BRIEF DESCRIPTION OF THE DRAWINGS
The objectives and advantages of the present invention will become apparent upon reading the following description and upon reference to the accompanying drawings in which:
FIG. 1 to FIG. 4 illustrate a method for preparing a palladium-containing layer on a porous substrate according to one embodiment of the present invention;
FIG. 5 shows TEM image and PXRD plot of the palladium nanoparticles according to one embodiment of the present invention;
FIG. 6 shows the argon flow rate (F) from the inner hollow portion via the PSS tube body to the outer environment before and after the surface-modifying process according to an embodiment of the present invention;
FIG. 7 shows the argon permeability of the PSS tube having the palladium-containing layer according to an embodiment of the present invention;
FIG. 8 shows the variation of concentrations of Pd(II) ions in the plating solutions with time during the electroless plating process; and
FIG. 9 shows the thermal history of the palladium-containing layer on the porous substrate for the hydrogen purification process according to an embodiment of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 to FIG. 4 illustrate a method for preparing a palladium-containing layer 10 on a porous substrate 12 according to an embodiment of the present invention. First, a cleaning process is performed to clean a top surface 12A of the porous substrate 12, e.g., a porous stainless steel tube, using a cleaning solution selected from the group consisting of de-ionized (DI) water, organic solvent, acidic solvent, alkaline solvent, and the combination thereof. Ultrasonic wave clean may be further applied to the cleaning process. The cleaned porous substrate 12 is then dried in an oven.
Referring to FIG. 2, a surface-modifying process is performed on the top surface 12A of the porous substrate 12 to form a planar surface 14 by filling holes or concaves of the porous substrate 12 with aluminum oxide particles 16 having particle sizes about 0.5 μm, and coating a sol-gel containing aluminum oxide or silicon oxide on the top surface 12A of the porous substrate 12. A thermal treating process is performed at a temperature between 350° C. and 550° C. to transform the sol-gel into the aluminum oxide (or silicon oxide) particles 16.
Referring to FIG. 3, a seeding process is performed on the planar surface 14 to adhere palladium nanoparticles 18 on the planar surface 14. The seeding process may include exposing the planar surface 14 of the porous substrate 12 in a colloidal solution having dispersed palladium nanoparticles 18. The dispersed palladium particles 18 can be derived from a palladium-containing species and a surfactant with/without a reducing reagent. The reducing reagents can be ethanol, alcohol, hydrazine and sodium borohydride
The palladium-containing species can be either cations selected from the group consisting of palladium(II), triamminepalladium(II), and tetraamminepalladium(II), or anions selected from the group consisting of tetrachloropalladium(II), Amminetrichloropalladium(II), and 1,2-ethanediyl(dinitrilo) tetraacetatepalladium(II). The surfactant can be either anions selected from the group consisting of sodium tetradecylsulfate, sodium tridecylsulfate, sodium dodecylsulfate (SDS), sodium undecylsulfate, sodium decylsulfate, sodium nonylsulfate, and sodium octylsulfate, or cations selected from the group consisting of octadecyltrimethylammonium brombide, cetyltrimethylammonium brombide (CTAB), myristyltrimethylammonium brombide, dodecyltrimethylammonium brombide, cetyltrimethylammonium chloride, and dodecyltrimethylammonium chloride.
If the surfactant is anionic, it is preferable to adjust the pH value of the colloidal solution to be lower than 7, e.g., pH=2. The aluminum oxide particles 16 are positively charged at pH=2, and the anionic surfactant carrying palladium is attracted by the positively charged aluminum oxide particles 16 to the planar surface 14. Furthermore, If the surfactant is cationic, it is preferable to adjust the pH value of the colloidal solution to be higher than 7, e.g., pH=12. The aluminum oxide particles 16 are negatively charged at pH=12, and the cationic surfactant carrying palladium is attracted by the negatively charged aluminum oxide particles 16 to the planar surface 14.
As such, the palladium is transferred from the colloidal solution to the planar surface by electrostatic force in the seeding process. Preferably, a pressure difference between the top surface 12A and a bottom surface 12B of the porous substrate 12 is generated during the seeding process to improve the mass-transferring efficiency from the colloidal solution to the seeding surface, and the pressure at the top surface 12A is higher than the pressure at the bottom surface 12B.
After the seeding process on the planar surface, it is preferable to expose the planar surface 14 of the porous substrate 12 in a complex ion solution including tetrachloropalladium(II) anions (PdCl42−) or tetraamminepalladium(II) cations (Pd(NH3)42+). The complex ion solution can provide some palladium atoms on the planar surface 14 in advance, and these palladium atoms serve as reactant in the subsequent electroless plating process. Consideration to the electrostatic attraction effect in mass-transferring process, it is preferable to use the complex ion solution including tetraamminepalladium(II) cations (Pd(NH3)42+) if the surfactant is anionic, and use the complex ion solution including tetrachloropalladium(II) anions (PdCl42−) if the surfactant is cationic.
Referring to FIG. 4, an electroless plating process is performed at 60° C. for 2 hours so as to form the palladium-containing layer 10 on the planar surface 14. The plating solution may include PdCl2, NH4OH, EDTA·2Na, and N2H4. In particular, the electroless plating process may be an osmosis electroplating process, which exposes the planar surface 14 of the porous substrate 12 in the plating solution and exposes the bottom surface 14B of the porous substrate 12 in a solution having an ionic or molecular concentration higher than the ion concentration of the plating solution employed in the electroless plating process. In addition, metal ion such as silver ion may be added in the electroplating solution such that the palladium-containing layer 10 could be a palladium alloy layer.
FIG. 5 shows Transmission Electron Microscope (TEM) image and Powder X-ray Diffraction (PXRD) plot of the palladium nanoparticles 18 according to one embodiment of the present invention. The palladium nanoparticles 18 can be prepared using different reactants such as Pd(OAc)2+SDS, PdCl2+CTAB+N2H4, and HPdCl3+SDS. The TEM image is used to estimate the sizes of the palladium nanoparticles 18, and the PXRD analysis is used to characterize the palladium nanoparticles 18, as shown in the following table. Obviously, the palladium nanoparticles 18 prepared by Pd(OAc)2+SDS have two size groups of about 2˜4 nm and 10˜30 nm.
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Particles size(nm) by
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Synthesis reactant
TEM
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Pd(OAc)2 + SDS
10~30 & 2~4
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PdCl2 + CTAB + N2H4
4~7 (FIG. 5)
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HPdCl3 + SDS
10~25
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FIG. 6 shows the argon (Ar) flow rate (F) from the inner hollow portion via the PSS tube body to the outer environment before and after the surface-modifying process according to an embodiment of the present invention. The surface-modifying process is performed in two ways: (1) the holes or concaves are filled with aluminum oxide and coated with aluminum oxide sol-gel (PSS\Al2O3\cAl2O3) thereon; (2) the holes or concaves are filled with aluminum oxide and coated with silicon oxide sol-gel (PSS\Al2O3\cSiO2). The argon flow rate of the PSS tube without undergoing surface-modifying process is higher that these of undergoing surface-modifying process. In addition, the argon flow rate of the PSS tube with silicon oxide sol-gel coating is higher than that with aluminum oxide sol-gel coating. In other words, the surface-modifying process using aluminum oxide filling and aluminum oxide coating has a better hole-filling (or concave-filling) ability.
FIG. 7 shows the argon permeability of the PSS tube having the palladium-containing layer 10 according to an embodiment of the present invention. The argon permeability of palladium-containing layer 10 prepared by traditional electroless plating process is much larger than that prepared by the electroless plating process with osmosis, i.e., the number of pin-holes in the palladium-containing layer 10 prepared by the electroless plating process with osmosis is fewer than that prepared by the simple electroless plating process without osmosis.
FIG. 8 shows variation of the Pd(II) ions concentration in the electroless plating solution. The electroless plating process is performed twice. The rate of Pd(II) ions consumed in a first electroless plating process right after the activation process, i.e., the seeding process, is much faster than that in a second electroless plating process, which is subsequent to the first electroless plating process. The variation of the Pd(II) ions concentration indicates that the rate of palladium deposition is significantly enhanced by the activation process according to the present invention.
FIG. 9 shows the thermal history of the palladium-containing layer 10 on the porous substrate 12 for the hydrogen purification process according to an embodiment of the present invention. Before the hydrogen purification process performs at about 350° C., a thermal treating process is preferably performed in an inert atmosphere such as nitrogen, argon, etc and in a mixture atmosphere of hydrogen and inert gas in sequence. In addition, it is also preferable to perform another thermal treating process in a mixture atmosphere of hydrogen and inert gas and in an inert atmosphere in sequence after the hydrogen purification process.
The hydrogen/nitrogen selectivity of the palladium-containing layer 10 is shown in the following table, and it is obvious the hydrogen/nitrogen selectivity of the palladium-containing layer 10 operating in accordance with the thermal history shown in FIG. 9 is higher than 6000. In particular, the thermal history may includes the processes of placing the porous substrate 12 having the palladium-containing layer 10 in a container containing nitrogen at a predetermined temperature, transferring a mixture gas containing hydrogen and argon into the container, transferring hydrogen into the container to actually perform the hydrogen purification process, transferring the mixture gas into the container, transferring nitrogen into the container and reducing the temperature in the container.
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Hydrogen flow
Nitrogen flow
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Test number
rate (ml/min)
rate (ml/min)
Selectivity
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1
181.1
0.03
6037
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2
215.1
0.03
7170
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3
225.0
0.03
7499
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4
224.7
0.03
7490
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5
232.9
0.03
7762
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1 mL/min = 4.5 × 10−4 mol/m2 · s = 2.6 × 10−6 mol/m2 · s · Pa0.5
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The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by those skilled in the art without departing from the scope of the following claims.
Patent Application
20080014355
