Designing a photocatalyst: Relationship between surface species and specific production of desired ROS

Academic Editor: Matthew Jones Received: 9 June 2025 Revised: 17 September 2025 Accepted: 20 September 2025 Published: 23 September 2025 Citation: Viale, F.E.; Elías, V.R.; Benzaquén, T.B.; Goya, G.F.; Eimer, G.A.; Ferrero, G.O. Designing a Photocatalyst: Relationship Between Surface Species and Specific Production of Desired ROS. Sustain. Chem. 2025,6, 31. https://doi.org/ 10.3390/suschem6040031 Copyright: © 2025 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/ licenses/by/4.0/). Article Designing a Photocatalyst: Relationship Between Surface Species and Specific Production of Desired ROS Fabrizio E. Viale 1, Verónica R. Elías 1, Tamara B. Benzaquén 1, Gerardo F. Goya 2, Griselda A. Eimer 1 and Gabriel O. Ferrero 1,* 1Centro de Investigación y Tecnología Química (CITeQ), Universidad Tecnológica Nacional (UTN)—Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), Maestro Marcelo López esq. Cruz Roja Argentina, Córdoba 5016ZAA, Argentina; [email protected] (F.E.V.); [email protected] (V.R.E.); [email protected] (T.B.B.); [email protected] (G.A.E.) 2Instituto de Nanociencia y Materiales de Aragón (INMA), Universidad de Zaragoza (UNIZAR), C/Mariano Esquillor Gómez s/n, 50018 Zaragoza, Spain; [email protected] *Correspondence: [email protected] Abstract Bimetallic mesoporous photocatalysts were synthesized via a wet impregnation method using SBA-15 as a support, and characterized by UV–visible diffuse reflectance spectroscopy, low-angle X-ray diffraction and N 2 physisorption. Among the tested materials, the Ti/Mn combination exhibited the highest photocatalytic activity in azo dye degradation. To understand this enhanced performance, catalysts with varying Mn loads and calcination ramps were evaluated. Additionally, experiments with radical scavengers (isopropanol, chloroform) and under N 2 insufflation were conducted to identify the active radical species. Catalysts prepared with low Mn content and higher calcination ramps showed the greatest activity, which significantly decreased with isopropanol, indicating hydroxyl radicals as the main reactive species. In contrast, samples with higher Mn content and quicker heating displayed reduced activity in the presence of chloroform, suggesting superoxide radical involvement. Spectroscopic analyses (XPS, UV–Vis DRS) revealed that increasing Mn load promotes the formation of Mn 2+ over Mn 4+ species and lowers the band gap energy. These findings highlight the direct correlation between synthesis parameters, surface composition and optical properties, providing a strategy for fine-tuning the performance of a photocatalyst. Keywords: heterogeneous photocatalysis; mesoporous silica; manganese; radical scavengers ; reactive oxygen species 1. Introduction Advanced oxidation processes (AOPs) are a group of emerging techniques aimed at degrading organic pollutants, with the goal of achieving complete mineralization into water and carbon dioxide. These processes have been extensively investigated and applied in various fields in recent years [ 1 , 2 ]. Among these techniques, photocatalytic methods stand out for their high efficiency and ease of implementation in aqueous systems, where the presence of a photocatalyst and light irradiation are sufficient to generate reactive oxygen species (ROS) that drive the degradation of organic compounds. Photocatalysis is promoted by the generation of photoinduced electrons (e − ) and holes (h + ) on the solid surface of a semiconductor upon irradiation [ 3 ]. These e − and h + are key in a series of chain redox reactions that yield a number of ROS, mostly hydroxyl (HO • ) radicals [ 4 ]. Sustain. Chem. 2025,6, 31 https://doi.org/10.3390/suschem6040031 Sustain. Chem. 2025,6, 31 2 of 18 Nevertheless, other species, such as singlet oxygen (O 21 ), hydrogen peroxide (H 2 O 2 ) or superoxide anion radical (O2•− ) are also in this group [3]. The superoxide radical has been largely studied in biological pathways due to its involvement in respiration and immune system processes [ 5 – 7 ], but its role in the degradation of organic substances has been relegated in favor of the study of hydroxyl radical chemistry [ 8 ]. Thus, understanding the reactivity of superoxide remains crucial, as its unique chemical properties distinguish it from hydroxyl radicals. Superoxides are unstable species due to their radical nature, but they are less reactive than hydroxyl ones [ 9 , 10 ]. Its chemistry is strongly related to its solvation ability in different media due to its dual state as a radical and as an anion [ 10 ]. It is not easy to generate and isolate a radical species, so its chemistry is yet to be fully understood [ 11 ]. In addition, due to the short half-life of superoxide anion radicals, their investigation and detection is a critical step in the study of photocatalytic processes. Therefore, it would be highly desirable to apply simple, reproducible, and quick spectrometric methods for its detection [ 12 – 14 ]. Furthermore, TiO 2 has been the most popular semiconductor in photocatalysis to produce ROS due to its favorable physicochemical properties and availability [ 15 ]. However, the practical application of TiO 2 in liquid-based media remains difficult. TiO 2 particles are very small and thus hard to recover from a liquid effluent [ 16 , 17 ]. Another issue related to TiO 2 is its band gap energy (3.2 eV), limiting its use under UV A -based applications. In order to overcome these challenges, impregnation of TiO 2 [ 17 ] and other photocatalytic species [ 18 , 19 ] over different supports, as well as development of bimetallic catalysts [ 20 , 21 ] have been applied in previous studies. Moreover, the synergic effect of bi- [ 22 ] and polymetallic photocatalysts [ 23 ] has been reported. The different TiO 2 species formed as a consequence of variable metal loading on Ti-based mesoporous photocatalysts have been studied [22]. Impregnation of TiO 2 particles over mesoporous supports such as SBA-15 silica leads to materials with a higher thermal and hydrothermal stability, yielding large surface areas. In addition, modifying synthesis conditions and varying metal loading can be of great significance due to the many different structures that may be developed on the surface. In this regard, the synthesis of solids by means of co-modification of SBA-15 with TiO 2 and other metallic species can be crucial to modify their properties as photocatalysts and, therefore, the selectivity toward the different generated ROS. Several authors have shown that different proportions of metal provide different structures [24,25], while varying calcination rates generate different silica–metal interfaces [26] that modify the catalyst photoactivity. Considering possible metal species to be used, manganese oxides (MnOx) stand out due to their wide availability and multiple possible configurations [ 27 ]. Copper oxides (CuOx) are another very versatile group of metallic oxides with large availability that is increasingly studied in association with TiO 2 [ 28 – 30 ]. Moreover, cobalt oxides (CoOx) can also strongly enhance the photoactivity of TiO 2 in the visible region, and interesting results have been observed for wastewater treatment [ 31 , 32 ]. Based on previously characterized mesoporous materials which showed ability to photogenerate different radical species [ 33 ], this research focuses on the development and optimization of mesoporous photocatalysts derived from SBA-15 silicates, modified with titanium and an additional transition metal (Co, Cu, or Mn), individually introduced to produce a series of bimetallic Ti/M catalysts. These materials were synthesized with varying nominal metal loadings and calcination heating rates in order to explore the effect of these parameters on the resulting physicochemical properties, metal–support interactions, and photocatalytic activity. The catalysts were evaluated for their ability to generate reactive oxygen species (ROS) under UV–Vis light and to degrade Acid Orange 7 (AO7), a commonly used azo dye model pollutant. Finally, to make a more exhaustive study of the photocatalytic process and analyze the selectivity towards the formation of Sustain. Chem. 2025,6, 31 3 of 18 two of the main ROS, • OH and O 2•− , reactions were carried out in the presence of specific radical quenchers: isopropanol for •OH [34] and chloroform for O2•− [35]. 2. Materials and Methods 2.1. Synthesis The synthesis followed the same steps as in a previous report [ 33 ], with the mesoporous silica support synthesized following the method for SBA-15-type materials [ 36 ]. For this synthesis process, Pluronic P123 (Sigma-Aldrich, St. Louis, MO, USA; 100%) was dissolved overnight in aqueous 2M solution HCl (Cicarelli, San Lorenzo, Argentina; 36.5–38%) under stirring at 40 ◦ C. Afterwards, tetraethoxysilane (TEOS) (Sigma-Aldrich, 98%) was added dropwise to the solution and further stirred at the same temperature for 20 h. The asobtained sol-gel underwent hydrothermal aging at 80 ◦ C for 24 h. The material was then filtered, washed, and dried for 24 h at 60 ◦ C. Finally, the Pluronic P123 surfactant was removed by calcining in a muffle furnace at 500 ◦C for 8 h, heating at a rate of 1 ◦C/min. A portion of the as-obtained mesoporous support was used for characterization techniques. Another three batches of this same material were modified via the wet impregnation method with transition metals Co, Cu, and Mn, using CuSO 4· 5H 2 O (Mallinckrodt, St. Louis, MO, USA; 99%), CoSO 4· 7H 2 O (Sigma-Aldrich, 98%), or MnSO 4· H 2 O (Merck, Darmstadt, Germany; 99%) as precursors, respectively. A nominal load of 10 wt.% of each metal was expected to be achieved by employing properly concentrated aqueous solutions of the aforementioned salts. The three batches were suspended in the respective solutions, and the solvent was removed with a rotovap at 70 ◦C. After this step, the supports were dried at 90 ◦C and calcined, heating at a rate of 4 ◦C/min to a temperature of 500 ◦C for 5 h. Afterwards, four different supports (the previously obtained materials and an unmodified silica batch) were impregnated with Ti, with titanium n-butoxide, Ti(OBu) 4 (Sigma-Aldrich, 97%) as a metal source in a concentration adequate to reach a fixed nominal load of 20 wt.%. The procedure was the same as in the previous impregnation, but using isopropanol (Sintorgan, Buenos Aires, Argentina; 99.5%) instead of water, removing the solvent with a rotovap at 55 ◦ C. The obtained materials were identified as Ti/Co/M, Ti/Cu/M, Ti/Mn/M, and Ti/M, where “M” represents the mesoporous support. Once the more photoactive bimetallic catalyst was determined, new batches were prepared by varying the synthesis conditions to tune the reaction selectivity for the production of different possible ROS. Therefore, the same impregnation method previously described was applied to load different nominal metal weights of 2.5, 5, and 10 wt.%, along with a fixed nominal load of 20 wt.% Ti. Moreover, calcination processes with different heating rates (4 and 8 ◦ C/min) were tested because of their influence on the development of different metallic species. The obtained materials were labeled as Ti/[Metal](x)/M R(y), where (x) represents the nominal wt.% of loaded metal and (y) the applied heating rate. 2.2. Characterization Unmodified and modified mesoporous silicas were characterized via low-angle X-ra y diffraction (XRD) through a PANalytical X-Pert Pro instrument with CuK α radiation ( λ= 1.541 8 Å) between 1.2 and 3 ◦ 2 Θ angles. Textural properties were characterized through a Micromeritics ASAP 2020 Plus sorption analyzer, using N 2 as the sorption gas and with the solid materials degassed at 350 ◦ C. The optical properties of the materials were established via UV–visible diffuse reflectance spectroscopy (UV–Vis DRS), obtained through a Jasco V650 spectrophotometer equipped with an integrating sphere. Metallic species were determined by X-ray photoelectronic spectroscopy (XPS) using a XPS KAlpha Thermo Scientific instrument equipped with a low-power AlK α X-ray source and Sustain. Chem. 2025,6, 31 4 of 18 a double-focusing hemispheric analyzer. CasaXPS (Version 2.3.26) software was used for spectra analysis. 2.3. Catalytic Experiments The photocatalytic experiments were carried out with a stirred batch photoreactor, irradiated over its top. The photoreactor consists of a 25 mL Teflon beaker which is positioned over a magnetic stirrer. The suspension is directly irradiated with a 390 nm UV/Vis LED 30 W lamp installed over a window in a fireproof coating-covered wood platform. The light is distributed between 360 and 420 nm, with a peak at 390 nm. Additionally, the relative radiometric optical power distribution is highly concentrated in this type of design, with at least 90% of the total light output confined within a 55 ◦ angle [ 37 , 38 ]. The LED is equipped with a large heat sink in order to avoid temperature increases, as seen in Figure 1. For each of the experiments, 20 mg of modified mesoporous material were suspended in 20 mL (for a catalyst concentration of 1 g/L) of AO7 (Sigma Aldrich, 99.5%) solution (20 ppm). Following a 45 min incubation period in the dark, during which the change in AO7 concentration was negligible—indicating that adsorption/desorption equilibrium had been reached—a sample was collected to determine the initial concentration (C 0 ). The experiment was then initiated by exposing the system to light for 3 h, and the AO7 concentration (C) was monitored by measuring the absorbance at 485 nm using a Persee T7DS UV/Vis spectrophotometer. The percentage degradation (X) was calculated using the following equation: X=(C0−C)×100 / C0 where Xis the degradation percentage, and Cis the AO7 concentration at a given reaction time. Figure 1. Components and disposition of the photoreactor. A control experiment is presented in Figure S1 (Supplementary Material), in which the concentration of AO7 was monitored under LED lamp irradiation for 3 h. The results show that the concentration remains constant, indicating that photolysis does not occur under these conditions. Finally, reactions in the presence of 8% vol. isopropanol (Sintorgan, 99.5%) or 4% vol. chloroform (Cicarelli, 99%)—used as specific scavengers of hydroxyl or superoxide radicals, respectively—were carried out in order to infer about selectivity towards the possibly involved ROS. Some ROS were also detected with an Attenuated Total Reflectance–Fourier-Transform Infrared (ATR-FTIR) spectrometer using a Thermo Scientific Nicolet iS10 equipped with a Smart ARK module. This was achieved by placing 3 mL of 1 g/L suspension of photocatalyst in water over the ATR mode ZnSe window. The LED lamp was placed on top of the module, irradiating the suspension. Under these conditions, spectra were recorded at 10 and 30 min. Sustain. Chem. 2025,6, 31 5 of 18 3. Results and Discussion 3.1. Characterization of Synthesized Materials The low-angle X-ray diffraction (XRD) technique was used to determine whether the synthesized support shows peaks consistent with planes present in SBA-15 mesoporous silicas. Thus, the pattern shown in Figure 2presents peaks at 1.60 and 1.84 ◦ , which can be assigned to the planes (110) and (200), respectively, typical of highly-ordered SBA-15 type materials [ 36 ]. The diffractograms of the post-synthesis catalysts (presented in Figure S2, Supplementary Material) show that these peaks remain after metal impregnation, thus the modified silicas retain the mesoporous structure. Figure 2. Low-angle X-ray diffraction (XRD) pattern of unmodified silica framework. Figure 3shows the N 2 adsorption–desorption isotherms of bare SBA-15, Ti/M, Ti/Cu/M, Ti/Co/M, and Ti/Mn/M. All of the studied materials exhibit a type IV isotherm, which is typical for mesoporous structures [ 39 ]. Unmodified mesoporous support presents a H1 hysteresis loop, which is consistent with that of silicas exhibiting highly-arranged uniform mesopores [ 39 – 41 ]. When the support is modified only with Ti, the material presents a H5 hysteresis loop, a type of loop associated with partially blocked mesopores [ 39 ], which has been found in post-synthesis modified SBA-15 materials [ 42 , 43 ]. The bimetallic mesoporous supports exhibit different types of hysteresis loops. Ti/Cu/M presents a H2b type of loop that is associated with pore-blocking phenomena due to a broad size distribution of pore neck widths and a narrow distribution of pore cavity sizes [ 39 , 44 ], which means Cu and Ti species are mainly deposited in the pore end points. This type of distribution has been found in other modified mesoporous supports [ 45 ], modified with Ti [ 46 ]. Ti/Co/M presents a H2b and H1 hybrid type of hysteresis loop, which is narrower and indicates a smaller distribution of neck widths [ 39 , 44 ]. Meanwhile, Ti/Mn/M presents a H2a type of loop, associated with materials that exhibit a wide distribution of pore body sizes but a narrower distribution of pore neck widths [ 44 ], which has been already reported in Zr [ 47 ] and polymer-modified [ 45 ] mesoporous materials. This distribution signals higher deposition of Mn and Ti species along the mesoporous channels. Sustain. Chem. 2025,6, 31 6 of 18 M Ti/M Ti/Cu/M Ti/Co/M Adsorbed quantity (cm³/g STP) Relative pressure Ti/Mn/M 0.2 0.4 0.6 0.8 Figure 3. N2adsorption–desorption isotherms for all the mesoporous materials. Table 1summarizes the textural properties of the materials, with the Brunauer– Emmett–Teller (BET) specific area, pore volume (PV), and pore diameter (PD) singled out. As it can be seen, bimetallic modification leads to larger presence of Ti along the channels, as Ti/M presents the lowest pore diameter with largely blocked pores, which is consistent with its H5 hysteresis loop. It should also be noted that Ti/Mn/M exhibits a remarkably smaller pore volume and pore diameter than the other bimetallic catalysts did, consistent with a high presence of both metals along the pores, according to its isotherm. Nevertheless, after metallic impregnation, a substantial decrease in pore diameter was observed in all synthetized materials. Table 1. Textural and optical properties of the mesoporous materials. Material SBET (m2g−1)aPD (nm) bPV (cm3g−1)bEg (eV) c M 816 5.8 1.03 --- Ti/M 485 4.6 0.52 3.51 Ti/Cu/M 399 5.5 0.55 3.44 Ti/Co/M 403 5.5 0.56 1.49 Ti/Mn/M 289 4.9 0.35 3.27 a Calculated from the linear part of the Brunauer–Emmett–Teller (BET) plot, b Estimated by the Barrett–Joyner–Halen da (BJH) formula using the desorption branch from the isotherm, c Estimated with the Kubelka–Munk equation as a function of the energy. The band gap energy (Eg) of the metal-modified photocatalysts was calculated from the UV–Vis DR spectra (Figure S3A, Supplementary Material) using the graphic method based on fitting the Kubelka–Munk (KM) equation as a function of the energy in eV (F igure S3 B, Supplementary Material). As can be observed in Table 1, a band gap reduction from 3.51 eV for Ti/M to 3.44 and 3.27 eV for Ti/Cu/M and Ti/Mn/M, respectively, is observed. Meanwhile, for Ti/Co/M, the band gap decreases to 1.49 eV. Sustain. Chem. 2025,6, 31 7 of 18 3.2. Photocatalytic Evaluation of Materials Figure 4shows the photocatalytic evaluation of the materials in AO7 removal. As seen in the figure, Ti/Mn/M shows the highest photocatalytic activity, while Ti/Co/M presents the lowest—even lower than that of Ti/M—despite its low band gap energy. There have been multiple studies focusing on the complex mechanisms involving cobalt, which can either enhance photoactivity or suppress it. According to Gonçalves et al. [ 48 ], photoactivity is favored when cobalt is present at proportions below 0.5% as a dopant in the catalyst. However, when cobalt content exceeds this threshold, it acts as a recombination center due to the low stability of the Co 3+ species formed, which markedly decreases photoactivity. This point is in agreement with Pradhan et al. [ 49 ], who via PL studies on pure Co 3 O 4 discovered high signal intensity related to high electron-hole recombination. Meanwhile, Dong et al. [ 50 ] evaluated the Co 3 O 4 /TiO 2 bimetallic support and concluded that Co 3 O 4 exhibits high hole-trapping capability, with electrons falling back to the valence band, transferring photogenerated holes from TiO 2 to Co 3 O 4 . Collectively, these findings support the view that cobalt acts as an electron–hole recombination center, thereby reducing photocatalytic efficiency. In the present study, the DRS spectrum of Ti/Co/M (Figure S3A) shows absorption peaks at 484.5 nm and 733 nm, which can be attributed to charge-transfer transitions in Co 3 O 4 . This behavior closely resembles that described in [ 50 ], indicating an accelerated recombination of photoexcited electrons and holes [ 51 ], along with a low redox potential that limits photocatalytic performance [33,52]. 0 102030405060708090100 AO7 removal (%) Metal couples Ti/Mn/M Ti/Co/M Ti/Cu/M Ti/M Figure 4. AO7 removal via photocatalytic activity of the tested mesoporous materials. Reaction conditions: pH = 5, T = 25 ◦C, catalyst concentration = 1 g/L, rotational speed = 600 rpm. These results, compared with previous studies involving mesoporous silica-based materials modified with transition metals such as Ti, Cr, Co, Fe, and Ni to induce AO7 degradation through photocatalysis, show that bimetallic photocatalysts based on Mn and Ti exhibit large activity in azo dye remotion. Out of the previously studied materials, only bimetallic Cr and Ti-modified MCM-41 and SBA-15 materials exhibited a better response as a photocatalyst, although the presence of hexavalent Cr poses a significant drawback due to its high toxicity and potential leaching into the liquid phase [ 16 , 51 ]. Another silica-based material prepared as a mixed Si-Nb oxide achieved similar removal amounts to those of this study in 180 min of light exposure, but employing a lamp with an emission light Sustain. Chem. 2025,6, 31 8 of 18 peaking in the UVB range, in addition to relying on niobium, a much rarer and more expensive metal than manganese and titanium [ 53 ]. Among non-silica-based materials, mesoporous anatase modified with Mo exhibits much reduced AO7 removal numbers when exposed to a 400 nm light source compared to activity at 365 nm in the same time lapse [ 54 ]. Even more diminished numbers are reported for photocatalysts based on Agmodified ZnO [ 55 ], with around 40% of AO7 removal under exposure to 400 nm light for 180 min. Therefore, Mo and particularly Ag do not demonstrate being very effective at shifting the absorbance peak from the semiconductors to wavelengths in the UV–visible border. Low AO7 removal numbers (approximately 40%) are observed for Ni-modified TiO 2 bilayer films intermittently exposed to 365 nm light for four hours in an eight-hour process [ 56 ]. It must also be added that one of the main benefits from these Mn and Ti-bas ed materials is the easy synthesis and modification process. Other photocatalysts with promising performance in AO7 removal, such as Bi-doped SrTiO 3 perovskites [ 57 ] and Co78Si8B14/g-C3N4nanocomposites [58], require highly-controlled synthesis processes at temperatures above 1000 ◦ C, making them very energetically demanding and less practical. As Ti/Mn/M proved to be the material with the highest photoactivity for AO7 removal, it was further studied. Materials synthesized with different Mn loads and calcined using two different heating rates were systematically evaluated to elucidate the selectivity towards ROS generation and correlate it with the synthesis conditions. Figure 5shows the results of the photocatalytic evaluation of the Ti/Mn(x)/M R(y) materials. The Ti/Mn(x)/M R4 materials show an AO7 removal activity that remains essentially independent of the Mn load on the material. On the other hand, Ti/Mn(2.5%)/M R8 is the material that shows the highest photoactivity, achieving AO7 removal close to 90%. However, as Mn load increases for Ti/Mn(x)/M R8 materials, AO7 degradation capacity becomes lower, remaining constant for 5 and 10 wt.% loads. Thus, a higher dispersion of more isolated Mn species would seem to favor the catalytic activity. Figure 5. Comparison of AO7 degradation (%) after 180 min of photocatalytic activity for Ti/Mn(x)/M R(y) materials. Reaction conditions: pH = 5, T = 25 ◦ C, catalyst concentration = 1 g/L, rotational speed = 600 rpm. It is well established that the degradation of AO7 dye is primarily driven by reactive oxygen species (ROS) generated by photocatalytic materials [ 59 – 62 ]. To elucidate the predominant ROS involved in the degradation process, Ti/Mn(x)/M R(y) materials were Sustain. Chem. 2025,6, 31 9 of 18 retested under the presence of different radical scavengers. Isopropanol was used as a hydroxyl radical scavenger, while chloroform acted as a superoxide radical scavenger, allowing for the selective inhibition of these radicals and enabling the assessment of their contributions to the AO7 degradation process. The results from these experiments are shown in Figure 6. The Ti/Mn(x)/M R4 materials (Figure 6A) exhibit a significant reduction in their AO7 degradation capacity in the presence of organic solvents. Notably, this decrease in photoactivity is particularly pronounced for the Ti/Mn(10%)/M R4 sample, where the degradation performance is similarly reduced in the presence of both isopropanol and chloroform. Thus, it can be inferred that Ti/Mn(x)/M R4 materials produce both hydroxyl and superoxide radicals, though it is only in the Ti/Mn(10%)/M R4 case that superoxide radicals are as active in AO7 degradation as hydroxyl radicals. On the other hand, Ti/Mn(x)/M R8 materials (Figure 6B) show a remarkable reduction of their AO7 removal capacity under aqueous isopropanol solution when their Mn load is relatively low, whereas for Ti/Mn(10%)/M R8 the photoactivity is largely reduced with both isopropanol and chloroform aqueous solutions. From these results, it can be inferred that Ti/Mn(x)/M R8 materials show largely hydroxyl radical activity when Mn load is relatively low (2.5 and 5 wt.%). However, when the Mn load is increased to 10 wt.%, the contribution of the superoxide radical pathway in the degradation of AO7 significantly increases, becoming comparable to that of the hydroxyl radical pathway. These results confirm Mn as a successful dopant for ROS production, which is influenced by the metal content in the material and the calcination rate [63]. Figure 6. Comparison of AO7 degradation (%) after 180 min of photocatalytic activity for (A) Ti /Mn(x)/M R4 and (B) Ti/Mn(x)/M R8 materials. Reaction conditions: pH = 5, T = 25 ◦ C, catalyst concentration = 1 g/L, rotational speed = 600 rpm. Based on this, Ti/Mn(2.5%)/M R8 and Ti/Mn(10%)/M R8 were defined as the most and least photoactive materials, respectively. In order to study the superoxide radical generation and confirm that, according to the literature, they are generated through surface reduction of molecular oxygen with photoinduced electrons from the catalyst [ 3 ], a test insufflating N 2 to induce O 2 displacement was performed. As can be seen in Figure 7, Ti/Mn(10%)/M R8 shows a larger reduction of its AO7 degradation capacity when N 2 displaces O 2 , which confirms the contribution of the superoxide radical pathway in this material, as seen in Figure 6. In order to detect the generation of superoxide species on the materials’ surface, aqueous suspensions of Ti/Mn(2.5%)/M R8 and Ti/Mn(10%)/M R8 were irradiated and analyzed by in situ testing with ATR–FTIR spectroscopy. 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