Catalysis Communications 10 (2008) 325–329

Contents lists available at ScienceDirect

Catalysis Communications

journal homepage: www.elsevier .com/locate /catcom

Self-activation and self-regenerative activity of trace Ru-doped plate-typeanodic alumina supported nickel catalysts in steam reforming of methane

Lu Zhou a,*, Yu Guo a, Qi Zhang b, Masayuki Yagi a, Huabo Li a, Jian Chen a, Makoto Sakurai a,Hideo Kameyama a

a Department of Chemical Engineering, Faculty of Engineering, Tokyo University of Agriculture and Technology, 24-16 Nakacho 2, Koganei-Shi, Tokyo 184-8588, Japanb Department of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, China

a r t i c l e i n f o

Article history:Received 25 July 2008Received in revised form 4 September 2008Accepted 9 September 2008Available online 4 October 2008

Keywords:Anodic alumina supportCH4

RuSteam reformingNiAl2O4

1566-7367/$ - see front matter � 2008 Elsevier B.V. Adoi:10.1016/j.catcom.2008.09.029

* Corresponding author.E-mail address: zhoulu1104@gmail.com (L. Zhou).

a b s t r a c t

A 17.9 wt.% Ni/Al2O3/alloy catalyst with porous anodic alumina support was subjected to steam reform-ing of methane after 700 �C steam purging and no reactivity was shown because of surface oxidation ofsintered Ni particles. When 0.05 wt.% Ru doped on this catalyst, it exhibited self-activation and self-regenerative activity, resulted from the hydrogen spillover over Ru and/or Ru–Ni alloy. Moreover, a97% CH4 conversion was kept for 20 h over the catalyst under electrical heating pattern.

� 2008 Elsevier B.V. All rights reserved.

1. Introduction

Although many reviews on steam reforming of methane (SRM)discuss the conventional process carried out on bead Ni catalysts inmulti-tubular reactors, due to their poor performance in gas diffu-sion, heat transmission and compact size [1], constructed wall-type reformers have become the focus of recent researches. Never-theless, the thermal endurance of plate-type catalysts employed bywall-type reactors has been presenting significant challenge todevelopment of long-life wall-type SRM reformers, because thehydrothermal environment during SRM usually results in delami-nation of the coating catalysts layer from base materials [2].

As an approach, we synthesized a plate-type porous anodic alu-mina supported Ni catalyst, denoted as 17.9 wt.% Ni/Al2O3/alloy[3,4]. It showed high heat resistance and no delamination of cata-lyst layer happened during a 40000-cycles thermal endurance test[5]. Moreover, this catalyst performed excellent SRM reactivity be-cause 97% CH4 conversion was maintained during 200 h stationarytest at 700 �C under F/W = 157000 mL/(h gcat) [3].

In contrast to large-scale use of reformers in industry under sta-tionary operating conditions, temperature and load varied fre-

ll rights reserved.

quently by daily start-up and shut-down (DSS) in the operationfor hydrogen production of PEFC in domestic use, and several hun-dred starts and stops are unavoidable over 10 years of operation.Between shut-down and start-up in the DSS operation, catalystbed is certainly purged by steam for securing the safety. Thus, cat-alyst must be tolerable to multiple cycles under such unusual tran-sient conditions without deterioration. However, some research [6]indicated that flowing steam alone could seriously deactivate alu-mina supported Ni catalysts, because Ni metal can be oxidized notonly by gaseous oxygen but also even in the presence of steam.This is the same case for our 17.9 wt.% Ni anodic catalysts [3].The catalyst, purged in steam at 700 �C for 2 h, showed negligibleactivity and re-reduction with H2 was required to regenerate it.Similar phenomenon was also evidenced by D. Li et al. [7].They reported that although a 16.0 wt.% Ni/Mg(Al)O catalystshowed excellent activity in the stationary SRM, it quickly deacti-vated due to the oxidation of Ni by steam in DSS SRM. But,Ni/Mg(Al)O catalysts doped with 0.1 wt.% Ru were found to beeffective for suppressing Ni oxidation and showed self-regenera-tive activity.

In this contribution, we prepared a 17.9 wt.% Ni catalyst dopedwith 0.05 wt.% Ru to investigate its SRM performance under steampurging. The author has found that trace Ru-doping was effectivefor suppressing the deactivation, while excellent catalytic behavioras self-regeneration and self-activation of this catalyst has beenfound.

85 µm

45 µm

Fe-Cr-Ni Alloy layer

Anodic Al2O3 layer

45 µm Anodic Al2O3 layer

Fig. 1. Cross-sectional photograph of anodic alumina support.

326 L. Zhou et al. / Catalysis Communications 10 (2008) 325–329

2. Experimental

2.1. Catalyst preparation

A commercial plate-type Al/Cr–Ni alloy/Al clad base mate-rial with an Al layer thickness of 45 and a Cr–Ni alloy layerthickness of 85 lm (Daido Steel Co., Ltd.) was used to preparethe support [3,4] in Fig. 1 (c-Al2O3/Cr–Ni alloy/Al2O3, the densityof the alumina and alloy layer were 1.70 and 7.54 g/cm3,respectively).

According to previous work [3,4], 17.9 wt.% Ni/Al2O3/alloy withan interfacial NiAl2O4 layer was prepared by a double impregna-tion method. The support was impregnated in an aqueous solutionof Ni (II) nitrate under ambient conditions. After drying at 120 �Covernight, the plate was calcined at 700 �C for 3 h. The impregna-tion and drying were then repeated, followed by calcination at500 �C for 3 h.

Ru/Al2O3/alloy was prepared by impregnation with RuCl3 solu-tion. The impregnated samples were dried at 120 �C overnight andcalcined at 500 �C for 3 h.

Ru-doped Ni catalysts, Ru/Ni/Al2O3/alloy, were prepared by thesequential impregnation of Ru onto the calcined Ni/Al2O3/alloy.

2.2. Catalyst characterization

Metal loadings were analyzed with an inductively-coupled plas-ma spectrometer (ICPS-7510, Shimadzu Corp.) and reported herebased on alumina quantity.

H2-TPR analyzes were performed on a ChemBET 3000 (Quanta-chrome Instruments, Co.) coupled with a thermal conductivity

0 50 100 150 200 250 300 350 400 450 500

0

50

100

Met

hane

con

vers

ion

[%]

Time on stream [h]

0 2 4 6 8 10 12 14 16

0

50

100

H2 re-reduction at 700oC

Steam purge

at 600oC or 700oC

0 50 100 150 200 250 300 350 400 450 500

0

50

100

Met

hane

con

vers

ion

[%]

Time on stream [h]

0 2 4 6 8 10 12 14 16

0

50

100

H2 re-reduction at 700oC

Steam purge

at 600oC or 700oC

Fig. 2. SRM reaction (a) (b) and H2-TPR (c) over

detector (TCD). After loading the sample into a U-shaped quartztube (i.d. 4 mm), the sample was purged with 70 mL/min Ar for1 h at 500 �C, and then cooled to room temperature. Next, the inletgas was switched to 70 mL/min 65% H2/Ar. When the system wasstable, the sample was heated from room temperature to 1000 �Cwith a heating rate of 10 �C/min. The TCD signals were calibratedusing Ag2O as a standard.

2.3. Activity experiments

SRM tests were carried out in a plug flow integrated reactor (i.d.10 mm). A plate-type catalyst (the loaded quantity of alumina lay-ers was about 76 mg) was cut into small pieces (about 5 mm2), andpacked into the reactor using quartz sands to dilute them with aloading density of 1.0 cm2-catalyst/(g-quartz sand). The reactiontemperature was monitored using a K-type thermocouple placedin the center of catalyst bed. N2 was introduced as the inner infer-ence gas for GC analyzes. In all SRM tests, the ratio of CH4/H2O/N2

in the feed was controlled at 1:3:2, while the CH4 flow was fixed at50 mL/min (i.e. F/W = 157000 mL/(h�g), excluding N2 and the quan-tity of interlayer alloy). Before each test, the catalyst was purgedwith N2 (100 mL/min) at 500 �C for 1 h, and then reduced in H2

(100 mL/min) at 500 �C for 1 h and at 800 �C for a further 2 h. Atthe outlet of the reactor, a cold trap was used to condense anywater from the product gas stream. The dry outlet gases were ana-lyzed by an on-line gas chromatograph (GC-2014AT, ShimadzuCorp.). The results obtained were evaluated in terms of CH4 conver-sion, carbon atom mass balance and H2 production.

CH4 conversionð%Þ ¼ ðCH4inlet � CH4outletÞCH4inlet

� 100%

C mass balanceð%Þ ¼ COoutlet þ CO2outlet þ CH4outlet

CH4inlet� 100%

H2productionðmLðmin gÞ�1Þ ¼ Fhydrogen produced

Mcatalyst

� CH4, CO and CO2: molar flow rate;� Fhydrogen produced: outlet gas flow of hydrogen produced (mL/

min);� Mcatalyst: catalyst quantity excluded the interlayer alloy (g).

0 200 400 600 800 1000

700 oC steam purged17.9 wt% Ni/Al

2O

3/alloy Cat.

600 oC steam purged17.9 wt% Ni/Al

2O

3/alloy Cat.

Sign

al [

-]

Temperature [oC]

Fresh 17.9 wt% Ni/Al2O

3/alloy Cat.

0 200 400 600 800 1000

700 oC steam purged17.9 wt% Ni/Al

2O

3/alloy Cat.

600 oC steam purged17.9 wt% Ni/Al

2O

3/alloy Cat.

Sign

al [

-]

Temperature [oC]

Fresh 17.9 wt% Ni/Al2O

3/alloy Cat.

fresh and steamed 17.9 wt.% Ni/Al2O3/alloy.

Table 1The quantity analyzes of the TPR results of fresh and steamed Ni/Al2O3/Alloy.

Sample Ni loading (wt.%) Ni2+ content (wt.%)

Fresh Cat. 17.9 17.9600 �C steamed Cat. 17.9 17.0700 �C steamed Cat. 17.9 9.36

L. Zhou et al. / Catalysis Communications 10 (2008) 325–329 327

3. Results and discussion

3.1. Deactivation of 17.9 wt.% Ni/Al2O3/Alloy under steam-purging SRM

In Fig. 2a, a satisfy CH4 conversion (97.3%) was kept for 500 hover the 17.9 wt.% Ni catalyst. Our previous researches [3,4] attrib-uted such excellent performance to the existence of interfacialNiAl2O4 layer. Due to a stronger Ni bonding to NiAl2O4 than alu-mina, the NiAl2O4 layer anchored the top metallic Ni particlesand thus alleviate their solid-state reactions with alumina supportwhich would aggravate the Ni oxidation with steam, meanwhileeffectively suppressed Ni sintering to maintain sufficient activemetallic Ni. However, in Fig. 2b, after subjected to 600 or 700 �Csteam purging, the catalyst deactivated quickly and a re-reductionwith H2 was required to regenerate its reactivity.

Fig. 2c showed H2-TPR profiles of fresh and steamed catalysts.The steamed catalysts were prepared as follows. After subjected

0 10 20 30 40

0

20

40

60

80

100

700 oC H2 re-reduction for 1h

Ru[0.05 wt%]-Ni[17.9 wt%] Cat. 0.1 wt% Ru Cat.

Met

hane

con

vers

ion[

%]

Time on stream [h]

700 oC steam purge for 2h

0 10 20

80

100

Met

hane

con

vers

ion

[%]

Time on s

80

100

80

1000

50

100

b

CH

con

vers

ion

and

C b

alan

ce[%

]

Fig. 3. SRM reaction over Ru-doped Ni catalyst after steam

to stationary SRM at 700 �C for ca. 3 h, methane and steam wereswitched off and the catalysts were then purged by 100 mL/minN2. When the catalysts temperature was stable at 700 or 600 �C,a 150 mL/min steam was introduced. After the catalysts subjectedto such steam purging for about 2 h, the steam was switched offand the catalysts were then cooled to room temperature underN2 purge. Then the steamed catalysts were soon subjected to theTPR analyzes. Although the H2 pre-reduction was conducted beforeSRM test, both steamed catalysts showed H2 consumption, reveal-ing the oxidation of Ni0 with steam into Ni2+ during steam purging.

The quantity analyzes of H2-TPR results coupled with Ni loadingwere presented in Table 1. The Ni loading remained the same as17.9 wt.% over fresh and steamed catalysts, indicating no loss ofcatalytic materials, whereas the steamed catalysts showed only17.0 wt.% (600 �C purge) and 9.36 wt.% (700 �C purge) existenceof Ni2+. It should be noted that the catalysts were exposed to steamfor as long as 2 h, and theoretically this would have made Ni0 beentotally oxidized. On the other hand, when only considering Ni oxi-dation over steamed catalysts, theoretically the higher tempera-ture as 700 �C steam-purging would have produced more Ni2+

content than that of 600 �C. However, the result is the opposite.Therefore, the most plausible explanation of this is that duringthe steam purging, metallic Ni particles were sintered to formlarge-sized particles and simultaneously surface the layer of Niparticles was oxidized to form the sintered structure with hard-

30 40 50

tream [h]

400oC steam purge

500oC steam purge

600oC steam purge

700oC steam purge

a

0 10 20 30 40 50 60

0

20

40

60

80

100

500

1000

1500

2000

2500

3000

3500

4000

C balance CH4 conversion

H2 p

rodu

ctio

n [m

Lm

in-1

g-1]

4

Time on stream [min]

H2 production

c

purging (a) and without pre-reduction (b) and (c).

328 L. Zhou et al. / Catalysis Communications 10 (2008) 325–329

to-reduce Ni2+ species while the core was kept as metallic Ni par-ticles [4].

3.2. Self-regenerative activity and self-activation of 0.05 wt.% Ru-doped 17.9 wt.% Ni/Al2O3/alloy under steam-purging SRM

SRM was carried out at 700 �C over 0.05 wt.% Ru doped Ni cat-alyst before and after steam purging at different temperature be-tween 400 and 700 �C. As shown in Fig. 3a, unlike the 17.9 wt.%Ni catalyst which showed a total deactivation just after the firststeam purging, when 0.05 wt.% Ru doped, the catalyst was regener-ated and showed a stable activity, regardless of the steam purgingfrequency and temperature.

In Fig. 3b, after the catalysts temperature was heated to 700 �Cin N2, a SRM reaction was carried out by flowing CH4 and steamover the calcined Ru-doped Ni catalyst without pre-reduction.High methane conversion was shown just after starting the reac-tion in Fig. 3c. After 15 min, the hydrogen productivity became sta-ble and no deactivation was found during 40 h operation. Also areference SRM test over 0.1 wt.% Ru/Al2O3/alloy was conducted inFig. 3b. In the absence of Ni, the activity of Ru catalyst gradually de-creased during the reaction regardless of the steam purging and H2

re-reduction, showing no self-regenerative activity. Besides, itmust be emphasized that Ru alone was not enough active withsmall loadings as 0.05 wt.% and the activity of Ru doped Ni cata-lysts depended mainly upon the Ni species.

In order to investigate the reason for the self-regenerative activ-ity and self-activation of Ru-doped Ni catalyst, H2-TPR analyzesover Ru-doped and undoped Ni/Al2O3/alloy, also Ru/Al2O3/alloywere carried out in Fig. 4. For the undoped Ni catalyst, three signif-icant reduction peaks were detected, reported as NiO at 470 �C,non-stoichiometric nickel aluminate (xNiO�Al2O3, � < 1) at 650 �Cand NiAl2O4 at 830 �C [3]. The H2-TPR profiles of Ru-doped Ni cat-alyst appeared similar to those of Ni/Al2O3/alloy, except for a sharpreduction peak centered at 170 �C and a magnified NiO peak aswell as the shifts of NiAl2O4 reduction temperature to lower value.

0 200 400

Anodic support

Ru [0.05 wt%]-Ni[17.9wt%]

Fresh Ni[17.9wt%] Cat.

Ru [0.1wt%] Cat.

H2 C

onsu

mpt

ion

[a.u

.]

Tempe

Fig. 4. H2-TPR spectra of Ru/Al2O3/alloy, R

In view of the 0.1 wt.% Ru catalyst, the reduction peak centered at170 �C for the Ru-doped Ni catalyst was considered to arise fromthe reduction of small clusters of RuOx laid over the Ni2+ surfaces.The availability of such Ru species was vital for Ru-doped Ni cata-lyst self-regenerative activity and self-activation. When Ru-dopedcatalyst subjected to the flow of CH4 and H2O, we considered thatCH4 decomposition over some of the RuOx species at 700 �C initi-ated the auto-reduction of the catalyst. As a result of CH4 decom-position over RuOx species, the hydrogen produced and thesubsequent formation of CO from the reaction between RuOx-ad-sorbed carbon and its labile oxygen could contribute to the Ni-oxide species reduction. Further reduction of Ru–Ni-oxide clusterswould be facilitated by increased availability of the hydrogen pro-duced from the Ru catalyzed CH4 reforming reaction followed bythe water gas shift reaction between CO and excess H2O. Accordingto these, the Ni2+ on this catalyst was self-activated by hydrogenspillover from Ru, while Ru–Ni alloy was formed on the surfaceof metallic Ni particles. It was considered that the formation ofRu–Ni alloy played a significant role on the self-regenerative activ-ity of the Ru-doped catalyst, in view of hydrogen spillover from thealloy could re-reduce and re-disperse the surface oxidized sinteredmetallic Ni particles during the steam purging [7].

3.3. SRM over 0.05 wt.% Ru-doped 17.9 wt.% Ni/Al2O3/alloy underelectrical heating pattern

Because of the alloy interlayer existence in the support (Fig. 1),it allows for an electrical heating pattern for our catalysts. We havereported that by applying the electrical heating method, the refor-mer start-up time would be expected to be shortened to withinjust few minutes [5]. Fig. 5 showed the SRM reactions under elec-trically heating pattern over 0.05 wt.% Ru-doped 17.9 wt.% Ni cata-lyst by using homemade electrically testing equipment (Fig. 5a). Acatalyst with 5 mm in width and 100 mm in length was used inthis test. During the experiments, the temperature of catalyst sur-face was controlled as 700 �C by adjusting the electrical current.

600 800 1000

Cat.

rature [oC]

u-doped and undoped Ni/Al2O3/alloy.

Gas inlet

Thermocouple entrance

Electrodes

Thermocoupleentrance

Electrodes

~

Gas outlet

+ -

Radiate Thermocouple

Plate Catalyst

Inlet Outlet

Gas inlet

Thermocouple entrance

Electrodes

Thermocoupleentrance

Electrodes

~

Gas outlet

+ -

Radiate Thermocouple

Plate Catalyst

Inlet Outlet

H2 pre-reduction at 800ºC after 15 min

H2 pre-reduction at 800ºC after 1 h

After H2 pre-reduction treatment (room temperature)

SRM testing at 700ºC for 2 h

SRM testing at 700ºC for 10 h

After SRM testing (room temperature) 0 2 4 6 8 10 12 14 16 18

0

20

40

60

80

100

C balance ovre 17.9 wt% anodic Ni catalyst

17.9 wt% anodic Ni catalyst Porous anodic alumina support

CH

4 con

vers

ion

and

C b

alan

ce[%

]

Time on stream [h]

Gas inlet

Thermocouple entrance

Electrodes

Thermocoupleentrance

Electrodes

~

Gas outlet

+ -

Radiate Thermocouple

Plate Catalyst

Inlet Outlet

Gas inlet

Thermocouple entrance

Electrodes

Thermocoupleentrance

Electrodes

~

Gas outlet

+ -

Radiate Thermocouple

Plate Catalyst

Inlet Outlet

H2 pre-reduction at 800ºC after 15 min

H2 pre-reduction at 800ºC after 1 h

After H2 pre-reduction treatment (room temperature)

SRM testing at 700ºC for 2 h

SRM testing at 700ºC for 10 h

After SRM testing (room temperature) 0 2 4 6 8 10 12 14 16 18

0

20

40

60

80

100

C balance ovre 17.9 wt% anodic Ni catalyst

17.9 wt% anodic Ni catalyst Porous anodic alumina support

CH

4 con

vers

ion

and

C b

alan

ce[%

]

Time on stream [h]

Fig. 5. SRM over Ru-doped Ni/Al2O3/alloy under electrical heating pattern using homemade electrically testing equipment.

L. Zhou et al. / Catalysis Communications 10 (2008) 325–329 329

The status of the catalyst under different stages during pre-reduc-tion and SRM reaction was shown in Fig. 5b. The conversion waskept at 97% over the catalyst, whilst neglect activity was exhibitedover the support in Fig. 5c. These concluded that the electricalheating pattern would never exert any unfavorable effect overthe SRM reactivity of anodic catalysts.

4. Conclusions

A 0.05 wt.% Ru-doped 17.9 wt.% Ni/Al2O3/alloy showed self-activation and self-regenerative activity performance during steampurging SRM at 700 �C. Moreover, no unfavorable effect was evi-

denced over the SRM reactivity of anodic catalysts when applyingthe electrical heating pattern.

References

[1] M. Zanfir, A. Gavrilidis, Chem. Eng. Sci. 58 (2003) 3947.[2] B.R. Johnson, N.L. Canfield, D.N. Tran, R.A. Dagle, X.S. Li, J.D. Holladay, Y. Wang,

Catal. Today 120 (2007) 54.[3] L. Zhou, Y. Guo, Q. Zhang, T.P. Tran, M. Sakurai, H. Kameyama, J. Chem, Eng.

Japan 41 (2008) 90.[4] L. Zhou, M. Sakurai, H. Kameyama, Appl. Catal. A 347 (2008) 200.[5] Q. Zhang, M. Nakaya, T. Ootani, H. Takahashi, M. Sakurai, H. Kameyama, Int. J.

Hydrogen Energy 32 (2007) 3870.[6] Y.S. Oh, H.S. Roh, K.W. Jun, Y.S. Baek, Int. J. Hydrogen Energy 28 (2003) 1387.[7] D. Li, T. Shishido, Y. Oumi, T. Sano, K. Takehira, Appl. Catal. A 332 (2007) 98.