CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

Available online at www.sciencedirect.com

ScienceDirect journal homepage: www.elsevier.com/locate/he

CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity Leilei Xu a,*, Fagen Wang b, Mindong Chen a,**, Dongyang Nie a, Xinbo Lian a, Zhenyu Lu a, Hanxiang Chen a, Kan Zhang a, Pengxiang Ge a a

Collaborative Innovation Center of the Atmospheric Environment and Equipment Technology, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, 210044, Nanjing, PR China b School of Chemistry and Chemical Engineering, Jiangsu University, 301 Xuefu Road, Zhenjiang, 212013, PR China

article info

abstract

Article history:

Ni based catalysts are usually used for catalyzing the CO2 methanation to produce syn-

Received 24 March 2017

thetic natural gas due to their low cost, though their catalytic activities cannot be com-

Received in revised form

parable with the noble metal counterparts. In order to address this challenge, a series of

2 May 2017

rare earth (La, Ce, Sm, and Pr) doped Ni based mesoporous materials had been facilely

Accepted 4 May 2017

fabricated by the one-pot evaporation induced self-assembly (EISA) strategy and directly

Available online xxx

employed as the catalysts for CO2 methanation. These mesoporous catalysts had been systematically characterized by means of X-ray diffraction, N2 physisorption, transmission

Keywords:

electron microscope, X-ray photoelectron spectroscopy, H2 temperature programmed

Rare earth

reduction, CO2 temperature programmed desorption, and so on. It was found that the Ni

CO2 activation

species were highly dispersed among the mesoporous framework and the strong metal-

Low-temperature activity

framework interaction had been formed. Thus, the thermal sintering of the metallic Ni

Ni catalyst

nanoparticles could be effectively suppressed under CO2 methanation conditions, prom-

CO2 methanation

ising these mesoporous catalysts with 50 h excellent catalytic stabilities without evident deactivation. Besides, the rare earth dopants could greatly increase the surface basicity of the catalysts and intensify the chemisorption the CO2. Further, the rare earth elements were also functioned as the electron modifiers, which was also helpful in activating the CO2 molecule. The apparent activation energies of CO2 could be obviously decreased by rare earth dopants. As a result, their low-temperature catalytic activity had been greatly intensified over these rare earth elements promoted catalysts. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (L. Xu), [email protected] (M. Chen). http://dx.doi.org/10.1016/j.ijhydene.2017.05.027 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

2

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

Introduction In recent decades, the utilization of carbon-rich fossil fuels, such as coal, crude oil, and natural gas, has been considered as the significant driving force of the advancement and prosperity of the global economy [1,2]. As a result, the CO2 concentration in the atmosphere has increased sharply, which causes the global warming and climate change owing to its greenhouse effect [3]. Hence, intensive efforts have been devoted to curbing the CO2 emission through various strategies, such as CO2 capture, separation, storage, and recycling utilization [4e9]. The recycling utilization of the CO2 through some chemical loops can be considered as a sustainable and promising route [10]. For CO2, it is a cheap, safe, and renewable carbon source, which has turned out to be an attractive C1 building block for the chemicals [2,11]. Up to now, various types of chemical transformation routes have been designed and proposed to utilize the CO2, such as the production of methane, methanol, and dimethyl ether (DME) via hydrogenation processes [2,12e14]. The methanation of CO2, also called the Sabatier reaction [15], is considered as the most advantageous reaction with respect to its thermodynamic feature, which can be conducted considerably faster than the reactions generating other hydrocarbons and alcohols [16e18]. It can theoretically realize the 100% conversion at low temperature under atmospheric pressure if highly efficient catalyst is successfully invented [14,17,19]. In 1990s, the Japanese scientists proposed the idea of global CO2 recycling, which consisted of the generation of electricity in deserts, the production of hydrogen and methane (CO2 methanation) on the coasts close to the deserts, and the utilization of CO2 in energy-consuming districts [18,20,21]. Specifically, the electricity was generated by solar cell in the deserts. Then, the electricity was employed for hydrogen production by electrolysis of seawater on the coasts close to the desserts and methane was formed by the hydrogenation of CO2 via methanation process. The methane was consumed as the fuel and the CO2 was recovered as well as recycled. In addition to this, the CO2 methanation can also be used to remove the trace amounts of CO2 in hydrogen feedstock for industrial ammonia synthesis [22,23]. Besides, CO2 methanation has potential utilization for providing the drinking water and fuels for the astronauts in the International Space Station and the future colonization of Mars by the recycling of CO2 from the breathing and the wasted H2 from the water electrolysis [10,23e26]. The CO2 methanation reaction is an exothermic process and the low reaction temperature favors the high CO2 conversion based on its thermodynamic characteristics [10,19]. However, the reduction of the fully oxidized carbon (þ4) to methane (4) is an eight-electron process with high kinetic barrier, which usually requires the efficient catalysts to realize high reaction rates and CH4 selectivity, especially at low temperature [2,9,10]. It has been confirmed that almost group VIII metals (Ru, Rh, Pd, Ni, Co, etc.) supported on various carriers (SiO2, Al2O3, ZrO2, TiO2, CeO2, etc.) are active toward CO2 methanation reaction [9,25,27e30]. The Ni based nonnoble metal based catalysts are preferable due to their low cost and availability [10,24]. However, the Ni-based catalysts

often exhibit poor catalytic activities at low temperature and insufficient stability at high temperature. Typically, the activation of CO2 at low temperature is still a big challenge due to its chemical inertness and thermodynamic stability [2]; the thermal sintering of metallic Ni active sites at high temperature easily takes place due to its low Tammann temperature (590  C, half of the melting point temperature), finally causing the rapid deactivation of the catalyst [31]. In the view of these challenges, enhancing the CO2 activation at low temperature and constructing strong metal (Ni)support interaction (SMSI) can be considered as the potential solutions [32,33]. Thus, various catalytic promotors, including alkaline-earth metals (Mg, Ca, etc.) [9,34e36] and rare earth metals (La, Ce, Sm, etc.) [8,18,37e42], have been added to enhance the activation of CO2 via modifying the surface basicity and metal-support interface. Compared with the alkaline-earth elements, the rare earth elements can not only act as the basic modifier but also regulate the electron property of the metallic active sites due to their unique d-orbit electron structures [8]. Zhi and Guo et al. reported the La-Ni/ SiC catalyst exhibited much better low-temperature catalytic activity and high CH4 selectivity than the pristine Ni/SiC catalyst with identical Ni loading [37,41]. They found that the La2O3 could improve the dispersion of the metallic Ni, intensify the CO2 chemisorption, and increase the d-electron density of the Ni [43], which would be in favor of the H2 dissociation and CO2 activation at low temperature. Cerium oxide (CeO2) is also widely employed as a structural and electronic promotor for Ni based catalysts by improving the Ni dispersion [44], tuning the metal-support interaction [45], and regulating the acidebase interaction through valence change between Ce3þ and Ce4þ [46]. Bian and Li et al. investigated the promoting effect of CeO2 over Ni/Al2O3 catalyst toward CO2 methanation [47]. They found that doping CeO2 can improve the Ni dispersion and surface basicity, which finally promoted the low-temperature activity. Samarium oxide (Sm2O3) is a basic metal oxide, which is usually used as the catalyst support and basic modifier [48]. Takano and Hashimoto et al. reported that the Ni-Zr-Sm catalyst exhibited much higher catalytic activity than Ni-Zr counterpart toward CO2 methanation [39]. They found that the presence of Sm3þ could partly substitute the Zr4þ in tetragonal ZrO2 lattice, whose oxygen vacancies might strongly interact with the oxygen atom in CO2 molecule and weaken the C]O bond strength. Thus, the hydrogenation of CO2 to CH4 over the Sm modified catalyst was finally enhanced. Besides, the praseodymium oxide (PrnO2n2) stands for a set of metal oxides as it possesses multiple meso-stable oxidation states, such as þ3 (Pr2O3), þ18/ 5 (Pr5O9), þ11/3 (Pr6O11), þ4 (PrO2), etc [49,50]. Similar to Ce cations, the valence alteration of Pr cations will also generate the oxygen vacancies, which will be beneficial to the activation of the CO2 molecule in CO2 methanation. Razali et al. once prepared a series of lanthanide doped nickel oxide based catalysts for CO2 methanation [51]. They found the Pr modified catalyst displayed the best catalytic performance by promoting the dispersion of Ni nanoparticles. Therefore, considering these merits, the rare earth oxides ought to be promising promotors for Ni based catalyst toward CO2 methanation. The construction of strong metal-support interactions (SMSI) is also an important concern for Ni based catalysts

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

because the thermal sintering of metallic Ni active sites can easily take place under CO2 methanation condition, where the catalyst bed temperature is usually higher than furnace temperature owing to the presence of hot-spot [52,53]. Thus, the Ni active sites are often confined in some particular structures, such as well-defined crystalline compound [54], rigid mesoporous framework [55], and coreeshell structure [56], where the Ni active sites can be effectively stabilized. Following this principle, we have already successfully prepared ordered mesoporous NiO-Al2O3 composite metal oxide with large surface area, big pore volume, and uniform mesoporous channel via one-pot evaporation induced selfassembly (EISA) strategy, where the Ni active sites are embedded among the ordered mesoporous Al2O3 matrix [57]. At a result, this catalyst performed much better catalytic activity and stability than both the Ni/Al2O3 supported catalyst and NiO-Al2O3 non-porous catalyst toward CO2 methanation by providing more accessible Ni active sites and stabilizing the catalysts via its confinement effect. However, its lowtemperature catalytic stability was not good enough and required further improvement. Herein, a series of rare earth elements (La, Ce, Sm, and Pr) doped mesoporous NiO-Al2O3 composite metal oxides were designed, fabricated, and employed as the catalysts for CO2 methanation. The incorporation of the rare earth elements could regulate the surface basicity and/or electron property, which can decrease the activation energy of the CO2. Thus, the rare earth doped catalysts displayed much better lowtemperature catalytic activity than the pristine NiO-Al2O3 catalyst. The catalysts were provided with excellent antisintering property owing to the formation of strong metalframework interaction. Therefore, all the mesoporous catalysts did not suffer evident deactivation after 50 h long time stability tests. The relationship between the catalytic performance and material structure was supported by various characterization techniques, which will be discussed in detail in the following text.

Experimental The preparation of the mesoporous rare earth elements doped NiO-Al2O3 catalysts The anhydrous ethanol (C2H5OH, Sinopharm Chemical Reagent Co. Ltd.), (EO)20(PO)70(EO)20 triblock copolymer (Pluronic P123, Mn ¼ 5800, SigmaeAldrich), nitric acid (HNO3, 67%, Sinopharm Chemical Reagent Co. Ltd.), aluminum isopropoxide (C9H21AlO3, 98þ%, SigmaeAldrich), nickel nitrate hexahydrate (Ni(NiO3)2,6H2O), SigmaeAldrich), lanthanum nitrate hexahydrate (La(NO3)3$6H2O, SigmaeAldrich), cerium nitrate hexahydrate (Ce(NO3)3,6H2O, SigmaeAldrich), samarium nitrate hexahydrate (Sm(NO3)3$6H2O, SigmaeAldrich), and praseodymium nitrate hexahydrate (Pr(NO3)3$6H2O, SigmaeAldrich) were employed as the solvent, structure directing agent, acid modifier, and precursors, respectively, which were directly used without further purification. Rare earth elements (denoted as Re, Re ¼ La, Ce, Sm, and Pr) doped mesoporous NiO-Al2O3 composite oxides were synthesized by one-pot evaporation induced self-assembly (EISA)

3

method according to the literatures reported elsewhere [58,59]. Based on the previously reported literatures, the loading amounts of the Ni active site and the doped rare earth oxides were usually controlled in wide ranges of 10.0e50.0 wt % and 1.0e16.0 wt%, respectively [8,39,41,60e63]. Therefore, the molar ratio between Ni and Al (nNi/nAl  100%) and the molar ratio between rare earth element and Al (nRe/ nAl  100%) in this research were fixed at 10% (ca. 9.5e10.3 wt% for metallic Ni) and 3% (ca. 8.3 wt% in oxide forms) for these materials, respectively. These mesoporous materials were denoted as MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr), where “MA”, “10”, and “3” stood for the “mesoporous alumina matrix”, “molar ratio of Ni/Al”, and “molar ratio of Re/Al”, respectively. For the specific synthesis procedure, 1.0 g P123 was dissolved in 20.0 mL anhydrous ethanol with vigorous agitation. Then, 1.6 mL nitric acid, 10 mmol aluminum isopropoxide, 1 mmol nickel nitrate hexahydrate, and 0.3 mmol rare earth element nitrate hexahydrate were sequentially added into the above P123-ethanol solution under vigorous agitation condition. After another 5 h vigorous agitation, the transparent solution was transferred into a 60  C convection oven with low relative humidity (<50%) to conduct the EISA process for 48 h. The xerogels in light green or yellowegreen colors were obtained, which were further calcined at 700  C for 5 h with 1  C/min ramping rate under static air atmosphere. Finally, the fresh MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) mesoporous composite metal oxide materials were obtained, which would be investigated as the catalysts for CO2 methanation. In order to investigate the role of rare earth dopants, the pristine mesoporous NiO-Al2O3 composite oxide material without rare earth modification was also fabricated by onepot EISA method. The fresh sample with identical Ni/Al ratio (10%) was calcined at 700  C. It was denoted as MA-10Ni, where “MA” and “10” also stood for the “mesoporous alumina matrix” and “Ni/Al molar ratio”, respectively.

Catalyst characterizations Small-angle and wide-angle powder X-ray diffraction (XRD) measurements were carried out by using an X'Pert Pro multipurpose diffractometer (PANalytical, Inc.) with Nifiltered Cu Ka radiation (0.15,046 nm) at room temperature   in the range of 0.5 e5.0 and 10.0 e80.0 , respectively. The XRD patterns were recorded under specific operation conditions: 40 kV voltage, 40 mA current setting, 0.02 step size, and 4 s count time. The crystalline phases of the XRD peaks were carefully assigned according to the Jade 6.5 software, where the PDF numbers of the standard JCPDS cards for the possible compounds, such as g-Al2O3 (PDF-#-10-0425), metallic Ni (PDF#-45-1027), etc., could be found. The nitrogen adsorptionedesorption measurements were conducted on a Quantachrome NOVA 2200e analyzer at 196  C. Prior to the tests, the samples ought to be degassed at 200  C for 4 h. The specific surface areas were calculated based on the BrunauereEmmetteTeller (BET) theory in the relative pressure range of 0.05e0.3; the pore volumes were calculated from the adsorption isotherm based on BarretteJoynereHalenda (BJH) theory; pore size distributions were calculated using adsorption branches of the isotherms according to BJH method.

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

4

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

Transmission electron microscopy (TEM), selected area electron diffraction (SAED), and energy-dispersive spectroscopy (EDS) measurements were carried out on the JEOL 2010F under a working voltage of 200 kV. Prior to analysis, the samples ought to be well dispersed in anhydrous ethanol with the assistance of ultrasonic method before loading on the carbon coated copper grids. Then, they were dried in a vacuum oven at 40  C for 30 min to remove the ethanol solvent. X-ray photoelectron spectroscopy (XPS) analyses of the catalysts were carried out on a VG ESCALAB 210 (Thermo Scientific) spectrometer. The fresh catalyst powder was loaded on sample holder with the conductive adhesive tape. The binding energies were calibrated using the C 1s line at 284.8 eV as the reference. The inductive coupled plasma atomic emission spectrometer (ICP-AES) was determined on the Optima 7300DV (Perkin Elmer) to obtain the accurate Ni/Al and Re/Al ratios. Prior to analysis, 0.05 g sample ought to be completely dissolved in the mixed acid solution (0.5 mL 37.5 wt% HCl þ 0.5 mL 68.0 wt% HNO3 þ 0.25 mL 40.0 wt% HF þ 0.2 g H3BO3 þ 50.0 g H2O) at ambient temperature with the assistance of ultrasonic treatment. H2 temperature-programmed reduction (H2-TPR) profiles were recorded by employing the Quantachrome ChemBET Pulsar TPR/TPD analyzer with the diluted hydrogen as reducing agent. Specifically, the sample (100 mg) was loaded in a U-shaped quartz reactor with the quartz wool as the holder. Before the H2-TPR measurement, the sample should be pretreated at 300  C for 1 h in He (50 mL/min) stream to remove the adsorbed moisture and impurities. The H2-TPR measurement was carried out under 5 vol% H2e95 vol% He (80 mL/min) stream with a heating rate of 10  C/min to 1100  C and the consumption of the hydrogen was recorded by a thermal conductivity detector (TCD). CO2 temperature-programmed desorption (CO2-TPD) measurements were carried out on the Micromeritics AutoChem II 2920 Chemisorption analyzer. The sample (100 mg) was purged by a He stream (50 mL/min) at 300  C for 1 h to clean the surface of the sample. After being cooled down to 25  C, the pretreated sample was exposed to CO2 atmosphere (50 mL/min) for 1 h to make the sample surface saturated with the chemisorbed CO2. Then, the sample was purged with He stream to remove the physically absorbed CO2 until the baseline was steady. Finally, the CO2-TPD was carried out with a heating rate of 20  C/min to 900  C under He stream (50 mL/min) and the amount of CO2 desorption was also recorded by the TCD.

Catalyst evaluation The CO2 methanation reaction was carried out under atmospheric pressure in a continuous flow fixed-bed quartz tube reactor (i.d. 10 mm), which was clearly depicted in Scheme 1. Specifically, the gas flows of the H2 and CO2 were separately controlled by two mass flow controllers. The reaction temperature was monitored and controlled by a thermocouple located at the center of the catalyst bed. Prior to the regular reaction, the catalyst (0.1 g) loaded at the center of the reactor was pretreated with H2/N2 (20/10 mL/min) mixed flow at the temperature of 800  C for 2 h with 1.5  C/min heating rate from

the room temperature. After reduction, the catalyst bed was purged with N2 to remove the chemisorbed H2 until the reactor was cooled down to 200  C. Then, the feed gases with a volume ratio of CO2/H2 ¼ 1/4 were directly introduced into the reactor without any dilution at a gas hourly space velocity (GHSV) of 15,000 mL/(g h). The catalytic performance over each catalyst was investigated in a desired reaction temperature range from 200 to 450  C at the interval of 50  C. 50 h labscale stability tests were conducted at 400  C with the GHSV of 15,000 mL/(g h) under atmospheric pressure. Before being introduced into the on-line GC system, the effluent product gases should flow through a cold trap to remove the generated water and mix with the N2 (5 mL/min) as an internal standard gas. Then, the mixture of product gases were injected via the online automatic six-port valve injectors and analyzed by the Agilent 7890B GC machine equipped with a packed column (TDX-01) for TCD and an alumina capillary column for FID. However, there was no C2þ hydrocarbon detected by the FID detector. Both the thermal conductivity conductor (TCD) and flame ion detector (FID) had been employed to detect the possible reaction products during the CO2 methanation. The profile peaks related with H2, N2 (internal standard), CO, CH4 could be observed in TCD signal; only CH4 peak could be observed in FID signal. Therefore, only CO byproduct could be detected during the CO2 methanation over MA-10Ni and MA10Ni3Ln catalysts. As a result, the CO2 conversion (abbreviated as CCO2 ) and CH4 selectivity (denoted as SCH4 ) could be calculated based on the following formulas. CCO2 ¼ FCO2 ; inlet  FCO2 ; outlet


 FCO2 ; inlet  100%

 SCH4 ¼ FCH4 ; outlet ðFCH4 ; outlet þ FCO; outlet Þ  100%

(1) (2)

In all formulas, the Fx, inlet and Fx, outlet represented the flow rate of the x species, which flowed into and out of the reactor, respectively.

Results and discussion Characterizations of the fresh catalysts XRD analysis Fig. 1(A) and (B) exhibited the small-angle (0.5-5 ) and wideangle (20e80 ) XRD patterns of the fresh MA-10Ni and MA10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts, respectively. It could be observed in Fig. 1(A) that the MA-10Ni without the doping of rare earth element showed a sharp peak at 0.8 and another broad peak around 1.4 in the small-angle range, which could be indexed as the (1 0 0) and (1 1 0) reflections, respectively. This indicated that the long-range ordered mesoporous channels with two dimensional p6mm hexagonal symmetry existed among the MA-10Ni sample [58]. However, in case of the MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) samples, they displayed much lower diffraction peak intensity than the MA-10Ni in the small-angle range, implying that the longrange orderliness of the mesoporous channels had been distorted after doping the rare earth elements. The reason for this phenomenon might be derived from the significant difference between the radiuses of Ni2þ (72 pm) and those of the

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

5

Scheme 1 e Schematic diagram of the catalyst evaluation system.

rare earth cations (La3þ 106.1 pm, Ce3þ 103.4 pm, Sm3þ 96.4 pm, and Pr3þ 101.3 pm). As a result, they cannot be well compatible with each other during the period of one-pot EISA, which might cause the distortion of the ordered mesoporous channels, especially after calcination at high temperature (700  C). Fig. 1(B) showed the corresponding wide-angle XRD patterns of these mesoporous catalysts. It could be observed that all of them displayed no obvious diffraction peak related with the crystalline phases of Al2O3, NiAl2O4, NiO, and rare earth oxides. One possible reason for this phenomenon was that all these species were in the amorphous state; another possible reason was that the crystalline phases related with the Ni species and rare earth elements were highly dispersed among the amorphous alumina mesoporous framework owing to the one-pot fabrication strategy.

N2 adsorptionedesorption analysis

Fig. 1 e (A) Small-angle and (B) wide-angle X-ray diffraction patterns of the fresh MA-10Ni and MA-10Ni3Re catalysts.

The isotherms and pore size distribution curves of the fresh MA-10Ni and MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts were showed in Fig. 2. As displayed in Fig. 2(I), all the isotherms were attributed to the IV type with H1-shaped hysteresis loops, suggesting that the mesoporous structures with facile pore connectivity had been well developed among these materials [64,65]. The rare earth element-free MA-10Ni sample displayed steep hysteresis loop with wide and parallel adsorptionedesorption brunches, which suggested the presence of the cylindrical mesopores with uniform pore diameter. In the case of the MA-10Ni3Re modified by rare earth elements, they still performed parallel adsorptionedesorption brunches, indicating their narrow pore size distributions. However, compared with MA-10Ni, the distance between adsorptionedesorption brunches was a bit narrower, suggesting that their pore volumes suffered some decline after the incorporation of these rare earth elements. Their corresponding pore size distribution curves were displayed in

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

6

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

Fig. 2 e Nitrogen adsorptionedesorption isotherms (column I) and corresponding pore size distribution curves (column II) of the fresh MA-10Ni and MA-10Ni3Re catalysts: (A) MA-10Ni; (B) MA-10Ni3La; (C) MA-10Ni3Ce; (D) MA-10Ni3Sm; (E) MA10Ni3Pr. Fig. 2(II). As observed, all the catalysts displayed greatly narrow pore size distributions in the range of 9.5e13.2 nm, which were located in the mesoporous scope (2.0e50.0 nm) according to its definition by IUPAC [64]. However, it was of great

interest to find that the introduction of the rare earth elements caused the slight enlargement of the pore diameters. Specifically, the samples modified with La, Ce, and Sm displayed pore diameter around 11.0 nm and the sample

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

7

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

process, which would influence the subsequent hydrolysis, condensation, and self-assembly stages. As a result, the ordered cylindrical mesoporous channels were distorted into the worm-like channels. Similar conditions had been also encountered in the previous literatures, where the EISA process could be substantially affected by the nature of the doped metal cations [66e68]. The SAED images (insets of Fig. 3 (a, d, g, j, m) of the all the fresh catalysts showed unclear diffraction rings, indicating the poor crystallinity of the mesoporous framework. Besides, the EDS profiles of fresh MA-10Ni and MA-10Ni3Re in Fig. 3 (c, f, i, l, o) exhibited featured peaks of Ni, Al, O, and/or Re (Re ¼ La, Ce, Sm, Pr) elements. This meant that the Ni active sites and rare earth modifiers had been successfully introduced into these mesoporous catalysts. The values of Ni/Al and Re/Al molar ratios based on the ICP-AES analyses were summarized in Table 1. It could be found that the actual Ni/Al and Re/Al molar ratios were very close to their respective theoretical values.

modified with Pr exhibited the pore diameter around 13.2 nm, which were a bit higher that of MA-10Ni (9.5 nm). Therefore, the presence of the rare earth elements could regulate the pore diameter by participating in the self-assembly process. Besides, their other structural properties, such as specific surface areas and pore volumes, were summarized in Table 1. The MA-10Ni still possessed high surface area (233.0 m2/g), big pore volume (0.42 cm3/g), and narrow pore size distribution (9.5 nm) after high temperature calcination at 700  C. As a comparison, the rare earth elements doped MA-10Ni3Re displayed a bit worse structural properties than the pristine MA10Ni, which might be caused by the incompatibility between the Ni2þ and rare earth cations because of the substantial difference in their cation diameters. Nevertheless, it was undeniable that the P123 template still played a critical role in constructing the mesoporous structure of MA-10Ni3Re materials.

TEM, SAED, and EDS analyses XPS analysis

Fig. 3 showed the TEM, SAED images and EDS profiles of the fresh MA-10Ni and MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts. For the MA-10Ni without rare earth modification, the alignment cylindrical pores viewed along [1 1 0] and [0 0 1] directions could be observed in Fig. 3 (a) and (b), respectively, suggesting the presence of long-range ordered mesostructure. However, in the case of the MA-10Ni3Re with rare earth modification, only worm-like mesoporous channels could be found. This indicated that the presence of rare earth element cations (La3þ, Ce3þ, Sm3þ, and Prxþ) could affect the formation of liquid crystal phase from the P123 template during the EISA

The XPS characterizations were carried out to investigate the surface chemical state of the fresh MA-10Ni and MA-10Ni3Re catalysts. As shown in Fig. 4(A), each sample regardless of the modification of rare earth element displayed the Ni 2p XPS profile with a 2p3/2 peak located at ca. 856.0 eV, which was together with an accompanying satellite peak at 862.0 eV. These are the featured peak for the Ni2þ ion in the form of NiAl2O4 spinel according to the pioneering literatures [68e70]. For the Ni species, there would be no difference between the “surface” and “bulk” Ni2þ in chemical coordination state due

Table 1 e Textual properties of the fresh, as-reduced, and 50 h spent MA-10Ni and MA-10Ni3Re catalysts. Samples

SBETa (m2/ g)

VBJHb (cm3/ g)

APDc (nm)

TRNi/Alf (%)

ARNi/Alg (%)

TRRe/Alh (%)

ARRe/Ali (%)

Eaj (kJ/mol)

Isotherm type

MA-10Ni MA-10Ni3La MA-10Ni3Ce MA-10Ni3Sm MA-10Ni3Pr ARd-MA-10Ni ARd-MA-10Ni3La ARd-MA-10Ni3Ce ARd-MA10Ni3Sm ARd-MA-10Ni3Pr SPe-MA-10Ni SPe-MA-10Ni3La SPe-MA-10Ni3Ce SPe-MA-10Ni3Sm SPe-MA-10Ni3Pr

233.0 82.8 86.2 94.3 68.9 148.9 76.0 82.8 83.4

0.42 0.25 0.26 0.25 0.21 0.30 0.23 0.24 0.23

9.5 11.1 11.1 11.2 13.2 8.6 11.0 9.5 10.0

10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0 10.0

9.70 9.27 9.56 9.58 9.57 e e e e

e 3.0 3.0 3.0 3.0 e 3.0 3.0 3.0

e 3.02 2.94 2.83 2.76 e e e e

75.2 66.6 68.6 67.5 64.2 e e e e

IV H1 IV H1 IV H1 IV H1 IV H1 IV H1 IV H1 IV H1 IV H1

66.1 139.0 74.0 76.7 82.3 63.3

0.20 0.27 0.23 0.23 0.22 0.20

12.3 8.5 11.0 9.6 9.5 12.2

10.0 10.0 10.0 10.0 10.0 10.0

e e e e e e

3.0 e 3.0 3.0 3.0 3.0

e e e e e e

e e e e e e

IV H1 IV H1 IV H1 IV H1 IV H1 IV H1

a b c d e f g h i j

SBET stands for the specific area calculated based on BrunauereEmmetteTeller theory. VBJH stands for the pore volume calculated based on BarretteJoynereHalenda theory. APD stands for average pore diameter. AR stands for as-reduced catalyst. SP stands for 50 h spent catalyst. TRNi/Al stands for the theoretical molar percent ratio between Ni and Al. ARNi/Al stands for the actual molar percent ratio between Ni and Al. TRRe/Al stands for the theoretical molar percent ratio between rare earth element (La, Ce, Sm, and Pr) and Al. ARRe/Al stands for the actual molar percent ratio between rare earth element and Al. Ea stands for the apparent activation of CO2.

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

8

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

Fig. 3 e TEM, SAED, and EDS measurements of the fresh MA-10Ni and MA-10Ni3Re catalysts: (a, b, c) MA-10Ni; (d, e, f) MA10Ni3La; (g, h, i) MA-10Ni3Ce; (j, k, l) OMA-10Ni3Sm; (m, n, o) OMA-10Ni3Pr.

to the unique advantage of the one-pot fabrication strategy. It was supposed that the Ni species were homogenously embedded among the mesoporous Al2O3 matrix. The small shoulder peak at 851.9 eV over the MA-10Ni3La might be caused the overlapping of La 3d5/2 peak with Ni 2p3/2 peak according to previous reports [41,71]. The 3d XPS profiles of the rare earth elements (La, Ce, Sm, and Pr) were exhibited in Fig. 4(B). As shown in Fig. 4 (B-I), the position of the La 3d5/2 peak is located at ca. 835.5 eV with a satellite peak at ca. 839.0 eV. The DE between them is about 3.5 eV, which was the characteristic feature of the La3þ in the state of La2(CO3)3 [72]. The formation of the surface La2(CO3)3 species could be attributed to the interaction between La2O3

and CO2 when the MA-10Ni3La catalyst was exposed to the atmosphere. Fig. 4 (B-II) showed the Ce 3d XPS profile of MA10Ni3Ce, displaying 3d5/2 peak at 882.8 eV and 3d3/2 peak at 901.1 eV, respectively. This indicated that the Ce species in the MA-10Ni3Ce catalyst were in the form of Ce4þ [73]. The Sm 3d XPS profiles in Fig. 4 (B-III) exhibited the 3d5/2 peak at 1083.6 eV and 3d3/2 peak at 1110.8 eV, respectively, indicating that the Sm element were presented as Sm3þ in MA-10Ni3Sm catalyst [74]. As observed in Fig. 4 (B-IV), the 3d XPS profile of Pr species in MA-10Ni3Pr displayed 3d5/2 peak at ca. 934.2 eV, which was different with the 3d5/2 peak in the form of PrO2 (935.0 eV) and Pr2O3 (932.9 eV) according to the pioneering literatures [73,74]. Thus, it was supposed that the valence

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

9

Fig. 4 e (A) Ni 2p and (B) Re 3d [Re ¼ (I) La, (II) Ce, (III) Sm, and (IV) Pr] XPS profiles of the fresh MA-10Ni and MA-10Ni3Re catalysts.

state of Pr cation is situated between þ3 and þ 4 in the form of PrOx (1.5 < x < 2.0), which might be the mixture of PrO2 and Pr2O3 oxides. The alteration of the valence states could generate the oxygen vacancies, which would contribute to the activation of the C]O bond of CO2 in methanation process.

H2-TPR and CO2-TPD analysis Fig. 5(A) showed the H2-TPR profiles of the fresh MA-10Ni and MA-10Ni3Re catalysts calcined at 700  C. For all the samples, there was no reduction peak observed at the temperature below 400  C, which was indicative of the absence of the disassociated NiO weakly bonded with the mesoporous framework [75e77]. Specifically, each sample only exhibited one

pronounced reduction peak at high temperature (750e830  C), indicating that the strong interaction between Ni species and mesoporous framework had been formed. The only one reduction peak in H2-TPR profile suggested the homogeneity of Ni species among the whole mesoporous framework due to their one-pot fabrication strategy. This phenomenon had been also observed in other one-pot prepared materials, such as nickel aluminate mesoporous materials, Ni based hydrotalcite-like compounds, etc., according to the previous reports [67,78e80]. Therefore, it was supposed that the Ni species were homogeneously embedded among the mesoporous Al2O3 matrix in the form of NiAl2O4 spinel phase according to the XPS analyses in Fig. 4(A), which only could be

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

10

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

temperature than monodentate CO2 [83]. Besides, compared with the MA-10Ni, the rare earth promoted MA-10Ni3Re samples exhibited much stronger CO2 desorption peaks, demonstrating that the incorporation of rare earth elements could greatly increase the number of surface basic sites. These results were well consistent with the pioneering reports, which also demonstrated the promoting roles of rare earth elements (La, Ce, Sm, and Pr) in enhancing the surface basicity [84e88]. The intensified surface basicity by rare earth elements would contribute to the activation of CO2 at low temperature during the methanation process.

Catalytic performances toward CO2 methanation The promoting effect of rare earth doping on catalytic activity The effect of the rare earth elements on the catalytic activity toward CO2 methanation had been investigated over MA-10Ni and MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts under specific conditions: H2/CO2 ¼ 4, GHSV ¼ 15,000 mL/(g h), 200e450  C, 1 atm. The results were displayed in Fig. 6. As can be observed in Fig. 6(A), with the increase of the reaction temperature from 200 to 400  C, the CO2 conversions rapidly increased and reached the maximum values at 400  C;

Fig. 5 e (A) H2-TPR and (B) CO2-TPD profiles of the fresh MA10Ni and MA-10Ni3Re catalysts.

reduced at high temperature (ca. 800  C) [67,77,81]. This might be the reason why the reduction peaks were located in the high temperature region. Moreover, compared with the pristine MA-10Ni, the reduction peaks of rare earth elements doped MA-10Ni3Re materials shifted to lower temperature regions. This suggested that the presence of the rare earth elements could promote the reducibility of the Ni2þ species in MA-10Ni3Re. The CO2-TPD was carried out to investigate the surface basicity of the fresh MA-10Ni and MA-10Ni3Re and their profiles were summarized in Fig. 5(B). The CO2 desorption peaks could be observed at two temperatures ca. 83.2  C and 450  C. Usually, the CO2 adsorbed on the weak basic site was desorbed at low temperatures; whereas that adsorbed on the strong basic site would be desorbed at higher temperatures. From the literatures [82,83], the low-temperature peaks might be attributed to the weak basic sites in the form of monodentate CO2 absorbed on the catalyst surface. The peaks with the hightemperature desorption might be assigned to the medium and/or strong basic sites in the form of bidentate CO2, which tended to be strongly bonded with the oxide surface via two lattice oxygen ions and exhibited much higher desorption

Fig. 6 e The curves of the (A) CH4 conversion and (B) CH4 selectivity versus reaction temperature over MA-10Ni and MA-10Ni3Re catalysts; reaction condition: H2/CO2 ¼ 4, GHSV ¼ 15,000 mL/(g h), 1 atm.

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

whereas, further increasing the temperature up to 450  C caused the decrease of the CO2 conversions. Moreover, it was worth noting that the trends of the CO2 conversion curves over these mesoporous catalysts were significantly different with that of the curve of equilibrium CO2 conversion, which gradually declined with the increase of the temperature due to methanation the thermodynamic feature of CO2 (CO2(g) þ 4H2(g) ¼ CH4(g) þ 2H2O(g), DG ¼ 173.1 þ 0.1983T kJ/ mol) [10,19]. For Gibbs free energy DG < 0, it was favorable for the generation of CH4 in the viewpoint of chemical equilibrium (ln K ¼ DG/RT), where the value of equilibrium constant K was positive. However, with the increase of temperature, the value of DG increased, which made the reaction shift towards the reactants. This accounted for the decrease of the CO2 equilibrium conversion at high temperature. The reason for the poor catalytic activity at low temperature was that the CO2 methanation was an eight-electron process for fully reducing the CO2 (þ4) into CH4 (4), which usually had great kinetic barrier to activate the stable CO2 molecule and required the efficient catalyst to decrease the activation energy [10]. Besides, it was noticeable in Fig. 6(A) that the rare earth elements doped MA-10Ni3Re catalysts displayed higher CO2 conversions than pristine MA-10Ni counterpart, especially at low temperature. For example, the CO2 conversions at 250  C over MA-10Ni3La (19.8%), MA-10Ni3Ce (14.5%), MA10Ni3Sm (16.1%), and MA-10Ni3Pr (21.8%) were nearly two or three times higher than that over MA-10Ni (7.3%). Thus, the rare earth elements played important roles in promoting the catalytic activity. The effect of the rare earth promotors on CH4 selectivity at different temperatures over MA-10Ni and MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts were reflected in Fig. 6(B). It could be observed that the equilibrium CH4 selectivity gradually decreased with the increase of the reaction temperature due to the reverse water-gas shift (RWGS) side reaction, which would generate the CO gas and decrease the CH4 selectivity [89,90]. As for the practical catalysts, their values of CH4 selectivity were a bit lower than the equilibrium value. Besides, it was noticeable that the MA-10Ni3Re catalysts promoted by rare earth elements performed higher CH4 selectivity than MA-10Ni. This indicated that the rare earth elements also displayed promoting roles in enhancing the CH4 selectivity. In order to intuitively illustrate the promoting effect of the rare earth dopants on the catalytic activity at different temperatures, the K value was defined, where K ¼ C/Co, C was the CO2 conversion of the MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalyst, and Co was the corresponding CO2 conversion of the pristine MA-10Ni catalyst at identical temperature. The K values (KLa, KCe, KSm, and KPr) versus reaction temperature over different MA-10Ni3Re catalysts were summarized in Fig. 7(A). As observed, with the increase of the reaction temperature from 200 to 450  C, the K values over these catalysts gradually decreased and approached 1.0, suggesting that the gap between C and Co gradually became smaller. The roles of rare earth dopants in promoting the catalytic activity were more effective at low temperature. This was because the CO2 methanation at low temperature was a kinetically determined process [10,89]. The presence of rare earth elements remarkably boosted the CO2 activation capacity in low temperature

11

Fig. 7 e (A) The curves of the K values versus reaction temperature over MA-10Ni3Re catalysts; (B) Arrhenius plots for CO2 reaction rate over MA-10Ni and MA-10Ni3Re catalysts; reaction condition: H2/CO2 ¼ 4, GHSV ¼ 15,000 mL/(g·h), 1 atm.

region. As a result, the CO2 activation energies over rare earth doped catalysts could be finally decreased. Besides, it was worth noting that the K values of rare earth doped catalysts were also different with each other and their values followed the below sequence: KPr > KLa > KSm > KCe. Therefore, according to their K values, the promoting abilities of the lowtemperature catalytic activities for these rare earth elements could be summarized as follows: Pr > La > Sm > Ce. The role of the rare earth elements in promoting the lowtemperature catalytic activities over MA-10Ni3Re catalysts toward CO2 methanation was further confirmed by the kinetic study. The Arrhenius plots of the MA-10Ni3Re and MA-10Ni catalysts were displayed in Fig. 7(B) and their corresponding apparent activation energies of CO2 were summarized in Table 1. As can be seen in Fig. 7(B), the slope value of the MA10Ni plot was bigger than those of the MA-10Ni3Re catalysts promoted by rare earth elements, suggesting its larger apparent activation energy value. Besides, it could be observed in Table 1 that the Ea values over MA-10Ni and MA-10Ni3Re catalysts followed the sequence: Eablank > EaCe > EaSm > EaLa > EaPr. The kinetic results

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

12

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

suggested that the presence of the rare earth elements played positive roles in decreasing the activation energy of the CO2 in methanation process, accounting for their enhanced lowtemperature catalytic activities. It was necessary to illuminate the specific roles of rare earth elements in lowering the activation of CO2 molecule during the methanation process. In the case of La, it was believed that it had dual roles of activating CO2 according to pioneering literature [41]. On the one hand, the basic La2O3 favored the chemisorption and dissociation of CO2, which had been confirmed by the CO2-TPD analysis. On the other hand, the La2O3 could increase the d-electron density of the surface metallic Ni atoms according to the previous reports [41,43,91], which would favor the activation of C]O bond in CO2 by promoting the d-p feedback bond. As a result, the C]O bond became weak and the NieC bond was finally formed, accounting for its excellent low-temperature catalytic activity. For the MA-10Ni3Ce catalyst, except for the surface basicity, its enhanced catalytic activity also might be attributed to the redox property of the Ce cations (Ce4þ and Ce3þ). Specifically, the some Ce4þ could be reduced into Ce3þ and the oxygen vacancies would be subsequently generated during the in situ reduction process [92]. The oxygen vacancies could catalyze the rate-determining step (CO2 activation) at relatively lower activation temperature according to the pioneering reports [27,93]. Similar to the La3þ in MA-10Ni3La, the valence of Sm element in MA-10Ni3Sm was also in þ3 based on the above XPS characterization. The excellent low-temperature catalytic activity of MA-10Ni3Sm was mainly due to its surface basicity. Besides, it was reported the presence of the Sm3þ could also generate the oxygen vacancies by substitution in Ni-Sm-Zr system, which had strong tendency to interact with oxygen in CO2 and weakened the C]O bond [39]. This was also greatly favorable in the enhancement of hydrogenation of CO2 to form CH4 via methanation process. As for the Pr element in MA-10Ni3Pr, it was adjacent to the Ce in the periodic table of elements, which endowed them with some common chemical features, such as the redox property. Based on the above XPS measurement, the valence Prxþ in MA-10Ni3Pr was between þ3 and þ 4, which might be caused by the coexistence of Pr2O3 and PrO2 species. Similar to Ce4þ, the Pr4þ could be partly reduced into Pr3þ under the pretreatment in H2/N2 (20/10 mL/ min) stream at 800  C [92]. The change of the valence from þ4 to þ3 would generate the oxygen vacancies, which would contribute to the activation of CO2 at low temperature. Overall, the doping of the rare earth elements could greatly promote the low-temperature catalytic activity by regulating the surface basicity and/or electron property of the MA-10Ni3Re catalysts.

50 h catalytic stability tests The development of the Ni based catalyst with long-term catalytic stability is also an important concern for CO2 methanation because of its exothermic feature, which would cause the thermal sintering of the metallic Ni active sites and subsequently rapid deactivation. Herein, 50 h lab-scale longterm stability tests were carried out over the MA-10Ni and MA10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts under given conditions: H2/CO2 ¼ 4, 400  C, GHSV ¼ 15,000 mL/(g,h), 1 atm. As can be seen in Fig. 8(A), the CO2 conversions over these

Fig. 8 e 50 h long-term stability tests over MA-10Ni and MA-10Ni3Re catalysts: (A) CO2 conversion and (B) CH4 selectivity; reaction conditions: H2/CO2 ¼ 4, GHSV ¼ 15,000 mL/(g,h), 400  C, 1 atm.

mesoporous catalysts did not suffered any deactivation after 50 h endurance tests. Their outstanding performances ought to be derived from the stabilization effect of the metallic Ni active sites by the confinement effect of the mesoporous framework, where the strong metal-framework interaction had been formed. It was supposed that the Ni species were homogeneously embedded among the mesoporous Al2O3 matrix via the one-pot EISA strategy. Thus, the mesoporous Al2O3 matrix would effectively inhibit the thermal sintering of the metallic Ni nanoparticles by controlling the free movement of the metallic Ni active sites during the processes of reduction and CO2 methanation reaction. Besides, the promoting effects of the rare earth elements on the catalytic activities could be also reflected in Fig. 8(A). Specifically, the MA10NiRe catalysts also displayed higher catalytic activities than MA-10Ni during the whole 50 h period of stability test following the below sequence: MA-10Ni3Pr > MA10Ni3La > MA-10Ni3Sm > MA-10Ni3Ce > MA-10Ni, well consistent with the results in Fig. 6(A). As regards the CH4 selectivity in Fig. 8(B), all the catalysts also displayed greatly stable CH4 selectivity during the whole 50 h stability tests without obvious decrease, suggesting that

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

13

the serious thermal sintering of the metallic Ni active sites had been effectively avoided. It had been reported that the H2 was usually activated and dissociated over the metallic Ni active sites during the CO2 methanation [10,17,57]. Garbarino et al. reported that the selectivity of CH4 was sensitive to the size Ni particles and the smaller Ni size could promise higher CH4 selectivity [94]. If the thermal agglomeration of the metallic Ni nanoparticles took place, the dissociation of the H2 into H over Ni surface would be severely influenced and the subsequent hydrogenation of the reaction intermediates (CO or formate) would be blocked. As a result, the CH4 selectivity would decrease due to the lack of the H source. Besides, the MA-10Ni3Re catalysts also performed a bit higher CH4 selectivity than MA-10Ni counterpart, demonstrating the promoting effect of the rare earth elements. Overall, these rare earth doped MA-10Ni3Ln catalysts displayed much higher low-temperature catalytic activities than the pristine MA-10Ni reference catalyst. It was found that the rare earth dopants (La, Ce, Sm, and Pr) played crucial roles in enhancing the low-temperature catalytic activity. However, compared with the noble metal based catalysts, the lowtemperature catalytic activities over these Ni based catalysts also had much room to improve [25,29,95]. For example, Tada et al. reported that the Ru/CeO2/Al2O3 catalyst could achieve the equilibrium CO2 conversion and almost 100% CH4 selectivity at 350  C [95]. Besides, Zhen et al. found that tiny amount of Ru doping could evidently promote the low-temperature catalytic activity and stability of the Ni/g-Al2O3 catalyst [96]. Thus, inspired by these pioneer researches, it was practical to dope tiny amount of noble metals (e.g. Ru) in the MA-10Ni3Ln catalysts to further improve their low-temperature activities in the future research from the cost-saving perspective.

Characterizations of the as-reduced and spent catalysts XRD analysis In order to investigate the anti-sintering properties of MA10Ni and MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts, the comparative analyses between the as-reduced and 50 h spent catalysts were carried out. Their XRD patterns were summarized in Fig. 9. For the XRD patterns of the as-reduced catalysts in Fig. 9(A), only g-Al2O3 diffraction peak could be observed over some samples and the metallic Ni diffraction peak was still absent after reduction at 800  C for 2 h. The appearance of the g-Al2O3 characteristic peaks (PDF-#-10-0425) [97] indicated that the phase transformation from amorphous to g phase took place at high temperature. As for the Ni species in MA10Ni and MA-10Ni3Re catalysts, it was believed that they were able to be reduced at 800  C according to the H2-TPR analyses in Fig. 5(A). Thus, the absence of the metallic Ni diffraction peak of these samples might be attributed to their high dispersion among the mesoporous framework. Considering the detection limit of the powder XRD (ca. 3.0 nm), the sizes of the metallic Ni nanoparticles over as-reduced catalysts ought to be less than 3.0 nm. Compared with the XRD patterns in Fig. 9(A), except for the g-Al2O3 diffraction peaks (PDF-#-10-0425), the tiny metallic Ni diffraction peaks (PDF#-45-1027) [98] could be observed over some spent catalysts in Fig. 9(B). However, it was difficult to calculate the crystal sizes of these metallic Ni nanoparticles by the Scherrer formula

Fig. 9 e X-ray diffraction patterns of the (A) as-reduced (AR-) and (B) 50 h spent (SP-) MA-10Ni and MA-10Ni3Re catalysts.

because of the diffraction peak broadening. This indicated that the serious thermal sintering of the metallic Ni active sites was effectively suppressed by the confinement effect of the mesoporous framework, accounting for no evident deactivation during the 50 h long-term stability tests.

N2 adsorptionedesorption analysis The N2 adsorptionedesorption analyses of the as-reduced and 50 h spent MA-10Ni and MA-10Ni3Re (Re ¼ La, Ce, Sm, and Pr) catalysts had been carried out to investigate the stability of the mesoporous structure. It could be observed in the column I of Fig. 10 that all the as-reduced samples exhibited IV H1 type isotherms and narrow pore size distributions, which were greatly similar to their corresponding fresh catalysts. However, as can be seen in Table 1, their specific surface areas, pore volumes, and average pore diameters experienced some decline after the reduction process at 800  C for 2 h. The reason for this might be derived from the thermal shrinkage of the mesoporous framework. It was worth mentioning that the thermal shrinkage had not caused the collapse of the mesoporous structure. Furthermore, as can be observed in the

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

14

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

Fig. 10 e Nitrogen adsorptionedesorption isotherms and corresponding pore size distribution curves (inset) of the asreduced (column I) and 50 h spent (column II) MA-10Ni and MA-10Ni3Re catalysts: (A) MA-10Ni; (B) MA-10Ni3La; (C) MA10Ni3Ce; (D) OMA-10Ni3Sm; (E) OMA-10Ni3Pr. Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

column II of Fig. 10, all the spent catalysts also displayed IV H1 shaped isotherms and narrow pore size distributions, which were almost identical to their corresponding as-reduced catalysts. The values of their textural properties were also summarized in Table 1. The spent catalysts only displayed a bit smaller specific surface areas and pore volumes than the corresponding as-reduced samples. This suggested that only slight thermal shrinkage of the mesoporous frameworks occurred during the 50 h stability tests under CO2 methanation condition. Therefore, the current MA-10Ni and MA10Ni3Re mesoporous catalysts were provided with sufficient thermal stability toward CO2 methanation.

Conclusions In present work, a series of rare earth elements (La, Ce, Sm, and Pr) doped Ni based mesoporous catalysts (MA-10Ni3Re) had been fabricated by the one-pot EISA strategy and investigated as the catalysts for CO2 methanation. The incorporation of the rare earth elements greatly enhanced the surface basicity and/or electron property of the catalysts, which contributed to the chemisorption and activation of the CO2. Thus, the MA-10Ni3Re catalysts displayed two or three times higher catalytic activities than the pristine MA-10Ni counterpart in low temperature region (200e250  C). Furthermore, the strong interaction between Ni active sites and mesoporous framework had been formed by confining the Ni species among the mesoporous Al2O3 matrix, which could effectively stabilize the metallic Ni nanoparticles via the confinement effect. As a result, there was no obvious deactivation observed over these mesoporous catalysts after 50 h stability tests. In conclusion, these rare earth elements doped MA-10Ni3Re mesoporous materials could be considered as a series of promising catalyst candidates with enhanced lowtemperature activities toward CO2 methanation.

Acknowledgements The authors sincerely acknowledge the financial support from National Natural Science Foundation of China (Grant No. 21503113, 21577065, 21503142, and 91543115), International ST Cooperation Program of China (2014DFA90780), Key Projects in the National Science & Technology Pillar Program of Jiangsu Province (BE2014602, SBE2014070928), and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.

references

[1] Olah GA, Goeppert A, Prakash GS. Chemical recycling of carbon dioxide to methanol and dimethyl ether: from greenhouse gas to renewable, environmentally carbon neutral fuels and synthetic hydrocarbons. J Org Chem 2008;74:487e98. [2] Wang W, Wang S, Ma X, Gong J. Recent advances in catalytic hydrogenation of carbon dioxide. Chem Soc Rev 2011;40:3703e27.

15

[3] Davis SJ, Caldeira K, Matthews HD. Future CO2 emissions and climate change from existing energy infrastructure. Science 2010;329:1330e3.
rey G, Serre C, Devic T, Maurin G, Jobic H, Llewellyn PL, [4] Fe et al. Why hybrid porous solids capture greenhouse gases? Chem Soc Rev 2011;40:550e62. [5] Mikkelsen M, Jørgensen M, Krebs FC. The teraton challenge. A review of fixation and transformation of carbon dioxide. Energy Environ Sci 2010;3:43e81. [6] Keith DW. Why capture CO2 from the atmosphere? Science 2009;325:1654e5. [7] Aaron D, Tsouris C. Separation of CO2 from flue gas: a review. Sep Sci Technol 2005;40:321e48. [8] Liu H, Zou X, Wang X, Lu X, Ding W. Effect of CeO2 addition on Ni/Al2O3 catalysts for methanation of carbon dioxide with hydrogen. J Nat Gas Chem 2012;21:703e7. [9] Park J-N, McFarland EW. A highly dispersed PdeMg/SiO2 catalyst active for methanation of CO2. J Catal 2009;266:92e7. [10] Wei W, Jinlong G. Methanation of carbon dioxide: an overview. Front Chem Sci Eng 2011;5:2e10. [11] Song C. Global challenges and strategies for control, conversion and utilization of CO2 for sustainable development involving energy, catalysis, adsorption and chemical processing. Catal Today 2006;115:2e32. [12] Arakawa H, Aresta M, Armor JN, Barteau MA, Beckman EJ, Bell AT, et al. Catalysis research of relevance to carbon management: progress, challenges, and opportunities. Chem Rev 2001;101:953e96. [13] Zhang Q, Zuo Y-Z, Han M-H, Wang J-F, Jin Y, Wei F. Long carbon nanotubes intercrossed Cu/Zn/Al/Zr catalyst for CO/ CO2 hydrogenation to methanol/dimethyl ether. Catal Today 2010;150:55e60. [14] Bonura G, Cordaro M, Cannilla C, Arena F, Frusteri F. The changing nature of the active site of Cu-Zn-Zr catalysts for the CO2 hydrogenation reaction to methanol. Appl Catal B Environ 2014;152:152e61. [15] Sabatier P, Senderens J. Direct hydrogenation of oxides of carbon in presence of various finely divided metals. CR Acad Sci 1902;134:689e91. [16] Inui T, Takeguchi T. Effective conversion of carbon dioxide and hydrogen to hydrocarbons. Catal Today 1991;10:95e106. [17] Aziz M, Jalil A, Triwahyono S, Ahmad A. CO2 methanation over heterogeneous catalysts: recent progress and future prospects. Green Chem 2015;17:2647e63. [18] Yamasaki M, Komori M, Akiyama E, Habazaki H, Kawashima A, Asami K, et al. CO2 methanation catalysts prepared from amorphous NieZreSm and NieZremisch metal alloy precursors. Mater Sci Eng A 1999;267:220e6. [19] Su X, Xu J, Liang B, Duan H, Hou B, Huang Y. Catalytic carbon dioxide hydrogenation to methane: a review of recent studies. J Energy Chem 2016;25:553e65. [20] Hashimoto K. Green materials-materials for global atmosphere conservation and abundant energy supply. Kinzoku Mater Sci Technol 1993;63:10. [21] Hashimoto K. Metastable metals for “green” materials for global atmosphere conservation and abundant energy supply. Mater Sci Eng A 1994;179:27e30.
chal F. Thermo-economic process model for [22] Gassner M, Mare thermochemical production of Synthetic Natural Gas (SNG) from lignocellulosic biomass. Biomass Bioenergy 2009;33:1587e604. [23] Brooks KP, Hu J, Zhu H, Kee RJ. Methanation of carbon dioxide by hydrogen reduction using the Sabatier process in microchannel reactors. Chem Eng Sci 2007;62:1161e70. [24] Frontera P, Macario A, Ferraro M, Antonucci P. Supported catalysts for CO2 methanation: a review. Catalysts 2017;7:59. [25] Lunde PJ, Kester FL. Carbon dioxide methanation on a ruthenium catalyst. Ind Eng Chem Process Des Dev 1974;13:27e33.

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

16

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

[26] Zhou R, Rui N, Fan Z, Liu C-j. Effect of the structure of Ni/TiO2 catalyst on CO2 methanation. Int J Hydrogen Energy 2016;41:22017e25. [27] Wang F, He S, Chen H, Wang B, Zheng L, Wei M, et al. Active site dependent reaction mechanism over Ru/CeO2 catalyst toward CO2 methanation. J Am Chem Soc 2016;138:6298e305. [28] Liu J, Li C, Wang F, He S, Chen H, Zhao Y, et al. Enhanced lowtemperature activity of CO2 methanation over highlydispersed Ni/TiO2 catalyst. Catal Sci Technol 2013;3:2627e33. [29] Karelovic A, Ruiz P. Mechanistic study of low temperature CO2 methanation over Rh/TiO2 catalysts. J Catal 2013;301:141e53. [30] Rosid SJM, Bakar WAWA, Ali R. Physicochemical study of supported cobaltelanthanum oxide-based catalysts for CO2/ H2 methanation reaction. Clean Technol Environ 2015;17:257e64. [31] Agnelli M, Kolb M, Mirodatos C. CO hydrogenation on a nickel catalyst.: 1. kinetics and modeling of a lowtemperature sintering process. J Catal 1994;148:9e21. [32] Wang G, Xu S, Jiang L, Wang C. Nickel supported on ironbearing olivine for CO2 methanation. Int J Hydrogen Energy 2016;41:12910e9. [33] Farmer JA, Campbell CT. Ceria maintains smaller metal catalyst particles by strong metal-support bonding. Science 2010;329:933e6. [34] Yang W, Feng Y, Chu W. Promotion effect of CaO modification on mesoporous Al2O3-Supported Ni catalysts for CO2 methanation. Int J Chem Eng 2016. http://dx.doi.org/ 10.1155/2016/2041821. [35] Guo M, Lu G. The effect of impregnation strategy on structural characters and CO2 methanation properties over MgO modified Ni/SiO2 catalysts. Catal Commun 2014;54:55e60. [36] Guo M, Lu G. The difference of roles of alkaline-earth metal oxides on silica-supported nickel catalysts for CO2 methanation. RSC Adv 2014;4:58171e7. [37] Yu Y, Jin G, Wang Y, Guo X. Synthesis of natural gas from CO methanation over SiC supported NieCo bimetallic catalysts. Catal Commun 2013;31:5e10. [38] Zhou L, Wang Q, Ma L, Chen J, Ma J, Zi Z. CeO2 promoted mesoporous Ni/g-Al2O3 catalyst and its reaction conditions for CO2 methanation. Catal Lett 2015;145:612e9. [39] Takano H, Izumiya K, Kumagai N, Hashimoto K. The effect of heat treatment on the performance of the Ni/(Zr-Sm oxide) catalysts for carbon dioxide methanation. Appl Surf Sci 2011;257:8171e6. [40] Perkas N, Amirian G, Zhong Z, Teo J, Gofer Y, Gedanken A. Methanation of carbon dioxide on Ni catalysts on mesoporous ZrO2 doped with rare earth oxides. Catal Lett 2009;130:455e62. [41] Zhi G, Guo X, Wang Y, Jin G, Guo X. Effect of La2O3 modification on the catalytic performance of Ni/SiC for methanation of carbon dioxide. Catal Commun 2011;16:56e9. [42] Gaudillere C, Navarrete L, Serra JM. CO2 hydrogenation on Ru/Ce based catalysts dispersed on highly ordered microchannelled 3YSZ monoliths fabricated by freeze-casting. Int J Hydrogen Energy 2017;42:895e905. [43] Xiancai L, Min W, Zhihua L, Fei H. Studies on nickel-based catalysts for carbon dioxide reforming of methane. Appl Catal A Gen 2005;290:81e6. [44] Trovarelli A, Deleitenburg C, Dolcetti G, Lorca J. CO2 methanation under transient and steady-state conditions over Rh/CeO2 and CeO2-promoted Rh/SiO2: the role of surface and bulk ceria. J Catal 1995;151:111e24. [45] Wang S, Lu GM. Role of CeO2 in Ni/CeO2eAl2O3 catalysts for carbon dioxide reforming of methane. Appl Catal B Environ 1998;19:267e77.

[46] Pan Q, Peng J, Sun T, Wang S, Wang S. Insight into the reaction route of CO2 methanation: promotion effect of medium basic sites. Catal Commun 2014;45:74e8. [47] Bian L, Zhang L, Xia R, Li Z. Enhanced low-temperature CO2 methanation activity on plasma-prepared Ni-based catalyst. J Nat Gas Sci Eng 2015;27:1189e94. [48] Zhang W, Liu B, Zhan Y, Tian Y. Syngas production via CO2 reforming of methane over Sm2O3La2O3-Supported Ni catalyst. Ind Eng Chem Res 2009;48:7498e504. [49] Burnham DA, Eyring L. Phase transformations in the praseodymium oxide-oxygen system: high-temperature Xray diffraction studies. J Phys Chem 1968;72:4415e24. [50] Ferro S. Physicochemical and electrical properties of praseodymium oxides. Int J Electrochem 2011. http:// dx.doi.org/10.4061/2011/561204. [51] Razali MH, Bakar WAWA, Buang NA, Marsin FM. CO2/H2 methanation on nickel oxide based catalysts doped with lanthanide series. Malays J Anal Sci 2005;9:492e6. [52] Duyar MS, Ramachandran A, Wang C, Farrauto RJ. Kinetics of CO2 methanation over Ru/g-Al2O3 and implications for renewable energy storage applications. J CO2 Util 2015;12:27e33. € rke O, Pfeifer P, Schubert K. Highly selective methanation [53] Go by the use of a microchannel reactor. Catal Today 2005;110:132e9. [54] Yan Y, Dai Y, He H, Yu Y, Yang Y. A novel W-doped Ni-Mg mixed oxide catalyst for CO2 methanation. Appl Catal B Environ 2016;196:108e16. [55] Du G, Lim S, Yang Y, Wang C, Pfefferle L, Haller GL. Methanation of carbon dioxide on Ni-incorporated MCM-41 catalysts: the influence of catalyst pretreatment and study of steady-state reaction. J Catal 2007;249:370e9. [56] Li Y, Lu G, Ma J. Highly active and stable nano NiOeMgO catalyst encapsulated by silica with a coreeshell structure for CO2 methanation. RSC Adv 2014;4:17420e8. [57] Xu L, Wang F, Chen M, Zhang J, Yuan K, Wang L, et al. CO2 methanation over a Ni based ordered mesoporous catalyst for the production of synthetic natural gas. RSC Adv 2016;6:28489e99. [58] Yuan Q, Yin A-X, Luo C, Sun L-D, Zhang Y-W, Duan W-T, et al. Facile synthesis for ordered mesoporous g-aluminas with high thermal stability. J Am Chem Soc 2008;130:3465e72. [59] Morris SM, Fulvio PF, Jaroniec M. Ordered mesoporous alumina-supported metal oxides. J Am Chem Soc 2008;130:15210e6. [60] Rivero-Mendoza DE, Stanley JN, Scott J, Aguey-Zinsou K-F. An alumina-supported Ni-La-Based catalyst for producing synthetic natural gas. Catalysts 2016;6:170. [61] Nizio M, Albarazi A, Cavadias S, Amouroux J, Galvez ME, Da Costa P. Hybrid plasma-catalytic methanation of CO2 at low temperature over ceria zirconia supported Ni catalysts. Int J Hydrogen Energy 2016;41:11584e92. [62] Song F, Zhong Q, Yu Y, Shi M, Wu Y, Hu J, et al. Obtaining well-dispersed Ni/Al2O3 catalyst for CO2 methanation with a microwave-assisted method. Int J Hydrogen Energy 2017;42:4174e83. [63] Wierzbicki D, Baran R, De˛bek R, Motak M, Grzybek T,
lvez ME, et al. The influence of nickel content on the Ga performance of hydrotalcite-derived catalysts in CO2 methanation reaction. Int J Hydrogen Energy 2017. http:// dx.doi.org/10.1016/j.ijhydene.2017.02.148. [64] Sing K, Everett D, Haul R, Moscou L, Pierotti R, Rouquerol J, et al. Reporting physisorption data for 1, 0x10-3 1, 2x10-3 1, 4x10-3 1, 6x10-3 1, 8x10-3 2, 0x10-3-8-6-4-2 0 2 4-1 K) I II III gas/solid systems with special reference to the determination of surface area and porosity. Pure Appl Chem 1985;57:603e19.

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 7

[65] Kruk M, Jaroniec M. Gas adsorption characterization of ordered organic inorganic nanocomposite materials. Chem Mater 2001;13:3169e83. [66] Yuan Q, Duan HH, Li LL, Li ZX, Duan WT, Zhang LS, et al. Homogeneously dispersed ceria nanocatalyst stabilized with ordered mesoporous alumina. Adv Mater 2010;22:1475e8. [67] Huang X, Xue G, Wang C, Zhao N, Sun N, Wei W, et al. Highly stable mesoporous NiOeY2O3eAl2O3 catalysts for CO2 reforming of methane: effect of Ni embedding and Y2O3 promotion. Catal Sci Technol 2016;6:449e59. [68] Murthy I, Swamy C. Catalytic behaviour of NiAl2O4 spinel upon hydrogen treatment. J Mater Sci 1993;28:1194e8. [69] Ng KT, Hercules DM. Studies of nickel-tungsten-alumina catalysts by x-ray photoelectron spectroscopy. J Phys Chem 1976;80:2094e102. [70] Oh Y-S, Roh H-S, Jun K-W, Baek Y-S. A highly active catalyst, Ni/CeeZrO2/q-Al2O3, for on-site H2 generation by steam methane reforming: pretreatment effect. Int J Hydrogen Energy 2003;28:1387e92.
M, Haverkamp R, Mims C, Moudallal H, [71] Stojanovic Jacobson A. Synthesis and characterization of LaCr1 xNixO3Perovskite oxide catalysts. J Catal 1997;166:315e23. [72] Milt V, Spretz R, Ulla M, Lombardo E, Fierro JG. The nature of active sites for the oxidation of methane on La-based perovskites. Catal Lett 1996;42:57e63. [73] Sarma D, Rao C. XPES studies of oxides of second-and thirdrow transition metals including rare earths. J Electron Spectrosc Relat Phenom 1980;20:25e45. [74] Uwamino Y, Ishizuka T, Yamatera H. X-ray photoelectron spectroscopy of rare-earth compounds. J Electron Spectrosc Relat Phenom 1984;34:67e78. [75] Richardson JT, Twigg MV. Reduction of impregnated NiO/aA12O3 association of A13þ ions with NiO. Appl Catal A Gen 1998;167:57e64. [76] Mile B, Stirling D, Zammitt MA, Lovell A, Webb M. The location of nickel oxide and nickel in silica-supported catalysts: two forms of “NiO” and the assignment of temperature-programmed reduction profiles. J Catal 1988;114:217e29. [77] Li C, Chen Y-W. Temperature-programmed-reduction studies of nickel oxide/alumina catalysts: effects of the preparation method. Thermochim Acta 1995;256:457e65. [78] Zhang L, Wang X, Chen C, Zou X, Ding W, Lu X. Dry reforming of methane to syngas over lanthanum-modified mesoporous nickel aluminate/g-alumina nanocomposites by one-pot synthesis. Int J Hydrogen Energy 2017. http:// dx.doi.org/10.1016/j.ijhydene.2017.03.140. [79] Sato K, Abe N, Kawagoe T, Miyahara S-i, Honda K, Nagaoka K. Supported Ni catalysts prepared from hydrotalcite-like compounds for the production of hydrogen by ammonia decomposition. Int J Hydrogen Energy 2017;10:6610e7. [80] Liu S, Chen D, Zhang K, Li J, Zhao N. Production of hydrogen by ethanol steam reforming over catalysts from reverse microemulsion-derived nanocompounds. Int J Hydrogen Energy 2008;33:3736e47. [81] Numaguchi T, Eida H, Shoji K. Reduction of NiAl2O4 containing catalysts for steam methane reforming reaction. Int J Hydrogen Energy 1997;22:1111e5.

17

[82] Li L, Song L, Wang H, Chen C, She Y, Zhan Y, et al. Water-gas shift reaction over CuO/CeO2 catalysts: effect of CeO2 supports previously prepared by precipitation with different precipitants. Int J Hydrogen Energy 2011;36:8839e49. [83] Razzaq R, Zhu H, Jiang L, Muhammad U, Li C, Zhang S. Catalytic methanation of CO and CO2 in coke oven gas over NieCo/ZrO2eCeO2. Ind Eng Chem Res 2013;52:2247e56. [84] Al-Fatesh AS, Naeem MA, Fakeeha AH, Abasaeed AE. Role of La2O3 as promoter and support in Ni/g-Al2O3 catalysts for dry reforming of methane. Chin J Chem Eng 2014;22:28e37. [85] Yoshimura Y, Kijima N, Hayakawa T, Murata K, Suzuki K, Mizukami F, et al. Catalytic cracking of naphtha to light olefins. Catal Surv Jpn 2001;4:157e67. [86] Pechimuthu NA, Pant KK, Dhingra SC, Bhalla R. Characterization and activity of K, CeO2, and Mn promoted Ni/Al2O3 catalysts for carbon dioxide reforming of methane. Ind Eng Chem Res 2006;45:7435e43. [87] Elkins TW, Neumann Br, Bau¨mer M, Hagelin-Weaver HE. Effects of Li doping on MgO-supported Sm2O3 and TbOx catalysts in the oxidative coupling of methane. ACS Catal 2014;4:1972e90. [88] Tankov I, Pawelec B, Arishtirova K, Damyanova S. Structure and surface properties of praseodymium modified alumina. Appl Surf Sci 2011;258:278e84. [89] Gao J, Liu Q, Gu F, Liu B, Zhong Z, Su F. Recent advances in methanation catalysts for the production of synthetic natural gas. RSC Adv 2015;5:22759e76. [90] Ghaib K, Nitz K, Ben-Fares FZ. Chemical methanation of CO2: a review. ChemBioEng Rev 2016;3:266e75. [91] Osaki T, Mori T. Kinetic studies of CO2dissociation on supported Ni catalysts. React Kinet Catal Lett 2005;87:149e56. [92] Gallego GS, Marı´n JG, Batiot-Dupeyrat C, Barrault J,
n F. Influence of Pr and Ce in dry methane Mondrago reforming catalysts produced from La1xAxNiO3d perovskites. Appl Catal A Gen 2009;369:97e103. [93] Xavier K, Sreekala R, Rashid K, Yusuff K, Sen B. Doping effects of cerium oxide on Ni/Al2O3 catalysts for methanation. Catal today 1999;49:17e21. [94] Garbarino G, Riani P, Magistri L, Busca G. A study of the methanation of carbon dioxide on Ni/Al2O3 catalysts at atmospheric pressure. Int J Hydrogen Energy 2014;39:11557e65. [95] Tada S, Ochieng OJ, Kikuchi R, Haneda T, Kameyama H. Promotion of CO2 methanation activity and CH4 selectivity at low temperatures over Ru/CeO2/Al2O3 catalysts. Int J Hydrogen Energy 2014;39:10090e100. [96] Zhen W, Li B, Lu G, Ma J. Enhancing catalytic activity and stability for CO2 methanation on NieRu/g-Al2O3 via modulating impregnation sequence and controlling surface active species. RSC Adv 2014;4:16472e9. [97] Dieuzeide M, Laborde M, Amadeo N, Cannilla C, Bonura G, Frusteri F. Hydrogen production by glycerol steam reforming: how Mg doping affects the catalytic behaviour of Ni/Al2O3 catalysts. Int J Hydrogen Energy 2016;41:157e66. [98] Bobadilla L, Romero-Sarria F, Centeno M, Odriozola J. Promoting effect of Sn on supported Ni catalyst during steam reforming of glycerol. Int J Hydrogen Energy 2016;41:9234e44.

Please cite this article in press as: Xu L, et al., CO2 methanation over rare earth doped Ni based mesoporous catalysts with intensified low-temperature activity, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.05.027