Linking Solid-State Reduction Mechanisms to Size-Dependent Reactivity of Metal Oxide Oxygen Carriers for Chemical Looping Combustion

The reactivity of copper oxide (CuO) particles in chemical looping combustion (CLC), a promising indirect combustion process that facilitates carbon capture, was investigated by measuring CuO phase transformations during reduction with methane. By comparing CuO reactivity to iron (α-Fe2O3) and cobalt (Co3O4) oxides using a continuous flow through system and complementary thermogravimetric analysis, we reveal a link between the solid-state reduction mechanism of CLC oxygen carriers and their size-dependent reactivity toward methane. Reactivity of CuO and Co3O4 is independent of the particle size, with reduction following the nucleation and nuclei growth (NNG) model, whereas α-Fe2O3 shows increased reactivity with decreasing particle size and reduction follows the unreacted shrinking core (USC) model. Supported by density functional theory (DFT) calculations comparing relative energies of formation for surface and bulk oxygen defects, we propose a conceptual framework for the size-dependence of metal oxide oxygen carriers for CLC. For oxygen carriers that reduce via the NNG model, where reduction initiates within the particle core, there will be no size dependence. For reduction via the USC model, where reduction initiates on the particle surface, reactivity will increase for smaller particles. These findings can guide development of metal oxide oxygen carriers for CLC by establishing trends in size-dependent behavior.