Effects of Water Gas Shift Gases on PdCu Alloy Membrane Surface

Effects of Water Gas Shift Gases on Pd-Cu Alloy Membrane Surface ... Department of Chemical Engineering, University of Colorado, Boulder, Colorado 803...
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Ind. Eng. Chem. Res. 2004, 43, 4188-4198

Effects of Water Gas Shift Gases on Pd-Cu Alloy Membrane Surface Morphology and Separation Properties Ames Kulprathipanja,† Go1 khan O. Alptekin,‡ John L. Falconer,† and J. Douglas Way*,§ Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309, TDA Research, Inc., Wheat Ridge, Colorado 80033, and Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado 80401

Palladium-copper alloy membranes (2-4 µm thick), which were deposited on porous ceramic tubular supports by electroless plating, separated H2 from a water gas shift (WGS) mixture at 623-723 K. However, within 120 min at 623 K, the total permeance increased and membrane selectivity decreased. H2, N2, or He did not change the transport properties of the membrane before WGS gas exposure. Annealing in CO2 and CO at 523-723 K increased the height of micronscale conical hillocks and defect sizes on the membrane surface by at least a factor of 3. Annealing in H2 and He after CO and CO2 exposure decreased the hillock heights by half. The hillocks were clusters of grains with defined valleys. The membrane defects allowed gases other than H2 to permeate through the membrane. Surface topology changes are partially due to the removal of C impurities by CO2 to form CO. Hillock heights on 25-µm-thick cast and rolled Pd-Cu alloy foils, which had no C impurities, increased by a factor of 4 at 723 K in the presence of CO2. The surface of the electroless-plated films was a factor of 3 rougher than the foils. Decreasing the membrane surface roughness, increasing the membrane thickness, and minimizing C impurities decreased membrane defect formation associated with surface rearrangement. Fewer vacancies and lattice defects in the alloy lattice may also make the foil more resistant to atom rearrangement than the electroless-plated membranes. The extent of WGS, CO disproportionation, and methanation reactions on the membrane increased at higher Cu alloy concentrations. Exposure to CO and CO2 segregated Pd to the feed side of the membrane and changed the membrane alloy composition and phase structure. The change in phase structure from bodycentered cubic to face-centered cubic decreased the H2 permeance through the membrane and may increase surface rearrangement. Introduction Palladium alloys have been studied as H2-separating membranes in applications that include hydrogenation and dehydrogenation reactions, H2 recovery from plant streams, and coal gasification.1-4 Many of these processes, including the water gas shift (WGS) reaction (eq 1) utilize the H2-separating properties of Pd alloy

CO + H2O f CO2 + H2

(1)

membranes in a membrane reactor to obtain conversions higher than thermodynamic equilibrium.2,5,6 Pd membranes have been operated at temperatures as high as 1023 K. In addition, combining reaction and separation into one unit can decrease the energy consumption and capital costs. Because H2 is an essential feedstock in the refining and chemical industry and is needed for fuel cells, this type of membrane reactor can be used in many applications.7-11 One disadvantage of Pd membranes is the high cost of Pd. Alloys require less Pd and thus lower the cost, but they also have many advantages for separation and stability. Pd has been alloyed with Ag, Cu, Au, and Ru.12-15 Pd-Cu alloys between 30 and 60 wt % Cu form * To whom correspondence should be addressed. Tel.: (303) 273-3519. Fax: (303) 273-3730. E-mail: [email protected]. † University of Colorado. ‡ TDA Research, Inc. § Colorado School of Mines.

the body-centered cubic (bcc) phase, which has higher H2 permeability, is more resistant to sulfur compounds, and can operate at lower temperatures than the facecentered cubic (fcc) phase of pure Pd.13,15,16 Below 573 K, pure Pd forms a palladium hydride, which embrittles as a result of lattice expansion. Alloying Pd also decreases the problem of membrane embrittlement caused by the pure Pd f palladium hydride phase transition during temperature cycling.17 Pd-Cu alloys with either the fcc or bcc phase structure do not embrittle at ambient temperature because they do not form a hydride phase. Pd and Pd alloy membranes have been reported with thicknesses of less than 5 µm.1,12,18,19 These thinner membranes require less Pd and are cheaper to produce. Though most Pd or Pd alloy membranes have been foils manufactured by casting and rolling, additional fabrication techniques are being used to decrease the membrane thickness. Recently, membranes were prepared by sputtering, chemical vapor deposition, and electroless plating. These membranes are usually supported on porous ceramic or stainless steel. Some membrane properties from recent studies are shown in Table 1.1,12,18,20 All of the membranes have similar H2 permeances (∼10-6 mol/s/m2/Pa) but have different properties due to composition, thickness, and the fabrication method. Uemiya et al.12 fabricated porous membranes by chemical vapor deposition, and the H2 transport through their membranes was hypothesized to be surface diffusion of H atoms. In contrast, H2 permeates

10.1021/ie030853a CCC: $27.50 © 2004 American Chemical Society Published on Web 06/12/2004

Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4189 Table 1. Recent Studies on Pd and Pd Alloy Membranes for H2 Separation membrane composition support material membrane thickness, µm preparation technique temperature, Κ H2 permeance, µmol/s/m2/Pa H2/N2 ideal selectivity reference

Roa et al.

Uemiya et al.

Uemiya et al.

Shu et al.

Edlund

Pd60Cu40 Al2O3 1.5 electroless plating 623 1.9 100 18

Pd Al2O3 3 CVD 773 3.1 240 12

Pt, Ru Al2O3 6, 3 CVD 773 0.5, 2.0 210, 120 12

Pd Al2O3 2 electroless plating 673 1 N2 undetectable 1

Pd60Cu40 none 15 chemically etched foil 673 5 N2 undetectable 20

through Pd and Pd alloy membranes fabricated by electroless plating or casting and rolling by solid-state diffusion of H atoms through the metal lattice.21 For most Pd membranes, H2 dissociates on the membrane surface, and H atoms diffuse through the membrane and recombine on the permeate side. Solidstate diffusion in the Pd alloy is the rate-limiting step for most operating conditions, but other possible ratelimiting steps include external mass-transfer resistance, reduced sticking probability due to surface contamination, and H2 desorption for membrane thicknesses of less than 1 µm.22 A decline in H2 permeation due to CO and H2O blocking H2 dissociation sites has been reported on Pd and Pd-Ag membranes.23,24 This study examined the effects of WGS reactants and products on the separation and catalytic properties of Pd-Cu alloy membranes prepared by electroless plating. Experimental Methods Membrane Preparation. The electroless-plated PdCu alloy membranes were fabricated using a procedure described previously.25 The Pd-Cu alloy films were prepared by successively depositing Pd and Cu onto asymmetric R-alumina tubular supports with a 20- or 50-nm-pore zirconia top layer (US Filter T1-70) and symmetric R-alumina tubular supports with a 200-nmpore top layer (CoorsTek GTC-998). The ceramic tubes were cleaned and activated by impregnating the top layer with a palladium acetate solution. The tubes were heated in air to oxidize the acetate and then reduced in flowing H2 to leave dispersed Pd metal in the pores of the top layer. A Pd film was formed by nucleation and growth at the Pd metal sites to a desired thickness. An osmotic pressure gradient was applied across the membrane by flowing an aqueous sucrose solution on the permeate side of the support during plating to reduce the porosity and promote surface homogeneity.26 After Cu was plated on top of the Pd film, intermetallic diffusion of the Pd and Cu metals was induced by annealing in a 5% H2/He gas mixture at 623-723 K to produce a uniform alloy film. The annealing process took approximately 5 days at 673 K for membranes of less than 5-µm thickness. Complete annealing was determined by a steady-state H2 permeance through the membrane for 2 days. The integrity of the Pd-Cu film was tested by pressurizing the membrane with N2, immersing it in a 2-propanol/H2O mixture, and checking it for gas leaks. In addition to the Cu plating process previously used,25 Cu was also deposited using a commercial process (Technic-Electroless Copper by Technic Inc.). Before annealing, this process produced a more uniform Cu layer, which increased selectivity but decreased H2 permeation. At least 10 days was necessary to anneal a 5-µm-thick membrane when the Technic Cu plating process was used.

Two 25-µm-thick Pd60Cu40 foils from Oremet Wah Chang, Inc. (Albany, OR), and Idatech, Inc. (Bend, OR), were also used to compare to the electroless-plated membranes. The Oremet Wah Chang foil was used for most of the characterization tests in this paper, unless otherwise stated, because of its availability at the time that this research was conducted. Membrane Characterization. Scanning electron microscopy (SEM; JEOL 840) and energy-dispersive X-ray spectroscopy (EDAX; Noran 5500) determined the film thickness and metal alloy composition of the membranes. The samples were sputtered with Au or C to reduce sample charging caused by the alumina support. The membranes were characterized after WGS exposures. Atomic force microscopy (AFM; Nanoscope II by Digital Instruments, Inc.) and X-ray diffraction (XRD) determined changes in the surface topology and lattice structure. The maximum vertical distance of the surface features measurable by AFM in contact mode was approximately 5 µm. The AFM images were analyzed using WSxM Scanning Probe Microscopy software. Siemens D500 XRD used monochromatic Cu KR radiation with a wavelength of 0.154 06 nm. The average grain size of the membranes was determined by the Scherrer equation from line broadening of the Pd-Cu alloy (110), (200), and (211) peaks. A 15-µm-thick Pd65Cu35 film was used for the AFM and XRD studies. Films of less than 4 µm thickness could not be removed from the tubular supports and handled without extensive damage. Total C analysis (Leco C-200) determined the bulk C content in the Pd-Cu films and foils. The samples were oxidized at over 1773 K, and the amount of carbon oxidation products was measured. Thermogravimetric analysis (TGA) was also conducted in a Shimadzu TGA-50 microbalance system, which allowed a continuous gas stream to be passed over a membrane sample while the temperature of the system was increased and the weight change monitored. Separation and Kinetic Measurements. The separation module consisted of a 2.5-cm-o.d. shell and a 0.6cm-o.d. inner tube, both made from stainless steel 316 tubing. The membrane was centered and attached to the tube by Swagelok fittings and graphite seals. The reactants entered through the tube, and gases permeated to the shell side. Heating tapes preheated the gases to 623 K, and a tube furnace heated the membrane module to 773 K. A digital flowmeter measured the permeate flow rate. The system pressure was controlled with a pressure control valve located downstream of the membrane module. Mass flow controllers fed H2, CO, CO2, He, and N2 to the system. Steam was introduced into the system with a high-pressure liquid pump and a boiler built from a stainless steel tube, band heaters, and insulation. After passing through the separation module, steam was condensed and collected to prevent condensation in the system lines. The feed gas mixture was directed either through the membrane module or

4190 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 Table 2. Electroless-Plated Pd and Pd-Cu Membrane Composition, Thickness, Pore Size of the Support Top Layer, and Usage Cu thickness, membrane wt % µm

pore size of the support top layer, nm

usage

35 ( 4 30 ( 5 25 ( 3 20 ( 3 90 ( 2 0 25 ( 3 20 ( 4

50 20 50 50 20 20 50 200

AFM, XRD C analysis binaries, SEM WGS mixture catalytic activity N2/CO2/H2 CO2/CO/He CO2/H2

1 2 3 4 5 6 7 8

15 ( 1 3(1 3(1 3(1 3(1 2(1 4(1 12 ( 1

through a bypass loop. Permeate and retentate streams were analyzed by a gas chromatograph equipped with a thermal conductivity detector and CO, CO2, and CH4 gas analyzers. H2/N2 ideal selectivity was determined by dividing the single gas fluxes at 623-723 K and 220-430 kPa. The permselectivity was calculated by dividing the permeances of gases in a mixture separation. Permeance is the flux divided by the transmembrane partial pressure difference. A linear pressure dependence was used because of the presence of small defects that allowed H2 permeation. Permeate and feed pressures and mixture composition were used to calculate the permselectivity.

Figure 1. SEM micrograph of a Pd75Cu25 alloy membrane.

Results Membrane Characterization. (a) Composition and Thickness. Membrane composition and thickness, the pore size of the support top layer, and how the membranes were used in this study are included in Table 2. EDAX and SEM measured the membrane compositions and thicknesses after various gas exposures except for membranes 1 and 8. Membrane 1 was electrolessly plated onto a ceramic tubular support but delaminated because of weak adhesion. The membrane was characterized before and after WGS exposure to compare the changes in membrane properties due to different gas treatments. Additional Pd and Cu were plated on membrane 8 after CO2/H2 exposure, so the alloy composition of the membrane after WGS exposure could not be directly obtained. Therefore, by comparison of the experimental conditions used to fabricate the membrane and earlier results of membranes made on GTC supports, its properties were approximated.25 Membranes 2-7 were exposed to WGS gases, broken, and then characterized. A SEM micrograph (Figure 1) of a Pd75Cu25 membrane (membrane 3) shows a cross section of a membrane deposited on an alumina porous support with a zirconia top layer. The gap between the support top layer and membrane may have formed when the tubular support was broken. The voids in the electroless-plated membrane probably formed during the nucleation and growth process. Measurements using EDAX showed that the composition of the top 1-µm surface region of the Wah Chang foil changed from 40 to 25 and 20 wt % Cu during CO2 and CO exposure, respectively. Annealing in H2 after CO exposure increased the Cu concentration of the foil to approximately 30 wt %. Therefore, the Cu concentrations in Table 2 for the feed side of the electroless-plated films exposed to carbon oxides may be lower than those in the bulk. Pd segregation to the surface region may explain the low Cu concentration after CO and CO2

Figure 2. AFM images of a Pd65Cu35 alloy film membrane annealed in H2 at 723 K and 220 kPa for 7 days: (a) twodimensional view; (b) three-dimensional view.

exposure because an alloy composition with 40 wt % Cu was normally targeted during fabrication. In addition, cross-sectional analysis by EDAX of membranes exposed to carbon oxides showed higher concentrations of Cu (50-70 wt %) near the support than those on the feed side. Cu must segregate to the support side because Pd was electrolessly deposited onto the support before Cu. The metals interdiffuse for the Pd-Cu alloy to form. After annealing in H2 but before exposure to CO and CO2, membrane 1 had a uniform alloy composition across the thickness of the membrane. The Cu concentration of membrane 5 was increased to 90 wt % to determine the effects of a high Cu alloy composition on separation and catalytic properties. Membrane 6 did not contain any Cu and was used to compare a pure Pd membrane with the Pd-Cu alloys. (b) Electroless-Plated Film Surface Topology. A 15-µm-thick Pd65Cu35 alloy membrane annealed in H2 at 723 K and 220 kPa for 7 days exhibited hillocks, micron-scale conical features protruding from the surface, and valleys, micron-scale indentations on the surface of the film. These are shown in the 5 × 5 µm AFM images of the feed side of the membrane in Figure 2. The light spots in Figure 2a are the hillocks and the darker spots the valleys (boundaries between separate hillocks). These conical hillocks and their heights are clearly seen in Figure 2b. The maximum hillock height of the sample is shown on the z axis of the threedimensional AFM image. The average grain size of the Pd65Cu35 film was approximately 100 nm before and after exposure to various WGS gases, as determined by the Scherrer equation. Though applicability of the Scherrer equation decreases above 80 nm, no change in the grain size was

Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4191 Table 3. Surface Roughness after Gas Exposures at 220 kPa for 7 days; 5 × 5 µm Sample Area surface roughness (rms), nm gas

Pd65Cu35 film

Pd60Cu40 foil

exposure temp, K

H2 CO2 He after CO2 H2 after CO2 CO2 after CO2 and H2 CO H2 after CO CO2 CO

67 ( 7 415 ( 43 201 ( 17 175 ( 25 143 ( 15 170 ( 10 190 ( 7 160 ( 10 150 ( 10

19 ( 4 148 ( 18 108 ( 12 78 ( 10 123 ( 30 14 ( 3 12 ( 2 24 ( 4 18 ( 2

723 723 723 723 723 723 723 523 523

Table 4. Maximum Hillock Height after Gas Exposures at 220 kPa for 7 days; 5 × 5 µm Sample Area maximum hillock height, µm gas

Pd65Cu35 film

Pd60Cu40 foil

exposure temp, K

H2 CO2 He after CO2 H2 after CO2 CO2 after CO2 and H2 CO H2 after CO CO2 CO

0.4 ( 0.1 2.5 ( 0.3 1.2 ( 0.1 0.9 ( 0.2 1.0 ( 0.1 1.1 ( 0.1 1.3 ( 0.3 1.1 ( 0.2 0.7 ( 0.2

0.18 ( 0.04 0.77 ( 0.13 0.76 ( 0.10 0.46 ( 0.10 0.79 ( 0.20 0.10 ( 0.02 0.09 ( 0.02 0.17 ( 0.05 0.12 ( 0.05

723 723 723 723 723 723 723 523 523

seen before and after WGS gas exposure. Therefore, the hillocks are not single grains (crystallites with the same bulk structure) because they are on the order of 400 nm in width. Electroless plating nucleates and grows grains formed from Pd seeds impregnated onto the support during activation. The voids, seen in Figure 1, may form the valleys between the hillocks seen in the AFM images. The valleys, if enlarged, can lead to membrane defect formation. A summary of the surface roughness and maximum hillock heights obtained from the three-dimensional AFM images for a 5 × 5 µm membrane sample area after various gas exposures at 523 and 723 K and 220 kPa are shown in Tables 3 and 4. Different sections of membrane 1 were used for each gas exposure though the samples were all first exposed to H2 at 723 K and 220 kPa for 7 days. The films were peeled off the support, and both sides of the membrane were exposed to the various gases. The surface roughness and maximum hillock heights in Tables 3 and 4 are on the retentate side and not the support side. The maximum hillock heights describe the vertical growth of the conical hillocks that protrude from the surface. The surface roughness is calculated by the root mean square (rms) of the differences between the mean surface height and individual hillock or valley heights using the AFM analytical software. The surface roughness is an average of the height differences and does not describe the variation in hillock heights of a particular sample. Therefore, a rougher surface has larger differences between the mean surface height and individual hillock or valley heights. In addition, because the hillocks grow horizontally and vertically, there are fewer hillocks per unit area. The tables include the results for the Pd65Cu35 electroless-plated film and the Pd60Cu40 Wah Chang foil. The data have a reproducibility of approximately (10% for a 5-8 quantity sample set. The Idatech foils were also exposed to H2 and CO2 and had

Figure 3. AFM images of a Pd65Cu35 alloy film membrane exposed to CO2 at 723 K and 220 kPa for 7 days: (a) twodimensional view; (b) three-dimensional view.

((10%) roughness and hillock heights similar to those of the Wah Chang foils. Exposing the delaminated electroless-plated film to pure CO2 at 723 K for 7 days coalesced the hillocks, which are seen as micron-size light spots, and produced larger hillock heights and deeper valleys, seen as the dark spots (Figure 3; note the difference in the z-axis scale from Figure 2). The CO2-exposed film is rougher than the original film because the hillocks increased in height and width. The maximum height of the hillocks increased from 0.43 to 2.5 µm. Interestingly, annealing the membrane at 723 K in He or H2 partially reversed changes caused by CO2. The maximum hillock heights decreased from 2.5 to 1.2 µm after He exposure and to 0.93 µm after H2 exposure. A second CO2 exposure had no effect on the hillock heights and surface roughness of the electroless-plated membrane. When a separate section of the original H2-annealed membrane was exposed to CO at 723 K for 7 days, the maximum hillock height increased from 0.43 nm to 1.1 µm, less than half the height of the CO2 exposure. The surface topology of the CO-exposed membrane was similar to that of the CO2-exposed membrane after H2 and He exposure (∼1 µm). H2 at 723 K did not reverse the surface changes caused by the CO. The film’s surface topology changed less when exposed to CO2 and CO for 7 days at 523 K rather than 723 K. (c) Cast and Rolled Foil Surface Topology. The cast and rolled foils were smoother than the electrolessplated films (Tables 3 and 4). The Pd60Cu40 foil surface roughness and hillock heights were less than 40% of those of the Pd65Cu35 film before CO2 exposure. The initial Wah Chang foil surface roughness and maximum hillock height were 19 nm and 0.18 µm, respectively. The Idatech foil had an initial surface roughness and hillock height of 31 nm and 0.19 µm, respectively. The initial film surface roughness and maximum hillocks heights were 67 nm and 0.43 µm, respectively. CO2 also changed the surface topology of the Pd60Cu40 foils (Figures 5a and 5b). Though Figure 5b of the foil after CO2 exposure may seem rougher than Figure 3b of the film after CO2 exposure, there is more than a factor of 3 difference in the z-axis scales. Exposing the Wah Chang foil to CO2 at 723 K increased the surface roughness from 19 to 148 nm. After CO2 exposure, the surface roughness and hillock height of the Idatech foil increased to 155 nm and 0.85 µm, respectively. In addition, unlike the electroless-plated film, the foils’ surface topology did not change following CO exposure. He and H2 partially reversed the effects of CO2 on the foils. Furthermore, unlike the electroless-plated film, a second CO2 exposure increased the hillock height and

4192 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004

Figure 4. AFM images of a Pd60Cu40 alloy foil membrane exposed to H2 at 723 K and 220 kPa for 7 days: (a) two-dimensional view; (b) three-dimensional view. Figure 6. XRD patterns of a Pd65Cu35 alloy membrane annealed in H2 at 723 K and 220 kPa for 7 days, a Wah Chang Pd60Cu40 foil unannealed and annealed in H2 at 723 K for 7 days, and a Idatech Pd60Cu40 foil annealed in H2 at 723 K for 7 days.

Figure 5. AFM images of a Pd60Cu40 alloy foil membrane exposed to CO2 at 723 K and 220 kPa for 7 days: (a) two-dimensional view; (b) three-dimensional view.

surface roughness to values similar to those of the first CO2 exposure (Tables 3 and 4). The foils’ surface topology did not change at 523 K. (d) Surface Area. The surface area of the films and foils, calculated using the AFM software, also corresponded well with the trends in surface roughness. However, the magnitudes of the change in the surface area were small because of the nanometer-scale surface features. The surface area of the foils and films annealed in H2 were 25 and 31 µm2, respectively, for a 5 × 5 µm sample area. Surface area and roughness both increased or decreased with different gas exposures except for the CO2- and CO-exposed film at 523 K. The lower temperature, which increased the surface area but decreased the surface roughness, produced more, but shorter hillocks, whereas the higher temperature produced fewer, but taller hillocks as described earlier. (e) Phase Structure. The Pd65Cu35 film and both foils were annealed in H2 for 7 days at 723 K and exposed to pure CO2 and CO at 723 K for 7 days. The XRD pattern showed that the film had a Pd-Cu bcc lattice parameter of 0.298 nm (Figure 6). The lattice parameter is the length of the unit cell of the cubic structure. A small amount of the fcc phase was also present with a lattice parameter of 0.377 nm. The bcc lattice parameter calculated from the (110) and (211) crystal planes parallel to the surface did not change after exposure to CO2 or CO; no indications of bulk carbide or oxide formation were detected. However, a stronger fcc peak for the film was detected after CO2, CO, and a second H2 exposure (Figure 7). The unannealed Wah Chang and Idatech Pd60Cu40 foils were both fcc but changed to bcc after annealing in H2. The Idatech foil crystal plane parallel to the surface was similar to the electroless-plated film; both have a larger bcc (110) orientation, whereas the Wah Chang foil was mainly bcc with a (211) orientation. Both of the foils’ bulk

Figure 7. XRD patterns of a Pd65Cu35 alloy membrane annealed in H2, CO2, and CO at 723 K and 220 kPa for 7 days.

crystal structures also saw a small increase in the fcc structure after exposure to CO2 or CO. (f) Carbon (C) Content. Total C analysis on a Pd70Cu30 film measured 8 wt % C in the membrane after H2 annealing, 5 wt % after CO annealing for 7 days at 723 K, and below the detection limit of 0.01 wt % after CO2 annealing for 7 days at 723 K. No C was detected in either foils after H2, CO2, or CO exposures. The C contents of three 10-mg film and foil samples were measured after each gas exposure and were within 1 wt % or less of each other. Furthermore, TGA determined that flowing CO2 over electroless-plated films and cast and rolled foils resulted in sample weight losses of approximately 6 and 0.2%, respectively. Sample sizes of approximately 6 mg were used and heated to 673 K. Membrane Transport Data. (a) Effect of CO and CO2. H2/CO2 permselectivity versus temperature and time for a 50% CO2/H2 mixture through a Pd75Cu25 membrane is shown in Figure 8. As stated earlier, the alloy compositions for the electroless-plated membranes were measured after CO2 exposure and had a high Pd concentration at the feed side surface region because of Pd segregation. Moreover, the 25 wt % Cu concentration is similar to the alloy composition measured by EDAX for the Pd60Cu40 Wah Chang foil exposed to CO2. Both H2 and CO2 permeate flows increased with temperature. The H2/CO2 permselectivity reached a maximum of 55 at 150 min and then decreased to 28 after 1200 min. Before introduction of CO2, the H2 permeance at 623 K was 2.5 × 10-7 mol/s/m2/Pa. The initial H2 permeance

Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004 4193

Figure 8. H2/CO2 permselectivity and temperature versus time for a Pd75Cu25 alloy membrane: total transmembrane pressure ) 420 kPa; 50% CO2/H2 feed composition.

Figure 9. H2 permeance and temperature versus time for a Pd75Cu25 alloy membrane: total transmembrane pressure ) 420 kPa; 50% CO2/H2 feed composition.

was 1 order of magnitude lower than that for membranes fabricated by Roa et al. (Table 1). A similar electroless plating technique was used to fabricate the membrane except for the use of the Technic Cu plating bath. Additional differences were membrane thickness and alloy composition. The Pd60Cu40 membrane fabricated by Roa et al. was 1.5 µm thick compared to 3 µm thick for the Pd75Cu25 membrane. A 40 wt % Cu alloy composition has the highest H2 permeability so any deviation in the alloy composition would decrease permeation.18 Introducing CO2 to the H2 feed at 20 min and 623 K decreased the H2 permeance by 2 orders of magnitude (Figure 9). After 200 min, the H2 permeance increased with temperature to 5.0 × 10-7 mol/s/m2/Pa at 723 K. An increase in the H2 permeance with temperature initially increased the H2/CO2 permselectivity, but the CO2 permeance increased with time and this decreased the permselectivity. After CO2 exposure, the membrane was exposed to CO and then H2O for approximately 20 h each. The H2 permeance increased to 1 × 10-6 mol/ s/m2/Pa during a 5% CO/H2 exposure at 723 K. A 7% H2O/H2 mixture decreased the H2 permeance through the membrane (Figure 10), but a 10% H2O/He mixture delaminated the film from the support, causing the membrane to fail within 60 min. The H2 and N2 single gas permeances and ideal selectivities for the same Pd75Cu25 membrane were determined between the different gas exposures described in the previous paragraph (Table 5). The mem-

Figure 10. H2 permeance and temperature versus time for a Pd75Cu25 alloy membrane: total transmembrane pressure ) 390 kPa; 7% H2O/H2 feed composition.

Figure 11. CO2 and CH4 concentration in the retentate after CO and during H2O/H2 exposure for a Pd75Cu25 alloy membrane: total transmembrane pressure ) 380 kPa; 7% H2O/H2 feed composition. Table 5. H2 and N2 Permeances and H2/N2 Ideal Selectivity versus Time at 723 K and 430 kPa Transmembrane Pressure for a Pd75Cu25 Alloy Membrane after Various Gas Exposures time, days 5 6 7 11 12 13 14

H2 permeance × 107, mol/s/m2/Pa

N2 permeance × 109, mol/s/m2/Pa

ideal selectivity H2/N2

2.3 ( 0.1 2.3 ( 0.1 2.8 ( 0.1 6.5 ( 0.3 5.4 ( 0.3 11 ( 0.6 8.6 ( 0.4

150 32 ( 3 32 ( 3 63 ( 6 11 ( 1 51 ( 5

gas He N2/H2 CO2/H2 CO2/H2 He CO/H2 H2O/H2

brane was exposed to the gas(es) listed in the last column of the table for at least 1 day before measurement of the H2/N2 ideal selectivity. After annealing of the membrane in He and a H2/N2 mixture at 723 K for 6 days, the N2 permeance was below the detection limit of 1.5 × 10-9 mol/s/m2/Pa. Binary combinations of CO2/ H2 and CO/H2 increased H2 and N2 permeances and decreased the ideal selectivity to 32 and 11, respectively. He flow after CO2 exposure increased the ideal selectivity to 63; both of the H2 and N2 single gas permeances decreased. The addition of a H2O/H2 mixture after CO exposure formed CO2 and CH4 (Figure 11) and also increased the ideal selectivity to 51. The decrease in the formation of the C products coincides with the reduction of C being removed by the H2O/H2 mixture on the membrane surface.

4194 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004

Figure 12. H2 permeance and temperature versus time for a pure Pd membrane: total transmembrane pressure ) 300 kPa; 25% N2, 25% CO2, and 50% H2 feed composition.

Figure 13. Nitrogen and CO2 permeance and temperature versus time for a pure Pd membrane: total transmembrane pressure ) 300 kPa; 25% N2, 25% CO2, and 50% H2 feed composition.

Table 6. He Permeance versus Time for a Pd75Cu25 Alloy Membrane Exposed to H2, CO2, and CO at 723 K time, min

gas

He permeance × 108, mol/s/m2/Pa

1500 1750 2750 3100 3400

2% H2/He He 50% CO2/He 10% CO/He He

3.0 ( 0.2 3.3 ( 0.2 5.1 ( 0.3 7.8 ( 0.4 7.6 +/-0.4

He permeance versus time for several binary mixtures through a Pd75Cu25 membrane showed that CO2 and CO exposure changed the membrane performance and not byproducts from reactions with H2 (Table 6). A 2% H2/ He mixture and pure He did not alter the total permeance through the membrane, but the addition of CO2 and CO increased the permeation as shown earlier with the H2 binaries. The He permeance in a 50% CO2/He binary reached a steady state, but the He permeance in a 10% CO/He mixture did not. After exposure of the membrane to CO2 and CO for a total of 1350 min, the He permeance was 2.5 times its original value. CO2 did not decrease the membrane selectivity or increase the H2 permeance of a 12-µm-thick electrolessplated Pd80Cu20 membrane. Though the initial H2 permeance of the membrane was only 9.8 × 10-9 mol/ s/m2/Pa because of the thickness and alloy composition, the membrane had a H2/N2 ideal selectivity of at least 150 and it did not change during exposure to a 50% CO2/ H2 mixture for 1800 min at 723 K and 1120 kPa. The permselectivity of the 3-µm-thick Pd75Cu25 membrane exposed to the same conditions except at 420 kPa transmembrane pressure decreased after 60 min at 723 K (Figure 8). The permeability (permeance × membrane thickness) of the Pd80Cu20 12-µm-thick membrane (1.2 × 10-13 mol/s/m/Pa) was similar to that of the Pd80Cu20 3-µm-thick membrane (1.5 × 10-13 mol/s/m/Pa). A pure Pd membrane exposed to a tertiary mixture of H2, N2, and CO2 showed transport characteristics similar to those of the Pd-Cu alloys. The H2 permeance was 1 order of magnitude higher than the N2 and CO2 permeances, and all three gas permeances increased with temperature (Figures 12 and 13). At 723 K, both the H2/N2 and H2/CO2 permselectivities reached maxima of approximately 15 but decreased to 8 after 2500 min. The H2 permeance increased to a maximum of 3.4 × 10-6 mol/s/m2/Pa at 1600 min, whereas the N2 and CO2 permeances reached maxima of approximately 2.5 × 10-7 mol/s/m2/Pa at 2200 min. The N2 permeance was slightly higher than the CO2 permeance at 723 K.

Figure 14. Total permeance and H2/N2, H2/CO, and H2/CO2 permselectivities versus time for a Pd80Cu20 alloy membrane: temperature ) 623 K; total transmembrane pressure ) 280 kPa; 5% CO, 20%, CO2, 35% H2O, and 40% H2 feed composition.

(b) Effect of the Alloy Composition. The H2 permeance of the pure Pd membrane was close to 2 orders of magnitude higher than that of the Pd80Cu20 membrane used for the WGS exposure because of the alloy composition and thickness. The large difference in H2 permeability is due to the variation in the diffusivity and solubility of the different Pd-Cu alloy compositions. Piper reported that diffusion coefficients for Pd-Cu alloys span more than 2 orders of magnitude.16 The increase in the diffusion coefficients is due to the change from the fcc to bcc phase structure as the Cu concentration is increased. However, increasing the Cu concentration in the Pd-Cu alloy also decreases the H2 solubility. Roa et al. have reported that the H2 permeability for an electroless-plated Pd membrane can be at least 4 times higher than that for a Pd80Cu20 membrane.18 (c) Effect of the WGS Mixture. A Pd80Cu20 membrane exposed to a WGS mixture (5% CO, 20% CO2, 35% H2O, and 40% H2) at 623 K (Figure 14) had an initial H2/N2 permselectivity of 45 and a total permeance of 5 × 10-8 mol/s/m2/Pa. The H2/CO and H2/CO2 permselectivities were initially 23 and 15, respectively. The permselectivities increased slightly with time, but then both decreased to approximately 15 as the total permeance increased to 1.8 × 10-7 mol/s/m2/Pa. The experiment was stopped before reaching steady state so that the membrane could be characterized. Membrane Catalytic Activity. The Pd-Cu membrane catalyzes the WGS (eq 1), CO disproportionation

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(eq 2), and methanation (eq 3) reactions.

2CO f C(s) + CO2

(2)

CO2 + 4H2 f CH4 + 2H2O

(3)

At 723 K and 340 kPa feed pressure, a Pd10Cu90 membrane converted 16% of the CO2 in a 50% CO2/H2 mixture to CO by the reverse WGS reaction, whereas a Pd75Cu25 membrane converted only 6%. A pure Pd membrane converted less than 0.5% of the CO2 in the feed. Discussion Effect of CO and CO2. (a) H2 Separation. Unlike the behavior reported for other pure Pd and Pd-Ag alloy membranes,5,6,23,24,27,28 introducing CO2 or CO to Pd and Pd-Cu alloy membranes prepared by electroless plating decreased the selectivity and increased the permeation over time. The conical hillocks on the Pd65Cu35 electroless-plated membrane grew to 2.5-6 times their original size during exposure to CO and CO2 (Figures 2 and 3). CO and CO2 enlarged valleys and formed membrane defects, which increased transport of molecules other than H2 through the membrane (Table 5). In membrane 3, the H2/CO2 permselectivity decreased with time as CO2 permeated through membrane defects (Figure 8). The initial increase in the permselectivity was due to the increase in the H2 permeance with temperature. H2 permeation by solution diffusion increased faster with temperature than the increase in the membrane defect size, which increased CO2 permeation. The formation of membrane defects during CO and CO2 exposure also explains the higher H2/N2 permselectivity before introduction of carbon oxides compared to the H2/CO2 and H2/CO permselectivities in Figure 14. Permeation through Knudsen size defects depends on the molecular weights of the individual molecules. Therefore, if permeating only by Knudsen diffusion, N2 should diffuse 1.3 times faster than CO2. The similar N2 and CO2 permeances at 623 K in Figure 13 may be because CO2 has a higher adsorption energy than N2 on the Pd surface. CO2 and N2 may transport through membrane defects by an additional transport mechanism such as surface diffusion. At 723 K, CO2 permeance was lower than N2, possibly because of the higher rate of CO2 desorption at higher temperatures. Unlike the results shown in Figure 13, Knudsen diffusion also predicts that CO2 and N2 permeance should decrease with increasing temperature. However, increasing temperature increased the size of the hillocks and membrane defects (Table 4), which increased the permeation of CO2 and N2. The increase in the H2 permeance (Figure 9) during CO2/H2 mixture exposure was not due to surface rearrangement caused by byproducts from side reactions because the He permeance also increased with time for a CO2/He mixture (Table 6). However, CO formed when both H2 and CO2 were exposed to the membrane because the Pd-Cu alloy promotes the reverse WGS reaction. CO, similar to CO2, increased the membrane defect sizes and decreased the membrane selectivity (Table 5). (b) Surface Roughness and Thickness. The electroless-plated film surface was 3 times rougher than the foil initially and after exposure to the WGS gases. The maximum hillock height of the foil after exposure to CO2

was 0.77 µm compared to 2.5 µm for the film. The higher hillock heights of the electroless-plated films partially explain the lower resistance of the films to WGS gases than the foils. Because hillocks that formed from electroless plating coalesce and grow vertically from the support, a minimum membrane thickness may be necessary to reduce the negative effects of valleys or membrane defects. Hillock heights of the Pd-Cu alloy films increased from 0.4 to 2.5 µm after exposure to CO2 at 723 K for 7 days. If the original film thickness was only 3 µm, valleys would penetrate through the entire membrane, form membrane defects, and decrease the membrane selectivity, as seen in the transport results. In contrast, a 12-µm-thick Pd80Cu20 membrane fabricated using the same electroless plating procedure did not show a decrease in the selectivity during CO2 exposure to the same temperature and more than 2 times higher feed pressure. The thicker membrane allowed the hillocks to grow without forming valleys that penetrated through the entire membrane. The permeance did not increase with surface rearrangement, because the overall change in the membrane thickness was averaged by both hillock and valley formation. Therefore, smoother and thicker membranes resist membrane defect formation due to surface rearrangement better than rougher or thinner membranes. Pd and Pd-Ag alloy membranes in other studies were more than 15 µm thick and were fabricated by electroless plating or obtained as foils. The Pd-Cu alloy membranes that deteriorated during CO and CO2 exposure were less than 4 µm thick and were made by electroless plating. (c) Alloy Composition. The low Cu concentration detected by EDAX at the top 1-µm surface region was due to exposure of carbon oxides. The target composition during membrane fabrication was 40 wt % Cu for all of the membranes except membranes 5 and 6. However, the membrane composition in most cases could not be verified until after breaking the membrane for characterization. The Pd65Cu35 film phase structure transformed from mainly bcc to some fcc in the presence of CO2 and CO (Figure 7). The phase structure changed because of Pd segregation to the surface region of the membrane. The fcc phase structure has a lower diffusion coefficient and is present at less than 30 wt % Cu and greater than 60 wt % Cu.29 The fcc phase formed not only at the feed side because of Pd segregation but also closer to the support side because of enrichment of Cu. The surface morphology rearrangement did not depend on the specific bcc phase orientation (110 vs 211) because both Wah Chang and Idatech foils were affected by CO2 exposure. The presence of Cu in the membrane is also not responsible for the decline in the membrane performance because the effect was also seen with pure Pd membranes that were prepared using our fabrication technique. (d) C Impurities. Removing C from the membrane structure may be partially responsible for the changes in the surface topology and selectivity. C was detected in a Pd70Cu30 membrane before CO2 and CO exposure, probably from the activation procedure and/or plating process. After CO2 exposure for 7 days at 723 K, C was not detected in the membrane, probably because it was oxidized to CO (eq 4). TGA completed with CO2 on both

C(s) + CO2 f 2CO

(4)

foils and films in this study supports the removal of C via eq 4. Moreover, exposure to CO deposited C and

4196 Ind. Eng. Chem. Res., Vol. 43, No. 15, 2004

formed CO2 on the Pd-Cu alloy membrane by the reverse reaction (eq 2). This would explain the small C content detected using total C analysis after CO but not CO2 exposure. C was also removed in the form of CO2 and CH4 when a H2O/H2 mixture was introduced to a membrane after CO exposure (Figure 11). The short decline in CO2 and CH4 formation at 200 min may be evidence of two types of C on the membrane surface or two different mechanisms of C removal. Most of the C from the plating process, but maybe not all, had already been removed from the membrane by CO2 before CO was introduced. Galuszka et al. reported that CO exposure formed filamentous C, which led to swelling and membrane failure in an electroless-plated Pd membrane.30 Membrane swelling by C may explain why the total permeance during 10% CO/He exposure did not reach a steady state after 350 min at 723 K (Table 6). Removing C with CO2 exposure increased the surface roughness (Table 3), hillock height (Table 4), and defect diameter, and it decreased the H2 selectivity (Figure 8). A second CO2 exposure did not change the film’s surface topology after H2 exposure possibly because CO2 had removed all C during the first exposure (Tables 3 and 4). Similarly, the foils did not have any detectable C content, and they exhibited a much lower degree of surface rearrangement than the electroless-plated films. CO2 increased the hillock heights on the foils to 0.77 µm compared to 2.5 µm for the films. However, unlike the film, a second CO2 exposure changed the foil’s surface topology after H2 exposure. Therefore, CO2 must have an additional effect on the Pd-Cu membranes besides removing C. The film’s surface may not have changed significantly after the second CO2 exposure because of the initially rougher surface than the foil. Aspects that may induce topology changes due to CO and CO2 exposure exposure are lattice stress relaxation, phase structure, grain coalescence, and metal sintering as discussed below. Surface Topology Rearrangement. (a) Lattice Stress Relaxation. Hillocks form on metal films generally as a result of mismatch stresses between the substrate and the film, because of thermal expansion.31 This is not the reason for the surface rearrangement of the Pd-Cu alloy foils and films because both were unsupported. Furthermore, because H2/N2 mixtures at 623 and 723 K did not decrease the membrane selectivity of supported films (Table 5 and Figure 14), thermal expansion probably did not form hillocks on the membrane. However, because these metal films were polycrystalline, stress relaxation at grain boundaries and membrane defects is possible. Intrinsic stresses form because of lattice defects or vacancies in a particular lattice structure. Lattice defects may include substitutional or interstitial impurities in the lattice. The annealing process did not remove the C impurities in the electroless-plated films as seen using total C analysis. In addition, atom stoichiometry is controlled during membrane fabrication and not the annealing process. An ordered metal structure in equilibrium has the highest atomic density and is much less likely to rearrange. In Pd60Cu40 alloys, the bcc phase has a higher atom density than the fcc phase.16 Because the films have a rougher surface (more grain boundaries and membrane defects), slightly higher surface area (sites for adsorption), and more impurities (from the electroless plating process) and undergo larger surface topology

changes than the foils during different gas exposures, the films may have a less ordered structure and undergo stress relaxation more readily. Fewer vacancies and lattice defects in the alloy lattice may make the foils more resistant to atom rearrangement than the films. Stress relaxation can be initiated by migration of atoms in the bulk material or creep processes along grain boundaries.32,33 Atom diffusion was detected in the Pd-Cu membranes through changes in the alloy composition and phase structure after exposure to different gases. Measurements using EDAX showed that the composition of the top 1-µm surface region of the foil changed from 40 to 25 wt % Cu after CO2 exposure. Stress relaxation may also change the macroscopic shape of the metal film by diffusional transport of atoms, as seen in the hillock formation with the PdCu alloys. Diffusion of atoms in metals has been observed through thermal relaxation31 but may also be promoted by specific gas exposures. Bonding interaction between the carbon oxides and metal surfaces seen in this study may promote atom mobility and therefore surface rearrangement. Introducing H2 and He after CO and CO2 exposure to the membranes also caused atom mobility and reduced hillock heights (Table 4). Another site of stress relaxation may be boundaries at sections of nonuniform alloy composition. The electroless-plated films were deposited as two distinct layers of Pd and Cu, which interdiffused during annealing. Because the metals were not mixed first and then rolled like foils, the alloy composition may vary spatially by 5 wt % Pd or Cu, as seen using EDAX, because of the nonuniformity in thickness of the deposition process. The differences in alloy composition within the film could result in more lattice defects and be partly responsible for the higher degree of surface topology changes in the films than the foils, which have a more uniform alloy composition. (b) Phase Structure. The phase transition from a bcc to fcc structure can also change the surface morphology and bulk structure integrity because of rearrangement of the lattice structure (Figure 7). The fcc structure of Pd60Cu40 metal has a 1.3 times bigger lattice parameter and a lower atomic density than the bcc structure. The structure changed because of Pd segregation to the feed side of the membrane. Hillocks also formed as ordered nanostructures on Pd films annealed in O2 at 1173 K.34 Palladium oxide hillocks approximately 1 µm high formed from 200-nm-thick Pd films. A 38% volume increase accompanying oxidation of Pd metal formed hillocks to relax the large compressive stresses due to the difference in volume between the Pd and PdO2 structures. As seen using XRD, bulk Pd-Cu oxide did not form on the membrane with CO2 or CO exposure. (c) Grain Coalescence. As seen in the AFM images of the Pd-Cu alloy membranes, hillocks and valleys formed during the electroless plating process. The electroless plating process produces grains, which coalesce to form hillocks. Membrane microstructure such as the grain size has been shown to affect the membrane performance and may also affect hillock formation. An increase in the Pd-Ag alloy membrane grain size from 18 to 62 nm increased the H2 permeance from 3.8 to 18 × 10-8 mol/s/m2/Pa and the H2/He permselectivity from 23 to 45.19 The grains were fabricated by magnetron sputter deposition and grew during a heat treatment at 773 K in He. The enlarged grain size appeared to

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increase the H2 permeation in the bulk. In contrast, Varma et al. reported that pure Pd membranes fabricated with 0.5-µm grain cluster diameters had 1.5 times higher H2 permeability than membranes with 7-µm diameters.35 The finer-grained cluster microstructure was hypothesized to increase H2 permeation through the grain boundaries. Their membranes were made by electroless plating using different osmotic conditions to vary the cluster sizes. (d) Metal Sintering. Metal sintering also rearranged the surface topology of Pd surfaces. Heemeier et al. reported rearrangement of Co, Rh, and Pd particles deposited on thin alumina films grown on a NiAl substrate at 603-903 K.36 The metal particles sintered and migrated into the substrate, causing the surface rearrangement. Pd nanoparticles also grew on CeO2 and TiO2 supports during CH3OH synthesis from CO and H2 at 473-873 K.37,38 Coke did not deposit significantly on Pd/CeO2, and particle growth was also explained by sintering. The electroless-plated Pd-Cu hillocks in this study enlarged at temperatures as low as 523 K, and metal sintering may increase the grain size during intermetallic diffusion of Pd and Cu. Before annealing, grain sizes of electroless-plated Pd-Cu membranes were approximately 20-50 nm.39 Grain sizes of approximately 100 nm were detected after 7 days in H2 at 723 K. No increase in the grain size was seen after CO2 and CO exposure. Furthermore, the reversibility of the hillock growth after H2 or He exposure makes metal sintering unlikely to be the cause for the hillock enlargement due to exposure of carbon oxides. Catalytic Activity. Cu-based alloys are used commercially as WGS and CO oxidation catalysts. The reverse WGS conversion was 10 times higher on a Pd75Cu25 membrane than on a pure Pd membrane. Therefore, Pd segregation to the membrane feed side during CO2 and CO exposure decreased the membrane catalytic activity. The catalytic reactivity of the Pd-Cu membrane may also explain the decrease in H2 permeance when CO2 was first introduced to H2 at low temperatures (Figure 9). CO2 may react with the dissociated H and C atoms on the membrane surface. The reaction of H atoms on the surface decreases the amount of H available to permeate through the membrane. In contrast, removing C from the surface through the production of CO (eq 4) increased the H2 permeation by increasing hillock heights and defect diameters. Introducing CO2 also segregated Pd to the membrane surface, decreased the catalytic activity, and increased the H2 solubility over time. Conclusions Pd-Cu alloy membranes separated H2 from a mixture of CO2, CO, and H2O. Over time in the presence of CO and CO2, valleys on the membrane surface expanded into membrane defects because of coalescence of hillocks, and molecules other than H2 then transported through the membrane, which decreased the selectivity. CO2 increased the hillock heights by a factor of 6 on Pd-Cu films and a factor of 2.5 on Pd-Cu foils. The widths of the valleys were reduced and the selectivity increased with membrane exposure to He and H2. Decreasing the membrane surface roughness and increasing the membrane thickness decreased the membrane defect formation associated with surface rearrangement. Surface topology changes are partially due to the removal of C impurities by CO2 to form CO.

Furthermore, the hillocks and valleys in the films and foils may also form because of stress relaxation by atom diffusion as seen in Pd segregation to the feed side of the membrane during exposure to CO and CO2. Fewer vacancies and lattice defects in the alloy lattice may make the foil more resistant to atom rearrangement than the electroless-plated films. Pd segregation in the presence of different gases altered the surface region and bulk alloy composition of the membranes. Changing the phase structure from bcc to fcc decreased the H2 permeability. The fcc structure also has a 1.3 times bigger lattice parameter and a lower atomic density than the bcc structure. The lattice rearrangement may change the surface morphology of the membrane. The use of Pd-Cu alloys instead of pure Pd increased side reaction conversion. Acknowledgment We gratefully acknowledge support from the CO2 Capture Project Consortium cofunded by the U.S. Department of Energy (U.S. DOE), the European Union, BP, and Norwegian Klimatek Agencies under Contract DE-FC26-01NT41145, Grant DE-FG26-03NT41792 from the DOE University Coal Research program, and Grant DE-FG03-99ER14363 from the U.S. DOE Office of Basic Energy Sciences to J.D.W. We also thank Dr. David Edlund of Idatech, Inc., and Oremet Wah Chang, Inc., for donating the Pd-Cu foils and Michael Block and Dr. Fernando Roa of the Colorado School of Mines for valuable discussions. Literature Cited (1) Shu, J.; Grandjean, B. P. A.; Kaliaguine, S.; Ciavarella, P.; Giroir-Fendler, A.; Dalmon, J. A. Gas permeation and isobutane dehydrogenation over very thin Pd/ceramic membranes. Can. J. Chem. Eng. 1997, 75, 712. (2) Damle, A. S.; Gangwal, S. K.; Venkataraman, V. K. A simple model for a water gas shift membrane reactor. Gas Sep. Purif. 1994, 8, 101. (3) Kikuchi, E. Membrane reactor application to hydrogen production. Catal. Today 2000, 56, 97. (4) Uemiya, S.; Sato, N.; Ando, H.; Matsuda, T.; Kikuchi, E. Steam reforming of methane in a hydrogen-permeable membrane reactor. Appl. Catal. 1990, 67, 223. (5) Basile, A.; Chiappetta, G.; Tosti, S.; Violante, V. Experimental and simulation of both Pd and Pd/Ag for a water gas shift membrane reactor. Sep. Purif. Technol. 2001, 25, 549. (6) Uemiya, S.; Sato, N.; Ando, H.; Kikuchi, E. The water gas shift reaction assisted by a palladium membrane reactor. Ind. Eng. Chem. Res. 1991, 30, 585. (7) Falconer, J. L.; Noble, R. D.; Sperry, D. P. In Catalytic membrane reactors; Falconer, J. L., Noble, R. D., Sperry, D. P., Eds.; Elsevier Science BV: Amsterdam, The Netherlands, 1995; p 669. (8) Hsieh, H. P. In Inorganic membrane reactorssa review; Hsieh, H. P., Ed.; American Institute of Chemical Engineers: New York, 1989; Vol. 85, p 53. (9) Berstein, L. A.; Lund, C. R. F. Membrane reactors for catalytic series and series-parallel reactions. J. Membr. Sci. 1993, 77, 155. (10) Armor, J. N. Catalysis with permselective inorganic membranes. Appl. Catal. 1989, 49, 1. (11) Itoh, N. In Simultaneous operation of reaction and separation by a membrane reactor; Itoh, N., Ed.; Elsevier: New York, 1990; Vol. 54, p 268. (12) Uemiya, S.; Kato, W.; Uyama, A.; Kajiwara, M.; Kojima, T.; Kikuchi, E. Separation of hydrogen from gas mixtures using supported platinum-group metal membranes. Sep. Purif. Technol. 2001, 22, 309. (13) McKinley, D. L. Method for Hydrogen Separation and Purification. U.S. Patent 3,439,474, 1969.

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Received for review December 4, 2003 Revised manuscript received April 29, 2004 Accepted May 4, 2004 IE030853A