Palladium Nanoparticle Generation within Microcellular Polymeric

Ind. Eng. Chem. Res. 2005, 44, 8521-8529

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Palladium Nanoparticle Generation within Microcellular Polymeric Foam and Size Dependence under Synthetic Conditions Alexandre Desforges,† Herve´ Deleuze,*,‡ Olivier Mondain-Monval,† and Re´ nal Backov*,† Centre de Recherche Paul Pascal, UPR 8641 CNRS, Avenue Albert Schweitzer, 33600 Pessac, France, and Laboratoire de Chimie Organique et Organome´ tallique, UMR 5802 CNRS, Universite´ Bordeaux I, 351 Cours de la libe´ ration, 33405 Talence Cedex, France

Microcellular polymer, known as polyHIPE, which is synthesized by polymerization of high internal phase emulsion, has been associated with heterogeneous nucleation. In this issue, we have generated palladium nanoparticles in situ by an impregnation-reduction method. The nanoparticle synthesis is very sensitive to conditions such as temperature, pH, metallic precursor, solvent, and surface chemistry of the support. We have tried to control the particle sizes and dispersion state by a careful choice of the reaction parameters and/or a suitable matrix functionalization. In a catalysis application aspect, we checked the efficiency of our supports via a hydrogenation reaction. Some of our supports offer good activity, even compared to commercial Pd/C, and also a satisfying reusability. Introduction Throughout the past decade, scientists showed a particular interest toward controlling the shapes and sizes of metallic or more generally inorganic particles.1 Continuous reduction of the size of a solid material to the monomer scale results in a quantum size effect at dimensions comparable to the length of the De Broglie electron, the wavelength phonons, and the mean free path of excitons, which means that the electronic bands of the bulk material become discrete states when nanometer size is obtained.2 This particular phenomenon renders inorganic nanoparticles quite attractive because of their physicochemical characteristics3 in catalysis, advanced electronic, nonlinear optics, or hydrogen adsorption with the singular case of palladium (Pd) nanoparticles.4 In this issue, a strong variety of media have been used in order to both control the nanoparticle sizes and offer a stabilizing effect while inhibiting aggregation phenomene. We can propose that a nonexhausting list of confinement media enables one to reach such requirements5 as porous membranes,6 microemulsions and micelles,7 vesicles,8,9 Langmuir monolayers and Langmuir-Blodgett films,10 or polymeric materials.11 Some authors have synthesized nanoparticles in solution, taking advantage of easier characterizations, prior to immobilizing them on a support.12 This fact comes from the huge industrial interest for solid-supported catalysis because of the easier purification step. However, if this two-step procedure enables one to reach a well-controlled model for catalysis, most of the research on this field has been performed using in situ nanoparticle generation, where, despite a onestep process, there are a few systematic studies of each parameter involved in the nucleation and growth mechanisms.13 As a matter of fact, the control of the general * To whom correspondence should be addressed. Tel.: 33(5) 56845630. Fax: 33(5) 56845600. E-mail: h.deleuze@ lcoo.u-bordeaux1.fr (H.D.), [email protected] (R.B.). † Centre de Recherche Paul Pascal, UPR 8641 CNRS. ‡ Laboratoire de Chimie Organique et Organome´tallique, UMR 5802 CNRS, Universite´ Bordeaux I.

procedure by using this one-step process is not such an easy task to reach, mainly because of the lack of knowledge for both nanoparticle nucleation and growth as for Pd-catalyzed reaction mechanisms.14 In this paper, we use macroporous organic matrices, referred to as polyHIPEs, first disclosed at Unilever15 and further developed by Sherrington,16 in order to promote or support Pd nanoparticle generation. This newly developed method for the preparation of first Pd on polyHIPE matrices is reported in detail. Control over the nanoparticle sizes and positions with several synthetic procedures is described. Finally, hydrogenation catalysis properties of those new nanofunctionalized monolith-type materials are provided and compared to the well-known Pd/carbon system. Experimental Section Materials. Styrene (99%), divinylbenzene (DVB; 80%, mixture of isomers), chlorobenzene (99%), hexanediol 1,6-dimethacrylate, sorbitan monooleate (SPAN 80), potassium persulfate (K2S2O8), sodium chloride, sodium borohydride, Pd/carbon black, and allyl alcohol (99%) were purchased from Aldrich. Palladium tetrachloropalladate and tetrahydrofuran (THF; 99+%) were purchased from Acros, and benzoyl peroxide (BPO; with 25% water) was purchased from Merck. All materials were used as received without further purification. Synthesis of Poly(styrene/DVB)/PolyHIPE (PHP). A total of 4.5 g of styrene, 1.2 g of DVB, 4.0 g of chlorobenzene as a porogen, and 2 g of SPAN 80 were mixed in a 500 mL hemispherical reactor vessel. The aqueous phase was prepared by mixing 0.16 g of K2S2O8 as an initiator, 0.5 g of NaCl, and 60 g of distilled water and then slowly added to the reactor mixture under mechanical stirring with a rod fitted with a D-shaped paddle, connected to an overhead stirrer motor (at approximately 300 rpm). The concentrated emulsion was then poured into a polyethylene bottle, and the polymerization was carried out by heating to 60 °C for 12 h using a temperature-controlled bath. The monolith is then cut into smaller pieces, washed with ethanol in

10.1021/ie040239e CCC: $30.25 © 2005 American Chemical Society Published on Web 02/10/2005

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Figure 1. Typical polyHIPE morphology: (a) SEM picture showing interconnected cells (scale bar represents 10 µm). (b) TEM picture of a previously polymerized epoxy film of material cut in thin slices (∼80 nm). The wall microporosity can be observed (scale bar represents 400 nm)

a Soxhlet apparatus for 2 days, and finally dried in air at ambient temperature. Synthesis of Pd/PolyHIPE Hybrid Material (Pd/ PHP). Dissolved oxygen was removed from water by bubbling argon gas for 30 min. Then K2PdCl4 (33.3 mg) was dissolved in 10 mL of water or a water/THF (1:1, w/w) mixture to get a final concentration of 10-2 mol/L. To fill up the matrix void space, we can force the solution to enter by applying and releasing a vacuum (a VAC prefix will be added to the Pd/PHP material name when this vacuum treatment is used). The reduction was performed using three distinguished methods. Spontaneous reduction (SR-PHP): PHP was added to the K2PdCl4 solution and was left for 15 days. Spontaneous reduction occurred with a change in color from white to deep purple, indicating Pd reduction. Reduction using sodium borohydride (BH4-PHP): a fresh NaBH4 solution was prepared by dissolving 130 mg of NaBH4 in 10 mL of water or a water/THF mixture. This solution was quickly added to the K2PdCl4/PHP solution under stirring, which resulted in a rapid color change from white/yellow to deep purple/black and a complete reduction within 20 min. Reduction by UV light irradiation (UVPHP): the K2PdCl4/PHP solution was placed under a mercury lamp irradiating at 354 nm at 250 W for a few hours to allow reduction to occur. The color change was still observed. All of the matrices were then washed in ethanol for 2 days and then allowed to dry in air. Catalysis: Hydrogenation of Allyl Alcohol. A 100 mL reaction vessel equipped with a condenser, a gas inlet, and a mechanical stirring apparatus, containing 50 mL of THF and Pd/polyHIPE (2% w/w) or the commercial Pd/C (5% w/w) was degassed with N2 for 15 min. Then the N2 gas was replaced by a H2 gas. Once all of the N2 had been replaced, a 2 mL solution of 1 M allyl alcohol in THF was added with a syringe. Hydrogenation took place under magnetic stirring and hydrogen pressure at ambient temperature. Aliquots were taken at 20 min intervals until complete hydrogenation and analyzed using gas-phase chromatography. Instrumentation. X-ray photoelectron spectroscopy (XPS) experiments were performed using Escalab VG 220i XL. Transmission electron microscopy (TEM) was performed using a CM10 Philips TEM operating at 60 kV. Sample preparations were performed by first polymerizing an epoxy resin inside the polyHIPE matrix and then cutting thin film of approximately 80 nm using an

ultramicrotone Ultracut E Reichert-Jung. The film was then placed on a copper TEM grid. Particle size distributions were determined by counting at least two micrographs of two different regions at 105× magnification. A minimum of 100 particles were taken into account. Scanning electron microscopy (SEM) was performed using a 515 Philips SEM. Powder X-ray diffraction (XRD) experiments were performed on a Philips PW1820/1710 powder diffractometer with Bragg-Brentano geometry (Cu KR1,2 radiation). Nitrogen adsorption/desorption measurements were performed at 77.3 K on a Micromeritics ASAP 2010 model. Samples were degassed at 100 °C overnight under vacuum prior to data collection. Surface area measurements utilized a multipoint adsorption isotherm collected over 0.05-0.20 P/P0 and were analyzed via the Brunauer-Emmett-Teller (BET) method.17 The average pore size distribution utilized the adsorption branch of the isotherm and were analyzed via the BarrettJoyner-Halenda method.18 Results and Discussion Preparation of the Porous Support. PolyHIPE is a material developed in the early 1980s by Unilever that is obtained by the polymerization of a concentrated inverse emulsion. This process leads to a cellular opencelled morphology. As described by several authors such as Williams or Cameron,19 this morphology can be tailored by the careful emulsion formulation. A wide range of preparation techniques and monomers are available.20 Possible applications for this material include adsorbents, catalysts, scavengers, peptide synthesis supports, and cell growth media.21 For this study, we have chosen the most studied system, which is certainly poly(styrene-co-DVB). A typical macroscale morphology is depicted in Figure 1a. Because of the high interconnecting cell size (connections between the cells are around 1 µm), the surface area is generally low (∼525 m2/g). By using a porogen component, phase separation between the porogen and the on-growing polymer22 induces a secondary porosity (mesoporosity). A TEM micrograph of a material prepared using chlorobenzene as a porogen is presented in Figure 1b, showing the obtained mesostructure. The corresponding BET surface area is around 90 m2/g, associated with pore sizes comprised between 10 and 80 nm with an average size of 20 nm.

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Figure 2. (a) XPS spectra showing the positions of the 3d5/2 and 3d7/2 peaks characteristic of metallic Pd particles. (b) XRD pattern performed on powder with the two main peaks at 2θ ) 40° and 46.5°. The XRD broad peak centered at 2θ ) 18° corresponds to the amorphous bulk organic matrices, whereas the two overscaled diffractions at 38.4° and 44.6° (2θ) correspond to the aluminum support holding the powder. Table 1. Sample Macroscopic Homogeneity over the Synthetic Conditions entry

method

solvent

[K2PdCl4]

initiator

% Pd (g/100 g of polymer)

nanoparticle mean size (nm)

sample homogeneity

1 2 3 4 5 6 7 8 9 10

BH4 VAC-UV UV SR VAC-SR SR SR SR SR SR

H/T H H/T H H H/T H/T H/T H/T H/T

10-2 10-2 10-2 10-2 10-2 10-2 10-2 10-2 10-2 5 × 10-2

K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 K2S2O8 (1%) BPO AIBN K2S2O8

3.8 6.7 ∼6 0.2 6 2.2 1.4 3 3.3 6

7.45 8.5 12.66

very bad bad medium very bad bad good good good good good

Nanoparticle Generation inside the Support. Basic Principle of Pd0 Generation. Basically, two major ways of generating nanoparticles are generally admitted, namely, the physical and chemical methods.23 In the first one, a macroscopic bulk metallic source will be broken into small nanometric particles. With this route and according to the literature, particles monodisperse in size are difficult to obtain.24 In the chemical method, the Pd source is a molecular species (generally salts) from where the Pd has to be reduced into zerovalent state particles. These particles will aggregate into bigger objects by a nucleation and growth process. The goal is to find a mean to stop the growth at the desired size.25 We will use a two-step process that consists of first impregnating our support with the Pd salt solution and then reducing the metallic cation Pd2+ to its zerovalent state. The support is supposed to bring steric stabilization against aggregation. However, the reduction method (rate of reduction, mechanism, etc.), the matrix (structure, surface groups, and hydrophobicity), or the environment (solvent and temperature) can play an important role.21,26 Characterization. For all procedures, Pd’s zerovalent nature and cubic-face-centered (cfc) metallic structure were checked with XPS and powder XRD patterns, respectively. At first, the XPS spectrum shows a Pd3d5/2 band at 336.8 eV, which is attributed to palladium oxide (PdO).27 When 1-2 nm of the material surface was dug with an electron beam, we observed a shift of those peaks toward characteristic Pd metallic 3d5/2 and 3d7/2 bands centered respectively at 335.2 and 340 eV (Figure 2a).27 Because there is no specific stabilizer, an oxide thin layer is spontaneously formed

19.8 13.8 14.4 12.0 11.9 12.1

onto the particle surface.28 In Figure 2b, we observe an XRD pattern performed on powder that depicts the diffractions centered at 2θ ) 40° and 46.5°, which are characteristic of metallic Pd materials that crystallize in the Fm3m cfc space group. Important Parameters. Because the polyHIPEs described in this study are obtained as monolith-type materials, it appears very difficult to cut small pieces within reproducible shapes. To overcome this issue, we decided to generate Pd nanoparticles using large pieces of PHP with a minimum of 0.5 cm in length. We have compared different parameters that influence the nanoparticle generation with an emphasis toward particle sizes and aggregation states. Beyond those specific nucleation and particle growth issues, we have studied the particles’ homogeneous or inhomogeneous repartition within the monolith host, as depicted in Table 1. The nanoparticle mean size and sample homogeneity were estimated by visual observation of a TEM micrograph of about 1 mm2. Pd loading was determined by elemental analysis. The PdII reduction procedure appears to be an important issue because we did generate Pd nanoparticles using three different routes. The first one is the spontaneous reduction (SR) induced by the radicals still present within the bulk porous matrices. In this issue, electron paramagnetic resonance (EPR) spectroscopy measurements were performed on the organic matrices. They reveal an EPR signal associated with a ∆Hpp of 6 G, which is characteristic of the presence of radicals as paramagnetic species.29 Free radicals are known to possibly act as reduction agents as well as strong adsorption sites.30 The second reduction process (BH4)

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Figure 3. (a) TEM micrograph and (b) associated size distribution for the BH4 method and with a 1:1 water/THF mixture as a solvent.

is based on the use of a strong reducing agent as Na(BH4), whereas the third way of reduction (UV) is reached upon UV irradiation. Solvent Effect. For each process, we have tried different solvents, from water to more hydrophobic mixtures. Because we are working with quite large samples, the impregnation step is crucial to obtain homogeneous materials. Polystyrene is a hydrophobic polymer, so it is difficult for water to fill up the support, whereas with a good solvent such as THF, the matrix will swell, thus rising to an easier impregnation process. We have found an alternative method to fill up the matrix in any case by applying a vacuum, to fill the porosity by capillarity. This vacuum has the effect of a better PdCl42- salt impregnation within the organic open-celled matrix. This process is essential with water as a solvent, whereas it becomes useless with a good solvent. We have compared water and a mixture of water and THF, which allows a good diffusion inside the matrix while still dissolving the Pd salt as solvents. Using a better solvent of the polymer, it enhances the salt diffusion within the porous matrix, which leads to a homogeneous sample, at both the macroscopic and nanoscopic scales. The diffusion of salt inside the matrix was estimated indirectly: A homogeneous distribution of the nanoparticles inside the support indicates a good swelling of the structure by the solvent used. BH4 Method. The method using BH4 in water both with and without vacuum treatment leads to a poor Pd nanoparticle anchoring at the organic surface and a strong inhomogeneity of the final organic-inorganic materials. This method appears to be a nonconvenient candidate able to generate reproducible samples. When the solvent to a water/THF mixture is changed, this method allows the reduction reaction to be performed within the monolith, thus leading to a reasonable amount of Pd but still with an inhomogeneous character of the samples. The values obtained for the response parameters are listed in Table 1 (entry 1). TEM observations (Figure 3a) show particles that are preferentially located onto the macropore surface and that do not diffuse into the mesoporosity. These nanoparticles are polydisperse in size, with a mean size distribution of 8-9 nm, and depict an aggregation tendency (Figure 3b). As a consequence, this method is not well suited for our kind of material. UV Method. To enhance the reduction process, we have performed a UV-light treatment mentioned as VAC-UV. In water, the VAC-UV method allows a good loading of Pd (Table 1, entry 2), but this process occurs mainly at the organic monolith outer part, leading to an inhomogeneous Pd repartition within the organic matrix. TEM micrograph (Figure 4a) and histogram diagram (Figure 4b) show two different types of nano-

particles with different sizes. The first type consists of particles of around 14 nm that are aggregated in large clusters. Considering the histogram of Figure 4a, nanoparticles associated with the agglomerate textures (Figure 4b) have not been taken into account because observation of the discrete nanoparticle size within such textures appears to be a difficult task. On the other hand, we can also observe a second type of particle that is monodisperse in size (∼7 nm) and homogeneously dispersed within the polymer matrix. This family of particles, which represent just a small part of the Pd in weight, are, in fact, formed during the vacuum treatment (this was checked by stopping the reduction at the end of this treatment). By applying a vacuum, we force the solution to fill up the smaller pores (mesoporosity) where the radical sites are mostly located (discussed below), leading to a better reduction kinetic. A possible explanation for the monodisperse particle size could be the confinement effect and the limited salt diffusion especially into the mesopores. However, we have not studied further this phenomenon because this method does not allow one to reach homogeneous materials and reproducible results under the current conditions. Concerning the UV methodology using water/THF (Table 1, entry 3), the amount of Pd loading appears to be within the same range when compared to the higher Pd nanoparticle amount reached using water as a solvent. However, this result is quite irreproducible, and the corresponding monoliths are inhomogeneous. By applying the UV technique over a period of 6 h, we generate Pd nanoparticles in a bulk solution (Figure 4c) that aggregates. These aggregates are made of nanoparticles with an average size of around 14 nm (Figure 4d). Here again, because syntheses are performed on large samples, it appears that the nanoparticle repartition is strongly dependent on the material depth, and even the aggregation state is changing, varying from well-dispersed (Figure 4e) to strongly aggregated (Figure 4c). Again, the non reproducibility of the samples synthesized with this method makes it a bad candidates for the generation of model materials. SR Method. For the VAC-SR sample in water (Table 1, entry 5), the amount of Pd generated is rather high, but beyond those quantitative characterizations, we did perform some qualitative investigations using mainly TEM experiments (Figure 5). A broad distribution of particle sizes is obtained (Figure 5b), with a strong aggregation tendency leading to the formation of Pd nanoparticle aggregates with sizes up to 100 nm (Figure 5a). Here, strong nanoparticle aggregation is obtained because of the low affinity between the solution and the matrix that renders their internal surfaces unable to


Figure 4. TEM micrographs and their associated size distributions for the UV method and using vacuum to fill the matrix: (a and b) with water as a solvent and using vacuum to fill the matrix; (c-f) with water/THF as a solvent.

Figure 5. TEM micrographs and their associated size distributions for the SR method: (a and b) with water as a solvent; (c and d) with water/THF as a solvent.

stabilize the on-growing nanoparticles. Because there are only a limited number of radicals that are available to the Pd precursor, this results in a majority of homogeneous nucleation.

For the Pd nanoparticle generation obtained via SR in a water/THF mixture (Table 1, entry 6), over a 15 day period, the better affinity between the solvent in use and the organic core allows better interfacial reac-

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Figure 6. TEM micrographs of Pd nanoparticles generated in (a) polyHIPE made with BPO as an initiator; (b) polyHIPE made with K2S2O8 as an initiator.

tions, and the slower method allows the solution to diffuse to the reduction sites. We can observe that the SR method offers a narrow nanoparticle size distribution (Figure 5d) associated with a homogeneous distribution of the nanoparticles into the organic macroporous system (Figure 5c). Furthermore, those results are reproducible, with loadings of around 2-2.2% Pd and nanoparticle sizes of around 14 nm. No improvement was obtained when the solvent was incorporated into the matrix under vacuum. This is the reason the VAC experiments are not mentioned in Table 1. We can explain those different behaviors by the important swelling of the matrix when it is impregnated with a good solvent such as the water/THF mixture. For instance, solvent impregnation by water alone provides 2 mL/g of water inserted within the matrices that reaches 4 mL/g when vacuum is performed. When water/THF is employed as a solvent, impregnation reaches 18 mL/g, which does not change upon the use of vacuum. If one compares these different methods, the best way to obtain nanoparticles with both a narrow size distribution and a minimized aggregation state appears to be by far the water/THF-SR method (Figure 5c). Furthermore, this methodology that requires neither any thermal treatment nor a vacuum process appears as a simple and soft method and enables one to reach reproducible materials. We believe that the BH4 and UV methods do not lead to satisfying results because of the impregnation time in use for this study. This choice of a short impregnation time was made to avoid additional SR reactions. Let us also note that radical reduction is strongly influenced by the pH conditions: Pd loading decreases as the pH decreases, and the reduction is strongly slowed below a pH value of 3. If we increase the pH up to 7, the salt solution begins to precipitate and inhomogeneous brown monoliths are obtained; XPS experiments reveal the presence of PdO nanoparticles. Polymerization Initiator Effect. Beyond control over the Pd nanoparticle sizes, another interesting feature is the possibility of tuning the overall particle position, i.e., near the surface of the macropores or within the mesopores. This control can be reached by using either a water-soluble initiator or an oil-soluble initiator during the emulsion process. Basically, if the initiator is in the water phase, polymerization will start at the edge of the droplets and then propagate to the center of the wall, hence forming more unreacted oligomer radicals in the mesopore. As a consequence, the particles should be formed at the surface of the macropore rather than within the mesopore structure (see Figure 6). By contrast, an oil-soluble initiator leads to the formation of radicals near the surface of the

Figure 7. Pd loading versus Pd salt concentration.

droplets. Therefore, we expect the formation of particles to occur in priority in the vicinity of the mesopores. With azobis(isobutyronitrile) (AIBN) (Table 1, entry 9) or BPO (Table 1, entry 8), we still obtain well-dispersed and quite monodisperse particles. However, particles seem to be a little smaller with an average 11-12 nm size while Pd loading is increased up to 3-3.3%. Those minor changes might come from the larger amount of accessible free-radical sites. Salt Concentration, Control over the Pd Loading. If the particle size and its aggregation state are the key parameters in explaining the activity of a catalyst, some studies have also demonstrated the influence of the Pd loading. For example, some reactions require a highly loaded support, while others are more efficient with low loadings, also depending on the targeted applications. In our case, the concentration of the Pd salt has been determined to be of primary importance in the control of the final Pd loading of the supports. When [K2PdCl4] is varied between 10-3 and 5 × 10-2 M (Table 1, entry 10), the final loading can be adjusted between 0.3% and 6% (see Figure 7). Moreover, the size of the nanoparticles is not affected by this change in the range of studied concentration (Figure 8). This is an easy and useful way to tailor our support to the suitable application. Finally, another way to control this loading is achieved by tuning the polymerization initiator concentration in the emulsion (Table 1, entry 7). By this means, one controls the number of growing oligomer chains created and then increases (or decreases) the probability for


Figure 8. TEM micrographs and their associated size distributions for the SR method with water/THF as a solvent: (a and b) [K2PdCl4] ) 10-2 M; (c and d) [K2PdCl4] ) 5 × 10-2 M.

Figure 9. Reaction rate versus time for the allyl alcohol hydrogenation, with Pd/PHP in monolith and powders forms, and a comparison with commercial Pd/C.

polymer chains to remain unreacted. This method has appeared to be less efficient than the other and more difficult to control. Catalysis. We have tested the hybrid material catalytic activity using the test reaction of allyl alcohol hydrogenation into 1-propanol.31 The comparison is made with 5% commercial Pd/C (Figure 9). The monoliths used were cubes of about 0.5 × 0.5 × 0.5 cm. In some cases, the support was grounded in powder of about 1-2 mm. We have used such quantities in which the reaction time is equivalent for both materials. In all cases, the expected hydrogenation reaction occurs. The total amount of Pd is more important in the case of the PHP material (0.4 mmol versus 0.25 mmol for Pd/C), meaning that Pd/C is more efficient under these conditions but our catalyst is not optimized here. First we can see the increase in activity when using the powder form instead of the monolith. This lower diffusion of the reactants inside the monolith host, which reduces the reaction kinetic, is quite surprising because of the large pores of the matrix but has already been mentioned for some other reactions.32 On the other hand, one advantage of Pd/PHP over Pd/C is that this

Figure 10. Evolution of the conversion at 20 min with the number of runs.

is a reusable catalyst. As demonstrated in Figure 10, we can perform several runs without loss of activity. There is even an increase in the reaction rate, which can be explained by the presence of an oxide layer at the surface of our catalyst, as pointed out by the XPS experiments. Indeed, this oxide has to be reduced to become active in this reaction, which is done by the flowing H2 gas. We can also note that, for the last run, we used 4 times more reactants, which means an increase of the catalytic activity, which can explain the difference in quantity of Pd used between Pd/PHP and Pd/C in Figure 9. This also means that a treatment prior to use is necessary to obtain the best results (for example, a H2 treatment or a storage in reductive solution). As a conclusion, we have synthesized Pd nanoparticles within polyHIPE matrices. The experimental conditions allow control of the particle sizes, aggregation states, and positions within the polymeric core. Those materials offer promising applications toward catalysis reaction with a certain degree of reusability. Next, work will be dedicated to Pd generation within modified polyHIPE matrices with a specific function, allowing both particle stabilization and catalysis under oxidative conditions.

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Received for review September 7, 2004 Revised manuscript received November 23, 2004 Accepted November 24, 2004 IE040239E

Palladium Nanoparticle Generation within Microcellular Polymeric

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