Enhancing Catalytic CO Oxidation over Co₃O₄ Nanowires by Substituting Co²⁺ with Cu²⁺

Co₃O₄ is an attractive earth-abundant catalyst for CO oxidation, and its high catalytic activity has been attributed to Co³⁺ cations surrounded by Co²⁺ ions. Hence, the majority of efforts for enhancing the activity of Co₃O₄ have been focused on exposing more Co³⁺ cations on the surface. Herein, we enhance the catalytic activity of Co₃O₄ by replacing the Co²⁺ ions in the lattice with Cu²⁺. Polycrystalline Co₃O₄ nanowires for which Co²⁺ is substituted with Cu²⁺ are synthesized using a modified hydrothermal method. The Cu-substituted Co₃O_4-Cux polycrystalline nanowires exhibit much higher catalytic activity for CO oxidation than pure Co₃O₄ polycrystalline nanowires and catalytic activity similar to those single crystalline Co₃O₄ nanobelts with predominantly exposed most active {110} planes. Our computational simulations reveal that Cu²⁺ substitution for Co²⁺ is preferred over Co³⁺ both in the Co₃O₄ bulk and at the surface. The presence of Cu dopants changes the CO adsorption on the Co³⁺ surface sites only slightly, but the oxygen vacancy is more favorably formed in the bonding of Co³⁺-O-Cu²⁺ than in Co³⁺-O-Co²⁺. This study provides a general approach for rational optimization of nanostructured metal oxide catalysts by substituting inactive cations near the active sites and thereby increasing the overall activity of the exposed surfaces.