Aqueous‐Phase Synthesis of Single‐Crystal Pd Seeds 3 nm in Diameter and Their Use for the Growth of Pd Nanocrystals with Different Shapes

This article describes an aqueous method for the synthesis of Pd seeds with a single‐crystal structure and a uniform diameter of 3 nm and their use for the growth of Pd nanocrystals with a variety of shapes. We have also investigated the effects of a number of parameters, including the temperature, reducing power of the reductant, and capping agent on the reduction rate of a Pd precursor, and thus the final size, size distribution, and morphology of the Pd seeds. By taking advantage of the coordination effect of Br^? ions with Pd²⁺ ions and their selective adsorption on the Pd(100) surface, Pd nanocrystals with a number of distinct shapes could be conveniently produced by varying the concentration of KBr added into the growth solution. This work provides a general and facile method for the green synthesis of Pd nanocrystals with controlled shapes, especially for the preparation of Pd nanocrystals with sizes in the sub‐10 nm regime.     

Adsorption of carbon on Pd clusters of nanometer size: a first-principles theoretical study

Adsorbed atomic C species can be formed in the course of surface reactions and commonly decorate metal catalysts. We studied computationally C adsorption on Pd nanoclusters using an all-electron scalar relativistic density functional method. The metal particles under investigation, Pd(55), Pd(79), Pd(85), Pd(116), Pd(140), and Pd(146), were chosen as fragments of bulk Pd in the form of three-dimensional octahedral or cuboctahedral crystallites, exposing (111) and (100) facets as well as edge sites. These cluster models are shown to yield size-converged adsorption energies. We examined which surface sites of these clusters are preferentially occupied by adsorbed C. According to calculations, surface C atoms form strongly adsorbed carbide species (with adsorption energies of more than 600 kJ mol(-1)) bearing a significant negative charge. Surface sites allowing high, fourfold coordination of carbon are overall favored. To avoid effects of adsorbate-adsorbate interaction in the cluster models for carbon species in the vicinity of cluster edges, we reduced the local symmetry of selected adsorption complexes on the nanoclusters by lowering the global symmetry of the nanocluster models from point group O(h) to D(4h). On (111) facets, threefold hollow sites in the center are energetically preferred; adsorbed C is calculated to be slightly less stable when displaced to the facet borders.     

A stepwise approach to the formation of heterometallic discrete complexes and infinite architectures

A strategy for the controlled construction of heterobimetallic discrete complexes and 1-D coordination networks is presented. The organic ligand based on the methanedithiolate group and the 4,5-diazafluorene moiety behaving as primary and secondary coordination poles respectively leads to the formation of a series of discrete metal complexes with various geometries via binding through the dithiolate site. The observed coordination geometries range from square-planar for Ni(ii) and Pd(ii) to distorted tetrahedral for Zn(ii) and Hg(ii). The square-planar Pd(ii) complex affords a discrete bimetallic trinuclear complex when treated with a capped Ni(ii) center. All three Ni(ii), Pd(ii) and Hg(ii) discrete complexes have been also shown to behave as metallatectons leading to the generation of infinite networks in the presence of bridging cations such as sodium.     

Low‐Coordinated Edge Sites on Ultrathin Palladium Nanosheets Boost Carbon Dioxide Electroreduction Performance

Electrochemical conversion of carbon dioxide (CO₂) to value‐added products is a possible way to decrease the problems resulting from CO₂ emission. Thanks to the eminent conductivity and proper adsorption to intermediates, Pd has become a promising candidate for CO₂ electroreduction (CO₂ER). However, Pd‐based nanocatalysts generally need a large overpotential. Herein we describe that ultrathin Pd nanosheets effectively reduce the onset potential for CO by exposing abundant atoms with comparatively low generalized coordination number. Hexagonal Pd nanosheets with 5 atomic thickness and 5.1 nm edge length reached CO faradaic efficiency of 94 % at ?0.5 V, without any decay after a stability test of 8 h. It appears to be the most efficient among all of Pd‐based catalysts toward CO₂ER. Uniform hexagonal morphology made it reasonable to build models and take DFT calculations. The enhanced activity originates from mainly edge sites on palladium nanosheets.     

Hydroxyapatite-supported palladium nanoclusters: a highly active heterogeneous catalyst for selective oxidation of alcohols by use of molecular oxygen

Treatment of a stoichiometric hydroxyapatite (HAP), Ca10(PO4)6(OH)2, with PdCl2(PhCN)2 gives a new type of palladium-grafted hydroxyapatite. Analysis by means of powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), energy-dispersive X-ray (EDX), IR, and Pd K-edge X-ray absorption fine structure (XAFS) proves that a monomeric PdCl2 species is chemisorbed on the HAP surface, which is readily transformed into Pd nanoclusters with a narrow size distribution in the presence of alcohol. Nanoclustered Pd0 species can effectively promote the alcohol oxidation under an atmospheric O2 pressure, giving a remarkably high turnover number (TON) of up to 236,000 with an excellent turnover frequency (TOF) of approximately 9800 h(-1) for a 250-mmol-scale oxidation of 1-phenylethanol under solvent-free conditions. In addition to advantages such as a simple workup procedure and the ability to recycle the catalyst, the present Pd catalyst does not require additives to complete the catalytic cycle. The diameters of the generated Pd nanoclusters can be controlled upon acting on the alcohol substrates used. Oxidation of alcohols is proposed to occur primarily on low-coordination sites within a regular arrangement of the Pd nanocluster by performing calculations on the palladium crystallites.     

Tripodal oxazoline-based homochiral coordination cages with internal binding sites

Homochiral coordination cages, which have two well-defined internal binding sites for ammonium and organoammonium ions, have been constructed by Pd(II)-mediated self-assembly of preorganized tripodal oxazolines containing pyridine pendant groups.     

Partially crystallized Pd nanoparticles decorated TiO2 prepared by atmospheric-pressure cold plasma and its enhanced photocatalytic performance

TiO2 decorated with partially crystallized Pd nanoparticles （Pd/TiO2-P） was successfully prepared by atmospheric-pressure dielectric barrier discharge cold plasma. The XRD and XPS analyses proved that Pd ions were reduced to partially crystallized metallic Pd nanoparticles in Pd/TiO2-P. The XPS spectra also indicated that an enhanced metal-support interaction was formed due to the existence of partially crystallized Pd nanoparticles with lower coordination number in Pd/TiO2-P. Photocatalytic activity of Pd/TiO2-P was much higher than that of TiO2 samples decorated with well crystallized Pd nanoparticles.     

Pd-Ga intermetallic compounds as highly selective semihydrogenation catalysts

The intermetallic compounds Pd(3)Ga(7), PdGa, and Pd(2)Ga are found to be highly selective semihydrogenation catalysts for acetylene outperforming established systems. The stability of the crystal and electronic structure under reaction conditions allows the direct relation of structural and catalytic properties and a knowledge-based development of new intermetallic catalyst systems. In the crystal structure of PdGa palladium is exclusively surrounded by gallium atoms. The alteration of the Pd coordination in PdGa leads to a strong modification of the electronic structure around the Fermi level in comparison to elemental Pd. Electronic modification and isolation of active sites causes the excellent catalytic semihydrogenation properties.     

Partially crystallized Pd nanoparticles decorated TiO_2 prepared by atmospheric-pressure cold plasma and its enhanced photocatalytic performance

TiO_2 decorated with partially crystallized Pd nanoparticles(Pd/TiO_2-P) was successfully prepared by atmospheric-pressure dielectric barrier discharge cold plasma. The XRD and XPS analyses proved that Pd ions were reduced to partially crystallized metallic Pd nanoparticles in Pd/TiO_2-P. The XPS spectra also indicated that an enhanced metal-support interaction was formed due to the existence of partially crystallized Pd nanoparticles with lower coordination number in Pd/TiO_2-P. Photocatalytic activity of Pd/TiO_2-P was much higher than that of TiO_2 samples decorated with well crystallized Pd nanoparticles.     

Design of highly active heterogeneous palladium catalysts for the activation of aryl chlorides in Heck reactions

In situ generation of highly active palladium species by intermediate dissolution of Pd from solid supported catalysts has been demonstrated to be a very successful approach for the activation of aryl chlorides in Heck reactions. The new ‘heterogeneous’ Pd catalysts act as reservoir for molecular Pd species with unsaturated coordination sphere in solution. Crucial Pd leaching and re-deposition onto the support can be controlled by optimization of reaction conditions and by the properties of the catalysts. Pd is re-deposited onto the support at the end of the reaction. The catalysts, palladium supported on activated carbon, on various metal oxides or fluorides and Pd complexes in zeolites, are easy to prepare, though the preparation conditions are crucial. The catalysts convert all aryl bromides completely within minutes (TON 100,000). Aryl chlorides (even deactivated ones) are converted with high yields, within 2-6 h. The catalysts belong to the most active ones in Heck reactions at all (including best homogeneous systems) and fulfill all relevant requirements for practical applications in laboratory and industry.     

Thermodynamics of pyridine coordination in 1,4-phenylene bridged bimetallic (Pd, Pt) complexes containing two N,C,N' motifs, 1,4-M2-[C6(CH2NR2)4-2,3,5,6

The thermodynamics of pyridine coordination in 1,4-phenylene-bridged binuclear palladium and platinum organometallic complexes [1,4-(MOTf)2-&{C6(CH2NR2)4-2,3,5,6}] (11, M =Pd, Pt; R =CH3, C2H5, R2 = -(CH2)5-) are measured by 1H NMR in DMSO-d6. The coordination of substituted pyridines by bimetallic complexes 11 or 12 in DMSO is found to proceed via two effectively independent metalligand binding events, and the association constants for pyridine coordination and rate constants for pyridine exchange are nearly identical to those measured previously on monometallic analogs. A linear free energy relationship between the association constant for pyridine coordination and the inductive Hammett constant of the pyridine substituent is observed, and the sensitivity (rho = -1.7 to -2.1) in DMSO depends only slightly on metal (Pd vs Pt) and spectator ligand (pincer dialkylamine vs triarylphosphine). The association constant for a particular pyridine ligand, however, varies by roughly 3 orders of magnitude across the series of metal complexes. The effective independence of the two coordination sites and the range of available thermodynamic and kinetic behaviors of the coordination guide the use of these versatile building blocks in metallosupramolecular applications.     

Pd／HM催化剂的制备及其表面酸性质

Ｐｄ／ＨＭ催化剂的制备及其表面酸性质徐忠民，苏运来，李全芝，胡家芬（复旦大学化学系，上海２００４３３）关键词钯／氢型丝光沸石，酸性质，氨－程序升温脱附，红外－程序升温脱附轻质烷烃的催化异构化是生产高辛烷值汽油的主要加工过程之一．自从Ｒａｂｏ等［１］发...     

CO adsorption and desorption on size-selected Pd(n)/TiO2(110) model catalysts: size dependence of binding sites and energies, and support-mediated adsorption

The nature of CO adsorption on Pd(n)/TiO(2)(110) (n = 1, 2, 7, 20) has been examined using temperature-programmed desorption (TPD), temperature-dependent helium ion scattering (TD-ISS), and X-ray photoelectron spectroscopy (XPS). All samples contain the same number of Pd atoms (0.10 ML-equivalent) deposited as different size clusters. The TPD and TD-ISS show that CO binds in two types of sites associated with the Pd clusters. The most stable sites are on top of the Pd clusters ("on-top" sites), however, there are also less stable sites, in which CO is bound in association with, but not on top of the Pd ("peripheral" sites). For saturation CO coverage over a fixed atomic concentration of Pd (present in the form of Pd(n) clusters of varying size), the population of CO in peripheral sites decreases with increasing cluster size, while the on-top site population is size-independent. This is consistent with what geometric considerations would predict for the density of the two types of sites, provided the clusters adsorb predominantly as 2D islands, which ISS results suggest to be the case. The XPS analysis indicates that CO-Pd binding is dominated by π-backbonding to the Pd(n) clusters. The results also show evidence for efficient support-mediated adsorption (reverse-spillover) of CO initially impinging on TiO(2) to binding sites associated with the Pd clusters.     

Design of coordination polymer as stable catalytic systems

Here we report the first example of catalytic metallogels, which are formed irreversibly in dimethylsulfoxide via the creation of cross-linked, three-dimensional coordination polymer networks by using transition-metal ions with multiple sites available for coordination and multidentate ligands. Conformational flexibility of the ligands and slow formation of the coordination polymers apparently favor the gelation. These metallogels are stable in water and most organic solvents and can catalyze the oxidation of benzyl alcohol to benzaldehyde by using their PdII moieties as the catalytic centers. The best catalytic turnover of the metallogel is twice that of [Pd(OAc)2] under similar reaction conditions.     

Investigation on the Coordination Mode of IL-Supported Diols Used as Phosphine-Free Ligands for Palladium Catalyzed Heck Reaction

The coordination mode of IL-supported diols used as phosphine-free ligands for palladium catalyzed Heck reaction has been investigated by tuning their compositions. The difference in coordination of these IL-supported diols with metal Pd in Heck reaction was related to the changes of the cation rings, leading to the different activities of Pd catalyst in the reaction. The experimental results indicated that the activities of Pd catalyst were affected mainly by n-electron density of the cation rings. Compared to pyridinium and piperidinium cations, imidazolium cations showed the best coordination to metal Pd. In the meantime, C-2 hydrogen and the length of alkyl side chains had impacts on the coordination as well.     

Divergent growth of coordination dendrimers on surfaces

Divergent growth of surface-initiated dendritic nanostructures on gold surfaces in a highly controlled, stepwise manner is demonstrated, using metal-organic coordination as the binding motif. The repeat unit for dendrimer growth was a branched, C3-symmetrical ligand building block bearing three bis-hydroxamate groups. The surface initiation sites for dendrimer growth were obtained by the formation of a mixed monolayer comprising isolated bis-hydroxamate disulfide anchor ligands and octanethiol (OT) at very low anchor/OT ratios. Following functionalization of the surface with spaced anchors, alternate immersion in solutions of Zr4+ ions and the branched ligand afforded surface-confined dendrimers of increasing generation, where the number of generations is conveniently controlled by the number of coordination binding sequences. The heights of different generation dendrimers are in excellent agreement with values predicted by molecular models, as well as with thicknesses of branched multilayers prepared by the same procedure on full anchor monolayers. At higher generation numbers, gradual dendrimer overlap and coalescence are observed, eventually resulting in a continuous overlayer and a transition from 3D to 1D growth. A mechanism for the development of dendritic coordination nanostructures on surfaces is discussed.     

C60Pd(Ph2PCH2PPh2)配合物的合成及光电性能研究

Fulerene complexe C60 Pd（ Ph2PCH2PPh2）was perpared by the method of ligand substitution via the reaction of C60 with Pd（Ph2PCH2PPh2）2 under condition of a nitrogen atmosphere and refluxing,and the title compound was appraised and characterized by methods of elemental analysis,FT-IR,UV-vis,XPS and XRD. The results showed that the structure of purposeful product was that the Ph2PCH2PPh2 took up two coordination sites of the central metal,and C60 took up another two sites in σ-π feeback pattern. The porperties on photoelectricity,redox and thermostability of the title complexe were studied. The results of studying on photoelectricity showed that the photovoltaic effect of（n+n）heterojunction electrode formed by C60Pd（Ph2PCH2PPh2）/ GaAs was supper,especially in the BQ/ H2Q redox couple,and the greatest value of photovoltaic potential was up to 174 mV. The photovoltaic performance of C60Pd（Ph2PCH2PPh2）/GaAs electrode at 1 μm for thickness of C60Pd（Ph2PCH2PPh2）film was the best.     

Ruthenium/TiO2-Catalyzed Hydrogenolysis of Polyethylene Terephthalate: Reaction Pathways Dominated by Coordination Environment

Polyethylene terephthalate (PET) hydrogenolysis can produce benzene, toluene, and xylene (BTX), where the selectivity control is challenging. We report a reaction pathway dictated by the Ru coordination environment by examining the binding geometries of adsorbates on differently coordinated Ru centers and their evolution during PET hydrogenolysis. A BTX yield of 77 % was obtained using a Ru/TiO₂ with a Ru coordination number of ca. 5.0 where edge/corner sites are dominant, while more gas and saturated products were formed for Ru/TiO₂ containing primarily terrace sites. Density functional theory and isotopic labelling revealed that under-coordinated Ru edge sites favor “upright” adsorption of aromatic adsorbates while well-coordinated Ru sites favor “flat-lying” adsorption, where the former mitigates ring hydrogenation and opening. This study demonstrates that reaction pathways can be directed through controlled reactant/intermediate binding via tuning of the Ru coordination environment for efficient conversion of PET to BTX.     

Palladium-catalyzed isomerization of 1-hexene

The isomerization of 1-hexene over Pd(acac)₂ + BF₃OEt₂ and Pd(acac)₂ + PPh₃ + BF₃OEt₂ catalyst systems proceeds by the consecutive-parallel scheme. Tetracoordinate square-planar Pd hydrides bearing two vacant coordination sites are likely to be catalytically active species for the isomerization of hexene-1. This revised version was published online in June 2006 with corrections to the Cover Date.     

Palladium - catalyzed dimerization of propene

Summary The insertion of the previously coordinated second molecule of propene into the Pd-C bond is the rate-determining step in the dimerization of propene with the Pd(acac)₂ + BF₃OEt₂ catalyst system. Tetracoordinated square planar Pd hydrides bearing two vacant coordination sites are likely to be catalytically active species in the dimerization of propene. The simplicity of the catalyst system composition might be of industrial importance.