Interactions between catalysts and oxygen carriers in chemical looping processes
This Dissertation considers the fundamentals and applications of materials composed of a metal catalyst supported on a reactive metal oxide, applied for chemical looping processes, using Ag/SrFeO3−δ as a model system. In chemical looping processes, lattice oxygen is provided from a metal oxide (termed the oxygen carrier) to generate purified oxygen, or, to react with a reducing gas (e.g. a hydrocarbon) to form oxygenated products, in the absence of gaseous oxygen. The oxygen carrier is then re-oxidised in air, in a separate step. Chemical looping has been investigated for a variety of selective oxidation reactions, either with the metal oxide oxygen carrier acting as both an oxygen reservoir and a catalyst for selective oxidation, or, by preparing particles composed of a metal catalyst supported on an oxygen carrier. During reduction, oxygen is transported from the lattice of the metal oxide to the surface of the metal catalyst. However, the interactions between the catalyst and the oxygen carrier, and the pathways taken by oxygen, are poorly understood, and hence form the subject of this Dissertation. To understand the influence of Ag on oxygen availability from SrFeO3−δ, kinetic studies were performed by measuring the rate of oxygen release and re-uptake from SrFeO3−δ with and without Ag (15 wt%) by reducing and oxidising the material in N2 and O2 (pO2 = 0.21 or 0.05 atm) in a packed bed. The presence of Ag increased the amount of oxygen released per redox cycle by more than three-fold at 500°C. From thermogravimetric experiments, interactions between Ag and SrFeO3−δ were shown to influence the thermodynamics of the material, with Ag increasing the concentration of oxygen vacancies at equilibrium, and hence lowering the oxygen stoichiometry under all investigated oxygen partial pressures (pO2 = 10−5−0.21 atm). Characterisation studies under simulated reaction conditions were performed to understand the changes in bulk and surface properties of Ag/SrFeO3−δ during chemical looping. In-situ X-ray diffraction under 5 vol% H2 showed that the presence of Ag allowed phase transformation of perovskite SrFeO3−δ to brownmillerite SrFeO2.5 at 300°C, as compared to 500°C for bare SrFeO3−δ. Near-ambient pressure X-ray photoelectron spectroscopy measurements under reduction and oxidation showed that oxygen removal from Ag/SrFeO3−δ was enhanced by the formation of reactive Ag-Ox surface species. Pathways for oxygen transport from the SrFeO3−δ to the surface of Ag were proposed, based on in-situ Raman spectroscopy and H2-TPR measurements. To further understand the mechanisms of oxygen transport in Ag/SrFeO3−δ, samples were prepared incorporating a layer of Na2CO3, either covering the surface of the particles, or in between the Ag nanoparticles and the SrFeO3−δ support. The Ag-Na2CO3-SrFeO3−δ materials were applied for a selective oxidation reaction, epoxidation of ethylene, demonstrating that an exposed Ag surface was necessary for reaction. The sample with an internal layer of Na2CO3 showed improved selectivity towards ethylene oxide during chemical looping epoxidation, as the carbonate layer mitigated complete combustion at the surface of SrFeO3−δ. Lastly, a series of Ag/SrFeO3−δ-based materials, modified with Cl and, or, Au, were investigated for the oxidation of propylene in a chemical looping mode. The materials containing Cl and Au showed unexpected selectivity towards propan-1-ol, with mechanisms proposed based on the reactions of possible intermediate products. In summary, this Dissertation demonstrates how the addition of metal catalysts to oxygen carriers can both enhance availability of lattice oxygen, and enable selective oxidation processes operated in a chemical looping mode. The interactions between the catalyst and oxygen carrier play a key role in determining kinetic and thermodynamic properties of the combined material. In particular, the transport of oxygen from the oxygen carrier to form reactive surface species greatly enhances reactivity under reducing and oxidising conditions. Hence, the work in this Dissertation expands the theoretical understanding regarding the behaviour of oxygen carriers during redox reactions, and opens new avenues to design materials for chemical looping processes.
chemical looping, catalysis, oxygen carriers, in-situ spectroscopy, kinetics
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