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Chem. Mater. 2006, 18, 4710-4712
Supported Pd Catalyst Preparation Using Liquid Carbon Dioxide Jaehoon Kim,† George W. Roberts,*,† and Douglas J. Kiserow†,‡ Department of Chemical and Biomolecular Engineering, North Carolina State UniVersity, Raleigh, North Carolina 27606-7905, and U.S. Army Research Office, Chemical Science DiVision, P. O. Box 12211, Research Triangle Park, North Carolina 27709-2211 ReceiVed June 21, 2006 ReVised Manuscript ReceiVed August 17, 2006
Preparation plays a key role in determining the chemical and physical properties of supported metal catalysts and, ultimately, their performance in catalytic reactions.1 Most commercial supported metal catalysts are prepared by impregnating a porous support with an aqueous solution containing a dissolved salt of the desired metal, followed by drying to remove the solvent and activation of the catalyst by high-temperature treatment. Traditional preparative methods have drawbacks, including slow diffusion of the metal salt into the pores of the support during impregnation because of the high viscosity of water, poor dispersion of metal, redistribution or inhomogeneous distribution of the active compound, and changes in the structure of the support during drying because of high capillary flow resulting from the high surface tension of water.1-4 Deficiencies such as these have led to a search for alternative methods of catalyst synthesis. For example, chemical vapor deposition (CVD) involves the adsorption of gaseous organometallic compounds onto the support, with subsequent heat treatment to activate the catalyst. Although CVD can produce high-quality supported catalysts on various supports,5 the utilization of CVD on an industrial scale is limited by the requirement for high volatility of the organometallic compound, the toxicity of some organometallic compounds and byproducts, the complexity of CVD processes, and high tool cost.6 Recently, preparation of supported metal catalysts using supercritical carbon dioxide (scCO2) has received considerable attention because of the high solubility of some organometallic compounds in scCO2.7-19 Catalysts with high metal * Corresponding author. E-mail:
[email protected]. Tel.: 1-919-5157328. † North Carolina State University. ‡ U.S. Army Research Office.
(1) Schwarz, J. A.; Contescu, C.; Contescu, A. Chem. ReV. 1995, 95, 477. (2) Neimark, A. V.; Kheifets, L. I.; Fenelonov, V. B. Ind. Eng. Chem. Prod. Res. DeV. 1981, 20, 439. (3) Neimark, A. V.; Fenelonov, V. B.; Kheifets, L. I. React. Kinet. Catal. Lett. 1976, 5, 67. (4) Cervello, J.; Garcia de la Banda, J. F.; Hermana, E.; Jimenez, J. F. Chem. Ing. Tech. 1976, 48, 520. (5) Serp, P.; Kalck, P.; Feurer, R. Chem. ReV. 2002, 102, 3085. (6) Iwasawa, Y. In Preparation of Solid Catalysts; Ertl, G., Kno¨zinger, H., Weitkamp, J., Eds.; Wiley-VCH: Weinheim, Germany, 1999; p 427. (7) Zhang, Y.; Kang, D. F.; Aindow, M.; Erkey, C. J. Phys. Chem. B 2005, 109, 2617. (8) Zhang, Y.; Kang, D. F.; Saquing, C.; Aindow, M.; Erkey, C. Ind. Eng. Chem. Res. 2005, 44, 4161.
dispersion and high activity have been prepared from scCO2.11,12,14,16 The unique physical properties of scCO2, including zero surface tension, low viscosity, and tunable density, make it a promising alternative, with the potential to overcome many of the constraints associated with traditional, water-based catalyst preparation techniques. This approach involves impregnation of a support using one or more organometallic compounds dissolved in scCO2. Impregnation is followed by pressure reduction to remove CO2 and by catalyst activation at an elevated temperature, either with or without a reducing agent. Catalyst synthesis via scCO2based methods does not require highly volatile organometallic compounds, as is necessary when using CVD techniques. This communication describes the first use of liquid carbon dioxide (L-CO2) for synthesis of supported metal catalysts. The extremely low surface tension of L-CO2 (from ∼ 4.5 dyn/cm at 0 °C to zero at the critical point) makes it an excellent wetting agent, even on very low surface energy substrates. This low surface tension may help to reduce the redistribution of metal compounds caused by the high surface tension of typical liquid solvents. In addition, the low surface tension of L-CO2 is advantageous in preventing capillary collapse of sol-gels and high-aspect-ratio structures during drying and facilitates the permeation of organometallic compounds into small pores. The low viscosity of L-CO2 (from ∼0.0994 cP at 0 °C to ∼0.044 cP at the critical point) can result in faster diffusion of organometallic compounds into the porous support and can help reduce solute concentration gradients. Higher solute diffusion rates combined with low surface tension make L-CO2 particularly suitable for deposition of metals onto highly porous supports or, more generally, onto surfaces with very small features. Moreover, the milder operating conditions (pressure less than 81.6 atm and temperature less than 30 °C) of a L-CO2-based technique relative to those of the scCO2-based techniques is advantageous. However, a L-CO2 technique can be used only when the metal precursor is sufficiently soluble in this medium. In this study, we demonstrate the use of palladium(II) hexafluoroacetylacetonate (Pd(hfac)2) dissolved in L-CO2 to deposit Pd nanoparticles onto various supports, including a (9) Zhang, Y.; Erkey, C. Ind. Eng. Chem. Res. 2005, 44, 5312. (10) Saquing, C. D.; Kang, D.; Aindow, M.; Erkey, C. Microporous Mesoporous Mater. 2005, 80, 11. (11) Lin, Y. H.; Cui, X. L.; Yen, C.; Wai, C. M. J. Phys. Chem. B 2005, 109, 14410. (12) Lin, Y. H.; Cui, X. L.; Ye, X. R. Electrochem. Commun. 2005, 7, 267. (13) Jiang, R. C.; Zhang, Y.; Swier, S.; Wei, X. Z.; Erkey, C.; Kunz, H. R.; Fenton, J. M. Electrochem. Solid-State Lett. 2005, 8, A611. (14) Haji, S.; Zhang, Y.; Kang, D. F.; Aindow, M.; Erkey, C. Catal. Today 2005, 99, 365. (15) Ye, X. R.; Lin, Y. H.; Wang, C. M.; Engelhard, M. H.; Wang, Y.; Wai, C. M. J. Mater. Chem. 2004, 14, 908. (16) Saquing, C. D.; Cheng, T. T.; Aindow, M.; Erkey, C. J. Phys. Chem. B 2004, 108, 7716. (17) Ohde, H.; Ohde, M.; Wai, C. M. Chem. Commun. 2004, 930. (18) Morley, K. S.; Licence, P.; Marr, P. C.; Hyde, J. R.; Brown, P. D.; Mokaya, R.; Xia, Y. D.; Howdle, S. M. J. Mater. Chem. 2004, 14, 1212. (19) Ye, X. R.; Lin, Y. H.; Wai, C. M. Chem. Commun. 2003, 642.
10.1021/cm061440d CCC: $33.50 © 2006 American Chemical Society Published on Web 09/02/2006
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Chem. Mater., Vol. 18, No. 20, 2006 4711
Figure 2. (a) High-resolution Pd 3d spectra of Pd deposited on a SiOx substrate. SEM images of Pd deposited on SiOx at reduction temperatures of (b) 75 and (c) 150 °C.
temperature was reached, this temperature was maintained for an additional 30 min. Figure 1. Schematic of supported Pd catalyst preparation using L-CO2 impregnation and deposition. (a) Preparation of Pd(hfac)2/L-CO2 solution. Catalyst support pellets are housed in a metal mesh. (b) Impregnation of the support by Pd(hfac)2 via venting gaseous CO2. (c) Introduction of H2. (d) Activation of catalyst via heating.
planar native oxide of silicon (SiOx), a low-surface-area R-alumina, and a high-surface-area γ-alumina. SiOx was purchased from Silicon Valley Microelectronics (San Jose, CA). The SiOx was in the form of nonporous wafers cut with approximate dimensions of 5 cm × 1 cm × 600 µm. Lowsurface-area (SA ) 14.4 m2/g, supplier value) R-alumina and high-surface-area (SA ) 220 m2/g, supplier value) γ-alumina were purchased from Alfa-Aesar (Ward Hill, MA). The alumina was in the form of cylindrical particles, with a 3.25 mm diameter and a 5.5 mm height. Figure 1 is a schematic representing the synthesis of a supported Pd catalyst using L-CO2 impregnation. The SiOx substrate was pretreated in an ultrasonic cleaner using acetone, methanol, and doubly distilled water and then dried with nitrogen. The alumina particles were heated at 300 °C for a minimum of 3 h in air. The pretreated supports were placed in the vessel. Palladium(II) hexafluoroacetylacetonate (Sigma-Aldrich, St. Louis, MO) was then charged to the vessel and L-CO2 (99.99% pure, National Welders, Charlotte, NC) was introduced at 69 atm and 28.5 °C (Figure 1a). The mixture was stirred for at least 12 h to ensure complete homogeneity of the solution. The deposition of Pd(hfac)2 onto the various supports was completed by slowly venting gaseous CO2 from the top of the vessel (Figure 1b) until the liquid was evaporated. A constant venting rate of 0.3 mL/ min was maintained using a pneumatic flow control valve (manufactured by Badger Meter, Inc., Milwaukee, WI), a computer regulated PID (PID ) proportional integral derivative) control loop, and a high-accuracy pressure transducer ((6.8 × 10-3 atm). The pressure in the vessel was then reduced to 1 atm by continuing to vent CO2. At this point, the vessel was purged with hydrogen at 3 atm (purity of 99.999%, National Welders, Charlotte, NC), after which the vessel was pressurized to 40.8 atm (Figure 1c). The reduction and activation of the deposited Pd precursor was then initiated by increasing the temperature of the vessel to a value between 45 and 150 °C (Figure 1d). Once the desired
Figure 2 shows the results for X-ray photoelectron spectroscopy (XPS, Riber LAS-3000, France) and scanning electron microscopy (SEM, Hitachi 4800, Japan) of Pd deposited on SiOx and reduced at various temperatures from 45 to 150 °C. The topmost spectrum in Figure 2a is pure metallic Pd foil and shows 3d5/2 and 3d3/2 doublet peaks that arise at binding energies of 340.9 and 335.6 eV, respectively. The Pd 3d doublet peak positions for Pd deposited on SiOx agree well with those of metallic Pd, and no peaks are observed at higher binding energies, indicating that deposited Pd(hfac)2 is reduced to metallic Pd at temperatures in the range 45-150 °C. As shown in panels b and c of Figure 1, monodisperse nanosize Pd particles were deposited on SiOx. The average Pd particle size was estimated using the microscope images and Matrox Inspector software (Matrox Electronic Systems Ltd., Canada). The average diameter is defined as the arithmetic average diameter of each hemispherically shaped particle in the images. The average Pd particle diameter at a reduction temperature of 75 °C is 5.9 ( 1.1 nm. When the reduction temperature was increased to 150 °C, the average Pd particle diameter increased to 7.9 ( 1.6 nm. Figure 3 shows SEM and scanning transmission electron microscope (STEM, Hitachi HD-2000, Japan) images of Pd particles on the low-surface-area R-alumina support and the high-surface-area γ-alumina support. The reduction temperature was fixed at 75 °C for these preparations. Metal loading was analyzed by an inductively coupled plasma emission spectrometer (ICP-ES, Perkin-Elmer Optima 2000DV, U.S.). Metal loading was controlled by adjusting the Pd(hfac)2 concentration in L-CO2. As metal loading on the support increased, Pd particle size on the R-alumina support also increased. When metal loading was 0.58 wt %, Pd particles with an average diameter of 22 ( 3 nm were deposited on the R-alumina support (Figure 3a). At a metal loading of 1.1 wt %, the average particle diameter was 60 ( 12 nm (Figure 3b). When using the γ-alumina support at a metal loading of 2.45 wt %, 4.4 ( 3.2 nm diameter Pd particles were deposited (Figure 3c). The distribution of particle diameters on γ-alumina is notably broader when compared to those on the R-alumina support. However, the average Pd particle size is much smaller on the γ-alumina.
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Figure 4. (a) EDX analysis of Pd deposited on an R-alumina support and a γ-alumina support. (b) Image of the core-shell structure of 0.58 wt % Pd on γ-alumina.
Figure 3. SEM images of Pd deposited on the R-alumina supports at metal loadings of (a) 0.58 and (b) 1.1 wt %. (c) STEM images of Pd deposited on the γ-alumina supports at a metal loading of 2.45 wt %.
There have been previous studies of the deposition of Pd nanoparticles from solutions of Pd(hfac)2 in supercritical CO2. Howdle et al.18 reported that Pd particles deposited on an extremely high-surface-area (770 m2/g) silica aerogel exhibited a very broad size distribution. Their Pd loading was 8 wt %, and the Pd particle size was generally in the range 6-60 nm, although particles larger than 100 nm were observed. In contrast, Wai et al.15 reported that Pd particles with a much narrower size distribution of 5-10 nm were deposited on multiwall carbon nanotubes. In that study, the Pd loading was 10 wt %. At this point, it is not clear what variables control the Pd particle size distribution. Metal loading, surface composition, surface area, and deposition and reduction conditions all may play a role. Figure 4a shows the Pd distribution along the radius of a pellet, as measured by energy dispersive X-ray spectroscopy (EDX, Hitachi 4800, Japan). With the R-alumina support, the Pd was distributed uniformly. When the γ-alumina support was used, an “egg-shell” structure resulted when the metal loading was low (0.58 wt %; Figure 4b). This distribution is probably the result of most of the Pd(hfac)2 molecules being adsorbed in the outer region of the pellet. If the adsorption of Pd(hfac)2 is strong and rapid and the concentration of Pd(hfac)2 is low, the impregnating solution can become depleted of Pd(hfac)2 before it can diffuse into the center portion of the pellet. For a midrange metal loading (2.45 wt %), the data in Figure 4a show that Pd is distributed
uniformly throughout the pellets. The uniform adsorption of Pd(hfac)2 is possibly a result of the low viscosity and the low surface tension of L-CO2. When the metal loading was increased to 3.98 wt %, there was an increase in the concentration of Pd in the outer regions of the pellets. This may be a result of additional Pd(hfac)2 precipitation near the surface during venting of L-CO2 at higher concentrations. In summary, uniform Pd distribution was obtained only when the lower surface area support was used, with all of the Pd(hfac)2 concentrations tested in this study (0.6-10.5 wt %), or when a midrange Pd(hfac)2 solution concentration (3.58.2 wt %) was used with the high-surface-area support. A simple method has been developed to prepare supported Pd catalysts with uniform Pd particle size and uniform distribution on R-alumina and γ-alumina support pellets. This method involves the use of L-CO2 to deposit Pd(hfac)2 onto the supports, followed by reduction/activation using hydrogen at relatively low temperatures. Particle diameters in the range 20-60 nm were obtained on an R-alumina support by adjusting the solution concentration. On a γ-alumina support, Pd diameters in the range 2-7 nm can also be controlled to some extent by varying the solution concentration. The Pd/R-alumina and the Pd/γ-alumina catalysts prepared from L-CO2 are being studied for the selective hydrogenation of the aromatic rings in polystyrene and have shown activity for this reaction. The ability to control the Pd particle size may have special importance for polymer hydrogenation reactions as a means of controlling the sequence-length distribution of the partially hydrogenated polymer. Acknowledgment. This research was performed while Jaehoon Kim held a National Research Council Research Associateship Award with the U.S. Army Research Office. The authors acknowledge the U.S. Army Research Office, DAAG5598-D-0003, for financial support and the National Science Foundation STC Program, CHE-9876674, for additional support. The authors also thank Dr. Wayne P. Robarge in the Department of Soil Science at the North Carolina State University for ICPES measurements. CM061440D
