Pd Shell Research Articles

Effect of Au and Ru in Au@Pd and Ru@Pd Core@Shell Nanoparticles for Direct Formic Acid Fuel Cells

The study of Pd-based core@shell bimetallic catalysts with Pd as the shell and another transition metal as the core has generated great interest in the field of PEM fuel cells because their electronic and chemical properties are completely different from pure Pd. As the anode catalyst in direct formic acid fuel cells (DFAFCs), an enhanced activity has been obtained for these Pd-based core@shell nanoparticles compare to pure Pd for formic acid oxidation [1,2]. However, there is a lack of understanding of how these added transition metals interact with Pd, namely by influencing their electronic structure and chemical property toward electrochemical oxidation of formic acid. It is important to fill this knowledge gap and find out a trend between the effect of the transition metals and the activity. This will help developing more efficient and less cost DFAFCs.In this study, Au@Pd and Ru@Pd core@shell nanoparticles with same thickness of Pd are synthesized via a two-step process. Non-supported nanoparticles are used for TEM and X-ray photoelectron spectroscopy (XPS) analysis. TEM is used to characterize the structure and morphology of the nanoparticles. XPS is used to differentiate the chemical shifts due to charge transfer between Pd and Au (or Ru). XPS analysis shows a different binding energy shift for Pd peak of Au@Pd and Ru@Pd compare to that of pure Pd nanoparticles. In addition, a change of the XPS valence band shape and down-shift of the d-band center are also observed. This different binding energy shift indicates that Au and Ru core interact with Pd shell and significantly influence the electronic structure of Pd. Electrochemical measurements (cyclic voltammetry and chronoamperometry) are used to investigate the electrochemical activity of carbon supported Au@Pd and Ru@Pd core@shell nanoparticles toward formic acid oxidation. The Pd loading of the carbon supported samples is 40wt%. Based on CV results shown in Figure 1, Au@Pd shows the best activity toward formic acid oxidation, which is about twice higher than that of Pd/C. The relationship between the electronic perturbation and electrochemical activity will be elucidated in this paper. Furthermore, a real direct formic acid fuel cell test will be also operated with these Au@Pd and Ru@Pd core@shell nanoparticles under the various fuel cell operating conditions.

Electrochemical Properties of Gold-Palladium Core-Shell Nanostructures for Ethanol Oxidation

Gold-palladium nanoparticles (NPs) have been used as catalysts due to their catalytic activity, selectivity and stability for a variety of reactions, such as the small organic molecule oxidations (i.e. ethanol). In the present contribution, the catalytic activity of Pd and Au-Pd core-shell (CS) NPs is investigated towards the ethanol oxidation (Figures 1A and 1B). The Au-Pd CS NPs were synthesised by a seed-mediated growth method, where PdCl4 2- is reduced at the surface of Au NPs in the presence of ascorbic acid. This method allows adjusting the average Pd shell thickness between 1 (CS1) and 10 (CS10) nm, by varying the amount of palladium chloride. Electrochemical studies in potassium hydroxide (KOH) containing electrolyte solutions with ethanol show that the current density of ethanol oxidation decreases as the shell thickness decreases. Correlation between catalytic activity and shell thickness will be discussed in terms of the structural parameters.

Facile Synthesis of Bimetallic Au@Pd Nanoparticles with Core-shell Structures on Graphene Nanosheets

In this paper, we report a simple one-step thermal reducing method for synthesis of bimetallic [email protected] nanoparticles with core-shell structures on the graphene surface. This new type of [email protected]–G composites is characterized by transmission electron microscopy, high resolution transmission electron microscopy, X-ray photoelectron spectroscopy and X-ray diffraction. It is found that [email protected] nanoparticles with an average diameter of 11 nm are well dispersed on the graphene surface, and the Au core quantity as well as the Pd shell thickness can be quantitatively controlled by loading different amounts of metallic precursors, and the involved core-shell structure formation mechanism is also discussed. The ternary Pt/[email protected]–G composites can also be synthetized by the subsequent Pt doping. The catalytic performance of [email protected]–G composites toward methanol electro-oxidation in acidic media is investigated. The results show that [email protected]–G composites exhibit higher catalytic activity, better stability and stronger tolerance to CO poisoning than Pd–G and Au–G counterparts.

Synthesis and electrocatalytic activity of Au@Pd core-shell nanothorns for the oxygen reduction reaction

Bimetallic core-shell nanostructures with porous surfaces have drawn considerable attention due to their promising applications in various fields, including catalysis and electronics. In this work, Au@Pd core-shell nanothorns (CSNTs) with rough and porous surfaces were synthesized for the first time through a facile co-chemical reduction method in the presence of polyallylamine hydrochloride (PAH) and ethylene glycol (EG) at room temperature. The size, morphology, and composition of Au@Pd CSNTs were investigated by transmission electron microscopy (TEM), X-ray diffraction (XRD), energy dispersive spectroscopy (EDX), EDX mapping, and X-ray photoelectron spectroscopy (XPS). The electrochemical properties of as-synthesized Au@Pd CSNTs were also studied by various electrochemical techniques. Au@Pd CSNTs exhibited remarkably high electrocatalytic activity and durability for the oxygen reduction reaction (ORR) in the alkaline media, owing to the unique porous structure and the synergistic effect between the Au core and Pd shell.

Ni@Pd core–shell nanoparticles modified fibrous silica nanospheres as highly efficient and recoverable catalyst for reduction of 4-nitrophenol and hydrodechlorination of 4-chlorophenol

In this study, a novel fibrous nano-silica (KCC-1) based nanocatalyst (Ni@Pd/KCC-1) was synthesized by modifying KCC-1 using Ni@Pd core–shell nanoparticles (NPs). The Ni@Pd/KCC-1 was used in the catalytic reduction of 4-nitrophenol (4-NP) and hydrodechlorination (HDC) of 4-chlorophenol (4-CP). KCC-1, prepared by hydrothermal method, exhibited a dandelion-like shape, high surface area, and easy accessibility of active sites. The Ni@Pd NPs possessed a magnetic nickel (Ni) core with palladium (Pd) shell structural composite. Thus, use of Ni led to the reduced consumption of Pd without sacrificing the overall catalytic performance, simultaneously making it reusable as it could be conveniently recovered from the reaction mixture by using an external magnetic field. Immobilization of the Ni@Pd NPs on KCC-1 nanospheres not only prevented their aggregation, but also significantly enhanced the accessibility of the catalytic active sites. The Ni@Pd/KCC-1 nanocatalyst displayed excellent catalytic activities for both the reduction of 4-NP and the HDC of 4-CP under green conditions. The above-mentioned approach based on fibrous KCC-1 and Ni@Pd NPs provided a useful platform for the fabrication of noble metal-based cost-effective nanocatalyst with easy accessibility, and acted as a promising candidate for numerous catalytic applications.

Methanol and ethanol oxidation on carbon supported nanostructured Cu core Pt–Pd shell electrocatalysts synthesized via redox displacement

Abstract Six different carbon-supported Cu core Pt–Pd shell (Cu@Pt–Pd) catalysts have been successfully synthesized by the galvanic replacement of Cu atoms by Pt4+ and Pd2+ ions at room temperature and their electrocatalytic activity for methanol and ethanol oxidation have been evaluated in acid media. Cu@Pt–Pd core shell nanoparticles with a narrow size distribution and an average diameter in the range of 3.1–3.5 nm were generated onto the carbon support. The compositional and the structural analysis of the as-prepared materials pointed out that the nanoparticles are formed by a Cu rich core covered by a Pt–Pd rich shell due to the interdiffusion of the metals after the galvanic replacement reaction. The electrocatalytic properties of the Cu@Pt–Pd electrodes in the electro-oxidation of methanol and ethanol was found to be dependent on the electrochemical surface area, lattice strain of the surface, composition and thickness of the Pt–Pd shell surrounding the Cu core. The optimum catalyst composition to obtain the best performance for methanol and ethanol electro-oxidation was determined to be Pt0.59Pd0.324Cu0.167/C (6.2 wt.% Pt, 2.2 wt.% Pd and 0.7 wt.% Cu). This catalyst has a greatly enhanced mass activity, lower onset potential and poisoning rate, and higher turnover number in the MOR and EOR reactions compared to a commercial Pt0.51Ru0.49/C (20 wt.% Pt and 10 wt.% Ru). Consequently, this simple preparation method is a viable approach to making a highly active catalyst with low platinum content for application in direct alcohol fuel cells (DAFCs).

Influence of shell thickness on thermal stability of bimetallic Al–Pd nanoparticles

Aluminum-based bimetallic core–shell nanoparticles have shown promising applications in civil and defense industries. This study addresses the thermal stability of aluminum–palladium (Al–Pd) core/shell nanoparticles with a varying shell thickness of 5, 6, and 7 A, respectively. The classic molecular dynamics (MD) simulations are performed in order to investigate the effects of the shell thickness on the ignition mechanism and subsequent energetic processes of these nanoparticles. The histograms of temperature change and structural evolution clearly show the inhibition role of the Pd shell during ignition. While the nanoparticle with a thicker shell is more thermally stable and hence requires more excess energy, stored as the potential energy of the nanoparticle and provided through numerically heating, to initiate the thermite reaction, a higher adiabatic temperature can be produced from this nanoparticle, thanks to its greater content of Pd. The two-stage thermite reactions are discussed with their activation energy based on the energy balance processes during MD heating and production. Analyses of the simulation results reveal that the inner pressure of the core–shell nanoparticle increases with both temperature and the absorbed thermal energy during heating, which may result in a breakup of the Pd shell.

Controlled overgrowth of Pd on Au nanorods

In this paper, we present a detailed, systematic study of the controlled overgrowth of Pd on Au nanorods. Pd nanoshells with fine-controlled dimensions and architectures were overgrown on single-crystalline Au nanorods through seed-mediated growth using H2PdCl4 as the Pd precursor and ascorbic acid as the reducing agent in the presence of cetyltrimethylammonium chloride (CTAC) or cetyltrimethylammonium bromide (CTAB) as the surface capping agent. The effects of surface capping agent, ascorbic acid to H2PdCl4 ratio, reaction temperature, and structure-directing foreign ions, such as Ag+, on the dimensions and architectures of the resulting Au@Pd core–shell heteronanostructures were systematically studied. At 30 °C, Au nanorods coated with polycrystalline Pd shells were obtained using CTAC as the surface capping agent, while single-crystalline Au@Pd core–shell nanocuboids formed in the presence of CTAB. The thicknesses of the polycrystalline and single-crystalline Pd shells were fine-controlled by adjusting the molar ratio of ascorbic acid to H2PdCl4. At an elevated reaction temperature of 60 °C, irregularly shaped and cylindrical co-axial core–shell nanorods were obtained in CTAC and CTAB, respectively. By introducing Ag+ ions into the Pd growth solution, Au nanorods coated with segregated Pd nanoislands and dumbbell-like core–shell heteronanostructures were obtained as a consequence of the underpotential deposition of Ag and sustainable galvanic replacements that concurred during the Pd overgrowth processes. The effects of the Pd shell dimensions and morphologies on the plasmonic properties of the Au@Pd core–shell nanostructures were also investigated.

Catalytic Activity of Pt Monolayer on Electrodeposited W-Ni Nanoparticles for the Oxygen Reduction Reaction

Proton Exchange Membrane Fuel Cells (PEMFCs) are promising solution for ever growing concern about fossil fuel availability and related environmental effects. However, high Pt catalyst loading in the cathodes is required due to the sluggishness of the oxygen reduction reaction (ORR) and Pt dissolution under fuel cell operating conditions, causing high cost of PEMFCs. The concept of Pt monolayer (PtML) electrocatalysts [1] offers a possible solution to these problems because it provides ultimate reduction in Pt loading and its complete utilization. We recently showed that Pt monolayer shell deposited on Pd or PdAu alloy core possesses higher activity and stability than pure Pt electrocatalysts [2]. Further reduction in nobel metal content can be achieved by using a less noble core that is stable under the oxidizing conditions of the ORR.Low cost refractory metals, such as W and/or its alloys with other transition metals, are excellent candidates as a material core for PtMLshell electrocatalysts, since they possess a good corrosion resistance under the acidic operating conditions of PEMFCs. It is well known that W can only be electrodeposited from aqueous solutions by induced co-deposition with iron group metals [3]. The morphology, composition, and the properties of the deposits depend strongly from the solution composition, pH, temperature and current density. In addition, the microstructure and the morphology of electrodeposited WNi binary nanocrystalline alloys could be tailored by applying galvanostatic or pulse electrochemical protocols.In this work we electrodeposited WNi alloys from citric bath solution at room temperature directly on functionalized carbon layer of gas diffusion layer (GDL) with geometric are of 25 cm2. Both, constant current mode and pulse deposition strategies are used to control the microstructure and the composition of the deposits. As obtained WNi nanostructures contained up to 50 at.% W, and possessed composite nanocrystalline - amorphous microstructure and very fine particles size distribution of 2-3 nm. Figure 1 presents the chronopotenciometric curve of deposited WNi nanocrystalline alloys at constant current of 3 mA/cm2. It is seen that the electrodeposition takes less than 7 minutes. The deposition is performed in a specially designed flow type cell that allows exchanging of the electrolytes. After rinsing the cell a Pd containing solution is introduced and part of the Ni atoms from the WNi alloys are galvanically displaced by Pd. Thus, we obtained Pd shell on WNi nanostructures. In consecutive steps we applied a well-established Cu UPD assisted protocol for PtML deposition on Pd [4] and obtained PtML/PdWNi/GDL electrocatalyst with total precious metal loading of 50 μg/cm2. As prepared electrode is assembled with Nafion 211 membrane and a standard anode with Pt loading of 100 μg/cm2, by hot pressing. The polarization curve measured on as fabricated MEA is presented in Figure 2. The mass activity of the catalyst of interest is 0.42 A/cm2, which is in good agreement with the US DOE targets for 2017. Furthermore, the accelerated stability test at 80 oC, showed that this MEA possessed an excellent stability. Figure 1. Chronopotenciometric curve of electrodeposited WNi nanoparticles directly on GDL at content current of 3 mA/cm2.Figure 2. Polarization curve measured on MEA prepared from PtML/Pd/WNi/GDL cathode, Nafion 211 membrane, and standard Pt anode with loading of 100 μg/cm2.AcknowledgmentWork at Brookhaven National Laboratory (BNL) is supported by US Department of Energy, Division of Chemical Sciences, Geosciences and Biosciences Division, under the Contract No. DE-AC02-98CH10886.

Enhanced Direct Oxidation of Formic Acid and Its Underlying Mechanism in the Ag-Pd Core-Shell Nanocatalyst: A First-Principles Study

Formic acid is an excellent in situ source of hydrogen for fuel cells, for it offers high energy density and remains as a liquid at standard temperature and pressure. So far, there has been a lack of reports regarding the underlying mechanism in enhancing the direct oxidation of formic acid on the bimetallic nano-catalyst. In this study, using spin-polarized density functional theory we examine the reactivity and selectivity of Ag-Pd core shell catalyst toward direct oxidation of formic acid. We find that the ligand effect in Ag-Pd core shell catalyst plays an important role in determining direct oxidation of formic acid rather than the strain effect. In particular, the thinnest Pd shell catalyst can boost the hydrogen production from direct oxidation of formic acid at room temperature and this result provides possibilities for the development of fuel cell devices.

Designer Titania-Supported Au–Pd Nanoparticles for Efficient Photocatalytic Hydrogen Production

Photocatalytic hydrogen evolution may provide one of the solutions to the shift to a sustainable energy society, but the quantum efficiency of the process still needs to be improved. Precise control of the composition and structure of the metal nanoparticle cocatalysts is essential, and we show that fine-tuning the Au-Pd nanoparticle structure modifies the electronic properties of the cocatalyst significantly. Specifically, Pd(shell)-Au(core) nanoparticles immobilized on TiO2 exhibit extremely high quantum efficiencies for H2 production using a wide range of alcohols, implying that chemical byproducts from the biorefinery industry can be used as feedstocks. In addition, the excellent recyclability of our photocatalyst material indicates a high potential in industrial applications. We demonstrate that this particular elemental segregation provides optimal positioning of the unoccupied d-orbital states, which results in an enhanced utilization of the photoexcited electrons in redox reactions. We consider that the enhanced activity observed on TiO2 is generic in nature and can be transferred to other narrow band gap semiconductor supports for visible light photocatalysis.

Facile Synthesis of AuPd Nanochain Networks on Carbon Supports and Their Application as Electrocatalysts for Oxygen Reduction Reaction

AbstractThe present work reports the facile synthesis and characterization of carbon‐supported porous Pd shell coated Au nanochain networks (AuPdNNs/C). By using Co nanoframes as sacrificial templates, AuPdNNs/C series have been prepared by a two‐step galvanic replacement reaction (GRR) technique. In the first step, the Au metal precursor, HAuCl4, reacts spontaneously with the formed Co nanoframes through the GRR, resulting in Au nanochain networks (AuNNs). The second GRR is performed with various concentrations of Pd precursor (0.1, 1, and 10 mM PdCl2), resulting in AuPdNNs/C. The synthesized AuPdNNs/C series are investigated as electrocatalysts for oxygen reduction reaction (ORR) in alkaline solution. The physical properties of the AuPdNNs/C catalysts are characterized by scanning electron microscopy (SEM), high‐resolution transmission electron microscopy (HRTEM), UV‐vis absorption spectroscopy, and cyclic voltammetry (CV). Rotating disk electrode (RDE) voltammetric studies show that the Au0.8Pd0.2NNs/C (prepared using 1 mM PdCl2) has the highest ORR activity among all the AuPdNNs/C series, which is comparable to commercial Pt catalyst (E‐TEK). The ORR activity of AuPdNNs/C is presumably due to the enhanced Pd surface area and high porosity of Pd nanoshells.

Nature of the two-step temperature-programmed decomposition of PdO supported on alumina

Temperature-programmed decomposition of PdO supported on high-surface-area alumina occurs in a two-step process starting near the temperature of bulk PdO decomposition, but slowing to near-zero rates as the bulk oxide decomposition temperature is traversed. Most of the oxide decomposes in a second step 40–60°C above the bulk PdO decomposition temperature. We present evidence suggesting that a thin Pd shell forms initially on the surface of PdO particles and propose that the resulting core–shell structure is connected with the observed meta-stability. Both the experimental results and order-of-magnitude estimates indicate that neither small particle effects (metal-support interaction or surface energy) nor low oxygen permeability through the Pd shell explain the two-step process. We tentatively ascribe the higher-temperature PdO decomposition step to the contribution of strain energy to the Pd shell imparted by epitaxy with the underlying PdO lattice.

Facile synthesis of Au-Pd core-shell nanocrystals with systematic shape evolution and tunable size for plasmonic property examination.

A facile synthetic method has been developed for the formation of Au-Pd core-shell nanocrystals in aqueous solution in just 0.5-2 h at 50 °C with systematic shape evolution from cubic to truncated cubic, cuboctahedral, truncated octahedral, and octahedral structures using octahedral gold cores. By adjusting the amounts of H2PdCl4, ascorbic acid, and sometimes surfactants and gold cores added, the particle morphology can be finely tuned, and Pd shells with ultrathin thicknesses have been achieved. Gold cores of three different sizes (35, 45, and 74 nm in opposite corner distance) were used to obtain a full range of particle sizes and shapes for a most complete examination of their plasmonic properties. Visual observations made during particle synthesis reveal that Au-Pd cubes are formed at a faster rate than that for the growth of octahedra. For the smaller cubes, cuboctahedra, and truncated octahedra prepared using 35 and 45 nm gold cores, the surface plasmon resonance (SPR) absorption band from the gold cores can be seen only when the Pd shell thickness is just 1 nm at the thinnest points of the particles. For small-sized Au-Pd octahedra, this band is observable at a Pd shell thickness of around 5 nm. For larger Au-Pd nanocrystals synthesized from 74 nm gold cores, the Au SPR band is more recognizable for all particle shapes, although octahedra still exhibit the most obvious band. The band shifts slightly to the red going from cubes to octahedra. Simulation spectra have been performed, and they roughly match with the experimental spectra. Au-Pd octahedra with two different core sizes and shell thicknesses have been used for hydrogen sensing by comparing their UV-vis spectra before and after hydrogen incorporation forming PdH. The results show that the shell thickness is more important in producing a larger spectral red-shift after hydrogen absorption.

Evidence for geometric effects in neopentane conversion on PdAu catalysts

Silica-supported Pd and Pd(shell)/Au(core) nanoparticles of a similar size were evaluated for neopentane conversion.

Magnetically recyclable core–shell nanocatalysts for efficient heterogeneous oxidation of alcohols

We describe the designed fabrication of magnetically recyclable core–shell Pd nanocatalysts for the efficient oxidation of alcohols under base-free reaction conditions in water.

Insights into the Electrooxidation of Formic Acid on Pt and Pd Shells on Au Core Surfaces via SERS at Dendritic Au Rod Electrodes

Highly surface-enhanced Raman scattering (SERS)-active Pt and Pd layers were fabricated using dendritic Au rod (DAR) structures, prepared by simple electrodeposition, as core substrates. Electroche...

Ultrahigh activity of Pd decorated Ir/C catalyst for formic acid electro-oxidation

A core–shell like Ir/C@Pd catalyst with ultrahigh activity toward formic acid electro-oxidation (FAEO) was prepared by decorating Pd shell on the Ir NPs for the first time. The structure has been demonstrated by XRD, XPS, TEM, EDS and electrochemical techniques. The mass-normalized current density at 0.08V (vs. SCE) of Ir/C@Pd for FAEO is 3756mAmg−1Pd, which is about 3.38 and 4.16 times higher than that of the PdIr/C and commercial Pd/C, respectively. The remarkable performance of Ir/C@Pd should be attributed to its unique core–shell like structure and the enhanced electronic coupling between the Ir and Pd, which was confirmed by XPS results.

Catalytic activities of sonochemically prepared Au-core/Pd-shell-structured bimetallic nanoparticles immobilised on TiO2 and its dependence on Pd-shell thickness

Catalytic activities of sonochemically prepared Au-core/Pd-shell-structured bimetallic nanoparticles (NPs) immobilised on TiO2 were evaluated. Comparing with the mixture of monometallic Au and Pd NPs on TiO2, core/shell-immobilised catalysts exhibited higher activities for the partial reduction of nitrobenzene (NB) to aniline (AN), suggesting that the synergistic effect originating from the core/shell structure enhanced the catalytic activities. In the case of high Au/Pd ratios, where the Pd-shell thickness was calculated to be 0.5 nm or lower, infrared spectroscopic measurements of adsorbed CO showed that the Au cores were successfully covered with Pd shells. It was found that a thin Pd shell of one layer or two layers of Pd atoms effectively catalysed the reduction of NB under ambient temperature, whereas the formation of AN was not confirmed on monometallic Au NP-immobilised catalysts.

Pulse electrodeposition to prepare core–shell structured AuPt@Pd/C catalyst for formic acid fuel cell application

A novel core–shell structured AuPt@Pd/C catalyst for the electrooxidation of formic acid is synthesized by a pulse electrodeposition process, and the AuPt core nanoparticles are obtained by a NaBH4 reduction method. The catalyst is characterized with X-ray powder diffraction and transmission electron microscopy, thermogravimetric analysis, cyclic voltammetry, CO stripping and X-ray photoelectron spectroscopy. The core–shell structure of the catalyst is revealed by the increase in particle size resulting from a Pd layer covering the AuPt core, and by a negative shift in the CO stripping peaks. The addition of a small amount of Pt improves the dispersion of Au and results in smaller core particles. The catalyst's activity is evaluated by cyclic voltammetry in formic acid solution. The catalyst shows excellent activity towards the anodic oxidation of formic acid, the mass activity reaches 4.4 A mg−1Pd and 0.83 A mg−1metal, which are 8.5 and 1.6 times that of commercial Pd/C. This enhanced electrocatalytic activity could be ascribed to the good dispersion of Au core particles resulting from the addition of Pt, as well as to the interaction between the Pd shell layer and the Au and Pt in the core nanoparticles.