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Al2O3SiO2 catalysts for CO2 methanation reaction
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Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction Shima Valinejad Moghaddam b, Mehran Rezaei a,b,*, Fereshteh Meshkani a, Reihaneh Daroughegi a a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran b Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
article info
abstract
Article history:
A series of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts with various SiO2/Al2O3
Received 2 May 2018
molar ratios were prepared by the sol-gel method for the carbon dioxide methanation
Received in revised form
reaction. The synthesized catalysts were evaluated in terms of catalytic performance and
17 August 2018
stability. The catalysts were studied using XRD, BET, TPR and SEM. The BET results indi-
Accepted 24 August 2018
cated that the specific surface area of the samples with composite oxide support changed
Available online xxx
from 254 to 163.3 m2/g, and an increase in the nickel crystallite size from 3.53 to 5.14 nm with an increment of Si/Al molar ratio was visible. The TPR results showed a shift towards
Keywords:
lower temperatures, indicating a better reducibility and easier reduction of the nickel oxide
Nanocatalyst
phase into the nickel metallic phase. Furthermore, the catalyst with SiO2/Al2O3 molar ratio
Sol-gel
of 0.5 was selected as the optimal catalyst, which showed 82.38% CO2 conversion and
Alumina-silica composite
98.19% CH4 selectivity at 350 C, high stability, and resistivity toward sintering. Eventually,
Methanation
the optimal operation conditions were specified by investigating the effect of H2/CO2 molar
Carbon dioxide
ratio and gas hourly space velocity (GHSV) on the catalytic behavior of the denoted catalyst. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Widespread use of energy sources and topical need to deduction carbon oxides emission have led to progress in methanation applications since 10 years ago [1,2]. On the other hand, carbon dioxide is the most common greenhouse gas in the atmosphere, and its uncontrolled emission leads to global warming and outstanding climate change [3,4]. Subsequently, serious actions need to be implemented to reduce the
amount of CO2 emerged by combustion of fossil fuels. Accordingly, CO2 separation, capture and its storage have gained attention in industries [5]. Besides, ethanol, methanol, formic acid, formates, dimethyl ether, higher hydrocarbons, etc. can be produced by means of carbon dioxide methanation [6,7]. The purification of hydrogen from carbon oxides in petrochemical and refining industries, where the high purity hydrogen is needed for the process is another application of methanation reaction [7,8].Various metals in methanation reaction including transition and noble metals as an active
* Corresponding author. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. E-mail address: [email protected] (M. Rezaei). https://doi.org/10.1016/j.ijhydene.2018.08.163 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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phase and Al2O3 [9], SiO2 [10], ZrO2 [11] and zeolites [12] as the support have been studied. Among these catalysts, the nickel based catalysts are considered as commercial catalysts, due to low price, high availability and acceptable catalytic performance [7,13]. Among the supports, alumina has unique characteristics such as high specific surface area and appropriate interaction with active metal [9,14]. These characteristics can be modified and lead to higher activity and stability in this reaction [15]. The performance of the Ni-based catalysts is related to various factors such as the content of Ni, type of promoters and carriers, catalyst synthesis method, and reaction conditions [16]. The physicochemical characteristics of the Nibased catalysts can be influenced by the catalyst preparation method. There are several methods for the preparation of nickel-based catalysts [17]. Among these methods, it is noteworthy to elucidate that the prepared samples by sol-gel route have higher stability and the dispersion of nickel is higher compared to the other preparation methods such as coprecipitation, deposition-precipitation, etc. [18]. In fact, solgel method can be considered as an effective method to produce powders with high purity, high surface area, chemical homogeneity and the ability of the control of the particle size [18e21]. The catalyst support has an important role on the morphology of the active phase, adsorption ability and catalytic features [22]. The strong interaction between Ni and Al2O3 and the formation of NiAl2O4 spinel with low degree of reducibility is one of the most important challenges in development of the appropriate catalysts for methanation reaction. The modification of the catalyst support is one of the best ways to overcome this problem and prepare a catalyst with higher activity and reducibility [23]. Different support modifiers (ZrO2, SiO2, MgO, La2O3, CeO2, and TiO2) have been used and the catalysts possessed better conversion, higher redox property, high thermal stability and resistance against sintering due to their excellent properties. Favorable aspects of using composite oxide supports are the inherent desirable properties and the synergetic effect of all the individual supports. On the other side, interactions between nickel and the composite oxide support exert high effect on the methanation reaction. Likewise, composite oxide supports have better chemisorption capability owing to better dispersion of nickel species [14,24,25]. Aldana et al. [24] reported that ceriazirconia composite oxide supported Ni-based catalysts prepared by the sol-gel method showed 80% CO2 conversion and 99.3% CH4 selectivity at 350 C. Ding et al. [26] noted that the synthesized Ni/CeO2eAl2O3 catalyst with 60 wt% of CeO2 showed excellent performance in methanation reaction. Zhang et al. [27] reported that the Ni/SiO2eAl2O3 catalyst prepared by the grinding-mixing method presented 76% CO conversion and 80% CH4 selectivity at 350 C. Chang et al. [28] prepared nickel catalysts on rice husk ash-alumina by incipient wetness impregnation method. They found that the reaction temperature of 500 C might be the optimum temperature for CO2 hydrogenation to give the maximum yield and selectivity of CH4. Cui et al. [29] noted that NieMg/ SiO2eAl2O3 catalysts prepared by combining co-precipitation and spray granulation showed 50% CO conversion and 87% CH4 selectivity at 350 C.
Alumina-silica composite oxide can be considered as a high potential catalyst support due to high thermal stability and high surface area at elevated temperatures. In addition, the silica particles restrict the contacts of Al2O3 particles by surrounding the geAl2O3, inhibiting its phase transition to aAl2O3 [30]. In this study, a series of nanocrystalline nickel catalysts supported on alumina-silica composite oxides with different SiO2 contents were prepared by the sol-gel method for the carbon dioxide methanation reaction.
Experimental Ni-based catalysts supported on alumina-silica composite oxides with various silica/alumina (SiO2/Al2O3) molar ratios were prepared by a facile sol-gel route. Ni(NO3)2.6H2O and Al(NO3)3.9H2O were used as Ni and Al salt precursors, respectively. Additionally, sodium silicate (NS) as a silica source, ethanol as a solvent, and propylene oxide (PO) as a network forming agent were used. Solvents are employed in this method as a media for the hydrolysis step in order to control the concentrations of constituents, which exerts a significant influence on the gelation kinetics. For this reason, solvent selection is a key factor in determining the rate of gel formation, its structure, and drying pattern [31]. Likewise, propylene oxide was used as a gelation promoter to facilitate the condensation procedure during the sol-gel method. 30Ni/Al2O3 and 30Ni/Al2O3.XSiO2 catalysts were prepared according to the method explained in our previous work [32]. Finally, the gel was calcined at 700 C with a heating rate of 3 C/min for 3 h under static air atmosphere. In all catalysts, the weight percent of the nickel was kept constant (30 wt%) and the SiO2 content was varied and the catalysts were denoted as 30Ni/Al2O3.XSiO2, where X represents the silica/ alumina (SiO2/Al2O3) molar ratio (X ¼ 0.25, 0.5, 0.75, 1 and 1.5).
Catalyst characterization The BET area, pore volume and pore size of the prepared catalysts were determined using a Belsorb mini II instrument. A PANalytical X'Pert-Pro instrument was employed for the Xray diffraction experiments.Hydrogen temperature programmed reduction (TPR) experiments were conducted using a Micrometrics chemisorb 2750 instrument. The detail conditions of the TPR analysis are reported in our previous work [32]. Scanning electron microscopy (SEM) analysis was done using a VEGA TESCAN microscope.
Catalytic tests The methanation reaction was carried out in a quartz micro reactor (Id.: 10 mm) at ambient pressure. Before reaction, the catalyst powders were pressed into tablets and then crushed into particles with a size in the range of 0.25e0.5 mm. Typically, 200 mg of the catalyst particles was charged into the reactor and reduced by a H2 stream (25 ml/min) at 600 C for 2 h. After that, the reactor was cooled to 200 C and then a mixture of H2 and CO2 with favorable ratio was entered the reactor as reactant feed. The analysis of the effluent gases was
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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performed by a gas chromatograph (Shimadzu). The reaction was performed at various reaction temperatures in the range of 200 and 500 C. The CO2 conversion (XCO2 ) and CH4 selectivity (SCH4) were determined using the following equations (Eqs. (1),(2)). XCO2 ¼
1
SCH4 ¼
CO2 CH4 þ CO þ CO2
CH4 CO þ CH4
100
(1)
3
aggregation of Ni species particles, leading to lowering the BET surface area [25]. This phenomenon associated with this point that alumina particle growth faced restriction after heat treatment and the addition of silica [28]. It is worth to illuminate this point that the catalysts with the composite oxide support exhibited broadened reflections showing the better dispersion of nickel species in the composite oxide catalysts [14].
N2 adsorption analysis
100
(2)
Results and discussion Characterization of Ni/Al2O3 and Ni/Al2O3·SiO2 catalysts XRD analysis The diffraction patterns of the 30Ni/Al2O3 catalyst, as well as those of 30Ni/Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios are displayed in Fig. 1. It is seen that different overlapped crystalline phases have been manifested on three marked diffraction peaks. The diffraction peaks observed at 37 , 44.2 , 64.4 were assigned to Al2O3 (code N. 00-047-1292). The cubic NiAl2O4 species were located at 37 , 44.2 , 64.4 (code N. 00-001-1299). The peaks corresponded to nickel oxide were not observed, because of the high dispersion of nickel oxide particles on the catalyst surface. In addition, the diffraction peaks corresponded to Ni2SiO4 species with the cubic structure were appeared at 37 , 44.2 , 64.4 (code N. 01070-2281) [30]. Furthermore, Ni-based composite oxide supported catalysts presented a new peak at 22 , which implied the existence of SiO2 species (01-082-1405). Totally, the peak intensities increased by increasing SiO2 concentration. These results were in agreement with the mean crystal size assigned to (440) plan of NiAl2O4 calculated by Scherer formula, Table 1. Indeed by an increment of SiO2/Al2O3 molar ratio from 0.25 to 0.75 crystallinity increased due to
The textural properties of the samples are reported in Table 1. It is noteworthy to mention that the textural and structural properties of the catalysts are highly influenced by processing parameters [33]. The 30Ni/Al2O3 catalyst possessed lower BET area compared to the Al2O3 support, due to partial blockage of the pores of the catalyst support by nickel oxide clusters. The catalysts also showed larger pore volume and pore diameter compared to alumina support. The pore volume of the samples with composite oxide support revealed a decreaseincrease-decrease trend with an increment of SiO2/Al2O3 molar ratio. The significant decrease in surface area and pore volume can be designated to the aggregation of Ni species particles, blocking of some mesopores of support by SiO2 and/ or partial collapse of the mesoporous structure with increasing SiO2 concentration, respectively [25,28,34,35]. Sample with SiO2/Al2O3 molar ratio of one, showed the highest surface area. Similarly, pore diameters of the samples showed a decrease-increase-decrease change, respectively. Pore diameters of the synthesized catalysts were between 4.8 and 8.2 nm, which confirmed the insignificant effect of support modifiers on the pore diameters [32]. The N2 adsorptiondesorption isotherms and the distribution of pore sizes are depicted in Fig. 2a,b, respectively. The isotherms are type IV with H2 type hysteresis loop [36]. The H2 type hysteresis loop is specific for mesoporous materials with not well-defined pore size and shape and also for the materials with bottleneck pore structure. The pore size distribution of the powders represented mesoporous structure with a distribution from 1.5 to 6 nm with a maximum centered at 3.5 nm. Furthermore, in isotherms the inflection points of hysteresis loops were relatively fixed at a relative pressure of 0.5 and then moved upward to a relative pressure of 0.55, which resulted from the increase in the pore diameters.
H2-TPR analysis
Fig. 1 e XRD patterns of the 30Ni/Al2O3 and 30Ni/ Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios calcined at 700 C.
Temperature programmed reduction (TPR) analysis was performed for studying of the relation between the reducibility and type of metal-support interaction, Fig. 3. The reducible NiO species could be classified into three denoted types: a, b and g. a-type species located in the temperature range of 300e550 C are assigned to bulk NiO particles with weak interaction with the carrier. betype species located at around 550e700 C are related to the bulk NiO or called Ni-rich phase with stronger interaction with Al2O3 than aetype species. gtype species seen at temperatures high than 700 C denoted to stable NiAl2O4 phase or Al-rich phase with the highest degree of interaction, indicating of the formation of hard reduced species [37]. Basically strong interactions are required for inhibiting the sintering of metal particles and carbon formation [38]. As is apparent in Fig. 3, one shoulder peak located between 360 and 520 C plus one distinguishing peak were
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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Table 1 e Physicochemical measurements of 30Ni/Al2O3 and 30Ni/Al2O3.XSiO2 catalysts. Catalysts Al2O3 30Ni/Al2O3 30Ni/Al2O3.0.25SiO2 30Ni/Al2O3.0.5SiO2 30Ni/Al2O3.0.75SiO2 30Ni/Al2O3$SiO2 30Ni/Al2O3.1.5SiO2
Surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Crystalline size (nm)
301.6 269.2 254 281 235.3 290.5 163.3
0.327 0.557 0.396 0.342 0.376 0.567 0.293
4.3 8.2 6.2 4.8 6.3 7.8 7.1
e 4.2 3.53 4.57 4.82 4.56 5.14
Fig. 3 e TPR profiles of the 30Ni/Al2O3 and 30Ni/Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios calcined at 700 C.
Fig. 2 e (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of 30Ni/Al2O3 and 30Ni/ Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios calcined at 700 C.
located at 550e950 C detected only for 30Ni/Al2O3 catalyst. For Ni-based composite oxide supported catalysts, all the samples represented a large reduction peak at 740 C and Tmax of g peaks shifted to the low temperatures, indicating the existence of the hard reduced NiAl2O4 species [14]. In 30Ni/ Al2O3 sample, the shoulder peak at moderate temperatures was related to the reduction of NiO species with a weak
interaction with the support. In all the samples containing SiO2, the second distinguished peak at high temperatures was attributed to the reduction of highly stable NiO species with intense interaction with spinel, most probably nickel silicates species in the composite oxides supported Ni-based catalysts [24]. With increasing in SiO2/Al2O3 molar ratio, an increase in the amount of H2 consumption on the composite oxides support confirmed a higher reducibility of NiO species with weaker metal-support interaction [14]. Since new interaction between silica and Al2O3 weakened the interaction between Ni species and Al2O3, hence the NiOeSiO2 interaction strengthened and the separation of NiO from NiAl2O4 facilitated [25,27,39].
Catalytic performance The catalytic performances of the catalysts are depicted in Fig. 4a,b, respectively. It is seen that at low temperatures below 250 C all the catalysts were inactive and no obvious carbon dioxide conversion was observed. The increase in reaction temperature up to 350 C significantly improved the conversion owing to sufficient H2 concentration and the maximum conversion was achieved at 350 C [22]. At higher temperature, a decline in conversion and selectivity was observed in all tested samples, which is related to the conversion of carbon dioxide to carbon monoxide via the reverse
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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performance was achieved due to the obtained improvements in reducibility of the catalysts, formed SiO2 network, optimal CO dissociation energy and weak metal-support interaction derived all from the SiO2 addition. On the other hand, the introduction of a metal oxide into Ni/Al2O3 structure caused weakened NieAl interactions and formed a large numbers of active Ni species, which improve the performance of the catalysts in the methanation reaction [27]. Further increase in SiO2 content reduced the fraction of small mesopores as shown in pore size distribution profiles and also partially filled some of the mesopores or the mesopores collapsed partially. It is worth to mention that increasing in nickel and SiO2/Al2O3 molar ratios can increase the Ni/NiO crystal size over the catalyst support. On the other hand, to achieve more reducibility, better stability, high catalytic performance and desirable dispersion there should be an optimum metal loading and SiO2/Al2O3 molar ratio. Besides, further increase in loadings can result in considerable agglomeration and final deactivation. Among the prepared catalyst, the 30Ni/Al2O3.0.5SiO2 catalyst possessed a maximum at the appropriate SiO2/Al2O3 molar ratio, and this catalyst showed the best performance (82.38% CO2 conversion and 98.19% CH4 selectivity at 350 C) compared to the other catalysts.
Effect of H2/CO2 ratio, GHSV, and stability
Fig. 4 e (a) CO2 conversion and (b) CH4 selectivity for H2/CO2 methanation reaction over 30Ni/Al2O3 and 30Ni/ Al2O3.XSiO2 catalysts with different Si/Al molar ratios, GHSV ¼ 9000 ml/gcat h, H2/CO2 molar ratio ¼ 3.5.
water gas shift and enhancement of the steam reforming reaction of methane at higher temperatures, respectively [11,26]. As can be seen, CH4 selectivity remained unaltered at low temperatures until 350 C, except for the 30Ni/ Al2O3.0.75SiO2 and 30Ni/Al2O3.1.5SiO2 catalysts, which presented a decrease-increase trend at low temperatures. These two composite oxide supported catalysts were calcined at 700 C and phase transformation of the alumina support occurred and resulted in catalyst deactivation. Under these conditions the Ni crystals were agglomerated and this thermal unstability led to low specific surface area. It is of great importance to mention that methanation reactions performed superior at low temperatures because of accessible H2 concentration as a desirable reducing media [40]. In general terms, the addition of silica to the support resulted in the enhanced catalytic performance and it significantly influenced the conversions, particularly at low temperatures [27]. Based on the results, higher catalytic
The effect of the H2/CO2 ratio on the catalytic characteristics of the 30Ni/Al2O3.0.5SiO2 at 350 C is illustrated in Fig. 5. As can be seen, an increase was observed in CO2 conversion and CH4 selectivity with the raise in H2/CO2 ratio from 3 to 4. The improve in CO2 conversion and CH4 selectivity is due to the presence of the sufficient hydrogen for carbon dioxide to react and hydrogenate carbonate species on the catalyst surface [41]. Indeed, H2 was taken into account as a limiting reactant at the stoichiometric ratio less than 4 [16]. If the H2/CO2 molar ratio reaches to the stoichiometric molar ratio of 4 in CO2 methanation reaction, carbon dioxide conversion increased to 78.26%. Moreover, CH4 selectivity almost showed negligible
Fig. 5 e Effect of the H2/CO2 molar ratio on the catalytic performance of the 30Ni/Al2O3.0.5SiO2 catalyst at 350 C, GHSV ¼ 12000 ml/gcat h.
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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gradual diminution at the intermediate molar ratio of 3.5, or it remained unchanged because of the full conversion of carbon dioxide to CH4 at 350 C. For this reason, carbon dioxide has not been converted to CH4 anymore. The influence of the GHSV on the catalytic performance of the 30Ni/Al2O3.0.5SiO2 catalyst at 350 C is plotted in Fig. 6. CH4 selectivity remained stable with the variations of the GHSV. CO2 conversion varied from 87.5% to 59%, when GHSV increased from 6000 ml/gcat h to 18000 ml/gcat h. This decrease is related to the decrease in contact time and the content of adsorbed reactants on the catalyst surface. The stability test for 30Ni/Al2O3.0.5SiO2 catalyst was conducted at 350 C , GHSV ¼ 12000 ml/gcat h and H2/CO2 molar ratio ¼ 3.5 and the results are shown in Fig. 7. The prepared catalyst exhibited high stability in CO2 conversion and CH4 selectivity, which is of great importance for industrial application.
CO methanation Fig. 8 compared the CO and CO2 methanation results of the 30Ni/Al2O3 catalyst and 30Ni/Al2O3.0.5SiO2 catalysts. As can be seen, the conversions of both methanation reactions showed a volcano-shaped trend. CO reached nearly 100% conversion at 350 C, while at the same temperature 73% conversion was observed in CO2 methanatio. CO conversions were far better than CO2 conversions because CO can react stronger with the surface of the catalyst. In addition, CO is more active than CO2 and had higher adsorption energy. At above 350 C, conversions of CO in CO methanation decreased due to the simultaneous occurrence of CO2 methanation and RWGS reaction. More importantly, the addition of silica to the catalyst mainly influenced the catalytic activity in CO2 methanation rather than CO methanation. Conversely, CH4 selectivities in CO2 methanation were higher than those of CO methanation, since CO turned into CO2 through below reactions including the water gas shift (Eq. (3)) and boudouard (Eq. (4)) reactions.
Fig. 6 e Effect of GHSV on 30Ni/Al2O3.0.5SiO2 catalyst performance for CO2 methanation, reaction conditions: 350 C, H2/CO2 molar ratio ¼ 3.5.
Fig. 7 e Catalytic stability of the 30Ni/Al2O3.0.5SiO2 catalyst for CO2 methanation at 350 C , GHSV ¼ 12000 ml/gcat h, H2/ CO2 molar ratio ¼ 3.5.
Fig. 8 e (a) Conversion and (b) CH4 selectivity of 30Ni/Al2O3 and 30Ni/Al2O3.0.5SiO2 catalysts, GHSV ¼ 9000 ml/gcat h, H2/CO2 molar ratio ¼ 3.5, H2/CO molar ratio ¼ 3.
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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CO þ H2 O/CO2 þ H2
(3)
SEM analysis
2CO/CO2 þ C
(4)
As illustrated by the SEM images of fresh and spent 30Ni/ Al2O3.0.5SiO2 catalysts in Fig. 9a,b, respectively, the presence of nanoscale size particles uniformly dispersed on the catalyst was observed in the SEM image of the fresh catalyst. The SEM image of the catalyst after stability test showed the existence of many agglomerates of the catalysts particles because of the sintering process during the reaction.
Incorporation of silica into the system, affected adversely the selectivities in CO2 methanation reaction. Similarly, SiO2 made little discrepancy in selectivities of CO methanation and enhanced them just at 200 and 300 C following a decreaseincrease-decrease trend.
Conclusion A series of alumina-silica composite oxide supported Ni-based catalysts with different SiO2/Al2O3 molar ratio synthesized by the sol-gel method and applied to carbon dioxide methanation process. The catalyst with 0.5 SiO2/Al2O3 molar ratio recognized as the optimal catalyst which performed the best among all the other tested samples. With increasing SiO2/Al2O3 molar ratio from 0.25 to 1.5, the catalytic performance greatly enhanced, specially at low temperatures due to the obtained improvements in reducibility of the catalysts, formed SiO2 network, optimal CO dissociation energy and weak metal-support interaction of the catalysts derived all from SiO2 addition. BET results manifested that the pore volume and the pore diameter of the samples with composite oxide support revealed a decrease-increase-decrease process with an increment of SiO2/ Al2O3 molar ratio. The significant decrease in surface area and pore volume can be designated to the aggregation of Ni species particles, blocking of some mesopore of support by SiO2 and/or partial collapse of the mesoporous structure with increasing SiO2 concentration, respectively. PSD plots demonstrated mesopore with the pore size variation from 1.5 to 6 nm with a maximum centered at 3.5 nm. TPR results showed a migration towards lower temperatures which can be pertained to the weaker interaction between NiO and the composite support. The catalyst remained greatly stable in the long term 10- h durability test, and this conclusion reached that the catalysts had strong resistivity toward sintering. When the H2/CO2 molar ratio approached to the stoichiometric value of 4, high increase was observed in CO2 conversion. With the increase of GHSV to higher values, CH4 selectivity approximately remained stable, whereas CO2 conversions diminished as a result of less residence time and lower contents of adsorbed reactant on the surface of the catalyst.
Acknowledgments The authors appreciate the support from University of Kashan (Grant No. 158426/245).
references
Fig. 9 e SEM images of (a) fresh and (b) spent 30Ni/ Al2O3.0.5SiO2 catalysts.
[1] Dutta A, Farooq S, Karimi IA, Khan SA. Assessing the potential of CO2 utilization with an integrated framework for producing power and chemicals. J CO2 Util 2017;19:49e57.
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Available online at www.sciencedirect.com
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Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction Shima Valinejad Moghaddam b, Mehran Rezaei a,b,*, Fereshteh Meshkani a, Reihaneh Daroughegi a a
Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran b Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan, Iran
article info
abstract
Article history:
A series of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts with various SiO2/Al2O3
Received 2 May 2018
molar ratios were prepared by the sol-gel method for the carbon dioxide methanation
Received in revised form
reaction. The synthesized catalysts were evaluated in terms of catalytic performance and
17 August 2018
stability. The catalysts were studied using XRD, BET, TPR and SEM. The BET results indi-
Accepted 24 August 2018
cated that the specific surface area of the samples with composite oxide support changed
Available online xxx
from 254 to 163.3 m2/g, and an increase in the nickel crystallite size from 3.53 to 5.14 nm with an increment of Si/Al molar ratio was visible. The TPR results showed a shift towards
Keywords:
lower temperatures, indicating a better reducibility and easier reduction of the nickel oxide
Nanocatalyst
phase into the nickel metallic phase. Furthermore, the catalyst with SiO2/Al2O3 molar ratio
Sol-gel
of 0.5 was selected as the optimal catalyst, which showed 82.38% CO2 conversion and
Alumina-silica composite
98.19% CH4 selectivity at 350 C, high stability, and resistivity toward sintering. Eventually,
Methanation
the optimal operation conditions were specified by investigating the effect of H2/CO2 molar
Carbon dioxide
ratio and gas hourly space velocity (GHSV) on the catalytic behavior of the denoted catalyst. © 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Widespread use of energy sources and topical need to deduction carbon oxides emission have led to progress in methanation applications since 10 years ago [1,2]. On the other hand, carbon dioxide is the most common greenhouse gas in the atmosphere, and its uncontrolled emission leads to global warming and outstanding climate change [3,4]. Subsequently, serious actions need to be implemented to reduce the
amount of CO2 emerged by combustion of fossil fuels. Accordingly, CO2 separation, capture and its storage have gained attention in industries [5]. Besides, ethanol, methanol, formic acid, formates, dimethyl ether, higher hydrocarbons, etc. can be produced by means of carbon dioxide methanation [6,7]. The purification of hydrogen from carbon oxides in petrochemical and refining industries, where the high purity hydrogen is needed for the process is another application of methanation reaction [7,8].Various metals in methanation reaction including transition and noble metals as an active
* Corresponding author. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. E-mail address: [email protected] (M. Rezaei). https://doi.org/10.1016/j.ijhydene.2018.08.163 0360-3199/© 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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phase and Al2O3 [9], SiO2 [10], ZrO2 [11] and zeolites [12] as the support have been studied. Among these catalysts, the nickel based catalysts are considered as commercial catalysts, due to low price, high availability and acceptable catalytic performance [7,13]. Among the supports, alumina has unique characteristics such as high specific surface area and appropriate interaction with active metal [9,14]. These characteristics can be modified and lead to higher activity and stability in this reaction [15]. The performance of the Ni-based catalysts is related to various factors such as the content of Ni, type of promoters and carriers, catalyst synthesis method, and reaction conditions [16]. The physicochemical characteristics of the Nibased catalysts can be influenced by the catalyst preparation method. There are several methods for the preparation of nickel-based catalysts [17]. Among these methods, it is noteworthy to elucidate that the prepared samples by sol-gel route have higher stability and the dispersion of nickel is higher compared to the other preparation methods such as coprecipitation, deposition-precipitation, etc. [18]. In fact, solgel method can be considered as an effective method to produce powders with high purity, high surface area, chemical homogeneity and the ability of the control of the particle size [18e21]. The catalyst support has an important role on the morphology of the active phase, adsorption ability and catalytic features [22]. The strong interaction between Ni and Al2O3 and the formation of NiAl2O4 spinel with low degree of reducibility is one of the most important challenges in development of the appropriate catalysts for methanation reaction. The modification of the catalyst support is one of the best ways to overcome this problem and prepare a catalyst with higher activity and reducibility [23]. Different support modifiers (ZrO2, SiO2, MgO, La2O3, CeO2, and TiO2) have been used and the catalysts possessed better conversion, higher redox property, high thermal stability and resistance against sintering due to their excellent properties. Favorable aspects of using composite oxide supports are the inherent desirable properties and the synergetic effect of all the individual supports. On the other side, interactions between nickel and the composite oxide support exert high effect on the methanation reaction. Likewise, composite oxide supports have better chemisorption capability owing to better dispersion of nickel species [14,24,25]. Aldana et al. [24] reported that ceriazirconia composite oxide supported Ni-based catalysts prepared by the sol-gel method showed 80% CO2 conversion and 99.3% CH4 selectivity at 350 C. Ding et al. [26] noted that the synthesized Ni/CeO2eAl2O3 catalyst with 60 wt% of CeO2 showed excellent performance in methanation reaction. Zhang et al. [27] reported that the Ni/SiO2eAl2O3 catalyst prepared by the grinding-mixing method presented 76% CO conversion and 80% CH4 selectivity at 350 C. Chang et al. [28] prepared nickel catalysts on rice husk ash-alumina by incipient wetness impregnation method. They found that the reaction temperature of 500 C might be the optimum temperature for CO2 hydrogenation to give the maximum yield and selectivity of CH4. Cui et al. [29] noted that NieMg/ SiO2eAl2O3 catalysts prepared by combining co-precipitation and spray granulation showed 50% CO conversion and 87% CH4 selectivity at 350 C.
Alumina-silica composite oxide can be considered as a high potential catalyst support due to high thermal stability and high surface area at elevated temperatures. In addition, the silica particles restrict the contacts of Al2O3 particles by surrounding the geAl2O3, inhibiting its phase transition to aAl2O3 [30]. In this study, a series of nanocrystalline nickel catalysts supported on alumina-silica composite oxides with different SiO2 contents were prepared by the sol-gel method for the carbon dioxide methanation reaction.
Experimental Ni-based catalysts supported on alumina-silica composite oxides with various silica/alumina (SiO2/Al2O3) molar ratios were prepared by a facile sol-gel route. Ni(NO3)2.6H2O and Al(NO3)3.9H2O were used as Ni and Al salt precursors, respectively. Additionally, sodium silicate (NS) as a silica source, ethanol as a solvent, and propylene oxide (PO) as a network forming agent were used. Solvents are employed in this method as a media for the hydrolysis step in order to control the concentrations of constituents, which exerts a significant influence on the gelation kinetics. For this reason, solvent selection is a key factor in determining the rate of gel formation, its structure, and drying pattern [31]. Likewise, propylene oxide was used as a gelation promoter to facilitate the condensation procedure during the sol-gel method. 30Ni/Al2O3 and 30Ni/Al2O3.XSiO2 catalysts were prepared according to the method explained in our previous work [32]. Finally, the gel was calcined at 700 C with a heating rate of 3 C/min for 3 h under static air atmosphere. In all catalysts, the weight percent of the nickel was kept constant (30 wt%) and the SiO2 content was varied and the catalysts were denoted as 30Ni/Al2O3.XSiO2, where X represents the silica/ alumina (SiO2/Al2O3) molar ratio (X ¼ 0.25, 0.5, 0.75, 1 and 1.5).
Catalyst characterization The BET area, pore volume and pore size of the prepared catalysts were determined using a Belsorb mini II instrument. A PANalytical X'Pert-Pro instrument was employed for the Xray diffraction experiments.Hydrogen temperature programmed reduction (TPR) experiments were conducted using a Micrometrics chemisorb 2750 instrument. The detail conditions of the TPR analysis are reported in our previous work [32]. Scanning electron microscopy (SEM) analysis was done using a VEGA TESCAN microscope.
Catalytic tests The methanation reaction was carried out in a quartz micro reactor (Id.: 10 mm) at ambient pressure. Before reaction, the catalyst powders were pressed into tablets and then crushed into particles with a size in the range of 0.25e0.5 mm. Typically, 200 mg of the catalyst particles was charged into the reactor and reduced by a H2 stream (25 ml/min) at 600 C for 2 h. After that, the reactor was cooled to 200 C and then a mixture of H2 and CO2 with favorable ratio was entered the reactor as reactant feed. The analysis of the effluent gases was
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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performed by a gas chromatograph (Shimadzu). The reaction was performed at various reaction temperatures in the range of 200 and 500 C. The CO2 conversion (XCO2 ) and CH4 selectivity (SCH4) were determined using the following equations (Eqs. (1),(2)). XCO2 ¼
1
SCH4 ¼
CO2 CH4 þ CO þ CO2
CH4 CO þ CH4
100
(1)
3
aggregation of Ni species particles, leading to lowering the BET surface area [25]. This phenomenon associated with this point that alumina particle growth faced restriction after heat treatment and the addition of silica [28]. It is worth to illuminate this point that the catalysts with the composite oxide support exhibited broadened reflections showing the better dispersion of nickel species in the composite oxide catalysts [14].
N2 adsorption analysis
100
(2)
Results and discussion Characterization of Ni/Al2O3 and Ni/Al2O3·SiO2 catalysts XRD analysis The diffraction patterns of the 30Ni/Al2O3 catalyst, as well as those of 30Ni/Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios are displayed in Fig. 1. It is seen that different overlapped crystalline phases have been manifested on three marked diffraction peaks. The diffraction peaks observed at 37 , 44.2 , 64.4 were assigned to Al2O3 (code N. 00-047-1292). The cubic NiAl2O4 species were located at 37 , 44.2 , 64.4 (code N. 00-001-1299). The peaks corresponded to nickel oxide were not observed, because of the high dispersion of nickel oxide particles on the catalyst surface. In addition, the diffraction peaks corresponded to Ni2SiO4 species with the cubic structure were appeared at 37 , 44.2 , 64.4 (code N. 01070-2281) [30]. Furthermore, Ni-based composite oxide supported catalysts presented a new peak at 22 , which implied the existence of SiO2 species (01-082-1405). Totally, the peak intensities increased by increasing SiO2 concentration. These results were in agreement with the mean crystal size assigned to (440) plan of NiAl2O4 calculated by Scherer formula, Table 1. Indeed by an increment of SiO2/Al2O3 molar ratio from 0.25 to 0.75 crystallinity increased due to
The textural properties of the samples are reported in Table 1. It is noteworthy to mention that the textural and structural properties of the catalysts are highly influenced by processing parameters [33]. The 30Ni/Al2O3 catalyst possessed lower BET area compared to the Al2O3 support, due to partial blockage of the pores of the catalyst support by nickel oxide clusters. The catalysts also showed larger pore volume and pore diameter compared to alumina support. The pore volume of the samples with composite oxide support revealed a decreaseincrease-decrease trend with an increment of SiO2/Al2O3 molar ratio. The significant decrease in surface area and pore volume can be designated to the aggregation of Ni species particles, blocking of some mesopores of support by SiO2 and/ or partial collapse of the mesoporous structure with increasing SiO2 concentration, respectively [25,28,34,35]. Sample with SiO2/Al2O3 molar ratio of one, showed the highest surface area. Similarly, pore diameters of the samples showed a decrease-increase-decrease change, respectively. Pore diameters of the synthesized catalysts were between 4.8 and 8.2 nm, which confirmed the insignificant effect of support modifiers on the pore diameters [32]. The N2 adsorptiondesorption isotherms and the distribution of pore sizes are depicted in Fig. 2a,b, respectively. The isotherms are type IV with H2 type hysteresis loop [36]. The H2 type hysteresis loop is specific for mesoporous materials with not well-defined pore size and shape and also for the materials with bottleneck pore structure. The pore size distribution of the powders represented mesoporous structure with a distribution from 1.5 to 6 nm with a maximum centered at 3.5 nm. Furthermore, in isotherms the inflection points of hysteresis loops were relatively fixed at a relative pressure of 0.5 and then moved upward to a relative pressure of 0.55, which resulted from the increase in the pore diameters.
H2-TPR analysis
Fig. 1 e XRD patterns of the 30Ni/Al2O3 and 30Ni/ Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios calcined at 700 C.
Temperature programmed reduction (TPR) analysis was performed for studying of the relation between the reducibility and type of metal-support interaction, Fig. 3. The reducible NiO species could be classified into three denoted types: a, b and g. a-type species located in the temperature range of 300e550 C are assigned to bulk NiO particles with weak interaction with the carrier. betype species located at around 550e700 C are related to the bulk NiO or called Ni-rich phase with stronger interaction with Al2O3 than aetype species. gtype species seen at temperatures high than 700 C denoted to stable NiAl2O4 phase or Al-rich phase with the highest degree of interaction, indicating of the formation of hard reduced species [37]. Basically strong interactions are required for inhibiting the sintering of metal particles and carbon formation [38]. As is apparent in Fig. 3, one shoulder peak located between 360 and 520 C plus one distinguishing peak were
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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Table 1 e Physicochemical measurements of 30Ni/Al2O3 and 30Ni/Al2O3.XSiO2 catalysts. Catalysts Al2O3 30Ni/Al2O3 30Ni/Al2O3.0.25SiO2 30Ni/Al2O3.0.5SiO2 30Ni/Al2O3.0.75SiO2 30Ni/Al2O3$SiO2 30Ni/Al2O3.1.5SiO2
Surface area (m2/g)
Pore volume (cm3/g)
Pore diameter (nm)
Crystalline size (nm)
301.6 269.2 254 281 235.3 290.5 163.3
0.327 0.557 0.396 0.342 0.376 0.567 0.293
4.3 8.2 6.2 4.8 6.3 7.8 7.1
e 4.2 3.53 4.57 4.82 4.56 5.14
Fig. 3 e TPR profiles of the 30Ni/Al2O3 and 30Ni/Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios calcined at 700 C.
Fig. 2 e (a) Nitrogen adsorption/desorption isotherms and (b) pore size distributions of 30Ni/Al2O3 and 30Ni/ Al2O3.XSiO2 catalysts with different SiO2/Al2O3 molar ratios calcined at 700 C.
located at 550e950 C detected only for 30Ni/Al2O3 catalyst. For Ni-based composite oxide supported catalysts, all the samples represented a large reduction peak at 740 C and Tmax of g peaks shifted to the low temperatures, indicating the existence of the hard reduced NiAl2O4 species [14]. In 30Ni/ Al2O3 sample, the shoulder peak at moderate temperatures was related to the reduction of NiO species with a weak
interaction with the support. In all the samples containing SiO2, the second distinguished peak at high temperatures was attributed to the reduction of highly stable NiO species with intense interaction with spinel, most probably nickel silicates species in the composite oxides supported Ni-based catalysts [24]. With increasing in SiO2/Al2O3 molar ratio, an increase in the amount of H2 consumption on the composite oxides support confirmed a higher reducibility of NiO species with weaker metal-support interaction [14]. Since new interaction between silica and Al2O3 weakened the interaction between Ni species and Al2O3, hence the NiOeSiO2 interaction strengthened and the separation of NiO from NiAl2O4 facilitated [25,27,39].
Catalytic performance The catalytic performances of the catalysts are depicted in Fig. 4a,b, respectively. It is seen that at low temperatures below 250 C all the catalysts were inactive and no obvious carbon dioxide conversion was observed. The increase in reaction temperature up to 350 C significantly improved the conversion owing to sufficient H2 concentration and the maximum conversion was achieved at 350 C [22]. At higher temperature, a decline in conversion and selectivity was observed in all tested samples, which is related to the conversion of carbon dioxide to carbon monoxide via the reverse
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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5
performance was achieved due to the obtained improvements in reducibility of the catalysts, formed SiO2 network, optimal CO dissociation energy and weak metal-support interaction derived all from the SiO2 addition. On the other hand, the introduction of a metal oxide into Ni/Al2O3 structure caused weakened NieAl interactions and formed a large numbers of active Ni species, which improve the performance of the catalysts in the methanation reaction [27]. Further increase in SiO2 content reduced the fraction of small mesopores as shown in pore size distribution profiles and also partially filled some of the mesopores or the mesopores collapsed partially. It is worth to mention that increasing in nickel and SiO2/Al2O3 molar ratios can increase the Ni/NiO crystal size over the catalyst support. On the other hand, to achieve more reducibility, better stability, high catalytic performance and desirable dispersion there should be an optimum metal loading and SiO2/Al2O3 molar ratio. Besides, further increase in loadings can result in considerable agglomeration and final deactivation. Among the prepared catalyst, the 30Ni/Al2O3.0.5SiO2 catalyst possessed a maximum at the appropriate SiO2/Al2O3 molar ratio, and this catalyst showed the best performance (82.38% CO2 conversion and 98.19% CH4 selectivity at 350 C) compared to the other catalysts.
Effect of H2/CO2 ratio, GHSV, and stability
Fig. 4 e (a) CO2 conversion and (b) CH4 selectivity for H2/CO2 methanation reaction over 30Ni/Al2O3 and 30Ni/ Al2O3.XSiO2 catalysts with different Si/Al molar ratios, GHSV ¼ 9000 ml/gcat h, H2/CO2 molar ratio ¼ 3.5.
water gas shift and enhancement of the steam reforming reaction of methane at higher temperatures, respectively [11,26]. As can be seen, CH4 selectivity remained unaltered at low temperatures until 350 C, except for the 30Ni/ Al2O3.0.75SiO2 and 30Ni/Al2O3.1.5SiO2 catalysts, which presented a decrease-increase trend at low temperatures. These two composite oxide supported catalysts were calcined at 700 C and phase transformation of the alumina support occurred and resulted in catalyst deactivation. Under these conditions the Ni crystals were agglomerated and this thermal unstability led to low specific surface area. It is of great importance to mention that methanation reactions performed superior at low temperatures because of accessible H2 concentration as a desirable reducing media [40]. In general terms, the addition of silica to the support resulted in the enhanced catalytic performance and it significantly influenced the conversions, particularly at low temperatures [27]. Based on the results, higher catalytic
The effect of the H2/CO2 ratio on the catalytic characteristics of the 30Ni/Al2O3.0.5SiO2 at 350 C is illustrated in Fig. 5. As can be seen, an increase was observed in CO2 conversion and CH4 selectivity with the raise in H2/CO2 ratio from 3 to 4. The improve in CO2 conversion and CH4 selectivity is due to the presence of the sufficient hydrogen for carbon dioxide to react and hydrogenate carbonate species on the catalyst surface [41]. Indeed, H2 was taken into account as a limiting reactant at the stoichiometric ratio less than 4 [16]. If the H2/CO2 molar ratio reaches to the stoichiometric molar ratio of 4 in CO2 methanation reaction, carbon dioxide conversion increased to 78.26%. Moreover, CH4 selectivity almost showed negligible
Fig. 5 e Effect of the H2/CO2 molar ratio on the catalytic performance of the 30Ni/Al2O3.0.5SiO2 catalyst at 350 C, GHSV ¼ 12000 ml/gcat h.
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gradual diminution at the intermediate molar ratio of 3.5, or it remained unchanged because of the full conversion of carbon dioxide to CH4 at 350 C. For this reason, carbon dioxide has not been converted to CH4 anymore. The influence of the GHSV on the catalytic performance of the 30Ni/Al2O3.0.5SiO2 catalyst at 350 C is plotted in Fig. 6. CH4 selectivity remained stable with the variations of the GHSV. CO2 conversion varied from 87.5% to 59%, when GHSV increased from 6000 ml/gcat h to 18000 ml/gcat h. This decrease is related to the decrease in contact time and the content of adsorbed reactants on the catalyst surface. The stability test for 30Ni/Al2O3.0.5SiO2 catalyst was conducted at 350 C , GHSV ¼ 12000 ml/gcat h and H2/CO2 molar ratio ¼ 3.5 and the results are shown in Fig. 7. The prepared catalyst exhibited high stability in CO2 conversion and CH4 selectivity, which is of great importance for industrial application.
CO methanation Fig. 8 compared the CO and CO2 methanation results of the 30Ni/Al2O3 catalyst and 30Ni/Al2O3.0.5SiO2 catalysts. As can be seen, the conversions of both methanation reactions showed a volcano-shaped trend. CO reached nearly 100% conversion at 350 C, while at the same temperature 73% conversion was observed in CO2 methanatio. CO conversions were far better than CO2 conversions because CO can react stronger with the surface of the catalyst. In addition, CO is more active than CO2 and had higher adsorption energy. At above 350 C, conversions of CO in CO methanation decreased due to the simultaneous occurrence of CO2 methanation and RWGS reaction. More importantly, the addition of silica to the catalyst mainly influenced the catalytic activity in CO2 methanation rather than CO methanation. Conversely, CH4 selectivities in CO2 methanation were higher than those of CO methanation, since CO turned into CO2 through below reactions including the water gas shift (Eq. (3)) and boudouard (Eq. (4)) reactions.
Fig. 6 e Effect of GHSV on 30Ni/Al2O3.0.5SiO2 catalyst performance for CO2 methanation, reaction conditions: 350 C, H2/CO2 molar ratio ¼ 3.5.
Fig. 7 e Catalytic stability of the 30Ni/Al2O3.0.5SiO2 catalyst for CO2 methanation at 350 C , GHSV ¼ 12000 ml/gcat h, H2/ CO2 molar ratio ¼ 3.5.
Fig. 8 e (a) Conversion and (b) CH4 selectivity of 30Ni/Al2O3 and 30Ni/Al2O3.0.5SiO2 catalysts, GHSV ¼ 9000 ml/gcat h, H2/CO2 molar ratio ¼ 3.5, H2/CO molar ratio ¼ 3.
Please cite this article in press as: Moghaddam SV, et al., Synthesis of nanocrystalline mesoporous Ni/Al2O3eSiO2 catalysts for CO2 methanation reaction, International Journal of Hydrogen Energy (2018), https://doi.org/10.1016/j.ijhydene.2018.08.163
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CO þ H2 O/CO2 þ H2
(3)
SEM analysis
2CO/CO2 þ C
(4)
As illustrated by the SEM images of fresh and spent 30Ni/ Al2O3.0.5SiO2 catalysts in Fig. 9a,b, respectively, the presence of nanoscale size particles uniformly dispersed on the catalyst was observed in the SEM image of the fresh catalyst. The SEM image of the catalyst after stability test showed the existence of many agglomerates of the catalysts particles because of the sintering process during the reaction.
Incorporation of silica into the system, affected adversely the selectivities in CO2 methanation reaction. Similarly, SiO2 made little discrepancy in selectivities of CO methanation and enhanced them just at 200 and 300 C following a decreaseincrease-decrease trend.
Conclusion A series of alumina-silica composite oxide supported Ni-based catalysts with different SiO2/Al2O3 molar ratio synthesized by the sol-gel method and applied to carbon dioxide methanation process. The catalyst with 0.5 SiO2/Al2O3 molar ratio recognized as the optimal catalyst which performed the best among all the other tested samples. With increasing SiO2/Al2O3 molar ratio from 0.25 to 1.5, the catalytic performance greatly enhanced, specially at low temperatures due to the obtained improvements in reducibility of the catalysts, formed SiO2 network, optimal CO dissociation energy and weak metal-support interaction of the catalysts derived all from SiO2 addition. BET results manifested that the pore volume and the pore diameter of the samples with composite oxide support revealed a decrease-increase-decrease process with an increment of SiO2/ Al2O3 molar ratio. The significant decrease in surface area and pore volume can be designated to the aggregation of Ni species particles, blocking of some mesopore of support by SiO2 and/or partial collapse of the mesoporous structure with increasing SiO2 concentration, respectively. PSD plots demonstrated mesopore with the pore size variation from 1.5 to 6 nm with a maximum centered at 3.5 nm. TPR results showed a migration towards lower temperatures which can be pertained to the weaker interaction between NiO and the composite support. The catalyst remained greatly stable in the long term 10- h durability test, and this conclusion reached that the catalysts had strong resistivity toward sintering. When the H2/CO2 molar ratio approached to the stoichiometric value of 4, high increase was observed in CO2 conversion. With the increase of GHSV to higher values, CH4 selectivity approximately remained stable, whereas CO2 conversions diminished as a result of less residence time and lower contents of adsorbed reactant on the surface of the catalyst.
Acknowledgments The authors appreciate the support from University of Kashan (Grant No. 158426/245).
references
Fig. 9 e SEM images of (a) fresh and (b) spent 30Ni/ Al2O3.0.5SiO2 catalysts.
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