Noble Metal Nanocolloids

Encyclopedia of Nanoscience and Nanotechnology

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Noble Metal Nanocolloids Ichiro Okura Tokyo Institute of Technology, Yokohama, Japan

CONTENTS 1. Introduction 2. Preparation and Characterization of Noble Metal Nanocolloids 3. Catalyses with Noble Metal Nanocolloids 4. Conclusion Glossary References

1. INTRODUCTION Much attention has been paid to nanosized metal colloids, for their specific properties are different from those of bulk metal compounds. Nanosized metal colloids are usually synthesized by a in-situ reduction method from a suitable metal precursor, such as chemical reduction, photoreduction, electrochemical reduction, or thermal decomposition. Many small molecular ligands, surfactants, and polymers have been used to stabilize metal colloids. Some metal colloids exist stably for a long time without coagulation in a solvent. The methods for suppressing coagulation are needed to maintain the stability of metal colloids in a sol state. Additives that do not cause coagulation are steric stabilizers. Polymers such as polyvinyl alcohol and polyvinyl pyrrolidone and surfactants have frequently been used. The mechanism of the steric stabilization or suppression of the coagulation is illustrated in Figure 1, in which large adsorbed molecules that prevent coagulation can be seen. Recently many polymer-stabilized noble metal nanocolloids have been studied intensively. Stable noble metal colloids have also been obtained by a unique preparation method without a stabilizer. In this case the ionic groups on the surface of this metal colloid suppress the coagulation of metal particles by electric repulsion. The colloidal particles prepared by reduction with citrate, for example, are surrounded by an electrical double layer arising from adsorbed citrate and chloride ions and the cations that are attracted to them. This results in a Columbic repulsion decay with the interparticle distance; the net result is shown in Figure 2. Thus the metal particles are stabilized in the dispersing medium. ISBN: 1-58883-064-0/$35.00 Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

A counteraction can be achieved by the above two methods, electrostatic stabilization and steric stabilization. For historical details and an extensive survey of the properties of noble metal nanocolloids, the reader is referred to previous reviews [1–3].

2. PREPARATION AND CHARACTERIZATION OF NOBLE METAL NANOCOLLOIDS Metal colloids can be prepared in two different ways: by dispersion of larger particles or by condensation of smaller units. Sols prepared by the dispersion method are rather unstable and consist of particles with a wide size distribution. To generate uniform particles it is necessary to use chemical methods such as the reduction of metal salts in solution.

2.1. Platinum Nanocolloid Application of platinum colloid in catalysis is regarded as an active research field. Many colloidal nanocatalysts have been studied intensively, and nanosized platinum metal colloids exhibit special catalytic properties different from those of conventional heterogeneous and homogeneous platinum metal catalysts. Typical platinum nanocolloids are listed in Table 1. Particle size can be kept between 1.0 to 40 nm through control of the preparation conditions. Though most platinum colloids in the table are stabilized with polymers, platinum nanoparticles without stabilizer are also prepared. The platinum colloid with electrostatic stabilization is prepared by the reduction of chloroplatinic acid with sodium citrate [20]. The platinum colloid obtained by this method is very stable and does not coagulate for several months if kept in a refrigerator. The following three points are important in the preparation of the platinum colloid: 1. The flask must be carefully washed, preferably with aqua regia. 2. Attention must be paid to the water used. Doubledistilled water is recommended. 3. The flask should always be sufficiently heated, and the solution must be kept boiling. Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 8: Pages (41–48)

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Noble Metal Nanocolloids

Unreacted chloroplatinic acid and excess sodium citrate can be removed by passing the solution through a column with the ion exchange resin Amberlite MB-1. Figure 3 shows the relationship between the particle size of the platinum colloid prepared by the above-mentioned method and the reflux time. The diameter of the platinum particles increases gradually with time and becomes constant at 32 Å. By this method, the particle size of the platinum colloid can be controlled just through control of the reflux time. The deviations in particle sizes are very small, and the particle size is homogeneous and almost uniform. From the results it seems that the number of colloid particles does not change during the growth process of the platinum colloid and that the particle size increases because of a gradual reduction in the amount of chloroplatinic acid adsorbed on the surface of the colloid.

2.2. Gold Nanocolloid Figure 1. Steric stabilization of metal colloid particles by polymers or surfactant molecules. (a) In the interparticle space the configurational freedom of the polymer chains of two approaching particles is restricted, causing a lowering of entropy. (b) The local concentration of polymer chains between the approaching particles is raised and the resulting higher local activity is osmotically counteracted by solvation. Reprinted with permission from [1], G. Schmmid, “Clusters and Colloids.” VCH, Weinheim, 1994, © 1994, Wiley-VCH.

Gold nanocolloid is a typical historical particle. The gold colloids, ranging from 2 to 150 nm in diameter, are available and are typical hydrophobic particles (Table 2). The properties and applications of gold nanoparticles are summarized in [3] with historical details. The major objective in using colloidal gold is the in-situ localization of cellular macromolecules. This information is used to elucidate biochemical properties and functions of cellular compartments and components. The silver-enhanced colloidal gold method can be used for both light and electron microscopy. Colloidal gold as a marker meets many requirements necessary for precise ultrastructural localization, distribution, and quantitation of macromolecules in living or fixed cells and tissues. During the last decade, scientific literature involving the use of colloidal gold as an immunocytochemical marker has increased, and this trend is expected to continue. The different methods for the synthesis of colloidal gold are based on controlled reduction of an aqueous solution of tetrachloroauric acid, with different reducing agents under various conditions.

2.3. Other Noble Metal Nanocolloids Other noble metal nanocolloids are also prepared as platinum and gold nanocolloids with electrostatic stabilization and steric stabilization. Nanocolloids of Rh, Pd, Ru, Ir, and Ag are shown in Tables 3–7.

2.4. Preparation and Characterization of Noble Bimetallic Nanocolloids

Figure 2. Electrostatic stabilization of metal colloid particles. Attractive van der Waals forces are outweighed by repulsive electrostatic forces between adsorbed ions and associated counterions at moderate interparticle separation. Reprinted with permission from [1], G. Schmmid, “Clusters and Colloids.” VCH, Weinheim, 1994, © 1994, Wiley-VCH.

Bimetallic particles have been a focus of increasing interest not only in catalysis in the fuel industry, but also as models for the study of the formation of alloys. They can be divided into two classes: alloy-like colloids consist of a homogeneous mixture of two metals with colloidal distribution and colloids with an inner nucleus of one metal, which is covered by a layer of the second metal. In the metal evaporation method, two types of preparation procedures are used [62], for example: (1) Half SMAD: A conventional catalyst was prepared, such as Pt/Al2 O3 , and this catalyst was treated with solvated Re atoms (in toluene). In this way the preformed Pt particles

43

Noble Metal Nanocolloids Table 1. Platinum nanocolloids. Particle size/nm 2.7 2.7–4.1 1.55–1.78 1.1–2.7 1.02–2.20 1.47–2.67 1.5 2.7 2.6 2.2 ∼30 ∼40 1.5 2.4 2.3 2.0 1.6 ∼2 2.1 27 ∼1
5 1.9–2.81 3.9 2.0 1.12 2.09–2.53 2.15–2.61 2.81 1.9 2.38 22 1.02–1.10 5 2.5 3.0 3.0–3.5 ∼3 15 3.2 2.9 7–11 80 ∼3 1.1–3.8 15

Stabilizer

Method

Ref.

PVA PVP PVP PVP PVPAA PNIPAAm PNIPAAm, PVP PNIPAAm PVP Poly(2-ethyl-2-oxazoline) Poly(1-vinylpyrrolidone-co-vinylacetate) Poly(2-hydroxypropyl-methacrylate) Poly(methylvinylether-co-maleic anhydride) Poly(methacrylic acid) Poly(1-vinylpyrrolidone-co-acylic acid) Poly(styrene sulfonic acid) Poly(2-acrylamido-2-methyl-1-propane sulfonic acid) Poly(vinyl phosphenic acid) PS-b-PMAA PS-b-PED PVP Polystyrene PNVF, PNVA, PNVIBA PVP PVP PVPAA PNVF PNVA PVP PNVIBA PNIPAAm PVP PVPAA

Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing EtOH Refluxing PrOH Refluxing PrOH Citrate Citrate Citrate Citrate LiBH(Et)3 Versatile method in supercritical CO2 KBH4 KBH4 H2 gas H2 gas UV irradiation Microwave-dielectric heating Versatile method in supercritical CO2

[4] [5] [6] [7] [8] [9] [10] [11] [12] [12] [12] [12] [12] [12] [12] [12] [12] [12] [13] [13] [5] [14] [15] [16] [17] [8] [18] [18] [18] [18] [18] [5] [8] [19] [20] [21] [22] [23] [29] [13] [13] [25] [26] [27] [28] [29]

Carbowax Octadecanthiol PMP, PTFE PS-b-PMAA PS-b-PED Polyacrylate Cyclodextrin 1,10-Phenanthroline PMP, PTFE

Note: PVP, Polyvinyl pyrrolidone; PNIPAAm, poly(N -isopropylacrylamide); PVA, polyvinyl alcohol; PMP, poly(4-methyl-1-pentene); PTFE, poly(tetrafluoroethylene); PS-b-PMAA, polystyrene-b-poly(methacrylic acid); PS-b-PED, polystyrene-b-poly(ethyleneoxide); PNVF, poly(N -vinylformamide); PNVA, poly(N -vinylacetamide); PNVIBA, poly(N -vinylisobutylamide); PVPAA, poly(1-vinylpyrrolidone-coacrylic acid).

would receive Re on their surface as a surface coating, but a homogeneous Pt-Re alloy should not form. (2) Full SMAD: The two metals were evaporated simultaneously, and layered structures or homogeneous alloy particles could form. Some bimetallic nanocolloids are listed in Table 8. Platinum-gold alloy colloids can be prepared by a method similar to the above-described method for preparing a platinum colloid [61]. For the alloy colloid, a mixture of the gold chloride acid solution and the chloroplatinic acid solution (1 g Pt+Au/liter) at an arbitrary ratio is used.

The composition of the prepared alloy colloid corresponds to the composition of the starting material metal. Reduction with sodium citrate is completed within 4 h. This can be confirmed from a visible spectrum. Figure 4 shows a visible spectra of platinum, gold, and platinum-gold alloy colloids with various compositions. As the gold colloid has a characteristic absorption peak at 520 nm, the gold colloid does not appear to be included in the alloy colloid. The colloid thus prepared is an alloy, because the spectrum of the alloy colloid is not a summary of spectra of the pure metal

44

Noble Metal Nanocolloids Table 4. Palladium nanocolloids. Particle size (nm) 5.3 1.9–2.7 3.4–7.8 5–6.8 1.3–2.1 3.4–5 3–10 4.8

Figure 3. Relationship between the particle size of the platinum colloid and reflux time. Reprinted with permission from [76], Namba and Okura, Hyomen 21, 450 (1983). ©1983, Koshinsha Co.

Stabilizer

Method

Ref.

PVP PVA PVP PS-b-PMAA PS-b-PEO PS-b-PMAA PS-b-PEO Ion exchange resin Tetraalkylammonium

Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing EtOH Refluxing EtOH KBH4 KBH4 NaBH4 Electrochemical

[53] [4] [7] [13] [13] [13] [13] [24] [56]

Table 5. Ruthenium nanocolloids. Particle size (nm) 1.3–1.8 1.28–1.76 3–20

Stabilizer

Method

Ref.

PVP PVP Ion exchange resin

NaBH4 NaBH4 NaBH4

[57] [58] [24]

Table 6. Iridium nanocolloids. Table 2. Gold nanocolloids. Particle size (nm) 0.82 2.6 3 3–17 4.2 5.2 5.7 8.5 5–12 10 12 12–64

∼3 ∼6 5 ∼6 2.5

Particle size (nm)

Color

Reducing agent

Ref.

Red

NaBH4 NaSCN White phosphorus Citrate NaBH4 White phosphorus Citrate Citrate White phosphorus EtOH Ascorbic acid Citrate

[30, 31] [32] [33] [34] [35, 36] [37] [38] [39] [40, 41] [42–44] [45–47] [48]

Stabilizer

Method

PS-b-PMAA PS-b-PMAA PS-b-PEO PS-b-PEO DOAC, ACT

KBH4 UV irradiation KBH4 UV irradiation Gas flow-cold trap method

Yellowish Red-orange Red Light brown Purple-brown Red Red Brown-red

14 1.1–1.4

Stabilizer

Method

Ref.

PVA PVP

Refluxing MeOH Refluxing alcohol

[4] [59]

Table 7. Silver nanocolloids. Particle size (nm) 5.0, 5.5 3.3 ∼4

Stabilizer

Method

Ref.

Sodiamoleate PS-b-PEO PS-b-PEO

NaBH4 KBH4 UV irradiation

[60] [13] [13]

Table 8. Bimetallic nanocolloids. [13] [13] [13] [13] [49]

Metals

Particle size (nm)

Stabilizer

Method

Ref.

Pd/Pt Pt/Rv Ag/Pt Pt/Re

1.5–2.5 2.0 3.2–11.3

PVP PVP

Refluxing EtOH Refluxing EtOH UV irradiation Metal evaporation

[63] [66] [64] [65]

Table 3. Rhodium nanocolloids. Particle size (nm) 3.5 0.8–4 3–7 4 4.3 3–3.5 2.24–5.7 3–20 ∼2

Stabilizer

Method

Ref.

PVP PVP PVP PVA Poly(methyl vinyl- ) PVP PVP PVP Ion exchange resin Surfactant

Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing MeOH Refluxing alcohol NaBH4

[51] [52] [53] [4] [4] [4] [54] [50] [24] [55]

Figure 4. Visible spectra of platinum-gold alloy colloids with various compositions. Reprinted with permission from [76], Namba and Okura, Hyomen 21, 450 (1983). ©1983, Koshinsha Co.

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Noble Metal Nanocolloids

colloids. The color of the alloy colloid is brownish black, whereas the color of the gold colloid is red. Platinumpalladium alloy colloids are prepared by a method similar to the above-described method [67]. In the case of palladium, 15 h is needed to complete the reduction with sodium citrate. The visible spectra of platinum, palladium, and these alloy colloids show alloy formation by the higher absorbance. The color of the alloy colloid is brownish black. A platinumpalladium alloy colloid with arbitrary metallic composition can be prepared.

3. CATALYSES WITH NOBLE METAL NANOCOLLOIDS The noble metal nanocolloids and alloy nanocolloids have been used in a sol state, in which the colloid is dispersed in the solvent, and in a gel state, in which the colloid is supported on the solid surface. Some examples of catalytic reactions in the sol state and the preparation and characterization of the noble metal nanocatalyst are described. Typical catalytic reactions with noble metal nanocolloids are listed in Table 9.

Table 9. Catalysis with noble metal nanocolloids. Metals

Catalytic reactions

Ru

Hydrogenation of Cyclooctene n-Heptene Citronellal o-Chloronitrobenzene Cyclohexene 1-Hexene Benzene Other alkenes Oxidation of alkane

Rh

Hydrogenation of 1-Hexene Cyclohexene Styrene Cyclooctene 1-Octene Other alkenes Hydroformylation Methanol carbonylation Hydrosilylation

Pt

Hydrogenation of Cinnamaldehyde Allyl alcohol Cyclohexene 1-Hexene −Ketoester Styrene Other alkenes Photoinduced hydrogen evolution Hydrogen peroxide decomposition

Pd

Hydrogenation of Cyclopentadiene Cyclohexene Dodecene Cyclooctene 1-Hexene Other alkenes and dienes

3.1. Hydrogenation Hydrogenation reactions have been the most extensively studied for measuring the activity of colloids. The six platinum group metals (Ru, Os, Rh, Ir, Pd, and Pt) have been investigated as unsupported catalysts. Alcohol solutions of these metals are active catalysts with varying degrees of selectivity for the deuteration of 1,2- and 1,6dimethylcyclohexenes. Both unsupported Pd and Pt colloids are active for selective hydrogenation of alkynes to alkenes.

3.2. Hydrogen Peroxide Decomposition Ionic platinum is not active for hydrogen peroxide decomposition. It is known that the metallic state of platinum is active for hydrogen peroxide decomposition, though the platinum ion is inert. When the platinum colloid shows a metallic character, the reaction proceeds [75]. Thus, the hydrogen peroxide decomposition reaction is a suitable model reaction. Hydrogen peroxide decomposition was carried out with platinum colloids with different particle sizes. The decomposition is easily carried out as follows by kinetic measurement: 2H2 O2 −→ 2H2 O + O2 As oxygen is generated in this reaction by the following equation, kinetic study is possible by measurement of either oxygen formation or hydrogen peroxide consumption by titration with potassium permanganate. The reaction obeys a first-order kinetics and is expressed by the following equation: ln C0 /C = kt where C0 and C are the hydrogen peroxide concentrations at times 0 and t, respectively, and k is a rate constant. Figure 5 shows the relationship between the rate constant k and the particle size of the platinum colloid. Although decomposition activity of hydrogen peroxide is not observed

Ref. [57] [57] [57] [58] [24] [68] [24] [69] [51, 53, 24] [51, 52, 24] [53, 24] [53] [70] [53, 24, 55] [50] [54] [71] [72, 73, 74] [9, 10, 11, 14] [12, 13, 24] [27, 24] [16, 17] [24] [6, 7, 27, 72, 74] [22, 27, 26] [75] [53] [13] [7] [7] [24] [24, 53]

Ir

Hydrogenation of methyl pyruvate

[59]

Pd/Pt

Hydrogenation of cycloocta-1,3-diene

[63]

Pt/Ru

Photoinduced hydrogen evolution

[66]

Note: PVP, Polyvinyl pyrrolidene; PNIPAAm, poly(N -isopropylacrylamide); PVA, polyvinyl alcohol; PMP, poly(4methyl-1-pentene); PTFE, poly(tetrafluoroethylene); PS-b-PMAA, polystyrene-b-poly(methacrylic acid); PS-b-PED, polystyrene-b-poly(ethyleneoxide); PNVF, poly(N -vinylformamide); PNVA, poly (N -vinylacetamide); PNVIBA, poly(N -vinylisobutylamide); PVPAA, poly(1-vinylpyrrolidene-co-acrylic acid).

when a colloid with a small particle size is used, the activity increases with increasing particle size. The activity increases remarkably at a particle size of about 16 Å. A platinum colloid with a particle size of 16 Å consists of crystalline particles that each contain 108 platinum atoms, and the platinum particles may be an amorphous cluster in a platinum colloid with a particle size of less than 16 Å, which is a precursor of the crystal. The number of active sites of platinum colloids can be determined by the poisoning method in which mercuric chloride (HgCl2 is used as an inhibitor. The addition of HgCl2 causes a remarkable decrease in activity. If one active site is inhibited by one HgCl2 molecule, the amount of HgCl2 obtained by the extrapolation corresponds to the

46

Figure 5. Relationship between catalytic activity for hydrogen peroxide decomposition and particle size. Reprinted with permission from [76], Namba and Okura, Hyomen 21, 450 (1983). © 1983, Koshinsha Co.

total active sites. For instance, in the case of platinum colloid prepared by reflex for 300 min, the number of active sites exposed on the surface is 4.2 × 1016 , and 37% of the platinum atoms are surface atoms. On the other hand, the activities of the hydrogen peroxide decomposition of platinumgold and platinum-palladium alloys are strongly dependent on the alloy composition. Gold colloid shows almost no activity. Even if up to 25% gold is added to platinum, the activity is almost the same as that of platinum alone. When 66% or more gold is added, the activity is almost lost. On the other hand, the activity decreases monotonously with the addition of palladium.

3.3. Photoinduced Hydrogen Evolution Recently, hydrogen evolution by photolysis of water has been used to convert solar energy into chemical energy. The photoinduced hydrogen generation reaction and a homogeneous system can be outlined as follows:

where D is an electron donor, S is a photosensitizer, and V is an electron carrier. In this reaction, the sensitizer is first photoexcited and reduces the electron carrier, and then the sensitizer is oxidized. The oxidized photosensitizer is reduced with the electron donor and returns to its former state. On the other hand, the reduced electron carrier gives an electron to the proton of water by the catalyst, resulting in the hydrogen evolution. The catalyst for the hydrogen evolution should be

Noble Metal Nanocolloids

highly active and should be soluble or able to be highly dispersed in the solvent. If a dispersed solid catalyst is used, the catalyst may prevent the light from reaching the photosensitizer, and the inefficient use of irradiation light would result in a lowering of the reaction efficiency. Widely used catalysts in this reaction are platinum colloid and the enzyme hydrogenase. These are highly active for the hydrogen evolution reaction and have little effect on irradiation light. The platinum colloid receives an electron from the electron carrier, serves as an electron pool, and gives the electron to the proton. The reaction pathway is shown in Figure 6. A comparison of the hydrogen evolution efficiencies for the platinum colloid and for other platinum catalysts shows that the hydrogen evolution efficiency is much higher for platinum colloid. This is an example of photoinduced hydrogen evolution when methyl viologen (electron carrier), ethylenediamine-teraacetic acid (electron donor), and Ru(bpy)2+ 3 (photosensitizer) are used. When a semiconductor such as titania is used as the photosensitizer, water is completely decomposed. The following RuO2 /TiO2 /Pt colloid has been synthesized for efficient photolysis of water (see Fig. 7), in which the charge separation proceeds and hydrogen and oxygen are obtained. The ruthenium oxide (catalyst for oxygen evolution) and the platinum (catalyst for hydrogen evolution) are supported on a colloidal semiconductor powder.

3.4. Immunochromatography with Au Colloid and Pt Colloid Immunochromatography is a method for detecting an antigen by an antigen-antibody reaction as illustrated in Figure 8. A complex (conjugate) is used in this method. The conjugate is a compound that binds with the antibody and metallic colloid, and the bare surface (the antibody not being adsorbed) is coated by bovine serum albumin. In this method, the conjugate and the antigen in the sample are reacted in advance, and a conjugate-antigen complex is formed. The conjugate-antigen complex is developed on a supported nitrocellulose film. Since the conjugate-antigen complex binds metallic colloid particles through the antigen, metallic colloid particles accumulate. When a gold colloid is used, a purple gold colloid color is observed. The detection sensitivity is ∼100 pg/ml. Sensitivity can be improved by platinum catalysis. For instance, a blue color appears when 3,3 ,5,5 -tetramethylbenzidine is oxidized with a platinum catalyst. The detection sensitivity in this case is 8 pg/ml.

Figure 6. Role of platinum colloid as an electron pool.

47

Noble Metal Nanocolloids

Figure 7. Mechanism of photodecomposition of water with RuO2 / TiO2 /Pt colloid.

3.5. Preparation and Characterization of Supported Platinum and Alloy Catalysts The metallic particles in a colloid are generally negatively charged. Therefore, the metallic particles are easily adsorbed to the carrier surface when the carrier contains polyvalent cations. Platinum and alloy colloids can be supported on an alumina plate as follows [20]. The hydrosol of the alumina plate is prepared as follows. Colloidal alumina is dispersed to aqueous acetic acid, and the hydrothermal reaction is allowed to proceed for 2.5 h at 200  C. The obtained sol is deionized with the use of an ion-exchange resin (Amberlite MB-1). When the metal colloid is added to the alumina plate sol thus prepared, the metallic particles are immediately adsorbed to the alumina surface. The supported metal catalyst is obtained as a precipitate. As the adsorption of the metallic particles to the alumina surface is very fast, the metallic particles that adsorb to the surface are heterogeneous. To avoid this, the alumina plate sol is diluted 10 times and the metal colloid is dropped with vigorous stirring. The alumina plate with adsorbed metallic particles is obtained as a precipitate. This precipitate is separated by decantation and centrifugation. The precipitate is washed with clear water and dried at 105  C. Figure 9 shows an electron micrograph of a supported platinum catalyst prepared by the above-described method. The size of the platinum particles is the same as that of the sol, and the platinum particles are supported homogeneously on the alumina plate. The figure shows that 0.48 wt% platinum is supported on the alumina plate. The loading of the metal can easily be changed by changing the amount of metal colloid. The platinum colloid supported on the alumina plate is active for hydrogenation of olefins and benzene and the H2 -D2 exchange reaction, as is a platinum catalyst prepared by the usual method. The thus-prepared catalyst has platinum particles of uniform sizes that are homogeneously dispersed on the surface. Such a platinum catalyst is suitable for studying the turnover numbers and selectivity change, depending on particle size. An alloy catalyst prepared by the

Figure 9. Electron microscopic picture of colloidal platinum on alumina. Reprinted with permission from [76], Namba and Okura, Hyomen 21, 450 (1983). © 1983, Koshinsha Co.

usual method may be different from an alloy catalyst prepared from a colloid. In the case of a platinum-gold alloy catalyst, it is known that only an extreme alloy (0–17% Pt, 98–100% Pt) is prepared because an immiscibility area exists in a catalyst prepared by the usual method. In a platinumgold alloy colloid, however, the metal composition in the bulk and that of the surface are almost the same. This has been confirmed by examination of visible spectra and by hydrogen peroxide decomposition reaction. When ethylene hydrogenation was carried out with the alloy colloid, the highest activity was observed when Pt/Au = 2 in the case of platinum-gold and at Pt/Pd = 1/2 in the case of platinumpalladium. The high activity is caused by an increase in the turnover number, not by the exposed metal atoms or the number of active sites. Colloidal alloy catalysts are expected to have excellent activity or selectivity because various metal combinations are possible.

4. CONCLUSION The nanotechnology of noble metal colloids has been making rapid progress in recent decades, and tailor-made nanocolloids of specific particle sizes have now become available. Such advances are most evident with the nanocolloids of platinum and gold as mentioned. Gold nanocolloids have been used particularly as labeling compounds of various biological materials. Catalytic reactions with platinum nanocolloids such as hydrogenation, decomposition, and hydrogen evolution have been the most extensively studied in a sol state and in a gel state. For example, platinum nanocolloids with specific sizes and Stabilizers are available for various degrees of selectivity in hydrogenation of alkenes and alkynes. Very recently, the interest in bimetallic nanocolloids has increased remarkably because of their possible use as a multifunctional catalyst, such as a fuel cell catalyst. Tailor-made multifunctional nanocolloids are strongly desired.

GLOSSARY Figure 8. Mechanism of immunochromatography with gold colloid.

Alloy A metal made by combining two or more metallic elements, especially to give greater strength or resistance to corrosion.

48

Noble Metal Nanocolloids

Coagulation A flocculate or cause of flocculate by the addition of an electrolyte to an electrostatic colloid. Hydrogenase An enzyme that promotes the reduction of protonation or the oxidation of hydrogen. Immunochromatography A kind of chromatography that uses the affinity of an antibody for an antigen. Stabilizer Substance added to chemical compounds to prevent deterioration or the loss of desirable properties. Turnover number The number of times that a catalyst or enzyme catalyzes during a given period of time.

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