The concentration of oxygen species over SiO2-supported Rh and Ru catalysts and its relationship with the mechanism of partial oxidation of methane to synthesis gas

Abstract

The partial oxidation of methane (POM) to synthesis gas over SiO2-supported Rh and Ru catalysts was studied by in situ microprobe Raman and in situ time-resolved FTIR (TR-FTIR) spectroscopies. The results of in situ microprobe Raman spectroscopic characterization indicated that no Raman band of Rh2O3 was detected at 500 °C over the Rh/SiO2 catalyst under a flow of a CH4/O2/Ar (2/1/45, molar ratio) mixture, while the Raman bands of RuO2 can even be detected at 600°C over the Ru/SiO2 catalyst under the same atmosphere. The experiments of in situ TR-FTIR spectroscopic characterizations on the reactions of CH4 over O2 pre-treated Rh/SiO2 and Ru/SiO2 catalysts indicated that the products of CH4 oxidation over Rh/SiO2 and Ru/SiO2 greatly depend on the concentration of O2− species over the catalysts. On the catalysts with high concentration of O2−, CH4 will be completely oxidized to CO2. However, if the concentration of O2− species over the catalysts is low enough, CH4 can be selectively converted to CO without the formation of CO2. The parallel experiments using in situ TR-FTIR spectroscopy to monitor the reaction of the CH4/O2/Ar (2/1/45, molar ratio) mixture over Rh/SiO2 and Ru/SiO2 catalysts show that the mechanisms of synthesis gas formation over the two catalysts are quite different. On the Rh/SiO2 catalyst, synthesis gas is mainly formed by the direct oxidation of CH4, while on the Ru/SiO2 catalyst, the dominant pathway of synthesis gas formation is via the sequence of total oxidation of CH4 followed by reforming of unconverted CH4 with CO2 and H2O. The significant difference in the mechanisms of partial oxidation of CH4 to synthesis gas over Rh/SiO2 and Ru/SiO2 catalysts can be well related to the difference in the concentration of O2− species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the two metals.