Reaction thermodynamics with soluble reductants

Equilibria involving reductive dissolution reactions add to the complexity of mineral solubility phenomena in just the way that pE-pH diagrams are more complicated than ordinary predominance diagrams, like that in Fig. 3.7. The electron activity or pE value becomes one of the master variables whose influence on dissolution reactions must be evaluated in tandem with other intensive master variables, like pH or p(H4Si04). Moreover, the status of microbial catalysis under the suboxic conditions that facilitate changes in the oxidation states of transition metals has to be considered in formulating a thermodynamic description of reductive dissolution. This consideration is connected closely to the existence of labile organic matter and, in some cases, to the availability of photons.26... 

From an engineering standpoint, the kinetics or rate of corrosion of a system is usually of primary importance. Corroding systems are not in equilibrium and therefore thermodynamic calculations cannot be applied. For metal corrosion to occur, an oxidation reaction (generally metal dissolution or oxide formation) and a cathodic reduction reaction (such as oxygen reduction) proceed simultaneously. In most normal water environments, the overall reaction for aluminium corrosion is reaction with water to form aluminium hydroxide and hydrogen. The aluminium hydroxide has very low solubility in water and precipitates as bayerite or boehmite, depending on the temperature of the water [2.8]. 

In contrast, when boron trifluoride etherate is substituted for the free boron trifluoride, only a trace of the hydrocarbon is formed, even after weeks of reaction.143 The unique effectiveness of boron trifluoride gas in promoting these reductions is believed to be due to several factors, including the ability of the coordinatively unsaturated boron center to rapidly and tightly coordinate with oxygen centers and to the thermodynamically favorable creation of a Si-F bond.1 A slight pressure of boron trifluoride gas must be maintained over the surface of the solution throughout the reaction because boron trifluoride has only limited solubility in the weakly coordinating dichloromethane solvent. 

Studies in this laboratory (69) of the water soluble ferri-heme model Fem(TPPS) in aqueous solution have shown that this species also undergoes reductive nitrosylation in solutions that are moderately acidic (pH 4-6) (Eq. (32)). The rate of this reaction includes a buffer dependent term indicating that the reaction of the Fem(TPPS)(NO) complex with H20 is subject to general base catalysis. The reaction depicted in Eq. (33) is not observable at pH values < 3, since the half-cell reduction potential for the nitrite anion (Eq. (1)) is pH dependent, and Eq. (33) is no longer thermodynamically favorable. 

Besides the effect of the electrode materials discussed above, each nonaqueous solution has its own inherent electrochemical stability which relates to the possible oxidation and reduction processes of the solvent,the salts, and contaminants that may be unavoidably present in polar aprotic solutions. These may include trace water, oxygen, CO, C02 protic precursor of the solvent, peroxides, etc. All of these substances, even in trace amounts, may influence the stability of these systems and, hence, their electrochemical windows. Possible electroreactions of a variety of solvents, salts, and additives are described and discussed in detail in Chapter 3. However, these reactions may depend very strongly on the cation of the electrolyte. The type of cation present determines both the thermodynamics and kinetics of the reduction processes in polar aprotic systems [59], In addition, the solubility product of solvent/salt anion/contaminant reduction products that are anions or anion radicals, with the cation, determine the possibility of surface film formation, electrode passivation, etc. For instance, as discussed in Chapter 4, the reduction of solvents such as ethers, esters, and alkyl carbonates differs considerably in Li or in tetraalkyl ammonium salt solutions [6], In the presence of the former cation, the above solvents are reduced to insoluble Li salts that passivate the electrodes due to the formation of stable surface layers. However, when the cation is TBA, all the reduction products of the above solvents are soluble. 

The primary challenge in commercialization of MCFC remains in the proper selection of materials for the cathode. The life expectancy of the electrode structure is aimed toward 40,000 hr for successful commercialization of MCFC. The following cathode properties were recognized as of fundamental importance with respect to the cell performance 1) high electronic conductivity at 650°C (cr > 1 S/cm) 2) low chemical reactivity and solubility in the electrolyte 3) thermodynamic stability at 650°C in carbonate electrolyte at different partial pressures of O2/CO2 mixtures 4) high electrocatalytic activity for the oxygen reduction reaction and 5) suitability for the fabrication of porous electrodes. ... 

The first example shows the synthesis of a C-C-bridged bis-macrocycle (Figure 2) [7]. The preparation of 1 is a condensation of a polyamine with a malonic ester derivative, in analogy to the procedure developed by Tabushi et al [8]. The tetraamide 1 is so insoluble that it precipitates and can be obtained practically pure from the reaction mixture. Its reduction to the octaamine takes place if the reaction with is done in diglyme (bis-(2-methoxyethyl)ether), in which 1 is partially soluble. The product 2 is an ideal ditopic ligand,since it has all the typical properties of 1,4,8,11-tetraazacyclotetradecane (cyclam), i. e. the thermodynamic and kinetical stability of its complexes and the C-C linkage between the two macrocyclic subunits does not reduce the coordination tendency of the amine nitrogens. 

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