Nanosized Pd Particles Research Articles

Formation and stabilization of nanosized Pd particles in catalytic systems: Ionic nitrogen compounds as catalytic promoters and stabilizers of nanoparticles

• Pd catalytic systems usually contain nanoparticles regardless of the type of pre-catalyst and require special stabilizers. • Ionic nitrogen compounds stabilize Pd nanoparticles and open up new fields of application in organic synthesis. • The structure of the ionic compound affects the stabilization mechanism and the efficiency of the catalysts. • The catalytic action of the ionic nitrogen compound can outweigh the action of the ligands. • The overall effect of stabilization can be highly dependent on the nature of catalysis and reaction conditions. Actual palladium catalysts in synthetic transformations in reaction mixtures are usually represented by dynamic catalytic systems that contain various interconvertible forms of metal particles, including molecular complexes, metal clusters, and nanoparticles. The low thermodynamic stability of Pd nanoparticles can lead to their aggregation and, as a consequence, to the deactivation of the catalytic systems. Therefore, stabilization of nanosized Pd particles is of key importance to ensure efficient catalysis. This review discusses the main pathways for the formation of Pd nanoparticles and clusters from various precatalysts in catalytic systems, as well as current views on the mechanisms of stabilization of these nanosized Pd particles using various types of ionic nitrogen compounds, such as ammonium, amidinium, azolium, and pyridinium salts. The use of ionic nitrogen compounds as specially added or in situ formed stabilizers, ligands, catalytic promoters, heterogenized catalysts (supported ionic liquid phase, SILP) and reaction media (ionic liquids) is exemplified by several important catalytic reactions. The main effects of ionic nitrogen compounds on catalytic processes are also discussed, including possible involvement in catalytic cycles and unwanted side reactions.

Enhanced electrocatalytic activity of urchin-like Nb2O5 microspheres by synergistic effects with Pd toward electrooxidation of ethylene glycol in an alkaline medium

Urchin-like niobium oxide (U-Nb2O5) microspheres are obtained using a low-temperature hydrothermal method. The nanosized palladium (Pd) particles loading onto U-Nb2O5 (Pd/U-Nb2O5) microspheres are successfully prepared using a reflux method. X-ray diffraction results imply that the U-Nb2O5 and Pd/U-Nb2O5 microspheres belong to the orthorhombic phase, and Pd loading does not change the crystalline phase. The average size of nanosized Pd particles loaded is 4.25 nm. In addition, the valence states of Pd on the surface of the Pd/U-Nb2O5 microspheres are mainly of a Pd° form. The catalytic activity of the different loading amount Pd/U-Nb2O5 electrocatalysts is carried out in a 0.5 M KOH and 1 M ethylene glycol (EG). When the usage of palladium acetate is 10 mg (the mass ration of Pd to Nb2O5 is 10 %), the Pd/U-Nb2O5 electrocatalyst exhibits high catalytic activity for the electrooxidation of EG in an alkaline medium. The current density of EG electrooxidation on the Pd/U-Nb2O5 modified glassy carbon electrode is higher than that on commercial Pd/C (10 %). Moreover, the Pd/U-Nb2O5 electrocatalyst exhibits strong anti-CO poisoning ability and good stability for EG oxidation in alkaline medium. After 6000 s, the maintained current density on the Pd/U-Nb2O5 electrocatalyst is 12.7 times that on commercial Pd/C. This remarkable performance of the Pd/U-Nb2O5 electrocatalyst is owing to the synergistic effect between Pd and Nb2O5. The calculation results imply that the synergistic effect improves electron transfer.

Geometric, electronic, and synergistic effect in the sulfonated carbon-supported Pd catalysts for the direct synthesis of hydrogen peroxide

Although carbon-supported palladium catalysts have been generally used for the direct synthesis of H2O2 (DSHP), the low H2O2 selectivity remains a challenge. In this work, it was systematically studied how the sulfonated carbon surface affects the physicochemical properties of Pd during catalyst preparation, and thereby affects the catalytic activity of Pd/C for the DSHP reaction. Nano-sized Pd particles (about 2–3 nm) with a high Pd2+/Pd0 ratio resulted in a high H2O2 selectivity. The sulfonated carbon material acted as an efficient solid acid support and enhanced the H2O2 selectivity. Fine-tuning the Pd properties on the sulfonated carbon support significantly improved the H2O2 selectivity by as much as 87 % in the absence of caustic promoters, which clearly demonstrates that the synergistic combination of the active metal properties and that of the carbon support are extremely important to realize a highly active and selective carbon-supported palladium catalyst for the DSHP reaction.

Pd nanoparticles stabilized with phosphine-functionalized porous ionic polymer for efficient catalytic hydrogenation of nitroarenes in water

Small palladium nanoparticles stabilized with phosphine-functionalized PIP displayed high catalytic activity for nitroarenes hydrogenation. Nano-size Pd particles, electron-donation effect of phosphine ligand, and surface wettability account for its excellent catalytic performance.

Palladium-loaded tantalum oxide modified Pt electrode toward electrochemical oxidation of ethylene glycol

The study of the microstructure and electrochemical property of palladium-loaded tantalum oxide (Pd/Ta2O5) particles is presented. Pd/Ta2O5 belongs to the orthorhombic phase. The palladium states onto the Pd/Ta2O5 surface are Pd0 and Pd2+ forms. From the morphology and size analysis of the loaded Pd particles using transmission electron microscopy, one can see that the nanosized-Pd particles are loaded onto the Ta2O5 surface. A Pd/Ta2O5 film-modified Pt (Pd/Ta2O5-Pt) electrode further exhibits excellent catalytic activity toward electro-oxidation of ethylene glycol (EG) in acid conditions. Density functional theory was also utilized to clarify the effect on microstructure of Ta2O5 after Pd loading showing that the density of states is increased and the density of electrons in the loading region is enlarged after Ta2O5 is loaded with Pd. Therefore, Pd loading improves the conductivity and electrochemical property of the Pd/Ta2O5-Pt electrode.

Two-dimensional Pd-nanosheets as efficient electrocatalysts for ethanol electrooxidation. Evidences of the C[sbnd]C scission at low potentials

Pd-nanosheets with a lateral size of ca. 5 nm have been prepared by a simple chemical method. The Pd-nanosheets obtained exhibited a 2D morphology in which Pd atoms are preferentially exposed at the surface of the particles. In addition, a preferential exposure of Pd atoms in 111 planes is observed. As a result, the Pd-nanosheets exhibit a very high catalytic activity for the electrooxidation of ethanol in alkaline electrolyte, superior to that of commercial nanosized Pd particles. In situ infrared studies conducted during the electrooxidation of ethanol reveal that the scission of the CC bond of ethanol takes place at the surface of Pd-nanosheets at low potentials (as low as 30 mV). However, at higher potentials, the formation of partially oxidized species, typically acetates, is the preferred reaction pathway.

Promoting Effect of Ni(OH)2 on Palladium Nanocrystals Leads to Greatly Improved Operation Durability for Electrocatalytic Ethanol Oxidation in Alkaline Solution.

Most electrocatalysts for the ethanol oxidation reaction suffer from extremely limited operational durability and poor selectivity toward the CC bond cleavage. In spite of tremendous efforts over the past several decades, little progress has been made in this regard. This study reports the remarkable promoting effect of Ni(OH)2 on Pd nanocrystals for electrocatalytic ethanol oxidation reaction in alkaline solution. A hybrid electrocatalyst consisting of intimately mixed nanosized Pd particles, defective Ni(OH)2 nanoflakes, and a graphene support is prepared via a two-step solution method. The optimal product exhibits a high mass-specific peak current of >1500 mA mg-1Pd , and excellent operational durability forms both cycling and chronoamperometric measurements in alkaline solution. Most impressively, this hybrid catalyst retains a mass-specific current of 440 mA mg-1 even after 20 000 s of chronoamperometric testing, and its original activity can be regenerated via simple cyclic voltammetry cycles in clean KOH. This great catalyst durability is understood based on both CO stripping and in situ attenuated total reflection infrared experiments suggesting that the presence of Ni(OH)2 alleviates the poisoning of Pd nanocrystals by carbonaceous intermediates. The incorporation of Ni(OH)2 also markedly shifts the reaction selectivity from the originally predominant C2 pathway toward the more desirable C1 pathway, even at room temperature.

Characterization and study on the thermal aging behavior of palladium–alumina catalysts

A series of Pd/Al2O3 catalysts were prepared by incipient wetness impregnation method. Palladium loading was varied in a range of 0.125–4.0 mass%. The catalytic performance of the samples was tested in model reaction of CO oxidation at oxygen excess. Catalysts were characterized by temperature-programmed reduction (TPR), electron paramagnetic resonance, and UV–visible spectroscopy. Thermal aging of chosen samples was performed at 1000 °C. The thermal aging behavior was studied using TPR, X-ray diffraction analysis, transmission electron microscopy, and X-ray photoelectron spectroscopy (XPS). It was shown that palladium stabilized in the form of dispersed surface Pd2+ species when Pd loading is 0.5 mass% and below. Samples with higher loading preferentially contain nanosized Pd particles. Agglomeration of Pd species during thermal aging was found to take place starting from Pd concentration of 1.0 mass%. In some cases, size of Pd particles exceeds 150 nm, which is in about 40 times higher comparing with the initial samples. According to XPS data, degree of Pd2+-alumina interaction in the aged samples is also increased.

High-selectivity electrochemical non-enzymatic sensors based on graphene/Pd nanocomposites functionalized with designated ionic liquids

Nano-sized Pd particles are uniformly dispersed on graphene nanosheets (GNSs) using a supercritical-fluid-assisted deposition technique to increase the electrochemical sensing properties. The incorporation of different kinds of ionic liquid (IL) can increase the electrode sensing current toward different analytes. Butylmethylpyrrolidinium–bis(trifluoromethanesulfonyl)imide (BMP–TFSI) IL is beneficial for glucose detection, whereas the electrode with butylmethylpyrrolidinium–dicyanamide (BMP–DCA) IL shows high sensitivity toward ascorbic acid (AA). The selective detection of glucose or AA from their mixture is for the first time demonstrated using a non-enzymatic electrode with the aid of an IL. Angle-resolved X-ray photoelectron spectroscopy analyses indicate that GNSs can create an aligned cation/anion orientation in the adsorbed IL film, with the anions preferentially occupying the topmost surface. As a result, the electrode sensitivity and selectivity are mainly determined by the IL constituent anions.

Pd size effect on the gas sensing properties of Pd-loaded SnO2 in humid atmosphere.

Pd particles of different nanosizes were loaded on the SnO2 surface by using different Pd precursors for the purpose of investigating the Pd size effect on gas sensing properties in humid atmosphere. One kind of Pd-loaded SnO2 nanoparticle was characterized by smaller Pd particles (2.6 nm) with high dispersion, while another kind was characterized by larger Pd particles (5-10 nm) with low dispersion. It was found that both kinds of Pd on the SnO2 surface let the mainly adsorbed oxygen species change from O(-) to O(2-) in humid atmosphere at 350 °C. In addition, the water vapor poisoning effect on electric resistance and sensor response was greatly reduced by loading Pd. Interestingly, for the CO response at 350 °C, Pd-SnO2 with small Pd size showed almost constant sensor response with varying humidity (0.5-4 vol % H2O). While the CO response of Pd-SnO2 with large Pd size even increased with increasing amount of water vapor. Moreover, the former CO response was increased from 300 to 350 °C, but the later response decreased with increase in operating temperature. These behaviors were analyzed by temperature programed reduction (TPR) in H2 and CO atmospheres, and they were supported by the different catalytic activities of different nanosized Pd particles.

The structure of nano-palladium deposited on carbon-based supports

Nano-palladium catalysts, prepared using the same procedure with the same metal content (3wt%) and two different supports, activated carbon (Pd/C) and activated carbon—multiwalled carbon nanotubes (Pd/C/CNT), are discussed. The simple technique of deposition reduction was applied in the preparation of these two types of Pd catalysts. TEM, XRD analysis, EXAFS signal analysis, and XANES were used for sample characterization. In both samples, transmission electron microscopy identified nanosized Pd particles with nearly spherical morphology but different sizes. The mean diameters of the particles on Pd/C and Pd/C/CNT were estimated to be 5.4nm and 7.8nm, respectively. The EXAFS signal analysis showed that Pd atoms on the particle surfaces were coordinated by 4 oxygens to form a PdO monolayer covering a metallic core. The XANES signal analysis indicated a smaller particle size for Pd/C (∅ 5nm) than for Pd/C/CNT (∅ 10nm), in good agreement with the TEM observations.

Nanocrystalline Pd/carbon nanotube composites synthesized using supercritical fluid for superior glucose sensing performance

With the aid of supercritical carbon dioxide (SCCO2), which has gas-like diffusivity, extremely low viscosity, and near-zero surface tension, the synthesized nano-sized Pd particles (approximately 5nm in diameter) are uniformly distributed on carbon nanotubes (CNTs). The decoration density of Pd nanoparticles on the CNTs can be simply controlled, without varying the particle size, by adjusting the Pd hexafluoroacetylacetonate precursor to CNT ratio. Since the Pd nanoparticles are highly dispersed and tightly anchored on the CNTs, the obtained composite shows enhanced electro-sensing performance toward glucose compared to that of the control sample prepared using a conventional deposition process (without SCCO2).

Energy & Chemicals from Renewable Resources by Electrocatalysis

The selective production of chemicals from renewable resources with contemporaneous release of energy is perhaps one of the most desired targets of sustainable chemistry. Here, we report an overview of our recent research efforts, where we have demonstrated that this can be achieved using renewable alcohols, by means of two electrochemical devices: direct fuel cells and electrolyzers. In either case, an aqueous solution of the fuel in the anode compartment is oxidized on a nanostructured electrocatalyst that promotes selectively the partial oxidation of the anolyte with high stability and fast kinetics. We have found that anode electrocatalysts based on nanosized Pd particles, alone or promoted by Ni-Zn phases as well as by CeO2 or TiO2, are able to accomplish this goal in alkaline environment when used in conjunction with commercially available cathode electrocatalysts and solid or liquid electrolytes. In an electrolyzer, containing an anode electrocatalyst similar to that employable in a DAFC, the electrolyte may be either an anion exchange-membrane or a solution of an alkali metal hydroxide (NaOH or KOH, for example) and the alcohol is converted to the corresponding alkali metal carboxylate, while hydrogen gas is produced at the cathode upon water reduction.

Sensing performance of zirconia-based gas sensor using titania sensing-electrode added with palladium

The sensing characteristics of a yttria-stabilized zirconia (YSZ)-based sensor using a sensing electrode (SE) consisting of rod-shaped TiO2 added with 1 wt.% of nano-sized Pd particles (abbreviated as TiO2(+ Pd)-SE) were evaluated, aiming at selective detection of hydrocarbons. The rod-shaped TiO2-SE without Pd addition (TiO2-SE) was found to give high sensitivities to both C3H8 and C3H6 which were used as representatives of alkane and alkene groups, respectively, in hydrocarbons (HCs), while TiO2(+ Pd)-SE gave selective detection of C3H8. The evaluation of catalytic activity to the gas-phase oxidation of HCs for the SE materials revealed that the oxidation of C3H6 was largely promoted by the Pd addition, leading to the high selectivity to C3H8 for the sensor using TiO2(+ Pd)-SE. From the results of polarization-curve measurements, the current value for the cathodic reaction of O2 was found to be slightly decreased by the Pd addition, while those for the anodic reaction of C3H8 and C3H6 were largely decreased. This is presumably due to the decrease in the actual concentration of C3H8 and C3H6 at the SE/YSZ interface owing to the promoted gas-phase oxidation of these HCs through the TiO2(+ Pd)-SE layer. XRD measurements as well as SEM observations confirmed that the added Pd particles were existing as PdO particles on the surface of the rutile-type TiO2 rods after the SE fabrication process. The stability test revealed that the C3H8 sensitivity of the sensor using TiO2(+ Pd)-SE gradually increased and reached the stable value after the 12 day operation. By considering the results of the complex-impedance analysis, we believed that this gradual increase in sensitivity was due to the decrease in the catalytic activity against the gas-phase oxidation of C3H8 in the TiO2(+ Pd)-SE layer.

수열흡착법을 이용한 나노팔라듐 점코팅 활성탄 분말의 합성 및 미세조직

Nano Pd spot-coated active carbon powders were synthesized by a hydrothermal-attachment method (HAA) using PVP capped Pd colloid in a high pressure bomb at <TEX>$250^{\circ}C$</TEX>, 450 psi, respectively. The PVP capped Pd colloid was synthesized by the precipitation-redispersion method. PVP capped Pd nano particles showed the narrow size distribution and their particle sizes were less than 8nm in diameter. In the case of nano Pd-spot coated active carbon powders, nano-sized Pd particles were adhered in the active carbon powder surface by HAA method. The component of Pd was homogeneously distributed on the active carbon surface.

Influence of CO2 and H2O on the separation of hydrogen over two types of Pd membranes: Thin metal membrane and pore-filling-type membrane

Abstract The influence of mixing CO2 and H2O on the permeation of H2 through the two different types of the Pd membranes was investigated. We used a thin metal membrane on an alumina support tube prepared by CVD and a pore-filling-type membrane that was constructed by embedding nano-sized Pd particles in the pores of an alumina support tube. The permeation rate at 573 K decreased with the increase of CO2 and H2O ratio. The decrease of the permeation rate of both gases on the pore-filling-type membrane was smaller than that on the CVD membrane. In particular, irreversible decrease of the flux appeared over the CVD membrane during mixing of CO2 at 573 K. That is, the permeation rate of H2 decreased in response to mixing with CO2 and was not easily recovered by the subsequent supply of pure H2 at the same temperature. This influence was due to the surface carbonous species produced by the reaction of CO2 and H2 as observed by XPS and TPD analysis. It was not easily desorbed at 573 K and blocked the surface sites for the adsorption of H2. At 673 K, the influence of CO2 was not observed, despite the formation of CO by the reverse water gas shift reaction because CO was easily desorbed from the membrane at this temperature.

A Simple Strategy for Fabrication of “Plum-Pudding” Type Pd@CeO2 Semiconductor Nanocomposite as a Visible-Light-Driven Photocatalyst for Selective Oxidation

The Pd@CeO2 semiconductor nanocomposite with “plum-pudding” structure has been fabricated successfully via a facile low-temperature hydrothermal reaction of polyvinylpyrrolidone (PVP)-capped Pd colloidal particles and cerium chloride precursor followed by a calcination process in air. Different characterization techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission scanning electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDX), UV–vis diffuse reflectance spectra (DRS), X-ray photoelectron spectra (XPS), photoluminescence spectra (PL), nitrogen adsorption–desorption, and electron spin resonance spectra (ESR), have been used to investigate the structure and properties of the Pd@CeO2 nanocomposite. It is found that the nanosized Pd particles are evenly dispersed into the matrix of CeO2, thus forming a plum-pudding structure, i.e., multi-Pd core@CeO2 shell configuration. This unique nanostructure endows the Pd@CeO2 nanocomposite with enhanced activity and selectivity toward the visible-light-driven oxidation of various benzylic alcohols to corresponding aldehydes using dioxygen as oxidant at room temperature and ambient pressure compared with a supported Pd/CeO2 nanocomposite and nanosized CeO2 powder. The formation of the multi-Pd core@CeO2 shell structure can be understood by a synergistic interaction of heterogeneous seeded growth process, monolayer-capped core coalescence, and shell re-encapsulation. Together with the previous report, it can be concluded that the intrinsic structure nature of noble metal colloids is able to play a key role in affecting the formation process of noble metal core@semiconductor shell nanocomposites, by which we can realize the design and preparation of different specific core–shell nanostructures with atomic scale accuracy. It is hoped that our current work could open promising prospects of the fabrication of multimetal core@semiconductor shell nanocomposites and their application to visible-light-driven selective organic transformations.

An electroless deposition technique for the synthesis of highly active and nano-sized Pd particles on silica nanosphere

A simple methodology to synthesize uniform and nano-sized Pd particles supported on silica support has been developed by the electroless deposition technique using Sn2+ ion, in which Pd precursor can be interacted directly with the Sn2+ ion, and subsequently transformed into Pd nanoparticles. Characterization by TEM analysis proved that the sizes of the Pd nanoparticles using the electroless deposition technique (eld-Pd/SiO2(Sn)) were quite uniform with smaller size compared with that prepared by the conventional impregnation method (imp-Pd/SiO2). Their catalytic applications for the hydrogenation of nitrobenzene and the direct synthesis of H2O2 from H2 and O2 gases were investigated and it is demonstrated that eld-Pd/SiO2(Sn) prepared by the electroless deposition technique was shown to perform significantly better activity than the imp-Pd/SiO2 owing to its uniform and nano-sized Pd particles.

Microwave-assisted synthesis and pulse electrodeposition of palladium nanocatalysts on carbon nanotube-based electrodes

The deposition of Pd nanoparticles prepared by microwave-assisted synthesis (MS) and pulse electrodeposition (PE) on networks of multiwall carbon nanotubes (CNTs) was investigated. The CNTs were grown directly on microscaled carbon paper using catalytic chemical vapor deposition. Both MS and PE methods enabled the quick formation of nanosized Pd particles over a CNT surface without any additional thermal reduction. Cyclic voltammetry and electrochemical impedance spectroscopy were used to examine the electrochemical behavior of the Pd catalysts. The Pd catalyst prepared with the MS method not only offers a higher active coverage for adsorption/desorption of hydrogen but also a more stable durability toward acid electrolytes when compared with that of the catalyst prepared with the PE method. The electrochemical surface area of the Pd catalyst was approximately 1.38 times than that of the Pt catalyst, which was also prepared with MS method. The equivalent series resistance for all the catalyst electrodes was kept between 2.07 and 2.25Ω after potential cycling. Based on the results, the Pd catalyst is found to be a feasible alternative to the Pt catalyst because of its low cost, durability, and high catalytic activity.

Pd-YSZ composite cathodes for oxygen reduction reaction of intermediate-temperature solid oxide fuel cells

Pd-Y 2O 3 stabilized ZrO 2 (YSZ) composite cathodes are prepared by conventional mechanical mixing and infiltration methods. In the case of infiltration, thermal decomposition and chemical reduction processes are used to form Pd particles on the YSZ scaffold. The phase structure, morphology and electrochemical performance of the Pd-YSZ composite cathodes are characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM) and electrochemical impedance spectroscopy (EIS). The performance of mechanically mixed Pd-YSZ composite cathodes is inadequate due to significant growth and sporadical distribution of Pd particles. The 5 wt.% Pd-loaded cathode prepared by infiltration-thermal decomposition process shows the lowest polarization resistance, i.e. between 0.042 Ω cm 2 and 1.5 Ω cm 2 in the temperature range of 850–600 °C, benefited from the formation of nano-sized Pd particles and the presence of well connected Pd network. The effect of Pd loading on the performance of the infiltrated-thermal decomposed Pd-YSZ composite cathodes is also evaluated, 5 wt.% Pd loading results in the lowest polarization resistances.