In situ studies of oxygen transport mechanisms in Ag/SrFeO3−δ materials for chemical looping catalysis

Abstract

Strontium ferrite (SrFeO_3−δ) impregnated with a noble metal (e.g. Ag) has been applied to catalyse selective oxidation reactions in a chemical looping mode, such as in ethylene epoxidation or ethanol dehydrogenation. The metal oxide donates oxygen to the reaction at the Ag catalyst, and then is re-oxidised in air in a separate step, however the mechanisms of oxygen transport are poorly understood. Here, we investigated the transport of oxygen within Ag/SrFeO_3−δ materials in situ to determine the mechanisms by which Ag improves the redox activity of SrFeO_3−δ. X-ray diffraction under 5 vol% H₂ showed that the Ag nanoparticles decreased the temperature required for phase transformation from perovskite to brownmillerite structure, SrFeO_2.5, from c. 500 °C for bare SrFeO_3−δ to c. 300 °C for Ag/SrFeO_3−δ. Near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) and Raman spectroscopy showed that Ag–O_x surface intermediates enhance the release and uptake of oxygen, confirming that Ag takes an active role in the reduction of Ag/SrFeO_3−δ.