Carbon dioxide reduction equilibria

For synthetic fuels or energy-storage media to be produced electrochemically, it is necessary that the carbon dioxide reduction be conducted at potentials only slightly (not more than by 0.2 V) more negative than the corresponding equilibrium potential. To do this requires extensive research aiming at refining the catalysts and the conditions for this process. 

One reaction that occurs in producing steel from iron ore is the reduction of iron(II) oxide by carbon monoxide to give iron metal and carbon dioxide. The equilibrium constant Kp for the reaction at 1000 K is 0.259. 

Hydrogen transfer reactions are highly selective and usually no side products are formed. However, a major problem is that such reactions are in redox equilibrium and high TOFs can often only be reached when the equilibria involved are shifted towards the product side. As stated above, this can be achieved by adding an excess of the hydrogen donor. (For a comparison, see Table 20.2, entry 8 and Table 20.7, entry 3, in which a 10-fold increase in TOF, from 6 to 60, can be observed for the reaction catalyzed by neodymium isopropoxide upon changing the amount of hydrogen donor from an equimolar amount to a solvent. Removal of the oxidation product by distillation also increases the reaction rate. When formic acid (49) is employed, the reduction is a truly irreversible reaction [82]. This acid is mainly used for the reduction of C-C double bonds. As the proton and the hydride are removed from the acid, carbon dioxide is formed, which leaves the reaction mixture. Typically, the reaction is performed in an azeotropic mixture of formic acid and triethylamine in the molar ratio 5 2 [83],... 

Equation (115) is the same as (6) studied by James and co-workers (62) in the CO reduction of RhCl3.] The labeling experiment also revealed information on the stability of the hydroxycarbonyl intermediate in (115). If this species, Rh—COOH, was formed in an equilibrium concentration, then proton transfer and the reverse reaction would lead to incorporation of labeled oxygen in the carbonyl ligand and therefore to the observation of doubly labeled C02. However, comparison of the abundances of the three isotopic carbon dioxide molecules found (masses 44, 46 and 48) with distributions calculated assuming (i) equilibrium formation of the hydroxycarbonyl and (ii) immediate decomposition of the intermediate clearly showed that the hydroxycarbonyl intermediate reacts to form C02 immediately after it is formed, with no indication of a substantial equilibrium or incorporation of lsO in the carbonyl ligand. 

A study of the velocity of reduction of carbon dioxide by carbon at 850° C. shows that the reaction is monomoleeular, and the same is true for the reverse reaction, namely the decomposition of carbon monoxide, which, however, proceeds 166 times more slowly. Undoubtedly, therefore, the reactions arc essentially surface phenomena, the rates varying directly with the partial pressure of the gas in either case. Since the decomposition of carbon monoxide is accompanied by a reduction m volume, increase of pressure should facilitate the reaction at constant temperature, and shift the equilibrium... 

As indicated by the equations, heat is absorbed and increase in volume accompanies both reactions, so that they are promoted by rise in temperature and reduction of pressure. Lang 2 found that the higher the temperature the greater is the proportion of carbon monoxide at 600° C. the product consists almost wholly of hydrogen and carbon dioxide, but at 1000° C. the composition of the equilibrium mixture expressed in atmospheres is... 

After the softening process, the pH is so high that reduction is necessary to prevent deposition of scales in distribution pipes. This can be accomplished inexpensively using carbon dioxide. We will therefore develop the method for determining the carboiuc acid necessary to set the water to the equilibrium pH. 

Producer gas is made by the reduction of carbon dioxide to carbon monoxide by carbon. With the total pressure 1 atmosphere, the equilibrium mixture of the two oxides at 1123° C contains 93.77 percent by volume of carbon monoxide, and 6.23 percent of carbon dioxide. What is the equilibrium constant for this reaction at this temperature What would be the composition of the mixture in equilibrium at this temperature if the total pressure ivere 2 atm ... 

S organism is inhibited by the hydrogen it produces Methanohacterium displaces the equilibrium by oxidizing hydrogen with reduction of carbon dioxide to methane. To survive in the ethanol-carbonate medium each organism requires the other. 

If a reacting species is involved in the reaction as a pure liquid or as a solid its fugacity can be taken to be unity and the corresponding term does not appear in the product. For example, in the reduction of carbon dioxide over solid carbon 2 CO — C — COa = 0, the equilibrium relationship would be... 

Figure 12.14 Variation ofthe equilibrium oxygen pressure as a function ofthe current passing trough the pump, (a) Reduction of flowing carbon dioxide for two temperatures of the sensor ...

Boudouard Equilibrium. Equilibria can be well demonstrated when gases are involved. Carbon is used to reduce iron oxide minerals for the production of iron metal in a blast furnace. However, carbon itself barely functions as a means of reduction but rather the carbon monoxide, CO. It results in a reaction of gaseous carbon dioxide with glowing carbon by a temperature of approximately 1000°C ... 

The reverse of this reaction, the reduction of carbon dioxide by carbon to form carbon monoxide is of considerable industrial importance being involved in the manufacture of producer gas, etc., and lias been carefully studied. The free energy change of this reaction is such that the equilibrium constant becomes equal to 1 at a temperature of about 1000° K (727° C.). Thus for the reaction ... 

FDH catalyzes the oxidation of formate to carbon dioxide, concomitant with the reduction of NAD+ to NADH (Fig. 16.6-4). Because of the favorable thermodynamic equilibrium of the reaction and the volatility of the reaction product, the enzyme is commonly applied for in situ regeneration of NADH during asymmetric synthesis of chiral compounds1131. 

This equilibrium has been studied. Using a catalyst consisting of 73 per cent charcoal and 27 per cent nickel, which resulted from carbonizing a mixture of sugar and nickel acetate, the decomposition of carbon monoxide into carbon and carbon dioxide was entirely suppressed and the catalyst maintained its activity for months in producing methane. Ferric oxide, vanadium pentoxide, and cerium oxide are promoters for the nickel-charcoal catalyst. Studies have also been made of the various reactions involved in the reduction of carbon monoxide and dioxide. ... 

In the same fashion as the early carnauba wax microemulsions, natural or synthetic wax micro- and miniemulsions stabilized by fatty soaps are being used for coating citrus fruit to avoid weight reduction and to lower internal carbon dioxide, with better performance than the conventional shellac or resin coatings [133]. Sometimes these are even more complex polyphasic systems involving both ionic and nonionic microemulsions in equilibrium with other phases [134,135]. 

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