氧气在不同粒径Pt/C催化剂上的电催化还原

在甲醇溶剂中,利用SnCl2作为还原剂,通过控制反应条件制备了带有不同粒径Pt粒子Pt/C催化剂。X-射线衍射和透射电镜的研究表明获得的Pt/C催化剂中Pt粒子具有高度的均一性和良好的分散度。电化学研究显示,对于氧气的电催化还原,Pt/C催化剂存在着明显的粒径效应。当Pt粒子粒径为3.2 nm时,Pt/C催化剂对氧气的电催化还原的质量比活性最佳。Pt/C催化剂对氧气的粒径效应可能与其表面含氧基团含量、Pt粒子的比表面积及其晶面结构相关。     

氧气在不同粒径Pt/C催化剂上的电催化还原

在甲醇溶剂中,利用SnCl₂作为还原剂,通过控制反应条件制备了带有不同粒径Pt粒子Pt/C催化剂。X-射线衍射和透射电镜的研究表明获得的Pt/C催化剂中Pt粒子具有高度的均一性和良好的分散度。电化学研究显示,对于氧气的电催化还原,Pt/C催化剂存在着明显的粒径效应。当Pt粒子粒径为3.2nm时,Pt/C催化剂对氧气的电催化还原的质量比活性最佳。Pt/C催化剂对氧气的粒径效应可能与其表面含氧基团含量、Pt粒子的比表面积及其晶面结构相关。     

Electrochemical processes on multi-component cathodic catalysts PtM and PtM<Subscript>1</Subscript>M<Subscript>2</Subscript> (M = Co,Ni, Cr): the effect of surface composition on the catalyst stability and its activity in O<Subscript>2</Subscript> reduction

The multi-component nanocatalysts based on platinum-transient metals alloys applied onto dispersed carbon material are considered as the most promising catalysts, which can be substituted for platinum in the fuel cell cathodes. The electrocatalytic activity of platinum in the PtM₁/C and PtM₁M₂/C alloys increases by several times with simultaneous increase in the stability. From the results obtained by structural and electrochemical methods, it is found that the synthesized binary and ternary catalysts are the metal alloys, whose surface is enriched in platinum as a result of surface segregation and subsequent chemical or electrochemical treatment. Under the corrosive attack, the less-noble metal, which has not entered into the alloy, dissolves, and the core-shell structures form. The properties of platinum in the shell differ from its properties in Pt/C due to the ligand effect of the core (metal alloy). As a result, the surface coverage with oxygen chemisorbed from water decreases in the binary and ternary systems. This causes an increase of the catalytic activity in the O₂ reduction reaction due to a decline in the effect of surface blocking against molecular oxygen adsorption and a decrease in the platinum dissolution rate, because the oxidation of platinum by water is the onset of corrosion process. For the catalytic systems studied, the mass activity decreases in the following order: 20% Pt in PtCoCr/C > 7.3% Pt in PtCo/C ≥ 7.3% Pt in PtCr/C and PtNi/C ≥ 40% Pt/C. The application of PtCoCr/C catalyst as the cathode in a low-temperature hydrogen-air fuel cell enabled one to reduce the platinum consumption by one half on retention of its performance.     

Methanol oxidation on unsupported and carbon supported Pt + Ru anodes

A novel Pt + Ru electrode material is shown to be highly active for the direct electro-oxidation of methanol in H₂SO₄ solutions and to show very little tendency to poison. X-ray photoelectron spectroscopy of this material before use as an anode showed that the ruthenium is oxidised and that there is an important surface concentration of oxidised platinum. After prolonged use as a methanol-oxidation anode, the concentration of oxidised platinum is somewhat increased and there is no evidence for any Pt-CO or Pt₂ = CO species; rather adsorbed formate is present. These data are consistent with Ru acting as a promoter of active surface oxygen. Dispersion of the Pt and Ru on a pure carbon support gives a much greater performance per gram of precious metal; however, the initial increase in overpotential is greater by over 100 mV. The differences in the catalytic behaviour of these two materials is discussed, and the importance of competing reactions is considered.     

The Direct Electron Transfer of Iron‐containing Metal Organic Frameworks (Mil‐100) and Its Catalysis for the Oxygen Reduction Reaction in Room‐Temperature Alkaline Media on a Glass Carbon Electrode

The most common electrocatalysts for the oxygen reduction reaction (ORR) are platinum‐based ones. This work demonstrates the performance of iron‐containing metal organic frameworks (MOFs) as non‐platinum‐based nano‐electrocatalysts for ORR in an alkaline medium. As a new non‐platinum catalyst to achieve the active sites for the ORR, Mil‐100 (Fe) nanoparticles were used in aqueous KOH by the rotating‐disk electrode method. The main objectives of this study are the investigations on the electron transfer number (n ), Tafel slope, and catalytic performance. The particles size of the obtained powders is in the nanoscale range (approximately 25 nm). The electron transfer number for the ORR on the surface of iron‐containing catalyst is approximately 4, and the Tafel slope of diffusion‐corrected kinetic current density is ~50.7 mV per decade at low overpotential. This work might extend a new non‐precious‐metal catalyst structure for ORR for use in low‐temperature fuel cells.     

In-situ infrared spectroscopy of buried organic monolayers: influence of the substrate on titanium reactivity with a Langmuir-Blodgett film

The reactivity of metals vapor deposited onto organic monolayers has historically been correlated to the metal/terminal organic group chemistry. Here we demonstrate that the chemical composition of the substrate unexpectedly plays a significant role as well. In particular, the reactivity of evaporated titanium toward a cadmium stearate Langmuir-Blodgett (LB) film was found to depend on the substrate upon which the LB film was deposited. Infrared spectra taken in a modified ATR (Kretschmann) geometry with a thin Au substrate showed large changes in peak shape, peak position, and peak width in the C-H stretching region, indicating titanium penetration into the LB film and decomposition of the original well-packed monolayer structure. LB monolayers formed on a platinum oxide (PtO(x)) surface showed remarkably small changes after Ti deposition, indicating only a slight increase in disorder and no significant metal penetration into the film. Films on SiO2 substrates showed reactivity between that of Au and PtO(x). These differences in reactivity can be correlated primarily with the amount of available oxygen associated with each substrate, including surface oxide layers and water incorporated within the LB film.     

Ta2O5 templated growth of droplet-like platinum particles by potentiodynamic polarization of tantalum in aqueous solution of hexachloroplatinic acid

By potentiodynamic polarization of mechanically polished tantalum in a diluted aqueous solution of hexachloroplatinic acid, droplet-like platinum microparticles were electrodeposited, embedded into the simultaneously formed Ta₂O₅ film. The roughness factor of platinum of 31 was achieved. Within the potential region of both hydrogen and oxygen underpotential deposition, in both acidic and alkaline solutions, the composite Pt/Ta₂O₅ electrode displayed an excellent electrochemical response characteristic of smooth polycrystalline platinum. The preparation method applied in this work presents an easy way to obtain an electrode surface combining the behaviour of smooth polycrystalline platinum with the behaviour of microdisc arrays. Its electrocatalytic effectiveness was demonstrated for an oxygen reduction reaction in alkaline solutions.     

Effect of surface oxygen groups of the supports on platinum dispersion in Pt/C catalysts

Summary A series of platinum catalysts supported on carbon black were prepared by the impregnation-reduction method. The original support was treated to ensure carbon blacks with the same porous texture but with a different amounts of surface oxygen groups. Platinum dispersion was calculated from TEM and XRD measurements. It was confirmed that the increase in the amount of surface oxygen groups led to a low metal dispersion.     

Tolerant-to-methanol cathodic electrocatalysts based on organometallic clusters

A new approach to the fabrication of catalytic systems based on hetero-and homometal-chalcogenide clusters of Pt-M-X type (M: Fe, Mn; X: S, Se, Te), which provides the reproducibility of catalyst composition and uniform distribution of catalyst over the carbon support, is proposed. Thus obtained catalysts are characterized using the XRD, TEM, and EDAX methods. The electrocatalytic activity of these systems in the oxygen reduction reaction, the role of the nature of chalcogenide atom and the atom of the second metal, which is the platinum partner, and the electrochemical behavior of nonplatinum chalcogenide systems are studied.     

Cyclic voltammetry of solid diphenylamine crystals immobilized on an electrode surface and in the presence of an aqueous solution

Diphenylamine solid crystals have been mechanically attached to platinum or gold electrodes by four different methods and investigated by cyclic voltammetry in the presence of aqueous acidic media. It is demonstrated that the oxidative dimerization and polymerization (observed in non-aqueous solutions or mixed solvent systems) can also be accomplished by using diphenylamine microcrystals in contact with the electrode metal in the presence of aqueous solutions. Formation of a thin polymer layer has been observed even in the areas of the electrode that were not in direct contact with the diphenylamine microparticles. This finding can be explained by surface diffusion of the organic molecules or by oversaturation of the solution close to the diphenylamine microcrystals. A gradual growth of the voltammetric peaks characteristic of the polymer has been observed, while the irreversible wave due to the formation of cation radicals and their dimerization has shown a stationary behaviour. The redox transformation of the oligomeric compound formed at the surface is accompanied by a clear colour change: at pH 0 the colourless reduced form is converted to the bright-blue (violet) oxidized form. Electronic Publication     

Virus crystals as nanocomposite scaffolds

The generation of long-range three-dimensional nanoscopic patterns is a major goal in materials chemistry. Here we report a strategy for creating such systems using virus crystals as scaffolds which can be infiltrated with metal specifically palladium and platinum. The inorganic component effectively packs within the porous macromolecular crystal architecture, providing a route for patterning these materials on the nanometer length scale. To verify the quality of the metal infiltration, SEM-EDX was used to determine the homogeneous distribution of metal across the crystal, and TEM was used to confirm that the metal was confined within the porous structure of the crystal.     

Oxygen electroreduction at catalysts PtM (M = Co,Ni, or Cr)

Bimetallic catalysts PtM (M = Co, Ni, or Cr) are synthesized. They exceed purely platinum commercial catalyst E-TEK (20 wt % Pt) in its mass activity (mA/mg_Pt) and specific activity (mA/c_Pt²) in the oxygen reduction reaction. According to XRD data, the high-temperature synthesis involving metal N₄-complexes, chloroplatinic acid, and XC72 carbon black as precursors, yields alloys (or solid solutions) of the metals. The higher activity of the bimetallic catalyst PtCo/C is likely to be caused by the practically entire formation of solid solutions (Pt₃Co and PtCo), unlike PtNi and PtCr where nickel and chromium exist also as oxides that decorate the electrode surface and partly block active centers. It is shown that the mechanism of the oxygen reduction reaction at the synthesized catalysts is similar to that of oxygen reduction at the purely platinum catalyst. The slow stage in the process is transfer of the 1st electron; at potentials more positive than 0.6 V the reaction mainly yields water. The higher electrocatalytic activity of the bimetallic systems is caused by the alloy formation, which leads to changes in the bond length between platinum atoms. The achieving of the optimal bond length, as a result of the alloy formation, provides appropriate conditions for dissociative adsorption of oxygen molecules; the surface coverage with oxygen-containing particles adsorbed from water (which block active centers for O₂ adsorption) decreased. The increase in the activity may also be caused by the formation of the “core-shell” structures whose surface is enriched with platinum whose surface properties are changed under the ligand action of the core formed by the metal alloy     

Catalysts with platinum–palladium nanoparticles on polymer matrix supports

The platinum–palladium/Nafion metal–polymer nanocomposites were synthesized by the chemical reduction of ions in the aqueous organic solutions of inverted microemulsions. The functional characteristics of the nanocomposites were studied by cyclic voltammetry, atomic force microscopy, and scanning electron microscopy. The nanocatalysts obtained exhibited high activity in the reactions of oxygen reduction and hydrogen oxidation. The influence of synthesis conditions on the catalytic activity of the metal–polymer nanocomposites was studied.     

Protecting structure analyses of organic molecule-protected nanoscopic noble metal clusters

The stabilizing structure of cationic surfactant-protected platinum clusters in water and tertiary amine-protected rhodium clusters in chloroform, prepared by photo- and hydrogen-reduction, respectively, was investigated. These nanoscopic noble metal clusters present a narrow size distribution and are stable. The structural information of protective organic molecules on the surface of metal clusters was studied by transmission electron microscopy and hydrodynamic radius measurements according to the Taylor dispersion method. The size of the entire cluster with the protective layer surrounding the metal surface, obtained as Stokes' radii by the Taylor dispersion method, is considered to be fairly consistent with the sum of the naked particle size, obtained by transmission electron micrographs, and the size of the adsorbed protective layer, supporting the conformational information.     

On the mechanism of electrochemical dispersion of platinum under the action of alternating current

The behavior of platinum electrodes in alkali solutions under the action of pulse alternating current was studied. A mechanism of electrochemical dispersion of platinum was proposed. An important step of the mechanism is the discharge and cathodic insertion of alkali metal cations into the crystalline lattice of platinum followed by the decomposition of intermetallic compound Pt _m Na upon the interaction with water. Dispersion is facilitated by the presence of an oxide film on the platinum surface formed in the anodic period of pulses, a periodical alternation of electrode polarity resulting in the electrochemical injection of vacancies from the metal bulk to the surface, and hydrogen and oxygen formation in the corresponding periods.     

Applications of self-assembled monolayers in materials chemistry

Self-assembly provides a simple route to organise suitable organic molecules on noble metal and selected nanocluster surfaces by using monolayers of long chain organic molecules with various functionalities like -SH,-COOH,-NH₂, silanes etc. These surfaces can be effectively used to build-up interesting nano level architectures. Flexibility with respect to the terminal functionalities of the organic molecules allows the control of the hydrophobicity or hydrophilicity of metal surface, while the selection of length scale can be used to tune the distant-dependent electron transfer behaviour. Organo-inorganic materials tailored in this fashion are extremely important in nanotechnology to construct nanoelctronic devices, sensor arrays, supercapacitors, catalysts, rechargeable power sources etc. by virtue of their size and shape-dependent electrical, optical or magnetic properties. The interesting applications of monolayers and monolayer-protected clusters in materials chemistry are discussed using recent examples of size and shape control of the properties of several metallic and semiconducting nanoparticles. The potential benefits of using these nanostructured systems for molecular electronic components are illustrated using Au and Ag nanoclusters with suitable bifunctional SAMs.     

透光导电金属及氧化铂半导体薄膜的制备与表征

通过PPh~3对“非保护型”铂金属纳米簇进行表面修饰,并将其萃取至甲苯中,制备的PPh~3修饰的Pt金属钠米簇于空气中可自发地在玻璃表面生长出均匀透光的金属钠米簇薄膜。该金属钠米簇薄膜经空气中加热处理后可转化为透光导电的氧化铂半导体薄膜。考察了金属钠米簇薄膜生长过程中UV-vis吸收光谱的变化。采用SEM和TEM等方法,表征了纳米簇的粒径及膜的多孔结构,由此解释了其透光原因。研究了薄膜的导电性与处理条件的关系,并采用XPS表征了处理过程中的物质变化。初步探索了PPh~3-Pt纳米簇自发成膜过程的机理,确定了氧气在此过程中的重要作用。     

直接有机小分子燃料电池氢钼/钨青铜修饰铂电催化剂

减少铂用量和提高铂的抗中毒能力,是直接有机小分子燃料电池实用化过程期待解决的难题,由此而引发了广泛的研究,迄今已先后建立了许多的解决方案.作者也提出了基于氢钼/钨青铜(HxMo(W)O3,0相似文献   

Electrophilic activation of alkenes by platinum(II): so much more than a slow version of palladium(II)

The electrophilic activation of alkenes by transition-metal catalysts is a fundamental step in a rapidly growing number of catalytic processes. Although palladium is the best known metal for this purpose, the special properties of its third-row cousin platinum (strong metal-ligand bonds and slow substitution kinetics) have enabled the development of transformations that are initiated by addition to the C=C bonds by protic carbon, nitrogen, oxygen, and phosphorus nucleophiles, as well as alkene or arene nucleophiles. Additionally, reactivity profiles, which are often unique to platinum, provide wholly new reaction products. This Review concerns platinum-catalyzed electrophilic alkene activation reactions, with a special emphasis on the mechanistic properties of known systems, on the differences between platinum and palladium catalysts, and on the prospects for the development of new systems.     

The mechanism of the platinum indicator electrode in argentometry

A study has been made of the behaviour of platinum and some other inert electrodes in silver nitrate titrations. Where the metal surface has been subjected to a reducing treatment, such as cathodic polarization, before use, the electrode will often function as a silver indicator electrode throughout the titration. It has been shown experimentally that this is due to the formation of a layer of metallic silver upon the electrode by interaction of the surface with silver ions in solution. If the metal surface undergoes such pretreatment that it is oxidized, then its potential normally remains at the oxide value during a silver nitrate titration until the silver ion concentration is sufficiently great for this value to be exceeded by the silver silver ion potential; formation of metallic silver then again takes place and from this point onwards the electrode behaves as a silver electrode. A detailed interpretation of the behaviour of platinum after various pretreatments has been made along these lines.