Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix

Fuel 147 (2015) 243–252

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Carbon dioxide reforming of methane over Ni nanoparticles incorporated into mesoporous amorphous ZrO2 matrix Xiaoping Zhang a,b, Qingde Zhang a, Noritatsu Tsubaki c, Yisheng Tan a, Yizhuo Han a,⇑ a

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, P.O. Box 165, Taiyuan 030001, China University of Chinese Academy of Sciences, Beijing 100039, China c Department of Applied Chemistry, University of Toyama, Toyama 930-8555, Japan b

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Ni NPs were well-dispersed on

amorphous mesoporous zirconia matrix.  The catalyst showed excellent catalytic activity and good anti-coke property.  The anchoring effect suppressed the sintering of Ni particles.  The increased contact area strengthened the interaction between Ni and ZrO2.

a r t i c l e

i n f o

Article history: Received 25 September 2014 Received in revised form 21 January 2015 Accepted 22 January 2015 Available online 3 February 2015 Keywords: Dry reforming Mesoporous Amorphous zirconia Interface

a b s t r a c t A series of mesoporous amorphous Ni–ZrO2 composite oxides with varying Ni contents were prepared by an improved co-precipitation/reflux digestion method and evaluated in the dry reforming of methane. The synthesis method enabled the preparation of uniformly sized Ni particles evenly dispersed within the porous amorphous zirconia matrix. The catalytic reaction results showed that mesoporous amorphous Ni–ZrO2-CR-15 catalyst (Ni loading of 15 wt%) exhibited the highest catalytic activity and excellent stability during the 80-h test among other studied Ni–ZrO2 catalysts. Rapid decline in the catalytic activity was observed for catalysts prepared by traditional impregnation and co-precipitation methods. The improved catalytic performance was attributed to the homogeneous distribution of the small Ni nanoparticles because of the high surface area of the amorphous structure and the strong interaction between Ni and ZrO2. Additionally, the confinement effect of the nano-amorphous structure was responsible for the superior thermal stability of the Ni nanoparticles. Furthermore, Ni–ZrO2-CR-15 catalyst displayed high resistance against carbon deposition owing to the presence of multiple interfaces between the metal and oxide support, absence of strong Lewis acid sites, and presence of different active centers for CO2 dissociation. Ó 2015 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel.: +86 351 4049747; fax: +86 351 4044287. E-mail address: [email protected] (Y. Han). http://dx.doi.org/10.1016/j.fuel.2015.01.076 0016-2361/Ó 2015 Elsevier Ltd. All rights reserved.

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1. Introduction Recently, there has been increased interest in dry reforming of methane owing to global concern regarding increasing CO2 emissions and efficient use of natural gas. Dry (CO2) reforming of methane (DRM) is an attractive and promising process for the conversion of non-desirable greenhouse gases into valuable syngas, which can be used as an intermediate in the manufacture of numerous chemicals such as ammonia, methanol, dimethyl ether, and synthetic petroleum. Furthermore, low H2/CO ratios (1) are more desirable for the Fischer–Tropsch and oxo-syntheses [1,2]. However, catalysts employed in DRM usually suffer from significant carbon deposition owing to the high thermodynamic potential [3]. Therefore, the development of stable and costeffective catalysts is a major challenge to advancing DRM processes on an industrial scale. Recently, numerous catalysts have been developed for DRM, evidencing that noble metals, such as Rh, Pt, Pd, and Ir, exhibit high activity and remarkable carbon resistance [4,5]. However, they are not suitable candidates for industrial applications because of their high cost. Alternatively, Ni-based catalysts have been considered the most promising candidates owing to their high activity and low cost. However, rapid deactivation resulting from the sintering of Ni particles and carbon deposition limits their industrial application [3,6]. Therefore, currently, much effort is invested in exploring stable and active Ni-based catalysts. As reported, carbon deposition is significantly suppressed on catalysts with small Ni particles [7,8]. However, Ni-based catalysts usually suffer from considerable sintering of active Ni nanoparticles under the typical conditions employed in DRM. Accordingly, numerous efforts have been focused on stabilizing the metal phase against sintering. Studies demonstrated that the sintering of active sites is influenced by several factors: well-defined structure, nature and porosity of the support, and the strong metal–support interaction [9–11]. Recent studies showed that the incorporation of Ni particles within the mesoporous support not only suppressed the sintering of Ni particles, but also strengthened the metal–support interaction because of the formation of additional interfaces [3,9,12]. Specifically, one of the important characteristics of mesoporous materials is the high surface area, which can afford improved dispersion of the Ni metal. Furthermore, Ni particles incorporated into a mesoporous matrix are stabilized because of the strong interaction between the metal and support. As exemplified in the literature, catalysts with Ni particles incorporated into mesoporous supports, such as SBA-16, MCM-41, TUD-1, mesoAl2O3, and meso-ZrO2, exhibited improved catalytic activity and coke resistance in DRM [3,9,13–15]. Similarly, an amorphous structure is believed to play a pivotal role in enhancing the activity and durability of the catalysts because of its unique surface chemical properties [16–18]. Undoubtedly, improved metal dispersion can be achieved by integration of the metal particles into high surface area amorphous materials. Furthermore, the strong basic character of terminal hydroxyls and improvement of surface defect centers on amorphous catalysts are important parameters that are responsible for superior catalytic activity and formation of small metal particles. As demonstrated, Au deposited on amorphous TiO2 exhibited higher dispersion than that deposited on anatase TiO2 owing to the physicochemical nature of the amorphous support [18]. Furthermore, the nanoscopic pore–solid architecture is another characteristic feature of the amorphous structure. When metal nanoparticles are embedded in a disordered mesoporous amorphous matrix, multiple interfaces are generated between the metal and support. Multiple contact areas can strengthen the interaction between metal nanoparticles and support, leading to improved thermal stability. Moreover, higher numbers of interfaces can

facilitate cooperativity between the metal and support [19,20]. Particularly, Bitter et al. [19] suggested that reactions occur within proximity of the metal (Pt) particle and support in DRM. Methane dissociates on the Pt particles, whereas carbon dioxide decomposes to CO and adsorbed oxygen species on the ZrO2 support. Then, CHx derived from methane dissociation reacts with oxygen species at the Pt–ZrO2 boundary. Therefore, the catalytic activity is closely associated with the length of perimeter, indicating the importance of interfaces on bi-functional catalysts. Zirconia is widely used as a catalytic support or promoter in numerous reactions because of its moderate acidity and basicity, high thermal stability, and surface oxygen mobility [21–23]. As reported [24], carbon deposition is suppressed on Pt–ZrO2 owing to the formation of active coke when compared with that on Pt– Al2O3 in methane reforming. Additionally, the low concentration of strong Lewis acid sites on ZrO2 afforded reduced carbon deposition. Although numerous mesoporous catalysts have been used in DRM, to date, amorphous zirconia catalysts have not been reported. The main reason is the poor thermal stability of the amorphous zirconia structure under severe DRM reaction conditions. The nano-amorphous structure suffers from considerable collapse owing to crystallization under harsh conditions [25]. In this study, a series of Ni–ZrO2 materials with different Ni contents were prepared by a co-precipitation strategy with a prolonged reflux digestion. The prepared materials possessed high surface areas, mesoporous amorphous structures, and high thermal stability. Their catalytic performance in DRM was investigated. For comparison, we also studied the effect of conventional preparation methods on morphology and catalytic performance of the resulting catalysts. 2. Experimental 2.1. Catalyst preparation Mesoporous amorphous Ni–ZrO2 catalysts with varying Ni contents were prepared by a combined co-precipitation and reflux digestion (CR) method (the catalysts are denoted as Ni–ZrO2–CR-x, where x refers to the loading (wt%) of Ni) adapted from a method reported elsewhere [14,26]. In a typical synthesis, stoichiometric quantities of pluronic P123 block copolymer (EO20PO70EO20, Sigma–Aldrich), Ni(NO3)2
6H2O, and ZrOCl2
8H2O were dissolved in deionized water with vigorous stirring. The molar ratio of the metal cation to P123 was 0.03. Following dissolution, the resulting solution was heated to 80 °C. An aqueous solution of 10 wt% KOH was added dropwise to the above solution with constant stirring. During the course of the co-precipitation reaction, the solution pH was maintained at 11. The obtained slurry was digested at 100 °C for 168 h. Subsequently, the precipitate was filtered, thoroughly washed with distilled water several times to remove any potassium impurities, and dried at 110 °C for 12 h. Finally, the resulting product was calcined for 4 h at 700 °C with a ramp of 1 °C/min under air atmosphere. For comparison, Ni–ZrO2 catalyst with a Ni loading of 15 wt% was prepared using a co-precipitation (C) method according to the above method in the absence of P123 and reflux digestion (the catalyst is denoted as Ni–ZrO2-C). Additionally, a conventional impregnation (IMP) method was used to prepare Ni–ZrO2 catalyst with a Ni loading of 15 wt% (the catalyst is denoted as Ni–ZrO2-IMP). The ZrO2 support was prepared by co-precipitation according to the literature [27]. 2.2. Catalyst characterization

at

Nitrogen adsorption and desorption isotherms were measured 196 °C on a TriStar 3000 using the static volumetric method.

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The specific surface areas were calculated using the Brunauer– Emmet–Teller (BET) method and the pore-size distributions were estimated using the Barrett–Joyner–Halenda (BJH) method on the desorption branch. Powder X-ray diffraction patterns (XRD) were recorded on a Rigaku MiniFlex II X-ray diffractometer, operating at 40 kV and 40 mA and using Cu Ka radiation (k = 0.15418 nm) and a scanning range of 2h 20–80.0°. The morphology and microstructure of the materials were observed on a transmission electron microscope (JEOL, JEM2100F), operating at 200 kV, with a 0.14-nm lattice resolution. H2 temperature-programmed reduction (H2-TPR) was conducted on a fixed-bed reactor system equipped with a gas chromatograph. Prior to analysis, the samples (100 mg) were pretreated at 500 °C for 1 h under flowing Ar (60 mL/min) to remove any moisture and adsorbed impurities. Following cooling of the reactor to 100 °C, a 10% H2/Ar (50 mL/min) mixed gas was introduced. The temperature of the reactor was linearly increased at a rate of 6 °C/min from 100 to 800 °C. Consumption of H2 in the reactant stream was analyzed online by a gas chromatograph equipped with a thermal conductivity detector. Hydrogen chemisorption measurements were carried out at 40 °C with a tp-5080 automated chemisorption analyzer, assuming that the adsorption stoichiometry is H/Ni = 1. Prior to measurement, the catalysts were reduced at 700 °C for 1 h using 10% H2/Ar. Fourier transform infrared (FTIR) spectroscopy was conducted on a Bruker Tensor 27 equipped with a diffuse reflectance chamber and a liquid nitrogen detector. Prior to the experiment, the samples were reduced by a 10% H2/Ar mixed gas at 700 °C in a fixed-bed reactor. Following sample loading in the diffuse reflectance cell, a 20% CO2/Ar (20 mL/min) mixed gas was introduced for catalyst surface analysis under CO2 adsorption conditions. The acidity of the samples was determined by temperature-programmed desorption of ammonia (NH3-TPD). Samples of about 100 mg were treated under flowing Ar at 500 °C for 1 h, then cooled to 50 °C and treated with a NH3 flow for 10 min. The samples were flushed with Ar gas for 1 h to eliminate the physisorbed ammonia. The desorption of ammonia was measured by a gas chromatograph equipped with a thermal conductivity detector at a constant heating rate of 6 °C/min up to 500 °C. Thermogravimetric differential thermal analysis (TG-DTA) measurements were performed on a Rigaku TG-8120 thermogravimetric analyzer; the samples were heated from room temperature to 850 °C at a rate of 20 °C/min under air atmosphere. CO2 temperature-programmed oxidation (CO2-TPO) experiments were conducted in a fixed-bed reactor. The sample (100 mg) was first loaded in a quartz tube. Then, a 10% CO2/Ar (50 mL/min) mixed gas was introduced. The temperature was raised from 100 to 800 °C at a rate of 5 °C/min using a temperature controller. The products were analyzed online by a mass spectrometer (Pfeiffer OmniStar).

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3. Results and discussion 3.1. Characterization of the reduced Ni–ZrO2 catalysts

3.1.1. XRD characterization Fig. 1(a) shows the XRD patterns of the reduced catalysts. A very broad X-ray diffraction band was observed at 2h 30° for the

2.3. Catalysis activity measurements The reforming reaction was carried out at 750 °C in a fixed-bed quartz microreactor (internal diameter: 8 mm) under atmospheric pressure. The temperature was monitored in real time using a thermocouple that was inserted in the center of the catalyst bed. The reactant gas mixture of CO2 and CH4 (molar ratio 1:1) was fed at a gas hourly space velocity (GHSV) of 24,000 mL/(g h). Typically, 0.5 g catalyst (20–40 meshes) was mixed with 1.5 g quartz sand (20–40 meshes) for each run. Prior to the reaction, the catalysts were reduced in situ at 700 °C for 1 h using 10% H2/Ar mixed gas (100 mL/min). The effluent gases from the reactor were analyzed online on a gas chromatograph fitted with a packed column (TDX-01) and a thermal conductivity detector.

Fig. 1. (a) XRD patterns, (b) N2 isotherms and (c) BJH pore-size distributions of ZrO2 and the reduced Ni–ZrO2 samples.

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Table 1 Physicochemical properties of the Ni–ZrO2 catalysts. Samples

Ni–ZrO2-CR-10 Ni–ZrO2-CR-15 Ni–ZrO2-CR-15-ETc Ni–ZrO2-CR-20 Ni–ZrO2-CR-25 ZrO2 Ni–ZrO2-C Ni–ZrO2-IMP a b c

Specific surface area (m2/g)

193 203 173 221 202 117 53 96

Pore volume (cm3/g)

0.31 0.37 0.43 0.35 0.35 0.20 0.16 0.23

Average pore diameter (nm)

5.0 5.6 8.5 5.2 5.2 6.5 9.3 8.0

Isotherm type

IV IV IV IV IV IV IV IV

H2 H2 H3 H2 H2 H2 H2 H3

Degree of reduction (%)

73 77 – 81 83 – 95 98

Ni surface area (m2/gcat)

2.2 3.5 – 3.7 4.0 – 3.3 3.1

Dispersion (%)

6.8 7.0 – 5.5 4.8 – 6.6 6.2

Particle size (nm) D1a

D2b

14.9 14.4 – 18.4 21.0 – 15.3 16.3

7.3 7.2 7.3 12.4 13.2 – 15.3 15.5

Calculated from H2 chemisorption, dNi (nm) = 1/D, D: dispersion. Calculated using Scherrer formula on the (1 1 1) peak of Ni. Spent catalyst subjected to 80-h DRM reaction.

Ni–ZrO2-CR samples that indicated the presence of amorphous zirconia. In contrast, Ni–ZrO2-IMP and Ni–ZrO2-C displayed characteristic diffraction peaks of monoclinic and tetragonal ZrO2 phases (m-ZrO2 and t-ZrO2). Characteristic diffraction peaks of Ni at 2h 44.5° and 51.8° were additionally observed in all samples. The intensity of the nickel diffraction peaks of the Ni–ZrO2-CR samples increased with increasing nickel loadings, suggesting the presence of larger particles in the samples with higher nickel loadings. Using the Scherrer equation, the average crystallite diameter of the Ni particles in the Ni–ZrO2-CR samples increased from 7.2 to 13.2 nm with increasing Ni loadings from 10 to 25 wt%. When compared with Ni–ZrO2-IMP and Ni–ZrO2-C, the diffraction peaks of Ni were weaker for Ni–ZrO2-CR-15, indicating the higher Ni dispersion within the mesoporous amorphous zirconia. Hence, based on the results above, crystallite size increased with increasing Ni contents; however, samples featuring a mesoporous structure exhibited higher Ni dispersion when compared with that of samples prepared by conventional impregnation and co-precipitation methods. The confinement effect exerted by the mesoporous amorphous structure prevents aggregation of the Ni nanoparticles [16].

3.1.2. Textural properties The N2 adsorption–desorption isotherms and pore-size distributions of the reduced catalysts are displayed in Fig. 1b and c. As shown in Fig. 1b, all Ni–ZrO2-CR samples exhibited type IV isotherms and H2-shaped hysteresis loops that are typical of a mesoporous structure. The H2-shaped hysteresis loops are associated with a more complex pore structure such as interparticle porosity or irregular tube-like porosity. ZrO2 displayed a similar isotherm that is indicative of the existence of a mesoporous structure. Although type IV hysteresis loops were also observed for both Ni–ZrO2-IMP and Ni–ZrO2-C, the small hysteresis loops suggested the presence of a poorly defined mesoporous structure (evidenced by the small-angle XRD patterns in Fig. S1). Furthermore, the poresize distributions of the Ni–ZrO2-CR samples, Fig. 1c, were very narrow, with a pore size centered at 5–7 nm. In contrast, the larger average pore diameters and broader pore-size distributions of Ni– ZrO2-IMP and Ni–ZrO2-C suggested the presence of a different pore structure and a reduced ordered pattern. The surface area, pore volume, and pore diameter of the samples are listed in Table 1. The surface areas of the Ni–ZrO2-CR catalysts increased to 221 m2/g with increasing Ni loadings up to 20 wt%, and then decreased with further Ni loading (25 wt%). However, Ni–ZrO2-CR-15 exhibited larger pore volume and average pore diameter when compared with other Ni–ZrO2-CR samples. Unlike the Ni–ZrO2-CR catalysts, the introduction of nickel via either impregnation or co-precipitation resulted in

catalysts with reduced specific surface areas and pore volume, which resulted in their poorly resolved mesoporous structure. 3.1.3. Morphology analysis Typical TEM images of the reduced Ni–ZrO2 catalysts are shown in Fig. 2. As observed, nickel nanoparticles with sizes of 10 nm were uniformly distributed within the amorphous ZrO2 matrix of Ni–ZrO2-CR-15 catalyst (Fig. 2a). A mesoporous amorphous feature was additionally observed, and was characterized by a sponge-like three-dimensional connecting network. The nickel particles were finely dispersed and no agglomerated particles were observed in the Ni–ZrO2-CR-15 catalyst. Fig. 2b shows that the Ni particles were uniformly dispersed over zirconia matrix with a narrow size range for Ni–ZrO2–CR-15. Moreover, it can be revealed that Ni particles size distribution did not change significantly even after catalytic reaction at 700 °C for 80 h (Fig. 2d), owing to the confinement effect of mesoporous amorphous structure. Ni–ZrO2-C displayed a ZrO2 support that was characterized by small uniform spherical particles, and aggregated Ni particles were observed (Fig. 2e and f). Large Ni particles up to 30 nm in size were observed that resulted in the poor distribution of the particles within the ZrO2 matrix. Ni–ZrO2-IMP featured a mixture of nano-sized particles of nickel and zirconia (Fig. 2g and h). However, it is difficult to accurately determine the size of the Ni particles because of the overlap between the Ni and ZrO2 particles. Nevertheless, the nickel crystallite diameter in Ni–ZrO2-IMP were larger (mean crystallite diameter: 15 nm) than those in Ni–ZrO2-CR-15 (7 nm) (Table 1) based on the XRD results. Therefore, Ni–ZrO2-CR-15 featured a high Ni dispersion owing to its nano-amorphous mesoporous structure. Abundant metal–support interfaces were formed upon embedment of small Ni particles within the mesoporous amorphous zirconia, thereby promoting strong interactions between Ni and ZrO2 [28]. 3.1.4. H2-TPR analysis The interaction between Ni and ZrO2 was evaluated by H2-TPR (Fig. 3). All Ni–ZrO2-CR catalysts displayed a major reduction peak with a small shoulder peak. The small peak at 500 °C was attributed to the reduction of highly dispersed NiO particles that had a weak interaction with the support. The intense peak observed within the temperature range of 660–700 °C was attributed to the reduction of NiO in the bulk of ZrO2 matrix, and had a strong interaction with ZrO2 [3,14]. With increasing nickel loadings, the main reduction peak shifted to higher temperatures, suggesting the presence of stronger interactions between Ni and ZrO2. The higher reduction peak might originate from the formation of Ni–Zr–O solid solutions to some extent in the case of higher nickel content. Ni–ZrO2-IMP displayed two broad peaks at 407 and

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Fig. 2. TEM images of (a) reduced Ni–ZrO2-CR-15, (c) spent Ni–ZrO2-CR-15, (e and f) Ni–ZrO2-C, and (g and h) Ni–ZrO2-IMP, with Ni particle size distributions for (b) reduced Ni–ZrO2-CR-15 and (d) spent Ni–ZrO2-CR-15; the encircled portion are Ni particles.

579 °C. The first peak was assigned to the reduction of NiO particles in weak interaction with the support, implying the aggregation of NiO. The second peak was related to NiO exhibiting a relatively strong interaction with the support. Similar profile could be observed for Ni–ZrO2-C, although in that case the second reduction peak shifted to a much lower temperature (514 °C), indicating the presence of weaker interactions between Ni and ZrO2. In summary, the interaction between the support and nickel oxide in Ni–ZrO2-CR was significantly stronger when compared with that in the

catalysts prepared via conventional impregnation and co-precipitation methods. Compared with Ni–ZrO2-C and Ni–ZrO2-IMP, the Ni–ZrO2-CR samples displayed a less pronounced peak at a low temperature, indicating that most of the NiO particles were well dispersed in the amorphous ZrO2 matrix. It is worth noting that NiO in Ni–ZrO2-CP catalysts was not reduced completely (Table 1). The lower degree of nickel oxide reduction indicated that NiO particles were incorporated into amorphous ZrO2 matrix and surrounded by ZrO2 particles. The

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Fig. 3. H2-TPR profiles of Ni–ZrO2 catalysts: (a) Ni–ZrO2-CR-10, (b) Ni–ZrO2-CR-15, (c) Ni–ZrO2-CR-20, (d) Ni–ZrO2-CR-25, (e) Ni–ZrO2-C, and (f) Ni–ZrO2-IMP.

reduction of NiO particles embedded in the ZrO2 matrix of Ni– ZrO2-CR was difficult because of the formation of multiple points of contact between Ni and ZrO2, resulting in strong interactions between Ni and ZrO2 [29]. Furthermore, the presence of a strong interaction between Ni and ZrO2 in all Ni–ZrO2 samples is evidenced by the high binding energy of the Ni2p3/2 peak centered at 856 eV (Fig. S2). The Ni2p3/2 peak of Ni–ZrO2-CR-15 was slightly shifted to a higher binding energy when compared with that of the other catalysts, thereby demonstrating the stronger interaction between Ni and ZrO2 in that particular sample, consistent with the H2-TPR results. 3.1.5. H2 chemisorption H2 pulse chemisorption results over various Ni–ZrO2 catalysts are presented in Table 1. For the Ni–ZrO2-CR catalysts, the surface area of nickel increased with increasing Ni loading; whereas the dispersion decreased with higher Ni loading due to the aggregation of Ni particles. Compared with Ni–ZrO2-C and Ni–ZrO2-IMP with the same Ni loading, the Ni–ZrO2–CR-15 sample exhibited higher Ni surface area and dispersion, illustrating the homogeneous distribution of the small Ni nanoparticles because of the high surface area of the amorphous structure and the strong interaction between Ni and ZrO2. Moreover, the average crystallite diameter of Ni calculated from H2 uptake over Ni–ZrO2-CR catalysts was larger than that determined by XRD. This can be possibly attributed to that some Ni particles entered into amorphous ZrO2 matrix and surrounded by ZrO2 particles [30,31]. Therefore, it was difficult to reduce the NiO particles completely under the prereduction conditions during H2 chemisorption, which is in consistent with the results of H2-TPR. 3.1.6. NH3-TPD The acidity of the samples was determined by NH3-TPD (Fig. 4). Ni–ZrO2-CP-15 only exhibited one desorption peak at 120 °C, indicating the presence of weak acid sites. Ni–ZrO2-IMP presented a major desorption peak at 129 °C and a minor desorption peak at 283 °C. Three peaks at 124 °C, 210 °C, and 360 °C were observed for Ni–ZrO2-C. Generally, the desorption peak at temperature within 100–250 °C can be ascribed to weak acid sites and the desorption peak at temperature above 250 °C is associated with strong acid sites. It is apparent that no strong acid sites on Ni– ZrO2-CP-15. In contrast, the presence of desorption peaks at high temperature on Ni–ZrO2-IMP and Ni–ZrO2-C demonstrated the existence of strong acid sites. Compared with Ni–ZrO2-IMP, the

Fig. 4. NH3-TPD profiles of Ni–ZrO2 catalysts.

desorption peak ascribed to strong acid sites on Ni–ZrO2-C shifted to a much higher temperature (360 °C), indicating the increase of strong acidity. It has been known that the strong acid sites are responsible for the formation of coke and polymer on the catalyst surface. Moreover, the increased acidity on Ni–ZrO2-IMP and Ni– ZrO2-C may hamper CO2 adsorption and activation leading to a lower activity. 3.1.7. FTIR analysis of the reduced catalysts CO2 adsorption onto the reduced catalysts was studied by in situ diffuse reflectance infrared spectroscopy to identify the surface chemical properties and catalytic activation of CO2; the FTIR spectra are shown in Fig. 5. The reduced catalysts displayed relatively different FTIR patterns. Ni–ZrO2-IMP displayed six main bands at 1635, 1570, 1515, 1442, 1415, and 1370 cm 1 in the region of 1700–1300 cm 1, Fig. 5a. The small peak at 1635 cm 1 was assigned to bicarbonate species (b-HCO3 ), which are formed by adsorption of CO2 onto basic OH groups. The peaks at 1515 and 1370 cm 1 were ascribed to monodentate species (m-CO23 ), which are formed by adsorption of CO2 onto highly basic coordinatively unsaturated (cus) O2 centers. The peak at 1570 cm 1 was ascribed to bidentate carbonate species (b-CO23 ) connected to acid–base pair sites (cus Zr4+–O2 centers). The bands at 1442 and 1415 cm 1 were attributed to the formation of polydentate carbonate species (p-CO23 ) [32], which are formed by adsorption of CO2 onto cus Zr4+ centers with strong Lewis acidity. The CO2 adsorption FTIR spectrum of Ni–ZrO2-C shows the presence of major polydentate carbonate species (1442 and 1415 cm 1) and minor other carbonate species. It is worth noting that only monodentate carbonate species (1545 and 1370 cm 1) were observed during CO2 adsorption onto Ni–ZrO2-CR-15. This implied the absence of Lewis acid sites (cus Zr4+ sites) because of the absence of polydentate carbonate species, which is consistent with the results of NH3-TPD. Generally, catalysts with a strong acidity always suffer from considerable carbon deposition [3,24]. The carbon deposition of large amounts on Ni–ZrO2-C and Ni–ZrO2-IMP, as discussed further in Section 3.3.1, was probably related to the catalysts’ Lewis acid sites. Based on the FTIR studies on the reduced samples exposed to CO2 flow at 400 °C and varying times, Ni–ZrO2-IMP displayed two peaks at 2015 and 1878 cm 1 that were assigned to linear and bridged adsorption of CO onto Ni, respectively (Fig. 5a) [33]. This indicated the dissociation of CO2 to CO over the catalyst. The dissociation became less prominent following exposure to CO2 for 6 min, and bands corresponding to CO were no longer observed

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coordinatively unsaturated zirconium cations [32]. However, surface lattice oxygen can be extracted by methane, leading to the formation of new oxygen vacancies during DRM [35]. Subsequently, surface carbonate species begin to decompose to form CO and surface oxygen species. This results in a cyclic consumption–replenishment of oxygen.

3.2. Catalytic performance 3.2.1. Effects of preparation method and Ni loading The CH4 and CO2 conversions of various catalysts at 750 °C are shown in Fig. 6. As observed in Fig. 6a, b, all Ni–ZrO2-CR samples, with varying Ni loadings, exhibited similar high initial activity except for Ni–ZrO2-CR-10. The low activity of Ni–ZrO2-CR-10 can be attributed to the less exposed active Ni sites, evidenced by its low Ni surface area (Table 1). Ni–ZrO2-CR-20 and Ni–ZrO2-CR-25 displayed slight reduced CH4 and CO2 conversions with increasing reaction times, demonstrating the relatively poor stability of the catalysts. Catalyst deactivation may be related to their lower Ni dispersion with high nickel loading that leads to the sintering of Ni particles and considerable coke accumulation [36]. In contrast, Ni–ZrO2-CR-15 catalyst exhibited high activity and long-term stability because of the presence of small Ni particles and sufficient surface active sites. The catalytic activity of Ni–ZrO2 catalysts prepared with different methods is presented in Fig. 6c and d. CH4 conversion on Ni– ZrO2-C and Ni–ZrO2-IMP decreased rapidly despite the initial high activity of Ni–ZrO2-IMP. Their initial activity was lower than that of Ni–ZrO2-CR-15, in good agreement with their lower dispersion and metal surface area. The superior performance of Ni–ZrO2-CR15 was closely associated with the characteristics of the amorphous mesoporous structure. Ni–ZrO2-CR-15 catalyst possessed a high surface area that contributed to the formation of smaller Ni particles. Smaller Ni particles are believed to be beneficial for enhancing activity and stability. Furthermore, the mesoporous framework stabilizes the active Ni species against sintering during DRM. The Ni-based catalysts prepared by impregnation and co-precipitation methods exhibited relatively poor activity and stability, which may be due to carbon deposition as discussed below. Fig. 5. FTIR spectra of CO2 adsorption onto the reduced Ni–ZrO2 catalysts: (a) Ni– ZrO2-IMP, (b) Ni–ZrO2-C, and (c) Ni–ZrO2-CR-15.

following CO2 exposure for 10 min owing to the depletion of oxygen vacancies. It is worth noting that the intensity of the band at 1515 cm 1 (corresponding to monodentate carbonate species) grew faster following the disappearance of CO, indicating that monodentate carbonate species are the main active intermediates of CO2 dissociation. In contrast, the relatively slight change in the intensity of peaks corresponding to polydentate carbonate species indicated their reduced activity towards CO2 dissociation. A similar phenomenon was observed for Ni–ZrO2-C with some differences. Although only active monodentate carbonate species were observed in Ni–ZrO2-CR-15, no bands corresponding to CO were observed, indicating the absence of CO2 dissociation. According to the mechanism of CO2 dissociation over zirconia-supported catalysts, the participation of lattice oxygen and oxygen vacancy sites is necessary for CO2 dissociation [34]. Therefore, it can be speculated that carbonate dissociation was prevented because of the absence of oxygen vacancy sites in Ni–ZrO2-CR-15; this was evidenced by the absence of cus Zr4+ sites. In other words, higher concentrations of surface oxygen vacancies exposes more

3.2.2. Stability test As shown in Fig. 7, Ni–ZrO2-CR-15 exhibited high activity and good stability during the 80-h long-term stability test; additionally, methane and carbon dioxide conversions were respectively over 85% and 88%. The CH4 and CO2 conversion values remained stable and no catalyst deactivation was observed during the 80-h reaction, demonstrating the high catalytic stability of Ni–ZrO2CR-15. The H2/CO ratio (1) suggested the minimal occurrence of the RWGS reaction on the catalyst. The excellent stability performance was attributed to the mesoporous amorphous structure and high dispersion of Ni, which generated multiple points of contact between Ni and ZrO2 [16,28,29]. According to Bitter et al. [19], CH4 decomposes on the metal surface and CO2 dissociates to CO and adsorbed oxygen species through the formation of carbonate in close proximity to the metal–support interface. Carbon deposition was diminished because of the spillover of oxygen species at the metal–support interface. It is believed that the formation of more metal/oxide boundaries promotes the oxygen transfer ability. Hence, the catalyst exhibited high resistance against carbon deposition and long-term stability. Furthermore, the sintering of Ni particles was limited owing to the ‘‘anchoring effect’’ exerted by the mesoporous structure [9,12,13,37]. Sintering usually results in catalyst deactivation.

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Fig. 6. Dependence of catalytic performance of the reduced Ni–ZrO2 catalysts on reaction time. Reaction conditions: T = 750 °C, CH4/CO2 = 1, GHSV = 24,000 mL/(g h).

Fig. 7. Stability test of Ni–ZrO2-CR-15 catalyst. (Reaction conditions: T = 700 °C, CH4/CO2 = 1, GHSV = 24,000 mL/g h.)

3.3. Characterization of carbon deposition on spent Ni–ZrO2 catalysts

3.3.1. TG-DTA analysis Carbon deposition on the spent Ni–ZrO2 catalysts was quantified by TG-DTA. As shown in Fig. 8a, the weight loss of the spent catalysts decreased in the following order: Ni/ZrO2-IMP > Ni– ZrO2-C > Ni–ZrO2-CR-15-ET. Ni–ZrO2-CR-15-ET only displayed minimal weight losses, demonstrating its higher de-coking performance when compared with that of conventionally impregnated and co-precipitated prepared samples. Fig. 8b shows the DTA profiles of the spent catalysts. The Ni–ZrO2-CR-15-ET catalyst

displayed two peaks centered at 294 and 610 °C corresponding to two types of carbon species. The first peak at 294 °C was attributed to reactive carbonaceous species (Ca) that can convert to CO. The second peak at 610 °C was ascribed to the formation of small amounts of graphitic carbon species (Cc) [16,37,38]. Spent Ni–ZrO2-C displayed a similar DTA profile with higher amounts of Cc. Compared with spent Ni–ZrO2-IMP, the lower amount of deposited carbon on Ni–ZrO2-C may be due to its poor activity. Spent Ni–ZrO2-IMP displayed two peaks centered at 500 and 591 °C that may be attributed to Cb and Cc species, respectively [39]. The absence of Ca species may be due to reduced interfacial contacts between the metal and support, as evidenced by TEM analysis (Fig. 2h). The oxygen species can only react with reactive carbonaceous species (Ca) to form CO and hydrogen when present within proximity of the interface between Ni particle and the support. Alternatively, some of the active carbon species can transform into relatively inert Cb species through subsequent dehydrogenation, polymerization, and rearrangement, leading to the formation of large amounts of graphitic carbon species (Cc) [9]. Both Cb and Cc species are associated with catalyst deactivation. The high carbon accumulation on Ni–ZrO2-IMP and Ni–ZrO2-C can be attributed to the presence of larger Ni particles and reduced interfacial contacts between Ni and the support. The high anti-coking performance of Ni–ZrO2-CR-15 is mainly attributed to the small Ni particle size and the mesoporous amorphous structure, which promotes intimate contact between Ni and ZrO2. The adsorption and dissociation of CO2 most likely occurred at the interface of nickel and ZrO2, resulting in the production of CO and active oxygen species. The carbon species derived from methane decomposition react with the active oxygen species to form CO and H2 [19,24,35,40]. Formation of Ni–ZrO2 interfaces facilitates migration of active

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shown in Fig. 8c, Ni–ZrO2-CR-15-ET and spent Ni–ZrO2-C catalysts showed two types of CO release peaks in accordance with the corresponding TG-DTA profiles. The two peaks were centered at 316 and 585 °C for the Ni–ZrO2-CR-15-ET catalyst and at 351 and 606 °C for the spent Ni–ZrO2-C catalyst. The peak at the lower temperature was attributed to reactive carbonaceous species (Ca), which can react with CO2 to form CO and hydrogen. The other peak at the high temperature was assigned to relatively inert Cc species [14,37]. The CO2-TPO profile of spent Ni–ZrO2-IMP shows the strongest CO release signal centered at 638 °C, indicating the formation of large amounts of coke on the Ni–ZrO2-IMP catalyst. Based on the TG-DTA results, this peak was mainly associated with both Cb and Cc species. Additionally, it was also observed that carbon deposited on Ni–ZrO2-CR-15 exhibited higher reactivity towards CO2 when compared with that on other catalysts, as evidenced from its lower CO release temperature. This demonstrates an enhanced ability to eliminate coke at low temperatures. As mentioned above, the superior performance of Ni–ZrO2-CR-15 towards coke removal was probably attributed to CO2 activation. FTIR analysis (Fig. 5) revealed the presence of active monodentate carbonate species on Ni–ZrO2-CR-15. In contrast, relatively inert polydentate carbonate species were observed on both Ni–ZrO2IMP and Ni–ZrO2-C. Therefore, the enhanced deposited carbon removal capability over Ni–ZrO2-CR-15 may be related to the different intermediates for CO2 dissociation.

4. Conclusions Mesoporous amorphous Ni–ZrO2 catalysts were prepared by a co-precipitation method with prolonged reflux digestion (Ni– ZrO2-CR catalysts). The preparation method afforded the synthesis of highly dispersed Ni nanoparticles that were well dispersed in a mesoporous amorphous zirconia matrix; the mesoporous amorphous structure exhibited excellent thermal stability at elevated temperatures. Compared with Ni–ZrO2 catalysts that were prepared by conventional impregnation and co-precipitation methods, the nanoamorphous Ni–ZrO2-CR catalysts exhibited superior activity, long-term stability, and high resistance against carbon deposition for the DRM reaction. The ‘‘anchoring effect’’ exerted by the mesoporous structure improved the dispersion of nickel nanoclusters and suppressed the sintering of Ni particles. Furthermore, the multiple points of contact between Ni particles and amorphous ZrO2 enhanced the oxygen transfer ability, leading to high activity and anti-coking capability. Additionally, carbon deposited on Ni– ZrO2-CR-15 was more reactive towards CO2 when compared with that deposited on the catalysts prepared by conventional methods under reforming reaction conditions.

Fig. 8. (a) TG, (b) DTA and (c) CO2-TPO profiles of spent catalysts: (1) Ni–ZrO2-CR15 after 80-h reaction, (2) Ni–ZrO2-C after 10-h reaction, and (3) Ni–ZrO2-IMP after 10-h reaction.

oxygen species, contributing to high resistance against carbon deposition [41]. The absence of strong Lewis acid sites (cus Zr4+ sites) was confirmed by FTIR analysis, and also played an important role in suppressing coke formation because carbon is prone to formation on acid sites [24].

Acknowledgments The authors are grateful for financial support from the Key Project of Energy Base of the Chinese Academy of Science (Contract No. 08YCA71921), the International S&T Cooperation Project of Shanxi Province, China (Contract No. 2008081012), and the Special-funded Program on National Key Scientific Instruments and Equipment Development of China (No. 2011YQ120039).

Appendix A. Supplementary material 3.3.2. CO2-TPO analysis CO2-TPO analysis was performed to further investigate the activity of the different coke species on the spent catalysts. As

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.fuel.2015.01.076.

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