Atomic dynamics of gas-dependent oxide reducibility.

Area of Science:

• Materials Science
• Surface Chemistry
• Catalysis

Background:

• Oxide reduction is crucial for metal production, catalysis, and energy technologies.
• Carbon monoxide (CO) and hydrogen (H2) are common reductants, but their atomic-scale reduction mechanisms are not fully understood.
• Distinguishing gas-specific pathways is vital for developing sustainable processes, especially replacing CO with H2 to reduce CO2 emissions.

Purpose of the Study:

• To directly visualize and differentiate the gas-dependent oxide reduction dynamics of Nickel Oxide (NiO) using environmental transmission electron microscopy.
• To elucidate the distinct atomistic pathways driven by CO versus H2 in NiO reduction.

Main Methods:

• Utilized environmental transmission electron microscopy (ETEM) for real-time, atomic-resolution imaging of gas-solid redox reactions.
• Investigated the reduction of NiO under reactive gas (CO and H2) and high-temperature conditions.

Main Results:

• CO-driven reduction results in surface nucleation and growth of metallic Nickel (Ni) islands, leading to self-limiting surface metallization.
• H2-driven reduction activates a coupled surface-to-bulk transformation, with protons infiltrating the lattice and promoting inward oxygen vacancy migration for bulk metallization.
• Oxygen vacancies generated by CO remain near the surface, forming a metallic Ni layer that hinders further reduction, unlike H2.

Conclusions:

• Revealed distinct atomistic reduction pathways for CO and H2 on NiO.
• Provided fundamental insights into gas-specific reduction mechanisms, crucial for optimizing metallurgical processes and catalyst design.
• Highlighted the potential of H2 as a cleaner reductant with different mechanistic outcomes compared to CO.