Partial oxidation of methane to syngas (POM) over Rh/SiO2 and Ru/SiO2 catalysts was investigated at 450-800 degreesC. Methane conversion and selectivity to both H-2 and CO were higher over Rh/SiO2 than over Ru/SiO2. The CO and H-2 selectivities substantially decreased with increasing GHSV over Ru/SiO2 at 500 degreesC, while these selectivities remained nearly constant over Rh/SiO2. Both the CH4 conversions and the CO and H-2 selectivities increased slightly over Rh/SiO2 at 700 degreesC as GHSV increases, while the CH4 conversions and the CO and H-2 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 degreesC. 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 degreesC 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. (C) 2004 Elsevier Inc. All rights reserved.