Catalytic Ozonation for Phenol Removal over Cobalt-doped α-MnO 2 Catalyst: Performance and Mechanism

In this paper, Cobalt-doped α-MnO 2 (i.e., Co-α-MnO 2 ) were synthesized through hydrothermal method. Phenol was employed as targeted pollutants to investigate the catalytic ozonation performance of Co-α-MnO 2 in terms of catalytic ability and mechanism. Results showed that that Co doped on α-MnO 2 signicantly improved the phenol removal increased to 97.47 % after 40 min, which was 16.46 %, 38.92 % higher than that of α-MnO 2 catalytic ozonation and single ozonation without catalyst. In order to investigate the effect of Co doping on the physicochemical properties of the catalysts, the synthesized α-MnO 2 and Co-α-MnO 2 (2% Co/Mn doping ratio) catalysts were characterized by XRD, TEM, BET and XPS techniques, including the phase, morphology, structural properties and dispersity of the surface active species. The larger specic surface area and pore volume, more crystal defect and oxygen vacancy, higher relative content of Mn 3+ and adsorbed oxygen (O ads ) as well as surface hydroxyl were obtained after Co doping on α-MnO 2 , which could result in higher catalytic oxidation performance of Co-α-MnO 2 . The inuence of masking agent showed that surface hydroxyl group and active free radicals (•OH and •O 2- ) were involved in the catalytic ozonation of phenol. This study could help recognize the role of surface hydroxyl groups and active free radicals and demonstrate the contribution of reactive oxygen species (ROS) on phenol removal in Co-α-MnO 2 systems.

changed by doping with Mo 6+ , leading to an increase in its speci c surface area and the number of catalytically active surface positions, which in turn improved the catalytic performance of the catalyst (Uematsu et al., 2016).
As a kind of transition metal, Co has a strong Co 2+ /Co 3+ redox cycle which can also promote electron transfer and thus improve catalytic oxidation e ciency (Anfar et al., 2021). Lv (Faleh et al., 2019). Additionally, some studies have showed that the synergistic role can be well presented when the radius of doped metal ions is close to that of metal ions in the catalyst (Kang et al., 2013). Therefore, due to the close ionic radius between Co 2+ and Mn 2+ , the catalyst with Co doping on the α-MnO 2 might have a better catalytic activity than that of the sole α-MnO 2 catalyst. However, it is not clear how in uence cobalt doping on its structure and physical and chemical properties.
In this article, α-MnO 2 catalyst doped with Co 2+ was prepared by hydrothermal method, and then investigate the catalytic ozonation activities on phenol removal. BET, XRD, XPS and FTIR were used to analyze the phase, morphology and structural properties of the synthesized catalysts. The catalytic ozonation mechanism of Codoped α-MnO 2 catalyst on phenol removal was explored in depth by the masking experiment of free radicals combining with catalyst structure characteristics. obtained from a Millipore Q water puri cation system. All reagents and chemicals were of analytical grade.

Synthesis of Catalysts
Co-doped α-MnO 2 were synthesized using a modi ed hydrothermal method, which was showed in Fig. 1. Codoped α-MnO 2 were synthesized via a one-step hydrothermal method according to a previous report (Hu et al., 2020). 36 mmol of MnSO 4 ·H 2 O and certain molar of CoSO 4 ·7H 2 O were dissolved in 50 mL of deionized water under stirring, and then the above mixed solution was added into 50 mL Potassium homologate solution (KMnO 4 was 20 mmol) dropwise, followed by stirring magnetically for about 30 min until the solution became homogeneous. After that, it was transferred into a 200 mL Te on-lined stainless-steel autoclave. The autoclave was kept at 160°C for 16 h in an oven and then cooled to room temperature, and then, the product was collected by ltration and fully rinsed several times with deionized water to remove K + , followed by drying at 105℃ for 8 h. In order to determine the optimal doping ratio, we prepared catalysts with different initial molar ratio of Co and Mn (0.1, 0.2, 0.3, 0.4 and 0.5 respectively). The sample with Co/Mn = 0.2 had the highest catalytic ozonation of phenol ( Fig S1). Therefore, catalysts with Co/Mn = 0.2 named Co-α-MnO 2 was the materials synthesized. The synthetic procedures for α-MnO 2 was similar to that for Co-doped α-MnO 2 with the exception of adding CoSO 4 ·7H 2 O to the initial solution.

Catalysts characterization
The catalyst was purged 5 h at 120 ℃ under nitrogen atmosphere protection and then determined the BET surface area and pore-size distribution on a Micromeritics ASAP2020 analyser when the sample was cooled.
Powder X-ray diffraction (XRD) analysis was carried out on a Bruker D8 ADVANCE Phaser using Cu Ka radiation (k = 0.15418 nm) with a LYNXEYE detector at 40 kV and 30 mA. X-ray photoelectron spectroscopy (XPS) was performed with an ESCALAB250 system equipped with an Al Kα excitation source and operated at 15 kW and 1486.6 eV. Fourier transformed infrared (FTIR) analyses were carried out using a NicoletiS10 FT-IR plus spectrophotometer in a wavelength range of 4000-400 cm −1 .

Catalytic ozonation activities
In this study, phenol was selected as the aim pollutant because it is one of the most common industrial wastewater contaminates and would cause serious ecological and environmental problems (Saputra et al., 2017). The phenol degradated by catalytic ozonation was performed in a self-regulating quartz reaction with 1 L aqueous solution showed in Fig. 2. The concentration of ozone was adjusted via controlling the current and ow rate of the ozone generator and measured by ozone detector. When the ozone concentration was stable and then bubbled into the integrated adsorption-catalytic ozonation reactor to start the reaction. The pneumatic panel was adopted to ensure that gaseous ozone was completely mixed with aqueous solution.
During the reaction, ozone concentration and ow rate were set at 3.0 mg/L and 3.0 L/min, respectively. The tail gas was absorbed by potassium iodide solution. 2 g catalyst was mixed with 1 L phenol wastewater (initial concentration was 400 mg/L; pH = 10; 293 ± 1 K) in the reaction system. The residual phenol concentration and COD concentration of water samples were measured to evaluate removal performance.  (81.01%) > single ozonation (58.55%). Phenol removals were also described with pseudo zero-order, rst order and second order kinetics, and the kinetic models were expressed as Eq. (1), Eq. (2) and Eq. (3).

Catalytic degradation of phenol
Where K 1 , K 2 , K 3 are the pseudo zero-order, rst-order and second-order rate constant, respectively (min −1 ); t represents the reaction time; C 0 and C t stand for the phenol concentration at 0 min and t min, respectively.
The reaction rate constants in different processes are showed in Table 1. The removal e ciency of catalytic ozonation on phenol well accords with the rst-order reaction kinetics, while that of single ozonation accords with the zero-order reaction kinetics. Based on rst-order kinetic tting, Co-α-MnO 2 achieved the highest reaction rate constant for phenol removal (0.092 min −1 ), which was 2.09 and 4 times higher than that in α-MnO 2 (0.044 min −1 ) and O 3 (0.023 min −1 ), respectively. It can be further veri ed that the doping of Co on α-MnO 2 e ciently improved the catalytic activities. The results might be attributed to the changes of catalyst construction, elementary composition and surface properties, et al., which will be detailly discussed in the following section.

Speci c surface and pore size distribution
The calculated results of speci c surface area and pore volume of α-MnO 2 and Co-α-MnO 2 are showed in Table 2. It can be seen that Co-α-MnO 2 catalyst has a bigger of speci c surface area (79.496 m 2 /g) and pore volume (0.0396 cm 3 /g) than that of α-MnO 2 The speci c surface area and pore volume are more, the better catalytic activity can be obtained. The reason is that the large speci c surface area and pore volume can promote the mass transfer rate between solid-liquid-gas (Ghuge and Saroha 2018). So, catalytic activity of Co-α-MnO 2 is more great than that of α-MnO 2 . N 2 adsorption-desorption isotherm showed that two kinds of catalysts belong to typically type (as showed in Fig. 4)

Crystal phase
The XRD results of α-MnO 2 and Co-α-MnO 2 catalysts are showed in Fig. 5. All patterns of the prepared α-MnO 2 and Co-α-MnO 2 catalyst samples can be indexed to body-centered tetragonal, which are same as that of the standard α-MnO 2 (JCPDS NO.44-0141). There are no additional peaks in XRD patterns be identi ed after Co doping, which indicates that the crystal structure of the preparation of test samples was the same as that of α-MnO 2 . This shows that Co doping enters the MnO 2 skeleton structure by replacing Mn or is embedded in the pore structure of the catalyst tunnel, rather than existing on the catalyst surface in the form of cobalt oxide with similar ionic radius to CO 2+ and Mn 2+ . However, the Co doping on α-MnO 2 would cause the deformation of α-MnO 2 lattice, which make the grain diameter decrease and the parameters a, b and c increase (as showed in Table 2

Surface chemical compositions
In order to further investigate the elemental valence on the catalyst surface, Mn 2P, O1s and Co2p of both α-MnO 2 and Co-α-MnO 2 characterized by XPS were presented in Fig. 5(b), Fig. 5(c) and Fig. 5(d), respectively. Li et al., 2018), respectively. As showed in Table 3, the relative content of Mn 3+ of Co-α-MnO 2 increases from 8.42-41.16% and the ratio of Mn 3+ /Mn 4+ increases from 0.092 to 0.70 compared with α-MnO 2 . According to the results of the Table 5, the main valence state of Co in the catalyst is Co 3+ , which indicates that the doped Co 2+ has a redox reaction in the catalyst preparation process, and promotes the increase of the ratio of

Surface functional groups properties
In order to expose the in uence of doping Co on surface functional groups of α-MnO 2 , the catalysts were characterized with FT-IR, and the results are present in Fig. 7. respectively. The results of masking experiments were showed in Fig. 9 and Fig. 10.

Analysis of the catalytic mechanism
The masking experiment results demonstrated that the presence of surface hydroxyl group and reactive oxygen species promoted the degradation e ciency of phenol. Therefore, the mechanism of catalytic ozonation of phenol by α-MnO 2 and Co-α-MnO 2 was analyzed from the surface of polyphase catalysis (as showed in Fig. 11).
According to the adsorption-desorption isotherm of the catalyst, the interaction between the adsorption material and the adsorption gas is relatively weak, indicating that the interaction between the catalyst and the adsorbent is relatively weak (as showed in Fig. 4)

Conclusion
Co-doped α-MnO 2 was prepared by hydrothermal method to be as the catalyst for phenol removal. The removal performance and mechanism by catalytic ozonation was obtained. The results showed that Co doping played a great role in promoting the e ciency of phenol removal by catalytic ozonation over Co-α-MnO 2 . After 40 min catalytic ozonation, the phenol removal rates by Co-α-MnO 2 reached 97.47%, which was 16.46%, 38.92% higher than that of α-MnO 2 catalytic ozonation and pure O 3 without catalyst respectively. The kinetic rate of Co-α-MnO 2 on phenol ozonation reaching 0.092min −1 , which was 2.09 times and 4 times of α-MnO 2 and ozonation without catalyst respectively. The higher catalytic activity of Co-α-MnO 2 catalyst could be attributed to a series of better properties in comparison with α-MnO 2 catalyst. Compared with α-MnO 2 , the doping of Co made Co-α-MnO 2 had much larger speci c surface area and pore volume, more crystal defect and oxygen vacancy, higher relative content of Mn 3+ and adsorbed oxygen (O ads ) and more surface hydroxyl.
The combined action of these factors nally improved the catalytic ozonation performance of Co-α-MnO 2 .
Masking experiments had showed that surface hydroxyl group and active free radicals (·OH and ·O 2− ) were involved in the catalytic ozonation of phenol. Furthermore, the primary reaction mechanism was proposed.
The catalyst achieved the regeneration of surface hydroxyl group and the initiation of free radical chain reaction. The doping of Co promoted these reactions by producing more surface hydroxyl groups, which promoted the catalytic ozonation process. Declarations