Volmer mechanism system

The hydrogen oxidation reaction (HOR) at the anode proceeds on Pt-based catalysts and is one of the simplest reaction systems. ° Nonetheless, fundamental information of the mechanism and kinetics of HOR is still lacking. The most common mechanisms are the so-called Heyrovsky-Volmer and Tafel-Volmer mechanisms involving the following steps ... 

To summarise, AC methods have proved most successful where the system is straightforward and can be modelled analytically. By measurement over a wide range of frequencies the constants for the reaction steps constituting the model can be established and, particularly if adsorbed species are involved, AC methods have proved very powerful indeed, with a major area of application being in the study of metal passivation, as discussed in detail elsewhere in the book. An example of this behaviour in practice is provided by the work of Conway s and Hillman s groups on chlorine evolution at platinum. Several mechanisms for this reaction have been proposed, including both Volmer and Heyrovsky types ... 

Both schemes have been observed in various systems. We consider hydrogen evolution on platinum from an aqueous solution in greater detail. In this system the Volmer-Tafel mechanism operates, the Volmer reaction is fast, the Tafel reaction is slow and determines the rate. Let us denote the rate constant for the Volmer reaction as ki(rj), that of the back reaction as k i(rj). Since the Volmer reaction is fast and in quasiequilibrium, we have ... 

Our chapter has two broad themes. In the first, we will consider some aspects of quantum states relevant to electrochemical systems. In the second, the theme will be the penetration of the barrier and the relation of the current density (the electrochemical reaction rate) to the electric potential across the interface. This concerns a quantum mechanical interpretation of Talel s experimental work of 1905, which led (1924-1930) to the Butler-Volmer equation. 

The effect of humic materials on the photolytic micellar system was evaluated in DR s photodegradation. DR solubilized within Tween 80 micellar solution with or without humic materials was determined. In order to calculate the quantum yield, the molar absorptivity of DR was determined by spectrophotometry. The determination of the quantum yield and reaction rates was examined through a pseudo first-order decay rate expression. Quenching and catalytic effects resulting from the humic substances were examined through Stem-Volmer analysis. A reaction mechanism of photolytic decay of DR solubilized within surfactant micelles in the presence of various amount of humic materials was proposed for this purpose. The effect of high and low concentration of humic materials has been accounted for by a designed model. 

The Stern—Volmer and Perrin equations, (v) and (vi), both allow a quantitative characterization of the efficiency of non-radiative energy transfers. They are useful for the comparison of various donoi -acceptor systems, although they provide no information concerning the nature of the excited state involved and the mechanism of the energy transfer process. More appropriate theories have been established to describe non-radiative energy transfers that occur when there is close resonance between the initial and final states. 

Another mathematical model based on the relative contributions of static and dynamic quenching mechanisms in heterogeneous systems was derived by Carraway etal [8]. The light intensity in their model is described by a series of Stern-Volmer relationships... 

Shetlar has derived non-linear relationships of the Stem-Volmer type suitable for systems in which quenching occurs by more than one mechanism. Rate constants for heavy atom fluorescence quenching of polynuclear aromatic hydrocarbons by 1-iodopropane in benzene have been found to decrease exponentially with the energy difference between the fluorescing state and the nearest lower triplet state (Dreeskamp et al.). Bromocyclopropane has been recommended as a heavy-atom quencher of excited singlet states since it is more photostable than simple alkyl bromides (Flemming, Quina, and Hammond). Laser studies with chlorophyll a have provided evidence for the interesting radiationless intermolecular process Tx + Sx -> T2 + S0 (Menzel). 

Note that this does not mean that the concentration profile of species A is equivalent to that of the E mechanism since it will be influenced by the chemical reaction through the surface boundary conditions. Thus, the chemical reaction affects the surface concentration of species B, which is related to that of species A through the Nernst equation (for reversible systems), or more generally, through the Butler-Volmer or Marcus-Hush relationships. Therefore, the surface concentration of species A, and as a consequence the whole concentration profile, will reflect the presence of the chemical process. 

The adsorption of hydrogen on metal electrodes such as platinum has been studied extensively in electrochemical systems over the last several decades. The mechanism for the hydrogen oxidation reaction on a Pt electrode in an acid electrolyte proceeds through two pathways, Tafel-Volmer and Heyrosky-Volmer, both of which involve the adsorption of molecular hydrogen followed by a... 

Lays out the fundamental concepts of electrochemistry, but with a particular focus on the theoretical aspects involved in the kinetics of electrode reactions. Covers various methods, yet with strong emphasis placed on impedance spectroscopy as well as voltamperometric methods. The main systems examined include redox reactions, electrosorption. Insertion and the Volmer-Heyrovsky mechanism. For master s degree level, engineering students and researchers. In 2000 and 2005 the same authors and publishing house produced two books compiling exercises on electrochemical kinetics (steady state and insertion method). 

Direct connection between a fluorescence quenching moiety and the excited species may lead to a loss of fluorescence due to two distinct, and eventually competing mechanisms, energy transfer (ET) or electron transfer (eT). In this introduction and the following chapter, we will not consider dynamic systems in which the association of the quencher with the excited species is not controlled. In this regard, the Stem-Volmer method for the study of fluorescence quenching will not be introduced. Information relevant to this method can be found in the literature. ... 

When a photoexcited molecule undergoes chemical reaction the fluorescence quantum efficiency is reduced, i.e., it is quenched. Fluorescence quenching provides an excellent example of how competition between chemical and physical processes may be used to establish both rate law and mechanism. The reaction of photoexcited acridine (A ) with amines in aqueous solution has been thoroughly studied.Addition of amines reduces the fluorescence efficiency. Data for an analogous system are presented as a Stern-Volmer plot in Fig. 6.6. The relative fluorescence efficiency, (pf c=0)l(pf c), is equal to the ratio of fluorescence intensities without and with quenchers lf c=0)jlf c). If this latter quantity is graphed against amine concentration, c, it appears that... 

Decisive evidence on whether static quenching is appreciable in a particular system can come from fluorescence lifetime measurements. If the mechanism represented by Equation (6.24) is correct, fluorescence emission occurs only from free A molecules, so the lifetime is unaffected by static quenching and is given by Equation (6.16) above, which may be written in the form of a lifetime Stem-Volmer relation (i.e., to/t[q] = 1 -f- fesroiQ]). A plot of to/t[q] against quencher concentration [Q] will be linear (despite the non-linearity of the corresponding plot of /o//[Q]) with unit intercept and slope whence the value of... 

Let us now address an interesting problem in colloid science and physical-chanical mechanics related to the contact interactions in disperse systems, namely, the possibility of spontaneous dispersion and the formation of thermodynamically stable colloid system. Originally, this problem was formulated by Max Volmer in 1927 [60,61] and later addressed by Rehbinder and Shchukin. Shchukin has made two principal contributions to the analysis of this problem [33,62-69]. The first is the detailed analysis of the conditions that make the process of spontaneous dispersion (at constant volume of the disperse phase, constant particle size, or constant number of particles) possible. Second, he proposed incorporating the entropy of mixing into the description of the conditions of spontaneous dispersion. The latter allows one to quantitatively estimate the concentration of the disperse phase in the disperse system formed. The analysis of the thermodynamics of spontaneous dispersion has important implications in the analysis of colloidal stability and in the control of various technological processes. 

When Sq < 2%, quantum dots are not formed, and the growth of heterostructures proceeds by the layer-by-layer growth mechanism. At very large values of the mismatch parameter the growth follows the Volmer-Weber mechanism. Then the critical wetting layer thickness is less than one monolayer, and the formation of three-dimensional islands occurs directly on the substrate surface. An example of such a system is the InAs/Si system, wfiere = 10.6%. In a system of silicon and chromium the lattice... 

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