Pd Aggregates Research Articles

Aggregation of phosphorescent Pd(II) and Pt(II) complexes with lipophilic counter-anions in non-polar solvents.

Phosphorescent cationic tridentate C^N^N (HC^N^N = 6-(2-R2,4-R1-phenyl)-2,2'-bipyridine; R1 = R2 = H or F, or R1 = OMe, R2 = H) cyclometallated Pd(II) complexes with an N,N-dimethyl-imidazol-allenylidene ancillary ligand (L) and their corresponding Pt(II) congeners have been synthesized and characterized, following the previously reported preparation of the complex [Pd(6-phenyl-2,2'-bipyridine)L]+. For these cationic Pd(II)/Pt(II) complexes with 2,3,4-tris(dodecyloxy)benzenesulfonate (LA-) counter-anions in mixed CH2Cl2/toluene solvents, uniform square flake or fibre-like aggregates were formed. Corresponding multicolour phosphorescence with obvious metal-metal-to-ligand charge transfer (MMLCT) features gradually changed from red to NIR by manipulating the various fractions of Pd/Pt species. Circular dichroism (CD) and circularly polarized luminescence (CPL) derived from the fibre-like Pd aggregates of [Pd(6-(2,4-difluorophenyl)-2,2'-bipyridine)L]+ in chiroptical CH2Cl2/limonene solvents were achieved with an isodesmic aggregation mode. Dispersive metallophilic interactions are claimed to be the driving force for these photo-functional aggregates.

Constitutionally Selective Dynamic Covalent Nanoparticle Assembly.

The future of materials chemistry will be defined by our ability to precisely arrange components that have considerably larger dimensions and more complex compositions than conventional molecular or macromolecular building blocks. However, exerting structural and constitutional control in the assembly of nanoscale entities presents a considerable challenge. Dynamic covalent nanoparticles are emerging as an attractive category of reaction-enabled solution-processable nanosized building block through which the rational principles of molecular synthetic chemistry can be extended into the nanoscale. From a mixture of two hydrazone-based dynamic covalent nanoparticles with complementary reactivity, specific molecular instructions trigger selective assembly of intimately mixed heteromaterial (Au–Pd) aggregates or materials highly enriched in either one of the two core materials. In much the same way as complementary reactivity is exploited in synthetic molecular chemistry, chemospecific nanoparticle-bound reactions dictate building block connectivity; meanwhile, kinetic regioselectivity on the nanoscale regulates the detailed composition of the materials produced. Selectivity, and hence aggregate composition, is sensitive to several system parameters. By characterizing the nanoparticle-bound reactions in isolation, kinetic models of the multiscale assembly network can be constructed. Despite ignoring heterogeneous physical processes such as aggregation and precipitation, these simple kinetic models successfully link the underlying molecular events with the nanoscale assembly outcome, guiding rational optimization to maximize selectivity for each of the three assembly pathways. With such predictive construction strategies, we can anticipate that reaction-enabled nanoparticles can become fully incorporated in the lexicon of synthetic chemistry, ultimately establishing a synthetic science that manipulates molecular and nanoscale components with equal proficiency.

An Imidazole‐Rich Pd(II)‐Polymer Pre‐catalyst for the Suzuki‐Miyaura Coupling: Stability Influenced by Dissolved Oxygen and Reactants Concentration

AbstractHerein a novel Pd(II)‐polymeric pre‐catalyst was prepared by coordinating Pd(II) ions to a low cost imidazole/carboxylate‐rich polymer. The material displays good activity in the Suzuki‐Miyaura coupling towards a range of aryl bromides and iodides in iPrOH/H2O mixtures at 25 or 60 °C. Catalyst longevity and recyclability proved to be sensitive to the concentration of the cross‐coupling partners. When the concentration of PhBr is high ([PhBr]=250 mmol L−1), inactive Pd(0) aggregates were detected. On the other hand, when the reaction was performed at 20‐fold dilution ([PhBr]=12.5 mmol L−1) the material could be reused up to 12 times without significant loss of catalytic activity. In this condition, minimum amount of Pd(0) was detected by XPS analysis and no Pd(0) aggregates were observed by XRPD. Importantly, it was found that the presence of oxygen is fundamental for avoiding formation of inactive Pd(0) aggregates.

Synthesis and characterization of a supported Pd complex on carbon nanofibers for the selective decarbonylation of stearic acid to 1-heptadecene: the importance of subnanometric Pd dispersion

Evaluation of the dispersion of Pd active sites on the catalyst performance during fatty acids decarbonylation to α-olefins was explored in this work. Pd subnanometric particles, clusters and aggregates were found to modulate the catalyst activity.

Template‐free anion‐controlled synthesis of Pd (II) nano‐aggregates for the antifouling polymerization of CO and ethylene

The reaction of [(domppp) Pd (OAc)2] [domppp = 1,3‐bis (di‐o‐methoxyphenylphosphino)propane] and imidazolium‐functionalized carboxylic acids containing various anions (Br−, PF6−, SbF6− and BF4−) resulted in the formation of nano‐sized Pd (II) aggregates under template‐free conditions. The rate of formation of aggregates can be modulated by changing the anion, affecting the rate of polymerization of CO and olefins without fouling. Herein, we describe the analysis of Pd (II) catalysts by dynamic light scattering, atomic force microscopy, X‐ray photoelectron spectroscopy and X‐ray crystallography, and co‐ and terpolymerization results including the catalytic activity, and bulk density and molecular weight of polymers.

Performance of Pd Cathode Catalyst Electrodeposited on Gas Diffusion Layer in Polymer Electrolyte Membrane Fuel Cells

Performance of a polymer electrolyte membrane fuel cell with a Pd cathode catalyst was systemically investigated in this study. The Pd catalyst was directly formed on a gas diffusion layer by using electrodeposition (Pd/GDL). The electrodeposition formed aggregates of Pd nanoparticles on a gas diffusion layer with the preferred orientation of Pd(111) and Pd(200). In addition, the Pd aggregates mainly formed on the top surface of the gas diffusion layer. The membrane electrode assembly was fabricated with Pd/GDL as the cathode. The performance of the membrane electrode assembly was investigated by varying hot pressing parameters and back pressures, and the operating condition for the polymer electrolyte membrane fuel cell was optimized. Notably, introducing back pressure increased operating current density at 0.6 V by up to 45%. Durability of the membrane electrode assembly was also examined. Negligible deterioration of surface morphology of the Pd catalyst was observed even after accelerated stress testing, except for a slight increase in particle size. The results indicate that deterioration of the Pd cathode catalyst was not a major factor affecting overall single-cell performance degradation.

Validation of an air-liquid interface toxicological set-up using Cu, Pd, and Ag well-characterized nanostructured aggregates and spheres.

Systems for studying the toxicity of metal aggregates on the airways are normally not suited for evaluating the effects of individual particle characteristics. This study validates a set-up for toxicological studies of metal aggregates using an air–liquid interface approach. The set-up used a spark discharge generator capable of generating aerosol metal aggregate particles and sintered near spheres. The set-up also contained an exposure chamber, The Nano Aerosol Chamber for In Vitro Toxicity (NACIVT). The system facilitates online characterization capabilities of mass mobility, mass concentration, and number size distribution to determine the exposure. By dilution, the desired exposure level was controlled. Primary and cancerous airway cells were exposed to copper (Cu), palladium (Pd), and silver (Ag) aggregates, 50–150 nm in median diameter. The aggregates were composed of primary particles <10 nm in diameter. For Cu and Pd, an exposure of sintered aerosol particles was also produced. The doses of the particles were expressed as particle numbers, masses, and surface areas. For the Cu, Pd, and Ag aerosol particles, a range of mass surface concentrations on the air–liquid interface of 0.4–10.7, 0.9–46.6, and 0.1–1.4 µg/cm2, respectively, were achieved. Viability was measured by WST-1 assay, cytokines (Il-6, Il-8, TNF-a, MCP) by Luminex technology. Statistically significant effects and dose response on cytokine expression were observed for SAEC cells after exposure to Cu, Pd, or Ag particles. Also, a positive dose response was observed for SAEC viability after Cu exposure. For A549 cells, statistically significant effects on viability were observed after exposure to Cu and Pd particles. The set-up produced a stable flow of aerosol particles with an exposure and dose expressed in terms of number, mass, and surface area. Exposure-related effects on the airway cellular models could be asserted.Graphical Electronic supplementary materialThe online version of this article (doi:10.1007/s11051-016-3389-y) contains supplementary material, which is available to authorized users.

Pd Nanoparticle Formation in Ionic Liquid Thin Films Monitored by in situ Vibrational Spectroscopy.

Ionic liquids (ILs) are flexible reaction media and solvents for the synthesis of metal nanoparticles (NPs). Here, we describe a new preparation method for metallic NPs in nanometer thick films of ultraclean ILs in an ultrahigh vacuum (UHV) environment. CO-covered Pd NPs are formed by simultaneous and by sequential physical vapor deposition (PVD) of the IL and the metal in the presence of low partial pressures of CO. The film thickness and the particle size can be controlled by the deposition parameters. We followed the formation of the NPs and their thermal behavior by time-resolved IR reflection absorption spectroscopy (TP-IRAS) and by temperature-programmed IRAS (TR-IRAS). Codeposition of Pd and [C1C2Im][OTf] in CO at 100 K leads to the growth of homogeneous multilayer films of CO-covered Pd aggregates in an IL matrix. The size of these NPs can be controlled by the metal fraction in the co-deposit. With increasing metal fraction, the size of the Pd NPs also increases. At very low metal content, small Pd carbonyl-like species are formed, which bind CO in on-top geometry only. Upon annealing, the [OTf](-) anion coadsorbs at the NP surface and partially displaces CO. Co-adsorption of CO and IL is indicated by a strong red-shift of the CO stretching bands. While the weakly bound on-top CO is mainly replaced below the melting transition of the IL, coadsorbate shells with bridge-bonded CO and IL are stable well above the melting point. Larger three-dimensional Pd NPs can be prepared by PVD of Pd onto a solid [C1C2Im][OTf] film at 100 K. Upon annealing, on-top CO desorbs from these NPs below 200 K. Upon melting of the IL film, the CO-covered Pd NPs immerse into the IL and again form a stable coadsorbate shell that consists of bridge-bonded CO and the IL.

Structural Diversity in Multinuclear PdII Assemblies that Show Low‐Humidity Proton Conduction

Systematic investigation on synergetic effects of geometry, length, denticity, and asymmetry of donors was performed through the formation of a series of uncommon Pd(II) aggregates by employing the donor in a multicomponent self-assembly of a cis-blocked 90° Pd(II) acceptor and a tetratopic donor. Some of these assemblies represent the first examples of these types of structures, and their formation is not anticipated by only taking the geometry of the donor and the acceptor building units into account. Analysis of the crystal packing of the X-ray structure revealed several H bonds between the counteranions (NO3 (-) ) and water molecules (OH⋅⋅⋅ON). Moreover, H-bonded 3D-networks of water are present in the molecular pockets, which show water-adsorption properties with some variation in water affinity. Interestingly, these complexes exhibit proton conductivity (1.87×10(-5) -6.52×10(-4) Scm(-1) ) at 296 K and low relative humidity (ca. 46 %) with activation energies of 0.29-0.46 eV. Moreover, the conductivities further increase with the enhancement of humidity. The ability of these assemblies to exhibit proton-conducting properties under low-humidity conditions makes these materials highly appealing as electrolytes in batteries and in fuel-cell applications.

Sonochemical synthesis of three-dimensional aggregates of pd nanoparticles with high electrocatalytic activity towards ethanol oxidation

Flower-like aggregates composed of (4.0±0.8) nm palladium(Pd) nanoparticles were prepared via ultrasonics in the palladium(II) chloride(PdCl2) H2O/EtOH(5/1, volume ratio) solution with the addition of a quantity of poly(vinyl pyrrolidone)(PVP) and sodium dodecyl sulfonate(SDS). The morphologies, crystal structures and the optical properties of the flower-like Pd nanostructures were characterized by X-ray diffraction(XRD). X-ray photoelectron spectroscopy(XPS). scanning electron microscopy(SEM). transmission electron microscopy(TEM). selected area electron diffraction(SAED) and UV-visible absorption spectroscopy, respectively. The mechanism of sonochemical reduction of Pd(II) ions was also investigated. The results show that the molar ratio of PVP to SDS affected the formation of the flower-like aggregates of Pd nanoparticles. Moreover, the electrocatalytic properties of Pd aggregates modified glassy carbon electrode for ethanol oxidation were also investigated by cyclic voltammetry(CV). This material exhibits remarkable electrocatalytic activity for ethanol oxidation in 1 mol/L KOH and appears as a promising candidate to be applied in direct ethanol fuel cells.

A Facile Strategy to Support Palladium Nanoparticles on Carbon Nanotubes, Employing Polyvinylpyrrolidone as a Surface Modifier

AbstractThis paper describes the development of a facile and environmentally friendly strategy for supporting palladium nanoparticles (Pd NPs) on multiwalled carbon nanotubes (MWCNTs) with ethanol as the solvent/reducing agent, potassium tetrachloropalladate(II) (K2PdCl4) as the Pd precursor, and polyvinylpyrrolidone (PVP) as the surface modifier. More specifically, our approach was based on the nonbonding interaction between MWCNTs and PVP, which is a cheap, nontoxic, and commercial polymer. As PVP can serve not only as a surface modifier but also as a stabilizing agent for Pd NPs, the utilization of additional functionalization steps, reducing agents, and stabilizers was not required to achieve uniform Pd deposition over the MWCNTs. Our results demonstrate that Pd NPs below 5 nm in diameter can be directly supported on MWCNTs by this route. Also, sequential Pd‐reduction steps can be employed to improve the coverage of Pd NPs at the MWCNT surfaces, although this can also lead to the formation of larger Pd particles or aggregates. The electrocatalytic activity for ethanol oxidation was investigated as a function of the composition and structure of the materials produced, in which MWCNTs decorated with Pd NPs of smaller sizes and lower coverages displayed the highest activities. The results described herein suggest that our approach may serve as a simple platform for the synthesis of MWCNTs decorated with metal NPs with well‐defined morphologies and uniform dispersion for electrochemical and catalytic applications.

Enhancement on Sensitivity and Stability of Methane Sensor by Increasing Pd Amount and Acid-Assisted Loading

Al2O3 was loaded with different amounts (5 wt%, 10 wt%, and 15 wt%) of Pd via isometric-impregnation. Transmission electron microscopy (TEM) demonstrated that the 15 wt% Pd/Al2O3 has numerous Pd nanoparticles formed on Al2O3. To decrease the hydrolysis rate of Pd(NO3)(2), which could cause the formation of Pd aggregates on Al2O3, determinate concentrations of HNO3 were mixed with the impregnation solution. The measuring results of TEM and Brunauer-Emmett-Teller-specific surface areas revealed that 15 wt% Pd loaded with 10 vol% HNO3 has high dispersion on Al2O3. The influences of the loading amounts of Pd and the acidity of the impregnated solution on the sensitivity and stability of gas sensors were investigated. Catalytic combustion gas sensors based on 15 wt% Pd/Al2O3 prepared from a 10 vol% HNO3-90 vork Pd(NO3)(2) impregnating solution exhibited high responses to methane and strong ability to resist organic silicon.

Solvothermal Preparation of Pd Nanostructures under Nitrogen and Air Atmospheres and Electrocatalytic Activities for the Oxidation of Methanol

Monodispersed Pd nanoparticles and their porous spherelike aggregates with different lattice parameters were solvothermally prepared under nitrogen and air atmospheres. Here PdCl(2) acted as Pd precursor and hexadecylamine played the roles of solvent, reductant, and surface capping agent. Oxygen existing in air resulted in the change of surface capping agent, which then induced the aggregation of Pd nanoparticles, and the incorporation of oxygen led to the expansion of its lattice parameter to 0.398 nm, whereas the standard lattice parameter in metallic Pd is 0.389 nm. The monodispersed nanoparticles presented better catalytic activity and stability for the oxidation of methanol than the sphere-like Pd aggregates.

Iodine in alluvial platinum–palladium nuggets: Evidence for biogenic precious-metal fixation

Synchrotron radiation-induced X-ray fluorescence (SR-XRF) spectrometry was applied to determine iodine concentrations in alluvial Pt–Pd aggregates with delicate morphological features from Córrego Bom Sucesso, Minas Gerais, Brazil. The millimetre-sized botryoidal and rod-shaped grains of alluvial Pt–Pd–Hg intermetallic compounds have surprisingly high concentrations of iodine, in the range from 10 to ~120μg/g. Because iodine is a strongly biophile element, known to be enriched in peatlands and plant remains in soils by microbial activity, its concentration in the Pt–Pd nuggets suggests biogenic precious-metal fixation in the aqueous alluvial milieu. Biogenic and inorganic processes, such as bioreduction and electrochemical metal accretion, could mutually have contributed to the growth of nanoparticles, formed on organic templates, to millimetric Pt–Pd nuggets.

Controlling the Adsorption Kinetics via Nanostructuring: Pd Nanoparticles on TiO2Nanotubes

Activity and selectivity of supported catalysts critically depend on transport and adsorption properties. Combining self-organized porous oxide films with different metal deposition techniques, we have prepared novel Pd/TiO(2) catalysts with a new level of structural control. It is shown that these systems make it possible to tune adsorption kinetics via their nanostructure. Self-organized TiO(2) nanotubular arrays (TiNTs) prepared by electrochemical methods are used as a support, on which Pd particles are deposited. Whereas physical vapor deposition (PVD) in ultrahigh vacuum (UHV) allows us to selectively grow Pd particles at the tube orifice, Pd/TiNT systems with homogeneously distributed Pd aggregates inside the tubes are available by particle precipitation (PP) from solution. Both methods also provide control over particle size and loading. Using in-situ infrared reflection absorption spectroscopy (IRAS) and molecular beam (MB) methods, we illustrate the relation between the nanostructure of the Pd/TiNT systems and their adsorption kinetics. Control over the metal nanoparticle distribution in the nanotubes leads to drastic differences in adsorption probability and saturation behavior. These differences are rationalized based on differences in surface and gas phase transport resulting from their nanostructure. The results suggest that using carefully designed metal/TiNT systems it may be possible to tailor transport processes in catalytically active materials.

Formation of palladium(0) nanoparticles at microbial surfaces

The increasing demand and limited natural resources for industrially important platinum-group metal (PGM) catalysts render the recovery from secondary sources such as industrial waste economically interesting. In the process of palladium (Pd) recovery, microorganisms have revealed a strong potential. Hitherto, bacteria with the property of dissimilatory metal reduction have been in focus, although the biochemical reactions linking enzymatic Pd(II) reduction and Pd(0) deposition have not yet been identified. In this study we investigated Pd(II) reduction with formate as the electron donor in the presence of Gram-negative bacteria with no documented capacity for reducing metals for energy production: Cupriavidus necator, Pseudomonas putida, and Paracoccus denitrificans. Only large and close-packed Pd(0) aggregates were formed in cell-free buffer solutions. Pd(II) reduction in the presence of bacteria resulted in smaller, well-suspended Pd(0) particles that were associated with the cells (called "bioPd(0)" in the following). Nanosize Pd(0) particles (3-30 nm) were only observed in the presence of bacteria, and particles in this size range were located in the periplasmic space. Pd(0) nanoparticles were still deposited on autoclaved cells of C. necator that had no hydrogenase activity, suggesting a hydrogenase-independent formation mechanism. The catalytic properties of Pd(0) and bioPd(0) were determined by the amount of hydrogen released in a reaction with hypophosphite. Generally, bioPd(0) demonstrated a lower level of activity than the Pd(0) control, possibly due to the inaccessibility of the Pd(0) fraction embedded in the cell envelope. Our results demonstrate the suitability of bacterial cells for the recovery of Pd(0), and formation and immobilization of Pd(0) nanoparticles inside the cell envelope. However, procedures to make periplasmic Pd(0) catalytically accessible need to be developed for future nanobiotechnological applications.

Preparation of Pd particles coated with mesoporous silica layers

Pre-formed polycrystalline Palladium aggregates (ca. 200 nm in diameter) were coated by thin mesoporous silica layers to form metal-mesoporous silica composite materials. To synthesize the composites, Pd aggregates were dispersed in an aqueous solution of C16H33N(CH3)3Br to which tetraethylorthosilicate (TEOS) was then added to give composite precipitates. The surfactant template in the precipitate was subsequently removed by extraction with an acetic acid/ethanol solvent. TEM images of the composite (containing as much as 65 wt% Pd) revealed that the Pd aggregates were fully surrounded by mesoporous silica layers (ca. 50 nm in thickness) having ordered pores that penetrate to large extent from the exterior to the surface of Pd aggregates. When the materials were used as catalysts for cinnamaldehyde hydrogenation in toluene, the reactant molecules were concentrated in the meso channels by a factor of ca. 3. Accordingly, the composites showed comparable or higher activity than that observed for neat Pd aggregates. This study highlights the preparation of unique metal-mesoporous silica composite materials by simple synthetic procedures and which are usable as active Pd catalysts.

Preparation of Colloidal BEA Zeolite Functionalized with Pd Aggregates as a Precursor for Low Dimensionality Sensing Layer

Preparation of Colloidal BEA Zeolite Functionalized with Pd Aggregates as a Precursor for Low Dimensionality Sensing Layer

Facile Fabrication and Unexpected Electrocatalytic Activity of Palladium Thin Films with Hierarchical Architectures

Cobalt thin films composed of a large number of nanopetals were fabricated on the glassy carbon (GC) substrate by cyclic voltammetric deposition of Co2+ ions on a glass carbon electrode (GCE). The hierarchical Co nanostructures were further used as the sacrificial template to acquire Pd (or Pt) thin film electrocatalysts with hierarchical architectures through the galvanic replacement reaction between Co nanopetals and chloropalladite (or tetrachloroplatinate). The as-prepared Pd (or Pt) thin films contain quantities of nanoparticles and many hollow Pd aggregates in the range of submicrometer to micrometer scale. The hollow Pd aggregates were found to burst in acidic solutions at potentials more negative than the hydrogen evolution potential since Pd absorbed too much hydrogen. As an electrocatalyst for the formic acid oxidation, the Pd thin films presented much higher catalytic activity than the Pt thin films with a similar architecture. An important reason is that the formic acid oxidation at the Pd nanostructures proceeds via a non-CO reaction pathway, while the reaction at the Pt nanostructures involves formation of CO-adsorbed species, which has been confirmed by the CO stripping voltammetric curves on Pd or Pt thin films. The as-prepared Pd thin films with hierarchical architectures are expected to be a promising electrocatalyst in direct formic acid fuel cells (DFAFCs).

Determination of the Surface Area of Pd and Nanometric Au Aggregates Supported on a Micrometric Solid Support by Thiol Adsorption and GC−MS

A new sensitive and specific method to measure gold and palladium surface areas using alkanethiol adsorption coupled with analysis by gas chromatography with mass spectrometry detection has been developed. The effectiveness of the method was tested with metallic samples having a known surface area. The results have also been compared with BET specific surface area measurements. The results obtained with both methods show a good correlation.