Stabilizing Ni on bimodal mesoporous-macroporous alumina with enhanced coke tolerance in dry reforming of methane to syngas

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Stabilizing Ni on bimodal mesoporous-macroporous alumina with enhanced coke tolerance in dry reforming of methane to syngas Qingxiang Maa,*, Yunxing Hana, Qinhong Weib, Shengene Makpala, Xinhua Gaoa, Jianli Zhanga, Tian-sheng Zhaoa a State Key Laboratory of High-efficiency Coal Utilization and Green Chemical Engineering, College of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, PR China b Department of Chemical Engineering, School of Petrochemical Technology and Energy Engineering, Zhejiang Ocean University, Zhoushan, 316022, Zhejiang, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Bimodal structure Methane Carbon dioxide Dry reforming of methane Nickel-based catalyst

The bimodal mesoporous-macroporous alumina support was prepared via evaporation-induced self-assembly method (EISA), in which the polystyrene latex spheres with 100 nm diameter size were employed as sacrificial templates to fabricate macropores. The mesoporous-macroporous Al2O3-supported Ni catalyst (Ni/MM-A) was prepared by incipient-wetness impregnation method and applied for dry reforming of methane (DRM) to syngas. Characterizations by XRD, H2-TPR, SEM, and TEM revealed that the long-range ordered mesopore structure together with existing macropores was fabricated by using the EISA method, and the Ni particles were mainly stabilized inside the mesoporous channels of Al2O3 support and the strong interaction between Ni and Al2O3 was constructed. The Ni/MM-A catalyst exhibited more excellent catalytic performance in DRM reaction than that over Ni/M-A without macroporous structure. Stability tests for 100 h DRM reaction further indicated that the Ni/ MM-A catalyst presented strong catalytic stability, which was significantly attributed to the macroporous structure that allowed the rapid mass transport, enhanced the tolerance to graphitic carbon and reduced the amount of graphitic carbon depositions.

1. Introduction In recent years, rapid industrial development consumed large amounts of non-renewable resources, such as coal, petroleum and natural gas, causing serious environmental problems. At present, carbon dioxide (CO2) emission becomes a hot topic because it acting as greenhouse gas can lead to the global warming. Currently, more and more researchers are aware of the seriousness of CO2 emission to the environment. To address this issue, some advanced technologies initiated by researchers have been applied to realize CO2 collection as well as conversion to useful energy chemicals [1–4]. For instance, CO2 from industrial tail gases can be removed and/or separated by CO2 capture and storage technology and then converted to energy chemicals, like methanol [5], light olefin and gasoline [6–8], by CO2 hydrogenation technology. In addition, CO2 dry reforming of methane (DRM) is a traditional and effective chemical technology, which simultaneously converts two types of potential carbon resources CO2 and CH4 (greenhouse gases) to useful syngas [9–14]. The produced syngas is readily converted to liquid fuels through gas-liquid technologies, such as Fischer-Tropsch synthesis and methanol synthesis [15,16]. ⁎

With the constant surge of shale gas, the DMR has recently been paid more attention again, because the abundant shale gas resources will ensure the sufficient supplies for natural gas so as to lower the input cost of DMR. However, the biggest barrier of the dry reforming reaction is the rapid deactivation of catalyst, which is caused by coke deposited on catalyst during reaction process [17–19]. Therefore, developing a robust catalyst exhibiting strong resistance to coke deposition during DMR reaction is the critical issue to this technology. In terms of DRM, numerous catalysts have been investigated. The catalysts basing on noble metals like Ru [20], Rh [21], Pt [22] and Pd [23] present outstanding catalytic performance and strong resistance to carbon deposition for DRM, but the high cost and limited resource of noble metals pose a concern in their industrial applications. Ni-based catalysts with low cost and wide availability have been confirmed to be the most effective catalyst for DRM reaction among all non-precious metal catalysts [24–26]. Unfortunately, the fatal problem of Ni-based catalysts is the rapid deactivation, which results from the carbon deposition as well as the sintering of metallic Ni particles. One of the main reasons resulting in Ni-based catalysts deactivation is the Ni particle size, as it is a critical factor to influence the carbon deposition [27,28].

Corresponding author. E-mail addresses: [email protected] (Q. Ma), [email protected] (T.-s. Zhao).

https://doi.org/10.1016/j.jcou.2019.10.010 Received 8 June 2019; Received in revised form 31 August 2019; Accepted 15 October 2019 2212-9820/ © 2019 Elsevier Ltd. All rights reserved.

Please cite this article as: Qingxiang Ma, et al., Journal of CO₂ Utilization, https://doi.org/10.1016/j.jcou.2019.10.010

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2.1.2. Synthesis of ordered mesoporous-macroporous alumina The ordered mesoporous-macroporous alumina (denoted as MM-A) was prepared by the same EISA method, and the difference is that a certain amount of uniform polystyrene latex spheres (diameter size: 100 nm) were added to the precursor solution of alumina. After drying, the precursor gel was calcined at same roasting conditions as that of MA, during which the polystyrene latex spheres played a role of macropore template, and meanwhile, could be removed in situ under high temperature.

Garcia Dieguez et al. investigated the Pt-modified Ni/Al2O3 catalyst for DMR reaction, and the result indicated that the smaller the Ni particle size, the easier the coke was suppressed and the higher the catalytic activity was obtained [29]. The Ni particle with small size can reduce the nucleation rate of coke effectively, thus inhibiting the formation of cokes deposited on catalyst [30,31]. Except for the small Ni particle size, the carbon deposition can also be restrained relying on the structure of the catalyst itself. In particular, catalyst with appropriate large pores contributes to the mass transfer, enhancing the coke tolerance and accelerating the catalytic reaction rate [32,33]. Mesoporous Al2O3, due to its low use-cost and excellent physicochemical properties, is the most extensively used support for catalytic reaction in industry as well as in laboratory. However, despite its high specific surface area, the only mesopores are not adequate to meet the requirement for rapid internal diffusion of reactants and products, resulting in low catalytic efficiency. Catalyst with a bimodal structure consisting of mesopore and macropore will relieve this situation because the macropore can allow rapid mass transport inside the pore channels [34]. Sang et al. prepared mesoporous-macroporous alumina supported Re2O7 catalyst for the metathesis of 1-butene and 2butene to synthesize propene, and the Re2O7/Al2O3 presented higher catalytic activity and longer lifetime than that of the only mesoporous Al2O3 supported Re2O7 catalyst [35]. Witoon et al. used hierarchical macro-mesoporous alumina-supported copper catalyst for catalyzing CO2 hydrogenation to methanol [36]. The Cu-based catalyst with bimodal pore structure showed excellent methanol selectivity and dimethyl ether selectivity, much higher than that over the mesoporous alumina supported Cu catalyst. The reason for high catalytic activity and selectivity over bimodal pore structure catalysts was mainly derived from the macro-mesoporous structure, on which the mass transfer was promoted and the reaction route towards key products was more likely guided [32]. In this work, the bimodal mesoporous-macroporous alumina supported Ni catalyst (Ni/MM-A) was prepared via evaporation-induced self-assembly method (EISA) and incipient-wetness impregnation method respectively, in which the polystyrene latex spheres with 100 nm diameter size were employed as sacrificial templates to fabricate macroporous structure. The Ni particles exhibited strong interaction with Al2O3 support, which was beneficial to the stability of Ni particles under harsh reaction conditions. Meanwhile, most of the Ni particles were embedded inside the pore channels of Al2O3, by which the coke depositions could be effectively suppressed. The catalytic behavior of the Ni/MM-A catalyst was investigated and compared with that of the mesoporous Al2O3 supported Ni catalyst (Ni/M-A) in the dry reforming of methane. The promoted effect of mesoporous-macroporous structure of the Ni/MM-A catalyst in enhancing catalytic performance and resisting coke depositions was also discussed in detail.

2.1.3. Catalyst preparation MA supported nickel catalyst (Ni/M-A) and MM-A supported nickel catalyst (Ni/MM-A) were prepared by incipient-wetness impregnation method using Ni(NO3)2·6H2O as nickel precursor. The nickel nitrate solution was drop by drop added to Al2O3 with an ultrasonic-assisted way. The impregnated samples were dried at 120 °C overnight. After that, the samples were calcined at 700 °C for 4 h with a temperature ramp of 2 °C/min under air atmosphere. Nickel loading amount on Ni/ M-A and Ni/MM-A catalysts was fixed at 6 wt. %.

2.2. Catalyst characterization The BET surface areas of samples were measured by N2 physisorption method at 77 K on an automated JW-BK122 F analyzer. The pore size distributions of samples were obtained by BJH method. Prior to tests, the samples were firstly outgassed in vacuum at 200 °C for 2 h. X-ray powder diffraction patterns (XRD) of the catalysts were carried out on a Bruker D8 Advance with Cu Kα radiation. The measurement was conducted in operating conditions of 40 kV and 40 mA, diffraction angle (2θ) from 10 to 80° at a scanning speed 2°/min. H2 temperature-programmed reduction (H2-TPR) experiments of samples were carried out on an AutoChem Ⅱ 2920 analyzer. For H2-TPR measurements, 50 mg of each sample was firstly purged in He flow of 30 mL/min at 200 °C for 2 h, followed by being cooled down to 50 °C. H2-TPR measurements were performed in H2/Ar flow (5 vol%, 30 mL/ min) from 50 °C to 900 °C with a heating rate of 10 °C/min. Hydrogen consumption was monitored by TCD monitor. Temperature-programmed hydrogenation (TPH) experiments of samples were also operated on the AutoChem Ⅱ2920 analyzer. Similar to H2-TPR, the used samples were pretreated at 200 °C for 2 h in flowing He (30 mL/min). Then, TPH tests were conducted in H2/Ar flow (5 vol%, 30 mL/min) from 50 °C to 900 °C with a ramping rate of 10 °C/min. The surface morphology of catalysts was observed by scanning electron microscopy (SEM) performed on JSM-7001 F instrument. Transmission electron microscope (TEM) images of samples were taken using a transmission electron microscope (HITACHI HT7700) operated at 200 eV. For preparation of specimens, the samples were dispersed in ethanol by an assist of ultrasonic oscillation. Then, the suspension droplets were dripped on a copper grid. After evaporation of ethanol, the copper grids loading with samples were used for tests. The chemical existing form of Ni species of catalysts was characterized by X-ray photoelectron spectroscopy (XPS) on an AXIS ULTRA DLDX-ray photoelectron spectroscopy spectrometer. To accurately detect the chemical transformation of Ni species after catalyst reduction, the semi-in situ XPS spectroscopy was carried out. These spectra obtained were calibrated by C1s peak with binding energy of 284.5 eV. The amounts of carbon deposition of the used catalysts were calculated by thermogravimetric (TG) analysis on a SETARAM SENSYS-TG thermogravimetric instrument. The used catalyst was put on a platinum sample cell and then heated to 800 °C with a heating rate of 10 °C/min in an air flow of 20 mL/min. The chemical property of the deposited carbon species on the used catalysts was investigated by Raman spectra on a Horiba DXR Raman spectrometer using a laser excitation line at 780 nm.

2. Experimental 2.1. Preparation of support 2.1.1. Synthesis of ordered mesoporous alumina The ordered mesoporous alumina (denoted as M-A) was synthesized via evaporation-induced self-assembly method (EISA), according to previous report [37,38]. In a typical preparation, an appropriate amount of surfactant (Pluronic P123, (EO)20(PO)70(EO)20, Aladdin) was dissolved in absolute ethanol under rigorous stirring for 1 h at room temperature. Then, the aluminum iso-propoxide and concentrated nitrate acid were added to above solution, and the mixed solution was stirred for another 5 h. After that, the mixture was put in a 60 °C drying oven for 72 h. Finally, the dry gel was calcined at 700 °C for 4 h with a heating rate of 2 °C/min under air atmosphere to obtain mesoporous Al2O3.

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2.3. Catalytic reaction

Table 1 BET surface area and pore structure of the samples.

The catalytic activity measurements were carried out under atmospheric pressure over a fixed-bed quartz reactor (6 mm). 0.1 g of catalyst (20–40 mesh) and 0.3 g of quartz and were uniformly mixed and loaded into the center position of quartz tube. Prior to reaction, the catalyst was in-situ reduced at 700 °C for 2 h under 5% H2/N2 flow (40 mL/min). Then the reactant gas CH4/CO2/Ar with a molar ratio of 45/45/10 was passed through the catalyst. Argon in reactant gas was used as internal standard for calculating CH4 and CO2 conversions. Catalytic activity was evaluated at different reaction temperatures (600 °C–800 °C) and at different Weight Hourly Space Velocities (15, 30, 45 and 115 L·g−1 h−1). The effluent gases consisting of CH4, CO2, CO and H2 were analyzed by on-line gas chromatography (GC-9560) equipped with a TCD detector. The conversions of CH4 and CO2, as well as molar ratio of H2/CO ratio were respectively calculated as follows:

CH 4 conv.(%) =

(CH 4 /Ar)in − (CH 4 /Ar)out × 100% (CH 4 /Ar)in

CO2 conv.(%) =

(CO2 /Ar)in − (CO2 /Ar)out × 100% (CO2 /Ar)in

H2 /CO molar ratio =

Samples

SBET (m2/g)

Vpore(cm3/g)

a

M-A MM-A Ni/MA Ni/MM-A

230.5 133.4 198.1 106.5

0.48 0.27 0.32 0.22

6.7 6.2 4.1 6.2

Dmesopore(nm)

b

Dparticle (nm)

6.2 5.5

a

Average pore size of the samples was decided by BHJ method. Metallic Ni particle size of the reduced Ni/MA and Ni/MM-A was calculated by XRD. b

7 nm and the average pore size is calculated to 6.7 nm obtained by BJH method shown in Table 1. The pore size distribution curve of MM-A displays not only the main mesopore but also a few macropores, them of which are around 6.2 nm and more than 50 nm, respectively. By using the polystyrene latex spheres, the bimodal mesoporous-macroporous structure was successfully constructed. When Ni was loaded onto M-A, the average pore size was reduced to 4.1 nm. This was due to a fact that parts of pore mouths of the mesopores might be plugged by nickel particles. After Ni impregnation, the BET surface areas of MA and MM-A were decreased from original 230.5 m2/g and 133.4 m2/g to 198.1 m2/g and106.5 m2/g, respectively, and the pore volumes being reduced from 0.48 cm3/g and 0.27 cm3/g to 0.32 cm3/g and 0.22 cm3/g separately. Low-angle XRD profiles of the prepared M-A, MM-A, Ni/M-A and Ni/MM-A are displayed in Fig. 2a. The XRD diffraction profiles present obvious diffraction peaks (100) at around 1°, indicating that the ordered mesoporous structure with hexagonal 6 mm symmetry was fabricated in the crystal framework of the samples. However, for Ni/M-A and Ni/MM-A samples, the (100) peak intensities are lower than those of M-A and MM-A. The results suggest that the long-range ordered mesoporous structures of Ni/M-A and Ni/MM-A were partially destroyed because the loaded Ni blocked part of mesopores as well as incorporated into the crystalline lattice of M-A and MM-A supports. Wide-angle XRD profiles of the Ni/M-A and Ni/MM-A catalysts show no NiO diffraction peaks, and the obvious peaks at 2θ = 37°, 45° and 65.5° are indexed to the reflection of NiAl2O4 in Fig. 2b. The formation of NiAl2O4 phase was due to the solid-state reaction between NiO and Al2O3, during which the introduced Ni was incorporated into the crystalline framework of Al2O3 by means of high-temperature calcination. When the Ni/M-A and Ni/MM-A catalysts were reduced (in Fig. 2c), two sets of new diffraction peaks positioned at (37.5°, 45.7°, 66.5°) and (44.5°, 52°) are clearly observed, which are in good agreement with the γ-Al2O3 phase and metallic Ni phase. Based on the XRD profiles, the crystalline sizes of metallic Ni of the reduced Ni/M-A and Ni/MM-A catalysts were calculated at around 6.2 nm and 5.5 nm, separately.

(H2 )out (CO)out

(CH4/Ar)in or out and (CO2/Ar)in or out represent the flow rates of CH4 and CO2 relative to Ar in the inlet gas and in the outlet gas, respectively; (H2)out and (CO)out stand for the flow rates of H2 and CO separately in the outlet gas. 3. Results and discussion 3.1. Catalyst characterization Nitrogen adsorption-desorption isotherms of the as-prepared supports and catalysts were exhibited in Fig. 1(a). It is clear that the isotherms of samples are classic type-IV adsorption curves with apparent H1 hysteresis loops, indicating the typical mesoporous materials for the samples. Particularly, the adsorption-desorption curves of the MA display abrupt steep and parallel tendency at about 0.6∼0.9 of the relative pressure P/P0, suggesting that the uniform mesopores were fabricated by EISA method [39]. For MM-A, except for the relative pressure at P/ P0 = 0.6∼0.9, the isotherms display a steep trend in the relative pressure P/P0 > 0.95, which was caused by the presence of the macropores. As expected, the polystyrene latex spheres indeed played a template role in fabricating macropores, meanwhile being removed in situ under high-temperature calcination. The pore size distribution curves of the samples are shown in Fig. 1(b). It can be seen that the pore size distribution curve of M-A shows a sharp peak located at around

Fig. 1. N2 adsorption-desorption isotherms (a) and pore size distribution curves (b) of the as-prepared Al2O3 supports and catalysts. 3

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Fig. 2. Low-angle XRD patterns (a) and Wide-angle XRD patterns (b) of the calcined samples; (c) Wide-angle XRD patterns of the reduced samples.

Fig. 3. SEM images of M-A (a), Ni/M-A (b), MM-A (c) and Ni/MM-A (d).

channels stacked together, implying that the mesoporous walls of the γAl2O3 were highly crystalline. This result is in good agreement with that of the low-angle XRD investigations. For the MM-A support and Ni/ MM-A catalyst (Fig. 3c and 3d), large amounts of mesopores and macropores exist on them, and the abundant macropores were due to the polystyrene latex spheres used as template for MM-A preparation.

The surface morphology of M-A, MM-A, Ni/M-A and Ni/MM-A are displayed in Fig. 3. It is clear that the cylindrical mesopores, like honeycomb, are formed on M-A (in Fig. 3a). After loading Ni on M-A support, these mesopores are still clearly visible (in Fig. 3b), and besides, the dense lines with various alignments are also observed distinctly. These numerous lines represent the hexagonal cylindrical 4

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Fig. 4. TEM images and HRTEM images of the samples M-A (a), Ni/M-A (b,c), MM-A (d) and Ni/MM-A (e,f).

In order to further verify the crystallinity of the as-prepared γ-Al2O3, additional proof for proving the crystalline γ-Al2O3 is also given by TEM investigations as in Fig. 4. Seen from TEM images in Fig. 4a and 4d for M-A and MM-A, the parallel alignments of lines reflect the ordered cylindrical mesoporous channels arranged uniformly [37], which accords well with the SEM and low-angle XRD results. The line width represents the actual mesoporous size, them of which being 7.4 nm and 8 nm, respectively. The TME images and HRTEM images of the Ni/M-A catalyst (in Fig. 4b and 4c) and Ni/MM-A catalyst (in Fig. 4e and 4f) indicate that the Ni-based particles are mainly dispersed inside the pore channels, and meanwhile, parts of Ni-based particles are also loaded outside the pores. From the results of TEM, it was inferred that the Ni particles could be stabilized by the confinement effect of the highly ordered mesopores of the Ni/M-A and Ni/MM-A catalysts, which was beneficial to the enhanced catalytic activity and stability in the DRM reaction. Metallic Ni rather than Ni-based oxide is responsible for the catalytic conversion of CH4 and CO2 to syngas. Thus, the reduction behaviors of the calcined Ni/M-A and Ni/MM-A catalysts were investigated by H2-TPR. As shown in Fig. 5, the reduction temperatures of the Ni/MA and Ni/MM-A catalysts reach 686 °C and 671 °C, respectively, which were attributed to the reduction of Ni2Al2O4 to metallic Ni [40]. However, no NiO reduction peak (below 500 °C) is observed [41]. The TPR results indicate that there was a strong interaction between NiO

Fig. 5. H2-TPR profiles of the Ni/M-A and Ni/MM-A catalysts.

and Al2O3. As for the formation of Ni2Al2O4 phase, it was due to the solid phase reaction occurring between Al2O3 and NiO, in which NiO was incorporated into the crystalline framework of Al2O3 under hightemperature calcination. However, the difference of reduction 5

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Fig. 6. XPS profiles of the Ni/M-A and Ni/MM-A catalysts (a) calcined and (b) reduced.

method also showed poor catalytic activity and stability, during which the catalytic activity decreased obviously within 10 h reforming reaction [45]. The excellent catalytic performances of Ni/M-A and Ni/MMA catalysts were mainly ascribed to the strong metal-support interaction between Ni and Al2O3 as well as the confinement effect of Ni particles by mesoporous channels. The metallic Ni particles, which were tightly anchored on Al2O3 and embedded into mesoporous channels, exhibited strong resistance to particle agglomeration and carbon deposition. The effect of reaction temperature on the catalytic activity was further investigated, and the catalytic activities of the Ni/M–A and Ni/ MM–A catalysts in a temperature range from 600 to 800 °C are displayed as in Fig. 8. With the increase of reaction temperature, the catalytic activity gradually increases. At the reaction temperature of 600 °C, the conversions of CH4 and CO2 are 39.2% and 53.1%, respectively. When reaction temperature is at 800 °C, the conversions of CH4 and CO2 are up to 96.9% and 98.3%. DRM is a strong endothermic reaction in thermodynamics, where the high temperature strikingly promoted the DRM reaction. Furthermore, it is found that as temperature increases the CH4 conversion is gradually approaching that of CO2 conversion. This is believed that high temperature is thermodynamically favorable to CH4 conversion because of the high dissociation energy of C–H bond in CH4 molecule [46]. Space velocity as a key parameter represents the production capacity in industrial applications, as it is directly related to the volume of production unit and therefore influences economic benefits. As shown in Fig. 9, the dependence of the catalytic performance of the Ni/M-A and Ni/MM-A catalysts with the Weight Hourly Space Velocity (WHSV) was studied. The various WHSV of 15, 30, 45 and 115 L·g−1 h−1 were investigated for DRM reaction under 700 °C. At a low WHSV of 15 L·g−1 h−1, the Ni/M-A and Ni/MM-A catalysts exhibit high catalytic activities, the conversions of CH4 and CO2 being 78.1% and 87.3% for Ni/M-A and 79% and 88.4% for Ni/MM-A. With the increase of space velocity, a notable decrease in CH4 and CO2 conversions is observed. When increasing the space velocity to 115 L·g−1 h−1, the catalytic activity rapidly decreases; however, the conversions of CH4 and CO2 are still very high especially for CO2 conversion up to 68.3% over Ni/MM-A catalyst. Based on the test results via temperatures and space velocities, the Ni/M-A and Ni/MM-A catalysts exhibited excellent catalytic performance under various reaction conditions. The reaction behavior at 700 °C suggests that the Ni/M-A and Ni/MM-A catalysts presented medium-low temperature speciality for DRM reaction. Moreover, the catalysts also showed considerable catalytic ability, even in a large space velocity of 115 L·g−1 h−1.The excellent catalytic behavior of the Ni/M-A and Ni/MM-A catalysts strongly gave a proposal for their potential applications in actual operating units.

temperature may depend crucially on the pore structure. For Ni/MM-A catalyst, it is composed of mesopores and macropores. The existing macropores, due to unencapsulated structure, preserved weak interaction with metal Ni particles, consequently resulting in low reduction temperature. In methane dry reforming reaction, the strongly interacting metallic Ni exhibited stronger ability of resisting sintering, inhibiting the particle agglomerations and thus preventing the Ni particle size from being increased. As reported previously, the smaller the metallic Ni particle size, the easier it is to resist the carbon depositions, because the small Ni particle can effectively restrain the coke nucleation [30]. The XPS spectra of calcined and in-situ reduced Ni/M-A and Ni/ MM-A catalysts are carried out to illustrate the chemical property of the surface Ni-based species. As shown in Fig. 6a, the XPS Ni 2p spectra of the calcined catalysts display two main peaks at 856 eV and 873.5 eV, along with a satellite peak at around 862 eV. The characteristic electron binding energy illustrates that the Ni existed in the form of bivalence [42]. For pure NiO, its binding energy is about 854.5 eV. The higher binding energies for Ni/M-A and Ni/MM-A catalysts suggested that the strong metal-support interaction between Ni species and Al2O3 was built. This result is consistent to that of XRD and TPR investigations. To accurately reveal the chemical transformation of Ni species during hydrogen reduction, the calcined catalysts were first reduced in situ by hydrogen, and then the XPS spectra were tested. The XPS profiles display that except for the inherent peaks of Ni2+, the reduced catalysts present a new peak at 852.8 eV (in Fig. 6b), which is assigned to the metallic Ni. The results reveal that the Ni2+ in NiAl2O4 was reduced to metallic Ni, but the Ni2+, due to strong interaction with support, was still preserved partially. It is worth mentioning that the Ni2+ played a role of slow-release agent during the dry reforming reaction, where the Ni2+ was slowly reduced to metallic Ni that could make up for the inactivated metallic Ni caused by coke deposition [43].

3.2. Catalytic performance To investigate the catalytic stability of the Ni/M-A and Ni/MM-A catalysts, the life time tests were carried out at 700 °C for 100 h, and the results are shown in Fig. 7. CO2 conversion is higher than that of CH4 conversion. This is because Reverse Water-gas shift (CO2 + H2 ⇌ CO + H2O) occurred together with main reaction under the reaction conditions, leading to higher CO2 conversion. Both Ni/M-A and Ni/MM-A catalysts keep good catalytic activities and stabilities during reaction. No obvious deactivation is observed after 100 h on stream. The conversions of CH4 and CO2 over Ni/MM-A catalyst are up to 73% and 83% respectively, higher than those of 70% and 81% over Ni/M-A catalyst. Zhang et al. reported surface-modified ZrO2 supported Ni catalyst for CO2 dry reforming [44]. The catalytic activity decreased constantly within 5 h reforming reaction, and the CO2 conversion went from 80% to 55%. A series of Ni/Al2O3 catalysts prepared by co-precipitation 6

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Fig. 7. Catalytic stability of the Ni/M-A and Ni/MM-A catalysts. (a) CH4 and CO2 conversions, (b) H2/CO molar ratio. Reaction conditions: T =700 °C, P = ambient pressure, CH4/CO2 = 1, WHSV = 30 L·g−1 h−1.

Fig. 10. TG profiles of the spent catalysts after DRM reaction for 100 h. Fig. 8. Effect of the reaction temperature on CH4 and CO2 conversions. Reaction conditions: P = ambient pressure, CH4/CO2 = 1, WHSV = 30 L·g−1 h−1, each temperature being tested for 20 h on stream.

3.3. Characterizations of the spent catalysts Catalyst deactivation caused by carbon deposition is the most fatal issue for DRM reaction. To reveal the carbon deposition, the TG analysis of spent catalysts was performed to quantify the content of carbon, and the results are shown in Fig. 10. The total weight loss on the spent Ni/ M-A catalyst is 8.8 wt%, which is larger than that of 5.3 wt% on the spent Ni/MM-A catalyst. From TG curves, there are two exothermic peaks observed, signifying two types of carbon deposited on metallic Ni active sites. The low-temperature exothermic peak at 150–250 °C was due to the removal of amorphous carbon, which was formed through methane decomposition in the early stage of DRM reaction and posed no threat to the DRM reaction. The amounts of amorphous carbon on spent Ni/MM-A and Ni/M-A were calculated to be 2.8 wt% and 4.7 wt %, respectively. The high-temperature exothermic peak at 500-700 °C was ascribed to the inert carbon, which tightly wrapped the Ni active sites and then deactivated the catalytic performance of metallic Ni. The amounts of inert carbon on spent Ni/MM-A and Ni/M-A were quantitatively estimated to be 2.5 wt% and 4.1 wt%. The TG results fully proved that the Ni/MM-A catalyst exhibited good coke resistance during DRM, better than that of Ni/M-A. The excellent catalytic peculiarity was mainly derived from the bimodal structure of Ni/MM-A catalyst, on which the macropores allowed the facile mass transport and thus enhanced the tolerance to coke. Kim et al. also gave similar report. The synthesized Ni-Co-Mn/Al2O3 catalyst with unique macroporous structure remarkably benefited the mass transport and effectively enhanced coke resistance in the DRM reaction, thereby promoting the catalytic performance and stability [32,33]. To further reveal the deposited carbon, the SEM and TME images of

Fig. 9. Effect of the space velocity on CH4 and CO2 conversions. Reaction conditions: P = ambient pressure, T =700 °C, CH4/CO2 = 1.

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Fig. 11. SEM and TEM images of the spent catalysts after DRM reaction for 100 h (a, c) Ni-MA and (b, d) Ni/MM-A.

The inert carbon, which results in the deactivation of catalyst during DRM reaction, usually possesses a certain of graphite degree. Raman spectra, as an effective characterization technique, were therefore performed to characterize the deposited carbon species. As shown in Fig. 12a, the Raman profiles of spent catalysts exhibit two characteristic peaks positioned at 1300 cm−1 and 1580 cm−1, which were ascribed to D band (lattice defects) and G band (stretching vibration of carbon atoms with sp2 hybridization), respectively [47,48]. The relative intensity (ID/IG) was used to determine the disorder degree in the graphite layer. For the spent Ni/M–A and Ni/MM–A catalysts, the ID/IG values were calculated as 1.14 and 1.23 respectively, indicating that the carbon species depositing on the spent Ni/M–A possessed higher graphite degree than that on the spent Ni/MM–A. Furthermore, the peak strength of the carbon species on the spent Ni/MM–A is lower compared to that on the spent Ni/M–A, implying the less content of carbon depositions. Combined with XRD analysis (in Fig. 12b), the spent Ni/M–A catalyst displays a diffraction peak at about 26°, which was designated to the graphite carbon species. Nevertheless, the peak is absent of the XRD pattern of the spent Ni/MM–A catalyst, meaning that the deposited carbon was amorphous and/or the carbon content was below the XRD detection line. Investigations by TG, SEM, Raman and XRD for the spent

the spent Ni-MA and Ni/MM-A catalysts after 100 h reaction are displayed in Fig. 11. It can be observed that a large amount of silk-like cokes was deposited on the surface of spent Ni/M-A catalyst (in Fig. 11a); however, the spent Ni/MM-A catalyst exhibited strong coke tolerance, with no silk-like coke being formed on it (in Fig. 11b). The formed coke is extremely stubborn and inert even through under harsh reaction atmosphere. The TEM image clearly displays that the formed coke presented nano-tube structure and firmly enclosed the Ni particles (in Fig. 11c), thus isolating the Ni catalytic active sites from CO2 and CH4. Moreover, the coke could weaken the interaction between metallic Ni and support by lifting the Ni particles from matrix. This could lead to the migration of Ni particles within the mesoporous channels or from inner channels to the outer surface of Al2O3. As a result, these Ni particles were easily agglomerated during DRM reaction. It is distinct that the Ni particles on the spent Ni/M-A catalyst occurred serious agglomerations, but still kept good dispersion on the spent Ni/MM-A catalyst. The metallic Ni particle size played a crucial role in DRM reaction because it significantly affected the catalytic behavior of catalyst. The Ni particles with smaller size exhibited stronger ability ineffectively suppressing carbon deposition, and thus it was beneficial to DRM reaction.

Fig. 12. Raman profiles (a) and XRD profiles (b) of spent catalysts after DRM reaction for 100 h. 8

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Declaration of Competing Interest The authors declare that there are no known conflicts of interest. Acknowledgements This work was supported by the financial support from National Natural Science Foundation of China (21766027, the East-West Cooperation Project of Ningxia Key R & D Plan (2017BY063) and State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2017-K11). References [1] L.L. Rao, R. Ma, S.F. Liu, L.L. Wang, Z.Z. Wu, J. Yang, X. Hu, Nitrogen enriched porous carbons from D-glucose with excellent CO2 capture performance, Chem. Eng. J. 362 (2019) 794–801. [2] S. Moret, P.J. Dyson, G. Laurenczy, Direct synthesis of formic acid from carbon dioxide by hydrogenation in acidic media, Nat. Commun. 5 (2014) 4017. [3] J.A. Mendoza-Nieto, Y. Duan, H. Pfeiffer, Alkaline zirconates as effective materials for hydrogen production through consecutive carbon dioxide capture and conversion in methane dry reforming, Appl. Catal. B Environ. 238 (2018) 576–585. [4] Z.-Q. Wang, Z.-N. Xu, S.-Y. Peng, M.-J. Zhang, G. Lu, Q.-S. Chen, Y. Chen, G.-C. Guo, High-performance and long-lived Cu/SiO2 nanocatalyst for CO2 hydrogenation, ACS Catal. 5 (2015) 4255–4259. [5] K. Larmier, W.-C. Liao, S. Tada, E. Lam, R. Verel, A. Bansode, A. Urakawa, A. Comas-Vives, C. Copéret, CO2-to-methanol hydrogenation on zirconia-supported copper nanoparticles: Reaction intermediates and the role of the metal-support interface, Angew. Chem. Int. Ed. 56 (2017) 2318–2323. [6] X. Wang, D.K. Wu, J.L. Zhang, X.H. Gao, Q.X. Ma, S.B. Fan, T.-S. Zhao, Highly selective conversion of CO2 to light olefins via Fischer-Tropsch synthesis over stable layered K-Fe-Ti catalysts, Appl. Catal. A Environ. 573 (2019) 32–40. [7] B.L. Liang, T. Sun, J.G. Ma, H.M. Duan, L. Li, X.L. Yang, Y.R. Zhang, X. Su, Y.Q. Huang, T. Zhang, Mn decorated Na/Fe catalysts for CO2 hydrogenation to light olefins, Catal. Sci. Technol. 9 (2019) 456–464. [8] J.F. Zhang, M. Zhang, S.Y. Chen, X.X. Wang, Z.L. Zhou, Y.Q. Wu, T. Zhang, G.H. Yang, Y.Z. Han, Y.S. Tan, Hydrogenation of CO2 into aromatics over a ZnCrOxzeolite composite catalyst, Chem. Commun. 55 (2019) 973–976. [9] Y. Lu, S.P. Jiang, S.P. Wang, Y.J. Zhao, X.B. Ma, Effect of the addition of Ce and Zr over a flower-like NiO-MgO (111) solid solution for CO2 reforming of methane, J. CO2 Util. 26 (2018) 123–132. [10] A.R. Kim, H.Y. Lee, J.M. Cho, J.H. Choi, J.W. Bae, Ni/M-Al2O3 (M=Sm, Ce or Mg) for combined steam and CO2 reforming of CH4 from coke oven gas, J. CO2 Util. 21 (2017) 211–218. [11] Q.H. Wei, G.H. Yang, X.H. Gao, N. Yamane, P.P. Zhang, G.G. Liu, N. Tsubaki, Ni/ Silicalite-1 coating being coated on SiC foam: a tailor-made monolith catalyst for syngas production using a combined methane reforming process, Chem. Eng. J. 327 (2017) 465–473. [12] J.W. Sun, Q.H. Feng, Q.K. Liu, S.F. Ji, Y.M. Fang, X.B. Peng, Z.J. Wang, An Al2O3coated SiC supported Ni catalyst with enhanced activity and improved stability for the production of synthetic natural gas, Ind. Eng. Chem. Res. 57 (2018) 14899–14909. [13] J.W. Sun, S. Wang, Y. Guo, M.G. Zou, Z.J. Wang, Carbon dioxide reforming of methane over nanostructured Ni/Al2O3 catalysts, Catal. Commun. 104 (2018) 53–56. [14] S.G. Wen, M.L. Liang, J.M. Zou, S. Wang, X.D. Zhu, L. Liu, Z.J. Wang, Synthesis of a SiO2 nanofibre confined Ni catalyst by electrospinning for the CO2 reforming of methane, J. Mater. Chem. A Mater. Energy Sustain. 3 (2015) 13299–13307. [15] Q.H. Lin, Q.D. Zhang, G.H. Yang, Q.J. Chen, J. Li, Q.H. Wei, Y. Tan, H.L. Wan, N. Tsubaki, Insights into the promotional roles of palladium in structure and performance of cobalt-based zeolite capsule catalyst for direct synthesis of C5-C11 isoparaffins from syngas, J. Catal. 344 (2016) 378–388. [16] M. Behrens, F. Studt, I. Kasatkin, S. Kühl, M. Hävecker, F. Abild-Pedersen, S. Zander, F. Girgsdies, P. Kurr, B.-L. Kniep, M. Tovar, R.W. Fischer, J.K. Nørskov, R. Schlögl, The active site of methanol synthesis over Cu/ZnO/Al2O3 industrial catalysts, Science 336 (2012) 893–897. [17] X.Q. Feng, J. Liu, P. Zhang, Q. Zhang, L.Y. Xu, L.P. Zhao, X.F. Song, L. Gao, Highly coke resistant Mg-Ni/Al2O3 catalyst prepared via a novel magnesiothermic reduction for methane reforming catalysis with CO2: the unique role of Al-Ni intermetallics, Nanoscale 11 (2019) 1262–1272. [18] A. Movasati, S.M. Alavi, G. Mazloom, Dry reforming of methane over CeO2-ZnAl2O4 supported Ni and Ni-Co nano-catalysts, Fuel 236 (2019) 1254–1262. [19] Z. Li, Z. Wang, S. Kawi, Sintering and coke resistant core/yolk shell catalyst for hydrocarbon reforming, ChemCatChem 11 (2019) 202–224. [20] S. Das, M. Shah, R.K. Gupta, A. Bordoloi, Enhanced dry methane reforming over Ru decorated mesoporous silica and its kinetic study, J. CO2 Util. 29 (2019) 240–253. [21] I.V. Yentekakis, G. Goula, M. Hatzisymeon, I. Betsi-Argyropoulou, G. Botzolaki, K. Kousi, D.I. Kondarides, M.J. Taylor, C.M.A. Parlett, A. Osatiashtiani, G. Kyriakou, J. Pedro Holgado, R.M. Lambert, Effect of support oxygen storage capacity on the catalytic performance of Rh nanoparticles for CO2 reforming of methane, Appl. Catal. B Environ. 243 (2019) 490–501.

Fig. 13. TPH profiles of the spent catalysts after DRM for 100 h reaction.

catalysts indicate that the Ni/MM–A catalyst, due to unique bimodal mesoporous-macroporous structure, exhibited strong coke tolerance so as to enhance the catalytic stability for DRM reaction. The deposited carbon on the spent catalysts presented different chemical properties. The TPH was carried out to investigate the activity of carbon species as in Fig. 13. The TPH profiles of the spent Ni/M-A and Ni/MM-A catalysts show two peaks, implying the different hydrogenation activities of carbon species. The low-temperature peak between 300 and 450 °C meant the removal of amorphous carbon due to the hydrogenation of amorphous carbon. Another peak between 500 and 700 °C was attributed to the hydrogenation of graphite carbon, and the latter exhibited considerable thermal stability [49]. It is clearly observed that there is more graphite carbon formed on the spent Ni/MA than that on the spent Ni/MM-A, as the peak area of graphite carbon over the spent Ni/M-A is larger than that over the spent Ni/MM-A. The inclination to form amorphous carbon on the spent Ni/MM-A catalyst is in line with the TG, SEM, TEM and XRD analyses.

4. Conclusions The bimodal mesoporous-macroporous alumina supported Ni catalyst (Ni/MM-A) was synthesized by evaporation-induced self-assembly method (EISA) method and incipient-wetness impregnation method respectively, in which the polystyrene latex spheres (diameter size: 100 nm) were used as sacrificial templates to fabricate macropores. Characterizations by XRD, pore size distribution curves, H2-TPR and TEM revealed that the prepared alumina presented ordered mesoporous structure along with existing macropores, and the metallic Ni was incorporated into the framework of Al2O3. Moreover, the well-dispersed Ni particles were mainly embedded into mesoporous channels and a strong metal-support interaction between Ni and Al2O3 was constructed. The prepared Ni/MM-A catalyst exhibited excellent catalytic activity in CO2 dry reforming reaction. Life time tests indicated that the Ni/MM-A catalyst showed good catalytic stability, better than that of the Ni/M-A catalyst. Combining the analysis of spent catalysts, the Ni/ MM-A catalyst showed strong resistance and tolerance to the formation of the graphitic carbon, which was attributed to the bimodal structure which not only contributed to the mass transport but also enhanced the resistance to the stubborn graphitic carbon. The TG and XRD results displayed that a large number of graphitic carbons were formed on the spent Ni/M-A catalyst.

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[22] D.C. Carvalho, H.S.A. de Souza, J.M. Filho, A.C. Oliveira, A. Campos, E.R.C. Milet, F.F. de Sousa, E. Padron-Hernandez, A.C. Oliveira, A study on the modification of mesoporous mixed oxides supports for dry reforming of methane by Pt or Ru, Appl. Catal. A Environ. 473 (2014) 132–145. [23] N. Koepfle, T. Goetsch, M. Gruenbacher, E.A. Carbonio, M. Haevecker, A. KnopGericke, L. Schlicker, A. Doran, D. Kober, A. Gurlo, S. Penner, B. Kloetzer, Zirconium-assisted activation of palladium to boost syngas production by methane dry reforming, Angew. Chem. Int. Ed. 57 (2018) 14613–14618. [24] Z.X. Wu, B. Yang, S. Miao, W. Liu, J.L. Xie, S. Lee, M.J. Pellin, D.Q. Xiao, D.S. Su, D. Ma, Lattice strained Ni-Co alloy as a high-performance catalyst for catalytic dry reforming of methane, ACS Catal. (2019) 2693–2700. [25] X.L. Yan, T. Hu, P. Liu, S. Li, B.R. Zhao, Q. Zhang, W.Y. Jiao, S. Chen, P.F. Wang, J.J. Lu, L.M. Fan, X.N. Deng, Y.-X. Pan, Highly efficient and stable Ni/CeO2-SiO2 catalyst for dry reforming of methane: effect of interfacial structure of Ni/CeO2 on SiO2, Appl. Catal. B Environ. 246 (2019) 221–231. [26] D.Y. Kalai, K. Stangeland, Y.Y. Jin, W.M. Tucho, Z.X. Yu, Biogas dry reforming for syngas production on La promoted hydrotalcite-derived Ni catalysts, Int. J. Hydrogen Energy 43 (2018) 19438–19450. [27] F.G. Wang, B.L. Han, L.J. Zhang, L.L. Xu, H. Yu, W.D. Shi, CO2 reforming with methane over small-sized Ni@SiO2 catalysts with unique features of sintering-free and low carbon, Appl. Catal. B Environ. 235 (2018) 26–35. [28] Z.J. Zuo, S.Z. Liu, Z.C. Wang, C. Liu, W. Huang, J. Huang, P. Liu, Dry reforming of methane on single-site Ni/MgO catalysts: importance of site confinement, ACS Catal. 8 (2018) 9821–9835. [29] M. García-Diéguez, I.S. Pieta, M.C. Herrera, M.A. Larrubia, L.J. Alemany, Nanostructured Pt- and Ni-based catalysts for CO2-reforming of methane, J. Catal. 270 (2010) 136–145. [30] J.H. Kim, D.J. Suh, T.J. Park, K.L. Kim, Effect of metal particle size on coking during CO2 reforming of CH4 over Ni-alumina aerogel catalysts, Appl. Catal. A Environ. 197 (2000) 191–200. [31] T.D. Gould, A. Izar, A.W. Weimer, J.L. Falconer, J.W. Medlin, Stabilizing Ni catalysts by molecular layer deposition for harsh, dry reforming conditions, ACS Catal. 4 (2014) 2714–2717. [32] W.Y. Kim, B.J. Lee, H. Park, Y.H. Choi, J.H. Kim, J.S. Lee, Ultrapermeable nickelcobalt-manganese/alumina inverse opal as a coke-tolerant and pressure-drop-free catalyst for the dry reforming of methane, ChemcatChem 10 (2018) 2214–2218. [33] T.H. Wang, Y. Fan, X.L. Wang, L.J. Chou, H. Lin, Selectivity enhancement of CoMoS catalysts supported on tri-modal porous Al2O3 for the hydrodesulfurization of fluid catalytic cracking gasoline, Fuel 157 (2015) 171–176. [34] K. Tao, Y. Zhang, S. Terao, Y. Yoneyama, T. Kawabata, K. Matsuda, S. Ikeno, N. Tsubaki, Chemical and spatial promotional effects of bimodal pore catalysts for methane dry reforming, Chem. Eng. J. 170 (2011) 258–263. [35] L. Sang, S.-L. Chen, G. Yuan, Z. Zhou, R. Li, A. Chen, M. Zheng, J. You, Preparation of mesoporous alumina with large pore size and their supported rhenium oxide

[36]

[37]

[38] [39]

[40]

[41]

[42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

10

catalysts in metathesis of 1-butene and 2-butene to propene, J. Nat. Gas Chem. 21 (2012) 352–359. T. Witoon, S. Bumrungsalee, M. Chareonpanich, J. Limtrakul, Effect of hierarchical meso-macroporous alumina-supported copper catalyst for methanol synthesis from CO2 hydrogenation, Energy Convers. Manage. 103 (2015) 886–894. Q. Yuan, A.-X. Yin, C. Luo, L.-D. Sun, Y.-W. Zhang, W.-T. Duan, H.-C. Liu, C.-H. Yan, Facile synthesis for ordered mesoporous γ-aluminas with high thermal stability, J. Am. Chem. Soc. 130 (2008) 3465–3472. S.M. Morris, P.F. Fulvio, M. Jaroniec, Ordered mesoporous alumina-supported metal oxides, J. Am. Chem. Soc. 130 (2008) 15210–15216. N. Wang, K. Shen, L.H. Huang, X.P. Yu, W.Z. Qian, W. Chu, Facile route for synthesizing ordered mesoporous Ni-Ce-Al oxide materials and their catalytic performance for methane dry reforming to hydrogen and syngas, ACS Catal. 3 (2013) 1638–1651. J. Shen, A.A.C. Reule, N. Semagina, Ni/MgAl2O4 catalyst for low-temperature oxidative dry methane reforming with CO2, Int. J. Hydrogen Energy 44 (2019) 4616–4629. Y.S. Park, M. Kang, P. Byeon, S.-Y. Chung, T. Nakayama, T. Ko, H. Hwang, Fabrication of a regenerable Ni supported NiO-MgO catalyst for methane steam reforming by exsolution, J. Power Sources 397 (2018) 318–324. L.L. Xu, H.L. Song, L.J. Chou, One-pot synthesis of ordered mesoporous NiO-CaOAl2O3 composite oxides for catalyzing CO2 reforming of CH4, ACS Catal. 2 (2012) 1331–1342. P.J. Tan, Z.H. Gao, C.F. Shen, Y.L. Du, X.D. Li, W. Huang, Ni-Mg-Al solid basic layered double oxide catalysts prepared using surfactant-assisted coprecipitation method for CO2 reforming of CH4, Chin J Catal 35 (2014) 1955–1971. M. Zhang, J.F. Zhang, Y.Q. Wu, J.X. Pan, Q.D. Zhang, Y.S. Tan, Y.Z. Han, Insight into the effects of the oxygen species over Ni/ZrO2 catalyst surface on methane reforming with carbon dioxide, Appl. Catal. B Environ. 244 (2019) 427–437. D. Yosef Kalai, K. Stangeland, Y.Y. Jin, Z.X. Yu, Active and stable hydrotalcite derived Ni catalysts for CO2 reforming of methane: comparison with catalysts by incipient wetness, J. CO2 Util. 25 (2018) 346–355. E. le Sache, L. Pastor-Perez, D. Watson, A. Sepulveda-Escribano, T.R. Reina, Ni stabilised on inorganic complex structures: superior catalysts for chemical CO2 recycling via dry reforming of methane, Appl. Catal. B Environ. 236 (2018) 458–465. E. le Saché, J.L. Santos, T.J. Smith, M.A. Centeno, H. Arellano-Garcia, J.A. Odriozola, T.R. Reina, Multicomponent Ni-CeO2 nanocatalysts for syngas production from CO2/CH4 mixtures, J. CO2 Util. 25 (2018) 68–78. Z.-Y. Lim, C.Z. Wu, W.G. Wang, K.-L. Choy, H.F. Yin, A novel and anti-agglomerating Ni@yolk-ZrO2 structure with sub-10 nm Ni core for high performance steam reforming of methane, RSC Adv. 5 (2015) 61925–61932. S. Das, M. Sengupta, A. Bag, M. Shah, A. Bordoloi, Facile synthesis of highly disperse Ni-Co nanoparticles over mesoporous silica for enhanced methane dry reforming, Nanoscale 10 (2018) 6409–6425.