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Supercritical CO2 for Making Nanoscale Materials Hiroaki Wakayama* and Yoshiaki Fukushima Toyota Central Research and DeVelopment Laboratory, Incorporated, Nagakute, Aichi, 480-1192, Japan
Nanoporous metals and oxides were synthesized by the templating process using supercritical solvents, which is called nanoscale casting using supercritical fluids (NC-SCF). Precursors dissolved in supercritical CO2 were attached to and coated on activated carbon templates. After removing the activated carbon templates, nanoporous materials replicating both the porous structures on a nanometer scale and the macroscopic shapes of the templates can be obtained. The surface areas of these nanoporous materials were much higher than those of the materials synthesized by a templating process using liquid solvents. The SCFs were also used for the synthesis of nanoparticles in microporous and mesoporous silicas with uniform pore sizes, FSM-16 (1.63.5 nm diam). Nanoparticles could be impregnated into the micropores and mesopores of FSM-16 by impregnation in a supercritical fluid solvent. On the other hand, nanoparticles could not be introduced into the smaller pores of FSM-16 by impregnation using liquid solvents. These results are based on the penetration difference into the nanoporous structures by supercritical solvents and liquid solvents. I. Introduction Porous materials have very attractive physicochemical properties based on their porous structures and surface characteristics. These materials have found application in adsorbents, catalysts, capacitors, and sensors. Many studies have been performed on controlling their porous structures with the use of templates.1-5 For instance, silica, alumina, or their complex oxide is polymerized around a template. After the removal of the templates, porous materials with pores, whose size corresponds to the size of the template, can be synthesized. The templating process is also important in industrial processes such as metal casting or plastic molding. In these processes, a template with a complex structure can be used. However, accurate replication is possible only above the millimeter scale. It is very difficult to replicate a fine template with a complex structure, because the high viscosity and the surface tension of conventional solvents, which are liquids, prevents penetration into the narrow gaps. Supercritical fluids6-9 have high diffusivity, low viscosity, absence of surface tension, and controllable solubility. They are not condensed in the liquid phase. Thus, they are expected to overcome the limitation of the diffusivity and mass transfer of conventional solvents and can transfer an effective amount of materials into very small spaces. We have developed a novel templating process using SCFs, called the nanoscale casting using supercritical fluids, i.e., the NC-SCF process.10-16 In this process, the precursors are dissolved in supercritical CO2 and attached to the template containing micropores or mesopores, such as activated carbon. After coating on the template, the templates are removed. Replicates of the oxide or metal can then be produced. A schematic drawing of the NC-SCF process is shown in Figure 1. The porous structures of the activated carbon are composed of randomly stacked graphene crystallites. The silica precursors dissolved in supercritical CO2 are carried into the pores and are hydrolyzed into the silica network through reactions with the adsorbed water and functional units on the surface of the activated carbon template. After removal of the activated carbon template, the pores with sizes corresponding to the sizes of the carbon crystallites appear in the silica replicate. In this paper, SCFs are shown to be excellent solvents for chemical reactions in pores on a nanometer scale. We demon* To whom correspondence should be addressed. Tel.: +81-56163-4762. Fax: +81-561-63-6137. E-mail:
[email protected].
Figure 1. Schematic drawing of the NC-SCF process.
strate a method to make porous materials using supercritical CO2 with highly porous activated carbon as the templates. Supercritical CO2 was also used to impregnate nanoparticles on porous supports. These results will be compared with the process using liquid solvents. II. Experimental Section II.1. Nanoscale Casting Using the SCFs (NC-SCF) Process. A high-pressure vessel (1000 mL) was used for the treatments in supercritical CO2. A diagram of the apparatus for the treatment in supercritical CO2 is shown in Figure 2. CO2 from a CO2 tank was liquefied through a cooling unit and compressed by a liquid pump. The CO2 was then preheated through a surge tank. Precursors along with a suitable entrainer, listed in Table 1, were placed in the vessel. Ten grams of the activated carbon templates (BW103; Toyobo Co., Ltd., or A-20 and M30; Osaka Gas Chemical Company) in a stainless steel basket was fixed in the upper part of the vessel. Supercritical CO2 was then pumped into the vessel up to 26-32 MPa. The
10.1021/ie050658r CCC: $33.50 © 2006 American Chemical Society Published on Web 11/04/2005
Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006 3329 Table 1. Experimental Conditions of the NC-SCF Process
silica titania alumina Pt Pt-Ru CaCO3 a
precursor
entrainer
temp (K)
pressure (mPa)
reaction time (h)
tetraethylorthosilicate titanium isopropoxide Al(acac)3a Pt(acac)2a Pt(acac)2 + Ru (acac)3a calcium 2-ethylhexanoate
2-propanol acetone acetone acetone methanol
393 423 423 423 423 423
26 32 30 32 32 32
2 2 24 24 24 24
acac ) acetylacetonate.
Table 2. BET Surface Areas of the Samples Synthesized by the NC-SCF Process and Liquid Solvent Process
NC-SCF process templating in liquid solvent
BET surface area (m2 g-1) Al2O3 Pt
method for removal of template
SiO2
TiO2
oxygen plasma calcination
447-889 480-1377
387 63
332
oxygen plasma calcination
540
124 51
268
treatment temperature was regulated at 393-423 K using an external electric heater. After a contact time of 2-24 h, the pressure was released by venting. The recovered samples were calcined in flowing air at 873 K for 6 h or treated in oxygen plasma for removal of the activated carbon templates. The preparation conditions are shown in Table 1. II.2. Impregnation of Nanoparticles. Supercritical CO2 was used to impregnate nanoparticles of metals and oxides on porous SiO2. Figure 3 shows the experimental apparatus for the impregnation in SCCO2. The precursors along with the entrainer were placed at the bottom of an autoclave. The nanoporous silica substrate with a uniform pore size, FSM-16,17 was placed in a basket fixed to the upper part of the autoclave. The closed autoclave was filled with CO2 and heated in an oil bath at 423 K and 32 MPa for 24 h. II.3. Characterization. Nitrogen sorption isotherms were obtained at 77 K on a QUANTACROME AUTOSORP-1-MP. Scanning electron microscope (SEM) observations were performed using a JEOL JSM-890 scanning electron microscope. Transmission electron microscope (TEM) images were obtained using a JEOL JEM-200EX transmission electron microscope. III. Results and Discussion III.1. Nanoscale Casting Using the SCFs (NC-SCF) Process. As seen in the SEM images of nanoporous materials synthesized by the NC-SCF process in Figure 4, the cloth and fibrous shape can be replicated. The surface areas of the samples prepared by the NC-SCF process and in the liquid coating are summarized in Table 2. The surface areas of the samples prepared by the NC-SCF process are much higher than those of the samples produced by the liquid coating. During the coating process in supercritical
Figure 2. Experimental apparatus for the NC-SCF process.
Pt-Ru
CaCO3
51
152
21
15
45
4
fluids, SCFs can carry precursors even into the micropores of the activated carbon. After removal of the activated carbon, the replicates have high surface areas. On the other hand, in the liquid coating process, the surface tension or wettability prevents penetration of the precursors into the micropores. Products would react only at the entrance of the micropores. After removal of the activated carbon, the samples have lower surface areas. In current nanotechnology, one of the most important and difficult tasks is how to connect nanoscale structures to macroscopic devices. This NC-SCF process can provide replication of both nanoscale structures and macroscopic shapes. Therefore, the NC-SCF process would be one answer to current nanotechnology. Figure 5 shows SEM images of the alumina replicates. The cloth, fibrous, and spherical shapes of the activated carbon templates can be replicated in the alumina samples. The morphology can be controlled through the choice of the shape of template. The SEM images of the Pt-Ru nanoporous material prepared by the nanoscale casting process using supercritical fluids (NCSCF) are shown in Figure 6. The Pt-Ru nanoporous materials retain the cloth and fibrous shape of the activated carbon template. The diameter of the Pt-Ru fibers is almost the same as that of the activated carbon fiber template. The samples consist of 10-20 nm diameter fused particles. The XRD patterns of nanoporous Pt-Ru synthesized by the NC-SCF process and the liquid solvent process show peaks from metallic Pt and RuO2 (Figure 7).
Figure 3. Experimental apparatus for the impregnation in SCCO2.
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Ind. Eng. Chem. Res., Vol. 45, No. 10, 2006
Figure 4. SEM images of nanoporous materials synthesized by the NS-SCF process: (a) activated carbon template, (b) silica, (c) titania, (d) alumina, (e) Pt, (f) Pt-Ru, and (g) CaCO3.
Figure 5. SEM images of nanoporous alumina synthesized by the NSSCF process.
Figure 7. XRD spectra of nanoporous Pt-Ru synthesized by (a) the NCSCF process and (b) the liquid solvent process.
Figure 6. SEM images of nanoporous Pt-Ru synthesized by the NCSCF process.
The electrocatalytic property of the Pt-Ru nanoporous materials was performed using them as the anode catalyst of the direct methanol fuel cells (DMFCs). Figure 8 shows the current-voltage curve of the Pt-Ru nanoporous material prepared by the NC-SCF process and the Pt-Ru sample prepared using a liquid solvent. It can be seen that the Pt-Ru nanoporous material is more active than the sample prepared using a liquid solvent. The high electrocatalytic activity of this sample would be attributed to its high surface area and the highly dispersed Pt and RuO2 crystallized domains in the sample. III.2. Impregnation of Nanoparticles. Nanoparticles of metals of Pt,18 Pd, and Rh can be impregnated on nanoporous silica with a uniform pore size, FSM-16. Figure 9 shows TEM images of the metal-impregnated FSM-16 in supercritical solvent
after the reduction treatment. The particles are highly dispersed and have a small distribution (Table 3). Metal nanoparticles were impregnated inside the pores. For the impregnation of Pt nanoparticles using an SC solvent, size-controlled Pt nanoparticles can be prepared in the FSM-16 nanoporous silica with a uniform pore size. The particle size linearly increased with the increase in the pore size of the FSM-16. Smaller Pt nanoparticles more efficiently catalyze the CO oxidation reaction. For the impregnation of Pt nanoparticles using a liquid solvent, Pt particles cannot be introduced into pores smaller than 3.5 nm. The combination of impregnation and the NC-SCF process is shown in Figure 10. Pt nanoparticles were first impregnated into the activated carbon. Thereafter, silica was coated and the activated carbon was removed. The sample prepared using SCF showed an excellent durability in the endurance test when compared to the sample prepared with the liquid solvent. After the treatment in air at 1173 K for 5 h, the Pt particle size of the sample prepared using SCF was 20 nm, whereas the Pt particle size of the sample prepared using the liquid solvent was 110 nm. The interaction between the Pt and the SiO2 of the sample prepared using SCF was potent. Therefore, the mobility of the Pt nanoparticles was limited. As a result, there was a lower possibility of sintering during the endurance test. These processes using SCFs can be applied to catalysts for exhaust gas clearing, capacitors, solar cells, and molecular
Table 3. Pt Particle Size/Distribution and Catalytic Activity of Samples Prepared in Supercritical Solvent
a
silica matrix
Pt particle size /standard deviation (nm/nm)
pore diameter of FSM-16 (nm)
temp for 50% conversion in CO oxidation (K)a
temp for 50% conversion in CO oxidation (K)b
C8FSM-16 C10FSM-16 C12FSM-16 C16FSM-16
1.5/0.15 1.7/0.19 2.3/0.10 2.9/0.16
1.6 2.1 2.4 3.5
477 495 575 724
825 809 816 743
Materials prepared using supercritical fluids. b Materials prepared using liquid solvents.
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IV. Conclusion Nanoporous oxides and metals were produced by the templating process using SCF, called the NC-SCF (nanoscale casting using supercritical fluids) process. They are novel nanoporous materials with a high surface area accurately replicating the microscale (nanometer) to macroscale (centimeter) structures of materials. SCFs are shown to be effective solvents for chemical reactions (ex, sol-gel) in nanospaces. The nanoscale fabrication process using SCFs is expected to be a versatile route to functional ceramics and catalysts and to be applicable to novel technology that includes energy exchange, storage, and environment clarification. Literature Cited
Figure 8. Current-voltage curves of nanoporous Pt-Ru synthesized by the NC-SCF process.
Figure 9. TEM images of metal-impregnated FSM-16 using supercritical solvent before and after reduction treatment: (a) Pt, (b) Rh, and (c) Pd.
Figure 10. Schematic drawing of the synthesis of Pt-supported silica using supercritical CO2.
sieves.19 Materials prepared using SCF had excellent properties when compared to the samples prepared using a liquid solvent.
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ReceiVed for reView June 8, 2005 ReVised manuscript receiVed September 27, 2005 Accepted September 30, 2005 IE050658R
