(140e) Morphological Evolution of Transition Metal Nanostructures with Oxygen Dissociation and Migration during Chemical Looping Processes

Transition metal are heavily used in chemical looping technologies because of their high oxygen capacity and high thermal reactivity. These oxygen activities result in the oxide formation and oxygen vacancy formation that affect the nanoscale crystal phases and morphologies within these materials and their subsequent bulk chemical behaviour. In this study, two selected earlier transition metals manganese and cobalt, as well as two selected later transition metals copper and nickel, are investigated with their undergoing cyclic redox reactions. We found Co microparticles exhibited increased CoO impurity when oxidized to Co₃O₄ upon cyclic oxidation; CuO redox cycles prefer to be limited to a reduced form of Cu₂O and an oxidized form of CuO; Mn microparticles were oxidized to a mixed phases of MnO and Mn₃O₄, which causes delamination during oxidation. For Ni microparticles, a dense surface were observed during the redox reaction. The atomistic thermodynamics methods and density functional theory (DFT) calculations are carried out to investigate the effect of oxygen dissociation and migration on the morphological evolution of nanostructures during the redox processes. Our results indicate that the earlier transition metals (Mn and Co) tend to have stronger interaction with O₂ than the later transition metals (Ni and Cu). Also, our modified Brønstedâ??Evansâ??Polanyi (BEP) relationship for reaction energies and total reaction barriers reveals that reactions of earlier transition metals are more exergonic and have lower oxygen dissociation barriers than those of later transition metals. In addition, it was found that for these transition metal oxides the oxygen vacancy formation energies increase with the depth. The oxide in the higher oxidation state of transition metal has lower vacancy formation energy, which can facilitate to form defective nanostructures. The fundamental understanding of these metal oxide reactions is essential in directing the development of metal oxide-based oxygen carriers for chemical looping applications.