Nickel oxide acts as a catalytic precursor, boosting hydrogen-based reduction kinetics by approximately two-fold.

Professor Zhou Xuyang from the School of Materials Science and Engineering at Xi'an Jiaotong University (XJTU), in collaboration with the Max Planck Institute for Sustainable Materials in Germany, has recently made significant progress in hydrogen-based green metallurgy and one-step metallurgical synthesis.

The research proposes and explains a solid-solid catalytic mechanism during the co-reduction of iron oxide and nickel oxide. They discovered that nickel oxide can act as a catalytic precursor, boosting hydrogen-based reduction kinetics by approximately two-fold while promoting the synchronous formation of iron-nickel alloys during the reduction process. The findings have been published in Nature Synthesis under the title Solid-solid catalysis for sustainable alloy synthesis.

The study reveals a previously underappreciated solid-solid catalytic mechanism in hydrogen-based metallurgy: a reducible metal oxide transforms in situ into an active metallic phase during the reaction, which continuously promotes hydrogen spillover, elemental partitioning, and rapid alloying through a dynamic metal-oxide interface.

This mechanism provides a new theoretical foundation for lowering hydrogen-based direct reduction temperatures, shortening processing times, reducing energy consumption, and achieving one-step alloy fabrication.

From an application perspective, this strategy is expected to offer new avenues for the green manufacturing of alloy systems such as nickel-bearing steels, stainless steels, Invar alloys, high-strength steels, and cryogenic engineering materials. By coupling oxide reduction with the alloying process, this method helps reduce the traditional metallurgical reliance on high-temperature melting and subsequent long-term homogenization treatments.

Using a mixed iron oxide and nickel oxide system as a model, the study proposes a new strategy of utilizing a reducible metal oxide as a catalytic precursor to accelerate hydrogen-based reduction. Experimental results show that at 700 C, the introduction of nickel oxide cuts the time required for iron oxide reduction to reach saturation by roughly half, approximately doubling the overall reduction kinetics compared to the uncatalyzed system. Under conditions simulating industrial continuous-heating direct reduction, adding nickel or nickel oxide also lowers the onset temperature of iron oxide reduction by at least 100 C.

Mechanistically, nickel oxide is preferentially reduced under hydrogen, generating nanoporous metallic nickel in situ. This porous nickel forms a dynamic metal-oxide interface with the adjacent iron oxide, which facilitates the dissociation of hydrogen molecules and transfers active hydrogen to the iron oxide surface via the hydrogen spillover effect, thereby accelerating oxygen removal and iron oxide reduction.

Compared to adding metallic nickel directly, the fine, porous nickel formed in situ from nickel oxide provides a larger interfacial contact area and higher catalytic activity, resulting in a much more pronounced kinetic acceleration.

Utilizing in situ synchrotron X-ray diffraction, scanning transmission electron microscopy (STEM), 4D-STEM, atom probe tomography, and theoretical calculations, the research team demonstrated that the iron-nickel alloy does not form slowly via traditional solid-state diffusion after the iron is completely reduced. Instead, it forms synchronously during the oxide reduction process.

The elemental partitioning and interfacial reconstruction at the interface between the nickel-rich phase and the wüstite-like (FeO) intermediate phase facilitate the entry of iron atoms into the face-centered cubic (fcc) nickel lattice. This directly generates the iron-nickel alloy phase, bypassing the limitations of body-centered cubic (bcc) iron phase nucleation and subsequent long-range diffusion found in traditional pathways.

Correlative characterization revealed a nanoscale distribution gradient of iron, nickel, and oxygen, alongside transient nickel enrichment at the metal-oxide reaction interface. These results demonstrate that the dynamic interface not only provides sites for hydrogen spillover, but also continuously drives the reduction of the wüstite-like intermediate phase and iron-nickel alloying through redox-driven elemental partitioning and defect-assisted diffusion.