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 Co
3O
4 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 Co
3O
4 catalyst with 50% propane conversion at 175 °C using a supercritical CO
2 anti-solvent precipitation method [
3]. Salek et al. obtained Co
3O
4 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 Co
3O
4 nanoparticles and obtained a Co
3O
4 catalyst with more abundant defects, surface Co
2+, and chemisorbed oxygen species, which presented a much higher activity than the commercial Pt/Al
2O
3 and Pd/Al
2O
3 catalysts [
5]. Analogously, Li et al. fabricated mesoporous Co
3O
4 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 Co
2+ and adsorbed oxygen species [
6]. Ren et al. constructed various 3D hierarchical Co
3O
4 nanocatalysts via a hydrothermal process and demonstrated that the hierarchical cube-stacked Co
3O
4 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 Co
3O
4 lattice with another metal cation, known as doping, is a potential way to improve the performance of Co
3O
4 catalysts. The chemical bonding at the surface of Co
3O
4 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 Co
3O
4 lattice enhanced propane oxidation reaction kinetics by promoting surface lattice oxygen activity and facilitating CO
2 desorption [
11]. The incorporation of Mn in Co
3O
4 increased surface Co
2+ 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 In
2O
3 induced the structural distortion of Co
3O
4 and promoted the catalytic performance of Co
3O
4 for CO oxidation [
13]. Baidya et al. demonstrated that 15% Fe-doped Co
3O
4 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.
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 M0.05Co, 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.% O
2+79 vol.% N
2), 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 CO
2 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,
where [CO
2] and [C
7H
8] represent the outlet CO
2 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.% O
2+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,
where [CO
2] and [C
3H
8] are the outlet CO
2 concentration and the initial propane concentration, respectively.