Pd-Ni Alloys Research Articles

Comparison of surface structure and segregation in AgAu and NiPd alloys

Abstract The structure and composition of a single-crystal AgAu(1 0 0) alloy surface was studied using low energy electron diffraction. The surface was found to be moderately enriched in silver, while the second atomic layer has an essentially bulklike composition. The first interlayer distance exhibits a small contraction, with a small expansion found in the second interlayer. This structure composition behavior is compared to that found in the low index surfaces of the NiPd alloy, which has compositional oscillations and a more complex surface relaxation behavior.

Molecular dynamic simulation studies of glass formation and atomic-level structures in Pd–Ni alloy

Based on the embedded-atom method, a constant-pressure, constant-temperature (NPT) molecular dynamics (MD) technique is applied to obtain an atomic description of glass formation process in Pd–Ni alloy. By using radial distribution function (RDF) and pair analysis (PA) methods, the structure and glass forming ability of this alloy is studied by quenching from the liquid at different cooling rates (1×10 11 K/s, 5×10 11 K/s, 6×10 11 K/s and 1×10 13 K/s). It is observed that the retention of amorphous structure requires extremely high cooling rates. The critical cooling rate of amorphous structures is 6×10 11 K/s. Structure analyses of the alloy in the simulations reveal the evolvement of the different clusters at various quenching rates during the quenching process.

Liquid properties of Pd–Ni alloys

Liquid properties of Pd–Ni metal alloys are computed by molecular dynamics (MD) simulation with the use of quantum Sutton–Chen potential (Q-SC) model. The thermodynamical, structural, and transport properties of the alloy are investigated. The melting temperatures for Pd–Ni system are predicted. The temperature and concentration dependence of diffusion coefficient and viscosity are reported. The transferability of the potential is tested by simulating the liquid state. The values of melting point are in excellent agreement with the experiment. Comparison of calculated structural and dynamical properties with the available experiments and other calculations shows satisfactory consistency.

Methane decomposition into hydrogen and carbon nanofibers over supported Pd–Ni catalysts

The effect of the addition of different metals (Cu, Rh, Pd, Ir, and Pt) into supported Ni catalysts on the catalytic performance of methane decomposition into pure hydrogen and carbon was examined. The addition of Pd brought about considerable increases in the catalytic life and in the accumulated yields of hydrogen and carbon at the complete deactivation of the catalyst, while modification with the other metals decreased the yields compared to those for Ni/SiO 2. For the Ni catalysts modified with Pd, the appropriate preparation conditions of the catalysts, the effect of catalytic supports, and the optimum reaction temperature on the methane decomposition were examined. The hydrogen yield attained a high value, 16,000 mol H 2 /mol Pd+Ni, when Pd and Ni were of a mole ratio Pd/(Pd+Ni)=0.5 and a total loading Pd+Ni=37 wt% on carbon nanofiber support. This yield is the highest among those reported so far. Methane decomposition over Ni catalysts modified with Pd produced carbon nanofibers with unique structures, i.e., branched carbon nanofibers. XRD studies on the Ni catalysts modified with Pd indicated the formation of Pd–Ni alloys. The alloys are responsible for the increase in the catalytic life and the formation of carbon nanofibers with unique structures.

Atomistic analysis of surface segregation in Ni–Pd alloys

Atomistic analysis and large-scale atomistic simulations using the Bozzolo–Ferrante–Smith (BFS) method for alloys were performed to investigate surface segregation in Ni–Pd alloys as a function of temperature, crystal face, and composition. Pd-enrichment of the first layer was observed in (1 1 1) and (1 0 0) surfaces, and enrichment of the top two layers occurred for (1 1 0) surfaces. In all cases, the segregation profile shows alternating planes enriched and depleted in Pd in a decreasing oscillatory profile. Based on atom-by-atom static energy calculations, an explanation for the observed surface features is presented in terms of competing chemical and strain energy effects.

Analysis of surface and bulk behavior in Ni–Pd alloys

The most salient features of the surface structure and bulk behavior of Ni–Pd alloys have been studied using the Bozzolo–Ferrante–Smith (BFS) method for alloys. Large-scale atomistic simulations were performed to investigate surface segregation profiles as a function of temperature, crystal face, and composition. Pd enrichment of the first layer was observed in (1 1 1) and (1 0 0) surfaces, and enrichment of the top two layers occurred for (1 1 0) surfaces. In all cases, the segregation profile shows alternated planes enriched and depleted in Pd. In addition, the phase structure of bulk Ni–Pd alloys as a function of temperature and composition was studied. A weak ordering tendency was observed at low temperatures, which helps to explain the compositional oscillations in the segregation profiles. Finally, based on atom-by-atom static energy calculations, a comprehensive explanation for the observed surface and bulk features will be presented in terms of competing chemical and strain energy effects.

Characterization of Ni–Pd alloy as anode for methanol oxidative fuel cell

Electrochemical deposition of Ni–Pd alloy films of various compositions from bath solution containing ethylenediamine (EDA) was carried out to use as anode material for methanol oxidative fuel cell in H 2SO 4 medium. Electronic absorption spectrum of bath solution containing Ni 2+, Pd 2+ ions and EDA indicated the formation of a four coordinate square planar metal–ligand complex of both the metal ions. X-ray diffraction (XRD) patterns of the deposited alloy films show an increase in Pd–Ni alloy lattice parameter with increase in Pd content, and indicate the substitution of Pd in the lattice. A nano/ultrafine kind of crystal growth was observed in the alloy film deposited at low current density (2.5 mA cm −2). X-ray photoelectron spectroscopic (XPS) studies on the successively sputtered films showed the presence of Ni and Pd in pure metallic states and the surface concentration ratio of Ni to Pd is less than bulk indicating the segregation of Pd on the surface. Electro-catalytic oxidation of methanol in H 2SO 4 medium is found to be promoted on Ni–Pd electrodeposits. The anodic peak current characteristics to oxidation reaction on Ni–Pd was found typically high when compared to pure nickel and the relative increase in surface area by alloying the Ni by Pd was found to be as much as 300 times.

Study of hydrogen electrosorption in Pd-Ni alloys by the quartz crystal microbalance

The electrochemical quartz crystal microbalance (EQCMB) method has been used to study the processes of hydrogen absorption/desorption in Pd-Ni alloy electrodes. It was found that hydrogen electrosorption is accompanied by an additional frequency shift, attributed to the stresses generated inside the alloy. The influence of stresses on the EQCMB response depends on the amount of absorbed hydrogen and the alloy composition. From the comparison of the EQCMB results with Pd-Ni alloy absorption capabilities data, it was concluded that the decrease of the hydrogen sorption capacity at a Ni content of ca. 25–30 at% is due to an excessive generation of stresses in the alloy lattice. Also, a dependency of the rate of hydrogen absorption in Pd-Ni alloys on potential is reported.

Effect of solute atoms on swelling in Ni alloys and pure Ni under He + ion irradiation

The effects of solute atoms on microstructural evolutions have been investigated using Ni alloys under 25 keV He + irradiation at 500 °C. The specimens used were pure Ni, Ni–Si, Ni–Co, Ni–Cu, Ni–Mn and Ni–Pd alloys with different volume size factors. The high number densities of dislocation loops about 1.5×10 22 m −3 were formed in the specimens irradiated to 1×10 19 ions/m 2, and they were approximately equivalent, except for Ni–Si. The mean size of loops tended to increase with the volume size factor of solute atoms. In a dose of 4×10 20 ions/m 2, the swelling was changed from 0.2% to 4.5%, depending on the volume size factors. The number densities of bubbles tended to increase with the absolute values of the volume size factor, and the swelling increased with the volume size factors. This suggests that the mobility of helium and vacancy atoms may be influenced by the interaction of solute atoms with them.

Hydrogen detection at high concentrations with stabilised palladium

In order to improve the stability to high hydrogen concentrations, of hybrid suspended gate field effect transistors (HSGFETs) with thin palladium films as sensitive layer, Pd–Ni and Pd–Ag alloys have been produced by co-evaporation techniques in UHV. In this paper, the preparation methods as well as hydrogen response measurements are presented. The observed results show that the Pd–Ni alloy is an appropriate material for hydrogen sensing at concentrations up to 2% H 2, even at room temperature. The response to 2% H 2 is around 500 mV at dry conditions. It is reduced to less than half of this value with moistened carrier gas, but at the same time, the desorption time is lowered. In contrast, the Pd–Ag alloy was not stable. A large drift of the sensor signal was observed and the morphology as well as the composition had changed after the test gas exposures.

Electronic structure of epitaxial NixPd1−x alloy films on Cu3Au(100)

We studied epitaxially grown single-crystalline chemically disordered ultrathin NixPd1−x alloy films on Cu3Au(100) by means of photoemission using magnetic circular dichroism in the angular distribution. We clearly observe a well-defined three-dimensional band dispersion in 15 monolayer films. On the basis of relativistic selection rules, this dispersion can be related to specific electronic states. Since the spin–orbit coupling (ΔEso) is smaller for Ni than for Pd, the alloy will assume an average value. We find that this average spin–orbit coupling clearly shows up in the electronic structure and correlates directly to the composition of the NiPd alloy film. The results are discussed in terms of a simple qualitative model.

Palladium and palladium alloy composite membranes prepared by metal-organic chemical vapor deposition method (cold-wall)

Thin Pd and Pd–Ni alloy membranes were prepared by the MOCVD method using an cold-wall technique. Pd(C 3H 5)(C 5H 5) and Ni(C 3H 5)(C 5H 5) were decomposed into densely aggregated metal crystallites that were to plug mesoporous nickel-stainless steel (Ni-SUS) or γ-Al 2O 3/α-Al 2O 3 supports. The use of highly volatile organometallic precursors enabled continuous and controlled deposition mode, which resulted in reproducible formation of thin impervious Pd or Pd–Ni alloy membranes. Pd(C 3H 5)(C 5H 5) and Nb(C 3H 5)(C 5H 5) 5 were deposited in a layer by layer deposition method. The H 2 permeance of the Pd/Ni-SUS membrane was 2.0–5.0×10 −2 cm 3 cm −2 cmHg −1 s −1 (723 K); the H 2/N 2 selectivity was 1600. The H 2 permeance of the Pd/Al 2O 3 was 1.5×10 −2 cm 3 cm −2 cmHg −1 s −1 (723 K); the H 2/N 2 selectivity was >1000.

Formation of filamentous carbon from 1,3-butadiene over a Ni-Pd/Al2O3 catalyst

The formation of carbon filaments from 1,3-butadiene over a Ni-Pd/Al2O3 catalyst at 400–450‡C was studied by high-resolution electron microscopy and X-ray diffraction analysis. The carbon filaments differ in morphology and in the arrangement of active metal inclusions: (1) abundant ∼ 1000 a thick filaments contain biconical alloy particles in the middle of the filament, (2) thin filaments of a thickness less than 500 a rolled in helical fashion contain oval-shaped alloy particles in the middle, and (3) in a few filaments that are less than 200 a thick, alloy particles are positioned at the ends. When the first two types of filaments grow, decomposition of the hydrocarbon occurs at the active sites of the surface with the formation of a carbide microphase with a hexagonal structure. This phase is called the active microphase. When the reaction stops, the active phase decomposes to form carbon and hexagonal nickel. Extensive defects like grain boundaries and stacking fault dislocations are present in the bulk of Ni-Pd alloy particles. The origin of these defects is related to the epitaxial effect of the active microphase particles on the alloy.

The determination of kinetics parameters of the hydrogen evolution on Pd–Ni alloys by ac impedance

The hydrogen evolution reaction (HER) at a Pd–Ni electrodeposited alloy electrode has been studied by means of ac impedance spectrum measurements and by dc polarization current behavior in 0.5 mol dm −3 NaOH solution at 25°C. The rate constants of the forward and backward reactions of the corresponding steps were estimated by a nonlinear fitting method. It has been found that the overall reaction proceeds through a Volmer–Heyrovsky mechanism, with Heyrovsky as the rate determining step. The coverage vs potential relation is discussed comparing the results of ac impedance calculations with the same of potential decay determinations.

Surface stress and chemical reactivity of Pt and Pd overlayers

Chemical properties of metals can be strongly modified by alloying. Whereas for association of transition metals having very different d band filling, i.e. located very far apart in the periodic table, the modified properties can be explained by direct electronic interaction, other effects have to be considered when considering metals in the same column of the periodic table, like Ni, Pd and Pt: viz the stress induced on the outer atoms by the substrate when their respective geometric parameters are very different. We will consider associations of Ni, Pd and Pt couples, the latter two having atomic radii more than 10% greater than Ni. Surfaces that were highly concentrated in one component were obtained either by surface segregation or by atomic beam deposition. Whatever the preparation method, the surface properties were comparable. Pt x Ni 1− x (1 1 1) alloys of respectively, 78 and 50 at.% Pt both present a well ordered (1 × 1) quasi complete Pt outer layer, the second layer mainly consisting of Ni. Nevertheless, they show very different chemisorptive properties with respect to H 2, CO, … and different reactivity for the 1,3-butadiene hydrogenation reaction. The interatomic distance in the substrate, which is respectively 2% and 5% lower than that of pure Pt, is the only parameter which is changed. Pd deposited on Ni(1 1 1), or Pd segregated at the surface of a Pd 8Ni 92(1 1 1) alloy, also exhibit a large enhancement of catalytic activity (for 1,3-butadiene hydrogenation) compared to pure Pd(1 1 1). We can again suggest that the Pd outer atoms, which are pseudomorphic with the substrate sites, have to sustain a noticeable stress. Pd deposited on Ni(1 1 0), or Pd segregated at the surface of a Pd 8Ni 92(1 1 0) alloy, also show a highly increased activity for the catalytic reaction considered, when compared to Pd(1 1 0). But, for this system a strain-induced buckling is observed, probably due to partial relaxation of strain at the surface. Pd segregates to a large and similar extent on the surfaces of PdNi and PdPt alloys having low Pd bulk content. But, while the catalytic reactivity of strained Pd atoms present at the surface of the PdNi alloy is greatly enhanced, the reactivity of unstressed Pd atoms present on the PdPt alloy is comparable to that of pure Pd. In conclusion, chemisorptive and catalytic properties of outer atoms can be more or less strongly modified with respect to the pure component depending upon the degree of stress they retain at the surface.

Hydrogen electrosorption in Ni–Pd alloys

Hydrogen absorption in Ni–Pd alloys has been investigated. The amount of absorbed hydrogen in alloys containing below 20 at.% of nickel is equal to the amount of hydrogen sorbed in pure palladium. Hydrogen absorption occurs in the range 0–40 at.% of nickel concentration. Cyclic voltammograms recorded at Ni–Pd alloys have characteristic peaks which overlap with the responses due to processes occurring on the surface at Ni and Pd atoms. Also some of the processes characteristic of the pure metals can be distinguished from the recorded voltammograms.

The surface processes at Pd–Ni alloy in acid and alkaline solutions

Potentiodynamic studies of surface processes at Pd–Ni electrode are made, in which the state of electrodeposited H and the nature of surface oxidation processes are investigated. In the anodic region of a potentiodynamic sweep at Pd–Ni electrodeposited alloy, processes of H ionization and the beginning of surface oxidation occur over somewhat overlapping potential range, since the onset of these processes are potentially different than that thermodynamically expected, owing to the irreversibility in the processes. In experiments at various sweep rates in acid and alkaline solutions some processes which involve diffusion are indicated. In acid solution, the first anodic peak is connected with oxidation of absorbed hydrogen and its diffusion to the surface of the electrode from the interior. In alkaline solution, the first anodic peak is attributed to the surface oxidation, but j p,a is not linear with the sweep rate, as for a surface process with a characteristic pseudocapacitance potential profile, but with the v 1 2 characteristic of a diffusion controlled process. It is presumably made from the bulk proton diffusion that comes from the anodic oxidation of M(OH) 2 to MOOH.

Evidence from SANS and H 2 solubilities for H-enhanced metal atom diffusion in Pd-Ni alloys

Evidence from SANS and H 2 solubilities for H-enhanced metal atom diffusion in Pd-Ni alloys

Effects of boron addition on microstructure and mechanical properties of Ti–Td–Ni high-temperature shape memory alloys

The application of Ti–Pd–Ni high-temperature shape memory alloys to practical use is restricted due to their low critical stress for slip at high temperatures. About 0.2 at.% boron was added to a Ti50Pd30Ni20 alloy to improve its high-temperature mechanical properties including high-temperature shape memory characteristics. The influence of boron on martensitic transformation temperatures, microstructure and high-temperature mechanical properties was studied. The results show that TiB2 precipitates with an average size larger than 1 μm are produced in Ti50Pd30Ni19.8B0.2 alloy due to proper heat treatments described in the text. These TiB2 precipitates seem to be distributed within grains, rather than to segregate to grain boundaries. The addition of boron is beneficial to high-temperature ductility of Ti–Pd–Ni alloy. The addition of boron has no noticeable influence on high-temperature shape memory characteristics of Ti50Pd30Ni20 alloy, and the reason for this is discussed.

Compositional changes in alloys during ion bombardment at elevated temperatures

Near-surface compositional changes in alloys during ion bombardment have been studied theoretically. The employed scheme operates with a stable target, and the effects of preferential sputtering, collisional mixing, radiation-enhanced diffusion, and Gibbsian and radiation-induced segregation are allowed for. High-fluence composition profiles were determined directly from a nonlinear integro-differential equation, after insertion of feasible input, by means of an efficient iteration procedure developed recently. The dependence of the composition profile on input parameters such as target temperature and defect mobility has been examined for Ni–Cu, Ni–Ge and Ni–Pd alloys and compared to experimental results.