Standardization protocol of process efficiencies and activation parameters in heterogeneous photocatalysis: relative photonic efficiencies ζr

The quantum yield Φ is crucial in homogeneous photochemistry; in heterogeneous photocatalysis this term remains elusive since the number of absorbed photons remains experimentally difficult to asses. A comprehensive method to standardize and compare process efficiencies in heterogeneous photocatalysis was proposed earlier by describing the relative photonic efficiency ζr (J. Photochem. Photobiol., A: Chem., 73 (1993) 11). The method of determining ζr is herein tested for the photocatalyzed degradation of phenol as the standard process and Degussa P25 TiO2 as the standard photocatalyst. The effects of light intensity, reactor geometry, pH, temperature, concentration of reactant and concentration of TiO2 on ζr were examined for the photodegradation of three substituted phenols: 2-methylphenol, 2,4-dimethylphenol and 4-chlorophenol. Relative photonic efficiencies of other phenolic substrates are reported for a given set of conditions. Efficiencies on varying the nature and the source of TiO2 for the photodegradation of phenol were also explored. These ζr are useful to assess process quantum yields once the actual quantum yield for a standard process (Φstand, for a given photocatalyst and a standard organic substrate) has been rigorously determined; thus Φ = ζrΦstand.