Surface Oxidation—Reduction Reactions

Surface oxidation-reduction reactions involve electron-transfer processes between surface species. Examples of these reactions are considered in Section 

The surface reactions in reductive dissolution processes are illustrated for a Fe(III) oxyhydroxide solid phase in Eqs. 3.53-3.55, with data shown in Fig. 3.10 for the organic ligand ascorbic acid. Other examples include Mn(IV) or Mn(III) oxyhydroxide solids and ligands, such as quinones, phenols, and inorganic oxyanions. Taking bimessite ( 5-Mn02) and selenite (SeOj ) as a case in point, one can adapt Eq. 3.53 in the form27 

This overall sequence differs a little from Eq. 3.53 (and from the reaction in Eq. 3.46) because two electrons are transferred when Mn(IV) is reduced to Mn(II) while Se(IV) is oxidized to Se(VI) to form selenate (SeO2 ). A similar situation occurs in the oxidation of As(III) to As(V) or Cr(III) to Cr(VI) on Mn oxyhydroxide solids.2728 At fixed proton and water concentrations, the rate law lor the formation of the surface species, =MnIV - OSeOOH, can still be described by Eq. 3.47, but with the three pseudo-rate coefficients, Kf = kf[H+], K, kh[H20, Kj kj H20, the last of which replaces k in Eq. 3.47. 

Equations 3.48 3.51 then can be applied with this minor modification. 

If a clear separation of time scales exists for Eqs. 4.52a and 4.52b, as compared to Eqs. 4.52c-4.52e, then the kinetics of surface oxidation-reduction can be decoupled from those of surface species detachment. For example, if Mn(II) oxidation is negligibly slow, Kb - 0 and Eqs. 4.52a and 4.52b can be solved approximately, as is described in either Section 3.4 in connection with Eqs. 3.48 and 3.49, or Section 4.3 in connection with Eq. 4.30. The first approach applies to constant [HSeO ], whereas the second one requires small deviations of the concentrations of the four chemical species in Eqs. 4.52a and 4.52b from their equilibrium values. 

---

In electroless deposition, the substrate, prepared in the same manner as in electroplating (qv), is immersed in a solution containing the desired film components (see Electroless plating). The solutions generally used contain soluble nickel salts, hypophosphite, and organic compounds, and plating occurs by a spontaneous reduction of the metal ions by the hypophosphite at the substrate surface, which is presumed to catalyze the oxidation—reduction reaction. 

Thus, Experiment 7 involved the same oxidation-reduction reaction but the electron transfer must have occurred locally between individual copper atoms (in the metal) and individual silver ions (in the solution near the metal surface). This local transfer replaces the wire middleman in the cell, which carries electrons from one beaker (where they are released by copper) to the other (where they are accepted by silver ions). 

Many dehydrogenase enzymes catalyze oxidation/reduction reactions with the aid of nicotinamide cofactors. The electrochemical oxidation of nicotinamide adeniiw dinucleotide, NADH, has been studied in depthThe direct oxidation of NADH has been used to determine concentration of ethanol i s-isv, i62) lactate 157,160,162,163) pyTuvate 1 ), glucose-6-phosphate lactate dehydrogenase 159,161) alanine The direct oxidation often entails such complications as electrode surface pretreatment, interferences due to electrode operation at very positive potentials, and electrode fouling due to adsorption. Subsequent reaction of the NADH with peroxidase allows quantitation via the well established Clark electrode. 

A unique CL reagent, /n.v(2,2 -bipyridyl)rut.hcnium(II) [Ru(bpy)32+] for the postcolumn CL reaction, was applied to HPLC detection. The oxidative-reduction reaction scheme of CL from Ru(bpy)32+ is shown in Figure 17. When the production of light following an oxidation of Ru(bpy)32+ to Ru(bpy)33+ at an electrode surface is measured, this CL reaction is termed electrogenerated chemiluminescence (ECL). The CL intensity is directly proportional to the amount of the reduc-tant, that is, the analyte. 

In addition, potentiometric titration methods exist in which an electrode other than an ion-selective electrode is used. A simple platinum wire surface can be used as the indicator electrode when an oxidation-reduction reaction occurs in the titration vessel. An example is the reaction of Ce(IV) with Fe(II) ... 

Various catalytic reactions are known to be structure sensitive as proposed by Boudart and studied by many authors. Examples are the selective hydrogenation of polyunsaturated hydrocarbons, hydrogenolysis of paraffins, and ammonia or Fischer-Tropsch synthesis. Controlled surface reactions such as oxidation-reduction reactions ° or surface organometallic chemistry (SOMC) " are two suitable methods for the synthesis of mono- or bimetallic particles. However, for these techniques. 

There existed oxidation-reduction reactions with the same reaction speed on the sulphide mineral surface in water. One is the self-corrosion of sulphide mineral. Another is the reduction of oxygen. If the equilibrium potential for the anodic reaction and the cathodic reaction are, respectively, E and, and the mineral electrode potential is E, the relationship among them is as follows ... 

Electrochemical Activation. In the electrochemical method, catalytic nuclei of metal M on a noncatalytic surface S may be generated in an electrochemical oxidation-reduction reaction,... 

The principal iron oxides used in catalysis of industrial reactions are magnetite and hematite. Both are semiconductors and can catalyse oxidation/reduction reactions. Owing to their amphoteric properties, they can also be used as acid/base catalysts. The catalysts are used as finely divided powders or as porous solids with a high ratio of surface area to volume. Such catalysts must be durable with a life expectancy of some years. To achieve these requirements, the iron oxide is most frequently dis-... 

Because chlorite is an anion, sorption of chlorite ions onto suspend particles, sediment, or clay surfaces is expected to be limited under enviromnental conditions. Thus, chlorite ions may be mobile in soils and leach into groundwater. However, chlorite (ions or salts) will undergo oxidation-reduction reactions with components in soils, suspend particles, and sediments (e.g., Fe, Mn ions see Section 6.3.2.2). Thus, oxidation-reduction reactions may reduce the concentration of chlorite ions capable of leaching into groundwater. 

A second way of expressing the same information is to give electrode potentials (Table 6-8). Electrode potentials are also important in that their direct measurement sometimes provides an experimental approach to the study of oxidation-reduction reactions within cells. To measure an electrode potential it must be possible to reduce the oxidant of the couple by flow of electrons (Eq. 6-62) from an electrode surface, often of specially prepared platinum. 

Adsorption by activated carbon is commonly employed for the removal of TNT from aq waste streams, eg, pink water formed in shell-loading operations. Low efficiency in regeneration of the carbon for reuse has led to a study of the factors involved (Ref 99), with conclusions as follows. The TNT is adsorbed at many of the numerous high-energy sites on the surface of the carbon. Basic materials, introduced during activation of the carbon by combustion and oxidation and also present at these sites, then induce oxidation-reduction reactions of the methyl with the nitro groups in the TNT. This is... 

Before we review the methods used to determine surface acidity, we wish to define the type of acidity that should be measured. An acid is an electron-pair acceptor. In our opinion, the term acid should be limited to this definition rather than broadening the term to include oxidizing agents as well. We agree with Flockhart and Pink (10) who suggest a clear distinction be made between Lewis acid-Lewis base reactions (which involve coordinate bond formation) and oxidation-reduction reactions (which involve complete transfer of one or more electrons). 

Essential Question How would the surface of the earth change if oxidation-reduction reactions did not occur ... 

The transfer of reactants from the bulk solution to the electrode interface and in the reverse direction is an ordinary feature of all electrode reactions. As the oxidation-reduction reactions advance, the accessibility of the reactant species at the electrode/electrolyte interface changes. This is because of the concentration polarization effect, that is, r c, which arises due to the limited mass transport capabilities of the reactant species toward and from the electrode surface, to substitute the reacted material to sustain the reaction [6,8,10,66,124], This overpotential is usually established by the velocity of reactants flowing toward the electrolyte through the electrodes and the velocity of products flowing away from the electrolyte. The concentration overpotential, r c, due to mass transport restrictions, can be expressed as... 

On the (assumed) much longer time scale over which SeOj and Mn2+ begin to appear in the aqueous-solution phase from the decomposition of = Mn" - 0Se020H, Eqs. 4.52c-4.52e can be solved under an appropriate imposed condition regarding the time variation of [=MnM - 0SeO2OH] based on the surface oxidation-reduction kinetics. For example, under steady-state conditions that yield constant concentrations of the adsorbed and dissolved selenite species, Eqs. 4.52a and 4.52b lead to a constant concentration of adsorbed selenate and therefore a constant rate of selenate detachment from the mineral surface (Eq. 4.52c). If the reasonable assumption is also made that the proton reaction with =MnH - OH equilibrates rapidly, then... 

Stumm, W., Aquatic Surface Chemistry, Wiley, New York, 1987. This edited volume offers advanced reviews of adsorption, surface oxidation-reduction, and mineral dissolution reactions with emphasis on metal oxyhydroxide adsorbents. 

Steps 1 to 3 represent the mechanism by which the reaction proceeds at steady state. If the catalyst is exposed to oxygen for a prolonged period of time, additional adsorption of oxygen represented by step 4 can take place. If such an oxidized catalyst is brought into contact with a reaction mixture, the reaction can proceed via a combination of steps 5 and 3 as well as via steps 1 to 3 depending upon the extent of surface oxidation. Reduction of the surface is represented by step 6. On a reduced surface the reaction can proceed via step 7 as well as via steps 1 to 3 depending upon the extent of reduction of the surface. 

Oxidation/Reduction Reactions. Reactions of chemicals via abiotic oxidation or reduction involve a transfer of electrons and result in a change in oxidation of the state of the product compared to its parent compound. As a general rule, reduction reactions are prevalent in soil sediments, while oxidation reactions are more important in surface waters and in the atmosphere.28... 

It is well-known that the photocatalytic oxidation-reduction reaction of Ti02 irradiated with UV light can decompose organic compounds adsorbed on the surface. This photocatalytic oxidation-reduction reaction originated from the discovery of water-splitting on TiC>2 semiconductor electrodes irradiated with UV light, which is known as the Honda-Fujishima effect and was reported in Nature in 1972 [1], This effect attracted considerable attention after the first oil crisis. 

Experimental Observations. The nearest we have come to demonstrating the existence of N—F ions is in the electrochemical oxidation-reduction reactions of HNF2. The oxidation has been carried out in water and in various polar organic solvents under acid conditions (15). The first step of this reaction is formation of the NF2 radical. The NF2 radical undergoes combination processes on the surface of the electrode rather than diffusing into the body of the solution before being involved in further reactions. The combination process on the electrode surface has been used to prepare various NF2 compounds by simultaneously generating other radical species—e.g.,... 

It is known that during the immersion of PS into the CUSO4 + HF bath, oxidation-reduction reactions between copper ions and silicon atoms from the silicon skeleton of the PS layer can occur [2,4]. This is conditioned by the high redox potential of Cu ions. Copper ions are reduced and Cu deposited via the electron exchange with silicon which is oxidized and dissolved in the fluoride-containing solution. The most important observation from this study for PS of 55% porosity is the crystalline structure of the deposited Cu grains very well faceted Cu crystals are formed at the PS surface and small Cu crystals are within the pore channels. 

---