Site uniformity

Easy chemical manipulation of all catalytic sites is an important and widely recognized zeolite characteristic. The near uniformity of the intracrystalline surface (in the absence of protic sites) provides an excellent opportunity to treat active sites uniformly. With protic zeolites, the presence of extra-framework alumina and silica-alumina phases and the need to optimize interaction between protic and Lewis acid sites make chemical manipulation complex, particularly with aluminum-rich zeolites. 

The model for an enzyme-catalyzed reaction is similar to that for a first-order reaction of a gaseous molecule adsorbed on a solid catalyst, which has a certain number of sites (uniformly active) per unit mass. The surface reaction goes from approximately first order at low partial pressure, when a small fraction of sites are covered, to nearly zero order at high partial pressure and high coverage. Derivations and examples for more complex surface reactions are given in Chapter 2. 

The addition polymerization of a vinyl monomer CH2=CHX involves three distinctly different steps. First, the reactive center must be initiated by a suitable reaction to produce a free radical or an anion or cation reaction site. Next, this reactive entity adds consecutive monomer units to propagate the polymer chain. Finally, the active site is capped off, terminating the polymer formation. If one assumes that the polymer produced is truly a high molecular weight substance, the lack of uniformity at the two ends of the chain—arising in one case from the initiation, and in the other from the termination-can be neglected. Accordingly, the overall reaction can be written... 

Single-Stack Acceptor. Simple charge-transfer salts formed from the planar acceptor TCNQ have a stacked arrangement with the TCNQ units facing each other (intermolecular distances of ca 0.3 nm (- 3). Complex salts of TCNQ such as TEA(TCNQ)2 consist of stacks of parallel TCNQ molecules, with cation sites between the stacks (17). The interatomic distance between TCNQ units is not always uniform in these salts, and formation of TCNQ dimers (as in TEA(TCNQ)2) and trimers (as in Cs2(TCNQ)Q can lead to complex crystal stmctures for the chainlike salts. 

Modem analytical techniques have not only aided glass producers, but also enabled suppUers of raw materials to produce a more consistent product. Closer monitoring of purity and on-site blending of raw materials are now used by many producers. A stable and uniform raw material is the key to the manufacture of high quaUty glass. 

The formation of anodic and cathodic sites, necessary to produce corrosion, can occur for any of a number of reasons impurities in the metal, localized stresses, metal grain size or composition differences, discontinuities on the surface, and differences in the local environment (eg, temperature, oxygen, or salt concentration). When these local differences are not large and the anodic and cathodic sites can shift from place to place on the metal surface, corrosion is uniform. With uniform corrosion, fouling is usually a more serious problem than equipment failure. 

Localized corrosion, which occurs when the anodic sites remain stationary, is a more serious industrial problem. Forms of localized corrosion include pitting, selective leaching (eg, dezincification), galvanic corrosion, crevice or underdeposit corrosion, intergranular corrosion, stress corrosion cracking, and microbiologicaHy influenced corrosion. Another form of corrosion, which caimot be accurately categorized as either uniform or localized, is erosion corrosion. 

When a battery produces current, the sites of current production are not uniformly distributed on the electrodes (45). The nonuniform current distribution lowers the expected performance from a battery system, and causes excessive heat evolution and low utilization of active materials. Two types of current distribution, primary and secondary, can be distinguished. The primary distribution is related to the current production based on the geometric surface area of the battery constmction. Secondary current distribution is related to current production sites inside the porous electrode itself. Most practical battery constmctions have nonuniform current distribution across the surface of the electrodes. This primary current distribution is governed by geometric factors such as height (or length) of the electrodes, the distance between the electrodes, the resistance of the anode and cathode stmctures by the resistance of the electrolyte and by the polarization resistance or hinderance of the electrode reaction processes. 

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. 

Because perspiration and bathing are commonly associated with sun exposure, the need to determine the SPE after bathing or long after appHcation to the body site is important. In use, the quantity of screen appHed and its uniform distribution over the exposed area control the achieved SPE. Methods for assessing the water-resistant or waterproof qualities of sunscreen products have been estabflshed by the EDA. 

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