Synthesis, Characterization, and Catalytic Properties of Colloidal

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Langmuir 1997, 13, 6465-6472

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Synthesis, Characterization, and Catalytic Properties of Colloidal Platinum Nanoparticles Protected by Poly(N-isopropylacrylamide) Chun-Wei Chen and Mitsuru Akashi* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890, Japan Received June 16, 1997. In Final Form: September 10, 1997X We have studied the formation of platinum nanoparticles in an ethanol-water mixture with an ethanol volume fraction of 0.6 by ethanol reduction of PtCl62- in the presence of poly(N-isopropylacrylamide) (PNIPAAm-Pt) by UV-visible spectrophotometry. As predicted by calculation, a maximum at 215 nm in the absorption spectrum of the PNIPAAm-Pt sol is observed. The Pt nanoparticles can be easily transferred into distilled water, and their average particle sizes as measured by transmission electron microscopy are 10-30 Å. The colloidal Pt nanoparticles that were protected by PNIPAAm with a mean molecular weight of 6000 exhibited inverse temperature dependent solubility and a cloud-point temperature of 34.2 °C in an aqueous solution. Because of the phase separation of PNIPAAm-Pt from the continuous phase containing the substrate above 38 °C, the temperature dependence of the reaction rate did not follow normal Arrhenius behavior in an aqueous hydrogenation of allyl alcohol. The mean molecular weight of PNIPAAm and the monomeric unit/Pt molar ratio significantly affected the reduction rate of PtCl62-, as well as the average diameter and the catalytic activity of the particles.

Introduction Colloidal metal particles are of continuing interest because of their fascinating catalytic,1 electronic,2 and optical properties.3 Owing to their extremely large surface area, monometallic and bimetallic noble metal colloids have been used as active catalysts in the hydrogenation of alkenes in biphasic or organic media,4 hydrosilylation of olefins in organic solutions,5 and the oxidation of carbon monoxide in aqueous solutions.6 On the other hand, ultrafine particles often agglomerate to form either lumps or secondary particles in order to minimize the total surface or the interfacial energy of the system.7 Therefore, it is very important to stabilize the particles against adverse agglomeration at both the synthesis and the usage stages. In order to accomplish this, two methods are commonly used: the first causes dispersion by electrostatic repulsion, and the second by the steric forces. The dispersion of fine particles in liquid media by surfactants has been studied intensively.8 Recently, a great deal of attention has been given to the synthesis of colloidal metal particles that are stabilized within a polymer matrix.9 Soluble polymers,1 block copolymer micelles,10 and polymeric cross-linked gels11 or * To whom correspondence should be addressed. X Abstract published in Advance ACS Abstracts, November 1, 1997. (1) (a) Hirai, H.; Chawanya, H.; Toshima, N. React. Polym. 1985, 3, 127. (b) Schmid, G., Ed.; Clusters and Colloids; VCH: Weinheim, 1994. (2) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (3) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 202. (b) Caro, D.; Agelou, V.; Duteil, A.; Chaudret, B.; Mazel, R.; Roucau, C.; Bradley, J. S. New J. Chem. 1995, 19, 1265. (4) (a) Larpent, C.; Brisse-Lemenn, F.; Patin, H. New J. Chem. 1991, 15, 361. (b) Boutonnet, M.; Kizling, J.; Touroude, R.; Maire, G.; Stenius, P. Appl. Catal. 1986, 20, 163. (5) Lewis, L. N. J. Am. Chem. Soc. 1990, 112 (16), 5998. (6) Haruta, M.; Kobayashi, T.; Sano, H.; Yamada, N. Chem. Lett. 1987, 405. (7) Chow, G. M.; Gonsalves, K. E. In Nanomaterials: Synthesis, properties and applications; Edelstein, A. S., Cammarata, R. C., Eds.; American Institute of Physics: Woodbury, NY, 1996; p 55. (8) (a) Real, M. J., Ed. Surfactants and Interfacial Phenomena, 2nd ed.; Wiley: New York, 1989. (b) Bo¨nnemann, H.; Braun, G.; Brijoux, W.; Brinkmann, R.; Tilling, S. A.; Seevogel, K.; Siepen, K. J. Organomet. Chem. 1996, 520, 143. (c) Liz-Marza´n, L. M.; Lado-Tourin˜o, I. Langmuir 1996, 12, 3585.

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solid-state resins12 are used in this system. Many research studies on colloidal metal particles that are protected by soluble polymers have focused on the control of particle sizes and shapes and their growth kinetics.13 Furthermore, the relationship between particle structures and catalytic activity and/or selectivity has also been studied.14 However, there is another advantage in the use of polymerprotected metal particles as catalysts; that is, the protective polymer serves not only as a stabilizer but as a functional component that assists the particle in its work as an active and selective catalyst. Toshima15 reported that neodymium ions, which were immobilized on protective polymers, did promote the catalytic activity of the palladium particles for the hydrogenation of acrylic acid. In another example, ultrafine Rh particles that were protected by a copolymer of methyl acrylate and N-vinyl2-pyrrolidone were immobilized on a polyacrylamide gel by forming an amide bond between the primary amino group of the support and the methyl acrylate residue in the protective polymer. The preferential hydrogenation of hydrophilic substrates by Rh particles was observed.16 In our previous paper,17 we reported the preparation of poly(N-isopropylacrylamide)-protected ultrafine platinum particles (PNIPAAm-Pt) by means of alcohol reduction. The colloidal dispersion was easily transferred to an aqueous phase, and the particle size was sharply distributed in a range between 10 and 50 Å. The average particle diameter was estimated to be 23.8 Å. The aqueous (9) (a) Hirai, H.; Toshima, N. In Tailored Metal Catalyst; Iwasawa, Y., Ed.; Reidel: Dordrecht, 1986. (b) Bradley, J. S.; Hill, E.; Leonowicz, M. E.; Witzke, H. J. Mol. Catal. 1987, 41, 59. (10) Antonietti, M.; Thu¨nemann, A.; Wenz, E. Colloid Polym. Sci. 1996, 274, 795. (11) Sergeev, G. B.; Gromchenko, I. A.; Petrukhina, M. A.; Prusov, A. N.; Sergeev, B. M.; Zagorsky, V. V. Macromol. Symp. 1996, 106, 311. (12) Nakamura, Y.; Hirai, H. Chem. Lett. 1976, 1197. (13) (a) Esumi, K.; Wakabayashi, M.; Torigoe, K. Colloids Surf., A 1996, 109, 55. (b) Curtis, A. C.; et al. Angew. Chem., Int. Ed. Engl. 1988, 27, 1530. (14) Clint, J. H.; et al. Faraday Discuss. Chem. Soc. 1993, 95, 219. (15) Teranishi, T.; Nakata, K.; Miyake, M.; Toshima, N. Chem. Lett. 1996, 277. (16) Ohtaki, M.; Komiyama, M.; Hirai, H.; Toshima, N. Macromolecules 1991, 24, 5567. (17) Chen, C.-W.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 1329.

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colloidal dispersion retained the inverse temperature dependent solubility of the protective PNIPAAm. Consequently, the PNIPAAm-Pt colloids underwent a reversible phase separation and a decrease in their reaction rate by a factor of 30% or more when the reaction temperature was increased from 20 to 40 °C. In this paper, we have analyzed the formation of the colloidal platinum nanoparticles that were protected by PNIPAAm by means of UV-visible spectrophotometry. We examined the average particle diameter and size distribution of the colloidal dispersions by electron microscopy after the nanoparticles were transferred into distilled water. We also compared the thermally induced phase separation of the colloidal Pt nanoparticles to the solution behavior of PNIPAAm by cloud-point measurement. Kinetic studies of the hydrogenation of allyl alcohol in an aqueous solution were performed over the colloidal platinum dispersions at different temperatures under atmospheric H2 pressure. Furthermore, the correlation between preparation parameters and catalytic activities for the Pt nanoparticles was analyzed in detail. Experimental Part Materials. Poly(N-vinyl-2-pyrrolidone) (PVP-K30, degree of polymerization 360, extra pure grade) and chloroplatinic acid (H2PtCl6‚6H2O, guaranteed reagent grade) were used without further purification as received from Nacalai Tesque, Inc. Distilled deionized water was used for all of our preparations. Ethanol was of a special grade that was purchased from Nacalai Tesque, Inc. Preparation Procedures. Synthesis of Poly(N-isopropylacrylamide). PNIPAAm with different mean molecular weights were obtained by radical polymerization at 60 °C under a nitrogen atmosphere according to the previously reported method.18 From gel permeation chromatographic (GPC) analysis, the numberaverage molecular weights of the polymers were ca. 3000, 4000, and 6000. Preparation of Aqueous Colloidal Pt Particles Protected by PNIPAAm (PNIPAAm-Pt) and PVP (PVP-Pt). The platinum particles with different monomeric unit/Pt molar ratios of 20, 40, 60, and 80 were prepared by refluxing a solution of chloroplatinic acid and PNIPAAm (mean molecular weight ) 6000) in ethanol/water as our previously reported method.17 A PNIPAAm with a mean molecular weight of 4000 or 3000 was used to prepare the Pt nanoparticles instead of a PNIPAAm with mean molecular weight of 6000. The same method was used to prepare ultrafine platinum particles that were protected by PVP (PVP-Pt). Evaporation the solvent from the dispersion solutions and then redissolving the residues in distilled water did give stable aqueous dispersions of PNIPAAm-protected Pt colloids. Physical Measurements. UV-Visible Spectrophotometry. The formation of Pt nanoparticles was followed by recording the UV-visible absorption spectra on a JASCO Model V-550 recording spectrophotometer as a function of refluxing time and the data were corrected for ethanol/water background absorption. Determination of Cloud-Point Temperature. The lower critical solution temperature (LCST) of the PNIPAAm in an aqueous solution was determined by measuring the absorbance at 500 nm on the same spectrophotometer in a 1.0-cm path-length quartz cell. The temperature was raised from 25 to 50 °C in 1.0 °C increments every 1.0 min and was monitored with a temperature controller (JASCO ETC-505T). The LCST was defined as the temperature at the initial break point in the resulting absorbance versus the temperature curve. The cloud-point temperature of the PNIPAAm-protected Pt colloidal dispersion was determined by the same method. Transmission Electron Microscopy (TEM). TEM images were obtained with a Hitachi H-700H microscope operating at an acceleration voltage of 150 kV at a magnification of 200 000. Specimens were prepared by first diluting the aqueous sols with distilled water to about 2.3 × 10-4 M [Pt] and then allowing a drop of the sols to be supported over a collodion film which was (18) Chen, M. Q.; Kishida, A.; Akashi, M. J. Polym. Sci., Part A: Polym. Chem. 1996, 34, 2213.

Chen and Akashi coated with a carbon layer and supported on a copper grid. We measured the size distributions of the particles from enlarged photographs of the TEM images using at least 150 counts. Hydrogenation of Allyl Alcohol in an Aqueous Solution. Catalytic hydrogenation reactions of allyl alcohol with at least three temperature cycles (40 °C f 20 °C f 40 °C f 20 °) under atmospheric pressure were performed as described in detail elsewhere.17 Temperature Dependence of the Reaction Rate. PNIPAAm-Pt or PVP-Pt sol was used to catalyze the hydrogenation of allyl alcohol in water at different temperatures. Samples of the reaction mixture (ca. 0.1 mL) were withdrawn during the hydrogenation at intervals and analyzed by gas chromatography in order to determine the allyl alcohol concentration. The rate constant was obtained from the slope of the straight line of ln(C0/C) against time and was used to construct the plot of rate constant (k) versus the temperature and the Arrhenius plot, namely, the plot of ln k versus the reciprocal of the absolute temperature.

Results and Discussion 1. Synthesis of Colloidal Pt Nanoparticles. It is relatively easy to synthesize the colloidal metal particles using the aqueous alcohol reduction method in the presence of poly(vinylpyrrolidone) (PVP) because the protective polymer is completely soluble in water up to the refluxing temperature in the absence of electrolytes. Poly(N-isopropylacrylamide) (PNIPAAm), however, exhibits a reversible phase transition at its lower critical solution temperature (LCST) in an aqueous solution.19 Moreover, cononsolvency can be observed when an organic solvent such as methanol or ethanol is mixed with an aqueous solution of PNIPAAm.20 Tirrell et al.21 found a decrease of ca. 40 °C in the LCST of PNIPAAm in the water-methanol mixture when the volume fraction of methanol was increased from 0 to 0.55. Above a volume fraction of methanol of 0.66, however, the LCST exceeded the boiling point of the mixture. In order to obtain a stable polymer-supported metal colloidal sol by means of the alcohol-reduction method, one should protect the protective polymer against precipitating upon refluxing the reaction mixture. We observed the solution behavior of PNIPAAm in ethanol/water mixture solvents. Our results showed that the PNIPAAm is soluble from room temperature to the boiling point of ethanol/water mixtures at ethanol volume fractions that are higher than 0.55. At the same time, water is of great importance in the preparation of colloidal metal sols by means of the alcohol-reduction method. Hirai et al.22 demonstrated that water is indispensable in regard to the reduction of Rh(III) ion with methanol in the presence of PVP. A similar dependence of reduction of PtCl62- on water was observed in the synthesis of PNIPAAm-Pt sols by means of ethanol reduction. Above a volume fraction of water of 0.1, stable colloidal Pt sols of dark brown were obtained in the presence of PNIPAAm with a mean molecular weight of 6000 in an induction period of 12 min. No color change was observed in 4 h, however, when ethanol was used as a solvent without water. Accordingly, we successfully synthesized the PNIPAAm-protected Pt nanoparticles when the ethanol volume fractions ranged from 0.6 to 0.9. Colloidal Pt nanoparticles were synthesized by ethanol reduction in the presence of PNIPAAm with different mean molecular weights at various molar ratios of monomeric unit/Pt. The optical and timing parameters for these (19) Hoffman, A. S. Artif. Organs 1995, 19 (5), 458. (20) Schild, H. G. Prog. Polym. Sci. 1992, 17,163. (21) Schild, H. G.; Muthukumar, M.; Tirrell, D. A. Macromolecules 1991, 24, 948. (22) Hirai, H. Makromol. Chem., Suppl. 1985, 14, 55.

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Table 1. Formation of Colloidal Pt Nanoparticles by Means of Ethanol-Reduction Method in the Presence of PNIPAAma mean mol wt

molar ratio of monomeric unit/Pt

induction period (min)

boiling duration (h)

color

av diameter (Å)

size distribution, B.D. (Å)

3000 4000 6000 6000 6000 6000

40/1 40/1 20/1 40/1 60/1 80/1

55 25 10 12 28 40

2.5 2.0 0.7 1.0 1.1 2.0

yellow-brown yellow-brown dark-brown dark-brown yellow-brown yellow-brown

14.7 20.9 26.7 23.8 21.2 20.3

5.7 6.3 6.9 6.3 6.3 6.2

a

For detail see Experimental Section.

syntheses are given in Table 1. We found that the degree of polymerization of the protective polymers and the molar ratio of monomeric unit/Pt influenced the reduction rate of the PtCl62- by ethanol, as well as the size distribution, stability, and the catalytic activity of the colloidal Pt nanoparticles. The reduction rate of the PtCl62- by ethanol did not show marked change as the ethanol volume fractions were increased from 0.6 to 0.9. However, the rate increased with an increase in the mean molecular weight from 3000 to 6000 at the same monomeric unit/Pt molar ratio of 40. Using PNIPAAm with mean molecular weights of 3000 and 4000 instead of one with 6000 also yielded stable colloidal Pt nanoparticles, but the induction periods were 55 and 25 min, respectively. At the same time, the color of the final PNIPAAm-Pt sols varied from yellow-brown to deep brown with an increase in the molecular weight. The color of the sols coincided with the particle size in the polymer-protected colloidal metal sols. Furlong et al.23 found that PVP-protected Pt sols with small and well-separated particles corresponded to yellowbrown colors and coagulated sols to black-brown ones. The same was true in this paper, as was demonstrated by the TEM measurements described below. The coordination of metal ions to protective polymers plays an important role in the synthesis of polymerprotected colloidal metal sols by the alcohol-reduction method. A different coordination ability in regard to PVP between platinum atoms and gold atoms has been used to determine the surface structure of Au/Pt bimetallic cluster particle that was protected by PVP.24 PNIPAAm shows high coordination affinity with both platinum ions and metal particles just as PVP serves as the protective polymer. The strong interaction of PNIPAAm with Pt ions results in a lower reduction rate by ethanol. On the other hand, the strong interaction of PNIPAAm with Pt particles prevents or slows the migration and agglomeration of the metal particles. Thus, the lower rate of reduction and the smaller particle sizes can be attributed to an increase in the number of molecules in the solution, when PNIPAAm with lower mean molecular weight was used in the synthesis of Pt colloids at the same monomeric unit/Pt molar ratio. It is suggested that an increase of the number of molecules may lead to an increase in the coordination affinity of PNIPAAm to either Pt ions or Pt in zero-valence states. Duteil et al.25 has reported that the reaction of Ru(cod)(cot) with hydrogen in the presence of nitrocellulose (NC) or cellulose acetate (CA) at 20 °C yields colloidal ruthenium sols. In the case of CA the smaller number of coordination sites and the poorer ligand properties of hydroxy and acetate groups compared to the nitro ones explained the increase in both the decomposition rate of Ru(cod)(cot) and the particle sizes. Just as surfactant concentration is used to both control the rate of particle growth and impart useful chemical

behavior (shape or catalytic activity, for example) to the final nanocrystal product,26 the particle sizes and shapes can be changed by altering the concentration of the capping polymer.27 The induction time necessary for the reduction of PtCl62- in the presence of PNIPAAm with a mean molecular weight of 6000 also varied with the monomeric unit/Pt molar ratio, requiring 10, 12, 28, and 40 min, for samples with ratios of 20, 40, 60, and 80, respectively. By increasing the molar ratio of the monomeric unit/Pt, the coordination affinity of polymer to metal ions is improved, thereby inducing a decrease in the rate of reduction of PtCl62-. The presence of protective polymer prevents the growth of the particles, and smaller agglomerates are formed with a decrease in the interactions among the particles. Thus, the increase in monomeric unit/Pt molar ratio inducing a decrease in the reduction rate of Pt ions and an increase in the protecting effect of the polymer against the growth of particles favors the formation of small particles, as described below. 2. Transferring Colloidal Pt Nanoparticles into Water. Reduction of PtCl62- in ethanol/water mixtures in the presence of PNIPAAm yields colloidal dispersions of PNIPAAm-protected platinum nanoparticles as transparent homogeneous solutions. The colloidal dispersions are stable in air for several months or longer. A major objective of this study is to relate the inverse temperature dependent solubility of the protective PNIPAAm and the catalytic activity of the colloidal Pt nanoparticles in water. The possibility of solvent exchange presents one of the main challenges to the preparation of metal colloids in the nanometer size range. This should allow for dispersions of the particles in both preparation and catalytic reaction solvents and avoid particle aggregation during the process. The adsorption of PNIPAAm on the surface of the Pt nanoparticles may be attributed to both hydrophobic interaction from the main chain and coordination from the amide groups. The interactions between the polymer and the metal surface can stabilize the Pt nanoparticles to such an extent that they could be isolated in a solid state. A brown gum of PNIPAAm-protected Pt nanoparticles was obtained after the ethanol/water mixture was removed by evaporation. When distilled water of the same volume was added, the particles were redispersed and were stable in air for several months. 3. Characterization of Colloidal Pt Nanoparticles. Optical Properties of the Colloidal Pt Nanoparticles. Colloidal dispersions of metals exhibit absorption bands or broad regions of absorption in the UV-visible range. These are due to the excitation of plasma resonances or interband transitions and are a characteristic property of the metallic nature of the particles.28 Figure 1 shows the absorption spectrum at various times of the reduction. In the progress of the metal-ion-to-colloid reduction by methanol, Duff et al.29 proposed a two-step reaction

(23) Furlong, D. N.; Launikonis, A.; Sasse, W. H. F.; Sanders, J. V. J. J. Chem. Soc., Faraday Trans. 1984, 80, 571. (24) Toshima, N.; Yonezawa, T. Makromol. Chem., Makromol. Symp. 1992, 59, 281. (25) Duteil, A.; Queau, R.; Chaudret, B.; Mazel, R.; Roucau, C. Chem. Mater. 1993, 5, 341.

(26) Lisiecki, I.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 4160. (27) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (28) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87 (24), 3881.

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Figure 1. UV-visible spectrum of a solution at various reduction times. The evacuated solution contained 2.7 × 10-4 M H2PtCl6‚6H2O, 2.0 × 10-4 M PNIPAAm, and ethanol/water (6/4, v/v). Spectra from 200 to 300 nm were amplified on the right upper corner.

mechanism where PtCl42- serves as the intermediate. In our synthesis, we suggested the following reduction of PtCl62-:

PtCl62- + CH3CH2OH f PtCl42- + CH3CHO + 2H+ + 2Cl- (a) PtCl42- + CH3CH2OH f Pt0 + CH3CHO + 2H+ + 4Cl- (b) The PtCl62- anion had a sharp absorption maximum at 260 nm, and the intensity of the peak was used to follow the reduction. The PtCl42- anion had an absorption peak at 220 nm and showed negligible absorbance at 260 nm by comparison.30 The absorption of the colloidal Pt0 sol was monitored at 500 nm. The absorption of PtCl62- at 260 nm decayed slowly during the first 10 min, while the absorption at 500 nm was not observed. The absorption of PtCl42- at 220 nm was observed even at the beginning of the boiling and showed a slight decrease in its intensity before the induction period. After an induction period of 12 min, the absorption of PtCl62- markedly decreased and the absorption of the sol at 500 nm also occurred. At this time, an obvious color change from yellow to brown was observed during the synthesis. This indicates that a higher concentration of Pt(II) precursor is required in order to build up a critical concentration of Pt0 atoms that is necessary for nucleation to occur. It is worthy to note that the adsorption of PtCl62- at 260 nm decreased rapidly after the development of the Pt0 colloid and no marked changes in the optical spectrum of the solution occurred after 20 min. We regard this as evidence for complete reduction of Pt(IV) ions into the Pt0 sol. Therefore, the reduction of Pt ions by ethanol is probably autocatalytic at the particle surfaces and similar to the hydrogenation of Pt(cod)2 to metallic platinum that was previously studied by Lee.31 An absorption peak at 215 nm was observed after the appearance of the sol color varied, and its intensity increased with an increase in colloid absorption at 500 nm. In contrast to the situation of the peak at 215 nm, the absorption maximum of PtCl42- at 220 nm decreased (29) Duff, D. G.; Edwards, P. P.; Johnson, B. F. G. J. Phys. Chem. 1995, 99, 15934. (30) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129. (31) Lee, T. R.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 2568.

Chen and Akashi

slightly in the induction period and disappeared with the appearance of the peak at 215 nm. Although the PtCl42was also continuously yielded from the reduction of the PtCl62- anion after the induction period, it was reduced to Pt0 rapidly. A peak at about 215 nm is expected in the absorption of colloidal platinum calculated by Creighton and Eadon according to the Mie theory.28 The predicted absorption maximum at 215 nm was also observed in the spectrum of a colloidal Pt sol that was prepared by the radiolytic reduction of K2PtCl4 solution of methanol in the presence of sodium polyphosphate.30 Although there is still a difference in the intensity of the peak between the experimental and the calculated spectra, the spectrum was found to be independent of particle size (mean diameter is 23.8, 18, and 70 Å for the present Pt colloid and two Pt colloids in ref 30, respectively). In addition, the PNIPAAm-Pt absorption spectrum was also observed after it was transferred into an aqueous solution. We found that the UV-visible spectra of the colloidal Pt nanoparticles before and after the transfer were quite similar, having absorption peaks at 215 nm and comparable intensity, which indicated the quantitative transfer of the Pt particles with a similar dispersion by this procedure. TEM Images of the Colloidal Pt Nanoparticles. Figure 2 shows the transmission electron micrographs and the size distributions of the colloidal Pt nanoparticles protected by PVP and PNIPAAm. Some of these results are also summarized in Table 1. Reduction of PtCl62- by aqueous ethanol in the presence of protective polymers yielded well-dispersed Pt nanoparticles, but the particle size and size distribution varied with both the stabilizing polymer in question and the reaction conditions. Thus, in the presence of PVP with a mean molecular weight of 40 000 and, using the same monomeric unit/Pt molar ratio of 40, the reduction of PtCl62- gave well-dispersed Pt particles with a mean diameter of 35.7 Å and a narrow particle size distribution (see Figure 2a). Figure 2b indicates that the average particle size (diameter) and size distribution (standard deviation) of the resulting platinum colloids in water were 35.5 and 7.2 Å, respectively, which are nearly the same as those originally prepared by ethanol reduction. Liu et al. also showed that the average particle size and the size distribution of the colloidal metal particles could be retained after a similar transferring procedure.32 It was tested that the transferring procedure does not affect the dispersion state of the PNIPAAm-Pt sol. An electron microscopic investigation of the aqueous PNIPAAm-Pt sols revealed that particles with diameters between 5 and 45 Å were present and almost no obvious changes in particle size and distribution were observed after aging for months in air. Correspondingly, the UVvisible spectra of the Pt sols only showed a slight change in intensity with time at room temperature. Parts c, d, and e of Figure 2 show the TEM images and the size distributions of the Pt nanoparticles that were protected by PNIPAAm with mean molecular weights of 3000, 4000, and 6000 at the same monomeric unit/Pt molar rate of 40. The average diameters and size distributions of the three PNIPAAm-Pt sols were as follows: 14.7 Å, 5.7 Å; 20.9 Å, 6.3 Å; 23.8 Å, 6.3 Å. The particle sizes of the Pt nanoparticles increased with increasing mean molecular weights from 3000 to 6000. A similar dependence of particle size on the mean molecular weight of the protective polymers has been observed in the preparation of colloidal (32) Liu, H.; Toshima, N. J. Chem. Soc., Chem. Commun. 1992, 1095. (33) Hirai, H.; Wakabayashi, H.; Komiyama, M. Chem. Lett. 1983, 1047.

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Figure 2. TEM micrographs and size distributions of platinum nanoparticles that were protected by PVP and PNIPAAm: PVPPt, before (a) and after (b) the transfer; PNIPAAm-Pt, (c) 3000 and 40, (d) 4000 and 40, (e) 6000 and 40, (f) 6000 and 20, (g) 6000 and 80 for mean molecular weight and monomeric unit/Pt molar ratio, respectively. The scale bar represents 500 Å for all parts of the figure.

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Figure 3. Plots of absorbance of the solutions of PNIPAAm and PNIPAAm-protected colloidal Pt nanoparticles in water at 500 nm as a function of temperature. A plot of the rate constant for the hydrogenation that was catalyzed at the same temperatures. Conditions: catalyst, PNIPAAm-Pt (Mn ) 6000, monomeric unit/Pt molar ratio ) 40); Pt, 0.008 mmol (13.4 mL); allyl alcohol, 2 mmol; Substrate/catalyst ) 250; solvent, water, 20 mL; hydrogen, 1 atm.

copper dispersion by reduction of copper sulfate with sodium tetrahydroborate.33 Both hydrophobic interaction and coordination affinity can be attributed to the adsorption of protective PNIPAAm onto the surfaces of the Pt nanoparticles. The protective effect of the PNIPAAm increases with the concentration in question and favors the formation of small particles. At a monomeric unit/Pt molar ratio of 20, the average diameter and standard deviation of the Pt particles are ca. 26.7 Å and 6.9 Å, respectively. Furthermore, some particles in Figure 2f stuck together because of the low polymer concentration, which reflected the poor protective effect of polymer against van der Waals interactions. At ratios higher than 40, well-dispersed particles with smaller average diameters were observed (Figure 2e,g). Solution Properties of PNIPAAm and PNIPAAmPt. Variable temperature UV-visible spectroscopy studies of 0.1 wt % aqueous solutions at 500 nm show that both PNIPAAm and PNIPAAm-Pt exhibited cloud-point temperatures. As shown in Figure 3, the PNIPAAm with a mean molecular weight of 6000 exhibited a lower critical solution temperature (LCST) of 34.8 °C, while the phase separation of PNIPAAm-Pt in aqueous solution was also clearly observed at 34.2 °C. It is well-known that the addition of an inorganic salt to an aqueous solution of a LCST polymer will perturb its cloud-point temperature.34 The effectiveness of the anion is predominant in salting out the polymer.35 The formation of chloride ions in the reduction of PtCl62- is profitable to rationalize the suppression of the cloud-point temperature of PNIPAAm in an aqueous solution, even though the adsorption of the polymers on the surfaces of the Pt particles does not contribute toward the changes that take place in the overall hydrophobic and hydrophilic interactions among the polymers. The endothermic transitions were also observed upon heating aqueous PNIPAAm and PNIPAAm-Pt solutions in a differential scanning microcalorimeter (DSC). Transition temperatures that were determined calorimetrically confirmed the effects of the chloride ions in salting the protective polymer out of the solution. 4. Catalytic Properties of Colloidal Pt Nanoparticles. Temperature Dependence of the Rate. We (34) Schild, H. G.; Tirrell, D. A. J. Phys. Chem. 1990, 94, 4352. (35) Guner, A.; Ataman, M. Colloid Polym. Sci. 1994, 272, 175.

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Figure 4. First-order time-conversion plots of the hydrogenation of allyl alcohol catalyzed by PNIPAAm-Pt in water at 30 and 40 °C. Conditions are the same as in Figure 3.

Figure 5. Plots of ln k versus the reciprocal of the absolute temperature for the hydrogenation of the allyl alcohol catalyzed by PVP-Pt and PNIPAAm-Pt in water.

determined first-order rates in substrate concentration for temperatures between 0 and 50 °C in the hydrogenation of allyl alcohol using PNIPAAm-Pt or PVP-Pt as the catalyst. Figure 4 presents examples of first-order regressions in the hydrogenation of allyl alcohol catalyzed by PNIPAAm-Pt nanoparticles at 30 and 40 °C. Hirai16 also observed the first-order kinetics with regard to the hydrogenation of acrylic acid that was catalyzed by the dispersed and immobilized ultrafine rhodium particles. In Figure 3 we have also given the data that is relevant to the dependence of the hydrogenation rate on the phase separation of the PNIPAAm-Pt catalyst. As illustrated, since the catalyst was in a solution at temperatures below 34 °C, hydrogenation proceeded at higher rates and the rate increased with increases in temperature. When the catalyst was phase separated (>38 °C), however, the rate decreased sharply. The rate constant at 40 °C was 1.36 × 10-3 min-1, and was even lower than that at 10 °C. Furthermore, the higher rate hydrogenation occurred when the solution was cooled to below 34 °C, as shown below. This demonstrated that the colloidal Pt nanoparticles that were protected by PNIPAAm possessed inverse temperature dependent solubility; therefore, the catalyst was active when in solution and was less active when phase separated. In order to show the contrast between the normal temperature dependence of the reaction rate and the temperature dependence of the rate when a temperaturesensitive PNIPAAm-Pt sol was used as the catalyst, the hydrogenation of allyl alcohol catalyzed by PVP-Pt nanoparticles in water was also evaluated at several different temperatures. Figure 5 gives the Arrhenius plots of the natural logarithm of rate constant vs 1/T for the hydrogenation reactions that were catalyzed by PVP-Pt and PNIPAAm-Pt sols. As can be seen from Figure 5,

Pt Nanoparticles Protected by PNIPAAm

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Figure 6. Schematic illustration of moderating the catalytic activity of PNIPAAm-Pt though solubility changes resulting from temperature changes.

Figure 8. Effects of monomeric unit/platinum molar ratio on the catalytic activity of PNIPAAm-Pt at 20 and 40 °C. Conditions: the mean molecular weight of PNIPAAm was 6000; other conditions are the same as in Figure 3.

Figure 7. Effects of the molecular weight of PNIPAAm on the catalytic activity of PNIPAAm-Pt at 20 and 40 °C. Conditions: monomeric unit/Pt ) 40; other conditions are the same as in Figure 3.

the straight line indicates the normal Arrhenius behavior for the PVP-Pt catalyst. However, the precipitation of PNIPAAm-Pt from the solution above 38 °C leads to a heterogeneous catalytic reaction and a marked decrease in the reaction rate. Here, apparently, catalytic hydrogenation did not follow normal Arrhenius-type kinetics. It is significant that the reaction rate increased steadily before the cloud-point showing Arrhenius-like behavior. The decrease in activity of the PNIPAAm-Pt sol above the cloud point shows that the catalyst is phase separated from the continuous phase that contains the substrate, as is illustrated in Figure 6. Effects of the Mean Molecular Weight. In the preparation of colloidal Pt nanoparticles, the mean molecular weight of PNIPAAm showed marked effects on the reduction rate of PtCl62- and the particle size. We also tested its effects on the catalytic activity in the hydrogenation of allyl alcohol below or above the cloudpoint temperature (Figure 7). When an experiment was performed at 20 °C, the Pt nanoparticles that were protected by PNIPAAm with a mean molecular weight of 6000 showed higher activity than these with a weight of 3000 or 4000. At the same monomeric unit/Pt molar ratio, the number of molecules increased with a decrease in the mean molecular weight, which is contributive to the formation of small particles. However, the increase in molecules that were adsorbed on the surface of the particles resulted in a decrease in the effective surface area for the catalysis. A study of hydration of acrylonitrile that had been catalyzed by PVP-protected copper colloids showed a similar dependence of catalytic activity on the mean molecular weight of PVP.33 At 40 °C, changes in the mean molecular weight of the protective polymer seemed to have less effects on the hydrogenation rate, because the catalyst was phase separated from the solution. It is worthy to note that the LCST of PNIPAAm is dependent on the chain length and generally increases with a decrease in mean molecular weight from 73 000 to 5400.34 In our cloud-point measurements of PNIPAAmPt, the final break point in the transmittance versus the

temperature curve was 38 °C for the PNIPAAm with a mean molecular weight of 6000, whereas the break point was higher than 40 °C for PNIPAAm with a mean molecular weight of 3000 or 4000. Therefore, it is reasonable to suggest that the higher activity of Pt nanoparticles that were protected by PNIPAAm with a mean molecular weight of 3000 or 4000 at 40 °C is associated with the incomplete precipitation of the catalyst from the solution. Effects of the Monomeric Unit/Pt Molar Ratio. Figure 8 summarizes the dependence of the hydrogenation rate on the monomeric unit/Pt molar ratio for PNIPAAm with a mean molecular weight of 6000. At 40 °C a slight decrease in rate was observed in a ratio range of 20-80. This decrease is probably due to the lower diffusion rate of the substrate through the polymer layer which covers the catalytic surface, especially for the catalyst in the phase-separation state above 38 °C. When the reaction solution was cooled to 20 °C, the protective polymer PNIPAAm was rehydrated and the catalyst redissolved. In this case, the catalytic hydrogenation proceeded at 20 °C with a higher rate than 40 °C except for the catalyst of the 20 monomeric unit/Pt molar ratio. At a monomeric unit/Pt molar ratio of 20, the amount of PNIPAAm is too low to protect the Pt nanoparticles efficiently, and some particles stick together as is shown in the TEM image (Figure 2f). Then, the dispersed PNIPAAm-Pt nanoparticles at the ratio of 20 showed lower activity in the solution below the cloud-point temperature. At the same time, the reaction rate tended to decrease with rising monomeric unit/Pt molar ratio in the range 40-80. This indicates that the protective PNIPAAm adsorbed on the surface of the Pt nanoparticles interferes with the diffusion of the allyl alcohol toward the catalytic sites. By selecting a substrate of higher miscibility with the protective polymers, one would expect to see an increase in the diffusion rate to the catalytic sites at the particle surfaces. The above results lead to the conclusion that the monomeric unit/Pt molar ratio is an important factor not only for the synthesis of stable well-dispersed PNIPAAm-Pt nanoparticles but in regard to improving the activity and the selectivity of the catalysts. Conclusions Colloidal platinum nanoparticles that were protected by poly(N-isopropylacrylamide) were synthesized in an ethanol/water mixture solvent by the reduction of PtCl62with ethanol. The volume fraction of the ethanol in the mixture should be in the range of 0.6-0.9 because of the cononsolvency of PNIPAAm in the mixture solvent. The reduction rate of PtCl62-, the protective effects of

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PNIPAAm, and the particle size of the resulting Pt nanoparticles are regulated by the monomeric unit/Pt molar ratio and the mean molecular weight. The PNIPAAm-Pt nanoparticles are easily transferred into distilled water through a dry state and retain their average diameters and particle distributions. The predicted maximum at 215 nm, which was calculated by Creighton and Eadon according to the Mie theory,28 in the absorption spectrum of colloidal platinum was observed in this system. The cloud-point temperature of PNIPAAm-Pt was clearly observed at 34.2 °C in an aqueous solution. Because of the phase separation of the catalyst from water above 38 °C, the temperature dependence of the reaction rate is different from normal Arrhenius behavior. The activities of the catalyst for states both in solution below LCST and phase separation above LCST are affected by the mean molecular weight of the PNIPAAm and the monomeric unit/Pt molar ratio.

Chen and Akashi

Finally, we can point out that the inverse temperature dependent solubility of PNIPAAm not only is used to moderate the activity of the protected Pt nanoparticles through a temperature change but also provides a novel method to recover the catalyst from aqueous solutions. Ongoing research in our group suggests that the catalyst can retain its average diameter and particle distribution though several recovery processes. Acknowledgment. C.-W. Chen is indebted to the Ministry of Education, Science, Sports, and Culture, Japan. This work was financially supported in part by a Grant-in-Aid for Scientific Research (No.08651055) from the Ministry of Education, Science, Sports, and Culture, Japan. The authors wish to thank Mr. M. Q. Chen and Mr. T. Taguchi for their help with our experiments. LA970634S