Pd Atomic Layers Research Articles

A Computational Perspective on Carbon-Carbon Bond Formation by Single Cu Atom on Pd(111) Surface for CO Electrochemical Reduction

This study focuses on the computational characterization of electrochemical C-C bond formation through the CO and CHO coupling process utilizing a dioxo-coordinated Cu single atom site ([CuO2]*) supported on a Pd(111) surface. The stable intermediate, [CuO2]*(CO)2, was identified as a tetradentate-and-tetrahedral species formed upon exposure to CO gaseous molecules. Electrochemically, the hydrogenation of the carbonyl group to CHO was found to be 0.87 eV, conceivably lower than the corresponding step for conventional Cu surfaces. This study observed a considerable charge transfer effect from the top layer of Pd atoms to the adsorbate moiety, especially at the TS structure. This phenomenon resulted in an accessible C-C bond formation barrier at 0.67 eV. Furthermore, the reaction energy of C-C bond formation was found to be exothermic at −0.21 eV, indicating a favorable chemical equilibrium condition. Considering the temperature effect and pressure of the gaseous molecules (CO, CO2, O2), the [CuO2]*(CO)2 intermediate was substantially populated at room temperature and was found to be chemically resilient under dry ambient conditions, as suggested by the kinetic modeling results.

Palladium atomic layers coated on ultrafine gold nanowires boost oxygen reduction reaction

Palladium-based nanocatalysts play an important role in catalyzing the cathode oxygen reduction reaction (ORR) for fuel cells working under alkaline conditions, but the performance still needs to be improved to meet the requirements for large-scale applications. Herein, Au@Pd core–shell nanowires have been developed by coating Pd atomic layers on ultrafine gold nanowires and display outstanding electrocatalytic performance towards alkaline ORR. It is found that Pd overlayers with atomic thickness can be coated on 3 nm Au nanowires under CO atmosphere and completely cover the surfaces. The obtained ultrafine Au@Pd nanowires exhibit an electrochemical active area (ECSA) of 68.5 m2/g and a mass activity of 0.91 A/mg (at 0.9 V vs. RHE), which is around 3.1 and 15.2 times higher than that of commercial Pd/C. The activity loss of the ultrafine Au@Pd nanowire after 10,000 cycles of accelerated degradation tests is only ∼20 %, demonstrating its much better stability compared to commercial Pd/C. Further characterizations combined with density functional theory (DFT) calculations demonstrate that the electronic interactions between Pd atomic layers and underlying Au can increase the electronic density of Pd and promote the efficient activation of oxygen, thus leading to the improved ORR performance.

Perpendicular magnetic anisotropy induced by 6p atomic layers: Crucial role of interface structural order

Two-dimensional atomic layers composed of metallic atoms with a high principal quantum number in valence shells are promising materials for applications in state-of-the-art magnetic devices due to the presence of a strong spin-orbit coupling. In this work we experimentally explored the effects of triggering perpendicular magnetic anisotropy (PMA) of ferromagnetic (FM) films by applying a series of $6p$-Pb, Bi and $4d$-Pd (reference) atomic layers with high-level atomic orbital and low-level lattice strain. Our research indicates that, compared with Pd atoms, Pb and Bi atoms can produce higher-strength PMA on adjacent FM films through orbital hybridization at the interface. Moreover, we demonstrate that the PMA induced by Pb and Bi atoms is highly sensitive to the ordering degree of the wetting layer which is determined by the interplay of surface free, cohesive, and strain energies within the grown materials; the Pb(Bi) atoms added on the wetting layer can enhance the PMA of the adjacent FM film only when the wetting layer maintains an ordered structure. This work clarifies the critical interface effects of $6p$-HMs on the FM layer, thus providing important clues to increase control over the spin-orbit interaction engendered by HMs through the interface.

Pd atomic layer coating enhances CO2 hydrogenation to CH3OH fuel on Cu@Pd(111) core-shell structure catalyst

CO2 hydrogenation to methanol can convert greenhouse gases and hydrogen into high value-added liquid fuels, which can be used as a promising hydrogen storage technology, it is of great significance for mitigating the greenhouse effect and energy crisis. In this paper, density functional theory is used to study the effect of Pd atomic layer coating on the activation energy barrier and selectivity in CO2 hydrogenation to methanol. The Pd atomic layer coating can effectively increase the adsorption strength of the intermediate substance and the catalyst surface, the adsorption energy of H* is increased from − 3.50 to − 3.63 eV, and the adsorption energy of COOH* increases by 0.59 eV, properly increasing the adsorption energy is conducive to the progress of catalysis. The Pd atom coating is conducive to the activation and hydrogenation of CO2 and the dissociation of hydrogen, the activation energy barrier of the rate-limiting step is reduced from 1.71 to 1.00 eV, and the energy barrier of H2 dissociation is reduced by 0.27 eV. Cu@Pd(111) enhanced the electron delocalization and increased the catalytic activity. Besides, Pd atom coating suppresses the production of by-product CO, the adsorption energy of CO* increased from − 0.77 to − 1.76 eV, and the energy barrier for CO* production increased by 0.80 eV, inhibition of by-products is conducive to further improved methanol selectivity.

Interfacial Electron Engineering of Palladium and Molybdenum Carbide for Highly Efficient Oxygen Reduction.

Interfacial electron engineering between noble metal and transition metal carbide is identified as a powerful strategy to improve the intrinsic activity of electrocatalytic oxygen reduction reaction (ORR). However, this short-range effect and the huge structural differences make it a significant challenge to obtain the desired electrocatalyst with atomically thin noble metal layers. Here, we demonstrated the combinatorial strategies to fabricate the heterostructure electrocatalyst of Mo2C-coupled Pd atomic layers (AL-Pd/Mo2C) by precise control of metal-organic framework confinement and covalent interaction. Both atomic characterizations and density functional theory calculations uncovered that the strong electron effect imposed on Pd atomic layers has intensively regulated the electronic structures and d-band center and then optimized the reaction kinetics. Remarkably, AL-Pd/Mo2C showed the highest ORR electrochemical activity and stability, which delivered a mass activity of 2.055 A mgPd-1 at 0.9 V, which is 22.1, 36.1, and 80.3 times higher than Pt/C, Pd/C, and Pd nanoparticles, respectively. The present work has developed a novel approach for atomically noble metal catalysts and provides new insights into interfacial electron regulation.

Revealing Active Sites for Hydrogen Evolution at Pt and Pd Atomic Layers on Au Surfaces.

Identification of the most active surface sites is one of the key tasks in the development of new electrocatalytic materials. This is in many cases both time and resource consuming due to methodological difficulties of in situ detection of centers of this kind. In this work, we use the recently developed approach based on the analysis of the tunneling current noise recorded by electrochemical scanning tunneling microscopy (n-ECSTM) to compare the nature of the most active hydrogen evolution catalytic sites in a system consisting of sub-monolayers of platinum on a Au substrate to the one of palladium on Au. Our n-ECSTM measurements performed under reaction conditions show that in striking contrast to Pd islands on gold, where the most active centers are located close to the boundary between Au and palladium atoms, all Pt ad-atoms contribute to the overall activity rather equally at pH 1. Methodological aspects related to the use of n-ECSTM in electrocatalytic research are also discussed.

Palladium Electrodeposition onto Pt(100): Two-Layer Underpotential Deposition.

Electrodeposition of the first Pd layers onto Pt(100) was investigated using cyclic voltammetry at a low scan rate (0.1 mV·s-1). Ultrathin films were characterized by cyclic voltammetry in 0.1 M H2SO4 solution and with ex situ AFM (atomic force microscopy). For the first time, we evidenced the underpotential character of the deposition of the first two Pd layers, characterized by a two-step mechanism, each step corresponding to the deposition of a complete Pd atomic layer. For thicker deposits, especially above 10 monolayers as equivalent thickness, the electrochemical characterization displays a strong irreversibility and a broadening of the adsorption/desorption peaks, associated with a reduction of long-range ordered flat areas. Ex situ AFM images are in agreement with this description. They show rough thick deposits and the growth of (100)-oriented rectangular shaped islands with their sides aligned with the two [011] and [0-11] perpendicular directions of the (100) Pt surface.

Hydrogen Sorption Kinetics on Bare and Platinum-Modified Palladium Nanofilms, Grown by Electrochemical Atomic Layer Deposition (E-ALD)

Nanofilms of Pd were grown using an electrochemical form of atomic layer deposition (E-ALD) on 100 nm evaporated Au films on glass. Multiple cycles of surface-limited redox replacement (SLRR) were used to grow deposits. Each SLRR involved the underpotential deposition (UPD) of a Cu atomic layer, followed by open circuit replacement via redox exchange with tetrachloropalladate, forming a Pd atomic layer: one E-ALD deposition cycle. That cycle was repeated in order to grow deposits of a desired thickness. 5 cycles of Pd deposition were performed on the Au on glass substrates, resulting in the formation of 2.5 monolayers of Pd. Those Pd films were then modified with varying coverages of Pt, also formed using SLRR. The amount of Pt was controlled by changing the potential for Cu UPD, and by increasing the number of Pt deposition cycles. Hydrogen absorption was studied using coulometry and cyclic voltammetry in 0.1 M H2SO4 as a function of Pt coverage. The presence of even a small fraction of a Pt monolayer dramatically increased the rate of hydrogen desorption. However, this did not reduce the films’ hydrogen storage capacity. The increase in desorption rate in the presence of Pt was over an order of magnitude.

(Invited) Improvement of Electron Transfer Rate at Metal/Organic Interfaces Using Pd Atomic Layers

Chemical modification of a metal electrode is one of promising methods to realize various electrochemical functions. For example, photocurrent generation may be achieved by surface modification of a metal electrode with chromophores. Electrocatalytic activity may be enhanced by immobilization of catalyst molecules on a metal surface. However, these molecule-induced functionalities are strongly affected by metal/organic interface properties. In particular, the electronic conductance at metal/organic interfaces may be one of essential issues to limit the efficiency of the electrochemical functions. In this work, the electron transfer kinetics at metal/organic interfaces was experimentally investigated using Cobalt porphyrin (CoTPP) modified Au electrodes. 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. The electron transfer rate for oxygen reduction reaction (ORR) on CoTPPs was thoroughly investigated for various modified electrodes using the rotating disk electrode (RDE) method. The interface structure between the Au electrode and the molecular layers was controlled with and without forming Pd monoatomic layers (Fig. (a)). The Pd overlayer on Au surface (Au/PdML) was prepared by the underpotential deposition (UPD) method. The Au and Au/PdML electrodes were self-assembly covered with various diisocyanide molecules such as TPDI (4,4’-terphenyl diisocyanide). CoTPPs were attached on these molecular layers. Figure (b) shows Tafel plots for ORR on CoTPPs on SAM-covered Au or Au/PdML electrode. Electron transfer rate on the Au/PdML was much faster than that on the Au while the Tafel slop was not affected by the presence of Pd overlayer. This result indicates that the performance of chemically functionalized electrodes can be improved by the metal/organic interface control. Figure 1

Diffusion of hydrogen interstitials in the near-surface region of Pd(111) under the influence of surface coverage and external static electric fields.

Past scanning tunneling microscopy (STM) experiments of H manipulation on Pd(111), at low temperature, have shown that it is possible to induce diffusion of surface species as well as of those deeply buried under the surface. Several questions remain open regarding the role of subsurface site occupancies. In the present work, the interaction potential of H atoms with Pd(111) under various H coverage conditions is determined by means of density functional theory calculations in order to provide an answer to two of these questions: (i) whether subsurface sites are the final locations for the H impurities that attempt to emerge from bulk regions, and (ii) whether penetration of the surface is a competing route of on-surface diffusion during depletion of surface H on densely covered Pd(111). We find that a high H coverage has the effect of blocking resurfacing of H atoms travelling from below, which would otherwise reach the surface fcc sites, but it hardly alters deeper diffusion energy barriers. Penetration is unlikely and restricted to high occupancies of hcp hollows. In agreement with experiments, the Pd lattice expands vertically as a consequence of H atoms being blocked at subsurface sites, and surface H enhances this expansion. STM tip effects are included in the calculations self-consistently as an external static electric field. The main contribution to the induced surface electric dipoles originates from the Pd substrate polarisability. We find that the electric field has a non-negligible effect on the H-Pd potential in the vicinity of the topmost Pd atomic layer, yet typical STM intensities of 1-2 VÅ(-1) are insufficient to invert the stabilities of the surface and subsurface equilibrium sites.

Effects of Chlorine in Titanium Oxide on Palladium Atomic Layer Deposition

The effect of residue chlorine on the synthesis of well-dispersed Pd nanoparticles on TiO2 supports using Pd atomic layer deposition (ALD) was investigated. The dispersion of Pd nanoparticles was compared over chlorine-containing and chlorine-free TiO2 supports prepared by selecting proper precursors. The detailed X-ray photoelectron spectroscopy and scanning transmission electron microscopy characterizations showed that higher dispersion of Pd nanoparticles was achieved on the chlorine-containing TiO2 surface than the chlorine-free TiO2. The preparation of TiO2 thin films and Pd nanoparticles was characterized by in situ FT-IR. The temperature required for complete deligation of palladium hexafluoroacetylacetonate decreased from 175 to 100 °C with the presence of chlorine on the TiO2 surface. Density functional theory calculations confirm that Pd–ligand bond strength could be weakened as Pd binds to the Cl sites. The water-gas-shift reaction was chosen as the model reaction, and the catalytic performance...

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.

Hydrogen sorption properties of bare and Rh-modified Pd nanofilms grown via surface limited redox replacement reactions

Pd nanofilms were grown using electrochemical atomic layer deposition (E-ALD) and used as a platform for investigations into changes in hydrogen absorption/desorption kinetics as a function of the coverage of Rh. Surface limited redox replacement (SLRR) reactions were used to form Pd atomic layers. That is, Pd atomic layers were grown on polycrystalline Au by first depositing a sacrificial Cu atomic layer using underpotential deposition (UPD), and then exchanging it for PdCl42− ions at open circuit potential (OCP). That cycle was then repeated 15 times to form one of the Pd nanofilms used in this study. Rh was deposited on the 15 cycle Pd films from a RhCl63− solution at constant potential or using an E-ALD procedure, similar to that employed to form the Pd nanofilms. Cyclic voltammetry (CV) of the Pd films modified with various coverages of Rh were compared with unmodified Pd nanofilms. The resulting CVs indicated that the presence of Rh enhanced the rates of hydrogen absorption and desorption into and out of the underlying Pd nanofilm. Rh overlayers formed at 0V for 60s produced the greatest kinetic enhancement. Two possible explanations for the observed behavior are proposed and discussed.

Formation of Palladium Nanofilms Using Electrochemical Atomic Layer Deposition (E-ALD) with Chloride Complexation

Pd thin films were formed by electrochemical atomic layer deposition (E-ALD) using surface-limited redox replacement (SLRR) of Cu underpotential deposits (UPD) on polycrystalline Au substrates. An automated electrochemical flow deposition system was used to deposit Pd atomic layers using a sequence of steps referred to as a cycle. The initial step was Cu UPD, followed by its exchange for Pd ions at open circuit, and finishing with a blank rinse to complete the cycle. Deposits were formed with up to 75 cycles and displayed proportional deposit thicknesses. Previous reports by this group indicated excess Pd deposition at the flow cell ingress, from electron probe microanalysis (EPMA). Those results suggested that the SLRR mechanism did not involve direct transfer between a Cu(UPD) atom and a Pd(2+) ion that would take its position. Instead, it was proposed that electrons are transferred through the metallic surface to reduce Pd(2+) ions near the surface where their activity is highest. It was proposed that if the cell was filled completely before a significant fraction of the Cu(UPD) atoms had been oxidized then the deposit would be homogeneous. Previous work with EDTA indicated that the hypothesis had merit, but it proved to be very sensitive to the EDTA concentration. In the present study, chloride was used to complex Pd(2+) ions, forming PdCl(4)(2-), to slow the exchange rate. Both complexing agents led to a decrease in the rate of replacement, producing more homogeneous films. Although the use of EDTA improved the homogeneity, it also decreased the deposit thickness by a factor of 3 compared to the thickness obtained via the use of chloride.

Investigation of the thermal diffusion during the formation of a quasicrystalline phase in thin Al-Pd-Re films

The layer mixing during the formation of the Al70Pd20Re10 icosahedral quasicrystalline phase in thin (55 nm) Al-Pd-Re layered film systems subjected to vacuum annealing has been studied. It is shown that a combined layer of Pd and Al atoms (with the Al3Pd2 phase dominating) is formed in the first stage (at 350°C), while the rhenium layer remains invariable. In the second annealing stage (at 450°C), the β′-AlPd phase is formed and the Re layer is diffused. In the third stage (700°C), Pd and Re atoms are uniformly distributed throughout the film with the formation of a quasicrystalline phase.

Hydrogen production from formic acid decomposition at room temperature using a Ag–Pd core–shell nanocatalyst

Formic acid (HCOOH) has great potential as an in situ source of hydrogen for fuel cells, because it offers high energy density, is non-toxic and can be safely handled in aqueous solution. So far, there has been a lack of solid catalysts that are sufficiently active and/or selective for hydrogen production from formic acid at room temperature. Here, we report that Ag nanoparticles coated with a thin layer of Pd atoms can significantly enhance the production of H₂ from formic acid at ambient temperature. Atom probe tomography confirmed that the nanoparticles have a core-shell configuration, with the shell containing between 1 and 10 layers of Pd atoms. The Pd shell contains terrace sites and is electronically promoted by the Ag core, leading to significantly enhanced catalytic properties. Our nanocatalysts could be used in the development of micro polymer electrolyte membrane fuel cells for portable devices and could also be applied in the promotion of other catalytic reactions under mild conditions.

Surface poisoning in the nucleation and growth of palladium atomic layer deposition with Pd(hfac)2 and formalin

Palladium (Pd) atomic layer deposition (ALD) can be performed with Pd(hfac)2 (hfac = hexafluoroacetyl-acetone) and formalin as the reactants. For Pd ALD on oxide surfaces, the nucleation of Pd ALD has been observed to require between 20 and 100 ALD cycles. To understand the long nucleation periods, this study explored the surface reactions occurring during Pd ALD nucleation and growth on hydroxylated Al2O3 substrates. In situ Fourier transform infrared (FTIR) spectroscopy on high surface area nanopowders was used to observe the surface species. The adsorption of Pd(hfac)2 on hydroxylated Al2O3 substrates was found to yield both Pd(hfac)* and Al(hfac)* surface species. The identity of the Al(hfac)* species was confirmed by separate FTIR studies of hfacH adsorption on the hydroxylated Al2O3 substrates. Isothermal loss of the Al(hfac)* species revealed second-order kinetics at 448–523K with an activation barrier of Ed=39.4kcal/mol. The lack of correlation between Al(hfac)* and AlOH* species during the loss of Al(hfac)* species suggested that the Al(hfac)* species may desorb as Al(hfac)3. After Pd(hfac)2 exposure and the subsequent formalin exposure on hydroxylated Al2O3 substrates, only hfac ligands from Pd(hfac)* species were removed from the surface. In addition, the formalin exposure added formate species. The Al(hfac)* species was identified as the cause of the long nucleation period because Al(hfac)* behaves as a site blocker. The surface poisoning by Al(hfac)* species was corroborated by adsorbing hfacH prior to the Pd(hfac)2 exposures. The amount of Pd(hfac)* species after Pd(hfac)2 exposures decreased progressively versus the previous hfacH exposure. Pd ALD occurred gradually during the subsequent Pd ALD cycles as the Al(hfac)* species were slowly removed from the Al2O3 surface. Ex situ transmission electron microscopy analysis revealed Pd nanoclusters that grew in size and dispersion with increasing number of Pd ALD cycles. These nanoclusters eventually coalesced to form a continuous Pd ALD film. Surface poisoning by the hfac ligands may help to explain the nucleation difficulties for metal ALD on oxide substrates using β-diketonate reactants.

Determining the Chemical Reactivity Trends of Pd/Ru(0001) Pseudomorphic Overlayers: Core-Level Shift Measurements and DFT Calculations

We have addressed the problem of determining a reliable experimental descriptor of surface chemical reactivity by measuring Pd and Ru 3d(5/2) core-level shifts of the Pd-n/Ru(0001) pseudomorphic overlayer system (n = 0-3) by high resolution X-ray photoelectron spectroscopy. We find a linear relationship between the calculated theoretical changes of the d-band center position projected on each Ru and Pd atomic layer (which is, according to the Hammer and Norskov d-band model, a good theoretical descriptor of chemical reactivity) and the corresponding core-level shifts, both for the Ru atomic planes and for the Pd overlayers. Core-level shifts, therefore, should be considered as reliable experimental descriptors of chemical reactivity in the same sense (and with similar limitations) of the theoretical descriptor d-band center. Final-state effect contributions to the shifts do not obscure this trend.

The electronic theory study of the influence of Pd on the passivation of Ti alloys

The electronic structure parameters of Ti alloys, such as density of states, Fermi energy level, environment-sensitive embedding energy etc, have been calculated by recursion method. The larger environment-sensitive embedding energy of Pd in the bulk of Ti alloy than that on the surface of Ti alloy indicates that Pd is apt to segregate on the surface. The negative formation energy of Pd clusters suggests that the distribution of Pd atoms on the surface of Ti alloy is in the form of atom clusters. The range of the local density of states of Pd is narrow, while that of Ti is wide. So a peak appears in the total density of states of Ti alloys between -20 and -15 eV. This peak has a significant effect on the Fermi level. It makes the Fermi energy of the surface Ti alloys with more Pd atoms low. So the microcells form between the area with more Pd atoms and the area with fewer or without Pd atoms on the surface of Ti alloys. Ti dissolves preferentially in the corrosive media. This leads to form a layer of Pd atomic clusters on Ti alloy surface. The Pd atomic layer serves as the electrocatalytic surface, facilitating the passivation of Ti alloys and improving the corrosion resistance.

Analysis of interfacial magnetic anisotropy in Co∕Pt and Co∕Pd multilayer films

Analysis of Co∕Pt and Co∕Pd multilayer films was performed to investigate the dependence of interfacial magnetic anisotropy (IMA) and induced magnetization on the thickness of the laminated Pd or Pt layers. For the Co∕Pd films, the IMA energy increased as the thickness of the laminated Pd layer was increased up to ∼0.8nm and depended proportionally on the amount of the induced magnetization. This suggests that the whole magnetic moment induced in a few Pd atomic layers contributed to the IMA. In contrast, for the Co∕Pt films, this proportional relationship only held when the thickness of the Pt layer was less than ∼0.2nm. Since one Pt monolayer is 0.225nm thick, the amount of the IMA energy in the Co∕Pt films is proportional to the number of Pt–Co atomic contacts. This difference between Co∕Pt and Co∕Pd multilayer films is attributed to the difference in the latent ferromagnetism between Pt and Pd.