Pd Shell Research Articles

Tuning the Oxygen Reduction Activity of Pd Shell Nanoparticles with Random Alloy Cores

Pd-based nanoparticles are promising candidates for non-Pt catalysts of the oxygen reduction reaction (ORR). Trends in ORR activity of Pd/Cu-alloy-core@Pd-shell nanoparticles are studied by calculating the oxygen binding energy on the Pd surface with different Cu compositions in the alloy core. Density functional theory calculations show that several properties of the nanoparticle surface, including the average oxygen binding energy, d-band center, and the net charge of Pd, are linearly related to the ratio of Cu in the core, demonstrating the capacity to tune ORR activity. Trends in oxygen binding of other core alloys are also studied and show similar linear trends with core composition, providing a design strategy for new ORR catalysts.

A Comprehensive Search for Stable Pt–Pd Nanoalloy Configurations and Their Use as Tunable Catalysts

Using density-functional theory, we predict stable alloy configurations (ground states) for a 1 nm Pt-Pd cuboctahedral nanoparticle across the entire composition range and demonstrate their use as tunable alloy catalysts via hydrogen-adsorption studies. Unlike previous works, we use simulated annealing with a cluster expansion Hamiltonian to perform a rapid and comprehensive search that encompasses both high and low-symmetry configurations. The ground states show Pt(core)-Pd(shell) type configurations across all compositions but with specific Pd patterns. For catalysis studies at room temperatures, the ground states are more realistic structural models than the commonly assumed random alloy configurations. Using the ground states, we reveal that the hydrogen adsorption energy increases (decreases) monotonically with at. % Pt for the {111} hollow ({100} bridge) adsorption site. Such trends are useful for designing tunable Pd-Pt nanocatalysts for the hydrogen evolution reaction.

Au/Pd core–shell nanoparticles for enhanced electrocatalytic activity and durability

Unique Au/Pd core–shell nanoparticles were synthesized via galvanic replacement of Cu by Pd on hollow Au nano-spheres. The catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. The Au/Pd nanoparticles exhibit superior formic acid oxidation (FAO) performance, lower CO-stripping potential, and long-term durability and stability compared to commercial Pd black due to enhanced electronic coupling between the Au core and Pd shell, which is confirmed by XPS results.

Preparation of highly active heterogeneous Au@Pd bimetallic catalyst using plant tannin grafted collagen fiber as the matrix

Au@Pd bimetallic nanoparticles (NPs) catalysts were synthesized by a seeding growth method using bayberry tannin grafted collagen fiber (BT-CF) as the matrix. In this method, Au3+ was first reductively adsorbed onto BT-CF to form Au NPs, and then they serve as the seeds for the over growth of Pd shell. The morphology of BT-CF-Au@Pd catalyst was observed by TEM and SEM, and the core–shell structure of the Au@Pd was confirmed by EDS and XRD. It was found that the as-prepared BT-CF-Au9@Pd3 catalyst showed excellent synergy effect in liquid-phase hydrogenation of cyclohexene, whose reaction time was three times faster than that catalyzed by BT-CF-Pd catalyst under the same conditions. Meanwhile, the BT-CF-Au9@Pd3 catalyst could be re-used four times without significant loss of activity. In the fourth run, the substrate conversion was still as high as 92.70%, much better than that by using commercial Pd/C catalyst (42.60%). Additionally, BT-CF-Au9@Pd3 catalyst exhibited high hydrogenation activity to various alkenes and nitro-compounds. For example, the TOF of allyl alcohol, styrene and nitrobenzene hydrogenations reached 10,980, 14,732 and 1379molmol−1h−1, respectively.

Au@Pd core–shell nanoparticles: A highly active electrocatalyst for amperometric gaseous ethanol sensors

Au@Pd core-shell nanoparticles (NPs) were synthesized using seed-mediated method, and three samples with different shell structures were obtained by tuning the Au core size. The shape-dependent catalytic activities of Au@Pd core-shell NPs toward ethanol oxidation in alkaline electrolytes were systematically explored. The electrochemical experiments showed that the Au@Pd-2 core-shell NPs (with Au core size of 9.8 nm and loose Pd shell) exhibited a distinctly higher activity and poison-resistance for the ethanol oxidation than other core-shell structures. Based on the Au@Pd-2 core-shell NPs and Pd NPs, two amperometric ethanol gas sensors were fabricated. During the sensor tests, the ethanol sensor based on Au@Pd-2 NPs showed good performance for ethanol in the range of 50-600 ppm with sensitivity of 178.5 nA ppm(-1) and less than 5% signal loss in 60 days. 2012 Published by Elsevier B.V.

“Ship‐in‐a‐Bottle” Growth of Noble Metal Nanostructures

AbstractNoble metal nanostructures are grown inside hollow mesoporous silica microspheres using “ship‐in‐a‐bottle” growth. Small Au seeds are first introduced into the interior of the hollow microspheres. Au nanorods with synthetically tunable longitudinal plasmon wavelengths and Au nanospheres are obtained through seed‐mediated growth within the microspheres. The encapsulated Au nanocrystals are further coated with Pd or Pt shells. The microsphere‐encapsulated bimetallic core/shell nanostructures can function as catalysts. They exhibit high catalytic performance and their stability is superior to that of the corresponding unencapsulated core/shell nanostructures in the catalytic oxidation of o‐phenylenediamine with hydrogen peroxide. Therefore, these hollow microsphere‐encapsulated metal nanostructures are promising as recoverable and efficient catalysts for various liquid‐phase catalytic reactions.

New Experimental Evidences of Pt–Pd Bimetallic Nanoparticles with Core–Shell Configuration and Highly Fine-Ordered Nanostructures by High-Resolution Electron Transmission Microscopy

In our facile synthesis method, poly(vinylpyrrolidone)-protected Pt and Pt–Pd bimetallic nanoparticles with controllable polyhedral core–shell morphologies are precisely synthesized by the reduction of Pt and Pd precursors at a certain temperature in ethylene glycol with silver nitrate as structure-controlling agent. The Pt nanoparticles exhibited well-shaped polyhedral morphology with highly fine and specific nanostructures in the size range of 20 nm. Important evidences of core–shell configurations of the Pt–Pd core–shell nanoparticles were clearly characterized by high-resolution transmission electron microscopy (HRTEM) measurements. The results of HRTEM images showed that the core–shell Pt–Pd nanoparticles in the size range of 25 nm with polyhedral morphology were synthesized with the thin Pd shells of ∼3 nm in thickness as the atomic Pd layers grown on the Pt cores. Very interesting characteristics of surface structure of Pt nanostructures and Pt–Pd core–shell nanostructures with surface defects were observed. The high-resolution TEM images of Pt–Pd bimetallic nanoparticles showed that the Frank–van der Merwe and Stranski–Krastanov growth modes coexist in the nucleation and growth of the Pd shells on the as-prepared Pt cores. It is predicted that the FM growth becomes the main favorable growth compared with the SK growth in the formation of the thin Pd shells of Pt–Pd core–shell nanoparticles. The experimental evidence of the deformations of lattice fringes and lattice-fringe patterns was found in Pt and Pt–Pd core–shell nanoparticles. The interesting renucleation and recrystallization at the attachments between the nanoparticles are revealed to form a good lattice match. In addition, our novel ideas of the largest surface-area superlattices and promising utilization of them are proposed for next generations of various fuel cells with low cost. Finally, the products of Pt–Pd core–shell nanoparticles can be potentially utilized as highly efficient catalysts in the realization of polymer electrolyte membrane fuel cell and direct methanol fuel cell using the very low Pt loading with better cost-effective design.

Two-Stage Melting in Core–Shell Nanoparticles: An Atomic-Scale Perspective

Microscopic understanding of thermodynamic behaviors of metallic nanoparticles is of significance for their applications in nanoscale catalysis and thermal energy storage. In this article, molecular dynamics simulations are used to investigate the thermal stabilities of Pt–Pd core–shell nanoparticles with different core sizes and shell thicknesses. Our study shows that a distinct two-stage melting occurs during the continuous heating of bimetallic nanoparticles. It has experienced a much broader temperature range compared with the melting of monometallic nanoparticles, although they have both developed from surface into interior. The temperature width for the two-stage melting is dependent not only on the bulk melting points of two component metals but also on the ratio of the shell thickness and core size. Furthermore, due to the melting of the Pd shell beforehand, the melting point of the Pt core is lower than that of the same size Pt nanoparticle not encapsulated by the Pd shell. This study provides a ...

Phase change of bimetallic PdCo electrocatalysts caused by different heat-treatment temperatures: Effect on oxygen reduction reaction activity

In the present paper, we report the importance of controlling the heat-treatment temperature, the factor that determines the fine structure (core–shell structure or alloy-phase structure) of PdCo bimetallic electrocatalysts. The dependence of the oxygen reduction reaction (ORR) activity of bimetallic PdxCo100−x nanoparticles on the various heat-treatment conditions employed in the preparation process has been investigated. Precise Rietveld refinements of high-resolution powder diffraction data and the fine structure parameters simulated using the extended X-ray absorption fine structure data reveal the following. At a heat-treatment temperature of 300°C, a phase-segregated PdCo core–shell structure (Pd shell on a Co core) is formed, while at higher temperatures (above 500°C), a PdCo alloy phase is formed and the core–shell structure is destroyed. The samples for which the core–shell structure had a high-compressive strain lattice structure with a Pd-rich phase caused a downshift of the d-band center, and eventually, the ORR activity of these samples was better than that of the alloy-phase samples formed by high-temperature heat treatment. Hence, the heat-treatment temperature is the most important factor to be considered in the design of PdCo alloy electrocatalysts.

Effect of Carbon Supports on Electrocatalytic Reactivity of Au–Pd Core–Shell Nanoparticles

The role of particle-substrate interactions on the reactivity of bimetallic nanostructures is investigated in the case of Au–Pd core–shell nanoparticles supported on Vulcan XC-72R (Vulcan). Core–shell nanostructures (CS) featuring 19 nm Au cores and Pd shells with thicknesses between ca. 1 and 10 nm were synthesized by controlled colloidal methods and subsequently incorporated in the carbon support. X-ray diffraction, energy dispersive X-ray analysis, and high resolution transmission electron microscopy confirmed the CS nature of the nanostructures, which remain unaffected upon incorporation onto the carbon matrix. Their electrochemical properties toward CO and HCOOH electro-oxidation were studied, using cyclic voltammetry and chronoamperometry. The results show that the CO stripping potential becomes independent of the average Pd lattice strain in the case of Vulcan supported CS. This behavior is significantly different to the trend observed in CS assemblies at In-doped SnO2 electrodes. Formic acid oxida...

Synthesis and Characterization of Surfactant Stabilized Trimetallic Au-Ag-Pd Nanoparticles for Heck Coupling Reaction

Colloidal dispersions of trimetallic nanoparticles (tnp) are synthesized by chemical method and compositionally four different tnp are prepared and particle sizes are characterized by UV-vis, HRTEM and XPS measurements. The XPS studies provide proof of the mode of binding that occurs in the nanoparticle surface. The catalytic activities of tnp are tested by Heck C-C coupling reaction. The product yields and recyclability of the recovered catalysts are studied. Tnp exhibited better catalysis than mono and bimetallic nanoparticles, which may be due to electronic effects of the Au-Ag core on to the Pd shell atoms.

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.

Epitaxial growth of Au@Pd core–shell nanocrystals prepared using a PVP-assisted polyol reduction method

Au@Pd core–shell nanocrystals were prepared using a two-step polyol reduction method. First, decahedral Au core seeds with small amounts of octahedral, triangular-platelike, and icosahedral Au particles were prepared by reducing HAuCl4·4H2O in diethylene glycol (DEG) under oil-bath heating in the presence of polyvinylpyrrolidone (PVP) as a polymer surfactant. Then Pd shells were overgrown epitaxially on Au core seeds by reducing Na2PdCl4 in EG with PVP, KBr, and H2O. The resultant crystal shapes were characterized using transmission electron microscopic (TEM), TEM-energy dispersed X-ray spectroscopic (EDS), and selected area electron diffraction (SAED) measurements. Effects of addition of KBr and H2O in the second step and reaction temperature for the yield of core–shell particles and their shape, size, and composition distributions were examined. Results show that both {111} and {100} facets of Pd shells were formed in the presence of KBr, depending on the shapes of Au core seeds, whereas only {111} facets were produced in the absence of KBr. New triangular-platelike, five-twin rod, decahedral, and icosahedral shapes of Au@Pd nanocrystals were prepared using triangular-platelike, decahedral, and icosahedral Au cores. When decahedral Au particles were used as seeds, monodispersed decahedral Au@Pd nanocrystals were prepared in high yield at low temperatures of 20–100 °C in PVP/EG solution. Au acts as a catalyst for Pd2+ reduction in the presence of PVP at low temperatures. This work demonstrates a simple two-step technique for the epitaxial growth of various shapes of Au@Pd nanocrystals.

Hollow Au@Pd and Au@Pt core–shell nanoparticles as electrocatalysts for ethanol oxidation reactions

Hybrid alloys among gold, palladium and platinum become a new category of catalysts primarily due to their enhanced catalytic effects. Enhancement means not only their effectiveness, but also their uniqueness as catalysts for the reactions that individual metals may not catalyze. Here, preparation of hollow Au@Pd and Au@Pt core–shell nanoparticles (NPs) and their use as electrocatalysts are reported. Galvanic displacement with Ag NPs is used to obtain hollow NPs, and higher reduction potential of Au compared to Ag, Pd, and Pt helps to produce hollow Au cores first, followed by Pd or Pt shell growth. Continuous and highly crystalline shell growth was observed in Au@Pd core–shell NPs, but the sporadic and porous-like structure was observed in Au@Pt core–shell NPs. Along with hollow core–shell NPs, hollow porous Pt and hollow Au NPs are also prepared from Ag seed NPs. Twin boundaries which are typically observed in large size (>20 nm) Au NPs were not observed in hollow Au NPs. This absence is believed to be due to the role of the hollows, which significantly reduce the strain energy of edges where the two lattice planes meet. In ethanol oxidation reactions in alkaline medium, hollow Au@Pd core–shell NPs show highest current density in forward scan. Hollow Au@Pt core–shell NPs maintain better catalytic activities than metallic Pt, which is thought to be due to the better crystallinity of Pt shells as well as the alloy effect of Au cores.

Location Effect of Pd Additives on the Detection of Reducing Gases for Nanoscale SnO2 Hollow Spheres based Gas Sensors

Abstract A water-in-oil microemulsion approach was established to synthesize Pd@SnO2 and SnO2@Pd core@shell nanocomposites in order to investigate the influence of the noble metal additive location on the sensing performance towards CO and H2 in dry and humid conditions at different temperatures. It turned out that the Pd additive, being either present on the outside of the shells or encapsulated by the SnO2 matrix, strongly influences the sensing performance. Especially, the inner shell Pd doped hollow spheres (Pd@SnO2) have shown very high signals towards CO in humid conditions at rather low sensing operation temperatures together with an almost linear response curve.

Electrochemical performance of Pd and Au–Pd core–shell nanoparticles on surface tailored carbon black as catalyst support

The present work studies the influence of the surface chemistry of carbon supports on the electrochemical behaviour of Pd and Au–Pd core–shell (CS) nanoparticles. Vulcan XC-72R was chemically modified by different acid treatments, inducing changes in the volume of the mesopores and surface density of oxygenated species. The CS nanostructures featuring 19 nm Au cores and 10 nm Pd shells were synthesized by colloidal methods and subsequently incorporated to the carbons supports. Pd nanoparticles were prepared by impregnating a Pd precursor into the modified carbons followed by reduction with sodium borohydride. The use of different preparation methods allowed the independent study of the effect of the support on the morphology/distribution of the nanoparticles and on the reactivity of the nanoparticles, through their interaction with organic molecules. The electrocatalysts were characterised by XRD, EDX, Raman spectroscopy and contact angle measurements. CO and formic acid (HCOOH) electro-oxidation were studied using cyclic voltammetry and chronoamperometry. The effect of the carbon support on the electrocatalytic activity was highly dependent on the method of preparation. Pd nanoparticles obtained by impregnation showed higher HCOOH oxidation currents when supported on the highly oxidised Vulcan support. This is due to the generation of smaller particle sizes (2.3 nm) as a result of the high density of oxygenated functional groups. On the other hand, the CS nanostructures are significantly less active in highly oxidised Vulcan as a results of specific chemical interactions which may be related to the formation of oxides. The implication of these findings towards rationalising particle–substrate interactions are briefly discussed.

Electrocatalytic Properties of Strained Pd Nanoshells at Au Nanostructures: CO and HCOOH Oxidation

The oxidations of carbon monoxide and formic acid at ultrathin Pd layers grown on Au nanoparticles were studied as a function of Pd thickness. Pd shells with thickness between 1 and 10 nm were grown on 19 nm Au nanoparticles by chemical reduction of H2PdCl4 with ascorbic acid. High-resolution transmission electron microscopy and X-ray diffraction confirm the core–shell configuration of the nanostructures. While the synthesis of pure Pd nanostructures led to a rather amorphous material, Pd nanoshells exhibited a polycrystalline structure confirming that Au nanostructures act as templates for Pd growth. Three-dimensional assemblies of nanoparticles were generated by alternate electrostatic layer-by-layer adsorption steps, involving poly-l-lysine and colloidal dispersions. Electrochemical studies in H2SO4 containing electrolyte solution demonstrate that CO coverage and anodic stripping potential are affected by the thickness of Pd nanoshells. In addition, the faradaic current density associated with HCOOH ox...

A SERS study of thiocyanate adsorption on Au-core Pd-shell nanoparticle film electrodes

We have systematically studied the adsorption of thiocyanate (SCN −) on gold-core palladium-shell nanoparticles (Au@Pd NPs) with different Pd shell thicknesses by surface-enhanced Raman scattering (SERS). When the Pd shell thickness is increased from one to ten atomic layers, the ν C N stretching frequency increases 6 cm −1 for S-bound SCN − and 10 cm −1 for N-bound SCN −. It infers that the C N stretching frequency is quite sensitive to electronic properties of the NP surface, but even more so when bonding to Pd occurs through the N atom (at negative potentials) than when it occurs through the S atom (at positive potentials). Data for a second probe, p-aminothiophenol (PATP), was compared to that of SCN −; however, PATP was found to be less sensitive than SCN − to the surface electronic properties of Au@Pd NPs.

Fabrication of Au–Pd Core–Shell Heterostructures with Systematic Shape Evolution Using Octahedral Nanocrystal Cores and Their Catalytic Activity

By using octahedral gold nanocrystals with sizes of approximately 50 nm as the structure-directing cores for the overgrowth of Pd shells, Au-Pd core-shell heterostructures with systematic shape evolution can be directly synthesized. Core-shell octahedra, truncated octahedra, cuboctahedra, truncated cubes, and concave cubes were produced by progressively decreasing the amount of the gold nanocrystal solution introduced into the reaction mixture containing cetyltrimethylammonium bromide (CTAB), H(2)PdCl(4), and ascorbic acid. The core-shell structure and composition of these nanocrystals has been confirmed. Only the concave cubes are bounded by a variety of high-index facets. This may be a manifestation of the release of lattice strain with their thick shells at the corners. Formation of the [CTA](2)[PdBr(4)] complex species has been identified spectroscopically. Time-dependent UV-vis absorption spectra showed faster Pd source consumption rates in the growth of truncated cubes and concave cubes, while a much slower reduction rate was observed in the generation of octahedra. The concave cubes and octahedra were used as catalysts for a Suzuki coupling reaction. They can all serve as effective and recyclable catalysts, but the concave cubes gave higher product yields with a shorter reaction time attributed to their high-index surface facets. The concave cubes can also catalyze a wide range of Suzuki coupling reactions using aryl iodides and arylboronic acids with electron-donating and -withdrawing substituents.

Catalytic performance of core–shell and alloy Pd–Au nanoparticles for total oxidation of VOC: The effect of metal deposition

The catalytic properties of palladium and gold nanoparticles deposited on mesoporous TiO2 are investigated in toluene and propene oxidation. The catalysts, containing Pd and Au deposited on mesoporous TiO2 have been prepared by different order of metal deposition (Pd(shell)–Au(core)/TiO2, Pd–Au(alloy)/TiO2 and Au(shell)–Pd(core)/TiO2. For both toluene and propene oxidation reactions, the catalytic activity was found significantly higher when palladium is deposited on already-deposited gold (Pd(shell)–Au(core)/TiO2). This enhanced activity could be explained by the core–shell morphology (Pd–shell and Au-core) observed by UV–vis spectra, TPR profiles and XPS spectra. It was suggested that the oxidation reaction follows a Langmuir–Hinshelwood mechanism where the molecules of oxygen and VOC are in competition for adsorption on the surface of catalyst. Operando DRIFT spectroscopy was carried out to test the catalytic activity in a mixture of volatile organic compounds (toluene and propene). It was demonstrated that there is a competition between the molecules of VOC for adsorption but also the toluene has an inhibition effect for oxidation of propene.