Reduction enhancement of Fe2O3 in physical mixtures with Pt/mordenite via Pt migration or ‘hydrogen spillover’

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.