Structure sensitivity definition

Attempts to determine how the activity of the catalyst (or the selectivity which is, in a rough approximation, the ratio of reaction rates) depends upon the metal particle size have been undertaken for many decades. In 1962, one of the most important figures in catalysis research, M. Boudart, proposed a definition for structure sensitivity [4,5]. A heterogeneously catalyzed reaction is considered to be structure sensitive if its rate, referred to the number of active sites and, thus, expressed as turnover-frequency (TOF), depends on the particle size of the active component or a specific crystallographic orientation of the exposed catalyst surface. Boudart later expanded this model proposing that structure sensitivity is related to the number of (metal surface) atoms to which a crucial reaction intermediate is bound [6]. 

Furthermore, ir-arene complexes of transition metals are seldom formed by the direct reaction of benzene with metal complexes. More usually, the syntheses require the formation of (often unstable) metal aryl complexes and these are then converted to ir-arene complexes. The analogous formation of w-adsorbed benzene at a metal surface via the initial formation of ff-adsorbcd phenyl, merits more consideration than it has yet been given. It is to be hoped that the recognition and study of structure-sensitive reactions will allow more exact definition of the sites responsible for catalytic activity at metal surfaces. The reactions of benzene, using suitably labeled materials, may prove to be useful probes for such studies. 

Asymmetric diarylmethanes, hydrogenolytic behaviors, 29 229-270, 247-252 catalytic hydrogenolysis, 29 243-258 kinetics and scheme, 29 252-258 M0O3-AI2O3 catalyst, 29 259-269 relative reactivity, 29 255-257 schematic model, 29 254 Asymmetric hydrogenations, 42 490-491 Asymmetric synthesis, 25 82, 83 examples of, 25 82 Asymmetry factor, 42 123-124 Atom-by-species matrix, 32 302-303, 318-319 Atomic absorption, 27 317 Atomic catalytic activities of sites, 34 183 Atomic displacements, induced by adsorption, 21 212, 213 Atomic rate or reaction definition, 36 72-73 structure sensitivity and, 36 86-87 Atomic species, see also specific elements adsorbed... 

Jacobs and Tompkins6 consider only structure-sensitive nucleation, which occurs at definite sites in the lattice where the activation energy is least, such as lattice defects and dislocations. The rate of nucleus formation thus depends on both the defect density and on the activation energy. 

These results indicate that, perhaps, a better definition of structure sensitive reactions would be those that occur over ensembles of surface atoms while structure insensitive reactions are those that are promoted by single atom active sites. 

Structural model definition Comparison of all potential model representations based on available data and parameter identifiability, sensitivity analysis to determine data elements that may affect parameter identifiability (missing values, data collection errors, etc.), and simulations to estimate predictive performance prior to covariate analysis. 

The difference between the two definitions is that the rates vary with time or position, while the TOF only depends on the availability of the sites present. The activity of the catalysts and the selectivity of the reaction depend on the characteristics of the material. Some important features of these reactions should be remembered. The first one is related to the sensitivity of the chemical reaction to the catalyst structure. According to Boudart s theory, the supported catalysts consist of metal particles of different and variable sizes. With the increase of particle diameter, the concentrations of metal atoms and exposed sites vary significantly, indicating that no changes in surface structure have happened. The structure sensitive reactions (SSR) are those in which the intrinsic reaction rate relative to the number of surface active sites, i.e., the frequency of the reaction varies with the particle sizes, which does not happen in structure insensitive reactions (SIR). This means that in the structure sensitive reactions, the frequency of... 

The main structure-sensitive characteristic of a liquid polymer is its viscosity. Figure 5.161 lists a definition for a liquid in terms of viscosity, t], the ratio of the tangential or shear-stress a to the rate-of-shear, de/dt. The sketch relates the varions quantities for an experiment consisting of a stationary base plate over which a mobile... 

Thus, recent experimental evidence seems to support the idea that growing small particles have maximum chemical reactivities, and certain sized/shaped small particles may have the highest reactivities. What size and/or shape varies with the metal in question and the reaction in question This information strongly supports three ideas (1) structure sensitivity in chemical reactions on metal surfaces is very important, (2) more than one atom is necessary to carry out at least some bond breaking processes, and (3) defect sites on growing small particles are extremely reactive (see Fig. 9). It has also been possible by pulsed laser vaporization to produce many types of gas phase metal clusters. Particularly interesting have been reactivity studies of niobium clusters Nb where X = 5-20. A definite cluster size dependence on reactivity was observed. Exposure... 

Instead of the definition in Eq. (7-82), the selectivity is often written as log k,). Another way to consider a selectivity-reactivity relationship is to compare the relative effects of a series of substituents on a pair of reactions. This is what is done when Hammett plots are made for a pair of reactions and their p values are compared. The slope of an LEER is a function of the sensitivity of the process being correlated to structural or solvent changes. Thus, in a family of closely related LFERs, the one with the steepest slope is the most selective, and the one with the smallest slope is the least selective.Moreover, the intercept (or some arbitrarily selected abscissa value, usually log fco for fhe reference substituent) should be a measure of reactivity in each reaction series. Thus, a correlation should exist between the slopes (selectivity) and intercepts (reactivity) of a family of related LFERs. It has been suggested that the slopes and intercepts should be linearly related, but the conditions required for linearity are seldom met, and it is instead common to find only a rough correlation, indicative of normal selectivity-reactivity behavior. The Br nsted slopes, p, for the halogenation of a series of carbonyl compounds catalyzed by carboxylate ions show a smooth but nonlinear correlation with log... 

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