Abstract: The partial oxidation of methane (POM) for the production of synthesis gas over Rh/SiO₂ catalyst was investigated by in situ Raman spectroscopy characterization, continuous flowing and pulse reaction evaluation focusing on the reaction mechanism of synthesis gas formation in the oxidation zone, i.e., the catalyst zone where O₂ is still available in gas phase. It was found that when a flow of CH₄:O₂:Ar = 2:1:45 at 600 °C was passed through the pre-reduced 4% Rh/SiO₂ catalyst, no bands associated with rhodium oxide could be detected on the catalyst by Raman spectroscopy. While Raman bands related to carbon species that originated from methane dissociation could be detected at the catalyst oxidation zone under working conditions. The results of pulse reaction of POM as well as steam reforming and CO₂ reforming of methane at 700 °C with a contact time less than 1 ms over the catalyst indicate that the formation of CO and H₂ is mainly resulted from the direct partial oxidation of CH₄ while the steam reforming and CO₂ reforming reactions play only a minor role in the oxidation zone of the catalyst bed. The pulse reaction with an isotopic gas mixture of CH₄:¹⁶O₂:H₂¹⁸O:He = 2:1:2:95 over the Rh/SiO₂ catalyst further indicated that the C¹⁶O percentage was higher than 92% of the total CO produced in the reaction. Based on these results, the conclusion, that the major reaction responsible for synthesis gas formation in the oxidation zone of Rh/SiO₂ catalyst bed is the pyrolysis of methane on reduced rhodium sites to form hydrogen and carbon adspecies followed by the coupling of two surface hydrogen atoms to H₂ and partial oxidation of surface carbon species to CO, is suggested.

Key words: rhodium, partial oxidation of methane, synthesis gas, pulse reaction, in situ Raman spectroscopy, isotope tracing