Modulating lattice oxygen activity of Ca2Fe2O5 brownmillerite for the co-production of syngas and high purity hydrogen via chemical looping steam reforming of toluene

The chemical looping steam reforming (CLSR) of biomass tar enables the process intensification for the co-preparation of syngas and high purity hydrogen. The practical application of brownmillerite-structured Ca2Fe2O5 is hindered by activity-related issues such as low fuel conversion and oxygen transfer capacity. Here, the doping of heteroatoms, e.g. Ni induces structural changes to the brownmillerite lattice, transforming it from a Pnma phase to a Pcmn one, with increased distortion of the FeO6 octahedra. The structural changes lead to the upwards shifts of the O 2p band of oxygen carrier, and subsequently improved lattice oxygen activity as well as oxygen transfer capacity. The formation of oxygen vacancy is a rate determining step during CLSR, while the Ni-doped Ca2Fe2O5 reduces the energy of oxygen vacancy formation and energy barrier for lattice oxygen migration through the bulk. During CLSR, Ca2Ni0.25Fe1.75O5 lead to significant improvement in syngas productivity, hydrogen purity and fuel conversion.