to suggest reading also his recent review ( 4 ) on photocatalysis,
which may be helpful for understanding the outline of studies
on photocatalysis.
II. PhotocatalysisA. DEFINITION
Although various definitions and interpretations of the term
“photocatalysis” have been proposed, “photocatalysis” or
“photocatalytic reaction”is defined, in this chapter, as a chemical
reaction induced by photoabsorption of a solid material, or
“photocatalyst,” which remains chemically unchanged during
and after the reaction. In other words, the solid acts catalytically,
without any changes in its composition or structure, under pho-
toirradiation, and this explanation may be consistent with most
other definitions. “Photocatalysis” is the conceptual name for
photocatalytic reactions. In this context, data can be obtained
by measuring consumption of the starting materials and/or for-
mation of reaction products initiated by photoirradiation and
then examining whether the photocatalyst or its properties have
been modified during the reaction. This seems to be a relatively
easy procedure. However, various problems are, in fact, encoun-
tered when trying to prove a given phenomenon as being
photocatalytic (Section V.C).
B. PHOTOCATALYSIS ANDCATALYSIS
The most significant difference between photocatalysis and
catalysis lies in their thermodynamics. In a general definition,
a catalyst reduces activation energy of a given chemical reaction
by changing the intermediate states and thereby accelerates the
reaction which proceeds spontaneously with negative Gibbs-
energy change, that is, catalysis is limited to thermodynamically
possible reactions. On the other hand, it is well known that pho-
tocatalysis can drive energy-storing reactions, for example,
splitting water into hydrogen and oxygen. In this sense,“photo-
catalysis”must be recognized as a concept completely different
from that of“catalysis.”Actually, apparent activation energy of
photocatalysis estimated by an Arrhenius plot has been reported
to be very small compared with that of catalytic reactions( 5 ).
PHOTOCATALYSIS BY INORGANIC SOLID MATERIALS 397