Bimetallic PdCu Research Articles

Investigation of Charge Transport in Percolating Network of PdCu Nanoclusters

Transport of charge carriers in percolating nanocluster devices based on bimetallic PdCu nanoclusters was investigated in this work. The device was fabricated by self-assembly of the nanoclusters between electrical electrodes inside an ultra-high vacuum compatible system. The average size of the produced nanoclusters was 7.3 nm, and the composition was Pd0.77Cu0.23. Systematic in situ current–voltage measurements as a function of temperature were performed which provide a conductance–temperature profile. The results are explained in terms of the charge carriers’ tunneling through small potential barriers at the junctions between nanoclusters. The results predict the size of the nanoclusters as well as the magnitude of the potential difference of the tunneling barriers. This investigation helps understanding the nature of the interface between the nanoclusters and the charge carrier transport within those devices to be utilized for optimizing gas sensing properties of PdCu nanocluster devices.

Preparation of Pd/Cu MCM-41 catalysts for hydrodechlorination: Influence of the synthesis procedure

A series of bimetallic mesostructured Pd/Cu MCM-41 catalysts were obtained by incipient wetness impregnation and direct hydrothermal synthesis using two different silica sources (silicates and Teos) and replacing the conventional hydrothermal treatment by the microwave hydrothermal one. Catalysts were tested in the hydrogen assisted dechlorination of CF3OCFClCF2Cl to CF3OCFCF2. The influence of the synthesis procedure on the supports and catalysts chemico-physical properties was investigated by XRD, N2 adsorption/desorption, HRTEM/EDS, and TPR. The incorporation of Pd and Cu in the course of MCM-41 synthesis, regardless of the hydrothermal treatment, did not destroy the typical hexagonal channel array and ordered pore system of MCM-41. However, the calcination for the removal of the template provoked the segregation of oxides, and thus large PdCu bimetallic particles are obtained after reduction. The impregnation leads to pore-occlusion, more remarkable for the sample obtained from silicates as silica source, and after reduction, both isolated monometallic Cu particles and large bimetallic PdCu particles coexist on the external surface of the support. The larger the metallic particles, the lower the conversion of CF3OCFClCF2Cl; while the presence of monometallic particles may decrease the selectivity in CF3OCFCF2, fostering the formation of CF3OCHCF2 by dehalogenation.

The effect of several parameters on catalytic denitrification of water by the use of H2 in the presence of O2 over metal supported catalysts

The present paper involves a detailed study of the selective catalytic reduction of nitrates in aqueous mediums by the use of H2 in the presence of O2 over monometallic and bimetallic supported catalysts. In this study, an attempt has been made to improve the denitrification efficiency (XNO3(-), SN2) of several catalysts by regulating some experimental parameters that are involved in the process. Therefore, the effects of the type of reactor (semi-batch reactor vs continuous flow reactor), the nature of the active phase (Pd, Cu, and Pd-Cu) and the particle size of γ-Al2O3 spheres (particle diameter = 1.8 mm and 3 mm) on catalytic activity and reaction selectivity, as well as the adsorption capacity of γ-Al2O3 spheres for nitrates, were examined. As the review indicates, most of the research has so far been conducted on batch or semi-batch reactors. This study successfully demonstrates the benefits of using a continuous flow reactor in terms of catalytic activity (XNO3(-), %) and reaction selectivity (SN2, %). Another important aspect of this study is the crucial role of bimetallic Pd-Cu clusters for the prevention of NH4(+) formation. Moreover, the use of 1.8 mm diameter γ-Al2O3 spheres as a support was proved to significantly enhance the catalytic performance of bimetallic Pd-Cu catalysts towards nitrate reduction compared to 3 mm diameter γ-Al2O3 spheres. This difference may be attributed to mass (NO3(-), OH(-)) transfer effects (external mass transfer phenomena).

Bimetallic PdCu nanoparticle decorated three-dimensional graphene hydrogel for non-enzymatic amperometric glucose sensor

A bimetallic PdCu nanoparticle (NP) decorated three-dimensional graphene hydrogel (PdCu/GE) was developed by a simple one-step hydrothermal method. The PdCu/GE hybrids exhibited an interconnected microporous framework with PdCu NPs dispersed and encapsulated within the GE layers. The PdCu/GE hybrids showed significant electrocatalytic activity toward glucose oxidation due to the synergistic effect of PdCu NPs and GE sheets in the alkaline solution containing chloride ions, presenting a substantial increase in the oxidation current and decrease in the onset potential of oxidation compared to the monometallic modified GE hybrids. At an applied potential of −0.4V, the PdCu/GE modified electrode with optimized bimetallic ratio presented quick respond to glucose oxidation with a wide linear range up to 18mM and a reproducible sensitivity of 48μA(mgmM)−1 in the presence of chloride ions. Furthermore, the PdCu/GE modified electrode exhibited high selectivity to glucose and resistance against poisoning by commonly interfering species such as dopamine, ascorbic acid, uric acid, acetamidophenol and some monosaccharides. The PdCu/GE hybrid hydrogels with 3D micropores were therefore promising for the future development of non-enzymatic amperometric glucose sensors with improved electrochemical performances.

Single atom alloy surface analogs in Pd0.18Cu15 nanoparticles for selective hydrogenation reactions

We report a novel synthesis of nanoparticle Pd-Cu catalysts, containing only trace amounts of Pd, for selective hydrogenation reactions. Pd-Cu nanoparticles were designed based on model single atom alloy (SAA) surfaces, in which individual, isolated Pd atoms act as sites for hydrogen uptake, dissociation, and spillover onto the surrounding Cu surface. Pd-Cu nanoparticles were prepared by addition of trace amounts of Pd (0.18 atomic (at)%) to Cu nanoparticles supported on Al2O3 by galvanic replacement (GR). The catalytic performance of the resulting materials for the partial hydrogenation of phenylacetylene was investigated at ambient temperature in a batch reactor under a head pressure of hydrogen (6.9 bar). The bimetallic Pd-Cu nanoparticles have over an order of magnitude higher activity for phenylacetylene hydrogenation when compared to their monometallic Cu counterpart, while maintaining a high selectivity to styrene over many hours at high conversion. Greater than 94% selectivity to styrene is observed at all times, which is a marked improvement when compared to monometallic Pd catalysts with the same Pd loading, at the same total conversion. X-ray photoelectron spectroscopy and UV-visible spectroscopy measurements confirm the complete uptake and alloying of Pd with Cu by GR. Scanning tunneling microscopy and thermal desorption spectroscopy of model SAA surfaces confirmed the feasibility of hydrogen spillover onto an otherwise inert Cu surface. These model studies addressed a wide range of Pd concentrations related to the bimetallic nanoparticles.

Bimetallic catalysts for the hydrogenation of aromatic nitro compounds

The catalytic activity of bimetallic Pd-Ni, Pd-Cu, and Pd-Ag catalysts in the reaction of nitrobenzene hydrogenation was studied. It was found that the rate of hydrogenation on the bimetallic catalysts was higher than that on palladium metal (palladium black), and it depended on catalyst preparation pro cedures. The catalytic activity of the bimetallic systems increased in the order Pd-Cu< Pd-Ag ≪ Pd-Ni.

Pt-Cu bimetallic catalysts obtained from layered double hydroxides by an anion-exchange route

Bimetallic Pt-Cu catalyst precursors have been prepared from NO3−‐Mg/Al layered double hydroxide (LDH) by anionic exchange with negatively charged copper hydroxy citrate colloids ([Cu(C6O7H3)x(OH)]yy(3x−1)−) and [PtCl6]2− species. Successive exchanges by varying the addition order of the two anionic species allowed removing NO3− almost completely and obtaining Cu/Pt molar ratios of 50.2 and 6.8 when Cu-containing colloids were introduced first (CuPt) or second (PtCu), respectively. After calcination and reduction treatments, the resulting materials were characterized by TPR and XPS analyses. Pt and Cu particles were homogeneously dispersed with the particle size of 2–7nm and the portion of Cu present as Pt-Cu alloy was 48.9% (CuPt) and 69.8% (PtCu), showing remarkably enhanced alloy formation by the PtCu exchange order. Using this PtCu material, total conversion in nitrate hydrogenation reaction was achieved with higher selectivity toward nitrogen than those reported with bimetallic Pd-Cu catalysts prepared from LDH precursors by impregnation and co-precipitation routes.

Reduction of nitrate on active carbon supported Pd-Cu catalysts

Bimetallic Pd-Cu catalysts supported on active carbon were tested for nitrate reduction in water. The catalysts were prepared either by successive incipient wetness impregnation or catalytic reduction and were characterized by HRTEM, XRD, TPR-H 2 and H 2-chemisorption. The method of preparation exerts an influence on the activity of Pd-Cu catalysts for nitrate reduction, but the effect on their selectivity to ammonia is negligible. The Cu/Pd atomic ratio and total metal content affect both activity and selectivity of the catalysts prepared by chemical reduction.

Synthesis and Characterization of Bimetallic PdM Nanoparticles (M =Ag, Cu) Oxygen Reduction Electrocatalysts

Electrochemical studies on the catalytic activity of bimetallic PdAg and PdCu compounds for the oxygen reduction reaction (ORR) in acid media are presented. The electrocatalytic activity for the ORR was evaluated using cyclic voltammetry (CV) and rotating-disk electrode (RDE) measurements in a 0.5 M H2SO4 electrolyte at 25 {degree sign}C. These materials were synthesized through PdCl2, AgNO3 and CuCl reductions with NaBH4 in a THF solution. The synthesized powder electrocatalysts were characterized by powder X-ray diffraction (XRD) and data analyzed with the software MDI jade 5.0. The XRD results show evidence of the alloy particles with 3nm as average sizes. The electrochemical current-potential behavior was compared with the Palladium and Platinum. Kinetics results show that the bimetallic catalysts containing Ag and Cu are more active for the ORR than the Pd pure, and less active than the nanometric Pt.

Microstructural Characterization of Colloid-Derived Bimetallic Pd-Cu Nanocatalysts Supported on γ-Al2O3 for Nitrate Reduction

Extended abstract of a paper presented at Microscopy and Microanalysis 2008 in Albuquerque, New Mexico, USA, August 3 – August 7, 2008

Vapor phase synthesis of supported Pd, Au, and unsupported bimetallic nanoparticle catalysts for CO oxidation

We report the vapor phase synthesis and characterization of supported Pd, Au and unsupported bimetallic nanoparticle catalysts for CO oxidation. The approach utilized in the present work is based on the laser vaporization/controlled condensation technique which uniquely combines the features of pulsed laser vaporization with the controlled condensation process from the vapor phase to synthesize nanoparticle catalysts of controlled size and composition. The results indicate that supported Pd/CeO 2, Au/CeO 2, and unsupported bimetallic CuPd, CuAu, and AuPd nanoparticle catalysts exhibit excellent activity for CO oxidation. The significance of the current method lies mainly in its simplicity, flexibility and the control of the different factors that determine the activity of the nanoparticle catalysts.

CO adsorption on monometallic Pd, Rh, Cu and bimetallic PdCu and RhCu monolayers supported on Ru(0 0 0 1)

This work presents a systematic periodic density functional study of the changes in CO adsorption on metallic Rh, Pd, Cu monolayers supported on Ru(0 0 0 1) relative to CO adsorption on their corresponding Rh(1 1 1), Pd(1 1 1), Cu(1 1 1) single metal surfaces. Moreover, CO adsorption on various compositions of bimetallic PdCu and RhCu overlayers, also supported on Ru(0 0 0 1) have been considered. The metallic support affects each metallic crystal surface in a different way. However, the changes in the ability of back-donation from each metal surface to the CO(2π*) antibonding molecular orbital explain the variation of the adsorption energies. On the contrary, the presence of Ru does not affect the CO adsorption geometry. For the supported bimetallic overlayers, the CO–Pd and CO–Rh bond strength is enhanced by the presence of Cu. It is suggested that this effect is due to charge transfer from Cu the other metals thus enhancing their ability to back-donate to CO. The study of adsorption on different sites of the different bimetallic monolayers indicates that the key factor controlling CO adsorption energy is the nature of the metallic atoms constituting the active sites. These findings have been confirmed for two different CO coverage situations.

Use of IR and XANES to Monitor Catalysts During Preparation, Regeneration and Reaction

The combined use of IR and XANES to gain information on the behaviour of catalytic solids under in situ conditions is examined. This approach provides details concerning the evolution of the surface and bulk of supported phases, allowing an interpretation of the chemical response of catalysts to reactive gas atmospheres. Examples will be given of bimetallic PdCu systems used in the hydrogenation of CO, PtRe during activation and regeneration (reduction/oxychlorination) treatments and Pt/BaO/Al2O3 catalysts used for NOx storage and reduction.

A simple method for absolute surface coverage determination for metal-film deposition

An accurate method of calibrating the absolute surface coverage delivered for metal deposition sources based on a simple ex-situ analysis utilizing atomic absorption spectroscopy with a sensitivity to coverages ≤0.10 ML is described. The usefulness of the method is demonstrated by two case studies involving growth of ultra-thin Pd films on highly oriented pyrolytic graphite (HOPG) and bimetallic CuPd film growth on HOPG. We demonstrate that traditional methods of characterizing thin-film growth mechanisms based on Auger electron spectroscopy or X-ray photo-electron spectroscopy which alone often provide misleading results, when used in combination with the ex-situ calibration technique described, allow definitive conclusions regarding both the film growth mode and the absolute amount of metal deposited. This technique allows absolute calibration of the quartz crystal microbalance (QCM) and provides a viable alternative for laboratories without in-situ QCM facilities.

Oxidation state of bimetallic PdCu catalysts during liquid phase nitrate reduction

The oxidation state and the structural properties of Al2O3-supported bimetallic PdCu catalysts during the catalytic reduction of KNO3 carried out in the aqueous phase were investigated by X-ray absorption spectroscopy. Under reaction conditions the noble metal component (Pd) was in a reduced state, while the less noble metal (Cu) was found to be partially oxidized. A PdCu phase was formed in the bimetallic catalysts, which appears to be located in small domains on the surface of Pd rich particles.

Pd/Cu alloys as hydrodechlorination catalysts: a medium-energy ion scattering study of surface composition under exposure to chlorinated hydrocarbons

Medium-energy ion scattering (MEIS) is a surface analytical technique with the unique ability to provide layer-by-layer compositional information for crystalline materials such as the bimetallic PdCu alloy investigated here. MEIS has been used to study the segregation effects induced by the adsorption of trichloroethene onto a Pd0.5Cu0.5(110) single crystal; this system is a model for industrially important hydrodechlorination catalysts. The PdCu (110) clean surface is shown to be slightly Pd-rich under the preparation conditions used, with oscillatory composition going into the bulk. Adsorption of trichloroethene caused Cu enrichment of the first atomic layer, although this is seen to decrease with higher temperature annealing. Comparable experiments using ethene indicate that adsorbed carbon and hydrocarbon species can give rise to considerable Pd segregation. The chemistry of trichloroethene adsorption on PdCu(110) is such that considerable residual carbon and hydrocarbons may be present and, because this induces an opposing effect, it is probable that the effect of adsorbed Cl on Cu segregation is larger than that observed directly. Copyright © 2000 John Wiley & Sons, Ltd.

Studies on the use of catalytic membranes for reduction of nitrate in drinking water

Hydrogenation of nitrate using supported bimetallic Pd-Cu or Pd-Sn catalysts provides a promising new alternative for denitrification of drinking water with several advantages over established technologies such as reverse osmosis or biological denitrification (Sell et al., 1992). One of the major problems of this technology is the formation of undesired side products, i.e. ammonium and nitrite, both subject to a limiting value far below the admissible nitrate content. Previous studies have shown that the particle size, among other effects, is a key parameter determining the selectivity of the catalyst, pointing to a detrimental effect of pore diffusion limitation (Hähnlein et al., 1998). Moreover, activity and selectivity depend on the concentration of dissolved hydrogen, i.e. high concentration means high activity, but at the same time favoring of ammonium formation due to further hydrogenation. In this paper a porous catalytic membrane acting as a hydrogen diffuser is proposed as a means to solve these problems by creating an efficient three-phase contact between the catalytic surface, dissolved nitrate, and hydrogen gas. Such a design offers the potential to control the activity and selectivity of the process through controlled dosage of hydrogen. Experimental data on the hydrogenation of aqueous nitrate using commercial ceramic membranes modified by insertion of palladium and copper or tin into the pore-structure through impregnation and MOCVD (metalorganic chemical vapor deposition) are presented and discussed together with characterization results obtained by SEM/EDAX and XPS.

Epitaxial Growth of Clusters on Defined Oxide Surfaces-HRTEM Studies

Pd and bimetallic PdCu particles (2-20 nm), epitaxially grown on oxide substrates were characterised by HRTEM, WBDF imaging, numerical analysis and image simulations. Their shape and the structure of the interface were determined according to the particle size. All the particles are octahbedra or tetrahedra depending on the symmetry of the substrate, limited by (100) and (111) faces. The ratio of interface energies was estimated as a function of the particle sizes. Deviations of the surface tension anisotrpy, compared to the macroscopic fcc crystals, were observed together with the sharpening of the angles between the limiting faces. It was shown that the particle strongly depends on the interactions between the metal and the oxide substrate and on the structure of the interface. Indeed, polyhedral shapes with re-entrant angles at the interface were found in particles with defects or disclocations at the interface. On the opposite, particles perfectly accommodated to the substrate at the atomic scale did not present any re-entrant angles.

Interaction of CO and NO with PdCu(111) Surfaces

A theoretical study of the structural parameters, interaction energies, and bonding mechanism of CO and NO to a Pd center located in two copper-rich bimetallic PdCu(111) surfaces and several coordination positions of the Pd(111) surface is reported. For CO, the bonding nature is predominantly covalent, and the analysis of bonding nature variations through the series is used to interpret the experimentally observed decrease of the CO/PdCu interaction energy with the increase in copper percentage of the alloy, the insensitivity of the C−O stretch frequency to the composition of the central Pd environment, and the linear correlation observed between the CO desorption energy and the X-ray photoelectron spectroscopy (XPS) core-level shift. For NO, the nature of the interaction varies from nearly covalent for pure Pd to a mixture of covalent plus ionic for copper-rich alloys; this parallels the expected growth of the ionic component of the bond with the decrease of the work function in going from Pd to Cu. A co...

Bimetallic PdCu and PdCu3Particles Prepared by Wet Impregnation-HRTEM Study of the Structure and the Interface with the MgO Substrate

Bimetallic particles of PdCu and PdCu3 are prepared by decomposition of orga- nometallic compounds on MgO micro-cubes. During the annealing between 350 and 400 °C, in reducing atmosphere, PdCu particles grow with the fcc structure, without defined shape. The particles epitaxially (001) oriented on MgO are dilated and accommodated to the substrate. After annealing at 450 ° C, the particles adopt the 03B2 ordered structure. The particles are limited at the edges by (100) and (110) faces. Most of them are oriented (001) on MgO with (110)PdCu ~ (100)MgO, which corresponds to a perfect accommodation of the lattices without deformation compared to the bulk, despite of the reduction in H2. The PdCu3 particles smaller than about 10 nm were found with the 03B1 ordered structure, without periodic anti-phase boundaries.