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Predicting, Fabricating, and Permeability Testing of Free-Standing Ternary Palladium-Copper-Gold Membranes for Hydrogen Separation† Kent E. Coulter,*,‡ J. Douglas Way,§ Sabina K. Gade,§ Saurabh Chaudhari,§ David S. Sholl,| and Lymarie Semidey-Flecha⊥ Southwest Research Institute, San Antonio, Texas 78228-0510, Colorado School of Mines, Golden, Colorado 80401, School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100, and Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 ReceiVed: April 30, 2010; ReVised Manuscript ReceiVed: August 11, 2010
For hydrogen from coal gasification to be used economically, novel separation processes that produce highpurity H2 must be developed. While binary palladium based alloys have shown promising pure hydrogen flux values, the search for optimal binary or ternary alloys is an involved and costly process due to the immense number of alloy variations that can be prepared and tested. In this paper an approach to identify, fabricate, and experimentally verify the hydrogen permeation performance of PdCu and PdAu binary alloys and ternary alloys of PdCuAu at various copper and gold concentrations to produce robust, poison-tolerant, hydrogen selective free-standing membranes is reported. This approach utilizes three primary tasks of (1) materials modeling and composition selection, (2) fabrication of high-performance binary and ternary alloy membranes, and (3) membrane testing and evaluation that are all operating independently and concurrently. For the binary Pd-Cu system, using a pure Pd membrane as the reference, both the theoretically predicted and experimentally measured additions of copper lowered the hydrogen permeability. For the binary Pd-Au system, the addition of gold slightly lowered permeability in the theoretical model but in the experimental testing was enhanced. For the ternary system, both experimental and theoretical permeabilities were depressed, with Cu exhibiting a larger influence on reducing the permeability. Introduction Hydrogen Separation Membranes in General. Coal gasification and fuel cells are two of our nation’s most promising technologies for the efficient production of clean electricity. At the heart of these technologies is hydrogen, but unfortunately, the ability to produce pure hydrogen has been a particular challenge that has impeded progress in both areas and will only become a more significant issue in the years ahead. Hydrogen is costly to produce or to separate from gas mixtures such as reactor effluent or waste streams due to the high capital and energy expenditures associated with compression, heat exchange, cryogenic distillation, and pressure swing adsorption (PSA).1 An affordable, tough, and selective hydrogen separating membrane, on the other hand, could significantly reduce these costs and ultimately replace traditional unit operations or be integrated into an existing process to recover hydrogen. A wide variety of membrane materials have been exploited to purify hydrogen streams. They can be divided into three general categories: microporous inorganic materials, dense organic (polymeric) materials, and dense inorganic materials. Microporous materials with mean pore diameters less than 2 nm may be made of silica, silicalite, or zeolite fabricated on porous media. The mechanism of separation is preferential sorption, †
Part of the “D. Wayne Goodman Festschrift”. * To whom correspondence should be addressed: Southwest Research Institute, P.O. Drawer 28510, San Antonio, TX 78228-0510; telephone, (210) 522-3196; fax, (210) 522-6220; e-mail,
[email protected]. ‡ Southwest Research Institute. § Colorado School of Mines. | Georgia Institute of Technology. ⊥ Oak Ridge National Laboratory.
and membranes can be produced with hydrogen permeances up to 1 × 10-5 mol m-2 s-1 Pa-1, with H2/N2 selectivities over 100. While these materials are very robust, the difficulty in synthesizing layers thin enough to produce acceptable fluxes while maintaining selectivity has limited their application.2,3 Dense polymeric membranes take advantage of the fact that hydrogen has a small kinetic diameter, giving it higher diffusivity than other molecules. Researchers in this field have therefore focused on increasing hydrogen diffusivity by altering the structure or rigidity of the polymer.4,5 However, these improvements in diffusivity tend to create losses in selectivity, and in reviews of the literature, there is a distinct upper bound in the relationship between permeability and selectivity that few polymer membranes have been able to surpass.6 Considerable research in the area of inorganic membranes for hydrogen gas separation for purification at high temperatures has taken place in recent years, much of which has been supported by the United States Department of Energy (DOE). Metal membranes appear to have significant advantages over ceramic and polymer membranes in terms of manufacturability, lifetime (durability), and ease of sealing for the former and higher operating temperatures and selectivity for the latter. Dense, pinhole-free membranes made of palladium, nickel, platinum, and the metallic elements in groups III-V of the periodic table are able to transport hydrogen in a dissociated form, and are thus capable of theoretically infinite selectivity. The superpermeable elements of niobium, tantalum, zirconium, and vanadium, in particular, exhibit high permeability for hydrogen.7 However, they have minimal catalytic activity for the hydrogen dissociation reaction and thus must be coated with a catalytic layer, which tends to dissolve into the base metal with time at temperature. Likewise, some perovskite/metal
10.1021/jp1039628 2010 American Chemical Society Published on Web 09/08/2010
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cermets are able to dissociate and transport hydrogen.8 These require high temperatures to become proton-conducting (typically 1173 K) and often have their permeability limited by the permeability of the metallic component. Pd-Based Metal Membranes. Pd-based membranes operate by dissociative adsorption of molecular hydrogen on membrane surfaces which comes about because of the powerful catalytic properties of palladium. Given the equilibrium between H2 molecules in the gas phase and hydrogen atoms within the surface (related by the respective rate constants for adsorption and dissociation) the concentrations of atomic hydrogen just within the metal is proportional to the square root of hydrogen partial pressure, according to Sieverts’ law. The dissociated atomic hydrogen diffuses through the membranes and is recombined into molecular hydrogen which desorbs on the other side of the membrane. The hydrogen permeation process is influenced by the surface topography, the purity of the metal, and its defect structure, e.g., grain boundaries and dislocations. Within the metal, the hydrogen occupies octahedral interstitial sites. At high hydrogen concentrations, above 293 K, a palladium hydride exists, and one of the problems associated with pure palladium as a membrane is hydrogen embrittlement and the distortion of the metal by repeated adsorption/desorption cycles of exposure.9 Alloy development has been critical in overcoming the problems of Pd embrittlement and in developing sulfur-tolerant Pd-based membranes. At higher temperature and hydrogen partial pressure, Pd interacts with hydrogen to form a β hydride phase that is unstable above 568 K. The formation of this high temperature hydride and instability at even higher temperatures causes serious alterations in the atom spacing of the metal lattice and the subsequent dimensional changes can distort the membrane making it mechanically weak and prone to rupture. Binary addition elements, having in general face-centered cubic (fcc) structures such as Ag, stabilize against β-hydride phase formation reducing the problem of embrittlement and yield a hydrogen permeability that is greater than that of pure Pd. However, Pd-Ag is rapidly and irreversibly poisoned by sulfur, and even with the best sulfur cleanup technologies, there is a reasonable likelihood that a process upset or a change in feedstock will expose the membrane to sulfur. Clearly, neither Pd nor Pd-Ag membranes, in and of themselves, are suitable for use with sulfur-containing feed gas.10 While a range of binary addition elements are known to stabilize Pd against embrittlement, Cu is a preferred binary constituent due to its low cost and reported tolerance to sulfur. A somewhat sulfur-tolerant body-centered cubic (bcc) phase Pd-40 wt % Cu alloy with a higher hydrogen permeance than pure Pd has been described previously.11 This alloy also avoids the hydride transition problem that plagues pure Pd. However, the optimum Pd-Cu composition is a bcc β-PdCu phase and is perilously close to the β-PdCu phase/R (fcc)+β(bcc) mixed phase stability boundary. This means that, with an increase in temperature, it can rapidly change structure, from the desirable bcc to the less desirable fcc phase, thus losing its hydrogen permeance and structural integrity. This structural change can be caused by a system upset that increases the temperature beyond the stability of the β phase or because of Pd-Cu segregation occurring over time. The hydrogen permeance of a range of Pd-Cu alloys over a wide range of temperatures has been studied by Howard et al.,12 and the increased permeability is attributed to the formation of the bcc structure. It has been suggested that electronic effects are the cause of this behavior.13 According to Roa et al.14 the
Coulter et al. highest hydrogen permeability is obtained at about 60 wt % Pd, as measured at 350 °C, while McKinley15 recommends 42 wt % Cu, and 58 wt % Pd to obtain a single phase β structure. Recently Huang et al16 completed thermodynamic modeling of the ternary Cu-Pd-H systems and were able to describe the Pd-H fcc miscibility gap. It is an important advantage of Pd-Cu alloys that they are resistant to the poisoning effects of sulfur at high temperatures, perhaps caused by the formation of impermeable sulfur compounds. Specifically, alloy compositions corresponding to the bcc crystalline phase exhibited decreases in hydrogen permeance of approximately an order of magnitude while those compositions corresponding to the fcc crystalline phase exhibited decreases in hydrogen permeance of less than 20%. Progress at National Energy Technology Laboratory (NETL) on S-tolerant hydrogen membranes has been reviewed by Killmeyer et al.17 As early as 1967, it was demonstrated that alloys of palladium with up to 20 wt % gold had higher hydrogen permeabilities than pure palladium and that membranes containing 40 wt % gold suffered almost no flux inhibition when used in streams containing hydrogen sulfide, as opposed to alloys with higher pure-gas permeability.15,18 These materials have the same stability and tolerance to typical reformate streams as the traditionally produced materials but, due to the method of fabrication, tend to have enhanced surface gold content and may not be entirely similar to homogeneous alloys.19 Since gold is currently three times more expensive than palladium, there may be economic constraints on its viability for industrial use, but for the purpose of this academic study, the material is included to study its influence on hydrogen permeation. There remains a need for a sulfur-tolerant, long-life, relatively low cost and high permeance Pd alloy and a hydrogen generation membrane of such an alloy that does not suffer from either the R-hydride/β-hydride transition problem, having a phase boundary close to the membrane operating point, or metal segregation under sulfur containing reformate with time. Approach. In this paper we will present results on selfsupporting, dense Pd alloy membranes that exhibited high hydrogen permselectivity and the ability to produce high-purity hydrogen feed streams needed for fuel cell applications. The combinations of PdAu and PdCu binary and PdCuAu ternary alloys that can be produced are theoretically infinite. We will show that the use of DFT modeling is an economical and efficient approach to identify promising alloy combinations, which can then be fabricated and evaluated to determine optimum alloy combinations. The development of thin PdCu, PdAu, and PdCuAu alloy membranes using advanced physical vapor deposition methods including magnetron sputter deposition will be presented, and the unique feature of these techniques is the ability to rapidly produce membranes of almost any alloy composition with good uniformity and large areas (up to 100 in2). The Colorado School of Mines (CSM) performed the screening via pure-gas permeation tests under controlled atmospheres to confirm that the targeted structures and compositions were produced and the results were used to guide and refine DFT-based modeling and guide the vacuum deposition effort. This cooperative approach has identified several promising ternary alloys in terms of pure-gas permeability and established a basic methodology for identifying new Pd-based ternary alloys. Experimental Section Modeling. All modeling was performed at the Georgia Institute of Technology (GT) using density functional theory (DFT).20 Plane wave DFT calculations were done using the
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TABLE 1: DFT Optimized Lattice Parameter of Alloys at the Composition of Interest in This Study system
lattice parameter (Å)
pure Pd Pd96Au4 Pd96Cu4 Pd74Cu26 Pd70Cu30 Pd70Cu26Au4
3.960 3.968 3.950 3.896 3.876 3.901
Vienna ab Initio Simulation Package (VASP)21 to characterize interstitial H in fcc Pd100-xMx and Pd100-x-yCuxMy (atom %) alloys. All calculations were performed using the generalized gradient approximation with the Perdew-Wang 91 exchangecorrelation functional22 and an energy cutoff of 233.7 eV. Total energy calculations used the residual minimization method for electron relaxation, accelerated using Methfessel-Paxton Fermilevel smearing with a width of 0.2 eV. Geometry relaxations were done using a conjugate gradient algorithm until the forces on all atoms were less than 0.03 eV/Å. A Γ-centered grid of 4 × 4 × 4 k points was used to sample reciprocal space. Each calculation used a 27-atom supercell with rhombohedral geometry (that is, 3 × 3 × 3 primitive fcc cells). We have a set of 27-atom supercells to independently describe Pd-rich alloys (atom % of Pd > 90%), Pd based binary alloys and PdCu based ternary alloys; further description of these supercells can be found in earlier publications.23,24 The lattice parameter of each alloy was optimized with DFT using the supercell that contained the desired alloy composition. For Pd96Au4, Pd96Cu4, Pd74Cu26, Pd70Cu30, and Pd70Cu26Au4, the DFT optimized lattice parameters can be found in Table 1. All subsequent calculations for each alloy are performed using the DFT optimized lattice constant. We calculate the classical binding energy and the zero point energy correction for small interstitial H in a series of octahedral and tetrahedral binding sites within the bulk of these fcc alloys as described in previous publications.23-25 At elevated temperatures, H diffuses across these membranes via a series of hops from one interstitial site to another via a transition state. In order to accurately describe the diffusion of H, we have also calculated the transition state found between adjacent octahedral and tetrahedral sites as previously described.23,24 A substitutionally random fcc alloy includes many structurally distinct interstitial sites in which H can reside. Simple characterization of these sites has been done to describe H properties within bulk materials.26,27 These simple methods of describing H do not offer a natural means to verify whether the model is sufficient to describe the available data and in turn if the data collected is sufficient to describe properly interstitial H. Instead we combined our DFT data with a cluster expansion method. The cluster expansion method provides a general mathematical framework based on a formally infinite series of characteristic figures such as pairs, triplets, four-body terms, etc., that when combined define the binding energy of H in a interstitial site based on its local environment.28-30 Because we wish to not only properly describe our DFT data but also use the information we have to attempt to predict H in interstitial sites for which no DFT data have been collected, we combine our cluster expansions with the leave-one-out (LOO) method. With LOO we are able to select a truncated form of the cluster expansion, among the set of all possible truncated expansions. This method typically generates a lattice model that uses a much smaller number of characteristic figures compared to the full cluster expansion. For each alloy, we perform the cluster expansion
analysis combined with LOO in order to generate a lattice model for the classical binding energy and the zero point energy correction separately for H in interstitial octahedral sites, tetrahedral sites, and transition state as described in previous publications23,24 for the alloys of interest in this work. Once the lattice models describing interstitial H for each alloy are defined, the macroscopic solubility, diffusivity, and permeability of H through the bulk alloy can be calculated using a combination of statistical mechanical calculations and kinetic Monte Carlo (KMC) simulations.26,31-33 These quantities can be calculated to high precision with minimal computational effort once the lattice model is defined. The solubility of dilute amounts of atomic H can be found using Sieverts’ law,34 which relates the interstitial concentration of atomic H to the gas phase H2 pressure as follows, Θ ) KsPH21/2 is the H/M ratio in the alloy and Ks is Sieverts’ constant. Although in general the octahedral sites tend to typically bind H more strongly than the tetrahedral sites,35 in our solubility analysis we include both contributions. At elevated temperatures H diffuses in an fcc metal through a series of hops from interstitial site to interstitial site via a transition state. Once all the sites have been characterized, a rate of hopping can be computed using quantum corrected harmonic transition state theory discussed in detail in previous publications.23,24 In order to predict H diffusivity in the materials of interest, the local hopping rates have to be linked with the long-range H transport across the bulk. This is done with a Kinetic Monte Carlo (KMC), which simulates the hopping dynamic within a lattice model with local hopping rates. The ability of a membrane to transport H is typically quantified in terms of either flux or permeability. The permeability, k, through a membrane is a function of the flux and the pressure drop across the membrane. When Siverts’ law applies, we can express the permeability of H as k ) (DsKs)/2. Here Ds is the diffusion constant obtained from KMC and Ks is the Sieverts’ constant. Membrane Fabrication. Southwest Research Institute (SwRI) led the membrane advancement effort through the development of materials using advanced physical vapor deposition methods. In this work, composite Pd-alloy membranes were deposited on an oxidized silicon wafer that was 6 in. in diameter. The composite Pd-alloy membranes were produced by dc magnetron sputtering from a Pd target with chips of Au and/or Cu placed on the surface in an argon atmosphere to synthesize PdAu, PdCu, and PdCuAu membranes. Target power and deposition time were varied in order to get the desired composition and thickness (1-30 µm). During the deposition process the sample holder was positioned at a distance of 4 cm from the target material. Coating deposition was then carried out at 780 W without substrate bias in order to limit the lattice stress in the Pd layers. With these deposition parameters, the film growth rate was ∼35 nm/min and the substrate temperature, as indicated by a thermocouple positioned in the back of the sample, never exceeded 323 K. Upon completion of the coating run the coated silicon wafer was removed from the vacuum chamber and stored under ambient conditions for a minimum of 24 h. The coating was then scored at the edge of the wafer around the entire perimeter, and the coating peeled from the wafer to form the free-standing membrane. Each individual membrane was then annealed in a flowing nitrogen environment at 673 K for 2 h. The membranes were then characterized for elemental composition, microstructure, and pinholes and delivered to Colorado School of Mines.
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Figure 1. Hydrogen permeability predictions for Pd96Au4, Pd96Cu4, Pd70Cu26Au4, Pd70Cu30, and Pd74Cu26, results normalized with respect to pure Pd.
At SwRI, coating structure and thickness was investigated by scanning electron microscopy on an Amray model 1645T. Elemental analysis was carried out using a Kevex Sigma Energy dispersive spectroscopy system. Compositions were determined at various positions on the substrate and on both sides of the released membrane to assess the compositional uniformity over the coating area. Pinholes were identified by laying the membrane over a light box and visually looking for light transmission. If a pinhole was identified the membrane was discarded. Test Protocol of Hydrogen Separation Membranes. Colorado School of Mines characterized and ran single-gas permeation experiments, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). Circular samples with a diameter of 2.35 cm were cut from each foil. The samples were then mounted in Millipore-type pressure test housings crafted of 316 stainless steel. Graphite gaskets were used to
make gastight seals, and mechanical support for the membrane was provided by disks cut from 0.5 µm Mott porous stainless steel sheeting. Zircar alumina paper was placed between the membrane and the porous metal support in order to prevent intermetallic diffusion at the elevated temperatures reached during testing. The combination of alumina paper and porous stainless steel disk has been shown to present negligible resistance to gas flow under test conditions, allowing for properties to be measured without consideration of support resistance.36 Membranes were heated to 573 K under flowing air at a rate of 1 K/min. Heating the membrane under mildly oxidizing conditions prevents low-temperature hydride phase formation, which may embrittle and crack the membrane.37,38 This oxidation also burns off organic surface contaminants and increases the available area for the hydrogen dissociation reaction, while possibly altering bulk lattice properties and transport rates from
TABLE 2: Deposition Process Parameters for Free-Standing Membranes run no.
composition
base press
run time
thickness (µm)
Ar pressure (Torr)
sput pwr (W)
sput volt
sput amp
Pd-18 Pd-23 Pd-31 Pd-44 Pd-46 Pd-50 Pd-51 Pd-62 Pd-74 Pd-75 Pd-93 Pd-95 Pd-99 Pd-101 Pd-102 Pd-105 Pd-107 Pd-112 Pd-119 Pd-121 Pd-126
Pd Pd/Cu 20% Pd/Cu20% Pd/Au 90/10% Pd/Au 90/10% Pd/Cu/Au 75/17/8% Pd/Cu/Au 75/17/8% Pd/Cu 75/25% Pd/Cu/Au 70/17/13% Pd/Cu/Au 70/17/13% Pd/Cu/Au 85/5/10% Pd/Cu/Au 85/5/10% Pd/Cu/Au 65/35/5% Pd/Cu/Au 65/35/5% Pd/Cu/Au 65/35/5% Pd/Cu 60/40% Pd/Cu 60/40% Pd/Cu 60/40% Pd-Cu 60/40% Pd-Cu 60/40% Pd-Au 90/10%
8-7T 1-6T 1.4-6T 1.5-7T 1.5-7T 2.5-7T 1.3-7T 2.3-7T 0.9-7T 2.0-7T 2.0-7T 2.1-7T 0.9-7T 2.4-7T 1.4-7T 0.9-7T 2.4-7T 3.0-7T 3.2-7T 1.3-7T 2.6-7T
5h 1 h 50 min 3 h 21 min 3 h 44 min 4 h 24 min 4 h 24 min 4 h 24 min 4 h 24 min 5h 5 h 10 min 5 h 10 min 4 h 53 min 4 h 53 min 5 h 20 min 5 h 15 min 3 h 10 min 3 h 10 min 3 h 55 min 3 h 55 min 3 h 55 min 12 h 25 min
5.3 5-7 9 10 10 10 7-10 10 10 11 10 10 10 10 7 7 7 7 7 27
2.0-3 4.8-4 1.0-3 6.5-4 5.8-4 7.6-4 9.4-4 0.9-3 1.8-3 1.6-3 1.3-3 1.4-4 1.4-3 1.4-3 1.4-3 1.4-3 1.4-3 1.4-3 1.4-3 1.4-3 1.4-3
1500 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780 780
403 497 496 464 485 495 498 510 477 475 485 484 520 515 516 506 514 522 550 544 463
3.6 1.5 1.7 1.8 1.72 1.72 1.65 1.4 1.72 1.72 1.72 1.72 1.5 1.5 1.5 1.5 1.5 1.5 1.4 1.4 1.7
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TABLE 3: EDS Determined Atomic % for Each Constituent Measured on Both Sides of the Membrane membrane ID Pd-18 Pd-31 Pd-23 Pd-62 Pd-107 Pd-105 Pd-112 Pd-119 Pd-121 Pd-44 Pd-46 Pd-126 Pd-95 Pd-93 Pd-51 Pd-75 Pd-50 Pd-74 Pd-102 Pd-101 Pd-99
side 1 composition Pd PdCu
PdAu
PdCuAu
side 2
standard deviation
average
% Pd
% Cu
% Au
% Pd
% Cu
% Au
% Pd
% Cu
% Au
% Pd
% Cu
% Au
100.0 83.7 78.6 64.5 57.4 47.5 50.8 44.4 42.5 95.0 95.3 94.3 83.9 83.4 78.8 76.5 76.9 69.8 57.9 55.5 55.2
0.0 16.3 21.4 35.5 42.6 52.5 49.2 55.6 57.5 0.0 0.0 0.0 8.9 10.2 16.7 19.1 18.8 21.3 39.5 40.9 40.8
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 5.0 4.7 5.7 7.2 6.4 4.5 4.3 4.3 8.9 2.6 3.6 4.0
100.0 NA NA 63.3 56.1 56.1 44.6 51.1 48.6 95.8 94.2 94.3 85.5 84.2 72.5 72.8 72.5 73.3 55.3 54.2 50.1
0.0 NA NA 36.7 43.9 43.9 55.4 48.9 51.4 0.0 0.0 0.0 7.9 8.9 23.7 20.0 24.2 19.5 41.2 41.4 47.4
0.0 NA NA 0.0 0.0 0.0 0.0 0.0 0.0 4.2 5.8 5.7 6.6 6.8 3.8 7.2 3.3 7.3 3.6 4.4 2.5
100.0 83.7 78.6 63.9 56.8 51.8 47.7 47.7 45.6 95.4 94.8 94.3 84.7 83.8 75.6 74.7 74.7 71.5 56.6 54.8 52.6
0.0 16.3 21.4 36.1 43.2 48.2 52.3 52.3 54.4 0.0 0.0 0.0 8.4 9.5 20.2 19.6 21.5 20.4 40.3 41.2 44.1
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 4.6 5.2 5.7 6.9 6.6 4.2 5.8 3.8 8.1 3.1 4.0 3.3
NA NA NA 0.85 0.89 6.03 4.43 4.73 4.29 0.54 0.79 0.04 1.17 0.59 4.49 2.64 3.13 2.45 1.89 0.92 3.65
NA NA NA 0.85 0.89 6.03 4.43 4.73 4.29 0.00 0.00 0.00 0.73 0.91 4.92 0.59 3.86 1.33 1.1 0.37 4.69
NA NA NA 0.00 0.00 0.00 0.00 0.00 0.00 0.54 0.79 0.04 0.44 0.31 0.43 2.05 0.73 1.12 0.71 0.55 1.05
the surface to the bulk [39, 40].39,40 Single-gas permeation experiments with industrial-grade (99.9+% purity) H2 and N2 were performed at pressure gradients of up to 689.5 kPa, with permeate flows measured by a soap film meter. The permeate side of the membrane was kept at ambient pressure (approximately 82 kPa), and no sweep gases were used during permeation testing. Temperature was stepped up in 50 K increments to 773 K, taking H2 and N2 flow measurements at each temperature. Results and Discussion Modeling. The predicted H permeabilities for Pd96Au4, Pd96Cu4, Pd74Cu26, Pd70Cu30, and Pd70Cu26Au4 were examined and are shown in Figure 1. In each case, the solubility is normalized with the DFT predicted results for pure Pd under the same conditions. In observing the PdCu binary alloys, we can clearly observe a decrease in H permeability with an increase in Cu content. At 623 K, Pd96Cu4 is predicted to reduce H permeability 35% compared to pure Pd, while Pd70Cu30 is predicted to have a reduction in H permeability of 96%. Although the addition of Au results in an overall reduction of H permeability compared to pure Pd, at 623 K Pd96Au4 is predicted to reduce H permeability merely 11% compared to pure Pd predictions of H permeability at the same conditions. Our DFT predictions suggest that both small additions of either Cu or Au reduce overall H permeability. Yet as we increase the temperature from Figure 1 we can observe that the reduction in H permeability for the Pd96Au4 alloys is reduced, ultimately resulting in comparable H permeability predictions for both this alloy and pure Pd. As discussed earlier the binary alloys with increased Cu content have the lowest H permeability in comparison to pure Pd. In the development of ternary alloys, the goals were to develop alloys that would improve H permeability with a small amount of a third metal additive. On comparison of H permeability for Pd70Cu26Au4 to H permeability predictions for Pd70Cu30 from Figure 1, the addition of 4 atom % of Au results in an overall improvement in H permeability. These results are a combination of a reduced Cu content and an added chemical
effect due to the third metal atom. On comparison of our results for the ternary alloys with H permeability predictions for Pd74Cu26, only Pd70Cu26Au4 results in an improved H permeability. Membrane Fabrication. When physical vapor deposition is utilized, the key factors that affect formation of a thin, dense, defect-free, alloy film are surface energy, roughness, and oxygen/moisture content of the backing material. By using thermally oxidized silicon wafers, we have been able to reduce the surface roughness while at the same time control surface chemistry and, more specifically, oxygen activity. Correspondingly, using vacuum processing conditions that have been optimized to minimize intrinsic film stress, we have produced pinhole-free alloy films over large areas for a range of thickness. Table 2 summarizes the coating processing parameters for each targeted composition in weight %. Table 3 presents the compositions in atomic percent to facilitate comparisons with the theoretical predictions. The compositions of the particular membrane are consistent from side to side with a range of stoichiometries covering the binary and ternary compositions used in the model. SEM analysis indicates that as fabricated, membranes are generally
Figure 2. Cross-sectional SEM image at 5000× magnification of a 33 µm thick, Pd90Au10 (by mass) membrane. The atomic composition of this membrane is 5.6 atom % Au.
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Figure 3. Hydrogen flux as a function of the square root of pressure gradient for a 13.2 µm thick, Pd83Cu10Au7 (atomic %) membrane.
of high quality, without visible surface features down to 5000× magnification, and with a dense, pinhole-free interior (Figure 2). Compositional analysis by EDS shows that membranes have little variation in average composition across their thickness, although from spot to spot on a given side compositions may vary by up to 6 atom %. Permeation Testing. All membranes tested had nitrogen fluxes below the detectable minimum of 2.8 × 10-5 mol m-2 s-1. Hydrogen fluxes displayed a linear relationship to the square root of pressure gradient across the range of temperatures tested (Figure 3). This suggests that, as a first approximation, the ratelimiting step in hydrogen transport across the membrane is bulk diffusion, rather than surface adsorption/desorption or surfaceto-bulk transport.41 A total of 25 Pd, PdCu, PdAu, and PdCuAu membranes were tested under these conditions. The pure palladium and binary PdCu and PdAu membranes had hydrogen permeabilities approximately corresponding to those cited in the literature. In ternary alloys, however, gold content has far less effect than copper content, which provides the best approximation of permeability for these membranes (Table 4). A selection covering the range of ternary alloy materials is presented in Figure 4, showing that in the face-centered cubic regime examined hydrogen permeability decreases with increased copper content. Comparison of Theoretical and Experimental Results. The measured and calculated permeabilities of the PdCu binary systems are in close agreement for an fcc structure; with small additions of Cu there is a rapid decrease in permeability. The decrease in permeability with the addition of Cu can be generalized as a Cu atom is replacing a Pd with a direct correlation between the availability of a Pd or Cu atom
Figure 4. Hydrogen permeability as a function of temperature for ternary PdCuAu membranes of varying alloy content.
determining the rate of permeation (Pd ) high permeation, Cu ) low permeation). Table 5 summarizes the experimental and theoretically calculated permeabilities normalized to pure Pd for a range of compositions. The italicized values are the theoretically predicted values. For the binary and ternary systems, the predicted and measured values are in general agreement. For example Pd-23 with a Pd/Cu atom % ratio of 78.6/21.4 has a measured relative permeability at 673 K of 0.13 while the theoretically predicted value for a Pd/Cu atomic ratio of 74.1/25.9 is 0.15. Another example is for Pd-50 which has a similar PdCuAu atomic ratio as the theoretically predicted system and the measured relative permeability is 0.2 at 673 K while the predicted is 0.33. With the PdCuAu data in Figure 5 the relative permeability is plotted as a function of the Pd atom % and the plotted trendline is quite tight with the outliers above the trendline having more Au replacing the Pd whereas the ones where the permeabilities below the trendline, more Cu is replacing Pd. The calculated permeability for the ternary system, while low, does show a similar trend to the measured values indicating qualitative validation of the model. Conclusions Binary PdCu and PdAu and ternary PdCuAu membranes for hydrogen separation membranes have been modeled, fabricated, and tested with measured permeabilities correlating with predicted permeabilities. For the binary Pd-Cu system, using pure Pd membrane as the reference, both the theoretically predicted and experimentally measured additions of copper lowered the hydrogen permeability. For the binary Pd-Au
TABLE 4: Comparison of Measured Ternary Alloy Hydrogen Permeabilities with Literature Permeabilities (columns with footnote a) for Binary Alloys H2 permeability × 108 (mol m-1 s-1 Pa-0.5)
Pd content (x, atom %)
Cu content (y, atom %)
Au content (z, atom %)
Pd1-yCuya
Pd1-zAuza
PdxCu1-xa
PdxAu1-xa
measured
55 56 74 72 75 72 83
41 41 22 26 19 20 10
4 3 4 2 6 8 7
0.1 0.1 0.5 0.4 0.6 0.6 0.8
1.3 1.3 1.3 1.3 1.3 1.2 1.3
1.0 0.6 0.3 0.3 0.2 0.1 0.4
0.6 0.7 1.1 1.1 1.1 1.0 1.2
0.1 0.1 0.3 0.3 0.4 0.6 0.8
a
Literature hydrogen permeabilities for binary alloys are from McKinley.15
Free-Standing Ternary Membranes
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Figure 5. Permeability plotted as a function of atomic percentage of Pd in a PdCuAu alloy. The unfilled diamond is the theoretically predicted permeability.
TABLE 5: Comparison of Theoretically Predicted (Italicized) and Experimentally Measured Permeabilities Relative to Pure Pd membrane ID Pd-18
atomic % (measured by EDS, SwRI) composition Pd PdCu
Pd-31 Pd-23 PdCu PdCu Pd-62 Pd-107 Pd-105 Pd-112 Pd-119 Pd-121 PdAu Pd-44 Pd-46 Pd-126 Pd-95 Pd-93 Pd-51 Pd-75 Pd-50 Pd-74 PdCuAu Pd-102 Pd-101 Pd-99
permeability relative to pure Pd
% Pd
% Cu
% Au
623 K
673 K
723 K
100.0 96.3 83.7 78.6 74.1 70.4 63.9 56.8 51.8 47.7 47.7 45.6 96.3 95.4 94.8 94.3 84.7 83.8 75.6 74.7 74.6 71.5 70.4 56.6 54.8 52.6
0.0 3.7 16.3 21.4 25.9 29.6 36.1 43.2 48.2 52.3 52.3 54.4 0.0 0.0 0.0 0.0 8.4 9.5 20.3 19.6 21.6 20.4 25.9 40.3 41.2 44.1
0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 3.7 4.6 5.2 5.7 6.9 6.6 4.1 5.7 3.9 8.1 3.7 3.1 4.0 3.2
1.00 0.65 0.04 0.12 0.13 0.04 0.13 0.01 0.10 0.04 0.03 0.05 0.89 1.24 1.01 0.94 0.68 0.79 0.29 0.43 0.32 0.64 0.18 0.14 0.09 0.08
1.00 0.67 0.05 0.13 0.15 0.04 0.16 0.16 0.12 0.05 0.04 0.05 0.90 1.24 1.26 1.11 0.66 0.81 0.32 0.47 0.33 0.62 0.20 0.14 0.11 0.08
1.00 0.69 0.00 0.00 0.17 0.05 0.20 0.20 0.13 0.05 0.06 0.06 0.90 1.25 1.41 1.04 0.73 0.85 0.33 0.54 0.38 0.68 0.23 0.11 0.13 0.09
system, the addition of gold caused permeability to become theoretically depressed but experimentally enhanced. The source of this divergence between theory and experiment is unknown. For the ternary system, both experimental and theoretical permeabilities were depressed, with Cu exhibiting a larger influence on the permeability. For the ternary systems the removal of Pd with the addition of the third material was equivalent to the impact of replacing a Pd atom in the binary system. Acknowledgment. The authors gratefully acknowledge support of this work from the U.S. Department of Energy (National Energy Technology Laboratory) through cooperative agreement No. DE-FC26-07NT43056.
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