Electronic Structure Of Pd Research Articles

PdCu/C Catalysts for the Electrochemical Oxidation of Renewable Polyalcohols

The development of the alkaline anion exchange membrane (AEM) for the direct liquid fuel cell opened up opportunities for the use of new fuels (that oxidized inefficiently in PEM fuel cells) and new catalysts (that corroded easily in PEM fuel cells). The first fuel studied in depth was ethanol because it oxidizes much more efficiently in alkaline media than acidic media. However, the same palladium catalyst that oxidizes ethanol in alkaline media can also be used to oxidize several other alcohol fuels including polyalcohols such as ethylene glycol (EG), propylene glycol (PG), and glycerol (G). Therefore, we recently demonstrated a flexible fuel direct liquid fuel cell that operated on alcohols such as ethanol, EG, PG, and G. However, these fuels all oxidize more slowly then ethanol, requiring significant kinetic improvement to be used practically. We tested various combinations of Pd:Cu and compared them in each polyalcohol fuel, EG, PG, and G. We observed that any quantity of Cu added to the Pd produced an increase in oxidation rate compared with pure Pd for all three polyalcohol fuels. We also observed a down-shift in d-band center as observed by XPS. We hypothesize that the addition of Cu to the Pd results in a combination of the electronic and bifunctional effects. The electronic effect is caused by perturbations in the electronic structure of Pd, the primary catalyst, induced by the Cu. The bifunctional effect is caused by the co-catalysis of Pd and Cu directly on the surface of the catalyst.

Stabilization of Palladium Nanoparticles on Nanodiamond-Graphene Core-Shell Supports for CO Oxidation.

Nanodiamond-graphene core-shell materials have several unique properties compared with purely sp(2) -bonded nanocarbons and perform remarkably well as metal-free catalysts. In this work, we report that palladium nanoparticles supported on nanodiamond-graphene core-shell materials (Pd/ND@G) exhibit superior catalytic activity in CO oxidation compared to Pd NPs supported on an sp(2) -bonded onion-like carbon (Pd/OLC) material. Characterization revealed that the Pd NPs in Pd/ND@G have a special morphology with reduced crystallinity and are more stable towards sintering at high temperature than the Pd NPs in Pd/OLC. The electronic structure of Pd is changed in Pd/ND@G, resulting in weak CO chemisorption on the Pd NPs. Our work indicates that strong metal-support interactions can be achieved on a non-reducible support, as exemplified for nanocarbon, by carefully tuning the surface structure of the support, thus providing a good example for designing a high-performance nanostructured catalyst.

Stabilization of Palladium Nanoparticles on Nanodiamond–Graphene Core–Shell Supports for CO Oxidation

AbstractNanodiamond–graphene core–shell materials have several unique properties compared with purely sp2‐bonded nanocarbons and perform remarkably well as metal‐free catalysts. In this work, we report that palladium nanoparticles supported on nanodiamond–graphene core–shell materials (Pd/ND@G) exhibit superior catalytic activity in CO oxidation compared to Pd NPs supported on an sp2‐bonded onion‐like carbon (Pd/OLC) material. Characterization revealed that the Pd NPs in Pd/ND@G have a special morphology with reduced crystallinity and are more stable towards sintering at high temperature than the Pd NPs in Pd/OLC. The electronic structure of Pd is changed in Pd/ND@G, resulting in weak CO chemisorption on the Pd NPs. Our work indicates that strong metal–support interactions can be achieved on a non‐reducible support, as exemplified for nanocarbon, by carefully tuning the surface structure of the support, thus providing a good example for designing a high‐performance nanostructured catalyst.

Clean Synthesis of an Economical 3D Nanochain Network of PdCu Alloy with Enhanced Electrocatalytic Performance towards Ethanol Oxidation.

A one-pot method for the fast synthesis of a 3D nanochain network (NNC) of PdCu alloy without any surfactants is described. The composition of the as-prepared PdCu alloy catalysts can be precisely controlled by changing the precursor ratio of Pd to Cu. First, the Cu content changes the electronic structure of Pd in the 3D NNC of PdCu alloy. Second, the 3D network structure offers large open pores, high surface areas, and self-supported properties. Third, the surfactant-free strategy results in a relatively clean surface. These factors all contribute to better electrocatalytic activity and durability towards ethanol oxidation. Moreover, the use of copper in the alloy lowers the price of the catalyst by replacing the noble metal palladium with non-noble metal copper. The composition-optimized Pd80 Cu20 alloy in the 3D NNC catalyst shows an increased electrochemically active surface area (80.95 m(2) g(-1) ) and a 3.62-fold enhancement of mass activity (6.16 A mg(-1) ) over a commercial Pd/C catalyst.

Electronic structures of cyclometalated palladium complexes in the higher oxidation states.

The electronic and redox properties of a series of cyclometalated Pd complexes with oxidation states of +2, +3 and +4 were examined using a range of currently available spectroscopic and electrochemical techniques. Changes in metal-ligand bond lengths were established by X-ray crystallography and correctly predicted by DFT calculations, from which the frontier orbitals and partial atomic charges can be obtained. X-ray absorption spectroscopy (XAS) revealed interesting XANES features that suggest a synergistic relationship between metal-ligand interactions. The electrochemical study of the Pd(ii) dimer was found to contain two sequential oxidative potentials indicative of a weak metal-metal interaction.

Promotion of palladium catalysis by silver for ethanol electro-oxidation in alkaline electrolyte

Bimetallic PdmAg alloy nanostructures (m being the atomic Pd/Ag ratio, m = 0.1–1.5), prepared through a simple co-reduction process, are employed as the catalysts toward ethanol electro-oxidation reaction (EOR). XPS results show that the electronic structure of Pd can be modified due to the presence of Ag, which is crucial for the enhancement of the catalytic performance of the PdmAg/C catalysts. It is found that the catalytic activity of Pd was strongly dependent on the composition of the PdmAg/C catalysts, with the best performance found with the Pd0.5Ag/C. The mass-specific activity (MSA) and intrinsic activity (IA) data of Pd0.5Ag/C is 3.6 and 2.4 times higher than that of the monometallic Pd/C catalyst, respectively, which might be ascribed to the electronic and synergistic effect. These findings would be promising in understanding the mechanism of EOR on Pd-based catalysts and designing the bimetallic catalysts for direct ethanol fuel cells and other applications.

Surface dynamics of the intermetallic catalyst Pd2Ga, Part II – Reactivity and stability in liquid-phase hydrogenation of phenylacetylene

The catalytic properties of unsupported Pd2Ga for the liquid-phase hydrogenation of phenylacetylene are investigated after different pre-treatments with focus on the stability of the catalyst during reaction. The surface of as-prepared Pd2Ga consists mainly on Pd and oxidized Ga species. Under the conditions of the liquid-phase hydrogenation of phenylacetylene, the intermetallic surface cannot be reformed in situ by reduction of Ga oxide. After a reductive pre-treatment in 5% H2/Ar at 400°C, an almost clean Pd2Ga surface can be obtained. Its hydrogenation activity is significantly lowered compared to elemental Pd, which is due to the intrinsic adsorption properties of the intermetallic surface. However, residues of H2O or O2 lead to oxidation of this surface. Excluding these impurities, the decomposition can be suppressed. In this case, the bulk material of Pd2Ga gets cracked during phenylacetylene hydrogenation. The controlled modification of the crystal and the electronic structure of Pd by formation of the intermetallic compound Pd2Ga are accompanied with a decreased stability.

Initial Interaction of Pd with MgO(100) Films

Growth and electronic structure of Pd on MgO(100) films, prepared on Mo(100) single crystal, were studied in situ under ultrahigh vacuum conditions by using ultraviolet photoelectron spectroscopy, Auger electron spectroscopy, high resolution electron energy loss spectroscopy, and low-energy electron diffraction. On the basis of valence band analysis, the Pd particles are of nonmetallic character at submonolayer-equivalent coverages. An interaction between the Pd particles and the MgO films has been observed, which may play a key role in chemisorption and catalysis. At higher coverages, the Pd epitaxially grows as (100) orientation. Thermal stability of the Pd particles depends on the coverage. Our experimental results agree with theory and suggest that surface oxygen vacancies on the MgO are active sites to bind the Pd particles. The presence of the properties of the nonmetallic particles is essential in catalytic reactions.

DFT study of small bimetallic palladium–copper clusters

Geometric and electronic structures of Pd n Cu m ( m + n ⩽ 6) clusters have been investigated by DFT calculations. Pd 4–6 and Pd n Cu m clusters with n ⩾ 2 and m + n ⩾ 4 are spatial whereas Pd 1Cu m and Cu m clusters favor planar shapes. Excess of electron density is shifted towards Pd atom in Pd–Cu bonds and, in spatial particles, towards less coordinated atoms. Bimetallic clusters are usually more stable than monometallic particles of the same size. Stability of bimetallic clusters of the same shape and composition increases linearly with increasing number of Pd–Cu bonds. Pd atoms mix with Cu at every composition, while Cu atoms prefer to segregate at high Cu content.

Site selectivity in chemisorption of C on Pd(211)

Results of ab initio electronic structure calculations of C on Pd(211), a vicinal of Pd(111), show a hierarchy of adsorption sites with adsorption energy ranging from $\ensuremath{-}8.21\phantom{\rule{0.5em}{0ex}}\text{to}\phantom{\rule{0.5em}{0ex}}\ensuremath{-}5.46\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ (0.33 ML coverage) and scaling almost linearly with the effective coordination at the site. In the most preferred site, C atoms sit almost under the Pd step atoms and influence drastically multilayer relaxations and surface registry of Pd(211). The adsorption energy at this site is $\ensuremath{-}9.10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$ for 0.17 ML coverage. The local densities of electronic states indicate strong hybridization between C $p$ states and neighboring Pd $d$ states leading to the formation of strong covalent $\mathrm{C}\mathrm{Pd}$ bonds. The depletion of the electronic density of states of the Pd surface atoms at the Fermi level suggests a poisoning effect of C which is found to be coverage dependent. We compare the changes in the electronic structure of Pd(211) on C adsorption with those on S adsorption.

Electronic Structure of the Palladium Hydride Studied by Compton Scattering

The hydrogen-induced changes in the electronic structure of Pd have been investigated by Compton scattering experiments associated with theoretical calculations. Compton profiles (CPs) of single crystal of Pd and β phase hydride PdH x ( x = 0.62–0.74) have been measured along the [100], [110] and [111] directions with a momentum resolution of 0.14–0.17 atomic units using 115 keV x-rays. The theoretical Compton profiles have been calculated from the wavefunctions obtained utilizing the full potential linearized augmented plane wave method within the local density approximation for Pd and stoichiometric PdH. The experimental and the theoretical results agreed well with respect to the difference in the CPs between PdH x and Pd, and the anisotropy in the CPs of Pd or PdH x . This study provides lines of evidence that upon hydride formation the lowest valance band of Pd is largely modified due to hybridization with H 1s-orbitals and the Fermi energy is raised into the sp-band.

Photoelectron spectroscopy studies of the Pd/Ru(0001) and (Cu + Pd)/Ru(0001)interfaces

The electronic structures of Pd and (Cu + Pd) overlayers onRu(0001) have been studied. Photoelectron emission spectroscopybased on synchrotron radiation and radiation from a Mg Kαx-ray source has been used to study the valence band of the surfaces and the Ru3d, Pd 3d and Cu 2p core levels. The interaction of Pd with Ru(0001) isstudied in the coverage range of submonolayer to several monolayers. Thecore level spectra of the Pd 3d5/2 level show a new component for Pdcoverages above 1 ML. In the submonolayer range the binding energy (BE)of the Pd 3d5/2 level is constant at 335.08 eV. A state characterizedas an interface state between the Pd adlayer and the Ru substrate isobserved at a BE of 1.22 eV. This state, which dominates the valenceband for submonolayer Pd coverages, has not been reported earlier andit is seen neither for clean Ru(0001) nor for multilayers of Pd. It is acharacteristic that the overlayer d bands are narrower in the low-coverage regime.Deposition of Cu, on a 0.4 ML Pd/Ru(0001) surface up to 1 ML totalcoverage of Pd and Cu, results in a mixed two-dimensional overlayer.

From x-ray-absorption near-edge structures to thed-hole population in Pd-Ag alloys

The charge transfer upon alloying and the screening of the core hole in the x-ray absorption process in Pd-Ag alloys are discussed according to different models and with respect to different quantities. The charge transfers are analyzed using the first-principles electronic structure and spectrum calculations based on the linear muffin-tin orbitals method. Experimentally, both Pd and Ag gain d charge and lose non-d charge in Pd-Ag compared to pure Pd and Ag. The theoretical result agrees with the experimental one for Pd whereas the calculated charge transfer for Ag is in contradiction with the experimental result. This discrepancy is attributed to the differences in the electronic structure of Pd and Ag requiring new interpretation of the Ag x-ray-absorption near-edge structures of Pd-Ag alloys.

Atomic and electronic structures ofPd13andPt13clusters

The electronic structures of ${\mathrm{Pd}}_{13}$ and ${\mathrm{Pt}}_{13}$ clusters of cuboctahedral and icosahedral symmetries are analyzed by the self-consistent spin-polarized density-functional scheme using the norm-conserving pseudopotential in the linear combination of atomic orbitals method. The electronic structure is discussed in relation to the shell structure of the giant atom. The stable atomic structure is discussed, considering the effects of the generalized gradient approximation of the exchange-correlation potential, the spin-orbit splitting, and the Jahn-Teller effect. The spin-orbit coupling effect depends on the symmetry of the whole cluster. It is the largest of these effects for Pt, and it is as large as the spin polarization effect for Pd. The electronic structure of Pd and Pt clusters of 13 atoms is comprehended as a complex of the shell structure and the localized Pd-$4d$ or Pt-$5d$ orbitals. The Jahn-Teller effect seems to be small to keep the shell structure. For both Pd and Pt metals the cuboctahedron clusters are more stable and have small spin polarization, $S=3$ for the Pd cluster and $S=4$ for the Pt cluster.

Electronic structure near the Fermi level and STM images ofPd(110)c(4 × 2)-benzene

The electronic structure ofPd(110)c(4 × 2)-benzene near the Fermi level ( E F ) was studied by photoemission spectroscopy (PES), metastable atom electron spectroscopy (MAES) and scanning tunneling microscopy (STM). Observed STM images of adsorbed benzene near E F consist of two elongated protrusions separated by a single nodal depression with C 2 symmetry. The benzene derived state near E F is newly observed by means of MAES, and it is assigned to the antibonding state hybridized between the 1e 1g molecular orbital of benzene and the Pd 4d orbitals from ab initio molecular orbital calculation. This state mainly contributes to the characteristic STM images. We conclude that thec(4 × 2)-benzene molecules are flat-lying and azimuthally rotated.

PHOTOEMISSION STUDIES OF Pd AND Pt IMPURITIES ON AND IN SILVER

The electronic structure of Pd and Pt atoms adsorbed on the (100), (110), and (111) surfaces of silver and of the related dilute alloys has been studied at different temperatures using ultraviolet photoelectron spectroscopy (UPS). The observed differences in lineshape of the virtual bound palladium 4d (platinum 5d) state on isolated atoms supported on the differently oriented surfaces in comparison with an impurity atom embedded in the first layer and in the bulk are well accounted for by a many-body calculation performed within the Anderson single impurity model. This analysis clearly indicates a stronger sd-hybridization in the bulk than at the surface, consistent with the change in coordination number.

Photoemission study of Pd and Pt impurities in and on silver

The electronic structures of Pd and Pt atoms adsorbed on the (110) surface of silver and of the related dilute Ag-(3 at.% Pd) and Ag-(3 at.% Pt) alloys have been studied at room temperature using photoelectron spectroscopy. The observed difference between the lineshape of the virtual bound palladium 4d (platinum 5d) state on the respective isolated atoms supported on the (110) surface and that of an impurity atom embedded in the bulk is well accounted for by a many-body calculation performed within the Anderson single-impurity model. This analysis clearly indicates stronger s-d hybridization in the bulk than at the surface, consistent with the change in coordination number.

Modification of the electronic structure of Pd by U films: chemisorption of CO

X-Ray and ultraviolet photoelectron spectroscopies (XPS and UPS), Auger electron spectroscopy (AES) and thermal desorption spectroscopy (TDS) have been used to study the coadsorption of CO and U on polycrystalline Pd. We investigated the modification of the electronic structure of Pd at low U coverages and the mode of CO chemisorption (dissociative vs. associative) as a function of surface U concentration. U adsorption results in the narrowing of the Pd 4d band. At low U coverages the density of states (DOS) at the Fermi level decreases and the U 5f electrons are localized. The CO saturation coverage at room temperature decreases with increasing U surface concentration, while CO adsorption at –165 °C is less affected. This is attributed mainly to a decrease of CO chemisorption energy but also to blocking of Pd adsorption sites by U. A. decrease in the heat of chemisorption for CO is explained by a change of the electronic structure of Pd by U. At low dosage U itself loses most of its reactivity, probably because of U—Pd solid-state bonding and below a critical U surface concentration CO dissociation becomes an activated process. High-temperature reaction between CO and U-covered surfaces leads to the partial transformation of U into a surface oxycarbide. In the presence of this compound some CO is chemisorbed strongly on the surface and is stable even at 300 °C, in contrast to CO on Pd metal. A similar effect has also been observed for classical promoters such as the alkali metals.

On the electronic structure of Pd and Pt overlayers: Example of the Nb(011) substrate

Pd and Pt monolayers on the (011) surface of bcc metals (Nb,Ta,Mo,W) resemble in many respects the faces of noble metals. We show that this behaviour can be reproduced in a simple tight-binding s-d electron model without invoking any additional mechanism.

Electronic Structure of Pd doped InP

ABSTRACTThe electronic structure of metal impurities in III-V semiconductors are interesting because of their ability to create deep centers. We use a spin unrestricted self-consistent local density theory in a cluster model to calculate the charge distributions, one electron energy spectra and the density of states for pure InP and Pd doped InP, substituted at the In site. The pure semiconductor gap is calculated to be 1.44 eV in excellent agreement with the experimental value of 1.41 eV. A small magnetic moment of 0.561μB appears on the Pd site. The impurity introduces a levels of t2 character at 0.01 and 0.04 eV above the valence band edge.