Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al
2O
3 catalysts with Rh loadings between 0.1 and 3 wt%. It was found that the ignition temperature of POM reaction increased with the decreasing of the Rh loadings in the catalysts. For the POM reaction over the catalysts with high (≥1 wt%) Rh loadings, steady-state reactivity was observed. For the reaction over the catalysts with low (≤0.25 wt%) Rh loadings, however, oscillations in CH
4 and reaction products (CO, H
2, and CO
2) were observed. Comparative studies using H
2-TPR, O
2-TPD and high temperature
in situ Raman spectroscopy techniques were carried out in order to elucidate the relation between the redox property of the Rh species in the Rh/Al
2O
3 with different Rh loadings and the performance of the catalysts for the reaction. Three kinds of oxidized rhodium species, i.e. the rhodium oxide species insignificantly affected by the support (RhO
x), that intimately interacting with the Al
2O
3 surface (Rh
iO
x) and the Rh(AlO
2)
y species formed by diffusion of rhodium oxides in to sublayers of Al
2O
3 [C.P. Hwang, C.T. Yeh, Q.M. Zhu, Catal. Today, 51 (1999) 93.], were identified by H
2-TPR and O
2-TPD experiments. Among them, the first two species can be easily reduced by H
2 at temperature below 350 °C, while the last one can only be reduced by H
2 at temperature above 500 °C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhO
x and Rh
iO
x species can be reduced by CH
4 in the reaction mixture. Compared to the Rh/Al
2O
3 with high Rh loadings, the catalysts with low Rh loadings contain more Rh
iO
x species which possess stronger RhO bond strength and are more difficult to be reduced than RhO
x by the reaction mixture. Higher temperature is therefore required to ignite the POM reaction over the catalysts with lower Rh loadings. The oscillation during the POM reaction over the Rh/Al
2O
3 with low Rh loadings can be related to the behaviour of Rh(AlO
2)
y species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction.
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