Search

Selected results of investigations focused on the changes to some mechanical properties of palladium and several palladium-based alloys caused by exposure to hydrogen have been collected and presented. The findings indicate that the mechanical properties of pure palladium are highly susceptible to alteration upon exposure to hydrogen. In the cases of alloying palladium with silver and copper, the alloys, as compared to palladium, appear to resist changes to mechanical properties. In the case of alloying palladium with manganese, the interesting order–disorder phenomenon plays an important role on the effects of hydrogen exposure on their mechanical properties. [ABSTRACT FROM AUTHOR]

Intermetallic diffusion between the Pd layer and the porous metal support can limit the hydrogen permeability, and ultimately give rise to serious problems in the long-term stability of Pd alloy hydrogen separation membranes. In this study, we developed a multi-layered Pd–Ag membrane having a uniform-microstructure and thermally stable alloy composition by DC magnetron sputtering and Ag up-filling heat treatment. This multi-layered Pd–Ag alloy membrane was introduced in order to prevent Ag segregation, to block the intermetallic diffusion from the porous stainless steel (PSS), and to improve the stability of the membrane. Experimental results showed that the homogeneous Pd–Ag alloy membranes acted as an extremely effective diffusion barrier and provided significant improvement on the thermal stability over a period of 2000 h. [ABSTRACT FROM AUTHOR]

Abstract: Thin PdAg alloy membranes with 20–25% Ag are being developed for hydrogen separation technology. Despite many investigations on such membranes as well as representative experimental and theoretical model systems, unresolved issues remain concerning the effect of the alloy surface structure and composition on adsorption and vice versa. Therefore, the interaction between hydrogen, carbon monoxide or oxygen with the surface of a PdAg model alloy was studied using periodic self-consistent density functional theory (DFT-GGA) calculations. In particular, the adsorption structure, coverage dependence and possible adsorption-induced segregation phenomena were addressed using Pd3Ag(111) model surfaces with varying degrees of surface segregation. In agreement with previous experimental and theoretical investigations, we predict Ag surface termination to be energetically favorable in vacuum. The segregation of Ag is then reversed upon adsorption of H, CO or O. For these adsorbates, the binding is strongest on Pd three-fold hollow sites, and hence complete Pd termination is favored at high coverage of H or CO, while 25% Ag may remain under oxygen because of the lower O-saturation coverage. CO adsorption provides a somewhat stronger driving force for Pd segregation when compared to H, and this may have implications with respect to permeation properties of PdAg alloy surfaces. Our predictions for high coverage are particularly relevant in underlining the importance of segregation phenomena to the hydrogen transport properties of thin PdAg alloy membranes. [Copyright &y& Elsevier]

Abstract: A theoretical model is proposed to describe hydrogen permeation in palladium and silver–palladium membranes in presence of a non-inert gas as CO; it is known indeed that hydrogen flux through palladium-based membranes drastically decreases when H2 is fed in mixtures containing carbon monoxide due to the interaction of the latter gas with the membrane surface. To model this process, the adsorption step of the well-known approach suggested by Ward and Dao has been suitably modified, since it must be considered as a competitive adsorption of the different non-inert molecules on the metal interface. In particular, the competitive adsorption of CO and H2 has been examined accounting for the spectrum of information available for CO adsorption on palladium, as well as for hydrogen in palladium and palladium–silver alloys. A validation of the model proposed has been performed through a comparison between several literature data and model calculations, over a rather broad range of operating conditions. A quite good agreement was obtained in the different cases; the model, thus, can be profitably used for predictive purposes. [ABSTRACT FROM AUTHOR]

Abstract: In this work a novel modelling approach based on Computational Fluid Dynamics (CFD) for the prediction of the gas separation process in a Pd–Ag membrane module for H2 purification is presented. With this approach, the pressure and velocity flow fields of the gas mixture and the species concentration distribution in the selected three-dimensional domain are simultaneously and numerically computed by solving the continuity, momentum and species transport equations, including a gas-through-gas diffusion term derived from the Stefan–Maxwell formulation. As a result, the H2 permeation calculations depend on the local determination of the mass transfer resistances offered by the gas phase and by the membrane, which is modelled as a permeable surface of known characteristics. The applicability of the model to properly predict the separation process under a wide range of pressure, feed flow rate, temperature and gas mixtures composition is assessed through a strict comparison with experimental data. The influence of inhibitor species on the module performance, that is obtained by implementing in the CFD model a suitable literature correlation, is also discussed. [Copyright &y& Elsevier]

Abstract: For many years enhancement of the hydrogen permeation rate through Pd and Pd-alloy membranes has been accomplished by decreasing the membrane thickness. This approach is based on the idea that the permeation rate is limited by the diffusion process through the membrane bulk material. Theoretical modeling suggests that as membrane thickness is reduced into the micrometer range the rate-limiting permeation mechanism may shift to desorption from the permeate surface. In order to test the model predictions, free-standing Pd–Ag alloy membranes (23wt% Ag) with a thickness on the order of 1μm have been microfabricated and their hydrogen permeation behavior has been experimentally determined. At temperatures between 523 and 723K, hydrogen permeation is limited by the diffusion process and is characterized by a membrane permeability pre-exponential factor of 5.51E−8±1.34E−8mol/m/s/Pa0.5 and an activation energy of 10.8±1.25kJ/mol. At temperatures below 500K, the activation energy increases appreciably. This transition is consistent with diffusion-limited permeation in the presence of non-ideal absorption behavior of hydrogen in the Pd–Ag alloy. A transition into a desorption-limited permeation regime is not observed under the operating conditions in this study. Additionally, two distinct failure modes have been observed for these microfabricated devices. The operating conditions leading to failure and cause of each failure mode are discussed. [Copyright &y& Elsevier]

Abstract: Current theoretical models for the permeation behavior of hydrogen through palladium and palladium alloys predict that low temperature operation of thin membranes will result in permeation which is no longer controlled by the diffusion of hydrogen through the metal lattice. Specifically, the process of desorption from the downstream surface is predicted to become the dominant resistance to mass transfer. However, these models neglect the non-ideal absorption behavior of hydrogen in palladium which typically occurs at temperatures below 300°C. In this work a model is developed which accounts for the non-ideal behavior of hydrogen in palladium–silver alloys (25% silver by weight) which are typically used for hydrogen purification. This model predicts that the diffusion-limited regime should be characterized by an increase in the activation energy to ∼32kJ/mol at temperatures below 200°C. This prediction is supported by data available in the literature for thick (>125μm) palladium–silver alloy membranes. [Copyright &y& Elsevier]

Abstract: Pure and mixed gas permeation tests were performed on Pd-based hydrogen selective membranes at different experimental conditions. In particular the permeance of pure hydrogen as well as of binary and ternary mixtures containing hydrogen, nitrogen and methane was measured, at temperatures ranging from 673 to 773K and at pressure differences up to 6bar. The membranes, supplied by NGK Insulators Ltd., Japan, were formed by a selective Pd–Ag layer (20wt% Ag) deposited on a tubular ceramic support, and showed very high hydrogen permeance and a practically infinite selectivity toward hydrogen. Interestingly, the permeance values measured in pure gas experiments resulted always higher than those obtained in permeation tests with gas mixtures; in the latter case, moreover, the permeate flux significantly deviates from Sieverts’ law based on the hydrogen partial pressure in the bulk gas phase. Both facts suggest the existence of non-negligible resistances to hydrogen transport in the gas phase itself, in addition to that offered by the metallic membrane. Experiments performed with increasing feed flow rates, showed also an increase in hydrogen permeance thus revealing the importance of the concentration polarization effects inside the module. Gas phase mass transport coefficients were calculated and used to determine the role of such a resistance in the overall mass transport process. The Sherwood number was also evaluated and was found to follow a boundary layer type of correlation. A general sensitivity analysis was performed in order to compare the effects on the transmembrane hydrogen flux of the two resistances, with different physical dimensions, offered by the gas phase and the metallic membrane. The concentration polarization number thus introduced allows for an a priori identification of the leading resistance at any operating conditions and gives clear indications on the actions required to improve the module performance. [Copyright &y& Elsevier]

Abstract: The performances and long-term stability in the 350°–450°C temperature range of a Pd–Ag alloy thin film membrane are reported. The membrane was prepared by electroless plating deposition (EPD) on the inner walls of a tubular alumina asymmetric support using a novel seeding procedure based on Pd-only complexes. A defect-free film with thickness around 12 microns is obtained. Scanning electron microscopy (SEM) images evidence the presence of aggregates of small crystallites of few hundred nanometers. The membrane shows a stable behaviour for at least 260h of time-on-stream in pure H2 and permeability tests indicate a high hydrogen flux with an activation energy of about 14kJ/mol. The permeability follows Sievert''s law with an exponent of 0.5, indicating a bulk diffusion of hydrogen as the rate-controlling step. There is an initial increase in the transmembrane H2 flux during the time-on-stream tests, due to hydrogen absorption in the Pd–Ag alloy which increases the permeation rate. The presence of two mechanisms of transport, within Pd–Ag alloy crystallites and in the intergrain boundary region, is discussed. The permeability behaviour using H2:N2 mixtures as the feed is also shown. The presence of N2 induces a large decrease in the H2 permeation rate, greater than that expected by dilution effect. The membrane is stable in the 350°–400°C temperature range with a high separation factor between H2 and N2, but at 450°C shows an initial fast decrease of the selectivity with time-on-stream up to reach a nearly constant value. The effect is interpreted in terms of formation of NH x species which inhibit hydrogen diffusion in the intergrain boundary region, as suggested also from literature data. [Copyright &y& Elsevier]

The present work deals with the study of palladium-silver (PdAg) and palladium-gold (PdAu) binary alloys over a broad range of temperatures and alloy compositions using density functional theory (DFT) to find possible conditions where the solubility of hydrogen (H) is significantly higher than that of pure palladium (Pd). Several alloy structures, such as Pd100-xAgxwith x 14.81, 25.93, 37.04, and 48.51, Pd100-xAuxwith x 14.81, 25.93, and 37.04, and Pd100-xCuxwith x 25.93 and 48.51 were considered. The lattice constants of these structures were optimized using DFT, and relaxed structures were used for the estimation of binding energy. It was found that the solubility of H in PdAg is higher than pure Pd with a maximum at approximately 30% Ag at 456 K. Also, the solubility of PdAu alloys was higher than pure Pd with a maximum at about 20% Au with a solubility 12 times higher than that of pure Pd. It was found that for a 3.7% H concentration in a PdAg alloy, a cell expansion of 0.15-0.2% occurs, which if ignored may affect the individual binding energy of the O-site by approximately 3.56% and may affect the predicted solubility by approximately 11.8%. [ABSTRACT FROM AUTHOR]

In this paper, a process for the microfabrication of a wafer-scale palladium—silver alloy membrane (Pd-Ag) is presented. Pd-Ag alloy films containing 23 wt% Ag were prepared by co-sputtering from pure Pd and Ag targets. The films were deposited on the unetched side of a 〈110〉-oriented silicon wafer in which deep grooves were etched in a concentrated KOH solution, leaving silicon membranes with a thickness of ca. 50 µm. After alloy deposition, the silicon membranes were removed by etching, leaving Pd-Ag membranes. Anodic bonding of thick glass plates (containing powder blasted flow channels) to both sides of the silicon substrate was used to package the membranes and create a robust module. The hydrogen permeability of the Pd-Ag membranes was determined to be typically 0.5 mol H[sub 2]/m² · s with a minimal selectivity of 550 for H[sub 2] with respect to He. The mechanical strength of the membrane was found to be adequate, pressures of up to 4 bars at room temperature did not break the membrane. The results indicate that the membranes are suitable for application in hydrogen purification or in dehydrogenation reactors. The presented fabrication method allows the development of a module for industrial applications that consists of a stack of a large number of glass/membrane plates. [ABSTRACT FROM AUTHOR]

Pd–Ag alloy coated V-15Ni composite membranes were prepared by co-sputtering of separate pure Pd and Ag targets using a DC multi-target sputtering system where voltage for each target can be controlled independently. The film composition was controlled by changing the target voltage. Hydrogen permeation characteristics of as-prepared composite membranes were investigated using the gas permeation technique in the temperature range of 423–673 K. The hydrogen permeation of the composite membranes was mainly bulk-diffusion limited at a temperature higher than 473 K. At 473 K or lower, the hydrogen permeability of the composite membranes increased with the Ag content until 30 at%. [Copyright &y& Elsevier]

Submicron-thick Pd-Ag alloy membranes, prepared on 4 nm pore γ-alumina support by magnetron sputter deposition, are nanocrystalline with a grain (crystallite) size of about 20 nm. The membranes show good selectivity for hydrogen over helium (about 4000 at 300°C). Hydrogen permeation is dominated by the surface reaction steps in 100–200°C with an activation energy of about 30 kJ/mol. Bulk diffusion resistance becomes important at higher temperatures (>200 °C). Grain size is the most critical parameter affecting the hydrogen permeance of the thin nanostructued Pd-Ag membranes. Increase in Pd-Ag grain size from about 20 to 60 nm results in a substantial improvement in hydrogen permeance with a higher apparent activation energy in 100–300°C. Grain growth appears to increase the hydrogen permeability in the bulk phase of the Pd-Ag membranes. Helium permeance through the grain boundary decreases with increasing temperature or hydrogen partial pressure due to grain expansion. Carbonation and the accompanied grain expansion have detrimental effects on the perm-selectivity of the Pd-Ag membranes. [ABSTRACT FROM AUTHOR]