Promoting Effect of Supports with Oxygen Vacancies as Extrinsic Defects on the Reduction of Iron Oxide
Javier C. Mora, Yoni C. M. Nederstigt, Josephine M. Hill, Sathish Ponnurangam
Abstract
The utilization of substrates that contain oxygen vacancies represents an effective approach to improve the reducibility of supported metal oxides and, thereby, increase the activity of metallic catalysts and the performance of oxygen carriers. However, the influence of different types of oxygen vacancies of the substrates on the reduction kinetics of supported metal oxides is not well understood. Here this influence was analyzed for supported iron oxide using temperature-programmed reduction (H2-TPR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). Kinetic parameters obtained by deconvolution of TPR profiles using a novel method indicate that supports containing oxygen vacancies as extrinsic defects (BaZr0.8Ce0.1Y0.1O3−δ and yttria-stabilized zirconia) promote the reduction of iron oxide with activation energies lower than for the reduction of pure iron oxide. The reaction mechanism is successfully described by a nucleation and nuclei growth model (Avrami–Erofeev) in a combined two- and three-step process with the formation of metallic iron at low temperatures (∼400 °C). In contrast, supports with oxygen vacancies as intrinsic defects (CeO2 and TiO2) inhibit the reduction due to complex kinetics associated with strong metal–support interactions. The mechanism by which oxygen vacancies affect the reducibility of iron oxides is rationalized in terms of different electronic/metal–support interactions.