Dual sites

The PTCR effect is complex and not fully understood in terms of the grain boundary states and stmcture. Both the PTCR effect and room temperature resistivities are also highly dependent on dopant type and ionic radius. Figure 11 (32) illustrates this dependence where comparison of the PTCR behavior and resistivity are made for near optimum concentrations of La ", Nd ", and ions separately substituted into BaTiO. As seen, lowest dopant concentration and room temperature resistivity are obtained for the larger radius cation (La " ), but thePTCR effect was sharpest for the smallest radius cation (Y " ), reflecting dual site occupancy of the Y " ion. 

Dual Sites When the numbers of moles of reactants and products are unequal, A <=> M -t- N, the mechanism is assumed to be... 

This equation gives (0) = 0, a maximum at =. /Km/K2, and (oo) = 0. The assumed mechanism involves a first-order surface reaction with inhibition of the reaction if a second substrate molecule is adsorbed. A similar functional form for (s) can be obtained by assuming a second-order, dual-site model. As in the case of gas-solid heterogeneous catalysis, it is not possible to verify reaction mechanisms simply by steady-state rate measurements. 

The model considers the noble-metal catalyzed oxidation reactions of CO, two hydrocarbons of differing reactivities and H2, and the reaction kinetics was described by the global rate expressions of the dual-site Langmuir-Hinshelwood type [2]. 

Figure 11 shows schematically how such sites may be formed on Mo/alumlna or CoMo/alumlna catalysts. The pair site contains a reduced metal next to an acidic metal cation, either Co or Mo. Prestimably this dual site can remove S, leaving an olefin or aromatic molecule attached to the cation. This Is then hydrogenated by hydrogen from the reduced metal component of the site. 

Sidik RA, Anderson AB. 2002. Density functional theory study of O2 electroreduction when bonded to a Pt dual site. J Electroanal Chem 528 69-76. 

Gomez-Sainero et al. (11) reported X-ray photoelectron spectroscopy results on their Pd/C catalysts prepared by an incipient wetness method. XPS showed that Pd° (metallic) and Pdn+ (electron-deficient) species are present on the catalyst surface and the properties depend on the reduction temperature and nature of the palladium precursor. With this understanding of the dual sites nature of Pd, it is believed that organic species S and A are chemisorbed on to Pdn+ (SI) and H2 is chemisorbed dissociatively on to Pd°(S2) in a noncompetitive manner. In the catalytic cycle, quasi-equilibrium ( ) was assumed for adsorption of reactants, SM and hydrogen in liquid phase and the product A (12). Applying Horiuti s concept of rate determining step (13,14), the surface reaction between the adsorbed SM on site SI and adsorbed hydrogen on S2 is the key step in the rate equation. 

They indicated that both single- or dual-site mechanisms could be postulated as follows. 

For the dual-site mechanism assume (as Mathur and Thodos did) that the reaction step can be written as ... 

Keywords Ferrierite, copper, TPD, heterogeneous dual sites. 

Keywords copper, FER, dual sites, carbonyl, cation site... 

Hadac, E. M., Ji, Z., Pinon, D. I., Henne, R. M., Lybrand, T. P., and Miller, L. J. (1999) A peptide agonist acts by occupation of a monomeric G protein-coupled receptor dual sites of covalent attachment to domains near TM1 and TM7 of the same molecule make biologically significant domain-swapped dimerization unlikely. J. Med. Chem. 42, 2105-2111. 

The authors proposed a dual-site mechanism whereby H20 is activated on the ceria at defects and CO adsorbs on Pt, the Pt-CO then reacts with the activated H20 to produce an intermediate on the metal, with decomposition of the intermediate occurring at the metal, as shown in Scheme 84. 

Sachtler [195] proposed a dual-site mechanism in which the hydrogen is dissociated on the Ni surface and then migrates to the substrate that is coordinated to the adsorbed dimeric nickel tartrate species. In their model, adsorption of modifier and reactants takes place on different surface atoms in contrast to Klabunovskii s proposal. Adsorbed modifier and reactant are presumed to interact through hydrogen bonding (Scheme 14.5). The unique orientation of adsorbed modifier molecules leads to a sterically favored adsorbed reactant configuration to achieve this bonding. 

For initial reaction rates r0 with a pure alcohol feed, all product partial pressures are zero, and pAO = yAO n, so that for the dual site... 

At low 7t, the denominator simplifies to unity in each case and both models are linear in n. For sufficiently high n, the parenthesis in the denominator approaches Ketv, the initial rate for the dual-site model then approaches zero, and that of the single-site model approaches a constant value. Thus the plot of the experimental data will indicate that the dual-site model is preferable if a maximum exists in the data, or that the single-site model is preferable if a horizontal high-pressure asymptote exists. Hence, for the data of Franckaerts and Froment (FI) shown in Fig. 2, the dual-site model is preferred over the single-site model. 

The data should lie along a straight line when plotted as (7i/r0)1/2 vs n, if the dual-site model is adequate. If, in addition, the dual-site model is to be preferred over the single-site model, the plot of njr0 vs n should be curved... 

An analysis of variance can also be used to test the adequacy of more theoretical models. For example, two models considered in Section III for pentane isomerization are the single-site and dual-site models of Eqs. (30) and (31). These were linearized to provide Eqs. (32) and (33). The overall fit of these equations to the data may now be judged by an analysis of variance, reported in Tables V and VI (K3). It is seen that Eq. (33) fits the data quite... 

The Q (initial reaction rate) plots were presented in Fig. 19. Note again that the data may be correlated well by straight lines for both models. The C2 values are correlated by the solid lines of Fig 20. Note that the dual-site values can again be correlated by a straight line, but that the single-site values of C2 show a definite curvature. Alternatively, the 0.975 atm value of the single-site C2 could be rejected, and the three high-pressure points... 

By using only simple hand calculations, the single-site model has been rejected and the dual-site model has been shown to represent adequately both the initial-rate and the high-conversion data. No replicate runs were available to allow a lack-of-fit test. In fact this entire analysis has been conducted using only 18 conversion-space-time points. Additional discussion of the method and parameter estimates for the proposed dual-site model are presented elsewhere (K5). Note that we have obtained the same result as available through the use of nonintrinsic parameters. 

The C2 plots of Fig. 26, however, indicate that a linear C2 dependence upon pressure will not be adequate. The use of the dual-site model will require three additional parameters for C2, in order that the model approximately describe the data. The single-site model, however, can be used if we take... 

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