•bcc PdCu/ceramic membrane were prepared by electroplating and alloying at 673–773 K.•Monitoring of deposition currents enables good alloy composition control.•Membrane operation under 1.1 MPa H2 at room temperature without leak growth.•Multiple H2/N2 exchanges at room temperature without leak growth.Concerns about limited durability at ambient temperatures are a major obstacle for the utilization of Pd-type membranes for H2 purification in local, on-demand H2 production. It is clear that pure Pd membranes are not suitable for applications that involve fast regular start-up and shut-down of purification systems but knowledge remains sparse about the performance of Pd alloy membranes in such situations. Here we investigated the stability of thin-layered, supported PdCu membranes with body-centered cubic (bcc) structure between room temperature and 673 K at H2 pressure differences ΔPH2 up to 1 MPa. Three PdCu layers with 46–50% Pd have been prepared by alternating electrodeposition of multiple Cu and Pd layers onto metallized ceramic membranes and subsequent alloying at 673–773 K. The H2 permeation rates of all membranes can be described by single permeation laws in the entire investigated temperature range. The nominal H2 permeability of a Pd48Cu52 membrane amounted to 2.3 × 10−9 mol m−1 s−1 Pa−0.5 at 673 K and ΔPH2 = 100 kPa with an ideal H2/N2 selectivity of 3382. The H2 fluxes and N2 leak rates of the three membranes were not affected by cycling between room temperature and 673 K under up to 1.1 MPa H2 which included more than a dozen H2/N2 exchanges at 298 K. These fluxes remained also stable through 100 h continuous operation in H2 at that temperature. The excellent low-temperature tolerance of these membranes is attributed to the marginal hydrogen solubility in bcc PdCu alloys rendering them promising membrane materials for H2 purification involving everyday operation at ambient temperatures.

•Outstanding water-gas shift conversion rates.•Strong influence of apatite cations on WGS reaction rate.•Facile reduction of oxidized platinum.•Methanation activity below detection limits.Platinum supported on calcium hydroxy (CaHAP) and fluoroapatite (CaFAP) exhibits excellent water-gas shift (WGS) activity comparable to the best Pt WGS catalysts on reducible oxides. The high activity of these unusual WGS catalysts is attributed to the parallel activation of CO on the noble metal and H2O on the ionic phosphates with formate species apparently being the major reaction intermediates. We have studied Pt supported on strontium hydroxy and fluoroapatites with varying cation and noble metal contents now in order to elucidate the role of the apatite cations and anions on WGS activity. Initial Pt dispersions approached 100% for loadings up to 3% (weight) with mean particle sizes of 1–2 nm according to H2 chemisorption and EXAFS results (coordination number of 6.6). TEM analyses show further that the Pt aggregates grew slightly during catalytic testing up to 723 K with 1.5–2.5 nm particles being most abundant afterwards. Moreover, Ptn+ reduction is much facilitated on these apatites as revealed by temperature programmed reduction coupled with in situ X-ray absorption spectroscopy (XAS). The WGS reaction rates in a reformate-type gas mixture increased strongly with Sr/P ratio in the apatite, with the rates over the most active Pt/SrHAP and Pt/SrFAP catalysts being among the highest reported to date. In particular, Pt/SrHAP and Pt/SrFAP rates surpassed the rates over analogous Ca2+ substituted catalysts by ca. 30% at 573 K. This demonstrates that apatite cations have a strong impact on WGS conversion over these systems opening a route to catalysts with potentially even higher WGS activity.Download high-res image (130KB)Download full-size image

•Hydrogen permeability improves with Au but declines with Cu amount.•Low-temperature hydride miscibility gap can be suppressed at room temperature.•In situ X-ray diffraction under H2 is powerful tool for examining alloy homogeneity.PdCuAu membranes are interesting tools for H2 separation especially from sulfur contaminated mixtures. We have studied the preparation of such membranes on ceramic supports via electroless plating and examined their permeation behavior. Alloying of the separately deposited metals was time consuming at 500 °C under H2. Hydrogen permeability improved and the associated activation energy became smaller with increasing Au and decreasing Cu content of the alloys. The low-temperature α/β hydride miscibility gap was examined by in situ synchrotron radiation X-ray diffraction (SR-XRD) under H2 employing PdCuAu samples alloyed at 800 °C under N2. The gap narrowed with decreasing Pd amount and was reduced stronger by Au addition. This two-phase regime extended beyond 125 °C in hydrogenated Pd87Cu7Au6. Results from Cu-rich alloys indicate that it can be completely suppressed at room temperature through proper balance of Au and Cu in alloys containing less than 75% Pd. Hence, the risk of embrittlement due to formation of incommensurate α and β hydride phases can be largely mitigated and Au addition to PdCu alloys benefits the low-temperature stability of such membranes. Moreover, the SR-XRD experiments under H2 uncovered distinct alloys with very similar lattice parameters but differing hydrogen solubility in two Cu-rich samples demonstrating that such in situ studies are very useful for probing the homogeneity of multicomponent alloys.

•PdCu membranes exhibit very good stability up to 650 °C.•Performance of PdAu membranes starts to deteriorate already at 550 °C.•Morphological deformation and compositional imbalance in PdAu layers above 550 °C.•Differing Cu and Au surface segregation impact high-temperature stability.•Stability of PdCuAu membranes depends on balance between Au and Cu.High-temperature stability of Pd-based membranes benefits their application in steam reformers and sulfur-contaminated H2 streams because both membrane reforming efficiency and sulfur tolerance of Pd alloys increase much with temperature. Hence, we investigated PdCu, PdAu, and PdCuAu membranes supported on porous ceramic tubes between 500 °C and 650 °C. Remarkably, PdCu membranes were much more stable than Au-containing ones. The H2 permeation rates of some PdAu and PdCuAu membranes declined at 550 °C with substantially increasing N2 fluxes. This was triggered by severe morphological deformation of the Au alloy films into stoichiometrically inhomogeneous, cavernous structures. The H2 fluxes of the PdCu membranes started to decline at 650 °C with leak flows increasing slightly. Moreover, the PdCu layer morphology remained dense and compositionally homogeneous even after testing for up to 4800 h between 500 and 650 °C. The strikingly different high-temperature stability can be understood by considering the divergent surface segregation tendencies of Cu and Au and their differing impact on hydrogen solubility in Pd alloys. As a result, Au may desorb much more easily from membranes than Cu leading to structural instability above 500 °C during operation in H2. The instability of PdAu membranes at high temperatures may be mitigated by addition of sufficient Cu to obtain ternary membranes with good H2 permeability and better thermal stability.

Water-gas shift (WGS) micro and membrane reactors are interesting components for compact H2 production and purification devices, but they require catalysts with very high activity for optimum efficiency to minimize catalyst bed thickness and mass transfer limitations. On the other hand, activation of H2O is known to be more challenging than CO in this reaction. Catalysts comprising ca. 2 nm large Pt particles on hydrophilic apatites are found to have very high WGS activity, with specific reaction rates exceeding those of a highly active Pt/CeO2 catalyst by up to 50% at 573 K. These apatite-supported catalysts exhibit stable CO conversions at 673 K without showing any CH4 formation tendencies up to 723 K. WGS activity increases with Ca/P ratio in the apatite, leveling off around Ca/P ≈ 1.75, and formate has been identified as the main reaction intermediate. The outstanding WGS performance is attributed to the superior activation of H2O on these ionic oxides due to coordination of H2O to Lewis acidic Ca2+ ions and H bonding to basic O atoms of PO43– units. This renders H2O molecules highly polarized and thus reactive on apatite surfaces with the ensuing formate-like intermediates being well stabilized through bonding to multiple Ca2+ ions, as well. Thus, apatites provide an intriguing alternative to increasingly expensive rare-earth oxides in high-performance noble-metal WGS catalysts not only for micro and membrane reactors.Keywords: activation energy; formate intermediates; H2O activation; high-temperature water-gas shift; ionic oxides; reaction rate

Addition of Ag is a promising way to enhance the H2 permeability of sulfur-tolerant PdCu membranes for cleanup of coal-derived hydrogen. We investigated a series of PdCuAg membranes with at least 70 atom % Pd to elucidate the interdependence between alloy structure and H2 permeability. Membranes were prepared via sequential electroless plating of Pd, Ag, and Cu onto ceramic microfiltration membranes and subsequent alloying at elevated temperatures. Alloy formation was complicated by a wide miscibility gap in the PdCuAg phase diagram at the practically feasible operation temperatures. X-ray diffraction showed that the lattice constants of the fully alloyed ternary alloys obey Vegard’s law closely. In general, H2 permeation rates increased with increasing Ag and decreasing Cu content of the membranes in the investigated temperature range. Detailed examination of the permeation kinetics revealed compensation between activation energy and pre-exponential factor of the corresponding H2 permeation laws. The origin of this effect is discussed. Further analysis showed that the activation energy for H2 permeation decreases overall with increasing lattice constant of the ternary alloy. The combination of these correlations results in a structure–function relationship that will facilitate rational design of PdCuAg membranes.Keywords: hydrogen separation; kinetic compensation; structure−function relationship; ternary alloy; Vegard’s law

Higher operation temperatures benefit H2 permeability and selectivity of metal membranes and they are interesting for e.g. water gas shift and steam reforming in membrane reactors. Hence the behaviour of PdAg–ceramic composite membranes has been investigated between 823 K and 923 K. The H2 flux of membranes with less than 10 μm thick alloy layers decreased continuously with time during operation under H2 at 873 K and above. This was accompanied by a steady increase of the activation energy for H2 permeation and the growth of Ag-depleted crystallites on the membrane surface. All phenomena could be reversed through annealing under N2 at 923 K. The textural and permeability changes are consistent with a segregation mechanism starting with metal sublimation from hydrogenated PdAg layers and subsequent metal resublimation. This implies an enhancement of the yet unknown metal activities in PdAg hydride phases over metallic PdAg alloys. Ramifications for application of thin-layered, supported PdAg membranes for H2 separation above 823 K are discussed.

Electroless plating is the technically most facile and most frequently studied method for preparation of PdAg/ceramic composite membranes. Limited high-temperature stability of such membranes requires alloying of sequentially deposited Pd and Ag layers far below their melting points, however. Here it is demonstrated that 600–800 h are needed for forming 2–4 μm thick, homogeneous alloy layers from Pd–Ag bilayers at 823 K under atmospheric H2 pressure. This is also the time scale on which the activation energy for H2 permeation becomes stable so that this characteristic can be employed for non-destructive, in-process monitoring of the alloying progress. High-temperature H2 permeation rates are shown to be less well suited for this purpose because they are not sufficiently sensitive to the homogeneity of PdAg membranes. The activation energies for the well-alloyed membranes indicate that diffusion through the bulk of the PdAg layer limits H2 permeation through these composite membranes. It is further shown that a fully alloyed Pd75Ag25 membrane tolerates temperature cycling under H2 well down to 373 K while H2/N2 exchanges at that temperature trigger a rapid growth of the N2 leak rate of that membrane. The defect formation is attributed to mechanical stress caused by the substantial expansion and shrinking of the alloy lattice during hydriding and dehydriding at low temperatures.Highlights► Alloying of 2–4 μm thick Pd-Ag bi-layers takes 600–800 h at 823 K under 100 kPa H2. ► Alloying can be monitored in process via activation energy for H2 permeation. ► Selectivity of 2–4 μm Pd75Ag25 membranes deteriorates during H2/N2 cycling at 373 K.

Knowledge about the (inter)dependence of permeation kinetic parameters on the stoichiometry of H2-selective alloys is still rudimentary, although uncovering the underlying systematic correlations will greatly facilitate current efforts into the design of novel high-performance H2 separation membranes. Permeation measurements with carefully engineered, 2–7 μm thick supported Pd100–xAgx membranes reveal that the activation energy and pre-exponential factor of H2 permeation laws vary systematically with alloy composition, and both kinetic parameters are strongly correlated for x ≤ 50. We show that this permeation kinetic compensation effect corresponds well with similar correlations in the hydrogen solution thermodynamics and diffusion kinetics of PdAg alloys that govern H2 permeation rates. This effect enables the consistent description of permeation characteristics over wide temperature and alloy stoichiometry ranges, whereas hydrogen solution thermodynamics may play a role, too, as a yet unrecognized source of kinetic compensation in, for example, H2-involving reactions over metal catalysts or hydrogenation/dehydrogenation of hydrogen storage materials.

A novel strategy for the preparation of supported PdAu alloy layers allows the facile and fast fabrication of highly permeable and selective H2 separation membranes from refractory metals via electroless plating and low-temperature alloying. Homogenous alloying of multiple, separately deposited Pd and Au layers with thickness in the nm range required less than one week at 773 K under atmospheric H2 as evidenced by X-ray diffraction and H2 permeation measurements. The H2 permeation rate JH2JH2 became stable within a day even, reaching 0.62 mol m−2 s−1 at 773 K and ΔPH2ΔPH2 = 100 kPa. The corresponding N2 leak rate remained constant during a 350 h experiment, resulting in an ideal H2/N2 selectivity of 1400 and demonstrating that such membranes tolerate extended operation at that temperature well.Research highlights► Facile preparation of nanostructured (Pd/Au)n multilayers by electroless plating. ► Pd and Au layers with nm range thickness readily alloy at 773 K. ► Fully functional supported PdAu membranes available within 24 h.

H2O2 was directly synthesized from the elements over supported Pd membranes in O2-saturated, aqueous H2SO4 (0.03 mol/l) between 303 and 348 K while feeding H2 through the membrane. The H2O2 yield increased with temperature despite H2O2 selectivity declining simultaneously due to faster H2O2 decomposition. Both H2O2 yield and selectivity improved with H2 feed pressure, however. Comparison of H2 consumption during H2O2 synthesis with the intrinsic membrane H2 permeability and the energetics of H2 oxidation on Pd surfaces revealed that the H2O2 productivity gain with temperature was primarily owed to the acceleration of reaction kinetics. The H2O2 selectivity enhancement with H2 feed pressure is attributed to the inhibition of direct H2O formation as a result of hindered O2 dissociation due to blocking of O atom adsorption sites on the membrane surface because of increasing occupation by H atoms. H2O2 decomposition and reduction on the Pd membrane surface accelerate much faster with temperature than H2O2 formation and need to be suppressed in order to maximize H2O2 yield and H2 utilization at higher temperatures.

The Pd–Cu phase diagram is discontinuous in the immediate vicinity of the body centered cubic (bcc) Pd47.3Cu52.7 alloy, which is a promising material for H2 separation membranes. Hence PdCu/ceramic composite membranes have been prepared by sequential electroless plating of the metals to investigate the influence of concomitant face centered cubic (fcc) phases on the behavior of such membranes. Structural and compositional segregation was observed during cycling of a bcc Pd48Cu52 membrane through the fcc/bcc miscibility gap between 723 and 873 K. Transient bcc and fcc phases with low Pd content also appeared in the initial formation stages of an fcc Pd63Cu37 membrane during alloying of Pd and Cu layers at 723 and 773 K. Both phenomena could be sources of persistent stoichiometric heterogeneity in supported bcc PdCu membranes, which considerably reduces their H2 permeability. An fcc/bcc Pd46Cu54 mixed phase membrane withstood 30 temperature cycles between room temperature and 673 K well, although a slight increase of the leak flow was observed. The widening of the leaks is tentatively attributed to mechanical stress caused by the incommensurate lattice expansion of hydrogenated fcc and bcc PdCu grains. A formation mechanism for the frequently observed pinholes in bcc PdCu membranes is proposed based on the discrepancy between the hydrogen solubility in bcc and fcc PdCu alloys. As a consequence coexisting fcc phases could reduce the life time of bcc PdCu membranes in H2 atmospheres.

Preparation of 3–5 μm thick, hydrogen-selective PdAu layers via sequential electroless plating of Pd and Au onto ceramic microfiltration membranes was investigated employing a cyanide-free Au plating bath. The Au deposition rate was strongly dependent on bath temperature and alkalinity reaching an optimum at 333 K and pH 10. Homogenous alloying of the separate metal layers under atmospheric H2 proved to be a protracted process and required approximately a week at 873 K for a PdAu layer as thin as 3 μm. After 300 h annealing at 823 K the 5 μm thick PdAu layer of a composite membrane still exhibited a Au gradient declining from 7.4 at.% at the top surface to 5.5 at.% at the support interface despite that the H2 permeation rate had become stable. Nonetheless, the membrane exhibited a very high H2 permeability of e.g. 1.3 × 10−8 mol m m−2 s−1 Pa−0.5 at 673 K, but it decreased much faster with temperature below 573 K than above, likely due to a change from bulk H diffusion-controlled to H2 adsorption or desorption-limited transport. The composite membrane withstood cycling between 523 and 723 K in H2 well showing that differing thermal expansion of the joined metallic and ceramic materials stayed within the tolerance range up to 723 K.

The permeation behavior of a 2.0 μm thick Pd95Ag5/ceramic composite membrane was investigated between 403 K and 823 K. Above 460 K H2 fluxes can be represented by an Arrhenius law with JH2=1.6±0.1 mol m−2 s−1 exp[−9.0±0.2 kJ mol−1/RT]JH2=1.6±0.1 mol m−2 s−1 exp[−9.0±0.2 kJ mol−1/RT] at ΔPH2=100 kPa with the initial ideal H2/N2 selectivity exceeding 4700 at 823 K. A discontinuity occurred in the H2 flux between 455 K and 420 K due to the opening of the miscibility gap in the Pd95Ag5Hx phase diagram and the appearance of the β Pd95Ag5 hydride phase. Below 420 K H2 fluxes can be represented by another Arrhenius law with JH2=1.7±0.2 mol m−2 s−1 exp[−7.2±0.3 kJ mol−1/RT]JH2=1.7±0.2 mol m−2 s−1 exp[−7.2±0.3 kJ mol−1/RT] at ΔPH2=100 kPa. Activation energies are consistent with diffusion limited H2 transport both above and below the permeation discontinuity. A gradual shift towards linear pressure dependence with decreasing temperature is attributed to an increasing impact of the mass flow resistance of the 2 mm thick ceramic support on the H2 permeation rates at low temperatures. A steeper increase of the pressure exponent n upon appearance of the β Pd95Ag5 hydride is associated with the lower hydrogen activity in this phase. The membrane exhibited good stability during 20 temperature cycles between 473 K and 673 K in H2 atmosphere and 3 cycles down to room temperature in N2, resulting only in a slight increase of the N2 leak rate from 0.44 × 10−3 mol m−2 s−1 to 0.54 × 10−3 mol m−2 s−1 at 673 K and ΔPN2=100 kPa. Monitoring of H2 on the permeate side after switching from H2 feed to N2 purge streams showed that between 5 h at 673 K and 9 h at 523 K were required for a complete release of H2 from the composite membrane. Analysis of the emitted H2 volumes showed that the H2 release on the permeate side was limited largely by the mass flow resistance of the porous ceramic support. H2 released on the membrane feed side exceeded that on the permeate side by 1–2 orders of magnitude and originated mainly from the stainless steel reactor shell. As a consequence the hydrogen content of supported Pd and PdAg layers could remain above the threshold for embrittlement after switching to inert atmospheres if not sufficient time is allowed for H2 release before cooling.

Defects in Pd and Pd–Ag membranes have been successfully sealed by directed electroless plating, which was achieved by feeding the metal source and the reducing agent from opposite directions to the defect zone. Optical microscopy showed that the surface texture of the metal layers was well preserved in the vicinity of cracks and pinholes, indicating that Pd deposition was effectively restricted to defect sites. The ideal H2/N2 selectivity could be improved by more than an order of magnitude during these point plating experiments while the very high H2 permeability of the membrane was completely retained, indicating that the overall metal layer thickness had not increased.

A water–gas shift (WGS) Pt/Ce0.6Zr0.4O2 catalyst has been prepared, which exhibits much faster kinetics than conventional high-temperature ferrochrome catalysts in the temperature range most suitable for operation of WGS Pd membrane reactors, i.e. above 623 K. The performance of the Pt catalyst was tested in a reactor furnished with a supported, 1.4 μm thick high-flux Pd membrane using feeds obtained by autothermal reforming of natural gas. CO conversion remained above thermodynamic equilibrium up to feed space velocities of 9100 l kg−1 h−1 at 623 K, Ptotal = 1.2 MPa and steam-to-carbon ratio S/C = 3, but H2 recovery decreased from 84.8% at GHSV = 4050 l kg−1 h−1 to 48.7% at the highest space velocity. This rapid decline of separation performance is attributed to slow H2 diffusion through the catalyst bed, suggesting that external mass flow resistance has a significant impact on the H2 permeation rate in such membrane reactors. This could be minimized by the development of WGS catalysts with even faster kinetics which would allow further reduction of the catalyst bed height.

The influence of high CH4 concentrations (8–31%) on the H2 permeation through a 2 μm thick Pd membrane was investigated between 473 and 823 K at 100 kPa pressure difference at the maximum H2 extraction limit, with the retentate H2 concentration always staying above 50%. The permeate flux was slightly higher in CH4/H2 mixtures than in equimolar N2/H2 mixtures. This is attributed to decomposition of CH4 on the Pd surface as indicated by trace amounts of C2H6. At 723 K and below H2 permeation remained stable during 6 h experiments in CH4/H2 mixtures, whereas it gradually decreased with time above that threshold. TPO, SEM and XRD revealed several types of carbon forming on the surface of membrane pieces during exposure to pure CH4. Following CH4 treatment below 723 K two types of carbon deposits were found, which are designated as carbidic carbon and disordered carbon aggregates. Carbonaceous filaments were observed after CH4 exposure above 723 K and graphite platelets after treatment at 823 K. Furthermore, metastable Pd0.9C0.1 and/or Pd0.85C0.15 phases were detected after CH4 treatment at 573, 623, and 773 K. Carbidization of the membrane was most severe at 623 K, but the carbides could be readily decomposed by heat treatment in N2 between 623 and 723 K. Mechanisms for deactivation of the Pd membrane during separation of CH4/H2 mixtures at elevated temperatures are discussed.

The N2 and H2 evolution, respectively, were monitored during deposition of Pd and Cu from electroless plating baths to obtain in-process control of the composition during preparation of 3–7 μm thick PdCu membranes on tubular ceramic substrates. Compositions estimated by gas evolution compare favorably to those measured in post-mortem XRD and EDS analyses, mostly differing by not more than 1 at.%. This result suggests that use of gas evolution measurements to enable in-process control of composition to within 1 at.% is feasible. Annealing experiments in an H2 atmosphere demonstrated that, at 893 K, only 48 h are needed to form a stoichiometrically homogeneous, 9.5 μm thick, face centered cubic (fcc) Pd63Cu37 membrane from sequentially deposited layers; at 723 K, the same transformation requires over 2 weeks. The appearance of transient body centered cubic (bcc) and fcc phases with lower Pd contents signaled compositional segregation in the initial stages of alloy formation at 723 and 773 K and could be a source of persistent stoichiometric heterogeneity particularly in bcc PdCu membranes. The H2 fluxes of fcc Pd58Cu42 and Pd70Cu30 membranes were JH2=(1.6±1.1) mol m−2 s−1 exp[(−24.8±0.4)kJ mol−1/RT]JH2=(1.6±1.1) mol m−2 s−1 exp[(−24.8±0.4)kJ mol−1/RT] and JH2=(3.7±0.6) mol m−2 s−1 exp[(−21.3±1.0)kJ mol−1/RT]JH2=(3.7±0.6) mol m−2 s−1 exp[(−21.3±1.0)kJ mol−1/RT], respectively, at 100 kPa H2 pressure difference.

H2 permeation hysteresis has been observed during cycling of a 3 μm thick supported PdCu membrane with ∼50 atom % Pd through the fcc/bcc (face-centered cubic/body-centered cubic) miscibility gap between 723 and 873 K. Structural investigations after annealing of membrane fragments under H2 at 823 K reveal retardation of the fcc(H) → bcc(H) transition, which is attributed to the occurrence of metastable hydrogenated fcc PdCu(H) phases. The H2 flux at 0.1 MPa H2 pressure difference in the well-annealed bcc single phase regime below 723 K can be described by JH2 = (1.3 ± 0.2) mol·m−2·s−1 exp[(−11.1 ± 0.6) kJ·mol−1/(RT)] and that in the fcc single phase regime above 873 K by JH2 = (7 ± 2) mol·m−2·s−1 exp[(−30.3 ± 2.5) kJ·mol−1/(RT)].

The interaction of pure CO2 with a 3 μm thin, supported Pd membrane has been investigated between 473 and 773 K. Diagnostic H2 permeation measurements indicate a reduction of the H2 flux after CO2 exposure at the lower and upper ends of this temperature range. Temperature-programmed oxidation and desorption in combination with scanning electron microscopy analyses reveal the dissociation of CO2 above 523 K, yielding molecularly and/or dissociatively adsorbed CO below 623 K and nanoscopic carbon deposits above 723 K. CO2 is obviously not inert over Pd surfaces at practical Pd membrane operation temperatures but could be kinetically stabilized in a narrow temperature window around 673 K.

Higher operation temperatures benefit H2 permeability and selectivity of metal membranes and they are interesting for e.g. water gas shift and steam reforming in membrane reactors. Hence the behaviour of PdAg–ceramic composite membranes has been investigated between 823 K and 923 K. The H2 flux of membranes with less than 10 μm thick alloy layers decreased continuously with time during operation under H2 at 873 K and above. This was accompanied by a steady increase of the activation energy for H2 permeation and the growth of Ag-depleted crystallites on the membrane surface. All phenomena could be reversed through annealing under N2 at 923 K. The textural and permeability changes are consistent with a segregation mechanism starting with metal sublimation from hydrogenated PdAg layers and subsequent metal resublimation. This implies an enhancement of the yet unknown metal activities in PdAg hydride phases over metallic PdAg alloys. Ramifications for application of thin-layered, supported PdAg membranes for H2 separation above 823 K are discussed.