Effect of support on redox stability of iron oxide for chemical looping conversion of methane

Abstract

The chemical looping processes utilize lattice oxygen in oxygen carriers to convert carbonaceous fuels in a cyclic redox mode while capturing CO 2. Typical oxygen carriers are composed of a primary oxide for active lattice oxygen storage and a ceramic support for enhanced redox stability and activity. Among the various primary oxides reported to date, iron oxide represents a promising option due to its low cost and natural abundance. The current work investigates the effect of support on the cyclic redox performance of iron oxides as well as the underlying mechanisms. Three ceramic supports with varying physical and chemical properties, i.e. perovskite-structured Ca 0.8 Sr 0.2 Ti 0.8 Ni 0.2 O 3 , fluorite-structured CeO 2 , and spinel-structured MgAl 2 O 4 , are investigated. The results indicate that the redox properties of the oxygen carrier, e.g. activity and long-term stability, are significantly affected by support and iron oxide interactions. The perovskite supported oxygen carrier exhibits high activity and stability compared to oxygen carriers with ceria support, which deactivate by as much as 75% within 10 redox cycles. The high stability of perovskite supported oxygen carrier is attributable to its high mixed ionic-electronic conductivity. Deactivation of ceria supported samples results from solidstate migration of iron cations and subsequent enrichment on the oxygen carrier surface. This leads to agglomeration and lowered lattice oxygen accessibility. Activity of MgAl 2 O 4 supported oxygen carrier is found to increase during redox cycles in methane. The activity increase is a consequence of surface area increase caused by filamentous carbon formation and oxygen carrier fragmentation. While higher redox activity is desired for chemical looping processes, physical degradation of oxygen carriers can be detrimental.

CO2 conversion to CO by auto-thermal catalyst-assisted chemical looping

“…The importance of the Ni catalyzed CH 4 reforming process is stressed here since the reduction of iron oxide by H 2 and CO is considerably faster than by CH 4 [35,36].…”

Ca1−A MnO3 (A = Sr and Ba) perovskite based oxygen carriers for chemical looping with oxygen uncoupling (CLOU)

“…Typical perovskite takes the form of ABO 3Àd , where A is a large cation of either the alkali earth or rare earth metal and B is a smaller transition metal cation [55]. As a support, mixed ionic and electronic conductive (MIEC) perovskites such as La 1Àx Sr x FeO 3 (LSF) have shown to enhance the redox activity of iron oxides by nearly two orders of magnitude [51][52][53][54]. Perovskite and perovskite supported iron oxide have also been explored as redox catalysts for syngas generation and water-splitting [42,56].…”

Reduction kinetics of lanthanum ferrite perovskite for the production of synthesis gas by chemical-looping methane reforming

“…The challenge of NiO based redox catalysts, however, is their high tendency for coke formation, high cost, and health concerns (Adanez et al, 2012;Neal et al, 2014). Fe-based oxides have the advantages of being cheaper, low agglomeration, high melting point and more environmentally benign (Cabello et al, 2014a;Adanez et al, 2012;Neal et al, 2014Neal et al, , 2015Shafiefarhood et al, 2014;Galinsky et al, 2015). However, iron oxide based redox catalysts are not particularly active for methane oxidation (Cabello et al, 2014a;Shafiefarhood et al, 2014), and tend to have low selectivity toward methane partial oxidation.…”