Keywords:Photocatalysis; Photo splitting; Solar energy utili-
zation; H 2 O, CO 2 , CO 32 ,N 2 ; Last Glacial Maximum.
I. IntroductionThe reactivity of the ubiquitous and abundant chemicals H 2 O,
CO 2 , and N 2 is of fundamental importance in its own right but
also with regard to applications related to climate change and
utilization of solar energy. A rather significant aspect of this
chemistry deals with the splitting of these molecules, for
example
2H 2 O!2H 2 þO 2 ð 1 ÞCO 2 !COþ1
2O 2 ð 2 ÞN 2 þ3H 2 !2NH 3 ð 3 ÞOwing to the simple composition and structure of H 2 O, CO 2 , and
N 2 , this chemistry might be expected to be simple, too. However,
due to the extreme stability of these species their splitting is
rather difficult to achieve, at least under ambient conditions
and requires a sophisticated mechanism and much energy. It is
an attractive idea to supply this energy by light (solar energy).
Nevertheless, the direct activation and cleavage need light of
extremely short wavelength (<200 nm), which is usually not
available at ambient conditions. Accordingly, some type of long-
wavelength sensitization is required to photochemically split
the very strong bonds in these molecules. Guidelines may be
derived from natural photosynthesis:
CO 2 þH 2 O!1
nðÞCH 2 OnþO 2 ð 4 ÞThe electrons that are provided by photosystem I are finally used
to reduce CO 2 to carbohydrates, while in photosystem II, water is
oxidized to oxygen. Intense research over many decades has par-
tially revealed the extremely complicated mechanism of natural
photosynthesis. It follows that it is obviously rather difficult to
imitate this in an artificial photosynthesis that is intended to
convert and store solar energy in simple but energy-rich
chemicals. Different approaches have been developed to solve
this problem( 1 ). It has been suggested to facilitate artificial pho-
tosynthesis by the assistance of redoxactive metal complexes in
homogeneous systems. Generally, photoredox reactions of metal
346 ARND VOGLER AND HORST KUNKELY