Simultaneously Depositing Pd−Ag Thin Membrane on Asymmetric

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Simultaneously Depositing Pd-Ag Thin Membrane on Asymmetric Porous Stainless Steel Tube and Application To Produce Hydrogen from Steam Reforming of Methane Jianhua Tong,*,†,‡ Lingling Su,‡ Yukari Kashima,† Ryuichi Shirai,† Hiroyuki Suda,‡ and Yasuyuki Matsumura§ Research Institute of InnoVatiVe Technology for the Earth (RITE), Kizu-cho, Soraku-gun, Kyoto 619-0292, Japan, Research Institute for InnoVation in Sustainable Chemistry, National Institute of AdVanced Industrial Science and Technology (AIST), Higashi, Tsukuba, Ibaraki 305-8565, Japan, and Research Institute for Ubiquitous Energy DeVices, National Institute of AdVanced Institute Science and Technology (AIST), Mirorigaoka, Ikeda, Osaka 536-8577, Japan

Thin Pd-Ag alloy composite membranes were simultaneously deposited on commercial asymmetric porous stainless steel (APSS) tubes, using a combined method of electroless plating and electroplating under mild conditions. The X-ray diffraction patterns proved that the Pd-Ag alloy structure was formed at room temperature. The permeation behavior was investigated in detail, revealing that the hydrogen permeation flux, which was as high as 0.28 mol/(m2 s), and the infinite hydrogen selectivity versus argon were achieved at a temperature of 773 K with a pressure difference of 100 kPa. The good membrane stability was proven by the temperature-changing cycles (11) and the gas-exchanging cycles (20). Electron probe microanalysis indicated that the Pd-Ag alloy composite membranes had a stable structure and a homogeneous Pd-Ag alloy layer. In addition, the membrane reactor for methane steam reforming was constructed and the reaction performance was tested. The methane conversion, which was as high as 80.72%, was achieved at a lower temperature of 773 K and a pressure of 500 kPa. 1. Introduction The Pd-Ag (palladium-silver) composite membrane with a thin Pd-Ag alloy layer on a porous substrate is one of the best membrane configurations for pure hydrogen separation, because of its higher hydrogen permeability, infinite theoretical hydrogen selectivity, good stability, relatively lower cost, and good resistance to hydrogen embrittlement.1-6 However, deposition of a thin and defect-free Pd-Ag alloy layer on a porous substrate is a tough process. The large thickness, the pinholes, and the uncontrollable composition greatly impair its superiority and prohibit its industrial application. Physical vapor deposition, magnetron sputtering, and chemical vapor deposition are common methods to prepare Pd-Ag composite membranes.7-10 However, the rigorous demand of having an ultrasmooth substrate prohibits their extensive applications, because the commercial substrates inevitably possess a large amount of surface roughness, especially for the porous stainless steel substrate. Electroless plating is a typical method that is used to deposit a thin palladium-based layer on porous substrates. Currently, this method has also been extensively applied to prepare the Pd-Ag alloy composite membrane.11-20 However, the preferential Ag deposition makes it too difficult to control * To whom correspondence should be addressed. Postal mailing address: Membrane Separation Processes Group, Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST), Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. Tel./fax: +81-29-861-4675. Email: [email protected]. † Research Institute of Innovative Technology for the Earth (RITE). ‡ Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology (AIST). § Research Institute for Ubiquitous Energy Devices, National Institute of Advanced Institute Science and Technology (AIST).

the Pd-Ag alloy composition. Therefore, the Pd and Ag layers were sequentially deposited in the normal case. After treatment at relatively higher temperatures for a long time period, the PdAg alloy can be formed. The electroplating method was successfully used to deposit the pure palladium membrane on the porous stainless steel substrate.21,22 However, few good results were obtained for depositing Pd-Ag alloy membrane by electroplating, because of the sensitive effects from the roughness, the pore structure, and the conductivity of the substrates.23,24 On the other hand, compared to other porous substrates (such as glass, alumina, and ceramics), porous stainless steel has a larger potential to be industrially applied as a substrate for preparing palladium-based composite membranes. The composite membrane, using porous stainless steel as the substrate, has relatively better thermal stability, because of the similar thermal coefficients of the palladium-based layer and the porous stainless steel substrate. In addition, it is easy to seal and construct modules using this type of composite membrane. However, the large surface pores and the roughness prohibit the deposition of a thin and defect-free palladium-based membrane on the porous stainless steel substrate.1,12,25-28 For example, Shu et al. found that the symmetric porous stainless steel with 0.2-µm (nominal) pores required the deposition of a palladium layer at least 15 µm thick, using electroless plating to form a dense, and impervious composite membrane.1,12 In our previous work, a commercial asymmetric porous stainless steel (APSS) tube with a relatively smooth top layer was used as the substrate and a thin and defect-free palladium membrane was prepared using an improved electroless plating method based on a multidimensional plating mechanism.29 In the present paper, the APSS tube was used as the substrate, and the improved electroless plating and the electroplating were

10.1021/ie050935u CCC: $33.50 © 2006 American Chemical Society Published on Web 12/15/2005

Ind. Eng. Chem. Res., Vol. 45, No. 2, 2006 649 Table 1. Composition of the Electroplating Baths Concentration (g/L) component

Pd95Ag5 bath

Pd90Ag10 bath

Pd80Ag20 bath

Pd(NH3)4Cl2‚H2O (NH4)2SO4 Ag2SO4 NH4NO3 KNO3

5.269 52.86 0.164 3.202 2.022

5.269 52.86 0.346 3.202 2.022

5.269 52.86 0.779 3.202 2.022

combined together. The thin and defect-free Pd-Ag composite membranes with controllable composition were prepared at room temperature. The hydrogen permeation behavior was investigated in detail. The methane steam reforming was tested in the membrane reactor from the as-prepared Pd-Ag alloy composite membrane. 2. Experimental Section An APSS tube (SUS316L, 10 mm in outer diameter (OD) and 50 mm in length) that had a smooth top layer was purchased from Nippon Seisen Co. One end of the tube was blind, and the other one was welded to an SUS316L plain tube. The preparation procedure for the Pd-Ag alloy composite membrane was described as follows: (1) To avoid intermetallic diffusion between the palladiumbased layer and the stainless steel fibers, a protective layer of silver, 0.25 µm thick, was electroplated on the surface of some APSS substrates. The silver plating solutions (Muden-Silver C1 and C2) were purchased from Okuno Chemical Industries (Japan). After mixing the C1 (50 mL) and C2 (200 mL) solutions well, the alkalinity of the mixed solution was adjusted to pH 10 using water. The silver electroplating was performed for 60 s at room temperature. (2) The silver-modified APSS substrate was activated by a process that is based on the multidimensional mechanism, as reported in our previous work.16,30 The aluminum hydroxide sol was prepared by titrating sodium carbonate solution (0.6 M) into aluminum nitrate solution (0.4 M). The resulting solid was peptized by nitric acid (1.6 M). The activating solution (Okuno, OPC-50) then was added to the aluminum hydroxide sol. The ratio between the activating solution and the aluminum hydroxide sol was ∼1:9. After strong stirring for 3 h, followed by treatment in an ultrasonic bath for another 3 h, the as-prepared palladium/aluminum hydroxide sol was introduced into the substrate pores from the outside of the tube via vacuum suction in an ultrasonic bath for 0.1-1 h. After the particles that were stuck on the substrate surface were completely removed, the APSS tube was dipped in the reducing solution (Okuno, OPC-150) for 0.5 h at a temperature of 313 K to change the Pd ions to palladium seeds. (3) The electroless plating of palladium on the activated APSS tube was performed in a commercial palladium plating solution (Okuno, Palla-Top) at 313-343 K with a pH value of 5-7. (4) The Pd-Ag alloy membranes, with a composition of 5, 10, and 20 mol % silver in Pd-Ag, were electroplated on the as-prepared Pd/APSS membranes. The compositions of the electroplating baths are shown in Table 1. The alkalinity of the baths was modulated to pH 10.5 using an ammonium aqueous solution before electroplating. The electroplating was performed at room temperature, with a voltage of 20-1500 mV for 0.1-5 h. The palladium and silver concentration in the plating solutions before and after membrane deposition were measured using inductively coupled plasma (ICP) spectroscopy (model ICPS7000, Shimadzu), which was applied to evaluate the apparent

Figure 1. Schematic of the gas permeation cell and membrane reactor for methane steam reforming.

thickness of the composite membranes (for the thickness of the Pd-Ag alloy layer, the apparent density calculated from the density and Ag/Pd ratio was used in this estimation). The membrane thickness was also examined using scanning electron microscopy (SEM). The structure of the membranes was measured using X-ray diffraction (XRD) (Rigaku, Japan). A copper anode (Cu KR radiation, 40 kV, 40 mA) was used as the X-ray source. The morphology of the membranes, such as three-dimensional images, was observed via laser-scanning confocal microscopy (LSCM) (Keyence model VK8500). The elemental distribution was analyzed using electron probe microanalysis (EPMA) (JEOL, model JXA-8100). The permeation tests of the as-prepared composite membranes were performed using a single-gas method in a double-tube permeation cell that was composed of the outside dense stainless steel tube and the inside membrane tube, as shown in Figure 1. The temperature of the permeation cell was increased to 773 K at a rate of 0.5 K/min under an argon atmosphere. Pure hydrogen then was introduced into the annulus space, and the membrane was activated for 1-6 h at 773 K. The hydrogen permeation behavior was tested at different temperatures with different hydrogen pressure differences, and the argon leak at 773 K with a pressure difference of 100 kPa was used to characterize the hydrogen selectivity. The membrane stability under the condition of temperature cycles was measured in an argon atmosphere. The gas-exchanging cycles between pure hydrogen and pure argon was also performed at 773 K to test the membrane stability. The membrane reaction of methane steam reforming was performed in the system that has been described in Figure 1. Eight grams of Ni/Al2O3 catalyst were packed into the annulus space, and the mixture of methane and steam was used as the feed in the retentate side. In the permeate side, the nitrogen was used as a sweep gas, to remove the permeated gas. The performance of the membrane reaction was measured under different operating conditions. 3. Results and Discussion 3.1. Effect of Substrate Property. The performance of the Pd-Ag alloy membranes, as a result of the electroplating

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Figure 2. X-ray diffraction (XRD) patterns (111 face) of Pd-Ag alloys with different compositions.

method, is strongly dependent on the property of the substrates. The symmetric porous stainless steel tubes (SPSS), having a nominal particle retention size of 0.5, 0.2, and 0.1 µm (from Mott Metallurgical Co.), and the asymmetric porous stainless steel (APSS) tubes with a top layer that consisted of stainless steel micrometer-sized fibers (rod diameters of 4, 3, 2, and 1 µm) from Nippon Seisen Co. were used to electroplate Pd-Ag membranes directly. On these unmodified substrates (without silver protection and palladium/aluminum hydroxide sol activation), preferential silver deposition was commonly observed, as reported by other researchers using the electroless plating technique.17,18 This means that the simultaneous Pd-Ag deposition cannot be directly fulfilled on these unmodified substrates. On the other hand, there was a preference for the membrane deposition to occur on the pore wall or on the stainless steel fibers. Membranes with large defects or pinholes were obtained in these cases. In this paper, to solve the aforementioned problems, the improved electroless plating method was used to modify the APSS substrate. Pre-deposition of the palladium membrane was performed on the activated APSS substrate (palladium/aluminum sol activation, based on the multidimensional plating mechanism). After that, on the surface of the palladium/activated APSS membranes, the Pd-Ag alloy layer, which has a controllable composition, can be electroplated at room temperature successfully. 3.2. Structure and Morphology of Pd-Ag Membranes. Figure 2 shows the 111-face peak of the Pd-Ag alloy composite membranes with different compositions prepared by the combined method of electroless plating and electroplating. For all three of these compositions, only the Pd-Ag alloy peak can be found after electroplating without any post-treatment, which indicates that the Pd-Ag alloy was formed at room temperature using this combined method. After activation at 773 K for 1 h in the pure hydrogen atmosphere, the 111-face peaks become narrower and the intensities become higher than those of the samples just after electroplating at room temperature. Moreover, with the increase in the amount of silver, the peak position shifts to a low-angle zone: the 111-face position of pure silver metal. Figure 3 shows the surface morphology of the fresh and used Pd-Ag/Pd/APSS membranes; for comparison, the images of fresh APSS substrate and the Pd/APSS membrane also are shown here. The palladium layer was continuously deposited

on the APSS substrate (Figure 3b), and there are no obvious defects on the Pd/APSS membrane surface. Figure 3c indicates that a Pd-Ag layer was plated on the Pd/APSS membrane continuously, the morphology of which is slightly different from that of the Pd/APSS membrane. However, the surface roughness after Pd-Ag deposition is almost same as that before Pd-Ag deposition, showing no preferential deposition on different places. The surface morphology after the hydrogen measurement (Figure 3d) indicates that a minor change occurred, but there still are no obvious defects. The cross section of the Pd-Ag/ Pd/APSS composite membrane with a nominal thickness of 5 µm (consisting of a 3-µm-thick palladium layer and a 2-µmthick Pd-Ag layer) was characterized using SEM (Figure 4). The membrane thickness, as determined using SEM analysis, is very consistent with the nominal values from the concentration change method. 3.3. Initial Process of Pd-Ag Membranes. Two Pd-Ag alloy composite membranes with a Pd80Ag20 layer 2 µm thick were electroplated on two Pd/APSS membranes from an improved electroless plating method. The palladium membrane deposited on the silver-modified APSS substrate is denoted as H-Pd/APSS and the related Pd-Ag membrane is denoted as H-Pd80Ag20/Pd/APSS. The palladium membrane that is deposited on the fresh APSS substrate (without silver protection) is denoted as L-Pd/APSS, and the related Pd-Ag membrane is denoted as L-Pd80Ag20/Pd/APSS. The H-Pd/APSS membrane has a hydrogen permeation flux of 0.278 mol/(m2 s), and infinite hydrogen selectivity (H2/Ar) is obtained at 773 K and a pressure difference of 100 kPa. The L-Pd/APSS membrane has a hydrogen permeation flux of 0.153 mol/(m2 s), and the hydrogen selectivity (H2/Ar) is only 83 under the same operating conditions. Figure 5 indicates that the hydrogen permeation flux of both the L-Pd80Ag20/Pd/APSS and H-Pd80Ag20/Pd/APSS membranes increases gradually in the period of the initial process. A time period of ∼30 h is required to initialize the Pd-Ag membrane, which is much longer than that (∼3 h) for pure palladium membranes. The hydrogen permeation flux of L-Pd80Ag20/Pd/APSS and H-Pd80Ag20/Pd/APSS membranes were tested to be 0.152 and 0.277 mol/(m2 s), respectively, at 773 K and a hydrogen pressure difference of 100 kPa. Moreover, the infinite hydrogen selectivity versus argon was obtained under the same conditions for both membranes. The results indicate that the 2-µm-thick Pd80Ag20 layer could remove the defects effectively while not increasing the hydrogen permeation resistance. On the other hand, the initial process of another PdAg membrane (Pd90Ag10/Pd/APSS) is shown in Figure 5. The steady hydrogen permeation flux of ∼0.321 mol/(m2 s) at 773 K and a pressure difference of 100 kPa was achieved within the short initial time of 3 h. The initial time of 3 h is much shorter than that for the other two Pd-Ag membranes. This phenomenon may be ascribed to the fact that there is no hydrogen permeation test before Pd90Ag10 deposition. The intermediate hydrogen tests for H-Pd/APSS and L-Pd/APSS membranes might change their surface structure to some extent, which makes initiation of the hydrogen permeation flux for L-Pd80Ag20/Pd/APSS and H-Pd80Ag20/Pd/APSS membrane more difficult. A more-detailed analysis will be given in our future investigation. 3.4. Performance of H-Pd80Ag20/Pd/APSS Membrane. Figures 6 and 7 show the hydrogen permeation flux of both H-Pd/APSS and H-Pd80Ag20/Pd/APSS membranes, respectively, at different temperatures versus the hydrogen pressure difference. It is obvious that the operating temperature and hydrogen pressure difference have a positive effect on the

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Figure 3. Morphology of substrates and Pd-Ag composite membranes: (a) fresh APSS substrate, (b) Pd/APSS membrane after H2 measurement, (c) fresh Pd80Ag20/Pd/APSS membrane, and (d) Pd80Ag20/Pd/APSS membrane after H2 measurement.

Figure 4. Scanning electron microscopy (SEM) image of the cross section of the Pd80Ag20/Pd/APSS composite membrane with a nominal thickness of 5 µm (consisting of a 3-µm-thick palladium layer and a 2-µm-thick PdAg layer).

hydrogen permeation flux. At the same temperature and pressure difference, these two membranes have similar hydrogen permeation fluxes (for example, a value of 0.278 mol/(m2 s) was obtained for H-Pd/APSS at 773 K and a pressure difference of 100 kPa, and a value of 0.277 mol/(m2 s) was obtained for H-Pd80Ag20/Pd/APSS under the same conditions). It indicates that the 2-µm-thick Pd80Ag20 layer did not impair the hydrogen permeance. The linearity of the curves is so good that all the correlation constants are >0.99, which indicates that proper exponent value (n) in the hydrogen permeation equation, J ) (Q/l)(PHn - PLn), is 1, rather than 0.5 for the bulk diffusion. The unit n value may result from the surface reaction and the Knudsen diffusion through membrane defects or the substrate. The infinite hydrogen selectivity can prove that the contribution of the Knudsen diffusion through membrane defects can be

Figure 5. Initial stages of Pd-Ag membranes electroplated on different substrates.

neglected. In regard to the substrate effect, we have checked it before the deposition of the palladium or Pd-Ag membranes. The involved substrates, such as fresh APSS, palladium/ aluminum hydroxide sol-activated APSS, and silver and palladium/aluminum hydroxide sol activated APSS, were treated at 773 K under the pure hydrogen atmosphere for 3 h. The pressure decrease through the substrates, while keeping the permeation flux equal to 0.5 mol/(m2 s), then was measured and all the values were 95.5% and increases gradually as the space velocity decreases. The value as high as 98% is obtained at a space velocity of 3000 1/h. The space velocity has a positive effect on the hydrogen production rate. Rates as high as 210 mL/min are achieved at the highest space velocity of 12 000 1/h in our experiments. Figure 15 shows the effect of the operating temperature on the methane conversion in the traditional reactor and the membrane reactor. It can be observed that the methane conversion in both reactors increases rapidly as the temperature increases. However, the difference of the methane conversion

Figure 16. Methane conversion versus sweep gas (N2) flux in the membrane reactor constructed from the Pd90Ag10/Pd/APSS composite membrane. Conditions: T ) 773 K, P ) 500 kPa, H2O/CH4 ratio ) 2, and space velocity (SV) ) 3000 1/h.

between the membrane reactor and the traditional reactor is very large. For example, the methane conversion in the membrane reactor is 3.4-5.7 times greater than that in the traditional reactor. Moreover, methane conversions as high as 89.23% were achieved at the lower operating temperature of 823 K in the membrane reactor. The greatly increased methane conversion in the membrane reactor is a result of the reaction equilibrium shift, because of the in situ removal of hydrogen through the thin Pd-Ag composite membrane. Figure 16 shows the effect of the sweep gas flux on the methane conversion in the membrane reactor. It can be observed that the methane conversion in the membrane reactor increases gradually as the sweep gas flux increases initially. In addition, the methane conversion in the membrane reactor is much higher than that in the traditional reactor under the same reaction conditions (18.21%). For example, the lowest methane conversion of 69.55% in the membrane reactor at the lowest sweep gas flux of 100 mL/min is still much higher than the methane conversion in the traditional reactor (18.21%). With further increases in the sweep gas flux, the methane conversion levels off rapidly. Table 2 shows the performance of membrane reactors constructed from different hydrogen separation membranes. It indicates that the performance of our Pd-Ag membrane reactor is relatively better than the others. 4. Conclusions A thin Pd-Ag alloy layer with controllable composition can be simultaneously electroplated on the surface of Pd/APSS membranes from improved electroless plating. The Pd-Ag alloy

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phase was formed at room temperature. The substrate property has an important effect on the performance of Pd-Ag alloy composite membranes. The hydrogen permeation behavior of the membrane (H-Pd80Ag20/Pd/APSS) was investigated in detail. A hydrogen permeation flux as high as 0.28 mol/(m2 s) was obtained at 773 K and a pressure difference of 100 kPa. Under the same conditions, the infinite hydrogen selectivity versus argon was obtained. The good membrane stability was proven by the temperature-changing cycles and the gasexchanging cycles. In addition, electron probe microanalysis (EPMA) indicated that this membrane had a stable membrane structure and a homogeneous Pd-Ag layer after long-term operation. Another Pd-Ag composite membrane of Pd90Ag10/ Pd/APSS with a hydrogen permeation flux of 0.321 mol/(m2 s) at 773 K and a pressure difference of 100 kPa was used as the membrane reactor for methane steam reforming. Better performance of the methane conversion (80.72%), CO2 selectivity (98%), and hydrogen production rate (210 mL/min) were obtained successfully. Acknowledgment The financial support of New Energy and Industrial Technology Development Organization (NEDO) and Japan Society for the Promotion of Science (JSPS) is gratefully acknowledged. Literature Cited (1) Shu, J.; Grandjean, B. P. A.; Ghali, E.; Kaliaguine, S. Simultaneous deposition of Pd and Ag on porous stainless steel by electroless plating. J. Membr. Sci. 1993, 77, 181. (2) Gobina, E. N.; Oklany J. S.; Hughes, R. Elimination of ammonia from coal-gasification streams by using a catalytic membrane reactor. Ind. Eng. Chem. Res. 1995, 34, 3777. (3) Paglieri, S. N.; Way, J. D. Innovations in palladium membrane research. Sep. Purif. Methods. 2002, 31, 1. (4) Tong, H. D.; Gielens, F. C.; Gardeniers, J. G. E.; Jansen, H. V.; van Rijn, C. J. M.; Elwenspoek, M. C.; Nijdam, W. Microfabricated palladiumsilver alloy membranes and their application in hydrogen separation. Ind. Eng. Chem. Res. 2004, 43, 4182. (5) Ma, Y. H.; Akis, B. C.; Ayturk, M. E.; Guazzone, F. Characterization of intermetallic diffusion barrier and alloy formation for Pd/Cu and Pd/Ag porous stainless steel composite membranes. Ind. Eng. Chem. Res. 2004, 43, 2936. (6) Kamakoti, P.; Morreale, B. D.; Ciocco, M. V.; Howard, B. H.; Killmeyer, R. P.; Cugini, A. V.; Sholl, D. S. Prediction of hydrogen flux through sulfur-tolerant binary alloy membranes. Science 2005, 307, 569. (7) Li, Z. Y.; Maeda, H.; Kusakabe, K.; Morooka, S.; Anzai, H.; Akiyama, S. Preparation of palladium-silver alloy membranes for hydrogen separation by the spray pyrolysis method. J. Membr. Sci. 1993, 78, 247. (8) Jayaraman, V.; Lin, Y. S. Synthesis and hydrogen permeation properties of ultrathin palladium-silver alloy membranes. J. Membr. Sci. 1995, 104, 251. (9) Xomeritakis, G.; Lin, Y. S. Fabrication of thin metallic membranes by MOCVD and sputtering. J. Membr. Sci. 1997, 133, 217. (10) O’Brien, J.; Hughes, R.; Hisek, J. Pd/Ag membranes on porous alumina substrates by unbalanced magnetron sputtering. Surf. Coat. Technol. 2001, 142-144, 253. (11) Uemiya, S.; Sato, N.; Ando, H.; Kude, Y.; Matsuda, T.; Kikuchi, E. Separation of hydrogen through palladium thin film supported on a porous glass tube. J. Membr. Sci. 1991, 56, 303. (12) Jemaa, N.; Shu, J.; Kaliaguine, S.; Grandjean, B. P. A. Thin Palladium Film Formation on Shot Peening Modified Porous Stainless Steel Substrates. Ind. Eng. Chem. Res. 1996, 35, 973. (13) Li, A. W.; Liang, W. Q.; Hughes, R. Characterisation and permeation of palladium/stainless steel composite membranes. J. Membr. Sci. 1998, 149, 259. (14) Roa, F.; Way, J. D. Influence of Alloy Composition and Membrane Fabrication on the Pressure Dependence of the Hydrogen Flux of Palladium-Copper Membranes. Ind. Eng. Chem. Res. 2003, 42, 5827. (15) Tong, J. H.; Matsumura, Y. Thin Pd membrane prepared on macroporous stainless steel tube filter by an in-situ multidimensional plating mechanism. Chem. Commun. 2004, 2460.

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ReceiVed for reView August 12, 2005 ReVised manuscript receiVed November 7, 2005 Accepted November 11, 2005 IE050935U