Influence of the encapsulation degree of Fe0 active sites on performance of garnets for chemical looping partial oxidation of CH4

It is of challenge to improve the coke resistance of Fe-based oxygen carriers (OC) due to the conflict of Fe-0 sites for CH4 activation but causing coke formation. The present work reported that carbon-tolerant of Y3Fe2Al3O12-T garnets (YFAO-T, T indicates calcination temperature) could be remarkably promoted by tuning calcination temperature for chemical looping partial oxidation of methane (CLPOM). This was because the encapsulation degree of Fe-0 (1-Fe-exposed(0)/Fe-total(0)) became larger with reduction from 19% to 79% due to decreased oxygen mobility leading to more surface oxygen-deficiency for YFAO-T calcined at higher temperature, which not only resulted in Fe-0 sites for CH4 activation but also accessible lattice oxygen of garnet shell for carbon oxidation formed on Fe-0 sites. Comparatively, the encapsulation of Fe-0 was destroyed leading to progressively naked Fe-0 with large particle size with reduction for YFAO-T calcined at lower temperature, which induced carbon deposition due to inaccessible lattice oxygen of garnets.

Automated summary

By tuning the calcination temperature, the coke resistance of Fe-based oxygen carriers can be improved. The encapsulation degree of Fe-0 is increased at higher temperature, allowing for CH4 activation and carbon oxidation reaction. Conversely, at lower temperature, the encapsulation of Fe-0 is destroyed, leading to carbon deposition.