Enhanced CH4 and CO oxidation over Ce1-xFexO2-Î´ hybrid catalysts

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Energy, Environmental, and Catalysis Applications

Enhanced CH4 and CO oxidation over Ce1-xFexO2-# hybrid catalysts by tuning the lattice distortion and the state of surface irons species Danyang Li, Kongzhai Li, Ruidong Xu, Xing Zhu, Yonggang Wei, Dong Tian, Xianming Cheng, and Hua Wang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b05409 • Publication Date (Web): 08 May 2019 Downloaded from http://pubs.acs.org on May 8, 2019

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ACS Applied Materials & Interfaces

Enhanced CH4 and CO oxidation over Ce1-xFexO

1

hybrid

2

catalysts by tuning the lattice distortion and the state of surface

3

irons species

4

Danyang Lia, b, Kongzhai Lia, b*, Ruidong Xua, Xing Zhua, c, Yonggang Weia, c, Dong Tianb,

5

Xianming Chenga, c, Hua Wanga, b, c

6

aState

7

Science and Technology, Kunming 650093, China

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bEngineering

9

Education, Kunming University of Science and Technology, Kunming 650093, China

Key Laboratory of Complex Nonferrous Metal Resources Clean Utilization, Kunming University of

Research Center of Metallurgical Energy Conservation and Emission Reduction, Ministry of

10

cFaculty

11

Kunming 650093, China

of Metallurgical and Energy Engineering, Kunming University of Science and Technology,

12 13

*Corresponding author: Kongzhai Li ([email protected]; [email protected]);

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ACS Paragon Plus Environment

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

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Abstract CeO2–Fe2O3 mixed oxides are very attractive as catalysts for catalytic oxidation. Herein,

3

we report the structural dependence of the Ce1-xFexO

4

oxidation via changing lattice distortion degrees, surface Fe2O3 states and oxygen vacancy

5

concentrations. The lattice distortion degree and oxygen vacancy concentration of Ce-Fe-O

6

solid solution can be tuned by changing the contents of Fe and the precipitation temperatures

7

in the preparation process. The precipitation at relatively high temperature (70 oC) promotes

8

the lattice distortion while lower temperature (0 oC) helps the formation of surface oxygen

9

vacancy. The in situ DRIFT/Raman experiments and the physicochemical characterization

10

suggest that both the CO and CH4 oxidations mainly follow a Mars-van Krevelen mechanism.

11

Both the lattice distortion and the surface iron species play crucial role in determining the

12

catalytic activity via affecting the redox property of the catalysts. The surface iron species,

13

combining with the oxygen vacancies, improve the catalytic performance by enhancing the

14

adsorption capacity of reactants and reducibility of catalysts. The lattice distortion of CeO2

15

contributes to the catalytic activity by tuning the oxygen mobility in the bulk, which promote

16

the re-oxidation rate of catalysts.

17

Key words: Methane combustion; CO oxidation; structural dependence; Ce1-xFexO

18

catalysts; lattice distortion

19

1. Introduction

:;

catalysts for CH4 combustion and CO

:;

20

The increasingly stringent emissions of CO, NOx, HC, and particulate matter arising

21

from the traditional fossil fuels used in stationary and mobile sources are emerging as an

2


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1

important issue due to its adverse environmental impact.1-4 Simultaneously, the use of natural

2

gas in vehicle, power generation and related heating applications is a promising program

3

launched in the world owing to the relatively abundant reserves, lower carbon intensity and

4

higher fuel efficiency.5-6 However, the release of unburned methane partly counteracts the

5

advantages because its warming effect is recognized to 25 times higher than that of CO2.6-8

6

Catalytic oxidation at relatively low temperatures is an effective and economical technology

7

for abating the emissions of unburned methane and CO,9-12 and considerable attentions have

8

been paid on the capable catalyst systems.3, 13-16

9

Among different types of catalysts, ceria has received intense applications in

10

heterogeneous catalysis owning to its remarkable redox property and high oxygen storage

11

capacity (OSC).11,

12

treatment for significant decrease of the surface area and therefore cannot meet the

13

requirements for practical use.19-21 Fortunately, the CeO2 shows improved performance after

14

doping or surface modification. It is generally reported that introducing other metal ions into

15

ceria lattice to form CeO2-based solid solutions would help the material with high oxygen

16

storage capacity, thermal stability, redox properties and thus superior catalytic performance.1,

17

22

18

competitive as a candidate for doping. Therefore, the Ce–Fe mixed-oxide catalysts have been

19

extensively applied in selective catalytic reduction of NOx reaction,2 synthesis gas

20

preparation,22 Fischer–Tropsch synthesis,23 catalytic soot oxidation,24-25 CO oxidation26 and

21

methane combustion.27

22

17-19

Nevertheless, the single CeO2 degrades quickly at high temperature

Fe2O3, which is environmentally friendly and low cost, is particularly attractive and highly

The nature of Fe–Ce mixed oxides is the key factor in determining the catalytic activity. 3



Page 4 of 48

1

According to previous reports,23-29 the formation of Ce-Fe-O solid solution is a frequently

2

proposed reactive site for many reactions. In Fisher-Tropsch synthesis and N2O

3

decomposition, Pérez-Alonso et al.23 observed an significantly enhanced catalytic activity of

4

Ce-Fe-O solid solution compared to the pure metal oxide counterparts. Li et al.22 use Fe-Ce

5

material as an oxygen storage material for chemical looping conversion of CH4 to synthesis

6

gas, and they attribute the relatively high reactivity of the mixed oxides to the enhanced

7

oxygen mobility of Ce-Fe-O solid solution. Similar phenomena are also observed in soot

8

oxidation,25 methane combustion,27 synthesis gas generation30 and CO oxidation26 over

9

Ce1-xFexO

:;

catalysts, in which the presence of CeO2–like solid solution strongly enhances

10

the reducibility, oxygen storage capacity and oxygen mobility of materials and thus greatly

11

improves the catalytic performance. Oxygen vacancy, which is created owing to the formation

12

of Ce-Fe-O solid solution, is proposed to be important in determining catalytic activity. Bao et

13

al.31-32 report that oxygen vacancy in the Ce–Fe–O system is the most reactive site towards

14

CO oxidation. Liu et al.33 and Tang et al.34 believe that the oxygen vacancy provides a

15

prerequisite for transferring oxygen from bulk to surface for reacting. It is reported that the

16

oxygen vacancies tend to distribute in the bulk or on the surface and are available to

17

adsorption purposes.32, 34 Our previous research also reveals that oxygen vacancy plays a very

18

important role in catalytic methane combustion,27 where the Ce0.7Fe0.3O

19

abundant oxygen vacancies exhibits high activity even at relatively high space velocity.

:;

sample with

20

Iron species (e.g., surface isolated particles of iron oxides and iron ions in the bulk) also

21

play an important role in determining the catalytic performance. Anushree et al.35 investigated

22

the activity for catalytic wet air oxidation over mesoporous Ce1-xFexO 4


:;

mixed oxides and

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1

found that the highly dispersed Fe2O3 particles provide augmented active sites to improve the

2

activity. Similar phenomena were also observed by Wang et al.2 and Luo et al.,36-37 and they

3

proposed that the surface dispersed iron species possesses higher activity than the bulk iron in

4

the Ce-Fe-O system. Protasova et al.38 and Li et al.39 observed that the surface Fe2O3 can be

5

reduced to create active sites for methane activation, and the concentration of active rates is

6

dominated by the dispersion of surface iron species. Our previous work by using in situ

7

Raman spectra also confirmed that methane tends to be activated on free Fe2O3 particles to

8

carbonaceous species in catalytic methane combustion.27

9

On the other hand, it is also reported that the chemical interaction between surface iron

10

species and the Ce–Fe solid solution strongly enhances the reducibility and catalytic activity

11

of the Ce–Fe–O materials by combining the redox couples of Fe3+/Fe2+ and Ce4+/Ce3+.19, 26, 39

12

Aneggi et al.40 believe that the high degree of Fe–O–Ce interaction can be established in two

13

ways, where one takes place through the sharing of oxygen anion based on the ceria-based

14

solid solution and the other takes place through close contact of Fe2O3 particles with ceria.

15

However, coupling mechanism among surface iron species, bulk lattice distortion and oxygen

16

vacancy is not clearly understood.

17

In the present work, two series of Ce1-xFexO

:;

catalysts with different characteristics in

18

lattice distortion degree, surface Fe2O3 state, Fe doping content, oxygen vacancy

19

concentration and specific surface area are prepared for CH4 combustion and CO oxidation.

20

Investigations on the various physicochemical properties were performed to understand the

21

chemical natures of these catalysts, which were corelated to their catalytic activity.

22

Particularly, the possible reaction mechanisms for CO and CH4 oxidation over Ce1-xFexO 5


:;


1

catalysts are discussed in detail by using the in situ infrared technology. The coupling of bulk

2

lattice distortion, surface iron sites and oxygen vacancy are identified to be the critical factor

3

for determining the catalytic activity.

4

2. Experimental Section

5

2.1. Catalyst preparation

6

The nanostructured Ce1-xFexO

:;

(x = 0, 0.05, 0.1, 0.2, 0.3, 0.4) oxides with different

7

FeGCe ratios were prepared by a co-precipitation method. The required amounts of

8

(NH4)2Ce(NO3)6 and Fe(NO3)3·9H2O were dissolved in ultrapure water with stirring at a

9

appropriate temperature (70 or 0 oC). A high precision cryostat (LH-1010) was employed to

10

assist the co-preparation process at 0 oC. The liquid medium of ethanol continuously

11

circulates in the cryostat to maintain the desired temperature. It should be highlighted that the

12

mixed solution is not frozen at 0 oC probably due to the supercooling of water and the

13

presence of cations in water. Then, a 10 % ammonia solution was dropped into the mixed

14

solution with a slow dropping speed. The mixtures were vigorously stirred for additional three

15

hours with the pH at 9-10. Thereafter, the resulting precipitates were washed for filtering with

16

deionized water and ethanol for three times, respectively, and then dried at 110 oC for 24 h.

17

Finally, the dried powders were heated in air at 500 oC for 2 h with a ramp rate of 2 oC/min.

18

The prepared sample with a precipitation temperature of 70 or 0 oC were labeled as

19

70-Ce1-xFexO

20

2.2. Catalyst characterizations

21

:; or 0-Ce1-xFexO :;, respectively.

X-ray powder diffraction (XRD) patterns were obtained at J = 10–120o by a Japan 6


Page 6 of 48

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1

Science D8 X-ray vertical diffractometer with a step size of 0.02 o, dwell time 0.3 s and step

2

scanning mode. The Rietveld refinement is realized by using GSAS program.

3

The Raman spectra were identified using a Thermo Fisher DXRxi Raman imaging

4

microscope. The 532 nm exciting wavelength was chosen and a low laser power (3 mW) was

5

kept in all cases to avoid sample heating.

6

The nitrogen adsorption-desorption isotherms were obtained at liquid N2 temperature

7

(77K) using a Quantachrome AutosorbiQ apparatus. The specific surface area of catalysts was

8

determined from the N2 adsorption isotherm applied the ?5

9

method.

5NO

77N

++ 5 (BET)

10

The hydrogen temperature program reduction was obtained on a Quantanchrome

11

Instrument equipped with a thermal conductivity detector. After a typical pretreatment

12

procedure, the catalyst (50 mg) was mounted in a U tube reactor and heated from room

13

temperature (RT) to 900 oC with a heating rate of 10 oC/min in flowing 10% H2/Ar (25

14

mL/min).

15

CH4-TPR over different catalysts was conducted on the CATLAB catalyst microreactor

16

system. Before the experiments, the catalysts (ca. 100 mg) were subjected to a flow of 10%

17

O2/Ar (25 mL/min) at 300 oC for 1 h, cooled down to RT and then switched to argon purging

18

for 30 min. After that, the pretreated sample was exposed to 5% CH4/Ar and heated at a

19

constant heating rate (5 °C/min) from RT to 600 °C. The gas stream was analyzed by an

20

online mass spectrometer. By replacing the 5% CH4/Ar with 5% CO/Ar, CO-TPR tests were

21

also measured.

22

The X-ray photoelectron spectroscopy (XPS) experiments were performed on a PHI 7



Page 8 of 48

1

5000 Versaprobe II system equipped with a monochromatic

+: T X-rays source. Spectra

2

were recorded after purging the samples at RT in vacuum (residual pressure < 10-7 Pa). The

3

spectra regions corresponding to Ce 3d, O 1s and Fe 2p core levels were recorded. All

4

photoelectron binding energies were calibrated using the C 1s peak of adventitious carbon

5

(284.8 eV) as a reference.

6

The microstructures of the catalysts were investigated by a JEOL JEM-2100 (UHR)

7

transmission electron microscopy (TEM) instrument, using a LaB6 filament operating at 200

8

kV.

9

Electron paramagnetic resonance (EPR) gives detailed information on the presence of

10

metal ions with unpaired electrons. EPR spectra recorded at low temperature (103 K) were

11

run on a JEOL-FA200 spectrometer. The calcined catalyst (30 mg) was placed inside the

12

quartz probe cell. The g factor was calculated by the equation V = W" where h is Planck

13

constant, H is the applied magnetic field, and W is Bohr magneton. Reference signals of Mn2+

14

ions in MnO crystals were used as the standard for the precise effective g-factor value.

15

2.3. In situ experiments

16

The in situ Raman spectroscopy experiment over the catalysts in CH4 atmosphere was

17

performed on the Thermo Fisher DXRxi Raman imaging microscope. Typically, the

18

pretreated catalyst (50 mg) are sealed in the in situ Raman cell with the reactant gas (5%

19

CH4/Ar) flowing through the sample. The temperature is ramped up at a heating rate of

20

10 °C/min from 25 °C to 600 °C, and the experimental data at each target temperature were

21

collected for at least three times. After cooling to room temperature, the spectra are also

22

collected during the cooling process. 8


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1

In situ diffuse reflectance infrared spectroscopy (DRIFTS) measurement was performed

2

on a Thermo Fisher FTIR spectrometer (iS50 FT-IR) equipped with a MCT detector cooled

3

by liquid nitrogen. The FTIR spectra were collected from 4000 to 1000 cm-1 at a resolution of

4

4 cm-1. Before the in situ DRIFTS experiments, 50 mg of catalysts were pretreated in the

5

reaction cell in flowing 10% O2/N2 (50 mL/min) at 300 oC for 30 min, and then cooled down

6

to room temperature before switching to N2. The DRIFTS spectrum of the pretreated catalyst

7

purged with N2 at RT was taken as the background spectrum. In adsorption experiments, the

8

IR spectra were recorded in a flow of 1% CO/N2 or 5% CH4/N2 (25 mL/min) at RT for 36

9

min. In catalytic oxidation testing, the reactant gas (1% CO-10% O2 or 1% CH4-20% O2)

10

flows through the pretreated sample with the temperature increasing from 25 to 500 oC with a

11

heating rate of 10 oC/min.

12

2.4. Catalytic activity tests

13

The catalytic activity measurement was performed in a fixed-bed quartz tubular reactor

14

with an inner diameter of 8 mm. Prior to each testing, the samples (30–40 mesh, 200 mg)

15

loaded in the quartz tube were exposed to the flowing 10% O2/Ar (100 mL/min) at 300 oC for

16

30 min to remove the possible impurities. The reactant gases, consisting of 1% CH4 and 20%

17

O2 (1% CO and 10% O2), balanced by Ar, were led continuously over the catalyst bed at a

18

flow rate of 100 mL/min. The reactor was heated through a temperature programmed route

19

from RT to 500 oC with a heating rate of 5 oC/min. An gas chromatograph (Agilent 7890A

20

GC system, produced by Agilent Co) was used to detect the amount of the effluent gas from

21

the reactor. The conversion was determined by the changes between the inlet and outlet gas

22

concentrations. The CH4 and CO conversion were calculated according to Eqs. (1) and (2) 9



Page 10 of 48

1

respectively. The [CH4]in and [CO]in are the amounts of inlet gases, and [CH4 ]out and [CO]out

2

are the amounts of outlet gases. The turnover frequencies (TOFs) for CH4 and CO oxidation

3

reactions were determined at a specified temperature (e.g. 325 oC for CH4 combustion and

4

120 oC for CO oxidation) according to Eqs. (3) and (4), respectively. The F (mol·s-1) is the

5

flow rate of CH4 or CO, X (%) is the CH4 or CO conversion at specified temperature, Mcat.

6

(g·mol-1) is the molar mass of the Ce1-xFexO

7

x(Ce3+) and x(Fe2+) are the ratios of Ce3+/Cetotal and Fe2+/Fetotal, respectively, while x(Ce) and

8

x(Fe) are the total content of Ce and Fe in different samples (obtained by XPS measurement).

9

CH4 conversion (%) =

10

[CH4]in

[CH4]out

[CH4]in

CO conversion (%) =

11

TOFCH4 (s -1) =

12

TOFCO (s -1) =

[CO]in

:; catalysts,

mcat. (g) is the amount of catalyst, the

(1)

× 100%

[CO]out

[CO]in

× 100%

(2)

FCH4XCH4Mcat. 3+

mcat.( (Ce

FCO XCOMcat. 3+

mcat.( (Ce

× 100%

(3)

× 100%

(4)

) × x(Ce) + (Fe2 + ) × x(Fe))

) × x(Ce) + (Fe2 + ) × x(Fe))

13

The kinetic measurement was conducted on the same fixed-bed reactor as mentioned

14

above to investigate the intrinsic activity of the catalysts. The reactor was operated under

15

different reaction conditions with a conversion of less than 20%. The outlet gas stream was

16

analyzed by the same as the aforesaid oxidation reaction.

17

3. Results

18

3.1. Structure of the Ce1-xFexO

catalysts

10


Page 11 of 48

1

Figures 1A and B show the XRD patterns of the prepared Ce1-xFexO

:;

catalysts. The two

2

pure CeO2 samples exhibit the fluorite cubic structure (space group Fm3m) with the

3

characteristic diffraction peaks at 28.5, 33.1, 47.4 and 56.3o.29, 32, 41 The diffractograms for all

4

the mixed oxides are similar with that of the single CeO2, but the diffraction peaks weaken

5

and broaden with the increase of Fe content, evidencing the decrease in grain size. The

6

diffraction peaks of Fe2O3 are absent in all the XRD patterns of Ce-Fe mixed oxides,

7

indicating that most of iron ions in both 70-Ce1-xFexO

8

incorporated into ceria lattice to form Ce–Fe–O solid solution.20, 27 It should be noted that free

9

Fe2O3 particles are detected on the Ce0.7Fe0.3O

:;

and 0-Ce1-xFexO

:;

and Ce0.6Fe0.4O

:; catalysts

:;

samples are

in the both series

10

by the Raman spectra (see Figures 1C and D). The absence of isolated Fe2O3 particles in XRD

11

measurement may be attributed to their small size and/or low content. [ CeO2

B

b

(111)

(111)

a

A

[ CeO2

[

28

29

30

[

Intensity (a.u.)

27

[

[

CeO2 X=0.05 X=0.1 X=0.2

[ [

X=0.4

40

50

C

60 20

463

30

40

2 Theta (deg.)

x=0.4 x=0.3

251

x=0.2 x=0.1

463

600

251

x=0.2

x=0.1

x=0.05

200

13

300

400

500

600

700

x=0.05 CeO2 0.1

200

Raman shift (cm-1)

x=0.4 x=0.3

CeO2 0.1

12

60

Fe2O3 449

600

50

D

Fe2O3 447

[

30

X=0.2 X=0.3

2 Theta (deg.)

29

X=0.1

X=0.4

30

28

X=0.05

X=0.3

20

27

CeO2

Intensity (a.u.)

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


300

Figure 1. XRD patterns and Raman spectra of various Ce1-xFexO

400

500

600

700

Raman shift (cm-1) :;

11


samples: 70-Ce1-xFexO

:;

(A, C) and


1 2

0-Ce1-xFexO

:; (B,

Page 12 of 48

D) samples. Inset shows the enlarged (111) reflections of CeO2.

It is very interesting that an obvious shift of CeO2 (111) diffraction peak towards higher

3

angle with increasing the iron content is observed over the 70-Ce1-xFexO

4

enlarged XRD patterns in Figure 1A), but this shift is almost undetectable for the

5

0-Ce1-xFexO

6

ions (0.064 nm for Fe3+ vs 0.101 nm for Ce4+) into the fluorite-type lattice of CeO2 to form a

7

Ce–Fe–O solid solution will result in the contraction of the unit cell, and thus the J degree

8

shifts to higher values.25, 42 Bao et al.32 also confirmed the distortion of the CeO2 lattice by Fe

9

doping via the K-edge XANES spectra. The absence of this shift for the 0-Ce1-xFexO

:;

:;

series (see the

samples (see inset b in Figure 1B). In general, the incorporation of smaller Fe3+

:;

10

samples indicates different doping mechanism of iron ions which will be discussed in the later

11

text with the help of XRD Rietveld refinement.

12

The surface structures of the Ce1-xFexO

:;

catalysts were investigated by using Raman

13

spectroscopy. As shown in Figures 1C and D, the prominent F2g Raman vibrational peak at

14

around 463 cm-1 is ascribed to oxygen symmetric breathing vibration around Ce4+, and the two

15

weak bands at around 251 cm-1 and 600 cm-1 can be assigned to the second-order transverse

16

acoustic (2TA) mode of CeO2 and the oxygen vacancy defect-induced (D modes) vibrational

17

peak, respectively.17, 36 Compared with the pure CeO2, this main band significantly attenuates

18

and broadens with the increase of Fe content, indicating lower crystallite size of ceria.19

19

Meanwhile, a sequential shift towards lower frequencies (red shift) is also observed over this

20

band, which is associated with the lattice contraction due to the cation doping.29, 42 This is

21

inconsistent with the observation by XRD measurement, where the lattice shrinkage of

22

0-Ce1-xFexO

:; is

undetectable. This phenomenon suggests that Fe3+ ions located on the surface 12


Page 13 of 48

1

for 0-Ce1-xFexO

2

results in the enhancement on the two weak bands at around 251 cm-1 and 600 cm-1, which

3

indicates an increase in oxygen vacancy concentration.18 Additionally, the segregated T:3 2O3

4

phase is observed on the Ce0.7Fe0.3O

5

According to the observations of XRD and Raman measurements, for both series, the

6

Ce1-xFexO

:; solid

solution may be higher than that in the bulk. The addition of iron also

7

samples represent a mixture of cubic Ce–Fe–O solid solution and free Fe2O3 nanoparticles.

:; (x Ce0.8Fe0.2O :;> Ce0.95Fe0.05O :;> CeO :;. Under the similar conditions, the

activity of the obtained Ce1-xFexO -8.3

:;

catalysts are much higher than that in literatures.11, 41, 46-47

A

-8.6

-8.4

22.9 kJ/mol

-8.5

ln k

ln k

-8.7 37.7 kJ/mol

-8.8

43.3 kJ/mol

1.56

-8.9

36.2 kJ/mol

-9.0 x=0.05 x=0.1 x=0.2 x=0.3 x=0.4

-8.9

1.60

1.64

-9.1 -9.2

x=0.05 x=0.1 x=0.2 x=0.3 x=0.4

1.50

1.68

26.7 kJ/mol

-7.6

35.8 kJ/mol

-8.4 60.6 kJ/mol

27.4 kJ/mol

x=0.05 x=0.1 x=0.2 x=0.3 x=0.4

2.5

37.0 kJ/mol

-8.2 63.8 kJ/mol

30.8 kJ/mol 56.2 kJ/mol

-8.4 2.4

x=0.05 x=0.1 x=0.2 x=0.3 x=0.4

-8.0

41.5 kJ/mol

-8.2

12

1.65

-7.8 23.3 kJ/mol

2.3

1.60

ln k

-7.8

11

1.55

-7.6

-7.4

-8.6

32.3 kJ/mol

B*

B

-8.0

41.9 kJ/mol

1000/T (K -1)

1000/T (K -1) -7.2

29.3 kJ/mol

48.6 kJ/mol

-8.8

31.0 kJ/mol

-8.6

-9.0

-8.7

A*

30.5 kJ/mol

ln k

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 18 of 48

2.6

2.30

2.35

2.40

2.45

2.50

2.55

1000/T (K -1)

1000/T (K -1)

Figure 4. Arrhenius plots for methane combustion (A, A*) and CO oxidation (B, B*) over 70-Ce1-xFexO 18


:;

Page 19 of 48 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


1 2

(A, B) and 0- Ce1-xFexO

:; (A*,

B*) catalysts.

The apparent activation energy (Ea) of different samples was calculated on basis of the

3

catalytic activity. According to the previous literatures,36,

4

reasonably considered to obey a first-order reaction mechanism when the CO or CH4

5

concentration is relatively low. The Arrhenius plots for CO oxidation and CH4 combustion as

6

well as the corresponding apparent activation energy over each catalyst are shown in Figure 4.

7

For both the series, the apparent activation energy value for either CH4 oxidation or CO

8

oxidation increases in the sequence of Ce0.6Fe0.4O

9

Ce0.9Fe0.1O

:;

< Ce0.95Fe0.05O

:;

:;

48

the oxidation of CO or CH4 is

< Ce0.7Fe0.3O

:;

< Ce0.8Fe0.2O

:;

Ce0.6Fe0.4O

:;

> Ce0.95Fe0.05O

:;

> CeO

:;


The

(see Table 2) regardless of the

Figures 6B and C show the EPR spectra of pure CeO2 and Ce1-xFexO 22

53

:;

samples. The

0.66

Page 23 of 48 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


1

spectra of pure CeO2 show a paramagnetic peak at g = 1.96 (see the upper inset), which is

2

attributed to the Ce3+ coupled with conduction electrons on ceria.19-20, 29 The 70-samples show

3

higher intensity of this peak than the 0-samples, indicating higher concentration of Ce3+ (see

4

the enlarged EPR spectra of Ce-Fe mixed oxides are shown in Figure S5). In addition, an

5

oxygen vacancy signal corresponding to a radical with O2- character (g = 2.0) is observed for

6

all the samples (see Figure S5), which is resulted from the interaction between surface Ce3+

7

and O2 from air (e.g. Ce3++O2 giving Ce4++O2-).45, 49 Interestingly, the O2- type radical signal

8

for the Ce0.6Fe0.4O

9

especially for the 70-Ce0.6Fe0.4O

:;

sample significantly broadens and weakens compared to others, :;

sample. The reason should be that large electron–electron

10

interactions at higher coverage causes decreasing electron relaxation times and conceals

11

certain signals.19, 45 Moreover, the incorporation of iron ions results in the appearance of an

12

additional new paramagnetic signal at g = 4.3, which can be ascribed to the isolated Fe3+

13

located in octahedral sites.20,

14

superposition of the Ce3+ and Fe3+ ions signals.

15

3.4. Reducibility

16

49

For all the Ce1-xFexO

:;

samples, each spectrum is a

Figures 7A and B show the H2-TPR profiles of the two series of Ce1-xFexO

:;

samples. In

17

the case of 70-Ce1-xFexO

18

peak in the range of 300-500 oC and a high-temperature reduction peak centered at 780 oC,

19

which are attributed to the reduction of surface and bulk ceria, respectively. The Ce1-xFexO

20

samples involve three well defined reduction peaks at ca. 360 (T0 630 (W0 and 800 oC (`0 and

21

each peak is gradually enhanced and shifts to lower temperature with the increase of Fe

22

content. According to the reduction behavior of CeO2-Fe2O3 mixed oxides reported in

:;

samples, pure CeO2 exhibits a broad low-temperature reduction

23


:;


1

literatures,26,

39

2

reduction of superficial layer Ce4+ and Fe3+, and the W peak is related to the overlapping

3

reduction of Fe3+ and Ce4+ in the Ce–Fe–O solid solution. The ` peak at high-temperature

4

corresponds to the further reduction of bulk iron oxides as well as the reduction of bulk Ce4+.

5

It is worth stressing that the Ce0.7Fe0.3O

6

Fe2O3 particles show higher reducibility (lower reduction temperature and higher peak

7

intensity) than the samples with pure solid solution phase. This suggests that the existence of

8

free Fe2O3 particles can improve the reducibility of Fe–Ce mixed oxides.

the low-temperature peak (T peak) should be attributed to the simultaneous

:;

and Ce0.6Fe0.4O

A

:;

A*

250

samples with the presence of

bulk reduction

Cumulative H2 uptake (mmol/g)

200

TCD signal (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

Page 24 of 48

f e d c b a

B

f

surface reduction 100 50 0

d c b a

50

400

600

800

bulk reduction

150

100

Temperature (oC)

B*

200

e

200

9

150

surface reduction

0 200

300

400

500

600

700

Temperature (oC)

10

Figure 7. H2-TPR profiles and cumulative H2 uptake of 70-Ce1-xFexO

11

x=0 (a), x=0.05(b), x=0.1(c), x=0.2 (d), x=0.3 (e) and x=0.4(f).

:; (A,

800

900

A*) and 0-Ce1-xFexO

:; (B,

B*)

12

Figures 7A* and B* reveal the cumulative hydrogen uptake as a function of reduction

13

temperature over different samples. One can see that the total hydrogen consumption of

14

Ce0.7Fe0.3O

:;

and Ce0.6Fe0.4O

:;

intuitively display higher values than other samples. The 24


Page 25 of 48

1

Ce0.6Fe0.4O

2

mmol/g for 70-Ce0.6Fe0.4O

3

the 70-Ce1-xFexO

4

0-Ce1-xFexO

:;

sample show the largest H2 consumption in the corresponding series (244

:;

:;

and 195 mmol/g for 0-Ce0.6Fe0.4O :;). Comparing the two series,

samples represent higher hydrogen consumption than the corresponding

:; samples.

(A) 6

a (B)

1.5

a

CO2

b

CO2

4

CO2 2

1.0

H2 0.5

CO 0

CO2 H2

4

CO

0 6

c

4

CO2 2

Intensity (*10-9 )

8

0.0

b

12

Intensity (*10-10 )

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


1.5 1.0 0.5 0.0

c

CO2

1.5 1.0

H2 0.5

CO 0 6

0.0

d

1.5

d

CO2

4

CO2 2

1.0

H2 CO

0.5

0

300

5

400

500

600

700

Temperature (oC)

800

0.0

100

200

300

6

Figure 8. CH4-TPR (A) and CO-TPR (B) profiles of Ce1-xFexO

7

70-Ce0.6Fe0.4O

8

:; (b),

0-Ce0.8Fe0.2O

:; (c)

and 0-Ce0.6Fe0.4O

400

500

Temperature (oC) :;

600

catalysts: 70-Ce0.8Fe0.2O

:;

(a),

:; (d).

Figure 8A shows the CH4-TPR profiles of the different Ce0.8Fe0.2O

:;

and Ce0.6Fe0.4O

:;

9

samples. All samples present an broad low-temperature CO2 peak with extension to around

10

600 oC, which can be attributed to the consumption of reactive oxygen by CH4.39 When the

11

temperature increases to higher than 600 oC, H2 and CO are gradually produced, indicating the

12

occurrence of methane partial oxidation.34 For both the series, the Ce0.6Fe0.4O

13

higher reactivity for methane oxidation to CO2 than the Ce0.8Fe0.2O

14

temperatures (

Enhanced CH4 and CO oxidation over Ce1-xFexO2-Î´ hybrid catalysts

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