Insights into Mechanism of Catalytic Ozonation over Practicable

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Insights into Mechanism of Catalytic Ozonation over Practicable Mesoporous Mn-CeOx/#-Al2O3 Catalysts zongwei wu, Guoquan Zhang, Ruoyu Zhang, and Fenglin Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04516 • Publication Date (Web): 25 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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Industrial & Engineering Chemistry Research

Insights into Mechanism of Catalytic Ozonation over Practicable Mesoporous Mn-CeOx/γ-Al2O3 Catalysts Zongwei Wu, Guoquan Zhang,* Ruoyu Zhang and Fenglin Yang Key Laboratory of Industrial Ecology and Environmental Engineering, Ministry of Education, Dalian University of Technology, Linggong road 2#, Dalian 116024, China.

KEYWORDS: Catalytic ozonation; Heterogeneous; Mesoporous alumina; Mn-Ce mixed oxides;

ACS Paragon Plus Environment

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ABSTRACT

The practicable mesoporous γ-Al2O3 supported manganese-cerium mixed oxides (Mn-CeOx/γAl2O3) catalysts were prepared in large quantities by a facile impregnation-calcination method. Characterization results demonstrated that the Mn-CeOx/γ-Al2O3 catalyst retained the mesoporous structure of γ-Al2O3 and existed in multi-valence redox couples of Mn4+/3+ and Ce4+/3+. The surface property, catalytic performance, reaction kinetics and mechanism of γ-Al2O3 and Mn-CeOx/γ-Al2O3 in catalytic ozonation of bromaminic acid (BAA) were investigated indetail. The protonated surface hydroxyl groups S − OH on Mn-CeOx/γ-Al2O3 were the active sites for ozone decomposition, and HO• and O• were main reactive oxygen species. The multivalence redox couples of Mn3+/4+ and Ce3+/4+ along with electron transfer between these redox couples and lattice oxygen resulted in the synergistically catalytic effect between Mn and Ce in Mn-CeOx. A catalytic ozonation mechanism over Mn-CeOx/γ-Al2O3 was tentatively proposed. The pilot-scale tests showed that the proposed catalytic ozonation has huge potential for the practical application in wastewater treatment.


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1. Introduction The heterogeneous catalytic ozonation has been proven to be an efficient strategy for the degradation and mineralization of refractory organic pollutants in water, due to its high efficiency and low negative effect on water quality.1–9 To date, a variety of transition metal oxides have been used in the catalytic ozonation, which exhibited different catalytic activities and ozonation decomposition mechanisms. Although the contradictory viewpoints have been reported on their catalytic activities, the following two possible mechanisms were approved: one is ozone decomposition producing surface bound O-radicals and HO• , while the other is both ozone and organic compounds are adsorbed on catalyst surface, and subsequently generate HO• and final HO• through electrons transformation from the catalysts to ozone and further interacting between the adsorbed species. Manganese oxides (MnOx) have been regarded as the promising active catalysts for the catalytic ozonation.5,10–21 The variation of oxidation states between Mn2+ and Mn4+ provides a good single electron-transfer redox reaction of Mn2+/Mn3+ or Mn3+/Mn4+, and oxygen mobility in the oxide lattice.5 This property enables MnOx possessing superior active sites for ozone to generate reactive oxygen species (ROS).11,22 Despite many research efforts have been devoted in the MnOx catalyzed ozonation for pollutant removal, its catalytic mechanism remains ambiguous. Ma et al.10,18,19 suggested that the surface-bound OH− ions or surface hydroxyl groups on MnOx are initiators for a chain reaction.18,19. However, Tong et al.23 demonstrated that the adsorption of organics on MnOx surface and subsequent attack of the aqueous or adsorbed ozone are responsible for organic pollutants removal. Thus, J. Nawrocki et al.6 described that the same catalysts frequently lead to different and sometimes even contradictory results, due to the


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competition between water, ozone and organics for catalytic and/or adsorptive active sites of solid catalysts. Cerium oxides (CeOx) have also been used as promising catalysts for catalytic ozonation.24,25 The special structure of 4f orbit and the redox pair (Ce4+/Ce3+) endow ceria the storing and releasing oxygen ability through oxygen vacancies formation, thus enhancing their catalytic effects for ozone decomposition.25–32 Additionally, the Mn–Ce mixed oxides have also been reported as effective catalysts for elimination of automobile exhaust, VOCs, and NOx.1,33–39 Faria et al.25 and Quinta-Ferreira′s group26,40 have identified Mn–Ce–O with a Mn/Ce molar ratio of 70/30 as a promising catalyst for catalytic ozonation of organic pollutants. However, the utilization of supported Mn-Ce mixed oxides for the catalytic ozonation of aqueous organic compounds is not much reported. Therefore, a deep understanding of the reaction mechanism of Mn-Ce oxides catalyzed ozonation is an essential requirement for a specific pollutant-catalystozonation oxidizing system. Herein, the practicable mesoporous alumina-supported Mn-Ce mixed oxides (Mn-CeOx/γAl2O3) were facilely prepared by a wetness impregnation-calcination method. The structure and property of Mn-CeOx/γ-Al2O3 were investigated by various characterization techniques. 1amino-4-bromoanthraquinone-2-sulfonic acid (BAA), a major intermediate for the synthesis of anthraquinone dyes, which usually resulted in serious water pollution due to its water-soluble nature was selected as model pollutant to evaluate the catalytic activity of Mn-CeOx/γ-Al2O3. In addition, the abundant functional groups of BAA molecular can provide more information regarding to the oxidative performance and mechanism of catalytic ozonation process. A detailed study is conducted to discuss the kinetics and mechanism of catalytic ozonation by EPR, FTIR, XPS analysis, ozone decomposition and utilization efficiency. The degradation intermediates


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were detected and a possible degradation pathway of BAA via Mn-CeOx/γ-Al2O3 catalyzed ozonation was proposed. The pilot-scale tests were also carried out and demonstrated that the Mn-CeOx/γ-Al2O3 catalysts can be easily prepared on a large scale, endowing the proposed catalytic ozonation a huge potential for the practical application in wastewater treatment. 2. Experimental Section The information of preparation process, SEM-EDS, XRD, BET, XPS, FTIR, and the point of zero charges (pHPZC) of γ-Al2O3 and Mn-CeOx/γ-Al2O3 were provided in the supplementary information S1. The procedures of catalytic ozonation, analytical methods and pilot-scale testing are provided in supplementary information S2. 3. Results and discussion 3.1 Characterization of Mn-CeOx/γ-Al2O3 SEM images shown in Fig. 1a and b revealed that both 300 ºC-activated γ-Al2O3 and MnCeOx/γ-Al2O3 possess the porous surface, while the latter is covered by the close-packed lamellar-like particles as reported the typical morphology of MnOx.11,40 EDS element analysis in Fig. S1 showed that the element composition of Mn and Ce wt% of Mn-CeOx/γ-Al2O3 is 1.37 and 1.17, respectively, which correlates well with the nominal loading molar ratio of Mn/Ce on the catalysts. Fig. 1c showed that both 300 ºC-activated γ-Al2O3 and Mn-CeOx/γ-Al2O3 displayed IV type isotherm with H3 type loop, which imply the mesoporous structure.11,41 The 300 ºC-activated γAl2O3 showed a broad hysteresis loop in the relative pressure (P/P0) range of 0.45−0.90, while the hysteresis loop of Mn-CeOx/γ-Al2O3 becomes narrower from 0.60 to 0.95, indicating the narrower pore size distribution. The single-modal pore size distribution in Fig. 1c is centered at ca. 6.2 and 8.9 nm for γ-Al2O3 and Mn-CeOx/γ-Al2O3, respectively. The BET surface area, total


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pore volume and average pore size were listed in Table S1. Mn-CeOx/γ-Al2O3 exhibits a lower BET specific

Figure 1 SEM images of a) 300 ℃-activated γ-Al2O3 and b) Mn-CeOx/γ-Al2O3, c) and d) N2 adsorption-desorption isotherms at −196 ºC and XRD patterns of 300 ºC -activated γ-Al2O3 and Mn-CeOx/γ-Al2O3, respectively. The inset in c) shows the corresponding pore size distribution.

surface area (264.8 vs. 336.1 m2 g–1) and a lower pore volume (0.44 vs. 0.57 cm3 g–1) than 300 ºC-activated γ-Al2O3, according well with the N2 adsorption/desorption isotherms. As seen from Fig. 1d, the XRD pattern of 300 ºC-activated γ-Al2O3 showed three principal broad peaks located at ca. 35.6º, 45.8º and 67.0º, which are attributed to the crystalline phase of γ-Al2O3.41 For comparison, the XRD pattern of commercial CeO2 was also given, which exhibits several characteristic diffraction peaks corresponding to (111), (200), (220), (311), (222), (400), (331), (420), (422) crystal planes of the cubic fluorite structure (JCPDS 34-0394).33,34,36 As for Mn-CeOx/γ-Al2O3, the diffraction peaks near 29.1º, 37.8º and 56.8º are verified for tetragonal αMnO2 (JCPDS 42-1169),11,33,42–45, and the peaks around 33.5° and 56° are attributed to Mn2O3


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(JCPDS 24-0508).33,42–45 Above results indicated that Ce and Mn oxides coexist, and the loaded Mn is presented mainly in multivalent oxidation states (MnOx), while the cerium is largely in CeO2 form. XPS was also carried out to further investigate the surface composition and valence state of Mn-CeOx/γ-Al2O3. As shown in Fig. 2a, obvious O 1s and relative weak Mn 2p and Ce 3d peaks are observed within the survey region. Two characteristic peaks centered at ca. 642.5 and 654.8 eV in Fig. 2b are attributed to Mn 2p3/2 and Mn 2p1/2, respectively.11,42-45 The XPS spectrum of Mn 2p3/2 region can be deconvoluted into two peaks corresponding to the binding energies of 641.5 and 643.0 eV, indicating that Mn element exists in Mn3+ and Mn4+.33,36,45 Additionally, the peak at 641.5 eV is sharper than that at 643.0 eV, implying that MnO2 is majority while Mn2O3 is a minor phase, although both the crystal patterns of α-MnO2 and Mn2O3 were detected in XRD

Figure 2 XPS spectra of fresh Mn-CeOx/γ-Al2O3 catalyst, a) Wide-range survey, b) Mn 2p region, c) Ce 3d region, and d) O 1s region.


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analysis, which accords with the data displayed in Table S2. The Mn4+ species have the optimal redox catalytic activity among the Mn-based catalysts,36,45 therefore, a large content of Mn4+ in Mn-CeOx/γ-Al2O3 is expected to be beneficial for enhancing the catalytic ozonation performance. The XPS core-level spectrum of Ce 3d shown in Fig. 2c can be separated into five pairs of spinorbit split doublets with u and v lines to depict the electronic transitions in Ce3+ and Ce4+. The marked v (882 eV), v0 (882.7 eV), v′′ (887.2 eV), v′′′ (897.8 eV), u (898.6 eV), u0 (900.7 eV), u′′ (905.7 eV) and u′′′ (917.5 eV) peaks are assigned to Ce4+, while the two photoelectron peaks labeled v′ (885.4 eV) and u′ (903.6) are associated with the Ce3+ species.36,37,45 The relative content of Ce3+ [Ce3+/(Ce3++Ce4+)] indicated that the chemical valence of Ce is mainly at Ce4+. The O 1s spectrum of Mn-CeOx/γ-Al2O3 displayed in Fig. 2d can be decomposed into two peaks: lattice oxygen (Olatt) at 529.8 eV and surface oxygen (Osurf) at 531.4 eV, corresponding to the oxygen defect states and the adsorbed hydroxyl,33,44 respectively. The Osurf is a crucial factor for HO• generation from ozone decomposition and plays a major role on the catalytic process.10,18,20 Surface atomic concentration and the ratio of Ce, Mn and O elements were quantitatively analyzed and summarized in Table S2. The results demonstrated that the weight ratio of Mn and Ce is 3.07% and 2.56%, respectively, which agrees well with that of EDS analysis and ICP measurement. 3.2 Catalytic ozonation for BAA degradation As seen from Fig. S2a, no obvious difference was observed in BAA decolorization by the sole ozonation and catalytic ozonation processes. The rapid decolorization of BAA by the sole ozonation can be attributed to the strong electrophilic nature of ozone, which tends to selectively attacks the unsaturated bonds and the aromatic moieties, especially the existence of electron donating amino group (–NH2).5 The control experiments such as adsorption, sole ozonation and


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Figure 3 a) The degradation profiles of BAA through adsorption, single ozone and catalytic ozonation. The kinetic simulation curves: (-----) pseudo-first-order, (──) combined pseudo-firstorder. b) The combined pseudo-first-order kinetic analysis of BAA degradation by the sole and catalytic ozonation. c) Change in UV–vis spectra of BAA via Mn-CeOx/γ-Al2O3 catalyzed ozonation. d) Comparison of TOC removal efficiency by the sole and catalytic ozonation. Experimental conditions: [BAA]0 = 50 mg L–1, catalyst loading = 1.0 g L–1, ozone flow rate = 12 L h–1 (4.0 mg min−1), Gaseous ozone concentration = 20 mg L–1, initial pH = 6.8, and temperature = 25 ºC. catalytic ozonation were conducted to provide a benchmark for catalytic performance of MnCeOx/γ-Al2O3. It is clear from Fig. 3a that approximately 10% of BAA was removed within 60 min by both γ-Al2O3 and Mn-CeOx/γ-Al2O3 adsorption, and no obvious improvement in removal efficiency was observed at 120 min. Adsorption elimination of BAA can be explained according to the open porous structure and the surface charge statue of γ-Al2O3 and Mn-CeOx/γ-Al2O3. Fig. S3 showed that an increscent shift of pHpzc from 8.31 for γ-Al2O3 to 8.57 is observed after MnCeOx components loading in pH 6.8 solution. According to the proton transfer reaction of surface hydroxyl group on metal oxide,19,34 both γ-Al2O3 and Mn-CeOx/γ-Al2O3 surface are positively charged, which is benefit to BAA adsorption through electrostatic attraction.


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MeOH + H → MeOH (pH < H )

(1)

MeOH + OH → MeO + H O(pH > H )

(2)

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BAA removal through the sole ozonation is only 32.7%. Nevertheless, nearly 100% BAA is eliminated by γ-Al2O3 and Mn-CeOx/γ-Al2O3 catalyzed ozonation within 120 min, demonstrating BAA elimination by the catalytic ozonation rather than adsorption. The catalytic ozonation kinetics is normally assumed to be the pseudo-first-order (Eq. 3),27,34 while a combined pseudo-first-order reaction can be expressed as Eq. 4,47 where kapp is the apparent pseudo-first-order rate constant (min–1), C01 and C02 are the initial BAA concentrations, respectively. = exp (− )

(3)

= exp − ! + exp (− )

(4)

The comparison of two kinetic models simulating for BAA degradation by different ozonation processes demonstrated that the pseudo-first-order kinetics do not fit well all three systems, while the combined pseudo-first-order kinetic model exhibits the higher correlation coefficients. The combined pseudo-first-order kinetic plots in Fig. 3b indicated that two parallel pseudo-firstorder reactions are responsible for BAA removal. The combined pseudo-first-order kinetic parameters are listed in Table 1. In all three cases, the values of kapp2 are much lower than those of kapp1, suggesting a quick oxidation during the first 30 min followed by a slower reaction. It is also worth noting that in both two stages, Mn-CeOx/γ-Al2O3 exhibits a higher kinetics constant than γ-Al2O3, implying a better catalytic activity of Mn-CeOx/γ-Al2O3. The time-dependent UVVis spectrum of BAA solution during Mn-CeOx/γ-Al2O3 catalyzed ozonation is displayed in Fig. 3c. The absorption peak at λmax = 485 nm disappears completely after 30 min, meanwhile, a significant decrease at 254 and 310 nm is also observed. The changes in UV and visible spectra


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indicated the cleavage of anthraquinone nuclear and the destruction of amino group. Another phenomenon is the faster reduction of maximum absorbance in visible region than those in UV region, implying the easy disruption of primary chromophore of BAA. The existence of absorbance peaks at UV spectra even after complete decolorization suggested that a certain amount of aromatic rings are more difficult to be mineralized than chromophore structure during the short-time catalytic ozonation.

Table 1 Pseudo-first-order kinetics constants of BAA degradation by different ozonation processes. The first stage Experiments

The second stage

kapp1 (min−1)

R2

kapp2 (min−1)

R2

Single Ozonation

9.71 × 10−3

0.976

5.47× 10−4

0.985

γ-Al2O3/Ozone

6.93 × 10−2

0.971

1.37 × 10−2

0.941

Mn-CeOx/γ-Al2O3/Ozone

9.17 × 10−2

0.967

1.73 × 10−2

0.995

As shown in Fig. 3d, only 9.6% of mineralization efficiency is achieved by the sole ozonation within 120 min. The reason may be that the partial intermediates can be transformed into the saturated compounds such as the short-chain and low molecular-weight carboxylic acids, which are low reactivity to the sole ozonation.7,26 Adding γ-Al2O3 brought a significant improvement in TOC removal rate (ca. 31.5%), according with the fact that γ-Al2O3 can effectively promote organics mineralization through surface reactions and ozone self-decomposition.1–3,6,48 The TOC removal rate was greatly enhanced in MnOx/γ-Al2O3, CeOx/γ-Al2O3 and Mn-CeOx/γ-Al2O3 catalyzed ozonation processes. Approximately 46.5% and 51.6% of TOC was removed with MnOx/γ-Al2O3 and CeOx/γ-Al2O3 at 120 min, respectively (Fig. S2b), which are higher than that obtained by single ozonation and γ-Al2O3 catalyzed ozonation. As expected, an obvious increase


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in TOC decay (ca. 64.7%) was obtained in Mn-CeOx/γ-Al2O3-catalyzed ozonation. By contrast, the Mn-CeOx component on γ-Al2O3 surface gives a relatively larger contribution to BAA mineralization (ca. 33.2%). 3.3. Ozone utilization efficiency It is well known that there exists the mass balance of ozone in gas and liquid phase during the sole ozonation and catalytic ozonation, which can be expressed as follow.24 [O3]T = [O3]O + [O3]R + [O3]C

(5)

where [O3]T, [O3]O, [O3]R and [O3]C represent the ozone concentration of totally applied, off gas, residual and the consumed, respectively. Thus, [O3]C can be calculated as: [O3]C = [O3]T − [O3]O − [O3]R

(6)

The utilization efficiency of ozone (Ru%) can be expressed as follows:49 t

Ru % =

"0 #($O3 %T $O3 %O )dt&$O3 %R t

"0 #$O3 %T dt

×100

(7)

where ν is gas flow rate and V is solution volume. The amount of ozone consumed per unit mass of TOC removal through the sole and catalytic ozonation can be calculated by Eq. (8).49 0

'=

() "1 *$+, %- ./ &($2+3%1 $2+3%0 )

(8)

As shown in Fig. 4a, [O3]R in γ-Al2O3 and Mn-CeOx/γ-Al2O3-catalyzed ozonation is lower than ozone alone at each time interval, while [O3]O in sole ozonation is higher than that in catalytic ozonation. Above results revealed that more dissolved ozone is rapidly decomposed or consumed by γ-Al2O3 and Mn-CeOx/γ-Al2O3. The Ru evolution in Fig. 4b demonstrated that the higher Ru was obtained in Mn-CeOx/γ-Al2O3-catalyzed ozonation during 30-120 min treatment. In different ozonation systems, the Ru value decreases gradually with the transformation of BAA


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Figure 4 a) [O3]R and [O3]O, b) Ru % and η in sole and catalytic ozonation, respectively.

and its intermediates into the saturated carboxylic acids, nevertheless, Ru values of Mn-CeOx/γAl2O3 catalyst are still higher than those of γ-Al2O3 and ozone alone. The η exhibited an opposite tendency in comparison to Ru. The η value of catalytic ozonation is lower than that of sole ozonation at each time interval, implying the efficient decomposition of ozone into ROS via γAl2O3 and Mn-CeOx/γ-Al2O3 catalysts. Generally, the aqueous ozone can be consumed through a combination of direct and indirect oxidation processes in catalytic ozonation system.1–6 In this work, the presence of Mn-CeOx/γ-Al2O3 catalyst leads to the higher Ru and lower η, demonstrating the adsorption and efficient decomposition of ozone on catalyst surface as well as the efficient mineralization of BAA by ROS rather than the direct ozone reaction. 3.3 Catalytic ozonation mechanism on Mn-CeOx/γ-Al2O3 catalyst It is accepted that the metal oxides tend to adsorb water molecules, which can dissociate into hydroxyls and further forming the surface hydroxyl groups at Lewis acid sites of metal oxide surfaces, and finally function as a Brønsted acid for catalytic ozone decomposition.1–5 Thus, the acidity-alkalinity property of catalyst surface plays an important role in adsorption and catalyzed ozone decomposition. The surface charge properties of γ-Al2O3 and Mn-CeOx/γ-Al2O3 were investigated during ozone decomposition by FTIR. As seen from Fig. 5, the FTIR spectra of the


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Figure 5 FTIR spectra of a) γ-Al2O3 and b) Mn-CeOx/γ-Al2O3 in unozonation, after treatment with ozone in solution and after use in aqueous mixture of ozone and BAA.

fresh γ-Al2O3 and Mn-CeOx/γ-Al2O3 display the surface hydroxyl groups and chemisorbed water at 3492 and 1640 cm–1, respectively. After the catalytic ozonation in the absence of BAA, a new and intense band was observed at 1381 cm–1 in both FTIR spectra, which is attributed to a surface oxide species generated from the interaction between ozone and the Lewis acid sites of γAl2O3.4,5 Meanwhile, the intensity of hydroxyl groups at 3492 cm–1 increases as compared to the fresh catalysts, indicating the formation of new surface hydroxyl groups through the interaction between catalyst and aqueous ozone.5,46 Interestingly, the feature peak at 1381 cm–1 was not observed for two catalysts after used in catalytic ozonation of BAA. Furthermore, the comparison in the surface hydroxyl group of γ-Al2O3 support and Mn-CeOx/γ-Al2O3 at 3492 cm– 1

implies that surface hydroxyl groups are still the active sites even though Mn-CeOx component


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Figure 6 EPR for Mn-CeOx/γ-Al2O3 catalyzed ozonation in the absence of BAA.

are loaded on γ-Al2O3 and the formed surface oxide species are responsible for BAA degradation. Above results agree with the fact that surface hydroxyl groups formed on Lewis acid sites of metal oxide serve as a Brønsted acid, which promote ozone decomposition to produce HO• .1–6 EPR spectroscopy was employed to characterize ROS generated during catalytic ozonation with DMPO for the detection of HO• and O• , while TEMP for identifying singlet oxygen (1O2). Fig. 6 showed the characteristic quartet line spectrum of DMPO- HO• accompanying sextuple line spectrum of DMPO-O• with the relative intensities of approximate 1:2:2:1 and 1:1:1:1:1:1, respectively, which indicates the generation of HO• and O• in Mn-CeOx/γ-Al2O3-catalyzed ozonation process. On the other hand, a specific scavenger for HO• , dimethyl sulphoxide (DMSO) was used to compare the remaining signals. Adding DMSO resulted in an obvious inhibition for DMPO-HO• adducts and the characteristic four-line spectrum disappeared in favor of the notable DMPO-O• signal. It is well known that TEMP reacting with 1O2 produces a nitroxide radical with the distinctive three-line EPR spectrum,50 while no characteristic three-line EPR signal was


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observed in present work. These results revealed that HO• and O• rather than 1O2 were the dominant ROS for BAA degradation in Mn-CeOx/γ-Al2O3-catalyzed ozonation process. To investigate the role of interfacial electron transfer on catalytic ozonation, XPS was carried out again to determine the surface chemical state of Mn-CeOx/γ-Al2O3 after used in catalytic ozonation. As seen in Fig. 7a, there is no obvious difference within the wide-range survey when compared to the fresh catalyst (Fig. 2). After catalytic ozonation, the surface content of Mn4+ and Ce4+ (Fig. 7b and c) increased from 42.44 to 59.05%, and 60.66% to 69.84%, while Mn3+ and Ce3+ decreased from 57.56% to 40.95%, and 39.34% to 30.16%, respectively. Similarly, the content of Olatt decreased from 69.81 to 50.46%, while that of Osurf increased from 30.19 to 49.54% (Fig. 7d). Above results demonstrated that Osurf increased during the catalytic ozonation, which agrees with the result displayed in Fig. 5. The high Ce4+ and Mn4+ would result in oxygen vacant sites formation and/or oxygen mobility by the electron transfer from Olatt to Ce4+ and Mn4+, due

Figure 7 XPS spectra of Mn-CeOx/γ-Al2O3 used in catalytic ozonation of BAA, a) Wide-range survey, b) Mn 2p region, c) Ce 3d region, and d) O 1s region.


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to the strong storing/releasing oxygen nature of Ce4+/Ce3+ redox couple.25–28 These oxygen vacant sites can promote ozone molecules adsorption, which would decompose via electron transfer from surface Ce3+ and Mn3+ to form HO• and O• . The proposed synergistic effect of Mn-CeOx component is illustrated in Scheme 1a. Both Brønsted acid and Lewis acid sites on catalyst surface are considered as the active centers for catalytic ozonation.1–5 Water and ozone as Lewis base can adsorb on γ-Al2O3 surface due to the existence of strong Lewis acid sites. The proposed mechanism of catalytic ozonation over Mn-CeOx/γ-Al2O3 is displayed in Scheme 1b. Firstly, water molecules are adsorbed onto the surface Lewis acid sites when γ-Al2O3 and Mn-CeOx/γ-Al2O3 are introduced into solution (pH = 6.8 < pHpzc, Fig. S2), and then dissociate into the protonated surface hydroxyl groups (S − OH , S refers to catalyst surface). Subsequently, the aqueous ozone molecules can interact with Brønsted acid S − OH forming surface complex S − OH − O via hydrogen bonding and/or electrostatic forces, due to the electrophilic and nucleophilic resonance structure of ozone

Scheme 1 Schematic illustrations of a) the synergistic effect of Mn-CeOx component. b) a probable mechanism of Mn-CeOx/γ-Al2O3 catalyzed ozonation.


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molecule.34,46,51,52 Finally, the surface complex S − OH − O is transformed to HO• and O• through the radical chain reactions (Eq. 9-15). On the other hand, CeOx can effectively promote oxygen vacancies formation via electron transfer between Olatt and surface Ce4+ and Mn4+, and then the adsorbed ozone molecules on these vacancies can dissociate into HO• and O• accompanying with electron transfer from Ce3+ and Mn3+ to aqueous ozone. Additionally, the partial intermediate products HO• generated on catalyst surface will diffuse into the bulk solution and decompose into HO• and O• .48,53 The laboratorial Mn-Ce-O catalysts have been reported to enhance catalytic ozonation of different wastewater under different operating conditions.25,26,40 Results showed that Mn-Ce-O catalyst with a Mn/Ce molar ratio of 70/30 exhibited outstanding excellent activity for catalytic ozonation of organic pollutants. The mechanism of catalytic ozonation over Mn–Ce–O develops mainly through surface reactions and the absence of an aqueous HO• pathway. By comparison, the Mn-CeOx/γ-Al2O3 catalyst also possessed excellent activity for catalytic ozonation, and more importantly, this catalyst can be produced in large quantities by a facile impregnation-calcination method using commercial industrial-grade γAl2O3 as support.

(9) S − OH + O → S − OH − O

(10)

S − OH − O → S − OH • + HO•

(11)

S − OH • + H O → S − OH + HO•

(12)

HO• + HO• → 2O• + HO•

(13)

• O + O• → O + O

(14)

• O• + H O → 2HO + O

(15)


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3.4 Intermediates and degradation pathway of BAA through catalytic ozonation The FTIR spectra of BAA and its final product generated during Mn-CeOx/γ-Al2O3 catalyzed ozonation are shown in Fig. 8a. The significant infrared peaks of BAA at ca. 1670, 3280 and 3379, 625, 1041-1238 cm–1 were attributed to the stretching vibrations of carbonyl (C=O), amino (N–H), C–Br and S=O, respectively.54 Other peaks at ca. 1528 and 1580 cm–1 corresponded to the breathing mode of anthracene ring, while the absorption peaks at ca. 721 and 741 cm–1 belonged to the C–H deformation mode of anthracene ring.45 As for the final product, two new FTIR peaks at ca. 1503 and 1578 cm–1 can be assigned to the skeleton stretching of benzene, suggesting the ring-opening and decolorization of BAA. The aromatic C–H deformation peak at ca. 860 cm–1 proved that the degradation intermediates are related to multi-substituted benzene derivatives. Moreover, the characteristic vibration peaks of N–H (3333 cm–1), C–Br (625 cm–1) and S=O (1041, 1149 and 1240 cm–1) implied that the final aromatic compounds product probably contained amino, bromo, hydroxyl and sulfonate groups.47 Some important information about intermediates generation and of BAA degradation pathway can also be obtained by monitoring pH changes during single and catalytic ozonation. Generally, ozone can cleave activated aromatic rings forming refractory carboxylic acids, which results in a low mineralization level. As seen from Fig. 8b, the solution pH always decreases in single ozonation and the final pH is lower than that of catalytic ozonation, which indicates the formation and accumulation of refractory carboxylic acids, leading to lower TOC removal. The pH value decreases in first 45 min for Mn-CeOx/γ-Al2O3 catalyzed ozonation, which should be ascribed to carboxylic acids formation during BAA degradation. Nevertheless, the larger final pH is mainly due to the generation of HO• and O• , which could mineralize the acidic intermediates effectively.


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Figure 8 a) FTIR analysis of BAA and its degradation product after catalytic ozonation by MnCeOx/γ-Al2O3. b) pH evolution during sole ozonation and catalytic ozonation over Mn-CeOx/γAl2O3.

The LC-MS spectra shown in Fig. S4 demonstrated that the HPLC peak at the retention time of 8.249 min declines gradually with the increasing treatment time, indicating BAA degradation through Mn-CeOx/γ-Al2O3 catalyzed ozonation. The identified intermediates and the corresponding MS fragment peaks are summarized in Table S3 and Fig. S5. On the basis of these intermediates, a possible degradation pathway of BAA is proposed and outlined in Fig. 9. In first stage, the oxidation process is initiated by the hydroxylation of BAA to form the colorless matters that the -SO3H and -NH2 are oxidized to hydroxyl. Another two possible pathways are minor routes, the C(3) and C(6) positions on the anthracene ring of BAA are attacked by HO• to form hydroxyl. The products in first stage are Salicylic acid (10), Dihydroxy-benzoic acid (12),


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Figure 9 Proposed BAA degradation routes by the Mn-CeOx/γ-Al2O3 catalyzed ozonation.

Trihydroxy-2-bromide-6-nitrobenzoic acid (13). In second stage, the products (10) and (12) will be further oxidized to form (14) and (15). Following degradation involved mainly aromatic ring cleavage, which leads to carboxylic acid (17) formation. Other products with –Br will break the aromatic ring and form (16) and (17). Further oxidation may generate (17) and (19) and then gradually transform into oxalic acid (18). On the other hand, the variation of low-molecularweight organic acids (formic, acetic and oxalic acid) concentrations and related ions (bromine, bromate and sulfate ion) during the sole ozonation and Mn-CeOx/γ-Al2O3 catalyzed ozonations were also analyzed by IC (Table S4). However, the retention time of formic and acetic acid (ca. 5.0 min) as well as bromate ion (ca. 4.6 min) is overlapping in IC spectra, thus it is difficult to determine their concentrations. The concentration of SO42– shown in Fig. 10 increases in the first 30 min and about 84.2% –SO3H of BAA was transformed to SO42– at 120 min. The concentration of oxalic acid and bromine was accumulated within 45 and 30 min of the sole ozonation and the Mn-CeOx/γ-Al2O3 catalyzed ozonation, respectively, and then both started to decrease and finally reach a stable value. The ultimate stable concentration of oxalic acid and bromine in Mn-CeOx/γ-Al2O3 catalyzed ozonation is lower than that of in the sole ozonation.


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Figure 10 The yielding of oxalic acid, bromine ion and sulfate ion during sole ozonation (dotted lines) and Mn-CeOx/γ-Al2O3 catalyzed ozonation (solid lines). The above differences indicated that the degradation efficiency of BAA was higher in MnCeOx/γ-Al2O3 catalyzed ozonation. Certainly, the removal of intermediates reflected the removal efficiency of TOC, which fully confirms that Mn-CeOx/γ-Al2O3 is an efficient catalyst for BAA mineralization via the catalytic ozonation process, as demonstrated in Fig. 3a. 3.5 Application in real wastewater treatments The multi-valence redox couples of Mn3+/4+ and Ce3+/4+ as well as the electron transfer between Mn3+/4+/ Ce3+/4+ and lattice oxygen (Olatt) of Mn-CeOx/γ-Al2O3 resulted in an enhanced catalytic ozonation performance. HO• and O• are the main reactive oxygen species contributed to highly oxidative ability. This catalyst can be easily prepared on a large scale through an impregnationpelletizing-calcination method with the commercial industrial alumina as support (Fig. S6 a-d). To investigate and evaluate the potential of practical application of these Mn-CeOx/γ-Al2O3 catalysts, we designed two different two-stage ozone oxidation towers (Fig. S6 e-g) and carried out a series of pilot-scale catalytic ozonation experiments. The established system exhibited excellent catalytic ozonation performance in the oxidative degradation of pharmaceutical and chemical industry wastewater (Fig. S7). Therefore, the proposed Mn-CeOx/γ-Al2O3 catalyzed


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Figure 11 a) TOC removal profile of BAA during the reuse of Mn-CeOx/γ-Al2O3 in ozonation, b) and c) XRD and N2 adsorption-desorption isotherms patterns of Mn-CeOx/γ-Al2O3 before and after ozonation, d) SEM image of Mn-CeOx/γ-Al2O3 after ozonation. The inset in c) shows the corresponding pore size distribution.

ozonation is a promising system and has enormous potential for the practical engineering applications in the field of wastewater treatment. 3.6 Stability of Mn-CeOx/γ-Al2O3 catalyst The stability of Mn-CeOx/γ-Al2O3 catalyst was investigated by repeated application in catalytic ozonation of BAA. Fig. 11a showed that TOC removal was slightly declined from ca. 64.7% to 57.2% in the first run, while it tends to nearly stable in the second and third run. The structural, textural and morphological profiles of Mn-CeOx/γ-Al2O3 after used in catalytic ozonation were also investigated by XRD, BET measurements and SEM (Fig. 11b-d). It is clearly observed that the Mn-CeOx/γ-Al2O3 catalysts still retain good crystallinity, porous structure and surface morphology after used in catalytic ozonation. Moreover, the practical pilot-scale application indicated that the Mn-CeOx/γ-Al2O3 catalyzed ozonation system exhibited good performance for


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oxidative degradation of pharmaceutical and chemical industry wastewater during successive operation. Above results indicated that Mn-CeOx/γ-Al2O3 is a robust catalyst for ozonation with high catalytic activity and good stability. 4. Conclusions The practicable Mn-CeOx/γ-Al2O3 can be facilely prepared via wetness impregnationcalcination method. This composite catalyst retained the mesoporous structure of γ-Al2O3 and possessed large surface area and pore volume. The presence of Mn-CeOx/γ-Al2O3 led to the decrease in residual ozone and the consumed ozone amount per unit mass of TOC removal as well as the increase of ozone utilization efficiency in catalytic ozonation process. BAA degradation by the sole ozonation and the catalytic ozonation both accorded with the combined pseudo-first-order kinetic model. The mechanism of Mn-CeOx/γ-Al2O3 catalyzed ozonation was explored by EPR, FTIR and XPS techniques. The protonated surface hydroxyl groups, S − OH were the active sites and played an important role on ozone decomposition into the active species, such as HO• and O• . The multi-valence redox couples of Mn4+/Mn3+ and Ce4+/Ce3+ as well as the electron transfer from Olatt to Ce4+ and Mn4+ brought a synergetic effect of Mn-CeOx component, which were also responsible for the improved catalytic activities. The degradation intermediates and the end products were analyzed by LC-MS and FTIR, and finally, BAA degradation pathways were tentatively proposed. In brief, the Mn-CeOx/γ-Al2O3 is an ideal and robust catalyst for catalytic ozonation in terms of high catalytic activity and good stability. More importantly, the Mn-CeOx/γ-Al2O3 catalyst can be facilely prepared on a large scale, which enables the proposed Mn-CeOx/γ-Al2O3 catalyzed ozonation system having huge potential for the practical applications in the field of wastewater treatment. ASSOCIATED CONTENT Supporting Information


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The preparation process of Mn-CeOx/γ-Al2O3, the characterization techniques including such as SEM-EDS, XRD, BET, XPS, FTIR, and the point of zero charges (pHPZC), catalytic ozonation procedure, analytical methods, BAA decolorization, intermediate product analysis, pilot results. AUTHOR INFORMATION Corresponding Author *G. Zhang. E-mail Address: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (Grant number 21437001) and the Programme of Introducing Talents of Discipline to Universities (Grant number B13012). REFERENCES (1) Ikhlaq, A.; Kasprzyk-Hordern, B.; Catalytic ozonation of chlorinated VOCs on ZSM-5 zeolites and alumina: formation of chlorides. Appl. Catal. B: Environ. 2017, 200, 274-282. (2) Ikhlaq, A.; Browna, D. R.; Kasprzyk-Hordern, B. Catalytic ozonation for the removal of organic contaminants in water on ZSM-5 zeolites. Appl. Catal. B: Environ. 2014, 154-155, 110122. (3) Ikhlaq, A.; Browna, D. R.; Kasprzyk-Hordern, B. Mechanisms of catalytic ozonation: An investigation into superoxide ion radical and hydrogen peroxide formation during catalytic ozonation on alumina and zeolites in water. Appl. Catal. B: Environ. 2013, 129, 437-449. (4) Bing, J.; Hu, C.; Nie, Y.; Yang, M.; Qu, J. Mechanism of Catalytic Ozonation in Fe2O3/Al2O3@SBA-15 Aqueous Suspension for Destruction of Ibuprofen. Environ. Sci. Technol. 2015, 49, 1690-1697.


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Electron transfer between Mn3+/4+/Ce3+/4+ and lattice oxygen resulted in a promising Mn-CeOx/γ-Al2O3 catalyzed ozonation system for the practical application.


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Insights into Mechanism of Catalytic Ozonation over Practicable

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