Thermodynamic and Catalytic Properties of ALD-Grown Fe2O3 and CeFeO3 Thin Films on High Surface Area Supports

Redox active materials like Fe₂O₃ are used as catalysts for a range of reactions such as selective oxidation, water gas shift, and oxidative dehydrogenation, just to name a few. They have also recently gained interest as oxygen carriers for chemical looping (CL) reactor schemes, such as CL H₂ production in which the oxide oxygen carrier is reduced at high temperature in one reactor and then re-oxidized with water, which also produces H₂, in a separate reactor. Since it is difficult to maintain high surface area for many redox active oxides, including iron oxide, the use of thin films on a separate support could potentially lead to more active systems. Confining the oxide film to two-dimensions also affects its thermodynamic redox properties which could in principle be used to tailor activity for specific reactions. In this talk we will present our recent results on using Atomic Layer Deposition (ALD) to grow conformal ultrathin (<2 nm) films of Fe₂O₃ and CeFeO₃ on a high surface area g‑Al₂O₃ support. The reaction engineering challenges involved in using ALD to grow oxide films on a high surface area material and how they can be overcome will first be discussed. We will then present thermodynamic data that shows how the heat of reduction of Fe₂O₃ varies as a function of film thickness and how this affects its reactivity for the water gas shift reaction. Finally, we will present results for the ALD growth of thin films of CeFeO₃, a material which is difficult to synthesize in bulk form. This material has a rich phase behavior in which both the Ce and Fe cations are redox active making it of interest as both a catalyst and as a support for metal catalysts.