• CO at 20% volume proportion in H2/CO mixed gas promotes Fe2O3 reduction at 850°C, enhancing overall reduction efficiency.
• ReaxFF MD simulations reveal that H2 reduces Fe2O3 faster initially than CO, but CO-induced cementite formation creates active carbon sites that facilitate further reduction.
• Internal CO generation from cementite can disrupt the dense surface layer, influencing reduction swelling and surface morphology.
• CO-reduced surfaces exhibit larger surface area, which accelerates subsequent H2 reduction, highlighting a synergistic effect in mixed gas reduction.