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Remediation and Control Technologies
Enhanced Performance and Conversion Pathway for Catalytic Ozonation of Methyl Mercaptan on Single-Atom Ag Deposited Three-Dimensional Ordered Mesoporous MnO2 Dehua Xia, Wenjun Xu, Yunchen Wang, Jingling Yang, Yajing Huang, Lingling Hu, Chun He, Dong Shu, Dennis Y. C. Leung, and Zhihua Pang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03696 • Publication Date (Web): 26 Oct 2018 Downloaded from http://pubs.acs.org on October 26, 2018
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Enhanced Performance and Conversion Pathway for Catalytic
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Ozonation of Methyl Mercaptan on Single-Atom Ag Deposited
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Three-Dimensional Ordered Mesoporous MnO2
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Dehua Xia a, b,, Wenjun Xu a, e,, Yuncheng Wang a, Jingling Yang a, Yajing Huang a,
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Lingling Hu a, Chun He a, b, *, Dong Shu c, Dennis Y.C. Leung d,**, Zhihua Pang e
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a School
of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China
9 b
10
Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Guangzhou, 510275, China
11 12
c
13
Guangdong Universities, School of Chemistry and Environment, South China Normal
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University, Guangzhou, 510006, China
Key Lab of Technology on Electrochemical Energy Storage and Power Generation in
d
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Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong
16 e
17
South China Institute of Environmental Science, Ministry of Environmental Protection (MEP), Guangzhou 510655, PR China
18 19 20
Corresponding author: School of Environmental Science and Engineering, Sun Yat-sen University, Guangzhou, 510275, China. Email address:
[email protected] (C. He);
[email protected] (D.Y.C. Leung). *
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Abstract
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In this study, Ag deposited three-dimensional (3D) MnO2 porous hollow
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microspheres (Ag/MnO2 PHMSs) with high dispersion of the atom level Ag species are
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firstly prepared by a novel method of redox precipitation. Due to the highly efficient
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utilization of downsized Ag nanoparticles, the optimal 0.3% Ag/MnO2 PHMSs can
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completely degrade 70 ppm of CH3SH within 600s, much higher than that of MnO2
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PHMSs (79%). Additionally, the catalyst retains a long-term stability and can be
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regenerated to its initial activity through regeneration with ethanol and HCl. The results
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of characterization of Ag/MnO2 PHMSs and catalytic performance tests clearly
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demonstrate that the proper amount of Ag incorporation not only facilitates the
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chemi-adsorption but also induces more formation of vacancy oxygen (Ov) and lattice
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oxygen (OL) in MnO2 as well as Ag species as activate sites to collectively favor to the
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catalytic ozonation of CH3SH. Ag/MnO2 PHMSs can efficiently transform CH3SH into
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CH3SAg/CH3S-SCH3 and then oxidize them into SO42- and CO2 as evidenced by in situ
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diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs). Meanwhile,
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electron paramagnetic resonance (EPR) and scavenger tests indicate that •OH and 1O2 are
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the primary reactive species rather than surface atomic oxygen species contributed to
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CH3SH removal over Ag/MnO2 PHMSs. This work presents an efficient catalyst of
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single atom Ag incorporated MnO2 PHMSs to control air pollution.
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Keywords: Single atom Ag; Catalytic ozonation; Chemi-adsorption; MnO2 hollow
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microspheres; Methyl mercaptan. 2
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1. Introduction
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Methyl mercaptan (CH3SH), one of the typical sulfur-containing volatile organic
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compounds (VOCs) with high toxicity and corrosive, is a representative odorous gas with
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a very low odor detection threshold around 0.4 ppb/v [1]. CH3SH is widely produced in
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urban waste, sewage treatment and industry wastes [2]. The presence of 5 ppm CH3SH in
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the atmosphere will give us an unpleasant feeling, while the exposure of high
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concentration of CH3SH can cause significant poisoning [3]. Till now, many
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conventional technologies for eliminating odorous VOCs have been widely applied,
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including adsorption, biological treatment, incineration, and plasma technology, but
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which may not suitable for CH3SH from industrial activities with the concentrations
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range from a few ppb to tens of thousands [4-8]. Currently, the catalytic conversion
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process (including catalytic oxidation, catalytic decomposition) that does not need any
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additional reagents (e.g. H2, O2) and produces low waste (e.g. H2S and inorganic
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hydrocarbons), is considered as a suitable technique for the complete removal of CH3SH
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[9, 10]. However, catalytic conversion process always operated under a higher
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temperature (over 300 °C) and the catalyst can easily deactivate due to coke deposit [11,
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12]. In comparison, catalytic ozonation is a more efficient advanced oxidation process for
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non-selective mineralization of CH3SH with different concentrations at room temperature,
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relying on the generated reactive oxygen species (ROS) as well as the ozone molecules
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[13]. Due to the efficient decomposition of VOCs, catalytic ozonation cause no great
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accumulation of intermediates on the catalysts and limited deactivation of catalysts [14]. 3
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Moreover, ozone is an electrophilic molecule and it especially reacts with high electronic
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density sites of molecules such as CH3SH, CH3S-SCH3 having carbon-carbon or
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carbon-sulfur bonds [15].
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During catalytic ozonation, the catalyst is important as it can accelerate
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decomposition of ozone and enhance utilization of ozone simultaneously. Till now,
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increasing interest has been paid in developing catalysts such as transition metal oxides
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(MnO2, CeO2), noble metal (Ag/SBA-15, Pd-Mn/Al2O3) and carbon materials
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(graphene), which exhibit promising performance for VOCs removal through catalytic
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ozonation [16-19]. Among them, the widely studied MnO2 is more promising for
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application, as it exhibits plentiful valence states, versatile structures (rod, wire, tubular,
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and sphere), and high stability [20-23]. In recent, a three-dimensional (3D) ɑ-MnO2
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porous hollow microsphere (PHMS) was reported as an efficient ozone catalyst to
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remove bisphenol A. Compared with nonporous MnO2, the high surface area of 3D
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ɑ-MnO2 can enhance its adsorption ability and prolong the retention of ozone, as well as
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the rich lattice oxygen of ɑ-MnO2 can promote the ozone decomposition [24-26]. The
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identified property of ɑ-MnO2 PHMSs infers its’ great potential for catalytic ozonation of
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CH3SH.
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To further enhance its catalytic ozonation property for CH3SH removal, the
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cooperation of noble metals like Ag atom into ɑ-MnO2 PHMSs provides a feasible
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strategy to modulate its structure. First, Ag can provide sufficient active sites and display
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better catalytic ozonation than other metals (Co, Ni, Fe, Mn, Ce, Cu, etc.) [27, 28]. 4
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Second, Ag is benefit for the removal of sulfur-containing VOCs because it has high
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affinity to S-H functional group [29, 30]. However, extensive application of the noble
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metal catalysts is still limited by their high cost. To save the cost of noble metal, reducing
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its deposited amount is a feasible method.
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Recently, single-atom catalysis has become a research hotspot, because the promoted
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dispersion of noble metal via downsizing a nanoparticle to the atomic level can maximum
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enhance its utilization [31]. Tang et al. used thermal treatment to make Ag nanoparticles
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to migrate along the external surfaces and insert in the tunnels of Hollandite-type MnO2
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nanorods, which can effectively catalyze oxidation of HCHO due to the high dispersion
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of sub-nanosized Ag and even single atoms [32]. However, the decoration of catalysts
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with highly dispersed noble-metal remains a tough task, because the Ag or Au
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nanoparticles with high surface energy easily agglomerate into large particles via
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Ostawald ripening [33]. Moreover, most noble-metal atoms are lost inside the bulk during
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synthesis through conventional methods such as coprecipitation or photo-deposition,
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where they are no longer effective as active sites [34, 35]. Furthermore, some studies also
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indicate that more tight contact can be achieved between atomic metal and MnO2 than
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that of Ag cluster and MnO2, thus can enhance the charge transfer of catalyst for catalytic
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reaction [36, 37].
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Inspired by the strategy for stabilizing single-atom Au on the surface of MnO2 rods
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via trapping by Chen et al. [38], a modified redox precipitation was developed to
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fabricate single-atom Ag-deposited 3D ɑ-MnO2 PHMSs for CH3SH removal in the 5
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present work. In this strategy, H2O2 etches the surface of ɑ-MnO2 through consuming
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released H+, thereby driving hydrolysis of AgNO3 into Ag(OH), which could be then
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reduced into metallic Ag and subsequently trapped into surface defects of α-MnO2
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PHMSs. The downsized single-atomic Ag NPs via this method was well-evidenced by
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HAADF-STEM. The enhanced performance of Ag/ɑ-MnO2 PHMSs is attributed to the
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highly dispersed Ag species, which not only facilitate chemi-adsorption of CH3SH, but
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also induce more vacancy oxygen and lattice oxygen in MnO2 as well as atomic Ag itself
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as activate site to collectively improve subsequent catalytic ozonation. Besides, the
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proposed mechanism for CH3SH mineralization process was revealed by in situ diffuse
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reflectance infrared Fourier transform spectroscopy (DRIFTS). The 3D structure catalyst
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with atomic Ag thus may offer great potential for improving the reaction activity to
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eliminate odorous VOCs, holding promise for environmental remediation.
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2. Experimental
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2.1 Preparation of Ag/MnO2 PHMSs
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MnO2 PHMSs were successfully synthesized by a self-template process at room
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temperature [39]. Ag/MnO2 PHMSs were prepared by a modified redox precipitation
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method (Text S1 and Figure S1). Different from Chen et al. work to use MnO2 nanorods
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[38], 1.0 g MnO2 PHMSs was ultrasonically dispersed in deionized water, and then a
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certain volume of AgNO3 solution (atomic Ag source) was added. The concentrated H2O2
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(0.5 g, 30 wt.%) was dropwise added to the mixture of MnO2 PHMSs and AgNO3. 6
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During adding H2O2, O2 was emitted. MnO2 (s) + H2O2 (aq) + 2H+ (aq) = Mn2+ (aq) +
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2H2O (l) + O2 (g), where the consumption of released H+ can trigger AgNO3 hydrolyze to
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Ag(OH): AgNO3 (aq) + H2O (l) = Ag(OH) (s) + HNO3 (aq). Because these two reactions
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occur simultaneously, the Ag species could be trapped by an in situ formed hole.
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Moreover, Ag(OH) could be further reduced into Ag nanoparticles by H2O2: O2 (g) + 2H+
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(aq) + 2e- ⇌ H2O2 (l) and Ag(OH) (s) + H+ (aq) + e- ⇌ Ag (s) + H2O (l). Later, the solid
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was dried at 200 °C for 2 h. Ag/MnO2 PHMSs prepared with different silver contents
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were labeled as x Ag/MnO2 (x represents the mass fraction of Ag, e.g., 0.3% Ag/MnO2).
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Moreover, H2O2 treated MnO2 was also prepared and the photo-deposition method was
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applied to synthesize PD-0.3% Ag/MnO2 as an aggregated-Ag deposited on MnO2 for
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comparison. The characterization methods are described in the supporting information
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(Text S2).
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2.2 Catalytic ozonation of CH3SH with Ag/MnO2 PHMSs
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The elimination of CH3SH with catalytic ozonation by Ag/MnO2 PHMSs was
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carried out using a continuous flow reactor as shown in Figure S2. The fixed bed quartz
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tube reactor (Φ1.5 cm) was padded with 0.1 g Ag/MnO2 PHMSs for each experiment.
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The CH3SH and N2 from bottle gas were premixed in a container to adjust the initial
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concentration of CH3SH maintained at 70 ppm. O3 was produced by an ozone generator
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(YDG, YE-TG-02PH) and the initial concentration of O3 was controlled at 1.5 mg/L. The
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concentrations of CH3SH and ozone in the inlet and outlet were continuously monitored 7
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by a CH3SH sensor (Detcon, DM-400IS) and an ozone analyzer (IDEAL-2000),
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respectively. The total gas stream velocity was kept at 0.1 L/min. A mixture of used
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catalyst and activated carbon was used to catalyze and absorb the residual ozone and
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exhaust gas, to ensure the emission of remaining gas meets the relevant standards of
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environmental protection. 0.050 g Ag/MnO2 was respectively added into 20 mL of
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trapping agents solution (20 mM of TBA, EDTA-Na, Ascorbic acid, Cr(VI), NaN3) and
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ultrasonicated for 30 min. The suspension was then loaded on non-woven fabric,
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followed by drying in air at 60 °C until water was completely removed. The next steps
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were the same as catalytic ozonation of CH3SH.
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3. Result and discussion
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3.1 Structural analysis of the catalysts
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In our strategy, the surface defects of the MnO2 PHMs can be created via H2O2
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etching and are filled by downsizing and confining Ag NPs simultaneously, thus leading
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to the well-dispersion of atomic Ag. The content of incorporated Ag in the MnO2 PHMSs
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was determined by ICP-OES, which are listed in Table S1. The corresponding contents
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are close to the nominal values. The crystalline structures were analyzed through XRD.
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As shown in Figure 1a, the MnCO3 showed the sharp characteristic reflections
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corresponding to the pure rhomb-centered hexagonal crystal phase of MnCO3 (JCPDS
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No.44-1472) [39]. Meanwhile, the main peaks of MnO2 solid spheres (2θ =28.8°, 37.3°
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and 56.7°) can be well indexed to the tetragonal α-MnO2 phase (JCPDS 44-0141) [40]. 8
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Similarly, the MnO2 PHMSs also match with the tetragonal α-MnO2 phase and no
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observation of MnCO3 peaks, confirming that MnCO3 core is completely dissolved by
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acid to form hollow structure. Moreover, after Ag deposition, the diffraction peaks of
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MnO2 PHMSs become stronger and wider, indicating microscopic stress exists and leads
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to the interaction of Ag with MnO2 support [41]. However, no diffraction peaks
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corresponding to Ag can be observed, presumably owing to the ultrafine-sized and high
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dispersion of Ag [42]. Moreover, the N2 adsorption and desorption isotherms for these
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samples (Fig. S3a) are of type IV curves with obvious hysteresis loops at a relative
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pressure P/P0 of 0.5-1.0, which is a character of capillary condensation occurring in
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mesoporous solids [35]. The pore size distribution curves in Fig. S3b indicates the hollow
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structure of both samples, while the average pore-size and specific surface area of 0.3%
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Ag/MnO2 PHMs (100.85 nm, 104.11 m2 g−1) are smaller than MnO2 PHMs (138.13 nm,
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124.13 m2 g-1), mainly due to the atomic Ag particles are highly dispersed and even inside
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the porous hollow structure of MnO2 (Table S1).
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As depicted in Figure 2a, the SEM images show that the 0.3% Ag/MnO2 PHMSs
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have regular 3D spherical shape, and the cavity of the broken MnO2 PHMSs confirms its
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hollow structure, indicating Ag incorporation does not break the 3D structure of MnO2.
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In contrast, some rough deposits appeared on the surface of PD-0.3% Ag/MnO2 PHMs,
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indicating the occurrence of agglomerated Ag nanoparticles (Figure 2b). To further
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inspect the defined structure of catalyst, HRTEM, HAADF-STEM, and EDS-mapping
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were utilized. The TEM in Figure 2c further confirms the 3D structure of MnO2 PHMSs. 9
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The HRTEM analysis (Figure 2d) of the optimal 0.3 % Ag/MnO2 PHMs reveals that the
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polycrystalline walls of MnO2 are grown along the [110] direction with lattice spacing of
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0.35 nm, but it is hard to find Ag nanoparticles within catalyst. Interestingly, due to the
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difference in element contrast between Ag and Mn, the Ag species looks brighter than the
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Mn in the dark field of Cs corrected HAADF-STEM (Figure 2e). According to
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Cs-corrected HAADF-STEM images (Fig. 2e), the bright dots (Ag) with particle size of
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approximately in 1 Ångstrom scale were well-dispersed in α-MnO2 PHMSs, which
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matches the van der Waals diameter of a single Ag atom [42, 43]. The HAADF-STEM
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image of 0.3% Ag/MnO2 PHMs at larger scale in Figure 2e shows no aggregation of the
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Ag species. The mapping images of Au, Mn, and O shown in Figure 2f-h and EDX data
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in Figure S4 indicates that Ag nanoparticles are homogenously dispersed on the MnO2
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PHMSs.
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3.2 Surface composition and reducibility
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XPS tests were conducted to further analyze the surface chemical compositions and
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element oxidation states of catalysts. As shown in Figure 3a, the Mn 2p3/2 region was
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resolved into sub-bands, three individual component bands at 640.6 eV, 641.6 eV and
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642.8 eV represent Mn2+, Mn3+ and Mn4+ respectively [44]. Table S2 displays that the
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surface molar ratio of Mn2+ + Mn3+ to Mn4+ in Ag/MnO2 PHMSs (1.06) is higher than
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that of MnO2 PHMSs (0.96), indicating the incorporated Ag leads to an increase in the
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content of Mn2+ and Mn3+. In general, the increase of surface Mn at low valence state can 10
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improve the amounts of crystalline defects and oxygen vacancies in MnO2, which play
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important roles as active sites for the catalytic ozonation to generate activated oxygen
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species [44]. Moreover, all the O-1s XPS spectra in Figure 3b have three peaks at 529.7
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eV, 531.1 eV and 532.1 eV, which are assigned to lattice oxygen (OL), adsorbed oxygen
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(Oads, such as O2 − , O − and OH group) and surface oxygen (H2O), respectively [45].
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Table S2 indicates that molar ratio of OL:Oads in Ag/MnO2 PHMSs (2.12) is higher than
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that of MnO2 PHMSs (1.78), suggesting the incorporated Ag can enhance the formation
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of OL. Tang et al reported that the Ag atoms have migrated along the surface and are
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highly dispersed on/in MnO2, and the stronger interaction between Ag and MnO2
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increase the density of crystalline [34]. Generally, OL with high mobility is more
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favorable than Oads for the formation of active oxygen species through the migration
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between surface lattice oxygen and oxygen vacancy with molecular oxygen [46].
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Additionally, there are two peaks belonging to Ag-3d5/2 (B.E. = 368 eV) and Ag-3d3/2
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(B.E. = 374 eV) orbital, which can be further separated and fitted into Ag(I) and Ag(0)
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species (Figure 3c). The Ag species is considered as an active site to activate substrates,
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which is beneficial to the chemi-adsorption and oxidation of CH3SH [38].
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Raman spectroscopy was used to further analyze the interaction between Ag and
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MnO2 (Figure 1b). MnO2 PHMSs shows a sharp band at 629 cm-1 ascribed to the
228
symmetric v2 (Mn-O) stretching vibrations of the MnO6 groups, and a weak band at 573
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cm-1 due to the stretching vibration v3(Mn-O) in the basal plane of [MnO6] sheets, as well
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as a weak band at 389 cm−1 attributed to the skeletal vibrations [47]. Comparatively, the 11
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peak intensity ratio between the v2(Mn-O) and v3(Mn-O) peaks decreased after Ag
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involved, and both peak red shift to 625 cm −1 and 560 cm −1, indicating the existence of
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disorder crystal defects, which are favorable to form oxygen vacancies[48]. Oxygen
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vacancies in MnO2 play key role in ozone destruction through transform O3 into
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intermediate O22- [49]. Moreover, the peak intensity and width of Ag/MnO2 PHMSs are
236
larger than that of MnO2 PHMSs. This is attributed to the stronger interaction of Ag and
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MnO2 support, which roots in the effect of surface stress induced by the larger radius of
238
Ag and increasing defects of MnO2 [48]. The results also confirm the surface stress is the
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reason that the diffraction peak in the XRD pattern of Ag/MnO2 widens (Figure 1a) after
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Ag deposition. As identified above, the stronger interaction between Ag and MnO2
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increase the density of crystalline defects and oxygen vacancies, which are beneficial for
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the adsorption, activation and migration of oxygen in the catalytic ozonation [50].
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H2-TPR measurements were carried out to explore the reducibility of Ag/MnO2
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PHMSs, because high reducibility of catalyst could favor to the catalytic ozonation [51].
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As shown in Figure 1c and Table S3, with the increase of reduction temperature, the
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sample will undergo the successive reduction of surface adsorbed oxygen species, and the
247
process of MnO2 →
248
deposition, the temperatures of MnO2 peaks decrease from 202 °C, 311 °C and 491 °C to
249
114 °C, 155 °C, and 253 °C, respectively. Same phenomena are also observed in other
250
study, the three peaks with decreased temperature are assigned to the successive
251
reduction of surface adsorbed oxygen species of MnO2 rather than the deposed limited
Mn2O3 →
Mn3O4 →
MnO, respectively [51]. After Ag
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Ag [52]. This indicates the Ag/MnO2 PHMs has stronger low-temperature reducibility
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than MnO2. Reference indicates that activated hydrogen on the Ag surface can easily
254
migrate to the surface of the MnO2 PHMSs and thus facilitate the reduction reaction at
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low temperatures [27, 53]. Meanwhile, the incorporated Ag also can activate surface
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lattice oxygen species of MnO2 PHMSs, which are easier to be desorbed and react with
257
hydrogen gas at low temperature [53]. Therefore, the Ag incorporation improves the
258
reducibility of surface adsorbed oxygen species and promotes the mobility of lattice
259
oxygen in MnO2. Accordingly, the better reducibility and higher oxygen mobility causes
260
more oxygen to be adsorbed and further excited to active oxygen, which would then be
261
involved in the reaction of Ag/MnO2 PHMSs.
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To investigate the interaction of Ag and MnO2 on the electron transfer progress and
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the redox ability, the CV curves of Ag/MnO2 PHMSs are studied (Text S3) [54]. As
264
shown in Figure 1d, the current of Ag/MnO2 PHMSs are much higher than that of MnO2
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PHMSs, verifying higher surface charge on Ag/MnO2 PHMSs. Moreover, with ozone
266
purging, the current intensity of Ag/MnO2 PHMSs was significantly increased and the
267
peak potential shifted more positive than MnO2 PHMSs, suggesting a reduction process
268
occurred between ozone and catalyst, and Ag/MnO2 PHMSs exhibit a more favorable
269
performance for interfacial electron transfer than MnO2 PHMSs.
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3.3 Catalytic removal of CH3SH with Ag/MnO2 PHMSs
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3.3.1 Surface adsorption of CH3SH by Ag/MnO2 PHMSs 13
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Because MnO2 PHMSs have high specific surface area and Ag has high affinity to
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CH3SH, the as-prepared Ag/MnO2 PHMSs were firstly employed for adsorb CH3SH to
275
test their adsorption ability (Figure 4a). In the presence of MnO2 PHMSs alone (124.13
276
m2g-1), the reaction reached adsorption equilibrium after 240 seconds and finally about
277
40.34% CH3SH was removed in 600 seconds. In contrast, Ag/MnO2 PHMSs displayed a
278
specific adsorption curve. The 0.1% Ag/MnO2 PHMSs exhibits lower adsorption in the
279
initial 400s than MnO2 PHMSs but over 43.8% removal of CH3SH within the same 600
280
seconds. Similar adsorption curve and higher adsorption performance (45%) was also
281
obtained by 0.3% Ag/MnO2 PHMSs. The lower adsorption in the initial period is due to
282
the lower surface area (104.11 m2g-1) of Ag/MnO2 PHMSs, but higher adsorption in the
283
following stage is mainly because the S-H functional group in CH3SH strongly absorbs
284
on Ag particle surface. The S-H bond of alkanethiols can be dissociated on Ag
285
nanoparticles to form alkanethiolate species and strongly chemisorbed on Ag surface
286
through S atom as an anchor [55]. Three BE peaks of S 2p in Fig. 3d attributed to S2-, S22-
287
and S6+ can be detected over the spent Ag/MnO2 PHMSs catalysts, indicating that various
288
intermediate and products of S species are generated over the surface of Ag/MnO2
289
PHMSs [56]. Obviously, the XPS results further supported the existence of Ag-S species
290
at S2p peak after the surface adsorption (Figure 3d), well coincident with the decrease of
291
Ag0/Ag+ mole ratio from 0.88 to 0.63 in 0.3% Ag/MnO2 PHMSs (Table S2). Meanwhile,
292
dimethyl disulfide (DMDS, CH3S-SCH3) was also detected because Ag could cleave the
293
S-H bond of CH3SH and two CH3S- molecular can easily form into DMDS [55, 56]. 14
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Although CH3S-SCH3 is an odor gas, the formation of CH3S-SCH3 from CH3SH act as a
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deodorization effect since the odor threshold of DMDS is 1/2 to 1/10 of that of CH3SH
296
[24]. This implies Ag/MnO2 PHMSs has favorable adsorption characteristic, which are
297
beneficial for the effective contact and reaction between gas molecules and active sites on
298
catalysts.
299 300
3.3.2 Catalytic ozonation of CH3SH by Ag/MnO2 PHMSs
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After the adsorption equilibrium was attained, the as-prepared Ag/MnO2 PHMSs
302
were also employed for catalytic ozonation of CH3SH from different aspects including
303
the effect of Ag content, relative humidity, and stability. As depicted in Figure 4a, within
304
600 seconds reaction, a 0.3% Ag/MnO2 PHMSs with rare Ag can realize the 95%
305
removal of CH3SH than pure MnO2 PHMSs (79%) and ozonation alone (28%), indicative
306
of enhanced catalytic effect from Ag incorporation. Meanwhile, the calculated
307
degradation rate of CH3SH (pseudo first-order kinetics, Table S1) by 0.3% Ag/MnO2
308
PHMSs (4.24 × 10-8 mol g-1s-1) is 1.3 times than that of MnO2 PHMSs (3.17 × 10-8 mol
309
g-1s-1), and 10.1 times than that of ozonation only (0.42 × 10-8 mol g-1s-1). Moreover, the
310
calculated turnover frequency (TOFs) of CH3SH for 3D-MnO2 is 0.0000437 s−1, while
311
that of 0.3% Ag/MnO2 is rapidly increased to 0.00228 s−1 (Table S4). In contrast, the
312
catalytic performance of H2O2-MnO2 PHMSs is similar to that of MnO2 PHMSs,
313
indicative of no influence by H2O2 modification onto MnO2 PHMs. As identified above,
314
incorporated Ag plays an important role to enhance its performance for catalytic 15
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315
ozonation. The further experiments of individual Ag catalysis were carried out in Ag/O3
316
system for CH3SH removal and the result was showed in Fig. S5a. Compared with
317
ozonation only (30%), the CH3SH removal efficiency was merely promoted by 36%
318
(0.36 mg Ag) and 47% (3.6 mg Ag) in Ag/O3 system in Ag/O3 system, while CH3SH
319
removal efficiency was promoted by 67% in Ag/MnO2 PHMSs/O3 system. Therefore, the
320
individual Ag also has a little contribution to the whole catalytic ozonation reaction.
321
The preparation method and Ag content on MnO2 PHMSs exhibit a great influence
322
on the catalytic activity of CH3SH removal. As depicted in Figure 4b, although
323
containing the same content of Ag, the optimum 0.3% Ag/MnO2 PHMSs (95%) prepared
324
by redox precipitation shows a better activity than PR-0.3% Ag/MnO2 PHMSs (76%)
325
prepared via photo-deposition. This confirms the feasibility of redox precipitation to
326
prepare efficient catalyst for catalytic ozonation. Moreover, the removal efficiency of
327
CH3SH was improved from 83% to 95% with Ag content increased from 0.1% to 0.3%.
328
This is because the increased Ag content meant higher CH3SH adsorption and higher
329
density of crystalline defect and oxygen vacancy to accelerate ozone decomposition [57].
330
However, CH3SH removal efficiency was reduced to 77.0% with Ag loading over 0.75%,
331
even worse than that of MnO2 PHMSs (79%). This is presumably due to the excess Ag
332
nanoparticles block the pore structures and cover the active sites of MnO2 PHMSs,
333
evidenced well by the greatly decreased specific surface area in Table S1 (75.29 m2g-1 for
334
0.75% Ag/MnO2 PHMSs).
335
Moreover, humidity is beneficial for the catalytic ozonation [58, 59], evidenced by 16
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the enhanced removal efficiency to ~100% of the 0.3% Ag/MnO2 PHMSs (Text S6 and
337
Figure S5b). Furthermore, the catalyst could also maintain a stable catalytic activity and
338
can be regenerated for continuous air purification (Text S7, Figures S7 and S8).
339 340
3.3.3 Role of reactive species
341
Generally, the catalytic ozonation is strongly related to the decomposed ozone
342
transform into atomic oxygen species (⁕O22 − ,⁕O-) or directly evolves into reactive
343
oxygen species (•O2 − , •OH, and 1O2) [60, 61]. Therefore, the ozone decomposition
344
efficiency by 0.3% Ag/MnO2 PHMSs was investigated to directly verify its catalytic
345
activity. As shown in Figure S9, 0.3% Ag/MnO2 PHMSs achieved higher ozone
346
decomposition efficiency (85.1%) than that of MnO2 PHMSs (67.3%), much faster than
347
the self-decomposition of ozone (negligible change). To identify the surface atomic
348
oxygen species caused ozone decomposition, Raman spectroscopy was conducted for
349
Ag/MnO2 PHMSs with O3 purging. As shown in Figure 1b, the catalyst did not exhibit
350
any peaks at approximately 828 and 938 cm − 1 after ozonation, which were usually
351
assigned to ⁕O22 − and surface ⁕O- respectively, indicating these species were not
352
involved in the catalytic ozonation [62]. However, the two diagnostic peaks of v2(Mn-O)
353
and v3(Mn-O) were blue-shifted after O3 purging, suggesting the energy of ozone could
354
efficiently activate Mn-O vibrations from ground state to the excited state [63]. Actually,
355
the activated Mn-O still can convert surface adsorbed O2/OH/H2O into reactive oxygen
356
species like •OH and •O2− [61]. Therefore, different chemical scavengers and a more 17
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357
sensitive electron spin resonance (EPR) technique were collectively utilized to analyze
358
the radicals. As shown in Figure 4c, the removal efficiency of CH3SH dramatically
359
decreased in the present of 5 mM tert-butanol (TBA, kTBAוOH = 6 × 108 M − 1s − 1),
360
indicating that •OH could dominate the catalytic ozonation process [54]. Coincidently,
361
the characteristic spectrum of the DMPO-•OH adducts (quartet lines with height ratio of
362
1:2:2:1) was observed to confirm the generation of •OH (Figure 4d). Similarly, when 5
363
mM AA (ascorbic acid) was added, the removal efficiency of CH3SH decreased a lot but
364
slightly better than TBA addition, indicating the moderate role of •O2 − for CH3SH
365
degradation [54]. The increasing characteristic spectrum of the DMPO-•O2− (1:1:1:1)
366
during catalytic ozonation was also observed (Figure 4e). Especially, the role of 1O2 was
367
always neglected in the previous study of catalytic ozonation. The generated •O2- is
368
proposed to recombine into 1O2, which prefer to oxidize electron-rich compounds like
369
phenols, thus it may be beneficial for CH3SH mineralization [64]. As seen in Figure 4c,
370
the addition of 5 mM FFA (furfuryl alcohol, a well-known scavenger for 1O2) greatly
371
inhibited the removal efficiency of CH3SH. The inhibition content by FFA (32.9%) is
372
similar with that of TBA addition (32.6%), which indicates the comparable role of 1O2
373
with that of •OH. The continuous generated 1O2 was evidenced well by its typical
374
three-line EPR spectrum (Figure 4f). In conclusion, radicals quenching experiments
375
identified that the •OH and 1O2 were the dominant ROSs for CH3SH decomposition.
376
Moreover, higher intensity of EPR peaks for Ag/MnO2 PHMSs were obtained than that
377
of MnO2 PHMSs, confirming that promoted ROS generation by incorporated Ag on 18
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MnO2 PHMSs.
379 380
3.3.4 Conversion route for CH3SH removal
381
To understand the mechanism of adsorption and ozonation of CH3SH over
382
Ag/MnO2 PHMSs, in situ DRIFTS is carried out to monitor time-dependent evolution of
383
the reaction intermediates and products over the catalyst surface, as shown in Figure 5a.
384
The CH3SH absorption bands appear once CH3SH comes in contact with the catalyst. In
385
the OH stretching region, a negative band at around 3400 cm−1 is observed, along with
386
adsorption IR bands at 963 cm − 1 due to the vibration of S-H, 1446 cm − 1 due to
387
deformation mode of CH3, 1464 cm − 1 due to bending vibration of CH3 and 2840~3000
388
cm-1 due to stretch vibration of C-H, respectively, indicating the adsorption of CH3SH
389
over Ag/MnO2 PHMSs [65]. Simultaneously, two outstanding band appear at 1309 and
390
1428 cm-1 corresponding to δs(CH3) and δas(CH3) species were observed to increase over
391
time, indicating intermediates of CH3S- and dimethyl disulfide (DMDS, CH3S-SCH3)
392
were formed [31]. This result suggested that S-H bond of CH3SH was broken and
393
deprotonated methyl mercaptan (CH3S-) tended to form into CH3S-Ag complex and
394
CH3S-SCH3, well evidencing the occurrence of chemi-sorption. The following reaction
395
can be proposed: CH3SH + Ag → CH3SAg → CH3S-SCH3 [34]. Some studies reported
396
that the S-H functional group in mercaptans or other sulfur compounds could strongly
397
adsorb on metal M and M+ (Cu, Ag, or Au), in which the S-H bond will be cleaved due to
398
the activation by metals [31, 58]. The XPS results (Figure 3d) also supported the 19
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399
existence of Ag-S species, as the mole ratio of Ag0/Ag+ decreased from 0.88 to 0.63 and
400
DMDS was greatly formed after surface adsorption (Table S5). Moreover, the other
401
absorption bands developed progressively can be assigned to the vibration of S=O and
402
S-O relevant to ν(S−O) of monodentate sulfonic acid (protonated acid CH3SO3H at 1158
403
cm−1 and deprotonated sulfonic acid CH3SO3− at 1215 cm−1) and SO42- at 1047 cm−1, as
404
well as stretch vibration of COO− at 1566 cm−1 [65, 66]. This suggests S-S and C-S bond
405
are also cleaved and thus induce the formation of CH3SO3-, SO42- and COO-. It is
406
proposed that Ag atom can activate the OL in MnO2 to generate active oxygen species at
407
ambient condition and these species could migrate to oxidize surface adsorbed CH3S- at
408
the perimeter of Ag nanoparticles [31, 32]. Consistently, XPS results also showed that
409
CH3SO3-, SO42- and COO- were greatly formed (Figure 3d), and the mole ratio of OL/Oads
410
decreased from 2.12 to 2.0 after adsorption due to the consumption of OL (Table S2).
411
During adsorption stage, it can be proposed that single atomic Ag provide active sites for
412
the chemisorption and partial oxidation of CH3SH, while MnO2 acts as a reservoir
413
providing OL species.
414
Once the adsorption equilibrium is achieved, ozone is purged to initiate the catalytic
415
ozonation. Figure 5b shows the IR spectra for Ag/MnO2 PHMSs with ozone purging in
416
time sequence. The decreased peak intensity of the OH groups at 3400 cm−1 can be
417
attributed to the consumption of OH groups for the generation of •OH, which is a critical
418
radical for CH3SH removal. Notably, the adsorption bands at 963, 1446, 1464 and
419
2840~3000 cm−1 corresponding to CH3S- species gradually decreased or shifted over 20
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time, indicating the consumption of accumulated chemisorbed CH3S-. Correspondingly,
421
the peak intensities of some intermediates (protonated and deprotonated sulfonic acid,
422
CH3SO3H and CH3SO3− at 1158, 1215 cm−1) and final products (sulfate species, SO42- at
423
1047 cm−1) significantly increase, indicating S elements in adsorbed CH3SH are
424
efficiently oxidized into SO3− and SO42− over the catalyst. Meanwhile, XPS results of S
425
2p (Figure 3d and Table S5) also indicate that both peaks of Ag-S species and
426
CH3S-SCH3 greatly decreased, while that of CH3SO3- and SO42- increased, further
427
confirming the S-S scission and chemisorbed CH3S- was largely oxidized into CH3SO3-
428
and SO42-. In parallel, some new peaks were also greatly intensified, corresponding to the
429
ν(COO−) at 1630 cm−1 and ν(C=O) at 1762 cm−1 of carboxylic acids, as well as the
430
νs(COO−) at 1340 cm−1 and the ν(C-O) at 1412 cm−1 of monodentate carbonate,
431
evidencing the C elements in the adsorbed CH3S- are also oxidized into carboxylic acids
432
(R-COO-) and carbonate (CO32-) over the catalyst [65, 66]. Especially, carbonate species
433
should be main intermediate product because the intensity of carbonate peaks is much
434
higher than that of carboxylic acids. Additionally, two appearing peaks at 2334 and 2362
435
cm−1 were assigned to the Fermi doublet of CO2 [67], indicating intermediate product can
436
be finally mineralized into CO2. Therefore, the mechanism of catalytic ozonation
437
involves two reactions: (1) oxidation, in which the CH3S-/CH3S-SCH3 was oxidized to
438
CH3SO3- and further SO42-, (2) S-C/S-S bond scission, leading to an increase in the
439
amount of carboxylic acids and carbonate species as well as finally CO2. Also, according
440
to IR spectra in time sequence, the normalized absorbance of intermediates (CH3S-, 21
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441
CH3S-SCH3) and final product (CH3SO3-, SO42-, R-COO-, CO32-) are illustrated in Figure
442
5c and 5d. The tendency of species evolution clearly indicated that the adsorption and
443
transformation of CH3S- are greatly boosted on Ag/MnO2 PHMSs.
444 445
3.3.5 Mechanism of adsorption and catalytic ozonation (electron transfer/redox couple)
446
XPS was also carried out to determine the chemical surface composition/state of
447
Ag/MnO2 PHMSs after adsorption and catalytic ozonation. After adsorption, XPS results
448
of Ag 3d (Figure 3c and Table S2) indicates the relative molar ratio of Ag0/Ag+ decreased
449
from 0.88 to 0.63, but limited change in the molar ratio of Mn2++Mn3+/Mn4+ from 1.06 to
450
1.02. This suggests the important chemisorption role of Ag rather than Mn during
451
adsorption stage. After catalytic ozonation, Figure 3c and Table S2 displays that the
452
molar ratio of Mn2++Mn3+/Mn4+ greatly decreased from 1.02 to 0.88, suggesting that
453
Mn2+/Mn3+ were the primary reactive sites and electrons were transferred from the
454
surface Mn2+/Mn3+ to ozone and result in the decomposition of ozone to ROSs. However,
455
XPS results of Ag 3d (Figure 3c, Table S2) display the molar ratio of Ag0/Ag+ slightly
456
increase from 0.63 to 0.69, but which is still lower than the initial 0.88, indicating that
457
chemisorbed CH3SAg was degraded but only partial Ag+ was reduced back to Ag0. This
458
may suggest that the Ag also work as active sites for the catalytic ozonation.
459
Generally, OL and OH groups play an important role in the oxidation reactions [25,
460
38]. After adsorption, the relative contents of OL negligibly reduced from 67.94 to
461
66.46% (Figure 3b, Table S2). The consumed OL is due to its activation into surface 22
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462
oxygen species by Ag nanoparticles, which plays important role in the partial oxidation
463
of chemi-sorbed CH3SAg. After catalytic ozonation, the relative contents of OL reduced
464
significantly from 66.46 to 60.11%, while that of Oabs increased from 33.19 to 37.56%
465
and Osurf increased from 0.35% to 2.32% (Figure 3b and Table S2). The remarkable
466
decrease in the OL content indicated its utilization took place during catalytic ozonation,
467
which reduce the oxidized Mn4+ to Mn3+/Mn2+ with O2 releasing, thus to complete the
468
redox reaction and maintain the electrostatic balance [61]. Consequently, the great
469
consumption of OL would result in the oxygen vacancies on the catalyst surface. It is
470
proposed that the oxygen-deficient bulk on the Ag/MnO2 PHMSs surface can be
471
replenished by the development of surface OH groups (large OH consumption in Figure
472
5b), thereby causing the increase of Oabs and Osurf through the enhanced adsorption,
473
activation and migration of surface absorbed O3 [61].
474
To sum up the whole catalytic process, a cycling reaction in the sequence of
475
Mn2+/Mn3+ → Mn4+ → Mn2+/Mn3+ was accomplished. During the course of oxidation,
476
the transfer of electrons from the surface Mn2+/Mn3+ (partial Ag) induced ozone
477
decomposition to reactive oxygen species, while the OL converted/reduced Mn4+ to Mn3+
478
vice versa. Such an electrostatic balance/redox couple between Mn3+/Mn4+ and OL/O2
479
played a key role in the catalytic activity. Moreover, it was suggested that the higher
480
activity of Ag/MnO2 PHMSs was related to the electronic effect of highly dispersed Ag
481
on Mn by increasing Mn3+ electron occupancy and dissociate molecular oxygen to
482
supplement the consumed OL during the reaction [63, 68]. To have full understanding of 23
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483
the above reactions, the conversion pathways are proposed in Text S7 and Figure 5f.
484 485
(1) Chemisorption:
486
CH3SH + Ag → CH3SAg or CH3S-SCH3 (Main)
487
CH3SAg + O2 → CH3SO3- or SO42- and R-COO- or CO32- (Partial)
488 489
(2) Catalytic ozonation:
490
≡Mn2+/3+(Ag)−OH + O3→≡Mn2+/3+(Ag)−OH−O3
491
≡Mn2+/3+(Ag)−OH−O3→≡Mn4+(Ag+) + OL + HO3•
492
≡Mn4+ + OL → ≡Mn3+ + OL → ≡Mn2+ + O2
493
HO3•+ HO3•→2•O2- + •OH
494
•O2- + H2O → 1O2
495
2•OH → •O2-
496 497
(3) Decomposition:
498
CH3SAg or CH3S-SCH3 + •OH or 1O2 or •O2-→ CH3SO3- → SO42- + HCOO- or CO32-
499
or CO2
500 501
Supporting Information
502
Additional experimental details. Materials and Synthesis of MnO2 PHMSs;
503
Characterization of Ag/MnO2 PHMSs; CV curves; Conclusion of characterization; 24
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Conclusion of pathway; Specific surface area and degradation rate constant of catalyst;
505
XPS analysis of Mn, O, Ag, S in Ag/MnO2; Analysis of H2-TPR; TOFs, Schematic
506
illustration of Ag/MnO2 preparation, Setup scheme, EDS of Ag/MnO2; Influence of
507
humidity on activity of Ag/MnO2 PHMSs; Reusability and regeneration of Ag/MnO2; O3
508
decomposition; IC analysis and FT-IR spectra of Ag/MnO2.
509 510
Acknowledgments
511
The authors wish to thank the National Natural Science Foundation of China (Nos.
512
51578556, 21673086, 41603097 and 21876212), Natural Science Foundation of
513
Guangdong
514
S2011010003416), Science and Technology Research Programs of Guangdong Province
515
(No. 2014A020216009) for financially supporting this work. Dr. Xia was also supported
516
by the Start-up Funds for High-Level Talents of Sun Yat-sen University
517
(38000-18821111).
Province
(Nos.
2015A030308005,
S2013010012927,
and
518 519
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for chemoselective hydrogenation of functionalized nitroarenes. Nat. Commun. 2014, 5,
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Thermally stable single-atom platinum-onceria catalysts via atom trapping. Science 2016,
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Immobilization of self-stabilized plasmonic Ag-AgI on mesoporous Al2O3 for efficient
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purification of industrial waste gas with indoor LED illumination. Appl. Catal. B:
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Catalytically active single-atom sites fabricated from silver particles. Angew. Chem., Int.
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low-temperature oxidatiion of formaldehyde. Environ. Sci. Technol. 2018, 52,
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Three-dimensional MnO2 porous hollow microspheres for enhanced activity as ozonation
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synthesize
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of MnO2 hollow nanospheres and their supercapacitive performance. New J. Chem. 2013,
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3493-3498.
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Confined-interface-directed
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graphene/amorphous carbon. Appl. Catal. B: Environ. 2018, 225: 291-297.
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wire-, tube-, and flower-like morphologies: highly effective catalysts for the removal of
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toluene. Environ. Sci. Technol. 2012, 46, 4034-4041.
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vacancy in birnessite-type MnO2 on room-temperature oxidation of formaldehyde in air.
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Appl. Catal. B: Environ. 2017, 204, 147-155.
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[46]Arandiyan, H.; Dai, H.; Deng, J.; Liu, Y.; Bai, B.; Wang, Y.; Li, X.; Xie, S.; Li, J.
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Three-dimensionally ordered macroporous La0.6Sr0.4MnO3 with high surface areas:
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Active catalysts for the combustion of methane. J. Catal. 2013, 307, 327-339.
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Palladium
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nanotubes
single-atom
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solution combustion synthesis as catalysts for the total oxidation of VOCs. Appl. Catal.
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catalysts by Ag in catalytic oxidation of toluene. Appl. Catal. B: Environ. 2013, 132-133,
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353-362.
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B.; Wang, Y.; Yu, G. Ball milling synthesized MnOx as highly active catalyst for gaseous
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POPs removal: significance of mechanochemically induced oxygen vacancies. Environ.
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Sci. Technol. 2015, 49, 4473-4480.
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Ultrasound-assisted nanocasting fabrication of ordered mesoporous MnO2 and Co3O4
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with high surface areas and polycrystalline walls. J. Phys. Chem. C 2010, 114,
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2694-2700.
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[51] Arandiyan, H.; Dai, H.; Deng, J.; Wang, Y.; Sun, H.; Xie, S.; Bai, B.; Liu, Y.; Ji, K.;
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Li, J. Three-dimensionally ordered macroporous La0.6Sr0.4MnO3 supported Ag
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nanoparticles for the combustion of methane. J. Phys. Chem. C 2014, 118, 14913-14928.
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oxidation of toluene over Ag/MnO2 catalysts. Appl. Sur. Sci. 2016, 385, 234-240.
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Chem. Eng. J. 2014, 252, 95-103.
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nanowires for highly efficient catalytic oxidation of carcinogenic airborne formaldehyde.
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ACS catalysis 2018, 8, 3435-3446.
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[55]He, D.; Wan, G.; Hao, H.; Chen, D.; Lu, J.; Zhang, L.; Liu, F.; Zhong, L.; He, S.;
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Luo, Y. Microwave-assisted rapid synthesis of CeO2 nanoparticles and its desulfurization
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processes for CH3SH catalytic decomposition. Chem. Eng. J. 2016, 289, 161-169.
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[56]Santos, V.P.; Pereira, M.F.R.; Órfão, J.J.M.; Figueiredo, J.L. The role of lattice
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oxygen on the activity of manganese oxides towards the oxidation of volatile organic
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compounds. Appl. Catal. B: Environ. 2010, 99, 353-363.
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manganese dioxide nanosheets for formaldehyde removal and regeneration performance.
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Chem. Eng. J. 2016, 306, 1172-1179.
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[58] Yi, D.; Huang, H.; Meng, X.; Shi, L. Adsorption–desorption behavior and
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mechanism of dimethyl disulfide in liquid hydrocarbon streams on modified Y zeolites.
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Appl. Catal. B: Environ. 2014, 148-149, 377-386.
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complete oxidation of formaldehyde in air with ozone over MnOx catalysts at room
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temperature. J. Hazard. Mater. 2012, 239-240, 362-369.
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[61] Afzal, S.; Quan, X.; Zhang, J. High surface area mesoporous nanocast LaMO3 (M =
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Mn, Fe) perovskites for efficient catalytic ozonation and an insight into probable catalytic
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mechanism. Appl. Catal. B: Environ. 2017, 206, 692-703.
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Trifunctional C@MnO catalyst for enhanced stable simultaneously catalytic removal of
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formaldehyde and ozone. ACS Catal. 2018, 8, 3164-3180.
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unsaturated manganese monoxide-containing mesoporous carbon catalyst in wet peroxide
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oxidation. ACS Catal. 2012, 2, 2577-2586.
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Environ. Sci. Technol. 2015, 49, 14392-14400.
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719
[68] Xu, R.; Wang, X.; Wang, D.; Zhou, K.; Li, Y. Surface structure effects in
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nanocrystal MnO2 and Ag/MnO2 catalytic oxidation of CO. J. Catal. 2006, 237, 426-430.
721
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1.0% Ag/MnO2 PHMSs 310
211
600
301
Intenstiy(a.u.)
310
211
310 110 200
220
211
310 400
22000
002
211 301
MnO2 solid sphere
411
600 521 002 541
v2(Mn-O) (625)
v3(Mn-O) (560)
14000 12000
0.3% Ag/MnO2
10000
v3(Mn-O) (573)
8000
MnCO3
v2(Mn-O) (632) 0.3% Ag/MnO2 + O3
16000
MnO2 PHMSs 002
600
301
v3(Mn-O) (568)
18000
0.3% Ag/MnO2 PHMSs 600 002
301
(b)
20000
Intensity (a.u.)
(a)
Page 36 of 41
v2(Mn-O) (629)
6000
MnO2
4000
2000
10
20
30
40 50 2 Theta(degree)
60
70
80
300
400
500
600
700
-1
800
900
Wavenumber (cm ) 0.020
(c)
peak 3
peak 2 peak 1
MnO2+O3
0.015
0.3% Ag/MnO2
peak 3
MnO2
0.010
Current/A
TCD signal
peak 1
(d)
0.3% Ag/MnO2+O3
0.3% Ag/MnO2 MnO2
0.005
0.000
peak 2 -0.005
-0.010
40
140
240
340
440
Temperature (C)
540
-0.2
640
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Potenial / V
722
Figure 1. (a) XRD patterns of MnCO3, MnO2 solid spheres, MnO2 PHMSs, 0.3%
723
Ag/MnO2 PHMSs and 0.75% Ag/MnO2 PHMSs; (b) Raman spectra and (c) H2-TPR
724
profiles; (d) CV curves of 0.3% Ag/MnO2 PHMSs and MnO2 PHMSs.
725 726 727
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Environmental Science & Technology
(b)
(a)
(h)
(c)
(g)
(f)
(d)
(e)
728 729
Figure 2. SEM images of 0.3% Ag/MnO2 PHMSs (a) and PD-0.3% Ag/MnO2 PHMSs
730
(b), TEM (c) and HRTEM (d) images of 0.3% Ag/MnO2 PHMSs, (e) Cs-corrected
731
HAADF-STEM images of 0.3% Ag/MnO2 PHMSs at the same district with partially
732
magnified pictures, (f − h) HAADF-STEM of 0.3% Ag/MnO2 PHMSs with element
733
distribution images corresponding to Mn, O and Ag.
734 735 736 737
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(a) Mn 2p
Page 38 of 41
(b) O 1s 4+
3+
MnO 2
Olat
MnO2
Mn
Mn
Oads
2+
Mn
3+
Mn
0.3% Ag/MnO2
4+
Mn
4+
Mn
Oads
Intensity (a.u.)
Intensity (a.u.)
3+
0.3% Ag/MnO2 (After adsorption)
2+
Mn
3+
2
adsorption) Oads (After Osurf
0.3% Ag/MnO
2
Olat
4+
Mn
2+
0.3% Ag/MnO
Olat
0.3% Ag/MnO2 (After catalytic ozonation)
Mn
0.3% Ag/MnO 2
Olat
2+
Mn
Mn
(After catalytic ozonation)
Oads O surf
Mn
638
640
642
Osurf
644
646
648
528
Binding Energy (eV)
530
532
534
Binding Energy (eV) Ag 3d5/2
(c) Ag 3d +
Ag
0.3% Ag/MnO2
Ag 3d3/2
0
Ag
0.3% Ag/MnO2
(d) S 2p
(After adsorption)
0
Ag
+
Ag
2-
S2 (Dimethyl disulfide)
6+
S (Sulfonic acid)
2-
Ag 3d3/2
+
Ag
0
Ag
+
+
366
0
Ag
0.3% Ag/MnO2
0
Ag
368
370
+
Ag
372
Binding Energy (eV)
6+
S (Sulfonic acid)
0.3% Ag/MnO2
(After catalytic ozonation) 6+
S (Sulfate)
2-
2S (Ag-S species) S2 (Dimethyl disulfide)
Ag 3d3/2 (After catalytic ozonation)
Ag
6+
S (Sulfate)
(C)
(After adsorption)
Ag
Ag 3d5/2
364
S (Ag-S species)
0.3% Ag/MnO2
Intensity (a.u.)
Intensity (a.u.)
Ag 3d5/2
(D)
0
Ag
374
376
162
378
164
166
168
170
Binding Energy (eV)
738
Figure 3. XPS spectra of Mn 2p (a), O 1s (b), Ag 3d (c), and S 2p (d) for fresh MnO2
739
catalyst, fresh 0.3% Ag/MnO2 PHMSs catalyst, 0.3% Ag/MnO2 PHMSs catalyst after 38
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740
deep adsorption of CH3SH, 0.3% Ag/MnO2 PHMSs catalyst after deep ozonation of
741
CH3SH. (b)
Catalytic ozonation
0.1% Ag/MnO2
40
Adsorption MnO2
0.3% Ag/MnO2 (N2)
H2O2-MnO2 0.3% Ag/MnO2
20
0.1% Ag/MnO2
60
1.0% Ag/MnO2
40
0.75% Ag/MnO2 0.5% Ag/MnO2 0.3% Ag/MnO2 0.2% Ag/MnO2
20
0.1% Ag/MnO2
Ozonation O3
0
100
0.3% Ag/MnO2 + O3
MnO2 + O3
Time (s)
*
*
3420
3440
300
400
*
3460
600
0
*
*
3480
100
200
-
O2
(d)
*
*
500
*
*
O3
3400
200
*
OH
3380
MnO2
0
-100
90 80
Catalytic Ozonation 0.3% Ag/MnO2 (No scavenger)
70
0.3% Ag/MnO2+AA
60
0.3% Ag/MnO2+FFA
50
0.3% Ag/MnO2+TBA
40
O3
30 20 10 0 0
100
200
300
Time (s)
400
500
600
40
20
0.3% PD-Ag/MnO2
*
MnO2 + O3
Time (s)
400
500
600
0 No Scavenger
(e)
3500
3520
3540
3460
3480
TBA
AA
1
O2
FFA
O3
(f)
0.3% Ag/MnO2 + O3
O3
*
Magnetic field (G)
300
0.3% Ag/MnO2 + O3
Intensity (a.u.)
0 -200
60
3500
Intensity (a.u.)
MnO2
80
CH3SH removal efficiency (%)
CH3SH removal efficiency (%)
0.3% Ag/MnO2 60
(c)
80
H2O2-MnO2
Intensity (a.u.)
CH3SH removal efficiency (%)
80
100
100
(a)
CH3SH removal efficiency(%)
100
100
MnO2 + O3
O3
3520
3540
Magnetic Field (G)
3400
3420
3440
3460
3480
3500
3520
Magnetic field (G)
742
Figure 4. (a) Performances in adsorption and catalytic ozonation of CH3SH in different
743
system; (b) Influence of different catalysts on CH3SH removal; (c) Catalytic ozonation of
744
CH3SH by 0.3% Ag/MnO2 PHMSs with/without addition of chemical scavengers (TBA:
745
Tert-butanol, AA: Ascorbic acid, FFC: Feri-furoic acid); Electron paramagnetic
746
resonance (EPR) signal of hydroxyl radical (d), superoxide radical (e) and singlet oxygen
747
(f).
748 749 750 751
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3540
3560
3580
3600
Environmental Science & Technology
Page 40 of 41
752 0.036
Adsorp. Equil.
(c)
Normalized absorbance (a.u.)
0.032 0.028
S-H CH3S
0.024
CH3SO3
CH3S-SCH3 -
2-
SO4
0.020
R-COOH
0.016 0.012 0.008 0.004 0.000 1
0.55
Normalized absorbance (a.u.)
0.50
2
3
Time (min)
4
5
(d)
0.45
S-H CH3S
0.40
CH3SO3
-
2-
SO4
0.35
2-
CO3
R-COOH
0.30 0.25 0.20 0.15 0.10 0.05
Adsorp. Equil.
0.00 0
(f)
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2
4
6
Time (min)
8
10
Page 41 of 41
Environmental Science & Technology
753
Figure 5. In situ DRIFTS spectra of (a) adsorption of CH3SH, (b) catalytic ozonation of
754
CH3SH with 0.3% Ag/MnO2 PHMSs; Species evolution of (c) adsorption of CH3SH, (d)
755
catalytic ozonation of CH3SH with 0.3% Ag/MnO2 PHMSs, (f) Adsorption and catalytic
756
ozonation pathway of CH3SH by Ag/MnO2 PHMSs.
757
Graphical Abstract
758 759 760
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