Microstructure analysis of (Ba,Ca)(Fe,Mg)O3-δ for rapid CO2 absorption by Mossbauer spectroscopy

The CO2 absorption properties and the microstructure of (Ba,Ca)(Fe,Mg)O3-delta have been studied by TGA, XRD, and Mossbauer spectrometry. Paramagnetic doublets of Fe-IV and Fe-III appeared in the Mossbauer spectra of cubic (Ba0.5Ca0.5)(Fe0.5Mg0.5)O3-delta heated in CO2 up to 600 degreesC, and a pair of sextets of tetrahedral Fe-III (H-in = 43 T) and octahedral Fe-III (H-in = 51 T) were produced above 800 degreesC, and an additional sextet characteristic of Fe-III in a spinel structure (H-in = 48 T) was observed at 1000 degreesC. On the other hand, a pair of sextets of tetrahedral and octahedral Fe-III of the orthorhombic (Ca0.95Ba0.05)(Fe0.5Mg0.5)O3-delta showed hardly any change after absorption of CO2. It is concluded that only a small portion of Mg entered the orthorhombic phase of (Ca0.95Ba0.05)(Fe,Mg)O3-delta and Mg preferred the octahedral B site of the perovskite lattice. The excess Mg formed separate CaO-MgO mixed oxide, and the primary mechanism of CO2-trapping is the formation of CaMg(CO3)(2).