Interface sites

This approach of using 2D and 3D monodisperse nanoparticles in catalytic reaction studies ushers in a new era that will permit the identification of the molecular and structural features of selectivity [4,9]. Metal particle size, nanoparticle surface-structure, oxide-metal interface sites, selective site blocking, and hydrogen pressure have been implicated as important factors influencing reaction selectivity. We believe additional molecular ingredients of selectivity will be uncovered by coupling the synthesis of monodisperse nanoparticles with simultaneous studies of catalytic reaction selectivity as a function of the structural properties of these model nanoparticle catalyst systems. 

Based on Au/TS-1, Delgass envisioned that both the simultaneous mechanism and sequential mechanism operated in parallel [152]. For the combination of Au sites and bare Ti-defect sites, propylene epoxidation proceeded by the sequential mechanism described above. However, the simultaneous mechanism dominated on Au-Ti interface sites (at least on the Au/TS-1 with a low Au loading of 0.01-0.06 wt%), where propylene epoxidation was accomplished by the attack of propylene adsorbed on H-Au-OOH species [153]. 

Assuming an equilibrium constant AOH for the adsorption of OH- on the interface sites, one can write ... 

However, this assumption is not necessarily justified. Even for a well-faceted nanoparticle there are a number of nonequivalent adsorption sites. For example, in addition to the low-index facets, the palladium nanoparticle exhibits edges and interface sites as well as defects (steps, kinks) that are not present on a Pd(l 1 1) or Pd(lOO) surface. The overall catalytic performance will depend on the contributions of the various sites, and the activities of these sites may differ strongly from each other. Of course, one can argue that stepped/kinked high-index single-crystal surfaces (Fig. 2) would be better models (64,65), but this approach still does not mimic the complex situation on a metal nanoparticle. For example, the diffusion-coupled interplay of molecules adsorbed on different facets of a nanoparticle (66) or the size-dependent electronic structure of a metal nanoparticle cannot be represented by a single crystal with dimensions of centimeters (67). It is also shown below that some properties are merely determined by the finite size or volume of nanoparticles (68). Consequently, the properties of a metal nanoparticle are not simply a superposition of the properties of its individual surface facets. 

There are a number of protein interface sites that have the properties of an enzyme structural zinc-binding site in that they are composed of four protein ligands and no water molecules. These sites may be involved in local protein conformation or be involved in stabilizing quaternary structure. The metal ion acting in this manner is not directly involved in the function of the protein but may still play an indirect role in aligning critical components of the protein for its intended function. 

Each corroding interface site will now have its amount of solid material, frac, reduced. Concentration contributions of M +, 02 and OH from each solid interface cell are distributed geometrically to adjacent electrolyte interface cells. 

It is assumed that 5-10% of the volume of material that is lost during dissolution over the time step is re-deposited at the surface of the material as corrosion product. Electrolyte interface sites are selected for corrosion product deposition according to voltage and concentration tolerances and here further ionic concentration contributions arise in Mn+ and H+. 

In the following sections, we investigate blend films spun from chloroform solution at a weight ratio of 50 50 (if not indicated differently). In films prepared in this way, the two polymers are well intermixed and hence the film contains a high density of interface sites. This makes it easier to detect signals due to the heterojunction. The thickness of the polymer films was —170 nm. 

This means that in these blends the spectral shape of the electroluminescence would be independent of whether process (a) or (b) in Fig. 2.1 occurs because ex-citons could quickly transfer towards a nearby interface site. In well-mixed blends it is therefore impossible to show that the exciplex is the primary product of charge capture and hence to prove that barrier-free capture occurs. We address these concerns here by presenting electroluminescence measurements from bilayer light-emitting diodes, where leakage currents are prevented, while a low density of interface sites (in fact the lowest possible) is maintained. 

In this model, the interface is assumed to be rough on the atomic scale, and a sizable fraction of the interface sites are available for growth to take place. Under these circumstances, the rate of growth is solely determined by the rate of atoms jumping across the interface (that is, the assumption is the process is controlled by the surface reaction rate and not diffusion). Using an analysis that is almost identical to the one carried out in Sec. 7.2.3, where the net rate of atom movement down a chemical potential gradient was shown to be [Eq. (7.26)]... 

A for the two histidines. The other two Cu" ions are found in surface exposed sites coordinated by two histidine e nitrogens and one or two water molecules. The putative cysteine ligand identified by spectroscopy and mutagenesis " does not bind Cu" in the structure and is quite distant from the metal-binding sites. It may be that this interaction occurs in solution between a cysteine residue from one dimer and a Cu" ion from a second dimer, and is precluded in the structure by crystal packing. The dimer interface site is proposed to deliver Ni" ions one at a time to the urease active site, and the other two sites are proposed to play a more secondary role, serving as reservoirs for Ni". ... 

The nature of the materials obtained by high temperature treatment of silica-alumina synthetic zeolites can be envisaged as follows. Such zeolites consist essentially of a mixture of 7-alumina and silica particles, the terminal silicons and aluminums sharing oxygens at the points of contact between alumina and silica particles. The exact coordination of the aluminum ions located at the interface is not known although it appears likely that they are in a three-coordinated state. This would correspond to the presence of the anhydride of HAIO2 at the interface sites. It further seems that the acid would be of the Lewis acid type rather than of the Brpnsted acid type, that is, it would have no proton associated with it. The data obtained on the thermal destruc-... 

Corrosion layer/PAM interface with gas caverns and pores along which the gas from the caverns leaves the plate [2] (a,b) images of gas caverns and channels (pores) at different interface sites ... 

Among the external portions of p-LG that have been postulated as potential sites for ligand binding are the outer surface near Trpl9-Argl24, the surface hydrophobic pocket in the groove between the a-helix and the p-barrel, a site near the aperture of the p-barrel and a site at the monomer-monomer interface of the protein dimer. The site near the aperture of the p-barrel is sensitive to Tanford transition, whereas the monomer-monomer interface site is dependent on the monomer/ dimer equilibrium. 

The difference of this view, in explaining the potential difference across biological interfaces, compared with that of the earlier workers, is profound. Thus, ion transport through the membrane is a consequence of the electrode potential differences referred to above. Electron transfer at the solid/liquid interface sites is rate determining (one of the two kinds of sites will dominate). The concentration of alkali metal ions on the two sides of the membranes may be the result, rather than the cause, of the potential... 

The fraction of available crystal interface sites, f, is presumed to increase (approaching 1) with decreasing temperature. 

X-Sb204. Vanadium, therefore, catalyzes the leoxidadon of antimony, reasonably that antimony situated in interface sites between Sb-oxide and vanadium-antimonate phases. 

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