Surface reactions trapping-desorption

Figure 6 Schematic potential energy diagrams for the interaction between O2 and Ag(l 11). Four panels are shown. In (a), the three states into which O2 can adsorb at the surfaces are depicted as a function of a reaction coordinate. In (b), the two potentials leading to direct inelastic scattering are shown. In (c), a trajectory representing a one dimensional representation of transient trapping-desorption in the O2 state is shown. In (d), two path ways leading to dissociative chemisorption are shown. From Kleyn et al. [45],...

Pulse radiolysis results (74) have led other workers to conclude that adsorbed OH radicals (surface trapped holes) are the principal oxidants, whereas free hydroxyl radicals probably play a minor role, if any. Because the OH radical reacts with HO2 at a diffusion controlled rate, the reverse reaction, that is desorption of OH to the solution, seems highly unlikely. The surface trapped hole, as defined by equation 18, accounts for most of the observations which had previously led to the suggestion of OH radical oxidation. The formation of H2O2 and the observations of hydroxylated intermediate products could all occur via... 

It has been proposed that the precursor state [81, 82] for the adsorption-desorption reaction consists of weakly physisorbed CO. This can be CO sitting on an occupied site (COad-CO) or on an sterically unfavorable Pt site. According to Ertl [81], the desorption process occurs through a trapping mechanism on such sites if the surface is saturated by chemisorbed CO the desorption channel involves either a COad-CO potential well or a Pt-CO attractive well which is sterically weakened by the presence of pre-absorbed CO . 

Note that plots, such as Figure 5.1, provide information only on the net outcome of chemical reactions. In the case of iron, a small addition does take place in estuaries as a result of desorption of Fe from the surfaces of riverine particles. As these solids move through the estuarine salinity gradient, the major cation concentrations increase and effectively displace the iron ions from the particle surfeces. Since this release of iron is much smaller than the removal processes, the net effect is a chemical removal of iron. Sedimentation of these iron-enriched particles serves to trap within estuaries most of the riverine transport of reactive iron, thereby preventing its entry into the oceans. 

Physical immobilization methods do not involve covalent bond formation with the enzyme, so that the native composition of the enzyme remains unaltered. Physical immobilization methods are subclassified as adsorption, entrapment, and encapsulation methods. Adsorption of proteins to the surface of a carrier is, in principle, reversible, but careful selection of the carrier material and the immobilization conditions can render desorption negligible. Entrapment of enzymes in a cross-linked polymer is accomplished by carrying out the polymerization reaction in the presence of enzyme the enzyme becomes trapped in interstitial spaces in the polymer matrix. Encapsulation of enzymes results in regions of high enzyme concentration being separated from the bulk solvent system by a semipermeable membrane, through which substrate, but not enzyme, may diffuse. Physical immobilization methods are represented in Figure 4.1 (c-e). 

The latter case (in contrast to the first one) will have no direct impact on the conduction. The former determines the appearance of a depletion layer at the surface of the semiconductor material, due to the equilibrium between the trapping of electrons in the surface states (associated with the adsorbed species) and their release due to desorption and the reaction with test gase. From the modeling point of view, it is described by the Poisson and electroneutrality equations. Out of the first two factors, one can calculate the dependence of the electron concentration s in the... 

Adsorption may proceed through a sequence of steps and the adsorbed species may be mobile or immobile. In general, the measured sticking coefficient corresponds to three successive processes (Figure 6.1) trapping of a molecule M into a precursor state Mp with a trapping constant k, desorption of Mp with a rate constant and transition from the precursor state to the chemisorbed state Ma with a rate constant k. As will be covered later, the precursor can be considered as a mobile state which is not associated with a particular site on the surface. These steps are summarized in the following set of reactions ... 

If the interaction with the surface is strong, the surface-bound molecule will be trapped near the bottom of the absorption well. The barrier for jumping to the neighbor well is then high and may therefore be higher than the barrier for the direct reaction. Thus the important aspects here are the steps involving adsorption, diffusion, reaction at the surface, and desorption. 

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