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Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline γ-Al2O3 in autothermal reforming of methane
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e9
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Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane Soodeh Sepehri a, Mehran Rezaei a,b,* 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:
In this study methane autothermal reforming (ATR) was investigated over Ni/Al2O3 and Ni/
Received 11 December 2016
Al2O3eCeO2 catalysts. The catalyst carriers were prepared through a facile one-step
Received in revised form
method, which produced mesoporous nanocrystalline carriers for Ni catalysts. The sam-
14 January 2017
ples were characterized by XRD, TPR, BET, TPO and SEM characterization techniques and
Accepted 16 January 2017
the catalytic activity and stability were also studied at different conditions (GHSV and feed
Available online xxx
ratio) in methane ATR. It was found that the nickel catalyst supported on 3 wt.% CeeAl2O3 exhibited higher activity compared to the catalysts supported on the Al2O3 and promoted
Keywords:
Al2O3 with 1 and 6 wt.% Ce. The results also showed that the nickel catalyst supported on
Autothermal reforming (ATR)
3 wt.% CeeAl2O3 possessed the highest resistance against carbon deposition in ATR
Ni based catalyst
reaction.
Solid state reaction
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cerium oxide
Introduction In recent years, synthesis gas composed of H2 and CO has attracted increasing attention as a new source of energy. Syngas can be a proper intermediate for H2 production and also a good feedstock for synthesis of other chemicals. Different processes such as steam reforming, autothermal reforming (ATR), dry reforming and partial oxidation have been suggested for producing syngas [1e6]. Recently, the ATR process has attracted special concerns since the process can provide its thermal requirements by combining the
endothermic partial oxidation and the exothermic steam reforming reactions [7]. Li et al. [8] investigated the alumina supported noble metal based catalysts (Rh, Pt and Pd) in the ATR reaction with a focus on their temperature profiles during the reaction. They have suggested that Rh is the most effective catalyst in terms of methane conversion. Due to the high cost of noble metals, they are not suitable for practical applications. Therefore Ni-based catalysts have been widely developed due to their low cost and high activity, though they suffer from sintering and coke deposition [9e12]. Horiuchi et al. [13] suggested that carbon deposition can be effectively suppressed when a metal oxide with a marked Lewis basicity is
* Corresponding author. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. Fax: þ98 3155559930. E-mail address: [email protected] (M. Rezaei). http://dx.doi.org/10.1016/j.ijhydene.2017.01.096 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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used as a support for a Ni catalyst. It is also found that CeO2 and ZrO2 have the potential of favoring carbon gasification because of their oxygen storage capacity [14]. Since the high costs of these materials make them inefficient to be applied as the supports in the commercial formulations, there are some proposals including the addition of these materials to the more practical supports as a promoter, such as addition of La, Ce, Sr and Zr on Ni/Al2O3 for autothermal reforming of methane [15] or CeO2 on Ni/Al2O3 for dry reforming of methane [16]. The results confirmed that the addition of CeO2 to the catalyst support can enhance the mobility of lattice oxygen, which leads to decreasing the carbon deposition and improving the thermal sintering of the catalytic active phase [17]. Yu et al. [18] have investigated the textural and catalytic performance of CeO2eMOx (M ¼ Mg2þ, Al3þ, Si4þ). They reported that Al2O3 is an effective surface stabilizer because of its enhanced textural properties. The results showed that the CeO2eAl2O3 possessed the highest activity in CO oxidation, due to its excellent textural characteristics, high homogeneity and redox properties [18]. There are several methods to synthesis alumina-based supports, such as solegel [19], precipitation and thermal decomposition methods [20]. In this study the promoted supports were prepared by a novel solid state synthesis method, which is a simple one-pot synthesis method including two steps (1) solid state reaction between ammonium carbonate and metal nitrates as precursors, and (2) calcination at elevated temperature. This procedure is a low cost and environmental friendly method since it does not require the addition of any surfactant, template or structuredirecting agent during the synthesis process. In this paper, we investigated the promoting effect of different percentages (0, 1, 3, 6 wt.%) of Ce on Ni/Al2O3 for ATR reaction using solid state reaction method to synthesize promoted supports.
pressure. All catalyst powders were pressed, ground, sieved to the 0.25e0.5 mm particle size, and reduced in hydrogen stream at 700 C for 3 h. The reaction mixture consisting of CH4, O2 and H2O (CH4:O2:H2O equal to 1:0.25:0.5) was passed over the catalyst bed. Brooks 5850 mass flow controllers were used to control the gas flow rates and also the water flow rates were controlled by a programmable syringe pump (IVAC P1000, UK). The activity measurements were carried out at a temperature range of 550 Ce700 C in steps of 50 C. The product stream was analyzed using an on-line Varian 3400 gas chromatograph equipped with a thermal conductivity detector and a Carboxen 1000 column. The exhaust passed through a cold-trap to remove its water content before each analysis.
Characterization In order to evaluate the specific surface area and pore size distribution, nitrogen physisorption isotherms were taken using a Tristar 3020 (Micromeritics) instrument. X-ray diffraction analysis (XRD) was done using a PANalytical X'Pert-Pro instrument. Temperature programmed reduction (TPR) analysis was performed to determine the type and relative proportion of reducible species on the prepared catalysts by using Micromeritics chemisorb 2750 instrument. In this analysis, 200 mg of the degassed catalyst was reduced in a gas mixture of H2:Ar (10:90) (20 ml/min) under a heating rate of 10 C/min. The H2 consumption was determined using a thermal conductivity detector (TCD). Temperatureprogrammed oxidation (TPO) of the spent catalysts was also conducted in a similar apparatus as described for TPR analysis by using a gas mixture of O2:He (5:95) (30 ml/min). Scanning electron microscopy (SEM) analysis was performed with a VEGA TESCAN microscope operated at 30 kV.
Results and discussion Materials and methods Characterization of the calcined samples Catalyst preparation Ni catalysts were prepared through the impregnation method, which is explained in our previous work in more details [21]. The catalyst supports were synthesized using a solid-state method by mixing a calculated content of Ce and Al salt precursors (Ce(NO3)3$6H2O, Al(NO3)3$9H2O) with ammonium carbonate ((NH4)2CO3/Al molar ratio ¼ 3) for 20 min using a mortar and pestle. During this process, the hydrate water of the mentioned precursors was released and the mixture was transformed into a pasty form. The prepared paste was then calcined with a ramp rate of 2 C/min at 700 C for 2 h. The Ni loading in all catalysts was chosen as 25 wt.%, while the Ce content was varied between 0, 1, 3 and 6 wt.% nominated as Ni/Al2O3, Ni/1Ce$Al2O3, Ni/3Ce$Al2O3 and Ni/6Ce$Al2O3.
Catalytic reaction A quartz fixed bed reactor was applied for autothermal reforming process at different temperatures. The process was carried out under continuous flow regime and atmospheric
Fig. 1 shows the XRD spectra of the catalysts. It is seen that the diffraction pattern for the unpromoted sample did not have a tangible difference with that of Ni/1Ce$Al2O3. Possible phases are NiO, Alumina, ceria and nickel aluminate, as indexed in this figure. The analysis confirmed the presence of poorly crystalline g-Al2O3 (2q ¼ 37.4, 46, 66.9 ) and NiO phases (2q ¼ 37.4, 43.3, 63.1, 75.6 ). The observed diffraction peaks at 2q ¼ 37, 45 and 65.5 are related to the NiAl2O4 spinel phase (JCPDS card No. 10-0339). The identification of the crystalline phases is complicated, due to overlapping of the NiO and Al2O3 characteristic peaks with the nickel aluminate [22]. CeO2 phase was not clearly observed in the Ni/1Ce$Al2O3 and Ni/ 3Ce$Al2O3 spectra due to the low Ce content. However the diffraction peaks of CeO2 were seen in Ni/6Ce$Al2O3 at 2q ¼ 28.7, 33.2, 47.7 and 56.6 . This phase is a cubic fluorite phase in accordance with the JCPDS Card No. 01-075-0076. It is also reported that CeO2 in the binary system with ZrO2 exhibits a cubic phase when the content of ceria is higher than 80%. For the ceria content of lower than 5% a monoclinic phase was reported [23]. The fluorite structure of ceria
Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Fig. 1 e XRD analysis of the calcined catalysts with various Ce loadings.
tolerates oxygen non-stoichiometry with high values without changing its structure [24]. According to the diffraction patterns, NiO sharp peaks reduced in terms of intensity as ceria increased in the catalysts, suggesting that ceria promotes the Ni dispersion and reduces the bulk NiO on the surface of the catalyst, which is assigned to the Ce redox properties. The Ce4þ/Ce3þ redox couple generates the surface and bulk oxygen vacancies, which form MeO surface complexes. This decreases the interaction of the metal-support and consequently improves the nickel dispersion. The nitrogen adsorption/desorption isotherms of all supports and catalysts are shown in Fig. 2. All isotherms revealed type-IV curves with H2 type hysteresis loops, which is characteristic of mesopores structure with ink-bottle or channellike pores. The narrow pore-size distribution further indicates that the calcined samples possess highly uniform mesopores. All supports showed pore sizes below 7 nm while
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pore sizes for all catalysts were around 6 nm. The decrease in pore size and pore volume of the samples after addition of Ni to the supports could be assigned to the partial blockage of pores by NiO clusters and/or partial collapse of mesoporous structure [25]. The results in Table 1 confirmed that the promoters improved the catalysts surface area and reduced pore volume compared with unpromoted catalyst. In order to evaluate the type and relative percentage of the different reducible species the TPR profiles of the catalysts were presented in Fig. 3. The relative percentage of reducible species was calculated by Origin software (using Gaussian multipeaks curve-fitting) and the results are presented in Table 2. Fig. 3 also included the deconvolution of the 25Ni/ 1Ce$Al2O3 reducibility profile. The TPR profiles show two main reduction peaks. The first reduction peak at lower temperature around 450 C is assigned to the bulk NiO, probably due to large aggregates of nickel oxide [26] as confirmed by XRD analysis, and the other one at higher temperature which is broader and can be disintegrated into three components. The first peak can be attributed to the reduction of weakly interacted Ni2þ ions with Al2O3 [27,28], while the reduction of Ni2þ species related to non-stoichiometric nickel aluminate (NiAlxOy) was observed at higher temperature, which is also nominated as amorphous surface spinel [29]. The other high temperature reduction peak can be assigned to stoichiometric nickel aluminate species (NiAl2O4) [28,29]. Table 2 shows how the intensity and the relative percentage of NiAlxOy peaks increase with increasing in Ce content. After adding 1 wt.% Ce to the catalyst, the reduction peaks of catalyst moved towards the direction of NiOeAl2O3 species with weak interaction, while no considerable change observed in percentage of NiAl2O4. This indicates that the interaction between the active component and ceria has appeared and it has weakened the interaction between the support and the active component, and made the active component easier to reduce, thus improving the reducibility of the catalyst [30]. As the ceria content increases to 3 and 6 wt.%, the relative percentage of strongly interacted species with alumina support increases. Meanwhile the weakly interacted NiO species with
Fig. 2 e Pore-size distributions and N2 adsorption/desorption isotherms of (a) supports and (b) catalysts with different Ce contents. Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Table 1 e Textural properties of supports and catalysts with different Ce contents. SBET (m2/g)
Catalysts
25Ni/Al2O3 25Ni/1Ce$Al2O3 25Ni/3Ce$Al2O3 25Ni/6Ce$Al2O3
Pore volume (cm3/g)
Pore size (nm)
Support
Calcined
Spent
Support
Calcined
Spent
Support
Calcined
Spent
220.4 221.8 222.1 234.4
144.1 180.6 155.3 220.1
121.0 125.9 115.6 117.1
0.49 0.47 0.53 0.49
0.33 0.25 0.31 0.38
0.31 0.30 0.30 0.24
6.1 6.4 6.6 6.0
5.3 5.4 6.1 5.2
7.0 6.5 6.9 6.1
Fig. 3 e TPR analysis of calcined samples.
Table 2 e Relative percentage and reduction temperatures of reducible species for catalysts with different Ce contents. Catalysts 25Ni/Al2O3 25Ni/1Ce$Al2O3 25Ni/3Ce$Al2O3 25Ni/6Ce$Al2O3
NiO 17.3% 11.9% 19.9% 19.2%
(450 (439 (447 (420
NiOeAl2O3
C) C) C) C)
32.1% 59.4% 48.2% 40.7%
(608 (600 (651 (646
C) C) C) C)
NiAlxOy 45.5% 23.2% 31.9% 40.1%
(698 C) (699 C) (739 C) (734 C)
NiAl2O4 5.1% (783 C) 5.5% (780 C) e e
alumina decreases. Those high reduction temperature species could be attributed to the well dispersed NiO on the support. It could be concluded that higher amount of Ce has higher effect on the dispersion of NiO on the catalyst surface than 1 wt.% Ce. It should be noted that the NiAl2O4 spinel phase has been eliminated by addition of 3 and 6 wt.% Ce. No peaks associated with ceria with strong interaction with alumina, CeO2eAl2O3, were found. The TPR profile of these species shows the reduction peak around 890 C and provides surface species of Ce3þ which form CeAlO3 solid solutions [31].
ATR catalyst performance A comparison of the CH4 conversion vs. temperature for various samples in ATR reaction is illustrated in Fig. 4. Generally, CH4 conversion raises with increasing temperature which is an indicative for endothermic reaction. The addition of CeO2 to the nickel catalysts does not have a direct effect on CH4 conversion. The catalysts with 1 and 6% CeO2 showed lower conversion than the catalyst with 3% CeO2. Yang et al. [32] were also observed a similar trend for the Ce promoted Ni/ Alumina steam reforming catalysts with different Ce loadings
Fig. 4 e Methane conversion of catalysts at different temperatures (Reaction conditions: GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), CH4:H2O:O2 ¼ 1:0.5:0.25).
Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Fig. 5 e (a) Short time stability of catalysts and (b) Long term stability of 25Ni/3Ce·Al2O3 at 700 C (Reaction conditions: GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), CH4:H2O:O2 ¼ 1:0.5:0.25). (0e2 wt.%). They found that the 1.02 wt.% Ce was an appropriate amount of promoter to obtain higher conversion, better carbon resistance and smaller nickel particle size and they showed that the higher amount of Ce led to lower conversion. A key consideration for nickel catalysts is to investigate carbon deposition and sintering which are studied through short and long term stability tests. The main causes of carbon deposition at the temperature range of 550e700 C are boudouard and methane cracking reactions. Fig. 5a indicates high stability of the catalysts during 4 h short term stability tests. Fig. 5b is also illustrated an appreciable stability for 25Ni/ 3Ce$Al2O3 catalyst within 20 h long term test. To determine the effect of increasing or decreasing steam reforming and partial oxidation reactions on catalytic performance in ATR process, we changed the S/C or O/C ratios and fixed other variables for 25Ni/3Ce$Al2O3 catalyst. The results are presented in Fig. 6a. As expected, the increase in S/C ratio led to improve the CH4 conversion, since steam forces the steam reforming reaction to proceed. Further increasing of
S/C ratio leads to further progress of steam reforming reaction which means that this reaction prevails over ATR reaction. The increase of O/C ratio had a similar effect on conversion with a higher effect (Fig. 6a), in a way that the methane conversion increases to near 100% when O/C ratio is equal to 1. At this ratio the feed started to spark because of the fast reaction of methane with oxygen at the bed inlet. It is also important to investigate the influence of GHSV on conversion which is an essential factor of catalytic performance. For this purpose the GHSV was varied between 104 and 7 104 ml/g h and other parameters were kept constant. Fig. 6b displays a slight decline in conversion with the increase of GHSV due to the decrease in amount of adsorbed reactants caused by a shortened contact time. H2/CO molar ratios vs. temperature for all samples were reported in Fig. 7. According to the results, this ratio starts from a maximum value at low temperature and ends in a minimum value at elevated temperature. This inverse relationship is assigned to the wateregas shift reaction which is
Fig. 6 e Methane conversion of 25Ni/3Ce·Al2O3 catalyst at various (a) S/C and O/C ratios (Reaction conditions: GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), Temperature ¼ 650 C) and (b) GHSV (Reaction conditions: CH4:H2O:O2 ¼ 1:0.5:0.25, Temperature ¼ 650 C). Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Fig. 7 e H2/CO ratios of catalysts with different Ce loadings at different temperatures.
more favorable at lower temperatures. The thermodynamic analysis has also predicted the similar trend [33]. Interestingly the H2/CO values for 25Ni/3Ce$Al2O3 are higher at each point, especially at 550 C which has an amount of 9.5. It could be concluded that 3 wt.% Ce has been more promotion effect on WGS reaction than 0, 1 and 6 wt.% Ce. It is also illustrated that Ni/Al2O3 revealed lower H2/CO ratio at any temperature. It could be concluded that Ce promotion of the catalysts increased H2/CO ratio.
Spent samples characterization The physisorption isotherms and pore size distributions of the spent samples and the spent 25Ni/3Ce$Al2O3 catalyst after long term stability are presented in Fig. 8. They are classified as IV type isotherms with H2-shaped hysteresis loops, which demonstrate that the samples have mesoporous structure even after reaction which can be related to the high thermal stability of the catalysts. The pore size distributions of the
Fig. 9 e SEM image of the spent 25 Ni/3Ce·Al2O3 sample. spent samples and spent 25Ni/3Ce$Al2O3 catalyst with long term stability are centered around 6.6 and 7.7 nm, respectively. The morphology of the spent 25Ni/3Ce$Al2O3 catalyst was studied using SEM analysis and the result is shown in Fig. 9. The SEM micrograph showed the existence of the agglomerates with soft and porous structure which contained tiny quasi-spherical particles. These agglomerates displayed a flake-like structure and an irregular morphology. Carbon formation was not detected on the surface of this spent sample.
Effect of ceria on carbon formation on ATR catalyst surface Since in normal conditions of ATR no carbonaceous species was observed on both promoted and unpromoted catalysts, in
Fig. 8 e N2 adsorption/desorption isotherms and pore-size distributions of (a) spent samples (b) spent 25Ni/3Ce·Al2O3 sample after long term stability. Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Fig. 10 e a) TPO and b, c) SEM analysis of spent catalysts with different Ce contents after harsh conditions (GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), CH4:H2O:O2 ¼ 6:2:1).
order to evaluate the effect of Ce on carbon formation on the catalyst surface during ATR reaction of methane the 25Ni/ Al2O3 and 25Ni/3Ce$Al2O3 catalysts were examined under harsh reaction conditions for 1 h (GHSV ¼ 2.2 104 (ml g1 h1), CH4:H2O:O2 ¼ 6:2:1). The TPO profiles of the spent Ni/Al2O3 and Ni/3Ce$Al2O3 catalysts are shown in Fig. 10. Oxidation of 25% metallic Ni made a valley at the beginning of these profiles. The 25Ni/Al2O3 catalyst represented a peak centered at the temperature of 673 C, associated with partly graphitized of carbon deposits [34], while carbon was not detected on the surface of 25Ni/3Ce$Al2O3 catalyst. This could be assigned to the CeO2 capability to promote the carbon gasification and its ability to store [35,36] and transfer [31,35] oxygen. Baidya and Cattolica [37] also investigated the effect of CeO2 on Ni catalysts during steam reforming of tars. They declared that the role of CeO2 in coke removal is due to the oxygen exchange from H2O via these reactions:
CeO2 þ C / Ce2O3 þ CO
(1)
Ce2O3 þ H2O / CeO2 þ H2
(2)
Conclusion The main goal of the present paper was to investigate the effect of addition of small amount of ceria to the Al2O3 as catalyst carrier for nickel catalysts. A simple solid state reaction method was employed to prepare the alumina and promoted alumina supports. The bare alumina support prepared through solid state reaction method exhibited high BET surface area (>230 m2/gr), while the addition of ceria improves the surface area in promoted catalysts. XRD results revealed the enhanced dispersion of NiO over 3 and 6 wt.% Ce promoted supports, which confirmed by TPR results, where 1 wt.% Ce improved the reducibility of the catalyst, but 3 and 6 wt.% Ce enhanced Ni dispersion by shifting other Ni species to non-stoichiometric nickel aluminate, NiAlxOy, species. The catalytic activity experiments results indicated that Ni/ 3Ce$Al2O3 sample had a better performance (45% at 550 C up to 73 at 700 C) during methane ATR, where produced H2-rich gas (H2/CO z 9.5) at 550 C. It also exhibited a significant stability during 20 h ATR experiment. The BET results of the spent catalysts indicated that the mesoporous structure of the catalysts remained after reaction which endorses the synthesis method of the supports. The Ni/3Ce$Al2O3 sample also passed the carbon formation examination during harsh
Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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conditions of ATR, where unpromoted catalyst represented coke deposition. [14]
Acknowledgements The authors gratefully acknowledge the support from University of Kashan for supporting this work by Grant No. 158426/177.
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CH4 by adding basic metal oxides to a Ni/Al2O3 catalyst. Appl Catal A General 1996;144:111e20. Roh H-S, Potdar HS, Jun K-W. Carbon dioxide reforming of methane over co-precipitated NieCeO2, NieZrO2 and NieCeeZrO2 catalysts. Catal Today 2004;93e95:39e44. Sepehri S, Rezaei M, Garbarino G, Busca G. Preparation and characterization of mesoporous nanocrystalline La-, Ce-, Zr-, Sr-containing NiAl2O3 methane autothermal reforming catalysts. Int J Hydrogen Energy 2016;41:8855e62. Luisetto I, Tuti S, Battocchio C, Lo Mastro S, Sodo A. Ni/ CeO2eAl2O3 catalysts for the dry reforming of methane: the effect of CeAlO3 content and nickel crystallite size on catalytic activity and coke resistance. Appl Catal A General 2015;500:12e22. Cheng Z, Wu Q, Li J, Zhu Q. Effects of promoters and preparation procedures on reforming of methane with carbon dioxide over Ni/Al2O3 catalyst. Catal Today 1996;30:147e55. Yu Q, Wu X, Tang C, Qi L, Liu B, Gao F, et al. Textural, structural, and morphological characterizations and catalytic activity of nanosized CeO2eMOx (M ¼ Mg2þ, Al3þ, Si4þ) mixed oxides for CO oxidation. J Colloid Interface Sci 2011;354:341e52. Akia M, Alavi SM, Rezaei M, Yan Z-F. Optimizing the solegel parameters on the synthesis of mesostructure nanocrystalline g-Al2O3. Microporous Mesoporous Mater 2009;122:72e8. Keshavarz AR, Rezaei M, Yaripour F. Preparation of nanocrystalline g-Al2O3 catalyst using different procedures for methanol dehydration to dimethyl ether. J Nat Gas Chem 2011;20:334e8. Sepehri S, Rezaei M. Preparation of highly active nickel catalysts supported on mesoporous nanocrystalline g-Al2O3 for methane autothermal reforming. Chem Eng Technol 2015;38:1637e45. Busca G, Lorenzelli V, Sanchez Escribano V. Preparation, solid-state characterization, and surface chemistry of highsurface-area nickel-aluminum (NixAl22xO32x) mixed oxides. Chem Mater 1992;4:595e605. Zhang F, Chen CH, Hanson JC, Robinson RD, Herman IP, Chan SW. Phases in ceriaezirconia binary oxide (1x) CeO2exZrO2 nanoparticles: the effect of particle size. J Am Ceram Soc 2006;89:1028e36. Kuznetsova T, Sadykov V. Specific features of the defect structure of metastable nanodisperse ceria, zirconia, and related materials. Kinet Catal 2008;49:840e58. Zhao A, Ying W, Zhang H, Ma H, Fang D. NieAl2O3 catalysts prepared by solution combustion method for syngas methanation. Catal Commun 2012;17:34e8. Kim P, Joo J, Kim H, Kim W, Kim Y, Song I, et al. Preparation of mesoporous Niealumina catalyst by one-step solegel method: control of textural properties and catalytic application to the hydrodechlorination of o-dichlorobenzene. Catal Lett 2005;104:181e9. € nicke D. Effect of alkali doping on catalytic Patcas F, Ho properties of alumina-supported nickel oxide in the selective oxidehydrogenation of cyclohexane. Catal Commun 2005;6:23e7. nchez-Sa nchez M, Navarro R, Fierro J. Ethanol steam Sa reforming over Ni/MxOyeAl2O3 (M ¼ Ce, La, Zr and Mg) catalysts: influence of support on the hydrogen production. Int J Hydrogen Energy 2007;32:1462e71. n-Go mez M. Nickel Juan-Juan J, Roman-Martinez M, Illa catalyst activation in the carbon dioxide reforming of methane: effect of pretreatments. Appl Catal A General 2009;355:27e32. Iriondo A, Barrio V, Cambra J, Arias P, Guemez M, SanchezSanchez M, et al. Glycerol steam reforming over Ni catalysts supported on ceria and ceria-promoted alumina. Int J Hydrogen Energy 2010;35:11622e33.
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[35] Zhang B, Tang X, Li Y, Cai W, Xu Y, Shen W. Steam reforming of bio-ethanol for the production of hydrogen over ceriasupported Co, Ir and Ni catalysts. Catal Commun 2006;7:367e72. [36] Wang S, Lu GM. Role of CeO2 in Ni/CeO2eAl2O3 catalysts for carbon dioxide reforming of methane. Appl Catal B Environ 1998;19:267e77. [37] Baidya T, Cattolica RJ. Improved catalytic performance of CaO and CeO2 promoted Ni catalyst on gasifier bed material for tar removal from producer gas. Appl Catal A General 2015;498:150e8.
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Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane Soodeh Sepehri a, Mehran Rezaei a,b,* 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:
In this study methane autothermal reforming (ATR) was investigated over Ni/Al2O3 and Ni/
Received 11 December 2016
Al2O3eCeO2 catalysts. The catalyst carriers were prepared through a facile one-step
Received in revised form
method, which produced mesoporous nanocrystalline carriers for Ni catalysts. The sam-
14 January 2017
ples were characterized by XRD, TPR, BET, TPO and SEM characterization techniques and
Accepted 16 January 2017
the catalytic activity and stability were also studied at different conditions (GHSV and feed
Available online xxx
ratio) in methane ATR. It was found that the nickel catalyst supported on 3 wt.% CeeAl2O3 exhibited higher activity compared to the catalysts supported on the Al2O3 and promoted
Keywords:
Al2O3 with 1 and 6 wt.% Ce. The results also showed that the nickel catalyst supported on
Autothermal reforming (ATR)
3 wt.% CeeAl2O3 possessed the highest resistance against carbon deposition in ATR
Ni based catalyst
reaction.
Solid state reaction
© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Cerium oxide
Introduction In recent years, synthesis gas composed of H2 and CO has attracted increasing attention as a new source of energy. Syngas can be a proper intermediate for H2 production and also a good feedstock for synthesis of other chemicals. Different processes such as steam reforming, autothermal reforming (ATR), dry reforming and partial oxidation have been suggested for producing syngas [1e6]. Recently, the ATR process has attracted special concerns since the process can provide its thermal requirements by combining the
endothermic partial oxidation and the exothermic steam reforming reactions [7]. Li et al. [8] investigated the alumina supported noble metal based catalysts (Rh, Pt and Pd) in the ATR reaction with a focus on their temperature profiles during the reaction. They have suggested that Rh is the most effective catalyst in terms of methane conversion. Due to the high cost of noble metals, they are not suitable for practical applications. Therefore Ni-based catalysts have been widely developed due to their low cost and high activity, though they suffer from sintering and coke deposition [9e12]. Horiuchi et al. [13] suggested that carbon deposition can be effectively suppressed when a metal oxide with a marked Lewis basicity is
* Corresponding author. Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran. Fax: þ98 3155559930. E-mail address: [email protected] (M. Rezaei). http://dx.doi.org/10.1016/j.ijhydene.2017.01.096 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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used as a support for a Ni catalyst. It is also found that CeO2 and ZrO2 have the potential of favoring carbon gasification because of their oxygen storage capacity [14]. Since the high costs of these materials make them inefficient to be applied as the supports in the commercial formulations, there are some proposals including the addition of these materials to the more practical supports as a promoter, such as addition of La, Ce, Sr and Zr on Ni/Al2O3 for autothermal reforming of methane [15] or CeO2 on Ni/Al2O3 for dry reforming of methane [16]. The results confirmed that the addition of CeO2 to the catalyst support can enhance the mobility of lattice oxygen, which leads to decreasing the carbon deposition and improving the thermal sintering of the catalytic active phase [17]. Yu et al. [18] have investigated the textural and catalytic performance of CeO2eMOx (M ¼ Mg2þ, Al3þ, Si4þ). They reported that Al2O3 is an effective surface stabilizer because of its enhanced textural properties. The results showed that the CeO2eAl2O3 possessed the highest activity in CO oxidation, due to its excellent textural characteristics, high homogeneity and redox properties [18]. There are several methods to synthesis alumina-based supports, such as solegel [19], precipitation and thermal decomposition methods [20]. In this study the promoted supports were prepared by a novel solid state synthesis method, which is a simple one-pot synthesis method including two steps (1) solid state reaction between ammonium carbonate and metal nitrates as precursors, and (2) calcination at elevated temperature. This procedure is a low cost and environmental friendly method since it does not require the addition of any surfactant, template or structuredirecting agent during the synthesis process. In this paper, we investigated the promoting effect of different percentages (0, 1, 3, 6 wt.%) of Ce on Ni/Al2O3 for ATR reaction using solid state reaction method to synthesize promoted supports.
pressure. All catalyst powders were pressed, ground, sieved to the 0.25e0.5 mm particle size, and reduced in hydrogen stream at 700 C for 3 h. The reaction mixture consisting of CH4, O2 and H2O (CH4:O2:H2O equal to 1:0.25:0.5) was passed over the catalyst bed. Brooks 5850 mass flow controllers were used to control the gas flow rates and also the water flow rates were controlled by a programmable syringe pump (IVAC P1000, UK). The activity measurements were carried out at a temperature range of 550 Ce700 C in steps of 50 C. The product stream was analyzed using an on-line Varian 3400 gas chromatograph equipped with a thermal conductivity detector and a Carboxen 1000 column. The exhaust passed through a cold-trap to remove its water content before each analysis.
Characterization In order to evaluate the specific surface area and pore size distribution, nitrogen physisorption isotherms were taken using a Tristar 3020 (Micromeritics) instrument. X-ray diffraction analysis (XRD) was done using a PANalytical X'Pert-Pro instrument. Temperature programmed reduction (TPR) analysis was performed to determine the type and relative proportion of reducible species on the prepared catalysts by using Micromeritics chemisorb 2750 instrument. In this analysis, 200 mg of the degassed catalyst was reduced in a gas mixture of H2:Ar (10:90) (20 ml/min) under a heating rate of 10 C/min. The H2 consumption was determined using a thermal conductivity detector (TCD). Temperatureprogrammed oxidation (TPO) of the spent catalysts was also conducted in a similar apparatus as described for TPR analysis by using a gas mixture of O2:He (5:95) (30 ml/min). Scanning electron microscopy (SEM) analysis was performed with a VEGA TESCAN microscope operated at 30 kV.
Results and discussion Materials and methods Characterization of the calcined samples Catalyst preparation Ni catalysts were prepared through the impregnation method, which is explained in our previous work in more details [21]. The catalyst supports were synthesized using a solid-state method by mixing a calculated content of Ce and Al salt precursors (Ce(NO3)3$6H2O, Al(NO3)3$9H2O) with ammonium carbonate ((NH4)2CO3/Al molar ratio ¼ 3) for 20 min using a mortar and pestle. During this process, the hydrate water of the mentioned precursors was released and the mixture was transformed into a pasty form. The prepared paste was then calcined with a ramp rate of 2 C/min at 700 C for 2 h. The Ni loading in all catalysts was chosen as 25 wt.%, while the Ce content was varied between 0, 1, 3 and 6 wt.% nominated as Ni/Al2O3, Ni/1Ce$Al2O3, Ni/3Ce$Al2O3 and Ni/6Ce$Al2O3.
Catalytic reaction A quartz fixed bed reactor was applied for autothermal reforming process at different temperatures. The process was carried out under continuous flow regime and atmospheric
Fig. 1 shows the XRD spectra of the catalysts. It is seen that the diffraction pattern for the unpromoted sample did not have a tangible difference with that of Ni/1Ce$Al2O3. Possible phases are NiO, Alumina, ceria and nickel aluminate, as indexed in this figure. The analysis confirmed the presence of poorly crystalline g-Al2O3 (2q ¼ 37.4, 46, 66.9 ) and NiO phases (2q ¼ 37.4, 43.3, 63.1, 75.6 ). The observed diffraction peaks at 2q ¼ 37, 45 and 65.5 are related to the NiAl2O4 spinel phase (JCPDS card No. 10-0339). The identification of the crystalline phases is complicated, due to overlapping of the NiO and Al2O3 characteristic peaks with the nickel aluminate [22]. CeO2 phase was not clearly observed in the Ni/1Ce$Al2O3 and Ni/ 3Ce$Al2O3 spectra due to the low Ce content. However the diffraction peaks of CeO2 were seen in Ni/6Ce$Al2O3 at 2q ¼ 28.7, 33.2, 47.7 and 56.6 . This phase is a cubic fluorite phase in accordance with the JCPDS Card No. 01-075-0076. It is also reported that CeO2 in the binary system with ZrO2 exhibits a cubic phase when the content of ceria is higher than 80%. For the ceria content of lower than 5% a monoclinic phase was reported [23]. The fluorite structure of ceria
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Fig. 1 e XRD analysis of the calcined catalysts with various Ce loadings.
tolerates oxygen non-stoichiometry with high values without changing its structure [24]. According to the diffraction patterns, NiO sharp peaks reduced in terms of intensity as ceria increased in the catalysts, suggesting that ceria promotes the Ni dispersion and reduces the bulk NiO on the surface of the catalyst, which is assigned to the Ce redox properties. The Ce4þ/Ce3þ redox couple generates the surface and bulk oxygen vacancies, which form MeO surface complexes. This decreases the interaction of the metal-support and consequently improves the nickel dispersion. The nitrogen adsorption/desorption isotherms of all supports and catalysts are shown in Fig. 2. All isotherms revealed type-IV curves with H2 type hysteresis loops, which is characteristic of mesopores structure with ink-bottle or channellike pores. The narrow pore-size distribution further indicates that the calcined samples possess highly uniform mesopores. All supports showed pore sizes below 7 nm while
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pore sizes for all catalysts were around 6 nm. The decrease in pore size and pore volume of the samples after addition of Ni to the supports could be assigned to the partial blockage of pores by NiO clusters and/or partial collapse of mesoporous structure [25]. The results in Table 1 confirmed that the promoters improved the catalysts surface area and reduced pore volume compared with unpromoted catalyst. In order to evaluate the type and relative percentage of the different reducible species the TPR profiles of the catalysts were presented in Fig. 3. The relative percentage of reducible species was calculated by Origin software (using Gaussian multipeaks curve-fitting) and the results are presented in Table 2. Fig. 3 also included the deconvolution of the 25Ni/ 1Ce$Al2O3 reducibility profile. The TPR profiles show two main reduction peaks. The first reduction peak at lower temperature around 450 C is assigned to the bulk NiO, probably due to large aggregates of nickel oxide [26] as confirmed by XRD analysis, and the other one at higher temperature which is broader and can be disintegrated into three components. The first peak can be attributed to the reduction of weakly interacted Ni2þ ions with Al2O3 [27,28], while the reduction of Ni2þ species related to non-stoichiometric nickel aluminate (NiAlxOy) was observed at higher temperature, which is also nominated as amorphous surface spinel [29]. The other high temperature reduction peak can be assigned to stoichiometric nickel aluminate species (NiAl2O4) [28,29]. Table 2 shows how the intensity and the relative percentage of NiAlxOy peaks increase with increasing in Ce content. After adding 1 wt.% Ce to the catalyst, the reduction peaks of catalyst moved towards the direction of NiOeAl2O3 species with weak interaction, while no considerable change observed in percentage of NiAl2O4. This indicates that the interaction between the active component and ceria has appeared and it has weakened the interaction between the support and the active component, and made the active component easier to reduce, thus improving the reducibility of the catalyst [30]. As the ceria content increases to 3 and 6 wt.%, the relative percentage of strongly interacted species with alumina support increases. Meanwhile the weakly interacted NiO species with
Fig. 2 e Pore-size distributions and N2 adsorption/desorption isotherms of (a) supports and (b) catalysts with different Ce contents. Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Table 1 e Textural properties of supports and catalysts with different Ce contents. SBET (m2/g)
Catalysts
25Ni/Al2O3 25Ni/1Ce$Al2O3 25Ni/3Ce$Al2O3 25Ni/6Ce$Al2O3
Pore volume (cm3/g)
Pore size (nm)
Support
Calcined
Spent
Support
Calcined
Spent
Support
Calcined
Spent
220.4 221.8 222.1 234.4
144.1 180.6 155.3 220.1
121.0 125.9 115.6 117.1
0.49 0.47 0.53 0.49
0.33 0.25 0.31 0.38
0.31 0.30 0.30 0.24
6.1 6.4 6.6 6.0
5.3 5.4 6.1 5.2
7.0 6.5 6.9 6.1
Fig. 3 e TPR analysis of calcined samples.
Table 2 e Relative percentage and reduction temperatures of reducible species for catalysts with different Ce contents. Catalysts 25Ni/Al2O3 25Ni/1Ce$Al2O3 25Ni/3Ce$Al2O3 25Ni/6Ce$Al2O3
NiO 17.3% 11.9% 19.9% 19.2%
(450 (439 (447 (420
NiOeAl2O3
C) C) C) C)
32.1% 59.4% 48.2% 40.7%
(608 (600 (651 (646
C) C) C) C)
NiAlxOy 45.5% 23.2% 31.9% 40.1%
(698 C) (699 C) (739 C) (734 C)
NiAl2O4 5.1% (783 C) 5.5% (780 C) e e
alumina decreases. Those high reduction temperature species could be attributed to the well dispersed NiO on the support. It could be concluded that higher amount of Ce has higher effect on the dispersion of NiO on the catalyst surface than 1 wt.% Ce. It should be noted that the NiAl2O4 spinel phase has been eliminated by addition of 3 and 6 wt.% Ce. No peaks associated with ceria with strong interaction with alumina, CeO2eAl2O3, were found. The TPR profile of these species shows the reduction peak around 890 C and provides surface species of Ce3þ which form CeAlO3 solid solutions [31].
ATR catalyst performance A comparison of the CH4 conversion vs. temperature for various samples in ATR reaction is illustrated in Fig. 4. Generally, CH4 conversion raises with increasing temperature which is an indicative for endothermic reaction. The addition of CeO2 to the nickel catalysts does not have a direct effect on CH4 conversion. The catalysts with 1 and 6% CeO2 showed lower conversion than the catalyst with 3% CeO2. Yang et al. [32] were also observed a similar trend for the Ce promoted Ni/ Alumina steam reforming catalysts with different Ce loadings
Fig. 4 e Methane conversion of catalysts at different temperatures (Reaction conditions: GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), CH4:H2O:O2 ¼ 1:0.5:0.25).
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Fig. 5 e (a) Short time stability of catalysts and (b) Long term stability of 25Ni/3Ce·Al2O3 at 700 C (Reaction conditions: GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), CH4:H2O:O2 ¼ 1:0.5:0.25). (0e2 wt.%). They found that the 1.02 wt.% Ce was an appropriate amount of promoter to obtain higher conversion, better carbon resistance and smaller nickel particle size and they showed that the higher amount of Ce led to lower conversion. A key consideration for nickel catalysts is to investigate carbon deposition and sintering which are studied through short and long term stability tests. The main causes of carbon deposition at the temperature range of 550e700 C are boudouard and methane cracking reactions. Fig. 5a indicates high stability of the catalysts during 4 h short term stability tests. Fig. 5b is also illustrated an appreciable stability for 25Ni/ 3Ce$Al2O3 catalyst within 20 h long term test. To determine the effect of increasing or decreasing steam reforming and partial oxidation reactions on catalytic performance in ATR process, we changed the S/C or O/C ratios and fixed other variables for 25Ni/3Ce$Al2O3 catalyst. The results are presented in Fig. 6a. As expected, the increase in S/C ratio led to improve the CH4 conversion, since steam forces the steam reforming reaction to proceed. Further increasing of
S/C ratio leads to further progress of steam reforming reaction which means that this reaction prevails over ATR reaction. The increase of O/C ratio had a similar effect on conversion with a higher effect (Fig. 6a), in a way that the methane conversion increases to near 100% when O/C ratio is equal to 1. At this ratio the feed started to spark because of the fast reaction of methane with oxygen at the bed inlet. It is also important to investigate the influence of GHSV on conversion which is an essential factor of catalytic performance. For this purpose the GHSV was varied between 104 and 7 104 ml/g h and other parameters were kept constant. Fig. 6b displays a slight decline in conversion with the increase of GHSV due to the decrease in amount of adsorbed reactants caused by a shortened contact time. H2/CO molar ratios vs. temperature for all samples were reported in Fig. 7. According to the results, this ratio starts from a maximum value at low temperature and ends in a minimum value at elevated temperature. This inverse relationship is assigned to the wateregas shift reaction which is
Fig. 6 e Methane conversion of 25Ni/3Ce·Al2O3 catalyst at various (a) S/C and O/C ratios (Reaction conditions: GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), Temperature ¼ 650 C) and (b) GHSV (Reaction conditions: CH4:H2O:O2 ¼ 1:0.5:0.25, Temperature ¼ 650 C). Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Fig. 7 e H2/CO ratios of catalysts with different Ce loadings at different temperatures.
more favorable at lower temperatures. The thermodynamic analysis has also predicted the similar trend [33]. Interestingly the H2/CO values for 25Ni/3Ce$Al2O3 are higher at each point, especially at 550 C which has an amount of 9.5. It could be concluded that 3 wt.% Ce has been more promotion effect on WGS reaction than 0, 1 and 6 wt.% Ce. It is also illustrated that Ni/Al2O3 revealed lower H2/CO ratio at any temperature. It could be concluded that Ce promotion of the catalysts increased H2/CO ratio.
Spent samples characterization The physisorption isotherms and pore size distributions of the spent samples and the spent 25Ni/3Ce$Al2O3 catalyst after long term stability are presented in Fig. 8. They are classified as IV type isotherms with H2-shaped hysteresis loops, which demonstrate that the samples have mesoporous structure even after reaction which can be related to the high thermal stability of the catalysts. The pore size distributions of the
Fig. 9 e SEM image of the spent 25 Ni/3Ce·Al2O3 sample. spent samples and spent 25Ni/3Ce$Al2O3 catalyst with long term stability are centered around 6.6 and 7.7 nm, respectively. The morphology of the spent 25Ni/3Ce$Al2O3 catalyst was studied using SEM analysis and the result is shown in Fig. 9. The SEM micrograph showed the existence of the agglomerates with soft and porous structure which contained tiny quasi-spherical particles. These agglomerates displayed a flake-like structure and an irregular morphology. Carbon formation was not detected on the surface of this spent sample.
Effect of ceria on carbon formation on ATR catalyst surface Since in normal conditions of ATR no carbonaceous species was observed on both promoted and unpromoted catalysts, in
Fig. 8 e N2 adsorption/desorption isotherms and pore-size distributions of (a) spent samples (b) spent 25Ni/3Ce·Al2O3 sample after long term stability. Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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Fig. 10 e a) TPO and b, c) SEM analysis of spent catalysts with different Ce contents after harsh conditions (GHSV ¼ 2.2 £ 104 (ml g¡1 h¡1), CH4:H2O:O2 ¼ 6:2:1).
order to evaluate the effect of Ce on carbon formation on the catalyst surface during ATR reaction of methane the 25Ni/ Al2O3 and 25Ni/3Ce$Al2O3 catalysts were examined under harsh reaction conditions for 1 h (GHSV ¼ 2.2 104 (ml g1 h1), CH4:H2O:O2 ¼ 6:2:1). The TPO profiles of the spent Ni/Al2O3 and Ni/3Ce$Al2O3 catalysts are shown in Fig. 10. Oxidation of 25% metallic Ni made a valley at the beginning of these profiles. The 25Ni/Al2O3 catalyst represented a peak centered at the temperature of 673 C, associated with partly graphitized of carbon deposits [34], while carbon was not detected on the surface of 25Ni/3Ce$Al2O3 catalyst. This could be assigned to the CeO2 capability to promote the carbon gasification and its ability to store [35,36] and transfer [31,35] oxygen. Baidya and Cattolica [37] also investigated the effect of CeO2 on Ni catalysts during steam reforming of tars. They declared that the role of CeO2 in coke removal is due to the oxygen exchange from H2O via these reactions:
CeO2 þ C / Ce2O3 þ CO
(1)
Ce2O3 þ H2O / CeO2 þ H2
(2)
Conclusion The main goal of the present paper was to investigate the effect of addition of small amount of ceria to the Al2O3 as catalyst carrier for nickel catalysts. A simple solid state reaction method was employed to prepare the alumina and promoted alumina supports. The bare alumina support prepared through solid state reaction method exhibited high BET surface area (>230 m2/gr), while the addition of ceria improves the surface area in promoted catalysts. XRD results revealed the enhanced dispersion of NiO over 3 and 6 wt.% Ce promoted supports, which confirmed by TPR results, where 1 wt.% Ce improved the reducibility of the catalyst, but 3 and 6 wt.% Ce enhanced Ni dispersion by shifting other Ni species to non-stoichiometric nickel aluminate, NiAlxOy, species. The catalytic activity experiments results indicated that Ni/ 3Ce$Al2O3 sample had a better performance (45% at 550 C up to 73 at 700 C) during methane ATR, where produced H2-rich gas (H2/CO z 9.5) at 550 C. It also exhibited a significant stability during 20 h ATR experiment. The BET results of the spent catalysts indicated that the mesoporous structure of the catalysts remained after reaction which endorses the synthesis method of the supports. The Ni/3Ce$Al2O3 sample also passed the carbon formation examination during harsh
Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096
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conditions of ATR, where unpromoted catalyst represented coke deposition. [14]
Acknowledgements The authors gratefully acknowledge the support from University of Kashan for supporting this work by Grant No. 158426/177.
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Please cite this article in press as: Sepehri S, Rezaei M, Ce promoting effect on the activity and coke formation of Ni catalysts supported on mesoporous nanocrystalline g-Al2O3 in autothermal reforming of methane, International Journal of Hydrogen Energy (2017), http:// dx.doi.org/10.1016/j.ijhydene.2017.01.096