Site density determinations

Hossain MM, Jonsson M. (2008) UOj oxidation site densities determined by one- and two-electron oxidants. J Nud Mater 373 186-189. 

TABLE 5,3 Site Densities Determined Using Grignard Method, Adopted from Ref. [12]... 

The correlation of TOF (h ) with the total Lewis and Br0nsted acid site density (determined from NHj-temperature-programmed desorption (TPD) measurements [83]) led to an unexpected conclusion the higher acid site density corresponds to the lower TOF. Such behavior may be explained only when taking into consideration limited accessibility of the acid surface sites as a result of physical blocking of a part of the active sites by the strongly chemisorbed... 

Surface hydrophobicity and acid sites density determines catalyst s activity selectivity. 

The density of Bronstcd and Lewis acid sites was determined by IR spectroscopy (Nicolet 710) of adsorbed pyridine, after desorption at 250°C, using the molar extinction coefficients previously obtained by Emeis [11]. The acid strength distribution of selected zeolites was studied by NH3-TPD in an Autochem 2910 Equipment (Micromeritics) coupled to a quadrupole mass spectrometer. First, NH3 was adsorbed at 175°C until saturation and then desorbed by increasing the temperature up to 800°C at a heating rate of 10°C/min. 

Corradini et al. examined in some detail by molecular mechanics15 and density functional studies100 the polymerization mechanism proposed by Cossee and the catalytic sites on TiC surfaces, including those proposed by Arl-man and Cossee13 and by Allegra.14 According to the calculations, for all these octahedral active sites a similar general mechanism of stereoselectivity occurs which is very similar to the one established several years later for stereospecific metallocenes (see previous section). The chirality of the site would determine a chiral orientation of the first C-C bond of the chain (for a A site,... 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

Surface site densities used in the computation of the oxide site concentrations presented in this paper were determined by either rapid tritium exchange or acquired from published values (18). Reported total site densities for hydrous metal oxides show relatively little variation generally they range by less than a factor of 3. Since [M], [SOM], [H] and x are known or can be determined from experimental data, uncertainties in estimates of the total site concentration are directly translated into uncertainties in the calculated partitioning coefficient. 

Nanocarbon emitters behave like variants of carbon nanotube emitters. The nanocarbons can be made by a range of techniques. Often this is a form of plasma deposition which is forming nanocrystalline diamond with very small grain sizes. Or it can be deposition on pyrolytic carbon or DLC run on the borderline of forming diamond grains. A third way is to run a vacuum arc system with ballast gas so that it deposits a porous sp2 rich material. In each case, the material has a moderate to high fraction of sp2 carbon, but is structurally very inhomogeneous [29]. The material is moderately conductive. The result is that the field emission is determined by the field enhancement distribution, and not by the sp2/sp3 ratio. The enhancement distribution is broad due to the disorder, so that it follows the Nilsson model [26] of emission site distributions. The disorder on nanocarbons makes the distribution broader. Effectively, this means that emission site density tends to be lower than for a CNT array, and is less controllable. Thus, while it is lower cost to produce nanocarbon films, they tend to have lower performance. 

Site Density and Entropy Criteria in Identifying Rate-Determining Steps in Solid-Catalyzed Reactions... 

We have already mentioned that Horiuti, Miyahara, and Toyoshima (7) determined the site densities of metal catalysts by using a theory similar to TST wherever the cases they consider are the same as the cases considered in classical TST development. We can now show this similarity in the light of Eq. (10) and the definition of C in Table I. Using our symbols where applicable, the equation Horiuti, Miyahara, and Toyoshima use is... 

Determining Site Density from High-Conversion Data... 

To calculate L values using Table I we need forward reaction rates, the number of molecules converted per unit area per second. These reaction rates are most easily obtained at low conversions. But frequently—often for practical purposes—high conversions are reported. We have developed a method for the determination of site densities in a certain class of systems even though the conversion and back reaction are both large. Also, the method can be used under certain conditions even if product isomerization is involved. We shall describe the theory here and in Section III,B apply it to the isomerization of 1-butene to cis- and trans-2-butene over silica-alumina. Although in this article we do not use this method to analyze any other systems, we present it in some detail because it may have potential for further use. 

Our problem has four parts. First, we must determine the rate law for an infinitesimal length of catalyst bed. Second, the resulting equation must be integrated over the length of the bed. Third, the values of certain of the system parameters must be determined using the integrated equation and experimental conversions. Finally, the site density is determined from the values of these parameters. 

For Example 9 the order is 0.7, suggesting a model for which the logL value is between that for Step 2 (0.5 order, log L = 15) and Step 1 (1.0 order, log L = 22). Thus, the rate-determining step may be a reaction on a partially filled surface. Since the log L value calculated in this way for 0.7 order is rather large, some surface mobility and/or rotation is indicated. The zero-order reactions of Examples 11 and 19 are clearly surface reactions for which expected site densities are obtained. For Example 11 Tottrup (25) suggested that the rate-determining step is C-O scission in adsorbed CO. 

In Section II,B,8 we discussed the question of determining site densities using high-conversion data. We developed a method applicable in the inter-conversion of three isomers when there is a common surface complex for the three possible reactions. We have tested this method using the conversion of 1-butene to cis- and rrans-2-butene over silica-alumina, a system that, according to Hightower and Hall, proceeds through a common surface complex (111). Their conclusion has been confirmed experimentally (112) and by semiempirical quantum-chemical calculations (113). 

Arai, Tominaga, and Tsuchiya (77) postulated for Example 26 that NO adsorbs on previously adsorbed oxygen. The logL values given in Table VII for this example cannot be applied directly to their reaction because they report fractional orders. But a combination of the information on orders and log L values suggests that the site density is low, that the surface is unsaturated with respect to both reactants, and that a surface reaction is rate determining. 

Site Density and Entropy Criteria in Identifying Rate-Determining Steps in Solid-Catalyzed Reactions Russell W. Maatman Organic Substituent Effects as Probes for the Mechanism of Surface Catalysis M. Kraus... 

A measurement system that is able to quantitatively determine the interactions of receptor and G protein has the potential for more detailed testing of ternary complex models. The soluble receptor systems, ([l AR and FPR) described in Section II, allow for the direct and quantitative evaluation of receptor and G protein interactions (Simons et al, 2003, 2004). Soluble receptors allow access to both the extracellular ligandbinding site and the intracellular G protein-binding site of the receptor. As the site densities on the particles are typically lower than those that support rebinding (Goldstein et al, 1989), simple three-dimensional concentrations are appropriate for the components. Thus, by applying molar units for all the reaction components in the definitions listed in Fig. 2A, the units for the equilibrium dissociation constants are molar, not moles per square meter as for membrane-bound receptor interactions. These assemblies are also suitable for kinetic analysis of ternary complex disassembly. 

---