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Articles Synthesis of Mesoporous ZnO (m-ZnO) and Catalytic Performance of the Pd/m-ZnO Catalyst for Methanol Steam Reforming Guangwei Xiong,† Laitao Luo,*,† Changquan Li,† and Xiaomao Yang‡ Institute of Applied Chemistry, Nanchang UniVersity, Nanchang, Jiangxi 330031, People’s Republic of China, and Institute of Applied Materials, College of Resource and EnViroment Management, Jiangxi UniVersity of Finance and Economics, Nanchang, Jiangxi 330013, People’s Republic of China ReceiVed October 9, 2008. ReVised Manuscript ReceiVed December 18, 2008
Mesoporous ZnO (m-ZnO) was successfully synthesized by a supermolecule-templeting approach under hydrothermal conditions, in which triblock co-polymer Pluronic (F-127) was introduced as a template reagent. The corresponding Pd/m-ZnO catalyst was prepared using the deposition-precipitation method, and its catalytic performance was determined in the methanol steam reforming process. Those products were characterized by means of Brunauer-Emmett-Teller surface area (BET), Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), and H2 temperature-programmed reduction (TPR). In comparison to the Pd/ZnO catalyst, the Pd/m-ZnO catalyst possessed the higher catalytic activity, H2 yield, and CO2 selectivity, which was attributed to the m-ZnO with a larger BET surface area (124.7 m2/g), uniform pore distribution, and stronger interaction with Pd.
1. Introduction The mesoporous materials with high Brunauer-EmmettTeller (BET) surface area, open mesoporous structure, and increased dispersion of active components have earned intensive interest from scientists in their numerous applications, including catalyst supports,1-3 since the mesoporous molecular sieve was first synthesized in the 1990s. Recently, methanol serving as a hydrogen source for fuel cells has become a research hotspot because the hydrogen energy is one way to solve the energy and environmental problems of the world. Literature studies concerning methanol steam reforming mainly focused on the Cu/ZnO catalyst.4-6 However, the rapid deactivation of the Cu/ ZnO catalyst at temperatures above 573 K and Pt electrode poisoning for its too high CO selectivity are barriers to its practical application in the steam reforming of the methanol process, such as on-board hydrogen manufacture for automobiles. Another interesting catalyst for methanol steam reforming is the Pd/ZnO catalyst, which possessed high activity and * To whom correspondence should be addressed: Institute of Applied Chemistry, Nanchang University, Nanchang, Jiangxi 330031, People’s Republic of China. E-mail:
[email protected]. † Nanchang University. ‡ Jiangxi University of Finance and Economics. (1) Sun, C.; Sun, J.; Xiao, G.; Zhang, H.; Qiu, X.; Li, H.; Chen, L. J. Phys. Chem. B 2006, 110, 13445–13452. (2) Joo, S. H.; Choi, S. J.; Oh, I.; Kwak, J.; Liu, Z.; Terasaki, O.; Ryoo, R. Nature 2001, 412, 169–172. (3) Jun, S.; Joo, S. H.; Ryoo, R.; Kruk, M.; Jaroniec, M.; Liu, Z.; Ohsuna, T.; Terasaki, O. J. Am. Chem. Soc. 2000, 122, 10712–10713. (4) Shen, J.-P.; Song, C. Catal. Today 2002, 77, 89–98. (5) Turco, M.; Bagnasco, G.; Costantino, U.; Marmottini, F.; Montanari, T.; Ramis, G.; Busca, G. J. Catal. 2004, 228, 43–55. (6) Turco, M.; Bagnasco, G.; Costantino, U.; Marmottini, F.; Montanari, T.; Ramis, G.; Busca, G. J. Catal. 2004, 228, 56–65.
selectivity for its high-temperature resistance, uneasy aging, and steady performance but lower dispersion of active secondary component Pd for a smaller surface area of ZnO. It was recently shown that mesoporous metal oxides with large surface areas and uniform pore distribution, e.g., SnO2 or Co3O4, possess better catalytic properties than nonporous samples of the same metal oxides.7,8 Clearly, the preparation methods of precipitation and template were used to enhance the surface area of mesoporous ZnO, whereas there is no report on mesoporous ZnO, which was prepared using a hydrothermal method and triblock co-polymer Pluronic F-127 (F-127) as the template reagent thus far. In this paper, F-127 was used as the template reagent and a novel hydrothermal approach was used to sybthesize mesoporous ZnO (m-ZnO). At the same time, the catalytic performance of a Pd-based catalyst was determined in the reaction of methanol steam reforming. In addition, m-ZnO and Pd/m-ZnO were characterized by means of Fourier transform infrared spectroscopy (FTIR), X-ray diffraction analysis (XRD), BET, and H2 temperature-programmed reduction (TPR). 2. Experimental Section 2.1. Preparation of m-ZnO. m-ZnO material was prepared under hydrothermal conditions using F-127 as the template reagent. In a typical experiment, a solution with Zn(CH3COO)2 · 2H2O (0.005 mol) and CO(NH2)2 (0.1 mol) were disolved in deionized water (100 mL) at room temperature and then F-127 (0.005 × 10-3 mol) was added with magnetic stirring to yield transparent solution. In (7) Wagner, T.; Waitz, T.; Roggenbuck, J.; Fro¨ba, M.; Kohl, C.-D.; Tiemann, M. Thin Solid Films 2007, 515, 8360–8363. (8) Waitz, T.; Tiemann, M.; Klar, P. J.; Sann, J.; Stehr, J.; Meyer, B. K. Appl. Phys. Lett. 2007, 90, 123108.
10.1021/ef8008376 CCC: $40.75 2009 American Chemical Society Published on Web 03/03/2009
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Figure 3. FTIR spectra of m-ZnO (a) before and (b) after calcination.
Figure 1. Nitrogen adsorption-desorption isotherm and the PSD for m-ZnO.
addition, the pH value of the solution is about 5, using acetic acid for pH adjustment. After the solution was stirred at the same temperature for another 2 h, the resulting solution was transferred into a hydrothermal kettle at 363 K for 24 h. The solid product was filtered off, washed with deionized water several times to neutral, and dried in an oven at 373 K overnight, followed by calcination in air under static conditions at 673 K for 2 h. 2.2. Preparation of Catalysts of Pd/m-ZnO and Pd/ZnO. The deposition-precipitation method was employed for the preparation of 2 wt % Pd/m-ZnO catalyst. Briefly, m-ZnO powder and H2PdCl6 (3.75 × 10-2 mol/L) solution were mixed to yield a suspension, and then Na2CO3 (0.6-0.7 mol/L) was slowly added to the suspension under vigorous stirring until the pH level was 10. After the solution was stirred at room temperature for 2 h, the resulting product was filtered off, washed with deionized water several times to remove Cl-, CO32-, and Na+, dried in an oven overnight, and then calcined in air under static conditions at 673 K for 2 h. To compare the catalytic performance, the 2 wt % Pd/ZnO catalyst was also synthesized on a non-mesoporous ZnO support under the same preparation conditions demonstrated in the preparation of the Pd/m-ZnO catalyst. 2.3. Catalytic Performance Measurement. Catalyst’s activity and selectivity of steam reforming of methanol was conducted with a fixed-bed flow reactor at atmospheric pressure using 0.1 g of the catalyst in the temperature range of 403-523 K. The catalyst was first reduced in a stream of H2 (30 mL/min) from room temperature to 623 K with a heating rate of 10 K min-1. It was kept at this temperature for 2 h before cooling to the reaction temperature. The mole ratio of H2O/CH3OH is 1:1, and the space velocity was controlled at 18 000 mL h-1 g-1. The components of the effluent passed through a condenser and were then introduced to a gas
Figure 2. (a) TEM and (b) SEM images of m-ZnO.
chromatogram (GC). The GC was equipped with two packed columns (Porapak Q, 5A molecule sieve) and one thermal conductivity detector (TCD). Three runs were performed to obtain a consistent result for every component peak area at each temperature point. The catalytic activity was expressed by the degree of CH3OH conversion, and hydrogen yield was calculated from the material balance. Meanwhile, CO2 selectivity [SCO2 (%)] was determined using the SCO2 (%) ) PCO2/(PCO2 + PCO) formula, where PCO2 and PCO are the partial pressures of CO2 and CO of gas production, respectively. 2.4. Characterization. Low- and wide-angle XRD patterns were obtained on a D8 Advance diffractometer (Bruker, Germany) with Ni-filtered Cu KR radiation (40 kV, 40 mA). N2 adsorption-desorption isotherms were collected on an ASAP 2020 (Micrometrics) surface area and pore size analyzer at -196 °C. Before the measurements, the samples were outgassed at 260 °C in a vacuum for 6 h. The BET method was used to calculate the specific surface areas using desorption data. The pore size distributions (PSDs) were derived from the adsorption branches of the isotherms using the BarrettJoyner-Halenda (BJH) method. TPR was performed in an automatic apparatus (Chemisorb 2750) equipped with a TCD. The fresh catalyst (0.1 mg) was submitted to a heat treatment (10 °C/min up to 600 °C) in a mixture gas flow (50 mL/min) of H2/Ar (5:95 vol %). Previous to the TPR experiment, the samples were heat-treated under inert atmosphere at 350 °C for 2 h. Fourier transform infrared (FTIR) spectra were obtained in the range of 400-4000 cm-1 on a Nicolet 5700 FTIR spectrometer using KBr pellets. Thermogravimetric analysis-differential thermogravimetry (TGA-DTG) was carried out in air flow using a DR-4P thermoanalyzer.
3. Results and Discussion 3.1. Surface Textural Properties of m-ZnO. Shown in Figure 1 is the N2 adsorption-desorption isotherm for the m-ZnO sample together with the corresponding PSD curve. It shows a type-IV adsorption-desorption isotherm with some
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Figure 4. (a) TGA curve and (b) corresponding DTG profile of m-ZnO.
Figure 5. Small- and wide-angle XRD patterns of m-ZnO.
contribution of type II, a feather of mesoporous material. There is a hysteresis of the m-ZnO support that first appears at low relative p/p0 ) 0.45, followed by a linear increase up to p/p0 ) 0.9 in both the adsorption and desorption branchs. The increase in slope at p/p0 ) 0.45 corresponds to capillary condensation, typical of mesoporous materials with uniform pore systems, while the further increase at higher relative pressures indicates substantial interparticle porosity. The resulting PSD (inset) of the sample indicates that the prepared m-ZnO is mainly mesoporous material with a narrow PSD (average pore diameter of about 6.8 nm). The BET surface area and pore volume of m-ZnO are about 124.7 m2/g and 0.31 cm3/g, respectively, which are far superior to those of non-mesoporous ZnO (9.0 m2/g and 0.016 cm3/g). Moreover, not only is the BET surface area of the as-synthesized m-ZnO much higher than that of ZnO but also the largest one among the reported mesoporous ZnO materials. Both the mesostructural order and the crystallinity within the pore walls are confirmed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM), as shown in Figure 2. The representative TEM image (Figure 2a) reveals that light parts correspond to the pores and dark parts correspond to ZnO particles with size of approximately 20 nm, which show good agreement with the value calculated from the XRD data. Among the ZnO particles, there are numerous pores with diameters in the range of between 6 and 8 nm, which roughly agrees with the result of the N2 adsorption-desorption observation. From the SEM image, one can see that there is no clear identifiable long-ranged structural order of these mesoporous channels. This shows that the templated solid structure in the present work is a randomly aggregated structure. 3.2. FTIR and TGA-DTG Analysis. The FTIR spectra for both fresh (curve a) and calcined (curve b) mesoporous ZnO are recorded in Figure 3. In curve a, there are two bands centered
Figure 6. XRD patterns of (a) Pd/m-ZnO and (b) Pd/ZnO catalysts.
Figure 7. TPR patterns of (a) Pd/m-ZnO and (b) Pd/ZnO catalysts.
at 2950 and 2875 cm-1, which are characteristic of the stretching vibrations of the C-H bond in hydrocarbons. Moreover, the bands in the range of 1551-1384 and 821-690 cm-1 are ascribed to stretching vibrations of CdO and C-O-C of F-127, respectively. From curve b, a consequence can be derived that
Synthesis of Mesoporous ZnO
Figure 8. Conversion and CO2 selectivity for methanol steam reforming over (a and b) Pd/m-ZnO and (c and d) Pd/ZnO catalysts.
Figure 9. Hydrogen yield over (a) Pd/m-ZnO and (b) Pd/ZnO catalysts.
the template reagent is decomposed by calcination at 400 °C, which is in good agreement with the results reported by Wan and Zhao.9 The TGA curve and corresponding DTG profile for the uncalcined ZnO, over the range of room temperature up to 600 °C in air, are shown in Figure 4. Three stages and a total weight loss of 27.8 wt % are observed in the TGA curve. The first stage below 200 °C (centered at 100 °C) leads to a 1.59 wt % weight loss that is assigned to desorption of adsorbed water and ammonia. The secend significant weight loss of 22.5 wt % between 200 and 300 °C, which is centered at 260 °C, can be attributed to the decomposition of the F-127 template in the nanochannels of the sample. From 300 to 400 °C (centered at 385 °C), there is a weight loss of 2.38 wt %, which is related to the removal of water molecules from the hydroxyls on zinc atoms and a small amount of residual organic fragments. In addition, no significant weight loss (less than 1 wt %) above 400 °C is observable, which is in good agreement with FTIR characterization, shown in Figure 3. 3.3. Structure of the Mesoporous ZnO and Catalysts. Small- and wide-angle XRD patterns are collected to study the pore structure and wall crystallinity (Figure 5). The small-angle pattern contains a low angle peak, which can be indicated by the mesostructure of ZnO. The wide-angle pattern (inset) shows that diffraction peaks appeared at 2θ ) 31.88, 34.48, 36.28, 47.58, 56.68, 62.88, and 67.98, corresponding to the ZnO (100), (002), (101), (102), (110), (103), and (112) planes reported in Joint Committee on Powder Diffraction Standards (JCPDS) card (36-1451), respectively. In general, the intensity and the full width at half-maximum (fwhm) of the peak of (002) plane can show the structural properties of the ZnO. The domain size (19.4
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nm) for the synthesized sample is then estimated using the Scherrer equation. This value is in good qualitative agreement with the value measured by TEM. The XRD patterns of the synthesized Pd/m-ZnO and Pd/ZnO catalysts are shown in Figure 6. As seen, both of the prepared catalysts shows a wurtzite-phase ZnO. The strong and clear peaks reveal the good crystallinity of the samples after loading 2 wt % Pd. Furthermore, peaks of metallic Pd and PdOx are not observed by XRD analysis, indicating that the average crystallite size of Pd is less than 1 nm and the dispersion of metallic Pd is very high. 3.4. TPR. Figure 7 depicts the H2 TPR spectra of the Pd/ m-ZnO and Pd/ZnO catalysts. The TPR profiles of the Pd/mZnO and Pd/ZnO exhibit low-temperature (LT) peaks (centered at 48 and 57 °C), which can be generally ascribed to the reduction of PdOx species weakly bound to the m-ZnO surface. In comparison to the TPR profiles of Pd/m-ZnO and Pd/ZnO, we found that the LT peak with a smaller reduction area may be related to the reduction of partial PdOx (Pd2x+ f Pd0) in the calcination process.9,10 Meanwhile, the high dispersion on a large m-ZnO surface area is formed using a hydrogen reduction treatment, and thus, metallic Pd acts as the active site of methanol steam reforming. As we know, methanol steam reforming is mainly carried out over the metal sites. Obviously, more amounts of metallic Pd are advantagious to catalytic reaction, which are in accordance with the results of catalytic activity. Furthermore, the shift of the LT peak to a lower temperature can reflect the effect of the metallic Pd loading, suggesting that there is a possible interaction between Pd nanoparticles and the m-ZnO support. It is quite noteworthy that no peak can be found at higher temperatures of Pd/m-ZnO, which further explains that Pd species mainly disperse on the m-ZnO surface but not or barely on the crystal lattice of ZnO. 4. Catalytic Activity The catalytic performances of catalysts in the methanol steam reforming process depend upon not only the nature of the active components but also the state of the supported metal/metal oxides,11 as further confirmed by Figures 8 and 9. The results show that supports have a great effect on the methanol conversion, H2 yield, and CO2 selectivity of Pd-based catalysts for their various structural characteristics. In comparison to the Pd/ZnO catalyst, the Pd/m-ZnO catalyst exhibits higher methanol conversion (99.9%) when the reaction temperature is 170 °C and the methanol conversion is invariant, with the temperature increasing from 170 to 250 °C, as shown in Figure 8, indicating excellent activity and stability of Pd/m-ZnO. The larger specific surface area of m-ZnO is one of the possible reasons that inhibits the aggregation of metallic Pd because of longer migration distances among Pd nanoparticles. Meanwhile, the CO2 selectivity also has a great relationship with the reaction temperature (Figure 9), and its variation trends of corresponding curves were almost the same as those of methanol conversion. It is well-known that the main reactions were CH3OH + H2O f CO2 + 3H2 and CH3OH f CO + 2H2. The increased CO2 selectivity is beneficial to H2 yield and the water-gas shift (WGS) reaction (CO + H2O f CO2 + H2). In other words, it (9) Wan, Y.; Zhao, D. Chem ReV. 2007, 107, 2821. (10) Enrique, R. C.; Josefa, M. R.; Lourdes, D. Appl. Catal., A 2004, 260, 9–18. (11) Lin, W.; Zhu, Y. X.; Wu, N. Z.; Xie, Y. C. Appl. Catal., B 2004, 50, 59–66. (12) Roggenbuck, J.; Schafer, H.; Tsoncheva, T.; Minchev, C.; Hanss, J.; Tiemann, M. Microporous Mesoporous Mater. 2007, 101, 335–341.
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can decrease CO selectivity for the formula SCO2 + SCO ) 1, where SCO2 and SCO are the selectivities of CO2 and CO, respectively. The H2 yield of Pd/m-ZnO are 1.12 and 1.15 mol h-1 gcat-1 at the temperature of 170 and 250 °C, respectively, far superior to those (0.91 and 0.75 mol h-1 gcat-1) of Pd/ZnO, as shown in Figure 9. 5. Conclusion m-ZnO was synthesized using a supermolecule-templeting approach under hydrothermal conditions, in which F-127 was introduced as the template reagent. The BET surface area
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and pore volume of m-ZnO are about 124.7 m2/g and 0.31 cm3/g, respectively. Not only is the BET surface area of m-ZnO much higher than that of ZnO but also the largest one among the reported pure ZnO mesoporous materials. In comparison to the Pd/ZnO catalyst, the Pd/m-ZnO catalyst exhibits a much better catalytic performance. When the reaction temperature is 170 °C, its methanol conversion, H2 yield, and CO2 selectivity are 99.9%, 1.12 mol h-1 gcat-1, and 94.4%, respectively, which are far superior to those (90.1%, 0.91 mol h-1 gcat-1, and 76.6%) of Pd/ZnO. EF8008376
