Partial oxidation of methane to H2 and CO over Rh/SiO2 and Ru/SiO2 catalysts

Partial oxidation of methane to syngas (POM) over Rh/SiO2 and Ru/SiO2 catalysts was investigated at 450–800 °C. Methane conversion and selectivity to both H2 and CO were higher over Rh/SiO2 than over Ru/SiO2. The CO and H2 selectivities substantially decreased with increasing GHSV over Ru/SiO2 at 500 °C, while these selectivities remained nearly constant over Rh/SiO2. Both the CH4 conversions and the CO and H2 selectivities increased slightly over Rh/SiO2 at 700 °C as GHSV increases, while the CH4 conversions and the CO and H2 selectivities decreased slightly over Ru/SiO2. Pulse reactions, transient reactions, and in situ microprobe Raman techniques were employed to investigate the oxidation of methane over Rh/SiO2 and Ru/SiO2 catalysts. CO was the main product when Rh/SiO2 was exposed to methane pulsing at 700 °C. No CO2 was detected during the first pulse over the Rh/SiO2 catalysts. However, CO2 was formed in every pulse over Ru/SiO2. Transient results showed that CO was formed prior to CO2 generation over Rh/SiO2 catalysts. CO2 was the primary product over Ru/SiO2 catalysts. In situ microprobe Raman spectroscopy at 450–600 °C demonstrated that Ru/SiO2 surfaces contained significantly larger amounts of metal oxide species than Rh/SiO2 during the POM reaction. The mechanisms of POM over the two catalysts are different. A direct oxidation process mainly occurs over the Rh/SiO2 catalyst, while the dominant pathway over the Ru/SiO2 catalyst is the indirect oxidation process.