Abstract:
The present invention provides a precious metal—metal oxide composite cluster, wherein said cluster is formed as a single particle by combining a precious metal portion comprising a single atom or an aggregate of a plurality of atoms consisting of one or more precious metals, and a metal oxide portion comprising a single molecule or an aggregate of a plurality of molecules consisting of one or more metal oxides, and wherein said particle has a particle size between 1 and 100 nm.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a precious metal—metal oxide composite cluster supplied to the production of a functional material containing a precious metal as a main component like a catalyst and so on.  
         [0003]     2. Description of the Related Art  
         [0004]     Precious metals, because of having superior corrosion resistance and superior high-temperature strength, are used not only as a component material of high-temperature equipment such as a glass-melting tank and a crucible, but are also used as a component material of various functional materials such as a catalyst and an electromagnetic material, because of having a unique electrical property, magnetic property and catalytic activity.  
         [0005]     One of means for applying precious metals to various functional materials, which recently captures attention, is a technique of applying an ultra-fine particle comprising precious metals. Here, the ultra-fine particle means an aggregate particle of a few precious metal atoms with a nanometric order. (Hereafter, such an ultra-fine particle is called a cluster in the present invention.)  
         [0006]     With the use of such a cluster, various functional materials can be produced by a method of absorbing and applying clusters on the surface of an appropriate support, and then drying and heat-treating them to remove the components of a protective agent into desired functional materials. For instance, when manufacturing a catalyst, a carrier is impregnated with a solution having the clusters dispersed therein, and is baked, where the above support is generally called the carrier that is a porous material such as alumina and carbon. Then, the catalyst consisting of fine cluster particles densely dispersed on the carrier can be produced. In addition, in the field of materials for optics, electricity and magnetism, a substrate or a powder both made of a polymeric material, a metal, a glass, a ceramic or the like, is used as the support.  
         [0007]     The present applicant has investigated a practical precious metal cluster.  
         [0008]     For instance, in Japanese Patent Laid-Open No. 11-151436, Japanese Patent Laid-Open No. 2000-279818 and Japanese Patent Laid-Open No. 2002-001095, the precious metal cluster is disclosed. (In these literatures, the cluster is called a colloid, which are synonymous.) These precious metal clusters comprise one or more metals, and the cluster particles are protected with a protective agent comprising a polymer such as polyvinylpyrrolidone (hereafter called PVP) and a salt such as a quaternary ammonium salt. Here, the protective agent means a compound which is chemically or physically bonded or adsorbed to the periphery of cluster particles in a metal colloid, inhibits an agglomeration of the cluster particles, and controls and stabilizes the particle size distribution into an appropriate range.  
         [0009]     In the field of functional materials, incidentally, it has been recently verified that precious metals when working with a metal oxide added as an auxiliary, show an additional function and a higher performance. An auxiliary metal oxide includes, for instance, cerium oxide known as an auxiliary metal oxide (a cocatalyst) to a platinum catalyst or a platinum/rhodium catalyst known as a catalyst for cleaning an exhaust gas of automobiles. Cerium oxide has the capability of occluding and discharging oxygen, and has the function of alleviating atmospheric variations of the exhaust gas.  
         [0010]     Such a function of an auxiliary metal oxide is not developed by only a metal oxide, but is often developed by an interaction with a precious metal. A catalyst containing the above described additional cerium oxide for cleaning an exhaust gas of automobiles, does not show the capability of occluding and discharging oxygen by only the capability of cerium oxide own, but by an interaction with platinum (platinum/rhodium) which is a main component.  
         [0011]     A functional material containing an auxiliary metal oxide can be produced with a method of making a support for adsorbing the clusters thereon from auxiliary metal oxides. A catalyst for cleaning an exhaust gas containing the above described cerium oxide as a cocatalyst, can employ cerium oxide for a carrier, and make the carrier absorb and carry precious metal clusters thereon into a catalyst.  
         [0012]     Another method can be considered which makes a support adsorb precious metal clusters, and then makes the support adsorb clusters comprising metal oxides in the similar way. The cluster of metal oxides, as is generally known, can be produced by a similar method to the method of manufacturing a metal cluster. The method is disclosed, for instance, in Japanese Patent Laid-Open No. 6-218276. Accordingly, by using the metal oxide clusters together with precious metal clusters, a functional material provided with auxiliary metal oxides can be produced.  
         [0013]     However, according to the present inventors, a functional material produced in the above described process may have insufficient characteristics. For instance, a catalyst for cleaning an exhaust gas, which contains cerium oxides in the above described example as an auxiliary metal oxide, can have low stability at a high temperature, and be deactivated by long hours of operation at a high temperature.  
         [0014]     In addition, as described above, a functional material provided with an auxiliary metal oxide develops the function of the auxiliary metal oxide through interaction with a precious metal, so that the auxiliary metal oxide is preferably arranged in the vicinity of the precious metal.  
         [0015]     The present invention was accomplished under the background described above. Accordingly, an object of the present invention is to disclose a precious metal cluster which is suitable for manufacturing a functional material comprising a precious metal as a main component and an auxiliary metal oxide, and to disclose a manufacturing method therefor.  
       SUMMARY OF THE INVENTION  
       [0016]     As a result of intensive investigations, the present inventors found a precious metal cluster combined with a metal oxide in a cluster particle, as a precious metal cluster capable of solving the above described problems.  
         [0017]     Specifically, the present invention provides a precious metal—metal oxide composite cluster forming a single particle which has a precious metal portion comprising a single atom or an aggregate of a plurality of atoms consisting of one or more precious metals, and a metal oxide portion comprising a single molecule or an aggregate of a plurality of molecules consisting of one or more metal oxides, mutually combined, and which has a particle size between 1 and 100 nm.  
         [0018]     The reason why deactivation occurs at a high temperature in the above described catalyst for cleaning an exhaust gas is considered to lie in growth of precious metal clusters due to connection between close clusters on a carrier through sintering in a high-temperature atmosphere. In a composite cluster according to the present invention, a metal oxide combined with a precious metal in a particle inhibits a sintering of precious metal particles on a carrier.  
         [0019]     In addition, a composite cluster according to the present invention has a precious metal particle and a metal oxide particle both in a nanometric order mutually combined and can show unique properties due to the fineness of the particle. For instance, when the composite cluster is applied to a catalyst, it promises the increase of a catalytic activity due to a drastic increase of the surface area. In addition, in the present invention, a precious metal is firmly combined with a metal oxide, and the composite cluster has a large contact area between both substances. Thus, compared to the carrier of a metal oxide having a precious metal adsorbed thereto or the carrier having a precious metal and a metal oxide separately adsorbed thereto, both conventional, the composite cluster has a larger contact area between both substances, and can more effectively develop an auxiliary function of the metal oxide due to the consequent interaction between both substances.  
         [0020]     Here, as for the structural aspect of the composite of the precious metal and the metal oxide according to the present invention, one may be an aspect shown in  FIG. 1 , where a plurality of precious metal atoms and a plurality of metal oxides are aggregated to form respectively a precious metal portion and a metal oxide portion and these parts are mutually combined. (Hereafter, a composite cluster in such a state is called a nanocomposite cluster.) Another may be an aspect shown in  FIG. 2 , where precious metal atoms and metal oxide molecules are arranged and combined at random. (Hereafter, a composite cluster in such a state is called a composite nano-cluster.)  
         [0021]     A composite cluster according to the present invention is constituted by a precious metal and a metal oxide, wherein the precious metal is any element selected from the group consisting of Pt, Au, Ag, Ru, Pd, Ir, Rh and Os, and the metal oxide is not particularly limited but an oxide of any metal selected from the group consisting of Be, Mg, Ca, Sr, Ba, Ra, Se, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Te, Re, Fe, Co, Ni, Cu, La, Ce, Pr, Nd, Pm, Sm, Gd, Th, Dy, Ho, Er, Tm, Yb, Lu, Zn, Cd, Hg, B, Al, Ga, In, Ti, Si, Ge, Sn, Pb, P, As, Sb, Bi, Se, Te and Po, can be applied. In addition, a cluster particle may comprise not only one sort of a precious metal or metal oxide, but also may comprise plural sorts of precious metals or metal oxides.  
         [0022]     A composite cluster according to the present invention is preferably provided with a protective agent on the surface. Though the effect of the protective agent was described above, by the addition of the protective agent, the cluster particles with small grain sizes can keep a suspended state while maintaining the grain sizes. In addition, when a catalyst is produced, the agent makes the grain size of a catalyst component small and increases the effective surface area of the catalyst. The applicable protective agent includes a polyvinylpyrrolidone, a polyvinylpolypyrrolidone, 1-acetyl-2-pyrrolidone, 1-n-butyl-2-pyrrolidone, N-tosyl pyrrolidone, 1-methyl-2-pyrrolidone, a N-vinylpyrrolidone/styrene, a polyacrylonitrile, a polyvinyl alcohol, a polyacrylic acid, a polyallylamine hydrochloride, citric acid, dodecanthiol, fluoro thiol, alkylthiol, mercaptopropionic acid, mercaptosuccinic acid, mercaptopropionylglycine, glutathione, trimethylammonium bromide, ethanolamine, N-(3-aminopropyl)diethanolamine, cyclodextrin, aminopectin, methyl cellulose, an alkylcarboxylic acid and alkylamine with the carbon number of 6-18, N,N-dimethyl-N-laurylamine oxide, alkylamine oxide with the carbon number of 6 to 20, n-tetradecyl sodium sulfate, sodium dodecyl sulfate, sodium decyl sulfate, sodium n-nonyl sulfate, sodium n-octyl sulphate, a sodium alkyl sulfate, a sodium polyoxyethylene alkylether sulfate, a polyoxyethylene alkylphenylether phosphate, triethanolamine lauryl sulfate, sodium lauroyl sarcosinate, sodium lauroylmethyltaurinate, alkyltrimethylammonium chloride, dialkyldimethylammonium chloride, stearyldimethylbenzylammonium chloride, stearyltrimethylammonium chloride, benzalkonium chloride, benzethonium chloride, polyoxyethylene sorbitan monooleate, ethyleneglycol monostearate, propyleneglycol monostearate, a polyoxyethylene glycerol monostearate and a polyoxyethylene sorbitan monostearate.  
         [0023]     In the next place, a method for producing a composite cluster according to the present invention will be described. A first method for producing a composite cluster according to the present invention is a method of mixing a solution having precious metal clusters dispersed therein with a solution having metal oxide clusters dispersed therein. This method is suitable for producing a composite cluster having the form of a nanocomposite cluster. A second production method is a method of, first, preparing a solution having dispersed metal oxide clusters therein, and dissolving a precious metal salt into the solution to reduce precious metal ions in the solution and simultaneously combine the reduced substance with the metal oxide clusters.  
         [0024]     In the first method, clusters of a precious metal and a metal oxide are each separately prepared. A method for preparing precious metal clusters comprises dissolving the salt of a metal constituting cluster particles into a solvent to ionize the salt, and adding a protective agent and a reducing agent to the solution, to reduce metal ions into cluster particles and simultaneously protect the formed cluster particles with the protective agent. The protective agent may be dissolved simultaneously with the metal salt. In order to disperse the cluster particles comprising a plurality of metals, a plurality of metal salts are dissolved in a solvent.  
         [0025]     There are following metal salts applicable as the raw materials. The applicable metal salts for preparing a platinum colloid include hexachloroplatinic acid, dinitrodiammineplatinum, dinitrodiammineplatinum nitrate, platinous chloride, platinic chloride, chloroplatinic acid and a chloroplatinate. The applicable metal salts for preparing a palladium colloid include palladium chloride, palladium nitrate and dinitrodiamine palladium. The applicable metal salts for preparing a gold colloid include chloroauric acid, a chloroaurate, potassium auric cyanide and potassium gold cyanide. The applicable metal salts for preparing a silver colloid include silver chlorate, silver nitrate, silver acetate and silver lactate. The applicable metal salts for preparing a ruthenium colloid include ruthenium chloride and ruthenium nitrate. The applicable metal salts for preparing a rhodium colloid include rhodium chloride, rhodium nitrate and rhodium acetate. The applicable metal salts for preparing an iridium colloid include hexachloroiridate and iridium trichloride. The applicable metal salts for preparing an osmium colloid include osmium oxide.  
         [0026]     In addition, as for a protective agent, the above described protective agents are applicable. Furthermore, as for a solvent, water or a mixed solvent of water and an organic solvent is applicable. The applicable organic solvent includes alcohols such as ethanol, ketones such as acetone, and esters such as ethyl acetate.  
         [0027]     The reduction of precious metal ions in a solvent is preferably conducted by adding a reducing agent. The reducing agent is not limited in particular so far as is applicable to a chemical reduction process, but has only to be able to reduce a mixed solution of a metal salt and a protective agent. Preferable reducing agents include alcohols such as ethanol, formic acid, hydrogen, hydrazine, amine, sodium borohydride and dimethylamine borane.  
         [0028]     On the other hand, a metal oxide cluster can be prepared by dissolving the salt of metals constituting metal oxides and a protective agent into a solvent, and then reducing and oxidizing the metal ions. The applicable metal salts to be dissolved include a nitrate, an acetate, a perchlorate, an alkoxide, a halide and an acetylacetonate.  
         [0029]     For a protective agent used in preparation for the metal oxide cluster, the protective agent similar to that used in preparation for a precious metal cluster is preferably applied. That is because when a different protective agent is applied, there is a fear of causing a reaction between the protective agents. In addition, as for methods of reducing metal ions and oxidizing the reduced substance, a method of applying an ultrasonic wave to a solution including metal ions is preferable. In the step, the metal ion in the solution is temporarily changed to a metal hydroxide and then changed to a metal oxide cluster. By mixing solutions of a precious metal cluster and a metal oxide cluster prepared in the above described steps, a composite cluster solution can be produced in which precious metals and metal oxides are combined.  
         [0030]     A second production method is a process of dissolving a precious metal salt and a protective agent in a solution of a metal oxide cluster, reducing the precious metal ion and simultaneously combining the reduced substance with the metal oxide. In the process, the metal oxide cluster solution can be prepared by a method similar to the above described method.  
         [0031]     In the process, applicable precious metal salts, metal salts and a protective agent to be added into a metal oxide cluster solution are similar to the precious metal salt and so on applied in the above described first method.  
         [0032]     In addition, as for reduction of precious metal salts, a method of adding a reducing agent is preferable similarly to the first method. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIG. 1  is schematic view showing form of composite cluster, which can be produced in the present invention.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0034]     Preferred embodiments according to the present invention will be now described below.  
         [0035]     First Embodiment: Dinitrodiammineplatinum in the amount of 0.1 g, PVP in the amount of 0.25 g and ethanol in the amount of 25 mL were mixed into 100 mL of water, and the solution was refulxed at 86° C. for 10 hours to prepare a platinum cluster. Aside from it, 0.3 g of cerium nitrate hexahydrate and 0.08 g of PVP were mixed into 50 mL of pure water, and the mixed solution was stirred, and irradiated with an ultrasonic wave having the frequency of 200 kHz at the output of 6 W/cm 2  for five hours, to prepare a cerium oxide cluster. Then, the prepared platinum cluster and cerium oxide cluster were mixed. By the above described operations, a platinum/cerium oxide composite cluster (a nanocomposite cluster in  FIG. 1 ( a )) was prepared.  
         [0036]     Second Embodiment: Dinitrodiammineplatinum in the amount of 0.1 g, PVP in the amount of 0.25 g and ethanol in the amount of 25 mL were mixed into 100 mL of water, and the solution was refulxed at 86° C. for 10 hours to prepare a platinum cluster. Cerium nitrate hexahydrate in the amount of 0.3 g, PVP in the amount of 1.0 g and urea in the amount of 0.05 g were mixed into 120 mL of a methanol-ethanol mixture solvent (with a mixture ratio of 5:7), and the mixed solution were stirred, and refluxed at 70° C. for seven hours to prepare a cerium oxide cluster. Then, the prepared platinum cluster and cerium oxide cluster were mixed. By the above described operations, a platinum/cerium oxide composite cluster (a nanocomposite cluster in  FIG. 1 ( a )) was prepared.  
         [0037]     Third Embodiment: Cerium nitrate hexahydrate in the amount of 0.3 g and PVP in the amount of 0.08 g were mixed into 50 mL of pure water, and the mixed solution was stirred, and irradiated with an ultrasonic wave having the frequency of 200 kHz and the output of 6 W/cm 2  for five hours, to prepare a cerium oxide cluster. Into 30 mL of the cerium oxide cluster solution, 0.1 g of dinitrodiammineplatinum, 0.25 g of PVP and 25 mL of ethanol were mixed, and the solution was refluxed at 86° C. for five hours so that platinum ions can be reduced. By the above described operations, a platinum/cerium oxide composite cluster (a composite nano-cluster in  FIG. 1 ( b )) was prepared.  
         [0038]     Fourth Embodiment: Dinitrodiammineplatinum in the amount of 0.1 g, PVP in the amount of 0.25 g and ethanol in the amount of 25 mL were mixed into 100 mL of water, and the solution was refulxed at 86° C. for 10 hours to prepare a platinum cluster. Ferric nitrate hexahydrate in the amount of 0.4 g and PVP in the amount of 0.08 g were mixed into 50 mL of pure water, and the mixed solution was stirred, and irradiated with an ultrasonic wave having the frequency of 200 kHz at the output of 6 W/cm 2  for five hours, to prepare a cerium oxide cluster. Then, the prepared platinum cluster and cerium oxide cluster were mixed. By the above described operations, a platinum/iron oxide composite cluster (a nanocomposite cluster) was prepared.  
         [0039]     Fifth Embodiment: Nickel nitrate in the amount of 0.3 g and PVP in the amount of 0.08 g were mixed into 50 mL of pure water, and the mixed solution was stirred, and irradiated with an ultrasonic wave having the frequency of 200 kHz at the output of 6 W/cm 2  for five hours, to prepare a nickel oxide cluster. Into 30 mL of the nickel oxide cluster solution, 0.2 g of aurichrolic acid, 0.25 g of PVP and 70 mL of ethanol were mixed, and the solution was refluxed at 86° C. for five hours so that gold ions can be reduced. By the above described operations, a gold/nickel oxide composite cluster (a composite nano-cluster) was prepared.  
         [0040]     Sixth Embodiment: Dinitrodiammineplatinum in the amount of 0.1 g, PVP in the amount of 0.25 g and ethanol in the amount of 25 mL were mixed into 100 mL of water, and the solution were refluxed at 86° C. for 10 hours to prepare a platinum cluster. Zirconium oxide nitrate in the amount of 0.2 g, PVP in the amount of 1.0 g and urea in the amount of 0.05 g were mixed into 120 mL of a methanol-ethanol mixed solvent, and the mixed solution was stirred, and refluxed for seven hours to prepare a zirconium oxide cluster. Then, the prepared platinum cluster and zirconium oxide cluster were mixed. By the above described operations, a platinum/zirconium oxide composite cluster (a nanocomposite cluster) was prepared.  
         [0041]     Seventh Embodiment: Molybdenum hexaammonium in the amount of 0.4 g and PVP in the amount of 0.08 g are mixed into 50 mL of pure water, and the mixed solution was stirred, and irradiated with an ultrasonic wave having the frequency of 200 kHz at the output of 6 W/cm 2  for five hours, to prepare a molybdenum oxide cluster. Into 30 mL of the molybdenum oxide cluster solution, 0.2 g of aurichrolic acid, 0.25 g of PVP and 70 mL of ethanol were mixed, and the solution was refluxed at 86° C. for five hours, so that gold ions were reduced. By the above described operations, a gold/molybdenum oxide composite cluster (a composite nano-cluster) was prepared.  
         [0042]     Eighth Embodiment: Chloroauric acid in the amount of 0.1 g, PVP in the amount of 0.25 g and ethanol in the amount of 25 mL were mixed into 100 mL of water, and the solution were refluxed at 86° C. for 10 hours to prepare a gold cluster. Sodium tungstate in the amount of 0.3 g, PVP in the amount of 1.0 g and urea in the amount of 0.05 g were mixed into 120 mL of a methanol-ethanol mixed solvent, and the mixed solution was stirred, and refluxed for seven hours to prepare a tungsten oxide cluster. Then, the prepared gold cluster and tungsten oxide cluster were mixed. By the above described operations, a gold/tungsten oxide composite cluster (a nanocomposite cluster) was prepared.  
         [0043]     Ninth Embodiment: Sodium tungstate in the amount of 0.3 g and PVP in the amount of 0.08 g were mixed into 50 mL of pure water, and the mixed solution was stirred, and irradiated with an ultrasonic wave having the frequency of 200 kHz at the output of 6 W/cm 2  for five hours, to prepare a tungsten oxide cluster. Into 30 mL of the tungsten oxide cluster solution, 0.1 g of dinitrodiammineplatinum, 0.25 g of PVP and 70 mL of ethanol were mixed, and the solution was refluxed at 86° C. for five hours, so that gold ions were reduced. By the above described operations, a platinum/tungsten oxide composite cluster (a composite nano-cluster) was prepared.  
         [0044]     Tenth Embodiment: Lanthanum acetate in the amount of 0.4 g and PVP in the amount of 0.08 g were mixed into 50 mL of pure water, and the mixed solution was stirred, and irradiated with an ultrasonic wave having the frequency of 200 kHz at the output of 6 W/cm 2  for five hours, to prepare a lanthanum oxide cluster. Into 30 mL of the lanthanum oxide cluster solution, 0.2 g of chloroauric acid, 0.25 g of PVP and 70 mL of ethanol were mixed, and the solution was refluxed at 86° C. for five hours, so that gold ions were reduced. By the above described operations, a gold/lanthanum oxide composite cluster (a composite nano-cluster) was prepared.  
         [0045]     Examination of catalytic performance: The platinum/cerium oxide composite cluster which was prepared in the first embodiment was absorbed and carried by a gamma-alumina carrier so that platinum occupies 1 wt %, and cerium oxide 20 wt % (both by ratio to the weight of the carrier), and the carrier was baked at 800° C. for 50 hours to prepare an almina catalyst carrying platinum-cerium oxide. In a similar way, a platinum/zirconium oxide composite cluster which was prepared in the sixth embodiment was absorbed and carried by the gamma-alumina carrier, and the carrier was baked to prepare an almina catalyst carrying platinum-zirconium oxide. Particle sizes of platinum, cerium oxide and zirconium oxide of these catalysts were 2 to 5 nm as a result of TEM observation.  
         [0046]     The performance of these catalysts was examined on propylene gas employed as a treatment object. As for an evaluation method, 0.4 g of a catalyst was weighed and filled in a reaction vessel, propylene gas was passed through the vessel under a condition of SV: 2000 h −1 , and the oxidation activity of the catalysts was evaluated by measuring a catalytic purging temperature T 50 .  
         [0047]     As a result, the catalyst prepared from a composite cluster in the first and the sixth embodiments showed T 50  of respectively 134° C. and 132° C., whereas a cerium oxide catalyst carrying platinum was examined in a similar way and showed T 50  of 160° C.