Comparative Study on the Mechanisms of Partial Oxidation of Methane to Syngas Over Rhodium Supported on SiO2 and γ-Al2O3

Abstract

A comparative study on the mechanisms of the partial oxidation of methane (POM) to syngas over SiO2- and γ-Al2O3-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 in 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 CH4 to syngas is the main pathway of the POM reaction over Rh/SiO2 catalysts, while the combustion--reforming mechanism is the dominant pathway of syngas formation over Rh/γ-Al2O3 catalysts. The results of TPR characterization indicate that Rh supported on γ-Al2O3 is more difficult to reduce than Rh supported on SiO2. The IR experiments of CO adsorption over Rh/SiO2 and Rh/γ-Al2O3 after the POM reaction reveal that the surface of the Rh/γ-Al2O3 catalyst contains more partially oxidized rhodium (Rh+) species as compared to the Rh/SiO2 catalyst. These results suggest that the significant difference in the mechanisms of the POM reaction over Rh/SiO2 and Rh/γ-Al2O3 catalysts can be related to the difference in the surface concentration of O2- species over the catalysts under the reaction conditions mainly due to the difference in oxygen affinity of the Rh species on the two supports.