Structure sensitive kinetic parameter

The kinetics of ethylene hydrogenation on small Pt crystallites has been studied by a number of researchers. The reaction rate is invariant with the size of the metal nanoparticle, and a structure-sensitive reaction according to the classification proposed by Boudart [39]. Hydrogenation of ethylene is directly proportional to the exposed surface area and is utilized as an additional characterization of Cl and NE catalysts. Ethylene hydrogenation reaction rates and kinetic parameters for the Cl catalyst series are summarized in Table 3. The turnover rate is 0.7 s for all particle sizes these rates are lower in some cases than those measured on other types of supported Pt catalysts [40]. The lower activity per surface... 

The combined use of the modem tools of surface science should allow one to understand many fundamental questions in catalysis, at least for metals. These tools afford the experimentalist with an abundance of information on surface structure, surface composition, surface electronic structure, reaction mechanism, and reaction rate parameters for elementary steps. In combination they yield direct information on the effects of surface structure and composition on heterogeneous reactivity or, more accurately, surface reactivity. Consequently, the origin of well-known effects in catalysis such as structure sensitivity, selective poisoning, ligand and ensemble effects in alloy catalysis, catalytic promotion, chemical specificity, volcano effects, to name just a few, should be subject to study via surface science. In addition, mechanistic and kinetic studies can yield information helpful in unraveling results obtained in flow reactors under greatly different operating conditions. 

Comment Whether it is used for simple monitoring, or to prove the existence of traces of a key intermediate (EXSY or PHIP), NMR spectroscopy has clearly developed into one of the catalytic community s most valuable analytical tools. NOE data will never afford a molecular picture which is quite as structurally exact as that from X-ray crystallography however, this latter method cannot determine kinetic parameters, recognize equilibria, mimic catalytic conditions (HP) or recognize when ion-pairing is important (PGSE). NMR is not a very sensitive method indeed, most methods are far more amenable to quantitative results. Nevertheless, its proven flexibility makes it indispensable. 

There has been a prevailing theory that oxidative degradation is accelerated by mechanical stress [100]. This theory is based on fracture kinetic work by Tobolsky and Eyring [101], Bueche [102, 103, 104], and Zhurkov and coworkers [105, 106, 107]. Their work resulted in an Arrhenius-type expression [108] sometimes referred to as the Zhurkov equation. This expression caused Zhurkov to claim that the first stage in the microprocess of polymer fracture is the deformation of interatomic bonds reducing the energy needed for atomic bond scission to U=U0-yo, where U0 is the activation energy for scission of an interatomic bond, y is a structure sensitive parameter and o is the stress. 

The previous analysis shown that the initial values of most of the kinetic parameters obtained from DFT calculations provide a good description of the reaction kinetics data collected over a wide range of conditions. The principal difference between the values of the final kinetic parameters used in the model and the initial values obtained from DFT calculations is that the fitted enthalpy changes for the formation of C2Ha transition states involved in cleavage of the C-C bond are lower than the initial values predicted from DFT calculations. This difference may be explained by the structure sensitivity of the system and/or by the inherent error of the DFT calculations. 

Nevertheless one might have expected that the kinetic data would reflect clearly the hydration properties of the added salt. This is not usually the case because charge-charge interactions are dominant. Moreover, structural effects often compensate and their influence on rate constants is minimised (p. 247). If, however, the reactants are neutral solutes, it might be anticipated that the kinetic parameters would be markedly sensitive to the particular salt added to the solution (p. 272). This prediction is borne out in practice, striking differences often being observed between the effects of added alkali metal cations and alkylammonium ions as the following examples show. 

Once we have deduced adequate kinetics expressions, we use the results of quantum-chemical computations to estimate the values of the relevant parameters. We make extensive use of resulfs of Hu and coworkers (31-33), who published a series of compufafional papers characterizing the Fischer-Tropsch reaction. This approach enables us to understand structure sensitivity and, more important, helps us to identify, for each mechanism, the catalyst structure that favors fhis mechanism. 

The oxidation of CO by either 02 or NO was studied by Peden et al. and Oh et al. over Rh, Pd, Pt,and Ir single crystals (90-92). The CO + 02 reaction was relatively insensitive to the atomic structure of the surface, and the specific activities and kinetic parameters agreed for both crystal surfaces and for alumina-supported catalysts. The Rh surfaces deactivated at high 02 pressures due to the formation of a near-surface oxide (91, 92). On the other hand, the CO + NO reaction was very sensitive to... 

The distinction between true and apparent activation enquiries is important to draw for several reasons. (1) In trying to understand how catalyst structure and composition affect activity, there are two factors to consider a thermochemical factor determining the concentration of reacting species, and a kinetic factor controlling their reactivity. Ea contains both, and only when E, and the relevant heats of adsorption are separated can their individual contributions be assessed. (2) Ea is not a fundamental characteristic of a catalytic system, because its value may depend on the reactant pressures used. As we shall see in Section 5.5, there are very helpful correlations to be drawn between kinetic parameters, reactant pressures and orders, and structure sensitivity in the field of hydrocarbon reactions. 

A recurring self-imposed task of many of the publications has been to identify the sites responsible for each type of reaction, so that structure-sensitivity has been a dominant theme. What has received much less attention is the possibility that, for example, particle size might determine the strength of hydrogen chemisorption, so that the use of constant operating conditions on a series of catalysts might produce results mainly decided by the surface concentration of hydrogen atoms. The dependence of kinetic parameters on particle size or other catalyst feature has been rarely examined. 

For many years it has been well known that CO electrooxidation on platinum is a structure-sensitive reaction. Studies with singlecrystal electrodes have shown that the kinetic parameters depend not only on the surface composition of the catalyst but also on the symmetry of the surface and that the presence of steps and defects alters significandy the reaction rate. As a consequence, the surface structure of the nanoparticles should also affect the performance for the oxidation of CO. Understanding how the different variables affect CO oxidation on Pt nanoparticles dispersed on carbon requires the control of the platinum surface in a similar way as has been achieved for single-crystal electrodes. In this sense, the influence of the surface site distribution on CO oxidation using nanoparticles of well-defined... 

The formation and growth of an electrodeposited phase is a complex process and many methods have been used to study it. The main feature of our laboratory scientific investigations is intensive study of the structure of poly- and monocrystalline refractory metal deposits, such as tungsten, molybdenum, rhenium, iridium, ruthenium and etc. We have shown that the structure of electrodeposited layers directly depends on the conditions of electrolysis [1]. Moreover this structures a most sensitive instrument to study the properties of our electrolyte, including purity of the melt, ion composition, dissipative ability, and kinetic parameters of electrodeposition. It was established that the reduction of oxygen in a chloride melt changed the direction of molybdenum growth texture from <111> to <001 > direction with a fine structure of epitaxial layers [2]. 

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