Pure Palladium Research Articles

Effect of Rhodium Distribution on Thermal Stability of Nanoporous Palladium–Rhodium Powders

Powders of nanoporous palladium and palladium alloy particles are of potential value for storage of hydrogen isotopes, as long as the pores remain stable over a useful range of temperatures and chemical environments. Rhodium alloys are known to have enhanced hydrogen storage and improved thermal stability versus pure palladium. However, the distribution of rhodium on pore and particle surfaces is critical to this. Pores are more ordered and thermally stable in rhodium-rich regions. Treatment of particles at elevated temperature under reducing conditions can cause rearrangement of Rh and Pd at the surface, but not a major change in Rh distribution throughout the particle. Heating in the presence of hydrogen causes more rapid pore rearrangement than heating in vacuum subsequent to hydrogen exposure, suggesting a direct chemical influence of hydrogen on mobility of surface atoms. These results provide a clear path to future improvements in the stability of nanoporous metals in reducing atmospheres.

Regular arrays of palladium and palladium-gold clusters supported on ultrathin alumina films: stability under oxygen

Grazing Incidence Small Angle X–ray Scattering (GISAXS) studies were conducted to determine the effect of oxygen on regular arrays of mono (Pd)– and bimetallic (Pd–Au) clusters. Unlike bimetallic Pd–Au particles, pure palladium clusters are not stable at a partial oxygen pressure of 10−6 mbar. A comparison of the experimental data with Fourier transforms of known distributions reveals that Pd clusters move and that their aspect ratio increases. These results are explained by the adsorption of oxygen that changes the equilibrium shape of the Pd particles and reduces the adhesion energy. In the case of bimetallic Pd–Au clusters, the presence of Au on the surface of the particle prevents adsorption of oxygen.

Networks of Ultrasmall Pd/Cr Nanowires as High Performance Hydrogen Sensors

The newly developed hydrogen sensor, based on a network of ultrasmall pure palladium nanowires sputter-deposited on a filtration membrane, takes advantage of single palladium nanowires' characteristics of high speed and sensitivity while eliminating their nanofabrication obstacles. However, this new type of sensor, like the single palladium nanowires, cannot distinguish hydrogen concentrations above 3%, thus limiting the potential applications of the sensor. This study reports hydrogen sensors based on a network of ultrasmall Cr-buffered Pd (Pd/Cr) nanowires on a filtration membrane. These sensors not only are able to outperform their pure Pd counterparts in speed and durability but also allow hydrogen detection at concentrations up to 100%. The new networks consist of a thin layer of palladium deposited on top of a Cr adhesion layer 1-3 nm thick. Although the Cr layer is insensitive to hydrogen, it enables the formation of a network of continuous Pd/Cr nanowires with thicknesses of the Pd layer as thin as 2 nm. The improved performance of the Pd/Cr sensors can be attributed to the increased surface area to volume ratio and to the confinement-induced suppression of the phase transition from Pd/H solid solution (α-phase) to Pd hydride (β-phase).

Significance of homologous temperature in softening behavior and grain size of pure metals processed by high-pressure torsion

High purity metals with low melting temperatures such as indium (99.999%), tin (99.9%), lead (99%), zinc (99.99%) and aluminum (99.99%) were processed using high-pressure torsion (HPT). An unusual softening behavior was observed in all these metals after processing by HPT at room temperature. Pure copper (99.99%) and palladium (99.95%) were used to simulate the softening behavior due to a thermal effect by processing and subsequently holding at the temperatures equivalent to room temperature of pure Al. It is shown that a hardness peak appears in any metal by static softening after processing by HPT at a homologous temperature of 0.32 which is equivalent to room temperature of pure Al. The contribution of dynamic softening on hardness decrease becomes more important as the homologous temperature and stacking fault energy increase. Microstructural examinations indicate that, although the stacking fault energy influences the rate of the microstructural evolution, the homologous temperature appears to be a dominant parameter to determine the steady-state grain size after processing by HPT.

Hydrogen permeation measurements of Pd and Pd–Cu membranes using dynamic pressure difference method

Hydrogen permeation across membranes is measured using a dynamic pressure difference method. In the method, a transient system for continuously monitoring hydrogen flux of a membrane is conducted. Three different membranes, consisting of two pure palladium (Pd) membranes with different thicknesses and one palladium-copper (Pd–Cu) membrane supported by porous stainless steel (PSS) tubes, are taken into account. Three different operating temperatures of 320, 350 and 380 °C as well as two different initial pressure differences of 5 and 10 atm are considered to evaluate the effects of the operating parameters upon the hydrogen permeation. The results suggest that a threshold of pressure difference is always exhibited at the end of the permeation process, regardless of which membrane is tested. The hydrogen permeation rate can be predicted well for the pressure exponent in the range of 0.1–1.0; however, the optimal pressure exponent is located between 0.5 and 0.8. The theoretical analysis indicates that the characteristic time of hydrogen permeation in the present system ranges from 245 to 460 s and the entire permeation period is longer than the characteristic time by an order of magnitude. In regard to the effect of membrane temperature on the permeation, the activation energies of the three membranes range from 11 to 18 kJ mol−1.

High Performance plasma sputtered PdPt fuel cell electrodes with ultra low loading

A PdPt (10 wt% Pt) catalyst is used to replace platinum at the cathode of a proton exchange membrane fuel cell membrane electrode assembly (PEMFC MEA) whereas pure palladium is used as the anode catalyst. The catalysts are deposited on commercial carbon woven web and carbon paper GDLs by plasma sputtering. The relations between the depth density profiles, the electrode support and the fuel cell performances are discussed. It is shown that the catalyst gradient is an important parameter which can be controlled by the catalyst depth density profile and/or the choice of electrode support. An optimised electrode structure has been obtained, which allows limiting the platinum requirement. Under suitable conditions of a working PEMFC (80 °C and 3 bar absolute pressure), very high catalysts utilization is obtained at both electrodes, leading to 250 kW g Pt −1 and 12.5 kW g Pd −1 with a monocell fitted with a PdPt (10:1 weight ratio) cathode and a pure Pd anode.

The effect of temperature and pressure on the hydrogen permeation through Pd-coated Ti 26Ni 21V 53 alloy membrane under different atmospheres

The paper introduces Ti 26Ni 21V 53 alloy membrane as a potential replacement for Pd-based membrane. The pressure–temperature–hydrogen permeation relationships of Pd-coated Ti 26Ni 21V 53 alloy membrane have been investigated under pure hydrogen and mixed hydrogen gas with carbon dioxide and carbon monoxide in a pressure range from 0.5 to 2 atm over a wide temperature range including the typical hydrogen membrane operating temperature range (350–450 °C). The experiments show that the hydrogen flux was directly proportional to the temperature at constant pressure according to an Arrhenius-type relationship, whereas it was not proportional to the square root of hydrogen partial pressure in little accordance with the Sievert's law. The hydrogen permeability values obtained for the Pd-coated Ti 26Ni 21V 53 alloy membrane under present conditions are higher than the pure palladium and those previously published in the Ti–Ni–V ternary systems.

Elastic constants of palladium hydride at elevated temperature and pressure.

Hydrogen atoms occupy the octahedral interstitial sites provided by metals with fcc lattices (such as palladium). Palladium hydride (PdHx) exists in two phases. At room temperature, the low concentration (PdHx=0.02) called α-phase has slightly larger lattice parameter compared to that of pure palladium. Above this concentration and up to (PdHx=0.6) the high concentration β-phase appears resulting in (α+β)-phase, a mixed phase region, and the lattice parameter becomes substantially greater than that of pure palladium. When x>0.6, α-phase disappears and the system becomes purely β-phase. The three phases coexist at the tri-critical point (temperature, pressure, and concentration). Resonant ultrasound spectroscopy (RUS) has been used to investigate the elastic constants of palladium hydride crystal and the equilibrium dynamics of the system around the tri-critical point temperature and pressure. A large decrease in shear modulus is noticed near this tri-critical point. The desire to have fundamental understanding of metal-hydrogen systems and to explore this for possible technological applications has been the motivation for research of these systems. This investigation includes the design and construction of high temperature and high pressure RUS cell.

Palladium thorn clusters as catalysts for electrooxidation of formic acid

Pure palladium thorn clusters were synthesized using the electrodeposition method. The clusters were composed of several thorns growing on one basis. Each thorn was composed of hexahedral units with decreasing sizes. The whole thorn was a single crystal along the <220> direction. The cluster was synthesized by square wave electrodeposition. By changing the deposition factors, a mixture of thorns and particles was synthesized, where each thorn grew on one basis and the thorn was composed of dodecahedral bases. Compared with the mixture, the cluster had higher activity toward the electrooxidation of formic acid, and also much higher activity than Pd powder, which was evidenced by the improved current density and onset potential of formic acid oxidation. The fact that pure thorn clusters had a higher catalytic activity than the mixture of thorns and particles proved that the higher activity was ascribed to the single crystal property of the thorns.

Production of Palladium Nanoparticles by Pulsed Laser Ablation in Water and Their Characterization

Nanosized particles of pure palladium were produced by pulsed laser ablation in aqueous solution, with and without the addition of sodium dodecyl sulfate as the surfactant agent. The work was motivated by the strong catalytic effect of Pd particles in many chemical reactions. The influence of experimental parameters, such as laser energy and beam focusing as well as surfactant concentration, on the laser ablation process, nanoparticle mass yield, and nanoparticle properties was investigated via plasma imaging, UV−visible spectroscopy, transmission electron microscopy/energy dispersive spectroscopy (TEM-EDS) analysis, and measurement of ζ potential. The surface-enhanced Raman scattering activity of nanoparticles was used for the monitoring of their adsorption capability on the metal surface during catalysis processes.

Reparation of palladium membrane over anodic TiO2 nanotube arrays on porous titanium

Pure palladium membrane has been successfully fabricated by photocatalytic deposition in fluoride electrolysis bath on TiO2 nanotube arrays which was in situ grown by anodic oxidation on the surface of asymmetric home-made compressed porous titanium foils. Results of SEM, XRD and XPS show that the completed membrane is homogeneous and its thickness is about 0.5 μm. After 8 times thermal shock test, no shedding or crack is observed on this composite material, which shows the excellent hot crisp resistance.

Electrodeposition of nanowires for the detection of hydrogen gas

Abstract

Experimental Evidence For The Nanocage Effect In Catalysis With Hollow Nanoparticles

Five different hollow cubic nanoparticles with wall length of 75 nm were synthesized from platinum and/or palladium elements. The five nanocatalysts are pure platinum nanocages (PtNCs), pure palladium nanocages (PdNCs), Pt/Pd hollow shell-shell nanocages (NCs) (where Pd is defined as the inner shell around the cavity), Pd/Pt shell-shell NCs, and Pt-Pd alloy NCs. These are used to catalyze the reduction of 4-nitrophenol with sodium borohydride. The kinetic parameters (rate constants, activation energies, frequency factors, and entropies of activation) of each shell/shell NCs are found to be comparable to that of pure metal NCs made of the same metal coating the cavity in the shell-shell NCs. These results strongly suggest that the catalytic reaction takes place inside the cavity of the hollow nanoparticles. Because of the nanoreactor confinement effect of the hollow nanocatalysts, the frequency factors obtained from the Arrhenius plots are found to be the highest ever reported for this reduction reaction. This is the reason for enhanced rate of this reaction inside the cavity. The importance of mechanism of the homogeneous and the heterogeneous nanocatalytic reactions occurring on the external surface of a solid nanoparticle are contrasted with those occurring on the nanocavity surface.

Preparation of palladium nanoparticles–titanium electrodes as a new anode for direct methanol fuel cells

Pd nanoparticle/Ti electrodes are prepared by electroless plating of palladium on titanium plates. The morphology and surface analysis of Pd nanoparticle/Ti electrodes are investigated using scanning electron microscopy and energy-dispersive X-ray spectroscopy, respectively. The results indicate that palladium nanoparticles are homogeneously deposited on the surface of titanium plates. The electro-catalytic activity of Pd nanoparticle/Ti electrodes in the methanol electro-oxidation is studied by cyclic voltammetry and chronoamperometry methods. The results show that the electro-catalytic oxidation of methanol on the Pd nanoparticle/Ti electrode improved compare to pure palladium electrode and confirmed the better electro-catalytic activity and stability of these new electrodes.

Thermally stable nanoporous palladium alloy powders by hydrogen reduction in surfactant templates

Nanometer-scale pores in metals used for hydrogen storage are expected to facilitate mass transport in the materials – in particular, the release of helium decay products when tritium is stored. Scalable methods for production of bulk powders of nanoporous metals typically use chemical reducing agents that can leave impurities in the product. Hydrogen gas can be used as the reducing agent in such procedures. This not only improves purity, but also expands the range of accessible pore and particle sizes and particle compositions. Powders of nanoporous palladium and its alloys with rhodium are synthesized by chemical reduction of chloride complexes by hydrogen in a concentrated nonionic aqueous surfactant at room temperature. Particle diameters are typically several micrometers and each particle is perforated by 2–3 nm pores, as determined by electron microscopy and nitrogen porosimetry. Alloys show major improvement in thermal stability and pore regularity compared to pure palladium. In addition to facilitating heavier isotope storage, the high surface areas of these materials may allow development of metal hydride batteries with high power density.

An Investigation on Solid State Reactions in Heat Treated Au/Pd Thin Films for Electrodes Applications

The interface between metals and oxides is particularly interesting. It has been the main topic of many research projects to illustrate that their properties are highly dependent on the structure of the interface. Poor adhesion between gold films and oxides is well known. On the basis of solid solution between the materials, the main effect of a Pd thin film as the adhesion layer between substrate and Au was investigated. Pure palladium and gold films were grown by electron beam evaporation sequentially on the SrTiO3 (001) single crystal substrates. After different annealing time (from 0.06 to 1.8 ks) at 500 degrees C, the growth sequence of the compositions between Au and Pd evolved as a results of the solid solution were investigated by means of X-ray photoelectron spectroscopy (XPS), X-ray diffraction. Meanwhile the surface morphology and the surface topography were examined respectively by scanning electron microscopy (SEM) and atomic force microscopy (AFM) observation. The effect of an intermediate Pd adhesion layer and its influence on the interfacial properties and morphology of the Au layer is investigated.

Pure Palladium and Palladium Phosphorus Depositions Used in ENEPIG and ENEP Surface Finishes – Comparison of Physical Properties and Their Influence on Soldering and Au Wire Bonding

As a surface finish, electroless nickel / electroless palladium / immersion gold (ENEPIG) has received increased attention for both packaging/IC-substrate and PWB applications. With a lower gold thickness compared to conventional electroless nickel / immersion gold (ENIG) the ENEPIG finish offers the potential for higher reliability, better performance and reduced cost.[1,2] This paper shows the benefits of using a pure palladium layer in ENEPIG and ENEP (Electroless Nickel / Electroless Palladium) surface finishes in terms of physical properties and in terms of gold wire bonding and solder joint integrity.

ENEP – A Cost-Effective Alternative for High Reliability Soldering Applications

Electroless nickel / immersion gold (ENIG) has been recognized within the electronics packaging industry as a reliable surface finish for soldering applications. Unlike surface finishes, such as OSP, immersion silver or immersion tin, which produce a Cu-Sn intermetallic upon soldering, the nickel layer of ENIG acts as a barrier to minimize copper dissolution during the soldering application. This function is especially important for assemblies that require multiple reflow steps. However, if not properly controlled, excessive corrosion of the nickel by the immersion gold step can also cause the well documented “black pad” effect, often resulting in solder joint reliability issues, including brittle fracture. As an alternative to ENIG, an electroless pure palladium layer over the electroless nickel (ENEP) provides a solderable surface without the noted risk of damage to the underling nickel layer. By eliminating the corrosive attack of the gold bath, the final layer stack would be suitable for high reliability soldering involving both IC substrate and PWB applications. In addition, because of the current high price of gold, replacing it with a pure palladium deposit can offer measureable cost savings for the ENEP process in comparison to ENIG. The paper summarizes an in-depth study of the effect on solder joint reliability caused by replacing the immersion gold by an electroless pure palladium layer. The study includes evaluations for both Pb-free (Sn-3.0Ag-0.5Cu) and eutectic SnPb (63Sn-37Pb) soldering applications. Results of investigations include: (1) cold ball pull testing to evaluate solder joint integrity, (2) SEM examinations of the underlying nickel surface, (3) IMC examinations to quantify nickel thickness degradation after multiple solder reflow cycles and (4) surface wetting through solder spread examinations. The paper discusses the relatively simple surface finish that, if proven effective in large-scale fabrication, may offer measureable performance and cost benefits in comparison to the more traditional ENIG finish.

수소 정제용 팔라듐 합금 분리막 연구

This study presented the effect of membrane thickness on hydrogen permeability. Microvoids on the surface of the membrane should not exist for the exact values of hydrogen permeability. Pd-Cu-Ni hydrogen alloy membranes were fabricated by Ni powder sintering, substrate plasma pretreatment, sputtering and Cu reflow process. And this leaded to void-free surface and dense film of Pd-Cu-Ni hydrogen alloy membrane. Hydrogen permeation test showed that hydrogen permeability increased from 2.7 to <TEX>$15.2ml/cm^2{\cdot}min{\cdot}atm^{0.5}$</TEX> as membrane thickness decreased from 12 to <TEX>$4{\mu}m$</TEX>. This represented the similar trend as a hydrogen permeability of pure palladium membrane based on solution-diffusion mechanism.

Studies of nanocrystalline Pd alloy films coated by electroless deposition

The microstructures of thin coating films of pure palladium and palladium alloys deposited from organic electrolytes onto different metallic substrates by electroless plating method have been investigated. The coatings are dense, pore-free 0.005–1 μm thick films with high adhesive strength to the substrate surface. X-ray spectral analysis, X-ray phase analysis, transmission and scanning electron microscopy were used to determine the composition and structure of alloy coatings of binary systems: Pd–Au, Pd–Ag, Pd–Ni, Pd–Pb, and ternary system Pd–Au–Ni. The coatings of Pd–Au, Pd–Ag, and Pd–Ni have a solid solution structure, whereas Pd–Pb is intermetallic compound. It has been found that the deposited films consist of nanocrystalline grains with sizes in the range of 11–35 nm. Scanning and transmission electron microscopy investigations reveal the existence of clusters formed by nanocrystalline grains. The origin for the formation of nanocrystalline structures of coating films is discussed.