H SPILLOVER ON NiO/CeO2–ZrO2 AND CH4 OXIDATION
23
TABLE 3
large surface area CeO2–ZrO2 solid solution did not change
the structure, and the reduced solid solution was easily re-
oxidized, even at room temperature. The NiO loading low-
ered the reduction temperature of the solid solution, and
reduction started in the presence of NiO species. Apparent
hydrogen spillover effect was suggested for NiO particles,
although a small amount of reduced NiO sites, which could
not be detected, might possibly show hydrogen spillover ef-
fect. The catalytic activities of the NiO catalyst supported
on the solid solution for the partial oxidation reaction of
methane increased with an increase in the oxygen storage
capacity of the support, which indicates that this reaction
proceeded by means of the redox mechanism. Ni particles
having a weak interaction with the support had a metal-
lic feature and produced large amounts of coke (carbon
nanotubes). On the other hand, the Ni particles having a
strong interaction had a cationic nature and produced a
small amount of coke.
The Amounts of Coke Produced during the Reaction and the
Results of XPS Measurement of the Catalyst
Amount of Coke
(mg/100 mg
of catal.)
Ratio of peak
intensitya I856eV
(I856eV + I
/
Catalyst
)
852eV
NiO(12.4)/ZrO2(GT)
45.32
4.53
26.51
33.59
0.37
0.50
0.43
0.44
NiO(12.4)/Ce0.07Zr0.93O2(GT)
NiO(12.4)/Ce0.13Zr0.87O2(GT)
NiO(12.4)/Ce0.25Zr0.75O2(GT)
Note. (GT) samples prepared by glycothermal method.
a Reduction was carried out at 673 K for 30 min; Binding energy of the
Ni 2p2/3 electron of NiO was 853.9 eV with a shoulder peak at 856 eV,
while that of Ni was 852.7 eV.
853 eV, and intensity of the 856-eV peak indicate that only
NiO species were present in this sample. This result is in
good accordance with the results of the FTIR spectrum of
adsorbed CO, where the presence of the cationic Ni species
was suggested.
The amounts of coke formed by the partial oxidation of
methane are also given in Table 3. Of all the catalysts exam-
ined, NiO(12.4)/Ce0.07Zr0.93O2(GT) produced the smallest
amount of coke during partial oxidation of methane. This
result suggests that coke formation is closely connected to
the oxidation state of the surface of the Ni species.
Figure 10 shows the TEM images of the coke formed by
the reaction. The dark particle in the lower photograph in
Fig. 10a was the Ni species encapsulated in the closed end of
a nanotube, suggesting that carbon nanotubes are formed
by the “tip growth” mechanism (28).
The significant heights of the TPR reduction peaks of
NiO supported on ZrO2(GT) and Ce0.25Zr0.75O2(GT) im-
ply that the NiO particles were reduced rapidly. In con-
trast, the medium height of the TPR reduction peak of NiO
supported on Ce0.07Zr0.93O2(GT) means that the NiO parti-
cles were reduced relatively slowly, suggesting that the NiO
particles had strong interaction with the support. This result
also is in good agreement with the data obtained from FTIR
of adsorbed CO and XPS measurement since these two
techniques suggested the presence of cationic Ni species.
Because of the strong interaction with the support, the Ni
particles cannot be “lifted off,” and therefore the catalyst
produced a relatively small amount of coke.
ACKNOWLEDGMENT
This work was partly supported by a Grant-in-Aid for Encouragement
of Young Scientists from the Ministry of Education, Science, Sports and
Culture, Japan (No. 11750674).
REFERENCES
1. Taylor, K. C., Catal. Rev.-Sci. Eng. 35, 457 (1993).
2. Summers, J. C., and Ausen, S. A., J. Catal. 58, 131 (1979).
3. Yao, H. C., and Yu Yao, Y. F., J. Catal. 86, 254 (1984).
4. Dissanayake, D., Rosynek, M. P., Kharas, K. C. C., and Lunsford, J.
H., J. Catal. 132, 117 (1991).
5. Tang, S., Lin, J., and Tan, K. L., Catal. Lett. 51, 169 (1998).
6. Diskin, A. M., Cunningham, R. H., and Ormerod, R. M., Catal. Today
46, 147 (1998).
7. Inui, T., Saigo, K., Ichino, K., Takami, T., Takeguchi, T., Iwamoto, S.,
and Fujii, Y., Ceram. Trans. 73, 39 (1997).
8. Inui, T., Ichino, K., Matsuoka, I., Takeguchi, T., Iwamoto, S., Pu, S.,
and Nishimoto, S., Korean J. Chem. Eng. 14, 441 (1997).
9. Fornaasiero, P., Di Monte, R., Rao, G. R., Kas˘par, J., Meriani, S.,
Trovarelli, A., and Graziani, M., J. Catal. 151, 168 (1995).
10. Fornaasiero, P., Balducci, G., Di Monte, R., Kas˘par, J., Sergo, V.,
Gubitosa, G., Ferrero, A., and Graziani, M., J. Catal. 164, 173
(1996).
11. Fornaasiero, P., Kas˘par, J., Sergo, V., and Graziani, M., J. Catal. 182,
56 (1999).
12. Vlaic, G., Di Monte, R., Fornaasiero, P., Fonda, E., Kas˘par, J., and
Graziani, M., J. Catal. 182, 378 (1999).
13. Fornaasiero, P., Fonda, E., Di Monte, R, Vlaic, G., Kas˘par, J., and
Graziani, M., J. Catal. 187, 177 (1999).
14. Fornaasiero, P., Hickey, N., Kas˘par, J., Dossi, C., Gava, D., and
Graziani, M., J. Catal. 189, 326 (2000).
15. Fornaasiero, P., Hickey, N., Kas˘par, J., Montini, T., and Graziani, M.,
J. Catal. 189, 339 (2000).
4. CONCLUSION
16. Hoang, H.-L., Berndt, H., and Lieske, H., Catal. Lett. 31, 165 (1995).
17. Shishido, T., and Hattori, H., Appl. Catal. A 146, 157 (1996).
18. Inoue, M., Sato, K., Nakamura, T., and Inui, T., Catal. Lett. 65, 79
(2000).
19. Inoue, M., Kominami, H., and Inui, T., Appl. Catal. A 97, L25
(1993).
Surface reduction of the large surface area CeO2–ZrO2
solid solutions prepared by the glycothermal method oc-
curred at 623–773 K, which was much lower than that
observed for the solid solution prepared by the conven-
tional method. Reduction and re-oxidation cycles of the 20. Sermon P. Q., and Bond, G. C., Catal. Rev. 8, 211 (1973).