Dogmas and misconceptions in heterogeneous photocatalysis. Some enlightened reflections

In a recent article, Ollis(1) analyzed heretofore reported photocatalyst kinetics of surface photochemical reactions that take place in heterogeneous systems and that rely heavily on the Langmuir-Hinshelwood (LH) kinetic model to interpret the experimental observations. This model assumes a fast adsorption/desorption equilibrium step and a subsequent slow surface step. His interesting analysis of the experimental results reported in 2000 by Emeline and co-workers,(2) Xu and Langford,(3) and Martyanov and Savinov(4) prompted our reexamination of the LH kinetic model along with several other dogmas that continue to propagate in the heterogeneous photocatalytic landscape. This short article discusses some of these issues and reexamines certain misinterpretations. Specifically, we reexamine (1) the a priori assumed validity of the LH kinetic model in heterogeneous photocatalysis, (2) the recombination of photogenerated free charge carriers on the solid (metal oxide) photocatalyst by the band-to-band recombination pathway, and (3) the mistaken assertion(1) that the kinetics of a heterogeneous photoreaction are either only first-order dependent or half-order dependent on photon flow (i.e., light irradiance).