Ti-Ce Nanocatalysts for Evaluation in the Photodegradation of Naproxen and Acetaminophen

Abstract

The pharmaceutical industry is a major source of pollution in wastewater effluents, characterized by chemical residues that are complex and difficult to degrade. Naproxen, a commonly detected drug in sewage effluents, exceeds safe concentrations for aquifers and is highly persistent, posing significant risks to aquatic life and ecosystems. This drug is known to cause long-term side effects in humans, such as gastrointestinal ulcers and nephrosis, associated with frequent and prolonged use. Additionally, the recent pandemic has led to a marked increase in drug consumption over a short period, exacerbating environmental contamination. Titanium dioxide has been extensively used as a photocatalyst in recent decades, proving effective in reducing these emerging pollutants. In this study, TiO₂ doped with cerium was synthesized using the sol–gel method, with cerium concentrations varied at 1, 3, 5, and 10% by weight. The resulting nanocatalysts were characterized through nitrogen physisorption, scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-Vis diffuse reflectance spectroscopy. Photocatalytic activity was assessed using a UV-Vis spectrophotometer to monitor the degradation of the drugs. XRD analysis confirmed the crystallinity and anatase phase of TiO₂. UV-Vis diffuse reflectance spectra indicated a decrease in bandgap energy of up to 3.00 eV compared to pure TiO₂. The materials demonstrated significant degradation of naproxen (NPX) and acetaminophen (ACTP), both prepared at 30 ppm, over a 6 h reaction period.

1. Introduction

The pharmaceutical industry significantly contributes to water pollution, with active pharmaceutical ingredients (APIs) permeating natural water bodies through their production, use, and disposal. This widespread dispersion has classified APIs as emerging pollutants posing global environmental challenges. Research by Beek et al. highlighted the detection of drug residues in 71 countries, identifying a total of 631 drugs and 16 pharmaceutical residues beyond acceptable limits [1,2]. The ecological impact of these residues includes bacterial resistance to antibiotics and disruption of aquatic life through mechanisms such as fish predation and endocrine disruption, manifesting as feminization of fish populations [3,4].

Naproxen, a widely recognized non-selective nonsteroidal anti-inflammatory drug and a bicyclic propionic acid derivative, exhibits ecosystem toxicity both directly and via its metabolites. Its adverse effects range from bioaccumulation and gastrointestinal issues to renal complications and endocrine disturbances [5]. Similarly, acetaminophen, one of the most commonly used over-the-counter medications, is largely present in wastewater, with human bodies excreting 58–68% of ingested doses [6].

The persistence of pharmaceutical pollutants in aquatic systems has necessitated the development of effective remediation strategies. Traditional methods like adsorption, biological treatments, and membrane-based processes are limited by factors such as sludge production and the need for frequent adsorbent regeneration [7]. Advanced oxidation processes (AOPs), which involve the generation of hydroxyl radicals to break down pollutants into non-harmful substances like water and carbon dioxide, offer a robust alternative. These radicals are produced through various methods, including chemical, electrochemical, and photochemical processes [8].

Photocatalysis, a method leveraging the unique properties of semiconductors like titanium dioxide (TiO₂) and zinc oxide (ZnO), is particularly promising due to its efficiency in generating electron–hole pairs essential for the catalytic breakdown of contaminants. TiO₂ and ZnO are widely recognized semiconductor photocatalysts; however, TiO₂ is preferred due to its superior chemical and photochemical stability under the experimental conditions [9]. TiO₂ exhibits strong resistance to photocorrosion and maintains its structural integrity during prolonged irradiation in aqueous systems [10], whereas ZnO is prone to dissolution and photocorrosion, particularly under acidic or reactive conditions [11]. Although ZnO can exhibit high electron mobility, its practical photocatalytic performance is often limited by its instability in aqueous environments. In contrast, TiO₂, especially in its anatase phase, has been widely reported to exhibit effective charge separation and efficient generation of reactive oxygen species under UV irradiation [12]. Furthermore, TiO₂ is considered the reference photocatalyst, with well-established mechanisms and widely validated modification strategies, which facilitate reproducibility and interpretation of results [13]. It also maintains stable activity over a broad pH range, unlike ZnO, whose performance decreases significantly in acidic environments. Although ZnO may offer advantages such as higher electron mobility or absorption efficiency in specific cases, these are often outweighed by its limited chemical stability [14,15]. Despite the effectiveness of TiO₂ and ZnO, their utility is often hindered by the rapid recombination of these electron–hole pairs. To overcome this limitation, doping TiO₂ with elements such as cerium (Ce) and lanthanum (La) has proven effective. CeO₂, known for its oxygen storage capabilities and redox properties, enhances photocatalytic activity [16].

This study evaluates the efficacy of Ce-TiO₂ nanoparticles in degrading common wastewater contaminants like naproxen (NPX) and acetaminophen (ACTP), focusing on their enhanced photocatalytic properties through the incorporation of cerium.

2. Results and Discussion

2.1. Characterization of Ti-Ce X Photocatalysts

2.1.1. Scanning Electron Microscopy (SEM)

Figure 1 presents the SEM micrographs of the Ti-Ce nanocomposites, where Ti-Ce1, Ti-Ce3, Ti-Ce5, and Ti-Ce10 display semicircular and amorphous morphologies with notable agglomeration. Noteworthy is that agglomeration intensifies with increasing Ce content, suggesting enhanced interaction within the Ti-Ce matrix. This statement is supported by other research where the presence and increase in metallic species induce morphological changes, including aggregation, particle growth, or the formation of denser structures, which reflect stronger interactions between phases, oxides, or dopants [17,18]. These interactions can be induced by the presence of oxygen vacancies in the matrix, which could enhance the interaction between species [19]. Likewise, the homogeneous incorporation of Ce during the synthesis method can lead to structures with larger aggregates [20].

2.1.2. Nitrogen Physisorption

The nitrogen physisorption analysis illustrated in Figure 2a shows Type IV isotherms with Type H2 hysteresis, indicative of mesoporous materials with quasi-cylindrical pore structures [21,22]. These features suggest variable pore sizes across samples, correlating with the degree of cerium incorporation (Figure 2b) [23,24].

2.1.3. X-Ray Diffraction (XRD)

XRD analysis confirms only the presence of the TiO₂ anatase phase with a tetragonal structure, evident from diffraction peaks at 2θ values corresponding to various crystal planes, as outlined in JCPDS No. 00-001-0562. As can be seen in Figure 3, no additional peaks attributed to cerium oxides were observed. This may be because the cerium species were well distributed within the crystalline structure of TiO₂ [25]. Even with high Ce concentrations (up to 10 wt.%), no significant peak shifts were observed, indicating that Ce cations could integrate into the TiO₂ matrix without altering its crystallographic structure significantly [26]. The intensity of the diffraction peak (101) decreases with increasing cerium content (Figure 3b), which is related to the fact that these metallic species could inhibit crystal growth (see

Table 1. Bandgap energy (Eg), crystallite size, d-spacing values and textural properties of Ti-Ce samples.

Samples	BET (m2/g)	Pore Diameter (nm)	Cell Parameter, a (Å)	Average Size Particle (nm)	Eg (eV)
Ti-Ce1	89.1	15.96	3.527	15 ± 2	3.28
Ti-Ce3	77.8	12.73	3.528	12 ± 1	3.16
Ti-Ce5	111.6	12.59	3.523	15 ± 3	3.03
Ti-Ce10	115.7	12.73	3.520	16 ± 2	3.00
TiO2	64.0	6.5	3.530	32 ± 6	3.24

) [27].

2.1.4. UV–Visible Spectroscopy

Figure 4 depicts the UV-Vis absorption spectra and band gap energies, where a shift towards the red region (from 3.00 to 3.24 eV) is observed for the Ti-Ce nanocatalysts compared to pure TiO₂ [28]. This shift suggests a modification in the bandgap due to Ce doping, affecting the electronic properties of TiO₂ [29,30].

2.1.5. Transmission Electron Microscopy (TEM)

TEM analyzes (Figure 5) reveal nanoglobular and other nanostructures within the nanocatalysts, confirming the homogeneous distribution of Ce within the titania matrix, which is critical for effective photocatalytic activity [31]. It can be observed that the presence of cerium metallic species in TiO₂ decreases the crystalline particle size, which is in agreement with the XRD results.

Table 2. Percentage by weight of nanocatalysts.

Nanocatalyst	Content %
	Ti	O	Ce
TiO2	60	40	-
Ti-Ce1	59	36	2
Ti-Ce3	63	29	4
Ti-Ce5	52	39	6
Ti-Ce10	51	39	10

shows the percentage by weight content of the species present in all synthesized materials, and these values are very close to the theoretical ones.

2.2. Photocatalytic Activity

The photocatalytic efficiency was quantified by monitoring the degradation of acetaminophen (ACTP) and naproxen (NPX) (Figure 6) [31]. The results show that Ti-Ce1 exhibits the highest degradation efficiency, potentially due to optimal Ce doping enhancing hydroxyl radical production, which is crucial for effective pharmaceutical degradation in wastewater treatments [32,33].

As can be seen in Figure 6, the degradation efficiency is observed under photocatalytic conditions using Ti-Ce 1% nanocatalyst, emphasizing their superior performance compared to photolysis under non-catalytic conditions. A notable reduction in degradation is seen, reinforcing the importance of catalyst and reaction conditions in achieving optimal performance, which highlights the critical role of Ti-Ce in accelerating the degradation process. This comparative analysis underscores the efficiency of the Ti-Ce nanocatalysts in environmental remediation, with ACTP and NPX exhibiting similar trends, though ACTP generally shows slightly higher degradation efficiencies.

The photocatalytic degradation mechanism of naproxen and acetaminophen using Ce-doped TiO₂ can be described based on the generation and participation of reactive oxygen species under irradiation. The excitation of the semiconductor produces electron–hole pairs (e⁻/h⁺) (Equation (1)), where the incorporation of Ce introduces intermediate energy states associated with the Ce⁴⁺/Ce³⁺ redox couple, which act as electron trapping centers, promoting charge separation and reducing recombination [34]. The photogenerated electrons react with dissolved oxygen to form superoxide radicals (${\mathrm{O}}_2^{·-}$) (Equation (2)), while the holes oxidize water or surface hydroxyl groups to generate hydroxyl radicals ($·\mathrm{O}\mathrm{H}$) (Equation (3)). These highly oxidative species are responsible for the degradation of pharmaceutical compounds through hydroxylation, aromatic ring cleavage, and progressive oxidation leading to partial or complete mineralization (Equation (4)), in agreement with mechanisms reported for doped TiO₂ photocatalytic systems [35,36,37]. This description represents the excitation of the semiconductor, the role of Ce as an electron trap, and the formation of reactive species responsible for contaminant degradation. When the dopant content is high, the cerium metal species become recombination sites for photogenerated positive holes, as they are negatively charged by acting as a trap for photogenerated electrons, thus decreasing photocatalytic activity.

$$\mathrm{T}\mathrm{i}{\mathrm{O}}_2/\mathrm{C}\mathrm{e}+\mathrm{h}\mathrm{\nu }\to {\mathrm{e}}^{-}+{\mathrm{h}}^{+}$$

(1)

$${\mathrm{e}}^{-}+{\mathrm{O}}_2\to {\mathrm{O}}_2^{·-}$$

(2)

$${\mathrm{h}}^{+}+{\mathrm{H}}_2\mathrm{O}\to ·\mathrm{O}\mathrm{H}+{\mathrm{H}}^{+}$$

(3)

$$·\mathrm{O}\mathrm{H}/{\mathrm{O}}_2^{·-}+\mathrm{D}\mathrm{r}\mathrm{u}\mathrm{g}\to \mathrm{C}{\mathrm{O}}_2+{\mathrm{H}}_2\mathrm{O}+\mathrm{I}\mathrm{n}\mathrm{t}\mathrm{e}\mathrm{r}\mathrm{m}\mathrm{e}\mathrm{d}\mathrm{i}\mathrm{a}\mathrm{t}\mathrm{e} \mathrm{p}\mathrm{r}\mathrm{o}\mathrm{d}\mathrm{u}\mathrm{c}\mathrm{t}\mathrm{s}$$

(4)

Furthermore, it can be observed that despite the presence of cerium doping species and the band-gap reduction, the catalytic photodegradation of both drugs is favored under UV irradiation at 254 nm. This is related, firstly, to the appropriate concentration of dopant that prevents the recombination of the photogenerated charges and, secondly, to the generation of hydroxyl radicals from the hydrogen peroxide formed during the photocatalytic process due to the interaction with a sufficiently energetic radiation source [38].

Figure 7 shows the kinetics of Ti-Ce nanocatalysts for the degradation of ACTP (a) and NPX (b), showing the effect of different nanocomposite concentrations (1%, 3%, 5%, and 10%) compared to photolysis as a control. In both cases, photolysis exhibits the lowest degradation rates, highlighting the limited impact of direct photodegradation without the catalyst. The degradation rates increase significantly with lower nanocatalyst concentrations, showing markedly improved performance due to the greater availability of active catalytic sites [39]. ACTP shows faster degradation kinetics across all concentrations compared to NPX, suggesting differences in the susceptibility of the compounds to photocatalytic breakdown, likely influenced by molecular structure or interaction with the catalyst. Ti-Ce nanocatalysts enhance the degradation process by generating reactive oxygen species under irradiation, with the results emphasizing their potential as efficient photocatalysts for environmental remediation. The data also suggest that doping concentration may provide optimal performance, balancing efficiency and material used [40,41].

3. Materials and Methods

3.1. Synthesis

The Ce-TiO₂ nanocatalysts were synthesized via the sol–gel method. In a three-neck flask, a mixture of ethyl alcohol and water at an 8:1 ratio was prepared, dissolving cerium nitrate to achieve cerium concentrations of 1%, 3%, 5%, and 10%. Solvent composition and the water-to-alkoxide molar ratio (R = [H₂O]/[alkoxide]) controlled the hydrolysis and condensation kinetics in TiO₂ sol–gel systems. Increasing R accelerates hydrolysis and affects nucleation and particle size, while moderate values (R ≈ 4–16) favor stable and homogeneous sols [42,43,44]. In this work, an ethanol/water mixture (8:1, v/v) was used to control the reaction. The use of an alcohol/water mixture moderated the hydrolysis rate of titanium alkoxides by reducing the effective water activity and improving phase compatibility, preventing uncontrolled precipitation and enabling a more uniform sol–gel process. This allowed a more controlled system and reproducible TiO₂ properties.

Subsequently, the mixture was heated to 70 °C, and titanium butoxide was added dropwise. The reaction was maintained under constant stirring and temperature for 24 h. Post-reaction, the material was dried at 100 °C, ground in an agate mortar, and finally calcined at 500 °C for 5 h with a heating ramp of 2 °C/min.

3.2. Characterization Techniques

3.2.1. Scanning Electron Microscopy (SEM)

A TESCAN MIRA3 microscope (Brno, Czech Republic) was utilized at 20.0 KV to achieve 30 to 60 kx magnification.

3.2.2. Nitrogen Physisorption

Conducted using a Micromeritics TriStar II Plus (Norcross, GA, USA) to assess adsorption isotherms and specific surface area via the BET method, and pore size distribution via the BJH method.

3.2.3. X-Ray Diffraction (XRD)

X-ray diffraction studies were carried out in Panalytical equipment, empyrean model (Malvern, UK) with Cu Kα radiation (λ = 0.154 nm), with a diffraction angle (2ϴ) from 10 to 90°, using a step of 0.03° and a time 3 s per step.

3.2.4. UV–Visible Spectroscopy

Diffuse reflectance spectra were obtained with a Shimadzu UV-2600 spectrophotometer (Kyoto, Japan), integrated with an integrating sphere. The spectra were analyzed using the Kubelka–Munk function to calculate the extinction coefficient (α), across wavelengths from 900 to 190 nm.

3.2.5. Transmission Electron Microscopy (TEM)

High-resolution imaging was done using a TITAN Cube 60-300 (FEI Company, Zaragoza, Spain) with a spherical aberration corrector at 80 kV.

3.2.6. Photocatalytic Activity Assessment

The photocatalytic degradation of acetaminophen and naproxen was tested in a 400 mL reactor containing 350 mL of the solution, with consistent agitation and air injection at 1 mg/min. The solution’s exposure to UV light varied by wavelength (254 nm, 356 nm, and solar irradiation > 400 nm) according to the Ce-TiO₂ composition (1, 3, 5, and 10 wt.%). Degradation kinetics were monitored by measuring the drug concentration at specific wavelengths (243 nm for acetaminophen and 271 nm for naproxen) using a Shimadzu UV-2600 spectrophotometer.

4. Conclusions

The TiO₂ and Ce-TiO₂ photocatalysts were successfully synthesized by the sol–gel method. Nanocatalysts show photodegradation activity because the Ce distribution on the TiO₂ surface and some Ce^4+/3+ cations in the TiO₂ matrix can improve the material’s surface redox properties, which facilitates the mobility of the photogenerated charges during photocatalytic activity. The effect of Ce dose on nanocatalysts could be related to the higher adsorption, and the 4f electron transition of Ce^4+/3+ ions.

Characterization of the material was successfully achieved. All these results confirmed the presence of the more active anatase crystalline phase of TiO₂. There was also evidence of the expected weight percentage values of each species present in the catalysts, calculated during the synthesis of the materials. The effect of the Ce was observed in the TEM images, in the modification of the particle size, which can be attributed to the presence of Ce-O-Ti bonds that inhibit the growth of crystals.

The best degradation efficiency was achieved with a low amount of cerium by reducing the recombination rate under UV radiation.