Ni/SBA-15 catalysts for CO methanation: effects of V, Ce, and Zr promoters

Abstract

Ni/SBA-15 catalysts with various promoters (V, Ce, and Zr) were prepared by an ultrasonic coimpregnation method and used in CO methanation. The addition of promoters played a significant role in improving the catalytic activity of the Ni/SBA-15 catalyst. This improvement could be explained by changes in the valences of the V and Ce promoter species through an oxidation–reduction shift cycle process (M^x+ ↔ M^y+, M = V, Ce), which could trigger electron transfer. This transfer enhanced the electron density of active Ni species and promoted CO dissociation. The Zr promoter could produce oxygen vacancies during calcination and reduction, thereby increasing the ability of CO to adsorb and dissociate. In addition, the formation of a Si–O–M bond (M = Zr, Ce, V) increased the interaction between the active species and support, which facilitated CO methanation. Under 1.0 MPa and a WHSV of 15 000 mL g⁻¹ h⁻¹, 10Ni–5V/SBA-15 exhibited the best catalytic performance (99.9% CO conversion; 95.5% CH₄ selectivity).

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1. Introduction

Since Sabatier and Senderens first discovered CO methanation in 1902, the reaction has been widely studied by researchers.^1–4 It is now used in eliminating trace amounts of CO from feed gas for ammonia synthesis. It is also used in forming methane-rich fuel, which has a high heating value and low CO content, as well as in processes related to Fischer–Tropsch synthesis.¹ In the 21^st century, the shortage of energy resources and environmental pollution caused by rapid consumption of fossil fuels has prompted many investigators to make great efforts to address these problems.⁵ Selectively catalytic methanation of CO is used to purify hydrogen for proton-exchange membrane fuel cells, which prevents their anodes from severe poisoning by CO in hydrogen-rich gas. The CO concentration in the gas is <50 ppm.⁶ Another effective method is production of synthetic or substitute natural gas (SNG) via syngas (CO and H₂) from coal and biomass.⁷ SNG is natural gas that has good combustibility and heating value. Therefore, development of viable and efficient catalysts for methanation is necessary.

Among the methanation catalysts, those based one noble metals such as Ru,⁸ Rh,⁹ Pd¹⁰ show better performance. But the scarcity and expense of noble metals restrict their large-scale applications. Because of their good activity and cost effectiveness, Ni-based catalysts have been extensively investigated and widely applied. However, Ni-based catalysts are vulnerable to sintering and coking, which may lead to their deactivation. Hence, many efforts have been made to enhance catalytic activity and stability, including selection of appropriate supports, addition of catalytic promoters, and search for advanced preparation methods. There have been extensive studies on catalyst supports such as Al₂O₃,¹¹ SiO₂,¹² TiO₂,¹³ ZrO₂ (ref. 14) and CeO₂.¹⁵ Takenaka et al.¹⁶ demonstrated that CO methanation performance of Ni-based catalysts on various supports are closely related to the type of catalytic supports. CO conversion at 250 °C increases in the order Ni/MgO < Ni/Al₂O₃ < Ni/SiO₂ < Ni/TiO₂ < Ni/ZrO₂. Addition of promoters can effectively enhance Ni dispersion, as well as increase the activity and stability of Ni-based catalysts. Wang et al.¹⁷ reported that use of ZrO₂ as promoter for Ni/SiO₂ catalyst significantly improves the catalytic activity, enabling complete conversion of CO into CH₄ at 240 °C in hydrogen-rich gas stream through methanation. Hayek et al.¹⁸ found that V and Ce are good electronic promoters for Rh- and Pd-based catalysts in CO hydrogenation for hydrocarbon production. Another study revealed that addition of V promoter could markedly enhance the performance of Ni/Al₂O₃ catalyst in CO methanation.¹⁹ Ren et al.²⁰ prepared Ni–Ce/Al₂O₃ catalysts by microwave heating method for the catalytic methanation of coke oven gas, the temperature gradient between the NiO and the alumina support caused by the difference in microwave energy absorption promoted the formation of a large amount of highly dispersed amorphous NiO, and then enhanced the catalytic activity and stability of this reaction. Mesoporous silica materials have been widely utilized as novel catalytic supports because of their large specific surface area, regular pore structure, and good hydrothermal stability. Zhang et al.²¹ reported that Ni-based MCM-41 catalysts, which they used in CO methanation, exhibited good activity and CH₄ selectivity. However, the Ni species was easily sintered by calcination and high-temperature reduction. The mesoporous silica material SBA-15 has thicker wall of hole, better hydrothermal stability, and stronger mechanical strength compared with MCM-41. Lu et al.²² found that NiO/SBA-15 catalysts prepared by heat treatment showed good activity and thermal stability in CO/CO₂ methanation; however, the CH₄ selectivity of the catalyst in methanation as compared with other existing catalysts needs further improvement.

In the present work, highly efficient Ni/SBA-15 catalysts were prepared by ultrasonic coimpregnation method, and the effect of Ni loading on CO methanation was studied. More importantly, we investigated the effects of the addition of the promoters V, Ce, and Zr on the catalytic activity and physicochemical properties of the Ni-based catalysts. The catalysts were characterized through X-ray diffraction (XRD) measurements, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FT-IR) spectroscopy, N₂ physisorption, Raman spectroscopy, H₂ temperature-programmed reduction (H₂-TPR), and H₂ temperature-programmed desorption (H₂-TPD) experiments. We aimed to explain the high performance of the catalysts. We found that addition of promoters could improve the low-temperature activity and high-temperature stability of the catalysts in CO methanation. 10Ni–5V/SBA-15 is a promising catalyst because of its high catalytic activity and high stability.

2. Experimental

2.1 Catalyst preparation

2.1.1 SBA-15 synthesis. The SBA-15 support was prepared according to previously reported methods.²² Triblock copolymer (Pluronic P123, 5800 g mol⁻¹ average, Sigma-Aldrich), a structure-directing agent, was dissolved in aqueous HCl solution (2 mol L⁻¹). Tetraethyl orthosilicate (98% purity, Sigma-Aldrich), the silicon source, was subsequently added. The resulting mixture was stirred for 24 h at 40 °C and then hydrothermally treated at mixture systems for 24 h. The solid product was filtered, washed, oven-dried overnight at 80 °C, and finally calcined at 550 °C for 6 h in air to remove the structure-directing agent.

2.1.2 Preparation of the catalysts Ni–(V, Ce, Zr)/SBA-15. Ni–(V, Ce, Zr)/SBA-15 were prepared by ultrasonic coimpregnation method. To prepare Ni/SBA-15 catalysts with various Ni loadings, a predetermined amount of nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) was dissolved in ethanol/water at 65 °C for 20 min under vigorous stirring. After addition of a specified mass of SBA-15, the mixtures were ultrasonicated for 30 min at 65 °C to well dispersed in a ultrasonic container and then stirred continuously at constant temperature until the solvent was almost removed. Next, the sample was dried at 90 °C overnight and finally calcined in a muffle furnace at 500 °C for 3 h. To prepare the Ni/SBA-15 catalysts with various V, Ce, and Zr promoters, a known amount of SBA-15 was added to solutions of Ni(NO₃)₂·6H₂O and Zr(NO₃)₄·5H₂O (NH₄VO₃ or Ce(NO₃)₃·6H₂O), respectively. Subsequent steps were the same as above. The catalysts are designated as xNi–mY/SBA-15 (Y = V, Ce, Zr), where x and m respectively represent the weight content of Ni and promoter species in the catalysts. All reagents were of AR grade (Sinopharm Chemical Reagent Co.).

2.2 Catalyst evaluation

The performance of the catalysts was studied by using a fixed-bed microreactor (40 cm length, 10 mm inner diameter). In each experiment, 0.4 g of catalyst sample (40–60 mesh) was used. Conditions consisted of reaction gases at a V(H₂)/V(CO) ratio of 3 : 1 at 1.0 MPa and total gas flow rate of 100 mL min⁻¹ (corresponding to a weight hourly space velocity (WHSV) of 15 000 mL g⁻¹ h⁻¹). Prior to the reaction, the catalysts were reduced in situ at 500 °C for 2 h under a stream of pure hydrogen (30 mL min⁻¹). The activity in CO methanation at 180–400 °C was examined, and the catalytic stability within 60 h at 500 °C was evaluated. Reaction products were analyzed on a gas chromatograph (GC900SD) equipped with a TDX-01 column and a thermal conductivity detector (TCD). Formulas for CO conversion (X_CO), CH₄ selectivity (S_CH4), and turnover frequency of CO methanation (TOF_CO) were calculated according to eqn (1)–(3):

X_CO (%) = (moles of CO reacted) × 100/(moles of CO supplied) (1)

S_CH4 (%) = (moles of CH₄ formed) × 100/(moles of CO reacted) (2)

TOF_CO (S⁻¹) = (moles of CO converted per second)/(moles of surface Ni atoms) (3)

2.3 Catalyst characterization

XRD measurements were performed on a Rigaku D/Max 2500 system equipped with a Cu Kα source (λ = 1.54056 Å) operated at 40 kV and 100 mA. The crystallite size was calculated by using the Scherrer formula.

N₂ adsorption–desorption isotherms at −196 °C were obtained on a Micromeritics ASAP 2020 specific surface area and porosity analyzer. Before measurement, the samples were degassed under vacuum at 300 °C for 4 h. The microscopic feature of the samples was observed by transmission electron microscopy (TEM) (JEM-2010F, JEOL, Japan).

Raman spectroscopic data were obtained using a Renishaw inVia microlaser Raman spectrometer, and the Raman spectra were excited by an argon ion laser (514.5 nm wavelength) with experimental power of 5 mW at a step length of 1 cm⁻¹ in the range between 200 and 1400 cm⁻¹.

XPS measurements were performed with a V.G. Scientific ESCALAB250 using Al Kα radiation (hν = 1486.6 eV). The instrument was calibrated internally by using the carbon deposit C(1s) (E_b = 284.7 eV).

FT-IR spectra of the samples were obtained on a Bruker TENSOR 27 spectrometer. Each 1 mg powdered sample was diluted with vacuum-dried IR-grade KBr.

H₂-TPR experiments were carried out with a Micromeritics AutoChem II 2920 analyzer. In a typical measurement, 50 mg of sample (40–60 mesh) was loaded into a U type quartz tube. A stream of pure Ar at 300 °C was introduced for 30 min to remove adsorbed H₂O and other volatile gases on the sample. The sample was then cooled to room temperature, and the gas was switched to a gaseous Ar mixture containing 10 vol% H₂ at a flow rate of 20 mL min⁻¹. Finally, the temperature was elevated to 900 °C at a rate of 10 °C min⁻¹. H₂ consumption was detected by using a TCD.

H₂-TPD experiments were carried out in the same system that was used in the H₂-TPR measurements. After the catalysts were reduced, H₂ desorption patterns were obtained from room temperature to 860 °C at a heating rate of 10 °C min⁻¹. The desorbed H₂ was detected by using a TCD. The number of surface Ni sites per unit mass of catalyst was calculated from the H₂-TPD results by assuming a H/Ni adsorption stoichiometry of 1 : 1. The peak area of the H₂-TPD profile was normalized according to that of the H₂-TPR profile of a standard CuO sample. The Ni dispersion (D) was calculated from eqn (4):

D (%) = (2 × V_ad × M × SF) × 100/(m × P × V_m × d_r) (4)

where V_ad (mL) represents the volume of chemisorbed H₂ at standard temperature and pressure (STP) from TPD measurements. M is the molecular weight of Ni (58 × 69 g mol⁻¹), and SF is the stoichiometric factor (the Ni/H molar ratio for chemisorption) which is taken as equal to 1. m is the sample weight (g); P is the weight fraction of Ni in the sample; V_m is the molar volume of H₂ (22 414 mL mol⁻¹) at STP; and d_r represents the degree of reduction of nickel species calculated from the H₂-TPR profile.

3. Results and discussion

3.1 Characterization of catalysts

3.1.1 XRD measurements. XRD patterns of the samples are shown in Fig. 1. The reduced catalysts 10Ni/SBA-15 and 10Ni/SBA-15 with promoters produced XRD peaks corresponding to metallic Ni crystals (JCPDS no. 65-2865) at 2θ values of 44.370°, 51.890°, and 76.410°. These peaks are respectively attributed to the (111), (200), and (220) planes of metallic Ni particles. There is a broad amorphous silica peak at around 23° for these samples. In addition, the intensities of peaks for metallic Ni phase in the catalysts with promoters are weaker than those of the 10Ni/SBA-15 catalyst. The full width at half maximum of the peak for Ni⁰ in the promoter-containing catalysts increased, suggesting that the promoters could effectively enhance Ni dispersion.

Fig. 1 XRD profiles of the reduced 10Ni/SBA-15, 5M/SBA-15 and 10Ni–5M/SBA-15 (M = V, Ce, Zr) samples.

The average crystalline sizes of metallic Ni in the catalysts (Table 1) were calculated from the XRD patterns through the Scherrer equation. Generally, highly dispersed Ni species lead to more numerous active sites on the catalysts. No new diffraction peaks on the promoter species appeared for all samples including the 5V/SBA-15, 5Ce/SBA-15 and 5Zr/SBA-15 reduced samples, indicating that the promoter species were highly dispersed on the surface of the SBA-15 support.

Table 1 Physicochemical properties of catalysts

Sample	Surface area^a (m^2 g^−1)	Pore volume^b (cm^3 g^−1)	Pore size^c (nm)	Ni average crystallite size^d (nm)	Ni dispersion^e (%)	TOFCO^f, 220 °C (×10^−3 S^−1)
a Calculated by the BET equation.b Pore volume obtained from the volume of nitrogen adsorbed at the relative pressure of 0.997.c BJH desorption average pore size.d Calculated by the XRD diffraction peak using the Scherrer equation.e Ni dispersion calculated based on H2-TPR and H2-TPD.f Calculated based on the Ni dispersion and the CO conversion at 220 °C.
SBA-15	594.6	0.99	5.7	—	—	—
10Ni/SBA-15	513.3	0.81	5.1	13.1	15.2	2.71
10Ni–5V/SBA-15	459.0	0.66	5.2	8.6	21.3	5.50
10Ni–5Ce/SBA-15	453.3	0.71	5.3	10.3	19.9	3.62
10Ni–5Zr/SBA-15	462.3	0.74	5.0	10.1	20.3	4.35

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3.1.2 FT-IR spectroscopy. Fig. 2 shows the FT-IR spectra of the samples. Asymmetrical stretching vibration of the Si–O–Si bond at about 1230 and 1091 cm⁻¹ and symmetrical stretching vibration at 803 cm⁻¹ may be discerned from the SBA-15 spectrum. The band at 961 cm⁻¹ is assigned to the asymmetrical stretching vibration of Si–OH (silanol group). The band at 463 cm⁻¹ corresponds to the bending vibration of Si–O bonds in a ring structure.²³ The asymmetric stretching vibration of the Si–O–Si bond of other samples shifted slightly toward lower wavenumbers relative to that in the SBA-15 spectrum (Fig. 2). These changes in asymmetric stretching vibration of the Si–O–Si bond are caused by addition of Ni and promoter species that are partially incorporated into the porous structures of SBA-15. The slightly lower intensity for the stretching vibration of the Si–OH bond in the Ni/SBA-15 sample at 961 cm⁻¹ compared with that for the stretching vibration in the SBA-15 sample suggests that the presence of Ni species in SBA-15 causes a structural change in the surface Si–OH group.²¹

Fig. 2 FT-IR spectra of the samples SBA-15, 10Ni/SBA-15, 20Zr/SBA-15 and 10Ni–5M/SBA-15 (M = V, Ce, Zr).

Moreover, bands at 961 cm⁻¹ of other samples with promoters correspond to vibrations of the Si–OH bond and it can be proved by the peak intensity change of 20Zr/SBA-15 at 961 cm⁻¹. From the FT-IR spectra, it was observed that the peak intensity of 20Zr/SBA-15 at 961 cm⁻¹ almost disappeared compared to SBA-15. The behavior indicates that the Zr–OH bond for the sample with the high Zr species loading almost consumes the Si–OH groups. The samples with promoters have smaller shoulder peaks and lower intensities compared with those in the spectra of SBA-15 and Ni/SBA-15. These differences are caused by the disappearance of Si–OH groups and the appearance of (Si–O)_nM=O species, suggesting that partial Si–OH groups were changed or consumed and transformed to the Si–O–M bond (M = Zr,^24,25 V,^26,27 Ce (ref. 28 and 29)).

3.1.3 Texture features. Fig. 3 presents the nitrogen adsorption–desorption isotherms and pore size distribution curves of the samples, and Table 1 lists details of the physical properties of the samples. These samples produced typical type IV isotherms with hysteresis loops, indicating that regular mesoporous structures existed in the samples.³⁰ The samples have high surface areas and pore volumes. SBA-15 loaded with metal species produced hysteresis loops that are markedly smaller than those of SBA-15, corresponding to a decrease in the specific surface area and pore volume. The specific surface area and pore volume of the SBA-15 support were 594.6 m² g⁻¹ and 0.99 cm³ g⁻¹, respectively. In addition, the 10Ni/SBA-15 catalyst had a pore size of 5.1 nm smaller than that of SBA-15, implying that some Ni species may enter and block the pores. The hysteresis loops for the catalysts with V, Ce, and Zr promoters are similar to an H1-type loop.^31,32 The catalysts with V, Ce, and Zr had smaller specific surface area and pore volume compared with those of Ni/SBA-15. These features indicate that the promoters occupy the support of the catalyst. Comparing to the SBA-15, not only the 10Ni/SBA-15 has double pores, but also the SBA-15 with V, Zr, Ce promoter show double pores, perhaps it can be revealed that addition of the species to SBA-15 contribute to the formation of the second pore. A possible explanation: the pore size at ∼6 nm is consistent with that of pure SBA-15 support, the second pore at ∼4 nm is caused by the partial Ni and promoter species located in the channel of SBA-15. Furthermore, the presence of the second pore demonstrates that the Ni species successfully embedded in the pores of mesoporous silica, which can improve the catalytic activity and stability.²¹ From the TEM images in Fig. 4, it is observed that the ordered mesostructure of the samples isn't destroyed by addition of active metallic and promoters. The pore size of the samples is ca. 5–6 nm, and there are smaller Ni species crystals located inside the pores of the SBA-15. Moreover, the crystalline size of Ni species for 10Ni–5V/SBA-15 sample is smaller than the 10Ni/SBA-15. The results is in good agreement with the conclusions of XRD and N₂ physisorption.

Fig. 3 The nitrogen adsorption–desorption isotherms (a) and pore size distribution curves (b) of the samples.

Fig. 4 TEM images of the 10Ni/SBA-15 (a) and (b) and 10Ni–5V/SBA-15 (c) and (d) catalysts.

3.1.4 Raman spectroscopy. The Raman spectra of the samples are presented in Fig. 5, the Ni/SBA-15 sample exhibits the Raman bands at 485, 807, and 970 cm⁻¹, which are assigned to tri-cyclosiloxane rings, siloxane bridges, and surface silanol groups, respectively.³³ For the Ni–V/SBA-15 sample, the 773, 846 and 956 cm⁻¹ bands may correspond to new VO_x species formed on the surface of the catalyst.³⁴ The band at 1025 cm⁻¹ is ascribed to the V=O bond stretching vibration of the isolated tetrahedral VO₂ species anchored on SBA-15. The shoulder band at 1065 cm⁻¹ is attributed to the perturbed silica vibrations for Si(–O⁻)_x functionalities, suggesting the formation of V–O–Si bond.³⁵ The Raman band at 952 cm⁻¹ for the Ni–Zr/SBA-15 sample is assigned to the Si–O–Zr linkages,³⁶ indicating that some Zr species interact with Si–OH to form Si–O–Zr bond. In addition, according to the literature,³⁷ the bands at 760, and 1147 cm⁻¹ may correspond to the intermediate phase of zirconia, and no Raman bands about tetragonal zirconia, monoclinic zirconia and zircon are observed. The results demonstrate that some Zr species on SBA-15 are amorphous zirconia or intermediate phase of zirconia, and are probably present as highly dispersed surface species. For the Raman spectrum of Ni–Ce/SBA-15 sample, the bands at 573 cm⁻¹, 1098 cm⁻¹ are assigned to the defect-induced mode and second-order longitudinal optical mode of CeO₂,³⁸ respectively. The band at ∼462 cm⁻¹ attributed to the F_2g mode of the cerium oxide overlaps with the Raman band of tri-cyclosiloxane rings. Based on the Raman features of Si–OH groups and Si–O–M (M = V, Zr) linkages, it can be concluded that the 983 cm⁻¹ band may attribute to the Si–O–Ce linkages. In a word, addition of the promoter species can lead to partially M–OH species combine with Si–OH groups to form Si–O–M bond, which is in agreement with the results of FT-IR spectroscopy. Interestingly, it is the formation of Si–O–M bond that increases the interaction between active species and support (see Section 3.1.6), which facilitates CO methanation.

Fig. 5 Raman spectra of the samples Ni/SBA-15, and Ni–M/SBA-15 (M = V, Ce, Zr).

3.1.5 XPS. Fig. 6(a) shows Ni 2p_3/2 XPS spectra of the reduced and used catalysts. Two Ni states in all catalysts can be inferred according to their binding energies (BE) in the Ni 2p_3/2 XPS spectra. The Ni 2p_3/2 peak of reduced 10Ni/SBA-15 at 852.8 eV (BE) is attributed to Ni⁰ species, and the Ni 2p_3/2 main peak at 855.8 eV with a shake-up satellite peak at 816.3 eV is assigned to the Ni²⁺ species.^19,39,40 For the 10Ni–5V/SBA-15 and the 10Ni–5Ce/SBA-15 reduced catalysts, the Ni 2p_3/2 binging energies show slight chemical shifts relative to that of Ni/SBA-15 reduced catalyst (Table 2). These shifts suggest that addition of V, Ce promoters could reduce the binding energy of Ni 2p_3/2 and increase the electronic density of nickel on the catalyst surface. The relative amounts of the various Ni oxidation states on the surface of the reduced catalysts is shown in Table 3. Compared with those of reduced 10Ni/SBA-15, the relative amounts of metallic Ni in the promoter-containing catalysts were higher. In particular, the relative amount of metallic Ni in reduced 10Ni–5V/SBA-15 was as high as 30.6%. Therefore, the promoter-containing catalysts had more numerous active Ni sites. In addition, it is observed that the Ni 2p_3/2 binging energies and the relative amount of metallic Ni of the used catalysts have almost no changes relative to their reduced counterparts. The behaviors suggest that there are no obvious changes in the Ni status (Ni species valence, Ni dispersion and number of Ni active sites) of the catalysts after the lifetime test.

Fig. 6 XPS spectra for the samples: (a) Ni 2p_3/2; (b) V 2p_3/2 and Ce 3d; (c) Zr 3d and O 1s.

Table 2 Binding energy values of the samples for XPS spectra

Samples	Binding energy/eV
	Ni 2p3/2	Zr 3d5/2	Ce 3d5/2		V 2p3/2
	Ni^2+	ZrI species (Zr^4+)	Ce^4+	Ce^3+	V^5+	V^4+	V^3+
a The error measurements is ±0.1 eV.
10Ni/SBA-15-reduced	855.8^a	—	—	—	—	—	—
10Ni/SBA-15-used	855.8	—	—	—	—	—	—
5Ce/SBA-15-reduced	—	—	882.4	885.5	—	—	—
10Ni–5Ce/SBA-15-reduced	855.6	—	882.5	885.8	—	—	—
10Ni–5Ce/SBA-15-used	855.6	—	882.5	885.9	—	—	—
5Zr/SBA-15-reduced	—	182.2	—	—	—	—	—
10Ni–5Zr/SBA-15-reduced	855.7	182.2	—	—	—	—	—
10Ni–5Zr/SBA-15-used	855.7	182.2	—	—	—	—	—
5V/SBA-15-reduced	—	—	—	—	517.4	516.0	515.1
10Ni–5V/SBA-15-reduced	855.5	—	—	—	517.6	516.4	515.5
10Ni–5V/SBA-15-used	855.5	—	—	—	517.6	516.4	515.5

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Table 3 Surface Ni, Zr, O compositions of the samples for the XPS spectra

Catalysts	Ni 2p3/2		Zr 3d5/2		O 1s
	Ni^0	Ni^2+	ZrI species	ZrII species	OI species	OII species
a Percentage of Ni, Zr and O species.
10Ni/SBA-15-reduced	16.9^a	83.1^a	—	—	—	—
10Ni/SBA-15-used	16.4	83.6	—	—	—	—
5Ce/SBA-15-reduced	—	—	—	—	—	—
10Ni–5Ce/SBA-15-reduced	25.0	75.0	—	—	—	—
10Ni–5Ce/SBA-15-used	24.2	75.8	—	—	—	—
5Zr/SBA-15-reduced	—	—	79.6^a	20.4^a	81.2^a	18.8^a
10Ni–5Zr/SBA-15-reduced	25.8	74.2	74.5	25.5	75.7	24.3
10Ni–5Zr/SBA-15-used	26.5	73.5	74.2	25.8	75.5	24.5
5V/SBA-15-reduced	—	—	—	—	—	—
10Ni–5V/SBA-15-reduced	30.6	69.4	—	—	—	—
10Ni–5V/SBA-15-used	30.1	69.9	—	—	—	—

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The V 2p_3/2 XPS spectrum of reduced 10Ni–5V/SBA-15 catalyst was fitted with peaks at 517.6, 516.4, and 515.5 eV (BE) in Fig. 6(b). These peaks correspond to V⁵⁺, V⁴⁺, and V³⁺ (ref. 41–43), respectively. It is observed that the 10Ni–5V/SBA-15 reduced catalyst has higher V 2p_3/2 binding energy (Table 2 and Fig. 6(b)) relative to the 5V/SBA-15 sample, implying the occurrence of electron transfer from V species to Ni,⁴⁴ and which further enhances the Ni electron cloud density. As the V⁴⁺ peaks are broader than those of V⁵⁺ and V³⁺ (Fig. 6(b)), high-valence V species can undergo reduction to V⁴⁺ and V³⁺ by reaction gas during CO methanation.⁴² Mori et al.⁴⁵ observed that the promoter V plays an important role in CO dissociation by interacting with charged V³⁺ centers and with oxygen atoms in chemisorbed CO. It was observed that the portion of V⁴⁺ in 10Ni–5V/SBA-15 used catalyst was increased compared with that in the 10Ni–5V/SBA-15 reduced catalyst, which should be caused by the oxidation of V³⁺ or the reduction of V⁵⁺ in CO methanation, and it has been proved by previous literature data.⁴⁶ In addition, based on the XPS, XRD, Raman spectroscopy, and H₂-TPR (see Section 3.1.6), there are the metallic Ni, Ni²⁺ species (nickel oxide), and the VO_x species (V₂O₅, VO₂, V₂O₃) on the surface of the 10Ni–5V/SBA-15 reduced catalyst. Combined with the results of Ni 2p_3/2 XPS spectra, it can be found that the addition of V promoter reduces the binding energy of Ni 2p relative to that of Ni/SBA-15 catalyst (Ni 2p from 855.8 for the 10Ni/SBA-15 catalyst to 855.5 for the 10Ni–5V/SBA-15), which may be caused by the coexistence of the V³⁺, V⁴⁺, and V⁵⁺ that increase the electron cloud density of metallic Ni atoms (Ni⁰ → Ni^δ−). Thus, V³⁺, V⁴⁺, and V⁵⁺ that coexist in reduced 10Ni/5V–SBA-15 could trigger electron transfer and promote CO dissociation through the oxidation–reduction shift cycle (V³⁺ ↔ V⁴⁺ ↔ V⁵⁺). These behaviors confirm that the catalyst 10Ni/5V–SBA-15 has high activity and high CH₄ selectivity.

XPS spectra of Ce 3d of the promoter-containing samples are shown in Fig. 6(b). For the high-resolution spectra of Ce 3d_5/2 and Ce 3d_3/2 of the samples with Ce promoter, ionization features were fitted with eight Gaussian distributions representing different states in the Ce 3d core-level XPS spectrum. Some investigations based on Ce 3d XPS analysis have revealed that fit for the Ce 3d Gaussian peak corresponds to Ce³⁺ and Ce⁴⁺ states.^47–49 Peaks in the Ce 3d XPS spectrum of the 10Ni–5Ce/SBA-15 reduced catalyst at 907.8 and 916.7 eV (BE) are ascribed to Ce 3d_3/2 ionization of Ce⁴⁺. The peak at 903.7 eV (BE) is assigned to Ce³⁺ 3d_3/2. Bands at 882.5, 884.3, 898.6, and 900.9 eV (BE) correspond to Ce⁴⁺ 3d_5/2, and the peak at 885.8 eV (BE) is attributable to Ce³⁺ 3d_5/2. So based on the XPS, XRD, Raman spectroscopy, and H₂-TPR (see Section 3.1.6), the surface of the 10Ni–5Ce/SBA-15 reduced catalyst shows the presence of metallic Ni, Ni²⁺ species (nickel oxide), highly dispersed CeO₂, and the reduced Ce³⁺ species. Moreover, it can be found that Ce species with various oxidation states (Ce³⁺ and Ce⁴⁺) co-exist in the reduced catalysts, which is attributed to the partial reduction of Ce⁴⁺ to Ce³⁺ during catalyst reduction. Therefore, in the activation process, it can be concluded that the CeO₂ species migrating to metallic Ni surface will be partially reduced into Ce³⁺ due to the catalytic action of Ni and H spillover, which can trigger electron migration and increase the density of the electron cloud of Ni species. These behaviors ultimately improve the ability of CO to dissociate and the catalytic performance.

Fig. 6(c) shows that Zr 3d and O 1s XPS spectra of the samples with Zr species. All the samples exhibit a spin–orbit doublet of the Zr 3d core level into 3d_5/2 and 3d_3/2 levels. It can be observed that there is an energy gap of 2.4 eV between Zr 3d_5/2 and Zr 3d_3/2, implying the existence of ZrO₂-like species.⁵⁰ Moreover, XPS peaks of Zr 3d core level correspond to two kinds of zirconium species, which are referred as species I (Zr_I) with low BE in the range 181.9–182.4 eV and species II (Zr_II) with higher BE in the range 183.2–183.9 eV, respectively. Among two kinds of zirconium species, the binding energy of Zr_I species in the reduced samples is similar to Zr⁴⁺ in pure ZrO₂. However, the binding energy of Zr⁴⁺ (182.2) in the reduced samples has slightly lower value relative to that of stoichiometric ZrO₂ (182.6 eV) and a shift of 0.4 eV between them indicates the existence of some oxygen deficiency,⁵¹ which are conducive to CO dissociation. The position of Zr_I species shifts toward the lower binding energy may be associated with the holes created by oxygen vacancies in the zirconia lattice.⁵² The Zr_II species (183.2–183.9 eV) is attributed to the formation of partially reduced Zr^δ+ sites. Meanwhile, the integrated area ratio of Zr_II/(Zr_II + Zr_I) for the 10Ni–5Zr/SBA-15 reduced sample (25.5%) in Table 3 is higher than that for the 5Zr/SBA-15 reduced sample (20.4%), revealing the increase of oxygen vacancies.⁵¹ The peak of the O 1s can be fitted into two peaks in Fig. 6(c): the peak at 531.1 eV corresponds to the O_I species (lattice oxygen), the peak at 532.5 eV belongs to the O_II species (adsorbed oxygen), respectively; and the 10Ni–5Zr/SBA-15 reduced catalyst has higher integrated area ratio of O_II/(O_II + O_I) than that of the 5Zr/SBA-15 (from 18.8% to 24.3%), the behavior also implies an increase in the number of oxygen vacancies, which is consistent with the conclusion of the Zr_II/(Zr_II + Zr_I). Comparing to the 10Ni–5Zr/SBA-15 reduced catalyst, there are no obvious changes for Zr 3d and O 1s spectra of the 10Ni–5Zr/SBA-15 used catalysts, suggesting that the Zr and O status of 10Ni–5Zr/SBA-15 catalyst surface has little changes after the lifetime test. In addition, combined with the results of XRD, Raman spectroscopy, and H₂-TPR (see Section 3.1.6), it can be concluded that metallic Ni, Ni²⁺ species (nickel oxide), amorphous zirconia or intermediate phase of zirconia containing partially reduced Zr^δ+ species are present on the surface of the 10Ni–5Zr/SBA-15 reduced catalyst. In a word, Zr species contributed to the generation of the oxygen vacancies during calcination and reduction, thereby enhancing the ability of CO to adsorb and dissociate. CO adsorbed on an oxygen vacancy and subsequently reacted with hydrogen, producing CH₄.^6,47,53 These observations show that Zr could improve the catalytic activity and CH₄ selectivity in CO methanation.

The addition of V, and Ce promoters reduce the binding energy of Ni 2p_3/2 (Fig. 6 and Table 2), and it is found that the catalytic performances of the Ni/SBA-15 catalysts with promoters are significantly improved at 240 °C with the decrease of the binding energy of Ni 2p_3/2 relative to the Ni/SBA-15. As we all know, there are 9.4 electrons existing in the 3d electron orbit of nickel atoms and 0.6 electron in the 4s electron orbit, thus, the 0.6 electron hole of the 3d electron orbit of nickel atoms can accept the electrons of other atoms or ions. On one hand, during the activation process, the promoter species M_xO_y (M = V, Ce) can transfer to the Ni⁰ surface, which will be partially reduced into low valence (V^<5+, Ce^<4+) on account of catalytic action of Ni and H spillover. The behaviors can reduce the binding energy of Ni 2p_3/2 and increase the electron density of Ni⁰ species. On the other hand, in the reaction process, the increase of the electron density of Ni⁰ species (Ni⁰ → Ni^δ−) can enhance the Ni–C bond and weaken the C–O bond of the Ni–C–O bond,⁵⁴ which improve the ability of CO to dissociate. In a word, a plausible explanation for the enhancement of CO methanation activity over promoter-containing catalysts is summarized as follows: changes in valences of the promoter species V and Ce (M^x+ ↔ M^y+, M = V, Ce) can trigger electron transfer and can enhance the electron density of Ni⁰ species (Ni⁰ → Ni^δ−), which weaken the C–O bond of the Ni–C–O bond. The promoter Zr species can induce the formation of oxygen vacancies during calcination and reduction. Such behaviors improve the ability of CO to dissociate and thus enhance the activity and selectivity of the catalysts with V, Ce, and Zr promoters in CO methanation.

3.1.6 H₂-TPR measurements. The interaction between NiO species and supports was investigated through H₂-TPR measurements. Fig. 7 presents H₂-TPR profiles of the samples, for the 5V/SBA-15 sample, the reduction peak at 400–600 °C corresponds to the reduction of VO_x species, and the amount of H₂ consumption during the reduction process indicates that the reduction of V₂O₅ to V₂O₃ occurs in this temperature range. The 5Ce/SBA-15 has reduction peak at 495 °C, which is attributed to the reduction of ceria species. It is observed that the 5Zr/SBA-15 has only a weak reduction peak at high temperature, which belongs to the partial reduction of Zr oxide species. For the catalysts, according to the peak positions, reduction of NiO species of the catalysts can be divided into three stages, namely, α-, β-, and γ-stages. The α-stage is assigned to the reduction of free NiO species interacting weakly with the support in the low-temperature range (300–360 °C). The β-stage is attributed to NiO species that moderately interact with the support. These species can be reduced between 370–510 °C. The γ-stage corresponds to NiO species that strongly interact with the support and undergo reduction at high temperature (520–630 °C). We found that T_max of reduction peaks of the three stages over the promoter-containing catalysts is slightly shifted to higher temperature relative to that of the 10Ni/SBA-15 catalyst. This shift is consistent with a previous finding⁵⁵ and implies stronger interaction between the NiO species and support. The result may be associated with the formed Si–O–M bond (confirmed by the FT-IR and Raman) via improving the acid strength and amounts of acid sites of the support can affect and increase the interaction between active species and the support,^17,26,29 thereby enhancing the activity and stability of the catalysts. Moreover, the dispersion of active metal on the support is strongly influenced by metal-support interaction. Areas of the reduction peaks of the catalysts with promoters are markedly larger than those of 10Ni/SBA-15. This difference suggests that the promoters could effectively improve Ni dispersion. Therefore, catalysts with the promoters V, Ce, and Zr possessed higher Ni dispersion, and more numerous active Ni species after reduction. Among the catalysts, 10Ni–5V/SBA-15 showed the highest Ni dispersion.

Fig. 7 H₂-TPR profiles of the 10Ni/SBA-15, 5M/SBA-15 and 10Ni–5M/SBA-15 (M = V, Ce, Zr) samples.

3.1.7 H₂-TPD measurements. Fig. 8 shows H₂-TPD profiles of all samples. 10Ni/SBA-15 produced only two peaks in the low-temperature range (<400 °C). In contrast, catalysts with V, Ce, and Zr promoters produced several peaks at lower (<400 °C) and higher (>400 °C) temperatures. The lower-temperature peaks can be assigned to H₂ that is weakly chemisorbed on a surface with highly dispersed Ni and a high density of defects. Such defects can act as traps during surface hydrogen diffusion and thus reduce the activation energy of H₂ dissociation.⁵⁶ The higher-temperature peaks can be due to H₂ that is strongly chemisorbed to the catalyst surface. It may also be due to desorption of hydrogen adsorbed on promoters with low valence or spillovered hydrogen, which enhance the storage capacity for H₂.^57–59 However, for the SBA-15 samples with promoters, there is no H₂ desorption peak for 5Zr/SBA-15 samples, but the 5V/SBA-15 and 5Ce/SBA-15 samples show a weak H₂ desorption peak at high temperature, which may be associated with the spillover of H₂.

Fig. 8 H₂-TPD profiles of the 10Ni/SBA-15, 5M/SBA-15 and 10Ni–5M/SBA-15 (M = V, Ce, Zr) samples.

Integrated areas for the catalysts with various promoters are markedly larger than that of the 10Ni/SBA-15 catalyst, demonstrating that catalysts with promoters had higher Ni dispersion and that more surface-dissociated hydrogen formed on the catalysts. Table 1 lists the dispersion of Ni calculated from the H₂-TPD and H₂-TPR results. Since 10Ni–5V/SBA-15 had the strongest ability for H₂ adsorption and highest Ni dispersion (21.3%), the promoter V had the best promoting effect. Luo et al.^57,60 found that H₂ chemisorbed on the Rh surface at high temperature over Rh–V/SiO₂ catalyst could reach and remain in the lower-valence V. During desorption, the stored hydrogen could flow back to Rh and then desorb from Rh. A similar observation in Ni–V/Al₂O₃ catalyst was reported by Liu et al.¹⁹ On the basis of these studies and combined with the results of H₂-TPD, it is hypothesized that the promoter Ce plays a similar role as V in causing the spread of H₂ from Ni to low-valence promoter species and in enhancing hydrogen storage and migration ability, which eventually improve the catalytic activity and selectivity (see Section 3.2.3). However, this phenomenon is not evident in the case of Zr promoter.

3.2 CO methanation over Ni–M/SBA-15

3.2.1 Catalytic activities. Fig. 9 shows the catalytic performance of Ni/SBA-15 catalysts in CO methanation at 1.0 MPa at a WHSV of 15 000 mL g⁻¹ h⁻¹ at 180–400 °C. As described in Fig. 9, 5Ni/SBA-15 had low CH₄ selectivity at 180–400 °C, and the amount of Ni species supported on the SBA-15 support was >5 wt%. The CO conversion and CH₄ selectivity was markedly greater than that of 5Ni/SBA-15. Thus, the catalytic activity and CH₄ selectivity increased with the increase in Ni loading; but when the amount of Ni species reached 20 wt%, the catalytic performance has almost no change. Furthermore, SBA-15 had larger specific surface area, which increased Ni dispersion and increased the number of active sites of catalysts with low Ni loading and thus improved their catalytic performance. The CO conversion and CH₄ selectivity on 10Ni/SBA-15 reached 99.8% and 89.2%, respectively. Notably, in terms of the stability test, the 10Ni/SBA-15 catalyst with low loading exhibits superior stability than 25Ni/SBA-15 (Fig. 9(c)), which is caused by that active Ni species with high loading is more likely to sintering. So, 10% of Ni on SBA-15 is chosen as the optimum loading for the further study of the role of promoters.

Fig. 9 CO conversion (a) and CH₄ selectivity (b) of the Ni/SBA-15 catalysts with different Ni loadings (reaction conditions: P = 1 MPa, WHSV = 15 000 mL g⁻¹ h⁻¹, H₂/CO = 3), (c) 60 hour stability results of the 10Ni/SBA-15 and 25Ni/SBA-15 catalysts (reaction conditions: P = 1 MPa, WHSV = 15 000 mL g⁻¹ h⁻¹, H₂/CO = 3).

3.2.2 Effects of V, Ce, and Zr promoters. Test results for catalysts with V, Ce, and Zr promoters are shown in Fig. 10. All catalysts exhibited higher activity, and the temperature for maximum CO conversion shifted to a temperature lower than that for maximum CO conversion over 10Ni/SBA-15. These changes indicate that the promoters improved the catalytic activity of Ni/SBA-15 catalysts. Based on Zr promoter, catalysts with various Zr loadings (3, 5, and 7 wt%) were used in CO methanation. When the Zr loading was 5 wt%, the 10Ni–5Zr/SBA-15 catalyst displayed the highest activity, and CH₄ selectivity reached 93.6%; we could achieve higher Ni dispersion and greater number of active sites than those of catalysts with other Zr loadings (Fig. 10). It is worth noting that the decrease in activity and CH₄ selectivity with increasing Zr loading may be ascribed to partial coverage of Ni active sites. Furthermore, we investigated the activity and CH₄ selectivity of the catalysts with V and Ce promoters (5 wt%). CO conversion over 10Ni–5Ce/SBA-15 reached 99.8%, and the CH₄ selectivity reached 92.1%. 10Ni–5V/SBA-15 exhibited the highest activity and CH₄ selectivity (99.9% CO conversion, 95.5% CH₄ selectivity). Compared with that of 10Ni/SBA-15, the performance of the catalysts with V, Ce, and Zr promoters significantly increased at 240 °C, indicating that catalysts with promoters exhibited high catalytic performance at low temperature.

Fig. 10 CO conversion (a) and CH₄ selectivity (b) of the Ni/SBA-15 catalysts with V, Ce, Zr promoters (reaction conditions: P = 1 MPa, WHSV = 15 000 mL g⁻¹ h⁻¹, H₂/CO = 3).

On the basis of detailed characterization, the improved catalytic performance of Ni/SBA-15 catalysts with V, Ce, and Zr promoters can be explained by several main factors. On one hand, formation of the Si–O–M bond (M = V, Ce, Zr) via addition of promoters can increase the interaction between Ni species and support. On the other hand, promoter species undergoing the oxidation–reduction shift cycle (M^x+ ↔ M^y+, M = V, Ce) can trigger electron transfer and significantly increase the electron cloud density of nickel species (Ni⁰ → Ni^δ−), thereby weakening the C–O bond of the Ni–C–O bond and improving the ability of CO to dissociate. Compared with promoter Ce species, the variation among different valences of V can produce stronger electronic effect during the oxidation–reduction shift cycle process (confirmed by XPS) since V metal oxide species are more easily reduced from high valence to low valence,¹⁸ consequently making the Ni/SBA-15 catalyst exhibits better catalytic performance. The promoter Zr species could cause the formation of oxygen vacancies during calcination and reduction, thus enhancing the ability of CO to adsorb and dissociate. Furthermore, all promoters could improve Ni dispersion, thereby increasing the number of active Ni species and the ability of H₂ to adsorb. In particular, V contributed the greatest improvement. In summary, Ni/SBA-15 catalysts with V, Ce, and Zr promoters exhibited higher catalytic performances than Ni/SBA-15. Among the catalysts, 10Ni–5V/SBA-15 showed the highest activity and CH₄ selectivity.

3.2.3 Catalyst stability. The catalyst for CO methanation not only exhibited superior catalytic performance, but could also remain stable in the long-duration tests. The stability of 10Ni–5V/SBA-15 at 550 °C under 1.0 MPa and a WHSV of 15 000 mL g⁻¹ h⁻¹ was evaluated. Fig. 11 shows that the CH₄ selectivity of 10Ni–5V/SBA-15 slightly decreased from 93.3 to 90.2% during 60 h of the test, whereas the CO conversion did not obviously change (from 99.8 to 99.2%). There are two possible reasons for such behavior: (1) the water gas shift reaction (CO + H₂O → CO₂ + H₂) slightly increases levels of the byproduct CO₂. (2) Carbon deposition at high temperature leads to a slight decrease in catalytic activity. 10Ni–5V/SBA-15 exhibited high activity and stability at high temperature, which because the promoter V prevents sintering of active species at high temperature.

Fig. 11 60 hour stability results of the 10Ni–5V/SBA-15 catalyst (reaction conditions: P = 1 MPa, WHSV = 15 000 mL g⁻¹ h⁻¹, H₂/CO = 3).

Furthermore, it is widely accepted that the lower vanadium species can effectively enhance the CO dissociation rate.⁴⁵ The obtained XPS spectra indicate that during CO methanation, the change in oxidation state of V gave rise to an electronic effect. It subsequently enhanced the density of the electron cloud of active Ni⁰ species through the oxidation–reduction shift cycle (V³⁺ ↔ V⁴⁺ ↔ V⁵⁺), which promotes CO dissociation and improves CH₄ selectivity. The shift cycle is due to the oxidation characteristics of V species and the reduction properties of the reactant gas. Thus, the results demonstrate that the catalyst 10Ni–5V/SBA-15 is a promising candidate for use in methanation.

3.3 Apparent activation energies

Sehested et al.⁶¹ reported that CO dissociation at the nickel active sites is the rate-determining step of CO methanation. During methanation, CO adsorbed on the metallic Ni atom surface dissociated and formed adsorbed carbon, which then combined with activated H and formed CH₄. Thus, the apparent activation energy of CO dissociation acted as the reaction activation energy. The relation between the logarithm of the activity of CO methanation and the reciprocal of temperature (Fig. 12) is described by the Arrhenius formula. The apparent activation energies of CO dissociation over the Ni-based catalysts are obtained from the slope of the trend lines. As given in Fig. 12, the apparent activation energies over various catalysts decrease in the order 10Ni/SBA-15 (131.72 kJ mol⁻¹) > 10Ni–5Ce/SBA-15 (120.36 kJ mol⁻¹) > 10Ni–5Zr/SBA-15 (115.61 kJ mol⁻¹) > 10Ni–5V/SBA-15 (103.53 kJ mol⁻¹). Therefore, addition of promoters enhanced the ability of CO to dissociate and reduced reaction energy barriers. XRD and XPS results together suggest that addition of V, Ce, and Zr promoters could improve the Ni dispersion and increase the Ni⁰ content of the catalyst surface, thus enhancing the active sites of metallic Ni. Changes in the valences of V and Ce promoter species could initiate electron transfer and could increase the electron cloud density of Ni⁰ species (Ni⁰ → Ni^δ−), thereby weakening the C–O bond of the Ni–C–O bond, increasing the ability of CO to dissociation, and reducing the reaction energy barriers.

Fig. 12 Arrhenius plots for CO methanation on the 10Ni/SBA-15 and 10Ni–5M/SBA-15 (M = V, Ce, Zr) catalysts.

Table 1 lists the calculated TOF values of the catalysts in CO methanation at 220 °C at low level of CO conversion and at high WHSV (15 000 mL g⁻¹ h⁻¹). TOF values of CO methanation over the catalysts increase follow the order 10Ni/SBA-15 (2.71 × 10⁻³ s⁻¹) < 10Ni–5Ce/SBA-15 (3.62 × 10⁻³ s⁻¹) < 10Ni–5Zr/SBA-15 (4.35 × 10⁻³ s⁻¹) < 10Ni–5V/SBA-15 (5.50 × 10⁻³ s⁻¹), further confirming that addition of promoters enhanced the activity and CH₄ selectivity of the catalysts in CO methanation.

4. Conclusions

Highly efficient Ni–(V, Ce, Zr)/SBA-15 catalysts for CO methanation were prepared by ultrasonic coimpregnation method. We examined the performance of the catalysts at 1.0 MPa at a WHSV of 15 000 mL g⁻¹ h⁻¹ at 180–400 °C. Notably, 10Ni/SBA-15 catalysts with V, Zr, and Ce promoters showed catalytic performances higher than that of 10Ni/SBA-15. Among the catalysts, 10Ni–5V/SBA-15 exhibited the highest catalytic activity and CH₄ selectivity (99.9% CO conversion, 95.5% CH₄ selectivity) and displayed high stability in a 60 h life test at 550 °C.

Characterization using H₂-TPR, H₂-TPD, XPS, and XRD measurements revealed that promoters enhanced the Ni dispersion. This enhancement led to enhanced exposure of Ni active sites, which improved the ability of H₂ to adsorb and dissociate, accounting for the excellent catalytic performances of the catalysts in CO methanation. Addition of V and Ce promoters to Ni/SBA-15 catalysts could effectively improve the ability of CO to dissociate via electronic effect in methanation. Zr promoter species led to the formation of oxygen vacancies during calcination and reduction, thereby increasing the ability of CO to adsorb and dissociate in CO methanation. These behaviors could lower the reaction energy barriers and could increase the catalytic activity and CH₄ selectivity. Results of FT-IR spectroscopy, Raman spectroscopy and H₂-TPR measurements, revealed that addition of promoters increased the interaction between the active species and support via the Si–O–M bond. All of the promoters could prevent sintering of active species, thereby contributing to enhancement of the activity and stability of the Ni–(V, Zr, Ce)/SBA-15 catalysts.

Because of its smallest Ni particle size, highest Ni dispersion, greatest number of Ni active sites, strongest H₂ adsorption ability, and strongest electronic effect, 10Ni–5V/SBA-15 had higher catalytic activity, as well as CH₄ selectivity and stability in CO methanation. Therefore, 10Ni–5V/SBA-15 may be a promising catalyst for CO methanation.

Acknowledgements

This work has been supported by a grant from the National Natural Science Foundation of China (21376159 and 21276169).

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