Tribo-Catalytic Degradation of Methyl Orange Dye via Cu/Al₂O₃ Nanoparticles

Abstract

In this study, we report, for the first time, the tribo-catalytic degradation of methyl orange (MO) using Cu/Al₂O₃ nanoparticles under mechanical stirring conditions. The hybrid catalyst was synthesized via a wet impregnation method and characterized through different techniques, confirming structural integrity and compositional uniformity. When subjected to friction generated by a PTFE-coated magnetic stir bar, Cu/Al₂O₃ nanoparticles exhibited high tribo-catalytic activity, achieving up to 95% MO degradation within 10 h under dark conditions. The observed activity surpasses that of alumina alone and is attributed to the synergistic effects between copper and alumina, facilitating charge separation and enhancing reactive oxygen species (ROS) formation. Tribo-catalytic efficiency was further influenced by stirring speed and contact area, confirming the key role of mechanical friction. Reusability tests demonstrated stable performance over five cycles, highlighting the material’s durability and potential for practical environmental remediation applications.

1. Introduction

In recent years, tribo-catalysis has attracted growing interest within the scientific community as an innovative and promising catalytic technology for the degradation of organic pollutants. It offers a sustainable and environmentally friendly approach by effectively converting mechanical energy, naturally present in the environment, into chemical energy capable of driving degradation reactions. This mechanical energy can originate from various ambient sources, such as vibrations [1], flowing water [2], or can be reproduced by artificial means, such as magnetic stirring [3,4]. Unlike conventional catalytic methods, such as photocatalysis [5,6,7,8] and piezocatalysis [9,10,11,12,13], which rely on light irradiation or structural deformation, tribo-catalysis operates through the tribo-electric effect. This physical phenomenon occurs when two materials of different natures come into contact and are then separated through a rubbing or frictional motion [2]. During this process, when two dissimilar materials come into contact and are subsequently separated through friction or mechanical movement, a transfer of electrons takes place between their surfaces [2]. This electron exchange results in an imbalance of electrical charges, causing one material to become positively charged while the other becomes negatively charged [14]. As a consequence, significant surface charge accumulation occurs on both materials, laying the foundation for subsequent redox reactions when these charges interact with surrounding molecules, such as water and oxygen, in the environment.

These tribo-electric charges, once transferred into an aqueous medium, interact with water and dissolved oxygen to produce reactive oxygen species (ROS) [15], including hydroxyl radicals (·OH) and superoxide anions (·O₂⁻), which are capable of oxidizing and breaking down complex organic compounds into less harmful substances. Due to its simplicity, environmental compatibility [16], and ability to function under ambient conditions without external energy sources, tribo-catalysis presents a particularly attractive approach for wastewater treatment applications [16].

In this context, one of the most critical environmental challenges is the removal of synthetic azo dyes from industrial effluents [17,18], particularly methyl orange (MO) [19,20]. MO is an anionic dye widely employed in the textile, printing, and chemical industries, as well as in analytical applications as a pH indicator. Its molecular structure, featuring an azo bond (–N=N–) and substituted aromatic rings, imparts high chemical stability, low biodegradability, and strong resistance to conventional treatment methods [21,22]. Furthermore, MO is known to be toxic to aquatic life and potentially harmful to human health, necessitating the development of more effective and sustainable remediation strategies [23].

Tribo-catalysis, although still relatively new compared to photocatalysis and piezocatalysis, has been increasingly explored for environmental remediation. Advances in tribo-catalysis have demonstrated its potential in degrading dyes, antibiotics, and other persistent organic pollutants [15,16,22], thereby positioning it as a sustainable alternative within the broader field of advanced oxidation processes. Nonetheless, a comprehensive understanding of the mechanisms, active materials, and structure–activity relationships is still developing, and systematic investigations remain limited.

Previous studies have shown that semiconductor oxides such as TiO₂ [24], ZnO [25,26], BiOIO₃ [27], CaCu₃Ti₄O₁₂ [28], and CdS [29] can act as efficient tribo-catalysts due to their capacity to generate electron–hole pairs under mechanical stress and their robustness in aqueous environments. The catalytic performance of such materials can be significantly improved by using mixed oxides or bimetallic systems, which leverage synergistic interactions between components. In this regard, alumina (Al₂O₃) offers chemical stability [30], high thermal resistance [31], and a large specific surface area [32] conducive to the adsorption and distribution of reactive species [33]. Furthermore, its excellent insulating properties enhance charge accumulation on the catalyst surface. Copper (Cu), on the other hand, exhibits favorable redox characteristics [34] and can promote electron transfer [34] and radical formation during catalytic processes [35].

The combination of these two materials into a hybrid Cu/Al₂O₃ catalyst offers the potential for enhanced tribo-catalytic activity through improved charge generation, better electron–hole separation, and more efficient interaction with the target pollutant.

In this work, we investigated, for the first time, the tribo-catalytic performance of Cu/Al₂O₃ for the degradation of methyl orange under standard magnetic stirring conditions, employing a sealed polytetrafluoroethylene (PTFE) magnetic stir bar. The mechanical energy generated from the friction between the catalyst particles and either the magnetic stir bar or the bottom surface of the reaction vessel was effectively utilized to induce tribo-electric charge accumulation on the catalyst surface, thereby initiating oxidative degradation of the dye molecules. By varying the size of the stir bar and the material composition of the reaction container, the overall dye removal efficiency was further enhanced. Additionally, the stability and reusability of the Cu/Al₂O₃ catalyst were also evaluated.

2. Experimental

2.1. Materials

All reagents were purchased from Sigma-Aldrich (Darmstadt, Germany) and used as received without further purification. The chemicals employed in this study included α-Al₂O₃ (alpha-alumina, 99%), copper nitrate (Cu(NO₃)₂, 99.9%), deionized water, methyl orange (MO, dye content 85%), hydrochloric acid (HCl, 0.1 M), sodium hydroxide (NaOH, 0.1 M), p-Benzoquinone (BQ, >98%), tert-butanol (TBA, >99%), silver nitrate (AgNO₃, >99%), and ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na, >99).

2.2. Synthesis

The copper catalyst supported by α-Al₂O₃ was synthesized by the wet impregnation technique, following the procedure described in [36], using α-Al₂O₃ characterized by a low specific surface area (0.57 sq. m/g) and negligible micropore volume (0.00 cub. mm/g). In a typical preparation, 300 mg of α-Al₂O₃ were dispersed in 20 mL of a 1 wt.% aqueous solution of copper nitrate (Cu(NO₃)₂). The suspension was continuously stirred for 2 h to ensure homogeneous impregnation. Subsequently, the solvent was removed using a rotary vacuum evaporator (Hei-VAP Core, Heidolph, Schwabach, Germany). The obtained material was dried overnight at 105 °C and then calcined in air at 500 °C for 3 h. The synthesis yield was 95.8 wt.% ± 1.2, with a reproducible copper loading of approximately 17 wt.% ± 2.7 in the form of Cu nanoparticles, as confirmed across three independent preparations.

2.3. Characterization of Cu/Al₂O₃ Nanoparticles

The Cu/Al₂O₃ catalysts were characterized using several analytical techniques to assess their structural, morphological, chemical, and thermal properties. X-ray diffraction (XRD) analysis was performed using a Bruker D2 Phaser diffractometer with Cu Kα radiation over a 2θ range of 10° to 80°, to determine the crystalline structure of the nanoparticles. FT-IR spectra were recorded using a Nicolet iS50 FT299 IR (Nicolet, Waltham, MA, USA). Raman spectra were obtained at room temperature with a microRaman spectrometer (Renishaw inVia; 514 nm excitation wavelength). The specific area and pore volume of α-alumina were obtained by N₂ adsorption–desorption at 77 K with a Kelvin 1042 V3.12, COSTECH Instruments (Costech Microanalytical, Tallinn, Estonia). The surface morphology and particle distribution were examined using Scanning Electron Microscopy (SEM) with a TESCAN VEGA LMH microscope operating at 230 V (Tescan, Brno, Czech Republic). The energy resolution of the detector was approximately 129 eV at Mn Kα. Additionally, a FEI Tecnai 20 Transmission Electron Microscope (TEM), operating at 200 kV with LaB₆ filament as the electron source, was employed for further characterization. X-ray Photoelectron Spectroscopy (XPS) was carried out using a Physical Electronics (PHI) VersaProbe II spectrometer equipped with an Al Kα radiation source to investigate the surface chemical composition and oxidation states of the elements.

2.4. Evaluation of Tribo-Catalytic Activity

The tribo-catalytic efficiency of Cu/Al₂O₃ nanoparticles was evaluated through the degradation of MO under magnetic stirring, performed in the dark and at room temperature. The concentration of MO was monitored by recording the absorption spectra using a UV–Vis spectrophotometer (Evolution 60S model). In a representative experiment, 50 mg of Cu/Al₂O₃ were dispersed in 30 mL of an aqueous MO solution with a concentration of 20 mg/L, contained in a glass beaker (75 mm in diameter, 95 mm in height). The suspension was stirred using a PTFE-coated cylindrical magnetic stir bar (∅ 8 mm × 35 mm) at a constant speed of 500 rpm under dark conditions. At regular intervals of 2 h, 3 mL aliquots were collected and centrifuged at 10,000 rpm for 5 min to separate the solid catalyst. The degradation ratio was then calculated according to Equation (1) [37,38]:

$$\mathrm{Degradation}\left(\%\right)={\displaystyle \frac{{\mathrm{C}}_0-{\mathrm{C}}_{\mathrm{t}}}{{\mathrm{C}}_0}}·100\%$$

(1)

where C₀ and C_t represent the initial concentration and the concentration at time t of MO, respectively.

3. Results and Discussion

3.1. Characterization of Nanoparticles

The XRD patterns of α-Al₂O₃ and Cu/Al₂O₃ samples are presented in Figure 1a, recorded over the 2θ range of 20–80°. The diffractogram of α-alumina (red line) shows sharp diffraction peaks at 2θ ≈ 25.6°, 35.1°, 37.8°, 43.3°, 52.5°, 57.5°, 66.5°, and 77.0° corresponding to the (012), (104), (110), (113), (024), (116), (214), and (300) planes of rhombohedral α-Al₂O₃ (JCPDS No. 98-004-0015). The absence of secondary reflections confirms its high crystallinity and phase purity. Similarly, the XRD pattern of the Cu/Al₂O₃ nanocomposite (Figure 1a, olive line) reveals diffraction peaks corresponding to face-centered cubic (FCC) Cu NPs [39]. The average crystallite size, calculated using the Scherrer equation, was found to be approximately 27.8 ± 2.5 nm. A slight shift in the 2θ values of Al₂O₃ was observed, which is likely due to the incorporation of Cu nanoparticles.

The nature of the chemical bonds of the synthesized nanocomposites was investigated by FT-IR spectroscopy, which enabled the identification of surface functional groups. Figure 1b presents the FT-IR spectra of pure α-Al₂O₃ and the Cu/Al₂O₃ composite material. Pure α-Al₂O₃ is typically characterized by absorption bands in the 500–800 cm⁻¹ range, corresponding to the Al–O-Al stretching vibration [40]. The IR spectrum of Cu/Al₂O₃ nanoparticles shows the characteristic peaks of Al–O-Al stretching. The broad band centered at 3431 cm⁻¹ is associated with the stretching vibrations of –OH groups and adsorbed water molecules, while the bands in the 1500–1700 cm⁻¹ range are attributed to bending vibrations of species adsorbed on the material surface [41]. The Raman spectrum of the α-Al₂O₃ sample under excitation of 514 nm is also reported in Figure 1. In particular, Figure 1c shows the characteristic vibrational modes of α-alumina (α-Al₂O₃) nanoparticles, with Raman bands detected at 378, 415, 572, 643, and 746 cm⁻¹. These frequencies correspond to the active Raman modes A₁g (around 415 and 643 cm⁻¹) and E₉ (approximately 378, 415, 572, and 746 cm⁻¹) [42,43,44].

The XPS spectra of Cu/Al₂O₃ NPs are illustrated in Figure 2. As shown in Figure 2a, the XPS survey scan revealed characteristic peaks corresponding to Cu 2p, Al 2p, and O 1s. The spectra were calibrated using the C 1s peaks at 284.8 eV as reference. Specifically, in Figure 2b, the Al 2p binding energy is observed at approximately 73.9 eV, confirming the presence of aluminum in the +3 oxidation state (Al³⁺) [45]. Figure 2c displays two distinct peaks at 932.3 eV and 952.7 eV, both assigned to Cu(0), confirming the presence of metallic copper species on the catalyst surface [46]. To exclude the occurrence of additional copper species, the O 1s spectrum was deconvoluted (Figure 2d), revealing two peaks centered at 530 eV and 532 eV. These features are attributed to lattice oxygen associated with aluminum (Al–O bonds) and Al–O–H bonds, respectively [47]. Overall, these spectroscopic results confirm the successful synthesis of Cu/Al₂O₃ nanoparticles with metallic copper species supported on alumina.

Figure 3a,b displays the surface morphology of Cu/Al₂O₃ NPs, characterized by a relatively rough texture and the absence of a regular surface relief. Additionally, some particle agglomerates were observed on the surface of the sample. Specifically, Figure 3b displays cluster-like structures with globular morphology, attributable to both the copper matrix and the alumina support (Figure 3c). The TEM inset in Figure 3c shows Al₂O₃-supported Cu NPs with irregular morphologies and an average particle size of approximately 65 nm, exhibiting a broad size distribution from 50 to 100 nm.

Finally, energy-dispersive X-ray spectroscopy (EDX) analysis confirmed the elemental composition of the material, namely aluminum (Al), oxygen (O), and copper (Cu), as shown in Figure 3d.

3.2. Tribo-Catalytic Efficiency of Cu/Al₂O₃ NPs

Tribo-catalysis induced by magnetic stirring using Cu/Al₂O₃ was investigated for the removal of MO as a model pollutant. Under continuous stirring at 500 rpm with a PTFE-coated magnetic bar in the presence of Cu/Al₂O₃, and in the absence of light, a gradual discoloration of the MO solution was observed. After 10 h of treatment, the solution was almost completely decolorized (95%), whereas alumina alone achieved only ~60% decolorization (see Figure 4a). In addition, when Cu nanoparticles alone were tested for MO degradation, only 41% removal was achieved after 10 h. Finally, to verify the tribo-catalytic activity of Cu/Al₂O₃ nanoparticles, a control experiment was performed in the absence of the catalyst (Figure 4a). Under these conditions, no degradation of MO was observed, and the slight decrease in concentration can be considered negligible, likely resulting from the mere friction between the PTFE rod and the glass beaker. These findings confirm that the presence of the catalyst is essential for the activation of the tribo-catalytic process. The relative concentration (C/C₀) of MO was evaluated as a function of stirring time, where C and C₀ correspond to the instantaneous and initial concentrations, respectively. As shown in Figure 4, more than 95% of MO was degraded within 10 h in the presence of Cu/Al₂O₃ nanoparticles. Kinetic analysis based on the pseudo-first-order model, Equation (2),

$$\ln \left({\displaystyle \frac{\mathrm{C}}{{\mathrm{C}}_0}}\right)=-\mathrm{kt}$$

(2)

confirmed an excellent linear fit (Figure 4b), indicating that MO degradation proceeds via pseudo-first-order kinetics, with an apparent rate constant of 0.318 h⁻¹. This behavior is further supported by the evolution of the UV–Vis spectra of the MO solution over time (Figure 4c,d). The gradual decrease in the intensity of the characteristic absorption peak at around 464 nm, with no significant spectral shifts, indicates the progressive degradation of the dye via cleavage of the azo bond.

Moreover, the disappearance of absorption bands in the UV region, together with the absence of new signals, suggests that the aromatic structures of MO were broken down and converted into simpler molecular species. To confirm the role of the magnetic bar, the effect of the material was also evaluated. Using a glass bar, the degradation rate after 10 h was only 4.01%, due to the limited electron transfer. In contrast, PTFE proved to be crucial; its low Fermi energy level enables substantial charge transfer with a high-Fermi-energy catalyst [48,49,50,51,52]. Operational parameters such as stirring speed and the contact area of magnetic bars significantly influence the efficiency of the tribo-catalytic process, as they affect the frictional dynamics between the materials involved. To evaluate this effect, the tribo-catalytic performance of Cu/Al₂O₃ was studied over a stirring speed range from 100 to 700 rpm (Figure 5).

An increase in stirring speed leads to more frequent and intense collisions between the catalyst and methyl orange (MO) molecules, thereby promoting the generation of reactive species and accelerating dye degradation. As shown in Figure 5a, the MO removal efficiency increases with stirring speed. This behavior is attributed to the higher number of frictional contacts per unit time between Cu/Al₂O₃ particles and the surface of the PTFE-coated magnetic stirring bar. The intensified friction facilitates the cleavage of azo bonds in the MO molecular structure, thereby improving the overall degradation process. Furthermore, the tribo-catalytic performance of Cu/Al₂O₃ particles slightly decreased when the stirring speed was increased to 700 rpm, which may be attributed to the partial dispersion of catalyst particles onto the beaker walls at higher stirring rates. All experiments were carried out in the dark to exclude any contribution from photocatalysis. Moreover, the effectiveness of the tribo-catalytic process is strongly influenced not only by stirring speed but also by the friction generated and the contact area between the magnetic stirring bar and the bottom of the beaker. A larger contact area between the PTFE-coated magnetic bar and the glass surface promotes the accumulation of Cu/Al₂O₃ particles at the solid–solid interface, thereby intensifying the catalytic activity through tribo-electric mechanisms. To experimentally investigate this effect, magnetic bars of different lengths were employed (Figure 5b). Increasing the bar length from 2.5 to 4.0 cm led to a pronounced enhancement in the degradation efficiency of methyl orange (MO), highlighting the critical role of contact area in tribo-catalytic activation. However, extending the length from 3.5 to 4.0 cm resulted in only a marginal improvement. Overall, these findings confirm that tribo-catalytic performance is positively correlated with contact area, although the incremental benefits tend to diminish beyond a certain threshold.

To further support this observation, a blank experiment was conducted in the absence of the magnetic bar. Under these conditions, no degradation of MO was observed, demonstrating that the mere presence of the catalyst is not sufficient. Mechanical friction generated by the movement of the PTFE-coated magnetic bar is crucial for triggering the tribo-catalytic process.

The effect of pH on the tribo-catalytic degradation of MO dye using the Cu/Al₂O₃ catalyst was systematically investigated at approximately pH 3, 6, and 9. The pH of the dye solutions was adjusted with 0.1 M NaOH or 0.1 M HCl to cover acidic, neutral, and alkaline conditions. At acidic pH values, the catalyst surface becomes positively charged, leading to electrostatic repulsion with the cationic form of MO, whereas under near-neutral conditions, attractive interactions occur with the anionic dye species. In contrast, at alkaline pH, the surface turns negatively charged, again causing repulsion with the anionic MO. These variations in surface charge and dye–catalyst interactions affect the adsorption process and, consequently, the efficiency of tribo-catalytic degradation. Neutral pH conditions, around 6, were identified as optimal. The effect of catalyst dosage on tribo-catalytic degradation performance is presented in Figure 6b. The Cu/Al₂O₃ catalyst demonstrates a progressive enhancement in the degradation of MO dye as the concentration increases from 10 mg/30 mL to 50 mg/30 mL. This improvement can be ascribed to the larger contact area established between Cu/Al₂O₃ particles and the stirrer, since a higher number of catalyst particles interact with the stirrer at increased dosages. As a result, the generation of active radicals is promoted, which is likely the key factor contributing to the enhanced degradation efficiency.

3.3. Possible Mechanism for Tribo-Catalytic Degradation of MO

The degradation of MO was achieved using Cu/Al₂O₃ nanoparticles under magnetic stirring conditions (as shown in Figure 7). It is proposed that frictional interactions between the Cu/Al₂O₃ nanoparticles and the surfaces of the reaction vessel, namely, the PTFE-coated magnetic stir bar and the glass beaker, induce the excitation of electron–hole pairs within the catalyst material [53,54,55,56,57]. The generated charge carriers (electrons and holes) migrate to the surface of the catalyst and transfer into the surrounding aqueous solution, where they initiate redox reactions that result in the formation of reactive oxygen species (ROS) such as hydroxyl radicals (·OH) and superoxide radicals (·O₂⁻).

These highly reactive species are responsible for the oxidative degradation of MO dye molecules. The tribo-catalytic degradation mechanism can be summarized as follows:

Cu/Al₂O₃ NPs + friction → Cu/Al₂O₃ NPs (e⁻ + h⁺)

-

Generation of hydroxyl radicals through hole-induced oxidation of hydroxide ions:

OH⁻ + h⁺ → ·OH

-

Formation of superoxide radicals by electron transfer to dissolved oxygen:

O₂ + e⁻ → ·O₂⁻

-

Oxidative attack on MO dye molecules by ROS:

OH (or ·O₂⁻) + MO → Degradation products

These findings demonstrate that Cu/Al₂O₃ acts as an effective tribo-catalyst, capable of converting mechanical frictional energy into chemical energy through the generation of redox-active species [58]. This highlights the significant potential of tribo-catalysis as a sustainable and efficient strategy for the degradation of organic pollutants and environmental remediation. On the other hand, to validate the proposed mechanism, reactive species trapping experiments were conducted. As shown in Figure 7b, the degradation efficiency of MO decreased upon the addition of specific scavengers, p-Benzoquinone (BQ; ·O₂⁻), tert-butanol (TBA; ·OH), and silver nitrate (AgNO₃; e⁻), while a pronounced inhibition was observed with ethylenediaminetetraacetic acid disodium salt dihydrate (EDTA-2Na; h⁺). The sequence of their effects followed the order of h⁺ > ·O₂⁻ > ·OH ≈ e⁻, confirming that holes are the primary active species in the tribo-catalytic process, with ·O₂⁻, ·OH, and e⁻ playing secondary roles.

3.4. Reusability of the Cu/Al₂O₃ Catalyst

Evaluating the long-term performance of the Cu/Al₂O₃ catalyst is essential for its practical application in environmental remediation. To this end, a reusability test was conducted using 30 mL of MO solution (20 mg/L), treated with 50 mg of Cu/Al₂O₃ under magnetic stirring (500 rpm) in the dark. Each tribo-catalytic cycle lasted 10 h. After each run, the catalyst was recovered, rinsed thoroughly with deionized water, and dried at 60 °C before reuse. As presented in Figure 8, the catalyst maintained high activity over five consecutive cycles, with MO degradation efficiency remaining at ~91%, compared to ~95% in the first cycle. This minimal performance loss highlights the excellent structural stability and reusability of Cu/Al₂O₃, underscoring its potential as a durable and scalable tribo-catalyst for repeated use in pollutant degradation systems.

Table 1. Comparison with the literature on tribo-catalytic degradation of organic dyes.

Catalysts	Dosage (mg/L)	Pollutants	Stirring Speed (rpm)	Degradation (%)	Degradation Time (h)	Kinetic Rate (h−1)	Cycle Degradation	Ref.
Ba(Zr0.05Ti0.95)O3	50	RhB 5 mg/L	1200	94.3	6	0.4482	3 (92%)	[59]
Ba0.8Sr0.2TiO3	30	RhB 5 mg/L	400	88.0	8	0.2613	3 (83%)	[60]
BaSrTiO3 NPs	30	MO 5 mg/L	300	80.0	24	-	-	[61]
BiOIO3	30	RhB 5 mg/L	500	98.5	12	0.3760	5 (96%)	[27]
CdS nanowires	30	RhB 10 mg/L	400	98.0	7	0.3200	5 (95%)	[62]
NiCo2O4	30	RhB 5 mg/L	400	98.6	56	0.0740	10 (91%)	[63]
PTFE	30	RhB 5 mg/L	500	97.0	12	0.0056	5 (90%)	[64]
Cu/Al2O3	50	MO 20 mg/L	500	95.0	10	0.3183	5 (91%)	This work

summarizes a head-to-head comparison of dye-degradation efficiencies across different catalysts and dyes. The catalyst developed in this work is inexpensive, easily reusable, and simple to synthesize, showing superior performance compared with many literature-reported systems and highlighting its promise for tribo-catalytic environmental remediation.

On the other hand,

Table 2. Copper-based catalytic technologies for pollutant removal.

Technologies	Catalyst	Pollutants	Efficiency (%)	Ref.
Photocatalysis	Sm2CuO4 nanostructures	MO	91.4	[65]
Electrocatalysis	Cu electrode	MO	100	[66]
Fenton-like	Cu@SB nanozyme	Amlodipine	100	[67]
PMS activation	CuFe2O4@GO	Dyes	>90	[68]
Tribo-catalyst	Cu/Al2O3	MO	95	This work

highlights different copper-based catalytic approaches for organic pollutant removal. Cu-oxide catalysts (e.g., Sm₂CuO₄) are efficient under visible-light photocatalysis, while Cu-based electrodes (e.g., Cu) achieve high performance in electrocatalysis. Moreover, Fenton-like systems (Cu@SB) and persulfate activation (CuFe₂O₄@GO) also demonstrate high efficiency. Finally, supported Cu/Al₂O₃ illustrates how tribo-catalysis can achieve a high performance (95%) and may represent a possible sustainable alternative, as it does not require light or external oxidants, and thus can emerge as a sustainable route for dye degradation.

4. Conclusions

This work demonstrated, for the first time, the tribo-catalytic efficiency of Cu/Al₂O₃ nanoparticles for the degradation of methyl orange (MO), utilizing only mechanical energy generated by magnetic stirring under ambient conditions and in the absence of light. The system achieved a remarkable 95% degradation of MO within 10 h, significantly outperforming pure α-Al₂O₃, which reached only 60% under the same conditions. The tribo-catalytic performance was strongly influenced by the stirring speed and the contact area between the PTFE-coated magnetic stir bar and the vessel surface. Increasing the stirring rate from 100 to 500 rpm and using longer magnetic bars (from 2.5 cm to 3.5 cm) notably enhanced dye removal efficiency by intensifying frictional interactions and tribo-electric charge generation. Control experiments confirmed that MO degradation was negligible in the absence of the catalyst or without mechanical stirring, ruling out contributions from photodegradation or spontaneous breakdown. Furthermore, the Cu/Al₂O₃ catalyst maintained high catalytic activity over five consecutive cycles, with only a slight decrease in performance from 95% to 91%, confirming its excellent structural stability and reusability. These findings highlight the potential of Cu/Al₂O₃ as an effective, robust, and sustainable tribo-catalyst for environmental remediation, particularly in the treatment of wastewater containing persistent organic pollutants.