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Chem. Mater. 2007, 19, 4776-4782
Synthesis of Porous r-Fe2O3 Nanorods and Deposition of Very Small Gold Particles in the Pores for Catalytic Oxidation of CO Ziyi Zhong,*,† Judith Ho,† Jaclyn Teo,† Shoucang Shen,† and Aharon Gedanken*,‡ Institute of Chemical and Engineering Sciences, 1 Pesek Road, Jurong Island, Singapore 627833, and Department of Chemistry and Kanbar Laboratory for Nanomaterials, Bar-Ilan UniVersity Center for AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan 52900, Israel ReceiVed April 30, 2007. ReVised Manuscript ReceiVed June 21, 2007
FeO(OH) nanorods were prepared by a mild hydrothermal synthesis using tetraethylammonium hydroxide (TEAOH) as the structure director. The as-prepared FeO(OH) was converted to porous R-Fe2O3 nanorods via calcination at 300 °C. The R-Fe2O3 nanorods have a pore size distribution in the range of 1-5 nm, which is ideal to house very small gold (Au) particles. Later, by employing our invented Au- colloid-method, in which lysine was used as the capping reagent and sonication was employed to facilitate the deposition of the Au particles, we inserted very small Au particles into these pores. The prepared Au/R-Fe2O3-nanorod catalyst exhibited higher catalytic activity than the Au/R-Fe2O3 (Fluka) catalyst.
1. Introduction In recent years, intensive attention has been paid to gold (Au) catalysis, evidenced by an exponential increase in publications on its applications.1-5 Initially, metallic Au was believed to be very stable and inert in catalysis. However, this status has changed since Haruta et al. observed the extraordinary high catalytic activity for CO oxidation on the supported-Au catalysts.1 Interestingly, the catalytic performance of Au catalysts is highly dependent on the Au particle size.6,7 When the Au particle size becomes very small (usually e5 nm), high activities for a number of reactions under mild conditions are reported.8-13 Both Haruta14 and Goodman6 reported that for CO oxidation, the optimum Au particle size should be between 2 and 4 nm. Other factors * Corresponding authors. E-mail:
[email protected] (Z.Z.);
[email protected] (A.G.) † Institute of Chemical Engineering and Sciences. ‡ Bar-Ilan University.
(1) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (2) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (3) Haruta, M. Catal. Today 1997, 36, 153. (4) Hutchings, G. J. Gold Bull. 1996, 29, 123. (5) Hutchings, G. J. Catal. Today 2002, 72, 11. (6) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (7) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (8) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M. Science 2003, 301, 935. (9) Kim, W. B.; Voitl, T.; Rodrigue-Rivera, G. J.; Dumesic, J. A. Science 2004, 305, 1280. (10) Enache, D. I.; Edwards, J. K.; Landon, P.; Solsona-Espiru, B.; Carley, A. F.; Herzing, A. A.; Watanabe, M.; Kiely, C. J.; Knight, D. W.; Hutchings, G. J. Science 2006, 311, 362. (11) Chowdhury, B.; Bravo-Sua´rez, J. J.; Date´, M.; Tsubota, S.; Haruta, M. Angew. Chem., Int. Ed. 2006, 45, 412. (12) Hughes, M. D.; Xu, Y. J.; Jenkins, P.; McMorn, P.; Landon, P.; Enache, D. I.; Carley, A. F.; Attard, G. A.; Hutchings, G. J.; King, F.; Stitt, E. H.; Johnston, P.; Griffin, K.; Kiely, C. J. Nature 2006, 437, 1132. (13) Corti, C. W.; Holloday, R. J.; Thompson, D. T. Appl. Catal. A. 2005, 291, 253. (14) Haruta, M.; Tsubota, S.; Kobayashi, T.; Kageyama, H.; Genet, M. J.; Delmon, B. J. Catal. 1993, 144, 175.
contributing to the enhanced catalytic activity include electronic effects such as the transition of Au particles from a metallic to a nonmetallic state when reducing Au particle size to or below ca. 5.0 nm, as well as the strong metalsupport interaction (SMSI).15 Although progress has been made recently in the preparation of supported-Au catalysts on silica,16-19 developing practical methods for the preparation of Au-based catalysts with good control of Au particle size and with high stability still remains a challenge. The current paper adds to the vast existing information on Au catalysis with the idea that, in order to provide a good control on the Au particle size and to improve the stability of the Au particle or lower their mobility, the pores of the catalyst support should be of the same dimension as the Au particles. Also, it is expected that the insertion of Au particles into the shallow pores can increase the perimeter distance of the Au-support interface, thus enhancing the catalytic performance for CO oxidation14. Among the various catalysts for CO oxidation, the catalysts of Au supported on R-Fe2O3 (hematite) exhibit almost the highest catalytic activity.1-4,14,20-22 Therefore, in order to further improve the catalytic performance of the Au/R-Fe2O3 catalysts, proper R-Fe2O3 should be prepared, e.g., with pores that are comparable to the size of the very small Au particles so as to accommodate or fix the gold in the pores. However, (15) (16) (17) (18) (19) (20) (21) (22)
Goodman, D. W. Catal. Lett. 2005, 99, 1. Budroni, G.; Corma, A. Angew. Chem., Int. Ed. 2006, 45, 3328. Liu, Y. H.; Lin, H. P.; Mou, C. Y. Langmuir 2004, 20, 3231. Chiang, C. W.; Wang, A.; Wan, B. Z.; Mou, C. Y. J. Phys. Chem. B 2005, 109, 18042. Zhu, H.; Liang, C.; Yan, W.; Overbury, S. H.; Dai, S. J. Phys. Chem. B 2006, 110, 10842. Cheng, W. H.; Wu, K. C.; Lo, M. Y.; Lee, C. H. Catal. Today 2004, 97, 145. Landon, P.; Ferguson, J.; Solsona, B. E.; Garcia, T.; Carley, A. F.; Herzing, A. A.; Kiely, C. J.; Golunski, S. E.; Hutchings, G. J. Chem. Commun. 2005, 3383. Khoudiakov, M.; Gupta, M. C.; Deevi, S. Appl. Catal. A. 2005, 291, 151.
10.1021/cm071165a CCC: $37.00 © 2007 American Chemical Society Published on Web 08/18/2007
Synthesis of Porous R-Fe2O3 Nanorods
so far, there are only a few successful reports on this kind of synthesis,23-25 including the synthesis of single-crystalline R-Fe2O3 nanotubes with inner diameters in the range of 4080 nm by reacting FeCl3 with NH4H2PO4 under hydrothermal conditions,23 the synthesis of hollow hematite nanowires (with a pore size of 15 nm) by a vacuum-pyrolysis route from β-FeOOH,24 and the hydrothermal synthesis of R-Fe2O3 hollow spheres in the presence of D-glucose.25 In a series of attempts, we first prepared porous γ-Fe2O3 (not R-Fe2O3) with high surface areas by the sonochemical method,26 and very recently, we prepared single crystals of R-Fe2O3 nanorods but failed to create pores inside.27 By using iron (III) ethoxide as the precursor and decylamine as the template, Bruce et al.28 first prepared ordered 2-dimensional (2D) and 3-dimensional (3D) mesoporous iron oxide with an average pore size of 7.0 nm, but the precursor used in the synthesis is quite expensive, limiting its large-scale application in catalysis. In the past decade, intensive efforts have also been invested in the synthesis of microporous and mesoporus silicalites via the use of tetrapropylammonium hydroxide (TPAOH) as the structure director.29 Moritz et al.30 also prepared 1-dimensional (1D) TiO2 using TPAOH as the structure director. However, so far, there is no report on the synthesis of R-Fe2O3 using similar strategies. Herein, we adapted this method to the synthesis of porous R-Fe2O3, in which, cheap Fe(NO3)3 was used as precursor and TEAOH was used as the structure director. The synthesis is a simple one-pot hydrothermal reaction and can be readily scaled up, and the as-prepared product, the R-FeO(OH) nanorods, can be converted to porous 1D R-Fe2O3 nanorods. Interestingly, the pore size in the R-Fe2O3 product is 1-5 nm. As already proven, these pores are ideal to host the very small Au particles. Indeed, by employing our invented method for the deposition of the Au particles, we inserted very small Au particles into these pores. The prepared Au/Fe2O3-nanorod catalyst exhibited higher catalytic activity than the Au/R-Fe2O3 (Fluka) catalyst. 2. Experimental Details All chemicals were purchased from Aldrich except for the commercial R-Fe2O3, which was from Fluka. Reference Au catalysts were purchased from the World Gold Council (WGC). The R-Fe2O3 support was synthesized by a hydrothermal method. Typically, 1.0 g of Fe(NO3)3‚9H2O was added to 10 mL of 20 wt % aqueous TEAOH solution. The added Fe(NO3)3‚9H2O was hydrolyzed within several minutes, and a brown slurry was formed. The mixture was stirred for a while and transferred into a screw-capped autoclave (23) Xiong, Y.; Li, Z.; Li, X.; Hu, B.; Xie, Y. Inorg. Chem. 2004, 43, 6540. (24) Jia, C. J.; Sun, L. D.; Yan, Z. G.; You, L. P.; Luo, F.; Han, X. D.; Pang, Y. C.; Zhang, Z.; Yan, C. H. Angew. Chem., Int. Ed. 2005, 44, 4328. (25) Titirici, M. M.; Antonietti, M.; Thomas, A. Chem. Mater. 2006, 18, 3808. (26) Srivastava, D. N.; Perkas, N.; Gedanken, A.; Felner, I. J. Phys. Chem. B 2002, 106, 1878. (27) Zhong, Z.; Lin, M.; Ng, V.; Ng, G. N. B.; Foo, Y.; Gedanken, A. Chem. Mater. 2006, 18, 6031. (28) Jiao, F.; Bruce, P. G. Angew. Chem., Int. Ed. 2004, 43, 5958. (29) (a) Rimer, J. D.; Fedeyko, J. M.; Vlachos, D. G.; Lobo, R. F. Chem.s Eur. J. 2006, 12, 2926. (b) Israelachvili, V. Intermolecular and Surface Forces; Academic Press: New York, 1992. (30) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 235.
Chem. Mater., Vol. 19, No. 19, 2007 4777 lined with a Teflon vessel. The autoclave was heated in an oven at a temperature of 80 °C. After the reaction, the precipitate was washed with deionized (DI) water four times, dried in a vacuum oven at 100 °C overnight, and calcined at 300 °C for 0.5 h. This synthesis can be readily scaled up due to its simplicity. Using an autoclave of 200 mL, we were able to prepare the R-Fe2O3 nanorods of ca. 12 g at one time. For catalyst preparation, we employed our recently invented Aucolloid-based method.31-35 In this technique, very small Au colloids were first produced by reduction of HAuCl4 with NaBH4 in the presence of the capping reagent (lysine) and the catalyst support. At the same time, during the reduction, ultrasonic irradiation (sonication) was applied which efficiently facilitated the deposition of the Au colloids on the catalyst supports. In a typical preparation, 0.50 g of R-Fe2O3 was added to 10 mL of DI water, followed by 1.5 mL of 0.01 M HAuCl4 and 1.5 mL of 0.01 M Lysine (Lys) (Au loading was ca. 0.5 wt %). The pH value of the suspension was then adjusted to 5-6 with 0.10 M of NaOH solution, after which it was subjected to a sonication for 20 s. During the sonication, freshly prepared NaBH4 (0.1 M, 5-10 times the Au molar number) was injected instantly. After the reaction, the endpoint pH value was measured. The suspension was changed in the dark immediately and was then washed with DI water four times with a centrifuge. The whole preparation for one catalyst including the solution preparation and the washing can be finished within 1 h. The measurement of the catalytic activity for CO oxidation was carried out in a fixed-bed microreactor. Prior to the test, the catalyst (50 mg) was treated in air at 300 °C for 1 h. According to our unpublished Fourier transform infrared (FTIR) results, the capping molecules of lysine were removed below 250 °C during this activation stage. After cooling to room temperature, a reactant gas containing 1% CO in air was passed through the catalyst bed. The outlet gas was analyzed on-line with a GC (Shimadzu-14B). For catalytic measurement at high gas hourly space velocity (GHSV) (400 000 h-1), 10 mg of the Au/R-Fe2O3 catalyst was diluted with 20 mg of R-Al2O3 prior to the catalytic run. The morphology and size of the products were observed employing a transmission electron microscope (TEM, Tecai TF20 Super Twin, 200 kV). Powder X-ray diffraction (XRD) analysis was conducted on a Bruker D8 Advance X-ray diffractometer with CuKR1 radiation, while the thermal analysis (TGA) was performed on a SDT 2960 instrument in air flow. The surface area was measured at 77 K (liquid nitrogen) on a Quantachrome Autosorb6B surface area and pore size analyzer. Before the BET surface area measurement, the samples were degassed at 200 °C for several hours. The infrared spectra for dried samples were recorded on a Bio-Rad Excalibur spectrometer employing a KBr disc method. X-ray photoelectron spectroscopy (XPS) spectra were recorded in an ESCALAB 250 spectrometer and Al KR radiation was used as the X-ray source. The C1s peak at 284.5 eV was used as a reference for the calibration of the binding energy (BE) scale.
3. Results and Discussion The influence of the experimental conditions on the morphology and structure are studied. The as-prepared iron (31) Zhong, Z.; Patskovskyy, S.; Bouvrette, P.; Luong, J. H. T.; Gedanken, A. J. Phys. Chem. B 2004, 108, 4046. (32) Zhong, Z.; Luo, J.; Ang, T. P.; Highfield, J.; Lin, J.; Gedanken, A. J. Phys. Chem. B 2004, 108, 18119. (33) Zhong, Z.; Subramanian, A. S.; Highfield, J.; Carpenter, K.; Gedanken, A. Chem.sEur. J. 2005, 11, 1473. (34) Zhong, Z.; Chen, F.; Subramanian, A. S.; Lin, J.; Highfield, J.; Gedanken, A. J. Mater. Chem. 2006, 16, 489. (35) Zhong, Z.; Lin, J.; Teh, S. P.; Teo, J.; Dautzenberg, F. M. AdV. Funct. Mater. 2007, 17, 1402.
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Figure 1. Influence of the molar ratio of TEAOH/Fe on the morphology of the FeO(OH) product. The molar ratio was 3/1 (A); 6/1 (B and C); and 12/1 (D). The reaction was conducted at 80 °C for 72 h.
oxide product exists in the form of FeO(OH) (see XRD results in Figure 4). Figure 1 shows the TEM images of the FeO(OH) products prepared at various TEAOH/Fe molar ratios. When the TEAOH/Fe ratio was below 3/1, no nanorods were observed except for the conventional agglomerates of the small particles (Figure 1A). Increasing the TEAOH/Fe molar ratio to 6/1 led to the near 100% formation of the FeO(OH) nanorods (Figure 1B and C). The highresolution TEM (HRTEM) image shows that the nanorods were well organized and crystallized on the surface (Figure 1C). It should be pointed out that little influence was observed on the aspect ratio by varying the TEAOH/Fe ratio. The impact of the reaction temperature and time on the morphology of the FeO(OH) product is shown in Figure 2. The formation of the FeO(OH) nanorods is a fast process. The initial suspension was a brown slurry composed of very small particles of Fe hydroxide (Figure 2A), but the FeO(OH) nanorods were formed after reaction at 80 °C for 0.5 h or even at temperatures as low as 40 °C (Figure 2C). These FeO(OH) nanorods are about 7-10 nm in diameter and 150200 nm in length. Usually, several nanorods bundle together due to the hydrophobic interaction. It is believed that the (Et)4N+ cations (dissociated TEAOH molecules) can adsorb on the iron oxide hydroxide particles. Thus, the ethyl groups will protrude out from these surfaces, causing them to be hydrophobic.29,30 The increase in the reaction temperature to 200 °C leads to a significant increase in diameter to 2050 nm (Figure 2D). In order to understand the reason for this change, two control experiments were conducted: two samples were prepared at 80 and 110 °C, respectively (Figure 3A and B). The nanorods prepared at 80 °C still had small diameters (Figure 3A). However, for the sample prepared at 110 °C, the diameters of the nanorods increased to 20-50 nm. Upon a closer observation, it was found that some of the large nanorods were actually composed of several small nanorods (Figure 3B). It is therefore clear that the increase
Zhong et al.
Figure 2. Impacts of the reaction time and temperature on the morphology of the final FeO(OH) product. In A and B, the TEAOH/Fe ratio was 12/1 and the reaction temperature was 80 °C, but the reaction time was 0.0 h in A and 0.5 h in B. In C and D, the molar ratio of TEAOH/Fe was the same as that in A and in B and the reaction time was 24 h, but the reaction temperature was 40 °C in C and 200 °C in D.
Figure 3. Several FeO(OH) nanorods merged into one larger FeO(OH) nanorod upon reactions at higher reaction temperatures or longer reaction time. In A, the reaction was conducted at 80 °C for 15 h, and in B, it was at 110 °C for 15 h. The molar ratio of TEAOH/Fe was 6/1.
in diameter is due to the merging of several smaller nanorods into a larger one. At high reaction temperatures, the adsorbed TEAOH molecules on FeO(OH) particles are probably reorganized or even gradually removed, thus enabling the merging process. The reaction conducted at a middle temperature such as 110 °C provides the possibility to detect intermediates of the merging process. Thermogravimetric analysis-differential thermal analysis (TGA-DTA), XRD, and FTIR were used to characterize the as-prepared FeO(OH) and its derived products calcined at various temperatures (Figures 4-6). The as-prepared iron oxide product was in the form of FeO(OH) (JCPDS-IC-00003-0249), and the calcined sample at 300 °C was identified as R-Fe2O3 by comparing its XRD pattern with that of the Fluka R-Fe2O3 (Figure 4, upper line). In the TGA-DTA curve, the total weight loss was 12.8% up to 300 °C (Figure 5), and two peaks at 71 and 265 °C were observed. This weight loss is a little higher than that of the weight loss from FeO(OH) to R-Fe2O3 (10.1%), indicating the existence of the TEAOH molecules in the FeO(OH) product. The
Synthesis of Porous R-Fe2O3 Nanorods
Figure 4. XRD results for the as-prepared FeO(OH) dried at 100 °C that was previously shown in Figure 3A and its derived products calcined at various temperatures.
Figure 5. TGA-DTA curves for the as-prepared FeO(OH) dried at 100 °C that was previously shown in Figure 3A.
Figure 6. FTIR results for the as-prepared FeO(OH) dried at 100 °C that was previously shown in Figure 3A and its derived products calcined at various temperatures.
existence of the TEAOH molecules in the FeO(OH) product was further confirmed by the FTIR spectra (Figure 6), which showed a weak C-H vibration at 2981 cm-1 for the samples calcined below 250 °C. The strong vibrations at 3144, 895, and 796 cm-1 are the characteristic vibrations in the FeO-
Chem. Mater., Vol. 19, No. 19, 2007 4779
(OH) product.36 These peaks disappeared after calcination at 250 °C, coinciding with the TGA-DTA results, in which a max peak located at 264.5 °C was observed. At the same time, we observed a sharp peak at 448 cm-1 for the sample calcined at 250 °C (Figure 6), indicating the formation of the R-Fe2O3 phase.37 Therefore, both the TGA-DTA and the FTIR results reveal that the TEAOH molecules existed in the FeO(OH) product. The TEAOH molecules could facilitate the hydrolysis of Fe(NO3)3 and bind with the iron oxide hydroxide particles as well. However, during the hydrothermal treatment and the washing procedure, most of the TEAOH molecules were removed. This is consistent with the observation in the synthesis of silicate using TEAOH as the structure director.29,30 Differing from Bruce’s method28 in the synthesis of mesoporous iron oxide, the R-Fe2O3 phase obtained here is free of contamination after the calcination at 300 °C. During the reaction, the iron hydroxide particles capped with structure director molecules probably first assembled into micellelike structures,29 which then led to the formation of the FeO(OH) nanorods. For the formation of the FeO(OH) nanorods, both the molar ratio of TEAOH/Fe and the TEAOH concentration are important. The former should be higher than 3, and the latter should be above 9 wt %. In one of our experiments, instead of using a 20 wt % TEAOH aqueous solution, we used a 9 wt % TEAOH aqueous solution. Even though the molar ratio of TEAOH/Fe was increased to 6, we still failed to obtain FeO(OH) nanorods. It is well-known that usually there is a minimum concentration required for the formation of certain micelles;29b thus, most probably a micelle mechanism was involved in the synthesis. Other important evidence for the formation of the micelles is that, for the sample structure director, the length of all prepared FeO(OH) nanorods was in the same range under various reaction conditions, e.g., at various TEAOH/ Fe ratios and reaction times (see the Supporting Information SI-1). This is different from the cases for the formation of CuO, Fe2O3, and TiO2 nanorods by assembly of the small particles one by one, in which the length of the formed nanorods always increases with the reaction time.27,38,39 Another role of the TEAOH molecules is to facilitate the hydrolysis of Fe(NO3)3. However, it should be pointed out that even in the absence of the TEAOH molecules, Fe(NO3)3 still can be hydrolyzed, although no nanorods are formed under the same hydrothermal conditions.27 The TEAOH molecules probably also promoted the formation of R-FeO(OH), but the reason is still not clear. Further calcination of the prepared 1D FeO(OH) is supposed to create some pores in the nanorods by removal of the residual TEAOH molecules and OH groups.29 Figure 7A presents the TEM image of the sample calcined at 300 °C. Compared to the smooth surface of the sample without (36) Socrates, S. Infrared Characteristic Group Frequencies, 2nd ed.; John Wiley & Sons: New York, 1994; p 74. (37) Cornell, R. M.; Schwermann, U. The Iron Oxide Structure, Properties, Reactions, Occurrence and Uses; VCH: Weinheim, 1996. (38) Zhong, Z.; Ang, T. P.; Luo, J.; Gan, H.; Gedanken, A. Chem. Mater.. 2005, 17, 6814. (39) Zhong, Z.; Chen, F.; Ang, T. P.; Han, Y.; Lim, W.; Gedanken, A. Inorg. Chem. 2006, 45, 4619.
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Figure 7. TEM images for the porous R-Fe2O3 support and the supportedAu catalysts. In A, the FeO(OH) product was calcined at 300 °C for 0.5 h, and in B, Au particles were supported on the porous R-Fe2O3. The image in C was the supported-Au catalyst shown in B but after the reaction. The big particles were R-Al2O3 used to dilute the catalyst in the reaction. The image in D was the supported-Au catalyst on Fluka R-Fe2O3 (black dots indicated the Au particles in C and D). The FeO(OH) product was prepared at 80 °C for 10 h with a molar ratio of TEAOH/Fe of 6/1.
annealing (Figures 1-3), the annealed sample (Figure 7A) possessed a lot of pores within the R-Fe2O3 nanorods, while still maintaining its 1D morphology. In Figure 7A, a porous R-Fe2O3 nanorod is marked by two dotted lines. The pores are 1-5 nm in size and are slightly smaller than the diameter of the nanorods (it can be seen more clearly in a higher magnification image in the Supporting Information SI-2). Different from the pore structures in TiO2 nanotubes and carbon nanotubes, these pores are open to the outer surface and almost isolated from each other. Thus, they should be very accessible to the small Au particles. After the calcination, the measured BET surface area increased markedly to 143 m2/g from the previous value of 30 m2/g for the uncalcined but degassed sample (Figure 8A). Degassing of the uncalcined sample at 200 °C still could not remove all of the residual TEAOH molecules and the OH groups from the dried 1D FeO(OH) product. Calcination at 300 °C could, however, achieve the complete removal (Figures 5 and 6). Deposition of the Au particles on the porous 1D R-Fe2O3 was carried out by employing our invented method, in which a capping reagent and ultrasonic irradiation were applied.31-35 The capping reagent (lysine) can effectively prevent the aggregation of the Au colloids and thus maintain their small size,32 while the sonication can facilitate the deposition and insertion of the small Au particles into the pores.40-42 As observed, most of the deposited Au particles were in the (40) Zhong, Z.; Prozorov, T.; Felner, I.; Gedanken, A. J. Phys. Chem. B 1999, 103, 947. (41) Gedanken, A.; Tang, X.; Wang, Y.; Perkas, N.; Koltypin, Y.; Laddau, M. V.; Vradman, L.; Herskowitz, M. Chem.sEur. J. 2001, 7, 4546. (42) Perkas, N.; Zhong, Z.; Chen, L.; Besson, M.; Gedanken, A. Catal. Lett. 2005, 103, 9.
Figure 8. Isotherms of N2 adsorption/desorption (A) and pore volume changes (B) before creating pores (bottom curves), after creating pores (top curves), and after deposition of the Au particles in the pores (middle curves). The pore size created in the 1D nanostructure is in the range of 1.0-5.0 nm and peaked at ca. 2.1 and 4.0 nm, respectively.
range of 1-5 nm (Figure 7B). Even after the reaction, there was little change on the Au particle size (Figure 7C). Interestingly, the surface area of the support catalyst decreased to 48 m2/g from the previous value of 143 m2/g after the deposition (Figure 8A). Changes in the pore volume are reflected in Figure 8B for the samples before and after the calcination, as well as for that of the calcined sample and after further deposition of the Au particles. The degassed only, uncalcined sample has small pores that peaked at 2.4 and 3.9 nm, respectively. After calcination, the pore volume increased markedly for the pores centered at 2.1 and 4 nm, respectively (Figure 8B). Further deposition of the Au particles led to the disappearance of the pore at 4 nm and a significant decrease in the Dv value for the pore centered at 2.1 nm, indicating that most of the small Au particles were housed or half-buried in these pores. As indicated in the literature, the Au particle size of 2-4 nm is critical for the high catalytic activity of the supported-Au catalysts. For comparison, we prepared an Au catalyst supported on the commercially available R-Fe2O3 (Fluka, shown in Figure 7D). It is seen that very small Au particles were deposited on the
Synthesis of Porous R-Fe2O3 Nanorods
outer surface of the Fluka R-Fe2O3 particles, as these particles have hardly any small pores. It should be pointed out that the Au particle size distributions are similar in the two cases. Recently, there are several reports published about the Au particles being deposited on some metal oxides nanorods.43-46 For example, Idakiev et al.43 deposited Au particles on TiO2 nanotubes by the deposition-precipitation method for the water-gas shift reaction, Gao et al.44 deposited Au particles on CeO2 and Pr6O11 nanorods with an Au particle size above 8 nm, and Shen et al. and Zhong et al.46 prepared Al2O3 nanorods and deposited very small Au particles on them. But, there is no evidence that the Au particles with sizes in the range of 2-4 nm were inserted in the hollow channels of the supports in these reports, unlike what we have achieved here. The catalytic performance of the supported-Au catalysts for CO oxidation was tested. The supported-Au catalyst on the porous R-Fe2O3 nanorods has a very high catalytic activity toward CO oxidation. Complete removal of CO at 30 °C was realized for a 3 % Au/R-Fe2O3-nanorod catalyst at a GHSV of 25 500 h-1. In order to eliminate the influence of the diffusion limitation, we lowered the Au loading in the catalyst to 0.5 wt % and measured the catalytic activity at a GHSV above 400 000 h-1 at 30 °C. The measured specific activity was 1.11 × 10-3 molCO‚g-1Au‚S-1 for the 0.5 wt % Au/R-Fe2O3-nanorod catalyst (Cat-1), which is close to the very high activity in the literature,22 while the specific activity for the 0.5 wt % Au/R-Fe2O3-Fluka (Cat-2) was 3.36 × 10-4 molCO‚g-1Au‚S-1 at 30 °C, only 1/3 the value of Cat-1. Under the same conditions, we tested the reference 1 wt % Au/R-Fe2O3 purchased from the World Gold Council (WGC), and its specific activity was ca. 4.87 × 10-4 molCO‚g-1Au‚S-1. Thus, the insertion of the very small Au particles in the pores enhances the catalytic performance of the Au catalysts, probably because of an increased contact between the Au particles and the support.14 One of the TEM images of the reacted catalyst is shown in Figure 7C. It is seen that after the reaction, there is almost no change in Au particle size. The measured XPS spectra are shown in Figure 9. In the two catalysts, the measured binding energies are lower than that (84.1 eV) of the metallic Au, indicating a slight negatively charged state of the Au particles. This is consistent with Goodman and Rousset’s observations,5-8,47 though Hutchings et al. and Deevi et al. observed Au cation species in the Au/Fe2O3 catalysts prepared by the coprecipitation (CP) method and the DP method, independently.48,49 In addition, compared to that of Cat-2, the binding energy of (43) Idakiev, V.; Yuan, Z. Y.; Tabakova, T.; Su, B. L. Appl. Catal. A 2005, 281, 149. (44) Huang, P. X.; Wu, F.; Zhu, B. L.; Gao, X. P.; Zhu, H. Y.; Yan, T. Y.; Huang, W. P.; Wu, H.; Song, D. Y. J. Phys. Chem. B 2005, 109, 19169. (45) Huang, P. X.; Wu, F.; Zhu, B. L.; Li, G. R.; Wang, Y. L.; Gao, X. P.; Zhu, H. Y.; Yan, T. Y.; Huang, W. P.; Zhang, S. M.; Song, D. Y. J. Phys. Chem. B 2006, 110, 1614. (46) (a) Shen, S. C.; Chen, Q.; Chow, P. S.; Tan, G. H.; Zeng, X. T.; Wang, Z.; Tan, R. B. H. J. Phys. Chem. C 2007, 111, 700. (b) Han, Y. F.; Zhong, Z.; Kanaparthi, R.; Chen, F. X.; Chen, L. W. J. Phys. Chem. C 2007, 111, 3163. (47) Arrii, S.; Morfin, F.; Renouprez, A. J.; Rousset, J. L. J. Am. Chem. Soc. 2004, 126, 1199.
Chem. Mater., Vol. 19, No. 19, 2007 4781
Figure 9. XPS spectra of Au4f and Fe2p for the 0.5 wt % Au/R-Fe2O3nanorod (Cat-1) and 0.5 wt % Au/R-Fe2O3-Fluka (Cat-2) catalysts.
Au4f in Cat-1 is ca. -0.25 eV shifted, though it is not very significant. There is probably a stronger interaction between the Au particle and the catalyst support in Cat-1 than in Cat2, because in Cat-1 many small Au particles are housed in the pores, thus having a closer contact with the support. This is possibly the reason for the higher catalytic activity for CO oxidation in the first catalyst.3,15 4. Conclusion In summary, we have successfully prepared porous R-Fe2O3 nanorods by the hydrothermal method using TEAOH as the structure director for the first time. The reaction temperature and time and the molar ratio of TEAOH/Fe influence the morphology of the product. The as-prepared product is R-FeO(OH) nanorods, and it can be converted to porous R-Fe2O3 nanorods via calcination at 300 °C. The created pore size is mainly distributed in the range of 1-5 nm and is centered at ca. 2.1 and 4.0 nm, respectively. The BET surface area measurement confirms that the small Au particles are inserted in these pores. The prepared Au catalyst (48) Khoudiakov, M.; Gupa, M. C.; Deevi, S. Appl. Catal. A 2005, 291, 151. (49) Hutchings, G. J.; Hall, M. S.; Carley, A. F.; Landon, P.; Solsona, B. E.; Kiely, C. J.; Herzing, A.; Makkee, M.; Moulijn, J. A.; Overweg, A.; Fierro-Gonzalez, J. C.; Guman, J.; Gates, B. C. J. Catal. 2006, 242, 71.
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exhibits a higher catalytic activity toward CO oxidation than the Au catalyst supported on a commercially available R-Fe2O3 Fluka, because of an enhanced interaction between the Au particles and the porous R-Fe2O3 support. Acknowledgment. This research was supported by the Agency for Science, Technology and Research in Singapore (A-STAR, ICES04-414001). Z.Z. thanks Dr. Frits M. Dautzenberg for his helpful advice, Ms. Z. Wang for measuring the
Zhong et al.
XPS spectra, and Drs. P. K. Wong, Armando Borgna, and Keith Carpenter for their kind support of this project. Supporting Information Available: TEM images of 1D FeO(OH) formed at 80 °C with various raction times. We also present HRTEM images of porous and 1D R-Fe2O3. This material is available free of charge via the Internet at http://pubs.acs.org. CM071165A
