Enhancing Catalytic CO Oxidation over Co3O4 Nanowires by Substituting Co2+ with Cu2+
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Abstract
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 Co3O4_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.
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