Nowadays, heterogeneous photocatalysis for water treatment and hydrogen production are topics gaining interest for scientists and developers from different areas, such as environmental technology and material science. Most of the efforts and resources are devoted to the development of new photocatalyst materials, while the modeling and development of reaction systems allowing for upscaling the process to pilot or industrial scale are scarce. In this work, we present what is known on the upscaling of heterogeneous photocatalysis to purify water and to produce green H2. The types of reactors successfully used in water treatment plants are presented as study cases. The challenges of upscaling the photocatalysis process to produce green H2 are explored from the perspectives of (a) the adaptation of photoreactors, (b) the competitiveness of the process, and (c) safety. Throughout the text, Green Chemistry and Engineering Principles are described and discussed on how they are currently being applied to the heterogeneous photocatalysis process along with the challenges that are ahead. Lastly, the role of automation and high-throughput methods in the upscaling following the Green Principles is discussed.

Nanocomposites with different ratios of titanium dioxide and bismuth vanadate [TiO 2 ]/[BiVO 4 ] give rise to compatible electronic band structure alignment at their interfaces to ensure enhanced photoactivated charge transfer under visible light. The sol-gel method and suitable post-synthesis thermal treatments were used to synthesize different compositions with stabilized anatase phase of tio 2 and monoclinic scheelite polymorph BiVo 4. Structural, electronic and optical characterizations were performed and the results were analysed as a function of the stoichiometry, in which both crystalline structures show a clear junction formation among their characteristic stacking planes. Photocatalytic and (photo) electrochemical responses of the nanocomposites were investigated and tested for the degradation of azo dyes (Acid Blue-113, AB-113) (~ 99%) under visible light radiation. The nanocomposite with a mass ratio of (1:10) shows the highest photocatalytic efficiency compared to the other compositions. HRTEM images showed marked regions in which both crystalline structures form a clear junction and their characteristic planes. However, the increase of BiVO 4 content in the network overcomes the photocatalytic activity due to the decrease in the reduction potential of the photo-generated electrons with high recombination rates. Due to rapid increase in population and the world's rapid industrialization, natural water bodies are being heavily contaminated with industrial wastes 1,2,3. Pollutants present in the industrial effluents are classified as toxins, carcinogens, mutagens, teratogens, and endocrine-disrupting agents 4. Particularly, the presence of dyes in industrial effluents leads to a decreased penetration of sunlight and a reduction of the photosynthetic activities in aquatic ecosystems, as well as an increase in the biochemical oxygen demand 5. The dyes are specified as dispersive dyes (polyester, nylon and acetate) 6 , reactive dyes (Safira dye, HEXL) 7 , sulphur dyes, and vat dyes (viscous) 8. The Azo dyes are colorants that fall under all of the above-mentioned categories. They contain one or more-N=N-groups in their chemical structure, which consists of highly substituted aromatic rings 9. To ensure water purification, several known methods are well-established: desalination, distillation, filtration, flocculation, reverse osmosis, and sedimentation 10,11 , etc. However, the chemical structure of azo dyes leads to recalcitrant characteristics, i.e., they degrade slowly under natural conditions, and they are also resistant to conventional wastewater treatment processes 6. For this reason, Advanced Oxidation Processes (AOPs), have received a widespread attention in recent years, since they are a very convenient water treatment method that eliminates these types of contaminants 5,7. Among AOPs, photocatalysis is considered as a green alternative to attain an efficient purification of polluted water 12,13. Furthermore, the implementation of photoactive nanomaterials in water treatment show outstanding resul...

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