Engineering the Quaternary Hydrotalcite Derived Ce-Promote Ni-Based Catalysts for Enhanced Low-Temperature CO₂ Hydrogenation into Methane

1. Introduction

Recently, CO₂ reduction and recycling has become one of the most research topics in neutral carbon economics due to the serious impact on global climate change led by the growing CO₂ emissions[1,2].To migrate the increasing authropogenic CO₂, methods like CO₂ capture, utilization and storage (CCUS)had been engaged [3]. Among these methods, the conversion of CO₂ with “green hydrogen” (hydrogen originates from sustainable energy, like wind, solar. Etc.) into synthetic natural gas (SNG) has been considered as one of the most promising and practical approaches for CO₂ utilization[4,5]. SNG, as a high value-added fuel product, plays a vital role as a raw material in the synthesis of syngas and many chemical products[6]. CO₂ hydrogenation to SNG reaction, which was also called Sabatier reaction, displayed a great potential in “Power to Gas” process[7]. However, this reaction went through an eight-electron process for CO₂ reduction into methane, which suffered an significant kinetic limitations [8]. To solve this puzzle, a catalytic material could achieve high reaction rates was required. Ni-based catalysts with high reaction activity and low-cost, aroused extensive interest of researchers[9]. While, in practical application, Ni-based catalysts always suffered from inferior activity at low temperatures[10]. Compared to CO₂ methanation reaction in a higher-temperature, the low-temperature CO₂ methanation showed advantages in two different aspects: (I) The competitive reaction like reverse water gas shift (RWGS) reaction could be effectively reduced at low temperatures, and the catalysts obtained a high CH₄ selectivity. (II) The sintering and carbon deposition problems of the catalyst usually occurred at high temperatures, low temperature condition was conducive to the catalysts stability[11,12,13]. Therefore, improving the low temperature activity of Ni-based catalysts is a current research hotspot[14].

Hydrotalcite (HT) was a kind of natural or synthetic ordered materials, which consisted of positively charged two-dimensional sheets of mixed hydroxides, and charge-compensating anions placed between the layers[15]. They were expressed as [M²⁺_1–xM³⁺_x(OH)₂] (A^n-)_x/n · mH₂O. (M²⁺ and M³⁺ were divalent and trivalent metals, respectively; x was the mole ratio of M³⁺/(M²⁺+M³⁺), Aⁿ⁻ was the interlayer anion)[7].The unique supramolecular structure provided great potential to disperse and tune active sites at the atomic scale[16]. Introduce a proportion of alkaline elements into the HT precursor could obtain a tunable alkaline site structure and promote the CO₂ adsorption, which would benefit for the CO₂-involving reaction. For instance, the addition of Mg element could improve the catalyst basic properties, result in the increase of the CO₂ adsorption capacity[17]. Moreover, small-size and heat-stable metal nanoparticles were highly dispersed on the calcined HT precursor surface after reduction, thus the catalyst stability and reducibility would get enhanced [10]. For example, Guo et al. synthesized the hydrotalcite-derived NiMgAl catalyst exhibited the excellent catalytic activity with CO₂ conversion of 91.8% at 250℃[18]. Therefore, it was a promising approach by fabricating the alkaline-assisted hydrotalcite-derived materials to obtain the efficient low-temperature catalyst for CO₂ methanation.

However, traditional NiMgAl catalyst derived from hydrotalcite also suffered from strong metal-support interaction[19], thus a long-time H₂ reduction process was needed to enhance reducibility. To overcome this drawback, quite a few reports were found to introduce a second metal (such as Mn, La and Y) to replace partial Mg element [20,21], thereby regulating the metal-support interaction. As reported by our group, the doping of Mn element could efficiently regulated the Ni and Mg(Mn)AlOx interaction, surface content of Ni⁰ species, and basic property[11]. Dominik Wierzbicki et al. supposed La could soften the interaction between Ni and the HT matrix, lead to the increase of Ni-species reducibility [22]. Sun et al. found the incorporation the 0.4 wt% Y strongly decreased the metallic nickel particle size and increased the medium-strength basic sites[23].

Besides the Mn, La,Y species, Ce, as a rare earth oxide, had also been a very attractive promoter for the CO₂ methanation due to its extraordinary ability to enhance metal dispersion as well as the thermal stability of the support [24,25]. Dębek R et al. found CeO₂ had a promoting effect on increasing surface basicity of the catalyst, which was attributed to the its high mobile oxygen capacity and redox activity [26]. Reducing CO₂ to CO process was the rate-determining step of CO₂ methanation. In addition, the oxygen vacancy provided by the CeO₂ could create an additional driving force for this process in the reducing atmosphere [27]. Zhang et al. considered that Ce³⁺ cations located in the AlCeO₃ solid solution could greatly promote the adsorption and activation of CO₂ and facilitate the formation of the intermediate, therefore the CO₂ conversion was significantly accelerated at low reaction temperatures[28]. In dry reforming of methane (DRM) reaction, Radosław Debek et al. incorporated Ce-species into the NiMgAl catalysts, which was found to promote the nickel species reducibility and introduce new strong oxygen species (low coordinated) and more medium-strength basic sites (Lewis acid-base pairs) [15,29].

Furthermore, a recent literature compared different Ni content (10.3, 16.2, 27.3, 36.8, 42.5 wt.%) on the performance of hydrotalcite-derived catalysts, the optimum Ni content was 42.5wt%. This result confirmed the higher amount of Ni introduced led to smaller crystallites, better reducibility and CO₂ adsorption capacity of the catalysts. H₂-TPR proved the Ni and hydrotalcite matrix interaction weakened with the increasing Ni contents, which had a positive effect on the catalytic activity[30]. However, according to our knowledge, quaternary hydrotalcite-derived Ce-containing Ni-Ce catalysts had not been widely used in CO₂ methanation. Herein, our goal was to study the structure-performance relationships between them. We prepared a series of Ce-promoted Ni/MgAlOx catalysts through co-precipitation method in this study. The low-temperature catalytic performance was investigated by varying Ce content from 0, 1, 5 to 10wt%. The optimal sample displayed superior CO₂ methanation activity with 80% CO₂ conversion at low temperature 250℃, and the CH₄ selectivity was close to 100%. Meanwhile the CO₂ conversion (91.7%) was not deactivated upon 80h operation at 300℃. Extensive characterization methods (XRD, BET, ICP SEM, TEM, XPS, H₂-TPR, CO₂-TPD) were used to deeply analyze the promoting effect of Ce on influencing the catalyst structure, morphology, surface properties, metal-support interaction, performances during CO₂ methanation.

2. Experimental

2.1. Catalyst Synthesis

All of the hydrotalcite precursors were prepared by the co-precipitation method. Ni content was kept at 40wt % and the Mg/Al molar ratio was fixed at 1 in all samples, while the loading of Ce varied from 0, 1, 5 to 10wt%. Firstly, the mixture of Ni(NO₃)₂·6H₂O, Mg(NO₃)₂·6H₂O, Al(NO₃)₃·9H₂O, Ce(NO₃)₃·6H₂O was dissolved in the deionized water to form a solution at 0.4 M. Then the mixed nitrate solution was added dropwise into a flask containing 280mL sodium carbonate solution (0.25M) at 60℃ under vigorous stirring. At the same time, keeping the slurry PH at 9.5-10 by adding the sodium hydroxide solution (1M). After co-precipitation, the slurry was vigorous stirred at 60℃ for 1h and then aged for 18 h at 60℃. The solid product was obtained by filtration, washed with deionized water for three times, dried at 60℃ overnight, and named as NiCe0-HT (without Ce), NiCex-HT (x=1,5,10). Finally, the obtained hydrotalcite precursors were calcined at 500℃ for 4 h, and labeled as NiCex-C (x=0,1,5,10).

2.2. Catalytic Experiments

CO₂ methanation reaction was performed in a miniature fixed-bed reactor at atmospheric pressure. 200 mg catalyst (40-60 mesh) mixed with 500 mg quartz sand were placed in the quartz tube (inner diameter of 6mm and a length of 40cm). Prior to the catalyst evaluation, the catalysts were reduced under 30ml/min H₂ at 700 ℃ for 2 h. After reduction, the catalyst bed was cooled down in the N₂ atmosphere to 180 ℃, then the mixture of 40ml/min H₂ and 10ml/min CO₂ (H₂: CO₂=4:1, GHSV=15000 ml/ g/ h) was introduced into the reactor. The catalytic test was carried out in the temperature range of 200-350 ℃ at a temperature interval of 25℃. The gas products were analyzed by an on-line SP-7890 gas chromatogram equipped with a thermal conductivity detector (TCD) (fitted with a TDX01 column). Taking the CO₂ conversion (X_CO2) and the CH₄ selectivity (S_CH4) as indicators to evaluate the activity of the catalysts, which were calculated by the following equations[31]:

$$X_{{\mathrm{C}\mathrm{O}}_2}=\left(\frac{F_{{\mathrm{C}\mathrm{O}}_2,\mathrm{i}\mathrm{n}}-F_{{\mathrm{C}\mathrm{O}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}}{F_{{\mathrm{C}\mathrm{O}}_2,\mathrm{i}\mathrm{n}}}\right)\times 100\%$$

(1)

$$S_{{\mathrm{C}\mathrm{H}}_4}=\left(\frac{F_{{\mathrm{C}\mathrm{H}}_4,\mathrm{o}\mathrm{u}\mathrm{t}}}{F_{{\mathrm{C}\mathrm{O}}_2,\mathrm{i}\mathrm{n}}-F_{{\mathrm{C}\mathrm{O}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}}\right)\times 100\%$$

(2)

where “F_{CO2, in}” and “F_{CO2, out}” refer to the inlet CO₂ gas flow and the outlet CO₂ gas flow respectively, “F_{CH4, out}” refer to the outlet CH₄ gas flow, mL/min.

The equations of CO₂ conversion rate (R_CO2, μmolCO₂/(g_catŸs)) and Turnover frequency (TOF, h^-1) are as follows[32,33].

$$R_{{\mathrm{C}\mathrm{O}}_2}=\frac{F_{{\mathrm{C}\mathrm{O}}_{2,}\mathrm{i}\mathrm{n}}\times X_{{\mathrm{C}\mathrm{O}}_2}}{m\times {\mathrm{V}}_{\mathrm{m}}}$$

(3)

where V_m represents the gas molar volume of 22.4 L/mol under standard conditions, X_CO2 refers to the conversion of CO₂ at 225 °C (X_CO2<15%), m denotes the quality of the catalyst (g)[11,34]

$$\mathrm{T}\mathrm{O}\mathrm{F}=\frac{F_{{\mathrm{C}\mathrm{O}}_{2,}\mathrm{i}\mathrm{n}}\times \delta }{N_{surface}\times {\mathrm{V}}_{\mathrm{m}}}$$

(4)

$${\mathrm{N}}_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}={\mathrm{N}}_{\mathrm{t}\mathrm{o}\mathrm{t}\mathrm{a}\mathrm{l}}\times D$$

(5)

where δ represents the methane yield and ${\mathrm{N}}_{\mathrm{s}\mathrm{u}\mathrm{r}\mathrm{f}\mathrm{a}\mathrm{c}\mathrm{e}}$ represents the molar amount of nickel atoms located on the catalyst surface, which is calculated by the nickel dispersion equation (D, %) based on the size of Ni nanoparticles after reduction (TEM reault).

The apparent activation energy of the catalyst was measured according to Arrhenius equation:

$$\mathrm{l}\mathrm{n}\mathrm{k}={lnk}_0-\frac{E_a}{RT}$$

(6)

where k is the reaction rate, k₀ is the pre-exponential factor, Ea is the apparent activation energy of the reaction, R is the gas reaction constant (8.314 J/(mol·K)), and T is the reaction temperature. The experimental conditions were carefully selected to obtain the catalytic data with CO₂ conversion below 15% for activation energy calculation, which effectively excluded the influence of diffusion limit[34].

2.3. Characterization of Catalysts

Powder X-ray diffraction (XRD) measurement was performed on Rigaku Ultma IV device equipped with a copper-based anode (Cu Kα radiation, λ=0.154 nm). The instrument settings were 35 kV× 30 mA. It operated in the 2θ range from 5° to 85° with the scanning speed of 2°·min ^-1, to identify the crystal phase and morphology of the sample.

The hydrogen temperature-programmed reduction (H₂-TPR) measurement was performed on a TP-5080 instrument equipped with a TCD detector (Tian jing, Xian quan) to investigate the reduction performance of the catalyst. Firstly, 50mg catalyst was placed in the quartz tube, then 30ml/min N₂ was put into the instrument for 1 h at 200 ℃ to remove the physical adsorbed impurities on the catalyst surface. After cooling down to 50 ℃ under argon, 28ml/min 10% H₂/N₂ mixture was introduced to reduce the catalyst, and the system was kept at 50 ℃ for 40min until the baseline was stable. Then the reactor temperature was linearly increased from 50℃ to 800℃ at a heating rate of 5℃/min.

The carbon dioxide temperature-programmed desorption (CO₂-TPD) was carried out on a Auto Chem II 2920 apparatus to determine the basic sites on catalyst surface. Firstly, 0.1g catalyst was placed in the quartz tube, 30ml/min H₂/Ar was put into the device for 10 min at room temperature to remove the residual gas in the tube. Then the catalyst was reduced in H₂/Ar at 700℃ for 2 h. After that, the sample was cooled down to 50℃ in N₂ atmosphere. After CO₂ adsorption of the catalyst for 1h, N₂ was put into to the system to remove the remained CO₂ in the gas phase and the physically adsorbed CO₂ on the catalyst surface. Finally, the system was heated from 50℃ to 800℃ at a linear heating rate of 10℃/min to desorb the chemisorbed CO₂ on the catalyst surface.

The N₂ adsorption-desorption analysis was carried out on the ASAP 2020 analyzer at -196℃. All samples were degassed at 300℃ for 6 h before the analysis to desorb contaminants and moisture. The specific surface area was characterized by adsorption isotherm according to the multiple Brunauere-Emmette-Teller (BET) equation. The pore size distribution and average pore diameter were determined by the Barrette-Joynere-Halenda (BJH) model. Inductively coupled plasma optical emission spectrometer (ICP-OES) was carried out by an Agilent 5110(OES) equipment to conduct elemental analysis of the catalysts.

X-ray photoelectron spectroscopy (XPS) measurements were performed over a Thermo Scientific K-Alpha spectrometer equipped with Monochromated Al Kα X-ray source, (hυ =1486.6 eV, 12 kV, 6 mA). The binding energy standard was C1s=284.80eV.

ZEISS Sigma 30 scanning electron microscope (SEM) was used to study the morphology of the catalyst precursor. The acceleration voltage was 3 kV and the magnification was 5W and 10W times. Moreover the morphology, metal dispersion and lattice spacing of the calcined and reduced catalysts were characterized by field emission transmission electron microscopy (TEM), which was performed on FEI Tecnai G2 F20 instrument with an acceleration voltage of 200 kV.

3. Results and Discussion

3.1. Texture Characteristics of the NiCex-C Catalysts

Figure 1a showed the XRD diffraction profiles of precursors with various Ce contents. Distinct diffraction peaks were observed at 2θ~11°, ~22°, ~34°, ~38°, ~46°, ~60°, ~61° for all the samples, which belonging to (003), (006), (012), (015), (018), (110), and (113) of hydrotalcite characteristic structure (JCPDS-22-0700)[11]. In particular, compared with Ce-free sample, obviously weaker and wider diffraction peaks were observed in other Ce-containing samples. This results indicated that the Ce doping could influence the crystallinity of the sample, and disturb the hydrotalcite plate layer structure[35]. Figure 1b showed XRD spectrum of samples after calcination at 500℃ for 4 h, it could be found that the hydrotalcite structure was destroyed and obtained the mixed metal oxide. The peaks at 2θ=37.3^°, 43.3^°, 62.9^°, 75.5^° were corresponded to (111), (200), (220) and (311) crystal planes of NiO，respectively (JCPDS-47-1049)[36]. After adding Ce element, the intensity of these characteristic peaks in the samples decreased significantly. Meanwhile, well-crystallized CeO₂ could be observed after calcination. And the peaks at 28.5^◦, 33.0^◦, 47.4^◦, and 56.3^◦ were indexed to (111), (200), (220) and (311) crystal planes of CeO₂ (JCPDS-34–0394)[37]. In special, the diffraction peaks corresponding to MgO and Al₂O₃ were absent. The Al and Mg species may form as a amorphous structure or being a part of the MgAl₂O₄ spinel, whose diffraction peaks were overlapped by the periclase[11,38].

Texture properties of the NiCex-C catalysts were investigated by N₂ adsorption-desorption analysis. As presented in Figure 2a, according to the IUPAC classification, all the samples displayed an IV N₂ adsorption-desorption isotherm (P/P₀>0.4) with H2 hysteresis loops, corresponding to the typical feature of mesoporous structure[3]. The pore structure might be caused by the stack of 2D structure of HT, which was conducive to the adsorption and activation of active gases, promoting the mass transmission process[39]. As shown in Figure 2b, all samples featured typical mesoporous structures with narrow pore size distributions, concentrated on 2 to 10 nm. In addition, the detailed textural properties of the NiCex-C catalysts were listed in Table 1. All the catalysts derived from hydrotalcite-like compounds displayed large specific surface area (>100 m²/g), which allowed for a better dispersion of surface nickel species[40]. The mean pore diameters of the catalysts were all slightly decreased after the introduction of Ce element. It mainly due to the Ce species on the external porous surface of the hydrotalcite crystallites, causing the blockage of partial smaller mesoporous [41]. On the other hand, there was no significant difference on the average pore volume among all samples.

3.2. Morphological Study and the Particle Size Analysis

The SEM images of the NiCe0-HT sample with the magnification of 10W and 5W were shown in Figure 3a,c, respectively. It was found that the NiCe0-HTexhibited platelet-like crystals that were aggregated as rosettes, referred to the typical characteristic for the hydrotalcite structure[42]. However, the morphology of the Ce-loading sample (NiCe5-HT) were found not changed apparently (Figure 3b,d). It further indicated that the hydrotalcite-like structure could be well generated, which was identical to the XRD results.

TEM measurements were used to observe the state of all the calcined and reduced NiCex-C catalysts. As shown in Figure 4, The dark spots were ascribed to the Ni nanoparticles dispersed on the frame of the mixed metal oxide. For all samples, the layered structure of the hydrotalcite precursors partially collapsed after calcination. The morphology and the Ni particle dispersion were varied due to the introduction of different Ce contents. For the Ce-containing catalysts, Ni nanoparticles dispersed evenly on the catalyst surface. While large clusters of Ni particles were observed on the surface of the NiCe0-C catalyst, the dispersion of Ni nanoparticles were poor. In addition, the well-defined lattice fringes assigned to the face-centered cubic Ni (111) surface were observed in HRTEM images for all samples, with an average lattice spacing of 0.2nm[4].

Moreover, the frequency distribution histogram of Ni particle-size for all samples were displayed in Figure 5, ranged from 5nm-10nm. The small size of Ni nanoparticles probably due to the confinement effect of HT precursor[43], which processed the ordered metal oxides grids would confine the growth of the Ni particles and achieved the highly dispersed Ni particles [44,45]. Meanwhile, the corresponding average particle size was found to follow the order: NiCe5-C (5.67nm) < NiCe1-C (6.82nm) < NiCe10-C (7.71nm) < NiCe0-C (9.06nm). Optimal Ce element loading (5 wt%) led to the lowest Ni particle-size. In a word, the statistics and TEM images proved that the aggregation of Ni particle in the NiCe0-C catalyst could be halted to some extent by the incorporation of Ce element. Which would play a positive effect on the uniform dispersion of the Ni nanoparticles.

3.3. Reducibility and Metal-Support Interaction Study

The reduction behavior of the NiCex-C (x=0,1,5,10) catalysts was evaluated by H₂-TPR analysis. H₂ consumption during the H₂-TPR experiments should be caused mainly by the reduction of NiO species. It was known that the reduction difficulty of NiO species might be determined by the metal-support interaction with the oxide support or the dispersion conditions over the support surface[37]. As shown in Figure 6, three types of the reduction peaks could be observed. The low-temperature reduction peak (~350℃) of NiO species can be ascribed to the weakly interacting with the support [46]. It could be found that the peak intensity (~350℃) gradually became weaker with the increasing of Ce contents. The peak temperature was decreased to 346℃ over NiCe1-C catalyst, compared to 360℃ for undoped NiCe0-C. This results indicated that after the addition of Ce element, this part of NiO was more easily reducible [47]. The minor shoulder peak (~510℃) was attributed to the reduction of a small amount of Ni²⁺ species, which are weakly bound to oxygen atoms at the Ni-O-Al (or Ce) interface, commonly reported in Ni/AlCeO-x catalysts[28]. Besides, for Ce-containing catalysts, this reduction peak also corresponded to the reduction of Ce⁴⁺ to Ce³⁺ species. [48]. The main reduction peak appeared at 700℃ for the NiCex-C catalysts, which was attributed to a stronger metal-support interaction between NiO species and the support matrix (Mg(Ce)AlO)[24]. To obtained sample with highly reduced, the reduction temperature in activity test was chosen to be about 700 ℃. Especially the reduction temperature of NiO in the NiCe5-C catalyst shifted to the lowest temperature of 690 ℃. Consequently, for these NiO species, the NiCe5-C catalyst displayed the best reduction behavior. However, when Ce element was incorporated into the catalyst, the high temperature peak (~700℃) became boarder with lower intensity. This result indicated that the strong metal-support interaction was weakened by the loading of Ce element[49]. Appropriate metal- support interaction would led to more Ni⁰ active sites [50], thus giving preferable low temperature catalytic activity.

3.4. Surface Basicity and Element Distribution Analysis

In CO₂ methanation process, reaction molecule CO₂ was not directly absorbed on metal nickel, but on surface basic sites (OH^- groups or alkali/alkaline-earth metal oxides), which was beneficial to activate CO₂ [19,51,52]. Hence, CO₂-TPD was carried out for the NiCex-C catalysts to determine the basic strength and CO₂ adsorption capacity. As shown inFigure 7, the desorption curves of all the samples could be divided into three types of Gaussian peaks (α, β, γ peak). The three reduction peaks occurred at 100~120, 140~170, 250~280℃, respectively[7,11]. Which corresponding to weak-strength basic sites (α and β peak), medium-strength basic sites (γ peak). The weak-strength basic sites was attributed to the rapid formation of bicarbonate which was due to the weak adsorption of CO₂ by hydroxyl group on the catalyst surface[14]. While the medium basic sites were assigned to Lewis basic sites (acid-base Ce⁴⁺- O²⁻ and metal-O^2-pairs) associated with CO₂ adsorption. These basic sites bound CO₂ sufficiently strongly for its activation and subsequent reduction[53]. Based on the literature[54], the medium basic sites played an important role in CO₂ methanation process. From the previous report, the surface charge would be disturbed by introducing hetero-ions into the hydrotalcite layer[55]. Thus, the incorporation of Ce element into the nickel-based hydrotalcite obviously influenced the distribution of basic sites. Detailed information of surface basic sites relative content for all reduced NiCex-C samples was displayed in Table 2. The proportion of the medium basic sites were calculated and were found to follow the order: NiCe5-C > NiCe1-C> NiCe0-C> NiCe10-C. The quantities of CO₂ adsorbed on basic sites over reduced NiCex-C catalysts were summarized in Table 3. The highest density of medium-strength basic sites was found on the NiCe5-C sample (0.79mmol CO₂/g_cat), suggesting that CO₂ could be more easily activated, and further affected the catalytic performance. While the incorporation of 10wt.% of Ce element could lead to a slight decrease in total basicity (0.80mmol CO₂/g_cat). The NiCe10-C exhibited the lowest CO₂ adsorption capacity and thus the least amount of CO₂ for the methanation. The correlation between the surface basic sites and its catalytic activity in the CO₂ methanation will be further explored in ubsequent section [56].

The information of the surface components for the NiCex-C catalysts was further probed by XPS measurements. It was known that the position of the most intense peak was used to confirm the oxidation state of the metallic element and to obtain the information about the charge density for its cations[27]. As displayed inFigure 8a, the main band of the Ni 2p_3/2 spectra for all the NiCex-C catalysts were deconvoluted into three peaks at ~ 853.8, ~856.6, and ~ 862.3 eV, corresponding to Ni⁰, Ni²⁺ (bulk NiO), and satellite peak of nickel species[40]. From the literature, the satellite peak was assigned to Ni²⁺ species in Ni-O-Ce interaction interface or Ni²⁺ species in unreducible NiAl₂O₄ spinel [57]. These Ni species at higher binding energy (~856 and~ 862eV) suggested the strong metal-support interaction[58]. Based on the peak fitting and peak area calculation, the detailed information of Ni⁰ species relative contents were listed in Table 4. Compared with the Ce-free sample, the Ni⁰ relative content increased. In particular, the maximum value 40.5% for the NiCe5-C catalyst, while the minimum value 31.5% for the NiCe0-C catalyst. It was shown that the strong interaction between Ni²⁺ and support was weakened by the Ce dopants, leading to more Ni⁰ species generated, which was consistent with the H₂-TPR results. Rare earth oxides might act as an electron donor so that more electrons were shifted to Ni species, resulting in an increase of d-electron density on the Ni surface. Similar to the reported (Sc, Y, Ce, and Pr) promoted NiMgAl catalysts for dry reforming of methane [59]. Meanwhile, this result also indicated that the introduction of Ce to the catalysts could promote the electron transfer and reduce more Ni²⁺ to Ni⁰. While further increased the incorporation of Ce contents (10%), the proportion of Ni⁰ shifted to the lower value (34.1%).

On the other hand, the two sets of Ce 3d spin–orbit coupling peaks, corresponding to 3d3/2 (labeled as u) and 3d5/2 (labeled as v) respectively, could be deconvoluted into eight peaks (Figure 8b). According to the literature, the oxidation states of cerium species mainly existed in the form of Ce³⁺ and Ce⁴⁺[1]. More precisely, the peak near 882.8eV (v), 887.8eV (v₂), 899.2eV (v₃), 901.9eV (u), 908.1eV(u₂) 917.5eV(u₃) could be attributed to the Ce⁴⁺ species in CeO₂[46]. However, for the NiCe1-C catalyst with lower Ce-loading, the u₃ peak near 917.5eV was not noticeable from XPS spectra. Besides, the peak near 885.3eV(v₁) 904.9eV (u₁) could be ascribed to the Ce³⁺state[60]. The Ce³⁺state indicated the reduction of Ce⁴⁺ due to the high temperature hydrogen reduction[41]. In order to keep charge balance of CeO₂, that oxygen vacancies were generated in this reduction process[46,61]. Thus the concentration of Ce³⁺ was in direct proportion to oxygen vacancies contents, which were the main active sites in CO₂ activation and absorption[8,62]. The concentrations of Ce³⁺ of all the NiCex-C catalysts were calculated by adding the areas under each deconvolution peak and dividing by the total peak area. The detailed molar ratio of Ce³⁺/ (Ce³⁺ + Ce⁴⁺) were tabulated in Table 4. It could be found the highest Ce³⁺ concentration of 23.2% for the NiCe5-C catalyst. Obviously higher than other Ce-containing catalysts. Demonstrating that there were more oxygen vacancies at the Ni–O-Ce interfaces. In a word, the NiCe5-C catalyst owed the most active sites for CO₂ absorption and activation as well as surface Ni⁰ sites for the H₂ molecular splitting. They may work synergistically and efficiently in CO₂ hydrogenation under this reaction condition.

3.4. Catalytic Activity and Stability in CO₂ Methanation Reaction

The catalytic performance of all prepared NiCex-C (x=0, 1, 5, 10) catalysts for CO₂ methanation reaction were investigated at gas hourly space velocity (GHSV) of inlet gas of 15,000 mL/g_cat/h with H₂/CO₂ molar ratio of 4.0 in the temperature range of 200-350℃. It was known that CO₂ methanation process was exothermic so the total conversion could not be reached to 100% at high reaction temperature (＞200℃)[63]. CO₂ methanation performances of the NiCex-C catalysts were shown in Figure 9. As shown in Figure 9a, CO₂ conversion increased for all the samples with increasing reaction temperature. It was displayed a significant difference in performance at 250℃. CO₂ conversion increased at 250℃ from 11.7% for NiCe0-C to 30.2% and 80.0% for NiCe1-C and NiCe5-C, respectively. The maximum CO₂ conversion was observed at 300℃.Then the catalysts reached CO₂ equilibrium conversion above 300 ℃. T₅₀ was used to identify the low-temperature activity of the samples[11]. The value of T₅₀ was 263.8℃ and 258.4℃ for Ce-free NiCe0-C catalyst and NiCe1-C catalyst, respectively. While it was obviously decreased to 238.6℃ on NiCe5-C catalyst, indicating the promotion effect from Ce. However, when further increasing Ce content, the value of T₅₀ increased to 277.9℃ for NiCe10-C catalyst. It could be found that the catalytic performance first increased and then decreased with the increase of Ce content. This excellent low-temperature performance of NiCe5-C was from two main aspects: (I) Abundant active Ni⁰ sites evenly dispersed over the NiMgAl mixed metal oxide surface (XPS and TEM results), which provided active metal sites for H₂ molecular dissociation and further promoted the hydrogenation process[37]. (II) The medium basic sites were conducive to the CO₂ adsorption and activation[18]. NiCe5-C possessed a highest amounts of medium-strength basic sites (CO₂-TPD result). Nevertheless, NiCe10-C exhibited the worst hydrogenation activity. This could have been mainly caused by the lowest amounts of total basic sites, as confirmed by CO₂-TPD. Furthermore, CO as the main by-product in CO₂ methanation process was due to reverse water gas shift (RWGS) process: CO₂ + H₂ = CO + H₂O. It was a endothermic reaction favorable at high temperature[64]. The CH₄ Selectivity of NiCex-C catalysts presented high value nearly 100% (Figure 9b), and few CO by-product could be found at low reaction temperature. The detailed product distributions of the tested catalysts in the CO₂ methanation reaction at reaction temperature of 225℃ were displayed in Table 5. At 225℃, The CO₂ conversion of the Ce-free sample was only 3.4%, while that of NiCe5-C reached a maximum value of 13.9%. The CO₂ conversion rate (R_CO2) and the TOF (Turnover frequency) value of NiCe5-C were 4.1 times and 3 times higher than that of NiCe0-C, respectively. It indicated that the low-temperature activity of Ni-based catalyst derived from hydrotalcite-like precursors could be significantly enhanced by moderate Ce doped.

The long-term stability of the catalyst was of great importance for industrial application. As shown inFigure 10, the stability of the NiCe5-C catalyst was investigated at 300 ℃ because of the best performance among all samples. After 80 h stability test, the NiCe5-C catalyst also exhibited high activity, with about 91.7% of CO₂ conversion and almost 100% of CH₄ selectivity. In addition, there was no obvious deviation for them under the 80h stability test. The NiCe5-C catalyst achieved high stability as well. It indicated that the well-dispersed nickel particles on the catalyst surface (TEM results) could enhance the anti-sintering ability of the NiCe5-C sample and avoid deactivation after long-term use.

Hydrogenation of CO₂ to methane was a first order reaction, and the reaction of H with CO₂ was the rate-controlling step for the this reaction[65]. The apparent activation energies of the samples were measured by the Arrhenius equation. As shown in Figure 11, take the logarithm of CO₂ conversion as x variable and the reciprocal of temperature as y variable to fit a straight line. The apparent activation energies of the catalysts were obtained by calculating the slop of the fitting lines. Compared to the NiCe0-C catalyst (98.13 kJ/mol), the apparent activation energy of the NiCe5-C catalyst was lower (96.04 kJ/mol). This result suggested the CO₂ methanation reaction was more facile on NiCe5-C catalyst. It could be attributed to the Ce element had a positive effect on promoting charge transfer from active metal to the CO₂ molecules[66]. Therefore, the reactant CO₂ molecules were more easily activated, which in favor of decreasing the energy barrier of H and CO₂ reaction.

3.5. The Descriptors of the Relationship between Catalytic Performance and Surface Basicity

On the basis of previous literature reports, the Lewis acid-base sites instead of strong basic site were involved in the CO₂ methanation mechanism[67,68]. Therefore, it was necessary to explore the relationship between catalytic activity and the number of medium basic sites. As plotted in Figure 12a, the highest CO₂ conversion at 225℃ was achieved over the NiCe5-C with the strongest medium-strength basic sites. In addition, the CO₂ conversion of the catalysts was positively correlated with the number of medium basic sites. In this contribution, we observed the turnover frequency of the catalysts almost displayed linear relationship with the content of medium-strength basic sites at 225℃ (Figure 12b). It showed that the TOF values increased with increasing amount of medium-strength basic sites. These results demonstrate that the variation of Ce content could effectively regulate the number of medium-strength basic sites on the surface of NiCex-C, and the medium-strength basic sites on the catalyst played a crucial role in achieving high catalytic activity for CO₂ methanation at low temperatures.

4. Conclusions

In this work, we demonstrated an extremely straightforward method of preparing Ce-promoted NiMgAl hydrotalcite-derived catalysts via co-precipitation operation, to promote CO₂ methanation at low temperatures. The NiCe5-C sample exhibited superior low-temperature activity for CO₂ methanation (CO₂ conversion: 80.0%, CH₄ selectivity >99%, GHSV=15,000mL/gcat/h, H₂/CO₂ =4, 250℃, 0.1Mpa), together with high stability. While the CO₂ conversion of the Ce-free sample was only 11.7% under the same conditions. At 225℃, TOF value of the NiCe5-C (1.19 h^-1) was three times higher than that of NiCe0-C (0.39 h^-1). The characterization results indicated that incorporation of appropriate amount of Ce into NiMgAl could weaken the strong metal- support interaction, thereby promoting the reduction of NiO species. Meanwhile, the Ce element could efficiently regulate the surface basic species. The medium-basic sites were found to have a good linear correlation with TOF. The NiCe5-C catalyst owed the highest ratio of Ni⁰/(Ni²⁺+Ni⁰) and rich medium-basic sites, was able to work more effectively on dissociating H₂ and activating CO₂, and further hydrogenated to methane. Our contribution provided a promising and practical thought to raise the catalytic performance in the CO₂ methanation at low-temperature.