Low temperature catalytic combustion of o-dichlorobenzene over supported Mn–Ce oxides: effect of support and Mn/Ce ratio

Abstract

A series of Mn–Ce mixed oxide catalysts supported on cordierite, TiO₂ or γ-Al₂O₃ carriers were tested for their catalytic performance towards hazardous gaseous o-dichlorobenzene combustion. The results indicated that superficial MnO_x species vary with carriers such as Mn₂O₃ on the cordierite, Mn₃O₄ on the TiO₂ and amorphous MnO_x on the Al₂O₃ support. The catalysts supported on cordierite with a 9 : 1 Mn/Ce the ratio presented high catalytic performance in a low temperature range of 150–300 °C, which is due to their better reducibility, well-dispersed manganese oxide and enrichment of available active oxygen species on the catalysts surface.

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1. Introduction

Chlorinated volatile organic compounds (CVOCs) are emitted to the atmosphere in waste gases from a wide range of industrial processes, and incineration of municipal and medical waste. Catalytic oxidation of CVOCs to carbon dioxide, HCl, and water is the best method for their destruction and V₂O₅/TiO₂ catalyst is one of the most commonly used catalysts.^1,2 However, the required operating temperature for V₂O₅/TiO₂ catalyst is typically 300–400 °C.^3,4 Therefore, it is necessary to develop a superior catalyst with high activity at low temperature. Manganese oxides (MnO_x) have been studied extensively⁵ as low-temperature CVOCs catalysts because a good dispersion of manganese oxides is important to the chlorobenzene catalytic oxidation. Ceria is also widely used as a CVOCs catalyst because of its oxygen storage and redox properties,^6,7 but CeO₂ deactivates quickly due to strong adsorption of HCl or Cl₂ produced from CVOCs decomposition on the surface to block the active sites. However, it was found that if chlorine or chloride ions adsorbed on the surface were removed or transferred rapidly enough, the CeO₂ catalyst stability could be improved. Wu and co-workers^8–10 developed a Mn–Ce mixed-oxide catalyst and found it to be an excellent low-temperature CVOCs catalyst.

Wang^11,12 found that MnO_x–CeO₂ catalysts with high Mn ratios presented highly stable activity. Zhu et al.^13,14 have developed a post-plasma catalysis system for removing methanol over Mn–Ce oxide catalysts with different Mn/Ce molar ratios at low temperatures. They found that Mn50Ce50 catalyst exhibited the best performance among the tested Mn–Ce catalysts. Therefore, it is very important to investigate the effect of the ratio rate of Mn/Ce on the catalysts properties. For industrial applications, active catalysts are usually supported on different carriers, such as TiO₂ (ref. 12, 15 and 16) and Al₂O₃,^17,18 which possess profound surface acid–base properties and provide high surface area, high thermal stability and strong mechanical strength.^19,20 Research and our previous studies have shown that the catalytic properties of supported catalysts remarkably vary with the carrier nature.^9,21–29 Comparative studies of manganese oxide catalysts supported on TiO₂, Al₂O₃, and SiO₂ had been performed for chlorobenzene oxidation by Liu et al.³⁰ They found that the activities of these supported manganese oxide catalysts were quite different. Jin³¹ found Mn–Ce oxides supported on TiO₂ and Al₂O₃ showed different selective catalytic reduction (SCR) activity at a different temperature range due to a different reaction pathway. Liu et al.²⁹ compared three different iron-containing catalysts supported on alumina, titania and beta zeolites and also found their different SCR performance. Therefore, it is very important that the effect of the carrier on the catalytic performance. To better understand the catalytic reaction of o-dichlorobenzene (o-DCB) over the supported Mn–Ce oxides catalysts, we studied the effect of carriers supported MnO_x–CeO₂ oxides and the atomic ratio of Mn/Ce on the complete catalytic oxidation of o-DCB. In addition, the possible cause of catalyst with the best activity was discussed based on the data from XRD, BET, H₂-TPR and XPS.

2. Experimental

2.1 Materials preparation

The Mn–Ce mixed oxides catalysts supported on different carriers such as cordierite (2MgO·Al₂O₃·2SiO₂) ceramics and TiO₂ and γ-Al₂O₃ powders, were prepared by the sol–gel method. The weight ratio of Mn plus Ce to supports was 25%. The atomic ratio of citric acid/(Mn + Ce) was 0.3, the atomic ratios of Mn/Ce of 3, 9, and 18 were designed. 25 mL of an aqueous solution containing a given amount of Mn and Ce nitrate, and citric acid was gradually heated to 80 °C and maintained at this temperature for 2 h with stirring, resulting in the formation of a yellowish sol, then 5 g of support was added with appropriate amounts of yellowish sol, then dried at 110 °C for 12 h, and calcined in air at 550 °C for 5 h. Finally, various catalysts were obtained. In the text or figures, x and 1 in Mn_xCe₁ indicate the mole proportion of Mn and Ce.

2.2 Characterization

The catalysts phase structure was determined using a D8-ADVANCE X-ray diffractometer (XRD) with Cu Kα radiation (Cu Kα = 0.15406 nm). X-ray photoelectron spectroscopy (XPS) was carried out with a Thermo ESCALAB 250 spectrometer using a monochromatic Al Kα (hν = 1486.6 eV) source. Charging of samples was corrected by setting the binding energy of adventitious carbon (C 1s) to 284.6 eV. All samples were not heated to remove possible surface hydroxides before analysis to avoid oxide reduction. The spectra were deconvoluted using the XPSPEAK program by curve fitting with a 90/10 Gaussian/Lorentian ratio after smoothing and subtraction of the Shirley-type background. H₂-temperature programming reduction (H₂-TPR) was investigated by heating samples (150 mg) in H₂ (5 vol%)/Ar flow (30 mL min⁻¹) at a 10 °C min⁻¹ heating rate from 50 to 700 °C. Hydrogen consumption was monitored with a thermoconductivity detector.

2.3 Catalytic activity

Catalytic experiments were performed with fixed-bed reactors made in-house working at atmospheric pressure. About 2 g of powder catalyst was used in each run. Gaseous o-DCB flow was introduced into the reactor by airflow passage through a vessel containing liquid o-DCB at constant temperature. Air was used both as the carrier gas and oxygen source. A K-type thermocouple was placed into the catalyst bed to monitor the reaction temperature. The feed flow through the reactor was set with a 500 ppm concentration of o-DCB, which was carried by air and metered by a mass flow controller. The total volumetric flow through the catalysis system was held constant at 150 mL min⁻¹ for all the measurements. The catalytic reaction was run from 150 to 300 °C by 50 °C steps, conversion was measured typically 30 min after a desired temperature was arrived at. The content of the o-DCB in the reactor inlet and outlet streams were sampled using acetone and analyzed by the gas chromatographic method by means of an FID detector. Each sampling was repeated twice and analyzed three times to assure reproducibility. The recycling experiments for understanding the catalysts stability were performed for longer reaction periods after evaluation of activity of the fresh catalysts under the same conditions with the continuous feeding the stream containing 500 ppm of o-DCB and maintaining a 150 mL min⁻¹ volumetric flow rate.

3. Results and discussion

3.1 Catalytic activity measurement

Fig. 1 shows catalytic activities of the various catalysts for o-DCB combustion as a function of temperature. It is important to stress that no other aromatic by-product was detected by GC-MS chromatography. Of those catalysts, Mn_xCe₁/cordierite had the best activity for o-DCB conversion at 200 °C. The conversions of o-DCB above 80% for Mn_xCe₁/cordierite were higher than that of 33% and 18% for Mn₉Ce₁/Al₂O₃ and Mn₉Ce₁/TiO₂, respectively. By contrast, Mn₉Ce₁/Al₂O₃ performed better combustion of o-DCB than Mn₉Ce₁/TiO₂. It could be observed that conversion of o-DCB depended on support materials and the cordierite was the best support for Mn–Ce oxide catalyst.

Fig. 1 Catalytic activities of various catalysts for o-DCB combustion.

The conversions of o-DCB over the Mn_xCe₁/cordierite catalysts were slightly different over the whole temperature range. There was an optimal Mn/Ce ratio. It was found that Mn₉Ce₁/cordierite catalyst showed a superior activity than the other Mn/Ce ratio catalysts. The conversion of o-DCB at 200 °C increased from 80% with an 18 : 1 Mn/Ce ratio in a 3 : 1 ratio with catalyst to 85% with a 9 : 1 Mn/Ce ratio to catalyst employed. In a word, Mn_xCe₁/cordierite catalysts present high activity for the low temperature combustion of o-DCB and Mn₉Ce₁/cordierite catalyst was considered as the most active catalyst, of which its T₈₅ (temperature at which 85% conversion was achieved) values for o-DCB combustion was 194 °C. The possible cause was explained from the following catalyst characterization.

3.2 Catalysts characterization

The XRD patterns of the catalysts are shown in Fig. 2. The main diffraction peaks almost corresponded to the characteristic peaks of various supports. Some slight differences could also be found from the XRD results. For Mn_xCe_1/cordierite catalysts, the weak diffraction peak at 2θ 33.8° was ascribed to Mn₂O₃, while for the MnO_x phase in Mn₉Ce₁/TiO₂ was Mn₃O₄. For Mn₉Ce₁/Al₂O₃ catalysts, no signals of MnO_x species appeared, indicating that MnO_x was amorphous or highly dispersed on the catalysts surface. As reported by Tang et al.,³² the phase composition of manganese and cerium mixed oxides strongly depended on the molar ratios of manganese and cerium oxides. Because the atomic ratio of Mn/(Mn + Ce) was designed to be larger than 0.75, it is highly possible to form crystallized Mn₂O₃ during the calcinations process. This agrees with Tang's report for catalysis support on the cordierite that was not applicable to the Al₂O₃ and TiO₂ supports. It could be concluded that the superficial MnO_x phases were influenced by the support and the Mn₂O₃ phase on the catalyst surface may be contributing to better performance for Mn_xCe/cordierite catalysts. In general, as a catalyst support, cordierite could provide the full availability of its surface compared to the Al₂O₃ or TiO₂ powders, which favours full contact between the MnO_x phase and the o-DCB molecule, leading to better performance for removal of o-DCB over Mn_xCe/cordierite catalysts.

Fig. 2 XRD patterns of various catalysts.

The reduction profiles for mixed Mn–Ce oxides catalysts, as well as for catalysts with different Mn/Ce ratios are shown in Fig. 3. The oxides behave different reducibility with various supports and Mn/Ce ratios. H₂-TPR profile on Mn₉Ce₁/cordierite showed two partly overlapped strong reduction peaks with maxima at 310 and 400 °C. It is reasonable to propose that the peak at low temperature could be assigned to Mn₂O₃ to Mn₃O₄ reduction and the one at high temperature to Mn₃O₄ to MnO reduction.^33–35 Compared with the Mn₉Ce₁/cordierite catalyst, a part of the oxide for the catalysts with 18 : 1 and 3 : 1 Mn/Ce ratios is more difficult to reduce, and hydrogen uptake was observed up to 450 °C, resulting from the fact that the proportion of Mn species interacted with the cordierite support increased, implying that the redox activity was reduced. Furthermore, the H₂ consumption was less compared with the reduction features of Mn₉Ce₁/cordierite, implying the catalyst with an optimal 9 : 1 Mn/Ce ratio had the best redox properties. However, the Mn₉Ce₁/Al₂O₃ and Mn₉Ce₁/TiO₂ catalysts had TPR patterns different from that of the Mn₉Ce₁/cordierite catalyst. Two reduction peaks at 460 and 550 °C were presented in the profile of the Mn₉Ce₁/TiO₂ catalyst, the reduction rate was slow in the beginning. The peak at 460 °C could be assigned to the reduction of Mn₃O₄ to MnO, and the reduction peak at 550 °C was possibly attributed to reduction of surface Ce⁴⁺ ions.⁸ The Mn₉Ce₁/Al₂O₃ catalyst presented the reduction over a rather broad temperature range (200–500 °C), which was related to the surface areas of the Mn₉Ce₁/Al₂O₃ catalyst. The TPR results showed that the support as well as the Mn/Ce ratio affected the reduction behavior of Mn–Ce oxide to a great extent. Moreover, the support had even stronger influence on the reduction profile of Mn–Ce oxide than different Mn/Ce ratios did. Mn₉Ce₁/cordierite represented the catalyst system with the lowest reduction temperature among the three supports (cordierite, TiO₂ and Al₂O₃).

Fig. 3 TPR profiles of catalysts with different supports and Mn/Ce ratios.

Fig. 4 presented XPS spectra of O 1s and Mn 2p in the Mn–Ce catalysts and the data are summarized in Table 1. Similar to the recent report concerning the oxidation states of manganese in the catalysts, the binding energies of Mn 2p_3/2 were found to be higher than those of recorded from pure MnO₂ and Mn₂O₃ due to the strong interaction between manganese and cerium oxides.³² The asymmetric Mn 2p_3/2 main metal peaks were found at 641.7–642.38 eV with an 11.7 eV 2p_3/2 to 2p_1/2 splitting. The binding energies around 642.4 and 641.1 eV could be attributed to the presence of Mn⁴⁺ and Mn³⁺ species, respectively.³² The broadening at about 644.0 eV is attributed to Mn 2p_3/2 energy losses.²⁹ The XPS O 1s spectra showed an oxygen peak at 529.5–530.0 eV binding energy eV, assigned to lattice.²⁹ A small peak at 530.8 eV attributed to defective oxide or carbonate species arising from reaction with CO₂ during air exposure and has been confirmed by inspection of the C 1s spectrum.²⁹ A broad shoulder at the higher binding energy region (531.8–532.2 eV) was evident in all samples and can be assigned to adsorbed hydroxyl groups,³⁶ which belonged to surface oxygen. The peak at 533.1–533.5 eV is ascribed to physisorbed water species. This also belongs to surface oxygen. By contrast, Mn₉Ce₁/TiO₂ had the lowest area contribution 26% from surface hydroxides in all the catalysts, Mn₉Ce₁/Al₂O₃ had the lowest 21% Mn³⁺ surface concentration of all catalyst species, and the Mn_xCe₁/cordierite had higher area contribution from surface hydroxides and Mn³⁺ species. Therefore, the higher concentration of Mn³⁺ species and surface hydroxides in Mn_xCe₁/cordierite could be another cause of their outperforming behavior in o-DCB oxidation.

Fig. 4 Curves fitted O 1s and Mn 2p XPS spectra of a Mn–Ce mixed oxide dispersed on various supports.

Table 1 Summary of the results of XPS analysis

Name	Mn3Ce1/cordierite		Mn9Ce1/cordierite		Mn18Ce1/cordierite		Mn9Ce1/Al2O3		Mn9Ce1/TiO2
	Peak BE/eV	%	Peak BE/eV	%	Peak BE/eV	%	Peak BE/eV	%	Peak BE/eV	%
O 1s A	529.52	11.0	529.59	22.0	529.87	23.0	529.83	7.0	530.09	55.0	O 1s lattice oxide
O 1s B	530.80	11.0	530.86	12.0	530.80	11.4	530.80	25.0	530.70	6.0	Carbonate species
O 1s C	532.15	54.0	532.22	48.0	532.04	45.2	531.85	49.0	531.82	26.0	Adsorbed hydroxyl groups
O 1s D	533.34	23.0	533.36	18.0	533.17	20.4	533.19	19.0	533.00	13.0	Physisorbed water
Mn 2p3 E	641.05	36.0	641.00	35.0	641.15	37.3	641.21	21.0	641.21	33.4	MnOOH
Mn 2p3 F	642.40	42.0	642.30	42.0	642.40	39.3	642.50	45.0	642.40	40.4	MnO2
Mn 2p3 G	644.05	22.0	644.02	23.0	644.07	23.4	644.24	34.0	644.10	26.2	Mn 2p3/2 energy losses

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Fig. 5 presents the experimental result of the stability test at 300 °C. This test was carried out after the evaluation of activity of the fresh catalysts. Mn_xCe₁/cordierite presented a stable 82% conversion. High stability was maintained at 300 °C for at least 30 h. During the stability test, no other organic was detected on Mn_xCe₁/cordierite within the FID detection limitation. It was demonstrated that the product selectivity remain almost constant, indicating that the active site structures were stable. Comparing the performance of the catalysts on different supports in Fig. 1, the most outstanding characteristic is lower combustion temperature with considerable higher conversion of o-DBC over Mn_xCe₁/cordierite catalysts than the Mn₉Ce₁/Al₂O₃ and Mn₉Ce₁/TiO₂ catalysts. To obtain deep insights into the reaction, Fig. 6A shows the catalytic process of o-DCB oxidation over Mn₉Ce₁/cordierite catalyst and Fig. 6B shows a possible reaction mechanism on the catalyst surface. The mechanism comprising two main steps is schematized (Fig. 6B): (a) absorption of o-DCB on the active Mn₉Ce₁ oxide species and their neighbouring lattice O; subsequently, reaction of adsorbed o-DCB species with the lattice O from Mn₉Ce₁ oxides to produce CO₂, CO, H₂O and HCl; (b) adsorption of the gas-phase oxygen on the surface of Mn₉Ce₁ oxide particles and migration of the adsorbed O back to the catalyst surface.

Fig. 5 Stability test result of the catalyst being used continuously.

Fig. 6 Schematic of (A) the catalytic process of o-DCB oxidation over Mn₉Ce₁/cordierite catalyst and (B) the possible reaction mechanism on the catalyst surface.

In this study, we highlight the effect of support and Mn/Ce ratio on the structure and catalytic activity of catalysts. We have tested the concentration change of o-DCB before and after the catalytic reaction; no other C-containing organics were detected on Mn₉Ce₁/cordierite within a detection limitation of FID. However, it was found that the reactivity for CO oxidation by CeO₂ addition was remarkably enhanced due to the active oxygen species generated on the CeO₂ surface, which directly participated in the reaction.³⁷ In addition, the final products of CO and CO₂ during o-DCB catalytic oxidation could be detected by gas chromatography (GC) equipped with a thermal conductivity detector.¹⁶ Within the performance limit of TCD and FID themselves, all MnCe catalysts for chlorobenzene catalytic combustion have more than 99.5% selectivity to carbon oxides (more than 98% CO₂ and trace CO)³⁸ and no other C-containing by-products were detected, which is similar to our observation, implying that a high selectivity for CO₂ and trace CO would be observed on Mn₉Ce₁/cordierite catalysts. However, the gas products of the catalytic reaction, especially HCl should be carefully detected to understand the roles of these catalysts in the reaction in our future study.

4. Conclusions

Mn₉Ce₁/cordierite catalysts showed high o-DCB oxidation activity. About 89% o-DCB conversion was obtained on the Mn₉Ce/cordierite catalyst at 200 °C, whereas Mn₉Ce₁/TiO₂ and Mn₉Ce₁/Al₂O₃ only performed below 37% of the conversion for o-DCB oxidation. In the case of Mn₉Ce/cordierite, XRD showed that Mn₂O₃ was the active manganese oxide species. Structure and property analysis by H₂-TPR, BET and XPS revealed that the Mn³⁺ species concentration and surface hydroxides, redox property, and dispersion of Mn–Ce oxide were obviously affected by the support materials. The higher surface Mn³⁺ species concentration and surface hydroxides, and the best redox properties could be attributed to the highest conversion of o-DCB over Mn₉Ce₁/cordierite.

Acknowledgements

This study was supported by the National Natural Science Foundation of China (No. 51374250) and the National Key Technology R & D Program of China (No. 2008BAC32B05).

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