Stepped surfaces, occurrence

Giovanoli and Briitsch [264] studied the kinetics of vacuum dehydroxylation of 7-FeO 0H(- -7 Fe203). It was not possible to demonstrate satisfactory obedience to a single kinetic expression. Microscopic examinations detected the occurrence of random nucleation over reactant surfaces and crystallographic indications of the specific structural reorganization steps, which occur at the reaction interface, are discussed. 

In voltammetric experiments, electroactive species in solution are transported to the surface of the electrodes where they undergo charge transfer processes. In the most simple of cases, electron-transfer processes behave reversibly, and diffusion in solution acts as a rate-determining step. However, in most cases, the voltammetric pattern becomes more complicated. The main reasons for causing deviations from reversible behavior include (i) a slow kinetics of interfacial electron transfer, (ii) the presence of parallel chemical reactions in the solution phase, (iii) and the occurrence of surface effects such as gas evolution and/or adsorption/desorption and/or formation/dissolution of solid deposits. Further, voltammetric curves can be distorted by uncompensated ohmic drops and capacitive effects in the cell [81-83]. 

The photodegradation rate dependence on phenol concentration in the TiOi/F system shows a plateau in the 3 X IO " M to 3 X 10 M range, whereas for naked TiO2, a maximum is reached around 2 X 10 M of phenol followed by a decrease. This behavior is rationalized by the possibility of reductive back reactions of intermediates formed after the first oxidation step. The presence of fluoride could limit the occurrence of this detrimental effect, reducing the interaction of the formed intermediates with the surface. Moreover, also the change of the oxidation pathway changes the amount of the products. 

The occurrence of O2 and O is not excluded, however. Shimizu et al. [291] detected, for the first time, Ag2+ with ESR techniques and postulated the existence of O2. Working with LEED, Bradshaw et al. [60] favoured the hypothesis that O is an active species. Indirect evidence for the presence of O at high temperatures (220° C) was obtained from IR spectra by Force and Bell [116]. It is interesting that preadsorption of chlorine, which is a well known moderator, completely destroys activity when it covers 25% of the surface. Kilty et al. [176] suggested that the first adsorption step is coupled to an ensemble of four adjacent silver atoms, viz. 

The El mechanism has, as the rate-determining step in solution, the ionisation of the reactant forming a carbonium ion which then decomposes rapidly. For heterogeneous catalytic reactions, the important features are the occurrence of the reaction in two steps and the presence on the solid surface of carbonium ions or species resembling them closely. Again, the kinetic characterisation by way of an unimolecular process is of little value. Even the relative rates of the two steps may be reversed on solid catalysts. A cooperation of an acidic and a basic site is also assumed, the reaction being initiated by the action of the acidic site on the group X. 

We here review the factors that control the kinetics of product formation through reaction at an active surface. This includes consideration of the availability of those adsorbed intermediates which participate in the rate-limiting step (this term is analogous to concentration in a homogeneous reaction) and the mobility of the same species, which may determine, or at least influence, the frequency of occurrence of the reaction situation. The discussion is given under three broadly interpreted general headings, between which there is considerable overlap. 

Common features in the various theoretical explanations of compensation behavior referred to in Section II, A, 1-7 are the occurrence of parallel reactions that are characterized by different values of the kinetic parameters (A, E) and/or a systematic change in the effective concentrations of reactants across the temperature interval used in the measurements of the Arrhenius parameters. Both influences are based on reaction models for which the kinetic behavior cannot be represented as a single desorption step and, indeed, the overall surface interactions could be much more complicated. 

---