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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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Environmental Science & Technology

Graphic Abstract

ACS Paragon Plus Environment


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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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‡

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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

2



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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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

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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

269

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,

273

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

274

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

275

mesoporous Ag/MnO2 catalyst.

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

277

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

278

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

279

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

280

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

281

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

282

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

283

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

284

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

286

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.

288 289



290

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

295 296



Acknowledgement

297

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

298

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

301

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

302

Project of China (grant No. 2013AA065304).

303 304 305 306 307 308

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15


Page 17 of 23


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.

16



457

Page 18 of 23

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


Page 19 of 23


(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



Page 20 of 23

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

19


Page 21 of 23


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


O 1s

A

529.3 530.9

Olatt

Oads

Relative Intensity (a.u.)

c

B

641.5 Mn

3+

642.7 Mn

Page 22 of 23

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


Binding Energy (eV)

b

a

471 472



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

21


Page 23 of 23


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


Three-Dimensional Ordered Mesoporous MnO2 ... - ACS Publications

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