Surface Of Palladium Nanoparticles Research Articles

One-step and one-pot method for synthesis of hybrid composite palladium-polypyrrole-carbon (Pd/PPy/C) nanomaterials

63 A onestep and onepot method to design compos� ite nanomaterials based on conjugated polymer, poly� pyrrole, and palladium nanoparticles by the redox reaction of the monomer and a palladium salt in non� aqueous and aqueous media was described previously in the works of our team (1-3). These composite materials were successfully tested as heterogeneous catalysts for the most known organic syntheses (Suzuki reaction, direct C-C conjugation, Sonogash� ira reaction, etc.) and showed excellent catalytic activ� ity. This fact made such materials interesting for fur� ther investigation. The main feature of palladium-polypyrrole (Pd/PPy) composite nanomaterials consists is that the formation of palladium nanoparticles within polypyr� role globules requires no additional stabilizers in the system. Therefore, no protective layers appear on the surface of palladium nanoparticles so that it remains catalytically active. At the same time, the polymer resulting from the chemical reaction has a porous structure that favors the diffusion of reactants to cata� lytically active centers.

Prominent Electronic and Geometric Modifications of Palladium Nanoparticles by Polymer Stabilizers for Hydrogen Production under Ambient Conditions

A remarkable effect from the modification of electronic and geometric properties of Pd nanoparticles by the use of polymer pendant groups bound to the surface of palladium nanoparticles is reported. The degree of electron promotion to the Pd nanoparticles under ambient conditions was found to be dependent on the availability of the lone pair electrons of the pendant groups.

Synthesis of Gold-Palladium Bimetallic Nanoparticles and Surface Catalytic Activity in Suzuki Coupling Reactions Using in FTIR Spectroscopy

Colloidal bimetallic gold core palladium shell nanoparticles were prepared by wet chemical method, in which Au(III) and Pd(II) ions in an aqueous solution in the presence of a cationic surfactant, Cetyltrimethylammonium bromide (CTAB). The structure and composition of the metallic nanoparticles were characterized by UV-Vis, HRTEM, SEM-EDX, XRD, XPS and FTIR. The catalytic activities of nanoparticles are tested on the surface chemical characterization study of Cetyltrimethylammonium bromide supported Au-Pd bimetallic nanoparticle catalyst, hereafter named Au-Pdnp, is reported. Such a catalyst was developed for the Suzuki coupling reaction and found excellent catalytic activity. Here we describe the catalytic performance and the FTIR studies provide proof of the mode of binding that occurs in the Palladium nanoparticle surface for the first time and also confirms the mechanism of the Suzuki reaction.

Displacement of Hexanol by the Hexanoic Acid Overoxidation Product in Alcohol Oxidation on a Model Supported Palladium Nanoparticle Catalyst

This work characterizes the adsorption, structure, and binding mechanism of oxygenated organic species from cyclohexane solution at the liquid/solid interface of optically flat alumina-supported palladium nanoparticle surfaces prepared by atomic layer deposition (ALD). The surface-specific nonlinear optical vibrational spectroscopy, sum-frequency generation (SFG), was used as a probe for adsorption and interfacial molecular structure. 1-Hexanoic acid is an overoxidation product and possible catalyst poison for the aerobic heterogeneous oxidation of 1-hexanol at the liquid/solid interface of Pd/Al(2)O(3) catalysts. Single component and competitive adsorption experiments show that 1-hexanoic acid adsorbs to both ALD-prepared alumina surfaces and alumina surfaces with palladium nanoparticles, that were also prepared by ALD, more strongly than does 1-hexanol. Furthermore, 1-hexanoic acid adsorbs with conformational order on ALD-prepared alumina surfaces, but on surfaces with palladium particles the adsorbates exhibit relative disorder at low surface coverage and become more ordered, on average, at higher surface coverage. Although significant differences in binding constant were not observed between surfaces with and without palladium nanoparticles, the palladium particles play an apparent role in controlling adsorbate structures. The disordered adsorption of 1-hexanoic acid most likely occurs on the alumina support, and probably results from modification of binding sites on the alumina, adjacent to the particles. In addition to providing insight on the possibility of catalyst poisoning by the overoxidation product and characterizing changes in its structure that result in only small adsorption energy changes, this work represents a step toward using surface science techniques that bridge the complexity gap between fundamental studies and realistic catalyst models.

Adsorption of hydrogen on palladium nanoparticle surfaces

AbstractThe behaviors of hydrogen (H) adsorbed on the palladium (Pd) nanoparticles (NPs) are examined with the modified analytic embedded‐atom method potentials and MORSE potentials. We study the effects of particle size and H coverage, and compare their adsorption properties of nanoparticle's facets with that of flat surfaces. We simulate the Pd truncated octahedron NPs with atoms from 38 to 2406 and the coverage of adsorbed H up to 1.0 monolayer (ML). Site preferences, adsorption geometries, adsorption energies, and bond lengths of HPd are calculated. We have also calculated the potential energy surface (PES). It is clear that the H atom binding to particle facets is quite stronger than that of flat surfaces when the particle size is smaller than 3.2 nm. We have found a significant variation that adsorption energies ascend gradually with increasing the particle size or surface coverage of H, and the adsorption energy varies about 0.6 eV for (111) facet and 0.3 eV for (100) facet as the coverage up to 1.0 ML. Our results are in reasonable agreement with the experimental values and other calculations. Copyright © 2009 John Wiley & Sons, Ltd.

Evidence for monoalkoxide species on the surface of palladium nanoparticles synthesized in ethylene glycol

Palladium nanoparticles consisting of metallic cores and encapsulating shells serve as an intriguing catalytic model system. This investigation is aimed at enhancing our understanding of the surface composition of palladium nanoparticles synthesized in polyol and exploiting their encapsulating shells to understand the catalytic properties of the nanoparticles more rationally. We investigated the surface structure of palladium nanoparticles by Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis (TGA). FTIR spectra showed the presence of monoalkoxide species with tilted geometry. TGA further confirmed the presence of these species along with some adsorbed byproducts formed during the reaction. Atomic force microscopy (AFM) shows the presence of diffused negatively charged coatings on the surface of the nearly spherical nanoparticles.

Size-Controlled Synthesis of Palladium Nanoparticles

Palladium nanoparticles protected by PVP were obtained first by reducing PdCl2 with NaBH4 solution. Then different size palladium nanoparticles with a large size range (1.49 nm ∼ 23.26 nm) were prepared by the multi-step reduction of PdCl2 by hydrogen adsorbed on the surface of palladium nanoparticles. Transmission electron microscopy (TEM) and x-ray diffraction (XRD) were used to characterize the nanoparticles. The increasing value of average diameter from the first to the eighth reduction reaction is about 0.58 nm, from the ninth to the fifteenth reduction reaction the value is about 2.44 nm.

Strong evidence of solution-phase catalysis associated with palladium leaching from immobilized thiols during Heck and Suzuki coupling of aryl iodides, bromides, and chlorides

A 3-mercaptopropyl-functionalized silica, SH-SBA-15, is used both as a support for palladium acetate, Pd-SH-SBA-15, and as a selective poison of soluble palladium complexes. Pd-SH-SBA-15 is used as a precatalyst for Heck and Suzuki couplings of an aryl iodide, bromide, and chloride under a range of reaction conditions. In all reactions in which metal-free SH-SBA-15 is added, the catalysis associated with either homogeneous palladium acetate or from Pd-SH-SBA-15 ceased. This strongly suggests that (i) SH-SBA-15 can be used as an effective scavenger and poison of soluble palladium and (ii) catalysis with Pd-SH-SBA-15 is solely associated with leached metal. Supporting evidence is given from hot filtration tests, self-quenching behavior of partially metalated Pd-SH-SBA-15, and poisoning by other solid-phase poisons such as poly(4-vinylpyridine), Quadrapure TU, and 3-mercaptopropyl-functionalized silica gel. It is postulated that the SH-SBA-15 poisons by over coordination of soluble monomeric or dimeric palladium, which rules out catalysis by palladium nanoparticle surfaces. SH-SBA-15 is an effective and selective poison of homogeneous palladium species and can be used as a test for heterogeneity of palladium catalysts developed for Heck and Suzuki reactions.

FTIR Study of the Mode of Binding of the Reactants on the Pd Nanoparticle Surface during the Catalysis of the Suzuki Reaction

In the Suzuki reaction between phenylboronic acid and iodobenzene catalyzed by palladium nanoparticles, our previous studies suggested that the phenylboronic acid adsorbs on the nanoparticle surface and then interacts with the iodobenzene that is present in solution. In the present study, FTIR is used to examine the change in the vibrational frequencies of phenylboronic acid in films with and without the addition of palladium nanoparticles. The large change in the B-O stretching frequency of phenylboronic acid from 1348 to 1376 cm(-1) in the presence of sodium acetate and palladium nanoparticles strongly suggests that the mode of binding of phenylboronic acid to the Pd nanoparticle surface involves a B-O-Pd type of bonding. Shifts in the B-C stretching mode and the out-of-plane phenyl C-C ring deformation bands associated with phenylboronic acid provide additional confirmations of the binding process. It is also shown that the phenylboronic acid needs to be in the deprotonated form in the presence of sodium acetate (phenylboronate anion) to bind to the palladium nanoparticle surface. No changes in the characteristic bands of iodobenzene were observed in films made in the presence of the palladium nanoparticles. The FTIR studies provide proof of the mode of binding that occurs in the nanoparticle surface for the first time and also confirms the mechanism of the Suzuki reaction that we proposed previously.

13C NMR and infrared evidence of a dioctyl-disulfide structure on octanethiol-protected palladium nanoparticle surfaces.

On 13C1-labeled octanethiol-protected 2.7 nm Pd nanoparticle surfaces, it has been observed that the 13C1 NMR of the alpha-carbon shows a peak centered around 38 ppm (with respect to tetramethylsilane (TMS)), which virtually coincides with that of the alpha-carbons in a dioctyl-disulfide molecule (39.3 ppm), and the corresponding 13C1 spin-spin relaxation becomes nonexponential. In addition, the infrared spectrum of the same sample shows that the ligands have a 100% gauche conformation, which is also consistent with a dioctyl-disulfide arrangement. By comparing with data obtained on 13C1-labeled octanethiol-protected 2.8 nm Au nanoparticles, we propose that a dioctyl-disulfide structure of the ligands is formed on the octanethiol-protected Pd nanoparticle surface, in contrast to the thiolate structure proposed on the Au nanoparticles. In addition, CO adsorption experiments show no sign of a PdS layer formed on the Pd nanoparticle surface. Furthermore, data taken over a period of more than 1 year show that the Pd nanoparticles are rather stable in organic solvents (for instance benzene), although slow degradation does happen and oxygen seems to play an important role in accelerating the degradation.