Abstract
Mixing Fe2O3 with Pt/NaMor of similar grain size, followed by grinding in a mortar and calcination in O2, leads to a remarkable enhancement of the reducibility of the Fe2O3 with hydrogen. The TPR profile of such mixtures is virtually identical with that of Fe2O3 onto which Pt was deposited chemically. It is concluded that in the ground and calcined 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% and 1%, TPR profiles are obtained showing two discrete peaks characterizing a Pt promoted and an unpromoted reduction of Fe2O3 respectively. No Pt migration occurs in mixtures of prereduced Pt/NaMor with Fe2O3; this shows that surface migration of P clusters is negligible, but transport of PtO2 either through the gas phase or via the surface is likely. 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 distinctly different from those of the mixtures calcined in O2. TPR also permits discrimination between the promotion of oxide reduction by migrating Pt and ‘true’ hydrogen spillover. The latter phenomenon requires transport of H atoms via protons and electrons and is realized with powder mixtures containing a semiconducting oxide, such as TiO2. Its TPR signature is a broad peak located between those for unpromoted and Pt promoted reduction. Physical mixtures of Fe2O3 and Pt/NaMor catalyze the reduction of acetic acid vapor to acetaldehyde via a Mars–van Krevelen mechanism. In this case Pt migration helps to regenerate oxygen vacancies in the Fe3O4 surface, whereas direct contact of CH3CO2H vapor with Pt results in the formation of methane and higher hydrocarbons. The promoting effect of Pt is not observed after prereduction of Pt/NaMor, because P does not migrate effectively under the conditions used.
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