Kinetics of reactions on surfaces

A. The Langmuir adsorption isotherm and kinetics of reactions on surfaces. Consider a molecule A coming from the gas phase and absorbed on an empty surface site S that then becomes occupied, A - - S A - S. The fraction of surface sites that are covered by A molecules is denoted B. (a) At equilibrium... 

The statistical mechanical formulation of the transition-state theory (TST) can also be applied to the kinetics of reactions on surfaces. Let us consider the following reaction ... 

Analyzing the rates of dissociation and desorption (as described in Chapter 2) yields kinetic parameters for the elementary steps (see Table 4.2). The attractive feature of static SIMS is that it provides a way to measure the kinetics of reactions on the surface, which would be very difficult to do with any other technique. Of course, such experiments require that SIMS operates under truly static conditions... 

These examples by no means exhaust the multiplicities and diversities of the kinetics of reactions on heterogeneous surface. The poisoning of catalytic reactions by means of foreign substances will be reviewed later in a section devoted to a discussion of catalyst modification. 

The kinetics of reactions on metal surfaces is strongly affected by the structures of reaction intermediates, especially by the incorporated metal adatoms in their structures (Sect 11.5). hi the mid 1970s, Falconer and Madix observed a surface- kinetic explosion for the decomposition of formate and acetate adsorbed on the Ni (110) surface [23, 24], Recently, with the help of STM, TPRS, and XPS, we were able to determine that Ni atoms are incorporated into the structures of the carboxylate intermediates. Remarkably, the incorporation of metal atoms into the carboxylate structure is an important aspect of the origin of the kinetic explosion. 

Kinetics of reactions on more than one site are more complex. A general formalism for single-step surface reactions and rate control by the reaction, adsorption of a reactant, or desorption of a product has been developed by Hougen and Watson ... 

The distribution of current (local rate of reaction) on an electrode surface is important in many appHcations. When surface overpotentials can also be neglected, the resulting current distribution is called primary. Primary current distributions depend on geometry only and are often highly nonuniform. If electrode kinetics is also considered, Laplace s equation stiU appHes but is subject to different boundary conditions. The resulting current distribution is called a secondary current distribution. Here, for linear kinetics the current distribution is characterized by the Wagner number, Wa, a dimensionless ratio of kinetic to ohmic resistance. 

The reaction interface can be defined as the nominal boundary surface between reactant and the solid product. This simple representation has provided a basic model that has been most valuable in the development of the theory of kinetics of reactions involving solids. In practice, it must be accepted that the interface is a zone of finite thickness extending for a small number of lattice units on either side of the nominal contact sur-... 

The drop of the voltammetric crurent is associated with Pt surface oxidation, and the drop on the negative-going mn is due to Reaction (12.9) (surface poisoning by CO) and the Tafehan kinetics of Reaction (12.8). Further, the shift between curves in Fig. 12.13a and b indicates that in the potential range between 0.5 and 0.6 V, methanol oxidation occms with zero or low level atop CO smface intermediate. The amplitudes on Fig. 12.13 on both scans nearly equal to each other indicate a high level of preferential (111) crystallographic orientation of the poly crystalline Pt surface used for this work, as inferred from data in [Adzic et al., 1982]. 

Adhesive force, non-Brownian particles, 549 Admicelle formation, 277 Adsorption flow rate, 514 mechanism, 646-647 on reservoir rocks, 224 patterns, on kaolinite, 231 process, kinetics, 487 reactions, nonporous surfaces, 646 surface area of sand, 251 surfactant on porous media, 510 Adsorption-desorption equilibria, dynamic, 279-239 Adsorption plateau, calcium concentration, 229... 

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