Hydrogenation of carbon dioxide in aqueous solution

Reaction of two gaseous compounds resulting in a liquid product are biased by a decrease in enthropy which -depending on the temperature-may make the whole process thermodynamically unfavourable. This is also the case for the hydrogenation of C02 to HCOOH (eq. 3.12) with AH°298 = -32 kJ mol-1 but AG°298= +33 kJ mol-1  

However, in aqueous solution hydration of the solutes makes the overall enthropy difference smaller, and reaction (3.13) becomes slightly exergonic with AG°298 = - 4 kJ mol-1  

Thermodynamics tells, therefore, that in water this reaction is likely to proceed we must not forget, though, that these data refer to standard conditions, and in order to eliminate the kinetic activation barrier at 25 °C highly active catalysts are needed. Unfortunately, the same catalysts can also be active in the reverse process, i.e. in the decomposition of formic acid to H2 and C02 at low pressures decomposition to CO and H20 (i.e. the reverse water gas shift) is rarely observed. 

Equation (3.13) can also be shifted to the right by further reactions of HCOOH which may be simply its neutralization with a base, or reactions with amines or alcohols, yielding formamides or formate esters, respectively. In this context it is worth recalling the C02/carbonate equilibrium (3.14) in water the distribution of the possible reactive species is highly dependent on the actual pH, temperature and pressure [286]  

The beneficial effect of water was observed in several experiments on reduction of C02. Inoue et al. were the first to discover that in the presence of a base (NaOH, NaHCOj, NMe3, NEt3, etc.) transition metal phosphine 

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Transfer hydrogenation and hydrogenolysis Hydrogenation of carbon dioxide in aqueous solution Hydrogenations of biological interest... 

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