Co3O4 polyhedral nanocage

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Kinetics, Catalysis, and Reaction Engineering

Self-template synthesis of a MnCeO#/Co3O4 polyhedral nanocage catalyst for toluene oxidation Yao Shan, Ning Gao, yingwen chen, and Shubao Shen Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00847 • Publication Date (Web): 12 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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Industrial & Engineering Chemistry Research

Self-template synthesis of a MnCeOδ/Co3O4 polyhedral nanocage catalyst for toluene oxidation Yao Shana, Ning Gaoa, Yingwen Chena*, Shubao Shena

a

College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech

University, Nanjing 211816, China

*Corresponding author. Yingwen Chen, College of Biotechnology and Pharmaceutical Engineering, Nanjing Tech University, Nanjing 211816, China

Address: Biotechnology Building No. 30 Puzhu South Road, Nanjing Tech University, Nanjing 211816, P. R. China Email: [email protected] Tel.: +86 25 58139922

Fax: +86 25 83587326

1

ACS Paragon Plus Environment

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

Abstract Herein, a novel nanocage catalyst of a porous MnCeOδ/Co3O4-NC polyhedral derived-from a Co-based zeolitic imidazolate framework (ZIF-67) was synthesized and its catalytic performance for toluene oxidation was evaluated. This composite of the MnCeOδ/Co3O4-NC catalyst presents a high toluene conversion of 95% at 230°C with a weight hourly space velocity (WHSV) of 40000 ml g-1 h-1. In addition, the reaction rate based on the catalyst surface area of the MnCeOδ/Co3O4-NC catalyst was 3.138×10-2 mmol m-2 h-1. Meanwhile, the MnCeOδ/Co3O4-NC catalyst also shows a lower apparent activation energy of the reaction (Ea) of 56.10 kJ mol-1. According to the characterization results, the specific surface area, the interaction between the MnCeOδ solid solution and Co3O4-NC, the low-temperature reducibility, and the concentration of surface active oxygen confinement that originates from the MnCeOδ/Co3O4-NC prove this catalyst with superior activity for toluene oxidation. Keywords Catalytic combustion; VOCs; ZIF-67; Solid solution; Toluene

2


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1. Introduction Volatile organic compounds (VOCs), as key precursors to air pollution, cause harm to both the environment and human health.1,2 As an aftertreatment system, catalytic combustion technology has aroused great attention owing to its economic feasibility, high efficiency and environmental friendliness.3,4 Currently, noble metals (Pt and Pd) and metal oxide catalysts have been used during the VOCs combustion process.2,5,6 Supported noble metals (Pt and Pd) have good catalytic activity at low temperature.7 However, a few obdurate encumbrances like the high cost and poor thermal stability are still required to surmount.8 Alternatively, non-noble metal catalysts (such as Mn-, Cu-, Co-, Ce- metal oxide catalysts) are considered effective catalysts of toluene oxidation with considerably lower cost and good activity.9-12 At present, the studies on the Ce-Mn oxide for VOCs combustion had been developed widely.13,14

MnOx-CeO2 has demonstrated superior performance

originating from a high oxygen storage capacity (OSC), more active oxygen species (for example, O2- , O22- and O-) and a strong synergistic effect.15,16 In addition, Co3O4 has a typical spinel structure, contains higher oxygen binding rate and weaker Co-O bond.17 Meanwhile, the higher activation of molecule oxygen and the lower bonds strength have positive effect on the improvement of catalytic performance.18,19 Thus, considering the advantages of above catalysts, developing a Mn-Ce-Co composite metal oxide for VOC conversion is of great interest. Currently, there are two main strategies for synthesizing Mn-Ce-Co composite metal oxides. One strategy is to use a one-step method to prepare the Mn-Ce-Co composite metal oxide catalyst in our previous work. With the doping of Mn and Co ions doped into the lattice of CeO2, Mn-Co-Ce mixed oxide catalyst was formed and the interaction among Co-Mn-Ce-O is helpful for the improvement of catalytic activity.20,21 On the other hand, apart from the catalyst composition, the surface area, morphology and pore structure of the composite material have important influence on catalytic performance. Dai et al.22 have found the Co3O4/3DOM ABO3 samples which were synthesized by the in situ PMMA-templating means possess better activity in the degradation of toluene. Feng et al.23 conducted an experiment of catalytic combustion of toluene using various 3



morphologies of cerium oxide. A conclusion was drawn that CeO2 hollow sphere possess outstanding catalytic performance which was characterized with largest surface area and high ratio of oxygen vacancies. Recently, pyrolysis of metalorganic frameworks (MOFs) has been introduced to prepare metal oxides with excellent physicochemical properties, which exhibits the potential for VOCs combustion in higher efficiency.24-26 Metal-organic frameworks (MOFs) is characterized with high inner surface areas, orderly crystalline structures, as well as diverse morphologies and architectures.27-29 Recently, many studies have been developed to the synthetic of novel nanoporous materials using MOFs as templates, which can achieve a high degree of nanoparticle dispersion, effectively preventing the agglomeration of the nanoparticles. In the study of Zhang et al.30 the deNOx catalyst of hollow porous MnxCo3−xO4 nanocages, which was prepared from the nanocube-like metal-organic frameworks, exhibited the high performance. Zamaro et al.31 developed hollow porous CeO2–CuO nanoparticles derived from the metal-organic framework (MOF) HKUST-1, which have high performance in oxidation carbon monoxide and it was characterized with highly dispersed CuO and CeO2 nanoparticles. In this study, basing on the previous research on the Mn-Ce-Co composite, we propose a facile and highly efficient synthesis of MnCeOδ/Co3O4 polyhedral nanocage catalysts using Co-based MOFs by a self-sacrificial template method and then evaluate their performance for toluene oxidation. In addition, the MnCeOδ/Co3O4 polyhedral nanocages were studied and compared with the Co3O4-NC and Co3O4 bulk catalysts. Numerous characterization results attempt to reveal the relationship between the catalytic performance and the surface structure of the catalysts. 2. Experiments 2.1 Experimental Section In this paper, we report the simple and robust method of MnCeOδ/Co3O4 polyhedral nanocage catalysts using Co-based MOFs by a self-sacrificial template modus. (Scheme 1). Synthesis of the ZIF-67 templates: First, the ZIF-67 templates as precursors were synthesized according to a previous report.32 Co (NO3)2 6H2O (0.45 4


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g, 1.55 mmol) was dissolved in 3.00 ml of deionized water. 2-Methylimidazole (5.50 g, 67.10 mmol) was dissolved in 20.00 ml of deionized water. Then, two solutions were mixed and vigorously stirred for 6 h. The final physical precipitates were separated by centrifugation, washed with methanol 3 times and vacuum dried at stable temperature of 80°C for 12 h. Synthesis of porous Co3O4-NC catalyst: the as-made ZIF-67 crystals were calcined in muffle at 350°C for 2 h at a heating procedure of 2°C·min-1. Finally, the black Co3O4 particles were finished. Synthesis of the porous MnCeOδ/Co3O4-NC catalyst: 0.40 g 50% Mn(NO3)2 and 0.55 g Ce(NO3)3·6H2O was first dispersed in 10 mL of methanol. ZIF-67 (0.60 g) was immersed in this solution for 10 min ultrasonically Then, the purple precipitates were collected using centrifugation and vacuum dried at 80°C for 12 h. Similar to Co3O4 preparation, the material was heated at 350°C for 2 h at a heating procedure of 2 °Cmin-1. Synthesis of Co3O4 bulk catalyst: Co(NO3)2·6H2O were vacuum dried at 80°C for 12 h. Similar to Co3O4 preparation, the material was calcined at 350°C for 2 h with a heating rate of 2 °Cmin-1.

Scheme 1 Synthesis route for the MnCeOδ/Co3O4-NC

2.2 Characterization In order to evaluate the prepared samples, the necessary methods were used to characterize the microstructure and surface properties of samples, such as Scanning electron microscopy (SEM), the X-ray diffraction (XRD), Transmission electron microscope(TEM), High Resolution Transmission Electron Microscope(HRTEM), N2 adsorption-desorption temperature-programmed

isotherms desorption

(Brunauer-Emmett-Teller, (O2-TPD),

H2


O2

temperature-programmed

reduction (H2-TPR),X-ray photoelectron spectroscopy (XPS). 5

BET),


2.3 Activity test Catalytic oxidation of toluene was implemented in an experimental system with a continuous-flow fixed bed (U-shaped quartz tube with an inner diameter of 7 mm) loading 500 mg of the as-prepared catalyst (40-60 mesh) and the operating temperature range from 160°C to 290°C. 1000ppm of toluene reaction gas is formed by mixing a certain concentration of standard gas with air. The toluene reactant gas with a flow of 333.30 ml min-1 was controlled to be GHSV of 40000 ml g-1 h-1. The effluent gases were detected by an on-line gas chromatograph (Shimazu GC-2014) which are equipped with flame ionization detector (FID). A Rtx-01 capillary column (30 m x 0.25 mm x 0.25 um) was used to detect the reactants, and a TDX-01 packed column (3 m x 3 mm) was used to analyze the product. In addition, each test temperature was held constant for approximately 30 minutes to ensure the accuracy of data. 3. Results and Discussion 3.1 Characterization studies

Figure 1 (a) XRD curves of the simulated ZIF-67 sample and as-synthesized ZIF-67 nanocrystals; (b) XRD patterns of the Co3O4 bulk, Co3O4-NC and MnCeOδ/Co3O4-NC catalysts.

XRD patterns of the simulated and synthesized ZIF-67 samples, and the Co3O4 bulk, Co3O4-NC and MnCeOδ/Co3O4-NC catalysts were shown in Figure 1. All diffraction peaks were almost consistent with the simulated pattern as well as the previous reports.33 These results indicate that the ZIF-67 nano crystals were successfully synthesized in this work. From Figure 1b, the XRD pattern of the Co3O4 6


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bulk, Co3O4-NC and MnCeOδ/Co3O4-NC materials displayed well-resolved reflections, which were in accordance with the pure cubic Co3O4 phase (JCPDS No. 43-1003). The diffraction peaks at 2θ = 19.0°, 31.3°, 36.8°, 38.5°, 44.8°, 55.7°, 59.3°, 65.2°, and 77.3° correspond to the crystal planes of Co3O4 (111), (220), (311), (222), (400), (422), (511), (440) and (533), respectively. Meanwhile, the diffraction peak of MnCeOδ/Co3O4-NC shows cubic fluorite CeO2 (JCPDS No. 34-0394) without manganese oxides. The diffraction peaks at 2θ = 28.6°, 33.1°, 47.5°, 56.3°, 59.1° and 76.7° correspond to the crystal planes of CeO2 (111), (200), (220), (311), (222) and (331). The lattice parameter of MnCeOδ/Co3O4-NC catalyst (5.4076 Å) show slight drop compared with that of pure cerium oxide (5.4113 Å), indicating that part of the manganese ions enter the cerium oxide phase and replace some of the cerium ions.34,35 Due to the smaller ionic radius of Mnn+. In Table 1, the grain size of the Co3O4 spinel phase of the three catalytic materials was calculated from the Scherrer equation. The self-template method helps to inhibit the growth of the MnCeOδ and Co3O4 crystal grains. In addition, the formation of MnCeOδ particles (5.80nm) also has a certain inhibitory effect on the growth of the Co3O4 crystallites.

0.24nm 311 Co 3O 4

Figure 2 FESEM patterns (a) ZIF-67 (b) Co3O4-NC (c) MnCeOδ/Co3O4-NC (d) TEM patterns of MnCeOδ/Co3O4-NC (e-f) HRTEM patterns of MnCeOδ/Co3O4-NC 7



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Figure 2 presents FESEM and TEM (HRTEM) images of three samples. Most of the formed ZIF-67 material is a regular diamond dodecahedron with a size of 500nm (Figure 2a), and the self-template prepared Co3O4-NC still retains this structure (Figure 2b). Seen from Figure 2c, the MnCeOδ are uniformly distributed on Co3O4 without agglomeration, which are consistent with the EDS-mapping of Mn, Ce, Co and O showing in Figure 3. Such a good dispersion of the MnCeOδ active components is believed to benefit from the calcination of the MOF self-template. During the roasting process, an anchor effect is demonstrated and the cluster phenomenon is weakened. The morphology and dispersion of the MnCeOδ/Co3O4-NC catalyst nanoparticles were clearly observed in the TEM and HRTEM images (Figure 2d, e and f). The lattice spacing of the support is 0.24nm, corresponds to the (311) crystal plane of Co3O4. The lattice spacing of the stripes was 0.31nm corresponding to the (111) surface parameter value of CeO2 of the fluorite structure. (a)

(b)

500 nm

500 nm

(d)

500 nm

(c)

500 nm

(f)

(e)

Ce

Mn

500 nm

Co

500 nm

O

Figure 3 EDS-mapping (a) Electron image of MnCeOδ/Co3O4-NC; (b) EDS layered image of MnCeOδ/Co3O4-NC; (c-f) the element mapping image of Mn, Ce, Co and O.

8


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Table 1 The specific surface area (SBET), pore volume (Vp), average pore size (DP), and crystallite size (DC) Catalyst

SBET/(m2 g-1)

Vp/(cm3 g-1)

Dp/ (nm)

Dc/ (nm)b

ZIF-67

1435.32

0.71

2.03

/

Co3O4 bulk

37.67

0.12

12.87

23.50

Co3O4-NC

55.10

0.33

6.42

21.20

MnCeOδ/Co3O4-NC

100.40

0.38

15.30

5.80a/18.90

CMC-0.2521

68.47

0.13

7.54

6.03

MnCeOδ21

50.19

0.13

10.23

6.72

a crystallite size of CeO were calculated by Scherrer equation. 2 b

crystallite size of Co3O4 were calculated by Scherrer equation..

Figure 4 N2 adsorption isotherms (a) and pore size distributions (b) of the porous Co3O4 bulk, Co3O4-NC and MnCeOδ/Co3O4-NC samples.

The pore size distribution and specific surface area of the as-prepared catalysts were investigated by the N2 adsorption–desorption isotherms (Figure 4), and the detailed data are listed in Table 1. In addition, ZIF-67 exhibits a large specific surface area of 1435.32m2g-1 and a micropore volume of 0.71cm3g-1. Such a favorable microstructure with high surface area and microporous property not only provides an efficient route for the good dispersion of the MnCe ions but also confines the reactants in the nanospace. Both of these characteristics are beneficial for the catalytic reaction. The isotherms of the Co3O4-NC and MnCeOδ/Co3O4-NC materials show a typical adsorption curve of type IV, typical of mesoporous materials in a relative 9



pressure (p/p0) range of 0.10-0.98 (Figure 4b ). These results demonstrate that most of the two materials have more than 20 nm slit holes. Notably, the incorporation of the MnCeOδ solid solution greatly increases the specific surface area of the material and further promotes the catalytic combustion performance of the catalytic material. Moreover, the SBET value of the MnCeOδ/Co3O4-NC catalyst synthesized by the self-template method is larger than that of the MnCeCo composite oxide, which has positive influence on the improvement of catalytic performance. (b)

Intensity (a.u.)

Ce 3d

11.8 eV

u

u''' u''

v

u。 v'''

u'

v''

v'

v。 MnCeO/Co3O4-NCs

920

915

Mn 2p

Intensity (a.u.)

(a)

910

905

Mn4+

MnCeO/Co3O4-NCs

900

895

890

885

880

665

660

(c)

Co 2p

Mn3+

655

Binding Energy (eV)

650

645

640

635

Binding energy (eV)

Co3+

Co2+

（ d（

O

O 1s O

Intensity (a.u.)

MnCeO/Co3O4-NCs 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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Co3O4-NCs

O MnCeO/Co3O4-NCs

Co3O4-NCs

Co3O4-bulk

Co3O4-bulk 805

800

795

790

785

780

775

536

534

532

530

528

526

Binding energy (eV)

Binding energy (eV)

Figure 5 XPS patterns of the catalysts: Co3O4 bulk, Co3O4-NC and MnCeOδ/Co3O4-NC. The XPS spectrum for Ce 3d (a) and Mn 2p (b) of MnCeOδ/Co3O4-NC; (c)The XPS spectrum for Co 2p; and (d) O 1s XPS spectrum

XPS spectra of the catalysts was shown in Figure 5: Co3O4 bulk, Co3O4-NC and MnCeOδ/Co3O4-NC. The peaks denoted by v°, v′, u°, and u′ are ascribe to Ce3+, while those marked by v, v″, v‴, u, u″, and u‴ are attributed to Ce4+ ions.34,36 verifying that Ce3+ and Ce4+ are coexisted in the samples. The Ce4+ species ratio was calculated to 10


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be 82% on the surface, indicating that The Ce4+ species dominate the surface. Figure 5b showed Mn 2p XPS spectra of MnCeOδ/Co3O4-NC catalysts. The binding energy of 641.69 and 653.80 eV corresponds to Mn3+, and the binding energy of 643.47 and 655.27 eV corresponds to Mn4+, and ΔE was calculated as 11.80 eV.37 The XPS spectrum for Co 2p (Figure 5c) presents two spin-orbit doublet peaks, two peaks around 779.50 eV and 795.50 eV can be attributed to Co 2p3/2 and Co 2p1/2, respectively. With the change in the chemical environment of the Co ions, the Co 2p3/2 and Co2p1/2 peak positions (BE values) are shifted to lower positions. There are abundant Co3+ species on the surface and correspond to Co2+/Co3+ in Table 2. As shown in Figure 5d, each asymmetric O 1s curve could be resolved into three peaks. The binding energy of 528.70, 531.20 and 532.58 eV were ascribed to lattice oxygen (Oα), surface absorbed oxygen (Oβ) and adsorbed molecular water or hydroxyl group (Oγ), respectively. Among them, lattice oxygen including O2-, surface absorbed oxygen contain O2-, O22- and O-.38,39 The Oβ groups derived from defective sites with an unsaturated structure have been recognized in the literature and are beneficial to benzene oxidation.40 The surface Oβ/Oα molar ratios are also listed in Table 2. The surface Oβ/Oα molar ratio (0.78) of the Co3O4-NC catalyst is higher than that of Co3O4 bulk (0.66), indicating that the adsorbed oxygen is more likely to be concentrated on a catalyst surface with a high specific surface area. Increasing of the surface adsorbed oxygen can effectively improve the ability of the catalyst to participate in the deep oxidation reaction.41,42

Figure 6 H2-TPR (a) and O2-TPD (b) profiles of the Co3O4 bulk, Co3O4-NC and 11



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MnCeOδ/Co3O4-NC samples. Figure 6a shows the reduction curves for the three samples. According to reports

in the literature, the reduction process of Co3O4 is divided into two stages.43 The first reduction peak at 297°C is for the reduction of high cobalt ions (Co3+-Co2+); the second reduction peak at 346°C can be ascribed to the reduction of cobalt ions (Co2+-Co0). Apparently, the reduction peak temperature (283°C, 330°C.) of the porous Co3O4-NC catalyst prepared from the template is lower than that of the Co3O4 bulk catalyst. The MnCeOδ/Co3O4-NC catalyst exhibits two overlapping reduction peaks at around 270°C and 320°C. The low temperature peak and the high temperature peak are attributed to the degradation of MnO2 to Mn3O4 and Mn3O4 to MnO, respectively.44,45

Compared with the other samples, the reduction peak of

MnCeOδ/Co3O4-NC shifted to lower temperatures, and the catalyst showed better reduction performance. This result indicates that the strong interaction between the MnCeOδ solid solution and the Co3O4 nanocages gives rise to an improvement in the low-temperature reducibility of MnCeOδ/Co3O4-NC. By quantifying the reduction peak in the H2-TPR, the H2 consumption of the catalyst can be calculated (as listed in Table 2). The results are basically in accordance with the order of the activity of the obtained catalyst. Table 2 H2 consumption, O2 desorption and surface elemental compositions of the samples Desorbed amount of O2 (mmol/g)

H2 consumption (mmol/g)

300°C

Total amount

Co3O4 bulk

5.08

0.05

0.02

0.05

0.94

0.66

Co3O4-NC

9.80

0.06

0.05

0.10

0.90

0.78

MnCeOδ/Co3O4-NC

12.60

0.07

0.08

0.15

0.90

0.83

Sample

Co2+/Co3+ molar ratio

Oβ/Oα molar ratio

The surface oxygen species and mobility of oxygen for catalysts were further investigated by O2-TPD (in Figure 6b), and the desorption amounts of O2 are summarized in Table 2. The MnCeOδ/Co3O4-NC catalyst exhibits three peaks, and the peak intensity indexing active oxygen is larger than that of Co3O4-NC and Co3O4 12


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bulk. It is obvious that introduction of the MnCeOδ component greatly enhanced the adsorbed oxygen desorption amounts, which were over 1.5 times higher than that from Co3O4-NC (Table 2). It was observed that abundant active oxygen species characterized on the the MnCeOδ/Co3O4-NC catalyst surface in the high valent form of O2-and O-. Compared to the Co3O4-NC sample, the MnCeOδ/Co3O4-NC catalyst presented similar signals in the O2 desorption curves, however, the shifting of peaks representing for active oxygen and lattice oxygen to lower temperatures was observed, which was accordance to those of the H2-TPR investigations. It can be speculated that lattice defects and oxygen vacancies were generated by Mn-O-Ce during the formation of the MnCeOδ/Co3O4-NC catalyst.46,49 It was reported that the capacity for oxygen activation could be improved when the catalyst was characterized with the lower oxygen desorption temperature and large desorption peaks intensities.47 3.2 Catalytic activity (a) 100

(b) -5.5 MnCeO/Co3O4-NCs Co3O4-NCs

/m

ol /m

ln k

ol

/m

kJ

-6.5

ol

40

kJ

.6 70

60

.1 56

-6.0

Co3O4-bulk

kJ

Toluene conversion (%)

80

.6 90

-7.0

20

MnCeO/Co3O4-NCs

Co3O4-NCs

Co3O4-bulk

0

-7.5

160

(c)

Toluene 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


180

200

220

240

260

Temperature(℃ )

280

1.9

300

2.0

2.1

2.2

2.3

1000/T (K-1)

100

90

80

70

60

0

200

400

600

800

1000

On-stream reaction time (min)

Figure 7 (a) Activity profiles and (b) Arrhenius curves of the three catalysts; (c) catalytic stability of the MnCeOδ/Co3O4-NC at a GHSV=40000 ml g-1h-1; and (d) normalized toluene oxidation rate 13



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according to the SBET value of the catalyst Figure 7 show the catalytic oxidation toluene of different materials. In Figure 7a,

the MnCeOδ/Co3O4-NC catalysts prepared from the template process have a higher toluene catalytic activity by comparison. This phenomenon is highly correlated with the formation of the MnCeOδ solid solution with a small size and good dispersion, resulting in more oxygen vacancies on the surface of the catalyst and synergy with the Co3O4 support. Compared with related studied (Table 4), The MnCeOδ/Co3O4-NC exhibits a better catalytic activity on toluene oxidation.3,8,48,49,50,51,52,53,54,55 Figure 7b is an Arrhenius plot for the catalytic oxidation of toluene by the three catalysts. The activation energies (Ea) of catalytic oxidation of toluene on the three catalysts are shown in Table 3. The MnCeOδ/Co3O4-NC catalyst has the smallest activation energy (Ea=56.10 kJ mol-1). For the same component of the catalyst, the activation energy is mainly determined by the number of surface active sites on the catalyst. This result is ascribed to the larger SBET value of the MnCeOδ/Co3O4-NC, which provides more active sites, and is consistent with the BET result and the active data. Table 3 Catalytic activity, reaction rates according to the SBET value of the catalyst (RS), activation energy (Ea), and the square of the correlation coefficient (R2)

Catalytic activity (°C) Sample

RS at 200°C

Ea R2

T20%

T50%

T95%

Co3O4 bulk

245

261

278

1.547×10-2

90.60

0.9987

Co3O4-NC

224

243

254

2.662×10-2

70.60

0.9986

MnCeOδ/Co3O4-NC

205

222

230

3.138×10-2

56.10

0.9963

(mmol

The Ea were calculated according to the formula: 𝑙𝑛𝑟 = ―

𝐸𝑎 𝑅

m-2h-1)

(kJ

mol-1)

1

× 𝑇 +𝑙𝑛𝐴, where r, R, A were

toluene reaction rate (μmol g−1 s−1), rate constant (s−1), and pre-exponential factor, respectively.

The stability of the MnCeOδ/Co3O4-NC catalyst at 240°C was investigated by controlling the reaction time in the range of 1200 min (Figure 7c). Obviously, the MnCeOδ/Co3O4-NC catalyst has good catalytic stability under the reaction conditions. Specifically, the different toluene oxidation reaction rates according to the SBET value 14


Page 15 of 23

of the catalyst (RS) are shown in Figure 7d and Table 3. The results demonstrate plenty of active sites can be generated on the MnCeOδ/Co3O4-NC surface, so that the catalyst exhibits excellent catalytic performance. Table 4 The oxidation of toluene over the noble-metal-free catalysts Samples

Reactants

Content (ppm)

3 DOM LSCO

Toluene

1000

Co/Sr-CeO2

Toluene

LaMnO3/δ-MnO2

WHSV T50(°C)

T90(°C)

References

20,000

237

253

（48）

1000

20,000

242

256

（49）

Toluene

1000

30,000

248

258

（8）

Ce0.03MnOx

Toluene

1000

40000

228

238

（50）

Co3O4

Toluene

1000

40000

262

268

（51）

Mn3O4

Toluene

1000

15000

245

270

(3)

Mn12Ce1-S

Toluene

1000

15000

237

277

(52)

Mn-Co(1:1)

Toluene

1000

30000

236

249

(53)

Ce(III)MnOx

Toluene

1000

36000

229

240

(54)

Co3O4-T2

Toluene

1000

37500

234

257

（55）

MnCeOδ/Co3O4-NC

Toluene

1000

40000

222

227

This work

(mL g−1 h−1)

100 95 3 % H2O in

90

Toluene 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


5 % H2O in

85

H2O off

80

Catalyst : 500mg Toluene : 1000 ppm WHSV : 40000 ml g-1 h-1

75 70 65 60 0

10

20

30

40

50

Time on-stream (h)

Figure 8 Effect of water vapor on the MnCeOδ/Co3O4 catalyst

As shown in Figure 8, toluene conversion dropped from 98% to 96% when the water vapor was added in 3 vol.%, and it continuously decreased to 93% when the 15



water vapor volume reaches 5 vol.%. Once the water vapor is off, the efficiency at 240°C recovered back to 98% finally. Water vapor adding affects the catalytic activity slightly at 240°C due to the competitive adsorption. The MnCeOδ/Co3O4 catalyst shows a good resistance to the presence of water vapor. The Co-MOF exhibites a larger specific surface area, which is favorable for the dispersion of MnCeOδ solid solution, and the increase of surface absorbed oxygen. Moreover, the interaction between Co-MOF and MnCeOδ inhibit the growth of MnCeOδ and Co3O4 crystal grains, and the more surface active sites will be exposed owing to the smaller crystal size. A certain lattice defects formed in ceria oxide by doping of manganese benefit the redox ability of MnCeOδ/Co3O4-NC.

Conclusions In this work, the Co3O4-NC and MnCeOδ/Co3O4-NC samples were prepared by a self-template method and an ultrasonic impregnation technique, and the Co3O4 bulk catalyst was prepared by the traditional method. The MnCeOδ/Co3O4-NC catalyst prepared by the template method shows a high specific surface area (100.40 m2·g-1) and a good dispersibility of the MnCeOδ component. Under a toluene concentration of 1000ppm and a space velocity (WHSV) of 40000 ml·g-1 h-1, T95% of the catalystic oxidation of toluene on the MnCeOδ/Co3O4-NC catalyst is 230°C. Compared with the Co3O4 bulk and Co3O4-NC catalysts, the MnCeOδ/Co3O4-NC have the smallest apparent activation energy (56.10 kJ·mol-1). It was concluded that the MnCeOδ solid-solution nanoparticles with good dispersion, high surface active oxygen concentration, superior reducibility at low temperatures, significant MnCeOδ to Co3O4 interactions, and a multiordered pore structure enable the excellent catalytic property of the MnCeOδ/Co3O4-NC.

Acknowledgments This research is financially supported by the Project of Science and Technology Department of Jiangsu Province (BE2016769), the Natural Science Foundation of China (No. 51172107), the Open fund by Jiangsu Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of 16


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Atmospheric Environment Monitoring and Pollution Control(AEMPC)

Supporting Information The directory of supporting information is as follows. The highlights of the article; The detailed characterization procedures and TG-DSC curve of the ZIF-67 crystal

material;

Raman

spectra

of

the

Co3O4

MnCeOδ/Co3O4-NC in this work.

17


bulk,

Co3O4-NC

and


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For Table of Contents Only

23


Co3O4 polyhedral nanocage

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