SBA-15

Journal of Environmental Chemical Engineering 6 (2018) 745–753

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Comparative study of Ni-Ce loading method: Beneficial effect of ultrasonicassisted impregnation method in CO2 reforming of CH4 over Ni-Ce/SBA-15

T

⁎

H.D. Setiabudia,b, , C.C. Chonga, S.M. Abeda, L.P. Tehc, S.Y. China,b a

Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Pahang, Malaysia Centre of Excellence for Advanced Research in Fluid Flow, Universiti Malaysia Pahang, 26300 Gambang, Kuantan, Pahang, Malaysia c School of Chemical Sciences and Food Technology, Faculty of Science and Technology, Universiti Kebangsaan Malaysia, 43600 UKM Bangi, Selangor, Malaysia b

A R T I C L E I N F O

A B S T R A C T

Keywords: Ni-Ce/SBA-15 CO2 dry reforming Ni-Ce loading methods Ultrasonic-assisted impregnation Metal-support interaction

A series of Ni-Ce/SBA-15 catalysts with 6 wt% Ce and 5 wt% Ni were synthesized using conventional impregnation (Ni-Ce/SBA-15(C-IM)), ultrasonic-assisted impregnation (Ni-Ce/SBA-15(US-IM)) and reflux-assisted impregnation (Ni-Ce/SBA-15(R-IM)) methods The samples were characterized using XRD, TEM, SEM, BET, FTIR, H2-TPR, XPS and TGA. The characterization results showed that Ni-Ce loading methods greatly influence the properties of Ni-Ce/SBA-15 whereby the homogeneity of metal dispersion and strength of metal-support interaction followed the order of Ni-Ce/SBA-15(C-IM) < Ni-Ce/SBA-15(R-IM) < Ni-Ce/SBA-15(US-IM). The smaller metal particle size and higher metal dispersion in Ni-Ce/SBA-15(US-IM) have led to the stronger metalsupport interaction and further decreased the surface area and porosity of the catalyst. The activity and stability of catalysts followed the order of Ni-Ce/SBA-15(C-IM) < Ni-Ce/SBA-15(R-IM) < Ni-Ce/SBA-15(US-IM), with the conversion of CH4 and CO2 over Ni-Ce/SBA-15(US-IM) was about 96.3% and 93.5%, respectively, and H2/ CO ratio of 1.02 at reaction temperature of 800 °C and almost remained constant during 48 h of reaction. The superior catalytic performance of Ni-Ce/SBA-15(US-IM) probably was related with the smaller metal particles, stronger metal-support interaction and more homogenous metal dispersion, which altered the properties of catalyst towards an excellent catalytic performance. The characterization of spent catalysts showed the lowest carbon formation in Ni-Ce/SBA-15(US-IM) catalyst, demonstrating the positive role of ultrasonic effect on alteration of catalyst properties towards carbon resistance. This study provides new perspective on the preparation of Ni-Ce/SBA-15 towards an excellent performance of CO2 reforming of CH4.

1. Introduction The global warming due to greenhouse gases emissions is a major concern of modern societies. In order to combat the problem of global warming, researchers had proposed the idea of producing the syngas from greenhouse gases. There are several ways can be used to produce syngas such as ethanol dry reforming [1], glycerol steam reforming [2], partial oxidation of CH4 [3,4] and CO2 reforming of CH4 [5–8]. Among the proposed synthesis routes, CO2 reforming of CH4 is the most promising process whereby two major greenhouse gases (CO2 and CH4) are consumed to produce syngas (CO and H2). In addition, the syngas produced has low equimolar ratio (1:1) which is preferred for the production of liquid hydrocarbon through Fischer-Tropsch synthesis [6]. The CO2 reforming of CH4 is expressed in the equation as below [9]: CH4 + CO2 → 2CO + 2H2 − 67.32T kJ/mol

⁎

ΔH° = 247.3 kJ/mol

ΔG° = 61770 (1)

The CO2 reforming of CH4 has been studied extensively using numerous types of catalysts including noble metal-based catalysts and transition metal-based catalysts [10]. However, transition metal-based catalysts, especially Ni-based catalyst is more feasible as it exhibits high catalytic activity, readily available and cost effective. In particular, Ni supported on SBA-15 has attracted intense interest for CO2 reforming of CH4 owing to the properties of SBA-15 that possesses hexagonal structure of mesopores with size of 4.6–30 nm, large surface area (600–1000 m2/g), thicker walls (3.1–6.4 nm) and high thermal stability [8,11,12]. However Ni-based catalyst is always accompanied by coke formation and sintering of Ni metal particles, which causes a severe deactivation of the catalyst [5]. Therefore, it is highly necessary to design stable and active Ni-based catalysts. It has been reported that the addition of promoters such as cerium oxide (CeO2) [5,7,13,14] and lanthanum oxide (La2O3) [15] played a significant role in improving the catalytic activity of the Ni-based catalyst and inhibit carbon deposition through modification of surface

Corresponding author at: Faculty of Chemical and Natural Resources Engineering, Universiti Malaysia Pahang, 26300, Gambang, Kuantan, Pahang, Malaysia. E-mail address: [email protected] (H.D. Setiabudi).

https://doi.org/10.1016/j.jece.2018.01.001 Received 28 September 2017; Received in revised form 19 December 2017; Accepted 1 January 2018 Available online 02 January 2018 2213-3437/ © 2018 Elsevier Ltd. All rights reserved.

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appropriate amount of Ce salt precursor, Ce(NO3)2·6H2O (SigmaAldrich, 99%) was mixed with SBA-15, and then was heated slowly at 80 °C under continuous stirring until nearly all the water had evaporated. The solid residue was dried overnight at 110 °C followed by calcination at 550 °C for 3 h to produce Ce/SBA-15(C-IM). Subsequently, similar procedure was used to synthesis Ni-Ce/SBA-15(CIM) by using Ce/SBA-15(C-IM) and Ni salt precursor, Ni(NO3)2 6H2O (Merck, 99%). For the US-IM method, Ce/SBA-15(US-IM) was prepared by mixing of SBA-15 with appropriate amount of Ce salt precursor, Ce (NO3)2.6H2O. The resulting slurry was immersed in the ultrasonic cleaner bath and was heated slowly at 80 °C until nearly all the water had evaporated. The solid residue was dried overnight at 110 °C followed by calcination at 550 °C for 3 h. Next, similar procedure was used to synthesis Ni-Ce/SBA-15(US-IM) by using Ce/SBA-15(US-IM) and Ni salt precursor, Ni(NO3)2 6H2O. Meanwhile, for the R-IM method, Ce/ SBA-15(R-IM) was prepared by mixing of SBA-15 with appropriate amount of Ce salt precursor, Ce(NO3)2·6H2O, and was refluxed at 80 °C for 6 h. Then, the resulting slurry was heated at 80 °C under stirring until nearly all the water had evaporated. The solid residue was dried overnight at 100 °C followed by calcination at 550 °C for 3 h. Subsequently, similar procedure was used to synthesis Ni-Ce/SBA-15(RIM) by using Ce/SBA-15(R-IM) and Ni salt precursor, Ni(NO3)2 6H2O. For all catalysts, the cerium content was fixed to 6 wt% and that of nickel to 5 wt%.

structure effects. In the case of CeO2, the redox properties of CeO2 (Ce4+ ↔ Ce3+) promotes the formation of oxygen vacancies, thus enhancing the mobility of surface oxygen whose contributes to lower carbon deposition of the metallic particles. Besides, the basic characteristic of CeO2 can suppress the carbon deposition by adsorbing the mild acidic CO2 species on the catalyst surface and further interacting with deposited carbon [16]. In addition, the presence of Ce in catalyst improves support interactions and enhances the dispersion of metal species over the support [17]. It was also found that high nickel dispersion inside the support channels was obtained in the presence of Ce, thus resulted in more stable reaction, with minor carbon nanotubes formation [6]. The catalyst preparation procedure is one of the main factors that can alter the physicochemical characteristics and influence the catalytic performance of the catalyst. Aghamohammadi et al. [7] studied the effect of ceria promotion and synthesis methods on Ni/Al2O3-CeO2 towards dry reforming of methane. They found that sol-gel method is more favored than sequential impregnation method with much improved catalytic activity and stability due to high dispersion of active phase. Albarazi et al. [18] investigated on three different synthesis routes: two sequential incipient wetness impregnation, one single impregnation and co-precipitation method for ceria-zirconia doped Ni/ SBA-15. They also claimed that different synthesis routes strongly influenced the properties of the catalyst. In addition, Kaydouh and coworkers [19] reported that the “two solvents” deposition method favors the dispersion of NiO particles entrapped in the porous channels of SBA15, thus inhibiting sintering and coke formation during the reaction. For Ni-Ce/SBA-15, CeO2 nanoparticles are also highly dispersed and the catalyst shows high activity and stability towards CO2 reforming of CH4. The dispersion of Ni particle inside the pores of SBA-15 was also reported by Gálvex et al. [20] for Ni/SBA-15 prepared by precipitation method. In their study, three preparation method where considered: incipient wetness impregnation, precipitation and precipitation in the presence of ascorbic acid as reducing agent. The results showed that Ni/ SBA-15 prepared by addition of ascorbic acid favors deposition of Ni particles inside the pores and thus resulted in higher activity, improved stability and enhanced selectivity towards CO2 reforming of CH4. Despite the fact that several studies have disclosed the positive role of Ce on the coke resistance and influential of synthesis strategies, however searching for a suitable catalyst preparation methods are still required in order to facilitate the uniform dispersion of active compound in the catalyst, and thus promote the catalytic activity and inhibit carbon deposition of the catalyst. Owing to the fact that the methodologies of catalyst preparation will significantly influence the properties and catalytic activity of the catalyst, thus, the objective of this study is to investigate the influence of Ce-Ni loading methods on the properties and catalytic activities of Ce-Ni/SBA-15 towards CO2 reforming of CH4.

2.2. Catalyst characterization The X-ray diffraction (XRD) analysis was recorded on powder diffractometer (Philips X’ Pert MPD, 3 kW) using a Cu Kα radiation (λ = 1.5405 Å). The primary crystallite size of NiO and CeO2 were calculated using the Scherrer equation at 2θ of 43.6° and 28.8°, respectively:

D=

0.9λ B cos θ

(2)

where λ is the X-ray wavelength corresponding to Cu-Kα radiation (0.15405 nm), B is the broadening (in radians) of the metal reflection and θ is the angle of diffraction corresponding to peak broadening. Transmission electron microscopy (TEM) was performed using JEM2100 Electron Microscope. The sample was dispersed in ethanol by sonication, and deposited on an amorphous, hollow carbon grid. The elemental distribution mapping of the catalyst was observed using a Scanning Electron Microscope (FEI Quanta 450 Electron Microscope) operating at 10 kV. The specific surface areas, total pore volume and average pore diameter were determined using AUTOSORB-1 model AS1 MP-LP instrument at −196 °C. Before each measurement, the sample was degassed in vacuum at 300 °C for 3 h. The Fourier Transform Infrared (FTIR) analysis was carried out using Thermo Nicolet Avatar 370 DTGS model in KBr matrix in order to study the chemical properties of catalysts and to identify the interaction of metal species with SBA-15. H2-TPR analysis was carried out using Micromeritics Chemisob 2920 Pulse Chemisorption in 10% H2/Ar at 10 °C/min. Prior to the chemisorption, 30 mg of the catalyst was reduced with pure H2 (20 mL min−1) at 850 °C for 1 h. The amount of hydrogen uptake was determined by injecting mixed gas (10% H2/Ar) periodically into the reduced catalyst. The X-ray photoelectron spectroscopy (XPS) analysis was conducted on a Kratos Analytical XSAM HS spectrometer. XPS data were analyzed using CasaXPS software. The amount of carbon deposited on the spent catalyst was determined using thermogravimetric analyzer (TGA Q500, TA Instruments), XRD (Philips X’ Pert MPD, 3 kW) analyses and TEM (JEM2100 Electron Microscope). TGA analysis was carried out under a mixture of air (20%O2/80% N2) with heating rate of 5 °C/min up to

2. Materials and methods 2.1. Catalysts preparation SBA-15 support was synthesized according to the method reported by Zhao et al. [21]. In concise, the triblock copolymer P123 (EO20PO70EO20, Aldrich) was dissolved in the solution of deionized water and 2 M hydrochloric acid solution under stirring. Then, tetraethyl orthosilicate (TEOS, Merck) was added dropwise to the previous solution and stirred at 40 °C for 24 h, and the precipitate product was obtained. The precipitate product was filtered, washed with deionized water and dried overnight at 110 °C. The sample was calcined at 550 °C for 3 h to remove the triblock copolymer. A series of Ni-Ce/SBA-15 catalysts were prepared by three types of preparation methods which are conventional wet impregnation method (C-IM), ultrasonic-assisted impregnation method (US-IM) and reflux followed with impregnation method (R-IM). For C-IM method, an 746

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900 °C. Meanwhile, the XRD analysis was carried out using a Cu Kα radiation (λ = 1.5405 Å).

Table 1 XRD analysis of SBA-15, Ni-Ce/SBA-15(C-IM), Ni-Ce/SBA-15(US-IM) and Ni-Ce/SBA15(R-IM).

2.3. Catalytic testing

Catalyst

The catalytic performance of catalyst towards CO2 reforming of CH4 was carried out in a stainless steel fixed bed reactor (length 417 mm, i.d. 11 mm) at atmospheric pressure and reaction temperature of 600, 700, 750 and 800 °C. Prior to the reaction, 0.2 g of catalyst was reduced in a H2 flow of 50 mL/min for 3 h at 700 °C. The feeding gas flow rate to the reactor was set at 50 mL/min, with a ratio of CH4:CO2:N2 = 1:1:1, and N2 was used as a carrier gas. The effluent gas was analyzed with an Agilent gas chromatography (AGILENT 6890 N) equipped with a thermal conductivity detector (TCD). The CH4 conversion, CO2 conversion, and product distribution ratio, H2/CO were calculated according to the following equations;

CO2 Conversion ,XCO2 =

FCO2, in − FCO2, out × 100% FCO2, in

(3)

CH 4 Conversion ,XCH4 =

FCH4, in − FCH4, out × 100% FCH4, in

(4)

FH2 H2 = CO FCO

Ni-Ce/SBA-15(C-IM) Ni-Ce/SBA-15(US-IM) Ni-Ce/SBA-15(R-IM) a

CeO2 crystallite size (nm)a

NiO crystallite size (nm)a

Fresh

Fresh

Spent

7.6 6.9 7.4

10.2 9.5 9.9

11.6 9.6 10.7

Determine by XRD (Scherer equation).

The presences of metal crystallites on the Ni-Ce/SBA-15 catalysts were characterized using wide-angle XRD, as shown in Fig. 1(B). The broad diffraction peaks at 2θ = 15–35° is assigned to reflection peak of the SiO2 framework of SBA-15 support [22]. The diffraction peaks at 2θ = 37.5°, 43.6° and 63.2° were attributed to the face-centered cubic crystalline nickel oxide (NiO), meanwhile the peaks at 2θ = 28.8°, 33.5°, 47.8°, 56.7°, and 70.1° were attributed to the cerium oxide (CeO2) with a face-centered cubic lattice [23]. The size of NiO and CeO2 crystallites were calculated using the Scherrer equation and the results are listed in Table 1. By referring to Table 1, it was observed that the NiCe/SBA-15(US-IM) catalyst results in a lower crystallites size in both NiO and CeO2, as compared to the other two methods. This result indicated that US-IM method leads to the formation of smaller size of NiO and CeO2 likely due to the synergetic effect of ultrasound that provides a uniform environment for the nucleation and growth of metal particles, thus preventing the agglomeration of metal particles. The formation of smaller metal particle with homogenous metal dispersion on NiCe/SBA-15(US) was confirmed by the SEM elemental mapping and TEM image as shown in Fig. 2. It was observed that both Ni and Ce can be clearly identified on the surface of Ni-Ce/SBA-15 with uniform and well-dispersed distributions. The positive role of ultrasound on the formation of smaller size of metal was also reported by Abdollahifar et al. [24] for Co/Y catalysts that synthesized using impregnation and ultrasound method. XRD, FESEM and TEM analysis revealed that the ultrasound method was more effective route to produce the small and/or uniform particles than impregnation method. In addition, ultrasound method can be an effective route to increase metal dispersion and prevent particle agglomeration. Similar phenomenon was also reported by Mahboob et al. [25] for Ni-Co/Al2O3-ZrO2. They found that the presence of ultrasound during impregnation method preventing the agglomeration of metals particles on the Al2O3 surface. With increasing sonication power and time, more uniform morphology with highly dispersed and small metals particles were obtained on the Al2O3 surface indicating the positive role of ultrasound on the distribution and size of metal particles. With the application of ultrasound, the turbulent flow and shockwaves produced drives the metal particles together at sufficiently high speeds to induce effective interaction at the point of collision, thus resulted in the homogeneity of particles onto the surface of SBA-15. Fig. 3 illustrates the N2 adsorption-desorption isotherm of SBA-15 and Ni-Ce/SBA-15 catalysts. All the Ni-Ce/SBA-15 catalysts showed a type-IV N2 adsorption-desorption isotherm with H1-type hysteresis loop, which is a typical feature of mesoporous materials and indicative a narrow distribution of relatively uniform cylinder pores according to the IUPAC classification [26]. The specific surface area, pore volume and pore diameter of SBA-15 and Ni-Ce/SBA-15 catalysts are shown in Table 2. It was observed that the introduction of Ni and Ce decreased the specific surface area, pore volume and pore diameter of the catalysts, indicating the blockage of the pores with the Ce and Ni species. By comparing the effect of the metal loading method, it was observed that the specific surface area, pore volume and pore diameter of catalysts were significantly decreased with the application of ultrasound energy. This result might be related with the presence of small particle of NiO and CeO2, and strong contact between metal and support that led the

(5)

where F is the molar flow rate for particular compound. 3. Results and discussion 3.1. Characterization of the catalysts Fig. 1(A) shows the low-angle XRD patterns of SBA-15 and Ni-Ce/ SBA-15 catalysts. The pattern of pure SBA-15 exhibited three peaks, indexed as (100), (110), and (200), which are a reflection of typical two dimensional, hexagonally ordered mesostructures (p6 mm), demonstrating the high quality of the mesopore packing [21]. The introduction of Ni and Ce with three different methods of metal loading (C-IM, US-IM and R-IM) resulted in a slight decreased of the (100) peak and eliminated the (110) and (200) peaks, indicating structural degradation of SBA-15. It is noted that Ni-Ce/SBA-15(US-IM) has a smaller decreased in peaks intensities as compared to the Ni-Ce/SBA-15(C-IM) and Ni-Ce/SBA-15(R-IM) which indicative that US-IM method has better dispersion of Ni and Ce as compared to the C-IM and R-IM. This result might be related to the presence of ultrasonic agitation which can rupture the metal particles and consequently decreased the particle size of metal and increase the metal dispersion on the surface of SBA-15.

Fig. 1. (A) Low- and (B) wide-angle XRD patterns of (a) SBA-15, (b) Ni-Ce/SBA-15(C-IM), (c) Ni-Ce/SBA-15(US-IM) and (d) Ni-Ce/SBA-15(R-IM).

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Fig. 2. (A) SEM elemental mapping images of Ni and Ce, and (B) TEM image of Ni-Ce/SBA-15(US).

Fig. 3. Nitrogen adsorption-desorption isotherms of (a) SBA-15, (b) Ni-Ce/SBA-15(C-IM), (c) Ni-Ce/SBA-15(US-IM) and (d) Ni-Ce/SBA-15(R-IM).

Fig. 4. H2-TPR profiles of (a) Ni-Ce/SBA-15(C-IM), (b) Ni-Ce/SBA-15(US-IM) and (c) NiCe/SBA-15(R-IM). The dotted lines represent the Gaussian peaks.

15(C-IM), the deconvoluted TPR profiles consist of four peaks centered at 350, 420, 490 and 710 °C. Meanwhile, the deconvoluted TPR profile of Ni-Ce/SBA-15(US-IM) and Ni-Ce/SBA-15(R-IM) consists of five peaks centered at 373, 438, 490, 540 and 748 °C for Ni-Ce/SBA-15(US-IM), and 383, 434, 480, 540 and 730 °C for Ni-Ce/SBA-15(R-IM). The appearance of multiple peaks in the H2-TPR profile indicated co-existence of different degree of metal-support interactions of NiO species in the catalyst. According to the literature [28], the peaks in temperature lower than 400 °C are assigned to the reduction of Ni2O3 or NiO species, while the peaks in temperature higher than 400 °C are attributed to the reduction of NiO species which interacted with the support. In brief, there are three reduction zones available at temperature higher than 400 °C. The peak at low temperature zone (400–500 °C) are attributed to the reduction of NiO species which have weak interaction with the support, whereas the peaks in the intermediate temperature zone (500 to 600 °C) are assigned to the reduction of NiO species having medium strength of interaction with the support [29]. Additionally, the peaks in the high temperature zone (> 600 °C) are attributed to the reduction of NiO species with strong interaction with the support [29]. Some researchers [30] claimed that the peak around 720 °C can be attributed to the reduction of Ce4+ to Ce3+ of very small CeO2 oxide crystalline or dispersed Ce phases. Besides, Zhang et al. [31] reported the formation of Ni-Ce-O species due to the introduction of Ni2+ into ceria lattice and distorted the CeO2 lattice. However, the peak appears at 470 °C is attributed to the reduction of NiO species dispersed on the silica support, possibly is Ni hydrosilicate species, which suggested that only a part of Ni species incorporated into Ni-Ce-O oxides. This behavior is

Table 2 BET analysis of SBA-15, Ni-Ce/SBA-15(C-IM), Ni-Ce/SBA-15(US-IM) and Ni-Ce/SBA15(R-IM). Catalyst

Surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

SBA-15 Ni-Ce/SBA-15(CIM) Ni-Ce/SBA-15(USIM) Ni-Ce/SBA-15(RIM)

856 377

0.999 0.701

7.45 7.36

357

0.638

7.08

414

0.750

7.17

NiO and CeO2 particles plugging the pores of SBA-15. Similar result was reported by Khoshbin and Highighi [27] for CuO-ZnO-Al2O3/HZSM-5 nanocatalyst. They found that the surface area of catalyst decreased markedly with the application of ultrasound as a result of the small particles of CuO-ZnO-Al2O3 and strong interaction between CuO-ZnOAl2O3 and HZSM-5. In contrast, Abdollahifar et al. [24] reported that utilization of ultrasound energy lead to the higher surface area due to the formation of small and well-dispersed Co particles on the surface of zeolite Y. Fig. 4 shows the H2-TPR profile of the Ni-Ce/SBA-15 catalysts with different preparation methods. It is known that nickel oxide species are reduced to metallic nickel species in one step (Ni2+O + H2 → Ni° +H2O), thus the H2 consumption peaks appear at different temperatures are attributed to the reduction of different species. For Ni-Ce/SBA748

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assigned to the asymmetric stretching vibrations of SieOeSi, SieO stretching vibration of SieOH groups, symmetric stretching vibrations of SieOeSi, and tetrahedral bending vibration of SieOeSi bonds, respectively [8]. The introduction of metals (Ni and Ce) decreased the intensity of the peak at 961 cm−1 and the peak becomes enveloped in the band at 1060 cm−1, indicating the incorporation of metal ions in the silica framework through the substitution of OeH with OeM to form SieOeM. Moreover, it was also observed that the introduction of metals decreased the intensity of OeH stretching vibration mode of SiOH involved in hydrogen interaction with adsorbed water molecules at 3400 cm−1 (Fig. 5(B)), indicating the substitution of OeH with OeM. The changes in the intensity of the peaks at 961 and 3400 cm−1 can be a direct evidence for the metal-support interaction and the changes was following the order of Ni-Ce/SBA-15(C-IM) < Ni-Ce/SBA-15(RIM) < Ni-Ce/SBA-15(US-IM). This result is in agreement with the result obtained by H2-TPR analysis. In CO2 reforming reaction, metalsupport interaction is one of the key influencing factors that affect the coke formation properties of the catalyst. Strong interactions between the metal and the support prevent the catalyst being susceptible to coking and deactivation. Therefore, according to the result observed, it is predicted that Ce/SBA-15(US-IM) catalyst will have higher stability and lower carbon formation as compared to Ce/SBA-15(C-IM) and Ce/ SBA-15(R-IM) catalysts. Fig. 6 shows the XPS spectra of O 1s for Ni/SBA-15 and Ni-Ce/SBA15. Both catalysts were composed of three overlapped peaks around 528, 531 and 533 eV. The first peak appears at 528 eV assigned to the lattice oxygen in NiO or Ce2O3 [34,35], while the peak around 531 eV assigned to the defect oxide or the surface oxygen ions with low coordination situation and weekly bonded oxygen species [36]. Moreover, the band around 533 eV related to the adsorbed oxygen species from hydroxyls species [36]. It was observed that the intensity of the peak at 528 eV decreased with the introduction of Ce might be due to the high dispersion of metal particles as a result of the formation of Ni (or Ce) eOeSi bonds, in agreement with the study reported by Li et al. [34] for Y2O3-promoted NiO/SBA-15. Furthermore, it was observed that the presence of Ce increased the intensity of the peaks around 531 and 533 eV, indicating more oxygen defects on the surface of Ni-Ce/SBA-15.

responsible for the facilitated reduction in Ni/Ce/SBA-16. Therefore, it should be noted that it is hard to ravel the reduction peak of the labile oxygen of ceria that may also contribute in the H2-TPR profile. Based on the H2-TPR results, the reducibility of Ni-Ce/SBA15 followed the order of Ni-Ce/SBA-15(C-IM) > Ni-Ce/SBA-15(R-IM) > NiCe/SBA-15(US-IM). Ni-Ce/SBA-15 prepared by C-IM method easily reduced at low temperature likely due to the weak metal-support interaction, while Ni Ni-Ce/SBA-15 prepared by US-IM method shifted the reduction peaks towards higher temperature closely related to the presence of strong metal-support interaction and well distribution of metals on the surface of SBA-15. The positive role of ultrasound in promoting strong metal-support interaction was also reported by Mahboob et al. [25] for Ni-Co/Al2O3-ZrO2 catalyst. The H2-TPR result showed that the reduction peak at 675–900 °C shifted towards higher temperature for ultrasound-assisted impregnation sample, indicating an improvement in the metal-support interaction. Contrary, Wang et al. [32] found that introduction of cerium promoted the reduction of nickel oxides on multi-walled carbon nanotubes (CNTs). Based on H2TPR results, the high reduction peak shifted to lower reduction temperature, meanwhile, the low reduction peak shifted to higher reduction temperature. The appearance of two movement trend may because of the alteration of the interaction between the metal oxides and CNTs by the addition of cerium. Apart from the positive role of ultrasound in promoting strong metal-support interaction, it was observed that the reflux method improved the metal-support interaction as compared to the conventional impregnation method. This might be related to an increased in the solubility of the metal precursor in boiling solvent, thus increased the adsorption-desorption process of metal ions on the surface of support. This observation is in agreement with Reinikainen et al. [33] for Co-Ru/SiO2 catalyst prepared by reflux method. They found that the reflux method increased the amount of tightly bound metal on the support by improved its metal precursor solubility and thus increased the activity of catalyst towards CO hydrogenation as compared to the impregnation method. Fig. 5 shows the FTIR spectra of Ni-Ce/SBA-15 with different preparation methods in the range of 1400–500 cm−1 and 3800–2500 cm−1. The bands in the range of 1400–500 cm−1 are attributed to the vibrations of the stretching and bending modes of SieO units, while the bands in the range of 3800–2500 cm−1 are associated with the OeH stretching vibration mode of Si-OH involved in hydrogen interaction of adsorbed water molecules. It was observed that the FTIR spectra of synthesized catalysts have similar trend in all methods indicating that all catalysts have same functional groups. As shown in Fig. 5(A), the bands observed at 1060, 961, 801 and 510 cm−1 were

3.2. Catalytic performance of the catalysts The effect of preparation methods on CO2 reforming of CH4 over NiCe/SBA-15 was studied in temperature range of 600–800 °C and the results are illustrated in Fig. 7. In order to clarify the role of Ni and Ce, reaction of bare SBA-15 and Ni/SBA-15 was carried out at 800 °C. In this case, Ni/SBA-15 was prepared by conventional wet impregnation

Fig. 5. FTIR spectra of KBr in the range of (A) 1400–500 cm−1 and (B) 3800–2500 cm−1 of (a) SBA-15, (b) Ni-Ce/SBA-15(C-IM), (c) Ni-Ce/SBA-15(US-IM) and (d) Ni-Ce/SBA15(R-IM).

Fig. 6. XPS spectra of O 1s of (a) Ni/SBA-15 and (b) Ni-Ce/SBA-15(US-IM).

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Fig. 7. Average (A) CH4 conversion, (B) CO2 conversion and (C) H2/CO ratio of SBA-15, Ni-Ce/SBA-15(CIM), Ni-Ce/SBA-15(US-IM) and Ni-Ce/SBA-15(R-IM) in CO2 reforming of CH4. Reaction conditions: mcat = 0.2 g, F = 50 mL/min, CH4:CO2:N2 = 1:1:1, P = 1 atm, time on stream = 5 h.

et al. [38] reported that strong metal-support interaction triggered the dissociation of CH4 on Ni surface to produce Cads and Hads that Cads reacted with O atoms in CeO2 to produce COx species. The H atoms desorbed from the catalyst surface as H2 or H2O. On the other hand, the presence of oxygen vacancies (Vac) in CeO2 may lead to the path of CO2 (g) + Vac → CO (ads) + O-Vac. Pal et al. [39] stated that the presence of strong Lewis basicity and oxygen vacancies in CeO2 due to the highly mobile surface oxygen and redox capability (Ce4+/Ce3+) in CeO2 can facilitate CO2 adsorption, minimize the CO formation and further enhance the catalytic activity. The catalytic performances of our catalysts at 800 °C are compared with the Ni-Ce/SBA-15 catalysts from the literatures prepared by βcyclodextrin-modified impregnation method (Ce = 10 wt%, Ni = 4.8 wt%) [5] and incipient impregnation method (Ce/Si molar ratio = 0.06, Ni = 12 wt%) [6], as shown in Fig. 7. It should be noted

method. It was observed that the reaction of bare SBA-15 shows lower activity (< 5% of CO2 and CH4 conversion), indicating that the metallic sites are necessary in the studied catalytic reaction. In thermodynamic considerations, a reaction is thermodynamically favorable when Gibbs free energy is negative (ΔG° < 0). As the ΔG° value is positive for CO2 reforming of CH4 at lower temperatures, thus reaction is not spontaneous. For all temperature studied, it was observed that increasing temperature leads to an increase in CO2 and CH4 conversion because of the endothermic nature of CO2 reforming of CH4. The CH4 conversion was slightly higher than that of CO2 above 700 °C might be due to the CH4 decomposition (CH4 → C + 2H2) at higher temperatures. Ni-Ce/ SBA-15(US-IM) exhibited the highest activity for CH4 and CO2 conversions compared to Ni-Ce/SBA-15(C-IM) and Ni-Ce/SBA-15(R-IM) catalysts. The CH4 and CO2 conversion reached 96.3% and 93.5%, respectively at 800 °C for Ni-Ce/SBA-15(US-IM), possessing the highest catalytic activity. The conversion of Ni/SBA-15 showed lower CH4 and CO2 conversion than Ni-Ce/SBA-15 catalysts indicating the positive role of CeO2 on the catalytic performance of the catalyst. The H2/CO ratios of all Ni-Ce/SBA-15 catalysts at 800 °C were almost 1. The H2/CO ratios obtained in the range of 0.8-1.15 is said to be suitable for the application of Fischer-Tropsch synthesis [16]. In brief, the catalytic performance of catalysts followed the order of SBA-15 < Ni/SBA-15 < NiCe/SBA-15(C-IM) < Ni-Ce/SBA-15(R-IM) < Ni-Ce/SBA-15(US-IM). According to Wang et al. [6], CO2 reforming of CH4 proceed with the adsorption of CH4 followed by decomposition of CH4 into CHx fragments on the Ni sites, while CO2 was adsorbed on the CeO2-SBA-15 support. As indicated in mentioned mechanism, the presence of high Ni dispersion which is accessible for the reactant molecules appears as a prerequisite to ensure good catalytic performances in CO2 reforming of CH4. Therefore, it is reasonable to assume that the excellent catalytic performance of Ce/SBA-15(US-IM) is closely associated with the availability of great quantity of active Ni sites on the surface of the catalyst. The presence of high quantity of active Ni sites was achieved by the presence of smaller size of Ni particles and well-dispersed Ni particles produced from a strong metal-support interaction, proved by the XRD, BET, FTIR and H2-TPR analysis. Han et al. [37] reported that the turnover frequency for the methane conversion on the Ni surface was dependent on the size of the Ni nanoparticles whereby the smallest nanoparticles (2.6 nm) exhibited a CH4 turnover frequency 4.1 times greater than that of the larger particles (17.3 nm). Additionally, Liu

Fig. 8. Comparison of the CH4 conversion (solid fill) and CO2 conversion (pattern fill) obtained at 800 °C in this study (GHSV = 15000 mL g−1 h−1, CO2/CH4 = 1, mcat = 0.2 g) and in the literatures (Ref. [5]: GHSV = 35000 mL g−1 h−1, CO2/CH4 = 1, mcat = 0.2 g; Ref. [6]: GHSV = 36000 mL g−1 h−1, CO2/CH4 = 1, mcat = 0.1 g).

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Fig. 9. (A) CH4 conversion, (B) CO2 conversion and (C) H2/CO ratio of stability tests over Ni/SBA-15, NiCe/SBA-15(C-IM), Ni-Ce/SBA-15(US-IM) and Ni-Ce/ SBA-15(R-IM) in CO2 reforming of CH4. Reaction F = 50 mL/min, conditions: mcat = 0.2 g, CH4:CO2:N2 = 1:1:1, P = 1 atm, T = 800 °C.

Khoshbin and Highighi [27] reported that employing ultrasound energy for preparation of CuO-ZnO-Al2O3/HZSM-5 has greatly influences the dispersion and catalytic activity of catalyst. CuO-ZnO-Al2O3/HZSM-5 prepared by combined co-precipitation-ultrasound method has higher dispersion and catalytic performance compared with impregnation and co-precipitation-physically mixing methods. Moreover, Abdollahifar et al. [24] found that Co/Y prepared by ultrasound method had a higher activity towards CO2 reforming of CH4 owing to its favorable structure properties which are smaller particle size, higher surface area and higher dispersion of active components. The favorable structure properties of catalyst resulted to the higher adsorption of CH4 and CO2 which it reinforced the reaction (CH4 + CO2 ↔ 2CO + 2H2). In addition, Shanmugam et al. [40] reported that highly dispersed and size controlled Ni nanoparticles through strong Ni-promoter interactions can be achieved by ultrasound treatment. Both CeO2 and ZrO2 promoters prevented the Ni particle sintering and inhibited the coke formation, thus, demonstrated superior catalytic performance in steam reforming of propylene glycol. Moreover, Rahmani et al. [41] reported that ultrasound-assisted impregnation hybrid synthesis method not only offers good dispersion of chromium metal particles by decreasing their particle size, but also promoted the chromium-support interaction and preserved catalyst sustainability (Fig. 8).

that CO2 conversion of Ni-Ce/SBA-15 reported in literatures was slightly higher than that of CH4 due to the contribution of the reversed water-gas shift (RWGS) reaction (CO2 + H2 → H2O + CO). Ni-Ce/SBA15(US-IM) shows promising catalytic activity with CH4 and CO2 conversions of 96.3% and 93.5%, respectively, indicating the positive role of ultrasonic waves during impregnation process. The high catalytic activity in CO2 reforming of CH4 was related to the presence of smaller metal particles, stronger metal-support interaction and highly dispersed metal particles upon irradiation with ultrasonic waves. Moreover, Ce/ SBA-15(US-IM) is more profitable in regards to the amount of metal loading used. This study clearly indicated that the method of introducing metal plays an important role in controlling the particle size and activity of the catalyst. This result in agreement with the studies reported elsewhere [27,28]. Sidik et al. [28] found that the different Ni-loading methods led to different structures and performance of Ni/MSN catalyst. By comparing with all preparation methods, Ni/MSN prepared by in-situ method showed the highest catalytic activity owing to its stronger metal-support interaction, smaller Ni° crystallite size, and higher number of structure defects and basic sites. It is noteworthy that the ultrasonic technique has been extensively used as an efficient method for preparation of homogeneous and dispersed metal particles.

Fig. 10. (A) TGA curves and (B) XRD patterns of spent (a) Ni-Ce/SBA-15(C-IM), (b) Ni-Ce/SBA15(US-IM) and (c) Ni-Ce/SBA-15(R-IM) at reaction temperature of 800 °C. (C) TEM image of spent NiCe/SBA-15(US-IM).

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promising catalyst. The irradiation of ultrasonic waves during impregnation process endowed the catalyst with smaller metal particles, stronger metal-support interaction and more homogenous metal dispersion. The smaller metal particles arose from the strong metal-support interaction would definitely be more advantageous to maintain the size of the metal at high reaction temperature, and thus ensuring high catalytic activity and better coke resistance. This study revealed that ultrasonic-assisted impregnation method (US-IM) successfully altered the properties of Ni-Ce/SBA-15 towards an excellent catalytic performance of CO2 reforming of CH4.

A long-term stability test of 48 h was performed for Ni/SBA-15 and Ni-Ce/SBA-15 catalysts at 800 °C. As depicted in Fig. 9, the CO2 and CH4 conversion for Ni/Ce-SBA-15(US-IM) was 95.09% and 97.72%, respectively, at the beginning of the reaction and almost remained constant during 48 h of reaction. However, the catalytic performance of other catalysts declined with time on stream due to the formation of large amount of coke via two routes, CH4 decomposition/cracking (CH4 ↔ C(s) + 2H2) occurs at temperature > 557 °C and Boudouard (2CO ↔ C(s) + CO2) reaction occurs at temperature < 700 °C [9]. A quantitative analysis of carbon formation over the spent Ni-Ce/SBA-15 catalysts was analyzed using TGA and the results are shown in Fig. 10(A). The weight loss at temperature lower that 200 °C was associated to the removal of adsorbed water, whereas the weight loss higher than 400 °C arose from the oxidation of carbon [28]. For all catalysts, the weight loss of the carbon residue followed a two-stage pattern indicating the different types of carbon were deposited. The weight loss at low temperature (< 600 °C) can be attributed to the oxidation of amorphous carbon, while the weight loss at high temperature (> 600 °C) can be attributed to the oxidation of the graphitic carbon [5]. The total weight loss of spent catalysts followed the order of Ni-Ce/SBA-15(US-IM) < Ni-Ce/SBA-15(R-IM) < Ni-Ce/SBA-15(CIM). The result obtained from TGA analysis in agreement with the XRD analysis of the spent catalyst shown in Fig. 10(B). The XRD pattern of the spent catalysts exhibited a peak at 2θ = 26.1°, which can be assigned to the crystallized graphitic carbon structure [28]. The intensity of the peak is the lowest for the Ni-Ce/SBA-15(US-IM) compared to the other catalysts, indicating a lower carbon formation on the surface of the catalyst. As shown in Table 1, NiO crystallite size for spent Ni-Ce/ SBA-15(US-IM) showed negligible differences as compared to the fresh sample, implying no agglomeration of Ni particles during reaction. Moreover, by comparing the TEM images of spent (Fig. 10(C)) with fresh Ni/Ce-SBA-15(US-IM) (Fig. 2(B)), it was observed that the hexagonal mesopores were retained even after the sample has been used in the CO2 reforming of CH4 for 48 h. In addition, no aggregation was observed and metal particles still maintained on the surface of catalyst. This result can be explained by the presence of strong metal-support interaction which prevents active metal particles lifted from the support thus minimizing the growth of the carbon formation. The positive role of strong metal-support interaction in coke reduction was also reported by Das et al. [17] for CeNiMg/Al catalyst. They found that well-dispersed nickel nanoparticels on the support provides better metal-support interaction, which plays an effective role in reducing coke. In addition, the presence of Ce, which acts as an oxygen donor, reduces further any coke deposition on the metal support.

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4. Conclusion In this study, a series of Ni-Ce/SBA-15 catalysts with 6 wt% Ce and 5 wt% Ni were prepared by three methods, which are conventional impregnation (Ni-Ce/SBA-15(C-IM)), ultrasonic-assisted impregnation (Ni-Ce/SBA-15(US-IM)) and reflux-assisted impregnation (Ni-Ce/SBA15(R-IM)) methods. The results showed that the different Ni-Ce loading methods significant influence the properties and catalytic activities of the catalyst. The characterization analyses showed that the homogeneity of metal dispersion and metal-support interaction followed the order of Ni-Ce/SBA-15(C-IM) < Ni-Ce/SBA-15(R-IM) < Ni-Ce/SBA15(US-IM). FTIR analysis revealed the presence of metal support interaction, Si-O-M through the substitution of OeH with OeM. At 800 °C, the activity and the stability of Ni-Ce/SBA-15 following the order of Ni-Ce/SBA-15(US-IM) (CH4 conversion = 96.3%, CO2 conversion = 93.5%, H2/CO = 1.02) > Ni-Ce/SBA-15(R-IM) (CH4 conversion = 93.0%, CO2 conversion = 91.3%, H2/CO = 1.07) > Ni-Ce/ SBA-15(C-IM) (CH4 conversion = 90.5%, CO2 conversion = 87.9%; H2/CO = 1.09). On the basis of the results obtained, ultrasonic-assisted impregnation method (US-IM) seems to be very effective to attain the 752

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