High Catalytic Activity of Platinum Nanoparticles Immobilized on

The spherical polyelectrolyte brushes consist of a solid core of poly(styrene) onto which ... In all cases, a delay time t0 has been observed, after w...
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Langmuir 2005, 21, 12229-12234

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High Catalytic Activity of Platinum Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes Yu Mei, Geeta Sharma, Yan Lu, and Matthias Ballauff* Physikalische Chemie I, University of Bayreuth, 95440 Bayreuth, Germany

Markus Drechsler Makromolekulare Chemie II, University of Bayreuth, 95440 Bayreuth, Germany

Thorsten Irrgang and Rhett Kempe Anorganische Chemie II, University of Bayreuth, 95440 Bayreuth, Germany Received August 3, 2005. In Final Form: October 6, 2005 We present a study on the catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes that act as carriers. The spherical polyelectrolyte brushes consist of a solid core of poly(styrene) onto which long chains of poly(2-methylpropenoyloxyethyl) trimethylammonium chloride are grafted. These positively charged chains form a dense layer of polyelectrolytes on the surface of the core particles (“spherical polyelectrolyte brush”) that tightly binds divalent PtCl6-2 ions. The reduction of these ions within the brush layer leads to nearly monodisperse nanoparticles of metallic platinum. The average size of the particles is approximately 2 nm. The composite particles exhibit excellent colloidal stability. The catalytic activity is investigated by photometrically monitoring the reduction of p-nitrophenol by an excess of NaBH4 in the presence of the nanoparticles. The kinetic data could be explained by the assumption of a pseudo-first-order reaction with regard to p-nitrophenol. In all cases, a delay time t0 has been observed, after which the reactions start. This time is shorter when the catalyst has already been used. All data demonstrate that spherical polyelectrolyte brushes present an ideal carrier system for metallic nanoparticles.

Introduction Metal nanoparticles exhibit different properties as compared to their bulk materials.1 Because of their high surface-to-volume ratio, metal nanoparticles of noble metals such as platinum or palladium are ideally suited as catalysts. Moreover, gold nanoparticles have been found to be catalytically active,2 which has led to a great number of investigations recently.3-5 However, such nanoparticles require a suitable support to prevent aggregation during the reaction to be catalyzed. For applications taking place at low temperatures and in an aqueous environment, polymers,6-10 dendrimers,11 microgels,12 and colloids13-16 * Corresponding uni-bayreuth.de.

author.

E-mail:

Figure 1. Scheme of the cationic spherical polyelectrolyte brush used in this study with the brush monomer of (2-methylpropenoyloxyethyl) trimethylammonium chloride.

Matthias.Ballauff@

(1) Henglein, A. Chem. Rev. 1989, 89, 1861. (2) Haruta, M. Catal. Today 1997, 36, 153. (3) Mohr, Ch.; Claus, P. Sci. Prog. 2001, 84, 311. (4) Haruta, M. Chem. Rec. 2003, 3, 75. (5) Yan, Z.; Chinta, S.; Mohamed, A. A.; Fackler, J. P., Jr.; Goodmann, D. W. J. Am. Chem. Soc. 2005, 127, 1604. (6) Kimura, Y.; Abe, D.; Ohnori, T.; Mizutani, M.; Harada, M. Colloids Surf., A 2003, 231, 131. (7) Vincent, T.; Guibal, E. Langmuir 2003, 19, 8475. (8) Adlim, M.; Abu Bakar, M.; Liew, K. Y.; Ismail, J. J. Mol. Catal. A: Chem. 2004, 212, 141. (9) Esumi, K.; Isono, R.; Yoshimura, T. Langmuir 2004, 20, 237. (10) Tsunoyama, H.; Sakurai, H.; Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2005, 127, 9375. (11) Scott, R. W. J.; Wilson, O. M.; Crooks, R. M. J. Phys. Chem. B 2005, 109, 692, and further citations given there. (12) Zhang, J.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2004, 126, 7908. (13) Mayer, A. B. R.; Mark, J. E. Colloid Polym. Sci. 1997, 275, 333. (14) Liang, Z.; Susha, A.; Caruso, F. Chem. Mater. 2003, 15, 3176. (15) Bedford, R. B.; Singh, U. G.; Walton, R. I.; Williams, R. T.; Davis, S. A. Chem. Mater. 2005, 17, 701, and further references given there. (16) Kidambi, S.; Bruening, M. L. Chem. Mater. 2005, 17, 301.

have been used as carrier systems. If the activity of the bare nanoparticles is to be studied in a quantitative manner, then the support should be totally inert. Moreover, no stabilizing agent should be induced that may alter or block the surface of the nanoparticles. However, the support should be sufficiently stable to ensure the reuse of the catalyst after the reaction. An additional problem is the formation of well-defined nanoparticles with a narrow size distribution that should take place directly on the support. Hence, a quantitative study of the catalytic activity of immobilized nanoparticles has a number of important prerequisites that should be met by the support under consideration. Recently, we showed that well-defined gold nanoparticles can be generated in spherical polyelectrolyte brushes.17 Figure 1 shows the structure of these particles (17) Sharma, G.; Ballauff, M. Macromol. Rapid Commun. 2004, 25, 547.

10.1021/la052120w CCC: $30.25 © 2005 American Chemical Society Published on Web 11/11/2005

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Figure 2. Schematic representation of the formation of platinum nanoparticles on the surface of the core-shell system. The core-shell system has a shell of poly((2-methylpropenoyloxyethyl) trimethylammonium chloride. The PtCl62- ions are immobilized within the brush layer. Reduction of the metal salt by NaBH4 leads to nanosized platinum particles.

in a schematic manner: long cationic polyelectrolyte chains are chemically grafted to colloidal polymer particles of ca. 100 nm diameter. The layer of polyelectrolyte chains affixed to the surface of the carrier particles is very dense; that is, the contour length Lc of the chains is much higher than their average distance on the surface of the carrier particle. In this way, a polyelectrolyte “brush” results that denotes a system of strongly interacting polymer chains grafted densely to a planar or curved surface.18 These spherical polyelectrolyte brushes are synthesized by photoemulsion polymerization by which polyelectrolyte chains are grafted from the surface of colloidal poly(styrene) particles.19-21 The particles are dispersed in water in which the concentration of the added salt as well as the pH can be controlled very well. Spherical polyelectrolyte brushes (SPBs) have been the subject of a number of recent studies.20-27 The main conclusion from these studies is the following: in saltfree solutions, the counterions are strongly confined within the brush layer attached to the surface of the core particles.22-25 The counterions derived from the synthesis can be replaced by suitable ions of noble metals (e.g. AuCl4or PtCl62-). This effect can now be used to remove a possible surplus of metal ions outside the brush particles by ultrafiltration. Subsequent reduction of the metal ions to the metal nanoparticles can be achieved by suitable reagents such as NaBH4.17 Figure 2 summarizes the method of generating nanoparticles on the surface of an SPB in a schematic manner.17 The advantages of this method of generating composite particles of a colloidal carrier and metal nanoparticles are at hand.17 The generation of nanoparticles can take place only within the brush layer and not outside. Hence, all particles are confined on the colloidal carrier. Moreover, no additional stabilizing agent is needed to keep the metal particles in the truly nanoscopic range (2-5 nm). There(18) Advincula, R. C., Brittain, W. J., Caster, K. C., Ru¨he, J., Eds.; Polymer Brushes; Wiley-VCH: Weinheim, Germany, 2004. (19) Guo, X.; Weiss, A.; Ballauff, M. Macromolecules 1999, 32, 6043. (20) Guo, X.; Ballauff, M. Langmuir 2000, 16, 8719. (21) Mei, Y.; Wittemann, A.; Sharma, G.; Ballauff, M.; Koch, Th.; Gliemann, H.; Horbach, J.; Schimmel, Th. Macromolecules 2003, 36, 3452-3456. (22) Guo, X.; Ballauff, M. Phys. Rev. E 2001, 64, 051406. (23) Das, B.; Guo, X.; Ballauff, M. Prog. Colloid Polym. Sci. 2002, 121, 34. (24) Jusufi, A.; Likos, C. N.; Ballauff, M. Colloid Polym. Sci. 2004, 282, 910. (25) Dingenouts, N.; Patel, M.; Rosenfeldt, S.; Pontoni, D.; Narayanan, T.; Ballauff, M. Macromolecules 2004, 37, 8152. (26) Mei, Y.; Ballauff, M. Eur. Phys. J. E 2005, 16, 341. (27) Wittemann, A.; Drechsler, M.; Talmon, Y.; Ballauff, M. J. Am. Chem. Soc. 2005, 127, 9688.

Table 1. Characterization of the Cationic Spherical Polyelectrolyte Brush Used in This Study label

charge

Ra [nm]

Lcb [nm]

Mwc [g/mol]

σd [nm-2]

De [nm]

Lc/R

MC1-30

positive

46

182

150 800

0.019

8.2

3.96

a

b

Core radius of polystyrene. Contour length of grafted chains determined from Mw. c Molecular weight of grafted chains as determined by viscosimetry. d Graft density on the surface of core particles. e Average distance between two neighboring graft points.

fore, the surface of the metal is free of strongly binding agents such as thiols that would be necessary to keep the nanoparticles from aggregating in free solution. In a recent study, we demonstrated that no coagulation takes place during the generation of the nanoparticles on the SPB.17 Moreover, the composite system has sufficient colloidal stability for several months and does not flocculate at elevated temperature. This is due to the strongly stabilizing effect of the polyelectrolyte chains that have not been removed by the process of reduction. In this article, we present the first systematic study of the catalytic activity of platinum nanoparticles generated on the surface of a cationic spherical polyelectrolyte brush. As a model reaction, we chose the reduction of pnitrophenol by sodium borohydride that can be easily monitored by UV/vis spectroscopy.9,28 Because this reaction has been used in other studies as well, the results given here can be directly compared to recent investigations that have employed different strategies of immobilization of the metal particles. Experimental Section Materials. H2PtCl6 and sodium borohydride (NaBH4) were received from Aldrich and used without further purification. 4-Nitrophenol (reagent grade) was obtained from Aldrich and used as received. Synthesis of the Cationic Polyelectrolyte Brushes. Cationic spherical polyelectrolyte brush MC1-30 was synthesized and characterized as described recently.17 Figure 1 displays the chemical structure of the monomer of (2-methylpropenoyloxyethyl)trimethylammonium chloride. All pertinent parameters, namely, the core radius R, the contour length Lc of the attached chains, andthe grafting density σ (number of polymer chains per unit area) are known from this analysis and summarized in Table 1. Synthesis of the Pt Composites Particles. After synthesis and purification of cationic SPBs, they were washed with an H2PtCl6 (10× volume) solution of a different concentration (2.5 × 10-4 or 2 × 10-3 mol/L), which ensures that all counterions are (28) Pradhan, N.; Pal, A.; Pal, T. Colloids Surf., A 2002, 196, 247.

High Catalytic Activity of Platinum Nanoparticles Table 2. Preparation of Pt Composites sample

brusha [g]

water [g]

NaBH4 [g]

H2PtCl6 [mol/L]

Pt1 Pt3

3.0 3.0

97.0 97.0

0.019 0.152

0.25 × 10-3 2.0 × 10-3

a

The solid content of MC1-30 was 15.22%.

replaced with PtCl62-. Afterward, the SPBs were subjected to ultrafitration with distilled water of 10× the volume to remove excess H2PtCl6 outside the polymer brushes. Then the PtCl62ions in SPBs were reduced by NaBH4 under an atmosphere of nitrogen. Here, a 10-fold excess of NaBH4 (Table 2) was added slowly to the suspension at room temperature with vigorous stirring. Typically, the addition of NaBH4 was done within 30 min. The onset of reaction could be seen from a slight discolorization of the suspension, which assumed a grayish color. After the last addition, the latices were stirred for another 1 h and then carefully washed again with distilled water in an ultrafiltration cell. This process removed not only the salts from synthesis but also a fraction of the free platinum particles not firmly bound to the particles. Two systems termed Pt1 and Pt3 were synthesized and used in this study. These systems differ with regard to the amount of platinum used in the synthesis of the nanoparticles. Table 2 summarizes the respective parameters in this synthesis. Catalysis. As a model reaction, we chose the reduction of p-nitrophenol by NaBH4 to p-aminophenol. In a typical run, a given amount of sodium borohydride (10 mmol/L) was added to a p-nitrophenol solution (0.1 mmol/L). The pH was adjusted to 10 with NaOH. After mixing these solutions, we added a given number of platinum particles to start the reduction. The process of the reduction was monitored by measuring the extinction of solution at 400 nm as a function of time. Methods. Cryo-TEM specimens were prepared by vitrification of thin liquid films supported on a TEM copper grid (600 mesh, Science Services, Munich, Germany) in liquid ethane at its freezing point. The specimen was inserted into a cryotransfer holder (CT3500, Gatan, Munich, Germany) and transferred to a Zeiss EM922 EFTEM (Zeiss NTS GmbH, Oberkochen, Germany). Examinations were carried out at temperatures around 90 K. The TEM was operated at an acceleration voltage of 200 kV. All images were recorded digitally by a bottom-mounted CCD camera system (UltraScan 1000, Gatan, Munich, Germany) and processed with a digital imaging processing system (Digital Micrograph 3.9 for GMS 1.4, Gatan, Muenchen, Germany).27 Dynamic light scattering measurements were performed with an ALV 4000 (Peters). The spectra of p-nitrophenol were measured with a Lambda 25 spectrometer (Perkin-Elmer). The number of pure platinum particles was determined by TGA using a Mettler Toledo STARe system. At first, the composite latex was dehydrated in a dry oven; then about 15 mg of solid composites was heated to 1000 °C under a 60.0 mL/min airflow with a heating rate of 10 °C/min. The theoretical specific surface area of platinum particles was estimated from these TGA results, the and particle size was estimated from TEM. For this calculation, the density of bulk platinum was used (F ) 21.45 × 103 kg/m3; ref 30.

Results and Discussion Synthesis of the Composite Particles. The synthesis of the cationic spherical polyelectrolyte brushes was carried out through photoemulsionpolymerization as described in previous papers.17-20 The cationic monomer (2-methylpropenoyloxyethyl)trimethylammonium chloride differs slightly from the monomer used previously.17 As demonstrated in Table 1, long chains of the cationic polyelectrolyte result from this procedure. In particular, the grafting density σ is rather high so that the average distance D between two chains is much smaller than the (29) Ghosh, S. K.; Mandal, M.; Kundu, S.; Nath, S.; Pal, T. Appl. Catal., A 2004, 268, 61-66. (30) Grigoriev, I. S., Meilikhov, E. Z., Eds.; Handbook of Physical Quantities; CRC Press: Boca Raton, FL, 1997; p 116.

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contour length Lc. Hence, the surface layer of polyelectrolyte chains is within the brush limit.18 The high number of monovalent counterions confined within the brush layer leads to strong stretching of the polyelectrolyte chains.27 This could be inferred from the measurement of the layer thickness L (Figure 1), which can be easily derived from the measured hydrodynamic radius RH. Recently, the strong stretching of the chains was visualized directly by cryogenic electron microscopy (cryo-TEM).27 All of these findings, which are in full accord with theory, demonstrate that the osmotic pressure of the confined counterions is the dominant effect in these systems.22-24 Replacing monovalent by divalent counterions therefore leads to strong shrinkage of the surface layer of polyelectrolyte chains because the number of counterions is roughly divided by a factor of 2. This again could be monitored quantitatively by DLS.22,26 The replacement of the chloride counterions’ reduction introduced by the synthesis with PtCl62- ions was done by washing the SPBs several times with a solution of H2PtCl6. The ion exchange is expected to proceed very quickly and efficiently because the PtCl62- ions are divalent. Hence, the release of roughly half of the osmotic pressure through the introduction of divalent ions instead of monovalent Cl- ions provides a strong driving force for the introduction of PtCl62- ions. In addition to this, complexation of the metal ions by the monomer units may further help to achieve the full exchange of ions, as has recently been found for the case of AuCl4- ions.17 The reduction to metallic nanoparticles can be done at room temperature through the addition of NaBH4. The onset of the reduction could be seen from a slight darkening of the suspensions. All systems remained stable during the reaction and the subsequent cleaning by ultrafiltration. In general, the composite particles exhibited the same colloidal stability as the SPBs. This is due to the fact that the polyelectrolyte chains are not degraded during the process of particle formation. Good stability was also found in subsequent studies of catalytic activity. Two different amounts of platinum have been introduced into the brush particles to yield the two systems termed Pt1 and Pt3 (Table 2). This was done by exchange of the counterions through washing the particles with a solution of H2PtCl6 of different concentrations (Pt1: 2.5 × 10-4 mol/L; Pt3: 2 × 10-3 mol/L). Figure 3a and b displays the TEM pictures of both systems. For the sake of comparison, only one latex particle is shown. These micrographs therefore present the composite particles in the dry state. Figure 4, however, displays the micrographs resulting from cryo-transmission electron microscopy (cryo-TEM). Here the composite particles are shown in situ, that is, directly in aqueous solution. Moreover, the total amount of platinum was determined by thermogravimetric analysis (TGA). This demonstrated that ca. 95% of the platinum ions were converted to nanoparticles. First, the micrographs demonstrate that Pt nanoparticles of ca. 2 nm diameter have been generated by the reduction of PtCl62- ions. For the system Pt1, we obtain 1.9 ( 0.4 nm whereas the system Pt3 has slightly larger particles (2.1 ( 0.4 nm). This difference is small and within the limits of error. Hence, increasing the number of platinum ions leads to an increase in the number of nanoparticles but not their size. This must be traced back to a nucleation mechanism related directly to the environment provided by the polyelectrolyte brush. The process of nucleation that seems to be supported by the brush layer leads to a rather high number of small particles. Moreover, the nucleation and growth of the particles must

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Figure 3. Transmission electron microscopy images of composite particles Pt1 (a) and Pt3 (b).

Figure 4. Cryo-transmission electron microscopy of Pt3 composite particles.

proceed practically instantaneously. This can be inferred from the rather narrow size distribution of the particles. Although the ordinary TEM micrographs display only particles in which the dry layer of polyelectrolyte chains has been collapsed, cryo-TEM shows the particles in situ. The contrast of the micrographs shown in Figure 3b is not sufficient to show the polyelectrolyte chains. It demonstrates, however, that practically all nanoparticles are located near the surface of the core particles. This finding can be understood from the fact that there must be a marked dispersion attraction between the nanoparticles and the cores. The process of particle formation can hence be envisioned as follows: The nucleation and growth of particles takes place within the brush layer. The nanoparticles cannot leave the brush layer attached to the surface of the core particles but migrate to their surface. No aggregation of the nanoparticles on the surface takes place, and the composite systems remain stable for a prolonged time. In conclusion, all prerequisites for a quantitative investigation of the catalytic activity of the nanoparticles are therefore given in this system. Having discussed the synthesis and structure of the composite particles, we now turn to the catalytic properties of these particles. As a model reaction, we used the reduction of p-nitrophenol by NaBH4. In this way, the resulting data can be directly compared to the data supplied in the literature.9,28,29 In all runs to be discussed here, the concentration of NaBH4 was chosen to exceed the concentration of p-nitrophenol by far. Thus, the kinetics of the reduction can treated as pseudo-first-order

Figure 5. Catalytic reduction of p-nitrophenol in the presence of Pt3 composite particles. The absorbance A(t) measured at different times t (in minutes) indicated in the graph is plotted against wavelength λ. The reaction conditions were as follows: [p-nitrophenol] ) 0.1 mmol/L, [NaBH4] ) 10 mmol/L, [Pt composites] ) 1.72 mg/L, and T ) 25 °C.

in phenol concentration, which simplifies the present analysis.28 More importantly, the large excess of NaBH4 takes into account the slow but noticeable hydrolysis of this reagent at pH 10. Because the concentration of platinum-composites particles in the system is very low, the absorption spectra of 4-nitrophenol are not disturbed by the presence of the platinum nanocomposites. The process of reduction can be monitored by measuring UV/vis spectra at different times t. Here the ratio of the extinction A(t) of the system at a given time t to the extinction A0 measured at t ) 0 gives the corresponding ratios c/c0 of the concentrations directly. Figure 5 displays a typical example of such an analysis. The characteristic peak of p-nitrophenol at 400 nm decreases with time, and a new peak at 290 nm appears that is due to p-aminophenol.9,28 We assumed that reduction rates were independent of the concentration of sodium borohydride because it was in excess compared to p-nitrophenol.29 The first approximation was to fit the kinetic data with a first-order rate law. Moreover, the apparent rate constant kapp was assumed to be proportional to the surface S of the metal nanoparticles present in the system

-

dct ) kappct ) k1Sct dt

(1)

where ct is the concentration of p-nitrophenol at time t

High Catalytic Activity of Platinum Nanoparticles

Figure 6. Influence of platinum composite particles’ (Pt3) concentration on the reduction of p-nitrophenol. The concentrations of the reactants were as follows: [p-nitrophenol] ) 0.1 mmol/L and [NaBH4] ) 10 mmol/L, with T ) 25 °C. The parameter of the different curves is the concentration of composite particles in the solution. O, 0.86 mg/L; 0, 1.72 mg/L; 4, 2.58 mg/L; and 9, 3.44 mg/L.

Figure 7. Analysis of the rate constant kapp obtained from Figure 6. (a) Rate constant kapp as a function of the concentration of platinum composites particles. (b) Rate constant kapp as a function of the surface area of platinum nanoparticles normalized to the unit volume of the system. 9, Pt3 system; b, Pt1 system.

and k1 is the rate constant normalized to S, the surface area of platinum nanoparticles normalized to the unit volume of the system. In all runs to be discussed here, linear plots of ln(c/c0) versus t have been obtained. Figure 6a displays a typical example of this analysis. Also, the values of the rate constant are found to increase linearly with the concentration of platinum composites particles. This is demonstrated in Figure 7. Additional experiments demonstrated that no reduction takes place without the catalyst. It is therefore evident that the conversion is solely due to the presence of the composite particles. A feature commanding attention is the delay time t0 resulting from the intercepts of the curves shown in Figure 6 with the abscissa. The reaction does not start immediately but only after some delay that may be due to the activation of the catalyst in the reaction mixtures. Here, additional experiments demonstrated that this delay time is greatly diminished when the catalyst particles have already been used once for this reaction. Parts a and b of Figure 7 show apparent rate constant as a function of the concentration of platinum composite particles and the total surface area of platinum particles in the system, respectively. Evidently, kapp is found to vary strictly linearly with the number of catalytic composite particles. In the case of system Pt1, the weight concentration of the composite particles was 5.6 mg/L whereas in the case of Pt3 only 1.72 mg/L has been used. Moreover, Figure 7b strongly suggests that the activity is related to the total surface S of the nanoparticles immobilized in the unit volume of the carrier suspension. Here the total surface of the Pt nanoparticles has been calculated from the thermogravimetric analysis and the average size of the particles as determined by TEM. The catalysis takes

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Figure 8. Influence of temperature on the kinetic constant k measured with composite system Pt1. The parameter of the different curves is temperature T. O, 283 K; 0, 288 K; 3, 293 K; b, 303 K; and 9, 313 K. The concentrations are as follows: [Pt1] ) 5.60 × 10-3g/L, [p-nitrophenol] ) 0.1 mmol/L, and [NaBH4] ) 10 mmol/L. The inset gives the delay time t0 of each run.

Figure 9. Arrhenius plot of the reaction rate k measured in the presence of the composite particles Pt1 (b) and Pt3 (9). The triangles (2) give the reciprocal delay time 1/t0.

place on the surface of the nanoparticles as expected, and kapp is expected to depend only on S and the temperature T. Figure 8 shows the dependence of the rate of reaction on temperature. Again, the pseudo-first-order kinetics is seen at all temperatures T. Moreover, the delay time t0 is reduced with increasing temperature as expected. Figure 9 demonstrates that the reaction constant k that follows from Figure 8 is fully described by a conventional Arrhenius expression. Both systems Pt1 and Pt3 are described by the same activation energy EA ) 43.7 kJ/ mol. The preexponential factor is different in both cases because different numbers of Pt composite particles have been used (see above). Figure 9 demonstrates that the mechanism that leads to the delay time t0 has the nearly the same activation energy (45 kJ/mol) as the reduction. This suggests that the delay of the reaction is related to the same rate-determining step as the reduction itself. One may speculate that this step is related to the replacement of adsorbed water molecules on the surface of the nanoparticles by the reactants. Further kinetic studies are necessary, however, to elucidate this point further. Comparison with Literature. Finally, we compare the activity of our nanoparticles stabilized by cationic SPB with the activity of Pt nanoparticles reported in the literature. Table 3 gives a survey of the studies suitable for this comparison. In all cases summarized in Table 3, Pt nanoparticles have been used for the reduction of p-nitrophenol with a large excess of NaBH4. As reported by S. K. Ghosh et al.,29 platinum nanoparticles with an average size of 20 ( 2.5 nm (Table 1 of ref 29) can be synthesized from micellar solutions. These particles were used as the catalyst in the reduction of p-nitrophenol by sodium borohydride. Gosh et al. obtained a rate constant of kapp ) 7.5 × 10-3 min-1 (1.25 × 10-4 s-1)

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Table 3. Catalytic Activity of the Pt Nanoparticles: Comparison with Literature sample

T [°C]

kappa [s-1]

k1b,c [s-1m-2L]

[Pt] [mmol/L]

dd [nm]

Sb,e [m2/L]

ref 29 Pt3 ref 9 Pt3

30 30 15 15

1.25 × 10-4 9.50 × 10-3 6.01 × 10-2 3.47 × 10-3

1.05 × 10-2 1.11 1.65 × 10-1 5.62 × 10-1

8.69 × 10-3 6.60 × 10-4 2.00 × 10-2 6.60 × 10-4

20 ( 2.5 2.1 ( 0.4 1.5 ( 0.28 2.1 ( 0.4

1.19 × 10-2 8.58 × 10-3 3.64 × 10-1 8.58 × 10-3

a Apparent rate constant. b Calculated from the data given in the respective papers. c Rate constant normalized to the surface of the particles in the system (eq 1). d Diameter of the platinum particles. e Surface area of platinum nanoparticles normalized to the unit volume of the system.

with 4.0 µg (8.69 × 10-3 mmol/L) of platinum catalyst at 30 °C (pure platinum; see Table 2 of ref 29). The concentration of NaBH4 was given as 13 mmol/L. In the present study, the rate constant is given by kapp ) 9.50 × 10-3 s-1 with 6.60 × 10-4 mmol/L platinum nanoparticles at the same temperature whereas [NaBH4] ) 10 mmol/L (see above). In the case studied by Gosh et al., the particle size was 20 ( 2.5 nm whereas the size is only 2.1 ( 0.4 nm in this investigation. Hence, the surface area is approximately 10 times larger for the same amount of platinum. Table 3 gives the rate constant reduced to the surface of the particles present in the unit volume of the solution. It demonstrates that the smaller particles used in the present study exhibit a considerably higher activity when the apparent rate constants are reduced to the surface of the particles. Esumi et al.9 synthesized platinum nanoparticles in the presence of poly(propyleneimine) dendrimers of various generations. These particles are narrowly distributed (1.5 ( 0.28 nm, Table 3 of ref 9) and were used as the catalyst in the reduction of p-nitrophenol by sodium borohydride. The rate constants were affected by the concentration and generation of dendrimers used in the preparation of platinum nanoparticles. In the case of thirdgeneration dendrimers (Table 5 of ref 9), Esumi and coworkers got the highest rate constant of kapp ) 6.01 × 10-2 s-1 with 2.0 × 10-2 mmol/L platinum nanoparticles (3.64 × 10-1 m2/L, recalculated from ref 9) at 15 °C. This value decreases, however, if more dendrimers are used for the stabilization of the particles (Table 5 of ref 9). In this study, a rate constant of kapp ) 3.47 × 10-3 s-1 with 6.60 × 10-4 mmol/L platinum catalyst (8.58 × 10-3 m2/L) was found for the same temperature. Table 3 demonstrates that the reduced rate constants k1 are of the same order of magnitude.

The comparison summarized in Table 3 seems to suggest that the absolute size of the Pt nanoparticles is the decisive factor. Nanoparticles of ca. 2 nm diameter are more active catalysts than 20 nm particles. The carrier systems, however, are quite different for the three cases under consideration here. Moreover, the data published by Esumi et al.9 point to the fact that the nature and amount of the stabilizing agent have a profound influence on kapp (Table 5 of ref 9). Hence, a fully quantitative comparison must use the same carrier system. Research along these lines is under way. Conclusions We have presented a study on the catalytic activity of platinum nanoparticles immobilized on spherical polyelectrolyte brushes. The divalent PtCl6-2 ions were bound as counterions within the brush layer and reduced to yield nearly monodisperse nanoparticles of metallic platinum (average size: 2 nm). High catalytic activity was found when photometrically monitoring the reduction of pnitrophenol by NaBH4 in the presence of the nanoparticles. The analysis of the kinetic data showed that the reaction is pseudo-first-order with regard to p-nitrophenol. The activation energy was found to be 43.7 kJ/mol. Moreover, the delay time t0 observed in all runs using fresh catalyst was found to have approximately the same activation energy. All data demonstrate that spherical polyelectrolyte brushes present an ideal carrier system for metallic nanoparticles. Acknowledgment. We thank the Deutsche Forschungsgemeinschaft, Sonderforschungsbereich 481, Bayreuth, and BASF-AG for financial support. LA052120W