## 2 Cu-Based Catalyst-Activated PMS Processes
### Cu-Based Designed Catalyst Materials
#### 2.4.3 Cu-Based DESs  
Recently, increasing environmental concerns have highlighted the need to develop sustainable and clean technologies. Researchers worldwide are actively working to create greener methods for synthesizing catalysts. Their efforts focus on minimizing the use of hazardous solvents during synthesis and purification, avoiding toxic substances, reducing energy consumption, and promoting synthesis under ambient conditions. In this context, DESs that act not only as solvents but also as structure-directing agents stand out [135]. DESs are liquid systems formed by the interaction of two or more components, usually a hydrogen bond donor (HBD) and a hydrogen bond acceptor (HBA), in a molar proportion where a eutectic mixture is formed, resulting in a liquid state with unusual solvent proper ties (a melting point lower than that of any of the individual components) [138]. There are different types of DESs depending on the combination of HBA and HBD used. DESs can be described by the general formula of Cat\({}^{+}\) X\({}^{-}\) zY, where Cat\({}^{+}\) refers to an ammonium, phosphonium, or sulfonium cation; X\({}^{-}\) generally represents a halide anion, and Y is Lewis or Bransted acid (z refers to the number of molecules Y interacting with the anion) [139]. DESs, first introduced by [PERSON] et al. [140], represent a new class of low-cost and environmentally friendly solvents with great potential to replace traditional catalysts and solvents. They have gained significant attention in recent years due to their remarkable properties, such as high solubility and conductivity, low toxicity, and easy recovery [139; 141]. One of the advantages of DESs is their high mutual compatibility with numerous metal salts. Thus, DESs can maximize the incorporation of metal salts used in synthesis into the final product, such as metal oxides [103]. For this reason, and because they are pure green solvents that are environmentally friendly, in recent years, they have proven to be very useful in the synthesis of catalytic materials. Specifically, there are still not many studies related to synthesized copper catalysts assisted by DESs and PMS. Table 2 highlights some of the copper catalysts synthesized with DESs. Among these, [PERSON] et al. [134] conducted a remarkable study in which CuO nanoparticles were synthesized using a hydrophobic DES, prepared by mixing DL-menthol with phenyl alcohol in a molar ratio of 1:1 at 70 \({}^{\circ}\)C. The ability of the resulting CuO nanoparticles to degrade Rh B was tested, and the experimental results showed that the CuO/PMS system (0.16 g\(\cdot\)L\({}^{-1}\) CuO and 0.13 g\(\cdot\)L\({}^{-1}\) PMS) successfully degraded 98.0% of a 20 mg\(\cdot\)L\({}^{-1}\) Rh B solution at pH 7.0 in 18 min. These findings indicate that the synthesized CuO nanoparticles exhibit excellent catalytic performance for the photodegradation of dye in the presence of PMS, outperforming other reported metal oxide-based catalysts. Recently, Cu-Ag nanostructures were synthesized using a one-step electrodeposition method, where the composition and morphology of Cu-Ag were adjusted by a DES composed of choline chloride and urea [142]. The characterization of this nanocatalyst in a single step confirmed that the electrodeposition of metals from a DES offers a cost-effective and environmentally friendly approach to prepare controllable bimetallic and multimetallic copper catalysts. However, there are some challenges associated with the use of DESs as catalysts. Their high viscosity can impact reaction kinetics by limiting mass transfer and the diffusion of reagents and products. Additionally, the stability and catalytic activity of DESs may be compromised due to their high sensitivity to water and other impurities [139]. To overcome these problems, research on the DES-assisted synthesis of copper-based catalysts is expanding and gaining significant attention.