Pd-Al Alloys Research Articles

TPD and XPS study of the CO adsorption on transition-SP metal systems: Pd and Al

CO adsorption properties of the bimetallic Pd/Al systems have been studied using thermally programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under UHV conditions. The samples were prepared by vacuum evaporation of Pd onto pure Al substrates. Measured TPD and XPS spectra of CO from bimetallic surfaces were compared to the ones of clean bulk metals. The deposited Pd ad-layers exhibited strong interaction with Al support, which determined the chemical activity of Pd towards interaction with CO. The formation of PdAl alloy of noble metal-like electronic structure with the d-band centered at 4.5eV below Fermi level was observed. The Pd 3d5/2 core level exhibited binding energy 1.8eV higher than that of the surface atoms of bulk Pd with further slight increase finishing at 2.1eV. The thermal desorption spectra of CO from the Pd/Al system consists of two peaks, both lower than those from Pd foil, indicating a distinct weakening in the strength of the CO adsorption bond. Heating induced alloying, an appearance of surface oxygen and a carbon contamination on bimetallic surfaces were observed. The correspondence of these effects with the character of electronic interaction between Pd and Al is discussed.

Enhanced rates of hydrogen absorption resulting from oxidation of Pd or internal oxidation of Pd-Al alloys

It is shown that a convenient method for obtaining very fast rates of H 2 absorption by Pd is by its oxidation or by the internal oxidation of Pd-M alloys where M=a solute more readily oxidizable than Pd such as Al. While the oxidation of Pd enhances the rates it does not affect its H 2 solubility because its oxidized layers are rapidly reduced upon exposure to H 2 even at low temperatures. The internal oxidation of Pd–M alloys, gives a matrix of pure Pd containing internal oxide precipitates, e.g., a Pd/alumina composite. The solubility of H 2 in the Pd matrix should be equivalent to that in pure Pd.

Inhibition by gaseous impurities of hydrogen absorption by Pd and by internally oxidized Pd–Al alloys

Absorption of H2 is a sensitive technique for the study of the effects of impurity gases on H2 chemisorption by Pd because chemisorbed is an intermediate for absorption. Since large amounts of hydrogen are absorbed by Pd, the absorption can be readily followed experimentally. The rates of H2 absorption by Pd foil in the presence of He, CO2 and CO gases are, in the order of increasing inhibition: He:CO2:CO. Surprisingly CO, which is strongly chemisorbed on Pd, was not as strong a poison for H2 chemisorption as anticipated. The effect of CO pre-adsorption followed by evacuation at 323 K did not affect the subsequent rates of H2 absorption nearly as much as absorption from (CO + H2) mixtures. For Pd foil, the simultaneous presence of H2 and CO led to inconveniently slow rates, but for high surface area Pd such as Pd-on-kieselguhr or Pd powder, the rates from (H2 + CO) mixtures were reasonably fast. After internal oxidation, Pd–Al alloys are converted into composites of alumina precipitates within Pd matrices. The rates of H2 absorption have been shown to be faster for these composites than for Pd foil. In the presence of gaseous impurities the rates of H2 absorption by internally oxidized Pd–Al alloys are faster than for pure Pd foil. The rates of H2 absorption from (H2 + CO) mixtures increase with % Al in the internally oxidized alloys and for alloys with Al greater than about 3%, the rates are quite fast.

Some microstructural aspects of hydriding internally oxidized Pd–Al alloys

Abstract Microstructural studies of internally oxidized Pd–Al alloys have been carried out using scanning electron microscopy. Pd nodules form by extrusion of Pd from the bulk while vacancies are transported from the surface during the formation of the internal oxide precipitates. The result is a dimpled honeycomb appearance in the subsurface region as seen along grain walls. Extensive intergranular cracking occurs during internal oxidation. When the internally oxidized alloys are hydrided/dehydrided, cracking is accentuated with transgranular cracks developing. Significant enhancement in the rates of hydriding and dehydriding have been observed after internal oxidation.

Truncated pseudopotentials for alloy calculations

In first-principles plane-wave pseudopotential calculations, there is a subtle interplay between the convergence of the wave function and the pseudopotential expansions. Changing the cutoff of the plane-wave expansion often causes a change in the calculated physical quantities. We discuss the origin of this problem and how to ameliorate it by introducing a simple truncation of the pseudopotential in the reciprocal space. By exploiting the arbitrariness inherent in pseudopotentials, the method also provides an extra handle to ``tune'' the pseudopotentials to yield the best possible agreement with the all-electron results for bulk systems. We demonstrate the benefits of this truncation using Pd as an example and illustrate the results by calculations for some Pd-Al alloys.

Thermal stability of the Pd-Al alloy Schottky contacts to n-GaAs

Thermal stability of the Pd-Al alloy Schottky contacts to n-GaAs have been investigated by X-ray diffraction (XRD), cross-sectional transmission electron microscopy (XTEM) and Auger depth profiling. Electrical characteristics of the Schottky diodes were assessed by current-voltage measurement. Four different alloy compositions, i.e. Pd 40Al 60, Pd 48Al 52, Pd 52Al 48 and Pd 68Al 32, were codeposited by dual electron-gun evaporation and then annealed by rapid thermal processing at temperatures 500–1000 °C for 20 s. The Al-rich films were chemically stable on GaAs up to 1000 °C, however, the contacts degraded and exhibited poor diode characteristics when interfacial diffusion was prominent and the interface became rough. The Pd-rich films were chemically less stable and the interfacial reaction occurred at a lower temperature with increasing Pd content. After high-temperature annealing, more As outdiffused into the Al-rich films. On the other hand, more Ga outdiffused into the Pd-rich films resulting in the chemical reaction and the formation of PdGa compound. The Pd 48Al 52 contact exhibited the best stability among all Pd-Al alloy metallizations on GaAs. A barrier height of 0.97 V and an ideality factor of 1.09 were obtained for Pd 48Al 52/ n-GaAs Schottky diode annealed at 900 °C for 20 s.

Hydrogen solubility in [formula omitted] composites prepared by internal oxidation of PdAl alloys

Internally oxidized PdAl alloys have been studied at 323 K using p H 2 -composition isotherms. Internal oxidation of Pd 0.97Al 0.03 alloys results in alumina precipitates within an essentially Pd matrix, i.e. internal oxidation results in a Pd Al 2O 3 composite. The dilute hydrogen solubilities (323 K) in the composite are enhanced compared to annealed, pure Pd. Solubility enhancements have been measured after oxidations at 983 K, 1073 K, 1173 K and 1273 K for different times. Thermal residual stresses develop when the internally oxidized alloys are cooled from the high oxidation temperatures. The solubility enhancements are attributed to the presence of these residual stresses about the ceramic particles. The solubility enhancements for completely internally oxidized alloys are greater after lower, rather than higher, oxidation temperatures. Transmission electron microscopy indicates that alumina precipitates are nanometer-sized after oxidation at the lower temperatures and coherent with the Pd matrix. After oxidation at 1273 K, they precipitate as triangular prisms with their lengths being much greater than their sides while after oxidation at 1073 K, their sizes are smaller and their lengths are similar to their sides. Theoretical analysis of solubility enhancements about spherical and cylindrical precipitates reveal that the thermal stresses about the former are greater. The results of the analysis are consistent with the thermal residual stresses as the principal source of the experimental solubility enhancements.

Neopentane conversion over Pd/γ-Al2O3

X-ray diffraction showed that during high temperature reduction at 600°C, chlorine-free Pd/γ-Al2O3 undergoes partial transformation to a Pd-Al alloy, which confirms results of other studies [9]. This evolution appears to have a large effect on the catalytic behaviour in the reaction of neopentane with hydrogen: the selectivity towards isomerization increases from <20 up to ∼80%. At the same time, the activation energy drops from ∼ 60 to ∼ 22 kcal/mol. These changes can be reversed by oxidation at 500°C followed by reduction at 300°C. The presence of residual chlorine (ex-PdCl2 precursor) appears to inhibit the Pd induced reduction of Al2O3 leading to Pd-Al alloy formation.

Palladium-induced charge transports with palladium/alumina/aluminum interface formation

Using XPS and XAES, we have investigated the Pd-Al 2 O 3 /Al interaction for thicknesses of the oxide layer from 0.6 to 3.0 nm. For oxide layers thinner than 1.2 nm, diffusion of Pd into the support and subsequent formation of a PdAl alloy under the oxide have been observed. Oxide layers thicker than 1.6 nm sufficiently reduce Pd diffusion. With the use of the Pd 3d line shag together with the parameters of the Auger transitions, the transfer of electrons from the terminating Al p orbitals at the oxide surface to the dispersed Pd particles has been proposed. This process creates a Pd/Al 2 O 3 interface dipole, the electric field of which attracts free electrons from elemental Al toward the sample surface

Strong evolution of the p-projected empty density of states in Pd-Al alloys: An M4,5 x-ray-absorption-spectroscopy investigation.

We present a near-edge x-ray-absorption investigation on the Pd ${\mathit{M}}_{4,5}$ edge of Pd, ${\mathrm{Pd}}_{0.85}$${\mathrm{Al}}_{0.15}$, PdAl, and ${\mathrm{PdAl}}_{3}$, and on the Ag ${\mathit{M}}_{4,5}$ edge of Ag. A comparison with theoretical results for the pure elements Pd and Ag indicates this kind of spectroscopy to be related to the p-projected density of states above the Fermi level via a 3d\ensuremath{\rightarrow}5p dipole allowed excitation. A direct estimation of the Ag ${\mathit{M}}_{4,5}$ core-hole lifetime width value (0.4\ifmmode\pm\else\textpm\fi{}0.05 eV) is furthermore accomplished. The dramatic changes of the investigated line shapes on going from pure Pd to an increasing dilution of Pd atoms into an Al matrix host suggest a strong participation of the p states to the formation of the chemical bonding which has been undervalued up to now.

Absorption of Hydrogen by Pd-Al and Pd-In Solid Solution Alloys*

Absorption of Hydrogen by Pd-Al and Pd-In Solid Solution Alloys*

Tailoring of Schottky barrier heights in Si and GaAs using metal alloys for infrared detectors

The tailoring of Schottky barrier heights in semiconductors for IR detectors is still a difficult task. Various approaches to modify barrier heights have been reported in the literature. A comparatively easier technique for tailoring barrier heights in Si and GaAs by adjusting the alloy composition of suitable metal alloys is proposed in this paper. The experimental verification of this technique, using Pd-Al alloy Schottky contacts on p-type Si, is also reported.

High-resolution transmission electron microscopy studies of structural modifications in pd-al2o3interfaces

Abstract High-resolution transmission electron microscopy (HRTEM), in conjunction with extensive image simulation, has been applied to topotaxial Al2O3-Pd interfaces formed by the internal oxidation of Pd-Al alloys. The Al2O3 phase has been characterized as η-Al2O3 from HRTEM images. We conclude that the Al2O3 is terminated by O ions after oxidation and by Al ions after a subsequent annealing in Al vapour. This structural modification is consistent with earlier H-trapping experiments. The origin of the ceramic precipitates in relation to the transmission electron microscopy and complementary X-ray measurements and the difficulties of applying HRTEM to an incoherent topotaxial interface are discussed.

Hydrogen and oxygen at metal/oxide interfaces

Using mostly electrochemical methods the chemical potential, diffusion coefficient, partial molar volume and the resistivity increment of hydrogen were determined at room temperature in internally oxidized PdAl, PdMg, PdZn and PdZr alloys. In all samples hydrogen is trapped at the formed metal/oxide interfaces, where irreversible trapping (with respect to anodic polarization) can be distinguished from reversible one. In the cases of Pd Al 2 O 3 and Pd MgO phase boundaries the oxide precipitates were characterized by electron microscopy with regard to their size and their orientation and, therefore, the coverage of the phase boundaries with irreversibly bound hydrogen could be calculated as 1.5·10 15 H/cm 2 or 7·10 14 H/cm 2, respectively, which is about one monolayer of the densest packed oxygen planes of the corresponding oxides. This equivalence and the irreversible nature of the bonding indicates the formation of OH bonds at the phase boundaries. During irreversible trapping a larger volume change is observed as compared to dissolution of hydrogen in regular octahedral sites of palladium. The measured partial molar volumes correspond to an increase of the phase boundary “thickness” of 0.3 to 0.9 Å between Pd and oxide. The irreversible component of hydrogen trapping disapears after annealing in the vapor of the alloying addition (Al, Mg, …) or in vacuum and appears again after a second treatment in air. As an explanation of this effect we propose that annealing in metal vapor or in vacuum, respectively, changes the composition at the phase boundary. At high oxygen activities oxygen segregates to the phase boundaries. In comparison to the oxygen in the oxide the segregated one is less strongly bound and, therefore, can be desorbed at low O-activities. Furthermore the missing bonds to the stable oxide allow the formation of OH bonds.

Tritium Mobility and Permeability in Pd-Al Alloys and the Influence and Structure of Segregational Overlayers

Tritium Mobility and Permeability in Pd-Al Alloys and the Influence and Structure of Segregational Overlayers

Electric field gradients in dilute aluminium alloys

Electric field gradients probed by111In, which are produced in dilute PdAl alloys, were studied with the PAC technique. Four different configurations are found which are probably due to metastable aluminium-rich PdAl phases.

Electric field gradient in dilute transitional alloys of Cu and Al

An attempt is made to explain the electric field gradient (EFG) and the asymmetry parameter η for Cu alloys with Ni, Pd, and Pt impurities and Al alloys with Sc, Ti, V, Cr, Mn, Fe, and Cu impurities. In Cu alloys, the valence effect contribution to the EFG is estimated using the Alfred and Van Ostenburg charge perturbation theory and the size effect contribution is adopted from that by Sagalyn and Alexander. Leading phase shifts are assumed to be η0 and η2 and these are determined by satisfying experimental data for residual resistivity and the Friedel sum rule. The effective charge on the impurity is estimated using the Friedel criterion of the bound state. It is found that the EFG increases with an increase in effective charge on the impurity. The size effect contribution dominates over the valence effect contribution. The agreement between the calculated and experimental values of the EFG and η for CuNi, CuPd, and CuPt is reasonably good. The calculations for the EFG and η for Al alloys are carried out following the charge perturbation theory due to Lautenschlager and Mrosan where the preasymptotic contribution is explicitly included through the energy dependence of phase shifts. Except for AlFe, the size effect contribution is greater than the valence effect contribution. The calculated and experimental values of the EFG and η are in reasonable agreement except for AlSc. Possible reasons for this are discussed.

CPA Calculation of the Electronic Structure of Transition Metal Alloys with Muffin-Tin Potential Model

A new method of calculation of the coherent potential approximation combined with the Korringa-Kohn-Rostoker method of the band structure calculation is proposed. The electronic structure of Fe–Al and Pd–Al alloys are calculated by this method. The concentration dependence of the electronic specific heat coefficient, the (high field) susceptibility and the nuclear spin-lattice relaxation time of the alloys is discussed in the light of the calculation.

Low‐Temperature Heat Capacities of Pd–Al Alloys at Low Al Contents

AbstractThe heat capacities of Pd–Al alloys up to 15 at% Al content were measured in the temperature range between 1.5 and 12 K. The results are discussed in terms of a band‐filling model. In this picture Al admixtures behave like Ag admixtures to Pd but contribute about three electrons to the host instead of one. Conclusions are drawn concerning the superconducting transition temperature in Pd–Al hydrides.

The effect of various types of treatment on the surface layer composition in binary palladium alloys

1. The reactions of oxidizing and reducing agents can markedly alter the surface compositions in binary alloys of palladium with aluminum, copper, and nickel. The treatment in hydrogen brings about a marked enrichment of the surface of Pd-Cu and Pd-Al alloys with respect to the low-melting metal. Air treatment covers the surface of Pd-Cu and Pd-Ni alloys with copper and nickel oxides. 2. Only catalytic dehydrogenation of cyclohexane to benzene leads to surface enrichment with respect to the second of the components in alloys of palladium with ruthenium and rhodium, redistribution of the metals between body and surface resulting in a sharp increase in the catalytic activity.