J. Phys. Chem. B 2005, 109, 2617-2624
2617
Preparation and Characterization of Ruthenium/Carbon Aerogel Nanocomposites via a Supercritical Fluid Route Ying Zhang,† Dafei Kang,‡ Mark Aindow,‡ and Can Erkey*,† Department of Chemical Engineering, EnVironmental Engineering Program, UniVersity of Connecticut, Storrs, Connecticut 06269, and Department of Materials Science and Engineering, Institute of Materials Science, UniVersity of Connecticut, Storrs, Connecticut 06269-3136 ReceiVed: July 21, 2004; In Final Form: NoVember 5, 2004
Carbon-aerogel-supported ruthenium nanoparticles were synthesized by impregnating carbon aerogels with Ru(acac)3 or Ru(cod)(tmhd)2 from supercritical carbon dioxide (scCO2) solutions, followed by thermal reduction of these precursors. Two different carbon aerogels with pore diameters of 4 and 21 nm were synthesized. The kinetics and the thermodynamics of impregnation of carbon aerogels with the ruthenium coordination complexes were studied. The approach-to-equilibrium data indicated very fast adsorption, and the adsorption isotherms were found to follow the Langmuir model. The impregnated carbon aerogel complexes were reduced thermally at different temperatures between 300 and 1000 °C in the presence of nitrogen. The resulting nanocomposites were characterized using transmission electron microscopy (TEM) and hydrogen chemisorption. TEM micrographs showed that the ruthenium nanoparticles were dispersed homogeneously throughout the porous carbon aerogel matrix, and the average sizes obtained under different conditions ranged from 1.7 to 3.8 nm. Once complete decomposition of the precursor had been achieved, the mean size of the ruthenium particles increased with increasing reduction temperature.
1. Introduction Carbon-supported ruthenium has been investigated extensively as a catalyst for many reactions including ammonia synthesis,1,2 ammonia decomposition,3 selective hydrogenation of D-glucose to D-sorbitol,4 hydrogenation of arabinonic acid and lactones to arabitol,5 hydrogenation of cinnamaldehyde,6 hydrogenation of carbon monoxide and carbon dioxide,7 and hydrodesulfurization of petroleum fractions.8 It has also been studied as an electrode component for electrochemical capacitors and was found to display a high energy density.9-11 Furthermore, the sensitive response of the ruthenium electrode to the direct oxidation of nitric oxide may be useful in the development of sensors for biomedical applications.12 It is well-known that the structure and properties of carbon supports may have a dramatic influence on the properties of carbon-supported metals.13 Carbon aerogels are a relatively new class of materials with low density, continuous porosity, high surface area, and high electrical conductivity. Carbon aerogels are synthesized by the pyrolysis of organic aerogels, which are produced by the polycondensation reactions of organic monomers such as resorcinol and formaldehyde. Because the properties of carbon aerogels, such as surface area, pore size, and density, can be controlled at the molecular level, carbon-aerogelsupported metal composites can be tailored for various applications. Metals can be incorporated into carbon aerogels by adding appropriate metal precursors to the reaction mixture for synthesis of organic aerogels.14-16 However, this method suffers from two major drawbacks. The first is the agglomeration of metal particles due to the high temperatures required for the pyrolysis of organic aerogels. The second is that the metallic precursors * Corresponding author. E-mail:
[email protected]. † Environmental Engineering Program. ‡ Institute of Materials Science.
in solution interfere with the polymerization chemistry resulting in organic aerogels with undesirable properties. Dresselhaus et al.17 developed a new strategy for the preparation of metaldoped carbon aerogels in which resorcinol is replaced with the potassium salt of 2,4-dihydroxybenzoic acid in the sol-gel process, producing K+-doped hydrogels. The potassium ions in the gel can be replaced with the desired metal ion through an ion-exchange process, and the gels can then be dried and carbonized to generate metal-doped carbon aerogels. Although a higher amount of metal could be incorporated into the carbon aerogel matrix with more uniform particle size distribution, agglomeration was still a problem. The particle size of copper obtained by this method ranged from 10 to 50 nm. Miller et al.9,10 developed a chemical vapor impregnation method to prepare ruthenium-loaded carbon aerogels and obtained highly dispersed ruthenium particles (2-3 nm) incorporated in the carbon aerogel matrix. An alternative is to use a supercritical fluid deposition method to incorporate metal nanoparticles into porous substrates. This supercritical fluid deposition process involves the dissolution of an organometallic precursor in a supercritical fluid and the exposure of a solid substrate to this solution. After impregnation of the substrate with the precursor, the organometallic precursor is converted to metal form by injecting a reducing agent such as hydrogen into the supercritical solution,18,19 or by first depressurizing the supercritical solution and then subjecting the precursor/substrate composite to hydrogen20,21 or heat treatment under inert atmosphere.22 Supercritical deposition technology was first demonstrated by Watkins et al.18,19 for the preparation of metallic films on different solid supports. Morley et al. incorporated palladium and silver particles into silica aerogel.20,21 Their results demonstrated large metal particle sizes and wide size distribution, which indicated a high degree of aggregation. More recently, highly dispersed Pt particles with an average
10.1021/jp0467595 CCC: $30.25 © 2005 American Chemical Society Published on Web 01/26/2005
2618 J. Phys. Chem. B, Vol. 109, No. 7, 2005 size as small as 1 nm were incorporated into carbon aerogels by our group through the supercritical deposition route followed by thermal reduction under an inert atmosphere.22 In this study, we were able to prepare ruthenium-incorporated carbon aerogels by the supercritical deposition method using two different organometallic Ru precursors, bis(2,2,6,6-tetramethyl-3,5-heptanedionato)(1,5-cyclooctadiene) ruthenium (Ru(cod)(tmhd)2) and ruthenium acetylacetonate (Ru(acac)3). We investigated both the kinetics and the thermodynamics of incorporation of organometallic precursors into porous substrates from supercritical carbon dioxide solutions to understand the incorporation process and the factors affecting the capacity of substrates. We also considered the factors that may influence the structure and size distribution of the ruthenium nanoparticles. These included: (i) the metal precursor complex used in the impregnation process; (ii) the pore structure of the carbon aerogels; (iii) the metal content; and (iv) the reduction temperature of the ruthenium precursors. The structures of the materials were characterized primarily by transmission electron microscopy (TEM) and hydrogen chemisorption techniques. 2. Experimental Section Materials. Carbon aerogels were synthesized in our lab. The synthesis details were described in a previous paper.22 Ru(acac)3 and Ru(cod)(tmhd)2 were purchased from STREM. All of the chemicals were used as received. Carbon dioxide (99.998%) and nitrogen (99.999%) were purchased from Airgas. Apparatus and Procedures. The setup for the preparation of supported ruthenium nanoparticles is given elsewhere.22 Briefly, the desired amount of ruthenium precursor, Ru(acac)3 or Ru(cod)(tmhd)2, a stirring bar, and the carbon aerogel were placed into a high-pressure vessel. The vessel was sealed and heated to 80 °C. Subsequently, it was charged slowly with CO2 up to a pressure of 27.6 MPa and kept under these conditions for 24 h. The vessel was then depressurized slowly through a restrictor into the atmosphere. After the vessel was cooled, the precursor/substrate composite was removed. The actual adsorbed amount of the precursor was determined by the weight change of the substrate using an analytical balance (Adventure model AR2140) accurate to (0.1 mg. The composite was then transferred into a quartz tube, which was introduced into a tube furnace. The impregnated precursor was reduced thermally at predetermined temperatures between 300 and 1000 °C in the presence of nitrogen gas with a flow rate of 100 cm3/min. The thermal decomposition was continued for 6 h. The ruthenium loading was calculated from the mass of precursor adsorbed during the impregnation step. TGA of the ruthenium precursors and the impregnated aerogels was carried out over the range 25-600 °C at a rate of 5 °C/min under nitrogen using a Hi-Res TGA 2950 Thermogravimetric Analyzer (TA Instruments). TGA was also performed on both the ruthenium-loaded and the naked carbon aerogels over the range of 100-800 °C at a heating rate of 5 °C/min under air. Hydrogen chemisorption measurements were carried out at 25 °C using an Omnisorp 100CX instrument. The sample was pretreated under hydrogen at 250 °C for 1 h to reduce any oxidized ruthenium, and subsequently evacuated to 10-5 bar at 300 °C for 1 h. The amount of chemisorbed hydrogen was determined by extrapolating the isotherms obtained in the first and repeat analyses to zero pressure, which corresponds to the total adsorption and the adsorption due to physisorption plus weak chemisorption, respectively. The values of the ruthenium dispersion were calculated by assuming a spherical geometry of ruthenium particle and Hads/Ru ) 1 stoichiometry.
Zhang et al. TABLE 1: Pore Structural Properties of Carbon Aerogelsa BET
BJH
CAs
Stot (m2/g)
D (nm)
Vtot (cm3/g)
Vmic (cm3/g)
t-plot Smic (m2/g)
Vmic/Vtot (%)
CA-21 CA-4
629.7 741.2
20.7 4.1
3.63 0.78
0.12 0.12
233 252.4
3.3 15.4
a The pore structure properties were obtained from nitrogen physisorption. In this table, Stot refers to total surface area calculated from the linear portion of the BET equation; D refers to average pore diameter from the Barrett, Joyner, and Halenda (BJH) method; Vtot refers to total pore volume; Vmic refers to micropore volume; and Smic refers to micropore surface area.
Specimens for TEM were prepared by transferring crushed fragments of the carbon aerogel samples onto copper mesh grids coated with a holey carbon film. These were examined in a JEOL 2010 FasTEM 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
