Oxygen Adsorption and Activation on Cobalt Center in Modified Keggin Anion-DFT Calculations

• Main Authors: Renata Tokarz-Sobieraj, Piotr Niemiec
• Format: Article
• Language: English
• Published: MDPI AG 2020-01-01
• Series: Catalysts
• Subjects: cobalt-modified heteropoly compounds; molecular oxygen activation; dft (density functional theory) calculations; nocv analysis
• Online Access: https://www.mdpi.com/2073-4344/10/2/144

The influence of the cobalt cation geometric environment on catalytic activity, namely, oxygen adsorption and its activation, was investigated by exploring two groups of systems. The first group was formed by cobalt cation complexes, in which the Co²⁺ was surrounded by water-H₂O or acetonitrile-CH₃CN solvent molecules. This represents heteropolyacids salts (Co_nH_3-nPW(Mo)₁₂O₄₀), where the Co²⁺ acts as a cation that compensates for the negative charge of the Keggin anion and is typically surrounded by solvent molecules in that system. The second group consisted of tungsten or molybdenum Keggin anions (H₅PW₁₁CoO₃₉ and H₅PMo₁₁CoO₃₉), having the Co²⁺ cation incorporated into the anion framework, in the position of one addenda atom. Detailed NOCV (Natural Orbitals for Chemical Valence) analysis showed that, for all studied systems, the &#963;-donation and &#963;-backdonation active channels of the electron transfer were responsible for the creation of a single Co-OO bond. Depending on the chemical/geometrical environment of the Co²⁺ cation, the different quantities of electrons were flown from the Co²⁺ <i>3d</i> orbital to the <i>&#960;*</i> antibonding molecular orbitals of the oxygen ligand, as well as in the opposite direction. In molybdenum and tungsten heteropolyacids, modified by Co²⁺ in the position of the addenda atom, activation of O₂ was supported by a &#960;-polarization process. Calculated data show that the oxygen molecule activation changed in the following order: H₅PMo₁₁CoO₃₉ = H₅PW₁₁CoO₃₉ &gt; Co(CH₃CN)₅²⁺ &gt; Co(H₂O)₅²⁺.