Polycrystalline Palladium Research Articles

Oxygen desorption from polycrystalline palladium: Thermal desorption of O2 from a chemisorbed layer of Oads in the course of the decomposition of PdO surface oxide and in the release of oxygen from the bulk of palladium

The desorption of oxygen from polycrystalline palladium (Pd(poly)) was studied using temperature-programmed desorption (TPD) at 500–1300 K and the amounts of oxygen absorbed by palladium (n) from 0.05 to 50 monolayers. It was found that the desorption of O2 from Pd(poly), which occurred from a chemisorbed oxygen layer (Oads), in the release of oxygen from a near-surface metal layer in the course of the decomposition of PdO surface oxide, and in the release of oxygen from the bulk of palladium (Oabs), was governed by repulsive interactions between Oads atoms and the formation and decomposition of Oads-Pd*-Oabs structures (Pd* is a surface palladium atom). At θ ≤ 0.5, the repulsive interactions between Oads atoms (ɛaa = 10 kJ/mol) resulted in the desorption of O2 from Pd(poly) at 650–950 K. At 0.5 ≤ n ≤ 1.0, the release of inserted oxygen from a near-surface palladium layer occurred during TPD in the course of the migration of Oabs atoms to the surface and the formation-decomposition of Oads-Pd*-Oabs structures. As a result, the desorption of O2 occurred in accordance with a first-order reaction with a thermal desorption (TD) peak at Tmax ∼ 700 K. At 1.0 ≤ n ≤ 2.0, the decomposition of PdO surface oxide occurred at a constant surface cover-age with oxygen during TPD in the course of the formation-decomposition of Oads-Pd*-Oabs structures. Because of this, the desorption of O2 occurred in accordance with a zero-order reaction at low temperatures with a TD peak at Tmax ∼ 675 K. At 1.0 ≤ n ≤ 50, oxygen atoms diffused from deep palladium layers in the course of TPD and arrived at the surface at high temperatures. As a result, O2 was desorbed with a high-temperature TD peak at T > 750 K.

Growth of nanoparticles in hydrogen-implanted palladium subsurfaces

Solid particles with nanometric dimensions are shown to grow in the opened subsurface of a polycrystalline palladium (Pd) hydrogen-implanted at around 500°C. The particles are Pd in main composition and densely grown on sloping walls of fissured grain boundaries or cracks. The average grain size increases from deeper to shallow regions, suggesting that a negative temperature gradient toward the surface existed along the crack walls. The nanoparticles are certain to arise from the condensation of Pd vapors on the walls, forcing us to assume that hydrogen atoms implanted in an overpopulation heated their implantation zone so strongly as to vaporize Pd.

Dynamic Impedance of Formic Acid Electrooxidation on Polycrystalline Palladium

The electrooxidation of formic acid on polycrystalline palladium electrodes in sulfuric acid is studied by dc voltammetry and dynamic electrochemical impedance spectroscopy. Impedance spectra with a frequency range of 5 decades with 10 points per decade, obtained with a multi-sine waveform at various scan rates are used to study the mechanism of the electrooxidation on Pd. The use of the dynamic impedance method allows for studying the active electrooxidation at varying potential and surface coverage. We also give some results showing that CO may not be the poisoning species nor present in the electrooxidation pathway.

X-ray photoelectron spectroscopy study of Pd oxidation by RF discharge in oxygen

The low-temperature oxidation of polycrystalline palladium by RF oxygen plasma was studied via X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Detailed information about the electronic states of palladium and oxygen was obtained based on the XPS curve fitting analysis of Pd3d and Pd3p + O1s lines. The results showed that Pd oxidation by oxygen plasma was different from Pd oxidation in pure O 2 at high temperature. SEM shows well-structured submicron PdO particles result from oxidation in pure O 2, whereas plasma oxidation results in the predominant formation of two-dimensional PdO structures covering the initial crystallites of the Pd foil. Further oxidation to a three-dimensional PdO phase occurs under prolonged treatment with oxygen plasma. The formation of a PdO x ( x > 1) species, characterized by a E b(Pd3d 5/2) = 338.0–338.2 eV value that is close to the Pd 4+ oxidation state, was also observed. This PdO x species was found to have low thermal stability ( T < 400 K). It is proposed that the PdO x species can be localized within the boundaries of crystallites.

Interaction between oxygen and polycrystalline palladium at O2 pressures of 10−6–10 Pa

The interaction between oxygen and polycrystalline palladium (Pd(poly)) at \( P_{O_2 } \) = 2.6 × 10−6–10 Pa and T = 300–1300 K was studied by the thermal desorption (TD) method. The interaction between O2 and Pd(poly) is governed by the O2 pressure and the sample temperature. At low pressures of \( P_{O_2 } \) (≤1.3 × 10−5 Pa), O2 is chemisorbed dissociatively on the Pd(poly) surface. During chemisorption, the Oads-surface bond energy and the O2 sticking coefficient gradually decrease as the surface coverage θ increases. At \( P_{O_2 } \) ≥ 10−2 Pa and T ≤ 500 K, after the saturation of the Oads layer (θ ∼ 0.5), Oads atoms penetrate under the surface layer of the metal to form surface palladium oxide. At \( P_{O_2 } \) ≥ 1 Pa and T > 500 K, after the saturation of the surface oxide film 2 ML in thickness (n ∼ 2), Oads atoms penetrate into the oxide film and then into the subsurface palladium layer and diffuse deep into the metal bulk. As a result, the oxygen uptake at 700 K is n ∼ 50. Upon heating, the surface oxides decompose, desorbing O2, which gives rise to a low-temperature TD peak with Tmax = 715 K. The release of oxygen inserted in the subsurface layers of palladium shows itself as a distinct high-temperature TD peak with Tmax ≥ 750 K.

Nanocontact-induced catalytic activation in palladium nanoparticles

We report the synthesis and catalytic studies of novel palladium nanostructures assembled from small nanoparticles by a surfactant-templated method. These one-dimensional nanomaterials comprise high-density nanocontacts of approximately 1 nm in contact length at the particle-particle interface. In contrast to dispersed Pd nanoparticles ( approximately 5 nm), the polycrystalline palladium nanowires exhibit enhanced ( approximately 200 times) catalytic reactivity towards carbon-carbon cross-couplings under mild conditions. Theoretical modeling studies suggest that the presence of nanocontacts triggers electron transfer and localized charge redistribution in the contact region. The charge redistribution causes the nanocontacts to become highly attractive to charged organic molecules, resulting in the facilitation of organic transformations.

Meltdown of palladium nanoparticles due to prolonged hydrogen ion bombardment

Nanometric crystalline particles of palladium are known to form on polycrystalline palladium under the bombardment of low-energy hydrogen ions. Mostly, the Pd particles disperse on an amorphous medium or in a matrix protruding from the target surface. Here evidence is presented that Pd nanoparticles originally present on the matrix surface are liquefied by bombarding hydrogen ions. The macroscopic target temperature for this to occur is around one-third of the melting point of Pd. This phenomenon is too unfamiliar to be described in terms of existing scientific models and an as-yet unknown process might underlie its occurrence.

Electronic properties of the interface between the organic semiconductor α-sexithiophene and polycrystalline palladium

The interface properties of α-sexithiophene (α-6T) and polycrystalline, atomically clean and contaminated palladium have been studied by combined core level X-ray photoemission spectroscopy and valence band ultraviolet photoemission spectroscopy. Our data indicate for the atomically clean palladium substrate a chemical reaction between the α-sexithiophene molecules and the substrate during the formation of a monolayer of flat lying α-6T molecules. We find an interface dipole of −1.2 eV and an injection barrier for holes of about 0.7 eV. In the case of ex-situ treated, contaminated palladium as metal electrode material we find a reduced interface dipole −0.4 eV and hole injection barrier 0.5 eV. By the comparison to the results of α-sexithiophene on gold we demonstrate the importance of the strength of chemical reactions at the interface.

Interaction of Niobium with Polycrystalline Palladium Surface. X-ray Photoemission Study

Evolution of niobium-palladium (Nb-Pd) interface alloy prepared by in situ Nb deposition on highly defect polycrystalline Pd surface at room temperature has been investigated using X-ray photoelectron spectroscopy. The binding energies of the Nb 3d5/2 and Pd 3d5/2 core-level lines and their line shapes and the kinetic energies and shapes of Pd M4,5N4,5N4,5 Auger spectrum have been used to characterize the Nb-Pd system. A two-step growth mode of the Nb adlayer has been observed: a two-dimensional (2D)-like growth in a submonolayer Nb concentration region and three-dimensional (3D) intermixing of Nb and Pd atoms at Nb loadings above the monolayer coverage. The latter growth mode resulted in appearance of rather homogeneous Nb-Pd alloy phase.

Catalytic and electrocatalytic production of H 2 from the water gas shift reaction over Pd in a high temperature proton-conducting cell-reactor

The water gas shift reaction (WGS) was studied in a proton-conducting cell-reactor at 873–1023 K and atmospheric total pressure. The proton conductor was a strontia–zirconia–yttria perovskite disk of the form: SrZr 0.95Y 0.05O 3 − α . A polycrystalline palladium film was used as the working electrode-catalyst. The reaction kinetics were evaluated with the cell operating under open circuit. Under closed circuit and with protons “pumped” away from the catalyst, the rate of H 2 formation increased to a moderate extent. The Faradaic efficiency ( Λ) was found to attain values as high as Λ = 8, while the highest rate enhancement ratio ( ρ) measured was ρ = 2. The Proton Transport Number (PTN) varied between 0.40 and 1.0.

Mechanisms of oxygen adsorption and desorption on polycrystalline palladium

The adsorption and desorption of oxygen on a polycrystalline palladium (Pd(poly)) surface (10-to 100-μm crystallites; ∼32% (100), ∼18% (111), ∼34% (311), and ∼15% (331)) at P O2 ≤ 1.3 × 10−5 Pa and T = 500–1300 K have been studied by TPD and mathematical modeling. The kinetics of O2 adsorption and desorption on Pd(poly) are primarily governed by the formation and decomposition of oxygen adsorption structures on the (100) and (111) crystallite faces. The O2 adsorption rate is constant at ϑ ≤ 0.15–0.25 owing to the formation of the p(2 × 2) structure with an Oads-surface bonding energy of D(Pd-O) = 364 kJ/mol on the (100) and (111) faces. The adsorption rate decreases with increasing coverage at ϑ ≥ 0.15–0.25 because of the growth, on the (100) face, of the c(2 × 2) structure, in which D(Pd-O) is reduced to 324 kJ/mol by lateral interactions in the adsorption layer. A high-temperature (∼800 K) O2 desorption peak is observed for ϑ ≤ 0.25, which is due to O2 desorption from a disordered adsorption layer according to a second-order rate law with an activation energy of E des = 230 kJ/mol. A lower temperature (∼700 K) O2 desorption peak is observed for ϑ ≥ 0.25, which is due to O2 released by the c(2 × 2) structure according to a first-order rate law with E des = 150 kJ/mol. At ϑ ≥ 0.25, there are repulsive interactions between Oads atoms on Pd(poly) (eaa = 5–10 kJ/mol).

Carbon monoxide adsorption on palladium electrodes modified with silver adatoms

Adsorption of carbon monoxide on palladium deposits and smooth polycrystalline palladium is studied in conditions of preliminary accumulation of various amounts of adsorbed silver (ϑAg) on them. It is discovered that carbon monoxide undergoes adsorption on palladium as it does on platinum, i.e. at the sites free of adsorbed silver, virtually without forcing out preliminarily adsorbed silver. At small and medium values of ϑAg in the region of electrodesorption of a mixed layer on palladium, as opposed to Pt/Pt, only two peaks are observed in an anodic voltammetric curve. The nature of the processes that are responsible for the presence of two peaks pertaining to the oxidation of a mixed adsorption layer is discussed.

Effect of H 2O presence on the propane decomposition reaction over Pd in a proton conducting membrane reactor

The reaction of C 3H 8 decomposition was studied in a proton conducting cell-reactor in the presence of H 2O vapor in the feed gas. The proton conductor was a strontia–ceria–ytterbia perovskite disk of the form: SrCe 0.95Yb 0.05O 3−α. Reaction temperature varied between 873 and 1023 K and a polycrystalline palladium film was used as the working electrode-catalyst. The H 2 produced by the reaction was electrochemically transported through the walls of the proton conducting disk to the outer reactor-chamber. Upon increasing the H 2O partial pressure from 0.3 to 2.8 kPa, a 4-fold increase on the reaction rate was observed. Upon proton “pumping” to the outer chamber, an up to 90% of the produced hydrogen was electrochemically separated from the reaction mixture.

The Use of Thin‐Layer Electroanalysis in the Study of the Chemisorption and Anodic Oxidation of Aromatic Molecules at Smooth Polycrystalline Palladium

AbstractThin‐layer electroanalytical chemistry has been used in the study of the chemisorption and anodic oxidation of hydroquinone and benzoquinone at smooth polycrystalline palladium electrode surfaces in aqueous sulfuric acid solutions. The results were reminiscent of those obtained previously on smooth polycrystalline platinum: i) At low aqueous‐solution concentrations, the diphenol is oxidatively chemisorbed to form surface‐coordinated benzoquinone oriented parallel to the surface. ii) At higher concentrations, the oxidative chemisorption occurs via CH activation to yield an edge‐oriented diphenolic species. iii) Chemisorption from benzoquinone solutions leads to species identical to those from hydroquinone solutions. iv) The extent of anodic oxidation of the chemisorbed organic depends upon the initial adsorbed‐molecule orientation: the flat‐adsorbed species are oxidized completely to carbon dioxide, whereas oxidation of the edge‐chemisorbed species yields other (unidentified) products that are chemisorbed upon regeneration of the oxide‐free surface.

Manipulating the activation barrier for H2(D2) desorption from potassium-modified palladium surfaces

In this work the permeation and desorption of hydrogen (deuterium) from potassium-modified Pd(111) and polycrystalline palladium surfaces have been studied in the temperature range from 350 to 523 K. Time-of-flight spectroscopy has been used to determine the translational energy distributions of associatively desorbing H(2)(D(2)) molecules as a function of the potassium coverage and additional isotropic O(2) and CO background pressures. It turned out that the energy distribution of the hydrogen desorption flux is thermalized for the clean Pd surfaces but hyperthermal for the potassium-covered surfaces. The activation barrier for adsorption was found to increase with the potassium coverage but to decrease again in the presence of coadsorbates such as O(2) or CO. Especially by choosing different isotropic CO pressures, the effective desorption barrier for hydrogen could be reversibly decreased and increased, which resulted in the equivalent changes of the mean kinetic energies of the desorbing H(2) molecules.

Catalytic Oxidation of Ethene on Polycrystalline Palladium: Influence of the Oxidation State of the Surface

The partial and total oxidation of ethene was studied on clean and oxidized polycrystalline Pd in a UHV high-pressure cell. The model catalyst was prepared under UHV conditions and characterized by thermal desorption and Auger and Raman spectroscopy. Starting from either a Pd or PdO surface, reaction to CO2 is the predominant pathway, and acetic acid is the main byproduct between 453 and 523 K. The route to partially oxidized products, in particular acetic acid, is more favoured on the PdO surface, while on Pd metal acetic acid is formed at lower temperatures than on PdO. On stoichiometric PdO activation for both partial and total oxidation occurs during an induction period, until under working conditions Pd metal, PdO, ‘PdC x ’ and isolated defects are coexistent on the surface. The presence of an extended metal surface favours the follow-up oxidation of byproducts, while the presence of PdO stabilizes acetic acid far beyond 473 K.

PHOTOELECTRONIC INVESTIGATION OF Pd ELECTRON STRUCTURE TRANSFORMATION AT OXYGEN ADSORPTION

Oxygen adsorption on the surface of polycrystalline palladium has been studied by the photoelectron spectroscopy method in a wide range of oxygen exposure and temperatures, starting with the earliest stages of adsorption to the formation of the surface compound of PdO -type. Based on the analysis of the experimental data and the calculation of the density of states carried out by the TB-LMTO-ASA method, the attempt to elucidate the electronic mechanism of adsorption and oxidation power of palladium was made.

Reaction and desorption kinetics of H 2 and H 2O on activated and non-activated palladium surfaces

The permeation/desorption of hydrogen and the reaction between oxygen and permeating/desorbing hydrogen from a polycrystalline palladium sample have been studied under ultra-high vacuum conditions. Furthermore, the effects of potassium on desorption and reaction has been investigated. Time-of-flight spectroscopy has been mainly used to determine the translational energy distribution of desorption and reaction products. These experiments were complemented by angular distribution measurements. It turned out that the associative hydrogen desorption is inactivated on the clean palladium, but activated on the potassium-covered surface. The water formation/desorption is inactivated on both, the clean and the potassium-covered surfaces.

XPS and atomic force microscopy analyses of thin Au and Cu films on Pd

AbstractThe surface properties of bimetallic systems have been studied recently due to possible applications in different areas, such as electrochemistry, magnetism, and catalysis. Understanding of the atomic and electronic structures is crucial for designing metallic devices. In this work, thin films of gold and copper were grown on a polycrystalline palladium substrate by sputtering. For both films, the thickness was estimated to be 10 nm. The morphology of the films was investigated by atomic force microscopy (AFM). XPS was employed in order to verify the possibility of alloy formation. Oxide formation was observed for the Cu film, but not for the Au film. The copper oxide (CuO) was removed by argon ion sputtering. Alloying was not observed in the Au/Pd or Cu/Pd films. Copyright © 2004 John Wiley &amp; Sons, Ltd.

Reaction intermediates in high temperature catalytic water formation studied with cavity ringdown spectroscopy

The rotational temperature and concentration of OH in the gas-phase outside a polycrystalline platinum catalyst has been measured using cavity ringdown spectroscopy. A mixture of H2 and O2 gases was used forming a stagnation point flow field outside the catalyst. The temperature of the catalyst was 1500 K, the total pressure in the chamber was 26 Pa, and the relative hydrogen concentration, αH2, was set to 8%. From a Boltzmann plot, the rotational temperature was determined to be 775±24 K, and the amount of OH was calculated from that temperature to be 1.5±0.2×1012 cm−3, 6.5 mm outside the catalyst. A similar experiment has also been performed using a polycrystalline palladium catalyst. However, due to lower amount of OH in the gas-phase outside the Pd catalyst, the number density could not successfully be determined. In this study we have shown that cavity ringdown spectroscopy successfully can be used to quantify desorbed intermediates within a catalytic reaction.