Abstract
<jats:p>Under Sabatier and Fischer-Tropsch reaction conditions, Ni catalysts are covered by a dense adsorbate layer dominated by carbon monoxide. To gain molecular-level insight into the structure of this layer and the surface phases that may form, CO adsorption on a Ni(111) model surface was investigated using near-ambient pressure scanning tunneling microscopy. Experiments were performed in-situ at CO pressures ranging from 5×10−9 to 50 mbar at room temperature. At 5×10−9 mbar, two c(4×2) configurations with 0.5 ML coverage were observed to co-exist on the surface, aligning with CO occupation of either hollow sites or alternating top and bridge positions. At 5 × 10−6 mbar, p( √ 7 × √ 7)R19.1◦ structure with a coverage of 0.57 ML formed, consistent with CO adsorption at a FCC or HCP hollow sites. At elevated pressures (1–50 mbar), the adlayer densifies to coverages of 0.62–0.77 ML, forming superstructures of variable-sized sub-cells where the smallest units appear structurally analogous to the p( √ 7× √ 7) elementary cell. Electric-field-induced and polarity-dependent surface modifications were observed, with negative sample bias (⪅ −1.9 V) promoting Ni layer growth and positive bias (⪆ 1 V) leading to layer disintegration. These restructuring effects provide a microscopic basis for the structural deterioration of Ni-based electrodes, such as those in solid oxide fuel cells, where applied operating potentials in CO-rich environments are known to compromise long-term interfacial stability.</jats:p>