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Mesoporous zirconia-modified clays supported nickel catalysts for CO and CO2 methanation
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Mesoporous zirconia-modified clays supported nickel catalysts for CO and CO2 methanation Huailiang Lu a, Xuzhuang Yang a,*, Guanjun Gao a, Kebing Wang b, Quanquan Shi c, Jie Wang a, Chenhui Han a, Jie Liu a, Min Tong a, Xiaoyuan Liang a, Changfu Li a a
School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia, 010021, PR China b College of Science, Inner Mongolia Agricultural University, Hohhot, Inner Mongolia, 010018, PR China c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China
article info
abstract
Article history:
The Ni catalysts supported on a new structure with zirconia nanoparticles highly dispersed
Received 9 August 2014
on the partly damaged clay layers has been prepared by the incipient wetness impregna-
Received in revised form
tion method and the new structure of the support has been prepared in one pot by the
10 September 2014
hydrothermal treatment of the mixture of the clay suspension and the ZrO(NO3)2 solution.
Accepted 16 September 2014
The catalytic performances for the CO and CO2 methanation on the catalysts have been
Available online 8 October 2014
investigated at a temperature range from 300 C to 500 C at atmospheric pressure. The catalysts and supports have been characterized by X-ray diffraction (XRD), transmittance
Keywords:
electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), nitrogen
CO methanation
adsorptionedesorption, and thermogravimetry and differential thermal analysis (TG-DTA).
CO2 methanation
It is found that the zirconia-modified clays have the typical bimodal pore size distribution.
SNG
Most of the pores with the sizes smaller than 10 nm are resulted from the zirconia pillared
Zirconia
clays and the mesopores with the sizes larger than 10 nm and the macropores with the
Clays
sizes larger than 50 nm are resulted from the partly damaged clay layers. The bimodal pore structure is beneficial to the dispersion of Ni on the layers of the zirconia-modified clays and the increase in Ni loading. The zirconia nanoparticles are highly dispersed on the partly damaged clay layers. Nickel oxide in cubic phase is the only Ni species that can be detected by XRD. The nickel oxide nanoparticles with the sizes of 12 nanometers or more are well dispersed on the zirconia-modified clay layers, which are observed to be buried in the stack layers of zirconia. The presence of nickel oxide in six different forms could be perceived on the new structure. Five of them except the Ni species that forms the spinel phase with Al in clays can be reduced to the active Ni species for the CO and CO2 methanation. But the activity of the Ni species is different, which is associated with the chemical environment at which the Ni species is located. The catalyst with the higher zirconia content, which also has the larger specific surface area and pore volume, exhibits the better catalytic performance for the CO or CO2 methanation. Zirconia in the catalyst is responsible for the dispersion of the Ni species, and it prevents the metallic Ni nanoparticles from sintering during the process of the reaction. In addition, it is also responsible for the reduction of the inactive carbon deposition. The catalyst with 15 wt.% zirconia
* Corresponding author. Tel.: þ86 471 4992982; fax: þ86 471 4992981. E-mail addresses: [email protected], [email protected] (X. Yang). http://dx.doi.org/10.1016/j.ijhydene.2014.09.076 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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content has the highest CO conversion of about 100% and the highest methane selectivity of about 93% at 450 C for CO methanation, and the catalyst with 20% zirconia content has the CO2 conversion of about 80% and the highest methane selectivity of about 99% for CO2 methanation at 350 C. The catalyst with 15 wt.% zirconia possesses promising stability and no distinct deactivation could be perceived after reaction for 40 h. This new catalyst has great potential to be used in the conversion of the blast furnace gas (BFG) and the coke oven gas (COG) to methane. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The “growth-at-any-cost” strategy in some countries results in serious environmental issues. In recent years, smog and haze have routinely been blanketing Beijing and other cities in China. The pollution is not limited to China d it affects the whole Northern Hemisphere through the atmospheric circulation [1]. The blast furnace gas (BFG) and the coke oven gas (COG), containing carbon monoxide, carbon dioxide, methane, hydrogen, and small solid particles etc. [2,3], may play an important role for air pollution if they are treated inadequately. Facing the environmental impact, Beijing has ordered more than 50 heavy-machinery and chemical companies to move to less developed regions [1], including several coke enterprises and its largest steel works [3]. However, such an action is not an ultimate solution but just the pollution transfer. The BFG and COG are normally combusted directly or combusted for electricity [4,5]. In recent years, they have been studied as raw materials to produce hydrogen [2,6], syngas [7], methanol [8,9], and methane [10,11], by which a high percentage of potential energy can be recovered and a large amount of greenhouse gas emissions can be reduced, and thus it has attracted great attentions of researchers. In some countries such as China the natural gas supplies are insufficient. It is reported that the total production capacity of synthetic or substitute natural gas (SNG) by plants under construction and planned projects in China will exceed 300,000 million Nm3 per year in the next ten years [12]. Accordingly, to produce SNG from BFG and/or COG is a good choice, which can not only alleviate the environmental pressure but also meet the market demand. The methanation reaction was first studied by Sabatier and Senderens in the beginning of the last century [13]. The earliest use of the technology of methanation was to remove the trace carbon oxides from the feed gas for ammonia synthesis [14]. In recent years, this reaction has also been studied for eliminating the trace CO in the hydrogen-rich gas, a fuel for the proton exchange membrane fuel cell (PEMFC), in order to prevent the anodes from being poisoned [15e18]. Another application of COx (x ¼ 1,2) methanation is to synthesize SNG. J. Kopyscinski et al. [19] have reviewed the production technology of SNG from 1950 to 2009, and mentioned that only one commercial SNG plant, the Great Plains Synfuels Plant of the Dakota Gasification Company (North Dakota, United States), has been commissioned since 1984, although several
demonstration or pilot plants have been constructed in Germany and Great Britain since the 1970s. Supported nickel catalysts are the most promising catalysts for COx (x ¼ 1,2) methanation due to their good catalytic performances and relatively low prices [10,20e29]. Metal oxides such as Al2O3 [30e35], SiO2 [15,18,36,37], TiO2 [38], ZrO2 [15,21,39e41] and CeO2 [10,42] are the most commonly used supports for nickel catalysts. Among them, Ni/SiO2 and Ni/ Al2O3 have been widely and extensively studied due to their good initial activities and relatively cheap prices [15,18,30e37]. However, nickel supported on silica or alumina often suffers from sintering and serious carbon deposition at high temperatures [32,43]. Some researchers have tried to dope zirconia in the catalyst to promote the dispersion of Ni species and prevent them from sintering [10,18,24,41,44], and achieved high activities and good stabilities. In our previous work [41], a nickel catalyst supported on a Al2O3@ZrO2 core/shell structure has been prepared for CO methanation. It has been found that zirconia on the surface of the core/shell structure promotes the dispersion and influences the reduction states of the nickel species on the support, and thus prevents the Ni species from sintering as well as from forming a spinel phase with alumina at high temperatures. On the other hand, some researchers have attempted to load Ni on porous supports on which the Ni species are supposed to be confined within the local spaces of the mesopores [45e49]. Xu et al. [45] have prepared a nickel catalyst supported on an ordered mesoporous alumina, but a large amount of the spinel phase NiAl2O4 have been formed during the calcination. Some researchers have prepared Niincorporated MCM-41 catalysts [46e48], but the Ni content cannot exceed 10 wt.% or extra Ni that cannot be incorporated in the frameworks of MCM-41will block the tunnels of the pores due to the narrow pore size (less than 10 nm). In another work of us [49], Ni species of 15 wt.% has been found to be well dispersed in the three dimensional interconnected mesopores composed by partly damaged clay layers resulted from acidleaching, which effectively prevents the Ni species from sintering during the methanation reaction. Volkonskoite belongs to family smectites and has abundant slit-like mesopores composed by plate-like clay layers with cations such as Naþ and Ca2þ in the interlayers. The clay layer is composed by an Al octahedral sublayer sandwiched by two Si tetrahedral sublayers [50]. The raw volkonskoite cannot be directly used as the support of catalyst due to its small specific surface area. The pillared process [50] and the acidleaching process [49,51] can remarkably increase the specific surface area of clays, but the pore structures of the two
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modified clays are quite different (Fig. 1). The pillared clays have intact clay layers with metal oxide pillars in the interlayers, but the acid leached clays have damaged clay layers with macro- or meso-openings on the layers. The reactants and products can only diffuse through the slit-like pores at the end sides of the clay layers of the pillared clays, but they can diffuse freely through the openings on the damaged clay layers and at the end sides of the clay layers of the acidtreated clays. In addition, part of Al in volkonskoite can be leached out by acid, which decreases the possibility of forming the spinel phase of nickel aluminate in the catalyst. Therefore, the layer-damaged clays have a great potential as a cheap and effective support for the nickel catalyst. In the present study, the zirconia-modified clays with a bimodal size distribution were prepared in one pot by the hydrothermal treatment of the mixture of the clay suspension and the zirconyl nitrate solution. The nickel catalysts supported on the zirconia-modified clays were prepared by the incipient wetness impregnation method. The catalysts and the supports were characterized by XRD, TEM, TPR, TG-DTA and nitrogen adsorptionedesorption. The activity and the CH4 selectivity of the catalyst for the CO or CO2 methanation were investigated. The aims of this study are to investigate the effects of the Zr content and pore structures of the zirconiamodified clays on the dispersion of the Ni species, the carbon deposition as well as the catalytic performances for CO and CO2 methanation, by which a possible catalyst that has high performances for the conversions of CO and CO2 to methane is to be developed.
Experimental Materials Volkonskoite was purchased from J & K Scientific. ZrO(NO3)2$2H2O, Ni(NO3)2$6H2O were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as received.
Catalyst preparation The clays of 5.0 g were dispersed into deionized water of 250 ml, continuously stirring for 24 h. An appropriate amount of the aqueous solution of zirconyl nitrate dihydrate was poured into the clay suspension with continuously stirring for 5 h, and then the suspension was transferred into autoclaves for the hydrothermal treatment at 150 C for 24 h. After washing with water and ethanol and drying at 100 C, the zirconia-modified mesoporous clays were then obtained after the calcination at 500 C for 5 h. The samples are labeled as
ZrO2-5-V, ZrO2-10-V, ZrO2-15-V and ZrO2-20-V. The number in the middle of the sample name stands for the zirconia content in the sample. The raw clays are labeled as VK. The supported nickel catalysts were prepared by the incipient wetness impregnation method. A suitable amount of the aqueous solution of Ni(NO3)2$6H2O was added into the support material. After standing for 24 h, the solids obtained were dried and calcined by a temperature programmed from room temperature to 500 Cat a heating rate of 2 C/min, and then kept at 500 C for 5 h. The nominal load of Ni in each of the catalyst is 15 wt.%. The catalysts are labeled as Ni/ZrO2-5V, Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V.
Characterization The X-ray diffraction (XRD) measurements were performed using a Bruker D8 Advance diffractometer with a Cu Ka radiation of wavelength 0.1541 nm operated at 40 kV and 40 mA with a scanning speed of 2 /min and a scanning angle (2q) range of 5e80 . The average crystallite size of the chemical was evaluated using the well-known Scherrer's equation (d ¼ 0.89l/bcosq). The N2 adsorptionedesorption isotherms were measured using a Micrometritics ASAP 2020 analyzer. The specific surface areas were calculated by the BrunauereEmmetteTeller (BET) equation from the N2 adsorption isotherm. The pore structure parameters were obtained from the adsorption branch using the BJH model. Prior to the experiment of adsorption, all samples were degassed under vacuum at 150 C for 12 h. The morphologies of the samples were observed using a scanning electron microscopy (SEM, HITACHI, S-4800), coupled with an X-ray energy dispersive spectroscopy (EDS) system to analyze the composition of the selected areas of the sample. The transmission electron microscopy (TEM) measurements were carried out using a JEM-2100 TEM operated at 100 kV. The temperature programmed reduction with hydrogen (H2-TPR) was performed with a Micromeritics AutoChem 2910 analyzer. Typically, the catalyst of 50 mg with the sizes between 0.25 mm and 0.42 mm was loaded into a quartz tube, and purged with N2 for 1 h at 300 C to remove the adsorbed H2O and other volatile gases on the sample. After cooling to 50 C, the inlet gas was switched to the gas mixture of 5% H2 balanced with N2 at a flow rate of 20 mL/min. Then, the temperature was elevated to 900 Cat a heating rate of 10 C/ min, in the meantime the hydrogen concentration in the effluent was monitored by a thermal conductivity detector (TCD). The thermogravimetric and differential thermal analyzer (TG-DTA), SII TG/DTA7300 model, was used to analyze the carbon deposition of the spent catalyst, operated in a temperature range from room temperature to 900 Cin air at a heating rate of 10 C/min. The EXTRA 600 System was used to analyze the data.
Methanation of COx (x ¼ 1,2)
Pillared clay
Layer damaged clay
Fig. 1 e Scheme of pillared clays and layer damaged clays.
The experiment for COx (x ¼ 1,2) methanation was performed in a fixed bed tubular reactor with a diameter of 10 mm at
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atmospheric pressure, and a thermocouple was installed in the middle of the catalyst bed to measure the temperature. The mixture of the catalyst of 0.5 ml with the sizes of 0.25e0.42 mm and quartz of 0.5 ml with the same sizes was loaded in the reactor in order to maintain the catalyst bed a constant temperature. The catalyst was reduced in hydrogen at 500 Cfor 3 h before the reaction. After cooling to the reaction temperature in N2, the feed gas with H2/CO ¼ 3 or H2/ CO2 ¼ 4 was introduced into the reactor at a certain flow rate, which was determined by the gas hourly space velocity (GHSV) and the load of the catalyst. The concentrations of CO, CO2, H2 and CH4 in the outlet gas were measured on-line by a GC1100 chromatograph equipped with a 2 m TDX01 column and a thermal conductivity detector. Other contents were measured on another chromatograph of the same model with a TCD detector and a Porapak Q column. The calculation formulas of the COx conversion and the methane selectivity are described as eq. (1) and eq. (2). COx conversion; ðXCOx ; %Þ :
XCOx ð%Þ ¼
FCOx ;in FCOx ;out 100% FCOx ;in (1)
CH4 selectivity; ðSCH4 ; %Þ :
SCH4 ð%Þ ¼
FCH4 100% FCOx ;in Fcox ;out (2)
where FCOx ;in , FCOx ;out and FCH4 are the mole flow rates, mol h1, of reactant in the inlet, reactant in the outlet and product in the outlet, respectively.
Results and discussion XRD The XRD patterns of the raw clays and the zirconia-modified clays are shown in Fig. 2a. The XRD pattern of the raw clays is in accordance with that of volkonskoite according to the International Centre for Diffraction Data (ICDD) PDF file No. 00-042-0619. The peaks at 21.8 and 26.8 of the 2 theta are
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ascribed to the diffractions of the (100) and (101) crystal faces of quartz, suggesting small amount of quartz in the raw clays. The aqueous solution of zirconyl nitrate is strongly acidic. Aluminum in the octahedral sublayers of clays can be leached out during the hydrothermal treatment of the mixture of the clay suspension and the zirconyl nitrate solution. Table 1 lists the contents of different elements in the catalysts. With increasing the Zr content from 2.57 at% to 18.96 at%, the content of Al decreases sharply from 20.73 at% to 9.56 at%; whereas, the content of Si remains almost constant. Therefore, the loss of Al from the octahedral sublayers of clays is evident. However, the fundamental phase structure of clays does not collapse because most of the characteristic peaks attributed to the clays, such as the peaks ascribed to the diffractions of the (100), (110) and (300) crystal faces of the clays, remain unchanged. The peaks attributed to the diffractions of the (005) crystal faces in the patterns of the zirconia-modified clays shift from the original 29.5 e28.1 , suggesting the enlargement of the distance between the (001) faces (d(001)). As a result, the peaks at the position of a in the patterns of the zirconia-modified clays should not be ascribed to the diffractions of the (001) faces but the (003) faces instead. In this case, the d(001) of the zirconia-modified clays increases to about 3 nm from the original 1.5 nm. In addition, with increasing the zirconia content, the peaks at the position a shift to the low angle direction, suggesting the d(001) increases with increasing the zirconia content in the clays. Zirconia in monoclinic phase also gives the strongest diffraction of the (111) crystal faces at 28.2 of the 2 theta. The reasons why the peaks at 28.2 in the patterns of the zirconia-modified clays are ascribed to the diffraction of the (005) faces of clays but not that of the (111) faces of zirconia are that the intensity of these peaks becomes weaker and weaker but not stronger and stronger and the peak positions slightly shift to the low angle direction but not remain unchanged with the increase of the zirconia content. Therefore, the sizes of the zirconia crystals in the modified clays must be very small or amorphous so that it cannot be detected by XRD. The diffraction peaks attributed to clays in the patterns of the nickel catalysts (Fig. 2b) are almost the same as those in
Fig. 2 e XRD patterns of the raw clays and the zirconia-modified clays (a) and nickel catalysts supported the zirconiamodified clays (b).
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Table 1 e Compositions of catalysts with different content of Zr. Si （at%） Ni （at%） Al （at%） Zr （at%）
Ni/ZrO2-5-V
Ni/ZrO2-10-V
Ni/ZrO2-15-V
Ni/ZrO2-20-V
57.59 19.11 20.73 2.57
55.54 19.12 20.40 4.94
57.09 17.03 18.07 7.81
54.72 16.76 9.56 18.96
the patterns of the zirconia-modified clays before loading with nickel (Fig. 2a), indicating that the process of loading nickel does not influence the phase structure of the supports. The nickel species in the catalysts is nickel oxide in cubic phase. The peaks at 37.3 , 43.3 , 62.9 and 75.4 of the 2 theta in the patterns of the nickel catalysts in Fig. 2b are attributed to the diffractions of the (111), (200), (220) and (311) crystal faces of NiO, respectively. The crystal sizes of NiO in the catalysts of Ni/ZrO2-5-V, Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V are 16.90 nm, 20.20 nm, 20.03 nm and 23.62 nm, respectively, calculated by Scherrer's formula according to the diffractions of the (200) crystal faces (See Table 2.).
TEM The micromorphology of the raw clays is shown in Fig. 3. The plate-like clay layers are clearly observed, which are similar to those in literature [52]. The images of the zirconia-modified clays are shown in Fig. 4aed. The plate-like clay layers can be observed in the images as well, which is consistent with the results of XRD. However, the damage or collapse of the clay layers is intensified with increasing the zirconia content. This is because the acidity of the mixture of the clay suspension and the zirconyl nitrate solution increases with increasing the amount of the zirconyl nitrate solution, which can result in increasing loss of Al from the clay layers by acid leaching. The Zr species is seldom observed on the surfaces of the clay layers of the samples ZrO2-15-V and ZrO2-10-V, but several clusters of Zr species with relatively larger size are observed on the clay layers of the sample ZrO2-5-V. The Zr species cannot be definitely identified on the clay layers of the sample ZrO2-20-V in image d because there are a lot of clay layer fragments mixing with it. However, the morphology of the
fine nanoparticles in image d is quite different from that of zirconia on the sample ZrO2-5-V. This is because the acidity of the precursor suspension not only influences the release of Al from the clay layers but also influences the crystallization of zirconia. The XRD results show that the d(001) of the clays in the zirconia-modified clays is about 3 nm, suggesting that the sizes of a large amount of zirconia in these samples are about 3 nm. This is also the reason that zirconia in these samples cannot be detected by XRD. In Fig. 4eeh, most NiO nanoparticles observed are embedded in the zirconia-modified clay layers. The sizes of NiO nanocrystals calculated by XRD are about 20 nm, but those observed by TEM seem to be larger. This is because that the size calculated by XRD is the average crystal size of NiO but the size observed by TEM is the particle size of NiO, which may be consisted of several nanocrystals.
Specific surface area and pore structure Table 2 lists the specific surface areas and pore volumes of the raw clays, the zirconia-modified clays and the nickel catalysts supported on the zirconia-modified clays, as well as the sizes of NiO nanocrystals in the catalysts. The specific surface area and pore volume of the VK are 63.97 m2/g and 0.11 cm3/g, respectively. After the modification by zirconia, the specific surface areas and pore volumes increase dramatically. With increasing the zirconia content, the specific surface areas increase from 89.45 m2/g to 164.12 m2/g and the pore volumes increase from 0.15 cm3/g to 0.19 cm3/g. After loading with Ni, the specific surface areas of the catalysts decrease by about 20% and the pore volumes decrease by about 15% compared with the zirconia-modified clays. Fig. 5a shows the nitrogen adsorptionedesorption isotherms of the raw clays and the zirconia-modified clays. All of
Table 2 e BET specific surface areas, pore volumes of supports and catalysts as well as the crystal sizes of nickel oxide on the catalysts. Samples VK ZrO2-5-V ZrO2-10-V ZrO2-15-V ZrO2-20-V Ni/ZrO2-5-V Ni/ZrO2-10-V Ni/ZrO2-15-V Ni/ZrO2-20-V
BET area, m2/g
Pore volume, cm3/g
Crystal size,a nm
63.97 89.45 117.16 155.35 164.12 73.78 93.25 121.90 133.38
0.11 0.15 0.16 0.17 0.19 0.13 0.13 0.14 0.16
e e e e e 16.90 20.20 20.03 23.62
a
Crystal sizes of nickel oxide are calculated by Scherrer's formula according to the diffractions of the (200) crystal faces.
Fig. 3 e TEM image of the raw clays of VK.
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Fig. 4 e TEM images of the zirconia-modified clays (aeZrO2-5-V; beZrO2-10-V; ceZrO2-15-V; deZrO2-20-V) and the nickel catalysts supported on the zirconia-modified clays (eeNi/ZrO2-5-V; feNi/ZrO2-10-V; geNi/ZrO2-15-V; heNi/ZrO2-20-V).
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Fig. 5 e Nitrogen adsorption and desorption isotherms of the raw clays, the zirconia-modified clays and the catalysts (a, c) and the pore size distributions of the zirconia-modified clays and the catalysts (b, d).
the five isotherms of VK, ZrO2-5-V, ZrO2-10-V, ZrO2-15-V and ZrO2-20-V are the type IV isotherms, suggesting there are mesopores in these samples. The hysteresis loop of the sample VK is attributed to the type H4 loop, which is associated with the narrow slit-like pores [53]. The hysteresis loops of ZrO2-5-V, ZrO2-10-V, ZrO2-15-V and ZrO2-20-V are similar to the type H3 loop, which is associated with the slit-shape pore structure composed by plate-like particles. The change of the hysteresis loop from the type H4 of the raw clays to the type H3 of the zirconia-modified clays implies the enlargement of the slit-like openings of the mesopores in the zirconiamodified clays, which is resulted from the pillaring effect by the zirconia nanoparticles as well as the perforation on the clay layers by the acid corrosion during the hydrothermal treatment. With increasing the zirconia content, the hysteresis loop is gradually close to the type H1 loop, which is often associated with the porous structure fabricated by regular particles [53], suggesting the plate-like clay layers gradually become almost uniform clay fragments due to acid increasingly leaching out Al from the octahedral sublayers. In addition, the pore volume almost proportionally increases with the zirconia content. The increase of the pore volume is resulted from the mesopores and macropores newly formed
by acid leaching. The Fig. 5b shows the profiles of the pore size distributions of the raw clays and the zirconia-modified clays. The VK has the mesopores with the sizes distributing in the range from 3 nm to 4 nm, which is originated from the slit-like pores in the raw clays. The small amount of the mesopores with the sizes from 4 nm to 30 nm in the raw clay is related to the agglomerates that are the assemblage of the clay particles rigidly jointed together [53]. The sample ZrO2-5-V has an obvious bimodal pore size distribution, one from 3 nm to 10 nm and the other from 10 nm to 20 nm. The former comes from the zirconia pillared clays and the latter is from the openings on the clay layers resulted from the acid corrosion. However, when the zirconia content in the sample exceeds 10 wt.% the pores in the sample are composed by a large amount of mesopores with the sizes between 3 nm and 5 nm, a quite amount of mesopores with the sizes from 5 nm to 50 nm and a small amount of macropores with the sizes larger than 50 nm. The mesopores with the sizes smaller than 5 nm are resulted from the zirconia pillared clays, which decrease by about 5 nm compared with those in the sample ZrO2-5-V. This is because extra zirconia in the samples with high zirconia content fills some of the spaces of the mesopores in the zirconia pillared clays. The mesopores larger than 5 nm and
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the macropores larger than 50 nm are resulted from the damaged clay layers by acid leaching. With the increase of the zirconia content, the amount of the mesopores and macropores resulted from acid leaching increased markedly. The isotherms of the nickel catalysts supported on zirconia-modified clays are attributed to the type IV isotherm as well, shown in Fig. 5c, and it suggests that mesopores exist in the catalysts. The hysteresis loops of the catalysts are attributed to the type H3 loop, which are consistent with those of the corresponding supports. The type H3 loop is associated with the slit-shape pore structure composed by plate-like particles. It suggests that zirconia and nickel oxide are most possibly attached on the clay layers, which is also evidenced by the TEM observation. Fig. 5d shows the pore size distribution profiles of the catalysts. The pore size distribution of the catalyst Ni/ZrO2-5-V changes greatly from that of the corresponding support. This is because the nickel species filled a part of the spaces of the mesopores between and on the clay layers of the support and gave rise to smaller mesopores with the sizes between 3 nm and 4 nm, and the larger Ni species as well as the fragments of the clay layers covered by zirconia jointed rigidly together to give rise to the mesopores larger than 4 nm and the macropores larger than 50 nm. Compared with the corresponding supports, the pore size distributions of the catalysts of Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V change slightly, which does not mean that the pores in
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corresponding support are well inherited by the catalyst but means that the pores newly formed in the catalyst after loading with Ni has the similar size distribution to the corresponding support.
H2-TPR The Fig. 6 shows the H2-TPR profiles of the as-prepared nickel catalysts supported on the zirconia-modified clays. The H2 reduction profiles of the catalysts can be classified into four bands, band a, band b, band g and band s. The peaks in the band a are attributed to the highly dispersed nickel species, such as the highly dispersed Ni2O3 and NiO on the surface of the supports [54]. According to the XRD results, there is only NiO in cubic phase in the catalysts. Therefore the Ni species in this band for all of the four catalysts is ascribed to the highly dispersed NiO. However, two small peaks can be obtained in the band a for the Ni/ZrO2-5-V catalyst by deconvolution, one centered at 356.8 C and the other peak at 373.7 C. The peak centered at 356.8 C is ascribed to the NiO highly dispersed on zirconia that is dispersed on the damaged clay layers and the one centered at 373.7 C is ascribed to the NiO highly dispersed on the exposed surfaces of the clay layers, because this support is fabricated by zirconia of about 5 wt.% highly dispersed on the clay layers so that the surfaces exposed may be zirconia or the clay layers. With increasing the zirconia content the
Fig. 6 e TPR profiles of the catalysts supported on zirconia-modified clays.
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clay surfaces exposed decreased dramatically, resulting in a dramatic decrease of the Ni species highly dispersed on the clay layers. Therefore the peaks centered at about 373 C cannot be identified in the samples with higher zirconia contents, which might be hidden in the larger peaks nearby if any. The peaks centered at about 400 C are generally ascribed to the reduction of the bulk NiO [54]. However, two peaks can also be obtained in the band b for each of the four catalysts by deconvolution, one located at 395 Ce425 C and the other located at 450 Ce470 C. The former is ascribed to the bulk NiO on zirconia and the latter is ascribed to the bulk NiO on the clay layers due to the same reason as in band a. From Table 3, it is observed that the ratio of the content of NiO on zirconia to that on the clay layers increases from 1.27 to 3.56 with increasing the zirconia content. In the band g the peaks located at about 520 Ce555 C are attributed to the reduction of NiO interacting strongly with the supports, which is also called fixed NiO. The contents of the fixed NiO in the catalysts of Ni/ZrO2-5-V, Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V are 11.88, 12.97, 5.77 and 21.81, respectively (See Table 3). At least a fraction of the fixed NiO is possible to be reduced to the active Ni during the reaction. The peaks in the band s from 620 C to 810 Care attributed to the reduction of nickel aluminate or nickel silicate [54,55], which cannot be reduced to active Ni during the reaction. It is evident that the content of nickel aluminate or nickel silicate in the catalyst decreased
Table 3 e Nickel species and content in the catalysts. Sample name Ni/ZrO2-5-V
Species reduced
Peak area
Dispersed NiO 481.9 984.9 Dispersed NiOa NiO on Zirconia 7626.1 NiO on clay 5988.9 Fixed NiO 2281.8 Nickel 1833.8 aluminate Ni/ZrO2-10-V Dispersed NiO 458.9 NiO on Zirconia 9073.3 NiO on clay 6085.0 Fixed NiO 2550.1 Nickel 1493.3 aluminate 1261.1 Ni/ZrO2-15-V Dispersed NiO NiO on zirconia 8619.3 NiO on clay 2628.0 Fixed NiO 808.9 Nickel 683.1 aluminate 1119.6 Ni/ZrO2-20-V Dispersed NiO NiO on zirconia 10,879.9 NiO on clay 3054.8 Fixed NiO 4459.2 Nickel 923.5 aluminate
a
Content of Peak species, % position, C 2.51 5.13 39.72 31.19 11.88 9.55
356.8 373.7 416.8 463.3 542.9 707.3
2.33 46.14 30.95 12.97 7.59
368.5 425.0 472.8 555.8 694.7
9.00 61.56 18.77 5.77 4.87
351.8 394.7 450.6 543.1 681.9
5.47 53.23 14.94 21.81 4.51
359.2 412.2 470.2 520.6 682.6
There are two types of the highly dispersed NiO on the surfaces of the Ni/ZrO2-5-V catalyst, and the one reduced at a lower temperature is dispersed on zirconia and the other one reduced at a higher temperature is dispersed on the clay layers.
from 9.55 wt.% to 4.51 wt.% with increasing the zirconia content from 5 wt.% to 20 wt.%.
Catalytic performance for CO and CO2 methanation The CO conversion, methane selectivity for CO methanation, CO2 conversion and methane selectivity for CO2 methanation of the catalysts are shown in Fig. 7aed. The CO conversions at 300 C increase from 18% to 39% with increasing the zirconia content from 5 wt.% to 20 wt.%. The CO conversions of the Ni/ ZrO2-5-V and Ni/ZrO2-10-V catalysts almost reach 100% at 350 C and 400 C but they decrease when the temperature is higher than 450 C, and are only about 90% at 500 C. However, the catalysts with higher zirconia content such as Ni/ZrO2-15V and Ni/ZrO2-20-V exhibit a better CO conversion of about 100% at higher temperatures. The methane selectivity for CO methanation increases first, then decreases for all of the four catalysts at the temperature range from 300 C to 500 C. But the temperature of the maximum methane selectivity of the catalyst is different from each other. It reaches the maximum at 350 C for Ni/ZrO2-5-V and Ni/ZrO2-10-V but at 450 C for Ni/ ZrO2-15-V and Ni/ZrO2-20-V. At the temperatures higher than 400 C, the methane selectivity for CO methanation on the catalysts with higher zirconia content is far better than that with low zirconia content. The catalyst of Ni/ZrO2-15-V exhibits the best methane selectivity for CO methanation in the whole temperature range, with the maximum of about 93% at 450 C. The CO2 conversions are not as good as the CO conversions on these catalysts. At the temperatures lower than 450 C, the catalysts with the lower zirconia contents exhibit better CO2 conversions. The Ni/ZrO2-5-V catalyst has the best CO2 conversion of 83% at 400 C. But the catalysts with higher zirconia content have better CO2 conversions at the temperatures higher than 400 C. The methane selectivity for CO2 methanation (Fig. 7d) at 300 C almost proportionally increases with increasing the zirconia content. The methane selectivity on Ni/ZrO2-20-V for CO2 methanation reaches the maximum of 99% at 350 C, then remains at 97% at the temperature range from 400 C to 500 C.The catalyst Ni/ZrO2-5-V that has the lowest zirconia content of 5 wt.% exhibits the lowest methane selectivity of about 88% in the temperature range from 400 C to 500 C. The Ni/ZrO2-15-V catalyst was used for the stability experiment due to its superior performance in CO and CO2 methanation. The experiment was performed at 450 C, with a GHSV of 10,000 h1 for CO methanation and with a GHSV of 12,000 h1 for CO2 methanation. Fig. 8a shows the time on stream of the catalyst for CO methanation and Fig. 8b shows that for CO2 methanation. This catalyst exhibits extremely good stability for CO methanation. The CO conversion remains at 100% and the methane selectivity only decreases by 2% in 40 h. However, the CO2 conversion on Ni/ZrO2-15-V decreases by 2% but the methane selectivity remains unchanged at about 94% in 40 h.
Carbon deposition on the catalyst The TG analysis on the used catalyst for CO and CO2 methanation is performed in order to investigate the condition of
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Fig. 7 e CO conversion (a), CH4 selectivity for CO methanation (b), CO2 conversion (c) and CH4 selectivity for CO2 methanation (d) of the catalysts.
carbon deposition on the surface of the catalyst. Fig. 9a shows the TG profile of the catalyst after 40 h on stream for CO methanation. The mass loss at the temperatures lower than 190 C is resulted from the moisture evaporation [56,57]. The mass gain at the temperature range from 190 C to 606 C is ascribed to the oxidation of the metallic nickel species in the catalyst. Assuming that all of the metallic nickel is oxidized to NiO, the mass gain of 0.354 mg should be the oxygen in NiO,
and thus the mass of NiO in the catalyst is 1.653 mg, accounting for 19.89% of the total mass of the dry catalyst. This result is in accordance with that measured by the EDX. The mass loss of 0.010 mg from 606 C to 655 C is ascribed to the active carbon deposited on the surface of the catalyst [56,57], accounting for 0.12% of the total mass of the dry catalyst. The active carbon is responsible for the formation of methane. Fig. 9b shows the TG profile of the catalyst after 40 h on stream
Fig. 8 e Time on stream of the catalyst of Ni/ZrO2-15-V for CO (a) and CO2 (b) methanation. GHSV ¼ 10,000 h¡1 and T ¼ 450 C for CO methanation; GHSV ¼ 12,000 h¡1 and T ¼ 450 C for CO2 methanation.
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Fig. 9 e TGA of the used Ni/ZrO2-15-V catalyst after 40 h on stream for CO methanation (a) and CO2 methanation (b).
for CO2 methanation. The mass loss at the temperature below 163 C is ascribed to the moisture evaporation [56,57], and the mass gain at the temperature range from 163 C to 592 C is ascribed to the oxidation of the metallic nickel species in the catalyst. The mass gain of 0.411 mg is the mass of oxygen in NiO. Therefore the mass of NiO in the catalyst is 1.918 mg, which accounts for 23.81% of the total mass of the dry catalyst. The mass loss of 0.010 mg from 592 C to 651 C is ascribed to the active carbon deposited on the surface of the catalyst [56,57], which accounts for 0.12% of the total mass of the dry catalyst. The TG analysis indicates that no inactive carbon deposited on the catalyst for both CO and CO2 methanation. This is why this catalyst exhibits good stabilities for the two reactions. The TEM observation provides supplementary evidence for the low carbon deposition on this catalyst. The Fig. 10a and b show the TEM images of the used catalyst of Ni/ZrO2-15-V after 40 h on stream for CO and CO2 methanation, respectively. The carbon deposition cannot be observed on the surface of the catalyst. However, the morphology of the used Ni/ ZrO2-15-V is quite different from that of the as-prepared catalyst (Fig. 4g). The insets in the upper right corners of image a and image b in Fig. 10 are the XRD profiles of the used
Ni/ZrO2-15-V catalyst for CO and CO2 methanation, respectively. The XRD results indicate that NiO dispersed on the zirconia-covered surfaces of the as-prepared catalyst has been reduced to metallic Ni nanoparticles during the reaction. The average size of the Ni crystals on the used Ni/ZrO2-15-V for CO methanation is 19.7 nm and that for CO2 methanation is 19.2 nm, calculated by the Scherrer's formula using the diffraction peak of the (111) crystal faces of Ni. Compared the Ni crystals observed in the TEM images in Fig. 10 with the average crystal size of NiO in the as-prepared catalyst calculated by XRD, it is obvious that the sizes of the metallic Ni nanoparticles grew slightly larger during the reaction, which might be resulted from the local sintering of the highly dispersed NiO on the zirconia-covered surfaces of the catalyst, however the sintering of the Ni nanoparticles in a remarkable scale has not occurred.
Discussion The nickel catalyst supported on zirconia-modified clays exhibits excellent catalytic performances for CO and CO2 methanation. The superior catalytic performance is resulted from the unique structure of the catalyst. The raw clays have
Fig. 10 e TEM images of the used Ni/ZrO2-15-V catalyst after 40 h on stream for CO methanation (a) and CO2 methanation (b). The insets in image a and image b are the XRD profiles of the used Ni/ZrO2-15-V after CO and CO2 methanation for 40 h, respectively.
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the small specific surface area and narrow slit-like pores that are not beneficial to the diffusion of the reactants and products; therefore they are seldom used as the support of a catalyst directly. The zirconyl nitrate solution is highly acidic. The Al in the octahedral sublayers of clays can be partly leached out by acid during the hydrothermal treatment when a suitable amount of the zirconyl nitrate solution is mixed with the clay suspension. As a result, a new structure with zirconia highly dispersed on the partly damaged clay layers is obtained in one pot. The new structure has a bimodal pore size distribution (Fig. 5b). Most of the mesopores with the sizes smaller than 10 nm are resulted from the zirconia pillared clays, which is evidenced by the peak shift of the diffractions of the (003) and (005) crystal faces of clays in the zirconiamodified clays (Fig. 2a). Apparently, zirconia nanoparticles play the role as pillars in addition to covering the partly damaged clay layers. The mesopores larger than 10 nm and the macropores larger than 50 nm come from the damaged clay layers by the acid leaching. The schematic diagram is shown in Scheme 1. The zirconia nanoparticles are very small and highly dispersed on the surfaces of the clay layers because they cannot be detected by XRD and only homogeneous fine nanoparticles can be observed on the damaged clay layers by TEM. Nickel oxide in cubic phase is the only nickel species that can be detected by XRD, and the NiO nanoparticles buried in the stack layers of zirconia nanoparticles on the surfaces of the clay layers can be observed by TEM. It seems to be that zirconia plays a role for the dispersion of the NiO nanoparticles, which is in consistence with the observations of other researchers [10,18,20,24,44]. Nickel oxide is reduced to metallic nickel by H2 before reaction. The metallic nickel
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nanoparticles grow slightly larger after the methanation reaction, which comes from the local sintering of the highly dispersed nickel oxide in the zirconia layers. However, the sintering that can result in the deactivation of the catalyst does not occur. Almost uniform Ni nanoparticles with an average size of about 20 nm, which are the active species for the CO and CO2 methanation reactions, can be observed on the clay layers of the zirconia-modified clays by TEM. Apparently, the highly dispersed zirconia prevents the Ni species from sintering in a remarkable scale. There are six types of Ni species in the catalyst, which are the highly dispersed NiO on zirconia or on the clay layers, the free NiO nanoparticles on zirconia or on the clay layers, the fixed NiO and nickel aluminate. The ratios of the highly dispersed NiO on zirconia to that on the clay layers as well as the free NiO on zirconia to that on the clay layers increase with increasing the zirconia content. The NiO species on zirconia is more readily to be reduced than that on the surface of the clay layers. The NiO species on zirconia has a close relation to the catalytic performance of the catalyst, which might give rise to more active Ni species after reduction. The amount of nickel aluminate that is hard to be reduced remarkably decreases with increasing the zirconia content, suggesting that zirconia prevents the formation of nickel aluminate. The NiO nanoparticles are not the active species for the CO or CO2 methanation but they give rise to the active metallic Ni nanoparticles after reduction by hydrogen. All NiO except that forming nickel aluminate with the residual Al in clays can be reduced to active metallic Ni, but the activity of the metallic Ni located at different sites is different, which has a close relation to its chemical environment. Most researchers believed that
Scheme 1 e Schematic preparation process of the zirconia-modified clays and the nickel catalyst supported on the zirconiamodified clays, as well as the evolution of the nickel species during the reaction.
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Ni on zirconia was more active than on other supports [10,18,20,24,44], and some of them even believed that both zirconia and Ni played important roles during the methanation [22,58]. The proofs that zirconia promotes the dispersion of Ni species and prevents them from sintering are sufficient in this study, but no evidence indicates that zirconia involves the reaction. In addition, zirconia might play an important role to reduce the inactive carbon deposition and thus enhance the stability of the catalyst. The methane selectivity for CO methanation exhibits a slight increase at low temperatures for each catalyst, then a sharp decrease for Ni/ZrO2-5-V or Ni/ZrO2-10-V and a slight decrease for Ni/ZrO2-15-V or Ni/ZrO2-20-V at high temperatures. This phenomenon is due to the thermodynamic equilibrium at the reaction conditions. The methane selectivity decreases from almost 100% to about 40% from 200 C to 600 C at 1 atm with the ratio of H2/CO ¼ 3 according to the thermodynamic calculation [59]. For the same reason, the methane selectivity for CO2 methanation maintains at a constant level from 350 C to 500 C for all catalysts.
Conclusions A new structure with zirconia nanoparticles highly dispersed on the partly damaged clay layers has been prepared in one pot by the hydrothermal treatment of the mixture of the clay suspension and the zirconyl nitrate solution. The new structure has the typical bimodal pore size distribution. Most of the pores with the sizes smaller than10 nm are resulted from the zirconia pillared clays and the mesopores with the sizes larger than 10 nm and the macropores with the sizes larger than 50 nm are resulted from the partly damaged clay layers. The nickel catalyst supported on the new zirconia-modified clays has been prepared by the incipient wetness impregnation. Nickel oxide in cubic phase is the only nickel species that can be detected by the XRD. The nickel oxide nanoparticles with the sizes of a dozen nanometers or more are well dispersed on the zirconiamodified clay layers. There are six types of the nickel oxide species on the new structure. Five of them except the nickel species that forms the spinel phase with aluminum in clays can be reduced to the active nickel species for the CO or CO2 methanation. But the activity of the metallic nickel species is different, which is associated with the chemical environment at which the metallic nickel species is located. The catalyst with higher zirconia content, which also has the larger specific surface area and pore volume, exhibits better catalytic performance for the CO or CO2 methanation. Zirconia in the catalyst is responsible for the dispersion of the nickel species, and it prevents the metallic nickel nanoparticles from sintering during the process of the reaction. In addition, it is also responsible for the reduction of the inactive carbon deposition.
Acknowledgment The work was financially supported by the key project of Inner Mongolia Education Department (NJZZ13002), the Natural Science foundation of Inner Mongolia (2014MS0207), the
project of Grassland Talent of Inner Mongolia and the project of the western light from the ODPC of Inner Mongolia.
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Mesoporous zirconia-modified clays supported nickel catalysts for CO and CO2 methanation Huailiang Lu a, Xuzhuang Yang a,*, Guanjun Gao a, Kebing Wang b, Quanquan Shi c, Jie Wang a, Chenhui Han a, Jie Liu a, Min Tong a, Xiaoyuan Liang a, Changfu Li a a
School of Chemistry and Chemical Engineering, Inner Mongolia University, Hohhot, Inner Mongolia, 010021, PR China b College of Science, Inner Mongolia Agricultural University, Hohhot, Inner Mongolia, 010018, PR China c State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, PR China
article info
abstract
Article history:
The Ni catalysts supported on a new structure with zirconia nanoparticles highly dispersed
Received 9 August 2014
on the partly damaged clay layers has been prepared by the incipient wetness impregna-
Received in revised form
tion method and the new structure of the support has been prepared in one pot by the
10 September 2014
hydrothermal treatment of the mixture of the clay suspension and the ZrO(NO3)2 solution.
Accepted 16 September 2014
The catalytic performances for the CO and CO2 methanation on the catalysts have been
Available online 8 October 2014
investigated at a temperature range from 300 C to 500 C at atmospheric pressure. The catalysts and supports have been characterized by X-ray diffraction (XRD), transmittance
Keywords:
electron microscopy (TEM), H2 temperature-programmed reduction (H2-TPR), nitrogen
CO methanation
adsorptionedesorption, and thermogravimetry and differential thermal analysis (TG-DTA).
CO2 methanation
It is found that the zirconia-modified clays have the typical bimodal pore size distribution.
SNG
Most of the pores with the sizes smaller than 10 nm are resulted from the zirconia pillared
Zirconia
clays and the mesopores with the sizes larger than 10 nm and the macropores with the
Clays
sizes larger than 50 nm are resulted from the partly damaged clay layers. The bimodal pore structure is beneficial to the dispersion of Ni on the layers of the zirconia-modified clays and the increase in Ni loading. The zirconia nanoparticles are highly dispersed on the partly damaged clay layers. Nickel oxide in cubic phase is the only Ni species that can be detected by XRD. The nickel oxide nanoparticles with the sizes of 12 nanometers or more are well dispersed on the zirconia-modified clay layers, which are observed to be buried in the stack layers of zirconia. The presence of nickel oxide in six different forms could be perceived on the new structure. Five of them except the Ni species that forms the spinel phase with Al in clays can be reduced to the active Ni species for the CO and CO2 methanation. But the activity of the Ni species is different, which is associated with the chemical environment at which the Ni species is located. The catalyst with the higher zirconia content, which also has the larger specific surface area and pore volume, exhibits the better catalytic performance for the CO or CO2 methanation. Zirconia in the catalyst is responsible for the dispersion of the Ni species, and it prevents the metallic Ni nanoparticles from sintering during the process of the reaction. In addition, it is also responsible for the reduction of the inactive carbon deposition. The catalyst with 15 wt.% zirconia
* Corresponding author. Tel.: þ86 471 4992982; fax: þ86 471 4992981. E-mail addresses: [email protected], [email protected] (X. Yang). http://dx.doi.org/10.1016/j.ijhydene.2014.09.076 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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content has the highest CO conversion of about 100% and the highest methane selectivity of about 93% at 450 C for CO methanation, and the catalyst with 20% zirconia content has the CO2 conversion of about 80% and the highest methane selectivity of about 99% for CO2 methanation at 350 C. The catalyst with 15 wt.% zirconia possesses promising stability and no distinct deactivation could be perceived after reaction for 40 h. This new catalyst has great potential to be used in the conversion of the blast furnace gas (BFG) and the coke oven gas (COG) to methane. Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction The “growth-at-any-cost” strategy in some countries results in serious environmental issues. In recent years, smog and haze have routinely been blanketing Beijing and other cities in China. The pollution is not limited to China d it affects the whole Northern Hemisphere through the atmospheric circulation [1]. The blast furnace gas (BFG) and the coke oven gas (COG), containing carbon monoxide, carbon dioxide, methane, hydrogen, and small solid particles etc. [2,3], may play an important role for air pollution if they are treated inadequately. Facing the environmental impact, Beijing has ordered more than 50 heavy-machinery and chemical companies to move to less developed regions [1], including several coke enterprises and its largest steel works [3]. However, such an action is not an ultimate solution but just the pollution transfer. The BFG and COG are normally combusted directly or combusted for electricity [4,5]. In recent years, they have been studied as raw materials to produce hydrogen [2,6], syngas [7], methanol [8,9], and methane [10,11], by which a high percentage of potential energy can be recovered and a large amount of greenhouse gas emissions can be reduced, and thus it has attracted great attentions of researchers. In some countries such as China the natural gas supplies are insufficient. It is reported that the total production capacity of synthetic or substitute natural gas (SNG) by plants under construction and planned projects in China will exceed 300,000 million Nm3 per year in the next ten years [12]. Accordingly, to produce SNG from BFG and/or COG is a good choice, which can not only alleviate the environmental pressure but also meet the market demand. The methanation reaction was first studied by Sabatier and Senderens in the beginning of the last century [13]. The earliest use of the technology of methanation was to remove the trace carbon oxides from the feed gas for ammonia synthesis [14]. In recent years, this reaction has also been studied for eliminating the trace CO in the hydrogen-rich gas, a fuel for the proton exchange membrane fuel cell (PEMFC), in order to prevent the anodes from being poisoned [15e18]. Another application of COx (x ¼ 1,2) methanation is to synthesize SNG. J. Kopyscinski et al. [19] have reviewed the production technology of SNG from 1950 to 2009, and mentioned that only one commercial SNG plant, the Great Plains Synfuels Plant of the Dakota Gasification Company (North Dakota, United States), has been commissioned since 1984, although several
demonstration or pilot plants have been constructed in Germany and Great Britain since the 1970s. Supported nickel catalysts are the most promising catalysts for COx (x ¼ 1,2) methanation due to their good catalytic performances and relatively low prices [10,20e29]. Metal oxides such as Al2O3 [30e35], SiO2 [15,18,36,37], TiO2 [38], ZrO2 [15,21,39e41] and CeO2 [10,42] are the most commonly used supports for nickel catalysts. Among them, Ni/SiO2 and Ni/ Al2O3 have been widely and extensively studied due to their good initial activities and relatively cheap prices [15,18,30e37]. However, nickel supported on silica or alumina often suffers from sintering and serious carbon deposition at high temperatures [32,43]. Some researchers have tried to dope zirconia in the catalyst to promote the dispersion of Ni species and prevent them from sintering [10,18,24,41,44], and achieved high activities and good stabilities. In our previous work [41], a nickel catalyst supported on a Al2O3@ZrO2 core/shell structure has been prepared for CO methanation. It has been found that zirconia on the surface of the core/shell structure promotes the dispersion and influences the reduction states of the nickel species on the support, and thus prevents the Ni species from sintering as well as from forming a spinel phase with alumina at high temperatures. On the other hand, some researchers have attempted to load Ni on porous supports on which the Ni species are supposed to be confined within the local spaces of the mesopores [45e49]. Xu et al. [45] have prepared a nickel catalyst supported on an ordered mesoporous alumina, but a large amount of the spinel phase NiAl2O4 have been formed during the calcination. Some researchers have prepared Niincorporated MCM-41 catalysts [46e48], but the Ni content cannot exceed 10 wt.% or extra Ni that cannot be incorporated in the frameworks of MCM-41will block the tunnels of the pores due to the narrow pore size (less than 10 nm). In another work of us [49], Ni species of 15 wt.% has been found to be well dispersed in the three dimensional interconnected mesopores composed by partly damaged clay layers resulted from acidleaching, which effectively prevents the Ni species from sintering during the methanation reaction. Volkonskoite belongs to family smectites and has abundant slit-like mesopores composed by plate-like clay layers with cations such as Naþ and Ca2þ in the interlayers. The clay layer is composed by an Al octahedral sublayer sandwiched by two Si tetrahedral sublayers [50]. The raw volkonskoite cannot be directly used as the support of catalyst due to its small specific surface area. The pillared process [50] and the acidleaching process [49,51] can remarkably increase the specific surface area of clays, but the pore structures of the two
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modified clays are quite different (Fig. 1). The pillared clays have intact clay layers with metal oxide pillars in the interlayers, but the acid leached clays have damaged clay layers with macro- or meso-openings on the layers. The reactants and products can only diffuse through the slit-like pores at the end sides of the clay layers of the pillared clays, but they can diffuse freely through the openings on the damaged clay layers and at the end sides of the clay layers of the acidtreated clays. In addition, part of Al in volkonskoite can be leached out by acid, which decreases the possibility of forming the spinel phase of nickel aluminate in the catalyst. Therefore, the layer-damaged clays have a great potential as a cheap and effective support for the nickel catalyst. In the present study, the zirconia-modified clays with a bimodal size distribution were prepared in one pot by the hydrothermal treatment of the mixture of the clay suspension and the zirconyl nitrate solution. The nickel catalysts supported on the zirconia-modified clays were prepared by the incipient wetness impregnation method. The catalysts and the supports were characterized by XRD, TEM, TPR, TG-DTA and nitrogen adsorptionedesorption. The activity and the CH4 selectivity of the catalyst for the CO or CO2 methanation were investigated. The aims of this study are to investigate the effects of the Zr content and pore structures of the zirconiamodified clays on the dispersion of the Ni species, the carbon deposition as well as the catalytic performances for CO and CO2 methanation, by which a possible catalyst that has high performances for the conversions of CO and CO2 to methane is to be developed.
Experimental Materials Volkonskoite was purchased from J & K Scientific. ZrO(NO3)2$2H2O, Ni(NO3)2$6H2O were purchased from Sinopharm Chemical Reagent Co. Ltd. All chemicals were used as received.
Catalyst preparation The clays of 5.0 g were dispersed into deionized water of 250 ml, continuously stirring for 24 h. An appropriate amount of the aqueous solution of zirconyl nitrate dihydrate was poured into the clay suspension with continuously stirring for 5 h, and then the suspension was transferred into autoclaves for the hydrothermal treatment at 150 C for 24 h. After washing with water and ethanol and drying at 100 C, the zirconia-modified mesoporous clays were then obtained after the calcination at 500 C for 5 h. The samples are labeled as
ZrO2-5-V, ZrO2-10-V, ZrO2-15-V and ZrO2-20-V. The number in the middle of the sample name stands for the zirconia content in the sample. The raw clays are labeled as VK. The supported nickel catalysts were prepared by the incipient wetness impregnation method. A suitable amount of the aqueous solution of Ni(NO3)2$6H2O was added into the support material. After standing for 24 h, the solids obtained were dried and calcined by a temperature programmed from room temperature to 500 Cat a heating rate of 2 C/min, and then kept at 500 C for 5 h. The nominal load of Ni in each of the catalyst is 15 wt.%. The catalysts are labeled as Ni/ZrO2-5V, Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V.
Characterization The X-ray diffraction (XRD) measurements were performed using a Bruker D8 Advance diffractometer with a Cu Ka radiation of wavelength 0.1541 nm operated at 40 kV and 40 mA with a scanning speed of 2 /min and a scanning angle (2q) range of 5e80 . The average crystallite size of the chemical was evaluated using the well-known Scherrer's equation (d ¼ 0.89l/bcosq). The N2 adsorptionedesorption isotherms were measured using a Micrometritics ASAP 2020 analyzer. The specific surface areas were calculated by the BrunauereEmmetteTeller (BET) equation from the N2 adsorption isotherm. The pore structure parameters were obtained from the adsorption branch using the BJH model. Prior to the experiment of adsorption, all samples were degassed under vacuum at 150 C for 12 h. The morphologies of the samples were observed using a scanning electron microscopy (SEM, HITACHI, S-4800), coupled with an X-ray energy dispersive spectroscopy (EDS) system to analyze the composition of the selected areas of the sample. The transmission electron microscopy (TEM) measurements were carried out using a JEM-2100 TEM operated at 100 kV. The temperature programmed reduction with hydrogen (H2-TPR) was performed with a Micromeritics AutoChem 2910 analyzer. Typically, the catalyst of 50 mg with the sizes between 0.25 mm and 0.42 mm was loaded into a quartz tube, and purged with N2 for 1 h at 300 C to remove the adsorbed H2O and other volatile gases on the sample. After cooling to 50 C, the inlet gas was switched to the gas mixture of 5% H2 balanced with N2 at a flow rate of 20 mL/min. Then, the temperature was elevated to 900 Cat a heating rate of 10 C/ min, in the meantime the hydrogen concentration in the effluent was monitored by a thermal conductivity detector (TCD). The thermogravimetric and differential thermal analyzer (TG-DTA), SII TG/DTA7300 model, was used to analyze the carbon deposition of the spent catalyst, operated in a temperature range from room temperature to 900 Cin air at a heating rate of 10 C/min. The EXTRA 600 System was used to analyze the data.
Methanation of COx (x ¼ 1,2)
Pillared clay
Layer damaged clay
Fig. 1 e Scheme of pillared clays and layer damaged clays.
The experiment for COx (x ¼ 1,2) methanation was performed in a fixed bed tubular reactor with a diameter of 10 mm at
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atmospheric pressure, and a thermocouple was installed in the middle of the catalyst bed to measure the temperature. The mixture of the catalyst of 0.5 ml with the sizes of 0.25e0.42 mm and quartz of 0.5 ml with the same sizes was loaded in the reactor in order to maintain the catalyst bed a constant temperature. The catalyst was reduced in hydrogen at 500 Cfor 3 h before the reaction. After cooling to the reaction temperature in N2, the feed gas with H2/CO ¼ 3 or H2/ CO2 ¼ 4 was introduced into the reactor at a certain flow rate, which was determined by the gas hourly space velocity (GHSV) and the load of the catalyst. The concentrations of CO, CO2, H2 and CH4 in the outlet gas were measured on-line by a GC1100 chromatograph equipped with a 2 m TDX01 column and a thermal conductivity detector. Other contents were measured on another chromatograph of the same model with a TCD detector and a Porapak Q column. The calculation formulas of the COx conversion and the methane selectivity are described as eq. (1) and eq. (2). COx conversion; ðXCOx ; %Þ :
XCOx ð%Þ ¼
FCOx ;in FCOx ;out 100% FCOx ;in (1)
CH4 selectivity; ðSCH4 ; %Þ :
SCH4 ð%Þ ¼
FCH4 100% FCOx ;in Fcox ;out (2)
where FCOx ;in , FCOx ;out and FCH4 are the mole flow rates, mol h1, of reactant in the inlet, reactant in the outlet and product in the outlet, respectively.
Results and discussion XRD The XRD patterns of the raw clays and the zirconia-modified clays are shown in Fig. 2a. The XRD pattern of the raw clays is in accordance with that of volkonskoite according to the International Centre for Diffraction Data (ICDD) PDF file No. 00-042-0619. The peaks at 21.8 and 26.8 of the 2 theta are
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ascribed to the diffractions of the (100) and (101) crystal faces of quartz, suggesting small amount of quartz in the raw clays. The aqueous solution of zirconyl nitrate is strongly acidic. Aluminum in the octahedral sublayers of clays can be leached out during the hydrothermal treatment of the mixture of the clay suspension and the zirconyl nitrate solution. Table 1 lists the contents of different elements in the catalysts. With increasing the Zr content from 2.57 at% to 18.96 at%, the content of Al decreases sharply from 20.73 at% to 9.56 at%; whereas, the content of Si remains almost constant. Therefore, the loss of Al from the octahedral sublayers of clays is evident. However, the fundamental phase structure of clays does not collapse because most of the characteristic peaks attributed to the clays, such as the peaks ascribed to the diffractions of the (100), (110) and (300) crystal faces of the clays, remain unchanged. The peaks attributed to the diffractions of the (005) crystal faces in the patterns of the zirconia-modified clays shift from the original 29.5 e28.1 , suggesting the enlargement of the distance between the (001) faces (d(001)). As a result, the peaks at the position of a in the patterns of the zirconia-modified clays should not be ascribed to the diffractions of the (001) faces but the (003) faces instead. In this case, the d(001) of the zirconia-modified clays increases to about 3 nm from the original 1.5 nm. In addition, with increasing the zirconia content, the peaks at the position a shift to the low angle direction, suggesting the d(001) increases with increasing the zirconia content in the clays. Zirconia in monoclinic phase also gives the strongest diffraction of the (111) crystal faces at 28.2 of the 2 theta. The reasons why the peaks at 28.2 in the patterns of the zirconia-modified clays are ascribed to the diffraction of the (005) faces of clays but not that of the (111) faces of zirconia are that the intensity of these peaks becomes weaker and weaker but not stronger and stronger and the peak positions slightly shift to the low angle direction but not remain unchanged with the increase of the zirconia content. Therefore, the sizes of the zirconia crystals in the modified clays must be very small or amorphous so that it cannot be detected by XRD. The diffraction peaks attributed to clays in the patterns of the nickel catalysts (Fig. 2b) are almost the same as those in
Fig. 2 e XRD patterns of the raw clays and the zirconia-modified clays (a) and nickel catalysts supported the zirconiamodified clays (b).
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Table 1 e Compositions of catalysts with different content of Zr. Si （at%） Ni （at%） Al （at%） Zr （at%）
Ni/ZrO2-5-V
Ni/ZrO2-10-V
Ni/ZrO2-15-V
Ni/ZrO2-20-V
57.59 19.11 20.73 2.57
55.54 19.12 20.40 4.94
57.09 17.03 18.07 7.81
54.72 16.76 9.56 18.96
the patterns of the zirconia-modified clays before loading with nickel (Fig. 2a), indicating that the process of loading nickel does not influence the phase structure of the supports. The nickel species in the catalysts is nickel oxide in cubic phase. The peaks at 37.3 , 43.3 , 62.9 and 75.4 of the 2 theta in the patterns of the nickel catalysts in Fig. 2b are attributed to the diffractions of the (111), (200), (220) and (311) crystal faces of NiO, respectively. The crystal sizes of NiO in the catalysts of Ni/ZrO2-5-V, Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V are 16.90 nm, 20.20 nm, 20.03 nm and 23.62 nm, respectively, calculated by Scherrer's formula according to the diffractions of the (200) crystal faces (See Table 2.).
TEM The micromorphology of the raw clays is shown in Fig. 3. The plate-like clay layers are clearly observed, which are similar to those in literature [52]. The images of the zirconia-modified clays are shown in Fig. 4aed. The plate-like clay layers can be observed in the images as well, which is consistent with the results of XRD. However, the damage or collapse of the clay layers is intensified with increasing the zirconia content. This is because the acidity of the mixture of the clay suspension and the zirconyl nitrate solution increases with increasing the amount of the zirconyl nitrate solution, which can result in increasing loss of Al from the clay layers by acid leaching. The Zr species is seldom observed on the surfaces of the clay layers of the samples ZrO2-15-V and ZrO2-10-V, but several clusters of Zr species with relatively larger size are observed on the clay layers of the sample ZrO2-5-V. The Zr species cannot be definitely identified on the clay layers of the sample ZrO2-20-V in image d because there are a lot of clay layer fragments mixing with it. However, the morphology of the
fine nanoparticles in image d is quite different from that of zirconia on the sample ZrO2-5-V. This is because the acidity of the precursor suspension not only influences the release of Al from the clay layers but also influences the crystallization of zirconia. The XRD results show that the d(001) of the clays in the zirconia-modified clays is about 3 nm, suggesting that the sizes of a large amount of zirconia in these samples are about 3 nm. This is also the reason that zirconia in these samples cannot be detected by XRD. In Fig. 4eeh, most NiO nanoparticles observed are embedded in the zirconia-modified clay layers. The sizes of NiO nanocrystals calculated by XRD are about 20 nm, but those observed by TEM seem to be larger. This is because that the size calculated by XRD is the average crystal size of NiO but the size observed by TEM is the particle size of NiO, which may be consisted of several nanocrystals.
Specific surface area and pore structure Table 2 lists the specific surface areas and pore volumes of the raw clays, the zirconia-modified clays and the nickel catalysts supported on the zirconia-modified clays, as well as the sizes of NiO nanocrystals in the catalysts. The specific surface area and pore volume of the VK are 63.97 m2/g and 0.11 cm3/g, respectively. After the modification by zirconia, the specific surface areas and pore volumes increase dramatically. With increasing the zirconia content, the specific surface areas increase from 89.45 m2/g to 164.12 m2/g and the pore volumes increase from 0.15 cm3/g to 0.19 cm3/g. After loading with Ni, the specific surface areas of the catalysts decrease by about 20% and the pore volumes decrease by about 15% compared with the zirconia-modified clays. Fig. 5a shows the nitrogen adsorptionedesorption isotherms of the raw clays and the zirconia-modified clays. All of
Table 2 e BET specific surface areas, pore volumes of supports and catalysts as well as the crystal sizes of nickel oxide on the catalysts. Samples VK ZrO2-5-V ZrO2-10-V ZrO2-15-V ZrO2-20-V Ni/ZrO2-5-V Ni/ZrO2-10-V Ni/ZrO2-15-V Ni/ZrO2-20-V
BET area, m2/g
Pore volume, cm3/g
Crystal size,a nm
63.97 89.45 117.16 155.35 164.12 73.78 93.25 121.90 133.38
0.11 0.15 0.16 0.17 0.19 0.13 0.13 0.14 0.16
e e e e e 16.90 20.20 20.03 23.62
a
Crystal sizes of nickel oxide are calculated by Scherrer's formula according to the diffractions of the (200) crystal faces.
Fig. 3 e TEM image of the raw clays of VK.
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Fig. 4 e TEM images of the zirconia-modified clays (aeZrO2-5-V; beZrO2-10-V; ceZrO2-15-V; deZrO2-20-V) and the nickel catalysts supported on the zirconia-modified clays (eeNi/ZrO2-5-V; feNi/ZrO2-10-V; geNi/ZrO2-15-V; heNi/ZrO2-20-V).
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Fig. 5 e Nitrogen adsorption and desorption isotherms of the raw clays, the zirconia-modified clays and the catalysts (a, c) and the pore size distributions of the zirconia-modified clays and the catalysts (b, d).
the five isotherms of VK, ZrO2-5-V, ZrO2-10-V, ZrO2-15-V and ZrO2-20-V are the type IV isotherms, suggesting there are mesopores in these samples. The hysteresis loop of the sample VK is attributed to the type H4 loop, which is associated with the narrow slit-like pores [53]. The hysteresis loops of ZrO2-5-V, ZrO2-10-V, ZrO2-15-V and ZrO2-20-V are similar to the type H3 loop, which is associated with the slit-shape pore structure composed by plate-like particles. The change of the hysteresis loop from the type H4 of the raw clays to the type H3 of the zirconia-modified clays implies the enlargement of the slit-like openings of the mesopores in the zirconiamodified clays, which is resulted from the pillaring effect by the zirconia nanoparticles as well as the perforation on the clay layers by the acid corrosion during the hydrothermal treatment. With increasing the zirconia content, the hysteresis loop is gradually close to the type H1 loop, which is often associated with the porous structure fabricated by regular particles [53], suggesting the plate-like clay layers gradually become almost uniform clay fragments due to acid increasingly leaching out Al from the octahedral sublayers. In addition, the pore volume almost proportionally increases with the zirconia content. The increase of the pore volume is resulted from the mesopores and macropores newly formed
by acid leaching. The Fig. 5b shows the profiles of the pore size distributions of the raw clays and the zirconia-modified clays. The VK has the mesopores with the sizes distributing in the range from 3 nm to 4 nm, which is originated from the slit-like pores in the raw clays. The small amount of the mesopores with the sizes from 4 nm to 30 nm in the raw clay is related to the agglomerates that are the assemblage of the clay particles rigidly jointed together [53]. The sample ZrO2-5-V has an obvious bimodal pore size distribution, one from 3 nm to 10 nm and the other from 10 nm to 20 nm. The former comes from the zirconia pillared clays and the latter is from the openings on the clay layers resulted from the acid corrosion. However, when the zirconia content in the sample exceeds 10 wt.% the pores in the sample are composed by a large amount of mesopores with the sizes between 3 nm and 5 nm, a quite amount of mesopores with the sizes from 5 nm to 50 nm and a small amount of macropores with the sizes larger than 50 nm. The mesopores with the sizes smaller than 5 nm are resulted from the zirconia pillared clays, which decrease by about 5 nm compared with those in the sample ZrO2-5-V. This is because extra zirconia in the samples with high zirconia content fills some of the spaces of the mesopores in the zirconia pillared clays. The mesopores larger than 5 nm and
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the macropores larger than 50 nm are resulted from the damaged clay layers by acid leaching. With the increase of the zirconia content, the amount of the mesopores and macropores resulted from acid leaching increased markedly. The isotherms of the nickel catalysts supported on zirconia-modified clays are attributed to the type IV isotherm as well, shown in Fig. 5c, and it suggests that mesopores exist in the catalysts. The hysteresis loops of the catalysts are attributed to the type H3 loop, which are consistent with those of the corresponding supports. The type H3 loop is associated with the slit-shape pore structure composed by plate-like particles. It suggests that zirconia and nickel oxide are most possibly attached on the clay layers, which is also evidenced by the TEM observation. Fig. 5d shows the pore size distribution profiles of the catalysts. The pore size distribution of the catalyst Ni/ZrO2-5-V changes greatly from that of the corresponding support. This is because the nickel species filled a part of the spaces of the mesopores between and on the clay layers of the support and gave rise to smaller mesopores with the sizes between 3 nm and 4 nm, and the larger Ni species as well as the fragments of the clay layers covered by zirconia jointed rigidly together to give rise to the mesopores larger than 4 nm and the macropores larger than 50 nm. Compared with the corresponding supports, the pore size distributions of the catalysts of Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V change slightly, which does not mean that the pores in
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corresponding support are well inherited by the catalyst but means that the pores newly formed in the catalyst after loading with Ni has the similar size distribution to the corresponding support.
H2-TPR The Fig. 6 shows the H2-TPR profiles of the as-prepared nickel catalysts supported on the zirconia-modified clays. The H2 reduction profiles of the catalysts can be classified into four bands, band a, band b, band g and band s. The peaks in the band a are attributed to the highly dispersed nickel species, such as the highly dispersed Ni2O3 and NiO on the surface of the supports [54]. According to the XRD results, there is only NiO in cubic phase in the catalysts. Therefore the Ni species in this band for all of the four catalysts is ascribed to the highly dispersed NiO. However, two small peaks can be obtained in the band a for the Ni/ZrO2-5-V catalyst by deconvolution, one centered at 356.8 C and the other peak at 373.7 C. The peak centered at 356.8 C is ascribed to the NiO highly dispersed on zirconia that is dispersed on the damaged clay layers and the one centered at 373.7 C is ascribed to the NiO highly dispersed on the exposed surfaces of the clay layers, because this support is fabricated by zirconia of about 5 wt.% highly dispersed on the clay layers so that the surfaces exposed may be zirconia or the clay layers. With increasing the zirconia content the
Fig. 6 e TPR profiles of the catalysts supported on zirconia-modified clays.
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clay surfaces exposed decreased dramatically, resulting in a dramatic decrease of the Ni species highly dispersed on the clay layers. Therefore the peaks centered at about 373 C cannot be identified in the samples with higher zirconia contents, which might be hidden in the larger peaks nearby if any. The peaks centered at about 400 C are generally ascribed to the reduction of the bulk NiO [54]. However, two peaks can also be obtained in the band b for each of the four catalysts by deconvolution, one located at 395 Ce425 C and the other located at 450 Ce470 C. The former is ascribed to the bulk NiO on zirconia and the latter is ascribed to the bulk NiO on the clay layers due to the same reason as in band a. From Table 3, it is observed that the ratio of the content of NiO on zirconia to that on the clay layers increases from 1.27 to 3.56 with increasing the zirconia content. In the band g the peaks located at about 520 Ce555 C are attributed to the reduction of NiO interacting strongly with the supports, which is also called fixed NiO. The contents of the fixed NiO in the catalysts of Ni/ZrO2-5-V, Ni/ZrO2-10-V, Ni/ZrO2-15-V and Ni/ZrO2-20-V are 11.88, 12.97, 5.77 and 21.81, respectively (See Table 3). At least a fraction of the fixed NiO is possible to be reduced to the active Ni during the reaction. The peaks in the band s from 620 C to 810 Care attributed to the reduction of nickel aluminate or nickel silicate [54,55], which cannot be reduced to active Ni during the reaction. It is evident that the content of nickel aluminate or nickel silicate in the catalyst decreased
Table 3 e Nickel species and content in the catalysts. Sample name Ni/ZrO2-5-V
Species reduced
Peak area
Dispersed NiO 481.9 984.9 Dispersed NiOa NiO on Zirconia 7626.1 NiO on clay 5988.9 Fixed NiO 2281.8 Nickel 1833.8 aluminate Ni/ZrO2-10-V Dispersed NiO 458.9 NiO on Zirconia 9073.3 NiO on clay 6085.0 Fixed NiO 2550.1 Nickel 1493.3 aluminate 1261.1 Ni/ZrO2-15-V Dispersed NiO NiO on zirconia 8619.3 NiO on clay 2628.0 Fixed NiO 808.9 Nickel 683.1 aluminate 1119.6 Ni/ZrO2-20-V Dispersed NiO NiO on zirconia 10,879.9 NiO on clay 3054.8 Fixed NiO 4459.2 Nickel 923.5 aluminate
a
Content of Peak species, % position, C 2.51 5.13 39.72 31.19 11.88 9.55
356.8 373.7 416.8 463.3 542.9 707.3
2.33 46.14 30.95 12.97 7.59
368.5 425.0 472.8 555.8 694.7
9.00 61.56 18.77 5.77 4.87
351.8 394.7 450.6 543.1 681.9
5.47 53.23 14.94 21.81 4.51
359.2 412.2 470.2 520.6 682.6
There are two types of the highly dispersed NiO on the surfaces of the Ni/ZrO2-5-V catalyst, and the one reduced at a lower temperature is dispersed on zirconia and the other one reduced at a higher temperature is dispersed on the clay layers.
from 9.55 wt.% to 4.51 wt.% with increasing the zirconia content from 5 wt.% to 20 wt.%.
Catalytic performance for CO and CO2 methanation The CO conversion, methane selectivity for CO methanation, CO2 conversion and methane selectivity for CO2 methanation of the catalysts are shown in Fig. 7aed. The CO conversions at 300 C increase from 18% to 39% with increasing the zirconia content from 5 wt.% to 20 wt.%. The CO conversions of the Ni/ ZrO2-5-V and Ni/ZrO2-10-V catalysts almost reach 100% at 350 C and 400 C but they decrease when the temperature is higher than 450 C, and are only about 90% at 500 C. However, the catalysts with higher zirconia content such as Ni/ZrO2-15V and Ni/ZrO2-20-V exhibit a better CO conversion of about 100% at higher temperatures. The methane selectivity for CO methanation increases first, then decreases for all of the four catalysts at the temperature range from 300 C to 500 C. But the temperature of the maximum methane selectivity of the catalyst is different from each other. It reaches the maximum at 350 C for Ni/ZrO2-5-V and Ni/ZrO2-10-V but at 450 C for Ni/ ZrO2-15-V and Ni/ZrO2-20-V. At the temperatures higher than 400 C, the methane selectivity for CO methanation on the catalysts with higher zirconia content is far better than that with low zirconia content. The catalyst of Ni/ZrO2-15-V exhibits the best methane selectivity for CO methanation in the whole temperature range, with the maximum of about 93% at 450 C. The CO2 conversions are not as good as the CO conversions on these catalysts. At the temperatures lower than 450 C, the catalysts with the lower zirconia contents exhibit better CO2 conversions. The Ni/ZrO2-5-V catalyst has the best CO2 conversion of 83% at 400 C. But the catalysts with higher zirconia content have better CO2 conversions at the temperatures higher than 400 C. The methane selectivity for CO2 methanation (Fig. 7d) at 300 C almost proportionally increases with increasing the zirconia content. The methane selectivity on Ni/ZrO2-20-V for CO2 methanation reaches the maximum of 99% at 350 C, then remains at 97% at the temperature range from 400 C to 500 C.The catalyst Ni/ZrO2-5-V that has the lowest zirconia content of 5 wt.% exhibits the lowest methane selectivity of about 88% in the temperature range from 400 C to 500 C. The Ni/ZrO2-15-V catalyst was used for the stability experiment due to its superior performance in CO and CO2 methanation. The experiment was performed at 450 C, with a GHSV of 10,000 h1 for CO methanation and with a GHSV of 12,000 h1 for CO2 methanation. Fig. 8a shows the time on stream of the catalyst for CO methanation and Fig. 8b shows that for CO2 methanation. This catalyst exhibits extremely good stability for CO methanation. The CO conversion remains at 100% and the methane selectivity only decreases by 2% in 40 h. However, the CO2 conversion on Ni/ZrO2-15-V decreases by 2% but the methane selectivity remains unchanged at about 94% in 40 h.
Carbon deposition on the catalyst The TG analysis on the used catalyst for CO and CO2 methanation is performed in order to investigate the condition of
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Fig. 7 e CO conversion (a), CH4 selectivity for CO methanation (b), CO2 conversion (c) and CH4 selectivity for CO2 methanation (d) of the catalysts.
carbon deposition on the surface of the catalyst. Fig. 9a shows the TG profile of the catalyst after 40 h on stream for CO methanation. The mass loss at the temperatures lower than 190 C is resulted from the moisture evaporation [56,57]. The mass gain at the temperature range from 190 C to 606 C is ascribed to the oxidation of the metallic nickel species in the catalyst. Assuming that all of the metallic nickel is oxidized to NiO, the mass gain of 0.354 mg should be the oxygen in NiO,
and thus the mass of NiO in the catalyst is 1.653 mg, accounting for 19.89% of the total mass of the dry catalyst. This result is in accordance with that measured by the EDX. The mass loss of 0.010 mg from 606 C to 655 C is ascribed to the active carbon deposited on the surface of the catalyst [56,57], accounting for 0.12% of the total mass of the dry catalyst. The active carbon is responsible for the formation of methane. Fig. 9b shows the TG profile of the catalyst after 40 h on stream
Fig. 8 e Time on stream of the catalyst of Ni/ZrO2-15-V for CO (a) and CO2 (b) methanation. GHSV ¼ 10,000 h¡1 and T ¼ 450 C for CO methanation; GHSV ¼ 12,000 h¡1 and T ¼ 450 C for CO2 methanation.
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Fig. 9 e TGA of the used Ni/ZrO2-15-V catalyst after 40 h on stream for CO methanation (a) and CO2 methanation (b).
for CO2 methanation. The mass loss at the temperature below 163 C is ascribed to the moisture evaporation [56,57], and the mass gain at the temperature range from 163 C to 592 C is ascribed to the oxidation of the metallic nickel species in the catalyst. The mass gain of 0.411 mg is the mass of oxygen in NiO. Therefore the mass of NiO in the catalyst is 1.918 mg, which accounts for 23.81% of the total mass of the dry catalyst. The mass loss of 0.010 mg from 592 C to 651 C is ascribed to the active carbon deposited on the surface of the catalyst [56,57], which accounts for 0.12% of the total mass of the dry catalyst. The TG analysis indicates that no inactive carbon deposited on the catalyst for both CO and CO2 methanation. This is why this catalyst exhibits good stabilities for the two reactions. The TEM observation provides supplementary evidence for the low carbon deposition on this catalyst. The Fig. 10a and b show the TEM images of the used catalyst of Ni/ZrO2-15-V after 40 h on stream for CO and CO2 methanation, respectively. The carbon deposition cannot be observed on the surface of the catalyst. However, the morphology of the used Ni/ ZrO2-15-V is quite different from that of the as-prepared catalyst (Fig. 4g). The insets in the upper right corners of image a and image b in Fig. 10 are the XRD profiles of the used
Ni/ZrO2-15-V catalyst for CO and CO2 methanation, respectively. The XRD results indicate that NiO dispersed on the zirconia-covered surfaces of the as-prepared catalyst has been reduced to metallic Ni nanoparticles during the reaction. The average size of the Ni crystals on the used Ni/ZrO2-15-V for CO methanation is 19.7 nm and that for CO2 methanation is 19.2 nm, calculated by the Scherrer's formula using the diffraction peak of the (111) crystal faces of Ni. Compared the Ni crystals observed in the TEM images in Fig. 10 with the average crystal size of NiO in the as-prepared catalyst calculated by XRD, it is obvious that the sizes of the metallic Ni nanoparticles grew slightly larger during the reaction, which might be resulted from the local sintering of the highly dispersed NiO on the zirconia-covered surfaces of the catalyst, however the sintering of the Ni nanoparticles in a remarkable scale has not occurred.
Discussion The nickel catalyst supported on zirconia-modified clays exhibits excellent catalytic performances for CO and CO2 methanation. The superior catalytic performance is resulted from the unique structure of the catalyst. The raw clays have
Fig. 10 e TEM images of the used Ni/ZrO2-15-V catalyst after 40 h on stream for CO methanation (a) and CO2 methanation (b). The insets in image a and image b are the XRD profiles of the used Ni/ZrO2-15-V after CO and CO2 methanation for 40 h, respectively.
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the small specific surface area and narrow slit-like pores that are not beneficial to the diffusion of the reactants and products; therefore they are seldom used as the support of a catalyst directly. The zirconyl nitrate solution is highly acidic. The Al in the octahedral sublayers of clays can be partly leached out by acid during the hydrothermal treatment when a suitable amount of the zirconyl nitrate solution is mixed with the clay suspension. As a result, a new structure with zirconia highly dispersed on the partly damaged clay layers is obtained in one pot. The new structure has a bimodal pore size distribution (Fig. 5b). Most of the mesopores with the sizes smaller than 10 nm are resulted from the zirconia pillared clays, which is evidenced by the peak shift of the diffractions of the (003) and (005) crystal faces of clays in the zirconiamodified clays (Fig. 2a). Apparently, zirconia nanoparticles play the role as pillars in addition to covering the partly damaged clay layers. The mesopores larger than 10 nm and the macropores larger than 50 nm come from the damaged clay layers by the acid leaching. The schematic diagram is shown in Scheme 1. The zirconia nanoparticles are very small and highly dispersed on the surfaces of the clay layers because they cannot be detected by XRD and only homogeneous fine nanoparticles can be observed on the damaged clay layers by TEM. Nickel oxide in cubic phase is the only nickel species that can be detected by XRD, and the NiO nanoparticles buried in the stack layers of zirconia nanoparticles on the surfaces of the clay layers can be observed by TEM. It seems to be that zirconia plays a role for the dispersion of the NiO nanoparticles, which is in consistence with the observations of other researchers [10,18,20,24,44]. Nickel oxide is reduced to metallic nickel by H2 before reaction. The metallic nickel
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nanoparticles grow slightly larger after the methanation reaction, which comes from the local sintering of the highly dispersed nickel oxide in the zirconia layers. However, the sintering that can result in the deactivation of the catalyst does not occur. Almost uniform Ni nanoparticles with an average size of about 20 nm, which are the active species for the CO and CO2 methanation reactions, can be observed on the clay layers of the zirconia-modified clays by TEM. Apparently, the highly dispersed zirconia prevents the Ni species from sintering in a remarkable scale. There are six types of Ni species in the catalyst, which are the highly dispersed NiO on zirconia or on the clay layers, the free NiO nanoparticles on zirconia or on the clay layers, the fixed NiO and nickel aluminate. The ratios of the highly dispersed NiO on zirconia to that on the clay layers as well as the free NiO on zirconia to that on the clay layers increase with increasing the zirconia content. The NiO species on zirconia is more readily to be reduced than that on the surface of the clay layers. The NiO species on zirconia has a close relation to the catalytic performance of the catalyst, which might give rise to more active Ni species after reduction. The amount of nickel aluminate that is hard to be reduced remarkably decreases with increasing the zirconia content, suggesting that zirconia prevents the formation of nickel aluminate. The NiO nanoparticles are not the active species for the CO or CO2 methanation but they give rise to the active metallic Ni nanoparticles after reduction by hydrogen. All NiO except that forming nickel aluminate with the residual Al in clays can be reduced to active metallic Ni, but the activity of the metallic Ni located at different sites is different, which has a close relation to its chemical environment. Most researchers believed that
Scheme 1 e Schematic preparation process of the zirconia-modified clays and the nickel catalyst supported on the zirconiamodified clays, as well as the evolution of the nickel species during the reaction.
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Ni on zirconia was more active than on other supports [10,18,20,24,44], and some of them even believed that both zirconia and Ni played important roles during the methanation [22,58]. The proofs that zirconia promotes the dispersion of Ni species and prevents them from sintering are sufficient in this study, but no evidence indicates that zirconia involves the reaction. In addition, zirconia might play an important role to reduce the inactive carbon deposition and thus enhance the stability of the catalyst. The methane selectivity for CO methanation exhibits a slight increase at low temperatures for each catalyst, then a sharp decrease for Ni/ZrO2-5-V or Ni/ZrO2-10-V and a slight decrease for Ni/ZrO2-15-V or Ni/ZrO2-20-V at high temperatures. This phenomenon is due to the thermodynamic equilibrium at the reaction conditions. The methane selectivity decreases from almost 100% to about 40% from 200 C to 600 C at 1 atm with the ratio of H2/CO ¼ 3 according to the thermodynamic calculation [59]. For the same reason, the methane selectivity for CO2 methanation maintains at a constant level from 350 C to 500 C for all catalysts.
Conclusions A new structure with zirconia nanoparticles highly dispersed on the partly damaged clay layers has been prepared in one pot by the hydrothermal treatment of the mixture of the clay suspension and the zirconyl nitrate solution. The new structure has the typical bimodal pore size distribution. Most of the pores with the sizes smaller than10 nm are resulted from the zirconia pillared clays and the mesopores with the sizes larger than 10 nm and the macropores with the sizes larger than 50 nm are resulted from the partly damaged clay layers. The nickel catalyst supported on the new zirconia-modified clays has been prepared by the incipient wetness impregnation. Nickel oxide in cubic phase is the only nickel species that can be detected by the XRD. The nickel oxide nanoparticles with the sizes of a dozen nanometers or more are well dispersed on the zirconiamodified clay layers. There are six types of the nickel oxide species on the new structure. Five of them except the nickel species that forms the spinel phase with aluminum in clays can be reduced to the active nickel species for the CO or CO2 methanation. But the activity of the metallic nickel species is different, which is associated with the chemical environment at which the metallic nickel species is located. The catalyst with higher zirconia content, which also has the larger specific surface area and pore volume, exhibits better catalytic performance for the CO or CO2 methanation. Zirconia in the catalyst is responsible for the dispersion of the nickel species, and it prevents the metallic nickel nanoparticles from sintering during the process of the reaction. In addition, it is also responsible for the reduction of the inactive carbon deposition.
Acknowledgment The work was financially supported by the key project of Inner Mongolia Education Department (NJZZ13002), the Natural Science foundation of Inner Mongolia (2014MS0207), the
project of Grassland Talent of Inner Mongolia and the project of the western light from the ODPC of Inner Mongolia.
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