SBA-15 catalysts prepared by urea co-precipitation for dry reforming of methane

Applied Catalysis A, General 554 (2018) 95–104

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Bimetallic Ni-Co/SBA-15 catalysts prepared by urea co-precipitation for dry reforming of methane Jinni Xin, Hongjie Cui, Zhenmin Cheng, Zhiming Zhou

T

⁎

State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai, 200237, China

A R T I C L E I N F O

A B S T R A C T

Keywords: Dry reforming of methane Ni-Co/SBA-15 Urea co-precipitation Structure-sensitive Coke- and sintering-resistance

A series of NixCo(10-x)/SBA-15 catalysts with constant metal loading (10 wt%) and different Co/Ni mass ratios (0/ 10–10/0) were prepared by urea co-precipitation and applied to the dry reforming of methane (DRM). The physicochemical properties of the catalysts were characterized in detail by various techniques (N2 physisorption, H2 chemisorption, XRD, TPR, ICP-OES, TGA, HRTEM and XPS), revealing that Ni and Co particles were embedded into the channel of SBA-15 and Ni was alloyed with Co. In addition, the bimetallic catalysts with a Co/Ni ratio (RCo/Ni) lower than 1 had relatively high metal dispersion (ca. 17–20%) and small metal particle sizes (ca. 4–5 nm). The DRM activity and stability of NixCo(10-x)/SBA-15 in terms of coke- and sintering- resistance were systematically examined and compared, which showed that the activity and stability of the catalysts depended strongly on the metal particle size, indicating the structure-sensitive character of the DRM reaction. Compared to monometallic Ni10/SBA-15 and Co10/SBA-15, bimetallic NixCo(10-x)/SBA-15 with RCo/Ni < 1 possessed improved activity and stability, which was mainly attributed to the small metal particle size, the synergetic effect of Co and Ni as well as the confinement effect of SBA-15 mesoporous channels. In particular, Ni9Co1/SBA-15 exhibited the highest activity, stability and H2/CO molar ratio (close to 1) during 50 h of operation at a gas hourly space velocity of 72000 mL/(gcat h) under 973 and 1073 K. Moreover, no coke deposition occurred on the catalyst, with only slight metal particle size growth to around 5 nm, indicating excellent coke- and sinteringresistance.

1. Introduction Dry reforming of methane (DRM), which makes use of two types of greenhouse gases, i.e., CH4 and CO2, is a promising technology for production of syngas that can be further transformed into liquid fuels through Fischer-Tropsch synthesis [1,2]. However, one of the major barriers for commercialization of the DRM process is the catalyst deactivation caused by metal sintering and coke deposition [3–5]. Although precious metal catalysts such as Pt, Pd, Ru and Rh normally possess high catalytic performance in DRM [6–8], the high cost limits their practical applications. In contrast, some non-precious metal catalysts such as Ni-based materials are competitive owing to their high activity and low cost, but unfortunately they are usually subject to deactivation. In recent years, many studies have been devoted to improving the DRM performance of non-previous catalysts in several ways, including utilization of high-surface-area supports [9,10], addition of second active metal into Ni (namely bimetallic catalysts) [11,12], addition of promoters [13,14] and encapsulation of Ni particles within a shell [15,16].

⁎

Compared to monometallic catalysts, bimetallic catalysts often show better DRM performance in terms of activity and stability, with examples including Ni-Fe/MgAl2O4 [17,18], Ni-W/carbide [19], Ni-Mo/ SBA-15 [20], Cu-Ni@SiO2 [21], Ni-Co/SBA-15 [22], Ni-Co/Al2O3 [23] etc. Several researchers [24–26] further compared the performance of bimetallic Ni-based catalysts and found that the Ni-Co catalysts were superior to others. For example, Zhang et al. [24] prepared a series of Ni-Me/Mg(Al)O (Me=Co, Fe, Cu, or Mn) catalysts using a coprecipitation method and claimed that the Ni-Co catalyst had superior activity and stability to other Ni-Me combinations. Yu et al. [25] reported that among the (Co, Cu, Mn or Zr)-promoted Ni catalysts derived from hydrotalcites, the initial DRM activity decreased in the order Ni-Co > NiCu > Ni-Mn > Ni-Zr; moreover, the Ni-Co catalyst showed excellent stability over 20 h. To date, supported Ni-Co bimetallic catalysts have mainly been prepared by impregnation method [11,22,26–30] and the obtained average metal particle size is normally above 8.0 nm [11,26–29], which exceeds the critical size of 7 nm needed for carbon formation [31,32]. In order to suppress coke deposition, the Ni-Co particle size should be

Corresponding author. E-mail address: [email protected] (Z. Zhou).

https://doi.org/10.1016/j.apcata.2018.01.033 Received 26 September 2017; Received in revised form 26 January 2018; Accepted 31 January 2018 Available online 02 February 2018 0926-860X/ © 2018 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic diagram of preparation of NixCo(10-x)/SBA-15.

autoclave. Finally, the resulting precipitate was filtered and washed with deionized water and absolute ethanol several times, dried at 353 K overnight, and calcined at 823 K for 6 h at a heating rate of 1 K/min.

smaller than the critical size, which could be achieved in two ways according to the literature [22,30,33]. The first way is the utilization of supports with high surface area that can facilitate the dispersion of metal particles. Indeed, even using the impregnation technique, highly dispersed Ni-Co catalysts with metal particle sizes as small as 3 nm for Ni-Co/SiO2 [30] and 3.3–6.9 nm for Ni-Co/SBA-15 [22] were prepared. However, the use of dispersing agents such as combination of oleic acid and oleylamine [30] and β-cyclodextrin [22] makes the preparation process somewhat complicated and cost-ineffective. The second way is the application of other preparation methods instead of impregnation. For example, a Ni-Co/CeO2-ZrO2 catalyst with an average metal particle size of 5.9 nm was prepared by a homogeneous precipitation method based on urea hydrolysis [33]. The advantage of this method is that the urea hydrolysis takes place slowly and uniformly, so that the precipitation process proceeds in a slow and controlled manner. Nevertheless, the stability of Ni-Co/CeO2-ZrO2 was poor, with about 50% decrease in the conversions of CH4 and CO2 after 80 h of operation at 823 K [33]; moreover, the average metal particle size of the spent catalyst increased up to 23 nm, indicating severe sintering. Considering the advantages of each method, it is reasonable to expect that if the two methods are combined, e.g., Ni and Co are deposited into the channels of SBA-15 via urea co-precipitation, the developed Ni-Co/SBA-15 would probably have extraordinary performance in the DRM reaction. Unfortunately, however, no such Ni-Co/SBA-15 catalysts are available to date. In this work, a series of Ni-Co/SBA-15 bimetallic catalysts with varying Co/Ni mass ratio were prepared by a simple urea co-precipitation method and applied to the DRM reaction. The structure-activity-stability relationships of the catalysts were systematically studied with emphasis on the effects of the Ni-o metal particle size and the alloying of Co with Ni. The results showed that the as-prepared Ni-Co/ SBA-15 with a low Co/Ni ratio had high activity and stability in terms of coke- and sintering-resistance.

2.3. Preparation of Ni-Co/SBA-15 catalysts A series of Ni-Co/SBA-15 catalysts with varying Co/Ni mass ratio (RCo/Ni) were prepared by a co-precipitation method with Ni(NO3)2 and Co(NO3)2 as the precursors and urea as the precipitator. The total loading of Ni and Co in each catalyst was prefixed at 10 wt% and the actual loading was determined by the inductively coupled plasma optical emission spectrometry. For simplicity, the developed catalysts were denoted as NixCo(10-x)/SBA-15, where x represented the mass fraction of Ni in the catalyst. The preparation scheme of NixCo(10-x)/ SBA-15 was depicted in Fig. 1. In a typical preparation (Ni6Co4/SBA-15 as an example), 0.33 g of Ni(NO3)2·6H2O and 0.22 g of Co(NO3)2·6H2O were first dissolved in 55 mL of deionized water, and then 1 g of SBA-15 powder was added and stirred for 1 h. Next, 1 g of urea was added into the above suspension, which was stirred at 363 K for simultaneously hydrolysis of urea and precipitation of nitrate salts. When the pH value of the suspension increased to 8, the heating and stirring process was terminated and the precipitate was aged for 6 h at room temperature. Finally, the precipitate was filtered and washed with deionized water several times, dried at 383 K overnight, and calcined at 773 K for 5 h with a heating rate of 2 K/min. Note that the RCo/Ni value of the catalyst was adjusted by varying the amount of precursors used. 2.4. Characterization of Ni-Co/SBA-15 catalysts The Brunauer-Emmett-Teller (BET) surface area, pore volume and pore size of SBA-15 and NixCo(10-x)/SBA-15 were acquired by N2 adsorption-desorption at 77 K (Micromeritics ASAP 2010). Before measurement the samples were degassed at 473 K under vacuum for 6 h. The metal-support interaction of catalysts was analyzed by H2 temperature-programmed reduction (TPR) using a Micromeritics AutoChem 2920 apparatus. About 50 mg of sample was first purged in Ar (30 mL/min) at 473 K for 0.5 h and cooled down to room temperature, and then the temperature was raised to 1173 K at a heating rate of 10 K/min in 10% H2/Ar (v/v, 30 mL/min). The metal dispersion of catalysts were measured by H2 chemisorption (Micromeritics AutoChem 2920) with the assumption of H/Ni(Co) stoichiometry of 1. The weighted sample was first reduced in 10% H2/Ar (v/v, 30 mL/min) at 1073 K for 2 h, and then purged with Ar at 1103 K for 0.5 h to remove H2. When the reactor was cooled to 308 K, H2 pulse chemisorption was carried out. The crystalline structure of catalysts was determined by Xray diffraction (XRD) (Rigaku D/Max 2550) using Cu Kα (λ = 0.15406 nm) radiation at small (2θ = 0.5–5°) and wide (2θ = 10–80°) angles. The actual metal loading of catalysts was obtained by inductively coupled plasma optical emission spectrometry (ICP-OES) (Agilent 725-ES). The fine structures of SBA-15 and NixCo(10x)/SBA-15 were characterized by high-resolution transmission electron microscopy (HRTEM) (JEOL JEM-2100). The surface phases of catalysts were analyzed by X-ray photoelectron spectroscopy (XPS) (Thermo

2. Experimental section 2.1. Materials and reagents Ni(NO3)2·6H2O (≥98.0%), Co(NO3)2·6H2O (≥99.0%), urea (≥99.0%), tetraethyl orthosilicate (TEOS, ≥98.0%), hydrochloric acid (HCl, 36.0–38.0%) and absolute ethanol (≥99.7%) were purchased from Sinopharm Group Chemical Reagent Co., Ltd., and Pluronic P123 (Mav = 5800) was from Sigma-Aldrich. All chemicals were used as received without further purification. 2.2. Preparation of SBA-15 Ordered mesoporous SBA-15 was prepared by a hydrothermal process with TEOS as the precursor and P123 as the template [34]. First, 30 g of deionized water and 120 g of HCl (2 mol/L) were mixed at 313 K, and then 4 g of P123 was added with stirring until P123 was completely dissolved. Next, 8.5 g of TEOS was injected into the above solution, and the mixture was stirred for 24 h at 313 K, followed by crystallization at 383 K for 24 h under static condition in a Teflon lined 96

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[40]: In the absence of external transfer limitations:

Scientific ESCALAB 250 Xi) using a monochromatic Al Kα source (hν= 1486.6 eV) and a pass energy of 40 eV. Binding energies of spectra were referenced to the C 1s binding energy set at 284.8 eV. The amount of carbon deposition on the spent catalyst was determined by oxidation conducted in a thermogravimetric analyzer (TGA) (PerkinElmer TGA4000). The TGA analysis was carried out at a heating rate of 10 K/min from room temperature to 1173 K under air atmosphere.

rCH4rpρc

⎧ cb,CH

4kc

< 0.15

⎨ (− Δ H)rCH4rpρc < 0.15 RT b hT b E ⎩

(6)

In the absence of internal transfer limitations: 2.5. Test of Ni-Co/SBA-15 catalysts

2

⎧ rCH4ρc rp < 1 ⎪ cb,CH4 De,CH4 ⎨ (− Δ H)rCH4ρc rp2 < ⎪ λT b ⎩

The DRM activity and stability of catalysts were evaluated in a quartz tube (i.d. = 8 mm) at atmospheric pressure. For each catalyst, two gas hourly space velocity (GHSV) values of 36,000 and 72,000 mL/ (gcat h) were applied, which were realized by varying the catalyst loading (0.1 g for 36,000 mL/(gcat h) and 0.05 g for 72,000 mL/(g h)) while maintaining the inlet flow rates of CH4 and CO2 at 30 NmL/min each. The reason for using two GHSVs is that 36,000 mL/(gcat h) is among the typical GHSVs for the DRM (20,000–40000 mL/(gcat h)) [10,22,35,36], while 72,000 mL/(gcat h) is located within a relatively high GHSV region [37,38], which is more interesting in view of practical applications. In a typical experiment, 0.1 g of catalyst with an average particle size of 0.34 mm (40–60 mesh) was premixed with 0.5 g of quartz sand of the same size, and then loaded into the center of the quartz reactor that is placed in a three-zone furnace. Before the DRM reaction, the catalyst was reduced with a temperature ramp of 10 K/min from room temperature to 1073 K in a stream of 30% H2/N2 (v/v, 100 mL/min), then held at 1073 K for 2 h. Two reaction temperatures, i.e., 973 and 1073 K, were used to test the catalysts, the former aiming at evaluation of the coke-resistant property of catalysts, while the latter at the sintering-resistant property. The reason lies in that the carbon deposition is favored thermodynamically at 873–973 K, whereas the sintering is facilitated at 1073 K or above [7,39]. The flow rate of the dry product gas leaving an ice trap was measured by using a soap film flowmeter, and the gas composition was analyzed on-line by a dual channel MicroGC (INFICON 3000) with a thermal conductivity detector (TCD). After each experiment, the overall carbon balance was calculated by comparing the carbon input (CH4 and CO2) with the carbon output in the product gas (CH4, CO2 and CO) plus the carbon deposition on the catalyst. The carbon balance for all experiments was found to be larger than 96% but lower than 100%. Indeed, a small amount of coke deposition was always observed on the reactor wall near the outlet (about 15 cm away from the catalyst bed), which, formed by the Boudart reaction, was not taken into account in the calculation of the carbon balance. The conversions of CH4 (α CH4 ) and CO2 (α CO2 ), the selectivities of H2 (SH2 ) and (SCO ), and the H2/CO molar ratio (R H2 /CO) are calculated by the following equations:

α CH4 =

FCH4,in−FCH4,out × 100% FCH4,in

(1)

α CO2 =

FCO2,in−FCO2,out × 100% FCO2,in

(2)

SH2 =

FH2,out × 100% 2 × [FCH4,in−FCH4,out]

(3)

SCO =

FCO,out × 100% [FCH4,in−FCH4,out] + [FCO2,in−FCO2,out]

(4)

R H2 /CO =

FH2,out FCO,out

RT b E

(7)

The notation of the variables and parameters in Eqs. 6 and 7 as well as the detailed calculation process are given in the Supplementary Material (Appendix A). The calculation results demonstrate that both external and internal mass and heat transfer limitations can be neglected in this study. 3. Results and discussion 3.1. Physicochemical properties of SBA-15 and Ni-Co/SBA-15 Fig. 2 shows the structure properties of SBA-15. The N2 sorption isotherm (Fig. 2(a)) is type IV with a H1 hysteresis loop at relative pressure (p/p0) = 0.6–0.8, characteristic of uniform mesoporous structure (inset in Fig. 2(a)) [41,42]. The BET surface area, pore volume and average pore diameter of SBA-15 are 775 m2/g, 1.05 cm3/g and 6.0 nm, respectively (Table 1). The small-angle XRD pattern of SBA-15 (Fig. 2(b)) shows three well resolved peaks at 2θ = 1.1, 1.8 and 2.0°, corresponding respectively to (100), (110) and (200) reflections associated with the two-dimensional hexagonal p6 mm structure [43,44]. The (100) lattice spacing (d100) is about 8.0 nm, from which the unitcell parameter (a0 = 2d100/ 3 ) [45] and the wall thickness (subtracting the pore diameter from a0) are calculated to be 9.2 and 3.2 nm, respectively. Such a large wall thickness contributes to the high thermal and hydrothermal stability of SBA-15 [26]. The highly ordered 2D hexagonal mesostructure of SBA-15 is further confirmed by the HRTEM images (Fig. 2(c)), from which the pore diameter and wall thickness are measured to be 5.9 and 3.4 nm, respectively, which are in good agreement with the data determined by N2 physisorption and XRD. The mesoporous structures of all catalysts are preserved after introduction of Ni and Co species (Fig. B1 and related discussion in Appendix B), but the BET surface areas and pore volumes of the catalysts decrease significantly in comparison with SBA-15 (Table 1), which is possibly caused by the occupation of the channels and surface of SBA15 by Ni and Co species. The actual metal loading and the Co/Ni mass ratio of various catalysts are close to the nominal values, demonstrating the effectiveness of the urea co-precipitation method for preparing NiCo/SBA-15 catalysts. Fig. 3 presents the XRD patterns of fresh NixCo(10-x)/SBA-15 without and with reduction. There exist obvious diffraction peaks in the smallangle range for all samples, indicating that the ordered mesoporous structure of the parent SBA-15 is not destroyed by the introduction of Ni and Co species. However, the (100) plane peak of the catalysts shifts toward a smaller angle (2θ < 1.1°) compared to SBA-15, reflecting a larger d(100)-spacing and an increased unit-cell parameter of the catalysts. This is caused by the substitution of Ni and Co into the mesoporous framework of SBA-15, considering that the bond lengths of Ni-O (1.93-2.07 Å) and Co-O (1.94-2.09 Å) [46] are larger than that of Si-O (1.63-1.65 Å) [47]. The lattice expansion owing to the incorporation of metals such as Ce [43], La [48] and Y [49], into the SBA-15 framework has been reported previously. For the catalysts without reduction (Fig. 3(a)), the broad peak at 2θ = 15–30° in the wide-angle range is related to the SiO2 amorphous phase, while the small amorphous peaks at 30–40° and about 60° are indexed to nickel and cobalt oxides [50].

(5)

where Fi,in and Fi,out are the inlet and outlet molar flow rates of species i, respectively. Under the above evaluation conditions, the mass and heat transfer limitations (both external (interphase) and internal (intraparticle)) are assessed by applying the well-known criteria as follows 97

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Fig. 2. (a) N2 adsorption-desorption isotherm (the inset is the pore size distribution), (b) small-angle XRD pattern, and (c) HRTEM images of SBA-15.

After reduction at 1073 K (Fig. 3(b)), the nickel and cobalt oxide phases disappear, but new peaks (2θ = 40–54°) assigned to metallic Ni and Co appear, indicating the reduction of nickel and cobalt oxides into Ni and Co. By zooming in the patterns from 40° to 47°, it is found that the peak (Ni(111) plane (2θ = 44.6°) for Ni10/SBA-15 and Co(111) (2θ = 44.2°) for Co10/SBA-15) shifts slightly to smaller angle with increasing RCo/Ni, implying the possible formation of the Ni-Co alloy [30,51]. The average crystallite sizes of the metal particles (dXRD) for all reduced catalysts are calculated by the Scherrer equation from the full width at half maximum (FWHM) of the (111) diffraction peak, which are in the range of 4.2–8.1 nm depending on RCo/Ni (Table 2). In general, a small doping of Co (0 < RCo/Ni < 1) helps to reduce the metal particle size. The HRTEM images of NixCo(10-x)/SBA-15 (Fig. B2 and related discussion in Appendix B) reveal that the ordered mesoporous structures of all catalysts are indeed well preserved and many particles are embedded into the channels of SBA-15. A comparison of the micromorphologies (top view) of SBA-15 and Ni9Co1/SBA-15 (Fig. 4) clearly shows the particles located inside the channels as indicated by the dashed circle. As summarized in Table 2, the TEM-derived average metal particle size is very close to the XRD-derived size, and Ni9Co1/ SBA-15 has the smallest metal particle size among all the catalysts

Table 1 Textural properties of SBA-15 and NixCo(10-x)/SBA-15. Catalysts

SBA-15 Ni10/SBA15 Ni9Co1/ SBA-15 Ni8Co2/ SBA-15 Ni6Co4/ SBA-15 Ni5Co5/ SBA-15 Ni3Co7/ SBA-15 Co10/SBA15 a b

SBET (m2/g)

Vporea (cm3/g)

dporea (nm)

Actual loadingb (wt%)

RCo/Ni

Ni

Co

nominal

actual

775 320

1.05 0.79

6.0 10.0

− 9.6

− −

− −

− −

316

0.72

8.7

8.9

1.1

0.11

0.12

294

0.85

14.0

7.7

2.0

0.25

0.26

327

0.79

9.2

5.5

3.9

0.67

0.71

285

0.68

10.2

4.6

4.8

1

1.04

303

0.67

8.3

2.8

7.1

2.33

2.54

283

0.62

8.3

−

9.7

−

−

Calculated from the desorption branch of the N2 isotherm by the BJH method. Determined by the ICP-OES analysis.

Fig. 3. XRD patterns of various catalysts: (a) without reduction; (b) with reduction.

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Table 2 Metal dispersion, particle size and carbon deposition of NixCo(10-x)/SBA-15. Catalyst

Spentd

Fresh

Ni10/SBA-15 Ni9Co1/SBA-15 Ni8Co2/SBA-15 Ni6Co4/SBA-15 Ni5Co5/SBA-15 Ni3Co7/SBA-15 Co10/SBA-15

H2 uptake (cm3/g)a

Dispersion (%)a

dchema (nm)

dXRDb (nm)

dTEMc (nm)

dXRDb (nm)

dTEMc (nm)

carbon depositione (wt%)

3.02 3.78 3.21 3.51 2.79 2.50 1.97

15.8 19.8 16.8 18.4 14.6 13.1 10.3

6.4 5.1 6.0 5.5 6.9 7.7 9.8

5.1 4.2 4.7 4.3 5.5 6.5 8.1

4.9 4.1 4.8 4.5 5.6 6.3 7.9

5.3 4.5 5.1 4.8 5.9 6.8 13.5

5.2 4.3 5.2 4.7 5.8 6.5 11.8

12.6 n.d. 1.2 n.d. 4.4 n.d. n.d.

a

Measured by H2 chemisorption. Calculated by the Scherrer equation from the XRD pattern. c Determined by the HRTEM analysis. d Reaction condition: 973 K, 0.1 MPa, 72,000 mL/(gcat h), CO2/CH4 (molar ratio) = 1, TOS = 30 h except 50 h for Ni9Co1/SBA-15, 7 h for Ni5Co5/SBA-15, 1.5 h for Ni3Co7/SBA-15 and 0.5 h for Co10/SBA-15. e Determined by the TGA analysis. b

Co10/SBA-15 Ni3Co7/SBA-15 Ni5Co5/SBA-15 Ni6Co4/SBA-15 Ni8Co2/SBA-15 Ni9Co1/SBA-15 Ni10/SBA-15

200

400

600

800

1000

Fig. 5. H2-TPR profiles of various catalysts.

interaction. For all monometallic and bimetallic catalysts, it can be seen that the area (or the amount of hydrogen consumption) of the hightemperature peak is much larger than that of the low-temperature peak, implying a larger proportion of small metal particles in the catalysts. This is in line with the small particle size of 4.2–8.1 nm as determined by the XRD analysis. Regarding the bimetallic catalysts, both the lowand high-temperature reduction peaks shift to higher temperatures with increasing RCo/Ni, suggesting the formation of Ni-Co alloy (NixCoyO) [22,53]. More detailed information about the Ni-Co alloy will be provided in the following HRTEM and XPS analysis. The H2 uptake, metal dispersion and average metal particle size of NixCo(10-x)/SBA-15 measured from H2 chemisorption are listed in Table 2. The metal dispersion is calculated based on the total metal loading [29]. The bimetallic catalysts with a low Co/Ni ratio (0 < RCo/ Ni < 1) have relatively high H2 uptake and metal dispersion as well as small particle size. Note that the chemisorption-derived metal particle size (dchem) is somewhat larger than that from XRD and HRTEM, probably because a minority of particles is confined in the inaccessible regions of pores, which leads to a lower H2 uptake per gram of metal and in turn a relatively larger particle size than the actual size. However, although the metal particle size varies slightly with the analysis method, the variation pattern of metal particle size with RCo/Ni is the same irrespective of the analysis method applied. Fig. 6 further shows the lattice fringe images of Ni10/SBA-15, Ni9Co1/SBA-15, Ni6Co4/SBA-15 and Co10/SBA-15 as well as the fast Fourier transform (FFT) patterns. Ni10/SBA-15 displays a lattice spacing of 2.026 Å (determined by FFT), corresponding to the (111) plane of face-centred cubic (fcc) Ni [54], while Co10/SBA-15 shows lattice

Fig. 4. Top-view HRTEM images of SBA-15 and Ni9Co1/SBA-15.

investigated. Fig. 5 shows the H2-TPR profiles of various NixCo(10-x)/SBA-15 catalysts. The monometallic Ni10/SBA-15 displays three reduction peaks at around 650, 850 and 910 K, which corresponds to the weak, moderate and strong interactions between metal and support, respectively [14,22,50,52]. The weakly bound NiO species are normally present in the form of large particles, whereas the strong bound NiO generally exist in fine particles [14,22]. For Co10/SBA-15, the main peak at about 990 K is attributed to the reduction of Co oxides with strong interaction with the support [8,53]. Compared to the monometallic catalysts, two reduction peaks are present for bimetallic NixCo(10-x)/SBA-15: the former at 660–740 K is assigned to the weak metal-support interaction, while the latter at 920–970 K to the strong 99

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Fig. 6. Lattice images and fast Fourier transformation (FFT) images (insets) of fresh catalysts.

spacings of 1.747 and 2.053 Å assigned to the (200) and (111) planes of fcc Co, respectively [51,55]. As regards the bimetallic catalysts, lattice spacings of 2.030 and 2.035 Å are present for Ni9Co1/SBA-15 and Ni6Co4/SBA-15, respectively, which are larger than that of Ni(111) but smaller than that of Co(111) and can be ascribed to the (111) plane of Ni-Co solid solution phase [28,51], indicating the formation of Ni-Co alloy. It should be noted that some Co particles of Co10/SBA-15 appear larger than the diameter of the channels, indicating that these particles are deposited on the external surface of SBA-15. However, these particles probably only account for a small proportion of the total metal particles because of the urea precipitation method used in this study (Fig. 1): the metal ions are first diffused into the channels of SBA-15, after which the urea precipitation process takes place. Therefore, a majority of the metal ions are precipitated and deposited in the channels of SBA-15. The alloying of Ni with Co is further evidenced by XPS analysis. As shown in Fig. 7(a), there are two peaks at the binding energies (BEs) of

852.4 and 856.3 eV in the Ni 2p XPS spectrum of Ni10/SBA-15, which are assigned to metallic Ni and NiO, respectively [23,56]. However, these peaks are shifted to higher BEs of 852.7 and 856.8 eV, respectively, for Ni6Co4/SBA-15. On the other hand, two peaks at 779.0 and 782.5 eV in the Co 2p XPS spectrum of Co10/SBA-15 (Fig. 7(b)), which are attributed to metallic Co and CoO, respectively [56,57], are negatively shifted to lower BEs of 778.5 and 781.7 eV, respectively, for Ni6Co4/SBA-15. The results indicate a charge transfer from Ni to Co, confirming the formation of Ni-Co alloy. Note that although all the samples are reduced ex situ at 1073 K for 2 h before the XPS analysis, the existence of NiO and CoO might be due to the oxidation by air during the sample transfer. It should be pointed out that the BE shifts of the Ni 2p and Co 2p peaks are comparable to the literature data. Wu et al. [22] reported that Ni-Co/SBA-15 had a Ni 2p BE shift of +0.1 eV relative to Ni/SBA-15 and a Co 2p BE shift of −0.3 eV relative to Co/ SBA-15. In another study made by Takanabe et al. [58], Ni-Co/TiO2 showed a Ni 2p BE shift of +0.1 eV relative to Ni/TiO2 and a Co 2p BE

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Ni 2p 856.3

852.4

Co 2p

782.5

Co10/SBA-15

779.0

Ni6Co4/SBA-15

890

Intensity / a.u.

Ni10/SBA-15

856.8

781.7

Ni6Co4/SBA-15

778.5

852.7

880 870 860 850 Binding energy / eV

810

840

800

790

780

Binding energy / eV

Fig. 7. (a) Ni 2p and (b) Co 2p XPS spectra of Ni10/SBA-15, Co10/SBA-15 and Ni6Co4/SBA-15.

shift of −0.1 eV relative to Co/TiO2. It has been found in the literature that the Ni particle size can be reduced to some extent by alloying with Co [59,60], which rationalizes the smaller metal particle sizes of Ni9Co1/SBA-15, Ni8Co2/SBA-15 and Ni6Co4/SBA-15 when compared to that of Ni10/SBA-15. Nevertheless, Ni5Co5/SBA-15 and Ni3Co7/SBA-15 have larger metal particle sizes than Ni10/SBA-15, suggesting that not all Co is alloyed with Ni and a relatively larger number of isolated Co particles are anticipated for the NixCo(10-x)/SBA-15 with a higher RCo/Ni.

[18,61]. Under a GHSV of 36,000 mL/(gcat h) (Fig. 8(a)), it appears that increasing the Co content in the catalysts generally leads to decreased conversions of CH4 and CO2 as well as a lower H2/CO ratio. Moreover, when RCo/Ni ≥ 1, the catalyst exhibits a dramatically reduced activity with time-on-stream (TOS), indicating poor stability in the DRM reaction. In particular, Co10/SBA-15 displays the lowest DRM activity and the worst stability, which can be ascribed to the intrinsic low activity of Co as the active metal for DRM [23,62] and the fast deactivation caused by Co oxidation [53,63] (see later discussion). From the above results it seems that the addition of Co is unfavorable for the DRM over Ni/SBA15. However, when the catalysts are tested at a high GHSV of 72,000 mL/(gcat h) (Fig. 8(b)), different results are obtained. At the GHSV of 72,000 mL/(gcat h), the conversions of CH4 and CO2 decrease as expected when compared to those at 36,000 mL/(gcat h), but the catalysts with a relatively low content of Co (0 < RCo/Ni < 1), i.e., Ni9Co1/SBA-15, Ni8Co2/SBA-15 and Ni6Co4/SBA-15, have higher DRM activity and yield larger R H2 /CO than Ni10/SBA-15. Considering that the conversions of CH4 and CO2 over the catalysts (RCo/Ni < 1) at the GHSV of 36,000 mL/(gcat h) approach the equilibrium values and mostly affected by the thermodynamic equilibrium, the data obtained

3.2. DRM performance of Ni-Co/SBA-15 catalysts First, all catalysts are tested at 973 K to evaluate the DRM activity and anti-coking property. The results are shown in Fig. 8 with the equilibrium conversions of CH4 (72.6%, calculated by Aspen Plus® assuming no carbon deposition) and CO2 (82.2%) indicated by dashed lines. For each NixCo(10-x)/SBA-15, it is found that the conversion of CH4 is lower than that of CO2 and the H2/CO molar ratio in the product gas is slightly lower than the stoichiometric ratio of 1, which is mainly owing to the reverse water-gas shift reaction occurring in parallel

Fig. 8. Comparison of various catalysts in DRM (conditions: 973 K, 0.1 MPa, CH4/CO2 = 1, (a) GHSV = 36,000 mL/(gcat h), (b,c) GHSV = 72,000 mL/(gcat h)).

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and Ni5Co5/SBA-15, and the peak intensity of the former is stronger than that of the latter, revealing more coke deposited on Ni10/SBA-15, which agrees well with the TGA and HRTEM analysis made on the spent catalysts. The above results clearly indicate that among all the catalysts studied here, Ni9Co1/SBA-15 has the best DRM performance in terms of catalytic activity and anti-coking ability: after 50 h of TOS at 973 K and at a GHSV of 72,000 mL/(gcat h), the conversions of CH4 and CO2 and the H2/CO molar ratio in the product gas are stabilized at about 59%, 71%, and 0.96, respectively. This performance is comparable to or better than that of most of the Ni-based catalysts reported so far under similar testing conditions (Table 3). The conversions CH4 and CO2 reported by Mo et al. [9] and Amin et al. [67] are higher than our results because an inert gas (N2 or He) was co-fed with CH4 and CO2 in their studies, which decreases the partial pressures of CH4 and CO2 and thus favors the DRM reaction according to Le Chatelier's principle. However, the introduction of N2 or He into the feed will actually reduce the throughput of reactant gases if the GHSV value remains the same as that without inert gas diluent, let alone an increase in the cost of downstream separation. Next, Ni9Co1/SBA-15 is evaluated at 1073 K and a GHSV of 72,000 mL/(gcat h) to examine the sintering-resistant property, and Ni10/SBA-15 serves as a reference. Both catalysts are tested for 50 h of TOS and the results are presented in Fig. 10. The two catalysts show good activity, selectivity and stability in the DRM reaction in addition to yielding syngas with a H2/CO ratio of about 1, despite the fact that the conversions of CH4 and CO2 are a little smaller than the equilibrium values (90.4% for CH4 and 94.9% for CO2). The good performance of the catalysts mainly arises from the confinement effect of the ordered mesoporous structure, which can effectively prevent the growth of metal particles within the channels. Compared to fresh Ni10/SBA-15 and Ni9Co1/SBA-15, the metal particle size of spent catalysts only slightly increases to 5.7 and 5.0 nm, respectively (Fig. B3 and related discussion in Appendix B). In addition, No coke deposition occurs on spent catalysts according to the TGA analysis, which is reasonable due to the high temperature used (1073 K). When comparing the two catalysts, Ni9Co1/SBA-15 has higher activity, selectivity and stability than Ni10/SBA-15, which, together with the results obtained at 973 K, demonstrates the synergetic effect of Ni and Co in the DRM reaction.

at 72,000 mL/(gcat h) is more appropriate for analyzing the structureactivity relationship of NixCo(10-x)/SBA-15. According to the conversion data of CH4 shown in Fig. 8(b), the initial reaction rate of CH4 can be calculated, which is used to compare the initial DRM activity of each catalyst. Fig. 8(c) correlates the initial rate of CH4 with the metal particle size of catalyst. It is obvious that the initial rate of CH4 or the DRM activity of catalyst decreases with increasing the metal particle size. For Ni5Co5/SBA-15, Ni3Co7/SBA-15 and Co10/SBA-15, their initial activities are lower than that of Ni10/ SBA-15, which might be ascribed not only to the metal particle size, but also to the lower amount of Ni in these catalysts as compared to Ni10/ SBA-15 together with the intrinsic low activity of Co. However, for Ni9Co1/SBA-15, Ni8Co2/SBA-15 and Ni6Co4/SBA-15, although the Ni content is still lower than that of Ni10/SBA-15, the initial activity is higher (inset in Fig. 8(c)); moreover, there is no correlation between the catalytic activity and the amount of Ni contained in the three catalysts. These results demonstrate the dependence of the DRM activity on the metal particle size, indicating the structure-sensitive character of the DRM reaction on NixCo(10-x)/SBA-15. Similar results have been reported by other researchers [64,65]. On the other hand, all NixCo(10-x)/SBA-15 undergo a decay in activity with time-on-stream, and the decay rate of the catalytic activity follows the same order as that of the metal particle size. The decay in activity is mainly caused by coke deposition rather than metal sintering because only a slight increase in metal particle size occurs for all the spent catalysts compared to the fresh ones, but a remarkable coke deposition appears on some spent catalysts such as Ni10/ SBA-15 and Ni5Co5/SBA-15 (Table 2), which is also reflected by the HRTEM images (Fig. B2 and related discussion in Appendix B). It is well known that the carbon deposition is a structure sensitive reaction [31,66], which is favored by increasing the metal particle size. Therefore, the NixCo(10-x)/SBA-15 catalysts with small metal particle sizes, ultimately originating from the Ni-Co alloy, have better coke-resistant performance, just as reflected from Table 2. It should be noted that no coke deposition is detected on spent Ni3Co7/SBA-15 and Co10/SBA-15 because only 1.5 h and 0.5 h of TOS are performed, respectively. The reason for deactivation of Ni3Co7/SBA-15 and Co10/SBA-15 is the oxidation of Co, because the CoO phase is absent in the fresh reduced Ni3Co7/SBA-15 and Co10/SBA-15 (Fig. 3(b)) but present in the spent ones (Fig. 9). It has been reported that the Ni-Co alloy can protect the alloyed Co from oxidation in DRM [58], but as analyzed above, Ni3Co7/ SBA-15 has a relatively large amount of isolated Co, which after oxidation will yield much CoO. In contrast, for the bimetallic NixCo(10-x)/ SBA-15 with a RCo/Ni < 1, no CoO phase is observed in the XRD patterns of the spent catalysts, indicative of no or few isolated Co particles in these catalysts. A further observation from Fig. 9 is that a peak at 2θ = 26.4° attributed to the deposited carbon occurs for Ni10/SBA-15

4. Conclusions NixCo(10-x)/SBA-15 (x = 0–10 wt%) bimetallic catalysts were successfully prepared by a urea co-precipitation method and applied to the DRM process for syngas production. The ordered mesoporous structures of the catalysts were confirmed by small-angle XRD and HRTEM analysis, and the formation of Ni-Co alloy was evidenced by XRD, H2-TPR, HRTEM and XPS. The Co/Ni mass ratio had a direct effect on the metal particle size, which in turn strongly influenced the activity and stability of the catalysts. It was found that both activity and stability (obtained at 973 K and a GHSV of 72,000 mL/(gcat h)) decreased in the sequence Ni9Co1/SBA-15 > Ni6Co4/SBA-15 > Ni8Co2/SBA-15 > Ni10/SBA-15 despite the largest amount of Ni in Ni10/SBA-15, which was just opposite to the order of their metal particle sizes, clearly indicating the structure-sensitive character of the DRM reaction. In contrast to heavy coke deposition on Ni10/SBA-15 and fast oxidation of Co on Co10/SBA15, bimetallic NixCo(10-x)/SBA-15, especially those with a small Co/Ni ratio (< 1), exhibited significantly enhanced coke and oxidation resistance, demonstrating the synergetic effect of Co and Ni. The best catalyst Ni9Co1/SBA-15 also possessed high sintering resistance due to the confinement effect of the ordered mesoporous channels of SBA-15, whose metal particle size was only slightly increased to 5 nm after 50 h of operation at 1073 K and a GHSV of 72,000 mL/(gcat h).

Fig. 9. XRD patterns of spent catalysts (condition: 973 K, 0.1 MPa, CH4/CO2 = 1, GHSV = 72,000 mL/(gcat h)).

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Table 3 Comparison of different Ni-based catalysts in the DRM reaction.a Catalyst

5%Ni/La2O3-SiO2 Ni0.15Co0.05/Mg(Al)O Rh0.1Ni10/BN 20%Ni2%Yb/Al2O3 10%Ni/Sm2O3-La2O3 10%Ni/MgO Ni1Co2/ZSM-5 La2NiO4/ZSM-5 Ni9Co1/SBA-15 a b

GHSV (mL/gcat h)

72,000 72,000 60,000 52,000 48,000 96,000 60,000 48,000 72,000

CH4/CO2

1:1:1(N2) 1:1 1:1 1:1:2(He) 1:1 1:1:2(N2) 1:1:3(N2) 1:1 1:1

DRM activityb

α CH4

α CO2

R H2/CO

80 68 69 81 50 35 57 63 59

85 64 86 89 56 58 63 71 71

0.9 ∼1 0.7 ∼1 − 0.85 0.88 − 0.96

Coke (wt%)

Ref.

n.d. (100 h) 1.8% (20 h) 5.9% (6 h) trace (20 h) 17% (26 h) 3.9% (4 h) 5% (12 h) 20% (36 h) n.d. (50 h)

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Fig. 10. Comparison of Ni9Co1/SBA-15 and Ni10/SBA-15 in DRM (condition: 1073 K, 0.1 MPa, CH4/CO2 = 1, GHSV = 72,000 mL/(gcat h)).

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