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High resistance to carbon deposition of silica-coated Ni catalysts in propane

stream reforming

Sakae Takenaka *, Yoshiki Orita, Hiroshi Umebayashi, Hideki Matsune, Masahiro Kishida *

Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, Moto-oka 744, Nishi-ku, Fukuoka 819-0395, Japan

1. Introduction

H2 is a clean fuel because CO2 is not emitted whenit is usedas a

fuel for polymer-electrolyte fuel cells (PEFCs). H2   has been

produced through steam reforming of CH4, which is a main

component of natural gas, followed by a water-gas shift reaction of 

CO. Supported Ni catalysts are generally used for the steam

reforming of CH4 [1–5]. These catalysts are, however, deactivated

by the sintering of Ni metal and by coke formation. The

development of Ni catalysts which are resistant to Ni metal

particle sintering and to coke formation is thus desired.

Steam reforming of CH4   is a promising technology for the

supply of H2 to stationary fuel cells due to an abundance of CH4 as

natural gas and as CH4 hydrate. H2 production facilities should be

constructed near natural gas pipelines due to energy costs

associated with storage and transportation of liquefied natural

gas. The energy requirement is due to the low liquefaction

temperature of CH4. Urban infrastructure exists for the distribution

of CH4 as utility gas but remote locations need alternative energy

sources for H2   production. Liquefied petroleum gas (LPG), which

mainly contains propane, is a promising alternative energy source

for H2  production because LPG is condensed at higher tempera-

tures than natural gas. The steam reforming of LPG should thus be

investigated for the potential use in fuel cells. Supported Ni

catalysts have been investigated forthe use in the steam reforming

of LPG but were found to be deactivated by coke formation  [6–10].

These reports indicated that the formation of coke is more

pronounced in the steam reforming of LPG than in the steam

reformingof natural gas. Thedevelopment of Ni catalysts with high

resistance to coke formation is thus desired.

Our group has previously reported excellent catalytic perfor-

mance for the partial oxidation of CH4  into synthesis gas using a

silica-coated Ni catalyst  [11]. The Ni metal nanoparticles within

the silica-coated Ni catalyst were not aggregated during the partial

oxidation of CH4 at 973 K because they were uniformly covered by

layers of silica. In addition, this catalyst was not deactivated by

coke formation during the partial oxidation of CH4. This study was

undertaken with the expectation that silica-coated Ni catalysts

would be effective for the steam reforming of LPG. The silica-

coated Ni metal catalyst was thus applied to the steam reforming

of propane.

2. Experimental

 2.1. Preparation of catalysts

The Ni catalyst coated with silica (denoted as coat-Ni hereafter)

was prepared by using a water-in-oil microemulsion [12–14]. The

Applied Catalysis A: General 351 (2008) 189–194

A R T I C L E I N F O

 Article history:Received 27 June 2008

Received in revised form 16 September 2008

Accepted 17 September 2008

Available online 1 October 2008

Keywords:

Propane steam reforming

Synthesis gas

Ni catalysts

Silica coating

Carbon deposition

A B S T R A C T

Thecatalytic performance of Ni metal catalystsuniformly covered with layersof silica for propane steamreforming was compared with the performances of Ni metal catalysts supported on substrates of 

magnesia (MgO), alumina (Al2O3) and silica (SiO2). Thesilica-coated Ni catalystsshowed high activity for

propane steam reforming at 873 K. This catalytic activity is higher than the activity of effective light

alkane steam reforming Ni/Al2O3   and Ni/MgO catalysts and we found that the catalytic activity of the

silica-supported Ni metal was poor. The coverage of Ni metal particles with silica thus improves their

catalytic activity for this reaction. Carbon deposition on silica-coated Ni catalysts during the steam

reforming of propane at 873 K waslowerthan that on Ni/MgO andNi/Al2O3. Ni K-edge XANES andEXAFS

spectra of silica-coated Ni catalyst showed a strong interaction between Ni metal and silica. This

interaction prevents the sintering of Ni metal particles during the propane steam reforming, which

results in the reduction of carbon deposition on silica-coated Ni catalysts. These properties of silica-

coated Ni catalysts result in high catalytic activity and improved catalyst stability.

 2008 Elsevier B.V. All rights reserved.

* Corresponding authors. Tel.: +81 92 802 2752; fax: +81 92 802 2752.

E-mail address:   takenaka@chem-eng.kyushu-u.ac.jp (S. Takenaka).

Contents lists available at ScienceDirect

Applied Catalysis A: General

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / a p c a t a

0926-860X/$ – see front matter    2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2008.09.017

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microemulsion was prepared by adding aqueous nickel nitrate

(Ni(NO3)2) into a solution of surfactant (polyoxyethylene (n = 15)

cetyl ether and cyclohexane). Nanoparticles of a compound

containing Ni cations were synthesized by the addition of 

hydrazine (N2H4H2O) to the microemulsion. Hydrolysis and

polycondensation of tetraethyl orthosilicate (TEOS) was done at

333 K by the addition of TEOS and aqueous ammonia (NH3) to the

microemulsion. The precipitates obtained were washed with

isopropanol several times andcalcined at 773 K for 2 h under an air

stream. To avoid an explosion involving the Ni metal precursor

during calcination of the catalysts in air, only a small amount of the

catalyst was calcined. The calcined samples were washed with

nitric acid (HNO3, 1.0 M) at room temperature in order to remove

any Ni species which were not covered by silica. After this

treatment, the Ni loading was estimated with X-ray fluorescence

spectroscopy (XRF). Silica-supported Ni catalysts (Ni/SiO2), mag-

nesia-supported Ni catalysts (Ni/MgO) and alumina-supported Ni

catalysts (Ni/Al2O3) were prepared with a conventional impreg-

nation method. Briefly, MgO (JRC-MGO4-500A from the Catalysis

Society of Japan), Al2O3   (JRC-ALO-8 from the Catalysis Society of 

 Japan) or SiO2, which was prepared by the hydrolysis and

polycondensation of TEOS, was added to aqueous Ni(NO3)2   at

353 K and then dried at 353 K. The dried samples were calcined at873 K for 5 h under an air stream. These catalyst samples were

pressed into a disc and crushed to 16–28 mesh prior to reaction

testing.

 2.2. Reaction procedure

Steam reforming of propane was done at atmospheric pressure

with a conventional gas flow system containing a fixed catalyst

bed. The catalyst samples, diluted with quartz sand, were packed

into a tubular quartz reactor (length = 300 mm and inner

diameter = 8 mm). The catalysts were reduced with H2  at 973 K

for 1 h before the reactions. After catalyst pretreatment, a mixed

gas of propane and steam, diluted with Ar was introduced into the

reactor at 873 K. Effluent gases from the catalyst bedwere sampledand analyzed by on-line gas chromatography. The yield of H2 and

selectivity to CO, CO2 or CH4 are defined as follows:

Yieldof H2 ð%Þ ¼  ð2 moles of H2 producedÞ

ðtotal moles of Hatoms in thefeedÞ  100

Selectivity to CO;  CO2 orCH4 ð%Þ

¼  ðmoles of CO;  CO2 orCH4Þ

ð3 molesof propane convertedÞ

  100

 2.3. Characterization of the catalyst samples

TEM images of the Ni catalysts were recorded with a JEOL JEM-

2000EX. Before TEM image collection, the catalyst samples werereduced with H2 at 973 K. The reduced samples were dispersed in

2-propanol at room temperature and then an aliquot of the

solution was dropped on a grid for the measurement of TEM

images.

Measurement of X-ray absorption spectra (X-ray absorption

near edge structure, XANES and extended X-ray absorption fine

structure, EXAFS) was done on the beam line, BL 7C, at the Photon

Factory in the Institute of Materials Structure Science for High

Energy Accelerator Research Organization, Tsukuba, Japan (Pro-

posal No. 2005G194). Ni K-edge XANES/EXAFS spectra of the Ni

catalysts were measured in transmission mode with a Si(1 1 1)

two-crystal monochromator at room temperature with a ring

energy of 2.5 GeV and a stored current of 250–450 mA. Before the

spectra were obtained, the catalysts were reduced with H2   at

973 K. After the treatment, the catalysts were packed into a

polyethylene bag under an Ar atmosphere. Analysis of the EXAFS

data was done using REX (Rigaku Co.), an EXAFS analysis program.

A Fourier transformation of the k3-weighted EXAFS oscillation was

done over the range of  k  = 3.5–15.5 A 1.

The quantity of carbon deposited on the Ni catalysts during the

steam reforming of propane was evaluated by thermogravimetric

analysis under an air stream. During the experiments, the catalyst

samples were heated to 1023 K under air, which resulted in the

gasification of deposited carbon.

3. Results and discussion

 3.1. Structure of the Ni species in the catalysts

Fig. 1 shows a TEM image of coat-Ni after reduction with H2

at 973 K as well as the TEM image of Ni/SiO2 for comparison. In

the TEM image of Ni/SiO2, small particles of 5–20 nm in

diameter are attached to the outer surface of the silica supports.

As described below, the small particles observed in the TEM

image for Ni/SiO2   are assigned to Ni metal. The TEM image of 

coat-Ni shows small particles with diameters of ca. 5 nm within

spherical silica particles of ca. 50 nm diameter. The small

particles in coat-Ni are also assigned to Ni metal as described

below. In the TEM image of coat-Ni, Ni metal particles are

always located at the center of the spherical silica particles. The

Ni metal particle in coat-Ni is thus stabilized within the

spherical silica particle, whereas Ni metal particles in Ni/SiO2,

are attached to the outer surface of silica.

Fig. 2 shows XRD patterns of coat-Ni, Ni/MgO, Ni/Al2O3 and Ni/

SiO2. The catalyst samples were reduced with H2 at 973 K before

measuring XRD patterns but were brought into the contact with

air, at room temperature, during the XRD measurement. Diffrac-

tion lines due to Ni metal crystallites are observed at about 44.5 8

Fig. 1. TEM images of Ni(10 wt%)/SiO2 (a) and coat-Ni(10 wt%) (b).

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and 52.08 in the XRD patterns for all the Ni catalysts. These peaks,

however, have a low intensity for coat-Ni and Ni/MgO but the

results do indicate that Ni metal crystallitesare present in allthe Ni

catalysts. The formation of a solid solution between NiO and MgO

is known to occur and the Ni species within the solid solution are

not easily reduced to Ni metal in the presence of H2, even at 973 K

[15]. The diffraction lines due to Ni metal in the XRD pattern of Ni/

MgO are, therefore, very small in intensity.

Fig. 3 shows the Ni K-edge XANES spectra of coat-Ni, Ni/MgO,

Ni/Al2O3   and Ni/SiO2. XANES spectra for the reference samples

(NiO and Ni foil) are also shown in Fig. 3. The XANES spectrum of 

NiO is markedly different from that of Ni foil, i.e. a strong

absorption at about 8350 eV is observed in the XANES spectrum of 

NiO, whereas the peak intensity at around 8350 eV for Ni foil is

very small. The XANES spectrum reveals useful information about

the oxidation state of the Ni species in the catalysts   [16]. The

XANES spectra of Ni/SiO2 and Ni/Al2O3 are similar to the spectrum

of Ni foil; therefore, most of the Ni species in Ni/SiO2 and Ni/Al2O3

are present as Ni metal. The XANES spectrum of Ni/MgO is more

similar in shape to thespectrumof NiOrather than the spectrum of 

Ni foil. A small shoulder peak at about 8333 eV, which is

characteristic of XANES spectrum for Ni foil, is found in the

XANES spectrum of Ni/MgO. This peak suggests that some Ni

species in Ni/MgOis present as Ni metal,although Ni species would

exist, mainly, in the oxidized state. The XANES spectrum of coat-Ni

resembles the spectrum of Ni foil more closely than the spectrum

of NiO. The peak at about 8350 eV in the XANES spectrum of coat-

Ni is slightly more intense than that for Ni foil. This indicates that

some Ni species in coat-Ni are in the oxidized state, while most Ni

species are present as Ni metal. The XANES spectrum of Ni/SiO2

shows that the fraction of oxidized Ni species to total Ni species incoat-Ni is higher than that in Ni/SiO2. The higher oxidation ratio

implies that theNi species in coat-Ni have a stronginteractionwith

the silica layers.

Fig. 4 shows Fourier transforms of Ni K-edge k3-weighted EXAFS

spectra (RSFs, radial structural functions) for coat-Ni, Ni/MgO, Ni/

Al2O3, Ni/SiO2 and Ni foil. A peak is observed at about 2.1 A in the

RSFs for all the Ni catalysts as well as for Ni foil. This peak is

assigned to the first Ni–Ni shell of Ni metal. The intensity of the

peak in the RSFs strongly depends on the type of Ni catalyst, i.e. the

peak intensity decreases in the following order: Ni/SiO2 > Ni/

Al2O3 > coat-Ni > Ni/MgO. The peak intensity due to the first shell

in the RSF for any metal is dependent on the crystallite size of the

metal [17,18]. The average crystallite size of Ni metal in the Ni

catalysts used in this study thus increases in the following order:Ni/SiO2 > Ni/Al2O3 > coat-Ni > Ni/MgO. Intense peaks are not

found in the range  R > 2.5 A in the RSF for Ni/MgO, although the

Fig. 2.   XRD patterns of Ni(10 wt%)/MgO, Ni(10 wt%)/SiO2, Ni(20 wt%)/Al2O3   and

coat-Ni (10 wt%).

Fig. 4. Fourier transforms of Ni K-edge  k3-weighted EXAFS spectra for different Ni

catalysts and Ni foil. (a) Ni(10 wt%)/MgO; (b) coat-Ni(10 wt%); (c) Ni(20 wt%)/

Al2O3; (d) Ni(10 wt%)/SiO2; (e) Ni foil.

Fig. 3. Ni K-edge XANES spectra of different Ni catalysts and reference samples (Ni

foil and NiO). (a) Ni foil; (b) Ni (10 wt%)/SiO2; (c) Ni (20 wt%)/Al2O3, (d) coat-Ni

(10 wt%); (e) Ni (10 wt%)/MgO; (f) NiO.

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RSF for NiO has an intense peak at around 2.7 A due to the second

shell (Ni–Ni) in NiO(the resultis notshown). Oxidized Ni species in

Ni/MgO are, therefore, not aggregated but highly dispersed in the

MgO supports, i.e. the formation of a Ni-Mg-O solid solution

occurs.

 3.2. Catalytic performance of coat-Ni for steam reforming of propane

Ni/MgO and Ni/Al2O3  catalysts are some of the most effective

catalysts for the steam reforming of light alkanes such as CH4,

ethane and propane. The catalytic performance of coat-Ni for the

steam reforming of propane was thus compared with the

performances of Ni/MgO and Ni/Al2O3. Propane conversion and

H2 yield in the steam reforming of propane over Ni/MgO, Ni/Al2O3

and coat-Ni catalysts with different Ni loading are listed in Table 1.

The quantity of catalyst loaded into the reactor was such that the

Ni content was equivalent in all the reactions. The results shown in

Table 1 were collected at 5 h of time on stream, from the reaction

over each catalyst. H2, CO, CO2 and CH4 were formed in addition to

trace amounts of ethylene and propylene during all the Ni catalyst

reactions. The H2  yield and propane conversion over coat-Ni and

Ni/Al2O3  increased with higher Ni loading. In contrast to coat-Ni

and Ni/Al2O3, the catalytic activity of Ni/MgO for the steamreforming of propane was lower when the Ni loading increased

from 10 to 20 wt%. The catalytic reactions of Ni (10 wt%)/MgO,

coat-Ni (10 wt%) and Ni (20 wt%)/Al2O3   were then examined.

Table 1 also shows theresults of propane steam reforming over the

Ni (10 wt%)/SiO2  catalyst, which was prepared by a conventional

impregnation method. The Ni/SiO2   catalyst did not show any

activity for the steam reforming of propane as no H2  or CO was

formedunder theexperimental conditions. It was reported that Ni/

SiO2 catalysts were easily deactivated for the steam reforming of 

methane [19,20]. The results shown in Table 1 were collectedat 5 h

of time on stream. Thus, the Ni/SiO2 would be deactivated at 5 h of 

time on steam. Likely, the silica coating of coat-Ni improves the

catalytic activity of the Ni metal for the steam reforming of 

propane.Steam reforming of propane over coat-Ni, Ni/MgO and Ni/Al2O3

catalysts was carried out with different W /F values (W , weightof Ni

atoms in the reactor; F , flow rate of reactants) in order to assess the

catalytic activity of each catalyst. Fig. 5 shows propane conversion

after 5 h of time on stream, as a function of  W /F  values from the

reactions over the coat-Ni, Ni/MgO and Ni/Al2O3  catalysts.   W /F 

values rangedfrom ca. 0.01 to 0.04. Allthe Ni catalyst reactionshad

lower propane conversion as the   W /F   ratio was reduced. The

propane conversion for coat-Ni was higher than for the other

catalysts over the whole   W /F  range, indicating that the coat-Ni

catalysts, generally, had the highest activity. As described

previously, the Ni metal crystallites in coat-Ni are smaller than

those in Ni/Al2O3, which also contributes to the higher catalytic

activity of coat-Ni. On the other hand, the Ni metal crystallites inNi/MgO are also small, but the fraction of Ni metal to all Ni species

in Ni/MgO is significantly lower than that in coat-Ni. This isconfirmed by the XANES and EXAFS spectra. The catalytic activity

of Ni/MgOfor the steam reforming of propane was thus lower than

the catalytic activity of coat-Ni.

Fig. 6  shows the change of propane conversion with time on

stream in the propane steam reforming over coat-Ni, Ni/MgO and

Ni/Al2O3. In these reactions, the molar ratio of propane to steam

was adjusted to 1/3. The change of H2 yield and selectivity to CO,

CO2 and CH4 as a function of time on stream in these reactions is

plotted in Fig. 7. The conversion of propane for coat-Ni was always

higher than the propane conversions for Ni/MgO and Ni/Al2O3

catalysts. The coat-Ni catalysts showed slight deactivation initially

and then stabilized after 3 h on stream. The catalytic activities of 

Ni/MgO and Ni/Al2O3  were also depressed slightly with time on

stream. The deactivation of these catalysts is due to carbondeposition during the reactions. In contrast to propane conversion,

the H2   yield for coat-Ni did not decrease appreciably during

propane steamreforming. The H2 yield andCH4 selectivity for coat-

Ni were higher than the H2 yield and CH4 selectivity of the other

catalysts. On the other hand, the CO2  selectivity of coat-Ni was

 Table 1

Steam reforming of propane over coat-Ni, Ni/MgO, Ni/Al2O3 and Ni/SiO2 at 873 K.

Catalyst N i loading (wt%) Co nver sion of C3H8  (%) H2 yield (%)

Coat-Ni 5 21 21

Coat-Ni 10 98 65

Ni/MgO 10 89 68

Ni/MgO 20 46 42

Ni/Al2O3   10 62 61

Ni/Al2O3   20 86 67

Ni/SiO2   10   <1 0

Composition of reactants, C3H8:H2O:N2:Ar = 1:3:1:15; flow rate = 100 ml min1,

loading mass of Ni in the reactor = 0.0025 g.

Fig. 5.   Change of propane conversion as a function of W/F in the propane steam

reforming over coat-Ni, Ni/MgO and Ni/Al2O3  catalysts at 873 K. P(C3H8) = 5 kPa,

P(H2O) = 15 kPa, P(N2) = 5 kPa and P(Ar) = 76 kPa.

Fig. 6.  Change of propane conversion with time on stream in the propane steam

reforming over coat-Ni, Ni/MgO and Ni/Al2O3   at 873 K. P(C3H8) = 5 kPa,

P(H2O) = 15 kPa, P(N2) = 5 kPa and P(Ar) = 76 kPa. Flow rate = 100 ml min1. Mass

of Ni in the reactor = 0.0013 g.

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hydrocarbons generally increases as the conversion of hydro-

carbons increases. The catalytic activity of coat-Ni was higher than

that of the other catalysts tested in this study. We thus concludethat the coat-Ni catalyst has a high resistance to carbon deposition

during the steam reforming of hydrocarbons. Carbon deposition

from hydrocarbonson thesupported Ni catalysts is more prevalent

when the Ni metal particle is larger   [21–23]. As previously

described, Ni metal particles in coat-Ni were smaller than those in

Ni/Al2O3. The Ni metal particles in coat-Ni are not aggregated

during the propane steam reforming because the metal particles

are covered with silica. Thus, coat-Ni catalyst has a high resistance

to carbon deposition during the steam reforming of propane. It is

reported that carbonis deposited as filamentous carbon during the

steam reforming of hydrocarbons over the supported Ni catalysts

[24,25]. Carbon atoms from hydrocarbons are deposited on the

surface of Ni metal particles, supported on carriers, during the

steam reforming of hydrocarbons, and then carbon atoms diffuseon the surfaces and/or in the bodies of the Ni metal particles,

resulting in the formation of filamentous carbon [26,27]. Ni metal

particles in the supported Ni catalysts are thus not present on the

carrier, but are present at the tip of filamentous carbon, when

carbons aredeposited on theNi catalysts. It shouldbe noted that Ni

metal particles in coat-Ni are covered with silica layers. The Ni

metal particles in coat-Ni strongly interact with silica, as shown by

its XANES spectrum. The strong interaction between silica and Ni

metal particles in coat-Ni should prevent detachment of Ni metal

particles from silica supports, which results in high resistance to

carbon deposition of coat-Ni catalysts during the steam reforming

of propane. These factors result in high catalytic activity and

improved stability of coat-Ni for the reaction.

4. Conclusion

Silica-coated Ni catalysts show high catalytic activity and

improved stability in the steam reforming of propane. The silica-coated Ni catalysts had reduced carbon deposition during propane

steam reforming, compared to Ni/Al2O3   and Ni/MgO, which are

some of the most effective catalysts for the steam reforming of 

hydrocarbons. The strong interaction of Ni metal particles with

silica, in the silica-coated Ni catalysts, prevents the sintering of Ni

metal and carbon deposition during the steam reforming of 

propane. These properties of the silica-coated Ni catalyst resulted

in excellent catalytic performance.

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S. Takenaka et al./ Applied Catalysis A: General 351 (2008) 189–194194