As of 2020, steel manufacture is estimated to be responsible for 11 per cent of global carbon emissions and 7 to 9 per cent of greenhouse gas emissions. To reduce emissions, researchers at the Max Planck Institute for Iron Research at Dusseldorf in Germany investigated how pores in a solid change its chemical reactions and showed how this could make steel manufacture more sustainable.
The conventional process involves using coal to reduce iron oxide in the ore to iron. The use of hydrogen as a reactant to produce steel is potentially more environmentally friendly than using carbon, but an industrial-scale switch to using hydrogen involves a lot of challenges. One is that the reaction needed to make steel using hydrogen is mysteriously sluggish. Xuyang Zhou and his colleagues have now identified the main cause and suggested a method to mitigate it. This work has been published in Physical Review Letters.
Steel is made through an electron-exchange or redox (reduction-oxidation) reaction in which iron oxide reacts with another material to produce steel and an oxide byproduct—CO2 if the oxidant is carbon, water if it is hydrogen. With the knowledge that removal of oxygen atoms during this process leaves behind nanometre-to-micrometre-scale pores in the iron oxide, using simulations and electron-microscopy observations, the researchers have shown that, when hydrogen is the reactant, water gets trapped in these pores and shifts the local equilibrium towards reoxidation, that is, removing electrons from the reduced iron, slowing the overall reaction.
Their findings have also offered a solution to this slowdown effect—if the pores are sufficiently interconnected to form channels, then the water formed would percolate out of the material before reoxidising it. “We should be able to achieve the required pore morphology by controlling temperature, pressure, and other parameters during the reaction,” said Zhou.