Boosting Toluene Oxidation Performance of CoAl₂O₄ Catalysts by Incorporating the Synergistic Co³⁺-Ov Sites

Abstract

Volatile organic compounds can aggravate the atmospheric pollution and health risks due to their high toxicity and photochemical reactivity. Herein, a series of cobalt aluminate spinel catalysts with high efficiency was fabricated via a cost-efficient solvothermal method. Plentiful oxygen vacancies with negative charge were introduced adjacent to the octahedrally coordinated Co³⁺ species in CoAl₂O₄ catalysts, thereby generating the synergetic Co³⁺-oxygen vacancy (Ov) sites, which facilitated the rapid activation and migration of oxygen species. Accordingly, the superior catalytic activity was observed for 1Al-1Co even with lower cobalt due to the presence of abundant Co³⁺-Ov sites, revealing the predominant roles of synergetic sites in the toluene oxidation. Moreover, the 1Al-1Co catalyst exhibited the optimal intrinsic catalytic performance with the lowest activation energy of 161.2 kJ·mol⁻¹ and the highest specific toluene reaction rate of 3.18 × 10⁻⁵ mmol·h⁻¹·m⁻². In situ DRIFTS results further verified that oxygen vacancies and active Co³⁺ species could synergistically boost highly reactive oxygen species, which rapidly oxidize benzoate into maleic anhydride, achieving the efficient complete oxidation of toluene.

1. Introduction

Volatile organic compounds (VOCs) refer to organic chemical compounds with low boiling points and high vapor pressure at room temperature, which are prevalently derived from industrial processes, solvent use, vehicle exhausts, and daily lives [1]. However, most VOCs such as benzene, formaldehyde, and toluene are not only highly toxic and carcinogenic, but also can participate in the generation of tropospheric ozone and secondary organic aerosol by photochemical reactions, leading to atmospheric pollution, which poses a serious threat to human health and the ecological environment [2,3]. Therefore, various destruction technologies including photocatalytic decomposition, catalytic oxidation, plasma catalysis, and biological degradation, have been investigated to eliminate VOCs, among which the catalytic oxidation is regarded as the potential technology due to the high efficiency, low consumption, and environmental friendliness [4,5].

Spinel oxides with a general formula of AB₂O₄ present a stable cubic structure consisting of A-O tetrahedra and B-O octahedra, which have been emerging as a hot research material in catalytic reactions on account of their diverse composition and electronic configuration [6,7,8]. However, traditional spinel materials suffer from inferior catalytic performance because of the low-density active sites and poor electron transfer [9,10]. Indeed, the electronic structure and coordination environment could be effectively modified by introducing heteroatoms to substitute the cations at the A/B sites in the spinel structure, thereby boosting the catalytic reaction activity [11,12]. Furthermore, the surface of spinel oxides was also refabricated by the tetrahedral-octahedral phase interface construction, leading to the lattice mismatch or defects, thus contributing to lowering the oxygen vacancy formation energy [13,14]. As an important lattice defect, oxygen vacancies can regulate the local electron structure and geometric configuration of the catalyst, which greatly improves the electron mobility and reduces the reaction free energy [15,16]. Meanwhile, the unsaturated coordination structure formed by the oxygen vacancy and surrounding metal cations (Mⁿ⁺-Ov) in the material can regulate the metal-oxygen bond and create new reaction sites, facilitating the catalytic performance [17,18]. Therefore, constructing the synergistic effect between active metal sites and oxygen vacancies in spinel catalysts is an effective method to regulate the catalytic efficiency of VOCs degradation.

In general, cobalt-based catalysts exhibit high activity and have become a potential alternative to noble metals in various catalytic reactions due to the abundant resource, low cost, and the reversible redox of Co²⁺/Co³⁺ species [19,20]. It has been found that surface Co sites of cobalt-based catalysts might undergo self-reconstruction to generate active octahedral Co³⁺ species, achieving a superior catalytic activity [21]. Thus, we selected the inert aluminum as the matching element and prepared a series of AlCo₂O₄ catalysts with abundant Co³⁺-Ov sites using a cost-efficient solvothermal method. It was found that abundant oxygen vacancies were generated in the vicinity of high-valence Co³⁺ species in cobalt aluminate spinel, thus generating the synergistic Co³⁺-Ov sites. Accordingly, the low-temperature reducibility and oxygen activation ability were promoted by the synergistic effect of oxygen vacancy and active Co³⁺ species, thus greatly enhancing the catalytic efficiency. The catalyst mechanism of toluene degradation was also elucidated by in situ DRIFTS spectra.

2. Results and Discussion

2.1. Structure and Surface State Characterization

According to the XRD pattern (Figure 1a), the 1Al-2Co, 1Al-1Co and 2Al-1Co catalysts showed obvious characteristic diffraction peaks at 2θ = 18.95°, 31.19°, 36.75°, 44.69°, 55.51°, 59.20°, 65.05°, corresponding to the crystal planes of (111), (220), (311), (400), (422), (511) and (440) of CoAl₂O₄ (JCPDS#44-0160) with the cubic crystal system [22]. It was proven that the spinel structure was successfully constructed via a mild solvothermal method. The crystallite sizes of CoAl₂O₄ samples were estimated from the Debye–Scherrer formula on the basis of the diffraction peaks ascribed to the (311) crystal plane. As shown in

Table 1. Physical and chemical properties of 1Al-2Co, 1Al-1Co, and 2Al-1Co catalysts.

Samples	Surface Area (m2·g−1)	Total Pore Volume (cm3·g−1)	Pore Diameter (nm)	Crystallite Size (nm)	Co2+/Co3+	Oads/Olatt
1Al-2Co	96.0	0.37	3.41	8.1	0.83	2.50
1Al-1Co	113.5	0.32	3.41	7.3	0.48	3.05
2Al-1Co	114.8	0.40	3.41	8.5	0.94	2.30

, the crystal sizes of the 1Al-2Co, 1Al-1Co, and 2Al-1Co samples are 8.1, 7.3, and 8.5 nm, respectively. The lower crystallite size indicates more grain boundaries or defects, demonstrating the high concentrations of defects in the 1Al-1Co sample [23,24]. Partially enlarged XRD patterns of 1Al-2Co, 1Al-1Co, and 2Al-1Co are displayed in Figure 1b. Obviously, the (311) plane peaks were shifted to a higher diffraction angle in the order of 2Al-1Co, Al-2Co, and 1Al-1Co, which demonstrated that there was compressive strain or lattice contraction in the CoAl₂O₄ crystal structure, probably due to the vacancy generation [25,26]. Therefore, it can be inferred that the vacancy defects increased in the order of 2Al-1Co, Al-2Co, and 1Al-1Co.

The nitrogen adsorption–desorption isotherms were measured to obtain the specific surface area and pore size distribution of the sample and are displayed in Figure 2. According to the IUPAC recommendations, the 1Al-2Co, 1Al-1Co, and 2Al-1Co samples exhibited type IV isotherms with H4-type hysteresis loops in the entire measured relative pressure (P/P₀) range [27]. As shown in Table 1, the specific surface areas of 1Al-2Co, 1Al-1Co, and 2Al-1Co samples were 96.0, 113.5, and 114.8 m²·g⁻¹, while the total pore volumes corresponded to 0.37, 0.32, and 0.40 cm³·g⁻¹, respectively. As shown in Figure 2b, there were small pores around 2.4 nm besides 3.4 nm for 1Al-2Co and 1Al-1Co, but the pore diameter of 2Al-1Co was 3.4 nm.

Raman spectroscopy was employed to further analyze the structural defects, such as lattice disorder or oxygen vacancy, in spinel catalysts. Raman spectra in Figure 3a showed that there were five distinct Raman vibrational bands for the 1Al-2Co catalyst. The peak located at 485 cm⁻¹ was attributed to the E_g vibrational mode corresponding to the symmetric bending motion of oxygen atoms in the tetrahedral sites [28]. Three F_2g vibrational modes at 523, 591, and 622 cm⁻¹ were assigned to the stretching vibration of Co/Al-O in the tetrahedral sites, while the A_1g symmetric mode at 688 cm⁻¹ was related to the vibration mode of Co³⁺-O²⁻ in the octahedral sites (CoO₆), which is a characteristic peak of the spinel structure [29]. It can be clearly seen from Figure 3 that 1Al-1Co and 2Al-1Co samples exhibited similar Raman spectra, but the A_1g Raman peaks shifted to a higher frequency in the sequence of 2Al-1Co, Al-2Co, and 1Al-1Co (Figure 3b), further verifying the plentiful defects in 1Al-1Co catalyst [30].

To further elucidate the surface elemental and chemical states of the spinel catalysts, X-ray photoelectron spectroscopy (XPS) was conducted. As displayed in Figure 4a, the peaks with binding energies at approximately 779.6, 530.9, 284.8, and 73.8 eV corresponded to Co 2p, O 1s, C 1s, and Al 2p, respectively. The detailed valence states of spinel catalysts were demonstrated in high-resolution XPS spectra. In Figure 4b, two obvious peaks of Co 2p_1/2 and Co 2p_3/2 were observed for the Co 2p XPS spectra of the three samples, which are caused by the spin–orbit re-splitting of Co 2p [31,32]. The fitted peaks at 797.9 and 783.2 eV were attributed to the tetrahedral Co²⁺ species, while those at 796.5 and 781.2 eV corresponded to the characteristic peaks of octahedral Co³⁺ species [33,34]. It has been demonstrated that the octahedral Co³⁺ sites could provide a more favorable local electronic structure for the catalytic reaction in comparison with the tetrahedral Co²⁺ sites, thus serving as the main catalytic active sites [31,35]. The relative content of Co³⁺ species was estimated from the area proportion of deconvoluted Co 2p XPS peaks and summarized in Table 1. Specifically, the order of the Co²⁺/Co³⁺ ratio was as follows: 1Al-1Co (0.48) < 1Al-2Co (0.83) < 2Al-1Co (0.94), and the 1Al-1Co sample possessed the highest content of octahedrally coordinated Co³⁺ species, thus improving the catalytic performance. It can be seen from the O 1s XPS spectra in Figure 4c that there were three obvious peaks at 529.9, 531.9, and 532.2 eV corresponding to lattice oxygen (O_latt), surface adsorbed oxygen (O_ads), and surface OH groups (O_OH), respectively [36]. It is reported that surface adsorbed oxygen is closely associated with the presence of oxygen vacancies [37,38]. Table 1 demonstrates that the O_ads/O_latt ratios of 1Al-2Co, 1Al-1Co, and 2Al-1Co were 2.50, 3.05, and 2.30, respectively, revealing the higher vacancy defects in the 1Al-1Co sample. Indeed, oxygen vacancies could be induced by a high-valence cobalt over CoAl₂O₄ spinel according to the Kroger–Vink notation as follows [39,40].

$$2{\mathrm{C}\mathrm{o}}_{\mathrm{C}\mathrm{o}}^{\times }+{\mathrm{O}}_{\mathrm{O}}^{\times }\to 2{\mathrm{C}\mathrm{o}}_{\mathrm{C}\mathrm{o}}^{•}+{\mathrm{V}}_{\mathrm{O}}^{″}+{\displaystyle \frac{1}{2}}{\mathrm{O}}_2$$

(1)

where the subscript and superscript correspond to the occupied lattice sites and the electric charge variation relative to that in a stoichiometric crystal, respectively. A positive or negative charge was indicated by a bullet (•) or prime (′), while there was no change in the charge, which was represented by the cross (×). Thus, the presence of cobalt with a higher oxidation state would yield a higher concentration of oxygen vacancies (V_O″). As a consequence, these provide strong evidence that the plentiful oxygen vacancies were generated in the vicinity of Co³⁺ sites in CoAl₂O₄ spinel, while the relative concentration exhibited an obvious increased tendency for 2Al-1Co, 1Al-2Co, and 1Al-1Co, among which there was the highest amount of synergistic Co³⁺-Ov sites in 1Al-1Co sample.

H₂-TPR profiles were employed to evaluate the redox properties of spinel catalysts. As displayed in Figure 5a, all cobalt aluminate spinel catalysts exhibited four kinds of hydrogen reduction peaks. Two reduction peaks centered at 277–290 and 381–390 °C could be ascribed to the reduction of Co³⁺ to Co²⁺. The former was associated with surface cobalt ions not interacting with the cobalt aluminate phase, while the latter was attributed to the reduction of Co³⁺ species in CoAl₂O₄. High-temperature peaks at 618–624 °C were identified as the reduction of Co²⁺ to metallic Co⁰ [11,41]. Low-temperature peaks at 138–146 °C were attributed to the reduction of the labile Co species near oxygen vacancies, where the Co-O bond strength is weaker [42,43]. It can be clearly seen that the 1Al-1Co sample exhibited a lower reduction temperature and higher hydrogen consumption in comparison with 1Al-2Co and 2Al-1Co, meaning a distinct improvement in low-temperature reducibility, probably due to the synergistic effect of the octahedral Co³⁺ species and oxygen vacancies.

O₂-TPD profiles of three spinel catalysts were also conducted and displayed in Figure 5b. It can be clearly seen that O₂-TPD profiles of all CoAl₂O₄ samples could be fitted into two desorption temperature regions around 340 and 500 °C, which were labeled as α-O and β-O. The α-O represented the surface lattice oxygen species, while the β-O species were correlated to the liberation mostly from the bulk spinel structure [44]. Peak area fitting analysis revealed that the relative content of lattice oxygen species in the 1Al-1Co sample (10.12%) was higher than that in 1Al-2Co (8.06%) and 2Al-1Co (7.75%), indicating easier activation and release of lattice oxygen species [45,46]. Accordingly, the synergistic Co³⁺-Ov sites could promote low-temperature reducibility and oxygen activation ability, which is of crucial importance in catalytic oxidation.

2.2. Catalyst Activity

Catalytic performance of CoAl₂O₄ catalysts was evaluated by selecting toluene oxidation as the representative reaction, and corresponding conversion curves are shown in Figure 6a. It can be clearly seen that the 2Al-1Co catalyst displayed the worst catalytic activity, and the temperatures for achieving 50% (T₅₀) and 90% (T₉₀) toluene conversion were 337 and 376 °C (

Table 2. Catalytic properties of toluene oxidation of 1Al-2Co, 1Al-1Co, and 2Al-1Co catalysts.

Samples	T50 (°C)	T90 (°C)	Rs at 310 °C (mmol·h−1·m−1)	Ea (kJ·mol−1)
1Al-2Co	333	344	3.18 × 10−5	177.0
1Al-1Co	325	337	2.90 × 10−5	161.2
2Al-1Co	337	376	2.58 × 10−5	175.2

). With increasing the Co/Al ratio from 1:2 to 1:1 and 2:1, the T₅₀ of 1Al-2Co and 2Al-1Co catalysts dropped to 325 and 337 °C, while T₉₀ decreased to 333 and 344 °C, respectively. Hence, the ranking of catalytic performance for toluene elimination was as follows: 1Al-1Co > 1Al-2Co > 2Al-1Co. Numerous studies have shown that cobalt oxides generally exhibit a superior low-temperature catalytic activity and are widely applied in toluene oxidation, but alumina is commonly used as the support due to its inertness in catalytic reaction. Thus, increasing the Co/Al proportion from 1:2 to 1:1 could boost the toluene degradation, but the worst catalytic efficiency was observed for 1Al-2Co, even with more cobalt in comparison with 1Al-1Co. This abnormal phenomenon could be attributed to the synergetic effect of oxygen vacancies and octahedral Co³⁺ species in 1Al-1Co, leading to a superior catalytic activity. Table S1 presents a comparison of the toluene oxidation activity between the catalyst prepared in this work and a series of other catalysts. It can be clearly seen that the as-synthesized 1Al-1Co catalysts exhibited a better degradation efficiency than the supported Co₃O₄ catalysts or CoAl₂O₄ catalysts in the literature. Furthermore, the comparable or superior catalytic activity was also observed for the 1Al-1Co catalyst in comparison with AAl₂O₄ spinel catalysts (A = Ni, Cu, Zn).

Kinetic tests of CoAl₂O₄ catalysts were also conducted to investigate the influence of the Co/Al atomic ratio on the intrinsic catalytic performance. According to the Arrhenius plot (Figure 6b), the apparent activation energy (Ea) of the 1Al-1Co sample was calculated to be 161.2 kJ·mol⁻¹, which was lower than that of the 1Al-2Co (177.0 kJ·mol⁻¹) and 2Al-1Co (175.2 kJ·mol⁻¹). The specific toluene reaction rate (R_s) at 310 °C was also measured to exclude the interference of the specific surface area on the catalytic activity. As shown in Table 2, the specific reaction rate of 1Al-1Co sample was 3.18 × 10⁻⁵ mmol·h⁻¹·m⁻², which was much higher than those of the 1Al-2Co (2.90 × 10⁻⁵ mmol·h⁻¹·m⁻²) and 2Al-1Co (2.58 × 10⁻⁵ mmol·h⁻¹·m⁻²). Consequently, the 1Al-1Co sample had the optimal intrinsic catalytic activity, revealing the vital role of the synergistic Co³⁺-Ov sites in promoting catalytic performance for CoAl₂O₄ catalysts. XRD patterns of 1Al-1Co-R catalyst after catalytic test were also performed and displayed in Figure S1. Obviously, the used 1Al-1Co-R catalyst exhibited similar XRD patterns to the fresh 1Al-1Co catalyst, indicating that the spinel structure was still retained.

In order to investigate the potential of 1Al-1Co catalyst in practical application, the variation of toluene conversion under heating–cooling cycles, different WHSV, and water vapor conditions was studied. Toluene conversion in three consecutive heating–cooling cycles was displayed in Figure 6c. It was evident that the 1Al-1Co sample maintained the almost unaltered efficiency of toluene degradation, indicating excellent tolerance to temperature fluctuations. Figure S2 shows that at a WHSV of 30,000 mL·g_cat⁻¹·h⁻¹, the catalytic efficiency of the 1Al-1Co catalyst was superior to that at higher WHSVs. With reducing the WHSV from 60,000 to 30,000 mL·g_cat⁻¹·h⁻¹, T₅₀ and T₉₀ decreased from 305 and 324 °C to 302 and 309 °C, respectively. When the WHSV was increased to 120,000 mL·g_cat⁻¹·h⁻¹, the higher temperature for toluene conversion with T₅₀ at 310 °C and T₉₀ at 348 °C was observed. The toluene conversion of the 1Al-1Co catalyst under different moisture conditions was also investigated. As shown in Figure 6d, the toluene conversion decreased from 92% to 86% with the introduction of 1 vol% water vapor into the reaction gas stream and then quickly recovered to about 90% once it was removed. Further increasing the water vapor to 3 vol% led to a continuous reduction to 82% in toluene conversion that recovered to 88% after removing the water vapor. These results illustrated that the 1Al-1Co catalyst possessed excellent water repellency and regeneration capability.

2.3. Reaction Mechanism

The catalytic degradation mechanism of toluene over the 1Al-1Co catalyst was dynamically monitored and clearly elucidated by in situ DRIFTS. As shown in Figure 7a, the C-O bending vibration attributed to benzyl alcohol was detected at 1024, 1087, and 1178 cm⁻¹, which might be due to the reaction between toluene and surface oxygen species [47,48]. Obvious characteristic peaks of benzaldehyde appeared at 1203, 1465, 1600, and 1656 cm⁻¹, and benzoic acid species were observed at 1355, 1423, 1499, and 1545 cm⁻¹ [49,50]. Moreover, the adsorption peaks of maleic anhydride species, as the main open-ring product of toluene oxidation, were also detected at 1240, 1306, and 1811 cm⁻¹, indicating that the benzene ring underwent a cleavage reaction [51,52]. The contour projection of the toluene transient reaction in Figure 7b further confirmed that the various oxidation intermediates, including benzyl alcohol, benzaldehyde, benzoic acid, and maleic anhydride, were generated during toluene degradation, and their intensity increased with extension of the reaction time, meaning a successive oxidation process. Therefore, it can be speculated that toluene was sequentially oxidized into the benzyl alcohol, benzaldehyde, and benzoate species by reacting with active oxygen species, and the latter two became the predominant intermediates [53]. Indeed, the ring-opening of benzoate into maleic anhydride has been reported to be the rate-limiting step during the toluene oxidation, as the further transformation of anhydride species into CO₂ and H₂O is a relatively straightforward process [47,54]. As mentioned above, abundant Co³⁺-Ov sites were successfully incorporated into CoAl₂O₄ catalysts via a mild solvothermal method and dominated the catalytic performance of toluene degradation, which was verified by a worse catalytic efficiency for 1Al-2Co even with more cobalt with respect to 1Al-1Co. Importantly, oxygen vacancies and octahedrally coordinated Co³⁺ species could synergistically promote the generation of the highly active and mobile oxygen species, which rapidly oxidize benzoate into maleic anhydride (Figure S3) [55]. Accordingly, the considerable accumulation of ring-opening product over the 1Co-1Al catalyst surface was observed, which was easily mineralized into CO₂ and H₂O, achieving the superior catalytic efficiency for toluene oxidation.

3. Experimental

3.1. Materials

All the reagents were used directly without further purification. Isopropanol and acetic acid solution were purchased from Tianjin Fuyu Fine Chemical Co., Ltd., Tianjin, China. Al(NO₃)₃·9H₂O and Co(NO₃)₂·6H₂O were obtained from National Pharmaceutical Chemical Reagent Co., Ltd., Shanghai, China.

3.2. Catalyst Preparation

Cobalt aluminate spinel catalysts were prepared via a simple solvothermal method by varying the atomic ratio of cobalt to aluminum. Specifically, 6.363 g Al(NO₃)₃·9H₂O and 4.937 g of Co(NO₃)₂·6H₂O were dissolved in 40 mL of isopropanol, and then 20 mL of acetic acid was added. After stirring for 30 min, the mixture solution was transferred into a 100 mL Teflon-lined stainless steel autoclave and placed into a preheating oven at 200 °C for 4 h. When cooled to room temperature, the precipitates were washed alternately using water and ethanol three times, then dried overnight at 60 °C in a vacuum drying oven, and finally calcinated at 400 °C for 4 h in a muffle furnace to obtain the spinel catalyst, labeled as 1Al-1Co catalyst. The atomic ratios of aluminum and cobalt were changed from 1:1 to 1:2 and 2:1, respectively, and the other steps remained unchanged to obtain the 1Al-2Co and 2Al-1Co catalysts.

3.3. Catalyst Characterization

X-ray diffraction (XRD) was conducted using a Rigaku/SmartLab SE type X-ray powder diffractometer with Cu Kα radiation at a scanning rate (2θ) of 8 °/min. N₂ adsorption–desorption was measured by the Quantachrome autosorb-iQ instrument at 77 K to evaluate the specific surface area and pore structure of the catalysts. An X-ray photoelectron spectrometer (Thermo Scientific K-Alpha, Waltham, MA, USA) was used to obtain the elemental composition and valence state of the sample surface with Al Kα radiation (hυ = 1486.6 eV). Raman spectroscopy was obtained by a Thermo Scientific DXR instrument (Thermo Scientific, Waltham, MA, USA), and the excitation light source was set to 532 nm. Hydrogen programmed reduction (H₂-TPR) and oxygen programmed temperature desorption (O₂-TPD) of the samples were acquired by an Automated Chemisorption Analyzer (Auto Chem III 2930, Micromeritics Instrument Corp., Norcross, GA, USA). The test temperature ranged from 50 to 750 °C at a heating rate of 10 °C/min, and the signals were monitored by TCD. In situ DRIFTS spectra were collected on a German Bruker INVENIO-S spectrometer (Bruker, Berlin, Germany) in the range of 1000–4000 cm⁻¹ by accumulating 12 scans at a resolution of 4 cm⁻¹. Prior to the measurements, the samples were pretreated in N₂ flow and the background spectrum was subtracted. After that, the toluene/O₂/N₂ mixture flow was introduced, and the spectra of the catalyst at 330 °C were recorded.

3.4. Activity Measurement

The catalytic oxidation of toluene on spinel catalysts was evaluated using a continuous flow fixed-bed catalytic reactor. The reaction gas consisted of 1000 ppm toluene and 20% oxygen, with nitrogen serving as the balancing gas. The gas passed through the fixed bed at a flow rate of 50 mL/min, corresponding to a weight hourly space velocity (WHSV) of 60,000 mL·g_cat⁻¹·h⁻¹. Prior to the catalytic test, the samples were pressed, crushed, and sieved into a 40–60 mesh, and then placed in a quartz tube with an inner diameter of 4 mm. The toluene concentration at the inlet and outlet was measured using a gas chromatograph (Techcomp GC 7980, Techcomp Scientific Instruments Co., Ltd., Hong Kong, China). The toluene conversion (X_toluene) and the reaction rate (R_s) were obtained by the following formula:

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

(2)

$${\mathrm{R}}_{\mathrm{s}}=\left({\mathrm{X}}_{\mathrm{t}\mathrm{o}\mathrm{l}\mathrm{u}\mathrm{e}\mathrm{n}\mathrm{e}}\mathrm{Q}{\mathrm{C}}_{\mathrm{f}}\right)∕\mathrm{W}$$

(3)

In the formula, C_inlet and C_outlet presented the inlet and outlet toluene concentrations in the reaction tube. Q, C_f, and W corresponded to the volumetric flow rate (mL/h), inlet toluene concentration (mol/mL), and mass (g), respectively. The apparent activation energy (Ea) was calculated from Arrhenius-type plots of toluene reaction rates.

4. Conclusions

In this contribution, we prepared a series of CoAl₂O₄ spinel catalysts with abundant Co³⁺-Ov sites by adjusting the molar ratio of cobalt to aluminum based on a mild solvothermal method. Abundant oxygen vacancies appeared in the vicinity of the octahedrally coordinated Co³⁺ species in CoAl₂O₄ spinel in company with the generation of the synergistic Co³⁺-Ov sites; meanwhile, their relative concentration exhibited an increased tendency for 2Al-1Co, 1Al-2Co, and 1Al-1Co. Accordingly, the redox property and the oxygen activation ability were greatly promoted by plentiful Co³⁺-Ov sites in 1Al-1Co. Catalytic activity displayed that a superior catalytic efficiency for toluene oxidation was observed for the 1Al-1Co catalyst, even with the lower cobalt in comparison with the 1Al-2Co catalyst, revealing the predominant roles of synergistic Co³⁺-Ov sites in toluene degradation. Furthermore, the 1Al-1Co catalyst had the highest specific surface reaction rate and the lowest reaction activation energy. In situ DRIFTS results exhibited the considerable accumulation of maleic anhydride over the 1Al-1Co catalyst, which indicated that the ring-opening product was easily generated due to the synergetic Co³⁺-Ov sites, thus achieving the efficient complete oxidation of toluene. This work provides a promising strategy to construct high-performance spinel catalysts by incorporating the synergistic Co³⁺-Ov sites for VOCs removal.