Understanding early stages of low-temperature hydrogen-driven direct co-reduction of Fe-Ni mixed oxide thin films at the near atomic scale

Abstract

Kinetic understanding of hydrogen co-reduction of multinary and multi-phase oxides is of interest for enhancing sustainability of alloy production and transition to a hydrogen-based economy. Benefits include decrease in energy consumption, enhanced kinetics, and conversion of oxides to alloys. Thin films provide a platform to study these processes as reactive co-deposition from multiple elemental, alloy or compound targets and precise oxygen flow control allow atomic mixing into various oxide phases which are well-defined nanoscale precursor structures for the subsequent reduction study at the near atomic scale. The early stages of hydrogen direct reduction of oxide thin films are investigated using a Fe50Ni50Ox thin film consisting of NiFe2O4 and NiO phases. After reduction at 280 C in pure H2 for different times, structural, morphological, and nanoscale changes were examined by different characterisation methods including atom probe tomography (APT). The low-temperature reduction is nucleation-limited marked by grain-boundary nucleation preceded by an incubation time of more than 5 min. APT revealed that the early-stages of the reduction involves phase separation into a Ni-rich FexNiy metallic phase and a transformed remaining oxide (magnetite, Fe3O4). Further reduction induces magnetite reduction and alloying into a nearly equiatomic FeNi alloy. The low-temperature reduction and alloying are facilitated by synergetic effects from the nanostructure of the film, and Ni autocatalytic effects through alloying and hydrogen spillover. The results pave the way for low-temperature formation of Fe-Ni alloy thin films with tunable compositions directly from oxides, and broaden the scope of hydrogen direct reduction of multinary oxides to thin-film platforms.