Ultrasound-Induced Construction of Cu_xCo_3−xO₄/Attapulgite for Catalytic Degradation of Toluene

Abstract

With the increasing demand for air pollution control, the development of efficient and stable catalysts to degrade hazardous VOCs such as toluene has become particularly important. Herein, various copper-doped attapulgite-supported cobalt oxide spinel composites (Cu_xCo_3−xO₄/ATP) were synthesized using an ultrasonic-assisted precipitation method. The results showed that the abundant Si-OH groups on the surface of ATP played a crucial role in anchoring Co, and the instantaneous high-energy input of ultrasonication facilitated the formation of Si-O-Co bonds in Co₃O₄/ATP. The doping of Cu ions induced the expansion of the Co₃O₄ lattice, resulting in a significant number of oxygen vacancies. The ultrasound-induced synthesized Cu_0.1Co_2.9O₄/ATP catalyst exhibited the best catalytic oxidation performance, achieving a 99% toluene degradation rate at 300 °C under a weight hourly space velocity (WHSV) of 10,000 mL·g⁻¹ h⁻¹ and initial toluene concentration of 1000 ppm, along with high stability during 12 h of continuous running. This work presents a new strategy for the cost-effective catalytic elimination of VOCs.

1. Introduction

The emissions of volatile organic compounds (VOCs) from various chemical industries pose a threat to the environment and health. This has led to increased pollution events such as haze and photochemical haze, which has seriously affected people’s lives and ecosystems [1]. To effectively remove these pollutants, catalytic combustion has become the main approach due to its low cost and high efficiency [2]. Supported metal catalysts play an essential role in tackling environmental issues. Generally, supported metal oxide catalysts are considered an ideal choice for providing stable and active sites [3]. The catalysts used in catalytic combustion include noble metal catalysts and non-noble metal catalysts [4,5]. Although noble metal catalysts have the advantages of low temperature and high activity, their high price and easy inactivation limit their industrial applications [6]. Alternatively, transition metal catalysts involving Mn, Cu, Co, and Ce elements have low cost, high catalytic activity, and good thermal stability [7,8]. Feng et al. prepared Fe, Cu, and Co composites, which formed a spinel-structured catalyst capable of efficient toluene oxidation [9].

Suitable support can not only serve as a physical support to disperse metals and interact with metal oxides but also influence catalytic activity and durability, which is attributed to the metal oxide–support interaction [10]. Catalytic performance is mainly determined by the number of active sites and the intrinsic activity of each site. The interaction between metal oxides and supports can improve both factors simultaneously, which is essential for the catalytic process [11]. Liu et al. adjusted the binary oxide–support interaction by dispersing CuO and CeO₂ on hydroxylated rough halloysite nanotubes. They found that the catalyst’s durability was enhanced due to the strong interaction between the oxides and the rough halloysite nanotubes [12]. Tong et al. demonstrated a crystal engineering strategy for adjusting strong electronic interactions in Fe₂O₃/ZrO₂ systems. Compared to the t-ZrO₂ phase, m-ZrO₂ exhibited a stronger electronic interaction with the iron oxide and iron species, generating more oxygen vacancies and transferring more electrons to facilitate the activation of intermediate products, thereby lowering the barrier of redox reactions [13].

Attapulgite (ATP) is a hydrated magnesium-aluminum silicate mineral with characteristic porosity and channels within the crystal run parallel to the fibers, providing a high surface area [14,15]. It has garnered considerable interest due to its abundant availability, cost effectiveness, and eco-friendliness [16]. ATP’s large specific surface area, excellent adsorption properties, and unique morphology and size promote the dispersion of active sites, making it a good support for catalysts [17]. Li et al. proposed a one-pot method for preparing Ce_1−xMn_xO₂/ATP with adjustable surface coverage, crystalline phase, and doping amount [18]. Liu et al. synthesized amorphous copper-manganese oxides using ATP as a carrier. However, the interaction between ATP and loaded oxides lacks investigation [19].

Ultrasonic treatment has been widely used in the preparation of materials due to its dispersal, cavitation, and surficial erosion effects [20]. The introduction of ultrasound can reconfigure the structure of the carrier, forming oxygen vacancies and metal-rich oxides, thus activating the metal carrier surface required for the successful construction of metal–support interaction catalysts [21]. Cano et al. developed a TiO₂/GO composite using an ultrasound-assisted technique to create surface oxygen vacancies and Ti³⁺ defects [22]. These defects enhanced metal–support interactions, increasing active sites and electron transfer. Kevin et al. prepared a stable Pd/a-TiO₂ catalyst with strong metal–support interactions through ultrasonic treatment, promoting active intermediate formation [21]. To the best of our knowledge, there are few reports on enhancing catalytic effects by ultrasonic induction in metal oxide-loaded attapulgite catalysts.

In this work, we successfully synthesized a series of Cu-doped Co₃O₄/ATP composite materials via ultrasonic precipitation by virtue of the cavitation effect and surface erosion, and they were employed for the catalytic oxidation reaction of toluene. The interaction between Co₃O₄ and ATP enhanced by ultrasound was investigated. The possible processes occurring during the catalytic degradation of toluene were studied using in situ DRIFTS technology. This work offers a new strategy for environmental purification and gives insight into the interactions between metal oxides and natural minerals.

2. Results and Discussion

2.1. XRD Analysis

The XRD patterns of ATP, Co₃O₄, Co₃O₄/ATP, U-Co₃O₄/ATP, and U-Cu_xCo_3−xO₄/ATP are presented in Figure 1a,b. ATP diffraction peaks appeared at 2θ = 8.49°, 13.90°, and 19.85°, corresponding to the crystal planes (110), (200), and (040) of ATP (JCPDS 21-0958), respectively. For Co₃O₄, the peaks at 2θ = 31.56°, 36.91°, 59.77°, and 65.62° corresponded to cubic Co₃O₄ (JCPDS#74-2120). The characteristic (311) reflection in the U-Co₃O₄/ATP sample at 2θ = 36.91° broadened and shifted to a low angle, which indicated that the lattice expansion characteristics of Co₃O₄ were initiated, suggesting there might be an interaction between Co₃O₄ and ATP synthesized by ultrasonic precipitation. As shown in Figure 1b, the peak intensity of the catalyst decreased significantly with an increase in the proportion of Cu doping and shifted to a low angle, indicating that the entry of Cu ions into the Co₃O₄ lattice caused lattice expansion and effectively delayed the growth of particles. By magnifying the main peak, it was observed that the shift was enhanced with an increase in the amount of Cu doping, which might have been caused by lattice defects generated by Cu doping [23]. However, in Figure 1b, no diffraction peaks corresponding to elemental Cu and its oxides are observed, indicating that Cu was uniformly doped into Co₃O₄ without forming bulk Cu crystals.

2.2. TEM Analysis

The morphology of the catalysts was examined by TEM and HRTEM. As shown in Figure 2a, upon loading Co₃O₄ on ATP, Co₃O₄ presented as nanoparticles, and significant agglomeration was observed. Figure 2b displays that in the U-Co₃O₄/ATP sample, Co₃O₄ nanoparticles were uniformly dispersed without noticeable agglomeration, indicating a distinct advantage of ultrasonic precipitation synthesis in the preparation of catalysts. In Figure 2c, Co₃O₄ nanoparticles in the U-Cu_0.1Co_2.9O₄/ATP sample are well dispersed, and their grain size appears smaller, further confirming the inhibitory effect of doping on crystal growth. The good dispersion and smaller crystal size increased the surface contact with toluene gas. In Figure 2d, the ordered lattice fringes of U-Cu_0.1Co_2.9O₄/ATP with an interplanar spacing of 0.286 nm corresponded to the Co₃O₄ (220) crystal plane [24].

2.3. FT-IR and Raman Analysis

The FT-IR spectra were utilized to observe the surface chemical bond states and functional groups of the synthesized catalysts. As shown in Figure 3, peaks at 3400–3500 and 1600–1700 cm⁻¹ were assigned to the stretching and binding of O-H bonds. The peaks near 1350–1450 cm⁻¹ were attributed to the stretching and bending vibrations of C-H from impurities in the support. For ATP, Co₃O₄/ATP, U-Co₃O₄/ATP, and U-Cu_0.1Co_2.9O₄/ATP, the surface Si-OH group’s asymmetric stretching vibrations and the stretching and bending vibrations of the Si-O-Si bonds appeared at 950–1100 cm⁻¹ [25]_. Importantly, Si-OH groups were anchoring sites for Co₃O₄, facilitating the formation of Si-O-Co bonds. The peak intensity of Si-O-Si bonds in U-Co₃O₄/ATP was significantly lower than that in ATP and Co₃O₄/ATP, indicating that more Co₃O₄ was anchored, forming Si-O-Co bonds in U-Co₃O₄/ATP [26]. These results validated that the introduction of ultrasound enhanced the Si-O-Co bond and the interaction between the oxide and support in U-Co₃O₄/ATP.

The structure and lattice distortions of the samples were further studied using Raman spectroscopy. As shown in Figure 3b, the characteristic peak located at about 667 cm⁻¹ corresponded to the A_1g vibrational mode of Co₃O₄ [27]. Specifically, the peak at 667 cm⁻¹ was related to the A_1g mode of the octahedral CoO₆ sites (Co³⁺-O²⁻) with stretching vibration, corresponding to the Raman vibration of Co(O_h)-O. Moreover, compared to Co₃O₄, the A_1g peak of Co₃O₄/ATP showed a noticeable redshift, indicating that U-Co₃O₄/ATP formed more crystal defects by anchoring Co₃O₄ onto Si-OH groups [26]. Additionally, the peak near 1000 cm⁻¹ was attributed to the Si-O bond. U-Co₃O₄/ATP demonstrated a pronounced characteristic peak of the Si-O-Co bond, suggesting that the introduction of ultrasound enhanced the interaction between the oxide and the carrier, consistent with the FT-IR analysis results. The above characterization indicated that the interaction between Co₃O₄ and ATP in U-Co₃O₄/ATP promoted the breakage of Co-O bonds, leading to substantial lattice distortion and the formation of oxygen vacancies, consistent with the XRD results [3]

2.4. H₂-TPR Analysis

The reducibility of Co₃O₄, Co₃O₄/ATP, U-Co₃O₄/ATP, and U-Cu_0.1Co_2.9O₄/ATP was evaluated through the H₂-TPR test, as shown in Figure 4. Two reduction peaks were present in the temperature range of 300–700 °C for the Co₃O₄/ATP sample. The first peaks at 375 °C for Co₃O₄/ATP and 341 °C for U-Co₃O₄/ATP were attributed to the reduction of Co³⁺ to Co²⁺ [28]. The second peaks at 532 °C for Co₃O₄/ATP and 537 °C for U-Co₃O₄/ATP were attributed to the reduction of Co²⁺ to Co⁰. Compared to Co₃O₄ and Co₃O₄/ATP, the reduction peaks for U-Co₃O₄/ATP shifted to lower temperatures. This suggested that the interaction between Co₃O₄ and ATP enhanced the low-temperature reducibility of U-Co₃O₄/ATP, consistent with the FT-IR and Raman analysis results. The U-Cu_0.1Co_2.9O₄/ATP sample showed only one reduction peak at 295 °C in the temperature range of 300–700 °C, corresponding to the reduction of Co³⁺ to Co²⁺. The disappearance of the reduction peak near 532 °C may have been due to the doping of Cu ions inhibiting the occurrence of the reduction of Co²⁺ to Co⁰.

2.5. XPS Analysis

The surface chemical states and elemental valence states of the prepared materials were further evaluated using XPS. Figure 5a presents the XPS survey spectra of Co₃O₄/ATP, U-Co₃O₄/ATP, and U-Cu_0.1Co_2.9O₄/ATP, where elements such as Co, Fe, C, O, and Si were detected. Notably, no Cu element was detected in U-Cu_0.1Co_2.9O₄/ATP, possibly due to the relatively low abundance of Cu. Figure 5b displays the XPS fine spectra of Co 2p. For all three samples, the Co 2p_3/2 and Co 2p_1/2 XPS spectra could be deconvoluted into two primary peaks and one satellite peak. The peaks at 779.79~780.24 eV and 781.42~781.82 eV in the Co 2p_1/2 XPS spectra were ascribed to Co³⁺ in the octahedral coordination and Co²⁺ in the tetrahedral coordination, respectively. After doping with Cu ions, a higher molar fraction of Co²⁺ was observed, suggesting the formation of more low-valent cobalt (Co²⁺) in the U-Cu_0.1Co_2.9O₄ catalyst. The presence of Co²⁺ may have promoted the generation of oxygen vacancies in the catalyst, which is beneficial for enhancing catalytic activity [29].

Figure 5c shows the XPS fine spectra of O 1s, containing three peaks, where 531.9 eV was attributed to lattice oxygen (O_lat) and 532.7 eV was attributed to surface adsorbed oxygen (O_ads). Notably, the O_lat peak of the U-Co₃O₄/ATP sample shifted toward a slightly higher binding energy, indicating a reduction in the electron density around O_lat. This suggested that the ultrasound-enhanced interaction between ATP and Co₃O₄ led to higher lattice oxygen activation [30]. Generally, the generation of surface-adsorbed oxygen is linearly correlated with the quantity of oxygen vacancies. It should be noted that larger O_ads/O_latt ratios for U-Co₃O₄/ATP and U-Cu_0.1Co_2.9O₄/ATP were identified, indicating that they were structurally prone to producing defects. As shown in Figure S1, the EPR representation of all samples showed a sharp EPR signal peak at g 2.003, which was mainly due to the Zeeman effect of the single electron trapped at the oxygen vacancy on the surface. It has been proven that the EPR signal is proportional to the oxygen vacancy concentration of the material. This indicated that U-Cu_0.1Co_2.9O₄/ATP contained more vacancies. Compared to Co₃O₄/ATP, doping with Cu may have promoted the concentration of oxygen defects and reactive oxygen species, aiding in the catalytic activity of the bimetallic oxides.

2.6. Catalytic Oxidation Performance

The catalytic oxidation of toluene over various catalysts was evaluated using toluene at a concentration of 1000 ppm, and the results are shown in Figure 6a. Both Co₃O₄/ATP and U-Co₃O₄/ATP reached T₉₉ at 320 °C. However, U-Co₃O₄/ATP exhibited significantly higher low-temperature oxidation activity than Co₃O₄/ATP. This indicated that the adsorption by the ATP support and its interaction with Co₃O₄ played a crucial role in improving the catalytic behavior. The T₉₉ temperatures for U-Cu_0.05CO_2.95O₄/ATP, U-Cu_0.1Co_2.9O₄/ATP, and U-Cu_0.2Co_2.8O₄/ATP were 310 °C, 300 °C, and 310 °C, respectively. Doping with Cu further improved the catalytic activity of U-Co₃O₄/ATP, which was higher than the previous reports [31,32,33], as seen in in Table S1, which was attributed to the defects introduced by ion doping providing active sites for toluene and O₂. Figure 6b shows the CO₂ yield of each catalyst. U-Cu_0.1Co_2.9O₄/ATP achieved the highest catalytic activity at 300 °C and had the same CO₂ yield (81%) as U-Cu_0.05CO_2.95O₄/ATP. U-Co₃O₄/ATP and U-Cu_0.05CO_2.95O₄/ATP reached CO₂ yields of 80% and 79% at 310 °C and 320 °C, respectively. With increasing temperature, the CO₂ selectivity significantly increased, indicating that the catalyst could quickly activate intermediates through electron transfer, promoting deep degradation of toluene. To further investigate the activity retention of the catalyst in different scenarios, different WHSVs were used to evaluate the catalyst at 300 °C, as shown in Figure 6c. When WHSV increased from 10,000 to 20,000 mL·g⁻¹ h⁻¹, the degradation rate of U-Cu_0.1Co_2.9O₄/ATP decreased, indicating that the contact time between the gas mixture and the catalyst surface was crucial for reaction activity, and shorter contact times resulted in poorer catalytic oxidation activity. To explore the stability of the samples, a continuous reaction was conducted for 12 h under a constant condition of 300 °C, as shown in Figure 6d. After 12 h of continuous reaction, the degradation rate and CO₂ yield of U-Cu_0.1Co_2.9O₄/ATP remained nearly unchanged. To investigate the intrinsic properties, the activation energies of the series catalysts were measured by controlling the reaction rate within the kinetic range. As shown in Figure S2, U-Cu_0.1Co_2.9O₄/ATP exhibited the lowest activation energy. The lower activation energy contributed to the oxidation of toluene. Therefore, it can be concluded that the U-Cu_0.1Co_2.9O₄/ATP catalyst had the best ability to catalyze the oxidation of toluene.

2.7. In-Situ DRIFTS Analysis

The reaction pathways in the degradation process of toluene over the U-Cu_0.1Co_2.9O₄/ATP catalyst were explored through in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) characterization, as shown in Figure 7. Initially, the sample was pre-treated at 200 °C under an N₂ atmosphere for 30 min to remove adsorbed substances from the surface. After cooling, in situ DRIFTS spectroscopy of toluene adsorption was performed at 30 °C for 30 min, as shown in Figure 7a. Following toluene adsorption on U-Cu_0.1Co_2.9O₄/ATP, absorption was observed at around 3032 cm⁻¹ for the C-H stretching vibrations of the toluene benzene ring, while the absorption at 2932 cm⁻¹ was attributed to methyl C-H stretching vibrations [34]. Stretching vibrations of the toluene ring skeleton (C=C) were observed at approximately 1459 and 1496 cm⁻¹ [35], and the C-H bending vibration of the methyl group was located at approximately 1031 cm⁻¹ [34], confirming that toluene was adsorbed by U-Cu_0.1Co_2.9O₄/ATP. With increasing time, a new peak emerged at around 1203 cm⁻¹ [36], corresponding to the C=C bond of the aromatic skeleton adsorbed on the surface of U-Cu_0.1Co_2.9O₄/ATP.

After reaching the adsorption equilibrium, the in situ cell was heated to 200 °C while toluene and air were introduced. In Figure 7b, peak values at approximately 1070 and 1462 cm⁻¹ were observed for benzyl alcohol, 1687 cm⁻¹ for benzaldehyde, and 1396 and 1520 cm⁻¹ for benzoic acid [36], respectively, suggesting that a single toluene was gradually oxidized to benzyl alcohol, benzaldehyde, and benzoic acid. Furthermore, C-H stretching vibrations of anisole were observed at around 1533 and 1543 cm⁻¹. A new peak characteristic of oxalic acid was observed at around 1726 cm⁻¹, indicating deep oxidation of toluene at 200 °C. The main degradation pathway of toluene during catalysis by U-Cu_0.1Co_2.9O₄/ATP could involve the following sequence: toluene → benzyl alcohol → benzaldehyde → benzoic acid → oxalic acid → carbon dioxide and water.

2.8. Catalytic Oxidation Degradation Mechanism

Figure 8 shows the reaction mechanism during the adsorption and catalytic degradation of toluene over the U-Cu_0.1Co_2.9O₄/ATP catalyst. In this process, the abundant Si-OH groups on the ATP surface play a crucial role in anchoring Co, facilitating the formation of Si-O-Co bonds. Simultaneously, the interaction between Co₃O₄ and ATP promotes the breakage of Co-O bonds in Co₃O₄, triggering substantial lattice distortion and the formation of oxygen vacancies. The further introduction of Cu ions increases the number of lattice distortions and oxygen vacancies [37]. When the catalyst contains a large number of oxygen vacancies, a considerable amount of active surface lattice oxygen species is generated, which are the sole oxygen species involved in the oxidation process of toluene. Hence, with the increase in oxygen vacancies, the oxygen species involved in the reaction gradually transfer from bulk lattice oxygen to surface lattice oxygen. The U-Cu_0.1Co_2.9O₄/ATP catalyst possesses abundant oxygen vacancies, leading to enhanced catalytic activity due to more active surface lattice oxygen species, thus lowering the activation energy for the oxidation of toluene. According to the Mars–van Krevelen mechanism, defects induced by ion doping play a dominant role in the adsorption, activation, and migration of molecular O₂. Toluene is initially adsorbed on the surface of the U-Cu_0.1Co_2.9O₄/ATP catalyst at low temperatures. As the temperature increases, toluene gradually oxidizes to benzyl alcohol, benzaldehyde, and benzoic acid and further decomposes into oxalic acid. Subsequently, these products are fully oxidized by lattice oxygen to CO₂ and H₂O and then leave the solid catalyst surface. Simultaneously, these products create oxygen vacancies and oxidize the surfaces of Co²⁺ to Co³⁺ species, forming a catalytic oxidation cycle [38].

3. Experimental

3.1. Materials

Cu(NO₃)₂·3H₂O, Co(NO₃)₂·6H₂O, and oxalic acid were purchased from Aladdin Ltd (Shanghai, China). All chemicals were used without further purification. Purified ATP powder was obtained from Xuyi, China.

3.2. Preparation of U-Cu_xCo_3−xO₄/ATP

First, 0.012 mol Co(NO₃)₂·6H₂O and Cu (NO₃)₂·3H₂O, 0.02 mol H₂C₂O₄, and 2 g of ATP were placed in a 50 mL beaker. They were stirred for 1 h to ensure they were thoroughly mixed. After stirring, the mixed solution was transferred to an ultrasonic reactor. The ultrasound frequency was set to operate for 10 s and then turn off for 10 s, with a power of 80 W. This cycle was repeated for 1 h. After the ultrasonic treatment, the mixed solution was centrifuged and filtered. The filtered solids were placed in an oven and dried at 80 °C for 12 h. After drying, the obtained solid samples were put into a muffle furnace and calcined at 400 °C for 4 h. The final composite was obtained and labeled as U-Cu_xCo_3−xO₄/ATP(x = 0~0.2), where x represented the molar ratio of Cu(NO₃)₂·3H₂O to Co(NO₃)₂·6H₂O.

3.3. Material Characterization

The samples’ crystal phase was identified through X-ray diffraction, utilizing Cu Kα radiation on a D/Max 2500 PC. The microstructure and morphology were probed via transmission electron microscopy using the JEM-2100 from JEOL (Tokyo, Japan) The UV–Vis diffuse reflectance spectra were recorded using a UV-2700 spectrophotometer. The chemical composition was analyzed through X-ray photoelectron spectroscopy (XPS) on the Thermo Scientific K-Alpha (Thermo Fisher, Waltham, MA, USA), equipped with an Al Kα X-ray source. The specific surface area was measured by the Brunner–Emmett–Teller (BET) method, employing the ASAP 300 (Micromeritics, Norcross, GA, USA). The Mott–Schottky (M–S) and transient photocurrent responses were assessed using the CHI 660D electrochemical workstation. Hydrogen temperature-programmed reduction (H₂-TPR) was carried out in a 10% H₂/Ar stream over a temperature range from 50 °C to 600 °C, with a heating rate of 10 °C/min, using the Auto Chem II 2920. The in situ DRIFTS (Diffuse Reflectance Infrared Fourier Transform Spectroscopy) was conducted on the Frontier FT-IR spectrometer (Thermo Fisher iS20, Waltham, MA, USA).

3.4. Catalytic Evaluation and Kinetic Test

Briefly, 1 g catalyst (sieved to 40~60 mesh) and 4 g quartz sand were evenly mixed and introduced into a quartz tube. In order to obtain different concentrations of toluene and WHSV, N₂ was bubbly injected into liquid toluene at different flow rates, and then fully mixed with dry air and flowed into the gas chromatographer. The catalytic performance of toluene oxidation was tested under WHSV from 10,000 to 20,000 mL·g⁻¹ h⁻¹. The toluene concentration was calculated according to the gas model. The central computer managed the reaction temperature and the velocity of the gas flow. Using a standard test procedure, the baseline toluene concentration was recorded using a gas chromatographer prior to the feed gas’s exposure to the catalyst. Subsequently, the flow was regulated through the catalyst to ascertain the post-reaction concentration. Concurrently, the temperature was incrementally raised from ambient to 350 °C, in increments of 25 °C, and held steady at each stage for a duration of 10 min to guarantee equilibrium of the reaction conditions. For kinetics measurement, the benzene conversion was kept below 10% to make sure the reaction was kinetic. The relative equations are as follows:

$${\mathrm{X}}_{\mathrm{T}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{e}}\left(\mathrm{\%}\right)={\displaystyle \frac{{[\mathrm{C}}_{\mathrm{i}\mathrm{n}}]-{[\mathrm{C}}_{\mathrm{o}\mathrm{u}\mathrm{t}}]}{\left[{\mathrm{C}}_{\mathrm{i}\mathrm{n}}\right]}}\times 100\mathrm{\%}$$

(1)

$${\mathrm{C}\mathrm{O}}_2 \mathrm{y}\mathrm{i}\mathrm{e}\mathrm{l}\mathrm{d}\left(\mathrm{\%}\right)={\displaystyle \frac{{{[\mathrm{C}\mathrm{O}}_2]}_{\mathrm{o}\mathrm{u}\mathrm{t}}}{\left[{\mathrm{t}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{e}}_{\mathrm{i}\mathrm{n}}\right]}}\times 100\mathrm{\%}$$

(2)

$$r=-kc=-\mathrm{A}\exp \left({\displaystyle \frac{-\mathrm{E}\mathrm{a}}{RT}}\right)c$$

(3)

where r, k, c, A, and Ea are the reaction rate (mol s⁻¹), rate constant (s⁻¹), concentration of the reactants, pre-exponential factor, and apparent activation energy (kJ mol⁻¹), respectively.

4. Conclusions

In summary, various Cu-doped Co₃O₄/ATP composites were successfully synthesized using the ultrasound-induced precipitation method. Ultrasonic treatment not only promoted the interaction between Co₃O₄ and ATP but also enhanced the low temperature reducibility of Co₃O₄. Cu doping further induced lattice expansion and generated oxygen vacancies in Co₃O₄, significantly improving the catalytic activity. When the molar ratio of Cu to Co was 0.1:2.9, the optimized U-Cu_0.1Co_2.9O₄/ATP catalyst demonstrated a 99% toluene degradation rate at 300 °C and WHSV of 10,000 mL·g⁻¹ h⁻¹, and it maintained high stability over 12 h running. This work demonstrates that ultrasound can tune the metal oxide–support interactions in creating efficient catalysts for VOC elimination, offering valuable perspectives for environmental catalysis.