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Ind. Eng. Chem. Res. 2008, 47, 3150-3155
Adsorption of Dimethyl(1,5-cyclooctadiene)platinum on Porous Supports in Supercritical Carbon Dioxide Ortrud Aschenbrenner,*,†,‡ Nicolaus Dahmen,† Karlheinz Schaber,§ and Eckhard Dinjus† Institute for Technical Chemistry, Forschungszentrum Karlsruhe, P.O. Box 3640, 76021 Karlsruhe, Germany, and Institute for Technical Thermodynamics and Refrigeration, UniVersity of Karlsruhe, Engler-Bunte-Ring 21, 76131 Karlsruhe, Germany
The adsorption of dimethyl(1,5-cyclooctadiene)platinum on porous supports in supercritical carbon dioxide was investigated. Silica gel tablets and a monolithic material were used as the solid support. An experimental method was developed for the investigation of adsorption kinetics where the substance was allowed to dissolve in the supercritical fluid before coming in contact with the solid support. Online UV spectroscopy was used to observe the decrease in bulk concentration in the supercritical solution. Adsorption experiments were carried out at 333 and 353 K at a constant pressure of 15 MPa. Additionally, the solubility of the platinum compound at the experimental conditions was determined. The influence of concentration, temperature, and support material on the adsorption was investigated. Introduction In recent years, there has been increasing interest in applications of supercritical fluids for syntheses such as the production of pharmaceuticals, thin metal coatings, and nanocomposites. Particularly, supercritical carbon dioxide has been explored as a nontoxic alternative solvent with favorable properties. Processes such as the deposition of substances on a solid support can be carried out in supercritical carbon dioxide. This has several advantages, such as the usually high purity of the produced coatings or particles, the easy separation of the unused precursor substance from the supercritical fluid, meaning very little loss of precursor substance, and the mild process conditions that are due to the low critical temperature of carbon dioxide. The deposition of dissolved compounds on solid support materials in supercritical fluids is a complex process. An important part of this process is the diffusion and adsorption of the dissolved substance onto the solid surface. To optimize the process conditions in the supercritical deposition process, knowledge of the influence of conditions such as pressure, temperature, and precursor concentration on the adsorption is essential. Some literature studies describe the influence of temperature and pressure on adsorption in supercritical carbon dioxide. Gla¨ser and Weitkamp1 investigated the adsorption of naphthalene on zeolite in supercritical CO2. They found that the uptake of naphthalene at a constant concentration and temperature was nearly independent of pressure. A different behavior was found for the adsorption of dimethylnaphthalene on zeolite in supercritical CO2, where the adsorbed amount of substance decreased with increasing pressure.2 Similar results were reported for the adsorption of various organic substances on active carbon, where a decrease in the adsorbed amount was caused by increasing pressure and increasing temperature.3 The only study on the adsorption of a metal precursor on a porous support in supercritical CO2 so far was carried out by * To whom correspondence should be addressed. E-mail:
[email protected]. Tel.: +44 114 2227572. Fax: +44 114 2227501. † Institute for Technical Chemistry. ‡ Current address: Department of Chemical and Process Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK. § University of Karlsruhe.
Zhang et al.4, who also investigated the kinetics of the adsorption. The adsorption was very fast, and the equilibrium loading increased with increasing precursor concentration. However, the process of adsorption in supercritical fluids is still largely unexplored.5 There is particularly little information on the kinetics of the adsorption process. In this study, the adsorption of an organometallic platinum compound on porous support materials in supercritical carbon dioxide was investigated. Special interest lies in the increase of the uptake with time. Therefore, a new experimental method was developed. The influence of support material, compound concentration, and temperature is reported. Materials and Methods Materials. High-purity grade 6.0 carbon dioxide (Messer Griesheim) was used in the experiments. Dimethyl(1,5-cyclooctadiene)platinum (Pt(cod)(me)2; Strem) with a purity of 99% was used without further purification. Two different support materials were used. A monolithic cordierite with washcoat, as it is typically used for catalytic converters in cars, was kindly provided by the Institute for Chemical Process Engineering at the University of Stuttgart. The material was used in pieces of 20 mm length and 7 mm × 7 mm width, containing 25 parallel channels per piece. Silica gel tablets with a diameter of 11 mm were obtained from Fluka. The tablets were cut in half, and four such halves were used for each experiment. The materials were dried at 120-140 °C for several days prior to use. Apparatus. A new experimental method was developed to measure adsorption kinetics in supercritical carbon dioxide. A scheme of the experimental setup is shown in Figure 1. The device consists of a fluid circuit with a gear pump (Ismatec), a 3 cm3 cell with 2 µm frits (Suprex) to contain the organometallic substance, a 3 cm3 adsorption cell (Suprex) containing the support material, and a high-pressure UV spectroscopy cell (Sitec). The adsorption cell can be connected to or disconnected from the circuit via a two-way valve (Valco). Carbon dioxide is supplied to the circuit via a syringe pump (Isco). The carbon dioxide supply is additionally connected with the two-way valve to allow for a separate filling of the adsorption cell. The UV spectroscopy cell is connected with a UV-vis spectrometer (Zeiss) so that the decrease of the
10.1021/ie071221h CCC: $40.75 © 2008 American Chemical Society Published on Web 04/02/2008
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Figure 1. Schematic of the experimental setup for the investigation of adsorption kinetics in supercritical solution. Legend: 1, cell; 2, adsorption cell; 3, UV spectroscopy cell; 4, gear pump; and 5, two-way valve.
concentration of the organometallic complex in the fluid can be monitored online. The apparatus was placed in a 45 dm3 water bath heated by a thermostat (Haake). The temperature was measured with a type K thermocouple (Thermocoax), and the pressure was measured with a pressure sensor (EBM Brosa). The maximum temperature was 353 K, and the maximum pressure was 30 MPa. The volume of the fluid circuit was 20.0 and 16.7 cm3 with and without the adsorption cell, respectively. Measurement. Prior to each experiment, the apparatus was heated to the experimental temperature and filled with pure carbon dioxide up to the experimental pressure. A UV spectrum was then taken and used as reference spectrum for the subsequent experiment. This procedure was necessary to obtain a sufficiently high accuracy of the online spectroscopic analysis. The adsorption cell was disconnected from the circuit and filled with a known amount of support material. The cell remaining in the circuit was then filled with a known amount of Pt(cod)(me)2. The concentration of this substance in the supercritical fluid, which could be calculated from the amount of substance and the amount of carbon dioxide at a given temperature and pressure present in the volume of the apparatus, had to be lower than the saturation concentration at the respective pressure and temperature. To ensure this, separate experiments were made to examine the solubility of the platinum compound in carbon dioxide as described in the next section. The circuit as well as the adsorption cell were then flushed with gaseous carbon dioxide, sealed, heated to the experimental temperature, and filled with carbon dioxide up to the experimental pressure. The gear pump was switched on with a flow rate of 5 cm3 min-1 to start a circulation and to ensure the mixing of the fluid in the circuit, bypassing the adsorption cell. The dissolution of the platinum compound in the supercritical fluid was observed online by monitoring the increase in concentration recorded by the UV spectrometer. A homogeneous solution was obtained when the concentration recorded by the spectrometer remained constant. This took about 1-2 h. The two-way valve was then switched, and the adsorption cell was connected to the circuit. At the same time, the automatic recording of data with the UV spectrometer was started. Spectra were recorded in intervals of 20 s to monitor the decrease in the concentration of the dissolved platinum compound. At the end of the experiment, the adsorption cell was disconnected from the circuit, and the carbon dioxide was released carefully. The support material was weighed to compare the weight gain with the obtained adsorption data and thus roughly to check the consistency of the experimental data. Experiments were performed at a pressure of 15 MPa and
Figure 2. Calibration curves for online UV spectroscopy of Pt(cod)(me)2 in carbon dioxide at 15 MPa: (a) 333 K and (b) 353 K.
temperatures of 333 and 353 K. The concentration of Pt(cod)(me)2 was varied. Equal experiments were carried out without support material to identify the decrease in compound concentration that arises from the added volume after connecting the adsorption cell to the circuit. Solubility Experiments. The solubility of Pt(cod)(me)2 in supercritical carbon dioxide at the experimental conditions (15 MPa and 333 and 353 K) was investigated in separate experiments. The adsorption cell was replaced by a sampling tube with a known volume, and the cell in the circuit was filled with an excess amount of the platinum compound. After obtaining a saturated solution, which could be observed with online UV spectroscopy, the sampling tube was disconnected from the circuit and decompressed into hexane, and its content was washed out with hexane into a known volume. The amount of Pt(cod)(me)2 in the sample was analyzed by UV spectroscopy in hexane. From this, the solubility in supercritical carbon dioxide could be calculated. Data Analysis. The UV absorption of Pt(cod)(me)2 at 285 nm was used for analysis of the concentration. This value presents a maximum in the UV spectrum at both 333 and 353 K at 15 MPa in carbon dioxide. Figure 2 shows the calibration curves for the platinum compound in carbon dioxide at 333 and 353 K and 15 MPa. For each experiment, the decrease in concentration versus time was calculated from the UV absorption data. The decrease in concentration due to the increase in fluid volume after connecting the adsorption cell to the circuit was determined from experiments without support material. Such experiments were performed for each concentration and temperature. The volume of the support material thereby was negligible as compared to the volume of the circuit. The resulting curves were approximated by a linear fit method according to eq 1. For the model, the initial and final concentrations of the dissolved substance, ci and cf, respectively, were calculated from the amount of Pt(cod)(me)2 used in the experiment, m0, and the
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Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 Table 2. Absolute Errors |∆B| in Pt(cod)(me)2 Uptake for Various Experimental Conditions and Adsorption Times t support material
temperature (K)
|∆B| (t < te) (mg g-1)
|∆B| (t g te) (mg g-1)
silica gel monolith monolith
333 333 353
4-8 7-16 9-12
4-5 7-8 9
Table 3. Solubility of Pt(cod)(me)2 in Carbon Dioxide
Figure 3. Decrease in concentration of Pt(cod)(me)2 in carbon dioxide due to the increase in volume of the apparatus after connecting the adsorption cell to the circuit; experiment with 30 mg of Pt(cod)(me)2 at 333 K and 15 MPa and no support material. Table 1. Fitting Parameters ts and te for the Model Description of the Decrease in Pt(cod)(me)2 Concentration Caused by the Increase in Volume after Connecting the Adsorption Cell to the Apparatusa temperature (K)
m0 (mg)
ts (s)
te (s)
333 353 353
20-60 20 30
120 60 120
400 300 600
circuit volume Vc (20.0 cm3) and Vc,0 (16.7 cm3) with and without the adsorption cell, respectively (eq 2):
{
c0(t) ) ci + cf
(for 0 < t < ts) (cf - ci)(t - ts) (te - ts) (for te < t) m0 ci ) Vc,0 cf )
m0 Vc
(for ts < t < te)
pressure (MPa)
CO2 density (kg m-3)
solubility (mg cm-3)
333 333 333 353 353 353
13.9 15.0 16.1 14.0 15.0 16.0
556.4 604.1 640.5 383.4 427.2 468.4
3.14 4.36 5.81 1.81 2.96 3.98
support material, ms. This gives the specific uptake B in grams of substance per gram of support material (eq 4):
ma ) (c0 - ca)Vc B)
a Parameters were obtained at different temperatures and different amounts (m0) of Pt(cod)(me)2 used in the experiment.
ci
temperature (K)
(1)
(2a) (2b)
Fitting parameters were the start time ts and the end time te of the decrease in concentration. The start time represents the time between switching the two-way valve and the visible registration of the decrease in concentration. This delay is caused by the distance between the adsorption cell and the spectroscopy cell. Figure 3 shows an example of the experimental curve together with the linear fit. The fitting parameters were found to be independent of the concentration of the platinum compound at 333 K, whereas they showed a significant dependence on concentration at the higher temperature of 353 K. This change in behavior with temperature may be caused by the temperature dependence of the properties of the supercritical solution, as the fitting parameters reflect both the diffusivity of the dissolved compound and the flow conditions that are related to fluid density and viscosity. The parameters obtained from the experiments are listed in Table 1. To account for the decrease in concentration due to the adsorption cell volume, the concentrations ca obtained for the adsorption experiments were subtracted from the concentrations c0 calculated from the linear model at the respective experimental conditions. The resulting concentrations were multiplied by the volume of the circuit Vc (20.0 cm3) to obtain the amount of adsorbed substance ma (eq 3) and divided by the amount of
ma ms
(3) (4)
Accuracy. The temperature in the experiments was kept constant within (0.5 K. The accuracy of pressure was (0.1 MPa. The reproducibility of the uptake curves was about (3 mg g-1. The calculated maximum relative errors are shown in Table 2. The errors are higher for the initial period of the uptake t < te because of the uncertainty in the concentration decrease arising from the increase in volume, which is calculated with the model for experiments without support material. Comparison of the uptake at the end of the experiments with the values calculated from the weight of the support material after decompression showed differences of up to 50%. However, parts of the adsorbed Pt(cod)(me)2 can be expelled from the support material during the decompression, or dissolved Pt(cod)(me)2 can precipitate on the support, so that the results obtained by weighing the support material can neither be taken as accurate nor true. Results and Discussion Solubility. The solubility data obtained for Pt(cod)(me)2 in carbon dioxide are presented in Table 3. Adsorption Equilibrium. In the experiments with monolithic support material, a constant concentration of Pt(cod)(me)2 in the apparatus was reached after 1-2 h of adsorption. The system was then assumed to be in equilibrium. The thus obtained equilibrium amounts of adsorbed Pt(cod)(me)2 and the corresponding Pt(cod)(me)2 concentrations in the fluid phase are shown in Figure 4. In the case of the silica gel tablets, equilibrium was not reached even after 3 h. Extra long runs were attempted but turned out to be inaccurate because of a gradual shift in the baseline of the UV spectroscopic analyzer. Table 4 gives an overview of the experimental conditions and results. According to Figure 4, the adsorption capacity was reached at a concentration of 0.75 mg cm-3 at 333 K. Higher concentrations did not result in a higher equilibrium loading at this temperature. However, the adsorbed amount of Pt(cod)(me)2 in equilibrium increases with increasing temperature at a constant concentration. While the adsorption capacity is ca. 37 mg g-1 at 333 K, it is obviously higher at 353 K. The adsorption isotherm, which represents the relation between the adsorbed
Ind. Eng. Chem. Res., Vol. 47, No. 9, 2008 3153 Table 4. Summary of Experimental Conditions and Results for Adsorption of Pt(cod)(me)2 on Porous Support Materials in Carbon Dioxide at 15 MPa material
temperature (K)
initial amount of Pt(cod)(me)2 (mg)
initial concentration of Pt(cod)(me)2 (mg cm-3)
equilibrium concentration of Pt(cod)(me)2 (mg cm-3)
equilibrium adsorption of Pt(cod)(me)2 (mg g-1)
monolith monolith monolith monolith monolith monolith silica gel silica gel silica gel silica gel silica gel
333 333 333 333 353 353 333 333 333 333 333
20 30 40 60 20 30 20 30 40 50 60
1.0 1.5 2.0 3.0 1.0 1.5 1.0 1.5 2.0 2.5 3.0
0.32 0.75 1.2 2.2 0.20 0.50 36a
a
These values were recorded at the end of the experiment but do not represent an equilibrium.
Figure 4. Adsorbed amount of Pt(cod)(me)2 versus concentration in the fluid phase in adsorption equilibrium for a monolithic support material in carbon dioxide at 333 and 353 K and 15 MPa.
Figure 5. Uptake of Pt(cod)(me)2 versus time for monolithic support material and silica gel tablets at initial amounts of 20 and 60 mg of Pt(cod)(me)2 in carbon dioxide at 333 K and 15 MPa.
amount and the concentration in the fluid phase, is shifted to higher equilibrium loadings for higher temperatures. This behavior is unexpected because adsorption is usually an exothermic process. Ryu et al.3 found a decrease in the adsorbed amount of volatile liquids in supercritical carbon dioxide with increasing temperature at a constant concentration and fluid density. However, the pressure was kept constant in our experiments so that the density of the supercritical fluid was significantly lower at the higher temperature. A decrease in density of the supercritical solution at constant temperature has been shown to result in a higher adsorption amount of the dissolved substance.2,3 It is possible that the effect of the decrease in density outweighs the effect of the increase in temperature in our experiments, resulting in a higher adsorbed
Figure 6. Uptake of Pt(cod)(me)2 versus time for silica gel tablets at various initial amounts of Pt(cod)(me)2 in carbon dioxide at 333 K and 15 MPa.
amount at higher temperatures. Another possible explanation for the increase in adsorption with temperature might be that the adsorbed amount is not dominated by adsorption equilibrium but by mass transport. Increasing the temperature enhances mass transport due to higher diffusion coefficients and therefore may result in an increase in the adsorbed amount. Influence of Support Material. The uptake curves for two different support materials at a temperature of 333 K for a high and a low concentration of Pt(cod)(me)2 are shown in Figure 5. For the monolithic support, the uptake increases sharply from the beginning at both concentrations. An adsorption equilibrium is reached after about 1 h. The silica gel tablets exhibit a much slower uptake of the platinum compound, and even after several hours, equilibrium is not reached. The rate of the uptake is higher for the monolithic material, whereas the total equilibrium loading and therefore the adsorption capacity are higher for the silica gel tablets. Table 5 lists some properties of the two support materials. An important difference is the geometric shape, which leads to different flow characteristics. Because of the parallel channels in the monolithic material, the fluid solution can flow through the channels, whereas the fluid can only flow over the outer surface of the compact silica gel tablets. For the same reason, the external surface area of the monolithic material is much higher than that of the silica gel tablets, leading to a higher contact area between solution and support. This is probably the reason for the higher uptake rate for the monolithic material as compared to the silica gel tablets. According to Table 5, the internal surface area of the porous structure is about 3.5 times higher for the silica gel tablets than for the monolithic material. This can explain the higher adsorption capacity of the silica gel tablets as compared to the
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Table 5. Properties of Support Materials Used in This Studya
shape mass average mass volume external surface area specific external surface area BET surface area a
monolith
silica gel
one block with parallel channels 400-450 mg 425 mg 0.48 cm3 0.00256 m2 0.00602 m2 g-1 59.5 m2 g-1
two porous tablets cut in half 760-800 mg 775 mg 0.76 cm3 0.000833 m2 0.00107 m2 g-1 217 m2 g-1
BET surface area was determined by nitrogen adsorption.
Figure 7. Uptake of Pt(cod)(me)2 versus time for monolithic support material at various initial amounts of Pt(cod)(me)2 in carbon dioxide at 333 K and 15 MPa.
Figure 8. Uptake of Pt(cod)(me)2 in carbon dioxide versus time for the monolithic support material at 333 and 353 K and 15 MPa at different absolute amounts of Pt(cod)(me)2 in the apparatus; respective concentrations of Pt(cod)(me)2 per mole of carbon dioxide are given in parentheses.
monolithic material. This indicates that the platinum compound dissolved in the supercritical fluid actually penetrates into the pores of the support material. Influence of Concentration. The uptake of Pt(cod)(me)2 versus time at 333 K is shown in Figure 6 for the silica gel tablets and in Figure 7 for the monolithic material. Both support materials exhibit a similar influence of concentration on the uptake curves. Increasing the concentration of the platinum compound in supercritical carbon dioxide increases both the rate of uptake and the equilibrium loading. However, there seems to be a limit for the uptake that cannot be exceeded with a further increase of concentration. In the case of the monolithic material, an increase in the amount of Pt(cod)(me)2 from 40 to 60 mg does not result in a higher uptake. The same is valid for the
silica gel tablets at an increase from 50 to 60 mg of Pt(cod)(me)2. Thus, the equilibrium loading increases with increasing concentration until it reaches a limit value. This behavior reflects the shape of the adsorption isotherm. The limit value is the maximum adsorption capacity for the respective support material at a given temperature. The value is higher for the silica gel tablets, probably due to the higher internal surface area as compared to the monolithic material (Table 5). Influence of Temperature. Figure 8 shows the influence of temperature on the uptake of Pt(cod)(me)2 on the monolithic material. For the same initial weight of the platinum compound, the uptake increases with increasing temperature. However, the density of the supercritical fluid strongly depends on the temperature. For pure carbon dioxide, the density is 427 kg m-3 at 353 K and 604 kg m-3 at 333 K.6 This means that the concentration of Pt(cod)(me)2 per amount of carbon dioxide is about 40% higher at 353 K. The respective concentration values also are given in Figure 8. The experiments with 30 mg of substance at 333 K and 20 mg of substance at 353 K represent similar concentrations of about 0.3 mmol of Pt(cod)(me)2 per mol of carbon dioxide. A comparison of the respective graphs shows that the equilibrium loading is similar in both cases. It can be concluded that the essential parameter for the adsorption is the relative amount of dissolved substance per amount of carbon dioxide rather than the absolute amount of substance in the fluid. However, the rate of uptake seems to be slightly lower at the higher temperature. Similarly, an initial concentration of ca. 0.45 mmol of Pt(cod)(me)2 per mol of carbon dioxide was obtained in the experiments with an initial weight of 40 mg of Pt(cod)(me)2 at 333 K (0.44 mmol per mol of carbon dioxide) and with 30 mg at 353 K (0.46 mmol mol-1). However, it can be seen from Figure 8 that at this concentration, the two uptake curves do not coincide, the uptake being higher at the higher temperature. This is due to the adsorption isotherms that show a higher equilibrium loading at higher temperatures, as discussed before. The equilibrium loading at 0.44 mmol mol-1 is virtually the same as that at 0.33 mmol mol-1 and represents the adsorption capacity at this temperature. At 353 K, however, there is still a significant increase of the equilibrium loading when the concentration of the platinum compound increases from 0.31 to 0.46 mmol mol-1, as the adsorption capacity is higher at this temperature. Conclusion The influence of concentration, temperature, and support material on the adsorption of dimethyl(1,5-cyclooctadiene)platinum in supercritical carbon dioxide was investigated. A new experimental method was developed where the substance was allowed to dissolve in the supercritical fluid before coming into contact with the solid support. When the two-way valve was switched, a homogeneous supercritical solution with a
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defined concentration of the organometallic compound entered the adsorption cell. Thus, the start of the uptake can be defined clearly. The monolithic support material shows a sharp increase in the uptake of the platinum compound already in the first few minutes. This is an advantage for processes comprising a reaction of the platinum compound on the surface of the support material. The rate of uptake can be increased by increasing the external surface area of the support material for a better contact between the fluid and the solid support. A higher temperature results in an increase in the adsorbed amount. The higher equilibrium loading for the silica gel tablets as compared to the monolithic support material is probably due to the higher internal surface area. This is important as it shows the ability of the dissolved compound to access the pores. Analyses of the loaded support material will have to be performed to confirm this conclusion. Acknowledgment This work was funded by the Ministry of Science, Research and the Arts as well as the Landesstiftung foundation of Baden Wuerttemberg (Az: 23-720.431-1.8/1). Nomenclature B ) specific uptake (g g-1) c0 ) concentration of dissolved substance in the fluid in experiments without adsorption (g cm-3) ca ) concentration of dissolved substance in the fluid in the adsorption experiments (g cm-3) ci ) initial concentration of dissolved substance in the fluid in experiments without adsorption (g cm-3)
cf ) final concentration of dissolved substance in the fluid in experiments without adsorption (g cm-3) m0 ) initial amount of solute in the experiment (mg) ma ) amount of adsorbed substance (mg) ms ) amount of support material (mg) t ) adsorption time (s) ts, te ) fitting parameters (s) Vc ) volume of the fluid circuit with adsorption cell (cm3) Vc,0 ) volume of the fluid circuit without adsorption cell (cm3) Literature Cited (1) Gla¨ser, R.; Weitkamp, J. Supercritical Carbon Dioxide as a Reaction Medium for the Zeolite-Catalyzed Alkylation of Naphthalene. Ind. Eng. Chem. Res. 2003, 42, 6294. (2) Iwai, Y.; Higuchi, M.; Nishioka, H.; Takahashi, Y.; Arai, Y. Adsorption of Supercritical Carbon Dioxide plus 2,6- and 2,7-Dimethylnaphthalene Isomers on NaY-Type Zeolite. Ind. Eng. Chem. Res. 2003, 42, 5261. (3) Ryu, Y.-K.; Kim, K.-L.; Lee, C.-H. Adsorption and Desorption of n-Hexane, Methyl Ethyl Ketone, and Toluene on an Activated Carbon Fiber from Supercritical Carbon Dioxide. Ind. Eng. Chem. Res. 2000, 39, 2510. (4) Zhang, Y.; Kang, D.; Aindow, M.; Erkey, C. Preparation and Characterization of Ruthenium/Carbon Aerogel Nanocomposites via a Supercritical Fluid Route. J. Phys. Chem. B 2005, 109, 2617. (5) Zhang, Y.; Erkey, C. Preparation of Supported Metallic Nanoparticles using Supercritical Fluids: A Review. J. Supercrit. Fluids 2006, 38, 252. (6) Span, R.; Wagner, W. A New Equation of State for Carbon Dioxide Covering the Fluid Region from the Triple-Point Temperature to 1100 K at Pressures up to 800 MPa. J. Phys. Chem. Ref. Data 1996, 25, 1509.
ReceiVed for reView September 11, 2007 ReVised manuscript receiVed February 4, 2008 Accepted February 20, 2008 IE071221H
