Oxidative Reforming of Methane over Rh-Containing Zeolites: Active Species and Role of Zeolite Framework

Rh ion-exchanged Y zeolite (Rh-Y) and amorphous silica-alumina (Rh-ASA) were prepared by the ion-exchange method. The Rh species were identified by UV-vis and IR spectroscopies, TEM, and H₂-TPR measurements. Isolated Rh cations were preferentially formed on the Y zeolite, while Rh oxide mainly existed on ASA. The catalytic activities of the prepared samples for oxidative reforming of methane were evaluated. The Rh oxide on Rh-ASA exhibited slightly higher methane conversion and CO yield than those of the isolated Rh cation on Rh-Y. On the other hand, the catalytic lifetime over Rh-Y was longer than that over Rh-ASA because the isolated Rh cations were protected from aggregation by electrostatic interaction with the zeolite framework. In addition, Rh-Y exhibited a high catalytic activity even at low Rh contents owing to its high dispersibility. On the basis of the experimental facts, we have successfully clarified the active Rh species for this oxidative reforming of methane and demonstrated the effectiveness of the zeolite framework for the stabilization of the active species. This article provides the important concept for the design of highly active catalysts in this catalytic system.