Low-Temperature Benzene Abatement over Active Manganese

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General Research

Low-Temperature Benzene Abatement over Active Manganese Oxides with Abundant Catalytic Sites Ke Xie, Dongrun Xu, Chao Li, Xiaona Liu, Xiaolei Hu, Zhen Ma, Xingfu Tang, and Yaxin Chen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b03370 • Publication Date (Web): 22 Aug 2019 Downloaded from pubs.acs.org on August 25, 2019

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Industrial & Engineering Chemistry Research

Low-Temperature Benzene Abatement over Active Manganese Oxides with Abundant Catalytic Sites Ke Xie,† Dongrun Xu,† Chao Li,† Xiaona Liu,† Xiaolei Hu,† Zhen Ma,† Xingfu Tang,†,§ Yaxin Chen†,* † Department § Jiangsu

of Environmental Science & Engineering, Fudan University, Shanghai 200433, China

Collaborative Innovation Center of Atmospheric Environment & Equipment Technology,

Nanjing University of Information Science & Technology, Nanjing 210044, China

Corresponding author: e-mail: [email protected]

ACS Paragon Plus Environment

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Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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ABSTRACT Benzene is a typical volatile organic pollutant, and catalytic oxidation is promising for its abatement at low temperatures. Here we prepared tunnel-structured hollandite-type manganese oxide (HMO) nanoparticles with abundant catalytically active sites (CASs). The HMO nanoparticles are more active than a traditional noble-metal catalyst, namely, Pt/Al2O3. The former catalyst can achieve 100% benzene conversion at 200 °C at a very high space velocity of 120 000 mL gcat-1 h-1. Results from relevant physicochemical characterizations and reaction kinetics studies demonstrate that the CASs are located at the tunnel openings of the HMO{001} surfaces. These CASs not only provide surface lattice oxygen species active for benzene oxidation, but also efficiently activate O2 during catalytic process. Moreover, HMO nanoparticles also have more CASs than HMO nanorods, thus leading to higher activity in benzene oxidation. This work may assist in the rational design of active transition metal oxide catalysts for eliminating volatile organic pollutants (such as benzene) efficiently at low temperatures.


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INTRODUCTION Benzene, as a typical volatile organic compound, is often released from various industrial plants for solvent production and chemical synthesis, and it can also be emitted from the engines of vehicles.1-3 Benzene is a carcinogen.4 In addition, benzene has a great contribution to the atmospheric pollution, especially to the formation of secondary organic aerosols.5 Therefore, it is extremely necessary to control benzene emission, for the sake of both human health and environmental protection. One promising approach to abating benzene is to use solid catalysts to transfer benzene into H2O and CO2 in the presence of O2 via the Eq. C6H6(g) + 7.5O2(g) E 6CO2(g) + 3H2O(g). Apparently, efficient catalysts that can catalyze the complete oxidation of benzene at low temperatures are highly desirable. Noble metal catalysts are able to activate O2 by donating their free or quasi-free electrons (at or near the Fermi level of metals) to the anti-bonding molecule orbitals ( * and *) of O2,6 thus leading to high activities in benzene oxidation.7-10 However, the high costs of noble metals constitute a problem for industrial application. An appealing alternative is to develop transition metal oxide catalysts with active surface lattice oxygen active for cleaving stable delocalized H bonds and strong C–H bonds of benzene according to the Marsvan Krevelen (M-K) mechanism.11-13 Among transition metal oxides, manganese oxides, typically hollandite-type manganese oxides (HMO) with a (4.7 Å × 4.7 Å) tunnel structure,14 demonstrate high catalytic activity in low-temperature oxidation of VOCs such as benzene11,12,15 and toluene,12,15,16 because their surface lattice oxygen is highly mobile and reactive. Besides, the strong hydrophobic property and the size-suited tunnel openings allow HMO to adsorb benzene molecules selectively in the presence of other VOCs, and make the produced H2O desorbed easily,12,15,17 thus facilitating the direct attack of benzene molecules by surface active oxygen.


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Depending on the surface structures, the lattice oxygen species on different surfaces of HMO show different levels of reactivity. The lattice oxygen on the {001} facets is much more active than that on the {100} or {110} facets owing to highly active O species at the tunnel openings on the {001} facets.18,19 Thus, the tunnel openings on the HMO{001} facets have been identified as catalytically active sites (CASs) in catalytic oxidation of VOCs including benzene.7,20,21 However, HMO often has a rod-shaped morphology enclosed by four {100} sidefacets and two {001} top-facets with a significantly high aspect ratio,22,23 i.e., only a minority of the reactive {001} facets are exposed on the HMO surfaces owing to a relatively higher surface energy than that of the {100} facets.7,24 Therefore, one strategy to enhance the activity of HMO is to increase the number of CASs by increasing the percentage of the exposed {001} facets. Herein, we prepared HMO nanoparticles (NPs) to expose more CASs responsible for benzene oxidation. Firstly, the bulk and surface structures of HMO NPs were determined by using synchrotron X-ray diffraction (SXRD) and high-resolution transmission electron microscopy (HRTEM). Next, we investigated the catalytic performance of HMO NPs, HMO nanorods, -MnO2, and Pt/Al2O3 (a highly active supported noble metal catalyst) in benzene oxidation. Finally, by combining Mn K-edge and L3-edge X-ray absorption spectra, H2 temperature-programmed reduction (H2-TPR), and O2 temperature-programmed desorption (O2TPD) data, we correlated the structures of HMO with the catalytic performance in benzene oxidation. Compared with the reported literature,24,25 we herein preciously identified the active sites of benzene oxidation on HMO, and intensively investigated the intrinsic nature of the excellent activity of HMO, which could assist the rational design of transition metal oxide catalysts for the abatement of VOCs such as benzene.


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EXPERIMENTAL SECTION Catalyst Preparation. HMO NPs were synthesized by a solid phase reaction method.25 In brief, a mixed solid powder containing KMnO4 (0.8 g, 5.0 mmol) and MnCl2·4H2O (1.5 g, 7.5 mmol) was fully ground in a mortar for ~20 min to ensure full reaction. The solid obtained was washed with deionized water sufficiently, and dried at 80 °C overnight. HMO nanorods and -MnO2 were made via a hydrothermal method by reacting KMnO4 with MnSO4 at 160 oC for 24 h,26 and by reacting (NH4)2S2O8 with MnSO4 at 140 oC for 12 h,25 respectively, and the resulting black materials were filtered, washed with deionized water, and dried at 110 oC for 24 h. Finally, these three samples were calcined at 400 °C in air for 4 h before activity tests and various characterizations. Pt/Al2O3 with a Pt loading of 0.5 wt% was also synthesized by impregnating Al2O3 with a potassium tetrachloroplatinate solution, and the obtained powders were dried at 80 oC

overnight, and calcined at 350 oC for 5 h.

Catalyst Characterization. SXRD patterns were obtained at BL14B of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 0.6883 Å. Transmission electron microscope (TEM) and HRTEM experiments were carried out on a JEM 2100F transmission electron microscope. X-ray absorption near-edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) spectra were recorded at the Mn K-edge at BL14W of SSRF with an electron beam energy of 3.5 GeV and a ring current of 200-300 mA. Soft X-ray absorption spectra at the Mn L3-edge and O K-edge were collected in a total electron yield mode at BL 4B9B of Beijing Synchrotron Radiation Facility (BSRF). Core-level X-ray photoelectron spectroscopy (XPS) and valence-band XPS experiments were conducted at BL 4B9B in the BSRF at a photon energy of 780 eV and 100 eV, respectively. All the data were obtained in an ultrahigh vacuum chamber equipped with a VG Scienta R4000 electron energy analyzer with a


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base pressure of

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4 × 10U mbar at room temperature. H2-TPR and O2-TPD experiments were

performed on an AutoChem1 II 2920 adsorption instrument (Micromeritics, USA) with a TCD. After pretreatment at 200 oC in 30 mL min-1 flow of He to remove surface weakly physically adsorbed substances, H2-TPR experiments were conducted in a 50 mL min-1 flow of 10 vol% H2 in Ar at a ramping rate of 10 °C min-1. For O2-TPD experiments, after saturation absorption of O2 at 50 oC in a 50 mL min-1 flow of 10 vol% O2 in He, the sample was firstly purged with He (50 mL min-1) and then conducted in a 50 mL min-1 flow of He at a ramping rate of 10 °C min-1. The contents of potassium, manganese and platinum were determined by X-ray Fluorescence Spectrometer (XRF) on a Bruker-AXS S4 Explorer, as listed in Table S1. Catalytic Evaluation. A fixed-bed quartz reactor was used to evaluate the catalysts for benzene oxidation under atmospheric pressure. The catalyst (50 mg, 40-60 mesh) was loaded for each run and was pretreated by 100 mL min-1 N2 at 200 oC for 0.5 h. The feed gas contained 40 ppm benzene, 20 vol% O2, and balance N2, and the total flow rate was 100 mL min-1 to get a high space velocity (SV) of 120 000 mL gcat-1 h-1. The reaction temperature increased from room temperature to 240 oC at a ramping rate of 5 oC min-1. Effluents were analyzed automatically with an Agilent 7890A gas chromatograph equipped with TCD and FID detectors. Reaction kinetics of benzene oxidation over HMO NPs and HMO nanorods were studied at 130 °C by controlling benzene conversion (XC6H6) to be below 20%. Benzene concentrations were set in the range of 40-70 ppm, and O2 concentrations were in the range of 2000-8000 ppm.

RESULTS AND DISCUSSION Structure and Morphology. The structures of HMO NPs and HMO nanorods were determined by SXRD (Figure 1). Both the diffraction patterns are indexed to Hollandite-type MnO2 with a tetragonal crystal structure (JCPDS-291020) and 4.7 Å × 4.7 Å square tunnels along the [001]


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axis.27 Furthermore, atomic occupancy and crystallographic parameters obtained from Rietveld refinement (Tables S2 and S3) indicate that both samples have almost the same structural features. Note that HMO often grows along the [001] direction and predominantly exposes four {100} side-facets with a lower surface energy, and the exposed percentage of two {001} topfacets with a higher surface energy is extremely low.22-26 According to the Scherrer’s equation, =

, where Dhkl and . represent the grain size in the normal direction and the full

widths at half maxima of the crystal (hkl) plane, respectively, Dhkl is inversely proportional to .. Thus, we can estimate the aspect ratios of both HMO samples by calculating the . ratios of the (002) or (200) peaks of both samples. As seen in the insets of Figure 1, a . ratio of the (002) peaks of HMO NPs to HMO nanorods is

1.5, higher than their (200) . ratio ( 1.0). This

observation suggests that HMO NPs have a significantly smaller aspect ratio than HMO nanorods, implying that HMO NPs expose a higher proportion of high-surface-energy {001} surfaces than HMO nanorods.


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(a)

HMO NP NP

(

)200

=1

rod

4

Intensity (10 )

1.0

( 0.5

NP

)002

= 1.5

rod

0.0

(b)

0.5

(002)

(411)

(521)

HMO nanorod

(301)

(310) (220)

(200)

(110)

4

1.0

(211)

1.5

Intensity (10 )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.0 -0.5 5

10

15

20

25

30

o

2 () Figure 1. SXRD patterns and Rietveld refinement analyses of HMO NPs (a) and HMO nanorods (b), showing the (002) and (200) . ratios of HMO NPs to HMO nanorods. The red dots are the experimental data and the black curves are the corresponding fitting data. The differential SXRD patterns are shown by the blue curves. The peak positions of all possible Bragg reflections are marked by the short vertical lines below the SXRD patterns.


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has a smaller aspect ratio. An enlargement in Figure 2d shows the structure of the typical (001) surface, where the regular white dots represent the tunnel structures, as judged from the structure feature and a corresponding model in the inset of Figure 2d. Hence, HMO NPs expose the abundant tunnel openings with a size of approximately 4.7 Å × 4.7 Å on the surfaces. The same results were obtained in the EXAFS spectrum of Figure S1. The amplitude due to the Mn-Mn shell in the Mn K-edge EXAFS spectrum of HMO NPs is weaker than that of HMO nanorods, suggesting that the coordinate number of the Mn-Mn shell of HMO NPs is smaller than that of HMO nanorods. These results manifest that HMO NPs have an average size smaller than HMO nanorods. Thus, the relatively small aspect ratio of HMO NPs allows the abundant tunnel openings to be exposed on the surfaces.

Catalytic Performance and Active Sites. Low-temperature oxidation of benzene (90~240 oC) was conducted over both HMO samples and Pt/Al2O3. As shown in Figure 3a, HMO NPs exhibit higher catalytic activity than HMO nanorods and Pt/Al2O3 under identical reaction conditions. The onset reaction temperatures (Tr) for benzene oxidation over HMO NPs is as low as 90 °C, and 100% XC6H6 is achieved at Tr =

200 °C, 40 oC lower than Tr ( 240 oC) required for 100%

XC6H6 over HMO nanorods and Pt/Al2O3. In the kinetic regime, HMO NPs shows XC6H6 of 13.6% at 120 oC, approximately four times as XC6H6 ( 3.6%) achieved over HMO nanorods. The catalytic stability of HMO NPs was also evaluated at 210 °C (Figure S2). The benzene conversion remained as high as 95% during the test lasting over 20 hours. There are potentially two reasons for the enhanced catalytic performance: the change of the nature of CASs and the increase of the number of CASs. In order to investigate which reason is possible, we studied the reaction kinetics at XC6H6 < 20%. As shown in Figure 3b, the Arrhenius profiles of benzene oxidation were plotted and the corresponding activation energies (Ea) were


10

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170 kJ mol-1, consistent with

calculated. Ea for benzene oxidation over Pt/Al2O3 is as high as

the previously reported values for this kind of catalysts,28 whereas a much lower energy barrier (Ea =

88 kJ mol-1) is required for benzene oxidation over both HMO samples. Thus, benzene

oxidation over HMO and Pt/Al2O3 may follow different reaction mechanisms. On the other hand, both HMO samples give the same Ea value (88 kJ mol-1), implying that benzene oxidation over both HMO samples follows a same mechanism in view of their same surface structures (Figure 2). In Figure S3, the reaction orders (k) of benzene and O2 for HMO NPs are -1 and 0.1, respectively, the same as the corresponding k values for HMO nanorods, implying that the nature of CASs of HMO NPs and HMO nanorods is same. Therefore, the different catalytic activity should originate from the number of CASs. The effects from the external and internal diffusion were eliminated by varying the linear velocity of the feed gas and the size of catalysts’ particles, respectively (Figure S4). (a)

100

HMO NP HMO nanorod Pt/Al2O3

80 60 40

6

6

XC H (%)

-MnO2

20 0 100

140

180

220

o

T ( C) (b) 140 10

R ( mol gcat-1s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


130

nano ±8k

Ea = 88

rod

J mo

l

1

E

a

0.1 2.40

120

HM O

HM O Ea = 88

T (oC)

2.45

=1

110

NP

±6k

J mo

l -1

-1

P t/

Al

70

±9

2.50

2O 3

kJ

mo

l -1 2.55

1000/T (K-1)


2.60

11


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Figure 3. (a) XC6H6 as a function of reaction temperature (T) over HMO NPs, HMO nanorods, Pt/Al2O3, and -MnO2. Reaction conditions: Benzene 40 ppm, O2 20 vol%, and balance N2; gas flow rate 100 mL min-1, SV 120 000 mL gcat-1 h-1. (b) Arrhenius plots for the reaction rates (R) in benzene oxidation and the corresponding Ea over the samples.

The value of the pre-exponential factor ( ) in the Arrhenius equation are positively proportional to the number of CASs.29 In this work, 3.0×1012, approximately four times as that ( =

of HMO NPs was calculated to be

7.1×1011) of HMO nanorods, and this ratio

almost equals the ratio of the intrinsic reaction rates in the kinetic regime (Figure 3a). Apparently, HMO NPs have more CASs than HMO nanorods, consistent with the fact that HMO NPs expose the {001} facets more than HMO nanorods, as evidenced by TEM and SXRD data. Hence, CASs of benzene oxidation over HMO are located on the {001} surfaces. In Figure 2d, there are two kinds of tunnel openings (2.3 Å × 2.3 Å and 4.7 Å × 4.7 Å) on the {001} facet. The small tunnel openings with the size of 2.3 Å × 2.3 Å are too small to efficiently adsorb and activate a relatively big benzene molecule (5 Å),30 as confirmed by the extremely low catalytic activity of -MnO2 with the 2.3 Å × 2.3 Å tunnel openings (also shown in Figure 3a). Hou et al.11 modified the big tunnel openings with the size of 4.7 Å × 4.7 Å by anchoring K+ on the centers of the tunnel openings, and found that the big tunnel openings modified by K+ altered the catalytic activity of HMO in benzene oxidation. By combining the literature and the above experimental evidence, we conclude that the big tunnel openings (which consist of the MnO6 octahedra and the K ions at the tunnel center) on the {001} facet are CASs, as modeled in the top-right inset of Figure 2d.


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Structure-Activity Relationship. Essentially, the nature and the number of CASs govern catalytic activity. The CASs of benzene oxidation are situated on the HMO{001} surfaces, which are composed of the big tunnel openings, closed by MnO6 octahedra and centered with K ions. As a consequence, the electronic structures and the numbers of Mn, O, and K ions of CASs play an important role in determining the catalytic performance in low-temperature benzene oxidation.

(b) 2.7 1.5

Intensity

(a) HMO NP HMO nanorod -MnO2

Intensity

MnSO4 1.0

0.5

Intensity

(E )

KMnO4

Normalized

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.0

1.8

HMO NP 2nd derivative

t2g

eg

0.9 0.0 1.8

HMO nanorod 2nd derivative

0.9 0.0 1.8

-MnO2 2nd derivative

0.9 0.0

6540

6550

6560

6570

639

Absorption Energy (eV)

642

645

648

Absorption Energy (eV)

Figure 4. (a) Mn K-edge XANES spectra of both HMO samples with three references. (b) Mn L3-edge X-ray absorption spectra and the second-derivative spectra.

Firstly, we investigate the electronic property and the number of surface Mn cations by using Mn K-edge XANES spectra and L3-edge X-ray absorption spectra, and core-level and valenceband XPS. Figure 4a shows the Mn K-edge XANES spectra of two HMO samples with three reference samples to determine the oxidation states of Mn. Often, the positions of white lines shift to higher energy with the increasing Mn oxidation states.31 Here the white lines of both


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HMO samples are located between -MnO2 (Mn4+) and MnSO4 (Mn2+), indicating the presence of Mn3+ in both cases. The average oxidation states of Mn of HMO NPs and HMO nanorods are +3.7 and +3.9, respectively, as judged from the curve of the oxidation states as a function of the white line in energy (Figure S5), which indicates that the former contains more Mn3+ than the latter. This agrees fairly well with the results of Mn core-level 2p XPS (Figure S6).25 According to the structure and the morphology features of both HMO samples, it is convincing that Mn3+ are mainly located on the {001} facets (which are part of CASs). The standard reduction potential (2 ) of Mn3+/2+ is 1.54 V, higher than that (23 = 1.22 V) of Mn4+/2+,32 and hence the presence of more Mn3+ species means the stronger oxidation ability towards benzene molecules, i.e., the Mn3+ species can accept the electrons transferred from carbon atoms of benzene molecule more readily. On the basis of the molecular orbital theory,33 the gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) directly determines the rate of the electron transfer between benzene molecules and CASs. To obtain information about the LUMO and HOMO of manganese ions, we collected Mn L3-edge soft X-ray absorption spectra and valence-band XPS, respectively. In Figure 4b, two peaks of three samples can be assigned to the 2p3/2Et2g and 2p3/2Eeg transitions, respectively.34 By considering the octahedral symmetrical geometry of Oh and in a high-spin electron configuration,35 the eg orbitals can be regarded as the LUMO of manganese ions for accepting electrons during benzene oxidation. From the second-derivative spectra of the samples in Figure 4b, the LUMO of HMO shifts down in energy of

0.6 eV with comparison to -MnO2, and similarly, the down-shifted HOMO of

HMO is also detected by using the valence-band XPS (Figure S7). We recently evidenced that the down-shifted LUMO of HMO is favorable for accepting electrons.25 Therefore, CAS of HMO containing Mn3+ with the down-shifted LUMO can efficiently activates benzene molecule,


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and one of the reasons for the higher activity of HMO NPs should be due to the presence of more Mn3+ on HMO surface. Next, we study the activity of the surface lattice oxygen of HMO by using XPS and H2TPR. Figure S8 depicts O 1s XPS of both HMO samples, where three kinds of surface oxygen species are detected according to the deconvoluted curves of their O 1s XPS. Three peaks with binding energies (BEs) centered at about 529, 531, and 533 eV are due to surface lattice oxygen (Olat), surface defect oxygen (Osd), and surface hydroxyls (OH-), respectively,36,37 and their relative abundances are calculated and listed in Table S4. The Osd abundance of HMO NPs is 38%, 7% larger than that of HMO nanorods, which is consistent with the presence of more surface Mn3+ of HMO NPs, as confirmed by the deconvoluted Mn 2p XPS in Figure S6.25 Such a high Osd abundance should result from the smaller aspect ratio of HMO NPs compared with that of HMO nanorods (Figures 1 and 2). To shed light on the difference of the surface oxygen species on the HMO{001} facets from that on HMO{100} facets, we used H2 as a probing molecule to investigate the oxidation ability of the two kinds of surface oxygen species with a temperature-programmed procedure (Figure S9), and the corresponding H2-TPR profiles in the low-temperature regimes are shown in Figure 5. Both HMO samples have a similar reduction property except for a slightly low-temperature shift of the reduction peak for HMO NPs, and the main reduction peak at T = 270-370 oC can be attributed to the reduction of the bulk MnO2 to MnO (Figure S9).21 A shoulder at the low temperature regime is discernible in both cases, due to the reduction of active surface oxygen species or Osd.21 The shoulder was deconvoluted into two reduction peaks at

160 and

210 oC

via curve-fitting, and the atomic percentages of the surface oxygen species with respect to the total lattice oxygen of the samples were calculated (Table S5). The data were calculated from the


15


amount of the corresponding consumed H2 according to the Eq. K0.6Mn8O16(s) + 7.7H2(g) 0.3K2O(s) + 8MnO(s) + 7.7H2O(g).21 In Table S5, the ratios of the Osd species are

2.5%,

approaching the reported value (3%) of surface active oxygen.17 In addition, two ratios of the peak areas of the reduction peak at 160 oC to the reduction peak at 210 oC are estimated to be 0.4 and 0.2 for HMO NPs and HMO nanorods, respectively. Considering the different aspect ratios of both HMO samples, two reduction peaks at

160 and

210 oC can be assigned to the

reduction of the Osd species of the {001} top-facets and {100} side-facets, respectively. Therefore, HMO NPs can provide much more active surface oxygen species to attack benzene molecules in catalytic oxidation than HMO nanorods. (a) HMO NP

TCD Signal

Osd(001) Osd(100)

= 0.4

Osd(100)

Osd(001) 0.7%

1.8%

(b) HMO nanorod Osd(001)

TCD Signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Osd(100)

Osd(100)

Osd(001)

2.0%

0.4% 120

160

= 0.2

200

240

280

o

T ( C) Figure 5. H2-TPR data of HMO NPs (a) and HMO nanorods (b) in the low temperature regime. The amount of the Osd species, and the ratios of the Osd species of the {001} top-facets to those of the {100} side-facets are also shown.


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Finally, we investigate the activation ability of CASs towards O2. According to the M-K mechanism, the consumed surface active oxygen generates surface oxygen vacancy (Vo), which needs to be replenished by O2 to finish the redox cycle. We carried out O2-TPD experiments to study the activation of O2 after saturate adsorption. In Figure 6, there are four peaks in the O2TPD profiles for both HMO samples after curve-fitting, and their abundances with respect to the total lattice oxygen of the samples are calculated and listed in Table S6 by assuming that a reaction Eq. 3K0.6Mn8O16(s)

0.9K2O(s) + 8Mn3O4(s) + 7.55O2(g) occurred during the O2-TPD

process at the temperature window of 25-700 oC.38 The low-temperature desorption peak started at

80 oC for both cases, and reached a maximum at

140 oC. These activated oxygen species

should be associated with the low-temperature activity of HMO in benzene oxidation in Figure 3a. As reported in our recent work,36 the potassium atoms at the HMO tunnel openings have high electronic density, and they are energetically favorable for activating O2 by charge transfer from potassium to the antibonding orbitals (H* and ]*) of O2, thus leading to a strong promotional effect on the activation of oxygen in catalytic oxidation.34 As a consequence, the low-temperature peak can be attributed to the desorption of the active oxygen species adsorbed on the isolated potassium atoms at CASs. As listed in Table S6, HMO NPs with abundant CASs have more active oxygen species, which are adsorbed and activated on the potassium atoms of the centers of CASs, 40% more than that of HMO nanorods at low temperatures. Similarly, the ratios of the second peak to the third peak in the O2-TPD profiles are determined to be

0.4 and

0.2 for HMO NPs and HMO nanorods, respectively, in good

accordance to the ratios obtained from the H2-TPR profiles. Thus, we deduce that the second and third peaks can be due to the oxygen evolution from the {001} top-facets and the {100} sidefacets. Note that by comparing the results of the H2-TPR profiles with the results of the O2-TPD


17


profiles of HMO NPs, the ratio of surface active oxygen species on the {001} facets is 5.6% after O2 adsorption, much higher than that (0.7%) before O2 adsorption. This strongly indicates that the {001} facets have a strong ability towards activating O2 and transferring the activated O2 into surface active lattice oxygen. The results above demonstrate that CASs on the HMO{001} facets can not only provide surface active lattice oxygen for benzene oxidation, but also efficiently activate O2 at low temperatures to replenish the Vo produced during the oxidation process to finish the catalytic cycles. Therefore, more abundant CASs of HMO NPs are responsible for its higher low-temperature activity than HMO nanorods. (a) TCD Signal

HMO NP Fit O(001) O(100)

= 0.4 O(100) O(001)

(b) HMO nanorod Fit

TCD Signal

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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O(001) O(100)

= 0.2 O(100) O(001)

150

300

450

600

o

T ( C) Figure 6. O2-TPD profiles of HMO NPs (a) and HMO nanorods (b). O(001) and O(100) refer to the oxygen evolution from the {001} top-facets and the {100} side-facets, respectively.

By combining the above evidence, we propose a possible reaction mechanism for low-


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temperature benzene oxidation. Firstly, the surface Mn3+ ions of the HMO’s CASs with the down-shifted LUMO can favorably accept the electrons transferred from benzene molecules, leading to the activation of benzene. Next, the surface lattice oxygen species of the HMO can readily react with the activated benzene molecules to produce CO2 and H2O, concomitant with the generation of the Vo. Finally, the potassium atoms with the high electron density at the centers of the CASs dissociate O2 so as to replenish the Vo and finish the redox cycle. From the dynamic reaction viewpoint, the electrons accepted by the Mn ions are easily transferred to the potassium atoms of CASs through the bridging oxygen atoms,34 thus increasing the electron density of the potassium atoms, which facilitate the activation of molecular oxygen. These factors are responsible for of the fact that HMO is more active .-MnO2 in benzene oxidation. Therefore, HMO NPs with abundant CASs exhibit excellent catalytic activity in benzene oxidation owing to the abundant active sites on the (001) facets and the specific electronic structure of the CASs, from which a high turnover frequency (TOF) of 6 × 10-3 s-1 can be achieved at 120 oC, much higher than the reported values (TOF = 5 × 10-4 s-1) over noble metal catalysts.39 This work could provide a strategy to rationally design transition metal oxide catalysts with high activity for efficiently controlling the emissions of volatile organic pollutants such as benzene at low temperatures. In summary, we have demonstrated that HMO nanoparticles with rich {001} facets are much more active than HMO nanorods in catalyzing benzene oxidation. This work demonstrates that the structures of the catalysts can influence the number of exposed active sites and thus catalytic activity. Although here we chose benzene oxidation to compare the activity, we found that the same catalyst with rich {001} facets also shows the high catalytic performance in ozone


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decomposition24 and selective catalytic reduction of NO by NH3.25 This elucidates that our result in this work is valid in a widely variety of catalytic reactions. ASSOCIATED CONTENT Supporting Information. Some related tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *(Y. C.) Phone: +86-21-31248935; e-mail: [email protected]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the NSFC (21777030) and National Engineering Laboratory for Mobile Source Emission Control Technology (NELMS2018B02). The SXRD patterns, the XPS and the X-ray absorption spectra were collected at SSRF and BSRF.

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Low-Temperature Benzene Abatement over Active Manganese

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