Highly Active and Stable Ni–SiO2 Prepared by a Complex

Article pubs.acs.org/IECR

Highly Active and Stable Ni−SiO2 Prepared by a ComplexDecomposition Method for Pressurized Carbon Dioxide Reforming of Methane Hua-Ping Ren,† Yong-Hong Song,† Qing-Qing Hao,† Zhong-Wen Liu,† Wei Wang,‡ Jian-Gang Chen,† Jinqiang Jiang,† Zhao-Tie Liu,*,† Zhengping Hao,†,§ and Jian Lu*,‡ †

Key Laboratory of Applied Surface and Colloid Chemistry (MOE) and School of Chemistry & Chemical Engineering, Shaanxi Normal University, Xi’an 710119, China ‡ Department of Catalytic Technology, Institute of Xi’an Modern Chemistry, Xi’an 710065, China § Research Center for Eco-Environmental Science, Chinese Academy of Sciences, Beijing 10085, China ABSTRACT: A series of Ni−SiO2 catalysts was synthesized by the complex-decomposition method using different amino acids as complexing agents and fuels and nickel nitrate and tetraethoxysilane as precursors of Ni and SiO2, respectively. For comparison, ammonium hydroxide and acetic acid were also used as complexing agents and fuels. Characterization by XRD, TEM, and N2 adsorption−desorption at low temperature indicated that the structural and textural properties of the Ni−SiO2 catalysts were strongly dependent on the complexing agent used. The dispersion of metallic Ni was increased by optimizing the complexing agent used. As revealed from H2-TPR patterns, Ni−SiO2 exhibited significantly different interactions between Ni and SiO2, the extent of which was influenced by the complexing agent used. The Ni−SiO2 catalysts were comparatively evaluated for carbon dioxide reforming of methane (CDR) under the following conditions: CH4/CO2 = 1.0, T = 750 °C, GHSV = 53200 mL· g−1·h−1, and P = 1.0 atm. The results indicate that the Ni−SiO2 materials prepared with glycine, alanine, serine, threonine, valine, and proline exhibited much higher activity and stability for CDR under atmospheric conditions than those prepared with lysine, acetic acid, and ammonium hydroxide. Moreover, the Ni−SiO2 material prepared with glycine was also tested at elevated pressures of 5.0 and 10.0 atm. The effect of pressure on the CDR performance was investigated. Importantly, a highly active and stable Ni−SiO2 material for pressurized CDR was obtained by tailoring the structure of Ni−SiO2 and adjusting the interactions between Ni and SiO2 by selecting the complexing agent. Thus, the main factors determining catalyst activity and stability for CDR were clearly revealed.

1. INTRODUCTION With the development of modern industry, the greenhouse effect has received increasing attention in recent years because of the large amount of greenhouse gases released into the atmosphere.1 Therefore, the utilization of greenhouse gases is an imminent problem.1−3 Thus, the catalytic carbon dioxide reforming of methane (CDR) is a promising process for converting potent greenhouse gases (CO2 and CH4) into syngas with a H2/CO molar ratio close to 1, which is appropriate for the synthesis of valuable chemicals and fuels.1,4 Furthermore, the direct utilization of natural gas (mainly composed of CH4) with a high content of CO2 avoids costly and complex gas-separation processes.5,6 Therefore, there has been great interest in CDR from the industrial and environmental standpoints in recent years.2,7,8 However, the severe deactivation of the catalyst caused by coke deposition and sintering of the metallic active sites at high reaction temperatures is still a challenge for its commercialization.9−11 Thus, developing highly stable catalysts is a crucial factor for CDR. Generally, natural gas is preserved and transported under high pressures. Moreover, the conversion of syngas into chemicals and fuels, such as in Fischer−Tropsch and methanol syntheses, is also commonly carried out at high pressures.12,13 Thus, from an economic viewpoint, it is more practical and © XXXX American Chemical Society

desirable, in terms of higher process efficiency, to perform the reforming of methane at high pressures.1,14 However, an overwhelming amount of research on CDR is performed under mild reaction conditions, namely, a higher CO2/CH4 molar ratio than the stoichiometric value of 1.0; high reaction temperatures; and, especially, atmospheric pressure. In addition, the reaction behavior of CDR at elevated pressures differs from that at atmospheric pressure. For example, coke deposition over the catalyst caused by the CO disproportionation reaction, which is promoted at high pressures, is a serious issue for pressurized CDR.15 Indeed, a few studies have focused on pressurized CDR. However, low activity and rapid deactivation of the catalysts are generally reported.13,16 Thus, a highly active and stable catalyst is still desirable for CDR at high pressures. It is well-known that noble metals (Rh, Pd, Pt, etc.) show a higher resistance to coking and thermal sintering than transition metals such as nickel.7,17 However, Ni-based catalysts are more practical in industry considering the high cost and limited reserves of noble metals. Thus, researchers have also Received: July 16, 2014 Revised: November 16, 2014 Accepted: November 18, 2014

A

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After the gel had been in air, a foam-like gray solid was obtained. Finally, the sample was calcined in air at 700 °C for 4 h in a furnace. The nickel content in the resulting Ni−SiO2 material was 10 wt %. The catalysts are abbreviated as Ni− SiO2−x, where x represents the complexing agent used. 2.2. Characterization Techniques. Powder X-ray diffraction (XRD) data were collected on an X-ray diffractometer (Bruker D8 Advance) using monochromatized Cu Kα radiation and operating at 40 kV and 40 mA. A step size of 0.02° and a scan rate of 0.2 s/step from 10° to 90° (2θ) were applied for all of the samples. The crystal sizes of NiO and Ni were estimated by Scherrer’s formula based on the 200 and 111 diffractions of the fresh and reduced catalysts, respectively. The surface areas, pore volumes, and pore size distributions of the samples were determined based on N2 adsorption− desorption isotherms measured with a BelSorp-Max apparatus (Bel Japan Inc.) at −196 °C. Prior to the measurements, each sample (100 mg) was degassed under a vacuum (10−2 kPa) at 300 °C for 10 h to remove any contaminants and physically adsorbed moisture. The surface area was calculated based on the Brunauer−Emmett−Teller (BET) method, and the pore size distribution was determined by the Barrett−Joyner− Halenda (BJH) equation using the adsorption branch. The surface analysis of the samples was performed by X-ray photoelectron spectrometry (XPS) (Kratos Analytical Ltd.) using an Al monochromatic X-ray source (Al Kα = 1486.6 eV) at room temperature in a high-vacuum environment (approximately 5 × 10−9 Torr). The C 1s binding energy of adventitious carbon (284.8 eV) was taken as an internal standard for correcting any charge-induced peak shifts. Transmission electron microscopy (TEM) images of the fresh and used catalysts were obtained on a JEM-2100 instrument (JEOL). The samples were prepared by ultrasonically suspending the catalyst powder in ethanol. A drop of the suspension was deposited on a carbon-enhanced copper grid and dried in air. Hydrogen temperature-programmed reduction (H2-TPR) was carried out on a Micromeritics Autochem 2920 instrument to probe the reduction behavior of the catalysts. A 50.0-mg sample loaded in a U-shaped quartz reactor was preheated in an Ar flow at 300 °C for 30 min and then cooled to room temperature. After the gas flow had been switched to 10 vol % H2 in Ar, H2-TPR was started at a heating rate of 10 °C·min−1 until the temperature reached 1000 °C. A downstream 2propanol/liquid N2 trap was used to retain the water generated during the reduction. The H2 consumption rate was monitored with a thermal conductivity detector (TCD) previously calibrated using the reduction of CuO as a reference. The amount of coke deposited on the used catalysts was determined by thermogravimetric differential scanning calorimetry (TG-DSC) on a Q1000DSC+LNCS+FACS Q600SDT thermogravimetric analyzer. The sample was heated from room temperature to 1000 °C at a rate of 10 °C·min−1 under an air atmosphere. 2.3. Reaction Procedure. The CDR experiments were carried out at 750 °C using a stainless steel fixed-bed reactor (i.d. = 8 mm). Prior to the reaction, 0.15 g of each catalyst (40−60 mesh) diluted with quartz sand (at a quartz sand/ catalyst volume ratio of 3.0) was reduced under the conditions of P = 1.0 atm, T = 700 °C, reducing time = 2.5 h, and 10% H2/ N2 flow rate = 50 mL·min−1. After the reactor had been purged with N2, the feed gas (CH4/CO2 molar ratio of 1.0) was switched on, and the reaction was started at 750 °C and a gas

paid attention to improving the coke resistance and antisintering properties of Ni-based catalysts, which are critical factors in catalyst stability toward CDR. For this purpose, several strategies have been tested. The first strategy is to adjust the acid−base properties of the support by using different basic oxides. As a result, the coke elimination reaction can be improved because basic centers are favorable for the chemisorption of CO2. Moreover, coke deposition can be alleviated by decreasing the amount of Lewis acidic sites by incorporating the basic centers.18 The second strategy is to generate bimetallic catalysts by adding a small amount of a noble metal, such as Pt, Ru, or Co, into Ni-based catalysts. In this case, the stability of the catalyst for CDR is increased by alleviating coke deposition.7,19,20 Finally, the structural and textural properties of Ni-based catalysts can be tailored by changing the preparation method.21,22 It has been confirmed that the dispersion of Ni can be increased by supporting Ni on mesoporous oxides, such as MCM-41, SBA-15, and TUD-1, and this allowed some progress in CDR.23−25 Moreover, Ni-based catalysts with ordered structures prepared by newly developed methods, such as improved evaporation-induced self-assembly10,18,26,27 and onespot coassembly,28 and a new type of Ni@SiO2 yolk−shell nanoreactor framework have also been applied in CDR.29 Although these catalysts mark some significant achievements for CDR, these processes are also performed under mild operating conditions of atmospheric pressure, higher CO2/CH4 molar ratio, and lower gas hourly space velocity (GHSV). With these considerations, Ni−SiO2 prepared by a complexdecomposition method was applied for CDR under severe conditions of T = 750 °C, CH4/CO2 = 1.0, GHSV = 53200 mL·g−1·h−1, and P = 1.0−10.0 atm. To reveal the impact of the complexing agent, acetic acid, ammonium hydroxide, and a series of amino acids were used as complexing agents. All of the catalysts were investigated for CDR under atmospheric conditions. With the optimized complexing agents, high activity and stability (whereby approximate equilibrium CH4 conversions of 85% were kept constant for 20 h) were obtained. Moreover, Ni−SiO2 was also tested at elevated pressures. The approximate equilibrium CH4 conversions (84%, 62%, and 46% at 1.0, 5.0, and 10.0 atm, respectively) were obtained over Ni− SiO2 prepared with glycine at high pressures. Importantly, a high stability in CDR was achieved even under the severe conditions of low CO2/CH4 molar ratios and high pressures. Thus, highly active and stable Ni−SiO2 for pressurized CDR was prepared by complex decomposition by optimizing the complexing agents.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. A series of Ni−SiO2 catalysts was prepared by the complex-decomposition method using nickel nitrate and tetraethoxysilane (TEOS) as the precursors of NiO and SiO2, respectively. Several amino acids, namely, glycine (Gly), alanine (Ala), serine (Ser), threonine (Thr), valine (Val), proline (Pro), and lysine (Lys), were used as complexing agents and fuels. For comparison, acetic acid (AA) and ammonium hydroxide (A) were also used as complexing agents and/or fuels. For the preparation of Ni−SiO2, nickel nitrate and TEOS were dissolved in ethanol, and the desired amount of complexing agent solution, for which the molar ratio of the complexing agent to nickel plus silicon was kept at 1.0, was added to the ethanol solution. Then, the solvent was evaporated on a hot plate at 60 °C until a viscous gel formed. B

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hourly space velocity (GHSV) of 53200 mL·g−1·h−1. The effluent products after condensation of the water with an ice− water trap were analyzed by online gas chromatography using a GC-9560 instrument (Shanghai Huaai Chromatographic Analysis Co., Ltd.) equipped with 5 A molecular sieve and Porapak Q capillary columns and a TCD.

given in Table 1. A clear dependence of the NiO crystal size on the complexing agent used was found. Ni−SiO2−Lys had the largest NiO crystal size (∼22 nm), whereas Ni−SiO2−Gly, Ni− SiO2−Ser, Ni−SiO2−Thr, Ni−SiO2−A, and Ni−SiO2−AA gave a much smaller crystal size (6.8−8.2 nm). It is well-known that the hydrolysis of TEOS can be accelerated in acidic or basic solution and that the rate of TEOS hydrolysis in basic solution is higher than that in acidic solution.31 Hence, for Ni−SiO2− Lys, a higher rate of TEOS hydrolysis was obtained because of the basicity of the lysine solution, which is also supported by the experimental phenomenon that a white and flocculent precipitate formed in solution when lysine was the complexing agent. Thus, it is expected that Ni2+ was not well dispersed and complexed with lysine, leading to a larger NiO crystal size for Ni−SiO2−Lys. Ni−SiO2−A showed an obviously different NiO crystal size from Ni−SiO2−Lys, although similar basicities were obtained in the ammonium hydroxide and lysine solutions. This result can be reasonably explained as the different complexing capacity of Ni2+ for NH3 and lysine. It is generally known that Ni2+ complexation is facile with NH3 to form Ni(NH3)62+, which can be used to explain the small NiO crystal size (7.5 nm) observed with Ni−SiO2−A. Moreover, various NiO crystal sizes were also found with Ni−SiO2−Gly, Ni− SiO2−Ser, Ni−SiO2−Thr, Ni−SiO2−Ala, Ni−SiO2−Pro, and Ni−SiO2−Val, which share similar acidities of the complexing agents (pH ∼6). This result can be explained based on the different structures and molecular sizes of amino acids. When the structures of various amino acids were carefully compared, a larger steric hindrance was shown for alanine, valine, and proline, leading to difficulty complexing with Ni2+, which causes a larger NiO crystal size compared to those of Ni−SiO2−Ala, Ni−SiO2−Pro, and Ni−SiO2−Val. However, the serine and threonine complexing agents showed greater steric hindrance than glycine, although similar NiO particles were achieved. This result can be attributed to −OH groups, which also have the ability to complex with Ni2+. 3.1.2. Structural Properties of the Reduced Catalysts. The XRD patterns of the reduced Ni−SiO2 materials are shown in Figure 2. Compared to the results in Figure 1, quite similar XRD patterns of amorphous silica at 2θ ≈ 23° were still observed for all of the reduced catalysts. However, prominent peaks were observed at 2θ ≈ 44.5°, 51.8°, and 76.3°, which correspond to the (111), (200), and (220) diffractions,

3. RESULTS 3.1. Structural and Textural Properties of the Catalysts. 3.1.1. Structural Properties of the Fresh Catalysts. XRD patterns of fresh Ni−SiO2 are presented in Figure 1. A

Figure 1. XRD patterns of fresh (a) Ni−SiO2−Lys, (b) Ni−SiO2−Val, (c) Ni−SiO2−Pro, (d) Ni−SiO2−Ala, (e) Ni−SiO2−Ser, (f) Ni− SiO2−Thr, (g) Ni−SiO2−Gly, (h) Ni−SiO2−AA, and (i) Ni−SiO2−A.

very broad peak at 2θ = 23° was observed for all of the catalysts irrespective of the complexing agent used, which indicates the amorphous nature of the silica.28 At the same time, all of the Ni−SiO2 materials showed well-separated diffractions at 2θ = 37.1°, 43.1°, 62.5°, 75.2°, and 79.0° that can be easily assigned to the (111), (200), (220), (311), and (222) lattice planes, respectively, of cubic NiO.30 However, the intensity and full width at half-maximum (FWHM) of the NiO peaks varied significantly depending on the complexing agent used, indicating the altered crystallinity of the catalysts. Moreover, the diffraction peak of (200) was applied to estimate the crystal size of NiO according to Scherrer’s formula, and the results are

Table 1. Summary of CDR Performance and Textural and Crystal Properties of Different Samples CH4 conversiona (%) sample

SBETb (m2·g−1)

Vpc (cm3·g−1)

Dpd (nm)

dNiOe (nm)

dNif (nm)

dispersiong (D, %)

initialh

finali

Ni−SiO2−Lys Ni−SiO2−Val Ni−SiO2−Pro Ni−SiO2−Ala Ni−SiO2−AA Ni−SiO2−Ser Ni−SiO2−A Ni−SiO2−Gly Ni−SiO2−Thr

78 637 595 606 261 558 62 552 620

0.10 0.51 0.52 0.51 0.13 0.38 0.16 0.39 0.54

5.3 3.2 3.5 3.4 2.0 2.7 10.7 2.8 3.5

22.4 13.7 12.5 10.8 7.0 8.2 7.5 7.3 6.8

25.6 18.3 14.2 11.9 10.9 9.0 8.1 7.6 7.3

3.8 5.3 6.8 8.2 8.9 10.8 12.0 12.8 13.3

83.9 84.0 84.3 85.3 63.8 84.9 81.2 84.5 85.5

1.7 79.5 83.4 84.3 14.4 82.2 49.4 83.8 85.0

Operating conditions: CH4/CO2 = 1.0, T = 750 °C, P = 1.0 atm, and GHSV = 53200 mL·g−1·h−1. bBET surface area. cTotal pore volume calculated by the BJH method with adsorption curves. dAverage pore diameter determined by the BJH method. eCalculated by Scherrer’s formula based on the (200) diffraction of fresh catalyst. f:Calculated by Scherrer’s formula based on the (111) diffraction of reduced catalyst. gDetermined by the equation D (%) = 97/dNi.31 hTOS = 0.5 h under atmospheric conditions. iTOS = 20.0 h under atmospheric conditions. a

C

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Ni. Moreover, the shoulder peaks at 855.35−855.91 eV and the satellite centered at 861.15−862.06 eV were ascribed to NiO, which is formed by oxidation upon air exposure before XPS analysis. When the XPS spectra were fitted, NiSiOx species could not be assigned based on the reference.34,35 Thus, on the basis of a combination of the XPS and XRD results, NiSiOx species were not formed over any of the Ni−SiO2 materials, and a very small amount of NiO formed over the reduced catalysts as a result of air exposure could not be detected by XRD. When the binding energies of Ni 2p3/2 over the reduced catalysts were compared, it was obvious that the binding energies of metallic Ni (approximately 852.51 eV) and Ni2+ (approximately 855.35 eV) over the reduced Ni−SiO2−Lys and Ni−SiO2−AA materials were clearly lower than those over the reduced Ni−SiO2−Gly and Ni−SiO2−Pro, namely, 852.87 and 855.91 eV, respectively. This result indicates that there was a stronger metal−support interaction (SMSI) between Ni and the support in Ni−SiO2−Gly and Ni−SiO2−Pro compared to those in Ni−SiO2−Lys and Ni−SiO2−AA. In the case of Ni− SiO2−Lys, the high hydrolysis rate of TEOS was due to the basicity of the lysine solution, which caused the serious aggregation of SiO2 sol particles. As a result, the Ni2+ was not well complexed with lysine. For Ni−SiO2−AA, acetic acid has only one carboxyl group, and it is difficult to correlate with Ni2+ and SiO2. These observations can be used to explain the weaker interactions between Ni and SiO2. On the contrary, the acids of glycine and proline are similar, which results in rates of TEOS hydrolysis catalyzed by glycine and proline that are much lower than that catalyzed by lysine. Thus, Ni2+ can be encapsulated in the framework of SiO2, which will increase the interactions between NiO and SiO2. 3.1.3. Textural Properties. The N2 adsorption−desorption isotherms and BJH pore size distribution are given in Figure 5. According to the IUPAC classification,36 Ni−SiO2−A, Ni− SiO2−AA, and Ni−SiO2−Lys exhibited type I adsorption isotherms, indicating that they are microporous materials. However, a type IV adsorption isotherm with an H2 hysteresis loop in the relative pressure range of 0.4−1.0 was observed for Ni−SiO2−Gly, Ni−SiO2−Pro, Ni−SiO2−Ala, Ni−SiO2−Val, Ni−SiO2−Thr, and Ni−SiO2−Ser, indicating the presence of mesopores. These results indicate the significant effect of the complexing agent on the porous properties of the Ni−SiO2 samples. The textural properties of the Ni−SiO2 materials are summarized in Table 1. It is clear that all of the samples had large BET surface areas (above 500 m2·g−1), except for Ni− SiO2−Lys, Ni−SiO2−AA, and Ni−SiO2−A. The BET surface areas of all samples were significantly influenced by the complexing agent used. The samples prepared with valine showed the highest surface area, whereas those prepared with ammonium hydroxide produced the smallest surface area. For all of the samples, the BET surface area and pore volume decreased in the order of Ni−SiO2−Val ≈ Ni−SiO2−Pro ≈ Ni−SiO2−Thr ≈ Ni−SiO2−Ala > Ni−SiO2−Ser ≈ Ni−SiO2− Gly ≫ Ni−SiO2−AA ≫ Ni−SiO2−Lys > Ni−SiO2−A. It is known that the hydrolysis of TEOS can be accelerated in acidic or basic solution and that the rate of TEOS hydrolysis in a basic solution is higher than that in an acidic solution.31 Thus, the smaller BET surface areas of Ni−SiO2−Lys and Ni−SiO2−A can be explained as the higher rate of hydrolysis of TEOS in a basic solution. Although similar pH values of approximately 6 were obtained for the other solutions, various BET surface areas and pore volumes were shown, which can be explained by the

Figure 2. XRD patterns for reduced (a) Ni−SiO2−Lys, (b) Ni−SiO2− Pro, (c) Ni−SiO2−Val, (d) Ni−SiO2−Ala, (e) Ni−SiO2−Ser, (f) Ni− SiO2−A, (g) Ni−SiO2−Thr, (h) Ni−SiO2−Gly, and (i) Ni−SiO2−AA.

respectively, of metallic Ni. Thus, under the pretreatment conditions used, nickel oxides can be effectively reduced to metallic Ni. To quantify the properties of the metallic Ni, the strongest diffraction peak at 2θ = 44.5° was applied to calculate the crystal size of Ni based on Scherrer’s formula, and the results are reported in Table 1. Moreover, the dispersion of Ni over different catalysts was also calculated based on the reported method,32 and the results are also included in Table 1. The crystal size of Ni in the reduced catalyst was apparently larger than that of NiO in the corresponding fresh catalyst shown in Table 1. This result can be explained by the aggregation of metallic Ni as a result of high-temperature reduction. To determine the size distributions of the Ni particles, the reduced Ni−SiO2−Gly, Ni−SiO2−Pro, Ni− SiO2−Lys, and Ni−SiO2−AA materials were observed by TEM, and typical results are provided in Figure 3. Although a difference in the Ni size, as determined by TEM and XRD, was obtained from Figure 3 and Table 1, its distribution was significantly influenced by the complexing agent used. In the cases of Ni−SiO2−Gly and Ni−SiO2−Pro, the former showed a relatively narrow distribution of Ni particles of ∼3−7 nm with an average size of 5.3 nm, which was slightly smaller than that of Ni−SiO2−Pro (entries a and b in Figure 3). On the contrary, Ni−SiO2−Lys clearly showed larger Ni particles (entry c in Figure 3). Moreover, although the particle size of the metallic Ni in Ni−SiO2−AA was similar to Ni−SiO2−Gly (Table 1), both the small and large Ni particles were obviously observed in Ni−SiO2−AA (entry d in Figure 3), leading to the smaller average particle size of the metallic Ni (Figure 2). Thus, the sizes and distributions of metallic Ni in the reduced catalysts were strongly influenced by the complexing agent used, and uniform Ni particles of approximately 5.0 nm were narrowly and uniformly distributed in Ni−SiO2−Gly. For these Ni species, no diffractions derived from Nicontaining oxides such as NiO or NiSiOx were observed in the reduced catalysts (Figure 2), indicating that the reduction conditions were appropriate for reducing the catalysts. However, it is possible that the XRD pattern was not observed because of low content and/or an unformed crystalline plane. To confirm this possibility, the reduced catalysts were subjected to XPS analyses, and typical Ni 2p3/2 spectra are shown in Figure 4. Generally, the binding energies of Ni 2p3/2 in metallic Ni and NiO are 852.4 ± 0.4 and 854.0 ± 0.4 eV, respectively.33 Thus, the peaks at 852.51−852.87 eV were ascribed to metallic D

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Figure 3. TEM images and particle size distribution of reduced catalysts: (a) Ni−SiO2−Gly, (b) Ni−SiO2−Pro, (c) Ni−SiO2−Lys, and (d) Ni− SiO2−AA.

differences in structure. The smaller BET surface area of Ni− SiO2−AA can be explained by the smaller molecular size.37 Moreover, a possible structure of the complex between Ni2+ and the amino acid is schematically shown in Figure 6. 3.1.4. Reduction Behavior of the Catalysts. The H2-TPR profiles of the catalysts are shown in Figure 7. It was confirmed that NiO was directly reduced to metallic Ni without going through intermediate steps. Thus, the reduction peaks in different temperature regions of hydrogen consumption can be reasonably attributed to the reductions of different Ni species.38 Generally, the peak at lower temperatures (10 nm) were found for Ni−SiO2−Ala, Ni−SiO2−Val, and Ni−SiO2− Pro. Moreover, a broad distribution of Ni particles, approximately 4−17 nm, was observed in Ni−SiO2−Pro (Figure 3b). This result can be well explained by the two reduction peaks presented in Ni−SiO2−Ala, Ni−SiO2−Val, and Ni−SiO2−Pro. 3.2. CDR Performance. 3.2.1. Results at Atmospheric Pressure. The time-on-stream (TOS) results for the CDR under the conditions of CH4/CO2 =1.0, T = 750 °C, GHSV = 53200 mL·g−1·h−1, and P = 1.0 atm are given in Figure 8. It is clear that nearly identical equilibrium initial CH4 conversions of approximately 85% were obtained for all of the Ni−SiO2 materials, except for Ni−SiO2−AA, indicating the high activity of Ni-based catalysts. Moreover, for Ni−SiO2−Gly, Ni−SiO2− Ala, Ni−SiO2−Val, Ni−SiO2−Ser, Ni−SiO2−Thr, and Ni− SiO2−Pro, the CH4 conversion was constant during a 20-h test, indicating their high stability for CDR. However, a linearly decreasing CH4 conversion was exhibited with Ni−SiO2−A, Ni−SiO2−Lys, and Ni−SiO2−AA with increasing TOS from 1 to 20 h. In the case of Ni−SiO2−AA, only a 63% initial CH4 conversion was obtained, and almost linearly decreased to 14% with increasing TOS until 20 h. 3.2.2. Results at Elevated Pressure. Based on the CDR performance at atmospheric pressure, all of the catalysts, Ni− SiO2−Gly, Ni−SiO2−Ala, Ni−SiO2−Val, Ni−SiO2−Ser, Ni− SiO2−Thr, and Ni−SiO2−Pro exhibited high activities and stabilities. To further reveal the impact of reaction pressure on

Figure 5. N2 adsorption−desorption isotherms.

Figure 6. Possible coordinating structure of Ni2+ with amino acids.

free NiO, whereas the peaks appearing at ∼500−750 °C can be assigned to the reduction of either smaller NiO particles or NiO particles having strong interactions with the SiO2, which are also supported by the XRD results (Figure 1). Thus, the reduction behavior of Ni−SiO2 was significantly influenced by the complexing agent used, and the temperature for the main reduction peak increased in the order Ni−SiO2−Lys < Ni− SiO2−A < Ni−SiO2−AA < Ni−SiO2−Ala ≈ Ni−SiO2−Pro ≈ Ni−SiO2−Val≈ Ni−SiO2−Gly ≈ Ni−SiO2−Ser ≈ Ni−SiO2− Thr. This result can be explained on the basis of the particle size of NiO and the interactions between NiO and SiO2. As shown in Table 1 and Figure 3, Ni−SiO2−Lys contained the largest NiO particles, whereas much smaller particles were obtained in the other catalysts, which agrees with the obviously F

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Figure 8. Time-on-stream CH4 conversion for Ni−SiO2 catalysts under atmospheric conditions.

the CDR performance of Ni−SiO2, Ni−SiO2−Gly was used as an example, and the CDR performances at different pressures are shown in Figure 9. Irrespective of the reaction conditions,

Figure 10. (a) TG and (b) DSC profiles of different catalysts after CDR under the following conditions: CH4/CO2 = 1.0, T = 750 °C, P = 1.0 atm, GHSV = 53200 mL·g−1·h−1, and TOS = 20 h.

Figure 9. Time-on-stream CH4 conversion for Ni−SiO2−Gly at different pressures.

the initial CH4 conversion was almost identical to the thermodynamic equilibrium value, namely, 84%, 59%, and 46% for 1.0, 5.0, and 10.0 atm, indicating that Ni−SiO2−Gly is highly active for CDR. Moreover, during the test, there was no observable catalytic deactivation under any conditions. However, a slightly increased pressure of the catalyst bed was observed at 5.0 atm after a TOS of 25 h, and blocking of the reactor, after a TOS of approximately 7 h, occurred at 10.0 atm. This result was ascribed to coke accumulation on the catalysts, which agrees with the results reported for pressurized CDR.

Figure 11. TG profiles of Ni−SiO2−Gly after a CDR test at different pressures.

characterization results of the fresh, reduced, and used catalysts will be provided. Except for Ni−SiO2−AA, all of the Ni−SiO2 catalysts showed a high initial CH4 conversion of approximately 84%, which is similar to the equilibrium conversion. Moreover, the estimated particle size of Ni in the reduced Ni−SiO2−AA was approximately 10.9 nm, which was clearly smaller than that for the reduced Ni−SiO2−Pro, Ni−SiO2−Val, and Ni−SiO2−Lys, leading to a higher dispersion of Ni (Table 1). However, as

4. DISCUSSION As indicated in catalyst preparation and characterization, all of the catalysts were composed of Ni and SiO2, and their nominal composition was kept the same. However, significantly different activities and stabilities for CDR were obtained, indicating the strong impact of complexing agent. Thus, to reveal the textural, structural, and reductive properties of catalysts in the various CDR performances, a detailed discussion together with the G

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Figure 12. TEM images and particle size distributions of (a) Ni−SiO2−Gly and (b) Ni−SiO2−Lys after a CDR test under the following conditions: CH4/CO2 = 1.0, P = 1.0 atm, T = 750 °C, GHSV = 53200 mL·g−1·h−1, and TOS = 20 h.

SiO2−AA, and Ni−SiO2−A, namely, 0.10, 0.13, and 0.16 cm3· g−1, respectively. This effect might lead to their sharp deactivation with increasing TOS. It is commonly known that severe coking is the crucial factor determining the stability of pressurized CDR. Thus, the used catalysts at different pressures were subjected to TG tests, and the results are given in Figure 11. As expected, the amounts of coking increased significantly under pressurized conditions, and the extent of coking was aggravated with increasing pressures.15,16 As shown in Figure 11, less than 3 wt % of weight loss was shown in the used Ni−SiO2−Gly at atmospheric pressure, while approximately 30 and 65 wt % of weight losses were obtained at the elevated pressures of 5.0 and 10.0 atm, respectively. Thus, correlated with the stability results of CDR, coking is still one of the main factors in catalyst deactivation during pressurized CDR. In addition to coke deposition, the sintering of metallic Ni at high temperature also plays an important role in determining catalyst stability for CDR, the extent of which is determined by the interactions between Ni and the support.23,37 As shown in the previous sections, the structural properties of the reduced catalysts and the reduction behavior of the catalysts, the interactions between Ni and SiO2 in Ni−SiO2−Gly are clearly stronger than those in Ni−SiO 2 −Lys. To verify this observation, Ni−SiO2−Gly and Ni−SiO2−Lys after a 20-h test for CDR at atmospheric pressure were subjected to TEM observations, and typical results are provided in Figure 12. Compared with the results for reduced Ni−SiO2−Gly (entry a in Figure 3), the average particle size of Ni was slightly increased to 5.9 nm for the used Ni−SiO2−Gly (entry a in Figure 12). Furthermore, only a limited variation in the distribution of metallic Ni over the reduced and used Ni− SiO2−Gly was observed, namely, 3−7 nm for reduced Ni− SiO2−Gly and 4−8 nm for used Ni−SiO2−Gly (Figures 3 and 12). On the contrary, when comparing these results with the

revealed in Figure 3d, it was clear that nonuniform Ni particles (both large and small Ni particles) were embedded in the SiO2 matrix, which can account for smaller Ni particles in the Ni− SiO2−AA, as determined from XRD patterns. Moreover, the very small NiO particles, which were incorporated into the framework of SiO2, were relatively difficult to be reduced. As shown in Figure 7, the integrated area of the H2-TPR peak for Ni−SiO2−AA was obviously smaller than that for other Ni− SiO2 catalysts. This result indicates that some smaller NiO was not reduced to metallic Ni with Ni−SiO2−AA under the same reduction conditions, which can be used to explain its low CDR activity. When catalytic stability is considered, only Ni−SiO2−Lys, Ni−SiO2−AA, and Ni−SiO2−A showed varying deactivation during the CDR test, whereas the other Ni−SiO2 catalysts exhibited no observable deactivation. Thus, there is no simple relationship correlating the porous properties of the catalysts with deactivation, indicating the less important role of such properties in determining CDR stability. It is well-known that deposited coke can deteriorate the activity of a catalyst for CDR by covering the active Ni sites. To further confirm this observation, the used catalysts were characterized by TG-DSC, and typical results are shown in Figure 10. Only one peak was observed with Ni−SiO2 catalysts for a CDR test, as shown from results of derivative of weight loss and DSC curves (Figure 10), indicating that there is only one type of coke. Moreover, as shown in Figure 10a, a weight loss of approximately 20% was found for the used Ni−SiO2−Lys and Ni−SiO2−AA at atmospheric pressure, which is consistent with their rapid deactivation. On the contrary, the high-stability catalysts Ni− SiO2−Gly and Ni−SiO2−Pro, showed less than 3 and 1 wt % of weight losses, respectively. This result also shows that coke can be easily deposited on catalysts that have larger Ni particles.37,40 Moreover, metallic Ni is more easily covered by deposited coke because of the smaller pore volume of Ni−SiO2−Lys, Ni− H

dx.doi.org/10.1021/ie502837d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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in situ with 3% O2/Ar at 750 °C for 1 h, and the results are given in Figure 13. A full restoration of the initial CH4 conversion was obtained over the regenerated catalyst at different pressures. Moreover, the regenerated catalysts also exhibited highly stable CDR performance, and a slight decrease in conversion at the end of the reaction was observed for regenerated Ni−SiO2−Gly. Thus, the sintering of Ni and the coke deposition contributed to the deactivation of Ni−SiO2− Gly for the pressurized CDR. However, deactivation caused by coke deposition is reversible, and activity can be restored by removing the coke. On the contrary, the sintering of Ni will cause irreversible deactivation.

5. CONCLUSIONS The structural properties, dispersion of Ni, and interactions between Ni and SiO2 of Ni−SiO2 were tailored using a complex-decomposition method with various complexing agents. This approach is very promising for preparing a highly efficient catalyst for CDR that can be operated under severe conditions of a stoichiometric CH4/CO2 ratio, elevated pressures, and high velocity. The structural properties of Ni− SiO2 were found to be influenced by the complexing agent used; specifically, Ni−SiO2−A, Ni−SiO2−Gly, and Ni−SiO2− Thr showed a higher dispersion of Ni (∼12%), whereas Ni− SiO2−Lys exhibited the lowest dispersion of Ni (∼4%). Depending on the structural, physical, and chemical properties of the complexing agents, the BET surface area and pore volume were also determined. Moreover, the interactions between the Ni and SiO2 of the Ni−SiO2 materials were significantly changed. As a result, different activities and stabilities for CDR were obtained for these Ni−SiO2 catalysts. Over the optimization, Ni−SiO2−Gly, Ni−SiO2−Thr, Ni− SiO2−Ser, Ni−SiO2−Ala, Ni−SiO2−Pro, and Ni−SiO2−Val showed high CDR performances at atmospheric pressure. Importantly, the effect of pressure on the CDR performance of Ni−SiO2−Gly was revealed. Under elevated pressures of 5.0 and 10.0 atm, the approximate equilibrium CH4 conversion and no observable deactivation were obtained using Ni−SiO2−Gly during a CDR test, although a slight plugging of the reactor of the catalyst bed was found. Based on the characterization results of the catalysts and CDR results at different pressures, the favorable formation of reactive coke and the superior antisintering properties of Ni−SiO2−Gly were revealed to be responsible for the high activity and stability toward CDR of this catalyst.

Figure 13. Time-on-stream CH4 conversions for fresh and in situ regenerated Ni−SiO2−Gly catalysts for a CDR test under the following conditions: T = 750 °C, GHSV = 53200 mL·g−1·h−1, CH4/CO2 = 1.0, and P = (a) 5.0 and (b) 10.0 atm.

results of the reduced Ni−SiO2−Lys (21.6 nm, entry c in Figure 3), a significant increase in the average particle size of Ni was observed (53.4 nm, entry b in Figure 12), indicating the obvious sintering of metallic Ni. These observations are in agreement with the CDR results. Irrespective of the CDR operating pressure (1.0−10.0 atm), there was no observable deactivation for Ni−SiO2−Gly (Figure 9), indicating its high stability for CDR. For atmospheric pressure, much less coke deposition (less than 3 wt %) and a limited variation in Ni particles in Ni−SiO2−Gly were found after a 20-h test (Figures 10 and 12). Thus, the high resistance of metallic Ni to coke deposition and sintering can be well explained by the high stability of Ni−SiO2−Gly. At elevated pressures of 5.0 and 10.0 atm, even though approximately 30 and 65 wt % of coke depositions, respectively, were obtained for Ni−SiO2−Gly, it still showed no observable deactivation during a CDR test (20 h for 5.0 atm and 7 h for 10.0 atm). However, during the reaction, the pressure of the catalyst bed increased to varying degrees, which can be ascribed to reactor plugging due to the large amount of coke deposition. Following this reasoning, inhibited coke deposition and decreased sintering of metallic Ni, especially coke deposition, on Ni− SiO2−Gly are responsible for its high CDR stability under pressurized conditions. To confirm this speculation, Ni−SiO2− Gly after 49- and 10-h tests at 5.0 and 10.0 atm was regenerated

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel.: +86 29 81530802. Fax: +86 29 81530727 (Z.-T. Liu). *E-mail: [email protected]. Tel.: +86 29 88291213. Fax: +86 29 88291213 (J. Lu). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the financial support of the National Natural Science Foundation of China (21176151, 21327011, 21306111), the Shaanxi Innovative Team of Key Science and Technology (2013KCT-17), and the Natural Science Foundation of Shaanxi Province (2012JZ2001). I

dx.doi.org/10.1021/ie502837d | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

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