US 20070101824 A1
A method for producing nanoparticles on a substrate using a metal precursor in an ionic liquid and microwave heating is described. The composite compositions are useful as catalysts for chemical reactions, fuel cell, supercapacitor and battery components, and the like.
1. A method for producing nanoparticles of metal deposited on a surface of a substrate which comprises:
(a) providing a solution of an ionic liquid in a reducing liquid solvent containing a precursor of the metal on the substrate; and
(b) exposing the metal precursor in the ionic liquid to microwaves so as to reduce the metal precursor to nanoparticles of the metal which are deposited on the substrate.
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11. A composite composition which comprises a substrate having nanoparticles of a metal deposited thereon.
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This application claims priority to U.S. Provisional Application Ser. No. 60/689,658, filed Jun. 10, 2005.
(1) Field of the Invention
The present invention relates to a method for producing metal nanoparticles on a solid surface of a substrate. In particular, the present invention relates to nanoparticles of a metal deposited on nanoparticles comprising a carbon or graphite in various forms such as carbon black, fibers and nanotubes, for instance.
(2) Description of the Related Art
U.S. Pat. No. 6,596,130 to Westman generally describes a process for microwave associated chemical transformation of organic compounds using ionic liquids (IL). This reference is incorporated herein in its entirety, particularly in reference to the ionic liquids. Microwave reactors are well known to those skilled in the art.
It is therefore an object of the present invention to provide a novel method for producing metallic nanoparticles on solid surfaces. In particular, it is an object of the present invention to provide a process and resulting composite which is economical and relatively easy to prepare. It is also an object of the present invention to provide a simple way of controlling metal nanoparticle formation on any substrate which is of the utmost importance for the performance of any industrial catalyst or electrocatalyst based on supported metals. These and other objects will become increasingly apparent by reference to the following description.
The present invention relates to a method for producing nanoparticles of metal deposited on a surface of a substrate which comprises: (a) providing solution of an ionic liquid in a reducing solvent, such as ethylene glycol, containing a precursor of the metal on the substrate; and (b) exposing the metal precursor in the ionic liquid to microwaves so as to reduce the metal precursor to nanoparticles of the metal which are deposited on the substrate. Preferably the substrate has a surface which comprises carbon on which the nanoparticles of the metal are deposited. Preferably the carbon is a graphite, a carbon black particle, a nanotube, or a carbon fiber. Also, preferably the carbon is a buckyball. Preferably the carbon has at least one dimension which is a nanodimension. Preferably the substrate is a nanoparticle which is less than 100 nanometers in at least one dimension. Preferably at least two of the metal precursors are provided in admixture in step (a).
The present invention also relates to a composite composition which comprises a substrate having nanoparticles of a metal deposited thereon. Preferably the nanoparticles of the metal are comprised of a noble metal alone or in combination with a transition metal. Preferably the nanoparticles of the metal are comprised of any metal alone or in combination with any other metal. Preferably the substrate is a nanoparticle having at least one dimension less than 100 nanometers. Preferably the substrate comprises a carbon. Preferably the substrate has a surface which comprises any solid on which the nanoparticles of the metal are deposited.
The substance and advantages of the present invention will become increasingly apparent by reference to the following drawings and the description.
The following description includes terms which are defined as follows:
The term “nanoparticle” is defined as a particle wherein at least one dimension is 100 nanometers or less, preferably 10 nanometers or less (1 nanometer equals 10−9 meters).
The term “ionic organic liquid” is defined as a liquid organic compound with a cation and an anion and which can be heated to a temperature up to or over 180° C. in order to reduce an ionic metal precursor.
The term “ionic metal precursor” means an ionic metal salt which can be reduced by microwave energy in the presence of the ionic organic liquid. The salt can be organic or inorganic.
The term “solution” means a liquid composition containing a reducing compound such as ethylene glycol and an ionic liquid at a concentration of between about 1 and 30%.
The term “substrate” means a solid material which has a surface on which metal nanoparticles can be deposited. Preferably the substrate is some form of carbon. Most preferably the substrate has at least one nanodimension of 100 nanometers or less. The metals are preferably noble metals alone or in combination with transition metals which can act as catalysts.
The term “microwave” means wave energy in the microwave spectrum. The most common frequency for microwave ovens sold for food uses is 2.45 GHz; however, higher or lower frequencies between 1 MHz and 300 GHz are in commercial use and are well known to those skilled in the art.
The term “reducing liquid” means an organic liquid which can function as a reducing agent in the ionic organic liquid in the presence of the microwaves. Such compounds are, for instance, ethylene glycol or other polyhydric alcohols, which do not volatilize in the presence of the microwaves. Other organic liquids are diethylene glycol and triethylene glycol.
Microwave dielectric heating has numerous advantages, such as rapid heating, higher reaction rate, and the reduction of reaction time compared to conventional oil-bath heating methods. Hence the microwave-assisted process has opened up the possibility of fast synthesis of organic and inorganic materials. From this perspective, ionic liquids (ILs) provide great advantages due to large organic positive ions with a high polarizability. Thus, ILs provide a good medium as well as good additive for absorbing microwave very well, leading to further high heating rate. By using the advantages of ILs in the microwave heating process, a fast and simple way of synthesizing carbon-supported electrocatalysts for fuel cell applications, batteries, supercapacitors, catalytic materials for chemical processing, and the like is provided. The ILs can be used with microwaves to synthesize the Pt-based catalysts supported on various carbons as well as to tune the size of Pt-based metals regardless of the content of active metal phase. This process can be applied to any metal.
The Pt/C (20 and 60 wt. % Pt on carbons) and PtM (M=Ru, Ni, Fe)/C (20 wt. % PtM on carbons) catalysts were synthesized by microwave dielectric heating of ethylene glycol (ACS grade, J. T. Baker) solutions of Pt and M precursors as a comparative test without the ionic liquids of the present invention. Four different carbon materials were used; Vulcan XC-72R carbon black (CB, Cabot Co.), graphite nanofiber (GNF, Nanomirae Inc.), as-produced single-wall nanotube (A-SWNT, CarboLex Inc.), and exfoliated graphite nanoplatelet (xGnP, Michigan State University; U.S. patent application Ser. No. 10/659,577, filed Sep. 10, 2003 (Publication No. US2004-0127621-A1, published Jul. 1, 2004), which is incorporated by reference). A typical preparation consists of the following procedures: For Pt/C or PtM/C catalysts, 40 mg of a carbon support was dispersed in 20 mL of ethylene glycol by ultrasonication for 20 min. 1 mL of ethylene glycol solution of 26 mg H2PtCl6.6H2O (Aldrich) or a 1:1 molar ratio of H2PtCl6.6H2O and other metal precursors (for example, RuCl3.3H2O) was added and mechanically stirred for 20 min. The beaker containing Pt precursor, carbon, and ethylene glycol was heated in a household microwave oven (1300 W) for 50 s. After cooling down to ambient temperature, the resulting suspension was filtered and washed with acetone and dried at 100° C. in a vacuum oven for 12 hrs. Catalysts with 20 and 60 wt. % Pt and PtRu loading were prepared by varying H2PtCl6.6H2O and the content of other metal precursors in ethylene glycol solution. The catalysts obtained are called as the Pt/C—N or PtRu/C—N, where N is no ionic liquid.
For Pt/C or PtRu/C catalysts assisted with ionic liquids (IL), 1-butyl-3-methyl-imidazolium hexafluorophosphate [(BMI) (PF6)] and 1-butyl-3-methyl-imidazolium acetate [(BMI)Ace) were purchased from Aldrich Chemical Co. and used as received. 0.025 mL˜1 mL of ((BMI) (PF6)] or [(BMI)Ace] was dissolved in 20 mL ethylene glycol prior to the dispersion of carbons. Other steps are the same as for Pt/C—N or PtRu/C—N catalysts. The catalysts synthesized with the addition of [(BMI) (PF6)] and [(BMI)Ace] are denoted as Pt (or PtM)/C—IM and Pt (or PtM)/C-M, respectively. [(BMI) (PF6)] is immiscible with ethylene glycol and [(BMI)Ace] is miscible with the solvent. IM-IL refers to [(BMI) (PF6)] and M-IL indicates [(BMI)Ace].
The prepared catalysts were examined by transmission electron microscopy (TEM) on a JEOL 2200FS and JEOL 100CX. For microscopic investigation, the catalyst samples re-dispersed in acetone were deposited on Cu grids covered with a holey carbon film. The particle size distribution of Pt/C and PtRu/C catalysts metal particles on carbons was manually and statistically determined by counting at least 120 particles in each sample from randomly chosen area in the TEM images with SIGMASCAN software.
Carbon Black (CB)-Supported Pt Catalyst
Morphologies and Pt size distribution of CB-supported Pt catalysts synthesized by microwave dielectric heating in the absence (1A) and the presence of ILs (0.5 mL; 1C, 1D) are shown in
Spiral Graphite Nanofiber (GNF)-Supported Pt Catalyst
TEM morphologies of Pt/GNF—N, Pt/GNF—IM, and Pt/GNF-M and the size distribution of Pt phase corresponding to each sample are shown in
The effect of ILs on the reduction of Pt size for GNF-supported catalysts is clearly shown in
As-Produced Single Wall Nanotube (a-SWNT)-Supported Pt Catalyst and Purified Multiwalled Carbon Nanotube (MWNT)-Supported Pt Catalyst
Pt deposition directly on a-SWNT was attempted by microwave heating process assisted with IM-IL (0.5 mL). As in
Purified but not oxidized MWNT was used as a support. The morphologies of Pt nanoparticles deposited on MWNT in the presence and the absence of an IL are shown in
Exfoliated Graphite Nanoplatelet (xGnP)-Supported Pt Catalyst
xGnP is attracting attention as a new reinforcing material for composites and a support for catalysts. xGnP is much more cost-effective than new carbon nanostructures such as carbon nanotubes, carbon nanohorns, and fullerenes being considered as breakthrough materials in nanotechnology area. xGnP has superior properties such as excellent mechanical, high corrosion and oxidation resistance and high crystallinity which are characteristics required as a support for the electrodes of fuel cell. Here, in spite of high inertness of its surface, xGnP could be very effectively deposited with nanosized Pt by microwave process. Hence it is worth while to evaluate xGnP-supported Pt-based catalyst for fuel cell application. These are microwave expanded and pulverized graphite nanoplatelets as described in U.S. Published Application No. 2004-0127621-A1.
The effect of IL content on the particle size of Pt particles is shown in
Pt/xGnP Nanocomposite with High Concentration of Pt
There are many studies underlining the difficulty of using conventional methods to prepare Pt catalysts with high metal loadings (>20 wt. %) and small particle size at the same time. The metal particle size for supported Pt catalysts with 10 wt. % and 30 wt. % Pt loading were 2.0 nm and 3.2 nm, respectively, but increased to 8.8 nm for a 60 wt. %. Pt catalyst. Therefore, the preparation of highly dispersed and loaded metal catalysts with small particle size has been a challenge. However, the challenge is accomplished by a simple method of adding IM-IL and M-IL in microwave-polyol process. The results for the sample with 60 wt. % Pt on xGnP are shown in
The Surface Area and the Dispersion of Pt Phase.
The specific surface area of Pt can be calculated by the Equation (1):
Assuming that all Pt particles are spherical, the surface average dispersion (D) of Pt, which is the ratio of the surface atoms to the total atoms within the nanoparticles, can be calculated by using Equation (2)˜(4) for particles with small size (d>24 dat):
PtM Alloy Catalyst Supported on xGnP
Strongly electropositive metals such as Au, Pt, Pd, Ag, and Rh can be reduced with a mild reducing agent under ordinary conditions, while more electronegative metals like Cu, Co, Ni, Fe, Sn, W, Cr, and Mo require a very strong reducing agent and frequently extreme conditions of temperature and pressure. The same principle is applied for the synthesis of PtM alloy particles, where M is metal in this case. Higher reaction temperatures and longer reactions are usually required for the preparation of carbon-supported bimetallic PtM (M=Co, Ni, Fe, Sn, Cr, W, and Mo) catalyst which are known to increase the activity of supported catalyst as an electrocatalyst for fuel cell system.
It was found that those bimetallic PtM nanoparticles can be successfully synthesized and deposited on various carbons rapidly with the help of a small quantity of IL which can assist to heat nonpolar solvents above their boiling point. Since the metal powder produced with polyol at a higher temperature is more crystalline than the sample reduced at lower temperature, IL brings another advantage. As an example, TEM images of bimetallic PtNi metal particles dispersed on xGnP included in
Pt and PtM catalysts can be deposited onto various carbon supports by microwave-assisted room temperature ionic liquid heating method. The size of Pt and PtM alloys supported on various carbons can be finely tuned by simply changing the amount of IL, regardless of the Pt and PtM loading level. An IL which is miscible with a reducing agent is more efficient in reducing the size of Pt and PtM than IL immiscible with the agent. The optimal catalytic performance of carbon-supported catalysts at a given concentration of active phase can be found. The improvement of catalytic activities of carbon-supported Pt catalyst is due to the enhanced surface area and dispersion of Pt phase.
The nanoparticle composites are useful as catalysts for chemical reactions, fuel cells, super capacitors and battery components. The very small size and uniformity of dispersion are highly effective for these uses.
It is intended that the foregoing description be only illustrative of the present invention and that the present invention be limited only by the hereinafter appended claims.