Structural and permeation kinetic correlations in PdCuAg membranes.

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.

Hydrogen-induced high-temperature segregation in palladium silver membranes.

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.

Nanoscale heterogeneity in alkyl-methylimidazolium bromide ionic liquids.

High-energy x-ray diffraction measurements and atomistic molecular dynamics (AMD) numerical simulations have been carried out on 1-alkyl-3-methylimidazolium bromide ionic liquids, C(n)mimBr, with n = 2, 4, and 6. Excellent agreement between experiment and simulation is obtained, including the region of the low-Q peak that has proved problematic in previous work in the literature. In the partial structure analysis of the AMD results, a distinct peak develops at the leading edge of the ring-ring pair distribution function and shifts to lower r with increasing alkyl chain length, indicating that the preferential parallel and antiparallel alignment of neighboring cation rings plays a larger role with increasing chain length. The ring-ring, anion-anion, and ring-anion partial structure factors are dominated by strong charge-ordering peaks around 1.1 Å(-1), corresponding to a distance between neighboring polar entities of D(2) = 5.7 Å. In contrast, the tail-tail S(Q) is dominated by the low-Q peak that rises and moves to lower Q with increasing chain length; the length scale of this structural heterogeneity D(1) increases from about 10 Å in C(2)mimBr to 14.3 Å in C(4)mimBr and 18.8 Å in C(6)mimBr. Both the length scale of the structural heterogeneity and its anomalous temperature dependence in the C(n)mimBr liquids studied here show considerable similarity to results in the literature for C(n)mimPF(6) liquids, indicating a remarkable insensitivity to the form and size of the anion. Our results are consistent with the concept of nanoscale heterogeneity with small, crystal-like moieties.

Structure of a prototypic ionic liquid: ethyl-methylimidazolium bromide.

High-energy X-ray diffraction measurements have been carried out on 1-ethyl-3-methylimidazolium bromide and complemented with molecular-dynamics simulations. Because the structure of the corresponding crystal is known, both the liquid and the crystal phases are simulated numerically. The liquid structure factor is dominated by an intense peak at 1.7 A(-1), associated mainly with the packing of the anions around the large cations. Analysis of the real-space correlations of the liquid shows that the Br(-) ions are distributed more symmetrically around the cation ring and move closer to the ring atoms compared with the crystal. Although the distribution of the anions around the cation in the first coordination shell of the liquid exhibits clear analogies with the crystal, the cation-cation partial distribution function of the liquid shows a significant component with lower distances between ring centers, with some pairs coming as close as 3.5 A in either parallel or antiparallel configurations. Finally, the presence of topological short-range order and charge ordering in the liquid is clearly demonstrated.

CO2 decomposition over Pd membrane surfaces.

The interaction of pure CO 2 with a 3 microm thin, supported Pd membrane has been investigated between 473 and 773 K. Diagnostic H 2 permeation measurements indicate a reduction of the H 2 flux after CO 2 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 CO 2 above 523 K, yielding molecularly and/or dissociatively adsorbed CO below 623 K and nanoscopic carbon deposits above 723 K. CO 2 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.

Permeation hysteresis in PdCu membranes.

H 2 permeation hysteresis has been observed during cycling of a 3 mum thick supported PdCu membrane with approximately 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 H 2 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 H(2) flux at 0.1 MPa H(2) pressure difference in the well-annealed bcc single phase regime below 723 K can be described by J(H2) = (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 J(H2) = (7 +/- 2) mol.m (-2).s (-1) exp[(-30.3 +/- 2.5) kJ.mol (-1)/( RT)].

Segregation and H2 transport rate control in body-centered cubic PdCu membranes.

The H2 permeation of a supported 2 microm thick Pd48Cu52 membrane was investigated between 373 and 909 K at DeltaP=0.1 MPa. The initial H2 flux was 0.3 mol.m(-2).s(-1) at 723 K with an ideal H2/N2 selectivity better than 5000. The membrane underwent a bcc-fcc (body-centered cubic to face-centered cubic) phase transition between 723 and 873 K resulting in compositional segregation. After reannealing at 723 K the alloy layer reverted to a bcc structure although a small fcc fraction remained behind. The mixed-phase morphology was analyzed combining X-ray diffraction with scanning electron microscopy-energy-dispersive spectroscopic analysis (SEM-EDS) measurements, which revealed micrometer-scale Cu-enriched bcc and Cu-depleted fcc domains. The H2 flux JH2 of the fcc Pd48Cu52 single phase layer prevailing above 873 K could be described by an Arrhenius law with JH2=(7.6+/-4.9) mol.m(-2).s(-1) exp[(-32.9+/-4.5) kJ.mol(-1)/(RT)]. The characterization of the H2 flux in the mixed-phase region required two Arrhenius laws, i.e., JH2=(1.35+/-0.14) mol.m(-2).s(-1) exp[(-10.3+/-0.5) kJ.mol(-1)/(RT)] between 523 and ca. 700 K and JH2=(56.1+/-9.3) mol.m(-2).s(-1) exp[(-25.3+/-0.6) kJ.mol(-1)/(RT)] below 454 K. The H2 flux exhibited a square root pressure dependence above 523 K, but the pressure exponent gradually increased to 0.77 upon cooling to 373 K. The activation energy and pressure dependence in the intermediate temperature range are consistent with a diffusion-limited H2 transport, while the changes of these characteristics at lower temperatures indicate a desorption-limited H2 flux. The prevalence of desorption as the permeation rate-limiting step below 454 K is attributed to the pairing of an extraordinarily high hydrogen diffusivity with a marginal hydrogen solubility in bcc PdCu alloys. These result in an acceleration of the bulk diffusion rate and a deceleration of the desorption rate, respectively, allowing the bulk diffusion rate to surpass the desorption rate up to relatively high temperatures.

Selenium/zeolite Y nanocomposites.

Confinement in molecular sieves is a promising strategy for fabricating nanostructured semiconductor assemblies with a highly uniform size and shape distribution. However, disorder effects often hamper the engineering of matrix-embedded cluster materials with specific properties. The host-guest interaction is a key factor for optimizing their structure, electronic characteristics, and stability. In this Account, we describe how the interplay between confined selenium and extra framework cations in zeolite hosts can be used to tailor these properties and to produce well-defined semiconductor nanocomposites with band gaps in the visible range.