Surface reactions single reactant

Bimolecular surface reactions reactants adsorption, 29 111-112 with single reactant, 29 108-109 1,1 -Binaphthyl, dehydrocyclization, 28 318 Binary oxides, 32 119 Binding energy, 32 160-162 chemisorbed sulfur, 37 281 hydrogen, sulfur effect, 37 295-296 shift, Pd, 37 62-64 ZnO/SiOj, 37 21-22 Binor-S, see Norbomadiene Biological systems, hydrogen in, activation of, 11 301... 

Bimolecular Surface Reaction with a Single Reactant... 

We studied the metathesis of propylene over W03-S102, an example of a single-reactant, bimolecular surface reaction. We calculated L for various conceivable rate-determining steps, including the step for which Eq. (34) applies (12). 

Fig. 12.5. (a) The region of multiple stationary-state behaviour for the Takoudis-Schmidt-Aris model of surface reaction, with = 10 3 and k2 = 2 x 10-3 (b), (c), and (d) show how the stationary-state reaction rate varies with the gas-phase pressure of reactant R for different values of p, giving isola, mushroom, and single hysteresis loop respectively. (Adapted and reprinted with permission from McKarnin, M. A. et al. (1988). Proc. R. Soc., A415, 363-87.)... 

It is rather desirable that investigations should be made of promoter action in reactions which only involve the decomposition or transformation of a single substance. When two substances interact, it is always possible that one component of the surface adsorbs one reactant and the other component adsorbs the other reactant. From what has been said about the localization of catalytic activity in special regions of the surface it will be seen that measurements of the total adsorption of the reactants by the separate constituents of the composite catalyst would throw no light on whether or not this actually occurred. 

Another apparatus that is very useful in studies of the mechanism of catalytic surface reactions is shown in Fig. 17. This is used in a molecular-beam surface scattering experiment (22b) in which a well-collimated beam of the reactant gas or gas mixture is scattered from a crystal surface and the products that are desorbed after a single scattering at a given solid angle... 

The values of As and s are not necessarily identical with, or to be identified as those calculated from, measurements for the overall reaction, A and E, since the latter are composite terms that may include contributions from the temperature dependences of c, c2, and /4S, as described in Appendix I. The surface reaction is not completely represented by the consideration of this single step (the surface collision) and rate expressions should be more realistically regarded as the resultant of several contributory factors in the sequence of interdependent (55, 119,120) processes required to convert the reactants into products. In general, the overall surface reaction is composite kinetic behavior and thus more complicated than many of the homogeneous processes that have attracted greatest interest. In the heterogeneous reactions,... 

The inadequacy of control experiments cannot be attributed to the lack of availability of electron microscopes. Yet only rarely were control experiments conducted to demonstrate that no single reactant or product gas was responsible for the observed surface modification. Furthermore, few investigations were conducted in which weight change was measured. Even more rare were investigations designed to determine the influence of variations in reaction parameters on the nature of the etching process. 

Figure 1 Schematic of the atomic beam dosing source used with REMPI detection by Murphy et al. to study the recombination of H [36, 37] and N [38] at metal single crystal surfaces. A single crystal surface is supported on a manipulator in the path of a collimated molecular beam. The beam supplies reactant molecules or atoms, produced using a microwave discharge in the glass nozzle, which react and recombine at the surface. The reaction products are ionised by the laser, which is focused in front of the surface (inset), and the resulting ions are timed into a microchannel plate detector.

This retardation of reaction rate with increase in reactant partial pressure is characteristic of catalytic reactions controlled by a surface reaction mechanism. Langmuir-Hinshelwood surface reaction rate mechanisms for single and dual site mechanisms are respectively ... 

In unimolecular surface reactions, a single reactant species is adsorbed and reacts on the surface. A good example is given by catalysed nucleophilic solvolyses discussed further in Sect. 2.1. The reaction scheme may then be written... 

The treatment here mirrors that in ref (1). Consider a single first-order reaction. In the conventional catalytic reaction, the reactant molecule diffuses to the external surface of the pellet, thence into a pore, at the internal surface of which the molecule adsorbs onto an active site, and reacts to form a product mdlecole, which desorbs and diffuses back out. If, as expected, external diffusion is relatively rapid, and if adsorption is not too strong, then the observed rate constant, with respect to the observed concentration at the pellet... 

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