Pd Core Level Research Articles

Study of carbon supported CuPd alloy nanoparticles with Pd-rich surface for the electrochemical formate oxidation and CO2 reduction

Carbon supported copper-palladium alloy nanoparticles (CuPd/C) with Pd-rich surface synthesized via the adsorbate induced surface segregation method were tested toward both the electrochemical oxidation of formate into electricity and CO2, and the reduction of CO2 to produce formate. An upshift of the d-band center and opposite binding energy shifts of Cu and Pd core levels were experimentally observed using X-ray photoelectron spectroscopy (XPS). This perturbation of the surface electronic structure of the CuPd/C nanoparticles leads to the lowering of the bonding strength of key adsorbate species for both the formate oxidation and electrochemical CO2 reduction, which increased the catalytic activity toward both reactions. Amongst the different compositions of CuxPd1-x/C, the 20:80 molar% of Cu:Pd concentration shows an important improvement toward formate oxidation (FO) compared to the other CuxPd1-y compositions and the monometallic Pd. On the other hand, CuxPd1-x/C catalyst with the 60:40 molar% of Cu:Pd concentration shows the highest faradaic efficiency for the electrochemical CO2 reduction toward the formate production. Therefore, by selecting a bimetallic CuxPd1-x/C alloy that (1) optimizes the formate oxidation current density and (2) the faradaic efficiency of the electrochemical reduction of CO2 into formate, a novel sustainable and regenerative power source (i.e., combining a direct formate fuel cell and a CO2 reduction unit into a single regenerative energy system) could be developed.

Electronic structure and chemical properties of Pd films supported by MgO

Pd films were deposited on MgO-covered Mg substrate by the thermal evaporation method under high vacuum conditions. The electronic and chemical properties of Pd structures with different Pd thicknesses were studied using X-ray photoelectron spectroscopy (XPS) and field emission-scanning electron microscopy (FE-SEM). For relatively low Pd thicknesses (<1 nm), separate Pd particles could be observed. At higher Pd thicknesses, continuous Pd thin films formed by agglomeration of Pd nanoparticles were found. With decreasing Pd thickness, a chemical state with a higher-binding-energy became more pronounced in the Pd core level spectra, demonstrating the electron-deficient nature of Pd atoms in contact with MgO substrate. Very small Pd particles with a pronounced core level shift were shown to be less active toward CO oxidation than thicker Pd films.

Electronic structure of PdAg(1 0 0) ordered surface alloys using synchrotron radiation

PdAg(1 0 0) ordered surface alloys have been prepared by e-beam evaporation technique in situ in UHV and were investigated using high energy photoemission and normal incidence X-ray standing wave (NIXSW) techniques with wiggler radiation. The line shapes of 3d core levels of Ag and Pd exhibited interesting changes with the increase of Pd concentration in the alloys. The Ag 3d core level exhibited a disorder induced broadening that gradually increased with Pd concentration. On the other hand Pd 3d core level exhibited an asymmetry on the high binding energy side of the peak which gradually increased with Pd concentration. Interestingly, Pd core level did not exhibit a broadening with increase of Pd content in the alloy.

Core-level shifts for surface bimetallic systems from first-principles theory: Pd-Mn structures on Pd(100)

The surface core-level shift (SCLS) for Pd(100) and Mn-induced Pd core-level shifts (CLSs) for PdMn surface alloys with different configurations formed on Pd(100) are calculated from first-principles theory. The shifts are calculated within the complete screening picture, which includes initial (core-electron energy-eigenvalue) and final state (relaxation due to screening) effects in the same scheme. The theoretical results are compared to recent experimental results for 1 monolayer Mn/Pd(100), formed at low temperature, and the ordered Pd(100)-$c(2\ifmmode\times\else\texttimes\fi{}2)$-Mn surface alloy formed upon annealing. The calculated core-level shifts are in all cases in good agreement with the experimental values. It is furthermore shown that the Pd(100)-$c(2\ifmmode\times\else\texttimes\fi{}2)$-Mn surface alloy as prepared experimentally is metastable, and predictions are made on the CLSs for energetically more stable structures that could be reached with further annealing.

Formation of Palladium Nanoclusters on Y-Zeolite via a Sonochemical Process and Conventional Methods

Abstract The preparation of Pd clusters was investigated by the sonochemical reduction of [Pd(NH3)4]2+ complexes on Y-zeolite in an aqueous solution at 20 °C. Through measurements of the UV–vis and X-ray photoelectron spectra, [Pd(NH3)4]2+ was found to be reduced by radicals formed from 2-propanol sonolysis to yield zero-valent Pd. It was confirmed that the binding energy of the Pd 3d core level on the Y-zeolite was shifted to a higher value in comparison with that of Pd bulk metal. The formation of Pd metal, however, could not be observed by TEM and EXAFS measurements, implying the formation of ultrafine Pd clusters. On the other hand, the results of XPS analyses for several standard Pd clusters of ca. 1nm prepared by the conventional thermal treatment indicated that the peak values of the Pd core level increased with decreasing particle size. The maximum difference in binding energy between these samples was 0.6 eV at the Pd 3d5/2 core level. This phenomenon was suggested to be attributed to a quantum size effect corresponding to a splitting of the band structure. In comparison with standard samples, the size of sonochemically formed Pd clusters was roughly estimated to be less than 1 nm and composed of several dozen Pd atoms.

Role of Pd–Al bimetallic interaction in CO adsorption and catalytic properties of bulk PdAl alloy: XPS, ISS, TDS, and SIMS study

CO adsorption on the bulk PdAl bimetallic systems has been studied using XPS, ISS, TDS, and SIMS under UHV conditions. Pd exhibits strong interaction with Al atoms resulting in the formation of noble metal-like electronic structure of PdAl alloy. The shifts of both Pd core levels and of Pd valence d-band centroid towards higher binding energies compared to bulk Pd were observed. The surface structure of the alloy changes with temperature (Al surface segregation), ion bombardment (preferential Al sputtering), and is also strongly affected by the presence of ambient CO. The Pd–Al bond is weakened upon the interaction with CO, which tends to dissociate on the surface even at room temperature, with carbon fraction bonding to Pd atoms and oxygen to Al. The TDS of CO spectra consisted of one to three desorption peaks, all lying lower than those from Pd foil, indicating a distinct weakening of the Pd–CO chemisorption bond. Moreover, the partial CO dissociation on PdAl was indicated both as adsorption capacity decay and CO 2 and H 2O production.

The electronic structure of PdO found by photoemission (UPS and XPS) and inverse photoemission (BIS)

The electronic structure of the 4d transition-metal oxide PdO is investigated by photoemission (UPS and XPS), inverse photoemission (BIS; ), and electron energy loss spectroscopy in reflection geometry (REELS; primary energy, ). The valence band spectra are compared to recent theoretical ab initio band-structure calculations. Good agreement between theory and experiment is found in the occupied part of the band structure down to 8 eV below as well as in the unoccupied part up to 6 eV above . This confirms the common view that the electronic structure of the 4d transition-metal oxides, e.g. PdO, can be explained in terms of a single-electron picture. Nevertheless correlation effects among the Pd 4d electrons are clearly visible in the spectra, as e.g. satellites of the Pd core level spectra. In order to explain the origin of these satellites we performed simple cluster model calculations and as a result we can explain one satellite in a screening picture by means of a charge transfer process. In addition radiation damage effects in PdO during the electron bombardment in the BIS experiments are reported. This is explained by the formation of the Pd -like states connected with oxygen loss due to the electron bombardment.

Circular dichroism in X-ray photoemission from Pd(111) and CO/Pd(111)

It is shown experimentally that in the soft X-ray region a large circular dichroism in the photoelectron angular distribution (CDAD) exists for both valence orbitals and core levels of CO molecules adsorbed on Pd(111). From theoretical consideration it follows that in the case of a spherically symmetric ground state wave function, like the 1σ and 2σ orbitals of CO, CDAD appears due to the lack of spherical symmetry in the final state. For carbonK-shell experimental results are compared to model calculations. Investigations at the Pd core levels proved that CDAD does also arise in X-ray photo-emission from non-magnetic crystals.

X-Ray Photoelectron Spectroscopic Studies of Palladium Dispersed on Carbon Surfaces Modified by Ion Beams and Plasmatic Oxidation

The effects of ion bombardment and r.f. plasma oxidation of graphite surfaces on subsequent growth and electronic properties of vacuum deposited palladium clusters have been investigated by methods of X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy with X-ray excitation (XAES). Due to the significantly increased density of surface defects on which the nucleation process occurs the bulk value of the Pd 3d core level binding energy is achieved at higher surface coverage by palladium on bombarded surfaces than on ordered graphite. Angle resolved photoelectron spectra of oxidized graphite surfaces reveal significant embedding of oxygen in graphite surface layers. The C 1s and O 1s photoelectron spectra are consistent with presence of two major oxygen species involving C-O and C=O type linkages which are not homogeneously distributed within the graphite surface layers. Two effects were observed on oxidized surfaces: an increase of palladium dispersion and interaction of the metal clusters with surface oxygen groups. Using the simple interpretation of the modified Auger parameter the relaxation and chemical shift contributions to the measured Pd core level shifts are estimated. In the region of low surface coverage by palladium the effect of palladium-oxygen interaction on Pd core level binding energy exceeds the effects of increased dispersity.

Tight-binding study of the electronic structure of Pd and Pt epitaxial overlayers on hcp transition metal surfaces

The electronic structures of Pd and Pt epitaxial monolayers on Re and Ru (0001) surfaces are studied within a self-consistent semi-empirical scheme. To this purpose an s—d-electron tight-binding Hamiltonian is treated by the recursion method. The over-layer d bands are shifted to higher binding energies with respect to the corresponding elemental metal position. No charge transfer between atoms is supposed in our model. The experimental Pd core-level shifts are not far from the calculated chemical shifts although the agreement is not perfect.

Pd-Ti bimetallics: A study of the electronic structure using x-ray photoelectron spectroscopy and x-ray-absorption near-edge structure.

We studied the core-level binding-energy shifts and the valence-band behavior of ${\mathrm{Pd}}_{3}$Ti and ${\mathrm{PdTi}}_{2}$ alloys using x-ray photoelectron spectroscopy (XPS) and x-ray-absorption near-edge structure (XANES). The observed Pd core-level shifts in the alloys, relative to the pure metal, were not consistent with the electronegativity predictions. It is proposed that upon alloying there is an increase in the number of sp-like conduction electrons and a decrease in the number of d electrons at the Pd site. This electron redistribution at the noble-metal site in the alloy results in a small net charge flow between Pd and Ti, in the direction in accord with electronegativity predictions. Pd ${\mathit{L}}_{3}$-edge and Ti K-edge XANES provide further evidence of electron redistribution upon alloying. Net charge transfer at the Pd site of ${\mathrm{PdTi}}_{2}$ has been estimated to be 0.29\ifmmode\pm\else\textpm\fi{}0.02 electron counts on the basis of a charge compensation model, results from XPS binding-energy shift, and XANES white-line areas. The observed Pd valence-band narrowing and shifts to a higher binding energy are also explained in terms of dilution and charge redistribution.

Mg/Pd and Ba/Pd interfaces with and without hydrogen

X-ray photoelectron spectroscopy was used to study alloying and interdiffusion of Mg and Ba overlayers and of their hydrides with Pd and with PdHx subtrates, respectively. Mg and Ba films were prepared by evaporation on Pd foils. To avoid the problem of very low reaction rate of gaseous H2 on alkaline metals we have prepared Mg- and Ba-hydrides by deposition of metallic Mg and Ba, respectively, on previously hydrided Pd-foils. The analysis of spectral changes in core levels of these metals (Mg 1s, Ba 3d and Pd 3d) compared with those of the bulk materials indicated the formation of a 1–3 monolayers thick interface alloy for the Mg/Pd system at 300 K, while the Ba/Pd system exhibited a considerably longer range of intermixing/alloying at 300 K. The range of Ba/Pd interdiffusion decreased at 120 K. Hydrogen diffused from the Pd substrate across the interface into the overlayers and inhibited intermixing of the metal components in both cases. From spectral changes of the Ba, Mg and the Pd core levels on PdHx substrate we conclude that all Ba was transformed into Ba-hydride. Contrary, part of the Mg layers remained metallic. The sticking coefficient of Mg vapour on Mg-hydride turned out to be low.

EXAFS and XPS studies of the influence of metal particle size, nature of support and H2 and CO adsorption on the structure and electronic properties of palladium

Abstract For the gas-phase hydrogenation of vinylacetylene the highly dispersed supported palladium particles show a low turnover number, regardless of the nature of the support (silica, γ-alumina, carbon). The peculiarities of their structural and electronic properties were studied by EXAFS and XPS. Regardless of the nature of the support (SiO 2 , C), and the size of the Pd metal particles (19–108 A), the Pd-Pd interatomic distances are close to those for the bulk metal. The quantitative correlation between turnover number and the binding energy, E b , of the Pd core level was found.

XPS investigation of the oxidation of the Al/Pd interface: The Al 2O 3/Pd interface

We have followed the oxidation of a polycrystalline Pd sample coated with thin aluminum films. The resulting overlayer was composed essentially of an Al 2O 3 overlayer coating the Pd substrate. At low Al doses, the very first layer was formed by PdO. Unlike 3D PdO, this Pd oxide exhibits partial decomposition under UHV at room temperature. Moreover, in all the cases examined here, our photoemission results could be explained only by the existence of small Pd aggregates included in the Al 2O 3 matrix. These clusters revealed several features of their electronic state: the Pd core levels exhibited a surface-like behaviour and their work function appeared to be lower than that of the Pd substrate, depending on their size.

X-ray photoemission lineshapes and energies in PdxCu1-x

Measurements of core and valence level X-ray photoemission spectra are reported for a series of ten PdxCu1-x alloys with varying compositions. The core level binding energy shifts display a distinct dichotomy in behaviour at x approximately=0.4. For x<or=0.4 the Pd shifts are zero within experimental error (+or-0.1 eV), which the authors interpret as resulting from the equality of the Fermi energy of the alloy and of Pd, which implies negligible charge transfer. For x>0.4, the Pd shifts are consistent with a Fermi level different from that of pure Pd. The systematics of the Pd core level lineshapes are shown to be in accordance with this interpretation.