Langmuir 1991, 7, 457-459
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Preparation of Bimetallic Pd-Pt Colloids in Organic Solvent by Solvent Extraction-Reduction Kunio Esumi,* Masayoshi Shiratori, Hidenori Ishizuka, Takafumi Tano, Kanjiro Torigoe, and Kenjiro Meguro Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received May 10, 1990. In Final Form: July 3, 1990 Pd-Pt bimetallic colloids were prepared by reduction of their corresponding salts extracted in organic solvents (cyclohexane, chloroform) with hydrazine or sodium boron hydride. Two extractants were used: trioctylphosphine oxide (TOPO) and distearyldimethylammonium chloride (DDAC). In both organic solvents, the particle size of Pd-Pt colloids depended on the feed molar fraction of Pd using hydrazine, but almost was independent on the feed molar fraction of Pd using boron hydride. The dispersion stability of Pd-Pt colloids increased as the particle size decreased.
Introduction The preparation of precious metal colloids has been studied exten~ivelyl-~ because of their wide use as electrical materials and their many applications, such as catalysts. Most of the work, however, has been limited to an aqueous solution in the presence of surfactants or water-soluble polymers as a protective colloid. Recent work has focused on preparation of metal colloids in organic solvents. Some workers4v5have made stable metal colloids in alcohol by adding polymer as a stabilizer, while several groupsG8 have prepared metal colloids by depositing metal atoms into organic solvents at low temperature, which leads to atom clustering moderated by solvation. We have developed a new methodgJOfor preparation of metal colloids in organic solvents that uses a solvent extraction; many metal salts are soluble in water and they can be easily transferred into organic solvents with extractants by means of solvent extraction. The extracted metal salts are reduced by reductants, resulting in the formation of metal colloids in the organic solvent. We have obtained platinum and gold colloids9J0in organic solvents by using the above technique, where the average diameters of platinum and gold are 2.0 and 6.8 nm, respectively. Thus prepared metal colloids can be applied in nonaqueous solvent, a more useful medium for carrying out organic reactions than water. In this work, we extended this extraction method for preparation of organo bimetallic colloids of Pd-Pt. The prepared colloids were characterized by means of electron microscopy, electron probe for microanalyses, and UVvis spectrophotometry. (1)Turkevich, J.; Stevenson, R. C.; Hiller, J. Discuss.Faraday SOC. 1951, 11, 55. (2)Takiyama, K. Bull. Chem. SOC.Jpn. 1958, 31, 944. (3) Wilenzick, R. M.; Russell, D. C.; Morriss, R. H.; Marshall, S. W. J . Chem. Phys. 1967,47, 533. (4) Hirai, H.; Ohtaki, M.; Kamiyama, M. Chem. Lett. 1986, 269. (5) Rampino, L. D.; Nord, F. F. J. Am. Chem. SOC.1941,63, 2745. (6) Cardenas-Trivino, G.;Klabunde, K.; Dale, E. B. Langmuir 1987, 3, 986. (7) Kimura, K.; Bandow, S. Bull. Chem. SOC.Jpn. 1983, 56,3578. (8) Andrews, M. P.; Ozin, G. A. J. Phys. Chem. 1986,90,2929. Jpn. 1988, (9) Meguro, K.; Torizuka, M.; Esumi, K. Bull. Chem. SOC. 61, 341.
(10) Meguro, K.; Tano, T.; Torigoe, K.; Nakamura, H.; Esumi, K. Colloids Surf. 1988/89, 34, 381.
0743-7463191f 2407-045~$02.50 f0
Experimental Section Materials. PdCl2 and H2PtCb were supplied by Tanaka Kikinzoku Kogyo K.K. As extractants, trioctylphosphineoxide (TOPO)and distearyldimethylammonium chloride (DDAC)were used as received. The TOPO was obtained from Wako Pure Chemical Industries, Ltd., and DDAC from Tokyo Kasei Industries, Ltd. As reductants, dehydrated hydrazine and sodium boron hydride, commercially available in extrapure grades, were used. The other chemicals were of extrapure grades. The water used in this experiment was purified by means of the Milli-Q water purification system. Procedure. The extraction procedure was as follows. PdCl2 or HzPtCb was dissolved in water and added to an organic solvent containing an extractant. In the case of TOPO, 0.6 mmol dm-3 of HCl was used to enhance the extractionratio. Then, the mixed solution was shaken at 25 "C for 12 h to attain an equilibrium of extraction. After the extraction, respective extracted PdCl2 and HzPtCb organic solutionswere mixed to make variousmixed ratios of them, and then their solutions were directly reduced by addition of areductant at room temperature. The added amount of the reductant was about 3 times that of the extracted salts. The particle size was determined by means of transmission electron microscopy with a Hitachi H-800 microscope. The magnificationof the electron microscope was 30000-100000. The particle size distribution and average diameter were obtained by counting particles on the basis of these enlarged electron micrographs. The elemental ratio of particles was determined by means of electron probe microanalyzer (EPMA). The UV-vis spectra of colloids in organic solventwere measured with a UV-vis spectrophotometer (220A, Hitachi Co.). The extraction ratio from aqueous solution to organic solvent was determined from the difference between the initial and the final concentrations before and after extraction in the aqueous solution by means of a UV-vis spectrophotometer. The stability of colloids thus prepared in the organic solvent was evaluated from the absorbance at 500 nm after centrifuging the colloids under a condition of 3000 rpm for 10 min. Here, a high absorbance indicates a high dispersion stability. Results and Discussion Four different systems were studied: TOPO-cyclohexane-hydrazine, TOPO-cyclohexane-NaBH4, DDACchloroform-hydrazine, and DDAC-chloroform-NaBH4. The extraction ratios from 0.5 mmol dm-3 of respective metal salts in an aqueous solution to cyclohexane by TOPO were 80% for HzPtC16 and 60% for PdC12, while those to 0 1991 American Chemical Society
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458 Langmuir, Vol. 7,No. 3, 1991
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Figure 1. Absorption spectra of Pd-Pt colloids prepared in a TOPO-cyclohexane-hydrazine system. chloroform by DDAC were nearly 100%. Feed molar fractions of Pd employed were 0,0.25,0.50,0.75, and 1.00 under total feed concentration of 0.3 mmol dm-3 and total volume of 20 mL. When the feed solutions were reduced with a reductant, the respective reaction ratios of both Pd and Pt in various feed solutions were almost the same, and about 98% with hydrazine and about 81% with NaBH4. Accordingly, the P d / P t ratios for the bimetallic colloids should be the same as that of feed solution. Actually, the P d / P t ratios estimated by EPMA were almost the same as those of the feed solutions. In order to confirm whether prepared colloids are bimetallic or individual ones, the UV-vis spectra of colloids in organic solvent were measured (Figure 1). Figure 1shows that respective spectra do not exhibit characteristic absorption bands, but the absorbancesof the Pd-Pt system are different from those of Pd and Pt alone in the TOPOcyclohexane-hydrazine system. The other systems showed a similar trend to that for the TOPO-cyclohexane-hydrazine system. These spectra are almost the same as those prepared by Turkevich et al. in aqueous solutions,ll suggesting that bimetallic colloidal particles of Pd-Pt are formed in the organic solvents. A typical EPMA result of the Pd-Pt colloids is given in Figure 2. By measuring X-ray photon counts vs energy, one can tell which elements are present and in what amounts. Figure 2 shows that Pd and Pt are present in the colloids, so the possibility seems to be ruled out that the samples are mixtures of individual Pd and Pt colloids. A similar microanalysis of Pd-Au colloidal particles was carried out by Michel and Schwartz12who reported that the composition of these colloids can be determined by analytical electron microscopy. Figure 3 demonstrates that the mean diameters of PdPt colloids prepared in TOPO-cyclohexanehydrazine and DDAC-chloroform-hydrazine are in the range of 15 and 30 nm, showing a maximum diameter a t feed molar fractions of Pd between 0.5 and 0.75, while those in TOPOcyclohexane-NaBH4 and DDAC-chloroform-NaE3H4 systems are almost independent of feed molar fraction of Pd and range between 5 and 9 nm. Apparently, the mean particle size is affected by the reductant; reduction by hydrazine provides larger particle size than by NaBH4. It is likely that since NaBH4 is a stronger reductant than (11) Miner, R.S.,Jr.; Namba, S.;Turkevich, J. Proceedings ofthe 7th International Congress on Catalysis; Seiyama, T., Tanabe, K., Eds.; Kodansha: Tokyo, 1981; p 160. (12) Michel, J. B.;Schwartz, J. T. Preparation of Catalysts IV; Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Elsevier Science Publishers: New York, 1987; p 669.
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Figure 2. Typical EPMA result of Pd-Pt colloids prepared in a TOPO-cyclohexane hydrazine system. E
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Figure 3. Effect of feed molar fraction of Pd on mean particle diameter of Pd-Pt particles for four systems: (e) DDACTOPO-cyclohexane-hydrazine; ( 0 ) chloroform-hydrazine; (0) DDAC-chloroform-NaBH,; ( 0 )TOPO-cyclohexaneNaEH4.
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Figure 4. Change in absorbance of Pd-Pt colloids as a function of feed molar fraction of Pd: ( 0 )DDAC-chloroform-hydrazine; (0)TOPO-cyclohexane-hydrazine; ( 0 ) DDAC-chloroformNaBH,; ( 0 )TOPO-cyclohexane-NaBH,. hydrazine, NaBH4 produces a larger number of nuclei, resulting in a smaller mean particle size. Further, with regard to the extractant, the usage of TOPO renders particle size of Pd-Pt smaller than that of DDAC. This result may be interpreted in terms of the different nature of interaction between extractant and metal salt: TOPO interacts with metal ions through coordinate bonds, while DDAC forms complexes by electrostatic interaction. Since a coordinate bond is generally stronger than an electrostatic bond, it is reasonable to assume that the reduction of metal
Bimetallic Pd-Pt Colloids in Organic Solvent
Langmuir, Vol. 7, No. 3, 1991 459
ions proceeds much more easily in the case of DDAC than with TOPO, due to favorable attack to metal ions by the reductant. Therefore, the particle size of Pd-Pt in a DDAC system becomes a little larger than that in a TOPO system. The stability of the colloids prepared is shown in Figure 4. TOPO-cyclohexane-NaBH4 and DDAC-chloroformNaBH4 systems provide a higher stability in the whole feed molar fraction range than TOPO-cyclohexane-hydrazine and DDAC-chloroform-hydrazine systems. In the former systems, the colloids were still dispersed for more than one month after the preparation, whereas in the latter systems, they precipitated within a few days. Since the mean particle size in the former systems is considerably smaller than those in the latter systems, this stability is likely to be predominantly governed by the particle size.
Summary Pd-Pt bimetallic colloids were prepared by reduction of their corresponding salts extracted in organic solvents with hydrazine or sodium boron hydride. Trioctylphosphine oxide (TOPO) and distearyldimethylammonium chloride (DDAC) were used as extractants. In TOPOcyclohexane-hydrazine and DDAC-chloroform-hydrazine systems, the particle size of Pd-Pt ranged in 20-30 nm and depended on the feed molar fraction of Pd, while in TOPO-cyclohexane-sodium boron hydride and DDACchloroform-sodium boron hydride systems they ranged in 5-8 nmand were independent on the feed molar fraction. The dispersion stability of Pd-Pt colloidal particles increased with decreasing particle size.
