Comparative study on the mechanisms of partial oxidation of methane to syngas over rhodium supported on SiO_2 and gamma-Al_2O_3

摘要

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO_2- and gamma-Al_2O_3-supported Rh catalysts was carried out using in situ time-resolved FTIR spectroscopy to follow the primary products of POM reaction over the catalysts. Experiments of catalytic performance evaluation and temperature-programmed reduction (TPR) characterization of the catalysts, as well as the situ FTIR spectroscopic study using CO to probe the oxidation state of Rh species over the catalysts were performed. It was found that the direct oxidation of CH_4 to syngas is the main pathway of the POM reaction over Rh/SiO_2 catalysts, while the combustion-reforming mechanism is the dominant pathway of syngas formation over Rh/gamma-Al_2O_3 catalysts. The results of TPR characterization indicate that Rh supported on gamma-Al_2O_3 is more difficult to reduce than Rh supported on SiO_2. The IR experiments of CO adsorption over Rh/SiO_2 and Rh/gamma-Al_2O_3 after the POM reaction reveal that the surface of the Rh/gamma-Al_2O_3 catalyst contains more partially oxidized rhodium (Rh~+) species as compared to the Rh/SiO_2 catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO_2 and Rh/gamma-Al_2O_3 catalysts can be related to the difference in the surface concentration of O~(2-) species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.