Catalytic partial oxidation of methane and isotopic oxygen exchange reactions over 18O labeled Rh/Gadolinium doped ceria

18O labeled catalysts, Rh/(Ce0.91Gd0.09)O2−x (Rh/GDC10) and Rh/γ-Al2O3 (Rh/ALU) were used to study the catalytic partial oxidation of methane (CPOM) and oxygen isotopic exchange reactions. During the CPOM tests, higher C18O than C16O concentrations were observed over 18O labeled Rh/GDC10 than Rh/ALU, which is explained by the higher oxygen storage capacity and oxygen mobility of the former catalyst. Similarly, Rh/GDC10 showed higher oxygen exchange rates than Rh/ALU during the isotopic exchange experiments. The oxygen exchange between the gas phase and the solid is limited by the oxygen mobility in/on the catalyst. This catalytic behavior is due to the fact that ceria has two stable oxidation states, Ce3+ and Ce4+ and the addition of Gd3+ to ceria lattice enhanced the oxygen mobility by the creation of oxygen vacancies. These higher oxygen exchange rates also correlate to higher concentrations of C18O than C16O during the CPOM experiments. Pulse experiments suggest that the reaction mechanism for the CPOM on Rh/GDC10 occurred through a mixed (direct and indirect) mechanism. The direct mechanism assumes that H2 and CO are primary reaction products formed in the oxidation zone at the catalyst entrance. Thus, CO formed from the reaction between lattice oxygen in Rh/GDC10 and adsorbed atomic carbon. CO2 is formed through an indirect mechanism, where CH4 reacts with O2 to form CO2 and H2O. CO forms through the reactions between 1) CO2 and CH4 and 2) CH4 and H2O.