Pd-Ni Alloys Research Articles

Massively parallel density functional calculations for thousands of atoms: KKRnano

Applications of existing precise electronic-structure methods based on density functional theory are typically limited to the treatment of about 1000 inequivalent atoms, which leaves unresolved many open questions in material science, e.g., on complex defects, interfaces, dislocations, and nanostructures. KKRnano is a new massively parallel linear scaling all-electron density functional algorithm in the framework of the Korringa-Kohn-Rostoker (KKR) Green's-function method. We conceptualized, developed, and optimized KKRnano for large-scale applications of many thousands of atoms without compromising on the precision of a full-potential all-electron method, i.e., it is a method without any shape approximation of the charge density or potential. A key element of the new method is the iterative solution of the sparse linear Dyson equation, which we parallelized atom by atom, across energy points in the complex plane and for each spin degree of freedom using the message passing interface standard, followed by a lower-level OpenMP parallelization. This hybrid four-level parallelization allows for an efficient use of up to $100\phantom{\rule{0.16em}{0ex}}000$ processors on the latest generation of supercomputers. The iterative solution of the Dyson equation is significantly accelerated, employing preconditioning techniques making use of coarse-graining principles expressed in a block-circulant preconditioner. In this paper, we will describe the important elements of this new algorithm, focusing on the parallelization and preconditioning and showing scaling results for NiPd alloys up to 8192 atoms and $65\phantom{\rule{0.16em}{0ex}}536$ processors. At the end, we present an order-$N$ algorithm for large-scale simulations of metallic systems, making use of the nearsighted principle of the KKR Green's-function approach by introducing a truncation of the electron scattering to a local cluster of atoms, the size of which is determined by the requested accuracy. By exploiting this algorithm, we show linear scaling calculations of more than $16\phantom{\rule{0.16em}{0ex}}000$ NiPd atoms.

Characterizations of precipitate phases in a Ti–Ni–Pd alloy

The microstructure of 46Ti–37.5Ni–16.5Pd (at.%) alloy was investigated by electron diffraction and high-resolution scanning transmission electron microscopy. The phase content and stability were determined at several different temperatures and times. Aging at 400 °C for 1 h results in a new phase (P1-phase), which is consumed by P-phase at longer aging times. At 450 °C, the P1-phase appears first, and then coexists with P-phase. At 500 °C, the entire alloy transforms into the P1-phase. At 550 °C, Ti3(Ni,Pd)4 phase begins to form.

Gas-phase hydrodechlorination of trichloroethylene over Pd/NiMgAl mixed oxide catalysts

Hydrotalcite derived NiMgAl mixed oxide (NiHT) catalysts with different NiMgAl molar ratio were synthesized and tested in the selective gas-phase hydrodechlorination of trichloroethylene to ethylene. The mixed oxide was further promoted by Pd in order to obtain Pd/NiHT catalysts. The catalysts performances for hydrodechlorination of trichloroethylene depend on the Ni/Mg/Al molar ratio, reduction temperature, and method of noble metal incorporation. The main products obtained were ethylene, methane, ethane, chloroethylene and traces of dichloroethylene. The higher the Ni amount in the NiHT support was, the higher were the catalytic activity and ethylene selectivity. Introduction of Pd increases the catalytic activity. Application of higher reduction temperatures improved the selectivity towards ethylene formation due to an increase in the concentration of surface metallic Ni. A better Pd-Ni surface interaction was obtained by impregnation of Pd on a previously reduced (823K) NiHT catalyst, resulting in higher ethylene selectivity (80%). The higher selectivity was associated to metallic Ni promoted by a Pd-Ni alloy, which was detected by HRTEM.

The spin Hall effect as a probe of nonlinear spin fluctuations

The spin Hall effect and its inverse have key roles in spintronic devices as they allow conversion of charge currents to and from spin currents. The conversion efficiency strongly depends on material details, such as the electronic band structure and the nature of impurities. Here we show an anomaly in the inverse spin Hall effect in weak ferromagnetic NiPd alloys near their Curie temperatures with a shape independent of material details, such as Ni concentrations. By extending Kondo's model for the anomalous Hall effect, we explain the observed anomaly as originating from the second-order nonlinear spin fluctuation of Ni moments. This brings to light an essential symmetry difference between the spin Hall effect and the anomalous Hall effect, which reflects the first-order nonlinear fluctuations of local moments. Our finding opens up a new application of the spin Hall effect, by which a minuscule magnetic moment can be detected.

Reversible hydrogen-storage at reduced temperatures in the intermetallic compound Mg6(Ni,Pd)

A rapidly-solidified Mg–Ni–Pd alloy is shown to exhibit reversible hydrogen-storage of 5 mass % H at a temperature of 473 K, nearly 100 K below temperatures typically required for Mg-based alloys, with an equilibrium absorption plateau near 1 atm H2 pressure. Additionally, a single plateau is observed in the pressure–composition isotherm where MgH2 and Mg2NiH4 form and decompose simultaneously during the absorption/desorption process. The increase in absorption plateau pressure to 100 kPa suggests a decrease in the free energy by as much as 15 kJ/mol resulting from either a decrease in enthalpy or an increase in entropy of reaction between the hydride and metallic phases. The improved thermodynamics are discussed in terms of the formation of a new equilibrium metallic phase, Mg6(Ni,Pd). The phase alters the thermodynamics of the hydriding reaction, allowing both MgH2 and Mg2NiH4 to form cooperatively in a single reaction. This method for reducing the free energy of the hydriding reaction is promising, and may prove useful in a variety of other metal-hydride hydrogen-storage systems.

Features of energy dependence of NiPd sputtering for various ion irradiation angles

MD simulation was applied to study disordered NiPd alloy sputtering under normal and oblique incidence of Ar ions at energy E 0 = 0.1–20 keV. Energy dependences of NiPd alloys and their components sputtering were studied and discussed for various incidence angles of ions interacting with either disturbed or undisturbed top layers of surfaces. We revealed and explained the essential differences in energy dependences and positions of NiPd single crystal sputtering yield maxima in oblique and normal ion incidence.

Electro-catalytic activity enhancement of Pd–Ni electrocatalysts for the ethanol electro-oxidation in alkaline medium: The promotional effect of CeO2 addition

Pd–Ni alloy catalysts with CeO2 modification are prepared for the ethanol oxidation reaction (EOR) in alkaline electrolyte through the extensively utilized deposition–precipitation route. The structures, morphologies, surface compositions and catalytic activities of the un-modified and Ce-modified Pd–Ni alloy catalysts have been characterized by XRD, TEM, temperature programmed reduction (TPR) and electrochemical measurements, respectively. Physical characterizations show that the 5wt.% of CeO2 addition (Ce5) can not only decrease the particle size of Pd–Ni but also promote the formation of surface Pd–Ni alloy species and amorphous NiO. CO stripping measurements and EOR results confirm that the surface Pd–Ni alloy and NiO species on Ce5 can modify the adsorption capacity toward ethanol, provide an oxygen source for CO oxidation at low potential, and thus accelerate the decomposition rate toward ethanol. In addition, the long-term durability of Ce5 is also improved due to its specific surface compositions. On the other hand, the excessive CeO2 may result in blocking the active sites and alternation in surface conditions in which the Pd–Ni rich surface species gradually converts to only Pd ones, thereby leading to the significant deterioration of EOR activity.

Ultrasonic-assisted synthesis of Pd–Ni alloy catalysts supported on multi-walled carbon nanotubes for formic acid electrooxidation

Pd–Ni alloys with different compositions (i.e. Pd 2Ni, PdNi, PdNi 2) dispersed on multi-walled carbon nanotubes (MWCNTs) are prepared by ultrasonic-assisted chemical reduction. The X-ray diffraction (XRD) patterns indicate that all Pd and Pd–Ni nanoparticles exist as Pd face-centered cubic structure, while Ni alloys with Pd. The transmission electron microscopy (TEM) images show the addition of nickel decreases the particle size and improves the dispersion. The X-ray photoelectron spectroscopy (XPS) spectra demonstrate the electronic modification of Pd by nickel doping. The electrochemical measurements reveal that the PdNi catalysts have better catalytic activity and stability for formic acid electrooxidation, among them PdNi/MWCNTs is the best. The performance enhancement is ascribed to the increase of electroactive surface area (EASA) and nickel doping effect which might modify the electronic structure.

Formation, structure, and magnetic changes at annealing of films deposited by laser sputtering of Ni and Pd

The electron microscopy investigations of Pd, Ni, and Ni-Pd alloy films are carried out. The films were produced by laser sputtering of one- and two-element targets. It is shown that, in the case of alternate deposition of nickel and palladium, polycrystalline films with a metastable hcp lattice are formed. The hcp lattice parameters increase monotonically with increasing palladium content in the film. As a result of the annealing, Ni and Ni-Pd alloy films acquire an equilibrium fcc structure. A positive deviation from Vegard’s law occurs for the dependence of the lattice parameter of the solid solution on the Pd concentration. The as-prepared Ni and Ni-Pd alloy films with the hcp structure are characterized by the absence of a magnetic moment. The transition into the ferromagnetic state occurs after annealing, and hysteresis is observed upon magnetization reversal.

Structure and magnetic state of the films deposited by laser ablation of composite nickel and palladium targets

Transmission electron microscopy, electron diffraction, and vibrating-sample magnetometry are used to show that a metastable hcp structure can form in both nickel and Ni-Pd alloy films during alternating sputtering of the Ni and Pd components of composite targets. The hcp lattice parameters increase monotonically when the palladium content in a sputtered target increases in the range 0–75%. The ratio of the hcp lattice parameters c/a is close to the ideal ratio for the hcp lattice (1.63) within the limits of experimental error. In the as-deposited state, nickel and Ni-Pd alloy films with an hcp structure have no magnetic moment. Upon annealing, the films transform into a ferromagnetic state with an fcc structure. The concentration dependence of the lattice parameter of the fcc solid solution a 0 is found to exhibit a positive deviation from Vegard’s law, which is characteristic of alloys with a concave liquidus line.

Novel nanocrystalline PdNi alloy catalyst for methanol and ethanol electro-oxidation in alkaline media

Nanocrystalline Pd 40Ni 60 alloy catalyst has been fabricated by dealloying a ternary Al 75Pd 10Ni 15 alloy in a 20 wt.% NaOH aqueous solution under free corrosion conditions. The microstructure and catalytic performance of the catalyst have been characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, and cyclic voltammetry. The Pd 40Ni 60 alloy consists of nanocrystals with sizes of 5–10 nm, and Pd/Ni elements exist in a solid solution form. Moreover, nanocrystalline zones, amorphous zones and lattice distortion can be observed in the Pd 40Ni 60 alloy. Electrochemical measurements demonstrate that, for equivalent mass Pd, Pd 40Ni 60 has an enhanced electrocatalytic performance towards methanol and ethanol oxidation in alkaline media than nanoporous Pd. The nanocrystalline Pd 40Ni 60 alloy is a promising catalyst towards alcohol oxidation in alkaline media for fuel cell applications.

The electrodeposition, magnetic and electrical characterisation of Palladium–Nickel alloys

A process for electrodepositing PdNi alloys and subsequent electrical and magnetic characterisations are reported. PdNi alloys are deposited on 0.019–0.021 Ω cm and 1–2 Ω cm n-type Silicon from a bath of Pd–ethylenediamine dichloride and Ni sulphate. Compositional analysis of films deposited at multiple deposition potentials from different bath concentrations showed that film composition can be estimated from the composition of the bath and the deposition potential used. This allows deposition of films with a range of Ni content from the same bath using different deposition potentials. The deposited films form excellent Schottky barriers on 1–2 Ω cm Si with leakage currents of the order of μA/cm 2 and a difference of about six orders of magnitude between forward and reverse bias currents. Films with Ni concentrations above 30% are observed to be ferromagnetic at room temperature and their saturation magnetisation is linearly correlated with the Ni content in the film.

Monodispersed Pd−Ni Nanoparticles: Composition Control Synthesis and Catalytic Properties in the Miyaura−Suzuki Reaction

We have successfully prepared a series of magnetically separable "quasi-homogeneous" Pd-Ni nanoalloy catalysts with tunable composition in a one-pot wet chemical route. We have evaluated the catalytic activity of these Pd-Ni alloy catalysts with different compositions through the Miyaura-Suzuki coupling reaction. These palladium/non-noble metal alloy catalysts show better catalytic activity than an equal amount of palladium nanoparticles. Furthermore, these catalysts exhibited excellent performance in superparamagnetism owing to its great advantage for reducing the usage of noble metal.

Crystal nucleation and growth in Pd–Ni alloys: a molecular simulation study

Using molecular simulation methods, we determine the microscopic mechanisms underlying crystal nucleation and growth (up to crystallites of a 2.5 nm radius) in various Pd–Ni alloys under ambient pressure. For all compositions, the critical nucleus is predominantly of the face-centered cubic structure. The concentration in Pd is higher in the nucleus than in the liquid alloy. However, our results show that the structure of the critical nucleus strongly depends on the composition. Increasing the mole fraction in Ni in the alloy leads to a decrease in the face-centered cubic character of the critical nucleus. Simulations of the growth of critical nuclei into large post-critical nuclei show that these trends also govern crystal growth of Pd–Ni. Throughout growth, the concentration in Pd in the nuclei always remains higher than in the liquid alloy. We also observe two different growth mechanisms. When the Pd mole fraction is greater than 0.5, we obtain large face-centered cubic post-critical nuclei with a few stacking faults. On the contrary, when Ni is predominant, the post-critical nuclei rapidly lose their structural identity during crystal growth, leading to highly polycrystalline nuclei.

Palladium–nickel alloys loaded on tungsten carbide as platinum-free anode electrocatalysts for polymer electrolyte membrane fuel cells

The strong interaction between PdNi alloys and WC makes PdNi/WC a novel Pt-free electrocatalyst for the anode hydrogen oxidation reaction of polymer electrolyte membrane fuel cells with activity and stability comparable to those of the conventional Pt/C catalysts.

Segregation and interlayer relaxation at the NiPd(111) surface

The composition and structure of the NiPd(111) alloy surface are measured using low energy electron diffraction (LEED). The diffracted intensities are analyzed using tensor-LEED methods with the average t-matrix approximation. The major composition results of the study are that the surface is enriched in palladium (≈80% Pd) while the second layer is enriched in nickel (≈28% Pd). No significant structural changes were found, with all measured interlayer displacements being smaller than the corresponding measurement uncertainties. Comparisons of these results are made with literature for other NiPd low-index surfaces, for the well-studied NiPt(111) surface, and with the growing body of theoretical literature for NiPd alloy surfaces.

Pure Ni and Pd-Ni Alloy Membranes Prepared by Electroless Plating for Hydrogen Separation

Pd-based composite membranes are the attractive membrane materials for hydrogen separation due to their high hydrogen permeability and infinite permselectivity. Thin pure Ni and Pd-Ni alloy membranes with high hydrogen permeation were prepared by the electroless plating method. It is difficult to prepare the dense pure Ni membranes with 1-2 μm thickness for hydrogen separation. However, Pd-Ni alloy membranes with several micrometers thickness showed good permeation performance. Hydrogen permeance of the Pd95Ni5 alloy membrane with fcc phase up to 3.1×10-6 mol/m2 s Pa and the ideal permselectivity over 600 were obtained at 773 K.

Electrocrystallization of Pd-Ni Alloys on Glassy Carbon Electrode

Using the Scharifker and Hills model and Heerman and Tarallo model, cyclic voltammetry and chronoamperometry were used to study the nucleation mechanism upon the electrodeposition of a Pd-Ni alloy from an electrolyte. The results show that the electrodeposition of the Pd-Ni alloy onto glassy carbon electrode consisted of nucleation and growth, and the nucleation process according to the Scharifker and Hills model follows three dimension-progressive nucleation, which is controlled by the diffusion of the electroactive species. By applying the Heerman and Tarallo model, the kinetic parameters associated with the crystal nucleation and growth processes were obtained. The nucleation rate constant and the density of active nucleation sites increased from 0.83 to 7.71 s - 1

Nanoporous PdNi Alloy Nanowires As Highly Active Catalysts for the Electro-Oxidation of Formic Acid

Highly active and durable catalysts for formic acid oxidation are crucial to the development of direct formic acid fuel cell. In this letter, we report the synthesis, characterization, and electrochemical testing of nanoporous Pd(57)Ni(43) alloy nanowires for use as the electrocatalyst towards formic acid oxidation (FAO). These nanowires are prepared by chemically dealloying of Ni from Ni-rich PdNi alloy nanowires, and have high surface area. X-ray diffraction data show that the Pd(57)Ni(43) nanowires have the face-centered cubic crystalline structure of pure Pd, whereas X-ray photoelectron spectroscopy confirm the modification of electronic structure of Pd by electron transfer from Ni to Pd. Electrocatalytic activity of the nanowires towards FAO exceeds that of the state-of-the-art Pd/C. More importantly, the nanowires are highly resistant to deactivation. It is proposed that the high active surface area and modulated surface properties by Ni are responsible for the improvement of activity and durability. Dealloyed nanoporous Pd(57)Ni(43) alloy nanowires are thus proposed as a promising catalyst towards FAO.

Lignin Gasification over Charcoal-supported Palladium and Nickel Bimetal Catalysts in Supercritical Water

Abstract Charcoal-supported palladium and nickel binary salt catalysts showed higher lignin gasification activity than the linear combination activities of the individual monometal catalysts, indicating a bimetallic synergetic effect. XRD and EXAFS analyses revealed that Pd–Ni alloy structures were formed during the lignin gasification over the catalysts, which provided higher activities for the gasification than the individual monometal catalysts.