Microstructure and reactivity evolution of La-Fe-Al oxygen carrier for syngas production via chemical looping CH4-CO2 reforming

The relationship between chemical looping CH4-CO2 reforming performance and the microstructure of oxygen carrier (OC) is very important for the rational design of OC. In this paper, we studied the structural evolution of La-Fe-Al (LFA-t, t = 900-1200 degrees C) OCs as thermal treatment and ten periodic CH4/CO2 redox cycles, and correlated to their reactivity and stability for syngas production. Different calcination temperature brought about great discrepancy in phase composition of LFA OCs: LaFeO3, Fe2O3, and alpha-Al2O3 at 900 degrees C, LaFexAl1-xO3 and La-hexaaluminate at 1000 degrees C, and monophasic La-hexaaluminate at 1100-1200 degrees C. During the CH4/CO2 redox process, the repeated phase separation occurred over LFA-900 and LFA-1000 accompanied by the appearance of metallic Fe and FeAl2O4, which resulted in serious CH4 pyrolysis. La-hexaaluminate showed good phase stability during CH4/CO2 redox process via the charge compensation mechanism. The large hexaaluminate crystalline of LFA-1200 inhibited the oxygen transport from the bulk to surface, which led to carbon deposition. LFA-1100 hexaaluminate OC with moderate crystal size exhibited excellent reactivity and stability for producing syngas with desirable H-2/CO ratio (similar to 2) during ten CH4/CO2 redox cycles thanks to high oxygen mobility and the reservation of hexaaluminate structure during redox process. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved