TRANSIENT RESPONSE OF CATALYST BED TEMPERATURE
413
observed. However, an endothermic response was detected catalysts exhibited a high catalytic activity in the decom-
only at the front edge of the catalyst bed over Rh/TiO2 and position of CH4 to give hydrogen and deposited carbon or
Rh/Al2O3 catalysts. Therefore, in the high methane con- CHx, even in the presence of oxygen. The temperature at
centration, compared to the oxidation of methane, decom- the front edge of the catalyst bed decreased upon introduc-
position of methane to give carbon or CHx would easily tion of a CH4/O2 pulse, and an increase in the temperature
proceed at the front edge of the catalyst bed over Rh/TiO2 at the rear end was observed, indicating that H2 formation
and Rh/Al2O3 catalyst.
and deposition of carbon probably took place via decompo-
Boucouvalas et al. (12, 13) reported that synthesis gas sition of CH4, and then deposited carbon or CHx generated
over Ru/TiO2 catalyst was, to a large extent, formed via the on the Rh surface was oxidized into COx. In the case of Rh-
direct partial oxidation scheme. CO and CO2 were proba- loaded catalysts, the reaction pathway depended strongly
bly formed by parallel routes via two different sites of the on the support materials.
catalyst.
When Ru/TiO2 catalyst was used, a sudden rise in the
temperature at the front edge of the catalyst bed was ob-
ACKNOWLEDGMENTS
served upon introduction of the 3-mL pulse of CH4 and O2.
This work was supported by a Grant-in-Aid on Priority Areas
However, the temperature drop at the front edge of the (09218255) from the Ministry of Education, Science, Sports, and Culture
of Japan. K. Nakagawa is grateful for his Research Fellowship from Japan
Society for the Promotion of Science (JSPS) for Young Scientists.
catalyst bed was observed for 1-mL pulses of CH4 and O2.
These results seemed to exhibit two possibilities of synthe-
sis gas formation routes over Ru/TiO2 catalyst. Catalytic
pathways depended on the reaction conditions such as con-
centrations of the reactant and flow rates.
REFERENCES
1
2
. Tsang, S. C., Claridge, J. B., and Green, M. L. H., Catal. Today 23, 3
The slight temperature increase of the rear edge of the
catalyst bed on Ir/TiO2 catalyst might be ascribed to the heat
conduction from the front and middle part of the catalyst
bed. Synthesis gas formation over Ir/TiO2 catalyst would
proceed basically via the two-step path. Methane combus-
tion would occur in the front edge of the catalyst bed. Since
a large amount of unreacted CO2 was observed, the total
heat of the reaction would be exothermic.
(
1996).
. Prettre, M., Eichner, CH., and Perrin, M., Trans. Faraday Soc. 43,
35 (1946).
3. Dissanayake, D., Rosynek, M. P., Kharas, K. C. C., and Lunsford, J. H.,
J. Catal. 132, 117 (1991).
. Vermeiren, W. J. M., Blomsma, E., and Jacobs, P. A., Catal. Today 13,
27 (1992).
. Nakamura, J., Umeda, S., Kubushiro, K., Kunimori, K., and Uchijima,
3
4
5
4
T., Sekiyu. Gakkaishi 36, 97 (1993).
On the other hand, when Rh/TiO2 and Rh/Al2O3 cata-
lysts were used, the temperature increase of the rear edge of
the catalyst bed might be caused by oxidation of unreacted
methane, CHx, H2 which were made from the decomposi-
tion of methane or oxidation of reduced Rh metal could be
possible due to a high concentration of oxygen. Probably,
hydrogen was produced mainly by decomposition of CH4
on the front edge of the catalyst bed, and then generated
H2 and CHx species were oxidized into H2O and COx in the
Rh/TiO2 and Al2O3 catalysts.
6. Nakagawa, K., Ikenaga, N., Suzuki, T., Kobayashi, T., and Haruta, M.,
Appl. Catal. A 169, 281 (1998).
7
. Nakagawa, K., Anzai, K., Matsui, N., Ikenaga, N., Suzuki, T.,
Kobayashi, T., Teng, Y., and Haruta, M., Catal. Lett. 51, 163 (1998).
. Hickman, D. A., and Schmidt, L. D., J. Catal. 138, 267 (1992).
. Hickman, D. A., Haupfear, E. A., and Schmidt, L. D., Catal. Lett. 17,
223 (1993).
8
9
1
1
0. Hickman, D. A., and Schmidt, L. D., Science 259, 343 (1993).
1. Bharadwaj, S. S., and Schmidt, L. D., Fuel Proc. Technol. 42, 109
(
1995).
2. Boucouvalas, Y., Zhang, Z., and Verykios, X. E., Catal. Lett. 40, 189
1996).
3. Boucouvalas, Y., Zhang, Z. L., Efstathiou, A. M., and Verykios, X. E.,
Stud. Surf. Sci. Catal. 101, 443 (1996).
1
1
1
1
1
1
(
5
. CONCLUSION
4. Buyevskaya, O. V., Wolf, D., and Baerns, M., Catal. Lett. 29, 249
(
1994).
5. Buyevskaya, O. V., Walter, K., Wolf, D., and Baerns, M., Catal. Lett.
8, 81 (1996).
A pulsed reaction technique of measuring the tempera-
ture jump of the catalyst bed afforded strong support that
synthesis gas was formed via a two-step reaction pathway,
which consisted ofmethane complete oxidation to give H2O
and CO2, followed by the reforming of methane with steam
and CO2 for Ir/TiO2 and Rh/SiO2 catalysts.
3
6. Tsipouriari, V. A., Efstathiou, A. M., and Verykios, X. E., J. Catal. 161,
31 (1996).
7. Rostrup-Nilsen, J. R., in “Catalysis, Science and Tecnology” (J. R.
Anderson and M. Boudart, Eds.), Vol. 5, pp. 1–117. Springer, Berlin,
1984.
On the other hand, when Rh/TiO2 and Rh/Al2O3 cata-
lysts were used, a different reaction pathway for synthe-
1
8. Claridge, J. B., Green, M. L. H., Tsang, S. C., York, A. P. E., Ashcroft,
A. T., and Battle, P. D., Catal. Lett. 22, 299 (1993).
sis gas formation can be proposed. Rh/TiO2 and Rh/Al2O3 19. Trimm, D. L., Catal. Rev. Sci. Eng. Chem. 16, 155 (1987).