Effect of Calcination Conditions on Co₃O₄ Catalysts in the Total Oxidation of Toluene and Propane

Abstract

Co₃O₄ catalysts were prepared via carbonate precipitation and subsequent calcination under specific conditions. The different catalysts were characterized as received using several techniques and tested in the total oxidation of toluene or propane. Calcination at low temperature or under dynamic conditions resulted in Co₃O₄ catalysts with small crystallite sizes and large surface areas. The performances of the Co₃O₄ catalysts appeared to be closely related to the low-temperature reducibility. The best catalyst, Co-350D, showed a toluene oxidation rate of 44.5 nmol g⁻¹ s⁻¹ at 200 °C and a propane oxidation rate of 54.0 nmol g⁻¹ s⁻¹ at 150 °C. Meanwhile, Co-350D exhibited excellent cycling stability and decent long-term durability in both reactions.

1. Introduction

Volatile organic compounds (VOCs), emitted from industrial manufacturing, automobile vehicles, and domestic activities and involved in the formation of ozone, photochemical smog, and secondary aerosols, are major contributors to air pollution [1]. Some of them are identified to be carcinogenic, teratogenic, and mutagenic, threatening human health [1]. Converting VOCs to harmless CO₂ and H₂O by catalytic oxidation is one of the most promising solutions. As the leading catalysts in the catalytic oxidation of VOCs, noble metals have been widely studied in the last decades [2]. However, viewing from an economic and sustainable perspective, transition metal oxides that exhibit comparable activity to noble metals could be a better choice. Among transition metal oxides, Co₃O₄ nanocatalysts have been proven to be very active in the total oxidation of toluene [3,4,5,6,7,8,9,10,11] and propane [12,13,14,15,16,17,18,19,20], two representative model VOCs. According to the well-accepted redox reaction mechanisms [21], the activity of Co₃O₄ is mainly governed by its reducibility and oxygen mobility, and these redox properties are generally related to the crystallite size, strain, structural defects, and surface cobalt oxidation state of Co₃O₄. Rational design and control of these properties lie in the preparation process. Precipitation is a commonly used method for Co₃O₄ synthesis with the advantages of simple steps, easy operation, and industrial scalability. The variation of precipitation conditions, such as precipitation agent [22], precipitation pH [23], or calcination procedure [24,25] could affect the physicochemical properties of Co₃O₄ and, consequently, the catalytic performance.

Recently, we have investigated the effect of the precipitation agent on Co₃O₄ catalysts in the total oxidation of toluene and propane and demonstrated carbonate as the most promising agent [26]. Straight after, we further singled out the optimum precipitation pH, 9.5 [27]. In addition to these, calcination conditions may be another important parameter that affects the physicochemical properties and catalytic performance of the catalysts. For instance, low calcination temperatures can be beneficial for obtaining small crystallite sizes and facilely accessible active sites [24,25]. By adjusting the calcination temperature, the spin states of Co³⁺ in LaCoO₃ catalysts can be accordingly tuned, which in turn determines the catalytic activity [28]. The content of surface oxygen vacancies in Co₃O₄/SiO₂ catalysts prepared via impregnation can be controlled by calcinating at various temperatures [29]. High temperature calcination oxidized excess surface Co²⁺ and lowered surface oxygen vacancies concentration. Shen et al. prepared Co₃O₄ by a solvothermal method and engineered the bond strength of Co–O in Co₃O₄ catalysts by annealing at 270, 300, 350, and 400 °C, respectively [3]. It was demonstrated that Co₃O₄ annealed at 300 °C exhibited the weakest Co–O strength, the highest surface oxygen species activity, and thus the best toluene oxidation performance. Wang et al. modulated oxygen vacancies in Co₃O₄ by controlling the calcination temperature (200–450 °C) [30]. The catalyst prepared via a hydrothermal method followed by calcination at 200 °C gave the best propane oxidation performance due to the abundance of oxygen defects. Liu et al. investigated the structural changes of CuO_x-CoO_y-CeO₂ ternary oxides in the calcination temperature range of 400 to 800 °C [31]. When calcined at 600 °C, the surface enrichment of Cu⁺, the improved interaction between Co₃O₄ and CeO₂, and the increased oxygen vacancies in CeO₂ were simultaneously achieved, giving the highest CO oxidation activity. Wu et al. found that calcination of CoCeO_x composites at high temperature resulted in much larger Co₃O₄ crystallites, lower surface areas, decreased reducibility, fewer surface cobalt species, and decreased amount of acid sites, and in turn inferior methane oxidation activity [32]. Li et al. synthesized Co₃O₄/CeO₂ composites by a modified citrate sol–gel method, in which the catalyst precursors were thermally treated with nitrogen first before calcination in air [33]. The results showed that the nitrogen-air two step annealing contributed to smaller grain size and higher surface area of Co₃O₄/CeO₂, and thereby better methane combustion performance. In addition, calcining in dynamic air is reported to eliminate excess reaction heat, avoiding hot spots and crystal sintering [34].

In this study, four Co₃O₄ catalysts were synthesized using the carbonate-precipitation method at a pH of 9.5, by varying the calcination temperature (350 °C vs. 550 °C) and calcination condition (static air vs dynamic air), to tune the structural and surface properties of the Co₃O₄ materials. The as-prepared catalysts were evaluated in the total oxidation of toluene and propane. Several characterization techniques were applied to probe the determining factor of the catalytic activity.

2. Results and Discussion

2.1. Structural and Textural Characterization

Figure S1 shows TG curve of the cobalt precursor in air flow. The small weight loss from 25 to 150 °C was attributed to the removal of physically adsorbed and chemically combined water. The large weight loss from 150 to 350 °C was due to the transformation of cobalt precursor into Co₃O₄. The slight gradual weight loss above 350 °C may arise from the perfection of the Co₃O₄ crystal. Based on the TG analysis, the cobalt precursor was calcined at 350 °C to produce Co₃O₄ with defective structure, and at 550 °C to generate Co₃O₄ with high crystallinity.

Figure S2 presents the FTIR spectra of the cobalt precursor and the cobalt oxide product. For the cobalt precursor, the broad band at 3495 cm⁻¹ was assigned to the stretching and bending modes of the H₂O and O–H groups [35,36,37]. The bands aroused by carbonate anions occurred at 1390 (ν(OCO₂), ν(CO₃)), 1070 (ν(C=O)), 829 (δ(CO₃)), 735 (δ(OCO)), and 679 cm⁻¹ (ρ(OCO)) [35,36,37]. The bands at 954 and 512 cm⁻¹ were attributed to δ(Co–OH) and ρ_w(Co–OH) [35,36,37]. After calcination, two bands typical for Co₃O₄ presented at 656 and 545 cm⁻¹ [38], accompanied by the disappearance of characteristic bands of cobalt carbonate hydroxide.

The chemical composition of the catalysts is listed in

Table 1. Chemical composition, average crystallite size, and textural data of the Co₃O₄ catalysts calcined under different conditions.

Catalysts	Co wt.% a	Na wt.% a	d (nm) b	a (Å) b	Strain (%) b	Sgeo (m2 g−1) c	SSA (m2 g−1) d	Vpore (cm3 g−1) d
Co-350S	72.7	0.12	16	8.088	0.58	64	61	0.281
Co-350D	72.2	0.11	12	8.090	0.74	81	81	0.365
Co-550S	73.9	0.17	51	8.085	0.17	19	20	0.032
Co-550D	73.3	0.11	29	8.086	0.33	34	33	0.116

^a Chemical composition determined by ICP-OES. ^b Average crystallite size(d), lattice constant(a) and strain estimated from XRD. ^c Geometrical specific area calculated by assuming all crystallites are spherical, S_geo = 6/(ρ∙d). ^d Specific surface area and total pore volume obtained from N₂ adsorption isotherms.

. The cobalt mass ratios of all catalysts were identical, close to that of stoichiometric Co₃O₄ (73.4 wt.%). The amounts of residual sodium in all catalysts were similar and were below 0.2 wt.%, thus excluding the possible poisoning effect of sodium on the catalysts [27].

Figure 1a gives the XRD patterns of the cobalt oxide catalysts calcined under different conditions. The diffraction peaks of all catalysts were well indexed to those of Co₃O₄ in the cubic spinel phase (a = 8.084 Å, Joint Committee on Powder Diffraction Standards (JCPDS), Card No. 74-2102). A higher calcination temperature led to sharper and stronger diffraction peaks due to the better crystallization of Co₃O₄. Meanwhile, calcination in dynamic air was conducive to obtaining broad and weak diffraction peaks. The average crystallite size(d), lattice constant(a), and strain of the catalysts were estimated by software MDI Jade 6.0 (Table 1). As expected, increasing the calcination temperature resulted in the sintering of Co₃O₄ crystallite, shrinkage of the Co₃O₄ lattice, and reduction in strain. The same trend was also observed when changing the calcination atmosphere from static air to dynamic air. Compared to other catalysts, smaller crystallite size, larger lattice constant, and more strain were observed for Co-350D, indicating the presence of more oxygen vacancies, point, or dislocation defects [39].

Raman spectra were obtained to further study the lattice distortion of the Co₃O₄ catalysts. As displayed in Figure 1b, five Raman peaks correspond to A_1g, F³_2g, F²_2g, E_g, and F¹_2g symmetries can be seen for all catalysts [18,40,41]. The calcination condition has a significant effect on the Raman spectra of Co₃O₄. For example, the peak positions of A_1g symmetry in all catalysts follow the order Co-350D (667 cm⁻¹) < Co-550D (670 cm⁻¹) < Co-350S (675 cm⁻¹) < Co-550S (688 cm⁻¹). The full width at half-maximum (FWHM) of A_1g symmetry decreases in the same order (

Table 2. Quantitative analysis of Raman spectra and CO-TPR of the Co₃O₄ catalysts calcined under different conditions.

Catalysts	Raman Spectra			CO2 Production in CO-TPR (mmol g−1)
	Peak Position of A1g (cm−1)	FWHM of A1g	Peak Ratios of F12g/A1g	100–300 °C	Total
Co-350S	675	24	0.073	3.3	16.2
Co-350D	667	31	0.099	4.7	16.1
Co-550S	688	11	0.036	1.5	16.7
Co-550D	670	25	0.089	2.0	17.0

). Thus, more defects caused by lattice distortion or residual stress of spinel crystal were present in Co₃O₄ catalysts calcined at low temperatures or under dynamic air [18], in line with the XRD analysis. In addition, the peak area ratios of F¹_2g/A_1g (tetrahedral Co³⁺–O²⁻/octahedral Co²⁺–O²⁻ [40]) were calculated and summarized in Table 2. The results demonstrated that Co-350D possessed the highest concentration of Co²⁺ and oxygen defects [24].

Figure 2 compares the N₂ adsorption-desorption isotherms and pore size distribution curves of the Co₃O₄ catalysts calcined under different conditions. Typical H3-type hysteresis loops due to capillary condensation in mesopores were observed and decreased in the order Co-350D > Co-350S > Co-550D > Co-550S. The calculated specific surface area and total pore volume follow the same tendency (Table 1). By assuming that all Co₃O₄ crystallites are spherical, the geometrical surface area of the catalysts was also estimated based on the XRD result (Table 1). The geometrical values are close to the SSA ones, implying that the Co₃O₄ nanoparticles are well dispersed without obvious aggregation. Regarding the pore size distribution, mesopores centered at ca. 19 and 31 nm were observed for Co-350D and Co-350S, respectively, whereas few mesopores can be seen for Co-550D and Co-550S. The results suggested that the Co₃O₄ catalysts suffered evident sintering when calcined at a higher temperature. Co-350D has the largest surface area (81 m² g⁻¹) and pore volume (0.365 cm³ g⁻¹), which can be beneficial to the adsorption of reactants, exposure of active sites, and diffusion of reaction products.

2.2. Reducibility

The reducibility of the as-prepared Co₃O₄ catalysts was investigated by CO-TPR, with the corresponding profiles shown in Figure 3. The CO₂ production curve matched perfectly with the CO consumption curve. The reduction of Co₃O₄ proceeds via two steps, the low-temperature peak was ascribed to Co³⁺→Co²⁺ and the higher one Co²⁺→Co⁰ [42], with total CO₂ production quantities of 16.1–17.0 mmol g⁻¹ (Table 2). By comparing the low-temperature reduction peaks, the reducibility of the catalysts is ranked as Co-350D > Co-350S > Co-550D > Co-550S. Meanwhile, the CO₂ production amount in 100–300 °C of each catalyst was calculated (Table 2). The result further revealed the best low-temperature reducibility of Co-350D, which may be related to its smallest crystallite size that can expose more reducible sites [14,24].

2.3. Catalytic Performance

The catalytic performance of the Co₃O₄ catalysts calcined in different conditions was evaluated in the total oxidation of toluene and propane, as presented in Figure 4. The reaction temperatures required for 10%, 50%, and 90% VOCs conversion (T₁₀, T₅₀, and T₉₀) were summarized in Table S1 to conveniently compare the catalytic performance. In terms of toluene oxidation, a significant difference in catalytic performance was observed in the low-temperature region. On the one hand, Co₃O₄ calcined at 350 °C performed much better than that calcined at 550 °C. On the other hand, Co₃O₄ calcined in dynamic air showed superior performance to that calcined in static air. The T₁₀ value increased in the order of Co-350D > Co-350S > Co-550D > Co-550S. When the reaction temperature is higher than 240 °C, these catalysts behaved similarly in toluene oxidation. Regarding propane oxidation, the catalytic performance of these catalysts follows the same trend as that in toluene oxidation in the whole reaction temperature range. Co-350D exhibited the best performance whether in toluene or propane oxidation, which was further evidenced by the reaction rate vs temperature profiles (Figure S3).

The catalytic stability of the optimal catalyst Co-350D was studied by performing three consecutive catalytic cycles. As shown in Figure 5, the light-off curves of the three cycles were almost coincident, no matter in toluene or propane oxidation, suggesting the excellent cycling stability of the catalyst.

Furthermore, the long-term catalytic durability of Co-350D was investigated in toluene oxidation. As displayed in Figure 6a, in a continuous operation condition at 250 °C, toluene conversion to CO₂ first decreased from 80% to 40% in 3 h and then became wavy around 53% in the next 60 h. Once the toluene stream was removed, a large quantity of CO₂ was produced immediately, probably due to the fast decomposition of the accumulated reactive coke [43]. It has been previously reported that when the formation rate of reactive coke was faster than the reaction rate, these accumulated compounds would be quickly oxidized to induce self-heating of the catalyst, thus causing the oscillation phenomenon in the VOCs conversion into CO₂ [44]. After the catalyst was fully regenerated, the durability test was continued for another 20 h. A similar oscillation curve occurred again, indicating the reproducibility of the experimental result.

Considering that water is always present in the catalytic oxidation process, either from the inlet gas or as a combustion product, the effect of water during the long-term durability test in propane oxidation over Co-350D was investigated. Figure 6b shows the evolution of propane conversion to CO₂ as a function of time-on-stream at 195 °C over Co-350D in normal conditions or under water vapor-rich conditions (~4.4 vol.%). In dry gas, the initial propane conversion to CO₂ was ca. 80%, and it dropped up to 68% in 30 h. After the catalyst was reactivated by cutting off the propane stream, the conversion was back to the original level and declined to 68% again in the next 20 h. Then, 4.4 vol.% moisture was sent to the feed gas stream, causing a decrease in conversion to 32%, which can be a result of the blocking of active sites [45] or the shift of reaction equilibrium [46]. After operating in water vapor for 10 h, water vapor was removed, the conversion was recovered to 64% and maintained at this value for 20 h. Finally, the activity of the catalyst was restored again by stopping the propane stream and was reduced to 64% in the final 65 h. To conclude, in 150 h of testing in propane oxidation, Co-350D exhibited considerable stability and tolerance to water vapor. The deactivation of the catalyst was reversible.

The characterization and catalytic test results indicated that the calcination condition played a crucial role in tuning the properties of the Co₃O₄ catalysts and in turn, affected the catalytic activity. As shown in Figure 7a, there is a proportional relationship between the VOCs reaction rate of the Co₃O₄ catalysts and their low-temperature reducibility. Similarly, the catalytic activity is closely related to the strain of the catalyst (Figure 7b). Calcination at lower temperatures and/or in dynamic air is an effective way to enhance the activity of the Co₃O₄ catalysts. The Co₃O₄ catalyst calcined at 350 °C in dynamic air has the most defective structure, the largest SSA, and the best reducibility. As a result, Co-350D is the most active catalyst in the total oxidation of both toluene and propane.

Table 3. Comparison of the catalytic performance in toluene and propane oxidation over various catalysts.

	Catalyst	WHSV (mL h−1 g−1)	VOC Content (vol.%)	T50/T90	Ref.
Toluene oxidation	Co3O4-HA	40,000	0.1	232/240	[3]
	Co3O4 microsphere	60,000	0.05	243/248	[4]
	Co3O4-cube	48,000	0.1	240/248	[5]
	Co3O4-S-160	60,000	0.05	236/250	[6]
	Co3O4-0.01	15,000	0.1	217/226	[7]
	3D-Co3O4	48,000	0.1	229/238	[8]
	Co3O4–PTA–L2	30,000	0.3	182/188	[9]
	N-Co3O4-200	60,000	0.1	208/218	[10]
	Co3−xO4−y	72,000	0.03	171/180	[11]
	Co-350D	40,000	0.1	230/257	This work
Propane oxidation	Co3O4-C100–550	12,000	0.8	200/213	[12]
	C-350	12,000	0.8	210/235	[13]
	CoDP	120,000	0.1	195/225	[14]
	Co3O4-H	9990	1.0	209/239	[15]
	Mic-Co3O4	120,000	0.8	262/374	[16]
	Co-SAS 10% water	15,000	0.5	175/200	[17]
	Co3O4-AC	240,000	0.3	237/250	[18]
	Co4Zr1	60,000	0.2	210/242	[19]
	Ca-Co3O4-Ac	120,000	0.2	236/260	[20]
	Co-350D	40,000	0.1	180/201	This work

summarized the catalytic performance of typical Co₃O₄ catalysts reported by other authors in order to compare them with the as-prepared Co-350D. As we can see, in terms of toluene oxidation, Co-350D exhibits similar performance to Co₃O₄-HA [3], hydrangea-like Co₃O₄ microsphere [4], Co₃O₄-cube [5], and Co₃O₄-S-160 [6], whereas it is inferior to Co₃O₄-0.01 [7], 3D-Co₃O₄ [8], Co₃O₄–PTA–L2 [9], N-Co₃O₄-200 [10], and Co_3−xO_4−y [11]. Nevertheless, it should be kept in mind that the T₅₀ and T₉₀ values used in this work are actually CO₂ yield, which has been previously emphasized to require a higher temperature than toluene conversion [46]. The total oxidation of toluene is a two-step process containing the oxidation of toluene to some intermediates (benzyl alcohol, benzaldehyde, and benzene) at low temperatures and further oxidation of these intermediates to CO₂ at higher temperatures [6]. Thus, the performance of Co-350D in toluene oxidation may be underrated. Moreover, the Co₃O₄ catalysts that performed better than Co-350D were mostly prepared by hydrothermal or hard-templating methods, using surfactants, and consuming much energy. Regarding propane oxidation, it is clear that Co-350D precedes Co₃O₄-C100–550 [12], C-350 [13], CoDP [14], Co₃O₄-H [15], Mic-Co₃O₄ [16], Co₃O₄-AC [18] Co₄Zr₁ [19], and Ca-Co₃O₄-Ac [20], while it keeps pace with Co-SAS 10% water [17]. The comparison results further proved the superiority of Co-350D in the total oxidation of toluene and propane.

3. Experimental

3.1. Catalyst Preparation

Two hundred mL of 0.4 M Co(NO₃)₂·6H₂O solution was mixed with 200 mL of 0.44 M Na₂CO₃ solution under vigorous stirring. Meanwhile, the pH of the mixture was adjusted to 9.5 by Na₂CO₃ solution and maintained at this value for 1 h. The obtained slurry was centrifuged and washed several times with distilled water. After drying at 80 °C overnight, the powder was divided into four parts. Two of them were calcined at 350 or 550 °C (2 °C min⁻¹) for 2 h in static air, and the other two were calcined at 350 or 550 °C (2 °C min⁻¹) for 2 h in dynamic air with a flow rate of 100 mL min⁻¹. For facile description, the as-prepared four samples were referred to as Co-350S, Co-350D, Co-550S, and Co-550D, respectively, where S represents static air calcination and D represents dynamic air calcination.

3.2. Catalyst Characterization

Thermogravimetry and differential thermal analysis (TG/DTA) experiments were carried out using a Setsys Evolution 12 calorimeter (SETARAM, Lyon, France). The cobalt precursor was heated from 25 to 800 °C at 10 °C min⁻¹ in an air stream.

Fourier Transform infrared (FTIR) spectra were obtained using a FT-IR C92712 spectrometer (PerkinElmer, Waltham, MA, USA) in an attenuated total reflectance mode from 4000 to 400 cm⁻¹.

The chemical composition of the cobalt oxides was measured using inductively coupled plasma optical emission spectroscopy (ICP-OES, Horiba Jobin Yvon, Paris, France). Prior to measurement, the powder was dissolved in a mixture solution of H₂SO₄ and HNO₃ at ca. 400 °C.

X-ray diffraction (XRD) patterns were recorded using a D8 Advance diffractometer (Bruker, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.154184 nm). Samples were scanned from 10° < 2θ < 80° with a step of 0.02° and counting time of 2 s per step.

Raman spectra were collected using a LabRam HR spectrometer (Horiba, Paris, France) with an Ar⁺ laser beam (λ = 514 nm) from 800 to 150 cm⁻¹.

N₂ adsorption–desorption isotherms were determined at −196 °C using a TRISTAR II apparatus (Micromeritics, Norcross, GA, USA). The samples were pre-degassed at 300 °C for 3 h. The specific surface area was calculated by the standard Brunauer–Emmett–Teller (BET) method, and the total pore volume and pore size distribution were obtained using the Barrett–Joyner–Halenda (BJH) method.

Temperature program reduction in CO (CO-TPR) was performed using a Micro-GC (SRA % GC-R3000, SRA Instruments, Milan, Italy). Typically, 0.03 g of the sample was pretreated in pure He at 300 °C for 0.5 h and then cooled down to 50 °C. The reduction experiment was performed in 0.3 vol.% CO/He gas mixture (100 mL min⁻¹), from 50 to 750 °C at 5 °C min⁻¹.

3.3. Catalytic Test

The catalytic test was performed in a U-shaped quartz reactor (i.d. = 4 mm). In each test, 0.15 g of catalyst was mixed with ca. 0.7 g of silicon carbide to avoid hot spot and maintain a catalytic bed height of 6 mm. The total flow rate was controlled at 100 mL min⁻¹, corresponding to a weight hourly space velocity (WHSV) of 40,000 mL g⁻¹ h⁻¹.

The reaction temperature program for toluene and propane oxidation was shown in Schematic S1. In toluene oxidation, T = 150 °C, the feed gas was composed of 1000 ppm of toluene and 21 vol.% O₂ balanced by N₂. The concentrations of CO and CO₂ were monitored using an online Rosemount Stream Gas Infrared Analyzer (Emerson Electric Co., St. Louis, MO, USA). In propane oxidation, T = 100 °C, the feed gas was composed of 1000 ppm of propane and 21 vol.% O₂ balanced by He. An online micro gas chromatograph (SRA % GC-R3000) equipped with two thermal conductivity detectors was used to monitor the gas effluents. The light-off curves during the cooling ramp were used to compare the catalytic performance.

The conversion of toluene/propane to CO₂ was calculated as follows (Equations (1) and (2)):

$${\mathrm{X}}_{{\mathrm{C}}_7{\mathrm{H}}_8}\left(\%\right)=\frac{[{\mathrm{CO}}_2]}{7\left[{\mathrm{C}}_7{\mathrm{H}}_8\right]}\times 100$$

(1)

$${\mathrm{X}}_{{\mathrm{C}}_3{\mathrm{H}}_8}\left(\%\right)=\frac{\left[{\mathrm{CO}}_2\right]}{3\left[{\mathrm{C}}_3{\mathrm{H}}_8\right]}\times 100$$

(2)

where [CO₂], [C₇H₈], and [C₃H₈] represent the outlet CO₂ concentration, the inlet toluene, and propane concentration, respectively.

The toluene/propane oxidation reaction rate (r, mol s⁻¹ g⁻¹) was calculated as follows (Equation (3)):

$$\mathrm{r}=\frac{\mathrm{X}·\mathrm{F}}{\mathrm{m}}$$

(3)

where X is the conversion of toluene/propane to CO₂, F is the flow rate of toluene/propane (mol s⁻¹) in the inlet gas, and m is the mass of the catalyst used (g).

4. Conclusions

In this study, different calcination conditions were applied to optimize the structural and textural properties of Co₃O₄ catalysts synthesized by carbonate precipitating at pH 9.5. The as-prepared catalysts exhibited high activity in the total oxidation of toluene and propane. Among them, the catalyst calcined at 350 °C under dynamic air (Co-350D) performed much better, with T₉₀ values of 257 and 201 °C in the toluene or propane oxidation, respectively. The superior catalytic performances of Co-350D could be attributed to their smaller crystallite size, larger surface area, abundant surface Co²⁺species, and good reducibility. Co-350D showed impressive cycling stability, whereas it deactivated slightly upon long-term durability tests due to the accumulation of reaction intermediates on the catalyst surface. In addition, propane oxidation over Co-350D appeared to be inhibited by the presence of 4.4 vol.% H₂O and the observed deactivation was reversible.