Effect of ceria morphology on the activity of MnO_x/CeO₂ catalysts for the catalytic combustion of chlorobenzene

Abstract

The effect of CeO₂ morphology on the performance of a MnO_x/CeO₂ catalyst was investigated for the catalytic combustion of chlorobenzene (CB), which was used as a model compound for chlorinated volatile organic compounds (CVOCs). The catalytic activity tests revealed that MnO_x/CeO₂ nanoparticles (NPS) achieved relatively higher CB conversions than MnO_x/CeO₂ nanorods (NR). The MnO_x/CeO₂ catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy, Brunauer–Emmett–Teller (BET) N₂ adsorption, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and hydrogen temperature-programmed reduction (H₂-TPR). The characterization of the MnO_x/CeO₂ catalysts indicated that CeO₂-NPS had a higher exposure of the (100) crystal plane and possessed more Mn⁴⁺ species, oxygen vacancies and surface-adsorbed oxygen. It was suggested that the CeO₂-NPS had a stronger interaction with MnO_x species, which resulted in greater catalytic activity in the combustion of CB. The catalytic activity of MnO_x/CeO₂-NPS could be attributed to higher concentrations of Mn⁴⁺ species, oxygen vacancies, and surface-adsorbed oxygen, which were associated with the exposed (100) crystal planes. Therefore, these results demonstrated that the catalytic performance of the MnO_x/CeO₂ catalyst was greatly affected by the CeO₂ morphology. Therefore, catalytic materials that have increased activity can be obtained through morphology-controlled synthesis.

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1. Introduction

Chlorinated volatile organic compounds (CVOCs) which are mainly generated from industrial processes, and medical waste are recognized as major contributors to air pollutants.^1,2 Generally, CVOCs can lead to photochemical smog and are highly toxic to humans and animals. Therefore, it is essential to develop practical and cost-effective methods to eliminate CVOCs in gases. Of the available techniques, catalytic combustion is one of the most effective technologies for the removal of destruction of CVOCs emissions due to its high removal efficiency and low operating temperature.

Among the catalysts used for the catalytic combustion of CVOCs, manganese oxides are known to be active catalysts on account of their high oxygen storage ability and redox properties.^3–8 Furthermore, catalysts based on MnO_x supported on metal oxides, such as MnO_x/CeO₂ and MnO_x/TiO₂ catalysts, have been reported to display higher catalytic performance for the oxidation of CVOCs compared with MnO_x catalysts alone.^9–11 Accordingly, the nature of the support material is critical to the catalytic performance of MnO_x-based catalysts.^12–15 In particular, the support material ceria has been widely utilized for the MnO_x-based catalyzed oxidation of chlorobenzene (CB) due to its high oxygen storage capacity (OSC) and reducibility in the oxidation of CVOCs. Reports have shown that the capacity for oxygen storage is dependent on the morphology of ceria.^10,16,17 Thus far, several studies document the shape effects of ceria in CO oxidation, ethanol steam reforming, and NO reduction with NH₃. Huang and co-workers¹⁸ investigated the catalytic performance of various Au-supported ceria shapes for CO oxidation. They found that predominantly exposed (100)/(110) surface structures of ceria nanorods are superior for dispersing and stabilizing Au nanoclusters. This leads to a higher redox activity for CO oxidation, while relatively lower reducing the dispersal of Au nanoclusters on the exposed (111) plane for ceria nanoparticles, as determined by its lower activity for CO oxidation. Soykal and co-workers¹⁶ also indicated that CeO₂ nanocubes were more active for ethanol steam reforming than ceria nanorods due to the presence of a higher exposure of the (110) plane for nanocubes.

Additional studies revealed that MnO_x/CeO₂ catalysts exhibited excellent catalytic performance in the combustion of CB. However, to the best our knowledge, little attention has been given to the morphology dependence of ceria nanomaterials in Mn species deposited on CeO₂ for the catalytic combustion of CB. Additionally, various ceria morphology supports may possess different capacities for oxygen storage,¹⁹ which can then influence the catalytic performance of Mn species. Herein, we present the effect of ceria morphology on a Mn-based catalytic system for the catalytic combustion of CB. Ceria nanorods and ceria nanoparticles were synthesized by hydrothermal and co-precipitation methods, respectively. A wet impregnation method was utilized to incorporate manganese oxides over CeO₂ nanomaterials. The catalytic performance of MnO_x/CeO₂ nanomaterials with two different shapes was examined in the catalytic combustion of CB.

2. Experimental section

2.1 Preparation

CeO₂ nanorods were prepared by the hydrothermal method as follows.²⁰ Ce(NO₃)₃·6H₂O (3.45 mmol) was dissolved in deionized water (5 mL) and mixed with an aqueous sodium hydroxide aqueous solution (C_NaOH = 6 M). The mixture was then stirred at room temperature for 30 min to obtain a milky slurry and subsequently transferred to a 50 mL autoclave, sealed, and heated at 100 °C for 24 h. After the hydrothermal treatment, the resulting precipitate was filtered, washed thoroughly with deionized water, and dried at 60 °C, and finally, calcination occurred at 400 °C for 4 h. The CeO₂ nanoparticles were prepared by the precipitation method that has been reported in the literature.²¹ Ce(NO₃)₃·6H₂O was dissolved in distilled water, and the pH value of the solution was rapidly adjusted to pH 12 with a 10% NaOH solution with stirring. The precipitate was filtered, washed with deionized water, and dried at 60 °C for 12 h, and then, calcination occurred at 400 °C for 4 h. The synthesized CeO₂ nanorods and CeO₂ nanoparticles were denoted as CeO₂-NR and CeO₂-NPS, respectively.

MnO_x/CeO₂ catalysts with different morphologies were prepared by incipient wetness impregnation. Manganese was loaded onto the CeO₂ support with 50% Mn(NO₃)₂. The manganese loading was selected as 11 wt% (Mn/(Ce + Mn) = 0.27, molar ratio). Following impregnation, the catalysts were aged at room temperature for 24 h and dried at 80 °C in air overnight, and calcination took place at 400 °C for 4 h. The MnO_x/CeO₂ nanorods and MnO_x/CeO₂ nanoparticles were denoted as MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS, respectively.

2.2 Characterization techniques

Transmission electron microscopy (TEM) tests were recorded on a JEM-2100 electron microscope operating at 200 kV. The samples were ultrasonically dispersed in ethanol and deposited on copper grids coated with lacey carbon. Energy dispersion spectra (EDS) were performed on a FALCON 80 X-ray energy-dispersive spectrometer that was used for the elemental analysis of the samples. X-ray diffraction (XRD) patterns of the MnO_x/CeO₂ and bare CeO₂ were collected on a Bruker D8 Advance X-ray diffractometer using Cu Kα radiation (λ = 1.5405 Å) operated at 40 kV and 50 mA. The diffraction patterns were taken in the 2θ angle of 20–70°. The BET surface areas of the catalysts were obtained from N₂ adsorption and desorption tests at 77 K using a NOVA2000e analyzer. The samples were degassed at 120 °C for 12 h before measurements were taken. The Raman spectra were recorded on a laser confocal microscopy Raman spectrometer (DXR, American Thermo Electron) operated at a wavelength of 532 nm. XPS measurements were carried out with a VG Multilab 2000 spectrometer using Al Kα (hν = 1253.6 eV) radiation. The TPR runs were conducted with a linear heating rate (10 °C min⁻¹) in a flow of 10% H₂ in argon at a flow rate of 40 mL min⁻¹. Hydrogen consumption was measured quantitatively by a thermal conductivity detector (TCD), which was calibrated by quantitative reduction of a given quantity of CuO to metallic copper.

2.3 Catalytic activity tests

Catalytic activity tests were performed in a fixed-bed flow reactor made of an 8 mm inner diameter quartz tube under an atmospheric pressure between 100–400 °C. Two-hundred milligrams of the (50–80 mesh) catalysts were placed in the reactor. The feed gas consisted of 5000 ppm CB, 20% O₂, and balanced N₂. The total flow rate was 100 mL min⁻¹, with the gas hourly space velocity (GHSV) at 20 000 h⁻¹. The effluent was analyzed by an on-line gas chromatograph (GC) equipped with a flame ionization detector (FID) for the quantitative analysis of CB.

3. Results and discussion

3.1 XRD and N₂ adsorption results

The BET surface areas of the supports and MnO_x/CeO₂ nanostructures are listed in Table 1. The BET surface area of CeO₂-NR and CeO₂-NPS were 113.4 and 127.7 m² g⁻¹, respectively. After loading MnO_x, the specific surface areas of the MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS decreased to 79.5 and 82.4 m² g⁻¹. Compared with the BET surface area of CeO₂-NR and CeO₂-NPS, a decrease in the specific surface areas of MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS can be attributed to pore blockage by MnO_x.²²

Table 1 The properties of the different catalysts

Sample	Surface area (m^2 g^−1)	D (CeO2) (nm)/by XRD^a	D (CeO2) (nm)/by TEM
a From the Scherrer equation, based on the (111) reflection of the CeO2.
CeO2-NR	113.4	—	—
CeO2-NPS	127.7	—	—
MnOx/CeO2-NR	79.5	15.1	13–90
MnOx/CeO2-NPS	82.4	9.8	10 ± 2

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X-ray diffraction patterns of MnO_x/CeO₂ nanostructures with different morphologies are shown in Fig. 1. All of the diffraction peaks were attributed to the cubic phase of CeO₂ (JCPDS 34-0394), but no peaks of manganese oxides were observed, indicating a high dispersion of MnO_x on the CeO₂ nanostructures. Moreover, diffraction peaks of MnO_x/CeO₂-NR were broader compared with those of MnO_x/CeO₂-NPS, suggesting the formation of a smaller size of MnO_x/CeO₂-NPS. CeO₂ crystallite sizes were calculated by the Scherrer equation, and the results (Table 1) indicated that CeO₂-NR and CeO₂-NPS were 15.1 and 9.8 nm, respectively, which are in agreement with the TEM results. Table 2 shows the peak intensities normalized with respect to the peak intensity of the (111) plane of ceria for the MnO_x/CeO₂-NPS and MnO_x/CeO₂-NR. As shown in Table 2, the (220) plane for both catalysts possessed similar relative intensities, while the (200) plane showed significantly higher diffraction for MnO_x/CeO₂-NPS, with a 45% relative intensity compared to 30% for MnO_x/CeO₂-NR. This indicates that the CeO₂-NPS morphology may expose more of the (100) plane compared with CeO₂-NR.

Fig. 1 XRD patterns of (a) CeO₂-NR, (b) CeO₂-NPS, (c) MnO_x/CeO₂-NR, and (d) MnO_x/CeO₂-NPS.

Table 2 XRD intensities for MnO_x/CeO₂ catalysts normalize with respect to CeO₂ (111) reflection

2θ	28.5	33.0	47.5	56.3
Respective planes	CeO2 (111)	CeO2 (200)	CeO2 (220)	CeO2 (311)
MnOx/CeO2-NR	100%	30%	56%	40%
MnOx/CeO2-NPS	100%	45%	57%	45%

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Si and co-workers reported that the formation energies for oxygen vacancies for ceria oxide surface follow the sequence (111) > (100).²³ Therefore, the (100) plane more readily form oxygen vacancies. It is also conceivable that the introduction of oxygen vacancies assists in improving catalyst performance significantly.¹⁶ Therefore, more (100) plane on the MnO_x/CeO₂-NPS may help improve the catalytic performance.

3.2 TEM and HRTEM results

The TEM images, structural models, and EDS spectra of synthesized materials are shown in Fig. 2. The two different CeO₂ nanomaterials maintain their original crystal shapes after deposition of MnO_x. MnO_x/CeO₂-NR has a uniform diameter of 10 ± 3 nm and a length within 13–90 nm. Fig. 2b shows the HRTEM image of MnO_x/CeO₂-NR. It can be observed that MnO_x/CeO₂-NR exposed the (100) and (111) planes, with an interplanar spacing of 0.27 nm and 0.31 nm, respectively. The structural model of the MnO_x/CeO₂-NR is shown in Fig. 2c. The single crystal nanorod grew along the [110] direction. Fig. 2c shows the MnO_x/CeO₂-NR hexangular prism enclosed by four (111) and two (100) planes.²⁰ The TEM image of MnO_x/CeO₂-NPS (Fig. 2e) reveals that the nanoparticles have a size of 10 ± 2 nm. The structural model of the MnO_x/CeO₂-NPS shows that MnO_x/CeO₂-NPS is approximately determined to be the truncated octahedra which was surrounded by eight (111) and six (100) planes.²⁴ The EDS spectra in Fig. 2d and h shows that the actual content of Mn element for MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS were 11.39 wt% and 16.50 wt%, respectively. In addition, the EDS spectra in Fig. 2d and h confirms the presence of Mn and suggests that the MnO_x species are highly dispersed on the surface of the CeO₂ nanostructure.

Fig. 2 TEM and HRTEM images of (a and b) MnO_x/CeO₂-NR, (e and f) MnO_x/CeO₂-NPS; the structural model of (c) MnO_x/CeO₂-NR, (g) MnO_x/CeO₂-NPS; EDS patterns of (d) MnO_x/CeO₂-NR, (h) MnO_x/CeO₂-NPS.

3.3 H₂-TPR results

Fig. 3 displays the H₂-TPR profiles of the CeO₂ and MnO_x/CeO₂ nanostructures. For CeO₂-NR, the reduction of CeO₂ starts at 350 °C and a broad peak centered at 490 °C, which is attributed to the reduction of the surface oxygen of CeO₂.²⁵ Meanwhile, CeO₂-NPS shows a similar reduction profile at a slightly higher temperature (540 °C), while the MnO_x/CeO₂ nanostructures showed different profiles of the reduction peaks. MnO_x/CeO₂-NR showed three distinct reduction peaks, with a shoulder peak at 368 °C. The peak at 227 °C could be attributed to reduction of MnO₂ to Mn₂O₃, and the peak at 302 °C, with a shoulder peak at 368 °C, was a further reduction of Mn₂O₃ to Mn₃O₄, while the peak at 475 °C ascribed to the combined reductions of Mn₃O₄ to MnO and surface oxygen removal of ceria.^24,26–30 A similar reduction profile was observed for MnO_x/CeO₂-NPS, except that the reduction peaks shifted to higher temperature regions, indicating that interaction between MnO_x and CeO₂-NPS is stronger than that with CeO₂-NR. Quantitative evaluation of the reduction peaks (Table 3) revealed that the H₂ consumption of the first two peaks on the MnO_x/CeO₂-NPS is greater than the MnO_x/CeO₂-NR, demonstrating that MnO_x/CeO₂-NPS has more surface oxygen and that a larger quantity of strongly interactive MnO_x species exist on the CeO₂-NPS.²⁶ Therefore, the more surface oxygen on CeO₂-NPS and the stronger interaction between MnO_x and CeO₂-NPS, result in the higher oxidative ability of the MnO_x species.³¹

Fig. 3 H₂-TPR profiles of (a) CeO₂-NR, (b) CeO₂-NPS, (c) MnO_x/CeO₂-NR, and (d) MnO_x/CeO₂-NPS.

Table 3 Hydrogen consumption of H₂-TPR tests

Sample	T (°C)	mmol g^−1	T (°C)	mmol g^−1
MnOx/CeO2-NR	227	0.77	302	0.89
MnOx/CeO2-NPS	245	0.86	331	1.14

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3.4 Raman spectra

The Raman spectra of CeO₂ and MnO_x/CeO₂ nanostructures are presented in Fig. 4. A distinct F_2g symmetry mode of the CeO₂ phase at approximately 462 cm⁻¹ can be observed in the Raman spectra of the CeO₂ nanostructures (Fig. 4a).³² Furthermore, two additional weak peaks, at approximately 600 and 1180 cm⁻¹, can also be observed in the Raman spectra of CeO₂ nanomaterials (Fig. 4a). These are attributed to the defect-induced (D) mode and the second-order longitudinal (2LO) mode, respectively.²⁸ A similar Raman spectrum was observed for MnO_x/CeO₂ nanostructures (Fig. 4b), except that the peak at 600 cm⁻¹ for oxygen vacancies displayed a blue shift to 650 cm⁻¹, and the peak at 1180 cm⁻¹ shifted to 1170 cm⁻¹. These shifts can be attributed to the Mn incorporation in the CeO₂ lattice. Additionally, the new peak at 577 cm⁻¹ can be assigned to the Mn–Ce–O bond on the MnO_x/CeO₂ nanostructures.

Fig. 4 Raman spectra of (a) CeO₂, (b) MnO_x/CeO₂ nanostructures, and (c) the peak intensity ratios of I₆₀₀/I₄₀₀ over CeO₂ (black column) and MnO_x/CeO₂ nanostructures (red column).

The Raman peak at 600 cm⁻¹ was related to the oxygen vacancies induced by the presence of Ce³⁺ ions, and the I₆₀₀/I₄₆₂ value (Fig. 4c) reflects the degree of the defect sites on nanostructures, such as the relative content of the oxygen vacancies.^32,33 From Fig. 4c, the intensity ratio I₆₀₀/I₄₆₂ follows the sequence: MnO_x/CeO₂-NPS > MnO_x/CeO₂-NR > CeO₂-NPS > CeO₂-NR, indicating that MnO_x/CeO₂-NPS possessed the greatest amount of oxygen vacancies. According to Fig. 4c, the I₆₀₀/I₄₆₂ value increased dramatically when manganese ions were doped into the ceria lattice. This indicated that Mn ions incorporated into the CeO₂ phase and thus led to an increase in the degree of defects. Clearly, MnO_x/CeO₂-NPS exhibited the higher value of I₆₀₀/I₄₆₂ compared to the MnO_x/CeO₂-NR nanostructure, demonstrating that the interaction between MnO_x and CeO₂-NPS is stronger than with CeO₂-NR. In summary, the Raman results reflect that the stronger interaction between MnO_x and CeO₂-NPS, and in turn, MnO_x/CeO₂-NPS incorporated into a greater oxygen vacancies, which is in accordance with the H₂-TPR result.

3.5 XPS results

The Mn 2p, O 1s, and Ce 3d XPS spectra of the MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS catalysts are shown in Fig. 5. As shown in Fig. 5a, the Mn 2p_3/2 spectra can be separated into three peaks. The binding energy at ca. 640.5, 641.8, and 643.7 eV corresponded to Mn²⁺, Mn³⁺, and Mn⁴⁺,³⁴ respectively. The concentration of Mn⁴⁺ on MnO_x/CeO₂-NPS is higher than on MnO_x/CeO₂-NR, as shown in Table 4. Liao and co-workers²⁶ reported that high Mn⁴⁺ could lead to oxygen vacant sites on catalysts. Accordingly, a higher ratio of Mn⁴⁺ on the CeO₂-NPS may lead to more oxygen vacancies.

Fig. 5 (a) Mn 2p, (b) Ce 3d and (c) O 1s XPS spectra of MnO_x/CeO₂ nanostructures.

Table 4 Surface chemical and states based on XPS analysis^a

Sample	Mn^4+ (%)^a1	Mn^3+ (%)^a2	Mn^2+ (%)^a3	Ce^3+ (%)	Olatt (%)	Oads (%)	Owat (%)
a a1 + a2 + a3 = 100%.
MnOx/CeO2-NR	15.6	38.8	45.5	26.4	40.0	23.0	36.9
MnOx/CeO2-NPS	21.1	37.0	41.9	29.0	58.0	31.0	6.9

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The Ce 3d spectra of the MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS are shown in Fig. 5b. The peaks at approximately U, U′′, U′′′, V, V′′, and V′′′ are attributed to Ce⁴⁺, and the Ce³⁺ can be fitted with the peaks at U′ and V′. The concentrations of Ce³⁺ on the MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS are 26.4%, and 29.0%, respectively. Liu and co-workers³⁵ suggested that a higher Ce³⁺ concentration can produce more oxygen vacancies on catalysts. The spectra indicates that MnO_x/CeO₂-NPS contain more oxygen vacancies than MnO_x/CeO₂-NR.

The O 1s spectra of MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS are shown in Fig. 5c. The O 1s profile can be fit into three peaks: lattice oxygen (O_latt) at 529.4–530.0 eV, surface-adsorbed oxygen (O_ads) at 531.0–531.7 eV and adsorbed molecular water (O_wat) at approximately 532.7 eV.³⁶ Quantitatively, it can be observed in Table 4 that the O_ads/O_wat + O_ads + O_latt ratio of MnO_x/CeO₂-NPS catalyst is higher than MnO_x/CeO₂-NR, indicating that MnO_x/CeO₂-NPS catalyst are rich in surface-adsorbed oxygen. Because O_ads is more active than O_latt due to its high mobility, the higher concentration of O_ads is helpful for the CB oxidation.³⁷

3.6 Characteristic of the used catalysts

The morphologies of the used MnO_x/CeO₂-NR and MnO_x/CeO₂-NPS catalysts were observed on TEM images in Fig. 6. The used MnO_x/CeO₂-NR has a uniform diameter of 11 ± 3 nm and a length within 13–95 nm while the MnO_x/CeO₂-NPS have a size of 9 ± 2 nm. This indicates that the grain size of the used catalysts basically remains unchanged after reaction. The catalysts still maintain their shapes after reaction, indicating that the MnO_x/CeO₂ nanostructures have structure stability during CB combustion.

Fig. 6 TEM images of the used catalysts: (a) MnO_x/CeO₂-NR, (b) MnO_x/CeO₂-NPS.

The XRD patterns of the used MnO_x/CeO₂ nanostructures are shown in Fig. 7. All typical diffraction peaks can be attributed to the ceria fluorite structure (JCPDS 34-0394), and no peaks of MnO_x were observed. This phenomenon demonstrates that a strong interaction between the MnO_x and CeO₂ nanostructures exists, which prevents the agglomeration of MnO_x during CB combustion.²⁸

Fig. 7 XRD patterns of the used catalysts: (a) MnO_x/CeO₂-NR, (b) MnO_x/CeO₂-NPS.

3.7 Catalytic activity and stability of catalysts

The catalytic performance of the bare CeO₂ supports and MnO_x/CeO₂ catalysts for the catalytic combustion of CB is shown in Fig. 8. As shown in Fig. 8, both CeO₂-NPS and CeO₂-NR supports exhibit poor activity for catalytic combustion of CB, although the CeO₂-NPS is slightly more active. After loading Mn, the activities of the Mn-supported catalysts were markedly enhanced, revealing the necessity for Mn in the catalytic combustion of CB. Additionally, a significant effect of support morphology on catalyst performance can be observed in Fig. 8. The MnO_x/CeO₂-NPS catalyst displayed higher activity than MnO_x/CeO₂-NR. At 275 °C, the CB conversions was 90.0% over MnO_x/CeO₂-NPS, while only 71.7% over MnO_x/CeO₂-NR. This indicates that MnO_x/CeO₂-NPS catalysts showed a better catalytic activity than the MnO_x-CeO₂ mixed oxides (90% conversion at 300 °C, Mn/(Mn + Ce) = 0.27, molar ratio) reported by Wang et al.¹

Fig. 8 MnO_x/CeO₂ catalysts for CB combustion, gas composition: 2500 mg m⁻³ CB, 20% O₂, N₂ balance; GHSV = 20 000 h⁻¹.

Because the obtained MnO_x/CeO₂ catalysts have different exposed planes, this may be one possible explanation for the observed difference in catalytic performance of the two catalysts. Because CeO₂-NPS support preferentially exposes the (100) plane and this plane is more likely to create anion vacancies and defects, it is reasonable that this support offers more oxygen vacancies.¹⁶ As detected by the Raman and XPS results, the MnO_x/CeO₂-NPS sample has more oxygen vacancies. Previous reports on this subject have described that the introduction of oxygen vacancies on support material were conducive to adsorbing oxygen and forming highly reactive atomic oxygen, which would significantly improve the catalyst performance.^21,38 Therefore, additional oxygen vacancies on the CeO₂-NPS support are a contributing factor affecting the catalytic activity in CB oxidation. In addition, the higher ratio of O_ads and stronger interaction between the Mn and the ceria support may be two additional factors to improve the catalytic performance. In conclusion, the performance of CeO₂-NPS might benefit from a stronger interaction between Mn and the ceria support, higher concentrations of oxygen vacancies, and surface-adsorbed oxygen.

Extended time stability tests of MnO_x/CeO₂ nanostructures with different support morphologies are shown in Fig. 9. Both catalysts show good stability after 30 h at 300 °C. Therefore, the two catalysts show good resistance to chlorine poisoning from CB.

Fig. 9 The stability tests over the MnO_x/CeO₂ nanostructures for CB combustion at 300 °C; gas composition: 2500 mg m⁻³ CB, 20% O₂, N₂ balance; GHSV = 20 000 h⁻¹.

4. Conclusion

Manganese catalysts loaded on a ceria support with two different morphologies, MnO_x/CeO₂ nanoparticles (NPS) and MnO_x/CeO₂ nanorods (NR), were examined for their performance on the catalytic oxidation of CB. The morphology of the ceria support was observed to have a strong effect on the catalytic oxidation of CB. The MnO_x/CeO₂-NPS catalyst presented better catalytic activity compared to the MnO_x/CeO₂-NR catalyst. HRTEM and XRD analysis indicated that the CeO₂-NPS support preferentially exposed the (100) plane and are more likely to form anion vacancies and defects. The XPS and Raman results displayed a morphology dependence on the concentration of Mn⁴⁺ species, surface-adsorbed oxygen, and oxygen vacancies. Higher catalytic activity of MnO_x/CeO₂-NPS may be attributed to abundant Mn⁴⁺ species, oxygen vacancies and surface-adsorbed oxygen, which are associated with exposed (100) crystal planes. Furthermore, the extended time stability tests showed that both catalysts demonstrate resistance to chlorine poisoning. Thus, MnO_x/CeO₂-NPS catalysts can be considered as a potential material for the abatement of CVOCs.

Acknowledgements

We gratefully acknowledge the funding from the National Natural Science Foundation of China (no. 20873097, 21071113, 21471120), Natural Science Foundation of Hubei Province (no. 2011CDA049), International Cooperation Foundation of Hubei Province (2012IHA00201), Educational Commission of Hubei Province of China (T201306).

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