Adsorption electron charge-transfer process

Recently, Kisza et al. [203] found that the total electrode reaction can be interpreted by a two-step two-electron charge transfer process with an intermediate adsorption ... 

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]. 

Another quantity that has to be taken into consideration is the density of states at the Fermi level, g E, and any alterations caused to it by the charge transfer process. The importance of g E on the adsorptive and catalytic properties of a metal surface has been stated by some investigators [131-133]. More specifically, the density of states defines the ability of the surface to respond to the presence of an adsorbate [132]. Theoretical calculations for the density of states function, g E), have been reported in the literature for certain metals [134]. The g E) function for the d metals Ru, Rh, and Pd is characterized by the participation of the d electrons. All three metals have a high density of states at the Fermi level (1.13 for Ru, 1.35 for Rh,... 

When the electroactive species or an intermediate adsorbs on the electrode surface, the adsorption process usually becomes an integral part of the charge transfer process and therefore cannot be studied without the interference of a faradaic current. In this situation, surface coverages cannot be measured directly and the role of an adsorbate must be inferred from a kinetic investigation. Tafel slopes and reaction orders will deviate substantially from those for a simple electron transfer process when an adsorbed intermediate is involved. Moreover the kinetic parameters, exchange current or standard rate constant, are likely to become functions of the electrode material and even the final products may change. These factors will be discussed further in the section on electrocatalysis (Section 1.4). 

The motivation behind the Symposium on Electron Spectroscopy and STM-AFM Analysis of the Solid-Liquid Electrochemical Interface was to assemble in one place some major players in electrochemical surface science. The obvious rationale was that such a gathering would help distill and focus future work to issues deemed most critical to further progress in the area. The processes that were discussed at the symposium included electrodeposition and electrocrystallization, passivation of metals and alloys, anodic dissolution of metals and semiconductors, oxidation of small molecules, assembly of semiconducting layers, hydrogen adsorption, and charge transfer at surface-modified electrodes. 

In the equivalent circuit analog, resistors represent conductive pathways for ion and electron transfer. As such, they represent the bulk resistance of a material to charge transport such as the resistance of the electrolyte to ion transport or the resistance of a conductor to electron transport. Resistors are also used to represent the resistance to the charge-transfer process at the electrode surface. Capacitors and inductors are associated with space-charge polarization regions, such as the electrochemical double layer, and adsorption/ desorption processes at an electrode, respectively. 

By analyzing the variation of peak position as a function of a scan rate, it is possible to gain an estimate for the electron transfer rate constants. Adsorption processes on electrode surface can be distinguished from charge transfer processes, as in former case cyclic voltammogram is symmetrical around potential axes. 

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