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Characterization of palladium nanoparticles protected with polymer as hydrogenation catalyst
REACTIVE & FUNCTIONAL POLYMERS
ELSEVIER
Reactive &Functional Polymers 37 (1998) 121-131
Characterization of palladium nanoparticles protected with polymer as hydrogenation catalyst Hidefumi Hirai *, Noboru Yakura, Yoko Seta, Shinya Hodoshima Department of Industrial Chemistry Faculty of Engineering, Science University of Tokyo, Kagurazaka Shinjuku-ku, Tokyo 162-8601, Japan
Received 4 August 1997; revised version received 31 October 1997; accepted 7 November 1997
Abstract Suspensions of palladium nanoparticles protected with poly(N-vinyl-2-pyrrolidone) (PVP) were prepared by refluxing the solution of palladium(II) chloride in methanol in the presence of PVP with various weight-average molecular weight (MW = 6 000,25 000, 175 000 and 574 000). The average diameter of palladium nanoparticles determined by transmission electron microscopy increased from 2.0 to 2.5 nm with increasing M, of PW The amount of PVP adsorbed on the palladium nanoparticles was determined by separation of palladium nanoparticle protected with PVP from dispersion medium using aminoethylated polyacIylamide gel. The amount of PVP adsorbed on palladium nanoparticles increased with increasing M, of PW? The sedimentation coefficient of palladium nanoparticles was measured with ultracentrifugation for estimation of the thickness of adsorbed layer of PVP on palladium nanoparticles. The thickness of adsorbed layer of PVP on palladium nanoparticles increased from 1.9 to 7.8 nm with increasing M, of PW! The higher molecular-weight fraction in PVP was more adsorbed on the palladium nanoparticles than the lower molecular-weight fraction. The catalytic activity of the suspension of palladium nanoparticles protected with PVP was determined in the hydrogenation of 1,3-cyclooctadiene at 30°C under 1 atm of hydrogen. The influence of adsorbed layer on the catalytic activity was investigated. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:
Palladium nanoparticles;
Poly(N-vinyl-2-pyrrolidone);
Protective polymer; Adsorbed layer; Hydrogenation
catalyst
1. Introduction
Metal nanoparticles have been rapidly developed into fields of physics, chemistry and biology [ 1,2]. Suspensions of metal nanoparticles were prepared by deposition of metallic vapor and by reduction of metal ion. Immediately after the formation of metal nanoparticles, a protective colloid or polymer *Corresponding author. Tel. +81 (3) 3260-4271, Fax: +81 (3) 5261-4631. 1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved PIZ S1381-5148(97)00169-7
is required to be added to the suspension in order to prevent coagulation and precipitation of the metal nanoparticles. A part of the protective polymer added exhibits the protective function by adsorption of the polymer on the metal nanoparticles and the other part of protective polymer dissolves in free state in the suspension of the metal nanoparticles. Both the amount of the polymer adsorbed on metal nanopartitles and the concentration of free polymer in the suspension are important for the properties of the suspension and the application of the metal nanopar-
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titles. However, there have been few reports on either the amount of the polymer adsorbed on metal nanoparticles or the concentration of free polymer in dispersion medium. The thickness of adsorbed layer of polymer on metal nanoparticles has a great influence on the dispersion stability and the behavior of the nanopartitles in the suspension. Recently, the combination of the scanning transmission microscopy (STM) and the high-resolution transmission electron microscopy (TEM) was used to determine the dimensions of palladium particles protected with tetraalkylammonium salt [3]. The difference between the diameter determined by STM and the diameter determined by TEM allows the estimation of the thickness of the protective tetraalkylammonium salt layer. However, the samples for STM were dried, which may change the structure, and consequently the STM image may not present the real structure of protected particles in suspension. Yonezawa et al. [4] reported that the entire size of the platinum nanoparticles protected with cationic surfactant in water and the rhodium nanoparticles protected with tertiary amine in chloroform were determined as Stokes’ radii from the diffusion coefficient of the nanoparticles by a Taylor dispersion method [5,6]. Garvey et al. [7] reported that the thickness of the adsorbed layer of the poly(viny1 alcohol) on styrene particles was determined from the sedimentation coefficient of the particles by ultracentrifugation. However, the tbickness of the adsorbed layer of polymer on the metal nanoparticles has never reported to our knowledge. Hirai et al. [8,9] reported that the noble-metal nanoparticles such as palladium, rhodium and platinum were formed by reducing the corresponding metal salts with alcohols in the presence of protective polymer such as poly(viny1 alcohol) and poly(N-vinyl-2-pyrrolidone) (PVP). These noblemetal nanoparticles acted as active and selective catalyst in the hydrogenation of olefins and dienes [8-l 11. Recently, we have found that a aminoethylated polyacrylamide gel (G(PAA-AE)) adsorbed the palladium nanoparticles protected with PVP and did not free PVP in the suspension. From this finding, the palladium nanoparticles protected with PVP were successfully separated from the dispersion medium. In the present study, we prepare the suspension of the palladium particles protected with PVP by
reduction of palladium(I1) chloride with methanol in the presence of PVP with various average molecular weights. The average diameters of the palladium nanoparticles without the adsorbed layer of PVP are determined by TEM. The thicknesses of adsorbed layer of the palladium nanoparticles protected with PVP are determined from the sedimentation coefficient of the particles by ultracentrifugation. Both the concentration of free PVP in the suspension and the amount of PVP adsorbed on palladium nanoparticles are determined by using separation of the palladium nanoparticles protected with PVP from the free PVP The catalytic activity of the suspension of palladium nanoparticles is examined in the hydrogenation of 1,3-cyclooctadiene. The influence of the adsorbed layer on the catalytic activity is discussed. 2. Experimental 2.1. Reagent Poly(N-vinyl-2-pyrrolidone)s K- 15, K-30, K-60 and K-90 (reagent grade, Tokyo Kasei Co.) were purified by dialysis. Palladium@) chloride (guaranteed grade, Kojima Chemical Co., Ltd.) was used without further purification. Methanol (guaranteed grade, Kanto Chemical Co.) and 1,3-cyclooctadiene (guaranteed grade, Tokyo Kasei Co.) were distilled and stored under nitrogen. Water was deionized, distilled and stored under nitrogen. Purified hydrogen (99.999%) from Nippon Oxygen Co. was used for catalytic hydrogenation. 2.2. Aminoethylatedpolyacrylamide gel The aminoethylated polyacrylamide gel (G(PAAAE)) for adsorption of palladium nanoparticles protected with PVP was prepared according to the procedure of Inman et al. [ 121. To 10 g of polyacrylamide gel beads (Bio-Gel P-100, the diameter of 45-85 pm, Bio-Rad Laboratories), 150 cm3 of 1,2-ethanediamine was added. The mixture was heated at 90°C with stirring for 4 h under nitrogen. After stirring this mixture, the gel beads were separated by filtration, washed repeatedly with 0.1 mol dme3 NaCl aqueous solution, with water and with methanol, and were dried in vacua. The amount of aminoethyl group in G(PAA-AE) beads was deter-
H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
mined by acid-base back titration. After the addition of 50 cm3 of 0.01 mol drne3 HCl aqueous solution to 50 mg of gel beads, the mixture was shaken for more than 3 h and filtrated. The excess HCl in the supematant was titrated with 0.01 mol dme3 NaOH aqueous solution. The amount of aminoethyl group in G(PAA-AE) beads was 1.3 mm01 dry gel g-l. 2.3. Preparation of palladium nanoparticle suspension All manipulations were carried out under nitrogen atmosphere. The suspensions of palladium nanoparticles were prepared by alcohol reduction method [8,9]. Palladium(II) chloride (5.9 mg, 0.033 mmol) was dissolved in methanol (25 cm3) and poly(N-vinyl-2-pyrrolidone) (PVP) (14.7 mg, 0.132 mm01 of monomeric units) was dissolved in methanol (20 cm3). The solution of PVP in methanol was added to the solution of palladium(I1) chloride in methanol. The mixture was heated with water bath and brought to reflux. After the solution was refluxed for 30 min, a solution of sodium hydroxide (2.6 mg, 0.066 mmol) in methanol (5 cm3) was added dropwise to the refluxing solution. The solution was refluxed for further 10 min to give a homogeneous dark brown suspension of palladium nanoparticles protected with PVP In this reaction, the molar ratio of N-vinyl-2-pyrrolidone residues to palladium (VP/Pd) was 4 and the molar ratio of sodium hydroxide to palladium was 2. For PVP with weight-average molecular weight (M,) of 6 000, the preparations of the suspension were carried out at VP/Pd ratios of 1,4,8 and 41. 2.4. Transmission electron microscopy Transmission electron micrographs of palladium nanoparticles were obtained with a JEM-2010 transmission electron microscope (Japan Electron Optics Laboratory), operated at 200 kV at a magnification of 100000. The diameters of 400 particles of palladium in arbitrarily chosen area were measured in an enlarged photograph (4 x 100 000 times). 2.5. Particle-size distribution and specific su$ace area
A particle-size distribution of palladium particles was represented by a histogram of 400 particles clas-
123
sified with particle diameter. The total surface area of palladium particles was obtained by summation of all the surface areas of particle-diameter classes. The total weight of the particles was determined by summation of all the weights of particle-diameter classes, calculated from the diameter and the density of particle. The specific surface area of palladium nanoparticles was calculated by dividing the total surface area by the total weight. 2.6. Gel permeation chromatography Molecular weights of poly(N-vinyl-2-pyrrolidone) (PVP) were determined by gel permeation chromatography (GPC). GPC was run on a 8010 high performance liquid chromatograph (Toso Co.) equipped with G 3000 PWm + G 5000 PWxt_ columns using 0.1 mol dmm3 sodium acetate methanol-water (l/4, v/v) solution as the mobile phase at a flow rate of 0.8 cm3 rnin-’ at 40°C. Poly(ethylene oxide)s (TSK standard poly(ethylene oxide), Toso Co., i&: 21000, 45 000, 85 000, 160 000, 340000, 570 000 and 860000) were used as the standard reference to molecule weight for PVP The GPC of free PVP was carried out as follows. The palladium nanoparticles protected with PVP were separated from the dispersion medium by adsorption on G(PAA-AE) for 6 h at 30°C. The supematant containing free PVP after the separation was dried in vacua. The resulting free PVP was dissolved in 0.1 mol dme3 sodium acetate methanolwater (l/4, v/v) solution for GPC sample. 2.7. UV-vis absorption spectroscopy UV-vis absorption spectra were measured at room temperature by a U-3300 autospectrophotometer (Hitachi Co.) equipped with a l-cm quartz cell. 2.8. Amount of PVP adsorbed on palladium nanoparticles
Amounts of PVP adsorbed on palladium nanopartitles were determined by measuring the concentration of PVP remaining in the supematant after the adsorption of palladium nanoparticles protected with PVP on G(PAA-AE) beads. G(PAA-AE) beads (30 mg) were swollen by the methanol-water (l/l, v/v)
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mixed solvent at pH 3 at 30°C for 24 h. The darkbrown methanol suspension of palladium nanoparticles (5 cm3) was added to the water (5 cm3). The resulting suspension of palladium nanoparticles in methanol-water (l/l, v/v) was added to the swollen G(PAA-AE) beads. After stirring the mixture for 6 h, the colorless supematant was separated from the dark-brown colored G(PAA-AE) by filtration. The concentration of PVP remaining in the colorless supematant was determined from the difference between absorbances at 220 nm and at 230 nm by using a U-3300 autospectrophotometer (Hitachi Co.). The amounts of PVP adsorbed on the palladium nanoparticles were obtained by the difference in the concentration of PVP between the suspension and the colorless supematant. The amount of the adsorbed PVP per the surface area of palladium nanoparticles (I’, g mp2) was calculated using the specific surface area of palladium nanoparticles. 2.9. Sedimentation coefJicient Sedimentation coefficient of palladium nanoparticles protected with PVP was determined by a CP 56 G ultracentrifuge (Hitachi Co.) equipped with a A 60 M rotor (Hitachi Co.) and a ABS 8 UV scanner (Hitachi Co.) for analytical ultracentrifugation. Sedimentation curves were taken 10 times every 3 min at 30 000 rpm (73 000 G) at 30°C by using absorption at 450 nm. 2.10. Thickness of adsorbed layer On the basis of the equation proposed by Garvey et al. [7], the sedimentation coefficient (S, s) of palladium nanoparticles protected with PVP in methanol can be expressed as the following equation:
s=
2R;(p1 - pc,>+2(3SR,2 + 3RtS2 + S3)(pz - PO)
9rl(R, + 6)
(1) where S is the thickness of adsorbed layer on palladium nanoparticles protected with PVP, Rt (m) is radius of palladium nanoparticles, n (g m-l s-l) is viscosity of methanol and po, p1 and p2 (g rne3) are densities of methanol, palladium and the adsorbed layer, respectively. Densities of methanol solution of various concentrations of PVP (~3, g mF3) were determined
pycnometrically with methanol at 30°C. The relationship between p3 and the concentration of PVP (Cl, g me3) was obtained as the following empirical formula: p3 =
po +
0.40 Cl
(2)
Assuming that the partial molar volume of PVP in the adsorbed layer equals to that in methanol solution, p2 =
(3)
P3
From Eqs. 2 and 3, pz = po + 0.40
c2
(4)
where C2 (g mp3) is the concentration of PVP in the adsorbed layer. The concentration of polymer in the adsorbed layer (Cz) is given by the literature [7] as follows:
where r (g rne2) is the amount of the adsorbed PVP per the surface area of palladium nanoparticles. Eq. 1 can be transformed to Eq. 6 by use of Eqs. 4 and5
(6) Using Eq. 6, the S value is calculated from the S value determined by ultracentrifugation, taking values of 12.02 x lo6 g mm3 [ 131 for pl, 0.7829 x lo6 g me3 [14] for po at 30°C and 0.5142 x 10T2 g m-l s-l [15] for Q at 30°C. Rt is determined by transmission electron microscopy. 2.11. Hydrogenation Catalytic hydrogenation of 1,3-cyclooctadiene was carried out by use of a atmospheric pressure hydrogenation apparatus. The reaction flask (50 cm3) immersed in a thermostated water bath at 30°C was evacuated and then flushed with hydrogen for 3 times. Methanol (19.3 cm3) and suspension of palladium nanoparticles (0.3 cm3) were then introduced into the flask and stirred to be saturated with hydrogen. A methanol solution (0.5 cm3) of 1,3-cyclooctadiene (1 mol dme3) was injected into the reaction flask to start the reaction. Hydrogen uptake under a constant atmospheric pressure was monitored with a gas burette.
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H. Hirai et al/Reactive & Functional Polymers37 (1998) 121-131
Initial rates of hydrogenation were calculated from the initial rates of hydrogen uptake. 3. Results and discussion 3.1. Preparation
of palladium
nanoparticle
suspension
A solution of poly(N-vinyl-2-pyrrolidone) (PVP) in methanol was added to a yellow solution of palladium(I1) chloride in methanol. After the solution was refluxed for 30 min, a methanol solution of sodium hydroxide was added to the refluxing solution, which caused a color change of the solution from light yellow to dark brown. Further refluxing for 10 min gave a dark-brown homogeneous suspension of palladium nanoparticles (the concentration of Pd, 0.66 mol dmd3). Formation of palladium nanoparticles protected with PVP is proposed in a similar mechanism as that of rhodium nanoparticles protected with poly(viny1 alcohol) [9]. The first step is formation of polymer complex due to the coordination of PVP to palladium ion. The second step is the reduction of palladium ion with methanol to form the palladium nanoparticles after the addition of sodium hydroxide as the following Eq. 7 [lo]: PdC12 + CH30H + 2NaOH + Pd + HCHO + 2HzO + 2NaCl
(7)
In the present study, the amount of the added sodium hydroxide was a stoichiometric amount for Eq. 7, that is, 2 molar equivalent to palladium@) chloride admitted. The resulting suspension was stable, with no coagulation and no precipitation on standing at 30°C for more than one month. When a solution of sodium hydroxide was added to a yellow solution of palladium(I1) chloride in the absence of PVP, the color of solution was varied from yellow to dark brown. The resulting solution was unstable, that is, all of the palladium particles were precipitated and the liquid phase turned colorless on standing at 30°C for 6 h. These facts clearly indicate that the palladium nanoparticles are protected with PVP The protective function can be expressed quantitatively in the term of ‘protective value’. The larger protective value, the stronger the protective function of the polymer is. Protective values of PVP, poly(viny1 alcohol) and polyacrylamide are 50, 5 and 1 [16].
Table 1 Molecular weight of poly(N-vinyl-2-pyrrolidone) (PVP) PVP
&
NV
M,JM,
K-15 K-30 K-60 K-90
3400 11000 56 000 148000
6OOQ 25 000 175 000 574000
1.8 2.3 3.2 3.9
Table 2 Average diameter of palladium particles determined by transmission electron microscopy WV
VP/Pd a
Average diameter (R2t) (m)
Standard deviation (nm)
6000 25000 175000 574000 6000 6000 6000
4 4 4 4 1 8 41
2.0 2.1 2.3 2.5 2.5 1.7 1.1
0.81 0.81 1.05 0.98 0.75 0.83 0.59
aMolar ratio of N-vinyl-2-pyrrolidone residues to PdCl2 in the preparation of Pd-particle suspension.
In the present study, PVP which has the largest protective value among these synthetic polymers was used. The average molecular weights of PVP determined by using gel permeation chromatography (GPC) are shown in Table 1. The average diameters and particle-size distributions of palladium nanopartitles obtained by transmission electron microscopy (TEM) are shown in Table 2 and Fig. 1. The diameters of palladium nanoparticles are mainly distributed in the range of smaller than 3 nm, where more than 80% of the whole particles is included. The average diameter of palladium nanoparticles, prepared using the amount of PVP at a molar ratio of N-vinyl-Zpyrrolidone residues to palladium (VP/Pd) of 4, increases from 2.0 mn to 2.5 nm with increasing I& of PVP from 6000 to 574 000. The increase in the average diameter of the palladium nanoparticles is ascribed to a decrease in the interaction of PVP molecules with palladium nanoparticles due to an increase in M, of PVP. According to literature [17], the magnitude of the interaction of PVP molecules with metal particles increases with decreasing molecular weight. The stronger interaction
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H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
uO123456709
0123456759
Diameter I nm
(a) VP/Pdr4,
A&
Diameter / nm
: 6ooa
0123456759
(b) VP/Pd=4,
-0
Diameter / nm (C) VPF’d.4,
Mw : 175000
bfw : sooo
1 2 3 4 5 6 7 8 9 Diameter I nm
(d) VP/Pd=4, M,,. : 574000
O”l 5 Lo j20 k-‘012345575
0
A
Diameter / nm (e) VP/Pd=l, A&
0
: 6000
9
012345679 Diameter / nm (f) VPIpd.9,
A&. : 6000
0123456759 Diameter / nm (g) VP/Pd=41, A&
Fig. 1. Particle-size with PVP.
: 6000
distributions
of palladium
particles
protected
of PVP molecule of smaller molecular weight probably prevents the adsorption of palladium(O) atom on the surface of palladium particles on the growing
stage of particles. Coordination of carbonyl groups of PVP to palladium(O) atom on the surface of the palladium nanoparticles was indicated by the shift of carbonyl stretching bands [lo]. The suspensions of palladium nanoparticles were prepared with various amounts of the PVP with M, = 6000. As the VP/Pd ratio increases from 1 to 41, the average diameter of palladium nanoparticles decreases from 2.5 to 1.1 nm. The decrease of the average diameter of palladium nanoparticles with increasing VP/Pd ratio is probably ascribed to a decrease in the number of palladium(O) atom adsorbed on the surface of palladium particles in the growing stage, because the number of palladium(O) atom with coordination of PVP molecule increased. 3.2. Amount of PVP adsorbed on palladium nanoparticles The amount of the polymer adsorbed on metal nanoparticles can be determined from the concentration of free polymer. The concentration of the free polymer is usually determined by the following procedure. The particles with adsorbed polymer are sedimentated by centrifugation, and the concentration of the free polymer in the supematant is measured. In the case of the metal nanoparticles protected with polymer, however, separation of the metal nanopartitles from dispersion medium is difficult, because the sedimentation of the metal nanoparticles requires a very large centrifugal force and the sedimentated metal particles are rapidly dispersed again on removal of the centrifugal force. The present authors have found that a aminoethylated polyacrylamide gel (G(PAA-AE)) adsorbed the palladium nanoparticles protected with PVP and did not free PVP in the suspension. The dark-brown suspension of palladium nanoparticles was added to the swollen G(PAA-AE) beads (colorless) in the methanol-water (l/l, v/v) mixed solvent at pH 3 and the mixture was stirred for 6 h. When the resulting mixture was kept to stand for 5 min, the dark brown G(PAA-AE) beads were sedimentated and the supematant turned colorless. This color change clearly indicates the adsorption of the palladium nanoparticles protected with PVP on the G(PAA-AE) beads. All of the added palladium nanoparticles were adsorbed on the G(PAA-AE) beads, because no pal-
H. Hirai et al./Reactive
200
220
240
280
Wavelength
280
& Functional Polymers 37 (1998) 121-131
300
/ nm
Fig. 2. UV-vis spectra of supematant (a) after treatment of original solution of PVP (MW = 6000) in methanol-water (l/l, v/v) with aminoethylated polyacrylamide gel (G(PAA-AE)) beads at 30°C for 6 h and the supematants of the suspensions after the adsorption of palladium particles protected with PVP on G(PAA-AE) beads at 30°C for 6 h. MWof PVP: 6 000 (b), 25 000 (c). 175 000 (c) and 574 000 (e).
ladium nanoparticles in the supematant could be detected by UV-vis absorption spectra in the range from 300 to 800 nm. The absorption of PVP in the region of shorter wavelengths than 230 nm was observed in the W spectrum of the supematant after the G(PAA-AE) beads adsorption. The W spectrum of the supematant after treatment of a PVP (Mw = 6000) methanol-water solution (l/l, v/v) with the swollen G(PAA-AE) beads at 30°C for 6 h is shown in Fig. 2a. The spectrum of the original solution of PVP in methanol-water before treatment with G(PAA-AE) beads coincided with that of the supematant depicted by Fig. 2a. This fact indicates that no PVP was adsorbed on the G(PAA-AE) beads. Concerning all the molecular weights of PVP used in the present study, no adsorption of PVP on G(PAA-AE) beads was confirmed by W-vis absorption spectra. Thus, the palladium nanoparticles protected with PVP were successfully separated from dispersion medium containing free PVP. W spectra of the supematant after adsorption of palladium nanoparticles prepared with various molecular weights of PVP at VP/Pd ratio of 4 on the swollen G(PAA-AE) beads for 6 h are shown in Fig. 2b-e. With increasing M, of PVP, the absorption of PVP in the supematant (Fig. 2b-e) decreased.
121
The decrease in the absorption of PVP with increasing M, of PVP indicates the increase in the amount of PVP adsorbed on palladium nanoparticles with increasing M, of PVP. The GPC graphs of the original PVP (dotted line) and the free PVP (solid line) which was contained in the supematant after adsorption of palladium particles protected with PVP on G(PAA-AE) beads are shown in Fig. 3. The GPC graphs of the free PVP indicate a decrease in peak at the higher molecularweight fraction as compared with the GPC graphs of the original PVP This decrease clearly indicates that the higher molecular-weight fraction in PVP was more adsorbed on the palladium nanoparticles than the lower molecular-weight fraction. The concentrations of free PVP and the amounts of PVP adsorbed on palladium nanoparticles (r, g rnp2) are shown in Table 3. The amounts of PVP adsorbed on palladium nanoparticles were determined from the concentration of the free PVP With increasing M, of PVP, the amount of PVP adsorbed on palladium nanoparticles increases and the number of the PVP molecules per palladium nanoparticle decreases. The main chain of the polymer protecting the metal particles in suspension is considered to consist of sequence of segments termed ‘trains’, ‘loops’ and ‘tails’ [18]. The trains are in direct contact with the surface of the metal particles, and loops and tails are the middle and free ends of polymer chains, respectively, both extending into bulk solution. Consequently, the trains interact with the surface of particles most strongly and cover most closely. According to the literature [19], when the amount of the adsorbed polymer is independent of molecular weight of the polymer, all polymer segments are in the form of trains. Consequently, the dependence of M, of PVP on the amount of PVP adsorbed on palladium nanoparticles indicates the existence of loops and tails in the adsorbed PVP As shown in Table 3, the numbers of the PVP molecules per palladium nanoparticle protected with PVP (M, = 6000) are 5.8, 6.7, 5.2 and 1.4 at VP/Pd ratios of 1, 4, 8 and 41, respectively. The decrease in the number of PVP adsorbed on each palladium particle prepared at VP/Pd ratio of 41 is probably due to the decrease in the surface area per palladium nanoparticle. The surface area per palladium nanoparticle prepared at VP/Pd ratio of 41
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H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
20
10
0
30
0
10
Retention time / min
(a)
VP/Pd
= 4,
20
(b)
30
0
VP/Pd
VP/Pd
??
4, 1u,:25000
10
Retention tlme / mln (C)
30
Retentlon time I mln
M,:6000
10
0
20
20
30
Retention time / mln
(d)
= 4, M,:175000
VP/Pd
= 4, &,:574000
Fig. 3. Gel permeation chromatograms of original PVP (dotted line) and free PVP (solid line) remained in the supernatant after the adsorption of palladium particles protected with PVP on G(PAA-AE) beads.
Table 3 Amount of PVP adsorbed on palladium particles and concentration of free PVP in suspension of palladium particles M,., of PVP
VP/Pda
Average diameter (2R,) (mu)
Surface area per Pd particle (mn2)
Amount of adsorbed PVP (r) x lo3 (g m-‘)
Number of PVP molecule per Pd particle b
Concentration OffreePVPC (mg dme3)
6000 25000 175 000 574000 6000 6000 6000
4 4 4 4 1 8 41
2.0 2.1 2.3 2.5 2.5 1.7 1.1
13 14 17 20 20 9 4
4.4 6.0 11.1 17.1 2.0 4.5 3.4
6.7 3.4 2.6 1.5 5.8 5.2 1.4
240 220 180 130 50 520 3000
a Molar ratio of N-vinyl-2-pyrrolidone residues to PdCl2 in preparation of Pd-particle suspension. b Number of PVP molecule adsorbed on one Pd particle by calculation from M,, of PW c Concentration of free PVP in suspension of Pd particles.
is as small as 31% of that at VP/Pd of 4, because the average diameter of the palladium particles decreases from 2.0 to 1.1 m-n. With increasing amount of the admitted PVP, the concentration of the free PVP increases remarkably, as shown in Table 3.
3.3. Thickness of adsorbed layer The diameter of the metal nanoparticles without the adsorbed layer of protective polymer can be measured by using TEM. The thickness of the adsorbed layer of protective polymer on the particles can be
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H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
Table 4 Thickness of adsorbed PVP and diameter of palladium particles including adsorbed layer of PVP b M, of PVP
6000 2.5000 175 000 574000 6000 6000 6000
VP/W a
4 4 4 4
1 8 41
Average diameter of Pd particles (2Rt) (mn)
Sedimentation coefficient (S) (lo-13 s)
Thickness of
Diameter of Pd particles b (OS) (nm)
DJ2 R,
adsorbed PVP (6) (nm)
2.0 2.1 2.3 2.5 2.5 1.7 1.1
23.1 24.3 24.7 26.6 32.0 17.5 12.1
1.9 2.7 4.7 7.8 2.2 1.9 0.5
5.8 7.4 11.7 18.1 6.9 5.5 2.0
2.9 3.5 5.1 7.2 2.7 3.2 1.8
a Molar ratio of N-vinyl-2-pyrrolidone residues to PdC12 in the preparation of Pd-particle suspension. b Calculated diameter (D,) of Pd particles including adsorbed layer of PVP
determined by a centrifugation method. One of the great advantage of centrifugation method is that the thickness of the adsorbed layer of protective polymer on the particles can be measured in dispersion state. The sedimentation coefficient (5) and the thickness (6) of adsorbed layer of PVP calculated from ultracentrifugation data are shown in Table 4. The thickness of adsorbed layer of PVP increases from 1.9 to 7.8 nm as i&. of PVP increases from 6000 to 574 000. Both the tails and the loops contribute to the thickness of adsorbed layer of PVP Increase in the thickness of adsorbed layer of PVP with increasing A4, of PVP is probably due to increase in the lengths of loops and tails extending into the dispersion medium. Cohen Stuart et al. [20] reported that the thickness of adsorbed layer of PVP increased with increasing M, of PVP for the adsorption of PVP on the inner surface of glass capillary. The thickness of adsorbed layer of PVP on the palladium nanoparticles prepared at VP/Pd ratio of 41 using PVP with M, = 6000 is smallest. The content of trains in PVP adsorbed on the palladium nanoparticles under this preparation condition is probably greater than those under the conditions at a smaller VP/Pd ratio. The diameter of palladium nanoparticles including adsorbed layer (D,) calculated from Eq. 8: D, = 2Rt + 2S
where 2Rt is the dium nanoparticles can be determined dium nanoparticles
(8) average diameter of the pallawithout adsorbed layer which by TEM. The diameter of pallaincluding adsorbed layer of PVP
(D,), the average diameter of palladium nanoparticles without the adsorbed layer of PVP (2RJ and the OS/2 Rt ratio are also summarized in Table 4. The diameter of palladium nanoparticles including adsorbed layer of PVP increases with increasing thickness of adsorbed layer of PVP. The diameters of palladium nanoparticles including the thickness of adsorbed layer of PVP are 1.8-7.2 times greater than the average diameter of palladium nanoparticles without the adsorbed layer.
3.4. Hydrogenation of 1,3-cyclooctadiene The suspension of palladium nanoparticles protected with PVP was used as catalyst for hydrogenation of 1,3-cyclooctadiene at 30°C in methanol under atmospheric hydrogen pressure. When the amount of hydrogen uptake became equimolar to the admitted amount of 1,3-cyclooctadiene in the hydrogenation using the palladium nanoparticles prepared at VP/Pd ratio of 4 with PVP of M, = 6000, the yields of cyclooctene and cyclooctane was 99.7% and 0.3%, respectively. In the previous paper [ 111, we reported that the hydrogenation of 1,3-cyclooctadiene gave cyclooctene in 99.9% yield using the palladium nanoparticles prepared at VP/Pd ratio of 41 with PVP of M, = 355000 (degree of polymerization, 3 200). The catalytic activity represented by the initial rate of hydrogenation per mol of palladium is shown in Table 5. The catalytic activity decreases by 19% with increasing M, of PVP from 6 000 to 574 000. 1,3-Cyclooctadiene is hydrogenated after the sub-
H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
130
Table 5 Hydrogenation of 1,3-cyclooctadiene using suspension of palladium particles Mw of PVP
VP/Pd a (m2 g-l)
Specific surface area b (nm)
Thickness of adsorbed PVP (Hz-mol Pd-mol-’ s-l)
Initial hydrogenation rate c
6000 25 000 175 000 574 000 6000 6000 6000
4 4 4 4 1 8 41
179 177 145 140 169 206 297
1.9 2.7 4.7 7.8 2.2 1.9 0.5
25.4 24.6 22.2 20.7 22.2 24.8 21.0
a Molar ratio of N-vinyl-2-pyrrolidone residues to PdC12 in the preparation of Pd-particle suspension. b Specific surface area of Pd particles determined from particle-size distributions shown in Fig. 1. CHydrogenation conditions: at 30°C under 1 atm. of hydrogen, [1,3-cyclooctadiene] = 25 mm01 dmm3, [Pd] = 3.3 pmol dnm3 in methanol (20 cm3).
strate diffuses into the adsorbed layer of PVP and is adsorbed on the surface of palladium nanoparticles. As M, of PVP increases from 6 000 to 574 000, the thickness of the adsorbed layer of PVP increases 4.1 times from 1.9 to 7.8 run as shown in Table 5. The reciprocal of the adsorbed layer thickness is probably proportional to the diffusion velocity of substrate, that is, the initial rate of hydrogenation. The reciprocal of the adsorbed layer thickness decreases by 75% from 0.53 to 0.13 nm-’ with increasing it4, of PVP from 6 000 to 574 000. The specific surface area of palladium nanoparticles decreases by 22% from 179 to 140 m* g-l with increasing M, of PVP from 6000 to 574000. A multiple correlation analysis on the data of the hydrogenation of 1,3-cyclooctadiene was carried out by using the catalytic activity (y, H2-mol Pd-mol-’ s-l) as criterion variate, and the specific surface area (~1, m* gg’) and the reciprocal of adsorbed layer thickness (~2, nm-‘) as explanatory variates, respectively. A multiple regression equation is obtained as follows: y = 0.0396~~ + 3.69x2 + 15.2
(9)
The multiple correlation coefficient is 0.845. Thus, the regression of the catalytic activity upon the specific surface area of palladium nanoparticles and the reciprocal of the adsorbed layer thickness is satisfactory. The variates y, x1 and x2 are standardized to dimensionless variates y*, x; and x;, respectively. Thus, Eq. 9 is transformed to Eq. 10. y* = 0.509x; + 0.354x;
(10)
The specific surface area contributes more strongly to the catalytic activity than the reciprocal of the adsorbed layer thickness, because the coefficient of x; is remarkably larger than that of x;, as shown in Eq. 10. In the hydrogenation using the palladium nanoparticles prepared at VP/Pd ratio of 4 with PVP of M, = 6000, dependence of the initial hydrogenation rate on the palladium concentration was found to be the first order. This result indicates that the catalytic activity is dependent on the specific surface area with the first order in the case of constant thickness of the adsorbed layer. According to previous paper [ll], the initial hydrogenation rate was zero order with respect to 1,3-cycloo-ctadiene concentration and first order with respect to palladium concentration using the palladium nanoparticles prepared at VP/Pd ratio of 41 with PVP of M,,, = 355 000 (degree of polymerization, 3 200). The contents of loops and tails in PVP adsorbed are probably large in the suspension prepared by using PVP with high molecular weight (M, = 355 000). The result suggests that loops and tails in the adsorbed PVP may have a small influence on the diffusion of 1,3-cyclooctadiene. The catalytic activity of the palladium nanopartitles prepared at VP/Pd ratio of 41 using PVP of M, = 6 000 is smallest in spite of the largest specific surface area of the palladium nanoparticles, as shown Table 5. The value of the thickness of the adsorbed layer on the palladium nanoparticles is smallest. The content of trains in the low molecular weight of PVP is probably large, and the trains in PVP may
H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
131
be compressed by the osmotic pressure due to the high concentration of the free PVP in the dispersion medium. The trains in PVP probably prevent the adsorption of 1,3-cyclooctadiene on the surface of palladium nanoparticles.
Koki Co.) for their aid to ultracentrifugation experiments. N.Y. also thanks T. Murakami and A. Yasuda (Johnson Polymer Co.) for useful discussions.
4. Conclusion
[l] G. S&mid, Clusters and Colloids, VCH, Weinheim, 1994. [2] P. Mulvancy, F. Grieser, D. Meisel, Surf. Sci. Ser. 38 (1991) 303. [3] M.T. Reetz, W. Helbig, S.A. Quaiser, U. Stimming, N. Breuer, R. Vogel, Science 267 (1995) 367. [4] T. Yonezawa, T. Tominaga, N. Toshima, Polym. Adv. Technol. 7 (1996) 645. [5] G.I. Taylor, Proc. R. Sot., London, Ser. A 219 (1953) 186. [6] G.I. Taylor, Proc. R. Sot., London, Ser. A 225 (1954) 473. [7] M.J. Garvey, T.H.F. Tadros, B. Vincent, J. Colloid Interface Sci. 49 (1974) 57. [8] H. Hirai, J. Macromol. Sci.-Chem. Al3 (1979) 633. [9] H. Hirai, Y. Nakao, N. Toshima, J. Macromol. Sci.-Chem. Al2 (1978) 1117. [lo] H. Hirai, H. Chawanya, N.Toshima, React. Polym. 3 (1983) 127. [ 1l] H. Hirai, H. Chawanya, N. Toshima, Bull. Chem. Sot. Jpn. 58 (1985) 682. [12] J.K. Imnan, H.M. Dintzis, Biochemistry 8 (1969) 4074. [13] The Chemical Society of Japan (Ed.), Kagaku Binran. Kisohen, Maruzen Ltd., Tokyo, 1975, p. 60. [14] S. Asahara, N. Tokura, M. Okawara, J. Kumanotani, M. Senoo (Eds.), Youzai Handbook, Koudansya Ltd., Tokyo, 1976, p. 329. [15] Kagaku Kougaku Kyoukai (Ed.), Bussei Teisuu, Vol. 7, Maruzen Ltd., Tokyo, 1969, p. 74. [ 161 H. Thiele, H.S. von Levem, J. Colloid Sci. 20 (1965) 679. [17] B. Jirgensons, Makromol. Chem., 6 (1951) 30. [18] T.F. Tadros, Polymer Colloids, Elsevier, Essex, 1985, p. 107. [19] R. Perkel, R. Ullman, J. Polym. Sci. 54 (1961) 127. [20] M.A. Cohen Stuart, J.W. Mulder, Colloid Surface 15 (1985) 49.
The palladium nanoparticles protected with PVP were successfully separated from dispersion medium by adsorption of the palladium nanoparticles protected with PVP on G(PAA-AE) beads. The concentration of free PVP in dispersion medium and the amount of PVP adsorbed on palladium nanopartitles were determined by using the G(PAA-AE) beads adsorption method. The amount of PVP adsorbed on palladium nanoparticles increased with increasing M, of PVP The concentration of free PVP in dispersion medium increased as the admitted amount of PVP increased. The higher molecularweight fraction in PVP was more adsorbed on the palladium nanoparticles than the lower molecularweight fraction. The thickness of adsorbed layer of PVP was successfully determined from sedimentation coefficient of palladium nanoparticles protected with PVP by the ultracentrifugation method. The thickness of adsorbed layer of PVP increased with increasing M, of PVP The hydrogenation activity for 1,3-cyclooctadiene using the suspension of palladium nanoparticles depended on specific surface area of palladium nanoparticles far more effectively than the thickness of adsorbed layer of PVP Acknowledgements The authors express their grateful acknowledgment to thank M. Morita and K. Takahashi (Hitachi
References
ELSEVIER
Reactive &Functional Polymers 37 (1998) 121-131
Characterization of palladium nanoparticles protected with polymer as hydrogenation catalyst Hidefumi Hirai *, Noboru Yakura, Yoko Seta, Shinya Hodoshima Department of Industrial Chemistry Faculty of Engineering, Science University of Tokyo, Kagurazaka Shinjuku-ku, Tokyo 162-8601, Japan
Received 4 August 1997; revised version received 31 October 1997; accepted 7 November 1997
Abstract Suspensions of palladium nanoparticles protected with poly(N-vinyl-2-pyrrolidone) (PVP) were prepared by refluxing the solution of palladium(II) chloride in methanol in the presence of PVP with various weight-average molecular weight (MW = 6 000,25 000, 175 000 and 574 000). The average diameter of palladium nanoparticles determined by transmission electron microscopy increased from 2.0 to 2.5 nm with increasing M, of PW The amount of PVP adsorbed on the palladium nanoparticles was determined by separation of palladium nanoparticle protected with PVP from dispersion medium using aminoethylated polyacIylamide gel. The amount of PVP adsorbed on palladium nanoparticles increased with increasing M, of PW? The sedimentation coefficient of palladium nanoparticles was measured with ultracentrifugation for estimation of the thickness of adsorbed layer of PVP on palladium nanoparticles. The thickness of adsorbed layer of PVP on palladium nanoparticles increased from 1.9 to 7.8 nm with increasing M, of PW! The higher molecular-weight fraction in PVP was more adsorbed on the palladium nanoparticles than the lower molecular-weight fraction. The catalytic activity of the suspension of palladium nanoparticles protected with PVP was determined in the hydrogenation of 1,3-cyclooctadiene at 30°C under 1 atm of hydrogen. The influence of adsorbed layer on the catalytic activity was investigated. 0 1998 Elsevier Science B.V. All rights reserved. Keywords:
Palladium nanoparticles;
Poly(N-vinyl-2-pyrrolidone);
Protective polymer; Adsorbed layer; Hydrogenation
catalyst
1. Introduction
Metal nanoparticles have been rapidly developed into fields of physics, chemistry and biology [ 1,2]. Suspensions of metal nanoparticles were prepared by deposition of metallic vapor and by reduction of metal ion. Immediately after the formation of metal nanoparticles, a protective colloid or polymer *Corresponding author. Tel. +81 (3) 3260-4271, Fax: +81 (3) 5261-4631. 1381-5148/98/$19.00 0 1998 Elsevier Science B.V. All rights reserved PIZ S1381-5148(97)00169-7
is required to be added to the suspension in order to prevent coagulation and precipitation of the metal nanoparticles. A part of the protective polymer added exhibits the protective function by adsorption of the polymer on the metal nanoparticles and the other part of protective polymer dissolves in free state in the suspension of the metal nanoparticles. Both the amount of the polymer adsorbed on metal nanopartitles and the concentration of free polymer in the suspension are important for the properties of the suspension and the application of the metal nanopar-
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H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
titles. However, there have been few reports on either the amount of the polymer adsorbed on metal nanoparticles or the concentration of free polymer in dispersion medium. The thickness of adsorbed layer of polymer on metal nanoparticles has a great influence on the dispersion stability and the behavior of the nanopartitles in the suspension. Recently, the combination of the scanning transmission microscopy (STM) and the high-resolution transmission electron microscopy (TEM) was used to determine the dimensions of palladium particles protected with tetraalkylammonium salt [3]. The difference between the diameter determined by STM and the diameter determined by TEM allows the estimation of the thickness of the protective tetraalkylammonium salt layer. However, the samples for STM were dried, which may change the structure, and consequently the STM image may not present the real structure of protected particles in suspension. Yonezawa et al. [4] reported that the entire size of the platinum nanoparticles protected with cationic surfactant in water and the rhodium nanoparticles protected with tertiary amine in chloroform were determined as Stokes’ radii from the diffusion coefficient of the nanoparticles by a Taylor dispersion method [5,6]. Garvey et al. [7] reported that the thickness of the adsorbed layer of the poly(viny1 alcohol) on styrene particles was determined from the sedimentation coefficient of the particles by ultracentrifugation. However, the tbickness of the adsorbed layer of polymer on the metal nanoparticles has never reported to our knowledge. Hirai et al. [8,9] reported that the noble-metal nanoparticles such as palladium, rhodium and platinum were formed by reducing the corresponding metal salts with alcohols in the presence of protective polymer such as poly(viny1 alcohol) and poly(N-vinyl-2-pyrrolidone) (PVP). These noblemetal nanoparticles acted as active and selective catalyst in the hydrogenation of olefins and dienes [8-l 11. Recently, we have found that a aminoethylated polyacrylamide gel (G(PAA-AE)) adsorbed the palladium nanoparticles protected with PVP and did not free PVP in the suspension. From this finding, the palladium nanoparticles protected with PVP were successfully separated from the dispersion medium. In the present study, we prepare the suspension of the palladium particles protected with PVP by
reduction of palladium(I1) chloride with methanol in the presence of PVP with various average molecular weights. The average diameters of the palladium nanoparticles without the adsorbed layer of PVP are determined by TEM. The thicknesses of adsorbed layer of the palladium nanoparticles protected with PVP are determined from the sedimentation coefficient of the particles by ultracentrifugation. Both the concentration of free PVP in the suspension and the amount of PVP adsorbed on palladium nanoparticles are determined by using separation of the palladium nanoparticles protected with PVP from the free PVP The catalytic activity of the suspension of palladium nanoparticles is examined in the hydrogenation of 1,3-cyclooctadiene. The influence of the adsorbed layer on the catalytic activity is discussed. 2. Experimental 2.1. Reagent Poly(N-vinyl-2-pyrrolidone)s K- 15, K-30, K-60 and K-90 (reagent grade, Tokyo Kasei Co.) were purified by dialysis. Palladium@) chloride (guaranteed grade, Kojima Chemical Co., Ltd.) was used without further purification. Methanol (guaranteed grade, Kanto Chemical Co.) and 1,3-cyclooctadiene (guaranteed grade, Tokyo Kasei Co.) were distilled and stored under nitrogen. Water was deionized, distilled and stored under nitrogen. Purified hydrogen (99.999%) from Nippon Oxygen Co. was used for catalytic hydrogenation. 2.2. Aminoethylatedpolyacrylamide gel The aminoethylated polyacrylamide gel (G(PAAAE)) for adsorption of palladium nanoparticles protected with PVP was prepared according to the procedure of Inman et al. [ 121. To 10 g of polyacrylamide gel beads (Bio-Gel P-100, the diameter of 45-85 pm, Bio-Rad Laboratories), 150 cm3 of 1,2-ethanediamine was added. The mixture was heated at 90°C with stirring for 4 h under nitrogen. After stirring this mixture, the gel beads were separated by filtration, washed repeatedly with 0.1 mol dme3 NaCl aqueous solution, with water and with methanol, and were dried in vacua. The amount of aminoethyl group in G(PAA-AE) beads was deter-
H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
mined by acid-base back titration. After the addition of 50 cm3 of 0.01 mol drne3 HCl aqueous solution to 50 mg of gel beads, the mixture was shaken for more than 3 h and filtrated. The excess HCl in the supematant was titrated with 0.01 mol dme3 NaOH aqueous solution. The amount of aminoethyl group in G(PAA-AE) beads was 1.3 mm01 dry gel g-l. 2.3. Preparation of palladium nanoparticle suspension All manipulations were carried out under nitrogen atmosphere. The suspensions of palladium nanoparticles were prepared by alcohol reduction method [8,9]. Palladium(II) chloride (5.9 mg, 0.033 mmol) was dissolved in methanol (25 cm3) and poly(N-vinyl-2-pyrrolidone) (PVP) (14.7 mg, 0.132 mm01 of monomeric units) was dissolved in methanol (20 cm3). The solution of PVP in methanol was added to the solution of palladium(I1) chloride in methanol. The mixture was heated with water bath and brought to reflux. After the solution was refluxed for 30 min, a solution of sodium hydroxide (2.6 mg, 0.066 mmol) in methanol (5 cm3) was added dropwise to the refluxing solution. The solution was refluxed for further 10 min to give a homogeneous dark brown suspension of palladium nanoparticles protected with PVP In this reaction, the molar ratio of N-vinyl-2-pyrrolidone residues to palladium (VP/Pd) was 4 and the molar ratio of sodium hydroxide to palladium was 2. For PVP with weight-average molecular weight (M,) of 6 000, the preparations of the suspension were carried out at VP/Pd ratios of 1,4,8 and 41. 2.4. Transmission electron microscopy Transmission electron micrographs of palladium nanoparticles were obtained with a JEM-2010 transmission electron microscope (Japan Electron Optics Laboratory), operated at 200 kV at a magnification of 100000. The diameters of 400 particles of palladium in arbitrarily chosen area were measured in an enlarged photograph (4 x 100 000 times). 2.5. Particle-size distribution and specific su$ace area
A particle-size distribution of palladium particles was represented by a histogram of 400 particles clas-
123
sified with particle diameter. The total surface area of palladium particles was obtained by summation of all the surface areas of particle-diameter classes. The total weight of the particles was determined by summation of all the weights of particle-diameter classes, calculated from the diameter and the density of particle. The specific surface area of palladium nanoparticles was calculated by dividing the total surface area by the total weight. 2.6. Gel permeation chromatography Molecular weights of poly(N-vinyl-2-pyrrolidone) (PVP) were determined by gel permeation chromatography (GPC). GPC was run on a 8010 high performance liquid chromatograph (Toso Co.) equipped with G 3000 PWm + G 5000 PWxt_ columns using 0.1 mol dmm3 sodium acetate methanol-water (l/4, v/v) solution as the mobile phase at a flow rate of 0.8 cm3 rnin-’ at 40°C. Poly(ethylene oxide)s (TSK standard poly(ethylene oxide), Toso Co., i&: 21000, 45 000, 85 000, 160 000, 340000, 570 000 and 860000) were used as the standard reference to molecule weight for PVP The GPC of free PVP was carried out as follows. The palladium nanoparticles protected with PVP were separated from the dispersion medium by adsorption on G(PAA-AE) for 6 h at 30°C. The supematant containing free PVP after the separation was dried in vacua. The resulting free PVP was dissolved in 0.1 mol dme3 sodium acetate methanolwater (l/4, v/v) solution for GPC sample. 2.7. UV-vis absorption spectroscopy UV-vis absorption spectra were measured at room temperature by a U-3300 autospectrophotometer (Hitachi Co.) equipped with a l-cm quartz cell. 2.8. Amount of PVP adsorbed on palladium nanoparticles
Amounts of PVP adsorbed on palladium nanopartitles were determined by measuring the concentration of PVP remaining in the supematant after the adsorption of palladium nanoparticles protected with PVP on G(PAA-AE) beads. G(PAA-AE) beads (30 mg) were swollen by the methanol-water (l/l, v/v)
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mixed solvent at pH 3 at 30°C for 24 h. The darkbrown methanol suspension of palladium nanoparticles (5 cm3) was added to the water (5 cm3). The resulting suspension of palladium nanoparticles in methanol-water (l/l, v/v) was added to the swollen G(PAA-AE) beads. After stirring the mixture for 6 h, the colorless supematant was separated from the dark-brown colored G(PAA-AE) by filtration. The concentration of PVP remaining in the colorless supematant was determined from the difference between absorbances at 220 nm and at 230 nm by using a U-3300 autospectrophotometer (Hitachi Co.). The amounts of PVP adsorbed on the palladium nanoparticles were obtained by the difference in the concentration of PVP between the suspension and the colorless supematant. The amount of the adsorbed PVP per the surface area of palladium nanoparticles (I’, g mp2) was calculated using the specific surface area of palladium nanoparticles. 2.9. Sedimentation coefJicient Sedimentation coefficient of palladium nanoparticles protected with PVP was determined by a CP 56 G ultracentrifuge (Hitachi Co.) equipped with a A 60 M rotor (Hitachi Co.) and a ABS 8 UV scanner (Hitachi Co.) for analytical ultracentrifugation. Sedimentation curves were taken 10 times every 3 min at 30 000 rpm (73 000 G) at 30°C by using absorption at 450 nm. 2.10. Thickness of adsorbed layer On the basis of the equation proposed by Garvey et al. [7], the sedimentation coefficient (S, s) of palladium nanoparticles protected with PVP in methanol can be expressed as the following equation:
s=
2R;(p1 - pc,>+2(3SR,2 + 3RtS2 + S3)(pz - PO)
9rl(R, + 6)
(1) where S is the thickness of adsorbed layer on palladium nanoparticles protected with PVP, Rt (m) is radius of palladium nanoparticles, n (g m-l s-l) is viscosity of methanol and po, p1 and p2 (g rne3) are densities of methanol, palladium and the adsorbed layer, respectively. Densities of methanol solution of various concentrations of PVP (~3, g mF3) were determined
pycnometrically with methanol at 30°C. The relationship between p3 and the concentration of PVP (Cl, g me3) was obtained as the following empirical formula: p3 =
po +
0.40 Cl
(2)
Assuming that the partial molar volume of PVP in the adsorbed layer equals to that in methanol solution, p2 =
(3)
P3
From Eqs. 2 and 3, pz = po + 0.40
c2
(4)
where C2 (g mp3) is the concentration of PVP in the adsorbed layer. The concentration of polymer in the adsorbed layer (Cz) is given by the literature [7] as follows:
where r (g rne2) is the amount of the adsorbed PVP per the surface area of palladium nanoparticles. Eq. 1 can be transformed to Eq. 6 by use of Eqs. 4 and5
(6) Using Eq. 6, the S value is calculated from the S value determined by ultracentrifugation, taking values of 12.02 x lo6 g mm3 [ 131 for pl, 0.7829 x lo6 g me3 [14] for po at 30°C and 0.5142 x 10T2 g m-l s-l [15] for Q at 30°C. Rt is determined by transmission electron microscopy. 2.11. Hydrogenation Catalytic hydrogenation of 1,3-cyclooctadiene was carried out by use of a atmospheric pressure hydrogenation apparatus. The reaction flask (50 cm3) immersed in a thermostated water bath at 30°C was evacuated and then flushed with hydrogen for 3 times. Methanol (19.3 cm3) and suspension of palladium nanoparticles (0.3 cm3) were then introduced into the flask and stirred to be saturated with hydrogen. A methanol solution (0.5 cm3) of 1,3-cyclooctadiene (1 mol dme3) was injected into the reaction flask to start the reaction. Hydrogen uptake under a constant atmospheric pressure was monitored with a gas burette.
125
H. Hirai et al/Reactive & Functional Polymers37 (1998) 121-131
Initial rates of hydrogenation were calculated from the initial rates of hydrogen uptake. 3. Results and discussion 3.1. Preparation
of palladium
nanoparticle
suspension
A solution of poly(N-vinyl-2-pyrrolidone) (PVP) in methanol was added to a yellow solution of palladium(I1) chloride in methanol. After the solution was refluxed for 30 min, a methanol solution of sodium hydroxide was added to the refluxing solution, which caused a color change of the solution from light yellow to dark brown. Further refluxing for 10 min gave a dark-brown homogeneous suspension of palladium nanoparticles (the concentration of Pd, 0.66 mol dmd3). Formation of palladium nanoparticles protected with PVP is proposed in a similar mechanism as that of rhodium nanoparticles protected with poly(viny1 alcohol) [9]. The first step is formation of polymer complex due to the coordination of PVP to palladium ion. The second step is the reduction of palladium ion with methanol to form the palladium nanoparticles after the addition of sodium hydroxide as the following Eq. 7 [lo]: PdC12 + CH30H + 2NaOH + Pd + HCHO + 2HzO + 2NaCl
(7)
In the present study, the amount of the added sodium hydroxide was a stoichiometric amount for Eq. 7, that is, 2 molar equivalent to palladium@) chloride admitted. The resulting suspension was stable, with no coagulation and no precipitation on standing at 30°C for more than one month. When a solution of sodium hydroxide was added to a yellow solution of palladium(I1) chloride in the absence of PVP, the color of solution was varied from yellow to dark brown. The resulting solution was unstable, that is, all of the palladium particles were precipitated and the liquid phase turned colorless on standing at 30°C for 6 h. These facts clearly indicate that the palladium nanoparticles are protected with PVP The protective function can be expressed quantitatively in the term of ‘protective value’. The larger protective value, the stronger the protective function of the polymer is. Protective values of PVP, poly(viny1 alcohol) and polyacrylamide are 50, 5 and 1 [16].
Table 1 Molecular weight of poly(N-vinyl-2-pyrrolidone) (PVP) PVP
&
NV
M,JM,
K-15 K-30 K-60 K-90
3400 11000 56 000 148000
6OOQ 25 000 175 000 574000
1.8 2.3 3.2 3.9
Table 2 Average diameter of palladium particles determined by transmission electron microscopy WV
VP/Pd a
Average diameter (R2t) (m)
Standard deviation (nm)
6000 25000 175000 574000 6000 6000 6000
4 4 4 4 1 8 41
2.0 2.1 2.3 2.5 2.5 1.7 1.1
0.81 0.81 1.05 0.98 0.75 0.83 0.59
aMolar ratio of N-vinyl-2-pyrrolidone residues to PdCl2 in the preparation of Pd-particle suspension.
In the present study, PVP which has the largest protective value among these synthetic polymers was used. The average molecular weights of PVP determined by using gel permeation chromatography (GPC) are shown in Table 1. The average diameters and particle-size distributions of palladium nanopartitles obtained by transmission electron microscopy (TEM) are shown in Table 2 and Fig. 1. The diameters of palladium nanoparticles are mainly distributed in the range of smaller than 3 nm, where more than 80% of the whole particles is included. The average diameter of palladium nanoparticles, prepared using the amount of PVP at a molar ratio of N-vinyl-Zpyrrolidone residues to palladium (VP/Pd) of 4, increases from 2.0 mn to 2.5 nm with increasing I& of PVP from 6000 to 574 000. The increase in the average diameter of the palladium nanoparticles is ascribed to a decrease in the interaction of PVP molecules with palladium nanoparticles due to an increase in M, of PVP. According to literature [17], the magnitude of the interaction of PVP molecules with metal particles increases with decreasing molecular weight. The stronger interaction
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H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
uO123456709
0123456759
Diameter I nm
(a) VP/Pdr4,
A&
Diameter / nm
: 6ooa
0123456759
(b) VP/Pd=4,
-0
Diameter / nm (C) VPF’d.4,
Mw : 175000
bfw : sooo
1 2 3 4 5 6 7 8 9 Diameter I nm
(d) VP/Pd=4, M,,. : 574000
O”l 5 Lo j20 k-‘012345575
0
A
Diameter / nm (e) VP/Pd=l, A&
0
: 6000
9
012345679 Diameter / nm (f) VPIpd.9,
A&. : 6000
0123456759 Diameter / nm (g) VP/Pd=41, A&
Fig. 1. Particle-size with PVP.
: 6000
distributions
of palladium
particles
protected
of PVP molecule of smaller molecular weight probably prevents the adsorption of palladium(O) atom on the surface of palladium particles on the growing
stage of particles. Coordination of carbonyl groups of PVP to palladium(O) atom on the surface of the palladium nanoparticles was indicated by the shift of carbonyl stretching bands [lo]. The suspensions of palladium nanoparticles were prepared with various amounts of the PVP with M, = 6000. As the VP/Pd ratio increases from 1 to 41, the average diameter of palladium nanoparticles decreases from 2.5 to 1.1 nm. The decrease of the average diameter of palladium nanoparticles with increasing VP/Pd ratio is probably ascribed to a decrease in the number of palladium(O) atom adsorbed on the surface of palladium particles in the growing stage, because the number of palladium(O) atom with coordination of PVP molecule increased. 3.2. Amount of PVP adsorbed on palladium nanoparticles The amount of the polymer adsorbed on metal nanoparticles can be determined from the concentration of free polymer. The concentration of the free polymer is usually determined by the following procedure. The particles with adsorbed polymer are sedimentated by centrifugation, and the concentration of the free polymer in the supematant is measured. In the case of the metal nanoparticles protected with polymer, however, separation of the metal nanopartitles from dispersion medium is difficult, because the sedimentation of the metal nanoparticles requires a very large centrifugal force and the sedimentated metal particles are rapidly dispersed again on removal of the centrifugal force. The present authors have found that a aminoethylated polyacrylamide gel (G(PAA-AE)) adsorbed the palladium nanoparticles protected with PVP and did not free PVP in the suspension. The dark-brown suspension of palladium nanoparticles was added to the swollen G(PAA-AE) beads (colorless) in the methanol-water (l/l, v/v) mixed solvent at pH 3 and the mixture was stirred for 6 h. When the resulting mixture was kept to stand for 5 min, the dark brown G(PAA-AE) beads were sedimentated and the supematant turned colorless. This color change clearly indicates the adsorption of the palladium nanoparticles protected with PVP on the G(PAA-AE) beads. All of the added palladium nanoparticles were adsorbed on the G(PAA-AE) beads, because no pal-
H. Hirai et al./Reactive
200
220
240
280
Wavelength
280
& Functional Polymers 37 (1998) 121-131
300
/ nm
Fig. 2. UV-vis spectra of supematant (a) after treatment of original solution of PVP (MW = 6000) in methanol-water (l/l, v/v) with aminoethylated polyacrylamide gel (G(PAA-AE)) beads at 30°C for 6 h and the supematants of the suspensions after the adsorption of palladium particles protected with PVP on G(PAA-AE) beads at 30°C for 6 h. MWof PVP: 6 000 (b), 25 000 (c). 175 000 (c) and 574 000 (e).
ladium nanoparticles in the supematant could be detected by UV-vis absorption spectra in the range from 300 to 800 nm. The absorption of PVP in the region of shorter wavelengths than 230 nm was observed in the W spectrum of the supematant after the G(PAA-AE) beads adsorption. The W spectrum of the supematant after treatment of a PVP (Mw = 6000) methanol-water solution (l/l, v/v) with the swollen G(PAA-AE) beads at 30°C for 6 h is shown in Fig. 2a. The spectrum of the original solution of PVP in methanol-water before treatment with G(PAA-AE) beads coincided with that of the supematant depicted by Fig. 2a. This fact indicates that no PVP was adsorbed on the G(PAA-AE) beads. Concerning all the molecular weights of PVP used in the present study, no adsorption of PVP on G(PAA-AE) beads was confirmed by W-vis absorption spectra. Thus, the palladium nanoparticles protected with PVP were successfully separated from dispersion medium containing free PVP. W spectra of the supematant after adsorption of palladium nanoparticles prepared with various molecular weights of PVP at VP/Pd ratio of 4 on the swollen G(PAA-AE) beads for 6 h are shown in Fig. 2b-e. With increasing M, of PVP, the absorption of PVP in the supematant (Fig. 2b-e) decreased.
121
The decrease in the absorption of PVP with increasing M, of PVP indicates the increase in the amount of PVP adsorbed on palladium nanoparticles with increasing M, of PVP. The GPC graphs of the original PVP (dotted line) and the free PVP (solid line) which was contained in the supematant after adsorption of palladium particles protected with PVP on G(PAA-AE) beads are shown in Fig. 3. The GPC graphs of the free PVP indicate a decrease in peak at the higher molecularweight fraction as compared with the GPC graphs of the original PVP This decrease clearly indicates that the higher molecular-weight fraction in PVP was more adsorbed on the palladium nanoparticles than the lower molecular-weight fraction. The concentrations of free PVP and the amounts of PVP adsorbed on palladium nanoparticles (r, g rnp2) are shown in Table 3. The amounts of PVP adsorbed on palladium nanoparticles were determined from the concentration of the free PVP With increasing M, of PVP, the amount of PVP adsorbed on palladium nanoparticles increases and the number of the PVP molecules per palladium nanoparticle decreases. The main chain of the polymer protecting the metal particles in suspension is considered to consist of sequence of segments termed ‘trains’, ‘loops’ and ‘tails’ [18]. The trains are in direct contact with the surface of the metal particles, and loops and tails are the middle and free ends of polymer chains, respectively, both extending into bulk solution. Consequently, the trains interact with the surface of particles most strongly and cover most closely. According to the literature [19], when the amount of the adsorbed polymer is independent of molecular weight of the polymer, all polymer segments are in the form of trains. Consequently, the dependence of M, of PVP on the amount of PVP adsorbed on palladium nanoparticles indicates the existence of loops and tails in the adsorbed PVP As shown in Table 3, the numbers of the PVP molecules per palladium nanoparticle protected with PVP (M, = 6000) are 5.8, 6.7, 5.2 and 1.4 at VP/Pd ratios of 1, 4, 8 and 41, respectively. The decrease in the number of PVP adsorbed on each palladium particle prepared at VP/Pd ratio of 41 is probably due to the decrease in the surface area per palladium nanoparticle. The surface area per palladium nanoparticle prepared at VP/Pd ratio of 41
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H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
20
10
0
30
0
10
Retention time / min
(a)
VP/Pd
= 4,
20
(b)
30
0
VP/Pd
VP/Pd
??
4, 1u,:25000
10
Retention tlme / mln (C)
30
Retentlon time I mln
M,:6000
10
0
20
20
30
Retention time / mln
(d)
= 4, M,:175000
VP/Pd
= 4, &,:574000
Fig. 3. Gel permeation chromatograms of original PVP (dotted line) and free PVP (solid line) remained in the supernatant after the adsorption of palladium particles protected with PVP on G(PAA-AE) beads.
Table 3 Amount of PVP adsorbed on palladium particles and concentration of free PVP in suspension of palladium particles M,., of PVP
VP/Pda
Average diameter (2R,) (mu)
Surface area per Pd particle (mn2)
Amount of adsorbed PVP (r) x lo3 (g m-‘)
Number of PVP molecule per Pd particle b
Concentration OffreePVPC (mg dme3)
6000 25000 175 000 574000 6000 6000 6000
4 4 4 4 1 8 41
2.0 2.1 2.3 2.5 2.5 1.7 1.1
13 14 17 20 20 9 4
4.4 6.0 11.1 17.1 2.0 4.5 3.4
6.7 3.4 2.6 1.5 5.8 5.2 1.4
240 220 180 130 50 520 3000
a Molar ratio of N-vinyl-2-pyrrolidone residues to PdCl2 in preparation of Pd-particle suspension. b Number of PVP molecule adsorbed on one Pd particle by calculation from M,, of PW c Concentration of free PVP in suspension of Pd particles.
is as small as 31% of that at VP/Pd of 4, because the average diameter of the palladium particles decreases from 2.0 to 1.1 m-n. With increasing amount of the admitted PVP, the concentration of the free PVP increases remarkably, as shown in Table 3.
3.3. Thickness of adsorbed layer The diameter of the metal nanoparticles without the adsorbed layer of protective polymer can be measured by using TEM. The thickness of the adsorbed layer of protective polymer on the particles can be
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H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
Table 4 Thickness of adsorbed PVP and diameter of palladium particles including adsorbed layer of PVP b M, of PVP
6000 2.5000 175 000 574000 6000 6000 6000
VP/W a
4 4 4 4
1 8 41
Average diameter of Pd particles (2Rt) (mn)
Sedimentation coefficient (S) (lo-13 s)
Thickness of
Diameter of Pd particles b (OS) (nm)
DJ2 R,
adsorbed PVP (6) (nm)
2.0 2.1 2.3 2.5 2.5 1.7 1.1
23.1 24.3 24.7 26.6 32.0 17.5 12.1
1.9 2.7 4.7 7.8 2.2 1.9 0.5
5.8 7.4 11.7 18.1 6.9 5.5 2.0
2.9 3.5 5.1 7.2 2.7 3.2 1.8
a Molar ratio of N-vinyl-2-pyrrolidone residues to PdC12 in the preparation of Pd-particle suspension. b Calculated diameter (D,) of Pd particles including adsorbed layer of PVP
determined by a centrifugation method. One of the great advantage of centrifugation method is that the thickness of the adsorbed layer of protective polymer on the particles can be measured in dispersion state. The sedimentation coefficient (5) and the thickness (6) of adsorbed layer of PVP calculated from ultracentrifugation data are shown in Table 4. The thickness of adsorbed layer of PVP increases from 1.9 to 7.8 nm as i&. of PVP increases from 6000 to 574 000. Both the tails and the loops contribute to the thickness of adsorbed layer of PVP Increase in the thickness of adsorbed layer of PVP with increasing A4, of PVP is probably due to increase in the lengths of loops and tails extending into the dispersion medium. Cohen Stuart et al. [20] reported that the thickness of adsorbed layer of PVP increased with increasing M, of PVP for the adsorption of PVP on the inner surface of glass capillary. The thickness of adsorbed layer of PVP on the palladium nanoparticles prepared at VP/Pd ratio of 41 using PVP with M, = 6000 is smallest. The content of trains in PVP adsorbed on the palladium nanoparticles under this preparation condition is probably greater than those under the conditions at a smaller VP/Pd ratio. The diameter of palladium nanoparticles including adsorbed layer (D,) calculated from Eq. 8: D, = 2Rt + 2S
where 2Rt is the dium nanoparticles can be determined dium nanoparticles
(8) average diameter of the pallawithout adsorbed layer which by TEM. The diameter of pallaincluding adsorbed layer of PVP
(D,), the average diameter of palladium nanoparticles without the adsorbed layer of PVP (2RJ and the OS/2 Rt ratio are also summarized in Table 4. The diameter of palladium nanoparticles including adsorbed layer of PVP increases with increasing thickness of adsorbed layer of PVP. The diameters of palladium nanoparticles including the thickness of adsorbed layer of PVP are 1.8-7.2 times greater than the average diameter of palladium nanoparticles without the adsorbed layer.
3.4. Hydrogenation of 1,3-cyclooctadiene The suspension of palladium nanoparticles protected with PVP was used as catalyst for hydrogenation of 1,3-cyclooctadiene at 30°C in methanol under atmospheric hydrogen pressure. When the amount of hydrogen uptake became equimolar to the admitted amount of 1,3-cyclooctadiene in the hydrogenation using the palladium nanoparticles prepared at VP/Pd ratio of 4 with PVP of M, = 6000, the yields of cyclooctene and cyclooctane was 99.7% and 0.3%, respectively. In the previous paper [ 111, we reported that the hydrogenation of 1,3-cyclooctadiene gave cyclooctene in 99.9% yield using the palladium nanoparticles prepared at VP/Pd ratio of 41 with PVP of M, = 355000 (degree of polymerization, 3 200). The catalytic activity represented by the initial rate of hydrogenation per mol of palladium is shown in Table 5. The catalytic activity decreases by 19% with increasing M, of PVP from 6 000 to 574 000. 1,3-Cyclooctadiene is hydrogenated after the sub-
H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
130
Table 5 Hydrogenation of 1,3-cyclooctadiene using suspension of palladium particles Mw of PVP
VP/Pd a (m2 g-l)
Specific surface area b (nm)
Thickness of adsorbed PVP (Hz-mol Pd-mol-’ s-l)
Initial hydrogenation rate c
6000 25 000 175 000 574 000 6000 6000 6000
4 4 4 4 1 8 41
179 177 145 140 169 206 297
1.9 2.7 4.7 7.8 2.2 1.9 0.5
25.4 24.6 22.2 20.7 22.2 24.8 21.0
a Molar ratio of N-vinyl-2-pyrrolidone residues to PdC12 in the preparation of Pd-particle suspension. b Specific surface area of Pd particles determined from particle-size distributions shown in Fig. 1. CHydrogenation conditions: at 30°C under 1 atm. of hydrogen, [1,3-cyclooctadiene] = 25 mm01 dmm3, [Pd] = 3.3 pmol dnm3 in methanol (20 cm3).
strate diffuses into the adsorbed layer of PVP and is adsorbed on the surface of palladium nanoparticles. As M, of PVP increases from 6 000 to 574 000, the thickness of the adsorbed layer of PVP increases 4.1 times from 1.9 to 7.8 run as shown in Table 5. The reciprocal of the adsorbed layer thickness is probably proportional to the diffusion velocity of substrate, that is, the initial rate of hydrogenation. The reciprocal of the adsorbed layer thickness decreases by 75% from 0.53 to 0.13 nm-’ with increasing it4, of PVP from 6 000 to 574 000. The specific surface area of palladium nanoparticles decreases by 22% from 179 to 140 m* g-l with increasing M, of PVP from 6000 to 574000. A multiple correlation analysis on the data of the hydrogenation of 1,3-cyclooctadiene was carried out by using the catalytic activity (y, H2-mol Pd-mol-’ s-l) as criterion variate, and the specific surface area (~1, m* gg’) and the reciprocal of adsorbed layer thickness (~2, nm-‘) as explanatory variates, respectively. A multiple regression equation is obtained as follows: y = 0.0396~~ + 3.69x2 + 15.2
(9)
The multiple correlation coefficient is 0.845. Thus, the regression of the catalytic activity upon the specific surface area of palladium nanoparticles and the reciprocal of the adsorbed layer thickness is satisfactory. The variates y, x1 and x2 are standardized to dimensionless variates y*, x; and x;, respectively. Thus, Eq. 9 is transformed to Eq. 10. y* = 0.509x; + 0.354x;
(10)
The specific surface area contributes more strongly to the catalytic activity than the reciprocal of the adsorbed layer thickness, because the coefficient of x; is remarkably larger than that of x;, as shown in Eq. 10. In the hydrogenation using the palladium nanoparticles prepared at VP/Pd ratio of 4 with PVP of M, = 6000, dependence of the initial hydrogenation rate on the palladium concentration was found to be the first order. This result indicates that the catalytic activity is dependent on the specific surface area with the first order in the case of constant thickness of the adsorbed layer. According to previous paper [ll], the initial hydrogenation rate was zero order with respect to 1,3-cycloo-ctadiene concentration and first order with respect to palladium concentration using the palladium nanoparticles prepared at VP/Pd ratio of 41 with PVP of M,,, = 355 000 (degree of polymerization, 3 200). The contents of loops and tails in PVP adsorbed are probably large in the suspension prepared by using PVP with high molecular weight (M, = 355 000). The result suggests that loops and tails in the adsorbed PVP may have a small influence on the diffusion of 1,3-cyclooctadiene. The catalytic activity of the palladium nanopartitles prepared at VP/Pd ratio of 41 using PVP of M, = 6 000 is smallest in spite of the largest specific surface area of the palladium nanoparticles, as shown Table 5. The value of the thickness of the adsorbed layer on the palladium nanoparticles is smallest. The content of trains in the low molecular weight of PVP is probably large, and the trains in PVP may
H. Hirai et al. /Reactive & Functional Polymers 37 (1998) 121-131
131
be compressed by the osmotic pressure due to the high concentration of the free PVP in the dispersion medium. The trains in PVP probably prevent the adsorption of 1,3-cyclooctadiene on the surface of palladium nanoparticles.
Koki Co.) for their aid to ultracentrifugation experiments. N.Y. also thanks T. Murakami and A. Yasuda (Johnson Polymer Co.) for useful discussions.
4. Conclusion
[l] G. S&mid, Clusters and Colloids, VCH, Weinheim, 1994. [2] P. Mulvancy, F. Grieser, D. Meisel, Surf. Sci. Ser. 38 (1991) 303. [3] M.T. Reetz, W. Helbig, S.A. Quaiser, U. Stimming, N. Breuer, R. Vogel, Science 267 (1995) 367. [4] T. Yonezawa, T. Tominaga, N. Toshima, Polym. Adv. Technol. 7 (1996) 645. [5] G.I. Taylor, Proc. R. Sot., London, Ser. A 219 (1953) 186. [6] G.I. Taylor, Proc. R. Sot., London, Ser. A 225 (1954) 473. [7] M.J. Garvey, T.H.F. Tadros, B. Vincent, J. Colloid Interface Sci. 49 (1974) 57. [8] H. Hirai, J. Macromol. Sci.-Chem. Al3 (1979) 633. [9] H. Hirai, Y. Nakao, N. Toshima, J. Macromol. Sci.-Chem. Al2 (1978) 1117. [lo] H. Hirai, H. Chawanya, N.Toshima, React. Polym. 3 (1983) 127. [ 1l] H. Hirai, H. Chawanya, N. Toshima, Bull. Chem. Sot. Jpn. 58 (1985) 682. [12] J.K. Imnan, H.M. Dintzis, Biochemistry 8 (1969) 4074. [13] The Chemical Society of Japan (Ed.), Kagaku Binran. Kisohen, Maruzen Ltd., Tokyo, 1975, p. 60. [14] S. Asahara, N. Tokura, M. Okawara, J. Kumanotani, M. Senoo (Eds.), Youzai Handbook, Koudansya Ltd., Tokyo, 1976, p. 329. [15] Kagaku Kougaku Kyoukai (Ed.), Bussei Teisuu, Vol. 7, Maruzen Ltd., Tokyo, 1969, p. 74. [ 161 H. Thiele, H.S. von Levem, J. Colloid Sci. 20 (1965) 679. [17] B. Jirgensons, Makromol. Chem., 6 (1951) 30. [18] T.F. Tadros, Polymer Colloids, Elsevier, Essex, 1985, p. 107. [19] R. Perkel, R. Ullman, J. Polym. Sci. 54 (1961) 127. [20] M.A. Cohen Stuart, J.W. Mulder, Colloid Surface 15 (1985) 49.
The palladium nanoparticles protected with PVP were successfully separated from dispersion medium by adsorption of the palladium nanoparticles protected with PVP on G(PAA-AE) beads. The concentration of free PVP in dispersion medium and the amount of PVP adsorbed on palladium nanopartitles were determined by using the G(PAA-AE) beads adsorption method. The amount of PVP adsorbed on palladium nanoparticles increased with increasing M, of PVP The concentration of free PVP in dispersion medium increased as the admitted amount of PVP increased. The higher molecularweight fraction in PVP was more adsorbed on the palladium nanoparticles than the lower molecularweight fraction. The thickness of adsorbed layer of PVP was successfully determined from sedimentation coefficient of palladium nanoparticles protected with PVP by the ultracentrifugation method. The thickness of adsorbed layer of PVP increased with increasing M, of PVP The hydrogenation activity for 1,3-cyclooctadiene using the suspension of palladium nanoparticles depended on specific surface area of palladium nanoparticles far more effectively than the thickness of adsorbed layer of PVP Acknowledgements The authors express their grateful acknowledgment to thank M. Morita and K. Takahashi (Hitachi
References