible Ni surface area, as determined by H₂ chemisorption. The 10NiSH sample, which exhibited a larger accessible Ni surface area, also demonstrated a higher CH₄ yield. This finding suggests that the enhanced activity is due to the increased availability of the metallic surface, rather than merely a higher bulk Ni content. In order to better contextualize the enhanced performance of the 10NiSH catalyst, it is essential to compare it with established Ni/oxide systems. With Ni/Al2O3 catalysts with high surface areas, improved CO methanation activity is strongly correlated with increased Ni nanoparticle dispersion, which leads to the formation of highly accessible Ni active sites that facilitate low-temperature reducibility through optimal metal–support interactions (Liu et al., 2015). Furthermore, Ni supported on hexaaluminate exhibits high metal-support interactions, inhibiting sintering and maintaining small Ni particle sizes even under high-temperature methanation conditions, resulting in superior catalyst stability (Online, 2013). It is demonstrated in this study that the 10NiSH catalyst has similar structure-property relationships like high Ni dispersion, as demonstrated by chemisorption of H₂ and narrow particle size distributions by TEM, as well as easily reducible surface NiO species, as demonstrated by TPR, which result in complete CO conversion and CH4 selectivity over 90%. Therefore, the catalytic behavior of 10NiSH aligns closely with these benchmark systems, and the amphoteric nature and basic sites of the Sr-hydroxyapatite support further contribute to CO activation and improved hydrogenation, establishing SH as an effective and competitive alternative support for efficient methanation catalysts.

3.2 Effect of reaction temperature

Reaction temperature is the most significant factor affecting CO methanation activity. At low temperatures, the activation energy barrier is higher than the dissociation energy barrier for CO. The CO molecules can obtain the required activation energy by increasing the reaction temperature, thus increasing their reactivity. All catalysts achieved the highest CO conversion and CH₄ selectivity at temperatures between 325 °C and 375 °C. CH₄ selectivity decreased slightly beyond this temperature range. Based on thermodynamics, Mutz et al. observed that the high-temperature and exothermic characteristics of CO methanation affect catalyst equilibrium and deactivation (Mutz et al., 2015). A high temperature promotes the WGS reaction and inhibits the CO methanation reaction. The CO conversion and CH₄ selectivity with temperature at a GHSV of 18,000 mL g⁻¹h^-1 are shown in Figure 6. However, the catalysts with different metal loading showed remarkably distinct performances. It is quite clear that CO conversion over 10NiSH is slightly higher than those of 5NiSH and 15NiSH at studied temperature light-off curves. When heated from 250 °C to 275 °C, the 10NiSH catalyst displays a sharp ascent in CO conversion from 22% to 86.0%, and a complete CO consumption is obtained above 325 °C. The CH₄ selectivity follows the same growth trend in all catalysts. Evidently, CO conversion increased as the temperature increased and peaked at 325 °C, while CH₄ selectivity and maximum yielded at 350 °C. Over 350 °C, the WGS reaction increases, suppressing the methanation reaction and promoting the conversion of CO into CO₂.

FIGURE 6

3.3 Effect of GHSV

Figure 7a presents how GHSV affects 10NiSH’s selectivity and its catalytic activity for CO methanation at 275 °C and H₂:CO ratio of 4. It illustrates that with increasing GHSV from 10,000 to 30,000 h^-1, the CO conversion decreased from 85.1% to 66.4%. As predicted, with increasing GHSV, the contact time or residence time for the catalyst in the reactor will be shorted, thus resulting in less reactivity and low conversion rates. At 18,000 h^-1 GHSV, CO conversion was 85%, and the selectivity of CH₄ was achieved. The CO conversion decreased rapidly when the GHSV was higher than 18,000 h^-1. To investigate the stability of the catalyst, 18,000 h^-1 was selected as the optimal GHSV based on the experimental results and the comparability of the data.

FIGURE 7

3.4 Stability test

Catalyst lifetime is a key consideration in the economical production of methane. Therefore, the stability of 10NiSH catalyst during CO methanation was further examined under optimized conditions (H₂:CO = 4, GHSV-18000h-1, 275 °C P = 0.1 MPa). CO conversion over10NiSH reached higher than 95% at 300 °C; therefore, 275 °C was chosen as the optimal temperature for further analysis of catalyst stability. The stability test of the 10NiSH catalyst (Figure 7b) reveals that CO conversion initially remains constant but gradually declines after 40 h, dropping to below 60% at 50 h. This moderate reduction is likely due to partial deactivation, possibly caused by slight Ni sintering or surface carbon accumulation, which are common in Ni-based methanation catalysts. CH₄ selectivity consistently stays above 95% for 50 h on stream. Supplementary Figure S4 presents the XRD patterns for both the fresh and used 10NiSH catalysts. The XRD pattern for the used catalyst indicated an increase in particle size after 50 h of reaction. We conducted regeneration and reuse experiments on the 10NiSH catalyst. Following the stability test (50-h test), the used catalyst was extracted from the reactor, calcined in air at 500 °C for 2 h to eliminate deposited carbon species, and then re-reduced under flowing H₂ at 450 °C for 2 h before reuse. The methanation activity of the regenerated catalyst is shown in Supplementary Figure S4. The regenerated 10NiSH catalyst exhibits a slightly reduced activity compared to its initial state, as evidenced by the CO conversion and CH₄ selectivity values (Figure 8). This suggests that the primary deactivation during TOS is reversible. We compared our work with some of the published in the literature on Ni-based catalysts for CO/CO₂ methanation and s\shown in Supplementary Table S2. The 10NiSH catalyst achieved complete CO conversion with 99% CH₄ selectivity at 350 °C under atmospheric pressure, demonstrating superior catalytic performance compared with several conventional Ni-supported catalysts.

FIGURE 8

An extensive in situ DRIFT study was conducted at various temperatures to track the evolution of intermediates and products and obtain insights into the CO methanation mechanism route over NiSH catalysts. CO methanation was initially performed at 225 °C and gradually increased the reaction temperature from 225 °C to 450 °C, and the results are displayed in Figure 9. At first, CO is associatively adsorbed over the metal catalyst in a methanation reaction, and the cleavage of C-O bond occurs. By dissociative adsorption, the adsorbed H_ads atoms contribute, and the dissociation of C-O bond happens over the active site, and the H_ads atoms are associative to form C-H. CO-associative methanation is extensively reported in metal catalysts. Typically, C-O and H-H are associatively adsorbed over a catalyst to form carbon-hydroxyl (COH_ad) or formyl (CHO_ad) intermediate species (Saravanan et al., 2024).

FIGURE 9

CO was molecularly absorbed in the range of 1900–2,200 cm^-1. CO-induced sub-carbonyl nickel species can be attributed to the band at 2,100 cm^-1, which can be detected in the low-coordinated steps, corners, or kinks consisting of nickel atoms. Increasing temperatures cause the carbon monoxide band to lower and gradually desorb on nickel, resulting in a gradual conversion to methane. This characteristic peak at 2,188 cm^-1 explains the existence of gaseous CO. The gaseous methane absorption line at 3,015 cm⁻¹ observed in the infrared wavelength range serves as an indicator of the catalyst activity during CO methanation. Interestingly, the methane gas phase peak intensity gradually increases with reaction temperature in agreement with the catalytic activity results. On 10NiSH, hydrogen-bonded hydroxyl groups are responsible for the broader stretching band that ranges from 3,600 to 3,400 cm^-1. As a result of CO methanation reaction, adsorbed H₂O molecules are responsible for the surface as well as the bending peak around 1,620 cm^-1 (González-Castaño et al., 2023). The formate species were identified as specific intermediates in the CO methanation at the stretching of the high-frequency adsorption band CH and OCO at v = 2,944–2,713 cm^-1. Additionally, the spectrum reveals the presence of monodentate carbonates at 1,480 cm^-1 and bidentate carbonates at 1,310 cm^-1. Hydrocarbonates are noted at 1,417 cm^-1 and 1,592 cm^-1, alongside formates at 1,330 cm^-1 and 2,841 cm^-1 on the surface of SH. The band at 2,361 cm^-1 represent the formed CO₂ gas and gradually decreases with reaction temperature. Notably, the bicarbonate species peaks gradually weaken at 200 °C, whereas the peaks at 1,367 and 1,580 cm^-1 that correspond to the bidentate formate species gradually intensify, indicating the possibility of *HCO₃ conversion by decomposition (Yang et al., 2022). The Hydrogenation of CO to methane on the surface of NiSH primarily passes via CHO or intermediates of CHO, H₂CO, and OCH₃ species, including CO species, adsorbed on the surface of the Ni divide into C and O atoms or formyl species generation. Once formyl species are formed, -CH₃ species reacts with hydrogen atoms to create CH₄. Hydrocarbonates and formates are signs of CO being hydrogenated to CH₄via the formate route over the 10NiSH catalyst. Moreover, the in-situ DRIFTS investigation was carried out at steady state reaction temperature during CO methanation (Figure 10). It is evident that the bands associated with bridging and linear carbonyls become gradually more intense throughout the 20-min stream period until eventually disappearing. In our research, the spectral characteristics of carbonyl, formate, carbonate, and methane in the gas phase correspond with the established ranges documented in the literature. This correspondence confirms that the identification of linear/bridged CO, formate intermediates, and carbonate species is fully in line with the known mechanistic studies involving Ni catalysts.

FIGURE 10

Based on the DRIFTS results, a simplified reaction mechanism is proposed and displayed in Figure 11. Initially, CO and H₂ molecules are adsorbed and activated on metallic Ni sites. Adsorbed CO interacts with surface hydroxyl species to form bicarbonate and carbonate intermediates, which are subsequently hydrogenated into formate species. The formate intermediates are further hydrogenated stepwise to produce methane, while oxygen species combine with hydrogen to form water. Finally, CH₄ and H₂O desorb from the catalyst surface, completing the methanation cycle. The strong correlation between the evolution of formate species and methane production provides clear evidence that the reaction predominantly follows the formate-assisted methanation pathway over the 10NiSH catalyst.

FIGURE 11

4 Conclusion

In this study, the CO methanation reaction was systematically investigated over Ni-loaded SH catalysts at light-off temperatures, highlighting the critical role of catalyst composition in performance. The bare SH support materials showed no catalytic activity, confirming that the active sites arose solely from the incorporated Ni species. X-ray diffraction analysis validated the retention of the Sr apatite crystal structure after Ni loading, indicating the structural stability of the catalyst framework. The catalytic performance was strongly influenced by Ni loading, with the 10 wt% NiSH catalyst achieving the best results, demonstrating over 80% CO conversion and complete selectivity toward methane formation at 300 °C and 0.1 MPa. TEM and chemisorption of H₂ demonstrate that the 10NiSH catalyst has structure-activity relationships, such as high Ni dispersion, which is accompanied by narrow particle size distributions, and easily reducible NiO species on the surface, which can result in complete CO conversion and more than 90% CH₄ yield. The basic sites of the Sr-hydroxyapatite support further contribute to CO activation and improved hydrogenation, establishing SH as an effective and competitive alternative support for efficient methanation catalysts. The proposed reaction mechanism, supported by DRIFT and reaction data, involves the initial adsorption of CO and H2 molecules onto the Ni surface. The metal catalyst weakens the bonds within these molecules, enhancing their reactivity and promoting the formation of intermediate species, such as COH. Subsequent hydrogenation of these intermediates leads to the selective production of methane. This mechanistic insight provides a molecular-level understanding of the catalytic process, enabling targeted improvements in catalyst formulations for efficient CO methanation.

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