Redox reaction interface

The large surface area of the nanomaterials increases the redox reaction interface. 

The potential of a metallic electrode is determined by the position of a redox reaction at the electrode-solution interface. Three types of metallic electrodes are commonly used in potentiometry, each of which is considered in the following discussion. 

Surface films are formed by corrosion on practically all commercial metals and consist of solid corrosion products (see area II in Fig. 2-2). It is essential for the protective action of these surface films that they be sufficiently thick and homogeneous to sustain the transport of the reaction products between metal and medium. With ferrous materials and many other metals, the surface films have a considerably higher conductivity for electrons than for ions. Thus the cathodic redox reaction according to Eq. (2-9) is considerably less restricted than it is by the transport of metal ions. The location of the cathodic partial reaction is not only the interface between the metal and the medium but also the interface between the film and medium, in which the reaction product OH is formed on the surface film and raises the pH. With most metals this reduces the solubility of the surface film (i.e., the passive state is stabilized). 

Friedrich et al. also used XPS to investigate the mechanisms responsible for adhesion between evaporated metal films and polymer substrates [28]. They suggested that the products formed at the metal/polymer interface were determined by redox reactions occurring between the metal and polymer. In particular, it was shown that carbonyl groups in polymers could react with chromium. Thus, a layer of chromium that was 0.4 nm in thickness decreased the carbonyl content on the surface of polyethylene terephthalate (PET) or polymethylmethacrylate (PMMA) by about 8% but decreased the carbonyl content on the surface of polycarbonate (PC) by 77%. The C(ls) and 0(ls) spectra of PC before and after evaporation of chromium onto the surface are shown in Fig. 22. Before evaporation of chromium, the C(ls) spectra consisted of two components near 284.6 eV that were assigned to carbon atoms in the benzene rings and in the methyl groups. Two additional... 

Thus in all corrosion reactions one (or more) of the reaction products will be an oxidised form of the metal, aquo cations (e.g. Fe (aq.), Fe (aq.)), aquo anions (e.g. HFeO aq.), Fe04"(aq.)), or solid compounds (e.g. Fe(OH)2, Fej04, Fe3 04-H2 0, Fe203-H20), while the other reaction product (or products) will be the reduced form of the non-metal. Corrosion may be regarded, therefore, as a heterogeneous redox reaction at a metal/non-metal interface in which the metal is oxidised and the non-metal is reduced. In the interaction of a metal with a specific non-metal (or non-metals) under specific environmental conditions, the chemical nature of the non-metal, the chemical and physical properties of the reaction products, and the environmental conditions (temperature, pressure, velocity, viscosity, etc.) will clearly be important in determining the form, extent and rate of the reaction. 

When the area A of the eleetrode/solution interface with a redox system in the solution varies (e.g. when using a streaming mercury electrode), the double layer capacity which is proportional to A, varies too. The corresponding double layer eharging current has to be supplied at open eireuit eonditions by the Faradaic current of the redox reaction. The associated overpotential can be measured with respect to a reference electrode. By measuring the overpotential at different capaeitive eurrent densities (i.e. Faradaic current densities) the current density vs. eleetrode potential relationship can be determined, subsequently kinetic data can be obtained [65Del3]. (Data obtained with this method are labelled OC.)... 

Passive electrodes serve only to conduct electrons to and from the interfaces. They do not take part in the redox reactions. 

In electrocatalysis, the major subject are redox reactions occurring on inert, nonconsumable electrodes and involving substances dissolved in the electrolyte while there is no stoichiometric involvement of the electrode material. Electrocatalytic processes and phenomena are basically studied in aqueous solutions at temperatures not exceeding 120 to 150°C. Yet electrocatalytic problems sometimes emerge as well in high-temperature systems at interfaces with solid or molten electrolytes. 

Heterogeneous ET reactions at polarizable liquid-liquid interfaces have been mainly approached from current potential relationships. In this respect, a rather important issue is to minimize the contribution of ion-transfer reactions to the current responses associated with the ET step. This requirement has been recognized by several authors [43,62,67-72]. Firstly, reactants and products should remain in their respective phases within the potential range where the ET process takes place. In addition to redox stability, the supporting electrolytes should also provide an appropriate potential window for the redox reaction. According to Eqs. (2) and (3), the redox potentials of the species involved in the ET should match in a way that the formal electron-transfer potential occurs within the potential window established by the transfer of the ionic species present at the liquid-liquid junction. The results shown in Figs. 1 and 2 provide an example of voltammetric ET responses when the above conditions are fulfilled. A difference of approximately 150 mV is observed between Ao et A" (.+. ... 

DCE interface in the presence of TPBCl [43,82]. The accumulation of products of the redox reactions were followed by spectrophotometry in situ, and quantitative relationships were obtained between the accumulation of products and the charge transfer across the interface. These results confirmed the higher stability of this anion in comparison to TPB . It was also reported that the redox potential of TPBCP is 0.51V more positive than (see Fig. 3). However, the redox stability of the chlorinated derivative of tetra-phenylborate is not sufficient in the presence of highly reactive species such as photoex-cited water-soluble porphyrins. Fermin et al. have shown that TPBCP can be oxidized by adsorbed zinc tetrakis-(carboxyphenyl)porphyrin at the water-DCE interface under illumination [50]. Under these conditions, the fully fluorinated derivative TPFB has proved to be extremely stable and consequently ideal for photoinduced ET studies [49,83]. Another anion which exhibits high redox stability is PFg- however, its solubility in the water phase restricts the positive end of the ideally polarizable window to < —0.2V [85]. 

The final internal boundary condition applies to the interface between phase 1 and phase 2, and relates the flux of species Red] and Red2, at the ITIES, to the rate of the second-order redox reaction occurring at the interface. 

It is well known that the selective transport of ions through a mitochondrial inner membrane is attained when the oxygen supplied by the respiration oxidizes glycolysis products in mitochondria with the aid of such substances as flavin mononucleotide (FMN), fi-nicotinamide adenine dinucleotide (NADH), and quinone (Q) derivatives [1-3]. The energy that enables ion transport has been attributed to that supplied by electron transport through the membrane due to a redox reaction occurring at the aqueous-membrane interface accompanied by respiration [1-5],... 

III. SELECTIVE ION TRANSFER AT THE W/0 INTERFACE COUPLED WITH REDOX REACTIONS BETWEEN FLAVIN MONONUCLEOTIDE IN W AND A FERROCENE DERIVATIVE IN O AND CO2 EVOLUTION [19,21]... 

In this section, redox reactions between NADH in an aqueous solution (W) and Q in an organic solution (O) at the W/O interface are investigated as a function of potential differences between W and O, w/o- The ion transfer at the W/O interface coupled with the redox reaction is also discussed. 

In order to investigate the redox reaction between NADH in W and Q in 1,2-dichlor-oethane (DCE), a current-scan polarogram was recorded at the interface of W and DCE, for which the compositions were as in Eq. (10), by scanning the current applied between W and O and measuring iiw/o- The Q employed in this study was chloranil (CQ) ... 

When toluquinone (TQ) was employed instead of CQ, any special currents other than the residual current were not observed as shown by curve 2 in Fig. 5. The difference in polarographic behaviors between TQ and CQ is attributable to the difference between the standard redox potential of the TQ/TQ couple, iiTQ/TQ- > and that of the CQ/CQ couple, E cq/cq- > in DCE since the potential range available for the appearance of polarographic wave due to the redox reaction at the W/O interface as in Eq. (11) depends strongly on the difference between the standard redox potential of the 01(W)/R1(W)... 

V. REDOX REACTIONS BETWEEN OXYGEN IN W AND A HYDROQUINONE DERIVATIVE IN 0 AT THE W/0 INTERFACE [23]... 

In the present section, in order to elucidate the essential part of chemistry in the respiration, the redox reaction between O2 in W and QH2 in O was investigated by adopting QH2 the structure and chemical property of which is well known, and the proton transfer at the W/O interface accompanied by the redox reaction was elucidated. The transfer of various ions other than protons coupled with the redox reactions was also discussed. 

The redox reaction between O2 in W and QH2 in DCE was investigated by current-scan polarography at the W/DCE interface by using a cell system as Eq. (17). The QH2 adopted was tetrachlorohydroquinone (CQH2) ... 

C. Redox Reaction Between H2O2 in W and CQH2 in DCE at the W/DCE interface... 

The redox reaction between O2 in W and CQEI2 in DCE controlled by (or coupled with) an ion transfer reaction at the interface was investigated by shaking W with DCE for 4h. 

In order to clarify the reason for the coupling of the redox reaction between O2 and CQH2 with the ion transfer at the W/DCE interface in system of Eq. (25), current-scan polarograms for ion transfers at the W/DCE interface (cf. curves 3 to 5 in Fig. 5) were compared with that for the interfacial redox reaction (cf. curve 1 in Fig. 8). From the comparison, it is clear that transfers of TPenA" " and TBA+ from W to DCE proceed at potentials in Range A where the polarographic wave due to the redox reaction... 

Proposed intermediates in the above reaction include atomic hydrogen [27, 28], hydride ions [29, 30], metal hydroxides [31], metaphosphites [32, 33], and excitons [34]. In general, the postulated mechanisms are not supported by direct independent evidence for these intermediates. Some authors [35] maintain that the mechanism is entirely electrochemical (i.e. it is controlled by electron transfer across the metal-electrolyte interface), but others [26] advocate a process involving a surface-catalyzed redox reaction without interfacial electron transfer. 

Metal/metal oxides are the materials of choice for construction of all-solid-state pH microelectrodes. A further understanding of pH sensing mechanisms for metal/metal oxide electrodes will have a significant impact on sensor development. This will help in understanding which factors control Nemstian responses and how to reduce interference of the potentiometric detection of pH by redox reactions at the metal-metal oxide interface. While glass pH electrodes will remain as a gold standard for many applications, all-solid-state pH sensors, especially those that are metal/metal oxide-based microelectrodes, will continue to make potentiometric in-vivo pH determination an attractive analytical method in the future. 

Photosystem I is a membrane pigment-protein complex in green plants, algae as well as cyanobacteria, and undergoes redox reactions by using the electrons transferred from photosystem II (PS II) [1], These membrane proteins are considered to be especially interesting in the study of monomolecular assemblies, because their structure contains hydrophilic area that can interact with the subphase as well as hydrophobic domains that can interact either with each other or with detergent and lipids [2], Moreover, studies with such proteins directly at the air-water interface are expected to be a valuable approach for their two-dimensional crystallization. 

In this chapter we introduce and discuss a number of concepts that are commonly used in the electrochemical literature and in the remainder of this book. In particular we will illuminate the relation of electrochemical concepts to those used in related disciplines. Electrochemistry has much in common with surface science, which is the study of solid surfaces in contact with a gas phase or, more commonly, with ultra-high vacuum (uhv). A number of surface science techniques has been applied to electrochemical interfaces with great success. Conversely, surface scientists have become attracted to electrochemistry because the electrode charge (or equivalently the potential) is a useful variable which cannot be well controlled for surfaces in uhv. This has led to a laudable attempt to use similar terminologies for these two related sciences, and to introduce the concepts of the absolute scale of electrochemical potentials and the Fermi level of a redox reaction into electrochemistry. Unfortunately, there is some confusion of these terms in the literature, even though they are quite simple. 

The thermodynamic feasibility of redox reactions at the semiconductor-electrolyte interface can be assessed from thermodynamic considerations. Since typical redox potentials for many redox couples encountered in electrolytes of natural or technical systems often lie between the band potentials of typical semiconductors, many electron transfer reactions are (thermodynamically) feasible (Pichat and Fox, 1988). With the right choice of semiconductor material and pH the redox potential of the cb can be varied from 0.5 to 1.5 V and that of the vb from 1 to more than 3.5 V (see Fig. 10.4). 

Segal, M. G., and R. Sellers (1984), "Redox Reactions at Solid-Liquid Interfaces", Advances in Inorganic and Bioinorganic Mechanisms 3, 97-129. 

---