A study was conducted to show CO2 hydrogenation as an alternative method for so-called artificial photosynthesis to produce fuels, such as formate/formic acid and methanol, with good selectivity and high efficiency. For CO2 hydrogenation, an inexpensive and green source of H2 is needed in contrast to the industrial reforming of natural gas. To obtain formic acid for the regeneration of H2 or use in fuel cells, additional acid must be added to neutralize the formate. As an alternative to the use of a base in CO2 hydrogenation, a Lewis acid is a useful additive for H2 release from formic acid. To reduce the cost, catalysis with earth-abundant metals such as Fe or Co is highly desirable, and considerable progress has been achieved. Innovative ligands with functional groups are capable of gaining or losing one or more protons, and photoresponsive ligands capable of undergoing a useful change in properties upon irradiation. Kinetic isotope effects and computational studies provide clear evidence for the involvement of a water molecule in the rate-determining heterolysis of H2 in CO2 hydrogenation that accelerates proton transfer through the formation of a water bridge. Solution pH alters the rate-determining step for H2 generation from formic acid with these bioinspired complexes.
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