Unraveling the thermodynamics and mechanism behind the lowering of direct reduction temperatures in oxide mixtures

Abstract

Hydrogen-based direct reduction offers a sustainable pathway to decarbonize the metal production industry. However, stable metal oxides, like Cr2O3, are notoriously difficult to reduce, requiring extremely high temperatures (above 1300 °C). Herein, we explain how reducing mixed oxides can be leveraged to lower hydrogen-based reduction temperatures of stable oxides and produce alloys in a single process. Using a newly developed thermodynamic framework, we predict the precise conditions (oxygen partial pressure, temperature, and oxide composition) needed for co-reduction. We showcase this approach by reducing Cr2O3 mixed with Fe2O3 at 1100 °C, significantly lowering reduction temperatures (by ∼200 °C). Our model and post-reduction structural and chemical analyses elucidate that the temperature-lowering effect is driven by the lower chemical activity of Cr in the Fe-Cr solid solution phase. This strategy achieves low-temperature co-reduction of mixed oxides, dramatically reducing energy consumption and CO2 emissions, while unlocking transformative pathways toward sustainable alloy design.