Electrochemical reaction mechanism

Lefebvre, M. C. Establishing the Link Between Multistep Electrochemical Reaction Mechanisms and Experimental Tafel Slopes 32... 

Electrochemical Reaction Mechanisms and Experimental Tafel Slopes... 

The chapter is divided into two subsections the first of which deals with the characterization of electrodes as prepared prior to any electrochemical treatment. The knowledge of the actual surface composition of the fresh electrodes is needed to optimize preparation conditions and to be able to correlate electrochemical performance with surface properties. In Section 3.2 the application of XPS to the elucidation of electrochemical reaction mechanisms will be demonstrated. Here XPS monitors possible changes after controlled electrochemical treatment. 

While characterization of the electrode prior to use is a prerequisite for a reliable correlation between electrochemical behaviour and material properties, the understanding of electrochemical reaction mechanisms requires the analysis of the electrode surface during or after a controlled electrochemical experiment. Due to the ex situ character of photoelectron spectroscopy, this technique can only be applied to the emersed electrode, after the electrochemical experiment. The fact that ex situ measurements after emersion of the electrode are meaningful and still reflect the situation at the solid liquid interface has been discussed in Section 2.7. 

Since the 1960s , various electrochemical methods such as linear potential sweep voltammetry, cyclic voltammetry etc. and various surface analysis apparatuses such as infrared spectra, X-ray photoelecfron spectroscopy etc. have been developed to investigate the electrochemical reaction mechanism involved in the flotation of sulphide minerals (Fuerstenau et al., 1968 Woods, 1976 Ahmed, 1978 Stm, 1990 Feng, 1989 Buckley, 1995 Arce and Gonzalez, 2002 Bulut and Atak, 2002 Costa et al., 2002). 

Again it seems not necessary to discuss the considerations of the chemical versus electrochemical reaction mechanism. It is clear from the extremely negative standard potential of silicon, from Eqs. (2) and (6), that the Si electrode is in all aqueous solutions a dual redox system, characterized by its OCP, which is the resultant of an anodic Si dissolution current and a simultaneous reduction of oxidizing species in solution. The oxidation of silicon gives four electrons that are consumed in the reduction reaction. Experimental results show clearly that the steady value of the OCP is narrowly dependent on the redox potential of the solution components. In solutions containing only HF, alternatively alkaline species, the oxidizing component is simply the proton H+ or the H2O molecule respectively. 

The stability of silicon electrodes contacting an aqueous electrolyte is a severe problem in regenerative solar systems. As mentioned previously, the standard electrode potential of a silicon element is negative enough to induce an electrochemical reaction mechanism, giving rise to an insulating surface silicon oxide in the absence of complexing reactants. On the... 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

The transfer coefficients are the ones determining how the electrode potential influences the electrochemical reaction rate or, in other words, the inclination of the relation between log I and the over-potential, also called the Tafel slope, of a multistep reaction. The coefficients are an important aid when unravelling the electrochemical reaction mechanisms, because the experimentally determined Tafel slope should correspond to the value that is calculated for the postulated sub-step sequence and RDS. 

Finally, to view electrolytic processes as heterogeneous systems does not seem at all sound, according to the description of the electrochemical reaction mechanism given in our introduction. If the first process, in accordance with the given exposition, is the discharge in the electrode boundary surface, and if the second is the separation on the electrode or the... 

In order to understand the manner in which the interfacial region influences the observed kinetics, especially in terms of the theoretical models discussed below, it is clearly important to gain detailed information on the spatial location of the reaction site as well as a knowledge of the mechanistic pathway. Information on the latter for multistep processes can often be obtained by the use of electrochemical perturbation techniques in order to detect reaction intermediates, especially adsorbed species [13]. Various in-situ spectroscopic techniques, especially those that can detect interfacial species such as infrared and Raman spectroscopies, are beginning to be used for this purpose and will undoubtedly contribute greatly to the elucidation of electrochemical reaction mechanisms in the future. 

It is common for homogeneous chemical reactions to accompany the electron-transfer step. Thus, an electrochemical reaction - mechanism may... 

In contrast to aliphatic amines, the anodic oxidation of aromatic amines shows a rather complex reaction pattern. Although extensive studies on the electrochemical reaction mechanism have been carried out, there are very few examples for the application of the anodic oxidation of aromatic amines to organic synthesis. 

Electrochemical Reaction Mechanisms and the Principle of Microscopic Reversihility... 

Williamson 1996), where r is in mol/m s, and Eh is in volts. The Eh-dependence of the rate suggests an electrochemical reaction mechanism rather than one involving site-specific adsorption of oxidants. That the log r versus log m02 plot in Fig 12.20 is linear for a wide range of O2 concentrations... 

Tafel slopes that are not infinite but are substantially greater than 118 mV dec- can be explained by (1) an arbitrary and trivial assumption that P < 1/2 (2) the effect (footnote f) of barrier-layer films such as oxide on Zr02 or Ti0242 (but this is usually only in the case of anodic reactions, particularly those involving valve-metal barrier oxide films) and (3) an electrochemical reaction mechanism where the rds is a chemical step and has a stoichiometric number, v, greater than 2 [refer to Eq. (1)]. This latter possibility will be developed in the next section in terms of a general multistep reaction mechanism. 

---