Interfacial electron-transfer rates

Leger C, Jones AK, Albracht SPJ, Armstrong FAA. 2002. Effect of a dispersion of interfacial electron transfer rates on steady state catalytic electron transport in [NiFe]-hydrogenase and other enzymes. J Phys Chem B 106 13058-13063. 

Interfacial electron transfer is the critical process occurring in all electrochemical cells in which molecular species are oxidized or reduced. While transfer of an electron between an electrode and a solvated molecule or ion is conceptually a simple reaction, rates of heterogeneous electron transfer processes depend on a multitude of factors and can vary over many orders of magnitude. Since control of interfacial electron transfer rates is usually essential for successful operation of electrochemical devices, understanding the kinetics of these reactions has been and remains a challenging and technologically important goal. 

Frechet and co-workers [32] studied the ability of the dendrimer shell to provide site isolation of the core porphyrin moiety, using benzene-terminated dendrimers Zn[G-n]4P (i.e. 6). From the cyclic voltammograms in CH2C12, the interfacial electron transfer rate between the porphyrin core and the electrode surface decreased with increasing dendrimer generation. However, small molecules like benzyl viologen could still penetrate the shell of 6 to access the porphyrin core as observed from the quenching of porphyrin fluorescence. Their results also revealed that the dendritic shell did not interfere electrochemically or photochemically with the porphyrin core moiety. 

Fig. 16.3 Quantum yield (QY) for electron and hole transfer to solution redox acceptors/donors as a function of the reduced variables y (related to the surface properties of the catalyst, i.e., ratio between interfacial electron transfer rate and surface recombination rate) and w (related to the ratio between surface migration currents of hole and electrons to the rate of bulk recombination), according to the proposed kinetic model [23],...

B. Effects of specific interfacial area of a microdroplet and liquid-junction potential on the interfacial electron transfer rate 188... 

C. Characteristic micrometer size dependence of the interfacial electron transfer rate 191... 

B. Effects of Specific Interfacial Area of a Microdroplet and Liquid-Junction Potential on the Interfacial Electron Transfer Rate... 

Equation 10 represents a simple relation useful for predicting interfacial electron transfer rates relevant to dye sensitization. At this time more detailed descriptions... 

Willig and co-workers used near-infrared spectroscopy to measure excited-state interfacial electron transfer rates after pulsed light excitation of cis-Ru(dcb)2(NCS)2-Ti02 in vacuum from 20 to 295 K [208]. They reported that excited-state electron injection occurred in less than 25 fs, prior to the redistribution of the excited-state vibrational energy, and that the classical Gerischer model for electron injection was inappropriate for this process. They concluded that the injection reaction is controlled by the electronic tunneling barrier and by the escape of the initially prepared wave packet describing the hot electron from the reaction distance of the oxidized dye molecule. It appears that some sensitizer decomposition occurred in these studies as the transient spectrum was reported to be similar to that of the thermal oxidation product of m-Ru(dcb)2(NCS)2. 

Several reports have addressed how interfacial electron transfer rate constants vary with thermodynamic driving force. The driving force is tuned by manipulating the conduction band edge through adsorption of specific cations, utilizing different semiconductors, or by keeping the semiconductor constant with a series of sensitizers with known formal potentials. A difficulty in these studies is that the position... 

Alternatively, the dynamics of trapping and detrapping of electrons localized in intra-bandgap states can control the overall reaction kinetics, which would not depend upon the sole interfacial electron transfer rate. 

The interactions with the surface and reaction kinetics have been studied in detail using various techniques, such as voltammetry, electrorefiectance measurements and surface enhanced Raman spectroscopy [139,140]. For monolayers on gold assembled from long chain thiols of the structure HS-(CH2) -C00H (with n > 9) the interfacial electron transfer rate exponentially decreases with chain length and the tunnelling parameter is in the... 

The rest of this chapter is organised as follows experimental techniques for measuring interfacial electron-transfer rates are described in Section 11.2, and Section 11.3 summarises theoretical descriptions and current experimental results. 

The highest interfacial electron transfer rate constant yet reported (about 14,000 s ) is for a c-type cytochrome from Aquifex aolicus This protein has a 62-amino acid linker domain by which it is usually anchored to the periplasmic side of the inner membrane this linker has a cysteine as the terminal residue before the signal region, and the sulfur atom provides an anchor point. The cytochrome adsorbs strongly onto a Au electrode that is already modified with a hexane-thiol SAM (note this requires that the molecules in the SAM move or vacate to allow this). The results are striking. 

Figure 4-12. Catalytic voltammetry of Paracoccus pantotrophus nitrate reductase (NarGH) adsorbed as a film on a PGE electrode at pH 6. (A) Increasing the electrode rotation rate from 0 to 3000 rpm removes the mass transport limitation of the catalytic response in 50 pM NO3 . (B) The enzyme s greater rate of chlorate reduction compared to nitrate reduction is reflected in greater distortion of the waveform through dispersion of sluggish interfacial electron transfer rates (see also Fig. 4-4C). Scan rate 10 mV s. Adapted from ref. 64. with permission.

There is a significant contrast here with Section 5.4.2(e), where we found that the results for reversible systems observed at spherical electrodes could be extended generally to electrodes of other shapes. This is true for a reversible system because the potential controls the surface concentration of the electroactive species directly and keeps it uniform across the surface. Mass transfer to each point, and hence the current, is consequently driven in a uniform way over the electrode surface. For quasireversible and irreversible systems, the potential controls rate constants, rather than surface concentrations, uniformly across the surface. The concentrations become defined indirectly by the local balance of interfacial electron-transfer rates and mass-transfer rates. When the electrode surface is not uniformly accessible, this balance varies over the surface in a way that is idiosyncratic to the geometry. This is a complicated situation that can be handled in a general way (i.e., for an arbitrary shape) by simulation. For UME disks, however, the geometric problem can be simplified by symmetry, and results exist in the literature to facilitate the quantitative analysis of voltammograms (12). 

A. Why Measure Fast Interfacial Electron Transfer Rate Constants And How 103... 

J. Moser, S. Punchihewa, P. P. Infelta, and M. Gratzel, 1991. Surface complexation of colloidal semiconductors strongly enhances interfacial electron-transfer rates. Langmuir 7, 3012—3018... 

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