on Ce-Doped Manganese Oxide Octahedral Molec

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 3

Insight into Low-Temperature Catalytic NO Reduction with NH on Ce-Doped Manganese Oxide Octahedral Molecular Sieves Xiaomin Wu, Xiaolong Yu, Xinyang He, and Guohua Jing

J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b01048 • Publication Date (Web): 05 Apr 2019 Downloaded from http://pubs.acs.org on April 5, 2019

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The Journal of Physical Chemistry

Insight into Low-Temperature Catalytic NO Reduction with NH3 on Ce-Doped Manganese Oxide Octahedral Molecular Sieves Xiaomin Wu,1,* Xiaolong Yu, 1 Xinyang He, 1 and Guohua Jing. 1 1 Department of Environmental Science & Engineering, College of Chemical Engineering, Huaqiao University, Xiamen, Fujian 361021, P. R. China.

Abstract: Oxygen vacancy defect (OVD) as one of the crystal defects is critical to the reactivity of heterogeneous catalysts surface for catalytic NO reduction with NH3 (NH3-SCR). Here, we propose a simple and feasible method to enhance OVDs through Ce doping manganese oxide octahedral molecular sieves (Ce-K-OMS-2), which was proven that Ce mainly entering the framework structure of K-OMS-2. The robust NH3-SCR catalytic test showed that the presence of Ce could lower the catalytic temperature and activity for NO conversion (~100%) across a wide temperature span of 140 °C to 230 °C. The characterization results indicated the distortion of octahedral units after Ce substitution and the subsequent introduction of OVDs. Moreover, we demonstrated by density functional theory (DFT+U) calculations that the unique nanostructure of Ce doping K-OMS-2 with OVDs in the framework can alter the local bond lengths and the density of states (DOS), thereby obviously accelerating the low-temperature NH3-SCR. NH3

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prefers to be adsorbed at the Ce site of Ce-K-OMS-2 while NO is much more likely to adsorb on the O site of Ce-K-OMS-2, following a Langmuir-Hinshelwood mechanism.

1. Introduction As a major air pollutant, nitrogen oxides (NOx, x=1 or 2) can cause serious environmental problems such as acid rain, ozone depletion, heavy haze and photochemical smog.1,

2

The

selective catalytic reduction (SCR) with NH3, as a conventional technique, has been widely used in the control of NOx emissions from stationary sources due to its high efficiency and SO2 tolerance. The most commonly used commercial V2O5-WO3/Mo2O3-TiO2 catalysts show good activity and selectivity at high temperatures (300-400oC).3, 4 However, owing to poor activity in the low temperature range, the SCR reactors can’t be located in the later portions of the dust collection and desulfurization devices in some power plants, sintering plants and gas turbines. Therefore, the desire to develop catalysts in this low temperature range has grown increasingly urgent. Generally, Mn based oxide catalysts, such as MnOx-CeO2,5 MnOx/TiO2,6, 7 Mn-Ce/Al2O3,8 and MnOx/carbon,9 are suitable for the low-temperature NH3-SCR system. In particular, cryptomelane-type manganese oxide octahedral molecular sieves (K-OMS-2) show strong adsorption and activation abilities with NH3 and NO.10, 11 K-OMS-2 is a porous manganese oxide with a composition of KMn8O16 and a structure consisting of 4.6 Å × 4.6 Å tunnels, due to a 2 × 2 arrangement of edge-shared MnO6 octahedra. Potassium ions reside in the tunnels to maintain the stability and charge balance on the structure of K-OMS-2.12 To further improve the catalytic activity, K-OMS-2 is usually doped by foreign metal ions, such as Co2+, Cu2+, Fe3+ and V5+.13, 14 The metal dopants can be substituted for K+ in the tunnel of K-OMS-2, or incorporated into the framework. Among them, cerium, with a redox cycle between the Ce4+ and Ce3+ states, can be


2

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manipulated to create efficient catalysts. The high activity of Ce doping into K-OMS-2 observed is usually attributed to the presence of Mn in high oxidation states (Mn4+/Mn3+) and oxygen vacancy defects (OVDs), which has been applied in CO oxidation,15 degradation of ciprofloxacin16 and removal of VOCs.17 However, to the best of our knowledge, the inclusion of Ce in the framework of K-OMS-2 (denoted as Ce-K-OMS-2) material for low-temperature NH3SCR and mechanism has not been reported. More importantly, we anticipate that Ce was substituted for Mn in the framework of K-OMS-2 can lead to the distortion of the octahedral units and an increase in OVDs, thereby enhancing the adsorption of NH3 and NO. In this work, Ce-K-OMS-2 was prepared with a reflux method and its performances for SCR were investigated. The effects of Ce doping on the morphology, structure and catalytic performance were systematically studied. Moreover, theoretical studies can complement these experimental observations. In particular, first-principles density functional theory (DFT+U) calculations can uncover the electronic and energetic structure-property relationships in these advanced materials. It was found that the surface OVDs significantly increased after the introduction of Ce into K-OMS-2. It was proven that the Ce-K-OMS-2 nanorods enhanced NH3 and NO adsorption compared to that of the pure K-OMS-2. Therefore, the effects of the structural properties of the Ce-K-OMS-2 nanorods, including OVDs, redox, and NO and NH3 adsorption properties, on their catalytic activity in the NH3-SCR were systematically investigated using both experimental results and theoretical calculations. 2. Experimental Section 2.1 Synthesis of catalysts The Ce-K-OMS-2 was prepared by using a reflux method.18 In a typical Ce-K-OMS-2 procedure, a solution of 0.4 M of KMnO4, 1.7 M MnSO4, a certain amount of Ce(NO3)3·6H2O


3


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and a 6.0 M solution of HNO3 were mixed and reacted under reflux at 100 °C for 24 h. The molar ratio between MnO4- and Mn2+ was fixed as 0.72-0.76. After the reaction, the sample was washed thoroughly with deionized water, and then dried at 100 °C overnight. The synthesized samples were denoted as K-OMS-2, Ce(0.06)-K-OMS-2, Ce(0.12)-K-OMS-2, Ce(0.24)-K-OMS2, and Ce(0.48)-K-OMS-2, where the number referred to the Ce/Mn molar ratio in the precursor solution of 0, 0.06, 0.12, 0.24 and 0.48, respectively. 2.2 Characterization X-ray photoemission spectroscopy (XPS) experiments were carried out on a Thermo Scientific Escalab 250Xi. The sample charging effects were compensated for calibrating all binding energies (BE) with the adventitious C 1s peak at 284.6 eV. The spectra were decomposed using XPSPEAK software. X-ray diffraction (XRD) patterns were measured on a Bruker D8 Advance diffractometer. Xray tubes were operated at 40 kV and 30 mA. The intensity data were collected during a 2θ range of 10-90°. The temperature-programmed reduction (H2-TPR) of H2 was performed on a chemi-sorption analyzer (Quanta chrome Chem BET & TPR/TPD), equipped with a thermal conductivity detector. For H2-TPR, 20-30 mg samples were pretreated with He at 300 °C for 1 h to remove the adsorbed carbonates and hydrates. After cooling down to room temperature and introducing a reduction agent of 5% H2/Ar with a flow rate of 50 mL/min, the temperature was then programmed to 900 °C, at a ramp of 10 °C/min. The specific surface areas were calculated from adsorbed nitrogen volume by an automatic volumetric apparatus, following standard Brunauer-Emmett-Teller (BET) theory.


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The morphology of the catalyst was observed by field emission scanning electron microscopy (FESEM, Hitachi SU8020) and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai G20). The K, Mn, Ce loading amounts of the catalysts were measured with an Optima 7000 DV inductively coupled plasma optical emission spectroscopy (ICP-OES). 2.3 Activity test The NH3-SCR reactions were performed in a fixed-bed quartz reactor (i.d. = 6 mm) under an atmospheric pressure. 200 mg of Ce-K-OMS-2 catalyst (40-60 mesh) was sandwiched between quartz wool layers in the tube reactor to charge for activity evaluation for each run. The feed gas was composed of 600 ppm NO, 600 ppm NH3, 6.0% O2 and balanced N2. The total flow rate was 600 mL/min. The reaction temperature was programmed from 100 °C to 300 °C. The concentrations of NO/NO2/NOx gases were determined by the flue gas analyzer (Testo 350). Finally, NO conversion was calculated using the following equation: NO conversion (%)  (1 

[NO]out )  100% [NO]in

(1)

where [NO]in and [NO]out represented the inlet and outlet concentration of NO, respectively. NO and NO2 were the only nitrogen containing oxides detected. For the SO2/H2O tolerance test, maintaining T = 150 °C, 200 mg catalyst was put into the reactor and the simulated real gas consisted of 600 ppm NO, 600 ppmNH3, 6.0% O2 and balanced N2; 10% H2O and 150 ppm SO2 were fed in for 150 min. 2.4 Computational details The K-OMS-2 and Ce-K-OMS-2 were modeled as follows, referring to the structural parameters from Suib’s reports.19 The K-OMS-2 primitive cell that belongs to the I4/m space


5


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group was first constructed. According to Cockayne’s work,20 K-OMS-2 is the K cation inside the 2×2 tunnels of α–MnO2, as shown in Figure 1(a, b). The Ce-doped K-OMS-2 model (1×1×2, K2Mn15CeO32) is built by replacing one of the Mn atoms with a Ce atom based on the K-OMS-2 model, as shown in Figure 1(c). The corresponding doping concentration by molar ratio is 6.25%. It should be noted that the outer electronic configurations of Mn, Ce and O atoms are 3d54s2, 4f15d16s2 and 2s22p4, respectively. Only Mn is a magnetic atom. Thereby, the magnetic arrangement of the Mn atoms is considered, and the ground-state magnetic structure of K-OMS2 is built:21 the Mn-Mn coupling between corner-sharing [MnO6] is antiferromagnetic, while the coupling between edge-sharing [MnO6] is ferromagnetic, as shown in Figure 1. Density functional theory (DFT) calculations were performed using CASTEP with ultrasoft pseudo potentials package provided by Materials Studio. The generalized gradient approximation (GGA) of the Perdew-Burke-Ernzerhof (PBE) functional was used to deal with the exchange correlation interactions. Kohn-Sham one-electron wave functions were expanded in a plane wave basis, with a high cutoff energy of 400 eV. K-point sampling was performed, using MonkhorstPack scheme, with a 2×2×4 sampling grid, based on convergence studies. A 3x3x4 MonkhorstPack mesh was used for density of state (DOS) calculations. A Gaussian smearing of 0.1 eV was used for the Fermi surface broadening. Relaxations of atomic positions and lattice vectors were performed until residualforces were 0.03 eV Å-1 or less. According to Zhou’s report,22 the (1 0 0) surfaces of K-OMS-2 facilitate the adsorption/activation of O2. To assess the effect of Ce-KOMS-2 on the catalytic performance of NH3-SCR, atomic slabs separated by 15 Å of vacuum are used to simulate the surface structure of K-OMS-2 and Ce-K-OMS-2 materials. For the property calculation, the parameter of on-site Coulomb interaction, U, was introduced to remove self-


6

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interaction errors. According to the suggestion of Duan and Song et al,21,

23

a Coulomb U

correction of 2.5 eV (Mn3d) and 5.0 eV(Ce4f) electrons is introduced.

Figure 1. The constructed 1×1×2 bulk of three materials: α-MnO2 (a), K-OMS-2 (b), Ce-KOMS-2 (c), oxygen vacancy formation on K-OMS-2 (d), and oxygen vacancy formation on CeK-OMS-2 (e) for calculation (purple (Mn), red (O), pink (K), and grey-white (Ce)). 3. Results and discussion 3.1 Characterization of catalysts To investigate the influence of Ce on the crystal structure and morphology, the corresponding X-ray diffraction (XRD) patterns are displayed in Figure 2. In all cases, the XRD patterns exhibited characteristic (110), (200), (310), (211), (301), (411), (600), (521), (002) and (541) reflections belonging to 2-theta angles 12.7, 17.9, 28.6, 37.4, 41.9, 49.9, 56.1, 60.2, 65.1 and 69. 6°, respectively, attributed to cryptomelane (KMn8O16, JCDPS 29-1020, tetragonal, I4/m). No indication of the other phase impurities attributed to manganese oxides. No diffraction


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peaks from Ce species were observed in Ce-K-OMS-2. It indicated that metal ions might be incorporated into the cryptomelane structure or exist as well dispersed metal oxides in low contents. With the increase of Ce doping, the intensity of the peaks decreased and the peak width broadened, indicating the interference of the crystallization of K-OMS-2 by Ce doping. The BET surface, pore volume and size of different amount of Ce doping in K-OMS-2 catalysts are listed in Table 1. The pure K-OMS-2 material with largest average pore size (38.7 nm) showed a low specific surface area (82 m2/g). The BET surface areas of Ce-K-OMS-2 catalysts first climbed, and then tended to be stable with the increase of Ce, indicating the introduction of Ce significantly modified K-OMS-2. The addition of Ce resulted in the formation of a poorly crystalline phase for the Ce-K-OMS-2 catalyst, introducing more defects, a lower pore size and enhancing the surface area. To confirm the position of Ce in the structure, the contents of K, Mn and Ce for all samples were determined by ICP. As is known, two other possibilities could be occurring during the nucleation and growth processes, where Ce species is found inside the structure of K-OMS-2 materials. Cerium ions could be substituted for K+ in the tunnel sites maintaining the structural integrity, which should result in a gradual loss in K+ with an increase in Ce concentration. On the other hand, the Ce species could compete with Mn in the framework of K-OMS-2 material. As shown in Table 1, the bulk K/(Mn+Ce) molar ratios of various Ce-K-OMS-2 catalysts were not notably decreased compared to the pure K-OMS-2 material. Meanwhile, the bulk Mole% Ce of different Ce-K-OMS-2 materials increased, whereas the Mn/(Mn+Ce) molar ratio decreased with an increase in the initial Ce content. Therefore, Ce resided in the framework sites rather than in tunnel sites, ascribing to Pauling’s coordination principle.24 In addition, bulk Mole% Ce of all materials were only ~2% despite the initial molar ratio of Ce in the reactants was higher than


8

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0.12. This is probably because the ionic radius of Cen+ (Ce4+=0.97 Å, Ce3+=1.14 Å) is larger than that of Mnn+ (Mn2+ = 0.83 Å, Mn3+ = 0.58Å, and Mn4+ = 0.53 Å).25 It would affect the properties as the substitution of Ce into the framework of K-OMS-2 structure during synthesis. Thereby, it can be concluded that it was not facile to substitute Ce for Mnn+ in the framework of K-OMS-2, although the increase of the initial preparation molar ratio of Ce largely led to a low content of Ce. In addition, Ce was partially incorporated into the K-OMS-2 lattice and replaced Mnn+ in the framework, ultimately resulting in the formation of small-sized particles with large surface areas. This process was particularly favored, leading to the formation of vacancy defects,15 as discussed in the coming sections. Table 1. Properties of varies Ce-K-OMS-2 samples. Sample

ICP

BET surface area

Total pore

Average pore

Mole% Ce

K/(Mn+Ce)

Mn/(Mn+Ce)

(m2/g)

volume(cm3/g)

size(nm)

K-OMS-2

0

0.08

1

82

0.79

38.7

Ce(0.06)-K-OMS-2

0.46

0.08

0.995

57

0.48

34.1

Ce(0.12)-K-OMS-2

1.86

0.11

0.980

136

0.61

17.9

Ce(0.24)-K-OMS-2

1.52

0.08

0.983

133

0.62

17.5

Ce(0.48)-K-OMS-2

2.54

0.11

0.972

137

0.69

16.5


9


(110) (200) ▼

▼

▼

(310) ▼

KMn8O16(K-OMS-2)

(211) ▼

(301) ▼

(411) ▼

(541) (600)(521) ▼ ▼ (002) ▼ ▼

Intensity (a.u.)

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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Ce(0.48)-K-OMS-2

Ce(0.24)-K-OMS-2 Ce(0.12)-K-OMS-2 Ce(0.06)-K-OMS-2 K-OMS-2

10

20

30

40

50

2 

60

70

80

90

Figure 2. XRD patterns of different Ce-K-OMS-2 catalysts. The FESEM images of Ce-K-OMS-2 samples with different content of Ce are illustrated in Figure 3. The pure K-OMS-2 displayed the typical fibrous morphology, with a length from 1 to 1.5μm and a diameter of about 10~30 nm as shown in Figure 3(a-b). The addition of Ce had little effect on the morphology. However, compared to pure K-OMS-2, the nanorod length of the catalysts was reduced with an increasing amount of cerium. Clusters were found between these short fibrous rods (Figure 3(c-f)). Further aggregations of fibrous rods with shorter length were observed on Ce(0.48)-OMS-2 to form structures resembling microspheres with rough surfaces (Figure 3(i-j)), which is attributed to the distortion of octahedral units after foreign metal substitution and the subsequent introduction of OVDs.26 Figure 4 shows HRTEM images of various Ce-K-OMS-2 nanorods. The TEM image confirmed that the Ce doped K-OMS-2 samples had a morphology of nanorods, as observed by FESEM. From the HRTEM image of one typical pure K-OMS-2 in Figure 4(a1-a3), well-defined lattice fringes were observed, and the fringe distance was approximately 0.50 nm, which corresponds to the interplane distance of (200) planes of K-OMS-2. For the Ce(0.06)-K-OMS-2


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material, it did not change the nanorods morphology and the lattice fringe was 0.50 nm as displayed in Figure 4(b1-b3). However, with an increase in the initial amount of Ce, the lattice fringes became complex and obscure. In the HRTEM image of Figure 4(c3), the Ce(0.12)-KOMS-2 sample had a poor crystal structure with more defects, as marked by the red arrows, and fringe distances approximately 0.69 nm, 0.50 nm, 0.31 nm and 0.24 nm which corresponds to the interplane distances of (110), (200), (310) and (210) planes of K-OMS-2, respectively, confirming the formation of a cryptomelane phase. Upon the further increase in the content of Ce, the nanorods became short and aggregated, with fringe distances around 0.69 nm and 0.24 nm, corresponding to (110), (211), as shown in Ce(0.24)-K-OMS-2 and Ce(0.48)-K-OMS-2. The MnO6 unit of K-OMS-2 was a relatively regular octahedron with average Mn-O bonds of 0.191 nm,27 while the coordination distance of Ce-O was 0.235 nm.28 The framework MnO6 octahedral units substituted by Ce should result in the production of point defects, such as OVDs, as illustrated in the corresponding model drawings in Figure 4. Hence, the perfect surface structures can be seen from the HRTEM image of pure K-OMS-2, while surface defect sites gradually increased with the increase of cerium concentrations, suggesting that OVDs become rich on the surfaces.10

a

c

e 200 nm

200 nm

1 um

1 um

200 nm

200 nm

200 nm

f

d

b

i

g

h 1 um

j

1um

1 um

Figure 3. FESEM images of the Ce-K-OMS-2 catalysts. (a-b: K-OMS-2, c-d: Ce(0.06)-K-OMS2, e-f: Ce(0.12)-K-OMS-2, g-h: Ce(0.24)-K-OMS-2, and i-j: Ce(0.48)-K-OMS-2).


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Figure 4. HRTEM images of the Ce-K-OMS-2 catalysts. (a1-a3: K-OMS-2, b1-b3: Ce(0.06)-KOMS-2, c1-c3: Ce(0.12)-K-OMS-2, d1-d3: Ce(0.24)-K-OMS-2, and e1-e3: Ce(0.48)-K-OMS-2).


12

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The corresponding XPS spectra of O 1s of Ce-K-OMS-2 samples are shown in Figure 5(a), and two kinds of surface oxygen species can be clearly observed. The binding energy of 529.0 to 530.0 eV is characteristic of the lattice oxygen (O2-) (hereafter denoted as Olatt) and the binding energy of 531.0 to 532.0 eV can be assigned to the surface adsorbed oxygen (hereafter denoted as Oads). It has been suggested that the Oads species are critical in the reaction of catalytic NH3SCR at low temperature, because of its higher mobility than the Olatt species.29 Hence, the corresponding ratio of Oads/Olatt in Ce-OMS-2 samples is calculated in Table 2. It was found that the surface Oads/Olatt increased with the increase of Ce doping (from 24.7% to 37.5%), implying that more Ce was incorporated into the framework of the K-OMS-2, resulting in more surface OVDs in a low coordination environment.29 Thereby, the addition of cerium to the K-OMS-2 causes the formation of more surface OVDs, which is consistent with the observation from the HRTEM images. Mixed-valency manganese ions are situated in the octahedral sites of cryptomelane.30 The Mn 2p3/2 can be separated into two peaks similar to some reports of Ce-K-OMS-2 oxides from Figure 5(b). The binding energies of 643.2-644.5 eV approximately and 641.6-642.4 eV could be attributed to the presence of Mn4+ and Mn3+ species, respectively. It has been proven that, when Ce ions replace manganese, some Mn3+ is converted into Mn4+ to maintain the charge balance. The relative concentration of Mn4+/Mn3+ in the Ce(0.12)-K-OMS-2 sample was calculated to be 67.14%, while it was only approximately 61.30% in the K-OMS-2 sample. Obviously, because of the addition of cerium, the Ce(0.12)-K-OMS-2 catalyst possessed more adsorbed oxygen species and Mn4+ on the surface than the pure K-OMS-2 sample.


13


(a)

Olatt

Oads

(b)

O 1s

Mn 2p Mn3+

Mn4+

Ce(0.48)-K-OMS-2

Intensity(a.u.)

Ce(0.48)-K-OMS-2

Intensity(a.u.)

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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Ce(0.24)-K-OMS-2

Ce(0.12)-K-OMS-2

Ce(0.24)-K-OMS-2

Ce(0.12)-K-OMS-2

Ce(0.06)-K-OMS-2

Ce(0.06)-K-OMS-2

K-OMS-2

534

533

K-OMS-2

532

531

530

529

528

527

660

656

Binding energy(eV)

652

648

644

640

636

Binding energy(eV)

Figure 5. XPS spectra of various Ce-K-OMS-2 catalysts over the spectral regions of O 1s (a), and Mn 2p (b). Table 2. The XPS results of various Ce-K-OMS-2 catalysts about O1s and Mn2p O1s

sample

Oads/Olatt (%)

BE(eV)

Mn2p3/2 BE(eV)

Mn4+/Mn3+ (%)

Mn4+

Mn3+

24.7

644.1

642.4

61.30

531.1

23.3

644.4

642.3

62.05

529.7

530.8

37.5

644.0

642.2

67.14

Ce(0.24)-K-OMS-2

529.7

531.0

34.4

644.3

642.3

65.28

Ce(0.48)-K-OMS-2

529.9

531.6

36.0

644.0

642.4

66.91

Olatt

Oads

K-OMS-2

529.9

531.6

Ce(0.06)-K-OMS-2

529.7

Ce(0.12)-K-OMS-2

The H2-TPR technique was applied to examine the reducibility of the Ce-K-OMS-2 catalysts (Figure 6). The parent K-OMS-2 displayed a broad peak in the temperature range 250350 °C, which was composed of two peaks, centered at 281 and 302 °C. These peaks could be attributed to the two-step reduction of Mn4+ to Mn3+ and further Mn3+ to Mn2+ at higher temperature.18,

31

After Ce doping, the reduction temperature of the Ce-K-OMS-2 materials

shifted to lower values at different degrees (see Figure 6). For Ce(0.12)-K-OMS-2, in particular, two continuous peaks decreased to 261 °C and 284 °C, respectively. The reduction temperature


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of metal oxides reflected the reducibility. Hence, the reducibility of the Ce-K-OMS-2 catalysts decreased in the sequence of Ce(0.12)-K-OMS-2 > Ce(0.48)-K-OMS-2 > Ce(0.24)-K-OMS-2 > Ce(0.06)-K-OMS-2 > K-OMS-2. With the increase of Ce, the reducibility of the Ce-K-OMS-2 catalysts first increased to the highest value, with moderate Mn2O3 phase presented, and then tended to be stable. We speculated that the introduction of Ce changed the combination intensity of manganese with oxygen in Ce-K-OMS-2.

268oC

Ce(0.48)-K-OMS-2

289oC

Intensity(a.u.)

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


274oC

301oC

261oC

Ce(0.24)-K-OMS-2

Ce(0.12)-K-OMS-2 284oC

271oC 281oC

220

240

260

280

301oC

Ce(0.06)-K-OMS-2 K-OMS-2

302oC

300

320

340

360

Temperature (oC)

Figure 6. The H2-TPR profiles of different Ce-K-OMS-2 catalysts. 3.2 Catalytic performance for NH3-SCR The catalytic NH3-SCR activity of the as-prepared catalysts with the GHSV=64000 h-1 is shown in Figure 7(a). K-OMS-2 and Ce(0.06)-K-OMS-2 achieved a NO conversion of 100% in the temperature range of 180–230 °C. The optimal reaction temperature shifted to lower temperature (140 oC) with an increaseing amount of Ce. When the initial Ce/Mn molar ratio was higher than 0.12, NO conversion reached 100% conversion in a broader temperature window of 140–230 °C. Compared with pure K-OMS-2, the Ce-doped K-OMS-2 catalyst had a lower operation temperature and a better performance in NH3-SCR, due to the distortion of octahedral units after Ce substitution and subsequent introduction of OVDs. In this work, Ce(0.12)-K-OMS-


15


2 was chosen as the optimum catalyst in this study, which is consistent with the above characterization. To further investigate the influence of SO2 and H2O during SCR, the K-OMS-2 and Ce(0.12)-K-OMS-2 catalysts were evaluated. As shown in Figure 7(b), in the presence of 10% H2O and 150 ppm SO2, the NO conversion for the K-OMS-2 catalyst sharply decreased to 40% during first few minutes, and to below 20% after 100 min. The Ce(0.12)-K-OMS-2 catalyst presented better stability and higher SCR activity than the pure K-OMS-2 catalyst. The Ce(0.12)K-OMS-2 catalyst still maintained approximately 50% NO conversion after 100 min. After SO2 and H2O were removed, the NO conversion of the two catalysts could recover to the previous level, indicating that the deactivation by SO2 and H2O was reversible. These results indicated that the addition of Ce could effectively inhibit the poisoning of catalysts in the presence of SO2 and H2O. Combined with the above results, it can be concluded that the distortion of octahedral units and formation of OVDs occurred after Ce substitution, which effectively protected the active Mn of the catalyst, and thus enhanced the thermal stability.

80

60

40

20

K-OMS-2 Ce(0.06)-K-OMS-2 Ce(0.12)-K-OMS-2 Ce(0.24)-K-OMS-2 Ce(0.48)-K-OMS-2

80 100 120 140 160 180 200 220 240 260 280 300 320

NO conversion (%)

(b) 100

(a) 100

NO conversion (%)

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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10% H2O+150 ppm SO2 on

80 H2O+ SO2 off

60 40 20 0

K-OMS-2 Ce(0.12)-K-OMS-2 0

20

Temperature (oC)

40

60

80

100

120

140

t (min)

Figure 7. The catalytic performance of NH3-SCR over Ce-doped K-OMS-2 catalysts (a), Effect of H2O and SO2 on the NH3-SCR activity (b) (Reaction condition: 600 ppm NO, 600 ppm NH3, 6% O2, 10% H2O (when used), 150 ppm SO2 (when used), N2 to balance and GHSV=64000 h-1).


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3.3 Computational details Our models consisted of a 1×1×2 supercell of α-MnO2 (a), K-OMS-2 (b), and Ce-K-OMS-2 (c) materials are shown in Figure 1. The bond lengths and band gap of the α-MnO2, K-OMS-2 and Ce-K-OMS-2 are summarized in Table 3. In this paper, K-OMS-2 and Ce-K-OMS-2 will be better understood, since they are derived from α-MnO2 after K+ was inserted into the tunnel of them. As our DFT calculation predicted, an underestimated band gap of α-MnO2 obtained with the DFT + U approach was 1.244 eV, which is in agreement with experimental observations (1.320 eV).32 For bond length, the average Mn-O bond length of our calculated α-MnO2 was 1.933 Å compared to the reference results (1.913 Å). The deviation of the calculated bond length from the experimental results was approximately 1.05%. The deviations of the calculated results are considered acceptable as the GGA functional tends to over-evaluate the lattice parameters and underestimate the width of the band gap. When K+ was doped into the α-MnO2, the average bond length was elongated to 1.959 Å. The increase of the distance between Mn and O may be related to a strong interaction of K+ ions in the 2×2 tunnels of α-MnO2. For Ce doped in the framework of K-OMS-2 material, the average Mn-O bond length was 1.947 Å, which is to be strained, as the size of Cen+ (Ce4+=0.97 Å, Ce3+=1.14 Å) is larger than Mnn+ (Mn3+ = 0.58 Å, and Mn4+ = 0.53 Å). However, the Ce-O bond length was 2.183 Å, which was elongated compared to the normal 2.360 Å, as illustrated in Table 3. Thereby, the average of Mn-O bond was strained after the inclusion of Ce and hence introduced a distortion to the octahedral MnO6 units. The density of states (DOS) of the pure OMS-2, K-OMS-2 and Ce-doped K-OMS-2 are depicted in Figure 8, respectively. As shown in Figure 8 (b, spin up and down), the α-MnO2 is a semiconductor with the Fermi level inside the band gap (1.244eV). The Fermi level is shifted to zero in each case. The partial density of states (PDOS) of α-MnO2 indicated that the lower


17


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valence band between -20 eV and -15 eV was mainly occupied by 2s states of O1 and O2, while the upper valence band between -7 eV and 0 eV was shared by Mn 3d states and 2p states of O1 and O2. With K+ doping, the conduction band of K-OMS-2 was partially filled (Figure 8(c)). This partial filling of the conduction band is a consequence of electronic charge transfer from K to α-MnO2. Integrating the projected DOS of K-OMS-2 up to the Fermi energy, the estimated band gap decreased slightly to 1.134 eV (for composition K1/8MnO2). Moreover, it can be seen that the DOS of K-OMS-2 obviously shifted to a lower energy for K doping on α-MnO2 (see Figure 8(a)), which can promote its redox properties for SCR reaction. For the Ce-K-OMS-2 material, however, Ce doping led to the asymmetrical DOS at the conduction band near the Fermi energy, as shown in Figure 8(d). It belonged to the 4f states of Ce (between 0 eV and 4 eV) when Ce was substituted to Mn, and ultimately decreased the magnetic moment of Mn from 3.31 μB to 3.28μB (see Table 3). The decreased magnetic moment on Mn atoms caused by Ce incorporated into the framework could lead to the weak ferromagnetic-ordering of the Ce-KOMS-2 materials.33 In addition, the new band at ~-29 eV and -12 eV was mainly occupied by 2s and 2p states of K, while other new bands belonged to 3s, 3p, 3d and 4f states of Ce (see Figure 8(d)). The 2p states of O1 and O2 were extended to lower energy levels to form a hybridize peak with Mn and Ce 3p states in the valence band region between -7 eV and -9 eV, indicating that the effect of electro negativity on oxygen atoms was enhanced after the introduction of Ce. In this study, the DOS of Ce-K-OMS-2 obviously shifted to lower energy, while the effect of electronegativity on oxygen atoms was enhanced after the introduction of Ce.


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Table 3. Average bond lengths, band gap, average magnetic moment, and oxygen vacancy formation energy Ev. (in eV) calculated via the DFT+U method. Model

Average bond length (Å)

Band gap (eV)

μ(Mn)

Ev (eV)

-

1.320

3.32

1.47

1.933

-

1.244

3.18

1.79

K-OMS-2

1.959

-

-

3.31

0.78

Ce-K-OMS-2

1.947

2.183

-

3.28

0.69

Mn-O

Ce-O

α-MnO2(Ref. 23, 32)

1.913

α-MnO2

Figure 8. The total density of states of α-MnO2 (a), the total and partial density of states of αMnO2 (b), K-OMS-2 (c), and Ce-K-OMS-2 (d), obtained by DFT + U approach. The Fermi level is expressed by the vertical solid line.


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To gain deep insight into the catalyst surface properties, oxygen vacancies with pure KOMS-2 and Ce-K-OMS-2 models through DFT+U calculations were investigated. The energy for oxygen vacancy generation was investigated by DFT+U calculations, the oxygen vacancy models of K-OMS-2 and Ce-K-OMS-2 after structure optimizations are displayed in Figure 1. The energy of oxygen vacancy formation (Ev) was calculated by the following equation: Ev= (Evac + 1/2 EO2) – Ebulk where Evac is the total energy of the system containing an oxygen vacancy, EO2 is the total energy of an isolated O2 in the gas phase, and Ebulk is the total energy of optimized clean slab structures. This equation indicates that the oxygen atoms move away from the surface to form the oxygen molecule and the oxygen vacancy. With our definition, a lower value of Ev indicates that the process is energetically preferable. The obtained Ev values for the K-OMS-2 and Ce-K-OMS-2 models were approximately 0.78 and 0.69 eV, respectively. These results indicated that the oxygen vacancy formation was more easily produced on the Ce-K-OMS-2 model than the pure K-OMS-2 model. Therefore, the calculations correspond well with the Oads XPS spectra in which the Ce-K-OMS-2 produced a higher Oads concentration because it requires less energy than what was required for oxygen vacancy formation. It should be noted that a correlation between oxygen vacancies and the catalytic behaviors has been established. In most cases, oxygen vacancies are believed to promote its redox properties for an SCR reaction. In our study, oxygen vacancies in the Ce-KOMS-2 catalyst mainly accelerated the catalytic SCR reaction. In this section, the K-OMS-2(1 0 0) and Ce-K-OMS-2(1 0 0) catalyst surfaces were employed, which corresponded to the major reactive surfaces of the K-OMS-2 and Ce-K-OMS-2 catalysts, respectively. Consequently, we studied the molecular adsorption abilities of the K-


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OMS-2(1 0 0) and Ce-K-OMS-2(1 0 0) surface catalysts for the NO and NH3 molecules. The molecular adsorption ability is defined by using adsorption energy term (Ed), which is calculated from Ed= Esurface+gas – Esurface – Egas where Esurface+gas is the total energy of the optimized adsorbate surface system and Esurface and Egas are the total energies of the naked surface and the gas molecule, respectively. Therefore, the more negative the Ed value is, the stronger the adsorption energy will be. It has been concluded that NH3 adsorption is one of the most critical steps in the NH3-SCR reaction.34 In Mn-based catalysts, L-acid site for NH3 adsorption acts more favorably than the Bacid site for NH3 adsorption.35, 36 Thus, we studied the NH3 adsorption at the Mn L-acid site of K-OMS-2(1 0 0), and both Mn and Ce L-acid sites of the Ce-K-OMS-2(1 0 0) catalysts, aiming to find the most favorable adsorption site. The optimized structures of NH3 adsorption at these three L-acid sites as well as the corresponding adsorption energies are presented in Figure 9. The obtained adsorption energies of NH3 molecules over the K-OMS-2(1 0 0), Mn sites of Ce-KOMS-2(1 0 0), and Ce sites of Ce-K-OMS-2(1 0 0) surface models were -1.15 eV, -1.21 eV and 1.31 eV, respectively. Obviously, it can be concluded that NH3 prefers to be adsorbed at the Ce atom in the Ce-K-OMS-2(1 0 0) surface, with an adsorption energy of -1.31 eV. Meanwhile, NO is much more likely to adsorb on the surface of Ce-K-OMS-2(1 0 0) with adsorption energy of 1.08 eV. Then, it can react with the adsorbed NH3,31,

36

indicating that the Langmuir-

Hinshelwood (L-H) mechanism should be dominant.


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Figure 9. NO and NH3 adsorption energy (eV) over the pure K-OMS-2(1 0 0) and Ce-K-OMS-2 (1 0 0) catalysts surface models. 3.4 Possible reaction mechanism As mentioned above, this work has clearly presented that Ce-doped K-OMS-2 plays an important role in the enhancement of NH3-SCR activity. The Ce can incorporate into the framework of the K-OMS-2, resulting in more active OVDs, a lower temperature of reducibility, a lower amount of energy required for oxygen vacancy formation, and stronger NO and NH3 adsorption energies compared with that of the pure K-OMS-2. Therefore, the probable reaction mechanism of Ce-K-OMS-2 was proposed in this study, as shown in Scheme 1. NH3 was adsorbed on the Ce L-acid site of Ce-K-OMS-2, while NO was easily adsorbed on the O site of Ce-K-OMS-2. Afterwards, the adsorbed NOx species reacted with the neighboring NH3 species to form N2 and H2O, following a Langmuir-Hinshelwood mechanism.37


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Scheme 1. The proposed NH3-SCR reaction mechanism over Ce-K-OMS-2 catalyst. 4. Conclusion The low-temperature NH3-SCR activity of the K-OMS-2 catalyst was significantly increased after the Ce substitution, due to more active OVDs formation. Among the as-prepared samples, Ce(0.12)-K-OMS-2 demonstrated remarkable deNOx performance, with elevated lowtemperature activity and a broadened temperature window from 140 °C to 230 °C. According to the characterization results, the Ce mainly entered the framework structure of K-OMS-2, significantly increasing the BET surface area from 81 m2/g for K-OMS-2 to 136 m2/g for Ce(0.12)-K-OMS-2. This was roughly because of the surface OVDs after the introduction of Ce into K-OMS-2. Moreover, the DFT+U calculations demonstrated that the unique nanostructure of Ce doping K-OMS-2 with an oxygen vacancy in the framework was crucial for the improvement of catalytic activity. We combined both theoretical and experimental evidence to provide a new physical insight into the significant effect due to the OVDs induced by the Ce ion substitution on the catalytic activity of K-OMS-2. The unique nanostructure of Ce doping KOMS-2 with OVDs in the framework can alter the local bond lengths and the DOS, thereby obviously accelerating the low-temperature NH3-SCR. The mechanism entailed the adsorption of NH3 on the Ce site of Ce-K-OMS-2, while NO was easily adsorbed on the O site of Ce-K-OMS2. After that, the adsorbed NOx reacted with the neighboring NH3 to form N2 and H2O, following a Langmuir-Hinshelwood mechanism.


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AUTHOR INFORMATION Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest ACKNOWLEDGMENT This research was supported by the Scientific Research Funds of Huaqiao University (Grant No. 600005-Z1824050). REFERENCES (1) Ma, J.; Chu, B.; Liu, J.; Liu, Y.; Zhang, H.; He, H., NOx promotion of SO2 conversion to sulfate: an important mechanism for the occurrence of heavy haze during winter in Beijing. Environ. Pollut. 2018, 233, 662-669. (2) Xue, L.; Gu, R.; Wang, T.; Wang, X.; Saunders, S.; Blake, D.; Louie, P. K. K.; Luk, C. W. Y.; Simpson, I.; Xu, Z.; et al. Oxidative capacity and radical chemistry in the polluted atmosphere of Hong Kong and Pearl River Delta region: analysis of a severe photochemical smog episode. Atmos. Chem. Phys. 2016, 16 (15), 9891-9903. (3) Wang, X.; Li, X.; Zhao, Q.; Sun, W.; Tade, M.; Liu, S., Improved activity of W-modified MnOx-TiO2 catalysts for the selective catalytic reduction of NO with NH3. Chem. Eng. J. 2016, 288, 216-222.


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TOC Graphic:


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on Ce-Doped Manganese Oxide Octahedral Molec

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