Photocatalysis constitutes one of the emerging technologies for a wide range of industrial processes, such as organic synthesis, photo-destruction of toxic compounds, and purification of drinking water [1]. However, the commercialisation of heterogeneous photocatalysis requires improved photo-reactors relative to those currently available. In particular the photonic efficiency needs to be improved. In this contribution, a novel reactor for multi-phase photocatalysis is presented, the so-called Internally Illuminated Monolith Reactor (IIMR). A monolith configuration provides a high geometrical surface area to support catalysts on, but till now only a design with coated fibers, using a monolith as support, has been described in the open literature [2]. In the concept of the IIMR, Figure 1, side-light emitting fibers are placed inside the channels of a ceramic monolith, equipped with a TiO2 photo-catalyst coated on the wall of each individual channel. To increase the side light emission intensity, the tips of the fibers are coated with a reflective aluminium coating. With the fibers tip-coated the light intensity measurements along the fiber bundle demonstrate that the side emission is rather uniform and over 95% of all photo-energy originating from the light source is emitted through the sides.   Figure 1: Internally Illuminated Monolith Reactor with a detailed scheme of the cross section of the monolith channels with the side light fibers inserted.

A priori the design described in this work, i.e. coating the catalyst on the walls of the monolith vs coating of the catalyst on the fiber surface, appears to have significant advantages. The governing rules to interpret the optics in OFR are the Snell's equation for light propagation along the fiber and the Lambert-Beer's law for the refracted light intensity inside the coated photocatalyst layer. In the titania layer coated on the surface of the fibers, the light transmission is in the opposite direction of the reactants diffusion. The light intensity is highest at the fiber-catalyst interface and attenuates with an exponential decay as it approaches the catalyst-reactant interface. Therefore, an optimal layer thickness is typically determined, where both sufficient light absorption and rapid reactant diffusion into the illuminated layer are satisfied. Furthermore, the TiO₂ coating on the optical fiber has two functions, namely to catalyze the surface reaction and to reflect part of incident light back into the optical fiber. Depending on the quartz fiber diameter and the coating material, different optimal coating layer thicknesses were found. Our design, with the fibers tip-coated with a reflective material, and the catalyst on the walls of the monolith, decouples the light propagation process in the fiber from the physical properties of the catalytic layer. With the side light fibers illuminating the coated walls of the monolith channels from the front, reactant concentration and light intensity decay in a similar direction, i.e. from the external surface towards the ceramic wall of the monolith channel. It is expected that this will positively affect the photonic efficiency.

The photonic efficiency achieved with the IIMR reactor operating in film flow in the selective photo-oxidation of cyclohexane (0.062) is lower than the one obtained with a top illumination reactor (0.151), whereas an Annular reactor and reactor configuration with side-light fibers immersed in a TiO₂ slurry reach a photonic efficiency of only 0.008 and 0.002, respectively.  These results are discussed on the basis of incident light flux and catalyst illumination affectivity, and compared with fiber optic concepts described in the literature, in which the fibers are coated with the catalyst.

At this time, experiments are conducted to apply the IIMR in other reactions and catalyst combinations, as well as in other flow regimes (Taylor flow). The preliminary results of these experiments will also be discussed.

[1]    M. R. Hoffmann, S. T. Martin, W. Choi, D. W. Bahnemann, D. W. , Chem. Rev. 95 (1995) 69-96.

[2]    H.F. Lin, K. T. Valsaraj, J. Appl. Electrochem. 35 (2005) 699-708.