Ba_xSr_1-xO/MgO nano-composite sorbents for tuning the transition pressure of oxygen: Application to air separation

BaO is a proven oxygen-selective sorbent for air separation via the BaO-BaO₂ oxygen sorption-desorption cycle. However, the low transition O₂ pressure necessitates a high vacuum that is required for desorption, hence limiting its application. Here, we report facile synthesis of new oxygen-selective Ba_xSr_1-xO/MgO nano-composite sorbents from which O₂ can be desorbed at moderate vacuum pressures. These sorbents are obtained from Ba_xSr_1-xMg(CO₃)₂ precursors synthesized by co-precipitation followed by calcination. It is shown that the transition O₂ pressure for the oxide-peroxide cycle is significantly increased by increasing the Sr²⁺/Ba²⁺ ratio. XRD results show that the crystal structures are preserved while the lattice constants are reduced upon substitution of Ba²⁺ by Sr²⁺ in both precursor carbonates and peroxides. Transmission electron microscopy/energy dispersive X-ray spectroscopy results show that Ba²⁺ and Sr²⁺ are mixed molecularly in Ba_xSr_1-xO/MgO nano-composite. We explain that the uniformly substituted Sr²⁺ in BaO/BaO₂ changes the dissociation energy of peroxide ions and the equilibrium of the reversible reactions, promoting decomposition of BaO₂. Cyclic O₂ sorption and desorption with Ba_xSr_1-xO/MgO nano-composite is demonstrated.