# Effect of the precipitation pH on the characteristics and performance of Co3O4 catalysts in the total oxidation of toluene and propane

Weidong Zhang, Kelsey Lassen, Claude Descorme, Jose Luis Valverde, A.

Giroir-Fendler

► To cite this version:

Weidong Zhang, Kelsey Lassen, Claude Descorme, Jose Luis Valverde, A. Giroir-Fendler. Effect of the precipitation pH on the characteristics and performance of Co3O4 catalysts in the total oxidation of toluene and propane. Applied Catalysis B: Environmental, 2021, 282, 10.1016/j.apcatb.2020.119566. hal-03058141

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# Effect of the precipitation pH on the characteristics and performance of Co₃O₄ catalysts in the total oxidation of toluene and propane

Weidong Zhang<sup>a</sup>, Kelsey Lassen<sup>b</sup>, Claude Descorme<sup>a</sup>, José Luis Valverde<sup>c</sup>, Anne Giroir-Fendler<sup>a,*</sup>

<sup>a</sup> Univ. Lyon, Université Claude Bernard Lyon 1, CNRS, IRCELYON, 2 avenue Albert Einstein, Villeurbanne, F-69622, France

<sup>b</sup> Department of Chemistry, University of Utah, Salt Lake City, UT, 84112-0850, USA

<sup>c</sup> Department of Chemical Engineering, Faculty of Chemical Science and Technology, University of Castilla-La Mancha, Avenida Camilo José Cela 12, 13005, Ciudad Real, Spain

*Corresponding author: Anne Giroir-Fendler (e-mail: anne.giroir-fendler@ircelyon.univ-lyon1.fr)

## Abstract

Co₃O₄ catalysts were prepared via carbonate precipitation at different pH, characterized using TG/DTA, FTIR, XRD, Raman, N₂ adsorption and TPR techniques, and tested in the total oxidation of toluene or propane. The precipitation pH may affect the composition of the cobalt oxide precursors as well as the physicochemical properties and the performance of Co₃O₄ catalysts. By controlling the precipitation pH at 9.5, a Co₃O₄ catalyst with the smallest crystalline size, the largest surface area, the most defective structure, and the best reducibility was obtained. The activity of the as-prepared Co₃O₄ catalysts in the oxidation of toluene was linearly dependent on the low temperature reducibility of the oxide. Among all catalysts, Co-9.5 exhibited the highest catalytic activity (T₁₀ = 197 °C and T₉₀ = 244 °C), excellent cycling-stability and good durability upon long-term activity test. In propane oxidation, residual sodium at the surface of the Co₃O₄ catalyst had a marked poisoning effect on the catalytic performance, evidencing the importance of washing procedure upon catalyst preparation.

**Key words:** pH; Precipitation method; Cobalt oxide; Toluene oxidation; Propane oxidation.
# 1. Introduction

Volatile organic compounds (VOCs) emissions are responsible for the formation of tropospheric ozone, secondary organic aerosols and photochemical smog [1-3]. Catalytic oxidation is an efficient technique for VOCs elimination. A variety of catalysts, including supported noble metals (Pt, Pd, Ru, Au and Ag) [4-9] and non-noble metal-based oxides (Co, Mn, Ce, Ni, Cu and Fe) [10-16] have been synthesized and applied for VOCs oxidation. Co₃O₄ is one of the most promising catalyst in terms of both activity and cost. Highly active Co₃O₄ catalysts generally feature large surface area [17,18], high reducibility [19,20], unique morphology [21,22], preferentially exposed crystal planes [10,20], and/or abundant oxygen vacancies [23,24]. In order to improve the catalytic activity of cobalt-based oxides in VOCs oxidation, different strategies such as porous structure building [17,25], solid solution formation [26,27], metal doping [28,29] or perovskite/hydrotalcite derived structures [30,31] have been proposed. Nevertheless, synthesizing Co₃O₄ catalysts with superior performance in VOCs oxidation, using simple components and set-ups under ambient conditions, is still a challenge. Preparation of Co₃O₄ catalysts via precipitation is simple, cheap and easy. As such, studying and understanding the impact of various factors upon precipitation is of the greatest importance, especially in order to establish an effective and valuable approach for the rational synthesis of low-cost and efficient Co₃O₄ catalysts.

Recently, our group has demonstrated that the nature of the precipitation agent may significantly influence both the redox properties of the Co₃O₄ catalysts and their catalytic performance [32]. Similarly, the pH of the solution during precipitation may also play an important role. Indeed, the pH fixed upon coprecipitation of CuO-CoOₓ-CeO₂ catalysts affected the synergistic effect between the different oxide components and modified the CO oxidation efficiency of the catalyst [33]. Zheng et al. found that both the catalytic activity and the stability of Co₃O₄ catalysts in methane combustion were influenced by the precipitation pH at the end of the preparation process [34]. Consequently and based on our previous results, demonstrating that sodium carbonate was the most appropriate precipitation agent, it was decided to further optimize the physicochemical properties of the Co₃O₄ catalysts by tuning the pH value upon carbonate-precipitation.
In this work, several Co₃O₄ catalysts were synthesized via carbonate-precipitation at different pH values. Toluene and propane were chosen as representative pollutants to test the catalytic performance of the as-prepared catalysts in the total oxidation of such VOCs. At first, the objective of this work was to evaluate the impact of precipitation pH on the physicochemical properties of the Co₃O₄ catalysts. In a second time, the modifications of the physicochemical characteristics of the Co₃O₄ catalysts were tentatively correlated with the evolution of their catalytic performance.

## 2. Experimental

### 2.1. *Catalyst preparation*

A series of Co₃O₄ catalysts **have** been prepared by precipitation using sodium carbonate as the precipitant but also to adjust and to maintain pH at the desired value.

Typically, a 0.2 M of Co(NO₃)₂·6H₂O (Sigma-Aldrich) aqueous solution was mixed under stirring at room temperature with a 0.22 M Na₂CO₃ (Sigma-Aldrich) aqueous solution. The pH of the mixture was adjusted to either 7.0, 7.5, 8.0, 8.5, 9.0, 9.5 or 10.0 using HNO₃ (Sigma-Aldrich) and Na₂CO₃. A constant pH was then maintained for 1 h. The slurry was recovered by centrifugation and washed several times with distilled water before drying overnight at 80 °C. Finally, the oxide precursor was successively calcined in static air at 200 °C for 1 h and at 500 °C for 1 h with a temperature ramp rate of 2 °C min⁻¹. The **obtained** black powder was then weighted to estimate the product yield. The obtained catalysts were referred to as Co-x, where x indicated the precipitation pH. The yield of Co-7.0 is only 16 %, suggesting that at pH 7 only a small amount of Co²⁺ can be precipitated.

For comparison with catalyst Co-9.5, two more Co₃O₄ catalysts were prepared. Catalyst Co-9.5-H₂O washed was prepared by washing the precipitates with lots of water until the conductivity of the supernatant solution remained constant to remove sodium ion, other synthesis condition was the same as that of catalyst Co-9.5; while catalyst Co-9.5-HNO₃ treated was prepared by treating the as-obtained Co-9.5 catalyst with 0.5 M of HNO₃ to remove surface sodium, and further washing and drying at 120 °C.
## 2.2. Catalyst characterization

The cobalt oxide precursors, after drying only at 80 °C, were characterized by thermogravimetric and differential thermal analysis (TG/DTA) using a SETARAM Setsys Evolution 12 calorimeter with alumina crucibles. The samples were heated under flowing air (50 mL min⁻¹) from 25 to 800 °C with a temperature ramp rate of 10 °C min⁻¹.

Fourier Transform infrared (FTIR) spectra for the cobalt oxide precursors and cobalt catalysts were recorded on a PerkinElmer FT-IR C92712 spectrometer in attenuated total reflectance mode from 4000 to 400 cm⁻¹ at a resolution of 4 cm⁻¹.

The chemical composition of the catalysts was determined by inductively coupled plasma-optical emission spectroscopy (ICP-OES, Horiba Jobin Yvon). Prior to determination, the samples were dissolved in a mixture of inorganic acids (H₂SO₄ and HNO₃) at ca. 400 °C.

X-ray diffraction (XRD) patterns were recorded on a Bruker D8 advance A25 diffractometer using a Cu Kα 1+2 radiation source (λ = 0.154184 nm). The samples were scanned from 10 to 80° with a step size of 0.02° and counting time of 2 s per step.

Raman spectra were obtained on a Horiba LabRam HR spectrometer equipped with an ion-argon laser (λ = 514 nm) and a CCD detector. A twenty seconds acquisition time was used for each spectrum and five spectra were accumulated from 150 to 800 cm⁻¹.

Nitrogen adsorption and desorption isotherms were recorded at -196 °C using a Micromeritics TRISTAR II apparatus. Before the analysis, the samples were outgassed under primary vacuum at 300 °C for 3 h. The specific surface area (SSA) was calculated by the standard Brunauer–Emmett–Teller (BET) method. The total pore volume and the pore size distribution were determined using the Barrett–Joyner–Halenda (BJH) method.

Temperature programmed reduction under H₂ (H₂-TPR) was performed on a commercial Micromeritics AutoChem 2950 HP unit. First, a 0.05 g sample was pretreated in flowing Ar at 350 °C for 0.5 h. The sample was subsequently cooled down to 45 °C and a 5 vol.% H₂/Ar gas mixture was then fed into the reactor. Finally, the reduction of the sample was performed at a temperature ramp rate of 10 °C min⁻¹ from
45 to 800 °C.

Temperature programmed reduction under CO (CO-TPR) was also studied using a Micro-GC (SRA % GC-R3000). First, a 0.03 g sample was pretreated in flowing He at 300 °C for 0.5 h. The sample was further cooled down to 50 °C and a 0.3 vol.% CO/He gas mixture was injected. Finally, the CO-TPR was carried out from 50 to 750 °C with a temperature ramp rate of 5 °C min⁻¹. During the reduction process, both CO and CO₂ concentrations were continuously monitored.

## 2.3. Catalytic performance evaluation

For each catalytic test, 0.15 g catalyst mixed with ca. 0.7 g SiC (Alfa Aesar) was packed inside a U-shaped quartz reactor with an inner diameter of 4 mm. A thermocouple was placed outside the reactor, close to the middle of the catalytic bed, to measure the reaction temperature. The reaction mixture was composed of 1000 ppm VOCs (toluene or propane), 21 vol.% O₂ and an inert gas balance (N₂ or He) to adjust the total flow to 100 mL min⁻¹.

Upon toluene oxidation, after the gas mixture was introduced into the reactor, the temperature was raised from room temperature to 150 °C and maintained for 0.5 h to stabilize the system. Subsequently, the temperature was increased to 350 °C at 2 °C min⁻¹ and maintained for 1 h. Finally, the reactor was cooled down to 150 °C at 2 °C min⁻¹ to complete the temperature cycle. The concentrations of CO and CO₂ were recorded online using a Rosemount Stream Gas Infrared Analyzer (Emerson Electric Co.).

For propane oxidation, the temperature program was similar to toluene oxidation, except that the temperature cycle started from 100 °C. Gases in the effluent were analyzed using an on-line micro gas chromatograph (SRA % GC-R3000) coupled with a thermal conductivity detector.

Toluene or propane conversion to CO₂ was calculated as follows:

$$X_{C_7H_8}(\%) = \frac{[CO_2]_{out}}{7[C_7H_8]_{in}} \times 100$$

$$X_{C_3H_8}(\%) = \frac{[CO_2]_{out}}{3[C_3H_8]_{in}} \times 100$$
where [CO₂]out, [C₇H₈]in and [C₃H₈]in represent the outlet CO₂ concentration and the inlet toluene and propane concentrations, respectively.

Three successive heating and cooling cycles were performed in both toluene and propane oxidation, with the aim of evaluating the catalyst stability.

The specific reaction rate (r, mol s⁻¹ g⁻¹) was calculated using the equation: r = X·F/m, where X represents toluene/propane conversion, F represents the inlet flow rate of toluene/propane (mol s⁻¹) and m represents the mass of the catalyst used (g). The intrinsic reaction rate (r', mol s⁻¹ m⁻²) was calculated by dividing r by the SSA (m² g⁻¹) of the catalyst.

**3. Results and discussion**

**3.1. Physical and chemical characterization of the cobalt oxide precursors and the**
**cobalt oxide catalysts**

To study the decomposition process of the cobalt oxide precursors prepared at various pH, TG/DTA analysis was performed. As visible from the TG curves in **Fig. 1**, two types of behaviors might be distinguished upon thermal decomposition of the cobalt oxide precursors. For the cobalt oxide precursors obtained at low precipitation pH (7.5–8.5), a very small weight loss (1.5–1.7 %) was observed between 25 and 150 °C, which was attributed to the removal of adsorbed water. In turn, a much higher weight loss (5.4–6.6 %) appeared in the same temperature range in the case of the cobalt oxide precursors precipitated at higher pH (9.0–10.0), indicating the presence of more water. On the opposite, in the higher temperature range from 150 to 500 °C, Co-9.0, Co-9.5 and Co-10.0 precursors underwent a lower weight loss compared to the **Co-7.5**, Co-8.0 and Co-8.5 precursors. Interestingly, the overall weight loss below 500 °C was very similar for all samples (26.6–28.0 %). On the DTA curves, endothermic peaks appeared for **Co-7.5**, Co-8.0 and Co-8.5 precursors, whereas exothermic peaks were observed for Co-9.0, Co-9.5 and Co-10.0 precursors.

In conclusion, TG/DTA analysis suggested that two different forms of cobalt oxide precursor might be produced depending on the precipitation pH value. In addition, 500 °C was evidenced to be an appropriate calcination temperature to ensure the total
decomposition of cobalt oxide precursors into stable cobalt oxide catalysts.

The FTIR spectra of the cobalt oxide precursors and the corresponding cobalt oxide
catalysts prepared at various pH were collected and displayed in **Fig. 2**. Two kinds of
FTIR spectra were clearly observed for the cobalt oxide precursors, similarly to what
could be observed from the TG/DTA results.

On the FTIR spectra of the **Co-7.0**, **Co-7.5**, Co-8.0 and Co-8.5 cobalt oxide precursors, the band at 3498 cm⁻¹ was attributed to the stretching vibration of water and O–H groups [35–37]. The band at 3377 cm⁻¹ was ascribed to the O–H groups interacting with carbonate anions, whose strength was reported to be proportional to the amount of carbonate anions incorporated into the structure [36]. The presence of carbonates was confirmed by additional bands at 1539 (ν(OCO₂)), 1346 (ν(CO₃)), 1068 (ν(C=O)), 835 (δ(CO₃)), 700 (δ(OCO)) and 662 cm⁻¹ (ρ(OCO)) [35–37]. Extra bands at 968 and 507 cm⁻¹ were characteristic of the Co–OH bending and rocking vibrations [35–37]. All these data suggested the formation of a cobalt hydroxide carbonate in the case of the **Co-7.0**, **Co-7.5**, Co-8.0 and Co-8.5 cobalt oxide precursors.

When precipitation was carried out at higher pH (9.0, 9.5 and 10.0), the bands related
to carbonate anions weakened and broadened, while the intensity of the bands
corresponding to cobalt hydroxide increased, suggesting a change in the overall cobalt
oxide precursor composition.

As observed above, cobalt oxide precursors existed in the form of cobalt hydroxide
carbonates, and the amounts of CO₃²⁻ and OH⁻ were somehow dictated by the
precipitation pH value. The global formula of the cobalt oxide precursors could be
deduced from the TG curves (Table 1). Obviously, Co-7.5, Co-8.0 and Co-8.5
precursors on one side, and Co-9.0, Co-9.5 and Co-10.0 precursors on the other side,
exhibited similar compositions, which could be reflected by their different colors (Fig.
S1). It appeared that the higher the precipitation pH, the higher the amount of OH⁻
groups in the cobalt oxide precursor. During the precipitation of the Co²⁺ cations, there
might be a competition between the CO₃²⁻ and OH⁻ anions. At lower pH, CO₃²⁻ anions
dominated in the solution and preferentially adsorbed on the oxide precursor nucleus.
At higher pH, OH⁻ anions dominated. Thus, depending on the precipitation pH, OH⁻
anions would partially substitute CO₃²⁻ anions in the cobalt hydroxide carbonate
structure, leading to different composition of the cobalt oxide precursors.
After calcination at 500 °C, the bands associated with the cobalt hydroxide carbonate vanished while two strong bands characteristic of the spinel Co₃O₄ phase appeared. The bands at 554 and 657 cm⁻¹ were due to the stretching vibration of octahedral Co³⁺ and tetrahedral Co²⁺, respectively [37].

The catalyst yields were calculated by assuming that all cobalt was in the form of Co₃O₄ and the results are listed in **Table 1**. In the pH range of 7.5-10.0, at least 85 % of the Co²⁺ ions were precipitated. When the pH value was set at 9.5, no loss of cobalt was observed at all (100 % yield).

### 3.2. Structural and textural characterization

**Fig. 3** shows the XRD patterns of the as-prepared cobalt oxide catalysts. For all samples, whatever the precipitation pH, characteristic diffraction lines of the cubic spinel Co₃O₄ phase (ICDD file 74-2102) were observed. A zoom in on the (311) plane diffraction line showed that the Co-9.5 catalyst exhibited the weakest and broadest diffraction line, indicating smaller and less crystalline oxide particles. The average crystallite sizes (d) and the lattice constants (a) of the different Co₃O₄ catalysts were calculated based on the six most intense diffraction lines ((111), (220), (311), (400), (511) and (440) phases). The results in **Table 2** suggested that the smallest crystallite size and the smallest lattice constant were observed for the Co-9.5 catalyst.

Raman spectra were further recorded to confirm the structure of the cobalt oxide catalysts and gain more information of the oxide lattice distortion. As displayed in **Fig. 4**, five bands corresponding to the 3 F₂g, E_g and A₁g Raman-active modes of the Co₃O₄ spinel structure were observed for all catalysts [38,39]. Among these bands, the bands at ca. 190 and 680 cm⁻¹ were attributed to the tetrahedral and octahedral Co sites, respectively [38,39]. A significant red shift, and a weakening and broadening of the Raman bands were detected for Co-9.0 and Co-9.5, suggesting the presence of more defects due to lattice distortion or residual stress in the spinel matrix [38,39]. In addition, the A₁₉₀/A₆₈₀ area ratio for the bands at 190 and 680 cm⁻¹ was calculated and listed in **Table 2**. This ratio increased with increasing precipitation pH up to 9.5, and then decreased. This finding demonstrated that Co-9.5 had the most defective structure among all catalysts, with the highest concentration of tetrahedral Co sites on the surface. The Co–O bond strength in such ensembles is lower, facilitating the formation of oxygen vacancies [40].
The textural properties of the cobalt oxide catalysts were studied by N₂ sorption. Fig. 5 shows the N₂ adsorption-desorption isotherms and pore size distributions for different catalysts. The overall nitrogen uptake first increased and then decreased as the precipitation pH increased. Co-9.5 showed the highest value. Accordingly, more mesopores centered ca. 23 nm were observed for the Co-9.5 catalyst. Co-9.5 was characterized by a type IV isotherm with a type H3 hysteresis loop, revealing that mesopores in the oxide grain originated from the agglomeration of smaller Co₃O₄ crystallites. The specific surface area (SSA) and total pore volume (Vp) of all catalysts were listed in Table 2. The SSA varied from 24 to 51 m² g⁻¹ and Co-9.5 possessed the largest one. The total pore volume of all samples was in the range of 0.07–0.32 m³ g⁻¹ and again Co-9.5 exhibited the largest one.

### 3.3. Reducibility of the cobalt oxide catalysts

Since it may strongly affect the performance of the Co₃O₄ catalysts in oxidation reaction, the reducibility of different catalysts was investigated by H₂-TPR and CO-TPR experiments. Fig. 6 shows the H₂-TPR profiles and Table 3 summarizes the quantitative results derived from these H₂-TPR curves.

All catalysts exhibited two main reduction peaks, with a total H₂ consumption ranging from 14.7 to 16.5 mmol g⁻¹. The first reduction peak corresponded to the reduction of Co(III) to Co(II), and the second one was assigned to the reduction of Co(II) to metallic Co, which is consistent with other studies [41–43]. Among all the catalysts prepared at different pH values, Co-9.5 was the most reducible one, as evidenced by the lowest onset reduction temperature and the largest H₂ consumption at low temperature, between 50 and 300 °C.

Fig. 7 shows the evolution of CO and CO₂ concentrations as a function of temperature upon CO-TPR experiments. The evolution of CO consumption curve was quite symmetrical to that of the CO₂ formation curve, suggesting that Co₃O₄ could be efficiently reduced by CO. As upon the H₂-TPR experiments, the two successive reduction peaks were ascribed to the Co³⁺→Co²⁺ and Co²⁺→Co⁰ reduction steps, respectively. The total CO₂ production ranged between 17.0 and 17.6 mmol g⁻¹ depending on the catalyst. The excellent low temperature reducibility of Co-9.5 was again confirmed by the low temperature reduction peak and the large CO₂ production in the temperature range 50–300 °C.
### 3.4. Catalytic performance

The catalytic performance of the as-prepared $Co_3O_4$ catalysts was evaluated using toluene and propane as model VOCs. To test the recyclability of the catalysts, three consecutive heating and cooling runs were systematically conducted. Considering the possible adsorption of VOCs and the formation of reaction intermediates over the $Co_3O_4$ catalysts, the effective conversion was measured from the $CO_2$ yield instead of VOCs conversion directly.

#### 3.4.1. Total oxidation of toluene

As shown in **Fig. S2a**, during the 3rd heating cycle, the conversion of toluene to $CO_2$ was higher than 100 % in the highest temperature region. This kind of artefact was due to the decomposition of the reaction intermediates formed and accumulated at the catalyst surface at lower temperature [44,45]. Therefore, for a reliable comparison between different catalysts, the light-off curve during the 3rd cooling cycle was preferred for a proper representation of the stabilized activity of the catalyst (**Fig. 8**). The temperatures for achieving 10, 50 or 90 % toluene conversion to $CO_2$ ($T_{10}$, $T_{50}$, and $T_{90}$, respectively) during the three cooling ramps were summarized in **Table S1**. The values were very similar along three runs, evidencing the very good cyclability of the $Co_3O_4$ catalysts. Moreover, it was clear that Co-9.5 exhibited outstanding performance in toluene oxidation, with the lowest $T_{10}$ and $T_{90}$ upon the third cooling run (197 and 244 °C, respectively). Co-8.0, Co-8.5 and Co-9.0 performed similarly, **followed by Co-7.5**, while **Co-7.0** and **Co-10.0** were the worst catalysts.

The toluene oxidation specific reaction rates (r, the amount of toluene converted to $CO_2$ per second and per unit of mass of the catalyst) and intrinsic reaction rates (r', the amount of toluene converted to $CO_2$ per second and per unit of SSA of the catalyst) on different $Co_3O_4$ catalysts at 208 °C, when the toluene conversion to $CO_2$ was systematically below 20 %, were calculated and listed in **Table 4**. The largest intrinsic rate, r' was measured with the Co-9.5 catalyst ($1.51 \text{ nmol m}^{-2} \text{ s}^{-1}$), which was at least twice the intrinsic rate measured with the other catalysts. This observation indicated clearly that the high SSA of the Co-9.5 catalyst was not the only explanation for the outstanding performance of this catalyst. The specific and the intrinsic reaction rates measured over different catalysts decreased in the same order: **Co-9.5 > Co-9.0 > Co-8.5 > Co-8.0 > Co-10.0 ≈ Co-7.5 ≈ Co-7.0**. The apparent activation energies ($E_a$) and
the pre-exponential factors (A) obtained from the Arrhenius plots, when the toluene conversions to CO₂ were still below 8% (Fig. S3a), were also listed in Table 4. It could be further observed from this data set that the natural logarithm of the pre-factor A increased linearly with increasing values of E_a (Cremer-Constable plot, Fig. S4), suggesting the existence of a compensation effect. Although neither any consensus nor real theoretical ground was found to explain such an effect, it is frequently observed when complex reactions are studied [46,47], involving different reaction steps and different catalytic sites. Compensation effect is an indication anyway that the calculated kinetic parameters are only apparent values, which cannot directly be used to compare activities.

In addition, in order to evaluate the long-term stability of the as-prepared Co₃O₄ catalysts, continuous toluene oxidation was performed over Co-9.5 at 236 and 205 °C. As displayed in Fig. 9, the initial toluene conversion at 236 °C was 83 %. It decreased to 67 % after 5 h and stabilized till the end of the test. A similar trend was observed when the reaction temperature was carried out at 205 °C. In both cases, when toluene was removed from the stream, a large amount of CO₂ was produced transiently. This phenomenon could be attributed to the decomposition of the carbonaceous intermediate species deposited on the surface of Co₃O₄ during the test [48], which species inhibited some of the active sites and deactivated the catalyst. When no more CO₂ was generated, toluene was fed again to the reactor. The evolution of the conversion was identical to the initial one, demonstrating that deactivation by adsorbed intermediates was reversible in this case.

Finally, the catalytic performance of the as-prepared optimal catalyst Co-9.5 was compared with recently published results. As shown in Table S3, catalyst Co-9.5 presented catalytic performance superior to catalysts T-0.5 (MnO₂-Mn₂O₃) [49] and Co₃O₄ microsphere [50], comparable to catalysts H (Co₃O₄) [51] and 3D-Co₃O₄ [21], and inferior to catalysts MnO₂@Co₃O₄ [52] and 0.2 wt% Pt/TiO₂ [53] in toluene oxidation.

### 3.4.2. Total oxidation of propane

**Fig. S2b and Fig. 10** show the light-off curves upon propane oxidation over the as-prepared cobalt oxide catalysts during the 3rd heating and cooling cycle, respectively. The light-off curves upon heating and cooling were very much the same, suggesting a
very limited accumulation of reaction intermediates in this case.

The Co₃O₄ catalysts stability upon cycling in propane oxidation was well-established by comparing the T₁₀, T₅₀, and T₉₀ values measured upon three successive propane oxidation cooling runs (**Table S2**). The catalytic performance of the Co₃O₄ catalysts in propane oxidation was quite different from that in toluene oxidation. Indeed, Co-9.5 was the best catalyst in toluene oxidation, while it was the worst one in propane oxidation. The catalytic performance in propane oxidation, although not markedly different followed the order: Co-9.0 > Co-10.0 > Co-8.0 ≈ Co-7.0 > Co-7.5 > Co-8.5 ≈ Co-9.5. All catalysts could achieve full conversion of propane below 270 °C.

For comparison purposes, the specific propane oxidation reaction rates and intrinsic reaction rates over Co₃O₄ catalysts at 150 °C, as the propane conversion to CO₂ was kept below 10 %, were calculated and listed in **Table 5**. Specific reaction rates ranged from 15.4 to 26.7 nmol g⁻¹ s⁻¹ and Co-9.5 demonstrated a much lower intrinsic reaction rate compared to other catalysts, probably due to its largest SSA. E_a and lnA values obtained from the Arrhenius plots (**Fig. S3b**) are also summarized in **Table 5**. A compensation effect was also observed in this case (**Fig. S4**).

Furthermore, a long-term durability test was performed over Co-9.5 in the oxidation of propane at both 205 and 185 °C. The results presented on **Fig. 11** revealed that the catalytic performance of Co-9.5 in propane oxidation was quite stable. After 28 h continuous reaction at 205 °C, the propane conversion only decreased from 44 % to 36 %. Only 3 % propane conversion decrease was observed when performing the reaction at 185 °C.

Finally, in order to understand better the different performance of the Co₃O₄ catalysts between toluene and propane oxidation, a special experiment was designed. After 3 oxidation cycles, some kind of temperature programmed oxidation was performed and, for that, the reaction temperature was raised from room temperature to 350 °C under a 21 vol.% O₂/He stream. The CO₂ production was recorded (**Fig. S5**) and it was observed that the amount of CO₂ produced after toluene oxidation was two orders of magnitude larger than after propane oxidation. This information indicated that upon toluene oxidation much more toluene and oxidation intermediates adsorbed and accumulated onto the surface of Co₃O₄. These experimental results could explain the different degree of deactivation observed over the Co₃O₄ catalysts in both toluene and propane oxidation.
The lower accumulation of propane and reaction intermediates on the surface of Co₃O₄ allowed a much better durability of Co₃O₄ in propane oxidation.

### 3.5. Effect of residual sodium

According to the characterization and the catalytic results, the precipitation pH was shown to impact strongly on the crystallite size, the specific surface area, the amount of oxygen vacancies, the reducibility of the as-prepared Co₃O₄ catalysts and the catalytic performance in both toluene and propane oxidation. A linear relationship was found between the intrinsic reaction rate over the Co₃O₄ catalysts in toluene oxidation at 208 °C and the low temperature (50–300 °C) H₂ consumption (Fig. 12). When the precipitation pH was controlled at 9.5, the Co₃O₄ catalyst exhibited the smallest crystallite size, the largest SSA, the largest amount of oxygen vacancies and the best reducibility. Co-9.5 also showed the best performance in toluene oxidation. However, Co-9.5 was the worst catalyst in propane oxidation.

Recently, Choya et al. showed that the presence of 0.4 wt.% residual sodium on a Co₃O₄ catalyst negatively affected the catalytic performance in methane combustion [54]. Similarly, Tang et al. found a negative effect of sodium on the catalytic oxidation of propane over a Co₃O₄ catalyst [38]. The same research group investigated the effect of alkali-metals in CO and propane oxidation over Co₃O₄ catalysts and also revealed negative impacts [55]. To test this hypothesis, the sodium content in the as-prepared Co₃O₄ catalysts was measured by ICP-OES. The sodium content varied as follows: Co-9.5 (0.60 wt.%) > Co-8.5 (0.35 wt.%) > Co-9.0 (0.21 wt.%) ≈ Co-8.0 (0.20 wt.%) > Co-7.5 (0.16 wt.%) > Co-10.0 (below 0.10 wt.%). Interestingly, the worst catalyst in propane oxidation demonstrated the highest sodium content. To further investigate the real impact of residual sodium on the catalytic performance of Co-9.5 in propane oxidation, a new batch of Co₃O₄ catalysts were synthesized. One batch was obtained by thorough washing of the precipitate with excess amount of water to remove sodium ion from the precipitate; while the other was prepared by treatment of the as-prepared Co-9.5 catalyst with 0.5 M of HNO₃ to remove surface sodium. The catalytic performance of the two catalysts in propane oxidation was tested and compared with the original Co-9.5 catalyst (Fig. S6). Catalytic performance of Co-9.5 catalyst was greatly improved by such treatments, due to the efficient removal of residual sodium by both methods. These results clearly evidenced the negative role of residual sodium
at the catalyst surface in propane oxidation. On the opposite, the treated Co-9.5 catalysts behaved worse in toluene oxidation. The intrinsic toluene oxidation activity at 225 °C over the Co-9.5, Co-9.5-H₂O washed and Co-9.5-HNO₃ treated catalysts are 4.8, 2.0 and 1.8 nmol m⁻² s⁻¹, respectively, suggesting that the decreased SSA (from 51 to 39 and 28 m² g⁻¹) is not the only reason for the worse performance of reference catalysts; the change of precursor composition (for the Co-9.5-H₂O washed catalyst) and the modification of the surface property (for the Co-9.5-HNO₃ treated catalyst) may work as well. The result indicates that toluene oxidation over Co₃O₄ was much less sensitive to the presence of residual sodium.

## 4. Conclusions

Several Co₃O₄ catalysts synthesized via sodium carbonate precipitation at different pH were tested in the total oxidation of toluene and propane. The characterization results revealed the key role of precipitation pH on the composition of the cobalt precursors (Co(OH)ₓ(CO₃)₀.₅(₂-x)·nH₂O). Upon calcination at 500 °C, all cobalt precursors transformed into cubic spinel Co₃O₄, with average crystallite sizes of 17–32 nm and surface areas of 24–51 m² g⁻¹. Among the as-prepared Co₃O₄ catalysts, Co-9.5 showed the best performance in toluene oxidation (r' = 1.51 nmol m⁻² s⁻¹ at 208 °C), which was attributed to the defective structure and the excellent reducibility of the oxide. However, the catalytic activity of Co-9.5 in propane oxidation was inhibited by the presence of residual sodium. Upon throughout washing of the cobalt precursor or treatment of the Co₃O₄ catalyst with nitric acid, the catalytic activity of the catalyst in propane oxidation was greatly improved. The catalytic activity in both hydrocarbons oxidation over Co-9.5 was stable upon temperature cycling but decreased upon long-term experiments due to the deposition of intermediate carbonaceous species, especially in the case of toluene oxidation.

## Acknowledgements

This work was financially supported by the Auvergne Rhône Alpes region (PIA project number 1900797501 – 10687), University Claude Bernard Lyon 1 and CNRS. We gratefully acknowledge the China Scholarship Council of P.R. China for ZHANG
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**Figure captions**

**Fig. 1.** TG/DTA curves obtained upon decomposition in flowing air (temperature ramp rate: 10°C min⁻¹) of the cobalt oxide precursors precipitated at different pH values.

**Fig. 2.** FTIR spectra of (a) the cobalt oxide precursors and (b) the corresponding cobalt oxide catalysts prepared at different pH values.

**Fig. 3.** XRD patterns of the cobalt oxide catalysts prepared at different pH values.

**Fig. 4.** Raman spectra of the cobalt oxide catalysts prepared at different pH values.

**Fig. 5.** (a) N₂ adsorption–desorption isotherms and (b) pore size distributions of the cobalt oxide catalysts prepared at different pH values.

**Fig. 6.** H₂-TPR profiles of the cobalt oxide catalysts prepared at different pH values.

**Fig. 7.** CO-TPR profiles of the cobalt oxide catalysts prepared at different pH values
(a) CO concentration and (b) CO₂ concentration as a function of temperature.

**Fig. 8.** Light-off curves during the 3rd cooling cycle upon toluene oxidation over the cobalt oxide catalysts prepared at different pH values.

**Fig. 9.** Toluene conversion to CO₂ at 205 and 236 °C as a function of time on stream over Co-9.5.

**Fig. 10.** Light-off curves during the 3rd cooling cycle upon propane oxidation over the cobalt oxide catalysts prepared at different pH values.

**Fig. 11.** Propane conversion to CO₂ at 185 and 205 °C as a function of time on stream over Co-9.5.

**Fig. 12.** Correlation between the intrinsic reaction rate (r') over Co₃O₄ catalysts in toluene oxidation at 208 °C and the H₂ consumption at low temperature (50–300 °C).
<figure><img src="image_2.png" /><figcaption>**Fig. 1.** TG/DTA curves obtained upon decomposition in flowing air (temperature ramp rate: 10°C min⁻¹) of the cobalt oxide precursors precipitated at different pH values.</figcaption></figure>

<figure><img src="image_3.png" /><figcaption>**Fig. 2.** FTIR spectra of (a) the cobalt oxide precursors and (b) the corresponding cobalt oxide catalysts prepared at different pH values.</figcaption></figure>
<figure><img src="image_4.png" /><figcaption>Fig. 3. XRD patterns of the cobalt oxide catalysts prepared at different pH values.</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Fig. 4. Raman spectra of the cobalt oxide catalysts prepared at different pH values.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>Fig. 5. (a) N₂ adsorption–desorption isotherms and (b) pore size distributions of the cobalt oxide catalysts prepared at different pH values.</figcaption></figure>

<figure><img src="image_7.png" /><figcaption>Fig. 6. H₂-TPR profiles of the cobalt oxide catalysts prepared at different pH values.</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>**Fig. 7.** CO-TPR profiles of the cobalt oxide catalysts prepared at different pH values
(a) CO concentration and (b) CO₂ concentration as a function of temperature.</figcaption></figure>

<figure><img src="image_9.png" /><figcaption>**Fig. 8.** Light-off curves during the 3rd cooling cycle upon toluene oxidation over the cobalt oxide catalysts prepared at different pH values.</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>**Fig. 9.** Toluene conversion to CO₂ at 205 and 236 °C as a function of time on stream over Co-9.5.</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>**Fig. 10.** Light-off curves of propane oxidation over the cobalt oxide catalysts prepared at different pH values during the 3rd cooling cycle.</figcaption></figure>
<figure><img src="image_12.png" /><figcaption>**Fig. 11.** Propane conversion to CO₂ at 185 and 205 °C as a function of time on stream over Co-9.5.</figcaption></figure>

<figure><img src="image_13.png" /><figcaption>**Fig. 12.** Correlation between the intrinsic reaction rate (r') over Co₃O₄ catalysts in toluene oxidation at 208 °C and the H₂ consumption at low temperature (50–300 °C).</figcaption></figure>
**Table captions**

**Table 1.** TG results on the as-prepared cobalt oxide precursors and oxide catalyst yield.

**Table 2.** Average crystallite sizes, lattice constants and textural data of the cobalt oxide catalysts prepared at different pH values.

**Table 3.** Quantitative data extracted from the H₂-TPR and CO-TPR experiments performed on the different cobalt oxide catalysts prepared at different pH values.

**Table 4.** Specific reaction rates (r) and intrinsic reaction rates (r') at 208°C, apparent activation energies (Ea), apparent pre-exponential factors (A) in toluene oxidation over cobalt oxide catalysts prepared at different pH values (reaction conditions: toluene concentration = 1000 ppm, O₂ concentration = 21 vol.%, WHSV = 40,000 mL h⁻¹ g⁻¹).

**Table 5.** Specific reaction rates (r) and intrinsic reaction rates (r') at 150°C, apparent activation energies (Ea) and apparent pre-exponential factors (A) in propane oxidation over cobalt oxide catalysts prepared at different pH values (reaction conditions: propane concentration = 1000 ppm, O₂ concentration = 21 vol.%, WHSV = 40,000 mL h⁻¹ g⁻¹).
<table><caption>**Table 1.** TG results on the as-prepared cobalt oxide precursors and oxide catalyst yield.</caption><thead><tr><th>Sample precursor</th><th>H<sub>2</sub>O weight loss (wt.%)<sup>a</sup></th><th>Total weight loss (wt.%)<sup>b</sup></th><th>Formula<sup>c</sup></th><th>Yield (%)<sup>d</sup></th></tr></thead><tbody><tr><td>Co-7.5</td><td>1.91</td><td>27.22</td><td>Co(OH)<sub>0.83</sub>(CO<sub>3</sub>)<sub>0.59</sub>•0.12H<sub>2</sub>O</td><td>85</td></tr><tr><td>Co-8.0</td><td>1.53</td><td>27.70</td><td>Co(OH)<sub>0.74</sub>(CO<sub>3</sub>)<sub>0.63</sub>•0.09H<sub>2</sub>O</td><td>88</td></tr><tr><td>Co-8.5</td><td>1.72</td><td>27.84</td><td>Co(OH)<sub>0.74</sub>(CO<sub>3</sub>)<sub>0.63</sub>•0.11H<sub>2</sub>O</td><td>91</td></tr><tr><td>Co-9.0</td><td>6.01</td><td>28.00</td><td>Co(OH)<sub>1.09</sub>(CO<sub>3</sub>)<sub>0.46</sub>•0.37H<sub>2</sub>O</td><td>96</td></tr><tr><td>Co-9.5</td><td>5.43</td><td>26.64</td><td>Co(OH)<sub>1.19</sub>(CO<sub>3</sub>)<sub>0.41</sub>•0.33H<sub>2</sub>O</td><td>100</td></tr><tr><td>Co-10.0</td><td>6.59</td><td>27.33</td><td>Co(OH)<sub>1.21</sub>(CO<sub>3</sub>)<sub>0.39</sub>•0.40H<sub>2</sub>O</td><td>89</td></tr></tbody></table>

<table><caption>**Table 2.** Average crystallite sizes, lattice constants and textural data of the cobalt oxide catalysts prepared at different pH values.</caption><thead><tr><th>Catalysts</th><th>d (nm)<sup>a</sup></th><th>a (Å)<sup>a</sup></th><th>A<sub>190</sub>/A<sub>680</sub><sup>b</sup></th><th>SSA (m<sup>2</sup> g<sup>-1</sup>)<sup>c</sup></th><th>V<sub>p</sub> (cm<sup>3</sup> g<sup>-1</sup>)<sup>c</sup></th></tr></thead><tbody><tr><td>Co-7.5</td><td>33</td><td>8.0877</td><td>0.023</td><td>27</td><td>0.07</td></tr><tr><td>Co-8.0</td><td>31</td><td>8.0876</td><td>0.062</td><td>27</td><td>0.08</td></tr><tr><td>Co-8.5</td><td>28</td><td>8.0887</td><td>0.067</td><td>28</td><td>0.11</td></tr><tr><td>Co-9.0</td><td>27</td><td>8.0870</td><td>0.086</td><td>29</td><td>0.26</td></tr><tr><td>Co-9.5</td><td>17</td><td>8.0865</td><td>0.091</td><td>51</td><td>0.32</td></tr><tr><td>Co-10.0</td><td>32</td><td>8.0897</td><td>0.041</td><td>24</td><td>0.22</td></tr></tbody></table>

<footnote><sup>a</sup> Weight loss of the cobalt oxide precursors upon TG analysis in the range 25–150°C.</footnote>

<footnote><sup>b</sup> Weight loss of the cobalt precursors upon TG analysis in the range 25–500°C.</footnote>

<footnote><sup>c</sup> Formula of the cobalt oxide precursors calculated from the TG results.</footnote>

<footnote><sup>d</sup> Estimated yield by assuming that all Co was converted into Co<sub>3</sub>O<sub>4</sub>.</footnote>

<footnote><sup>a</sup> Average crystallite sizes and lattice constants calculated from XRD patterns.</footnote>

<footnote><sup>b</sup> Peak area ratios of the F<sup>1</sup><sub>2g</sub> and A<sub>1g</sub> vibration modes on the Raman spectra.</footnote>

<footnote><sup>c</sup> Specific surface areas and total pore volumes obtained from N<sub>2</sub> adsorption-desorption isotherms.</footnote>
<table><caption>**Table 3.** Quantitative data extracted from the H₂-TPR and CO-TPR experiments performed on the different cobalt oxide catalysts prepared at different pH values.</caption><thead><tr><th rowspan="2">Catalysts</th><th colspan="2">Low temperature<br>reduction peak position<br>(°C)</th><th colspan="2">H₂ consumption<br>(mmol g<sup>-1</sup>)</th><th colspan="2">CO₂ production<br>(mmol g<sup>-1</sup>)</th></tr><tr><th>H₂-TPR</th><th>CO-TPR</th><th>Total</th><th>50–300 °C</th><th>Total</th><th>50–300 °C</th></tr></thead><tbody><tr><td>Co-8.0</td><td>284</td><td>330</td><td>15.0</td><td>3.1</td><td>17.3</td><td>1.9</td></tr><tr><td>Co-8.5</td><td>289</td><td>310</td><td>16.5</td><td>3.4</td><td>17.0</td><td>1.7</td></tr><tr><td>Co-9.0</td><td>280</td><td>299</td><td>14.7</td><td>3.3</td><td>17.3</td><td>2.2</td></tr><tr><td>Co-9.5</td><td>270</td><td>299</td><td>16.0</td><td>3.9</td><td>17.4</td><td>2.3</td></tr><tr><td>Co-10.0</td><td>291</td><td>309</td><td>15.8</td><td>3.0</td><td>17.6</td><td>2.2</td></tr></tbody></table>

<table><caption>**Table 4.** Specific reaction rates (r) and intrinsic reaction rates (r') at 208 °C, apparent activation energies (Eₐ), apparent pre-exponential factors (A) in toluene oxidation over cobalt oxide catalysts prepared at different pH values (reaction conditions: toluene concentration = 1000 ppm, O₂ concentration = 21 vol.%, WHSV = 40,000 mL h⁻¹ g⁻¹).</caption><thead><tr><th>Catalysts</th><th>r (nmol g<sup>-1</sup> s<sup>-1</sup>)</th><th>r' (nmol m<sup>-2</sup> s<sup>-1</sup>)</th><th>E<sub>a</sub> (kJ mol<sup>-1</sup>)</th><th>lnA</th></tr></thead><tbody><tr><td>Co-7.0</td><td>4.1</td><td>-</td><td>165</td><td>22</td></tr><tr><td>Co-7.5</td><td>6.2</td><td>0.2</td><td>166</td><td>23</td></tr><tr><td>Co-8.0</td><td>9.7</td><td>0.4</td><td>186</td><td>28</td></tr><tr><td>Co-8.5</td><td>19.6</td><td>0.7</td><td>167</td><td>24</td></tr><tr><td>Co-9.0</td><td>21.9</td><td>0.8</td><td>171</td><td>25</td></tr><tr><td>Co-9.5</td><td>76.9</td><td>1.5</td><td>172</td><td>26</td></tr><tr><td>Co-10.0</td><td>4.6</td><td>0.2</td><td>215</td><td>34</td></tr></tbody></table>
<table><caption>**Table 5.** Specific reaction rates (r) and intrinsic reaction rates (r') at 150°C, apparent activation energies (Ea) and apparent pre-exponential factors (A) in propane oxidation over cobalt oxide catalysts prepared at different pH values (reaction conditions: propane concentration = 1000 ppm, O₂ concentration = 21 vol.%, WHSV = 40,000 mL h⁻¹ g⁻¹).</caption><thead><tr><th>Catalysts</th><th>r (nmol g<sup>-1</sup> s<sup>-1</sup>)</th><th>r' (nmol m<sup>-2</sup> s<sup>-1</sup>)</th><th>E<sub>a</sub> (kJ mol<sup>-1</sup>)</th><th>lnA</th></tr></thead><tbody><tr><td>Co-7.0</td><td>15.4</td><td>-</td><td>86</td><td>6.4</td></tr><tr><td>Co-7.5</td><td>21.8</td><td>0.8</td><td>78</td><td>4.6</td></tr><tr><td>Co-8.0</td><td>23.6</td><td>0.9</td><td>77</td><td>4.2</td></tr><tr><td>Co-8.5</td><td>15.8</td><td>0.6</td><td>78</td><td>4.3</td></tr><tr><td>Co-9.0</td><td>26.7</td><td>0.9</td><td>76</td><td>4.2</td></tr><tr><td>Co-9.5</td><td>16.8</td><td>0.3</td><td>77</td><td>4.1</td></tr><tr><td>Co-10.0</td><td>19.5</td><td>0.8</td><td>83</td><td>5.7</td></tr></tbody></table>
Graphical Abstract

<figure><img src="image_14.png" /></figure>