Chemistry Letters 2001
461
and H2 at 873 K with a small amount of carbon deposition. The
other supported Ni catalysts yielded CO2 and H2O via complete
oxidation of methane. To give high activity for partial oxidation
of methane, metallic Ni species is required.5 On basic metal
oxides or Al2O3, nickel oxide was stabilized and could be
reduced to metallic nickel at lower temperatures only with great
difficulty.5,11 These findings suggest that active species of the Ni
catalyst for partial oxidation seems to be metallic nickel. NiO on
the oxidized diamond might be easily reduced to metallic Ni, due
to a weak interaction between the support and loaded NiO. In the
fresh catalyst, the oxidic nickel species, stabilized on TiO2,
Al2O3, MgO, La2O3, and SiO2 surface, did not exhibit catalytic
activity until it was reduced to metallic form at 923 K.
Figure 2 shows the temperature dependence of the catalytic
activity of a Ni/oxidized diamond catalyst for the partial oxida-
tion of methane. When the temperature was increased from 673
to 973 K, synthesis gas was formed above 873 K. Below 823 K,
only the complete oxidation of methane occurred. The Ni/oxi-
dized diamond catalyst was activated and produced synthesis gas
above temperature of 823 K. Furthermore, carbon deposition
was not observed for Ni/Oxidized diamond catalyst at the reac-
tion temperature of 973 K. Carbon deposition on the Ni/oxidized
diamond catalyst was measured under an isothermal reaction at
973 K with a thermobalance. These results seem to indicate that
the synthesis gas production via the partial oxidation of methane
proceeded basically by means of a two-step path consisting first
of methane combustion to give CO2 and H2O, followed by the
methane reforming with both CO2 and H2O.
The effect of supports on the partial oxidation of methane
seemed to be related to the catalytic activity of the CO2 reforming
reaction.9 CO2 reforming was carried out with oxidized diamond-
supported nickel catalysts, and the relation between the activity in
the partial oxidation and the CO2 reforming was compared. Prior
to the reaction, catalysts were reduced with H2 at 873 K for 1 h.
Table 1 illustrates the results of the CO2 reforming of methane
with a Ni/oxidized diamond catalyst. CH4 and CO2 conversions
were greatly affected by the support of nickel. In all the cases, the
CO2 conversion was higher than that of CH4. The synthesis gas
yields increased with increasing reaction temperatures from 673 to
1073 K. Oxidized diamond support exhibited significant catalytic
activity with a small amount of carbon deposition. In the dehy-
drogenation of ethane over the Cr2O3/oxidized catalyst, the cat-
alytic dehydrogenation of ethane was promoted in the presence of
CO2, as compared with the absence of CO2.2 The significant
effect of the support might be ascribed to the activation of CO2
with metal oxides used as supports. These results indicate that
oxidized diamond is useful as a novel support of catalyst, and sug-
gesting that the surface properties of oxidized diamond have a
potential possibility for producing unique reaction fields in the
catalytic activation of methane to synthesis gas.
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
1
2
3
4
5
6
T. Ando, K. Yamamoto, M. Ishii, M. Kamo, and Y. Sato, J. Chem.
Soc., Faraday Trans., 89, 3635 (1993).
K. Nakagawa, C. Kajita, N. Ikenaga, T. Kobayashi, M. N.- Gamo, T.
Ando, and T. Suzuki, Chem. Lett., 2000, 1100.
A. T. Ashcroft, A. K. Cheetham, J. S. Foord, M. L. H. Green, C. P.
Grey, A. J. Murrell, and P. D. F. Vernon, Nature, 344, 319 (1990).
S. S Bharadwaj and L. D. Schmidt, Fuel Process. Technol., 42, 109
(1995).
S. C. Tsang, J. B. Claridge, and M. L. H. Green, Catal. Today, 23, 3
(1996).
T. Hayakawa, H. Harihara, A. G. Andersen, K. Suzuki, H. Yasuda, T.
Tsunoda, S. Hamakawa, A. P. E. York, Y. S. Yoon, M. Shimizu, and
K. Takehira, Appl. Catal. A, 149, 391 (1997).
7
8
K. Tomishige, Y. Chen, and K. Fujimoto, J. Catal., 181, 91 (1999).
K. Nakagawa, N. Ikenaga, T. Suzuki, T. Kobayashi, and M. Haruta,
Appl. Catal. A, 169, 281 (1998).
9
K. Nakagawa, K. Anzai, N. Matsui, N. Ikenaga, T. Suzuki, Y. Teng,
T. Kobayashi, and M. Haruta, Catal. Lett., 51, 163 (1998).
10 K. Nakagawa, N. Ikenaga, Y. Teng, T. Kobayashi, and T. Suzuki, J.
Catal., 186, 405 (1999).
11 K. Nakagawa, N. Ikenaga, Y. Teng, T. Kobayashi, and T. Suzuki,
Appl. Catal. A, 180, 183 (1999).