Effect of Rh loading on the performance of Rh/Al2O3 for methane partial oxidation to synthesis gas

Catalytic performance for partial oxidation of methane (POM) to synthesis gas was studied over the Rh/Al2O3 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 CH4 and reaction products (CO, H-2, and CO2) 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/Al2O3 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 (RhOx), that intimately interacting with the Al2O3 surface ((RhOx)-O-i) and the Rh(AlO2)(y) species formed by diffusion of rhodium oxides in to sublayers of Al2O3 [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 H2 at temperature below 350 degrees C, while the last one can only be reduced by H2 at temperature above 500 degrees C. The ignition temperatures of POM reaction over the catalysts are closely related to the temperature at which most of the RhOx and (RhOx)-O-i species can be reduced by CH4 in the reaction mixture. Compared to the Rh/Al2O3 with high Rh loadings, the catalysts with low Rh loadings contain more (RhOx)-O-i species which possess stronger Rh-O bond strength and are more difficult to be reduced than RhOx 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/Al2O3 with low Rh loadings can be related to the behaviour of Rh(AlO2)(y) species in the catalyst switching cyclically from the oxidized state to the reduced state during the reaction. (c) 2007 Elsevier B.V. All rights reserved.