Pd Dendrimer-encapsulated Nanoparticles Research Articles

Single atoms and small clusters of atoms may accompany Au and Pd dendrimer-encapsulated nanoparticles

We report the presence of small clusters of atoms (<1 nm) (SCs) and single atoms (SAs) in solutions containing 1-2 nm dendrimer-encapsulated nanoparticles (DENs). Au and Pd DENs were imaged using aberration-corrected scanning transmission electron microscopy (ac-STEM), and energy dispersive spectroscopy (EDS) was used to identify and quantify the SAs/SCs. Two main findings have emerged from this work. First, the presence or absence of SAs/SCs depends on both the terminal functional group of the dendrimer (-NH2 or -OH) and the elemental composition of the DENs (Au or Pd). Second, dialysis can be used to remove the majority of SAs/SCs in cases where a high density of SAs/SCs are present. The foregoing conclusions provide insights into the mechanisms for Au and Pd DEN synthesis and stability. Ultimately, these results demonstrate the need for careful characterization of systems containing nanoparticles to ensure that SAs/SCs, which may be below the detection limit of most analytical methods, are taken into consideration (especially for catalysis experiments).

Composition-dependent catalytic activity of bimetallic PtPd dendrimer-encapsulated nanoparticles having an average size of 1.7 nm for hydrolytic dehydrogenation of ammonia borane

We investigated composition-dependent catalytic activity of bimetallic PtPd dendrimer-encapsulated nanoparticles (DENs) that had a uniform size of ∼1.7nm for hydrolytic dehydrogenation of ammonia borane (AB). The PtPd DENs, composed of seven different Pt: Pd ratios, were synthesized using hydroxyl-terminated sixth-generation polyamidoamine dendrimers as a molecular template. The dendrimer-templating method allowed for synthesizing bimetallic PtPd DENs with controllable nanoparticle composition while fixing the size of the nanoparticles uniformly at ∼1.7 nm. Compared with monometallic Pt and Pd DENs, the bimetallic PtPd DENs showed superior catalytic activity for the hydrolytic dehydrogenation of AB. Furthermore, the bimetallic PtPd DENs exhibited composition-dependent activity with the maximum activity (i.e., average turnover frequency=108.5±15.9molH2·molatom Pt+pd−1·min−1) at a Pt: Pd ratio of 1:1 for the catalytic hydrolysis of AB.

Dendrimer-Encapsulated Pd Nanoparticles, Immobilized in Silica Pores, as Catalysts for Selective Hydrogenation of Unsaturated Compounds.

Heterogeneous Pd‐containing nanocatalysts, based on poly (propylene imine) dendrimers immobilized in silica pores and networks, obtained by co‐hydrolysis in situ, have been synthesized and examined in the hydrogenation of various unsaturated compounds. The catalyst activity and selectivity were found to strongly depend on the carrier structure as well as on the substrate electron and geometric features. Thus, mesoporous catalyst, synthesized in presence of both polymeric template and tetraethoxysilane, revealed the maximum activity in the hydrogenation of various styrenes, including bulky and rigid stilbene and its isomers, reaching TOF values of about 230000 h−1. Other mesoporous catalyst, synthesized in the presence of polymeric template, but without addition of Si(OEt)4, provided the trans‐cyclooctene formation with the selectivity of 90–95 %, appearing as similar to homogeneous dendrimer‐based catalysts. Microporous catalyst, obtained only on the presence of Si(OEt)4, while dendrimer molecules acting as both anchored ligands and template, demonstrated the maximum activity in the hydrogenation of terminal linear alkynes and conjugated dienes, reaching TOF values up to 400000 h−1. Herein the total selectivity on alkene in the case of terminal alkynes and conjugated dienes reached 95–99 % even at hydrogen pressure of 30 atm. The catalysts synthesized can be easily isolated from reaction products and recycled without significant loss of activity.

Dendrimer-encapsulated Pd nanoparticles as catalysts for C-C cross-couplings in flow microreactors.

The inner walls of glass microreactors were functionalized with dendrimer-encapsulated Pd nanoparticles. The catalysts were efficient for the Heck-Cassar (copper-free Sonogashira) and Suzuki-Miyaura (SMC) cross-coupling reactions. For the Heck-Cassar reaction between iodobenzene and phenylacetylene, the catalytic system exhibited a high turnover frequency (TOF) and on average required milder reaction conditions as compared to other continuous flow cross-couplings. A study of the substituent effect of para-substituted aryl halides revealed a beneficial effect of electron-withdrawing side groups for the SMC. Moreover, a reaction constant (ρ) of 1.5, determined from the Hammett plot, indicated a possible rate-determining step other than the oxidative addition.

Synthesis, characterization, and electrocatalysis using Pt and Pd dendrimer-encapsulated nanoparticles prepared by galvanic exchange

In this report we present the synthesis and characterization of Pt and Pd dendrimer-encapsulated nanoparticles (DENs) using the method of galvanic exchange. Sixth-generation hydroxyl-terminated poly(amidoamine) dendrimers were used to prepare Cu DENs composed of 55 atoms. In the presence of either PtCl42− or PdCl42−, the less noble Cu DENs oxidize to Cu2+ leaving behind an equal-sized DEN of Pt or Pd, respectively. DENs prepared by direct reduction with BH4−, which is the common synthetic route, and those prepared by galvanic exchange have the same composition, structure, and properties as judged by UV-vis spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, and electrochemical methods. However, the galvanic exchange synthesis is much faster (3 h vs. 96 h), and the yield of reduced DENs is significantly higher (nearly 100% in the case of galvanic exchange).

Nanoparticle Size Determination by 1H NMR Spectroscopy

High-resolution solution (1)H NMR spectroscopy has been used to characterize the size of Pd dendrimer-encapsulated nanoparticles (DENs). The Pd nanoparticles measured by this technique contain 55, 147, 200, or 250 atoms, and they are encapsulated within sixth-generation, hydroxyl-terminated poly(amidoamine) PAMAM dendrimers (G6-OH). Detailed analysis of the NMR data shows that signals arising from the innermost protons of G6-OH(Pd(n)) decrease significantly as the size of the encapsulated nanoparticles increase. A mathematical correlation between this decrease in the integral value and the theoretical number of Pd atoms in the nanoparticle is extracted. It enables the elucidation of the size of Pd DENs by (1)H NMR spectroscopy. NMR pulse-field gradient spin-echo experiments demonstrate that G6-OH with and without DENs have identical hydrodynamic radii, which excludes the presence of dendrimer/nanoparticle aggregates.

Quantitative Analysis of the Stability of Pd Dendrimer-Encapsulated Nanoparticles

The stability of Pd dendrimer-encapsulated nanoparticles (DENs) in air-, N(2)-, and H(2)-saturated aqueous solutions is reported. The DENs consisted of an average of 147 atoms per sixth-generation, poly(amidoamine) dendrimer. Elemental analysis and UV-vis spectroscopy indicate that there is substantial oxidation of the Pd DENs in the air-saturated solution, less oxidation in the N(2)-saturated solution, and no detectable oxidation when the DENs are in contact with H(2). Additionally, the stability improves when the DEN solutions are purified by dialysis to remove Pd(2+)-complexing ligands such as chloride. For the air- and N(2)-saturated solutions, most of the oxidized Pd recomplexes to the interiors of the dendrimers, and a lesser percentage escapes into the surrounding solution. The propensity of Pd DENs to oxidize so easily is a likely consequence of their small size and high surface energy.

Dendrimer-encapsulated Pd nanoparticles anchored on carbon nanotubes for electro-catalytic hydrazine oxidation

This study describes a facile method for preparing amine-terminated poly(amidoamine) (G4-NH 2)-encapsulated palladium nanocatalysts with carbon nanotubes as a support. The Pd 2+/G4-NH 2 complexes are synthesized by a template approach in which PdCl 4 2 - ions are extracted into the interior of G4-NH 2 dendrimers, and then bonded on multi-walled carbon nanotubes (MWNTs) support through dendrimer covalent linkage. Conversion of Pd 2+/G4-NH 2 into dendrimer-encapsulated nanoparticles was achieved by the addition of NaBH 4. Transmission electron microscopy (TEM), X-ray diffraction (XRD), UV–vis spectroscopy confirmed that the size-monodisperse Pd nanoparticles anchored on MWNTs was about 2 nm in diameter. The obtained Pd/G4-NH 2/MWNT nanocomposites exhibit high electro-catalytic activity for the hydrazine oxidation by cyclic voltammetry.

Dendrimer-Encapsulated Pd Nanoparticles versus Palladium Acetate as Catalytic Precursors in the Stille Reaction in Water

The performance of several palladium precatalysts, namely, palladium(II) acetate, palladium(0) nanoparticles encapsulated into poly(amidoamine) (PAMAM) dendrimers (Pd DENs), and palladium(II)-PAMAM complexes, in the Stille reaction between trichloro(phenyl)stannane and iodoarenes in water is compared. The reactivity of Pd DENs is similar or inferior to that of palladium(II) acetate, although the presence of the dendrimer suppresses the formation of homocoupling products and allows catalyst recycling. It is suggested that the reaction catalyzed by Pd DENs occurs via palladium species which are leached from the nanoparticle but which remain coordinated to the dendritic macromolecule.

NMR Characterization of Fourth-Generation PAMAM Dendrimers in the Presence and Absence of Palladium Dendrimer-Encapsulated Nanoparticles

High-resolution solution NMR spectroscopy has been used to characterize the structure of Pd dendrimer-encapsulated nanoparticles (DENs), consisting of approximately 55-atom nanoparticles encapsulated within fourth-generation, hydroxyl-terminated poly(amidoamine) PAMAM dendrimers (G4-OH). Detailed analysis of 1D and 2D NMR spectra of dendrimers with (G4-OH(Pd(55))) and without (G4-OH) nanoparticles unambiguously demonstrate that single nanoparticles are encapsulated within individual dendrimers. This conclusion is based on the following results. First, the NMR data show that signals arising from the innermost methylenes of G4-OH(Pd(55)) are more highly influenced by the presence of the Pd nanoparticles than are the terminal functional groups. This means that DENs are encapsulated within dendrimers rather than being adsorbed to their surface, as would be the case for aggregates consisting of multiple dendrimers and nanoparticles. Second, extraction of DENs from within their dendrimer hosts results in an increase in the NMR intensity associated with the interior methylenes, which corroborates the previous point. Third, NMR pulse-field gradient spin-echo experiments demonstrate that G4-OH and G4-OH(Pd(55)) have identical hydrodynamic radii, and this finding excludes the presence of dendrimer/nanoparticle aggregates.

Enhanced stability of charged dendrimer-encapsulated Pd nanoparticles in ionic liquids

Highly stable dendrimer-encapsulated Pd nanoparticles in ionic liquids were prepared for the first time by using charged PAMAM dendrimers as templates, which could maintain hydrogenation efficiency for up to at least 12 recycles.

Dendrimer-Encapsulated Pd Nanoparticles as Aqueous, Room-Temperature Catalysts for the Stille Reaction

We report that dendrimer-encapsulated Pd nanoparticles having a diameter of approximately 1.7 nm are effective and general catalysts for coupling aryl halides to organostannanes (the Stille reaction) under mild conditions. The significant results of this study are that the Stille reaction is catalyzed by dendrimer-encapsulated Pd nanoparticles in very good yield, in aqueous solution at 23 degrees C, and using only 0.100 atom % of Pd as catalyst.

Synthesis, Characterization, and Stability of Dendrimer-Encapsulated Palladium Nanoparticles

Here we report on the synthesis, physical and chemical properties, and stability of Pd nanoparticles encapsulated within poly(amidoamine) (PAMAM) dendrimers. Specifically, amine- and hydroxyl-terminated PAMAM dendrimers ranging in generation from 4 to 8 were studied. Under appropriate conditions, addition of K2PdCl4 results in covalent attachment of the PdCl3- hydrolysis product of this complex to tertiary amines within the dendrimers. Reduction with NaBH4 results in conversion of dendrimer-encapsulated PdCl3- to nearly size monodisperse, encapsulated, zerovalent Pd nanoparticles. Details regarding the Pd species present in solution and within the dendrimer prior to reduction are reported, as is the maximum Pd2+ loading of the dendrimers. Dendrimer-encapsulated Pd nanoparticles undergo oxidation in air, but this process is slowed significantly when coordinating ions are removed from solution. In the absence of O2, dendrimer-encapsulated Pd nanoparticles are stable indefinitely. The oxidation product is not PdO, but rather Pd ions coordinated to the dendrimer interior. Dendrimer generation does not affect the rate of Pd oxidation. The dendrimer itself undergoes irreversible oxidation in the presence of O2. Finally, the oxidation of dendrimer-encapsulated Pd nanoparticles is reversible. Specifically, H2 gas can be used to re-reduce partially oxidized Pd nanoparticles without changing their average size.

Extraction of Monodisperse Palladium Nanoparticles from Dendrimer Templates

We describe a strategy for extracting nearly size-monodisperse, 1.7-nm-diameter Pd nanoparticles from within dendrimer hosts. The approach involves two steps. First, an aqueous solution of dendrimer-encapsulated Pd nanoparticles is prepared, followed by the addition of a toluene solution containing an n-alkanethiol. When the two-phase system is stirred, n-alkanethiols presumably self-assemble onto the surface of the Pd nanoparticles, extract them from the dendrimer, and transport them to toluene phase. UV-vis spectroscopy indicates that the Pd nanoparticles, in the form of monolayer-protected clusters, are transported into the toluene phase. FT-IR spectroscopy shows that the dendrimers remain in the aqueous phase. High-resolution transmission electron microscopy shows that the average size and size distribution of the particles do not change during the extraction process.

Dendrimer-Encapsulated Pd Nanoparticles as Fluorous Phase-Soluble Catalysts

Dendrimer-Encapsulated Pd Nanoparticles as Fluorous Phase-Soluble Catalysts

Homogeneous Hydrogenation Catalysis with Monodisperse, Dendrimer-Encapsulated Pd and Pt Nanoparticles.

Extraordinarily stable, monodisperse noble metal nanoparticles can be prepared by using dendrimers as both templates and stabilizers. Dendrimer-encapsulated Pd nanoparticles (see the schematic representation) exhibit high catalytic activity for the hydrogenation of alkenes in water. The catalytic activity and selectivity of these materials can be controlled by adjusting the dendrimer generation.