Mn-doped CeO2 flower-like microstructures have been synthesized by a facile method, involving the precipitation of metallic alkoxide precursor in a polyol process from the reaction of CeCl3·7H2O with ethylene glycol in the presence of urea followed by calcination. By introducing manganese ions, the composition can be freely manipulated. To investigate whether there was a hybrid synergic effect in CH4 combustion reaction, further detailed characteristics of Mn-doped CeO2 with various manganese contents were revealed by XRD, Raman, FT-IR, SEM, EDS, XPS, OSC, H2-TPR, and N2 adsorption-desorption measurements. The doping manganese is demonstrated to increase the storage of oxygen vacancy for CH4 and enhance the redox capability, which can efficiently convert CH4 to CO2 and H2O under oxygen-rich condition. The excellent catalytic performance of MCO-3 sample, which was obtained with the starting Mn/Ce ratios of 0.2 in the initial reactant compositions, is associated with the larger surface area and richer surface active oxygen species.
Metal oxide is an important catalyst support in many catalytic reactions and serves especially as a substitute for the transitional noble catalyst in the activation of hydrocarbon. Recently, as an important functional metal oxide, CeO2 has attracted increasing interest with respect to its tremendous applications in environmental catalysis due to its oxygen vacancy defects and high oxygen storage capacity associated with the facile Ce4+/Ce3+ redox cycle [
In general, the activity of a catalyst is associated with various structural factors, such as chemical composition, surface modifications, specific surface area, surface oxygen vacancies, and preferential exposure of reactive facets [
Herein, we report the synthesis of Mn-doped CeO2 flower-like microstructures via thermal decomposition of metallic alkoxide precursors, which were synthesized in ethylene glycol-mediated process. Preliminary CH4 catalytic oxidation experiments indicated that the Mn-doped CeO2 samples showed strikingly higher catalytic activity than pure CeO2, owing to the rich surface active oxygen species and large surface area.
All chemicals and solvents were of analytical grade and used as received without further purification or modification. Mn-doped CeO2 catalysts were synthesized by a facile polyol-based precursor and annealing method. The prepared catalysts were Mn-doped CeO2 powder with the Mn/Ce molar ratios of 0.1, 0.15, 0.2, and 0.3. As to the synthesis of Mn-doped CeO2 (marked as MCO-1, Mn:Ce = 0.1), typically, 0.04 g of Mn(CH3COO)2·4H2O, 0.6 g of CeCl3·7H2O, 0.5 g of CO(NH2)2, and 2 g of poly(vinylpyrrolidone) (PVP; K-30) were dissolved into 50 mL of EG in a 150 mL round flask to form a clear solution. The resulting solution was then heated to 180°C under reflux and constant stirring. The clear solution became opaque after 20 min and the reaction was stopped after refluxing for 1 h. Upon finishing the reaction, the sample was allowed to cool naturally, after which the final product was collected by centrifugation and washed with absolute ethanol several times and then dried in air at 80°C. Thermal decomposition of the precursors was achieved in muffle furnace at a heating rate of 6°C/min up to 450°C maintained for 2 h in static air.
Mn-doped CeO2 catalysts with different Mn contents (Mn:Ce = 0.15, 0.2, and 0.3, which were marked as MCO-2, MCO-3, and MCO-4, resp.) were prepared under identical conditions, in order to investigate the role of composition in the catalytic activities. For comparison, pure CeO2 was also prepared.
Phase purity was examined by X-ray diffraction (XRD) on a Bruker D8-Advance powder X-ray diffractometer with Cu K
The oxygen storage capacity (OSC) experiments were performed on a PCA-1200 instrument by pulse technique. The samples were pretreated at 550°C in flowing H2 for 45 min and then in flowing Ar for 20 min and subsequently cooled to 200°C in Ar atmosphere. The reduced samples were oxidized at 300°C with pulse of O2 periodically at an interval of 3 min until a constant value being obtained for the peak intensity. The O2-OSC values were determined by the amount of O2 consumed during the O2 pulse.
The temperature-programmed reduction measurements under a H2 environment (H2-TPR) were performed with a PCA-1200 instrument. Typically, the catalyst sample (30 mg) was pretreated under an O2 stream at 300°C for 30 min. After the sample cooled to room temperature, a flow of H2/Ar was introduced into the samples at a flow rate of 30 mL/min, and the temperature was increased to 900°C at a rate of 10°C/min.
The activity measurements were carried out in a continuous flow fixed-bed microreactor at atmospheric pressure. In the experiments, 200 mg of catalyst was loaded into a stainless steel reactor with a gas mixture typically containing 1% CH4, 21% O2, and 78% N2 at the flow rate of 23.4 mL/min and the space velocity of 4000 h−1 in the temperature range from 250 to 500°C. A portion of the product stream was extracted periodically from the reactor with an automatic sampling valve and analyzed using a gas chromatograph with a thermal conductivity detector.
To investigate whether there was a hybrid synergic effect in CH4 combustion reaction, we acquired detailed information on the structure and local atomic composition of the Mn-doped ceria with various Mn contents. Fundamental characteristics of the samples were revealed by FT-IR, XRD, SEM, EDS, XPS, Raman, BET, OSC, and H2-TPR.
A polyol-based precursor route was developed to synthesize the Mn-doped CeO2 samples with various Mn contents. Figure
Typical FT-IR spectrum of the as-obtained MCO-1 precursor.
XRD was employed to ascertain the change in bulk crystal structure for CeO2 support. Figure
Lattice parameters and the proportions of O′′ and Mn3+ of pure CeO2 and Mn-doped CeO2 samples calculated from XRD and XPS, respectively.
Samples | Lattice parameter (Å) | O′′/(O′′ + O′) (%) | Mn3+/(Mn2+ + Mn3+) (%) |
---|---|---|---|
Pure CeO2 | 5.4113 | 39.5 | — |
MCO-1 | 5.4059 | 44.7 | 44.9 |
MCO-2 | 5.3983 | 45.4 | 45.1 |
MCO-3 | 5.3938 | 50.6 | 56.1 |
MCO-4 | 5.4015 | 45.9 | 56.8 |
XRD patterns of Mn-doped CeO2 samples with different Mn contents.
Raman scattering is an effective tool for the investigation of the effects of doping on nanomaterials, as the incorporation of dopants leads to shifts of the lattice Raman vibrational peak positions. Figure
Raman spectra of pure CeO2 and Mn-doped CeO2 samples with different Mn contents. Peak positions (
The detailed morphologies and microstructures of the as-obtained Mn-doped CeO2 with different Mn contents were characterized by SEM. As shown in Figure
Actual Mn/Ce ratios of as-synthesized Mn-doped CeO2 samples based on the EDS results.
Samples | MCO-1 | MCO-2 | MCO-3 | MCO-4 |
---|---|---|---|---|
Starting Mn/Ce ratio | 0.1 | 0.15 | 0.2 | 0.3 |
Actual Mn/Ce ratio | 0.062 | 0.070 | 0.078 | 0.087 |
SEM images of the Mn-doped CeO2 samples with different Mn contents: (a) MCO-1, (b) MCO-2, (c) MCO-3, and (d) MCO-4 and (e) pure CeO2.
Typical EDS spectrum (a) and elemental mapping (b) of the as-synthesized MCO-1 sample.
XPS was performed in order to further illuminate the surface composition and the chemical state of the elements existing in Mn-doped CeO2 samples with various Mn contents. The XPS results suggest that the surface Mn/(Mn + Ce) ratios were rather low, even when the amount of initial Mn content in the raw material reached up to 30 mol% (surface Mn/(Mn + Ce) ≈ 0.072), which coincided with the EDS results. As displayed in Figure
XPS region spectra of (a) Ce 3d, (b) O 1s, and (c) Mn 2p for pure CeO2 (i) and Mn-doped CeO2 samples with different Mn contents: (ii) MCO-1, (iii) MCO-2, (iv) MCO-3, and (v) MCO-4.
The O 1s XPS spectra for the Mn-doped ceria samples early show the existence of two states of surface oxygen atoms (Figure
The Mn 2p XPS spectra of Mn-doped CeO2 samples are displayed in Figure
As discussed above, the MCO-3 sample contains more amount of Mn3+ species and oxide defects (O′′), which are expected to exhibit different reduction properties and catalytic oxidation activities subsequently. These results provide an evidence of existence of redox equilibrium of Mn3+ + Ce3+
The textural properties of the as-synthesized Mn-doped CeO2 samples were further investigated by measuring adsorption-desorption isotherms of N2 at 77 K, as shown in Figure
BET surface areas, average pore sizes, and total pore volumes of Mn-doped CeO2 with various Mn contents.
Samples | BET surface area (m2/g) | Average pore size (nm) | Total pore volume (cm3/g) |
---|---|---|---|
MCO-1 | 56.0 | 13.04 | 0.100 |
MCO-2 | 75.7 | 8.12 | 0.085 |
MCO-3 | 91.0 | 7.90 | 0.109 |
MCO-4 | 74.2 | 16.64 | 0.167 |
The N2 adsorption-desorption isotherms of Mn-doped CeO2 catalysts with various Mn contents: (a) MCO-1, (b) MCO-2, (c) MCO-3, and (d) MCO-4.
In redox catalysis, the role of ceria is usually to act as an oxygen transferring component. The oxygen storage capacity (OSC) and the reducibility are important characteristics to determine its catalytic properties. The OSC results for all samples are displayed in Table
Oxygen storage capacity (OSC) of pure CeO2 and Mn-doped CeO2 with various Mn contents.
Samples | Pure CeO2 | MCO-1 | MCO-2 | MCO-3 | MCO-4 |
---|---|---|---|---|---|
OSC ( |
89.5 | 111.5 | 133.0 | 166.2 | 155.4 |
H2-TPR was performed to explore the oxygen reactivity and reducibility of the pure ceria and Mn-doped ceria. Generally, the reducibility of oxide is associated with the formation of oxygen vacancies: the more susceptibly reducible an oxide is, the easier it can generate oxygen vacancies [
H2-TPR profiles of pure CeO2 and Mn-doped CeO2 catalysts with different Mn contents.
Catalytic oxidation of CH4 to CO2 and H2O under oxygen-rich condition plays an important role in energy supply using natural gas and here is used as a probe reaction to evaluate the catalytic performance of the Mn-doped CeO2 samples. Figure
The reaction temperatures at CH4 conversions of 10% (
Samples | Pure CeO2 | MCO-1 | MCO-2 | MCO-3 | MCO-4 |
---|---|---|---|---|---|
|
418 | 387 | 370 | 339 | 360 |
|
— | 457 | 446 | 405 | 432 |
(a) Plots of CH4 conversion versus temperature over pure CeO2 and Mn-doped CeO2 with various manganese contents; (b) catalytic stability test of Mn-doped CeO2 with various manganese contents.
Catalytic stabilities for CH4 oxidation were further investigated over these Mn-doped CeO2 samples at 450°C for 82 h. As shown in Figure
In summary, Mn-doped CeO2 flower-like microstructures have been successfully synthesized via a polyol-based precursor process at 180°C and subsequent direct thermal decomposition at 450°C. The resultant samples were used as catalysts in CH4 combustion under oxygen-rich condition. Catalytic results suggest that the Mn-doped CeO2 show relatively higher activity. The fundamental characteristics of the Mn-doped CeO2 samples with different Mn contents were revealed to investigate whether there was a hybrid synergic effect in CH4 combustion reaction. SEM results reveal that the doping Mn ions in CeO2 did not change the total flower-like morphology. BET, XPS, OSC, and H2-TPR results suggest that the higher surface area, more Mn3+, richer surface active oxygen species, and higher bulk oxygen mobility are responsible for the better performance of the MCO-3 sample. The as-obtained doped CeO2 should be a promising material for environmental application and is expected to be useful in many other application fields. It is also believed that this synthetic method may provide a powerful synthetic technology for the future application of binary/ternary metal oxide nano/microstructures.
The authors declare that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the National Natural Science Foundation of China (NSFC) (21306223) and the Fundamental Research Funds for the Central Universities (2012QNA09).