Efficiency and mechanism of pollutant degradation and bromate inhibition by faceted CeO2 catalyzed ozonation: experimental and theoretical study

Journal Pre-proofs Efficiency and mechanism of pollutant degradation and bromate inhibition by faceted CeO2 catalyzed ozonation: experimental and theoretical study Xixi Chen, Hai Yang, Chaktong Au, Shuanghong Tian, Ya Xiong, Yu Chang PII: DOI: Reference:

S1385-8947(20)30471-X https://doi.org/10.1016/j.cej.2020.124480 CEJ 124480

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

18 November 2019 30 January 2020 16 February 2020

Please cite this article as: X. Chen, H. Yang, C. Au, S. Tian, Y. Xiong, Y. Chang, Efficiency and mechanism of pollutant degradation and bromate inhibition by faceted CeO2 catalyzed ozonation: experimental and theoretical study, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124480

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Efficiency and mechanism of pollutant degradation and bromate inhibition by faceted CeO2 catalyzed ozonation: experimental and theoretical study Xixi Chena, Hai Yangc, Chaktong Auc, Shuanghong Tiana,b,*, Ya Xionga,b,*, Yu Changd, e a.School

of Environmental Science & Engineering, Sun Yat-sen University, Guangzhou 510275,

P. R. China. b.Guangdong

Provincial Key Laboratory of Environmental Pollution Control and Remediation

Technology, Guangzhou 510275, PR China c.Hunan

Provincial Key Laboratory of Environmental Catalysis & Waste Recycling, Hunan

Institute of Engineering, Xiangtan 411104, PR China d.School

of Environment, South China Normal University, University Town, Guangzhou

510006, China e.

Guangdong Provincial Key Laboratory of Chemical Pollution and Environmental Safety &

MOE Key Laboratory of Theoretical Chemistry of Environment, South China Normal University, Guangzhou 510006, China

* Corresponding author: Tel.: +86 20 84115556; fax: +86 20 39332690. E-mail address: [email protected] (Shuanghong Tian); E-mail address: [email protected] (Ya Xiong).

1

ABSTRACT Reduction of potential carcinogenic bromate formation is a big challenge for the application of ozone in the treatment of Br−-containing wastewater. Three CeO2 nanocrystals individually with exposed (100), (110), and (111) facets were prepared and adopted as catalysts for the ozonation of Br—containing wastewater. It is found that faceted CeO2 greatly inhibited the bromate formation and simultaneously enhanced the pollutant removal. The catalytic activity follows the order of (100) > (110) > (111). In the presence of CeO2(100), there is 71.4±2.9% of sulfamethoxazole (SMZ) degradation (only 24.9±1.8% in ozone alone). Moreover, the generation of bromate was reduced by 29.6%, 38.8%, 52.4%, 66.4%, respectively, when the initial Br− concentration is 1.0, 2.0, 34.0, 65.0 mg L-1. Meanwhile, the antibacterial-active groups in most of the identified intermediates were destroyed and much lower biotoxicity of the treated wastewater is observed. The good reducibility and Lewis acidity of CeO2(100) favors the activation of ozone and the redox cycle of ceria. DFT calculations show that ozone facilely dissociates into surface O and O22- (O22-⇌•O2-) through interaction with the (100) facet. The experimental results demonstrate that •O2−, its derivatives (i.e. •OH and 1O2) and surface O are generated and get involved in SMZ degradation. It is noted that •O2− also functions as a key reductant to convert Ce4+ to Ce3+ which reduce HBrO/BrO− and BrO3− to Br− and therefore inhibit the formation of bromate. These results indicate that CeO2(100) catalyzed ozonation is a promising advanced oxidation process for the treatment of Br-containing wastewater, and crystal facet engineering is an efficient strategy to enhance the catalytic performance. 2

Keywords: ceria; crystal facet; catalytic ozonation; bromate inhibition; DFT calculation 1. Introduction With a high redox potential of 2.08 eV, ozone is widely applied for water disinfection and oxidative degradation of pollutants [1,2]. However, ozonation could result in selectively oxidation of bonds in refractory organic molecules, leading to the accumulation of toxic intermediates and increase of biotoxicity [3,4]. In addition, bromide (Br−) is commonly present in water sources at concentrations ranging from 10 to >1000 μg L-1 in fresh water and about 67 mg L-1 in seawater [5]. When treating water that contains bromide, single ozonation would lead to the formation of carcinogenic bromate (BrO3−), which has been classified as carcinogen of 2B level by World Health Organization (WHO) [6]. Strategies such as limiting the ozone concentration, decreasing pH, and using ammonia or hydrogen peroxide were proposed to control bromate formation [7], but they need to consume additional chemicals and even the additives may have negative effects on ozonation efficiency. Heterogeneous catalytic ozonation has been timely developed for wastewater treatment because the technique can overcome the limitations of ozonation, such as selective oxidation of certain bonds, formation of unwanted products and low ozone utilization [8]. However, only a few studies have been conducted to investigate the control of BrO3− formation during heterogeneous catalytic ozonation. In recent years, catalysts such as LaCoO3 and LaFeO3 [9], C3N4/LaCoO3 [10], MnOx/Al2O3 [11], βFeOOH/Al2O3 [12], Fe-Al LDH/Al2O3 [13], Ce-MCM-48 [14], Fe-Cu-MCM-41 [15], CexZr1-xO2 [16], CeO2 [17], and CeO2/RGO [18] were studied to inhibit BrO3− 3

formation, and some of them could even simultaneously enhance the removal of organic pollutants. In the literatures, the enhanced degradation efficiency of organic pollutants was commonly attributed to the promotion of •OH generation [9–16]. In contrast, totally different or even obviously contradictive mechanisms were proposed for BrO3− elimination in catalytic ozonation [9–17]. The bromate elimination was attributed to the reduction of bromate by H2O2 or [Fe-H2O2]s [9,10], as well as Fe2+ on the surface of βFeOOH/Al2O3 and Fe-Al LDH/Al2O3 [12,13]. Li et al. and Chen et al. pointed out that the catalysts accelerated the decomposition of O3 to produce more •OH radicals and less aqueous O3 is available for the sequential oxidation of Br− to BrO− [14,15]. Yang et al. held another viewpoint that the competitive reaction between organics and Br− with •OH lowers the chance for the oxidation of Br− to BrO3− by •OH [16]. Zhang et al. drew a totally different conclusion that •OH generation was inhibited by the catalysts, and therefore the oxidation of Br− by highly reactive •OH to BrO3− decreased [17]. It is noted that the above researchers mainly focused on the mechanism of •OH formation. Little attention was paid to the generation and roles of other reactive oxidation species (ROS) such as •O2−, 1O2, and surface O, which are usually formed by chain reactions during catalytic ozonation and play key roles in the oxidation processes [19]. Ignoring them would likely lead to the contradictory remarks. In addition, as an initial step of the chain reactions, the way of ozone adsorption and decomposition on a catalyst surface is important for proper deduction of mechanism. However, it is extremely difficult to observe and analyze the atomic changes, which is therefore ignored in 4

previous investigations [9–17]. CeO2 is the most abundant rare earth oxide. Due to its facile redox cycle of Ce4+/Ce3+, rich acid and base chemistry, and structure stability, CeO2 or modified CeO2 have been becoming attractive catalysts in organic synthesis, CO oxidation, VOC combustion, water-gas shift reaction, three way catalytic converters, as well as many other catalysis [20,21]. In recent decade, CeO2 as active components or supports have been widely investigated as ozonation catalysts to inhibit the bromate formation or strengthen the refractory organic pollutants removal or both [14,16,18,19,22–29]. It was found that the efficiency of ceria catalysts in ozonation was greatly affected by the redox behavior of Ce3+/Ce4+ and the defective surface sites (Ce3+ and the accompanied oxygen vacancies on the surface). Fluorite CeO2 typically has exposed (111), (110), and (100) facets. Due to the big discrepancy in atom arrangement, the three facets display significant difference in redox ability and acid properties [21], which would greatly affect the catalytic performance and mechanism in the ozonation of Br−-containing wastewater. To the best of our knowledge, the effects of the exposed crystal planes of CeO2 on the inhibition of bromate formation have never been reported. The goal of this study is to disclose the mechanism of organic degradation and bromate inhibition during catalytic ozonation over faceted CeO2 nanocrystals. For such an end, three kinds of CeO2 nanocrystals individually with exposure of (100), (110), and (111) facets were prepared, and for the first time used as catalysts for the ozonation of Br−-containing wastewater. Sulfamethoxazole (SMZ), a commonly used antibiotic in mariculture and widely detected in natural waters, was selected as model refractory 5

organic pollutants. Actually, SMZ has been employed as a probe molecule in numerous advanced oxidation processes (AOPs), especially catalytic ozonation [8,30–34]. The surface properties of CeO2, reactive oxidation species ROS (i.e. O3, H2O2, •OH, •O2−, 1O

2,

surface O), oxidation intermediates (e.g., HBrO/BrO−, BrO3−, and bromated

organics), and the bromine balance were investigated. Finally, the atomistic details of ozone adsorption and decomposition on each faceted surface were illustrated by density functional theory (DFT) calculations. 2. Material and methods 2.1. Reagents and Chemicals. Cerium (III) nitrate hexahydrate, Cerium trichloride heptahydrate, 2,2,6,6tetramethylpiperidine (TEMP) were purchased from Tianjin Baishi Chemical Reagent Co., Ltd. (China). Sulfamethoxazole (SMZ), 5,5-dimethyl-1-pyrroline N-oxide (DMPO), dimethyl sulfoxide (DMSO) were obtained from Aladdin Chemistry Co., Ltd. All reagents were of analytical reagent grade and used without further purification. CeO2 nanocubes with primarily exposed (100) crystal facets and nanorods with (110) and (100) were prepared as described by Hu et al. [35]. Nanorods with (111) and (100) were developed by the same method of preparing CeO2 nanocubes with (100) except that using CeCl3 as the CeO2 precursor instead of Ce(NO3)3. After the hydrothermal treatment, the CeO2 nanocrystals were rinsed with 0.1 M NH4OH to remove residual Na+. Herein, the CeO2 sample with (100), (110) and (111) facets is denoted as NC100, NR110, and NR111, respectively. 2.2. Characterization Methods 6

The morphology of powder samples was observed using a Field-Emission scanning electron microscope (SEM, Gemini500, Zeiss/Bruker) and a high-resolution transmission electron microscope (HRTEM, JEM-2010HR, JEOL) operated at 200 kV. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ESCALAB 250, Thermo-VG Scientific (UK) system provided with software for data acquisition and analysis. The specific BET surface area of powder samples was measured by N2 physical adsorption at 77 K using an autoadsorption system (Autosorb6, Quanta chrome). 2.3. Analysis Methods The concentration of SMZ was quantified by high performance liquid chromatography (HPLC) instrument (Shimadzu LC-15C) with a UV detector set at 265 nm (Wondasil C18 column, 4.6×150 mm, particle size of 5 μm). The mobile phase consisted of water/methanol (30:70, v/v%) with pH 2.8 adjusted by phosphoric acid. The flow rate of the mobile phase was 0.8 mL min-1 and the injection volume was 20 μL. The total organic carbon (TOC) was determined by a TOC analyzer (Shimadzu, Japan). Gaseous ozone concentration was measured with an online ozone analyzer (IDEAL-2000, IDEAL tech Inc. China), and aqueous ozone concentration was determined by the indigo method [36]. Reactive oxygen species (ROS) like •OH, •O2−, and 1O2 were detected by the electron spin resonance (ESR) technique at ambient temperature (Bruker A300-10-12 spectrometer, Germany). The details of ESR experiments are described in Text S2. H2O2 concentration was determined according to the method of Schick et al. [37]. Organic intermediates identification was performed by 7

UPLC/MS/MS using UPLC (Ultmate 3000)-Orbitrap-Fusion-Lumos High-resolution Mass instrument (ThermoFisher). Chromatographic separation was performed using a Hypersil GOLD column (100×2.1 mm, particle size of 1.9 μm, Waters, USA) maintained at 30 oC at a flow rate of 0.4 mL min-1. The mobile phase was composed of A (acetonitrile) and B (water containing 0.01% formic acid). The gradient elution was set by changing the B volume percentage to that of the whole mobile phase linearly as follows: 0-3 min，1%-30% B；3-6 min，30%-40% B；6-9 min，40% B；9-15 min, 40%-60% B; 15-19 min，60%-90% B ,19-23 min, 99% B; 23-23.01min, 99%-1% B; 23.01-27min, 1% B. The injection volume was 10 μL. Mass spectrometric analysis was performed in positive- or negative- ion mode using an electrospray ionization (ESI) source over a mass scan range of m/z 50–800. The operation parameters are: capillary voltage, 4.0 kV; desolvation temperature, 350 oC; flow rate of cone gas (N2), 8.0 L min-1; spay pressure, 45.0 psi. Br− and BrO3− were determined by ion chromatography (882 Compact IC plus, Metrohm, Swiss). HBrO/BrO− was detected by a phenolderivatization method described by Zhang et al. [17]. The leached Ce ions in the solution in the ceria-mediated catalytic ozonation within 30 min were determined using inductively Coupled Plasma Optical Emission Spectrometry (ICP-OES, Optima 8300, PerkinElmer). The incident power is 1300 W, the flow rate of plasma gas and nebulizer gas is 12.0 and 0.55 L min-1, respectively. 2.4. Catalytic ozonation. Tests were conducted in batch mode. A 500 mL glass column reactor was used, and ozone produced by an ozone generator (YE-TG-01PII, Nanjing YDG ozone Co., Ltd., 8

China) with a concentration of 4 mg L-1 and a flow rate of 200 mL min-1 was bubbled into 300 mL of ultrapure water inside the reactor. After the aqueous ozone concentration reached 4 mg L-1, the ozone bubbling was stopped. Then a desired amount of catalyst, SMZ, and Br−-containing water were quickly and simultaneously added into the stock ozone solution to start the reaction. The initial concentration of SMZ and Br− was controlled at 10 mg L-1 and 2 mg L-1, respectively, unless specified otherwise. A magnetic stirrer was used to continuously agitate the suspension. The reaction temperature was controlled at 25 °C using a thermostatic bath. At fixed intervals, 2 mL of suspension was collected from the reactor, purged with N2 to remove residual O3, and passed through a 0.22 μm Millipore filter before analysis. 2.5. Computational methods Density functional theory calculations were performed following the projector augmented plane-wave method as implemented in the Vienna ab initio simulation package (VASP) [38–40]. The generalized gradient approximation (GGA) proposed by Perdew, Burke, and Ernzerh (PBE) was used for exchange-correlation potential [39]. To diminish the error of electron self-interaction in the Kohn-Sham formalism, a U value of 5.0 eV was added to the Ce 4f state [40]. The cutoff energy for the plane wave was set at 420 eV and the Brillouin zone was sampled by 2×2×1 k-points with Gamma center. The energy criterion was set to 10-5 eV in iterative solution of the Kohn-Sham equation. The vacuum layer in the slab model was more than 12 Å, in which all atoms could relax. All the structures were relaxed with the conjugate gradient algorithm until the residual forces on the atoms declined to 9

less than 0.01 eV/Å. 3. Results and discussion 3.1. Surface properties of faceted CeO2 The morphology and exposed crystal facet of the prepared ceria samples were investigated. As shown in Fig. 1 & Table S2, the size of NC100, NR110, and NR111 is 8–30, (7–12) × (15–120), and (10–30) × (20–180) nm with specific surface area of 65, 111 and 56 m2 g-1, respectively. The HRTEM observations show that NC100 is enclosed by {100} planes, NR110 primarily by {110}, while NR111 mainly by {111} planes (Fig. 1). The corresponding lattice distance of {100}, {110}, and {111} planes is 0.27, 0.19 and 0.32 nm, respectively. These observations agree well with the literature [35]. It is worth noting that NR111 mainly with {111} planes were developed when using CeCl3 as the CeO2 precursor instead of Ce(NO3)3 during the preparation of NC100. It is deduced that chloride functioned as a capping agent, which interacted with the {111} facets of CeO2 and lowers the surface energy, thus leading to a larger percentage of {111} facets. In catalytic ozonation, CeO2 activity is strongly affected by its defect property, redox ability and surface acidity which are primarily determined by the exposed crystal facets. To estimate the defect concentration of NC100, NR110, and NR111, we acquired Raman spectra over the three catalysts using visible (514 nm) and UV (325 nm) excitation laser lines under ambient conditions (Fig. 2). According to Guo et al. [41] and Wu et al. [42], the use of visible laser can obtain information reflecting the entire CeO2 sample while UV only the outer layers. When visible laser is used, an intense 10

band at 460 cm-1 attributable to the Raman mode of F2g symmetry in cubic fluorite structure, and two weak bands at 598 cm-1 and 836 cm-1 corresponding to oxygen vacancies (OVs) and η2 peroxide species (η2-O22-) are observed, respectively (Fig. 2a) [41,42]. The A598/A460 and A836/A460 ratio of integrated peak areas can be used to quantify the relative concentration of oxygen vacancies (OVs) and peroxide species, respectively, and the results are listed in Table S1. The NR110 catalyst possesses the highest concentration of OVs (A598/A460=0.0225), followed by NC100 and NC111, showing an order of NR110 > NC100 > NR111. Despite the A836/A460 ratios are significantly low, it can still be seen that NC100 has the highest A836/A460 ratio (0.0041), indicating that it is relatively easy for an adsorbed O2 on NC100 to capture two electrons from CeO2 to become peroxide species. When UV laser (325 nm) is used, the spectral features are quite different (Fig. 2b). The oxygen vacancy band at ca. 598 cm-1 has intensity comparable to that of F2g mode at ca. 460 cm-1. It indicates that the concentration of OVs on the outer surface of CeO2 follows the order of NC100 > NR110 > NR111, different from that of entire samples as suggested according to the results of visible laser excitation. It is believed that the OVs on the outer surface play a more important role than those in the bulk since the reactions occur on the surface of catalysts in heterogeneous catalysis. This is the first example of UV Raman being used in the investigation of catalytic ozonation. The redox property of CeO2 in catalytic ozonation is mainly determined by its reducibility (i.e., from Ce4+ to Ce3+) because the reverse oxidation step (i.e., from Ce3+ to Ce4+) is relatively facile. Fig. 2c shows the H2-TPR of NC100, NR110, and NR111. 11

The temperature peaks below 500 °C stem from the reduction of surface CeO2 while the large peak above 500 °C the reduction of bulk CeO2 [43]. The NC100, NR110, and NR111 samples exhibit two overlapped peaks below 500 °C. The two (295 °C, 372 °C) of NR110 are lower than those of NC100 (386 °C, 452 °C) or NR111 (370 °C, 448 °C), indicating NR110 has a surface that is the most reducible, in accord with the literature [21]. As shown in Table S2, the normalized H2-consumption below 500 °C follows the order of NR110 > NC100 > NR111. In this study, the specific surface area of NR110 is about 50% larger than that of NC100 and NR111. To eliminate the effect of surface area, the H2 consumption per unit surface area was calculated and the results are listed in Table S2. Then the order of the normalized H2-consumption (in terms of per peak area) below 500 °C changes to NC100 > NR110 > NR111, which agrees with the results of Desaunay et al. [44]. The redox ability of NC100, NR110, and NR111 without or with ozone was investigated by electrochemical method. The CV curves of NC100, NR110, and NR111 film electrodes in a 0.5 M Na2SO4 solution without and with ozone are shown in Figs. 2d & e. Without O3 purging, NC100 and NR110 exhibit an oxidation peak at 0.94 V and 0.79 V, respectively. The lower oxidation peak of NR110 indicates that the oxidation of Ce3+ to Ce4+ in NR110 is easier than that in NC100. With O3 purging, there is significant increase of current intensity. Compared to the case of O3 alone (0.13 V), the NC100/O3, NR110/O3, NR111/O3 cases show shift of reduction peak potential to 0.19, -0.24, -0.19 V. The positive shifts suggest the occurrence of a reduction process between ozone and the electrodes, and that the interfacial transfer of electrons is more 12

favorable in NR110 than in NC100 or NR111. The surface acidity of NC100, NR110, and NR111 nanocrystals was investigated by NH3-TPD technique. As shown in Fig. 2f, the NH3-desorption peaks at 50–210 °C, 210–338 °C, and >338 °C are attributed to weak, moderate and strong acid sites, respectively. The amounts of NH3 desorbed from these acid sites per unit mass were calculated and are listed in Table S2. The results reveal that the amount of NH3 desorption from the weak and medium acid sites follows an order of NR110 > NC100 > NR111. It is apparent that NR110 possesses the largest amount of weak and medium acid sites, followed by NC100 and NR111. As for the amount of strong acid sites, NC100 is the largest. The amounts of adsorbed NH3 per unit surface area of CeO2 were also calculated and are listed in Table S2. The amounts of NH3 adsorbed on the weak acid sites are all ca. 0.05 cm3 mcat-2 for the three samples. The amount of medium acid sites of NC100 is slightly higher than that of NR110, while that of NR111 is negligible. The good Lewis acidity of NC100 and NR110 would favor ozone activation due to the Lewis base character of the two terminate oxygen atoms of ozone [45]. 3.2. BrO3− formation and SMZ degradation in ozonation and catalytic ozonation To investigate the catalytic activity of NC100, NR110, and NR111 for the removal and mineralization of SMZ, as well as for the inhibition of bromate generation, we adopted specific cases, namely, ceria alone, O3 alone and ceria/O3 in a batch mode. To exclude the effect of SBET on the catalytic activity, the dosage of 0.127 g for NC100, 0.075 g for NR110 and 0.148 g for NR111 were used to keep the CeO2 dosages with the same surface areas of 8.3 m2 in 300 mL reaction solution. As shown in Figs. 3a & 13

b, the extent of SMZ removal within 15 min by using NC100, NR110, NR111 as adsorbents was less than 8.3%, whereas in the case of O3 alone, it was 62.5±2.9%. The Fukui functional calculations and the experimental results show that the S-N and S-C bonds in SMZ molecules are easily cleaved by ozone molecules. That is why SMZ molecules can be efficiently removed by O3 alone [31–34]. After the introduction of NC100, NR110 or NR111 into the ozonation system, the removal and mineralization of SMZ was significantly enhanced. The enhancement follows the order of NC100/O3 > NR110/O3 > NR111/O3. In the case of NC100/O3, 71.4±2.9% of SMZ was degraded within 3 min and 33.3±1.4% of TOC was removed within 30 min, much higher than those of O3 alone (24.9±1.8% and 10.4±1.5%, respectively). No leached Ce ions in the solution were detected by ICP-OES after 30 min catalytic ozonation in the presence of NC100, NR110 or NR111. It indicates that the SMZ degradation was contributed by heterogeneous catalysis. Although O3 alone can remove most of SMZ molecules within 15 min, the introduction of ceria catalysts furthermore fastened the oxidation of SMZ and obviously enhanced the TOC removal. It is reported that the catalysts of ironmanganese silicate, gamma-Ti-Al2O3, Fe3O4/Co3O4 did not show superiority in SMZ removal but significantly enhanced the minerization of SMZ by ozone [31,32,34]. The enhancement was ascribed to the facts that more hydroxyl radicals were produced with iron-manganese silicate or Fe3O4/Co3O4 while more surface atomic oxygens and superoxide radicals with gamma-Ti-Al2O3 [31,32,34]. Herein some reactive oxygen species might also form with the catalyst of the faceted ceria, which will be discussed in the mechanism part. As mentioned above, CeO2 crystal facets of {100}, {110}, and 14

{111} orientations are different in atom arrangement as well as in unsaturated coordination sites [46]. As for the CeO2 samples, Ce4+ is six-fold coordinated on the {100} and {110} facets, seven-fold coordinated on {111} facet. Generally, the saturated coordination number of Ce4+ in CeO2 is eight. A lower coordination number of Ce means stronger Lewis acidity because of the tendency of obtaining a coordinatively saturated surface. Hence, the Ce4+ sites on the surface of NC100 and NR110 would exhibit stronger acid strengths and bind adsorbates more strongly than those on NR111 with exposed {111} facet. In addition, the {100} surface consists of repeating O-Ce-O-Ce units with a pure Ce layer exposed on one side and the {110} surface has both exposed O and Ce ions on each side. As for a CeO2 surface of {111} orientation, it is an “oxygen-terminating” structure with O-Ce-O-O-Ce-O repeating units [46]. According to the atom arrangement of each facet, the exposed number of undercoordinated Ce with stronger Lewis acidity follows the order of {100} > {110} > {111}. In addition, theoretical calculations have shown that the formation energy of OVs follows the sequence of {110} <{100} < {111} [47]. Among the three faceted ceria, NR110 is the highest in the amount of oxygen vacancies (OVs) followed by NC100 and NR111, which well agrees with the calculation results. However, on the outer surface, NC100 has the highest amount of OVs (as indicated by the results of UVRaman spectroscopic analysis). It is known that the creation of OVs is usually accompanied by the formation of Ce3+ for charge balance. The rich OVs and/or Ce3+ in NC100 and NR110 indicate that NC100 and NR110 tend to have more Lewis acid sites. These results are in accord with the above experiments. As for the three faceted ceria, 15

NR110 per unit mass possessed the most weak and medium acid sites and followed by NC100. Whereas, the medium and strong acid sites obey another sequence of NC100 > NR110 > NC111 when calculated by per unit surface area. The good Lewis acidity of NC100 and NR110 derived from the atom arrangement on each crystal facet favored ozone activation due to the Lewis base character of the two terminate oxygen atoms of ozone. Actually, it has been reported that the amount of Ce3+ on the surface have positive relationship with the catalytic efficiency because Ce3+ has high electron density than Ce4+ and the ozone is electron deficient [24,26]. The facile formation of OVs or Ce3+ implies that NC100 and NR110 have good reducibility, which has been proved by the H2-TPR and CV measurement. With ample surface defects and appropriate redox ability and acidity, NR110 and NC100 show outstanding catalytic activities in the removal and mineralization of SMZ. It is noted that the catalytic activities follow an order of NR110-0.15g > NC100 > NR111 in which the dosage of NR110 was enhanced to 0.15 g to achieve a comparison on the basis of similar catalyst mass (the mass of NC100 and NR111 was 0.127 g and 0.148 g, respectively). Nonetheless, the surface area of NR110-0.15g was double that of NC100 and NR111 (i.e., 8.3 m2). The results indicate that the specific surface area of CeO2 also has an influence on catalytic activity. This is understandable because a larger surface offers more active sites for the reaction. The formation of BrO3− in ozonation using O3 alone and that by means of catalytic ozonation over NC100, NR110 and NR111 was investigated (Fig. 3c). The results reveal that in all the cases BrO3− concentration quickly increases within the initial 10 min, and then reaches a plateau or gradually slows down. In the case of O3 alone, the 16

production of BrO3− was the highest, reaching an amount of 3.16±0.06 μmol L-1. With the introduction of NC100, NR110, and NR111 into the ozonation system, the formation of BrO3− is only 1.87±0.11, 2.53±0.09, and 2.82±0.12 μmol L-1, respectively. The presence of the CeO2 catalysts obviously inhibits the formation of BrO3−, and the inhibition efficiency follows an order of NC100 > NR110 > NR111. This order is similar to that of the catalytic activities in the removal and mineralization of SMZ. When the dosage of NR110 was raised to 0.15 g, only 0.71±0.12 μmol L-1 of BrO3− was produced in 60 min. The results demonstrate that both NC100 and NR110 are potential catalysts for simultaneous degradation of organic pollutants and inhabitation of bromate formation. In this study, samples of synthetic ground water (Br−, 1 or 2 mg L-1), marine culture water (Br−, 34 mg L-1), and marine water (Br−, 65 mg L-1) were investigated. The inhibition of BrO3− at various initial Br− concentrations in O3 and NC100/O3 processes is shown in Fig. 3d. The BrO3− concentration in the NC100/O3 case was always notably lower than that in the case of O3 alone. Taking the case of O3 alone as control, the reduction percentage of BrO3− in the NC100/O3 case reaches 29.6%, 38.8%, 52.4%, and 66.4% at an initial Br− concentration of 1.0, 2.0, 34.0, 65.0 mg L-1, respectively. The results show that the presence of faceted CeO2 can efficiently inhibit the formation of bromate in ozone treatment of marine culture water or marine water. 3.3. Mechanism of enhanced SMZ degradation and Bromate inhibition 3.3.1. Transformation of bromine species In ozonation, HBrO/BrO− is a critical intermediate affecting BrO3− formation [48]. 17

We focused on the concentration changes of Br−, HOBr/BrO−, and BrO3−, which are the three major bromine-containing species in the reaction solution, as depicted in Figs. 4a & b, and Fig. 3c, respectively. The results indicate good mass balance on total bromine throughout the reaction in the oxidation processes (Fig. 4c). In all cases the critical intermediate HBrO/BrO− forms quickly due to the ample presence of aqueous ozone in the initial 10 min, and then its concentration slightly increases or fluctuates, reaching 5.34±0.27, 6.69±0.43, and 11.58±0.38 μmol L-1 at 60 min in the case of NC100/O3, NR110/O3, and NR111/O3, respectively. Compared to the extent of BrO3− formation as illustrated in Fig. 3c, it can be detected that in the presence of faceted CeO2, Br− is mainly transferred to HBrO/BrO− and only a small portion of HBrO/BrO− is oxidized to BrO3−. The results demonstrate that NC100, NR110 and NR111 inhibit the oxidation of HBrO/BrO− to BrO3−. Moreover, the Br− concentration in the NC100/O3 case is the highest, followed by that of NR110, both of which are higher than that in the case of O3 alone. The results indicate that the presence of NC100 and NR110 also inhibits the oxidation of Br− to HBrO/BrO−. It is apparent that the decrease of BrO3− generation is a result of the restricted oxidation of Br− and HBrO/BrO−. 3.3.2 Determination of organics oxidation intermediates In the catalytic ozonation system, organics would be oxidized into various intermediates, even into bromated organics disinfection byproducts (Br-DBPs). To determine the intermediates during catalytic ozonation, the degraded samples were analyzed using an Orbitrap-Fusion-Lumos High-resolution mass spectrometer at both negative ([M-H]−) and positive ([M+H]+) mode. Not counting SMZ (m/z=251.092 53), 18

14 intermediates at negative mode and 2 at positive mode were detected at the reaction time of 5 min, as listed in Tables S3 & S4. The intensities of the evolved intermediates versus reaction time are shown in Fig. S1. Organics can react with HOBr and produce toxic Br-DBPs. These Br-DBPs might be oxidized to release Br−, which could be another bromate inhibition way. It is fortunate that only two Br-DBPs (P124(-) and P164(-)) were detected in minute amount (Fig. S1) and to oxidize Br-DBPs is more difficult (due to the presence of the electronwithdrawing Br) than to oxidize DBPs, it is deduced that bromate inhibition by the oxidation of Br-DBPs could be ignored. As shown in Fig. 5, the detection of 14 non-bromated intermediates evidences the occurrence of a series of degradation reactions, including (i) hydroxylation of the benzene ring or isoxazole heterocycle (P172(-), P179(+), P254(-), P268(-), P298(-), P300(-), P316(-)), (ii) oxidation of the amine groups at the benzene ring (P202(+), P226(-), P282(-), P300(-)); (iii) oxidation of the methyl groups at the isoxazole heterocycle (P298(-), P316(-)); (iv) cleavage of the C-S or S-N bonds or -SO2 (P156(-), P172(-), P177(-), P179(+), P179(-)); and (v) oxidation of benzene or isoxazole rings (P196(-), P202(+), P226(-)). The para-aminobenzene sulfonamide in the SMZ molecules are the main reactive parts to generate antibacterial activity [49]. It is reasonable to consider that the amine groups at the benzene ring and sulfonamido groups on the SMZ molecules are the reactive functional groups. Among the fourteen non-bromated intermediates, six of them (enclosed inside the boxes of orange dash line) are with the sulfonamido groups destroyed, while eight of them (enclosed inside the boxes of blue dash line) are with the sulfonamide groups intact. 19

However, of the eight with amide groups, four of them undergo oxidation to become unreactive nitro groups and the other four become hydroxylated. Because of the introduction of the activated hydroxyl groups, the intermediates would become more easily oxidized. Overall, all the detected intermediates were further oxidized and almost disappeared after 10 min (Fig. S1). It is worth noting that much lower biotoxicity is observed in the case of NC100/O3/SMZ/Br− than in the case of O3/SMZ/Br− or O3/SMZ (TEXT S1 & Fig. S2). It is attributed to the outcome of less remaining SMZ, higher mineralization, almost no presence of toxic bromated organic intermediates, and less presence of toxic BrO3− in the wastewater treated by NC100/O3/SMZ/Br−. 3.3.3. Model Reduction of HBrO/BrO− and BrO3− over faceted CeO2 As discussed above, the oxidation of HBrO/BrO− to BrO3− was inhibited over the CeO2 catalysts. It was reported that CeO2 can catalyze the oxidation of alkylbenzenes by BrO3− [50]. Despite the authors just focused on the oxidized products and did not mention the transformation of BrO3−, it is certain that BrO3− was reduced. Inspired by this work, we are curious whether CeO2 can by itself reduce HBrO/BrO− and BrO3− or catalyze H2O2 and O3 to realize the reduction of HBrO/BrO− and BrO3−. To shed light on the mechanism of bromate inhibition, six model reactions were performed in aqueous solution at pH=4.6. They were the reduction of HBrO/BrO− and BrO3− in the cases of NC100 alone, NC100/H2O2, and NC100/O3. Fig. 6a & b show that NC100 by itself can reduce HBrO/BrO− to Br− and BrO3− to HBrO/BrO− and further to Br−, respectively. Fig. 6c & d show that NC100 can also catalyze the reduction of HBrO/BrO− and BrO3− by H2O2 but the extent of reduction is lower than that of NC100 20

alone. The presence of NC100 can also enhance the reduction performance in the case of NC100/O3 although the amount of reduced species is less than that of the NC100 alone and NC100/H2O2 cases (Fig. 6e & f). These results demonstrate that the inhibition of bromate formation in the presence of faceted CeO2 (as shown in Fig. 3c & d) is possibly due to the reduction of HBrO/BrO− and BrO3−. 3.3.4. Roles of reactive oxidation species In the catalytic ozonation process, the decomposition of aqueous ozone might produce certain reactive oxidation species (ROS), such as H2O2, •OH, •O2−, 1O2 and surface O, which play key roles in the oxidation of organics, inhibition of bromate formation, and redox cycle of the catalysts. The changes of ozone concentration in the aqueous solutions versus reaction time in ozonation and catalytic ozonation are shown in Fig. 7a. Ozone promptly decomposes in the initial 15 min. It is obvious that NC100 is the most efficient to catalyze ozone decomposition, followed by NR110 and then NR111. The high decomposition of ozone can be attributed to the acidity and reducibility of NC100. Through Lewis acid and base interaction, ozone adsorption (via the two terminate oxygen atoms) is promoted, whereas with good reducibility, the activation of O3 is favored via electron transfer from the catalyst to ozone. H2O2 is an important ROS and usually present during ozonation. As shown in Fig. 7b, a considerable amount of H2O2 was detected after 0.5 min, and then its concentration decreased with reaction time. Among the four processes, the extent of H2O2 generation follows the order of O3 > NR111/O3 > NR110/O3 > NC100/O3. During 21

ozone decomposition, H2O2 is produced via the reaction between O3 and OH− and the combination of H2O• or •OH during ozone decomposition [17]. With an oxidation potential of 1.76 V vs NHE, H2O2 could react with aqueous ozone (2.08 V), •OH (2.80 V), •O2− (1.35 V), or other oxidants [25, 51]. In addition, CeO2 could also catalyze these reactions [52]. Herein the lower H2O2 concentration in the presence of faceted CeO2, especially NC100, is plausibly due to H2O2 decomposition under the influence of the catalysts. In Figs. 7c–e, the presence of •OH, •O2− and 1O2 within a reaction time of 10 min over the faceted CeO2 as an example were compared in terms of the relative intensity of ESR signals. With the addition of DMPO trapping agents, a characteristic “1:2:2:1” pattern DMPO-•OH adduct was observed over the three faceted CeO2 samples (Fig. 7c), confirming the presence of •OH radicals. The signal intensity of •OH over NC100, NR110, and NR111 is 5.71×104, 6.01×104 and 1.03×104, respectively, revealing that NR110 is the most efficient in catalyzing O3 decomposition to generate •OH. It is known that •OH can unselectively oxidize SMZ and Br−. In the case of adding DMPO/DMSO, an ESR “1:1:1:1:1:1” pattern characteristic of DMPO/DMSO-•O2− emerged over NC100, NR110, and NR111, proving that •O2− is also generated in the catalytic ozonation system (Fig. 7d). The signal intensity of •O2− over NC100, NR110, and NR111 is 11.09×104, 4.99×104 and 2.78×104, respectively. Among the three, NC100 is the highest in •O2− generation, producing •O2− ca. 2 and 4 times that of NR110 and NR111, respectively. The addition of TEMP resulted in the appearance of the characteristic “1:1:1” pattern of TEMP-1O2. This verifies the presence of 1O2, and 22

among the three, NC100 is the highest in 1O2 generation. The time profile of ESR signal intensity over various faceted CeO2 are displayed in Figs. 7f–g. The signal intensities of DMPO-•OH, DMPO/DMSO-•O2− and TEMP-1O2 EPR at other reaction time over various faceted CeO2 show the similar trend to that at 10 min. The results show that NC100 favor to produce •OH, especially •O2− and 1O2. The surface O formed by interaction of CeO2 and O3 has been proved by Raman spectra [19]. The oxidation potentials of O3, H2O2, •OH, •O2−, 1O2, and surface O are 2.08, 1.76, 2.80, 1.35, 2.20 and 2.43 V [51]. respectively, which played different roles in the SMZ degradation and inhibition of bromate formation. According to the results in the literatures and the identified ROS of O3, H2O2, •OH, •O2−, 1O2 in our experiments, the mechanistic reactions involved in catalytic ozonation over the faceted CeO2 are suggested and compiled in Table S5. First, •O3 forms by transferring an electron from the electron-excess surface sites of CeO2 to the electron-deficient ozone molecule (Eq. S1) [53]. In the simplest case, these electron-excess sites may be Ce3+ or OVs as observed experimentally in the Raman spectra of CeO2 (Figs. 2a & b). O3 may also decompose into a surface O (*O) and an oxygen molecule when it was adsorbed on stronger Lewis acid sites (Eq. S2) [25,32,45,54]. Herein the surface O has not been observed experimentally but is logically suggested by the following DFT theory calculation. Also, ozone can react with OH− to produce •O2− and HO2• (Eq. S3) [55– 57]. Then •O3, *O, and •O2− initiate the following chain reactions. Namely, the interaction of •O3 with H+ (Eq. S4) and that of *O with H2O (Eq. S5) would produce •OH which is the highest in oxidation ability among ROS [58,59]. The reaction of *O 23

with an O3 molecule would generate •O2− and release O2 (Eq. S6) [58]. Superoxide (•O2−) can be obtained by the reaction between O3 and OH− (Eq. S3), and that between *O and O3 (Eq. S6) [55–58]. Then •O2− can initiate many chain reactions (Eqs. S7–13) to generate a number of ROS, e.g. HO2• (Eq. S7), H2O2 (Eq. S8), •OH and 1O2 (Eq. S9), and •O3 (Eq. S10) [51,57,60–64]. The experimental results show that the three faceted ceria all enhanced both SMZ removal and bromate inhibition, and the enhancement follows the order of NC100 > NR110 > NR111. The relationship between atom arrangement and coordination on each crystal facet, the surface properties (e.g. defects of OVs, reducibility, and acidity), and the catalytic activity has been discussed above. Herein we would further explore the difference in the catalytic activity from the point view of redox reactions. Obviously, the transformation of ozone depends on the surface composition of catalyst, which would greatly affect the wastewater treatment efficiency. Despite the amount of •OH produced on NC100 is slightly less than that on NR110, higher amounts of •O2−, 1O2 and *O are generated on NC100, which can also oxidize SMZ and the degradation intermediates. Especially, NC100 has large amount of strong Lewis acid sites, which favor the dissociation of O3 into *O, which has been verified by the following DFT calculations. *O can more readily oxidize the aliphatic hydrophilic organics containing carboxylic groups (e.g. oxalate) than •OH [25,32], which might explain the higher TOC removal by NC100. Less ROS were produced by NR111, which leads to the worst performance among three faceted ceria. There are several reasons for the bromate inhibition by faceted ceria. Firstly, O3 24

dominates the initial oxidation of Br − to HOBr/BrO −, which is a critical intermediate affecting bromate formation [6]. The presence of three faceted all promoted the decomposition of ozone and the remained aqueous ozone concentration follows the order of NC1001:2) inhibited bromate formation by reducing HOBr/BrO−to Br− whereas H2O2 at low concentration promoted bromate formation due to the enhanced due to the hydroxyl radical production from the reaction between HO2− and O3 [48]. It is obvious that the H2O2/O3 molar ratio is much lower than 1:2 and the extent of H2O2 generation follows the order of O3 > NR111/O3 > NR110/O3 > NC100/O3 as shown in Fig. 7a &b. Therefore, the lower H2O2 concentrations in the presence of faceted ceria inhibited the bromate information in the sequence of NC100>NR110>NR111. Finally, •O2− is believed to play an important role in the bromate inhibition. Since the redox potential of Ce4+/ Ce3+ is 1.77 V, aqueous •O2− (1.35 V in H+ form) can reduce Ce4+ to Ce3+ (Eqs. S14 & S15) [14,65–68]. In the case of NC100/O3, the ample presence of •O2− guarantees the realization of Ce4+ reduction to Ce3+ (Eqs. S14 & S15). Although it is proposed that Ce3+ can also catalyze H2O2 in the presence of H+ to generate reactive •OH (Eq. S16), most importantly, Ce3+ takes a key role in inhibiting the formation of bromate. Previous model reactions showed that Ce3+ of fluorite CeO2 could reduce BrO3− and HBrO/BrO− by itself or in the presence of H2O2 or O3 (e.g., Eqs. S17 & S18). Pelle et al. reported that Ce3+ 25

reduced BrO• to HOBr (Eq. S19) and Br• to Br− (Eq. S20) [69]. Actually, it has been suggested that a lower BrO3− formation by inhibiting the oxidation of Br- and HBrO/BrO− through the circulating reactions of Ce3+/Ce4+ [14]. Due to more •O2− available in the presence of NC100 and then NR110, and the facile reduction of Ce4+ to Ce3+ in both catalysts, Ce3+ are easily recovered for bromate inhibition in NC100 and then NR110 during the catalytic ozonation. Based on the three above-discussed minimization pathways, the formation of bromate from Br− can be inhibited in the sequence of NC100>NR110>NR111. With the reverse Ce3+-to-Ce4+ step facile in the oxidation environment, the Ce4+/Ce3+ redox cycle enables the outstanding catalytic performance of CeO2 in ozonation systems. 3.3.5 DFT theory calculation Catalytic ozonation is closely related to the adsorption and decomposition of ozone on the surface of a catalyst. We used density functional theory (DFT) calculations to understand ozonation activity on the surface of faceted CeO2. First, we investigated and compared ozone interaction between the CeO2(100), CeO2(110), and CeO2(111) surfaces. Two adsorption modes were considered, (i) adsorption via a single terminate oxygen atom on a Ce atom (mode 1) and (ii) adsorption via the two terminate oxygen atoms on two neighboring Ce atoms in a bridged manner (mode 2). The results of mode 1 are shown in Fig. 8. There is transfer of electrons from the (100) surface to the three oxygen atoms (O1, O2, O3) of the ozone molecule, leading to O3 dissociation and formation of *O and *O22- without any energy barrier (Eq. S21). The process is different from the transformation of O3 on catalyst surfaces proposed in previous 26

reports (i.e., Eqs. S1 & S2) [25,29,45,54]. Peroxide (*O22-) can readily transform into superoxide radicals (•O2−) (Eq. S22). Then surface O and •O2− initiate the sequential chain reactions for catalytic ozonation and bromate inhibition (Eqs. S5–22). The electron density difference depicted in Fig. 8d shows that there is transfer of electrons from the adjacent Ce atoms of (100) facet to O1, O2, O3, and there is bond formation between O1 and O3 (i.e., forming O22-). Fig. 8g2 depicts the charge density difference of O1–O3 in the optimized structure of an ozone molecule adsorbed on a (100) facet. The hybridization between s-orbit and p-orbit of the O1–O3 systems confirms the generation of peroxide ions. For the adsorption on (110) and (111) facets, the ozone molecule just adsorbs on the surface without decomposition despite there is electron transfer from surface to ozone (Figs. 8b, c, e & f). It suggests the formation of •O3 on (110) and (111) (Eq. S1), and the generated •O3 could also initiate chain reactions. The adsorption energy of ozone on (100), (110), and (111) is -10.23, -0.64, and -0.25 eV, respectively. The data reveal that the tendency of ozone adsorption on the facets follows the order of (100) > (110) > (111). Illustrated in Figs. 8g1, h & i are the partial density of states (PDOS) of O1, O2 and O3. The intense peaks of O1 and O3 on (100) appear in the range of -6 to -2 eV, while those of O2 in the range of -3 to 0 eV, verifying that *O on (100) is more active than *O22-. For the PDOS of O1, O2, and O3 on the (110) and (111) surfaces, the electron states do not alter as significantly as those on (100). The results for mode 2 adsorption are depicted in Fig. S4. The adsorption energy of ozone on (100), (110), and (111) is -10.49, -0.88, and -0.25 eV, respectively. It is noted that the adsorption energy of ozone on (100) and (110) are lower than those of mode 1. 27

The results indicate that mode 2 is preferred for ozone adsorption on (100) and (110). Nonetheless, disregard of adsorption modes, once ozone is adsorbed on the (100) facet, it promptly dissociates to surface O and O22-. It is apparent that the chain reactions that lead to catalytic ozonation and bromate inhibition are favored on the (100) facet. Furthermore, we investigated and compared ozone interaction with *O on CeO2(100), CeO2(110), and CeO2(111), labeled as O-(100), O-(110), and O-(111) for easy reference. The adsorption energy of ozone on O-(100), O-(110), and O-(111) is 17.61, -2.12, and -1.54 eV, respectively, much lower than those without *O. The theoretical results confirm that ozone adsorption on O-(100), O-(110), and O-(111) is more favored in comparison with that on (100), (110), and (111). Figs 9a, d, d1 & g illustrate the formation of peroxide corresponding to O2 and O3 and a surface O in ozone interaction with O-(100). It reveals ozone activation on O-(100) leads to ozone decomposition into a surface oxygen atom and a peroxide. As for O-(110) and O-(111), it is ozone physisorption without decomposition (Fig. 9). The adsorption of O3 is through the interaction of a terminate oxygen with a Lewis acid site on the surface of metal oxide catalysts [15, 45]. Without any energy barrier, ozone adsorbed on (100) readily decomposes to surface O and O22-. Bing et al. also reported that ozone decomposed to give surface O when it was adsorbed on stronger Lewis acid sites [45]. The large discrepancy in atom arrangement and coordination number of atoms on each facet signify the tendency of ozone adsorption, and the tendency order is (100) > (110) > (111). The theoretical results are in accord with the experimental data as the catalytic activity follows the same sequence. The outcomes of 28

the present study confirm that the catalytic activity of CeO2 in ozonation is determined by the redox behavior of CeO2 and surface-O3 interaction. 4. Conclusions In summary, three CeO2 nanocrystals individually with exposed (100), (110), and (111) facets were prepared and adopted as catalysts for the ozonation of Br−-containing wastewater. Among them, CeO2(110) is the highest in the amount of oxygen vacancies (OVs), best in reducibility, and most enriched with weak and medium acid sites (on per unit mass basis). However, on the outer surface, CeO2(100) has the highest amount of OVs (as indicated by the results of Visible-Raman spectroscopic analysis), the largest amount of strong and medium acid sites and consumes the largest amount of hydrogen per unit surface area. The good reducibility and Lewis acidity of CeO2(100) and CeO2(110) favors the activation of ozone and the redox cycle of ceria. The catalytic activity of the faceted CeO2 towards pollutants degradation and bromate formation inhibition follows the order of (100) > (110) > (111). It is suggested that five reactive oxygen species (ROS) i.e. O3, •OH, •O2−, 1O2, and surface O contributed to the oxidization of the pollutants. Especially, •O2−, as the precursors of most ROS, promotes the sequential chain reaction of catalytic ozonation. •O2− also functions as a key reductant to recover Ce3+, which can reduce HBrO/BrO− and BrO3− and therefore inhibit the bromate formation in system. DFT calculation and ESR results show that •O2− the most readily formed over CeO2(100). The experimental and theoretical results demonstrate that CeO2(100) catalyzed ozonation is a promising advanced oxidation for the treatment of Br-containing wastewater, and crystal facet engineering is an efficient 29

strategy to enhance the catalytic performance.

Appendix A. Supplementary data Determination of SMZ degradation intermediates by LC/Ms/Ms; intensity of SMZ degradation intermediates in mass spectra against the reaction time; toxicity evaluation of the treated water; analysis methods of reactive oxygen species; reusability of the catalysts; DFT calculation of the adsorption of ozone on faceted CeO2 by double terminated oxygens bridge-adsorbed on two Ce atoms (mode 2).

Declaration of Competing Interest 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.

Acknowledgements The research is supported by the National Natural Science Foundation of China (21677180, 21777196, 21976215) and Science and Technology Research Programs of Guangzhou City (2019).

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Figure captions Fig. 1. SEM images of (a) NC100, (b) NR110, (c) NR111, TEM images of (d) NC100, 38

(e) NR110, (f) NR111, and high-resolution TEM (HRTEM) images of (g) NC100, (h) NR110, (i) NR111.

Fig. 2. (a) Visible Raman spectra with excitation laser at 514 nm, (b) UV Raman spectra with excitation laser at 325 nm, (c) H2-TPR, (d) CV curves without O3 purging, (e) CV curves with O3 purging, and (f) NH3-TPD of NC100, NR110, and NR111

Fig. 3. Catalytic activity assessment of NC100, NR110 and NR111: (a) SMZ removal, (b) SMZ mineralization, (c) bromate formation with an initial Br− concentration of 2.0 mg L-1, (d) inhibition of bromate formation versus initial Br− concentration within the reaction time of 20 min. Conditions: Initial solution pH=6.3, SMZ=10 mg L-1, O3=4 mg L-1, NC100=0.127 g, NR110=0.075 g, NR111=0.148 g unless specified otherwise, catalyst dosages with the same surface areas=8.3 m2, reaction solution volume=300 mL.

Fig. 4. Evolution of (a) Br−, (b) HBrO/BrO− and (c) total Br.

Fig. 5. Initial degradation pathway of SMZ in NC100/O3.

Fig. 6. Model reduction of (a) HBrO/BrO− in NC100/HBrO process, (b) BrO3− in NC100/HBrO process, (c) HBrO/BrO− in NC100/H2O2/HBrO or H2O2/HBrO process, (d) BrO3−

in NC100/H2O2/BrO3− or H2O2/BrO3− process, (e) HBrO/BrO− in

NC100/O3/HBrO process, and (f) BrO3−

in NC100/O3/BrO3−. Conditions: Initial

solution pH=4.2, SMZ=10 mg L-1, O3=4 mg L-1, NC100=0.127 g; BrO3−=105 μmol or HBrO/BrO−=25 μmol in all the processes without H2O2; H2O2=50 μmol; BrO3−=100 μmol in NC100/H2O2/BrO3− or /H2O2/BrO3− process; HBrO/BrO−=100 μmol in NC100/H2O2/HBrO or /H2O2/HBrO process.

Fig. 7. (a) Concentration of ozone, (b) H2O2 versus time, (c) DMPO-•OH EPR spectra, 39

(d) DMPO/DMSO-•O2− EPR spectra, (e) TEMP-1O2 EPR spectra at a reaction time of 10 min as an example, Time profile of (f) DMPO-•OH signal intensity, (g) DMPO/DMSO-•O2− EPR signal intensity, (h) TEMP-1O2 EPR singal intensity over various faceted CeO2.

Fig. 8. Ozone adsorbed on (a) (100), (b) (110), and (c) (111), charge density difference of ozone adsorbed on (d) (100), (e) (110), and (f) (111), PDOS of O1, O2, and O3 in the initial state of ozone adsorbed on (g1) (100), (h) (110), and (i) (111), and (g2) PDOS of O1–O3 in mode 1 adsorption on the (100) facet having a single terminate oxygen atom connected to a Ce atom.

Fig. 9. Ozone interaction with the *O of (a) O-(100), (b) O-(110), and (c) O-(111), charge density difference of ozone interaction with the *O of (d) O-(100) (side view), (d1) O-(100) (top view), (e) O-(110), and (f) O-(111), and (g) PDOS of O2–O3 in ozone interaction with *O of O-(100).

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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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☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Highlights 

CeO2 (100) significantly enhanced pollutant degradation and bromate inhibition.



•O2− played an important role in the bromate inhibition.

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DFT results demonstrate ozone decomposes into surface O and O22- on CeO2(100).

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