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Ordered mesoporous alumina supported nickel based

catalysts for carbon dioxide reforming of methane

Leilei Xu a,b, Huahua Zhao a,b, Huanling Song a, Lingjun Chou a,*a State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences,

No. 18, Tianshui Middle Road, Lanzhou, Gansu 730000, PR Chinab Graduate School of Chinese Academy of Sciences, Beijing 100049, PR China

a r t i c l e i n f o

Article history:

Received 13 September 2011

Received in revised form

13 January 2012

Accepted 21 January 2012

Available online 6 March 2012

Keywords:

Ordered mesoporous alumina

Confinement effect

Carbon dioxide reforming

Methane

a b s t r a c t

Ordered mesoporous alumina facilely synthesizedvia improved evaporation-induced self-

assembly (EISA) strategy was provided with large specific surface area, big pore volume,

uniform pore size and excellent thermal stability. The obtained mesoporous material was

used as the carrier of the Ni based catalysts for carbon dioxide reforming of methane.

These mesoporous catalysts performed high catalytic activity and long stability. Typically,

the catalytic conversions of the CH4 and CO2 were greatly close to the equilibrium

conversion and no deactivation was observed during the 100 h long lifetime test. The

advantageous structural properties of ordered mesoporous alumina contributed to high

dispersion of the Ni particles among the mesoporous framework, which further accounted

for the good catalytic activity due to more accessible Ni active sites for the reactants. The

confinement effect of the mesopores could effectively prevent the thermal sintering of

the Ni nanoparticles to some extent, committed to its long-term catalytic stability. Besides,the mesoporous catalysts possessed enhanced ability to withstand coke, although not any

modifiers had been added. Properties of the coke over the mesoporous catalyst were also

carefully investigated. Therefore, the ordered mesoporous alumina was a promising

catalyst support for the carbon dioxide reforming with methane.

Copyright 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

With the development of the modern industry, large amountsof the greenhouse gases are released into the atmosphere,

eventually leading to the global warming, namely, green-

house effect[1]. Carbon dioxide and methane are commonly

recognized as two sorts of the most important greenhouse

gases committed to the greenhouse effect [2]. How to effec-

tively realize the mitigation of these two greenhouse gases,

especially carbon dioxide, becomes an imminent problem to

be solved [1,3]. The catalytic process of carbon dioxide

reforming of methane (CRM) provides a potential route to

simultaneously transform these two greenhouse gases into

more valuable synthesis gas. Therefore, this process was ofa great interest from the industrial as well as environmental

standpoint in recent years [4e9]. Furthermore, compared

with partial oxidation (2/1) and steam reforming (3/1) of CH4,

CRM produces synthesis gas with a lower H2/CO ratio (1/1),

which is more favorable in oxo synthesis, hydroformylation,

synthesis of various oxygenated derivatives, etc [10,11].

Whereas, the rapid deactivation of the catalysts deriving from

the carbon deposition and the thermal sintering of the

* Corresponding author.Tel.: 86 931 4968066; fax: 86 931 4968129.E-mail address:[email protected](L. Chou).

Available online atwww.sciencedirect.com

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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 7 4 9 7 e7 5 1 1

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doi:10.1016/j.ijhydene.2012.01.105
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metallic active centers hinders the large scale industrial

application[5,12].

In the previous studies, most of the supported VIII metal

catalysts such as Ni, Co, Ru, Rh, Ir and Pt are commonly

consideredto be active catalyststowardsCRM reaction [13e22].

Although noble catalysts show excellent activity as well as

good resistance to coke, yettheirlow availabilityand high price

make them unsuitable for the extensive industrial application.Therefore, the research interests of dry reforming catalysts

turn to non-precious metals, especially Ni. Nickel is an ideal

candidate due to its high initial catalytic activity, inherent

availability and low cost. However, nickel based catalysts was

more inclined to suffer from serious carbon deposition and

thermal sintering of the active centers, finally causing the

deactivation of catalystsand plugging of the reactor. Generally,

the coke is causedviathe Boudouard reaction (2CO C CO2)

and methane decomposition reaction (CH4 C 2H2). The

facile thermal agglomeration of the Ni active sites originates

from the low Tammann temperature (also known as the initial

sintering temperature) around 590 C, which is much lower

than the reaction temperature (600 C) for CRM reaction[23,24]. Hence, preparing Ni based catalysts with excellent

stability as well as good activity remains a great challenge.

In order to develop catalysts satisfying above demands,

lots of strategies have been employed, including altering the

acid-base or redox properties of carriers by adding various

modifiers[25e28], introducing a second active metal compo-

nent (e.g. Co, Ru, Rh, Pt) [9,17,29e32], and controlling the

dispersion of the Ni via special preparation methods, such as

plasma treatment [33,34], etc. Apart from utilizing various

catalysts preparation arts, more and more researchers have

devotedtheir enthusiasm to designing new catalytic materials

in recent years based on the belief: the structure of the

material will finally determine its catalytic performance.In this research context, the mesoporous materials

emerged. Since the ordered mesoporous silica material (MCM-

41) was firstly reported in 1992, the interests of the meso-

porous materials have been rapidly extended to many

research areas, including their potential applications in cata-

lyticfield [35e38]. Non-silica mesoporous materials, especially

metal oxide or metal composite oxide mesoporous materials,

might as well develop rapidly owing to eminent catalytic

performances and good carriers [39e50]. Nickel based cata-

lysts supported on mesoporous materials normally present

high dispersion due to the large specific surface areas, big pore

volumes as well as uniform pore sizes of the carriers. Conse-

quently, the highly dispersed Ni particles over the meso-porous framework are able to provide more accessible or

exposed Ni active centers for the reactants, accounting for

the good activity for these materials. As mentioned above, one

major reason causing the deactivationof the CRM catalyst was

the thermal sintering of the Ni particles due to the reduction

in the number of the active centers. However, confining

the growth of nickel particles and localizing the active

component at specific location is not so easy. The sintering of

Ni particles seems to be inevitable under severe reduction and

reaction conditions. Recently, researchers have found that the

thermal sintering of the supported metal particles could be

effectively controlled when loaded on mesoporous materials.

This phenomenon occurring on mesoporous support might be

attributable to the confinement effect, which could limit the

growth of Ni particles during the reaction and promote the

catalytic stability [50,51]. The seriously thermal agglomeration

of the Ni nanoparticles is effectively avoided since the Ni

atoms are stabilized by confinement effect of the meso-

porous framework, eventually making for the long stability of

the catalyst. Therefore, mesoporous materials promised ideal

catalyst candidates for CRM reaction.In our present work, ordered mesoporous alumina (OMA)

with large specific surface area, big pore volume and uniform

pore structure was facilely synthesized via improved

evaporation-induced self-assembly (EISA) strategy. A series of

Ni based catalysts supported on OMA were also prepared by

incipient wetness impregnation method. These catalysts

behaved good catalytic performances with high catalytic

activity as well as long lifetime stability. To the best of our

knowledge, there was almost no report on Ni based catalyst

supported on OMA as catalyst for CRM reaction. More details

about these mesoporous catalysts for CRM reaction would be

described amply in the main article.

2. Experimental

2.1. OMA preparation

Ordered mesoporous alumina (OMA) powders were synthe-

sizedvia improved evaporation-induced self-assemble (EISA)

by fine control the volatile process according to the previous

literature [52,53]. In a typical synthesis process, approximately

1.0 g of (EO) 20(PO) 70(EO) 20 triblock copolymer (Pluronic P123,

from SigmaeAldrich) was dissolved in 20.0 mL anhydrous

ethanol with vigorously stirring. Then, 1.6 mL of 67wt% nitric

acid and approximately 10 mmol aluminum isopropoxide(C9H21AlO3, 98%, from SigmaeAldrich) were added into the

above solution with vigorous stirring. The final mixture was

covered with PE film and stirred at room temperature for at

least 5 h. Finally, transferred the mixture to a Petri dish,

covered the Petri dish with holed PE film, and finally put the

covered Petri dish into a 60 C drying oven to undergo the

solvent evaporation process. A light yellow solid was obtained

after 48 h fine control EISA process. Calcined the final solid by

slowly increasing temperature (1 C/min ramping rate) to

700 C and kept at 700 C for 5 h. Ultimately, the OMA with

large specific surface area, big pore volume and narrow pore

size distribution was obtained.

2.2. Catalyst preparation

Nickel catalysts supported on ordered mesoporous alumina

containing X wt % (X wt % mNi/(mNi mAl2O3) 100%,

denoted as X%Ni/OMA in the following text) were prepared

via incipient wetness impregnation method assisted with

3 h ultrasound treatment using nickel nitrate hexahydrate

(Ni (NO3)2$6H2O, from Shanghai NO.2 Reagent Factory, China)

as the precursor of nickel. After impregnation, the catalyst

precursors were dried under the irradiation of the infrared

lampandthendriedina60 Covenfor24h.Finally,thecatalyst

precursorswerecalcinedat700 C for5 h. Allcatalysts obtained

were pressed, crushed and sieved through 20e

40 meshes.

http://dx.doi.org/10.1016/j.ijhydene.2012.01.105http://dx.doi.org/10.1016/j.ijhydene.2012.01.105http://dx.doi.org/10.1016/j.ijhydene.2012.01.105http://dx.doi.org/10.1016/j.ijhydene.2012.01.105

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2.3. Catalyst characterization

Powder X-ray diffraction (XRD) measurements were per-

formed using an XPert Pro multipurpose diffractometer

(PANalytical, Inc.) with Ni-filtered Cu Ka radiation

(0.15046 nm) at room temperature from 10.0 to 80.0 (wide-

angle range) and 0.6e5.0 (small angle range). Measurements

were conducted using a voltage of 40 kV, current setting of40 mA, step size of 0.02, and count time of 4 s. Crystallite

sizes of Ni particles were calculated using Scherrer equation:

D ( h k l ) Kl/(bcosq), where K was the shape factor of the

average crystalline,l, the wavelength (1.54056 Afor Cu Ka),b,

defined by a relationship between the full wave at half

maximum (FWHM) of the sample and that of a quartz stan-

dard, and q was the peak position.

The nitrogen adsorption and desorption isotherms at

196 C were recorded on an Autosorb-iQ analyzer (Quan-

tachrome Instruments U.S.). Prior to the tests, samples were

degassed at 200 C for 4 h. The specific surface areas were

calculated via the BET method in the relative pressure rangeof

0.05e

0.3; the single-point pore volume was calculated fromthe adsorption isotherm at a relative pressure of 0.990; pore

size distributions were calculated using adsorption branches

of nitrogen adsorption-desorption isotherms by BJH method.

TEM images were taken on the JEM-2010 (Japan) high-

resolution transmission electron microscopy under a working

voltage of 200 kV. The samples were dispersed in absolute

ethanol by moderate sonication at concentration of 5 wt %

solid. A Lacy carbon-coated 200 mesh TEM micro-grid was

dipped into the sample suspension and then dried under

vacuum at given temperature for a while prior to analysis.

X-ray photoelectron spectroscopy (XPS) analyses of the

catalysts were performedon an ESCALAB 210(VG Scientific Ltd)

spectrometer. The fresh catalyst was placed on sample holderand pressed into self-supported wafer. An Mg target was used

as the anode of the X-ray source with a power of 200 W. The

pass energy of the analyzer was 30 eV in a step increment of

0.05 eV. The binding energies were calibrated using the Si (2P)

line at 103.4 eV as the reference. Near-surface compositions

were calculated from peak areas using the sensitivity factors,

which were provided in the software of the instrument.

Thermogravimetri-differential scanning calorimetry

(TG-DSC) measurements were carried out on a NETZSCH STA

449F3 thermogravimetric analyzer from room temperature to

900 C with the rate of 10 C/min under air atmosphere.

H2 temperature-programmed reduction (TPR) measure-

ments were performed on an AMI-100 unit (Zeton-Altamirainstrument) employing hydrogen as reducing agent. The

samples (250 mg) were loaded in a U-shaped quartz reactor.

Prior to the TPR measurements, samples were pretreated at

300 C for 0.5 h in flowing He (50 mL/min) to remove any

moisture and other adsorbed impurities. After cooling the

reactor to the room temperature, a 5% H2-He (50 mL/min) gas

mixture was introduced. The catalyst was heated to 1300 C at

a rate of 20 C/min and the hydrogen consumption was

measured using an AMETEK (LC-D-200 Dycor AMETEK) mass

spectrometer.

Temperature programmed hydrogenation (TPH) charac-

terizations were also operated on the same device as H2-TPR.

The spent catalyst (40 mg) was submitted to a heat treatment

(10 C/min, up to 900 C) in a gas flow (50 mL/min) of the

mixture 5% H2eHe. The CH4signal of the effluent gases was

detected by the mass spectrometer.

2.4. Catalytic activity evaluation

Catalytic tests were performed at atmospheric pressure

(1 atm) in a vertical fixed-bed continuous flow quartz reactor(8 mm, i.d.). The whole reaction evaluation system was con-

sisted of a mass flow controller unit (MT50-4J METRON

Instruments), a reactor unit, and an analysis unit (SP-6800A

GC). The reaction temperature increased from 600 C to 800 C

at a 50 C increment. The analysis for the effluent gas was

carried out after stabilizing for 1 h at each studied tempera-

ture and gas hourly space velocity (GHSV). Typically, 100 mg X

%Ni/OMA catalyst diluted with 350 mg quartz sand (20e40

meshes) was used in each run. Prior to the reaction, the

catalyst was reduced in situ in a mixed flow of H2 and N2(H2: N2 10:20 mL/min) with a heating rate of 1.5 C/min to

800 C, and maintained at 800 C for 120 min. Before intro-

ducing in the reaction gases, the catalyst bed was purged withN2for half an hour to remove the absorbed hydrogen. Then,

the reaction mixture was fed into the reactor via flow

controller unit. The effluent mixed gases were cooled in an

ice-water trap to remove the gaseous water generating via

reverse water-gas shift (RWGS) reaction. The separation and

quantification of the products were achieved on an on-line

chromatograph equipped with TDX-01 packed column.

3. Results and discussion

3.1. Structure characterization of OMA materials

3.1.1. XRD analysis

Small-angle X-ray diffraction (SXRD) was usually considered

as the evidence for the formation of the ordered meso-

structure [35,36]. As shown in Figs. 1 and 2 (1), the OMA

calcined at 700 C exhibited a extremely strong diffraction

peak around 1.0 and one weak peak around 1.6, which could

be attributed to p6mm hexagonal symmetry based on the

observation in TEM images (see Fig. 3). Part (2) of theFig. 1

presented the wide-angle X-ray diffraction (WXRD) pattern

for the above-mentioned sample. Calcination at 700 C gave

rise to the ordered mesoporous framework with g phase

alumina (JCPDS Card No. 10e0425)[52]. Therefore, the ordered

mesoporousg alumina with crystalline wall was successfullyprepared based on the SXRD and WXRD characterization

results.

3.1.2. Nitrogen adsorption-desorption analysis

Similar to the XRD analysis, the nitrogen physisorption could

also offer the bulk information of the porous materials. The

nitrogen adsorption and desorption isotherms (inFig. 2) of the

ordered mesoporous alumina calcined at 700 C performed

typical type IV curves with H1 shaped hysteresis loops, sug-

gesting the presence of the uniform cylindrical mesopores.

The inlet of theFig. 2was the pore size distribution curve of

the above-mentioned OMA sample. As shown in the figure,

the material displayed narrow pore size distribution around

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9.0 nm, which was located in the size range (2e50 nm) of the

mesopore defined by IUPAC. Besides,the textural propertiesof

the OMA material were summarized in Table 1. OMA treated

at 700 C had a large BET specific surface area of 224.957 m 2/g

and a big pore volume of 0.538 cm3/g. The large specific

surface area and narrow pore size distribution combined with

excellent thermal stability promised its potential applicationin the field of catalysis.

3.1.3. TEM analysis

In order to further confirm the presence of the ordered mes-

opores for OMA, images of TEM (inFig. 3) for the sample were

taken. The highly ordered hexagonal arrangement of the

pores along [0 0 1] (Fig. 3(b)) direction and the alignment of

cylindrical pores along [1 1 0] (Fig. 3(a), (c), (d)) direction were

distinctly observed, illustrating that the ordered mesopores

with p6mm hexagonal symmetry indeed existed among the

skeleton of the material. The characterization results of the

TEM images were well consistent with the SXRD and N2

adsorption and desorption analysis characterization results.

3.2. Characterization of as-prepared X%Ni/OMA

catalysts

3.2.1. XRD analysis

The XRD patterns of the as-prepared X%Ni/OMA catalysts

calcined at 700 C were shown inFig. 4. Apart from 15%Ni/

OMA, all the samples exhibited similar profiles, performing

three distinct diffraction peaks regardless of the loading of the

Ni. For the sample of 3%Ni/OMA, only three typical diffraction

peaks of g phase alumina (JCPDS Card No. 10e0425) was

detected. On the contrary, the diffraction peaks of the NiO

were absent, suggesting the high dispersion of the NiO speciesamong the mesoporous frameworkof OMA. However, with the

increase of the Ni loading from 3% to 10%, the location of the

three pronounced diffraction peaks gradually migrated

towards the low angle, which was clearly observed by the

relative positions of the peaks marked with the dotted line.

The reason for this might derive from the overlapping of the

typical diffraction peaks for NiO (JCPDS Card No. 78e0429) and

g-Al2O3 (JCPDSCard No.10e0425) species, which were adjacent

to each other for the position of diffraction peaks. There were

no evident characteristic diffraction peaks of the NiO appear-

ing even as the Ni loading was as high as 10 wt%, further

confirming the high dispersion of the NiO among the meso-

porous framework. For 15%Ni/OMA, the typical diffractionpeaks of NiO were clearly observed, but the crystallite size of

the NiO was still difficult to calculate by the Scherrer equation

dueto the peak broadening.All theevidenceindicated that NiO

was highly dispersed among the mesoporous skeleton.

3.2.2. Nitrogen adsorption-desorption analysis

The nitrogen adsorption-desorption isotherms as well as pore

size distributions of as-synthesized X%Ni/OMA calcined at

700 C were displayed inFig. 5. As shown inFig. 5(1), all the

samples presented IV type isotherms with H2 shaped

hysteresis loops, suggesting the presence of the ink-bottle

shaped mesopores among the mesoporous framework.

Compared with the OMA carrier with H1 shaped hysteresis

Fig. 1 e (1) Small-angle X-ray diffraction and (2) Wide-angle

X-ray diffraction patterns of ordered mesoporous alumina

calcined at 700 C.

Fig. 2 e Nitrogen adsorption-desorption isotherm and pore

size distribution of ordered mesoporous alumina calcined

at 700 C.


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loop, the H2 shaped hysteresis loops of the as-synthesized X%

Ni/OMA catalysts indicated that the uniform cylindrical-

shaped mesopores were deformed into ink-bottle shaped

pores after the loading of NiO and during the subsequentcalcination process of the catalyst precursors. Besides, the

pore size distribution curves for the samples were clearly

exhibited inFig. 5(2). All the catalysts performed extremely

narrow pore size distribution around 5.50 nm. In contrast with

the OMA carrier, their average pore diameters were relatively

smaller than that of the OMA, illustrating the occurrence of

the shrinkage of the mesoporous skeleton after the loading of

NiO and the following high temperature (700 C for 5 h)

calcination. Furthermore, the characterization results of the

structural properties of the above samples were alsosummarized in Table 1. It could be observed that all the

samples were still provided with large specific surface areas

upto 213.268 m2/g and big pore volumes up to 0.300 cm3/g. The

average pore diameters of the catalysts were in the range of

5.50 nme6.50 nm without exception. In addition, it was

noteworthy that the specific surface areas and pore volumes

Fig. 3 e Images of transmission electron microscopy (TEM) of ordered mesoporous alumina calcined at 700 C.

Table 1e Textural properties of the ordered mesoporous alumina calcined at 700 C, as-prepared X%Ni/OMA catalyst,as-reduced 10%Ni/OMA, the used 10%Ni/OMA and the endurance-tested 10%Ni/OMA.

Samples Specific surfacearea (m2g1)

Pore volume (cm3g1) Average porediameter (nm)

Isotherm type

OMA 224.957 0.538 9.457 IV H1

3%Ni/MA 212.169 0.300 5.576 IV H2

5%Ni/MA 213.268 0.293 5.597 IV H2

7%Ni/MA 194.651 0.289 5.595 IV H2

10%Ni/MA 173.685 0.251 5.619 IV H2

15%Ni/MA 157.866 0.229 6.484 IV H2

as-reduced 10%Ni/MAa 159.187 0.255 5.607 IV H2

used 10%Ni/MAb 114.945 0.370 5.633 IV H2

endurance-tested 10%Ni/MAc 113.202 0.352 6.548 IV H2

a The 10%Ni/OMA catalyst was in situ reduced under H2/N2(H2:N2 10:20 mL/min) atmosphere at 800 C for 2 h.

b The used catalyst was the catalyst tested under the conditions: CH4/CO2 1, GHSV 15000 mL/(g.h), 1 atm, and temperature from 600 C to

800 C with the increment of 50 C and stayed at each temperature stage for 70 min.

c The 10%Ni/OMA material was used as catalyst of CRM reaction for 100 h long-term stability test; Reaction conditions: CH 4/CO2 1,

GHSV 15000 mL/(g.h), 700

C, 1 atm.


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almost suffered from a bit decline as the NiO loading

increased. Generally, all the mesoporous catalysts still

preserved large specific areas, big pore volumes and narrow

pore size distributions even after a second high temperature

catalyst calcination process, fully demonstrating the

enhanced thermal stability of the OMA.

3.2.3. TEM analysisThe morphology analysis of the X%Ni/OMA catalysts was

performed. 7%Ni/OMA and 10%Ni/OMA catalysts were

selected as representatives and their TEM images were dis-

played inFig. 6. Compared with the as-prepared OMA mate-

rial, the orderliness of the mesopores for X%Ni/OMA catalysts

was subjected to some damages during the second calcination

process (at 700 C for another 5 h) of the catalysts preparation,

which had been reflected in the shapes of the hysteresis

loops in N2 adsorption-desorption analysis as discussed

above. However, the mesopores along [1 1 0] (Fig. 6(a) and (c))

and [0 0 1] (Fig. 6(b) and (d)) directions for 7%Ni/OMA and 10%

Ni/OMA catalysts were still visible, demonstrating excellent

thermal stability of OMA carrier. Besides, it was worth notingthat no evident NiO particles were observed in the images,

further illuminating the high dispersion of the NiO among the

mesoporous frameworks. Overall, the characterization results

of TEM were in good agreement with those of XRD and N 2adsorption-desorption analyses.

3.2.4. H2-TPR analysis

TPR technique was a potent means for determining the

interactions between metal and support for metal oxide sup-

ported catalysts. H2-TPR profiles of the as-prepared X%Ni/

OMA catalysts with diverse Ni content were clearly shown in

Fig. 7. All the samples except for 15%Ni/OMA performed

semblable profiles of hydrogen reduction, displaying only oneapparent reduction peak in the region from 860 C to 965 C

regardless of the Ni content. No obvious reduction peak

located in the range of 300e400 C could be observed, sug-

gesting the absence of the dissociated or free NiO not inter-

acting with the mesoporous framework [54]. There were evidences that the intense interaction between the Ni species

and the support had been established. In addition to this, it

could be distinctly observed that the TPR patterns the

following order of the maximum peak temperature: 3%Ni/

OMA (963 C) 5%Ni/OMA (924 C) 7%Ni/OMA (882 C)

10%Ni/OMA (866 C). This indicated that the Ni-OMA interac-

tions were greatly affected by the Ni loading. Generally, the

lower of the Ni loading was, the stronger the Ni-OMA inter-

action was. The reason for this might be derived from thefacile formation of the NiAl2O4spinel-like species in the case

of lower Ni loading and relatively higher alumina percentage.

As for 15%Ni/OMA, apart from the maximum uptake around

902 C, another visible shoulder peak centered at 584 C were

also detected, reflecting that the NiO species which had rela-

tively weak interaction with the mesoporous framework

existed. Generally, the result of H2-TPR analysis was in good

agreement with the XRD characterization.

3.2.5. XPS analysis

Being sensitive to the composition of the top surface layers,

XPS measurement was employed in determining the state of

the surface nickel species in the catalysts. The XPS profiles of

Fig. 4 e Wide-angle X-ray diffraction patterns of X%Ni/OMA

catalysts: (a) 3%Ni/OMA, (b) 5%Ni/OMA, (c) 7%Ni/OMA, (d)

10%Ni/OMA, (e) 15%Ni/OMA.

Fig. 5 e (1) Nitrogen adsorption-desorption isotherms and

(2) pore size distributions of X%Ni/OMA catalysts.


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Ni element for the as-prepared X%Ni/OMA were displayed in

Fig. 8. As shown in the figure, the intensities of the XPS curves

gradually became stronger as the Ni loading increased. From

the XPS spectra of the Ni, it was also observed that the posi-

tions the Ni2p3/2peak for all the catalysts were located in the

range of 855.40e856.30 eV. It was well known that the binding

energy of the Ni2p3/2in pure NiO was about 853.30 eV, which

was less than the binding energiesof theNi2p3/2 in X%Ni/OMA

catalysts[55]. Therefore, the oxidation state of the surface Ni

elements was presence in the form of Ni2. Furthermore, the

binding energies of the Ni2p3/2 of all samples were much

higher than 853.30 eV, suggesting that the Ni2 species did notexist in the form of free NiO and the strong interaction

between Ni2 species and OMA supporter did exist. Pioneer

works pointed out that Ni2p3/2 peak of the Ni species with

a binding energy at 856 eV and an accompanying shake-up

satellite peak at 862 eV were characterized for NiAl2O4spinel

[55,56]. As for the profiles for X%Ni/OMA with the Ni loading

not morethan15 wt% ((a)w (e)), Ni2p3/2 peakscentered around

856 eV (typically, 855.43 eVe856.23 eV) and a satellite peak

around 862 eV could be observed, implying that the NiAl2O4spinel-like species indeed formed on the surface of the cata-

lysts. Previous study results had confirmed that NiO species

was facile to form strong metalesupport interaction (SMSI) in

the Ni/Al2O3 catalyst [57,58]. Thus, the NiO species having

SMSI and NiAl2O4 spinel-like species coexisted in the fresh

catalyst. All the evidences above mentioned could account for

the high temperature reduction peaks in H2-TPR analysis.

Besides, it was worth noting that with the increase of Ni

loading from 3 wt% to 15 wt%, the binding energies of Ni2p3/2and Ni2p1/2peaks gradually migrated from 856.23 to 855.43 eV

and from 873.68 to 872.98 eV, respectively. As regards the X%

Ni/OMA catalysts, the alteration in the binding energies

directly embodied the change in thestrength of theinteraction

between NiO species and OMA supporter. In other words, as

the Ni containing increased, the relationship between the NiO

species and OMA carrier became weaker, which had beenelaborated detailedly in the part of TPR analysis. Overall, the

XPS analysis was well consistent with the H2-TPR analysis.

3.3. Catalytic performances of the CRM reaction over X%

Ni/OMA catalysts

3.3.1. Effect of reaction temperature

Blank test was performed prior to regular catalytic experi-

ments and behaved almost no catalytic activity even at

temperature as high as 800 C. The catalytic activities of the X

%Ni/OMA with diverse Ni containing (where, X 3, 5, 7, 10, 15)

in the CRM reaction at different temperatures under given

reaction conditions (GHSV 15000 mL/(g.h), CH4/CO2 1,

Fig. 6 e TEM images of the X%Ni/OMA catalysts: (a) and (b) 7%Ni/OMA, (c) and (d) 10%Ni/OMA.


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1 atm) were shown in Fig. 9. Asobserved in the Fig. 9 (1) and (2),

the conversions of CH4, CO2 were greatly dependent on the

reaction temperatures. Specifically, with the temperatures

elevated, the conversions of the CH4 and CO2 increased,

reflecting the strong endothermic feature of the CRM reaction

[4e6]. Hence, all the catalysts performed their respective

highest catalytic activity at 800 C in the temperature range

examined. Besides, the connection between H2/CO ratio and

reaction temperature were also detailedly studied and depic-

ted inFig. 9(3). Overall, the H2/CO ratios for all the reaction

temperatures studied were lower than the stoichiometric

ratio (1: 1) of the CRM reaction. The reason for this was that

CRM reaction was always accompanied by the reverse water-

gas shift (RWGS) reaction (CO2 H2 CO H2O), which was

responsible for this phenomenon[9,26,27]. H2, one of the two

main products of the CRM reaction, was partly consumed in

the RWGS reaction. As a result, the actual H2/CO ratio was

relatively lower than the stoichimetric ratio (1: 1). Further-

more, it was of great interest that the H2/CO ratio was found to

be elevated as the rise of the reaction temperature according

toFig. 9(3), which was in good agreement with the thermo-

dynamics tendency of the RWGS side reaction.

In addition to this, the relationship between the catalytic

activity and Ni mass percentage for X%Ni/OMA catalysts also

Fig. 7 e H2-TPR profiles of the X%Ni/OMA catalysts with

different Ni contents: (a) 3%Ni/OMA, (b) 5%Ni/OMA, (c) 7%

Ni/OMA, (d) 10%Ni/OMA, (e) 15%Ni/OMA.

Fig. 8 e XPS spectra of the Ni element in X%Ni/OMA

catalysts with different Ni contents: (a) 3%Ni/OMA, (b) 5%

Ni/OMA, (c) 7%Ni/OMA, (d) 10%Ni/OMA, (e) 15%Ni/OMA.

Fig. 9 e The curves of the (1) CH4conversion, (2) CO2conversion, (3) H2/CO ratios versus Ni wt% at various

reaction temperatures; Reaction conditions: CH4/CO2 [ 1,

GHSV [ 15000 mL/(g.h), 1 atm.


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sintering of the metal did not occur. As for the 15%Ni/OMA,

thermal sintering of the metallic active centers might have

taken place during severe reduction and reaction process

based on the XRD patterns (refer toFig. 4) presented above. In

addition, the connections between the H2/CO ratio and the

GHSV as well as the Ni content were also presented inFig. 10

(3). Nevertheless, their mutual relationships were extremely

complicated and no rule could be kept to, waiting for furtherinvestigation.

3.3.3. Long-term stability test of the catalyst

The long stability of the X%Ni/OMA catalyst was examined

under specific reaction conditions: CH4/CO2 1, 700 C,

GHSV 15000 mL/(g.L), 1atm. The 10%Ni/OMA was chosen as

the representative catalysts for the long lifetime test. As dis-

played in theFig. 11, the 10%Ni/OMA catalyst exhibited both

high catalytic activity and excellent catalytic stability

throughout the 100 h long stability test. Specifically, the

catalyst performed over 80% and 81% conversions of the CH 4and the CO2, respectively, both of which had reached the

thermodynamic equilibrium conversions[5]. No deactivationwas observed during the100 h time on stream. Besides, the H2/

CO ratio also remained steady and oscillated around 0.80. It

was of interest that the conversion of CO2 was a bit higher

than that of CH4. The reason for this might attribute to the

concomitance of the RWGS reaction[9,26,27]. CO2rather than

CH4 was consumed in this reaction, accounting for the

phenomenon mentioned above. Besides, the RWGS reaction

also committed to the lower H2/CO ratio than stoichiometric

ratio (1: 1) due to the consumption of H 2. Overall, the X%Ni/

OMA catalysts without any modification still possessed

favorable catalytic stability, demonstrating the advantage of

the mesoporous catalysts.

3.4. Characterization of the spent X%Ni/OMA catalysts

3.4.1. Comparative analysis of the WXRD patterns for the

as-reduced, the used and the endurance-tested 10%Ni/OMA

The comparative analysis of the WXRD for the as-reduced,

the used and the endurance-tested catalysts was conducted.

The 10%Ni/OMA catalyst was chosen as representative and

their corresponding patterns were displayed inFig. 12. Here,

the as-reduced catalyst was the catalyst reduced at 800 C for

2 h in a mixed flow of H2 : N2 (10: 20 mL/min) and cooled

to room temperature in the N2 protective stream; the

used catalyst was the catalyst tested under given conditions

(CH4/CO2 1, GHSV 15000 mL/(g.h), 700 C, 1 atm) and

temperature from 600 C to 800 C with the increment of 50 Cand stayed at each temperature platform for 70 min; the

endurance-tested catalyst was the catalyst went through

100 h long lifetime test under given condition as mentioned

above. Dissimilar to the as-prepared 10%Ni/OMA (seeFig. 4),

typical diffraction peaks of NiO disappeared for the as-

reduced sample and the characteristic diffraction peaks of

Ni (JCPDS Card No. 87-0712) appeared after reduction.

Compared with the as-reduced 10%Ni/OMA, the used sample

still presented similar Ni diffraction intensity after under-

going different temperature stages. Its average D (2 0 0)(D was

the crystallite size) of the Ni particles was only 14.46 nm

calculated according to Scherrer equation and preserved

nano-sized state. As for the 100 h endurance-tested 10%Ni/OMA, its WXRD pattern of Ni was also parallel to the used

sample, suggesting that the thermal sintering of the Ni

nanoparticles (15.52 nm) during the long lifetime test was

effectively suppressed. The confinement effect of the

mesopores was supposed to contribute to the stabilization of

the metallic nanoparticles[50,51]. Besides, the graphic carbon

diffraction peaks with strong intensities were distinctively

observed in the patterns for both the used and 100 h

endurance-tested 10%Ni/OMA. The significant amount of the

carbon deposition might derive from the surface Lewis acidity

of the OMA, which might cause the badly coke on the catalyst

surface. Moreover, the intensity of graphic carbon peak for

the used sample was a little stronger than that of theendurance-tested sample, implying that the amount of the

carbon deposition had little relationship with the reaction

time. As mentioned above, the good catalytic activity of the

Fig. 11e Long term stability test over the 10%Ni/OMA

catalyst; Reaction conditions: CH4/CO2 [ 1, GHSV [ 15000

mL/(g.h), 700 C, 1 atm.

Fig. 12e Wide-angle X-ray diffraction patterns for 10%Ni/

OMA catalyst after different treatments: (a) the as-reduced

catalyst, (b) the used catalyst, (c) the endurance-tested

catalyst.


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10%Ni/OMA was successfully maintained for the whole 100 h

time on stream in despite of the presence of considerable

amount of the coke according to the observation of XRD,

illuminating that this kind mesoporous catalyst possessed

enhanced capacity of tolerating coke.

3.4.2. Comparative analysis of the textural properties for the

as-reduced, the used and the endurance-tested 10%Ni/OMAIn order to further confirm the thermal stability of the meso-

porous skeletons for the X%Ni/OMA catalysts, the structural

properties of the as-reduced, used and endurance-tested

samples were also carefully characterized. Similar to the

above section, the 10%Ni/OMA catalyst was also selected as

a representative and their characterization results were

depicted inFig. 13. As shown in theFig. 13(1), the as-reduced,

used and endurance-tested 10%Ni/OMA entirely performed IV

type isotherms with H2 shaped hysteresis loops, which were

the typical characteristic for the mesoporous materials with

ink-bottle shaped mesopores. As mentioned above, the as-

prepared 10%Ni/OMA also performed IV type isotherms

affiliated with H2 shaped hysteresis loops (refer toFig. 5(1)).

Therefore, the damage of the framework of the as-reduced,

used and endurance-tested 10%Ni/OMA during the severe

reduction and reaction conditions was effectively prevented

to some extent. Besides, the pore size distributions of the

samples were also given inFig. 13. As shown in theFig. 13(2),

all the samples possessed very narrow pore size distributions

around 6.0 nm, once again demonstrating the presence of theuniform mesopores after various thermal treatments. Overall,

all the evidences mentioned above suggested that the X%Ni/

OMA mesoporous catalysts were also provided with enhanced

thermal stability to withstand rigorous reduction and reaction

conditions.

Furthermore, the characterization results of the textural

properties of the as-reduced, used and endurance-tested 10%

Ni/OMA were simultaneously summarized in Table 1.

Compared with the as-prepared 10%Ni/OMA, the as-reduced

sample suffered a bit decline in the specific surface area

from 173.685 m2/g to 159.187 m2/g; however, their pore

volumes and average pore diameter performed similar values,

typically0.251 cm3/g and 5.619 nm for as-prepared sample and0.255 cm3/g and 5.607 nm for as-reduced sample, respectively.

This phenomenon again indicated that the mesoporous

structure of the 10%Ni/OMA catalyst was not damaged under

rigorous reduction condition. Whereas,compared with the as-

reduced 10%Ni/OMA, the specific surface areas of the used

and endurance-tested samples suffered further decrease. The

decline in the surface areas might be caused by the carbon

deposition, which blocked some of the mesoporous channels

to some degree. On the contrary, it was of great interest that

their pore volumes and average pore diameters were greatly

improved after the CRM reaction. The reason for these might

also stem from the coke deposited on the catalyst surface. It

was well known that the carbonmaterialscommonly behavedlarge surface areas and big pore volumes [60]. Although the

coke on the catalyst surface would bring on the decrease in

the surface areas of the catalysts due to the blockade of the

mesopores, yet their own textural properties might also

contributed to the structural properties of the used and

endurance-tested 10%Ni/OMA catalysts. However, the specific

mechanism for the effect of the coke on the structural

Fig. 13 e Isotherms (1) and pore size distributions (2) of the

10%Ni/OMA catalyst after different treatments: (a) the as-

reduced catalyst, (b) the used catalyst, (c) the endurance-

tested catalyst.

Fig. 14e TG-DSC analysis of the 100 h endurance-tested

10%Ni/OMA catalyst.


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features of the spent catalysts remained unclear and

demanded further investigation. Generally, all the indications

described above demonstrated that the mesostructures of the

catalysts were not damaged under harsh reduction and

reaction conditions. Hereby, the OMA material was equipped

with reinforced thermal stability to act as a support of the

catalysts.

3.4.3. TG-DSC analysis of the endurance-tested 10%Ni/OMA

The properties of the carbon deposition over 10Ni/OMA cata-

lyst after 100 h long-term stability test at 700 C were dis-

played inFig. 14. As for the TG curve, its general trend was

downward as the temperature increased. Nevertheless, thecurve primitively underwent minor rise in the region from

200 C to 500 C, suggesting that the oxidation of the metallic

Ni occurred. It was well known that the weight loss of the TG

curve for the spent catalysts of the CRM reaction indicated the

removal of the deposition carbon from the catalyst. The TG

curve showedthat the weight loss of the coke over thecatalyst

was 19%. The DSC profile indicated that the deposition carbon

could be burned out in a temperature range between 200 C

and 780 C. One pronounced exothermic peaks at 667 C and

two weak shoulder peaks around 343 C, 517 C were observed

in theDSC profile, implying that there were at least three sorts

of coke deposited on the surface of the catalyst. The weak

exothermic peak around 343 C might derive from thecombustion of the amorphous carbon, which contributed to

the formation of the synthesis gas [61,62]. Another weak

shoulder exothermic peak around 517 C might be attributed

to the intermediate state carbon deposition between amor-

phous and whisker carbon. As regards the intense peak

at 667 C, it could be ascribed to the whisker type carbon

Fig. 15e TPH profile of the 100 h endurance-tested 10%Ni/

OMA catalyst.

Fig. 16e

TEM pictures of the 100 h endurance-tested 10%Ni/OMA catalyst.


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(also called carbon nanotube, filament carbon), which was

responsible for the deactivation of the catalyst based on the

literatures[62e64]. However, as mentioned above, the cata-

lytic activity of the 10Ni/OMA catalyst was not seriously

affected during the long-term stability test even though the

carbon deposition was as high as 19%, performing enhanced

capacity of tolerating the coke.

3.4.4. TPH analysis of the endurance-tested 10%Ni/OMA

In order to further verify the hypothesis on the species of

carbon deposition based on the DSC curve, temperature-

programmed hydrogenation (TPH) of the endurance-tested

10%Ni/OMA was conducted. TPH technique was usually

used to study the types of the coke deposited on the catalyst.

The TPH profile of the endurance-tested catalyst was shown

inFig. 15. There were three apparent peaks centered at 373 C,

440 C, 590 C observed in the figure. The first peak around

373 C might be the amorphous carbonaceous species, which

was related to Ca according to the pioneer literature

[46,47,62,65,66]. The amorphous carbon (Ca) might be the

reaction intermediate, which was responsible for the COformation[61,62]. Besides, part of the Ca could be converted

less active Cb through further dehydrogenation and poly-

merization and re-arrangement of Ca [24,67,68]. The second

overlapped peak centered at 440 C ought to be attributed to

Cb, which could be further gasified, might encapsulate on the

surface, or dissolved in or encapsulate the Ni crystallite. The

last peak around 590 C was identified as the whisker type

carbon (also known as carbon nanotube, filament carbon),

which had the lowest reactivity compared with other coke

species towards hydrogenation and conduced to the deacti-

vation of the catalyst[69,70]. Hence, the characteristic result

of the TPH was in good agreementwith the inference basedon

the TG-DSC analysis.

3.4.5. Morphology analysis of the endurance-tested 10%Ni/

OMA

In order to further confirm the morphology of the coke as well

asthe thermal stability of the OMA carrier, TEM analysis of the

100 h endurance-tested 10%Ni/OMA was performed. The

images were depicted in Fig. 16. Similar to the as-prepared

10%Ni/OMA catalyst (refer to Fig. 6 (c) and (d)), the meso-

pores along [1 1 0] and [0 0 1] directions for the endurance-

tested sample were still observable fromFig. 16 (a) and (b),

respectively, suggesting that mesostructure of the 10%Ni/

OMA catalyst had not been destroyed during the processes of

reduction and 100 h long-term stability test. Besides, as showninFig. 16(c) and (d), the main coke residue over the catalysts

was carbon nanotubes. Moreover, it could be observed that

the carbon nanotubes were mainly distributed outside of the

mesopores (see Fig. 16 (a)) and no Ni nanoparticles was

encapsulated by the nanotubes according toFig. 16(c) and (d).

Consequently, the deactivation of the catalyst deriving the

coverage of the Ni active centers was effectively avoided,

further accounting for the 100 h long-term stability. Besides, it

was of interest that no obvious amorphous carbon was found.

The reason for this might be that the amorphous carbon was

uniformly distributed among the mesopores. Generally, the

observation of the TEM was well consistent with the analyses

of TG-DSC and TPH.

4. Conclusion

Ordered mesoporous alumina was facilely via improved

evaporation-induced self-assembly strategy. The obtained

mesoporous material possessing large specific surface area,

big pore volume, uniform pore size and favorable thermal

stability was employed as the support as the Ni based cata-lysts for CRM reaction. These mesoporous catalysts per-

formed high catalytic activity and long catalytic stability

toward this reaction. The mesoporous framework of the X%

Ni/OMA catalysts played a critical role to endow the catalysts

with these merits, specifically, which would provide more

accessible Ni active centers for the reactants and stabilized

the Ni active sites by the confinement effect during the

reaction. Besides, these mesoporous catalysts behaved

enhanced capacity of tolerating carbon deposition. It was also

observed that carbon nanotube was the main form of the

coke, which would not cause the deactivation of the catalyst

during 100 h lifetime test. Due to these favorable advantages,

ordered mesoporous alumina promised an ideal catalystcarrier for CRM and even other reactions.

Acknowledgements

The authors sincerely acknowledge the financial support from

the National Basic Research Program of PR China (No.

2011CB201404) and the National Natural Science Foundation

of China (No. 21133011).

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