Catalytic Combustion of Lean Methane Assisted by Electric Field over

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

Catalytic Combustion of Lean Methane Assisted by Electric Field over MnCo Catalyst at Low Temperature x

y

Ke Li, Dejun Xu, Ke Liu, Hong Ni, Feixiang Shen, Ting Chen, Bin Guan, Reggie Zhan, Zhen Huang, and He Lin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00496 • Publication Date (Web): 03 Apr 2019 Downloaded from http://pubs.acs.org on April 3, 2019

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The Journal of Physical Chemistry

Catalytic Combustion of Lean Methane Assisted by Electric Field over MnxCoy Catalyst at Low Temperature †

†

†

†

†

†

Ke Li , Dejun Xu , Ke Liu , Hong Ni‡, Feixiang Shen , Ting Chen , Bin Guan , Reggie †

†

†

Zhan , Zhen Huang , He Lin *

† The Key Laboratory for Power Machinery and Engineering of the Ministry of Education, Shanghai Jiao Tong University, Shanghai, PR China, 200240

‡Chinese Academy of Environmental Sciences, Beijing, PR China, 100012

*Corresponding author: He Lin Dongchuan Road No.800, Min Hang District, Shanghai, P.R.China 200240 Tel.: +86 21 34207774; fax: +86 21 34205959. E-mail: [email protected]

ACS Paragon Plus Environment

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Abstract Electric field was introduced into the catalytic oxidation of lean methane at low temperature over MnxCoy catalyst. Mn1Co5 exhibited the best catalytic performance with the light off temperature (T50) as low as 271 ℃ in the electric field, nearly 60 ℃ lower that in conventional reaction system. The electric field promoted the formation of octahedrally coordinated Mn3+ with active oxygen species released from the reduction of octahedrally coordinated Co3+ in Co3O4 spinel. And octahedrally coordinated Mn3+ sites was proved to be the main active sites for methane catalytic oxidation. With in-situ FTIR technique, it was found that the oxygen species from catalyst bulk instead of gaseous oxygen will adsorb on the octahedrally coordinated Mn3+ sites in the electric field, promoting the activation of CH4 at low temperature. The dehydroxylation process will be accelerated through the formation of CoO(OH) species that will quickly convert due to the enhanced reducibility of Co3+ in the electric field, weakening the inhibition of produced hydroxyl species on active sites. Based on the experimental results, the mechanism of catalytic oxidation of CH4 over MnxCoy catalyst in electric field was proposed.


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1. Introduction In recent years, demand in compressed natural gas (CNG) is increasing for its widespread availability and lower emissions of nitrogen oxides, sulfur and toxic hydrocarbons, along with the rapid growth of global energy demand and the increasingly strict emissions regulations. However, the unburned methane (CH4) in tail gas of natural gas automobile has a global warming potential 23 times higher than that of CO21. The concentration of CH4 existed in ventilation air and natural gas automobile is only 0.1%~0.5%, which is much lower than the inflammability limit of CH4 in the air (5%~15% CH4). And the temperatures the exhausted gas from natural gas automobile are below 500℃ in most conditions2. Besides the industrial importance of CH4 combustion, CH4 is commonly used as model compound in catalytic combustion research, because it is more difficult to burn than most of the hydrocarbons, or volatile organic compounds (VOCs) in general3-4. Low concentration CH4 treatment at low temperature therefore is of great interest due to the environmental concern and academic research. Catalytic combustion of methane (CCM) is regarded as a promising and effective method to reduce CH4. The researches of CCM over the last decade show that noble metal-based catalysts, especially the Pd-based ones2, 5-14，can reveal excellent catalytic property at low temperature but it is too expensive for large-scale industrial applications. So catalysts based on transition metal (such as Fe, Cu, Co, Ni, Mn, Cr etc) 15-22 become more appealing for its low cost and relatively outstanding performance, and in particular the Co-based catalyst15-16, 18-20, 23-27 is a good CCM candidate. J. Chen et al27



reported that the Co-Cr catalyst with Co:Cr = 1:2 exhibited the most promising catalytic activity for their higher valence and coordination number. L.F. Liotta et al.25 contributed the high catalytic activity to the presence of finely dispersed Co3O4 species in good contact with CeO2 as well as the cooperation between both oxides. And Choudhary et al. had observed high catalytic activity of methane oxidation over MnZrO2.28 In addition, Li et al.26 have studied the CCM activities of Co-Mn catalyst prepared by co-precipitation method. Although the sample with a Co/Mn ratio of 5:1 exhibited rather high catalytic activity, the light off temperature of methane is higher than 300 ℃. Besides, the concentration of CH4 tested in the experiment is 1%, which is much higher than that of ventilation air and natural gas automobile. Moreover, the most recent standards for natural gas motivated vehicles (the EURO VI and EPA greenhouse gas legislation) set more stringent methane emission limits with not exceeding 0.10 g km-1 for passenger cars and 0.16 g km-1 for light commercial vehicles.29 These strict limits entail active oxidation catalysts with higher noble metal loading 4 times higher compared to after treatment systems for gasoline vehicles.30-31 Recently, a novel catalytic technology introducing electric field into the conversional reaction process has been widely investigated.32-41 Sugiura. et al.40 reported the promotion effect of electric field on the catalytic oxidative coupling of methane over Ce-W-O catalysts system. Kazumasa Oshima et al.38 found that the efficiency of 1% wt Pd/Ce0.5Zr0.5O2 catalyst on steam reforming of CH4 is 0 at 150 ℃, but the 40.6% efficiency is available in 4mA direct-current (DC) electric field. Oxidative coupling of methane proceeding at 423 K in the electric field over Zr-La2O3


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is also investigated by Tanaka et al.42 And Y. Sekine et al.43 discovered that it is the synergetic effect of electric field with catalyst that improved the reaction of steam reforming of CH4. In our previous study

44

, we investigate the CCM over Pd based

catalysts in the electric field and realize the light off of lean methane at 261 ℃ over 1%wt Pd dopped catalysts. Further application of the electric field over non-noble metals based catalyst, which is promising for methane oxidization, has not been studied. Therefore, is proposed and investigated in this study for catalytic oxidation of lean methane with the electric field. In this study, a series of MnxCoy catalysts were prepared by Self-Propagating HighTemperature Synthesis method (SHS)45, and the electric field was applied in catalytic combustion of ultra-lean CH4 to enhance the low-temperature catalytic activity. The synergetic effect between electric field and the catalyst was thoroughly investigated from the structural and surface point of view. The pathway of methane oxidation in the electric field was intrinsically clarified with an in-situ technique.

2. Experimental 2.1 Catalyst Synthesis The MnxCoy catalysts in this study were prepared by Self-Propagating HighTemperature Synthesis method (SHS). Stoichiometric amounts of 50% Mn(NO3)2 aqueous solution and Co(NO3)2•6H2O (AR grade) were first dissolved in the deionized water, and then stoichiometric glycine (CH2NH2COOH) was appended into the precursor mixture as fuel. After stirred on a heating plate until the solution was



transparent and harmonious, the mixture was transferred into a corundum crucible and was therewith put into a muffle furnace, which would maintained at 400 ◦C to promptly ignited and run out in minutes, followed by maintaining in the muffle furnace for 4h. The resulting powers would go through a process of compaction, grinding and sieving to be 40-80 meshes particles as test samples. The catalysts are identified as MnxCoy and here x and y represents the mole ratio of Mn and Co. 2.2 Catalyst Characterization The physicochemical properties of MnxCoy catalysts were obtained by various characterization methods. The X-ray diffraction (XRD) measurement was obtained on a computerized Rigaku D/max-2200/PC X-ray diffractometer with a Ni-filtered u Kα (k = 0.1528) radiation to examined the crystalline structure of the catalysts and the dispersion of the active species. The recorded 2θ range was 20-90°. The X-ray photoelectron spectroscopy (XPS) analyses were conducted to obtain the concentration of active species on the surface, which was proceeded on a RBD upgraded PHI 5000C ESCA scientific electron spectrometer (Perkin-Elmer) equipped with a Mg Kα radiation (h = 1253.6 eV) as the excitation source. The binding energies of Mn2p, Co2p and O1s were normalized using Cs 1s (284.8 eV). To evaluate the reducibility of the catalysts, Hydrogen temperature-programmed-reduction (H2-TPR) was carried through a fixedbed flow reactor by a gas chromatograph (GC 2014) equipped with a TCD detector. The examined catalysts was firstly preprocessed in the N2 atmosphere at 400 ℃ for 1 hour, and then tested in a gas atmosphere containing 95% N2 and 5% H2 from 20 ℃ to 600 ℃ at a programmed temperature increasing rate of 10 ℃/min. The in situ diffuse


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reflectance infrared Fourier transform spectroscopy (in-situ DRIFTS) was applied to explore the absorption and oxidation of CH4 on the catalyst surface by a FTIR spectrometer (Thermo Nicolet 6700). And the examined catalysts was firstly pretreated in the N2 atmosphere at 450 ℃ for 2 hour. The spectra was recorded in a range of 4000800 cm-1 at a spectra resolution of 1 cm-1. 2.3 Catalyst Evaluation Catalyst performance test was evaluated in a fixed-bed flow reactor, as shown in Figure 1. The catalyst was filled in the middle of the quartz tube reactor, which was installed in a tubular furnace whose temperature was set by a PID controller. There were two stainless steel electrodes installed at two sides of the catalyst and connected to a DC power supply with a constant-current output from 0 mA to 100 mA. And there is another a K-type thermocouple inserted into the catalyst bed to measure the catalyst temperature so as to prevent the temperature deviation.

Figure 1. Experimental setup of catalytic process combining with electric field

The operating conditions were as follows: 0.2% CH4, 1.0 % O2, and N2 as balance gas, with a total gas flow rate of 150 ml/min corresponding to a gas hourly space velocity (GHSV) of 30000h-1. The activity data were obtained at a steady condition by



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a high-resolution Thermo Nicolet 6700 FTIR spectrometer at a sampling rate of 1Hz. The catalytic activity was calculated in terms of the CH4 conversion according to Equations 1.

CH 4 

[CH 4 ]in  [CH 4 ]out  100% [CH 4 ]out

(1)

3. Results and discussion 3.1 Catalytic Activity Figure 2 presented the methane oxidation activities of MnxCoy catalysts. According to the results, methane conversion over MnxCoy catalyst increased with Co content. And there is a significant improvement of activity with Co content higher than 50%.Thus the catalysts can be divided into two groups. Catalysts with Co/Mn molar ratio higher than 1:1 are assigned to group-A while the others drawn in group B. Mn1Co5 catalyst exhibited the best catalytic performance with low light-up temperature (T50) of 326 ℃, while the activity of Mn1Co3 is nearly the same. With electric field introduced into the catalytic system, activity of group-A catalysts were slightly improved while methane conversion over group-B catalysts is significantly promoted. And the light off temperature of Mn1Co5 is reduced to as low as 271 ℃. This indicates that catalysts with high Co content exhibit a better synergistic effect with electric field on methane oxidation. However, when the percent of Co/Mn is higher than 5:1, there will be a decrease in the catalytic activity as well as synergistic effect with electric field, as shown in Fig. S1.The input voltage depend on the electric property of the catalysts and varied with the change of reaction temperature, as listed in table 1.


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100 Solid：Catalytic Reaction Open：Catalytic Reaction in EF

Methane 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


Mn1Co5

80

Mn1Co3 Mn1Co1

60

Mn3Co1 Mn5Co1

40

Mn1Co5 in EF Mn1Co3 in EF Mn1Co1 in EF

20

Mn3Co1 in EF Mn5Co1 in EF

0 100

200

300

400

500

600

Temperature(℃)

Figure 2. CH4 conversion efficiency over MnxCoy catalyst with different ratios of Mn/Co; Reaction conditions: [CH4] = 0.2%, [O2] = 1%, N2 as Balance Gas; GHSV = 30,000 h−1.The input current is 20 mA.

Table.1 Test results of CH4 oxidation over MnxCoy catalyst with and without electric field Without EF

Sample Mn1Co5 Mn1Co3 Mn1Co1 Mn3Co1 Mn5Co1

With EF

Input parameter with EF

T20/℃

T50/℃

T90/℃

T20/℃

T50/℃

T90/℃

Current(mA)

Votage(V)

280

326

390

203

270

347

20

78-247

390

234

278

354

20

71-211

482

347

399

480

20

62-178

490

369

421

490

20

54-151

535

393

450

535

20

34-112

293 353 375 393

330 404 425 450

Figure 3 shows the Arrhenius plots of MnxCoy catalyst with different ratios of Mn/Co with or without electric field, and Table.S1 lists the calculated kinetic parameter. And the Ea of catalysts for methane oxidation reaction are lowered with the introduction of electric field. The Ea over Mn1Co5 is 45.39 kJ·mol-1 , far less than that of 65.06 without electric field. There is an obvious difference of activation energies (Ea) over catalysts from group A and B, which may be due to the different catalyst crystal structural of each group.



4

2.5

(a)

2.0

(b)

3

1.5

2

1.0 0.5

lnk(s-1)

lnk(s-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

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Mn1Co5

0.0

Mn1Co3

-0.5

Mn1Co1

Mn1Co5 0

Mn1Co3 Mn1Co1

Mn3Co1

-1.0

1

-1

Mn5Co1

-1.5

Mn3Co1 Mn5Co1

-2

1.3

1.4

1.5

1.6

1000/T(K-1)

1.7

1.8

1.3

1.4

1.5

1.6

1000/T(K-1)

1.7

1.8

Figure 3. (a) Arrheniu plots upon MnxCoy catalyst with different ratios of Mn/Co; (b) Arrheniu plots upon MnxCoy catalyst with different ratio of Mn/Co with electric field (The input current is 20 mA)

3.2 Catalyst Characterization 3.2.1

XRD results

The XRD analysis results are presented in Figure 4. Figure 4a shows the XRD patterns of MnxCoy catalyst with different ratios of Mn/Co. It is clear that those patterns fell into two categories. The peaks at 2θ = 31.271°, 36.845°, 44.808°, 59.353°, 65.231° for pure Co sample belong to Co3O4 (JCPDS: 43-1003). So it can be deduced that the characteristic peaks of group A were all assigned to Co3O4 phase, which means that Co element in those four catalysts existed in the form of pure Co3O4 cube spinel structure. Meanwhile, no MnO2 or Mn3O4 phase could be observed obviously in the XRD patterns, indicating that Mn element has dispersed well into the Co3O4 lattice and there was no conglomeration of MnOx. And the peak intensities of the four catalysts decreased obviously with the increase of Mn/Co ratio, demonstrating that the crystallite size also decreases and the number of crystallite defection may increases with the increase of the


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quantity of adulterated manganese. However, catalysts from group B exhibit different characteristic patterns. The patterns at the position of 2θ = 28.880°, 31.015°, 32.315°, 36.085°, 44.446° , 58.510°, 59.840° and 64.651°. are all belong to Mn3O4 according to the JCPDS: 24-0734Correspondingly, no CoO or Co3O4 phase could be observed obviously, indicating that Co element has dispersed well into the Mn3O4 lattice. To investigate the effect of electric field over the catalytic structure, the catalyst samples were treated in electric field in pure N2 at 300 ◦C for 5 hours. As shown in Figure 4b, there were no obvious differences in characteristic patterns over treated catalysts, demonstrating that electric field did not change the catalyst crystal structure. As the Co3O4 and Mn3O4 are the typical spinel crystal structure, the possible electron migration induced by the electric field may occur on manganese and cobalt ions distributed in the octahedral sites and tetrahedral sites, which cannot be detected by XRD method.

Co3O4

(a)

Mn

(b)

Co

Mn1Co5

Mn1Co5

Mn1Co5

Intensity(a.u.)

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


Mn1Co3 Mn1Co1

Mn3Co1

Mn3Co1

Mn3Co1

Mn1Co1

Mn5Co1

Mn1Co1

Mn

20

25

30

35

40

45

50

55

60

65

70

75

80

20

25

30

Figure 4.

35

40

45

50

55

60

65

70

75

80

2θ(°)

2θ(°)

(a) XRD patterns of MnxCoy catalyst with different ratios of Mn/Co; (b) XRD patterns of fresh and

treated Mn1Co5, Mn3Co1 and Mn1Co1sample, treated catalysts in electric field in pure N2 at 300 ◦C for 5h.



40

=19.1

Percent Count(%)

Percent Count(%)

40 30 20 10 0

12

14

16

18 20 Dp(nm)

22

24

20 10 0 10 12 14 16 18 20 22 24 26 28 Dp(nm)

26

(d) 40

=17.8

Percent Count(%)

Percent Count(%)

40 30 20 10 0

=18.9

30

(a)

20 10 0

12

14

16

18 20 Dp(nm)

22

24

40

26

12 14

16 18 20 Dp(nm)

22 24

26

(e)

40 Percent Count(%)

=21.7

30 20 10 0 10 12 14 16 18 20 22 24 26 28 Dp(nm)

(c)

=17.2

30

(b)

Percent Count(%)

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

=18.0

30 20 10 0 10 12 14 16 18 20 22 24 26 28 Dp(nm)

(f)

Figure 5. TEM images of different catalysts: (a) fresh Mn1Co5; (b) fresh Mn1Co1; (c) fresh Mn3Co1; (d) treated Mn1Co5; (e) treated Mn1Co1; (f) treated Mn3Co1.

The TEM morphologies of fresh and treated MnxCoy catalysts are shown in Figure 5a-f. The MnxCoy catalysts display amorphous shape with average size from 16 mm to 22 mm. The particle size of the sample exhibit a slight decrease after treated in the electric field, and reduced value decreases with the increment of Mn ratio. This indicates that the decrease of the particle size may be related to the behavior of Co oxide in the electric field. Figure 6a-f shows the HRTEM images of the fresh and treated MnxCoy catalysts. The measured d spacing values of 0.470 nm and 0.425 nm on fresh


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Mn1Co5 image correspond to (110) plain of Co3O4. No lattice fringe of crystal phase attributed to Mn oxides is observed, indicating that most Mn species enter the lattice of Co3O4 spinel. Inversely, lattice fringes of 0.272 nm and 0.318 nm consistent with the (103) and (102) plain of Mn3O4 were observed on images of fresh Mn1Co1 and Mn3Co1, suggesting the entrance of Co species into Mn3O4 spinel. It can be concluded that catalysts from group A and B exhibit quite different structural morphologies, leading to different performance for methane oxidation. Figure 6d shows another lattice with dspacing values at around 0.20 nm, ascribed to the (111) plain of CoO. This indicates that the Co3O4 was partly reduced in the electric field, resulting in the decrease of the particle size. However, no obvious changes was found between images of fresh and treated catalysts from Group B, which may be accounting for the poor activitypromotion phenomenon of the catalysts in the electric field.

(a)

(d) CoO

0.470 nm （103）

0.201 nm

Co3O4

0.470 nm

（111）

（110）

Co3O4

(b)

(e)

0.273 nm

（103）

Mn3O4

（112）

（112） 0.318 nm

Mn3O4


0.319 nm


(c)（103）

(f)

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（103）

（112）

0.272 nm

0.273nm

Mn3O4

0.317 nm

Mn3O4 （112） 0.318 nm （112） 0.318 nm

Figure 6. HRTEM images of different catalysts: (a) fresh Mn1Co5; (b) fresh Mn1Co1; (c) fresh Mn3Co1; (d) treated Mn1Co5; (e) treated Mn1Co1; (f) treated Mn3Co1.

3.2.2

XPS results

XPS spectra of MnxCoy catalyst with different Mn/Co ratios were carried out to obtain the local atomic environments and valence states of the fresh sample and the sample treated in electric field in pure N2 at 300 ◦C for 5 hours. The Mn (2p) XPS spectra of MnxCoy catalyst are shown in the Figure 7a. Generally, the Mn 2p peaks are composed of two main characteristic peak, which are Mn 2p3/2 and Mn 2p1/2 peaking at about 641.1eV and 652.4eV respectively.46-47 Since the difference of the binding energy values between Mn2+, Mn3+ and Mn4+ are not obvious, the Mn 2p3/2 characteristic peak was divided into three components, associated with the binding energy of 642.01 eV, 642.7 eV and 645.7 eV respectively.23, 48-49 The peak of Mn4+ indicates the existence of MnO2 or Mn3O4, which were not detected in the XRD spectrum due to their low concentration and high similarity with the characteristic pattern of Mn3O4. Mn3+ predominates the surface atomic environments of Mn element in all MnxCoy catalyst, Moreover, the different Mnx+ ion concentration distributed in the different Mn/Co ratio sample could be explained by the easy formation of solid solutions


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between cobalt and manganese oxides.49 The Mn4+ ion and Mn3+ concentration increased with the decrease of Mn/Co ratio and was consistent with sequence of the catalytic efficiency on the methane combustion. Over the treated catalysts, only Mn3+ ion concentration of treated catalysts increases apparently while the ratio of Mn4+ is not varied comparing with the fresh sample, which demonstrated that high Mn3+ ion concentration may be an important reason for the improvement of the catalytic performance. In addition, the concentration of Mn2+ decreased accompanied with the increase of Mn3+, demonstrating that the formation of Mn3+ in the electric field is from the oxidation of Mn2+ And the ratio of Mn3+/( Mn3++ Mn2+) is not higher than 66.7%, indicating that formation of Mn3+ is from oxidation of octahedral Mn2+ rather than tetrahedral Mn2+, which may be due to the stability of the latter ion. Figure 7b shows that the Co (2p) spectra were ascribed to two main peak of Co 2p3/2 and Co 2p1/2 with binding energy at around 780.3 and 795.8 eV. To investigate oxidation state of Co surface species, Co 2p3/2 spectra were de-resolved into two asymmetric peaks of Co3+ and Co2+ component. The observed binding energies centered at 780.3-780.7 eV and 779.2-779.6 eV are associated with the tetrahedral Co2+ and octahedral Co3+ in Co3O4.50-52 And there was a satellite peak at 786.7 eV characteristic for octahedral Co2+ cation in Co3O4 or Co2+ in CoO. As no CoO pattern was observed in XRD results, The satellite peak can be ascribed to the octahedral Co2+ cation, which was not common in Co3O4,50 further proving the existence of the ionic equilibrium among manganese and cobalt ions. As it shown in Fig.4(b), Co2+ was the major species on the surface of the fresh sample, in accordance with the result of



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Todorova, S. et al.49 The Co2+/( Co2++ Co3+) ratio value increases with the decrease of Mn/Co ratio, Some researches indicated that the presence of Co2+ is helpful to the formation of the oxygen vacancy and deficiency which would facilitate the mobility and release of the lattice oxygen, leading to the promotion of the catalytic oxidation performance.23, 53 The concentration of Co2+ and Mn4+ increases synchronously with the decrease of Mn/Co ratio and the Co2+ atom concentration of MnxCoy catalyst are all higher than the original percentage of 33.3% in Co3O4, which suggests that the replacement of Co3+ by Co2+ and Mn3+ may occurred in the catalyst synthesis progress.47, 49 Co3+ + Mn2+  Co2+ + Mn3+

(2)

It is noteworthy that the Mn3+ proportion on the of group A catalyst increased obviously after treated in the electric field, while that of group B catalyst varied very little. This indicates that the reduction of Co3+ in the electric field is easier to occur in group A catalysts with Co3O4 spinel crystal structure, while group B catalysts with Mn3O4 spinel crystal structure and less Co3+ is less active in the electric field. As indicated in the Figure 7c, O 1s spectra of the catalysts could be resolved into two asymmetric peaks, and the characteristic peak with the lower binding energy (529.7-529.8 eV) was assigned to lattice oxygen (denoted as OL, such as O2-, etc.) and the other one with the higher binding energy (531.0-531.5 eV) was assigned to surface chemisorbed oxygen (denoted as OA, such as O22-, O-, OH-, CO32-,etc.).23, 54 It is well known that high concentration of surface chemisorbed oxygen is beneficial for the catalytic activity of oxidation reactions.23, 53 OA covered a pretty large proportion in the


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surface oxygen species, which is a main reason for the better catalytic performance of MnxCoy catalyst in the most metallic oxide catalysts. It is interesting catalysts from group A and group B had a close OA/(OA+OL) value , which may be attributed to the similar crystal structural of Co3O4 and Mn3O4. The OA/(OA+OL) ratios of the treated catalysts from group A sample increased obviously, which may be an important reason for the improvement of catalytic property in electric field. Since the sample were treated in the experimental atmosphere without O2, the increase of surface chemisorbed oxygen can only be ascribed to the transformation of lattice oxygen through the reduction process of the metal oxide in the electric field. It can be seen from the results above that Mn3+ and Mn4+ is hard to be reduced in the electric field,23 demonstrating that electric field could facilitate the transformation process of lattice oxygen to surface chemisorbed oxygen through promoting the reduction of Co3+ to Co2+. Mn 2p

Mn1Co5

Mn 2p

Mn1Co3

Mn1Co1

Mn 2p

Mn3Co1

Mn 2p

Mn 2p

660

655

Mn4+

Mn4+

Mn3+ Mn2+

Mn 2p

Mn1Co5

Mn3+

Mn 2p

Mn1Co3

Mn 2p

Mn1Co1

Mn2+

Intensity(a.u.)

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


Mn4+

Mn3+ Mn2+

Mn4+

Mn3+ Mn2+

Mn 2p

Mn3+ Mn2+

Mn 2p

Mn5Co1

Mn4+

650

645

Binding Energy(eV)

Mn4+

Mn4+

Mn4+

Mn3Co1 Mn

640

660

(a) ACS Paragon Plus Environment

Mn5Co1

655

650

4+

Mn

4+

645

Binding Energy(eV)

Mn3+ Mn2+

Mn3+ Mn2+

Mn3+ Mn2+

Mn3+ Mn2+

Mn3+ Mn2+

640

Page 18 of 39

Co2+

Co 2p

Mn1Co5

Co

2+

Co3+

Co 2p

Mn1Co5*

Co 2p

Mn1Co3

Co2+

Co3+

Co 2p

Mn1Co3*

Co 2p

Mn1Co1

2+

Co3+

Co 2p

Mn1Co1* Co2+

Co 2p

Mn3Co1

Co2+

Co3+

Co 2p

Mn3Co1*

Co 2p

Mn5Co1*

Co 2p Mn5Co1

800

795

Co

Co2+

790 785 780 Binding Energy（eV）

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

Intensity(a.u.)


Co3+

800

775

(b)


795

790

Co3+

Co2+

Co3+

Co3+

Co2+

Co3+

Co2+

Co3+

785

780

Binding Energy（eV）

775

Page 19 of 39

Mn1Co5

O 1s Mn1Co3

O 1s

O 1s

OA

OL

O 1s

OA

Mn1Co3*O

L

Mn1Co1* OL OA

O 1s

OA

Mn1Co5* OL

OA

OA

OA

Mn5Co1

O 1s

OA

Mn1Co1

O 1s Mn3Co1

O 1s

OL

Intensity(a.u.)

O 1s

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


Mn3Co1*

OL

OA

OA

O 1s

OA

Mn5Co1* OL OA

OA

535 534 533 532 531 530 529 528 527 Binding Energy（eV）

535 534 533 532 531 530 529 528 527 Binding Energy（eV）

(c) Figure 7. XPS spectra of fresh and treated MnxCoy catalyst(termed as MnxCoy*) with different ratios of Mn/Co, treated in electric field at 300 ◦C in N2 atmosphere for 5h: (a) Mn (2p), (b) Co (2p), (c) O (1s)

Table.2 Surface atomic composition and atomic ratio of Mn1Co5 and Mn3Co1 determined by XPS O(1s)

atomic ratio(%)

Co (2p)

sample OA

OL

Mn4+

Mn3+

Mn2+

Co3+

Co2+

Mn5Co1

47.3

52.7

8.5

47.9

43.5

36.8

63.2

Mn5Co1*

50.2

49.8

10.0

44.9

45.1

35.6

64.4

Mn3Co1

44.2

55.8

10.6

48.3

41.1

35.2

64.8

Mn3Co1*

48.8

51.2

10.8

45.1

44.1

34.5

65.5

Mn1Co1

39.1

60.9

12.8

49.0

38.2

32.7

67.3

Mn1Co1*

44.2

55.8

13.3

45.5

41.2

31.1

68.9

Mn1Co3

45.9

54.1

13.9

56.9

29.2

31.2

68.8



Page 20 of 39

Mn1Co3*

60.2

39.8

14.1

52.6

33.3

25.4

74.6

Mn1Co5

47.2

52.8

13.6

57.5

28.9

30.1

69.1

Mn1Co5*

65.4

34.6

16.8

51.9

31.3

23.2

76.8

* Catalyst treated in the electric field

3.2.3

H2-TPR

H2-TPR was performed to investigate the reducibility of MnxCoy catalyst with or without electric field, a key indicator reflecting the oxidation ability of catalyst. As shown in the Figure 8, the spectra of pure Mn oxide consisted of two reduction peaks (α and β), which were assigned to the reduction of highly dispersed MnO2 and Mn3+ to their corresponding metals respectively.46,

49, 55-56

And the small α-peak area was

consistent with the little amount of MnO2 in XPS spectra, which is also an evidence for the existence of MnO2. The spectra of Co catalyst also exhibited two reduction peaks (μ and ν), and the μ-peak at around 310 ℃ was ascribed to the reduction of Co3O4 to CoO while the ν-peak at around 380 ℃ was ascribed to the reduction of CoO to Co0.2425, 49, 54, 57

Since the reduction temperature internal of cobalt and manganese oxide are

very close, it is difficult to separate their reduction peak entirely.46, 49 Considering the reduction peak position of pure Co and pure Mn catalyst, the spectra of Mn1Co5 and Mn1Co3 all include three peaks, in which the first one was assigned to the reduction of MnO2, the second one to the reduction of Co3O4 to CoO and the last one to the reduction of CoO to Co0 and Mn3+ to Mn2+. The reduction peaks of catalysts from group A all shifted to the lower temperature with the decrease of Mn/Co ratio, which is regarded as a positive factor of the better catalytic activity for methane oxidation. As for Mn1Co1,


Page 21 of 39 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


Mn3Co1 and Mn5Co1, the first reduction peak was also ascribed to the reduction of MnO2, and the second one to a combination of Co3O4 to CoO, Mn3+ to Mn2+ and CoO to Co0. Although the first reduction peak of group B catalysts was at relatively low temperature, the second and third reduction peak located at the higher temperature. The movement of u and β peak with the variation of Co/Mn ratio exhibit the same trending with the catalytic activity, indicating that the reduction Co3O4 to CoO and Mn3+ to Mn2+may be mainly responsible for the catalytic activity of methane oxidation. When the experiment was performed in the electric field, the α-peak of all MnxCoy catalyst shifted to lower temperature of around 200 ℃, indicating that the reduction of MnO2 was enhanced in the electric field. However, Mn4+ is not possibly the active sites for methane considering the fact that catalysts from group B, though exhibit improved capability of Mn4+ in the electric field, did not show obvious promotion in activity. In addition, the reduction peak of Mn3+ to Mn2+ and Co3+ to Co2+ over catalysts from group A both shifted two lower temperature, but the H2 consumption of Mn3+ reduction increased significantly in the electric field, while that of Co3+ exhibit a corresponding decrease. This is in good accordance with the XPS results that the equation 2 will occur in the electric field, leading to formation of more Mn3+ in catalysts from group A. By contrast, reduction peak of catalysts from group B show no such changes. It can be proposed that the octahedral Mn3+ in Mn3O4 spinel is the active sites for methane oxidation in the electric field and exhibit the most intense correlation with catalytic activity of Group A catalysts, followed Co3O4.



ν

Co3O4

μ

H2 Consumption

Mn1Co5

α

Mn1Co3

β μ

Mn1Co1

ν

β

α

Mn1Co3

ν

Mn1Co1

Mn5Co1

Mn5Co1

100

Mn1Co5

Mn3Co1

Mn3Co1

Mn3O4

(b)

ν

μ

(a)

Co3O4

H2 Consumption

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

Page 22 of 39

β

α 200

300

400

500

β

100

600

μ α

Mn3O4

200

300

400

500

600

Temperature(℃)

Temperature(℃)

Figure 8. H2-TPR spectra of MnxCoy catalyst (a) without the electric field; (b) with the electric field

3.2.4

In-situ DRIFTS Studies

The in-situ DRIFTS studies were performed to investigate the mechanism of methane oxidation over MnxCoy catalyst with and without the electric field. Figure 9a shows the spectra of CH4 adsorption without the electric field. Only bands of CH4 (3016 cm-1 and 1304 cm-1)26,

58

were detected, indicating that the

chemisorption and activation of CH4 can hardly proceed in the absence of oxygen over the MnxCoy catalyst, which is different from our previous conclusion of Pd based catalysts. When electric field was introduced into the reaction, as shown in Figure 9b, the band of CH4 (3016 cm-1 and 1304 cm-1) decreased slightly with temperature, indicating that the absorption of CH4 could occur in electric field under the oxygen-free condition.59-61 And weak bands of formates (1540 cm-1 and 1390 cm-1) and carbonates (1490 cm-1, 1341 cm-1 and 1040 cm-1)58,

62

were detected with the increase of

temperature, suggesting that CH4 could be partially oxidized to formates and further to carbonates in electric field without gaseous O2. Since no bands of O2- (around 1200-


Page 23 of 39 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


1050 cm-1) 26, 58, 63-67could be detected, the only possibility of the oxygen species source is the lattice oxygen in spinel structure, demonstrating that electric field is conductive to the transformation process of lattice oxygen to surface chemisorbed oxygen, which is also proved in XPS analysis results. The band of hydroxyl (1000 cm-1) did not appeared, demonstrating that hydroxyl could not be easily formed in the absence of O2. The spectra of CH4 oxidation without electric field is shown in Figure 9c. The intensity of CH4 decreased with temperature while the band at 1550 cm-1 attributed to formates (HCO or HCOO) was observed at 300℃, And band peaked at 1341 cm-1 and 1220 cm-1assigned to carbonates were obvious below 350 ℃.26, 58, 62, 68 Both bands of formats and carbonates turned weak above 400 ℃ where the efficiency of CH4 oxidation approaches 100%, due to quick formation of CO2. Junhua Li et al.26 proposed that the formation of O2- was mostly due to the interactions between the active sites on surface and gaseous O2. The band at 1135 cm-1 corresponding to the O2- formed by the absorbed O2 appeared at 250 ℃,26 weakened at 350 ℃ and vanished at 400 ℃, demonstrating that O2- may not participate in the CH4 oxidation at 200 ℃ and might be quickly consumed by hydrocarbons at relatively higher temperature. The broad band at 1000 cm-1 could be attributed to hydroxyl on MnO(OH),69 and the band peaked between 3300 cm-1 and 3400 cm-1 was a sign of hydroxyls associated with dopant trivalent cations, indicating that the hydroxyl species will adsorb on the Mn3+. Junhua Li et al obtained the similar result and proposed the potential behavior of hydroxyl species in the reaction process, which will be further discussed in the discussion part. When electric field was introduced in the CH4 oxidation, as shown in Figure 9d,



no changes was observed over the band of O2- species, suggesting that electric field may not accelerate the absorption of gaseous O2. The reaction at Compared with the spectra of CH4 oxidation without electric field presented in Figure 9c, the band intensity of formates (1543 cm-1) and carbonates (1460 cm-1, 1341 cm-1 and 1012 cm-1)62 is lower, indicating oxidation of formates and the decomposition of carbonates in electric field respectively. And the band assigned to CO2 appeared at lower temperature of 300 ℃. The band of hydroxyl on MnO(OH) (1005 cm-1) and hydroxyls associated with dopant trivalent cations were not observed in the spectra, while band of hydroxyls associated with dopant trivalent cations appeared at 250 ℃. It indicates that the OH group may mainly be attached on the Co2+ and form CoOOH species in the oxidation process under the electric field, which is different from the conventional reaction process.

3016

(a)

3016

500℃

3200

Absorbance(a.u.)

350℃ 300℃

1304

1040

350℃ 300℃ 250℃

250℃

200℃

200℃

150℃

1600

1490 1390 1341 1540

400℃

400℃

3000

(b) 450℃

450℃ Absorbance

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

Page 24 of 39

1400

Wavenumber(cm-1)

1200

1000

3200

3000


1600

1400

Wavenumber(cm-1)

1200

1000

Page 25 of 39

(d)

(c) 2342 3284

3000 500℃

3284

1630 1559 1418 1301 1050 1000

3000

2342 1630

500℃

1559 1418 1301 1050 1000

Absorbance

450℃

450℃

Absorbance

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


400℃ 350℃

400℃ 350℃

300℃

300℃

250℃

250℃

200℃

3200

2800

2400

200℃

1600

Wavenumber/cm-1

1200

3200

2800

2400

1600

Wavenumber/cm-1

1200

Figure 9. In situ DRIFTS spectra of Mn1Co5 catalyst: (a) CH4 adsorption, (b) CH4 adsorption in electric field, (c) CH4 oxidation in electric field (d) CH4 oxidation in electric field

3.3 Discussion In the last decades, many studies focused on the mechanism of CH4 oxidation over metal oxide catalysts. L-H models, E-R models, and MvK models are proposed as the possible kinetic models for methane oxidation. It is widely accepted that oxygen from gaseous phase adsorbed on the catalyst surface and form active sites for the methane oxidation at low temperature, following the E-R models, while MvK redox cycle is involved in reaction at higher temperature. In this study, the DRIFT results show that the methane adsorption and activation occur at lower temperature in the electric field without the existence of O2- species formed from the gaseous O2. Since the lattice oxygen in the catalyst bulk is more liable to migrates to the catalyst surface due to high mobility in the electric field, the methane oxidation process in the electric field may also follow MvK models at low temperature. And the oxidation rate is proved to boost significantly with bulk oxygen available for the reaction. The behavior of hydroxyl group is another factor influencing the oxidation rate due to its inhibiting effect on the surface vacancies. Junhua Li et al investigate the migration



of hydroxyl group over manganese cobalt oxides and proposed that hydroxyl group may transfer from octahedrally coordinated Co3+ to tetrahedrally located Mn2+ and form MnO(OH) species which is unstable and further be oxidized to stable MnO4 tetrahedron. They assumed that the above process is faster than the dehydroxylation steps occurred over cobalt cations, which can prove the promotional effects of manganese. In this study, similar results can also be concluded from the DRIFTS spectra for CH4 oxidation without electric field. However, with the introduction of electric field, hydroxyl group was possibly attached on the Co2+ rather than Mn2+. This may be due to formation of more octahedrally coordinated Co2+ through reaction shown as equation 2. Though the formed CoO(OH) with Co3+ at octahedral sites is more stable than MnO(OH) with tetrahedrally located Mn3+, dehydroxylation of CoO(OH) will be accelerated in the electric field due to enhanced reducibility of Co3+ in the electric field. And this may be another factor for the promoted CH4 oxidation in the electric field. As discussed above, the promotion effect of electric field for the methane catalytic oxidation mainly reflects in two aspects: One is the formation of active sites from active oxygen species in the support through MvK models at low temperature (< 300 ℃), plays a leading role for the promoted catalytic performance in the electric field. Comparatively, the formation of active sites would occur at high temperature with the dissociation of gaseous O2 through ER models in conventional reaction way. The other role the electric field played in the oxidation process is to lead the dehydroxylation process to a faster pathway with the CoO(OH) as an important intermediates. And the attachment of the hydroxyl group on the active sites which will inhibit the oxidation


Page 26 of 39

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rate was eliminated due to the quick reduction of CoOOH species in the electric field. Based on the discussion above, the reaction pathway of methane catalytic oxidation over MnxCoy (x/y

Catalytic Combustion of Lean Methane Assisted by Electric Field over

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