Engineering Crystal Facet of Î±-MnO2 Nanowire for Highly Efficient

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Engineering Crystal Facet of #-MnO Nanowire for Highly Efficient Catalytic Oxidation of Carcinogenic Airborne Formaldehyde Shaopeng Rong, Pengyi Zhang, Fang Liu, and Yajie Yang ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b00456 • Publication Date (Web): 14 Mar 2018 Downloaded from http://pubs.acs.org on March 15, 2018

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ACS Catalysis

Engineering Crystal Facet of α-MnO2 Nanowire for Highly Efficient Catalytic Oxidation of Carcinogenic Airborne Formaldehyde

Shaopeng Rong ab, Pengyi Zhang ab*, Fang Liu a, Yajie Yang a a

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China.

b

Beijing Key Laboratory for Indoor Air Quality Evaluation and Control, Beijing 100084, China

ACS Paragon Plus Environment

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ABSTRACT: The activity of exposed crystal facets directly determines its physicochemical properties. Thus, acquiring a high percentage of reactive facets by crystal facet engineering is highly desirable for improving the catalytic reactivity. Herein, single-crystalline α-MnO2 nanowires with major exposed high-index {310} facets were synthesized via a facile hydrothermal route with the assistance of capping agent of oxalate ions. Comparing with other two low-index facets ({100} and {110}), the resulting α-MnO2 nanowires with exposed {310} facets exhibited much better activity and stability for carcinogenic formaldehyde (HCHO) oxidation, making 100% of 100 ppm of HCHO mineralized into CO2 at 60 ℃, even better than some Ag supported catalysts. The density functional theory (DFT) calculations were used to investigate the difference in the catalytic activity of α-MnO2 with exposed {100}, {110} and {310} facets. The experimental characterization and theoretical calculations all confirm that the {310} facets with high surface energy can not only facilitate adsorption/activation of O2 and H2O, but also be beneficial to generation of oxygen vacancies, which result in significantly enhanced activity for HCHO oxidation. This is a valuable report on engineering surface facets in the preparation of α-MnO2 as highly efficient oxidation catalysts. This study deepens the understanding of facet-dependent activity of α-MnO2 and points out a strategy to improve their catalytic activity by crystal facet engineering. KEYWORDS: Crystal facet engineering, α-MnO2, Formaldehyde, Indoor air, Density functional theory


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1. INTRODUCTION

Crystal facet engineering is known to induce various physical and chemical performance in functional materials due to the distorted electronic structure and different exposed atoms in the surface of crystals with different exposed facets

1-3

. Many applications such as heterogeneous

catalysis, molecule adsorption, gas sensing and ion detecting, energy conversion and storage are very sensitive to surface atomic structures. Thus, engineering surface structures and exposing specific facets with high energy and reactivity as we wish, is becoming a promising research direction. In theory, the reactivity and activity of facets are proportional to their surface energy, the facets with high surface energy are usually more reactive in heterogeneous reaction 4. However, the facets with high surface energy usually vanish in the bulk of crystals due to the fast growth rate, and the thermodynamically stable facets preferentially predominate on the surface so as to minimize the total surface energy of crystals. Thus, how to expose specific facets on the surface of crystal is still a challenging topic of nanomaterials. Up to now, many efforts have been made to tailor the exposure of crystal facets, one of the basic strategy is to select appropriate morphology-controlling/capping agents, which preferentially adsorb on some surface to decrease surface energy and inhibit the growth of the facet. It is well known that solvents, impurities and additives in solution can substantially influence the final shape of the crystals

5,6

. Due to the

difference in the surface atomic arrangement, the surface affinity for the solvent to each orientation may be not the same, which can certainly affect the growth rates to the crystal surfaces and hence the final shape of the crystal 7. So far, this strategy has been well used in synthesizing some transitional metal oxides such as TiO2, Fe2O3 and WO3. For example, Yang et al. made a breakthrough in the synthesis of anatase TiO2 with 47% of exposed {001} facets using hydrofluoric acid as a morphology controlling agent 8. Subsequently, even higher percentage


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(89%) of exposed {001} facets has been obtained by simply using high concentration of hydrofluoric acid as the {001} facet capping agent 9. α-Fe2O3 crystals with major {110} and minor {001} facets can be selectively etched by oxalic acid along the [001] direction with phosphate ions as capping agent preferentially adsorbed on {110}, so that the resultant discs are reversely terminated with major {001} and minor {110}

10

. Urea acting as both etching and

capping agent is an appropriate candidate for preparing WO3 octahedron with {111} surface facets from irregular commercial WO3 particles 11. Formaldehyde (HCHO) is one of the priority indoor air pollutants emitted from furnishing and building materials 12, causing profound impacts on human health. In 2010, the World Health Organization has classified HCHO as group 1 carcinogen for humans, and set an indoor guideline value of 0.1 mg/m3 for HCHO

13

, which is also adopted as the Chinese national

standard. In China, the HCHO concentration in 70% of the newly built or remodeled houses exceeded the safety standards 14, with an average concentration of 0.238 mg/m3. Previous studies showed that indoor HCHO concentration was positively correlated with temperature and absolute humidity 15. Especially in summer, the high temperature and humidity will accelerate the emission of HCHO, causing the serious HCHO pollution in indoor environment

15, 16

. Thus,

safe and efficient removal of indoor HCHO has a great practical demand. Catalytic oxidation of HCHO to CO2 by using catalysts is considered as a promising technology for indoor HCHO elimination, and the high-efficiency catalyst is the key factor to the above technology. Noble metal-based catalysts exhibit superior activity at low temperature

17

, and supported Pt catalysts

have showed the ability to completely transform HCHO into CO2 at room temperature. However, there is no doubt that the high price and scarce resources restrict its wide application. Among various non-noble transition metal oxides, manganese dioxide (MnO2) have been widely studied


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for HCHO oxidation. However, to date, for non-noble catalysts, it is very difficult to realize complete mineralization of HCHO into CO2 at low temperature. Zhou et al. 18 constructed a coreshell α-MnO2 -MnO2@L-MnO2 catalysts through oriented growth of layered-MnO2 nanosheets (L-MnO2) over α-MnO2 nanotubes. However, the HCHO removal ratio within 1 h was only 14.8%, almost no CO2 was generated at room temperature. With loading of 1wt% Pt, Pt/αMnO2-MnO2@L-MnO2 achieved mineralization of 92.1% HCHO. Ciotonea et al.,

19

demonstrated that the ball-milling treatment could improve the activity of tunnel-type cryptomelane (α-MnO2), achieving complete conversion of HCHO at 170 °C. Through cerium doping 20, creation of manganese vacancy 21 and surface pits 22, our group significantly enhanced the catalytic activity of layered-type birnessite (δ-MnO2), the complete conversion temperature of HCHO was reduced to 100 °C under the gas hourly space velocity (GHSV) of 90 L/gcat.h. Recently, we fabricated a three-dimensional framework consisted with ultrathin δ-MnO2 nanosheets and α-MnO2 nanorods, achieving 45% of 100 ppm of HCHO continuously converted into CO2 at room temperature and complete conversion at 90 °C

23

. Though under much lower

GHSV, i.e. 60 L/gcat.h, the complete conversion over hollow KxMnO2 was achieved at 80 °C 24; and under 30 L/g.h, over graphene-MnO2 the complete conversion was achieved at 65 °C 25. As a matter of fact, normally the HCHO concentration in the polluted indoor environmental is at low level (e.g., 0.1-1 mg/m3), and the typical face velocity of air filtering systems is as high as 1–3 m/s. So, besides reducing the complete conversion temperature for high concentration HCHO, it is more meaningful to develop the catalysts which could achieving high efficiency for low-concentration HCHO at high air velocity at room temperature. Recent breakthrough in facet engineering gives us a hint that engineering high energy surface may be an effective approach to enhance the performances of MnO2 crystals. In this work, α-MnO2 nanowires with exposed high-


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index {310} facets on the side walls were synthesized via a facile hydrothermal route. The resulting α-MnO2 single-crystalline nanowires with exposed {310} facets exhibited superior activity for the oxidation of HCHO, achieving complete conversion of 100 ppm HCHO at 60 °C under the GHSV of 90 L/gcat.h and 86 % conversion of 0.53 mg/m3 HCHO at room temperature under the GHSV of 600 L/gcat.h. This is the first report on engineering surface structures and facets control in the preparation of α-MnO2 as highly efficient oxidation catalysts.

2. EXPERIMENTAL SECTION

2.1 Catalysts preparation

Synthesis of α-MnO2-310: α-MnO2-310 were synthesized by hydrothermal redox reaction between KMnO4 and (NH4)2C2O4. In a typical procedure, 20 mmoL of KMnO4 and 10 mmoL (NH4)2C2O4·H2O were dissolved into 70 mL deionized water under vigorous magnetic stirring. The mixture solution was then transferred to a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was heated in electric oven and kept at 180 °C for 24 h. After the autoclave cooled naturally to room temperature, the precipitates were collected by centrifugation, washed several times with deionized water, and then dried at 105 °C. Synthesis of α-MnO2-110: α-MnO2-110 were prepared by the same synthesis route as αMnO2-310, but the reductant of (NH4)2C2O4 was replaced by (NH4)2SO4. Synthesis of α-MnO2-100: α-MnO2-100 were prepared by hydrothermal redox reaction of MnSO4·H2O, (NH4)2S2O8, and (NH4)2SO4 26. In a typical procedure, 8 mmoL of MnSO4·H2O, 8 mmoL of (NH4)2S2O8, 15 mmoL of (NH4)2SO4, and 8 mmoL KNO3 were dissolved in 40 mL of deionized water under vigorous magnetic stirring. Then the above solution was transferred to a 100 mL Teflon-lined stainless-steel autoclave. The autoclave was heated in electric oven and


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ACS Catalysis

kept at 120 °C for 20 h. After the autoclave cooled naturally to room temperature, the precipitates were collected by centrifugation, washed several times with deionized water and dried at 105 °C. 2.2 Catalysts characterization Field emission scanning electron microscopy (FESEM) images were obtained with a Hitachi S5500 field emission scanning electron microscope (Hitachi, Japan). Transmission electron microscopy (TEM) images were recorded on a JEM-2100 electron microscope (JEOL, Japan). X-ray diffraction (XRD) patterns were performed on a Bruker D8-Advance X-ray diffractometer (Bruker, Germany) using a Cu Kα radiation source. The scanning speed was 8° min-1, and the tube voltage and current were 40 kV and 40 mA, respectively. The chemical states of the catalyst surface elements were examined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Fisher, USA) at a pass energy of 30 eV, equipped with Al Kα as an exciting X-ray source. Binding energies were corrected for surface charging by referring to the energy of the C 1s peak at 284.8 eV. The XPS spectrums were processed by the XPSPEAK41 software, and the ratio of elements with different valence states was all calculated based on the peak areas. The Brunauer-Emmett-Teller (BET) specific surface area measurements were determined on an ASAP 2020 nitrogen adsorption apparatus (Micromeritics Instruments, USA). All the samples were degassed at 300 °C for 4 h before test. The total specific surface area (SBET) was determined by a multipoint BET method, the single-point pore volume was estimated from the amount adsorbed at a relative pressure of 0.99. Thermogravimetric (TG) analysis was performed using TGA/DSC 1 STARe (Mettler Toledo, Switzerland) with the heating rate of 5 °C min-1 from 25 °C to 950 °C in N2 gas (20 mL min-1). Laser Raman spectra were measured on


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confocal Raman microscope spectrometer (inVia, Renishaw, Britain) using Ar+ laser (532 nm) as the excitation source. Hydrogen temperature programmed reduction (H2-TPR) was performed on an AutoChem II 2920 adsorption apparatus (Micromeritics, USA) equipped with a TCD detector. About 30 mg of sample was loaded and pretreatment in the 50 ml/min He at 105 °C for 30 min. After cooling down to room temperature, the analysis gas was changed to 5% H2/Ar with a flow rate of 50 mL/min and the temperature was then programmed up to 600 °C at a ramp of 5 °C /min. Oxygen temperature programmed desorption (O2-TPD) was also performed on an AutoChem II 2920 adsorption apparatus (Micromeritics, USA). About 30 mg sample was firstly purged with He at 105 °C for 30 min to remove surface adsorbed water and followed by cooling down to room temperature. Then, the catalyst was purged with 50 ml/min O2 at room temperature for 30 min. After that, the sample was purged with 50 mL/min He for 30 min to remove the physisorbed O2 and stabilize the detector baseline. Finally, the temperature was programmed rise to 950 °C from 25 °C at a ramp of 5 °C/min in the He stream. 2.3 Catalytic activity tests As for catalytic activity test, HCHO conversion efficiency of various catalysts were investigated in a fixed bed flow reactor which was similar to that reported in our previous publication 23. Briefly, 100 mg catalyst (40-60 meshes) was loaded in a quartz tube reactor (i.d. = 6 mm). 100 ppm gaseous HCHO was generated by flowing synthetic air (21% O2/79% N2) over paraformaldehyde in glass bottle which was immersed in a water bath kept at 28 °C. The total flow rate was 150 mL/min with the corresponding gas hourly space velocity (GHSV) of 90 L/gcat·h, and the relative humidity (RH) was controlled as 70% generated by bubbling air into the water. The CO2 generated from the oxidation of HCHO was determined by a GC-2014 gas


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ACS Catalysis

chromatograph (Shimadzu, Japan) equipped with a methanizer and a flame ionization detector (FID). The HCHO conversion was calculated as follows:

× 100% % =

(1)

Where [HCHO]in is the inlet HCHO concentration of the feed gas, and [CO2]out is the outlet CO2 concentration determined by gas chromatograph. Furthermore, the activity for low-level HCHO at room temperature was also tested in the same system. In this case, HCHO was generated by flowing compressed air over HCHO solution kept in a thermostated water bath and the inlet concentration was set at 0.53 mg/m3. The total flow rate was 1 L/min, with the corresponding GHSV of 600 L/gcat·h. The relative humidity was kept at 55% by passing compressed air through a water bubbler. The concentration of HCHO was determined by MBTH method 12. 2.4 Theoretical calculation method The {310}, {110} and {100} surface slabs of α-MnO2 compound were considered to study by first principles density functional theory (DFT) calculations as implemented in Vienna Abinitio Simulation Package (VASP) (PAW) pseudopotentials

28

27

with plane wave basis sets and projector-augmented wave

. The electronic structure was calculated using the Generalized

Gradient Approximation (GGA) of Perdew-Burke-Ernzerhof with Hubbard U corrections (PBE+U) 29. Previous work

30, 31

demonstrated a good description of lithium intercalation, band

gaps and magnetic interactions when PBE+U is applied in the fully-localized limit, which we employ (U − J) = 5.2 eV in this work as well. To obtain the equilibrium lattice parameters by relaxation of the bulk cell a cutoff for the plane wave basis set of 520 eV was used to avoid Pulay stress. According to the results of XRD, the α-MnO2 crystal occurs in the tetragonal space group I4/m (# 87) with lattice parameters a =


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9.784 Å, b = 9.784 Å and c = 2.863 Å. All subsequent calculations were performed based upon the obtained equilibrium lattice constants using a cutoff of 400 eV. A vacuum thickness greater than 15 Å was applied to avoid the interaction between the slab and its periodical images. All the ions were iteratively relaxed by conjugate gradient algorithm until the absolute value of force on each ion converged below to 0.05 eV/ Å. The surface energy (γ) of surface slabs is defined and calculated as follows:

γ=

!" #$!% &'

(2)

where, Es is the energy of a slab containing n formula units and Eb is the total energy per formula unit of bulk α-MnO2. A is the area of the slab surface and the factor of 2 reflects the fact that there are two surfaces for each slab. Formation energy (Evo) of oxygen vacancy is defined as follows: ()* = (+,- − (/012 + 1/2(

(3)

where Edef is the system energy with the loss of one oxygen atom (O), Ebulk the energy of a slab without the loss of an oxygen atom, and ( the energy of an O2 molecule in the gas phase. To understand the absorptive properties of those surface slabs, an additional molecule is allowed to adsorb on the surface and its adsorption energy (Eads) is calculated from the formula: (678 = (9*9:1 − (/:;*1,=01,

(4)

where, Eads is adsorption energy of water molecule, Etotal, Ebasic and (>*1,=01, are the total energy of the surface containing the adsorbate, of clean surface, and of isolated molecule, respectively.

3. RESULTS AND DISCUSSION

3.1 Characterization of α-MnO2


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ACS Catalysis

α-MnO2 single-crystalline nanowires with exposed different facets on the side walls were prepared through a facile hydrothermal redox reaction. The XRD patterns shown in Figure 1 reveals that all the as-synthesized samples are indexed to the pure α-MnO2 (JCPDS, PDF#440141), which is a type of well-defined 2 × 2 tunnel structure with a tunnel size of ∼4.6 Å and is composed of double chains of edge-shared MnO6 octahedra and corner-sharing double chains. FESEM images (Figure 2) show that all of the samples are characterized by the morphology of nanowires, and further observation we find that these nanowires all present rectangular cross section. The α-MnO2-310 and α-MnO2-100 exhibit similar size with diameter of 15-20 nm and length in the range of several micrometers. But α-MnO2-110 shows a larger diameter in the range of 45-50 nm. Because the α-MnO2 usually grows along the [001] direction (c-axis), so the four side walls will be the major exposed facets. With the help of the high-resolution TEM (HRTEM), we determined the exposed crystal facets on the side walls. As shown in Figure 3 b & c, the lattice distance along the growth axis is 0.31 nm, corresponding to the (310) facet of α-MnO2. Thus, the other side perpendicular to it is (310) facets. Since the four side walls are equivalent, so we can conclude that the α-MnO2 nanowires grow along the [001] direction with four {310} facets exposed on the side walls. Moreover, the corresponding selected-area electron diffraction (SAED) pattern could be indexed as the diffraction spots of the [310] zone of a α-MnO2 single crystal. Similar, the other two α-MnO2 samples were exposed {110} (Figure 3 f & g) and {100} (Figure 3 j & k) facets, respectively. Comparing the intensities of XRD diffraction peaks in (310), (110) and (100), we can find that their strength has a different relative ratio. In the case of αMnO2 with the major exposed {310} facet, the intensity of XRD diffraction peak corresponding to (310) is the strongest among the diffraction peaks in (310), (110) and (100). In the cases of αMnO2-100 and α-MnO2-110, we also find the similar situation that the relative intensity of


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diffraction peak corresponding to the major exposed facet obviously increased. The success in controlling synthesis of α-MnO2 with major high-index {310} exposed facets allowed us to significantly enhanced catalytic activity. It is well known that a facet with high percentage of unsaturated atoms always possesses a superior reactivity to that with a low percentage of unsaturated atoms

32, 33

. These high-index facets usually possess high density of under-

coordinated atoms, such as steps, edges, and kinks, serving as active sites 34.

Figure 1. XRD patterns of α-MnO2 with different exposed facets.


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ACS Catalysis

Figure 2. SEM images of (a, b) α-MnO2-310, (c, d) α-MnO2-110 and (e, f) α-MnO2-100.


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Figure 3. TEM and HRTEM images of (a-c) α-MnO2-310, (e-g) α-MnO2-110 and (i-k) α-MnO2100; Atomic structure of the (d) {310}, (h) {110} and (l) {100} surface of α-MnO2. Small (red) spheres are oxygen and large (purple) are manganese.

3.2 Catalytic activity for HCHO oxidation Figure 4 shows the catalytic performance of HCHO on three α-MnO2 samples with different exposed facets. As shown in Figure 4a, we can find that the catalytic activity exhibits facetdependent, and the activities followed the order of {310} > {110} > {100}. Under the GHSV of 90 L/gcat.h, the α-MnO2 with the exposed {310} facet could completely transform 100 ppm HCHO into CO2 at as low as 60 °C, which was significantly lower than those of α-MnO2 with the {110} (130 °C) and {100} (150 °C) exposed facets. Moreover, according to Arrhenius plots of the catalysts for HCHO oxidation (Figure 4b), α-MnO2-310 shows the lowest apparent activation energy (Ea) values, suggesting HCHO was more easily oxidized over α-MnO2-310. Interestingly, even under much higher GHSV, α-MnO2-310 could still completely transform HCHO at less than 100 ℃. From Figure 4c, under very high GHSV (600 L/gcath), the complete oxidation temperature is 90 ℃ for α-MnO2-310, which is still lower than that for many metal oxides tested at a lower GHSV. Compared with other MnO2-based catalysts for the oxidation of HCHO summarized in Table 1, it can be found that the α-MnO2-310 showed the best catalytic performances among them, the complete conversion temperature of α-MnO2-310 in this work is much lower than any other kinds of MnO2-based catalysts reported in literatures. In addition, the catalytic activity of α-MnO2-310 even better than some Ag supported catalysts. Such as Bai et al used Ag/Co3O4 to oxidize HCHO, the complete oxidization of HCHO was obtained at 70 ℃ (GHSV = 30 L/gcat·h)

35

. Tang et al., used MnOx-CeO2 as supporter to load Ag, the complete

oxidization of HCHO was obtained at even higher temperature 100 ℃ (GHSV = 30 L/gcat·h) 36.


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ACS Catalysis

Moreover, the long-term catalytic stability of α-MnO2-310 was also performed. As shown in Figure 4d, the α-MnO2-310 could complete oxide HCHO into CO2 at 60 ℃, and during the 48 h test periods, no deactivation was observed. The above-mentioned results further indicate that the α-MnO2 with exposed high-index {310} facet not only exhibits higher catalytic activity, but also better stability. In addition, taking the practical condition of air cleaning system into account, the long-term catalytic performance in the real environmental situations was evaluated at an inlet HCHO concentration of 0.53 mg/m3 and a GHSV of 600 L/gcat·h at room temperature. As shown in Figure 4e, during the 10 h test periods, the HCHO removal efficiency by α-MnO2-310 remained as high as 86%. We have previously reported a kind of cerium modified birnessite-type MnO2, which exhibited high activity for HCHO removal at low temperature, obtaining ~52% removal efficiency in the same test situations

20

. Comparing the HCHO removal efficiency in the real

environmental situations, we can conclude that the α-MnO2-310 catalyst showed a much better HCHO removal activity than cerium modified MnO2. The results mentioned above further indicate that the α-MnO2-310 not only has high activity at high temperature but also owns significant activity for HCHO removal at room temperature. It is possible for α-MnO2-310 to be used for long-term elimination of HCHO at real environmental situations.


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Figure 4. (a) Temperature-dependent of HCHO conversion over α-MnO2 with different exposed facets (HCHO concentration = 100 ppm, 21%O2, N2 as balance gas, RH = 70%, GHSV= 90 L/gcat·h). (b) Arrhenius plots for HCHO oxidation over α-MnO2 with different exposed facets. (c) Temperature-dependent of HCHO conversion over α-MnO2-310 under the different GHSV (HCHO concentration = 100 ppm, 21%O2, N2 as balance gas, RH = 70%). (d) Stability test of the α-MnO2-310 under 60 ℃ (HCHO concentration = 100 ppm, 21%O2, N2 as balance gas, RH = 70%, GHSV= 90 L/gcat·h). (e) Performance of the α-MnO2-310 at environmental situations. (HCHO concentration = 0.53 mg/m3, GHSV= 600 L/gcat·h, RH = 55%, room temperature)


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ACS Catalysis

Table 1. Survey of catalytic performance of MnO2 catalysts for the oxidation of HCHO. T50% (℃ ℃)

T100% (℃ ℃)

93

140

85

160

Ramsdellite

100

>160

MnOOH

105

>160

α-MnO2

90

150

140

>200

γ-MnO2

125

150

δ-MnO2

58

80

Pyrolusite

150

180

110

140

140

160

Catalysts

Test conditions

Birnessite Cryptomelane HCHO = 100 ppm; GHSV = 60 L/gcat.h

β-MnO2

37

HCHO = 170 ppm; GHSV = 100 L/gcat.h

Cryptomelane


Todorokite

Reference

38

39

MnOx/SBA-15


107

125

40

Porous Birnessite


85

100

41

MnOx-CeO2


>80

100

42

Ag/MnOx-CeO2


~70

100

36

Hollow KxMnO2


~57

80

24

Graphene−MnO2


~47

65

25

100

140

150

180

90

130

α-MnO2 β-MnO2


3D-MnO2

43

Ce-MnO2 (1:10)


~70

100

20

δ-MnO2


55

110

21

Surface pits δ-MnO2


62

100

22

125

150

100

130

35

60

α-MnO2-100 α-MnO2-110 α-MnO2-310


This work.


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3.3 Effect of oxalate ions on the synthesis of α-MnO2-310 The current synthesis yielded a special α-MnO2 nanowires with exposed high-index {310} facets, it is important to understand the effects of key synthetic parameters on the crystal growth and its formation mechanism. The former may be clarified by investigating the role of reaction precursors. Compared with the synthesis route of α-MnO2-310 and α-MnO2-110, we can find that the difference in the precursors resulting in the different exposed facets. We also used NH4F and NH4Cl to replace (NH4)2C2O4, the final samples are all α-MnO2 nanowires (Figure S1) with exposed {110} facets (Figure S2). It is well known that the key in controlling the exposed crystallographic facets is to alter the relative stability of every facet during crystal growth. Surface adsorption of capping agents (impurities or additives in the reaction medium), which selectively interact with the different crystalline facets, has been expected to be an efficient strategy in designing exposed facets

32

. Therefore, oxalate ions acting as capping agent is an

appropriate candidate for preparing α-MnO2 nanowires with exposing {310} surface facets. During the crystal growth, oxalate ions selectively adsorb on the surface of {310} facets, hence reduce its surface free energy and inhibit the crystal growth along the corresponding direction. Due to the barrier effect of capping agent, the {310} facets with high surface energy will not disappear during the crystal growth. As shown in Scheme 1, compared with other reaction precursors, we can find that oxalate ions are the key factor influencing the exposed facets. Furthermore, we also investigated the effects of other parameters including the ratio of MnO4-/C2O42-, hydrothermal temperature and hydrothermal time. In the typical synthesis, the MnO4-/C2O42- ratio was 2, at which the product was dominated by α-MnO2 nanowires. When MnO4-/C2O42- was 3, the products were still α-MnO2 nanowires (Figure S4a). However, when MnO4-/C2O42- < 2, the MnO(OH) nanorods appear in the products (Figure S4c). As the MnO4-


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/C2O42- ratio decreased to 0.5, the product was composed of MnO(OH) nanorods and MnCO3 nanocubes, as demonstrated by XRD (Figure S3) and SEM images (Figure S4d). Insufficient KMnO4 cannot provide enough oxidation ability, so the final products become manganese oxide with low Mn valence. It indicates that a relatively high concentration of oxalate ions was unfavorable for crystal growth of α-MnO2, and the final products exhibit lower catalytic activity (Figure S5). Besides, it is generally believed that temperature can affect crystal growth in several ways, all of them resulting in smaller crystal size at lower temperature. At low temperature (90 ℃), layered structure δ-MnO2 nanosheets can be obtained (Scheme 1). However, a higher temperature (150 ℃) is preferable for the anisotropic growth of crystal, and results in a product with higher aspect ratios. With the increase of hydrothermal temperature, the phase changed from δ-MnO2 to α-MnO2 (Figure S6), and the morphology changed from the agglomerates of nanosheets to interlaces of nanowires (Figure S7). Moreover, the catalytic activity for the HCHO oxidation was also improved with the increase of hydrothermal temperature (Figure S8). The effect of hydrothermal time on the formation of α-MnO2 nanowires was also investigated. As shown in Figure S9, the sample taken after 1.5 h hydrothermal treatment, can be indexed to that of layered structure δ-MnO2. After a further prolonged reaction time (≥3 h), only nanowires are obtained and the XRD patterns of the samples are indexed to the pure α-MnO2 (Figure S10). With the extension of reaction time, the crystallinity of nanowires gradually increases, and the catalytic activity of HCHO increases accordingly (Figure S11). According to the XRD patterns of the intermediates (1.5 h and 3 h), a phase transformation from δ to α-MnO2 must have taken place in the formation α-MnO2. Among the several crystal forms of MnO2, δ-MnO2 has a layer structure, which tends to curl with the driving force of increasing temperature and pressure 44.


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Scheme 1. The effects of synthesis parameters on the formation of α-MnO2 and its exposed facets.

Based on the above experimental results, the formation mechanism can be concluded as follows (Scheme 1): (1) a large number of tiny primary [MnO6] units appear first as a result of the reduction of the KMnO4 molecules with oxalates under hydrothermal conditions; (2) through a condensation reaction [MnO6] units will form sheets of δ-MnO2; (3) the layer structure of δMnO2 tends to curl and collapse in elevated temperature and pressure; if a moderate amount of K+ or NH4+ exist, the layer structure will directly collapse into 2×2 tunnel structure of α-MnO2 or 2×1 tunnel structure of γ-MnO2. Otherwise, in the absence of enough cations, it will directly collapse into 1×1 tunnel structure of β-MnO2 44. Along with the phase transformation, the curling lamellar structures will grow into nanowires, the shorter nanowires may re-dissolve into the solution phase, and the longer ones further grow into much longer ones. Meanwhile, oxalate ions selectively adsorb on the surface of {310} facets and inhibit the crystal growth along the


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corresponding direction. Finally, the α-MnO2 nanowires with the {310} exposed facets are formed. 3.4 Surface physicochemical analysis Many applications such as heterogeneous catalysis, gas sensing, molecule adsorption, energy conversion and storage are very sensitive to surface atomic structures, thus the surface physicochemical properties are directly related to the properties of the materials. The surface chemical states are investigated by XPS, and its spectra are displayed in Figure 5. As shown in Figure 5b, the Mn 2p3/2 bands were chosen to fit the curve, the ratio of Mn3+ and Mn4+ was calculated by their peak areas

45

, and the quantitative analysis on the XPS spectra were

conducted and the results are given in Table 2. The Mn3+/Mn4+ ratios follow the order: α-MnO2310 > α-MnO2-110 > α-MnO2-100. Correspondingly, the average oxidation state (AOS) of Mn also followed the same order, which was calculated according to the binding energy difference of Mn 3s (Figure 5c) 46. Because of the electrostatic balance once Mn3+ appeared in the catalyst, the oxygen vacancies would be formed 47. In other words, the higher ratio of Mn3+/Mn4+ means the higher content of oxygen vacancies, which play important roles as active sites for oxidation reactions. The α-MnO2-310 owned the highest Mn3+/Mn4+ ratio, indicating that it had largest amount of oxygen vacancies to maintain electrostatic balance due to Mn3+.


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Figure 5. XPS spectra of α-MnO2-100, α-MnO2-110 and α-MnO2-310: (a) survey spectra, (b) Mn 2p3/2, (c) Mn 3s, and (d) O 1s.

Table 2. Properties of different synthesized MnO2 catalysts. Samples α-MnO2-100 α-MnO2-110 α-MnO2-310

SBET (m2/g) 87.9 119.5 135.3

Pore volume (cm3/g) 0.54 0.51 0.47

XPS 3+

Mn /Mn 0.13 0.45 0.82

4+

Oads/Olatt

AOS a

0.49 0.40 0.52

3.89 3.71 3.55

a

The average oxidation state (AOS) of Mn was calculated according to the binding energy difference (△E) of Mn 3s in XPS through an empirical formula, i.e. AOS = 8.956–1.126 ×△E.

The O 1s spectra shown in Figure 5d can be deconvoluted into two peaks at 529.6-530.1 eV and 531.2-531.7 eV, corresponding to lattice oxygen (Olatt) and surface adsorbed oxygen species (Oads, such as O2−, O− and -OH group) 48. From Figure 5d and Table 2, the α-MnO2-310 owned


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the most abundant Oads, indicating that it possessed the highest density of reactive site. Oads tends to participate into the oxidation reaction due to its high activity and the interaction with reactants 49

. In other hand, the higher Oads always means strong adsorption capacity for water and oxygen，

which is confirmed by the DFT calculation in the following section. Moreover, the binding energy of Olatt exhibits different shifts. For α-MnO2-100, α-MnO2-110 and α-MnO2-310, the Olatt peak shifts towards lower binding energies in turn, indicating the weaker interaction between Mn and O atom. It is understandable that the presence of oxygen vacancies increases the electron density of lattice oxygen, thus the binding energy of lattice oxygen decreases.

Figure 6. Raman spectra over α-MnO2 with different exposed facets. The inset shows the corresponding Mn–O bond force constant of α-MnO2 with different exposed facets.

The Raman spectra of α-MnO2 with different exposed facets are displayed in Figure 6a. The Raman band around 650 cm-1 can be attributed to the symmetric stretching vibrations (ν2(Mn-O)) perpendicular to the direction of the MnO6 octahedral double chains

50

. Interestingly, three α-


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MnO2 samples exhibit different Raman shift of the Mn-O stretching vibration mode. According to Hooke's law 51, the following equation was used to calculate the Mn-O bond force constant (k), which represents the Mn–O bond strength.

?=

@

2

B &A= C

(5)

where ω is the Raman shift (cm-1), c is light velocity, µ is effective mass. As shown in the inset of Figure 6, the Mn-O force constant (k) decreases in the sequence α-MnO2-100 > α-MnO2-110 > α-MnO2-310, which corresponds well with the catalytic performance observed in Figure 4. This result reveals that the high active exposed facets always lead to a weakening of the Mn-O bonds, thus increasing the lattice oxygen reactivity and finally resulting in higher activity for HCHO. The XPS and Raman results are further confirmed by H2-TPR and O2-TPD. H2-TPR experiments were performed to investigate the reducibility of the samples and distinguish the catalytic activity of surface adsorbed oxygen. With the increase of reduction temperature, the sample will undergo the reduction of surface adsorbed oxygen species, and the process of MnO2 → Mn2O3 → Mn3O4 → MnO 52. As shown in Figure 7a, the temperatures corresponding to the reduction process followed the order: α-MnO2-310 < α-MnO2-110 < α-MnO2-100. The α-MnO2310 shows the reduction peak at the lowest temperature, indicating the most mobile oxygen species both at the surface and in the bulk. Accordingly, the high oxygen mobility causes more oxygen to be adsorbed and further excited to active oxygen, which would then be involved in the reaction. In addition, the onset reduction temperature of the samples (the inset of Figure 7a) followed the order: α-MnO2-310 < α-MnO2-110 < α-MnO2-100. The easy reduction of the αMnO2-310 may be derived from the formation of more surface oxygen vacancies, thus greatly enhancing the mobility of oxygen species.


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Figure 7. (a) H2-TPR and (b) O2-TPD of α-MnO2 with different exposed facets.

The results of O2-TPD are displayed in Figure 7b. Generally, the O2-TPD spectra can be separated into three regions: low temperature (600 °C), which correspond to the release of surface oxygen molecules/active surface oxygen, sub-surface lattice oxygen and bulk lattice oxygen, respectively 53. In all regions, the oxygen desorption temperature follows the same order: α-MnO2-310 < α-MnO2-110 < αMnO2-100. Moreover, the α-MnO2-310 released largest amount of surface adsorbed oxygen species, as can be seen from the desorption peaks at 178 °C. Since oxygen molecules are usually adsorbed at the oxygen vacancies of an oxide material, it is reasonable to deduce that α-MnO2310 owned largest amount of oxygen vacancy and exhibited strong oxygen adsorption and activation capacity. 3.5 Surface energies and oxygen vacancy formation To further understand the difference of exposed facets of α-MnO2 and its effect on HCHO oxidation, we carried out the first-principle calculation using VASP program, based on densityfunctional theory (DFT). As shown in Figure 8, the {100} and {110} surface, with low surface energies of 0.58 and 0.60 J/m2, respectively, are thermodynamically stable facets. While, the


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high-index {310} facet exhibits the highest surface energies of 0.72 J/m2, is thermodynamically unstable. According to Wulff construction theory, these facets with high surface energy will vanish in the bulk of crystals as a result of minimizing the total surface energy of crystals. In the present case, the {100} and {110} facets have similar low surface energies, thus in most experiments on nanostructured α-MnO2 only {100} and {110} facets were clearly observed. However, due to higher stability, lower surface energy and less active sites, {110} and {100} facets are usually less reactive, displaying much poorer performance. It is generally considered that facets with a higher percentage of under-coordinated atoms are usually more reactive in heterogeneous reactions. And these reactive facets usually have a relatively high surface energy. Meanwhile, a high density of surface under-coordinated atoms does cause a high surface energy for the crystal facet. Thus, exposing high-energy facets to the surface, in the most cases, can effectively enhance their performance in practical applications. Among the three basic facets, {310} facets with the highest surface energy are usually considered to possess the best performance, which has been proven by the catalytic performance (Figure 4).


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Figure 8. The surface energy and formation energy of oxygen vacancy over α-MnO2 with different exposed facets. Small (red) spheres are oxygen and large (purple) are manganese.

Oxygen vacancy is suggested to be the active site for gaseous HCHO decomposition and its activity largely depends on the property and concentration of the oxygen vacancy 20. Therefore, we focus on the formation energies of a single O defect at the α-MnO2 surface, and the formation energies of oxygen vacancy obtained for each facet is shown in Figure 8. Theoretical studies have shown that the formation energy of oxygen vacancies for {310} (()* 310= 0.33 eV) is ACS Paragon Plus Environment

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much lower than that of {100} (()* 100= 1.10 eV) and {110} (()* 110= 0.96 eV), which means that oxygen vacancies are easier to form on {310} facets, i.e., more oxygen vacancies may be formed over the α-MnO2-310. Theoretical result is consistent with the above-mentioned XPS, Raman, H2-TPR and O2-TPD results. The lower oxygen vacancy formation energies at {310} facets demonstrate a mechanism for its better catalytic performance for HCHO oxidation. 3.6 Adsorption of oxygen and water molecule at the exposed facet surface According to Mars-van Krevelen mechanism, organic molecules adsorbed on the catalyst surface are oxidized by surface lattice oxygen, and the resultant oxygen vacancies are subsequently replenished by gas-phase O2. The strong adsorption of O2 over catalysts surface is favorable for catalytic oxidation of HCHO. Therefore, the adsorption energy of O2 with each surface slabs was calculated by DFT. As shown in Figure 9, the facets of {100} and {110} exhibit a similar adsorption capacity to O2 molecular with very high adsorption energy, indicating the adsorption of O2 over both facets are weak physical adsorption. However, the additional O2 is strongly bonded with the surface of {310} as the O-O (of adsorbed O2) bond distance is 1.24 Å. The longer bond length indicates that the bond of O2 can be more easily broken and activated. Therefore, the strong adsorption of O2 on {310} facets is beneficial to the quickly replenish of surface active oxygen species consumed by the oxidation of HCHO, which is favorable for catalytic oxidation of HCHO.


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Figure 9. The adsorption energy of O2 and H2O over α-MnO2 with different exposed facets. Small (red) spheres are oxygen, small (cyan) spheres are hydrogen and large (purple) are manganese.

Water not only widely exists in the atmosphere, but also exhibits co-catalysis effect in many chemical reactions. Our previous research found that the presence of water could strongly enhance the catalytic oxidation of HCHO

23

. Water will play two roles in HCHO removal, i.e.,

one is to promote the adsorption of HCHO via hydrogen bond, and the other is to compensate the consumed hydroxyl group on the catalyst surface and promote the further oxidation of HCHO intermediates into CO2

23, 54-57

. Therefore, the adsorption energy of water molecule with each

surface slabs is calculated and given in Figure 9. In general, the H2O prefers to bond with Mn atoms of all surface slabs, and the adsorption energies of {100}, {110} and {310} facet are -0.73, -0.47 and -0.8 eV, respectively. It shows that the {310} surface strongly interacted with H2O,


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and the additional H2O is strongly bonded with its surface as the Mn-O (of adsorbed H2O) bond distance is 2.15 Å. Especially, the short bond length can promote the transfer of more electrons from α-MnO2 {310} surface to H2O, resulting in the easy activation of H2O molecules. The easy adsorption and activation of H2O was favorable for the generation of surface hydroxyl, which can enhance the catalytic oxidation of HCHO. Due to the differences of the adsorption ability of water, the samples will reflect different water content. In order to verify the above assumptions, the weight loss and evolution of water molecules from three different samples were determined with TG. As shown in Figure S12, the weight loss of the three samples decreased dramatically as the temperature increased. The weight loss below 100 °C can reflect the surface water content, and the water content followed the order: α-MnO2-310 > α-MnO2-100 > α-MnO2-110, which is consist with the theoretical calculation result very well. Because of its higher-water content and better water adsorption ability, α-MnO2-310 showed excellent activity for HCHO oxidation.

4. CONCLUSION

In summary, we have demonstrated a facile hydrothermal route with the assistance of capping role of oxalate to fabricate single-crystalline α-MnO2 nanowires with exposed highindex {310} facets on its side walls. The resulting α-MnO2 with exposed {310} facets exhibited superior activity for the oxidation of HCHO, which could completely transform 100 ppm HCHO into CO2 at 60 ℃ under the GHSV of 90 L/gcat.h and achieving 86 % conversion of 0.53 mg/m3 HCHO at room temperature under the GHSV of 600 L/gcat.h. The experimental characterization and theoretical calculations all confirm that the {310} facets with high surface energy are favorable for the formation of oxygen vacancies. Exposure of {310} facet can induce the generation of more oxygen vacancies, which favors the formation of surface active oxygen


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species and thus enhances the catalytic activity. Besides, the DFT simulations clearly show that the exposed {310} facets are beneficial to the adsorption and activation of O2 and H2O, which facilitates the enhancement of HCHO oxidation activity. These findings could provide a facile and effective strategy for exposing the reactive facets and deep understanding of facet-dependent catalytic activity of α-MnO2.

ASSOCIATED CONTENT

Supporting information Figure S1, XRD pattern of the samples prepared with different reaction precursor; Figure S2, TEM and HRTEM images of samples prepared with different reaction precursor; Figure S3, XRD pattern of the samples prepared with different KMnO4/(NH4) C2O4 ratios; Figure S4, SEM images of the samples prepared with different KMnO4/(NH4) C2O4 ratios; Figure S5, temperature-dependent of HCHO conversion over the samples prepared with different KMnO4/(NH4) C2O4 ratios; Figure S6, XRD pattern of the samples prepared with different reaction temperature; Figure S7, SEM images of the samples prepared with different reaction temperature; Figure S8, temperature-dependent of HCHO conversion over the samples prepared with different reaction temperature; Figure S9, XRD pattern of the samples prepared with different reaction time; Figure S10, SEM images of the samples prepared with different reaction time; Figure S11, temperature-dependent of HCHO conversion over the samples prepared with different reaction time; Figure S12, thermogravimetry (TG) curves of the different samples.

AUTHOR INFORMATION

Corresponding Author


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* E-mail: [email protected] (P.Y.Z.). Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT

This work was financially supported by National Natural Science Foundation of China (Nos. 21677083, 21521064) and Suzhou-Tsinghua innovation guiding program (No. 2016SZ0104). The theoretical calculation is completed on the “Explorer 100” cluster system of Tsinghua National Laboratory for Information Science and Technology.

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(20) Zhu, L.; Wang, J. L.; Rong, S. P.; Wang, H. Y.; Zhang, P. Y. Cerium Modified Birnessitetype MnO2 for Gaseous Formaldehyde Oxidation at Low Temperature. Appl. Catal., B: Environ. 2017, 211, 212-221. (21) Wang, J. L.; Li, J. G.; Jiang, C. J.; Zhou, P.; Zhang, P. Y.; Yu, J. G. The Effect of Manganese Vacancy in Birnessite-type MnO2 on Room-temperature Oxidation of Formaldehyde in Air. Appl. Catal., B: Environ. 2017, 204, 147-155. (22) Wang, J. L.; Zhang, G. K.; Zhang, P. Y. Layered Birnessite-type MnO2 with Surface Pits for Enhanced Catalytic Formaldehyde Oxidation Activity. J. Mater. Chem. A. 2017, 5, 5719-5725. (23) Rong, S. P.; Zhang, P. Y.; Yang, Y. J.; Zhu, L.; Wang, J. L.; Liu, F. MnO2 Framework for Instantaneous Mineralization of Carcinogenic Airborne Formaldehyde at Room Temperature. ACS Catal. 2017, 7, 1057-1067. (24) Chen, H. M.; He, J. H.; Zhang, C. B.; He, H. Self-Assembly of Novel Mesoporous Manganese Oxide Nanostructures and Their Application in Oxidative Decomposition of Formaldehyde. J. Phys. Chem. C. 2007,111, 18033-18038. (25) Lu, L.; Tian, H.; He, J. Hui.; Yang, Q. W. Graphene-MnO2 Hybrid Nanostructure as a New Catalyst for Formaldehyde Oxidation J. Phys. Chem. C. 2016, 120, 23660-23668. (26) Hou, J. T.; Liu, L. L.; Li, Y. Z.; Mao, M. Y.; Lv, H. Q.; Zhao, X. J. Tuning the K+ Concentration in the Tunnel of OMS-2 Nanorods Leads to a Significant Enhancement of the Catalytic Activity for Benzene Oxidation. Environ. Sci. Technol. 2013, 47, 13730-13736. (27) Kresse, G.; Furthmüller. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. J. Phys. Rev. B. 1996, 54, 11169-11186. (28) Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector Augmented-Wave Method. Phys. Rev. B. 1999, 59, 1758-1775.


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Graphical Table of Content


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Figure 1. XRD patterns of α-MnO2 with different exposed facets. 261x183mm (300 x 300 DPI)


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Figure 2. SEM images of (a, b) α-MnO2-310, (c, d) α-MnO2-110 and (e, f) α-MnO2-100. 216x243mm (300 x 300 DPI)


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Figure 3. TEM and HRTEM images of (a-c) α-MnO2-310, (e-g) α-MnO2-110 and (i-k) α-MnO2-100; Atomic structure of the (d) {310}, (h) {110} and (l) {100} surface of α-MnO2. Small (red) spheres are oxygen and large (purple) are manganese. 319x170mm (300 x 300 DPI)


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ACS Catalysis

Figure 4. (a) Temperature-dependent of HCHO conversion over α-MnO2 with different exposed facets (HCHO concentration = 100 ppm, 21%O2, N2 as balance gas, RH = 70%, GHSV= 90 L/gcat·h). (b) Arrhenius plots for HCHO oxidation over α-MnO2 with different exposed facets. (c) Temperature-dependent of HCHO conversion over α-MnO2-310 under the different GHSV (HCHO concentration = 100 ppm, 21%O2, N2 as balance gas, RH = 70%). (d) Stability test of the α-MnO2-310 under 60 ℃ (HCHO concentration = 100 ppm, 21%O2, N2 as balance gas, RH = 70%, GHSV= 90 L/gcat·h). (e) Performance of the α-MnO2-310 at environmental situations. (HCHO concentration = 0.53 mg/m3, GHSV= 600 L/gcat·h, RH = 55%, room temperature) 261x183mm (300 x 300 DPI)


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Scheme 1. The effects of synthesis parameters on the formation of α-MnO2 and its exposed facets. 316x170mm (300 x 300 DPI)


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ACS Catalysis

Figure 5. XPS spectra of α-MnO2-100, α-MnO2-110 and α-MnO2-310: (a) survey spectra, (b) Mn 2p3/2, (c) Mn 3s, and (d) O 1s. 261x184mm (300 x 300 DPI)


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Figure 6. Raman spectra over α-MnO2 with different exposed facets. The inset shows the corresponding MnO bond force constant of α-MnO2 with different exposed facets. 261x183mm (300 x 300 DPI)


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ACS Catalysis

Figure 7. (a) H2-TPR and (b) O2-TPD of α-MnO2 with different exposed facets. 261x92mm (300 x 300 DPI)


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Figure 8. The surface energy and formation energy of oxygen vacancy over α-MnO2 with different exposed facets. Small (red) spheres are oxygen and large (purple) are manganese. 169x199mm (300 x 300 DPI)


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ACS Catalysis

Figure 9. The adsorption energy of O2 and H2O over α-MnO2 with different exposed facets. Small (red) spheres are oxygen, small (cyan) spheres are hydrogen and large (purple) are manganese. 254x151mm (300 x 300 DPI)


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Graphical Table of Content

420x235mm (300 x 300 DPI)


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Engineering Crystal Facet of Î±-MnO2 Nanowire for Highly Efficient

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