Flexible structural transformation and high oxygen-transfer capacity of mixed inverse spinel magnesium manganese oxides during methane chemical looping combustion

• Published in: Fuel processing technology 2022-07, Vol.232, p.107262, Article 107262
• Main Authors: Son, Namgyu, Park, Byung Hyun, Kim, Sujeong, Kim, Minkyu, Park, No-Kuk, Ryu, Ho-Jung, Baek, Jeom-In, Kang, Misook
• Format: Article
• Language: English
• Subjects: CH4–CLC reaction; Crystal structure; Flexible structural transformation; Fluidized bed combustion; Jahn-Teller effect; Magnesium; Manganese; Methane; Mixed inverse spinels; Oxidation; Oxygen transfer; Oxygen vacancies; Oxygen-transfer capacity; Roasting; Spinel; Transformations; Valence
• ISSN: 0378-3820; 1873-7188
• DOI: 10.1016/j.fuproc.2022.107262

This study aimed at elucidating the active species in spinel crystals composed of Mg and Mn calcined at various temperatures and to determine the relationships among their crystal structure, oxidation state, and oxygen-transport capacity. The particles were obtained as mixed inverse spinel [Mg1.5-xMn2+y]td[MgxMn(3+ or 4+)1.5-y]ohO4 polyhedra, which were strongly distorted owing to the Jahn–Teller effect. Their structural transformation to Mg0.43Mn0.57O occurred via reduction with methane (CH4) and the significant shrinkage of the M–O bonds caused the migration of cationic metals into the inner crystal. This structural transformation created maze-like large spaces (50–75 nm); however, during oxidation, oxygen species re-filled these spaces and the structure completely attained its original mixed inverse spinel [Mg0.73Mn2+0.27]td[Mg0.77Mn(3+,4+)1.23]ohO4 polyhedral shape. The oxygen-transfer capacity of MMO-1100, calcined at 1100 °C, was improved by 8.10% in the CH4-CO2/air redox system, and the abundance ratio of [(Mn3+) + (Mn4+)]oh/(Mn2+)td, corresponding to the oxygen-transferring redox species, was 4.5. Upon 1000 cyclic voltammetry cycles, the current density related to the oxygen-transfer capacity of the MMO-1100 particles was unchanged, suggesting that the oxygen-transfer capacity remained stable even after the 1000th redox cycle. [Display omitted] •Successful synthesis of a mixed inverse spinel [MgMn]td[MgMn]ohO4 oxygen carrier•A reversible MMO-1100 structure which completely restores oxygen vacancies•MMO-1100 oxygen carrier with improved oxygen-transfer ability of 8.10%