N-Doped TiO₂-Nb₂O₅ Sol–Gel Catalysts: Synthesis, Characterization, Adsorption Capacity, Photocatalytic and Antioxidant Activity

Abstract

TiO₂-based semiconductors are formidable photocatalysts for redox reaction applications. Although N-doped TiO₂-Nb₂O₅ catalysts have already been explored in the literature, studies on their antioxidant activity are scarce, and systematic investigations on the effects of synthesis parameters over a wide range of %Nb and NH₄OH concentrations are limited. In addition, the relationship between optimal pH and %Nb has not yet been adequately explored. In the present work, the sol–gel synthesis of N-doped TiO₂-Nb₂O₅ catalysts was optimized using a design of experiments approach focused on photocatalysis, adsorption, and antioxidant applications. The samples were characterized by TGA, SEM/EDS, XRD, PZC tests, photoacoustic spectroscopy, and N₂-adsorption/desorption experiments. The salicylic acid (SA) degradation tests and DPPH radical scavenging assays demonstrated the superior photocatalytic activity (up to 72.9% SA degradation in 30 min, pH 5) and antioxidant capacity (IC₅₀ = 88.9 μg mL⁻¹) of pure TiO₂ compared to the N-doped TiO₂-Nb₂O₅ catalysts. The photocatalytic activity, however, proved to be intensely dependent on the pH and %Nb interaction, and at pH 3, the 25Nb-1N-400 catalyst promoted more significant SA degradation (59.9%) compared to pure TiO₂ (42.8%). In the methylene blue (MB) adsorption tests, the catalysts N-doped TiO₂-Nb₂O₅ showed removals at least seven times greater than TiO₂ catalysts without Nb.

1. Introduction

The control of water bodies’ contamination and the search for more efficient technologies for the remediation of organic pollution in water have been the subject of several recent studies [1]. This type of pollution often involves the presence of dyes, pharmaceuticals, pesticides, pollutants of emerging concern, and so-called POPs (persistent organic pollutants), which may be very harmful to human life and aquatic ecosystems [1,2,3,4]. Advanced oxidative processes (AOP) have been widely explored for the degradation of organic contaminants, mainly photocatalytic processes [5,6]. Photocatalysis is based on the formation of photogenerated electron/hole pairs, which can react directly with organic molecules and even completely mineralize them [5]. The strong dependency of these processes on the catalyst characteristics has motivated several efforts focused on photocatalyst improvement [7,8].

Titanium dioxide (TiO₂) is the most used photocatalyst, standing out for its chemical stability, low band gap (3.2 eV), photocatalytic activity under UV light, low toxicity, and environmentally benign characteristics [8,9,10,11]. Nevertheless, in the last decades, niobium pentoxide (Nb₂O₅) has stood out as an n-type semiconductor with a low band gap (3.4 eV) that may be applied as an alternative photocatalyst. While TiO₂ forms hydrocolloids in water and has increased hydrophilicity under UV radiation [9,12], Nb₂O₅ forms hydrocolloids of low stability in water, facilitating its separation [9,13,14]. About 98% of the world’s niobium reserves are located in Brazilian territory, and 90% of all niobium sold worldwide comes from Brazil, followed by Canada and Australia [15].

Many efforts have recently been made in semiconductors’ doping, mixed oxides synthesis, and heterojunction formation technologies to improve photocatalytic activity in organic pollutants degradation [5]. TiO₂-Nb₂O₅-based photocatalysts have already been applied in the degradation of compounds such as anti-inflammatories [16], dyes [17], herbicides [18], and cyanide oxidation [19]. Although pure titanium dioxide performs better than pure niobium pentoxide in some cases [20,21], studies indicate that the combination of these two semiconductors can bring benefits, such as increased absorption of visible light radiation, considerable improvement in the photocatalytic activity of TiO₂ [22,23], increased surface acidity of the catalysts, [23], greater sensitivity of the composite to UV radiation [18], and better charge separation in the heterojunction [24].

Previous studies on the TiO₂-Nb₂O₅ combination have not yet exhausted the investigation regarding the variation in the Ti/Nb proportions. Sacco et al., for example, prepared Pt–TiO₂–Nb₂O₅ by simply mixing the oxides in isopropanol at a fixed 1:1 ratio and calcining it at 650 °C [16]. Rafael et al. assessed the effects of Nb₂O₅ doping with small percentages of Ti (0.5–5.0%) in dyes degradation, reporting increased discoloration percentage as the Ti content increased [17]. Ferrari-Lima et al. carried out a fascinating study regarding the application of an N-doped TiO₂-Nb₂O₅ catalyst to the photocatalytic degradation of aromatic compounds, obtaining promising results for the N-doped Ti-Nb catalyst calcined at 500 °C. N-doping was performed by the addition of NH₄OH during the sol–gel synthesis. The work, however, was restricted to the study of the 50%TiO₂/50%Nb₂O₅ composition without evaluating the effect of the percentages of each phase [25]. Sedneva et al. studied the effects of %Nb (0.35% to 65%) and calcination temperature (80 to 1150 °C) on photoactivity, phase composition, and crystallite size. However, no assessment was made regarding the effect of solution pH on the catalyst performance [26]. There is a significant lack of studies addressing the impacts of pH variation on N-doped TiO₂-Nb₂O₅ catalysts’ performance. Many relevant works on the Ti-Nb combination reported in the literature [22,25,26,27,28,29,30] did not discuss the effect of pH on the photocatalytic activity of the oxides. However, it is known that the pH directly affects the surface charges of the catalyst and the potential of the valence and conduction bands [31,32,33].

In this context, the present work proposes a systematic study of the TiO₂-Nb₂O₅ combination applied to salicylic acid degradation, considering a wide range of Ti:Nb ratios combined with the effect of the synthesis conditions and solution pH. It uses experimental design methodologies focused on optimizing %Nb and other synthesis and experimental parameters to improve the photocatalytic activity of the synthesized materials and statistically confirm the significance of the observed effects. A radical quenching study was conducted to understand better the main routes involved in the photocatalytic degradation process. As a differential, the present study focused on evaluating whether the addition of Nb favors the photocatalytic activity of the material in different pH conditions, considering not only the effects of pH on the catalysts’ PZC but, mainly the impact of pH variation on the potentials of the valence and conduction bands of the oxides, given that previous works on N-doped TiO₂-Nb₂O₅ catalysts [22,25,26,27,28,29,30] have not addressed this topic in depth.

Salicylic acid (SA) is an emerging pollutant generally found in higher concentrations in wastewater treatment plants [34] and sewage sludge [35]. This compound is also frequently used as a model pollutant for studying photocatalytic processes [36,37,38], mainly replacing dyes [39]. For this reason, SA was chosen as a model pollutant for evaluating the synthesized photocatalysts.

Recently, the number of studies concerning the antioxidant activity of photocatalysts has grown, with a view to reducing food oxidation or neutralizing the harmful effects of free radicals on cells [40,41,42,43]. To the best of the authors’ knowledge, no investigation has been carried out on the antioxidant activity of N-doped TiO₂-Nb₂O₅ semiconductors. In this context, the antioxidant activity of N-doping TiO₂-Nb₂O₅ catalysts was studied in the present work using DPPH scavenging tests. As for the adsorption capacity of the catalysts, this was also evaluated by methylene blue (MB) adsorption tests, given that the MB molecule is already widely used in adsorption and photocatalysis studies [14,23,44,45,46,47].

2. Results and Discussion

2.1. Characterization Results

All synthesized catalysts were visually very similar, as a fine white powder. The SEM images (Figure 1) revealed a very rough and irregular microscopic surface, like tiny particles deposited over larger ones, as described by [25]. EDS spectra confirmed the presence of Nb in the catalyst in a mass percentage close to the theoretical Nb content.

The TGA indicated close to 30% mass loss by heating the samples to 400 °C (Figure S1 in Supplementary Materials). After calcination at 400 °C, most of the catalysts became crystalline, as shown in Figure 2. According to the X-ray diffractograms, pure TiO₂ calcined at 400 °C (0Nb-0N-400) was composed predominately by anatase phase (PDF#71-1167) and a small fraction of rutile (PDF#78-1510) (Figure 2h, peak around 27°). N-doping seems to have prevented rutile formation (Figure 2h) [25] and caused a slight displacement of peaks (Figure 2g,h). As for the pure Nb₂O₅ calcined at 400 °C (Figure 2a,b), minimal differences were observed in the diffractograms of undoped and N-doped samples, being possible to detect the Nb₂O₅ pseudohexagonal phase (TT-Nb₂O_5, PDF#28-0317) in both diffractograms.

In Figure 2c–f, the crystallinity of the mixed oxides calcined at 400 °C decreased as the Nb content increased. The widening of anatase peaks in the 25%Nb samples (Figure 2e,f) is evident compared to the TiO₂ samples calcined at the same temperature. The presence of Nb⁺⁵ ions in the structure stresses the TiO₂ lattice, hindering the growth of crystallites and reducing the crystallinity of the sample, as observed by [27]. The Nb incorporation also contributed to preventing rutile formation. Silva et al. observed that Nb doping could postpone anatase–rutile transformation at almost 150 °C [48]. According to Castro et al., this postponing effect of Nb addition can be observed even in samples calcined at higher temperatures, around 700 °C [49]. The 25Nb-0N-400 and 25Nb-2N-400 diffractograms did not evidence peaks associated with any niobium phase, while 75Nb-0N-400 and 75Nb-2N-400 catalysts proved to be amorphous, with no characteristic peaks (Figure 3c,d).

N-doping seems to have caused a modest increase in crystallinity in the sample 25Nb-2N-400 (Figure 2e), as confirmed by the crystallite sizes estimated for the 25%Nb catalysts (from 10.2 nm in the undoped sample to 16.1 nm in catalysts produced with NH₄OH, see

Table 1. Catalysts’ textural properties, identified phases, percentages, and crystallite size.

Catalyst	SBET m2 g−1	Slangmuir m2 g−1	Vtotal mm3 g−1	Identified Phases	dm (nm)
0Nb-0N-400	18	154	45	Anatase	27.0
				Rutile	41.3
0Nb-2N-400	18	229	49	Anatase	23.5
25Nb-0N-400	86	395	132	Anatase	10.2
25Nb-2N-400	86	601	178	Anatase	16.1
75Nb-0N-400	29	230	71	Amorphous	-
75Nb-2N-400	27	276	78	Amorphous	-
100Nb-0N-400	22	260	76	TT-Nb2O5	32.5
100Nb-2N-400	20	215	72	TT-Nb2O5	32.1
0Nb-0N-800	4	14	5	Rutile	96.1
0Nb-2N-800	3	19	6	Rutile	99.4
25Nb-0N-800	0	0	0	Rutile	77.3
				TiNb2O7	61.8
25Nb-2N-800	0	0	0	Rutile	79.6
				TiNb2O7	92.9
75Nb-0N-800	0	0	0	TiNb2O7	-
				Ti2Nb10O29	-
75Nb-2N-800	0	0	0	TiNb2O7	-
				Ti2Nb10O29	-
100Nb-0N-800	0	0	0	Monoclinic Nb2O5	60.57
100Nb-2N-800	2	8	2	Monoclinic Nb2O5	54.63

). Raba et al. reported a similar result, stating that the synthesis catalyzed in an alkaline medium tends to increase crystallites [13]. However, the crystallite size of pure TiO₂ and Nb₂O₅ catalysts slightly decreased after N-doping.

Calcination at 800 °C strongly impacted the crystalline structure (Figure 3), causing a relevant increase in crystallinity in all samples, as confirmed by de crystallite size estimations (Table 1). The anatase phase was transformed into rutile (Figure 3g,h) in the TiO₂ catalyst, while the monoclinic phase (PDF#72-1121) was the main phase identified in the Nb₂O₅ sample (Figure 3a,b). In the present work, obtaining the Nb₂O₅ monoclinic phase at a relatively low temperature was possible compared to reports [50]. In the mixed oxide catalysts, two additional phases were detected: the TiNb₂O₇ phase (PDF#39-1407) in the 25%Nb samples (Figure 3e,f) and the Ti₂Nb₁₀O₂₉ phase (PDF#40-0039) in the 75%Nb catalysts (Figure 3c,d).

The formation of a pure TiO₂ phase (rutile) and a mixed phase (TiNb₂O₇) in the 25%Nb catalysts suggests that Nb is not homogeneously dispersed in the sample. According to [51], niobium tends to segregate on the surface of Nb-doped TiO₂ (0.18 to 0.018% atom Nb) under oxidizing atmospheres. Based on this, Silva et al. stated that TiNb₂O₇ formation at 800 °C indicates niobium segregation in the catalyst surface [49]. Silva et al. estimated 2 mol% Nb₂O₅ as the maximum limit from which a second phase is formed, probably with the occurrence of surface segregation [48]. The second phase could not be visualized in the sample calcined at 400 °C, probably due to the low crystallinity [49].

The thermal treatment at 800 °C caused a drastic decrease in the specific surface area and pore volume (Table 1). The combination of Ti and Nb and calcination at 400 °C produced oxides with a higher specific area (S_{BET,25Nb-0N-400} = 86 m² g⁻¹, and S_{BET,75Nb-0N-400} = 29 m² g⁻¹) than pure oxides (S_{BET,0Nb-0N-400} = 18 m² g⁻¹ for TiO₂ and S_{BET,100Nb-0N-400} = 22 m² g⁻¹). This evidences that Nb incorporation into titania favors an increase in the S_BET, producing S_BET values four times as high as synthesized TiO₂ specific area. This increase in surface area was expected, along with a decrease in surface energy and increased stability of nanoparticles [48]. Considering the area determined by the Langmuir model, it was possible to notice that, in general, the use of NH₄OH led to an increase in the specific surface area of the catalysts. The same behavior was not noticed in the specific area values determined by the BET model.

The analysis of the adsorption isotherms (Figure S3) indicated that the TiO₂ samples presented isotherms similar to type III, and the TiO₂ catalyst prepared with the addition of NH₄OH showed a slight H3 hysteresis loop. Vaizoğullar observed a similar type of isotherm for his MoS₂ photocatalyst, associating it with the unrestricted adsorption in multilayer and the H3 hysteresis with the presence of slit-like pores in the catalyst [52]. The undoped 25%Nb sample (Figure S3c) showed a characteristic type IV isotherm associated with mesoporous materials with pores larger than 4 nm [53,54]. The other samples showed isotherms with intermediate characteristics between these two.

The Rietveld refinement performed (Figure 4), based on the work of [55] on the TT-Nb₂O₅ phase, resulted in the structure represented in Figure 5a. The refinement of the 25% sample indicated an increase and slight distortion in the cell parameters of the anatase phase after Nb doping (

Table 2. Cell parameters obtained in Rietveld refinement.

χ2	Sample	Phase	a	b	c	Alfa	Beta	Gamma	Volume
1.271	0Nb-0N-400	anatase	3.784329	3.784329	9.511708	90	90	90	136.219
		rutile	4.59335	4.59335	2.959285	90	90	90	62.438
2.901	25Nb-0N-400	anatase	3.805582	3.805582	9.524768	90	90	90	137.942
5.244	100Nb-0N-400	TT-Nb2O5	6.221601	29.05318	3.926213	90	90	90	709.691

) due to the difference in the ionic radius of Nb, evidencing Nb atoms incorporation into TiO₂ structure [48].

As for the optical properties of the catalysts, there was no significant variation in the band gap of the samples (Figure S4).

Table 3. Band gap values estimated for samples calcined at 400 °C.

	Band Gap Energy (Eg)
Sample	Direct	Indirect
0Nb-0N-400	3.04	2.94
0Nb-1N-400	3.04	2.94
0Nb-2N-400	3.04	2.93
12.5Nb-0N-400	3.04	2.98
12.5Nb-1N-400	3.16	3.04
12.5Nb-2N-400	3.17	3.01
25Nb-0N-400	3.12	2.98
25Nb-1N-400	3.18	3.03
25Nb-2N-400	3.17	3.00
100Nb-0N-400	3.14	3.01
100Nb-2N-400	3.11	2.99

summarizes the estimated values for some catalysts calcined at 400 °C. Undoped and N-doped TiO₂ samples showed the lowest direct (3.04 eV) and indirect (2.93–2.94 eV) band gap energies. On the other hand, catalysts with 12.5 or 25% Nb showed a slightly higher band gap, closer to the values presented by Nb₂O₅ samples (3.11–3.14 eV). Nitrogen doping caused different effects on the band gap value depending on the %Nb: it was indifferent to TiO₂, causing an increase in the band gap for mixed oxides and a decrease for Nb₂O₅.

The band gap values observed in the present work are in accordance with literature reports. Kumari et al. calcined Nb₂O₅ samples at different temperatures and registered band gap values between 3.06 and 3.09 eV for the samples with an orthorhombic structure [50]. Rafael et al. synthesized catalysts with higher Nb₂O₅ content (95–99.5%) with band gap energies in the range from 3.10 and 3.35 eV [17], while Ücker obtained Nb₂O₅/TiO₂ catalysts with band gaps of 3.4 eV [29].

In the plots of photoacoustic signal vs. wavelength (Figure 6), two samples stand out: 0Nb-0N-400 and 25Nb-1-400. They presented the two most intense absorption bands in the graph, the 25Nb-1N-400 sample with a more intense and narrow absorption band and pure TiO₂ (0Nb-0N-400) with a broader absorption band, including the region between 400 and 420 nm.

2.2. Photocatalytic Activity Tests

2.2.1. The 2⁴-Factorial Design: Screening Tests

In the first set of experiments, the catalysts calcined at 800 °C presented the lowest removal percentages (Figure 7a). For 25%Nb catalysts, the probable cause for this behavior would be the complete conversion of anatase to rutile (Figure 3) and the drastic decrease in the specific surface area (Table 1) after calcination at 800 °C. Although combining anatase and rutile in optimal proportions is beneficial for the photocatalytic process, higher rutile contents reduce the catalyst performance due to the inferior photocatalytic activity of rutile [56]. Its lower activity is closely related to the decrease in specific surface area and increase in pore size, resulting in a limited number of available active sites [56].

Catalysts containing 75% of niobium, in turn, presented inferior results, regardless of the calcination temperature (Figure 7a), probably because they are amorphous at 400 °C, and, despite becoming crystalline at 800 °C, they suffered a drastic decrease in their specific surface area. Statistical analysis of results confirmed the strong effect of T_calc and %Nb on the photocatalytic activity. The effects estimate of the 2⁴-factorial design indicated significant linear effects of the calcination temperature, niobium percentage, and pH (Figure 7b). The first two were negative, and the last positive, suggesting that decreased T_calc or %Nb and increased pH favors SA degradation. T_calc was the most significant effect, followed by T_calc interaction with %Nb and %Nb linear effect. Thus, T_calc and %Nb combined were the main determinants for the photocatalytic activity of the samples (Figure 7), given their direct impact on the critical properties of photocatalysts, such as crystalline structure (Figure 2 and Figure 3), specific surface area (Table 1), charge separation, and superficial acidity, among others. Supplementary Materials reunites all the response surfaces, effect estimates, and the ANOVA table obtained for the 2⁴-factorial design. Regarding the model adjustment, a linear model was fitted to the data (R² = 0.96405 e R²adj = 0.91461) with no significant curvature or lack of fit (Table S1).

Significant interaction effects were detected involving T_calc and %Nb, and T_calc and pH. The response surface plotted as a function of %Nb and T_calc (Figure 7a) evidences the T_calc/%Nb interaction effect, given that decreasing %Nb potentializes the impact of calcination temperature.

N-doping presented a negative interaction effect with the calcination temperature, slightly improving the photocatalytic performance of catalysts calcined at 400 °C (Figure S1b).

Based on the analysis of the first block of experiments, it was clear that using catalysts with lower Nb content and calcined at lower temperatures radically favored the degradation of SA. Therefore, a new set of experiments was carried out using 0% to 25%Nb catalysts in the same pH and NH₄OH:M range. As for the calcination temperature, despite its significant effect on the degradation of SA, increasing T_calc just caused a sharp reduction in the removal efficiency, even when considering interactions with other factors. The negative effect of the calcination temperature on the surface area also impacted this decision-making. That said, calcination temperature was excluded from the analysis, and T_calc was set at 400 °C, the lowest temperature required to remove organic residues from the catalyst (see TGA).

2.2.2. The 3³-Factorial Design: Optimization

Additional tests were conducted in the second block of experiments using a 3³-experimental design. The response surface obtained (R² = 0.93463 R²_adj = 0.82765) was able to predict the experimental results with reasonable accuracy (Figure 8a). The model was significant with 95% confidence (p = 0.000386 < 0.05, F_Calculated = 8.73 > F_{Table, 0.05,18,11} = 2.67), while its lack of fit was not significant (p = 0.178122 > 0.05, F_Calculated = 3.28874 < F_{Table, 0.05,8,3} = 8.85). The ANOVA table is in the Supplementary Materials.

The effects estimates highlighted the relevance of the interaction between %Nb and pH (Figure 8b), followed by their respective linear and quadratic effects. The response surfaces at different pHs (Figure 9a–c) evidence that the %Nb effect is pH dependent. At pH 5 and 7, TiO₂ catalysts, notably 0Nb-0N-400, showed better results than mixed oxides, achieving 72.9% SA degradation in 30 min at pH 5. The better activity of the 0Nb-0N-400 catalyst may be related to the rutile phase in the samples, given that rutile in small amounts combined with the anatase phase can prevent the recombination of electron-hole pairs [57]. On the other hand, the samples containing Nb at pH 5 and 7, showed degradations between 44.4% and 57.3%, the latter being obtained by the catalyst 12.5Nb-1N-400 at pH 5.

At pH 3 (Figure 9a), the catalysts with higher %Nb present the best results. The photocatalyst 25Nb-1N-400 achieves 59.9% SA degradation compared to 42.8% degradation produced by 0Nb-0N-400. Surface plots in the function of pH and NH₄OH:M ratio (Figure 9d–f) also confirm this behavior. While TiO₂ catalysts’ activity (Figure 9d) suffers a significant dependence on pH and is almost independent of N-doping, 25%Nb catalysts’ performance (Figure 9f) depends on both pH and N-doping, with a maximum removal at pH 3 and NH₄OH:M ratio close to one.

Different reports have already been made in the literature regarding the effects of the Nb₂O₅ and TiO₂ combination. According to Rafael et al. [17], increasing Ti content in Ti-doped Nb₂O₅ catalyst promoted bromophenol blue degradation. It is important to emphasize that the tests were conducted at pH 5.5 [17]. On the other hand, Ahmad et al. (2008) observed that increasing Nb-doping in TiO₂ samples from 0.5 to 1.0 atom% improved the photocatalytic activity of the material in the degradation of 2-chlorophenol under sunlight [30], while Sacco et al. reported that Pt-TiO₂-Nb₂O₅ prepared with Ti:Nb = 1:1 presented a similar performance to Pt-TiO₂ in diclofenac degradation [16]. In the present case, as indicated in the response surfaces (Figure 9), only a minimal %Nb can be used without impact on the photocatalytic activity at pH 5 and 7. Although the TiO₂-Nb₂O₅ combination is beneficial for charge separation, the excessive increase in the Nb/Ti ratio (above 0.2) can lead to a decreased photocatalytic activity, given that an excessive number of heterojunctions can act as recombination sites for electrons and holes [24]. In this sense, Shiraishi et al. affirmed that Nb⁺⁵ loading in TiO₂ is only beneficial at minimal concentrations, around 0.1%, due to the formation of NbO mononuclear species. Higher Nb⁺⁵ contents produced polynuclear Nb₂O₅ species, decreasing the activity of Au/Nb⁵⁺/TiO₂ catalysts synthesized by the authors [28].

In contrast, Sedneva et al. [26] identified the optimal ratio in much higher niobium concentrations, between 15 and 30% Nb. The percentage of 25%Nb identified as optimal at pH 3 in the present work is within the range described by Sedneva [26], although the authors did not mention the effect of pH in their study. Many relevant works on the Ti-Nb combination [22,25,26,27,28,29,30] do not discuss the effect of pH on the photocatalytic activity of the oxides. Therefore, there is still much to be explored regarding this topic.

Given the electrostatic interactions between the organic molecule and the catalyst surface, it is necessary to ponder that the pH of the solution can directly affect the photocatalysis degradation efficiency of organic compounds like SA. Figure 10 shows the net specific surface proton excess vs. pH plots obtained from the potentiometric titration for samples 0Nb-0N-400 and 25Nb-1N-400. The point of zero charge determined for 0Nb-0N-400 was equal to 6.3, meaning that at pH 3 and 5 tests, the catalyst surface is predominantly positively charged (pH < pH_pcz), and the SA is negatively charged (pH > pKa = 2.98 [58]), a condition favorable for SA adsorption.

The addition of Nb to TiO₂ led to a decrease in the pH_pcz of the catalyst (Figure 10b), restricting the pH range in which the electrostatic attraction between the negatively charged SA molecules (pH > pKa = 2.98 [58]) and the positively charged photocatalyst surface (pH < pH_pcz = 5.2) is more intense. Nonetheless, even when the surface of the 25Nb-1N-400 catalyst is positively charged (pH < pH_pcz), the values of net specific surface proton excess are lower than in the TiO₂ catalyst in the same condition.

Such observations, however, do not justify the results obtained from the photocatalytic degradation of SA, given that, based on the values indicated in Figure 10, the photocatalytic activity of the 0Nb-0N-400 catalyst is expected to increase at pH 3, due to the more intense interaction between the SA and the catalyst. Nevertheless, the opposite was obtained experimentally. Additional quenching tests were performed better to understand the SA degradation mechanism by N-doped TiO₂-Nb₂O₅ catalysts.

2.3. Quenching Tests: Contribution of Radicals

Photolysis tests (without catalyst addition) were initially performed at pH 3 and 7 (Figure 11a,d). The results obtained in the absence of scavengers (leftmost columns) confirmed the increase in photolytic degradation of SA when pH increases from 3 to 7, as previously reported [59]. Thus, in part, a more significant degradation of AS at pH 7 in photocatalytic tests is expected due to the considerable contribution of photolysis.

The values of photolytic degradation obtained experimentally (red) were used to estimate the contribution of the photocatalysis process alone (blue), discounting photolysis. In addition, photolysis tests were performed in the presence of scavengers (ISOP, BZQ, and AO) to assess their effect on photolysis. AO, the hole (h⁺) scavenger, did not significantly affect the photolytic degradation of SA, which was expected since no photogenerated holes participated in the photolysis reaction. The observed variation was minimal and could be attributed to experimental fluctuations. Isopropyl alcohol, a hydroxyl radical (^●OH) scavenger, only affected photolysis at pH 3. The presence of benzoquinone, a superoxide (O₂^●−) radical scavenger, on the other hand, practically nullified photolysis both at pH 3 and pH 7.

In the photocatalytic tests with the 25Nb-1N-400 catalyst (Figure 11b,e) with no scavengers, discounting the estimated contributions of photolysis (red) and adsorption (white), it is evident how much the photocatalysis (blue) is relevant at pH 3, in contrast with pH 7. In tests with scavengers, the participation of hydroxyl radicals and holes is much more significant at pH 3 than at pH 7. The more significant participation of holes at pH 3 can be directly related to the better adsorption of AS molecules due to the surface charges of the catalyst. Even so, it is also necessary to consider that photogenerated holes contribute to hydroxyl radicals’ formation.

On the other hand, for the 0Nb-0N-400 catalyst, the participation of hydroxyl radicals and the superoxide radical is much more relevant at pH 7. This seems to suggest that each catalyst favors a different degradation pathway. The reaction mechanism was analyzed from the perspective of the conduction band (CB) and valence band (VB) potentials to distinguish between the oxidation and reduction pathways. It is possible to estimate the band energy position using Equation (1) [60]:

$$E_{CB}^0=\chi \left(S\right)-E^e-\frac{1}{2}{E}_g$$

(1)

In Equation (3), $E_{CB}^0$ is the conduction band edge position at the point of zero charge (PZC), $E_g$ is the band gap energy of the semiconductor, and $E^e$ corresponds to the energy of free electrons in the hydrogen scale (~4.5 eV) [60]. The term $\chi \left(S\right)$ is the electronegativity of the semiconductor, which can be calculated by the geometric mean of the electronegativity of the constituent atoms [60]:

$$\chi \left(S\right)=\sqrt[N]{{\chi }_1^n{\chi }_2^s\dots {\chi }_{n-1}^p{\chi }_n^q}$$

(2)

where ${\chi }_n$ is the electronegativity of the constituent atom, $n$ is the number of species, and $N$ is the total number of atoms. Using Equation (2) and the electronegativity of Nb, Ti, and O atoms described by [61] and [60], the electronegativity of Nb₂O₅ and TiO₂ obtained were 6.23 eV and 5.83 eV, respectively. Applying these values and the $E^e$ values obtained experimentally (Table 3) to Equation (1), $E_{CB}^0$ estimated for TiO₂ and Nb₂O₅ were $E_{CB, Ti{O}_2}^0=-0.13$ eV and $E_{CB, N{b}_2{O}_5}^0=0.23$. Valence band energy position ($E_{VB}^0$) can be obtained from Equation (3) [17]:

$$E_{VB}^0=E_{CB}^0+E_g$$

(3)

From Equation (3), the obtained values were $E_{VB, Ti{O}_2}^0=2.8$ and $E_{VB, N{b}_2{O}_5}^0=3.24$. However, the position of the bands varies with pH, shifting negatively with increasing pH [31,32,33]. Considering that the values obtained for the positions of the valence and conduction bands refer to the pH_pcz, the following correction was made:

$$E_{CB}=E_{CB}^0+0.059\left(p{H}_{PCZ}-pH\right)$$

(4)

$$E_{VB}=E_{VB}^0+0.059\left(p{H}_{PCZ}-pH\right)$$

(5)

For N-doped Nb₂O₅, pH_pzc obtained experimentally was 6.0, while for pure TiO₂, the pH_pzc value was 6.3; for N-doped TiO₂, it was equal to 5.8. The corrected values are described in Figure 12.

As can be seen in the comparison between the catalyst’s band positions, photogenerated electrons in TiO₂ CB tend to migrate to Nb₂O₅ CB, while holes formed in Nb₂O₅ VB migrate to the VB of TiO₂, favoring charge separation in the heterojunction. As can be seen from the analysis of the radical’s redox potentials [62], TiO₂ VB potential is more positive than OH⁻/^●OH (+1.89 vs. NHE) and H₂O/^●OH (+2.72 vs. NHE) redox potentials [62], making hydroxyl radical generation thermodynamically favorable at both pHs. On the other hand, Nb₂O₅ CB potential is less negative than O₂/O₂^●− reduction potential, making its formation unfavorable at both pHs. This suggests that, in the case of the mixed oxide catalyst, the photocatalytic process takes place more by the oxidative route, with significant participation of photogenerated holes and hydroxyl radicals formed by the oxidative pathway.

For pure TiO₂, its conduction band potential at pH 7 (-0.17 eV) is slightly more negative than the O₂/O₂^●− reduction potential (−0.13 vs. NHE) [62], allowing the 0Nb-0N-400 catalyst to promote reduction pathway reactions at pH 7 (Figure 11f), with the contribution from both O₂^●− radical and ^●OH radicals formed from O₂^●−.

Reusability Tests

Reuse tests were carried out using the 25Nb-1N-400 catalyst, in 60 min lasting tests. At the end of each reuse cycle, a certain amount of SA stock solution was added to the medium to reestablish the initial concentration of the pollutant. The results obtained (Figure 13) indicated a reduction in the percentage of SA degradation, which became stabilized after the fourth cycle. In the last cycle, the catalyst reached only 73.9% of its degradation capacity presented in the first test. This reduction can be partly attributed to the accumulation of intermediaries formed by the degradation of the SA. The main reaction products of SA degradation by hydroxyl radicals have already been described in the literature as 2,3-dihydroxybenzoic acid, 2,5-dihydroxybenzoic acid, and catechol [63,64,65,66,67]. These compounds compete with SA for hydroxyl radicals, reducing the SA photocatalytic degradation rate.

Despite this reduction in degradation percentages, it can be said that the catalyst degradation capacity has been reasonably preserved, in view of the competition between SA and its by-products for the hydroxyl radicals formed. The reuse tests indicated the possibility of applying the synthesized catalysts in processes of longer duration, as well as in conditions where different organic compounds are simultaneously present.

2.4. DPPH Assays: Antioxidant Capacity

In the DPPH scavenging tests (Figure 14), TiO₂ catalysts showed the highest inhibition percentages, with N-doping clearly showing a detrimental effect on their antioxidant activity. For the mixed TiO₂-Nb₂O₅ catalysts, on the contrary, the increase in the concentration of NH₄OH used in the synthesis favored the antioxidant activity, more pronounced in the catalysts with higher %Nb.

The catalyst with the best antioxidant activity was pure TiO₂ (0Nb-0N-400), with an estimated IC₅₀ equal to 88.9 μg mL⁻¹ (R² = 0.9932). However, even at considerably higher catalyst concentrations (up to 2 mg mL⁻¹), it was impossible to exceed 58.3% inhibition (Figure 15).

2.5. Methylene Blue Adsorption Tests

Although some catalysts have presented low specific surface areas, it is known that the adsorption capacity of a material does not depend exclusively on this property but also on the characteristics of its surface chemistry [45,46,47]. Thus, the following adsorption tests were carried out to provide an overview of their adsorption capacity and support future in-depth studies on their surface characteristics and applicability. Methylene blue was used as adsorbate given that it is a molecule already widely used in adsorption and photocatalysis studies [23,44], applied in evaluating adsorption and cation exchange capacity and surface area of clays [45,46,47], and is also recognized for its ability to interact with Nb₂O₅ acidic sites during adsorption [14]. The results of adsorption tests carried out according to the 3²-experimental design are represented in Figure 13a. The response surface obtained was able to predict the experimental results with considerable accuracy. The ANOVA table is in the Supplementary Materials. The analysis of the effects confirmed the significance of all factors, highlighting the linear and quadratic effects of %Nb (Figure 16b).

Catalysts containing Nb removed more significant percentages of MB, at least seven times higher than TiO₂ catalysts (

Table 4. Methylene blue removal by adsorption.

Sample	MB Removal %
0Nb-0N-400	10.14
0Nb-1N-400	0.69
0Nb-2N-400	9.61
12.5Nb-0N-400	71.37
12.5Nb-1N-400	84.41
12.5Nb-2N-400	84.44
25Nb-0N-400	74.56
25Nb-1N-400	90.49
25Nb-2N-400	88.60
100Nb-0N-400	4.85
100Nb-2N-400	6.82

). Likewise, the TiO₂-Nb₂O₅ catalysts also showed better results than Nb₂O₅.

According to [68], the main mechanism of MB adsorption on Nb₂O₅ is the interaction between the dye molecules and Lewis and Bronsted acidic sites present on the catalyst. Metallic ions with reduced coordination on the oxide surface, like Nb⁺⁵, can act as strong Lewis acidic sites [69] and interact with certain nucleophilic parts of methylene blue molecules, such as the electron pairs of nitrogen and sulfur, favoring adsorption [14]. In this sense, Ferraz et al. [23] reported that pure Nb₂O₅ showed a slightly higher MB removal capacity than the mixture of TiO₂-Nb₂O₅ oxides. Such results, however, differ from those currently observed (Table 4). Although the difference in the surface area of the catalysts (Table 1) may have contributed to these results, the fact that the mixed oxides showed a removal capacity between 7 and 9 times greater than that of TiO₂ and between 10 and 15 times greater than that of pure Nb₂O₅, compared to about a fourfold increase in surface area, suggests a strong influence of the surface characteristics of the materials.

Incorporating Nb in TiO₂ may increase surface acidity [22] and improve methylene blue adsorption [23]. MB adsorption results strongly suggest that the TiO₂-Nb₂O₅ combination contributed to an increased number of acidic sites on the catalyst surface [22], improving the adsorption of dye molecules. This fact is encouraging for dye-sensitization applications, given that the efficiency in this process is directly dependent on the efficiency of dye adsorption [70], making room for additional studies on the application of synthesized catalysts and their surface characteristics.

3. Materials and Methods

3.1. Chemicals

Acetylsalicylic acid (ASA, ≥98%-supplied by Biotec, São Paulo, Brazil), acetonitrile (HPLC—supplied by J.T. Barker, Ciudad de México, México), NbCl₅ (CBMM), titanium isopropoxide (Aldrich, São Paulo, Brazil), Tween 20 (Synth), isopropyl alcohol (ISOP, Êxodo científica, Sumaré, São Paulo, Brazil), p-benzoquinone (BZQ, Êxodo científica, Sumaré, São Paulo, Brazil), ammonium oxalate (AO, Êxodo científica, Sumaré, São Paulo, Brazil), and ammonium hydroxide (NH₄OH), phosphoric acid (Biotec), potassium phosphate monobasic (KH₂PO₄, Dinâmica, São Paulo, São Paulo, Brazil).

3.2. Synthesis of the Catalysts

The TiO₂-Nb₂O₅ catalysts were synthesized through a sol–gel methodology using different percentages of niobium (%Nb) in the total number of moles of metal ions (M = Ti +Nb). Titanium isopropoxide and NbCl₅ were used as precursors, and due to their high reactivity, both reagents were manipulated inside a glovebox containing an inert atmosphere (Ar).

NbCl₅ was dissolved into isopropanol and stirred until the solution became colorless. After this interval, Tween 20 and titanium isopropoxide were added to the mixture, considering the ratio of 30 g of Tween 20 for each mol of M (M = Ti⁴⁺ + Nb⁵⁺) and respecting the %Nb determined by the experimental design. Finally, ultrapure water (15 mol of H₂O for each mol of M) mixed with different concentrations of NH₄OH (0, 1, or 2 mol for each mol of M) was added slowly, dropwise, to the mixture under intense agitation. The gel formed was left to age for 72 h at 25 °C. The precipitate obtained was filtered, washed with ultrapure water, dried at 60 °C overnight, and finally calcined at different temperatures (400 °C, 600 °C, or 800 °C) for five hours. The calcination process comprehended a heating rate of 1 °C min⁻¹ and constant temperature steps every 100 °C, lasting 30 min.

The NH₄OH: M molar ratio and the calcination temperature used in the synthesis step were varied following the DoE (see Section 2.4). NH₄OH was used in the synthesis since it reduces gelation time [71] and affects drying and calcination steps of sol–gel synthesis [72]. The effect of NH₄OH addition was observed in the almost immediate formation of the gel in the present work, mainly in the case of Nb₂O₅, which presented the slowest gelation process. Regarding the use of Tween 20, reports indicated adding Tween 20 in the synthesis can help in shape control [73].

The catalysts’ nomenclature was given by percentage of Nb, NH₄OH:M molar ratio, and calcination temperature; e.g., 0Nb-2N-800 represents TiO₂ catalyst (0% Nb), synthesized with 2 mols of NH₄OH per mol of Ti and calcined at 800 °C, while 25Nb-0N-400 corresponds to the catalyst containing 25% Nb, prepared with no addition of NH₄OH and calcined to 400 °C.

3.3. Characterization of the Photocatalysts

Rigaku MiniFlex 600 diffractometer (40 kV and 15 mA, Rigaku, Wilmington, MA, USA) was used to perform X-ray diffractometry (XRD) analysis of the catalysts, with Cu Kα radiation (k = 1.5406 Å), in step scan mode, with a step of 0.02° and a time per step of 2 s at a scanning interval of 2θ = 3–90°. The data were compared with the JCPDS (Joint Committee on Powder Diffraction Standards) database.

The thermogravimetric analyses were performed in TA Instruments equipment, model SDT Q-600 (Oxford, TESCAN, Saint Petersburg, Russia), with an analysis temperature range of 303 to 1073 K and a heating rate of 10 K min⁻¹ in a nitrogen atmosphere and 100 mL min⁻¹ flow. Surface characterizations were performed using scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) (Tescan Scanning Electron Microscope, Vega 3 LMU equipped with dispersive energy detector—EDS-Oxford, AZTec Energy X-Act).

The textural properties of the materials were determined from N₂ adsorption–desorption isotherms at -196 ºC. Firstly, samples were degasified using a micromeritics Smart Vacprep (Micromeritics Instrumen, Norcross, GA, USA), and secondly, N₂ adsorption–desorption isotherms were obtained using a Micromiretics TriStar II Plus adsorption analyzer. Surface areas and pore volume were determined using TriStar II Plus Version 3.03. The degassing of the samples was carried out based on the following heating program: up to 30 °C, at 10 °C/min and holding time of 3.3 min; up to 90 °C, at 5 °C/min and holding time of 30 min; and, finally, up to 150 °C, at 5 °C/min and holding time of 480 min.

The catalysts’ point of zero charge (PZC) was determined using a potentiometric titration methodology [74]. The preparations for the analyzes included intense boiling of the water used in the solutions and purification of the analyzed catalysts by repeated washing steps with ultrapure water. A 50% NaOH stock solution was prepared and filtered to remove carbonates. For each titration, 50 mg of purified catalyst was suspended in 50 mL of electrolyte solution (NaNO₃, 0.005 M, and 0.5 M). Titration was then performed with NaOH and HNO₃ solutions, 0.1 M each, with additions of 50 μL, down-titration first, followed by up-titration. The pH of the suspension was recorded after reaching equilibrium. The correlation between pH and proton concentration was determined by a calibration curve, as described by [74]. The catalysts were also characterized by photoacoustic spectroscopy, as previously described in [75]. Direct and indirect band gap energies were estimated from the Tauc plot, as described in [76].

3.4. Desing of Experiments (DoE)

Initially, a 2⁴-factorial design with 3 replicates of the central point was performed as a screening step to assess the most relevant factors affecting the N-doped TiO₂-Nb₂O₅ catalysts synthesis and photocatalytic performance. The factors and levels considered are described in

Table 5. Factors and levels considered in the 2⁴-factorial design—Photocatalytic tests.

Level (Coded Variables)	%Nb (Molar)	NH4OH:M (Molar Ratio)	Tcalc (°C)	pH (Photocatalytic Tests)
−1	25	0	400	3
0	50	1	600	5
+1	75	2	800	7

. The %Nb (Equation (6)) represents the percentage of niobium in the total number of mols of metallic ions, M:

$$\%\mathrm{Nb} =\frac{n_{Nb}}{n_M}·100= \frac{n_{Nb}}{n_{Nb}+n_{Ti}}·100$$

(6)

Thus, %Nb = 0 represents pure TiO₂, and %Nb = 100 represents pure Nb₂O₅. NH₄OH:M is the molar ratio between ammonium hydroxide and M. The calcination temperature used as the final synthesis step is indicated by T_calc (°C). The fourth factor considered (Table 5) was the solution pH during the photocatalytic tests. The chosen pH range varied between 3 and 7, given that pH values close to 3 tend to maximize direct electron transfer processes involving SA, which are described as the main pathway responsible for its degradation [37].

Based on the results obtained in this initial screening step (2⁴-factorial design), a new set of experiments was carried out, following a 3³-factorial design, to optimize the photocatalytic activity of the catalysts. The factors and levels considered are described in

Table 6. Factors and levels considered in the 3³-factorial design—Photocatalytic tests.

Level (Coded Variables)	%Nb (Molar)	NH4OH:M (Molar Ratio)	pH (Photocatalytic Tests)
−1	0	0	3
0	12.5	1	5
+1	25	2	7

.

The removal of methylene blue by adsorption was also studied following a 3²-experimental design, with factors and levels indicated in

Table 7. Factors and levels considered in the 3²-factorial design—Adsorption tests.

Level (Coded Variables)	%Nb (Molar)	NH4OH:M (Molar Ratio)
−1	0	0
0	12.5	1
+1	25	2

.

3.5. Photocatalytic Activity Tests

The photocatalytic activity of the synthesized catalyst was assessed in degradation tests of a model pollutant (salicylic acid or SA, C₀ = 50 mg L⁻¹). The solution was magnetically stirred, the reaction temperature was kept around 20 °C, and atmospheric air was continuously bubbled inside the reactor. The solution pH was adjusted to pH = 3, 5, or 7, using HCl and NaOH solutions, and the catalyst concentration in all tests was equal to 1.0 g L⁻¹, similar to that reported in other works [77,78,79]. In each experiment, the catalyst used and the pH value of the solution were determined according to the experimental design (see Section 2.4). Before every photocatalytic test, the suspension formed by the SA solution and the catalyst was kept in the dark for 60 min. After this interval, the Hg vapor lamp (250 W) without a bulb (34.10 mW cm⁻²) was turned on, initiating the photocatalysis step. The suspension samples were collected during the experiment, centrifuged, and had their SA concentration determined by high-performance liquid chromatography.

3.6. High-Performance Liquid Chromatography (HPLC)

SA was quantified in the liquid samples by High-Performance Liquid Chromatography (HPLC, YL Clarity 9100, UV-VIS detector, monitoring at 210 nm, Young In Chromass Co., Gyeonggi-do, Coreia) using a C-18 column (Luna, 5 μm, 150 × 4.6 mm, Phenomenex). The mobile phase was 40/60 acetonitrile/phosphate buffer (pH = 2.8, 50 mM) at 1 mL min⁻¹, 30 °C. The methodology currently used was adapted from the work of [36], and the validation tests are described in detail in previous work [59].

3.7. Quenching Experiments

Quenching tests were performed to determine the main active species in SA degradation. Isopropyl alcohol (ISOP, 2 mM), benzoquinone (BZQ, 2 mM), and ammonium oxalate (AO, 2 mM) were used as ^●OH, O₂^●− and h⁺ scavengers, respectively [80,81]. The experimental conditions were identical to those described in Section 3.5. Tests were carried out both in the absence (photolysis) and in the presence of the catalyst (photocatalysis) at two different pHs (3 and 7). The catalysts used were those with the most promising results in the experiments of the 3³-factorial design.

3.8. DPPH Assays

The antioxidant activity of the catalysts was measured in DPPH radical inhibition assays [82,83]. In the tests, 1 mL of DPPH solution, 100 μM, was added to 3 mL of catalyst suspension, 1 mg mL⁻¹. After 30 min the dark, the suspension was centrifuged, and the sample was analyzed using a UV–Vis Spectrophotometer at 517 nm. The %DPPH inhibition was determined by Equation (7).

$$\%\mathrm{DPPH} \mathrm{inhibition}=\frac{\left(Ab{s}_{blank}-Ab{s}_{sample}\right)}{Ab{s}_{blank}}·100$$

(7)

3.9. Methylene Blue Adsorptions Tests

The adsorption capacity of the N-doped TiO₂-Nb₂O₅ catalysts was evaluated in equilibrium adsorption tests at pH3 using methylene blue as the model molecule. Each catalyst (1 g L⁻¹) remained in contact with 10 mL of MB solution (10 mg L⁻¹) for 24 h to reach equilibrium. At the end of this interval, samples were collected and centrifuged and had the MB concentration determined by UV–Vis Spectrometry at 665 nm. The results were evaluated using the 3²-factorial design (Table 3, Section 2.4).

4. Conclusions

N-doped TiO₂-Nb₂O₅ photocatalysts were synthesized through a sol–gel methodology. The characterization of the catalysts indicated a strong effect of the calcination temperature on the properties of the catalysts, notably reducing the specific surface area and inducing phase transformation. These characteristics reflected directly on the photocatalytic activity of the catalysts, which decreased with increasing calcination temperature. In turn, the addition of Nb to TiO₂, in the molar ratio of 25:75 (Nb:Ti), was able to promote an increase of more than four times in the S_BET surface area, which went from around 18 m² g⁻¹ to 86 m² g⁻¹.

According to the experimental design analysis, another relevant property considered was the niobium content (%Nb) of the catalysts, whose impact on photocatalytic activity was strongly pH dependent. This interaction between pH and %Nb has not been addressed before in studies about N-doped TiO₂-Nb₂O₅ catalysts. The addition of Nb to TiO₂ proved favorable for the photocatalytic activity at pH 3, but negative at pH 5 and 7. Pure TiO₂ catalyst showed maximum SA removal at pH 5 (72.9%), which decreased to about 42.8% at pH 3 in 30 min of the photocatalytic test. In the same time interval, catalyst 25Nb-1N-400 promoted approximately 49.0% degradation at pH 5, which increased to 59.9% at pH 3, surpassing the performance of TiO₂ in the same condition. The results of the SA degradation tests indicated that the addition of Nb was only favorable in a specific pH range, close to three.

The solution pH was decisive for electrostatic interactions between surface/organic molecules and for the displacement of CB and VB potentials as the pH increased, directly impacting the formation of radicals and the degradation of organic molecules. The TiO₂ and Nb₂O₅ combination proved interesting for charge separation and oxidative pathway applications. Even so, future studies can be carried out to improve the applicability of the catalysts, in actual conditions of varied concentrations of pollutants, in different water matrices, with monitoring of the levels of toxicity, formation of by-products, degree of mineralization, and evaluating the use of different concentrations of catalysts.

In the antioxidant activity tests, catalysts without the addition of Nb were superior, with IC₅₀ = 88.9 μg mL⁻¹ for the catalyst 0Nb-0N-400. However, in the catalyst’s comparison, N-doping improved the antioxidant activity, suggesting future studies optimizing the N content in TiO₂-Nb₂O₅ catalysts. On the other hand, the combination TiO₂-Nb₂O₅ excelled in methylene blue adsorption tests. The mixed oxides achieved a removal capacity between 7 and 9 times greater than that of TiO₂ and between 10 and 15 times greater than that of pure Nb₂O₅, compared to about a fourfold increase in surface area, suggesting a strong influence of the surface characteristics of the materials. According to literature reports, due to its structural characteristics, the MB molecule is adsorbed on catalysts by interactions with acidic sites of the surface. The adsorption results currently obtained open space for future in-depth studies on the surface characteristics of the synthesized catalysts.