Metal Migration from Xeolites onto lron Oxide: An Alternative to Hydrogen Spillover

摘要

The reduction of physical mixtures of Fe_2O_3 with transition metal-containing zeolites was studied. Two zeolites (Na-mordenite and Na-ZSM-5), each loaded with one of four metals (Pd, Rh, Co, Ni), were mixed with Fe_2O_3, ground, and subjected to various treatments. Temperature-programmed reduction of these mixtures was compared with that of Fe_2O_3 onto which the transition metals had been deposited by wet impregnation. The extent by which the reduction of Fe_2O_3 is enhanced in the mixtures depends on the pretreatment and the nature of the metal, while the zeolite structure has little effect. Reduced metal clusters do not show observable migration. Crucial is the migration of metal oxide clusters out of the zeolite onto Fe_2O_3. Mobility of PtO_2 is high on zeolite walls and the Fe_2O_3 surface. PdO is rather immobile in the zeolite. Rhodium oxides. Rh_2O_3 or RhO_2, migrate through zeolite channels but appear rather immobile on the surface of Fe_2O_3. The arrival of Pt and Rh on Fe_2O_3 has been detected by transmission electron microscopy and X-ray energy dispersive spectrometry. On reduction of the oxide particles on the Fe_2O_3 surface at low temperature, H spillover and significant reduction enhancement of Fe_2O_3 take place. No H spillover through zeolite channels has been detected.