| The present work focuses on the studies of the mechanism of the partial oxidation of methane (POM) to synthesis gas over SiO2-supported Rh, Ru, Ir catalysts and the effect of calcination temperatures on the catalytic performance of Rh/Al2O3 for POM to synthesis gas. The catalysts were characterized by N2 adsorption at low temperature, H2 (or O2,CO) chemisorption, XRD, XPS and temperature-programmed techniques (H2-TPR, O2-TPD, TPSR). Catalytic performance of the catalysts for POM to synthesis gas was also investigated in detail. Comparative studies using in situ time-resolved FTIR spectroscopy, in situ microprobe Raman spectroscopy, pulsed reactions of CH4, temporal analysis of products techniques have been carried on the noble metal catalysts in order to elucidate the primary products of POM, the nature of active species under reaction atmosphere, deposited carbon species and oxygen affinity of the catalysts. Over H2-reduced Rh/SiO2 catalyst, CO is the primary product of the POM reaction, and direct oxidation of CH4 to synthesis gas is the dominant pathway for the POM reaction. Unlike Rh/SiO2, over H2-reduced Ru/SiO2 catalyst, CO2 is the primary product of POM reaction, and the dominant reaction scheme of the POM to synthesis gas is via combustion-reforming mechanism. CO is the primary product of POM reaction over the fresh H2-reduced Ir/SiO2 catalyst, while the mechanism of CO formation of CO under the steady state reaction conditions may be different from that on the fresh catalyst. No Raman band of metal oxide species are detected on the SiO2-supported catalysts under the POM reaction conditions, however Raman band of carbon species can be observed over Rh/SiO2 and Ir/SiO2 catalysts. These results clearly indicated that most of the surface active species are in the metallic state under the POM reaction conditions. The results of pulsed reactions of CH4 in which He containing a trace amount of O2 was used as carrier gas indicated that Ru species is more easily oxidized than Rh and Ir species under the same reaction conditions. These results suggested that the significant difference in the mechanism of the POM reaction over SiO2-supported Rh, Ru and Ir catalysts can be related to the difference in the surface concentration of O2- species over the catalysts under the reaction conditions. The primary product of the POM reaction over Rh/Al2O3 is closely related to the calcination temperatures of the catalysts. CO is the primary product over Rh/Al2O3 calcined at 600℃, while CO2 is the primary product over Rh/Al2O3 calcined at 900℃. Based on the results of TPR and XPS characterizations, the difference in the POM performance of the Rh/Al2O3 catalysts calcined at 600 and 900℃may have resulted from the formation of Rh species of different redox property, which affects the concentration of O2- species on the surface of the catalysts under the reaction conditions.
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