Adjacent-site mechanism

Whilst, in principle, kinetic measurements should allow a differentiation between the two possible mechanisms, it must be noted that in catalytic hydrogenation reactions relatively few examples are sufficiently clear cut to allow this differentiation to be made. Thus, for example, it is quite commonly found that the experimentally observed orders of reaction are zero in the unsaturated substrate A and unity in hydrogen. Such results are readily interpreted by the adjacent-site mechanism by assuming A to be much more strongly adsorbed than hydrogen or by the Rideal— Eley type of mechanism. Clearly, kinetic measurements alone are insufficient for the establishment of mechanism. 

Much work has been undertaken to understand the steps and intermediates by which the reaction occurs on the heterogeneous catalyst surface. However, the exact mechanism is not fully established. One approach assumes a first-step adsorption of carbon monoxide on the catalyst surface followed by a transfer of an adsorbed hydrogen atom from an adjacent site to the metal carbonyl (M-CO) ... 

Ensemble or Third-Body Ejfects. These effects concern the selective blockage of a particular adsorption site by adatom deposition. This can be advantageous when the reaction mechanism contains parallel paths that can be affected differently by blocking particular sites. In some cases, the undesired reaction needs more than one free adjacent site (ensemble), and can be inhibited by blocking particular sites without decreasing the reactivity of the surface for the catalyzed reaction. 

It is now widely accepted that COads electro-oxidation proceeds via the F-H mechanism, which includes the reaction steps of CO adsorption, water sphtting (15.18), and COads + OHads recombination (15.19) on two adjacent sites to yield CO2 ... 

Surface diffusion is yet another mechanism that is often invoked to explain mass transport in porous catalysts. An adsorbed species may be transported either by desorption into the gas phase or by migration to an adjacent site on the surface. It is this latter phenomenon that is referred to as surface diffusion. This phenomenon is poorly understood and the rate of mass... 

C5 cyclization requires stricter geometric conditions than aromatization. This is in favor of the dual-site mechanism of C5 cyclic reactions (25). All metals catalyzing it have an fee lattice, and their atomic diameter lies between 0.269 and 0.277 nm. These two criteria must be fulfilled simultaneously. With such a distance between the two sites, the screening of the C—C bond adjacent to the preferably adsorbed tertiary C atom becomes evident. Figure... 

If hydrogen occupies all sites, the dual-site mechanism may operate over two adjacent /2g sites 42). The importance of active site periodicity and the screening of the adjacent C—C bond is valid in this case, too. This (assumedly adsorbed) hydrogen does not participate in C5 cyclic reactions. There is some indication, however, that it might be mobilized for cyclobutane ring opening 97, 97a). 

Step 2. It then reacts either with another molecule on an adjacent site (dualsite mechanism), with one coming from the main gas stream (single-site mechanism), or it simply decomposes while on the site (single-site mechanism). 

For a bimolecular reaction proceeding by an adjacent site (Langmuir— Hinshelwood) mechanism ... 

This final hybrid mechanism may be responsible for the formation of the dimer Ion of the dodecanucleotlde (1) or of water clusters (17). Each molecular unit ejects Intact and then Interacts with other molecules In the near surface region to form the cluster entitles. In the case of (H20)2 clusters our calculations Indicate that the two H2O molecules originate from mostly adjacent sites on the surface (15). This Is a consequence of the relatively weak H2O-H2O Interaction. Ionic clusters such as (H20)H+, however, can form from an H2O molecule and an H Ion that were further apart on the surface. 

For a vacancy mechanism, in a self-diffusion process, the jump frequency, T, of an atom to a given adjacent site is given by [4]... 

The mechanism of the fourth category of bimolecular surface steps is peculiar to redox reactions catalysed by metals and semiconductors. Here both reactants sit on the surface, not necessarily on adjacent sites, and the electrons are transferred from the reducing to the oxidising species through the solid catalyst. The rate therefore depends not only on the concentrations at the surface but also on the potential taken up by the catalyst, and this potential in turn is a function of the concentrations of the electroactive species present. Equations (28) and (29) fail to represent the kinetics in these cases because khel is no longer independent of concentration. These kinetics must accordingly be treated by an electrochemical method of analysis and this is done in Sect. 4.1. 

Fig. 4.9 Diagram of electron-transfer mechanisms between adjacent sites separated by a potential-energy barrier.

If the q/RT part of Equation 4.12 is increased, then Aj will no longer be controlled by collisions between adsorbed molecules. As q/RT increases, A decreases and is approaching the spacing between adjacent sites, and a hopping mechanism is... 

Selective oxidation of CO in hydrogen over different catalysts has been extensively examined. Most research to date has occurred with formulations that include a precious metal component supported on an alumina carrier. The catalyst-mediated oxidation of CO is a multistage process, commonly obeying Langmuir-Hinshelwood kinetics for a single-site competitive mechanism between CO and 02. Initially, CO is chemisorbed on a PGM surface site, while an 02 molecule undergoes dissociative chemisorption either on an adjacent site or on the support in order for surface reaction between chemisorbed CO and O atoms to produce C02. 

Abstract We consider a possible realization of the position- and momentum-correlated atomic pairs that are confined to adjacent sites of two mutually shifted optical lattices and are entangled via laser-induced dipole-dipole interactions. The Einstein-Podolsky-Rosen (EPR) "paradox" [Einstein 1935] with translational variables is then modified by lattice-diffraction effects. We study a possible mechanism of creating such diatom entangled states by varying the effective mass of the atoms. 

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