Structural and Chemical Dynamics of Cu/NiO/Cu(100) Surfaces During CO₂ Hydrogenation

Understanding the dynamic restructuring of bimetallic catalysts under reaction conditions is key to improving their performance and stability. Here, we unveil the drastic changes in the structure and surface composition of copper-encapsulated NiO islands supported on Cu(100) during CO₂ hydrogenation. Using in situ low-energy electron microscopy (LEEM), x-ray photoemission electron microscopy (XPEEM), and spatially resolved x-ray absorption spectroscopy (µ-NEXAFS), we show that NiO gradually reduces under hydrogenation conditions, despite being buried beneath a ∼3 nm Cu overlayer. Furthermore, the reduced Ni exhibits distinct spatial redistribution depending on whether CO is present in the CO₂/H₂ reaction mixture. Density functional theory (DFT) calculations establish an adsorbate-dependent rule: C-bound intermediates thermodynamically favor Ni segregation, whereas O-bound species stabilize a Cu-terminated surface. Near-ambient pressure (NAP) LEEM directly visualizes size-dependent NiO island reduction governed by adsorbed H_ads availability and local geometry. Together, these results show how reaction microenvironments can be used to steer bimetallic surface composition and the active sites during CO₂ hydrogenation.