Connecting experimental degradation kinetics to theoretical models for photocatalytic reactors: The influence of mass transport limitations

Catalytic microreactors offer great opportunities to measure reaction kinetics, and parameters influencing the reaction. Although microreactors are quite useful for characterizing catalysts, it is important to understand the relative contributions of mass transport and intrinsic kinetics to the apparent reaction rate. In this paper, we demonstrate the importance of accounting for mass transport limitations in the photocatalytic degradation of Bisphenol A over titanium dioxide. Using analytical scaling laws available from literature and numerical simulations, we provide guidelines for the use of microreactors in characterizing (photo)catalysts. These guidelines identify the mass transport limited regime, or the reaction rate limited regime. The photocatalytic degradation of Bisphenol A was found to be mass transport limited at high light intensities (photon fluxes of above 25 mW/cm²). Neglecting the influence of mass transfer limitations in fitting kinetic data resulted in the exponent of reaction rate (β) with respect to light intensity to be β~0.25, while including these effects gave an exponent directly proportional to the light intensity (β~1). These findings stress the importance of a correct inclusion of mass transport limitations. A simple analysis of the transverse Péclet number and second Damköhler number, to quantify the transport and reaction rates, is presented for our laminar flow reactor to illustrate the different limiting regimes.