Morphology of Nanostructured Platinum in Mesoporous

Nov 4, 2005 - dispersed Pt nanoparticles and nanonetworks are fabricated from ... rous silica HMM-1 of 1D channel system11 and Pt nanonetworks...
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J. Phys. Chem. B 2006, 110, 1638-1646

Morphology of Nanostructured Platinum in Mesoporous MaterialssEffect of Solvent and Intrachannel Surface Kuei jung Chao,* Yen po Chang, Ya chieh Chen, Angelia S. Lo, and Ting hao Phan Department of Chemistry, National Tsing-hua UniVersity, Hsinchu 30013, Taiwan ReceiVed: August 24, 2005; In Final Form: NoVember 4, 2005

An intrachannel surface of host silica was functionalized through the reaction of surface silanol groups with silanes to generate a monolayer of positively charged groups, and together with the strongly adsorbed and negatively charged PtCl62-, resulting in nanostructured platinum-mesoporous silica composites. The highly dispersed Pt nanoparticles and nanonetworks are fabricated from (CH3O)3Si(CH2)3N(CH3)3+Cl- functionalized mesoporous silica MCM-48 with H2PtCl6 in ethanol and water solvent, and characterized by PXRD, XAS, TEM, and N2 adsorption. The solvent of H2PtCl6 solution is found to affect the mobility of Pt precursors and the resulting morphology of nanostructured metallic Pt. The effect of the intrachannel surface properties on the incorporation and the morphology of nanostructured Pt on the deposition of Pt(NH3)4Cl2 and H2PtCl6 on Al-doped or C-coated mesoporous silica MCM-41 is also studied relative to that on pure silica MCM-41.

Introduction Nanostructured metals with controlled size and shape exhibit unique electronic, optical, magnetic, and catalytic properties relative to the bulk metal.1-6 For the nanostructured platinum, the interest mainly focuses on catalytic applications.2,6 Preparation of Pt nanobars, nanoparticles, and nanowires has been extensively studied through template synthesis or the host-guest method recently. The metal precursors can be either reduced or decomposed in the presence of organic capping reagents in solution7,8 and within nanochannels of inorganic porous substrates such as FSM-16, MCM-41, SBA-15, and MCM-48 mesoporous silicas.9-12 Such porous substrates possess 1D and 3D channels of uniformly sized mesopores with tunable pore size in the range 2-30 nm13,14 and have been employed as hosts of nanometals. Platinum nanoparticles and/or surfactant-coated Pt nanoparticles were also dispersed in reactant mixtures to produce mesoporous silica and found to play the role as template for self-organization of the silica matrix.15-17 A few papers have reported the formation mechanism of Pt nanowires in mesoporous silica HMM-1 of 1D channel system11 and Pt nanonetworks or nanobars in 3D cubic mesoporous silica MCM-4816 investigated by TEM. They suggested that the migration of Pt species plays the key role for the fabrication of platinum nanowires and nanonetworks in addition to the structures of host mesoporous silicas. In this study, anionic PtCl62- complexes in either ethanolic or aqueous solution of H2PtCl6 were ion-exchanged with Clions of (CH3O)3Si(CH2)3N(CH3)3+Cl- (TPTAC) functionalized mesoporous silica MCM-48, and the materials underwent subsequent H2 reduction to produce Pt nanoparticles and nanonetworks. The solvent factor on the migration of an ionic platinum species on the positively charged intrachannel surface as well as on the morphology of nanostructured Pt fabricated into mesochannels was illustrated. An alternative method was used to influence the migration of metal precursors in the mesochannels, and the intrachannel surface of MCM-41 silica * To whom correspondence should be addressed. Phone: 886-3-5715131 ext 33377. Fax: 886-3-572-0964. Email: [email protected].

was modified by Al doping and C coating before metal incorporation through ion exchange and incipient wetness of Pt(NH3)4Cl2 and H2PtCl6. Characterization techniques such as nitrogen adsorption, powder X-ray diffraction (PXRD), extended X-ray absorption fine structure (EXAFS), and transmission electron microscopy (TEM) with electron diffraction (ED) were used to examine the structure, shape, and size of nanostructured platinum and the pore properties of mesoporous materials before and after metal incorporation. Experimental Section MCM-48 Synthesis. High-quality mesoporous silica MCM48 was synthesized following the literature procedure.19 A measured quantity (23.3 g) of colloidal silica Ludox HS-40 (39.5 wt % SiO2, 0.4 wt % Na2O, and 60.1% H2O; Du Pont) solution was preheated and stirred at 70 °C in an Erlenmeyer flask for 2 h. The 74 mL aqueous solution of 1 M NaOH was then slowly added into the preheated solution under stirring. The mixture was continuously stirred at 70 °C for 2 h, cooled to room temperature, and then stored in a polypropylene (PP) bottle to give a sodium silicate solution of 9.2 wt % SiO2, 2.4 wt % Na2O, and 88.4 wt % H2O as silica source. In a separate PP bottle, 9.5 g of cationic surfactant CTAB (C16H33N(CH3)3Br, Aldrich) and 1.7 g of neutral surfactant C12EO4(C12H25(OCH2CH2)4OH, Acros) were dissolved in 132 mL of deionized water and stirred at 40 °C for 30 min. The silica mix was quickly poured into the surfactant solution to give a solution with molar composition of 5 SiO2/1.25 Na2O/0.85 CTAB/0.15 C12EO4/400 H2O. The mixture was vigorously stirred in a PP bottle for 30 min, then heated at 100 °C for 48 h in a preheated oven. The mixture was cooled to room temperature and adjusted to pH ) 10 with acetic acid. The mixture was heated again at 100 °C in a preheated oven for another 48 h and cooled to room temperature. The precipitate was filtered, washed with 1500 mL deionized water and 250 mL ethanol, and then dried at 60 °C for 24 h in a preheated oven to give MCM-48-as. The MCM48-as was heated from 180 to 540 °C at a heating rate of 1 °C/min, held at 540 °C for 8 h, and then cooled to room temperature to give MCM-48-c. The functionalization was

10.1021/jp0547820 CCC: $33.50 © 2006 American Chemical Society Published on Web 01/05/2006

Morphology of Pt/MCM-41 and -48 carried out through the reaction of surface silanol groups with TPTAC silane (N-trimethoxysilylpropyl-N,N,N-trimethylammonium chloride, 50 wt % in methanol, Gelest) to produce a monolayer of positively charged propyltrimethylammonium (PTA+) groups on the intrachannel surface of MCM-48-m as developed in our group.12,20 Decreased IR absorption of silanol groups at 3750 cm-1 accompanied by the appearance of absorption peaks of C-H bonds (2900-3000 cm-1), C-N bonds (∼1490 cm-1), and Si-C bonds (∼1250 cm-1) indicated that the surface silanol groups were reacted with TPTAC and formed -O-Si(CH2)3N(CH3)3+ groups. Incorporation of Pt into MCM-48. For metal incorporation, 0.5 g of dried MCM-48-m was mixed with 19 mL ethanolic or aqueous solution of 0.1 M H2PtCl6 (98.5%, Showa) and stirred at room temperature for 12 h. The solid product was filtered, washed with some ethanol or deionized water, and then dried at room temperature for 5 d to give yellow Pt/MCM-48al-as and Pt/MCM-48wa-as samples, which were further reduced by heating at a ramping rate of 0.5 °C/min from room temperature to 300 °C and held at 300 °C for 5 h in H2 flow (100 mL/min) to produce dark brown Pt/MCM-48al-r and Pt/MCM-48wa-r. To obtain a silica-free Pt sample, a Pt/MCM-48wa-r sample was treated with 10 wt % hydrofluoric acid. The black-colored Pt sample was recovered by centrifugation and washing with ethanol, then it was suspended in ethanol and supported on a carbon-coated copper grid for TEM measurement. Si- and Al-MCM41 Synthesis. Mesoporous silica MCM41 was synthesized by the procedure described previously21 involving an aqueous solution of sodium silicate (Na2O ) 7.58.5%, SiO2 ) 25.5-28.5%; Merck) and cationic surfactant CTAB in the molar ratio of 1.0 SiO2/0.58 CTAB/215 H2O. After stirring the solution for 30 min, a sulfuric acid (H2SO4, Centrified) solution was added to the above solution to adjust the pH to 10. The resulting mixture was sealed in a PTFElined autoclave and then heated at 100 °C for 1 d. The solid product was washed with deionized water, centrifuged, and dried at 70 °C. To remove the organic template, it was further heated to 540 °C at a heating rate of 0.5 °C/min and held at 540 °C for 1 h under N2 flow (200 mL/min), followed by another 10 h at 540 °C under air (200 mL/min). Al-MCM41 was synthesized with a similar procedure by an addition of sodium aluminate (Al/Na ) 0.58, Wako) solution into the reactant mixture to give a molar ratio of 1.0 SiO2/0.05 Al/0.55 CTAB/80 H2O and was heated at 100 °C for 6 d. C-MCM41 Synthesis. The C-MCM41 material was synthesized following the carbonization procedure in NaY zeolite channels as described in the literature.22 The calcined SiMCM41 was dried under vacuum at 150 °C and then adsorbed furfuryl alcohol (98%, Riedel-deHae¨n) vapor at 40 °C for 12 h in a vacuum line. Later, 1 g of pretreated Si-MCM41 powder and 10 mL furfuryl alcohol were mixed and stirred for 7 d. The resulting solid was filtered, followed by washing with mesitylene (99%, TCI) to remove furfuryl alcohol on the external surface, and dried at 70 °C. The furfuryl alcohol in Si-MCM-41 was polymerized by heating at 80 °C for 24 h and then at 150 °C for 8 h, followed by carbonization at 700 °C for 3 h under N2 flow (200 mL/min) at ramping rate of 5 °C/min. Incorporation of Pt into Si- and Al-MCM41. The introduction of Pt into silicate and aluminosilicate-MCM41 materials was carried out following the procedure described in the literature.23 Before Pt introduction, the calcined Si-MCM41 sample was treated in an aqueous solution of 3 × 10-3 M NH4OH (33%, Riedel-deHae¨n), and the slurry was stirred for 1

J. Phys. Chem. B, Vol. 110, No. 4, 2006 1639 h, then the solid was recovered by filtration, washed with deionized water, and dried at 70 °C. A measured quantity (3 g) of NH4OH treated Si-MCM41 was ion-exchanged with 300 mL of aqueous solution of 0.06 g Pt(NH3)4Cl2‚H2O (99%, Strem) and stirred for 3 h at ambient temperature. The solid product was collected by filtration, washed with deionized water, and dried at 70 °C to get the as-synthesized form of 1.0 Pt/ Si-MCM41 sample. The ion-exchanged Pt precursor was oxidized by heating at 320 °C in a stream of O2 (1000 mL/ min) for 2 h at a ramping rate of 0.5 °C/min and then reduced with a stream of H2 (200 mL/min) at 400 °C for 2 h at a ramping rate of 1 °C/min. These treatments gave the reduced 1.0 Pt/ Si-MCM41 sample. The number 1.0 represents the weight percent of platinum on the dry MCM-41. Later, 1 g of reduced 1.0 Pt/Si-MCM41 was immersed by incipient wetness in an aqueous Pt(NH3)4Cl2 solution and dried at 70 °C to get the as-synthesized form of 10 Pt/Si-MCM41, of which the Pt precursor was oxidized and then reduced on the reduced form of 10 Pt/Si-MCM41 by the same procedures applied to the 1.0 Pt/Si-MCM41 sample. The introduction of Pt into Al-MCM41 was carried out following similar procedures24 as for dried and NH4OH-treated Si-MCM41 on Al-MCM41 sample to get the reduced forms of 1.0 Pt/Al-MCM41 and 10 Pt/Al-MCM41 samples. Incorporation of Pt into C-MCM41. The introduction of Pt into C-MCM41 was carried out following the procedure described in the literature for the Pt metal-filled carbon nanotubes.25 A measured quantity (0.45 g) of C-MCM41 was slurried in an ethanolic solution of H2PtCl6, then was stirred for 3 h at ambient temperature, heated at 80 °C to remove the ethanol, and dried at 70 °C to get the as-synthesized form of 23 Pt/C-MCM41 sample. The impregnated Pt precursor was reduced under H2 flow (200 mL/min) and heated from 70 to 400 °C at a heating rate of 1 °C/min, then held at 400 °C for 2 h on the reduced 23 Pt/C-MCM41 sample. The preparation methods are summarized in Table 1. X-ray Absorption Spectroscopy (XAS). XAS measurements were performed on the wiggler beamline BL17C of NSRRC, Taiwan, with storage ring energy of 1.5 GeV and a beam current between 100 and 200 mA. The XAS spectra were collected in transmission mode at the Pt LIII edge. The energies of the spectra were scanned from 200 eV below the Pt LIII edge (11 564 eV) to 1200 eV above the edge covering the XANES and EXAFS regions for analysis. Platinum foil was used as a reference and was measured simultaneously with the samples for energy calibration between scans. For the in situ XAS measurements, the as-synthesized sample was mounted on an in situ cell26 and heated under hydrogen flow, then cooled to room temperature for XAS measurements. After taking the X-ray absorption spectra of the in situ reduced sample, the in situ cell was opened under ambient conditions for more than 1 h. Another measurement was then carried out on the air-exposed sample. The UWXAFS 3.0 software package27 and the FEFF8 program28 were employed in EXAFS data fitting. Powder X-ray Diffraction (PXRD). PXRD measurements were carried out either on a MAC Science MXP18AHF XRD machine with a Cu KR radiation source (λ ) 1.54 Å) and a two-circle powder diffractometer or on the wiggler beamline BL17A of NSRRC, Taiwan, with the X-ray radiation wavelength equal to 1.333 Å. Transmission Electron Microscopy (TEM) with Electron Diffraction and N2 Adsorption Measurements. TEM micrographs and electron diffraction patterns were collected using a JEOL JEM-2010 electron microscope. Samples for TEM

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TABLE 1: Sample Designations and Preparation Methods for Pt/MCM-48 and Pt/MCM-41 Samples sample MCM-48-as MCM-48-c MCM-48-m Pt/MCM-48wa-as Pt/MCM-48-al-as Pt/MCM-48wa-r Pt/MCM-48al-r Pt/MCM-48wa-r-air Pt/MCM-48al-r-air Si-MCM41 1.0 Pt/Si-MCM41 10 Pt/Si-MCM41 Al-MCM41 1.0 Pt/Al-MCM41 10 Pt/Al-MCM41 hiPt/Al-MCM41 C-MCM41 23 Pt/C-MCM41

preparation method hydrothermal synthesis,a as-synthesized sample calcined sample surface functionalization using TPTACb Pt introduced via ion exchange with H2PtCl6 in waterc Pt introduced via ion exchange with H2PtCl6 in alcoholc reduced sample (H2, 300 °C, 5 h) exposed to air after reduction, RT hydrothermal synthesis using sodium silicate as silica sourcec,d Pt introduced via ion exchange with Pt(NH3)4Cl2 (aq) on NH4OH treated-Si-MCM41c,e incipient wetness with Pt(NH3)4Cl2 (aq) on 1.0 Pt/Si-MCM41c hydrothermal synthesis using sodium aluminate and sodium silicatec,d Pt introduced via ion exchange with Pt(NH3)4Cl2 (aq) on Al-MCM41c,f incipient wetness with Pt(NH3)4Cl2 (aq) on 1.0 Pt/Al-MCM41c incipient wetness repeatedly with Pt(NH3)4Cl2(aq) on 10 Pt/Al-MCM41c carbonization on Si-MCM41 c,g Pt introduced via impregnation of H2PtCl6 in ethanolc,h

Figure 1. Small-angle PXRD patterns of Pt/MCM-48 samples.

a ref 19. b ref 12, 20. c This study. d ref 21. e ref 23. f ref 24. g ref 22. ref 25. All Pt/MCM-41 samples are reduced in H2 flow at 400 °C for 2 h.

h

measurements were embedded in resin and ultra-microtomed into slices with thicknesses of ∼50 nm. Nitrogen sorption measurements at 77 K were carried on a Micromeritics ASAP 2010 system. The pore diameters of samples were determined by the BJH (Barrett-Joyner-Halenda) method on the basis of desorption branches of nitrogen isotherms. Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES). The samples were digested with mixed acids, and their elemental compositions Si/Al ratio and Pt content were determined by ICP-AES on Jarrell-Ash ICAP-9000. Results and Discussion XRD and TEM on Pt/MCM-48. In the small-angle PXRD patterns of Pt/MCM-48s and MCM-48-c or m (Figure 1), no significant change in peak positions was observed at 2θ ) 1-5°, indicating that the pore structure of MCM-48 remains unchanged after functionalization as well as after Pt incorporation. The PXRD intensities of mesoporous structure were found to increase after template removal and decrease after Pt incorporation; a likely explanation is the difference of the scattering power of the silicate wall and the pore filling materials as observed on MCM-41 via organic adsorption reported by Marker et al.29 In the high-angle region of PXRD, Pt/MCM-48s samples display four peaks typical of fcc crystalline Pt (Figure 2, inset). The diffraction peaks of XRD profile of Pt/MCM-48wa-r (prepared from an aqueous solution of 0.1 M H2PtCl6) are narrower than those of Pt/MCM-48al-r (prepared from an ethanolic solution of 0.1 M H2PtCl6) (Figure 2), which suggests that larger metal particles could be detected on the Pt/MCM-48wa-r sample. The Scherrer equation has been applied in estimating the average particle size by measuring the full widths at half-maximum

Figure 2. PXRD patterns (solid line) for Pt(111) and (200) diffraction peaks of Pt/MCM-48al-r (a) and Pt/MCM-48wa-r (b) with the deconvoluted (dotted line) and fitted (open cycle) peaks; and their high-angle PXRD patterns (inset) (NSRRC, Wiggler A, λ ) 1.327 Å).

(fwhms) of the XRD Pt(111) diffractions on both samples. The size of the platinum particle was calculated to be 3.2 nm (fwhm

Morphology of Pt/MCM-41 and -48

Figure 3. TEM images and SAD patterns of Pt/MCM-48al-r (a), Pt/ MCM-48wa-r (b) (scale bar ) 50 nm) - (e), (f) for magnification in a square of (a).

≈ 2.30°) and 2.6 nm (fwhm ≈ 2.90°) on Pt/MCM-48wa-r and Pt/MCM-48al-r, respectively. For a direct observation of the morphology and size of metallic Pt in Pt/MCM-48 samples, TEM measurements were performed. In TEM images of Pt/MCM-48al-r, the platinum nanoparticles with size of ∼3.0 nm were uniformly distributed in the channels of MCM-48 (Figure 3a,f). The electron diffraction pattern (Figure 3a(inset)) consisted of rings attributed from the randomly oriented Pt nanoparticles. ICP-AES analysis reveals a Pt content of ∼17.2 wt % on Pt/MCM-48al-r, which is higher than that (∼7.6 wt %) on Pt/MCM-48wa-r. For Pt/ MCM-48wa-r, the platinum metal is in the form of a nanonetwork in the channels of MCM-48, as shown in Figure 3b. The electron diffraction pattern of these Pt nanonetworks also consists of rings attributed from the randomly oriented and nanostructured Pt (Figure 3e). Interestingly, a closer look at this image shows that the Pt nanonetworks exhibit tripod morphology and reflect the Ia3d structure of MCM-48 pore (Figure 3ce). The ED patterns of the inset of Figure 3c,d even display single-crystal-like platinum nanonetworks formed in asymmetric bicontinuous channels of MCM-48. A TEM image of silicaremoved Pt isolated from Pt/MCM-48wa-r is shown in Figure 4 and indicates a nanonetwork structure. An EDX analysis of this Pt sample shows that almost all of the mesoporous SiO2 has been dissolved with 10 wt % HF. The observed peaks at 2.1, 9.4, and 11.1 keV are assigned to Pt M, LR, and Lβ lines,

J. Phys. Chem. B, Vol. 110, No. 4, 2006 1641

Figure 4. TEM image (a) and EDX pattern (b) of template-free Pt nanonetwork isolated from Pt/MCM-48wa-r.

respectively; and confirm the existence of Pt nanonetworks in Pt/MCM-48wa-r. The results above show the solvent effect of incorporating Pt in MCM-48 mesopores. The different surface tension possessed by the ethanol and water solution in mesochannels determines the mobility of platinum precursors in the ionexchange process followed by the reduction step. The platinum ions tend to form smaller particles in ethanol solution, while Pt nanonetworks have been obtained in the presence of water because of the migration of platinum ions on the surface being comparable to the reduction rate, and the reduced platinum metal in the channels tends to accumulate and give Pt tripods of high local concentration, especially at the intersections of channels. The requirement of water as solvent also was reported in the formation of platinum networks inside the nonfunctionalized cubic MCM-48 host30and photon-reduced platinum nanowires inside the channels of organic-inorganic mesoporous silica hexagonal HMM-1.11 Sakamoto et al.11 suggested that the proper migration of Pt ions might facilitate the growth of Pt nanoparticles to nanowires. EXAFS and N2 Adsorption on Pt/MCM-48. The Fourier transforms (FT) of k3-weighted EXAFS spectra and curve-fitting analyses of Pt/MCM-48wa and Pt/MCM-48al samples are shown in Figure 5. Figure 5b shows that as-prepared Pt/MCM48wa-as has one large peak derived from 5.1 Pt-Cl bonds with

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Figure 5. Fourier transforms of Pt LIII-edge k3-weighted EXAFS data for Pt foil (a), Pt/MCM-48wa-as (b), Pt/MCM-48al-r (c), Pt/MCM48wa-r (d), Pt/MCM-48al-r-air (e), and Pt/MCM-48wa-r-air (f). Solid lines and open circles are the experimental and fitted results, respectively. Note that the phase shifts were not corrected.

a bond length of 2.33 Å as listed in Table 2. It is suggested that the PtCl62- complex is adsorbed on MCM-48 without any covalent bonding between the metal precursor and the support. After reduction at 300 °C for 5 h under H2 treatment, the FT (EXAFS) spectrum of Pt/MCM-48wa-r shows one first-shell peak derived from 8.6 Pt-Pt bonds with a bond length of 2.77 Å (the value for Pt foil). Similarly, a Pt-Pt distance of 2.77 Å and N(Pt-Pt) ) 8.7 are calculated on the EXAFS spectrum of Pt/MCM-48al-r. Those results show that the platinum of Pt/ MCM-48wa-r and Pt/MCM-48al-r has been reduced to the metallic state. Using a spherical model with face-centered cubic structure gives average particle sizes of 2 ( 1 nm for Pt/MCM48wa-r and Pt/MCM-48al-r.

Figure 6. N2 adsorption isotherms (a) and pore size distributions (b) of MCM-48 and Pt/MCM-48 samples.

In addition, the accessibility of air to nanonetwork Pt was detected by the EXAFS of the Pt LIII edge on Pt/MCM-48war-air. It shows that nearly all the reduced Pt atoms remain in zero oxidation state after exposure to air at room temperature (Figure 5f). However, the reduced form of the Pt nanoparticles in Pt/MCM-48al-r was found to be partially oxidized during exposure to air at room temperature, as shown by the existence of a Pt-O coordination shell in Figure 5e. The curve-fitting analysis of the Pt/MCM-48al-r-air spectrum gives a Pt-Pt distance of 2.77 Å with N(Pt-Pt) ) 6.7 and a Pt-O distance

TABLE 2: Pt LIII Edge EXAFS Results of Pt/MCM-48 and Pt/MCM-41 Samples particle size (nm) sample

shell

Ra (Å)

Na

Pt foil Pt/MCM-48al-r Pt/MCM-48al-r-air

Pt - Pt Pt - Pt Pt - Pt Pt - O Pt - Cl Pt - Pt Pt - Pt Pt - Pt Pt - Pt Pt - Pt Pt - Pt Pt - Pt

2.78 2.77 2.77 2.03 2.33 2.77 2.77 2.76 2.77 2.75 2.75 2.77

12 8.7 6.7 0.9 5.1 8.6 8.6 8.3 10.5 6.2 9.1 10.5

Pt/MCM-48wa-as Pt/MCM-48wa-r Pt/MCM-48wa-r-air 1.0 Pt/Si-MCM41 10 Pt/Si-MCM41 1.0 Pt/Al-MCM41 10 Pt/Al-MCM41 23 Pt/C-MCM41

EXAFSb

TEM

PXRDc

2(1 1 ( 0.5

3.0

2.6

2 ( 1.1 2 ( 1.1 2.0 4.7 1.3 2.3 4.7

2.9-3.3

3.2

∼1-2 8 1 2 8 nm, and such sizes are considered to be too large to be located within the pore of the MCM-41 structure. Pt/Al-MCM41. Ion-exchange capacity could also be produced by incorporating aluminum into the wall of MCM-41.24 In this study, an aluminum-containing MCM-41 sample was prepared by adding an aqueous solution of sodium aluminate to the reactant mixture to give the Al-MCM41 sample with Si/Al ) 30 as detected by ICP-AES. It exhibits an ordered structure and pore size similar to that of Si-MCM41 as detected by PXRD (Figure 7) and TEM (Figure 10b) and listed in Table 3. The channel structure of Al-MCM41 is stable under platinum introduction through Pt(NH3)42+ ion exchange and impregnation as shown in TEM images of Figure 10e,f,h. The platinum clusters in Al-MCM41s were found as nanoparticles, nanobars, and nanowires depending on their content (Figure 10). The ionic platinum precursors on Pt/MCM41 samples can be considered as migrating relatively smoothly in “water” solvent and are similar to that on the Pt/MCM-48 sample (as has been described in the previous section) and aggregate together to produce nanobar and nanowires during reduction. N2 adsorption and TEM also indicate that a large portion of the 1D mesochannels of Al-MCM41 have been filled with nanobars and nanowires rather than nanoparticles. Pt/C-MCM41. Platinum was loaded by impregnation of a H2PtCl6 ethanolic solution. The TEM image of 23 Pt/ C-MCM41 (Figure 10i) does not reveal wirelike Pt metal, but

Morphology of Pt/MCM-41 and -48

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Figure 10. TEM images of Si-MCM41 (a), Al-MCM41 (b), C-MCM41 (c), 1.0Pt/Si-MCM41 (d), 1.0Pt/Al-MCM41 (e), 10Pt/ Al-MCM41 (f) 10Pt/Si-MCM41 (g), hiPt/Al-MCM41 (h), and 23Pt/ C-MCM41 (i).

pore structure, intrachannel surface properties, and the solvent used in dissolving the metal precursors. Most likely, the formation of anisotropic morphology as Pt nanowires or nanonetworks becomes the dominant process when the migration of ionic Pt precursors in the intrachannel void space is comparable to the reduction. The resulting shape and size of nanostructured metal could be controlled by tuning the diffusion of metal species in mesochannels during host-guest synthesis. Figure 9. TPR of Pt/Si-MCM41-as (a) and wide-angle PXRD patterns of reduced Pt/MCM-41 (Rigaku DMAX II, λ ) 1.54 Å) (b).

rather very fine particles of sizes either