## 1 Introduction
During photocatalytic and catalytic reactions, Gibbs free enthalpy is positive (\(\Delta\)G \(>\) 0) and negative (\(\Delta\)G \(<\) 0), respectively [29]. In general, the photocatalytic process involves five subsequent steps: (1) pollutants transfer to photocatalyst surface, (2) adsorption of pollutants on the surface, (3) photonic activation and decomposition of adsorbed molecule, (4) reaction product's desorption and (5) removal of reaction products from the photocatalyst surface [30]. According to widely accepted theory, the base of the photocatalytic process is generation of electron-hole pairs on the photocatalyst surface as a result of the light with equal or higher energy than the energy gap of the photocatalyst. The formed electron-hole pairs dissociate into electrons in conduction band and holes in the valance band responsible for oxidation and reduction of adsorbed pollutants on photocatalyst surface [31]. Due to high effectiveness photocatalysis depending on photocatalytic activity of catalyst, light intensity and wavelength, dihydroxybenzene isomers' initial concentration, medium pH and co-existing substances [32], this process found many applications in phenolic compound degradation using many materials--i.e., Au-ZnO [33], TiO\({}_{2}\)[34; 35], ZnO [36], TiO\({}_{2}\)/graphene/heteropoly acid nanocomposite [37] or MnOx/TiO\({}_{2}\) nanocomposite [38]. It is reported that the modification with CNT enhanced the photocatalytic performance of CNT-TiO\({}_{2}\), Ag\({}_{3}\)PO\({}_{4}\)/CNT, CNT/g-C\({}_{3}\)N\({}_{4}\), BiOI/CNT and Au@CNT@TiO\({}_{2}\) composites [39; 40; 41; 42; 43], as well as TiO\({}_{2}\)/CNT anodes for H\({}_{2}\) generation [44]. The main aim of this work was to explore the aspects of the solar photocatalysis process intensification in optimal pilot scale reactor geometries. The solar photocatalysis is a surface phenomenon, highly dependent on the irradiation intensity reaching photocatalyst surface. In that manner, one way to intensify the process is the proper reactor design. This process can be conducted efficiently in some rather simple reactor configurations such as flat-plate reactor types. However, the use of compound parabolic collector (CPC)-based photoreactors is highly recommended by many researchers [45; 46] since the light capture in reaction space is better than in flat-plate systems. The CPC-based configuration assumes closed tubular reactors, whereby in the case of slurry TiO\({}_{2}\) applications, incident light concentrated inside the tube may be utilized all over its diameter. The novelty presented in current study includes the use of immobilized photocatalyst inside the tube where it is fully exposed to incident irradiation; the goal was to estimate the intensification in comparison with a flat-plate cascade reactor (FPCR). Another method of intensification is the intervention in photocatalyst formulation to shift the absorption to the visible region and lower the energy gap. The latter is extensively investigated among researchers worldwide. However, a need for large scale immobilized photocatalysts for flat-plate reactors may limit the use of numerous doped TiO\({}_{2}\) due to price and small-scale production methods (e.g., anodization). In the current study, only commercially available components were used; TiO\({}_{2}\)-based film made from TiO\({}_{2}\) P25 was modified by the addition of multi-walled carbon nanotubes (MWCNT). The photocatalytic film is simple to prepare ensuring the profitability of full-scale solar photocatalysis. The highlight of the study was the simple methodology for quantitative estimation of solar photocatalysis intensification. Intensification indices were calculated via kinetic modeling, and the results were presented. The applied modeling allows a clearly comparison of different photocatalyst and reaction geometries as was shown in previous work [46; 47].