Incorporation of platinum nanoparticles in ordered mesoporous carbon

Incorporation of platinum nanoparticles in ordered mesoporous carbon

Journal of Colloid and Interface Science 305 (2007) 204–208 www.elsevier.com/locate/jcis Note Incorporation of platinum nanoparticles in ordered mes...

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Journal of Colloid and Interface Science 305 (2007) 204–208 www.elsevier.com/locate/jcis

Note

Incorporation of platinum nanoparticles in ordered mesoporous carbon Kjell Wikander a,∗ , Ana B. Hungria b , Paul A. Midgley b , Anders E.C. Palmqvist a,c , Krister Holmberg a , John M. Thomas b,d a Division of Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers University of Technology, SE-412 96 Göteborg, Sweden b Department of Materials Science and Metallurgy, University of Cambridge, Pembroke Street, Cambridge CB2 3QZ, UK c Competence Centre for Catalysis, Chalmers University of Technology, SE-412 96 Göteborg, Sweden d Davy-Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albermarle Street, London W1X4BS, UK

Received 26 August 2006; accepted 28 September 2006 Available online 5 October 2006

Abstract Platinum nanoparticles were incorporated within the pore system of ordered mesoporous carbon (OMC) by impregnating the carbon with a water-in-oil (w/o) microemulsion containing dissolved platinum salt followed by reduction of the platinum ions in situ inside the carbon pore system. The procedure provides preparation of metallic nanoparticles from hydrophilic precursors inside the hydrophobic carbon support structure with simultaneous control of the maximum metal particle size. Electron tomography was used to verify the presence of platinum nanoparticles inside the carbon material. © 2006 Elsevier Inc. All rights reserved. Keywords: Platinum; Nanoparticles; Microemulsion; Mesoporous; Carbon; Electron tomography

1. Introduction During recent years there has been substantial progress in the field of nanoparticle synthesis and a variety of methods have been developed. Among these, the water-in-oil (w/o) microemulsion method, developed during the 1980s [1–3], have shown to be useful for preparation of nanoparticles of single metals, e.g. noble metals such as Pt [4,5] and Ag [6,7] but also transition metals such as Cu [8], as well as bi-metallic alloys [9,10] with good control of the nanoparticle size and shape. To keep the nanoparticles dispersed after synthesis they are often deposited on a high surface area support material that is added to the nanoparticle dispersion [5]. In the case when the support is an ordered mesoporous material one drawback with this procedure is that the nanoparticles are in the same size regime as the pore openings of the support. Hence, for an ordered mesoporous carbon (OMC) support the nanoparticles have low probability to enter the pore system and instead become mainly deposited on the exterior of the support * Corresponding author. Fax: +46 31160062.

E-mail address: [email protected] (K. Wikander). 0021-9797/$ – see front matter © 2006 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2006.09.077

particles [11]. Further, since carbon materials are highly hydrophobic synthesis of metallic nanoparticles from hydrophilic precursors inside the carbon pores requires a medium that is able to wet the OMC pore walls and simultaneously can introduce the hydrophilic precursor to avoid that the metal particles deposit solely on the carbon exterior. Studies of the introduction of platinum nanoparticles in OMC materials have been done using incipient wetness techniques, where the metal precursor was introduced via an acetone solution and later reduced within the OMC structure [12]. In an alternative method an aqueous solution of the metal salt was introduced to the ordered mesoporous silica template prior to formation of the OMC [13]. In the work presented here a new approach is described for synthesis of platinum nanoparticles in situ inside high surface area OMC material by impregnation of the hydrophobic OMC particles with a water-in-oil (w/o) microemulsion containing dissolved platinum salt in its water pools. During the subsequent reduction reaction the platinum ions form platinum nanoparticles in situ in the OMC material. The great advantage with the w/o microemulsion is that it wets the entire carbon support surface and at the same time introduces nanosized water droplets containing dissolved platinum salt. Further, the wa-

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ter droplets act as nanoreactors and provide size control to the formed platinum nanoparticles. After finished reduction reaction and washing steps electron tomography is employed to evidence the successful formation of platinum nanoparticles and their distribution throughout the OMC support material [14–16]. 2. Materials and methods 2.1. Materials Chemicals used as-received from Aldrich were 1-butanol (99%), HCl (concentrated, 37 M), tetraethyl ortosilicate (TEOS, 98%), p-toluenesulfonic acid (99%), furfuryl alcohol (99%), H2 PtCl6 (99%), sodium bis(2-ethylhexyl)sulfosuccinate (AOT, 98%), n-heptane (p.a.), NaBH4 (99%), tetrahydrofuran (THF, 99%), ethanol (s.p.). HF (48% ) was from Riedel–de-Haën and Pluronic P123 (block copolymer, EO20 PO70 EO20 ) was from BASF Corp. (Mount Olive, New Jersey). A 5 wt% Pt/C reference material was from ETEK. Deionized water was used in all cases. 2.2. Ordered mesoporous carbon synthesis A cubic ordered mesoporous silica (OMS) material was prepared according to the published synthesis method of KIT-6, with 100 ◦ C as the hydrothermal treatment temperature [17]. The corresponding carbon replica was synthesized using furfuryl alcohol as carbon precursor. Typically 0.73 g of the prepared and calcined OMS was immersed in 0.5 M p-toluenesulfonic acid (EtOH) for 1 h at RT after which the material was collected and washed with a small volume of ethanol before drying at 80 ◦ C in air [18]. A volume of furfuryl alcohol equal to the accessible OMS pore volume, as measured using N2 sorption, was then added to the silica, impregnating its pore system, while the sample was thoroughly mixed with a spatula. The impregnated sample was kept at 80 ◦ C in air for 2 h enabling the carbon precursor to polymerize. The polymerized carbon precursor network was then carbonized in situ in the silica pores under flowing N2 by slow (2 ◦ C/min) temperature increase from RT to 800 ◦ C where it was kept for 2 h before cooling down to RT. The obtained silica–carbon composite was immersed in HF (conc) for 12 h to remove the silica network. The remaining carbon material was filtered off, washed with large quantities of water and dried at 80 ◦ C. 2.3. Incorporation of platinum nanoparticles into the ordered mesoporous carbon The diameter of the water droplets in the w/o microemulsion is governed by the water-to-surfactant molar ratio, W , and affects the size of the nanoparticles formed inside it [2,19]. Here, a (w/o) microemulsion was prepared by mixing deionized water, n-heptane, and AOT in the weight ratio 10:60:30, which corresponds to a W of approximately 8. Hexachloroplatinic acid, H2 PtCl6 , was used as platinum precursor and sodium borohydride, NaBH4 , acted as reducing agent in the

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reaction. The reactants were dissolved in separate microemulsions having the same water, oil and surfactant composition. An initial platinum salt concentration of 0.125 M was used in the water solution and the initial NaBH4 concentration in water was 0.625 M. The platinum salt-containing microemulsion was added to the OMC powder in an amount to completely fill the pore volume of the OMC. The mixture was thoroughly mixed by means of a spatula giving rise to a wet powder, which was kept in a closed vessel. After 2 h of equilibration 0.20 ml of the NaBH4 -containing microemulsion was added and the reduction reaction was allowed to proceed over night. THF was finally added to the material to destabilize the microemulsion system by removal of the surfactants and deposition of the nanoparticles. The Pt/OMC was subsequently filtered and dried at room temperature. 2.4. Characterization Nitrogen adsorption and desorption isotherms were recorded at −196 ◦ C using a Micromeritics Tristar on the OMS and OMC materials that previously had been dried at 200 ◦ C in vacuum for 2 h to remove adsorbates. Pore size distributions (PSDs) of the materials were calculated from the nitrogen adsorption branch using the BJH method [20]. The specific surface area was calculated with the BET method [21] using the isotherm data from the relative pressure range of 0.05–0.20. TEM imaging of all the prepared materials was done with a JEOL JEM-1200 EX II operated at 120 kV. The TEM specimens were prepared by dispersing a small sample amount in ethanol and placing one drop of the dispersion on a holey carbon coated copper grid (3.0 mm, 300 mesh, Pelco) and allowing the solvent to evaporate. Imaging and elemental analysis of the OMC material was performed to determine the carbon, silicon, and oxygen content using a Leo Ultra 55 FEG SEM equipped with an Oxford Inca EDX system operated at 20 kV with a WD = 10 mm (EDX). Before introduction into the SEM vacuum chamber a small sample amount was fixed onto a stainless steel sample holder by means of silver glue. The distribution of platinum nanoparticles inside the OMC pores was determined using electron tomography. High angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) tomography was performed on a 200 kV FEI Tecnai F20 STEM/TEM electron microscope. Specimens were prepared by depositing the particles onto a copper grid (3.0 mm, 300 mesh, Agar) supporting a holey carbon film. To minimize the possibility of contamination during imaging acquisition, the powder was deposited directly on the grid (after being heated on a hot plate at 100 ◦ C for 1 h) eliminating the need for any solvent. A series of HAADF STEM images was acquired between −70◦ and 74◦ every 2◦ . Image alignment and reconstructions were performed using the FEI software Inspect 3D and visualized using Amira. The XPS experiments were performed on a Perkin-Elmer PHI 5500 using non-monochromatic MgKα radiation. The samples were first analyzed by a wide-scan analysis of the 0– 1100 eV region of binding energy and then by a multi-scan

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Fig. 1. (a) Nitrogen sorption isotherms for the OMS template and the OMC replica. Both materials show Type IV hysteresis. (b) The BJH pore size distribution of the OMS and OMC materials is centered around 7.7 and 3.1 nm, respectively. (c) TEM micrograph of the OMS material, (d) SEM micrograph of the OMC material.

analysis of the Pt4f and C1s regions. The relative peak intensities of the reference Pt/C material were used in the calculation of the platinum loading in the Pt/OMC material. 3. Results and discussion 3.1. Preparation of ordered mesoporous carbon The measured nitrogen adsorption–desorption isotherms in Fig. 1a show type IV hysteresis and Fig. 1b confirms that both the OMS and OMC materials have narrow pore size distributions in the mesoporous range. The OMS material has a BET surface area of 777 m2 /g, a BJH pore volume of 1.04 cm3 /g, and a narrow pore size distribution centered around 7.7 nm while the OMC replica has a BET surface area of 1623 m2 /g, a BJH pore volume of 1.49 cm3 /g, and a pore size distribution centered around 3.1 nm. The high structural order of the OMS and OMC materials is evidenced with TEM and SEM, respectively, as illustrated in Figs. 1c and 1d. The OMC material was free from silica as shown with SEM-EDX. The mesoorder of the OMS and OMC materials was confirmed by SAXS analysis, which showed a well-resolved (211) peak corresponding to d-values of 94.9 and 85.7 Å, respectively, see supporting information. 3.2. Incorporation of platinum nanoparticles in the ordered mesoporous carbon The platinum loading in the prepared Pt/OMC material was measured to 1.2 wt% with SEM-EDX in good agreement with

the added amount of platinum salt in the w/o microemulsion. The electron tomography shows that there is a fairly homogeneous distribution of platinum nanoparticles throughout the OMC material. Fig. 2a shows a single HAADF STEM image from the tomography tilt series at 0◦ where a significant number of well-dispersed nanoparticles can be seen, each with a diameter of ∼3 nm in agreement with earlier BF TEM results (not shown here). There are also some larger particles, which are likely aggregates of the smaller ones. Inspection of slices through the tomographic reconstruction of the tilt series shows that the nanoparticles are distributed both at the interior and the exterior surfaces of the OMC material. Two illustrative pictures are shown in Figs. 2b and 2c. Fig. 2d shows the full 3D tomographic reconstruction viewed from different angles (∼60◦ separation) about a common tilt axis. The nanoparticle distribution is more clearly seen in Movie 1 showing the complete 3D tomographic reconstruction, see supporting information. X-ray photoelectron spectroscopy (XPS) was used as a complement to the electron tomography measurements to estimate the platinum loading in the outermost part of the OMC particles. Comparing the relative area of the Pt4f and C1s peaks of the Pt/OMC material to those of the reference Pt/C material of known composition (5 wt% platinum, ETEK) shown in Fig. 3 the platinum loading of the Pt/OMC was found to be 1.1 wt%. This value is very close to the bulk composition measured with SEM-EDX and corroborates the even nanoparticle distribution observed with the electron tomography.

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Fig. 2. High-angle annular dark-field electron tomography obtained by scanning transmission electron microscopy. (a) Single HAADF STEM image acquired at 0◦ tilt. The impression of the mesoporous template can be seen by viewing at glancing angles parallel to the arrows labelled ‘m.’ (b) and (c) show two parallel 1-nm thick slices (separated by a depth of 35 nm) taken from the reconstructed tomogram. (d) Projections of the 3D reconstruction viewed at 60◦ intervals about a common tilt axis. The carbon surface is coloured blue, the platinum nanoparticles are red.

Fig. 3. XPS spectrum of the Pt/OMC material. The inset shows the comparison of the Pt4f peaks of the Pt/OMC and the reference Pt/C materials.

4. Conclusions In this study a new generic w/o microemulsion-based method for preparation and deposition of metallic nanoparticles from hydrophilic precursors in situ inside ordered mesoporous hydrophobic carbon has been demonstrated. By introducing platinum in the form of platinum ions dissolved in the water droplets of a w/o microemulsion to the porous material a high degree of pore penetration is achieved and the subsequent in situ

reduction and deposition leave elemental platinum nanoparticles deposited relatively evenly throughout the pore system of the OMC as evidenced by electron tomography. Acknowledgments The authors thank BASF Corp., Mount Olive, New Jersey, for donating Pluronic P123. Financial support from Mistra (The Swedish Foundation for Strategic Environmental Re-

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search)/Jungner Centre via the programme “Fuel Cells in a Sustainable Society” is greatly appreciated. A.E.C. Palmqvist thanks the Swedish Research Council for a senior researcher grant. P.A. Midgley thanks the Isaac Newton Trust for financial support; he thanks the Leverhulme Trust and the Royal Academy of Engineering for a Senior Research Fellowship. A.B. Hungria is funded by an EC Marie Curie Research Fellowship. Supporting information The online version of this article contains additional supporting information. Please visit DOI:10.1016/j.jcis.2006.09.077. References [1] M. Boutonnet, J. Kizling, P. Stenius, G. Maire, Colloids Surf. 5 (1982) 209. [2] M.P. Pileni, J. Phys. Chem. 97 (1993) 6961. [3] M.A. Lopez-Quintela, Curr. Opin. Colloid Interface Sci. 8 (2003) 137. [4] H. Härelind Ingelsten, R. Bagwe, A. Palmqvist, M. Skoglundh, C. Svanberg, K. Holmberg, D.O. Shah, J. Colloid Interface Sci. 241 (2001) 104.

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