J. Phys. Chem. 1986, 90,4765-4767
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Further Studies on the Synthesis of Finely Divided Platlnum John Turkevich, Robert S. Miner, Jr., Chemistry Department, Princeton University, Princeton, New Jersey 08544
and L. Babenkova* Institute of Organic Catalysis and Electrochemistry, Alma Ata, USSR (Received: November 14, 1985; In Final Form: May 27, 1986)
An investigation was made of the effect of pH and of starting platinum complexes on the synthesis of monodisperse platinum particles by citrate reduction. The antitumor drug cis-platin does not readily produce colloidal particles, and these lack activity for hydrogen peroxide decomposition. The growth of platinum,particles by both citrate reduction ahd hydrogen gas treatment was also studied.
Introduction Previous work in the Princeton laboratory has established that monodisperse platinum sol of 3.2-3.8-nmdiameter can be prepared by the reaction of sodium citrate with hexachloroplatinum(1V) acid at 100 OC.I4 The cited publications give the details of this preparation and the characterization of this product by electron microscopy (Figure l ) , sedimentation analysis, optical absorption, electron diffraction, and catalytic properties for a variety of reactions. In the present investigation the “standard” procedure of citrate reduction of the chloroplatinate ion was modified by changing the starting pH, using different platinum compounds, and introducing platinum particles as nuclei. Experimental Procedure Two experimental procedures were used. In one, the solution containing the appropriate platinum complex (in a concentration equivalent to 60 mg of metallic platinum in 1000 mL of water) was brought to its boiling point, and 120 mL of a 1 wt % solution of sodium citrate was added. The solution was then refluxed for at least 4 h, during which time samples were taken to determine changes in optical absorbance. In the second procedure, hydrogen gas was bubbled through a glass fritted disk into a solution containing the platinum complex and colloidal platinum particles. Optical absorbance was determined at 600 nm on a Bausch and Lomb Spectronic 20 spectrophotometer. The absorbance of the platinum sol is metallic and is constant in the visible r e g i ~ n .The ~ reaction was considered complete when the absorbance reached a constant value. An approximate idea of the completion of the reduction could be obtained from the final absorbance at 600 nm, which for complete reduction should be at least 0.50 for a platinum sol containing 60 mg of Pt in 1 L of solution. The instability of a given preparation was judged by a drop in absorbance or, in extreme cases, by the appearance of particles visible to the eye. The particle diameter was calculated from the sedimentation constant by using for density the value for bulk metallic platinum (21.45 g/cm3). For particles incompletely reduced, this density value gave diameters smaller than 3.2 nm. Such results were treated with caution since electron microscopy indicated that these particles were organometallic agglomerates with electron density different from that of metallic platinum and represented the precursor to the metal particle.’ High-resolution electron mi(1) Aika, K.; Bann, L. L.; Okura, I.; Namba, S.; Turkevich, J. J. Res. Imt. Catal., Hokkaido Univ. 1976, 24, 54. (2) Gonzales-Tejuca, L.; Aika, K.;Namba, S.; Turkevich, J. J . Phys. Chem. 1977, 81, 1399. (3) Turkevich, J.; Bann, L. L.; Wall, J. H. Perspectives in Catalysis; Larssen, Ed.; Lund, Sweden, 1980; p 58. (4) Turkevich, J.; Miner, R. S., Jr.; Okura, I.; Namba, S.; Zakharina, N. In Perspectives in Catalysis; Larssen, Ed.; Lund, Sweden, 1980; p 111. ( 5 ) Miner, R. S., Jr.; Namba, S.; Turkevich, J. Proc. 7th. Int. Cong. Catal., Tokyo 1980, 160. (6) Turkevich, J. Plenary Lecture at the 8th Ibero-American Symposium on Catalysis, La Rabida, Spain, July 1982.
0022-3654/86/2090-4765$01 .50/0
croscopy was carried out on certain preparation^.^ The electron micrographs so obtained permitted imaging of the lattice spacing of the particles and identifying them as metallic platinum. The hexachloroplatinic acid and tetraammineplatinum(I1) dichloride were obtained from Engelhard Industries, and sodium citrate was obtained from C. G. Fischer Scientific Co. Potassium tetrachloroplatinate(I1) was prepared by the procedure given by Brauers and the bis(nitrito)diammineplatinum(II) by the procedure from the same s o ~ r c e . cis~ and trans-dichlorodiammineplatinum(I1) were obtained from Aldrich Chemical Co. of Milwakee, WI.
Experimental Results pH Effect. The pH of the starting standard preparation of sodium citrate-chloroplatinic acid was varied by the addition of either 0.1 N NaOH or HCl. The range covered was 4.0-10. The course of the absorbance with time is given by Figure 2. It is seen that after a short induction period the absorbance at 600 nm rises, goes through an inflection point, and then levels off. This constant value is greater than 0.50 for pH 4.0-6.6. In alkaline solutions it is low, having at value of 02.0-0.25 indicating incomplete production of metallic platinum particles. We must be dealing with organometallic particles or platinum clusters so small that the metallic character is lacking. During the course of many preparations the starting pH changes: pH 4.0 changes to 3.6; pH 6.6 remains unchanged; pH 7.8 drops to 7.0; and pH 10 decreases to 7.0 in the first 40 min and then rises slowly to 8.1 in 280 min. Sedimentation constants using the density of metallic platinum give the following values for the particle diameter produced at different pH values: 26.5 nm at pH 4.0; 7.0 nm at pH 4.6; 3.7 nm at pH 5.5; 2.7 nm at pH 7.8; and 4.0 nm at pH 10. The last two preparations are suspicious because of the low absorbance of the final product. We conclude that there is a small range of pH between 5.5 and 6.6 which gives satisfactory preparations. Higher acidity gives larger particles, while an increase in basicity results in an incomplete reaction. Effect of Starting Platinum Compounds. The standard preparation using the sodium salt of hexachloroplatinum(1V) acid gives particles of 3.8-nm diameter as determined by sedimentation analysis. The absorbance at 600 nm is 0.66. If potassium hexachloroplatinate(1V) is used, the resulting product is different (Figure 3). The final absorbance is 1.1, and sedimentation analysis indicates two types of particles with “diameters” of 5.4 and 7.0 nm. It is interesting to note that the presence of a small amount of potassium ion in a system containing a stoichiometric excess of sodium ion from sodium citrate exerts such an appreciable effect on the process of particle formation. This may be associated with the low solubility of potassium hexachloro(7) Turkevich, J.; Stevenson, P. C.; Hillier, J. Discuss. Faraday SOC.1951, 8, 348.
(8) Brauer, G.Handbuch der Preparativen Anorganischen Chemie; Enke Verlag: Stuttgart; p 1368. (9) Brauer, G.Ibid., p 1375.
0 1986 American Chemical Society
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986
4766
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(i) Figure 1. Particle size distribution of standard platinum sol at different times of preparation based on electron microscopy of L. L. Ban.
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platinum(1V) (0.48 g per 100 mL of water), while the sodium salt has a solubility that is 2 orders of magnitude higher (53 g per 100 mL of water at 15 "C). It is possible that the potassium ions are firmly bound to the chloroplatinate moiety in the organometallic polymer that serving as the precursor of the metal particle. Potassium tetrachloroplatinate(I1) with a slightly higher solubility (0.93 g per 100 mL water) than its hexavalent analogue is also reduced more rapidly to give a colloidal solution with an absorbance of 0.70 and particles with diameters of 8.2 nm from sedimentation analysis and 3.2 nm by electron microscopy. This
discrepancy is surprising. When the absorbance of the solution was greater than 0.5, at 600 nm there has been better concordance between electron microscopy and sedimentation analysis. The complex cation tetraammineplatinum(I1) does not form a colloid when treated with sodium citrate at 100 "C. Furthermore, addition of this cation to a standard 3.2-nm sol precipitates the latter. The neutral complex bis(nitrito)diammineplatinum(11) also does not form a colloid under the same conditions. It was of some interest to investigate the citrate reduction of cis-dichlorodiammineplatinum(11), a very important drug in Cancer therapy, and that of its inactive trans isomer.I0 The reaction was carried out at 90 O C with a nitrogen gas purge to avoid any action of oxygen. The course of the reaction was monitored by absorbance at 600 and 400 nm. As Figure 4 shows, the reaction is extremely slow at 90 O C . Also, the product has low catalytic activity for hydrogen peroxide decomposition. Colloidal platinum must play a very minor role in cancer therapy, though there is an interesting difference in the behavior of the two isomers. It has been found" that catalytic activity is associated with a compound formation between cis platinum and the cytosine moiety of the DNA. This reaction takes place slowly a t physiological temperatures. The trans platinum compound, though reacting with cytosine, shows no catalytic activity for hydrogen peroxide decomposition. Thus, on the basis of these experiments, only negatively charged complexes readily form a colloid when treated with sodium citrate at 100 "C. Growth of Platinum on Platinum. It would be highly desirable to grow in a controlled way platinum on the original monodisperse platinum sol. This would give a set of monodisperse preparations of varied diameter, useful in determining the effect of size on particle properties. Previous attempts were unsuccessful in growing platinum on platinum particles by reduction of hexachloroplatinate solution on standard platinum sol. The reducing agents tried were sodium citrate at 100 OC and hydroxylamine hydrochloride at room temperature. In both cases spontaneous nucleation and coagulation took place. Difficulty was also experienced in growing gold or palladium on platinum nuclei. The problem may be due to the small size of the platinum nuclei, allowing spontaneous nucleation to take place. Two approaches were used to overcome these difficulties. In one, a more easily reducible platinum complex such as potassium tetrachloroplatinum(I1) was used. In one experiment, 140 mL of standard 3.2-nm sol (containing 7 mg of Pt) was mixed at room temperature with 120 mL of 1 wt % sodium citrate and 960 mL (10) Rosenberg, B.;van Camp, L.;Trosco, J. E.; Mansour, V. N. Nature (London) 1965, 222, 385. (1 1 ) Turkevich, J.; Xu, Z. Proc. 8rh In?. Congr. Catal., West Berlin, 1984,
4, 791.
Synthesis of Finely Divided Platinum of potassium tetrachloroplatinum(I1) solution (containing 60 mg of Pt). The solution was gently boiled under reflux, and the absorbance followed with time. After 4.5 h, the absorbance reached a constant value of 0.75. Ultracentrifugation showed the presence of two sets of particles-the predominant fraction having a sedimentation constant of 614 S,corresponding to particle diameter of 6.1 nm, and a small fraction with sedimentation constant of 310 S, corresponding to a particle diameter of 4.7 nm. The value predicted by stoichiometry of the system is 6.8 nm. Thus growth of the 3.2-nm diameter particles was carried out. In another experiment a smaller number of nuclei were used for growth in an attempt to obtain larger particles. Fifty-eight milliliters of the standard 3.2-nm sol (3 mg of Pt) was used. The final absorbance was 0.6. The sedimentation constant was 909 S, corresponding to a particle diameter of 8.0 nm. The predicted value was 8.8 nm. This again is taken as evidence of growth. Other experiments carried out by mixing a t the boiling point or by using other than 3.2-nm standard sol as nuclei gave discordant results. In another approach hydrogen gas was bubbled through a platinum sol in the presence of a soluble platinum salt. It was hoped that the hydrogen would be adsorbed by the platinum particles and the soluble platinum salt would be reduced on the particle, thereby making the particle grow. It was first established that hydrogen gas does not produce colloidal sols when bubbled through solutions of potassium tetrachloroplatinate(II), tetraammineplatinum(I1) dichloride, or bis(nitrit0)diammineplatinum(I1); 140 mL of standard 3.2-nm platinum sol (3 mg of Pt) was used as nuclei for growth. Addition of tetraammineplatinum(I1) dichloride produced a precipitate in the standard sol with and without hydrogen gas. The positively charged divalent tetraammineplatinum ion undoubtedly neutralizes the negative
The Journal of Physical Chemistry, Vol. 90, No. 20, 1986 4767 charge on the colloidal platinum particles, causing coagulation. The addition of the neutral bis(nitrito)diammineplatinum(II) (60 mg of R)produced on hydrogen treatment at room temperature a gradual darkening of the colloidal sol with an absorbance of 0.72 after 6 h. The sedimentation constant was 1874 S, corresponding to a particle diameter of 11.4 nm. The expected diameter was 6.2 nm. The particles partially coagulated after standing 1 month. This behavior can be rationalized by assuming that growth took place but was followed by coalescence of two particles to form doublets. The neutral platinum complex was more strongly adsorbed than the large excess of citrate ion present. If potassium tetrachloroplatinum(I1) is used as growth medium, the absorbance increases from an initial value of 0.38 to 0.78. The sedimentation constant was 407 S, and the final diameter was 5.3 nm. The calculated value was 6.8 nm. Thus the potassium tetrachloroplatinate(I1) is suitable as a growth medium not only with citrate but also with hydrogen as a reducing agent. The citrate process is simpler and more reliable, while the hydrogen gas reduction is inherently more interesting since it represents an autocatalytic reduction of a platinum ion to platinum metal on a platinum metal catalyst.
Acknowledgment. This investigation was part of the USAUSSR program in Chemical Catalysis. We wish to acknowledge financial support from the National Science Foundation. We also appreciate the determination of the sedimentation constants by Barbara Bamman of the Biochemistry Department of Princeton University and the ultrahigh resolution electron microscopy of Lazlo L. Ban of Cities Service Research Center at Hightstown, NJ. Registry No. Pt, 7440-06-4; potassium tetrachloroplatinate(II), 10025-99-7; potassium hexachloroplatinum(IV), 1692 1-30-5; sodium hexachloroplatinum(IV), 16923-58-3; citrate, 126-44-3.
