Elementary processes occurring

Due to the fast kinetics of adsorption/desorption reactions of inorganic ions at the oxide/aqueous interface, few mechanistic studies have been completed that allow a description of the elementary processes occurring (half lives < 1 sec). Over the past five years, relaxation techniques have been utilized in studying fast reactions taking place at electrified interfaces (1-7). In this paper we illustrate the type of information that can be obtained by the pressure-jump method, using as an example a study of Pb2+ adsorption/desorption at the goethite/water interface. 

Heterogeneous catalysis is clearly a complex phenomenon to understand at the molecular level. Any catalytic transformation occurs through a sequence of elementary steps, any one of which may be rate controlling under different conditions of gas phase composition, pressure, or temperature. Furthermore, these elementary processes occur catalytically on surfaces that are usually poorly understood, particularly for mixed oxide catalysts. Even on metallic catalysts the reaction environment may produce surface compounds such as carbides, oxides, or sulfides which greatly modify... 

The following elementary process occurring either on one site or, as shown, on two sites is called a Rideal or a Rideal-Eley mechanism ... 

Magee (22) proposed that three consecutive elementary processes, occurring in about 10 18 sec., yield OH and HsO as the primary chemical... 

Changes in the state of the adsorbent-adsorbate system which, at the atomic-molecular level, is described by the lattice-gas model are caused by variations in the occupancy of its individual sites as a result of the elementary processes. The following elementary processes occur on the adsorbent surface adsorption and desorption of the gaseous phase molecules, reaction between the adspecies, migration of the adspecies over the surface and their dissolution inside the solid. The solid s atoms are capable of participating in the chemical reactions with the gaseous phase molecules, as well as migrating inside the solid or on its surface. 

AC s is few so that any elementary process occurs independently of the other processes and does not affect the distribution pattern of the remaining system s components. 

Phenol-water clusters are good models for the investigation of the photoinduced elementary processes occurring in living matter. Intracluster hydrogen transfer processes in phenol-water (Ph-W) complexes have extensively been studied in recent years, see refs [11-14] for reviews. Phenol-ammonia (Ph-A) clusters also have served as easily accessible and versatile models of intracluster hydrogen transfer dynamics [14,16]. It has been inferred by several authors that intracluster proton transfer occurs more readily in Ph-A clusters than in Ph-W clusters, but it has been a matter of debate whether the hydrogen or proton transfer occurs in the S excited state, or in the cluster cation, or in both [12,14],... 

In this way, fall-off curves for the rate coefficient, k, are characterized by three numbers k°, k°° and Fin. k° is the rate coefficient at low concentrations. For example, this formalism has been used in a compilation of kinetic parameters of elementary processes occurring in the middle atmosphere [65]. 

Bradley considers that the following elementary processes occur in the high-temperature pyrolysis of hydrocarbons unimolecular initiation by carbon—carbon bond rupture, radical dissociation by /3-bond breaking plus simultaneous radical isomerization (via 1—5, 1—4 and forbidden 1—3 and 1—2 hydrogen shifts), H abstraction by hydrogen, methyl and alkenyl radicals, addition of hydrogen, methyl and alkenyl radicals to unsaturated molecules, combination two by two of methyl and alkenyl radicals. 

Until now, we have been interested in the types of elementary process occurring in the system. The classes of species undergoing these processes must also be defined. Let us first apply the elementary processes to the... 

The second line of research in catalysis has been aimed at determining the structure and composition of catalysts and interpreting catalysis in terms of the elementary processes occurring on the catalyst surface. Pursuit of these objectives has depended heavily on the availability of instrumentation and has advanced at a rate closely correlated with the rate of introduction of... 

Nevertheless, the dynamical study of the elementary processes occuring in the course of a reaction remains useful and complementary to the static study of the potential surface, even though it is incomplete and does not lead to the reaction rate. In particular, the comparison of dynamical trajectories with the static minimum-energy path is very instructive. As we mentioned in Chap. A for CHj + H2— the initial conditions seem to play a crucial part in the shape of dynamical trajectories only certain specific initial conditions lead to trajectories close to the minimum energy path most dynamical trajectories are much more complex than this path. Furthermore, deviations may result from the fact that for a given potential surface in several dimensions the optimum path is most often drawn approximately under the assumption that the evolution of the system can be represented by the sliding of a mass point on the potential surface. This model is generally unsuitable for constrained systems ... 

Any solid state reaction, however complex, must resolve itself into interactions between pairs of solid phases, the elementary processes occurring successively or simultaneously to give a variety of intermediate and final products. Because the entropy change is small, all solid state reactions are exothermic. This property forms the basis of the heating curve method for detecting reactivity in solid mixtures. 

On the other hand, if we are to understand radiation chemistry and discharge chemistry at all we must understand them in terms of elementary process, instead of in terms of over-all effects which can be so readily observed. We establish the reality of such elementary processes for the most part on an essentially intuitive basis. We are of the opinion that we know that certain elementary processes occur in radiation chemistry or in discharge chemistry because of observations made, for example, in cloud-chamber experiments or in mass spectroscopy. We conclude... 

What elementary processes occur before the formation of a PbS04 crystal layer These processes were studied using a rotating lead disc electrode at low anodic polarization. The obtained curves are presented in Fig. 2.9b [21]. When the electrode is rotated during the anodic polarization, the current increases rapidly. No cathodic current flows when the anodic polarization is stopped and the direction of the potential scan is reversed. There are no Pb + ions to be reduced as they have been thrown out into the bulk of the solution as a result of the rotation of the electrode. Pb ions survive in the solution for a certain period of time before reacting with ions to form the PbS04 phase. This indicates that the... 

Fig. 6.4 Fundamental architectures and elementary processes occurring in operating OLEDs...

The calculated transition state of 77 kJ/mol for step (2) implies that in this cycle of steps, that hydrogen addition may be the rate-limiting step. While the results from theoretical predictions suggest that we can speculate as to the MARI and the rate-limiting step, we can not be sme unless we carry out full simulations of the elementary processes occurring simultaneously to establish the actual kinetic outcome. 

Unlike the hydrogen-chlorine reaction, that of hydrogen with oxygen follows the chain branching mechanism. For this reason, the summation of the elementary processes occurring in the detonation wave... 

The 3-process invrdves auto- and cross-conelation terms and since it is assumed to be a weighted sum of elementary processes occurring in a variety of local environments, it may be rather broad in the frequency domain. The a-process is also a weighted sum of processes, and relaxes the portion of <11/(O) M(r)) which remains after the /3-process has occuned. It is emphasized that although the a- and -processes have different mechanisms for motion, the two processes are inter-related since the same dipoles ccmtribute to both. This leads to a conservation relation Ae = Ae + Ae, 2) If the a-process becomes so slow that it cannot be followed in an accessible time-scale, then only the /3-process will be observed experimentally. The system will be in a non-equilibrium state, the glassy state (T < Tg). Equations (3) — (7) and (14) — (23) must be modified to accommodate the situation that v Q(kk) (0 and < a(kk ) (0 do not decay. We obtain (Williams and Watts, 1971a Williams, 1972b). 

Figure 1 Elementary processes occurring during PA signal generation. The absorbed photon energy is partly transformed into heat and acoustic energy.

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