Catalysts for advanced oxidation processes: Deep eutectic solvents-assisted synthesis - A review

New catalyst synthesis techniques, including green materials, are extensively studied for heterogeneous photocatalytic advanced oxidation processes (AOPs) on spotlight of sustainable development. Deep eutectic solvents (DESs) started to be used in this field as environmentally friendly alternative to ionic liquids (ILs). During the catalyst synthesis, DESs can act as stabilizers, capping agents, structure directing agents, templates, hydrolyzing agents, etching agents, intercalators, and latent supramolecular catalysts. Importantly, DESs have exhibited the ability to significantly influence catalyst morphology, functionalization and photocatalytic properties (confirmed both for classic UV lamps and light emitting diode (LED)), including band-gap modification. DESs positive effect was proved for a variety of materials, including metal oxides, metalorganic (MO) complexes and doped materials, MXene (MAX phase etching in DES environment), inorganic-organic hybrids, carbo-catalysts. Substantial enhancements were obtained for modification of photocatalytic materials like TiO2, ZnO, MnO2, iron oxides, ceria oxides, CdS, bismuth based photocatalysts and biochar modification. In this aspect, a particular role of DESs was confirmed for synthesis of nanomaterials in a form of nanoparticles, nanopowders or nanosheets. Effectiveness was further increased by oxidants such as hydrogen peroxide, persulfates and Fenton process. Effective application of DES-modified catalysts was confirmed for degradation of dyes (Rhodamine B, Reactive orange 16, Safranine, Orange II, methylene blue), pharmaceuticals and antibiotics (Cefixime, Tetracycline, Oxytetracycline, Flumequine, Sulfamethaxazole), PFASs (Perfluorooctanoic acid) and Cr(VI). This reveals high potential of DES based photocatalysts for environmental engineering and remediation. There are still remaining a significant gaps in our understanding of the roles and impacts of DESs in AOPs. Furthermore, there is an absence of data regarding the recovery of DESs in the catalyst synthesis processes applied in AOPs. Addressing this aspects is vital for economic and environmentally friendly applications. As research progresses, it is essential to unravel the intricacies of DES-mediated catalyst synthesis and their broader consequences.