Growth and Structural Characterization of VOx/Co3O4(111) Model Catalysts

This study presents the first successful preparation and structural characterization of model systems based on VO_x epitaxially grown on Co₃O₄(111)/Au(111) thin films, designed to mimic catalytically active powder VO_x/Co₃O₄ materials. The model systems were prepared under ultrahigh vacuum (UHV) conditions by physical vapor deposition and oxidation at elevated temperatures and characterized by a combination of scanning tunneling microscopy (STM) and infrared reflection absorption spectroscopy (IRAS) employing CO as the probe molecule for different adsorption sites. Three distinct VO_x-related structural motifs were identified for systematically changing V loadings: (1) V atoms incorporated into Co₃O₄(111) at the position of tetrahedrally coordinated Co²⁺ lattice sites, (2) two-dimensional triangular VO_x islands composed of a single V–O layer and terminated by V³⁺ ions, and (3) three-dimensional truncated triangular VO_x particles terminated by V⁴⁺ ions and consisting of alternating O–V planes comprising three V and two O layers. In addition, vanadyl (V⁵⁺═O) groups were observed on both types of VO_x islands. The formation and distribution of these structures strongly depend on vanadium loading: at low coverage, embedded V atoms and 2D VO_x islands dominate, whereas higher V loadings result in the growth of extended 3D VO_x structures.