Effects of Calcination Temperature and Calcination Atmosphere on the Performance of Co₃O₄ Catalysts for the Catalytic Oxidation of Toluene

Abstract

A series of Co₃O₄ catalysts were synthesized and derived from Co-BTC (BTC = 1,3,5-benzenetricarboxylic acid). The effects of different calcination temperatures and calcination atmospheres on the catalytic activity of the materials were investigated. The characteristics of the catalysts were investigated by using various techniques, including X-ray diffraction, N₂ adsorption–desorption measurements, scanning electron microscopy, X-ray photoelectron spectroscopy, and H₂ temperature-programmed reduction. The findings demonstrated that an increase in calcination temperature caused a higher agglomeration of grains, reduced the specific surface area, and influenced the contents of the active substance Co³⁺ and surface-adsorbed oxygen of the catalyst. The catalyst pretreated under the N₂ atmosphere showed a more uniform particle distribution, better low-temperature reducibility, and the highest catalytic activity. The in situ DRIFTS results indicated that toluene was decomposed successively to benzaldehyde, benzoic acid, bicarbonate, and carbonate species and was eventually broken down into small molecules of CO₂ and H₂O as the temperature increased.

1. Introduction

Volatile organic compounds (VOCs) are generally defined as the sum of a range of carbon-based organic air pollutants with boiling points ranging from 50 to 260 °C and a Reid vapor pressure greater than 10.3 Pa at a normal temperature (293.15 K) and pressure (101.325 kPa) [1,2]. VOCs have the characteristics of easy volatility, easy diffusion, and easy poisoning and have become major pollutants in the atmosphere [3,4]. Toluene is usually considered to be one of the most typical representatives of VOCs, with a wide range of sources and easy poisoning characteristics in various types of air pollution, such as ozone pollution, fine particulate matter pollution, and secondary aerosol pollution, and it is often an important precursor of direct or indirect harm to the environment and human body, causing widespread concern in society [5,6]. Among the reported methods for toluene removal, catalytic oxidation is considered to be one of the most effective and economical ways to remove toluene, due to its low energy consumption, fast reaction rate, high feasibility, and lack of secondary pollution [7,8].

The choice of catalyst in catalytic oxidation plays a key role in the degradation efficiency of VOCs. The metal catalysts currently used for the degradation of toluene can be divided into noble metal catalysts and non-noble metal oxide catalysts [9,10]. Although noble metal catalysts exhibit outstanding performance for the catalytic removal of VOCs at low temperatures, their susceptible poisoning and high costs have limited their widespread application. Therefore, transition metal oxides have been considered as an alternative for the oxidation of VOCs, due to their relatively low cost, superior reducibility, and anti-poisoning ability [11,12]. Among the non-noble metal oxide catalysts, Co₃O₄ has the advantages of low cost and being environmentally friendly [13,14]. In related studies, Co₃O₄ catalysts showed excellent catalytic activity in the degradation process of toluene, and the high activity of Co₃O₄ catalysts is related to the presence of mobile oxygen in their spinel-type structures, which gives them good low-temperature reduction ability and a large number of oxygen vacancies and adsorbed oxygen content on the surface [15,16]. However, the specific surface area, pore structure, and physical and chemical properties of Co₃O₄ catalysts also play an important role in the degradation of toluene [17]. Therefore, most of the research is devoted to the synthesis of Co₃O₄ catalysts with different structures by different preparation methods [13,17]; however, traditional methods are not conducive to controlling the particle size and structure within the catalyst [18]. Therefore, a simple and controllable method is needed to prepare Co₃O₄ catalysts to achieve high-efficiency degradation of toluene.

In recent years, metal–organic frameworks (MOFs) have attracted widespread interest due to their large specific surface areas, flexible and tunable structures, and versatile properties [19,20]. The nature of the MOFs—namely, that their highly ordered metal nodes are well separated by long chains of organic ligands [21]—also allows the nanometal oxides prepared by direct pyrolysis of MOFs to have highly dispersed metal active centers while still retaining their original pore structure [20]. This provides a very effective method for the design of Co₃O₄ catalysts. It has been demonstrated that Co₃O₄ catalysts derived from Co-MOFs showed good degradation of toluene. Juan Lei and his co-workers synthesized Co₃O₄ catalysts with octahedral, dodecahedral, and rod shapes using three different Co-MOFs; among them, the ZSA-1-Co₃O₄ catalyst with an octahedral shape showed the best toluene degradation activity, with 90% toluene conversion reached at 249 °C using a gaseous hourly space velocity (GHSV) of 40,000 mL·g⁻¹·h⁻¹ for 1000 ppm of toluene [22].

However, the monometallic catalysts derived from MOFs are generally vulnerable to clustering or sintering during pyrolysis, which leads to larger metallic particles and poorer dispersion, resulting in the reduced catalytic activity of the catalyst [23]. Juan Lei and his colleagues investigated the effects of different calcination temperatures on the catalytic activity of Co₃O₄ catalysts and found that the catalysts exhibited different physical and chemical properties under different calcination temperatures, with Co₃O₄ prepared at 350 °C having the largest specific surface area and pore volume and the highest Co³⁺/Co²⁺ and O_ads/O_latt values, meaning the highest catalytic activity in the catalytic degradation of toluene [24]. Xiaodong Zhang and his co-workers prepared Mn₂O₃ catalysts in different pyrolysis atmospheres and found that the catalytic activity of the Mn₂O₃ catalyst pretreated with Ar and then with O₂ had the highest Mn³⁺ and adsorbed oxygen content, and its catalytic activity was superior to that of the Mn₂O₃ catalysts treated with either Ar or O₂ alone [25]. However, most of the existing studies have been limited to investigating the effects of the calcination temperature on the catalysts or the effects of the calcination atmosphere on the catalysts; the synergistic influences of the calcination temperature and atmosphere on the physical and chemical properties of materials have not been comprehensively analyzed.

Herein, we provide a simple method using Co-MOFs as a precursor to obtain honeycomb–spherical structures and Co₃O₄ catalysts for the degradation of toluene. In order to determine the influence mechanism of the Co₃O₄ catalysts formed by the calcination temperature and calcination atmosphere on the degradation activity of toluene, the ideal synthetic route for the synthesis of Co₃O₄ materials using Co-BTC as a precursor was explored to provide a reference for the synthesis process of metal oxides derived from MOFs. Finally, the catalyst was subjected to in situ IR tests under different atmospheric conditions to explore the toluene degradation process and intermediate products.

2. Experimental Section

2.1. Preparation of Catalysts

2.1.1. Synthesis of Co-BTC

Co-BTC was synthesized by a simple solvothermal method. Typically, 0.01 mol of BTC was added to 120 mL of a mixture of DMF, ethanol, and water, with a volume ratio of 8:1:1. After the mixture was completely dissolved, 0.012 mol of Co(NO₃)₂·6H₂O was added to the mixed solution and stirred for 30 min with a magnetic stirrer. Then, the resulting solution was poured into a 200 mL Teflon-lined autoclave and heated at 150 °C for 24 h. After cooling naturally to room temperature, the product was separated using sand core filters. The collected precipitate was added to 30 mL of ethanol and stirred using a magnetic stirrer for 30 min to remove the unreacted chemicals from the material, before being filtered by repeated rinsing with ethanol. The obtained product was dried at 80 °C overnight to obtain the Co-BTC material.

2.1.2. Synthesis of Co₃O₄ Catalysts

Co-BTC was calcined in air at different temperatures (350 °C, 450 °C, and 550 °C) for 2 h at a heating rate of 2 °C·min⁻¹; the products were named Co₃O₄-350, Co₃O₄-450, and Co₃O₄-550, respectively. Then, the Co-BTC was calcined at 450 °C in N₂ for 2 h at a heating rate of 2 °C·min⁻¹ to obtain Co₃O₄-N₂. Additionally, the sample of Co-BTC was first treated at 450 °C in N₂ for 1 h and then treated at 350 °C in O₂ for 1 h, and the product was named Co₃O₄-N₂-O₂. As a comparison, 0.012 mol of the Co(NO₃)₂·6H₂O was treated at 450 °C in N₂ for 1 h and then treated at 350 °C in air for 1 h to obtain Co₃O₄-P.

2.2. Characterization of Materials

The thermal stability of Co-BTC was tested via thermogravimetric analysis (TGA) using an STA-449 F4 Jupiter thermal analyzer (Netzsch, Germany). The weight change in the catalyst was recorded from room temperature to 750 °C under different air and N₂ conditions, at a heating rate of 2 °C·min⁻¹. Powder X-ray diffraction (XRD) patterns were recorded on a Bruker D8 Advance powder diffraction system using Cu-Kα radiation in a 2θ range of 5–90°, at a scanning rate of 5°·min⁻¹. The catalysts’ specific surface areas, pore volumes, and pore size distribution were determine using a Micromeritics ASAP 2460 Automated Gas Sorption analyzer. Surface areas were determined using the Brunauer−Emmett−Teller (BET) method; pore volumes and pore size distribution were determined using the Barrett−Joyner−Halenda (BJH) method. A ZEISS Sigma 300 scanning electron microscope (SEM) (Oberkochen, Germany) was operated at an accelerating voltage of 15 kV. The catalyst was deposited on a clean silicon wafer, mounted on the aluminum holder using carbon tape, and sputter-coated with gold to minimize sample charging. X-ray photoelectron spectroscopy (XPS) analysis was performed using a Thermo Fisher Scientific spectrophotometer with Al Kα (1486.6 eV) as an X-ray excitation source. Survey scans and narrow scans were carried out at pass energies of 100 eV and 30 eV, respectively. C1 s was used as a reference and assigned a binding energy of 284.8 eV. The Micromeritics AutoChem II 2920 chemical adsorption apparatus was used for a hydrogen temperature-programmed reduction (H₂-TPR) experiment. Before the H₂-TPR experiment, 0.1 g of catalyst was pretreated under an Ar atmosphere at 150 °C for 1 h. The temperature was then cooled to 50 °C and the reduction process began, proceeding in a temperature range of 50–800 °C in 10% vol. H₂/Ar (30 mL·min⁻¹). In situ diffusion reflectance infrared Fourier-transform spectrophotometry (DRIFTS; Nicolet is 50) was employed to investigate the reaction process of toluene oxidation over Co₃O₄-N₂-O₂. The catalyst was squeezed into a thin slice in the DRIFT cell and tested in situ using absorbance; it was then activated to remove surface impurities under Ar for 1 h at 200 °C, and then cooled to room temperature. Toluene (200 ppm) was introduced until the peaks remained unchanged. Subsequently, Ar or air was used to record changes in IR peaks at different temperatures under different gas atmospheres.

2.3. Catalytic Testing

The catalytic oxidation of toluene was carried out in a fixed-bed flow reactor at atmospheric pressure. First, 100 mg of catalyst was packed into a quartz tube 10 cm in length, with an inner diameter of 2 cm; the ends were held in place by quartz fiber, and the reaction temperature was controlled using a tubular furnace (KTF-0.5-06). Toluene was injected from a single-channel syringe pump (SPLab01) into the gasification chamber, and the gasified toluene was carried through 100 mL·min⁻¹ air into a quartz tube (the gas flow was controlled using a gas flowmeter). For toluene, the reacted gas was detected using gas chromatography (GC112A), and the detection conditions were as follows: high-purity N₂ was used as a carrier gas; a flame ionization detector (FID) was used; the column temperature was 150 °C; the gasification chamber temperature was 250 °C; the detector temperature was 250 °C; and the chromatographic column was a capillary column. For CO₂, the reacted gas was detected using a multifunctional gas analyzer (GT-2000). Toluene’s conversion (η) and mineralization (μ) were calculated according to the following equations:

$$\eta =\frac{C_0-C_i}{C_0}\times 100\%$$

$$\mu =\frac{C_{{\mathrm{C}\mathrm{O}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}}{C_{{\mathrm{C}\mathrm{O}}_2,\mathrm{i}\mathrm{n}}}\times 100\%$$

where C₀ (ppm) and C_i (ppm) are the toluene concentrations of the inlet and outlet gases, respectively, while $C_{{\mathrm{C}\mathrm{O}}_2,\mathrm{i}\mathrm{n}}$ (ppm) and $C_{{\mathrm{C}\mathrm{O}}_2,\mathrm{o}\mathrm{u}\mathrm{t}}$ (ppm) represent the concentration of carbon dioxide at the time of complete oxidation of toluene and the concentration of carbon dioxide at the outlet after the reaction, respectively.

Reaction rates over catalysts and apparent activation energies (E_a) were calculated for toluene conversions lower than 20%, in line with the practice previously described in the literature [24].

3. Results and Discussion

3.1. Structural Analysis

The XRD patterns of all catalysts are presented in Figure 1. It can be seen that the air-calcined Co₃O₄ catalysts in the 2θ range of 5–90° displayed similar patterns, which corresponded to the standard Co₃O₄ sample (Co₃O₄-PDF#74-2120). The characteristic diffraction peaks of Co₃O₄ at 19°, 31.3°, 36.8°, 44.8°, 59.4°, and 65.2° corresponded to the (111), (220), (311), (400), (511), and (440) crystal faces, respectively. This showed that the air-calcined Co-BTC ended up with a Co₃O₄ crystal-phase structure. The Co₃O₄-N₂ nanomaterial exhibited faint diffraction peaks at 2θ of 36.6°, 42.6°, and 61.8°, which corresponded to the (111), (200), and (220) lattice planes, respectively, of pure CoO (CoO-PDF#75-0533). It was found that the Co in the Co₃O₄-N₂ catalyst existed mainly in the form of CoO, due to the fact that, in the absence of oxygen, Co mainly combines with oxygen bonds to form CoO. From Figure 1 and Tables S1 and S2, it can clearly be seen that the peak intensity of Co₃O₄ was enhanced and its width narrowed with increasing air calcination temperature, indicating that with the increase in calcination temperature, O₂ oxidized the metal clusters in the Co-BTC skeleton more fully, resulting in better crystallization of Co₃O₄ and the formation of metal oxide clusters with larger particles. By comparing the diffraction peaks of the metal oxides formed in different calcination atmospheres, it was found that the oxides calcined with N₂ had lower peak intensities and larger peak widths, especially for the Co₃O₄-N₂ catalyst, indicating that the Co₃O₄-N₂ catalyst had a smaller particle size.

The N₂ adsorption−desorption isotherms and BJH pore size distributions of the Co₃O₄ catalysts are shown in Figure S2. The catalysts all exhibited type IV isotherms with H3-type hysteresis loops, indicating that the catalysts had similar mesoporous structures [26,27].

Table 1. Specific surface area and pore structure data of Co₃O₄ catalysts.

Samples	BET Surface Area (m2·g−1)	Average Pore Size (nm)	Pore Volume (cm3·g−1)
Co3O4-550	8.7	4.9	0.01
Co3O4-450	16.8	9.4	0.04
Co3O4-350	40.2	17.9	0.18
Co3O4-N2-O2	57.2	21.25	0.3
Co3O4-N2	161.4	11.51	0.46

shows the specific surface area, average pore size, and pore volume data for the different catalysts. It can be seen that the influence of the calcination temperature on the pore structure of the material was significant. With the increase in temperature, the specific surface area, average pore size, and pore volume of the material all decreased as a result of agglomeration and expansion between crystals at high calcination temperatures (Figure 2). In addition, the calcination atmosphere also affected the pore structure of the material. It was found that the oxides calcined under an N₂ atmosphere had a larger specific surface area. Co₃O₄-N₂ had the largest specific surface area (161.4 m²·g⁻¹), and this was attributed to the carbonization of the organic ligands in Co-BTC under the N₂ atmosphere, which enabled the material to maintain its original skeletal structure. It was also found that Co₃O₄-N₂-O₂ had the largest average pore size, which facilitated the transfer process between reactants and products.

The microstructures of the catalysts are presented in Figure 2. It can be seen that all catalysts exhibited a honeycomb-like spherical structure. From Figure 2a–c, it can also be seen that the pore structure inside the Co₃O₄-350 catalyst spheres was composed of a number of aggregated grains. As the air calcination temperature was increased, the surface of these grains gradually became smooth, and the particle size also showed signs of expansion, indicating that sintering and agglomeration between the grains occurred at high temperatures, resulting in a decreasing specific surface area of the catalyst, which was consistent with the results of the XRD and BET characterizations. The Co₃O₄-N₂-O₂ catalyst spheres showed no obvious aggregation between the grains and exhibited a highly discrete and uniform distribution; this indicated that the catalyst exhibited more roughness and possessed a larger specific surface area, facilitating the exposure of the Co active sites in the catalyst, and thereby increasing the chance of contact with toluene and improving the catalytic oxidation activity of the catalysts.

3.2. Surface Composition and Reduction Behavior

To obtain further information about elemental chemical states and surface oxygen species, the Co 2p_3/2 and O 1s XPS spectra of the samples were obtained, as shown in Figure 3a,b, respectively. The Co 2p_3/2 XPS peak could be fitted to two component peaks and two satellite peaks, with the main peaks at binding energies of 779.5 eV and 781.29 eV corresponding to the Co³⁺ and Co²⁺ species, and the main peaks at binding energies of 785.68 eV and 789.02 eV corresponding to the Co³⁺ and Co²⁺ satellite peaks [28,29]. The Co³⁺ and Co²⁺ contents and the corresponding Co³⁺/Co²⁺ ratios were calculated from the peak areas, and the data are presented in

Table 2. XPS surface element analysis of catalysts prepared at different calcination temperatures.

Samples	Co3+(%)	Co2+(%)	Co3+/Co2+	Oads(%)	Olatt(%)	Oads/Olatt
Co3O4-350	53.29	46.71	1.14	41.46	58.54	0.70
Co3O4-450	51.69	48.31	1.06	36.43	63.57	0.57
Co3O4-550	50.28	49.72	1.01	33.06	66.94	0.49

and

Table 3. XPS surface element analysis of catalysts prepared with different atmospheres.

Samples	C(%)	O(%)	Co(%)	Co3+/Co2+	Oads/Olatt
Co3O4-350	29.90	47.34	22.75	1.14	0.70
Co3O4-N2-O2	27.55	50.04	22.40	1.18	0.74
Co3O4-N2	64.02	23.06	12.92	0.96	1.92

. The ratio of Co³⁺/Co²⁺ increased in the order of Co₃O₄-N₂ < Co₃O₄-550 < Co₃O₄-450 < Co₃O₄-350 ≈ Co₃O₄-N₂-O₂, where Co₃O₄-350 and Co₃O₄-N₂-O₂ exhibited the highest values of Co³⁺/Co²⁺, indicating the presence of abundant Co³⁺ species in Co₃O₄-350 and Co₃O₄-N₂-O₂. Previous studies have shown that Co³⁺ usually acts as the active center of toluene and can effectively promote the degradation of toluene [30].

Figure 3b shows the O 1s XPS spectra of the samples, which could be divided into two peaks at binding energies of 529.36 eV and 531.03 eV, respectively. This could be ascribed to lattice oxygen (O_latt) and adsorbed oxygen species on oxygen vacancies (O_ads) [31], where the Co₃O₄-N₂ catalyst shifted the position of the oxygen species peak, so that the peaks with binding energies at 529.66 eV and 531.54 eV represented lattice oxygen species and adsorption oxygen, respectively. The O_ads/O_latt molar ratios were determined based on the area-fitted curve peaks of the O 1s XPS spectra; these are listed in Table 2 and Table 3. It can be seen that the ratio of O_ads/O_latt decreased as the calcination temperature of the material increased, indicating that high-temperature calcination was not conducive to the accumulation of adsorbed oxygen in the material. Previous studies have shown that surface-adsorbed oxygen (O_ads) is related to surface defects and that higher contents of O_ads facilitate the uptake and activation of gas-phase oxygen molecules by the catalyst, promoting the degradation of pollutants [32,33]. In addition, it is worth noting that Co₃O₄-N₂ had the highest O_ads/O_latt molar ratios. However, from Figure S3 and Table 3, it can also be seen that the surface C, O, and Co contents of the Co₃O₄-350 and Co₃O₄-N₂-O₂ catalysts were essentially the same, with the highest elemental content being that of O, accounting for about 50% of the total. In contrast, the Co₃O₄-N₂ catalyst had the largest amount of elemental C but only 23% of the total amount of O. This was because the Co₃O₄-N₂ catalyst was prepared in the absence of oxygen, so most of the organic matter in the ligand was stored as carbon inside the catalyst, and the metal Co inside the material could not be fully oxidized, resulting in a lack of sufficient O in the catalyst itself, so that the Co₃O₄-N₂ catalyst had the highest O_ads/O_latt molar ratios but the lowest total amount of O_ads compared with other catalysts. This phenomenon can also be seen in Figure S1.

The reducibility of a sample was highly related to its catalytic performance. Figure 4 illustrates the H₂-TPR profiles of all synthesized catalysts. Three distinct reduction peaks were found for all catalysts, in which the low-temperature-region reduction peak indicated the reduction of Co³⁺ to Co²⁺, the medium-temperature-region reduction peak indicated the reduction of Co²⁺ to Co⁺, and the high-temperature-region reduction peak indicated the reduction of Co²⁺ to Co⁰ [34]. From Figure 4a, it can be seen that the reduction peak of the catalyst gradually moved towards the high-temperature region as the calcination temperature increased, indicating that the reduction effect of the material deteriorated under high-temperature calcination conditions; this may be related to the particle size of the catalyst, as the XRD results indicated that the particle size of the catalyst became larger as the calcination temperature increased. It has been reported in the literature that the reduction peak tends to move towards the high-temperature region as the particle size increases [35]. Figure 4b shows the reduction peaks of the catalysts prepared in different calcination atmospheres. A comparison reveals that Co₃O₄-N-O₂ had the lowest reduction temperature. This finding may be related to the specific surface area of the material and the abundance of Co³⁺ active centers, i.e., the high specific surface area of the material made it easier to expose a large number of Co³⁺ active sites on the external surface. Compared with Co₃O₄-N₂-O₂, the reduction peak of Co₃O₄-N₂ was very weak, because the Co in the Co₃O₄-N₂ catalyst existed mainly in the form of CoO and, thus, lacked a sufficient number of Co³⁺ active sites.

3.3. Catalytic Performance

Figure 5a,b show the temperature dependence of the catalytic efficiency in toluene conversion for all as-prepared catalysts under the conditions of toluene concentration = 1000 ppm and WHSV = 60,000 mL·g⁻¹·h⁻¹. It can be seen from the graph that no catalyst showed any significant toluene degradation below 210 °C; subsequently, as the temperature increased, the catalysts’ active sites were gradually activated and the toluene degradation rate rapidly increased. From Figure 5a, it can be seen that increasing the calcination temperature of the catalyst did not promote catalytic activity, because this increase in calcination temperature also caused the particle size of the catalyst to gradually increase, resulting in a decreased specific surface area of the catalyst, which was not conducive to the exposure of the active sites. From Figure 5b, it can be seen that the different calcination atmospheres of the materials also had an effect on the degradation of toluene. The Co₃O₄-N₂-O₂ catalyst exhibited the best catalytic effect, while the Co₃O₄-N₂ catalyst had the poorest, i.e., the worst degradation of toluene. This was because the catalytic activity of the catalyst was not only related to the specific surface area of the material—the Co³⁺ active sites and surface-adsorbed oxygen content were also important influencing factors. The degradation activity of the catalyst for toluene decreased as follows: Co₃O₄-N₂-O₂ > Co₃O₄-350 > Co₃O₄-450 > Co₃O₄-N₂ > Co₃O₄-550. The best toluene degradation rates were produced by Co₃O₄-N₂-O₂. The T₁₀, T₅₀, and T₉₀ temperatures required for 10%, 50%, and 90% toluene conversion were 221 °C, 234 °C, and 245 °C, respectively. In combination with the above characterization, these findings demonstrate that the catalytic activity of the catalysts was determined by a combination of the specific surface area, the morphology, the contents of Co³⁺ active sites and surface-adsorbed oxygen, and the redox properties of the material. In short, we found that a large specific surface area, homogeneous grain distribution, high Co³⁺ and surface-adsorbed oxygen contents, and excellent redox properties promoted the catalytic activity of the material.

The apparent activation energy (Ea) of the catalysts was calculated by applying the linear fitting of Arrhenius plots at a conversion rate of less than 20% (as shown in Figure 5c). The calculated results are listed in Table S3 in the following order: Co₃O₄-N₂-O₂ (39.65 kJ·mol⁻¹) < Co₃O₄-350 (42.06 kJ·mol⁻¹) < Co₃O₄-450 (47.84 kJ·mol⁻¹) < Co₃O₄-N₂ (67.27 kJ·mol⁻¹) < Co₃O₄-550 (70.43 kJ·mol⁻¹). The lowest Ea value corresponds to the highest possibility of toluene decomposition. These results were consistent with those of toluene conversion.

In order to explore the advantages of Co₃O₄ catalysts derived from Co-MOF in terms of catalytic activity and mineralization rate, we carried out a test in which Co₃O₄-N₂-O₂ (with the best catalytic activity) was used as the catalyst. The toluene conversion rates and CO₂ yields were measured for both the Co₃O₄-N₂-O₂ and Co₃O₄-P catalysts. As can be seen in Figure 5d,e, we found that the Co₃O₄-N₂-O₂ catalyst achieved obviously higher conversion rates at lower temperatures than the Co₃O₄-P catalyst, in terms of both toluene degradation efficiency and mineralization efficiency. The toluene degradation efficiencies of Co₃O₄-N₂-O₂ catalysts at 50% and 90% were 234 °C and 245 °C, respectively, and these values were broadly consistent with the temperatures corresponding to toluene mineralization efficiencies at 50% and 90% (236 °C and 246 °C, respectively). This showed that the Co₃O₄-N₂-O₂ catalyst could adequately convert toluene molecules to CO₂ and H₂O at the appropriate temperature, thereby reducing the emission of intermediate products during the reaction.

Figure 5f shows results of a continuous test of the catalytic oxidation of toluene over the Co₃O₄-N₂-O₂ catalyst. The test results demonstrated that the toluene degradation efficiency of the catalyst remained above 99% for 50 h at 260 °C, without any significant decline, indicating that the catalytic activity of the Co₃O₄-N₂-O₂ catalyst does not change over time and exhibits good catalytic stability when operated for a long period at an appropriate temperature, suggesting great potential for its usage in practical applications.

3.4. Mechanism for Degradation of Toluene over Co₃O₄-N₂-O₂

To further investigate the possible reaction pathway of toluene oxidation on the Co₃O₄-N₂-O₂ catalyst and any intermediates in the degradation of toluene, in situ DRIFT spectra of the catalyst at different temperatures under Ar and air conditions were obtained, as shown in Figure 6. It was found that, after a period of introducing toluene at room temperature, characteristic peaks of toluene appeared: at 3091 and 3035 cm⁻¹, attributed to unsaturated C-H bonds on the benzene ring; and at 2932 cm⁻¹, attributed to saturated C-H bonds on toluene’s methyl groups. In addition. 1585 and 1495 cm⁻¹ were found to be the benzene ring skeleton vibration peaks of toluene [36,37,38], indicating that toluene was adsorbed on the surface of the catalyst. Figure 6a shows that the characteristic peak of toluene did not change significantly with increasing temperature under an Ar atmosphere, meaning that the catalyst had little degradation effect on toluene under an Ar atmosphere. Conversely, under air-supply conditions, as shown in Figure 6b, the characteristic peak of toluene disappeared with the increase in temperature. At 140 °C, new bands appeared at 2840 and 2721 cm⁻¹, which could possibly be attributed to adsorbed benzaldehyde [39]; in addition, bicarbonate species (1356 cm⁻¹) also started to appear [40]. New characteristic peaks also appeared at 1563 and 1340 cm⁻¹ when the temperature reached 200 °C; these were attributed to benzoic acid and carbonate species, respectively [41,42]. As the temperature increased, benzaldehyde was further oxidized to benzoic acid, and bicarbonate species were gradually oxidized to carbonate species. The characteristic peaks of 1431 cm⁻¹ for carboxylate species such as benzoic acid, and of 2340 and 2360 cm⁻¹ for CO₂, became progressively larger as the temperature increased under an air atmosphere [38], meaning that more and more benzoic acid and CO₂ were generated and accumulated on the catalyst’s surface as the temperature increased.

The above analysis clearly demonstrates that the toluene molecules in the air were adsorbed on the surface of the catalyst, and that the catalytic oxidation effect of the lattice oxygen of the catalyst on toluene adsorbed on the catalyst’s surface was not significant. The degradation process of toluene molecules is shown in Figure 7. The toluene molecules were first adsorbed on the catalyst’s surface and then gradually degraded by the interaction with the adsorbed oxygen. With increasing temperature, the toluene molecules were gradually oxidized into benzaldehyde, benzoic acid, and bicarbonate and carbonate species, and finally decomposed into small molecules of CO₂ and H₂O [42].

4. Conclusions

Different Co₃O₄ catalysts were prepared under different calcination temperatures and calcination atmospheres, and their degradation effects on toluene were compared. It was found that increasing the air calcination temperature intensified the agglomeration of the internal grains of the catalyst and reduced the contents of Co³⁺ active sites and surface-adsorbed oxygen; this was not conducive to the improvement of the catalytic activity of the material. However, pretreatment with N₂ significantly improved the internal structure of the catalyst, making the internal grains of the catalyst more uniformly distributed; this promoted the exposure of active sites and improved the degradation efficiency of toluene. The Co₃O₄-N₂-O₂ catalyst exhibited superior toluene degradation activity and better toluene mineralization rates compared with Co₃O₄-P; it also exhibited stable catalytic activity, indicating its great potential for practical applications. Finally, the degradation process of toluene on the surface of Co₃O₄-N₂-O₂ catalyst was investigated; it was found that benzaldehyde, benzoic acid, and bicarbonate and carbonate species are intermediate products in the degradation process of toluene, which are ultimately decomposed into small molecules of CO₂ and H₂O with increased temperature.