Gold Palladium Research Articles

Carbon-Supported Palladium–Gold Bimetallic Disperse Systems Formed in Aqueous Solutions at 110°С

Various palladium–carbon composites have been manufactured by autoclaving at 170°С to be used as precursors for manufacturing bimetallic particles. The morphology of the manufactured items was comprehensively studied by scanning electron microscopy; the ultrafine metal palladium was found to have particles sizes lying in the range 30–120 nm. The specifics of hydrothermal reduction of gold(III) chloro complexes by palladium–carbon composites at 110°С have been studied. An appreciable increase in gold(III) reduction rate was observed with the use of a palladium–carbon composite relative to the rate observed for ultrafine metallic palladium. Gold is reduced on a palladium–carbon composite to an individual metallic phase.

Glycerol Oxidation Using MgO‐ and Al2O3‐supported Gold and Gold–Palladium Nanoparticles Prepared in the Absence of Polymer Stabilizers

AbstractAu and AuPd nanoparticles supported on MgO and Al2O3 were employed for the selective aqueous phase oxidation of glycerol under basic conditions. Catalysts were prepared by sol‐immobilization without the addition of a stabilizing agent such as polyvinyl alcohol (PVA), which is generally added to stabilize the noble metal sol prior to immobilization. The obtained materials prepared with and without stabilizing agent were active for glycerol oxidation and showed similar catalytic performances—implying that the stabilizing polymer is not required to obtain active materials. Depending on the support used, it was possible to tailor the selectivity towards the desired oxidation products by using catalysts prepared with or without stabilizing agent. PVA‐free Au/γ‐Al2O3 exhibited a remarkably high selectivity towards tartronic acid (40 % at 97 % conversion), which was not observed for Au/γ‐Al2O3 prepared with PVA (27 % at isoconversion). Selective glycerol oxidation performed under base‐free conditions over AuPd/MgO catalysts also corroborated the previous results that the presence of a stabilizing polymer is not required to prepare active catalysts by sol‐immobilization. Thus, a facile way to circumvent the inherent drawbacks encountered by the use of polymer stabilizers during catalyst preparation is presented herein. Experimental results suggest that the presence of the polymer stabilizers can affect the reaction pathways and control selectivity.

Ab initio studies of ethanol dehydrogenation at binary AuPd nanocatalysts

Density Functional Theory (DFT) simulations have been performed to study ethanol anaerobic oxidation at 10-atom binary gold–palladium clusters with varying compositions, ranging from 0% to 100% gold content. For each case, we have studied the loss of two hydrogen atoms, the desorption of acetaldehyde, and recombination of hydrogen into gas-phase H2. The results show that alloying produces sizable changes in the reaction barriers and in the stability of the intermediate species, which are related to different intrinsic reactivities of palladium and gold.

TiO2 Supported gold–palladium catalyst for effective syngas production from methane partial oxidation

The CO and H2 (syngas) production from methane partial oxidation (MPO) using TiO2 supported Au-Pd bimetallic catalysts are discussed. The supported Au, Pd and Au-Pd bimetallic nanoparticles were prepared by an incipient wetness impregnation or co-impregnation method and were characterized. The supported Au-Pd catalyst was selective for reforming and was more active compared to bare TiO2, TiO2 supported Au only, or Pd only catalysts. The catalyst properties before and after MPO reaction were investigated, including the stability of the TiO2 support, and the stability of Au and Pd nanoparticles. The supported Au-Pd catalyst has the highest TON (TONH2 = 23 at 650 °C), compared to supported Au catalyst (TONH2 < 1) and supported Pd catalyst (TONH2 < 1). Based on the analysis of outlet gases from the MPO reaction in a flow reactor, synergetic effects between Au and Pd, where Pd activates CH bonds while gold modulates the behavior of oxygen at the catalyst surface, are shown to contribute to MPO. Additionally, the Au-Pd/TiO2 material shows long-term activity (>12 h) for the MPO reaction at 600 °C.

Disclosure of the Surface Composition of TiO2-Supported Gold–Palladium Bimetallic Catalysts by High-Sensitivity Low-Energy Ion Scattering Spectroscopy

It is well-known that there is a critical relationship between the surface composition and activity for a bimetallic catalyst. However, the surface is normally reconstructed under different conditions, which makes the surface more complicated. In this work, TiO2-supported colloidal PdxAuy-NPs with different atomic ratios were prepared and treated under oxidation, reduction, and actual catalytic reaction conditions. The surface composition and structure were measured using X-ray photoelectron spectroscopy (XPS) and high-sensitivity low-energy ion scattering spectroscopy (HS-LEIS). Phase diagrams of the surface compositions under various conditions as a function of the bulk composition were established. Oxidation induces dealloying and enrichment of PdO on the surface, while H2 reduction results in realloying with just slight surface enrichment of Pd. Gold helps Pd to resist oxidation and enhances its activity for CO oxidation at low temperature.

Descriptor study by density functional theory analysis for the direct synthesis of hydrogen peroxide using palladium–gold and palladium–mercury alloy catalysts

New concepts combining thermodynamic and kinetic parameters for designing catalysts with high selectivity.

Evolution of surface catalytic sites on thermochemically-tuned gold-palladium nanoalloys.

Nanoscale alloying constitutes an increasingly-important pathway for design of catalysts for a wide range of technologically important reactions. A key challenge is the ability to control the surface catalytic sites in terms of the alloying composition, thermochemical treatment and phase in correlation with the catalytic properties. Herein we show novel findings of the nanoscale evolution of surface catalytic sites on thermochemically-tuned gold-palladium nanoalloys by probing CO adsorption and oxidation using in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) technique. In addition to the bimetallic composition and the support, the surface sites are shown to depend strongly on the thermochemical treatment condition, demonstrating that the ratio of three-fold vs. bridge or atop Pd sites is greatly reduced by thermochemical treatment under hydrogen in comparison with that under oxygen. This type of surface reconstruction is further supported by synchrotron high-energy X-ray diffraction coupled to atomic pair distribution function (HE-XRD/PDF) analysis of the nanoalloy structure, revealing an enhanced degree of random alloying for the catalysts thermochemically treated under hydrogen. The nanoscale alloying and surface site evolution characteristics were found to correlate strongly with the catalytic activity of CO oxidation. These findings have significant implications for the nanoalloy-based design of catalytic synergy.

Structural Properties of Catalytically Active Bimetallic Gold–Palladium Nanoparticles Synthesized on Rutile Titania Nanorods by Pulsed Laser Deposition

Gold–palladium nanoparticles with an average size of 3.5 nm and controlled compositions were grown on rutile titania nanorods using pulsed laser deposition. The structural properties of the as-synthesized nanoparticles and their interface with the oxide support were studied at the atomic scale using aberration-corrected transmission electron microscopy (TEM). High resolution TEM imaging showed that both polycrystalline Au–Pd particles with droplet morphology and single-crystalline particles with truncated octahedron morphology were stabilized on the nanorods during laser deposition depending on the degree of epitaxy between the nanoparticles and the oxide support. By analyzing the single-crystalline particles formed in the presence of a strong epitaxy, 11 epitaxial relationships between the Au–Pd nanoparticles and their support were identified with the two dominant ones being: Au–Pd (111) [101] // r-TiO2(110) [111] and Au–Pd (100) [101] // r-TiO2 (110) [110]. Catalytic testing of the bimetallic titania...

Vinyl Acetate Formation on Au/Pd(100) Alloy Surfaces

The reaction between ethylene and acetate species on oxygen-covered Au/Pd(100) alloy surfaces is explored using reflection–absorption infrared spectroscopy and temperature-programmed desorption (TPD). Distinctly different behavior is found for alloys with less than ~ 0.5 monolayers (ML) of gold, where the alloy contains Pd‒Pd bridge sites, and alloys with gold coverages above ~ 0.5 ML, where only isolated Pd sites are available. Ethylene reacts with η2 acetate species on alloys with low gold coverages where it is found that acetate removal rate increases linearly with gold coverage with a concomitant decrease in carbon monoxide formation, indicating that the selectivity also increases. A different acetate species forms on the isolated palladium sites present on the alloy with high gold coverages, which exhibits infrared peaks at ~ 1450 and 1590 cm−1, as opposed to the characteristic frequency of ~ 1414 cm−1 found for the η2 acetate. It is confirmed that the infrared features are due to adsorbed acetate species by using TPD, but this form of adsorbed acetate is found to be completely unreactive with ethylene, even in a relatively high background pressure of ~ 2 Torr, compared with the η2 acetate species that react rapidly in the presence of ethylene pressures of ~ 10−4 Torr. The structure is assigned to a unidentate acetate species with a strong Pd‒O bond, in which the C=O group can interact with an adjacent gold atom and such a structure would rationalize the lack of reactivity with ethylene because ethylene insertion into the Pd‒O bond of this species would be prohibited. This result is surprising because previous catalytic measurements on model gold–palladium alloys under realistic conditions have shown that high-gold coverage Au/Pd(100) alloys are the most active. A possible explanation for this discrepancy is that the alloys become enriched by palladium under reaction conditions until the gold coverage is reduced below ~ 0.5 ML to yield a very catalytically active surface.

Increased sensitivity of Au-Pd nanolayer on tapered optical fiber sensor for detecting aqueous Ethanol

BackgroundOptical fiber sensors gains popularity as an alternative and a better solution compared to its electronics counterpart. Especially in detecting hazardous materials, optical fiber sensors prove to have many advantages such as miniaturization, remote yet real-time sensing and immune to electromagnetics interference.MethodsIn this paper, we used a 40um tapered standard multi-mode fiber coated with gold (Au), palladium (Pd) and mixtures of Au-Pd nano-composites to detect hazardous ethanol in its various concentration in aqueous environment. Interrogation were made possible using UV-VIS light source, and changes in absorbance and intensity were recorded via spectrometer.Results and ConclusionEthanol ranging from 20% till 100% were investigated. It was found that the composition of nanomaterial coating on the developed sensors strongly affected the sensing performance. From the experiments carried out, the sensor with a gold palladium nanocomposite layer with a 2: 1 ratio of gold to palladium produced the highest sensitivity, which is 0.74/vol% concentration of Ethanol. The fabricated fiber sensor also exhibits fast response and recovery time of 13 s and 6 s respectively.

Alloying of AuNPs with palladium: A promising tool for tuning of selectivity for epoxide in oxidation of styrene using molecular oxygen

• Epoxidation of styrene on gold and Au-Pd alloy nanoparticles using molecular oxygen . • Lowest catalyst quantity 0.038 mmol%. • 98% conversion with 75% selectivity for epoxide. • A green protocol with general applicability. Present investigation, deals with synthesis of gold and gold palladium alloy nanoparticles and their application to epoxidation of styrene using molecular oxygen. Synthesized AuNPs@ Resin and bimetallic Au-Pd NPs@ Resin of average size <5 nm were analyzed by various techniques such as X-ray diffraction (XRD), SEM-EDX, TEM-EDS, inductively coupled plasma atomic emission spectroscopy (ICP-AES) and X-ray photoelectron spectroscopy (XPS). Results showed that alloying of Au with Pd enhanced selectivity of the catalyst towards epoxidation. Au/Pd molar ratio (1/1) provided 98% conversion of styrene with 75% selectivity towards styrene oxide with a TOF of 0.181 (s −1 ) in terms of gold present in catalyst. The catalyst is promising candidate for industrial application as it showed remarkable stability, reusability and general applicability.

Inside Back Cover: Highly Active Gold and Gold–Palladium Catalysts Prepared by Colloidal Methods in the Absence of Polymer Stabilizers (ChemCatChem 15/2017)

Inside Back Cover: Highly Active Gold and Gold–Palladium Catalysts Prepared by Colloidal Methods in the Absence of Polymer Stabilizers (ChemCatChem 15/2017)

Gold–Palladium Bimetallic Catalyst Stability: Consequences for Hydrogen Peroxide Selectivity

During application, electrocatalysts are exposed to harsh electrochemical conditions, which can induce degradation. This work addresses the degradation of AuPd bimetallic catalysts used for the electrocatalytic production of hydrogen peroxide (H2O2) by the oxygen reduction reaction (ORR). Potential-dependent changes in the AuPd surface composition occur because the two metals have different dissolution onset potentials, resulting in catalyst dealloying. Using a scanning flow cell (SFC) with an inductively coupled plasma mass spectrometer (ICP-MS), simultaneous Pd and/or Au dissolution can be observed. Thereafter, three accelerated degradation protocols (ADPs), simulating different dissolution regimes, are employed to study the catalyst structure degradation on the nanoscale with identical location (IL) TEM. When only Pd or both Au and Pd dissolve, the composition changes rapidly and the surface becomes enriched with Au, as observed by cyclic voltammetry and elemental mapping. Such changes are mirrored by ...

Synergistic Effects of Gold-Palladium Nanoalloys and Reducible Supports on the Catalytic Reduction of 4-Nitrophenol.

Herein we report on the catalytic activity of mesoporous nickel, iron, cerium, cobalt, and manganese oxides prepared using KIT-6 as a hard template via evaporation-assisted precipitation. The mesoporous metal oxides (MMOs) were characterized and used as heterogeneous catalysts in the reduction of 4-nitrophenol (4-Nip) by sodium borohydride (BH4-). Furthermore, polyamidoamide (PAMAM) dendrimers were used to synthesize gold-palladium nanoalloy particles. The size of AuPd/PAMAM was found to be 3.5 ± 0.8 nm in diameter before being immobilized on the aforementioned mesoporous metal oxides and used as catalysts in the reduction of 4-Nip. Prior to catalytic evaluation, the reduction profiles of the mesoporous metal oxides were investigated by hydrogen-temperature-programmed reduction (H2-TPR) and showed that mesoporous metal oxides can be easily reduced at lower temperatures and that the immobilization of gold-palladium nanoalloy particles lowers their reduction temperatures. Mesoporous cobalt and manganese oxides showed catalytic activity toward 4-Nip reduction, and the activity was enhanced after immobilization of the gold-palladium nanoalloys. Isolation of nanoparticles activity was achieved by immobilization of the gold-palladium nanoalloys on the inert silica support. From this we postulated an electron relay mechanism for the reduction of 4-nitrophenol. With the use of power rate law we showed that 4-Nip reduction follows pseudo-first-order kinetics.

Deactivation Behavior of Supported Gold Palladium Nanoalloy Catalysts during the Selective Oxidation of Benzyl Alcohol in a Micropacked Bed Reactor

Highly active, supported Au–Pd catalysts have been tested for catalyzing benzyl alcohol oxidation in a silicon-glass micropacked bed reactor. The effects of Au–Pd composition and anion content during catalyst preparation on catalyst deactivation were studied, and a relationship between the deactivation rate and the amount of Cl– and Au used in the catalyst formulation was found. While Au aids in enhancing the selectivity to the desired product and the Cl– ions help the formation of uniform 1–2 nm nanoparticles, higher amounts of Au and Cl– become detrimental to the catalyst stability once a certain amount is exceeded. Loss of small (1–2 nm) metal nanoparticles was evident in all catalysts studied, accompanied by agglomeration and the formation of larger >10 nm particles. A secondary deactivation mechanism characterized by the formation of an amorphous surface film was observed via transmission electron microscopy in catalysts with high Cl– and Au and was associated with the detection of carbon species on ...

Highly Active Gold and Gold–Palladium Catalysts Prepared by Colloidal Methods in the Absence of Polymer Stabilizers

AbstractSupported gold and gold–palladium nanoparticles were found to be effective catalysts for the selective oxidation of glycerol and benzyl alcohol. The properties and stabilities of catalysts are often sensitive to factors such as the dimensions, shape, and composition of the metal nanoparticles. Although colloidal methods provide an easy and quick way to synthesize supported metal catalysts, they typically involve the use of polymers such as polyvinyl alcohol and polyvinylpyrrolidone as steric stabilizers, which can sometimes be detrimental in subsequent catalytic reactions. Herein, we report the synthesis of supported gold and gold–palladium nanoparticles without the addition of stabilizing polymers. The catalysts prepared with and without the addition of polymers performed very similarly in the selective oxidation of glycerol and benzyl alcohol, which suggests that polymers are not essential to make active catalysts for these reactions. Thus, this new stabilizer‐free method provides a facile and highly effective way of circumventing the inherent problems of polymer stabilizers in the preparation of gold and gold–palladium catalysts.

Tuning Synergistic Effect of Au–Pd Bimetallic Nanocatalyst for Aerobic Oxidative Carbonylation of Amines

The activation and utilization of carbon monoxide is of crucial importance to C1 chemistry. Various catalytic transformation processes have been developed and studied in the last century, and oxidative carbonylation of amines is one of them. Catalysts that have been identified to date for the oxidative carbonylation of amines generally show relatively low activity and/or selectivity. Herein, a metal–organic framework (MOF), i.e., MOF-253 prepared from AlCl3·6H2O and 2,2′-bipyridine-5,5′-dicarboxylic acid, was employed as a support of gold–palladium bimetallic nanoparticles (Au–Pd/MOF) for the oxidative carbonylation of amines under mild conditions. Compared to palladium or gold monometallic catalysts, higher catalytic activity (turnover frequency up to 2573 h–1) and selectivity in the carbonylation of amines were achieved by Au–Pd/MOF bimetallic catalysts through adjusting the molar ratio of gold and palladium within the framework. A breathing effect of Au–Pd/MOF in the catalytic process was further obser...

PdAu Alloy Nanoparticle Catalysts: Effective Candidates for Nitrite Reduction in Water

Well-defined palladium–gold nanoparticles (PdAuNPs) with randomly alloyed structures and broadly tunable compositions were studied in catalytic nitrite (NO2–) reduction. The catalysts were synthesized using a microwave-assisted polyol coreduction method. PdxAu100–xNPs with systematically varied compositions (x = 18–83) were supported on amorphous silica (SiO2) and studied as model catalysts for aqueous NO2– reduction in a batch reactor, using H2 as the electron donor. The reactions followed pseudo-first-order kinetics for ≥80% NO2– conversion. The PdxAu100–xNP-SiO2 catalysts showed a volcano-like correlation between NO2– reduction activity and x; the highest activity was observed for Pd53Au47, with an associated first-order rate constant of 5.12 L min–1 gmetal–1. Alloy NPs with greater proportions of Au were found to reduce the loss in catalytic activity due to sulfide fouling. Density functional theory calculations indicate that this is because Au weakens sulfur binding at PdAuNP surfaces due to atomic e...

Bimetallic Nanoparticles with Exotic Facet Structures via Iodide-Assisted Reduction of Palladium

Iodide is arguably the most challenging halide to control as a shape-directing additive in metal nanoparticle synthesis and the addition of iodide during bimetallic nanoparticle growth often leads to inhomogeneously stellated products. Through judicious control of low micromolar concentrations of iodide ions in solution in a seed-mediated approach, alloyed gold–palladium tetradecapod nanoparticles have been synthesized with a mixture of both well-defined convex and concave surfaces. Notably, these particles are uniform and symmetrical, and this unusual combination of convex and concave features in a single nanostructure is not simply an artifact of intersecting spikes, as would be the case with stellated particles. Further, an important new role for iodide in catalyzing the reduction of palladium ions is identified, particularly at the edge sites of the growing gold nanoparticles. This differs from the commonly accepted theory that iodide slows metal ion reduction, and thus opens up promising new routes to the synthesis of other bimetallic nanoparticles with exotic shapes and surface structures.

Multifaceted Gold-Palladium Bimetallic Nanorods and Their Geometric, Compositional, and Catalytic Tunabilities.

Kinetically controlled, seed-mediated co-reduction provides a robust and versatile synthetic approach to multimetallic nanoparticles with precisely controlled geometries and compositions. Here, we demonstrate that single-crystalline cylindrical Au nanorods selectively transform into a series of structurally distinct Au@Au-Pd alloy core-shell bimetallic nanorods with exotic multifaceted geometries enclosed by specific types of facets upon seed-mediated Au-Pd co-reduction under diffusion-controlled conditions. By adjusting several key synthetic parameters, such as the Pd/Au precursor ratio, the reducing agent concentration, the capping surfactant concentration, and foreign metal ion additives, we have been able to simultaneously fine-tailor the atomic-level surface structures and fine-tune the compositional stoichiometries of the multifaceted Au-Pd bimetallic nanorods. Using the catalytic hydrogenation of 4-nitrophenol by ammonia borane as a model reaction obeying the Langmuir-Hinshelwood kinetics, we further show that the relative surface binding affinities of the reactants and the rates of interfacial charge transfers, both of which play key roles in determining the overall reaction kinetics, strongly depend upon the surface atomic coordinations and the compositional stoichiometries of the colloidal Au-Pd alloy nanocatalysts. The insights gained from this work not only shed light on the underlying mechanisms dictating the intriguing geometric evolution of multimetallic nanocrystals during seed-mediated co-reduction but also provide an important knowledge framework that guides the rational design of architecturally sophisticated multimetallic nanostructures toward optimization of catalytic molecular transformations.