  Cobalt-porphyrin catalyzed reductive decomposition of CO2 to CO is
investigated based on Koper's water facilitated CO2 reduction mechanism using a
simple but accurate protocol based on thermodynamics. In our protocol, accurate
predictions of standard redox potentials and free energy differences are
achieved by combining strengths of both density functional theory and
experimental observations. With the proposed protocol, we found that the proton
transfer from H2O takes place at -0.80 V vs. RHE at pH=3 through a concerted
pathway and, as a result, the key intermediate for the CO generation, i.e.,
[CoP-COOH]- is formed. Since the redox potential of the proton transfer agrees
well with experimentally observed CO2 reduction potential, we successfully
clarified that H2O plays an important role in the reductive decomposition of
CO2 to CO. This result is valuable not only for understanding the
cobalt-porphyrin catalyzed reductive decomposition of CO2 but also as a guide
for the development of new catalysts.
