Stabilized Pd Nanoparticles Research Articles

Electrocatalytic Activity and Stability of Supported Palladium Nanoparticles

The overall performance of the polymer electrolyte membrane fuel cell (PEMFC) essentially depends on the catalytic activity of its core-part: the membrane electrode assembly (MEA). Palladium (Pd), one of the platinum group elements, is a potential substitute for Pt to be applied as the electrocatalyst, since it has more abundant reserves and a lower price compared to Pt. Pd is stable under extremely corrosive conditions similar to Pt.Nanosized metal particles perform considerably different as compared to the corresponding bulk materials. Furthermore, some reactions catalyzed by metal particles show a strong particle-size sensitivity. In previous work of our group, particle-size dependent activity of metal particles for electrocatalytic reaction was observed, such as Au nanoparticles supported on HOPG for oxygen reduction reaction (ORR) [1] and Pd nanoislands supported on Au(111) surface for hydrogen evolution reaction (HER) [2]. Beside the particle-size effects, support effects (e.g. ligand effect, strain effect) play important roles on electrocatalytic activity as well [3].In order to research the particle-size effects and support effects on the activity and stability of Pd nanoparticles, highly oriented pyrolytic graphite (HOPG) and nitrogen doped HOPG (N-HOPG) [4] were used in this work. Due to the weak interaction of HOPG with metal deposits, the substrate-related effects can be eliminated. N-HOPG is rich in electronegative nitrogen functional groups, and the roughness of N-HOPG is much higher than the untreated HOPG before the nitrogen implantation process. Pd nanoparticles were electrochemically deposited by double-pulse electrochemical deposition on both substrates. Pd nanoparticles are located mainly at the defects and step-edges on HOPG surface, but on the N-HOPG particles are distributed homogeneously. For ORR, the smaller particles have a negative particle-size effect. Pd nanoparticles supported on N-HOPG do not show enhanced activity for ORR. For HER, the case is much more difficult for the behavior of hydrogen absorption of Pd. The content of absorbed hydrogen atoms in Pd NPs is influenced by the particle size. The stability of Pd nanoparticles on HOPG is influenced not only by the particle size, but also by the pH value of the electrolyte. Pd particles on N-HOPG show poorer stability as compared to the pure HOPG insofar as they dissolved rapidly in potential cycling measurements.[1] Brülle, T.; Ju, W.; Niedermayr, P.; Denisenko, A.; Paschos, O.; Schneider, O.; Stimming, U., Molecules 2011, 16 (12), 10059-77.[2] Pandelov, S.; Stimming, U., Electrochimica Acta 2007, 52 (18), 5548-5555.[3] T. Bligaard, J.K. Nørskov, Electrochimica Acta, 52 (2007) 5512-5516.[4] Favaro, M.; Perini, L.; Agnoli, S.; Durante, C.; Granozzi, G.; Gennaro, A., Electrochimica Acta 2013, 88, 477-487.

Enhanced hydrogenation of ethyl levulinate by Pd–AC doped with Nb2O5

Gamma-valerolactone (GVL), as sustainable feedstock for high-value chemicals and fuel, has been produced by hydrogenation of levulinic acid (LA) or ethyl levulinate (EL). In this work, Pd nanoparticles supported on Nb2O5-doped activated carbon (AC) were prepared via wet incipient impregnation with PdCl2 followed by reduction with NaBH4. The dispersed niobia plays a bifunctional role in the catalytic process: stabilizing Pd nanoparticles and acting as an acidic co-catalyst. This synergistic effect between niobia and Pd leads to an unprecedented high activity of supported Pd catalysts in EL hydrogenation. The synergy is correlated with both the niobia loading and calcination temperature, with 10 wt% Nb2O5 calcined at 500 °C showing the best performance (EL conversion of 87% and GVL selectivity of 97%) under mild reaction conditions (100 °C and 0.5 MPa H2).

C2H2 Treatment as a Facile Method to Boost the Catalysis of Pd Nanoparticulate Catalysts

A facile method to boost the catalysis of Pd nanoparticulate catalysts by simple C2H2 treatment is developed. During the C2H2 treatment, Pd nanoparticles serve as active catalysts to polymerize C2H2 into trans-polyacetylene. The deposition of trans-polyacetylene layer on Pd nanoparticles makes their surface hydrophobic. Such a hydrophobic surface modification helps to accumulate more hydrophobic substrates during catalysis, making the modified Pd nanoparticulate catalysts more active than untreated catalysts in the catalytic reaction involving hydrophobic substrates. Moreover, the coating of polyacetylene on Pd creates encapsulated Pd nanocatalysts, stabilizing Pd nanoparticles against sintering or aggregation. Since the catalytic polymerization of polyacetylene on Pd is not facet dependent, the development is readily applied to enhance the catalysis of commercial Pd nanoparticulate catalysts by simple C2H2 treatment.

Critical Effects of Ligand Integration in Creating Palladium-Incorporated Porous Polymer Composites

This Article reports on the ligand integration effect of poly(amidoamine) (PAMAM) dendrimers working in the solid composite systems, which is significant for stabilizing Pd nanoparticles (PdNPs). To carry forward this research, the fourth generation of PAMAM dendrimer (G4) was modified with polymerizable functions and tied to polymer networks constituting porous composites through the polymerization-induced phase separation (PIPS) strategy. The obtained results indicate that creation of PdNPs is strongly affected by the G4 template, and, consequently, unforeseen critical control of their sizes was definitely performed. Also, the crucial importance from perspective of catalytic applications is strong suppression of Pd leaching for the G4-utilized porous polymer composites (G4-p). This fact means that the catalytic event of G4-p entirely occurs on the polymer support. All of these characteristics are in marked contrast to those of the composite that was fabricated with a nonintegrated, small molecule ligand system. On the basis of the present study, a primary mechanism that describes how Pd and ligand associate mutually during PIPS is proposed, where the Pd reduction and polymer propagation concurrently occurred.

Supramolecular assembling of molecular ion-ligands on graphite-based solid materials directed to specific binding of metal ions

Molecular ligands with a particular topology, based on a polar heteroaromatic nucleus linked to hydrophilic (metal-complexing) substituents, form very stable assemblies with activated carbons (ACs) due to strong π–π interactions between the planar pyrimidine moiety and the arene centers of the AC surfaces. Heteroatomic functions at the edges of the AC graphitic sheets do not alter the interaction mode with the ligands nor the stability of the assemblies. That results in the formation of stable hybrids, AC/ligand, whose surface chemical properties, specially the acid base behavior and the affinity to metal ions, reflect those of the non-conjugated, hydrophilic moieties of the ligands. Consequently, such hybrid materials show metal ion adsorptivities that are superior to those of the parent AC and correlate with the metal-binding abilities of the non-conjugated substituents. Significant amounts of meso- and macropores are required in the parent AC to efficiently transfer the chemical properties of the ligand non-conjugated substituents to the hybrid AC/ligand. After adsorption of metal ions, new hybrids, AC/ligand/metal, emerge with interesting potential applications. Thus, an AC/triamine ligand/Pd(II) hybrid showed high catalytic activity in hydrogenation reactions due to the formation of highly stable Pd nanoparticles at the AC surface during the first reaction cycle.

Peptide-Nanofiber-Supported Palladium Nanoparticles as an Efficient Catalyst for the Removal of N-Terminus Protecting Groups.

Sonication-induced tryptophan- and tyrosine-based peptide bolaamphiphile nanofibers have been used to synthesize and stabilize Pd nanoparticles under physiological conditions. The peptide bolaamphiphile self-assembly process has been thoroughly studied by using several spectroscopic and microscopic techniques. The stiffness of the soft hydrogel matrix was measured by an oscillatory rheological experiment. FTIR and circular dichroism (CD) experiments revealed a hydrogen-bonded β-sheet conformation of peptide bolaamphiphile molecules in a gel-phase medium. The π-π stacking interactions also played a crucial role in the self-assembly process, which was confirmed by fluorescence spectroscopy. Electron (SEM and TEM) and atomic force microscopy (AFM) studies showed that the peptide bolaamphiphile molecules self-assemble into nanofibrillar structures. Pd nanoparticles were synthesized in the hydrogel matrix in which redox-active tryptophan and tyrosine residues reduce the metal ions to metal nanoparticles. The size of the Pd nanoparticles are in the range of 3-9 nm, and are stabilized by peptide nanofibers. The peptide-nanofiber-supported Pd nanoparticles have shown effective catalytic activity for the removal of N-terminus protecting groups of amino acids and peptides.

Synthesis, Characterization, and Application of PVP-Pd NP in the Aerobic Oxidation of 5-Hydroxymethylfurfural (HMF)

PVP stabilized Pd nanoparticles (Pd NP) were prepared in ethylene glycol at three different NaOH/Pd ratios and characterized by XRD, transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). The Pd NP were applied in the aerobic oxidation of 5-hydroxymethylfurfural (HMF) into 2,5-furandicarboxylic acid (FDCA) in an alkaline aqueous solution (T = 90 °C, pO2 = 1 bar). The highest yield of FDCA (90 %) was obtained in the presence of Pd NP with a mean diameter of 1.8 nm, prepared with a 4:1 molar ratio of NaOH:Pd. Thus prepared Pd NP could be stored in their alkaline synthesis solution under ambient conditions and showed consistently good catalytic performances after up to one month storage time, in contrast to Pd NP prepared with 2:1 and 1:1 ratio and stored in neutral or acidic environment.

The Influence of N‐doped Carbon Materials on Supported Pd: Enhanced Hydrogen Storage and Oxygen Reduction Performance

N-doped graphene has become an important support for Pd in both hydrogen storage and catalytic reactions. The molecular orbitals of carbon materials (including graphene, fullerene, and small carbon clusters) and those of the supported Pd species will hybrid much stronger as N dopants are introduced, owing to the increased electrostatic attraction at the interface. This enhances the carbon substrates' catching force for the supported Pd, preventing its leaching and aggregation in many practical applications. The better dispersion and stabilization of Pd nanoparticles, which are induced by various carbon supports with N-doping, are pleasing to us and could increase their efficiency and facilitate their recycling during various reaction processes in several fields.

Selective hydrogenation of phenylacetylene over a nano-Pd/α-Al2O3 catalyst

Selective hydrogenation of phenylacetylene is an important reaction for increasing the purity of the styrene monomer. In this study a series of supported nano-Pd/α-Al2O3 catalysts were prepared and applied to the selective hydrogenation of phenylacetylene. The preparation procedure consisted of two steps: synthesis of colloidal Pd nanoparticles by a reduction-by-solvent method, followed by impregnation onto α-Al2O3. In the first step, NaBH4 was used as a reducing agent to reduce Pd2+ ions in the C2H5OH/H2O solvent system, and polyvinylpyrrolidone (PVP) was used as a protecting agent to stabilize Pd nanoparticles. The prepared colloids and catalysts were characterized by N2 physisorption, CO chemisorption, XRD, UV–vis, FT-IR, ICP and HRTEM techniques. It was found that the amounts of NaBH4, C2H5OH and PVP had effects on the Pd particle size, which in turn influenced the catalyst performance. The selective hydrogenation of phenylacetylene was sensitive to the structure of the nano-Pd/α-Al2O3 catalysts. In the case where the Pd particle size varied from 3.8 to 6.6nm, the specific activity and the selectivity to styrene increased with increasing particle size, but when the particle size increased in the range of 6.6–12.1nm, the specific activity increased slowly and the selectivity to styrene decreased gradually. Nano-Pd/α-Al2O3 can be reused several times without loss of activity and selectivity and no distinct variation in the Pd particle size was observed, indicating its good stability in the selective hydrogenation of phenylacetylene.

In situ synthesis of palladium nanoparticle on functionalized graphene sheets at improved performance for ethanol oxidation in alkaline media

Dimethyldiallylammonium chloride (DMDAAC) modified reduced graphene oxide supported Pd nanoparticles (Pd/DMDAAC-RGO) were fabricated by polyol microwave heating method. The Pd/DMDAAC-RGO hybrid were characterized by transmission electromicroscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD) analysis and electrochemical tests. High Pd metal loadings, up to 80wt. % with a mean size of 1.8nm, were densely in situ decorated on DMDAAC-modified RGO surfaces. Compared with traditional carbon-based Pd catalysts, Pd/DMDAAC-RGO exhibits better activity and stability for ethanol oxidation in alkaline media with the same Pd content on the electrode. This improved activity indicates that DMDAAC plays a crucial role in the dispersion and stabilization of Pd nanoparticles on RGO sheets and DMDAAC-RGO are able to an alternative support for Pd immobilization in direct ethanol fuel cells and other catalytic devices.

Role of Phenolic Groups in the Stabilization of Palladium Nanoparticles

For carbon supported Pd catalysts, the surface properties of activated carbons (ACs) are closely related with the Pd particle size. In this study, phenolic groups were adjustably introduced on ACs by hydrothermally treating ACs under different temperatures (433–513 K). Pd/ACs catalysts were prepared by the wetness impregnation method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), CO chemisorption, and H2-temperature-programmed reduction (TPR). The results revealed that the size of Pd nanoparticles (NPs) was highly dependent upon the amount of phenolic groups. The density functional theory (DFT) study suggested that the enhanced binding between palladium clusters and surface functional groups (SFGs) modified carbon nanotubes (CNTs) in the sequence: CNTs–O > CNTs–OH > CNTs–COOH > CNTs. Both the experimental and theoretical results suggested that phenolic groups on the surface of ACs play a vital role in the stabilization of Pd NPs, which provides insight into how to treat/or choose carbon supports for the preparation of small noble metal particles.

Catalytic hydrodechlorination of trichloroethylene in water with supported CMC-stabilized palladium nanoparticles

In this work, we developed and tested a new class of supported Pd catalysts by immobilizing CMC (carboxymethyl cellulose) stabilized Pd nanoparticles onto alumina support. The alumina supported Pd nanoparticles were able to facilitate rapid and complete hydrodechlorination of TCE (trichloroethylene) without intermediate by-products detected. With a Pd mass loading of 0.33 wt% of the alumina mass, the observed pseudo first order reaction rate constant, kobs, for the catalyst was increased from 28 to 109 L/min/g when CMC concentration was raised from 0.005 to 0.15 wt%. The activity increase was in accord with an increase of the Pd dispersion (measured via CO chemisorption) from 30.4% to 45.1%. Compared to the commercial alumina supported Pd, which has a lower Pd dispersion of 21%, our CMC-stabilized Pd nanoparticles offered more than 7 times greater activity. Pre-calcination treatment of the supported catalyst resulted in minor drop in activity, yet greatly reduced bleeding (<6%) of the Pd nanoparticles from the support during multiple cycles of applications. The presence of DOM (dissolved organic matter) at up to 10 mg/L as TOC had negligible effect on the catalytic activity. The alumina supported CMC-stabilized Pd nanoparticles may serve as a class of more effective catalysts for water treatment uses.

Thermoregulated poly(ethylene glycol) biphasic system with Pd nanoparticle catalysts for selective hydrogenation of cinnamaldehyde

Poly(ethylene glycol) (PEG)-stabilized Pd nanoparticles were prepared by simple hydrogen reduction of PdNa 2 Cl 4 · x H 2 O in the presence of PEG 4000 (PEG with an average molecular mass of 4000 g/mol). These were shown to be efficient and recyclable catalysts for the selective hydrogenation of cinnamaldehyde in a thermoregulated PEG biphasic system, which allows for an efficient homogeneous catalytic reaction, easy biphasic separation and catalyst reuse. Under optimized reaction conditions, the conversion of cinnamaldehyde and the selectivity of hydrocinnamaldehyde (HCAL) were 99% and 98%, respectively. The PEG 4000 -stabilized Pd nanoparticle catalyst could be easily separated from the product by phase separation and reused eight times without evident loss of activity and selectivity. 将具有“高温混溶、室温分相”功能的聚乙二醇4000(PEG 4000 )与甲苯-正庚烷组成的两相体系用于纳米钯催化的肉桂醛选择性加氢反应中. 在优化的反应条件下, 14;肉桂醛转化率和氢化肉桂醛选择性分别为99%和98%. 钯纳米催化剂经简单分相即可与产物分离, 14;且循环使用8次, 14;其活性和选择性基本保持不变. 14; Poly(ethylene glycol)-stabilized Pd nanoparticles were demonstrated to be efficient and recyclable catalysts for the selective hydrogenation of cinnamaldehyde in a thermoregulated PEG biphasic system.

Palladium nanoparticles supported on a triptycene-based microporous polymer: highly active catalysts for CO oxidation

Pd nanoparticles were well dispersed on a new triptycene-based microporous polymer support by the gas bubbling-assisted membrane reduction (GBMR) method. The stability of Pd nanoparticles is improved by the porous support, and the materials show excellent performance for CO oxidation.

Palladium catalytic systems with hybrid pyrazole ligands in C–C coupling reactions. Nanoparticles versus molecular complexes

This paper reports the comparison of the chemoselectivity of two different Pd catalytic systems, namely molecular and colloidal systems, in C–C coupling reactions. For this purpose, new hybrid pyrazole derived ligands containing alkylether, alkylthioether or alkylamino moieties have been synthesized and used to form Pd(II) complexes and to stabilize Pd nanoparticles (Pd NPs). With the aim of studying the coordination mode of the ligands and further to understand their role in catalysis, both types of Pd species were characterized by appropriate techniques. In C–C coupling reactions promoted by different Pd colloidal systems, several reports evidenced that active species are molecular catalysts leached from Pd NPs. The most important feature of this work relies on the differences observed in the output of C–C coupling reactions, depending on the colloidal or molecular nature of the catalyst employed. Thus, molecular systems carry out typical Suzuki–Miyaura cross-coupling, together with the dehalogenation of the substrate in different proportions. In contrast, Pd NPs catalyze either Suzuki–Miyaura or C–C homocoupling reactions depending on the haloderivative used. Interestingly, Pd NPs catalyze the quantitative dehalogenation of 4-iodotoluene. Differences observed in the chemoselectivity of these two catalytic systems support that reactions carried out with Pd NPs stabilized with the hybrid pyrazole ligands employed here take place on the surface of the colloids.

Pd Nanoparticles in the Thermoregulated Ionic Liquid and Organic Biphasic System: An Efficient and Recyclable Catalyst for Heck Reaction

A thermoregulated ionic liquid and organic biphasic system composed of ionic liquid [CH3(OCH2CH2)16N+Et3][CH3SO3 −] (ILPEG750) with ILPEG750-stabilized Pd nanoparticles and organic solvents, which allows for not only a highly efficient homogeneous catalytic reaction, but also an easy biphasic separation and reuse of catalyst attributed to the thermoregulated phase-transition property of the system, was employed for Heck reaction. The ILPEG750-stabilized Pd nanoparticle catalyst exhibited high efficiency, more importantly, after reaction the Pd nanoparticle catalyst could be separated from products by simple phase separation and recycled for six times without evident loss of activity. A thermoregulated ionic liquid and organic biphasic system containing [CH3(OCH2CH2)16N+Et3][CH3SO 3 - ] (ILPEG750)-stabilized Pd nanoparticles was applied to Heck reaction for the first time. The Pd nanoparticle catalyst exhibited high efficiency and good recyclability.

Synthesis of Palladium Nanoparticles Supported on Mesoporous N-Doped Carbon and Their Catalytic Ability for Biofuel Upgrade

We report a catalyst made of Pd nanoparticles (NPs) supported on mesoporous N-doped carbon, Pd@CN(0132), which was shown to be highly active in promoting biomass refining. The use of a task-specific ionic liquid (3-methyl-1-butylpyridine dicyanamide) as a precursor and silica NPs as a hard template afforded a high-nitrogen-content (12 wt %) mesoporous carbon material that showed high activity in stabilizing Pd NPs. The resulting Pd@CN(0.132) catalyst showed very high catalytic activity in hydrodeoxygenation of vanillin (a typical model compound of lignin) at low H(2) pressure under mild conditions in aqueous media. Excellent catalytic results (100% conversion of vanillin and 100% selectivity for 2-methoxy-4-methylphenol) were achieved, and no loss of catalytic activity was observed after six recycles.

Time-efficient microwave synthesis of Pd nanoparticles and their electrocatalytic property in oxidation of formic acid and alcohols in alkaline media

In this study time-efficient size controlled synthesis of stable Pd nanoparticles was carried out using microwave heating method by the decomposition of palladium chloride with glucose in aqueous medium using poly(ethyleneglycol) as capping agent. The benefit of the synthesis is that it was achieved in only 20 s. The synthesized Pd nanoparticles were characterized by UV–Visible spectroscopy, transmission electron microscopy, X-ray diffraction, and particle size analysis. The relative rates of electro-oxidation of formic acid, methanol and ethanol measured by cyclic voltammetry showed that efficiency of Pd nanoparticles as catalyst followed the order: formic acid < methanol < ethanol. The current–voltage characteristics improved with increase in either electrolyte (NaOH) or fuel concentrations but decreased with further increase in NaOH concentration.

Pd nanoparticles encaged in nanoporous interpenetrating polymer networks: A robust recyclable catalyst for Heck reactions

A novel type of Heck reaction catalyst composed of hydrophilic interpenetrating polymer networks (IPNs) and palladium (Pd) nanoparticles was prepared by simultaneous crosslinking of polyvinyl alcohol and polyacrylamide. The mesh sizes of the IPNs are one order of magnitude smaller than the average sizes of the uniformly dispersed Pd nanoparticles, which functions well to stabilize Pd nanoparticles and prevent aggregation. The Pd particles in the IPNs can catalyze Heck coupling reactions with high activities and be recycled over 20 times, providing more sustainability than any other polymer-stabilized Pd catalyst reported in the literature. The confined reactions inside the IPN nanopores in neat water may provide a green route for C–C coupling reactions.

Pd nanoparticles deposited on poly(lactic acid) grafted carbon nanotubes: Synthesis, characterization and application in Heck C–C coupling reaction

Herein we described the synthesis of a novel f-CNTs-Pd nanocatalyst by covalent grafting of poly(lactic acid) (PLA) onto carbon nanotubes (CNTs) and subsequent deposition of Pd nanoparticles. Prior to grafting of PLA, CNTs were oxidized with a mixture of HNO 3/H 2SO 4 and successively activated with thionyl chloride. The PLA grafted CNTs ( f-CNTs) were then used as platform for in situ deposition of Pd nanoparticles. The formation of f-CNTs-Pd nanocatalyst was analyzed by UV–vis, FTIR and Raman spectroscopy, powder XRD, energy dispersive spectroscopy and thermogravimetric analysis. The morphologies of the nanocatalyst were characterized using scanning and transmission electron microscopes. The f-CNTs stabilized Pd nanoparticles are found to be more effective in the promotion of Heck cross-coupling reaction between aryl halides and n-butyl acrylate. The f-CNTs-Pd nanocatalyst was regenerated for three cycles of reaction without any significant loss in its activity.