Chem. Mater. 1999, 11, 1381-1389
1381
Preparation of Platinum Colloids on Polystyrene Nanospheres and Their Catalytic Properties in Hydrogenation† Chun-Wei Chen, Takeshi Serizawa, and Mitsuru Akashi* Department of Applied Chemistry and Chemical Engineering, Faculty of Engineering, Kagoshima University, 1-21-40 Korimoto, Kagoshima 890-0065, Japan Received January 4, 1999. Revised Manuscript Received March 17, 1999
Well-dispersed platinum colloids have been prepared on polystyrene nanospheres with surface-grafted poly(N-isopropylacrylamide) (PNIPAAm) via the reduction of PtCl62- by ethanol. A range of monodisperse nanospheres was obtained by emulsifier-free dispersion copolymerization of styrene with a PNIPAAm macromonomer. The surface compositions of the polystyrene nanospheres before and after immobilization of Pt colloids have been investigated by X-ray photoelectron spectroscopy. Peak fitting of the C 1s core-line spectra provides evidence for the presence of PNIPAAm chains at the surface of polystyrene nanospheres. These immobilized Pt colloids were found to be active and stable heterogeneous catalysts for the hydrogenation of allyl alcohol in water. Both the particle size and activity of platinum colloids showed a marked dependence on the level of surface PNIPAAm. The average diameter of an immobilized Pt colloid is 15.0 Å and largely unchanged after seven cycles in hydrogenation, as confirmed by TEM. The immobilized Pt colloid shows considerable increased activity as compared with the PNIPAAm-stabilized colloidal Pt sol. An unusual temperature dependence of catalytic activity was observed due to the heating-induced insolubility of PNIPAAm chains.
Introduction Continuous reduction of the size of a solid material to the nanometer scale results in quantum size effects at dimensions comparable to the length of the de Broglie electron, the wavelength of phonons, and the mean free path of excitons.1 Nanosized metal and semiconductor particles possess unique electronic, optical, and catalytic properties that are obviously different from bulk macrocrystallites.2,3 These nanoparticles are presently under intense study for potential uses in optoelectronic devices,4 in ultrasensitive chemical and biological sensors,5 and as catalysts in chemical and photochemical reactions,6 etc. They have a characteristic high surface* Corresponding author. Fax: 81-99-255-1229. E-mail: akashi@ apc.eng.kagoshima-u.ac.jp. † This paper is part XXV in the series of the study on Graft Copolymers Having Hydrophobic Backbone and Hydrophilic Branches. Part XXIV is as follows: Chen, C.-W.; Serizawa, T.; Akashi, M. Langmuir (submitted for publication). (1) (a) Fendler, J. H.; Meldrum, F. C. Adv. Mater. 1995, 7, 607. (b) Weller, H. Angew. Chem., Int. Ed. Engl. 1993, 32, 44. (c) Schmid, G.; Maihack, V.; Lantermann, F.; Peschel, S. J. Chem. Soc., Dalton Trans. 1996, 589. (2) (a) Clusters and Colloids; Schmid, G., Ed.; VCH: Weinheim, 1994. (b) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (3) (a) Alivisatos, A. P. Science 1996, 271, 933 and references therein. (b) Henglein, A. Chem. Rev., 1989, 89, 1861. (4) Colvin, V. L.; Schlamp, M. C.; Alivisatos, A. P. Nature 1994, 370, 354. (5) (a) Emory, S. R.; Nie, S. J. Phys. Chem. B 1998, 102, 493. (b) Bruchez, M., Jr.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P. Science 1998, 281, 2013. (c) Chan, W. C. W.; Nie, S. Science 1998, 281, 2016. (6) (a) Toshima, N.; Nakata, K.; Kitoh, H. Inorg. Chim. Acta 1997, 265, 149. (b) Schmidt, T. J.; Noeske, M.; Gasteiger, H. A.; Behm, R. J. J. Electrochem. Soc. 1998, 145, 925.
to-volume ratio, and consequently large fraction of the metal atoms that are exposed at surfaces are accessible to reactant molecules and available for catalysis. They are often coated with an organic shell to prevent them from agglomeration due to van der Waals forces.7 Noble metal colloids stabilized by synthetic polymers and their catalytic properties were described by Nord and co-workers as far back as the 1940s.8 The synthesis of catalytically active metal colloids by aqueous alcohol reduction of metal salts in the presence of protective polymer was first reported by Hirai and co-workers.9 The colloidal particles of 1-3 nm mean diameter with narrow size distributions showed high activity and selectivity for the hydrogenation of olefins and dienes, the hydration of acrylonitrile, and the light-induced hydrogen generation from water.9,10 Polymer-stabilized noble metal colloids on supports have great potential for environmental and industrial processes, but many established methods suffer from complex steps for the immobilization and decreased catalytic activity.11,12 These synthesis routes are based on a two-step process involving synthesis of metal colloids and immobilization (7) Reetz, M. T.; Helbig, W.; Quaiser, S. A. Chem. Mater. 1995, 7, 2227. (8) Rampino, L. D.; Nord, F. F. J. Am. Chem. Soc. 1941, 63, 2745. (9) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Chem. 1978, A12, 1117. (10) (a) Hirai, H.; Chawanya, H.; Toshima, N. React. Polym. 1986, 3, 127. (b) Toshima, N.; Takahashi, T.; Yonezawa, T.; Hirai, H. J. Macromol. Sci., Chem. 1988, A25 (5-7), 669. (11) Ohtaki, M.; Komiyama, M.; Hirai, H.; Toshima, N. Macromolecules 1991, 24, 5567. (12) Wang, Q.; Liu, H.; Wang, H. J. Colloid Interface Sci. 1997, 190, 380.
10.1021/cm9900047 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/24/1999
1382 Chem. Mater., Vol. 11, No. 5, 1999
Figure 1. Schematic formation of platinum colloids on polystyrene nanospheres with surface-grafted poly(N-isopropylacrylamide).
via covalent interaction or ligand coordination between the protective polymers and supports. Duff et al.13 reported the immobilization of polymer-stabilized colloidal platinum on oxide supports. The supported catalyst showed lower activity than the colloidal platinum in the sol state, although its activity was enhanced through removal by oxidation of the protective polymer from the particle surfaces after a calcination process. It is well-known that the protective polymer of the metal colloid serves not only as a stabilizer but also as a functional component that influences the activity and selectivity in catalysis.14 Therefore, it is necessary to develop a method to immobilize metal colloids on support simply without destroying their protecting shell of polymers. In the present study, a new approach has been developed so that the protective polymers are covalently bound to polystyrene nanosphere supports by the macromonomer technique. A range of monodisperse nanospheres was obtained by emulsifier-free dispersion copolymerization of styrene with a poly(N-isopropylacrylamide) (PNIPAAm) macromonomer. By using the PNIPAAm chains on the surface of polystyrene nanospheres as the capping agent, well-dispersed platinum colloids were synthesized in situ on the polymeric support via ethanol reduction of ionic platinum (see Figure 1). We report here the preparation and characterization of the platinum colloids on polystyrene nanospheres by TEM, FTIR, and XPS together with catalytic properties in the aqueous hydrogenation of allyl alcohol. The effects of the level of surface PNIPAAm on the particle size and catalytic activity of platinum colloids were investigated. The catalyst exhibited the unusual temperature dependence of activity due to the thermoresponsive property of PNIPAAm. A preliminary account of this part of the work has been published.15 Experimental Section Materials. The synthesis of poly(N-isopropylacrylamide)coated polystyrene nanospheres (PS-PNIPAAm) via emulsionfree dispersion polymerization and their characterization by TEM and dynamic light scattering (DLS) were performed as described in a previous paper.16 The PNIPAAm macromonomer containing a styrene end group was prepared by the reaction of p-(chloromethyl)styrene with the hydroxyl group-terminated oligoNIPAAm, which was obtained by the free radical polymerization of NIPAAm using 2-mercaptoethanol as a chain transfer agent in the presence of AIBN. The molecular weight (13) Duff, D. G.; Mallat, T.; Schneider, M.; Baiker, A. Appl. Catal. A: Gen. 1995, 133, 133. (14) (a) Chen, C.-W.; Akashi, M. Langmuir 1997, 13, 6465. (b) Teranishi, T.; Nakata, K.; Miyake, M.; Toshima, N. Chem. Lett. 1996, 277. (15) Chen, C.-W.; Chen, M.-Q.; Serizawa, T.; Akashi, M. Chem. Commun. 1998, 831. (16) Chen, M.-Q.; Kishida, A.; Akashi, M. J. Polym. Sci. Part A: Polym. Chem. 1996, 34, 2213.
Chen et al. and molecular weight distribution of the PNIPAAm macromonomer were 3600 and 2.1, respectively, determined using gel permeation chromatography (GPC). The surface compositions of the obtained polystyrene nanospheres were studied by XPS. The surface density of PNIPAAm on the polystyrene nanoparticles is referred to as the level of surface PNIPAAm. The particle sizes were examined by transmission electron microscopy. The particle size distribution is expressed by the ratio Dw/Dn, namely, the polydispersity index (PDI). Dw and Dn are the weight average diameter and the number average diameter of particles, respectively, and they can be calculated from the following equations: N
∑d
4 i
Dw )
i)1
(1)
N
∑d
3 i
i)1 N
∑d Dn )
i)1
N
i
(2)
where N is the total number of particles and di is the diameter of the ith particle. H2PtCl6‚6H2O, platinum-activated carbon (Pt/C, 5% of Pt) and ethanol (Nacalai Tesque, Inc.) were used as received. The commercial polystyrene beads (750 nm in diameter) were obtained from Polysciences, Inc. Preparation of Pt Colloids on Polystyrene Nanospheres. The synthesis of platinum colloids on polystyrene nanospheres (PS-PNIPAAm/Pt) by alcohol reduction of PtCl62is similar to that of PNIPAAm-stabilized platinum sol (PNIPAAm/Pt). For example, H2PtCl6‚6H2O (0.01 mmol Pt) and PS-PNIPAAm (45.2 mg, 0.4 mmol as PNIPAAm monomeric unit) were added in an ethanol/water mixed solvent (6/ 4, v/v, 38 mL), and the solution was then refluxed at 90 °C in an oil bath. After an induction period of 25 min, the color of the mixture changed suddenly from pale yellow to dark gray. The reaction mixture was boiled for 85 min before being allowed cooling. After separating the nanospheres from the reaction mixture by centrifugation (7000 rpm, 10 min) and redispersing them in water, we studied the activity and stability of the Pt colloids on the nanospheres for the aqueous hydrogenation of allyl alcohol. On occasion, the commercial polystyrene bead (45.2 mg) was used instead of the PSPNIPAAm nanosphere in the catalyst preparation (PS/Pt). In this case, the induction period was 10 min. Hydrogenation of Allyl Alcohol. Catalytic hydrogenation reactions of allyl alcohol at 25 °C under atmospheric pressure were performed as follows: into a 50 mL flat-bottom flask 15.2 mL of PS-PNIPAAm/Pt solution (the amount of platinum metal ) 4 × 10-6 mol) was pipetted as well as 2.8 mL of water. The mixture was agitated with a magnetic stirrer at room temperature under 1 atm of hydrogen. After a steady volume of the gas phase was attained, the reaction as initiated by an addition of 2 mL of 1.0 mol/dm3 water solution of the substrate to the agitated mixture (the final volume of the liquid phase was 20 mL) and the hydrogen uptake was monitored by a gas buret during the reaction. A sample was taken 10 min after the addition of the substrate and analyzed by analytical gas chromatography (GC). The GC analysis was carried out on a Yanaco model G2800-F gas chromatograph equipped with a 2.5 m × 3.4 mm PEG 6000/Shimalite TPA column using an electronic-integration technique (Hitachi model D-2000 electronic integrator). 1-Propanol was the only product, and the reaction rate was calculated according to the substrate conversion detected by GC. In an attempt to check the temperature dependence of the catalytic activity, the hydrogenation was also carried out at other temperatures ranging from 10 to 50 °C.
Pt Colloids on Polystyrene Nanospheres
Chem. Mater., Vol. 11, No. 5, 1999 1383
Table 1. Particle Sizes and Distributions of Polystyrene Nanospheres and Platinum Colloids on Their Surfacesa polystyrene nanospheres sample PS-PNIPAAm-2 PS-PNIPAAm-3 PS-PNIPAAm-4 PS-PNIPAAm-5 PS beadsb
PNIPAAm size (mol %) (nm) 2 3 4 5 0
600 500 450 400 750
platinum colloids
PDI
IP (min)
1.01 1.01 1.02 1.02
