N. Serpone et A. Salinaro, Terminology, relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Part I: Suggested protocol (technical report), PUR A CHEM, 71(2), 1999, pp. 303-320
The term photocatalysis is one amongst several in a quagmire of labels used
to describe a photon-driven catalytic process; a simple description of pho
tocatalysis is proposed herein. Other labels such as quantum yield and/or q
uantum efficiency used in solid/liquid and solid/gas heterogeneous photocat
alytic systems to express process efficiencies have come to refer (incorrec
tly) to the ratio of the rate of a given event to the rates of incident pho
tons impinging on the react or walls and typically for broadband radiation.
There is no accord on the expression for process efficiency. At times quan
tum yield is defined; often, it is ill-defined and more frequently how it w
as assessed is not described. This has led to much confusion in the literat
ure, not only because of its different meaning from homogeneous photochemis
try, but also because the description of photon efficiency precludes compar
ison of results from different laboratories owing to variations in light so
urces, reactor geometries, and overall experimental conditions. The previou
sly reported quantum yields ore in fact apparent quantum yields, i.e. lower
limits of the true quantum yields. We address this issue and argue that an
y reference to quantum yields or quantum efficiencies in a heterogeneous me
dium is inadvisable until the number of photons absorbed by the light harve
ster (the photocatalyst) is known. A practical and simple alternative is pr
oposed for general use and in particular for processes employing complex re
actor geometries: the concept of relative photonic efficiency (xi(r)) is us
eful to compare process efficiencies using a given photocatalyst material a
nd a given standard test molecule. A quantum yield can subsequently be calc
ulated since Phi = xi(r) Phi(phenol), where Phi(phenol) denotes the quantum
yield for the photocatalyzed oxidative transformation of phenol used as th
e standard secondary actinometer and Degussa P-25 TiO2 as the standard phot
ocatalyst. For heterogeneous suspensions (only), an additional method to de
termine quantum yields Phi is also proposed.