Positive Effects of CeO₂ Promoter and Co-Reactant/CO on Methanation of CO₂-Rich Gas over Ni/SBA-15 Catalyst

Abstract

NiO/SBA-15 catalysts modified by CeO₂ were synthesized and studied for the hydrogenation of CO₂-rich gas. Effect of CeO₂ promoter on the physicochemical properties of the catalysts was investigated by BET, XRD, Raman spectroscopy, SEM, TEM, H₂-TPR and CO₂-TPD methods. The 4 mass% CeO₂ addition improved reduction and dispersion of Ni, resulting in its higher activity in methanation, reaching CO₂ conversion of 90% at 350°C. Furthermore, the performance of the prepared catalysts in methanation reaction was enhanced by adding small amount of CO (1 mol%) in the feedstock.

Fig. 11 CO₂ conversion in methanation of mixture of 19 mol% CO₂ with 1 mol% CO (case I - dashed lines) and with 1 mol% N₂ (case II - solid lines) on Ni4Ce/SBA catalyst.

1. Introduction

It was observed that global temperature has increased in parallel with the atmospheric concentration of carbon dioxide that has been constantly rising, from 375 ppm in 2001 to 390.5 ppm in 2011, 403.53 ppm in 2016, and 500 ppm within the next 50 years.^1,2) As a result, the highest temperature which was about 1.1°C higher than the pre-industrial level was recorded in 2016 since 1880. Therefore, if greenhouse gas emissions cannot be properly reduced, the global temperature increase will soon reach the red line at which sea level rises more rapidly and extreme weather will happen more frequently.²⁾ The elimination of CO₂ has been attracting interest from the environmental perspective with various methods.³⁾

Methanation of CO₂, which is abundant in the exhaust gas and a “zero-calorific value” greenhouse gas,⁴⁾ for on-site production of CH₄ fuel could be a promising and effective strategy for CO₂ emissions mitigation.^5,6) Currently, nickel-based catalysts are good candidates because of its lower cost and comparable catalytic activity for methanation⁶⁾ despite exhibiting lower stability and resistance against coke deposition compared to the noble metal catalysts.^7,8) Moreover, using various traditional supports, such as γ-Al₂O₃, SiO₂, TiO₂ has been shown to be an effective way to improve the activity of Ni-based catalysts.⁹⁾ Basically, high nickel loading is always desirable, however, the optimal value has been found to be strongly dependent on the nature of supports since excessive loading can cause blockage of their channels.¹⁰⁾ It has been reported that NiO can be loaded up to 10 mass% on Zn_xMg_1−xAl₂O₄,⁶⁾ 20 mass% on Al₂O₃,¹¹⁾ 20.7 mass% on ZrO₂¹²⁾ and especially up to 50 mass% on ordered mesoporous SBA-15¹³⁾ thanks to its excellent features such as high surface area, large pore volume, thermal stability and uniform pore size distribution. Nevertheless, it has been reported that Ni particles could not stay stable inside the mesoporous channels of the silica support at elevated temperatures due to their naturally weak interaction.^14,15) Fortunately, this interaction could be strengthened by using another oxide such as TiO₂, ZrO₂, or CeO₂.^16,17) Among them, CeO₂ is used as a catalyst support in CO oxidation, NO_x purification reaction, and water gas shift reaction.¹⁸⁾ Besides that, CeO₂ has been widely studied as a promising promoter due to its excellent capability of: (i) stabilizing metal particles against thermal sintering,¹⁹⁾ (ii) modifying the structural and electronic properties of Ni,²⁰⁾ (iii) enhancing the interaction between Ni particles and support,²¹⁾ thus, (iv) controlling the growth of the active metal.²²⁾ In addition, it was reported that CeO₂ could promote effectively the adsorption and activation of CO₂ molecules on the MgAl₂O₄ supported NiCeO₂ZrO₂ catalyst due to its oxygen exchange capacity.²³⁾ Besides, CeO₂-modified NiO catalysts are generally less prone to coke deposition.²⁴⁾ The interaction of Ni²⁺ with CeO₂ leads to the formation of Ce₂O₃ and oxygen vacancies, enhancing dissociative adsorption of CO₂. This, in turn, improves the coke resistance of CeO₂-doped catalysts, as confirmed by Li et al.²⁵⁾

In this study, the effect of CeO₂ promoter on the physico-chemical properties and catalytic performance of the Ni-based catalysts supported on SBA-15 was particularly investigated. Besides, adding a small amount of co-reactant/CO into the reaction feedstock to improve the process efficiency of CO₂ methanation in terms of catalytic performance was also studied.

2. Experimental

SBA-15 was synthesized by the combination of sol–gel and hydrothermal methods according to the report by authors:²⁶⁾ 4 gram of Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol) (P123) (Sigma-Aldrich) was diluted by deionized water to form 105 mL solution; afterthat, 8.8 mL of Tetraethyl orthosilicate (TEOS) (≥99.9%, Merck) was added drop by drop to the above solution with vigorous stirring, followed by adding 24.2 mL of HCl (≥99.9%, Merck); after further stirring for 30 minutes, the mixture was transferred to a Teflon autoclave for hydrothermal treatment at 60°C for 24 hours. After that, the solid was filtered off, washed many times with deionized water and ethanol, and calcined at 550°C for 10 hours to obtain SBA-15.

The CeO₂-modified 50 mass% NiO/SBA-15 catalyst was prepared by the co-impregnation method from the precursor solutions using Ni(NO₃)₂·6H₂O (99.8 mass%, Prolabo) and Ce(NO₃)₃·6H₂O (99.8 mass%, Merck) at room temperature for 5 hours with ultrasound assistance. After stirring at 80°C and 100°C for 2 hours at each temperature, the sample was calcined in air at 600°C for 4 hours. The obtained catalysts were denoted as NixCe/SBA, where x = 4–5 mass%. The Ce/Si atomic ratio was in the range of 0.028–0.036, being below the limited value of 0.04.²⁷⁾

The characteristics of obtained catalysts were determined by different methods. X-ray powder diffraction (XRD) using Bruker D2 Phaser powder diffractometer with CuKα radiation (λ = 0.15406 nm) was used to investigate the crystalline structure of the catalysts. Raman spectroscopy measurements were carried out at room temperature with a laser Raman spectrometer (Invia, Renishaw). Morphology of catalysts and particle size of the active phase was characterized by scanning electron microscopy (SEM) on the FE-SEM JEOL 7401 instrument and transmission electron microscopy (TEM) on the TEM JEM 1400 of Jeol USA instrument. The hydrogen temperature-programmed reduction (H₂-TPR) measurement was performed in a fixed-bed reactor at atmospheric pressure by using a Gas Chromatograph GOW-MAC 69-350 with a TCD detector and the reduction gas of 10 mol% H₂/N₂ was introduced. The temperature of the reactor was raised linearly from 50 to 900°C at a heating rate of 10°C·min⁻¹. The carbon dioxide temperature-programmed desorption (CO₂-TPD) experiment was carried out to determine the basicity of the catalyst. This technique was carried out in a series of steps. Firstly, catalyst was reduced in H₂ at 450°C for 1 hour and cooled down to room temperature, followed by CO₂ adsorption for 1 hour. After purging with N₂ for 1 hour at 50°C to remove gas-phase and physically adsorbed CO₂, the sample was heated from 50 to 700°C at a rate of 10°C·min⁻¹. The desorbed CO₂ was then monitored online by using a Gas Chromatograph GOWMAC 69-350 with a thermal conductivity detector (TCD).

Methanation of CO₂-rich gas was tested in a standard micro-flow reactor with and internal diameter of 10 mm under atmospheric pressure and a temperature range of 250–350°C. The loaded catalyst (0.2 grams) was mixed with quartz (0.8 grams, 99.95 mass%, Sigma-Aldrich) with a size range of 0.25–0.5 mm. The CO₂/H₂ molar ratio was 1/4 and gas hourly space velocity (GHSV) was 15,000 h⁻¹. For common experiments, the CO₂ concentration in feed was 20 mol%; for studying the effect of CO, the mixture of 19 mol% CO₂ + 1 mol% CO (case I) and 19 mol% CO₂ + 1 mol% N₂ (case II) were used. Before carrying out the reaction, the catalyst was reduced in-situ in H₂ flow at 450°C for 4 hours. The reaction mixture was analyzed using the Agilent 6890 Plus Gas Chromatograph equipped with a TCD detector (capillary column HP-PLOT MoleSieve 5A) and an FID detector (capillary column DB624).

The CO₂ adsorption capacity at the typical reaction temperature of the samples reduced was also assessed. Firstly, 0.2 grams of the catalyst was reduced in H₂ at 450°C for 4 hours and cooled down to a typical reaction temperature of 350°C. Then, the adsorption gas of 60,000 ppm CO₂ in N₂ was introduced. CO₂ concentration with time is continuously recorded online by CO₂ sensor (GMP251, Vaisala) connected with the computer using Senko Sensor Monitor. The adsorption process terminated when CO₂ concentration in the output reached 60,000 ppm. The CO₂ adsorption capacity ($\textit{mg}_{\textit{CO}_{2}}\cdot g_{\textit{cat}}^{ - 1}$) was calculated by eq. (1).   

\begin{equation} A_{\textit{CO${_{2}}$},\textit{ads}} = \frac{\biggl(C_{o}\cdot t - \displaystyle\int\limits_{0}^{t}C (t)\biggr)\cdot 10^{-3}\cdot \frac{V}{3600}\cdot M_{\textit{CO${_{2}}$}}}{22.4m_{\textit{cat}}} \end{equation} (1)

where: C_o is the CO₂ concentration in the input, 60,000 ppm; t is the saturated adsorption time, s; C(t) is the function CO₂ concentration by time in the output; V is the flow of the adsorption gas, l·h⁻¹; $M_{CO_{2}}$ is the CO₂ molar mass of 44.01 g·mol⁻¹; m_cat is the loaded catalyst mass.

3. Results and Discussions

3.1 Physico-chemical characteristics

It could be observed from the low-angle XRD pattern for bare SBA-15 support (Fig. 1) that there were two-dimensional SBA-15 an intensive main diffraction peak 2θ = 0.9° and two small peaks at 2θ of 1.61° and 1.84°, indexed as the corresponding (100), (110) and (200) reflections of two dimensional, ordered hexagonal meso-structure.²⁸⁾ These demonstrated that a two-dimensional SBA-15 was successfully synthesized. The low-angle diffraction pattern of the as-synthesized Ni/SBA-15 catalyst exhibited the main peak corresponding to (100) reflection with low intensity and shifts toward a larger angle. The remaining two reflections almost disappeared. The same phenomenon was observed on Fe/SBA-15, TiO₂/SBA-15, NiO/SBA-15 catalysts^29–31) and boron-doped Ni/SBA-15 catalysts.³²⁾ This was explained by the presence of the nitrate salt (Ni(NO₃)₂) in an amorphous state leading to good dispersion of metal on the surface of supports.³⁰⁾

Fig. 1

The low angles XRD patterns of SBA-15 and Ni/SBA-15.

The characteristic peaks of NiO crystals appear sharply at 2θ = 37.3, 43.3 and 62.8° (JCPDS card No. 47-1049) while diffraction peaks of CeO₂ were not observed on XRD patterns of the CeO₂-doped catalysts (Fig. 2). This fact indicated the high crystallinity of NiO and amorphous or well-dispersed state of CeO₂ on the surface. Aghamiri et al.³³⁾ also believed that Ce affected the structure of the catalyst leading to an increase in the Ni dispersion on SiO₂ surface. XRD patterns of the catalysts reduced at 450°C for 4 hours in the H₂ flow of 2 L·h⁻¹ showed a new phase assigned to metallic Ni crystallites detected at 2θ = 44.9, 51.8, and 76.4° (JCPDS card No. 87-0712). Furthermore, it was clearly seen that the diffraction peaks of the NiO phase almost were not observed, indicating that Ni²⁺ ions in the NiO phase were fully reduced to metallic Ni at 450°C for 4 hours in hydrogen. The size of the NiO and Ni crystallites was calculated at the strongest intensity peak corresponding to 2θ = 43.3° and 44.9° respectively by Scherrer formula³⁴⁾ and was presented in Table 1. It was shown that the adding 4–5 mass% CeO₂ to Ni/SBA catalyst led to a decrease in crystallite sizes of NiO as well as Ni supported on SBA-15. The results of N₂ sorption isotherms (seen in Fig. 3) showed that all materials exhibited Type IV adsorption isotherms with Type-H1 hysteresis loops.³⁵⁾ The loop size decreased with adding nickel and/or ceria macropore character, indicating that nickel and ceria have been attached to the channels of SBA-15.

Fig. 2

XRD patterns of as-synthesized catalysts ((a) Ni/SBA and (c) Ni4Ce/SBA) and catalysts reduced in H₂ at 450°C for 4 hours ((b) Ni/SBA and (d) Ni4Ce/SBA).

Table 1 Textural properties of the prepared materials.

Fig. 3

Nitrogen adsorption-desorption isotherms of the samples ((a) SBA-15; (b) Ni/SBA; and (c) Ni4Ce/SBA).

SEM images (Fig. 4) showed that catalyst morphology consisted of cocoon-shaped particles of SBA-15 support with 200–500 nm in diameter and 600–800 nm in length. Their surface was highly covered by spherical NiO particles of dozen nanometres in size. There was no noticeable change in surface morphology and macroscopic structure of the Ni/SBA-15 catalyst by adding CeO₂. The SEM micrograph of the CeO₂-doped catalyst (Fig. 4(c)) showed that the cocoon-shaped particles were stacked together. As a result, the surface area of the CeO₂-doped sample was lower than that of undoped one (176.1 vs. 214.7 m²·g⁻¹).

Fig. 4

SEM images of SBA-15 support (a) and as-synthesized catalysts Ni/SBA (b) and Ni4Ce/SBA (c).

Additionally, from the TEM images of the prepared catalysts (Fig. 5), well-ordered hexagonal channels of a few nm in diameter could be observed. NiO particles of several and dozen nanometres were found to be located both inside and outside of the nanochannels of the support. Similar results were observed in the previous studies.³⁶⁾

Fig. 5

TEM images of as-synthesized catalysts Ni/SBA (a), Ni4Ce/SBA (b), and Ni5Ce/SBA (c).

Figure 6 showed the reduction behavior of both CeO₂-doped and undoped catalysts and 4 mass% CeO₂/SBA-15. The H₂-TPR profile of 4 mass% CeO₂/SBA sample multiplied by 30 times showed that CeO₂/SBA sample was reduced unsignificantly. The ceria subsurface oxygen was reduced at high temperatures around 800°C.³⁷⁾ For Ni-based catalysts, the reduction peaks of free NiO species, appearing at a low temperature of 350–370°C,³⁸⁾ were observed in all catalysts. Comparing to Ni/SBA-15 catalyst, the maximum reduction temperature in H₂-TPR patterns shifted to a lower value (340°C versus 366°C) and bigger reduction peaks were observed in Ce-modified catalysts. These facts indicated that NiO species are more easily and better reduced by introducing CeO₂. Besides, two reduction peaks in the range of 480–580°C and 600–740°C observed in H₂-TPR profiles of Ce-modified catalysts mighty be attributed to the strong interaction of NiO with CeO₂ and SBA-15.^22,39) These phases of nickel should be crucial for obtaining sintering-resistant nickel particles during the calcination and reduction process. The good dispersion of NiO particles in the CeO₂-doped catalyst is explained by the strong interaction of Ni²⁺ with CeO₂ forming Ce³⁺ ions and oxygen vacancies that dispersed NiO particles better on the catalyst surface.⁴⁰⁾

Fig. 6

H₂-TPR spectra of catalysts.

The raman spectra of Ni/SBA and Ni4Ce/SBA were also illustrated in Fig. 7. Two bands at around 490 cm⁻¹ (α) and 1070 cm⁻¹ (β), a strong band at around 465 cm⁻¹ and a stretch band at 900–1200 cm⁻¹ were detected on raman spectra of Ni/SBA and NiCe/SBA catalysts, respectively. According to the authors,³²⁾ for SBA-15 support and NiO/SBA-15 catalyst, two bands detected in raman spectra at about 487.3 cm⁻¹ and 1068.7 cm⁻¹ were attributed to the cyclic tetrasiloxane rings and Si-OH stretching mode, respectively. So, two bands detected on raman spectrum of Ni/SBA at about 490 cm⁻¹ (α) and 1070 cm⁻¹ (β) were attributed to the corresponding cyclic tetrasiloxane rings and Si-OH stretching mode, typical for SBA-15 support.^36,41) However, compared to Thielemann’s report,⁴¹⁾ the blue shift of β band (1070 vs. 970 cm⁻¹) in this case indicated the formation of Si–O–Ni bond.³⁸⁾ For the raman spectrum of Ni4Ce/SBA sample, the band at around 465 cm⁻¹ was attributed to the F2g mode of the cerium oxide. This overlapped with the raman band of tri-cyclosiloxane rings of SBA-15³⁸⁾ or pure cubic CeO₂⁴²⁾ or the symmetric “breathing” vibrations of the oxygen anions around the cerium cation of crystalline CeO₂ of fluorite structure. The band stretching between 900–1200 cm⁻¹ in raman spectrum of Ni4Ce/SBA catalyst might include the band at 983 cm⁻¹, attributed to the Si–O–Ce linkages³⁸⁾ and the bands at 1098 cm⁻¹, assigned to the second-order longitudinal optical mode of CeO₂.⁴³⁾ The formation of Si–O–Ce bond improved the interaction between active species and support, which facilitated CO methanation.³⁸⁾

Fig. 7

Raman spectra of catalysts.

CO₂-TPD patterns (Fig. 8) showed that all samples exhibited the weak (α), moderate (β), and strong basic sites (γ), corresponding to three CO₂ desorption zones in the range of 115–150°C; 200–300°C and 500–600°C, respectively. In particular, the number of weak basic sites was dominant. This changed noticeably while the amount of average and strong basic sites decreased when CeO₂ was added. The higher basicity of the NiCe/SBA sample, as seen in Table 1, was explained by the adsorption of CO₂ on surface Ce⁴⁺ and Ce³⁺ ions to form carbonate CO₃²⁻ species.⁴⁴⁾ The good migration ability of oxygen in the crystal and the electron delocalization formed by the oxygen vacancy could increase the electron density in the nano-CeO₂ structure^45,46) that increased the strength and the amount of the basic sites.⁴⁷⁾ Another cause was the high dispersion of CeO₂ in catalysts as demonstrated in XRD analysis. The decrease in particle size of CeO₂ was proved to be favorable for the increase of basicity of catalysts.⁴⁸⁾

Fig. 8

CO₂-TPD patterns of catalysts.

3.2 Activity of catalysts

The results in Fig. 9 showed that the CO₂ conversions increased when catalyst was modified with 4 mass% CeO₂ at all the temperature values in range of 250–350°C. Besides, the reaction temperature was lowered at the same CO₂ conversion (e.g. 50%) when Ni/SBA was replaced by Ni4Ce/SBA, from 325 down to 312°C. The CH₄ selectivity of CeO₂-doped catalyst reached over 99% at all temperatures. It could be explained that the modification of CeO₂ led to increasing of CO₂ adsorption capacity and catalytic activity as a consequence.⁴⁹⁾ According to authors,^45,46) in the CeO₂ crystal, oxygen has a good migration ability and the electron delocalization formed by the oxygen vacancy that can increase the electron density in the nano-CeO₂ structure and the strength and the amount of the basic sites. The obtained results (Fig. 10) are well proven that at a typical reaction temperature of 350°C, the CO₂ adsorption capacity of Ni4Ce/SBA catalyst was slightly higher than that of Ni/SBA (10.9 compared to 10.4 $\textit{mg}_{\textit{CO}_{2}}.g_{\textit{cat}}^{ - 1}$, respectively).

Fig. 9

CO₂ conversion in solo-methanation of 20 mol% CO₂ gas on Ni/SBA catalysts modified with varying in CeO₂ contents.

Fig. 10

CO₂ adsorption by time at 350°C on the reduced catalysts (T = 350°C; m_cat = 0.05 g; 6 mol% CO₂ + 94 mol% N₂; GHSV = 15,000 h⁻¹).

On the other hand, Boreskov et al.⁵⁰⁾ showed that smaller crystal size of Ni led to a higher of CO₂ conversion, and the optimal values was in the range of 10–20 nm, corresponding to the highest catalytic activity. Meanwhile, the results in Table 1 showed that the increase in CeO₂ content from 3 to 4 mass% led to a reduction in the crystallite size of reduced Ni^o (from 23.3 to 20.5 nm) and an increase of CO₂ conversion consequently. However, if CeO₂ content was continually increased to 5 mass%, the catalyst activity was dropped. This could be explained that when the CeO₂ content was high, the Ni sites were occupied by the larger amount of produced O species, then the reaction conversion decreased.⁵¹⁾

It could be observed from Fig. 11 that adding of 1 mol% of CO into the CO₂ feedstock stream led to an enhancement of CO₂ conversion as well as a drop in reaction temperature of methanation. Moreover, the reaction temperature of CO₂ hydrogenation without CO (case II) was 313°C while it was only 290°C in case I (with 1% CO in the feedstock stream) at the conversion of 50% on Ni4Ce/SBA catalyst. The CH₄ selectivity still reached 98–99%. This result was also confirmed by Tada et al.⁵²⁾ It could be explained by thermal dynamic data of following reactions:⁵³⁾   

\begin{align} &\text{CO$_{2}$(g)} + \text{H$_{2}$(g)} \rightleftharpoons \text{CO(g)} + \text{H$_{2}$O (g)}, \\ &\Delta \text{H} = +41\,\text{kJ${\cdot}$mol$^{-1}$} \end{align} (2)

  

\begin{align} &\text{CO(g)} + \text{3H$_{2}$(g)} \rightleftharpoons \text{CH$_{4}$(g)} + \text{H$_{2}$O(g)},\\ &\Delta \text{H} = -206\,\text{kJ${\cdot}$mol$^{-1}$} \end{align} (3)

  

\begin{align} &\text{CO$_{2}$(g)} + \text{4H$_{2}$(g)} \rightleftharpoons \text{CH$_{4}$(g)} + \text{2H$_{2}$O(g)},\\ &\Delta \text{H} = -165\,\text{kJ${\cdot}$mol$^{-1}$} \end{align} (4)

Fig. 11

CO₂ conversion in methanation of mixture of 19 mol% CO₂ with 1 mol% CO (case I - dashed lines) and with 1 mol% N₂ (case II - solid lines) on Ni4Ce/SBA catalyst.

The reaction (2) is endothermic so that it takes place at high temperature. The reaction (3), which is exothermic, easily occur by presence of a small amount of CO in feedstock stream. Furthermore, heat released from this reaction causes the temperature in the reaction (2) decreased and the reaction (2) shifts forward. As a result, both CO₂ conversion and CH₄ selectivity increase. It is also observed that the conversion of CO in the mixture reaches 100% at 250°C on Ni4Ce/SBA catalyst.

4. Conclusion

The catalytic activity of NiO/SBA-15 in methanation of carbon dioxide was enhanced when modified by CeO₂. Adding CeO₂ led to an increase in CO₂ conversion and decrease in reaction temperature due to the enhancement of the dispersion of metallic Ni^o, improvement of the NiO reducibility and CO₂ adsorption. However, the conversion decreased when CeO₂ content was higher than 4 mass%. In addition, adding a small amount of CO lowered the temperature of CO₂ methanation, showing a promotional effect of co-reaction/CO in the CO₂ hydrogenation.

Acknowledgments

This research was supported by Vietnam Academy of Science and Technology under the grant No. ĐLTE00.10/18-19.

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