ORIGINAL PAPER

Carbon Dioxide Reforming of Methane over Nickel CatalystSupported on MgO(111) Nanosheets

Luming Zhang • Lin Li • Jinlin Li •

Yuhua Zhang • Juncheng Hu

Published online: 15 November 2013

� Springer Science+Business Media New York 2013

Abstract MgO nanosheets possessing the (111) facet as

the main surface were synthesized and the Ni catalyst

supported on MgO(111) nanosheets was investigated for

the carbon dioxide reforming of methane. The catalytic

activity and carbon deposition were compared between Ni/

MgO(111) and Ni/MgO(commercial) catalysts. The results

showed that Ni/MgO(111) performed at a higher activity as

well as a longer stability. From the characterization results,

the improved catalytic performances of Ni/MgO(111) were

suggested to be closely associated with both the high dis-

persion of active Ni particles owing to the strong metal-

support interaction and the large amount of basic sites of

MgO(111) due to its unusual surface properties.

Keywords Carbon dioxide reforming of methane �MgO(111) � Nickel catalyst � Activity � Stability

1 Introduction

Recently, considerable attention has been paid to the carbon

dioxide reforming of methane (CRM) as this reaction is

attractive in converting two low-cost greenhouse gases to

valuable syngas with a more desirable H2/CO for Fischer–

Tropsch process [1, 2]. Compared to noble metals, the nickel

is the suitable catalyst due to its lower cost, high activity and

good selectivity. However, up to now the major drawback of

the nickel catalyst remains the rapid deactivation originating

from the sintering of the metal active sites, as well as the

carbon deposition [3–5]. Therefore, developing an

improved Ni-based catalyst with high activity and high

stability for CRM is extremely important.

Previous studies have confirmed that two main proper-

ties of catalyst affect the carbon deposition: surface struc-

ture and surface basicity [6, 7]. The size of the Ni particle

has a significant effect on inhibiting the coke, and smaller

Ni particles perform better to suppress carbon deposition.

Besides, it has also been suggested that carbon deposition

can be attenuated, as the active metal is supported on a

carrier with Lewis basicity [8]. Therefore, the basic sites of

MgO are in favor of the adsorption and activation of CO2,

which would be beneficial to accelerate the gasfication of

the surface carbonaceous species and then, prohibit the

formation of inactive carbon species [4, 9].

The average surface energy of MgO is 2.39 J m-2 for

MgO(100) and 3.08 J m-2 for MgO(111) [10]. Generally,

MgO(100) is the sole surface generated by current chem-

ical methods because of its low surface energy. Moreover,

MgO samples possessing the (111) facet as the primary

surface is one example of the rock salt type Tasker III

surface [11]. The surface of MgO(111), consisting of

alternating polar monolayers of O2- and Mg2?, creates a

strong electrostatic field perpendicular to the (111) surface.

There are large amounts of medium basic sites on

MgO(111) which can be attributed to surface O2- Lewis

basic sites. From recent theoretical estimation, the MgO

with nanosheet morphology that possesses (111) surfaces

on both sides of the sheet is predicted to have zero net

polarity as a result of the high symmetry [11].

As reported, the surface O2- sites and the medium basic

sites are the main active sites for some reactions [12, 13].

L. Zhang � L. Li (&) � J. Li (&) � Y. Zhang � J. Hu

Key Laboratory of Catalysis and Materials Science of the State

Ethnic Affairs Commission & Ministry of Education, College of

Chemistry and Materials Science, South-Central University for

Nationalities, Wuhan 430074, China

e-mail: lilinchem@126.com

J. Li

e-mail: jinlinli@aliyun.com

123

Top Catal (2014) 57:619–626

DOI 10.1007/s11244-013-0220-1

Hu et al. has investigated methanol adsorption on the

surface of MgO(111) nanosheets at low temperature. The

results showed that methanol can be decomposed on the

surface of MgO(111) nanosheets at low temperature. The

formed surface C=O species during methanol decomposi-

tion can be oxidized quickly to CO2 by the oxygen anions

on the surface of MgO(111). So the MgO(111) nanosheets

are likely to serve as nickel catalyst supports for CRM

because of their unique surface properties. According to the

literature [14], chemical nature of a support material plays

a major role both in forming active Ni metal particles and

may be also in the reaction mechanism.

In this paper, MgO(111) nanosheets with unique

exposed facets were synthesized according to the literature

[12]. They were employed as a support for nickel catalysts

in CRM reaction. Hereby, this catalyst would probably

simultaneously display excellent catalytic activity and

stability. Several technologies were used to characterize

the catalysts, such as transmission electron microscopy

(TEM), physisorption of N2, temperature programmed

desorption of CO2 (CO2-TPD), X-ray photoelectron spec-

troscopy (XPS), chemisorption of hydrogen (H2-TPD),

Raman spectra and thermogravimetry analysis (TG), and

the relationship between the structure and the performance

of catalysts was discussed in detail.

2 Experimental

2.1 Catalyst Preparation

All the chemicals were purchased from Sinopharm

Chemical Reagent Corporation and were used without

further purification. MgO(111) nanosheets were synthe-

sized according to the literature [12]. Typically, 3.6 g Mg

ribbon was dissolved in 159 ml anhydrous methanol. Then,

10.4 g 4-methoxybenzyl alcohol was added to the mixture

while stirring for 5 h. After that, a solution of water

(5.4 ml) and methanol (110 ml) was added dropwise into

the stirring solution. The mixture was stirred for 12 h

before being transferred to an autoclave. 1 MPa Ar was

imposed to the autoclave after purging for 10 min. The

mixture was kept at 265 �C and 9.6 MPa for 15 h, and then

the solvent was removed in the supercritical state. The

obtained white powder was finally calcined at 500 �C, for

6 h.

Ni/MgO(111) catalysts (wt. Ni% = 6 %) were prepared

by the impregnation method. A certain amount of nickel

acetylacetonate was dissolved into 20 ml of tetrahydrofu-

ran and then, MgO(111) nanosheets (1 g) were impreg-

nated into the solution. After being stirred for 2 h at 35 �C

under vacuum, the mixture was transferred into an oven at

100 �C. The obtained samples were calcined at 500 �C, for

8 h. As a comparison, 6 % Ni/MgO (commercial, CM) was

prepared by the same procedure.

2.2 Catalyst Evaluation

The catalysts were tested in a fixed bed continuous-flow

reactor at atmospheric pressure. A 200 mg catalyst

(90–120 lm) was diluted with 2 g carborundum. The

overall length of the reaction tube is about 90 cm with the

inner diameter about 1.2 cm, and the height of the catalytic

mass is about 3.5 cm. Prior to the reaction, the catalyst was

reduced at 650 �C, for 10 h in pure H2 with a flow rate of

50 ml/min. Then, we switched H2 to the mixture of CH4

and CO2 (CH4/CO2 = 1:1) and the space velocity was kept

at 36,000 ml/h.g(cat.). Activity tests were performed at

different temperatures increasing from 450 to 650 �C, in

steps of 50 �C. For the stability tests, the temperature was

kept at 650 �C for 10 h, or 100 h on stream. The outlet

gases were analyzed online by an Agilent MicroGC 3000A.

All of the conversion values reported in this paper were

taken after the carbon balance.

2.3 Catalyst Characterization

TEM images of the samples were obtained with a FEI

Tecnai G20 instrument. Before TEM analysis for the

reduced catalysts, the catalysts were reduced at 650 �C for

10 h by pure H2 in tube furnace, and cooled down to room

temperature under H2 atmosphere. Then the samples were

immersed into ethanol as soon as possible. The surface area

was obtained by physisorption of N2, using the Brunauer-

Emmett-Teller (BET) model at -196 �C, in a relative

pressure range of 0.05–0.30. Surface basicity of the two

supports (MgO(111) and MgO(CM)) was measured by

CO2-TPD. The sample was preheated at 400 �C, for 1 h

and then cooled to room temperature in flowing He. The

pretreated sample was exposed to CO2 for 1 h, followed by

purging with He for 30 min. The temperature was then

raised to 600 �C, at a rate of 10 �C min-1, under He

stream. XPS results were obtained with an American

thermal electron VG Multilab spectrophotometer, cali-

brated by C 1s (284.6 eV). The metal dispersion was

determined by H2-TPD. Prior to absorption of H2, the

catalyst was reduced by H2 at 650 �C, for 10 h. The degree

of reduction was measured by oxygen titration. The sample

was reduced by H2 at 650 �C, for 10 h and then reoxidized

at 450 �C, by O2 until there was no further consumption of

O2 detected by TCD. The reduction degree of the catalyst

was calculated by assuming stoichiometric reoxidation of

Ni to NiO. The carbon deposited on spent catalysts was

analyzed by Laser-Raman spectra on Renishaw Confocal

Raman Microspectroscopy and TG on NETZSCH TG

209F3 instrument, respectively.

620 Top Catal (2014) 57:619–626

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Fig. 1 TEM Images of

MgO(111) (a, b), Calcined

Ni/MgO(111) (c, d), Reduced

Ni/MgO(111) Catalyst and Ni

particle size distribution (e),

Reduced Ni/MgO(CM) Cata-

lyst and Ni particle size

distribution (f)

Top Catal (2014) 57:619–626 621

123

3 Results

3.1 Characterization of Supports and Catalysts

The TEM images of the prepared MgO(111) nanosheets,

along with the calcined and reduced catalysts of Ni/

MgO(111) and Ni/MgO(CM) were shown in Fig. 1. As can

be seen from Fig. 1a, MgO(111) showed a nanocrystalline

structure with a plate like shape of the thickness within

3–5 nm. Besides, the HRTEM image (Fig. 1b) of the

support with the lattice spacing 0.245 nm showed that the

MgO nanosheets mainly exposed the well-defined (111)

planes, which confirmed the presence of MgO(111) support

with an unusual surface. From the Fig. 1c, Ni/MgO(111)

catalyst also exhibited the similar plate like shape and the

nickel species were dispersed well on the support. In

addition, it can be seen from Fig. 1d that the exposed (111)

surfaces remained in support of Ni/MgO(111). This

observation confirmed the maintaining of (111) lattice with

an unusual surface. Images and Ni particle size distribution

of reduced catalysts (Fig. 1e, f) reflected the different Ni

particle size between Ni/MgO(111) and Ni/MgO(CM)

catalysts. From Fig. 1e and f, it could be found that the Ni

particle size of Ni/MgO(111) was much smaller than that

of Ni/MgO(CM) catalyst, which was approximately in

agreement with the particle sizes of chemisorption results

(shown in Table 1).

Figure 2 showed the CO2-TPD profiles of MgO(111)

and MgO(CM). From Fig. 2, the MgO(111) sample

exhibited two desorption peaks. The first peak is a big peak

centered at about 187 �C and the second peak is a small

peak centered at about 545 �C. According to the literature

[11] desorption of CO2 at temperatures lower than 160 �C

was attributed to the weak basic sites caused by surface OH

groups. The desorption of CO2 at temperatures ranging

from 160 to 400 �C on the other hand, was assigned to

Mg2?/O2– pairs, while strong basic sites attributed to the

under-coordinated O2- were observed at temperatures

above 400 �C in CO2-TPD. The results in Fig. 2, revealed

that MgO(111) were primarily covered by Mg2?/O2– pairs

with medium basicity, and the surface basic sites of

MgO(111) were much more than those of MgO(CM).

The XPS spectra of Ni2p for calcined catalysts Ni/

MgO(CM) and Ni/MgO(111) were compared in order to

investigate the electronic donor intensity of the nickel-

based catalysts and the interaction between support and

nickel metal, as shown in Fig. 3. The main binding energy

of Ni 2p3/2 for Ni/MgO(CM) catalyst was located at

854.52 eV, and the binding energy of Ni 2p3/2 for Ni/

MgO(111) was shifted to 855.22 eV. The lower binding

energy of nickel species suggested that the electronic donor

intensity of the nickel active species in the Ni/MgO(CM)

catalyst was stronger, and the Ni/MgO(CM) catalyst could

easily give off electrons. Also, it implied that the interac-

tion between NiOx species and MgO(CM) support was

weaker, which was similarly reported in the literature [15].

On the other hand, the nickel active species of the Ni/

MgO(111) sample had the higher binding energy

(855.22 eV, for the main peak). It suggested the weaker

electronic donor intensity of the Ni/MgO(111) catalyst,

which also indicated that the interaction between NiOx

species and MgO(111) support was stronger due to the

unique surface of MgO(111). The stronger metal-support

Table 1 Physicochemical properties of Ni/MgO(111) and Ni/MgO(CM)

Catalysts SBET

(m2 g-1)

Uptake amount (lmol g-1) Dispersion

(%)aReduction

degree (%)bCorrected

dispersion (%)cParticle

size (nm)d

H2 O2

Ni/MgO(111) 70 54.5 170.3 10.7 33.3 32.1 3.1

Ni/MgO(CM) 21 43.5 315.1 8.5 61.6 13.8 7.2

a Determined by hydrogen chemisorption, assuming H/Nisurface = 1b Calculated by oxygen titration. Assuming 2Ni0 ? O2 = 2NiO and the total amount of reduced Ni = 2 9 (O2 consumption)c Corrected dispersion = Dispersion/Reduction degreed Estimated by using the corrected dispersion (D) data, assuming d = 1/D [16]

Fig. 2 CO2-TPD profiles of MgO(111) and MgO(CM)

622 Top Catal (2014) 57:619–626

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interaction may be helpful to suppress the growth of Ni

particles in the reaction.

The physicochemical properties of Ni/MgO(CM) and

Ni/MgO(111) catalysts were listed in Table 1. Nitrogen

physisorption was employed to determine the surface areas

of the catalysts after calcination. From Table 1, the surface

area of Ni/MgO(111) (70 m2g-1) was larger than that of

Ni/MgO(CM) (21 m2g-1). In addition, the amount of H2

desorption of Ni/MgO(111) catalyst was larger than that of

Ni/MgO(CM) catalyst, which was an indication of the

different amount of Ni available for CRM reaction. The O2

uptake, which was related to the reduction degree, was

higher for Ni/MgO(CM) compared to Ni/MgO(111).

Nickel particle size has been calculated from the value of

corrected dispersion by assuming spherical metal particles.

From Table 1, we could conclude that after reduction the

average Ni particle size of Ni/MgO(111) was much smaller

than that of Ni/MgO(CM), and the dispersion of Ni metal

on MgO(111) support was much higher than that on

MgO(CM). In addition, we could find that the reduction

degree of Ni/MgO(CM) was higher than that of Ni/

MgO(111), which was probably due to the smaller particle

size of Ni metal on MgO(111) and the stronger interaction

between nickel and MgO(111) support.

3.2 Catalytic Performance of Different Catalysts

The catalytic performances of Ni/MgO(111) and Ni/

MgO(CM) catalysts at different reaction temperatures were

shown in Table 2. As shown in Table 2, the CH4 and CO2

conversions on Ni/MgO(CM) were both lower than those

on Ni/MgO(111) catalyst at all the temperature steps tested.

Moreover, for the two catalysts, the conversions showed a

similar increasing trend with the temperature rising from

450 to 650 �C. Compared to the CH4 conversion, the

conversion of CO2 was higher in all cases due to the

simultaneous presence of reverse water–gas shift reaction

[17, 18]. The reverse water–gas shift reaction also resulted

in the ratio of H2/CO below 1, and the values of H2/CO on

Ni/MgO(111) catalyst were all higher than those on Ni/

MgO(CM) catalyst, indicating the better selectivity of Ni/

MgO(111) catalyst.

In order to elucidate the effect of support on the initial

conversions of CH4 and CO2, we have calculated the

turnover frequency (TOF) of CH4 and CO2, at the initial

reaction (450 �C) on the basis of the amount of H2

desorption listed in Table 1. Under 450 �C, the TOF of

CH4 and CO2 over Ni/MgO(CM) catalyst were 0.11 and

0.15 S-1. The TOF of CH4 and CO2 over Ni/MgO(111)

catalyst were 0.12 and 0.23 S-1, respectively. From the

results, it could be found that at 450 �C the TOF of CH4

over Ni/MgO(111) catalyst was a little higher than that

over Ni/MgO(CM) catalyst, and the TOF of CO2 over Ni/

MgO(111) catalyst exhibited superiority than that over Ni/

MgO(CM) catalyst to some extent.

The evaluation of the stability was performed at 650 �C

(GHSV = 36,000 ml/g.h, CH4/CO2 = 1:1, 1 atm). As can

be seen from Fig. 4a and b, the degree of deactivation for

the two catalysts was different. After 10 h of reaction, the

Ni/MgO(CM) catalyst underwent an obvious decrease of

about 16 % for methane conversion, and the Ni/MgO(111)

showed much higher stability with nearly no decrease for

methane conversion. As to the CO2 conversion, Ni/

MgO(111) catalyst also displayed a much more excellent

catalytic performance than Ni/MgO(CM) and there was

nearly no decrease for the Ni/MgO(111) catalyst. At the

same time, it was found that the conversions of CH4 and

CO2 were both higher for the stability test compared to the

results in Table 2 under the same reaction conditions. The

lower values in Table 2 could be attributed to the carbon

formation during the catalysts tests at different tempera-

tures. In addition, the long stability of Ni/MgO(111)

Fig. 3 XPS of Ni/MgO(CM) (a) and Ni/MgO(111) (b)

Table 2 Catalytic performances of Ni/MgO(111) and Ni/MgO(CM)

at different temperatures

Reaction

temperature (�C)

Conversion of

CH4 (%)

Conversion of

CO2 (%)

H2/CO

Ni/

MgO

Ni/

MgO

Ni/

MgO

Ni/

MgO

Ni/

MgO

Ni/

MgO

(CM) (111) (CM) (111) (CM) (111)

450 4.3 5.7 5.8 11 0.6 0.6

500 7.3 16 13.1 20.4 0.55 0.62

550 9.3 25.7 15.3 33.2 0.49 0.65

600 19.8 35.9 27.7 46.4 0.53 0.68

650 32.6 46.2 45.7 58.5 0.61 0.71

Reaction conditions: CH4/CO2 = 1:1, GHSV = 36,000 ml/(g h),

1 atm

Top Catal (2014) 57:619–626 623

123

catalyst has also been tested for 100 h, as shown in Fig. 4c.

From Fig. 4c, even after 100 h of reaction, the conversions

of methane and carbon dioxide still remained high values,

which indicated the good stability of Ni/MgO(111)

catalyst.

3.3 Analysis of the Spent Catalysts

The carbon deposition over Ni/MgO(CM) and Ni/

MgO(111) catalysts after 10 h of reaction at 650 �C were

investigated by TG, as shown in Fig. 5. From Fig. 5, the

total weight loss was about 48.0 and 37.8 % for Ni/

MgO(CM) and Ni/MgO(111), respectively, which sug-

gested that the amount of carbon deposited on Ni/

MgO(111) was obviously less than that on Ni/MgO(CM).

The amounts of coke formed per gram of the catalyst after

Fig. 4 Conversions of CH4 (a) and CO2 (b) over Ni/MgO(111) and Ni/MgO(CM) for 10 h; long term stability of Ni/MgO(111) for 100 h

(c) (Reaction conditions: CH4/CO2 = 1:1, GHSV = 36,000 ml/(g h), T = 650 �C, 1 atm)

Fig. 5 TG analysis of the spent catalysts after 10 h reaction

624 Top Catal (2014) 57:619–626

123

10 h, for Ni/MgO(CM) and Ni/MgO(111) were 923 and

608 mg, respectively. Usually, carbonaceous species have

several different types [19]. Active carbonaceous species

formed in the CRM might be the intermediates of reaction,

which can be eliminated by CO2 during the following

process of reaction, and the graphite carbon is the one

leading to the final deactivation.

The various deposited carbon species of used catalysts

were also investigated by Raman spectroscopy and the

results were displayed in Fig. 6. The D (1,357 cm-1) and

D*(1,620 cm-1) bands are induced by disorder carbon

species closely associated with the disorder-induced

vibration of C–C bond, and the G (1,588 cm-1) and

G*(2,700 cm-1) bands are Raman active for sp2 carbon

networks attributed to graphite carbon [20–22]. For the two

spent catalysts, the intensity of D band was higher than that

of G band, indicating that there were plenty of defective

carbon species. Furthermore, the ratio between ID and IG

bands are close to 1.04, for the used Ni/MgO(CM) catalyst,

while for the sample of used Ni/MgO(111) catalyst after

10 h, the ratio value rises to 1.87. The ID/IG ratio of used

Ni/MgO(111) catalyst being 1.87, reflected that the amount

of defective carbon species is much higher than that of

graphite which was responsible for the deactivation of the

catalyst.

In order to further investigate the metal sintering of the

two catalysts, TEM analysis of the spent Ni/MgO(111) and

Ni/MgO(CM) catalysts after 10 h of reaction at 650 �C was

performed. As shown in Fig. 7, carbon nanotubes could be

easily found in the images of spent catalysts, which were

produced due to the carbon deposition. In addition, the

metal sintering was also a crucial factor to affect the per-

formance of catalysts. From Fig. 7, over the spent Ni/

MgO(111) catalyst, most of the Ni particles were between

6 and 8 nm along with a few larger ones of 10–20 nm. On

the contrary, it could be observed that most of the Ni par-

ticles over the spent Ni/MgO(CM) catalyst were larger than

20 nm, suggesting that a serious metal sintering occurred

during the reaction. The difference of metal sintering may

be related to the different metal-support interaction (shown

in XPS results), and the stronger metal-support interaction

in the Ni/MgO(111) catalyst can prohibit the growth of Ni

particles in the reaction, which plays an important role to

the stability of catalytic performance.

4 Discussion

The above experimental results indicated that Ni/

MgO(111) catalyst exhibited much more excellent perfor-

mance than Ni/MgO(CM) catalyst. This was probably

caused by the surface effects and small size effects of

nanocrystalline. As reported in literature [1, 7], the con-

version of CH4 is mainly from its dissociation on active

metal nickel particles, which may not be covered by carbon

deposition at the initial reaction. Therefore, the conversion

of CH4 on Ni/MgO(111), performed much higher than that

on Ni/MgO(CM), due to the higher dispersion of Ni metal

Fig. 6 Raman spectra of coke deposits over spent catalysts after 10 h

reaction

Fig. 7 TEM images of spent

catalysts: Ni/MgO(111) (a) and

Ni/MgO(CM) (b)

Top Catal (2014) 57:619–626 625

123

on MgO(111). However, the conversion of CO2 has two

ways. The first is the conversion of CO2 adsorbed on Ni

particles, and the second is the dissociation of CO2

adsorbed on oxygen basic sites of basic support. Conse-

quently, the high concentration of surface O2- Lewis basic

sites on MgO(111) surfaces, ensures the high catalytic

performance to the absorption and activation of carbon

dioxide.

It is well-known that the deactivation of Ni-based cat-

alysts for CRM mainly results from the carbon deposition

and the sintering of active Ni particles [14]. From the TEM

results of the spent catalysts, the metal sintering of Ni/

MgO(111) was better than that of Ni/MgO(CM). From the

TG and Raman results, we could make out that the amount

of carbon deposited on Ni/MgO(111) was obviously less

than that on Ni/MgO(CM), and there existed more amor-

phous carbon and defective graphite species on the used

Ni/MgO(111) catalyst. It was believed that smaller Ni

particles had strengthened capability to suppress carbon

deposition, which indicated that anchoring the nickel par-

ticle on the support became particularly important. It is

important because the Ni/MgO(111) catalyst containing

smaller Ni particles, due to its strong metal-support inter-

action, could effectively suppress the coke deposition and

the metal sintering.

In addition, it has been reported that the properties of

support, especially basicity, are very important to the

suppression of coke deposition in CRM [4, 9]. As shown in

the Fig. 2 of CO2-TPD, MgO(111) contained plenty of the

medium basic sites and was in favor of the chemisorption

of CO2, which would accelerate the elimination procedure

of surface active carbonaceous species coming from the

dehydrogenation of CH4 and CO disproportionation and

thusly, avoiding the formation of inactive species. Thus,

the high concentration of surface Lewis basic sites on

MgO(111) surface not only ensures the high catalytic

performance to the absorption and activation of CO2, but

also helps to suppress the carbon deposition.

Therefore, polar anionic surfaces of MgO(111) are of

particular importance to provide a possible explanation for

the excellent catalytic performances of Ni/MgO(111) cat-

alyst observed in the CRM.

5 Conclusions

In summary, MgO nanosheets with the high ionic (111)

facet as the major surface were employed as support for

nickel catalysts in CRM. From the characterization and

evaluation results, it can be concluded that the excellent

catalytic activity and stability of Ni/MgO(111) catalyst

should be attributed to the high dispersion of active Ni

particles, small Ni particle size, large amount of medium

basic sites and the strong interaction between metal and

support.

Acknowledgments Financial supports of this work by the South-

Central University for Nationalities (CZZ12002) are greatly

appreciated.

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