Pd Coverage Research Articles

Vinyl Acetate Synthesis over Model Pd−Sn Bimetallic Catalysts

Pd−Sn bimetallic model catalysts were prepared as alloy films on a Rh(100) substrate via physical vapor deposition. The surface composition, structure, and chemisorption properties were studied by low energy ion scattering spectroscopy (LEIS), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), infrared reflection adsorption spectroscopy (IRAS), and temperature programmed desorption (TPD). The ordered surface alloy of c(2 × 2) was formed after annealing the Pd−Sn mixtures to 700 K as evidenced by LEED and LEIS showing a 50% surface concentration of Pd. This ordered surface arrangement was further confirmed by IRAS, LEED, and TPD studies using CO as a probe molecule, in which the surface Pd atoms are completely isolated by Sn atoms. The surface Pd composition was determined to be 0.5 monolayers (ML). The catalytic properties of this Pd−Sn surface were tested with respect to vinyl acetate (VA) synthesis by ethylene acetoxylation that showed a maximum in the VA formation rate at a surface Pd coverage of 0.5 ML, i.e., a c(2 × 2) surface arrangement. This is consistent with our previous proposal that a pair of suitably spaced, isolated Pd monomers is the more efficient site for VA synthesis.

Pd adsorption on Si(1 1 3) surface: STM and XPS study

Pd-induced surface structures on Si(1 1 3) have been studied by scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS). In the initial process of the Pd adsorption below 0.10 ML, Pd silicide (Pd 2Si) clusters are observed to form randomly on the surface. By increasing the Pd coverage to 0.10 ML, the clusters cover the entire surface, and an amorphous layer is formed. After annealing the Si(1 1 3)-Pd surface at 600 °C, various types of islands and chain protrusions appears. The agglomeration, coalescence and crystallization of these islands are observed by using high temperature (HT-) STM. It is also found by XPS that the islands correspond to Pd 2Si structure. On the basis of these results, evolution of Pd-induced structures at high temperatures is in detail discussed.

Electrocatalytic Reactivity of Pd Monolayers and Monatomic Chains on Au

Nanostructured surfaces were prepared by electrodeposition of Pd monolayers and monatomic chains onto vicinally stepped Au single-crystal surfaces. The regular arrangement of the steps served to obtain an ordered bimetallic surface with the steps acting as template for the deposition of Pd. Products formed during electrocatalytic hydrogenation of simple unsaturated hydrocarbons and oxidation of CO were detected quantitatively by differential electrochemical mass spectrometry. By variation of both Pd coverage and step density of the substrate surface, the formation of different reaction products was attributed to the respective adsorption sites systematically. Pd step sites were found to strongly influence hydrogenation of simple unsaturated organic adsorbates. Hydrogen adsorption at monatomic Pd chains was not observed. Surprisingly, CO adsorbed on Pd terraces was found to be oxidized at a lower potential than CO on Pd steps.

Pd, Co and Co–Pd clusters on the ordered alumina film on NiAl(1 1 0): Contact angle, surface structure and composition

We have investigated the structure and morphology of Co and Pd clusters grown at room temperature on an alumina film on NiAl(1 1 0) by scanning tunneling microscopy, low energy ion scattering and Auger electron spectroscopy. We have also studied the clusters after annealing to 300 °C and Pd clusters deposited at 300 °C. Mixed Co–Pd clusters obtained by sequential deposition at room temperature were also studied. Pure Co deposited at room temperature forms a single type of clusters, most or all of them with close-packed planes parallel to the oxide surface. Their shape can be approximated by truncated spheres with a high contact angle of 115–125°. These clusters are stable upon annealing up to 300 °C. Pd clusters deposited at room temperature grow in two different modes. At the reflection domain boundaries the clusters grow in their thermodynamically favorable shape. The clusters do not have a single crystallographic orientation and their shape can be approximated by a truncated sphere with a high contact angle of about 110°, especially at very low coverages (below 0.05 ML). At the antiphase domain boundaries, the Pd clusters grow in (1 1 1) orientation and on some of them small (1 1 1) facets appear at their tops already at low coverages. For higher coverages of Pd, the majority of Pd clusters are rather flat with a large Pd(1 1 1) facet on top. The clusters’ shape at the antiphase domain boundaries differs from the thermodynamically favorable one, due to kinetic limitations, especially at higher coverages. Annealing the Pd clusters to 300 °C leads to re-structuring of these Pd clusters. They transform into higher and more rounded clusters and a thin disordered alumina film is formed on top of the clusters. When Pd is deposited at 300 °C, about 16% of the Pd clusters have a steep slope and rounded tops. The rest of the Pd forms lower clusters, goes subsurface and is covered by a disordered alumina film. When Co and Pd are deposited sequentially, Pd covers the Co clusters forming a shell. The resulting mixed clusters are still truncated spheres with a lowered contact angle. For deposition in the reverse order (first Pd and then Co) we found that Co forms an alloy with Pd already at room temperature.

A spectro-microscopic study of the reactive phase separation of Au + Pd and O on Rh(1 1 0)

Reorganization of Au + Pd submonolayers on a Rh(1 1 0) surface occurring during the water formation reaction has been observed and characterized by low energy electron microscopy (LEEM) and X-ray photoemission electron microscopy (XPEEM). The results demonstrate segregation of Au + Pd and oxygen into separate surface phases, the morphology and size of the O and Au + Pd patterns being governed by the reaction parameters and adsorbate coverage. At moderate Au + Pd coverages and temperatures in the range 760–860 K, lamellar periodic Au + Pd/O micro-structures are generated. The results are interpreted in terms of kinetic and thermodynamic considerations.

Electrocatalytic hydrogenation and oxidation of aromatic compounds studied by DEMS: Benzene and p-dihydroxybenzene at ultrathin Pd films electrodeposited on Au( hkl) surfaces

Differential electrochemical mass spectrometry (DEMS) was used to investigate the electrocatalytic hydrogenation and oxidation of benzene and p-dihydroxybenzene (hydroquinone, H 2Q) chemisorbed on ultrathin Pd films electrodeposited at Au(332) and Au(111) surfaces. At low sub-monolayer coverages on Au(332), the Pd ad-atoms preferentially adsorb on the step sites. At higher (but still sub-monolayer) coverages, Pd also occupies terrace sites; on Au(111), only terrace sites are available. The electrochemical reactivities of the subject compounds at the Pd-decorated steps and terraces were then compared. No hydrogenation products were detected by DEMS for either benzene or H 2Q, but a considerable degree of electrodesorption of (intact) benzene occurred near the hydrogen-evolution region (HER). Anodic oxidation of both compounds yielded only CO 2 as the volatile (DEMS-detectable) product, although it took up to three anodic cycles to attain exhaustive (complete) oxidation. The anodic oxidation of benzene was also accompanied by potential-induced desorption of (unreacted) benzene particularly in the case of the stepped surface. Electrodesorption at the HER was more facile at the terrace sites than from the step sites; the opposite was true for electrodesorption at the more positive potential. Contrary to hydrogen, which does not adsorb at such monoatomic rows, both aromatic species adsorb when the nominal Pd coverage just corresponds to step decoration.

Nanoscale Tunable Proton/Hydrogen Sensing: Evidence for Surface-Adsorbed Hydrogen Atom on Architectured Palladium Nanoparticles

By gradually decreasing the Pd concentration on Au nanoparticles from complete to incomplete surface coverage, in which well-dispersed nanoparticles are eventually obtained on the Au, we have demonstrated that the electrochemical response of hydrogen at Pd is critically dependent on the size and structure of the Pd surface. A gradual increase in the electrochemical signal attributed purely to the oxidation of adsorbed hydrogen atoms can be achieved as the Pd coverage becomes dispersed and the ratio of hydrogen atom adsorption sites compared to the absorption sites rises.

Atomic-scale studies on the growth of palladium and titanium on GaN(0 0 0 1)

We have used elevated-temperature scanning tunnelling microscopy (STM) to investigate the initial stages of growth of Pd and Ti on GaN(0 0 0 1). Deposition of Pd onto a Ga-rich GaN(0 0 0 1) surface followed by annealing at ∼600 °C leads to the formation of an ordered Pd/Ga alloy layer, followed by the nucleation and growth of hexagonal Pd nanocrystals. When Pd is deposited on a less Ga-rich surface, the Pd initially wets the surface. With increased Pd coverage the wetted 2D islands coalesce into 3D islands without a wetting layer. The 3D islands have hexagonal shapes with a flat (1 1 1) top surface, and STM images of the nanocrystal tops show that they are reconstructed. A separate series of experiments was performed investigating Ti deposition on GaN(0 0 0 1), where the Ti reacts with GaN to form TiN. Deposition of Ti onto GaN(0 0 0 1) followed by annealing in UHV results in the formation of 2D irregular TiN islands, which can be transformed into more regular triangular TiN nanocrystals upon annealing in an ammonia atmosphere. This study shows overall that the growth modes of Pd and Ti are strongly influenced by the stoichiometry and cleanliness of the GaN substrate. Therefore, to create reliable electrical contacts between metals and GaN, the atomic level order and stoichiometry of the GaN substrates needs to be controlled.

Buildup of the InSe/M interface (M [dbnd] Pd, Au) studied by X-ray photoemission and X-ray absorption spectroscopy

The electronic properties of InSe/M (M Pd, Au) interfaces have been studied by X-ray photoemission measurements. For the InSe/Pd interface, it has been found that Pd atoms diffuse into the InSe lattice at early stages of Pd coverage, acting as acceptor centers. As the Pd coverage increases, a Pd–InSe reaction determines the electronic behaviour of the interface. However, for Pd coverages higher than 1 ML, the barrier formation tends to be controlled by an emerging bulklike Pd overlayer. Despite the atomic structure of this system is far from that expected for an ideal Schottky one, the final electronic barrier value is close to that expected for an abrupt InSe/Pd Schottky interface. On the contrary, the InSe/Au system appeared to behave as a quasi-ideal abrupt Schottky interface. Annealing processes performed at temperatures higher than 600 K alter this scheme, as revealed by X-ray absorption spectroscopy measurements, enhancing diffusion of Au atoms into InSe. In any case, the electronic barrier results to be determined by the Au overlayer formed.

Infrared reflection absorption study of carbon monoxide adsorption on Cu(1 0 0)– c(2 × 2)–Pd surfaces formed by palladium vacuum-depositions at various temperatures

Using infrared reflection absorption spectroscopy (IRRAS) and temperature programmed desorption (TPD), we investigated carbon monoxide (CO) adsorption and desorption behaviors on atomic checkerboard structures of Cu and Pd formed by Pd vacuum deposition at various temperatures of Cu(1 0 0). The 0.15-nm-thick Pd deposition onto a clean Cu(1 0 0) surface at room temperature (RT) showed a clear c(2 × 2) low-energy electron diffraction (LEED) pattern, i.e. Cu(1 0 0)– c(2 × 2)–Pd. The RT-CO exposure to the c(2 × 2) surfaces resulted in IRRAS absorption caused by CO adsorbed on the on-top sites of Pd. The LEED patterns of the Pd-deposited Cu(1 0 0) at higher substrate temperatures revealed less-contrasted c(2 × 2) patterns. The IRRAS intensities of the linearly bonded CO bands on 373-K-, 473-K-, and 673-K-deposited c(2 × 2) surfaces are, respectively, 25%, 22%, and 10% less intense than those on the RT-deposited surface, indicating that Pd coverages at the outermost c(2 × 2) surfaces decrease with increasing deposition temperature. In the initial stage of the 90-K-CO exposure to the RT surface, the band attributable to CO bonded to the Pd emerged at 2067 cm −1 and shifted to higher frequencies with increasing CO exposure. At saturation coverage, the band was located at 2093 cm −1. In contrast, two distinct bands around 2090 cm −1 were apparent on the spectrum of the 473-K-deposited surface: the CO saturation spectrum was dominated by an apparent single absorption at 2090 cm −1 for the 673-K-deposited surface. The TPD spectra of the surfaces showed peaks at around 200 and 300 K, which were ascribable respectively to Cu–CO and Pd–CO. Taking into account the TPD and IRRAS results, we discuss the adsorption–desorption behaviors of CO on the ordered checkerboard structures.

The Interaction between Pd and CeO2(111) surface: A first-principle study

The adsorption properties of Pd on CeO2(111) surface are studied using the first principle projector-augmented-wave (PAW) method based density functional theory (DFT) within generalized gradient approximation (GGA) and with the inclusion of on-site Coulomb interaction (DFT+U). It is found that there exist different adsorption features for different coverages of Pd: (1) For one monolayer (ML) Pd adsorption on CeO2(111) surface, Pd prefers to be adsorbed on the atop O site leaning toward the Ce-bridge site; while, for the 1/4 ML adsorption, Pd prefers to be adsorbed on the O-bridge site leaning toward the atop subsurface O site. (2) The interaction between the adsorbed Pd atoms is very strong when the coverage is one ML; on the other hand, there is almost no interaction between the Pd atoms for the 1/4 ML Pd adsorption, correspondingly, the interaction between Pd adatoms and CeO2(111) substrate is stronger for lower coverage adsorption (1/4 ML) than that for higher coverage adsorption (1 ML). (3) The adsorption of Pd disturbes the CeO2(111) surface structure in the vicinity of the adsorption site. (4) The Pd adsorption makes the Pd/CeO2 more active as compared with the clean CeO2(111) and bulk Pd metal, and there may exist some active sites at the Pd/CeO2 interface. These studies may lead to a better understanding for the Pd/CeO2 catalysts and give some hints to improve the efficiency of TWC.

Scanning tunneling microscopy study of Pd growth on Ge(001)

The interaction of Pd with the Ge(001) surface was studied as a function of Pd coverage and temperature using scanning tunneling microscopy. At 300K, initial Pd adsorption displaces Ge from the surface which then combines with additional Pd to form tetramers. At least some of the Pd moves subsurface inducing dimer vacancy complexes. As the temperature was increased, Pd–Ge alloy phases were observed on the surface. At 475K, these formed small three-dimensional clusters that were seen together with two-dimensional Ge islands created from Ge ejected from the surrounding vacancy rich terraces. When several Pd monolayers were deposited at 675K, larger faceted clusters were observed on the surface. High-resolution images revealed a hexagonal structure on the facets with the periodicity of a (3×3)R30° reconstruction of Pd2Ge(0001). This reconstruction was assigned to Pd adtrimers that make the Pd2Ge(0001) surface stoichiometric. At higher temperatures most of the Pd tended to move subsurface. After depositing 100 ML Pd at 300K and annealing to 1030K, however, very large three-dimensional clusters were observed. Atomic resolution images of the surfaces of these clusters revealed a nearly rectangular surface unit cell consistent with a (121−2) reconstruction of the PdGe(110) surface. Unlike recent results for Au and Pt on Ge(001), under no conditions were ordered metallic chains observed. The results were more similar to Ag on Ge(001) where three-dimensional cluster formation was also favored. The results reinforce recent suggestions that relativistic effects in the electronic structure of the 5d metals make them behave more like each other in low coordination than the corresponding 4d metals.

Alloying and dealloying in pulsed laser deposited Pd films on Cu(100)

Using surface x-ray diffraction we have studied the geometric structure of ultrathin Pd films grown on Cu(001) at room temperature by pulsed laser deposition in the coverage regime between 0.4 and about 4 monolayers (ML). Up to about 2 ML, the interface formation is characterized by an alloying-dealloying mechanism, where Pd atoms are incorporated into the Cu substrate for less than half-filled layers, but expelled if the Pd coverage is close to a complete layer. In this case the top layer is composed of Pd. Above 2 ML, Pd agglomeration sets in characterized by Pd-rich alloy layers covered by Pd layers. Interlayer spacings linearly depend on the Pd concentration $(x)$ in the ${\mathrm{Pd}}_{x}{\mathrm{Cu}}_{1\ensuremath{-}x}$ alloy layers in agreement with continuum elasticity considerations. Our results have important implications for modeling strain relaxation.

Methane conversion and SO 2 resistance of LaMn 1− xFe xO 3 ( x = 0.4, 0.5, 0.6, 1) perovskite catalysts promoted with palladium

Lanthanum perovskites were synthesized by malic acid complexation and different amounts of palladium were added by dry-wet impregnation. Measurements of methane conversion on fresh and SO 2-treated perovskite catalysts were made between 25 and 500 °C. Methane conversion on fresh catalysts commenced at ∼300 °C, and the best activities were achieved with low loadings of palladium. The B-site metal combinations Mn 0.4Fe 0.6 and Mn 0.6Fe 0.4 showed better conversion activity than Mn 0.5Fe 0.5 and Fe. All fresh perovskite catalysts except those with 10% Pd gave 100% conversion of methane at 500 °C. The resistance of 10% Pd perovskite catalysts to sulfur was generally better than that of catalysts with low Pd loadings. This was due to the high Pd coverage on the perovskite surface. Fresh and SO 2-treated LaMn 0.4Fe 0.6O 3 perovskite catalysts promoted with 2 and 2.5% of palladium exhibited the highest activities for methane conversion.

The nature of the active site for vinyl acetate synthesis over Pd–Au

The surface composition of a Pd–Au alloy was determined using low energy ion scattering spectroscopy (LEISS). A stable surface composition was found between 700 and 1000 K with substantial enrichment in Au compared to the bulk. Infrared reflection absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) were used to investigate the surface adsorption sites and ensembles. The isolated Pd sites, PdAu 6 for Pd–Au on Mo(1 1 0) and for Pd/Au(1 1 1), and PdAu 4 for Pd/Au(1 0 0), were observed by controlling the Pd amount and the annealing temperature. Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of Au in a Pd–Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two, non-contiguous, suitably spaced Pd monomers. The results show that the role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product while inhibiting the formation of undesirable reaction by-products.

Low temperature CO induced growth of Pd supported on a monolayer silica film

Nucleation, growth and sintering of Pd deposited on an ultra-thin silica film were studied by scanning tunneling microscopy and infrared reflection absorption spectroscopy. No preferential nucleation of Pd on the silica surface was observed both at 90 and 300 K deposition. When adsorbed on Pd clusters formed at 90 K, CO causes a strong sintering effect even at this temperature. The results are rationalized on the basis of a high mobility of Pd carbonyl-like species on the silica film. At a given Pd coverage, the extent of CO induced sintering cannot be achieved by annealing in vacuum. In addition, vacuum sintering, which commences above 700 K, goes simultaneously with interdiffusion of Pd and support.

The epitaxial growth of Pd electrodeposition on Au(1 0 0) studied by LEED and RHEED

The epitaxial growth of Pd adlayers electrochemically deposited onto Au(1 0 0) has been studied by LEED, RHEED and AES. For the first 6 ML, the Pd deposits grow pseudomorphically on Au(1 0 0) with a lateral expansion of 4.5% with respect to bulk Pd. The strain in the expanded commensurate (1 × 1) Pd layers on Au(1 0 0) begins to be relieved at the Pd coverage between 6 and 9 ML range by formation of a compressed Pd film with respect to Au(1 0 0) surface and the compression increases continuously with thickness. At ca. 20 ML Pd the lattice constant of the film approaches to the bulk Pd and three-dimensional Pd islands develop since around 30 ML coverage. No superstructure due to the Pd–Au surface alloy can be found for coverages from monolayer up to 30 ML Pd on Au(1 0 0). A c(2 × 2) phase has been observed on the Pd-deposited Au(1 0 0) electrodes, which is ascribed to an ordered Cl adlayers adsorbed on Pd adlayers rather than a Pd–Au surface alloy.

The Promotional Effect of Gold in Catalysis by Palladium-Gold

Acetoxylation of ethylene to vinyl acetate (VA) was used to investigate the mechanism of the promotional effect of gold (Au) in a palladium (Pd)-Au alloy catalyst. The enhanced rates of VA formation for low Pd coverages relative to high Pd coverages on Au single-crystal surfaces demonstrate that the critical reaction site for VA synthesis consists of two noncontiguous, suitably spaced, Pd monomers. The role of Au is to isolate single Pd sites that facilitate the coupling of critical surface species to product, while inhibiting the formation of undesirable reaction by-products.

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).

CO adsorption and thermal stability of Pd deposited on a thin FeO(1 1 1) film

We have studied adsorption of CO on Pd deposited on a thin FeO(1 1 1) film grown on a Pt(1 1 1) substrate using temperature programmed desorption, infrared reflection absorption spectroscopy and molecular beam techniques. At sub-monolayer Pd coverage, where formation of Pd(1 1 1) single layer islands upon heating is suggested on the basis of scanning tunneling microscopy studies, the CO desorption temperature is 70–100 K lower than on a Pd(1 1 1) single crystal, which agrees well with the results of ab initio calculations. However, annealing to 600 K results in strong reduction of the CO adsorption capacity, which is rationalized in terms of significant Pd migration through the film as revealed by angular resolved photoelectron spectroscopy measurements using synchrotron radiation. The results demonstrate the complexity of Pd interaction with the thin FeO(1 1 1) film at elevated temperatures.