Comparison of Different Metal Doping Effects on Co₃O₄ Catalysts for the Total Oxidation of Toluene and Propane

Abstract

Metal-doped (Mn, Cu, Ni, and Fe) cobalt oxides were prepared by a coprecipitation method and were used as catalysts for the total oxidation of toluene and propane. The metal-doped catalysts displayed the same cubic spinel Co₃O₄ structure as the pure cobalt oxide did; the variation of cell parameter demonstrated the incorporation of dopants into the cobalt oxide lattice. FTIR spectra revealed the segregation of manganese oxide and iron oxide. The addition of dopant greatly influenced the crystallite size, strain, specific surface area, reducibility, and subsequently the catalytic performance of cobalt oxides. The catalytic activity of new materials was closely related to the nature of the dopant and the type of hydrocarbons. The doping of Mn, Ni, and Cu favored the combustion of toluene, with the Mn-doped one being the most active (14.6 × 10⁻⁸ mol g_Co⁻¹ s⁻¹ at 210 °C; T₅₀ = 224 °C), while the presence of Fe in Co₃O₄ inhibited its toluene activity. Regarding the combustion of propane, the introduction of Cu, Ni, and Fe had a negative effect on propane oxidation, while the presence of Mn in Co₃O₄ maintained its propane activity (6.1 × 10⁻⁸ mol g_Co⁻¹ s⁻¹ at 160 °C; T₅₀ = 201 °C). The excellent performance of Mn-doped Co₃O₄ could be attributed to the small grain size, high degree of strain, high surface area, and strong interaction between Mn and Co. Moreover, the presence of 4.4 vol.% H₂O badly suppressed the activity of metal-doped catalysts for propane oxidation, among them, Fe-doped Co₃O₄ showed the best durability for wet propane combustion.

1. Introduction

Hydrocarbons emitted from both industrial manufacture and motor vehicle exhaust cause a lot of atmospheric pollution. Catalytic combustion is one of the most promising countermeasures for the elimination of hydrocarbons. As an inexpensive and active catalyst, Co₃O₄ has attracted much interest in the past decade as a substitute for the noble metal catalysts.

Liu et al. prepared nanocrystalline cobalt oxide by a soft reactive grinding procedure, the catalyst shows high specific rate for propane total oxidation benefiting from a high concentration of superficial electrophilic oxygen (O⁻) species [1]. Garcia et al. synthesized ordered Co₃O₄ via a nanocasting method using KIT-6 as the hard template. The good activity of these catalysts in the total oxidation of toluene and propane was correlated with both the high surface area and the presence of oxygen vacancies instead of the ordered structure [2]. Marin et al. produced very active Co₃O₄ catalyst with 50% propane conversion at 175 °C using a supercritical CO₂ anti-solvent precipitation method [3]. Salek et al. obtained Co₃O₄ with high porosity by moderate calcination of CoO(OH) derived from aqueous precipitation. This catalyst showed a superior activity for CO and propane total oxidation [4]. By using an acetic acid leaching strategy, Tang and his co-workers modified the surface structure and chemistry of Co₃O₄ nanoparticles and obtained a Co₃O₄ catalyst with more abundant defects, surface Co²⁺, and chemisorbed oxygen species, which presented a much higher activity than the commercial Pt/Al₂O₃ and Pd/Al₂O₃ catalysts [5]. Analogously, Li et al. fabricated mesoporous Co₃O₄ via nitric acid treatment, and the as-prepared catalyst showed enhanced activity in toluene oxidation due to higher specific surface areas, more weak acidic sites, and a greater amount of surface Co²⁺ and adsorbed oxygen species [6]. Ren et al. constructed various 3D hierarchical Co₃O₄ nanocatalysts via a hydrothermal process and demonstrated that the hierarchical cube-stacked Co₃O₄ microspheres exhibited best activity for toluene oxidation [7].

Nevertheless, it is still a challenge to increase the catalytic activity of Co₃O₄ for different VOCs to broaden the feasible range and meet the need of realistic application.

Substituting a small fraction of cobalt in a Co₃O₄ lattice with another metal cation, known as doping, is a potential way to improve the performance of Co₃O₄ catalysts. The chemical bonding at the surface of Co₃O₄ could be modified by metal doping, which may in turn induce lattice imperfection and create oxygen vacancies that are beneficial to catalytic reaction. The effect of metal dopant features, including oxidation state, ionic radius, electronegativity, etc., on the physicochemical properties and redox ability of cobalt-based catalysts was extensively studied theoretically and practically [8,9,10].

Ni doping into a Co₃O₄ lattice enhanced propane oxidation reaction kinetics by promoting surface lattice oxygen activity and facilitating CO₂ desorption [11]. The incorporation of Mn in Co₃O₄ increased surface Co²⁺ concentration and active oxygen, contributing to high activity in catalytic combustion of 1,2-dichlorobenzene (o-DCB) and retarding chlorination of o-DCB [12]. The doping of In₂O₃ induced the structural distortion of Co₃O₄ and promoted the catalytic performance of Co₃O₄ for CO oxidation [13]. Baidya et al. demonstrated that 15% Fe-doped Co₃O₄ can achieve superior catalytic activity and stability towards CO oxidation [14].

Although there have already been some reports on metal-doped Co₃O₄ catalysts that present desirable activity, the comparative study of different metal dopant effects on the catalytic activity of Co₃O₄ in different hydrocarbons oxidation is still limited and needs to be further explored.

Recently, we have found that the precipitation agent could affect the catalytic activity of Co₃O₄. Among the investigated precipitation agent, sodium carbonate proved to be the most promising one. Considering this fact, in this study, the effect of metal-doping based on carbonate precipitation method on the physicochemical properties and redox ability of Co₃O₄ was investigated.

Herein, we prepared four metal-doped (Mn, Cu, Ni, and Fe) Co₃O₄ by a coprecipitation method. A comparative study of the catalytic behavior of pure and metal ion-substituted Co₃O₄ was investigated for the total oxidation of toluene and propane. A small amount of metal incorporated into the Co₃O₄ lattice affected the structure and redox property of Co₃O₄ and in turn led to different catalytic behavior as a function of the reactant used.

2. Results and Discussion

2.1. Chemical Composition, Structure, and Textural Properties

The chemical compositions of the as-prepared samples were determined by inductively coupled plasma optical emission spectroscopy (ICP-OES) measurement. The results are listed in

Table 1. Chemical composition, structural, and textural properties of the Co₃O₄ and M_0.05Co catalysts.

Catalysts	Co	M/Co	D (nm) a	Strain (%) a	a (Å) a	SSA	Vpore
	(wt.%)	(at.%)				(m2 g−1) b	(cm3 g−1) b
Co3O4	74.7	-	29.4	0.34	8.084	30	0.08
Mn0.05Co	68.1	4.5	19.1	0.53	8.098	47	0.19
Cu0.05Co	68.3	5.6	26.6	0.38	8.093	29	0.07
Ni0.05Co	67.6	5.3	22.5	0.45	8.096	35	0.11
Fe0.05Co	68.0	5.9	27.1	0.37	8.094	34	0.09

^a Average crystalline size (D), strain, and lattice constant (a) obtained from XRD analysis. ^b Specific surface area (SSA) and total pore volume (V_pore) calculated from N₂ adsorption-desorption isotherm.

. The cobalt mass ratio of pure cobalt oxide was 74.7 wt.%, close to the stoichiometric value of Co₃O₄ (73.4 wt.%). The substituted metal to cobalt molar ratios were similar to the theoretical one (5%), indicating the success of the coprecipitation process.

Powder X-ray diffraction (XRD) patterns of pure cobalt oxide and metal-doped cobalt oxides are shown in Figure 1. It is observed that all metal-doped cobalt oxides exhibited characteristic diffraction peaks corresponding to Co₃O₄ spinel structures (lattice constant a = 8.0840 Å, PDF # 74-2120) identical to pure cobalt oxide, no matter what kind of substituted metal was used, indicating that the doped metals were uniformly distributed in the samples, either in the lattice or on the surface. However, looking at the enlarged patterns for all M-Co₃O₄ samples, an evident shift of the most intense peak (311) to lower angles could be seen, suggesting an increase in the lattice constant due to metal substitution. The average crystalline sizes, strains, and the lattice constants of all samples were calculated and are listed in Table 1. Compared to pure Co₃O₄, all M-Co₃O₄ samples presented a smaller crystalline size, a bigger strain, and a larger lattice constant, especially for the Mn_0.05Co sample. This result demonstrates that some M ions were successfully introduced into the crystal lattice of Co₃O₄, leading to a lattice expansion of the spinel, thus generating a smaller crystalline size.

Figure 2 shows the Fourier transform infrared (FTIR) spectra of Co₃O₄ and M_0.05Co samples. Two characteristic peaks at ca. 655 and 550 cm⁻¹ (with shoulders at higher frequencies), corresponding to vibrations of Co³⁺ in tetrahedral sites and Co²⁺ in octahedral sites in Co₃O₄ spinel structure respectively [15], were observed for all samples, in agreement of XRD analysis. However, some new peaks appeared in the case of Mn_0.05Co and Fe_0.05Co, which could be assigned to vibration modes of Mn–O and Fe–O, respectively [16,17]. In addition, the presence of Ni–O or Cu–O vibration modes could not be excluded since they were expected to be located in the same region of Co–O vibration modes and could be overlapped [18,19]. This phenomenon revealed that not all metal ions were successfully inserted into the matrix of Co₃O₄ lattice, some of them might form amorphous MO_x species and distribute on the surface.

The N₂ adsorption–desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distributions of undoped and doped Co₃O₄ samples are displayed in Figure 3. The type IV isotherm curves and H1-shaped hysteresis loops indicated the presence of slit-type mesopores originated from the accumulation of nanoparticles. The textural parameters of the different samples are summarized in Table 1. The specific surface areas (SSA) of all samples sit in the range of 30 to 47 m² g⁻¹. Among all samples, Mn_0.05Co showed the largest SSA (47 m² g⁻¹) and biggest pore volume (0.19 cm³ g⁻¹), followed by the Ni_0.05Co and Fe_0.05Co samples. It should be noted that the average crystallite size of Cu_0.05Co was smaller than that of pure Co₃O₄, while the SSA for Cu_0.05Co was lower than that for pure Co₃O₄, indicating a strong agglomeration of the crystallites.

Raman spectra were recorded to investigate the effect of metal doping on the lattice distortion of the spinel. As shown in Figure 4, five characteristic bands at 665, 601, 507, 463, and 187 cm⁻¹ corresponding to stretching mode of spinel Co₃O₄ (A_1g, F³_2g, F²_2g, E_g, and F¹_2g symmetry) were clearly observed for pure Co₃O₄ [5,20]. Compared to pure Co₃O₄, the Raman bands of the M_0.05Co samples weakened and broadened because of the dissolution of M into the Co₃O₄ lattice, which induced lattice expansion and lattice distortion, as revealed by XRD analysis. The peak position and full width at half maxima (FWHM) of the most intense peak are listed in

Table 2. Raman and temperature-programmed reduction in hydrogen (H₂-TPR) analysis of the Co₃O₄ and M_0.05Co catalysts.

Catalysts	Raman Peak a (cm−1)	FWMH a	Total H2 Uptake (mmol g−1)	H2 Uptake Below 300 °C (mmol g−1)
Co3O4	667	34	16.0	3.0
Mn0.05Co	672	37	15.0	2.5
Cu0.05Co	667	34	13.7	8.1
Ni0.05Co	660	46	15.0	3.2
Fe0.05Co	684	24	13.8	2.1

^a Peak position and FWMH of the most intense Raman peak.

. Raman spectrum of Ni_0.05Co underwent a red-shift, while that of Fe_0.05Co underwent a blue-shift. Ni_0.05Co exhibited the largest FMWH while Fe_0.05Co exhibited the smallest one.

2.2. Reducibility

To investigate the effect of metal doping on the reducibility of Co₃O₄, H₂-TPR experiments were carried out; the results are shown in Figure 5, whereas the quantitative results of hydrogen consumption are summarized in Table 2. For pure Co₃O₄, the reduction initiated at 195 °C and finished at 400 °C. Two sequent peaks corresponding to the reduction of Co³⁺→Co²⁺ and Co²⁺→Co⁰ were clearly observed at 291 and 383 °C, respectively. However, after metal doping, the positions of these two peaks clearly changed. Moreover, some small peak shoulders that may arise from the reduction of doping species appeared (indicated by the arrows). In the case of Cu_0.05Co and Ni_0.05Co, both peaks shifted towards low temperature range, especially for Cu_0.05Co. The reduction of the Cu-doped sample started from 170 °C and ended at 360 °C, suggesting that the presence of Cu or Ni facilitated the Co₃O₄ reduction. Regarding Mn_0.05Co and Fe_0.05Co, the low-temperature peak appeared to be similar to that of the pure Co₃O₄, while the high-temperature peak moved to higher temperature zone, indicating that Mn or Fe doping could hinder the formation of metallic cobalt, which is beneficial for the redox circle during reaction between Co³⁺ and Co²⁺ [21]. The total hydrogen consumption amount decreased for all the metal-doped samples, suggesting the nonstoichiometric nature of the samples with metal doping. By comparing the hydrogen uptake below 300 °C, one can conclude that the low temperature reducibility ranks in the sequence Cu_0.05Co > Ni_0.05Co > Co₃O₄ > Mn_0.05Co > Fe_0.05Co. Based on the above reduction behaviors, Cu or Ni doping enhanced the reducibility of Co₃O₄, whereas Mn or Fe doping worsened it. This way, Cu_0.05Co was the most reducible sample.

2.3. Catalytic Performance

2.3.1. Catalytic Activity and Stability in Toluene Oxidation

The catalytic performance of all the samples was evaluated in both toluene and propane oxidation. Regarding the toluene oxidation, three continuous heating–cooling loop tests were performed. Figure S1a presented the light-off curves of pure Co₃O₄ and metal-doped Co₃O₄ catalysts during the third continuous heating run. In the high conversion stage, toluene conversion to CO₂ exceeds 100% due to the accumulation of intermediates during the low conversion period [22]. Considering this fact, the light-off curves during the third continuous cooling run were used and are displayed in Figure 6, whereas the light-off temperatures at 10, 50, and 90% toluene conversion (T₁₀, T₅₀, and T₉₀) were listed in

Table 3. Light-off temperatures at 10, 50, and 90% toluene conversion upon the cooling process.

Catalysts	1st Run			2nd Run			3rd Run
	T10	T50	T90	T10	T50	T90	T10	T50	T90
Co3O4	223	235	256	223	235	256	223	235	256
Mn0.05Co	203	224	254	204	224	255	204	224	255
Cu0.05Co	211	228	251	211	228	251	211	228	251
Ni0.05Co	206	226	250	206	226	250	206	226	250
Fe0.05Co	234	244	270	234	244	270	234	245	270

. The difference among the three catalytic cycles was imperceptible, demonstrating the excellent cycle stability of the Co₃O₄ and M_0.05Co catalysts. All catalysts were able to achieve full conversion of toluene to CO₂ below 280 °C. After doping with Mn, Cu, or Ni, the catalytic performance of pure Co₃O₄ was obviously improved, being that the light-off curve shifted to lower temperature. However, the incorporation of Fe into Co₃O₄ resulted in a worse performance. Notably, the catalytic performance enhancement by metal doping is more pronounced in the low temperature stage than in the high one. In addition, the concentrations of CO produced during the toluene oxidation process over all catalysts were below 110 ppm (Figure S2a). Among them, Fe_0.05Co with poor catalytic efficiency showed inferior selectivity. Nevertheless, the calculated carbon balances at 300 °C for all catalysts were higher than 95%, indicating the decent selectivity of the as-prepared cobalt-based catalysts.

In order to better compare the activity, the catalytic reaction rates of all catalysts at 210 °C were calculated and are listed in

Table 4. Reaction rates (r) at 210 °C, apparent pre-exponential factors (A), apparent activation energies (E_a), and coefficient of determination (R²) for toluene oxidation over various catalysts (reaction conditions: toluene concentration = 1000 ppm, O₂ concentration = 21 vol.%, and WHSV = 40,000 mL h⁻¹ g⁻¹).

Catalysts	r (10−8 mol gCo−1 s−1)	lnA	Ea (kJ mol−1)	R2
Co3O4	1.5	30.5 ± 0.8	196 ± 3	0.995
Mn0.05Co	14.6	18.7 ± 0.5	142 ± 2	0.998
Cu0.05Co	6.4	40.6 ± 1.2	231 ± 5	0.997
Ni0.05Co	10.2	24.3 ± 1.2	165 ± 5	0.993
Fe0.05Co	0.5	35.8 ± 0.9	222 ± 4	0.997

. This activity ranked as follows, Mn_0.05Co > Ni_0.05Co > Cu_0.05Co > Co₃O₄ > Fe_0.05Co. Moreover, the activation energies (E_a) and pre-exponential factors (A) were estimated based on the slopes and intercepts of the Arrhenius plots (Figure S3a). Diverse values of the activation energy ranging from 142 kJ mol⁻¹ to 231 kJ mol⁻¹ were obtained. The activation energies and pre-exponential factors here showed did not correlated with the activity, which can be explained by the compensation effect (Figure S4) [23,24].

The long-term stability of the catalysts was assessed at 20–25% of toluene conversion after the first cooling run. As shown in Figure 7, the conversion declined for all catalysts to some extent after 5 h, remaining invariable throughout the test. The Mn_0.05Co, Cu_0.05Co, and Ni_0.05Co catalysts exhibited similar stability, with ~15% decrease in toluene conversion during the whole process. The Co₃O₄ and Fe_0.05Co catalysts deactivated more severely, ~20% of toluene conversion drop was observed. In order to figure out the reason causing catalyst deactivation, a post-calcination treatment in air flow was carried out after 24 h stability test; the CO₂ evolution as a function of time on stream was plotted in Figure S5. The maximum in the CO₂ concentration could arise from the catalytic oxidation of adsorbed toluene or intermediate into CO₂ [25]. The time for reaching this maximum CO₂ concentration was dependent on the catalytic activity of the different catalysts and likely on the reaction temperatures (varied from 215 to 240 °C). The higher the reaction temperature, the faster the CO₂ desorption on the catalyst surface. The area of this peak reflected the amount of toluene/intermediate adsorbed in the catalyst. Among these catalysts, Fe_0.05Co exhibited the largest peak area, suggesting its excellent CO₂ adsorption capacity. To verify the reversibility of this deactivation for catalyst Co₃O₄, a stability test was conducted after the calcination treatment (Figure S6). Identical stability curve was observed after the calcination step, demonstrating that it is possible to completely regenerate the catalyst.

2.3.2. Catalytic Activity and Water Resistance in Propane Oxidation

On the other hand, the light-off curves of the propane oxidation for pure Co₃O₄ and metal-doped Co₃O₄ catalysts during the cooling run are displayed in Figure 8.

Table 5. Light-off temperatures at 10, 50, and 90% propane conversion upon cooling process in normal condition or in the presence of 4.4 vol.% water.

Catalysts	Normal Condition			In 4.4 vol.% Water
	T10	T50	T90	T10	T50	T90
Co3O4	161	199	226	185	218	245
Mn0.05Co	162	201	227	178	215	245
Cu0.05Co	180	225	257	206	248	287
Ni0.05Co	170	213	245	199	243	290
Fe0.05Co	169	210	241	184	219	255

summarized the temperatures at which 10, 50, and 90% of propane conversion was reached. Propane can be converted into CO₂ below 280 °C for all of the catalysts. However, totally different trends in the catalytic performance of propane and toluene oxidation were observed. For propane oxidation, pristine Co₃O₄ was the most active catalyst. The doping with Cu, Ni, or Fe was detrimental for the catalytic performance of pure Co₃O₄, being that the light-off curve shifted to higher temperature. This detrimental phenomenon was especially remarkable over Cu_0.05Co. Fortunately, the catalytic performance remained practically unaltered after the incorporation of Mn. The concentrations of CO and propene produced during the propane oxidation process over all catalysts were 0 ppm (no shown) and <5 ppm (Figure S2b), respectively, suggesting the excellent selectivity of the catalysts, which was further confirmed by the good carbon balance throughout the catalytic system (>99%).

Likewise, the catalytic reaction rates of all catalysts in the propane oxidation at 160 °C were calculated and are listed in

Table 6. Reaction rates (r) at 160 °C, apparent pre-exponential factors (A), apparent activation energies (E_a), and coefficient of determination (R²) for propane oxidation over various catalysts (reaction conditions: propane concentration = 1000 ppm, O₂ concentration = 21 vol.% and WHSV = 40,000 mL h⁻¹ g⁻¹).

Catalysts	r (10−8 mol gCo−1 s−1)	lnA	Ea (kJ mol−1)	R2
Co3O4	6.1	4.2 ± 0.2	76.2 ± 0.8	0.999
Mn0.05Co	6.1	3.0 ± 0.3	72.0 ± 0.9	0.999
Cu0.05Co	3.0	0.8 ± 0.1	66.8 ± 0.4	0.999
Ni0.05Co	4.2	2.3 ± 0.2	70.9 ± 0.7	1.000
Fe0.05Co	4.2	4.5 ± 0.2	78.6 ± 0.7	0.999

. They followed the order, Co₃O₄ ≈ Mn_0.05Co > Fe_0.05Co ≈ Ni_0.05Co > Cu_0.05Co. In addition, from Arrhenius plots (Figure S3b), various E_a (67−79 kJ mol⁻¹) were obtained (Table 6). Compensation effect appeared again (Figure S4) [23,24], namely, the higher the value of E_a, the higher the value of lnA.

As in every oxidation process, water is inevitably present either as a part of the inlet stream or as a combustion product, which would alter reaction equilibrium and in turn cause deactivation, due to either the competitive adsorption between H₂O and reactant molecules or the formation of inert hydroxyls groups on the catalyst surface [26]. Therefore, the effect of presence of H₂O on the catalytic oxidation of propane was investigated. As shown in Figure 9, a decrease in propane conversion was observed for all studied catalysts when 4.4 vol.% of H₂O was present in the reactant gas. Their T₅₀ values were 9–30 °C higher than those obtained in the dry reaction condition. Among these catalysts, Fe_0.05Co presented the best water tolerance for propane oxidation since it maintained good activity to a certain extent. Ni_0.05Co was the most sensitive sample to H₂O. On the other hand, Mn_0.05Co still showed desirable catalytic performance, with T₁₀ as low as 178 °C.

2.4. Discussion

By metal doping, smaller crystalline sizes and larger specific surface areas were obtained for the Mn_0.05Co catalyst. XRD and Raman analysis demonstrated that other metal (Mn, Cu, Ni, or Fe) was inserted into the structure of Co₃O₄. This was corroborated by lattice expansion and broadening of Raman bands. Furthermore, FTIR spectra revealed that there were some metal oxides (Mn, Fe) distributed on the surface of Co₃O₄. H₂-TPR results confirmed the strong interaction between M and Co in the Mn_0.05Co catalyst, as the reduction behavior changed much after M-doping.

The effect of the metal doping on the catalytic performance of all the catalysts in the toluene and propane oxidation reactions was not always the same. A catalyst that was active for toluene oxidation may not be good for propane oxidation. That was the case, for example, for catalyst Cu_0.05Co. This may be due to the different nature of these two reactant molecules (type of C–C bonds and molecular sizes) and the different reaction mechanisms involved for these two reactions [27]. It is generally accepted that the total oxidation of toluene over Co-based catalysts proceeds through a Mars–van Krevelen mechanism: toluene first reacts with the mobile lattice oxygen species to produce CO₂ and H₂O accompanied by the reduction of Co³⁺ to Co²⁺, meanwhile O₂ molecules in air replenish the reduced catalyst and complete the redox cycle Co³⁺ ⇋ Co²⁺ [28,29]. The promoted catalytic performance of Cu_0.05Co and Ni_0.05Co for toluene oxidation could be attributed to the strong synergistic effect between cobalt and doped metal cations. Lattice oxygen could be much more easily abstracted by hydrogen from Cu_0.05Co and Ni_0.05Co than from Co₃O₄, increasing the mobility of lattice oxygen and resulting in better oxidation ability. For propane oxidation, the amount of surface adsorbed oxygen is reported to be the determining factor [30,31]. Thus, metal doping may influence the chemical status of surface/chemisorbed oxygen in M_0.05Co, thus leading to a different catalytic behavior in propane oxidation.

Anyhow, benefitting from its big strain and large surface area, Mn_0.05Co is the best catalyst prepared in this study. It is efficient for both toluene and propane oxidation reactions. Hopefully, it could be considered as a potential industrial catalyst for removing hydrocarbon pollutants by oxidation processes.

3. Experimental

3.1. Catalyst Preparation

Cobalt oxide and M-doped (Mn, Fe, Ni, and Cu) cobalt oxide catalysts were prepared by a coprecipitation method using sodium carbonate as the precipitant. All chemicals were obtained from Sigma-Aldrich and used as received. Twenty millimoles of Co(NO₃)₂·6H₂O and 1 mmol of second metal salt (Mn(Ac)₂·4H₂O, Cu(NO₃)₂·3H₂O, Ni(NO₃)₂·6H₂O, or Fe(NO₃)₃·9H₂O) were dissolved in 100 mL of distilled water. Twenty-two millimoles of Na₂CO₃ was dissolved in 100 mL of distilled water. The resulting Na₂CO₃ aqueous solution was added to the metal salts aqueous solution. After being stirred at room temperature for 1 h, the solid was separated and fully washed by centrifugation and redispersion. The obtained wet cake was dried at 80 °C overnight and then calcined in a muffle furnace at 200 °C for 1 h and at 500 °C for 1 h with a temperature ramp rate of 2 °C/min⁻¹. The final products were referred to as M_0.05Co, where M is the doped metal and 0.02 is the atomic ratio of M:Co.

3.2. Catalyst Characterization

The chemical composition of the samples was measured by ICP-OES (Horiba Jobin Yvon, Paris, France). Prior to the determination, the metal oxides were dissolved in a mixture of inorganic acids (H₂SO₄ and HNO₃).

FTIR spectra were recorded using a FT-IR C92712 spectrometer (PerkinElmer, Waltham, MA, USA) in attenuated total reflectance mode at an instrument resolution of 1 cm⁻¹ over a range of 400 to 4000 cm⁻¹.

Nitrogen adsorption−desorption isotherms were obtained using a TRISTAR II apparatus (Micromeritics, Norcross, GA, USA) at −196 °C. Before analysis, each sample was pretreated at 300 °C for 3 h under primary vacuum. The specific surface areas of the samples were determined by the standard Brunauer–Emmett–Teller (BET) procedure. The total pore volume and the pore size distribution were calculated using the BJH method.

Powder X-ray diffraction (XRD) patterns were recorded on a D5005 diffractometer (Bruker, Karlsruhe, Germany) equipped with a Cu Kα radiation (λ = 0.154184 nm) and a graphite monochromator on the diffracted beam. Samples were scanned from 10° < 20 < 80° with a step size of 0.02° and a counting time of 2 s per step.

Raman spectra were recorded by a LabRam HR spectrometer (Horiba, Paris, France) using Ar⁺ laser beam of 514 nm wavelength for an excitation.

Temperature-programmed reduction experiments in hydrogen (H₂−TPR) were performed on a commercial Micromeritics AutoChem 2950 HP (Micromeritics, Norcross, GA, USA) unit with TCD detection. In each test, 0.05 g of sample was pretreated under 50 mL/min of Ar flow at 350 °C for 0.5 h. After cooling down to 45 °C, reduction was performed under 50 mL/min of 5 vol.% H₂/Ar from 45 to 800 °C at a rate of 10 °C/min. A trap cooled with isopropyl alcohol/liquid nitrogen slurry (−80 °C) was applied to remove water that could distort the TCD signal.

3.3. Catalytic Activity Tests

For each test, 150 mg of catalyst mixed with ~600 mg of silicon carbide was packed inside a U-shaped reactor (220 mm in length and 4 mm in internal diameter) with a bed height of 6 mm.

For the toluene oxidation tests, the reactant gas mixture, composed of 1000 ppm toluene and synthetic air (21 vol.% O₂+79 vol.% N₂), with a total flow of 100 mL/min, was fed into the reactor before being heated from room temperature to 150 °C (5 °C/min) and held at this temperature for 0.5 h to stabilize the system. Then, a second temperature ramp of 2 °C/min was run until 350 °C and held at this temperature for 1 h. Next, the reactor was cooled down to 150 °C (2 °C/min). Three consecutive heating–cooling catalytic cycles were performed to evaluate the catalytic stability. The concentrations of CO and CO₂ were in situ recorded by a Rosemount Xtreme Gas Infrared Analyzer (Emerson Electric Co., St. Louis, MO, USA). The toluene conversion was calculated as follows,

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

(1)

where [CO₂] and [C₇H₈] represent the outlet CO₂ concentration and the initial toluene concentration, respectively.

Regarding the propane oxidation, after 100 mL/min of the reactant gas mixture (0.1 vol.% propane +21 vol.% O₂+79 vol.% He) was introduced into the reactor at room temperature, the reactor was heated from room temperature to 100 °C (5 °C/min) and held at this temperature for 0.5 h to stabilize the system. Subsequently, the temperature was increased from 100 °C to 350 °C (2 °C/min) and held at this temperature for 1 h. Next, the reactor was cooled down to 100 °C (2 °C/min). Gas effluents were analyzed by an online micro gas chromatograph (SRA % GC-R3000) coupled with a thermal conductivity detector. The propane conversion was calculated as follows,

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

(2)

where [CO₂] and [C₃H₈] are the outlet CO₂ concentration and the initial propane concentration, respectively.

4. Conclusions

Cobalt oxides doped with Mn, Cu, Ni, or Fe were successfully synthesized via simple carbonate coprecipitation method. XRD and Raman analysis proved the formation of the cubic spinel phase Co₃O₄ after the incorporation of dopants. The Mn_0.05Co sample exhibited the smallest crystalline size and biggest strain. The MnO_x and FeO_x phases could be detected on the surface of Mn_0.05Co and Fe_0.05Co by FTIR spectra. The H₂–TPR study demonstrated the strong interaction between doped metal and cobalt. Cu_0.05Co appeared to be the most reducible sample. The catalytic activities were investigated for the total oxidation of toluene and propane. Mn_0.05Co exhibited undoubtedly the highest catalytic activity in both reactions. However, the activity for toluene and propane oxidation for the other catalysts was different. Cu_0.05Co and Ni_0.05Co were more active in toluene oxidation than pure Co₃O₄, whereas the opposite occurred for propane oxidation. Although the doping of Fe lowered the activity of pure Co₃O₄ in both cases, it allowed for the acquisition of better water-resisting catalysts for propane oxidation. Different mechanisms and different rate-determining factors are supposed to explain this activity variation as a function of the molecules to be oxidized.