Active sites, nonuniform distribution

Active centers, nature of, 10 96 Active site, 27 210-221 in catalysts, 17 103-104, 34 1 for olefin chemisorption, 17 108-113 dual-site concept, 27 210 electrical conductivity, 27 216, 217 ESCA, 27 218, 219 ESR, 27 214-216 infrared spectroscopy, 27 213, 214 model, 27 219-221 molybdena catalyst, 27 304-306 Mdssbauer spectroscopy, 27 217, 218 nonuniform distribution, transport-limited pellets, 39 288-291... 

An important aspect concerning catalytically active membrane reactors, is the distribution of the active phase within the membrane system. Modem modification techniques (van Praag et al. 1989, Lin, de Vries and Burggraaf 1989) allow control over the catalyst distribution and preferential deposition of the active phase at different places in the membrane (top layer/support) system. Studies on conventionally used plate-shaped and cylindrically-sha-ped catalytically active pellets (Vayenas and Pavlou 1987a, b, Dougherty and Verykios 1987) have shown that nonuniformly activated catalysts (catalysts with nonuniform distribution of active sites according to a certain profile)... 

Polyethylene with a narrow molecular weight distribution MJMn = 2 Ziegler catalysts, in contrast, produce polyethylene with value of MJMn of 5-10 because the nonuniformity of the active sites... 

D. Nonuniform Distribution of Active Sites within Transport-Limited Pellets... 

The nonuniform distribution of the proton-active centers in zeolites can be measured by temperature-controlled desorption of adsorbed organic bases. The bases that are adsorbed on the centers of highest activity require the highest temperature for desorption. The IR spectra of adsorbed bases such as ammonia and pyridine give information about the nature of the adsorption centers. For example, the pyridi-nimn ion is indicative of proton-donor sites. NMR and ESR spectroscopy are also useful for elucidating the nature of acid centers. 

The values of isomer shifts (S= 1.08 and 1.05-1.07 mm s relative to a-Fe) and quadrupole splittings (A q = 3.0-3.1 and 2.3-2.4mms see Table 17.3) obtained for the two doublets allowed those components to be correlated with the two cation-binding sites in the GS active center (sites n2 and n I, respectively [42-44]). As mentioned above, these sites of bacterial GSs have different coordination environments, as well as a correspondingly lower (for site n2) and higher (for site nl) affinity to the cation. The latter difference is in line with the nonuniform distribution of Co" between the spectral components (as the areas of quadrupole doublets I and 2 in each spectrum corresponding to different Co" forms are significantly different see Fig. 17.6). 

The factor (1 — y)/[ + active sites that are vacant, as shown in Chapter 5. This factor represents the profile of sites within the pellet that are available for both the main and deactivation reactions. If this profile is treated as the activity profile for the rate constant kp, the result obtained in Section 4-8 for the internal effectiveness factor for a pellet with a nonuniform activity distribution can be applied (Section 5-6) ... 

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. 

An interesting consequence of the highly nonuniform electrostatic potential and distribution of the molecular species is that the local activity coefficients of the chemical species taking part in chemical equilibria depend on their exact location at the interface. As an example, Figure 2.8 shows that the oxidation fraction of the osmium sites is a nonuniform function of the distance to the electrode. The consequences of this finding for the electrochemical response will be discussed in Section 2.3.4. 

Although the matrix may have a well-defined planar surface, there is a complex reaction surface extending throughout the volume of the porous electrode, and the effective active surface may be many times the geometric surface area. Ideally, when a battery produces current, the sites of current production extend uniformly throughout the electrode structure. A nonuniform current distribution introduces an inefficiency and lowers the expected performance from a battery system. In some cases the negative electrode is a metallic element, such as zinc or lithium metal, of sufficient conductivity to require only minimal supporting conductive structures. 

If the metallocene is linked to the support first, bonding occurs on various adsorption sites, giving nonuniform supported species and much broader molecular weight distributions in the polymer product than those obtained in the corresponding homogeneous catalysis. Furthermore, a large part of the metallocene may be destroyed by acidic centers of the support then the activity of the supported catalyst is much lower than in the case of the homogeneous system. 

The benefits of nonuniform activity distributions (site density) or diffusive properties (porosity, tortuosity) within pellets on the rate of catalytic reactions were first suggested theoretically by Kasaoka and Sakata (Ml). This proposal followed the pioneering experimental work of Maatman and Prater (142). Models of nonuniform catalyst pellets were later extended to more general pellet geometries and activity profiles (143), and applied to specific catalytic reactions, such as SO2 and naphthalene oxidation (144-146). Previous experimental and theoretical studies were recently discussed in an excellent review by Lee and Aris (147). Proposed applications in Fischer-Tropsch synthesis catalysis have also been recently reported (50-55,148), but the general concepts have been widely discussed and broadly applied in automotive exhaust and selective hydrogenation catalysis (142,147,149). 

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