Highly catalytic spherical carbon nanocomposites allowing tunable activity via controllable Au-Pd doping

We report the synthesis of highly catalytic spherical carbon composite particles with Au-Pd bimetallic nanoparticle doping using a microwave-assisted technique that allows control over the location of the nanoparticles (NPs), putting them into stable interior, but still near-surface locations (within a 100 nm thick shell). First, composite particles with Pd NPs inside of nanoporous carbon spheres (CSs) were synthesized. Subsequent immersion of the composite particles in HAuCl(4) solutions containing PVP led to an addition of Au near the Pd. Au-Pd/CS composites with Au:Pd atomic ratios varying from 0.4 to 4.6 were prepared. The growth of Au and its location relative to the carbon's surface and the Pd are discussed. The catalytic activity towards the reduction of 4-nitrophenol is tunable via the Au:Pd atomic ratio. Optimizing the composition increases the activity a hundredfold over that of the corresponding monometallic Pd/CS. The catalytic activity arises from the synergy between different contributing mechanisms, here especially the interaction between the carbon matrix and metals, metal-metal interfaces, and the hydrogen absorption capabilities of Pd.     

Catalytic activity and regeneration property of a Pd nanoparticle encapsulated in a hollow porous carbon sphere for aerobic alcohol oxidation

A core-shell composite consisting of a palladium (Pd) nanoparticle and a hollow carbon shell (Pd@hmC) was employed as a catalyst for aerobic oxidation of various alcohols. The core-shell structure was synthesized by consecutive coatings of Pd nanoparticles with siliceous and carbon layers followed by removal of the intermediate siliceous layer. Structural characterizations using TEM and N(2) adsorption-desorption measurements revealed that Pd@hmC thus-obtained was composed of a Pd nanoparticle core of 3-6 nm in diameter and a hollow carbon shell with well-developed mesopore (ca. 2.5 nm in diameter) and micropore (ca. 0.4-0.8 nm in diameter) systems. When compared to some Pd-supported carbons, Pd@hmC showed a high level of catalytic activity for oxidation of benzyl alcohol into benzaldehyde using atmospheric pressure of O(2) as an oxidant. The Pd@hmC composite also exhibited a high level of catalytic activity for aerobic oxidations of other primary benzylic and allylic alcohols into corresponding aldehydes. The presence of a well-developed pore system in the lateral carbon shell enabled efficient diffusion of both substrates and products to reach the central Pd nanoparticles, leading to such high catalytic activities. This core-shell structure also provided high thermal stability of Pd nanoparticles toward coalescence and/or aggregation due to the physical isolation of each Pd nanoparticle from neighboring particles by the carbon shell: this specific property of Pd@hmC resulted in possible regeneration of catalytic activity for these aerobic oxidations by a high-temperature heat treatment of the sample recovered after catalytic reactions.     

Effect of Pd nanoparticle size on the catalytic hydrogenation of allyl alcohol

We report a particle size dependence for the rate of hydrogenation of allyl alcohol using 1.3-1.9 nm Pd dendrimer-encapsulated nanoparticle (DEN) catalysts. For particles with diameters of <1.5 nm and containing <147 Pd atoms, the modulation in catalytic activity is due to the electronic properties of the particle. For the larger particles, 1.5-1.9 nm in diameter and containing an average of 147-250 Pd atoms, the size effect is a result of geometrical constraints. Specifically, the hydrogenation reaction is shown to occur preferentially on the face atoms of the larger nanoparticles.     

Synthesis of ultra-small platinum,palladium and gold nanoparticles by Shewanella loihica PV-4 electrochemically active biofilms and their enhanced catalytic activities

Ultra-small nanoparticles (USNPs) of noble metals have a great potential in a variety of applications due to their high surface areas and high reactivity. This work employed electrochemically active biofilms (EABs) composed of a single bacterium strain of Shewanella loihica PV-4 and successfully synthesized USNPs of noble metal Au, Pd, and Pt. The synthesized USNPs had a size range between 2 and 7?nm and exhibited excellent catalytic performance in dye decomposition. The results of this work shine light on the use of EABs in nanoparticle synthesis.     

Selective hydrogenation by Pd nanoparticles embedded in polyelectrolyte multilayers

Alternating adsorption of poly(acrylic acid) and a polyethylenimine-Pd(II) complex on alumina and subsequent reduction of Pd(II) by NaBH4 yield catalytic Pd nanoparticles embedded in multilayer polyelectrolyte films. The polyelectrolytes limit aggregation of the particles and impart catalytic selectivity in the hydrogenation of alpha-substituted unsaturated alcohols by restricting access to catalytic sites. Hydrogenation of allyl alcohol by encapsulated Pd(0) nanoparticles can occur as much as 24-fold faster than hydrogenation of 3-methyl-1-penten-3-ol. Additionally, the nanoparticle/polyelectrolyte system suppresses unwanted substrate isomerization, when compared to a commercial palladium catalyst. Selective diffusion through poly(acrylic acid)/polyethlyenimine membranes suggests that hydrogenation selectivities are due to different rates of diffusion to nanoparticle catalysts. First-order kinetics are also consistent with a diffusion-limited mechanism. Further exploitation of the versatility of polyelectrolyte films should increase selectivity in hydrogenation as well as other reactions.     

Size controlled synthesis of Pd nanoparticles in water and their catalytic application in C-C coupling reactions

Catalytically active Pd nanoparticles have been synthesized in water by a novel reduction of Pd(II) with a Fischer carbene complex where polyethylene glycol (PEG) was used as stabilizer. PEG molecules wrap around the nanoparticles to impart stability and prevent agglomeration, yet leave enough surface area available on the nanoparticle for catalytic activity. Our method is superior to others in terms of rapid generation and stabilization of Pd nanoparticles in water with a cheap, readily available PEG stabilizer. The size of the nanoparticles generated can be controlled by the concentration of PEG in water medium. The size decreased with the increase in the PEG: Pd ratio. This aqueous nano-sized Pd is a highly efficient catalyst for Suzuki, Heck, Sonogashira, and Stille reaction. Water is used as the only solvent for the coupling reactions.     

Argon glow discharge plasma‐reduced palladium nanoparticles supported on activated carbon for Suzuki and Heck coupling reactions

Palladium nanoparticles supported on activated carbon were prepared by argon glow discharge plasma reduction (Pd/C‐P) without any chemical reducing agents and protective agents. The as‐prepared Pd/C‐P catalyst was characterized using nitrogen adsorption–desorption, X‐ray diffraction and transmission electron microscopy analyses. The results showed that the palladium nanoparticles reduced by plasma are well dispersed with a smaller particle size than commercial Pd/C. Pd/C‐P exhibited a high catalytic activity in Suzuki and Heck coupling reactions. Moreover, there was no obvious loss of catalytic activity even after eight repeated cycles, showing good reactivity and recyclability.     

Dendron‐Functionalized Core–Shell Superparamagnetic Nanoparticles: Magnetically Recoverable and Reusable Catalysts for Suzuki C?C Cross‐Coupling Reactions

A metallodendron functionalized with dicyclohexyldiphosphino palladium complex was synthesized. The metallodendron was grafted onto core–shell superparamagnetic nanoparticles (γ‐Fe₂O₃/polymer, 200–500 nm) to give optimal catalytic reactivity in cross‐coupling reactions. The grafted nanoparticles were used as recoverable and reusable catalysts for Suzuki C? C cross‐coupling reactions. They showed remarkable reactivity towards iodo‐ and bromoarenes under mild conditions, and unprecedented reactivity towards chloroarenes. On completion of the catalytic reaction, the catalysts were readily recovered by using a simple magnet to attract the superparamagnetic grafted nanoparticles. Catalysts were recovered more than 25 times with almost no discernable loss of reactivity.     

Correlating Oxygen Reduction Reaction Activity and Structural Rearrangements in MgO-Supported Platinum Nanoparticles

We develop a multi-scale approach towards the design of metallic nanoparticles with applications as catalysts in electrochemical reactions. The here discussed method exploits the relationship between nanoparticle architecture and electrochemical activity and is applied to study the catalytic properties of MgO(100)-supported Pt nanosystems undergoing solid-solid and solid-liquid transitions. We observe that a major increment in the activity is associated to the reconstruction of the interface layers, supporting the need for a full geometrical characterisation of such structures also when in-operando.     

Facile approach to the electrochemical synthesis of palladium-reduced graphene oxide and its application for Suzuki coupling reaction

Electrochemically codeposited palladium nanoparticles (Pd NPs) and reduced graphene oxide (ERGO-Pd) were used as catalyst for Suzuki cross coupling reactions. The catalyst was characterized by various analytical techniques. The mean particle size of Pd was found to be 5.7 ± 1.8 nm. The ERGO-Pd catalyst demonstrated excellent catalytic activity and recyclability for Suzuki cross coupling reactions. The remarkable reactivity of the ERGO-Pd catalyst toward cross-coupling reactions is attributed to the high degree of the dispersion of Pd NPs on reduced graphene oxide with narrow size distribution from 3 to 9 nm.     

Synthesis of PVP stabilized Cu/Pd nanoparticles with citrate complexing agent and its application as an activator for electroless copper deposition

A simple method has been developed to synthesize Cu/Pd nanoparticles in aqueous solution in ambient condition with the addition of complexing agent, trisodium citrate. UV-vis spectra confirmed the complexing behavior of trisodium citrate and metal ions. The particles synthesized with trisodium citrate were well dispersed with particle size ranging between 3-4 nm while the particles without trisodium citrate were larger and aggregated, as demonstrated by transmission electron microscopy (TEM). X-ray diffraction patterns (XRD) indicated the formation of bimetallic nanoparticles without impurities in the complexing agent-supplemented system. In contrast, large amounts of PdO and Cu(OH)(2) were precipitated along with the formation of particles in the complexing agent-free system. X-ray photoelectron spectroscopy (XPS) revealed small amounts of oxidized Pd on the surface of particles and the existence of zerovalent Cu and oxidized Cu in particles with trisodium citrate. With a simpler process for electroless copper deposition, the Cu/Pd nanoparticle activator with less Pd metal used exhibited comparable catalytic activity to conventional Pd/Sn colloidal activator. In summary, application of Cu/Pd nanoparticles synthesized with the complexing agent as an activator suggested a novel, simpler and inexpensive process in PCB industry.     

Catalytic properties of supported gold nanoparticles in organic syntheses

Gold nanoparticles exhibit unique properties due to their ability to form aggregates of atoms of diverse morphology shapes and sizes of which depend, to a considerable extent, on specific features of the nearest environment. The nature of gold nanoparticles varies in a wide range: from the particles with pronounced Lewis acidic properties to the negatively charged particles bearing a formal zero-valence charge. The most examples of new reactions catalyzed by gold nanoparticles include unsaturated compounds and strong nucleophiles (such as amines) as substrates. This short review provides a digest of the catalytic properties of gold nanoparticles. The main attention is paid to the possible role of certain forms of the metal in catalytic reactions. Of special interest are reactions in which effects of synergism of gold and other active species or second metals present in the catalyst are revealed or a size effect is established.     

Novel method for the investigation of single nanoparticle reactivity

The reactivity of planar surfaces has been well investigated but there are many new unexplored aspects involved in the reactivity of nanoparticle surfaces. Most investigative methods only measure the average properties of the particles present on the surface. In order to investigate the local reactivity of nanoparticles, the STM tip electrode can be used as a local sensor. The feasibility of this approach is demonstrated by detecting some of the hydrogen (through the hydrogen oxidation reaction current at the tip) which is evolved at a single Pd particle on the Au(111) substrate at a constant distance from the tip. In principle, it is thereby possible to determine the reactivities of surfaces on a nanometre scale. To avoid interference from different reactive particles it is essential to use electrodes with a very low particle density. Hence, in this investigation electrodes with single Pd particles which were deposited from the STM tip onto the substrate are used. Results of potential-dependent measurements at single metal nanoparticles will be shown and the possibilities and limitations of the approach will also be discussed.     

碳纳米粒子支撑的钯纳米催化剂在甲酸氧化中的电催化活性

采用化学还原法制备了碳纳米粒子支撑的钯纳米结构(Pd-CNP). 透射电镜表征显示在Pd-CNP纳米复合物中,金属Pd呈菜花状结构,粒径约20~30 nm。它们由许多更小的Pd纳米粒子（3~8 nm）组成. 电化学研究表明,虽然Pd-CNP的电化学活性面积比商业Pd黑低40%（可能原因是部分Pd表面被一层碳纳米粒子覆盖）,但其对甲酸氧化却表现出更好的电催化活性：质量比活性和面积比活性都比Pd黑高几倍. 催化活性增强的原因可能是碳纳米粒子支撑的Pd纳米结构具有特殊的层次化结构,可以形成更多的活性位,以及表面位更利于反应进行.     

Selective Hydrogenation of Acrolein Over Pd Model Catalysts: Temperature and Particle‐Size Effects

The selectivity in the hydrogenation of acrolein over Fe₃O₄‐supported Pd nanoparticles has been investigated as a function of nanoparticle size in the 220–270 K temperature range. While Pd(111) shows nearly 100 % selectivity towards the desired hydrogenation of the C=O bond to produce propenol, Pd nanoparticles were found to be much less selective towards this product. In situ detection of surface species by using IR‐reflection absorption spectroscopy shows that the selectivity towards propenol critically depends on the formation of an oxopropyl spectator species. While an overlayer of oxopropyl species is effectively formed on Pd(111) turning the surface highly selective for propenol formation, this process is strongly hindered on Pd nanoparticles by acrolein decomposition resulting in CO formation. We show that the extent of acrolein decomposition can be tuned by varying the particle size and the reaction temperature. As a result, significant production of propenol is observed over 12 nm Pd nanoparticles at 250 K, while smaller (4 and 7 nm) nanoparticles did not produce propenol at any of the temperatures investigated. The possible origin of particle‐size dependence of propenol formation is discussed. This work demonstrates that the selectivity in the hydrogenation of acrolein is controlled by the relative rates of acrolein partial hydrogenation to oxopropyl surface species and of acrolein decomposition, which has significant implications for rational catalyst design.     

Synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles

In this article we describe the synthesis, characterization, and applications of dendrimer-encapsulated nanoparticles (DENs). These materials are synthesized by a template approach in which metal ions are extracted into the interior of dendrimers and then subsequently chemically reduced to yield nearly size-monodisperse particles having dimensions of less than 3 nm. Monometallic, bimetallic (including core/shell), and semiconductor nanoparticles have been prepared by this route. The dendrimer component of these composites serves not only as a template for preparing the nanoparticle replica but also to stabilize the nanoparticle, makes it possible to tune solubility, and provides a means for immobilization of the nanoparticle on solid supports. These materials have a number of potential applications, but the focus here is on catalysis. Homogeneous catalytic reactions, including hydrogenations, Heck coupling, and Suzuki reactions, in water, organic solvents, biphasic fluorous/organic solvents, and liquid and supercritical CO2 are discussed. In many cases it is easy to recycle catalytic DENs. DENs can also be immobilized on supports, such as silica and titania, and used for heterogeneous catalysis. Bimetallic DENs are shown to have particularly interesting catalytic properties. In addition to a discussion of current progress in this field, a number of intriguing questions related to the properties and potential applications of these materials are examined.     

Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion

Mixtures of nanosized platinum and palladium particles have been prepared by reduction of salt-containing microemulsion droplets using hydrazine as the reducing agent. To avoid possible negative effects of the presence of sulfur compounds during the preparation the microemulsion was made using the sulfur-free nonionic polyoxyethylene 4 lauryl ether surfactant. Transmission electron microscopy showed that the as-prepared mixtures contained crystalline platinum particles of fairly homogeneous size (20 to 40 nm) with adsorbed amorphous palladium particles 2 to 5 nm in size. Catalyst samples were prepared by depositing the nanoparticles on a gamma-Al(2)O(3) support followed by heating in air at 600 degrees C. Alloyed particles of platinum and palladium with sizes ranging from 5 to 80 nm were obtained during the heating. The majority of the particles had the fcc structure and their compositional range was dependent upon the Pt:Pd molar ratio of the microemulsion. A catalyst prepared from a microemulsion with a 20:80 Pt:Pd molar ratio showed the highest catalytic activity for CO oxidation, while pure platinum and palladium catalysts showed higher sulfur resistance. These results differ from the performance of conventional wet-impregnated catalysts, where a 50:50 Pt:Pd molar ratio resulted in the highest catalytic activity as well as the highest sulfur resistance.     

Effect of elemental composition of PtPd bimetallic nanoparticles containing an average of 180 atoms on the kinetics of the electrochemical oxygen reduction reaction

PtPd bimetallic nanoparticles containing an average of 180 atoms and composed of seven different Pt:Pd ratios have been prepared within sixth-generation, hydroxyl-terminated, poly(amidoamine) dendrimers. Transmission electron microscopy indicates that the sizes of all seven nanoparticle compositions are within +/-0.2 nm of one another and the calculated size. Single-particle energy-dispersive spectroscopy shows that the elemental composition is determined by the ratio of the Pt and Pd precursor salts used to prepare the nanoparticles. Cyclic voltammetry and rotating disk voltammetry measurements show that the Pt:Pd ratio of the nanoparticles determines their efficiency for the oxygen reduction reaction (ORR). The maximum activity for the ORR occurs at a Pt:Pd ratio of 5:1, which corresponds to a relative mass activity enhancement of 2.4 compared to otherwise identical monometallic Pt nanoparticles.     

金属核及氧化铈的物理化学性质对一氧化碳氧化的影响

氧化铈独特的氧化还原性能使其适合用作氧化反应中的催化剂或载体.氧化铈负载的过渡金属纳米粒子或孤立的单原子提供了金属-载体界面,从而降低了去除界面氧原子的能耗,提供了可以参与ManVanKulvian氧化过程的活性氧物种.CO氧化是测试氧化铈负载催化剂还原性的主要探针反应,并且它常见于在相对低温下消除CO的各种应用中.在过量H2中优先氧化CO(PROX)反应可控制CO浓度达到超低水平,以防止氢氧化电催化剂中毒.催化剂在CO氧化反应中的活性和在PROX反应中对CO和H2的选择性取决于金属物种的种类和分散性、CeO2的结构和化学性质以及催化剂的合成方法.在这篇综述中,我们总结了最近发表的关于CeO2负载的金属纳米粒子和单原子催化CO氧化和PROX反应的相关工作;以及不同的负载金属和同种金属在普通CeO2表面上的反应性.我们还总结了密度泛函理论计算中提出的最可能的反应机理;并且讨论了各种负载型金属在PROX反应中影响CO氧化选择性的因素.     

Highly Selective Pd–Cu/α-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Catalysts for Liquid-Phase Hydrogenation: The Influence of the Pd: Cu Ratio on the Structure and Catalytic Characteristics

The structure and catalytic characteristics of a series of Pd–Cu/α-Al₂O₃ catalysts with Pd: Cu ratio varied from Pd₁–Cu_0.5 to Pd₁–Cu₄ were studied. The use of α-Al₂O₃ with a small surface area (S_sp = 8 m²/g) as a support made it possible to minimize the effect of diffusion on the catalytic characteristics and to study the structure of Pd–Cu nanoparticles by X-ray diffraction (XRD) analysis. The XRD analysis and transmission electron microscopy (TEM) data indicated the formation of uniform bimetallic Pd–Cu nanoparticles (d = 20–60 nm), whose composition corresponded to a ratio between the metals in the catalyst, and also the absence of monometallic Pd⁰ and Cu⁰ nanoparticles. The study of catalytic properties in the liquid-phase hydrogenation of diphenylacetylene (DPA) showed that the activity of the catalysts rapidly decreased with the Cu content increase; however, in this case, the yield of a desired alkene compound significantly increased. The selectivity of alkene formation on the catalysts with the ratios Pd: Cu = 1: 3 and 1: 4 was superior to the commercial Lindlar catalyst.