- Email: [email protected]
Al molar ratio
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen production by steam reforming of ethanol over mesoporous NieAl2O3eZrO2 xerogel catalysts: Effect of Zr/Al molar ratio Seung Ju Han a, Yongju Bang a, Jeong Gil Seo b, Jaekyeong Yoo a, In Kyu Song a,* a
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea b Department of Environmental Engineering and Energy, Myongji University, Nam-dong, Cheoin-gu, Yongin-si, Gyeonggi-do 449-728, South Korea
article info
abstract
Article history:
A series of mesoporous NieAl2O3eZrO2 xerogel catalysts (denoted as Ni-AZ-X ) with
Received 18 October 2012
different Zr/Al molar ratio (X) were prepared by a single-step epoxide-driven solegel
Received in revised form
method, and they were applied to the hydrogen production by steam reforming of ethanol.
7 November 2012
The effect of Zr/Al molar ratio of Ni-AZ-X catalysts on their physicochemical properties
Accepted 10 November 2012
and catalytic activities was investigated. Textural and chemical properties of Ni-AZ-X
Available online 6 December 2012
catalysts were strongly influenced by Zr/Al molar ratio. Surface area of Ni-AZ-X catalysts decreased with increasing Zr/Al molar ratio due to the lattice contraction of ZrO2 caused by
Keywords:
the incorporation of Al3þ into ZrO2. Interaction between nickel oxide species and support
Hydrogen production
(Al2O3eZrO2) decreased with increasing Zr/Al molar ratio through the formation of NiO
Steam reforming of ethanol
eAl2O3eZrO2 composite structure. Acidity of reduced Ni-AZ-X catalysts decreased with
Mesoporous NieAl2O3eZrO2 xerogel
increasing Zr/Al molar ratio due to the loss of acid sites of Al2O3 by the addition of ZrO2.
catalyst
Acidity of Ni-AZ-X catalysts served as a crucial factor determining the catalytic perfor-
Zr/Al molar ratio
mance in the steam reforming of ethanol; an optimal acidity was required for maximum
Acidity
production of hydrogen. Among the catalysts tested, Ni-AZ-0.2 (Zr/Al ¼ 0.2) catalyst with an intermediate acidity exhibited the best catalytic performance in the steam reforming of ethanol. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With increasing demand for new and clean energy, hydrogen has attracted much attention as a promising energy carrier, because it possesses high energy density (ca. 120.7 kJ/g) and burns without emitting any environmental pollutants [1e4]. There are various resources for hydrogen production such as coal, natural gas, propane, methanol, and ethanol. Among these
resources, ethanol has been considered as a practical resource, because it is biodegradable, low in toxicity, and free from catalyst poisons such as sulfur [4]. Moreover, ethanol becomes more abundantly available due to the increasing ethanol production by bio-chemical processes [5]. Thus, ethanol has served an environmentally friendly resource for hydrogen production. Reforming technologies for hydrogen production from ethanol can be classified into steam reforming, auto-thermal
* Corresponding author. Tel.: þ82 2 880 9227; fax: þ82 2 889 7415. E-mail address: [email protected] (I.K. Song). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.057
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
reforming, and partial oxidation [6]. Among these technologies, ethanol steam reforming has been extensively investigated due to its high hydrogen yield compared to the other reforming technologies [7]. The target reaction of ethanol steam reforming is represented in Eq. (1). However, it is known that ethanol steam reforming reaction is accompanied by several reactions, including ethanol dehydration (Eq. (2)), ethanol decomposition (Eq. (3)) followed by methane steam reforming (Eq. (4)), and ethanol dehydrogenation (Eq. (5)). Among these reactions, ethanol dehydration reaction is mainly responsible for low hydrogen yield due to the formation of polymeric carbon species [8]. On the other hand, water-gas shift reaction (Eq. (6)) is an advantageous reaction in the steam reforming of ethanol because it enhances hydrogen yield. o
C2H5OH þ 3H2O / 2CO2 þ 6H2 (DH
298 K
¼ 174 kJ/mol)
(1)
C2H5OH 4 C2H4 þ H2O (DHo298 K ¼ 45 kJ/mol)
(2)
C2H5OH 4 CH4 þ CO þ H2 (DHo298
(3)
K
¼ 49 kJ/mol)
CH4 þ H2O 4 CO þ 3H2 (DHo298 K ¼ 205 kJ/mol)
(4)
C2H5OH 4 C2H4O þ H2 (DHo298 K ¼ 68 kJ/mol)
(5)
CO þ H2O 4 CO2 þ H2 (DHo298
(6)
K
¼ 41.2 kJ/mol)
Therefore, developing an efficient ethanol steam reforming catalyst which can not only promote hydrogen production but also suppress undesired ethanol dehydration reaction is of great importance. Conventionally, nickel catalysts supported on alumina (Ni/Al2O3) have been widely used in the steam reforming of ethanol due to their high dehydrogenation activity and low cost [9,10]. However, it is known that Ni/Al2O3 catalysts suffer from significant deactivation caused by carbon deposition on their acid sites during the ethanol steam reforming reaction [11]. For this reason, many attempts have been made to increase both catalytic activity and durability of Ni/Al2O3 catalysts through modification of supporting materials [12]. Alkaline oxides such as MgO and CaO have been widely used as additives or promoters, because they can suppress the coking reaction by neutralizing acid sites of Al2O3 [12]. Addition of lanthanide oxides can also increase the catalytic activity by promoting gasification reaction of dissolved carbon species on the surface of nickel catalysts [13]. Among various metal oxides, ZrO2 is known to be the most effective additive for Ni/Al2O3 catalysts in the steam reforming of ethanol, because ZrO2 can not only enhance the stability of the catalysts but also promote adsorption and dissociation of water on the surface of nickel catalysts [14e16]. On the other hand, Ni/Al2O3 xerogel catalysts have also been investigated as efficient catalysts for hydrogen production by reforming reactions due to their high surface area and welldeveloped mesoporous structure [16,17]. Therefore, a systematic investigation on optimizing ZrO2 addition to NieAl2O3
1377
xerogel catalyst for hydrogen production by steam reforming of ethanol would be worthwhile. In this work, a series of mesoporous NieAl2O3eZrO2 xerogel catalysts (Ni-AZ-X ) with different Zr/Al molar ratio (X ) were prepared by a single-step epoxide-driven solegel method, and they were applied to the hydrogen production by steam reforming of ethanol. The effect of Zr/Al molar ratio on the physicochemical properties and catalytic activities of mesoporous Ni-AZ-X catalysts in the steam reforming of ethanol was investigated.
2.
Experimental
2.1. Preparation of mesoporous NieAl2O3eZrO2 xerogel catalysts A series of mesoporous NieAl2O3eZrO2 xerogel catalysts with different Zr/Al molar ratio were prepared by a single-step epoxide-driven solegel method, according to the similar methods reported in the literatures [18,19]. Known amounts of aluminum precursor (aluminum nitrate nonahydrate, SigmaeAldrich, 98%) and zirconium precursor (zirconium oxynitrate hydrate, SigmaeAldrich, 99%) were dissolved in ethanol (58 ml). The total amount of two precursors was 0.04 mol. An appropriate amount of nickel precursor (nickel nitrate hexahydrate, SigmaeAldrich, 97%) was then dissolved in the solution containing aluminum and zirconium precursors with vigorous stirring. Propylene oxide (29 ml) as a gelation agent was then added into the resulting solution. Upon maintaining the solution for a few minutes, deprotonation of hydrated metal salts and subsequent cross-linking of metal ion complex occurred for the formation of NieAl2O3eZrO2 composite gel. After the gel was aged for 2 days, it was washed with ethanol two times in order to minimize the destruction of pore structure of the catalyst by removing foreign substances in the gel with ethanol. The product was then dried at 80 C in a convection oven for 5 days. The resulting powder was finally calcined at 550 C for 5 h to obtain a mesoporous NieAl2O3eZrO2 xerogel catalyst. The prepared mesoporous NieAl2O3eZrO2 xerogel catalysts were denoted as Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4), where X represented Zr/Al molar ratio. Nickel loading of Ni-AZ-X catalysts was fixed at 10 wt%.
2.2.
Characterization
Nitrogen adsorptionedesorption isotherms of Ni-AZ-X catalysts were obtained with an ASAP-2010 (Micromeritics) instrument. Chemical compositions of Ni-AZ-X catalysts were determined by ICP-AES (ICPS-1000IV, Shimadzu) analyses. Crystalline structures of calcined and reduced catalysts were investigated by XRD (D-Max2500-PC, Rigaku) measurements using Cu-Ka radiation (l ¼ 1.541 A) operated at 50 kV and 100 mA. In order to examine the interaction between nickel species and support, temperature-programmed reduction (TPR) measurements were conducted in a conventional flow system with a moisture trap connected to a thermal conductivity detector (TCD) at temperatures ranging from room temperature to 1000 C with a ramping rate of 5 C/min. For the TPR measurements, a mixed stream of H2 (2 ml/min) and
1378
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
2.3.
Hydrogen production by steam reforming of ethanol
Hydrogen production by steam reforming of ethanol was carried in a continuous flow fixed-bed reactor under atmospheric pressure. Each calcined catalyst (100 mg) was reduced with a mixed stream of H2 (3 ml/min) and N2 (30 ml/min) at 550 C for 3 h. Ethanol and water were sufficiently vaporized by passing through a pre-heating zone, and they were continuously fed into the reactor together with N2 carrier (30 ml/min). Feed composition was fixed at C2H5OH:H2O:N2 ¼ 1:6:24.5. Total feed rate with respect to catalyst weight was maintained at 23,140 ml/h∙g. Catalytic reaction was carried out at 500 C. Reaction products were periodically sampled and analyzed using an on-line gas chromatograph (ACME 6000, Younglin) equipped with a thermal conductivity detector (TCD). Ethanol conversion, hydrogen yield, and carbon-containing product selectivity were calculated according to the following equations. Fi,in and Fi,out are molar flow rates of the specific species (i) at the inlet and outlet of the reactor, and ni is a stoichiometric factor of carbon-containing product. Ethanol conversion ð%Þ ¼
Hydrogen yield ð%Þ ¼
Si;carboncontaining
FEtOH;in FEtOH;out 100 FEtOH;in
FH2 ;out 100 3 FEtOH;in FEtOH;out
Ni-AZ-0.4
Volume adsorbed (A. U.)
N2 (20 ml/min) was used for 100 mg of catalyst sample. Dispersion of metal species on the reduced catalysts was examined by TEM-EDX analyses (Tecnai F20, FEI). Prior to the TEM analyses, each catalyst was preliminarily reduced with a mixed stream of H2 (3 ml/min) and N2 (30 ml/min) at 550 C for 3 h. NH3-TPD experiments were conducted to determine the acid property of reduced catalysts (BELCAT-B, BEL Japan). 0.07 g of each catalyst charged into the TPD apparatus was reduced at 550 C for 3 h with a mixed stream of H2 (2.5 ml/ min) and Ar (47.5 ml/min). After cooling the catalyst to room temperature, a mixed stream of NH3 (2.5 ml/min) and He (47.5 ml/min) was introduced into the reactor at 50 C for 50 min to saturate acid sites of the catalyst. Physisorbed ammonia was removed at 100 C for 1 h under a flow of He (50 ml/min). After cooling the sample, furnace temperature was increased from 50 C to 900 C at a heating rate of 5 C/min under a flow of He (30 ml/min). The desorbed ammonia was detected using a TCD (thermal conductivity detector). The amount of carbon deposition in the used catalysts was determined by CHNS elemental analyses (CHNS 932, Leco).
Ni-AZ-0.3
Ni-AZ-0.2
Ni-AZ-0.1
Ni-AZ-0
0
3.1.
Physicochemical properties of Ni-AZ-X catalysts
0.6
0.8
1.0
Fig. 1 e Nitrogen adsorptionedesorption isotherms of NiAZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C.
isotherm measurements as represented in Fig. 1. All the NiAZ-X catalysts exhibited type-IV isotherms with H2-type hysteresis loops, indicating the presence of well-developed mesopores [20]. Detailed physicochemical properties of NiAZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts are summarized in Table 1. The observed Zr/Al molar ratios of Ni-AZ-X catalysts were quite similar to the designed values. Surface area of NiAZ-X catalysts decreased with increasing Zr/Al molar ratio. This might be due to the increased textural density of Ni-AZ-X catalysts caused by the addition of ZrO2 into NieAl2O3 composite [21]. The lowest surface area of Ni-AZ-0.4 catalyst could be explained by higher surface energy of Zr4þ than Al3þ; the coalescence of structure occurred for the minimization of surface energy, resulting in the decrease of surface area [22,23]. However, pore volume and average pore diameter of Ni-AZ-X catalysts showed no consistent trend with respect to Zr/Al molar ratio.
(7)
(8)
(9)
Results and discussion
0.4
Relative pressure (P/P0)
Table 1 e Detailed physicochemical properties of Ni-AZ-X catalysts calcined at 550 C for 5 h. Catalyst
Zr/Al molar ratioa
Surface area (m2/g)b
Pore volume (cm3/g)c
Average pore diameter (nm)d
Ni-AZ-0 Ni-AZ-0.1 Ni-AZ-0.2 Ni-AZ-0.3 Ni-AZ-0.4
0 0.07 0.15 0.25 0.39
339 336 320 292 214
0.66 0.62 0.63 0.49 0.28
5.5 5.3 5.7 5.0 3.9
ni Fi;carbon-containing product 100 product ð%Þ ¼ 2 FEtOH;in FEtOH;out
3.
0.2
Textural properties of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts were examined by nitrogen adsorptionedesorption
a b c d
Determined by ICP-AES measurement. Calculated by the BET equation. BJH desorption pore volume. BJH desorption average pore diameter.
1379
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
XRD and TPR results of Ni-AZ-X catalysts
486
Fig. 2 shows the XRD patterns of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C. In the Ni-AZ-0 catalyst, three diffraction peaks indicative of NiAl2O4 phase (solid lines in Fig. 2) were observed, as reported in the literatures [16,24]. However, the peak intensity of NiAl2O4 gradually decreased with increasing Zr/Al molar ratio. Instead, the diffraction peak of tetragonal ZrO2 (1 1 1) (dashed line in Fig. 2) became strong with increasing Zr/Al molar ratio. It is noticeable that XRD peak of tetragonal ZrO2 (1 1 1) in the Ni-AZ-X (X ¼ 0.1, 0.2, 0.3, and 0.4) catalysts shifted to the higher diffraction angle than its original angle (2q ¼ 30.2 ), representing the lattice contraction of ZrO2 caused by the incorporation of Al3þ ions A) is into ZrO2. This is because the radius of Zr4þ ion (¼0.84 A) [25]. Thus, it can be larger than that of Al3þ ion (¼0.54 inferred that NiOeAl2O3eZrO2 composite structure was developed in the Ni-AZ-X catalysts by the simultaneous solidestate reaction among NiO, Al2O3, and ZrO2 [26]. Interestingly, diffraction peaks corresponding to NiO (closed circles in Fig. 2) suddenly appeared in the Ni-AZ-0.4 catalyst. This is because some Ni2þ ions hardly interacted with Al2O3 when an excess amount of ZrO2 was added into the Ni-AZ-X catalysts. In order to elucidate the metalesupport interaction in the Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts, TPR measurements were carried out as shown in Fig. 3. All the catalysts exhibited a reduction band at relatively high temperature within the range of 568e638 C. On the other hand, Ni-AZ-X (X ¼ 0.2, 0.3, and 0.4) catalysts showed an additional reduction band at relatively low temperature ranging from 454 C to 486 C. It has been reported that the band appeared at high temperature is related to the reduction of nickel oxide species strongly interacted with Al2O3 [27], while the band appeared at
ZrO2 (1 1 1)
NiAl2O4
Ni-AZ-0.4
Hydrogen consumption (A. U.)
3.2.
568
572 464
Ni-AZ-0.3 585 454
Ni-AZ-0.2 590
Ni-AZ-0.1
638
Ni-AZ-0
0
200
400
600
Temperature
800
1000
(oC)
Fig. 3 e TPR profiles of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C.
low temperature is associated with the reduction of nickel oxide species weakly interacted with ZrO2 [28,29]. It is noticeable that reduction peak temperature of nickel oxide species interacting with Al2O3 decreased with increasing Zr/Al molar ratio, while that of nickel oxide species interacting with ZrO2 increased with increasing Zr/Al molar ratio. In other words, reducibility of nickel oxide species in the Ni-AZ-X catalysts increased with increasing Zr/Al molar ratio. From these results, it can be inferred that the addition of ZrO2 suppressed the interaction between nickel oxide and support (Al2O3 or Al2O3eZrO2) through the formation of NiOeAl2O3eZrO2 composite structure.
NiO Ni (1 1 1) Ni (2 0 0)
Ni (2 2 0)
Ni-AZ-0.4 Ni-AZ-0.4
Ni-AZ-0.2
Ni-AZ-0.1
Intensity (A. U.)
Intensity (A. U.)
Ni-AZ-0.3 Ni-AZ-0.3
Ni-AZ-0.2
Ni-AZ-0.1
Ni-AZ-0 Ni-AZ-0
20
30
40 50 60 2 Theta (degree)
70
80
Fig. 2 e XRD patterns of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C.
20
30
40
50
60
70
80
2 Theta (degree) Fig. 4 e XRD patterns of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C.
1380
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
Fig. 5 e TEM images of (a) Ni-AZ-0, (b) Ni-AZ-0.2, and (c) Ni-AZ-0.4 catalysts reduced at 550 C, and EDX maps with distribution of (I) Ni, (II) Al, and (III) Zr.
Fig. 4 shows the XRD patterns of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C. All the reduced catalysts showed diffraction peaks corresponding to metallic Ni (solid lines in Fig. 4). Furthermore, diffraction peaks indicative of NiO and NiAl2O4 were not observed, indicating that nickel species in all the Ni-AZ-X catalysts were completely reduced into metallic nickel during the reduction process employed in this work. TEM images of Ni-AZ-0, Ni-AZ-0.2, and Ni-AZ-0.4 reduced at 550 C, and EDX maps with distribution of Ni, Al, and Zr are represented in Fig. 5. It was observed that density of Al decreased while that of Zr increased with increasing Zr/Al molar ratio, in accordance with ICP-AES analyses (Table 1). It is noteworthy that Al and Zr were homogeneously distributed throughout the catalysts without any significant aggregation regardless of the different Zr/Al molar ratio.
3.4.
This can be explained by the fact that acidity of the catalysts is mainly related to Al2O3 surface, because large amount of acid sites exists in Al2O3 rather than ZrO2 [24,30e34]. This result can also be deduced by the fact that the addition of ZrO2
Ni-AZ-0.4
NH3 desorbed (A. U.)
3.3. Crystalline structure and metal dispersion of reduced Ni-AZ-X catalysts
Ni-AZ-0.3
Ni-AZ-0.2
Ni-AZ-0.1
Ni-AZ-0
Acidity of reduced Ni-AZ-X catalysts
Fig. 6 shows the NH3-TPD profiles of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C. Acidity of reduced NiAZ-X catalysts was calculated from peak area as summarized in Table 2. It was observed that acidity of the catalysts monotonically decreased with increasing Zr/Al molar ratio.
100
200 300 400 500 600 700 800 900 Temperature (oC)
Fig. 6 e NH3-TPD profiles of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C.
1381
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
200
Table 2 e Acidity of reduced Ni-AZ-X catalysts. Acidity (mmol-NH3/g)a
Ni-AZ-0 Ni-AZ-0.1 Ni-AZ-0.2 Ni-AZ-0.3 Ni-AZ-0.4
131 124 101 86 79
a Calculated from peak area of NH3-TPD profiles in Fig. 6.
decreased surface area of the catalysts (Table 1), resulting in the decrease of acid sites of the catalysts [35].
160 Hydrogen yield (%)
Catalyst
120
3.5. Hydrogen production by steam reforming of ethanol over Ni-AZ-X catalysts
Ni-AZ-0.4
80
Ni-AZ-0.3 Ni-AZ-0.2 Ni-AZ-0.1
40
Ni-AZ-0
0 0
Fig. 7 shows the hydrogen yields with time on stream over NiAZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol at 500 C. Both Ni-AZ-0 and Ni-AZ-0.1 catalysts experienced a catalytic deactivation resulting from the significant carbon deposition on the catalyst surface (Table 3). However, Ni-AZ-X (X ¼ 0.2, 0.3, and 0.4) catalysts showed a stable performance with time on stream. It is believed that the addition of ZrO2 improved the catalytic stability by inhibiting coke formation [12]. Detailed catalytic performance of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol at 500 C after a 900 min-reaction is summarized in Table 3. Complete conversion of ethanol was achieved in all the catalysts under the conditions of high reaction temperature and excess amount of steam. This is in good agreement with the previous reports [6,36]. Selectivity for C2H4 over Ni-AZ-X catalysts decreased in the order of Ni-AZ-0 (9.8%) > Ni-AZ-0.1 (1.6%) > Ni-AZ-0.2 (0%) ¼ Ni-AZ-0.3 (0%) ¼ Ni-AZ-0.4 (0%). This can be explained by the fact that large amount of acid sites in the supports promoted the dehydration of ethanol to ethylene via one-step elimination mechanism [37]. The amount of carbon deposition decreased in the order of Ni-AZ0 (28.8%) > Ni-AZ-0.1 (21.5%) > Ni-AZ-0.2 (3.5%) > Ni-AZ-0.3 (2.6%) > Ni-AZ-0.4 (1.3%) (Table 3). This might be because ZrO2 in the Ni-AZ-X catalysts hindered both ethanol dehydration reaction (Eq. (2)) and ethylene formation reaction related to coking [38]. It was also observed that CO/CO2 molar ratio increased with increasing Zr/Al molar ratio (with decreasing acidity) in the order of Ni-AZ-0 (0.045) < Ni-AZ-0.1 (0.049) z NiAZ-0.2 (0.053) ¼ Ni-AZ-0.3 (0.053) < Ni-AZ-0.4 (0.240). This
200
400
600
800
1000
Time (min) Fig. 7 e Hydrogen yields with time on stream in the steam reforming of ethanol over Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts at 500 C. All the catalysts were reduced at 550 C for 3 h prior to the reaction.
result indicates that water-gas shift reaction (Eq. (6)) was suppressed with increasing Zr/Al molar ratio (with decreasing acidity). It has been reported that the increased surface acidity promotes the water-gas shift reaction by increasing the number of adsorbed CO species [39]. Therefore, it can be inferred that an excess amount of ZrO2 hindered water-gas shift reaction in the steam reforming of ethanol. Fig. 8 shows the hydrogen yields over Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol obtained after a 900 min-reaction, plotted as a function of Zr/ Al molar ratio and acidity of the catalyst. Hydrogen yields showed a volcano-shaped curve with respect to Zr/Al molar ratio and acidity of the catalyst. Hydrogen yield decreased in the order of Ni-AZ-0.2 > Ni-AZ-0.3 > Ni-AZ-0.1 > Ni-AZ0.4 > Ni-AZ-0. Among the catalysts tested, Ni-AZ-0.2 catalyst exhibited the best catalytic performance in terms of hydrogen yield. Although acidity of the catalyst was not the sole factor determining the catalytic performance in the steam reforming of ethanol, it greatly affected the reaction path and stability [6,12]. Relatively more acidic catalysts (Ni-AZ-0 and Ni-AZ-0.1) exhibited low hydrogen yield because undesired ethanol
Table 3 e Catalytic performance of Ni-AZ-X catalysts in the steam reforming of ethanol at 500 C after a 900 min-reaction. Catalyst
Ethanol conversion (%)
Hydrogen yield (%)
Selectivity for C2H4 (%)
CO/CO2 molar ratio
Amount of carbon deposition (wt%)a
Ni-AZ-0 Ni-AZ-0.1 Ni-AZ-0.2 Ni-AZ-0.3 Ni-AZ-0.4
100 100 100 100 100
104 123 137 130 119
9.8 1.6 0 0 0
0.045 0.049 0.053 0.053 0.240
28.8 21.5 3.5 2.6 1.3
a Determined by CHNS elemental analysis.
1382
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
140
a
140
Ni-AZ-0.2
b Ni-AZ-0.3
Ni-AZ-0.1 Ni-AZ-0.4
110
Hydrogen yield (%)
Hydrogen yield (%)
Ni-AZ-0.3
130 120
Ni-AZ-0.2
130 Ni-AZ-0.1
120 Ni-AZ-0.4
110
Ni-AZ-0
Ni-AZ-0
100
100
90
90 0
0.1
0.2
0.3
0.4
Zr/Al molar ratio
70
80
90
100
110
120
130
140
Acidity of catalyst ( mol-NH3/g)
Fig. 8 e Hydrogen yields over Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol obtained after a 900 min-reaction, plotted as a function of (a) Zr/Al molar ratio and (b) acidity of the catalyst.
dehydration reaction occurred on the acid-rich surface, resulting in a severe coke formation [37,38]. On the other hand, relatively less acidic catalysts (Ni-AZ-0.3 and Ni-AZ-0.4) showed low hydrogen yield due to low reactivity for water-gas shift reaction [39]. In the aspect of textural properties and reducibility, it was revealed that the addition of ZrO2 decreased surface area and increased reducibility of the catalysts. As these two factors affect the catalytic activity in an opposite way, it is believed that high surface area of Ni-AZ-0 and Ni-AZ-0.1 compensates low reducibility, and high reducibility of Ni-AZ-0.3 and Ni-AZ0.4 compensates low surface area [40]. Consequently, it is concluded that catalytic performance of Ni-AZ-X catalysts was well correlated with acidity regardless of the effect of textural properties and reducibility. Among the catalysts tested, Ni-AZ-0.2 catalyst with an intermediate acidity showed the maximum hydrogen yield.
4.
Conclusions
A series of mesoporous NieAl2O3eZrO2 (Ni-AZ-X ) xerogel catalysts with different Zr/Al molar ratio (X) were prepared by a single-step epoxide-driven solegel method. The prepared catalysts were applied to the hydrogen production by steam reforming of ethanol. The effect of Zr/Al molar ratio on the catalytic performance was investigated. It was found that all the Ni-AZ-X catalysts exhibited a mesoporous structure. Surface area of the catalysts decreased with increasing Zr/Al molar ratio due to the incorporation of ZrO2. Reducibility of NiAZ-X catalysts increased with increasing Zr/Al molar ratio through the formation of NiOeAl2O3eZrO2 composite structure. Acidity of Ni-AZ-X catalysts monotonically decreased with increasing Zr/Al molar ratio. In the hydrogen production by steam reforming of ethanol, hydrogen yields over Ni-AZ-X catalysts showed a volcano-shaped curve with respect to Zr/ Al molar ratio and acidity of the catalysts. Among the catalysts tested, Ni-AZ-0.2 catalyst with an intermediate acidity exhibited the best catalytic performance. It is concluded that an optimal Zr/Al molar ratio of NieAl2O3eZrO2 xerogel catalyst was required for maximum production of hydrogen by steam reforming of ethanol.
Acknowledgments This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (20110031575).
references
[1] Haryanto A, Fernando S, Murali N, Adhikari S. Current status of hydrogen production techniques by steam reforming of ethanol: a review. Energy Fuels 2005;19:2098e106. [2] Llorca J, Piscina PR, Sales J, Homs N. Direct production of hydrogen from ethanolic aqueous solution over oxide catalysts. Chem Commun 2001:641e2. [3] Schrope M. Which way to energy utopia? Nature 2001;414: 682e4. [4] Youn MH, Seo JG, Lee H, Bang Y, Chung JS, Song IK. Hydrogen production by auto-thermal reforming of ethanol over nickel catalysts supported on metal oxides: effect of support acidity. Appl Catal B Environ 2010;98:57e64. [5] Balat M, Balat H. Recent trends in global production and utilization of bio-ethanol fuel. Appl Energy 2009;86:2273e82. [6] Ni M, Leung DYC, Leung MKH. A review on reforming bioethanol for hydrogen production. Int J Hydrogen Energy 2007; 32:3238e47. [7] Rabenstein G, Hacker V. Hydrogen for fuel cells from ethanol by steam-reforming, partial-oxidation and combined autothermal reforming: a thermodynamic analysis. J Power Sources 2008;185:1293e304. [8] Rass-Hansen J, Christensen CH, Sehested J, Helveg S, Rostrup-Nielsen JR, Dahl S. Renewable hydrogen: carbon formation on Ni and Ru catalysts during ethanol steamreforming. Green Chem 2007;9:1016e21. [9] Vizcaı´no AJ, Lindo M, Carrero A, Calles JA. Hydrogen production by steam reforming of ethanol using Ni catalysts based on ternary mixed oxides prepared by coprecipitation. Int J Hydrogen Energy 2012;37:1985e92. [10] Wang W, Wang Y. Steam reforming of ethanol to hydrogen over nickel metal catalysts. Int J Hydrogen Energy 2010;34: 1285e90. [11] Rossetti I, Biffi C, Bianchi CL, Nichele V, Signoretto M, Menegazzo F, et al. Ni/SiO2 and Ni/ZrO2 catalysts for the steam reforming of ethanol. Appl Catal B Environ 2012;117:384e96.
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
[12] Sa´nchez-Sa´nchez MC, Navarro RM, Fierro JLG. Ethanol steam 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. [13] Fatsikostas AN, Kondarides DI, Verykios XE. Steam reforming of biomass-derived ethanol for the production of hydrogen for fuel cell applications. Chem Commun 2001:851e2. [14] Hegarty MES, O’Connor AM, Ross JRH. Syngas production from natural gas using ZrO2-supported metals. Catal Today 1998;42:225e32. [15] 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. [16] Seo JG, Youn MH, Cho KM, Park S, Lee SH, Lee J, et al. Effect of Al2O3eZrO2 xerogel support on hydrogen production by steam reforming of LNG over Ni/Al2O3eZrO2 catalyst. Korean J Chem Eng 2008;25:41e5. [17] Tang S, Ji L, Lin J, Zeng HC, Tan KL, Li K. CO2 reforming of methane to synthesis gas over solegel-made Ni/geAl2O3 catalysts from organometallic precursors. J Catal 2000;194: 424e30. [18] Gao YP, Sisk CN, Hope-Weeks LJ. A sol-gel route to synthesize monolithic zinc oxide aerogels. Chem Mater 2007;19:6007e11. [19] Bang Y, Lee J, Han SJ, Seo JG, Youn MH, Song JH, et al. Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous nickel-alumina xerogel catalysts prepared by a single-step carbon-templating sol-gel method. Int J Hydrogen Energy 2012;37:11208e17. [20] Kim P, Kim Y, Kim C, Kim H, Park Y, Lee JH, et al. Synthesis and characterization of mesoporous alumina as a catalyst support for hydrodechlorination of 1,2-dichloropropane: effect of catalyst preparation method. Catal Lett 2003;89: 185e92. [21] Faro AC, Souza KR, Camorim VLDL, Cardoso MB. Zirconiaalumina mixing in alumina-supported zirconia prepared by impregnation with solutions of zirconium acetylacetonate. Phys Chem Chem Phys 2003;5:1932e40. [22] Dominguez JM, Hernandez JL, Sandoval G. Surface and catalytic properties of Al2O3eZrO2 solid solutions prepared by sol-gel methods. Appl Catal A Gen 2000;197:119e30. [23] Hawa T, Zachariah MR. Coalescence kinetics of unequal sized nanoparticles. J Aerosol Sci 2006;37:1e15. [24] Bang Y, Seo JG, Youn MH, Song IK. Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous NieAl2O3 aerogel catalyst prepared by a singlestep epoxide-driven sol-gel method. Int J Hydrogen Energy 2012;37:1436e43. [25] Youn MH, Seo JG, Jung JC, Park S, Song IK. Hydrogen production by auto-thermal reforming of ethanol over nickel catalyst supported on mesoporous yttria-stabilized zirconia. Int J Hydrogen Energy 2009;34:5390e7.
1383
[26] Breval E, Deng Z, Chiou S, Pantano CG. Sol-gel prepared Nialumina composite materials. J Mater Sci 1992;27:1464e8. [27] Kim P, Joo JB, Kim H, Kim W, Kim Y, Song IK, et al. Preparation of mesoporous Ni-alumina catalyst by one-step sol-gel method: control of textural properties and catalytic application to the hydrodechlorination of o-dichlorobenzene. Catal Lett 2005;104:184e9. [28] Seo JG, Youn MH, Park S, Lee J, Lee SH, Lee H, et al. Hydrogen production by steam reforming of LNG over Ni/Al2O3eZrO2 catalysts: effect of ZrO2 and preparation method of Al2O3eZrO2. Korean J Chem Eng 2008;25:95e8. [29] Youn MH, Seo JG, Cho KM, Jung JC, Kim H, La KW, et al. Effect of support on hydrogen production by auto-thermal reforming of ethanol over supported nickel catalysts. Korean J Chem Eng 2008;25:236e8. [30] Pines H, Haag WO. Alumina: catalyst and support. I. Alumina, its intrinsic acidity and catalystic activity. J Am Chem Soc 1960;82:2471e83. [31] Krokidis X, Raybaud P, Gobichon AE, Rebours B, Euzen P, Toulhoat H. Theoretical study of the dehydration process of boehmite to g-Alumina. J Phys Chem B 2001;105:5121e30. [32] Kim Y, Kim C, Kim P, Yi J. Effect of preparation conditions on the phase transformation of mesoporous alumina. J Noncryst Solids 2005;351:550e6. [33] Onfroy T, Li W-C, Schu¨th F, Kno¨zinger H. Surface chemistry of carbon-templated mesoporous aluminas. Phys Chem Chem Phys 2009;11:3671e9. [34] Kwak JH, Hu JZ, Kim DH, Szanyi J, Peden CHF. Pentacoordinated Al3þ ions as preferential nucleation sites for BaO on g-Al2O3: an ultra-high-magnetic field 27Al MAS NMR study. J Catal 2007;251:189e94. [35] Cutrufello MG, Ferino I, Monaci R, Rombi E, Solinas V. Acidbase properties of zirconium, cerium and lanthanum oxides by calorimetric and catalytic investigation. Top Catal 2002; 19:225e40. [36] Wang W, Wang YQ. Thermodynamic analysis of steam reforming of ethanol for hydrogen generation. Int J Energy Res 2008;32:1432e43. [37] Mattos LV, Jacobs G, Davis BH, Noronha FB. Production of hydrogen from ethanol: review of reaction mechanism and catalyst deactivation. Chem Rev 2012;112:4094e123. [38] Choong CKS, Huang L, Zhong Z, Lin J, Hong L, Chen L. Effect of calcium addition on catalytic ethanol steam reforming of Ni/Al2O3: II. Acidity/basicity, water adsorption and catalytic activity. Appl Catal A Gen 2011;407:155e62. P, Levec J, Pintar A. Effect of structural and acidity/ [39] Djinovic basicity changes of CuOeCeO2 catalysts on their activity for water-gas shift reaction. Catal Today 2008;138:222e7. [40] Rangel MC, Berrocal GP, Maciel CG, Assaf JM. Comparison between zirconia and alumina supports for nickel-based catalysts for hydrogen production. Adv Chem Res 2011;10: 341e59.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogen production by steam reforming of ethanol over mesoporous NieAl2O3eZrO2 xerogel catalysts: Effect of Zr/Al molar ratio Seung Ju Han a, Yongju Bang a, Jeong Gil Seo b, Jaekyeong Yoo a, In Kyu Song a,* a
School of Chemical and Biological Engineering, Institute of Chemical Processes, Seoul National University, Shinlim-dong, Kwanak-ku, Seoul 151-744, South Korea b Department of Environmental Engineering and Energy, Myongji University, Nam-dong, Cheoin-gu, Yongin-si, Gyeonggi-do 449-728, South Korea
article info
abstract
Article history:
A series of mesoporous NieAl2O3eZrO2 xerogel catalysts (denoted as Ni-AZ-X ) with
Received 18 October 2012
different Zr/Al molar ratio (X) were prepared by a single-step epoxide-driven solegel
Received in revised form
method, and they were applied to the hydrogen production by steam reforming of ethanol.
7 November 2012
The effect of Zr/Al molar ratio of Ni-AZ-X catalysts on their physicochemical properties
Accepted 10 November 2012
and catalytic activities was investigated. Textural and chemical properties of Ni-AZ-X
Available online 6 December 2012
catalysts were strongly influenced by Zr/Al molar ratio. Surface area of Ni-AZ-X catalysts decreased with increasing Zr/Al molar ratio due to the lattice contraction of ZrO2 caused by
Keywords:
the incorporation of Al3þ into ZrO2. Interaction between nickel oxide species and support
Hydrogen production
(Al2O3eZrO2) decreased with increasing Zr/Al molar ratio through the formation of NiO
Steam reforming of ethanol
eAl2O3eZrO2 composite structure. Acidity of reduced Ni-AZ-X catalysts decreased with
Mesoporous NieAl2O3eZrO2 xerogel
increasing Zr/Al molar ratio due to the loss of acid sites of Al2O3 by the addition of ZrO2.
catalyst
Acidity of Ni-AZ-X catalysts served as a crucial factor determining the catalytic perfor-
Zr/Al molar ratio
mance in the steam reforming of ethanol; an optimal acidity was required for maximum
Acidity
production of hydrogen. Among the catalysts tested, Ni-AZ-0.2 (Zr/Al ¼ 0.2) catalyst with an intermediate acidity exhibited the best catalytic performance in the steam reforming of ethanol. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
With increasing demand for new and clean energy, hydrogen has attracted much attention as a promising energy carrier, because it possesses high energy density (ca. 120.7 kJ/g) and burns without emitting any environmental pollutants [1e4]. There are various resources for hydrogen production such as coal, natural gas, propane, methanol, and ethanol. Among these
resources, ethanol has been considered as a practical resource, because it is biodegradable, low in toxicity, and free from catalyst poisons such as sulfur [4]. Moreover, ethanol becomes more abundantly available due to the increasing ethanol production by bio-chemical processes [5]. Thus, ethanol has served an environmentally friendly resource for hydrogen production. Reforming technologies for hydrogen production from ethanol can be classified into steam reforming, auto-thermal
* Corresponding author. Tel.: þ82 2 880 9227; fax: þ82 2 889 7415. E-mail address: [email protected] (I.K. Song). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijhydene.2012.11.057
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
reforming, and partial oxidation [6]. Among these technologies, ethanol steam reforming has been extensively investigated due to its high hydrogen yield compared to the other reforming technologies [7]. The target reaction of ethanol steam reforming is represented in Eq. (1). However, it is known that ethanol steam reforming reaction is accompanied by several reactions, including ethanol dehydration (Eq. (2)), ethanol decomposition (Eq. (3)) followed by methane steam reforming (Eq. (4)), and ethanol dehydrogenation (Eq. (5)). Among these reactions, ethanol dehydration reaction is mainly responsible for low hydrogen yield due to the formation of polymeric carbon species [8]. On the other hand, water-gas shift reaction (Eq. (6)) is an advantageous reaction in the steam reforming of ethanol because it enhances hydrogen yield. o
C2H5OH þ 3H2O / 2CO2 þ 6H2 (DH
298 K
¼ 174 kJ/mol)
(1)
C2H5OH 4 C2H4 þ H2O (DHo298 K ¼ 45 kJ/mol)
(2)
C2H5OH 4 CH4 þ CO þ H2 (DHo298
(3)
K
¼ 49 kJ/mol)
CH4 þ H2O 4 CO þ 3H2 (DHo298 K ¼ 205 kJ/mol)
(4)
C2H5OH 4 C2H4O þ H2 (DHo298 K ¼ 68 kJ/mol)
(5)
CO þ H2O 4 CO2 þ H2 (DHo298
(6)
K
¼ 41.2 kJ/mol)
Therefore, developing an efficient ethanol steam reforming catalyst which can not only promote hydrogen production but also suppress undesired ethanol dehydration reaction is of great importance. Conventionally, nickel catalysts supported on alumina (Ni/Al2O3) have been widely used in the steam reforming of ethanol due to their high dehydrogenation activity and low cost [9,10]. However, it is known that Ni/Al2O3 catalysts suffer from significant deactivation caused by carbon deposition on their acid sites during the ethanol steam reforming reaction [11]. For this reason, many attempts have been made to increase both catalytic activity and durability of Ni/Al2O3 catalysts through modification of supporting materials [12]. Alkaline oxides such as MgO and CaO have been widely used as additives or promoters, because they can suppress the coking reaction by neutralizing acid sites of Al2O3 [12]. Addition of lanthanide oxides can also increase the catalytic activity by promoting gasification reaction of dissolved carbon species on the surface of nickel catalysts [13]. Among various metal oxides, ZrO2 is known to be the most effective additive for Ni/Al2O3 catalysts in the steam reforming of ethanol, because ZrO2 can not only enhance the stability of the catalysts but also promote adsorption and dissociation of water on the surface of nickel catalysts [14e16]. On the other hand, Ni/Al2O3 xerogel catalysts have also been investigated as efficient catalysts for hydrogen production by reforming reactions due to their high surface area and welldeveloped mesoporous structure [16,17]. Therefore, a systematic investigation on optimizing ZrO2 addition to NieAl2O3
1377
xerogel catalyst for hydrogen production by steam reforming of ethanol would be worthwhile. In this work, a series of mesoporous NieAl2O3eZrO2 xerogel catalysts (Ni-AZ-X ) with different Zr/Al molar ratio (X ) were prepared by a single-step epoxide-driven solegel method, and they were applied to the hydrogen production by steam reforming of ethanol. The effect of Zr/Al molar ratio on the physicochemical properties and catalytic activities of mesoporous Ni-AZ-X catalysts in the steam reforming of ethanol was investigated.
2.
Experimental
2.1. Preparation of mesoporous NieAl2O3eZrO2 xerogel catalysts A series of mesoporous NieAl2O3eZrO2 xerogel catalysts with different Zr/Al molar ratio were prepared by a single-step epoxide-driven solegel method, according to the similar methods reported in the literatures [18,19]. Known amounts of aluminum precursor (aluminum nitrate nonahydrate, SigmaeAldrich, 98%) and zirconium precursor (zirconium oxynitrate hydrate, SigmaeAldrich, 99%) were dissolved in ethanol (58 ml). The total amount of two precursors was 0.04 mol. An appropriate amount of nickel precursor (nickel nitrate hexahydrate, SigmaeAldrich, 97%) was then dissolved in the solution containing aluminum and zirconium precursors with vigorous stirring. Propylene oxide (29 ml) as a gelation agent was then added into the resulting solution. Upon maintaining the solution for a few minutes, deprotonation of hydrated metal salts and subsequent cross-linking of metal ion complex occurred for the formation of NieAl2O3eZrO2 composite gel. After the gel was aged for 2 days, it was washed with ethanol two times in order to minimize the destruction of pore structure of the catalyst by removing foreign substances in the gel with ethanol. The product was then dried at 80 C in a convection oven for 5 days. The resulting powder was finally calcined at 550 C for 5 h to obtain a mesoporous NieAl2O3eZrO2 xerogel catalyst. The prepared mesoporous NieAl2O3eZrO2 xerogel catalysts were denoted as Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4), where X represented Zr/Al molar ratio. Nickel loading of Ni-AZ-X catalysts was fixed at 10 wt%.
2.2.
Characterization
Nitrogen adsorptionedesorption isotherms of Ni-AZ-X catalysts were obtained with an ASAP-2010 (Micromeritics) instrument. Chemical compositions of Ni-AZ-X catalysts were determined by ICP-AES (ICPS-1000IV, Shimadzu) analyses. Crystalline structures of calcined and reduced catalysts were investigated by XRD (D-Max2500-PC, Rigaku) measurements using Cu-Ka radiation (l ¼ 1.541 A) operated at 50 kV and 100 mA. In order to examine the interaction between nickel species and support, temperature-programmed reduction (TPR) measurements were conducted in a conventional flow system with a moisture trap connected to a thermal conductivity detector (TCD) at temperatures ranging from room temperature to 1000 C with a ramping rate of 5 C/min. For the TPR measurements, a mixed stream of H2 (2 ml/min) and
1378
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
2.3.
Hydrogen production by steam reforming of ethanol
Hydrogen production by steam reforming of ethanol was carried in a continuous flow fixed-bed reactor under atmospheric pressure. Each calcined catalyst (100 mg) was reduced with a mixed stream of H2 (3 ml/min) and N2 (30 ml/min) at 550 C for 3 h. Ethanol and water were sufficiently vaporized by passing through a pre-heating zone, and they were continuously fed into the reactor together with N2 carrier (30 ml/min). Feed composition was fixed at C2H5OH:H2O:N2 ¼ 1:6:24.5. Total feed rate with respect to catalyst weight was maintained at 23,140 ml/h∙g. Catalytic reaction was carried out at 500 C. Reaction products were periodically sampled and analyzed using an on-line gas chromatograph (ACME 6000, Younglin) equipped with a thermal conductivity detector (TCD). Ethanol conversion, hydrogen yield, and carbon-containing product selectivity were calculated according to the following equations. Fi,in and Fi,out are molar flow rates of the specific species (i) at the inlet and outlet of the reactor, and ni is a stoichiometric factor of carbon-containing product. Ethanol conversion ð%Þ ¼
Hydrogen yield ð%Þ ¼
Si;carboncontaining
FEtOH;in FEtOH;out 100 FEtOH;in
FH2 ;out 100 3 FEtOH;in FEtOH;out
Ni-AZ-0.4
Volume adsorbed (A. U.)
N2 (20 ml/min) was used for 100 mg of catalyst sample. Dispersion of metal species on the reduced catalysts was examined by TEM-EDX analyses (Tecnai F20, FEI). Prior to the TEM analyses, each catalyst was preliminarily reduced with a mixed stream of H2 (3 ml/min) and N2 (30 ml/min) at 550 C for 3 h. NH3-TPD experiments were conducted to determine the acid property of reduced catalysts (BELCAT-B, BEL Japan). 0.07 g of each catalyst charged into the TPD apparatus was reduced at 550 C for 3 h with a mixed stream of H2 (2.5 ml/ min) and Ar (47.5 ml/min). After cooling the catalyst to room temperature, a mixed stream of NH3 (2.5 ml/min) and He (47.5 ml/min) was introduced into the reactor at 50 C for 50 min to saturate acid sites of the catalyst. Physisorbed ammonia was removed at 100 C for 1 h under a flow of He (50 ml/min). After cooling the sample, furnace temperature was increased from 50 C to 900 C at a heating rate of 5 C/min under a flow of He (30 ml/min). The desorbed ammonia was detected using a TCD (thermal conductivity detector). The amount of carbon deposition in the used catalysts was determined by CHNS elemental analyses (CHNS 932, Leco).
Ni-AZ-0.3
Ni-AZ-0.2
Ni-AZ-0.1
Ni-AZ-0
0
3.1.
Physicochemical properties of Ni-AZ-X catalysts
0.6
0.8
1.0
Fig. 1 e Nitrogen adsorptionedesorption isotherms of NiAZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C.
isotherm measurements as represented in Fig. 1. All the NiAZ-X catalysts exhibited type-IV isotherms with H2-type hysteresis loops, indicating the presence of well-developed mesopores [20]. Detailed physicochemical properties of NiAZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts are summarized in Table 1. The observed Zr/Al molar ratios of Ni-AZ-X catalysts were quite similar to the designed values. Surface area of NiAZ-X catalysts decreased with increasing Zr/Al molar ratio. This might be due to the increased textural density of Ni-AZ-X catalysts caused by the addition of ZrO2 into NieAl2O3 composite [21]. The lowest surface area of Ni-AZ-0.4 catalyst could be explained by higher surface energy of Zr4þ than Al3þ; the coalescence of structure occurred for the minimization of surface energy, resulting in the decrease of surface area [22,23]. However, pore volume and average pore diameter of Ni-AZ-X catalysts showed no consistent trend with respect to Zr/Al molar ratio.
(7)
(8)
(9)
Results and discussion
0.4
Relative pressure (P/P0)
Table 1 e Detailed physicochemical properties of Ni-AZ-X catalysts calcined at 550 C for 5 h. Catalyst
Zr/Al molar ratioa
Surface area (m2/g)b
Pore volume (cm3/g)c
Average pore diameter (nm)d
Ni-AZ-0 Ni-AZ-0.1 Ni-AZ-0.2 Ni-AZ-0.3 Ni-AZ-0.4
0 0.07 0.15 0.25 0.39
339 336 320 292 214
0.66 0.62 0.63 0.49 0.28
5.5 5.3 5.7 5.0 3.9
ni Fi;carbon-containing product 100 product ð%Þ ¼ 2 FEtOH;in FEtOH;out
3.
0.2
Textural properties of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts were examined by nitrogen adsorptionedesorption
a b c d
Determined by ICP-AES measurement. Calculated by the BET equation. BJH desorption pore volume. BJH desorption average pore diameter.
1379
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
XRD and TPR results of Ni-AZ-X catalysts
486
Fig. 2 shows the XRD patterns of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C. In the Ni-AZ-0 catalyst, three diffraction peaks indicative of NiAl2O4 phase (solid lines in Fig. 2) were observed, as reported in the literatures [16,24]. However, the peak intensity of NiAl2O4 gradually decreased with increasing Zr/Al molar ratio. Instead, the diffraction peak of tetragonal ZrO2 (1 1 1) (dashed line in Fig. 2) became strong with increasing Zr/Al molar ratio. It is noticeable that XRD peak of tetragonal ZrO2 (1 1 1) in the Ni-AZ-X (X ¼ 0.1, 0.2, 0.3, and 0.4) catalysts shifted to the higher diffraction angle than its original angle (2q ¼ 30.2 ), representing the lattice contraction of ZrO2 caused by the incorporation of Al3þ ions A) is into ZrO2. This is because the radius of Zr4þ ion (¼0.84 A) [25]. Thus, it can be larger than that of Al3þ ion (¼0.54 inferred that NiOeAl2O3eZrO2 composite structure was developed in the Ni-AZ-X catalysts by the simultaneous solidestate reaction among NiO, Al2O3, and ZrO2 [26]. Interestingly, diffraction peaks corresponding to NiO (closed circles in Fig. 2) suddenly appeared in the Ni-AZ-0.4 catalyst. This is because some Ni2þ ions hardly interacted with Al2O3 when an excess amount of ZrO2 was added into the Ni-AZ-X catalysts. In order to elucidate the metalesupport interaction in the Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts, TPR measurements were carried out as shown in Fig. 3. All the catalysts exhibited a reduction band at relatively high temperature within the range of 568e638 C. On the other hand, Ni-AZ-X (X ¼ 0.2, 0.3, and 0.4) catalysts showed an additional reduction band at relatively low temperature ranging from 454 C to 486 C. It has been reported that the band appeared at high temperature is related to the reduction of nickel oxide species strongly interacted with Al2O3 [27], while the band appeared at
ZrO2 (1 1 1)
NiAl2O4
Ni-AZ-0.4
Hydrogen consumption (A. U.)
3.2.
568
572 464
Ni-AZ-0.3 585 454
Ni-AZ-0.2 590
Ni-AZ-0.1
638
Ni-AZ-0
0
200
400
600
Temperature
800
1000
(oC)
Fig. 3 e TPR profiles of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C.
low temperature is associated with the reduction of nickel oxide species weakly interacted with ZrO2 [28,29]. It is noticeable that reduction peak temperature of nickel oxide species interacting with Al2O3 decreased with increasing Zr/Al molar ratio, while that of nickel oxide species interacting with ZrO2 increased with increasing Zr/Al molar ratio. In other words, reducibility of nickel oxide species in the Ni-AZ-X catalysts increased with increasing Zr/Al molar ratio. From these results, it can be inferred that the addition of ZrO2 suppressed the interaction between nickel oxide and support (Al2O3 or Al2O3eZrO2) through the formation of NiOeAl2O3eZrO2 composite structure.
NiO Ni (1 1 1) Ni (2 0 0)
Ni (2 2 0)
Ni-AZ-0.4 Ni-AZ-0.4
Ni-AZ-0.2
Ni-AZ-0.1
Intensity (A. U.)
Intensity (A. U.)
Ni-AZ-0.3 Ni-AZ-0.3
Ni-AZ-0.2
Ni-AZ-0.1
Ni-AZ-0 Ni-AZ-0
20
30
40 50 60 2 Theta (degree)
70
80
Fig. 2 e XRD patterns of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts calcined at 550 C.
20
30
40
50
60
70
80
2 Theta (degree) Fig. 4 e XRD patterns of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C.
1380
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
Fig. 5 e TEM images of (a) Ni-AZ-0, (b) Ni-AZ-0.2, and (c) Ni-AZ-0.4 catalysts reduced at 550 C, and EDX maps with distribution of (I) Ni, (II) Al, and (III) Zr.
Fig. 4 shows the XRD patterns of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C. All the reduced catalysts showed diffraction peaks corresponding to metallic Ni (solid lines in Fig. 4). Furthermore, diffraction peaks indicative of NiO and NiAl2O4 were not observed, indicating that nickel species in all the Ni-AZ-X catalysts were completely reduced into metallic nickel during the reduction process employed in this work. TEM images of Ni-AZ-0, Ni-AZ-0.2, and Ni-AZ-0.4 reduced at 550 C, and EDX maps with distribution of Ni, Al, and Zr are represented in Fig. 5. It was observed that density of Al decreased while that of Zr increased with increasing Zr/Al molar ratio, in accordance with ICP-AES analyses (Table 1). It is noteworthy that Al and Zr were homogeneously distributed throughout the catalysts without any significant aggregation regardless of the different Zr/Al molar ratio.
3.4.
This can be explained by the fact that acidity of the catalysts is mainly related to Al2O3 surface, because large amount of acid sites exists in Al2O3 rather than ZrO2 [24,30e34]. This result can also be deduced by the fact that the addition of ZrO2
Ni-AZ-0.4
NH3 desorbed (A. U.)
3.3. Crystalline structure and metal dispersion of reduced Ni-AZ-X catalysts
Ni-AZ-0.3
Ni-AZ-0.2
Ni-AZ-0.1
Ni-AZ-0
Acidity of reduced Ni-AZ-X catalysts
Fig. 6 shows the NH3-TPD profiles of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C. Acidity of reduced NiAZ-X catalysts was calculated from peak area as summarized in Table 2. It was observed that acidity of the catalysts monotonically decreased with increasing Zr/Al molar ratio.
100
200 300 400 500 600 700 800 900 Temperature (oC)
Fig. 6 e NH3-TPD profiles of Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts reduced at 550 C.
1381
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
200
Table 2 e Acidity of reduced Ni-AZ-X catalysts. Acidity (mmol-NH3/g)a
Ni-AZ-0 Ni-AZ-0.1 Ni-AZ-0.2 Ni-AZ-0.3 Ni-AZ-0.4
131 124 101 86 79
a Calculated from peak area of NH3-TPD profiles in Fig. 6.
decreased surface area of the catalysts (Table 1), resulting in the decrease of acid sites of the catalysts [35].
160 Hydrogen yield (%)
Catalyst
120
3.5. Hydrogen production by steam reforming of ethanol over Ni-AZ-X catalysts
Ni-AZ-0.4
80
Ni-AZ-0.3 Ni-AZ-0.2 Ni-AZ-0.1
40
Ni-AZ-0
0 0
Fig. 7 shows the hydrogen yields with time on stream over NiAZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol at 500 C. Both Ni-AZ-0 and Ni-AZ-0.1 catalysts experienced a catalytic deactivation resulting from the significant carbon deposition on the catalyst surface (Table 3). However, Ni-AZ-X (X ¼ 0.2, 0.3, and 0.4) catalysts showed a stable performance with time on stream. It is believed that the addition of ZrO2 improved the catalytic stability by inhibiting coke formation [12]. Detailed catalytic performance of Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol at 500 C after a 900 min-reaction is summarized in Table 3. Complete conversion of ethanol was achieved in all the catalysts under the conditions of high reaction temperature and excess amount of steam. This is in good agreement with the previous reports [6,36]. Selectivity for C2H4 over Ni-AZ-X catalysts decreased in the order of Ni-AZ-0 (9.8%) > Ni-AZ-0.1 (1.6%) > Ni-AZ-0.2 (0%) ¼ Ni-AZ-0.3 (0%) ¼ Ni-AZ-0.4 (0%). This can be explained by the fact that large amount of acid sites in the supports promoted the dehydration of ethanol to ethylene via one-step elimination mechanism [37]. The amount of carbon deposition decreased in the order of Ni-AZ0 (28.8%) > Ni-AZ-0.1 (21.5%) > Ni-AZ-0.2 (3.5%) > Ni-AZ-0.3 (2.6%) > Ni-AZ-0.4 (1.3%) (Table 3). This might be because ZrO2 in the Ni-AZ-X catalysts hindered both ethanol dehydration reaction (Eq. (2)) and ethylene formation reaction related to coking [38]. It was also observed that CO/CO2 molar ratio increased with increasing Zr/Al molar ratio (with decreasing acidity) in the order of Ni-AZ-0 (0.045) < Ni-AZ-0.1 (0.049) z NiAZ-0.2 (0.053) ¼ Ni-AZ-0.3 (0.053) < Ni-AZ-0.4 (0.240). This
200
400
600
800
1000
Time (min) Fig. 7 e Hydrogen yields with time on stream in the steam reforming of ethanol over Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts at 500 C. All the catalysts were reduced at 550 C for 3 h prior to the reaction.
result indicates that water-gas shift reaction (Eq. (6)) was suppressed with increasing Zr/Al molar ratio (with decreasing acidity). It has been reported that the increased surface acidity promotes the water-gas shift reaction by increasing the number of adsorbed CO species [39]. Therefore, it can be inferred that an excess amount of ZrO2 hindered water-gas shift reaction in the steam reforming of ethanol. Fig. 8 shows the hydrogen yields over Ni-AZ-X (X ¼ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol obtained after a 900 min-reaction, plotted as a function of Zr/ Al molar ratio and acidity of the catalyst. Hydrogen yields showed a volcano-shaped curve with respect to Zr/Al molar ratio and acidity of the catalyst. Hydrogen yield decreased in the order of Ni-AZ-0.2 > Ni-AZ-0.3 > Ni-AZ-0.1 > Ni-AZ0.4 > Ni-AZ-0. Among the catalysts tested, Ni-AZ-0.2 catalyst exhibited the best catalytic performance in terms of hydrogen yield. Although acidity of the catalyst was not the sole factor determining the catalytic performance in the steam reforming of ethanol, it greatly affected the reaction path and stability [6,12]. Relatively more acidic catalysts (Ni-AZ-0 and Ni-AZ-0.1) exhibited low hydrogen yield because undesired ethanol
Table 3 e Catalytic performance of Ni-AZ-X catalysts in the steam reforming of ethanol at 500 C after a 900 min-reaction. Catalyst
Ethanol conversion (%)
Hydrogen yield (%)
Selectivity for C2H4 (%)
CO/CO2 molar ratio
Amount of carbon deposition (wt%)a
Ni-AZ-0 Ni-AZ-0.1 Ni-AZ-0.2 Ni-AZ-0.3 Ni-AZ-0.4
100 100 100 100 100
104 123 137 130 119
9.8 1.6 0 0 0
0.045 0.049 0.053 0.053 0.240
28.8 21.5 3.5 2.6 1.3
a Determined by CHNS elemental analysis.
1382
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
140
a
140
Ni-AZ-0.2
b Ni-AZ-0.3
Ni-AZ-0.1 Ni-AZ-0.4
110
Hydrogen yield (%)
Hydrogen yield (%)
Ni-AZ-0.3
130 120
Ni-AZ-0.2
130 Ni-AZ-0.1
120 Ni-AZ-0.4
110
Ni-AZ-0
Ni-AZ-0
100
100
90
90 0
0.1
0.2
0.3
0.4
Zr/Al molar ratio
70
80
90
100
110
120
130
140
Acidity of catalyst ( mol-NH3/g)
Fig. 8 e Hydrogen yields over Ni-AZ-X (X [ 0, 0.1, 0.2, 0.3, and 0.4) catalysts in the steam reforming of ethanol obtained after a 900 min-reaction, plotted as a function of (a) Zr/Al molar ratio and (b) acidity of the catalyst.
dehydration reaction occurred on the acid-rich surface, resulting in a severe coke formation [37,38]. On the other hand, relatively less acidic catalysts (Ni-AZ-0.3 and Ni-AZ-0.4) showed low hydrogen yield due to low reactivity for water-gas shift reaction [39]. In the aspect of textural properties and reducibility, it was revealed that the addition of ZrO2 decreased surface area and increased reducibility of the catalysts. As these two factors affect the catalytic activity in an opposite way, it is believed that high surface area of Ni-AZ-0 and Ni-AZ-0.1 compensates low reducibility, and high reducibility of Ni-AZ-0.3 and Ni-AZ0.4 compensates low surface area [40]. Consequently, it is concluded that catalytic performance of Ni-AZ-X catalysts was well correlated with acidity regardless of the effect of textural properties and reducibility. Among the catalysts tested, Ni-AZ-0.2 catalyst with an intermediate acidity showed the maximum hydrogen yield.
4.
Conclusions
A series of mesoporous NieAl2O3eZrO2 (Ni-AZ-X ) xerogel catalysts with different Zr/Al molar ratio (X) were prepared by a single-step epoxide-driven solegel method. The prepared catalysts were applied to the hydrogen production by steam reforming of ethanol. The effect of Zr/Al molar ratio on the catalytic performance was investigated. It was found that all the Ni-AZ-X catalysts exhibited a mesoporous structure. Surface area of the catalysts decreased with increasing Zr/Al molar ratio due to the incorporation of ZrO2. Reducibility of NiAZ-X catalysts increased with increasing Zr/Al molar ratio through the formation of NiOeAl2O3eZrO2 composite structure. Acidity of Ni-AZ-X catalysts monotonically decreased with increasing Zr/Al molar ratio. In the hydrogen production by steam reforming of ethanol, hydrogen yields over Ni-AZ-X catalysts showed a volcano-shaped curve with respect to Zr/ Al molar ratio and acidity of the catalysts. Among the catalysts tested, Ni-AZ-0.2 catalyst with an intermediate acidity exhibited the best catalytic performance. It is concluded that an optimal Zr/Al molar ratio of NieAl2O3eZrO2 xerogel catalyst was required for maximum production of hydrogen by steam reforming of ethanol.
Acknowledgments This work was supported by the Global Frontier R&D Program on Center for Multiscale Energy System funded by the National Research Foundation under the Ministry of Education, Science and Technology, Korea (20110031575).
references
[1] Haryanto A, Fernando S, Murali N, Adhikari S. Current status of hydrogen production techniques by steam reforming of ethanol: a review. Energy Fuels 2005;19:2098e106. [2] Llorca J, Piscina PR, Sales J, Homs N. Direct production of hydrogen from ethanolic aqueous solution over oxide catalysts. Chem Commun 2001:641e2. [3] Schrope M. Which way to energy utopia? Nature 2001;414: 682e4. [4] Youn MH, Seo JG, Lee H, Bang Y, Chung JS, Song IK. Hydrogen production by auto-thermal reforming of ethanol over nickel catalysts supported on metal oxides: effect of support acidity. Appl Catal B Environ 2010;98:57e64. [5] Balat M, Balat H. Recent trends in global production and utilization of bio-ethanol fuel. Appl Energy 2009;86:2273e82. [6] Ni M, Leung DYC, Leung MKH. A review on reforming bioethanol for hydrogen production. Int J Hydrogen Energy 2007; 32:3238e47. [7] Rabenstein G, Hacker V. Hydrogen for fuel cells from ethanol by steam-reforming, partial-oxidation and combined autothermal reforming: a thermodynamic analysis. J Power Sources 2008;185:1293e304. [8] Rass-Hansen J, Christensen CH, Sehested J, Helveg S, Rostrup-Nielsen JR, Dahl S. Renewable hydrogen: carbon formation on Ni and Ru catalysts during ethanol steamreforming. Green Chem 2007;9:1016e21. [9] Vizcaı´no AJ, Lindo M, Carrero A, Calles JA. Hydrogen production by steam reforming of ethanol using Ni catalysts based on ternary mixed oxides prepared by coprecipitation. Int J Hydrogen Energy 2012;37:1985e92. [10] Wang W, Wang Y. Steam reforming of ethanol to hydrogen over nickel metal catalysts. Int J Hydrogen Energy 2010;34: 1285e90. [11] Rossetti I, Biffi C, Bianchi CL, Nichele V, Signoretto M, Menegazzo F, et al. Ni/SiO2 and Ni/ZrO2 catalysts for the steam reforming of ethanol. Appl Catal B Environ 2012;117:384e96.
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 8 ( 2 0 1 3 ) 1 3 7 6 e1 3 8 3
[12] Sa´nchez-Sa´nchez MC, Navarro RM, Fierro JLG. Ethanol steam 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. [13] Fatsikostas AN, Kondarides DI, Verykios XE. Steam reforming of biomass-derived ethanol for the production of hydrogen for fuel cell applications. Chem Commun 2001:851e2. [14] Hegarty MES, O’Connor AM, Ross JRH. Syngas production from natural gas using ZrO2-supported metals. Catal Today 1998;42:225e32. [15] 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. [16] Seo JG, Youn MH, Cho KM, Park S, Lee SH, Lee J, et al. Effect of Al2O3eZrO2 xerogel support on hydrogen production by steam reforming of LNG over Ni/Al2O3eZrO2 catalyst. Korean J Chem Eng 2008;25:41e5. [17] Tang S, Ji L, Lin J, Zeng HC, Tan KL, Li K. CO2 reforming of methane to synthesis gas over solegel-made Ni/geAl2O3 catalysts from organometallic precursors. J Catal 2000;194: 424e30. [18] Gao YP, Sisk CN, Hope-Weeks LJ. A sol-gel route to synthesize monolithic zinc oxide aerogels. Chem Mater 2007;19:6007e11. [19] Bang Y, Lee J, Han SJ, Seo JG, Youn MH, Song JH, et al. Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous nickel-alumina xerogel catalysts prepared by a single-step carbon-templating sol-gel method. Int J Hydrogen Energy 2012;37:11208e17. [20] Kim P, Kim Y, Kim C, Kim H, Park Y, Lee JH, et al. Synthesis and characterization of mesoporous alumina as a catalyst support for hydrodechlorination of 1,2-dichloropropane: effect of catalyst preparation method. Catal Lett 2003;89: 185e92. [21] Faro AC, Souza KR, Camorim VLDL, Cardoso MB. Zirconiaalumina mixing in alumina-supported zirconia prepared by impregnation with solutions of zirconium acetylacetonate. Phys Chem Chem Phys 2003;5:1932e40. [22] Dominguez JM, Hernandez JL, Sandoval G. Surface and catalytic properties of Al2O3eZrO2 solid solutions prepared by sol-gel methods. Appl Catal A Gen 2000;197:119e30. [23] Hawa T, Zachariah MR. Coalescence kinetics of unequal sized nanoparticles. J Aerosol Sci 2006;37:1e15. [24] Bang Y, Seo JG, Youn MH, Song IK. Hydrogen production by steam reforming of liquefied natural gas (LNG) over mesoporous NieAl2O3 aerogel catalyst prepared by a singlestep epoxide-driven sol-gel method. Int J Hydrogen Energy 2012;37:1436e43. [25] Youn MH, Seo JG, Jung JC, Park S, Song IK. Hydrogen production by auto-thermal reforming of ethanol over nickel catalyst supported on mesoporous yttria-stabilized zirconia. Int J Hydrogen Energy 2009;34:5390e7.
1383
[26] Breval E, Deng Z, Chiou S, Pantano CG. Sol-gel prepared Nialumina composite materials. J Mater Sci 1992;27:1464e8. [27] Kim P, Joo JB, Kim H, Kim W, Kim Y, Song IK, et al. Preparation of mesoporous Ni-alumina catalyst by one-step sol-gel method: control of textural properties and catalytic application to the hydrodechlorination of o-dichlorobenzene. Catal Lett 2005;104:184e9. [28] Seo JG, Youn MH, Park S, Lee J, Lee SH, Lee H, et al. Hydrogen production by steam reforming of LNG over Ni/Al2O3eZrO2 catalysts: effect of ZrO2 and preparation method of Al2O3eZrO2. Korean J Chem Eng 2008;25:95e8. [29] Youn MH, Seo JG, Cho KM, Jung JC, Kim H, La KW, et al. Effect of support on hydrogen production by auto-thermal reforming of ethanol over supported nickel catalysts. Korean J Chem Eng 2008;25:236e8. [30] Pines H, Haag WO. Alumina: catalyst and support. I. Alumina, its intrinsic acidity and catalystic activity. J Am Chem Soc 1960;82:2471e83. [31] Krokidis X, Raybaud P, Gobichon AE, Rebours B, Euzen P, Toulhoat H. Theoretical study of the dehydration process of boehmite to g-Alumina. J Phys Chem B 2001;105:5121e30. [32] Kim Y, Kim C, Kim P, Yi J. Effect of preparation conditions on the phase transformation of mesoporous alumina. J Noncryst Solids 2005;351:550e6. [33] Onfroy T, Li W-C, Schu¨th F, Kno¨zinger H. Surface chemistry of carbon-templated mesoporous aluminas. Phys Chem Chem Phys 2009;11:3671e9. [34] Kwak JH, Hu JZ, Kim DH, Szanyi J, Peden CHF. Pentacoordinated Al3þ ions as preferential nucleation sites for BaO on g-Al2O3: an ultra-high-magnetic field 27Al MAS NMR study. J Catal 2007;251:189e94. [35] Cutrufello MG, Ferino I, Monaci R, Rombi E, Solinas V. Acidbase properties of zirconium, cerium and lanthanum oxides by calorimetric and catalytic investigation. Top Catal 2002; 19:225e40. [36] Wang W, Wang YQ. Thermodynamic analysis of steam reforming of ethanol for hydrogen generation. Int J Energy Res 2008;32:1432e43. [37] Mattos LV, Jacobs G, Davis BH, Noronha FB. Production of hydrogen from ethanol: review of reaction mechanism and catalyst deactivation. Chem Rev 2012;112:4094e123. [38] Choong CKS, Huang L, Zhong Z, Lin J, Hong L, Chen L. Effect of calcium addition on catalytic ethanol steam reforming of Ni/Al2O3: II. Acidity/basicity, water adsorption and catalytic activity. Appl Catal A Gen 2011;407:155e62. P, Levec J, Pintar A. Effect of structural and acidity/ [39] Djinovic basicity changes of CuOeCeO2 catalysts on their activity for water-gas shift reaction. Catal Today 2008;138:222e7. [40] Rangel MC, Berrocal GP, Maciel CG, Assaf JM. Comparison between zirconia and alumina supports for nickel-based catalysts for hydrogen production. Adv Chem Res 2011;10: 341e59.