Pd Surface Layer Research Articles

Colorimetric Plasmonic Hydrogen Gas Sensor Based on One-Dimensional Nano-Gratings

Plasmonic hydrogen gas sensors have become widely used in recent years due to their low cost, reliability, safety, and measurement accuracy. In this paper, we designed, optimized, and fabricated a palladium (Pd)-coated nano-grating-based plasmonic hydrogen gas sensor; and investigated using the finite-difference time-domain method and experimental spectral reflectance measurements, the calibrated effects of hydrogen gas exposure on the mechano-optical properties of the Pd sensing layer. The nanostructures were fabricated using DC sputter deposition onto a one-dimensional nano-grating optimized with a thin-film gold buffer to extend the optical response dynamic range and performance stability; the color change sensitivity of the Pd surface layer was demonstrated for hydrogen gas concentrations as low as 0.5 vol.%, up to 4 vol.%, based on the resonance wavelength shift within the visible band corresponding to the reversible phase transformation. Visual color change detection of even the smallest hydrogen concentrations indicated the high sensitivity of the gas sensor. Our technique has potential for application to high-accuracy portable plasmonic sensors compatible with biochemical sensing with smartphones.

Magneto-optical surface plasmon resonances on perpendicular magnetic thin films consisting of CoPt/ZnO/Ag stacked nanolayers

Magneto-plasmonic phenomena on CoPt/ZnO/Ag stacked nanolayers were investigated in polar Kerr excitation geometry. The nanolayer displays an ideal square-shaped out-of-plane magnetic hysteresis loop with a large polar Kerr activity. The surface plasmon resonances (SPRs) induce drastic magneto-optical (MO) responses with a narrow linewidth and a sharp reversal of the Kerr polarity. The CoPt/ZnO/Ag nanolayer is a candidate for high-performance chemical sensor elements. For example, an MO‒SPR sensor with a Pd surface layer was applied for hydrogen detection. The Pd layer acts as a transducer for both hydrogen detection and plasmon response. Substantial MO responses to the exposure of hydrogen gas can be observed. The results show that the polarized sensing signal is not affected by the intensity of the incident light. The use of the new type of MO‒SPR element with polar Kerr activity leads to a stable chemical sensing system with a simple measurement configuration.

Elastic strain effects on the catalytic response of Pt and Pd thin films deposited on Pd–Zr metallic glass

Abstract

Controlling the stability of a Fe–Ni reforming catalyst: Structural organization of the active components

Fe–Ni catalysts present high activity in dry reforming of methane, with high carbon resistance, but suffer from deactivation via sintering and Fe segregation. Enhanced control of the stability and activity of Fe–Ni/MgAl2O4 was achieved by means of Pd addition. The evolution of the catalyst structure during H2 Temperature Programmed Reduction (TPR) and CO2 Temperature Programmed Oxidation (TPO) was investigated using time-resolved in situ X-ray diffraction (XRD). During reduction of Fe–Ni–Pd supported on MgAl2O4, a core shell alloy forms at the surface, where Fe–Ni is in the core and Fe–Ni–Pd in the shell. A 0.2wt% Pd loading or Ni:Pd molar ratio as high as 75:1 showed the best performance in terms of both activity and stability of the catalyst at 1023K and total pressure of 101.3kPa. Experimental results and DFT calculations showed that Pd addition to bimetallic Fe–Ni reduces the tendency of Fe to segregate to the surface of the alloy particles under methane dry reforming (DRM) conditions, due to the formation of a thin Fe–Ni–Pd surface layer. The latter acts as a barrier for Fe segregation from the core. Segregation of Fe from the trimetallic shell still occurs, but to a lesser extent as the Fe concentration is lower. This Ni:Pd molar ratio is capable of controlling the carbon formation and hence ensure high catalyst activity of 24.8mmols−1gmetals−1 after 21h time-on-stream.

C2H2-Induced Surface Restructuring of Pd–Ag Catalysts: Insights from Theoretical Modeling

In this paper, the influence of acetylene chemisorption on the equilibrium distribution of atoms in the (111) surface layer of Pd–Ag alloys is studied. All possible surface layer compositions are considered from a statistical mechanics based model with parameters determined from first-principles computations. Surface restructuring is investigated at three different temperatures. In conditions relevant for the catalytic acetylene selective hydrogenation, the equilibrium surface distribution of alloy atom type strongly deviates from the smooth binomial random case and specific ensembles are favored, with sharp transition zones between them as a function of Ag content. Indeed, the chemisorption energy of acetylene on Pd rich ensembles provides a contribution to the free energy that counterbalances the entropic gain from the random alloy distribution. The molecule coverage and the associated energetic term are weakened by thermal effects and lateral interactions between adsorbates.

Influence of phosphorus concentration on the state of the surface layer of Pd–P hydrogenation catalysts

Phase composition and surface layer state of the Pd–P hydrogenation catalyst formed at various P/Pd ratios from Pd(acac)2 and white phosphorus in a hydrogen atmosphere were determined. Palladium on the catalyst surface is mainly in two chemical states: as Pd(0) clusters and as palladium phosphides. As the P/Pd ratio increases, the fraction and size of palladium clusters decrease, and also the phase composition of formed palladium phosphides changes: Pd3P0.8 → Pd5P2 → PdP2. The causes of the modifying action of phosphorus on the properties of palladium catalysts for hydrogenation of unsaturated compounds were considered.

Evolution of Pd catalyst structure and activity during catalytic oxidation of methane and ethane

Methane and ethane oxidation in alkane-rich mixtures over Pd powder in temperature range of 300–450°C has been studied using combined mass-spectrometry and thermogravimetry. Fresh Pd powder was found to have very low stationary initial catalytic activity. The activity gradually rose with time on stream reaching a certain value where oscillations of the reaction rate began. The oscillation phases with relatively high catalytic activity were accompanied by a periodic accumulation/removal of carbon in Pd bulk, whereas during the low activity phases a periodic accumulation/removal of oxygen in Pd surface layer took place. The correlation between Pd oxidation state and catalytic activity was drawn. The metallic Pd had the lowest catalytic activity in alkane oxidation at high partial pressure of O2 and while the highest catalytic activity was observed at low partial pressure and high conversion of O2. Microscopic study of Pd samples revealed that the catalyst evolution caused changes in Pd surface morphology resulting in formation of sponge-like structure. In turn, such morphological changes were found to increase duration and intensity of the high activity phase.

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.

Fluorine adsorption on transition metal surfaces: A DFT study

On the basis of Density Functional Theory calculation, the adsorption of fluorine was investigated on a) Cu(111), Au(111), Pd(111) and Pt(111) surfaces, b) on Pd monolayer surfaces over Cu(111), Au(111) and Pt(111) surface, and c) on the surfaces of Pd(111) monocrystal with inserted metal monolayer (M = Cu, Au, Pt) underneath the first Pd surface layer. The results evidenced that the adsorption did not cause significant structural parameters of metallic substrate. The strongest adsorption amounting to -4.49 eV was calculated in the case of Cu(111) surface. The Cu(111) and Au(111) surface atoms interact with F adatoms by the mediation of sp-band exclusively, while the surface atoms of Pt and Pd-based surfaces interact with F adatoms additionally by the mediation of d-band, too. In the case of Pt(111) and PdML/M(111) surfaces, the binding energies are correlated with the d-band center positions, which indicated a significant contribution of covalent interaction. These results confirmed that the nature of surface interaction of highly electronegative F atom with metallic surfaces depends significantly on the nature of the metal substrate.

Dealloyed PdCu3 thin film electrocatalysts for oxygen reduction reaction

The catalytic activity of electrochemically dealloyed PdCu3 thin films for oxygen reduction reaction (ORR) in acidic media has been studied by using a rotating disk electrode (RDE). The dealloyed PdCu3 thin films show a ∼2.0 fold increase in the specific oxygen reduction activity over pure Pd thin films. The structure of electrochemically dealloyed PdCu3 thin films has been investigated at an atomic scale by synchrotron-based anomalous X-ray diffraction (AXRD). AXRD reveals that a Pd enriched surface layer is formed in the dealloyed film and a compressive lattice strain exists in this Pd surface layer. The enhanced catalytic activity of dealloyed Pd–Cu films for the ORR is primarily due to the compressive strain in the surface layer. We compare the structure–composition–catalytic activity relationships in dealloyed Pd–Cu thin films to related results on dealloyed Pt–Cu thin films. These studies show that dealloying and the resulting structure and the ORR activity are dependent on the nature of the noble component of alloy.

Effects of the PdO nanoflake support on electrocatalytic activity of Pt nanoparticles toward methanol oxidation in acidic solutions

We prepared PdO nanoflake thin films on carbon cloths by reactive sputtering deposition, and studied electrocatalytic performance of Pt nanoparticles electrodeposited on the PdO thin film in methanol oxidation reaction (MOR) in acidic media. The PdO nanoflake thin film exhibits a cyclic voltamperometric (CV) behavior similar to metallic Pd electrodes because a metallic Pd surface layer is formed on the PdO thin film electrode under the CV measurement condition. The nanoflake morphology of the PdO electrode provides a large surface area for Pt nanoparticle loading, resulting in a large electrochemical active surface area (ESA). The Pt/PdO electrode has a high electrocatalytic activity toward MOR, and the good electrocatalytic performance of the Pt/PdO electrode is ascribed to a high CO tolerance and the large ESA. We suggest that the high CO tolerance of the catalyst on the Pt/PdO electrode is a result of the synergism of the bifunctional mechanism and the electronic effect. XPS analyses indicate that negative charge transfer occurs from the PdO support to the Pt nanoparticles, indicating the presence of the electronic effect. Pt nanoparticles on the PdO support can greatly alleviate the nanoflake damage during the CV measurement, which results from anodic dissolution of metal Pd from the PdO support in acidic media.

Local structure at interfaces between hydride-forming metals: A case study of Mg-Pd nanoparticles by x-ray spectroscopy

The structure at the interface between elements or phases that exhibit different hydrogen (H) binding energies exerts a profound influence on the thermodynamics of H in nanophase materials. In this paper, we study the local structure at the Mg/Pd interface in Mg nanoparticles with partial Pd coating, and we map its evolution in response to annealing and H sorption. This task is accomplished by x-ray photoelectron spectroscopy and x-ray absorption spectroscopy, also including in situ experiments, with the support of crystallographic information from x-ray diffraction. It is shown that the initial Pd surface layer reacts with Mg at relatively low temperatures, leading to irreversible formation of a Mg-rich intermetallic phase Mg${}_{6}$Pd. Due to the high Mg-H binding energy, this phase reversibly transforms, upon H absorption, into a nanophase mixture of magnesium hydride and a Pd-rich intermetallic with H in solid solution, MgPdH${}_{\mathrm{\ensuremath{\delta}}}$. These reversible structural changes are discussed with reference to recent calculations that highlight their relevance to the thermodynamics of the metal-hydride transition. The picture drawn here might be relevant to other multiphase materials presently investigated in the field of hydrogen-related science and technology.

Magnetic Properties and Relaxation of Vanadium Monolayer on Pd(001) Surface

We investigated the magnetism of vanadium monolayers on a Pd(001) surface. The electronic structure and the magnetic properties of the V/Pd(001) system were determined with the use of the full-potential linearized augmented plane-wave method within the general gradient approximation. Three magnetic configurations were studied: non-, ferro-, and antiferromagnetic. From the total energy calculations, we found that the V/Pd(001) system is the most stable in the antiferromagnetic configuration. The importance of relaxation on the magnetic properties of the systems was also studied. It was found that the Pd(001) surface covered with a V monolayer undergoes considerable relaxation in which the spacing between Pd layers increases in all three magnetic configurations. Contrary to the Pd interlayer spacing, the distance between the V overlayer and the topmost Pd layer is reduced. The interlayer spacing between the V overlayer and the Pd surface layer is the largest for the antiferromagnetic configuration. In the relaxed antiferromagnetic structure, the magnitude of the calculated magnetic moments on the V atoms was <TEX>$1.31\;{\mu}_B$</TEX>. The presence of the vanadium monolayer does not affect the paramagnetic properties of the Pd(001) surface.

In Situ Generation of Pd/PdO Nanoparticle Methane Combustion Catalyst: Correlation of Particle Surface Chemistry with Ignition

Decomposition of a fuel-soluble precursor was used for in situ generation of Pd/PdO nanoparticles, which then catalyzed ignition of the methane/O2/N2 flow. To help understand the relationship between particle properties and activity, the composition, structure, and surface chemical state of the particles were determined by a combination of high-resolution transmission electron microscopy (HRTEM), electron diffraction, scanning transmission electron microscopy/energy dispersive X-ray spectroscopy (STEM/EDX), and X-ray photoelectron spectroscopy (XPS). The particles, collected under methane-free conditions, were found to be primarily crystalline, metallic Pd, with TEM results showing a narrow size distribution around 8 nm and scanning mobility particle sizing measurements (SMPS) indicating a median particle size of ∼10 nm. The ignition temperature was lowered ∼150 K by the catalyst, and we present evidence that ignition is correlated with formation of a subnanometer oxidized Pd surface layer.

Correlation of H 2 heat of adsorption and ethylene hydrogenation activity for supported Re@Pd overlayer catalysts

Alumina-supported bimetallic overlayer Pd on Re (Re@Pd) catalysts were synthesized using the directed deposition technique. Hydrogen chemisorption, TEM, EDS, XRD, and ethylene hydrogenation studies were used to characterize the catalysts and provide indication of electronic modification of the Pd surface layer due to the overlayer particle structure. First principles computation and single crystal studies of Pd overlayers on Re in the literature have shown that electronic modification of the Pd overlayer is observed and leads to decreased binding strength for chemisorbed species such as H 2, C 2H 4, and CO. Measured hydrogen chemisorption isotherms indicated that Pd was deposited on the Re and not as pure isolated Pd particles. H 2 heats of adsorption, as determined by chemisorption, indicated that the Re@Pd overlayer catalysts were lower than either pure Pd or Re. The Re@Pd catalysts were slightly less active for ethylene hydrogenation than pure Pd but displayed similar apparent activation energies and H 2 and C 2H 4 reaction orders. A linear correlation between turnover frequency and maximum heat of H 2 adsorption was observed for the Pd and Re@Pd catalysts. This suggests an electronic modification of the Re@Pd catalyst surface compared to Pd as predicted in the literature by first principles computational studies.

Nanotopography and electrochemical impedance spectroscopy of palladium deposited on different electrode materials

The objective of this work was to describe the characteristics of chemically and electrochemically deposited Pd surface layers on HOPG and polycrystalline gold electrode, using in situ ECSTM and EIS measurements, and SEM-EDX element analysis. Pd surface layers were deposited, in successive voltammetric cycles, and anodically dissolved in 0.01 M HCl+0.01 M (NH4)2PdCl4 aqueous electrolyte. Both of the electrode materials used in the study were treated as standard testing electrodes: (i) HOPG for STM/ECSTM measurements, and (ii) polycrystalline Au as the well known working electrode in various electro-analytical applications. The elements’ surface analysis and nano-surface pictures were used to interpret the EIS diagrams and electrical equivalent circuits. Pd chemical and electrochemical deposition on the HOPG surface was compared with the same process on the polycrystalline gold electrode, on which palladium can be electrodeposited only by means of electrochemical cathodic deposition. Surface topographies of the electrodeposited palladium layers on HOPG and Au were completely different. The equivalent electrical circuits were fitted and the surface roughness of the investigated electrodes calculated. Relations between the surface topography, EIS and SEM-EDX, and interface model of the electrolyte solution ∣ electrodeposited Pd layer ∣ matrix electrode were proposed.

Equilibrium and non-equilibrium surface structures of Al/Pd (0 0 1)

It has been demonstrated that the equilibrium state of Al deposited on Pd (0 0 1) and annealed to 900 K is a (2 × 2)p4g structure induced by the stress related to the formation of a pure Pd surface layer over a c(2 × 2) Al–Pd second layer. This state can be changed by exposure to oxygen (at room temperature) which causes the Al to migrate to the surface layer with a concentration of up to 50% as measured by non-thermal removal of oxygen. In this process the surface is exposed to H 2 at 375 K and the oxygen is removed. The oxygen-covered surface assumes a (1 × 1) structure. Once this surface is annealed to 700 K the oxygen is desorbed. Annealing to higher temperatures (900 K) returns the surface to the initial (2 × 2)p4g structure. This cycle has been followed for five cycles without significant loss of Al demonstrating the trapping power of the second layer structure.

Nanocrystalline structure in Pd40Ni40P20 alloy

Abstract The formation of a nanocrystalline structure with a grain size of 20–40 nm in the surface layer of amorphous Pd 40 Ni 40 P 20 alloy was observed when crystallization occurred in vacuum. The surface layer was found to be depleted in phosphorus. The nanocrystalline structure with a grain size of 2–20 nm also forms upon crystallization of this amorphous alloy in the electron microscope.

The Influence of Support on the Low-Temperature Activity of Pd in the Reaction of CO Oxidation: 2. Adsorption Properties and Reactivity of Adsorbed Species

Adsorption properties of Pd supported on γ-Al2O3, TiO2, SiO2, and adsorbed CO and oxygen low-temperature reactivities have been studied using IR spectroscopy, TPD, and pulse titration techniques. The results obtained are compared with the Pd surface structure data. At saturation coverages, weakly bound forms of CO (both linear and bridge-bonded) are the most reactive, their properties depending upon the type of support. Effect of Pd surface layer rearrangement due to CO and/or oxygen adsorption has also been found to be of importance. For mixed carbon monoxide–oxygen adlayer (θCOand θOare comparable), an overall Pd surface disordering occurs to wipe out all support effects.

The Influence of Support on the Low-Temperature Activity of Pd in the Reaction of CO Oxidation: 1. The Structure of Supported Pd

The morphology of small Pd clusters on SiO2, TiO2, and Al2O3and the influence of CO and the reaction mixture CO+O2on their fine structure have been investigated by TEM and EXAFS. For all supports, Pd particles have a quite narrow size distribution with mean diameters in the range 15–25 Å, whereas their morphology and spatial homogeneity of location on the support are different. EXAFS data acquired in controlled-atmosphere cells have shown distortion of Pd surface layers to depend upon the nature of the support and/or the adsorbate. Disordering of Pd surface structure suggests appearance of the defect centers of CO and oxygen adsorption.