Surface Electron Transfer

Let us see now what happens in a similar linear scan voltammetric experiment, but utilizing a stirred solution. Under these conditions, the bulk concentration (C0(b, t)) is maintained at a distance S by the stilling. It is not influenced by the surface electron transfer reaction (as long as the ratio of electrode area to solution volume is small). The slope of the concentration-distance profile [(CQ(b, t) — Co(0, /))/r)] is thus determined solely by the change in the surface concentration (Co(0, /)). Hence, the decrease in Co(0, t) duiing the potential scan (around E°) results in a sharp rise in the current. When a potential more negative than E by 118 mV is reached, Co(0, t) approaches zero, and a limiting current (if) is achieved ... 

Hyperthermal surface ionisation (HSI) is an ultrasensitive tuneable selective ion source [222,223] which is based on the very effective ionisation of various hyperthermal molecules upon their scattering from a surface with a high work function, such as rhenium oxide. Molecule-surface electron transfer constitutes the major and most important HSI mechanism for GC-MS. 

This is another indication of the large potential sensitivity of the CV and LSV methods. Equation (2.46) shows that, in order to increase the measurable area under the voltammogram, the scan rate need only be increased. This will be applicable so long as the kinetics of the surface electron transfer reaction are sufficiently fast. 

In the simplest catalytic reaction scheme (Figure 2.16) a fast and reversible couple, P/Q serves as catalyst (mediator) for the reduction (taken as an example, transposition to oxidation being straightforward) of the substrate A. Instead of taking place at the electrode surface, electron transfer to A occurs via... 

OXftmik of solution) > (electrode surface) MASS TRANSPORT O felectrode surface) "t" riQ, - - (electrode surface) ELECTRON TRANSFER... 

Examination of the behaviour of a dilute solution of the substrate at a small electrode is a preliminary step towards electrochemical transformation of an organic compound. The electrode potential is swept in a linear fashion and the current recorded. This experiment shows the potential range where the substrate is electroactive and information about the mechanism of the electrochemical process can be deduced from the shape of the voltammetric response curve [44]. Substrate concentrations of the order of 10 molar are used with electrodes of area 0.2 cm or less and a supporting electrolyte concentration around 0.1 molar. As the electrode potential is swept through the electroactive region, a current response of the order of microamperes is seen. The response rises and eventually reaches a maximum value. At such low substrate concentration, the rate of the surface electron transfer process eventually becomes limited by the rate of diffusion of substrate towards the electrode. The counter electrode is placed in the same reaction vessel. At these low concentrations, products formed at the counter electrode do not interfere with the working electrode process. The potential of the working electrode is controlled relative to a reference electrode. For most work, even in aprotic solvents, the reference electrode is the aqueous saturated calomel electrode. Quoted reaction potentials then include the liquid junction potential. A reference electrode, which uses the same solvent as the main electrochemical cell, is used when mechanistic conclusions are to be drawn from the experimental results. 

Miller RDJ, Mclendon G, Nojik AJ, Schmickler W, Willing F (1995) Surface electron transfer Processes, VCH Publishers, New York... 

RJ.D. Miller, G.L. McLendon, A.J. Noszik, W. Schmickler, F. Willig Surface Electron Transfer Processes, VCH, New York, 1995. 

R. J. Dewayne Miller, G. McLendon, A. J. Nozik, W. Schmickler, and Frank Willig, Surface Electron Transfer, VCH Publishers, New York (1995). Advanced discussions. 

Fig. 9.17. Schematic diagram of the potential energy surfaces for an electron transfer reaction the generalized coordinates xand y correspond to inner and outer sphere modes, respectively. (Reprinted from R. J. D. Miller, G. L. McLendon, A. J. Nozik, W. Schnickle, and F. Willig, Surface Electron Transfer Processes, p. 58, copyright 1995 VCH-Wiley. Reprinted by permission of John Wiley Sons, Inc.)...

Let us consider the surface electron-transfer reaction occurring at a single electrode maintained at a fixed potential, where both O and R are soluble and the reaction shown is rate-controlling (i.e., no other processes limit this heterogeneous reaction) ... 

When the size of the particle decreases, the electron-hole separation is expected to be inefficient. As has been postulated in the 2D ladder reaction model stated above, the recombination rate is reciprocally proportional to the volume of the particle. That is, the recombination rate is proportional to the probability at which an electron encounters a hole. Since the surface electron transfer reaction competes with the recombination reaction, the transfer efficiency decreases with the decrease of the size of the particle, if the rate of the surface electron transfer does not change significant . ... 

The underlying motivation of the work presented in this paper is to provide a theoretical understanding of basic physical and chemical properties and processes of relevance in photoelectrochemical devices based on nanostructured transition metal oxides. In this context, fundamental problems concerning the binding of adsorbed molecules to complex surfaces, electron transfer between adsorbate and solid, effects of intercalated ions and defects on electronic and geometric structure, etc., must be addressed, as well as methodological aspects, such as efficiency and reliability of different computational schemes, cluster models versus periodic ones, etc.. 

Electron transfer processes are at the heart of electrochemistry, and often the focus is on events at electrode surfaces. While the theory for electron-transfer in solution [94], and at metal surfaces [95] is rather extensive, a comprehensive theory for electron transfer at metal oxide-organic interfaces [96, 97] is still under development. This section is devoted to a discussion of some of the key elements of the surface electron transfer in dye-sensitized solar cells, illustrated by results from recent calculations. 

Both the cluster and the periodic calculations indicate a similarity to the Newns-Anderson model for metal adsorbates, in that both energy shifts, and broadenings need to be included in models of electron transfer, as shown schematically in Fig. 13. It will be a challenge in the near future to incorporate the increasingly accurate calculations of the crucial electronic coupling-strength parameter in existing dynamical models of the surface electron transfer processes. 

The implications of the obtained structural and electronic information on the binding and the surface electron transfer models in dye-sensitized solar cells have been discussed. Calculated strong binding, and strong electronic surface-adsorbate interactions, is consistent with experimentally observed ultrafast photoinjection processes in stable dye-sensitized electrochemical devices. It will be important, however, to combine results from explicit calculations of... 

The data in Figure 7.13 show reductive-dissolution kinetics of various Mn-oxide minerals as discussed above. These data obey pseudo first-order reaction kinetics and the various manganese-oxides exhibit different stability. Mechanistic interpretation of the pseudo first-order plots is difficult because reductive dissolution is a complex process. It involves many elementary reactions, including formation of a Mn-oxide-H202 complex, a surface electron-transfer process, and a dissolution process. Therefore, the fact that such reactions appear to obey pseudo first-order reaction kinetics reveals little about the mechanisms of the process. In nature, reductive dissolution of manganese is most likely catalyzed by microbes and may need a few minutes to hours to reach completion. The abiotic reductive-dissolution data presented in Figure 7.13 may have relative meaning with respect to nature, but this would need experimental verification. 

Oxidative dissolution of metal-sulfides (e.g., pyrite, FeS2) is a complex process involving surface adsorption of the oxidant (Fe3+, 02), surface electron transfer, and surface product formation and detachment. The overall oxidation process, without considering the detailed mechanisms, is demonstrated below using pyrite (FeS2) (Evangelou, 1995b) ... 

The activation energy is least when the CO bond is weakened because of its interaction with the metal surface. Electron transfer from the metal surface into antibonding molecular orbitals weakens the CO bond. This is a very general feature of dissociative adsorption. The dissociation energy of adsorbed molecules decreases when they are adsorbed with considerable electron transfer from the metal surface to the adsorbate. 

In contrast to the aforementioned systems, relatively low translational energy thresholds for fragmentation are observed in the scattering of H2, N2+, and O2+ on Ni(lll), O2+ on Ag(lll), ° H2+ on Cu(lll), and OCS+ on Ag(lll). In each of these systems, a dissociative neutralizar tion mechanism has been assigned. Dissociative neutralization occurs when a surface electron transfers to an incident cation, whereupon a repulsive... 

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