Polycrystalline Pd Research Articles

Effect of Gold Deposition on the Activity of Palladium Electrode for Propane-1,3-diol Oxidation

Abstract The Au deposition on polycrystalline Pd electrode and the electrocatalytic oxidation of propane-1,3-diol on the Au-deposited Pd electrodes are investigated. Structure analyses show that the surface of Pd is partially covered by Au nanoparticles. Electrochemical measurements reveal a significant improvement in the catalytic activity of Pd electrode by Au deposition for propane-1,3-diol oxidation in alkaline solution. The Au-modified Pd electrodes show oxidation potential similar to Pd electrode but considerably higher peak current density than Pd and Au electrodes.

Direct measurement of hydrogen dislocation pipe diffusion in deformed polycrystalline Pd using quasielastic neutron scattering.

The temperature-dependent diffusivity D(T) of hydrogen solute atoms trapped at dislocations-dislocation pipe diffusion of hydrogen-in deformed polycrystalline PdH(x) (x∼10(-3) [H]/[Pd]) has been quantified with quasielastic neutron scattering between 150 and 400 K. We observe diffusion coefficients for trapped hydrogen elevated by one to two orders of magnitude above bulk diffusion. Arrhenius diffusion behavior has been observed for dislocation pipe diffusion and regular bulk diffusion, the latter in well-annealed polycrystalline Pd. For regular bulk diffusion of hydrogen in Pd we find D(T)=D(0)exp(-E(a)/kT)=0.005exp(-0.23 eV/kT) cm(2)/s, in agreement with the known diffusivity of hydrogen in Pd. For hydrogen dislocation pipe diffusion we find D(T)≃10(-5)exp(-E(a)/kT) cm(2)/s, where E(a)=0.042 and 0.083 eV for concentrations of 0.52×10(-3) and 1.13×10(-3)[H]/[Pd], respectively. Ab initio computations provide a physical basis for the pipe diffusion pathway and confirm the reduced barrier height.

Structural studies of thin Pd films loaded with hydrogen

In this work variable energy positron annihilation spectroscopy was employed for investigation of defects created in Pd films electrochemically charged with hydrogen. The development of hydrogen-induced defects in nanocrystalline, polycrystalline and epitaxial Pd films were compared. It was found that absorbed hydrogen causes plastic deformation and increases defect density in all Pd films studied. Moreover, buckling was observed in nanocrystalline and polycrystalline films loaded above certain critical hydrogen concentration.

Controlled overgrowth of Pd on Au nanorods

In this paper, we present a detailed, systematic study of the controlled overgrowth of Pd on Au nanorods. Pd nanoshells with fine-controlled dimensions and architectures were overgrown on single-crystalline Au nanorods through seed-mediated growth using H2PdCl4 as the Pd precursor and ascorbic acid as the reducing agent in the presence of cetyltrimethylammonium chloride (CTAC) or cetyltrimethylammonium bromide (CTAB) as the surface capping agent. The effects of surface capping agent, ascorbic acid to H2PdCl4 ratio, reaction temperature, and structure-directing foreign ions, such as Ag+, on the dimensions and architectures of the resulting Au@Pd core–shell heteronanostructures were systematically studied. At 30 °C, Au nanorods coated with polycrystalline Pd shells were obtained using CTAC as the surface capping agent, while single-crystalline Au@Pd core–shell nanocuboids formed in the presence of CTAB. The thicknesses of the polycrystalline and single-crystalline Pd shells were fine-controlled by adjusting the molar ratio of ascorbic acid to H2PdCl4. At an elevated reaction temperature of 60 °C, irregularly shaped and cylindrical co-axial core–shell nanorods were obtained in CTAC and CTAB, respectively. By introducing Ag+ ions into the Pd growth solution, Au nanorods coated with segregated Pd nanoislands and dumbbell-like core–shell heteronanostructures were obtained as a consequence of the underpotential deposition of Ag and sustainable galvanic replacements that concurred during the Pd overgrowth processes. The effects of the Pd shell dimensions and morphologies on the plasmonic properties of the Au@Pd core–shell nanostructures were also investigated.

Hydrogen Interaction with Defects in Nanocrystalline, Polycrystalline and Epitaxial Pd Films

Hydrogen interaction with defects and structural development of Pd films with various microstructures were investigated. Nanocrystalline, polycrystalline and epitaxial Pd films were prepared and electrochemically loaded with hydrogen. Structural changes of Pd films caused by absorbed hydrogen were studied by in-situ X-ray diffraction combined with acoustic emission and measurement of electromotorical force. Development of defects during hydrogen loading was investigated by positron annihilation spectroscopy. It was found that hydrogen firstly fills open volume defects existing already in the films and subsequently it occupies also interstitial sites in Pd lattice. Absorbed hydrogen causes volume expansion, which is strongly anisotropic in thin films. This introduces high stress into the films loaded with hydrogen. Acoustic emission measurements revealed that when hydrogen-induced stress achieves a certain critical level rearrangement of misfit dislocations takes place. The stress which grows with increasing hydrogen concentration can be further released by plastic deformation and also by detachment of the film from the substrate.

Simple Electrodeposition of Dendritic Pd Without Supporting Electrolyte and Its Electrocatalytic Activity Toward Oxygen Reduction and H2O2 Sensing

AbstractMetallic palladium (Pd) electrocatalysts for oxygen reduction and hydrogen peroxide (H2O2) oxidation/reduction are prepared via electroplating on a gold metal substrate from dilute (5 to 50 mM) aqueous K2PdCl4 solution. The best Pd catalyst layer possessing dendritic nanostructures is formed on the Au substrate surface from 50 mM Pd precursor solution (denoted as Pd‐50) without any additional salt, acid or Pd templating chemical species. The Pd‐50 consisted of nanostructured dendrites of polycrystalline Pd metal and micropores within the dendrites which provide high catalyst surface area and further facilitate reactant mass transport to the catalyst surface. The electrocatalytic activity of Pd‐50 proved to be better than that of a commercial Pt (Pt/C) in terms of lower overpotential for the onset and half‐wave potentials and a greater number of electrons (n) transferred. Furthermore, amperometric i–t curves of Pd‐50 for H2O2 electrochemical reaction show high sensitivities (822.2 and −851.9 µA mM−1 cm−2) and low detection limits (1.1 and 7.91 µM) based on H2O2 oxidation H2O2 reduction, respectively, along with a fast response (<1 s).

Electro-deposition of Pd on Carbon paper and Ni foam via surface limited redox-replacement reaction for oxygen reduction reaction

Pd nanostructured catalysts were electrodeposited by surface-limited redox replacement reactions using the electrochemical atomic layer deposition technique. Carbon paper and Ni foam were used as substrates for the electrodeposition of the metal. Supported nanostructured Pd electrodes were characterized using electrochemical methods and scanning electron microscopy. Carbon paper and Ni foam produced good quality deposits with some agglomeration on Ni foam. The EDX profiles confirmed the presence of Pd particles. Cyclic voltammograms of the electrodeposited Pd on substrates showed features characteristic of polycrystalline Pd electrodes. In the acidic electrolyte a very weak oxygen reduction reaction (ORR) activity was observed on Pd/Carbon paper electrode when compared to Pd/Ni foam electrode. The Pd/Ni foam electrode showed improved ORR activity in alkaline medium.

Nanopatterning Palladium Surface Layers through Electrochemical Deposition and Dissolution of Zinc in Ionic Liquid

Cracklike nanopatterned palladium surface layers have been produced by a green chemistry method based on in situ electrochemical deposition-dissolution of zinc (Zn-ECDD) in an ionic liquid bath. During the cathodic process, reactive Zn was electrochemically deposited onto a polycrystalline Pd substrate. During the subsequent anodic process, Zn was removed from the substrate through electrochemical dissolution. Scanning electron microscope (SEM) measurements showed that repetitive Zn-ECDD mediated by potential cycles results in the nanopatterning of Pd surface layers, characterized by uniform crack appearance with well-distributed concave spacings separated by nanowidth cracks. Energy-dispersive X-ray microscopy (EDX) studies revealed that the nanopatterned surface layers chemically contain a small amount of Zn. A mechanism based on the development of stress induced by the Zn-ECDD on Pd surfaces was proposed to be responsible for the nanopatterning of Pd surface layers. Electrochemical oxidation of formic acid and reduction of nitrite were studied as model reactions to demonstrate potential applications of the nanopatterned Pd electrode to electrocatalysis and electrochemical determination of environmental contaminants. Highly improved electrochemical responses were obtained on the nanopatterned Pd for the two reactions, compared to the untreated Pd.

Synthesis of a Au-on-Pd Heteronanostructure Stabilized by Citrate and its Catalytic Application

The preparation of Au-on-Pd heteronanostructure (HNS) using citrate-stabilized polycrystalline Pd nanoparticles (NPs) as the seeds is described. The resulting Au-on-Pd HNS is characterized and it is found that the formation of Au-on-Pd HNS depends greatly on a ratio between Pd seeds and AuCl4− ions added and the optimal molar ratio is 10:1. If fewer AuCl4− ions are added (Pd/Au ratio is 100:1), the growth of Au NPs only occurs on part of the Pd seeds’ surface. The addition of more AuCl4− ions (Pd/Au ratio is 5:1) hinders the growth of Au NPs on the Pd seeds’ surface. To demonstrate the catalytic performance, the electrochemistry oxidation of ethanol and the reduction of p-nitrophenol by NaBH4 are chosen to examine the catalytic activity of Au-on-Pd HNS. Pd seeds, Au NPs, and poly(vinyl pyrrolidone) (PVP)-stabilized PdAu nanoalloy are used as the references for comparison. In the first reaction, the catalytic reactivity of Au-on-Pd HNS is better than that of corresponding pure Pd or Au NPs, while the opposite occurs for the latter reaction. The catalytic activity of Au-on-Pd HNS is much higher than that of PVP-stabilized PdAu nanoalloy.

The Role of Defects in the Local Reaction Kinetics of CO Oxidation on Low-Index Pd Surfaces.

The role of artificially created defects and steps in the local reaction kinetics of CO oxidation on the individual domains of a polycrystalline Pd foil was studied by photoemission electron microscopy (PEEM), mass spectroscopy (MS), and scanning tunneling microscopy (STM). The defects and steps were created by STM-controlled Ar+ sputtering and the novel PEEM-based approach allowed the simultaneous determination of local kinetic phase transitions on differently oriented μm-sized grains of a polycrystalline sample. The independent (single-crystal-like) reaction behavior of the individual Pd(hkl) domains in the 10–5 mbar pressure range changes upon Ar+ sputtering to a correlated reaction behavior, and the reaction fronts propagate unhindered across the grain boundaries. The defect-rich surface shows also a significantly higher CO tolerance as reflected by the shift of both the global (MS-measured) and the local (PEEM-measured) kinetic diagrams toward higher CO pressure.

Evolution of concentration profiles in Pd–Cu systems studied by SNMS technique

Interdiffusion in polycrystalline Pd–Cu thin film system was investigated in the temperature range of 120–310 °C, by means of SNMS depth profiling technique. Depending on the annealing parameters, an almost complete homogenization was detected even at temperatures, where the volume diffusion was frozen in. In the Pd layer, the detected Cu profiles and their time evolution were typical C-kinetic regime grain boundary profiles at short annealing times. Because of the much higher grain boundary diffusivity on the Cu-rich side, the saturation of Cu grain boundaries is reached in very short times. In Pd-side, from the time evolution of C-type depth profiles, we determined the Cu grain boundary diffusion coefficients at different temperatures. The high concentration level in Cu-side could be interpreted supposing moving boundaries that leave behind regions with high Pd concentration.Though the intermetallic phase formation in this system under similar conditions was proved, the shape of our depth profiles does not show any effects of a reaction layer formation near the original interface. Our findings remain compatible with the previous results, if we suppose an unconventional type of growth mode: phase formation takes place in and around grain boundaries, by means of transport along moving grain boundaries/interfaces.

Hydrogen-induced microstructural changes of Pd films

The development of the microstructure in nanocrystalline, polycrystalline and epitaxial Pd films loaded with hydrogen is investigated. Structural changes in Pd films loaded with hydrogen were characterized by positron annihilation spectroscopy combined with electron microscopy and X-ray diffraction. It was found that hydrogen charging causes plastic deformation which leads to an increase of the defect density in all Pd films studied. Moreover, the formation of buckles was observed in nanocrystalline and polycrystalline Pd films loaded above a certain critical hydrogen concentration. Buckling leads to detachment of the film from the substrate and this is accompanied with in-plane stress relaxation and plastic deformation of the film.

The Electrochemical Atomic Layer Deposition of Pt and Pd Nanoparticles on Ni Foam for the Electro-Oxidation of Alcohols

Electrodeposition of Pt and Pd metal by surface limited redox replacement reactions was performed using the electrochemical atomic layer deposition. Carbon paper and Ni foam were used as substrates for metal deposition. Supported Pt and Pd electrodes were characterised using electrochemical methods and scanning electron microscopy (SEM). Cyclic voltammograms of the electrodeposited Pt and Pd on substrates showed features characteristic of polycrystalline Pt and Pd electrodes respectively. Ethanol oxidation in alkaline media was highly favoured on Pd/Ni foam than on Carbon paper while methanol oxidation was favoured more on Carbon paper than on Ni foam. Carbon paper and Ni foam produced good quality deposits and the EDX confirmed the presence of Pt and Pd particles.

Hydrogen induced changes in the optical properties of Pd capped V thin films

Abstract Optical properties of 50 nm thick single crystal vanadium films deposited on double side polished MgO substrates have been obtained from spectrophotometric measurements. The films were coated with a polycrystalline Pd layer (5 nm thick) to protect them from oxidation and to favor absorption of atomic hydrogen. Electrical resistance was recorded while hydrogen pressure was increased slowly up to 750 mbar keeping temperature constant. Simultaneously, under normal incidence of non-polarized radiation [350–950 nm], transmittance spectra of this Pd/V/MgO system were measured. These were numerically inverted to obtain the spectral behavior of the Pd, V, PdH y and VH x dielectric functions at 22 and 140 °C, and at 750 mbar. Hydrogen concentration in V film was first determined from a resisto-optical method. Finally, we demonstrate the possibility to determine the concentration in the Pd and the V layers independently, solely using the changes in the optical transmission.

Experimental measurement of acoustic plasmons in polycrystalline palladium

An experimental study of collective oscillations in Pd covering the region of very low energy and momentum transfers is reported. Through Dynamic Electron Scattering spectroscopy, structure factor spectra were measured from 80 K to 298 K on a bulk polycrystalline Pd sample. Here we report the first experimental evidence of damped acoustic plasmons and their evolution to the single-particle excitation continuum. The acoustic plasmons follow a linear dispersion and are experimentally shown to be a separate and distinct resonance mode from acoustic surface plasmons. Calculations of the dielectric function employed a model that incorporates complete mixing of two conduction bands with contributions from both interband and intraband transitions. The model was used in computational studies that focused on specific experimental results to aid the characterization and understanding of the plasmon behavior. We found that the Pd acoustic plasmon energy matched the longitudinal phonon anomaly that has sparked numerous theoretical reports on the possible energetic coupling of these modes. Further experimental evidence of plasmon and phonon dynamical processes are found in the linewidth analysis of the data. The primary decay mechanism of the plasmons is interpreted to be strong phonon-assisted interband transitions. Further spectral features and the plasmon velocity are also reported.

Silicon Oxide Surface Segregation in CO Oxidation on Pd: An in situ PEEM, MS and XPS Study.

The effect of silicon oxide surface segregation on the locally-resolved kinetics of the CO oxidation reaction on individual grains of a polycrystalline Pd foil was studied in situ by PEEM, MS and XPS. The silicon oxide formation induced by Si-impurity segregation at oxidizing conditions, was monitored by XPS and its impact on the global and local (spatially resolved) kinetics of the CO oxidation was determined by MS and PEEM. The results reveal a drastic inhibiting effect of silicon oxide on the Pd reactivity towards CO oxidation, manifested both in the collapse of the global CO2 formation rate and in the modified local reactive properties of individual Pd micrograins. The presence of adsorbed oxygen on the Pd surface effectively enhances the silicon segregation to the Pd surface.Graphical

In situ FTIR spectroscopic studies of ethylene glycol electrooxidation on Pd electrode in alkaline solution: The effects of concentration

We investigated the electrooxidation of ethylene glycol (EG) on polycrystalline Pd electrode in alkaline by in situ FTIR reflection spectroscopy, and focused on the effects of EG concentration (2mM–1M) on the reaction pathways. Voltammetric result shows the reaction order of EG is near 0.5. In situ FTIR results demonstrated that the reaction pathways and the product contribution strongly depend on the EG concentration. The oxidation degree of EG decreases with increasing EG concentration, and the main products gradually varies from CO32-, C2O42-, glyoxylate, to glycolate. Bridge-bonded CO (COB), generated from dissociative adsorption of EG, can be easily oxidized at low EG concentration. These facts were attributed to the competitive adsorption of EG itself and reaction intermediates with water, which inhibits the formation of oxygen species from water dissociation, a key oxygen donor for deep oxidation of EG. Even though, EG is still much better than ethanol as candidate fuel for alkaline direct alcohol fuel cells. The present study is of importance for understanding reaction mechanism of EG electrooxidation.

Photoelectron spectroscopic study on the interaction of l-cysteine with the silver-based dental alloy

The electronic structure of the silver-based dental alloy MC12 and its constituent metals and their interaction with l-cysteine have been investigated by ultraviolet photoelectron spectroscopy (UPS) using synchrotron radiation. The UPS spectra of l-cysteine on polycrystalline Ag, Cu, Pd, and Au substrates have been measured to understand the interaction of l-cysteine with the dental alloy surface. It was found that the electronic states of MC12 originate dominantly from Cu 3d states and Pd 4d states around the top of the valence bands, while the 4–7-eV electronic structure of MC12 originates from the Ag 4 d5/2 and Ag 4 d3/2 states. For l-cysteine, it was found that a new peak at 2 eV is observed for the thin films on Ag, Cu, and Au, while the structure around 2 eV on MC12 is similar to that on Pd. The shift of the 5-eV peak is observed for the thin films on Cu, Pd, and Au, but not on Ag and MC12. These results indicate that the interaction of l-cysteine with MC12 may be dominantly due to the Pd-S, Cu-S, and Ag-O bonds, while the contribution of the Ag-S bond is small.

Ethanol and Methanol Oxidation Activity of PtPb, PtBi, and PtBi2 Intermetallic Compounds in Alkaline Media

The electrocatalytic activities of a wide range of intermetallic bulk compounds in the ethanol (EtOH) and methanol (MeOH) oxidation reactions in alkaline media have been studied, and the results have been compared to those of pure polycrystalline Pt and Pd electrodes. The PtPb, PtBi, and PtBi2 intermetallic compounds appeared to be the most promising electrocatalysts among all of the examined bulk electrodes. The current densities at −0.2 V (vs. Ag/AgCl KCl satd.) for PtPb, PtBi, and PtBi2 in EtOH were 21.4, 7.73, and 6.03 mA cm−2, respectively. All of the other intermetallic bulk electrodes exhibited current densities less than 2.5 mA cm−2. PtPb had the lowest onset potential of −0.58 V for EtOH oxidation, which was 20–30 mV less than that of pure Pt and Pd. The current densities of PtPb were at least seventeen times larger than those of pure Pt and Pd. PtBi and PtBi2, which have onset potentials of −0.51 and −0.45 V, respectively, for the EtOH oxidation reaction exhibited extremely stable oxidation currents of 4.8 and 3.3 mA cm−2 during the constant-potential electrolysis of EtOH. The same behavior was also observed for the MeOH oxidation with PtPb, PtBi, and PtBi2 intermetallic compounds.

Electroreduction of nitrous oxide on platinum and palladium: Toward selective catalysts for methanol–nitrous oxide mixed-reactant fuel cells

The kinetics of N2O electroreduction in the absence and presence of methanol was studied between 295 and 333K on polycrystalline Pt and Pd electrodes in 0.1M NaOH. In the absence of methanol the reduction of N2O on Pd is more facile than on Pt as shown by the approximately four times lower apparent activation energy and lower Tafel slope (Pt: 0.111±0.019Vdec−1, Pd: 0.084±0.007Vdec−1 at 295K). Two different electroreduction mechanisms are proposed for Pt and Pd with and without participation of underpotential deposited hydrogen, respectively. The selectivity of Pt and Pd electrodes toward both N2O electroreduction and methanol (0.5 and 1M) oxidation at 295K was also investigated. Pt based electrocatalysts are promising candidates for the anode of a mixed reactant CH3OH–N2O fuel cell due to inhibition of N2O reduction by chemisorbed methanol. Pd on the other hand is a selective cathode electrocatalyst since N2O reduction takes place fairly actively in the presence of 1M methanol, while methanol oxidation is inhibited.