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Hydrogen production by catalytic partial oxidation of cokeoven gas in BaCo0.7Fe0.2Nb0.1O3Ld membranes with surfacemodification

Hongwei Cheng a,*, Xionggang Lu a, Dahai Hu a, Yuwen Zhang a, Weizhong Ding a,*,Hailei Zhao b

aShanghai Key Laboratory of Modern Metallurgy and Materials Processing, Shanghai University, Number 275 Mailbox, 149 Yanchang Road,

Shanghai 200072, People’s Republic of ChinabDepartment of Inorganic Nonmetallic Materials, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China

a r t i c l e i n f o

Article history:

Received 9 August 2010

Received in revised form

26 September 2010

Accepted 1 October 2010

Available online 20 October 2010

Keywords:

Hydrogen production

Oxygen-permeation membrane

Surface-coating

Coke oven gas

Partial oxidation

* Corresponding authors. Tel./fax: þ86 21 563E-mail addresses: hwcheng@188.com (H.

0360-3199/$ e see front matter ª 2010 Profedoi:10.1016/j.ijhydene.2010.10.002

a b s t r a c t

The perovskite-type oxygen separation membranes BaCo0.7Fe0.2Nb0.1O3�d (BCFN) combined

with Ce0.8Re0.2O2�d (Re ¼ Sm, Gd) surface modification layers was investigated for hydrogen

production by partial oxidation reforming of coke oven gas (COG). The Ce0.8Re0.2O2�d materials improve the oxygen permeation flux of the BCFN membrane by 8e31%

under the COG atmosphere at 875 �C. The high oxygen permeation flux achieved using the

Ce0.8Gd0.2O2�d surface-coating layer in this work is quite encouraging with a maximum

value reaching 21.9 ml min�1 cm�2 at 900 �C. Characterization of the membrane surfaces by

SEM and XRD after 100 h long life test show that the Ce0.8Gd0.2O2�d surface-coating layer on

the permeation side can dramatically withstand corrosion of the hash strong reductive

working conditions, which will be promising surface modification material in the catalytic

partial oxidation reforming of COG using oxygen-permeable ceramics.

ª 2010 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.

1. Introduction oxidation of CH4 in COG using mixed ionic and electronic

Both near- and long-term hydrogen production options are

being explored urgently tomeet the needs of the development

of proton-exchange membrane fuel cells as the power source

for transportation, industrial, and residential needs [1e4].

Coke oven gas (COG), a by-product generated in the process of

producing coke from coal at a high temperature of 800 �C, hasattracted constantly increasing attention as one of the most

promising sources of hydrogen production [5,6]. By reforming

and the water gas shift reaction, the amount of hydrogen

produced from COG will be several times more than that of

original hydrogen in the COG. Among them, catalytic partial

3 8244.Cheng), wzhding@shu.edssor T. Nejat Veziroglu. P

conductingmembrane is regarded as an efficient and low-cost

method to produce hydrogen [7e9]. The oxygen-permeable

ceramics technology enables the combination of oxygen

separated from air and partial oxidation of hydrocarbons in a

single unit. This technology has been gaining considerable

research interest due to its advantages compared to the

conventional process, such as continuous production of

oxygen with infinite permeation selectivity, energy conser-

vation and its effective reduction in cost for both plant

construction and production.

Currently, one of the critical targets for the development of

membrane reactor is improving its oxygen-permeation rate

u.cn (W. Ding).ublished by Elsevier Ltd. All rights reserved.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 2 8e5 3 8 529

and structural stability under reductive environment [10e12].

It is well known that the oxygen transport in a membrane is

limited mainly by oxygen-ion diffusion in the membrane bulk

and oxygen exchange on the surfaces of the membrane.

Generally, when there is an oxygen chemical potential

gradient across a membrane and the oxygen permeation

through the membrane is controlled only by bulk diffusion,

the oxygen permeation flux ðjO2 Þ obeys the Wagner equation:

jO2¼ RTsesi

16F2Lðse þ siÞln�PO2 ;high

PO2 ;low

�(1)

where R is the gas constant, T is the temperature, se is elec-

tronic conductivity, si is oxide ion conductivity, F is the

Faraday constant, L is the membrane thickness, and PO2 ;high

and PO2 ;low are the oxygen partial pressures at the higher

(oxygen feed side) and lower sides (sweep side), respectively

[13]. Therefore, the oxygen permeation flux can be signifi-

cantly improved by decreasing themembrane thickness when

the oxygen permeation is mostly limited by bulk diffusion,

or by coating the membrane with a catalytic layer to enhance

oxygen surface exchanges when the oxygen permeation is

mostly limited by the oxygen surface-exchange kinetics.

During the past several years, extensive efforts have

been made on reporting applying porous catalytic layers on

either side or both sides of the membrane to enhance the

oxygen flux. This enhancement was caused by the modifica-

tion of oxygen potential drops at gasemembrane interfaces

or the potential slope in the membrane due to the improve-

ment of oxygen exchange rates by porous catalytic

layers [14e16]. Wang et al. [17] reported that the oxygen

permeation flux of the Ba0.5Sr0.5Co0.8Fe0.2O3�d membranes

coated by a GdBaCo2O5þd layer showed significant enhance-

ment. Zhu et al. [18] presented a new cobalt-free

BaCe0.15Fe0.85O3�d membrane and found that

Ba0.5Sr0.5Co0.8Fe0.2O3�d porous layer coated on the membrane

surfaces could effectively accelerate the surface oxygen

exchange rate, thus greatly improve the oxygen permeability

of the membrane. Chen et al. [19] proposed a new configura-

tion of Ba0.5Sr0.5Co0.8Fe0.2O3�d membrane with layered

Ba0.5Sr0.5Co0.2Fe0.8O3�d as the thin-film and showed that the

oxygen permeation flux was about 3.5 times higher than that

of the Ba0.5Sr0.5Co0.2Fe0.8O3�d membrane under the same

conditions. Kusaba et al. [20] investigated the oxygen

permeation phenomena of the La0.1Sr0.9Co0.9Fe0.1O3�d dense

disks with different surface areas and the results indicated

that increase in surface area at the sweep side was more

effective to increase the oxygen permeation flux than that at

the oxygen feed side. Furthermore, the increase of the oxygen

surface-exchange rate may be enhanced also by depositing

a catalytically active substance (e.g. Pt, Ag or Pr oxide) onto

the surface of the membrane, which may be porous [21,22].

Recently, our research group has applied disk-type

BaCo0.7Fe0.2Nb0.1O3�d (BCFN)membrane by solid state reaction

to the partial oxidation of COG for hydrogen production, and

have found that catalyst has great influence on the oxygen

permeation flux and reaction effluent gas composition

[7e9,23e25]. It was reported that the use of a hydrotalcite

precursor Ni/Mg(Al)O catalyst could form highly dispersed

metal particles on the surface of the catalyst and substantially

reduce coke formation [23]. The addition of small quantities

of rare earth elements, which have basic properties, can

change the hydrotalcite characteristic after thermal treat-

ment by increasing the basicity of hydrotalcite [24]. For the

practical applications of the membrane, it must possess

sufficiently high oxygen permeability and sustainable struc-

tural stability to withstand the intense reductive atmosphere

at high temperature. This work continues previous research

and focuses on mixed-conducting membranes and their

applications. We hope to develop a kind of porous surface-

coating layer on the membrane, which can be used for partial

oxidation of COG possessing high oxygen permeability and

reducing the corrosion on the surface of the membrane

under the harsh working conditions such as syngas, carbon

dioxide and water vapor. The doped or pure ceria have been

thoroughly studied and proven to be the mixed ionic/elec-

tronic conductors [26,27]. In addition, the ceria-based mate-

rials have been receiving a great deal of attention due to their

higher ionic conductivity than that of the stabilized zirconia

and their better stability at reductive atmosphere than that of

the perovskite-related oxides [28e30]. In view of this, themain

objective of the present work was to estimate the effect of

surface modification of dense BCFN membranes using

Ce0.8Re0.2O2�d (Re ¼ Sm, Gd) on the oxygen permeation flux

under the condition of COG. The influence of reaction

temperature on the reaction effluent gas composition and the

structural stability of the membrane reactors was also

examined.

2. Experiment

2.1. Materials preparation

The BaCo0.7Fe0.2Nb0.1O3�d (BCFN) membrane was obtained

using the samemethod as that described in our previous work

[7e9]. The pure or doped ceria Ce0.8Re0.2O2�d (Re ¼ Sm, Gd)

coating materials were prepared by using co-precipitation of

the Ce3þ and Sm3þ or Gd3þ nitrates at pH 12.0 controlled by

adding an aqueous solution containing NH3$H2O. The

precipitate was filtered, dried and calcined in air at 650 �C for

2 h. For simplicity, the CeO2, Ce0.8Sm0.2O2�d and Ce0.8Gd0.2O2�d

samples will hereafter be denoted as C, CS and CG, respec-

tively. The Ce0.8Re0.2O2�d-coated BCFN membranes (named

BCFNeC, BCFNeCS and BCFNeCG) were prepared with spin-

coatingmethod [31]. The coating paste was amixture of 9 wt%

of Ce0.8Re0.2O2�d powder, 90 wt% of terpineol, 0.2 wt% of

carbon fiber, 0.3 wt% of glycol and 0.5 wt% of ethyl cellulose.

The Ce0.8Re0.2O2�d layer was coated on the permeation side of

the BCFN membrane. Post-heat treatment was conducted to

remove the organic additives at 950 �C for 2 hwith heating and

cooling rate 2 �C min�1.

TheNi/Ce0.75Zr0.25O2/Mg3(Al)O catalyst was prepared by co-

precipitation and impregnation methods. The MgeAl mixed

oxide was prepared by using hydrotalcite as the precursor

with the atomic ratio of Mg/Al ¼ 3/1. MgeAl hydrotalcite

precursor was prepared by co-precipitation of the nitrates of

Mg2þ and Al3þ at PH 10.0 controlled by addition of an aqueous

solution containing NaOH and Na2CO3, followed by aging for

12 h at 65 �C. The precipitate was washed with de-ionized

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 ) 5 2 8e5 3 8530

water and dried in air at 110 �C overnight. The precursor

was calcined by increasing the temperature from ambient

temperature to 500 �C at a rate of 5 �Cmin�1 and kept at 500 �Cfor 15 h, followed by calcination at 800 �C for 5 h to give

a white powder, Mg3(Al)O mixed oxide. Subsequently, the

powder of Mg3(Al)O was dipped in an aqueous solution of Ce

(NO3)3$6H2O and Zr(NO3)4$5H2O for overnight at room

temperature, followed by drying in air at 110 �C and calcined

at 800 �C for 5 h to given Ce0.75Zr0.25O2/Mg3(Al)O sample.

Finally, the Ni/Ce0.75Zr0.25O2/Mg3(Al)O catalyst was prepared

by impregnation of Ce0.75Zr0.25O2/Mg3(Al)O with an aqueous

solution of Ni(NO3)2$6H2O for overnight at room temperature,

followed by drying in air at 110 �C and calcined at 850 �Cfor 5 h. Themass fraction of Ce0.75Zr0.25O2 in the Ce0.75Zr0.25O2/

Mg3(Al)O sample was 10 wt%. The Ni mass fraction in the Ni/

Ce0.75Zr0.25O2/Mg3(Al)O catalyst also was 10 wt%.

2.2. Sample characterization

X-ray diffraction (XRD, Rigaku D/Max-2550) was used to

characterize the phase evolution of the coating materials and

membranes.

The changes of the morphology of the coating materials

and membranes were observed using a scanning electron

microscope (SEM, JEOL JSM-6700F). The samples were broken

into fragments for the cross-section SEM images. The

compositions of the membranes before and after the reaction

were determined using an energy-dispersive X-ray spectro-

scope (EDXS, OXFORD INCA).

2.3. Experimental setup

The schematic diagram of the partial oxidation of COG in

BCFN membrane reactor combined with a surface-coating

layer and the gas flowchart was presented in Fig. 1. The gas

flow rates were controlled by mass flow controllers. One side

of the membrane was exposed to compressed air and the

other side to He or COG (59.89%H2, 30.09%CH4, 7.04%CO

Fig. 1 e Schematic diagram of the membran

and 2.98%CO2). On the permeation side of the membrane, H2,

CH4, CO and CO2 in the outlet gas, in which the water

was removed by Mg(ClO4)2, were analyzed by a Varian CP 3800

gas chromatography (GC) with a thermal conductivity

detector (TCD).

Prior to the start of a test, the discoidmembranewas sealed

into the reactor with a silver seal. One side of the membrane

was exposed to compressed air (He) and the other side to

He (air). Gas tightness of membrane was ensured by moni-

toring nitrogen and no nitrogen leakage in He. The effective

inner surface area and the thickness of the membrane disc

were controlled around 1.3 cm2 and 1.0 mm, respectively.

A total of 1.0 g of 20e40 mesh the catalyst was directly placed

on the membrane. A K-type thermocouple was placed at the

center of the bed to monitor the reaction temperature. The

oxygen permeability of the discoid sample was determined

from the content of CO and CO2 in the reacted gas, and the

amount of H2O was evaluated from the balance of hydrogen

before and after the reaction. The flow rate of outlet gas was

measured by a soap-membrane flow meter. The conversion

of CH4, the selectivity of H2, CO, CO2 and H2O were defined

as follows:

CH4 conversion; % ¼ FinCH4

� FoutCH4

FinCH4

� 100; H2 selectivity;

% ¼ FoutH2

� FinH2

2�FinCH4

� FoutCH4

�� 100;

CO selectivity; % ¼ FoutCO � Fin

CO

FinCH4

� FoutCH4

� 100; CO2 selectivity;

% ¼ FoutCO2

� FinCO2

FinCH4

� FoutCH4

� 100;

H2O selectivity; % ¼ 2FinCH4

þ FinH2

� FoutH2

� 2FoutCH4

2�FinCH4

� FoutCH4

� � 100

where Fini and Fout

i were the flow rates of the inlet and outlet

gas i, respectively.

e reactor system and gas flow direction.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 2 8e5 3 8 531

3. Results and discussion

3.1. Characterization of materials

The XRD patterns of the Ce0.8Re0.2O2�d calcined at 650 and

950 �C are presented in Fig. 2(a). The results showed that

all doped ceria samples crystallized to a single phase of

cubic fluoride structure, the same as the original substance

of pure ceria. Fig. 2(b) shows the X-ray diffraction patterns of

Ce0.8Re0.2O2�d-coated BCFNmembranes. The results indicated

that no other phase than cubic perovskite and fluoride ceria

was found in the X-ray diffraction patterns of the samples.

Fig. 3 shows SEM micrographs of the membranes and

coating material powder. For the fresh BCFN membrane, the

ceramic grains with clear boundaries were visible as shown in

Fig. 3(a). The Ce0.8Gd0.2O2�d powder consists of isometric

particles with a rather narrow particle size distribution

ranging from 80 to 120 nm (Fig. 3(b)). As seen in Fig. 3(c),

Ce0.8Gd0.2O2�d layer was porous due to the volatilization of

organic solution during the post-heat treatment. This porosity

Fig. 2 e XRD patterns of the samples: (a) the Ce0.8Re0.2O2Ld

calcined at 650 or 950 �C for 2 h; (b) the BCFN membrane,

pure CeO2 and the Ce0.8Re0.2O2Ld-coated BCFN membranes

calcined at 950 �C for 2 h.

would provide adequate diffusion pathway near the surface

and increase the effective surface area of Ce0.8Gd0.2O2�d layer.

From the cross section of Ce0.8Gd0.2O2�d-coated BCFN

membrane (Fig. 3(d)), we can distinguish the porous

Ce0.8Gd0.2O2�d-coating layer, which had approximately

10 mm thickness, from the dense BCFN membrane. All the

other synthesized powders and Ce0.8Re0.2O2�d-coated BCFN

membranes showed similar particle size distribution and

morphology.

3.2. Oxygen permeation of the membranes

Fig. 4(a) presents the influence of different membrane thick-

nesses on the oxygen permeation flux through BCFN

membranes under the He atmosphere at 725e900 �C. It was

found that the oxygen permeation flux increased rapidly with

decreasing thickness of the membrane. Simultaneously,

when the reaction temperature increased from 725 to 900 �C,the change of the experimental condition resulting from

the oxygen permeation flux of the membrane with 0.6 mm

thickness enhanced gradually from 1.2 to 2.8 ml min�1 cm�2,

which is a high value under the analogous experimental

conditions among all the plate membrane reported in Refs.

[32e34].

In order to identify the rate-limiting step of oxygen trans-

port through the membrane, we also studied the oxygen

permeation of the membranes with 1.0 mm thickness and

different surface-coating layers on the sweep side, as

shown in Fig. 4(b). It can be seen that the addition of porous

catalytic materials Ce0.8Re0.2O2�d on the membrane surface

can sharply improve the oxygen permeation flux. The oxygen

permeation flux through uncoated BCFN membrane was

1.1 ml min�1 cm�2 at 725 �C, whereas the value through the

membrane coated using Ce0.8Gd0.2O2�dwas 1.3mlmin�1 cm�2,

whichwas 18%higher than that of uncoated BCFNmembrane.

Thus, it is reasonable to consider that the rate-determining

steps of the oxygen permeation in the BCFN membrane are

both the oxygen exchange on the surface and the bulk

diffusion.

3.3. Partial oxidation of COG in the membrane reactors

To obtain a detailed performance of the membrane reactor,

the catalytic partial oxidation reaction of COG was studied in

a disk-shape BCFN membrane reactor coated with a porous

layer and packed with Ni/Ce0.75Zr0.25O2/Mg3(Al)O catalyst. In

order to clarify the performance of the Ce0.8Re0.2O2�d layer to

partial oxidation of CH4 in COG, a blind experiment was also

carried out in a BCFN membrane reactor without coating.

3.3.1. Effect of coatingsThe influence of various surface-coating layers on the

performance of the membrane reactor at 875 �C is shown in

Fig. 5. In Fig. 5(a), it can be clearly seen that the BCFN

membrane showed significant enhancement in both oxygen

permeation flux and CH4 conversion during the entire reac-

tion process by coating Ce0.8Re0.2O2�d layer on the permeation

side. The average oxygen permeable flux through uncoated

BCFNmembrane was 15.6 ml min�1 cm�2, whereas the values

through the membranes coated using CeO2, Ce0.8Sm0.2O2�d

Fig. 3 e SEM images of the samples: (a) surface of the BCFN membrane; (b) Ce0.8Gd0.2O2Ld calcined at 650 �C for 2 h; (c) the

surface and (d) cross section of the fresh Ce0.8Gd0.2O2Ld-coated BCFN membrane calcined at 950 �C for 2 h.

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 ) 5 2 8e5 3 8532

and Ce0.8Gd0.2O2�d increased to 16.8, 18.4 and

20.4 ml min�1 cm�2, respectively, i.e. 8%, 18% and 31%

higher than that of uncoated BCFN membrane. In the mean-

time, the corresponding conversion of CH4 increased from

96.9 to 97.9, 98.3 and 99.4%, respectively. Wang et al. [17]

proposed that a coating layer might change the “effective”

oxygen pressure imposed on the membrane surface. There-

fore, we concluded that the increase of the oxygen fluxes in

the samples should be the fast desorption ability of the

coating layer can lower the “effective” oxygen pressure on the

permeation side of the membranes as well as the increase of

the effective surface area.

Furthermore, the changes of oxygen permeation flux and

reactant conversion will affect the products yield. The influ-

ence of Ce0.8Re0.2O2�d surface-coating layers on the selectivity

of H2 and CO is shown in Fig. 5(b). In the BCFN membrane

reactor, the average selectivity of H2 and CO were 81.9 and

97.1%. However, the average selectivity of H2 and CO lowered

to 78.2 and 93.6%, 72.1 and 93.9%, 65.7 and 91.4% in the

BCFNeC, BCFNeCS and BCFNeCG membrane reactors,

respectively. This means that increasing the oxygen desorp-

tion area by porous surface-coating layer resulted in an

increase in the oxygen permeability flux, and the increment

of oxygen supplied by the coated membrane was not only

reacted with CH4, but also with H2 and CO, which is causing

the enhancement of CO2 and H2O yields, as shown in Fig. 5(c).

3.3.2. Effect of reaction temperatureFig. 6(a) shows the influence of the reaction temperature on

the reforming of the COG in BCFNeCG membrane reactor. It

can be seen that the performance of the membrane reactor

was greatly affected by the operating temperature. For a dense

ceramic membrane, at constant COG and air flow rates, the

oxygen permeation flux can be changed as a function of

temperature, and this change will effect the methane

conversion and product selectivity. Furthermore, the varia-

tion in the product compositions will have influence on the

oxygen partial pressure at the permeation side, which will

alter the oxygen permeation flux in turn. This is a self-

consistent process between the oxygen permeation and

the catalytic partial oxidation reaction [7,23,35,36]. As the

temperature increased from 750 to 900 �C, the changes of

the reforming reactions resulting from the oxygen permeation

flux increased from 10.4 to 21.9 ml min�1 cm�2 and the CH4

conversion increased from 62.0 to 99.5%, while the selectivity

of CO decreased gradually from 102.6 to 89.1%. According

to the definition of CO selectivity, it exceeded 100% from 750

to 800 �C due to partial conversion of the original CO2 in COG to

CO through dry reforming (CH4 þ CO2 ¼ 2CO þ 2H2). However,

the effect of reaction temperature on the selectivity of H2

and H2O was diversified. When the reaction temperature

increased from 750 to 800 �C, the selectivity of H2 increased

from 76.2 to 81.0%, while the selectivity of H2O dropped from

23.8 to 18.9%, whereas the selectivity of H2 decreased and the

selectivity of H2O rose with the elevated temperature higher

than 800 �C.The variations in the components of COG before and after

reforming in the BCFNeCG membrane reactor at different

temperatures are shown in Fig. 6(b). It can be seen that at

different reaction temperatures, the average yields of H2 and

CO were 88.3e104.2 and 26.2e35.3 ml min�1, respectively. It is

also clearly observed that the amount of CH4 after reforming

Fig. 4 e Effect of reaction temperature on oxygen

permeation fluxes through (a) the BCFN membranes with

different thicknesses and (b) Ce0.8Re0.2O2Ld-coated BCFN

membranes with 1.0 mm thickness. Reaction conditions:

He flow rate, 100 ml minL1; air flow rate, 300 ml minL1.

Fig. 5 e Effect of the coatings in the BCFN membrane

reactors on (a) the CH4 conversion and oxygen permeation

flux; (b) and (c) products selectivity of the reforming

reaction. Reaction conditions: COG flow rate, 100 ml minL1;

air flow rate, 300 ml minL1; membrane thickness, 1.0 mm;

temperature, 875 �C.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 2 8e5 3 8 533

decreased gradually and the amplification of the original

hydrogen in COG was 1.5e1.8 times. Furthermore, the

amounts of H2 and CO after reforming increased in the

beginning, but decreased later, while the amounts of CO2 and

H2O behaved in a contrary trend with increasing reaction

temperature.

3.3.3. Membrane stabilityFor practical applications, the technical innovation on the

catalytic partial oxidation of hydrocarbon compounds by

using an oxygen-permeable ceramic membrane reactor is

strongly dependent upon the development of the mixed con-

ducting membrane material having the potential for high

oxygen permeability and structural stability under reductive

atmosphere. Recently, Makoto et al. [37] reported that the

BCFN membrane reactor presented long-term stable opera-

tion performance more than 300 h for partial oxidation of

methane. However, compared with methane, the major

component of COG was hydrogen, the oxygen-permeable

membrane exposed to COG was easier to be destroyed [7,23].

Therefore, in order to understand the influence caused by the

surface-coating layer on the structural stability of the

membrane under COG atmosphere, a long-term test was

performed in BCFN and BCFNeCG membrane reactors packed

with Ni/Ce0.75Zr0.25O2/Mg3(Al)O catalyst at 850 �C.From Fig. 7(a), it can be seen that the oxygen permeation

flux and CH4 conversion kept constant during a 100 h run in

both BCFN and BCFNeCG membrane reactors. In the BCFN

membrane, the average oxygen permeation flux and CH4

Fig. 7 e 100 h stability of the BCFN and BCFNeCG

membrane reactors for the partial oxidation of methane in

COG: (a) the CH4 conversion and oxygen permeation flux;

(b) and (c) products selectivity of the reforming reaction.

Reaction conditions: COG flow rate, 100 ml minL1; air flow

rate, 300 ml minL1; membrane thickness, 1.0 mm;

temperature, 850 �C.

Fig. 6 e Effect of the reaction temperature on (a) the oxygen

permeation flux, conversion and selectivity of the

reforming reaction; and (b) variation of components in COG

before and after reforming. Reaction conditions: COG flow

rate, 100 ml minL1; air flow rate, 300 ml minL1; BCFNeCG

membrane thickness, 1.0 mm.

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 ) 5 2 8e5 3 8534

conversion were 14.2 ml min�1 cm�2 and 95.5%, while the

corresponding average values increased to 18.7mlmin�1 cm�2

and 98.9% in BCFNeCG membrane, respectively. In our

previous work, the performance of BCFN membrane reactor

packed with NiO/MgO catalyst showed a slightly decrease,

and the CH4 conversion decreased from 95 to 90% over 100 h

operation [8]. This difference may indicate that the Ni/

Ce0.75Zr0.25O2/Mg3(Al)O catalyst derived from hydrotalcite

precursor showed satisfactory catalytic activity and good

resistance to carbon formation [24]. Furthermore, the selec-

tivity of the products also kept invariant during the reaction

period (Fig. 7(b) and (c)), which suggested that the BCFN and

BCFNeCG membrane reactors combined with the catalyst

presented excellent stability.

3.4. Characterization of the used membrane

The used BCFN and BCFNeCGmembranes after long-term test

were characterized by SEM, EDXS and XRD. For the surface

and cross section of the fresh membrane, ceramic grains with

clear boundaries were visible as shown in Figs. 3(a) and 8(f),

respectively. But, the morphology of the used membrane

surface exposed to the reforming gas changed after reaction.

From Fig. 8(a) and (c), it can be seen that the permeation side of

the BCFNmembrane surfacewas destroyed. The cross-section

image of themembrane near the permeation side showed that

the damaged porous layer was up to 80 mm in depth. However,

under the same reaction condition, the SEM image of the used

BCFNeCG membrane surface exposed to the reforming gas

Fig. 8 e The SEM images of the BCFN and BCFNeCGmembranes: (a) and (b) are cross sections near the permeation side of the

used BCFN and BCFNeCG membranes, respectively; (c) and (d) are the surface of the used BCFN and BCFNeCG membranes

exposed to COG, respectively; (e) is the surface of the used BCFNeCG membrane with partly removed the coating; (f) is cross

sections of the fresh BCFN membrane. The white broken line frames in the images show the areas of the EDXS analysis.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 2 8e5 3 8 535

had little change (Fig. 8(b)), the morphology of the cross

sectionwas compact and different from that of themembrane

before reaction (Fig. 3(d)). It is noteworthy that the BCFNeGC

membrane away from the reaction side was kept at the same

morphology and structure as the fresh membrane (Fig. 8(b)

and (e)). The characterization results demonstrated that the

Ce0.8Gd0.2O2�d surface-coating layer on the reaction side could

dramatically reduce the corrosion of the COG atmosphere to

the membrane surface.

Table 1 shows the EDXS results of the tested BCFN and

BCFNeCG membranes. The compositions of the cross section

of the fresh BCFN membrane was about Ba/Co/Fe/

Nb ¼ 52.1:33.9:9.9:4.1. Segregation of the constituent metals

was not observed on the cross section of a fresh membrane.

After the long-term experiment, the content of Co increased

on the permeation side of the BCFN membrane, which was

similar to the results of our previous work [23]. The elemental

compositions of positions 2, 3 and 8 in Table 1 were similar to

those of the fresh membrane. Especially, the bulk of the

BCFNeCG membranes tested during 100 h reaction period

showed the approximate composition as the freshmembrane.

This revealed that the decomposition of the BCFN and

BCFNeCG materials occur on the membrane surface in depth

of only several or 10 mm. In the region of the Ce enrichment on

both cross section and surface of membrane (positions 4, 6

and 7 in Fig. 8), CeO2 was the main phase detected by XRD

(Fig. 9(b)).

The XRD patterns of the fresh and used membranes were

shown in Fig. 9. In comparison to the fresh membrane, some

new phases, such as CoO and/or Co formed on the reaction

side of the used BCFN and BCFNeCG membrane surfaces,

while only BaCO3 formed on the air side of the membranes.

However, a typical cubic perovskite structure was still kept in

the middle section of the BCFNeCG membrane (Fig. 9(e)). This

Table 1 e EDX results of the fresh, used BCFN and BCFNeCG membranes.

Membrane Position (No.) Ba (mol%) Co (mol%) Fe (mol%) Nb (mol%) Ce (mol%)

Used BCFN Permeation cross section (1) 54.69 42.88 2.43 0 0

Permeation cross section (2) 51.98 37.81 8.93 1.28 0

Permeation surface (5) 55.95 40.79 3.26 0 0

Used BCFN-CG Permeation cross section (3) 50.82 36.57 9.12 3.49 0

Permeation cross section (4) 24.46 16.97 0 0 58.57

Permeation surface (6) 30.51 14.72 3.52 0 51.25

Permeation surface (7) 27.31 16.74 2.87 0 53.08

Bulk (8) 51.11 35.83 9.14 3.92 0

Fresh BCFN Cross section (9) 52.12 33.91 9.85 4.12 0

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 ) 5 2 8e5 3 8536

might be one reason why the membrane disk could operate

steadily for a long time. In our experiments, the small

changes of the microstructure and phase composition near

the BCFN membrane surfaces did not result in a sharp

reduction of performance. For a steady operation with more

than 100 h, the BCFNeCG membrane reactor was suitable

for the partial oxidation of methane in COG atmosphere.

It is well known that the reduction of the mixed-conduct-

ing membrane under syngas atmosphere is inevitable. In

fact, the increase of the oxygen permeation flux might

restrain the destruction of the membrane to a deeper extent.

The destruction process progresses into the membrane up to

a certain depth (d ), corresponding to the lowest value for

the oxygen activity ðaO2 Þ, where the oxide is thermodynami-

cally stable [38,39]. Fig. 10 shows the schematic illustration of

the profile in oxygen chemical potential mO2across the

membrane at a steady-state condition. For a membrane, the

lowest value for the oxygen activity must be constant. When

the oxygen permeation flux through the membrane is larger

ðJ0O2> J00O2

Þ and the oxygen chemical potential across the

Fig. 9 e XRD patterns of the fresh, used BCFN and BCFNeCG

membranes: (a) and (b) are permeation side of the used

BCFN and BCFNeCG membranes, respectively; (c) and (d)

are air side of the used BCFN and BCFNeCG membranes,

respectively; (e) bulk of the used BCFNeCG membrane and

(f) fresh BCFN membrane.

bulk of the membrane is higher ðm0O2

> m00O2Þ, the destruction

process progresses into the membrane up to a certain depth

is lower, i.e. d1 < d2 as shown in Fig. 10. For instance, in our

previous study, the structure changes run deeper into the

bulk of the BCFN membrane up to 200 mm after just 45 h

reaction period when the Ni/La2O3�gAl2O3 catalyst was used

and the maximum of the oxygen permeation flux was

only 7.5 ml min�1 cm�2 [7]. Since the unwanted chemical

reductions would be a main cause for the instability of

the reactive membrane materials, increasing oxygen flux

through the membrane could restrain the chemical reduc-

tions and thus promote the chemical stability of membrane.

Therefore, in our present work, both the Ce0.8Re0.2O2�d mate-

rials and the increased oxygen permeation flux are beneficial

to promoting the stability of the membrane.

Fig. 10 e Schematic illustration of the profile in oxygen

chemical potential mO2across the membrane at a steady-

state condition.

i n t e r n a t i o n a l j o u rn a l o f h y d r o g e n en e r g y 3 6 ( 2 0 1 1 ) 5 2 8e5 3 8 537

4. Conclusions

Hydrogen amplification of COG by partial oxidation of

methanewas investigated by using a BCFNmembrane reactor

combined with Ce0.8Re0.2O2�d surface-coating layers. The

oxygen permeation fluxes for the BCFN membranes surface-

modified by the layers on the permeation side were 8e31%

higher than that for uncoated BCFNmembrane under the COG

atmosphere at 875 �C. The maximum oxygen permeation

flux reached 21.9 ml min�1 cm�2 in the Ce0.8Gd0.2O2�d-coated

BCFNmembrane reactor at 900 �C, which is a high value under

the analogous experimental conditions among all the plate

membrane reported in the literature. At optimized reaction

conditions, 99.4% CH4 conversion, 65.7% H2 selectivity, 91.4%

CO selectivity have been achieved at 875 �C. Characterizationof the membrane surfaces by SEM and XRD after 100 h long

life test showed that the Ce0.8Gd0.2O2�d surface-coating layer

on the permeation side can dramatically withstand the

corrosion of COG, which showed a potential application for

hydrogen production fromCOG by partial oxidation reforming

of methane in the coated BCFN membrane reactor.

Acknowledgments

The financial supports received from the National High

Technology Research and Development Program of People’s

Republic of China (No. 2006AA11A189), the Science and

Technology Commission of Shanghai Municipality (No.

0952NM01400), the National Natural Science Foundation of

People’s Republic of China (No. 51004069) and Shanghai

Postdoctoral Science Foundation (No. 10R21412700) are greatly

appreciated.

r e f e r e n c e s

[1] Weinert JX, Liu SJ, Ogden JM, Ma JX. Hydrogen refuelingstation costs in Shanghai. Int J Hydrogen Energy 2007;32:4089e100.

[2] Cheekatamarla PK, Finnerty CM, Cai J. Internal reformingof hydrocarbon fuels in tubular solid oxide fuel cells. Int JHydrogen Energy 2008;33:1853e8.

[3] Xu JK, Zhou W, Li ZJ, Wang JH, Ma JX. Biogas reforming forhydrogen production over nickel and cobalt bimetalliccatalysts. Int J Hydrogen Energy 2009;34:6646e54.

[4] Kim HW, Kang KM, Kwak HY. Preparation of supported Nicatalysts with a core/shell structure and their catalytic testsof partial oxidation of methane. Int J Hydrogen Energy 2009;34:3351e9.

[5] Joseck F, Wang M, Wu Y. Potential energy and greenhousegas emission effects of hydrogen production from coke ovengas in U.S. steel mills. Int J Hydrogen Energy 2008;33:1445e54.

[6] Shen J, Wang ZZ, Yang HW, Yao RS. A new technology forproducing hydrogen and adjustable ratio syngas from cokeoven gas. Energy Fuels 2007;21:3588e92.

[7] Zhang YW, Li Q, Shen PJ, Liu Y, Yang ZB, Ding WZ, et al.Hydrogen amplification of coke oven gas by reforming ofmethane in a ceramic membrane reactor. Int J HydrogenEnergy 2008;33:3311e9.

[8] Yang ZB, Ding WZ, Zhang YY, Lu XG, Zhang YW, Shen PJ.Catalytic partial oxidation of coke oven gas to syngas in anoxygen permeation membrane reactor combined with NiO/MgO catalyst. Int J Hydrogen Energy 2010;35:6239e47.

[9] Cheng YF, Zhao HL, Teng DQ, Li FS, Lu XG, Ding WZ.Investigation of Ba fully occupiedA-site BaCo0.7Fe0.3�xNbxO3�d

perovskite stabilized by low concentration of Nb for oxygenpermeation membrane. J Memb Sci 2008;322:484e90.

[10] Diethelm S, Van herle J, Middleton PH, Favrat D. Oxygenpermeation and stability of La0.4Ca0.6Fe1�xCoxO3�d (x ¼ 0, 0.25, 0.5) membranes. J Power Sources 2003;118:270e5.

[11] Park CY, Lee TH, Dorris SE, Balachandran U. Hydrogenproduction from fossil and renewable sources using anoxygen transport membrane. Int J Hydrogen Energy 2010;35:4103e10.

[12] Waindich A, Mobius A, Muller M. Corrosion ofBa1�xSrxCo1�yFeyO3�d and La0.3Ba0.7Co0.2Fe0.8O3�d materialsfor oxygen separating membranes under Oxycoal conditions.J Memb Sci 2009;337:182e7.

[13] Watanabe K, Takauchi D, Yuasa M, Kida T, Shimanoe K,Teraoka Y, et al. Oxygen permeation properties of Co-freeperovskite-type oxide membranes based on BaFe1�yZryO3�d.J Electrochem Soc 2009;156:E81e5.

[14] Teraoka Y, Honbe Y, Ishii J, Furukawa H, Moriguchi I.Catalytic effects in oxygen permeation through mixed-conductive LSCF perovskite membranes. Solid State Ionics2002;152e153:681e7.

[15] Lee TH, Yang YL, Jacobson AJ, Abeles B, Milner S. Oxygenpermeation in SrCo0.8Fe0.2O3�d membranes with porouselectrodes. Solid State Ionics 1997;100:87e94.

[16] Watanabe K, Yuasa M, Kida T, Shimanoe K, Teraoka Y,Yamazoe N. Preparation of oxygen evolution layer/La0.6Ca0.4CoO3 dense membrane/porous support asymmetricstructure for high-performance oxygen permeation. SolidState Ionics 2008;179:1377e81.

[17] Wang YF, Hao HS, Jia JF, Yang DL, Hu X. Improving theoxygen permeability of Ba0.5Sr0.5Co0.8Fe0.2O3�d membranesby a surface-coating layer of GdBaCo2O5þd. J Eur Ceram Soc2008;28:3125e30.

[18] Zhu XF, Cong Y, Yang WS. Oxygen permeability andstructural stability of BaCe0.15Fe0.85O3�d membranes. J MembSci 2006;283:38e44.

[19] Chen ZH, Shao ZP, Ran R, Zhou W, Zeng PY, Liu SM. A denseoxygen separation membrane with a layered morphologicstructure. J Memb Sci 2007;300:182e90.

[20] Kusaba H, Shibata Y, Sasaki K, Teraoka Y. Surface effect onoxygen permeation through dense membrane of mixed-conductive LSCF perovskite-type oxide. Solid State Ionics2006;177:2249e53.

[21] Figueiredo FM, Kharton VV, Viskup AP, Frade JR. Surfaceenhanced oxygen permeation in CaTi1�xFexO3�d ceramicmembranes. J Memb Sci 2004;236:73e80.

[22] Leo A, Liu SM, Diniz da Costa JC. The enhancement of oxygenflux on Ba0.5Sr0.5Co0.8Fe0.2O3�d (BSCF) hollow fibers usingsilver surface modification. J Memb Sci 2009;340:148e53.

[23] Cheng HW, Zhang YW, Lu XG, Ding WZ, Li Q. Hydrogenproduction from simulated hot coke oven gas by usingoxygen-permeable ceramics. Energy Fuels 2009;23:414e21.

[24] Cheng HW, Lu XG, Zhang YW, Ding WZ. Hydrogenproduction by reforming of simulated hot coke oven gas overnickel catalysts promoted with lanthanum and cerium ina membrane reactor. Energy Fuels 2009;23:3119e25.

[25] Cheng HW, Lu XG, Liu X, Zhang YW, Ding WZ. Partialoxidation of simulated hot coke oven gas to syngas overRueNi/Mg(Al)O catalyst in a ceramic membrane reactor.J Nat Gas Chem 2009;18:467e73.

[26] Zhang TS, Ma J, Chan SH, Hing P, Kilner JA. Intermediate-temperature ionic conductivity of ceria-based solid solutions

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 ) 5 2 8e5 3 8538

as a function of gadolinia and silica contents. Solid State Sci2004;6:565e72.

[27] Shimonosono T, Hirata Y, Ehira Y, Sameshima S, Horita T,Yokokawa H. Electronic conductivity measurement of Sm-and La-doped ceria ceramics by HebbeWagner method. SolidState Ionics 2004;174:27e33.

[28] Mori T, Drennan J, Lee JH, Li JG, Ikegami T. Oxide ionicconductivity and microstructures of Sm- or La-doped CeO2-based systems. Solid State Ionics 2002;154e155:461e6.

[29] Marina OA, Bagger C, Primdahl S, Mogensen M. A solid oxidefuel cell with a gadolinia-doped ceria anode: preparation andperformance. Solid State Ionics 1999;123:199e208.

[30] Mogensen M, Sammes NM, Tompsett GA. Physical, chemicaland electrochemical properties of pure and doped ceria.Solid State Ionics 2000;129:63e94.

[31] Shen ZG, Lu PX, Yan G, Hu X. Enhancing the oxygenpermeability of Ba0.5Sr0.5Co0.8Fe0.2O3�d membranes bycoating RBaCo2O5þd (R ¼ Pr, Nd, Sm, Gd) layers. Mater Lett2010;64:980e2.

[32] Luo HX, Wei YY, Jiang HQ, Yuan WH, Lv YX, Caro J, et al.Performance of a ceramic membrane reactor with highoxygen flux Ta-containing perovskite for the partialoxidation of methane to syngas. J Memb Sci 2010;350:154e60.

[33] Ishihara T, Tsuruta Y, Todaka T, Nishiguchi H, Takita Y. Fedoped LaGaO3 perovskite oxide as an oxygen separatingmembrane for CH4 partial oxidation. Solid State Ionics 2002;152e153:709e14.

[34] Shao ZP, Yang WS, Cong Y, Dong H, Tong JH, Xiong GX.Investigation of the permeation behavior and stability ofa Ba0.5Sr0.5Co0.8Fe0.2O3�d oxygen membrane. J Memb Sci 2000;172:177e88.

[35] Zhang C, Chang XF, Dong XL, Jin WQ, Xu NP. The oxidativestreamreformingofmethane tosyngas ina thintubularmixed-conductingmembrane reactor. J Memb Sci 2008;320:401e6.

[36] Ikeguchi M, Mimura T, Sekine Y, Kikuchi E, Matsukata M.Reaction and oxygen permeation studies in Sm0.4Ba0.6Fe0.8Co0.2O3�d membrane reactor for partial oxidation of methaneto syngas. Appl Catal A Gen 2005;290:212e20.

[37] Makoto H, Domen K, Hara M, Tatsumi T. Ba1.0Co0.7Fe0.2Nb0.1

O3�d dense ceramic as an oxygen permeable membrane forpartial oxidation of methane to synthesis gas. Chem Lett2006;35:1326e7.

[38] Lein HL, Wiik K, Grande T. Kinetic demixing anddecomposition of oxygen permeable membranes. Solid StateIonics 2006;177:1587e90.

[39] Bouwmeester HJM. Dense ceramic membranes for methaneconversion. Catal Today 2003;82:141e50.