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Properties of Ni/Y2O3 and its catalytic performance inmethaneconversion to syngas

Huimin Liu, Dehua He*

Innovative Catalysis Program, Key Laboratory of Organic Optoelectronics and Molecular Engineering of Ministry of Education,

Department of Chemistry, Tsinghua University, Beijing 100084, PR China

a r t i c l e i n f o

Article history:

Received 9 June 2011

Received in revised form

28 July 2011

Accepted 7 August 2011

Available online 13 September 2011

Keywords:

Ni/Y2O3 catalyst

Partial oxidation of methane

Syngas

* Corresponding author. Tel/Fax: þ86 10 6277E-mail address: hedeh@mail.tsinghua.ed

0360-3199/$ e see front matter Copyright ªdoi:10.1016/j.ijhydene.2011.08.025

a b s t r a c t

Ni/Y2O3, with Y2O3 support prepared by the conventional precipitation method, was

prepared by an impregnation method. The physicochemical properties of Y2O3 and Ni/Y2O3

were characterized by BET, CO2-TPD, NH3-TPD, TPR, XRF and TGA, and compared with

those of g-Al2O3 and Ni/g-Al2O3, respectively. The catalytic performance of Ni/Y2O3 in the

reaction of partial oxidation of methane (POM) to syngas was evaluated and compared with

that of Ni/g-Al2O3 catalyst, too. The results showed that, Y2O3 was a basic support with few

acidic sites while g-Al2O3 was an acidic support. NiO particles supported on Y2O3 were

more easily to be reduced than those supported on g-Al2O3. In the partial oxidation of

methane, Ni/Y2O3 catalyst showed high catalytic activity and exhibited better catalytic

stability than Ni/g-Al2O3. After POM reaction at 700 �C for 550 h, methane conversion

decreased little and only 2.2 wt% carbon was deposited on Ni/Y2O3 catalyst. Ni/Y2O3 was

stable in POM even after a series of reaction temperature variations within the temperature

range of 400 w 800 �C.

Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights

reserved.

1. Introduction Ni-based catalysts have been widely studied because of their

Catalytic conversion of methane to syngas (and also to

hydrogen gas) is an important route for the effective utiliza-

tion of natural gas [1e4]. Generally, there are three ways for

the conversion of methane to syngas, including the steam

reforming of methane (SRM), the carbon dioxide reforming of

methane (CRM) and the partial oxidation of methane (POM).

SRM is a traditional industrial route, whereas CRM and POM

are potential technical routes and have attracted more and

more attentions in the recent 20w 30 years. Particularly, POM

has overwhelmed SRM due to its obvious advantages, such as

high energy efficiency [5], suitable H2/CO ratio for methanol

synthesis and FischereTropsch processes.

In the partial oxidation of methane, compared with noble

metal-based catalysts, such as Rh, Ru, Ir, Pd, Pt [6e10],

3346.u.cn (D. He).2011, Hydrogen Energy P

good catalytic performances as well as the low costs.

However, Ni-based catalysts are suffered from deactivation

due to carbon deposition in POM [11,12]. Lots of methods have

been employed to improve their abilities to resist carbon

deposition. Reports showed that the acidic-basic properties of

Ni-based catalysts could affect the amount of carbon depos-

ited. Choudhary et al. [13,14] discovered that, no obvious

carbon was deposited on Ni/CaO after 15 h reaction in POM.

Miao et al. [15] modified Ni/Al2O3 catalyst with Li2O and La2O3,

and the obtained LiNiLaOx/Al2O3 exhibited improved ability to

resist carbon deposition during the 50 h life-test in POM.

Similar results were also obtained on ZrO2, MgO and La2O3,

which were also used as the supports of Ni-based catalysts in

POM [16e18]. Except oxide supports, non-oxide supports, such

as SiC and Si3N4, were also employed in POM. Shang et al. [19]

ublications, LLC. Published by Elsevier Ltd. All rights reserved.

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7e1 4 4 5 414448

studied the catalytic performance of nitrified Ni/SiC in the

partial oxidation of methane, and the results showed that,

compared with Ni/SiC, less carbon was deposited on the

nitrified Ni/SiC. Ni/Si3N4 also exhibited good catalytic perfor-

mance in POM [20], and almost no carbon was deposited.

However, it is not the truth that, the more basic of catalysts,

the more able to resist carbon deposition, because methane

coupling reaction might occur on catalysts with the supports

of strong basicity [21]. Therefore, searching suitable supports

for POM is still an important issue.

Y2O3, an important metal oxide, has been applied in a wide

range of areas because of its optical, thermal and chemical

stabilities [22e24]. In the recent years, Y2O3 has also been

applied in some catalytic reactions. Yao et al. [25] showed that

the addition of a small quantity of Y2O3 and K2O to Ag/Al2O3

could adjust the electronic density of adjacent silver atoms

and produce proper adsorbed oxygen species, and the

conversion of propylene and the selectivity to propylene oxide

were high on the obtained AgeY2O3eK2O/Al2O3 catalyst in the

reaction of epoxidation of propylene. Costa et al. [26] studied

Pd/CeO2 and Pd/Y2O3 in the partial oxidation of ethanol, and it

was found that the selectivity to CO was higher over Pd/Y2O3

catalyst, since Pd/Y2O3 catalyst favored the transformation of

ethoxy species to acetate whereas Pd/CeO2 catalyst facilitated

the further oxidization of CO to CO2. Wu et al. [27] employed

different Rh supported catalysts in the steam reforming of

ethanol (SRE), and the order of catalytic activity was:

Rh/Y2O3 > Rh/CeO2 > Rh/La2O3 > Rh/Al2O3. The activity of

Rh/Y2O3 in SRE was promoted due to the surface oxygen

vacancies of Y2O3. Yttria stabilized zirconia (YSZ) was also

used as supports in the steam reforming of ethanol [28], the

selective reduction of NO [29], the carbon dioxide reforming of

methane [30] and the partial oxidation of methane [31].

Santos et al. added a small amount of Y2O3 to Al2O3, and

the obtained Ni/Al2O3eY2O3 catalyst presented higher cata-

lytic activity than Ni/Al2O3 in the methane autothermal

reforming reaction (ATR) [32]. Fu et al. [33] used different

oxides, including Y2O3, to modify Al2O3 support, and found

that the ability to resist carbon deposition of the prepared Ni-

based catalysts (measured after 1 h TOS of ATR) decreased in

the following order: Ni/CaOeAl2O3 > Ni/MgOeAl2O3 > Ni/

TiO2eAl2O3 > Ni/CeO2eAl2O3 > Ni/La2O3eAl2O3 > Ni/

Y2O3eAl2O3 > Ni/Fe2O3eAl2O3 > Ni/Al2O3. Wang et al. [34]

employed Y2O3 promoted metallic Ni catalyst in POM. The

catalyst was acid treated nickel sponge and modified with

Y2O3 by an impregnationmethod. The results showed that the

conversion of CH4 and the selectivities to H2 and CO were

increased on the Y2O3 modified Ni sponge. However, the

amount of carbon deposited was not analyzed and long time

life-test was not reported by Wang et al. [34]. What’s more, in

this case, Y2O3 was not used as catalyst support, instead, only

a small amount of Y2O3 was used (the content of Y2O3 was in

the range of 1.38 w 10.1%). On the other hand, Y2O3 was also

used as a support of Ru catalyst in the partial oxidation of

methane [35], and Nishimoto et al. reported that Ru/Y2O3 was

an active catalyst in POM and no carbon was deposited after

10 h reaction. However, the physicochemical properties of

Y2O3 were not revealed or longer time life-test was not

reported either. In the previous studies of Y2O3 as supports, no

matter Rh/Y2O3 in SRE, Ni/Al2O3eY2O3 in ATR, or Ru/Y2O3 in

POM, the relationship between the properties of Y2O3 and the

catalytic performance of the relevant catalysts was not

investigated intensively. However, the properties of Y2O3

might exert an influence on the performance of Ni/Y2O3

catalyst in POM. Therefore, in this paper, the physicochemical

properties of Y2O3 and Ni/Y2O3, as well as the catalytic

performance of Ni/Y2O3 catalyst in POM, were studied and

compared with those of Al2O3 and Ni/Al2O3, respectively.

2. Experimental section

2.1. Catalyst preparation

Y2O3 was prepared by the conventional precipitation method.

200 mL Y(NO3)3$6H2O aqueous solution (0.25 M) containing

0.05 mol Y(NO3)3 was added dropwise into 145 mL NH3

aqueous solution (2.5 wt%) under the conditions of pH 10w 11

and vigorous stirring. After 0.5 h of stirring, the white

precipitate of Y(OH)3 was aged at room temperature for 12 h.

Then, the precipitate was filtered andwashed thoroughlywith

deionizedwater till the pH of the filtratedmother liquor was 7.

Afterward, the Y(OH)3 hydrogel was dried at 110 �C for 12 h in

air and then calcined at 500 �C for 5 h in air. Ni/Y2O3 catalyst

was prepared by impregnating Ni(NO3)2∙6H2O aqueous solu-

tion (analytical grade reagent, provided by Shantou Xilong

chemical factory) with the above obtained Y2O3 support at

room temperature for 10 h. After water being removed by

vaporization, the precursor of Ni/Y2O3was then dried at 110 �Cin air for 12 h and calcined at 650 �C in air for 5 h.

For comparison, Ni/g-Al2O3 was also prepared with the

impregnation method, and the detailed information was

available in our previous paper [36].

2.2. Catalyst characterization

The specific surface areas of the catalysts and the supports

were measured by N2 adsorption-desorption with the BET

method on a Micromeritics ASAP 2010 C analyzer. The crys-

talline phases of the Ni particles and the supports were

investigated by X-ray diffraction (XRD). The reduction

behaviors of the catalysts were characterized by Temperature

programmed reduction (TPR). The amounts of carbon depos-

ited on the catalysts were evaluated by thermogravimetric

analysis (TGA). The acidic properties of the catalysts were

measured by NH3-temperature programmed desorption (NH3-

TPD). The actual contents of Ni on the catalysts were

measured by X-ray fluorescence spectrometer (XRF). The

detailed experimental procedures of BET, XRD, TPR, TGA, XRF

and NH3-TPD were consistent with those in our previous

paper [36].

The basic properties of the catalysts were measured by

CO2-temperature programmed desorption (CO2-TPD). The

CO2-TPD profiles were measured by Quantachrome adsorp-

tion instrument (Chembet-3000 TPR/TPD). The catalysts (0.1 g)

were firstly treated in highly pure He (99.999%, 110 mL/min) at

500 �C for 0.5 h. After that, the catalysts were saturated with

flowing highly pure CO2 at 100 �C, and then flushed with

highly pure He (110 mL/min) to remove the physically adsor-

bed CO2. Finally, the desorption of CO2 was carried out in

10 20 30 40 50 60 70 80

Ni/Y2O3 (550h POM)

Ni/Y2O3 (125h POM)

Ni/Y2O3 (10 min POM)

NiO/Y2O3

Y2O3

+

+ ****

****

*****

*

*

Ni+Y2O3*

Inte

nsity

(a.u

.)

2 Theta degree

Fig. 1 e XRD patterns of Y2O3 support and Ni/Y2O3 catalyst.

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 en e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7e1 4 4 5 4 14449

flowing highly pure He (110mL/min) from 100 �C to 800 �Cwith

a heating ramp of 15 �C/min. Nevertheless, carbonate

(Y2O2CO3) may be formed on Y2O3 and Ni/Y2O3 due to the

preservation of the samples in air [37]. In order to eliminate

the influence of Y2O2CO3 on the CO2-TPD profiles, CO2-TPD

profiles obtained by pretreating catalysts at 700 �C for 0.5 h

before CO2 adsorption was used as reference.

2.3. Catalytic performance

The apparatus used to evaluate the catalytic performance of

Ni/Y2O3 in POM was the same as that in our previous paper

[36]. It was conducted in a fixed-bed quartz reactor with an

inner diameter of 5 mmunder atmosphere. A sample (0.1 g) of

the catalysts was packed in the center of the quartz reactor.

A furnace was used to heat the quartz tube reactor. A ther-

mocouple, placed in the center of the furnace, was used to

control the temperature of the furnace. Another two ther-

mocouples, installed in the quartz tube reactor and contacted

Table 1 e Textural properties of supports and amount of carbo

Catalyst/support Specific surfaceareas (m2/g)a

Pore sizedistributions

(nm)b

Y2O3 50.8 2e8

g-Al2O3e 142.5 2e8

Ni/Y2O3 43.4 5e20

Ni/g-Al2O3e 116.9 2e8

a Measured by N2 adsorption-desorption with the BET method.

b Measured by BJH.

c Measured by XRF.

d Measured by TGA.

e Data was cited from Ref. [36].

with the inlet and outlet sides of the catalyst bed, were used to

measure the temperatures of the inlet and outlet sides of the

bed. Before reaction, the catalyst was reduced with 20 mol%

H2/Ar at 600 �C for 2 h, then the temperature was raised to the

reaction temperature (700 �C), and then themixture gas of CH4

and O2 with a molar ratio of 2/1 was introduced into the

reactor at a total flow rate of 165mL/min (The concentration of

CH4 is 66.7mol% and the concentration of O2 is 33.3mol%, and

no inert gas was introduced). The effluent gas was firstly

cooled down in a cool trap (about 0 �C) to remove water in the

products and the gaseous products were analyzed by two on-

line gas chromatographs (GC), both equipped with TDX-01

columns and connected with TCD detectors. The GC-A with

Ar as the carrier gas was used to analyze the volume ratio of

H2/CO in the products, and the GC-B with H2 as the carrier gas

was used to analyze the components of CH4, CO, O2 and CO2 in

the products. The relative amount of the gases in the products

was calculated by the normalization method, and the data

obtained by these two GC were linked by CO amount. The

equations were shown as follows:

Conversion of CH4 ¼�Fin�CH4

� Fout�CH4

��Fin�CH4

� 100%

Selectivity of CO ¼ Fout�CO=�Fin�CH4

� Fout�CH4

�� 100%

Selectivity of H2 ¼ Fout�H2=2=

�Fin�CH4

� Fout�CH4

�� 100%

F� volume flow rate; mL=min

3. Results and discussion

3.1. Crystal structures and textural properties ofsupports and catalysts

The XRD pattern of Y2O3 support is shown in Fig. 1. It could be

seen that the Y2O3 prepared by the conventional precipitation

method was cubic Y2O3 [01-082-2415]. The result was consis-

tent with those reported by Lin et al. [22] and Wang et al. [38],

who found that cubic Y2O3 could be obtained by calcining

Y(OH)3 hydrogel at temperatures higher than 500 �C. The

crystalline phase of g-Al2O3 was also confirmed by XRD, and

n deposition.

Actual contentof Ni (wt %)c

TOS(h) Amount of carbondeposition (%)d

e e e

e e e

5.28 125 1.9

550 2.2

5.45 50 3.1

100 200 300 400 500 600

Inte

nsity

(a.u

.)

Temperature (oC)

Al2O3

Ni/Al2O3

Ni/Y2O3

Y2O3

Fig. 3 e NH3-TPD profiles of Y2O3, g-Al2O3 and relevant

supported Ni catalysts. Data of Al2O3 was cited from Ref.

[36].

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7e1 4 4 5 414450

the result was shown in our previous paper [36]. The textural

properties of Y2O3 and g-Al2O3, measured by N2 adsorption-

desorption isotherm method, are listed in Table 1. The

specific surface area of Y2O3 was 50.8 m2/g, lower than that of

g-Al2O3 (142.5 m2/g). However, both Y2O3 and g-Al2O3 exhibi-

ted IV-type isotherms with pore size distributions within the

range of 2 w 8 nm (Fig. 2).

After loading of NiO on Y2O3 (the actual content of Ni was

measured by XRF and listed in Table 1), the XRD pattern of

NiO/Y2O3 (before reduction) is shown in Fig. 1. It could be seen

that no peak attributed to NiO phase was observed, implying

that NiO particles were well dispersed on Y2O3. The phase of

cubic Y2O3 could be clearly observed in the XRD pattern of

NiO/Y2O3. The textural properties of Ni/Y2O3 and Ni/g-Al2O3

are listed in Table 1. Both catalysts exhibited IV-type

isotherms (Fig. 2). However, for both catalysts, the specific

surface areas decreased somewhat compared with the rele-

vant supports. The specific surface area of Ni/Y2O3 was

43.4 m2/g and that of Ni/g-Al2O3 was 116.9 m2/g. Besides, for

Ni/g-Al2O3, the average pore size didn’t change compared

with g-Al2O3, while for Ni/Y2O3, the pore size increased to the

range of 5 w 20 nm.

3.2. Acidic and basic properties of supports andcatalysts

The acidic properties of Y2O3, g-Al2O3 and the relevant sup-

ported Ni catalysts were measured by NH3-TPD, and the

profiles are shown in Fig. 3. For Y2O3 support, there was

a broad and weak NH3 desorption peak at the temperature of

125 w 300 �C, whereas for g-Al2O3 support, broad NH3

desorption peaks with high intensity were observed within

the temperature range of 125 w 500 �C. Besides, the area of

NH3 desorption peaks on g-Al2O3 was much larger than that

on Y2O3. That is to say, there weremore acidic sites on g-Al2O3

than on Y2O3. For NiO/g-Al2O3, the area and the intensity of

the desorption peaks of NH3 decreased a little compared with

g-Al2O3 support. For NiO/Y2O3, the area and the intensity of

0.0 0.2 0.4 0.6 0.8 1.0Relative pressure (P/P

0)

Volu

me

adso

rbed

, cm

3/g

-STP

Al2O3

Ni/Al2O3

Y2O3

Ni/Y2O3

Fig. 2 e Adsorption-desorption isotherms of Y2O3, g-Al2O3

and relevant supported Ni catalysts Data of Al2O3 was cited

from Ref. [36].

the desorption peak of NH3 changed little after the impreg-

nation of NiO on Y2O3.

The basic properties of Y2O3, g-Al2O3 and the relevant

supported Ni catalysts were measured by CO2-TPD, and the

profiles are shown in Fig. 4. It could be seen that, for Y2O3

support, there was a broad CO2 desorption peak with high

intensity at 110 w 550 �C, on the contrary, a small and weak

CO2 desorption peak was observed on g-Al2O3 support.

Nevertheless, the CO2 desorption peak for Y2O3 was centered

at about 250 �C while that for g-Al2O3 was centered at about

180 �C. What’s more, the area of CO2 desorption peak on Y2O3

was much larger than that on g-Al2O3. That is to say, in

comparison to g-Al2O3, there were more basic sites on Y2O3

with stronger basic strength. For NiO/Y2O3 and NiO/Al2O3

catalysts, the number of basic sites and the strength of

basicity changed little compared with the relevant supports.

However, in the present research, wide and broad CO2

desorption peaks were also observed at 550 w 800 �C on the

CO2-TPD profiles of Y2O3 and NiO/Y2O3. In order to investigate

100 200 300 400 500 600 700 800

Inte

nsity

(a.u

.)

Temperature (oC)

Y2O3

Ni/Y2O3

Al2O3

Ni/Al2O3

Fig. 4 e CO2-TPD profiles of Y2O3, g-Al2O3 and relevant

supported Ni catalysts.

100 200 300 400 500 600 700 800 900

Treated at 700oC before CO2 adsorption

Inte

nsity

(a.u

.)

Temperature (oC)

Treated at 500oC before CO2 adsorption

Fig. 5 e CO2-TPD profiles of Y2O3 at different pretreatment

conditions.

100 200 300 400 500 600 700 800 900 950 950 950

Inte

nsity

(a.u

.)

Temperature (oC)

950oC

Al2O3

Ni/Al2O3

Ni/Y2O3

Y2O3

Fig. 6 e TPR profiles of Ni/Y2O3 and Ni/g-Al2O3 Data of Al2O3

was cited from Ref. [36].

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 en e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7e1 4 4 5 4 14451

the attribution of CO2 desorption peak on Y2O3 and NiO/Y2O3

at 550 w 800 �C, a comparative CO2-TPD experiment was

carried out. In the comparative CO2-TPD experiment, Y2O3

was treated at 700 �C for 0.5 h in the flowing He under atmo-

spheric pressure before CO2 adsorption. The CO2-TPD profile

obtained is shown in Fig. 5. Obviously, the CO2 desorption

peak at 550 w 800 �C disappeared on the profile obtained by

pretreating Y2O3 at 700 �C before CO2 adsorption. This implied

that the CO2 desorption peak of Y2O3 at 550w 800 �C could not

be ascribed to the basic properties of Y2O3. The result was in

accordance with that of Sato et al.’s [37]. Sato et al. [37]

reported that, on the CO2-TPD profile of Y2O3, no CO2

desorption peaks could be observed at temperatures higher

than 500 �C. Then, what’s the reason for the desorption of CO2

on Y2O3 and NiO/Y2O3 at 550 w 800 �C in the present experi-

ment? Sato et al. [37] also reported that, CO2 desorbed at

temperatures above 500 �C could be attributed to the decom-

position of carbonate. In the present research, since Y2O3 and

NiO/Y2O3 were preserved in air before the CO2-TPD experi-

ment, Y2O2CO3might be formed on Y2O3 due to the adsorption

of CO2 in the air. Miao et al. [39] had proposed that Y2O2CO3

would decompose at about 640 �C. Then, it’s speculated that

the desorbed CO2 peak at 550w 800 �C on the CO2-TPD profiles

of Y2O3 and NiO/Y2O3 could be attributed to the decomposi-

tion of Y2O2CO3 on the surface of Y2O3 and NiO/Y2O3.

3.3. Reduction behaviors of supports and catalysts

The reduction behaviors of Y2O3, g-Al2O3 and the relevant

supported Ni catalysts (NiO/Y2O3 and NiO/g-Al2O3) were

measured by TPR, and the results are shown in Fig. 6. Clearly,

g-Al2O3 was irreducible under the conditions employed. As for

Y2O3, a weak reduction peak centered at about 600 �C was

observed. It has been reported that, Y2O3 could be partially

reduced due to the lattice oxygen on its surface [40]. For NiO/g-

Al2O3, two reduction peaks were observed at 400w 650 �C (the

peak was weak) and 850 �C, respectively. The peak at

400 w 650 �C was attributed to the reduction of NiO that

weakly interacted with g-Al2O3 and the peak at about 850 �Cwas ascribed to the reduction of NiAl2O4 [41]. For NiO/Y2O3,

three reduction peaks, centered at 280 �C, 400 �C and 500 �Crespectively, were observed. Generally speaking, Ni-based

catalysts would exhibit different reduction peaks, which

depend on the nature of the supports [42,43]. Bulk reduction

peaks of free NiO particles that don’t interact with supports

generally appear at about 400 �C, and the interactions between

NiO and supports decrease the tendency of Ni cation to be

reduced to metallic Ni [44]. Therefore, it could be speculated

that, the two reduction peaks on NiO/Y2O3 centered at 400 �Cand 500 �C could be attributed to the reduction of bulk NiO and

the reduction of NiO that weakly interacted with Y2O3. As for

the reduction peak onNiO/Y2O3 at about 280 �C in TPR, it could

be ascribed to the reduction of surface NiO particles promoted

by the surface oxygen vacancies of Y2O3 [45].

3.4. Catalytic performance of Ni/Y2O3

The catalytic performances of Ni/Y2O3 and Ni/g-Al2O3 in POM

are shown in Fig. 7. It could be seen that the conversion of CH4

on Ni/Y2O3 catalyst at the initial stage of reaction was about

82%,with CO selectivity andH2 selectivity higher than 90%. On

Ni/g-Al2O3, the initial conversion of CH4 was about 82%, too,

and CO selectivity and H2 selectivity were also higher than

90%. However, CH4 conversion on Ni/g-Al2O3 decreased from

82% to 79% after 50 h reaction in POM. By comparison, on

Ni/Y2O3 catalyst, CH4 conversion decreased little even after

125 h reaction in POM. Further prolonging the reaction time

over Ni/Y2O3 to 550 h, still no obvious deactivation occurred

(Fig. 8). That is to say, Ni/Y2O3 showed high stability in POM.

In order to further examine the stability of Ni/Y2O3 catalyst

over a wide range of reaction temperatures, the reaction

temperatures were varied periodically during POM. Firstly, the

initial reaction temperaturewas 700 �C and kept for 24 h. Then

it was increased to 800 �C and kept at 800 �C for 24 h, and then

decreased to 700 �C, 600 �C, 500 �C and 400 �C successively, and

at each temperature it was kept for 24 h. Finally it was raised

again to 700 �C and kept for 24 h. The results are shown in

Fig. 9. The data for the first 24 h were similar to those in Figs. 7

and 8, and the catalytic performance of Ni/Y2O3 catalyst

changed little at 700 �C after a series of reaction temperature

variations. This indicates that Ni/Y2O3 was rather stable in

0 20 40 60 80 100 12060

65

70

75

80

85

90

CH

4 con

vers

ion

(%)

Time on stream (h)

Ni/Y2O3

Ni/Al2O3

0 20 40 60 80 100 1200

20

40

60

80

100

0

20

40

60

80

100

Ni/Y2O3Ni/Al2O3

CO

sel

ectiv

ity (%

)

Time on stream (h)

H2 s

elec

tivity

(%)

Ni/Al2O3

Ni/Y2O3

Fig. 7 e Comparison of catalytic performance of Ni/Y2O3

and Ni/g-Al2O3 in POM Data of Ni/g-Al2O3 was cited from

Ref. [36].

0 20 40 60 80 100 120 140 16020

40

60

80

100

700 oC400 oC500 oC600 oC700 oC800 oC

Con

vers

ion

or s

elec

tivity

(%)

Time on stream (h)

700 oC

CH4 conversion

CO selectivity

H2 selectivity

Fig. 9 e Stability test of Ni/Y2O3 in POM (Reaction

temperature varied).

100 CO selectivity

i n t e rn a t i o n a l j o u r n a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7e1 4 4 5 414452

POMwithin the temperature range of 400 w 800 �C. Besides, itcould be seen that, higher catalytic activities were observed at

higher reaction temperatures. H2 selectivity was higher at

lower reaction temperatures (400 w 700 �C), while CO selec-

tivity was higher at higher reaction temperature (800 �C). The

0 100 200 300 400 50020

40

60

80

100

Con

vers

ion

or s

elec

tivity

(%)

Time on stream (h)

CH4 conversion

CO selectivity

H2 selectivity

Fig. 8 e Stability test of Ni/Y2O3 in POM (550 h TOS).

reasonwas probably that water-gas shift reactionwas favored

at low temperatures [46,47].

3.5. Effects of GHSV (gas hourly space velocity) on thecatalytic performance of Ni/Y2O3 catalyst

Fig. 10 shows the effects of GHSV (in the range of 9� 104 h�1 to

24 � 104 h�1) on the catalytic performance of Ni/Y2O3 in POM.

It could be discovered that, CH4 conversion increased slightly

with the increase of GHSV, whereas CO selectivity and H2

selectivity increased apparently with the increase of GHSV.

The results were similar with that reported byWang et al. [31].

POM is an exothermic reaction, that is, it could be influenced

by heat transfer [48]. The increase in GHSV could result in

more heat generation per unit time due to the release of

reaction heat, and meanwhile, it could also improve the heat

transfer in the catalyst bed. There was a balance between heat

generation and heat transportation out of the reaction system.

At higher GHSV, more heat energy released, which could lead

to an increase in reaction temperature and finally higher

8 10 12 14 16 18 20 22 2450

60

70

80

90

CH4 conversion

H2 selectivity

Con

vers

ion

or s

elec

tivity

(%)

GHSV (×104 h-1)

26

Fig. 10 e Effects of GHSV on the catalytic performance of Ni/

Y2O3 in POM Reaction conditions: 700 �C, atmospheric

pressure, CH4/O2 [ 2.

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 en e r g y 3 6 ( 2 0 1 1 ) 1 4 4 4 7e1 4 4 5 4 14453

conversion of methane [49]. The reason for the selectivities to

H2 and CO increase with GHSV was probably due to the

reduction of secondary reactions, which resulted in higher

selectivities to the target products, H2 and CO.

3.6. Crystalline structures of Ni/Y2O3 and anti-carbondeposition properties of Ni/Y2O3

The crystalline structures of Ni/Y2O3 catalyst after POM reac-

tion with different time on streams are shown in Fig. 1. It can

be seen that after 10 min reaction in POM, the diffraction

peaks attributed to the phases of Ni or NiO could not be

observed. This implies that Ni particles were still well

dispersed on the Ni/Y2O3 catalyst after the reduction and

10 min reaction in POM. However, after 125 h and 550 h

reaction in POM, the diffraction peaks of Ni phases were

detected on Ni/Y2O3, indicating Ni particles aggregated

somewhat during POM reactions.

From the XRD patterns in Fig. 1, no diffraction peak

attributed to graphic carbon was observed on the three used

Ni/Y2O3 catalysts after different reaction time in POM. This

means that no obvious carbon deposition occurred on these

catalysts. TGA was carried out to evaluate the amount of

carbon deposited on Ni/Y2O3 and Ni/g-Al2O3 after different

reaction time in POM, and the results are listed in Table 1. It

could be seen that the amount of carbon deposited on Ni/g-

Al2O3 was 3.1% after 50 h reaction in POM, whereas that on Ni/

Y2O3 was only 1.9% after 125 h reaction and 2.2% after 550 h

reaction in POM. The amount of carbon deposited on Ni/Y2O3

was much less than that on Ni/g-Al2O3 catalyst. Namely, Ni/

Y2O3 possessed stronger ability to resist carbon deposition.

Guo et al. [50] reported that the acidic-basic properties of

catalysts could affect the ability of Ni-based catalysts to resist

carbon deposition. The more basic of the catalysts, the more

chances for the reaction between deposited carbon and

adsorbed CO2, and the less chances for Boudouard reaction

(CO / CO2 þ C) [51,52]. Then it could be inferred that the

basicity of Y2O3 contributed to the less amount of carbon

deposited on it in POM reaction.

4. Conclusions

Y2O3, prepared by the conventional precipitationmethod, was

a basic support with more basic sites and almost no acidic

sites. After loading of Ni, the prepared Ni/Y2O3 catalyst

showed relatively high catalytic activity and high selectivities

to CO and H2 in POM reaction. Ni/Y2O3 was a stable and

potential catalyst in POM, and it showed strong capacity to

resist carbon deposition, after 550 h POM reaction, negligible

amount of carbon deposited.

Acknowledgement

We acknowledge the financial support of this work from

National Basic Research Program of China (973 Program,

2011CB201405), NSFC (21073104, 20921001) and Analytical

fund of Tsinghua University.

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