Platinum, particles

Co-decomposition of Ru(77 -C8H]o)( -C8Hi2) and Pt(dba)2 leads to the formation of bimetallic Ru .-Pti j, particles. Platinum-rich particles are fee, whereas ruthenium-rich ones are hep. There is a critical composition Ru3-Pt, for which most of the particles are twinned. In this case, the particles are monodisperse and very small (1.1 nm). This composition corresponds roughly to the limit of solubility of ruthenium in the platinum lattice for bulk alloys. The particles adopt a twinned fee structure with the twinning wall lying in a (111) plane located in the middle of the particle. The homogeneity in the size and shape of the twinned particles suggests a well-defined atomic organization, namely, a twinned truncated octahedron for the particles which can also be described as well-defined clusters. 

Due to their small size, nanoparticles have a very high specific surface area and dispersion, the latter being defined as the ratio of the number of surface atoms to the total number of atoms in the particle. Platinum particles with 2 nm diameter, e.g., have a surface area of 140 mVg and a dispersion of 50 %. Note that also the relative concentration of atoms located at corners, edges, and faces is strongly size dependent (Fig. These surface atoms are not equivalent to each other and frequently play different roles in catalysis. In the following, we focus on metal nanoparticles,[ -ioi as one can find good reviews about metal oxide [n-i6] and semiconductor [17-23] nanopartides elsewhere. 

The catalyst layer is either applied to the gas diffusion media (Mode 1) or to the membrane (Mode 2). In either case, the objective is to place the catalyst particles, platinum or platinum alloys, in close proximity of the membrane. 

In these equations the electrostatic potential i might be thought to be the potential at the actual electrodes, the platinum on the left and the silver on the right. However, electrons are not the hypothetical test particles of physics, and the electrostatic potential difference at a junction between two metals is nnmeasurable. Wliat is measurable is the difference in the electrochemical potential p of the electron, which at equilibrium must be the same in any two wires that are in electrical contact. One assumes that the electrochemical potential can be written as the combination of two tenns, a chemical potential minus the electrical potential (- / because of the negative charge on the electron). Wlien two copper wires are connected to the two electrodes, the... 

Each newly cleaved mica surface is very clean. Flowever, it is known that mica has a strong tendency to spontaneously adsorb particles [45] or organic contaminants [46], which may affect subsequent measurements. The mica sheets are cut into 10 nun x 10 nun sized samples using a hot platinum wire, then laid down onto a thick and clean 100 nun x 100 nun mica backing sheet for protection. On the backing sheet, the mica samples can be transferred into a vacuum chamber for themial evaporation of typically 50-55 mn thick silver mirrors. 

The catalysts with the simplest compositions are pure metals, and the metals that have the simplest and most uniform surface stmctures are single crystals. Researchers have done many experiments with metal single crystals in ultrahigh vacuum chambers so that unimpeded beams of particles and radiation can be used to probe them. These surface science experiments have led to fundamental understanding of the stmctures of simple adsorbed species, such as CO, H, and small hydrocarbons, and the mechanisms of their reactions (42) they indicate that catalytic activity is often sensitive to small changes in surface stmcture. For example, paraffin hydrogenolysis reactions take place rapidly on steps and kinks of platinum surfaces but only very slowly on flat planes however, hydrogenation of olefins takes place at approximately the same rate on each kind of surface site. 

Precious Meta.1 Ca.ta.lysts, Precious metals are deposited throughout the TWC-activated coating layer. Rhodium plays an important role ia the reduction of NO, and is combiaed with platinum and/or palladium for the oxidation of HC and CO. Only a small amount of these expensive materials is used (31) (see Platinum-GROUP metals). The metals are dispersed on the high surface area particles as precious metal solutions, and then reduced to small metal crystals by various techniques. Catalytic reactions occur on the precious metal surfaces. Whereas metal within the crystal caimot directly participate ia the catalytic process, it can play a role when surface metal oxides are influenced through strong metal to support reactions (SMSI) (32,33). Some exhaust gas reactions, for instance the oxidation of alkanes, require larger Pt crystals than other reactions, such as the oxidation of CO (34). 

F1 NMR of chemisorbed hydrogen can also be used for the study of alloys. For example, in mixed Pt-Pd nanoparticles in NaY zeolite comparaison of the results of hydrogen chemisorption and F1 NMR with the formation energy of the alloy indicates that the alloy with platinum concentration of 40% has the most stable metal-metal bonds. The highest stability of the particles and a lowest reactivity of the metal surface are due to a strong alloying effect. 

The dehydrogenation reaction is an extremely rapid endothermic reaction which converts alkylcyclohexanes to aromatics almost quantitatively. It is promoted by the catalyst platinum function and is so rapid that it is normally limited by diffusion into the catalyst particle. 

The deliberate raising of the electrical potential of titanium, either by the attachment of discrete particles of a noble metal, such as platinum or palladium, at the surface, or by the application of positive direct current to force the formation of a protective film, is dealt with at a later point. The electrochemical aspect of the corrosion of titanium is comprehensively treated in a number of papers ... 

Platinum, palladium and the normal alloys of platinum used in industry are easily workable by the normal techniques of spinning, drawing, rolling, etc. To present a chemically clean surface of platinum and its alloys after fabrication, they may be pickled in hot concentrated hydrochloric acid to remove traces of iron and other contaminants —this is important for certain catalytic and high-temperature applications. In rolling or drawing thin sections of platinum, care must be taken to ensure that no dirt or other particles are worked into the metal, as these may later be chemically or elec-trolytically removed, leaving defects in the platinum. 

In some cases, the catalyst is a solid substance on whose surface a reactant molecule can be held (adsorbed) in a position favorable for reaction until a molecule of another reactant reaches the same point on the solid. Metals such as iron, nickel, platinum and palladium seem to act in this way in reactions involving gases. There is evidence that in some cases of surface adsorption, bonds of reactant particles are weakened or actually broken, thus aiding reaction with another reactant particle. 

Platinum and zinc have the same number of atoms per cubic centimeter. Would thin sheets of these elements differ in the way they scatter alpha particles Explain. 

FIGURE 1.6 Rutherford s model of the atom explains why most u particles pass almost straight through the platinum foil, whereas a very few—those scoring a direct hit on the nucleus—undergo verv large deflections. Most of the atom is nearly empty space thinly populated by the atom s electrons. The nuclei are much smaller relative to their atoms than shown here. 

Determine the particle emitted and write the balanced nuclear equation for each of the following nuclear transformations (a) carbon-14 to nitrogen-14 (b) neon-19 to fluorine-19 (c) gold-188 to platinum-188 (d) uranium-229 to thorium-225. 

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