Catalytic ozonation of dichloromethane at low temperature and even room temperature on Mn-loaded catalysts

Five Mn-loaded catalysts were synthesized on γ-Al2O3, TiO2, ZrO2, nano γ-Al2O3 and nanoZrO2 supports. The catalytic ozonation of DCM (dichloromethane) was evaluated under industrial conditions (i.e., temperature, O3 input, H2O and SO2 content). According to results, >90% DCM conversion without O3 residue was achieved for all samples at 120 °C and an O3/DCM ratio of 6. At 20–120 °C, the highest Mn3+ content, abundant surface oxygen species and more weak acid sites led to the best performance of Mn/nanoAl2O3 (M/A-II). At 20 °C and 120 °C, 80% and 95% DCM can be degraded respectively on M/A-II at 20 °C with matched surface oxygen species and acidity. An O3/DCM ratio of 6 was optimal for performance and economy. For the effects of complex exhaust, both H2O and SO2 deactivated M/A-II. The H2O-induced deactivation was recoverable and also removed surface-deposited chlorine-containing species, enhancing the HCl selectivity. Finally, the Cl equilibrium of the reaction was comprehensively analyzed.


Introduction
Achieving sustainable green and low-carbon development requires the secondary utilization of waste (e.g., upgrading waste plastics to arenes, 1 using biomass to produce organonitrogen chemicals 2 ) and the reduction of polluting waste (including solid, liquid and gaseous) emissions. Simultaneously, the control of pollutants can also lessen the pressure of waste utilization and conversion. As highly toxic and hard to degrade organic gas pollutants, chlorinated VOCs deserve attention. As a typical species of chlorinated VOCs, dichloromethane (DCM, CH 2 Cl 2 ) has been identied as priority pollutant for strict control in the US, EU, and China. 3 Due to its exceptional organic solubility, DCM has been frequently used in painting, 4 chemical synthesis pharmaceuticals, 5 synthetic resin 6 etc. According to studies, direct exposure and accumulation of highly toxic DCM 7,8 can cause central nervous system damage, cancer, and even genetic mutations. 5,9 DCM emitting into atmosphere will result in photochemical smog and aerosols, as well as forming more powerful radicals that deplete stratospheric ozone. 10 Hence, DCM degradation is an urgent issue to be addressed. Catalytic degradation is the dominant (when DCM < 1000 ppm) destructive technology because of its high degradation rate, low costs, and products controllability.
Most research use noble metals and transition metals as DCM catalysts. The noble metals, i.e., Rh, Ru, Pt, and Pd, are usually loaded on a single or synthetic supports such as TiO 2 , Al 2 O 3 , CeO 2 , SiO 2 , HZSM-5, Ti x Sn 1−x , etc. [11][12][13][14][15] The improved dispersion and formation of ligand bonds between noble metals and supports allowed for efficient DCM degradation. Ru exhibits better capabilities among all these noble metals, while PCDD/Fs-containing by-products was inevitable. 13 Transition metals oxides possess multiple valence states, stronger interactions with appropriate others, and better resistance to chlorine-containing species for DCM degradation. Constantly emerging materials with desired catalytic properties include Nb-CeNTs, 16 X 4 /TiO 2 (X = V, Ce, Mn), 17 Ce/TiO 2 . 18 However, the T 90 (temperature required to convert 90% DCM) of above catalysts were generally above 400°C. To obtain high degradation rates at lower temperatures, several catalyst modication attempts had been made. [19][20][21][22][23] For example, Wang et al. reported that spinel-type CoCr 2 O 4 with high specic surface area prepared by sol-gel method declined T 90 of DCM conversion to 257°C. 21 Xia et al. investigated the effect of sulphating treatment on the iron titanate catalysts, which attained T 90 of DCM conversion declining from 300°C to 250°C. 22 However, the lowest T 90 of DCM conversion over catalysts with transition metals was reported to be ∼200°C. 23 Recently, it has become appealing to employ active molecule O 3 in conjunction with simpler and cheaper catalysts for pollutants degradation. In comparison with conventional catalytic oxidation, O 3 can further lower the activation energy of VOCs oxidation, allowing effective VOCs degradation at temperatures below 200°C or even at ambient temperature, 24 and facilitating catalyst stability. 25 Ozone decomposition increases the quantity of high-valent metal ions and reactive oxygen species (e.g., -O 2 and -OH) on the catalyst surface, contributing to deeper oxidation and increased activity. 26 Among a series of transition metals, O 3 typically performs well over Mn-based catalysts, effectively degrading compounds such as NO, 27 formaldehyde, 28 benzene, 29 toluene, 30 and chlorobenzene. 31,32 However, seldom studies focused on catalytic ozonation of DCM. 33 Generally, low-temperature DCM oxidation over Mn-based catalysts usually exhibits poor performance. Low DCM conversion, negligible mineralization rate, and Cl 2 poisoning of the catalyst due to the active Deacon reaction are commonly reported 17,34,35 as well as lacking considering real industry conditions. Supported catalysts with large surface area and matched surface acidity and redox ability are good choices for catalytic ozonation of DCM. Therefore, it is worthwhile to explore the O 3 coupling of Mn-loaded catalysts to achieve DCM degradation under reality conditions.
Herein, Mn-loaded catalysts were prepared on industrially common supports (g-Al 2 O 3 , TiO 2 , ZrO 2 ) using an evaporationdrying impregnation method. Nano g-Al 2 O 3 and nano ZrO 2 supports were also selected to evaluate the effect of metal dispersion on catalytic behaviours. Firstly, catalytic ozonation of DCM over ve Mn-loaded catalysts with selectivity of target products and by-products were investigated at temperature of 20-120°C. Next, the stability and byproducts of the bestperforming catalysts in DCM degradation at 120°C was evaluated in deep. The effects of H 2 O and SO 2 on catalytic ozonation of DCM are investigated. Finally, the visible Cl equilibrium of the DCM conversion was evaluated.

Catalyst preparation
In the present experiment, manganese based catalysts were prepared by impregnation method in the present experiment. Unpretreated g-Al 2 O 3 , nano g-Al 2 O 3 , TiO 2 , ZrO 2 , and nano ZrO 2 (AR, Aladdin) were used as supports. Around 4 wt% of Mn(CH 3 COO) 2 (AR, 98%, Aladdin) and 3 g support powder was added successively in 30 mL ethanol solution. Aer stirring at 800 rpm for 1 h at room temperature, the mixture was stirred in a water bath at 60°C with suitable speed until nearly dry. The nearly dry samples were moved to the quartz boat and baked at 110°C for 12 h. The solid was calcined at 450°C for 3 h under static air atmosphere, following by grinding and sieving into 40 to 60 meshes. Corresponding to the order of above 5 supports, the obtained catalysts are named as M/A-I, M/A-II, M/T, M/Z-I, M/Z-II.

Experimental setup and activity measurement
The 0.1 g prepared catalyst mixed with suitable amount of quartz sand (40-60 mesh) were loaded in the reactor with a total gas volume of 250 mL min −1 (10% O 3 /O 2 , 100 ppm DCM and equilibrium gas N 2 ) to maintain a constant gas hourly space velocity (GHSV) around 30 000 h −1 . The mole ratio of O 3 /DCM was controlled between 1.5 and 8 (Fig. 1).
The concentrations of DCM, HCl, CHCl 3 , CH 3 Cl, CHOH and other byproducts from outlet were determined by FT-IR ue gas analyzer (Gasmet FTIR DX4000, Finland). The CO and CO 2 in the off gas were rst converted into CH 4 and then analyzed by FID detector in an online gas chromatograph (GC9790-II, Zhejiang Fuli Co., Ltd). The ozone residue was measured by a low concentration ozone analyzer (BMT-932-1, OSTI Inc.; 0-100 ppm, ±0.1 ppm). In particular, the Cl 2 was absorbed in 30 mL NaOH solution (0.01 M) within xed time (e.g. one minute), and the solution was determined by residual chlorine concentration meter (1 mg L −1 , Hana, HI93701). According to eqn (1), the concentration of Cl 2 could be obtained ClO − .
The conversion rate of DCM, the selectivity of HCl, Cl 2 , the yield of CO, CO 2 , CO x (i.e. the total amount of CO and CO 2 ), the ratio of C in-out and the ratio of Cl in-out were dened as: where the in and out at subscript refer to the initial and outlet concentrations of reactants or products. The C out and Cl out represent the total concentration of all detected carboncontaining compounds and chlorine-containing compounds in the exhaust, respectively. The concentration unit for DCM, HCl, Cl 2 , CO, CO 2 , CO x , C out and Cl out are ppm. Fig. 1 The diagram of the DCM activity testing test system.   Table 1. The O 3 /DCM molar ratio was xed to be 6. The ozone residue at the outlet maintainedzero (<0.1 ppm) for all the tested conditions as shown in Fig. 2(a). As displayed in Fig. 2(a), the DCM conversion of M/A-II was the highest from 20 to 120°C. At 20-100°C, the DCM conversions of ve different catalysts were ranked as M/A-II $ M/A-I > M/Z-II $ M/Z-I > M/T, indicating the nano catalyst supports were slightly effective than the non-nano ones. In particular, the DCM conversion of M/A-II reached 80% at 20°C and continued to rise as the temperature increased. While, the other four catalysts showed a decrease in DCM conversion as the temperature elevating from 20 to 60°C, which should attribute to the by-products generation and will be explained in section 3.  17) and less than 10% over 30MnAl. 35 Mn-Co@Z loaded with a dual active component also failed to achieve 50% DCM conversion at 360°C

Results and discussion
. 34 Obviously, the addition of ozone can greatly decrease the needed reaction temperature of DCM. Fig. 2(b) shows the yields of CO and CO 2 . The yield of CO x (i.e., CO + CO 2 ) was approximately equal to the conversion of DCM, indicating effective DCM-catalyzed ozonation and carbon equilibrium. Overall, the higher CO yield may be caused by the limited deep oxidation at low temperatures especially less than 80°C. 31 The highest CO 2 yield on M/A-II, reaching 39.18% at 120°C and results are shown in Table 1, corresponding to the best activity performance, but still lots of CO emitted. Although Xiang et al. 33 obtained slightly higher CO 2 yields over 1/8Cu-HU-MnO 2 than those produced here over M/A-II, the same DCM conversion was also obtained at a lower ozone dosage usage of O 3 /DCM of 6. With the elevation of temperature, the HCl selectivity increased slightly.
The O 3 input directly affects the pollutants conversion and products selectivity. Herein, the variation in DCM conversion, HCl, Cl 2 , CH 3 Cl, and CHCl 3 selectivity over M/A-II with increasing molar ratio of O 3 /DCM were examined, as the results presented in Fig. 2(e) and (f). Clearly, the DCM conversion continuously increased from ∼25% to ∼97% as the molar ratio of O 3 /DCM rose from 1.5 to 6. Finally, the DCM conversion uctuated in the range of 97-99% in O 3 /DCM of 7-8.
The Cl 2 selectivity was much higher than the HCl selectivity. Although the HCl selectivity increased a little when O 3 /DCM reached 6, the Cl 2 selectivity was still much higher and exhibited more obvious elevation at the initial stage, which indicating that O 3 promoted the Deacon reaction. 31,32 Fig. 2(f) presents that CH 3 Cl and CHCl 3 both diminish with the increase of O 3 /DCM. The CHCl 3 selectivity surpassed the CH 3 Cl selectivity along the O 3 /DCM range, and CHCl 3 could not fall to zero even with O 3 / DCM of 8. Fortunately, O 3 residuals maintained almost zero at all tested conditions. In summary, O 3 /DCM = 6 was the optimal conditions in this study both considered the economic and DCM degradation performance.
3.1.2 Other byproducts distribution. The generation of organic byproducts signies incomplete degradation of VOCs. Due the Cl substitution, Cl-VOCs oxidation tends to produce Clcontaining organic byproducts, which might pose more serious  hazards. An FTIR analyzer was used to fully comprehend the distribution of these byproducts during the catalytic ozonation of DCM. Fig. 3(a)-(c) shows the concentrations of the three main non-chlorinated organic compounds with a function of temperature. Both CHOH and HCOOH were below 1 ppm. Comparatively, CH 3 OH was the predominant byproduct of DCM degradation below 80°C, which should originate from hydrodechlorination and oxygenation of DCM. 36 Similar to the rst step of the DCE reaction mechanism on Mn/ZSM-5-27, CH 3 OH was most likely produced by -OH species replacing the chlorine atom of DCM, and the alcohol species gradually decreased with increasing temperature. 36 Aerwards, under the oxidation of reactive oxygen species and O 3 and its decomposition products on the catalyst surface, CH 3 OH was rst converted to intermediate products such as formaldehyde (or carbonxylic-type species 2 ) and formic acid (or R-COO − ), 33 and then formed CO x by further oxidation. Apparently, all the four catalysts produced higher CH 3 OH concentrations than those on M/A-II. On M/A-I and M/T, the CH 3 OH concentration was higher at 20-40°C, but dropped with increasing temperature, accompanied by a rapid rebound of DCM conversion ( Fig. 2(a)). This phenomenon may be caused by the accumulation of continuously generated CH 3 OH with high boiling point (64.7°C) on the catalysts, which competes with the degradation of DCM. For M/ Z-I and M/Z-II, although the concentration of CH 3 OH was less than 10 ppm and declined to ∼1 ppm at 60°C, but the DCM conversion rebounded until 80°C (Fig. 2(a)). The decreasing CO yields in Fig. 2(b) together with the increase in CH 3 OH suggested a weaker recovery ability of M/Z-I and M/Z-II from the effects of CH 3 OH. Fig. 3(d) and (e) presents the dominant chlorinated organic byproducts, i.e., CHCl 3 and CH 3 Cl. Obviously, CHCl 3 is higher than CH 3 Cl but all less than 3 ppm, which is consistent with the results of catalytic ozonation of chlorobenzene, 32 but contrary to the previous catalytic oxidation. 37 This may be related to the dissimilarity between the degradation mechanism of DCM catalytic ozonation and catalytic oxidation. The minimal byproducts on M/A-II suggested higher selectivity for the target product. In contrast, M/A-I and M/T possessed not only large amounts of CH 3 OH, but higher concentrations of CHCl 3 . In sum, the above byproducts reduced to ca. 1 ppm on the ve catalysts when the temperature >100°C.

Stability of DCM ozonation on M/A-II at 120°C and 20°C
As mentioned above, M/A-II not only attained such high DCM conversion at 20-120°C, but also produced negligible organic byproducts, which veries its superiority in catalytic ozonation of DCM. To evaluate the stability at 120°C, Fig. 4 demonstrates variations of multi-parameters with function of time. Clearly, DCM conversion maintained above 95% during a long-time test of about 300 minutes. Throughout the whole reaction process, no O 3 residue was measured in the exhausts at an accuracy 0.1 ppm, implying no risk of ozone contamination. Cl 2 was the main product should be related to the abundance of weakly acidic sites. As shown in Fig. 4(b), the CO 2 yield varies from around 35% to 40% and carbon equilibrium was achieved within the absolute value of 6% ratio of C in-out .
Moreover, the redox performance of M/A-II at 20°C was comparable to catalysts with complex preparation methods and even to catalysts doped noble metal for O 2 catalytic oxidation at about 280°C. 14,18 As shown in Fig. S2, † the DCM conversion of about 80% at 20°C was also stable for about 120 min. Interestingly, the Deacon reaction for Cl 2 formation typically required a higher temperature, 7 but Cl 2 selectivity still attained 60-70% with a low HCl selectivity, ca. <5%, which should attribute to the promotion effects from O 3 . However, aer 60 min, the byproduct CH 3 OH emerged and continually rose, which might attribute to the accumulation of intermediates on catalyst surface. The simultaneous larger decrease of CO yield in Fig. S2(b) † also demonstrated that more CH 3 OH was produced without further deep oxidation. Overall, the too low temperature is not favorable for desorption of the deposits and induced catalyst deactivation in redox ability.    40 The N 2 adsorption and desorption isotherms and pore size distribution curves of the ve Mn-loaded catalysts are displayed in Fig. S5. † They all belong to type IV isotherms with H3-type hysteresis loops. The pore size for all these catalysts was mostly dispersed across 50 nm, representing the coexistence of mesoporous and macroporous characteristics, which could improve mass transfer properties and gas molecule diffusions. 41 (Table  3), which was advantageous for ozone decomposition and thus promoted catalytic ozonation. 31,33 The Mn 2p 3/2 spectra possesses two typical peaks centralized at 642.1-642.6 eV and 644.1-644.9 eV, corresponding to Mn 3+ and Mn 4+ , respectively. 30 Since a higher percentage of Mn 3+ implies a lower average oxidation state (AOS) and more oxygen vacancies, catalysts with nano-support (M/A-II and M/Z-II) possess better O 3 decomposition. 43 The highest percentage of O sur. was obtained for M/A-II, ca. 82.64%, which was consistent with the following O 2 -TPD results. Besides, the effective promotion of high-density oxygen vacancies in the activated adsorption of surface oxygen species O sur. by O 2 molecules allows deep oxidation of VOCs. 44 In particular, O sur. was more critical at low temperatures. 24 Therefore, M/A-II attained the most effective performance in degrading DCM.

Structure and surface properties
On the basis of the mechanism of catalytic ozonation of VOCs over Mn-based catalysts, 26,33,36,45 and byproducts measured by the ue gas analyzer (mentioned in section 3.1.2), we propose a possible mechanism for the catalytic ozonation of   , etc.), followed by further oxidation to form CO x . In addition, Mn 4+ is restored to Mn 3+ by an electron transfer reaction (eqn (15)), achieving the maintenance of charge balance on the catalyst surface and the replenishment of surface adsorbed oxygen.

Redox ability, oxygen species mobility, and surface acidity
As the H 2 -TPR proles in Fig. 7(a), M/Z-II and M/T possess the large assembled peaks and the three peak tips correspond to the process of three valence changes of MnO x . 30 The lower detected reduction temperatures in the latter two steps indicated better redox properties due to high dispersion. In addition, M/A-II merged the three reduction peaks into one peak with an increased percentage of peak area in the low temperature region. Interestingly, the H 2 uptake values (mmol g cat −1 ) estimated in Table 4  ) at 300-650°C and bulk lattice oxygen (O 2− ) above 650°C. 20,30 According to quantitative estimates in Table 4, the quantitative O 2 desorption of M/A-II was the highest both in the low (6.27 a.u. g cat −1 ) and medium (8.13 a.u. g cat −1 ) temperature region, illustrating the high content and mobility of surface adsorbed oxygen species and surface lattice oxygen. 20,33 These characteristics promoted Deacon reaction, 47 resulting in the highest DCM conversion rate and strong resistance to chlorine-containing species. The distribution of surface acidity and acidic are crucial factors in catalytic degradation of VOCs. The NH 3 -TPD proles in Fig. 7(c) possess weak, medium, and strong acidity based on the temperature of the NH 3 desorption peak. 33,48 Table  4 further validates that the weak acid sites ratio decreased as the order: M/A-II > M/Z-II > M/T, which was associated with the breakage of C-Cl bonds, 33 thereby contributing to the highest activity on M/A-II. Additionally, the synthesis of HCl and Cl 2 was closely related to acidity and redox properties. 7,48-50 The stronger acidity of M/Z-II and M/T corresponded to their higher HCl selectivity. Although the acidity of M/A-II was weaker, it obviously had the largest peak area with a more abundant acid center. Moreover, the better low-temperature redox ability together with the promotion of O 3 could contribute to the improvement of Cl 2 selectivity, which was probably due to the Deacon reactions 2HCl + 1/2O 2 = Cl 2 + H 2 O and 2HCl + O 3 = Cl 2 + H 2 O. 32 The Py-IR proles presented in Fig. S6 † demonstrate that Lewis acid sites (1445 and 1610 cm −1 ) predominated over Brønsted acid sites (1540 cm −1 ) acid sites for all these catalysts, hence facilitating the breakage of the C-Cl bond and ozone decomposition. 18,30 As reported, the matched degree of acidity and redox ability within reaction temperature range was also important. 19 The abundant weak acidity of M/A-II could better match with the lower H 2 reduction temperature and higher O 2 mobility, thus explaining its excellent DCM degradation ability.
DCM adsorption is the rst step for catalytic ozonation. The desorption properties of DCM over the catalyst were revealed using DCM as a probe molecule. As shown in Fig. 7(d), the DCM desorption below 200°C decreased as the order: M/A-II > M/Z-II > M/T. M/T only possessed higher DCM desorption at 567 and 739°C, demonstrating stronger adsorption affinity of DCM, which was disadvantageous for DCM conversion at such low temperature. Interestingly, the temperature points for M/A-II between DCM-TPD, O 2 -TPD, and NH 3 -TPD proles were very close, contributing to its excellent performance at lower temperature.

Effect of SO 2 and H 2 O
SO 2 and H 2 O are inescapable in operational situations. Consequently, the effects of SO 2 and H 2 O on DCM conversion and the selectivity of HCl and Cl 2 were investigated. Fig. 8(a) presents the effect of H 2 O alone on catalytic activity. Aer introduction of 1 vol% H 2 O, DCM conversion rate gradually declined by 8% over the rst 30 min and remained steady at 82%. Interestingly, the HCl selectivity was signicantly increased by 20%, while Cl 2 selectivity decreased by the same amount. All these values returned back aer removal of moisture. Aer feeding 3 vol% H 2 O into the system, the DCM conversion rapidly decreased to 70%, showing a stronger competition impact. The HCl selectivity substantially rose to 70%, whereas the Cl 2 selectivity slightly declined to ∼30%. Therefore, Cl equilibrium was thus attained by HCl and Cl 2 , indicating the absence of signicant organic chlorinated byproducts. In contrast, only 60-70% Cl equilibrium was achieved under dry conditions. Similarly, the recovery phenomena recurred aer the removal of water. H 2 O exerts a cleaning effect on the chlorine species (detailed in section 3.7) deposited on catalyst surface, hence limiting the synthesis of organic chlorinated byproducts. 20 Besides, H 2 O could serve as a source of H for HCl production and retard the Deacon reaction. 51 On the whole, H 2 O exhibited both positive and negative effects on catalytic ozonation of DCM. In the future, catalysts with a high hydrophobic performance and activation property should be developed to enlarge the positive effect and inhibit the negative effect.
The dynamic experimental curves of the individual effects of SO 2 are displayed in Fig. 8(b). Clearly, severe deactivation was detected in the SO 2 to DCM conversion process. Initially, only 2% drop in DCM conversion was observed and maintained for 30 min aer the low concentration (25 ppm) of SO 2 was introduced into the reaction gas path. However, the curve dropped signicantly aer 30 min and the dropping rate gradually increased. More seriously, DCM conversion continued to fall at a slightly slower rate aer removing SO 2 . Next, higher concentrations (75 ppm) of SO 2 was added, accelerating the rate of decline until the DCM conversion reached ∼20%. Aer 30 min of tolerance to low SO 2 concentrations, the active site on the M/ A-II was progressively deactivated by the coverage of S(IV) species, 47 bisulfate species, 52 and trace sulte species 53 (Fig. S7 †). The deactivation of the sample was further conrmed by comparing the XPS images of Mn 2p 3/2 (in Fig. S9(d) and (e) †), which showed a decrease in Mn 3+ and Mn 4+ and the presence of a small amount of Mn 2+ (Table S1 †). Lin et al. concluded that SO 2 could increase the yield of DCM and HCl during catalytic ozonation of chlorobenzene over MnO x /Al 2 O 3 . 32 In combination with the present experiments, it was further conrmed that SO 2 could not enhance but rather inhibit catalytic ozonation of DCM. Surprisingly, the sharp enhancement of Cl 2 selectivity with decreasing catalyst activity could be attributed to the presence of several active sites of Deacon reaction still present. Furthermore, the emergence of CHOH, HCOOH and CH 3 OH at the end of the test (Fig. S8 †) 32 revealed the coverage of a large number of active sites by suldes. 7,36 In summary, the M/A-II catalyst can endure brief exposure to low concentrations of SO 2 . However, once the tolerable threshold was reached, the constant deposition of sulfate would cover the catalyst surface active sites and diminish its redox capacity. Future research on catalyst should focus on enhancing their capacity to transfer sulfate. Fig. 8(c) depicts the change in catalyst performance resulting from simultaneous introduction of water vapor and SO 2 . Following the addition of 1 vol% H 2 O and 25 ppm SO 2 , the DCM conversion declined rapidly by 10%. and kept falling to below 70%. During the rst cut-off of H 2 O and SO 2 , the DCM conversion curve exhibited a parabolic shape. The rst increase in DCM conversion is due to the re-exposure of H 2 O adsorption sites that compete with DCM. The subsequent decrease was a result of the continued negative effect of SO 2 erosion. Surrounded by 1 vol% H 2 O and 25 ppm SO 2 and higher concentrations of them, HCl production was increased to varying degrees, while Cl 2 remained almost unchanged. Thus, even with the high concentration of SO 2 , the clearance of chlorinecontaining species by H 2 O helped to achieve the visible Cl equilibrium on M/A-II again. Interestingly, the DCM conversion remained at ∼50% despite a pre-existing rapid reduction, during the feeding of 3 vol% H 2 O and 75 ppm SO 2 . In spite of the deposition of S species as well (Fig. S7 †), the DCM conversion rose and stabilized at ∼62% during the elimination high concentrations of SO 2 and H 2 O. It could be inferred that the faster adsorption of water molecules shielded the active site on catalysts surface from sulfation, thus could postpone or even prevent the SO 2 erosion.

Further discussion for Cl balance of DCM decomposition
The present experiment achieved C equilibrium within the error range, but the nal Cl-product was 30-40% different from the Cl content of the imported DCM (the detailed description and data are in the text of ESI and Fig. S10 †). Herein, the surface Cl elements of the M/A-II catalyst aer stability testing and the byproducts undetected by the ue gas analyzer of the exhaust gas were analyzed. The deactivation induced by the deposition of chlorine-containing species during the catalytic oxidation of CVOCs tends to occur on Mn-loaded catalysts. 17,34,35 According to Fig. S9, † the abundant oxygen vacancies brought by ozone. 47 were unable to prevent the deposition of inorganic chlorinecontaining species (e.g., MnCl 2 ) and organic chlorinecontaining species on the catalyst surface. But a deep oxidation of DCM at low temperatures was achieved, i.e., the CO x yield was very close to the DCM conversion in Fig. 2(b). 31,32 Simultaneously, the stability of the degraded DCM was substantially improved. As shown in Fig. S9(b), † more chlorinecontaining species accumulate on the catalyst surface at 20°C due to the slow transfer. Comparatively, fewer chlorinecontaining species on the catalyst surface at 120°C could stabilize the reaction for a longer time ( Fig. 4 and S2 †). Moreover, the clearance of chlorine-containing species (especially inorganic chlorine-containing species) deposited on the catalyst surface by H 2 O veried the remarkable increase of HCl in section 3.5. Unexpectedly, multi-carbon byproducts CCl 4 , C 2 H 4 Cl 2 , and C 3 H 5 Cl (their generation mechanism could be referred to Fig. 6 in the review 49 ) at the ppb level were detected in the off-gas ( Fig. S11 and Table S2 †) aer stability reactions at both 20°C and 120°C. Tiny amounts of byproducts should be avoided in future studies. For instance, catalysts can be prepared using hydrothermal methods to improve product selectivity. 20,54 The absorption of the stainless steels tube for test 31,51 and the greening of the emulsion tube in the present experiment indicate that part of the chlorine-containing species was captured in the pipeline. A comprehensive analysis of the visible Cl equilibrium revealed the presence of four streams of chlorinated products (Fig. 9), including Cl 2 and HCl in the offgas and adsorbed in the pipeline, the chlorine-containing species deposited on the catalyst surface and chlorinated byproducts all at the ppb level.

Conclusions
In conclusion, around 4 wt% Mn-loaded catalysts were synthesized on g-Al 2 O 3 , nanoAl 2 O 3 , TiO 2 , ZrO 2 , and nanoZrO 2 supports using a simple impregnation method and evaluated for the catalytic ozonation of DCM. Firstly, the highly dispersed active species enabled all ve catalysts to achieve more than 90% DCM conversion at 120°C, mostly yielding of CO x , Cl 2 and HCl. Among them, M/A-II exhibited the maximum DCM conversion and CO 2 yield with almost little byproduct. The larger specic surface area and multi-scale pores of M/A-II favored the degradation of DCM to some extent. In addition to the XPS pattern and O 2 -TPD curve analyses, the facilitation of O 3 decomposition by Mn 3+ and the promotion and enhancement of the Deacon reaction and chlorine species resistance by surface oxygen species also explained the best performance of M/A-II. Secondly, an increase in O 3 input (i.e., an increase in the O 3 /DCM ratio) an improved DCM conversion and a reduced byproduct at 120°C. The optimal ratio of O 3 / DCM was selected to be 6 at 120°C, considering the economic and degradation effects. Strikingly, when the O 3 /DCM ratio was 6, M/A-II maintained over 95% of the DCM degradation rate at 120°C for about 300 min and 80% of the DCM degradation rate at 20°C for approximately 120 min. Based on the results of H 2 -TPR, NH 3 -TPD and DCM-TPD, a good match between the superior DCM low temperature desorption performance, more surface weak acid sites and lower reduction temperature allowed for excellent DCM conversion even at lower temperatures. This provides an effective reference for the design of DCM degradation catalysts under moderate conditions. Thirdly, when the H 2 O and SO 2 concentrations increased, deactivation on M/A-II intensied. The reversible deactivation caused by H 2 O also cleaned up the chlorine-containing species (mainly inorganic chlorine-containing species) deposited on the catalyst surface, thus enhancing the HCl selectivity and achieving a visible Cl equilibrium. M/A-II can tolerate a low concentration of SO 2 briey. Nevertheless, once the tolerance threshold he for S species deposition on the surface is reached, the catalyst deactivated irreversibly. The competitive adsorption of H 2 O in the presence of both SO 2 and H 2 O was a protection of the active site and retarded the deposition of S species. Finally, there are two reasons for the difficulties in achieving visible Cl ideal equilibrium (DCM with both HCl and Cl 2 ) for DCM degradation. First, the accumulation of chlorine-containing species on the catalyst surface. Second, the chlorine-containing adsorbed in the stainless steel and emulsion tubes.

Conflicts of interest
There are no conicts to declare.