Fe-substituted Ba-hexaaluminate with enhanced oxygen mobility for CO2 capture by chemical looping combustion of methane

While Fe-based oxygen carriers (OC) are regarded to be promising for chemical looping combustion (CLC), the decrease of CO2 selectivity during deep reduction process and the severe agglomeration of Fe2O3 often occur after multiple redox cycles due to the low oxygen mobility. Herein, Fe-substituted Ba-hexaaluminates (BaFexAl12-xO19, denoted as BF(x)A-H, x = 1 and 2) prepared by a modified two-step method exhibited not only higher amount of converted oxygen (O-t) and CH4 conversion (77% and 81% vs. 17% and 75%) than those prepared by the traditional co-precipitation method (BF(x)A-C, x = 1 and 2) but also high CO2 selectivity above 92% during the nearly whole reduction from Fe3+ to Fe2+. Furthermore, the BF(x)A-H exhibited the excellent recyclability during 50 cycles. The better performance was ascribed to the markedly enhanced oxygen mobility which resulted from dominant occupancy of Fe cations in Al(5) sites (Fe-5: 71% and 70% vs. 49% and 41%) in mirror planes of hexaaluminate leading to larger amount of lattice oxygen coordinated with Fe-5 (O-Fe-5) (0.45 and 0.85 mmol/g vs. 0.31 and 0.50 mmol/g). The improvement of oxygen mobility also favored the preservation of chemical state of Fe cations in hexaaluminate structure in the re-oxidation step, resulting in the excellent recyclability of BF(x)A-H. (C) 2018 Science Press and Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier B.V. and Science Press. All rights reserved