Preparation and Characterization of Polymer-Stabilized Rhodium Sols

J. Phys. Chem. B 1999, 103, 1651-1659

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Preparation and Characterization of Polymer-Stabilized Rhodium Sols. I. Factors Affecting Particle Size G. W. Busser, J. G. van Ommen, and J. A. Lercher* Department of Chemical Technology, Catalytic Processes and Materials Group, P.O. Box 217, 7500 AE Enschede, The Netherlands ReceiVed: August 11, 1998; In Final Form: December 22, 1998

Preparation and characterization of polymer-stabilized rhodium sols with average metal particle diameters between 1 nm and 3.5 nm are described. The interaction of the rhodium ions with the polar polymer before reduction is one of the most important factors in preparing stable metal sols. With poly(ethylene oxide) the interaction was so weak that only large metal particles precipitated and with polyethyleneimine the interaction was so strong that the precursor could not be reduced. The best polymers were polyvinyl-2-pyrrolidone (PVP) and poly-2-ethyloxazoline. With PVP as a stabilizer it was shown that the reduction rate determined the particle size. The primary particle size in the final sol was the smallest when hydrogen was used as a reducing agent and 1-butanol as a solvent. When colloids were prepared in an alcohol/water mixture in which the alcohol functions as a reducing agent, the size increased with increasing molecular weight of the alcohol. The stability of the colloids decreased with increasing solubility of the polymer.

Introduction Beginning with early reports of Turkevich et al.1 the topic of controlled preparation of monodisperse nanoscale metal particles has maintained its fascination over the years (e.g., refs 2-12). The fundamental interest arises from the fact that the small metal particles in a sol show properties different from atomically dispersed and bulk metals.13-16 Their electronic, magnetic, and optical properties can be fine-tuned.13-19 Applications include such diverse fields as electroless metal deposition20,21 and catalysis.22-29 Generally, the sols can be stabilized via Coulombic forces (electrostatically stabilized colloids) or via steric protection (sterically stabilized colloids). Sterically stabilized colloids are preferred, as they can be used in aqueous and nonaqueous media without particle coagulation. In polymer-stabilized colloidal suspensions the polymer acts as a steric stabilizer by balancing the van der Waals forces that cause coagulation of the particles.30 The adsorption of the polymer on the metal cluster/particle is considered irreversible, as simultaneous desorption of all polymer segments is statistically unlikely.31 Thus, conceptually, one should be able to subtly control the size and distribution of the metal particles via the reduction/nucleation process, provided sufficient stabilization by the polymer exists. However, detailed information on preparation procedures to obtain monodisperse sols are scarce. In their classic paper, Turkevich et al.1 show that the particle size and distribution are determined by temperature, metal concentration, and reducing agent. These results were updated some 40 years later by Kirkland et al.,32 who completed the characterization also with transmission electron microscopy (TEM) and UV-vis spectroscopy. In the present paper we describe the preparation of polymerstabilized rhodium sols that are used as catalysts for hydrogenation in the liquid phase.33 The most important parameters affecting the particle size are explored by varying the preparation * Author for correspondence. Tel. +31-534892860, Fax +31-534894683.

conditions, with a special emphasis on the role of the reduction process. Characterization of the precursors and final sols includes high-resolution transmission electron microscopy (HRTEM), 13C NMR, and X-ray absorption spectroscopy (XAS). Experimental Section Chemicals. Rhodium chloride (RhCl3‚3H2O, high purity), PVP, poly-2-ethyloxazoline (POX), poly(ethylene oxide) (PEO), and polyethyleneimine (PIM) (all special grade) were obtained from Aldrich; methanol, ethanol, 1-propanol, 1-butanol, and D2O (pro analysis) from Merck. The chemicals were used without further purification. Preparation. Preparation with Various Reducing Agents. Rhodium chloride (0.057 mmol) and polymer (2.0 mmol) monomer units of PVP (MW ) 40 000) were dissolved in water (20 mL). This mixture was heated at 373 K for 2 h to establish a good interaction between the metal salt and the polymer. Then, it was mixed rapidly with 130 mL of alcohol (methanol, ethanol, 1-propanol, or 1-butanol) and heated to 348 K, 353 K, 373 K, and 393 K, respectively. Heating at that temperature was continued for 48 h. Subsequently, the colloidal solutions were cooled with liquid nitrogen, the solvent was evaporated under vacuum, and the materials were redissolved in 1-butanol. To prepare a colloid reduced by hydrogen, a solution of rhodium chloride in water was prepared as described above. This mixture was treated with hydrogen at 7 bar and 343 K for 1 h. Preparation with Various Polymers. Colloids stabilized by a high-molecular-weight (MW ) 36 0000) polyvinyl-2-pyrrolidone (PVP+), POX, PEO, and PIM were prepared using the above-described method with hydrogen as a reducing agent. All colloids are designated according to the stabilizing polymer and reducing agent (see Table 1). The principal chemical structures of the polymers are shown in Figure 1. Physicochemical Characterization. TEM. For TEM a drop of the colloidal solution was placed on a carbon-covered copper

10.1021/jp9833449 CCC: $18.00 © 1999 American Chemical Society Published on Web 02/24/1999

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TABLE 1: Properties of the Colloids colloid

polymer

Rh/PVP/MeOH PVP (MW ) 40 000) Rh/PVP/EtOH (MW ) 40 000) Rh/PVP/PrOH (MW ) 40 000) Rh/PVP/BuOH (MW ) 40 000) Rh/PVP/H2 (MW ) 40 000) Rh/PVP+/H2 PVP+ (MW ) 360 000) Rh/POX/H2 POX (MW ) 50 000) Rh/PEO/H2 PEO (Mw ) 100 000) Rh/PIM/H2 PIM (MW ) 250 000) a

reducing agent

average particle sizea (nm)

methanol

3.5 (0.5)

ethanol 1-propanol 1-butanol hydrogen hydrogen

2.3 (0.8) 2.5 (0.7) 3.3 (0.6) 1.3 (0.7) 1.5 (0.7)

hydrogen

1.2 (0.7)

hydrogen hydrogen

severe coagulation no reduction

Determined by TEM; numbers in brackets are standard deviations.

Figure 2. Cell used for XAS experiments.

Figure 1. Polymers used for stabilization.

grid (Balzers) and analyzed with a transmission electron microscope JEOL 200 CX, which was operated at 250 kV. Particle size distributions were determined by optical inspection of the photographs counting at least 75 particles. In this way, at least three samples per colloid were studied. Additionally, HRTEM was performed at the National Centre for High-Resolution Electron Microscopy in Delft, The Netherlands. Measurements were performed using a Philips CM 30 T electron microscope operated at 300 kV. NMR Spectroscopy. Interactions of rhodium chloride with the carbonyl group of the polymer were studied with 13C NMR. Spectra of concentrated solutions (in D2O) of colloid precursors were recorded at ambient conditions with a Bruker spectrometer operating at 62.9 MHz, using 5-mm high-resolution liquid probes with internal 2H lock and 1H decoupling. XAS. XAS measurements were performed to determine the average rhodium particle size in the colloids and to get information about the electronic state and neighboring atoms of the rhodium atoms. Measurements were performed at the National Synchrotron Light Source, Brookhaven National Laboratory, Upton, New York, beamline X23A2 equipped with a Si (311) double crystal monochromator that required no detune. Additionally, some spectra were measured at SRS (Daresbury, UK) station 9.2 equipped with a Si (220) monochromator detuned to 50% to remove high-energy harmonics. Concentrated solutions of catalyst (( 1.4 × 10-2 mmol Rh/g) were poured into a glass cell containing polyimide windows (KAPTON) and an optical path length of 1 cm (Figure 2). The samples could be flushed with hydrogen or inert.

Figure 3. Influence of Rh(III) concentration on chemical shift of the peak from the carbonyl carbon atom measured with 13C NMR spectroscopy. Concentration of PVP: A-E, 4 M; concentration of Rh(III): (A) 0 M; (B) 0.3 M; (C) 0.8 M; (D) 1.6 M; (E) 2.4 M.

Extended X-ray absorption fine structure (EXAFS) spectra at the Rh-K edge (23 220 eV) were recorded at liquid N2 temperature or at 343 K, the spectral resolution being ( 1 eV. The EXAFS analysis was performed by standard procedures.34 The k2-weighted spectra were Fourier transformed within the limits 4 < k < 16. The EXAFS of the first Rh-Rh shell were fitted using phase-shift and amplitude functions obtained from a Rh foil under the assumption of plane waves and single scattering. From this fit, the Rh-Rh coordination number (accuracy ( 15%), Rh-Rh distance (accuracy ( 1%), and Debye-Waller factor (accuracy ( 15%) were obtained. From the average coordination number an average particle size was determined using the correlation described by Kip et al.,35 assuming a spherical particle shape. The relative error in the resulting particle size was estimated to be ( 30%. Additionally, reduction of some samples was monitored by recording spectra in the X-ray absorption near-edge structure (XANES) range (from -20 eV to 75 eV relative to the edge) during treatment with hydrogen (T ) 343 K). Results The preparation of Rh sols involves three elementary steps that determine the properties of the final stabilized sol: (a) the

Polymer-Stabilized Rhodium Sols

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Figure 4. Particle size distributions of colloids prepared using PVP as a stabilizer. Reducing agents: (A) methanol, (B) ethanol, (C) propanol, (d) 1-butanol.

mixing of the Rh salt and the polymer (preparation of the precursor), (b) the reduction of the metal salt to metal particles, and (c) the stabilization of these metal particles by the polymer. Balancing the three factors is important to tailor the particle size and stability (time that the sol does not coagulate under a set of reaction conditions) of the sol. Thus, evidences for the elementary processes during these three steps will be provided and discussed with respect to the implications for the final material properties. Evidence for Interaction of Rh3+ with PVP. The primary condition to obtain small particles is to disperse the metal ions well over the polymer and to fix the location on the polymer in that state. As the salt is per se well dispersed in the aqueous solution, one would speculate that dispersion on the polymer requires a relatively strong interaction between the polar nucleophilic groups of the polymer and the metal cations. In the absence of such an interaction one would expect that in the subsequent reduction step uncontrolled nucleation in the liquid

phase would take place. Experimentally, the anchoring was performed by refluxing the aqueous metal salt with the polymer for 2 h. At first, evidences of the interaction between the nucleophilic groups of PVP with Rh3+ are addressed. For this, varying concentrations of rhodium chloride (0-2.4 M) in water were mixed with approximately the same amount of PVP (appr. 4 mM solution of the polymer in water) and the resulting solution was characterized by 13C NMR spectroscopy (see Figure 3). The 13C NMR spectrum of pure PVP showed a signal from the carbonyl group between 177 ppm and 179 ppm, consisting of at least five peaks. Following Ebdon et al.36 and Cheng et al.,37 we attribute these to signals from carbonyl groups with different surroundings (result of the tacticity of the polymer containing an asymmetric carbon atom). These signals seemed to shift to lower field upon addition of increasing concentrations of rhodium chloride. Also, the shape of the signal varied with increasing concentration. At the highest rhodium chloride

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Figure 5. Particle size distributions of colloids prepared using H2 as a reducing agent. Stabilizers: (A) PVP, (B) PVP+; (C) POX.

concentration, only two distinct peaks were observed. This suggests that the individual peaks shift to a different extent, resulting in an overlap at these high rhodium chloride concentrations. The lower integral intensity suggests that a part of the carbonyl groups do not contribute to the NMR signal. We speculate that this is affiliated with the presence of insoluble parts of the polymer due to the high concentration of Rh salts. Influence of Reduction Conditions. After addressing the evidence for a direct interaction between the Rh ions and the functional groups of the polymer, we turn to the role of the reduction process for the size and size distribution of the Rh sol. In the reduction process alcohol is converted to the corresponding aldehyde9,10 according to RhCl3 + 1.5 CnH2n+2OH Rh +1.5 CnH2nO + 3 HCl. In analogy to metal catalyst precursors dispersed on solid supports, it can be expected that a stronger reducing agent and a better dispersed Rh precursor should lead to smaller particles. The average particle sizes and standard deviations of Rh in sols prepared with various alcohols are compiled in Table 1. The particle size distributions are depicted together with the TEM graphs in Figure 4. The distribution curves were narrow and the particle size increased with increasing molecular weight of the alcohol used as reductant. However, reduction with methanol appeared to produce unusually large particles. Similar sizes for PVP-stabilized Pt and Pd particles reduced by methanol, ethanol, and 1-propanol have been reported by Teranishi et al.38,39 When hydrogen was used for reduction (see Figure 5A), the resulting metal particles were the smallest observed, (around 1 nm).

To characterize the metal particles more extensively, the EXAFS of the various samples were measured. Since the colloids were prepared with the objective to use them as catalysts in hydrogenation reactions, all spectra were recorded before and after treatment with hydrogen at 343 K. It was difficult to obtain high-quality spectra of all samples at liquid nitrogen temperature because of crack formation during the solidification process. Therefore, some of the spectra were recorded at 343 K and compared with spectra of the reference materials at 343 K. The higher temperature mainly affects the Rh-Rh contribution, which is lower when recorded at a higher temperature. As a result, the Rh-O and Rh-Cl contributions are more pronounced compared with the Rh-Rh contribution when recording under these conditions. Figure 6A shows the k2-weighted Fourier transforms (k ) 4-16) of Rh/PVP/H2 at 343 K, before treatment with hydrogen. In Figure 6B, Rh/PVP/H2 is compared with the reference materials rhodium chloride and rhodium oxide. The Fourier transform of the spectrum of Rh/PVP/H2 shows a very small Rh-Rh contribution, indicative of the presence of very small particles. Additionally, contributions of Rh-Cl and Rh-O are clearly visible. Fourier transforms of spectra of Rh/PVP/MeOH and Rh/PVP/ BuOH at 343 K are depicted in Figure 7. The former sample has a much smaller amplitude than the latter, implying the presence of smaller particles. This is surprising, because TEM photographs indicate that both samples contain similar particle sizes (see Figure 4).

Polymer-Stabilized Rhodium Sols

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Figure 6. Weighted Fourier transform of Rh/PVP/H2 before reduction with hydrogen (recorded at 343 K). (A) Comparison with a rhodium foil; (B) comparison with rhodium oxide and rhodium chloride.

Figure 7. Weighted Fourier transforms of Rh/PVP/BuOH and Rh/ PVP/MeOH before reduction with hydrogen (recorded at 343 K).

TABLE 2: Results from Fitting Analysis of EXAFS Data; N ) coordination number; ∆σ2 ) Debye-Waller Factor; r ) Rh-Rh Interatomic Distance

colloid Rh/PVP/H2 Rh/PVP/EtOH Rh/PVP/PrOH Rh/PVP/BuOH Rh/PVP/MeOH

N

∆σ2 (Å2)

r (nm)

sizea (nm)

7.1 8.4 8.6 10.1 6.4

20 × 8.3 × 10-4 12 × 10-4 8.8 × 10-4 1.5 × 10-4

2.649 2.677 2.673 2.676 2.661

1.3 1.9 1.9 3.3 1.2

10-4

number of atoms per particlea 34 104 104 674 28

a Determined using the method published by Kip et al. (ref 35) assuming spherical particles (R ) 0.5).

Figure 8 shows the radial distribution function of samples with different sizes after hydrogen treatment (20 min, 343 K, atmospheric pressure) recorded at liquid nitrogen temperature. With increasing size, an increase in Rh-Rh contribution is observed, although no additional contributions are visible. Additonal contributions are also nearly absent in the radial distribution functions from spectra obtained at 343 K (Figure 9), again confirming the disappearance of a substantial part of the Rh-Cl and Rh-O contributions after reduction. From the fourier transforms of spectra of the reduced samples (recorded at liquid nitrogen temperature), the first-shell RhRh contribution has been isolated (1.6-3 Å), back transformed, and subjected to a one-shell fitting procedure. The phase-shift and amplitude functions were obtained from a Rh foil. In Table 2, the resulting fitting parameters N (coordination number), r (Rh-Rh distance), and ∆σ2 (Debye-Waller factor) are tabulated. From the coordination number, an average particle size was calculated following the suggestions in ref 35. The average

particle sizes determined by EXAFS are generally in good agreement with those determined by TEM, except for the size of Rh/PVP/MeOH. For this sample the particle size determined by TEM (3.5 nm) was larger compared with the size determined by EXAFS (1.2 nm). Thus, the sample was reexamined with HRTEM to determine whether the 3.5-nm crystals might be composed of smaller loosely agglomerated crystals. Figure 10 shows a typical metal particle that shows multiple orientation of the lattice fringes, indicating indeed particle agglomerates. In Figure 11, XANES of samples with different particle sizes before treatment with hydrogen are compiled. For comparison, XANES of reference compounds (rhodium foil, rhodium oxide, and rhodium chloride) are shown in Figure 12. The features of the spectrum of Rh/PVP/1/H2 were very similar to those of rhodium chloride, with slight contributions of metallic rhodium. The XANES of Rh/PVP/BuOH were very similar to that of the rhodium foil, whereas the spectrum of Rh/PVP/MeOH indicated contributions of rhodium oxide and rhodium foil. After reduction with hydrogen at 343 K, all spectra showed only the characteristics of metallic rhodium (Figure 13), that is, two well-defined peaks were present. Note that the width of the first peak increased with decreasing particle size, whereas the intensity of the second peak increased in intensity with increasing particle size. Role of Polymer. Using hydrogen as reductant to realize the smallest Rh particles, the effect of the polymer upon the particle size and reduction behavior was explored. Two parameters were varied: the size of the polymer strands for PVP and the polarity of the functional groups, by choosing a range of different polymers. The TEM graph and the determined size distribution of the colloid stabilized by the high-molecular-weight PVP (PVP+) are shown in Figure 5B. The metal particles were slightly larger compared with those seen with the standard PVP (Figure 5A, Table 1). With POX as stabilizer, Rh particles with an average size of 1 nm were prepared (Figure 5C, Table 1). PEO was ineffective in stabilizing the particles, and severe coagulation was observed after reduction with hydrogen. Also, PIM was was found to be unsuitable for preparing colloidal Rh metal. The color of the material remained yellow during treatment with hydrogen, indicating that reduction of the Rh3+ was prevented. Stability of Colloids in Presence of Hydrogen. The influence of hydrogen treatment (7 bar, 343 K, 1 h) on the stability of materials prepared was tested in 1-butanol and water. Figure 14 shows the TEM graphs after these experiments (compare with Figures 5A and 4D). The size (distribution) of materials

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Figure 8. k2-Weighted Fourier transforms of colloids after reduction (recorded at 100 K) compared with Rh foil. (A) Rh/PVP/H2; (B) Rh/PVP/ MeOH; (C) Rh/PVP/EtOH; (D) Rh/PVP/PrOH; (E) Rh/PVP/BuOH.

Figure 9. k2-Weighted Fourier transforms of colloids after treatment with hydrogen (recorded at 343 K) compared with Rh foil.

with 1-butanol as solvent was hardly changed by the hydrogen treatment. In water as solvent, however, exposure to hydrogen led to the formation of a black deposit indicating coagulation (Figure 15). Discussion As outlined above, one can conceptually differentiate three key factors that influence the size and stability of the Rh sol: (a) the mixing of the Rh salt and the polymer (preparation of the precursor), (b) the reduction of the metal salt to metal particles, and (c) the stabilization of these metal particles by the polymer. With respect to the first factor, the strength of interaction will improve the fixation of Rh ions on the polymer strands. The prime interactions occur between the (partly accessible because of solvation) Rh3+ ion and the carbonyl oxygen and the nitrogen having strong electron-pair donor properties. A priori, one expects the carbonyl group to be the more important for interactions, as it is less sterically hindered than the basic

Figure 10. HRTEM photograph of Rh/PVP/MeOH.

nitrogen. This is confirmed by 13C NMR measurements indicating a significant change in the NMR spectrum of the carbonyl carbon upon addition of rhodium chloride, whereas hardly any effects were observed for the other carbons. The 13C NMR signal of the carbonyl group (Figure 3) consists of several peaks that can be attributed to carbonyl groups with different surroundings as a result of the tacticity of the polymer containing an asymmetric carbon atom.36,37 There is a gradual shift of these carbonyl peaks to a lower field upon increasing the rhodium concentration. This indicates loose ionic-type coordination of rhodium cations to the carbonyl group and the establishing of a sorption equilibrium of rhodium chloride. Similar types of interactions were observed with NMR, for example for supramolecular complexes (host-guest interactions).40 If we assume that for a particular polymer a certain degree of fixation of Rh is reached, the size of the Rh metal particles will depend on the rate at which it is reduced and the chance to undergo migration to a neighboring metal atom/particle. For

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Figure 11. XANES of colloids before reduction. (A) Rh/PVP/H2; (B) Rh/PVP/MeOH; (C) Rh/PVP/BuOH; (D) Rh foil.

Figure 14. TEM photographs of colloids dissolved in 1-butanol after hydrogen treatment (7 bar H2; 343 K; 1 h). Colloids: (A) Rh/PVP/H2; (B) Rh/PVP/BuOH.

Figure 12. XANES of the reference materials. (A) Rhodium oxide; (B) rhodium chloride; (C) rhodium foil.

Figure 15. TEM photograph of Rh/PVP/H2 dissolved in water after hydrogen treatment (7 bar H2; 343 K; 1 h).

Figure 13. XANES of colloids after treatment with hydrogen. (A) Rh/PVP/H2; (B) Rh/PVP/MeOH; (C) Rh/PVP/BuOH; (D) Rh foil.

realizing small particles, nucleation must be faster than the particle growth. Figure 4 shows that the particle size increases with increasing molecular weight of the alcohol. Since the reducing agent has to interact with the metal complex during the reduction, the elementary process will be influenced by the ligands surrounding the metal ion and the reducing agent. Bulky ligands and reducing agents and a strong interaction of the ligands with the metal ion, limit the interaction and, hence, can retard reduction, which results in an increase of the particle size. Considering the results with PVP, the difference in particle size must result from the differences of the reducing agents. As the steric environment of the reducing alcohol groups hardly varies from methanol to 1-butanol, the decreasing reduction potential of the alcohols41,42 is seen as the most drastic difference between the reductants. Note in this context that Reetz and Helbig43 have reported size-selective synthesis of transition

metal clusters in an electrochemical cell. That study shows, in line with the present observations, that the metal particle size can be lowered by increasing the reduction potential at the cathode of the cell. Thus, we conclude that the decrease in the reduction potential from methanol to 1-butanol causes decreasing reduction rates (and, thus, nucleation rates) and consequently larger metal particles. Using only the results from TEM measurements compiled in Figure 4, the trend that increasing reduction potential leads to smaller particle size does not to apply when methanol is used as a reducing agent. However, EXAFS results suggest the presence of small particles, in line with the expectations. In this context it should be emphasized that the two techniques yield complementary information. EXAFS provides primarily information on the average local arrangement around the absorbing atom, whereas TEM allows the estimate the overall size of particles. Thus, we conclude that with methanol as a reducing agent small clusters of metal atoms (approximately 1 nm in size) have been formed that agglomerate to larger entities. A similar result has been reported by Nakao and Kaeriyama3 using also the two techniques for characterizing Rh and Pt/Rh clusters. Indeed, with HRTEM (Figure 10) it is seen that in the methanolreduced sample, complex Rh particles consisting of several individual crystallites exist. Teranishi et al.38,39 report results in preparing PVP-stabilized Pt and Pd colloids with methanol, ethanol, and 1-propanol. Particles produced in methanol are found to be the largest,

1658 J. Phys. Chem. B, Vol. 103, No. 10, 1999 whereas the ones prepared in ethanol and 1-propanol are smaller and similar in size. On the basis of these results, they conclude that the reduction rate increases with increasing molecular weight of the alcohol. Our EXAFS and HREM results reported in this study clearly demonstrate that reduction by methanol led to particles consisting of very small primary particles and that particles made by reduction with 1-butanol led to particles substantially larger than those prepared with ethanol and 1-propanol, that is, that the reduction rate increases from butanol to methanol, in agreement with the results of Hilmi et al.41 and Leungand Weaver.42 Whereas the results reported here are in line with the expected reduction behavior of the alcohols, we think that the results of Teranishi et al.38,39 stem from a secondary sintering of particles prepared in methanol or illresolved TEM images. Apparently, the very small particles are insufficiently stabilized in methanol. In that context Napper30 pointed out that stabilization of metal clusters by polymers strongly depends on the extent of the solvation of the polymer. When the polymer dissolves very well, the interactions of the polymer molecules with the solvent increase at the expense of the interactions of the polymer functional groups with the metal surface. Since the solvation of PVP increases in the order 1-butanol < 1-propanol < ethanol < methanol, the relative stability of the metal particles in the solvents investigated will be the lowest in methanol. As the interaction of the polymer with the metal is dominated by electrostatics, the strength will be lower the lower the oxidation state of the metal is. Treatment of the sol with hydrogen can thus influence the interaction by changing the oxidation state. The Fourier transform of the EXAFS spectrum of the colloid containing small (1 nm) particles before hydrogen treatment shows a rather substantial abundance of Rh-Cl and Rh-O contributions (Figure 6B). This suggests that a considerable amount of chloride has not been removed during preparation (which involved heating in butanol and reduction at 343 K in 7 bar H2 for 1 h). In this respect, the preparation by refluxing in alcohols [Rh/PVP/MeOH (348 K) and Rh/PVP/BuOH (393 K); Figure 7] seems to be far more effective. Comparing the XANES of the samples in Figure 11 with reference XANES of rhodium chloride, rhodium oxide, and rhodium foil in Figure 12 indicates the presence of chlorine and oxygen in Rh/PVP/H2. In contrast, Rh/PVP/MeOH resembles mainly a combination of rhodium oxide and rhodium metal, suggesting that a fraction of the metal was oxidized. Rh/PVP/BuOH in contrast showed mainly features of bulk rhodium. This suggests that smaller particles are less reduced or reoxidize more easily when the particle size is small, in agreement with ref 44. The XANES after hydrogen treatment confirm the decrease of Rh-Cl and Rh-O contributions and clearly resemble the characteristics of a rhodium foil (compare Figures 12 and 13). However, the spectra show differences for different particle sizes. The white line broadened with decreasing particle size, whereas the second peak decreased in intensity. The broadening of the white line is attributed to the presence of small quantities of oxygen, because the white line of rhodium oxide is also very broad (Figure 12). The effect of particle size on the intensity of the second peak has been reported by Resasco et al.45 for Rh/TiO2 and was thought to be mainly due to a difference in the number of neighbors around the absorber atom. In conclusion, the XAS results show that with increasing particle size the metallic character of the particle increases. The interaction of the rhodium particle with the polymer consists

Busser et al. TABLE 3: Estimation of Particle Loadings on PVP (2.5 wt % Rh on PVP) particle size (nm)

amount of atoms per particlea

amount of polymer molecules per particle (MW ) 40 000)

amount of polymer molecules per particle (MW ) 360 000)

1 1.5 2 2.5 3.5 4.5

18 50 122 259 825 1947

1.8 4.9 12.0 25.6 81.5 192

0.2 0.6 1.3 2.8 9.0 21.4

a Determined as described by Kip et al. (ref 35) assuming spherical particles.

mainly of an electrostatic attraction between the free electron pairs of the oxygen of the carbonyl group and positively charged rhodium atoms on the surface. This may be similar to the interaction between rhodium cations and PVP in the precursor. Treatment with hydrogen leads to a decrease of fraction of oxidized surface Rh atoms and, therefore, to a decreased stability. This leads to coagulation in a polar solvent (water) in which the interaction of the metal surface with the polymer is already low because of the strong interaction of the polymer with the solvent molecules. An increasing molecular weight of PVP (PVP+) leads to colloids that contain particles that are slightly larger than those stabilized by a lower-molecular-weight polymer. This suggests that the particle size is not only determined by the metal (salt) loading of functional groups, but also by the amount of metal ions coordinated to one particular polymer string. To give an indication of this loading, in Table 3 an estimation of the amount of polymer molecules per particle as a function of the particle size is tabulated for polymer molecular weights of 40 000 and 360 000. Using a molecular weight of 40 000 and a metal loading of 2.5 wt %, it can be estimated that particles of 1 nm (containing approximately 18 atoms according to ref 40) are surrounded by two polymer strings. Note that a large fraction of the metal surface has to be surrounded by the polymer to obtain sufficient stabilization.30 Therefore, the low number of polymer strings per particle indicates that multiple adsorption21,46 of the polymer on the metal surface is mandatory for sufficient stabilization. With POX as a stabilizer, with about the same molecular weight as PVP, particles with similar size as with PVP are obtained. The polymer contains the same type of functional groups, which leads us to believe that this is a crucial factor in stabilizing the metal particle. The steric structure of the polymer (rings, type of backbone) is obviously of less importance. PEO was not a good stabilizer. Possibly, the interaction with functional groups containing only oxygen atoms as potential electron donors, is weaker. This is supported by results reported for the preparation of rhodium particles stabilized by (vinyl alcohol) and polymethylvinyl ether. Particle size of colloids stabilized by these polymers, being prepared under similar conditions as when using PVP,9 are found to be significantly larger. In the presence of PIM, rhodium chloride could not be reduced. Apparently, the interaction with the nitrogen-containing functional group was too strong. Conclusions Rhodium colloids with particles in the size range of 1-3.5 nm stabilized by PVP (MW ) 40 000) in alcoholic solutions were successfully prepared. One of the most important parameters determining particle size was found to be the reduction rate. The highest reduction rate resulted in the smallest particles.

Polymer-Stabilized Rhodium Sols Second, the nature of the stabilizing polymer strongly affected the resulting material. Strong coordination (with polyvinylpyridine) resulted in complexes that could not be reduced, whereas weak coordination (with PEO) gave unstable colloids. Moderate coordination (with PVP and POX) led to the best results. The materials have a higher stability as the oxidation state of the metal increases and the polarity of the solvent decreases. Therefore, we speculate that the interaction between the colloidal rhodium particles and the polymer is similar to that between the precursor and the polymer. The main stabilization is then achieved through interactions between positively charged rhodium atoms on the surface of the metal particle and the carbonyl group of the polymer. Acknowledgment. We are grateful for experimental help by Ing. Marc Smithers and Dr. Thomasz Kachlicki (TEM measurements), Dr. John van Duynhoven (NMR measurements) all of the University of Twente, Enschede, The Netherlands, and Dr. P. J. Kooyman of the National Centre for High Resolution Electron Microscopy, Delft University of Technology, Delft, The Netherlands (High Resolution Transmission Electron Microscopy). XAFS measurements were carried out at the National Synchrotron Light Source (Beamline X23A2), Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Division of Materials Sciences and Division of Chemical Sciences. Additional XAFS measurements were carried out at SRS (station 9.2), Daresbury, U.K. This work has been partly financed by the Onderzoeks Stimulerings Fonds of the University of Twente, Enschede, The Netherlands. This work was performed under the auspices of NIOK, The Netherlands Institute for Catalysis (report number UT-99-1-04). References and Notes (1) Turkevich, J.; Cooper Stevenson, P.; Hillier, J. Discuss. Faraday Soc. 1951, 11, 55. (2) Furlong, D. N.; Launikonis, A.; Sasse, W. H. F.; Sanders, J. V. J. Chem. Soc. Faraday Trans.1 1984, 80, 571. (3) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1986, 110, 82. (4) Toshima, N. Takahashi, T.; Hirai, H. Chem. Lett. 1985, 1245. (5) Meguro, K.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 341. (6) Bo¨nnemann, H.; Brijoux, W.; Brinkmann, R.; Dinjus, E.; Joussen, T.; Korall, B. J. Mol. Catal. 1992, 74, 323. (7) Bo¨nnemann, H.; Brijoux, W.; Brinkmann, R.; Dinjus, E.; Joussen, T.; Korall, B. Angew. Chem. Int. Ed. Engl. 1991, 30, 1312. (8) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129. (9) Hirai, H. J. Macromol. Sci. Chem. 1979, A13, 633. (10) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci. Chem. 1978, A12, 1117. (11) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1992, 4, 1234.

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