An active and stable nickel-based catalyst with embedment structure for CO2 methanation

Journal Pre-proof An active and stable nickel-based catalyst with embedment structure for CO2 methanation Yiming Chen (Investigation) (Data curation) (Writing - original draft), Baocheng Qiu (Investigation) (Data curation), Yi LiuWriting - reviewi and editing), Yi Zhang (Supervision)Writing - reviewi and editing)

PII:

S0926-3373(20)30216-2

DOI:

https://doi.org/10.1016/j.apcatb.2020.118801

Reference:

APCATB 118801

To appear in:

Applied Catalysis B: Environmental

Received Date:

27 September 2019

Revised Date:

3 February 2020

Accepted Date:

21 February 2020

Please cite this article as: Chen Y, Qiu B, Liu Y, Zhang Y, An active and stable nickel-based catalyst with embedment structure for CO2 methanation, Applied Catalysis B: Environmental (2020), doi: https://doi.org/10.1016/j.apcatb.2020.118801

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An active and stable nickel-based catalyst with embedment structure for CO2 methanation Yiming Chen, Baocheng Qiu, Yi Liu*, Yi Zhang*

State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China.

AUTHOR INFORMATION Corresponding Authors Email: [email protected] (Y. L.)

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*

[email protected] (Y. Z.)

Highlights

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Ni@HZSM-5 catalyst was synthesized by hydrothermal method using conventional Ni/SiO2 catalyst. The Ni@HZSM-5 showed high activity and excellent stability during 40 h CO2 methanation reaction. The nickel active phase of the Ni@HZSM-5 catalyst donates more electrons to zeolite. Successfully preventing the formation of gaseous Ni(CO)x results in excellent stability.

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Graphical abstract

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ABSTRACT: The CO2 methanation catalyst with high activity and stability has attracted significant attention from industrial and academic community. In this work, a highly active and stable Ni@HZSM-5 catalyst was synthesized by hydrothermal method using conventional Ni/SiO2 1

catalyst as unique silicone source. The Ni@HZSM-5 showed high activity and excellent stability during 40 h CO2 methanation reaction, comparing to the conventional Ni/SiO2 and Ni/HZSM-5 catalysts prepared by impregnation method. After a prolonged reaction, the Ni@HZSM-5 still maintain similar nickel content and the structure of active nickel to fresh catalyst, due to the special embedment structure. Meanwhile, it is found that the nickel active phase of the Ni@HZSM-5 catalyst donates more electrons to zeolite, resulting in higher BE values, lower

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wavenumber and weak intensity of CO bands. Hence, the Ni@HZSM-5 catalyst can prevent the

formation of volatile metal-molecule intermediates (gaseous Ni(CO)x), which resulted in serious sintering and loss of the supported nickel in conventional Ni catalysts. All obtained catalysts were

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characterized by XRD, SEM-EDS, TEM, XRF, BET, TGA, in situ CO-DRIFTS and in situ XPS.

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Keywords: CO2 methanation; Nickel; Hydrogenation; Nickel Carbonyl; Embedment structure

1. INTRODUCTION

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In 2018, the total emissions of CO2 reached a record as high as 33.1 billion tons, which brought a series of environmental and social issues. The CO2 mitigation and efficient utilization

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has become a promising solution in the last years. So far, one of the most efficient approach to recycling carbon dioxide is CO2 methanation process [1-5], which can convert CO2 into methane (Synthetic Nature Gas, SNG) with commercial and environmental benefits, using renewable water electrolysis to provide H2 [6, 7]. Therefore, the development of CO2 methanation catalyst with high activity and stability has attracted more and more attention from industrial and academic 2

community. As reported by many researchers, CO2 methanation reaction is considered to follow two steps, including the conversion of CO2 to CO in the reverse water gas shift (RWGS) reaction and the subsequent hydrogenation of CO or atomic Cads species [8-10]. Many studies have reported that metals such as Ni [8, 11-16], Fe[17], Ru [18], Rh [19], and Pd [20] have ability to catalyze the methanation of carbon dioxide. Although noble metal catalysts have a higher activity and stability,

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the high price limits their usage in industrial process. Therefore, it is highly necessary to find ways

of fabricating supported transition metal catalysts with excellent activity and stability

simultaneously for CO2 methanation. Ni based catalyst along with porous materials support has

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been extensively applied in CO or CO2 methanation, because of its high activity and low price

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[21-23]. However, Ni-based catalysts always suffer from deactivation due to the carbon deposition,

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sintering of metal particles and Ni loss during the methanation reaction [24]. Therefore, how to alleviate deactivation phenomena has become a main research field for CO2 methanation process.

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It has been reported that CO2 methanation over Ni catalyst was sensitive to the catalysts’ physical and chemical properties [10, 25]. The optimum electronic donor intensity and particle size of

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nickel active phase on the catalyst can prevent the disproportionation of CO and form less carbon deposition [26, 27]. Especially, sintering of the supported nickel, such as on Ni/SiO2 catalyst,

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would accelerate the carbon deposition resulting in fast deactivation [27]. In addition, the formation of gaseous nickel-carbonyl species, depending on the interaction between CO and Ni particles, can also lead to catalyst deactivation [28, 29]. The mobile nickel-carbonyl species will cause the loss of active nickel species and accelerate the growth of the supported metal particles [30]. Moreover, the agglomeration and thermal degradation or sintering of the catalyst could 3

rapidly decrease the surface area and hence, reducing the catalyst activity and lifetime [31-33]. Therefore, suppressing the formation of surface nickel carbonyls and catalyst sintering could enhance the activity and stability for CO2 methanation over nickel based catalysts [34, 35]. Herein, we synthesized Ni@HZSM-5 catalyst with embedment structure by hydrothermal synthesis method, which use conventional Ni/SiO2 as unique silicon source to stabilize the nickel active phase and keep high catalytic activity at the same time, without using any promoter. In

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order to explore the reasons for its high stability and activity, we also compared the conventional

nickel supported catalysts, i.e. Ni/HZSM-5 and Ni/SiO2, in the CO2 methanation reaction. All prepared catalysts were characterized by XRD, SEM-EDS, TEM, XRF, BET, TGA, in situ XPS

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and in situ CO-DRIFTS, and the effects of this embedment structure on the properties of nickel

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active phase as well as catalytic performance were investigated.

2.1. Catalyst preparation

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2. EXPERIMENTAL

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The conventional Ni/SiO2 catalyst was prepared by the wet impregnation method using commercial silica gel (SiO2) as support (99.2 %, Qingdao Haiyang Chemical Co., China; Specific

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surface area 455 m2·g-1, pore volume 1.06 cm3·g-1, average diameter of pore 9.6 nm). The metal loading of Ni was 10 wt %. Before the impregnation of SiO2 with Ni precursors, the SiO2 was

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pre-calcined at 673 K for 2 h in the atmosphere of air. Then the aqueous solution (3.2 ml) of nickel nitrate (Ni(NO3)26H2O, 1.65 g) was impregnated onto the pre-calcined SiO2 support (3 g) by incipient-wetness impregnation method followed by drying in air at 393 K for 12 h. After that, the sample was calcined at 673 K for 2 h in the atmosphere of air. The obtained sample was denoted as Ni/SiO2. 4

The Ni@HZSM-5 catalyst with embedment structure was prepared by a hydrothermal method using conventional Ni/SiO2 catalyst as the only source of silicon in the crystallization of HZSM-5. The tetrapropylammonium bromide (TPABr) was applied as the structure-directing agent. The molar ratio of the reactants in the precursor solution was SiO2: Al(NO3)3·9H2O: TPABr: NH3·H2O: H2O = 1: 0.025: 0.1: 1.5: 24. Firstly, Al(NO3)3·9H2O was dissolved by H2O in a teflon-lined stainless steel autoclave, followed by the addition of precursor catalyst power under

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stirring. Then TPABr and NH3·H2O were added dropwise. The reaction mixture was further vigorously stirred at room temperature for 6 h, and the initial pH value is 8.7. Finally, the autoclave was sealed and hydrothermally crystallized at 453 K for 72 h. After cooling the

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autoclave naturally, the product was separated by filtration and washed with 100 ml deionizer

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water and 100 ml ethanol for 3~5 times to neutral state. The as-synthesized sample was dried at 393 K for 12 h, and the final calcinations process was performed in air from room temperature to

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823 K with a heating rate of 2 K·min-1 and maintained at 823 K for 6 h to remove the organic

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template. This method allowed us to successfully embed the nickel oxide particles into the zeolite crystal.

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For comparison, the conventional 10 wt. % Ni/HZSM-5 was also prepared by impregnation method using homemade HZSM-5. Firstly, the pure HZSM-5 was synthesized via a traditional

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hydrothermal method. The template, Al source, NH3·H2O, molar ratio of all reactants, and synthesis procedure were same as that in the preparation of Ni@HZSM-5 catalyst, except that the Si source was pure SiO2. The as-synthesized sample was then dried at 393 K for 12 h and calcined at 823 K for 6 h in air. Secondly, the obtained pure HZSM-5 support (3 g) was impregnated with excessive Ni(NO3)26H2O aqueous solution (20 ml, 0.28 M). Thereafter, the suspension was left to 5

rest 6 h at ambient temperature before it was continuously stirring and heating at 333 K to remove the residual water. The resulting sample was then dried at 393 K for 12 h and calcined at 673 K in air for 2 h. All obtained samples were pressed into pellets (10 MPa), crushed and sieved to retain 20-40 mesh particles for reaction tests. The molar ratios of Si/Al for Ni@HZSM-5 and Ni/HZSM-5 as determined by XRF was 37.4 and 39.1, respectively, only slightly lower compared with the synthetic solution with a nominal

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Si/Al ratio of 40. 2.2. Catalytic activity test

The catalysts were tested in a fixed-bed tubular quartz microreactor under atmospheric

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pressure. Prior to reaction, 0.3 g catalysts mixed with 0.6 g quartz sand were reduced in situ by

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pure H2 for 3 h at 673 K and atmospheric pressure, followed by cooling down to 353 K in N2. Then, the feed gas (CO2:H2:N2=23:72:5) was introduced to start the reaction at 673 K, 0.1 MPa,

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and GHSV= 3600 h-1. The product effluent gas was analyzed by online gas chromatography

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(GC-2014C, Shimadzu) equipped with a TCD detector and Active Carbon column. The reaction was continuously carried out for 40 h. For all catalysts, only CH4 and CO were observed as

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C-based reaction products and no other hydrocarbons were detected. The reaction conversion was calculated by the relative proportion of CO2 and N2 in the gas

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after the reaction. CO2 conversion, CH4 and CO selectivity were calculated as follows:

𝑋𝐶𝑂2 (%) = (1 −

(𝐶𝑂2⁄𝑁2 )𝑜𝑢𝑡𝑙𝑒𝑡 ) × 100 (𝐶𝑂2⁄𝑁2 )𝑖𝑛𝑙𝑒𝑡

𝑆𝐶𝐻4 (%) =

𝐶𝐻4 × 100 𝐶𝐻4 + 𝐶𝑂

𝑆𝐶𝑂 (%) =

𝐶𝑂 × 100 𝐶𝐻4 + 𝐶𝑂 6

where Xi and Si are conversion and product selectivity, respectively. In this work, the catalytic results of all catalysts have been confirmed by double repeated tests. The conversions of all samples were in the deviation range of ±5 % and the selectivity were in the deviation range of ±1 %. 2.3. Catalyst characterization Powder X-ray diffraction (XRD) measurements of the fresh (i.e. pre-reduced and passivated

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one) and used catalysts were determined on an X-ray diffractometer (SHIMADZU LabX

XRD-6000) with Cu Kα radiation (λ=0.154 nm) at 40 kV and 30 mA. The diffraction patterns were recorded in the range 5° < 2θ < 80°. The scan speed is 5°/min. The scan mode is continuous

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scan and the sampling pitch is 0.02°. X-ray fluorescence (XRF) measurements were conducted on

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a spectrometer (XRF-1800, Shimadzu) to determine the actual Ni metal loading of all fresh and

The

hydrogen

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used catalysts. temperature

programmed

reduction

(H2-TPR)

measurements

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(FINESORB-3010) were carried out in a quartz tube reactor using 0.1 g as-prepared catalysts. The reducing gas, a mixture of 10 % H2 diluted by Ar, was fed via a mass flow controller at 50 mL/min.

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The temperature was raised from 298 to 1073 K at a heating rate of 10 K/min. The specific surface areas of catalysts were determined by N2 adsorption-desorption measurements (Micrometrics

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ASAP 2020) at 77 K by employing the Brunauer-Emmet-Teller (BET) method. The sample was degassed at 453 K for 6 h prior to the measurement. The pore properties were analyzed by the HK or BJH methods. CO2 temperature programmed desorption (CO2-TPD) was carried out on Quantachrome automated gas sorption analyzer. The pre-reduced sample was heated up to 673 K under N2 flow, 7

re-reduced under H2 flow (30 mL/min) at 673 K for 1 h, cooled to room temperature under N2 flow and exposed to CO2 stream (30 mL/min) at room temperature for 30 min. The physisorption of CO2 was removed by N2 purging at room temperature for 2 h. Then, the temperature was increased with heating rate of 5 K/min under N2 flow (30 mL/min) from 303 K to 773 K and the desorbed CO2 was monitored by TCD. The size and morphology of the obtained catalysts were characterized by scanning electron

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microscope (SEM-EDS, Zeiss Supra55) and high-resolution transmission electron microscope

(HRTEM, JEOL, JEM-2100). The SEM sample was evenly applied to the conductive paste. EDS

data of all fresh and used Ni catalysts were acquired by using Oxford MAX-50 EDS detector. The

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specimen of HRTEM was prepared by ultrasonically suspending the sample in ethanol. A drop of

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the suspension was dropped into copper grids and dried in air.

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Thermogravimetric (TG) analysis was conducted on a differential thermal analyzer (DTG-60, Shimadzu) and the experimental results were recorded and analyzed on a TA-60WS thermal

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analysis workstation. The sample of 5-10 mg was placed in a crucible. The temperature was increased from 298 K to 1073 K with a heating rate of 10 K/min, and the air flow rate was set to

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50 mL/min.

The in situ X-ray photoelectron spectroscopy (XPS) analysis was performed using a

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SHIMADZU AXIS Supra X-ray photoelectron spectrometer with an Al Kα X-ray resource. The peaks were fitted by Gaussian-Lorentzian curves after Shirley or Tougaard background subtraction. All of the binding energies were calibrated by the C1s peak at 284.8 eV. Prior to experiment, the pre-reduced samples were re-reduced in situ in a H2 stream at 673 K for 1 h in the catalytic chamber. After reduction, the sample was cooled down to room temperature under an N2 8

atmosphere and transferred to the analysis chamber under vacuum conditions for measurements. The in situ diffuse reflectance infrared Fourier transform spectra (in situ CO-DRIFTS) experiments were carried out on a NICOLET 6700 spectrometer. The pre-reduced sample (15 mg) was placed in an infrared cell with a ZnSe window and reduced in situ for 1 h in a H2 gas flow at 673 K and atmospheric pressure. Subsequently, the system was cooled to RT. After introduction of carbon monoxide at RT for 0.5 h, the catalysts were flushed by a N2 stream for 1 h. All spectra

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were collected with a resolution of 4 cm−1 and accumulation of 32 scans. 3. RESULTS AND DISCUSSION 3.1 Reaction performance

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The catalytic test results of all prepared catalysts are shown in Fig. 1 and Table 1. The

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Ni@HZSM-5 catalyst exhibits good CO2 conversion (66.2 %) and excellent stability during CO2

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methanation reaction with the highest methane selectivity (99.8 %). In contrast, the conventional Ni/HZSM-5 and Ni/SiO2 rapidly deactivated after 5 h reaction, especially for the Ni/SiO2 catalyst,

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which lost its 67 % CO2 conversion after 10 h reaction. Furthermore, higher CO selectivity (3.3 % and 5.2 %) was obtained on the conventional Ni/SiO2 and Ni/HZSM-5 catalysts. It is believed that

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more CO formation may lead to preferential CO disproportionation during the reaction, forming more carbon deposition. Meanwhile, higher CO concentration inside the catalyst will accelerate

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the formation of nickel-carbonyl species on the active Ni particles, causing Ni active sites to be ineffective or lost, as gas phase carbonyl species can desorb from the surface of nickel particles and diffuse out of the catalyst [30, 34]. Based on above results, the Ni@HZSM-5 catalyst not only guarantees relatively high CO2 conversion in methanation reaction, but also greatly enhances the stability of the catalyst. As can 9

be seen from the reaction results, the possible reasons are that the Ni@HZSM-5 catalyst generates less CO or different adsorbed CO species during the reaction. And the probability of the sintering and loss of the nickel active phases is greatly reduced due to the specific embedded structure of Ni@HZSM-5. 3.2 Characterizations 3.2.1 Physical adsorption

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The BET specific surface area, pore volume and average pore diameter of pure HZSM-5 and as-prepared catalysts are shown in Table 1. The specific surface area of HZSM-5 is 301 m2/g,

while the specific surface area of Ni/HZSM-5 is decreased to 293 m2/g. The decrease in the BET

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specific surface area of Ni/HZSM-5 is mainly due to the blocking of the zeolite micropores by

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active nickel crystallites. Meanwhile, the Ni@HZSM-5 catalyst has a specific surface area of 313

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m2/g, which is higher than that of the Ni/HZSM-5 catalyst. It is believed that, inside the Ni@HZSM-5 catalyst, the HZSM-5 was newly formed to encapsulate supported nickel particles

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from original Ni/SiO2 catalyst, which avoided blocking micropores of newly formed zeolite and resulted in higher specific surface area.

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3.2.2 SEM and TEM images

The morphology of the Ni@HZSM-5 catalyst was also determined by SEM. As shown in Fig.

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2, it can be found that the classical crystalline of ZSM-5 was successfully synthesized by hydrothermal method using Ni/SiO2 catalyst as unique silicon source, indicating relatively good uniformity and high crystallinity [36]. The TEM images of fresh Ni@HZSM-5 and Ni/HZSM-5 catalysts are shown in Fig. 3 and Fig. 4a, c. It can be clearly seen that the supported nickel in fresh Ni@HZSM-5 catalyst is clustered inside zeolite crystal as 30-60 nm clusters (Fig. 3a and Fig. 4a), 10

which is very similar to fresh Ni/SiO2 catalyst (Fig. 4e). Because we used the Ni/SiO2 catalyst as unique silicon source to synthesize the zeolite, the nickel cluster was formed on the silica support prior to formation of zeolite frameworks. Therefore, it is believed that the newly formed zeolite frameworks should grow surrounding the nickel particles during the hydrothermal synthesis. In contrast, as shown in Fig. 3b and Fig. 4c for Ni/HZSM-5 catalyst, the supported nickel particles are anchored on the surface of the HZSM-5, and the nickel particles are present separately rather

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than clustered groups in the range of 5-17 nm. The average size of nickel particle in fresh Ni@HZSM-5 catalyst is 7.6 nm (Table 2), which is very similar to fresh Ni/SiO2 catalyst (7.3 nm, Fig. 4e and Table 2), calculated from the statistical TEM data over 50 particles. As illustrated, for

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Ni@HZSM-5 and Ni/SiO2 catalyst, both morphology of clusters and particle size of supported

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nickel are very close to each other, proving that the supported nickel particles (or clusters) were

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embedded into newly-formed zeolite crystalline as designed. These findings indicate that the size and distribution of Ni clusters in the Ni/SiO2 precursor tend to remain in the newly formed zeolite

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crystals after the hydrothermal synthesis process for the Ni@HZSM-5. For used catalysts, the Ni@HZSM-5 catalyst maintained the nickel cluster structure and

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density of nickel cluster (Fig. 4b), as well as slightly larger nickel particles than fresh one, as shown in Table 2. This should be the reason of slight deactivation during initial stage of CO2

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methanation over Ni@HZSM-5 catalyst. However, both Ni/HZSM-5 and Ni/SiO2 catalysts show obviously enlarged nickel particle size and carbon deposit, as well as sparse nickel distribution. which appears as much far separated nickel particle or clusters and lower density of nickel cluster for Ni/SiO2 catalyst. It is suggested that the significant sintering and loss of the supported nickel cause the deactivation of these catalysts and the enlarged nickel particles promote the CO 11

selectivity [28], as compared in Fig. 1 and Table 1. Therefore, the special structure of the Ni@HZSM-5 catalyst, in where the active nickel particles were encapsulated by zeolite framework, would contribute to the stability during CO2 methanation (Fig. 1). 3.2.3 XRD patterns Fig. 5 shows the XRD patterns of various catalysts before and after CO2 methanation reaction at 673 K. The Ni@HZSM-5 and Ni/HZSM-5 catalysts, including fresh and used samples, show

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distinct peaks of HZSM-5 and metallic nickel species. We can clearly see that the used Ni@HZSM-5 catalyst shows relatively intensive metallic nickel peaks than fresh Ni@HZSM-5

catalyst (Fig. 5), suggesting that the size of nickel particles were slightly increased after CO2

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methanation. The result is well agreed with TEM data in Table 2. And this should be the reason of

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slight deactivation during initial stage of CO2 methanation over Ni@HZSM-5 catalyst.

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After 10 h methanation reaction, the intensity of metallic nickel peak dramatically decreased for the used Ni/HZSM-5 and Ni/SiO2 catalysts. The intensity/half peak width of XRD peaks is not

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only dependent on the particle size, but also the content of crystal phase. Although TEM statistical data in Table 2 show that the Ni particle size were increased after reaction for both Ni/HZSM-5

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and Ni/SiO2 catalyst, the serious loss of Ni during reaction leads to the much lower Ni content in used catalysts. Hence, we cannot observe the sintering of used Ni/SiO2 or Ni/HZSM-5 catalysts

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from XRD patterns. As shown in XRF and EDS results (Table 2 and Fig. S1), it can be clearly seen that the Ni mass fraction of Ni/HZSM-5 and Ni/SiO2 catalysts remarkably decreased after reaction, indicating the loss of active metallic nickel. In contrast, the used Ni@HZSM-5 catalyst shows relatively intensive metallic nickel peaks, suggesting most of the supported nickel species were maintained after CO2 methanation. Therefore, it exhibited excellent stability during 40 h 12

reaction. 3.2.4 Nickel content In order to analyze the nickel content, all fresh and used catalysts were characterized by XRF and EDS. X-ray fluorescence analysis is a technique widely used in the determination of major and trace elements in silicon-containing materials [37]. As shown in Table 2 and Fig. S1, it can be clearly seen that the Ni mass fraction of Ni/HZSM-5 and Ni/SiO2 catalysts remarkably decreased

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after reaction, indicating that the deactivation of these two catalysts is closely related to the loss of

active metallic nickel. This conclusion is in good agreement with TEM and XRD results. It is worth noting that the Ni@HZSM-5 catalyst has no obvious decrease of nickel content after 40 h

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CO2 methanation, which should be the reason for its good stability in CO2 methanation reaction.

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3.2.5 CO-DRIFTS

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The dissociation of CO2 into the adsorbed CO as a crucial intermediate for CH4 synthesis has been found on the supported nickel catalysts by many researchers [8, 9, 38]. The formed carbon

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monoxide during the CO2 methanation would lead to the formation of Ni(CO)x, and their high mobility leads to the loss of the metallic nickel and sintering of nickel particles, resulting in poor

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long-term stability of the catalyst [30, 34, 39]. In order to further investigate the formation of Ni(CO)x on the different catalysts, in situ CO-DRIFT were performed on three catalysts, and the

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results are shown in Fig. 6.

As reported, the adsorbed CO bands on nickel-based catalyst are in the range of 2100-1750

cm−1 [10, 25, 40], in which the bridge-bound CO (1950-1750 cm−1) is associated to CO adsorption on the close-packed Ni sites, whereas linear-bound CO (2100-1950 cm−1) refers to CO connected on Ni defect sites [41]. As shown in Fig. 6, for Ni/SiO2 catalyst, the CO bands at 2060 cm−1 and 13

2031 cm−1 can be attributed to Ni(CO)x, which is linearly/terminally adsorbed CO atop a single nickel atom and generally accepted as main reason of the loss of CO hydrogenation activity over Ni-based catalysts [10, 30, 40, 42]. In addition, the strong and broad adsorbed CO band located at 1936 cm−1 should be attributed to the bridge-band CO on the close-packed Ni sites [40-43]. And the CO band at 1853 cm−1 can be attributed to the three-fold carbonyl species, i.e. CO species bound to three neighbouring Ni atoms

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[10, 40, 42], which are advantageous to the hydrogenation of CO and contributed to the highest

initial CO conversion of Ni/SiO2 catalyst. It is considered that the large pore size of Ni/SiO2

catalyst provides good diffusion rate of the formed gaseous Ni(CO)x than micropores of zeolite. As

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a result, the accelerating formation of gaseous Ni(CO)x leads to heavy sintering of nickel particles

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stability during CO2 methanation.

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and loss of the supported nickel, as proved by XRD, TEM, XRF and EDS, resulting the worst

However, for Ni/HZSM-5 and Ni@HZSM-5 catalysts, the spectra are quite different to that

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of Ni/SiO2 catalyst. Firstly, the intensity of adsorbed CO bands significantly decreased especially for Ni@HZSM-5 catalyst, suggested relatively lower CO adsorption ability of these two catalysts.

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It is believed that this would contribute to reducing the formation of gaseous Ni(CO)x and the disproportionation of CO during CO2 methanation. Secondly, all of the CO bands shift to lower

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wavenumbers than that of Ni/SiO2 catalyst. The bands of Ni(CO)x species shift to 2044 cm-1 and 2005 cm-1, and the bridge-bands of the adsorbed CO shift to 1924 cm-1 and 1850 cm-1, respectively. As the Ni(CO)x species located at lower wavenumber are more stable than that located at higher wavenumber [40], the red shift of Ni(CO)x bands would benefit to reduce the formation of gaseous nickel carbonyls and contributed to preventing the nickel loss and sintering induced by Ni(CO)x. 14

Thus, the Ni/HZSM-5 and Ni@HZSM-5 catalysts show the enhanced stability in Fig. 1. Furthermore, for Ni@HZSM-5 catalyst, the intensity of Ni(CO)x species is much lower than that of Ni/HZSM-5 catalyst, suggesting lower concentration of Ni(CO)x species on the surface. As reported by Y. Bai et al. [30], there is a critical concentration of Ni(CO)4 on the surface of Ni particles. And at a suitable formation rate of Ni(CO)4, these gaseous molecules neither cause significant growth of the nickel particles nor are easily removed from the catalyst. It is believed

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that the stable and less amount of Ni(CO)x species on the surface of Ni@HZSM-5 catalyst prevent the sintering and loss of the supported nickel, as demonstrated by XRD, TEM, XRF and EDS,

realizing the excellent stability as illustrated in Fig. 1 and Table 2. Meanwhile, comparing to the

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Ni/HZSM-5 catalyst, the much lower intensity of CO band at 2044, 2005 and 1924 cm-1 for

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Ni@HZSM-5 catalyst indicates the different properties of active nickel phase duo to the specific

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structure of Ni@HZSM-5 catalyst, in where the active nickel particles were surrounded by zeolite frameworks.

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3.2.6 In situ XPS

To clarify the nickel active phase, in situ XPS was applied to probe the chemical states of

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surface Ni species in all catalysts. All samples were in situ re-reduced before characterization. As shown in Fig. 7, for Ni/SiO2 catalyst, the binding energy at 871.1 and 853.0 are assigned to Ni02p1/2

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and Ni02p3/2, while the binding energy at 873.2 and 856.0 are Ni2+2p1/2 and Ni2+2p3/2, respectively. And all peaks in green region are satellite peaks of Ni [25, 44, 45]. For Ni/HZSM-5 and Ni@HZSM-5 catalysts, all nickel species shift to the higher BE values than that of Ni/SiO2 catalyst in this study, suggesting that the nickel species on Ni@HZSM-5 catalyst donate more electrons to zeolite support than to silica support [25, 44]. This may result in a lower wavenumber 15

of CO band on these two catalysts, as illustrated in Fig. 6. Furthermore, as compared in Fig. 7, the BE values of Ni@HZSM-5 catalyst is also higher than that of Ni/HZSM-5 catalyst, implying that this embedment structure forms different nickel species compared with Ni/HZSM-5 catalyst, leading to more donation of electrons from nickel species to zeolite support. 3.2.7 H2-TPR To further evaluate the reduction behavior of various catalysts, we performed H2 temperature

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programmed reduction (H2-TPR) examination. The H2-TPR results of various catalysts are

compared in Fig. 8. It can be seen that there is a broad and strong hydrogen consumption peak

from 630 K to 800 K for Ni/SiO2 catalyst, indicating the existence of some hardly reduced nickel

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hydrosilicates on the surface of catalyst [27]. For Ni/HZSM-5 catalyst, the reduction peak is

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similar to that of Ni/SiO2 catalyst, except that it is a little sharp and shift to slightly higher

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temperature, due to the different support and larger nickel particle size. In contrast, the Ni@HZSM-5 catalyst exhibits a sharp peak at much higher temperature (675 K), followed by a

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broad peak locate at 700-850 K. The different H2-TPR curves between Ni@HZSM-5 and other two catalysts proved the existence of many hardly reduced nickel species in Ni@HZSM-5. It is

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considered that the nickel species connecting with zeolite frameworks in the Ni@HZSM-5 catalyst should be hard reduced and can donate more electrons to zeolite, resulting in higher BE

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values, lower IR wavenumber and lower intensity of the adsorbed CO bands. Those are very helpful to avoid forming gaseous Ni(CO)x and contribute to the excellent stability during CO2 methanation. 3.2.8 CO2-TPD The basicity of different samples was measured by CO2-TPD. In Fig. 9, CO2-TPD profiles 16

show that three samples contain different CO2 desorption peaks with very weak intensity. According to literatures, CO2 might be desorbed from three different basic sites: weak, which is provided by surface OH groups; medium, which is assigned to metal-oxygen pairs; and strong, which is associated with strongly basic adsorbed O2- species [25, 40, 46]. In contrast, there is only one CO2 desorption peak for three catalysts, which is 430 K for Ni/SiO2, 550 K for Ni/HZSM-5, and 540 K for Ni@HZSM-5. The above peaks are all in the temperature range of weak and

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medium adsorption sites, because these catalysts do not contain any basic promoter besides silica or zeolite. As previously reported [40, 47], this kind of CO2 desorption peak should associate with

the desorption of various form of carbonates or CO2 linear complexes, which have weak or

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moderate thermal stability and adsorbed namely on O-terminated Ni metallic sites or unreduced

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NiO particles, as well as extra-framework Al (EFAL) of zeolite. Thus, the different properties of

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the supported nickel, as confirmed by CO-DRIFTS, XPS and H2-TPR, induced different behavior of CO2 desorption. It is believed that CO2 is linearly adsorbed via electrostatic interactions onto

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Ni0 species of Ni/SiO2 catalyst [40], which desorbed at lower temperature. In contrast, Ni/HZSM-5 and Ni@HZSM-5 catalysts formed more NiO species on the catalysts due to their

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low reducibility, as compared in H2-TPR. Then more CO2 should be adsorbed on the NiO species because metal-oxygen pairs can provide medium basic sits [25, 40], resulting in higher CO2

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desorption temperature. Meanwhile, the different chemical state of metallic nickel and nickel ions on two catalysts (Ni/HZSM-5 and Ni@HZSM-5), as proved by in situ XPS, could also influence the desorption of adsorbed CO2, leading to slightly higher desorption temperature of Ni/HZSM-5 catalyst. Moreover, the specific structure of Ni@HZSM-5 catalyst, in where the active nickel particles were surrounded by zeolite frameworks, would result in different Al species to that of 17

Ni/HZSM-5 catalyst. Wang et al. [48] reported that the AlF siting and distribution are highly affected by the properties of initial silicon source, and the formation of different Al species has a significant effect on the catalytic environment as well as performance. In our previous work [49], we also found that the Al species in Co@HZSM-5 Fischer-Tropsch synthesis catalyst are different from Co/HZSM-5 catalyst due to this special synthesis method and embedment structure. Hence, we speculated that the presence of Ni on the SiO2 source will affect the distribution of Al (in the

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straight and sinusoidal channels, or in the channel intersections) during the synthesis of

Ni@HZSM-5 and the local chemical environment around Al atoms in Ni@HZSM-5 would be

different from the Ni/HZSM-5. And this would lead to the different adsorbed CO2 on Al species,

-p

which would be desorbed at different temperature. In a word, the medium CO2 desorption

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temperature and relatively high intensity of CO2 desorption peak, comparing to that of Ni/SiO2,

listed in Table 1 and Table 2.

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3.2.9 TGA of used catalysts

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contribute to the good enough CO2 conversion and the highest TOF of Ni@HZSM-5 catalyst, as

As the coke formation remarkably influence the stability of Ni catalysts in CO2 methanation,

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the TGA was used to characterize the used catalysts. As shown in Fig. 10, the weight loss of all samples locates between 600 K and 900 K, which should be associated with coke combustion.

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And only 5 % weight loss is found for the Ni@HZSM-5 catalyst, which is much lower than those of Ni/HZSM-5 and Ni/SiO2 catalyst. It is considered that the optimum electrons density of nickel active phase, as proved by XPS results, enhances the resistance of coke formation for Ni@HZSM-5 catalyst. In addition, the less Ni(CO)x formation successfully prohibit the excessive sintering of nickel particles, contributing to decreasing the coke formation on Ni@HZSM-5 18

catalyst [28]. Due to a synergic effect, Ni@HZSM-5 catalyst simultaneously realized good activity and stability during CO2 methanation reaction. 3.3 Discussion For CO2 methanation on the supported nickel catalysts, CO2 firstly dissociate to the adsorbed CO and then CO is hydrogenated to CH4 [38, 46]. As a crucial intermediate for CH4 synthesis in CO2 methanation, the properties of the adsorbed CO are very helpful to clearly investigate the

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activity and stability of different catalysts. Comparing the in situ CO-DRIFT spectra of three catalysts as illustrated in Fig. 6, the Ni/SiO2 catalyst shows the strongest bridged adsorbed CO

bands (1936 cm-1, 1853 cm-1), which is higher active for CO hydrogenation. Meanwhile, besides

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the property of CO adsorption, the highest dispersion of supported nickel and good reducibility

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realized the highest initial CO2 conversion of the Ni/SiO2 catalyst (Fig. 1). But its TOF is the lowest as listed in Table 2, because of the weak CO2 adsorption as illustrated in Fig. 9. However,

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the large pore size and higher wavenumber Ni(CO)x species (2060 cm-1 and 2031 cm-1 in Fig. 6),

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which is advantageous to forming gaseous Ni(CO)x, accelerate the sintering and loss of the supported nickel, as confirmed by the results of TEM, XRD, XRF and EDS. Meanwhile, the

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sintered nickel particle leads to more carbon deposition as illustrated by TGA [26]. Therefore, the Ni/SiO2 catalyst exhibits the worst stability during CO2 methanation as shown in Fig. 1.

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As reported by Y. Bai et al. [30], there is a critical concentration of Ni(CO)4 on the surface of

Ni particles. And at a suitable formation rate of Ni(CO)4, these gaseous molecules neither cause significant growth of the nickel particles nor are easily removed from the catalyst. It is believed that the stable and less amount of Ni(CO)x species on the surface of Ni@HZSM-5 catalyst prevent the sintering and loss of the supported nickel, as demonstrated by XRD, TEM, XRF and EDS, 19

realizing the excellent stability as illustrated in Fig. 1 and Table 2. Meanwhile, comparing to the Ni/HZSM-5 catalyst, the much lower intensity of CO band at 2044, 2005 and 1924 cm-1 for Ni@HZSM-5 catalyst indicates the different properties of active nickel phase duo to the specific structure of Ni@HZSM-5 catalyst, in where the active nickel particles were surrounded by zeolite frameworks. The much lower intensity of CO band at 2044, 2005 and 1924 cm-1 (Fig. 6) and the highest BE values of the active nickel phase (Fig. 7) for Ni@HZSM-5 catalyst suggest that the

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Ni@HZSM-5 catalyst is disadvantageous to forming gaseous Ni(CO)x species during CO2

methanation. In addition, the lowest CO selectivity of Ni@HZSM-5 also indicate the less formation of Ni(CO)x species (Table 1) [30]. Moreover, CO2-TPD result in Fig. 9 illustrate

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different behavior of CO2 desorption between Ni@HZSM-5 and Ni/HZSM-5 catalysts, and also

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prove the different properties of metallic nickel or nickel oxide on the Ni@HZSM-5 catalyst.

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Therefore, due to the specific embedment structure, the Ni@HZSM-5 catalyst maintained the initial nickel content, reduced the sintering of nickel particles and prevented carbon deposition, as

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confirmed by various characterization results. This would contribute to the best stability in this work. Generally, ZSM-5 is a considerably versatile support material which can disperse nickel

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particles well to achieve a promising performance in catalytic reactions [22]. Li and Peng et al. investigated nickel-based catalysts with various supports (ZSM-5, SBA-15, MCM-41, Al2O3, and

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SiO2) for CO2 methanation at 473-723 K, H2/CO2= 4:1, and GHSV = 2400 h-1, and they found that Ni/ZSM-5 catalyst displayed the most active catalytic properties with excellent stability and no deactivation up to 100 h [22]. In contrast, the Ni/HZSM-5 catalyst in our present work rapidly deactivated after 5 h reaction. That's probably because our catalyst was performed in CO2 methanation at higher GHSV (3600 h-1) and lower H2/CO2 ratio (H2/CO2= 3:1). Carbon deposition 20

on the catalyst can be avoided by increasing the ratio of H2/CO2 because hydrogen react with the dissociated carbon and prevent the formation of stable carbon [31]. In other words, the lower H2/CO2 ratio will lead to the poor hydrogen content in reactants and decreased the degree of hydrogenation, which will accelerate the formation of carbon and decrease the CO2 conversion and CH4 selectivity [23, 50]. In order to clearly finding out the difference of all catalysts,

we used H2/CO2 ratio of 3 and higher GHSV to fasten the deactivation of catalysts

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during CO2 methanation. Hence, the catalytic results in our work have further demonstrated

that the Ni@HZSM-5 catalyst is an efficient catalyst for CO2 methanation, which exhibited enhanced stability even at lower H2/CO2 ratio.

-p

Meanwhile, besides the excellent stability, the Ni@HZSM-5 catalyst without any promoters

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exhibited relatively strong CO2 desorption peak at medium temperature (Fig. 9) and the highest

lP

TOF (Table 2), contributing to the good enough CO2 conversion and CH4 selectivity as compared to the data from references (Table 1).

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4. CONCLUSIONS

The Ni@HZSM-5 catalyst with specific embedment structure was successfully synthesized

ur

by a simple hydrothermal synthesis using conventional Ni/SiO2 as unique silicon source, which simultaneously realized higher CO2 conversion (66.2 %) and methane selectivity (99.8 %) at 673

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K. As the nickel particles were surrounded by the zeolite frameworks on this catalyst, the nickel active phases donated more electrons to zeolite, resulting in higher BE values, lower wavenumber and weak intensity of CO bands, those are disadvantageous to forming gaseous Ni(CO)x on the nickel active sites. Therefore, the lower formation ability of gaseous Ni(CO)x did not cause significant growth of the nickel particles or loss of nickel active phase, guaranteeing the best 21

stability during CO2 methanation in this study. Besides the excellent stability, the Ni@HZSM-5 catalyst exhibited good activity and selectivity without adding any promoters. Therefore, it is believed that this simple and effective preparation method is useful for synthesizing embedment structured catalyst to realize excellent reaction performance.

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Author Contribution Statement

Yiming Chen: Investigation, Data curation, Writing-Original draft preparation. Baocheng Qiu: Investigation, Data curation.

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Yi Liu: Writing- Reviewing and Editing.

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Yi Zhang: Supervision, Writing- Reviewing and Editing.

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Declaration of interests

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☒ The authors declare that they have no known competing financial interests or personal

Notes

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relationships that could have appeared to influence the work reported in this paper.

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The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of P. R. China (91334206, 21606011), and China Postdoctoral Science Foundation (2016M591051 and 2017T100029).

22

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24

Figures and Tables

100

10Ni@HZSM-5 10Ni/HZSM-5 10Ni/SiO2

90

70 60 50

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Converision (%)

80

40

20

5

10

-p

30 15 20 25 30 Time on stream (h)

35

40

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Figure 1. CO2 conversion with time on stream over various Ni catalysts. Reaction conditions: 673

lP

K; CO2:H2:N2=23:72:5; GHSV= 3600 h-1; 0.1 MPa. The conversions of all catalysts were in the

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na

deviation range of ±5 %, bases on the double repeated experiment for each sample.

25

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na

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Figure 2. SEM image of the as-synthesized Ni@HZSM-5 catalyst

26

a)

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30-60 nm clusters

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-p

b)

na

lP

5-17 nm nanoparticles

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Figure 3. TEM images of (a) fresh Ni@HZSM-5 and (b) fresh Ni/HZSM-5 catalysts. Fresh means

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pre-reduced and passivated sample.

27

(c)

(d)

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(b)

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re

-p

(a)

(f)

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na

(e)

Figure 4. TEM images of all catalysts: (a) fresh Ni@HZSM-5, (b) used Ni@HZSM-5, (c) fresh Ni/HZSM-5, (d) used Ni/HZSM-5, (e) fresh Ni/SiO2 and (f) used Ni/SiO2.

28

  (a) 

☆



☆

Intensity (a.u.)



◆ : ZSM-5 ☆ : Ni Ni@HZSM-5

☆

Ni/HZSM-5 Ni/SiO2 Ni - PDF#04-0850

ro of

NiO - PDF#44-1159 ZSM-5 - PDF#44-0003 20

 (b)

30

☆



60

70

80

◆ : ZSM-5 ☆ : Ni ☆ Ni@HZSM-5 ☆

re

 

40 50 2Theta (degree)

-p

10

Intensity (a.u.)

Ni/HZSM-5

lP

Ni/SiO2

20

30

ur

10

na

Ni - PDF#04-0850

NiO - PDF#44-1159

ZSM-5 - PDF#44-0003

40 50 2Theta (degree)

60

70

80

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Figure 5. XRD patterns of Ni@HZSM-5, Ni/HZSM-5 and Ni/SiO2 catalysts. (a) fresh

catalysts, (b) used catalysts.

29

Ni@HZSM-5 Ni/HZSM-5 Ni/SiO2

1936

0.01

2031

1853

1850

2044 2005

2050

2000 1950 1900 1850 Wavenumbers (cm-1)

1800

-p

2100

1924

ro of

Intensity

2060

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na

lP

re

Figure 6. In situ CO-DRIFT spectra of Ni@HZSM-5, Ni/SiO2 and Ni/HZSM-5 catalysts.

30

2p 1/2

Ni@HZSM-5

2p 3/2 856.2

873.9

880.2

861.7 872.0

853.5

Ni/HZSM-5

ro of

856.1 873.8

879.8

861.2 871.3

853.2

879.5

870

na

880

873.2 871.1

lP

Ni/SiO2

re

-p

Intensity (a.u)

Ni0 Ni2+ Satellite peak

860.5

860

856.0 853.0

850

Binding Energy (eV)

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Figure 7. In situ XPS spectra of Ni@HZSM-5, Ni/HZSM-5 and Ni/SiO2 catalysts.

31

Intensity (a.u.)

Ni@HZSM-5

Ni/HZSM-5

500

600

700 800 900 Temperature (K)

1000

ro of

Ni/SiO2

re

-p

Figure 8. H2-TPR profiles of Ni@HZSM-5, Ni/HZSM-5 and Ni/SiO2 catalysts.

lP

Intensity (a.u.)

Ni@HZSM-5

na

Ni/HZSM-5

ur 400

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300

Ni@SiO2

500 600 Temperature (K)

700

Figure 9. CO2-TPD profiles of Ni@HZSM-5, Ni/HZSM-5 and Ni/SiO2 catalysts.

32

TG

14 12

weight loss (%)

95 90

10 Ni/SiO2 Ni/HZSM-5 Ni@HZSM-5

8 6

85 4

Exothermicity (μV mg-1)

100

2

80 DTA

ro of

0

75 400

600 800 Temperature (K)

1000

-p

Figure 10. TGA curves of the used Ni/SiO2, Ni@HZSM-5 and Ni/HZSM-5 catalysts under

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air flow.

33

Table 1. Reaction results and structural parameters of various Ni catalysts. Pore size c (nm)

N.A.

N.A.

301

0.122

0.39

74.3

96.7

3.3

455

1.060

9.6

Ni/HZSM-5

68.4

94.8

5.2

293

0.125

0.42

Ni@HZSM-5

66.2

99.8

0.2

313

0.131

0.43

Ni/USY d

72.6

95

5

N.A.

N.A.

N.A.

77.5

98

2

N.A

N.A

N.A

97

3

N.A

N.A

N.A

99

N.A.

299.8

0.231

3.082

CO2 Conv.Sel. (%) (%) CH4

HZSM-5

N.A.

Ni/SiO2

Ni/ZrO2-P

e

NiO-ZrO2-CNT f 55 Ni/HZSM-5

g

76

ro of

CO

Surface area a Pore volume b (m2/g) (cm3/g)

Catalyst

Reaction conditions: 673 K; CO2:H2:N2=23:72:5; GHSV= 3600 h-1; 0.1 MPa. The data were taken at 1 h of time on stream. a Calculated

by the BET method.

desorption pore volume for zeolite and BJH desorption pore volume for SiO2.

c HK

desorption average pore diameter for zeolite and BJH desorption average pore diameter

-p

b HK

for SiO2.

from ref. [40]: Ni loading is 14 wt %, CO2:H2= 1:4 and T=723 K.

e Data

from ref. [25]: Ni loading is 10 wt %, CO2:H2:N2=8:32:10 and T=673 K.

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d Data

Data from ref. [51]: Ni loading is 5 wt %, CO2:H2:He=1:5:94 and T=673 K.

g

Data from ref. [22]: Ni loading is 10 wt %, CO2:H2= 1:4 and T=673 K.

lP

f

Ni (wt.%) a

Ni (wt.%) b

Ni particle size (nm) c

Fresh

Used

Fresh

Used

Fresh

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Table 2. The characterization results of the supported nickel for various catalysts. Catalyst

Used

Ni dispersion (%) d

TOF (10-2 s-1) e

10.59

10.50

12.87

11.94

7.6

9.2

15.1

11.1

Ni/HZSM-5 g

10.29

8.97

12.26

8.03

10.2

14.3

11.7

10.5

9.68

6.53

12.75

6.86

7.3

15.1

15.8

9.2

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Ni@HZSM-5 f

Ni/SiO2 g a

XRF results;

b

EDS results; c Calculated from TEM images;

d

Calculated based on the

nickel particle size of fresh catalysts; e Calculated from H2 uptake with ratio of Hadsorbed/Nisurface=1 and initial CO2 conversion; f 40 h methanation reaction; g 10 h methanation reaction.

34