Ceria imparts superior low temperature activity to nickel catalysts for CO2 methanation

Xinpeng Guoab, Hongyan Heb, Atsadang Traitangwongb, Maoming Gongb, Vissanu Meeyoob, Ping Lid, Chunshan Li*bc, Zhijian Peng*a and Suojiang Zhangb
aSchool of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China. E-mail: pengzhijian@cugb.edu.cn
bCAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex Systems, The National Key Laboratory of Clean and Efficient Coking Technology, Beijing Key Laboratory of Ionic Liquids Clean Process, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China. E-mail: csli@ipe.ac.cn; Fax: +86 10 82544800; Tel: +86 10 82544800
cZhongke Langfang Institute of Process Engineering, Hebei Province, PR China
dState Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering, School of Chemistry and Chemical Engineering, Ningxia University, Yinchuan, 750021, PR China

Received 17th June 2019 , Accepted 27th August 2019

First published on 28th August 2019

Carbon dioxide methanation is well known to achieve the catalytic conversion of the CO2 molecule into value-added chemicals. At present, the important challenge is the development of efficient methanation catalysts at low reaction temperature. In this study, Ni–Al layered double hydroxides (LDHs) were used as catalyst precursors and modified by CeO2 for CO2 methanation. By the optimization of CeO2 loading, the reaction activity was improved, especially at low temperature. Among the prepared catalysts, the NiAl-MO/CeO2-5 catalyst exhibited the highest catalytic activity with a CO2 conversion of 91% at 250 °C. The addition of CeO2 was beneficial to the formation of small particles with good dispersion and provided appropriate basic sites and oxygen vacancies, which were conducive to the catalytic performance. DFT calculation and in situ DRIFTS results further verified that the catalyst with an appropriate amount of CeO2 was favorable for CO2 adsorption and conversion. The reaction mechanism results demonstrated that the high amount of formate intermediate species can accelerate the reaction and a certain amount of ceria in the catalyst could provide a suitable metal–support interaction, which was beneficial for CO2 methanation. The LDH as a catalyst precursor integrated with CeO2 shows a promising effect on CO2 methanation at low temperature.

1. Introduction

In recent years, carbon dioxide emissions that result from the burning of large amounts of fossil fuels have led to an ecological and environmental crisis. Several efforts have been devoted to CO2 utilization due to the energy and environment problem. Currently, catalytic hydrogenation of CO2 is receiving significant attention and it provides promising strategies to produce methane or other hydrocarbon compounds.1–3 As an excellent energy carrier, not only can methane be directly used as a clean fuel, but it can also make full use of excess hydrogen generated from H2-rich coke oven gas or renewable energy sources (solar energy, hydropower, biomass).4,5 Therefore, CO2 methanation is a sustainable solution to consume greenhouse gas and produce valuable chemicals. CO2 hydrogenation to methane has been investigated over supported noble metal catalysts (Pd, Ru, and Rh),6–12 which had excellent catalytic performance, especially at relatively mild temperatures. According to the literature, with a H2/CO2 ratio of 4, the rate of reaction was 0.318 × 10−2 molCH4 molRh−1 s−1 for the Rh50–Pd50 catalyst at 200 °C.9 The CO2 conversion could reach almost 100% for the optimized Ru/TiO2 catalyst prepared by a polygonal barrel-sputtering method below 200 °C.12 However, these catalysts are not widely used because of their high cost. Among the potential catalysts of methanation, much attention has been paid to nickel-based catalysts due to their comparable activity and economic attraction.13–16

The major hurdle in the development of the Ni-based catalyst is to enhance the low-temperature performance due to the limits of thermodynamic equilibrium. The performance of methanation catalysts is closely related to the metal dispersion, basic property, and the interaction between active metals and supports, which are mainly affected by the synthesis methods and support properties. To increase the catalytic activity at low temperature, a promising approach is to design a catalyst with well-dispersed active metal and suitable oxide supports by developing new materials with a unique preparation method. Layered double hydroxides (LDHs) as a layered material include divalent and trivalent metal cations which could be effective for the synthesis of highly active metal oxide catalysts with high surface area, uniform dispersion and suitable basic character.14,17,18 The metal catalysts derived from the LDH precursor were widely applied to various catalytic reactions, such as methanol reforming,18 CO2 methanation,19 water-gas shift,20 dimethyl ether synthesis, etc.21

On the other hand, metal oxides such as Al2O3, SiO2, TiO2, and CeO2 are widely used as supports in the literature to promote nickel-based catalysts' activity for methanation.12,22–27 Among these, the reducible oxide CeO2 as the structural and electronic promoter could enhance the catalytic performance by partial reduction of the support, strong metal–support interaction, and the formation of well-dispersed Ni metal.24,28 CeO2 as a basic oxide support is conducive to the absorption and activation of CO2, which can effectively improve the methanation activity.24 In addition, a large number of oxygen defects on the surface of CeO2 can also improve the CO2 adsorption. Research on ceria promoting nickel-based catalysts derived from LDHs for CO2 methanation has not been reported in the literature, especially the role of CeO2 in the reaction. Although several studies have investigated the reaction mechanism of CO2 methanation, the adsorption and activation intermediates, which are affected by the support and active metal component, are still under debate.

Herein, we developed an innovative catalyst by integrating LDHs with CeO2 that showed superior activity and stability at low-temperature CO2 methanation. The effects of ceria loading amount on the methanation activity of the Ni-based catalysts were investigated. The correlations between the catalytic performance and the physicochemical properties which focus on the basic sites, redox property and metal–support interaction were investigated. To further understand the reaction mechanism, the adsorption phases and reaction intermediates were determined by DRIFTS measurements.

2. Experimental section

2.1. Catalyst preparation

All chemicals of analytical grade were purchased from Xilong Co., Ltd. and used without further pretreatment. The procedure for catalyst preparation mainly includes two steps. The first step is the preparation of the LDH precursor by a hydrothermal synthesis method.19 In the next step, ceria was deposited on the LDH precursor through a hydrothermal treatment. Briefly, the aqueous solution containing Ni(NO3)2·6H2O and Al(NO3)3·9H2O was rapidly added to the alkali mixture including NaOH and Na2CO3 with continuous stirring. The obtained suspension was then transferred to a Teflon autoclave, sealed and subjected to a hydrothermal process at 110 °C for 48 h. After filtration, washing, and drying, the LDH precursor was obtained.

0.4 g of LDH precursor was dispersed in a mixed solvent containing 80 ml of ethanol and 2.0 g of PVP with stirring and ultrasound for 1 h at room temperature. Then ammonia (28 wt%) was added to the mixture while stirring constantly. Finally, different molar amounts of Ce(NO3)3·6H2O (0, 2, 5, 10, 30 mmol) aqueous solution were introduced to the above solution, and stirring was maintained for 0.5 h before transferring to a Teflon autoclave. After hydrothermal treatment at 90 °C for 18 h, the product was processed by filtration, washed several times with water and ethanol, and then dried at 60 °C for 12 h and calcined at 550 °C for 4 h in air. The obtained catalysts with different ceria contents were labeled as NiAl-MO/CeO2-x, where x is the molar amount of Ce(NO3)3·6H2O (x = 0, 2, 5, 10, 30).

2.2. Catalyst characterization

X-ray diffraction (XRD) measurements of the Ni-based catalysts were performed using a Rigaku Smart Lab X-ray diffractometer with Cu Kα radiation from 5° to 90°.

The specific surface area and pore properties of the catalysts were measured using a Micromeritics ASAP 2460 instrument. Before analysis, all the catalysts were outgassed under vacuum at 350 °C for 6 h. The pore size distribution of the catalysts was determined by the Barrett–Joyner–Halenda (BJH) method.

Transmission electron microscope (TEM) images were obtained to observe the morphologies and structures of the reduced samples by employing a JEM-2100F system at 200 kV. The catalyst was uniformly dispersed in ethanol to prepare the TEM sample, which was dipped into a holey carbon-supported grid for observation. Ni loadings of the NiAl-MO/CeO2-x catalysts were determined by inductively coupled plasma atomic emission spectrometry (ICP-AES, Shimadzu ICPE-9000).

The H2 and CO2 chemical adsorption properties were measured using an Autochem II 2920 Chemisorption Apparatus with a quartz tube reactor and a TCD detector. Prior to measurement, approximately 50 mg samples were pretreated in a flowing stream of hydrogen at 500 °C for 1 h. After cooling to 50 °C, the adsorption gas was introduced and the samples were heated with a rate of 10 °C min−1. The Ni dispersion of the NiAl-MO/CeO2-x catalysts was measured using the H2 pulse adsorption results. The H2-TPR measurements for the catalysts were performed with the same instruments as for CO2-TPD. 50 mg samples placed in a quartz tube reactor were heated to 900 °C under flowing 10% H2–Ar.

X-ray photoelectron spectroscopy (XPS) of the reduced nickel-based catalyst was performed using an ESCALAB 250Xi spectrometer with Al Kα radiation. The binding energies (BEs) of the spectra were referenced to the C 1s peak of aliphatic C atoms at 284.6 eV.

In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) spectra were recorded on a Bruker Tensor II spectrometer. The catalyst was reduced in situ in a flowing stream of hydrogen at 450 °C for 1 h before the testing. Then, the reduced samples were cooled to the appropriate temperature and exposed to the CO2 or reaction mixture gas. Subsequently, the FTIR spectra corresponding to adsorption and reaction intermediates were obtained.

2.3. Theoretical calculation

First-principles calculations of the CO2 adsorption on NiAl-MO/CeO2-0 and NiAl-MO/CeO2-5 were performed through the projector augmented wave (PAW) method by using the Vienna Ab initio Simulation Package (VASP).29 The exchange-correlation functional with Perdew–Burke–Ernzerhof (PBE) parameterization was implemented in the structure optimization.30 We used the GGA + U (U = 4.5 eV for Ce) method31,32 for the binding of CO2 adsorption on NiAl-MO/CeO2-5.

2.4. Catalyst evaluation

The behavior of the nickel-based catalysts for CO2 methanation was investigated. Methanation was carried out in a fixed-bed reactor with 8 mm inner diameter under atmospheric pressure. A total of 0.5 g catalyst was mixed with some quartz sand and charged in the reactor. Before the reaction, the catalyst was reduced in situ at 500 °C for 4 h under a hydrogen atmosphere and cooled to the reaction temperature. Then the reactant mixture of H2 and CO2 (4[thin space (1/6-em)]:[thin space (1/6-em)]1, v/v) was fed into the reactor at a GHSV of 2400 h−1. The catalytic evaluation was carried out from 150 °C to 450 °C and the outlet gases were analyzed online using a gas chromatograph (GC6890, TCD detector, 2 m TDX01 column).

The catalyst activity was measured by testing the CO2 conversion and CH4 selectivity, which were calculated as follows:

image file: c9cy01186b-t1.tif(1)
image file: c9cy01186b-t2.tif(2)
where [CO2]in/out was the molar flux of CO2 in the feed or outlet and [CH4] was the flow rate of the product CH4 in the outlet.

3. Results and discussion

3.1. Structural and morphological study of the NiAl-MO/CeO2-x catalysts

Fig. 1 shows the XRD patterns of the LDH precursors and calcined and reduced NiAl-MO/CeO2-x samples. The XRD patterns of the LDH precursors (Fig. 1a) showed typical characteristic diffractions with good crystalline structure at about 2θ = 11.3°, 22.7°, 34.4°, 38.8°, 46.3°, 61.3° and 62.7°, which can be indexed to a nickel–aluminum LDH phase similar to that of previous research.33 The peaks with the strongest intensity at 2θ = 11.3°, 22.7°, and 34.4° correspond to the (003), (006) and (012) phase of the LDH precursor, respectively. Therefore, this demonstrated that the Ni–Al LDHs were successfully prepared. In addition, the diffraction peaks of CeO2·xH2O could also be clearly detected in the LDH precursors. In Fig. 1b, the NiAl-MO/CeO2-0 catalyst showed significant NiO diffraction peaks around 2θ = 7.3°, 43.3°, 62.9°, 75.4°, and 79.4°. The other calcined NiAl-MO/CeO2-x (x = 2, 5, 10, 30) catalysts showed the presence of NiO and CeO2 crystallites, revealing the formation of NiO particles on the CeO2 support. XRD results presented low-intensity NiO diffraction peaks and high amount of CeO2 crystalline phase with the increase of CeO2 content in the calcined catalysts. With the increase of CeO2 content in the catalyst, the amount of NiO decreased correspondingly, so the strength of the NiO peak decreased. The diffraction peak of NiO could hardly be detected in the NiAl-MO/CeO2-30 catalyst due to the high CeO2 content.
image file: c9cy01186b-f1.tif
Fig. 1 XRD patterns of the (a) as-synthesized LDH precursors and (b) calcined and (c) reduced nickel-based catalysts.

The pattern for all the reduced catalysts showed three kinds of diffraction peaks corresponding to metallic Ni, NiO, and CeO2. High-intensity Ni diffraction peaks were observed for the reduced catalysts (2θ = 44.5°, 51.8°, and 76.4°), indicating that NiO was readily reduced to metallic Ni. Nickel particles were inevitably oxidized when exposed to air. Thus the diffraction peaks of nickel oxide could be detected. The increased content of CeO2 in the catalysts reflected the enhanced intensity of the CeO2 diffraction peak, while for Ni the peak was broad and barely visible. It is worth noting that the NiAl-MO/CeO2-x (x = 2, 5) catalysts had relatively weak and wide diffraction peaks for metallic Ni, indicating that the Ni crystallite size of the catalysts doped with CeO2 was smaller than that of the NiAl-MO/CeO2-0 catalyst.28 The result revealed that nickel metal interacted with ceria during the high-temperature reduction process, making it easier to form small particles with good dispersion, and the reduction of CeO2 could produce some oxygen vacancies due to its redox properties, which could form the metal–oxygen complex.34 In addition, the composition of the metal elements in the reduced catalyst was determined based on ICP-AES (Table 1). The measured compositions of the metal element were close to the theoretical calculation. The Ni loading determined by the proportion of LDH precursors and CeO2 was in the broad range of 11.0–80.3 mol%.

Table 1 Physical properties of the NiAl-MO/CeO2-x catalysts
Catalyst SBETa (m2 g−1) Pore volumea (cm3 g−1) Mean pore diametera (nm) Metal compositionb (mol%) nH2c (mmol per g cat) Ni particle sized (nm) Ni dispersione (%)
Ce Al Ni
a Values determined by N2 adsorption–desorption.b Values calculated based on ICP-AES.c Calculated according to H2-TPD.d Obtained by TEM.e Calculated by H2 pulse adsorption.
NiAl-MO/CeO2-0 115 0.30 10.3 19.7 80.3 3.52 13.5 18.6
NiAl-MO/CeO2-2 107 0.24 8.79 33.1 14.5 52.4 3.20 12.7 20.7
NiAl-MO/CeO2-5 102 0.20 8.30 49.9 8.4 41.7 2.26 10.8 21.5
NiAl-MO/CeO2-10 91.2 0.19 8.19 64.8 6.10 29.1 2.06 10.4 20.3
NiAl-MO/CeO2-30 71.6 0.11 7.55 86.7 2.25 11.0 1.65 10.1 16.2

Fig. 2 shows the N2 adsorption–desorption isotherm and the corresponding pore size distribution of NiAl-MO/CeO2-x catalysts. All the catalysts presented the mixture of type IV isotherm and type II isotherm.59 With the increase of CeO2 content, the volume of the ink-bottle shaped pores was found to be decreased, indicating that the pore volume of the catalyst was decreasing gradually. The pore size distribution plots reflected the pore size directly. They revealed that most of the pores were assigned to the mesoporous range (2–50 nm) with a narrow and monomodal distribution. The physical properties of NiAl-MO/CeO2-x catalysts are summarized in Table 1. The BET surface areas, pore volumes, and mean pore diameters of the catalysts were all slightly decreased after the introduction of CeO2 to the nickel–aluminum hydrotalcite precursor. XRD results revealed that the addition of CeO2 made the metal particles smaller. Thus, the metal particle was more likely to enter the pore channel of catalysts, which inevitably reduced the pore size and pore volume. The high content of CeO2 nanoparticles could partially cover the surface of the Al2O3 support, and thus the surface area was correspondingly reduced.

image file: c9cy01186b-f2.tif
Fig. 2 Isotherm plot and pore size distributions of the NiAl-MO/CeO2-x catalysts.

TEM images of the NiAl-MO/CeO2-x catalysts were obtained to investigate the morphology as shown in Fig. 3. The TEM images showed that the Ni and CeO2 nanoparticles could be recognized by their characteristic lattice fringes. With the increase of CeO2, the catalyst morphology would undergo some changes and the high density of the CeO2 made it difficult to identify the nickel spheres. It revealed that the metal nanoparticles had small particles and better dispersion in the catalysts with the presence of CeO2, which was consistent with the XRD result. The edges of the structures were gradually blurred due to the coating of CeO2 on the surface of the catalyst. It was confirmed that CeO2 with obvious nanocrystals coated the nickel supported on porous aluminum oxide. The EDX elemental mapping of the NiAl-MO/CeO2-5 catalyst is shown in Fig. 3a(f). The mapping shows that the Ni, Al, Ce and O elements are uniformly distributed, indicating a strong interaction between the active Ni metal and the support. The strong interaction effect could influence the growth of Ni nanoparticles and the Ni dispersion, thus affecting the activity of the catalysts. Further research was carried out from the H2-TPR characterization.

image file: c9cy01186b-f3.tif
Fig. 3 a. TEM images of (a) NiAl-MO/CeO2-0, (b) NiAl-MO/CeO2-2, (c) NiAl-MO/CeO2-5, (d) NiAl-MO/CeO2-10 and (e) NiAl-MO/CeO2-30 catalysts and (f) the EDX elemental mapping of NiAl-MO/CeO2-5. b. TEM image and size distribution of the NiAl-MO/CeO2-x catalysts.

In addition, the TEM image of NiAl-MO/CeO2-5 at high magnification and the Ni nanoparticle diameter distribution of NiAl-MO/CeO2-x catalysts are also displayed in Fig. 3b. The TEM image at high magnification further shows the clear characteristic lattice fringes of Ni and CeO2 nanoparticles. It could be observed that the CeO2 with a large lattice was distributed in the outer layer of the catalyst. It was revealed that the nickel supported on porous aluminum oxide could be coated by CeO2. Particle size distribution was characterized by statistical analysis of about 150 nickel nanoparticles arbitrarily selected from the TEM image. It showed that the nanoparticles had a very broad particle size distribution in the range of 5–21 nm for these series of NiAl-MO/CeO2-x catalysts as shown in Fig. 3b. Notably, with the enhancement of CeO2 in the catalyst, the average Ni particle size decreased slightly from 13.5 nm to 10.1 nm (Table 1), which had no significant change due to the variation of CeO2 content. Relatively, the Ni particle sizes of NiAl-MO/CeO2-2 and NiAl-MO/CeO2-5 catalysts had uniformly normal distribution, and the Ni nanoparticles of less than 12 nm accounted for the majority of the particle distribution of about 70% in the NiAl-MO/CeO2-5 catalyst. As for the other three Ni-based metal oxide catalysts, the Ni nanoparticle diameter distribution was nonuniform, particularly for the NiAl-MO/CeO2-30 catalyst. This further indicated that the Ni nanoparticles exhibited high dispersion with high amount of small particles in the catalysts with a suitable content of CeO2. The difference in particle size and distribution might cause fundamental changes in catalytic properties for CO2 methanation. Moreover, the dispersion of Ni was estimated using H2 pulse adsorption. These series of NiAl-MO/CeO2-x catalysts show a comparable Ni dispersion and it varied slightly with CeO2 content, which was in the range of 16.2–21.5%. Among these catalysts, the NiAl-MO/CeO2-5 catalyst exhibited a relatively higher Ni dispersion than the catalysts with excess CeO2.

3.2. XPS measurement

XPS was performed to understand the surface electronic and chemical properties of the reduced NiAl-MO/CeO2-x catalysts, as shown in Fig. 4. The Ni 2p XPS results showed that the peak of the Ni 2p spectrum was at 852.6 eV and 856.3 eV, corresponding to Ni0 2p3/2. The peak position at 853.7 eV, 855.5 eV, and 861.0 eV could be attributed to Ni2+ 2p3/2, indicating the presence of NiO in the reduced catalysts, which was in agreement with the XRD analysis. The re-oxidation of reduced Ni metals was a possible reason for the formation of the Ni2+ state. Furthermore, the presence of Ni2+ species revealed the strong interaction between Ni and the CeO2 support, and the existence of CeO2 could cause an increase in the electron density on the Ni metal. In addition, the binding energy shifted toward higher values with the addition of increasing CeO2 into the catalysts. It also indicated that the strong interaction between Ni metal and the CeO2 support significantly influences the electronic environment of Ni.
image file: c9cy01186b-f4.tif
Fig. 4 The XPS result of the NiAl-MO/CeO2-x catalyst for Ni 2p, Ce 3d, O1s, and Al 2p.

The XPS spectrum of Ce 3d featured at 875–920 eV indicated that the catalysts were primarily present as Ce4+ along with some Ce3+ in the surface region. The XPS results revealed that the peaks at 882.6, 887.8, 898.7, 901.5, 905.1, 908.3, and 917.1 eV corresponded to the Ce4+ species in CeO2. The peaks at 881.8, 885.5, 898.7, 903.7 eV were characteristic of Ce3+, which indicated the reduction of Ce4+ due to the high temperature hydrogen reduction. They were also associated with the formation of oxygen vacancies and the interaction between nickel species and CeO2 support.24,28,34 It also proved the existence of oxygen vacancies. The oxygen vacancies were expected to promote the absorption and activation of CO2.35

The peak at 529–530 eV in the O 1s spectrum revealed the existence of metal oxides (nickel oxide and the lattice oxygen in the CeO2 support) and the binding energy slightly shifted to the low binding energy with the addition of CeO2 in the catalysts due to the increment of the lattice oxygen. Furthermore, a slight increment in the binding energy shoulder at around 528.5 eV revealed the existence of oxygen vacancies.36 The XPS spectrum of O 1s exhibited a peak at 531.5–533.0 eV, which could be mainly assigned to the surface oxygen due to CO2 adsorption on the surface of the catalysts. This peak had a large proportion in the NiAl-MO/CeO2-5 catalyst, indicating that this catalyst had a strong ability to adsorb CO2, while for all the catalysts no change in the Al 2p binding energy was observed, revealing that the state of surface Al did not change due to the addition of CeO2. All in all, the variation of the state of electrons may change the surface properties of the catalyst and affect the active sites on the catalyst surface.

3.3. Adsorption and reduction properties

CO2 thermodesorption experiments were performed to investigate the relative amounts and strength of basic sites in the catalysts. The CO2-TPD profiles of the NiAl-MO/CeO2-x catalysts are shown in Fig. 5. The desorption peaks could be roughly classified into three types according to the temperature: 50–130 °C, 130–300 °C and >300 °C, which correspond to weak, medium and strong basic sites, respectively. The desorption peak at low temperature was due to the CO2 desorption from weak basic sites, attributed to the basic CeO2 support, the unreduced Ni sites on the support and the presence of surface hydroxyl groups on the metal oxides.35,37 Addition of CeO2 led to an increase in the amount of CO2 desorption and the desorption peak shifted to higher temperatures. That is to say, with the increase of CeO2, the CO2 adsorption capacity and basic strength increased at low temperature (50–130 °C). This indicated that CO2 adsorption capacity and basic strength were related to the basicity of CeO2. As a weak acid, carbon dioxide is usually attached to the surface basic sites of the catalyst.38 Therefore, the addition of the moderate basicity of CeO2 can provide additional CO2 adsorption sites to the catalyst. The medium basic sites could be assigned to CO2 emissions from Ni particles and various forms of carbonates adsorbed onto CeO2.37,39 Three desorption peaks with the maximum CO2 desorption temperature of 90 °C, 174 °C and 250 °C were observed for the NiAl-MO/CeO2-0 catalyst and the latter two peaks belong to the medium basic sites. With the increase of CeO2 in the catalysts, the medium basic sites changed from two peaks of the NiAl-MO/CeO2-0 catalyst to one peak in the NiAl-MO/CeO2-x catalysts (x = 2, 5, 10). Furthermore, the intensity of the medium temperature peak initially increased until x = 5 and then decreased gradually with the further increase of CeO2 content, and finally disappeared in the NiAl-MO/CeO2-30 catalyst. Among all the catalysts, the NiAl-MO/CeO2-5 catalyst had the highest concentration of medium basic sites. For the NiAl-MO/CeO2-30 catalyst with the highest CeO2 content, only one desorption peak at about 95 °C was detected. Due to the small amount of nickel content in the NiAl-MO/CeO2-30 catalyst, the number of medium basic sites assigned to CO2 desorption from Ni particles was decreased correspondingly. This demonstrated that the exposed metal Ni gradually decreased with increasing amount of CeO2, and the metal–support interaction made the basic sites dispersed uniformly. As a result, the addition of CeO2 provided both weak and moderate basic sites, which significantly affected the basic properties of the NiAl-MO/CeO2-x catalysts. The weak and medium basic sites can be involved in the adsorption and activation of CO2, which is conducive to the formation of reactive intermediates.40
image file: c9cy01186b-f5.tif
Fig. 5 CO2-TPD profiles for the NiAl-MO/CeO2-x catalysts.

The H2 adsorption properties obtained in the temperature range from 50 °C to 650 °C are presented in Fig. 6. The H2-TPD profiles of NiAl-MO/CeO2-x catalysts exhibited three H2 desorption peaks with a maximum temperature of 88 °C, 171 °C and about 263 °C, respectively. For all the catalysts, one H2 desorption peak was observed at 88 °C and the relative peak strength did not have a significant difference, but the peak shifted to a higher temperature for the NiAl-MO/CeO2-10 and NiAl-MO/CeO2-30 catalysts. This suggested that the high content of CeO2 in the catalysts enhanced the hydrogen adsorption capability at low temperature, therefore the H2 desorption was more difficult at about 88 °C. Except for the NiAl-MO/CeO2-30 catalysts, all the catalysts had a significant H2 desorption peak at about 171 °C. This desorption peak was attributed to the hydrogen desorption on the surface of nickel metal.41 Thus, the NiAl-MO/CeO2-0 catalyst with the highest nickel content had a relatively strong H2 desorption peak at 171 °C. Therefore, we can infer that too much CeO2 is not conducive to the adsorption of H2, which can reduce the activity of the catalyst. It is worth noting that there was a poor intensity peak at about 263 °C for the NiAl-MO/CeO2-x catalysts in the profile, attributed to the strong nickel bonding on the surface of the catalyst. Previous research revealed that H2 adsorption on the catalysts was considered to be an activated process.41 The relative intensity of the third broad peak for the NiAl-MO/CeO2-5 catalyst was higher than those of the other catalysts and the peak moved towards a higher temperature, indicating that the NiAl-MO/CeO2-5 catalyst had more activation sites and strong adsorption ability of hydrogen. That is to say, an appropriate CeO2 content could reduce the overall H2 desorption capacity of the catalysts, but the interaction between nickel metal and the CeO2 support could introduce new hydrogen adsorption sites with different strengths, which was conducive to improving the catalytic activity. From Table 1, it is observed that the amount of H2 uptake for the NiAl-MO/CeO2-x catalysts decreased with the addition of CeO2, which was due to the electronic effect.42 According to the literature, adding CeO2 to Pd/Al2O3 catalysts could also decrease the amount of H2 desorption and the phenomenon became more pronounced with high amount of CeO2.43

image file: c9cy01186b-f6.tif
Fig. 6 H2-TPD profiles for the NiAl-MO/CeO2-x catalysts.

The reducibility of the NiAl-MO/CeO2-x catalysts was investigated by H2-TPR and presented in Fig. 7. Two types of reduction peaks in the range of 100–800 °C were observed for the NiAl-MO/CeO2-x catalysts. The reduction peaks at low temperature (100–300 °C) were assigned to the reduction of NiO species, which had a weak interaction with the support, while the reduction peaks of the NiO species that interacted strongly with the support could be detected at high temperature (300–800 °C). For the NiAl-MO/CeO2-10 and NiAl-MO/CeO2-30 samples, two connected peaks centered at about 200 °C were observed, which revealed that the nanoparticles were not uniform and the interaction with the CeO2 support was weak. In addition, compared to the other catalysts, the latter reduction peaks of these two catalysts significantly shifted to higher temperatures, indicating a relatively difficult to reduce nickel species and larger reduction hindrance in the catalysts. Among the catalysts, it was observed that NiAl-MO/CeO2-5 had a higher fraction of reduction peak centered at about 200 °C, which was attributable to the uniformly dispersed metal species with small particle size.44 This observation was also consistent with the TEM result. Notably, the reduction peaks of NiAl-MO/CeO2-5 slightly shifted to higher temperatures as compared with the NiAl-MO/CeO2-x (x = 0, 2), suggesting the enhanced interaction between NiO and the support. All in all, the addition of a certain amount of CeO2 to the catalyst could provide suitable metal–support interaction, which was beneficial for CO2 methanation.

image file: c9cy01186b-f7.tif
Fig. 7 H2-TPR profiles of the NiAl-MO/CeO2-x catalysts.

3.4. Catalytic performance

The effect of different contents of CeO2 on catalytic activity is illustrated in Fig. 8. The CO2 conversion reached a maximal value and then decreased gradually with the increase of reaction temperature, and the CH4 yield curve had a similar trend due to the high selectivity of about 99%. A certain amount of CeO2 in the catalysts considerably improved the catalytic performance at low temperature (<250 °C). It is worth noting that the activities of the NiAl-MO/CeO2-2 and NiAl-MO/CeO2-5 catalysts were better than that of the NiAl-MO/CeO2-0 (without CeO2) catalyst at each temperature below 250 °C. However, the catalytic performance of NiAl-MO/CeO2-x (x = 10, 30) catalysts was inferior to that of others, indicating the negative effect of excess CeO2. The NiAl-MO/CeO2-5 catalyst had the best catalytic activity with a CO2 conversion of 91% at 250 °C. The superior activity of the NiAl-MO/CeO2-5 catalyst was primarily due to the existence of small particles with good dispersion in the catalysts with the addition of a certain amount of CeO2, which were confirmed by XRD, TEM and BET results. According to CO2-TPD analysis, the addition of basic CeO2 can provide both weak and moderate basic sites, which could bring additional CO2 adsorption sites to the catalyst. The NiAl-MO/CeO2-5 catalyst had the highest concentration of medium basic sites and various strengths of hydrogen adsorption sites, while too much CeO2 was not conducive to the adsorption of H2, and the medium basic sites were also gradually decreased. Based on the XPS analysis, the existence of oxygen vacancies in the catalyst directly increased the active sites and promoted CO2 absorption. The comparison of catalytic results between the NiAl-MO/CeO2-x catalyst and the reference results are summarized in Table 2. The NiAl-MO/CeO2-5 catalyst had catalytic activity with CO2 conversion of 91% at 250 °C, which appeared to be the superior one among the listed catalysts in the low-temperature CO2 methanation. Most of the reported catalysts were less reactive than the NiAl-MO/CeO2-5 catalyst even at low space velocity and high temperatures. Metal oxide integrated with LDH precursor is a successful strategy to design a promising catalyst with excellent catalytic performance.
image file: c9cy01186b-f8.tif
Fig. 8 The catalytic activity of NiAl-MO/CeO2-x catalysts with different contents of CeO2. (a) CO2 conversion, (b) CH4 selectivity and yield.
Table 2 Comparison of activities of CO2 methanation between NiAl-MO/CeO2-x catalyst and literature references
Catalyst GHSV nCO2/nH2 Synthesis CO2 conversion (%) Reaction temperature (°C) Ref.
AE: ammonia evaporation, EGR: ethylene glycol reduction, RM: reverse microemulsion.
NiAl-MO/CeO2-5 2400 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Hydrothermal 91 250 This work
NiWMgOX 2000 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Precipitation 28 250 37
Ni–Al hydrotalcite 30[thin space (1/6-em)]000 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Coprecipitation 50 250 45
10% Ni@MOF-5 2000 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Impregnation 47 280 46
Ni/Ce0.5Zr 0.5O2 20[thin space (1/6-em)]000 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4.5 Impregnation 20 260 40
Ni/CeO2–ZrO2 20 L g−1 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 AE 55 275 47
Ni–Ce-CN 10 L g−1 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Sol–gel 50 305 48
Ru/Al2O3 1000 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 EGR 37 250 49
Pd–Mg/SiO2 3273 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 RM 59 450 11
5% Ni/Ce0.72Zr0.28O2 21[thin space (1/6-em)]000 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Sol–gel 80 350 50
10% Ni/CeO2 10[thin space (1/6-em)]000 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Impregnation 87 300 23
Ni@UiO-66 1650 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]5.2 Double solvent 48 350 51
Ni/ZSM-5 2400 h−1 1[thin space (1/6-em)]:[thin space (1/6-em)]4 Impregnation 70 400 52

Fig. 9 shows the long-term stability of the NiAl-MO/CeO2-x catalysts for CO2 methanation, which was investigated at 300 °C under atmospheric pressure. For all the catalysts, the CO2 conversion had no obvious decline during the 120 h test. The carbon balance was about ±5% and CO was the only by-product. This suggested that the addition of CeO2 did not influence the stability of the catalysts. The NiAl-MO/CeO2-x catalysts showed excellent catalytic stability for CO2 methanation and promising potential for practical application.

image file: c9cy01186b-f9.tif
Fig. 9 Catalytic stability test of the NiAl-MO/CeO2-x catalysts at 300 °C.

3.5. In situ DRIFTS measurements

3.5.1. CO2 adsorption studies. DRIFTS measurements were performed under a CO2 atmosphere from 150 °C to 450 °C for the NiAl-MO/CeO2-x catalyst (Fig. 10). For the NiAl-MO/CeO2-0 catalyst, an increase in the temperature to 350 °C led to the emergence of CO (2002 cm−1), and the intensity of the CO peak increased as the temperature rose to 450 °C. The results provided information on how CO2 is adsorbed on the catalyst. The presence of CO was due to the adsorption of CO2 on Ni particles or thermal decomposition of CO2 at high temperatures. According to the literature, the bands observed in the range of 1600–1000 cm−1 could be attributed to carbonate-like species.53 With increasing temperature, the bands associated to carbonate-like species centered at 1345 cm−1 1472 cm−1, and 1545 cm−1 decreased slightly. The main peak of CO2 adsorption on the NiAl-MO/CeO2-0 catalyst centered at about 1345 cm−1, which was assigned to monodentate carbonates. With increasing temperature, the intensity of the band originating from bidentate carbonates (1597 cm−1) increased significantly, revealing the strong interactions with the catalyst surface.54 The band centered at 1076 cm−1 was ascribed to the polydentate carbonate. This peak did not have sufficient thermal stability and decreased gradually with the increase of temperature. It confirmed that the main intermediate adsorbed on the surface of the NiAl-MO/CeO2-0 catalyst was monodentate carbonates.
image file: c9cy01186b-f10.tif
Fig. 10 DRIFTS spectra of CO2 adsorption over the NiAl-MO/CeO2-x catalyst at different temperatures.

Fig. 10 also displays the spectrum of the NiAl-MO/CeO2-5 catalyst at different temperatures after the introduction of CO2. The band in the range from 1200 cm−1 to 2000 cm−1 could be attributed to different kinds of carbonates. The main peak of CO2 adsorption on the NiAl-MO/CeO2-5 catalyst centered at 1599 cm−1 was assigned to the bidentate carbonates on Ce(III) and the species displayed high thermal stability. In contrast, the species with low intensity in the NiAl-MO/CeO2-0 catalyst increased with increasing adsorption temperature. This suggested that an appropriate amount of CeO2 can probably improve the CO2 adsorption capacity and stabilize the bidentate carbonates, allowing the hydrogenation reaction to proceed. The stable bands at 1291 cm−1 and 1371 cm−1 were related to the bidentate carbonates and monodentate carbonates adsorbed on Ce (IV), respectively.53 The peak of “free” carbonate can only be detected at 1446 cm−1 when the temperature was below 300 °C, revealing that carbonate could be desorbed at high temperature. In addition, a weak band at 1850 cm−1 belongs to carbonyls adsorbed onto Ni0 sites. Significantly, the formation of the bands at 1599 cm−1, 1371 cm−1 and 1291 cm−1 (carbonate-like adsorbed on Ce(III) and Ce (IV)) was preferred over the bands at 1850 cm−1 and 1446 cm−1. The high intensity peak of bidentate carbonates on Ce(III) (1599 cm−1) revealed that CO2 was more likely to be adsorbed on surface oxygen sites of Ce(III) compared to other sites. Various carbonates adsorbed on the NiAl-MO/CeO2-5 catalyst revealed that the catalyst had an effective CO2 adsorption capacity, which was confirmed by the CO2-TPD result (Fig. 5).

As shown in Fig. 11, the main intermediates adsorbed on the surface of the NiAl-MO/CeO2-0 catalyst were monodentate carbonates at 1345 cm−1. CO2 adsorption on the NiAl-MO/CeO2-x (x = 2, 5, 10) catalyst was different from the adsorption on the NiAl-MO/CeO2-0 catalyst. Some new characteristic peaks were detected due to the adsorption of CO2 on CeO2. A band at about 1386 cm−1 of the NiAl-MO/CeO2-2 catalyst was relevant to the symmetric stretch of a bidentate carbonate adsorbed onto the CeO2.50,55 This revealed that CO2 could adsorb on the CeO2 support with basic sites by available oxygen to form the Ce–OCO2 intermediate species.56 The bands centered at 1637 and 1219 cm−1 were assigned to hydrogen carbonates. In addition, a broad band at 1051 cm−1 was detected due to the presence of bidentate carbonates. The main CO2 adsorption intermediates of the NiAl-MO/CeO2-5 catalyst were the bidentate carbonates adsorbed on Ce at 1599 cm−1 and 1291 cm−1, and the intensity peak of bidentate carbonates in NiAl-MO/CeO2-5 was higher than that of NiAl-MO/CeO2-2 and NiAl-MO/CeO2-0 (without CeO2) catalysts, revealing that appropriate amount of CeO2 is conducive to CO2 adsorption. The spectra of CO2 adsorption on NiAl-MO/CeO2-10 catalyst observed in the 1600–1000 cm−1 range are attributed to thermodynamic unstable polydentate carbonate (1091, 1048 cm−1), thermodynamic stable monodentate carbonates on Ce (IV) (1375 cm−1) and weak stable bidentate carbonates (1584, 1290 and 1528 cm−1), respectively. In general, the monodentate carbonates have the highest thermal stability compared with other types of carbonates. The presence of excess CeO2 could probably destabilize carbonate-like species, which was expected to be unstable to take part in the CO2 methanation. It was an important reason for the low activity of the NiAl-MO/CeO2-10 and NiAl-MO/CeO2-30 catalysts.

image file: c9cy01186b-f11.tif
Fig. 11 DRIFTS spectra measured at 150 °C for CO2 adsorption after 30 min.

Density functional theory (DFT) calculations were performed to calculate the CO2 adsorbed on the surface of the NiAl-MO/CeO2-0 (110) and NiAl-MO/CeO2-5 (110) catalysts. First, the stable adsorption configuration of CO2 was investigated, and the optimized adsorption states are shown in Fig. 12. Fig. 12a shows the optimized structures of CO2 on the surface of NiAl-MO/CeO2-0 (110). In the calculations, the C–Ni bond length is 1.83 Å and two O–Ni bond lengths are 2.16 and 2.01 Å, <OCO = 140.37°. In Fig. 12b, direct bonding could be observed between Ce and O of CO2 resulting from the oxygen vacancy on the surface of CeO2. In the optimized structure, an oxygen vacancy is present on the surface and it has the lowest binding energy. The Ce atom and Ni atom share one oxygen atom, the Ce–O bond length is 2.39 Å, and the Ni–O bond length is 1.81 Å. The length of another Ni–O bond is 2.03 Å, C–O bond length is 1.51 Å, C adsorption on Ni has a bond length of 1.70 A, <OCO = 174.34° (Fig. 12(b)). For CO2 adsorption of NiAl-MO/CeO2-0 and NiAl-MO/CeO2-5 catalysts, the corresponding binding energy is −1.05 eV and −0.41 eV, respectively. It can be seen that the NiAl-MO/CeO2-5 (110) catalyst has lower CO2 binding energy and longer C–O bond length (1.51 Å) than the NiAl-MO/CeO2-0 (110) catalyst, which contributed to the adsorption and activation of CO2. According to the comparison of adsorption bond length and adsorption energy, the NiAl-MO/CeO2-5 catalyst is favorable for CO2 conversion.

image file: c9cy01186b-f12.tif
Fig. 12 The optimized structures of adsorbed CO2 on (a) NiAl-MO/CeO2-0 (110) and (b) NiAl-MO/CeO2-5 (110), where pink and red spheres denote Al and O atoms and gray, blue and yellow represent C, Ni and Ce atoms respectively.
3.5.2. Methanation conditions. The DRIFTS spectra of NiAl-MO/CeO2-x catalysts collected under methanation conditions from 150 °C to 450 °C are presented in Fig. 13. The typical intermediate species monodentate carbonates, hydrogen carbonates, and bidentate carbonates were still detected on the NiAl-MO/CeO2-x catalyst with the introduction of hydrogen. At the same time, some new species were observed with increasing temperature. For the NiAl-MO/CeO2-0 catalyst, the absorption bands relevant to bridged carbonates (1726 cm−1) and polydentate carbonates (1369 cm−1) become clearly invisible with increasing temperature, indicating the high temperature instability property of these absorption species. Operando IR spectra presented that monodentate carbonate (1435 cm−1) increased gradually with increasing temperature, accompanied by the formation of methane (1305 cm−1) for the NiAl-MO/CeO2-0 catalysts. It was indicated that monodentate carbonate played an important role in the formation of methane. The absorption band identified as formate (1595 cm−1) was visible at 200 °C, and the intensity increased until 300 °C and then decreased gradually. This indicated that the formate formation was due to hydrogen dissociation and CO2 adsorption on Ni nanoparticles for the NiAl-MO/CeO2-0 catalyst.57,58 An increase in the temperature to 200 °C led to a significant increase in the intensity of the CH4 (1305 cm−1) peak for NiAl-MO/CeO2-5 and NiAl-MO/CeO2-2 catalysts, while for the NiAl-MO/CeO2-0 and NiAl-MO/CeO2–10 catalysts, the CH4 adsorption peak's temperature was higher than that of the other two catalysts, which appeared at 250 °C. This phenomenon suggested that methane was more likely to form on the NiAl-MO/CeO2-2 and NiAl-MO/CeO2-5 catalysts compared with the other catalysts; thus the appropriate amount of CeO2 can significantly improve the catalyst activity. The catalyst evaluation result showed that a small amount of CO can be detected at about 350 °C, which was consistent with the results of the DRIFTS spectra.
image file: c9cy01186b-f13.tif
Fig. 13 DRIFTS spectra of CO2 methanation over the NiAl-MO/CeO2-x catalyst at different temperatures.

The OCO stretching vibration of bidentate carbonates adsorbed on CeO2 was observed at 1577 cm−1 and the band of the monodentate formates adsorbed onto Ni0 appeared at 1345 cm−1 for the NiAl-MO/CeO2-2 catalyst. It clearly appeared that the bidentate carbonates centered at 1577/1581/1584 cm−1 for the NiAl-MO/CeO2-x (x = 2, 5, 10) catalyst gradually decreased and even vanished at the same time as the hydrogen carbonate (1441 cm−1) increased, since the bidentate carbonates were unstable at high temperature and the hydrogen carbonate was formed due to the bidentate carbonates' hydrogenation. It seemed that hydrogen carbonate was more readily generated in the catalysts modified with ceria. Notably, the NiAl-MO/CeO2-5 catalyst had a relatively high amount of formate species (1368 cm−1) with a lower amount of hydrogen carbonate than other catalysts. Hydrogen carbonate could be an active participant in the formation of formates. In Fig. 13, the formates as the main adsorption peak were detected at 1345–1370 cm−1 for NiAl-MO/CeO2-x (x = 2, 5, 10) catalysts, suggesting that adsorbed formates were the critical main intermediate species of the methanation reaction. It is interesting to note that the peak intensity of formate (1345/1368/1370 cm−1) in the NiAl-MO/CeO2-x catalysts increased until 300 °C before decreasing with increasing temperature. It should be noted that the concentration of the adsorbed formates appeared to be directly related to the formation of CH4. Formates could be hydrogenated as intermediates to produce methane through formaldehyde and methoxy species.55 The high amount of formate species with a low amount of hydrogen carbonate could be generated under the methanation conditions for the NiAl-MO/CeO2-5 catalyst, which can accelerate the methanation reaction and improve the catalytic performance at low temperature. In the methanation condition, the presence of excess ceria induced the visible appearance of polydentate carbonate species (1077 cm−1) on CeO2 in the NiAl-MO/CeO2-10 catalyst. The methane peak of the NiAl-MO/CeO2-10 catalyst became clearly visible at 300 °C, and the temperature was higher than those of the NiAl-MO/CeO2-2 and NiAl-MO/CeO2-5 catalysts. It means that under methanation conditions, the appropriate amount of ceria can more actively participate in the synthesis of methane.

3.6. Reaction mechanism

Based on the DRIFTS observations, a reaction mechanism for CO2 methanation over the NiAl-MO/CeO2-x catalyst is shown in Fig. 14. CO was sourced from CO2 adsorption and decomposition on Ni particles at high temperature. Hence the main reaction route did not include the CO intermediate. CO2 could react with the surface hydroxyls and the surface oxygen sites on the support to produce hydrogenated carbonates and carbonates, respectively. The hydrogenated carbonates and carbonate species combined with dissociated H2 on the Ni0 particles to generate formates species. In the presence of hydrogen, the addition of CeO2 could introduce more adsorption sites, leading to more adsorbed formates as revealed by the band at 1368 cm−1 for the NiAl-MO/CeO2-x catalyst. Therefore, the methanation reaction had a lower activation energy and faster reaction rate over the NiAl-MO/CeO2-x catalyst, which was conducive to improving the catalytic performance for CO2 methanation at low temperature. The formates species are further hydrogenated with multi-steps on metallic Ni to form –CH2OH and –CH3. Finally, the –CH3 further reacted with the dissociated hydrogen to form methane.
image file: c9cy01186b-f14.tif
Fig. 14 Proposed reaction mechanism for CO2 methanation on NiAl-MO/CeO2-x catalysts.

According to the proposed CO2 methanation mechanism, we could observe that both active Ni and the mixed metal oxide support sites could participate in the CO2 hydrogenation process. Therefore, the close interaction between the two sites in the catalyst was essential to accelerate the methanation process. It was concluded that metal–support interaction in NiAl-MO/CeO2-x catalysts played an important role in improving the catalyst property for CO2 methanation at low temperature.

4. Conclusion

A series of NiAl-MO/CeO2-x catalysts prepared by integrating LDH precursors with CeO2 were developed for low-temperature CO2 methanation. Appropriate CeO2 addition can significantly improve the catalytic properties at low temperature. The NiAl-MO/CeO2-5 catalyst exhibited the highest catalytic activity with a CO2 conversion of 91% at 250 °C. The high activity of the NiAl-MO/CeO2-5 catalyst was attributed to its highly dispersed small Ni metal nanoparticles and the presence of oxygen vacancies with the addition of a certain amount of CeO2. The addition of CeO2 could provide both weak and moderate basic sites, which are involved in the activation of CO2. Therefore it could affect the types of intermediate and the methanation process. DFT calculation and in situ DRIFTS results further verified that the catalyst with the appropriate amount of CeO2 was favorable for CO2 adsorption and conversion. The reaction mechanism results demonstrated that the high amount of formate intermediate species can accelerate the reaction. The metal–support interaction introduced appropriate active sites and improved the dispersion of nanoparticles, which could also accelerate the methanation reaction. While too much CeO2 was not conducive to the adsorption of H2, it also destabilized the carbonate-like species and decreased the amount of medium basic sites. Active participation of metal oxide integrated with LDH precursor is a successful strategy to design catalysts with excellent catalytic performance.

Conflicts of interest

There are no conflicts to declare.


This work was supported by the Major Program of National Natural Science Foundation of China (21890762), the NSFC-NRCT joint project (No. 51661145012), the Foundation of State Key Laboratory of High-efficiency Utilization of Coal and Green Chemical Engineering (Grant No. 2017-K08), the Key Research Program of Frontier Sciences, CAS (No. QYZDB-SSW-SLH022), the National Natural Science Foundation/China (No. 21706258), and the K. C. Wong Education Foundation (No. GJTD-2018-04). The authors are grateful for the assistance from teacher Hui Wu of the Analysis and Test Centre, Institute of Process Engineering, Chinese Academy of Sciences.


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