Ind. Eng. Chem. Res. 2005, 44, 4161-4164
4161
Supported Platinum Nanoparticles by Supercritical Deposition Ying Zhang,† Dafei Kang,‡ Carl Saquing,† Mark Aindow,‡ and Can Erkey*,† Departments of Chemical Engineering and Metallurgy and Materials Engineering, Institute of Materials Science, University of Connecticut, Storrs, Connecticut 06269
Supported nanoparticles of platinum on a wide variety of substrates, including carbon aerogel, carbon black, silica aerogel, silica, γ-alumina, and Nafion 112 film, were synthesized via a supercritical fluid route. The porous substrates and Nafion film were impregnated with an organometallic precursor, dimethyl(1,5-cyclooctadiene)platinum(II) (PtMe2COD), from a supercritical carbon dioxide (scCO2) solution at 80 °C and 27.6 MPa. After depressurization, the impregnated organometallic precursor was reduced to elemental platinum by heat treatment in the presence of nitrogen gas. The resulting nanocomposites were investigated by transmission electron microscopy (TEM), which revealed uniformly dispersed platinum particles on each of the substrates with average particle sizes ranging from 1.2 to 6.4 nm and a narrow particle size distribution. A comparison of nanocomposites produced under different conditions showed that both the metal contents and the particle sizes are controllable. 1. Introduction Supported metal nanocomposites have unique electronic, optical, electrooptical, electrochemical, and catalytic properties1 that are directly related to the specific concentration, size, and distribution of the metal particles within their host environment. There are several ways to synthesize supported nanoparticles,2 including impregnation,3 deposition-precipitation,3 chemical vapor impregnation,4 sol-gel processing,5-7 or microemulsion generation using organic stabilizing agents.2 However, control over particle size, distribution, and metal concentration in the composite is challenging.1 Here, we utilize a supercritical fluid as a processing medium to incorporate metal nanoparticles into different substrates. A supercritical fluid (SCF) is a fluid that has been heated and compressed above its critical temperature and pressure. The thermophysical properties of a SCF are intermediate between those of a gas and a liquid and can be adjusted by slight changes in temperature and/or pressure. Among the supercritical fluids, supercritical carbon dioxide (scCO2) (Tc ) 31.1°C, Pc ) 71.8 bar) is particularly attractive for a wide variety of applications because it is chemically inert, nontoxic, and environmentally acceptable and leaves no residue in the treated medium. Such properties have been exploited in the development of novel processes for the synthesis of nanostructured materials.1,8-14 We recently demonstrated that platinum (Pt) nanoparticles could be incorporated into mesoporous carbon aerogels (CAs) via a supercritical fluid route.15,16 In this method, CAs were contacted with a solution of an organometallic precursor, dimethyl(1,5-cyclooctadiene)platinum(II) (PtMe2COD), dissolved in scCO2. After adsorption onto the CA surface, the precursor was reduced, resulting in CA-supported Pt nanoparticles with a very uniform size distribution. In this study, we demonstrate that this technique also results in a very uniform distribution of platinum nanoparticles on a * To whom correspondence should be addressed. Tel.: 860486-4601. Fax: 860-486-2959. E-mail:
[email protected]. † Department of Chemical Engineering. ‡ Department of Metallurgy and Materials Engineering.
wide variety of substrates other than CAs. Specifically, we prepared nanoparticles of platinum supported by carbon black (CB), silica aerogel (SA), silica (SiO2), γ-alumina (γ-Al2O3), and Nafion 112 film (Nafion) by impregnation with PtMe2COD from scCO2 followed by thermal reduction. The morphology of these supported metal nanocomposites was characterized by highresolution transmission electron microscopy (HRTEM). 2. Experimental Section 2.1. Materials. Resorcinol (99%), formaldehyde (37%), and sodium carbonate (99.99%) were purchased from Aldrich. PtMe2COD was purchased from Strem. Carbon aerogel substrates with an average pore diameter of 20 nm were manufactured in-house. The details of the synthesis route for this CA is described elsewhere.15 Carbon black powder (Vulcan XC-72R) was purchased from Cabot International. The silica aerogel (random 0.2-2.0-cm pieces) was purchased from MarkeTech International, Inc. Both silica and γ-alumina pellets were donated by Saint-Gobain NorPro, Inc. All of the chemicals were used as received except for the alumina. Before γ-Al2O3 was used as a substrate, it was dried at 300 °C for 2 h. 2.2. Preparation of Supported Nanoparticles. A schematic diagram of the setup used for impregnation is given in Figure 1. The 54-mL vessel is custom manufactured from stainless steel and is fitted with two sapphire windows (1-in. i.d.; Sapphire Engineering, Inc.), poly(ether ether ketone) O-rings (Valco Instruments, Inc.), a T-type thermocouple assembly (Omega Engineering, DP41-TC-MDSS), a pressure transducer (Omega Engineering, PX300-7.5KGV), a vent line, and a rupture disk assembly (Autoclave Engineers). For each run, a certain amount of organometallic precursor PtMe2COD, a stirring bar, and a certain amount of substrate were placed into the vessel. A stainless steel screen was used to separate the substrate from the stirring bar. The vessel was sealed and heated to 80 °C by a circulating heater/cooler (Fischer Scientific Isotemp Refrigerated Circulator model 90) via a machined internal coil. It was then charged slowly with CO2 from a syringe pump (ISCO, 260D) to a pressure of 27.6 MPa
10.1021/ie050345w CCC: $30.25 © 2005 American Chemical Society Published on Web 04/27/2005
4162
Ind. Eng. Chem. Res., Vol. 44, No. 11, 2005
ages. Chemical microanalysis was performed in situ using an EDAX Phoenix atmospheric thin-window energy-dispersive X-ray spectrometer (EDXS). 3. Results and Discussion
Figure 1. Schematic diagram for supercritical impregnation.
and kept at these conditions for 24 h. During this process, all precursor added to the vessel was dissolved in scCO2 or adsorbed into the substrate. The vessel was then depressurized slowly (0.46 MPa/min) through a restrictor into the atmosphere. After the vessel had cooled, the precursor/substrate composite was removed. The amount of precursor adsorbed was determined by the weight change of the substrate using an analytical balance (Adventure model AR2140) accurate to (0.1 mg. Subsequently, the precursor/substrate composite was placed in an alumina process tube (Cole-Parmer) with dimensions of 25 mm (i.d.) × 28 mm (o.d.) × 1219.2 mm (length), and the tube was placed into a tube furnace (model F1125 Thermolyne). The impregnated organometallic precursor was reduced thermally at 200 or 300 °C in the presence of nitrogen gas with a flow rate of 100 cm3 min-1 for 6 h. TGA scans of pure organometallic precursor and of precursor-substrate composites under a nitrogen atmosphere were used to select the minimum thermal reduction temperatures. 2.3. Determination of Metal Content by Acid Digestion. Samples of supported platinum were digested with 5 mL of HNO3 and 5 mL of HCl in a hotblock tube at 95 °C for 4 h. After being kept at room temperature overnight, 2 mL of HF was added to the sample solutions and digested at 95 °C for 2 h. The resulting solutions were analyzed by inductively coupled plasma spectroscopy/optical emission spectroscopy (ICP/OES, Perkin-Elmer, Optima 3300XL with AS 91 autosampler) for their metal content. 2.4. Transmission Electron Microscopy (TEM) Analysis. The morphology of these supported metal nanocomposites was characterized by high-resolution transmission electron microscopy (HRTEM). Specimens for HRTEM examination were prepared by carefully crushing the metal/substrate samples with a mortar and pestle set. The platinum-impregnated Nafion film was frozen using liquid nitrogen before being crushed. The resulting powders were suspended in a volatile solvent, except for the SA-supported platinum particles, and ultrasonicated to obtain a uniform suspension. One or two drops of this suspension were deposited onto a copper mesh grid coated with a holey carbon film (Quantifoil Micro Tools GmbH). This approach produced structural changes in the SA-supported particles, and so, for this material, the particles were transferred by dry dipping of the grid into the powdered sample. The HRTEM specimens were allowed to dry completely before examination in a JEOL 2010 FasTEM instrument operating at 200 kV. This instrument is equipped with a high-resolution objective lens pole-piece (spherical aberration coefficient Cs ) 0.5 mm), giving a pointto-point resolution of
