Deposition of Platinum Nanoparticles, Synthesized in Water-in-Oil

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Langmuir 2002, 18, 1811-1818

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Deposition of Platinum Nanoparticles, Synthesized in Water-in-Oil Microemulsions, on Alumina Supports Hanna Ha¨relind Ingelsten,*,†,‡ Jean-Christophe Be´ziat,†,‡,⊥ Kristina Bergkvist,†,‡ Anders Palmqvist,†,‡ Magnus Skoglundh,†,‡ Hu Qiuhong,§ Lena K. L. Falk,| and Krister Holmberg†,‡ Competence Centre for Catalysis, Department of Applied Surface Chemistry, Department of Applied Physics, and Department of Experimental Physics, Chalmers University of Technology, SE-412 96 Go¨ teborg, Sweden Received July 16, 2001. In Final Form: November 30, 2001 Platinum nanoparticles were prepared in water-in-oil microemulsions and deposited on γ-alumina using two different methods. In the first method, the alumina support was added to the particle suspension and the microemulsion was subsequently destabilized by addition of tetrahydrofurane, whereby the particles were deposited on the alumina support. In the other method, the platinum nanoparticles were transferred to an aqueous solution were they were redispersed by a stabilizing surfactant prior to addition of the alumina support. The size of the microemulsion droplets and of the unsupported platinum particles was in the range of a few nanometers as measured by a dynamic light scattering technique (photon correlation spectroscopy). The size of the unsupported platinum nanoparticles and of the particles deposited on alumina was studied by transmission electron microscopy. Both methods for platinum particle deposition resulted in some degree of particle agglomeration, the first probably because of too-fast destabilization of the microemulsion and the second due to inefficient redispersion of the Pt particles when transferred to the aqueous phase. All samples investigated showed high catalytic activity for CO oxidation by oxygen. The highest activity was found for those samples prepared via the redispersion method where a relatively weak interaction was achieved between the redispersed Pt particles and the alumina.

Introduction Traditionally, supported catalysts have been produced by wet impregnation of the support material, using watersoluble metal salts, followed by calcination and reduction. This results in well-dispersed catalysts with high activity and good thermal stability. The particle size of the active phase is usually in the nanometer range but with a quite broad size distribution and a low degree of control over the particle size. This renders the interpretation of sizedependent catalytic phenomena very difficult. To achieve a deeper understanding of details in reaction mechanisms and kinetics of catalytic processes, well-defined catalysts in the nanometer regime are an indispensable tool. Several techniques have been investigated to prepare catalysts with better control over particle size, shape, and interparticle distance, for example, electron beam lithography,1-2 colloidal lithography,3 and spin-coating techniques.4 Another route to prepare nanosized metal particles is to use water-in-oil (w/o) microemulsions where a metal precursor is reduced to metallic particles in the water * To whom correspondence should be addressed. Tel: +46 31 772 29 59. Fax: +46 31 772 29 67. E-mail: hannahi@ surfchem.chalmers.se. † Competence Centre for Catalysis. ‡ Department of Applied Surface Chemistry. § Department of Applied Physics. | Department of Experimental Physics. ⊥ Present address: RENAULT - DIV - DIMat, Centre Technique de Lardy - CTL L16 1 41, 1 alle´e Cornuel, F-91510 Lardy, France. (1) Johansson, S.; Wong, K.; Zhdanov, V. P.; Kasemo, B. J. Vac. Sci. Technol., A 1999, 17, 297. (2) Wong, K.; Johansson, S.; Kasemo, B. Faraday Discuss. 1996, 105, 237. (3) Hanarp, P.; Sutherland, D.; Gold, J.; Kasemo, B. NanoStructured Materials 1999, 12, 429. (4) Gunter, P. L. J.; Niemantsverdriet, J. W.; Riberio, F. H.; Somorjai, G. A. Catal. Rev.sSci. Eng. 1997, 39 (1&2), 77.

pools. The particle size can be almost precisely controlled by an appropriate choice of the water-to-surfactant molar ratio.5-13 Further, the size distribution of the prepared particles is very narrow, with diameters ranging from a few to 20 or 30 nm,5-11 that is, the same particle size range as wet-impregnated catalysts. In some systems, it has been found that not only the size but also the shape of the particles can be affected using microemulsion templates.12-13 Important advantages, compared to other techniques for controlled nanoparticle preparation, are that the particles can be formed at atmospheric pressure and at room temperature and that large sample volumes relatively easily can be obtained. In the literature, two main strategies to transfer nanoparticles prepared in w/o microemulsions to a support material have been reported. In the first method, described by Boutonnet et al.,6-7 the support is added to the nanoparticle suspension and the microemulsion is destabilized by addition of a solvent, for example, tetrahydrofurane. The catalyst suspension is then stirred, filtered, washed, and calcined. In the second method, described by (5) Jo¨nsson, B.; Lindman, B.; Holmberg, K.; Kronberg, B. Surfactants and polymers in aqueous solution; Wiley: Chichester, 1998; Chapter 18, 20. (6) Boutonnet, M.; Kizling, J.; Touroude, R.; Marie, G.; Stenius, P. Catal. Lett. 1991, 9, 347. (7) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209. (8) Pillai, V.; Kumar, P.; Hou, M. J.; Ayyub, P.; Shah, D. O. Adv. Colloid Interface Sci. 1995, 55, 241. (9) Sjo¨blom, J.; Lindberg, R.; Friberg, S. E. Adv. Colloid Interface Sci. 1996, 95, 125. (10) Friberg, S. E.; Buraczewska, I.; Sjo¨blom, E. Adv. Chem. Ser. 1979, 177, 205. (11) Pileni, M. P. J. Phys. Chem. 1993, 97, 6961. (12) Pileni, M. P. Langmuir 1997, 13, 3266. (13) Pileni, M. P.; Tanori, J.; Filankembo, A. Colloids Surf., A 1997, 123-124, 561.

10.1021/la0110949 CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002

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Kim et al.14-15 and Ikeda et al.,16 the support is also synthesized within the microemulsion. A metal alkoxide precursor of the support metal oxide is added to a microemulsion containing ammonium hydroxide yielding a metal hydroxide. Subsequently, a microemulsion containing a precursor of the active metal, for example, a metal salt solution, is added and the metal ions are reduced to metal nanoparticles. The latter technique has the drawback that some of the metal particles become embedded in the supporting metal oxide, which reduces the available surface area of the active phase.15 In this study, we have investigated the deposition of Pt nanoparticles, prepared in w/o microemulsions, on γ-alumina. Two different methods for deposition have been used. The first one resembles the method described by Boutonnet et al.6-7 In the second method, the Pt particles were transferred from the microemulsion to an aqueous phase and the suspension was stabilized by a surfactant prior to the deposition step. Anionic, cationic, and nonionic surfactants were studied as dispersants in the aqueous phase. The objective of this work was to prepare Pt/Al2O3 catalyst samples and study the Pt particle size, degree of agglomeration, and the catalytic activity for CO oxidation of the final catalysts. Our main interest was to follow the whole procedure from the formation of Pt particles to the performance of the final catalyst, with a special emphasis on the techniques used for the deposition of the Pt particles on the γ-Al2O3. Experimental Section Chemicals. The platinum nanoparticles were prepared using the following chemicals: sodium bis(2-ethylhexyl)sulfosuccinate (AOT) (99%), technical tetra(ethylene glycol)monododecyl ether (C12E4), hexachloroplatinic(IV) acid (H2PtCl6) (99.995%), hydrazine monohydrate (98%), n-heptane (99%), cyclohexane (99%), and tetrahydrofurane (THF) (99+%), from Sigma-Aldrich. In the photon correlation spectroscopy (PCS) measurements, homologue pure surfactants, tetra(ethylene glycol)monododecyl ether (C12E4), penta(ethylene glycol)monododecyl ether (C12E5), and hexa(ethylene glycol)monododecyl ether (C12E6), from NIKKOL, Japan, were used. The Pt nanoparticles were redispersed in water using the following surfactants: cetyltrimethylammonium chloride (CTAC) (25% in aq), Sigma-Aldrich; the alcohol ethoxylate, Berol OX 91-4 (C9-11E5.5), Akzo Nobel Surface Chemistry AB; and the oleyl ether carboxylic acid, AKYPO RCO 105 O (C16E11CH2COOH), supplied by Kao Chemicals GmbH. Acetic acid (>90%), KEBO Lab, was used to prepare cetyltrimethylammonium acetate (CTAAc) from CTAC, and ammonia (25% in aq, pro analysi) and nitric acid (65% in aq, pro analysi), Merck, were used to adjust the pH. The Pt particles were deposited on γ-Al2O3, puralox NGA-180, grain size 80-100 µm, 180 m2/g, Condea. Platinum Nanoparticle Synthesis. Platinum nanoparticles were prepared in w/o microemulsions with cyclohexane or n-heptane as the continuous oil domain. A solution containing the platinum complex, PtCl62-, was added to a mixture of the oil and 15 wt % surfactant (C12E4 or AOT). Since it has previously been shown that the particle size is dependent on the waterto-surfactant molar ratio (w/s) and that this dependence is linear up to w/s values of approximately 10,5,17-18 we have chosen to prepare the particles with w/s ) 4 to obtain nanoparticles of about 5 nm. The platinum concentration in the water domain (14) Kim, W. Y.; Hanaoka, T.; Kishida, M.; Wakabayashi, K. Appl. Catal., A 1997, 155, 283. (15) Kim, W. Y.; Hayashi, H.; Kishida, M.; Nagata, H.; Wakabayashi, K. Appl. Catal., A 1998, 169, 157. (16) Ikeda, M.; Takeshima, S.; Tago, T.; Kishida, M.; Wakabayashi, K. Catal. Lett. 1999, 58, 195. (17) Lisiecki, I.; Bjo¨rling, M.; Motte, L.; Ninham, B.; Pileni, N. P. Langmuir 1995, 11, 2385. (18) Chen, D.-H.; Yeh, J.-J.; Huang, T.-C. J. Colloid Interface Sci. 1999, 215, 159.

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Figure 1. Schematic illustrations of interaction between a surfactant-coated Pt particle and the alumina surface: (a) interaction when the alumina surface is negatively charged, at high pH, and (b) interaction when the alumina surface is positively charged, at low pH. was varied between 0.5 and 16 wt % Pt. After the mixture was equilibrated for 1 h under stirring, the platinum complex was reduced by addition of hydrazine monohydrate (N2H4/Pt, 10:1 molar ratio) to the microemulsion and Pt nanoparticles were formed within a few minutes. The particle suspension was kept under stirring for 12 h to ensure complete reduction to metallic platinum and then further processed, as described below, for the deposition on alumina. The kinetics of the formation of Pt nanoparticles in w/o microemulsions based on alcohol ethoxylates and AOT, the microemulsion droplet size, and the Pt particle size have previously been reported.19 Deposition of Platinum Nanoparticles on γ-Al2O3. The nonionic C12E4 is preferred over the anionic AOT as the surfactant in the preparation of Pt particles because it allows a broader variation of the Pt concentration in the water pools of the microemulsion. Hence, platinum nanoparticles were synthesized in C12E4-based microemulsions and deposited on alumina according to the following two methods. Destabilizing the Microemulsion Using THF. The first method of deposition has been described elsewhere.6-7 The support material, γ-Al2O3 (calcined in air at 600 °C, 2 h), was added to the Pt nanoparticle suspension under vigorous stirring after which THF was added dropwise in order to slowly break the microemulsion and allow the particles to stick onto the support material. The amount of γ-Al2O3 added corresponded to 98 wt % of the final catalysts. The rate of THF addition was found to be a crucial step since the particles tended to agglomerate if THF was added too fast. The volume of THF added was 3 times the volume of the microemulsion, and the rate was about 0.25 mL THF/(h mL microemulsion). Once the THF had been added, the suspension was stirred vigorously for 12 h and subsequently filtered, washed with THF in order to remove the surfactant, dried at room temperature for 12 h, and finally calcined in air (1 h, 550 °C). This preparation method is denoted the THF method below. Redispersion in an Aqueous Surfactant Solution. The objective of the second deposition method was to suppress platinum particle agglomeration and to facilitate the adhesion between the Pt nanoparticles and the alumina support. Alumina is an amphoteric substrate, which promotes adsorption of either positively or negatively charged species depending on the pH (see Figure 1). In water, OH groups on the alumina surface are the most important sites for adsorption, and alumina acts as acid or base depending on the pH. By stabilization of the Pt particles with an ionic surfactant in the form of an aqueous colloidal suspension, their adhesion on the alumina support can be governed by electrostatic interaction through a suitable choice of pH. To transfer the Pt particles to an aqueous phase, they were separated from the oil domain using ultracentrifugation (Sorvall Instruments Centrifuge model RC-5B) for 45-120 min and a relative centrifugal force of 12 000-17 000g. After the supernatant was decanted, an aqueous surfactant solution was added and the particles were dispersed by vigorous agitation. The dispersion process was repeated twice since total removal of the (19) Ha¨relind Ingelsten, H.; Bagwe, R.; Palmqvist, A.; Skoglundh, M.; Svanberg, C.; Holmberg, K.; Shah, D. O. J. Colloid Interface Sci. 2001, 241, 104.

Platinum Nanoparticles on Alumina Supports supernatant was difficult to achieve. A series of slurries of γ-Al2O3 (calcined in air at 600 °C, 2 h) were prepared, and pH was adjusted by ammonia or nitric acid to 5, 7, 9, and 11. Aqueous suspensions of surfactant-stabilized Pt particles were correspondingly adjusted to pH values of 5, 7, 9, and 11 and added dropwise to the alumina slurry with the same pH under vigorous stirring. The mixed suspensions were stirred for 30 min, freeze-dried, and calcined in air (550 °C, 1 h). The suspensions were mixed in a ratio so as to yield a composition of 98 wt % γ-Al2O3 and 2 wt % Pt in the final catalysts. This method of preparation is denoted the redispersion method below. Three different surfactants were used to stabilize the Pt particles in the aqueous phase. These surfactants were chosen to fulfill several requirements: (a) since interaction between the alumina and the stabilized Pt particles was the topic of investigation, an anionic, a cationic, and a nonionic surfactant were used. (b) The particles were to be stabilized in an aqueous solution for which reason the surfactants needed to be watersoluble. (c) The surfactants should not contain any inorganic matter that could poison the final catalyst, such as sulfur or halogens. (d) A relatively long carbon chain would lead to strong intersurfactant interaction, thus to firmly attach the surfactant layer on the Pt particle surface. The cationic surfactant CTAAc was obtained by ion exchange of CTAC according to a previously described method.20 The anionic surfactant sodium undeca(ethylene glycol)monododecyl ether monocarboxymethyl was obtained by deprotonization of the corresponding acid, C16E11CH2COOH. The nonionic surfactant alcohol ethoxylate (C9-11E5.5) was used as obtained. Since these surfactants have differently charged headgroups, they can be expected to give rise to different interactions with the alumina support. Characterization Methods. The microemulsion droplet size was studied by PCS, which is a dynamic light scattering method. Microemulsions (with n-heptane as the oil domain) were prepared as described above, and PCS measurements were performed for microemulsions without Pt complex (only oil, surfactant, and water) and for microemulsions containing Pt complex (0.5 and 3 wt % Pt) in the water pools. PCS measurements were also carried out on the reduced Pt particles redispersed in an aqueous surfactant solution. The experimental equipment and analytical details used for the PCS measurements have been described previously.19 The Pt particle size was analyzed by transmission electron microscopy (TEM). The nanoparticle suspensions, that is, before deposition on alumina, were applied on copper grids, and micrographs were taken in a JEOL 200CX instrument. The supported samples were prepared using either of the following two procedures. (i) The Pt/Al2O3 samples were carefully ground in an agate mortar and then dispersed in ethanol. A droplet of the dispersion was deposited on a holey carbon film supported by a copper grid. (ii) The Pt/Al2O3 samples were dispersed in ethanol, in an ultrasonic bath, and subsequently coated on a Si wafer and heated to 90 °C. A protective polymer film (about 100 nm thick) was coated on the sample, which was then etched with HF to get the replica. The specimens were characterized in a Philips CM200 TEM equipped with a field emission gun (FEG) or in a JEOL 2000-FX. The ζ-potentials of redispersed Pt particles and of the alumina slurry were determined as a function of pH. The particlesurfactant-water suspensions and the alumina slurry were diluted with water, and the pH was adjusted using ammonia or nitric acid solutions. The electrophoretic mobility of the particles was measured as a function of pH using a Delsa 440 Zeta potential meter, and ζ-potentials were calculated using the Smoluchowski equation.21 The amount of platinum in the final catalyst samples was determined using either X-ray fluorescence (XRF) (Philips PW 1404) for powder samples or atomic absorption spectroscopy (AAS) (SpectraAA-30, Varian) for powders solubilized with LiBO2 fusion using the lithium metaborate method.22 (20) Sepu´lveda, L.; Cabrera, W.; Gamboa, C.; Meyer, M. J. Colloid Interface Sci. 1987, 117 (2), 460. (21) Shaw, D. J. Introduction to Colloid and Surface Chemistry, 4th ed.; Butterworth-Heinemann: Oxford, 1992; Chapter 7. (22) Medlin, J. H.; Suhr, N. H.; Bodkin, J. B. At. Absorp. Newsl. 1969, 8, 25.

Langmuir, Vol. 18, No. 5, 2002 1813 Table 1. Microemulsion Droplet Size Measured by PCS, Standard Deviation 0.1 nm, and Pt Particle Size Observed by TEM, Standard Deviation 0.5 nma surfactant, and aqueous domain

microemulsion droplet radius (nm)

AOT, H2O AOT, 0.85 wt % Pt C12E4, H2O C12E4, 0.5 wt % Pt C12E4, 3 wt % Pt C12E5, H2O C12E5, 0.5 wt % Pt C12E5, 3 wt % Pt C12E6, H2O C12E6, 0.5 wt % Pt C12E6, 3 wt % Pt

3.1 2.8 3.1 3.1 3.2 4.2 4.3 4.4 5.9 6.5 5.3

Pt particle radius (nm) 2.2 1.9 2.3 2.0 2.5 1.5 2.2

a The microemulsions were based on n-heptane and 15 wt % surfactant.

Catalyst Evaluation. The catalytic activity for CO oxidation was studied in a continuous-flow reactor system described elsewhere.23 The reactor consisted of a vertical quartz tube with a sintered quartz filter in the center, on which the sample was placed. The gases, H2 (99.998%), CO (99.997%), O2 (99.998%), and Ar (99.9997%), were introduced via mass flow controllers (Bronkhorst Hi-Tech) above the sample, and the temperature was measured inside the catalyst bed using a thermocouple (Thermo-coax, K-type) in contact with the quartz filter. The gas composition, after the catalyst bed, was probed using a quartz capillary and continuously measured by a quadrupole mass spectrometer (Balzers QMS 200). The m/e signals analyzed were 28 (CO), 32 (O2), 40 (Ar), and 44 (CO2). After prereduction with 1% H2 in Ar (400 °C, 15 min, 40 mL/ min), the catalyst samples (100 mg) were stabilized in the reaction mixture, 1% CO and 10% O2 balanced with Ar to maintain a total flow rate of 40 mL/min, for 10 min at 400 °C. The extinction and light-off performance was then investigated under a cooling and heating ramp, respectively (400 °C f 100 °C f 400 °C, 10 °C/ min), at a total flow rate of 40 mL/min (corresponding to a space velocity of 2400 h-1).

Results Platinum Nanoparticle Synthesis. The results from the PCS and TEM analyses of the microemulsion droplets and platinum particles are summarized in Table 1. The relaxation times, obtained from the PCS measurements, correspond to diffusion constants24-26 that can be converted into hydrodynamic radii of the microemulsion droplets.27 It was found that microemulsion droplets based on nonionic surfactants were slightly larger than those based on AOT and that the radii ranged from 2.8 to 6.5 nm (see Table 1). Addition of PtCl62- to the water pools of the AOT-based microemulsions resulted in a slight decrease of the droplet hydrodynamic radius, whereas the droplet size was unaffected or slightly increased when a nonionic surfactant was used to form the microemulsion. The reproducibility of these PCS experiments has previously been evaluated in a series of measurements, and the standard deviation was calculated to 0.1 nm.19 In Figure 2, a TEM micrograph of platinum prepared in an AOT-based microemulsion shows particles of 4.4 ( 1 nm in average diameter. The size of the primary particles (23) Jansson, J. J. Catal. 2000, 194, 55. (24) Berne, B.; Pecora, R. Dynamic Light Scattering; Wiley-Interscience: New York, 1976; pp 56-65. (25) Pusey, P. N.; Tough, R. J. A. In Dynamic Light Scattering: Applications of Photon Correlation Spectroscopy; Pecora, R., Ed.; Plenum Press: New York, 1985; pp 85-179. (26) Mandel, M. In Dynamic Light Scattering: The Method and Some Applications; Brown, W., Ed.; Clarendon Press: Oxford, 1993; pp 319371. (27) Atkins, P. W. Physical Chemistry; Oxford University Press: Oxford, 1990; p 765.

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Figure 2. TEM micrograph of primary particles of Pt prepared in AOT-based microemulsion. The average diameter is 4.4 ( 1 nm (the scale bar represents 40 nm).

of Pt was found to increase slightly with increasing PtCl62concentration in the water pools for microemulsions based on nonionic surfactants (Table 1). For all syntheses studied, it was found that the size of the microemulsion droplets as determined by PCS measurements was larger than that of the Pt particles formed within them. This is not surprising since the PCS measurements give the hydrodynamic radius, which is larger than the particle size obtained from TEM because it also includes the thickness of one monolayer of surfactants. Deposition of Platinum Nanoparticles on γ-Al2O3. During the deposition of Pt particles on γ-Al2O3 using the THF method, some of the Pt particles formed agglomerates of primary particles, which coalesced during calcination. This can be seen on the TEM micrographs in Figure 3, for a series of deposited Pt particles prepared with varying Pt complex concentrations in the water pools of microemulsions based on the nonionic surfactant. The TEM micrographs indicated smaller and more evenly distributed agglomerates of primary Pt particles when the PtCl62concentration in the microemulsion droplets was higher. The samples prepared with low Pt complex concentration (2 and 6 wt % Pt) in the water pools showed large agglomerates of coalesced Pt particles in the range 200800 nm, unevenly distributed on the alumina surface. Platinum particles prepared in a microemulsion with 10 wt % Pt in the aqueous domain were evenly distributed on the alumina and formed agglomerates in the range of 5-30 nm. Increasing the Pt complex concentration in the water droplets even further resulted in approximately the same particle size and particle distribution as in the 10 wt % sample. The amount of Pt, measured by XRF, in the final Pt/Al2O3 samples prepared by the THF method was found to be close to 2 wt % Pt. The size range of the

calcined supported samples and the results from the chemical analyses of these samples are summarized in Table 2. During the development of the THF method, it was found that the rate of THF addition was crucial for the result, too fast an addition causing more agglomeration of the primary Pt particles. The rate finally adapted was 0.25 mL THF/(h mL microemulsion). In the alternative approach to deposit the Pt particles on γ-Al2O3, the particles were transferred, via an ultracentrifugation step, to an aqueous phase in which they were stabilized with a surfactant. One of the reasons for this approach was to enable the use of varying pH to alter the surface charge of the γ-Al2O3 support and the redispersed particles. To study the effect of surface charge on the deposition, the ζ-potentials of the redispersed Pt particles, as well as the γ-Al2O3 slurry, were measured as a function of pH. The result is shown in Figure 4. The alumina sample showed an isoelectric point slightly below pH 9, as expected. The ζ-potential of the Pt particles redispersed in water with the nonionic surfactant (C9-11E5.5) was zero throughout the whole pH range. The Pt particles redispersed in water with the anionic surfactant (C16E11CH2COONa) showed a negative ζ-potential at low pH values which decreased with increasing pH and reached a point of zero charge around pH 9. The Pt particles redispersed with the cationic surfactant (CTAAc) showed an isoelectric point around pH 5, the ζ-potential being positive for lower pH values and negative for higher ones. The samples were not compensated for the variation in salinity, which may affect the absolute values of the ζ-potential; however, the trends and the zero intersections should not be affected. The PCS measurements of redispersed Pt particles indicated particle agglomeration with hydrodynamic radii around 50 nm for Pt particles stabilized with anionic and nonionic surfactant and around 100 nm for Pt particles stabilized with cationic surfactant. This indicated that the redispersion was not complete for any of the surfactants studied but was better for the anionic and nonionic surfactants than for the cationic surfactant. Figure 5 shows TEM micrographs of Pt particles on alumina prepared by redispersion with nonionic surfactant at pH 5 and 11, anionic surfactant at pH 5, and cationic surfactant at pH 11. The TEM analysis showed that smaller (diameter around 10-50 nm) and more evenly distributed Pt particle agglomerates were generally obtained for the samples prepared at pH 11 as compared with those prepared at pH 5. However, agglomerates in the range 100-200 nm of coalesced platinum particles were also observed for the pH 11 samples. The samples with Pt particles redispersed with cationic surfactant showed large areas without any platinum particles,

Table 2. Range of Pt Particle Diameters Determined by TEM and Chemical Analyses (XRF and AAS) of Calcined Pt/ Al2O3 Samples Prepared Using C12E4-Based (w/s ) 4) Microemulsions Containing Various Organic Solvents as Main Constituents solvent

Pt in water pool (wt %)

Pt particle deposition method

Pt particle diameter (nm)

Pt content in catalyst (wt %)

cyclohexane cyclohexane cyclohexane cyclohexane n-heptane n-heptane n-heptane n-heptane n-heptane n-heptane

2.0 6.0 10.0 16.0 2.0 2.0 2.0 2.0 2.0 2.0

THF method THF method THF method THF method redisp, pH 5, C9-11E5.5 redisp, pH 11, C9-11E5.5 redisp, pH 5, CTAAc redisp, pH 11, CTAAc redisp, pH 5, C16E11CH2COONa redisp, pH 11, C16E11CH2COONa

15-25, clusters 200-800 5-100 5-30 5-40 25-50 10-50 30-50 30-50 15-25 15-25

1.6a 2.0a 2.1a 2.3a 0.43b 0.43b 0.43b 0.44b 0.44b 0.43b

a

XRF. b AAS.

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Figure 3. TEM micrographs of calcined Pt/Al2O3 samples prepared by the THF method. The Pt particles were formed in microemulsions based on C12E4 with (a) 6 wt % Pt in the water pools, (b) 10 wt % Pt in the water pools, and (c) 16 wt % Pt in the water pools.

whereas the samples redispersed with anionic and nonionic surfactants showed more evenly distributed Pt particles. The amount of Pt in the samples prepared by the redispersion method was analyzed using AAS, and the results indicated that the Pt content in these samples was very similar for all samples and about 0.4 wt % (see Table 2). It is likely, however, that the absorption measured in the AAS was somewhat reduced due to the presence of acid in the samples prepared by the lithium metaborate method.22 Hence, the concentration of Pt was probably somewhat higher than 0.4 wt %. Catalyst Evaluation. The catalytic activity for CO oxidation was studied for the alumina-supported Pt particles after calcination. All samples showed good catalytic activity for CO oxidation. The light-off and extinction behavior was similar to that of systems prepared by wet impregnation, that is, catalysts prepared by adsorption of metal precursors such as PtCl62- on the support in aqueous solutions, followed by drying and

calcination.28-30 The samples prepared using the THF method exhibited similar activity with 50% conversion (T50,CO) around 200 °C and complete conversion above 250 °C. The activity for the samples prepared by the redispersion method, using the three different types of surfactants (anionic, cationic, and nonionic) at pH 5 and pH 11 during the deposition procedure, is shown in Figure 6 for heating ramps and in Figure 7 for cooling ramps and summarized in Table 3. Figure 6 shows that the samples prepared using the nonionic surfactant at pH 5 and the anionic surfactant at pH 11 exhibited the highest activity for CO oxidation with T50,CO at 151 and 156 °C, respectively. The activity of the (28) To¨rncrona, A.; Skoglundh, M.; Thorma¨hlen, P.; Fridell, E.; Jobson, E. Appl. Catal., B 1997, 14, 131. (29) Drewsen, A.; Ljungqvist, A.; Skoglundh, M.; Andersson, B. Chem. Eng. Sci. 2000, 55, 4939. (30) Carlsson, P.-A.; Thorma¨hlen, P.; Skoglundh, M.; Persson, H.; Fridell, E.; Jobson, E.; Andersson, B. Top. Catal. 2001, 16/17, 343.

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Figure 4. ζ-Potential as a function of pH for γ-alumina (O) and Pt particles redispersed in water containing cationic surfactant CTAAc (]) and CTAC (×), anionic surfactant C16H11CH2COONa (0), and nonionic surfactant C9-11E5.5 (/).

samples prepared using the cationic surfactant at pH 5, the nonionic surfactant at pH 11, and the anionic surfactant at pH 5 was somewhat lower with T50,CO at 166, 170, and 170 °C, respectively. The lowest activity for CO oxidation was found for the sample prepared using the cationic surfactant at pH 11 (T50,CO at 179 °C). Furthermore, the increase in CO conversion with temperature was slower for the samples redispersed with the nonionic surfactant, at both pH 5 and pH 11, compared to the other samples. Figure 7 shows the CO conversion as a function of temperature during cooling ramps. For all samples, the extinction process was shifted to lower temperatures compared to the corresponding light-off process. The most conspicuous result is the extinction temperatures (T50,CO at 109 and 115 °C) for the samples prepared using the nonionic surfactant. Discussion Particle growth in microemulsions has been studied for a large number of elements, including the precious metals,6-9,12 and the sizes of the microemulsion droplets and metal particles can be controlled to some extent. The droplet sizes obtained in this work, with and without hexachloroplatinic complex dissolved in the water pools, are in the range of a few nanometers. The TEM micrographs of the unsupported primary particles of Pt indicate particle sizes similar to those obtained for the microemulsion droplets by PCS, even though some TEM micrographs also show loose agglomeration of particles (not shown here). It is likely that this agglomeration occurs during the sample preparation, that is, as the suspension is dried on the TEM grid, rather than in the suspension. The TEM analyses of the deposited Pt particles after drying and calcination reveal significantly larger agglomerates of Pt particles, compared to unsupported Pt particles both for the THF and the redispersion method. One may assume that the particle agglomeration occurs during the deposition, drying, and/or calcination steps of the preparation procedure. The crucial step for the THF method seems to be the addition of tetrahydrofurane, since if the addition is too fast, the Pt particles can form agglomerates already in suspension. The drying step can also contribute to particle agglomeration since during this step the solvent can facilitate the transport of primary

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platinum particles. For Pt/Al2O3 samples prepared from microemulsions with high concentrations of Pt complex in the water pools (10 and 16 wt % Pt), the THF method gave fairly small and evenly distributed agglomerates of Pt particles. However, for the samples with lower Pt complex concentrations in the water pools larger agglomerates of coalesced Pt particles were observed. In the redispersion method, the crucial step seems to be the redispersion of the centrifuged particles. The PCS measurements of the redispersed suspensions show that the particles are not fully redispersed but that agglomerates are present even before deposition on γ-Al2O3. The agglomerates found on the γ-Al2O3 can easily coalesce during the calcination giving rise to larger crystallites of Pt and to neck formation between Pt particles as found by TEM. The adhesion of the Pt particles to the alumina support is most probably dependent on electrostatic interaction between the surfactant headgroups and the alumina surface.31 If the interactions are too weak, the Pt particles will be loosely bound to the surface, which may lead to migration of particles during a drying step, possibly resulting in formation of larger agglomerates. The freezedrying used in this method is, however, less likely to contribute to particle agglomeration since sublimation of water does not facilitate transport of particles to a large extent. On the other hand, a weak interaction may also lead to the Pt particles penetrating further into the alumina pore system before attaching to its surface during the deposition step. The ζ-potential for the particles redispersed with the nonionic surfactant is close to zero for all pH values investigated, and it is likely that the interaction between the redispersed Pt particles stabilized by nonionic surfactant and the alumina surface is weak. The Pt particles, redispersed with the anionic surfactant, are negatively charged at low pH and uncharged at pH 11. The TEM analysis indicates better dispersion and smaller particles at pH 11. Platinum particles redispersed with the cationic surfactant show an isoelectric point around pH 5. Since the alumina surface has its isoelectric point at pH 9, it is likely that the interaction between the redispersed particles and the alumina surface is relatively weak both at pH 5 and at pH 11. This result was quite different from that obtained with the corresponding chloride (CTAC) which showed a positive ζ-potential over the entire pH range studied. One possible explanation to the difference between CTAAc and CTAC can be specific adsorption of acetate ions at the platinum surface, which has been reported previously by Fukuda and Aramata.32 It is also possible that the acetate ions could replace the CTA+ ions which would cause less stable particles and, hence, a higher degree of agglomeration. The CO oxidation experiments show a slower increase with temperature for the two samples redispersed with the nonionic surfactant, compared to the other samples (see Figure 6). This is probably due to mass transport limitations in the alumina pore structure, implying that the Pt particles or particle agglomerates in these samples are small and distributed in smaller pores of the alumina support. Figure 7 shows the extinction processes, and particularly interesting is the large hysteresis (compared with the corresponding light-off experiment) found for the samples prepared with the nonionic surfactant. This hysteresis is also indicative of the platinum particles being located inside the smaller pores of the alumina support, (31) Førland, G. M.; Rahman, T.; Høiland, H.; Børve, K. J. J. Colloid Interface Sci. 1996, 182, 348. (32) Fukuda, T.; Aramata, A. J. Electroanal. Chem. 1999, 467, 112.

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Figure 5. TEM micrographs of Pt particles deposited on γ-alumina using the redispersion method: (a) the nonionic surfactant, C9-11E5.5, at pH 5, (b) C9-11E5.5, at pH 11, (c) the anionic surfactant, C16H11CH2COONa, at pH 5, and (d) the cationic surfactant, CTAAc, at pH 11.

favoring the oxidation by suppressing self-poisoning by CO and utilizing the high thermal retention of the support.29 Of the two samples prepared with the anionic surfactant, the highest activity was found for the one prepared at pH 11, whereas the sample prepared at pH 5 showed higher activity than that prepared at pH 11 for the cationic system. The three samples with the highest activity in Figure 7 were all prepared in systems where a weak interaction between Pt particles and γ-Al2O3 was present during the deposition. Concluding Remarks The objective of this study was to investigate the deposition of Pt nanoparticles, synthesized in microemulsions, on a porous γ-alumina support. Two different methods have been used for this purpose, one previously described in the literature where the support was added to the microemulsion, which subsequently was broken by addition of THF.6-7 In the other method, the platinum particles were transferred from the w/o microemulsion to an aqueous phase and stabilized by a surfactant prior to

Figure 6. Conversion of CO as a function of temperature over Pt/Al2O3 samples prepared by the redispersion method during the heating ramp: CTAAc, pH 5 (O) and pH 11 (3); C16E11CH2COONa, pH 5 (4) and pH 11 (]); and C9-11E22, pH 5 (/) and pH 11 (0).

addition of the support. Both methods for Pt particle deposition seemed to cause agglomeration of the particles, the first probably because Pt particles agglomerate in

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Figure 7. Conversion of CO as a function of temperature over Pt/Al2O3 samples prepared by the redispersion method during the cooling ramp: CTAAc, pH 5 (O) and pH 11 (3); C16E11CH2COONa, pH 5 (4) and pH 11 (]); and C9-11E22, pH 5 (/) and pH 11 (0). Table 3. CO Oxidation Light-Off and Extinction Temperaturesa for the Catalyst Samples Prepared by the Redispersion Method sample

T50, heating ramp (°C)

T50, cooling ramp (°C)

C9-11E5.5, pH 5 C9-11E5.5, pH 11 CTAAc, pH 5 CTAAc, pH 11 C16E11CH2COONa, pH 5 C16E11CH2COONa, pH 11

151 170 166 179 170 156

109 115 141 155 146 133

a

That is, temperature at 50% conversion of CO (T50,CO).

suspension if the addition of THF is too fast and the second due to inefficient redispersion of the primary particles of Pt when transferred to the aqueous phase. However, the calcined Pt/Al2O3 catalyst samples showed good CO oxidation performance for both methods of preparation studied. The concept of stabilizing the Pt particles in the

aqueous solution and steering their deposition on the alumina surface by controlling the electrostatic interaction between the species present on the two surfaces seems promising and worth further studies. The choice of surfactant and the pH used in the deposition of the particles on the support proved to be very important factors in the preparation of these catalysts. In general, it seemed advantageous to achieve a relatively weak interaction between the Pt particles and the γ-Al2O3 during the deposition since the samples prepared in this way showed the highest catalytic activity. A stronger electrostatic interaction between the Pt particles and the γ-Al2O3 support, on the other hand, seemed to cause more agglomeration of Pt particles and less effective catalysts. Further work is needed, however, to enable better control of the Pt deposition and distribution on the alumina surface. Acknowledgment. The authors acknowledge Dr. Christer Svanberg at the Department of Experimental Physics, Chalmers University of Technology, for support and advice during the PCS measurements, Dr. Kjell Jansson at the Department of Inorganic Chemistry, Stockholm University, for performing some of the TEM micrographs, and Dr. Bertil Magnusson at Eka Chemicals AB for performing the XRF measurements. We also thank Akzo Nobel Surface Chemistry AB and Kao Chemicals GmbH for providing some of the surfactants and Condea for supplying the γ-alumina. This work has been performed within the Competence Centre for Catalysis, which is financially supported by the Swedish National Energy Administration and the following member companies: AB Volvo, Johnson Matthey CSD, Saab Automobile AB, Perstorp AB, Eka Chemicals AB, MTC AB, and Swedish Space Corporation. LA0110949