Pd Monolayer Research Articles

Importance of Ligand Effect in Selective Hydrogen Formation via Formic Acid Decomposition on the Bimetallic Pd/Ag Catalyst from First-Principles

The critical role of the Ag–Pd ligand effect (which is tuned by changing the number of Pd atomic layers) in determining the dehydrogenation and dehydration of HCOOH on the bimetallic Pd/Ag catalysts was elucidated by using the spin-polarized density functional theory (DFT) calculations. Our calculations suggest that the selectivity to H2 production from HCOOH on the bimetallic Pd/Ag catalysts strongly depends on the Pd atomic layer thickness at near surface. In particular, the thinnest Pd monolayer in the Pd/Ag system is responsible for enhancing the selectivity of HCOOH decomposition toward H2 production by reducing the surface binding strength of specific intermediates such as HCOO and HCO. The dominant Ag–Pd ligand effect by the substantial charge donation to the Pd surface from the subsurface Ag [which significantly reduce the density of state (particularly, dz2–r2 orbital) near the Fermi level] proves to be a key factor for the selective hydrogen production from HCOOH decomposition, whereas the expansive (tensile) strain imposed by the underlying Ag substrate plays a minor role. This work hints on the importance of properly engineering the surface activity of the Ag–Pd core–shell catalysts by the interplay between ligand and strain effects.

Influence of strain and metal thickness on metal-MoS₂ contacts.

MoS2 and other transition metal dichalcogenides are considered as potential materials in many applications including future electronics. A prerequisite for these applications is to understand the nature of the MoS2 contact with different metals. We use semi-local density functional theory in conjunction with dispersion corrections to study the heterostructures composed of Pd and Pt monolayers with (111) orientation grown pseudomorphically on MoS2(001). The interface properties are mapped as a function of the number of deposited overlayers, as well as a function of tensile and compressive strains. Although we show that the dependence of the contacts on strain can be fully explained using the d-band model, we find that their evolution with the number of deposited metal layers is markedly different between Pd and Pt, and at variance with the d-band model. Specifically, the Pt/MoS2 heterostructures show an anomalous large stability with the deposition of two metal monolayers for all investigated strains, while Pd/MoS2 exhibits a similar behavior only for compressive strains. It is shown that the results can be rationalized by accounting for second-nearest-neighbor effect that couples MoS2 with the subsurface metal layers. The underpinnings of this behavior are attributed to the larger polarizability and cohesive energy of Pt compared to Pd, that leads to a larger charge-response in the subsurface layers.

Electron Transfer Kinetics at Co Porpyrin-Modified Electrodes with Controlled Interfaces

Electrocatalysis has been extensively studied in the fields of both fundamental research and applications such as fuel cells. Chemical modification of an electrode surface is quite beneficial to non-catalytic metals to introduce catalytic activity for a particular electrochemical reaction. For example, metalloporphyrin-modified electrodes are known to act as electrocatalysts for oxygen reduction reaction (ORR). Chemical design of molecular catalysts is clearly one of key issues to realize highly efficient electrocatalysts. However, the electron transfer rate at such a chemically modified electrode is significantly affected by electronic coupling between the metal and molecular catalysts. In this work, a gold electrode surface was self-assembly modified with Cobalt porphyrin (CoTPP) having ORR activity. To control the connection between Au and CoTPPs, bridge monolayers were formed on the electrode using various thiol- or isocyanide-molecules, and CoTPPs were then immobilized on top of the bridge layers. For isocyanide-monolayers, the electrode-molecule junctions were also controlled by forming Pd- or Pt-atomic layers on the Au surface. The electron transfer rate for ORR was thoroughly investigated for such various CoTPP-modified electrodes using the rotating disk electrode (RDE) method. The junction between the electrode and the molecular monolayers was controlled as shown in Fig. (a). The Pd monoatomic layer on an Au surface (Au/PdML) was prepared by the underpotential deposition (UPD) method. The Au and Au/PdML electrodes were self-assembly covered with TPDI (4,4’-terphenyl diisocyanide) molecules. As shown in Fig. (b), the adsorption geometry of TPDIs, measured by PM-IRRAS, was on-top configuration on Au and bridge on Au/PdML. The 2120 cm-1 peak found on both electrodes was attributed to be the unbound NC streching mode. Figure (c) shows Tafel plots for ORR on the TPDI-CoTPP monolayer formed on each electrode. Electron transfer rate on the Au/PdML was much faster than that on the Au. The elementary step of the electron transfer between O2molecule and CoTPP is not affected much by the presence of the Pd monolayer, as the Tafel slop is nearly identical for both plots. Therefore, this result indicates that the electron transfer rate at the electrode-molecule interface was significantly enhanced by controlling the adsorption geometry, leading to the enhancement of the ORR activity of the electrode.Figure (a) Schematic illustration of TPDI-CoTPP monolayers on metal electrodes. (b) PM-IRRAS spectra of TPDI-SAMs on Au and Au/PdML electrodes. (c) Tafel plots for the ORR on TPDI-CoTPP monolayer formed on Au and Au/PdML electrodes.

Volcano-like behavior of Au-Pd core-shell nanoparticles in the selective oxidation of alcohols.

Gold-palladium (AuPd) nanoparticles have shown significantly enhanced activity relative to monometallic Au and Pd catalysts. Knowledge of composition and metal domain distributions is crucial to understanding activity and selectivity, but these parameters are difficult to ascertain in catalytic experiments that have primarily been devoted to equimolar nanoparticles. Here, we report AuPd nanoparticles of varying Au:Pd molar ratios that were prepared by a seed growth method. The selective oxidation of benzyl alcohol was used as a model reaction to study catalytic activity and selectivity changes that occurred after varying the composition of Pd in bimetallic catalysts. We observed a remarkable increase in catalytic conversion when using a 10:1 Au:Pd molar ratio. This composition corresponds to the amount of Pd necessary to cover the existing Au cores with a monolayer of Pd as a full-shell cluster. The key to increased catalytic activity derives from the balance between the number of active sites and the ease of product desorption. According to density functional theory calculations, both parameters are extremely sensitive to the Pd content resulting in the volcano-like activity observed.

Catalytic properties of Au electrodes modified by an underlayer of Pd

Bimetallic catalysts have manifold technological applications; their reactivity can greatly exceed that of the original single metals. In this work, we investigate an ideal model-system consisting of a complete monolayer of Pd underneath the surface of Au(111). First, we investigate the stability of this system, and then its interaction with hydrogen. We analyze in detail the energetics and the electronic interactions for the Volmer step of the hydrogen evolution reaction (HER), and the further absorption below the surface layer of gold. We combine Density Functional Theory based on computational techniques with the theory of electrocatalysis.

Role of caveolin-1 in pulmonary microvascular endothelial cells injury induced by lipopolysaccharide in rat

To investigate the role of caveolin-1 (Cav-1) in the modulation of rat pulmonary microvascular endothelial cells (RPMVEC) injury induced by lipopolysaccharide (LPS). Cultured RPMVEC were randomly divided into time-dependent injury group induced by LPS and intervention group in which cells were pretreated by protein kinase A inhibitor (PKI). In the time-dependent injury group, monolayers of cells were constructed to determine permeability changes after 10 μg/mL LPS challenge for 0, 1, 3, 6, 12 and 24 hours with the method of Evans blue-labeled albumin flux across the monolayer (Pd). Western blotting was used to determine the Cav-1 expression after LPS stimulation and the phosphorylation-Cav-1 (p-Cav-1) expression after LPS challenge for 0, 10, 30, 60, 90, 120 minutes. In the intervention group, after pre-treatment with 10 μmol/L PKI for 30 minutes, RPMVECs were challenged with 10 μg/mL LPS, and the expression of p-Cav-1 was determined 30 minutes after LPS challenge, the permeability and the Cav-1 protein expression were assessed by Pd and Western blotting, respectively. Non-stimulation group and single PKI simulation group served as controls. Western blotting revealed that the expression of Cav-1 protein was elevated at 1 hour (2.97 ± 0.07), peaking at 3 hours (3.77 ± 0.37), then it lowered gradually, but it was still higher at 24 hours (1.45 ± 0.18) when compared with 0 hour group (1.12±0.08) with significant differences (F=178.047, P=0.000). After RPMVEC monolayers were challenged by LPS for different periods (0, 1, 3, 6, 12 and 24 hours), there were significant increases in a time-dependent manner in Cav-1 expression in the permeability as measured by Pd [(99.67 ± 4.32)%, (118.17 ± 2.32)%, (159.00 ± 2.61)%, (141.17 ± 2.64)%, (120.17 ± 2.79)% and (108.83 ± 2.04)%, F=345.869, P=0.000] which was similar to the changes in Cav-1 expression. LPS also increased Cav-1 phosphorylation in a time-dependent manner occurring at 10 minutes (2.41 ± 0.11), peaking at 30 minutes (2.83 ± 0.10), and then it decreased gradually, finally returned to basal levels (1.03±0.04) at 120 minutes (1.04 ± 0.04) after LPS treatment with significant difference (F=519.417, P=0.000). When PKI was pre-treated with RPMVEC the expression of Cav-1 was significantly increased (5.07 ± 0.22 vs. 3.81 ± 0.23, P<0.01), and p-Cav-1 (3.93 ± 0.23 vs. 2.77±0.10, P<0.01), and RPMVEC monolayers permeability was enhanced [(184.17 ± 5.49)% vs. (151.50 ± 3.08)%, P<0.01] after being challenged. Up-regulated expression of Cav-1 and phosphorylation-Cav-1 that may be modulated by protein kinase A signal pathway plays an important role in RPMVEC permeability injury as induced by LPS.

Is platinum necessary for efficient hydrogen evolution? – DFT study of metal monolayers on tungsten carbide

In this work WC-supported metal monolayers (Cu, Ru, Rh, Pd, Ag, Ir, Pt and Au) are investigated using Density Functional Theory in order to establish general trends regarding monolayer stability, electronic structure and reactivity. Using calculated hydrogen–metal bond energies and available data on the exchange current densities (j0) for hydrogen evolution reaction (HER) volcano-type curve is obtained enabling prediction of HER j0 for the entire series of MML/WC systems not considered so far as HER electrocatalysts. Among investigated surfaces, CuML/WC(0001) and RhML/WC(0001) are identified as promising HER electrocatalysts with (i) HER exchange current density matching the one of Pt and (ii) stability in electrochemical environment under HER conditions. Provided results point to a general conclusion that Pt might not be necessary for efficient catalysis of hydrogen electrode reactions – superior catalysts can be obtained by rational design approach with suitable choice of overlayer/support system not involving Pt at all.

Monolayer palladium supported on molybdenum and tungsten carbide substrates as low-cost hydrogen evolution reaction (HER) electrocatalysts

Active and low-cost hydrogen evolution reaction (HER) electrocatalysts are needed to minimize capital costs associated with large-scale hydrogen production from water electrolysis. Catalysts based on monolayer (ML) amounts of precious metals supported on carbides are a promising concept for this purpose. In the current study Pd supported on tungsten carbide (WC) and molybdenum carbide (Mo2C) were evaluated for HER activity. Carbide foils were synthesized using temperature programmed reaction of W or Mo in a CH4/H2 atmosphere. Physical vapor deposition was used to deposit Pd on WC or Mo2C while X-ray Photoelectron Spectroscopy (XPS) was used to determine the Pd surface coverage. Linear sweep voltammetry and chronopotentiometry were used to evaluate the HER activity and electrochemical stability of the catalysts, demonstrating the possibility of using ML Pd on either WC or Mo2C as active, stable and lower-cost HER catalysts.

Growth of magnetic nanowires on self-organized stripe templates: Fe on Pd–O/W(110)

The structural and magnetic properties of Fe-Pd nanostructures grown on regular alternating Pd and oxygen stripes on the W(110) surface are studied using SPELEEM methods. The self-organized Pd-O stripe template, formed at about 1000 °C and aligned with the [001] direction, has an average period of only few tens nm and is preserved upon quenching the sample temperature to ambient conditions. Fe is shown to preferentially bond to the Pd stripes, forming an Fe-Pd surface alloy within the stripe phase. The magnetic easy axis is found to be aligned perpendicular to the Fe-Pd stripe axis.

Graphene moiré structure grown on a pseudomorphic metal overlayer supported on Ru(0001)

A versatile method is demonstrated to modify graphene–metal interaction by overlaying a pseudomorphic monolayer of transition metal on the substrate metal. Using this method, sample-sized, high-quality graphene has been prepared on a pseudomorphic monolayer of Co and Pd deposited on Ru(0001) respectively, and studied by scanning tunneling microscopy, low energy electron diffraction, Auger electron spectroscopy, and density function theory calculations. Graphene develops moiré patterns on the two pseudomorphic monolayer surfaces, with a periodicity that is identical to that of graphene on Ru(0001) but completely different from graphene on Co(0001) and Pd(111). STM measurements, supported by DFT calculations, indicate that graphene on the two surfaces exhibits distinctly different corrugation from that of graphene on Ru(0001), in order of decreasing height CoML/Ru>Ru>PdML/Ru, suggesting that this method can be used to tune the interaction strength between graphene and a metal substrate.

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.

Magnetic coupling of single Co adatoms to a Co underlayer through a Pd spacer of variable thickness

We have performed a combined experimental and theoretical investigation of Co atoms on top of a Pd spacer on Co/Ir(111). Using spin-polarized scanning tunneling microscopy and spectroscopy we first studied the morphology and the spin-resolved electronic properties of mono- and double-layer Pd on Co/Ir(111). We find pseudomorphic growth with two stackings of the Pd monolayer, which can easily be distinguished electronically by measuring the differential tunneling conductance. Spin contrast is achieved on both the Pd mono- and double-layer and on Co adatoms on top, which means that the electron density above the surfaces and adatoms is spin-polarized. Indeed, based on our ab initio calculations, we find that the surface Pd atoms of mono- and double-layer carry an induced magnetic moment of about 0.3 and 0.2 ${\ensuremath{\mu}}_{\mathrm{B}}$, respectively; Co layers adsorbed on top of a Pd mono- and double-layer possess a magnetic moment of about 2.0 ${\ensuremath{\mu}}_{\mathrm{B}}$/atom. In accordance with our experimental observations of Co atoms on the Pd spacer, the calculations also show that Co adlayers are ferromagnetically coupled to the Co/Ir(111) underneath. The size of the magnetic exchange coupling is reduced by a factor of three from a mono- to double-layer Pd spacer between the Co layers.

A UPD-spontaneous redox approach to the preparation of monolayer palladium on reduced graphene oxide-supported gold nanoparticles for ethanol electrooxidation in alkaline media

Abstract Monolayer Pd on electrochemical-reduced graphene oxide supported Au nanoparticles (m-Pd–Au/ERG) was designed by a simple and effective route, which produces electrocatalyst with a considerably low Pd loading and good electrocatalytic performance for ethanol oxidation. The as-prepared m-Pd–Au/ERG nanocompostie was characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy. The results show that high-density m-Pd–Au nanoparticles are dispersed on the surface of ERG uniformly. The electrochemical activities of the as-prepared nanocomposites toward ethanol oxidation in alkaline media were investigated by using cyclic voltammetry and chronoamperometric technique. The results demonstrate that the m-Pd–Au/ERG catalyst shows much higher electrocatalytic activity, stronger tolerance to CO than the m-Pd–Au and b-Pd–Au/ERG catalysts for ethanol oxidation.

The Adsorption of Oxygen and Coadsorption of CO and Oxygen on Structurally Well‐Defined PdAg Surface Alloys

The dissociative interaction of oxygen with structurally well-defined monolayer Pd(x)Ag(1-x)/Pd(111) surface alloys of different compositions, with well-known distributions of the respective surface atoms (A. K. Engstfeld et al., Phys. Chem. Chem. Phys. 2012, 14, 10754-10761), and the coadsorption of/reaction with CO on oxygen pre-covered surfaces were studied by high-resolution electron energy loss spectroscopy (HREELS) and temperature-programmed desorption/reaction spectroscopy (TPD/TPR). The impact of geometric ensemble effects as well as electronic ligand and strain effects on the adsorption and reaction behaviour of the respective species on the bimetallic surfaces is elucidated and compared with related systems such as CO adsorption on similar surfaces and oxygen adsorption on a Pd(67)Ag(33)(111) bulk alloy surface. The data show a clear dominance of ensemble effects on the oxygen adsorption and CO coadsorption behaviour, with oxygen adsorption limited to threefold-hollow sites on Pd(3) sites, while the combined electronic effects, as evident from modifications in the adsorption and reaction characteristics on the Pd sites, are small.

Reduced Pd density of states in Pd/SAM/Au junctions: the role of adsorbed hydrogen atoms

Experiments have shown that a Pd monolayer deposited electrochemically on a Au-supported self-assembled monolayer (SAM) of 4-mercaptopyridine (Mpy) exhibits a strongly reduced Pd local density of states (LDOS) at the Fermi energy (E(f)). Understanding the origin of this modified electronic structure is crucial for the use of the sandwich design as a platform for future nanoelectronics. Here we suggest that hydrogen adsorption might be the origin of the modified electronic properties. We performed periodic density functional theory calculation to explore the influence of hydrogen adsorption on the geometric and electronic structure of a Pd/Mpy/Au(111) complex. Dissociative adsorption of H(2) on a Pd monolayer on top of a Mpy SAM is a strongly exothermic process leading to atomic hydrogen atoms preferentially located at the hollow sites. Due to the formation of a strong Pd-H bond the Pd-SAM interaction realized via one-fold N-Pd bonds is substantially weakened. Upon hydrogen adsorption, the Pd LDOS becomes significantly modified exhibiting a drastic reduction of the density of states at E(f). The calculated spectra are in a good agreement with the experiment for a hydrogen coverage corresponding to two monolayers which is still thermodynamically allowed.

Formation, atomic distribution and mixing energy in two-dimensional PdxAg1−x surface alloys on Pd(111)

The formation and atom distribution in two-dimensional Pd(x)Ag(1-x)/Pd(111) monolayer surface alloys were studied by high resolution scanning tunnelling microscopy (STM) with chemical contrast. From short-range order (SRO) parameters, we calculate preferences for like or unlike nearest neighbours to elucidate the mixing behaviour of the two components for various sub monolayer Ag surface contents. In the regime of low Ag surface contents (<40% Ag), the system shows a weak tendency towards phase separation, high Ag coverages (>60% Ag) result in a disperse distribution of the atoms in the surface. Effective pair interactions (EPIs) were derived by comparing the measured distribution with distributions obtained using Monte Carlo (MC) simulations. From the EPIs, we derived a function for the mixing energy, which can describe the change from clustering to a disperse distribution. The effects of the resulting surface atom distributions and of the Ag coverage dependent surface mixing/demixing on catalytic reactions are discussed.

Synthesis of bimetallic Pt-Pd core-shell nanocrystals and their high electrocatalytic activity modulated by Pd shell thickness

Bimetallic Pt-Pd core-shell nanocrystals (NCs) are synthesized through a two-step process with controlled Pd thickness from sub-monolayer to multiple atomic layers. The oxygen reduction reaction (ORR) catalytic activity and methanol oxidation reactivity of the core-shell NCs for fuel cell applications in alkaline solution are systematically studied and compared based on different Pd thickness. It is found that the Pd shell helps to reduce the over-potential of ORR by up to 50 mV when compared to commercial Pd black, while generating up to 3-fold higher kinetic current density. The carbon monoxide poisoning test shows that the bimetallic NCs are more resistant to the CO poisoning than Pt NCs and Pt black. It is also demonstrated that the bimetallic Pt-Pd core-shell NCs can enhance the current density of the methanol oxidation reaction, lowering the over-potential by 35 mV with respect to the Pt core NCs. Further investigation reveals that the Pd/Pt ratio of 1/3, which corresponds to nearly monolayer Pd deposition on Pt core NCs, gives the highest oxidation current density and lowest over-potential. This study shows for the first time the systematic investigation of effects of Pd atomic shells on Pt-Pd bimetallic nanocatalysts, providing valuable guidelines for designing high-performance catalysts for fuel cell applications.

Surface structure determination of Pd on W(100) using X-ray photoelectron diffraction

Ferromagnetic ordering in non-ferromagnetic transition metals induced by structural effects such as lattice expansion and stacking faults have been predicted by several theoretical studies. Different surfaces such as Nb(100), W(100) and C(0001) have been suggested as potential templates which could drive a Pd ultrathin film to a stable ferromagnetic state, depending on growth packing, film thickness and lateral lattice parameter. In this work we present an X-ray photoelectron diffraction (XPD) study of sub-monolayer to a few layers of Pd grown on W(100) and its surface structure determination. XPD patterns of Pd 3d and W 4f peaks were measured using synchrotron radiation with photon energy of 1810eV provided by the SXS beamline at the Brazilian Synchrotron Light Laboratory. Our results indicate that the first few monolayers of Pd grow on W(100) with bcc packing and lateral lattice parameter dramatically changed in comparison with the Pd(fcc) bulk values. Based on these results we also present a discussion on the magnetic properties of this system derived from spin-polarized DFT calculation.

Hydrogen Electrocatalysis on Single Crystals and on Nanostructured Electrodes

We investigate hydrogen evolution on plain and nanostructured electrodes with a theory developed by us. On electrodes involving transition metals the most strongly adsorbed hydrogen is often only a spectator, while the reaction proceeds via a weakly adsorbed species. For Pt(111) the isotherms for both species are calculated. We explain why a nanostructure consisting of a monolayer of Pd on Au(111) is a good catalysts, and predict that Rh/Au(111) should be even better. Our calculations for a fair number of metals are in good agreement with experiment.

Characterization of Ultra-Thin Films of Pd Deposited on Au(111)

Ultra-thin films (1 and 3 monolayers) of Pd were deposited on the Au(111) surface and then characterized by X-ray photoelectron spectroscopy (XPS), X-ray excited Auger spectroscopy (XAES), low-energy electron diffraction (LEED), and X-ray photoelectron diffraction (XPD). For the 1 ML Pd film annealed at 450 °C, XPS and XAES results indicated that Pd had diffused into the Au substrate. For the 3 ML Pd film deposited at room temperature, the comparison between experimental and theoretical XPD results indicated approximately 30% of the surface was formed by 2 ML Au layers, and 70% of the surface, by 1 ML Au layers.