Three-Dimensional Ordered Mesoporous MnO2-Supported Ag

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Three-Dimensional Ordered Mesoporous MnO2 Supported Ag Nanoparticles for Catalytic Removal of Formaldehyde Bingyang Bai, Qi Qiao, Hamidreza Arandiyan, Junhua Li, and Jiming Hao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03342 • Publication Date (Web): 02 Dec 2015 Downloaded from http://pubs.acs.org on December 3, 2015

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Graphic Abstract

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Three-Dimensional Ordered Mesoporous MnO2 Supported

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Ag Nanoparticles for Catalytic Removal of Formaldehyde

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Bingyang Bai,*,† Qi Qiao,† Hamidreza Arandiyan,‡ Junhua Li,*,§ and Jiming Hao§

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State Key Laboratory of Environmental Criteria and Risk Assessment and Key

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Laboratory of Eco-Industry of the Ministry of Environmental Protection, Chinese

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Research Academy of Environmental Sciences, Beijing 100012, China

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of New South Wales, Sydney, NSW 2052, Australia

Particles and Catalysis Research Group, School of Chemical Engineering, University

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§

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of Environment, Tsinghua University, Beijing 100084, China

State Key Joint Laboratory of Environment Simulation and Pollution Control, School

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Abstract

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Three-dimensional (3D) ordered mesoporous Ag/MnO2 catalyst was prepared by

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impregnation method based on 3D-MnO2, and used for catalytic oxidation of HCHO.

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Ag nanoparticles are uniformly distributed on the polycrystalline wall of 3D-MnO2.

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The addition of Ag does not change the 3D ordered mesoporous structure of the

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Ag/MnO2, but does reduce the pore size and surface area. Ag nanoparticles provide

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sufficient active site for the oxidation reaction of HCHO, and Ag (111) crystal facets in

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the Ag/MnO2 are active faces. The 8.9% Ag/MnO2 catalyst shows a higher normalized

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rate (10.1 nmol·s-1·m-2 at 110 °C) and TOF (0.007 s-1 at 110 °C) under 1300 ppm of

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HCHO and 150000 h-1 of GHSV, and its apparent activation energy of the reaction is

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the lowest (39.1 kJ/mol). More Ag active sites, higher low-temperature reducibility,

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more abundant surface lattice oxygen species, oxygen vacancies and lattice defects

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generated from interaction Ag with MnO2 are responsible for the excellent catalytic

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performance of HCHO oxidation on the 8.9% Ag/MnO2 catalyst. The 8.9% Ag/MnO2

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catalyst remained highly active and stable under space velocity increasing from 60,000

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h-1 to 150,000 h-1, under initial HCHO concentration increasing from 500 ppm to 1300

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ppm, and under the presence of humidity, respectively.

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Introduction

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Formaldehyde (HCHO) emitted from the widely used building and decorative

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materials, oil paint and textile, etc. is becoming a major indoor pollutant, and it also has

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photochemical activity in the atmosphere.1 Long-term exposure to indoor air containing

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even a few ppm of HCHO may lead to serious and hazardous effects on human health.

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2

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automobile exhaust gas. The efficiency of removal for conversional absorbing materials

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was not excellent because of the limited capacities.2,3 Currently, the catalytic materials

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used in catalytic oxidation of HCHO are mainly oxide-supported noble metal (Ag, Pt,

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Pd and Au) catalysts, such as Ag/Co3O4,2 Ag/SBA-15,4 Ag/HMO,5 Ag/SiO2,6

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Ag/CeO2,7 Ag/MnOx-CeO2,8 Pt/MnOx-CeO2,9 Pt/TiO2,10-14 Pt/MnO2,15 Pt/Fe2O3,16

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Pt/SiO2,17 Pd-Mn/Al2O3,18 Pd/TiO2,19 Pd/Beta,20Au/Fe2O3,21 Au/ZrO2,22 Au/CeO2,23-25

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Au/CeO2-Co3O4,26 Au/Co3O4-CeO2,27 etc. In addition, several metal oxide catalysts for

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the oxidation of HCHO have been reported such as MnOx-SnO2,28 KxMnO229,

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Co/Zr,30and mesoporous Co-Mn,31 Cr2O332 and 3D-Co3O4,3 etc.

Therefore, it is essential to remove and transfer to CO2 in indoor air and industrial and

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Three-dimensional (3D) ordered MnO2 has been used as an effective

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environmentally friendly material for volatility organic compounds (VOCs) because of

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its special structure and large surface area, which make it more advantageous than a

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nonporous metal oxide.33-35 Previously, our group reported that 3D-MnO2 has an

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excellent catalytic performance of ethanol oxidation.33 After further study, we found

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that 3D-MnO2 sample has also much better HCHO oxidation property,36 and is superior

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to our reported 3D-Co3O4.3 Thus, 3D-MnO2 catalyst has a certain development potential

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for catalytic oxidation of VOCs.

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The development of effective catalysts for complete oxidation of low-concentrations

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of HCHO at lower temperature, even room temperature is still a challenging subject to

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be solved, and catalytic performance for VOCs oxidation of oxide-supported noble

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metal catalysts is better than that of transition metal oxide catalysts as a whole.

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Although the catalysts loading Pt nanoparticles have super HCHO catalytic activity,10-

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12

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nanoparticles. Relatively inexpensive noble metal Ag can also provide sufficient active

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sites for oxidation reaction. Our previous literatures reported the oxidation performance

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of VOCs on the mesoporous Ag/Co3O4,2 Ag/CeO2 nanopheres7 and Macroporous

the precursors of Pt nanoparticles are much more expensive than the ones of Ag

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Ag/La0.6Sr0.4MnO3,37 which confirm that oxide-supported Ag catalysts are promising.

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In this paper, three-dimensional ordered mesoporous Ag/MnO2 with Ag

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nanoparticles were prepared by excessive impregnation method based on 3D-MnO2,

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and used for the catalytic oxidation of HCHO. The factors such as low-temperature

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reducibility, interaction between the noble metal and support, surface oxygen species,

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lattice defects, etc. have been proven to have extremely favorable effects on catalytic

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performance.37-40 We hope that the addition of Ag improves the oxidation activity of

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HCHO at low temperature because of the change of the factors. The catalysts are

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characterized by some physical and chemical techniques, and their catalytic

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performance for HCHO oxidation was evaluated.

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Experimental

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Catalysts preparation. The detailed preparation processes of mesoporous KIT-6

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(ia3d) silica, 3D-MnO2 and mesoporous Ag/MnO2 catalysts are described in the

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Supporting Information.

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Catalyst characterizations. All of the samples were characterized by techiniques

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such as X-ray diffraction (XRD), N2 adsorption-desorption (BET), scanning electron

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microscopy (SEM), transmission electron microscopy (TEM), hydrogen temperature-

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programmed reduction (H2—TPR), X-ray photoelectron spectroscopy (XPS) and laser

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Raman spectra (Raman). The detailed methods are described in the Supporting

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Information.

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Catalytic Evaluation. The catalytic activity for HCHO oxidation was tested in a

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fixed bed quartz tube reactor (Φ10 mm) with 0.2 g catalyst (40-60 mesh). HCHO gas

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was generated by using a N2 bubbler. The detailed reaction condition and calculation

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formulas of HCHO conversion and turnover frequency (TOF) are shown in the

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Supporting Information.

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3.1. Structural analysis

Results and discussion

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Figure 1 show that all catalysts show diffraction peaks at 28.7°, 37.3°, 42.8°, 56.7°,

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59.4°, 64.8° and 72.3° (2θ), which correspond to the (110), (101), (111), (211), (220),

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(002) and (301) planes, respectively.33 This result indicates that the samples have a β-

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MnO2 crystalline phase, which corresponds to pyrolusite with a rutile structure. Ag 3

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crystalline phases are not found for the catalysts in the diffraction peaks, which indicate

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that Ag are uniformly dispersed in 3D-MnO2 support. With the addition of Ag, The (110)

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peak intensity of Ag/MnO2 becomes stronger and wider. The result indicates that a

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microscopic stress exists in the catalyst and lead to the interaction between Ag and

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support.

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N2 adsorption-desorption isotherms all have hysteresis rings and display type Ⅳ

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isotherms (See Figure S1, Supporting Information). These results indicate that the

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addition of Ag cannot affect the mesoporous characteristic of 3D-MnO2. 32, 41 The inset

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of the top left corner in Figure S1 shows the pore size distribution of Barrett-Joyner-

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Halenda. All catalysts have two pore sizes of maximum distribution at 3.7 and 11.6 nm,

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because the 3D-MnO2 replicated the double pore structure of the KIT-6 template.42 The

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physical parameters of all samples are displayed in Table 1. The addition of Ag

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decreases BET surface areas and pore diameters.

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From Figure S2 of the Supporting Information, STEM images of 3D-MnO2 and

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Ag/MnO2 further confirms that the Ag addition does not change 3D ordered

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mesoporous structure. Ag/MnO2 has a rough surface with white spots (See Figure S2c,

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Supporting Information) and its STEM images display many black spots on the

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crystalline walls (See Figure S2d, Supporting Information). The white or black spots

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are real Ag nanoparticles, which exhibits the excellent dispersion (Table 1).

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Figure 2 clearly displays that all samples possess ordered mesoporous characteristics

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and polycrystalline walls.33 The crystalline walls thicken and pore diameters narrow in

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the presence of Ag. The 3D-MnO2 possesses a surface lattice spacing of 0.311 and 0.240

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nm for the (110) and (101) crystal planes, respectively (Figure 2b). Compared with the

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3D-MnO2, Ag/MnO2 catalysts all show homogeneous distribution of Ag nanoparticles

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on the surface of polycrystalline walls, which is consistence with STEM results.

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Although many “black balls” are exhibited in the TEM images (Figure 2a,b) of 3D-

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MnO2, these ones are not Ag nanoparticles. They are skeleton intersections viewed

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along the [111] or [100] direction according to the structure analysis of 3D-MnO2 in

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our previous literature.33 Except the skeleton intersections, smaller “black balls” stand

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for Ag nanoparticles on the surface of 3D-MnO2 catalyst. Some Ag nanoparticles are

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also dispersed on the skeleton intersections. Ag nanoparticles in the Figure 2c-d have

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been marked by white circles. From TEM images tested at the same condition, density 4

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of Ag nanoparticles over the 8.9% Ag/MnO2 sample (Figure 2e) is larger than that over

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the 4.3% Ag/MnO2 sample (Figure 2c), which is consistent with their Ag loading.

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According to Figure 2d and 2f, the 4.3% Ag/MnO2 has smaller Ag nanoparticle about

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3 nm, while the 8.9% Ag/MnO2 has larger Ag nanoparticle about 5 nm. High Ag

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concentration can result in the aggregation and growth of Ag cluster during the

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preparation. The 8.9% Ag/MnO2 exhibits (110) crystal planes of 3D-MnO2 and Ag (111)

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planes with a lattice spacing of 0.236 nm (Figure 2f). The growth of the (110) planes is

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favorable to enhance catalytic oxidation ability.

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3.2. Chemical adsorption and surface composition

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From Figure 3, the 3D-MnO2 pattern shows reduction peak 1 at 320 °C, peak

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2 at 350°C and peak 3 at 425 °C, corresponding to the reduction of MnO 2 to

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Mn2 O3 , Mn 2 O 3 to Mn 3 O4 and Mn 3 O4 to MnO, respective, because of the

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existence of a disproportionation reaction. 33 The 4.3% Ag/MnO 2 catalyst has peak

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1 at 78 °C which belongs to H2 spillover. 2,7,8 Its peak 2 at 95°C, peak 3 at 144 and

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167 °C and peak 4 at 238 °C correspond to the reduction reaction like the 3D-

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MnO2 . With the Ag content increase, the 8.9% Ag/MnO 2 has the reduction peaks

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to be similar with the 4.3% Ag/MnO 2 . The addition of Ag obviously shifts the

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reduction peaks of 3D-MnO2 to the lower temperatures. Ag/MnO 2 catalysts have

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better low-temperature reducibility because of the Ag addition. The H 2 —TPR patterns

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of the 4.3% Ag/MnO 2 and 8.9% Ag/MnO 2 all have H2 spillover phenomenon

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because of the existence of Ag nanoparticles. The hydrogen spillover can affect the

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interaction between Ag and 3D-MnO 2 support, and possesses a behavior to

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adsorb, activate and migrate hydrogen. Activated hydrogen on the Ag surface can

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easily migrate to the surface of MnO 2 support and participate in reduction

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reaction. 2 Meanwhile, the Ag addition can also activate surface lattice oxygen

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species of 3D-MnO 2 support, which are easier to be desorbed and react with

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hydrogen gas at low temperature. From Table S1 in the Supporting Information,

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the H 2 consumption (1.77 mmol/g) of peak 1 for 8.9% Ag/MnO 2 is lower than

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that (1.97 mmol/g) for 4.3% Ag/MnO 2 , which indicates that the 8.9% Ag/MnO 2

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possesses less effects of H 2 spillover, probably because the activation of more

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surface lattice oxygen species provides some electrons for Ag nanoparticles.

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Table S1 shows that the Mn 3+ /Mn 4+ mole ratios of the catalysts increase with the 5

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increase of Ag content, indicating the 8.9% Ag/MnO 2 has greater quantities of

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Mn3+ cations. In addition, Figure S3 in the Supporting Information shows that the

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8.9% Ag/MnO 2 has a much higher initial H 2 consumption rate because of stronger

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low-temperature reducibility. Many literatures reported that better reducibility

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are favorable to enhance the catalytic oxidation reaction. 32,35,37 Therefore, the

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8.9% Ag/MnO 2 exhibit excellent catalytic performance for HCHO oxidation.

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Figure 4A shows that O1s has signals displayed in BE at 529.3 and 530.9 eV, which

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correspond to surface lattice oxygen (Olatt) and surface adsorbed oxygen (Oads),

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respectively.33,43 Table 1 displays that the Olatt /Oads ratios of 8.9% Ag/MnO2 (1.6) is

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larger than that of 4.3% Ag/MnO2 (1.3) and 3D-MnO2 (1.0), which indicates that the

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Olatt of Ag/MnO2 catalyst increases with the increase of Ag content. The 8.9% Ag/MnO2

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has more abundant surface lattice oxygen species due to the presence of Ag in the top-

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layer, which can enhance oxidation ability of HCHO. Figure 4B displays that Mn 2p2/3

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has two components displayed in BE at 641.5 and 642.7 eV, which correspond to the

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surface Mn3+ and Mn4+ ions, respectively.33,34 Table 1 displays that the Mn3+/Mn4+ ratios

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of 3D-MnO2, 4.3% Ag/MnO2 and 8.9% Ag/MnO2 is 0.7, 1.1 and 1.4, respectively,

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indicating the 8.9% Ag/MnO2 has more Mn3+ ions, which is consistent with the H2 —

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TPR result. The increase of surface Mn3+ ions can improve the amount of oxygen

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vacancies.43 From Figure 4C, a signal displayed at BE = 368.7 eV demonstrated that

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the Ag0 nanoparticles exist in Ag/MnO2,2,5,7 which is consistence with STEM and TEM

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results.

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Raman pattern of the catalysts is displayed in Figure S4 of the Supporting

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Information. 3D-MnO2 shows a sharp band at 645 cm−1 due to the symmetric v2 (Mn—

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O) stretching vibration of the MnO6 groups.44 It is indicative of rutile-type structure

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with an interstitial space. The weak band at 349 cm−1 is attributed to the skeletal

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vibrations. After the Ag addition, the peaks of 4.3 % Ag/MnO2 and 8.9 % Ag/MnO2 at

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645 cm−1 shift to lower wave number at 455 and 453 cm−1, respectively (red shifts),

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which indicate the existence of disorder crystal defects. The crystal defects are

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favorable to form oxygen vacancies. The peak intensity and peak width of 8.9 %

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Ag/MnO2 are larger than that of 4.3 % Ag/MnO2 because of a stronger interaction of

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Ag and support. This interaction roots in the effect of surface stress for the surface

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structure, which may be consistence with TiO2 sample because of the similar structure 6

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with a rutile type.45 The result further confirms a microscopic stress is the reason that

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the (110) peak of XRD pattern for the Ag/MnO2 widens with the addition of Ag. The

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literature reported that red shifts and larger peak width can cause the increase of oxygen

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vacancies.44 Therefore, the 8.9 % Ag/MnO2 possesses stronger interaction Ag with Mn,

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more lattice defects and oxygen vacancies, which is coincident with the XRD, XPS

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result. The oxygen vacancies are beneficial for adsorption, activation and migration of

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oxygen in the oxidation reaction of HCHO.

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3.3 Activity test

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The addition of Ag obvious improves the catalytic performance of HCHO at low

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temperature, because Ag can provide sufficient active sites for the HCHO oxidation

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reaction. From Figure S5 of the Supporting Information, the complete conversion of

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8.9% Ag/MnO2 displays at 110 °C, attributing to more Ag active sites (0.019 mmol)

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shown in Table 1.

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From Figure 5, 3D-MnO2 has a TOF of 0.000024 s-1 at 110 °C due to only Mn4+ ions

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as active phase.33 The TOFs of 4.3% Ag/MnO2 is 0.0002 s-1 at 30 °C and 0.0037 s-1 at

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110 °C. The TOFs of 8.9% Ag/MnO2 is 0.0005 s-1 at 30 °C and rapidly increases to

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0.007 s-1 at 110 °C. Compared with reported catalysts, their TOFs for HCHO oxidation

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were 0.005 s-1 on Ag/CeO2 at 110 °C,7 0.0039 s-1 on Ag/Al2O3 and 0.0035 s-1 on Ag/SiO2

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at 100 °C,6 respectively. These values are lower than the TOF of 8.9% Ag/MnO2.

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Normalized rates of the samples is shown in Figure S6 of the Supporting Information.

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Although the normalized rate of 3D-MnO2 is the lowest (only 2.0 nmol·s-1·m-2 at

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110 °C), the rate is still larger than the normalized rate of reported 3D-Co3O4 (1.0

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nmol·s-1·m-2 at 110 °C).3 The result confirms that the catalytic activity of 3D-MnO2 is

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superior to that of 3D-Co3O4. By calculation, the rates at 110 °C of several catalysts are

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2.9 nmol·s-1·m-2 on Ag-HMO,5 1.6 nmol·s-1·m-2 on Ag/MnOx-CeO2,8 0.7 nmol·s-1·m-2

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on Au/Co3O4-CeO2,27 0.3 nmol·s-1·m-2 on Pt/SiO2 17 and 8.8 nmol·s-1·m-2 on Ag/CeO2,7

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respectively, which are much lower than that (10.1 nmol·s-1·m-2 at 110 °C) of 8.9%

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Ag/MnO2. From Figure S7 of the Supporting Information, 3D Ag/MnO2 still has a

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better low-temperature catalytic activity than 2D Ag/MnO2 and Ag/MnO2 nanorods.

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Table S2 of the Supporting Information shows comparison the catalytic activity of 3D

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8.9 wt% Ag/MnO2 catalyst with the reported catalysts mentioned in the introduction.

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Although 3D mesoporous 8.9% Ag/MnO2 is superior to some Ag-supported catalysts 7

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or others for HCHO oxidation, it cannot completely convert HCHO into CO2 and H2O

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at room temperature, unlike Pt, Pd-supported catalysts such as Pt/TiO210 and Pd/TiO219

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which have better low-temperature catalytic activities than that of 3D Ag/MnO2 because

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of a more expensive Pt as active sites. However, cheap and promising catalyst

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substitutes, such as 3D Ag/MnO2 completely converting HCHO at lower temperature,

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still need to be developed to reduce the production cost.

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A Koros-Nowak test from Table S3 of the Supporting Information shows that the

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TOF almost remained unchanged when Ag loading amount decrease to 4.45 wt.%,

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confirming that there were no heat and mass transfer limitations in the catalyst system.

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For kinetics measurement, the HCHO conversion is kept below 15%. The apparent

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activation energy of the reaction over 8.9% Ag/MnO2, 4.3% Ag/MnO2 and 3D-MnO2

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is 39.1, 45.3 and 56.6 kJ/mol, respective (Figure S8, Supporting Information), The 8.9%

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Ag/MnO2 has the lowest activation energy, indicating that HCHO molecules are easier

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to be activated.

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With the increase of space velocity and HCHO concentration, the HCHO conversion

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at 110 °C gradually decreases (See Figure S9 and S10, Supporting Information).

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However, the 8.9% Ag/MnO2 catalyst still remains highly active and stable under high

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space velocity and initial HCHO concentration. The increase of humidity scarcely

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reduces the catalytic activity and stability at 110 °C on the 8.9% Ag/MnO2 catalyst (See

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Figure S11, Supporting Information), which indicates that the presence of water vapor

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does not lead to a negative effect for HCHO oxidation reaction. On the contrary, the

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existence of moisture can probably enhance the catalytic activity and stability of HCHO

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oxidation. This case is similar with Pt/TiO2 catalyst.10 The HCHO conversion for 8.9%

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Ag/MnO2 catalyst slightly reduces when humidity increases to 75%, attributing that

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larger humidity can cause Ag active sites to be covered by water. However, the 8.9%

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Ag/MnO2 still kept a better activity and stability in humid air. Overall, three-

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dimensional ordered mesoporous Ag/MnO2 has much better catalytic performance,

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implying that they should be potential catalysts for HCHO oxidation.

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According to the characterization analysis, Ag nanoparticles are distributed in the

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Ag/MnO2 sample. The Ag addition only reduces the pore size and surface area, but not

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change the 3D ordered mesoporous structure of Ag/MnO2, which are conducive to the

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diffusion of reactants and products. Ag nanoparticles provide sufficient active sites for 8

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the oxidation reaction of HCHO, while 3D-MnO2 support only acts as a reservoir

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providing oxygen species.38 More Ag active sites can improve the catalytic performance

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for HCHO oxidation because of the exposure of Ag (111) crystal faces, which can

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promote the breakage of H2, O2 and NO bonds, and increase the bond strength of H, O,

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N and C atoms.46 TEM results have confirmed the 8.9% Ag/MnO2 catalyst exposes Ag

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(111) crystal facets, which are also active faces in the 3D Ag/MnO2 sample. From XRD

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results, a microscopic stress exists in the Ag/MnO2 and lead to the interaction between

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Ag and Mn. Raman results also confirmed that this interaction roots in the effect of

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surface stress for the surface structure. From H2—TPR analysis, the existence of the

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interaction between Ag and Mn causes a better low-temperature reducibility of

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Ag/MnO2 catalyst. More literatures2,3,5,7,8,9,12 have demonstrated that better low-

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temperature reducibility can enhance the catalytic ability of HCHO oxidation. From

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XPS results, the 8.9% Ag/MnO2 has more abundant surface lattice oxygen species,

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which are favorable to the adsorption and activation of HCHO molecules in the gas

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phase. The addition of Ag changes the distribution of original Mn4+ in 3D-MnO2

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support and increases the amount of Mn3+ ions. Larger ratio of Mn3+/Mn4+ is favorable

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to form more oxygen vacancies.43 In addition, the interaction Ag with Mn generated by

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a microscopic stress for surface structure leads to the presence of lattice defects from

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Raman analysis, which easily produce certain oxygen vacancies. In one word, the

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enhancement of the impact factors such as active sites, low-temperature reducibility,

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surface lattice oxygen species, oxygen vacancies and lattice defects generated from

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interaction Ag with MnO2 improves the catalytic ability for HCHO oxidation on the 3D

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mesoporous Ag/MnO2 catalyst.

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The critical part of the mechanism for metal-catalyzed oxidation is the complex

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interaction of oxygen with the metal surface.47 In the process of HCHO oxidation,

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thermal activation can enhance the depletion of the number of active oxygen species

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(Oact). Oact is not necessarily present on the surface after reoxidation by O2. It can

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migrates from surface lattice sites at the perimeter of Ag nanoparticles. Finally, surface

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lattice sites are supplemented by oxygen in the gas phase. 48 The active oxygen species

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surrounding Ag on Ag/MnO2 catalyst are the ones primarily consumed by the reaction

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of HCHO oxidation. More oxygen vacancies on Ag/MnO2 catalyst can provide a

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transfer station for the adsorption, activation and migration of oxygen species. The 9

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activation and migration mechanism of oxygen species on the oxygen vacancies of

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Ag/MnO2, like a Ag/Co3O4 catalyst,2 depend on the redox cycles of Mn4+/Mn3+ and

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Ag+/Ag0 after the oxygen species are consumed.

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Corresponding Author

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*B.B.: e-mail, [email protected]; tel., +86 10 84914902.

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*J.L.: e-mail, [email protected]; tel., +86 10 62771093.

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Notes

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The authors declare no competing financial interest.

AUTHOR INFORMATION

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Acknowledgement

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This study was supported by State Key Laboratory of Environmental Criteria and

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Risk Assessment, Chinese Research Academy of Environmental Science. This study

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was also supported by State Environmental Protection Key Laboratory of Sources and

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Control of Air Pollution Complex, the National Natural Science Fund of China (grant

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No. 21325731&51478241&21221004) and the National High Science & Technology

302

Project of China (grant No. 2013AA065304).

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Table and Figure Captions

446 447 448

Table 1 The physical parameters, chemical and surface compositions of the samples

449

Figure 1. XRD pattern of the samples

450

Figure 2. TEM image of (a, b) 3D-MnO2, (c, d) 4.3% Ag/MnO2 and (e, f) 8.9%

451

Ag/MnO2

452

Figure 3. H2 —TPR patterns of the samples

453

Figure 4. XPS patterns of (a) 3D-MnO2, (b) 4.3% Ag/MnO2 and (c) 8.9% Ag/MnO2

454

Figure 5. TOFs of HCHO Oxidation under the condition of HCHO 500 ppm and GHSV

455

60000 h-1 for 3D-MnO2 and 4.3% Ag/MnO2, and HCHO 1300 ppm and GHSV 150,000

456

h-1 for 8.9% Ag/MnO2.

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457

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Table 1 The physical parameters, chemical and surface compositions of the samples

Sample

Ag

Ag

Active sites

Surface

Pore

Surface element molar

content

dispersion[b]

number[c]

areas

diameter

ratio[e]

ABET (m3/g)

DP (nm) [d]

(wt.%)[a]

D(%)

n (mmol)

Mn3+/Mn4+

Olatt /Oads

3D-MnO2

0



1.38

87

3.8/11.6

0.7

1.0

4.3%Ag/MnO2

4.3

19.6

0.016

76

3.7/8.6

1.1

1.3

8.9%Ag/MnO2

8.9

11.8

0.019

61

3.7/6.3

1.4

1.6

458

[a] The Ag content detected by ICP-AES; [b] Ag dispersion estimated by Ag nanoparticle size from

459

the TEM results; [c] Active sites number of 3D-MnO2 calculated by Mn4+ions from the H2-TPR

460

result, and active sites number of Ag/MnO2 calculated by Ag content and Ag dispersion; [d] Pore

461

size obtained from the N2 adsorption isotherms and TEM images; [e] Surface element molar ratio

462

calculated by peak areas of XPS.

17

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(101) (211)

(110) (111) Relative intensity

8.9%Ag/MnO2

(220) (301) (002)

4.3%Ag/MnO2

3D-MnO2

463 464

10

20

30

40

50

2

60

70

80



Figure 1. XRD pattern of the samples

18

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465

466 467

Figure 2. TEM image of (a, b) 3D-MnO2, (c, d) 4.3% Ag/MnO2 and (e, f) 8.9%

468

Ag/MnO2

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peak 1

3D-MnO2 4.3% Ag/MnO2 8.9% Ag/MnO2 TCD Signal

peak 2

peak 3

peak 4

peak 2 peak 1

469 470

35

135

235 335 o Temperature / C

peak 3

435

535

Figure 3. H2 —TPR patterns of the samples

20

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O 1s

A

529.3 530.9

Olatt

Oads

Relative Intensity (a.u.)

c

B

641.5 Mn

3+

642.7 Mn

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4+

Mn 2p3/2 c b

a 638 639 640 641 642 643 644 645 646 647 648

526

528

530

532

534

Binding Energy (eV)

536

Relative Intensity (a.u.)

Binding Energy (eV)

b

a

471 472

Relative Intensity (a.u.)

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368.7

C

365

Ag 3d

c b a 367

369

371

373

375

377

Binding Energy (eV)

Figure 4. XPS patterns of (a) 3D-MnO2, (b) 4.3% Ag/MnO2 and (c) 8.9% Ag/MnO2

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7 6

3

10 TOF / s-1

5

3D-MnO2 4.3% Ag/MnO2

4

8.9% Ag/MnO2

3 2 1 0 20

30

40

50

60 70 80 90 o Temperature / C

100 110 120

473 474

Figure 5. TOFs for HCHO oxidation under the condition of HCHO 500 ppm and GHSV

475

60000 h-1 for 3D-MnO2 and 4.3% Ag/MnO2, and HCHO 1300 ppm and GHSV 150,000

476

h-1 for 8.9% Ag/MnO2

477

22

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