Significantly enhanced oxygen transfer capacity by the oxygen delivery channels formed in the inverse spinel Cu_xMg_2-xMn_yTi_1-yO_4.0particle

Abstract

Summary Particles containing Cu and Mn as oxygen transfer species in the development of alternative oxygen carriers for commercialized NiO‐based particles used in CH4 chemical looping combustion were investigated in this study. To maximize their oxygen transfer capacity, divalent Cu and tetravalent Mn were maintained in their respective oxidation states, which resulted in their reduction capacities not being canceled by each other. To prevent abrasion and sintering caused by collision and aggregation between 2 metals at high temperatures, they were stably fixed in an Mg1.0Ti1.0O3.0 ilmenite support. As the result, the Cu‐substituted particle transferred to a pseudobrookite Mg1.0Ti2.0O5.0 structure, and the particles with Cu2+ and Mn4+ substituting Mg2+ and Ti4+, respectively, changed to an inverse spinel Mg2.0Ti1.0O4.0 structure, and not an ilmenite structure. In the hydrogen temperature‐programmed reduction test, the reduction temperatures of the Cu and Mn ions in CuxMg2‐xMnyTi1‐yO4.0 particles were significantly lower by 200°C or higher than those in the particles containing only Cu or Mn. Furthermore, adsorption curves of CH4 and CO were shifted to low temperatures, and the adsorption concentrations were greatly increased for CuxMg2‐xMnyTi1‐yO4.0. For the cycling test for the CH4‐CO2/air redox system, the oxygen transfer capacities in particles containing only Cu or Mn were 3.4% or 3.1%, respectively, but this increased to 9.4% in Cu1.5Mg0.5Mn0.5Ti0.5O4.0. In CuxMg2‐xMnyTi1‐yO4.0, the Mn species was present in the tetravalent state, and it participated in the CH4 combustion reaction with the Cu ion, resulting in improved oxygen transfer ability. Finally, this study demonstrated an important mechanism, in which the Cu2+ and Mn4+ ions were reduced while combusting CH4, and the Mg2+ and Ti4+ transfer oxygen to the reduced Cu and Mn through the inverse spinel lattice of CuxMg2‐xMnyTi1‐yO4.0. All metal species were reduced to like a dominos in the reduction system, creating a regular oxygen delivery channel in the crystal during the CH4‐CO2/air redox reaction, and resulting in improved oxygen transfer capacity.