Carbon and Oxygen Reaction Pathways of Co_2 Reforming of Methane over Ni/La_2O_3 and Ni/Al_2O_3 Catalysts Studied by Isotopic Tracing Techniques

摘要

Steady-state tracing techniques, using isotopically labeled molecules, were applied to study mechanistic aspects of the carbon and oxygen reaction pathways to form CO over Ni/La_2O_3 and Ni/Al_2O_3 catalysts. Over the Ni/La_2O_3 catalyst, it was found that under steady-state reaction conditions, the quantity of reversibly adsorbed CH_4 and the active carbon-containing intermediate species in the carbon pathway to form Co originating from CH_4 is higher than the respective quantities derived from the Co_2 molecule. Over the Ni/Al_2O_3 catalyst, much smaller quantities of reversibly adsorbed CH_r and active carbon-containing species, originated from the CH_4 molecule, which lead to Co formation were detected. It was also determined that a large quantity of oxygen atoms, originating from the La_2O_3 support of the Ni/La_2O_3 catalyst, participate in the reaction scheme. It is concluded that La_2O_2CO_3, which form by the interaction of La_2O_3 and CO_2, may decompose to produce Co or provide oxygen species which react with carbon accumulated on Ni crystallites due to CH_4 cracking to produce Co. The latter process is very fast over the Ni/La_2O_3 catalyst, as compared to carbon accumulation, and this imparts this catalyst its special stability characteristics.