Predicting CO adsorption sites in Co-doped zeolite via density functional theory calculations

Abstract

The interaction of CO with Co/MFI and the structural features of stabilization of extra-framework Co-species in the straight channel of the zeolite were investigated by using density functional theory (DFT) calculations, employing the ω_CO/r_CO correlation for calculations of CO stretching frequencies. Various locations of two Al atoms are considered depending on the number (from 1 to 4) of silicon atoms separating former atoms. The calculated CO stretching frequencies of 2200-2195 cm⁻¹ found for models with the divalent cobalt cation located near closely spaced Al atoms (k = 1, 2) in 10R-str zeolite channel corresponds most closely to the experimental frequencies. The probability of а stabilization of the divalent cation in models with a distant arrangement of two Al atoms in the straight channel of the MFI is rather low. On the contrary, an isolated arrangement of Al atoms assists the formation of more probable structures of the binuclear Co^II-oxo and Co^III-oxo cations. The highest values of the calculated CO stretching frequency (2206-2226 cm⁻¹) correspond to the models with [Co^III-(O₂)-Co^III]²⁺ oxo cation. According to DFT calculations, these Co^III-binuclear clusters are the most likely active sites of redox reactions on Co/MFI catalysts.