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At least 883 records · Page 49Linked to original sources
PubMed2005

Recyclable and reusable Pd(OAc)2/DABCO/PEG-400 system for Suzuki-Miyaura cross-coupling reaction.

[reaction: see text] A stable and efficient Pd(OAc)2/DABCO (triethylenediamine) catalytic system for Suzuki-Miyaura cross-coupling reaction has been developed. In the presence of Pd(OAc)2 and DABCO, coupling of aryl halides with arylboronic acids was carried out smoothly to afford good to excellent yields and high turnover numbers (TONs) (the maximal TONs were up to 960,000 for the reaction of 1-iodo-4-nitrobenzene with phenylboronic acid) using PEG-400 as the solvent. Moreover, the Pd(OAc)2/DABCO/PEG-400 system could be recycled and reused five times without any loss of catalytic activity for aryl iodides and bromides.

Journal Article↗
PubMed2005

Pd(II)-bipyridine catalyzed conjugate addition of arylboronic acid to alpha,beta-unsaturated carbonyl compounds.

[Reaction: see text]. A Pd(II)-catalyzed conjugate addition of arylboronic acid to alpha,beta-unsaturated ketones, aldehydes, esters, etc. in the presence of 2,2'-bipyridine was developed. A mechanism involving transmetalation, insertion of the carbon-carbon double bond into the C-Pd bond, and protonolysis of the resulting C-Pd bond is proposed. The reaction conditions are mild and the yield is high. The presence of 2,2'-bipyridine is crucial for the reaction to inhibit beta-hydride elimination.

Journal Article↗
PubMed2006

(IPr)Pd(acac)Cl: an easily synthesized, efficient, and versatile precatalyst for C-N and C-C bond formation.

A very straightforward synthesis of (IPr)Pd(acac)Cl from two commercially available starting materials, Pd(acac)2 and IPr.HCl [acac = acetylacetonate; IPr = N,N'-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], has been developed. The resulting complex, (IPr)Pd(acac)Cl (1), has proven to be a highly active PdII precatalyst in the Buchwald-Hartwig and the alpha-ketone arylation reactions. A wide range of substrates has been screened, including unactivated, sterically hindered, and heterocyclic aryl chlorides.

Journal Article↗
PubMed2005

Structure and deformations of Pd-Ni core-shell nanoparticles.

Homogeneous collections of Pd-Ni core-shell nanoparticles have been prepared by decomposition of metal-organic compounds and studied by several electron microscopy techniques: transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), energy-filtered microscopy (EFTEM), and by X-ray photoelectron spectroscopy (XPS). The physical and chemical properties of the Pd shell are supposed to depend on its electronic properties, which are influenced by the presence of the Ni core and by the deformation in the Pd lattice. Here, the interfacial structure of Pd/Ni and the lattice deformations in the core and the shell are studied in detail. The catalytic properties of the pure metal and the bimetallic particles, toward CO oxidation, have been investigated.

Journal Article↗
PubMed2005

A molecular beam study of the NO + CO reaction on Pd(111) surfaces.

Nitric oxide (NO) reduction with carbon monoxide (CO) on the Pd(111) surface was studied under isothermal conditions by molecular beam techniques as a function of temperature, NO:CO beam composition, and beam flux. Systematic experiments were performed under transient and steady state conditions. Displacement of adsorbed CO by NO in the transient state of the reaction was observed at temperatures between 375 and 475 K for all the NO:CO compositions studied. NO accumulation occurs on Pd(111) surface under steady state conditions, below 475 K, due to stronger chemisorption of NO. The steady state reaction rates attain a maximum at about 475 K, nearly independent of beam composition. N2 was found to be the major product of the reduction, along with a minor production of N2O. The production of N2 and N2O indicates molecular and dissociative adsorption of NO on Pd(111) at temperatures up to 525 K. Postreaction TPD measurements were performed in order to determine the nitrogen coverage under steady-state conditions. Finally, the results are discussed with respect to the rate-controlling character of the different elementary steps of the reaction system.

Journal Article↗
PubMed2005

Nanostructured, Gd-doped ceria promoted by Pt or Pd: investigation of the electronic and surface structures and relations to chemical properties.

Nanostructured ceria doped with other rare earth elements is a good oxygen ion conductor, which gives rise to various catalytic applications such as the construction of membranes for syngas production by partial oxidation of methane. This article focuses on the Gd-doped cerium dioxides, which can be modified with Pt or Pd to enhance the reactivity of the lattice oxygen in interaction with methane. The aim of the work is the elucidation of correlations between the structural, electronic, and chemical properties of these nanomaterials. Detailed studies were performed for a series of samples with and without surface modification by noble metals using a complex combination of physicochemical methods: XRD, TEM, CH(4) TPR, XPS, SIMS, and FTIR spectroscopy of adsorbed CO. XPS and TPR data revealed that surface modification with noble metals enhances the reducibility of the doped ceria support, where the effect is more pronounced for Pd than for Pt. The formation of highly cationic Pd species due to strong metal support interactions provides a possible explanation for this behavior. Furthermore, the results obtained in the present work for the Gd-doped ceria system are compared to those obtained previously for the Pr-doped ceria system.

Journal Article↗
PubMed2005

Interaction of Pd and PdCl2 with cellulose: a theoretical investigation.

The pyrolytic fragmentation of cellulose in the presence of atomic palladium (Pd) and palladium(II) chloride (PdCl2) has been studied with use of hybrid density functional theory and cellobiose as a model for cellulose. The configuration changes in the host, rearrangement of geometries of the products, and the respective reaction energetics for different fragmentation pathways are analyzed. While Pd is found to undergo insertion at the beta-1,4-linkage oxygen (O1)-carbon (C-1) of the rings, Pd(II) chloride is observed to promote the cleavage of the chain as well as rearrangement of the rings. A detailed mechanism for the formation of levoglucosan from one of the fragments following the interaction with PdCl2 is also highlighted.

Cellobiose↗
PubMed2005

Realization of a particle-in-a-box: electron in an atomic Pd chain.

Well-defined Pd chains were assembled from single atoms on a NiAl(110) surface with the tip of a scanning tunneling microscope. The electronic properties of the chains were determined by spatially resolved conductance measurements, revealing a series of quantum well states with parabolic dispersion. The particle-in-a-box states in Pd chains show higher onset energy and larger effective mass than those in Au chains investigated before, reflecting the influence of elemental composition on one-dimensional electronic systems. The intrinsic widths and spectral intensities of Pd induced states provide information on lifetime and spatial localization of states in the atomic chain.

Letter↗
PubMed2005

In situ studies of methanol decomposition and oxidation on Pd(111) by PM-IRAS and XPS spectroscopy.

Methanol decomposition and oxidation on Pd(111) at millibar pressure were studied by in situ polarization-modulation infrared reflection absorption spectroscopy (PM-IRAS), on-line gas chromatography and pre- and postreaction X-ray photoelectron spectroscopy (XPS). Various dehydrogenation products such as methoxy CH3O, formaldehyde CH2O, formyl CHO, and CO could be spectroscopically identified. Methanol oxidation proceeds via dehydrogenation to formaldehyde CH2O, which either desorbs or is further dehydrogenated to CO, which is subsequently oxidized to CO2. Carbonaceous overlayers that are present during the reaction may favorably affect the selectivity toward CH2O. The reaction takes place on metallic Pd, and no indications of an involvement of Pd surface oxide were observed.

Journal Article↗
PubMed2006

An ab initio periodic study of NiO supported at the Pd(100) surface. Part 1: The perfect epitaxial monolayer.

The epitaxial monolayer of NiO on Pd(100) has been theoretically simulated using a hybrid-exchange GGA-DFT Hamiltonian and a slab model, periodic in two dimensions. This "perfect" system is an essential reference for the simulation of nonstoichiometric two-dimensional phases which are formed during the deposition of nickel on palladium in the presence of oxygen. The adequacy of the computational procedure is discussed, especially as concerns the use of a thermal smearing technique which has been adopted to improve the convergence properties of the SCF procedure and to prevent the onset of nonphysical spin-polarized solutions. The equilibrium configuration corresponds to antiferromagnetic order in the overlayer, with oxygens on top of the surface Pd atoms; the ad-film is slightly corrugated with Ni closer to the surface by 0.1 A with respect to O. The interaction energy is quite small, 0.20 eV/NiO unit (about 5 kcal mol(-1)); correspondingly, the electronic and magnetic properties of the Pd slab and the isolated NiO monolayer are only marginally affected by the interaction.

Journal Article↗
PubMed2006

Comparative theoretical study of formaldehyde decomposition on PdZn, Cu, and Pd surfaces.

Methanol steam reforming, catalyzed by Pd/ZnO (PdZn alloy), is a potential source of hydrogen for on-board fuel cells. CO has been reported to be a minor side product of methanol decomposition that occurs in parallel to methanol steam reforming on PdZn catalysts. However, fuel cells currently used in vehicles are very sensitive to CO poisoning. To contribute to the understanding of pertinent reaction mechanisms, we employed density functional slab model calculations to study the decomposition of formaldehyde, a key intermediate in methanol decomposition and steam reforming reactions, on planar surfaces of Pd, Cu, and PdZn as well as on a stepped surface of PdZn. The calculated activation energies indicate that dehydrogenation of formaldehyde is favorable on Pd(111), but unfavorable on Cu(111) and PdZn(111). On the stepped PdZn(221) surface, the dehydrogenation process was calculated to be more competitive to formaldehyde desorption than on PdZn(111). Thus, we ascribe the experimentally observed small amount of CO, formed during steam reforming of methanol on the Pd/ZnO catalyst, to occur at metallic Pd species of the catalyst or at defect sites of PdZn alloy.

Journal Article↗
PubMed2006

Adsorption of small Au(n) (n = 1-5) and Au-Pd clusters inside the TS-1 and S-1 pores.

We used a hybrid quantum-mechanics/molecular-mechanics (QM/MM) approach to simulate the adsorption of Au(n)() (n = 1-5), AuPd, and Au(2)Pd(2) clusters inside the TS-1 and S-1 pores. We studied nondefect and metal-vacancy defect sites in TS-1 and S-1 for a total of four different environments around the T6 crystallographic site. We predict stronger binding of all clusters near Ti sites in Ti-substituted framework compared to adsorption near Si sites-consistent with the experimental finding of a direct correlation between the Ti-loading and the Au-loading on the Au/TS-1 catalysts with high Si/Ti ratio. The cluster binding is also stronger near lattice-metal vacancies compared to fully coordinated, nondefect sites. In all the cases, a trend of binding energy (BE) versus Au cluster size (n) shows a peak at around n = 3-4. Our results show that there is enough room for the attack of H(2)O(2) on the Ti-defect site even with Au(1-4) adsorbed-a result that supports the possibility of H(2)O(2) spillover from the Au clusters to the adjacent Ti-defect sites. Mulliken charge analysis indicates that in all the cases there is electron density transfer to adsorbed clusters from the zeolite lattice. In the case of both gas-phase and adsorbed Au-Pd clusters, all the Pd atoms were positively charged, and all the Au atoms were negatively charged due to the higher electron-affinity of Au. We also found a correlation between the BE and the charge transfer to the clusters (the higher the charge transfer to the clusters, the higher the BE), and a universal correlation was found for Au(2-5) when BE and charge transfer were plotted on a per atom basis. A relatively larger charge transfer to the adsorbed clusters was found for the Ti sites versus the Si sites, and for the defect sites versus the nondefect sites. The trends in the BE were corroborated using Gibbs free energy of adsorption (DeltaG(ads)), and the implications of DeltaG(ads) in sintering of Au clusters are also discussed. Our results confirm that electronic factors such as cluster-charging are potentially important support effects for the Au/TS-1 catalyst.

Journal Article↗
PubMed2005

Electrochemical properties of monolayer-protected Au and Pd nanoparticles extracted from within dendrimer templates.

The electrochemical properties of Au and Pd monolayer-protected clusters (MPCs), prepared by dendrimer-templating and subsequent extraction, are described. Differential pulse voltammetry was used to estimate the size of the MPCs, and the results were compared to microscopic data and calculated values. Purification of the extracted Au and Pd nanoparticles was not required to obtain well-defined differential pulse voltammetry peaks arising from quantized double-layer charging. The calculated sizes of the nanoparticles were essentially identical to those determined from the electrochemical data. The capacitance of the particles was independent of the composition of core metal. Transmission electron microscopy data overestimated the size of the smallest Pd nanoparticles because of inadequate point-to-point resolution.

Journal Article↗
PubMed2006

Modification of the surface adsorption properties of alumina-supported Pd catalysts for the electrocatalytic hydrogenation of phenol.

The electrocatalytic hydrogenation (ECH) of phenol has been studied using palladium supported on gamma-alumina (10% Pd-Al2O3) catalysts. The catalyst powders were suspended in aqueous supporting electrolyte solutions containing methanol and short-chain aliphatic acids (acetic acid, propionic acid, or butyric acid) and were dynamically circulated through a reticulated vitreous carbon cathode. The efficiency of the hydrogenation process was measured as a function of the total electrolytic charge and was compared for different types of supporting electrolyte and for various solvent compositions. Our results show that these experimental parameters strongly affect the overall ECH efficiency of phenol. The ECH efficiency and yields vary inversely with the quantity of methanol present in the electrolytic solutions, whereas the presence of aliphatic carboxylic acids increased the ECH efficiency in proportion to the chain length of the specific acids employed. In all cases, ECH efficiency was directly correlated with the adsorption properties of phenol onto the Pd-alumina catalyst in the studied electrolyte solution, as measured independently using dynamic adsorption isotherms. It is shown that the alumina surface binds the aliphatic acids via the carboxylate terminations and transforms the catalyst into an organically functionalized material. Temperature-programmed mass spectrometry analysis and diffuse-reflectance infrared spectroscopy measurements confirm that the organic acids are stably bound to the alumina surface below 200 degrees C, with coverages that are independent of the acid chain length. These reproducibly functionalized alumina surfaces control the adsorption/desorption equilibrium of the target phenol molecules and allow us to prepare new electrocatalytic materials to enhance the efficiency of the ECH process. The in situ grafting of specific aliphatic acids on general purpose Pd-alumina catalysts offers a new and flexible mechanism to control the ECH process to enhance the selectivity, efficiency, and yields according to the properties of the specific target molecule.

Journal Article↗
PubMed2006

Molecular adsorption at well-defined electrode surfaces: hydroquinone on Pd(111) studied by EC-STM.

The interaction of hydroquinone (H2Q) with well-defined Pd(111) surfaces at preselected potentials in dilute H2SO4 has been studied by molecule-resolved electrochemical scanning tunneling microscopy (EC-STM). H2Q spontaneously undergoes oxidative chemisorption to benzoquinone (Q), which adopts a slightly tilted parallel orientation. Evidently, the surface coordination is through the quinone pi-electron system. At potentials within the double-layer region, a close-packed well-ordered Pd(111)-(3 x 3)-Q adlattice was formed. A potential excursion to 0.7 V, a potential at which the solution-phase Q/H2Q redox reaction takes place, introduced disorder into the organic adlayer; this positive-potential-induced order-to-disorder phase transition is reversible because the ordered (3 x 3)-Q adlattice was regenerated when the potential reverted to 0.4 V. When the potential was poised at 0.2 V, a potential at which hydrogen evolution was initiated, an appreciable fraction of Q was (hydrogenatively) desorbed; the remnant Q molecules were agglomerated in small islands that retained the (3 x 3) symmetry of the full adlayer. Two possible structural models of the Pd(111)-(3 x 3)-Q adlattice are described.

Journal Article↗
PubMed2006

Plasmonic properties of supported Pt and Pd nanostructures.

The plasmonic properties of nanodisk arrays of Pt, Pd, and, for comparison, Ag are studied over a large size and spectral range and analyzed theoretically by an electrostatic model. Pt and Pd nanodisks exhibit broad localized surface plasmons with a higher sensitivity of the plasmon to the disk aspect ratio compared to Ag. Extinction cross-sections are generally about 50% smaller for Pt and Pd. The spectral plasmon positions, line-widths, and extinction cross-sections are well reproduced by the model.

Computer Simulation↗
PubMed2000

An efficient Pd-catalyzed route to silyl esters

[formula: see text] A one-step, highly selective catalytic route to silyl esters is reported. Commercially available silanes with Si-H functionality were converted to silyl or siloxy esters in the presence of Pd(OAc)2 under mild reaction conditions. This protocol was found to be equally applicable for the modification of multiple silicon centers in one framework and lead to the corresponding polysilyl esters in high yields. A comparison of catalytic efficiency of Pd(OAc)2 versus Pd on carbon was also undertaken.

Journal Article↗
PubMed2001

Mizoroki-Heck type reaction of organoboron reagents with alkenes and alkynes. A Pd(II)-catalyzed pathway with Cu(OAc)2 as an oxidant.

[reaction: see text]. In contrast to the Pd(0)-catalyzed mechanism by Uemura, Mizoroki-Heck type reaction of boronic acids is found to proceed under a Pd(II)-mediated pathway using a catalytic amount of Pd(OAc)2 in the presence of Cu(OAc)2 as an oxidant. Treatment of a variety of alkenes with boronic acids, boronates, and sodium tetraphenylborate furnishes beta-arylated and alkenylated products in good to excellent yields. The reactions with norbornene, norbornadiene, and diphenylacetylene are also performed to give 1:2 or 2:1 coupling products.

Journal Article↗