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Al2O3 catalysts for hydrogen production by steam reforming of ethanol
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 ) 7 5 1 6 e7 5 2 2
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Catalytic properties of Ag promoted ZnO/Al2O3 catalysts for hydrogen production by steam reforming of ethanol Meng-Nan Chen a,b, Dong-Yun Zhang a,c, Levi T. Thompson b, Zi-Feng Ma a,* a
Department of Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, China Hydrogen Energy Technology Laboratory, Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA c Department of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China b
article info
abstract
Article history:
Ag promoted ZnO/Al2O3 catalysts were prepared by using the incipient wetness impreg-
Received 26 January 2011
nation method. The catalytic properties of steam reforming reaction for hydrogen
Received in revised form
production on the prepared catalysts were evaluated with H2O:C2H5OH molar ratios of 3:1
10 March 2011
at 450 C and atmospheric pressure. Ag promoted ZnO/Al2O3 catalysts show higher SRE
Accepted 20 March 2011
catalytic activity than ZnO/Al2O3 catalysts. H2 and CH3CHO are the major products on Ag
Available online 22 April 2011
promoted catalysts, and C2H4 is also produced probably due to acid sites on Al2O3. SRE
Keywords:
from that on ZnO/Al2O3 catalysts. A method based on thermogravimetry (TG), differential
Hydrogen production
scanning calorimetry (DSC) and mass spectrometry (MS) was used to analysis the coking
Steam reforming of ethanol
behavior on catalyst surface. The surfaces of Ag promoted ZnO/Al2O3 catalysts show two
ZnO/Al2O3 catalyst
different types of coking, and suffer higher coke deposition during the steam reforming
Ag promotion
reaction.
Reaction mechanism
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
mechanism on Ag promoted ZnO/Al2O3 catalysts, which contains CeC scission, is different
reserved.
1.
Introduction
SRE has been recognized as an attractive and promising alternative route for hydrogen production because ethanol can be produced from the fermentation of biomass or renewable raw materials [1,2]. A variety of catalysts were developed for SRE including ZnO, Rh/CeO2, Pd/Al2O3 and nanocrystalline CuOeZnOeAl2O3 [3e8]. Among these, ZnO based catalysts have demonstrated attractive performance for hydrogen production by SRE. Llorca et al. [4] reported that C2H5OH conversion and H2 selectivity could reach 100% and 85% for SRE over ZnO catalyst at 450 C with the H2O:C2H5OH molar ratio of 13:1 and gas hourly space velocity (GHSV) of 5000 h1. SRE over 1% Pd/Al2O3 catalyst was reported by Basagiannis et al. [7], and the C2H5OH conversion and H2
selectivity reached 100% and 38%, respectively, at 400 C and H2O:C2H5OH molar ratio of 3:1. Nanocrystalline CuOeZnOeAl2O3 catalyst was prepared through sol-gel process, impregnation method (IMP) and a combination of both preparative procedures (ISG), respectively [8], and ethanol conversion of 100% and higher H2 selectivity ranging from 57.7 to 74.6% were obtained at 500 C with different contact times. Casanovas et al. [9] described the SRE over a 2.8% Pd/ZnO and a 2.8% Pd/SiO2 catalyst at a H2O:C2H5OH molar ratio of 13:1 and GHSV of 5200 h1. 70% and 40% of H2 selectivities were found on Pd/ZnO and Pd/SiO2 catalysts at 450 C. Based on the literature search, less report have been found for SRE over Ag based catalysts. Herein, we reported Ag promoted ZnO/Al2O3 catalysts for hydrogen production from SRE firstly. The cause of deactivation and coking on the prepared catalyst were
* Corresponding author. Tel.: þ86 21 54742894; fax: þ86 21 54747717. E-mail address: [email protected] (Z.-F. Ma). 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.2011.03.128
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 ) 7 5 1 6 e7 5 2 2
studied by using a thermal analysis based on thermogravimetry, differential scanning calorimetry and mass spectrometry. The different SRE mechanism among these prepared catalysts was also presented.
2.
Experimental
2.1.
Catalyst preparation
Ag promoted ZnO/Al2O3 (ZnOeAg/Al2O3) catalysts were prepared by using the incipient wetness impregnation method. First, g-Al2O3 (99%, Alfa, 3 mm, 80 m2/g) support was calcined at 700 C in air for 5 h to remove any absorbed species from the surface. A solution of Zn(NO3)2∙6H2O, (99%, Alfa) in de-ionized water was added to the calcined g-Al2O3 in a dropwise fashion to fill the pore volume. The concentration of the precursor solution was adjusted to the desired ZnO loading. Second, the resulting solids were thoroughly mixed using a vortex mixer, dried at 80 C overnight and then calcined at 500 C in air for 5 h to form ZnO/Al2O3 catalysts with different ZnO loadings of 2 wt% and 10 wt% (labeled as ZnAl(2) and ZnAl(10) respectively). Finally, the solution of AgNO3 in de-ionized water was added to the prepared ZnAl(2) and ZnAl(10) catalysts in a dropwise fashion to control the Ag loadings of 2 wt%, and the heat treatment process mentioned above was repeated to obtain ZnOeAg/Al2O3 catalysts (labeled as AgZnAl(2) and AgZnAl(10) respectively).
2.2.
Catalysts characterization
X-ray diffraction (XRD) patterns of the prepared catalysts were collected on a Rigaku Miniflex Diffractor using CuKa radiation at a scan rate of 7 /min. The BET surface areas and pore volumes of the catalysts and supports were determined by N2 physisorption isotherms which measured on Micromeritics ASAP 2020. All samples were degassed under vacuum at 350 C for 2 h prior to the measurements. Coke analysis for the spent catalysts was carried out on TG-DSC-MS system (Netzsch STA 449 F3 thermal gravimetric analyzer with process mass spectrometry from AMETEK). A quarts capillary tube was used as the interface between thermal gravimetric analyzer and process mass spectrometry. The TG-DSC system vacuumized after the sample was loaded in order to remove CO2 in the furnace. N2 of 5 ml/min was used as protective gas, and 20% O2/N2 mixture of 50 ml/min was used as purge gas. The TG-DSC and MS program were started simultaneously. The temperature region for TG-DSC was from room temperature to 700 C, and the MS program was set to detect m/z of 44 refer to CO2.
2.3.
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125e250 mm prior to use so that the L/Dp was 60 and D/Dp was 16. The reactor was heated to 450 C in N2 (80 ml/min) at temperature ramp rate of 10 C/min. For Ag promoted catalysts, mixture of 20 ml/min H2 and 80 ml/min N2 was fed into the reactor for 1 h to pre-treat the catalyst. After pretreatment, the H2OeC2H5OH mixture was pumped into a vaporizer at 0.02 ml/min by a liquid pump, and then mixed with 80 ml/min N2. A buffer tank was placed to ensure stable flow before the inlet of the reactor. The products were collected, separated and analyzed using two on-line SRI-8610C gas chromatographs (GC). One GC used Ar as the carrier gas, and was equipped with Carboxen 1000 and Hayesep D separation columns, and thermal conductivity (TCD) and flame ionization (FID) detectors. The Carboxen 1000 column was used to separate the permanent gases and the Hayesep D column was used to separate the hydrocarbons. The other GC used He as carrier gas, and was equipped with Carboxen 1000 column and TCD for CO and CO2 separation and detecting. Tubing between the reactor outlet and GC inlet was heated to 130 C to prevent condensation of any products. Ethanol conversion (Xethanol) and products selectivities (Si) were calculated according to following equations: Xethanol ¼
Si ¼
Fethanol in Xethanol out Fethanol in
Fi produced nðFethanol in Fethanol out Þ
(1)
(2)
Where n is 2, 1, 2/3 and 3 for CO, C2 species, acetone and H2, respectively.
3.
Results and discussion
3.1.
XRD patterns
Fig. 1 shows the XRD pattern of g-Al2O3 support and Ag promoted ZnO/Al2O3 catalysts. No peaks was attributed to ZnO on AgZnAl(2) catalyst, indicating these domains were
Catalytic activity evaluations
The catalytic activity of the prepared catalysts for SRE was evaluated using a fixed-bed quartz reactor with inner diameter of 4 mm at 450 C and atmospheric pressure. For all catalytic runs, H2O:C2H5OH molar ratio of 3 and GHSV of 30,000 h1was maintained by diluting catalysts with inert SiO2. To satisfy the plug flow condition (L/Dp > 50 and D/Dp > 10) [10], catalysts and inert SiO2 were sieved to
Fig. 1 e X-ray diffraction patterns for the Ag promoted ZnO/ Al2O3 catalysts and g-Al2O3.
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amorphous. ZnO characteristic peaks were clearly identified on AgZnAl(10) catalyst, and the crystallite size of ZnO was 12 nm Al2O3 and Ag2O peaks were also identified on Ag promoted ZnO/Al2O3 catalysts, and their crystallite sizes were 7 nm and 3 nm, respectively.
3.2. Catalytic activities and steam reforming reactions analysis Fig. 2 shows the results of C2H5OH conversion as a function of time on stream (TOS) for ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts at 450 C. It can be seen that C2H5OH conversion was relatively high for ZnAl(2) and AgZnAl(2) catalysts, and this could be attributed to its uncovered Lewis acid sites on Al2O3, which could accelerate the dehydration of C2H5OH (Equation (3)). CH3 CH2 OH/C2 H4 þ H2 O
(3)
It could also be observed from Fig. 2 that the C2H5OH conversion on Ag promoted ZnO/Al2O3 catalysts was higher than ZnO/Al2O3, indicating that Ag would promote the catalytic activity of ZnO/Al2O3 catalyst. Deactivation can be found on all catalysts for first 180 min. The deactivation on these catalysts probably could be related to sintering or coking on the surface of catalysts. The BET surface area and pore volume of g-Al2O3 support, fresh and spent catalysts were summarized in Table 1. The surface areas and pore volume for fresh ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts were slightly lower than that of g-Al2O3 support. This probably was due to the blockage of the pore structure of Al2O3 support by ZnO and Ag2O during impregnation. For the spent ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts, it could be observed that the surface areas and pore volume were slightly lower than fresh catalysts as well. This was probably attributed to the coke deposition on catalysts surface during steam reforming reaction. Figs. 3 and 4 show the selectivity of steam reforming reaction products as a function of TOS for ZnAl(2), ZnAl(10),
Fig. 2 e Ethanol conversions as a function of time on stream for ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3.
Table 1 e BET results of support, fresh and spent catalysts. Sample
g-Al2O3 Fresh ZnAl(2) Fresh ZnAl(10) Spent ZnAl(2) Spent ZnAl(10) Fresh AgZnAl(2) Fresh AgZnAl(10) Spent AgZnAl(2) Spent AgZnAl(10)
BET surface area
Pore volume
2
m /g
cm3/g
94 90 90 75 70 88 87 72 70
0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2
AgZnAl(2) and AgZnAl(10) catalysts at 450 C, respectively. For ZnAl(2) catalyst (Fig. 3a), C2H4 and CH3COOH were main products, and this probably was due to dehydration and CH3CHO oxidation (Equations (3) and (4)) on uncovered Al2O3. For ZnAl(10) catalyst (Fig. 3b) on the other hand, initial selectivity for CH3CHO reached 80%, indicating that C2H5OH dehydrogenation (Equation (5)) was the major reaction during
Fig. 3 e Products selectivity as a function of time on stream for ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3. (a) ZnAl(2) catalyst. (b) ZnAl(10) catalyst.
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 ) 7 5 1 6 e7 5 2 2
Fig. 4 e Products selectivity as a function of time on stream for Ag promoted ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3. (a) AgZnAl(2) catalyst. (b) AgZnAl(10) catalyst.
SRE on this catalyst. For both ZnAl(2) and ZnAl(10) catalysts, selectivity for H2 and CH3CHO decreased along with TOS, indicating that the basic reactive sites, which could enhance dehydrogenation, were deactivated during the reaction. Thus the C2H4 formation, which was due to acid sites, was increased on both catalysts. H2 and CH3CHO selectivity on ZnAl(2) catalyst could be found decreased by w10% at the first 3 h in Fig. 3a. However, that was less than 5% for ZnAl(10) catalyst in Fig. 3b. Furthermore, ZnAl(10) catalyst also showed good long-term stability. The H2 and CH3CHO selectivity only decreased by 5∼10% after 20 h reaction. These results indicated that increasing ZnO loading could decrease the loss of catalytic activity and improve the long-term stability for the catalyst. For the products on both ZnAl(2) and ZnAl(10) catalysts, no CO or CH4 was produced, suggesting that CH3CHO decomposition (Equation (6)) was not favored at 450 C on these catalysts. This result agreed with that reported on ZnO/SiO2 catalysts [5]. CH3COCH3 production was also detected on ZnAl(2) and ZnAl(10) catalysts. This was probably due to Equations (7) or (8).
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CH3 CHO þ H2 O/CH3 COOH þ H2
(4)
CH3 CH2 OH/CH3 CHO þ H2
(5)
CH3 CHO/CH4 þ CO
(6)
2CH3 CHO þ H2 O/CH3 COCH3 þ CO2 þ 2H2
(7)
2CH3 COOH/CH3 COCH3 þ CO2 þ H2 O
(8)
For AgZnAl(2) catalyst (Fig. 4a), it could be observed that compared with ZnAl(2) catalyst, H2 and CH3CHO selectivities were increased to 40% and 50%, respectively. C2H4 selectivity also was found much lower on AgZnAl(2) catalyst. For AgZnAl(10) catalysts (Fig. 4b), H2 selectivity was higher compared with ZnAl(10) catalyst as well, however CH3CHO selectivity was similar. Fig. 5 summarized the calculated H2 production rate for both Ag promoted and unpromoted catalysts. It could be clearly observed that H2 production rate was increased dramatically after the Ag promotion, suggesting that Ag could significantly improve H2 production for SRE on ZnO/Al2O3 catalysts. Furthermore, for Ag promoted ZnO/Al2O3 catalysts in Fig. 4, CO was detected in the products. The selectivity to CO was decreased from 25% to 10% while ZnO loading increased. These results indicated that the first step of SRE on Ag promoted ZnO/Al2O3 catalysts was similar as ZnO/Al2O3 catalysts, which was dehydrogenation of C2H5OH to CH3CHO, and Ag could promote this reaction at the same temperature. Dehydration of C2H5OH to C2H4 also occurred on all of the prepared catalysts. However, for the second step of SRE, the pathway through which CH3CHO was converted on Ag promoted ZnO/Al2O3 catalysts was different. It could be proposed that CH3CHO was further converted to CO and H2 in the present of water on Ag promoted ZnO/Al2O3 catalysts (by Equations (6), (9) and 10). Unlike ZnO/Al2O3 catalysts, this may probably due to CeC scission on Ag site. Liberatori et al. [11] reported that on Ag/Ni/Al2O3 catalyst, the same amount of CO and CH4 was produced at 350 C because of the decomposition of CH3CHO (Equation (6)), and then CH4 was converted to CO and CO2 by steam reforming or water gas shift (Equations (9) and 10) as temperature increased.
Fig. 5 e Hydrogen production rate for both Ag promoted and unpromoted ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3.
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CH4 þ H2 O/CO þ 3H2
(9)
CO þ H2 O/CO2 þ H2
(10)
Take all these results into account, the mechanism of steam reforming reaction over ZnO/Al2O3 catalysts could be proposed as follow: C2H5OH initially absorbed on its surface and evolved to CH3CHO, and then it could be oxidized to CH3COOH or converted to CH3OCH3. This mechanism was also confirmed by other researchers [12e17]. For Ag promoted ZnO/Al2O3 catalysts, Ag could improve dehydrogenation of C2H5OH, which was the reason for the significant increase of CH3CHO and H2 formation in Figs. 4a and 5. The Ag promoted catalysts also had less C2H4 production than unprompted catalysts, indicating that Ag had positive effect for suppressing C2H5OH dehydration. For CH3CHO conversion, Ag promoted ZnO/Al2O3 catalysts favored CeC scission, and could convert CH3CHO to CO and H2. Though the decomposition of CH3CHO could greatly enhance the H2 production, a CO-rich stream would be produced after the reaction. Therefore, the products need further conversion if it was applied to CO-sensitive devices such as PEM fuel cells. Lower CO selectivity was found on high ZnO loading Ag promoted catalyst, indicating that ZnO could probably suppress CO production for these catalysts.
3.3.
Coke deposition analysis
Coking behaviors of ZnAl(10) and AgZnAl(10) catalysts for SRE were studied. The TG-DSC curves for spent ZnAl(10) catalyst were shown in Fig. 6. The TG curve showed that the weight loss was started from 350 C and ended at 500 C with total weight loss of 3.5 wt%, and a big exothermic peak at 440 C was detected on the DSC curve. The MS spectra which collected simultaneously with TG-DSC analysis was shown in inset within Fig. 6, it could be found that the COþ 2 (m/z ¼ 44) signal increased at 300 C and maximized at 450 C. These results indicated that there was 3.5 wt% of coke deposited on the surface of ZnAl(10) catalyst. According to the results in Fig. 3, which we proposed that the deactivation was occurred on basic reactive sits, we suggest that the coke could possibly be located on ZnO sites. Furthermore, the deposited coke
Fig. 6 e TG-DSC curves for spent ZnAl(10) catalyst and its MS spectra (inset).
could be probably because of the polymerization of C2H4 [18e20], and it might be assigned to filamentous or graphitic carbon [21e23]. The TG-DSC results for spent AgZnAl(10) catalyst were shown in Fig. 7. The total weight loss of AgZnAl(10) catalyst on TG curve was 5.4 wt%, and that was higher that ZnAl(10) catalyst (3.5 wt%, Fig. 6). These results suggested that AgZnAl(10) catalyst suffered higher coke deposition than ZnAl(10) catalyst. This coking behavior is similar to the results reported by Liberatori [11], They found out that higher coke deposition was detected on Ag/Ni/Al2O3 catalyst than unpromoted Ni/Al2O3 catalyst for SRE at 600 C. For the DSC cure in Fig. 7, compared with ZnAl(10) catalyst, the similar exothermic peak at 440 C appeared for AgZnAl(10) catalyst. However a small exothermic peak at 240 C, which did not appear on ZnAl(10) catalyst, was detected. The MS spectra for spent AgZnAl(10) catalyst was shown in inset within Fig. 7. Corresponding to the DSC peaks at 240 C and 440 C, it could be observed that two COþ 2 (m/z ¼ 44) peaks were detected at 240 C and 440 C. This indicated that both exothermic peaks on DSC curve were caused by combustion of carbonaceous deposition on the surface of the catalyst. Comparing the exothermic peaks in Figs. 6 and 7, it could be proposed that there were two types of coke on AgZnAl(10) catalyst surface. According to Ordonez’s report [24], the differences in coking behavior should be mainly caused by different active site. Therefore, the peak at 440 C in both Figs. 6 and 7 could be attributed to the coke deposited on ZnO site, which was caused by polymerization of C2H4, while the peak at 240 C in Fig. 7 was coke deposited on Ag site. The coke deposition at lower temperature could be assigned to monoatomic carbon which was highly reactive and could be removed easily [23,25e27]. Based on the results and analysis of steam reforming reactions described in Section 3.2, CeC scission of CH3CHO could occur on Ag site, producing CH3 and CHO*, and CH3 could convert to coke consequently (Equations 11 and 12). It can be concluded that the low-temperature deposited coke on Ag site could probably be formed by decomposition of CHx species [20]. CH3 CHO/CH3 þ CHO
(11)
CH3 /CHx þ coke
(12)
Fig. 7 e TG-DSC curves for spent AgZnAl(10) catalyst and its MS spectra (inset).
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4.
Conclusion
Ag promoted ZnO/Al2O3 catalysts were synthesized successfully by incipient wetness impregnation method, and all of them were very active for SRE at 450 C. H2 and CH3CHO were the major products on these catalysts, indicating that the SRE reaction take place by initial dehydrogenation of C2H5OH to CH3CHO. For ZnO/Al2O3 catalysts, CH3CHO could be converted to CH3OCH3 and CH3COOH. For Ag promoted ZnO/Al2O3 catalysts, Ag could improve C2H5OH conversion and H2 selectivity dramatically, and would suppress C2H5OH dehydration as well. Unlike ZnO/Al2O3 catalysts, the CeC scission of CH3CHO was occurred on Ag promoted ZnO/Al2O3 catalysts. A significant amount of CO was produced on promoted catalysts, which was due to CH3CHO decomposition. The CO selectivity could be reduced by increasing ZnO loading. Deactivation was detected on both Ag promoted and unpromoted ZnO/Al2O3 catalysts, and it was due to site blockage by coking. Increasing ZnO loading could prevent initial catalytic loss and improve long-term stability for the catalyst. Ag promoted ZnO/Al2O3 catalysts suffered higher coke deposition than ZnO/Al2O3 catalysts. The coke deposited on ZnO/Al2O3 catalysts was probably caused by C2H4 polymerization; while the coke on Ag promoted ZnO/Al2O3 catalysts was formed from both C2H4 polymerization and CHx decomposition.
Acknowledgment This work was supported by the Hydrogen Energy Technology Laboratory at the University of Michigan, National Basic Research Program of China (2007CB209705), Science and Technology Commission of Shanghai Municipality (09XD1402400, 10520708900).
Nomenclature
SRE GHSV TOS m/z L Dp D Xethanol Si Fi
steam reforming of ethanol gas hourly space velocity, h1 time on stream, min mass to charge ratio height of catalyst bed, mm catalyst particle size, mm internal diameter of reactor, mm ethanol conversion selectivity for species i molar flow rate for species i, mol/s
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Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Catalytic properties of Ag promoted ZnO/Al2O3 catalysts for hydrogen production by steam reforming of ethanol Meng-Nan Chen a,b, Dong-Yun Zhang a,c, Levi T. Thompson b, Zi-Feng Ma a,* a
Department of Chemical Engineering, Shanghai Jiaotong University, Shanghai 200240, China Hydrogen Energy Technology Laboratory, Department of Chemical Engineering, University of Michigan, Ann Arbor, MI 48109, USA c Department of Material Science and Engineering, Shanghai Institute of Technology, Shanghai 201418, China b
article info
abstract
Article history:
Ag promoted ZnO/Al2O3 catalysts were prepared by using the incipient wetness impreg-
Received 26 January 2011
nation method. The catalytic properties of steam reforming reaction for hydrogen
Received in revised form
production on the prepared catalysts were evaluated with H2O:C2H5OH molar ratios of 3:1
10 March 2011
at 450 C and atmospheric pressure. Ag promoted ZnO/Al2O3 catalysts show higher SRE
Accepted 20 March 2011
catalytic activity than ZnO/Al2O3 catalysts. H2 and CH3CHO are the major products on Ag
Available online 22 April 2011
promoted catalysts, and C2H4 is also produced probably due to acid sites on Al2O3. SRE
Keywords:
from that on ZnO/Al2O3 catalysts. A method based on thermogravimetry (TG), differential
Hydrogen production
scanning calorimetry (DSC) and mass spectrometry (MS) was used to analysis the coking
Steam reforming of ethanol
behavior on catalyst surface. The surfaces of Ag promoted ZnO/Al2O3 catalysts show two
ZnO/Al2O3 catalyst
different types of coking, and suffer higher coke deposition during the steam reforming
Ag promotion
reaction.
Reaction mechanism
Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
mechanism on Ag promoted ZnO/Al2O3 catalysts, which contains CeC scission, is different
reserved.
1.
Introduction
SRE has been recognized as an attractive and promising alternative route for hydrogen production because ethanol can be produced from the fermentation of biomass or renewable raw materials [1,2]. A variety of catalysts were developed for SRE including ZnO, Rh/CeO2, Pd/Al2O3 and nanocrystalline CuOeZnOeAl2O3 [3e8]. Among these, ZnO based catalysts have demonstrated attractive performance for hydrogen production by SRE. Llorca et al. [4] reported that C2H5OH conversion and H2 selectivity could reach 100% and 85% for SRE over ZnO catalyst at 450 C with the H2O:C2H5OH molar ratio of 13:1 and gas hourly space velocity (GHSV) of 5000 h1. SRE over 1% Pd/Al2O3 catalyst was reported by Basagiannis et al. [7], and the C2H5OH conversion and H2
selectivity reached 100% and 38%, respectively, at 400 C and H2O:C2H5OH molar ratio of 3:1. Nanocrystalline CuOeZnOeAl2O3 catalyst was prepared through sol-gel process, impregnation method (IMP) and a combination of both preparative procedures (ISG), respectively [8], and ethanol conversion of 100% and higher H2 selectivity ranging from 57.7 to 74.6% were obtained at 500 C with different contact times. Casanovas et al. [9] described the SRE over a 2.8% Pd/ZnO and a 2.8% Pd/SiO2 catalyst at a H2O:C2H5OH molar ratio of 13:1 and GHSV of 5200 h1. 70% and 40% of H2 selectivities were found on Pd/ZnO and Pd/SiO2 catalysts at 450 C. Based on the literature search, less report have been found for SRE over Ag based catalysts. Herein, we reported Ag promoted ZnO/Al2O3 catalysts for hydrogen production from SRE firstly. The cause of deactivation and coking on the prepared catalyst were
* Corresponding author. Tel.: þ86 21 54742894; fax: þ86 21 54747717. E-mail address: [email protected] (Z.-F. Ma). 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.2011.03.128
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studied by using a thermal analysis based on thermogravimetry, differential scanning calorimetry and mass spectrometry. The different SRE mechanism among these prepared catalysts was also presented.
2.
Experimental
2.1.
Catalyst preparation
Ag promoted ZnO/Al2O3 (ZnOeAg/Al2O3) catalysts were prepared by using the incipient wetness impregnation method. First, g-Al2O3 (99%, Alfa, 3 mm, 80 m2/g) support was calcined at 700 C in air for 5 h to remove any absorbed species from the surface. A solution of Zn(NO3)2∙6H2O, (99%, Alfa) in de-ionized water was added to the calcined g-Al2O3 in a dropwise fashion to fill the pore volume. The concentration of the precursor solution was adjusted to the desired ZnO loading. Second, the resulting solids were thoroughly mixed using a vortex mixer, dried at 80 C overnight and then calcined at 500 C in air for 5 h to form ZnO/Al2O3 catalysts with different ZnO loadings of 2 wt% and 10 wt% (labeled as ZnAl(2) and ZnAl(10) respectively). Finally, the solution of AgNO3 in de-ionized water was added to the prepared ZnAl(2) and ZnAl(10) catalysts in a dropwise fashion to control the Ag loadings of 2 wt%, and the heat treatment process mentioned above was repeated to obtain ZnOeAg/Al2O3 catalysts (labeled as AgZnAl(2) and AgZnAl(10) respectively).
2.2.
Catalysts characterization
X-ray diffraction (XRD) patterns of the prepared catalysts were collected on a Rigaku Miniflex Diffractor using CuKa radiation at a scan rate of 7 /min. The BET surface areas and pore volumes of the catalysts and supports were determined by N2 physisorption isotherms which measured on Micromeritics ASAP 2020. All samples were degassed under vacuum at 350 C for 2 h prior to the measurements. Coke analysis for the spent catalysts was carried out on TG-DSC-MS system (Netzsch STA 449 F3 thermal gravimetric analyzer with process mass spectrometry from AMETEK). A quarts capillary tube was used as the interface between thermal gravimetric analyzer and process mass spectrometry. The TG-DSC system vacuumized after the sample was loaded in order to remove CO2 in the furnace. N2 of 5 ml/min was used as protective gas, and 20% O2/N2 mixture of 50 ml/min was used as purge gas. The TG-DSC and MS program were started simultaneously. The temperature region for TG-DSC was from room temperature to 700 C, and the MS program was set to detect m/z of 44 refer to CO2.
2.3.
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125e250 mm prior to use so that the L/Dp was 60 and D/Dp was 16. The reactor was heated to 450 C in N2 (80 ml/min) at temperature ramp rate of 10 C/min. For Ag promoted catalysts, mixture of 20 ml/min H2 and 80 ml/min N2 was fed into the reactor for 1 h to pre-treat the catalyst. After pretreatment, the H2OeC2H5OH mixture was pumped into a vaporizer at 0.02 ml/min by a liquid pump, and then mixed with 80 ml/min N2. A buffer tank was placed to ensure stable flow before the inlet of the reactor. The products were collected, separated and analyzed using two on-line SRI-8610C gas chromatographs (GC). One GC used Ar as the carrier gas, and was equipped with Carboxen 1000 and Hayesep D separation columns, and thermal conductivity (TCD) and flame ionization (FID) detectors. The Carboxen 1000 column was used to separate the permanent gases and the Hayesep D column was used to separate the hydrocarbons. The other GC used He as carrier gas, and was equipped with Carboxen 1000 column and TCD for CO and CO2 separation and detecting. Tubing between the reactor outlet and GC inlet was heated to 130 C to prevent condensation of any products. Ethanol conversion (Xethanol) and products selectivities (Si) were calculated according to following equations: Xethanol ¼
Si ¼
Fethanol in Xethanol out Fethanol in
Fi produced nðFethanol in Fethanol out Þ
(1)
(2)
Where n is 2, 1, 2/3 and 3 for CO, C2 species, acetone and H2, respectively.
3.
Results and discussion
3.1.
XRD patterns
Fig. 1 shows the XRD pattern of g-Al2O3 support and Ag promoted ZnO/Al2O3 catalysts. No peaks was attributed to ZnO on AgZnAl(2) catalyst, indicating these domains were
Catalytic activity evaluations
The catalytic activity of the prepared catalysts for SRE was evaluated using a fixed-bed quartz reactor with inner diameter of 4 mm at 450 C and atmospheric pressure. For all catalytic runs, H2O:C2H5OH molar ratio of 3 and GHSV of 30,000 h1was maintained by diluting catalysts with inert SiO2. To satisfy the plug flow condition (L/Dp > 50 and D/Dp > 10) [10], catalysts and inert SiO2 were sieved to
Fig. 1 e X-ray diffraction patterns for the Ag promoted ZnO/ Al2O3 catalysts and g-Al2O3.
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amorphous. ZnO characteristic peaks were clearly identified on AgZnAl(10) catalyst, and the crystallite size of ZnO was 12 nm Al2O3 and Ag2O peaks were also identified on Ag promoted ZnO/Al2O3 catalysts, and their crystallite sizes were 7 nm and 3 nm, respectively.
3.2. Catalytic activities and steam reforming reactions analysis Fig. 2 shows the results of C2H5OH conversion as a function of time on stream (TOS) for ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts at 450 C. It can be seen that C2H5OH conversion was relatively high for ZnAl(2) and AgZnAl(2) catalysts, and this could be attributed to its uncovered Lewis acid sites on Al2O3, which could accelerate the dehydration of C2H5OH (Equation (3)). CH3 CH2 OH/C2 H4 þ H2 O
(3)
It could also be observed from Fig. 2 that the C2H5OH conversion on Ag promoted ZnO/Al2O3 catalysts was higher than ZnO/Al2O3, indicating that Ag would promote the catalytic activity of ZnO/Al2O3 catalyst. Deactivation can be found on all catalysts for first 180 min. The deactivation on these catalysts probably could be related to sintering or coking on the surface of catalysts. The BET surface area and pore volume of g-Al2O3 support, fresh and spent catalysts were summarized in Table 1. The surface areas and pore volume for fresh ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts were slightly lower than that of g-Al2O3 support. This probably was due to the blockage of the pore structure of Al2O3 support by ZnO and Ag2O during impregnation. For the spent ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts, it could be observed that the surface areas and pore volume were slightly lower than fresh catalysts as well. This was probably attributed to the coke deposition on catalysts surface during steam reforming reaction. Figs. 3 and 4 show the selectivity of steam reforming reaction products as a function of TOS for ZnAl(2), ZnAl(10),
Fig. 2 e Ethanol conversions as a function of time on stream for ZnO/Al2O3 and Ag promoted ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3.
Table 1 e BET results of support, fresh and spent catalysts. Sample
g-Al2O3 Fresh ZnAl(2) Fresh ZnAl(10) Spent ZnAl(2) Spent ZnAl(10) Fresh AgZnAl(2) Fresh AgZnAl(10) Spent AgZnAl(2) Spent AgZnAl(10)
BET surface area
Pore volume
2
m /g
cm3/g
94 90 90 75 70 88 87 72 70
0.4 0.3 0.2 0.2 0.2 0.2 0.2 0.2 0.2
AgZnAl(2) and AgZnAl(10) catalysts at 450 C, respectively. For ZnAl(2) catalyst (Fig. 3a), C2H4 and CH3COOH were main products, and this probably was due to dehydration and CH3CHO oxidation (Equations (3) and (4)) on uncovered Al2O3. For ZnAl(10) catalyst (Fig. 3b) on the other hand, initial selectivity for CH3CHO reached 80%, indicating that C2H5OH dehydrogenation (Equation (5)) was the major reaction during
Fig. 3 e Products selectivity as a function of time on stream for ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3. (a) ZnAl(2) catalyst. (b) ZnAl(10) catalyst.
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Fig. 4 e Products selectivity as a function of time on stream for Ag promoted ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3. (a) AgZnAl(2) catalyst. (b) AgZnAl(10) catalyst.
SRE on this catalyst. For both ZnAl(2) and ZnAl(10) catalysts, selectivity for H2 and CH3CHO decreased along with TOS, indicating that the basic reactive sites, which could enhance dehydrogenation, were deactivated during the reaction. Thus the C2H4 formation, which was due to acid sites, was increased on both catalysts. H2 and CH3CHO selectivity on ZnAl(2) catalyst could be found decreased by w10% at the first 3 h in Fig. 3a. However, that was less than 5% for ZnAl(10) catalyst in Fig. 3b. Furthermore, ZnAl(10) catalyst also showed good long-term stability. The H2 and CH3CHO selectivity only decreased by 5∼10% after 20 h reaction. These results indicated that increasing ZnO loading could decrease the loss of catalytic activity and improve the long-term stability for the catalyst. For the products on both ZnAl(2) and ZnAl(10) catalysts, no CO or CH4 was produced, suggesting that CH3CHO decomposition (Equation (6)) was not favored at 450 C on these catalysts. This result agreed with that reported on ZnO/SiO2 catalysts [5]. CH3COCH3 production was also detected on ZnAl(2) and ZnAl(10) catalysts. This was probably due to Equations (7) or (8).
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CH3 CHO þ H2 O/CH3 COOH þ H2
(4)
CH3 CH2 OH/CH3 CHO þ H2
(5)
CH3 CHO/CH4 þ CO
(6)
2CH3 CHO þ H2 O/CH3 COCH3 þ CO2 þ 2H2
(7)
2CH3 COOH/CH3 COCH3 þ CO2 þ H2 O
(8)
For AgZnAl(2) catalyst (Fig. 4a), it could be observed that compared with ZnAl(2) catalyst, H2 and CH3CHO selectivities were increased to 40% and 50%, respectively. C2H4 selectivity also was found much lower on AgZnAl(2) catalyst. For AgZnAl(10) catalysts (Fig. 4b), H2 selectivity was higher compared with ZnAl(10) catalyst as well, however CH3CHO selectivity was similar. Fig. 5 summarized the calculated H2 production rate for both Ag promoted and unpromoted catalysts. It could be clearly observed that H2 production rate was increased dramatically after the Ag promotion, suggesting that Ag could significantly improve H2 production for SRE on ZnO/Al2O3 catalysts. Furthermore, for Ag promoted ZnO/Al2O3 catalysts in Fig. 4, CO was detected in the products. The selectivity to CO was decreased from 25% to 10% while ZnO loading increased. These results indicated that the first step of SRE on Ag promoted ZnO/Al2O3 catalysts was similar as ZnO/Al2O3 catalysts, which was dehydrogenation of C2H5OH to CH3CHO, and Ag could promote this reaction at the same temperature. Dehydration of C2H5OH to C2H4 also occurred on all of the prepared catalysts. However, for the second step of SRE, the pathway through which CH3CHO was converted on Ag promoted ZnO/Al2O3 catalysts was different. It could be proposed that CH3CHO was further converted to CO and H2 in the present of water on Ag promoted ZnO/Al2O3 catalysts (by Equations (6), (9) and 10). Unlike ZnO/Al2O3 catalysts, this may probably due to CeC scission on Ag site. Liberatori et al. [11] reported that on Ag/Ni/Al2O3 catalyst, the same amount of CO and CH4 was produced at 350 C because of the decomposition of CH3CHO (Equation (6)), and then CH4 was converted to CO and CO2 by steam reforming or water gas shift (Equations (9) and 10) as temperature increased.
Fig. 5 e Hydrogen production rate for both Ag promoted and unpromoted ZnO/Al2O3 catalysts at 450 C, H2O:C2H5OH molar ratio of 3.
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CH4 þ H2 O/CO þ 3H2
(9)
CO þ H2 O/CO2 þ H2
(10)
Take all these results into account, the mechanism of steam reforming reaction over ZnO/Al2O3 catalysts could be proposed as follow: C2H5OH initially absorbed on its surface and evolved to CH3CHO, and then it could be oxidized to CH3COOH or converted to CH3OCH3. This mechanism was also confirmed by other researchers [12e17]. For Ag promoted ZnO/Al2O3 catalysts, Ag could improve dehydrogenation of C2H5OH, which was the reason for the significant increase of CH3CHO and H2 formation in Figs. 4a and 5. The Ag promoted catalysts also had less C2H4 production than unprompted catalysts, indicating that Ag had positive effect for suppressing C2H5OH dehydration. For CH3CHO conversion, Ag promoted ZnO/Al2O3 catalysts favored CeC scission, and could convert CH3CHO to CO and H2. Though the decomposition of CH3CHO could greatly enhance the H2 production, a CO-rich stream would be produced after the reaction. Therefore, the products need further conversion if it was applied to CO-sensitive devices such as PEM fuel cells. Lower CO selectivity was found on high ZnO loading Ag promoted catalyst, indicating that ZnO could probably suppress CO production for these catalysts.
3.3.
Coke deposition analysis
Coking behaviors of ZnAl(10) and AgZnAl(10) catalysts for SRE were studied. The TG-DSC curves for spent ZnAl(10) catalyst were shown in Fig. 6. The TG curve showed that the weight loss was started from 350 C and ended at 500 C with total weight loss of 3.5 wt%, and a big exothermic peak at 440 C was detected on the DSC curve. The MS spectra which collected simultaneously with TG-DSC analysis was shown in inset within Fig. 6, it could be found that the COþ 2 (m/z ¼ 44) signal increased at 300 C and maximized at 450 C. These results indicated that there was 3.5 wt% of coke deposited on the surface of ZnAl(10) catalyst. According to the results in Fig. 3, which we proposed that the deactivation was occurred on basic reactive sits, we suggest that the coke could possibly be located on ZnO sites. Furthermore, the deposited coke
Fig. 6 e TG-DSC curves for spent ZnAl(10) catalyst and its MS spectra (inset).
could be probably because of the polymerization of C2H4 [18e20], and it might be assigned to filamentous or graphitic carbon [21e23]. The TG-DSC results for spent AgZnAl(10) catalyst were shown in Fig. 7. The total weight loss of AgZnAl(10) catalyst on TG curve was 5.4 wt%, and that was higher that ZnAl(10) catalyst (3.5 wt%, Fig. 6). These results suggested that AgZnAl(10) catalyst suffered higher coke deposition than ZnAl(10) catalyst. This coking behavior is similar to the results reported by Liberatori [11], They found out that higher coke deposition was detected on Ag/Ni/Al2O3 catalyst than unpromoted Ni/Al2O3 catalyst for SRE at 600 C. For the DSC cure in Fig. 7, compared with ZnAl(10) catalyst, the similar exothermic peak at 440 C appeared for AgZnAl(10) catalyst. However a small exothermic peak at 240 C, which did not appear on ZnAl(10) catalyst, was detected. The MS spectra for spent AgZnAl(10) catalyst was shown in inset within Fig. 7. Corresponding to the DSC peaks at 240 C and 440 C, it could be observed that two COþ 2 (m/z ¼ 44) peaks were detected at 240 C and 440 C. This indicated that both exothermic peaks on DSC curve were caused by combustion of carbonaceous deposition on the surface of the catalyst. Comparing the exothermic peaks in Figs. 6 and 7, it could be proposed that there were two types of coke on AgZnAl(10) catalyst surface. According to Ordonez’s report [24], the differences in coking behavior should be mainly caused by different active site. Therefore, the peak at 440 C in both Figs. 6 and 7 could be attributed to the coke deposited on ZnO site, which was caused by polymerization of C2H4, while the peak at 240 C in Fig. 7 was coke deposited on Ag site. The coke deposition at lower temperature could be assigned to monoatomic carbon which was highly reactive and could be removed easily [23,25e27]. Based on the results and analysis of steam reforming reactions described in Section 3.2, CeC scission of CH3CHO could occur on Ag site, producing CH3 and CHO*, and CH3 could convert to coke consequently (Equations 11 and 12). It can be concluded that the low-temperature deposited coke on Ag site could probably be formed by decomposition of CHx species [20]. CH3 CHO/CH3 þ CHO
(11)
CH3 /CHx þ coke
(12)
Fig. 7 e TG-DSC curves for spent AgZnAl(10) catalyst and its MS spectra (inset).
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4.
Conclusion
Ag promoted ZnO/Al2O3 catalysts were synthesized successfully by incipient wetness impregnation method, and all of them were very active for SRE at 450 C. H2 and CH3CHO were the major products on these catalysts, indicating that the SRE reaction take place by initial dehydrogenation of C2H5OH to CH3CHO. For ZnO/Al2O3 catalysts, CH3CHO could be converted to CH3OCH3 and CH3COOH. For Ag promoted ZnO/Al2O3 catalysts, Ag could improve C2H5OH conversion and H2 selectivity dramatically, and would suppress C2H5OH dehydration as well. Unlike ZnO/Al2O3 catalysts, the CeC scission of CH3CHO was occurred on Ag promoted ZnO/Al2O3 catalysts. A significant amount of CO was produced on promoted catalysts, which was due to CH3CHO decomposition. The CO selectivity could be reduced by increasing ZnO loading. Deactivation was detected on both Ag promoted and unpromoted ZnO/Al2O3 catalysts, and it was due to site blockage by coking. Increasing ZnO loading could prevent initial catalytic loss and improve long-term stability for the catalyst. Ag promoted ZnO/Al2O3 catalysts suffered higher coke deposition than ZnO/Al2O3 catalysts. The coke deposited on ZnO/Al2O3 catalysts was probably caused by C2H4 polymerization; while the coke on Ag promoted ZnO/Al2O3 catalysts was formed from both C2H4 polymerization and CHx decomposition.
Acknowledgment This work was supported by the Hydrogen Energy Technology Laboratory at the University of Michigan, National Basic Research Program of China (2007CB209705), Science and Technology Commission of Shanghai Municipality (09XD1402400, 10520708900).
Nomenclature
SRE GHSV TOS m/z L Dp D Xethanol Si Fi
steam reforming of ethanol gas hourly space velocity, h1 time on stream, min mass to charge ratio height of catalyst bed, mm catalyst particle size, mm internal diameter of reactor, mm ethanol conversion selectivity for species i molar flow rate for species i, mol/s
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