Improved activity and stability for chlorobenzene oxidation over ternary Cu-Mn–O-Ce solid solution supported on cordierite

A series of CuMnOx/CeO2/cordierite and CuMnCeOx/cordierite catalysts prepared by a complex method with citric acid were investigated for the performance of chlorobenzene (CB) oxidation. The effects of the molar ratio of Mn/Cu, transition metal oxide loading, calcination temperature and time were investigated as the main investigation factor for the performance. Meanwhile, XRD, SEM, BET, H2-TPR, O2-TPD and XPS were conducted to characterize the physicochemical properties of these catalysts. The results demonstrated that CuMnOx/CeO2/cordierite catalysts prepared by step-by-step synthesis with the Cu/Mn molar ratio of 5:2 exhibited a high activity (T90 = 350 °C), owing to the incorporation of CuO and MnOx for forming CuMn2O4 spinel oxide supported on CeO2 surface. More importantly, CuMnCeOx/cordierite catalysts prepared by one-step exhibited the highest oxidation activity (T90 < 300 °C) attributed to the low H2 reduction temperature and desorption energy of surface oxygen, and the formed Cu-Mn–O-Ce solid solution and CeO2 promoted the high dispersion of CuMnOx in the supported catalysts. In addition, the possible oxidation mechanism was described to demonstrate the by-products generation and oxygen transfer of CuMnCeOx catalysts.


Introduction
Volatile organic compounds (VOCs) with complex components produced by different types of industrial processes have attracted wide attention. VOCs are important precursors for the formation of ozone/smog and harmful for human health, but difficult to degrade effectively (Jiang et al. 2018;Pan et al. 2020;Tu et al. 2022). Among VOCs, chlorinated volatile organic compounds (CVOCs) are classified as an important group, which has attracted more concern from researchers owing to their high toxicity, strong thermal stability, poor biodegradation, and easy bioaccumulation (He et al. 2019;Li et al. 2020;Ye et al. 2021).
The technologies developed for removing CVOCs can be divided into two categories: destructive strategies (regenerative thermal oxidizer, regenerative catalytic oxidizer, and non-thermal plasma) and recuperative technologies (adsorption, absorption, condensation, and separation). Among them, catalytic oxidation is considered as a cost-effective method to control CVOCs due to its advantages of low reaction temperature, high catalytic efficiency, low energy consumption, and less secondary pollution (Zhang et al. 2018). However, domestic industrial catalysts still suffer from some problems, such as low oxidation activity, poor stability, and chlorine poisoning and deactivation. Therefore, it is important for the development of the effect wide-spectrum catalysts to reduce the degradation temperature of CVOCs, with the benefits of improving the performance of oxidation and purification systems and reducing cost.
Compared with the traditional granular or powder catalysts, the supported catalysts with a honeycomb structure have been increasingly used in industry due to their advantage of reducing bedding pressure and high-quality transfer efficiency. Supports can effectively affect the morphological structure and active components of catalysts. The physicochemical properties of supports can also affect the deposition of active elements and particle dispersion, thus influence the durability and antitoxicity.
The recent studies on catalysts for CVOCs purification have focused on the zeolites, noble metals, and transition metal oxides. Zeolites are often used in CVOCs oxidation due to their acid properties, high specific surface area, excellent adsorption, and abundant pore structure (Serati-Nouri et al. 2020). However, the by-products (polychlorinated compounds) created by these catalysts have limited their wide applications. Noble metals catalysts are mainly composed of Pt (Gu et al. 2019a, b), Pd (Liu et al. 2019), Ru (Li et al. 2018), and others, and they have the advantages of low reaction temperature and high activity for C-Cl and other bonds. However, noble metal catalysts still encounter some challenges, such as deactivation by chlorine poisoning or coking from waste gases, higher cost, and the formation of undesired polychlorinated benzenes (Zhang et al. 2020a, b). Transition or rare metal oxides catalysts, such as Ni , Co (Xiang et al. 2019), Cr (Deng et al. 2020;Zhang et al. 2020a, b), Mn (Wang et al. 2021), Fe (Qin et al. 2022;Tian et al. 2022), Cu (Huang et al. 2021), and Ce (Gu et al. 2019a, b), not only can resist the inactivation caused by chlorine poisoning, but also can exhibit high activity and stability by improving other metal oxides or by supporting solid acids as supports. Although their activities are still lower than those of noble metal catalysts, transition metal oxides catalysts have attracted increasing attention for improving the performance of CVOC oxidation.
In single Cu or Mn oxide, there often remains some problems, such as low oxidation activity and gradual deactivation. Therefore, Cu-Mn mixed oxides have been synthesized to improve the activity of VOCs oxidation. Wang et al. (2009) synthesized a series of MnO x -CeO 2 mixed oxide catalysts by sol-gel method with different Mn/Ce molar ratios for CB oxidation; they found that MnO x -CeO 2 could exhibit high activity against CB, where MnO x (0.86)-CeO 2 catalyst was identified as the optimum catalyst for completely oxidizing CB to HCl, H 2 O, CO 2 , and trace amounts of CO at 254 °C. Their work also showed that when the Mn/Ce ratio increased, MnO x -CeO 2 mixed catalyst had more surfaceactive oxygen, which could quickly remove adsorbed Cl over the catalysts surface, due to abundant oxygen vacancy and high oxygen storage capacity of CeO 2 . CeO 2 nanorods and nanoparticles were also prepared by hydrothermal method, and MnO x was incorporated to MnO x /CeO 2 catalysts (Zhao et al. 2014). Both types of MnO x /CeO 2 exhibited high oxidation activity at a low range of reaction temperature. Furthermore, MnO x /CeO 2 nanoparticles exhibited a high conversion efficiency and strong Cl resistance, due to the exposure to more crystal planes (100) and containing more Mn 4+ , oxygen vacancies, and surface oxygen. He et al. (2015a, b) prepared a series of mesostructured CuMnCeO x (Cu a Mn 0.3-a Ce 0.7 O x (a = 0.1, 0.15, 0.2)) powder catalysts with different amounts of Cu-Mn incorporating by a ureaassistant coprecipitation method. They found that the Mn and Cu phases entered the CeO 2 matrix with a fluorite-like structure to form the Cu-Mn-O-Ce solid solution. Among them, the Cu 0.15 Mn 0.15 Ce 0.7 O x exhibited high activity (90.0% at 250 °C) in catalytic oxidation of CB, owing to the oxygenenriched properties of Ce and enhanced removal of Cl over the catalyst surface. The incorporation of CuO and MnO x also effectively prevented the formation of chlorination by-products.
However, so far, there are few studies on the preparation of monolithic transition metal oxides for CVOCs oxidation. The development and synthesis of monolithic catalysts with excellent oxidation activity and stability are of great significance in industrial application. We have researched the effect of CuA x Mn 2-x O 4 /cordierite (A = Cr, Fe, Ce; x = 0.25, 0.5, 1) catalysts for catalytic oxidation of CB (Huang et al. 2021). CuCe 0.25 Mn 1.75 O 4 /cordierite catalyst exhibited high activity and good stability owing to CeO 2 nano-rods structure conducive to increase the amount of O ads and the strength of weak acidity. However, in order to further improve performance, understand the effect of CeO 2 with a support, and reaction mechanism, the supported CuMnCeO x /cordierite and CuMnO x /CeO 2 /cordierite catalysts were prepared by a novel method with mutual comparison. To investigate the effects of different Mn/Cu molar ratios, amounts of transition metal oxide incorporating, and calcination temperatures and times on the catalytic oxidation of CB, XRD, SEM, BET, H 2 -TPR, O 2 -TPD and XPS were conducted to characterize the physicochemical properties of the samples. In situ DRIFT and GC-MS were also applied to determine the relationship between the preparation method and the oxidation activity. Catalysts with high thermal stability, specific surface area, oxidation ability, and good resistance to chlorine poisoning were identified in this research.

Catalyst preparation
A 30 wt.% CuMnCeO x catalyst supported on cordierite with one-step synthesis was prepared by the citric acid complex method. Specifically, 7.81 mL of Mn (NO 3 ) 2 (50.0 wt.%) was added to a 100 mL beaker containing 5.05 g of Ce (NO 3 ) 3 ·6H 2 O, 3.25 g of Cu(NO 3 ) 2 ·3H 2 O and 2.05 g of citric acid. The solution was stirred until complete dissolution. The ratio of Mn/Cu molar was 5:2, and total metal ion/ citric acid molar was 6:1. The above solution was gradually heated to 80 °C for 7 h until a sol formed. Then, 5.0 g of cordierite (5-12 mesh) was separately impregnated in the sol for 2 h and then removed. This catalyst was gradually dried at 80 °C for 6 h and calcined at 450 °C for 4 h. The other catalysts with different incorporating amounts, calcination temperature and time were all synthesized following the same method described above. For comparison, a series of CuMnO x /CeO 2 /cordierite with step-by-step synthesis were also prepared. Specifically, 20 mL of distilled water was added to a 100 mL beaker containing 4.0 g of Ce (NO 3 ) 3 ·6H 2 O, and they were stirred until complete dissolution. 5.0 g cordierite was separately impregnated in the above solution for 2 h and then removed. These 20 wt.%CeO 2 /cordierite was dried at 80 °C for 6 h and calcined at 450 °C for 4 h. CuMnO x /CeO 2 /cordierite was obtained by the citric acid complex method described above.

Catalyst characterization
The crystal structures of the prepared samples were analyzed by powder X-ray diffraction (XRD, D/max-RB, Japan) with Cu Kα radiation in the 2θ range of 10-80° with a scanning rate of 4°/min. The surface morphologies of these samples were observed by a scanning electron microscopy (SEM) performed at 15.0 kV. The surface areas of these samples were measured using the Brunauer-Emmett-Teller (BET) method, and the pore size distributions and pore volume were obtained using the Barrett-Joyner-Halenda (BJH) method. H 2 -temperature-programmed reduction (H 2 -TPR) was carried out for 50 mg of these samples from 50 to 800 °C with a gas chromatograph equipped with TCD. O 2 -temperature-programmed desorption (O 2 -TPD) was measured on the same instrument as H 2 -TPR, and the desorption profiles were recorded online under He flow. X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Thermo ESCALAB 250 system equipped with Al Kα radiation (1486.6 eV), operating at 84 W with an energy pass of 40 eV. The in situ DRIFT spectra of the catalyst were recorded by using a Nicolet FTIR-6700 spectrometer equipped with a MCT detector. The DRIFT cell equipped with CaF 2 windows was used as the reaction chamber to heat catalysts to 600 °C. All the spectra were collected in the range of 4000-1200 cm −1 at the resolution of 4 cm −1 and 32 scans. In each measurement, the catalysts were first pretreated at the temperature of 400 °C under He flow (99.99%, 30 mL/min) for 1 h. After cooling down, 500 ppm CB and 10 vol.% carrier gas (O 2 and N 2 ) were introduced, followed by a series of heat treatments to 255 °C, 275 °C and 300 °C. The off-gas (various gaseous products) was detected using an adsorption tube (Tenan TA) at 300 °C for 30 min. The adsorbed compounds were released in a thermal desorption instrument and were identified and injected into the MS (AMETEK TILON LC-D200, Ametek).

Catalytic activity measurement
Catalytic oxidation of CB was tested using a continuous fluid-solidified bed with a steel material tube (I.D. = 80 mm). In each test, 3.2 g of catalyst (12-16 mesh) was applied. The feed stream containing 500 ppm CB and air was passed through the catalyst at 800 mL/min (10,000 h −1 of GHSV). The composition of the mixed gas was analyzed by an on-line FID gas chromatograph using a Restek Rtx-1 (0.25 × 0.25 μm × 30 m) stainless steel tube. According to the concentrations of chlorobenzene inlet (C in ) and outlet (C out ) during the reaction, the CB conversion was calculated as an index to measure the activity of these catalysts as follows: The values of T 50 and T 90 represented the temperature with light-off (50.0%) and complete oxidation (90.0%) of CB conversion.

Structural and morphology properties
The XRD patterns of CuMnO x /CeO 2 /cordierite catalysts with different Mn/Cu molar ratios and CuMnCeO x /cordierite catalysts were prepared with various calcining temperatures and times. As shown in Fig. 1a, the peaks allocated at 2θ = 10. 5, 21.8, 26.4, 28.5, and 29.6° corresponded to cordierite (PDF#12-0303) in the CuMnO x /CeO 2 /cordierite. With the increase in Mn/Cu molar ratios, the intensity of the cordierite diffraction peak decreased first and increased later, because of that, the cordierites were covered by numerous active components on the surface. Simultaneously, the samples with weaker characteristic diffraction peaks of cordierites exhibited high CB oxidation performance, due to the high dispersion of active components. The peaks observed at 2θ = 28.6, 33.1, 47.5, and 56.3° were attributed to (111), (200), (220) and (311) crystals of CeO 2 (PDF#34-1400), respectively; the peaks at 2θ = 37.3, 42.8, 35.6 and 38.7° correspond to MnO 2 (PDF#81-2261) and CuO (PDF#48-1548), respectively. Moreover, the characteristic diffraction peak of spinel CuMn 2 O 4 (PDF#34-1400, 2θ = 35.6°) could be found with the increase in the molar ratio of Mn/Cu, but the diffraction peak of CeO 2 decreased, indicating that the particle size and crystal structure of oxides could be affected by the Mn/Cu molar ratio. The XRD pattern of the CuMnCeO x /cordierite catalysts synthesized with different calcining temperatures and times was shown in Fig. 1b. Compared with the supported catalyst, the CuMnCeO x powder could only be detected for the diffraction peaks of CeO 2 without CuO or MnO x. As for CuMnCeO x /cordierite, the characteristic diffraction peaks were also assigned mostly to cordierites and no obvious CeO 2 could be found. This suggests that CeO 2 was highly dispersed on the surface or particles with the synergistic interaction in the Cu-Mn binary oxides formed a ternary Cu-Mn-O-Ce solid solution. When the calcining temperature increased to 550 °C, the peaks at 2θ = 35.9° could be analyzed to Cu 1.5 Mn 1.5 O 4 (PDF#35-1172), probably due to the melting of partial sintering on the microstructure caused by high temperature. With the increase in calcining temperature and time, cordierite peaks at 2θ = 54.4° shifted to the left, from 54.4° at 350 °C to 54.1° at 550 °C, and they shifted from 54.3° at 3 h to 54.1° at 5 h. The above results showed that these catalyst structure could have been changed, potentially related to the obvious different pore structures of catalysts.
To further reveal the surface structure and distributions of CuMnO x /CeO 2 /cordierite and CuMnCeO x /cordierite catalysts, the SEM was used to observe. As indicated in Fig. 2a, the CuMnO x /CeO 2 /cordierite showed some clumps with a smooth surface, which is possibly related to the slippery surface of cordierite, indicating that the CeO 2 was distributed inside the cordierite for CeO 2 /cordierite. Meanwhile, some rod-like and irregular morphology could be detected on the surface, which is attributed to CuO or MnO x active components.
The CuMnCeO x /cordierite catalysts prepared by a complex method with different calcination temperatures and times were displayed in Fig. 2b, c, d, e, f. Notably, the active components of CuMnCeO x oxides were distributed on the cordierite surface in the form of uniform and dense particles. However, the surface structure of these samples synthesized with various calcination temperatures and times were different. First, the catalyst surface formed lumps of spherical particles, especially for CuMnCeO x /cordierite calcined at 450 °C for 4 h (Fig. 2c). When the calcination temperature increased from 350 °C (Fig. 2b) to 550 °C (Fig. 2d), a small number of needles and flakes formed with some melting. Meanwhile, SEM observation revealed that abundant filaments with smooth surfaces and melting phenomenon appeared in the samples when the calcining time increased from 3 to 5 h, mainly owing to that the citric acid template calcined at different temperatures and times. Among them, the CuMnCeO x /cordierite catalyst prepared at 450 °C for 4 h exhibited a preferable surface morphology with a large amount of uniformly distributed spherical particulate and the highest surface areas (35.5 m 2 /g). It did not form the large lumps of dispersed particles and neither exhibit melting phenomenon, which was consistent with the higher oxidation performance. In addition, the occurrence of melting could be easily found at 550 °C, which was associated with the analysis results of surface area and oxidation activities.
The pore-size distribution curves and N 2 adsorption/ desorption isotherms of CuMnO x /CeO 2 /cordierite and CuMnCeO x /cordierite were shown in Fig. 3. Both catalysts demonstrated Type IV isotherms. The CuMnCeO x /cordierite catalyst exhibited a more pronounced H4 hysteresis loop with a relative pressure (P/P 0 ) from 0.7 to 1.0, indicating the characteristics of mesoporous materials and the adsorption behavior from single layer to multi-layer of capillary condensation. The adsorption amount in the high-pressure region increased rapidly, implying that the distribution of pore size was relatively wide, large, and mainly macropores, which was conducive to the adsorption of reactants and the removal of reaction products during catalytic oxidation. Meanwhile, the specific surface area, pore-volume, and average pore size for these catalysts were shown in Table 1. CuMnCeO x /cordierite exhibited a larger surface area, porevolume, and pore size than CuMnO x /CeO 2 /cordierite, due to the high CuMnO x distribution promoted by CeO 2 . However, the surface area, pore-volume, and pore size of CuMnCeO x /cordierite reduced to varying degrees, and these variations were associated with calcining temperature and time. The surface area reductions decreased in the order of 450 °C > 350 °C > 550 °C, which was consistent with the decline of CB oxidation activities. Notably, more active sites with high surface area were consistent with the improvement of CB oxidation, because larger pore volumes and sizes were conducive to the active adsorption and diffusion of VOCs in these catalysts (Huang et al. 2015b, a Fig. 3 The pore-size distribution curves and N 2 adsorption/desorption isotherms of CuMnO x /CeO 2 /cordierite and CuMnCeO x /cordierite exhibited the largest surface area, pore-volume, pore size and preferable surface morphology, so it exhibited a higher activity than others (Fig. S1).

H 2 -TPR and O 2 -TPD analysis
In order to investigate the reducibility of CuMnCeO x / cordierite, catalysts incorporated with different amounts of CeO 2 , H 2 -TPR and O 2 -TPD profiles were compared. As shown in Fig. 4a, two obvious H 2 reduction peaks appeared in the low-temperature region of 150-300 °C, due to the excellent oxygen migration ability and synergistic effect of cerium oxide. The reduction peak at 230 °C was possibly ascribed to the reduction of surface oxygen, while the reduction peak at 266 °C was due to the reduction peak of the mix of CuO and MnO x . H 2 reduction peak of CeO 2 was difficult to detect, indicating that incorporation of cerium oxide probably promoted the interaction between CuO and MnO x (Li et al. 2011). With the increase in CeO 2 amount, the H 2 reduction peak further shifted to low temperature, indicating that its redox ability was further enhanced. This is possibly because the increase in CeO 2 content improved the synergistic effect of ternary Cu-Mn-O-Ce solid solution, and the amount of active oxygen species on the surface further increased. Moreover, the H 2 peak concentrations of catalysts grew with the increase of CeO 2 supporting (1.99%, 3.06% and 4.32%), which was conducive to the oxidation performance (Hu et al. 2008). Figure 4b showed the O 2 -TPD profile of CuMnCeO x / cordierite incorporated with different amounts of CeO 2 . In general, transition metal oxides have three oxygen desorption peaks: the desorption peak of oxygen in the low-temperature range of 50-350 °C was the adsorbed oxygen (O 2 − ); the medium-temperature range of 350-750 °C was the chemisorbed oxygen (O − ) adsorbed in the oxygen vacancies; the high-temperature range above 750 °C was the lattice oxygen (O 2− ). He et al. (2015a, b) also found CuMnCeO x /cordierite with three oxygen desorption peaks. The desorption peaks at 250 °C and 480 °C were ascribed to the adsorbed oxygen (O 2 − ) and the chemisorbed oxygen (O − ) adsorbed at the oxygen vacancies, respectively. The temperature of oxygen desorption (O 2 − and O − ) gradually shifted to a lower temperature region. With the increase in CeO 2 incorporating, the peak area of adsorbed oxygen (O 2 − ) decreased, and chemisorbed oxygen (O − ) increased gradually, indicating that it was conducive to the O 2 activation over the catalyst surface with more CeO 2 incorporating. Meanwhile, the peak area of oxygen desorption at 717 °C slowly decreased and even disappeared with 20 wt.% CeO 2 incorporated, implying that more active oxygen species could be generated to improve the oxidation activity.

XPS analysis
The XPS measurements of CuMnCeO x /cordierite were performed to elucidate the chemical states of its surface element. Ce 3d, Mn 2p, Cu2p, and O 1 s spectra of catalysts were incorporated with different amounts of CeO 2 . Figure 5a showed the XPS spectra of Ce 3d. The spectral peaks at 886.14 eV and 905.05 eV were assigned to 3d 5/2 and 3d 3/2 of Ce 3+ , respectively, and the rest were characteristic peaks of Ce 4+ . When the characteristic peak of Ce shifted to low binding energy, the peak area of Ce increased. With the increase in CeO 2 incorporating from 10 to 20 wt.%, the whole concentration of Ce 3+ and Ce 4+ increased gradually, while the proportions of Ce 3+ /Ce 4+ decreased from 32.1 to 22.9%. Regarding XPS spectra of Mn 2p (Fig. 5b), the Mn 2p 3/2 peak at 642.4 eV indicated the presence of Mn 3+ on the surface. The peak at 645.9 eV confirmed the presence of surface Mn 4+ , and the XPS spectra of Mn 2p of CuMnCeO x /cordierite did not significantly change with the varying CeO 2 incorporating, possibility due to that CeO 2 did not have a significant effect on MnO x . It is generally believed that higher valence MnO 2 has stronger catalytic activity for VOCs. The coexistence of Mn 4+ and other valences Mn will make the catalyst surface rich in oxygen vacancies and unsaturated chemical bonds, which is more conducive to oxidation reaction . According to the XPS spectra of Cu 2p 2/3 (Fig. 5c), the peak at 935.9 eV was ascribed to Cu 2+ . The characteristic peak area of Cu 2+ gradually expanded with the CeO 2 increase, indicating that increase in CeO 2 caused an expansion of CuO particle size. According to XPS spectra of O 1 s (Fig. 5d), the peaks at 529.51 eV, 531.9 eV, and 533.2 eV were ascribed With the increase of CeO 2 , the proportion of O sur /O latt grew from 18.3%, 24.4% to 27.6%, and the high oxidation performance was owing to more surface active oxygen (O sur ).
With the increase of CeO 2 incorporating, more Ce 3+ over the surface can form more oxygen vacancies to maintain the electroneutrality balance of the catalyst surface (Lu et al. 2017), thus the catalysts exhibited stronger oxidation performance. However, the surface lattice oxygen (O latt ) decreased, because part of the surface lattice oxygen existed in the bulk phase with the excessive CeO 2 incorporation.

In situ DRIFT measurements
In situ DRIFT measurements of CB adsorption and oxidation on CuMnCeO x /cordierite were illustrated in Fig. 6. The bands at 1240 and 1300 cm −1 corresponded to phenate species (Huang et al. 2015b, a). After the CB was initially absorbed onto the catalysts during the oxidation, an adsorption species as phenate was formed (He et al. 2015a, b). The bands at 1379 and 1481 cm −1 were ascribed to carbonate bidentate and C = C degenerate stretching vibrations of benzene (Gao et al. 2022). The bands at 1530 and 1618 cm −1 corresponded to maleate related species (possibly involve chlorine) (Jin et al. 2022) and phenolic species (Huang et al. 2015b, a). The weak band at 1650 cm −1 corresponded to cyclohexanone or benzoquinone species observed at 300 °C (Sun et al. 2016). The band at 1682 cm −1 corresponded to aldehyde-type species (Sun et al. 2016). Noteworthy, when the reaction temperature increased, the band decreased at 1682 cm −1 while the band increased at 1530, 1618 and 1650 cm −1 . When the number of aldehyde-type species produced during the reaction decreased, the numbers of maleate species, phenolic species and cyclohexanone or benzoquinone species increased. Moreover, in the hydroxyl section, a negative band appeared at 3740 cm −1 at 300 °C. This indicated that the hydroxyl on the CuMnCeO x /cordierite catalyst surface interacted with CB, forming hydroxyl-bonded OH (Gao et al. 2022).

Catalytic activity of chlorobenzene oxidation and mechanism
The oxidation activities for CB oxidation over CuMnO x / CeO 2 /cordierite catalysts with different Mn/Cu molar ratios were investigated (Fig. 7a). The CB conversions curves all displayed the "S-type." A certain performance of CB decomposing over the supports of cordierite was low and could be excluded even at high reaction temperature. As for CuMnO x / CeO 2 /cordierite, the catalysts with the Mn/Ce molar ratios of 5:2 and 2:1 exhibited better performances than others, with CB conversion of 94.1% and 91.8% (T 90 = 350 °C), respectively. Meantime, when the reaction temperature increased to 400 °C and above, these samples with different Mn/Cu molar ratios exhibited similar CB conversions and remained at a high conversion. For the CeO 2 /cordierite, when the reaction temperature was lower than 350 °C, it also showed a similar CB conversion as CuMnO x /CeO 2 /cordierite, owing to CB adsorption on the surface. When the reaction temperature increased to 350 °C and above, the CB conversion increased slowly, which was markedly worse than CuMnO x / CeO 2 /cordierite. To improve CB oxidation activities, a series of CuMnO x / CeO 2 /cordierite (n(Mn)/n(Cu) = 5:2) with step-by-step and CuMnCeO x /cordierite catalysts with one-step synthesis were prepared with the increase of CuMnO x incorporation and 20 wt.% CeO 2 supporting ( Fig. 7b and c). The CB conversion of these samples increased consistently with the growing reaction temperature and CuMnO x incorporating. The CB conversions over CuMnO x /CeO 2 /cordierite mainly occurred at 250-350 °C, and the T 90 was at 350 °C. However, the performance of CB oxidation has not been continuously improved with the increase of reaction temperature and CuMnO x incorporating. As for CuMnCeO x /cordierite, the CB conversion was mainly exhibited at 200-300 °C; the CB conversion achieved above 90.0% at 300 °C, and the T 90 was 50 °C lower than that of CuMnO x /CeO 2 /cordierite with stepby-step synthesis. The results showed that the CuMnCeO x / cordierite catalyst with 20 wt % CuMnO x and 20 wt.% CeO 2 incorporating prepared by the one-step synthesis exhibited the highest oxidation activity (above 99.9% CB conversion at 300 °C), indicating the advantages of simpler preparation and lower reaction temperature. Fig. 6 In situ DRIFT spectra of CB oxidation over CuMnCeO x / cordierite catalyst To obtain a stronger performance of catalysts and a lower cost, a series of CuMnCeO x /cordierite catalysts were synthesized with the 20 wt % CuMnOx supporting and different amounts of CeO 2 (Fig. 7d). When the incorporating amount of CeO 2 was reduced from 20 wt.% to 10 wt.%, the CuMnCeO x /cordierite catalysts still displayed a high oxidation performance for CB decomposing. However, the oxidation activity decreased significantly as the incorporating amount of CeO 2 further reduced to 5 wt.% and 1 wt.%, and the T 90 increased from 300 to 350 °C or 400 °C. Among them, the CuMnO x /cordierite without CeO 2 incorporating exhibited the lowest activity. Therefore, it is reasonable to conclude that CuMnCeO x /cordierite catalyst synthesized by one-step with CuMnO x and CeO 2 together exhibited the highest oxidation performance and lower cost for CB conversion (T 90 < 300 °C).
It was worth to mention that calcination temperature was an important parameter to affect the activity and structure of these catalysts. As such, the CuMnCeO x /cordierite catalysts prepared with different calcination temperatures and times were investigated for their catalytic performance of CB oxidation (Fig. S1). While all catalysts exhibited good oxidation performance, the catalyst prepared at 450 °C for 4 h displayed the highest activity. The CB degradation varied with the changing calcination temperature and time, which is in good agreement with SEM results.
The repeatability and stability of the CuMnCeO x /cordierite catalysts were also investigated (Fig. S2). These catalysts demonstrated good repeatability for CB oxidation with three times, and T 90 were all 300 °C without declining activities. Meanwhile, the stability of CuMnCeO x /cordierite catalysts was observed to understand the chlorine-resisting with 24 h and reaction temperature at 300 °C. Fig. S2b showed that the CuMnCeO x /cordierite catalyst could maintain over 99.0% of CB oxidation without obvious chlorine poisoning, indicating that the properties of the prepared catalyst were stable sufficiently to be used for industrial application.
In order to evaluate the CB by-products and explore the possible oxidation mechanism and catalyst route, the outlet gas was absorbed with a kind of VOCs adsorption column (Tenan TA) for 30 min, and the column was released by a thermal analyzer and then injected into MS. The main byproducts of the CuMnCeO x /cordierite were chloropropane (C 3 H 7 Cl) and hexane (C 6 H 14 ), especially a large amount of chloropropane, indicating that the aromatic ring of CB was effectively cracked, and a chlorination reaction occurred over these catalysts ). The complete oxidation of CB (99.9%) over the CuMnCeO x /cordierite achieved at 300 °C, indicating that a strong synergistic interaction appeared in Cu-Mn-O-Ce solid solution during the catalyst oxidation process. According to the Mars-van Krevelen mechanism, this can be explained by the oxygen transfer mechanism, oxygen mobility through the redox cycles of Cu 2+ /Cu, Mn 4+ /Mn 3+ , and Ce 4+ /Ce 3+ (Fig. 8).
First, CuO decomposed the oxygen species, which participated in the oxidation of CB, and Cu achieved oxygen species from Mn 2 O 3 to reoxidized CuO; second, the surface lattice oxygen and surface adsorbed oxygen species from CeO 2 made the regeneration from MnO 2 to Mn 2 O 3 ; third, Ce 2 O 3 was transformed to CeO 2 via the molecular oxygen in the gas phase. Part of the Cl generated during the oxidation process returned to the catalyst surface and was adsorbed over oxygen vacancies or part of the active sites to replace oxygen. When the reaction temperature further increased, the chlorine species adsorbed over the surface reacted with O 2 in the gas phase to generate Cl 2 via Deacon reaction, and thereby the catalyst could maintain stable and efficient oxidation activity for a long time. Based on the results of in situ DRIFT measurements and GC-MS, the CB oxidation route was initially from the dechlorination, aromatics were broken into the phenate species, and then transferred into cyclohexanone or benzoquinone species. The cyclohexanone or benzoquinone species was finally oxidized into the maleate species. Part of the maleate species were oxidized by reactive oxygen species to hexane, and the other part were combined with chemisorbed chlorine to produce chloropropane. As the reaction temperature increased, these hexane and chloropropane molecules were oxidized by reactive oxygen species to form HCl, CO 2 , and H 2 O (Fig. 8).

Conclusion
In this study, the effects of different preparation conditions on the performance of CuMnCeO x supported on cordierite for CB oxidation were investigated. The oxidation activity and structural morphology of these catalysts were influenced by the synthetic methods and conditions with active component of metal oxide. Among them, the CuMnCeO x / cordierite prepared by one-step synthesis exhibited the highest catalytic performance for CB oxidation (T 90 < 300 °C), which was 50 °C lower than the CuMnO x /CeO 2 /cordierite with the step-by-step synthesis. The higher oxidation activities of CuMnCeO x /cordierite correspond to the high specific surface area, stronger dispersion of active components on cordierite, more active oxygen species, and more formed Fig. 8 The proposed chlorobenzene destruction mechanism over CuMnCeO x /cordierite catalyst Cu-Mn-O-Ce solid solution. The synergistic cooperation in Cu-Mn-O-Ce ternary oxides significantly improved the activity, repeatability and stability of supported catalysts without obvious chlorine poisoning. XPS measurement revealed that the increase in CeO 2 caused more Ce 3+ on the surface to form more oxygen vacancies to maintain the electroneutrality balance and more surface adsorbed oxygen (O sur ), but the lattice oxygen (O latt ) decreased. Furthermore, the possible oxidation mechanism was described to illustrate the by-products generation and oxygen transfer of CuMnCeO x /cordierite catalysts. The good catalytic performance with lower cost of these catalysts shows the large potential in industrial application.