Comparison of Quadratic vs. Langmuir–Hinshelwood Kinetics for Formic Acid Mineralization in a Photocatalytic Film

A plane reactor illuminated by LEDs was used to study the kinetics of the photocatalytic mineralization of formic acid in a TiO2 film. Two of the most widespread types of kinetics were considered to see if their popularity is deserved. More specifically, one-parameter quadratic-type and Langmuir-Hinshelwood-type kinetics were compared against the concentration-time experimental data at different levels of illumination. Closed-form solutions, which allow for the calculation of substrate concentration over time, were derived for the application of the integral method of kinetic analysis. The considered factors, which affect the reaction rate, were the substrate concentration and the rate of photon absorption (RPA) and were varied in order to investigate most of the possible kinetic regimes. The possible onset of limitations due to external and internal mass transfer and transport of the photons was analyzed and discussed. Thanks to the absence of such limitations in the system under examination, it was possible to appraise the "intrinsic" kinetics directly. Both the models were apt to fit the observed decrease in the substrate concentration with time, even if with different soundness. However, substantial differences between the two models were evidenced in the capabilities to reliably reproduce the effects of the RPA.