Effects of calcination temperatures on the catalytic performance of Rh/Al2O3 for methane partial oxidation to synthesis gas

Comparative studies using high temperature in situ Raman spectroscopy, TPSR, pulsed reactions of CH4, H2-TPR and O2-TPD techniques were carried out on Rh/Al2O3 calcined at 600 and 900 °C to elucidate the effect of calcination temperatures on the nature of the Rh species in the catalysts and its relationship with the catalytic performance of Rh/Al2O3 for partial oxidation of methane (POM) to synthesis gas. The results indicate that significant difference in the POM performance of Rh/Al2O3 catalysts calcined at 600 and 900 °C can be attributed to the formation of rhodium species of different nature after the samples were calcined in the air at above temperatures. Calcining a Rh/Al2O3 sample in the air at 900 °C caused not only the formation of more than 81% of rhodium species irreducible by H2 at the temperature below 600 °C, but also affected the redox property of the RhOx species reducible at the temperature below 250 °C. Compared to the rhodium species in the Rh/Al2O3 calcined at 600 °C, those in the catalyst calcined at 900 °C possesses stronger Rh–O bonds and higher oxygen affinity. These factors would affect the concentration of oxygen species on the two catalysts under the POM reaction conditions and consequently affect the primary products of the reaction. The results of TPSR and in situ Raman characterizations on the POM reaction over the catalysts indicate that, Rh/Al2O3 in fully oxidized form are mainly responsible for complete oxidation CH4 to CO2 and H2O. The catalysts abruptly changed its oxidation state at the temperature when POM reaction starts. The results of high temperature Raman characterizations also suggest that a rhodium species in which Rh may substitute for the Al site of Al2O3 was formed in the Rh/Al2O3 sample calcined in air at 900 °C. This rhodium species is difficult to be reduced to Rh metal by H2 at temperature below 600 °C.