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Constructing La2B2O7 (B = Ti, Zr, Ce) Compounds with Three Typical Crystalline Phases for Oxidative Coupling of Methane: the Effect of Phase Structures, Super Oxide Anions and Alkalinity on the Reactivity Junwei Xu, Yan Zhang, Xianglan Xu, Xiuzhong Fang, Rong Xi, Yameng Liu, Renyang Zheng, and Xiang Wang ACS Catal., Just Accepted Manuscript • Publication Date (Web): 26 Mar 2019 Downloaded from http://pubs.acs.org on March 26, 2019

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is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

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Constructing La2B2O7 (B = Ti, Zr, Ce) Compounds with Three Typical Crystalline Phases for Oxidative Coupling of Methane: the Effect of Phase Structures, Super Oxide Anions and Alkalinity on the Reactivity Junwei Xu1, Yan Zhang1, Xianglan Xu1, Xiuzhong Fang1, Rong Xi1, Yameng Liu1, Renyang Zheng2, Xiang Wang1, *

1 Key

Laboratory of Jiangxi Province for Environment and Energy Catalysis, College of

Chemistry, Nanchang University, Nanchang, 330031, PR China

2 Research

Institute of Petroleum Processing (RIPP), SINOPEC,18 Xueyuan Road,

Haidian district, Beijing 100083, China

 Corresponding author. E-mail: [email protected] (X. Wang)

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Abstract: To probe the phase structure-reactivity relationship of A2B2O7 catalysts for oxidative coupling of methane (OCM), three model La2B2O7 compounds with Ti4+, Zr4+ or Ce4+ B site have been purposely designed. By decreasing the rA/rB ratios in the order of La2Ti2O7 > La2Zr2O7 > La2Ce2O7, typical monoclinic layered perovskite, cubic ordered pyrochlore and disordered defective cubic fluorite phase are formed, respectively. The reaction performance of the catalysts based on CH4 conversion and C2 product yield follow the order of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. It has been discovered that superoxide O2- is the active oxygen species detected on all the catalysts and responsible for the OCM reaction, whose amount follows also the sequence of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. Moreover, the surface alkalinity related to the superoxide anions observes the same order. This testifies that the amount of surface superoxide O2- determines the OCM reaction performance over the La2B2O7 compounds. On the basis of the characterization results, the formation of active O2- species could follow two pathways. For La2Zr2O7 and La2Ce2O7 possessing intrinsic 8a oxygen vacancies, O2- anions are formed by activating the oxygen species entering into the vacancies in the bulk and then migrating to the catalyst surface. For La2Ti2O7 possessing no oxygen vacancies, they are formed directly by transforming the O2 molecules adsorbed on its surface. Usually, the former pathway


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generates more abundant O2- species than the latter one. La2Ce2O7 displays not only promising reaction performance at low temperature region, but also potent sulfur and lead poisoning resistance, thus having the potential for application after further optimization.

Keywords: Oxidative coupling of methane, surface O2- anions, La2B2O7 compounds, different crystalline phases, structure-reactivity relationship.

1. Introduction

Oxidative coupling of methane into ethylene is an attractive route to convert natural gas directly into value-added chemical products.1 Over past decades, people have investigated many catalysts to achieve sufficient CH4 conversion and C2 selectivity simultaneously to industrialize this important resource utilization reaction.2 Among these catalysts, Li/MgO, which was first reported by Ito et al.,3 is a representative one with promising performance, but it deactivates quickly due to the fast Li+ cation vaporization at elevated temperatures.4, 5 Rare earth oxide-based catalysts also show good activity for 3 ACS Paragon Plus Environment

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OCM reaction, due to the existence of reactive oxygen sites generated by surface oxygen vacancies, but they have low C2 product yield because of low selectivity.6, 7 At present, it is commonly agreed that Mn/NaWO4/SiO2 is the most applicable catalyst, over which the optimal C2 product yield of ~27% can be obtained.8 However, high temperature above 700 oC is always needed to initiate the reaction, and the optimal C2 yield is usually obtained around 800 oC.9, 10 In addition, the nature of the catalytic active sites is not yet understood clearly due to its complex compositions. Summarizing the results over all the studied OCM catalysts up to now, the one-way C2 yield is still below 30%, the minimum required yield to implement this important reaction in industrial scale. Therefore, there is still strong incentive to understand deeper the active sites requirements and seek catalysts with higher performance for this reaction, especially those can function at a relatively lower temperature region. It is commonly accepted that two typical steps are involved in an OCM reaction, which include a heterogeneous step to activate CH4 molecules into gas phase CH3· radicals on the active surface electrophilic oxygen sites, and a followed homogeneous gas phase step to combine two CH3· radical into an C2H6 molecule, which is then dehydrogenated 4 ACS Paragon Plus Environment

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into an C2H4 molecule.11-13 In spite of some still existing controversies on the active sites for catalytic OCM, most of the researchers agreed that the surface active oxygen sites is of great importance for methane conversion and C2 product formation.14-16Activation of O2 molecules to generate desirable surface oxygen species plays a key role to govern the formation of CH3· radicals and the subsequent oxidative dehydrogenation of C2H6.11 It was reported previously that surface electrophilic oxygen anions, e.g., O-, O2- and O22, are very important oxygen species contributing to C2 product selectivity 14-16. In contrast, the surface O2- lattice oxygen usually results in deep oxidation of the products.14-16 However, the mechanisms and pathways for the generation of surface active electrophilic oxygen species involving the cycle between gas phase O2 and surface oxygen vacancies have not been understood very clearly. A2B2O7 compounds possess some unique properties, such as superior thermal stability, good oxygen mobility and oxygen ion conductivity etc. Therefore, they have been investigated as functional materials for various applications such as magnetic, electrical and optical materials.17-21 The investigation on their catalytic properties are much less documented. Our former studies have demonstrated that A2B2O7 compounds with 5 ACS Paragon Plus Environment

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different A or/and B site compositions displayed very good performance for several high temperature reactions, including methane reforming and OCM reaction.22-24 It has been substantiated that an effective OCM catalyst usually possesses surface electrophilic oxygen sites and also has certain surface alkalinity.15, 16, 25, 26 A typical A2B2O7 compound containing trivalent rare earth A site can provide surface alkalinity, which matches mostly the requirement of a good OCM catalyst, together with its mobile oxygen species. Therefore, our former work has testified that a La2Ce2O7 catalyst displays very good performance for OCM reaction, especially at low temperature region.25 The study on using A2B2O7 compounds for OCM can be traced back to 1980s but with very limited publications.27-31 Although people have correlated their catalytic performance to the substitution of A sites, the change of B-O bond strength and the deficiency in B sites, the OCM reaction performance and phase structure relationship has not yet been investigated in detail and elucidated disntinctly.28, 30, 31 It has been proved that the phase structure of A2B2O7 oxides can be altered by adjusting the rA/rB radius ratio.32 In detail, if the ratio is over 1.78, monoclinic layered perovskite crystalline phase (space group P21) will be formed;32 If the ratio is between 1.46 and 1.78, 6 ACS Paragon Plus Environment

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a stably ordered face-centered cubic pyrochlore phase (space group Fd3m) will be formed;17, 22, 32 and if the ratio is below 1.46, a disordered cubic defective-fluorite phase (space group Fm3m) will be formed.17, 22, 32 In the lattice matrix of the latter two phases, intrinsic 8a oxygen vacancies are present, which can enhance the oxygen migration in the structure evidently and is believed to be beneficial to the OCM reaction.25, 26 Aiming to understand the above mentioned fundamental issues and seek for improved catalysts more applicable in industry, we have purposely designed and prepared three model La2B2O7 catalysts in this work with three typical crystalline phases by changing the B site cations (Ti4+, Zr4+ and Ce4+) but fixing the La3+ cation A site. It has been proved that La2B2O7 catalysts possessing a typical monoclinic layered perovskite (La2Ti2O7), cubic pyrochlore (La2Zr2O7) or disordered defective cubic fluorite (La2Ce2O7) phase has been successfully synthesized. The OCM reaction performance over the samples follows the sequence of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. By using various characterization methods, the effects of phase structure change on the surface property and reactivity have been discussed and correlated.


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2. Experimental 2.1 Catalyst synthesis A series of La2B2O7 catalysts with varied B site cations (Ti4+, Zr4+ and Ce4+) were synthesized by a citric acid sol-gel method, using La(NO3)3.6H2O (99.99%), Ti(OC4H9)4 (99.9%), Zr(NO3)4.5H2O (99.99%), Ce(NO3)3.6H2O (99.99%) and citric acid (99.8%) as the precursors. While La(NO3)3.6H2O, Zr(NO3)4.5H2O and Ce(NO3)3.6H2O were dissolved directly into a certain amount of DDI water directly to get stable solutions, Ti(OC4H9)4 was dissolved first in an appropriate quantity of nitric acid aqueous solution to prevent the hydroxylation of Ti4+, which can generate Ti(OH)4 precipitation. Afterwards, the stoichiometric molar ratio of B site salts: La(NO3)3.6H2O: citric acid = 1:1:1.2 were mixed into uniform aqueous solutions and then stirred completely till clear solutions were formed. NH3.H2O solution (22 ~ 25 wt%) was then dripped slowly to adjust their pH around 2. Afterwards, the samples will be dried in a water bath at 80 oC under constant stirring until viscous gels were formed. The gels were aged for 12 h at 130 oC, followed by calcination at 800 oC for 4 h in air atmosphere to get rid of the citric acid to achieve the


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final La2B2O7 (B= Ti, Zr and Ce) catalysts. The catalysts are named La2Ce2O7, La2Zr2O7 and La2Ti2O7 according to the element compositions.

2.2. Reaction performance evaluation OCM reaction was used to test the reactivity of the three model La2B2O7 (Ti4+, Zr4+ and Ce4+) catalysts. The reaction was carried out in a fixed-bed straight quartz tube reactor having a length of 300 mm and an internal diameter of 10 mm at 1 atm. Since OCM is a strong exothermic reaction, to avoid any hot-spot effects, typically, 200 mg catalyst was mixed with the same weight of quartz sand before loading into the reactor, and the reaction temperature was controlled by inserting the thermal couple into the reactor, with the head touching the catalyst bed. The flow rates of the CH4, O2 reactants and N2 balance gas were controlled separately by three mass flow controllers to get a mixed flow with a CH4 : O2 : N2 volume ratio of 4:1:5. The total rate of the flow is 60 mL min-1, resulting in a WHSV of 18000 mL h-1 g-1. The reaction generally starts from 550 oC with a 50 oC gap until 800 oC for the reactivity tests. To get steady state kinetic data, all measurements were taken after 1 hour’s stabilizing at a certain temperature. The OCM products were monitored by two online GC, with a GC9310 equipped with a TDX-01 column and a TCD 9 ACS Paragon Plus Environment

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to analyze CO, CO2 and CH4; and a GC9310II equipped with a Propak Q column and a FID to analyze CH4, C2H6, C2H4, C3H8, C3H6. The methane conversion (XCH4), C2 selectivity (SC2) and C2 yield (YC2) were quantified by using the standard normalization method on the basis of carbon balance, which are defined in the following equations:

XCH4 =

[2 ∗ (C2H4 + C2H6) + 3 ∗ (C3H6 + C3H8) + CO + CO2] × 100% (CH4)in

2 ∗ (C2H4 + C2H6)

Sc2 = 2 ∗ (C2H4 + C2H6) + 3 ∗ (C3H6 + C3H8) + CO + CO2 × 100%

YC2 = XCH4 × SC2 × 100%

(1)

(2)

(3)

2.3. Catalyst characterization

The bulk and surface properties of the catalysts were explored by different means to discern the effects induced by the replacement of the B site cations. The texture properties of the catalysts were measured with N2 sorption method at 77K on an ASAP2020 instrument. The phase compositions of the catalysts were analyzed by XRD on a Bruker AXS D8 Focus X-ray diffractometer system with Cu Kα radiation (40kV and


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30mA). Raman spectra were performed on a Renishaw in Via instrument with a 532 nm semiconductor laser as excitation. The HAADF-STEM images and elemental phase mapping were obtained by using a Tecnai F30 transmission electron microscope equipped with an Oxford EDX detector operated at 300 keV. H2-TPR was performed on a FINESORB 3010C instrument equipped with a thermal conductivity detector (TCD). In each experiment, 50 mg sample was calcined at 400 oC in a high purity air flow for 30 min to remove any possible impurities, and then cooled down to room temperature in an ultrahigh purity Ar flow and kept for 30 minutes. Afterwards, it was heated from room temperature to 800 oC with a rate of 10 oC/min in a 10% H2/Ar gas mixture flow. O2-TPD and CO2-TPD were measured on a micromeritics Auto Chem 2920 apparatus equipped with a TCD. In each experiment, 50 mg sample was calcined at 400 oC in a He flow for 30 min to remove any possible impurities, and then cooled down to 50 oC. At the same temperature, it was exposed to an O2 or CO2 flow for 1h to saturate the surface completely. After purged by a He flow to remove any physically absorbed O2 or CO2 for 30 min, it was heated from room temperature to 800 oC with a rate of 10 oC/min in an ultra-high purity He flow. EPR technique was performed on a JEOL FA-200 EPR Spectrometer. For the 11 ACS Paragon Plus Environment

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experiment, a sample was pretreated with 100 Torr oxygen at 450 °C for 30 min and then cooled down to room temperature, which was followed by evacuation before collecting EPR spectra at 77K in liquid N2. In situ DRIFT experiments were conducted on a Bruker Vertex equipped with an MCT detector and KBr windows. XPS spectra were performed with a PerkinElmer PHI1600 system using a single MgKα X-ray source operated at 300 W and 15 kV voltage.The equipment models, operation conditions, experimental procedures and parameters are described in detail in the accompanied supporting information file. Depending on necessity, some of the sample treatment details are also included in the related results and discussion section.

3. Results and discussion 3.1 La2B2O7 phase structure analysed by N2 sorption and XRD N2 sorption has been emplyed to identify the texture properties of the La2B2O7 catalysts. Figure 1 displays that both La2Ce2O7 and La2Zr2O7 possess a Type II isotherm and an H3 hysteresis loop, which indicates that these two samples have similar texture structure. However, La2Ti2O7 exhibits a Type IV isotherm and with a hysteresis loop type between


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H1 and H2, testifying that it has different texture property from the other two samples. The presence of hysteresis loops indicates the mesoporous structure of the samples, which is also confirmed by the pore size distribution profile. As listed in Table 1, all the samples have slightly different average pore volumes and pore sizes. In contrast, the difference in specific surface area is more distinct, among which La2Ce2O7 possesses the largest and La2Zr2O7 possesses the smallest surface area. This implies that the changing of B site cations has some impact on the texture peroperties of the prepared catalysts. Compared with the fresh catalysts, Figure S1 displays that the N2 sorption isotherms and pore size distribution profiles have no essential change for all the used catalysts after high temperature OCM reaction. However, the surface areas have some decrease, the average pore volumes have nearly no change and the average pore sizes have some increase for all the spent samples. This implies that all the samples have relatively stable structure and good thermal resistance due to the formation of A2B2O7 compounds with a fixed formula and stable crystalline phases.


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200 180

0.008

(a)

160

-1 -1

140 La Ce O 2 2 7

-1

120 100 80 La2Zr2O7 60 40

La2Ti2O7 La2Zr2O7

0.006

La2Ce2O7

0.005 0.004 0.003 0.002 0.001 0.000

20 La2Ti2O7 0 0.0

(b)

0.007

dV/dW (cm g nm )

2

Volume Adsorbed (cm /g)

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

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-0.001 0.2

0.4

0.6

0.8

0

1.0

20

Relative pressure(P/P0)

40

60

80

100

Pore size(nm)

Figure 1 N2 sorption measurement of the fresh catalysts (a) isotherms, (b) pore size distribution profiles.

Table 1 Physical chemical properties of the fresh and spent La2B2O7 catalysts Catalysts

BET surface area

Pore volume

Pore size

(m2 g-1)

(cm3 g-1)

(nm)

fresh

spent

fresh

spent

fresh

spent

La2Ti2O7

16.3

10.6

0.09

0.08

15.2

17.6

La2Zr2O7

9.7

7.4

0.06

0.06

16.9

22.5

La2Ce2O7

22.5

16.8

0.11

0.13

16.4

26.1

The phase compositions of the La2B2O7 catalysts were identified by XRD. As presented in Figure 2, all the fresh samples display intensive diffraction peaks, indicating they are 14 ACS Paragon Plus Environment

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well crystallized. After careful analysis, it is concluded that no diffraction features relating to the individual metal oxides, such as La2O3, TiO2, ZrO2 and CeO2, are detected, testifying that complete solid reaction occurred during the calcination process at 800 oC. As depicted in Figure S2(a), XRD patterns of the used catalysts display no evident change in comparison with that of the corresponding fresh samples, indicating these La2B2O7 compounds are very stable for the high temperature OCM reaction. As discussed in the introduction section, the bulk phase structures of A2B2O7 compounds are governed by the rA/rB radius ratios.32 For all the La2B2O7 synthesized here, the radius of the 8-fold coordinated A site La3+ is 1.16 Å and the radii of the 6-fold coordinated B site Ti4+, Zr4+ and Ce4+ are 0.61, 0.72 and 0.87 Å, respectively. Hence, the rA/rB ratios of La2Ti2O7, La2Zr2O7 and La2Ce2O7 are 1.90, 1.61 and 1.33. According to the phase formation rule discussed above,32 monoclinic layered perovskite, cubic pyrochlore and disordered defective cubic fluorite phase will be theoretically generated for La2Ti2O7, La2Zr2O7 and La2Ce2O7 in sequence. Indeed, the diffraction features of the samples matches well with the standard JCPDS cards of monoclinic layered perovskite La2Ti2O7 (JCPDS #28-0517), cubic pyrochlore (JCPDS #71-2363) and disordered defective cubic 15 ACS Paragon Plus Environment

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fluorite (JCPDS #04-12-6396), proving that the desired model catalysts with typical phase compositions have been successfully synthesized. XPS results in Figure S3 show that La in all the samples is in trivalent state, Ti, Zr and Ce in the related samples are predominantly in tetravalent state, which testifies indirectly the formation of La2B2O7 compounds possessing varied crystalline phases for all the catalysts. 2500

3000

(a) La2Ti2O7

Intensity (a.u.)

Intensity (a.u.)

(b) La2Zr2O7

2500

2000

1500

1000

500

2000 1500 1000 500

0

0

JCPDS #28-0517

10

20

30

40

50

60

70

80

JCPDS #71-2363

10

20

30

2  ()

(c) La2Ce2O7 2000

1500

1000

500

0

JCPDS #04-12-6396

10

20

30

40

40

2  ()

2500

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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50

60

70

80

2  ()


50

60

70

80

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Figure 2 XRD patterns of the freshly calcined La2B2O7 catalysts. (a) La2Ti2O7, (b) La2Zr2O7, (c) La2Ce2O7.

To further confirm the phase structure, XRD Rietveld refinement method is used to identify accurately the lattice parameters of the samples in Table 2, with the refinement patterns depicted in Figure S4. As demonstrated by the three same side lengths of 5.573 Å and intersection angles of 90 o, La2Ce2O7 has obviously a cubic unit cell similar to cubic fluorite CeO2,33-35 which matches its disordered defective cubic fluorite phase composition. In contrast, La2Zr2O7 has doubled side lengths of 10.746 Å but has still three intersection angles of 90o, which matches the typical characteristics of a cubic pyrocholre phase.17 However, La2Ti2O7 possesses three different side lengths and a special β intersection angle of 98.6o, which are the typical lattice parameters for a monoclinic layered perovskite unit cell.36 In summary, the XRD Rietveld refinement results are in good accordance to the phase analysis results, indicating that the phase structure of La2B2O7 compounds can be strongly influenced by changing the metal cations at B site.


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Table 2 Lattice proprieties of the fresh La2B2O7 catalysts obtained by XRD Rietveld refinement.

Catalyst

rA/rB

s

ratio

Lattice parameter a (Å)

b (Å)

c (Å)

α (o)

β (o)

γ (o)

Crystallit

Crystallite

e sizes a

phase

(nm) Monoclini La2Ti2O7

1.90

12.971

5.524

7.787

90.0

98.6

90.0

17.4

c layered perovskite

La2Zr2O

Cubic 1.61

10.746

10.746

10.746

90.0

90.0

90.0

20.2

7

La2Ce2 O7

1.33

5.573

5.573

5.573

90.0

90.0

90.0

16.1

pyrochlor e Disordered defective cubic fluorite

a Calculated by Scherrer equation from the XRD patterns 3.2 Structure identification of the catalysts by Raman and STEM Mapping To discern the structure change of the La2B2O7 compounds by B site replacement, the catalysts have been further studied by Raman, with the spectra depicted in Figure 3(a). Previous studies have indicated that Raman spectroscopy is very effective to provide 18 ACS Paragon Plus Environment

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explicit information to distinguish A2B2O7 compounds with a monoclinic layered perovskite, cubic pyrocholre and disordered defective cubic fluorite phase.37 As formerly reported, La2Ti2O7 shows theoretically 37 Raman bands due to the phonon modes of multiple La-O and Ti-O vibrations.36 The peaks at 342, 371, 405, 430, 448, 520, 541, 559, 609, 793 cm-1 correspond to the typical monoclinic layered perovskite crystalline structure.38 The bands at 154, 244 and 342 cm-1 are most likely due to La-O bond vibration. The stretching of octahedral O-Ti-O bonds can be found at 445 (Eg mode), 610 and 793 cm-1. The peaks at 130 (B1g mode) and 820 (B2g mode) cm-1 are similar to the Ti-O bond stretching vibration for the six-fold coordinated Ti in rutile TiO2.39 The detection of these fine Raman peaks has confirmed the formation of monoclinic layered perovskite phase in La2Ti2O7 catalyst. Based on group theory, an ordered pyrochlore has typically 6 active Raman modes distributed as A1g + Eg + 4F2g.40, 41 The most intensive peak at 300 cm-1 can be ascribed to the vibrations of B-O bond in Eg mode. The peak at 500 cm-1 can be assigned to the A1g mode relating to the O-B-O bending vibrations in octahedral BO6 unit. The four F2g modes at 395, 520, 601 cm-1 are most likely due to the mixed B-O and A-O bond 19 ACS Paragon Plus Environment

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stretching with O-B-O bending vibrations.22 The evident observation of all the active Raman modes for La2Zr2O7 strongly testifies the existence of a well ordered pyrochlore phase in its bulk structure. For La2Ce2O7 catalyst, the Raman band at 453 cm-1 is assigned to the F2g mode of cubic fluorite structure, and the band at 576 cm-1 is ascribed to the surface oxygen vacancies, which is generated by the random distribution of La3+ and Ce4+ cations in the lattice structure.35, 36 The weak band at 256 cm-1 could be assigned to the disordering of oxygen ions in the sublattices.35 In summary, the Raman results support the XRD analysis, proving the successful preparation of the desired model catalysts with the three typical crystalline phases for A2B2O7 compounds by varying the B site cations. It is mentioned here that Figure S2(b) proves that the Raman spectra of the spent catalysts have no any evident difference from that of the corresponding fresh samples, confirming that these La2B2O7 catalysts are very stable with potent heat resistance during the OCM reaction process operating at high temperatures.


Page 20 of 74

Page 21 of 74

As discussed above, surface electrophilic oxygen species could be in different forms and are indispensible for OCM reaction. For instance, O- species is observed on the surface of Li/MgO and Na/CaO catalysts;42 O2- superoxide species is detected on the surface of Y2O3-CaO and LaOF catalysts;43, 44 and O22- peroxide species is observed on the surface of some lanthanide oxide-based catalysts such as La2O3 and Sm2O3.45-47 All these surface oxygen species in different forms are proposed to be important active and selective sites for OCM reaction. 2000

100000

Oxygen vacancy

(b)

(a)

F2g

576

80000

1500

La2Ce2O7 Eg

60000

302

40000

F2g

La2Zr2O7 793

820

0

900

A1g F2g

500 396

A1g 559 540 Eg 405371 520 445430

610

La2Ti2O7

1000

F2g 520

596

B2g

20000

1075

453

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

ACS Catalysis

800

700

600

500

400 Raman shift (cm-1)

244 154 342

300

200

130

1295

La2Zr2O7

1170 1184

1310 1290

100

1078

1000 La Ce O 2 2 7

500

B1g

1186

1320

1360

La2Ti2O7 0 1400 1350

1300

1250

1200

1116

1150

Raman shift (cm-1)

1100

1065

1050

1000

Figure 3 (a) Raman spectra of the freshly calcined La2B2O7 catalysts, (b) Magnified Raman spectra from 1400 to 1000 cm-1.

To identify the surface active oxygen species clearly, the Raman spectra of the three catalysts are characterized. In detail, the catalysts were first heated to 450 oC and kept


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Page 22 of 74

for 30 min to remove any possible impurities in a 30 ml min-1 He flow, which were then exposed to a 30 ml min-1 5% O2/Ar gas mixture flow for 30 min to saturate the surface completely at the same temperature. Afterwards, the samples were cooled to the room temperature in a He flow and analyzed by Raman. The shift region of 1400-1000 cm-1 are particularly enlarged and displayed in Figure 3(b). Several peaks attributed to the surface adsorbed dioxygen species can be detected for all the samples. For La2Ti2O7, the signals at 1065, 1116 and 1184 cm-1 can be attributed to O2- species, and the signals at 1290 and 1310 cm-1 could be attributed to the surface CO32- species.48, 49 For La2Zr2O7, the signals at 1078 and 1170 cm-1 can be ascribed to O2- species, and the signals at 1295 cm-1 could be assigned to the surface CO32- species.48, 49 For La2Ce2O7, the signals at 1075 and 1186 cm-1 can be attributed to O2- species, and the signals at 1360 and 1320 cm-1 could be assigned to the surface CO32- species.48,

49

Therefore, on all the three

catalysts, surface superoxide O2- anions have been detcted. It was formerly reported that O22- peroxide species showed Raman bands at ~750 cm1,

~800 cm-1 and ~900 cm-1.48, 49 Apparently, no any Raman band matchs O22- species

is observed for all the three catalysts. On the basis of Raman results, it is proposed that 22 ACS Paragon Plus Environment

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ACS Catalysis

superoxide O2- is the only type of electrophillic active oxygen sites present on the surfaces of all the three catalysts, which plays the crucial role for the OCM reaction.

(a) La2Ti2O7

(b) La2Zr2O7

La

50 nm

Ti

(c) La2Ce2O7

La

50 nm

Zr

O

O

La

200 nm

Ce

O

Figure 4 HAADF STEM mapping images of (a) La2Ti2O7, (b) La2Zr2O7 and (c) La2Ce2O7.

To identify the element distribution in the three catalysts with varied B sites, HAADFSTEM mapping images have been obtained and displayed in Figure 4. Obviously, the images have demonstrated that Ti, Zr or Ce element in the corresponding samples 23 ACS Paragon Plus Environment

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Page 24 of 74

distributes very homogeneously with La and O elements, testifying strongly the formation of uniform and chemically stable A2B2O7 composite oxides in the bulk of all the samples. The particle morphology and size distribution of the catalysts were investigated by TEM technique. As shown in Figure S5, the mean particle sizes of the catalysts are 24.7, 32.8 and 21.2 nm for La2Ti2O7, La2Zr2O7 and La2Ce2O7, respectively. In comparison with the crystallite sizes of the samples measured by XRD in Table 2, the average particle sizes are a little larger, indicating that slight aggregation occurred for all the samples.

3.3 Reaction performance tests of the catalysts Figure 5 displays the reaction performance on the catalysts. By increasing the temperature from 550 to 800 oC, CH4 conversion, C2 product selectivity and yield follows the order of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. Apparently, La2Ce2O7 possesses the best OCM reaction performance in all the samples, over which 17.2% C2 yield can even be gotten at 800 oC. In contrast, La2Ti2O7 exhibits the lowest performance, on which only 6.0% C2 yield is achieved at the same temperature.


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ACS Catalysis

To better understand the reaction behaviors, the C2H4 selectivity over all the catalysts is compared particularly in Figure 5(d). Interestingly, the selectivity also follows the order of La2Ce2O7 > La2Zr2O7 > >La2Ti2O7 at different temperatures, substantiating that the surface active site composition of La2Ce2O7 and La2Zr2O7 is much more favorable than that of La2Ti2O7 for the direct production of valuable ethylene, which comes from the dehydrogenation of the initially formed C2H6.11, 25 In order to estimate the contribution of the B site metal oxides to the OCM reaction, the reactivity of the individual TiO2, ZrO2 and CeO2, which were also prepared by the same sol-gel method and calcined at 800 oC, have been tested. As depicted in Figure S6, XRD and Raman results have proven that pure phase rutile TiO2, baddeleyite ZrO2 and cubic fluorite CeO2 have been successfully achieved, but with very low surface area (Table S1) due to high temperature calcination. Figure S7 demonstrates that all the pure oxides display extremely low CH4 conversion, C2 product selectivity and yield. This strongly testifies that without forming La2B2O7 compounds, all the individual B site metal oxides possess low reaction performance for OCM. In other words, only if Ti4+, Zr4+ and Ce4+


ACS Catalysis

cations combine with La3+ to form La2B2O7 compounds, active OCM catalysts can then be fabricated. 30

100

(a)

(b)

La2Ce2O7

20 La2Ti2O7 10

C2 Selectivity ()

CH4 Conversion ()

80

La2Zr2O7

La2Ce2O7

60

La2Zr2O7

40

La2Ti2O7 20

0

550

600

650

700

750

0

800

550

600

o

650

700

750

800

Temperature (oC)

Temperature ( C)

30

50 La2Ti2O7 La2Zr2O7 La2Ce2O7

(c)

20

C2H4 Selectivity ()

40

C2 Yield ()

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 26 of 74

(d)

30

La2Ce2O7 La2Zr2O7

10

La2Ti2O7 0

550

600

650

700

Temperature (oC)

750

800

20

10

0

550

600

650

700

750

800

Temperature (oC)

Figure 5 The OCM reaction performance over the La2B2O7 catalysts. (a) CH4 conversion, (b) C2 selectivity, (c) C2 yield, (d) C2H4 selectivity. Reaction conditions: 0.200g catalysts under 60 mL min-1 CH4: O2: N2=4:1:5 flow rate, WHSV=18000 mL h-1 gcat-1.


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ACS Catalysis

Sulfur and lead compounds are often found at low percentage in natural gas,50 which are well known poisoning components for many catalysts. Since La2Ce2O7 catalyst shows good OCM performance at low temperature region (below 750 oC) and has certain application potential, its resistence to these poisoning agents has been tested. Figure 6(a) displays the sulfur tolerance test of La2Ce2O7. In detail, the catalyst was kept first at 750 oC for 5 hours in the typical oxidative coupling feed adopted in this study, which exhibited stable CH4 conversion, C2 product selectivity and yield. Afterwards, 100 ppm SO2 was introduced into the reaction flow, which decreases the reaction performance slightly. In addition, during the continued ~35 hours‘ test, no futher performance dropping was observed. After removing SO2 from the feed, the CH4 conversion, C2 product selectivity and yield can be restored completely. This indicates that SO2 molecules adsorbed reversibly on La2Ce2O7 surface and occupied only a small amount of active sites for the reaction, which can be desorbed easily when SO2 is removed from the reaction feed, and poses no any permanent damage to the catalyst. Generally, individual La and Ce oxides can react with sulfur oxides at high temperatures to form sulfates or sulfites.50, 51 It is apparent that after forming La2Ce2O7 compound, the La and Ce cations 27 ACS Paragon Plus Environment

ACS Catalysis

have been protected by the potent and stable La2B2O7 lattice, thus La2Ce2O7 showing potent sulfur resistance during high temperature OCM reaction. Figure 6(b) shows the lead poisoning test of the catalyst. Prior to the experiment, a certain amount of La2Ce2O7 was mixed with Pb(NO3)2 acqueous solution, followed by drying at 120 oC and calcining at 800 oC to get the final sample polluted by ~1% PbO. By increasing the reaction temperature from 550 to 750 oC, it is observed the CH4 conversion, C2 product selectivity and yield have only negligible decrease (dashed lines), demonstrating that this catalysts possesses also potent resistance to lead deactivation.

100

100

(a)

CH4 conversion

CH4 Conv, C2 sel and C2 yield ()

CH4 Conv, C2 sel and C2 yield ()

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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C2 selectivity

80

C2 yield SO2 off

SO2 on

60

40

20

0

0

10

20

30

40

50

55

(b)

CH4 conversion C2 selectivity C2 yield

80

60

40

20

0

550

600

650

Temperature (oC)

Time on stream (h)


700

750

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ACS Catalysis

Figure 6 Poisoning tests of La2Ce2O7 catalyst. (a) Sulfur poisoning test at 750 oC with 100 ppm SO2, (b) Lead poisoning test. Dashed line: catalyst polluted by 1% PbO; Solid line: fresh catalyst.

3.4 H2-TPR study on the redox property H2-TPR was adopted to probe the redox behaviours of the freshly calcined La2B2O7 samples. Figure 7 displays that the three La2B2O7 catalysts show similar reduction behaviours but also have some differences. La2Ti2O7 displays one broadened reduction peaks at ~ 342 oC and La2Zr2O7 exhibits one reduction peak at ~ 345 oC. La2Ce2O7 shows two wide reduction peaks at ~ 346 and ~ 538 oC. Theoretically, if all the lattice oxygen of an A2B2O7 compound can be reduced completely, the O/(A+B) molar ratio is 1.75. However, the quantified O/(La+B) ratios of all the samples in Table 3 are below 0.1, much less than 1.75, substantiating that only a small quantity of active surface oxygen can be reduced for the catalyst. Our former study has shown that the originally reducible Sn-O bonds in pure SnO2 become almost nonreducible below 800 oC when La2Sn2O7 pyrochlore compound is formed.24 The reason


ACS Catalysis

lies in the strengthening and stabilization of the Sn-O bonds in the lattice matrix of the pyrochlore phase.24 It is concluded that the same phenomenon has occurred to all the La2B2O7 catalysts in this study. 12.5 346

H2- comsumption (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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538

12.0 La2Ce2O7

345

11.5 La2Zr2O7 11.0

342 La2Ti2O7

10.5

200

300

400

500

600

700

Temperature (oC)

Figure 7 H2-TPR profiles of La2B2O7 catalysts

Therefore, the peak at ~ 350 oC could be attributed to the reducing of a small quantity of surface electrophilic oxygen such as O2- for the catalysts.14 The quantity of this part of oxygen species is in the order of La2Ce2O7 > La2Zr2O7 > La2Ti2O7, well consistent with the OCM performance exhibited in Figure 5. Notably, for La2Ce2O7, an additional peak positioned at ~ 538 oC might be ascribed to the reduction of a small quantity of surface lattice O2-, which relates to the cycle of Ce4+ into Ce3+.25 On the basis of H2-TPR and


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ACS Catalysis

Raman results, it is rational to deduce that the active surface electrophilic oxygen sites are of great importance for the OCM reaction.

Table 3 The H2-TPR quantification results of the La2B2O7 catalysts. Catalysts

Temperature

H2

Temperature

H2

(oC)

consumption

(oC)

consumption

amount

amount

(mmol g-1)

(mmol g-1)

O/(La+B)

La2Ti2O7

342

0.13

-

-

0.01

La2Zr2O7

345

0.27

-

-

0.04

La2Ce2O7

346

0.33

538

0.12

0.08

3.5 O2-TPD measurement of the oxygen property To further understand the oxygen properties, O2-TPD experiments have been conducted for the three catalysts. Figure 8 depicts that all the samples display several oxygen desorption peaks, demonstrating the existing of various types of oxygen species having differed chemical environments. As formerly reported, the peaks at 200~400 oC are ascribed to desorption of loosely bounded surface oxygen, and the peaks at 400~600 oC

can be ascribed to the part of oxygen species that benefits to CH4 activation and C2 31 ACS Paragon Plus Environment

ACS Catalysis

selectivity for OCM reaction. 16, 52 In addition, any peaks above 600 oC are attributable to the desorption of lattice oxygen.

3.484

Loosely bounded oxygen 180

3.480

O2- desorption (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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3.476

OCM reactive oxygen

233

88

313 452

3.472 3.468 91

568

La2Ce2O7

213 290

3.464

448

La2Zr2O7

3.460 84 3.456

La2Ti2O7

50 100

200

300

400

500

600

700

800

Temperature (oC)

Figure 8 O2 -TPD study on the La2B2O7 catalysts.

As discussed above, La2Ti2O7 possesses a monoclinic layered perovskite phase and has no any intrinsic oxygen vacancy.17, 22, 32 In contrast, La2Zr2O7 owns a cubic pyrochlore phase and La2Ce2O7 possesses a disordered defective cubic fluorite phase, in which an intrinsic 8a oxygen vacancy is present in a unit cell, and which can improve the amount of both loosely bounded surface oxygen and OCM reactive oxygen species significantly.17, 22, 32

Furthermore, in comparison with cubic pyrochlore phase, the oxygen mobility in a

disordered defective cubic fluorite phase can be evidently increased because of the


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ACS Catalysis

random distribution of the A and B site cations.17, 22, 32 Hence, it is not surprise to find that the total oxygen desorption amount quantified in Table 4 follows the sequence of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. In other words, the much larger amount of desorbed oxygen species observed for La2Zr2O7 and La2Ce2O7 is closely related to their inherent 8a oxygen vacancies in the lattice structure. However, since La2Ti2O7 lacks any intrinsic 8a oxygen vacancies, it possesses only a very small amount of desorbed oxygen species.

Table 4 The O2-TPD quantification results of the La2B2O7 catalysts. Catalysts

O2 Desorption Amount (μmol/g) Loosely bounded

OCM

Lattice

surface oxygen

reactive

oxygen

species

oxygen

(< 400 oC)

species

Total oxygen

(500~600 oC)

(400~500 oC) La2Ti2O7

2.1

-

-

2.1

La2Zr2O7

18.5

1.8

-

20.3

La2Ce2O7

53.7

4.8

1.8

60.3


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3.6 CO2-TPD measurement of the surface alkaline sites on the catalysts Former studies have pointed out the correlations between the surface alkalinity and C2 product yield for OCM reaction.15,

16, 52, 53

Therefore, the alkalinity of the three model

catalysts has been probed by CO2-TPD technique. It is well known that surface active alkaline sites with moderate strength are beneficial to both CH4 activation and C2 selectivity, but which are inactive at low temperature due to the adsorption of CO2 molecules. By increasing the reaction temperature, these alkaline sites start to be progressively available for methane activation due to CO2 desorption. However, the strong alkaline sites are easily to capture CO2 to form stable carbonates, which are not active for OCM reaction.54 Therefore, it is rational to believe that the moderate alkaline sites contribute mainly to the OCM reaction. Figure 9(a) shows that multiple alkaline sites with varied strengths exist on all the three catalysts’ surfaces. While all the alkaline sites owning differed strength are active to activate the CH4 molecules and beneficial to the formation of C2 products, the moderate strength alkaline sites are more favorable for C2H4 selectivity and formation.25, 26, 55 Hence, according to the CO2 desorption temperatures, the peaks are divided into three categories. 34 ACS Paragon Plus Environment

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Page 35 of 74

In brief, the peaks below 250 oC, in the range of 250 ~ 600 oC and over 600 oC are defined to correspond to weak, moderate and strong alkaline sites in sequence, which are deconvoluted and integrated in Table 5. Apparently, the total amount of alkaline sites has an order of La2Ce2O7 >La2Zr2O7 > La2Ti2O7. The quantity of the moderate alkaline sites, which is of great importance for the C2 product formation, follows basically the same order, which is in good accordance to the C2 yield obtained over the catalysts. For clear comparison, Figure 9(b) depicts particularly the C2 yield obtained at 800 °C against the quantities of the moderate surface alkaline sites of the catalysts. 20

3.495

83

276 428 778

567 85

80

127 201

3.465 50 100

La2Ce2O7

231 306

3.475 3.470

La2Ce2O7

15

3.485 3.480

(b)

162

200

443

607

732

C2 Yield (%)

3.490

Strong (a)

Moderate

Weak

CO2- desorption (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

ACS Catalysis

300

387 400

700 500

600

700

10

La2Ti2O7

La2Zr2O7 300

La2Zr2O7

5

La2Ti2O7

0

800

Temperature (oC)

5.6

20.5

Moderate alkaline sites amount (mol/g)

28.5

Figure 9 CO2-TPD study on the La2B2O7 catalysts. (a) CO2-TPD profiles, (b) C2 yield at 800 oC vis. intermediate alkaline sites amount.


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It noted here that the surface alkalinity is believed to be related closely to the chemisorbed oxygen anions such as O2-, O22-, O- and surface defects.54, 56, 57 For instance, by doping Li+ into MgO lattice, Li+O- electron pair defects were generated, which improved both the surface alkalinity and active oxygen amount.57 Therefore, it is easy to understand that both the amount of surface alkaline sites and active oxygen species follows the same sequence for the three La2B2O7 catalysts.

Table 5 The CO2-TPD quantification results of the La2B2O7 catalysts. Catalyst s

CO2-TPD quantification results Strong alkaline sites

Total amount

Weak

Moderate alkaline

alkaline sites

sites

amount

amount

(μmol/g)

(μmol/g)

La2Ti2O7

15.0

5.6

0.6

21.2

La2Zr2O7

23.8

20.5

2.3

46.6

La2Ce2O7

27.3

28.5

3.0

58.8

(μmol/g)

amount (μmol/g)

In summary, CO2-TPD results have demonstrated that by varying the B site cations, different amount of surface alkaline sites with changed strengths can be generated. 36 ACS Paragon Plus Environment

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ACS Catalysis

Particularly, surface alkaline sites with moderate strength, which is beneficial to the C2 product formation, can be optimally tuned, thus achieving catalysts with better reaction performance. Taking into account of Raman, O2-TPD and CO2-TPD results, it is apparent that the generation of moderate surface alkaline sites is closely related to the chemisorbed oxygen species, which is important for OCM reaction.

3.7 EPR study on the active oxygen species over the catalysts As demonstrated by Raman and O2-TPD results, active surface superoxide O2- species is detected in all the three La2B2O7 catalysts with different phase structures. To further elucidate this issue, all the samples are subjected to EPR analysis. In detail, the samples were pretreated with 100 Torr oxygen at 450 °C for 30 min and then cooled down to room temperature, which were followed by evacuation at the same temperature, and then analyzed by EPR. It is well known that EPR is an extensively used technique to identify any chemical species possessing unpaired electrons, such as O2- and O- anions, and single electrons trapped by oxygen vacancies.26, 58-60 Figure 10 shows that three peaks at g=1.994，2.000 and 2.042 can be observed for all the La2B2O7 catalyst but with varied 37 ACS Paragon Plus Environment

ACS Catalysis

intensities. The g values is well comparable with O2- at gxx=1.994, gyy=2.006, gzz=2.040 found on La2O3 by Lunsford et al,58 and at gxx=2.0001, gyy=2.0045, gzz=2.0685 found on SrF2/La2O3 by Wang et al.59 Therefore, it is concluded that the EPR signals detected on the La2B2O7 catalysts can be attribued to surface superoxide O2- anions, which confirms the Raman results in Figure 3(b). For better comparison, the integrated areas of the three peaks are listed in Table S2 for the samples. The total relative area of each sample follows the sequence of La2Ce2O7 > La2Zr2O7 > La2Ti2O7. It is apparent that La2Ce2O7 with a disordered defective cubic fluorite phase possesses the largest amount of O2-, and La2Ti2O7 with a monoclinic layered perovskite phase possesses the least amount of O2-. This implies that the O2quantities are different for the samples due to the phase structure variation. 800 600 400

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

Page 38 of 74

La2Ti2O7

gyy=2.000

La2Zr2O7 La2Ce2O7

200

gzz=2.042

0

-200 -400 -600 -800 300

gxx=1.994 305

310

315

320

325

330

335

Magnetic field (mT)


340

345

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ACS Catalysis

Figure 10. EPR spectra of the La2B2O7 catalysts.

3.8 Probing the reactive species and intermediates on the catalysts with In situ DRIFTS Probing the reactive oxygen species on the catalysts It was formerly reported by Lunsford et al that O2- superoxide species can be stabilized at 200 oC on MgO-based catalysts and is the active oxygen sites for OCM.61 Over CaO/Y2O3 OCM catalyst, Osada et al found that O2- can even be stabilized up to 750 oC.43 Moreover, over Na2O2, SrO2 and BaO2 catalysts, Otsuka et al proved that O22- peroxide could be another type of reactive oxygen species for OCM reaction.62 In this study, Raman and EPR have substantiated that O2- is the only type of active electrophilic oxygen species detected on all the La2B2O7 catalysts. To further confirm that it is the active oxygen species for OCM reaction, in situ DRIFTS experiments have been performed on La2Ce2O7, the optimal catalyst in this study, with the spectra displayed in Figure 11 and 12. Notably, for more complete information, the corresponding full range spectra are also included in Figure S8 and S9 in the supporting information file.


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In detail, the La2Ce2O7 sample was pretreated at 550 oC in a 30 mL min-1 ultra-high purity Ar feed for 1 hour to clean any surface impurities and then switched to a 5% O2/Ar gas mixture flow at the same temperature. As shown in Figure 11 (A), three peaks at 1120, 1068 and 1010 cm-1 can be observed. With the increasing of exposing time, while the intensities of the 1120, and 1068 cm-1 peaks remained unchanged, the intensity of the 1010 cm-1 peak increased evidently from 0 to 30 min. It was formerly reported that O-O bond stretching vibration mode of surface O2- species is IR active and shows signals at 1015-1160 cm-1.44, 49, 63 In addition, surface CO32- species is also IR active in this region. 43, 48, 62

However, based on the intensity change, it is rational to assign the 1010 cm-1

peak to O2- superoxide species, and the other two bands to surface carbonates. To confirm this, the same batch of catalyst was consecutively switched to a 30 mL min-1 5% H2/Ar flow at the same temperature. In comparison with the (d) curve in Figure 11 (A), the (a) curve in Figure 11 (B) demonstrates that the 1010 cm-1 peak decreased evidently after H2 introduction. As depicted in Figure 11B (b-d), with the extension of time to 5, 15 and 30 min, this peaks disappeared completely, indicating it relates to some oxygen species reactive with H2. Therefore, there is no doubt that this band should correspond 40 ACS Paragon Plus Environment

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to the surface O2- anions, as also detected by Raman and EPR. However, with the extension of the exposure time in 5% H2/Ar flow, the intensities of the 1120 and 1068 cm-1 peaks are almost constant, testifying that the corresponding chemical species is inert to H2 and should be assigned to surface carbonates. Afterwards, the sample was switched back to the 5% O2/Ar flow. Figure 11B (e-g) shows that the reduced O2- species can be restored, as testified by the intensity increase of the 1010 cm-1 peak. This strongly demonstrates that on the surface of La2Ce2O7, reactive superoxide O2- is present, which is believed to play a vital role for OCM on the La2B2O7 catalysts in this study. It is well accepted that an efficient OCM catalyst requires certain surface alkalinity, which could relate closely to the chemisorbed oxygen anions such as O2-, O22-, O- and surface defects.54, 56, 57 Therefore, In situ DRIFTS experiments have been performed on La2Ce2O7 by exposing it to CO2. In detail, the La2Ce2O7 catalyst was treated first in a 5% O2/Ar feed at 550 oC for 30 min to completely create surface superoxide O2- anions. Afterwards, it was switched to a 5% CO2/Ar gas mixture feed with a rate of 30 mL min-1 at the same temperature. In comparison with the spectroscopy collected at 0 min, Figure 11 (C) shows that the IR bands of surface carbonate species at 1120, 1068 cm-1 increases 41 ACS Paragon Plus Environment

ACS Catalysis

gradually with the increasing of the exposing time, which is accompanied by the decreasing of the 1010 cm-1 band corresponding to superoxide O2- species. No formation of new surface carbonate species can be observed, testifying that surface superoxide O2is the only type of active oxygen species. Furthermore, this proves also that the moderate alkaline sites detected by CO2 -TPD are intimately related to the superoxide O2- species.

0.08

0.12

(B)

(A)

0.07

2-

CO3

0.06

1120

0.10 0.08

Absorbance

Absorbance

d

0.03

c

0.02

b

O2 1010

f e

0.04

d c

0.02

a 1250

1200

1150

1100

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0.00 1300

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Wavenumber (cm-1)

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-

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c

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CO3

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0.01 0.00 1300

1120

-

O2 1010

0.05 0.04

2-

CO3

1068 -

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

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1000

950

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Wavenumber (cm-1)


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Figure 11 In situ DRIFTS spectra of La2Ce2O7. (A) Treated in 5% O2/Ar gas mixture flow at 550 oC. (a) 0 min, (b) 5 min, (c) 15 min, (d) 30 min. ( B) First treated in 5% H2/Ar gas mixture flow at 550 oC. (a) 0 min, (b) 5 min, (c) 15 min, (d) 30 min. Then switched to 5% O2/Ar gas mixture flow at 550 oC. (e) 5 min, (f) 15 min, (g) 30 min. (C) Treated in 5% CO2/Ar gas mixture flow at 550 oC. (a) 0 min, (b) 5 min, (c) 15 min, (d) 30 min.

Probing the possible reaction pathway on the catalysts In order to elucidate the possible reaction pathway for OCM reaction on the catalysts, the reaction between surface O2- species and CH4 was first investigated. In detail, the same batch of La2Ce2O7 catalyst was treated in a 5% O2/Ar feed at 550 oC for 30 min to completely restore the surface superoxide O2- anions. Afterwards, it was switched to a 5% CH4/Ar gas mixture feed with a rate of 30 mL min-1 at the same temperature. In comparison with the spectroscopy collected at 0 min, Figure 12 (A) shows that two new peaks at 1307 and 957 cm-1 are detected on the spectra collected at 5, 15 and 30 min. The 1307 cm-1 band can be ascribed to CH4 in gas phase, 44, 49, 63 and the 957 cm-1 band can be assigned to C2H4 in gas phase.64


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It is observed that the 1010 cm-1 band corresponding to superoxide O2- species decreases gradually with the increasing of the exposing time, which is accompanied by the formation of C2H4, testifying that the reaction occurs between CH4 and surface O2species. In addition, the concentration of C2H4 in the in situ cell increases obviously with the extension of the reaction time, as evidenced by intensity increase of the 957 cm-1 band. This strongly substantiates that the surface O2- anions is the predominant active oxygen species, which reacts with CH4 to form the coupling product. In contrast, the intensities of the 1120 and 1068 cm-1 CO32- bands remain nearly unchanged, testifying that surface carbonates are not active for OCM reaction. Finally, the in situ reaction was also studied by introducing the whole composition gas feed into the IR cell. In detail, the same batch of La2Ce2O7 catalyst was treated again in the 5% O2/Ar flow at 550 oC for 30 min to completely oxidize the surface. As depicted in Figure 12B (a), the surface O2- species was completely regenerated. After this, the gas feed was switched to a 2% CH4 + 0.5%O2 + 2.5%N2 + 95% Ar balance gas mixture flow having a rate of 30 mL min-1. Again, the formation of C2H4 is evidently observed, whose concentration improves with the extension of reaction time. Due to the presence of gas 44 ACS Paragon Plus Environment

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phase O2, the intensity of the superoxide O2- band remained constant during the reaction, testifying the O2- consumption and regeneration loop can proceed kinetically fast and reach equilibrium at the reaction temperature.

0.12

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(A)

0.08 0.06 0.04 0.02

CO3 CH4 1307

1120

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C2H4

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O2 1010

Absorbance

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Absorbance

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d c

CH4 1307

CO3 1120

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Figure 12 In situ DRIFTS spectra of La2Ce2O7. (A) First treated in 5% O2/Ar gas mixture at 550 oC for 30 min (a), then switched to 5% CH4/Ar gas mixture flow at 550 oC. (b) 5 min, (c) 15 min, (d) 30 min. (B) Treated in 2% CH4+0.5%O2+2.5%N2+95%Ar gas mixture flow at 550 oC. (a) 0 min , (b) 5 min, (c) 15 min, (d) 30 min.

3.9 XPS study on the surface oxygen properties of the catalysts The surface oxygen properties were further investigated by XPS for the catalysts. Figure 13 displays that multiple and overlapped O 1s spectra are detected for all the


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samples, testifying the existing of surface oxygen sites having varied chemical environments. According to the Raman, EPR and In situ DRIFTS results, superoxide O2is the only type of active oxygen species observed on the La2B2O7 catalysts. Based on this, the O1s peaks of each sample have thus been deconvoluted, integrated and quantified in Table 6. According to Figure S10, a peak at 289.6 eV attributable to surface carbonate is detected for all the samples.65 Based on the rule that the FWHM of a single O1s peak should be within the width of 1.8 - 2.0 eV,14 the O1s spectra of the La2B2O7 catalysts can be deconvoluted into three peaks by assuming a peak owning 80% Gaussian and 20% Lorentzian shape.14, 16, 25, 26 The oxygen species of the samples are then attributed to O2- (533.0 ~ 533.2 eV), CO32- (531.5 ~ 532.0 eV), and lattice O2- (528.6 ~ 529.2 eV), as shown in Figure 13.


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35000

40000

(a) La2Ti2O7

(b) La2Zr2O7

2-

O

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Binding Energy (eV)

Binding Energy (eV)

Intensity (a.u.)

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O2

10000 536

534

532

530

528

0

526

0.03

Binding Energy (eV)

0.07 - 2-

0.13

O2 /O ratio

Figure 13 XPS O1s spectra of the La2B2O7 catalysts. (a) La2Ti2O7, (b) La2Zr2O7, (c) La2Ce2O7, (d) C2 yield at 800 oC vis O2-/O2- ratios.

The ratios of the surface superoxide O2- to the lattice oxygen O2- are calculated and listed in Table 6, which follows the order of La2Ce2O7 > La2Zr2O7 > La2Ti2O7, fitting well to the H2-TPR, O2-TPD results and the OCM reaction performance. For clarification, the C2 product yield at 800 oC vis the surface O2-/O2- ratio is depicted in Figure 13(d). Apparently, a sample owning a bigger amount of superoxide O2- sites generally has a


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better reaction performance. In addition, this testifies the formation of the surface active superoxide O2- sites is closely related to the inherent 8a oxygen vacancies in their phase structures.

Table 6 Curve-fitting and quantification results of XPS O1s spectra Catalysts

O1s B.E., FWHM (eV) and relative amount (%)

O2-/O2-

O2-

CO32-

O2-

La2Ti2O7

533.2/2.0/2.3

531.5/1.7/21.7

528.7/2.0/76.0

0.03

La2Zr2O7

533.1/2.0/4.5

531.6/1.8/35.2

529.4/1.8/60.3

0.07

La2Ce2O7

533.0/2.0/5.4

531.6/1.7/53.0

528.7/2.0/41.6

0.13

4. Discussion As mentioned above, the phase structure of A2B2O7 complex oxides can be altered with the rA/rB radius ratio.32 La2Ti2O7 has a rA/rB ratio of 1.90, hence possessing a monoclinic layered perovskite-type structure and belonging to space group P21. This compound can be classed into a homologous series of AnBnO3n+2 with n=4,66 which possesses parallel layers of four distorted TiO6 octahedra connected by corners.67 By a cleavage plane along 48 ACS Paragon Plus Environment

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c axis, two kinds of La cations can be found. As shown in Scheme 1(a), the one is topped with corner-sharing distorted TiO6 octahedra, and the other is located in the interstice between the two octahedra layers.67 La2Zr2O7 has a rA/rB ratio of 1.61, thus a stable and ordered face-centered cubic pyrochlore phase belonging to space group Fd3m has been formed.17, 22, 32 The formula of an ideal pyrochlore A2B2O7 structure can also be written as A2B2O6 O′. With the origin fixed on B site, four crystallographically non-equivalent sites are occupied by each atoms, that is, A site at 16d (1/2,1/2,1/2 ), B site at 16c (0,0,0), one O at 48f (x,1/8,1/8) and another O′ at 8b (3/8,3/8,3/8). Compared with the ordered cubic fluorite phase, such as in CeO2, an 8a (1/8,1/8,1/8) site is unoccupied, thus resulting in ordered inherent oxygen vacancies in its structure. To maintain charge neutrality, it results in a face-centred cubic pyrochlore structure with doubled side lengths compared to a cubic fluorite structure, which can improve the oxygen mobility of the catalysts.22 As shown in Scheme 1(b), according to the local coordination environments, four sublattices can be observed. They are AO6O2′, BO6, OA2B2 and O′A4, where A- and B- cations are 8- and 6-fold coordinated with O anions.17, 22 In the AO6O2′ sublattice, the 8-fold coordinated A site cation is located 49 ACS Paragon Plus Environment

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within a disordered cube and coordinated to six O anions spaced equally and two O′ anions, in which the bond length of A-O′ is slightly shorter than that of A-O.17, 68 In the BO6 sublattice, the smaller 6-fold coordinated B site cation surrounded by two 8a inherent oxygen vacancies is located within a distorted octahedral and coordinated to six O anions at equal distance.17,

68

The pyrochlore structure can be varied by the 48f oxygen’s x

parameter. For an ideal fluorite structure, the x value is 0.375. Whereas, for the pyrochlore structure, it varies in the range of 0.3125≤x La2Zr2O7 > La2Ti2O7, which well matches the reaction performance, indicating the abundance of surface O2- sites is a critical factor to affect the reaction performance. (3) CO2-TPD results have substantiated that the quantities of total surface alkaline sites and moderate alkaline sites follow the order of La2Ce2O7 > La2Zr2O7 > La2Ti2O7, which is also well consistent with the catalytic performance. In situ


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DRIFTS results have testified that the moderate alkaline sites are intimately related to the surface O2- sites. (4) The formation of active surface O2- species on La2B2O7 catalysts could proceed via two pathways. For La2Zr2O7 and La2Ce2O7 possessing intrinsic 8a oxygen vacancies, it is formed by activating the oxygen species entering into the oxygen vacancies in the lattice matrix, and then migrates to the surface. For La2Ti2O7 possessing no intrinsic oxygen vacancies, the active oxygen species forms directly by transforming the O2 molecules adsorbed on its surface. Usually, the former pathway generates more abundant and facile surface electrophilic oxygen sites than the latter one. (5) The surface electrophilic O2- species determines the OCM reactivity of the La2B2O7 compounds in this study. La2Ce2O7 owns the highest amounts of superoxide O2- sites, thus having the optimal reaction performance in the three catalysts.

Associated content 56 ACS Paragon Plus Environment

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Supporting Information

Supporting information is provided that includes additional details concerning experimental information; XRD patterns and Raman spectra of the used La2B2O7 catalysts; XPS spectra of the surface metal cations on the La2B2O7 catalysts; Rietveld refinement XRD profiles of the La2B2O7 catalysts; TEM images and particle size distributions of the La2B2O7 catalysts ; XRD patterns and Raman spectra of the B site metal oxides; N2-BET specific surface area results for the B site metal oxides; The OCM reaction performance over the B site metal oxides; EPR quantification results of the La2B2O7 catalysts; The full range In situ DRIFTS spectra of La2Ce2O7; XPS C1s spectra of surface carbonates for the La2B2O7 catalysts.

Author information

Corresponding Author * E-mail: [email protected] (X. Wang)


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Notes The authors declare no competing financial interest. Acknowledgement The authors acknowledge deeply the financial supporting by the National Natural Science Foundation of China (21567016, 21666020), the Natural Science Foundation of Jiangxi

Province

(20181ACB20005,

20181BCD40004,

20181BAB203017,

20171BAB213013) and the Education Department of Jiangxi Province (GJJ150016, GJJ150085, KJLD14005).

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