Atomic dynamics of gas-dependent oxide reducibility

Understanding oxide reduction is critical for advancing metal production^1,2, catalysis^3,4 and energy technologies⁵. Although carbon monoxide (CO) and hydrogen (H₂) are widely used reductants, the mechanisms by which they work are often presumed to be similar, both involving lattice oxygen removal^6-9. However, because of growing interest in replacing CO with H₂ to lower CO₂ emissions, distinguishing gas-specific reduction pathways is critical. Yet, capturing these atomic-scale processes under reactive gas and high-temperature conditions remains challenging. Here we use environmental transmission electron microscopy, which is capable of real-time, atomic-resolution imaging of gas-solid redox reactions^10-16, to directly visualize the gas-dependent oxide reduction dynamics in NiO. We show that CO drives surface nucleation and the growth of metallic Ni islands, leading to self-limiting surface metallization. Conversely, H₂ activates a coupled surface-to-bulk transformation, where protons from dissociated H₂ infiltrate the oxide lattice to promote the inward migration of surface-generated oxygen vacancies and enabling bulk metallization. By contrast, oxygen vacancies formed by CO remain confined near the surface, where they rapidly form a metallic Ni layer that inhibits further reduction. These results reveal distinct atomistic pathways for CO and H₂ and provide insights that may guide metallurgical processes and catalyst design.