Understanding the Contribution of Surface Temperature and Hydrogen Radicals to Hydrogen Plasma Reduction of Iron Oxide

Microwave-powered, atmospheric-pressure plasmas have attracted attention to increase the reactivity of hydrogen for decarbonized reduction of iron oxide. However, the processes are often operated at high temperatures where reactions involve molecular hydrogen, in addition to any plasma-activated species such as atomic hydrogen. In this work, a plasma source was developed by coupling microwave radiation from a solid-state amplifier to an antenna surrounded by gas flow, to produce a free jet that enables treatment of a material surface at low temperatures (<500 °C). The surface temperature during plasma treatment was measured by infrared pyrometry, and control experiments confirmed that reduction by molecular hydrogen at these temperatures was kinetically suppressed. We thus were able to study the reduction of iron oxide at low temperature (∼280 to 500 °C) and the effect of various process conditions. The observed trends were understood in terms of the surface temperature and transport of the plasma-activated species, namely atomic hydrogen. Decoupling these various contributions enabled kinetic analysis and the extraction of an apparent activation energy of 50 kJ/mol for the overall reduction by atomic hydrogen at atmospheric pressure, free from molecular hydrogen and diffusional effects. The results show that reduction is enhanced by atomic hydrogen, but surface temperature continues to play a predominant role, which can guide low-temperature hydrogen plasma reduction of iron or other metal oxides for sustainable and on-demand production of critical resources such as steel.