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Kinetics, Catalysis, and Reaction Engineering

Highly active Ni-based catalyst derived from double hydroxides precursor for low temperature CO2 methanation Xinpeng Guo, Zhijian Peng, Mingxiang Hu, Cuncun Zuo, Atsadang Traitangwong, Vissanu Meeyoo, Chunshan Li, and Suojiang Zhang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01619 • Publication Date (Web): 20 Jun 2018 Downloaded from http://pubs.acs.org on July 2, 2018

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Industrial & Engineering Chemistry Research

Highly active Ni-based catalyst derived from double hydroxides precursor for low temperature CO2 methanation Xinpeng Guoa,b, Zhijian Penga, Mingxiang Huc, Cuncun Zuob, Atsadang Traitangwongb,Vissanu Meeyoob, b

Chunshan Lib*, Suojiang Zhang b

a School of Engineering and Technology, China University of Geosciences, Beijing 100083, PR China CAS Key Laboratory of Green Process and Engineering, State Key Laboratory of Multiphase Complex System, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, PR China. c State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

Abstract The Ni nanoparticles supported on Al2O3 metal oxides with different Ni2+/Al3+ molar ratios from 1/1 to 6/1 (denoted as NixAL-MO) derived from Ni-Al hydrotalcite precursors (NixAl-LDHs) were prepared and applied to CO2 methanation at low temperature. By adjusting the Ni2+/Al3+ ratios, the alkaline and reducibility of the catalysts were designed and the catalysts showed efficient catalytic performance at low temperature. The Ni5AL-MO catalyst demonstrated the highest CO2 conversion of 89.4% at 250 °C. The superior catalytic property was related to the basic property, readily reducible NiO species and the cooperation of metal nanoparticles and basic sites. The reaction intermediate species formed on catalyst surface were identified by in situ FTIR analysis and reduction mechanism of CO2 was revealed. Moreover, the Ni5Al-MO catalyst had excellent resistance to carbon deposition and metal sintering after long reaction duration. The NixAl-LDHs as catalyst precursor shows a promising potential for CO2 methanation at low temperature. Keywords: methanation; layered double hydroxides; low temperature *

Corresponding author: Chunshan Li. TeL/FaX: +86-10-82544800; E-mail: [email protected]

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction With the economic growth the concentration of carbon dioxide in the atmosphere has risen significantly. Carbon dioxide as the primary greenhouse gas has raised much attention due to its contribution to greenhouse effect and the resulting environmental problems.1 Moreover, as a kind of renewable, abundant and cheap carbon resource, CO2 application in fuel engineering or organic chemistry is very important basic step in C1chemistry.2-4 The CO2 utilization is hampered due to its high thermodynamic stability and thus particular chemistry and technology methods are required. Among many methods for CO2 processing (capture, sequestration, conversion, etc.), catalytic methods may provide promising strategies.5-7 CO2 catalytic hydrogenation to useful methane is one of research focus since it involves promoting energy regeneration and potential environmental/commercial value.8-11 Among the potential catalysts of methanation are the Group VIII metals, of which nickel gain much attention because of its good activity and relatively low cost for industrial application.12-15 Currently, the challenges have been focused on the design of Ni-based catalysts in CO2 methanation to enhance the low-temperature activity. Thermodynamically, the low temperature is conducive to the exothermic methanation and low temperatures can save energy as well as prevent the sintering of the active species. Specifically, the supports with sufficient surface area, appropriate basicity and redox property as well as potential support-metal interactions could achieve excellent catalytic activity for CO2 methanation.16,17 Therefore, it is an effective way to improve the low temperature catalytic activity of nickel-based catalyst by applying novel materials to introduce alkaline sites and readily reducible species in the catalyst.18-20 According to previous report, alkali species binded CO2 molecule on the support hydrogenated with dissociated hydrogen to produce CH4.21-23 Substantially, the catalytic property of


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alkali and reduction species is determined by its nature characteristic such as the category, strength and quantity of alkaline species onto a support.24-26 That is to say, the alkalinity and reducibility of nickel catalyst vary with support materials and preparation methods. Especially, very little research related to the design of the reducibility and alkaline sites with enhanced low-temperature CO2 methanation catalytic activity. Therefore, the influence of alkaline and reduction property on catalytic activity and its catalytic mechanism would be a valuable research. Layered double hydroxides (LDHs) are an anionic clay mineral with general molecular formula [M2+1-xM3+x (OH)2](An-)x/n·mH2O. The two-dimensional LDHs are composed of the divalent and trivalent metal ions and they can be distributed highly ordered in the hydroxide layers.27,28 The LDHs materials are widely used in the field of catalysis due to their versatility in chemical composition and structural architecture.29-32 Also, LDHs can be converted into mixed oxides with sufficient surface areas, high dispersion of the elements, excellent reducibility and the most important, a tunable alkaline property.27 Therefore, it is a promising strategy to improve the catalytic performance by adjusting the reduction property and alkaline property. In this work, the metal oxides powder catalysts with a series of Ni2+/Al3+ ratio were prepared by hydrothermal synthesis method and attempted in CO2 methanation. For comparison, Ni/Al2O3 catalyst was prepared by traditional impregnation method. We aimed to investigate the relationship between catalytic performance and physicchemical properties which focused on the alkaline and redox property of the catalyst. Moreover, the reaction pathway of CH4 formation and the long reaction duration of Ni5Al-MO catalyst were also investigated. 2. Experimental



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2.1. Preparation of the catalysts The NixAl-LDH precursors with various Ni2+/Al3+ ratios (from 1/1 to 6/1) were synthesized

utilizing

a

hydrothermal

synthesis

method.27,28

Commercial

Ni(NO3)2·6H2O (≥98.0%) received from Xilong Co., Ltd. was employed as nickel source. Other chemicals including Al(NO3)3·9H2O, NaOH and Na2CO3 were also purchased from Xilong Co., Ltd. The preparation process of the LDHs precursors was as follows: Ni(NO3)2·6H2O and Al(NO3)3·9H2O with various molar ratios of Ni2+/Al3+ were dissolved in 100 mL deionized water in appropriate concentrations. Then the aqueous solution was added quickly into a certain amount of mixture solution containing excessive NaOH and Na2CO3 at room temperature under vigorous stirring for 30 min. Then the resulting suspension was transferred into a sealed Teflon autoclave for hydrothermal treatment at 110 °C for 48 h. The obtained solid precipitate was separated by filtration and washed thoroughly with water and ethanol until the filtrate solution was neutral. Then the sample dried at 60 °C for 12 h and the NixAl-LDH precursor was obtained. Finally, the catalyst precursor was reduced under H2/N2 (1/9, v/v) atmosphere by a temperature programmed from room temperature to 500 °C for 4 h with the phase transformation from NixAl-LDH to Ni nanoparticles supported on Al2O3 metal oxides which was denoted as NixAl-MO. For comparison, Ni catalyst supported on Al2O3 as reference sample was prepared by an impregnation method and denoted as Ni/Al2O3. 2.2. Catalyst Characterization X-ray diffraction (XRD) patterns of the catalysts was analysed on a Rigaku Smart Lab X-ray powder diffractometer with Cu Kα radiation and the diffractograms were recorded with 2θ ranged from 5 to 90o. Brunauer–Emmett-Teller (BET) specific surface area, pore volumes and pore mean


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diameters of catalysts was determined by using a micromeritics ASAP 2460 instrument. In order to guarantee the accuracy of the measuring data, the catalysts were outgassed at 350 °C for 6 h under vacuum before analysis. Transmission electron microscope (TEM) images of the catalysts were performed using a JEM-2100 system at an acceleration voltage of 200 kV. During the TEM observation, the chemical position of the catalyst was also analyzed by EDX. The samples were dispersed via ultrasonic in ethanol and a holey carbon-supported grid was dipped into the suspension for observation. The reduction behavior of the catalysts was measured by H2 temperature programmed reduction (H2-TPR). It was tested by AutochemⅡ2920 Chemisorption Apparatus (Micromeritics). Prior to H2-TPR measurement, the samples were pretreated under He at 500 °C for 1 h before cooling down to 50 °C in He flow. The gas of 10% H2-Ar was then switched to the samples for 1 h at 120 C. The temperature was increased to 900 °C at a heating rate of 10 °C/min. The basicity of the catalyst was examined by CO2 temperature-programmed desorption (CO2-TPD) by using the same apparatus as H2-TPR. Approximately 50 mg samples were fixed in a quartz tube reactor with flowing He at 500 °C for 1 h and cooling to 50 °C prior to the measurements. Then CO2 was supplied and the sample was placed under a stream of He with a heating rate of 10 K min−1 up to 650 °C. Thermo-gravimetric/differential thermal analysis (TGA/DTA) of the spent catalyst was examined in air atmosphere at a rate of 10 °C/min-1 on a simultaneous Shimadzu thermal analyzer. The catalysts were heated from RT to 700 °C and the carbon deposition could be obtained by weight loss. In situ FTIR spectroscopy of the methanation catalyst was carried out in a Nicolet 6700 spectrometer equipped with a cell. The samples were pressed into a disk and



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activated at 500 °C for 1 h in hydrogen stream. Then the catalysts cooled to corresponding temperature (150 °C, 200 °C, 250 °C, 300 °C) and scanned to get a background spectrum in flowing Ar. Subsequently, the sample was exposed to a reaction gas (4% CO2-16% H2-80% Ar) for 1 h. The FTIR spectra were collected with accumulating 50 scans and a resolution of 4 cm−1 2.3 Catalytic evaluation Catalytic tests were evaluated at atmospheric pressure in a fixed bed reactor with an inner diameter of 8 mm in the temperature range of 150-400°C. The reactor was loaded with 0.5 g catalyst and fixed vertically inside a furnace equipped with a temperature controller. Prior to reaction, the catalysts were reduced in situ by a gas mixture of H2/N2 (2/3,v/v) with a flow rate of 150 ml/min for 2 h at 500 °C and then the temperature cooled to 150 °C in Ar. Then a mixture of H2, CO2 and Ar (molar ratio of H2: CO2 : Ar = 12:3:5) with a total flux of 40 mL/min was introduced into the reactor at an hourly space velocity of 2400 h-1 and all gases flow rates were controlled by mass flow controllers. The analysis of the outlet gases was performed online by a gas chromatograph (GC6890) equipped with a TCD detector and 2 m TDX01 column. The CO2 conversion and methane selectivity were calculated by the following equations: CO conversion % =

CH selectivity % = Where [CO2]in, was the

$%

× 100

× 100

(1) (2)

molar feed rate of reactant CO2, [CO2]out and [CH4]

represent the molar flow rate of the CO2 and CH4 product in the outlet. The reaction rates at a low reaction temperature (250 °C) over NixAl-MMO catalysts were evaluated. The reaction equation is described as equation (3). R=

'×( ) × '* × 10+


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where V is the Volume flow rate of reactant CO2 fed into the reaction(mL/min), X refers to the conversion of CO2 at 200 °C, m represents the quality of the catalyst (g), and Vm denotes the molar volume of CO2 (22.4 L/mol). 3. Results and discussion 3.1．X-ray diffraction of nickel based catalysts

Figure 1. (a) XRD patterns of the as-synthesized precursors (b) XRD patterns of the NixAl-MO and Ni/Al2O3 catalysts.

The XRD patterns of LDHs precursors with various Ni2+/Al3+ ratios are shown in Figure 1a. The XRD patterns showed characteristic peaks at 2θ = 11.7°, 23.6°, 35.2°, 39.7°, 46.3°, 61.3°, 62.7°, 66.7°, confirming the formation of LDHs on the precursors, indicating the formation of NixAl-LDHs. And the peaks at 11.7°, 23.6°, 35.2° could be indexed to (003), (006) and (012) of an LDH phase, respectively. With increasing of Ni2+/Al3+ ratio, the peaks were shifted slightly to lower angles due to increased Ni loading with large crystalline interplanar spacing and the lattice expansion. And at the same time the peak intensities were also increased which could be explained that the addition of Ni promoted the formation of LDHs phase. However, when the ratio of Ni2+/Al3+ ≥ 5, the additional phase was formed at 43.5°, 51.9°, indicating that the increase of Ni2+/Al3+ ratio destroyed the complete layered crystal structure and Ni2+/Al3+=5 is the transition state of pure LDHs phase. After treatment of the NixAl-LDH precursors at 500 °C, the Ni-based catalysts supported on Al2O3 were observed. As shown in Figure 1b, it could be noted that the XRD patterns of the catalysts displayed metallic NiO diffraction peaks at 37.2°, 44.5°, 62.9°, 75.3°, 79.4°. While no characteristic reflection of Al2O3 was found, due to the amorphous form of Al2O3 and the superposition with NiO peaks. The XRD patterns showed that the diffraction peaks became more intense and sharper by increasing the Ni2+/Al3+ ratio ACS Paragon Plus Environment


from 1/1 to 6/1. However, the samples with Ni2+/Al3+ ratio x ≥ 5 displayed the presence of splitting peaks at 43.3°, suggesting the slight lattice distortion and the crystal phase transition of the Ni-based catalyst. As a result, it could be concluded that crystal structure of Ni-based catalyst is significantly affected by Ni2+/Al3+ ratios. What is more, the diffraction peaks of NixAl-MO catalysts were similar to the Ni/Al2O3 catalyst which was synthesis by an impregnation method. It suggests that both of the methods could obtain nickel based catalyst supported on Al2O3. 3.2. N2 adsorption-desorption study

Figure 2. The N2 adsorption-desorption isotherms and distributions of pore size of NixAl-MO catalysts

Table 1. The BET surface areas, pore volume and mean pore size of the NixAl-MO catalysts

Figure 2 shows the N2 adsorption-desorption isotherms and pore size distribution curves of the catalysts. According to the IUPAC classification, the catalysts displayed type IV N2 adsorption-desorption isotherms with a H2 hysteresis loops, indicating the presence of mesoporous structure in the catalyst. The pore size distribution curves displayed that the samples had mesoporous structure with different range of pore size distribution and the Ni5Al-MO catalyst had a broader pore size distribution with an average value of about 26.03 nm. The specific surface areas, pore volumes and pore diameter of the catalysts are summarized in Table 1. It can be seen that the BET surface area increased firstly and then decreased after introduction of Ni. The decrease in the BET surface area was due to a large amount of nickel oxide nanoparticles blocked the pores. However, the pore size was slightly decreased with the increasing of Ni2+/Al3+ ratio and then showed increasing trend. With the increase of Ni loading, the pore volume


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had hardly changed. In general, the surface area and pore diameter had reverse trend, so the surface area increased and the pore diameter decreased. Furthermore, compared with the series of NixAl-MO catalysts, the Ni/Al2O3 catalyst had highest surface area (259.74 m2/g) and smallest pore diameter (7.217 nm). 3.3. TEM images

Figure 3-1. TEM images of (1) NiAl-MO, (2) Ni2Al-MO, (3) Ni3Al-MO, (4) Ni4Al-MO, (5) Ni5Al-MO, (6) Ni6Al-MO, (7) Ni/Al2O3 catalysts.

Figure 3-2. The TEM image of Ni5Al-MO catalyst and corresponding EDX elemental mapping. (A) the EDS layered image (B) the TEM image of Ni5Al-MO catalyst

The morphology of the catalysts was examined using TEM, as shown in Figure 3-1. It could be seen that the NiO nanoparticles were highly dispersed within the investigated catalysts. The TEM images displayed that all the metal oxides catalysts are porous lamellar hexagonal structure. With the increase of Ni2+/Al3+ ratio the laminated hexagonal structures of the catalyst tended to be incomplete and the edges of the hexagonal structures changed irregular and ambiguous due to the crystal phase transition and structure overlap phenomenon, which was in agreement with the XRD result. It was noted that with the increase of the Ni2+/Al3+ molar ratios from 1/1 to 6/1, the average size of NiO nanoparticles was almost unchanged (8.4 nm). In addition, for comparison, Ni particles supported on Al2O3 were also prepared by the impregnation method. Although the XRD pattern of Ni/Al2O3 catalyst was similar to NixAl-MMO samples, the TEM images varied widely. The TEM images of Ni/Al2O3 catalyst displayed that the metal nanoparticles have an irregular shape with uneven distribution and the NixAl-MO catalysts had relatively higher metal nanoparticles dispersion than Ni/Al2O3 catalyst. Figure 3-2 presented the energy-dispersive X-ray



(EDX) elemental mapping of Ni5Al-MO catalyst. It was observed that the Ni, Al and O elements were highly dispersed in Ni5Al-MO catalyst. Especially, the Ni active sites had been observably introduced into the Al2O3 metal oxides. 3.4. H2-TPR results

Figure 4. H2-TPR profiles of the NixAl-MO and Ni/Al2O3 catalysts.

The reducibility of the nickel-based catalyst was investigated by H2-TPR (Figure 4). For the NixAl-MO series catalysts, two reduction peaks with a broad temperature range from 150 to 800 °C were observed, suggesting that two different kinds of reduction species existed in the catalysts. The first reduction peak centered at around 210 °C was basically identical to the reduction of highly dispersive NiO species which were weakly interacted with the support, and the reduction temperature was lower than that of pure NiO particles (350 °C). Therefore, the material prepared by hydrothermal synthesis method, enhanced the reducibility of metal cation. As in the case of NixAl-MO catalysts, the reduction peak at a relatively high temperature around 580 °C was assigned to the reduction of NiO species which had a strong interaction with the support. With increasing of Ni2+/Al3+ ratio, the second reduction peak shifted to lower temperature and Ni5Al-MO catalyst had the most reducible species, indicating that a relatively high Ni2+/Al3+ ratio was beneficial to the formation of more readily reductive NiO species. The reduction peak of the Ni5Al-MO catalyst had the lowest temperature, which indicated that the metal oxide species of the catalyst had excellent activity and they were facile to reduction. For Ni/Al2O3 catalyst, only one unique reduction peak at a relatively high temperature (650 °C) is identified, indicating a relatively hard to reduce Ni2+ species entered in the Al2O3 structure. 3.5. CO2-TPD results


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Figure 5. CO2-TPD profiles of the NixAl-MO and Ni/Al2O3 catalysts

The basic strength and CO2 adsorption capacity of the NixAl-MO and Ni/Al2O3 catalysts were investigated by using CO2-TPD (Figure 5.). The profiles of NixAl-MO catalysts exhibited several CO2 desorbed peaks in the wide temperature range, implying that the basic sites with several strengths were widely distributed on the catalyst surface. For all the NixAl-MO catalysts two main CO2 desorption peaks were observed, a small desorption peak at about 90 °C and a broad one at about 180 °C. The former peak was assigned to CO2 desorption from weak basic sites, which is due to the unreduced metal sites on the catalyst support or the presence of a certain amount of hydroxyl groups on the metal oxides. While the latter one belonged to weak basicity attributing to the desorption of CO2 that adsorbed on the Ni particles.33,34 However, with the increase of Ni2+/Al3+ molar ratio, the NixAl-MO catalysts (x=2-5) displayed two medium temperature peak with fairly low density at approximately 285 °C and 340 °C, corresponding to medium basic sites. What is more, the NixAl-MO catalysts (x=2-5) exhibited a broad CO2 desorption peak with strong basic sites at about 545 °C. However, when the molar ratio increased to 6, partial medium and strong desorption peaks disappeared only two weak basic sites existed on the surface of Ni6Al-MO catalyst. Similarly, for NiAl-MO catalyst, only two weak basic peaks less than 200°C could be detected. Thus, with Ni2+/Al3+ molar ratio changed, the basic property of NixAl-MO catalysts had one transition state, which was according with the XRD result. The total amount of CO2 desorption decreased gradually with increased Ni content (table1). Thus, the amount of surface basic sites decreased correspondingly. According to previous report, the weak and medium basic sites are both favorable to the activation of CO2, while the strong basic sites on



catalysts cannot participate in the reaction.35 Therefore, from the CO2-TPD results, we can intuitively find out that the significant impact of Ni loading on CO2 desorption behavior. 3.6. Catalytic activity

Figure 6. (a) CO2 conversion and (b) CH4 selectivity for CO2 methanation at different reaction temperature over the as-prepared NixAl-MO, Ni/Al2O3 catalysts (GHSV: 2400 h−1, H2/CO2=4:1).

CO2 conversions and CH4 selectivity over NixAl-MO (x=1 to 6) catalysts along with reference Ni/Al2O3 were presented in Figure 6a. NiAl-MO presented a relatively poor activity of CO2 methanation at reaction temperature lower than 250 °C. However, the catalyst with higher Ni loadings exhibited enhanced activities compared to Ni/Al2O3 catalyst especially at low temperature. The CO2 conversion could reach to 89.4% over Ni5Al-MO catalyst with 99% selectivity to CH4 at reaction temperature of 250 °C. However, further increasing Ni loading to Ni2+/Al3+ =6 could lead to an obvious decrease in CO2 conversion, perhaps because the aggregation of surface metal particles36,37 and the transition point was consistent with the XRD and CO2-TPD results. Notably, the CO2 conversion increased with increasing reaction temperature and then decreased slightly at high temperature. This is because the CO2 methanation is an exothermic reaction, the reaction is controlled by the thermodynamic equilibrium and the CO2 can convert to CO due to the reverse water gas shift reaction at high temperature. The temperature values obtained at 50% CO2 conversion were 216 °C, 230 °C, 270 °C and 328 °C for Ni5Al-MO, Ni2Al-MO, NiAl-MO and Ni/Al2O3 catalyst, respectively. Compared to the catalytic activity of


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NixAl-MO catalysts, the Ni/Al2O3 catalyst was observed a poor CO2 conversion, implying that the homogeneous hydrothermal synthesis method improved the catalytic performance for CO2 methanation at low temperature. What is more, for Ni5Al-MO catalyst CO2 conversion reached the maximal value (89.4% at 250 °C) which is a remarkably superior activity for CO2 methanation to the catalysts reported previously, such as supported noble-metal and nickel based catalysts.38-40 Therefore, the Ni5Al-MO catalyst displayed apparently enhanced low-temperature catalytic performance for CO2 methanation. It can be seen from Figure 6b that the increasing Ni2+/Al3+ molar ratio had no effect on CH4 selectivity, while for Ni/Al2O3 catalyst the selectivity decreased slightly at high temperature. It indicated that the NixAl-MO catalysts synthesized by hydrothermal synthesis method had superior selectivity to CH4 than the Ni/Al2O3 catalyst prepared by impregnation method. According to CO2-TPD analysis, considering that Ni/Al2O3 catalyst had only two weak basic sites and the series of NixAl-MO catalysts possessed both the weak and moderate basic sites, the moderate basic sites would be one of the necessary properties of nickel-based catalyst for CO2 methanation. Among the researched catalysts for CO2 methanation, the Ni/Al2O3 catalyst had the highest surface area, but the inferior activity (Table. 1). It was indicate that the surface area was not a key to enhance catalyst activity. On the other hand, the facility in the reduction of active species in NixAl-MO catalysts should be one of the reasons for the high activity of the catalyst, which was observed in the H2-TPR analysis (Figure 4).

Figure 7. The reaction rate as a function of the Ni2+/Al3+ molar ratio for the NixAl-MO catalysts



(Reaction temperature: 200 °C, GHSV = 2400 h−1, CO2 conversion < 30%).

To further study the structure-activity relationship, the reaction rates of the Nix/Al-MO catalysts were investigated at low reaction temperature. Figure 7 shows the relationship between the reaction rate and the Ni2+/Al3+ molar ratio of the NixAl-MO catalysts. The results displayed that increasing the Ni2+/Al3+ molar ratio could improve the reaction rate, while an apparent decline was observed when the ratio was increased to 6. Noticeably, the Ni5Al-MMO catalyst possessed the maximum reaction rate, suggesting that the essential role of the cooperative catalysis between Ni and Al2O3 base sites for CO2 methanation, which determined the progress of CH4 formation at low temperature. Furthermore, the series NixAl-MO catalysts possessed Ni nanoparticles with similar surface structure and average size displayed rather different CH4 formation rate, indicating the basic property and reducing property played an important role in low-temperature methanation process. The adjustment of the Ni2+/Al3+ ratios is actually the adjustment of the reducing property as well as the cooperative effect between Ni and the base site, which enhanced low-temperature catalytic activity. 3.7. Catalytic stability

Figure 8. Durability test for CO2 methanation over Ni5Al-MO catalyst at 250 °C. Reaction conditions: CO2/H2 = 1:4, GHSV=2400 h-1.

Figure 9. Characterization for the spent Ni5Al-MO catalyst: (a) XRD, (b) TG/DTA

The durability test for the Ni5Al-MO catalyst was investigated at 250 °C and 2400 h-1 due to its relatively excellent catalytic activity for CO2 methanation. Figure 8


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shows the time courses of CO2 conversion over Ni5Al-MO catalyst. The catalyst had high activity and no obvious signs of catalyst deactivation during 120 h stability test and the selectivity kept an extremely high value of 100%. XRD and TG/DTA analysis was performed to obtain more insight into the physicochemical property of the spent catalysts during the 120 h stability test. Figure 9a shows the XRD patterns of the fresh Ni5Al-MO and spent catalysts, respectively. No deposited carbon diffraction peaks were confirmed to Ni5Al-MO catalyst from XRD analysis, indicating that the deposited carbon was amorphous. However, the intensity of the diffraction peaks slightly increased at 2θ = 44.3°, 51.75° and 75.3° over Ni5Al-MO catalyst. The Ni particle sizes of the catalysts before and after the stability test was 20.97 nm and 22.22 nm, respectively, which were calculated by Scherrer equation. It was suggested that the metal particles were partially sintered or aggregated as also confirmed by the increase in pore sizes. What is more, there is no significant change in the diffraction peak of the fresh and spent catalysts, indicating that the structure of the catalyst is stable and the catalyst has excellent anti-sintering performance. To study carbon deposition during the 120 h methanation reaction, TG analysis was performed in air. As shown in Figure 9b, the first weight decrease less than 250 °C was attributed to the elimination of physical adsorbed water and some absorbed CO2. The increase at 250-400 °C owing to the oxidation of metal nickel and then a slight weight loss was observed for the combustion of the carbon deposition. The results revealed that there was no significant deposited carbon on the spent catalyst and the catalyst had promising resistance to carbon deposition. The anti-deposition performance of Ni5Al-MO catalyst could be attributed to its moderate surface alkalinity according to CO2-TPD result. 3.8. Reaction Mechanism investigation of CO2 methanation



Figure 10. In situ FTIR spectra for CO2 methanation over (a) NiAL-MO and (b) Ni5AL-MO catalysts at various reaction temperature.

The in situ FTIR analysis was used to study the adsorption intermediates and reaction pathway during CO2 methanation process. The Ni5AL-MO and NiAL-MO catalysts were selected due to the two catalysts provided the apparently different catalytic activities. Figure10 shows the FTIR spectra in the region between 1000 cm-1 and 4000 cm-1 after passing H2 and CO2 over Ni5AL-MO and NiAL-MO catalyst at different temperature. Overall, the different nickel based catalysts have different FTIR spectra due to the different Ni2+/Al3+ molar ratios. The NiAL-MO catalyst exhibited one predominant band at around 1340 cm-1, which was attributed to monodentate formate species and the band did not exhibit significant variations throughout the reaction temperature as shown in Figure 10a. Thus, the monodentate formate species was stable and necessary during the methanation. The adsorption peak at 1620 cm-1 was assigned as hydrogen atom when the reaction temperature at 150 °C. As the temperature increased to 200 °C, the peak of hydrogen atom disappeared and another band was observed at around 1600 cm-1, assignable to bidentate formate species. However, by increasing the temperature to 300 °C, the formate species was replaced by carbonate species (1560 cm-1), thus, the formate species were quite active over NiAl-MO catalyst under this condition. Meanwhile, as the temperature increased from 100 °C to 300 °C, the intensity of methanol band at 1070 cm-1 decreased significantly, meaning that some of the methanol species converted to methane gas. Thus, the CO2 methanation over the NiAL-MO catalyst proceeds via the formation of formate and carbonate intermediate. In contrast, the surface intermediates of Ni5Al-MO catalyst displayed in Figure10b only involved hydrogen carbonate species (1530 cm-1 and 1690 cm-1)without any


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intermediate either carbonate or formate species. As the reaction temperature increased from 150 °C to 300 °C, the signal peak (1530 cm-1 and 1690 cm-1) of the reaction intermediates gradually increased, indicating that the increase of temperature provided more active bites and enhanced the reaction speed. What is more, a relatively high amount of methanol could be detected at 3630 cm-1 and the peak intensity decreased slightly. In the hydrogen atmosphere, carbonates can be hydrogenated into formate species and further produced methanol intermediates during CO2 methanation.18,41 It was suggested that the hydrogen carbonate was quite stable over Ni5Al-MO catalyst and formate species was readily convert to methanol in the methane production under this condition. The in situ FTIR spectra measurements demonstrated that the CO2 methanation over Ni5AL-MO and NiAL-MO catalysts via the formation of hydrogen carbonate species and formate species/carbonates, respectively. According to previous studies, base sites can activate stable CO2 to carbonate/hydrocarbonate species at appreciable rates and active metal provided reactive H-species.9,42 Different intermediates represent different rates of reaction, thus Ni5AL-MO catalyst had higher CO2 activation rate than NiAL-MO catalyst, which was achieved by its moderate active basic site density. And the basic sites could induce the optimal tradeoff between H2 activation and CO2 activation.

Figure 11. Schematic representation of CO2 methanation by the NixAL-MO catalysts

On the basis of the in situ FTIR analysis the existence of intermediates under reaction conditions are revealed. Based on the results, the Figure 11 represented the reaction mechanism that the CO2 methanation on the NixAL-MO catalysts. In the reduction process, H2 is dissociated by active metallic Ni. The dissociated hydrogen atom attacks Al-O bond, which can generate the surface hydroxyl. Then the activated CO2 will react with the surface hydroxyl and atomic hydrogen to produce bicarbonate ACS Paragon Plus Environment


species and formate species, respectively. And the bicarbonate species similarly combine to surface hydroxyl to form carbonate which can also convert to formate by react with hydrogen atom. At last the hydrogen will multi-step reaction with formate species to produce gas phase methane and water. The reaction intermediates changed from formate to methanol with the increase of reaction temperature. This phenomenon indicates that different intermediates require different activation temperature and contribute in different elementary steps.2 Raising the reaction temperature facilitates the existence of carbonate species for the NiAL-MO catalyst. It was also observed that with the bicarbonate species forming, the Ni5AL-MO catalyst had a higher CO2 conversion. 4. Conclusions The methanation of CO2 was performed over highly-dispersed Ni catalysts supported on metal oxides prepared by hydrothermal synthesis method with various Ni2+/Al3+ molar ratios. The highest catalytic activity with CO2 conversion of 89.4% and CH4 selectivity of 99% was achieved over Ni5Al-MO catalyst at low temperature of 250 °C. Based on the results of XRD and TG analysis, it can be concluded that Ni5Al-MO catalyst possessed a great resistance to carbon deposition and excellent stability. This material appeared to be significantly enhanced catalytic activity at low temperature, due to the basic property, readily reducible NiO species and the cooperation of metal nanoparticles and base site. However, the surface area was not a key to enhance catalyst activity. The adjustment of the Ni2+/Al3+ ratios is actually the adjustment of the reducing property as well as the base site. What is more, the reaction intermediate species formed on the surface of the catalyst also played a key role in catalytic activity of low temperature which was identified by the in situ FTIR analysis. Different intermediates represent different rates of reaction, Ni5AL-MO


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catalyst had higher CO2 activation rate than NiAL-MO catalyst due to its moderate active basic site density. Acknowledgement This work was supported by the National Science Fund for Excellent Young Scholars (No.21422607), NSFC-NRCT joint project (No.51661145012), and Key Research Program of Frontier Sciences, CAS (NO. QYZDB-SSW-SLH022).



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Table and Figure Captions Table 1. The BET surface areas, pore volume and mean pore size of the NixAl-MO catalysts Figure 1. (a) XRD patterns of the as-synthesized precursors (b) XRD patterns of the NixAl-MO and Ni/Al2O3 catalysts. Figure 2. The N2 adsorption-desorption isotherms and distributions of pore size of NixAl-MO catalysts Figure 3-1. TEM images of (1) NiAl-MO, (2) Ni2Al-MO, (3) Ni3Al-MO, (4) Ni4Al-MO, (5) Ni5Al-MO, (6) Ni6Al-MO, (7) Ni/Al2O3 catalysts. Figure 3-2. The TEM image of Ni5Al-MO catalyst and corresponding EDX elemental mapping. (A) the EDS layered image (B) the TEM image of Ni5Al-MO catalyst Figure 4. H2-TPR profiles of the NixAl-MO and Ni/Al2O3 catalysts. Figure 5. CO2-TPD profiles of the NixAl-MO and Ni/Al2O3 catalysts Figure 6. (a) CO2 conversion and (b) CH4 selectivity for CO2 methanation at different reaction temperature over the as-prepared NixAl-MO, Ni/Al2O3 catalysts (GHSV: 2400 h−1, H2/CO2=4:1). Figure 7. The reaction rate as a function of the Ni2+/Al3+ molar ratio for the NixAl-MO catalysts Reaction temperature: 200 °C, GHSV = 2400 h−1, CO2 conversion < 30%). Figure 8. Durability test for CO2 methanation over Ni5Al-MO catalyst at 250 °C. Reaction conditions: CO2/H2 = 1:4, GHSV=2400 h-1. Figure 9. Characterization for the spent Ni5Al-MO catalyst: (a) XRD, (b) TG/DTA Figure 10. In situ FTIR spectra for CO2 methanation over (a) NiAL-MO and (b) Ni5AL-MO catalysts at various reaction temperature. Figure 11. Schematic representation of CO2 methanation by the NixAL-MO catalysts



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Table 1. The BET surface areas, pore volume and mean pore size of the NixAl-MO catalysts

a

b

Catalyst

SBET (m2/g)a

NiAl-MO Ni2Al-MO Ni3Al-MO Ni4Al-MO Ni5Al-MO Ni6Al-MO Ni/Al2O3

126.302 158.044 181.691 147.940 135.970 92.906 259.740

Pore volume(cm3/g)a 0.667 0.709 0.658 0.580 0.642 0.542 0.469

Mean pore diameter (nm)a 21.358 18.277 14.438 15.807 19.174 24.073 7.217

nCO2(mmol g-1cat)b 2.125 1.373 1.116 1.087 0.823 0.531 0.485

Obtained from N2 adsorption-desorption Calculated according to CO2-TPD.

Figure 1. (a) XRD patterns of the as-synthesized precursors (b) XRD patterns of the NixAl-MO and Ni/Al2O3 catalysts.

Figure 2. The N2 adsorption-desorption isotherms and distributions of pore size of NixAl-MO catalysts


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2

1

6

5

4

3

7

Figure 3-1. TEM images of (1) NiAl-MO, (2) Ni2Al-MO, (3) Ni3Al-MO, (4) Ni4Al-MO, (5) Ni5Al-MO, (6) Ni6Al-MO, (7) Ni/Al2O3 catalysts.

Al

O

Ni

A

B

Figure 3-2. The TEM image of Ni5Al-MO catalyst and corresponding EDX elemental mapping. (A) the EDS layered image (B) the TEM image of Ni5Al-MO catalyst

Figure 4. H2-TPR profiles of the NixAl-MO and Ni/Al2O3 catalysts.



Figure 5. CO2-TPD profiles of the NixAl-MO and Ni/Al2O3 catalysts

Figure 6. (a) CO2 conversion and (b) CH4 selectivity for CO2 methanation at different reaction temperature over the as-prepared NixAl-MO, Ni/Al2O3 catalysts (GHSV: 2400 h−1, H2/CO2=4:1).

Figure 7. The reaction rate as a function of the Ni2+/Al3+ molar ratio for the NixAl-MO catalysts Reaction temperature: 200 °C,GHSV = 2400 h−1, CO2 conversion < 30%).


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Figure 8. Durability test for CO2 methanation over Ni5Al-MO catalyst at 250 °C. Reaction conditions: CO2/H2 = 1:4, GHSV=2400 h-1.

Figure 9. Characterization for the spent Ni5Al-MO catalyst: (a) XRD, (b) TG/DTA

Figure 10. In situ FTIR spectra for CO2 methanation over (a) NiAL-MO and (b) Ni5AL-MO catalysts at various reaction temperature.



Figure 11. Schematic representation of CO2 methanation by the NixAL-MO catalysts


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The following graphic will be used for the TOC:

Schematic representation of CO2 methanation by the NixAL-MO catalysts


Highly active Ni-based catalyst derived from ... - ACS Publications

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