and is the main driving power of environmental reactivity. Among
catalyzed photolysis those reactions that follow substrate absorp-
tion are limited mainly to those cases when solar radiation
induces photodegradation of a pollutant that is enabled or
accelerated by some natural or anthropogenic catalyst.
The most essential pathways of pollutant photodegradation
start from photocatalyst absorption followed by photosensitization
or photoassisted reactions. The former consists in the energy or
charge transfer from an excited photosensitizer to the substrate
(quencher) molecule, whereas the latter results in generation of
a“catalyst for one cycle,”called photoinitiator. To guarantee the
continuity of the charge transfer reactions both photosensitizers
and photoinitiators should be recycled; the common practice is
their regeneration by means of adequate electron donors and/or
acceptors. These are not restored in a subsequent redox process
but destroyed by irreversible chemical conversion; thus they are
called sacrificial donors and acceptors, respectively.
The most common sacrificial electron acceptor in the environ-
ment is molecular oxygen, whereas the main sacrificial donors
are organic compounds. In consequence, the self-cleaning pro-
cesses consist in oxidation of organic pollutants by molecular oxy-
gen in its triplet ground state; the reactions are driven by energy
from solar radiation. In nature, many different photoinitiators or
photosensitizers are reactive, but the most common environmen-
tal photosensitizers include humic substances (HS), whereas the
best photoinitiators are transition metal complexes.
FIG. 1. Classification of photocatalysis.METAL COMPLEXES AS SOLAR PHOTOCATALYSTS 295