Reduction potentials, methane

A calculation of the temperature dependence of the free energy for the reactions in Eqs. (15)-(18), and hence the electrochemical potential, showed that with an increase in temperature, formic acid formation became more unfavorable.4 In the case of formaldehyde, methanol, and methane formation, the calculation indicated a positive shift in the reduction potential, but of very small magnitude ca. 30 mV for a temperature change from 300 to 500 K, and ca. 20 mV from 500 to 1200 K.4... 

Table 6. (Fluoren-9-ylidene)methane derivatives as probases Reduction potentials...

Values of kp emerging from these studies indicate that the (fluoren-9-ylidene)-methane dianions will deprotonate acids in the range pK, 1117 at useful rates. Furthermore there is an indication (Fig. 4) that the kinetic basicity of such EGB s may be roughly correlated with the second reduction potential, E (2). This possible correlation needs further rigorous examination as more reliable kinetic information is obtained the k values used in Fig. 4 are not corrected for the competition from reproportionation. 

Fig. 4. Plot of log kp vs second reduction potential for fluoren-9-ylidene-methane probases listed in Table 6. (Numbers against aberrant points correspond to entries in Table 6)...

Table 9.3 Standard Transformed Gibbs Energies (in kJ moE ) of Reactions and Standard Apparent Reduction Potentials (in volts) at 289.15 K, 1 bar, pH 7, and Ionic Strength 0.25 M for Reactions Involved in the Methane Monooxygenase Reaction...

The third largest class of enzymes is the oxidoreductases, which transfer electrons. Oxidoreductase reactions are different from other reactions in that they can be divided into two or more half reactions. Usually there are only two half reactions, but the methane monooxygenase reaction can be divided into three "half reactions." Each chemical half reaction makes an independent contribution to the equilibrium constant E for a chemical redox reaction. For chemical reactions the standard reduction potentials ° can be determined for half reactions by using electrochemical cells, and these measurements have provided most of the information on standard chemical thermodynamic properties of ions. This research has been restricted to rather simple reactions for which electrode reactions are reversible on platinized platinum or other metal electrodes. 

In multielectron transfer processes, the reduction of CO2 can yield formic acid, carbon monoxide, formaldehyde, methanol, or methane that is, the primary electrochemical process supplies Ci compounds. These reactions can proceed at reasonable reduction potentials between —0.24 and —0.61 V (NHE) (Equations (6.12-6.16) the reduction potentials, E°, refer to pH 7 in aqueous solutions versus NHE), while the formation of the C02 radical anion is estimated to take place at —2.1 V.104 Reduction of CO (in the presence of H + ) supplies CH2" radicals that may yield methane directly or leads to higher hydrocarbons (e.g., ethene or ethane) by recombination.24,105 Efficient formation of ethene (together... 

Another nuclear heat application system that offers reduction potential is the conversion of coal by gasification to other synthetic fuels. The gasification process selected is dependent on what type of fuel is desired. Hydro-gasification is appropriate for H2 or CH4 production, whereas partial oxidation and steam gasification are better suited for methanol production. The latter process has been proposed for a nuclear coal reforming system with methane reformer and steam gasifier based on a 450 MW(th) HTGR. With an input of 93 t/h of water and 34.5 t/h of coal and 27,490 Nm /h of methane, the methanol production rate would be 101 t/h plus 68 MW(e) electricity [31]. 

Wetlands and rice paddies are major sources of methane and emit approximately up to 50% of annual methane to the atmosphere. Microbial pathways involved in production of methane from wetlands and aquatic systems are discussed in detail in Chapter 5. Until all electron acceptors (oxygen, nitrate, iron and manganese oxides, and sulfate) with higher reduction potentials are exhausted, no methane will be produced. Potentially all these electron acceptors can be present in the same soil profile with electron acceptors with higher reduction potentials utilized in surface layers and the electron acceptors with lower reduction potentials utilized in lower depths (Figure 10.33). 

Iron and manganese, which serve as important electron acceptors, especially in mineral soils, can regulate methane production (an important greenhouse gas). Until electron acceptors with higher reduction potential (e.g., Fe(III), Mn(IV)) are exhausted, no methane will be produced. In wetland soils, methane production is inhibited in zones where intense cycling of iron occurs. 

Paulsen KE, Liu Y, Fox BG, Lipscomb JD, Mtlnck E, Stankovich MT. 1994. Oxidation-reduction potentials of the methane monooxygenase hydroxylase component from Methylosinus trichosporium OB3b. Biochemistry 33 713-722. 

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