Structural changes of hydrotalcite-based Co-containing mixed oxides with calcination temperature and their effects on NO_x adsorption: A combined experimental and DFT study

The structural changes of Co-containing hydrotalcite-based mixed oxides and their effects on the NO_x adsorption were investigated using experiments and density functional theory (DFT) calculation. Co-Al and Co-Mg-Al hydrotalcite-like compounds were synthesized using the co-precipitation method and calcined at various temperatures between 550 and 850 °C. The X-ray diffraction and X-ray photoelectron spectroscopy results indicated that high-temperature calcination thermally decomposed the spinel phases, which were derived from the hydrotalcite-like compounds, to cobalt monoxide and decreased the amount of Co³⁺ ions on the surface of the mixed oxides. The NO_x adsorption experiments performed at 150 °C presented that the calcination of the Co-Al and Co-Mg-Al hydrotalcite-like compounds at lower temperature was beneficial for the NO_x uptake, and the Co-Mg-Al oxides calcined at 550 °C presented the highest NO_x uptake (0.326 mmol/g) of all analyzed samples and also good cyclic stability and H₂O resistance. The effects of the calcination temperature were further scrutinized using a DFT method and the results of the DFT calculation demonstrated that the adsorption of NO molecules was more favorable over the spinel phases than over the cobalt monoxide. This work is the first study to perform the DFT calculation on the NO_x adsorption over the mixed oxides derived from the hydrotalcite-like compounds, and our works are expected to be helpful for developing high-performance NO_x adsorbents by identifying the advantageous structures of the mixed oxides for NO_x adsorption.