Low-temperature catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts

Catalysis Communications 9 (2008) 2158–2162

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Low-temperature catalytic combustion of chlorobenzene over MnOx–CeO2 mixed oxide catalysts Xingyi Wang *, Qian Kang, Dao Li Laboratory for Advanced Materials, Research Institute of Industrial Catalysis, East China University of Science and Technology, P.O. 396, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 13 February 2008 Received in revised form 9 April 2008 Accepted 19 April 2008 Available online 7 May 2008 Keywords: Chlorinated VOCs Chlorobenzene MnOx–CeO2 CeO2 Catalytic combustion

a b s t r a c t MnOx–CeO2 mixed oxide catalysts prepared by sol–gel method were tested for the catalytic combustion of chlorobenzene (CB), as a model of chlorinated aromatic volatile organic compounds (CVOCs). MnOx– CeO2 catalysts with the different ratio of Mn/Ce + Mn were found to possess high catalytic activity for catalytic combustion of CB, and MnOx(0.86)–CeO2 was the most active catalyst, on which the complete combustion temperature (T90%) of chlorobenzene was 236 °C. The stability of MnOx–CeO2 catalysts in the CB combustion was investigated. MnOx–CeO2 catalysts with high Mn/Ce + Mn ratios present high stable activity, which is related to their high ability to remove Cl species adsorbed and a large amount of active surface oxygen. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Chlorinated volatile organic compounds (CVOCs) are emitted to the atmosphere in waste gases from a wide range of industrial processes, the incineration of municipal and medical waste. Conventional technology for the destruction of chlorinated organic compounds is direct thermal combustion. It usually requires temperatures exceeding 850–1000 °C and leads to the formation of small amounts of toxic polychlorinated aromatic compounds (i.e., polychlorinated dibenzodioxins [PCDDs] and dibenzofurans [PCDFs]), in addition to the standard combustion by-products (i.e., CO, NOx, and SOx). Stringent environmental regulations are in place in several countries limiting PCDD/PCDF emissions [1]. Currently, the catalytic oxidation of CVOCs to carbon dioxide, HCl, and water is the best method for their destruction. Catalytic oxidation can be efficiently performed at temperatures between 250 and 550 °C. The major advantage of catalytic combustion is that it can efficiently treat very dilute pollutants which cannot be thermally combusted without additional fuel. Therefore, a lowtemperature catalytic combustion process offers significant cost saving when compared to a thermal process. Most reported studies of the catalysts for CVOC catalytic combustion have focused on three types of catalysts based on noble metals [2–3], transition metals [4–5] and zeolites [6–7]. Metal catalysts have been widely used because of their ability to catalyze * Corresponding author. Tel./fax: +86 21 64253372. E-mail address: [email protected] (X. Wang). 1566-7367/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2008.04.021

oxidation, but they are susceptible to the deactivation by HCl and Cl2. The activities of zeolite materials are related to their acid properties. However, the problem of the formation of by-product poly-chloro-species on these catalysts is yet to be solved. Therefore, developing high performance catalysts for eliminating chlorinated CVOCs is still necessary and urgent. In our previous work [8,9], CeO2 was found to be a very active catalyst for CVOCs catalytic combustion, and its activity was higher than that of all other catalysts reported [10–14]. However, its activity quickly diminished due to the strong adsorption of HCl or Cl2 produced from the decomposition of CVOCs on CeO2 surface to result in the blockade of active sites. We have found that if chlorine or chloride ions adsorbed on CeO2 surface were removed or transferred rapidly enough, the stability of the catalyst could be improved and increased. Among the catalysts for CVOCs catalytic combustion, such as noble metals, transition metal oxides and solid acid catalysts, generally, transition metal oxide catalysts can resist deactivation by chlorine poisoning. Manganese oxide, supported or unsupported, is a very important catalyst and can be used directly in the catalytic reactions [15–19], such as the oxidation of carbon monoxide, methane and hydrocarbons, the decomposition of ozone, N2O and H2O2, the selective catalytic reduction (SCR) of NO. It has been found that, the presence of Mn in the Mn/Ce and Mn/Cu oxide catalysts can enhance their catalytic activities for CO oxidation and SCR of NO, respectively. In the present work, MnOx was doped into the CeO2 catalyst, resulting in the obvious improvement of its activity and stability for catalytic combustion of chlorobenzene.

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2. Experimental

CeO2

2.1. Catalysts preparation 300

400 500

600

MnOx

Intensity (a.u.)

MnOx, CeO2, and MnOx–CeO2 mixed oxide catalysts were prepared by the sol–gel method: an aqueous solution containing Mn and/or Ce nitrate, and citric acid (citric acid/(Mn + Ce) = 0.3, Mn/ Ce + Mn ratio as 0, 0.27, 0.50, 0.86, and 1) was gradually heated to 80 °C and kept at this temperature for 6 h with stirring, resulting in the formation of a yellowish gel. It was then dried at 110 °C for 12 h, crushed, calcined in air at 550 °C for 5 h and sized to 20– 40 mesh. Mn/Ce + Mn ratio in MnOx–CeO2 is noted as MnOx(X)– CeO2 in the text, and XMn in Figs. 1–5.

0.86Mn 0.50Mn

0.27Mn

CeO2

2.2. Catalysts characterization

100

200

300

400

500

600

Temperature (°C ) Fig. 2. TPR profiles of MnOx–CeO2 catalysts with different ratios of Mn/Mn + Ce.

530.0

528.9

531.9 533.5

CeO2 0.27Mn

Intensity (a.u.)

Powder X-ray diffraction pattern. The powder X-ray diffraction patterns (XRD) of samples were recorded on a Rigaku D/Max-rC powder diffractometer using Cu Ka radiation (40 kV and 100 mA). The diffractograms were recorded in the 2h range of 10–80° with a 2h step size of 0.01° and a step time of 10 s. Nitrogen adsorption/desorption. The nitrogen adsorption and desorption isotherms were measured at 196 °C on an ASAP 2400 system in static measurement mode. Samples were outgassed at 160 °C for 4 h before the measurement. Specific surface area was calculated using BET model. Thermogravimetric analysis. Coke formation was evaluated with thermogravimetric analysis (TGA) using a PerkinElmer Pyris Diamond TG/TGA Setaram instrument. The fresh and used MnOx– CeO2 samples were heated up to 800 °C from room temperature (heating rate of 10 °C min1) in a N2/O2 stream. X-ray photoelectron spectroscopy. The XPS measurements were made on a VG ESCALAB MK II spectrometer by using Mg Ka (1253.6 eV) radiation as the excitation source. Charging of samples was corrected by setting the binding energy of adventitious carbon (C1s) at 284.6 eV. Powder samples were pressed into self-supporting disks, loaded into a sub-chamber, and evacuated for 4 h, prior to the measurements at 25 °C. Temperature programming reduction. H2-temperature programming reduction (H2-TPR) was investigated by heating MnOx–CeO2 samples (150 mg) in H2 (5 vol.%)/Ar flow (30 ml min1) at a heating rate of 10 °C min1 from 20 to 750 °C. The hydrogen consumption was monitored by thermo-conductivity detector (TCD). Before H2-TPR analysis, the samples were heated for 60 min in Ar flow at 500 °C, and then treated in O2 at room temperature for 30 min.

531.3 0.50Mn

0.86Mn 529.8 MnOx 534

531

528

525

Biding energy (eV) Fig. 3. The binding energy of O1s of MnOx–CeO2 catalysts with different ratios of Mn/Mn + Ce.

100

80

0.86Mn 0.50Mn 0.27Mn CeO2

60

MnOx blank

30

40

MnOx 0.86Mn 0.50Mn

CB Conversion (%)

Intensity (a.u.)

0.86 Mn

40

20 0.27Mn

CeO2 20

30

40

50

60

70

80

2 theta (°) Fig. 1. XRD patterns of MnOx–CeO2 catalysts with different ratios of Mn/Ce + Mn.

0 0

100

200

300

400

500

600

Temperature (°C) Fig. 4. The activity of over the fresh MnOx–CeO2 catalysts for CB catalytic combustion, gas composition: 1000 ppm CB, 10% O2, N2 balance; GHSV = 15,000 h1.

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100

CeO2 MnOx 0.86 Mn

400 60

300

40

200

100

Temperature (°C)

CB Conversion (%)

80

500

20 0 0 0

100

200

300

400

500

600

Time (min) Fig. 5. The stability of catalysts for CB catalytic combustion at different temperature, gas composition: 1000 ppm CB, 10% O2, N2 balance; GHSV = 30,000 h1.

2.3. Catalytic activity measurement Catalytic combustion reactions were carried out in a continuous flow micro-reactor constituted of a quartz tube of 4 mm of inner diameter at atmospheric pressure. Two hundred milligrams catalyst was placed at the bed of the microreactor. The feed flow through the reactor was set at 50 cm3 min1 and the gas hourly space velocity (GHSV) was maintained at 15,000 h1. The feed stream to the reactor was prepared by delivering liquid chlorobenzene (CB) with a syringe pump into dry air, which was metered by a mass flow controller. The injection point was electrically heated to ensure the complete evaporation of CB. The concentration of CB in the reaction feeds was set at 1000 ppm. The temperature of reaction was measured and controlled with a thermocouple located just at the bed thermal point of the reactor. Effluent gases were analyzed at a given temperature (to maintain 10 min) on-line by using gas chromatographs (GC), one equipped with FID for the quantitative analysis of the organic chlorinated reactant, the other with TCD for the quantitative analysis of CO and CO2. The concentrations of Cl2 and HCl were analyzed by the effluent stream bubbling through a 0.0125 N NaOH solution. Then, Cl2 concentration was determined by the titration with ferrous ammonium sulphate (FAS) with N,N-diethyl-p-phenylenediamine (DPD) as an indicator [20]. The concentration of chloride ions in the bubbled solution was determined by using a chloride ion selective electrode [21]. 3. Results and discussion Details of the physical characterizations of these catalysts by XRD profiles are shown in Table 1 and Fig. 1. It can be seen that the diffractograms of CeO2 show the diffraction peaks (28.6, 33.3, 47.5, 56.5, and 59.2°) of cerianite with a fluorite-like structure. For MnOx(0.27)–CeO2 and MnOx(0.50)–CeO2, only CeO2 phase with

the fluorite-like structure is observed on the XRD profiles. The intensity of reflections from different crystal planes of CeO2 phase, however, is lowed gradually with the increase of the amount of Mn. Moreover, for MnOx–CeO2 catalysts, the diffraction peaks of cubic fluorite structure are slightly shifted to higher values of Bragg angles in the range of 28.62–29.01°. This shift indicates that part of manganese species can enter into the fluorite lattice to form MnCeOx solid solutions [22]. As reported in Ref. [23], the ionic radius of Mn3+ (0.066 nm) is smaller than that of Ce4+ (0.094 nm), and the incorporation of Mn3+ into the fluorite lattice would result in the decrease in the lattice parameter, lined with the results shown in Table 1. It also can be seen that appreciable decreases of ceria particle size parallel an increase in Mn amount. For MnOx(0.86)–CeO2, the diffraction peaks corresponding to CeO2 phase almost disappear, due to a high dispersion of Ce species into MnOx matrix, whereas there appear two small peaks at 32.9 and 37.2°, indexes of incipient a-Mn2O3 crystallization in the form of bixbyite [24] and MnO2 crystallization in the form of pyrolusite [25–26], respectively. For pure MnOx, there appear only reflections from a-Mn2O3 phase. From XPS results (not shown), it can be seen that three kinds of Mn species, Mn2+ (640.9 eV), Mn3+ (641.8 eV), and Mn4+ (642.5 eV) [27] exist on the surface of all containing-Mn catalysts under study. The fact that one, or more of MnO2, a-Mn2O3, and MnO phases were not observed on MnOx–CeO2 catalysts may be due either to the formation of MnCeOx ‘solid solutions’ with cubic fluorite structure or to the occurrence of very small crystals of manganese oxides. Surface areas of catalysts were tested by BET. It can be seen that the surface areas of MnOx and CeO2 are about 9 m2/g. The surface areas of MnOx–CeO2 catalysts are much more than those of pure oxides. Here, it can be seen that the incorporation of CeO2 into MnOx or MnOx into CeO2 with a small amount can promote the dispersion of matrix oxides to a large extent. The results of TPR analyses for MnOx–CeO2 catalysts are shown in Fig. 2. There are two peaks resulting from the reduction of Mn ions belonging to different structures/phases. H2-TPR profile of pure MnOx shows two overlapped strong reduction peaks at 380 and 480 °C. Assuming that MnO is the final reduction state [28] from different Mn species existing in the initial MnOx, it is possible to propose that two strong broad peaks correspond to the reduction of very small MnO2 particles and Mn3+ cations belonging to a crystalline Mn3O4 phase [29], respectively. This observation is in good accordance with the previous reports that the reduction of bulk MnO2 and/or Mn2O3 takes place through a distinct two-step process [30–31]: the first step involves the reduction of MnO2 to Mn3O4, and the second step represents the reduction of Mn3O4 to MnO. The reduction of pure CeO2 occurs mainly at 517 °C in the range of experimental temperature, associated with the reduction of surface Ce4+ ions [26]. H2 consumption on CeO2 is much smaller than MnOx. Compared with the reduction features of pure MnOx and CeO2, the reduction temperatures of MnOx–CeO2 mixed oxides

Table 1 The properties of MnOx–CeO2 catalysts Mn content (at%)

Mn/Ce + Mn (at/at)

d111 (CeO2) (Å)

Surface area (m2/g)

Ceria particle size dCeO2 (nm)a

Lattice parameter (Å)

Oxygen distribution (%)b Olatt

Osur

Oads

CeO2 27 50 86 MnOx

0 0.150 1.00 6.00 –

3.1250 3.1293 3.1100 3.0889 –

9 48 53 65 9

13.9 6.7 5.9 5.6 –

5.41 5.42 5.39 5.35 –

51.5 52.4 43.0 41.1 45.4

10.4 11.7 14.0 20.1 20.3

2.3 2.9 3.7 0.9 3.3

a b

The value estimated by the Scherrer equation, applied to the h1 1 1i reflection of the cerianite. The value by XPS analysis.

X. Wang et al. / Catalysis Communications 9 (2008) 2158–2162

systematically shift to lower temperature regions. H2-TPR profile of the catalysts with a lower amount of Mn, such as MnOx(0.27)–CeO2 and MnOx(0.50)–CeO2, exhibits two broad reduction peaks around 260 and 350 °C, respectively. The former can be related to the presence of ‘‘isolated” Mn4+ ions, ‘‘embedded” into the surface defective positions of the ceria lattice. Their reduction is promoted by a high degree of coordinative unsaturation and, perhaps, by neighboring Ce4+ ions, which, in turn, undergo reduction to Ce3+ species [32]. For MnOx(0.86)–CeO2, the peak related to the reduction of very small MnO2 particles well-dispersed across the ceria matrix [26] appears at about 322 °C, and the second step reduction occurs at higher temperature, due to the decrease of the amount of Ce species. Fig. 3 presents XPS measurement of the binding energy of O1s of oxygen species on the surface of catalysts under study, and the data are summarized in Table 1. For pure CeO2, the spectrum of O1s has two peaks at 528.9–530 eV, assigned to the lattice oxygen. Two shoulder peaks at ca. 531.9 and 533.5 eV are observed, which can be attributed to adsorbed oxygen or/and weekly bonded oxygen species (active oxygen) [33], or to surface oxygen by hydroxyl species and/or adsorbed water species presser as contaminants at the surface, respectively. With the addition of Mn, the binding energy of O1s of lattice oxygen on the MnOx–CeO2 catalysts shifts to high values because of ‘‘Mn O” electron-transfer processes [26], and the amount of lattice oxygen decreases. The binding energy of O1s of active oxygen, at first, is increased from 531.9 to 531.3 eV with the increase of the Mn/Mn + Ce ratio from 0.27 to 0.50. After that, it does not change, even for MnOx. However, the amount of active oxygen is increased with the increase of the Mn/Ce + Mn ratio (see Table 1), with agreement of the increase of H2 consumptions in Fig. 2 (according to the area under H2-TPR profile). Arena [26] found that ‘‘Mn O” electron-transfer processes enable the formation of very reactive electrophilic oxygen species ¼   (e.g., O 2 , O2 , O , O ) that are active oxygen species. Combined with their possessing high oxygen mobility (TPR peak appearing at lowtemperature), it can be expected that the MnOx–CeO2 catalysts with high Mn/Ce + Mn ratio, will be very active in the CVOC oxidation. In order to check whether some reactions under thermal combustion condition could take place, ‘‘Blank test”, with 3 mm crushed quartz glass (40–60 mush) in the reactor, was carried out. As shown in Fig. 4, the conversion of CB without catalyst could occur only above 450 °C to a lower extent. Can Li [34] found no significant oxidation of CB at temperature up to 500 °C in a blank reactor experiment. MnOx–CeO2 catalysts appear to be superior for CB catalytic combustion, of which MnOx(0.86)–CeO2 has the best activity, and its complete combustion temperature of CB is only 236 °C at GHSV 15,000 h1, which is much lower than those of all catalysts reported in previous literatures, including noble metal-based catalysts [35–37] and MnOx/TiO2–Al2O3 or TiO2 catalysts [34,38]. With the decrease of Mn/Mn+Ce ratio, the activity of MnOx–CeO2 catalysts lowers gradually. For pure MnOx, the conversion of CB reaches 90% at 406 oC, higher than that for the MnOx–CeO2 catalysts, except MnOx(0.27)–CeO2. CeO2, at first, presents a higher activity for CB combustion than MnOx, but it deactivates gradually from 25% at 200 °C to 8% at 300 °C in the conversion of CB. With the further increase of temperature, the conversion of CB on CeO2 increases gradually, and reaches 90% at 555 °C, which is much higher than the temperature for total conversion of trichloroethylene on CeO2 [9]. The incorporation of MnOx into CeO2 greatly improves the activity and stability, depending on the Mn/Ce + Mn ratio. For MnOx(0.27)–CeO2, the temperature which need for 50% conversion of CB, shifts to 240 °C from 438 °C for CeO2. Moreover, the deactivation occurs at higher temperature and to a smaller extent. With

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the increase of Mn amount, the MnOx–CeO2 catalysts does not deactivate over a short time period. Within the limit of FID, all the catalysts studied in this experiment provide more than 99.5% selectivity to carbon oxides (more than 98% CO2 and trace CO) and no other detectable C-containing by-products are found. This is very different from that for noble metal-based catalysts [34–36], where substantial amount of polychlorinated benzene are formed during catalytic combustion of CB. The Cl balance reached 80–85%, which means that the deposition of Cl species on the surface of catalysts occur. The stability of MnOx(0.86)–CeO2, MnOx, and CeO2 catalysts was investigated for the CB combustion. The results from the activity tests conducted when the temperature was varied stepwise are presented in Fig. 5. In these activity tests, the activity stability was studied at different temperatures, that is, the change of activities were observed in 60 min at a given temperature and plotted as conversion of CB vs. the reaction temperature and reaction time. During the test, the activity of every catalyst was evaluated with a step by step temperature from 50 to 500 °C. As shown, CeO2 presents an unstable activity and the activity drop for CB catalytic combustion increases at 100–200 °C, and decreases at 250–350 °C to a small extent, probably due to fairly low activity. After that, CeO2 achieves a low stable activity without a substantial decrease in conversion. There is not a significant drop in the activity observed on MnOx, although MnOx shows fairly low activity. The combination of MnOx with CeO2 really promotes the activity on the various temperature step, but the activity of MnOx(0.86)–CeO2 is not stable until 300 oC, at which MnOx(0.86)–CeO2 achieves a high stable activity without any decrease in conversion. The range of temperature in which the activity significantly drops on MnOx(0.86)–CeO2 is almost identical with that on CeO2, i.e. 125–300 °C. It is necessary to test the stability at 350 °C in a long period for practical use. It can be found that the stable activity can be achieved within 0.5–1 h over MnOx–CeO2 catalysts with the Mn/ Ce + Mn ratios of 0.86, 0.5, 0.27, MnOx, and CeO2, on which the stable conversion of CB within 20 h is about 98, 76, 50, 45, and 19%, respectively. Compared with the activity of the fresh MnOx–CeO2 catalysts shown in Fig. 4, it can be seen that the level of the activity drop of catalysts parallels the amount of Ce species in catalysts. So, the deactivation of catalysts during the combustion of CB is related to the poisoning of Ce species, probably due to the strong adsorption of HCl or Cl2 produced from the decomposition of CB on the CeO2 surface to result in the blockade of active sites [9]. In this experiment, the used MnOx(0.86)–CeO2 and CeO2 were passed with CB-free air and Ar at 350 °C for 12 h. The activity of the used MnOx(0.86)–CeO2 was recovered almost to the level of the fresh catalyst in air, but not in Ar. The removal of Cl species can be processed by the reaction of oxygen species with Cl species to form Cl2 (detected in the effluent with MC). The activity of the used CeO2 catalyst can be recovered in air only to a smaller extent, but not in Ar. CeO2 is active in the decomposition of chloro-containing compound [8–9]. In the CB catalytic combustion, chlorobenzene adsorption and C–Cl bond dissociation occur at low-temperatures on CeO2 [9], and the resulting fragments can react with active oxygen to produce CO2 and H2O. Chlorine species need be removed as Cl2 or HCl from the surface of catalysts, or chlorine species block the active sites so as to deactivate catalysts. In this study, the removal of chlorine species was promoted by elevating temperature (such as 350 °C). It may be deduced that the removal of Cl species is a slow step so that it determines the rate of CB catalytic combustion. The incorporation of Mn makes active oxygen increase (see Table 1) and improve the activity towards CB catalytic combustion. On the one hand, it is more important that the removal of chlorine species from the surface of MnOx–CeO2 catalysts is significantly

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promoted by the increase of Mn amount. For MnOx(0.86)–CeO2, the removal of chlorine species is rapid enough to maintain the active sites accessible to reactant molecules at 350 °C. To obtain more insight into the promotion of MnOx, the physico-chemical properties of different catalysts were characterized. The results show that after being used for 20 h at 350 °C for the combustion of CB, the surface area of all of MnOx–CeO2 catalysts almost does not change, no phase transformations are observed in their XRD patterns and the obvious deposition of coke on the catalyst surface was not detected by the EDS and TG techniques. However, the change of binding energy of elements on the surface was observed by XPS, depending on the amount of Mn. For the deactivated CeO2, the binding energy of Ce3d shifts substantially to higher values, and that of O1s of lattice oxygen increases from 528.9 to 530.2 eV, whereas the spectra of Ce3d and Mn2p for MnOx–CeO2 catalysts almost do not change. The binding energy of O1s of lattice oxygen shifts from 529.3 to 530.0 eV and 529.8 to 530.0 eV, for the used MnOx(0.50)–CeO2 and MnOx(0.86)–CeO2, respectively. Moreover, Cl2p peak appearing at around 198 eV on XPS spectra confirms the presence of chloride on all of used catalysts under study. The strongly electro-negative chlorine species adsorbed on CeO2 sites make the transition of charge density from Ce atom to Cl atom, which results in the shift of binding energy. The binding energy shifts of all species on surface of MnOx(0.86)–CeO2 is slight, due to the weak adsorption of chlorine species. The extent of the shift in binding energy of Ce3d or O1s of lattice oxygen may reflect the strength of chlorine species adsorption. Therefore, the deactivation of catalysts is related to the strong adsorption of HCl or Cl2 produced from the decomposition of CB. From the above results. It can be concluded that oxygen species can efficiently react with Cl adsorbed with moderate strength. For pure CeO2 or MnOx–CeO2 catalysts with low amount of Mn, surface oxygen mobiles readily (see the first reduction peak showed in Fig. 2), but the amount of surface oxygen is small and the adsorption of Cl species is strong. So, these catalysts need high temperature to remove Cl adsorbed, and present a low stable activity. For MnOx(0.86)–CeO2, there is more active oxygen species to remove Cl adsorbed with the moderate strength on the surface of catalysts during the reaction, so that more active sites can be accessible. Therefore, the stability MnOx–CeO2(0.86) in CB catalytic combustion is promoted at 350 °C, due to a decrease in chlorine species deposit. 4. Conclusion In summary, it has been found that MnOx–CeO2 catalysts present high activity for the low-temperature catalytic destruction of chlorobenzene. The main products of chlorobenzene catalytic destruction over the MnOx–CeO2 catalysts are HCl, Cl2, CO2, and trace CO, and polychlorinated compounds has not been detected. MnOx–CeO2 catalysts with high Mn/Ce + Mn ratios present high stable activity, which is related to their high ability to remove Cl species adsorbed and a large amount of active surface oxygen.

Acknowledgements We would like to acknowledge the financial support from National Basic Research Program of China (Nos. 2006AA06Z379 and 2004CB719500), National Natural Science Foundation of China (No. 20377012) and the Commission of Science and Technology of Shanghai Municipality (Nos. 05QMX1413 and 06 JC14020). References [1] R. Weber, T. Sakurai, H. Hagenmeier, Appl. Catal. B: Environ. 20 (1999) 249. [2] M. Taralunga, B. Innocent, J. Mijoin, P. Magnoux, Appl. Catal. B: Environ. 75 (2007) 139. [3] S. Scirè, S. Minicò, C. Crisafulli, Appl. Catal. B: Environ. 45 (2003) 117. [4] F. Bertinchamps, M. Treinen, P. Eloy, A.M. Dos Santos, M.M. Mestdagh, E.M. Gaigneaux, Appl. Catal. B: Environ. 70 (2007) 360. [5] F. Bertinchamps, C. Gregoire, E.M. Gaigneaux, Appl. Catal. B: Environ. 66 (2006) 1. [6] M. Taralunga, J. Mijoin, P. Magnoux, Catal. Commun. 7 (2006) 115. [7] M. Guillemot, J. Mijoin, S. Mignard, P. Magnoux, Appl. Catal. A: Gen. 327 (2007) 211. [8] Q.G. Dai, X.Y. Wang, G.Z. Lu, Catal. Commun. 8 (2007) 1645. [9] Q.G. Dai, X.Y. Wang, G.Z. Lu, Appl. Catal. B: Environ. 81 (2008) 192. [10] M. Magureanu, N.B. Mandache, J. Hu, R. Richards, M. Florea, V.I. Parvulescu, Appl. Catal. B: Environ. 76 (2007) 275. [11] M. Guillemot, J. Mijoin, P. Magnoux, Appl. Catal. B: Environ. 75 (2007) 249. [12] M.E. Swanson, H.L. Greene, Appl. Catal. B: Environ. 52 (2004) 91. [13] R. Lopez-Fonseca, J.I. Gutierrez-Ortiz, A.A. Aranzabal, J.R. Gonzalez-Velasco, Appl. Catal. B: Environ. 41 (2003) 31. [14] X.T. Sayle, D. Sayle, Phys. Chem. Chem. Phys. 7 (2005) 2936. [15] M.C. Alvarez-Galvan, V.A. de la Pena O Shea, J.L.G. Fierro, P.L. Arias, Catal. Commun. 4 (2003) 223. [16] C.N. Costa, V.N. Stathopoulos, V.C. Belessi, A.M. Efstathiou, J. Catal. 197 (2001) 350. [17] R. Craciun, B. Nentwich, K. Hadjiivanou, H. Knözinger, Appl. Catal. A: Gen. 243 (2003) 67. [18] G. Qi, R.T. Yang, J. Phys. Chem. B 108 (2004) 15738. [19] M. Kramer, W.F. Maier, Appl. Catal. A: Gen. 302 (2006) 257. [20] J.R. Gonzalez-Velasco, A. Aranzabal, R. Lopez-Fonseca, R. Ferret, J.R. GonzalezMarcos, Appl. Catal. B: Environ. 24 (2000) 33. [21] R. Lopez-Fonseca, A. Aranzabal, J.I. Alvarez-Uriarte, J.R. Gonz-alez-Velasco, Appl. Catal. B: Environ. 30 (2001) 303. [22] M. Machida, M. Uto, D. Kurogi, T. Kijima, Chem. Mater. 12 (2000) 3158. [23] D. Terribile, A. Trovarelli, C. De Leitenburg, A. Primavera, G. Dolcetti, Catal. Today 47 (1999) 133. [24] L. Dimesso, L. Heider, H. Hahn, Solid State Ionics 123 (1999) 39. [25] F. Arena, E. Alongi, P. Famulari, A. Parmaliana, G. Trunfio, Catal. Lett. 107 (2006) 39. [26] F. Arena, G. Trunfio, J. Negro, B. Fazio, L. Spadaro, Chem. Mater. 19 (2007) 2269. [27] E. López-Navarrete, A. Caballero, A.R. González-Elipe, M. Ocaña, J. Eur. Ceram. Soc. 24 (2004) 3057. [28] F. Kapteijn, L. Singoredjo, A. Andreini, Appl. Catal. B: Environ. 3 (1994) 173. [29] F. Arena, T. Torre, C. Raimondo, A. Parmaliana, Phys. Chem. Chem. Phys. 3 (2001) 1911. [30] J. Carnö, M. Ferrandon, E. Björnbom, S. Järås, Appl. Catal. A: Gen. 155 (1997) 265. [31] J. Trawczyns´ki, B. Bielak, W. Mis´ta, Appl. Catal. B: Environ. 55 (2004) 277. [32] S.T. Hussain, A. Sayari, F. Larachi, Appl. Catal. B: Environ. 34 (2001) 1. [33] H.C. Yao, Y.F. Yu Yao, J. Catal. 86 (1984) 254. [34] Y. Liu, Z. Wei, Z. Feng, M. Luo, P. Yang, C. Li, J. Catal. 202 (2001) 200. [35] A. Musialik-Piotrowska, B. Mendyka, Catal. Today 90 (2004) 139. [36] R.W. van den Brink, P. Mulder, R. Louw, Catal. Today 54 (1999) 101. [37] M. Taralunga, J. Mijoin, P. Magnoux, Appl. Catal. B: Environ. 60 (2005) 163. [38] Y. Liu, M. Luo, Z. Wei, Q. Xin, P. Yang, C. Li, Appl. Catal. B: Environ. 29 (2001) 61.