Pseudomorphic Monolayer Of Pd Research Articles

Borohydride electrooxidation reaction on Pt(111) and Pt(111) modified by a pseudomorphic Pd monolayer

The borohydride oxidation reaction (BOR) on Pt(111) electrode modified by the electro-deposition of one pseudomorphic Pd layer (Pd1ML/Pt(111)) has been studied in sodium hydroxide solution using cyclic voltammetry in static and dynamic conditions. The results are compared to those obtained at Pt(111) single crystal. Pd1ML/Pt(111) shows an onset potential slightly shifted negative and the BOR current density is overall larger than observed at Pt(111), demonstrating the beneficial influence of the pseudomorphic Pd monolayer towards the BOR. Both surfaces are however very sensitive to surface poisoning by BHx,ad adsorbed intermediates and deactivation by accumulation of hardly-soluble BOR products (BOx species). The forced convection created by rotation of the RDE can nevertheless limit the accumulation of BOR products in the interfacial region, which results in more sustainable BOR performances. Additionally, a modification of the surface signature in sulfuric acid is observed for the two electrodes after BOR, which could suggest that either BOx or BHads species remain and block the electrode surface, or that the surface has been reconstructed/de-structured in the strong reducing environment of BOR experiments.

Potential of zero free charge of Pd overlayers on Pt(1 1 1)

Differential capacitance measurements of Pd overlayers on a Pt(1 1 1) electrode in dilute aqueous NaF solutions have been performed as a function of film thickness in order to determine the potential of zero free charge (pzfc). The pzfc of the first, pseudomorphic Pd monolayer on Pt(1 1 1) is −0.21 V versus SCE. By increasing the amount of deposited Pd, a clear shift of the pzfc to more positive values is observed. After deposition of an equivalent of 10 monolayers, the value approaches that of a massive Pd(1 1 1) electrode (−0.12 V versus SCE). The pzfc's for the various Pd coverages are correlated with surface structure information, derived from STM images (R. Hoyer, L.A. Kibler, D.M. Kolb, Electrochim. Acta 49 (2003) 63). Variations in the pzfc are discussed in the context of an electronic modification by the underlying substrate and are compared with corresponding data for Pd overlayers on Au(1 1 1).

Alloying of Pd thin films with Nb(0 0 1)

Annealing at elevated temperatures (1000–1600 K) of at least 10 ML thick Pd films deposited on Nb(0 0 1) has been found to result in a substrate capped by a pseudomorphic monolayer of Pd. This 1 ML thick Pd cap layer was characterised with a combination of UPS and DFT-calculations. UPS, RHEED and AES show that this cap layer protects the Nb(0 0 1) surface against (oxygen) contamination, which is a well known problem of Nb substrates. AES sputter profiling indicates that a major part of the Pd material in excess of the pseudomorphic monolayer is dissolved in the Nb lattice just below the surface. XPD shows that these dissolved Pd atoms occupy substitutional sites in the substrate. The analysis of the XPS-anisotropy also provides some information about the concentration and positions of the Pd and Nb atoms in the alloyed samples.

Electronic structure of Pd thin films on Re(0001) studied by high-resolution core-level and valence-band photoemission

The electronic structures of Pd thin films grown epitaxially on a Re(0001) single-crystal surface are investigated with high-resolution photoelectron spectroscopy. A clear splitting of the Pd $3{d}_{5∕2}$ core level is observed as the coverage of Pd increases from submonolayer to multilayer. The peak at higher binding energy is assigned to emission from the Pd layer at the interface with the Re substrate, while the other is from ``bulk'' Pd. The observed valence-band spectrum of the pseudomorphic Pd monolayer on Re clearly revealed a reduction in the density of states near the Fermi level and shifting of the $d$-band center to higher binding energy. It is possible to reconcile the seemingly contradictory core level and valence-band shifts based on the charge-density maps from the earlier full-potential linearized augmented-plane-wave band-structure calculation by Wu and Freeman. Filling of the Pd $d$ band by electron donation from the substrate does not occur. Rather, the correct physical picture for the electronic modification is substrate-induced charge polarization in the Pd layer

Surface (electro-)chemistry on Pt(1 1 1) modified by a Pseudomorphic Pd monolayer

The formic acid and methanol oxidation reaction are studied on Pt(1 1 1) modified by a pseudomorphic Pd monolayer (denoted hereafter as the Pt(1 1 1)–Pd 1 ML system) in 0.1 M HClO 4 solution. The results are compared to the bare Pt(1 1 1) surface. The nature of adsorbed intermediates (CO ad) and the electrocatalytic properties (the onset of CO 2 formation) were studied by FTIR spectroscopy. The results show that Pd has a unique catalytic activity for HCOOH oxidation, with Pd surface atoms being about four times more active than Pt surface atoms at 0.4 V. FTIR spectra reveal that on Pt atoms adsorbed CO is produced from dehydration of HCOOH, whereas no CO adsorbed on Pd can be detected although a high production rate of CO 2 is observed at low potentials. This indicates that the reaction can proceed on Pd at low potentials without the typical “poison” formation. In contrast to its high activity for formic acid oxidation, the Pd film is completely inactive for methanol oxidation. The FTIR spectra show that neither adsorbed CO is formed on the Pd sites nor significant amounts of CO 2 are produced during the electrooxidation of methanol.

Kinetics of Oxygen Reduction on an Epitaxial Film of Palladium on Pt(111),

The kinetics of the oxygen reduction reaction on epitaxial thin films of Pd on Pt(111) was studied using the rotating ring-disk electrode technique. The results for the pseudomorphic monolayer of Pd are compared to those of the unmodified Pt(111) surface, both surfaces having nominally identical 2D structures. In an electrolyte containing a nonabsorbing anion, 0.5 M HClO4, the pseudomorphic layer of Pd has a somewhat lower (ca. factor of 2) activity than Pt(111). In an electrolyte containing an anion which is strongly adsorbing on Pt(111), 0.5 M H2SO4, the inhibition due to anion absorption is even greater on the pseudomorphic monolayer of Pd than on the unmodified Pt(111) surface. This additional inhibition is attributed to the difference in the potential of zero charge (pzc) between the two surfaces, the Pt(111) having the more positive pzc.

Theoretical analysis of hydrogen chemisorption on Pd(111), Re(0001) andPdML/Re(0001),ReML/Pd(111)pseudomorphic overlayers

Gradient-corrected density-functional theory (DFT-GGA) periodic slab calculations have been used to analyze the binding of atomic hydrogen on monometallic Pd(111), Re(0001), and bimetallic ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001)$ [pseudomorphic monolayer of Pd(111) on Re(0001)] and ${\mathrm{Re}}_{\mathrm{ML}}/\mathrm{P}\mathrm{d}(111)$ surfaces. The computed binding energies of atomic hydrogen adsorbed in the fcc hollow site, at 100% surface coverage, on the Pd(111), Re(0001), ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001),$ and ${\mathrm{Re}}_{\mathrm{ML}}/\mathrm{P}\mathrm{d}(111)$ surfaces, are -2.66, -2.82, -2.25, and -2.78 eV, respectively. Formal chemisorption theory was used to correlate the predicted binding energy with the location of the d-band center of the bare metal surfaces, using a model developed by Hammer and N\o{}rskov. The DFT-computed adsorption energies were also analyzed on the basis of the density of states (DOS) at the Fermi level for the clean metal surfaces. The results indicate a clear correlation between the d-band center of the surface metal atoms and the hydrogen chemisorption energy. The further the d-band center is from the Fermi level, the weaker is the chemisorption bond of atomic hydrogen on the surface. Although the DOS at the Fermi level may be related to the location of the d-band, it does not appear to provide an independent parameter for assessing surface reactivity. The weak chemisorption of hydrogen on the ${\mathrm{Pd}}_{\mathrm{ML}}/\mathrm{R}\mathrm{e}(0001)$ surface relates to substantial lowering of the d-band center of Pd, when it is pseudomorphically deposited as a monolayer on a Re substrate.

Growth mechanism and structure of ultrathin Pd films vapor-deposited on Ta(110)

The structure and growth of Pd films formed by vapor-deposition on Ta(110) have been studied using AES and LEED over a wide range of substrate temperatures. The thermal stability of monolayer, bilayer and trilayer Pd was examined also. At 300 K, the growth of Pd on Ta(110) is best described by a Frank-van der Merwe (layer-by-layer) growth mode. Initially, the first Pd layer grows in 2D pseudomorphic islands up to a Pd coverage of θ Pd = 0.65 (defined relative to the atomic density of the (111) surface of bulk Pd, θ Pd = 1). Subsequent deposition of Pd atoms induces a phase structural phase transition; for 0.65 < θ Pd< 1.0 the monolayer growth proceeds through formation of 2D islands of Pd fcc (111) structure. At 125 K, the growth mode of Pd/Ta(110) is similar, but seems to deviate slightly from the layer-by-layer mode. For a substrate temperature of 500 K the growth mechanism changes. The first layer is still pseudomorphic initially, but the phase transition to the Pd fcc(111) structure occurs at θ Pd = 0.82. After completion of the Pd monolayer, additional Pd deposition results in the formation of 3D crystallites 2–3 layers thick on top the first Pd layer, i.e., a Stranski-Krastanov growth mode occurs at 500 K. Annealing studies show that the pseudomorphic Pd monolayer is stable to very high temperatures (up to 1350 K), but the growth of Pd crystallites occurs upon heating thicker Pd layers above 370 K.

Ｐｄ／Ｍ（Ｍ：Ｗ，Ｍｏ，Ｔａ，Ｎｂ）原子界面とＰｄ成長層のＦＩＭ観察

The growth processes and the atomic structure of Pd ultra-thin films vapor deposited on the substrate metals (W, Mo, Ta, Nb), and a hydrogen interaction with the Pd monolayer film were investigated by a field ion microscope (FIM) with atomically high resolution. The Pd monolayer on the substrate metals exhibits a pseudomorphic structure which has an atomic arrangement identical to the substrate and is strongly bound with the substrate metal atoms. The desorption field strengths of the pseudomorphic Pd monolayer on W (011), Mo (011), Ta (011) and Nb (011) planes were measured. The fields depend strongly on the evaporation fields of the substrate metals and are higher than the evaporation field of Pd. In the growth of the Pd films, the two different Pd structures are observed. The observed structures are the Pd overlayer grown with the Pd inherent atomic structure and the stable pseudomorphic Pd underneath. Hydrogen interactions with the pseudomorphic Pd monolayer grown on W (011) and Mo (011) planes were also studied.

Chemisorption of hydrogen on ultrathin Pd films on Mo (100)

The H 2 adsorption properties of ultrathin Pd films deposited on Mo(100) have been studied with Auger electron spectroscopy (AES) and temperature-programmed desorption (TPD). The pseudomorphic monolayer of Pd on Mo(100) has a greatly reduced H 2 uptake capacity at 150 K compared to bulk Pd. Desorption from the Pd monolayer occurs from a chemisorption state with a desorption temperature of about 290 K. Thicker Pd films also have a reduced H 2 uptake, until θ Pd≥5.5. H 2 desorption from the low-temperature hydride state that is characteristic of bulk Pd only occurs for Pd films with θ Pd≥5.5.

Chemisorption of CO on ultrathin films of Pd on Mo(100)

The altered bonding that exists at bimetallic interfaces can affect the properties of metal monolayers and thin films. We have probed the chemisorptive properties of ultrathin films of Pd on Mo(100) by studies of CO adsorption on these surfaces. Our investigations were carried out using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), temperature programmed desorption (TPD), and high resolution electron energy loss spectroscopy (HREELS). The heat of adsorption of CO is reduced from 36.5 kcal/mol on Pd(100) single crystal surfaces to 20 kcal/mol on the pseudomorphic Pd monolayer. In addition, a 34% reduction in the initial sticking probability of CO occurs on the Pd monolayer at 150 K relative to thick (20 layers) Pd films. Both the heat of adsorption and the sticking probability of CO increase with Pd film thickness indicating that the chemistry of these surfaces can be “tuned”. However, the origin of this tuning is not clear. On the Pd monolayer, HREELS shows that hollow and bridging sites are populated first, followed by atop sites at higher CO coverages. This is consistent with the site preference on bulk Pd(100) surfaces. The bonding of CO at bridging sites is clearly destabilized on the Pd monolayer and to a lesser extent on thicker films. The vibrational spectra of chemisorbed CO are shown to be a sensitive probe of the structure of Pd films of this type. The alterations in CO adsorption on the Pd monolayer and thin films on Mo(100) compared with that on bulk Pd surfaces are consistent with those observed for ultrathin Pd films on other early transition metal substrates such as Nb, Ta, and W. CO bonding on the pseudomorphic monolayer of Pd on Mo(100), which has a highly strained PdPd lattice, is not weakened as much as on the relatively less strained Pd monolayers on Nb(110) and Ta(110), indicating that lattice strain is not sufficient to account for the origin of the weakened CO interactions.

The interaction of ultrathin films of Ni and Pd with W(110): an XPS study

The interaction of ultrathin films of Ni and Pd with W(110) has been examined using X-ray photoelectron spectroscopy (XPS) and the effects of annealing temperature and adsorbate coverage (film thickness) are investigated. The XPS data show that the atoms in a monolayer of Pd or Ni supported on W(110) are electronically perturbed with respect to the surface atoms of Pd(100) and Ni(100). The magnitude of the electronic perturbations is larger for Pd than for Ni adatoms. Our results indicate that the difference in Pd(3 d 5 2 ) XPS binding energies between a pseudomorphic monolayer of Pd on W(110) and the surface atoms of Pd(100) correlates with the variations observed for the desorption temperature of CO (i.e., the strength of the PdCO bond) on these surfaces. A similar correlation is seen for the Ni(2 p 3 2 ) XPS binding energies of Ni/W(110) and Ni(100) and the CO desorption temperatures from the surfaces. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of: (1) variations that occur in the NiNi and PdPd interactions when Ni and Pd adopt the lattice parameters of W(110) in a pseudomorphic adlayer; and (2) transfer of electron density from the metal overlayer to the W(110) substrate upon adsorption. Measurements of the Pd(3 d 5 2 ) XP binding energy of Pd/W(110) as a function of film thickness indicate that the PdW interaction affects the electronic properties of several layers of Pd atoms.