REDUCTION ENHANCEMENT OF FE2O3 IN PHYSICAL MIXTURES WITH PT MORDENITEVIA PT MIGRATION OR HYDROGEN SPILLOVER/

Mixing Fe2O3 with Pt/NaMor of similar grain sizer followed by grinding in a mortar and calcination in O-2, leads to a remarkable enhancement of the reducibility of the Fe2O3 with hydrogen. The TPR profile of su ch mixtures is virtually identical with that of Fe2O3 onto which Pt wa s deposited chemically. It is concluded that in the ground and calcine d mixtures Pt migration from the zeolite to the iron oxide is crucial. Upon varying the amount of deposited Pt in Pt/Fe2O3 between 0.001% an d 1%, TPR profiles are obtained showing two discrete peaks characteriz ing a Pt promoted and an unpromoted reduction of Fe2O3 respectively. N o Pt migration occurs in mixtures of prereduced Pt/NaMor with Fe2O3; t his shows that surface migration of Pt-0 clusters is negligible, but t ransport of PtO2 either through the gas phase or via the surface is li kely. Pt migration is also detectable at room temperature in mixtures stored for weeks in a moist atmosphere; in this case the data suggest surface migration of hydrated Pt2+ ions; the TPR profiles are distinct ly different from those of the mixtures calcined in O-2. TPR also perm its discrimination between the promotion of oxide reduction by migrati ng Pt and 'true' hydrogen spillover. The latter phenomenon requires tr ansport of H atoms via protons and electrons and is realized with powd er mixtures containing a semiconducting oxide, such as TiO2. Its TPR s ignature is a broad peak located between those for unpromoted and Pt p romoted reduction. Physical mixtures of Fe2O3 and Pt/NaMor catalyze th e reduction of acetic acid vapor to acetaldehyde via a Mars-van Krevel en mechanism. In this case Pt migration helps to regenerate oxygen vac ancies in the Fe2O3 surface, whereas direct contact of CH3CO2H vapor w ith Pt results in the formation of methane and higher hydrocarbons. Th e promoting effect of Pt is not observed after prereduction of Pt/NaMo r, because Pt-0 does not migrate effectively under the conditions used .