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Autothermal reforming of methane over Ni catalysts supported on nanocrystalline MgO with high surface area and plated-like shape
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 3 6 ( 2 0 1 1 ) 1 1 7 1 2 e1 1 7 1 7
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Autothermal reforming of methane over Ni catalysts supported on nanocrystalline MgO with high surface area and plated-like shape Mehran Rezaei*, Fereshteh Meshkani, Abolfazl Biabani Ravandi, Behzad Nematollahi, Atiyeh Ranjbar, Narges Hadian, Zeinab Mosayebi Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran
article info
abstract
Article history:
Autothermal reforming of methane (ATRM), combination of partial oxidation and steam
Received 30 April 2011
reforming was performed over MgO supported Ni catalysts. The preparation of MgO via
Received in revised form
surfactant-assisted precipitation method led to obtain a nanocrystalline carrier for nickel
9 June 2011
catalysts. The results demonstrated that methane conversion is significantly increased
Accepted 11 June 2011
with increasing the Ni content (5, 7, 10 and 15%Ni) and methane conversion of 15%Ni/MgO
Available online 13 July 2011
was higher than that of other catalysts with lower Ni loading in all operation temperatures.
Keywords:
to the fact that water-gas shift reaction was thermodynamically unfavorable at elevated
Autothermal reforming
temperatures. This catalyst also exhibited stable catalytic performance during 50 h time on
Syngas
stream. Furthermore, the influences of varying GHSV and feed ratio on activity of 15%Ni/
Nickel catalyst
MgO catalyst were investigated.
In addition, increasing the system operation temperatures led to decrease in H2/CO due
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Synthesis gas (Syngas) is a mixture of hydrogen and carbon monoxide used in a variety of petrochemical and metallurgical process. Syngas is used primarily for the synthesis of methanol and Oxochemicals [1]. There are three major thermochemical reforming techniques used to produce syngas from hydrocarbon fuels, i.e. steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR) [2]. The most conventional method for production of hydrogen is steam reforming [3e6] that generates a high H2/CO ratio [7] but this process is highly endothermic and requires high reaction temperature, usually higher than 700 C on supported nickel catalysts [8]. This reaction is shown in Eq. (1).
CH4 þ H2 O/CO þ 3H2
DH298K ¼ þ206kJ=mol
(1)
Partial oxidation of methane (POX) is an alternative process for synthesis gas and hydrogen production. Partial oxidation process avoids the costs associated with superheated steam and approach to equilibrium can occur at a short contact time more selectively than steam reforming [9]. Partial oxidation of methane is an exothermic reaction outlined in Eq. (2). CH4 þ 0:5O2 /CO þ 2H2
DH298K ¼ 36 kJ=mol
(2)
ATR is an economical process for hydrogen production, which combines a highly exothermic reaction (POX) and a highly endothermic reaction (Steam reforming) [10]. This process can avoid the explosion dangers in partial oxidation
* Corresponding author. Tel.: þ98 361 5912469; fax: þ98 361 5559930. E-mail address: [email protected] (M. Rezaei). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.056
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 3 6 ( 2 0 1 1 ) 1 1 7 1 2 e1 1 7 1 7
of methane, lessen the additional steam cost in steam reforming, shorten start-up time and control the product composition (H2/CO ratio) by changing the steam and oxygen content in feed stream. Furthermore, the higher methane conversion and hydrogen yield can be achieved by a combination of these two processes. All these advantages demonstrate that the ATR process can be carried out in a most energy-efficient and safe manner [11]. Different types of catalysts have been employed for ATR process, namely: supported nickel and noble metal catalysts and transition metal carbide catalysts. Among these catalysts, supported nickel catalysts are cheaper than other employed catalysts, and thus commercially attractive [12]. The common carriers of Ni catalysts in methane reforming are a-alumina, calcium and magnesium aluminates. However, the rate and extent of nickel catalyst deactivation is high because of carbon formation on the catalyst surface [13]. So many studies have focus on Ni catalysts supported over various supports [14e17]. These studies demonstrated that the catalyst support plays an important role in the activity and stability of nickel supported catalysts in ATR process. It is well known that the carbon deposition can occur more easily on larger nickel particles than smaller ones [18]. It was found that the reduction of NiO in NiO-MgO solid solution was more difficult than that of pure nickel oxide. This phenomena leads to the formation of small nickel particles on the catalysts surface. In this paper, nanocrystalline magnesium oxide with high surface area was synthesized with a facile synthesis method and employed as a catalyst support for Ni catalysts in ATRM reaction and the effects of Ni loading, GHSV and feed ratio on the activity of prepared catalysts were investigated.
2.
Experimental
2.1.
Preparation of support
The magnesium oxide support was prepared through the wet precipitation route. The preparation procedure was reported previously in details [19]. In brief, firstly polyvinyl alcohol (PVA, MW: 70000) was dissolved in water at 90 C and under vigorous stirring to form a transparent solution. After that the Mg(NO3)2. 6H2O was dissolved in water containing PVA. The metal ion to PVA monomer unit mole ratio (M/PVA) was chosen as 1:3. An aqueous ammonia (25 wt.%) was added dropwise at room temperature to the resulting viscous liquid mixture under rapid stirring by careful pH adjustment to 10.5. After precipitation, the resulting slurry was subsequently aged at 80 C. for 20 h followed by filtering, washing, drying at 80 C for 24 h and calcination at 700 C for 4 h.
2.2.
Preparation of catalyst
The Ni/MgO catalysts were prepared by the wet impregnation method. An aqueous solution of Ni(NO3)2.6H2O was used as nickel precursor for preparing Ni/MgO catalysts. After impregnation at room temperature the resulting materials were dried at 80 C and subsequently calcined at 500 C for 2 h in static air atmosphere.
2.3.
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Characterization
The surface areas (BET) were determined by nitrogen adsorption at 196 C using an automated gas adsorption analyzer (Tristar 3000, Micromeritics). The nickel dispersion was measured by H2 chemisorption at 40 C, assuming that chemisorption stoichiometry is H/Ni ¼ 1 and that a Ni atom occupies 0.065 nm2 on a Ni particle [20]. Temperature programmed reduction (TPR) analysis was used for evaluating the reduction properties of prepared catalysts. In the TPR measurement, the fresh catalyst (200 mg) was subjected to a heat treatment (10 C/min) in a gas flow (30 ml/min) containing a mixture of H2:Ar (10:90). Before the TPR experiment, the samples were heat treated under an inert atmosphere at 350 C for 3 h. Temperature-programmed hydrogenation (TPH) of the spent catalysts was also carried out. The spent catalyst (25 mg) was subjected to heat treatment (10 C/min up to 800 C) in a gas flow (30 ml/min) containing a mixture of H2:Ar (10:90). Before the TPH experiment, the samples were heat treated under an inert atmosphere at 300 C for 3 h. Surface morphology of samples was investigated by using scanning electron microscopy (SEM, Vega@Tescan) and transmission electron microscopy (TEM, JEOL JEM-2100UHR).
2.4.
Catalytic evaluation
The ATR reaction was carried out in a continuous flow reactor made of a 7-mm-i.d. quartz tube at different temperatures under atmospheric pressure. The reactor was charged with 50 mg of the prepared catalyst. The catalyst particle size employed in catalyst test was between 0.28 and 0.48 mm. Prior to the reaction, the catalyst was reduced in situ at 650 C for 4 h in flowing H2 (30 ml/min) and cooled down to 500 C in a flow of Ar (30 ml/min). After that, a reactant gas feed consisting of a mixture of CH4 and O2 was introduced into the reactor. Water was fed by means of a syringe pump. The activity tests were performed at different temperatures, ranging from 500 C to 700 C in steps of 50 C that were kept for 30 min at each temperature. A cold trap was used for separating water vapor from the tail gas and the product was analyzed by a gas chromatoghraph equipped with a TCD detector and a Carboxen 1000 column.
3.
Results and discussion
The TEM and SEM images of MgO calcined at 700 C are shown in Fig. 1a and b, respectively. As can be seen, the prepared support shows a nanocrystalline structure with a plate like shape. The SEM analysis also demonstrates that the MgO consists of irregularly shaped quasi-two-dimensional micronsized particles (flakes). The structural properties of catalyst support and calcined catalysts are presented in Table 1. It is seen that the catalyst support shows a high surface area and nanocrystalline structure. In addition, by increasing Ni content the specific surface area of the calcined catalysts decreased. The H2 chemisorption analysis also revealed that increasing in Ni content leads to decreasing in Nickel dispersion. The comparison of catalytic performance of the prepared catalysts with different Ni loadings in ATRM reaction at
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Fig. 2 e (a) CH4 conversion and (b) H2/CO ratio of catalysts with different Ni loadings at different temperatures, GHSV [ 1.2 3 105 ml/g.h, for CH4:H2O:O2 [ 1:0.5:0.25. Fig. 1 e (a) TEM and (b) SEM image of catalyst support. different temperatures ranging from 500 C to 700 C for CH4:H2O:O2 ¼ 1:0.5:0.25 is presented in Fig. 2a and b in terms of CH4 conversion and H2/CO production ratio, respectively. It is seen that the methane conversion increased by increasing Ni loading from 5%wt. to 15% wt. in operation temperatures. The
results shown in Fig. 2b demonstrated that H2/CO ratio for all the samples is more than three, due to the water gas shift reaction. According to the results the observed higher H2/CO ratio (>6.0) at low temperature, suggested that the water-gas shift reaction occurred extensively as reported in the literature [21]. In addition, increasing the system operation
Table 1 e Structural properties of samples. Support
S
2 BET(m
MgO Catalyst
5%Ni/MgO 7%Ni/MgO 10%Ni/MgO 15%Ni/MgO
g1)
116 S
BET
(m2 g1)
Pore volume (cm3 g1)
0.69 Pore volume (cm3 g1)
Pore size (nm)
21.7 Pore size (nm)
Crystallite size (nm) (111)
(200)
(220)
17.5
12.8
13.7
Ni area (m2 g1Ni)
Ni size (nm)
Dispersion (%)
Calcined
Calcined
Calcined
Reduced
Reduced
Reduced
58.0 48.1 46.9 44.1
0.469 0.423 0.419 0.399
29.82 31.54 32.62 34.40
3.1 4.1 4.8 4.6
11.0 11.5 14.1 20.8
9.1 8.7 7.1 4.8
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11715
temperatures led to decrease in H2/CO due to the fact that water-gas shift reaction was thermodynamically unfavorable at elevated temperatures. Fig. 3a shows the stability of ATRM reaction on the Ni/MgO catalysts at 700 C. All these catalysts showed a high stability without any decrease in CH4 conversion during the reaction time of 300 min. In addition, the long term stability of the 15% Ni/MgO catalyst showed very high stability during 50 h time on stream (Fig. 3b). In order to evaluate the influence of adding O2 in ATRM reaction, the experiment was carried out at a constant feed ratio of H2O/CH4 ¼ 0.5 and increasing the ratio of O2/CH4 from 0.25 to 1.5. Fig. 4a presents the CH4 conversion for the 15%Ni/ MgO catalyst. The results show that the CH4 conversion increases when the ratio of O2/CH4 increases until O2/CH4 ¼ 1.0. The increase in CH4 conversion with O2 addition can be related to combustion of part of methane with all O2 in the feed. In addition, it can be clearly observed that as O2/CH4 ratio increases more than one, the conversion of CH4 significantly decreases, which could be related to oxidation of nickel. In addition, as it can be seen in Fig. 4b, at a constant feed ratio of O2/CH4 ¼ 0.25 the CH4 conversion increases when the ratio
Fig. 4 e CH4 conversion of 15%Ni/MgO catalyst a) H2O/ CH4 [ 0.5, varying the O2/CH4 feed ratio, b) O2/CH4 [ 0.25, varying the H2O/CH4 feed ratio, Reaction conditions: GHSV [ 1.2 3 105 ml/g.h and temperature: 700 C.
Fig. 3 e (a) Short time stability of catalysts with different Ni loadings and (b) Long term stability of 15%Ni/MgO at 700 C, GHSV [ 1.2 3 105 ml/g.h, for CH4:H2O:O2 [ 1:0.5:0.25.
Fig. 5 e Effect of GHSV on the catalytic activity of 15%Ni/ MgO catalyst, Reaction temperature: 700 C.
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Fig. 6 e TPR profiles of the 15%Ni/MgO catalyst. Fig. 8 e TPH profile of spent 15%Ni/MgO catalyst after 50 h of reaction. of H2O/CH4 increases until H2O/CH4 ¼ 2.0, which the CH4 conversion becomes 100%. The effect of gas hour space velocity (GHSV) on the catalytic performance of 15%Ni/MgO catalyst was studied by
maintaining the reaction temperature and feed ratio in the system constant (T ¼ 700 C and CH4:H2O:O2 ¼ 1:0.5:0.25). Fig. 5 shows that increasing the GHSV causes a decrease in methane conversion due to decrease of contact time and also amount of adsorbed reactants. Fig. 6 presents the results of temperature programmed reduction (TPR) of the 15%Ni/MgO catalyst which shows two resolved reduction peaks due to the varying degrees of interactions between NiO and MgO. The small peak at around 420 C is related, in accordance with the literature [22], to reduction of bulk nickel oxide, which has a low interaction with magnesium oxide. The other peak at more than 800 C is attributed to NiO strongly interacting with MgO and formation of NiO-MgO solid solution. SEM images of fresh and spent 15%Ni/MgO catalyst after 50 h time on stream at 700 C with different magnification are shown in Fig. 7a and b respectively. In SEM image of fresh 15%Ni/MgO catalyst finely dispersed Ni particles can be seen. It is also clear from the Fig. 7b that whisker type carbon was deposited over the spent 15%Ni/MgO catalyst. Temperature Programmed Hydrogenation (TPH) was carried out after 50 h of reaction on the 15%Ni/MgO catalyst (Fig. 8). The TPH profile of this catalyst clearly shows that three kinds of carbon species deposited on the catalyst surface during ATRM reaction. One small peak is observed at a temperature lower than 100 C could be related to superficial carbidic carbon (Ca) [23], that shows high reactivity towardhydrogenation. The peaks which are observed at different temperatures between 250 and 350 C might be amorphous carbon on the nickel sites while the major peak is observed at temperature above 600 C identified as whisker-type carbon [24,25].
4. Fig. 7 e SEM image of (a) Fresh and (b) spent 15%Ni/MgO catalyst after 50 h of reaction.
Conclusion
Ni catalysts with various nickel loadings supported on nanocrystalline magnesium oxide with high surface area were
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 3 6 ( 2 0 1 1 ) 1 1 7 1 2 e1 1 7 1 7
employed in autothermal reforming of methane. The results presented in this article demonstrated that the increase in Ni content brought about a positive effect on the activity of reduced catalysts and the methane conversion of 15%Ni/MgO catalyst was higher than that observed on the catalysts with lower nickel content. The small metal particle size constitutes the ability of MgO-based catalysts to show high activity and stability during the reaction time of 300 min. In addition, 15% Ni/MgO catalyst showed very high stability during 50 h time on stream. The results showed that the feed ratio plays a decisive role on catalytic activity during ATRM reaction and the increase in O2/CH4 feed ratio until O2/CH4 ¼ 1 increases the CH4 conversion. The activity results also indicate that increasing the GHSV leads to a decrease in CH4 conversion due to decrease of contact time and also amount of adsorbed reactants. SEM image of spent 15%Ni/MgO catalyst after running the reaction for 50 h showed that whisker carbon was formed over this catalyst. TPH analysis also reveals that different kinds of carbon deposited over the 15%Ni/MgO catalyst during ATRM reaction.
references
[1] Therdthianwong S, Therdthianwong A, Siangchin C, Yongprapat S. Synthesis gas production from dry reforming of methane over Ni/Al2O3 stabilized by ZrO2. Int J Hydrogen Energy 2008;33:991e9. [2] Zahedi Nezhad M, Rowshanzamir S, Eikani MH. Autothermal reforming of methane to synthesis gas: modeling and simulation. Int J Hydrogen Energy 2009;34:1292e300. [3] Xu J, Froment GF. Methane steam reforming, methanation and water-gas shift. 1 Intrinsic kinetics. AIChE J 1989;35: 88e96. [4] Hou K, Hughes R. The kinetics of methane steam reforming over Ni/aeAl2O3 catalyst. Chem Eng J 2001;82:311e28. [5] Laosiripojana N, Assabumrungrat S. Methane steam reforming over Ni/CeeZrO2 catalyst: influences of CeeZrO2 support on reactivity, resistance toward carbon formation, and intrinsic reaction kinetics. Appl Catal A Gen 2005;290: 200e11. [6] Pan L, Wang S. Methanol steam reforming in a compact plate-fin reformer for fuel-cell systems. Int J Hydrogen Energy 2005;30:973e9. [7] Dias JAC, Assaf JM. Autothermal reforming of methane over Ni/geAl2O3 catalysts: the enhancement effect of small quantities of noble metals. J Power Sources 2004;130:106e10. [8] Halabi MH, de Croon MHJM, van der Schaaf J, Cobdenb PD, Schouten JC. Modeling and analysis of autothermal reforming of methane to hydrogen in a fixed bed reformer. Chem Eng J 2008;137:568e78.
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[9] Hickman DA, Haupfear EA, Schmidt LD. Synthesis gasformation by direct oxidation of methane over Rh monoliths. Catal Lett 1993;17:223e37. [10] Cai X, Dong XD, Lin W. Autothermal reforming of methane over Ni catalysts supported on CuOeZrO2eCeO2eAl2O3. J Nat Gas Chem 2006;15:122e6. [11] Cai X, Dong XD, Lin W. Effect of CeO2 on the catalytic performance of Ni/Al2O3 for autothermal reforming of methane. J Nat Gas Chem 2008;17:98e102. [12] Chen L, Lu Y, Hong Q, Lin J, Dautzenberg FM. Catalytic partial oxidation of methane to syngas over Ca-decorated-Al2O3supported Ni and NiB catalysts. Appl Catal A Gen 2005;292: 295e304. [13] Dantas SC, Escritori JC, Soares RR, Hori CE. Effect of different promoters on Ni/CeZrO2 catalyst for autothermal reforming and partial oxidation of methane. Chem Eng J 2010;156: 380e7. [14] Takeguchi T, Furukawa S, Inoue M, Eguchi K. Autothermal reforming of methane over Ni catalysts supported over CaOeCeO2eZrO2 solid solution. Appl Catal A Gen 2003;240: 223e33. [15] Ayabe S, Omoto H, Utaka T, Kikuchi R, Sasaki K, Teraoka Y, et al. Catalytic autothermal reforming of methane and propane over supported metal catalysts. Appl Catal A Gen 2003;241:261e9. [16] Hoang DL, Chan SH. Experimental investigation on the effect of natural gas composition on performance of autothermal reforming. Int J Hydrogen Energy 2007;32:548e56. [17] Takehira K, Shishido T, Wang P, Kosaka T, Takaki K. Autothermal reforming of CH4 over supported Ni catalysts prepared from MgeAl hydrotalcite-like anionic clay. J Catal 2004;221:43e7. [18] Rostrup-Nielsen JR, Sehested J, Norskov JK. Hydrogen and synthesis gas by steam- and CO2 reforming. Adv Catal 2002; 47:65e139. [19] Meshkani F, Rezaei M. Facile synthesis of nanocrystalline magnesium oxide with high surface area. Powder Technol 2009;196:85e6. [20] Lemaitre JL, Menon PG, Delannay F. In: Delannay F, editor. Characterization of heterogeneous catalysts, vol. 15. New York: Marcel Dekker Inc; 1984. [21] Souza MMVM, Schmal M. Autothermal reforming of methane over Pt/ZrO2/Al2O3 catalysts. Appl Catal A Gen 2005; 281:19e24. [22] Dong WS, Roh HS, Liu ZW, Jun KW, Park SE. Hydrogen production from methane reforming reactions over Ni/MgO catalyst. Bull Korean Chem Soc 2001;22:1323e7. [23]. Bartholomew CH. Carbon deposition in steam reforming and methanation. Catal Reviews-Sci Eng 1982;24:67e112. [24] Rezaei M, Alavi SM, Sahebdelfar S, Bai P, Liu X, Yan ZF. CO2 reforming of CH4 over nanocrystalline zirconia-supported nickel catalysts. Appl Catal B Env 2007;77:346e54. [25] Nematollahi B, Rezaei M, Khajenoori M. Combined dry reforming and partial oxidation of methane to synthesis gas on noble metal catalysts. Int J Hydrogen Energy 2011;36: 2969e78.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Autothermal reforming of methane over Ni catalysts supported on nanocrystalline MgO with high surface area and plated-like shape Mehran Rezaei*, Fereshteh Meshkani, Abolfazl Biabani Ravandi, Behzad Nematollahi, Atiyeh Ranjbar, Narges Hadian, Zeinab Mosayebi Catalyst and Advanced Materials Research Laboratory, Chemical Engineering Department, Faculty of Engineering, University of Kashan, Kashan, Iran
article info
abstract
Article history:
Autothermal reforming of methane (ATRM), combination of partial oxidation and steam
Received 30 April 2011
reforming was performed over MgO supported Ni catalysts. The preparation of MgO via
Received in revised form
surfactant-assisted precipitation method led to obtain a nanocrystalline carrier for nickel
9 June 2011
catalysts. The results demonstrated that methane conversion is significantly increased
Accepted 11 June 2011
with increasing the Ni content (5, 7, 10 and 15%Ni) and methane conversion of 15%Ni/MgO
Available online 13 July 2011
was higher than that of other catalysts with lower Ni loading in all operation temperatures.
Keywords:
to the fact that water-gas shift reaction was thermodynamically unfavorable at elevated
Autothermal reforming
temperatures. This catalyst also exhibited stable catalytic performance during 50 h time on
Syngas
stream. Furthermore, the influences of varying GHSV and feed ratio on activity of 15%Ni/
Nickel catalyst
MgO catalyst were investigated.
In addition, increasing the system operation temperatures led to decrease in H2/CO due
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Synthesis gas (Syngas) is a mixture of hydrogen and carbon monoxide used in a variety of petrochemical and metallurgical process. Syngas is used primarily for the synthesis of methanol and Oxochemicals [1]. There are three major thermochemical reforming techniques used to produce syngas from hydrocarbon fuels, i.e. steam reforming (SR), partial oxidation (POX), and autothermal reforming (ATR) [2]. The most conventional method for production of hydrogen is steam reforming [3e6] that generates a high H2/CO ratio [7] but this process is highly endothermic and requires high reaction temperature, usually higher than 700 C on supported nickel catalysts [8]. This reaction is shown in Eq. (1).
CH4 þ H2 O/CO þ 3H2
DH298K ¼ þ206kJ=mol
(1)
Partial oxidation of methane (POX) is an alternative process for synthesis gas and hydrogen production. Partial oxidation process avoids the costs associated with superheated steam and approach to equilibrium can occur at a short contact time more selectively than steam reforming [9]. Partial oxidation of methane is an exothermic reaction outlined in Eq. (2). CH4 þ 0:5O2 /CO þ 2H2
DH298K ¼ 36 kJ=mol
(2)
ATR is an economical process for hydrogen production, which combines a highly exothermic reaction (POX) and a highly endothermic reaction (Steam reforming) [10]. This process can avoid the explosion dangers in partial oxidation
* Corresponding author. Tel.: þ98 361 5912469; fax: þ98 361 5559930. E-mail address: [email protected] (M. Rezaei). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.06.056
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 3 6 ( 2 0 1 1 ) 1 1 7 1 2 e1 1 7 1 7
of methane, lessen the additional steam cost in steam reforming, shorten start-up time and control the product composition (H2/CO ratio) by changing the steam and oxygen content in feed stream. Furthermore, the higher methane conversion and hydrogen yield can be achieved by a combination of these two processes. All these advantages demonstrate that the ATR process can be carried out in a most energy-efficient and safe manner [11]. Different types of catalysts have been employed for ATR process, namely: supported nickel and noble metal catalysts and transition metal carbide catalysts. Among these catalysts, supported nickel catalysts are cheaper than other employed catalysts, and thus commercially attractive [12]. The common carriers of Ni catalysts in methane reforming are a-alumina, calcium and magnesium aluminates. However, the rate and extent of nickel catalyst deactivation is high because of carbon formation on the catalyst surface [13]. So many studies have focus on Ni catalysts supported over various supports [14e17]. These studies demonstrated that the catalyst support plays an important role in the activity and stability of nickel supported catalysts in ATR process. It is well known that the carbon deposition can occur more easily on larger nickel particles than smaller ones [18]. It was found that the reduction of NiO in NiO-MgO solid solution was more difficult than that of pure nickel oxide. This phenomena leads to the formation of small nickel particles on the catalysts surface. In this paper, nanocrystalline magnesium oxide with high surface area was synthesized with a facile synthesis method and employed as a catalyst support for Ni catalysts in ATRM reaction and the effects of Ni loading, GHSV and feed ratio on the activity of prepared catalysts were investigated.
2.
Experimental
2.1.
Preparation of support
The magnesium oxide support was prepared through the wet precipitation route. The preparation procedure was reported previously in details [19]. In brief, firstly polyvinyl alcohol (PVA, MW: 70000) was dissolved in water at 90 C and under vigorous stirring to form a transparent solution. After that the Mg(NO3)2. 6H2O was dissolved in water containing PVA. The metal ion to PVA monomer unit mole ratio (M/PVA) was chosen as 1:3. An aqueous ammonia (25 wt.%) was added dropwise at room temperature to the resulting viscous liquid mixture under rapid stirring by careful pH adjustment to 10.5. After precipitation, the resulting slurry was subsequently aged at 80 C. for 20 h followed by filtering, washing, drying at 80 C for 24 h and calcination at 700 C for 4 h.
2.2.
Preparation of catalyst
The Ni/MgO catalysts were prepared by the wet impregnation method. An aqueous solution of Ni(NO3)2.6H2O was used as nickel precursor for preparing Ni/MgO catalysts. After impregnation at room temperature the resulting materials were dried at 80 C and subsequently calcined at 500 C for 2 h in static air atmosphere.
2.3.
11713
Characterization
The surface areas (BET) were determined by nitrogen adsorption at 196 C using an automated gas adsorption analyzer (Tristar 3000, Micromeritics). The nickel dispersion was measured by H2 chemisorption at 40 C, assuming that chemisorption stoichiometry is H/Ni ¼ 1 and that a Ni atom occupies 0.065 nm2 on a Ni particle [20]. Temperature programmed reduction (TPR) analysis was used for evaluating the reduction properties of prepared catalysts. In the TPR measurement, the fresh catalyst (200 mg) was subjected to a heat treatment (10 C/min) in a gas flow (30 ml/min) containing a mixture of H2:Ar (10:90). Before the TPR experiment, the samples were heat treated under an inert atmosphere at 350 C for 3 h. Temperature-programmed hydrogenation (TPH) of the spent catalysts was also carried out. The spent catalyst (25 mg) was subjected to heat treatment (10 C/min up to 800 C) in a gas flow (30 ml/min) containing a mixture of H2:Ar (10:90). Before the TPH experiment, the samples were heat treated under an inert atmosphere at 300 C for 3 h. Surface morphology of samples was investigated by using scanning electron microscopy (SEM, Vega@Tescan) and transmission electron microscopy (TEM, JEOL JEM-2100UHR).
2.4.
Catalytic evaluation
The ATR reaction was carried out in a continuous flow reactor made of a 7-mm-i.d. quartz tube at different temperatures under atmospheric pressure. The reactor was charged with 50 mg of the prepared catalyst. The catalyst particle size employed in catalyst test was between 0.28 and 0.48 mm. Prior to the reaction, the catalyst was reduced in situ at 650 C for 4 h in flowing H2 (30 ml/min) and cooled down to 500 C in a flow of Ar (30 ml/min). After that, a reactant gas feed consisting of a mixture of CH4 and O2 was introduced into the reactor. Water was fed by means of a syringe pump. The activity tests were performed at different temperatures, ranging from 500 C to 700 C in steps of 50 C that were kept for 30 min at each temperature. A cold trap was used for separating water vapor from the tail gas and the product was analyzed by a gas chromatoghraph equipped with a TCD detector and a Carboxen 1000 column.
3.
Results and discussion
The TEM and SEM images of MgO calcined at 700 C are shown in Fig. 1a and b, respectively. As can be seen, the prepared support shows a nanocrystalline structure with a plate like shape. The SEM analysis also demonstrates that the MgO consists of irregularly shaped quasi-two-dimensional micronsized particles (flakes). The structural properties of catalyst support and calcined catalysts are presented in Table 1. It is seen that the catalyst support shows a high surface area and nanocrystalline structure. In addition, by increasing Ni content the specific surface area of the calcined catalysts decreased. The H2 chemisorption analysis also revealed that increasing in Ni content leads to decreasing in Nickel dispersion. The comparison of catalytic performance of the prepared catalysts with different Ni loadings in ATRM reaction at
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Fig. 2 e (a) CH4 conversion and (b) H2/CO ratio of catalysts with different Ni loadings at different temperatures, GHSV [ 1.2 3 105 ml/g.h, for CH4:H2O:O2 [ 1:0.5:0.25. Fig. 1 e (a) TEM and (b) SEM image of catalyst support. different temperatures ranging from 500 C to 700 C for CH4:H2O:O2 ¼ 1:0.5:0.25 is presented in Fig. 2a and b in terms of CH4 conversion and H2/CO production ratio, respectively. It is seen that the methane conversion increased by increasing Ni loading from 5%wt. to 15% wt. in operation temperatures. The
results shown in Fig. 2b demonstrated that H2/CO ratio for all the samples is more than three, due to the water gas shift reaction. According to the results the observed higher H2/CO ratio (>6.0) at low temperature, suggested that the water-gas shift reaction occurred extensively as reported in the literature [21]. In addition, increasing the system operation
Table 1 e Structural properties of samples. Support
S
2 BET(m
MgO Catalyst
5%Ni/MgO 7%Ni/MgO 10%Ni/MgO 15%Ni/MgO
g1)
116 S
BET
(m2 g1)
Pore volume (cm3 g1)
0.69 Pore volume (cm3 g1)
Pore size (nm)
21.7 Pore size (nm)
Crystallite size (nm) (111)
(200)
(220)
17.5
12.8
13.7
Ni area (m2 g1Ni)
Ni size (nm)
Dispersion (%)
Calcined
Calcined
Calcined
Reduced
Reduced
Reduced
58.0 48.1 46.9 44.1
0.469 0.423 0.419 0.399
29.82 31.54 32.62 34.40
3.1 4.1 4.8 4.6
11.0 11.5 14.1 20.8
9.1 8.7 7.1 4.8
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temperatures led to decrease in H2/CO due to the fact that water-gas shift reaction was thermodynamically unfavorable at elevated temperatures. Fig. 3a shows the stability of ATRM reaction on the Ni/MgO catalysts at 700 C. All these catalysts showed a high stability without any decrease in CH4 conversion during the reaction time of 300 min. In addition, the long term stability of the 15% Ni/MgO catalyst showed very high stability during 50 h time on stream (Fig. 3b). In order to evaluate the influence of adding O2 in ATRM reaction, the experiment was carried out at a constant feed ratio of H2O/CH4 ¼ 0.5 and increasing the ratio of O2/CH4 from 0.25 to 1.5. Fig. 4a presents the CH4 conversion for the 15%Ni/ MgO catalyst. The results show that the CH4 conversion increases when the ratio of O2/CH4 increases until O2/CH4 ¼ 1.0. The increase in CH4 conversion with O2 addition can be related to combustion of part of methane with all O2 in the feed. In addition, it can be clearly observed that as O2/CH4 ratio increases more than one, the conversion of CH4 significantly decreases, which could be related to oxidation of nickel. In addition, as it can be seen in Fig. 4b, at a constant feed ratio of O2/CH4 ¼ 0.25 the CH4 conversion increases when the ratio
Fig. 4 e CH4 conversion of 15%Ni/MgO catalyst a) H2O/ CH4 [ 0.5, varying the O2/CH4 feed ratio, b) O2/CH4 [ 0.25, varying the H2O/CH4 feed ratio, Reaction conditions: GHSV [ 1.2 3 105 ml/g.h and temperature: 700 C.
Fig. 3 e (a) Short time stability of catalysts with different Ni loadings and (b) Long term stability of 15%Ni/MgO at 700 C, GHSV [ 1.2 3 105 ml/g.h, for CH4:H2O:O2 [ 1:0.5:0.25.
Fig. 5 e Effect of GHSV on the catalytic activity of 15%Ni/ MgO catalyst, Reaction temperature: 700 C.
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Fig. 6 e TPR profiles of the 15%Ni/MgO catalyst. Fig. 8 e TPH profile of spent 15%Ni/MgO catalyst after 50 h of reaction. of H2O/CH4 increases until H2O/CH4 ¼ 2.0, which the CH4 conversion becomes 100%. The effect of gas hour space velocity (GHSV) on the catalytic performance of 15%Ni/MgO catalyst was studied by
maintaining the reaction temperature and feed ratio in the system constant (T ¼ 700 C and CH4:H2O:O2 ¼ 1:0.5:0.25). Fig. 5 shows that increasing the GHSV causes a decrease in methane conversion due to decrease of contact time and also amount of adsorbed reactants. Fig. 6 presents the results of temperature programmed reduction (TPR) of the 15%Ni/MgO catalyst which shows two resolved reduction peaks due to the varying degrees of interactions between NiO and MgO. The small peak at around 420 C is related, in accordance with the literature [22], to reduction of bulk nickel oxide, which has a low interaction with magnesium oxide. The other peak at more than 800 C is attributed to NiO strongly interacting with MgO and formation of NiO-MgO solid solution. SEM images of fresh and spent 15%Ni/MgO catalyst after 50 h time on stream at 700 C with different magnification are shown in Fig. 7a and b respectively. In SEM image of fresh 15%Ni/MgO catalyst finely dispersed Ni particles can be seen. It is also clear from the Fig. 7b that whisker type carbon was deposited over the spent 15%Ni/MgO catalyst. Temperature Programmed Hydrogenation (TPH) was carried out after 50 h of reaction on the 15%Ni/MgO catalyst (Fig. 8). The TPH profile of this catalyst clearly shows that three kinds of carbon species deposited on the catalyst surface during ATRM reaction. One small peak is observed at a temperature lower than 100 C could be related to superficial carbidic carbon (Ca) [23], that shows high reactivity towardhydrogenation. The peaks which are observed at different temperatures between 250 and 350 C might be amorphous carbon on the nickel sites while the major peak is observed at temperature above 600 C identified as whisker-type carbon [24,25].
4. Fig. 7 e SEM image of (a) Fresh and (b) spent 15%Ni/MgO catalyst after 50 h of reaction.
Conclusion
Ni catalysts with various nickel loadings supported on nanocrystalline magnesium oxide with high surface area were
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 3 6 ( 2 0 1 1 ) 1 1 7 1 2 e1 1 7 1 7
employed in autothermal reforming of methane. The results presented in this article demonstrated that the increase in Ni content brought about a positive effect on the activity of reduced catalysts and the methane conversion of 15%Ni/MgO catalyst was higher than that observed on the catalysts with lower nickel content. The small metal particle size constitutes the ability of MgO-based catalysts to show high activity and stability during the reaction time of 300 min. In addition, 15% Ni/MgO catalyst showed very high stability during 50 h time on stream. The results showed that the feed ratio plays a decisive role on catalytic activity during ATRM reaction and the increase in O2/CH4 feed ratio until O2/CH4 ¼ 1 increases the CH4 conversion. The activity results also indicate that increasing the GHSV leads to a decrease in CH4 conversion due to decrease of contact time and also amount of adsorbed reactants. SEM image of spent 15%Ni/MgO catalyst after running the reaction for 50 h showed that whisker carbon was formed over this catalyst. TPH analysis also reveals that different kinds of carbon deposited over the 15%Ni/MgO catalyst during ATRM reaction.
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