(27a) Decontamination of Opaque Fluids Using Heterogeneous Photo Catalysis: Critical Role of Mobile Electron Holes on Semiconductor Surfaces

Heterogeneous photocatalysis is a technique to destroy pollutants in water solutions with a big promise, using a semiconductor and near UV-irradiation (1). However, decontamination of opaque fluids using photocatalysts and near UV irradiation involves major technical challenges, mainly because the resistance for the light transmission in a slurry, and then it is necessary to use the catalyst attached to a surface. This study considers a thin TiO₂ layer placed inside and in the bottom of a CREC-Photoreactor cell containing the opaque fluid. In this new setup, an external UV-lamp was placed below the unit, facing the interface support-TiO₂ film and the TiO₂ film received irradiation before photons could reach the opaque fluid. This new photoreactor cell is employed for the photocatalytic degradation of malic and malonic acids, typical apple juice components (2). The reactor setup was run for 30 minutes in the dark in order to reach absorption equilibrium on the TiO₂layer surface. Once adsorption equilibrium was reached, the lamp was turned on and the photocatalytic degradation was initiated. Samples were taken every hour for 15 hours to analyze the change in malic and malonic acid concentrations during the photocatalytic conversion, and were analyzed using HPLC and TOC.

Conversion of organic species can only proceed through the “dark side” of the TiO₂ layer, which is in direct contact with the fluid.  Under the selected operating conditions both external mass transfer limitations and photolysis are found to be negligible. It was found that the relatively high fluid circulation in the near thin film region, estimated at  24 and 47 cm/s respectively, promotes high  mixing and negligible external mass transfer on the overall photoconversion rate. Macroscopic radiation balances shows that 92% of near UV radiation is absorbed by the ‘back side” of the TiO₂-film.

Photocatalytic degradation experiments with 10, 20, 30 and 40 ppm malic acid initial concentrations, show that malonic acid is a main intermediate. Complete malic acid conversion occurs after 5-8 hours of irradiation. Kinetic modeling of malic and malonic acid photodegradation with kinetic parameter estimation is performed using both an “in series” and an “in series-parallel” reaction networks, represented by a set of differential equations of the Langmuir – Hinshelwood type (3). The “in series-parallel” reaction network displays better ability for predicting CO₂ formation, showing maximum quantum yields of 14.2%. According to literature, the required time for holes and electrons to reach the 1.3 10^-6 m agglomerate surface as in the present study, is significantly 1.3x10^-10s and 6.5x10^-10s respectively. This is significantly smaller than the 8.9x10^-5s expected charges lifetime as quoted by (4) Wilke, K. and Breuer, H.D (1999).

Malic acid photoconversion allows one to demonstrate that Langmuir-Hinshelwood kinetics  in conjunction with adequate reaction pathways such as the “in series-parallel" (RN2) network are required for photocatalytic kinetic modeling. However, “in series-parallel" provides a better description of the CO₂formed.

Given that in the CREC-Photoreactor cell with a thin TiO₂ film, photocatalysis can only proceed via the transfer of mobile “h⁺” sites from the irradiated side to the “dark side’, this study demonstrates the significance of this step on the overall photocatalysis mechanism.  

References.

(1)               de Lasa H, Serrano B, Salaices M. Photocatalytic Reaction Engineering. Springer New York. 2005.

(2)               Duffy S, Churey J, Worobo R, Schaffner DW. Analysis and modeling of the variability associated with UV inactivation of Escherichia coli in apple cider. J. Food Prot. 2000, 63, 1587-590.

(3)               Moreira J, Serrano B, Ortiz A, de Lasa H. A unified kinetic model for phenol photocatalytic degradation over TiO2 photocatalysts. Chem. Eng. Sci. 2012, 78, 186-203.

(4)               Wilke K, Breuer H D,  The influence of transition metal doping on the physica and photocatalytic properties of titania, J. of Photochemistry and photobiology, A, Chemistry, 1999, 121, 49 – 53.