Terminology, relative photonic efficiencies and quantum yields in heterogeneous photocatalysis. Part I: Suggested protocol (technical report)

Citation
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
Citations number
57
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Chemistry
Journal title
PURE AND APPLIED CHEMISTRY
ISSN journal
0033-4545 → ACNP
Volume
71
Issue
2
Year of publication
1999
Pages
303 - 320
Database
ISI
SICI code
0033-4545(199902)71:2<303:TRPEAQ>2.0.ZU;2-O
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 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.