Comparative investigation on catalytic ozonation of VOCs in different types over supported MnOx catalysts

Journal Pre-proof Comparative Investigation on Catalytic Ozonation of VOCs in Different Types over Supported MnOx Catalysts Guanyi Chen, Zhi Wang, Fawei Lin, Zhiman Zhang, Hongdi Yu, Beibei Yan, Zhihua Wang

PII:

S0304-3894(20)30206-5

DOI:

https://doi.org/10.1016/j.jhazmat.2020.122218

Reference:

HAZMAT 122218

To appear in:

Journal of Hazardous Materials

Received Date:

13 December 2019

Revised Date:

26 January 2020

Accepted Date:

1 February 2020

Please cite this article as: Chen G, Wang Z, Lin F, Zhang Z, Yu H, Yan B, Wang Z, Comparative Investigation on Catalytic Ozonation of VOCs in Different Types over Supported MnOx Catalysts, Journal of Hazardous Materials (2020), doi: https://doi.org/10.1016/j.jhazmat.2020.122218

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Comparative Investigation on Catalytic Ozonation of VOCs in Different Types over Supported MnOx Catalysts Guanyi Chen1, Zhi Wang1, Fawei Lin1*, Zhiman Zhang1, Hongdi Yu1, Beibei Yan1, Zhihua Wang2* School of Environmental Science and Engineering, Tianjin University/Tianjin Key Lab of Biomass/Wastes Utilization, Tianjin 300072, P.R. China State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, P.R. China *

Corresponding author: Tel.: +86-022-87401929; fax: +86-022-87402075 *

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Corresponding author: Tel: +86-0571-87953162; fax: 0571-87951616

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Email addresses: [email protected] (F. Lin); [email protected] (Z. Wang)

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Graphic Abstract

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2

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1

Highlights: 

Relationships between catalytic ozonation behavior of CB and support property were explored.



The difficulty of degradation decreased as the order: DCM > DCE > CB > PhH.



Co-existence of DCE significantly improved CB conversion to reach totally degradation.



DCM, carboxyl and formic acid were detected as critical intermediates for CB ozonation.

ABSTRACT: This paper conducted catalytic ozonation of CB (chlorobenzene) over a series of MnOx based catalysts

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with different supports (Al2O3, TiO2, SiO2, CeO2, and ZrO2) at 120 oC. Mn/Al2O3 exhibited highest CB conversion efficiency, ca. 82.92%, due to its excellent textual properties, O2 desorption, redox ability, and desirable surface adsorbed oxygen species and acidity. O3 conversion all approached nearly 100%, with

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residual <10 ppm. Mn/Al2O3 was further employed to investigate effect of temperature, O3/CB, and space

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velocity on CB conversion. Hereafter, catalytic ozonation of single and binary VOCs in different types was

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performed, i.e., CB, DCE (dichloroethane), DCM (dichloromethane), and PhH (Benzene). Conversion results demonstrated aromatics degraded easier than alkanes and more carbon atoms decreased difficulty,

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as CB~PhH > DCE~DCM, and DCE > DCM; but chlorinated substitution increased difficulty, as PhH > CB. Catalytic co-ozonation of CB/DCE indicated that DCE significantly improved CB conversion to reach

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totally degradation at low O3 input, but inhibited DCE conversion, especially at higher ratio of DCE/CB. Co-ozonation improved ozone utilization efficiency, and maintained the original property of catalyst. By

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contrast, CB/PhH co-ozonation displayed very mild effects. Finally, critical intermediates during catalytic CB ozonation, i.e., DCM, carboxyl and formic acid, were detected from mass spectrum results. Keywords: Catalytic ozonation, chlorinated VOCs, alkanes and aromatics, Mn/Al2O3, co-ozonation

1. Introduction Volatile organic compounds (VOCs) have severe hazard to human health and environment, and have been proved to be important precursors for the formation of ozone, secondary organic aerosols and photochemical smog [1, 2]. VOCs mainly origin from industrial manufacture, solvent use, transportation, habitation, etc. China’s government has released a comprehensive VOCs treatment project from key industries, including petrochemical industry, chemical industry, industrial spraying, package printing, oil storage and distribution, industrial parks and clusters, etc. Additionally, waste incineration also contributes

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to VOCs emissions [3], which should assign to organized discharge. No matter which kind of emission source, VOCs emission possesses characteristics of unstable concentration and co-existence of various types. Among varieties of VOCs, chlorinated volatile organic compounds (CVOCs) such as

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dichloromethane (DCM), dichloroethane (DCE), chlorobenzene (CB), 1,2-dichlorobenzene (o-DCB) are

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especially difficult to be naturally degraded due to their high chemical stability and low biodegradability,

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and can accumulate in environment, thereby causing persistent pollution [4-6]. Hence, elimination of CVOCs has been widely concerned, and researchers devoted to develop efficient technologies for CVOCs

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removal.

VOCs removal technologies can be categorized into two series: recovery and degradation, which

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mainly include adsorption, non-thermal plasma, photocatalysis, regenerative combustion, catalytic oxidation, and biological degradation [7-9]. Catalytic oxidation is recognized as one of the most efficient

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technologies for low concentration VOCs degradation due to multicomponent adaptability, high efficiency and mineralization rate [2, 6, 10]. VOCs catalytic oxidation is strongly correlated with highly efficient catalysts. Generally, noble metal such as Au, Pt, Pd, Ru, etc. demonstrates excellent catalytic activities at low temperature. However, these noble catalysts still face with problems of high cost, chlorine species accumulation, and inclining to sinter, which resist their industrial application [11, 12]. Alternatively, transition metal oxides-based (MnOx, Fe2O3, Co3O4, CuO and V2O5) catalysts possess variable valance

states and higher resistance to chlorine species, as well as low cost, therefore attracting widely investigation [13-15]. MnOx was most concerned for catalytic oxidation of benzene [16], toluene [17], ethanol, ethyl acetate [18], chlorinated aromatics [19], et al. Researchers also conducted several kinds of modification for MnOx based catalysts, such as morphology regulation [17, 20], supports introduction [21-23], metal modification [5, 24], perovskites alternation [25]. Desirable conversion efficiency can be usually attained over these catalysts, just with different demanded temperature. However, catalyst deactivation induced by coke, chlorine species, and byproducts accumulation were always inevitable.

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Ozone assistance significantly decreases apparent activation energy, and thereby improving conversion at lower temperature, alleviating catalyst sintering, and attaining high stability [26, 27]. Ozone addition could induce several improvements on catalyst surface, including activating more oxygen species

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with high mobility and converting metal ions to higher valance state, which finally promotes catalytic

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activity [28]. Similarly, noble metals and transition metal oxides were investigated as highly efficient

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catalysts for VOCs degradation by catalytic ozonation, including Pt, Ag, Cu, Co, Mn, Ce, etc. [27, 29-31]. Among them, manganese oxides were most commonly researched due to its strong reducibility and multiple

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oxidation states, as well as superior performance in ozone decomposition [32-34]. Unsupported manganese oxides (MnOx) were synthesized by redox [35], precipitation [26, 27], nano-replication [36], and

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evaporation-to-dryness [37] for catalytic ozonation of HCHO, PCDD/Fs, C6H5Cl, C6H6, respectively. Generally, supported catalysts exhibit higher surface area that is beneficial to catalytic behavior. It was

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found that manganese acetate exhibited better performance than other precursors, which displayed better dispersion, higher oxygen mobility, and higher Mn3+ ratio, i.e., more oxygen vacancies [38]. Hence, this paper selected MnOx based catalysts derived from manganese acetate with different supports to investigate catalytic activity of VOCs in different types by ozone. So far, catalytic ozonation mainly focused on benzene and toluene, while few literatures reported CVOCs ozonation expect for PCDD/Fs and their congeners. Especially, researchers usually devoted to

develop new catalysts to improve catalytic ozonation performance of single component, but ignored coexistence of various VOCs. Guo et al. found that catalysts displayed totally different activity between toluene and chlorobenzene [39]. However, catalytic co-ozonation performance of multiple VOCs has not been reported. It is important to explore catalytic behaviors over different types of VOCs, which helps to instruct catalyst design for real application. This paper mainly includes three parts: 1) catalyst screening from CB ozonation; 2) comparison of VOCs in different types; 3) catalytic co-ozonation of CB/DCE and CB/PhH. Firstly, MnOx based catalysts supported on Al2O3, TiO2, SiO2, CeO2, and ZrO2 were synthesized

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to compare their activity in CB ozonation. Several kinds of characterizations were employed to obtain textual properties, crystal structure, surface elements, redox properties, surface acidity, and O2 desorption abilities. Secondly, the optimal candidate was further utilized for DCM, DCE, and PhH ozonation, which

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represented chlorinated alkanes with different carbon atoms, non-chlorinated aromatics, respectively. CB

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could represent chlorinated aromatics. Finally, catalytic co-ozonation of CB and DCE, as well as CB and

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PhH were performed to detect the interaction between different types of VOCs. Variation of surface changes and adsorbed species after long-term ozonation were analyzed.

2.1 Catalyst preparation

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2. Experimental section

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Al2O3, TiO2, SiO2, CeO2, and ZrO2 were used as support materials. Al2O3 (gamma-phase, Alfa Aesar), TiO2 (AEROXIDE@TiO2 P25, Evonik degussa), and SiO2 (AEROSIL 380) were purchased from

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commercial company without any pretreatment. CeO2 was prepared by direction calcination of cerium nitrate (Ce(NO3)2·6H2O, AR, 99.0%, Sinopharm, China) at 450 oC for 2h in a tube furnace under static air. ZrO2 was prepared by urea precipitation method. The zirconyl nitrate hydrate solution (ZrO(NO3)2 xH2O, Meryer, Zr: 27.3 wt.%) with a concentration of 1.0 mol/L was added into urea solution with a concentration of 1.6 mol/L. The mixture was stirred vigorously at 90 oC with hot distilled water continuously addition to maintain the liquid volume until pH came back to neutral. After dried overnight at 110 oC, the sample was

crushed into powder for two-step calcination in a tube furnace under static air, i.e., 100 oC for 1 h and then heated to 450 oC for 3 h. The supported MnOx catalysts were synthesized by impregnation method. 0.4456 g Mn(CH3COO)2·4H2O (AR, 99.0%, Aladdin) was dissolved in 20 mL ethyl alcohol solution, following by addition of 1 g support. After stirring at room temperature for 1 h, the mixture was stirred in a water bath at 60 oC. When the solution was evaporated to nearly dryness, the beaker was transferred to oven and dried at 110 oC overnight. Finally, the sample was calcined at 450 oC for 3 h with a heating rate of 1 oC/min in a tube furnace under static air. The obtained catalysts were sieved to 40~60 mesh for activity tests and

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labeled as Mn/Al2O3, Mn/TiO2, Mn/SiO2, Mn/CeO2, and Mn/ZrO2, respectively. 2.2 Gaseous catalytic oxidation

Catalytic ozonation of CB, DCM, DCE, and PhH was performed in a fixed bed quartz reactor (i.d. 8

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mm) with continuous gas flow, as shown in Fig. S1. The cylinder gas was supplied by Ludong Gas Co.,

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Ltd. (N2-99.99%, O2-99.999%). O2 first flowed into a DBD reactor (Dielectric Barrier Discharge, HTU500E, 1 g/h, Canada AZCO) to generate ozone. A separated N2 with a flow rate of 16 mL/min flowed

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through a bubbling saturator maintained at 5 oC to generate gaseous CB. DCM, DCE, and PhH were

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supplied from cylinder gas balanced with N2, and VOC concentration was 500 ppm in cylinder. The VOC contained gas was mixed with N2 and O3/O2 before flowing into the reactor. Typically, the initial

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concentration of CB, DCM, DCE, and PhH was fixed at 100 ppm except for investigation on concentration, while CB concentration exhibited a certain fluctuation due to instability of bubbling. The total flow rate

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was controlled by individual MFC (Mass Flow Controller, Sevenstar Electronics Co., Ltd., Beijing) and maintained at approximately 100 mL/min. A micro electrical furnace was arranged to provide reaction temperature. Considering CB condensation, the heat-tracing pipeline was arranged and maintained at 120 o

C from the exit of bubbling saturator to GC analyzer. Except for investigation on effect of relevant

parameters, 0.025 g of catalyst was loaded in the reactor, reaction temperature was 120 oC, and molar ratio of O3/CB was 10.0. In the next corresponding sections, catalyst dosage, temperature, and O3 input were

varied for investigation of its effects. The ozone input was monitored continuously by an ozone analyzer (BMT-964BT, OSTI, Inc., 0~100 g Nm3, ±0.1 g/Nm3) with a separated O3/O2 gas from ozone generator. An online GC (Gas Chromatograph, GC9790II, Zhejiang Fuli Co., Ltd) was equipped with two individual FID detectors for the measurement of CB and COx concentrations. CO and CO2 were firstly converted into CH4 in the built-in converter of GC and then could be analyzed in the FID detector. The residual ozone in outlet flue gas was measured by a low concentration ozone analyzer (FGD2-C-O3, 0~200 ppm, ±0.1 ppm). The off-gas was absorbed by

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0.0125 mol/L NaOH solution for 150 min, and the concentration of Cl− (as resulted from Cl2 and HCl, Eq. (1)) and ClO− (as resulted from Cl2, Eq. (2)) was measured by using an ion chromatograph instrument (ICAP-MS-QC, Thermo Fisher Scientific). Prior to experiments, gas mixture flowed through catalyst bed

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at target temperature for 2~6 h to stabilize CB, DCM, DCE, and PhH concentration. All results were

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obtained after more than 2 h to ensure stabilization, which were averaged by last ten points.

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Cl2 + NaOH = NaCl + NaClO + H2O HCl + NaOH = NaCl + H2O

(1) (2)

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VOC (CB, DCM, DCE, and PhH) conversion, O3 conversion, mineralization efficiency (Min. eff.), and COx/HCl selectivity were calculated as the following equations, respectively.

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VOC conv. = ([VOC]initial - [VOC]outlet) / [VOC]initial × 100%

(3) (4)

Min. eff. = [CO2]outlet / (N × [VOC] initial) ×100%

(5)

CO2 sel. = [CO2]outlet / ([CO] outlet + [CO2]outlet) ×100%

(6)

CO sel. = [CO]outlet / ([CO] outlet + [CO2]outlet) ×100%

(7)

HCl sel. = [HCl]outlet / ([CB]initial - [CB]outlet)×100%

(8)

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O3 conv. = ([O3]initial - [O3]outlet) / [O3]initial × 100%

where [VOC]initial and [VOC]outlet are initial and outlet CB, DCM, DCE, and PhH concentration in ppm, respectively. [O3]initial and [O3]outlet are the initial and outlet O3 concentration in ppm, respectively. N is the

number of carbon atom in target VOC molecule. [CO]outlet and [CO2]outlet are the CO and CO2 outlet concentration in ppm, respectively. [HCl]outlet is the HCl outlet concentration in ppm. 2.3 Catalyst characterization The pore structure parameters of catalysts were measured through an automated gas sorption analyzer (Quantachrome), which were degassed at 250 oC prior to experiment. The surface area, total pore volume, and average pore diameter were determined by Brunauer–Emmett–Teller (BET) model from N2 adsorptiondesorption isotherms and Barrett–Joyner–Halenda (BJH) method from the desorption branch, respectively.

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The X-ray diffractometer (XRD) patterns were collected by an X’Pert Pro MRD (PA-Nalytical, Netherlands) diffractometer using Cu Kα radiation ( =0.154056 nm) with an X’celerator detection system. Data was collected at 40 kV and 40 mA from 10 to 80o (2θ, diffraction angle).

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The X-ray photoelectron spectroscopy (XPS) was conducted using a photoelectron spectrometer

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referenced to the C 1s line at 284.6 eV.

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(Thermo Scientific Escalab 250Xi) with a standard Al Kα source (1486.6 eV). All binding energies were

The temperature-programmed reduction (H2-TPR) and ammonia temperature-programmed desorption

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(NH3-TPD) were performed by Quantachrome chembet TPD/TPR. The oxygen temperature-programmed desorption (O2-TPD) were all conducted using Micromeritics Autochem II 2920 TPD/TPR. For H2-TPR,

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~50 mg of catalyst was charged in the U-tube. The catalyst was firstly heated to 150 oC (10 oC/min) under 50 mL/min of pure He (>99.999%) and kept for 1 h to remove adsorbed species on catalyst surface. When

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temperature was reduced to 50 oC, the flow gas was switched to 10% H2/He (30 mL/min). After stabilization, the typical TPR process was started by a ramp of 10 oC /min up to 800 oC. The hydrogen uptake was quantified by referring to the results of copper oxide. For NH3/O2-TPD, 50 mg/100 mg of catalyst was loaded and purged at 150 oC (10 oC/min) for 1 h under 30 mL/min of pure He (>99.999%). The temperature was then cooled down to 50 oC. Subsequently, the flow gas was switched to 50 mL/min of 7%NH3/He and 5%O2/He, respectively. After adsorption for 1 h, the flow gas was switched to 30 mL/min of pure He and

stabilized for another 1 h. Then, TPD was initiated by a ramp of 10 oC/min up to 800 oC. TG-MS (Thermogravimetry-Mass Spectra) measurements of spent Mn/Al2O3 catalysts after 200 min ozonation reaction with different O3 input were conducted to investigate surface species accumulated on catalyst surface. A thermal analyzer (Netzsch Sta 449 F3) connected with a mass spectrometer (Netzsch Qms 403 Aeolos). ~15 mg spent catalyst was firstly purged at 50 oC for 30 min with 50 mL/min pure Ar flow, following by heating to 1000 oC with a heating rate of 10 oC/min under same atmosphere. The weight loss signal and MS signal of desorbed gas were obtained instantaneously.

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3. Results and discussion 3.1 Catalytic ozonation of CB 3.1.1 Effect of catalyst support

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Catalytic ozonation of CB was performed over five MnOx catalysts supported on Al2O3, TiO2, SiO2,

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CeO2, and ZrO2, respectively. Catalytic behaviors, including CB conversion, O3 conversion, and COx

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selectivity, are presented in Fig. 1, and detailed values are tabulated in Table 1. Clearly, Mn/Al2O3 exhibited the highest CB conversion, ca. 82.92%, and the other catalyst decreased as the order: Mn/TiO2 >

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Mn/SiO2 > Mn/CeO2 > Mn/ZrO2. Hereafter, Mn/Al2O3 was selected as optimal catalyst for further investigation. Nevertheless, all these five catalysts possessed higher than 70% of CB conversion at this

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specific condition. Table 1 also presents CB oxidation with absence of ozone, which only reached 27.99% until 360 oC, verifying strong promotion effect of ozone on CB oxidation, especially at low temperature.

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Mn/CeO2 exhibited lower CB conversion but with the highest CO2 selectivity, ca. 69.02%. By contrast, CO2 selectivity of Mn/Al2O3 was much lower, ca. 63.74%. However, CO2 selectivity seemed to be no strongly relevant with catalysts, which were relatively stable between 60~70%. As expected, O3 conversion all approached nearly 100%, with residual concentration less than 10 ppm, indicating effective decomposition. The O3 utilization efficiency was calculated based on Eq. (9) to be 81.23%, 81.65%, 79.64%, 78.15%, and 78.97% for these five catalysts, respectively. This value took CB conversion

efficiency, CO2 selectivity, and O3 residual concentration into consideration. Mn/Al2O3 and Mn/TiO2 also exhibited the highest O3 utilization efficiency. Fig. S2 demonstrates increasing tendency in residual O3 along with the 140 min-stability tests, but relatively stable CB conversion and CO2 selectivity were observed. This indicates parts of O3 did not participate in CB ozonation but intermediates accumulation weakened its decomposition behavior. All the experiments exhibited carbon balance (atomic ratio of (CO2+CO+CBoutlet×6)/(CBinitial×6)) between 95% and 100% (not shown here). Sometimes carbon balance higher than 100% was also observed due to unstable CB concentration. Only HCl was detected from

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absorbed solution without Cl2. Table 1 also tabulates HCl selectivity based on balance of original Cl in CB. The stainless-steel pipe connected behind the catalytic reactor inclined to adsorb these generated chlorinecontained species [40]. Therefore, the actual HCl yield should be higher than detected value, which explains

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lower HCl selectivity in Table 1. Nevertheless, HCl and Cl2 can be adsorbed together on the internal wall

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CB conv.

CO2 sel. 100

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80

CO sel.

80

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CB conv./O3 conv. (%)

100

O3 conv.

60

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60

40

40

20

20

0

COx sel. (%)

lower than HCl so that it was totally adsorbed.

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of pipe, thus HCl might be the only chlorine-contained inorganic products, and perhaps Cl2 yield was much

0 Mn/Al2O3 Mn/TiO2

Mn/SiO2

Mn/CeO2

Mn/ZrO2

Catalysts Fig. 1. Catalytic performance of CB ozonation over MnOx with different supports. Molar ratio of O3/CB:

10.0, temperature: 120 oC, catalyst dosage: 0.025 g. Table 1 Catalytic activity of CB ozonation over MnOx with different supports

Catalyst

CB conc.

O3/CB

ppm

O3 deco. rate

CB conv.

CB conv. rate

CO2 sel.a

Min. eff.

Catalyst dosage

HCl sel.

×10-5 mol gcat-1 min-1

%

×10-5 mol gcat-1 min-1

%

%

g

%

Temp. o

C

114

9.9

73.68

65.68

43.00

0.025

30

100

10.0

76.11

66.17

54.91

0.025

60

107

10.0

80.33

65.29

56.88

0.025

90

107

9.3

63.74

57.20

0.025

100

9.9

65.26

62.43

51.35

0.025

150

107

2.3

23.48

0.44

72.17

11.85

0.025

120

107

4.7

49.28

0.91

65.18

27.76

0.025

120

107

7.1

70.38

1.31

63.68

41.41

0.025

120

107

9.3

82.92

1.48

63.74

57.20

0.025

107

11.6

86.72

1.61

63.57

62.69

0.025

120

107

14.0

91.73

1.70

114

10.0

80.50

3.26

96

10.0

85.31

0.78

110

10.0

84.89

114

0

0.86

114

0

114

0

114

0

114

0

Mn/TiO2

95

10.0

Mn/SiO2

110

9.7

Mn/CeO2

109

Mn/ZrO2

104

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32.81

32.81

120

120

64.19

64.06

0.025

120

62.75

55.54

0.0125

120

63.58

56.22

0.05

120

63.53

60.28

0.1

120

97.81

1.71

0.025

120

4.43

95.80

2.40

0.025

180

11.36

93.01

5.85

0.025

240

14.81

89.94

13.88

0.025

300

85.35

23.06

0.025

360

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1.48

0.42

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15.91

82.92

27.99 76.93

1.24

66.63

55.70

0.025

33.39

120

19.41

73.92

1.41

66.78

50.74

0.025

30.44

120

9.9

17.21

73.04

1.38

69.02

49.18

0.025

25.17

120

10.1

19.25

71.90

1.24

68.95

50.51

0.025

27.55

120

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16.20

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Mn/Al2O3

15.91

3.1.2 Effect of operation parameters Temperature, O3 input amount, and catalyst dosage are critical operation parameters. Fig. 2 presents effect of these parameters on CB ozonation performance over Mn/Al 2O3 catalyst, and detailed values are also tabulated in Table 1. Considering low temperature application, temperatures from 30 oC to 150 oC were studied (Fig. 2(a)). Initially, temperature rising gradually promoted CB conversion, ca. from 73.68%

at 30 oC to 82.92% at 120 oC. When temperature further increased to 150 oC, obvious decrease from 82.92% to 65.26% was observed. Generally, temperature rising could accelerate reaction rate to improve CB conversion by ozone. However, ozone decomposition also accelerated at higher temperature. Excessive ozone decomposition caused insufficient ozone for CB conversion, thereby lowering CB conversion at 150 o

C. Eq. (9) demonstrates the reaction stoichiometry of CB ozonation. The stoichiometric molar ratio of

O3/CB should be 14 that can completely convert CB into CO2 and HCl. Therefore, molar ratio of O3/CB was investigated from zero to 14.0, as shown in Fig. 2(b). Linear growth in CB conversion was observed

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when the molar ratio of O3/CB increased from zero to 9.3. CB conversion further improved with more ozone input but exhibited slower increasing tendency, and finally attained a 91.73% efficiency when the molar ratio of O3/CB was 14.0. Clearly, CB conversion was strongly correlated with ozone concentration,

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but its utilization efficiency decreased as the post slower increasing tendency. Interestingly, increasing

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ozone input did not promote CO2 selectivity. CO2 selectivity attained highest value of 72.17% at a lowest

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molar ratio of O3/CB, ca. 2.3. Subsequently, it decreased to 65.18% and remained stable at approximately 63%. Eq. (9) indicates that lower CO2 selectivity consumes less ozone. Therefore, the slower increasing

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tendency in CB conversion and decreasing CO2 selectivity at higher ozone input demonstrated gradually decreased ozone utilization efficiency. Fig. 2(c) demonstrates effect of GHSV (gas hour space velocity) on

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CB conversion and CO2 selectivity. No obvious changes were detected with different GHSV, indicating catalyst dosage was not the critical factor for catalytic performance. As shown in Table 1, CO2 selectivity

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maintained stable, while CB conversion firstly increased and then decreased when catalyst dosage increased from 0.05 to 0.1 g. According to catalytic ozonation mechanism [38], ozone decomposition on catalyst surface was critical initial step for CB oxidation. Too much catalyst dosage induced excessive ozone decomposition without further utilization for CB conversion and therefore inducing lower CB conversion. CB conversion rate was also listed in Fig. 2(c) for comparison, which distinctly elevated as GHSV increasing. It should be noted that CO2 selectivity always maintained at similar level, ca. 60%~70%, under

these varied parameters and also above five catalysts. C6H5Cl + (14−x)O3 → (6−x)CO2 + xCO + HCl + (14−x)O2 + 2H2O

80

60

40

(c)

CB conv. CO sel.

100

100

CO2 sel.

80

60

40

0

30

60

90

120

150

180

3

80

2

60

40 1

0

0

0

CB conv. rate CO2 sel.

20

20

20

CB conv. CO sel.

0.0

2.3

4.7

7.1

9.3

11.6

23700

37800

58900

-1

GHSV (h )

Molar ratio of O3/CB

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Temperature (oC)

0 14900

14.0

CB conv. rate (×10-5 mol g-cat-1 min-1)

CO2 sel. CB conv./COx sel. (%)

CB conv./COx sel. (%)

(b)

CB conv. CO sel.

100

CB conv./COx sel. (%)

(a)

(9)

Fig. 2. Effect of operation parameters on catalytic performance of CB ozonation. (a) Temperature, (b) Molar ratio of O3/CB, (c) Gas hour space velocity. 3.2 Catalyst characterization

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3.2.1 Textual properties

Table 2 presents textural properties of these five MnOx catalysts with different supports. These five

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supports possessed great difference in BET surface area, pore volume, and diameter. SiO2 and Al2O3

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exhibited better pore structures, thus Mn/SiO2 and Mn/Al2O3 also acquired higher surface area and pore volume. MnOx loading always occupied some original pores from supports and induced decrease in surface

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area and pore volume, while the average pore diameter presented uncertain changeable tendency. In comparison, MnOx loaded in SiO2 support caused most drastic reduction in surface area and pore volume,

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ca. from 555 to 308 m2 g-cat-1 and from 1.17 to 0.61 mL g-cat-1, approximately half reduction. Also, MnOx

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loaded in ZrO2 support induced decrease in surface area from 83 to 14 m2 g-cat-1. But, pore volume increased comparing with support, which might attribute to pores reformation by removal of surface impurities and restructuring. Fig. S3 presents detailed nitrogen adsorption/desorption isotherms and pore size distribution. All these catalysts possessed type I isotherms. Obviously, Mn/Al2O3, Mn/SiO2, and Mn/CeO2 possessed bigger hysteresis loop, and former two assigned to type H3 and Mn/CeO2 assigned to type H2. Mn/TiO2 and Mn/ZrO2 also exhibited very weak type H3 hysteresis loop. Mn/Al2O3 possessed wider pore size distribution from 5 nm to 35 nm, while some small pores were observed for Mn/TiO2, Mn/SiO2, and

Mn/ZrO2. Bigger pore diameter is favorable for diffusion and transport, thereby promoting ozonation rate. Above all, Mn/Al2O3 possessed the highest pore volume and pore diameter, as well as second highest surface area among these five catalysts. This should contribute to its excellent catalytic ozonation performance. Table 2 Textural properties of MnOx with different supports. BET surface area/ m2 g-cat-1

Pore volume b / mL g-cat-1

Avg. pore diameter c / nm

Mn/Al2O3

237 (289)

0.92 (0.96)

14.7 (9.7)

Mn/TiO2

58 (85)

0.12 (0.34)

3.8 (18.7)

Mn/SiO2

308 (555)

0.61 (1.17)

3.0 (11.6)

Mn/CeO2

80 (86)

0.22 (0.24)

7.9 (9.9)

Mn/ZrO2

14 (83)

0.02 (0.01)

3.4 (2.2)

a

Values in parentheses are for corresponding pristine support materials. BJH desorption cumulative pore volume. c BJH desorption average pore diameter.

-p

b

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Catalyst

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3.2.2 Crystal structures

Comparison of XRD patterns between MnOx catalysts and corresponding supports are presented in

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Fig. 3. Al2O3 and SiO2 supports exhibited typical XRD patterns of PDF#29-0063 and PDF#27-0605, respectively. TiO2 support confirmed coexistence of anatase and rutile phase, but anatase phase was

na

dominant. CeO2 support was composed of the fluorite structure of cubic CeO2 (111), (200), (220), (311), (222), (400), and (331). ZrO2 support exhibited tetragonal phase. Standard XRD patterns of Mn2O3

ur

(PDF#41-1442) and MnO2 (PDF#24-0735) were labelled below in each figure. Clearly, after MnOx loading,

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diffraction peaks relating to MnOx were not detected for Mn/Al2O3, Mn/TiO2, Mn/CeO2, and Mn/ZrO2, indicating excellent dispersion in supports. Some weak diffraction peaks of Mn2O3 and MnO2 could be observed in Mn/SiO2, and Mn2O3 was dominant. The relatively smooth XRD pattern in related range was beneficial to present diffraction peaks of doping metal oxides. Furthermore, MnOx loading also caused slight decrease in peak intensity, indicating metal incorporation. Diffraction peaks of CeO2 shifted towards higher angle after MnOx loading, which signified Ce4+ reduction to Ce3+ and increase in ionic radius (Fig.

Intensity (a.u.)

(a)

Al2O3

(b)

☆

Intensity (a.u.)

3(d)) [41].

★ ☆

☆ ★ ★

☆

★

☆ ★ ★

☆

Mn/Al2O3

☆ Anatase ★ Rutile

☆☆

☆

☆☆

☆

Mn2O3 MnO2 10

20

30

40

50

60

70

TiO2

Mn/TiO2 Mn2O3 MnO2

80

10

20

30

50

60

70

80

2 theta (o)

(c)

ro of

2 theta (o)

40

(d)

(111)

☆

★

★

★

★

Mn/SiO2

10

20

30

40

50

60

70

(222)

80

10

(331) (400)

Mn/CeO2

Mn2O3 MnO2

20

30

40

50

60

70

80

2 theta (o)

na

2 theta (o)

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★Mn2O3 ☆MnO2

CeO2

re

SiO2

(311)

-p

Intensity (a.u.)

Intensity (a.u.)

(220)

(200)

(e)

Intensity (a.u.)

Jo

ur

(101)

(112) (211)

(110)

ZrO2

(202)

Mn/ZrO2 Mn2O3 MnO2 10

20

30

40

50

60

70

80

2 theta (o)

Fig. 3. XRD patterns of MnOx catalysts and corresponding supports. 3.2.3 Surface elements Catalytic activity is usually correlated with valance state of Mn species and oxygen species, thus XPS

was investigated for these five catalysts. Mn 2p XP spectra are presented in Fig. 4(a) with two characteristic peaks center at ~653.0 eV and ~641.0 eV that assign to Mn 2p1/2 and Mn 2p3/2, respectively [42]. After deconvolution by Gaussian functions, two or three peaks were obtained, corresponding to Mn2+, Mn3+, and Mn4+ from lower to higher binding energies [43]. Table 3 provides detailed binding energies and proportions calculated by area integration of these three peaks. Mn2+ was only observed for Mn/Al2O3 and Mn/ZrO2, but their proportions of Mn4+ were both much higher than other three catalysts, ca. 43.3% and 41.0%, respectively. This indicates that Mn/Al2O3 and Mn/ZrO2 exhibited higher average oxidation state

ro of

(AOS) of Mn (Mn 3s exhibited very weak signal, not shown). Generally, oxygen vacancy can be implied from AOS of Mn, which is created for charge balance as presence of Mn 3+ [44]. In other words, higher AOS, i.e., lower Mn3+ proportion, represents less oxygen vacancies. Therefore, Mn/Al2O3 and Mn/ZrO2

-p

possessed less oxygen vacancies. It was reported that more oxygen vacancies and surface defects were

re

beneficial to ozone decomposition [44]. However, Mn/Al2O3 and Mn/ZrO2 displayed the highest and lowest

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CB conversion. It is not easy to find the relationship between oxygen vacancies and CB ozonation performance due to too many other factors related to supports. Lattice oxygen (Olat.), surface oxygen (Osur.),

na

and hydroxyl oxygen (OOH) were attained after deconvolution from O 1s XP spectra (Fig. 4(b)) [43]. Most obviously, Mn/Al2O3 possessed much higher surface oxygen proportion than other four catalysts, ca. 84.6%

ur

(shown in Table 3), which was beneficial to ozone decomposition. This testifies more surface-active

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oxygen species of Mn/Al2O3, contributing to its best catalytic performance in CB ozonation.

(a)

(b)

Mn3+

Mn 2p

Olat.

O 1s

Osur. Mn/ZrO2

Mn4+ Mn2+

Mn/ZrO2

Mn/CeO2

Intensity (a.u.)

Intensity (a.u.)

Mn/CeO2

OOH

Mn/SiO2

Mn/SiO2 Mn/TiO2

Mn/TiO2

Mn/Al2O3

660

655

650

645

640

635

536

Binding energy (eV)

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Mn/Al2O3

534

532

530

528

Binding energy (eV)

Fig. 4. XP spectra of MnOx with different supports.

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Table 3 Mn 2p3/2 and O 1s binding energies, valance and species distribution. Mn 2p3/2 Catalyst

O 1s

Mn4+

Mn3+

Mn2+

Mn4+/Mn (%)

B.E. (eV)

Mn3+/Mn (%)

B.E. (eV)

Mn/Al2O3

643.8

43.3

641.7

53.8

639.5

Mn/TiO2

644.5

34.2

641.6

Mn/SiO2

644.3

39.1

Mn/CeO2

644.4

31.2

Mn/ZrO2

643.7

41.0

OOH

B.E. (eV)

Olat./O (%)

B.E. (eV)

Osur./O (%)

B.E. (eV)

OOH/O (%)

2.9

529.4

7.5

531.2

84.6

533.1

7.9

65.8

529.6

71.6

530.6

28.4

641.8

60.9

530.0

2.9

532.8

61.2

533.4

35.9

641.6

68.8

529.3

57.6

530.7

39.9

534.4

2.6

529.6

58.6

530.9

38.0

533.3

3.4

lP

na 641.8

55.6

639.7

3.4

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3.2.4 Redox behavior

Osur.

Mn2+/Mn (%)

re

B.E. (eV)

Olat.

Redox behaviors of catalysts are also correlated with catalytic performance [34]. Generally, Al2O3,

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TiO2, SiO2, and ZrO2 are all inert supports with negligible H2-TPR reduction areas [45]. CeO2 reduction usually occurred at above 450 oC [46]. Therefore, these reduction peaks in Fig. 5 mainly originated from MnOx reduction. Commonly, two step reduction of MnOx, MnO2 to Mn2O3 to MnO, contributed to two characteristic peaks, first peak at 287~322 oC and second peak at 398~432 oC [46]. Besides, they were also assigned to surface adsorbed oxygen and lattice oxygen species, respectively. In comparison, except for Mn/Al2O3, the second peaks possessed higher area, i.e., H2 uptake, indicating less surface adsorbed

oxygen comparing with lattice oxygen. Conversely, Mn/Al2O3 possessed higher H2 uptake from first reduction peak than second peak, implying more surface adsorbed oxygen species. These findings are consistent with XPS results in Fig. 4 and Table 3. Table 4 tabulates quantitative amounts of H2 uptake from Fig. 5, which were calculated by integration and standardized by CuO reduction. Distinctly, Mn/Al2O3 exhibited the highest H2 uptake among these five catalysts, ca. 1.86 mmol g-cat-1. Actually, Mn loadings were constant for these five catalysts. These difference in H2-TPR should relate to interaction between active components and supports. Table 4 also calculated the molar ratio of H2 uptake to reducible

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species, i.e., Mn species in MnO2. However, Mn/Al2O3 possessed a H2/M ratio higher than 1.0, ca. 1.18. On one hand, this demonstrates better redox behavior of Mn/Al2O3. On the other hand, excessive H2 uptake might originate from surface adsorbed oxygen. Other catalysts all possessed H2/M ratio lower than 1.0.

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The relatively higher H2 uptake of Mn/CeO2 was correlated with the excellent redox ability of CeO2

re

support [45, 47]. Except for Mn/CeO2, other four catalysts possessed sequentially decreasing tendency in

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H2 uptake, in consistent with the order of their CB conversion. It can be concluded that abundant oxygen species and oxidation sites of Mn/Al2O3 promote CB ozonation.

o

289 C

H2 uptake (a.u.)

na

287 oC

Mn/ZrO2 398 oC

Mn/CeO2 320 oC

o

427 C

Mn/SiO2

ur Jo

400 oC

315 oC

429 oC

Mn/TiO2

322 oC

432 oC

Mn/Al2O3 100

200

300

400

500

600

o

Temperature ( C)

Fig. 5. H2-TPR profiles of MnOx with different supports. Table 4 Amount of H2 uptake in H2-TPR over MnOx with different supports. Catalyst 10%Mn/Al2O3

Fir. Red. Peak/ oC

Sec. Red. Peak/ oC

H2 uptake/ mmol g-cat-1

H2/M ratio*

322

432

1.86

1.18

*

10%Mn/TiO2

315

429

0.80

0.51

10%Mn/SiO2

320

427

0.65

0.42

10%Mn/CeO2

289

398

1.16

0.74

10%Mn/ZrO2

287

400

0.58

0.37

Ratio between the amounts of hydrogen consumed and the reducible species in molar.

3.2.5 Surface acidity and oxygen adsorption NH3-TPD was investigated to evaluate surface acidity of MnOx catalysts with different supports. Fig. 6(a) presents NH3-TPD profiles and divided into three sections corresponding to weak, moderate and strong acid sites, respectively. It was pointed out that NH4+ ions (Brønsted acid sites) were less thermostable than

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coordinated NH3 molecules (Lewis acid sites) [48]. This demonstrates that NH3 desorption at lower temperature (peak area Ⅰand Ⅱ) represents Brønsted acid, while higher temperature desorption (peak area Ⅲ) signifies Lewis acid sites. Mn/Al2O3 exhibited average distribution in peak area Ⅰ, Ⅱ, and Ⅲ. By contrast,

-p

Mn/TiO2, Mn/SiO2, and Mn/CeO2 possessed very weak NH3 desorption in peak area Ⅲ. Mn/ZrO2 displayed

re

strong desorption in peak area Ⅲ, but very weak in peak area Ⅰand Ⅱ, resulting in its less NH3 desorption. Table 5 lists the quantitative amounts of NH3 desorption. Except for Mn/SiO2, other four catalysts exhibited

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sequentially decreasing tendency, which was in consistent with the order of their CB conversion. Previous works found that VOCs molecules were readily adsorbed on Brønsted acid sites, while Lewis acid sites

na

were strongly correlated with ozone decomposition activity and beneficial to generate active oxygen species

ur

[38, 49, 50]. Recent literatures also proposed that Brønsted acid sites were related to removal of chlorinated species formed during Cl-VOCs oxidation, while Cl-VOCs molecules mainly adsorbed on Lewis acid sites

Jo

[51-53]. Brønsted acid sites can activate C−Cl bond and provide hydride to form HCl, which is beneficial to desorption of chlorinated species [54, 55]. Several literatures also reported to improve catalytic performance by enhancing of Brønsted acid sites [56, 57]. Mn/Al2O3 possessed higher NH3 desorption, as well as abundant Brønsted and Lewis acid sites, which should be an explanation for its best performance of catalytic CB ozonation. O2-TPD was performed after O2 adsorption to investigate the interaction and activation between

oxygen molecules and catalyst surface. O2 desorption profiles are presented in Fig. 6(b). They were divided into two parts with a boundary of 400 oC, which represented surface adsorbed oxygen and lattice oxygen, respectively [16, 58]. Clearly, Mn/Al2O3 and Mn/TiO2 possessed larger amounts of surface adsorbed oxygen. Mn/CeO2 and Mn/ZrO2 also exhibited weak desorption before 400 oC but without obvious peaks, but it also negligible for Mn/SiO2. Mn/SiO2, Mn/CeO2 and Mn/ZrO2 had a distinct desorption peak located at nearly 550 oC. Mn/Al2O3 and Mn/TiO2 also exhibited O2 desorption at temperature higher than 400 oC. On the whole, Mn/Al2O3 possessed largest O2 desorption area within the entire desorption temperature

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window, corresponding to its highest O2 desorption amount, ca. 0.32 mmol g-cat-1, as shown in Table 5. Mn/ZrO2 achieved similar O2 desorption amount with Mn/TiO2, but the desorption peak of Mn/TiO2 centralized at lower temperature than Mn/ZrO2. This demonstrates Mn/TiO2 possessed more active oxygen

-p

species after O2 adsorption. Above all, for Mn/Al2O3 and Mn/TiO2, the higher desorption amount of surface

re

adsorbed oxygen species from O2-TPD implies more active oxygen species, which further resulted in better catalytic performance of CB ozonation.

(b)

na

NH3 desorption (a.u.)

Mn/ZrO2

Mn/CeO2

Jo

ur

Mn/SiO2

100

200

300

400

500

Mn/TiO2

Olat.

Osur.

Mn/ZrO2

O2 desorption (a.u.)

Ⅲ

Ⅱ

Ⅰ

lP

(a)

Mn/CeO2 Mn/SiO2

Mn/TiO2 Mn/Al2O3

Mn/Al2O3

600

700

800

100

200

300

500

Temperature ( C)

Temperature ( C)

Fig. 6. NH3-TPD and O2-TPD profiles of MnOx with different supports. Table 5 Amount of NH3 and O2 desorption over MnOx with different supports. NH3 desorption/ mmol g-cat-1

O2 desorption/ mmol g-cat-1

Mn/Al2O3

5.77

0.32

Mn/TiO2

3.13

0.16

Catalyst

400

o

o

600

700

800

Mn/SiO2

9.67

0.04

Mn/CeO2

2.83

0.09

Mn/ZrO2

1.99

0.15

3.3 Comparison for catalytic ozonation of VOCs in different types Commonly, chlorobenzene is not individually existed in flue gas and waste gas. Therefore, catalytic performance of other VOCs with same catalyst should be investigated. In this paper, DCE (dichloroethane) and DCM (dichloromethane) were selected as typical model chlorinated alkanes with different carbon atoms. Besides, PhH (benzene) was also investigated to compare the catalytic performance between

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chlorinated and non-chlorinated aromatics. The reaction stoichiometry of CB, DCE, DCM, and PhH ozonation can be observed from Eqs. (9)~(12), respectively. Generally, stoichiometric ozone demand is mainly related to numbers of carbon and hydrogen atoms. Presence of chlorine also neutralizes partial ozone

-p

demand. Clearly, the stoichiometric ozone demand for these four VOCs decreased as the order: PhH > CB >

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DCE > DCM. The conversion efficiencies for CB, DCE, DCM, and PhH were all tightly linked with O3 input, but exhibited different activities, as shown in Fig. 7. Fig. 8(a) collects conversion profiles varied

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with O3 input of these four VOCs for clear comparison. All these four VOCs attained conversion higher than 80% at O3/VOC molar ratio ranging from 7.0 to 9.0, and DCE reached firstly. Catalytic performances

na

of CB and PhH were very similar. Clearly, DCE and DCM conversions were higher than CB and PhH at

ur

same ozone input. But, both DCE and DCM exhibited quite low conversion when the molar ratio of O3/VOC was lower than 5.0. Especially, DCE and DCM conversions only reached 67.21% and 42.03% at

Jo

their stoichiometric ozone input ratio, ca. 5.0 and 2.0, respectively. CB conversion attained 91.73% at its stoichiometric ozone input ratio, ca. 14.0. Both CB and PhH conversions also exceeded 80% at O3/VOC of 8.5, lower than their stoichiometric ozone input ratio, ca. 14.0 and 15.0, respectively. Therefore, catalytic ozonation of aromatics is easier than that of alkanes. The difficulty of degradation decreased as the order: DCM > DCE > CB > PhH. It should be noted that DCE and DCM conversions increased slower when O3/VOC surpassed 5.0, especially for DCM, indicating further improvement faced with bigger obstacle.

Above all, aromatics degraded easier than alkanes, as CB~PhH > DCE~DCM in conversion, and chlorinated substitution increased difficulty, as PhH > CB in conversion, as well as more carbon atoms decreased difficulty, as DCE > DCM in conversion. These observations are consistent with previous works on catalytic oxidation by oxygen [59, 60]. C2H4Cl2 + (5−x)O3 → (2−x)CO2 + xCO + (5−x)O2 + 2HCl + H2O

(10)

CH2Cl2 + (2−x)O3 → (1−x)CO2 + xCO + (2−x)O2 + 2HCl

(11)

C6H6 + (15−x)O3 → (6−x)CO2 + xCO + (15−x)O2 + 3H2O

(12)

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Fig. 7 also presents COx selectivity varied with ozone input for these four VOCs. Except for DCM, other three VOCs possessed similar CO2 selectivity within studied conditions, ca. 60~70%. Besides, CO2 selectivity was slightly higher at less ozone input. By contrast, CO2 selectivity of DCM was much lower at

-p

approximately 20~30%, and higher ozone input firstly decreased and then increased CO2 selectivity, as

re

shown in Fig. 7(c). Totally, CO2 selectivity at the O3/VOC of 10.0 decreased as the order: PhH > CB >

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DCE > DCM, as shown in Fig. 8(b), which was almost consisted with conversion efficiency. Different molecule structures for various kinds of VOCs might contribute to this difference [59, 61-63]. Furthermore,

na

Fig. 8(b) also demonstrates that O3 conversions of DCE and DCM exhibited lower value than CB and PhH. Over-stoichiometric ozone input for DCE and DCM ozonation finally resulted in lower ozone utilization

ur

efficiency and higher ozone residual. (a)100

DCE conv./COx sel. (%)

Jo

80

CB conv./COx sel. (%)

(b) 100

CB conv. CO sel. CO2 sel.

60

40

DCE conv. CO sel. CO2 sel.

80

60

40

20

20

0

0 0.0

2.3

4.7

7.1

9.3

Molar ratio of O3/CB

11.6

14.0

0.0

1.1

2.0

3.0

3.9

Molar ratio of O3/DCE

4.8

10.0

(d)100

DCM conv. CO sel. CO2 sel.

80

60

40

PhH conv. CO sel. CO2 sel.

80

PhH conv./COx sel. (%)

DCM conv./COx sel. (%)

(c)100

60

40

20

20

0

0 0.0

1.1

2.0

2.9

3.9

4.8

0.0

10.0

1.0

2.0

3.1

3.9

4.8

7.9

10.4

Molar ratio of O3/PhH

Molar ratio of O3/DCM

VOC conv./COx sel./O3 conv. (%)

80

60

40

20

VOC conv. O3 conv.

100

80

60

CO sel. CO2 sel.

40

re

VOCs conv. (%)

(b)

CB DCE DCM PhH

100

-p

(a)

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Fig. 7. Effect of O3 input on CB, DCE, DCM, and PhH conversion, as well as COx formation over Mn/Al2O3 catalyst. Temperature: 120 oC, catalyst dosage: 0.025 g.

20

0

0 2

4

6

8

10

12

14

lP

0

CB

PhH

DCM

DCE

Molar ratio of O3/VOCs

na

Fig. 8. Comparison of catalytic performance on CB, DCE, DCM, and PhH conversion (a), O3 conversion, as well as COx formation Mn/Al2O3 catalyst. (b) Molar ratio of O3/VOC: 10.0, temperature: 120 oC, catalyst dosage: 0.025 g. 3.4 Catalytic co-ozonation of CB/DCE and CB/PhH

ur

Catalytic co-ozonation of VOCs mixtures is necessary for real conditions. Previous works have

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explored catalytic co-oxidation of various VOCs by oxygen [2, 59, 64]. A mixture of VOCs exhibited different electronic and steric structures, inducing varying behaviors on oxidation. For instance, competitive adsorption resulted in mutual inhibition between 1,2-DCE, TCE, and n-hexane, while n-hexane greatly improved HCl selectivity for chlorinated VOCs oxidation [65]. By contrast, promotion effects were also reported that ethanol could promote toluene oxidation [66]. However, few literatures reported catalytic oxidation of VOCs mixtures by ozone. In this paper, catalytic co-ozonation of CB/DCE and CB/PhH were performed, respectively, to investigate mutual effect between chlorinated aromatics and chlorinated alkanes,

as well as chlorinated and non-chlorinated aromatics. Fig. 9 describes catalytic behaviors of co-ozonation over Mn/Al2O3. Meanwhile, CB and DCE conversion under single compound conditions were listed as the dot lines for comparisons. Interestingly, CB conversion was greatly improved by DCE addition. Quite high conversion of 97.9% was attained at O3/(CB+DCE) of 4.1 and further increased to 100% (Fig. 9(a)), which distinctly surpassed single CB ozonation. As above results, DCE conversion was higher than CB during single ozonation under same molar ratio of O3/VOC. However, CB conversion obviously surpassed DCE during co-ozonation, and even

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inhibited DCE conversion, as the decreased value compared with red dot line in Fig. 9(a). To investigate these interesting observations, CB concentration was further elevated in the mixture of CB and DCE, as shown in Fig. 9(b)~(c). Elevation of CB concentration simultaneously reduced CB and DCE conversion at

-p

same O3 input. Finally, when CB concentration elevated to 75 ppm with coexistence of 25 ppm DCE, DCE

re

conversion was still negligible until the molar ratio of O3/(CB+DCE) reached 5.8 (Fig. 9(c)). Subsequently,

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it was motivated at O3/(CB+DCE) of 7.7, and further increased to 76.4% at O3/(CB+DCE) of 11.6. Even CB conversion could not attain such high value at lower O3 input with elevation of CB concentration, it

na

still reached nearly 100% at O3/(CB+DCE) of 12.5 and 11.6, respectively, as shown in Fig. 9(b)~(c). The distance between dot lines (single ozonation) and columns (co-ozonation) clearly demonstrates that CB

ur

conversion was greatly improved, while DCE conversion was inhibited especially at higher ratio of DCE/CB. Fig. 10(a) clearly demonstrates that DCE addition significantly promoted CB conversion,

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especially when DCE became dominant compound in the mixture. In turn, CB addition obviously restrained DCE conversion, especially when CB concentration was higher, as shown in Fig. 10(c). This finding provides a new insight for catalytic co-ozonation of VOC mixtures containing chlorinated aromatics and alkanes, and addition of alkanes significantly promoted degradation of aromatics. As mentioned above, the degradation difficultly of DCE was much harder than CB, resulting in priority of CB conversion. Furthermore, electronic and steric factors should be key role in this different behaviors. CB possesses higher

polarity, while the alternately situated chlorine atoms in DCE molecule causes no polar character, resulting in easier interaction between CB molecule and catalyst surface compared with DCE. Besides, the DCE accessibility to the active sites on catalyst surface is difficult due to its tetrahedral structure [67]. In short, it was the competition of adsorption between CB and DCE on the active sites on catalyst surface, causing attenuation of DCE ozonation. Therefore, additional ozone input was required to attain equivalent ozonation efficiency. Catalytic co-ozonation of CB and PhH was also performed with equivalent concentration, ca. 50 ppm.

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CB and PhH conversions varied with O3 input were presented in Fig. 9(d). These two compounds exhibited similar catalytic behavior at same O3 input during single ozonation as shown from dot lines. Co-ozonation relatively suppressed PhH conversion, while CB possessed the priority again. However, these promotion

-p

and inhibition effects were very mild compared to the mixture of CB and DCE. This conclusion can be

re

further clearly verified from Fig. 10(b) and Fig. 10(d). Finally, CB and PhH attained desirable conversion

lP

when O3/(CB+PhH) was higher than 10.0, ca. 90.3~97.3% for CB and 85.5~95.8% for PhH, surpassing single ozonation. In this point of view, co-existence of CB and PhH promoted each other, attaining quite

CB conv.

100.0

99.3

97.9

78.2

100.0

82.4

68.1

DCE conv.

100

83.3

40

80

99.8

96.2

92.3

58.0

60

DCE conv.

CB conv.

100.0

ur

80

CB conv.

DCE conv.

Jo

CB/DCE conv. (%)

100

(b)

DCE conv.

CB conv.

CB/DCE conv. (%)

(a)

na

high value that could not reach for single ozonation at close stoichiometric O3 input.

73.9 62.6

60 49.2

46.2

40

26.1

34.8 25.4

20

20

5.6

13.9 2.7

0

5.0

0 2

4.1

6.1

8.2

10.3

Molar ratio of O3/(CB + DCE)

12.2

2.1

4.1

6.3

8.3

10.5

Molar ratio of O3/(CB + DCE)

12.5

(c)

CB conv.

(d)

DCE conv.

CB conv.

DCE conv.

PhH conv.

CB conv.

100.0

98.3

100

CB conv.

PhH conv.

100

97.3 95.8 90.3

80

76.4 69.6

60 52.2 42.1

40

CB/PhH conv. (%)

CB/DCE conv. (%)

88.6

85.5

80

77.9 69.2 61.8

60

50.8 44.3

40 32.2

26.9

24.0

20

20

14.6 3.3

15.1

5.3

4.0

0

0 1.9

3.8

5.8

7.7

9.7

1.9

11.6

3.9

5.9

7.8

9.9

11.7

Molar ratio of O3/(CB + PhH)

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Molar ratio of O3/(CB + DCE)

100

80

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(b) 100

CB conv. (%)

80

60

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CB conv. (%)

(a)

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Fig. 9. Catalytic co-ozonation performance of CB/DCE and CB/PhH over Mn/Al2O3 catalyst. Temperature: 120 oC, catalyst dosage: 0.025 g. (a) 25 ppm CB and 75 ppm DCE; (b) 50 ppm CB and 50 ppm DCE; (c) 75 ppm CB and 25 ppm DCE; (d) 50 ppm CB and 50 ppm PhH. Column: co-ozonation values; Dot line: single ozonation values.

40

25 CB + 75 DCE 50 CB + 50 DCE 75 CB + 25 DCE 100 CB

0 0

2

4

6

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20

8

10

12

60

40

20

100 CB 50 CB + 50 PhH

0

14

0

2

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PhH conv. (%) 100 DCE 25 CB + 75 DCE 50 CB + 50 DCE 75 CB + 25 DCE

20

0 4

6

8

12

14

80

40

2

10

100

60

0

8

(d)

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DCE conv. (%)

80

6

Molar ratio of O3/(CB + PhH)

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Molar ratio of O3/(CB + DCE)

(c)

4

10

Molar ratio of O3/(CB + DCE)

12

14

60

40

20

100 PhH 50 CB + 50 PhH

0 0

2

4

6

8

10

12

14

Molar ratio of O3/(CB + PhH)

Fig. 10. Comparison of VOC conversion between co-ozonation and single ozonation over Mn/Al2O3

catalyst. Temperature: 120 oC, catalyst dosage: 0.025 g. (a) Effect of DCE presence on CB conversion; (b) Effect of PhH presence on CB conversion; (c) Effect of CB presence on DCE conversion; (d) Effect of CB presence on PhH conversion. To further explore the effect of DCE addition on CB ozonation, dynamic measurement of CB conversion varied with DCE addition and O3 increase was conducted. The results are shown in Fig. 11, which was divided into four sections: (Ⅰ) 50 ppm CB single ozonation with 500 ppm O3, (Ⅱ) equivalent DCE addition with unchanged O3, (Ⅲ) O3 increased to 750 ppm, and (Ⅳ) O3 further increased to 1000 ppm to achieve O3/(CB+DCE) of 10.0. After stabilization of stage Ⅰ, 50 ppm DCE addition firstly decreased

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CB conversion from ~80% to ~65%, then recovered to initial value and maintained stable, ca. ~81%. DCE conversion also increased from ~33% to ~37% and suddenly begun to decrease continuously. At the initial time of stage Ⅱ, DCE addition without O3 change resulted in decrease of O3/(CB+DCE) from 10.0 to 5.0,

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thereby CB conversion decreased firstly. But, afterwards, the promotion effect from DCE elevated CB conversion again. DCE conversion gradually decreased to ~10% at the final period of stage Ⅱ. Thus, DCE

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addition firstly grabbed partial ozone for its degradation, but finally it still failed in this competition with

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CB. That’s to say, O3 was mainly assigned to CB in stage Ⅱ, and to some extent, the molar ratio of O3/CB still remained near 10.0, explaining similar conversion value with stage Ⅰ. O3 input increased to 750 ppm

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in stage Ⅲ, which further promoted CB conversion to almost 100%, and DCE conversion greatly increased to ~40%. DCE conversion finally attained ~75% in stage Ⅳwith 1000 ppm O3. These observations in stage

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Ⅲ and Ⅳ demonstrate that DCE conversion was strongly correlated with CB conversion, that is, DCE

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conversion initialized until CB attained nearly complete degradation, in consistent with Fig. 9. Interestingly, variation of four conditions did not cause any influence on CO2 selectivity, which remained quite stable. It was speculated that lower CO2 selectivity should attribute to short residence time on catalyst surface. Researchers tried to employ multi-walled carbon nanotubes to prolong time for contact time between CB, ozone, and catalysts, which could improve CO2 selectivity [68]. Next, catalysts with novel morphology that can provide more space for ozonation might promote CO2 selectivity, as well as degradation efficiency. Besides, catalyst modification that can enhance CO oxidation at low temperature should be a good choice.

50 DCE 50 CB 750 O3

80

50 DCE 50 CB 1000 O3

60

40

20

CB conv. DCE conv. CO2 sel.

0 0

100

200

Time (min)

300

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CB/DCE conv./CO2 sel. (%)

50 DCE 50 CB 500 O3

50 CB 500 O3

100

Fig. 11. Dynamic measurement of CB conversion variation with DCE addition and further O3 increase. Catalyst: Mn/Al2O3, temperature: 120 oC, catalyst dosage: 0.025 g.

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3.5 Catalyst stabilities 3.5.1 Conversion efficiency and CO2 selectivity

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Catalyst stabilities are critical for real application. Low temperature catalytic reaction always induced

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passivation problem due to byproducts and intermediates accumulation without heating decomposition [37, 38, 43]. Especially, chlorinated compounds further exacerbate this problem, which will produce large

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amounts of acid species, such as HCl and Cl2 [5, 22, 69]. Therefore, single ozonation of CB, DCM, DCE, and co-ozonation of CB and DCE were investigated over Mn/Al2O3 with O3/VOC of 10.0 and the stability

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results are presented in Fig. 12. Fortunately, these compounds all exhibited stable activity within testing

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time, and slight fluctuation might originate from unstable concentration due to bubbling effect. CO2 selectivity of DCM decreased from ~55% and finally stabled at ~35%, while other compounds all exhibited stable effect in CO2 selectivity. Lab-scale ozone generator cannot guarantee too long-time stable operation, but these observations also indicate that catalytic ozonation is a promising technology for VOCs degradation, especially for chlorinated compounds. It was reported that presence of O3 not only accelerated VOCs oxidation by reducing activation energy, but also significantly promoted oxidation of surface accumulated compounds [26, 70]. Therefore, even at low temperature, catalytic activity and stability were

enhanced obviously by O3 addition. (a)

(b) 100

DCM conv./CO2 sel. (%)

CB conv./CO2 sel. (%)

100

80

60

40

80

60

40

20

20

CB conv. CO2 sel.

0 0

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200

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400

DCM conv. CO2 sel.

0 0

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400

500

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200

80

60

500

40

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40

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400

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CB/DCE conv./CO2 sel. (%)

DCE conv./CO2 sel. (%)

100

0

300

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(d)

(c)

0

200

Stabilization Time (min)

Stabilization Time (min)

20

CB conv. DCE conv. CO2 sel.

0

0

Stabilization Time (min)

100

200

300

400

500

Stabilization Time (min)

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Fig. 12. Stabilities of single ozonation of CB, DCM, DCE, and co- ozonation of CB/DCE. Catalyst: Mn/Al2O3, molar ratio of O3/VOC: 10.0, temperature: 120 oC, catalyst dosage: 0.025 g, CB/DCE coozonation: 50 ppm CB + 50 ppm DCE. 3.5.2 Variation of surface properties after catalytic ozonation

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Spent catalysts after above stability tests were investigated by XPS analysis, as well as fresh Mn/Al2O3

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was listed together in Fig. 13 for comparison. Region of Mn 2p3/2 was deconvoluted into two or three peaks by Gaussian functions. Single components ozonation resulted in two characteristic peaks of Mn3+ and Mn4+ with the disappearance of Mn2+ observed in fresh Mn/Al2O3. However, spent catalyst of binary components ozonation presented co-existence of Mn2+, Mn3+ and Mn4+. Fig. 13(d) summarized variation of Mn4+ proportions calculated by integration. Interestingly, CB ozonation caused distinctly decrease in Mn4+ from 43.3% to 17.9%, indicating reduction of Mn4+ to lower valance state after stability tests. By contrast, proportion of Mn4+ increased from 43.3% to 52.6% after DCE ozonation. Thus, it can be verified that

surface Mn ions participated in VOCs oxidation cycle. Previous literatures also found that O3 promoted VOCs oxidation by accelerating metal ions redox cycle and further generating active oxygen species, such as V4+ to V5+ [71], Mnn+ to Mn(n+2)+ [43, 72], Co3+ to Co4+ [73]. Eqs. (9)~(10) have indicated that the equivalent ratios of O3/CB and O3/DCE are 14.0 and 5.0, respectively. O3 input ratios, ca. both 10.0, were less and excessive for CB and DCE, respectively. Therefore, insufficient ozone input for CB resulted in Mn4+ reduction, while excessive ozone input for DCE caused increase of surface Mn ions state. This can also elucidate that co-ozonation of CB and DCE finally resulted in a middle value of Mn4+ proportion, ca.

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47.9%. However, it can be observed that Mn4+ decreased after DCM ozonation, while O3 input ratio was even much higher than its equivalent ratio, ca. 2.0. This might attribute to its maximum difficulty in degradation [59], thereby reducing surface Mn ions even with too much excessive O3 input.

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Fig. 13(b) presents O 1s XP spectra of fresh and spent catalysts. For most spent catalysts, lattice

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oxygen (Olat.) and hydroxyl oxygen (OOH) disappeared with only surface oxygen (Osur.) observed. Ozone decomposition produced large amounts of O radicals, ca. O2−, O22− or O−, corresponding to surface adsorbed

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oxygen species [68, 74]. Less Mn4+ ratios of spent catalysts after CB and DCM ozonation corresponded to

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decrease in Olat.. However, spent catalyst after DCE ozonation exhibited higher Mn4+ ratio, and therefore increase in Osur. should attribute to large amounts of reactants, byproducts, and products accumulation,

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which were also contribute to Osur. for spent catalysts after CB and DCM ozonation. Furthermore, it also demonstrates that Olat. was consumed during catalytic ozonation process. Even for spent catalyst after co-

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ozonation of CB and DCE, Olat. was observed, but the proportion of Osur. also increased from 84.6% to 94.4%, as shown in Fig. 13(d). The presence of Mn2+ and Olat. in spent catalyst after co-ozonation of CB and DCE, as well as minimal changes compared with fresh catalyst demonstrate high ozone utilization efficiency. Co-ozonation can maintain the original property of catalyst, which is beneficial to catalyst stability and possess longer lifetime. Fig. 13(c) presents Cl 2p XP spectra after stability. Two peaks at 198 eV and 200 eV were detected for

spent catalysts after CB ozonation and CB/DCE co-ozonation, similar with previous literatures [5, 75]. Interestingly, only CB contained reactants resulted in Cl species appearance on catalyst surface. Coozonation of CB and DCE exhibited lower intensity than CB single ozonation. Single ozonation of DCM and DCE caused negligible signals. These observations indicate that CB ozonation tended to deposit chlorine species, while chlorine contained alkane, DCE, exhibit super chlorine removal ability. And coozonation of CB and DCE was also beneficial to remove surface adsorbed chlorine species. Next, surface adsorbed species were evaluated by TG-MS measurements of spent catalysts after CB ozonation. Mn 2p

(b)

Mn3+

O 1s OOH

Mn4+

Spent_CB+DCE

Spent_DCE

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Spent_DCE

Spent_DCM

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Spent_DCM

Spent_CB

650

645

640

Fresh

635

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655

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Spent_CB

Fresh

536

534

Binding energy (eV)

Cl 2p

100

530

528

100

100

94.4

84.6 Proportion (%)

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Intensity (a.u.)

Spent_DCM

Mn4+/Mn Osur/O 100

(d)

200 eV 198 eV

Spent_CB+DCE

532

Binding energy (eV)

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(c)

Olat.

Spent_CB+DCE

Intensity (a.u.)

Intensity (a.u.)

Mn2+

660

Osur.

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(a)

Spent_DCE

80

60

52.6 43.3

47.9 40.2

40

Spent_CB

17.9

20

204

202

200

198

196

Binding energy (eV)

194

192

190

0 Fresh

CB

DCE

DCM

CB+DCE

Fig. 13. Variation of surface properties after catalytic ozonation over Mn/Al2O3. (a) Mn 2p, (b) O 1s, (c) Cl 2p, (d) variation of proportions.

3.5.3 TG-MS measurements To compare the species changes and byproducts formation on catalyst surface after CB ozonation, FTIR spectra were collected and presented in Fig. S4. The catalysts after 200 min duration tests were selected as spent samples. The bands around 1630 cm-1 were assigned to C=O vibration of carbonyl and carboxyl groups [5, 76]. However, fresh and spent catalysts all possessed this band with some changes in intensity. Therefore, it might originates from δ(HOH) modes of adsorbed water or byproducts formation (formic acid) [77, 78]. To some extent, C=O in fresh Mn/Al2O3 should originate from surface adsorbed

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oxygen and hydroxyl radicals. As presented in Fig. S4(a), distinct increasing tendency of intensity can be observed in range of wavenumber higher than 3000 cm-1, indicating abundant in hydroxyl radicals. Therefore, the hydroxyl radicals should contribute to this strong intensity of C=O in fresh Mn/Al 2O3.

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However, for spent catalyst, the hydroxyl radicals were consumed a lot, confirmed by XPS results, and

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therefore decreasing intensity of C=O, which should also include new adsorbed species after ozonation.

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Except for Mn/SiO2, some new peaks at 1040~1100 cm-1, 1383 cm-1, and 2900~2980 cm-1 corresponding to C-H species [16] emerged after CB ozonation, representing byproducts formation on catalyst surface.

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Hereafter, TG-MS measurements over two Mn/Al2O3 spent catalysts with O3/CB of 2.5 and 10.0, respectively, were performed to investigate products desorption from catalyst surface. As expected, higher

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O3 input caused higher weight loss, as described from Fig. 14(a). Almost 15% TGA weight loss was observed for catalyst after 200 min reaction with O3/CB of 10.0, while this value was approximately 10%

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for O3/CB of 2.5. This verifies that more O3 input improved species adsorption on catalyst surface, which not only included reactants, CB and oxygen contained species, but also covered intermediates and byproducts formed during reaction. To further detect these adsorbed species, on-line MS signals were obtained as shown in Fig. 14(b)~(e). In comparison, CO desorption peaks were centralized at higher temperature than CO2 desorption peaks. Higher O3 input induced COx species formation, thereby more CO and CO2 desorption. More CO2 desorption peaks emerged at higher temperature for the spent catalyst with

O3/CB of 10.0, indicating stronger adsorption strength. But, CO desorption shifted towards lower temperature at higher O3 input. This could be plausible explanation for slightly higher CO2 selectivity at lower O3 input, as shown in Fig. 2(b). Interestingly, DCM desorption was detected, while MS signals of CB were both negligible. More importantly, DCM desorption increased when O3/CB elevated from 2.5 to 10.0, verifying more DCM generated during CB ozonation. Other chlorinated byproducts were also checked but with negligible signal. Furthermore, MS signals of HCl and Cl2 were also negligible in Fig. S5(a)~(b). Therefore, DCM should be a major chlorine contained byproduct during CB catalytic ozonation, which

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contributes to the chlorine species detected in Cl 2p XP spectra in Fig. 13(c). Besides, carboxyl and formic acid were detected from MS signals, as shown in Fig. S5(c)~(d). These compounds and DCM should assign to intermediates during CB ozonation process. Absence of CB provided evidence that catalytic CB

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ozonation followed Eley-Rideal (E-R) mechanism [79]. Firstly, O3 decomposed into free radicals (O2−, O22−

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or O−), along with oxidation of Mn ions on catalyst surface. CB was then destructed by these oxidized free

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radicals and Mn ions with higher valance state. Cl was dissociated and recombined to produce DCM, while benzene ring was destroyed and coupled with O radicals to form carboxyl and formic acid. These

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intermediates can be further converted into final products, ca. CO2, CO, H2O, and HCl, etc. Finally, these final products desorbed from catalyst surface, with little residual on catalyst surface, to finish one catalytic

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ozonation cycle.

TGA weight loss (%)

(a) 100

O3/CB=2.5 O3/CB=10

Spent catalyst

95

90

85

80 200

400

600

800

1000

o

(b) 5.0E-11

O3/CB=2.5 O3/CB=10

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(c) 1.0E-11

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Temperature ( C) Spent catalyst

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365 C

400

600

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O3/CB=2.5 O3/CB=10

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2.0E-13

0

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600

Temperature (°C)

800

645 oC

0

200

400

600

800

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Temperature (°C)

(e)

1.0E-14

Spent catalyst

O3/CB=2.5 O3/CB=10

CB QMID (m/z=112)

(d)

800

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Temperature (°C)

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200

480 oC

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750 oC

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CO2 QMID (m/z=44)

CO QMID (m/z=28)

650 C

0

DCM QMID (m/z=85)

375 oC

o

O3/CB=2.5 O3/CB=10

1000

0

200

400

600

800

1000

Temperature (°C)

Fig. 14. TGA weight loss curves of spent Mn/Al2O3 catalysts with different O3 input (a), and desorption gas MS signals (b)~(e).

4. Conclusions Catalytic ozonation of CB was performed over a series of MnOx catalysts supported on Al2O3, TiO2, SiO2, CeO2, and ZrO2, at 120 oC with O3/CB molar ratio of 10.0. The highest CB conversion efficiency was attained at 82.92% over Mn/Al2O3, while other catalysts decreased as the order: Mn/TiO2 > Mn/SiO2 > Mn/CeO2 > Mn/ZrO2. O3 all decomposed nearly 100%, with residual less than 10 ppm. Interestingly, all catalysts exhibited relatively consistent CO2 selectivity of CO + CO2, ca. 63.74%~69.02%. Mn/Al2O3 was further employed to investigate effect of operation parameters on CB conversion. CB conversion increased

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and then decreased as temperature rising, with an optimal condition of 120 oC. Linear improvement in CB conversion was observed with more O3 input before O3/CB of 10.0. GHSV had negligible effect, but too high GHSV also slightly reduced CB conversion. Mn/Al2O3 possessed excellent textual properties, highest

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proportion of surface adsorbed oxygen species, highest redox ability, second high surface acidity, and

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largest O2 desorption, which synthetically contributed to its best catalytic performance in CB ozonation.

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Catalytic ozonation of CB, DCE, DCM, and PhH were performed over Mn/Al2O3 for comparison. DCE and DCM only attained conversion efficiency of 67.21% and 42.03% at their stoichiometric O3/VOC,

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ca. 5.0 and 2.0, respectively, while it reached 91.73% for CB, at its stoichiometric O3/CB, ca. 14.0. PhH exhibited higher conversion than CB, ca. 91.77% > 82.92% at O3/VOC of 10.0. In conclusion, aromatics

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degraded easier than alkanes, as CB~PhH > DCE~DCM in conversion, and chlorinated substitution increased difficulty, as PhH > CB in conversion, as well as more carbon atoms decreased difficulty, as DCE >

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DCM in conversion. CO2 selectivity of DCM was much lower at approximately 20~30%, while others all exhibited similar CO2 selectivity, ca. 60~70%. Catalytic co-ozonation of CB/DCE and CB/PhH were performed, respectively, to investigate mutual effect between chlorinated aromatics and chlorinated alkanes, as well as chlorinated and non-chlorinated aromatics. DCE addition significantly improved CB conversion, especially at higher ratio of DCE/CB. Most interestingly, CB attained quite high conversion of 97.9% at O3/(CB+DCE) of 4.1 and further increased to

100%. However, DCE conversion was inhibited that initialized until almost completely degradation of CB. PhH addition in CB conversion performed similar results, but these effects were very mild. O3 assistance and co-ozonation of CB/DCE were beneficial to maintain the original property of catalyst, thereby exhibiting excellent stability even at lower temperature. Finally, DCM, carboxyl and formic acid were detected as critical intermediates during catalytic CB ozonation from TG-MS results.

Author statement Guanyi Chen: Conceptualization, Validation, Project administration.

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Zhi Wang: Investigation, Data Curation, Visualization, Roles/Writing - original draft. Fawei Lin: Conceptualization, Methodology, Visualization, Investigation, Writing - Review

Zhiman Zhang: Investigation, Formal analysis.

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Hongdi Yu: Investigation, Data Curation.

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& Editing, Resources, Funding acquisition, Supervision.

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Beibei Yan: Resources, Project administration.

Zhihua Wang: Conceptualization, Methodology, Supervision.

Acknowledgment

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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This work was supported by the National Key Research and Development Program of China

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(2018YFC1900105) and National Natural Science Foundation of China (51906175). References

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