||I U P A C
Organizations & People
Pure Appl. Chem.,
Vol. 71, No. 2, pp.303-320, 1999
COMMISSION ON PHOTOCHEMISTRY
Terminology, relative photonic efficiencies
and quantum yields in heterogeneous photocatalysis. Part I: Suggested protocol
Serpone and Angela Salinaro
Abstract: The term photocatalysis is
one amongst several in a quagmire of labels used to describe a photon-driven
catalytic process; a simple description of photocatalysis is proposed
herein. Other labels such as quantum yield and/or quantum
efficiency used in solid/liquid and solid/gas hetero-geneous photocatalytic
systems to express process efficiencies have come to refer (incorrectly)
to the ratio of the rate of a given event to the rate of incident
photons impinging on the reactor walls and typically for broadband radiation.
There is no accord on the expression for process efficiency. At times
quantum yield is defined; often, it is ill-defined and more frequently
how it was assessed is not described. This has led to much confusion
in the literature, not only because of its different meaning from homogeneous
photochemistry, but also because the description of photon efficiency
precludes comparison of results from different laboratories owing to
variations in light sources, reactor geometries, and overall experimental
conditions. The previously reported quantum yields are in fact apparent
quantum yields, i.e. lower limits of the true quantum yields.
We address this issue and argue that any reference to quantum yields
or quantum efficiencies in a heterogeneous medium is inadvisable
until the number of photons absorbed by the light harvester
(the photocatalyst) is known. A practical and simple alternative is
proposed for general use and in particular for processes employing complex
reactor geometries: the concept of relative photonic efficiency
is useful to compare process efficiencies using a given photocatalyst
material and a given standard test molecule. A quantum yield can subsequently
be calculated since f = xr
the quantum yield for the photocatalyzed oxidative transformation of
phenol used as the standard secondary actinometer and Degussa P-25 TiO2
as the standard photocatalyst. For heterogeneous suspensions (only),
an additional method to determine quantum yields f is also proposed.
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