Synergistic effects of binary oxygen carriers during chemical looping hydrogen production

Abstract

The use of binary oxygen carrier allows for the materials of enhanced activity or stability during chemical looping process. However, the lack of mechanical understanding of the origin of the improvements hindered the rational design and control of the doping process in the oxygen carrier production. Here, we synthesized a series of M0.6Fe2.4Oy (M = Ni, Cu, Co, Mn) binary spinel materials and carried out various characterization techniques to study how the dopants influenced the material phase change, the oxygen transfer as well as the chemical looping performance. The results showed the chemical looping reactivity can be related to the oxygen transformation between lattice oxygen and oxygen vacancy, which was determined by the redox properties of both dopants and iron. The metal in tetrahedral site for Cu, Mn, Ni-doped sample were relatively stable, limiting oxygen transformation ability. In comparison, Co dopant promoted the reducibility of iron in tetrahedral site as well as metals in other sites, making almost all lattice oxygen rapidly transformed to oxygen vacancy during reduction. This was the main cause for the subsequent high hydrogen production rate (average ∼0.02 mmol. g−1.s−1) and yield (∼15.9 mmol.g−1). Upon cycling, the phase separation of single oxides from Co0.6Fe2.4Oy and Mn0.6Fe2.4Oy spinels led to the decreased ability of oxygen transformation. However, the performance was extremely stable for Cu0.6Fe2.4Oy with reversible phase change between spinel and (Fe, Cu) wusitite by the Cu-Fe interaction. Based on the current results, this work points to a promising Cu-Co co-doping material with both good reactivity and stability.