Temperature profile of catalyst bed during oxidative steam

reforming of methane over Pt-Ni bimetallic catalysts

Baitao Li a, Shigeru Kado a, Yuya Mukainakano a, Mohammad Nurunnabi a,Toshihiro Miyao b, Shuichi Naito b, Kimio Kunimori a, Keiichi Tomishige a,*

a Institute of Materials Science, University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-8573, Japanb Department of Applied Chemistry, Faculty of Engineering, Kanagawa University, 3-27-1, Rokkakubashi,

Kanagawa-ku, Yokohama, Kanagawa 221-8686, Japan

Received 20 October 2005; received in revised form 30 January 2006; accepted 7 February 2006

Available online 9 March 2006

Abstract

The catalyst bed temperature during oxidative reforming of methane (CH4/H2O/O2/Ar = 40/30/20/10) at 1123 K and atmospheric pressure was

investigated by infrared thermography over g-Al2O3 supported bimetallic Pt-Ni catalysts prepared by different impregnation methods: co-

impregnation and sequential impregnation. The thermographical results clearly demonstrated that the catalyst bed temperature was strongly

dependent on the preparation method. The bimetallic catalyst prepared from the sequential impregnation method exhibited much higher resistance

to hot spot formation in oxidative reforming of methane. Temperature programmed reduction (TPR) with H2 revealed that the addition of Pt by a

sequential impregnation method greatly promoted the reduction of Ni species; furthermore, infrared spectra of CO adsorption suggests that the

surface composition of Pt on the catalyst prepared by the sequential method is much higher than that for the catalyst prepared by the co-

impregnation method. The surface enrichment of Pt is responsible for the effective overlap between the combustion and reforming zones, and this

can enhance the inhibition of hot spot formation.

# 2006 Elsevier B.V. All rights reserved.

Keywords: Oxidative steam reforming of methane; Pt; Ni; Bimetallic catalyst; Thermograph; Hot spot

www.elsevier.com/locate/apcata

Applied Catalysis A: General 304 (2006) 62–71

1. Introduction

In recent years, there has been much interest in the

production of hydrogen for highly efficient generation of

electricity using fuel cells, where one of the candidates for the

hydrogen source is the natural gas composed mainly of

methane [1]. At the same time, much attention has been paid to

the production of synthesis gas by natural gas reforming which

is related to gas to liquid (GTL) technology [2,3].

Steam reforming of methane (Eq. (1)) and other light

hydrocarbons is the primary and most important commercial

method of synthesis gas production [3–5]. Because of its

endothermic nature, external energy cost is necessary.

CH4 þ H2O!COþ 3H2 ðDH�298 K ¼ þ206 kJ=molÞ (1)

* Corresponding author. Tel.: +81 29 853 5030; fax: +81 29 853 5030.

E-mail address: tomi@tulip.sannet.ne.jp (K. Tomishige).

0926-860X/$ – see front matter # 2006 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2006.02.025

In the conventional process, the heat is supplied from the

outside the reactor wall by methane combustion, which makes

the efficiency of the production process influenced by the heat

supplying method. On the other hand, it is well-known that an

internal heat supply is more energy-efficient than an external

heat supplying system. A famous internal heat supplying

system for natural gas reforming is the autothermal reforming

process (called ATR), which has been developed by Haldor

Topsoe [6,7]. The ATR process consists of the non-catalytic

partial oxidation (Eq. (2)), combustion of methane (Eq. (3)),

and catalytic reforming of methane (Eq. (1)).

CH4 þ 12O2!COþ 2H2 ðDH�298 K ¼ �36 kJ=molÞ (2)

CH4 þ 2O2!CO2 þ 2H2O ðDH�298 K ¼ �803 kJ=molÞ (3)

One of the important points in ATR is to inhibit the contact

of the reforming catalysts with oxygen which causes the

sintering of support materials and the aggregation of metal

particles. Such results are connected to catalyst deactivation

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–71 63

and the large temperature gradient in the catalyst bed [8,9].

Catalytic partial oxidation of methane, shown in Eq. (2), has

advantages such as mild exothermicity, high conversion, high

selectivity, suitable H2/CO ratio and very short residence time

[10–12]. However, even a low conversion to CO generates a

large amount of heat, which leads to the hot spot formation, as

reported previously [13,14]. Oxidative steam reforming of

methane, which is a combination of a catalytic combustion and/

or partial oxidation reaction with reforming reaction, is an

effective and economic internal heat supply process [15–17].

Since the combustion reaction proceeds more rapidly than the

reforming reaction, in the usual cases it occurs near the

catalyst bed inlet where the temperature becomes very high

within a very small thickness; especially when Ni catalyst is

utilized, it can easily be oxidized in the presence of oxygen

and can lose its reforming activity [18–20]. On the other

hand, in the catalyst bed after oxygen is consumed, Ni

species can maintain the metallic state and contribute to the

reforming reaction. This will reasonably result in a very large

temperature gradient because the exothermic and the

endothermic regions are separated [21–23]. Recently, our

group used an infrared thermograph to monitor the surface

temperature and found that Rh and Pt catalysts exhibited

flatter temperature profiles in oxidative steam reforming of

methane reaction than Ni and Pd catalysts [24,25]. This

phenomenon is due to the overlap of the combustion zone with

the reforming zone.

In this research, the oxidative steam reforming of methane

was investigated by infrared thermography over monometallic

Ni and Pt catalysts and over bimetallic Pt-Ni catalysts prepared

by two methods: co-impregnation method and sequential

impregnation method. Various characterization techniques

were applied to determine the surface structures of these

catalysts. The relationship between the catalyst structure and

the bed temperature is discussed in detail.

2. Experimental

2.1. Catalyst preparation

g-Al2O3 support was prepared by calcining commercially

available JRC-ALO-1 (Catalysis Society of Japan,

SBET = 143 m2/g, grain size of 2–3 mm) at 1123 K in the air

for 3 h. The support thus obtained had a specific surface area

of 110 m2/g. Then it was crushed and sieved to particle

sizes between 180 and 250 mm before the impregnation with

the metal components. Supported monometallic Ni and Pt

catalysts were prepared by a wet impregnation method using

Ni(NO3)2�6H2O (Wako Pure Chemical Industries, Ltd., 99.9%)

or H2PtCl6�6H2O (Soekawa Chemical Co., Ltd., 99.9%),

respectively. After removal of the solvent by the evaporation at

353 K, the resulting product was dried in an oven at 383 K

overnight. Subsequently, the sample was calcined at 773 K in

the air for 3 h. Bimetallic Pt-Ni catalysts were prepared by

two methods. One is the sequential (two-step) impregnation

method. The calcined monometallic nickel catalyst was

reduced at 1123 K for 0.5 h under H2 flow, followed by the

impregnation of Pt using the precursor of Pt(C5H7O2)2�H2O

(Soekawa Chemical Co., Ltd., 99.9%) acetone solution. After

the removal of the acetone solvent, the catalyst was dried at

383 K overnight and next calcined in air at 573 K for 3 h.

The catalyst thus prepared is denoted as Pt/Ni/Al2O3. The other

is the co-impregnation method; here the aqueous solution

mixture of Ni(NO3)2�6H2O and H2PtCl6�6H2O was used a

precursor. After co-impregnation, the calcination procedure

was the same as that of monometallic catalyst. This catalyst

thus obtained is denoted as Pt + Ni/Al2O3. The value in

parentheses represents the weight percent of metallic compo-

nents in the catalyst.

2.2. Activity test and thermographical observation

Methane reforming catalytic reaction was conducted under

atmospheric pressure in a continuous fixed-bed quartz reactor

(i.d. 4 mm). Details are explained in our previous report [26].

Pictures of catalyst granules with 180–250 mm diameter in the

reactor and one example of a thermographical image are

illustrated in Fig. 1. The quartz reactor had an axial thermowell

(i.d. 1.5 mm) containing a chromel–alumel thermocouple

located at the outlet of the catalyst bed that was used for the

temperature control. An electronic furnace was connected with

this thermo-controller; it had a window (15 mm � 18 mm) for

the observation of the temperature profile of catalyst granules.

The temperature profile was measured with infrared thermo-

graph equipment (TH31, NEC San-ei Instruments Ltd.). As

shown in Fig. 1, the radial temperature gradient was rather flat in

the A cross section, indicating that the gas flowing condition is

close to the plug flow. Therefore, we only show the temperature

profile along the B cross section in Section 3. The catalyst

(0.08 g) was reduced in the hydrogen flow (30 ml/min, 100% H2)

at 1123 K for 0.5 h in the reactor prior to every activity test. After

the reduction, the feed gases with partial pressure ratio CH4/H2O/

O2/Ar = 40/30/20/10 were introduced to the catalyst bed at

various contact times W/F (W (g) = catalyst weight, F (mol/

h) = total flow rate of gases). CH4, O2, Ar and H2 (Takachiho

Trading Co., Ltd.) were research grade and they were used

without further purification. Steam was obtained by vaporizing

distilled water supplied by a feeding pump (MT2111, Moleh

Ltd.). An iced water trap was located at the reactor exit to remove

the steam contained in the effluent gas. The effluent gas was

collected from the sampling port using a micro-syringe and then

analyzed by gas chromatography (Shimadzu GC-14A). This

instrument was equipped with a flame ionization detector (FID)

and a thermal conductivity detector (TCD). The concentrations

of CO, CO2 and CH4 were determined on an FID-GC equipped

with a methanator, using a stainless steel column packed with

Gaskuropack 54; the concentration of H2 was determined by

TCD-GC using a stainless steel column packed with molecular

sieve 13X.

The activity test of methane combustion was performed using

air containing 2% CH4 as the reactant gas (W/F = 0.40 g h/mol)

with 0.09 g catalyst. The measurement was carried out from 523

to 1123 K at the temperature intervals of 50 K. The analysis

method was the same as that described above.

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–7164

Fig. 1. Picture of catalyst granules in the quartz glass tube (a) before reaction and (b) one example of thermographical image.

Table 1

Catalytic performance over nickel catalysts with different loading amounts

Catalyst W/F

(g h/mol)

CH4 conversion

(%)

H2/CO CO selectivity

(%)

Ni (2.6) 0.07 >99.9 2.7 82

0.12 >99.9 2.7 82

0.24 >99.9 2.7 83

0.80 >99.9 2.6 86

Ni (0.9) 0.12 94 2.9 78

Ni (0.3) 0.12 67 2.3 76

Reaction conditions: CH4/H2O/O2/Ar = 40/30/20/10; temperature 1123 K; total

pressure 0.1 MPa; catalyst weight 0.08 g; H2 pretreatment at 1123 K.

2.3. Catalyst characterization

The surface area of the catalysts was determined by the BET

method using a Gemini (Micromeritics) instrument.

Powder X-ray diffraction (XRD) patterns of the reduced

samples (H2 flow, 1123 K, 0.5 h) were collected on a Philips

X’pert diffraction-meter using Cu Ka (l = 0.154 nm) gener-

ated at 40 kV and 20 mA. Scanning was conducted over the

range of 2u = 5–758.Transmission electron microscope (TEM, JEM2010) mea-

surements were collected over the reduced catalyst (H2 flow,

1123 K, 0.5 h). Samples were dispersed in 2-propanol by

supersonic waves and put on Cu grids for the TEM observations

under atmosphere.

Temperature programmed reduction (TPR) measurements

were conducted in a fixed bed quartz reactor containing 0.01 g

of sample at a heating rate of 10 K/min, from room temperature

to 1273 K, in 5% hydrogen diluted in argon (30 ml/min) as a

reducing gas. Water produced during the reduction was

removed using a cold trap with frozen acetone (about

173 K). The TPR profile was monitored continuously with

an on-line TCD-GC. H2 consumption was estimated from the

integrated peak area of the reduction profiles.

Infrared (IR) spectra of CO adsorption on the catalysts were

measured at room temperature with an FT-IR spectrometer

(Nicolet, Magna-IR 550 spectrometer) in transmission mode

using a quartz glass IR cell with CaF2 windows connected to a

vacuum-closed circulating system. Fresh catalyst was reduced

in hydrogen flow ex situ at 1073 K for 0.5 h. Then the reduced

sample was pressed into a disk (20 mm Ø) at a weight of about

0.12 g. After reduction in situ at 773 K for 1 h, CO (0.1 kPa)

was exposed to the catalyst at room temperature and IR spectra

were recorded at room temperature under vacuum.

3. Results and discussion

3.1. Catalytic performance and catalyst bed temperature

profile over monometallic Ni and Pt catalysts in oxidative

steam reforming of methane

Oxidative steam reforming of methane was conducted under

the condition of CH4/H2O/O2/Ar = 40/30/20/10 which corre-

sponds to the combination of the combustion gas (CH4/O2 = 10/

20) and the steam reforming gas (CH4/H2O = 30/30). In this

part, we examine the catalytic performance and the bed

temperature profile over Ni/Al2O3 and Pt/Al2O3 with various

metal loading amounts. The catalytic performance details in

terms of the methane conversion, H2/CO ratio and CO

selectivity over Ni/Al2O3 are listed in Table 1 and the

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–71 65

Fig. 2. Temperature profiles of the catalyst bed in oxidative steam reforming of

methane over monometallic Ni catalysts measured by using IR thermography.

(A) W/F dependence over Ni (2.6) catalyst: W/F (g h/mol) = 0.07 (bold solid

line), 0.12 (dotted line), 0.24 (thin solid line), 0.80 (broken line). (B) Ni loading

amount dependence at W/F = 0.12 g h/mol: Ni (0.3) (dotted line), Ni (0.9) (thin

solid line), Ni (2.6) (bold solid line). Reaction conditions: CH4/H2O/O2/

Ar = 40/30/20/10; temperature 1123 K; total pressure 0.1 MPa; catalyst weight

0.08 g; H2 pretreatment at 1123 K.

Table 2

Catalytic performance over platinum catalysts with different loading amounts

Catalyst W/F

(g h/mol)

CH4 conversion

(%)

H2/CO CO selectivity

(%)

Pt (0.9) 0.12 >99.9 2.5 80

Pt (0.2) 0.12 99 2.6 83

Pt (0.1) 0.07 90 2.3 90

0.12 91 2.4 88

0.24 92 2.4 87

0.80 96 2.7 85

Pt (0.03) 0.12 84 2.0 92

Reaction conditions are the same as those in Table 1.

temperature profiles along the catalyst bed are shown in Fig. 2.

From the comparison at W/F = 0.12 g h/mol, one can clearly

observe that methane conversion decreased with decreasing the

loading amount of Ni. Especially, Ni (2.6) showed high

methane conversion even at low W/F, for example 0.07 g h/

mol, where the reaction almost reached the equilibrium.

Fig. 2(A) shows the effect of W/F on the catalyst bed

temperature profile over Ni (2.6). At W/F = 0.07 g h/mol, the

exothermic profile near the catalyst bed inlet was present and

the maximum reached about 1323 K; the temperature

decreased steeply after the maximum. This behavior in the

temperature profile was observed in oxidative reforming of

methane, which corresponds to the combination of the

exothermic reactions, such as methane combustion and partial

oxidation of methane, with the endothermic reaction, such as

steam and dry reforming of methane [25,26]. The exothermic

reaction zone is usually located near the catalyst bed inlet,

which is due to the rates of the exothermic reactions being

higher than those of the endothermic reactions. The tempera-

ture decrease after the maximum is assigned to the endothermic

reaction. It can be found that the maximum decreased with

increasing in W/F and with decreasing the total flow rate of

reactant gases. At W/F = 0.24 g h/mol, the exothermic profile

was observed only at the half part near the inlet, while the

endothermic profile was found at the half part near the outlet.

This profile clearly indicates that the exothermic and

endothermic zones exist separately. Furthermore, at W/

F = 0.80 g h/mol, almost no exothermic zone was observed,

which means that the heat produced by the oxidation of

methane near the catalyst bed inlet can be applied to the

endothermic reaction directly, suggesting that the endothermic

zone can effectively overlap the exothermic zone. The W/F

dependence of the catalyst bed temperature revealed that hot

spot can easily form at higher flow rate condition (low W/F).

Fig. 2(B) shows the effect of the loading amount of Ni on the

temperature profile at W/F = 0.12 g h/mol. In addition, the

catalyst performance details of various Ni catalysts are also

listed in Table 1. Methane conversion decreased with

decreasing Ni loading amount. The temperature peak near

the catalyst bed inlet over Ni (2.6) was higher than that over Ni

(0.9), and the declination after the maximum over the former

was more dramatic than that of the latter. These can be reflected

by the methane combustion activity of Ni (2.6) being higher

than that of Ni (0.9) [26]. Furthermore, the peak of Ni (0.3) was

close to that of Ni (0.9) and the temperature after the peak was

higher than that of Ni (0.9), which is corresponding to its lower

reforming activity; these behaviors are also observed in

methane conversion, as listed in Table 1. It is important to

find that the maximum temperature in the catalyst bed cannot

be decreased even if the loading amount of Ni is lowered. This

result demonstrates that the monometallic Ni is not suitable to

the oxidative steam reforming of methane because of its facility

to form hot spots, which agrees with our previous reports [26].

Catalytic performance over monometallic Pt catalysts is

presented in Table 2. At W/F = 0.12 g h/mol methane conver-

sion increased with increasing loading amount; over Pt (0.1)

catalyst, it slightly increased with the increasing W/F. The

results of catalyst bed temperature profile over Pt catalyst are

shown in Fig. 3. Over Pt (0.1), one can clearly observe that the

maximum temperature increased with decreasing W/F, and that

such behavior was similar to that over nickel catalysts. Fig. 3(B)

shows the effect of loading amount of Pt on the catalyst bed

temperature profile at W/F = 0.12 g h/mol; the temperature

peak decreased with increasing Pt loading amount. If one

compares Fig. 2(B) with Fig. 3(B), one will find that the peak

over Ni catalysts cannot be decreased by changing the amount

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–7166

Fig. 3. Temperature profiles of the catalyst bed in oxidative steam reforming of

methane over monometallic Pt catalysts measured by using IR thermography.

(A) W/F dependence over Pt (0.1) catalyst: W/F (g h/mol) = 0.07 (bold solid

line), 0.12 (dotted line), 0.24 (thin solid line), 0.80 (broken line). (B) Pt loading

amount dependence at W/F = 0.12 g h/mol: Pt (0.03) (thin solid line), Pt (0.1)

(bold solid line), Pt (0.2) (broken line), Pt (0.9) (dotted line). Reaction

conditions are the same as those in Fig. 2.

Fig. 4. Methane conversion as a function of reaction temperature in the

methane combustion. Reaction conditions: CH4/air = 2/98, W/F = 0.40 g h/

mol, total pressure 0.1 MPa, catalyst weight 0.09 g, H2 pretreatment 1123 K.

(*) Pt (0.1); (~) Pt (0.9); (*) Ni (2.6).

Table 3

Catalytic performance over Pt-Ni bimetallic catalysts prepared by different

methods

Catalyst W/F

(g h/mol)

CH4 conversion

(%)

H2/CO CO selectivity

(%)

Pt (0.1) + Ni (2.6) 0.07 >99.9 2.7 82

0.12 >99.9 2.7 81

0.24 >99.9 2.7 82

0.80 >99.9 2.8 82

Pt (0.1)/Ni (2.6) 0.07 >99.9 2.7 82

0.12 >99.9 2.8 81

0.24 >99.9 2.7 81

0.80 >99.9 2.7 82

Reaction conditions are the same as those in Table 1.

of Ni loaded; in the case of Pt, however, the peak can be

decreased by increasing the loading amount of Pt. Furthermore,

the temperature on Pt catalyst was usually lower than that on Ni

catalyst. If one considers Pt (0.9) and Ni (0.3) catalysts with the

identical mole-based loading (4.5 � 10�5 mol/g-catalyst), one

finds that the Pt catalyst exhibited much lower bed temperature

that the Ni catalyst. These results indicate that Pt catalysts have

high reforming activity and high resistance to the hot spot

formation. Next, we evaluated the combustion activity over the

Ni and Pt catalysts (Fig. 4). It is clear that Pt catalysts have

higher combustion activity than Ni catalysts considering the

differences of loading amount. Pt catalysts showed a low

temperature profile in oxidative reforming of methane in spite

of their high combustion activity. This behavior can be

explained by the overlap between exothermic and endothermic

reaction zones, which is related to the oxygen affinity [27] of

the metal species. Ni species with higher oxygen affinity are

easily oxidized in the presence of gas phase oxygen and thus,

lose their reforming activity [28] and mainly show the

combustion activity, although the activity was not so high.

Therefore, the effective overlap between exothermic and

endothermic zones is difficult. In contrast, Pt with lower oxygen

affinity [27] maintains the metallic state even in the presence of

gas phase oxygen. The stable metal state enables the overlap

between two reaction zones over the Pt catalysts. However, in

fact, higher resistance to hot spot formation needs a sufficient

loading amount of Pt, as shown in Fig. 3(B).

3.2. Catalytic performance and catalyst bed temperature

profile over Pt-Ni bimetallic catalysts

Because of the high cost and limited availability of

platinum, it is important to develop a catalyst with high

resistance to hot spot formation that uses only a slight amount

of platinum. Here, we investigated the additive effect of a small

amount of Pt to Ni catalysts on the temperature profile during

the oxidative steam reforming of methane. Two preparation

methods for Pt-Ni bimetallic catalysts were developed: co-

impregnation method (denoted as Pt + Ni) and sequential

impregnation method (Pt/Ni). Table 3 summarizes the catalytic

performance over two bimetallic catalysts. It was found that

both of two catalysts showed very high methane conversion

and H2/CO ratio where the reactions almost reached the

equilibrium level; however, the bed temperature profiles were

quite different. Fig. 5(A) presents the results over Pt (0.1) + Ni

(2.6) catalyst prepared by the co-impregnation method. The

temperature maximum was located near the catalyst bed inlet

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–71 67

Fig. 5. Temperature profiles of the catalyst bed in oxidative steam reforming of methane over bimetallic catalysts prepared by (A) co-impregnation method Pt

(0.1) + Ni (2.6) and (B) sequential impregnation method Ni Pt (0.1)/Ni (2.6) measured by using IR thermograph. W/F (g h/mol) = 0.07 (bold solid line), 0.12 (dotted

line), 0.24 (thin solid line), 0.80 (broken line). Reaction conditions are the same as shown in Fig. 2.

and it increased with decreasing W/F. A comparison between

Ni (2.6) (Fig. 2(A)) and Pt (0.1) + Ni (2.6) (Fig. 5(A)) shows

that the temperature maximum decreased by about 20 K with

the addition of Pt by co-impregnation method; however, the

effect was small. On the other hand, the additive effect of Pt

during the sequential impregnation method was found to be

much more remarkable. Fig. 5(B) shows the result of the Pt

(0.1)/Ni (2.6) catalyst. No obvious exothermic peak was found

at W/F = 0.24 g h/mol. The temperature maximum was about

80 K lower than that over Ni (2.6) at W/F = 0.07 g h/mol.

These results strongly confirmed that the additive effect of

platinum by the sequential impregnation method was much

more effective than that by the co-impregnation method in

terms of the inhibition of hot spot formation. It should be noted

that the catalyst bed temperature of Pt (0.1)/Ni (2.6) was lower

than that of the individual monometallic catalysts, Pt (0.1) and

Ni (2.6), which proved the existence of a synergetic effect

between Pt and Ni species. Similar behavior was observed in

the case of oxidative CO2 reforming of methane [21,23].

Furthermore, this synergetic effect is strongly dependent

on the preparation methods and the interaction between Pt

and Ni species, which will be discussed later based on the

characterization results.

Next, we investigated the effect of Ni loading amount over

Pt/Ni bimetallic catalyst prepared by the sequential impreg-

Table 4

Catalytic performance over Pt-Ni bimetallic catalysts with different nickel

loading amounts prepared by sequential impregnation method

Catalyst W/F

(g h/mol)

CH4 conversion

(%)

H2/CO CO selectivity

(%)

Pt (0.1)/Ni (0.9) 0.07 >99.9 2.7 82

0.12 >99.9 2.7 82

0.24 >99.9 2.7 83

0.80 >99.9 2.6 85

Pt (0.1)/Ni (0.3) 0.07 >99.9 2.6 81

0.12 >99.9 2.6 81

0.24 >99.9 2.6 81

0.80 >99.9 2.6 83

Reaction conditions are the same as those in Table 1.

nation method. The catalytic performance (Table 4) remained

high even over Pt (0.1)/Ni (0.3), higher than that over the

individual monometallic catalysts, Ni (0.3) and Pt (0.1) as

presented in Tables 1 and 2, respectively. The catalyst bed

temperature profiles for Pt (0.1)/Ni (0.9) and Pt (0.1)/Ni (0.3)

catalysts are shown in Fig. 6. A comparison between Fig. 5(B)

and Fig. 6 shows that the catalyst bed temperature decreased

Fig. 6. Temperature profiles of the catalyst bed in oxidative steam reforming of

methane over bimetallic catalysts with different nickel loading amounts pre-

pared by sequential impregnation method. (A) Pt (0.1)/Ni (0.9), (B) Pt (0.1)/Ni

(0.3). W/F (g h/mol) = 0.07 (bold solid line), 0.12 (dotted line), 0.24 (thin solid

line), 0.80 (broken line). Reaction conditions are the same as those in Fig. 2.

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–7168

Table 5

Catalyst properties of monometallic and bimetallic catalysts

Catalyst BET surface areaa (m2/g-catalyst) Particle size of Nia (nm) Metal dispersionb (%) Reduction degreec (%)

XRDd TEMe

Pt (0.1) 109 – – – –

Ni (2.6) 104 5.2 � 0.6 4.8 � 0.4 19 102

Pt (0.1) + Ni (2.6) 101 5.6 � 0.6 4.5 � 0.4 17 96

Pt (0.1)/Ni (2.6) 104 4.8 � 0.6 5.0 � 0.4 20 104

a Reduced in H2 flow at 1123 K, 0.5 h.b Determined by XRD results [34].c Determined from TPR results.d Evaluated by Ni(2 0 0) at 51.88 [32]. Calculated from Scherrer equation, using the half-width at half-height of the peak [33].e Calculated according to Ref. [34]. Mean diameter ðdsÞ ¼

Pnid

3i =P

nid2i , in which ni is the number of particles having a characteristic diameter di (within a given

diameter range).

significantly with decreasing Ni loading amount. An important

and meaningful temperature profile was observed over Pt (0.1)/

Ni (0.3), which gave a comparable temperature profile to that of

Pt (0.2) (Fig. 3(B)) at W/F = 0.12 g h/mol. This tendency

suggests that bimetallic catalyst with lower nickel loading

prepared by sequential method is a good substitute for a Pt

catalyst with higher metal component content.

Fig. 7. (A) XRD patterns of various catalysts reduced in H2 flow, 1123 K, 0.5 h. (a)

(&) Ni. (B) TEM images of various catalysts reduced in H2 flow, 1123 K, 0.5 h.

4. Catalysts characterization

BET surface area, metal dispersion, and particle size are

compiled in Table 5. The BET surface areas of the catalysts

after reduction pretreatment were slightly lower than that of the

support material after the pre-calcination; this decrease during

the catalyst preparation and pretreatment procedure is thought

Pt (0.1); (b) Ni (2.6); (c) Pt (0.1) + Ni (2.6); (d) Pt (0.1)/Ni (2.6). (*) g-Al2O3;

(a) Ni (2.6); (b) Pt (0.1) + Ni (2.6); (c) Pt (0.1)/Ni (2.6).

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–71 69

Fig. 8. Temperature-programmed reduction profiles for fresh catalysts: (a) Ni

(2.6); (b) Pt (0.1) + Ni (2.6); (c) Pt (0.1)/Ni (2.6); (d) Ni (2.6)-calcined at 573 K

again after reduction at 1123 K.

Fig. 9. IR spectra of CO adsorption on various catalysts at room temperature:

(a) Ni (2.6); (b) Pt (0.1) + Ni (2.6); (c) Pt (0.1)/Ni (2.6); (d) Pt (0.1).

to be due to the moderate sintering of g-Al2O3. The XRD

patterns of the reduced catalysts (Fig. 7(A)) shows the presence

of the diffraction peaks assigned to g-Al2O3 with a strong

intensity; no peak assigned to a-Al2O3 was found. Over all

catalysts, no peak that could be assigned to Pt metal was

observed. Over Ni-containing catalyst, the characteristic peak

assigned to Ni metal (2u = 44.58) [29–31] was overlapped by

that of g-Al2O3; therefore, the second strongest peak at 51.88was applied to estimate the particle size of Ni phase. It was

found that the peaks at 51.88 were rather broad, suggesting that

the Ni metal phase is highly dispersed. In the case of Pt-Ni

bimetallic catalysts, one would expect that the diffraction angle

should be shifted by alloy formation. However, it is difficult to

distinguish the peak shift by XRD data because of the low and

broad peak intensity of Ni phase and the low concentration of Pt

in Ni phase. The results of the size of Ni metal phase estimated

by XRD peak broadening are listed in Table 5. The particle size

had no significant difference among the three Ni-containing

catalysts. In order to estimate the particle size of metal phase

more precisely, we measured the TEM images over reduced

catalysts (Fig. 7(B)). The average particle sizes of three

catalysts are also listed in Table 5 and the results are in good

agreement with those by XRD. The characterization results

obtained by XRD and TEM indicate that the effect of particle

size is not an important factor in determining the bed

temperature profile in oxidative steam reforming of methane.

Therefore, we think that the surface temperature can be

reflected by the surface properties of the catalysts, for example,

the reduction degree or the surface composition. To confirm this

hypothesis and to obtain more information on the catalyst

surface, we conducted temperature programmed reduction and

IR spectra of CO adsorption.

TPR were performed to evaluate the catalyst reducibility.

Fig. 8(a) shows the TPR profile of the calcined Ni (2.6). The

consumption of H2 was observed in the temperature range 700–

1200 K and the reduction degree of Ni reached almost 100%

(Table 5). This H2 consumption peak is assigned to the

reduction of highly dispersed NiO that strongly interacted with

the Al2O3 surface [35]. On the other hand, in the case of Pt

(0.1) + Ni (2.6), the reduction peak started at about 40 K lower

than Ni (2.6), suggesting that the addition of Pt promoted the

NiO reduction, as was observed in the previous report [36–38].

Fig. 8(c) shows the TPR profile of Pt (0.1)/Ni (2.6); an

impressive sharp peak at 560 K was found [30,39], which is

quite different from the case of Fig. 8(a). The evident

temperature shift is considered to be due to the different Ni

species on the bimetallic and monometallic Ni catalysts. It

should be noted that in a sequential impregnation method, Ni

catalyst was reduced before the Pt impregnation; during the

preparation procedure, the structure of Ni may be changed

drastically. Therefore, as a reference we measured TPR of Ni

(2.6) calcined at 573 K after the reduction at 1123 K. Here, the

calcination temperature (573 K) was adjusted to the preparation

condition of Pt (0.1)/Ni (2.6). Fig. 8(d) presented a strong peak

at 620 K, 60 K higher than that in Fig. 8(c). TPR profiles

confirmed that the additive effect of Pt during the sequential

method is more remarkable than that in the case of co-

impregnation, and that the reduction peak shift to the lower

temperature in Pt (0.1)/Ni (2.6) originated from the sequentially

impregnated platinum.

It is known that Pt-Ni alloy exists in the supported Pt-Ni

bimetallic catalysts [40–42]. The difference in the temperature

profile during oxidative steam reforming of methane can be

caused by the surface structure and composition of Pt-Ni alloy

particles. In order to evaluate these surface properties of alloy

particles, we measured FT-IR spectra of CO adsorption. Over

Ni (2.6) catalyst (Fig. 9(a)), the band at 2070 cm�1 is attributed

to the multi-carbonyl species typical of well-dispersed Ni0 [43],

while the band at 2048 cm�1 is due to the linearly adsorbed CO

B. Li et al. / Applied Catalysis A: General 304 (2006) 62–7170

[43,44]. The band at the lower frequency, i.e., 1930 cm�1, is

assigned to the bridged adsorbed CO [45]. For the mono-

metallic platinum catalyst (Fig. 9(d)), only one band was

observed at 2070 cm�1; this is assigned to linear CO [46–48].

We found that the band of the linear CO on Pt has almost the

same position as that of multi-carbonyl species of Ni.

Therefore, the relative intensity of the band was taken into

account for the peak assignment in the bimetal catalysts. In the

case of Pt (0.1) + Ni (2.6) catalyst (Fig. 9(b)), two broad bands

at 2070 and 2048 cm�1 were observed at the same intensity,

which was similar to the spectrum of Ni (2.6). On the other

hand, over Pt (0.1)/Ni (2.6) catalyst (Fig. 9(c)), the intensity of

the 2070 cm�1 band was much higher than that of the

2048 cm�1 band, suggesting that a significant amount of Pt

atoms are located on the catalyst prepared by the sequential

impregnation method. The similarity of Pt (0.1) + Ni (2.6) and

Ni (2.6) indicates that the ratio of Pt on the surface in Pt

(0.1) + Ni (2.6) is much lower than that in Pt (0.1)/Ni (2.6). This

means that the sequential impregnation method is more

effective than the co-impregnation method in terms of the

surface modification, even with a small amount of Pt.

A surface segregation phenomenon of Pt atoms was often

found in Pt-Ni alloy [49–51]. However, this effect is not so

significant on the catalyst prepared by the co-impregnation

method, due to the very low composition of Pt to Ni (molar ratio

Pt/Ni = 1/100). In contrast, the surface concentration of Pt is

enhanced by the sequential impregnation method, although the

average composition of Pt to Ni was just the same as that for the

co-impregnation method. The effect of preparation methods

can be explained on the basis of the TPR profiles. From the TPR

profile of Pt (0.1) + Ni (2.6) (Fig. 8(b)), one can deduce that Ni

interacted with Al2O3 support surface and that the Ni species is

reduced slowly even in the presence of Pt. Since the reducibility

of Pt is high, Ni species is gradually reduced after the reduction

of Pt, suggesting that Pt species can be covered with reduced Ni

species. This would induce the low composition of surface Pt.

In contrast, the TPR profile of Ni species without Pt (Fig. 8(d))

gave a sharp peak at much lower temperature than that of Ni

(2.6), suggesting that NiO formed on the surface and that the

interaction between NiO species and the support surface was

rather weak. The sharp peak at much lower temperature in Pt

(0.1)/Ni (2.6) (Fig. 8(c)) showed that the introduction of Pt

promoted the reduction of this NiO species, and that NiO

particles were reduced directly to Ni metal particles, thus

inhibiting the mixing of Pt with Ni species and leading to the

high surface composition of Pt. Further investigations are

necessary for the structural analysis of the Pt-Ni bimetallic

catalysts; however, we can conclude that the addition of Pt to

Ni/Al2O3 catalysts by the sequential impregnation method is

very effective for the inhibition of the hot spot formation in

oxidative reforming of methane and that the preparation

methodology ensured the surface enrichment of Pt.

5. Conclusions

(1) In the oxidative steam reforming of methane (CH4/H2O/O2/

Ar = 40/30/20/10), Ni catalysts showed the presence of an

exothermic reaction at the catalyst bed inlet; the

endothermic reaction zone and the exothermic zone are

present separately, exhibiting a large temperature gradient

in the catalyst bed. The maximum of the bed temperature

cannot be decreased by the decrease of Ni loading amount.

(2) M

onometallic Pt catalyst gave a much smaller temperature

gradient than Ni catalysts, attributable to the overlap

between the endothermic and exothermic reaction zones.

The inhibition of hot spots is dependent on the loading

amount of Pt, and the catalyst with a small amount of Pt did

not have a significant effect.

(3) T

he addition of small amount of Pt to Ni/Al2O3 was

effective for the inhibition of hot spot formation. The two

bimetallic Pt-Ni catalysts investigated showed different

temperature profiles. The effect of adding small amounts of

Pt to Ni/Al2O3 was strongly dependent on the preparation

method and the addition by a sequential impregnation

method had a more remarkable effect than that by a co-

impregnation method.

(4) C

haracterizations of monometallic and bimetallic catalysts

by means of XRD, TEM, TPR and FT-IR of CO adsorption

suggest that the introduction of Pt by a sequential

impregnation leads to the efficient surface modification

even with a small amount of Pt, showing the high Pt surface

composition.

Acknowledgement

This study was supported by the Industrial Technology

Research Grant Program (05A43002C) from the New Energy

and Industrial Technology Development Organization (NEDO)

of Japan.

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