Reaction Mechanism for Partial Oxidation of Methane to Synthesis Gas over Rh/SiO2 Catalyst

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 O-2 is still available in gas phase. It was found that when a flow of CH4:O-2:Ar = 2:1:45 at 600 degrees 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 degrees C with a contact time less than 1 ms over the catalyst indicate that the formation of CO and H-2 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:O-16(2):(H2O)-O-18:He = 2:1:2:95 over the Rh/SiO2 catalyst further indicated that the (CO)-O-16 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 H-2 and partial oxidation of surface carbon species to CO, is suggested.