Langmuir 2006, 22, 2827-2831
2827
Easy Replication of Pueraria Lobata toward Hierarchically Ordered Porous γ-Al2O3 Chengzhang Li and Junhui He* Functional Nanomaterials Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences (CAS), Chaoyangqu Datunlu Jia 3, Beijing 100101, China ReceiVed December 7, 2005. In Final Form: January 19, 2006 Hierarchically ordered porous alumina was prepared via a facile immersion-fuming-calcination process using Pueraria lobata as template. The as-prepared alumina inherited nearly all morphological features of the template, as shown by SEM observations. It also contains abundant mesopores based on nitrogen adsorption-desorption measurements. The crystalline phase of the as-prepared alumina was ascertained to be γ-alumina by analyzing its XRD pattern. Pt nanoparticles were in situ synthesized in the γ-alumina matrix and annealed at different temperatures in N2 atmosphere. TEM observations showed that Pt nanoparticles supported by the as-prepared alumina have significantly high thermal stability.
Introduction Template synthesis is an important approach to fabrication of porous materials with ordered structures and morphologies. For example, ordered macroporous materials were prepared using self-assembled microspheres,1 emulsion droplets,2 and starch sponges.3 Ordered macroporous materials with multimodal pores were synthesized by dual templating,4 and by a combination of micromolding, colloidal templating, and cooperative assembly of a surfactant.5 Many biological materials have hierarchically ordered porous structures of multiple pore sizes ranging from the nanoscale to the macroscale. This fact made them an attractive template, recently, for preparation of hierarchically ordered porous artificial materials. For example, Davis and co-workers employed bacteria to synthesize silica fibers with ordered tubes inside.6 Later, the same group also prepared zeolite fibers with hierarchical structure in a similar way.7 Dong et al. replicated the cellar structures of bamboo and pine with β-zeolite nanocrystals,8 and Valtchev et al. prepared micro-/meso-/macroporous structures of β-zeolite using a silica-containing vegetal template (Equisetum arVense).9 He and a co-worker applied the sol-gel process to worm silks, producing porous filaments of ZrO2 and TiO2.10 Porous alumina is of much interest and importance because of its wide applications in catalysis (as both catalysts and catalyst supports), adsorption, and separation. The presence of macropores * To whom correspondence should be addressed. Tel: +86-10-64860285. Fax: +86-10-64860285. E-mail:
[email protected]. (1) (a) Johnson, S. A.; Ollivier, P. J.; Mallour, T. E. Science 1999, 283, 963. (b) Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. AdV. Mater. 2000, 12, 693. (c) Velev, O. D.; Lenhoff, A. M.; Kaler, W. W. Science 2000, 287, 2240. (2) (a) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (b) Imhof, A.; Pine, D. J. AdV. Mater. 1998, 10, 697. (3) Zhang, B.; Davis, S. A.; Mann, S. Chem. Mater. 2002, 14, 1369. (4) (a) Davis, S. A.; Burkett, S. L.; Mendelson, N. H.; Mann, S. Nature 1997, 385, 420. (b) Holland, B. T.; Abrams, L.; Stein, A. J. Am. Chem. Soc. 1999, 121, 4308. (5) Yang, P.; Deng, T.; Zhao, D.; Feng, D.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (6) Davis, S. A.; Burkett, S. L.; Mendekson, N. H.; Mann, S. Nature 1997, 385, 420. (7) Zhang, B.; Davis, S. A.; Mendelson, N. H.; Mann, S. Chem. Commun. 2000, 781. (8) Dong, A.; Wang, Y.; Tang, Y.; Ren, N.; Zhang, Y.; Yue, Y.; Gao, Z. AdV. Mater. 2002, 14, 926. (9) (a) Valtchev, V.; Smaihi, M.; Faust, A.-C.; Vidal, L. Angew. Chem., Int. Ed. 2003, 42, 2782. (b) Valtchev, V.; Smaihi, M.; Faust, A.-C.; Vidal, L. Chem. Mater. 2004, 16, 1350. (10) He, J.; Kunitake, T. Chem. Mater. 2004, 16, 2656.
in mesoporous alumina is important and useful especially for the treatment of bulky molecules, because textural mesopores and interconnected macropores should efficiently transport guest species to framework binding sites.11 For example, in the hydrocracking reaction of residue from Athabasca bitumen, a catalyst containing 15% of alumina that has macropores (0.1100 µm) gave larger +525 °C conversion, higher yields of distillates, and greater total product hydrodesulfurization, hydrodenitrogenation, and microcarbon residue conversion as compared to a standard reference catalyst.12 Very recently, Rambo and co-worker synthesized biomorphic R-Al2O3 ceramics via three consecutive high-temperature processes: pyrolysis of rattan in nitrogen atmosphere (800 °C), Al-vapor infiltration in a vacuum (1200-1600 °C) and oxidation/sintering (1600 °C).13 Pueraria lobata belongs to the bean family. It is a deciduous twining vine that spreads rapidly and covers everything in its path with a dense tangle of hairy stems and large trifoliate leaves. It is native to eastern Asia, probably to China originally, and can be found now in many other parts of the world. Its stem has hierarchically ordered porous structure. Therefore, it must be very interesting to use the stem as a template for preparation of hierarchically porous metal oxides that are attractive catalyst supports. In the current work, a facile immersion-fuming-calcination process was developed by which ordered hierarchically porous γ-Al2O3 was fabricated via replication of Pueraria lobata. The replica not only inherited all-level morphological features of Pueraria lobata, but also contained abundant mesopores. It was surprising that Pt nanoparticles in situ synthesized showed significantly high thermal stability. Experimental Section Materials. Pueraria lobata (Pl) was picked from Dabie Mountain, China. HNO3 (65-68%) and ammonia solutions (25-28%) were purchased from Beihua Fine Chemicals. Aluminum wire and hydrochloric acid (36-38%) were purchased from China Medical Cooperation Shanghai Chemicals and Tianjin Third Chemical Reagents Plant, respectively. H2PtCl6‚6H2O was bought from Shenyang Jin-Ke Reagents. Sodium borohydride (g98.0% NaBH4) was obtained from Tianjin Huan-Wei Fine Chemicals. Aqueous (11) Blin, J.-L.; Le´onard, A.; Yuan, Z.-Y.; Gigot, L.; Vantomme, A.; Cheetham, A. K.; Su, B. L. Angew. Chem., Int. Ed. 2003, 42, 2872. (12) Ternan, M. Energy Fuels 1998, 12, 239. (13) Rambo, C. R.; Sieber, H. AdV. Mater. 2005, 17, 1088.
10.1021/la0533213 CCC: $33.50 © 2006 American Chemical Society Published on Web 02/15/2006
2828 Langmuir, Vol. 22, No. 6, 2006 AlCl3 (1.0 M, pH < 1, in aqueous HCl) was prepared by dissolving aluminum wire (99.999%) in hydrochloric acid (ca. 10%). Replication of Pl. The stem of Pl was decorticated and transversely cut into slices. The density of Pl slices was estimated to be ca. 0.51 g/cm3. To remove any possible metal ions, the Pl slices were immersed in aqueous HNO3 (2.6 M) for 24 h, and rinsed repeatedly with distilled water until the pH value was nearly 7. They were dried in a vacuum at room temperature overnight. The acid-treated Pl (ATPl) slice was immersed in aqueous AlCl3 for 70 min, placed in air for 6 h, and exposed to excessive NH3 gas evaporating from ammonia solution for 3 h. After being dried at 50 °C overnight, the specimen was placed in a Tianjin Zhonghuan SX2-5-12 programmable furnace and calcined at 800 °C. A white alumina slice was finally obtained. Incorporation of Pt Nanoparticles into Alumina Slice. The as-prepared alumina slice was immersed in aqueous H2PtCl6 (1 mM) for 10 min, and rinsed in absolute ethanol for ca. 4 s. After reduction in aqueous NaBH4 (100 mM) for 10 min, the specimen turned gray. It was then rinsed with distilled water for 1 min and dried in a vacuum overnight. The alumina slice with Pt nanoparticles was annealed under nitrogen gas for 30 min at 400, 600, 700, and 800 °C, respectively. Characterization. The AT-Pl slice and the alumina slice were coated with gold by a Hitachi E-1010 Ion Sputter and observed on a Hitachi S-4300 field emission scanning electron microscope (FESEM). In addition, the latter species’ compositions were confirmed by electron dispersive spectroscopy (EDS). The alumina slice was grinded into powder and characterized by X-ray powder diffraction using a Rigaku Dmax-RB X-ray diffractometer with graphite monochromatized Cu KR radiation (λ)1.5406 Å). Nitrogen adsorption-desorption isotherms were measured using the volumetric method with a Micromeritics ASAP 2010 accelerated surface area analyzer (measurable diameter range 1.7-50 nm) at -196 °C. The as-prepared alumina was dried at 200 °C before analysis. The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) method using a linear plot over the range P/P0 ) 0.06-0.20 (eight points collected). Pore size distributions were calculated from the adsorption branch of the isotherm using the Barrett, Joyner, and Halenda (BJH) method. Pore volumes were determined from the amount of N2 adsorbed at P/P0 ) 0.99. A Pt-supported alumina slice was crushed, to which a small amount of absolute ethanol was added. A drop of the obtained dispersion was transformed onto a carboncoated copper grid. The alumina slices with Pt nanoparticles before and after annealing were observed on a JEOL JEM-200CX transmission electron microscope (TEM) at an acceleration voltage of 150 kV. Diameters of 100 Pt nanoparticles on TEM images of each specimen were measured and analyzed using SigmaPlot 2001.
Results and Discussion 1. Alumina Replica with Hierarchical Pores. The AT-Pl slice is white in the center and yellow in all other parts, as shown in Figure 1a. Vessels are visible in the digital image (pointed by arrows). Figure 1b shows a one-fourth section of the AT-Pl slice. Four different concentric areas were noticed and labeled from center to surface as A, B, C, and D. A is pith (ca. 1.5 mm in diameter) that is made up of honeycomb parenchyma cells. The mean size of the cells was estimated to be ca. 101.1 and 63.8 µm from SEM observations of transverse and longitudinal sections, respectively. Surrounding the pith is B which has a thickness of ca. 260 µm. It is composed of a vascular bundle. Vascular tracheids of ca. 13.2 µm in diameter were aligned longitudinally. C consists of reticulate vessels, the big (pointed by a thick arrow) and small (pointed by a thin arrow) ones have mean diameters of ca. 133.0 and 18.6 µm, respectively. D is made up of irregular rectangular thin-walled cells and a small number of scattered secretory lumens (pointed by dotted arrows). Almost every thin-walled cell is filled with a starch grain which is polyhedral in shape. The starch granule was removed from the cell by acid-treatment, leaving an empty lumen, as revealed by SEM observation.
Li and He
Figure 1. Digital (a and c) and SEM (b and d) images of acidtreated Pl (a and b) and alumina (c and d) slices. In parts b and d, the scale bar represents 200 µm.
A white alumina slice was finally obtained (Figure 1c) after calcination. Its density was estimated to be 0.43 g/cm3, indicating its porosity. The SEM overview of the alumina slice is shown in Figure 1d, which is similar to that of the AT-Pl slice. It also has four different concentric areas that are labeled as H, I, J and K, respectively. Clearly, they were derived, respectively, from A, B, C, and D in Figure 1b. As compared with the AT-Pl slice, the alumina slice decreased by ca. 41% in thickness and ca. 50% in diameter, respectively. The detailed structures of the alumina slice were studied by SEM at higher magnifications. Figure 2a,b shows magnified images of the transverse and longitudinal sections of H in Figure 1d, respectively. Clearly, H has a similar honeycomb structure to that of the pith in the AT-Pl slice. The mean size of alumina cells was estimated to be ca. 56.9 µm (Figure 2a) and 43.1 µm (Figure 2b). Therefore, these cells have different dimensions along the transverse and longitudinal directions. Figure 2c,d contains magnified images of the transverse and longitudinal sections of I in Figure 1d. A microtube-array morphology of alumina was observed (Figure 2c) which is the same as that of vascular bundle. The diameter of the alumina microtubes is also uniform and was estimated to be ca. 8.1 ( 2.5 µm. The microtubes are straight and run-through from the top to bottom surfaces of the alumina slice (Figure 2d). Magnified images of the transverse and longitudinal sections of J in Figure 1d are shown in Figure 2e,f, respectively. Both large and small alumina microtubes were observed, and their mean diameters were estimated to be ca. 67.5 and 8.6 µm, respectively (Figure 2e). These features were clearly inherited from the large and small vessels of C in Figure 1b. In Figure 2f, ordered closely packed pores were observed on the wall of the large alumina microtube. They are replicas of the pores in the wall of reticulate vessels. The mean diameter of the pores was estimated to be ca. 2.7 µm. Figure 2g shows a magnified cross-sectional image of K in Figure 1d. Many small (ca. 10.3 µm) and a few larger (ca. 26.1µm) pores were observed. They were produced by replication of parenchymatous tissue (D in
Easy Replication of Pueraria Lobata
Langmuir, Vol. 22, No. 6, 2006 2829
Figure 3. N2 adsorption-desorption isotherms (a) and the corresponding pore-size distributions (b) of alumina slice. (Closed triangles and open triangles represent adsorption and desorption, respectively, in part a.)
Figure 2. Magnified SEM images of transverse (a, c, e, g) and longitudinal (b, d, f, h) sections of H, I, J, and K in Figure 1d.
Figure 1b). Magnified images of the longitudinal section of K in Figure 1d show that the small pores are in fact thin-walled alumina cavities. From the above discussion, it was concluded that the alumina slice possesses a structure similar to that of the AT-Pl slice. Mesoporosity of the alumina replica was revealed by N2 adsorption-desorption measurements. Obtained isotherms are shown in Figure 3a. They are of typical type IV with a hysteresis loop, representative of mesoporosity based on the IUPAC nomenclature. The hysteresis loop seems to lie between types H1 and H4. Adsorption and desorption branches are almost vertical and nearly parallel over an appreciable range of gas uptake in type H1, whereas they are nearly horizontal and parallel over a wide range of relative pressure (P/P0) in type H4.14 Type H1 is related to mesopores consisting of agglomerates (an agglomerate is defined as an assemblage of alumina nanocrystals (14) Sing, K. S. W.; Everett, D. H.; Haul, R. A. W.; Moscou, L.; Pierotti, R. A.; Rouque´rol, J.; Siemieniewska, T. Pure Appl. Chem. 1985, 57, 603.
rigidly joined together).14 Type H1 also implies good pore connectivity with channel or ink-bottle pores.15 On the other hand, Type H4 suggests narrow slitlike pores. Thus, the mesopores in the as-prepared alumina slice may have mixed shapes rather than a single one. It is particularly noteworthy that the as-prepared alumina (calcined at 800 °C) has BET surface area and mesopore volume (175.19 m2/g and 0.34 cm3/g) higher than previously reported values (169 m2/g. and 0.30 cm3/g, calcined at 500 °C).16 Its pore size distribution obtained from the adsorption branch of the isotherms is narrow as shown in Figure 3b. The average pore diameter calculated by the BJH method is 6.13 nm. The above results suggest that the as-prepared alumina contains abundant mesopores. Thus, it has a hierarchically porous structure. Compared with the BET surface area and pore volume of conventional γ-alumina as catalyst supports (usually in the ranges 70-360 m2/g and 0.3-1.5 cm3/g, respectively17), the as-prepared alumina would be suitable for catalyst supports. Besides, the presence of biomorphic macropores in the as-prepared alumina (not in conventional γ-alumina) would allow transportation of bulk guest molecules to reaction sites, which could not occur in conventional γ-alumina. It should be emphasized that the equipment used has a measurable pore range of 1.7-50 nm and could only reveal partial micropores of the material. Therefore, it is not appropriate to discuss the micropores of the material in detail. 2. Composition and Crystalline Phase of Alumina Replica. To obtain pure alumina, it is essential to use ash-free templates. Thus, it is necessary to find a proper temperature at which the AT-Pl slice can be completely removed. When the AT-Pl slice was calcined at 400 °C, a residue (17.8 wt % of the AT-Pl slice) was obtained, which was similar in shape to the AT-Pl slice. When calcination was carried out at 600 °C, however, no remains were noticed. Therefore, the AT-Pl slice is an ash-free template at 600 °C. The AlCl3 precursor was adsorbed into the AT-Pl slice via immersing and reached the walls of vascular tracheids, vessels, (15) Ren, T.; Yuan, Z.; Su, B. Langmuir 2004, 20, 1531. (16) Zhang, H.; Hardy, G. C.; Khimyak, Y. Z.; Rosseinsky, M. J.; Cooper, A. I. Chem. Mater. 2004, 16, 4245. (17) Zhu, H. Preparation and Application Technology of Catalyst Support, 1st ed.; Petroleum Industry Press: Beijing, 2002; pp 373-386.
2830 Langmuir, Vol. 22, No. 6, 2006
Li and He
Figure 4. X-ray diffraction pattern of alumina slice.
and parenchyma cells, etc. Upon exposure to ammonia gas, Al(OH)3 and NH4Cl were formed in the walls, and mixed with template compositions (i.e., cellulose and protein, etc.18). After calcination, the template was completely removed, accompanied by decomposition of NH4Cl into NH3 and HCl and transformation of Al(OH)3 into Al2O3. Hence, a pure alumina replica of the template was produced. The purity of the alumina replica was confirmed by electron dispersive spectroscopy (EDS). The EDS spectrum showed two strong peaks centered at 0.493 and 1.504 keV that were attributed to oxygen and aluminum. Small peaks attributed to Au were also noticed. Au had been sputtered onto the specimen for SEM observations. It is necessary to clarify the crystalline phases of the as-prepared alumina. As shown in its XRD pattern (Figure 4), diffraction peaks appear at 2θ ) 66.76°, 45.48°, 19.08°, 36.62°, and 33.12° and can be assigned to the (440), (400), (111), (311), and (220) planes of γ-alumina (JCPDS card no. 48-367). The average size of alumina nanophase was estimated by using the Scherrer formula from the half-height width of the (440) peak. It is ca. 5.48 nm, indicating that the alumina slice is a sinter of alumina nanocrystals. This is at least partially consistent with the above nitrogen adsorption-desorption results. 3. Thermal Stability of Pt Nanoparticles Supported on γ-Alumina Replica. It is interesting and important to investigate the loading and thermal stability of metal nanoparticles on the as-prepared γ-alumina replica. Pt nanoparticles were first synthesized in situ and then annealed at different temperatures. Figure 5a shows a TEM image of alumina-supported Pt before annealing. The gray matrix is alumina, and the small black dots are Pt nanoparticles that are well dispersed in the alumina matrix. Figure 5b shows the histogram of Pt nanoparticles. The mean diameter and standard deviation of Pt nanocrystals were estimated to be 3.24 and 0.93 nm, respectively. It is also noted in Figure 5a that the alumina matrix contains mesopores whose shapes are varied, consistent with the analysis of the N2 adsorptiondesorption isotherms. The mean diameter of the mesopores measured by TEM is ca. 7.59 nm, which is slightly larger than the aforementioned BJH average pore diameter (6.13 nm) in the absence of Pt nanoparticles. Pt nanoparticles (ca. 3.24 nm) probably occupied some small mesopores in γ-alumina replica, lessening the percentage of small mesopores in the total mesopores.10 The Pt-supported alumina slices were annealed under nitrogen gas for 30 min at 400, 600, 700, and 800 °C, respectively. TEM images obtained from the four specimens were similar to Figure 5a. The diameters of Pt nanoparticles were obtained, respectively, as the following: 3.29 ( 0.71 nm (400 °C), 3.67 ( 1.01 nm (600 °C), 4.40 ( 1.11 nm (700 °C), and 5.58 ( 1.84 nm (800 °C). (18) Zhang, C.; Wang, X. Shanxi Forest Sci. Tech. 1999, 1, 24.
Figure 5. TEM image of alumina slice containing Pt nanoparticles before annealing (a) and histogram of Pt nanoparticles (b).
Figure 6. Dependence of mean diameter (d) and standard deviation (σ) of Pt nanoparticles on annealing temperature.
Figure 6 shows the relationship of the mean diameter and standard deviation of Pt nanoparticles with annealing temperature. The particles transferring model is often used to account for sintering of supported metals.19 In this model, metal microcrystals are supposed to exist in a quasiliquid state when the temperature is beyond their Tamman temperature (0.4 times the metal melting point in Kelvin). Thus, the metal microcrystals could transfer, collide, and grow up on the surface of support over their Tamman temperature. Because of their smaller sizes, Pt nanocrystals must have an even lower Tamman temperature than Pt microcrystals (535.8 °C). Thus, Pt nanocrystals probably still existed in their solid state and hardly grew up at 400 °C, which was proved by the negligible increase of the mean diameter and standard deviation from room temperature to 400 °C. In the range 400600 °C, the mean diameter and standard deviation increased slightly faster. In this range, smaller Pt nanoparticles might be in the quasiliquid state and begin to transfer and grow. Although the mean diameter and standard deviation increased evidently faster in the range 600-800 °C than in the range 400-600 °C, the increase was not significant. It is possible that Pt nanoparticles were located within the mesopores formed by alumina nanocrystals. The confining effect of alumina mesopores on Pt nanoparticles may make them less easy to transfer and grow. Clearly, it is difficult for Pt nanoparticles to grow up beyond the confining mesopores of the γ-alumina matrix. These results indicate that the γ-alumina replica effectively immobilized and isolated Pt nanoparticles. Therefore, Pt nanoparticles in the as(19) Hughes, R. DeactiVation of Catalysts, 1st ed.; translated by Ding, F., Yuan, N.; Science Press: Beijing, 1990; pp 63-72.
Easy Replication of Pueraria Lobata
prepared γ-alumina matrix have high thermal stability. In contrast, Pd nanoparticles loaded on the surface of R-Al2O3 were less stable, and sintering occurred via traditional ripening and coalescence mechanisms.20
Conclusions In summary, hierarchically ordered porous γ-alumina was facilely fabricated using Pueraria lobata as template by a novel immersion-fuming-calcination process. The unique structure of the Pueraria lobata stem made it an attractive template for preparation of hierarchically ordered porous materials. Although the immersion-fuming-calcination process was very simple, nearly all level morphological features of the template were (20) Liu, R.-J.; Crozier, P. A.; Smith, C. M.; Hucul, D. A.; Blackson, J.; Salaita, G. Appl. Catal., A 2005, 282, 111.
Langmuir, Vol. 22, No. 6, 2006 2831
precisely replicated, including those on both macro- and mesoscales. Other important advantages of the current approach are that small metal nanoparticles of narrow size distribution can be readily in situ synthesized in the γ-alumina replicas and that these nanoparticles demonstrate significantly high thermal stability. Further efforts to control the characteristics of the materials are currently carried out in our laboratory by systematic investigations. These obtained γ-alumina materials would have promise in applications such as catalysts, catalyst supports, absorbents, and separation materials. Acknowledgment. This work was supported by the National Natural Science Foundation of China (Grant 20471065), the Hundred Talents Program of CAS, and the President Fund of CAS. LA0533213
