High temperature oxygen sorption on metal oxides with defects as sorbents for air separation

Metal oxides with oxygen nonstoichiometry (vacancies) have been studied as sorbents for high temperature air separation. The principle of the oxygen sorption/desorption on these materials is based on the fact that the oxygen nonstoichiometry in their structures varies with the oxygen partial pressure (PO2) of the surrounding gas and temperature. This dissertation is dedicated to understanding and improving the fundamental oxygen sorption properties of these metal oxides.; Two metal oxides, La0.1Sr0.9Co0.9Fe 0.1O3-delta (LSCF1991) with a cubic perovskite structure, and SrCoFeOx (SCF111) with an orthorhombic layered-perovskite structure were selected as candidate materials for this study. The LSCF1991 provides high oxygen sorption capacity, fast adsorption, and relatively slow desorption rate at temperatures of 400--900°C in the PO2 range from 10-4 to 0.21 atm. To understand and improve the oxygen sorption capacity and kinetics of LSCF1991, several main factors were investigated. A linear driving force model was adopted to calculate the surface reaction rate of the sorption and desorption period. The surface reaction rates were found to be a function of PO2 and temperature, and determined by the crystalline size rather than the aggregate size. The disorder-order phase transition can enhance the oxygen sorption capacity and desorption rate of LSCF 1991. Addition of other cations into the structure yields effects on the oxygen sorption properties of LSCF1991.; The comparison of oxygen sorption properties between SCF111 and LSCF1991 shows that SCF111 is more promising as sorbents for air separation at temperatures above 800°C. The sorption mechanism is proposed and can well explain the oxygen sorption property differences between these two structures. The synthesis methods were found to have critical effects on the oxygen sorption properties of SCF111.; Heat of sorption, one of the important properties of sorbent materials, was studied by simultaneous DSC-TGA and fixed-bed measurements. For LSCF1991, the net heat of the oxide during the sorption process is the combination of heat produced from two processes, the endothermic heat from the phase transition and the exothermic heat from the oxygen sorption. By taking advantage of the phase transition during the sorption process, it is desirable to find air separation process with minimum heat effects.