## 3 Investigating Methodologies for Assessing PMS Activation with Copper-Based Catalysts
### EPR and ESR Methods  
To investigate PMS activation, the frequently used methods are EPR and ESR. The reason they are used is that they allow for the study of ROS due to their unpaired electrons. However, the immediate identification of active species with ESR or EPR is challenging because of their short life span. Hence, spin-trapping agents are introduced to interact with active species, forming stable spin complexes that can be readily quantified as paramagnetic species using EPR or ESR [146]. For \(SO_{4}^{\bullet-}\), \(HO^{\bullet}\), and \(O_{2}^{\bullet-}\), 5,5-dimethyl-1-pyrroline-1-oxide (DMPO) is commonly chosen as the alternative capture agent, thus forming DMPO-SO\({}_{4}^{\bullet-}\), DMPO-\(HO^{\bullet}\), and DMPO-\(O_{2}^{\bullet-}\) spin complexes [85; 146]; \({}^{1}O_{2}\) can be trapped by 2,2,6,6-tetramethyl-4-piperidine (TEMP) to create the spin adduct TEMP-\({}^{1}O_{2}\)[147]. After the adducts are formed, hyphenate splitting constants (HPCs) are calculated employing specialized software associated with EPR technology, and these constants are subsequently assessed against the literature or databases to detect active species [146]. An example of a characteristic EPR spectrum is shown in Figure 5. As explained by [PERSON] et al., Figure 5a illustrates that the TEMP-\({}^{1}O_{2}\) spin adduct shows intense and distinct signals after the addition of the catalyst compared to when only PMS is present [148]. In addition, other points to note from the EPR spectra shown in Figure 5 include that, in the case of DMPO-\(HO^{\bullet}\) adduct, a quaternion characteristic peak of 1:2:2:1 (HPC:\(\alpha\)N = 14.9 G, \(\alpha\)\(\beta\)-H = 14.9 G) typically appears (Figure 5b); for DMPO-\(SO_{4}^{\bullet-}\) admixtures, the characteristic peaks normally indicate a six-component characteristic peak of 1:1:1:1:1:1 (HPC:\(\alpha\)N = 13.51 G, \(\alpha\)\(\beta\)-H = 9.93 G, \(\alpha\)\(\gamma\)-H1 = 1.34 G, \(\alpha\)\(\gamma\)-H2 = 0.88 G) (Figure 4b) [146; 149]. In the case of detecting the DMPO-\(O_{2}^{\bullet-}\) adduct, the EPR spectrum is typically characterized by a six-component peak with a ratio of 1:1:1:1:1:1 (HPC:\(\alpha\)N = 14.3 G, \(\alpha\)\(\beta\)-H = 11.2 G, \(\alpha\)\(\gamma\)-H1 = 1.3 G) [146]. For the TEMP \({}^{1}O_{2}\) adduct, a ternary characteristic peak of 1:1:1 (HPC:\(\alpha\)N = 16.3 G) typically appears (Figure 5a) [146; 150]. Furthermore, by analyzing the EPR and ESR peak intensity at different reaction times, the evaluation of various radical species can be performed. For instance, at the beginning of the reaction involving the activation of PMS by \(ZnFe_{2}O_{4}\) or by \(MnO_{2}\) for the elimination  
Figure 5: Characteristic EPR spectra are shown based on (**a**) TEMPO for detecting \(O_{2}\) and (**b**) DMPO as a scavenger for \(HO^{\bullet}\) and \(SO_{4}^{\bullet-}\) radicals generated during the PMS activation process, using a Cu–Fe Prussian blue precursor (CuFe-PBA\({}^{\bullet}\)) as a catalyst. Adapted from [148] copyright 2023 with permission from Elsevier (Amsterdam, The Netherlands). of phenol, only \(HO^{\bullet}\) is formed (1 min). Subsequently, both \(SO_{4}^{\bullet-}\) and \(HO^{\bullet}\) are generated, and the intensity of DMPO-\(SO_{4}^{\bullet-}\) increases progressively as the reaction continues for up to 30 min [151]. Subsequently, only the DMPO-\(HO^{\bullet}\) signals persist, as observed at the beginning of the reaction; this indicates that \(HO^{\bullet}\) plays a significant role at both the beginning and the end of the reaction. In some of the preceding publications, the importance of \({}^{1}O_{2}\) was not acknowledged. It remains uncertain whether this oversight resulted from the absence of \({}^{1}O_{2}\) formation in those systems or simply because the authors did not contemplate its potential generation. Newer studies have examined the involvement of \({}^{1}O_{2}\) and demonstrated its significance for certain Cu-based materials [32].