When the catalytic reduction of carbon dioxide is truly homogeneous (occurs in the
solution), electrochemical and photochemical systems may have much in common. The
means of electron delivery differs, of course, with photoinduced electron transfer
processes serving the role of the electrode in the photochemical system. Many of the
catalyst systems studied so far, cobalt and nickel macrocycle systems, for example, work
in both kinds of experiments. In both approaches, the ultimate source of these electrons
is an issue. Sacrificial reagents (generally organic compounds that become oxidized) are
commonly used and one of the challenges is to replace these reactions with processes that
are less costly and wasteful. For aqueous systems, it would be highly desirable to use the
water oxidation half-reaction, i.e.
H20 =2e-+1/2 02 + 2H+
for this purpose so that the overall reaction would be
COZ + 2 Hz0 + CHsOH + 3/2 OzThe challenge remains the effective
development and deployment of water oxidation catalysts.
Electrocatalysis Photocatalytic Reduction
At present, electrochemical reduction of CO2 yields carbon monoxide, formate, methane,
etc. with good current efficiencies and, in photochemical systems, quantum yields for
carbon monoxide (and/or formate) are up to 40%.
Electrochemical and photochemical electron sources in the presence of proton
sources can avoid use of expensive H2, but both need:
Faster catalytic processes, more stable catalytic systemsDevelopment of
useful second half reaction, i.e. elimination of sacrificial reagent/useful
anode reaction
