Enhancing the capacity of oxygen carriers for selective oxidations through phase cooperation: bismuth oxide and ceria–zirconia

Abstract

Oxygen carriers are a class of materials, typically solid oxides, that can reversibly store and release oxygen for a variety of applications in energy and chemical processes, e.g. chemical looping combustion (CLC) and chemical looping air separation (CLAS). In recent years, growing interest in these materials have been focused on their use in chemical looping selective oxidations. A method for enhancing the oxygen-carrying capacity of oxygen carriers for use in selective oxidations is presented. In this approach, one material that is selective and active in the reaction is deposited on the surface of a second material acting as a reservoir of oxygen and as a support. Here, the approach has been investigated using the selective combustion of hydrogen in the presence of ethylene, an important step in the oxidative dehydrogenation of ethane. Bismuth oxides, supported on a range of ceria–zirconia materials, were made into particulate oxygen carriers and studied for their activity, selectivity and oxygen-storage capacity. STEM-EDS imaging showed that the bismuth phase was spread uniformly over the surface of the nanocrystalline particles. XPS measurements indicated that the surface was enriched with bismuth oxide. It was found that the presence of ceria in the support supplied lattice oxygen additional to that provided by the bismuth oxide, without affecting the selectivity of bismuth oxide towards the combustion of H₂. In other words, the surface chemistry was decoupled from the bulk properties of the support, thus simplifying the design and formulation of selective oxygen carriers. This demonstrates a readily-applicable generic approach for the design of oxygen carriers for selective oxidations.