of photocatalytic reaction obeys the first-order rate law, but this is
only formal and does not mean the mechanism of monomolecular
reaction with activation energy.
One of the most significant points that we must consider in scien-
tific studies, not limited to studies on photocatalysis, is distinction
between“evidence”and“consistency,”as least as far as the author
thinks. In other words, it is necessary to recognize every fact to be
a“necessary condition”but not a“sufficient condition”in a strict sci-
entific sense. For example, the fact that a reaction rate obeys the
first-orderratelawgivingalinearrelationinaplotofdataasinFig.6
is only a necessary condition for a monomolecular reaction in homo-
geneous phase and also a necessary condition for heterogeneous
photocatalyticreactionin diffusion-limitedconditionsorthatinsur-
face-reaction limited conditions with a Henry-type adsorption or a
Langmuir-type adsorption in the lower-concentration region.
B. LANGMUIR–HINSHELWOODMECHANISM
The term“Langmuir–Hinshelwood mechanism”has often been
used in discussion of the mechanism of photocatalytic reaction in
suspension systems, but, as far as the author knows, there has
Time (min)
0 10 20 30 40
0
1
2
3
4
5
0
2
4
[A] / mmol dm – (slope) =k
–3
ln([A
] / [A]) 0
FIG. 6. First-order kinetic analysis for a reaction consuming a sub-
strate A. Plot of logarithm of relative consumption (in the present plot,
ratio of initial concentration of substrate A and concentration of A at a
given time).
PHOTOCATALYSIS BY INORGANIC SOLID MATERIALS 407