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Ind. Eng. Chem. Res. 2000, 39, 2124-2126
RESEARCH NOTES Palladium-Modified Yttria-Stabilized Zirconia Membranes Jinsoo Kim and Y. S. Lin* Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0171
The reservoir method was applied to produce a continuous palladium (Pd) phase inside the porous yttria-stabilized zirconia (YSZ) membranes. Thin porous YSZ membranes were prepared from YSZ suspension, and the pores were modified with Pd by the liquid-phase impregnation method. Helium permeation experiments showed that the pore size/porosity of the YSZ top layer decreases with the number of Pd impregnations. The number of helium permeation reductions was about three times for the four-time Pd-impregnated YSZ top layer. The Pd phase inside the suspensionderived YSZ layer became continuous after four Pd impregnations, which was confirmed by electrical conductivity measurements. Introduction Inorganic membranes generally possess superior chemical, structural, and thermal stability and long life in applications relative to polymeric membranes. Also, they can have good catalytic and electrical properties and withstand very rigorous cleaning conditions such as steam sterilization and high back-flush capacity. Because of these advantages, they can find potential applications in various fields such as high-temperature gas separation and catalytic membrane reactors as well as conventional applications in filtration and purification of the environmental and food industries.1 Among the inorganic membrane materials, yttriastabilized zirconia (YSZ) is a well-known material as a fast ionic conductor with a very good chemical and structural stability. It has a cubic fluorite structure, which is stable between room temperature and 3000 K. YSZ has received the most attention in the past 2 decades because of its use as an electrolyte in hightemperature solid oxide fuel cells (SOFC) and oxgen sensors.2-4 Porous YSZ membranes can be used as ultrathin dense ceramic or metallic membrane supports and as filters for conventional liquid-phase filtration. Such applications are attractive because of their good chemical stability and high ionic conductivity. Also, they can be used for ceramic-metal composite membranes because of the proper thermal expansion coefficient with respect to some dense membrane materials. Recently, dense dual-phase composite membranes consisting of YSZ and Pd were reported to give much improved oxygen permeation flux.5,6 However, they were prepared by solid-state powder pressing and sintering and the thickness was in the range of 0.25-2 mm.5,6 If the porous YSZ membranes are modified properly with metal, they can be used as a base for thin dual-phase membranes. In the present paper, suspension-derived macrosporous YSZ membranes were modified with palladium by the reservoir method. If a continuous Pd phase is formed inside the YSZ layer, the Pd-modified YSZ membranes may offer a number of applications requiring a thin film * Corresponding author.
conducting both electrons and oxygen ions. For example, this membrane composite can be used as the base for growing thin, gastight dual-phase YSZ/Pd membranes with high oxygen permeability.7 This paper reports on the synthesis and electrical and gas transport properties of the palladium-modified YSZ membranes. Experimental Section Supported YSZ membranes were prepared by dipcoating of a stable colloidal YSZ suspension on macroporous R-alumina disks (diameter, 20 mm; thickness, 2 mm; pore diameter, 0.23 µm; porosity, 50%). A more detailed experimental procedure was reported elsewhere.8 In brief, a stable YSZ suspension was prepared by ballmilling of commercially available YSZ powder (Tosoh, TZ-8Y) in acetone. Before the colloidal solutions were dipcoated, poly(vinyl alcohol) (PVA) was added to the solutions as a drying control chemical additive (DCCA). The dipcoated disks were dried in the drying oven under a controlled atmosphere (relative humidity, 40-50%; temperature, 40 °C). After drying, the suspensionderived YSZ membranes were heat-treated at 1000 °C. These dipcoating, drying, and heat treatment processes were repeated. The YSZ top layer was modified with palladium by the reservoir method. The palladium solution was prepared by dissolution of 0.25 g of palladium acetate (Alfa, MW) 673.46) in a mixture of 0.3 mL of concentrated HCl (Aldrich, 35 wt %) and 10 mL of acetone. The supported YSZ membranes were soaked in the Pd solution for 30 min to fully saturate both the R-alumina disk and the porous YSZ top layer. After that, they were taken out and placed on a nonporous plate with the YSZ layer facing upward for drying for 1 day. After drying, the Pd impregnated YSZ membranes were calcined at 500 °C for 2 h under a hydrogen flow at the heating/ cooling rate of 4 °C/min. These soaking, drying, and calcination processes were repeated to deposit an appropriate amount of Pd inside the porous YSZ membranes. Room-temperature helium permeation experiments were conducted to check the extent of pore modification with the number of Pd impregnations. Helium permeance was measured by changes in the upstream and
10.1021/ie990674m CCC: $19.00 © 2000 American Chemical Society Published on Web 05/11/2000
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Figure 1. XRD patterns of Pd-modified YSZ membranes.
downstream pressures, while the helium flow rate was fixed. Then, permeance data were plotted versus an average pressure. The presence of palladium inside porous YSZ membranes and its crystal-phase structures of Pd/YSZ membranes was analyzed by X-ray diffraction (XRD) (Siemens Kristalloflex D500 diffractometer, with CuKR in the range of 2θ from 20° to 65°). The electrical conductivities of supported membranes were measured in the temperature range from 600 to 900 °C in air with a four-point DC method. Results and Discussion Crack-free supported YSZ membranes could be prepared by dipcoating of the stable YSZ suspension on R-alumina supports. The YSZ layer thickness was about 5 µm after one dipcoating and 10 µm after two dipcoatings, respectively. The average pore diameter of unsupported YSZ membranes was 100 nm when measured by a mercury porosimeter. More detailed results on the YSZ membranes were reported elsewhere.8 The R-alumina/YSZ two-layer membranes were put in the Pd solution, followed by drying, which promoted the concentration of the metal cations inside the top layer of the membranes. The thin YSZ top layer has a smaller pore size compared to that of the thick alumina support layer. During the drying process, the evaporation begins from the top layer. As the Pd solution in the YSZ layer is dried, the impregnated solution inside the alumina layer fills the pores of the YSZ top layer by the capillary pressure which results from the difference in the pore sizes. During the calcination, the Pd precursor changed into the pure metal Pd. Figure 1 shows the XRD patterns of the Pd-modified YSZ membranes with the number of impregnations. The peaks at 2θ values of 40° and 47° indicate pure Pd. From these XRD patterns, they clearly show two phases of YSZ and Pd. This indicates that all of the Pd solution was changed into pure metal Pd during the calcination step. The Pd peak intensity increases with the number of Pd impregnations, indicating an increase of Pd loading inside the porous suspension-derived YSZ layer. EDS cross-section mapping of the Pd-modified YSZ composite membranes shows that the majority of Pd is deposited in the YSZ top layer.7 Helium permeation experiments were conducted before and after Pd modification to monitor the pore size changes on the membranes. Because most of the Pd modification takes place inside the YSZ top layer,
Figure 2. Helium permeance of the alumina-supported YSZ and Pd-modified YSZ membranes.
helium permeation reduction is mainly caused by the pore size decrease at the top layer. The permeation data were fitted to the following equation,9
(F/L) ) R + βPAV
(1)
where the permeance is defined as F/L ) Q/S(Ph - Pl) with Q being the molar gas flow rate, L the disk thickness, S the permeation area of the membrane, and Ph and Pl the pressures at upstream and downstream. The permeability coefficients R and β are attributed to the Knudsen and viscous flow, respectively. The permeability data for the support and the support/top layer were respectively measured for the R-alumina disk (before dipcoating) and the same disk but after being dipcoated with a YSZ layer. This disk was used for the additional permeation experiments after Pd impregnation. Figure 2 shows the helium permeance as a function of an average pressure for Pd-modified suspension-derived YSZ membranes. The permeance data are from the two-layer membrane, consisting of a 2-mm thick R-alumina layer and a 10-µm thick YSZ top layer before and after modification with Pd. The permeance of the two-layer membrane decreased with Pd impregnation. The helium permeability reductions were 23% for the four-time Pd-impregnated suspensionderived YSZ membrane. The permeance data for the YSZ top layer and Pd-modified YSZ top layer were calculated by using the resistance-in-series model [15]. The pressure at the interface of the top layer and support in the composite membrane (Pm) at the constant flow rate was calculated by the following equation,9
Pm )
[( ) Rs βs
2
()
+ 2Pl
]
Rs 2Q + Pl2 + βs βs S
1/2
-
Rs βs
(2)
where permeation coefficients Rs and βs were calculated from the permeability data of the support only. Thus, a permeation value for the supported top layer at an average pressure of PAV ) (Ph + Pm)/2 was obtained using the following equation:
F/L (top layer) ) Q/S(Ph - Pm)
(3)
The calculated helium permeance of the top layer was presented in Figure 3 for a Pd-impregnated suspensionderived YSZ top layer. From the top-layer permeability, we can see a substantial decrease in helium permeance
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tions and the measured conductivity is through the connected metal Pd phase. The measured total electrical conductivity is still very low compared to that of the pure metal Pd (about 104 S/cm). This is in part because the Pd phase deposited in the YSZ pores is porous. Its cross-sectional area is much smaller than the value used in calculating the conductivity of metal. It is reported that the electrical conductivity of a porous YSZ layer was about 10 times lower than that of a dense YSZ sample because of the porosity.8 Because the Pd phase is formed inside the porous YSZ layer, this would give an even lower electrical conductivity for the deposited Pd phase as compared to that of a dense thin Pd layer.
Figure 3. Helium permeance of the top layer (YSZ or Pd-modified YSZ).
Conclusions Suspension-derived porous YSZ membranes were effectively modified with Pd by the reservoir method. Multiple Pd impregnations inside YSZ membranes resulted in a pore reduction as well as an increase of Pd concentration. Helium permeation experiments showed that the average permeability decreased from 3.13 × 10-5 to 7.34 × 10-6 mol/(m2 s Pa) after four Pd impregnations inside a suspension-derived YSZ membrane. The XRD results indicate that the prepared membranes have pure YSZ and Pd phases. For Pdimpregnated suspension-derived YSZ membranes, the Pd phase became continuous after four impregnations, which was confirmed from the electrical conductivity data. Acknowledgment This work was supported by the National Science Foundation (CTS-9502437). One of the authors (J. Kim) would like to acknowledge the financial support from the North American Membrane Society in the form of a fellowship.
Figure 4. Electrical conductivity of Pd-modified YSZ membranes.
of the YSZ layer due to the Pd modification. The helium permeance was reduced by about 3-fold after four Pd impregnations for suspension-derived YSZ membranes. From Figure 3, the suspension-derived YSZ layer shows a larger slope compared to the other Pd-modified YSZ layers, which indicates that both Knudsen diffusion and viscous flow are important for permeation. However, the Pd-modified membranes have much lower slopes. Therefore, the Knudsen diffusion becomes more important compared to viscous flow with pore modification. Figure 4 shows the electrical conductivities of the YSZ membranes as a function of temperature. For Pdimpregnated suspension-derived YSZ membranes, the electrical conductivity of pure YSZ and two- and threetimes Pd-impregnated YSZ membranes is oxygen ionic conductivity of YSZ, with an activation energy of about 86 kJ/mol. These are consistent with the literature data.10 The electronic conductivity of YSZ is in the range of 10-4-10-6 S/cm, several orders of magnitude lower than its oxygen ionic conductivity under the same conditions. Thus, the contribution of the electron conduction to the total conductivity in these three YSZ membranes is negligible. These show that Pd after two or three impregnationd did not form a continuous phase. The total electrical conductivity of the four-time Pdimpregnated suspension-derived YSZ membrane is 3 orders of magnitude higher than that of the other membranes, as shown in Figure 4. This indicates that the Pd phase became continuous after four impregna-
Literature Cited (1) Bhave, R. R. Inorganic Membrane Synthesis, Characterization and Application; Van Nostrand Reinhold: New York, 1991. (2) Stevens, R. Zirconia and Zirconia Ceramics, 2nd ed; Magnesium Electron Ltd.: Twidkenham, U.K., 1986. (3) Hammou, A.; Guinder, J. Solid Oxide Fuel Cells. In CRC Handbook of Solid State Electrochemistry; Gellings, P. J., Bouwmeester, H. J. M., Eds.; CRC Press: New York, 1996. (4) Hibino, T.; Kuwahara, Y.; Otsuka, T.; Ishida, N.; Oshima, T. NOx Detecting Using the Electrolysis of Water Vapor in a YSZ CellsPart II Electrochemical Oxygen Pump. Solid State Ionics 1998, 107 (3-4), 217. (5) Chen, C. S.; Boukamp, B. A.; Bouwmeester, Cao, G. Z.; Kruidhof, H.; Winnubst, A. J. A.; Burggraaf, A. J. Microstructural Development, Electrical Properties and Oxygen Permeation of Zirconia-Palladium Composites. Solid State Ionics 1995, 76, 23. (6) Mazanec, T. J.; Cable, T. L.; Frye, J. G. Electrocatalytic Cells for Chemical Reactions. Solid State Ionics 1992, 53-56, 111. (7) Kim, J.; Lin, Y. S. Synthesis and Oxygen Permeation Properties of Thin YSZ/Pd Composite Membranes. AIChE J. 2000, in press. (8) Kim, J.; Lin, Y. S. Synthesis and Characterization of Suspension Derived Porous Ionic Conducting Ceramic Membranes. J. Am. Ceram. Soc. 1999, 82, 2641. (9) Lin, Y. S.; Burggraaf, A. J. Experimental Studies on Pore Size Change of Porous Ceramic Membranes after Modification. J. Membr. Sci. 1993, 79, 65. (10) Park, J. H.; Blumenthal, R. N. Electronic Transport in 8 mole percent Y2O3-ZrO2. J. Electrochem. Soc. 1989, 136, 2867.
Received for review September 9, 1999 Revised manuscript received April 10, 2000 Accepted April 19, 2000 IE990674M
