Partial oxidation of methane to synthesis gas over oxidized diamond catalysts

Applied Catalysis A: General 264 (2004) 65–72

Partial oxidation of methane to synthesis gas over oxidized diamond catalysts Hiro-aki Nishimoto a , Kiyoharu Nakagawa a,b,c,e , Na-oki Ikenaga a , Mikka Nishitani-Gamo b,d , Toshihiro Ando b,e , Toshimitsu Suzuki a,b,∗ a

d

Department of Chemical Engineering, Kansai University, 3-3-35 Yamate, Suita, Osaka 564-8680, Japan b High Technology Research Center, Kansai University, Suita, Osaka 564-8680, Japan c Japan Science Technology Corporation (JST), Tsukuba, Ibaraki 305-0044, Japan Department of Applied Chemistry, Faculty of Engineering, Toyo University, Kujirai, Kawagoe, Saitama 350-8585, Japan e National Institute for Materials Science (NIMS), 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan Received in revised form 22 November 2003; accepted 19 December 2003

Abstract Oxidized diamond (O-dia) acted as an excellent support material for the partial oxidation of methane to synthesis gas. The O-dia gave the best performance among the support materials of Ni- and Co-loaded catalysts for the partial oxidation of methane at a low temperature. A nickel (3 wt.%)/O-dia catalyst afforded a high CH4 conversion of 26.5% at a CH4 to O2 ratio of 5 to give nearly a 1 to 2 ratio of CO and H2 with selectivities of 70% at 873 K. Cobalt (3 wt.%)/O-dia catalyst exhibited a slightly lower catalytic activity than that of Ni (3 wt.%)/O-dia catalyst, where CH4 conversion was 23.0%. No change in the support was observed in the oxidation atmosphere at a high temperature. Carbon deposition of the catalyst surface in the reaction of CH4 was examined by thermogravimetric analyses. Carbon deposition occurred on the nickel (3 wt.%)/O-dia catalyst at below 923 K, and above this temperature no carbon deposition was seen. However, on the cobalt-loaded catalyst no carbon deposition was observed at any reaction temperature. © 2004 Elsevier B.V. All rights reserved. Keywords: Oxidized diamond; Partial oxidation; Methane; Synthesis gas

1. Introduction Diamond has been considered to be inert in relation to most chemicals. However, diamond surface is easily oxidized by oxidants such as HNO3 , HClO4 , or H2 O2 , and is also oxidized with O2 at an elevated temperature to form C–O–C ether-type linkages and C=O carbonyl-type structures [1,2]. Oxidized diamond (O-dia) surface has recently received attention as a new and unique material phase of carbon, being an apparent pseudo-solid carbon-oxide. Sillica (SiO2 ) has long been known as a support material of a catalyst, since it showed weak interaction between support and loaded metal or metal oxide, in addition to its higher surface area. Carbon belongs to the first row element of group 14 elements in the periodic table. No carbon oxide solid phase exists above room temperature. O-dia contains a C–O–C or ∗ Corresponding author. Tel.: +81-6-6368-0865; fax: +81-6-6388-8869. E-mail address: [email protected] (T. Suzuki).

0926-860X/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apcata.2003.12.029

C–OH bond on the top surface, as shown in Scheme 1. Such functional groups could be regarded as a pseudo-solid carbon oxide phase. From this viewpoint, use of O-dia as a catalyst support has attracted the attention of the present authors. We have first reported the use of an O-dia-supported Cr2 O3 catalyst in the dehydrogenation of ethane [3]. Further studies on the use of an O-dia-supported catalyst under oxidative atmosphere were performed and we found that a Ni-loaded O-dia catalyst exhibited significant catalytic activity in the partial oxidation of methane to give CO and H2 [4]. As a method of converting the natural gas to a raw material for chemicals, initial conversion of natural gas into synthesis gas (H2 , CO) must be performed by reforming reactions using H2 O (Reaction 1) or CO2 (Reaction 2), and the partial oxidation reaction (Reaction 3). Steam reforming reaction of methane: CH4 + H2 O
CO + 3H2 ,

0 H298 = +206 kJ/mol

(1)

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(a)

O

O

(b) Scheme 1. (a) Hydrogenated diamond and (b) oxidized diamond.

CO2 reforming reaction of methane: CH4 + CO2 → 2CO + 2H2 ,

catalyst is reported to be an active catalyst for the partial oxidation of methane at temperatures higher than 823 K [10]. Ni/Ce-ZrO2 catalyst [11], NiO/barium hexaaluminate catalyst [12], and Ni/Mg-Al oxide catalyst [13] has been reported to be active catalysts for the partial oxidation of methane. It was reported that NiO/MgO solid solution catalyst exhibited high activity and selectivity in the partial oxidation of methane at 973 K [14,15]. On the other hand, only a few papers have been published on the cobalt-loaded catalysts. It is reported that Co/Yb2 O3 catalyst was highly active, selective, and much more productive [16]. Recently, Co/Mg/Al2 O3 [17], Co/La/Al2 O3 [18], and Co/ThO2 catalyst [19] were reported to be active catalysts for the partial oxidation of methane. Partial oxidation of methane with Co/MgO was reported to exhibit high catalytic activity at a high loading level of Co and at a high temperature [20]. Co/MgO catalyst provided a CO yield of 93% and a H2 yield of 90% at 1173 K with very high stability in the CO2 reforming of methane [21,22]. We have previously reported that Ir/TiO2 catalyst afforded the highest activity for the partial oxidation of methane into synthesis gas selectivity of over 80% at 873 K, without any carbon deposition. The synthesis gas production proceeded basically via a two-step path consisting of methane combustion to give CO2 and H2 O, followed by the reforming of methane with CO2 and H2 O [23–27]. This paper deals with the catalytic activities of nickel/Odia (the oxidation state of Ni is not simple; hereafter refer to the catalyst as Ni compound-loaded catalyst) and cobalt/O-dia catalysts (cobalt-loaded catalyst) in the partial oxidation of methane.

0 H298 = +247 kJ/mol

(2)

2. Experimental

Partial oxidation reaction of methane: CH4 + 21 O2 → CO+2H2 ,

0 H298 = −36 kJ/mol

(3)

Recently, increasing attention has been paid to the conversion of methane to synthesis gas by partial oxidation of methane. Nickel-loaded catalysts are very effective for the partial oxidation of methane to synthesis gas because Ni is abundant resource. However, remarkable carbon deposition occurs during the reaction over Ni-loaded catalysts. Lunsford and co-workers [5] carried out the partial oxidation of methane over an Ni-loaded Al2 O3 catalyst, and obtained high selectivities of synthesis gas. To avoid this carbon deposition, several researchers have reported the use of catalysts containing nickel with Ni/␥-Al2 O3 catalyst prepared by the sol–gel method [6] or LiLaNiO/␥-Al2 O3 catalyst [7]. Ni/Ca1−x Srx TiO3 perovskite catalyst showed high activity for CH4 combustion at around 873 K. At 1073 K, catalytic performance suddenly changed to high synthesis gas selectivities [8]. LaNix Fe1−x O3 perovskite catalysts are efficient catalysts for synthesis gas production from methane and oxygen with no deactivation and no coke formation at 1073 K during 250 h [9]. Ni/CeO2

2.1. Catalyst preparation Commercially available powdered diamond (less than 0.5 ␮m in diameter; General Electric Company) was used for the study. Before oxidation, in order to prepare the homogeneous surface conditions, the diamond powder was first hydrogenated at 1173 K for 1 h under pure H2 stream. The hydrogenated diamond powder was then oxidized at 723 K for 1 h under O2 stream (O2 /Ar = 1/4). The O-dia is known to have oxygenated species with C–O–C and C=O structures, and the existence of these functional groups were confirmed by diffuse reflectance FT-IR. The supported group 8–10 metal catalysts were prepared by impregnating an aqueous solution of Co(NO3 )2 ·6H2 O, RuCl3 ·nH2 O, Pd(CH3 COO)2 , IrCl4 ·H2 O, (NH3 )2 Pt(NO2 )2 , RhCl3 ·H2 O (Mitsuwa Pure Chemicals), Fe(NO3 )3 ·9H2 O, and Ni(NO3 )2 ·6H2 O (Wako Pure Chemical Industries Ltd.) onto O-dia. Water was removed by evaporation under reduced pressure. Each dried catalyst was calcined at 723 K in air for 5 h. The other catalyst supports used were Al2 O3 (JRC-ALO-4), SiO2 (Merck), MgO (500 A; Ube Industries,

H.-a. Nishimoto et al. / Applied Catalysis A: General 264 (2004) 65–72

Ltd.), TiO2 (P25; Japan Aerosil Co.), graphite (Wako Pure Chemical Industries Ltd.), activated carbon (Wako Pure Chemical Industries Ltd, Darco G-60), and La2 O3 . The catalyst support of La2 O3 was prepared by the thermal decomposition of La(CH3 COO)3 ·3/2H2 O (Wako Pure Chemical Industries Ltd.) at 873 K under air for 5 h. Catalysts were prepared by impregnating metal salts onto respective oxides. These supported catalysts were dried and calcined at 873 K for 5 h in air prior to the reaction.

(a)

(b)

(c)

2.2. Catalytic reaction

(d)

(e) 5

The reaction was carried out with a fixed-bed flow-type quartz reactor (i.d. 8 mm × 350 mm) operated at atmospheric pressure. For a 60 mg of catalyst, 25 ml/min of CH4 and 5 ml/min of O2 were introduced at temperature ranges of 673–973 K. Products were analyzed with an online high speed gas chromatograph equipped with CP-Sil 5 and Mol-plot columns (PC-Chrom, M200 Chromato Analyzer).

(f) 4000

2.3. Characterization Fourier-transform infrared spectra were recorded on a Jeol JIR 7000 in a diffuse reflectance mode. A KBr standard powder was used as the reference. Sixty-four scans were accumulated for each spectrum at a resolution of 2 cm−1 , and observed spectra were converted into Kubelka–Munk function units. Carbon deposition behavior was traced by using a thermogravimetric analyzer (Shimadzu, TGA-50). A 13 mg portion of a sample was placed on the TG pan. Heating was programmed at a heating rate of 20 K/min to a desired temperature under an Ar atmosphere. After the the sample reached the desired temperature, Ar flow was switched to CH4 -Ar (CH4 = 15, Ar = 20 ml/min) mixture, and a carbon weight increase in the sample was monitored.

3. Result and discussion 3.1. Diffuse reflectance FT-IR spectroscopic analysis IR spectra of hydrogenated diamond, O-dia, fresh nickel/ O-dia and cobalt/O-dia catalyst, and nickel/O-dia and cobalt/ O-dia catalysts after the reaction are shown in Fig. 1. The spectrum of hydrogenated diamond contained bands in the region from 2800 to 2970 cm−1 (Fig. 1a), which were ascribed to C–H stretching vibrations of sp3 hybridized bonding. The O-dia showed significant peaks in the region from 900 to 2000 cm−1 (Fig. 1b). The band centered in the region 900–1350 cm−1 was assigned to C–O–C stretching vibrations of a cyclic ether or cyclic ester, and the doublet band in the region 1650–1850 cm−1 was assigned to C=O stretching vibrations of cyclic ketone, lactone, or carboxylic anhydride structure. The infrared spectrum of the fresh catalyst of nickel/O-dia is shown in Fig. 1c. Nickel/O-dia catalyst

67

3500

3000

2500

2000

1500

Wavenumber/cm

1000

500

-1

Fig. 1. Diffuse reflectance FT-IR spectra of oxidized diamond-loaded catalyst: (a) hydrogenated diamond at 1173 K; (b) oxidized diamond (O-dia) at 723 K; (c) nickel (3 wt.%)/O-dia: fresh; (d) nickel (3 wt.%)/O-dia: after reaction (CH4 /O2 = 5, 973 K); (e) cobalt (3 wt.%)/O-dia: fresh; (f) cobalt (3 wt.%)/O-dia: after reaction (CH4 /O2 = 5, 973 K).

after the CH4 ·O2 reaction at 973 K for 2 h was subjected to IR analysis and the spectrum is shown in Fig. 1d. The nickel/O-dia catalyst after the reaction showed the peaks in the region from 900 to 2000 cm−1 (Fig. 1d), indicating that oxygen species on the diamond species remained on the surface. The broad absorption at 700–500 cm−1 is ascribed to nickel oxide. Cobalt/O-dia exhibited a spectrum similar to that of Ni/Odia catalyst except for two absorptions ascribed to Co–O bonds at 600 and 680 cm−1 . However, intensities of absorptions ascribed to O-dia decreased after the reaction, indicating that surface oxygen on diamond is transferred during the reaction, even under oxidative conditions. Absorptions ascribed to metal oxides in both Ni- and Co-loaded catalyst remained after the reaction, indicating that oxide phases exist after the reaction. These results clearly show that O-dia support in the nickel/O-dia catalyst did not burn off during the partial oxidation of methane at 973 K, since excess methane was supplied to the system. 3.2. Effect of group VIII metals on the performance of the partial oxidation of methane Fig. 2 shows CH4 conversion and product selectivities in the partial oxidation using several group 8–10 metal oxide-supported O-dia catalysts. In order to avoid accidental explosion of the CH4 and O2 mixed gas, the reaction was conducted at a CH4 /O2 ratio of 5. Under such conditions, carbon formation tends to proceed more severely

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H.-a. Nishimoto et al. / Applied Catalysis A: General 264 (2004) 65–72 30 O-dia

CH4 conversion / %

TiO2 Al2O3 20

Active Carbon MgO La2O3

10

SiO2

0 673

Fig. 2. Partial oxidation of methane over various metal-supported oxidized diamond-loaded catalysts. Reaction temperature: 873 K; flow rate: 30 ml/min (CH4 /O2 = 5); catalyst: 60 mg; metal-loading level: 5 wt.%.

than the stoichiometric feed ratio. Thus, performance of the support could easily be evaluated. CH4 conversion and selectivities to CO and H2 were the highest with the nickel/O-dia catalyst. However, carbon deposition was observed on the nickel/O-dia catalyst at 873 K. Cobalt/O-dia and ruthenium/O-dia catalysts afforded slightly lower CH4 conversions to give CO and H2 with selectivities above 60% for the partial oxidation of methane at 873 K. Rhodium/O-dia and palladium/O-dia catalysts exhibited a moderate activity in the partial oxidation of methane. Rapid carbon deposition occurred on the palladium/O-dia catalyst in the initial stage of the reaction at 873 K, indicating that these catalysts would be deactivated by carbon deposition. The iridium/O-dia catalyst afforded a low activity in the partial oxidation of methane. The high CO2 selectivity is a characteristic feature of iron/O-dia and platinum/O-dia catalysts, indicating that complete oxidation occurred.

873

Fig. 3. Effect of temperature on the CH4 conversion over nickel-loaded various metal oxide-supported catalysts for the partial oxidation of methane. Flow rate: 30 ml/min (CH4 /O2 = 5); catalyst: 60 mg; Ni-loading level: 3 wt.%.

air flow. Ni oxides on the O-dia might be reduced, due to a weak interaction between the support and loaded Ni oxides. The oxidic Ni species in the fresh catalyst, on La2 O3 , Al2 O3 , MgO, TiO2 , active carbon, and SiO2 surface, did not exhibit catalytic activity in the partial oxidation of methane at 873 K. Fig. 4 shows the effect of support materials on the partial oxidation of methane over cobalt-loaded catalysts. Similar to the case of nickel-loaded catalysts, only O-dia-supported cobalt exhibited a high catalytic activity at 873 K. Cobalt/Odia exhibited higher CH4 conversion at lower temperatures as compared to the nickel/O-dia catalyst. This seems to be a result of the higher oxidation capability of cobalt oxides. Other support materials did not exhibit activity for synthesis gas in the temperature ranges examined. Co-loaded La2 O3 and MgO showed higher methane conversion at lower temperatures with high CO2 selectivity, indicating

3.3. Effect of support and temperature on the activity of Ni- and Co-loaded catalysts

30

CH4 conversion / %

Fig. 3 shows the effects of various supports of nickel (3 wt.%) on the partial oxidation of methane in the temperature ranges of 673–873 K. As support materials, typical oxides for partial oxidation (Al2 O3 , La2 O3 , and MgO) are selected; TiO2 loaded with iridium was reported to be the best support for this reaction [25]; and SiO2 and activated carbon are selected to allow comparisons with O-dia catalyst. The conversion of methane varied in regard to the supports; the order of their activities was: O-dia > La2 O3 > Al2 O3 > MgO > TiO2 > active carbon > SiO2 at 873 K. The conversion of methane was the highest with nickel/O-dia catalyst at 873 K. Only the nickel/O-dia catalyst afforded CO and H2 at 873 K with a small amount of carbon deposition. In order to give high activity for the partial oxidation of methane, metallic Ni seems to be required as an active species. Ni species of nickel/O-dia catalyst before the reaction is nickel oxide since the catalyst was prepared by calcination in an

773 823 723 Temperature / K

O-dia TiO 2 Al 2O 3 Active Carbon MgO La2O 3 SiO 2 Graphite

20

10

0 673

723 773 823 Temperature / K

873

Fig. 4. Effect of temperature on the CH4 conversion over cobalt-loaded various metal oxide loaded catalysts for the partial oxidation of methane. Flow rate: 30 ml/min (CH4 /O2 = 5); catalyst: 60 mg; Co-loading level: 3 wt.%.

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Table 1 Effect of metal-loading level on the partial oxidation over nickel and cobalt/O-dia at 873 K Metal content (wt.%)

Conversion (%) CH4

Selectivity (%)

H2 /CO (ratio)

CO

CO2

H2

0 0.5 1.0 3.0 5.0

6.1 19.1 20.2 26.5 24.0

0 66.4 65.2 73.2 67.0

100 33.6 34.8 26.8 33.0

0 80.9 78.4 97.5 77.0

– – – 2.7 2.3

Co 0.5 1.0 3.0 5.0

9.0 8.4 22.9 22.5

– – 68.0 67.2

100 100 32.1 32.8

– – 77.6 79.5

2.3 2.4

Ni

Catalyst: 60 mg; flow rate: 30 ml/min (CH4 /O2 = 5); space velocity: 30,000 ml h−1 /g-catalyst/h.

that complete oxidation of methane proceeded on these catalysts. Cobalt-loaded catalyst exhibited lower activity as compared to nickel-loaded catalyst. Hydrogen pre-treatment of cobalt-loaded catalyst was examined. Ni-loaded SiO2 and Co-loaded Al2 O3 , which exhibited poor activities in the partial oxidation of methane with freshly prepared catalyst, showed high activities, indicating that reduced nickel or cobalt species are required for this reaction. 3.4. Effect of metal-loading level on the partial oxidation of methane Table 1 illustrates the effects of Ni- and Co-loading level on the CH4 conversion and product concentrations. The partial oxidation proceeded with a loading level as low as 0.5 wt.% of Ni and the highest CH4 conversion of ca. 27% was obtained with a loading level of 3.0 wt.%. O-dia support alone showed small activity in the complete oxidation (Reaction 4) at 873 K.Complete oxidation reaction of methane: CH4 + 2O2 → CO2 + 2H2 O, 0 H298 = −801 kJ/mol

(4)

On the other hand, higher loading levels (above 3.0 wt.%) were required in the partial oxidation of methane with Co-loaded catalyst. At lower loading levels, Co/O-dia catalyst afforded complete oxidation to give CO2 and H2 O. Higher selectivity to H2 was obtained with Ni/O-dia catalyst from lower temperature regions. This seems to show that the partial oxidation of methane proceeded through different catalytic pathways for Ni- and Co-loaded O-dia catalysts at 873 K. 3.5. Effect of reaction temperature of nickel/O-dia and cobalt/O-dia catalysts on the partial oxidation of methane Fig. 5 shows the temperature dependence of the catalytic activity of nickel/O-dia catalyst for the partial oxidation

Fig. 5. Effect of temperature on the CH4 conversion and product concentrations over nickel-loaded/O-dia catalyst. Flow rate: 30 ml/min (CH4 /O2 = 5); catalyst: 60 mg; Ni-loading level: 3 wt.%.

of methane. Below 773 K, only the complete oxidation of methane occurred, and synthesis gas was formed above 823 K. The nickel/O-dia catalyst was activated and produced synthesis gas above 823 K. Furthermore, nickel/O-dia catalyst showed CH4 conversion of ca. 32% at 973 K without carbon deposition. When the temperature was increased from 873 to 973 K, the product concentration of CO2 decreased. The reason for this seems to be a reverse water gas shift reaction (Reaction 5) and an equilibrium shift to the CO side. It seems reasonable that the reaction proceeded favorably at a higher temperature due to the endothermic nature: H2 + CO2
CO + H2 O,

0 H298 = +41 kJ/mol

(5)

Cobalt/O-dia catalyst exhibited similar behavior to that of nickel/O-dia catalyst in the partial oxidation, and showed high catalytic activities to synthesis gas above 823 K (Figs. 5 and 6). As described above, cobalt-loaded catalyst afforded higher methane conversion even at temperatures as low as 673 K, where only CO2 was obtained indicating that complete oxidation proceeded on this catalyst. 3.6. Stability of nickel/O-dia and cobalt/O-dia catalyst Fig. 7 shows effect of time-on-stream on the partial oxidation of methane at the reaction temperature of 873 K and 973 K. At the reaction temperature of 973 K, the catalytic activity was maintained for 10 h with a high CH4 conversion of ca. 32%. On the other hand, CH4 conversion and CO selectivity at the reaction temperature of 873 K decreased during the reaction period of 10 h, probably due to carbon deposition. Fig. 8 shows effect of time on stream on the partial oxidation of methane on cobalt/O-dia catalyst. During the reaction period of 10 h, no deactivation of the activity was seen,

H.-a. Nishimoto et al. / Applied Catalysis A: General 264 (2004) 65–72

Conversion / %

70

40 30 20 10 0

Selectivity / %

100

Fig. 6. Effect of temperature on the CH4 conversion and product concentrations over cobalt-loaded/O-dia catalyst. Flow rate: 30 ml/min (CH4 /O2 = 5); catalyst: 60 mg; co-loading level: 3 wt.%.

75 CO

H2

25 0

indicating differences in the reaction mechanisms between Ni- and Co-loaded catalyst at 873 K. Carbon deposition on group 8–10 metal (Ni, Ru, Rh, Pd, Ir, Pt, Fe, Co) loaded O-dia catalysts were measured under an isothermal reaction at 873 K with H2 -Ar mixed gas after hydrogen pre-treatment at the same temperature using a TG. Results are shown in Table 2. Carbon deposition was observed on nickel/O-dia catalyst at 873 K but no carbon deposition was detected at 973 K. Rapid carbon deposition occurred on Palladium/O-dia catalyst at the reaction at 873 K. Other metal species loaded on O-dia did not form carbon under the conditions employed. Fig. 9 shows scanning electron microscope images of: (a) nickel/O-dia catalysts fresh, (b) after the partial oxidation

CO selectivity / %

100 90 80 70

CO2

50

0

1

2

3

4 5 6 7 Time-on-stream / h

8

9

10

Fig. 8. Effect of time-on-stream on the CH4 conversion and CO selectivity over cobalt/O-dia catalyst. Flow rate: 30 ml/min (CH4 /O2 = 5), Catalyst: cobalt (3 wt.%)/O-dia, 60 mg.

for 2 h at 873 K, and (c) 973 K. No specific carbon formation was observed after the reaction at the 973 K as seen in Fig. 9c. On the other hand, on nickel/O-dia catalyst formation of a ‘whisker’-type carbon was observed, as shown in Fig. 8b. Carbon deposition through Boudouard reaction (Reaction 6) is thermodynamically favored below 1173 K [26,27]. However, Reaction 7 is endothermic at this temperature. When CO was introduced instead of CH4 , a small amount of carbon formation was seen for nickel-loaded O-dia catalysis, indicating that the source of carbon during the partial oxidation seems to be CH4 . The fact that no carbon deposition was observed for the cobalt-loaded catalyst indicates that Reaction 7 may not proceed during the partial oxidation as described above. Much higher activity for complete oxidation for cobalt-loaded catalyst suggests that complete oxidation followed by reforming reactions are the reaction pathways.

CH4 conversion / %

60 40

Table 2 TG analyses of O-dia-supported catalysts for CH4 decomposition at 873 K

30

Catalyst

Carbon deposition rate (C ␮mol (g-catalyst/min))

Carbon deposition (C ␮mol)

20

Pd/O-dia Ni/O-dia Co/O-dia Ru/O-dia Ir/O-dia Rh/O-dia Pt/O-dia Fe/O-dia

1318 1290 n.d. n.d. n.d. n.d. n.d. n.d.

1208 1006 n.d. n.d. n.d. n.d. n.d. n.d.

873 K

10

973 K

0 0

1

2

3 4 5 6 7 Time-on-stream / h

8

9

10

Fig. 7. Effect of time-on-stream on the CH4 conversion and CO selectivity over nickel/O-dia catalyst. Flow rate: 30 ml/min (CH4 /O2 = 5); catalyst: nickel (3 wt.%)/O-dia, 60 mg.

Prior to the reaction, catalysts were reduced with H2 at 873 K for 1 h. Flow rate: 30 ml/min (CH4 /Air = 1), reaction time: 1 h.

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973 K in the partial oxidation of methane to synthesis gas (CH4 /O2 = 5). No deactivation of the nickel/O-dia catalyst was observed without carbon deposition during the reaction at 973 K for 10 h. O-dia was not burned off during the CH4 –O2 reaction for 10 h. Cobalt/O-dia catalyst showed similar but slightly lower activity in the partial oxidation of methane. Carbon deposition was observed for nickel/O-dia catalyst at 873 K, but no carbon deposition was seen for cobalt/O-dia catalyst. A different reaction pathway for these catalysts is suggested. These results indicate that O-dia is useful as a novel support material even in oxidative conditions.

Acknowledgements This work was supported by Grant-in-Aid for Scientific Research No. 10555283 from the Japan Society for the Promotion of Science (JSPS). K. Nakagawa is grateful for his fellowship from JSPS for Young Scientists.

References

Fig. 9. Scanning electron microscope images showing the carbon deposited on the surface of the nickel (3 wt.%)/O-dia catalyst. Top: (a) fresh catalyst, center: (b) catalyst after the reaction with CH4 /O2 at 873 K for 2 h, bottom: (c) catalyst after the reaction with CH4 /O2 at 973 K for 2 h.

Detailed studies on the mechanism of O-dia-supported catalyst appear in a following paper. 2CO → C + CO2 ,

0 H873 = −173 kJ/mol

(6)

CH4 → C + 2H2 ,

0 H873 = +100 kJ/mol

(7)

4. Conclusion Nickel/O-dia catalyst afforded high CH4 conversion of 32% into CO and H2 with the selectivities over 75% at

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