Adsorption of CO2 on Amorphous and Crystalline Zirconia: A DFT and Experimental Study

Herein, we study the structural and electronic origins of molecular adsorption using experiments and density functional theory (DFT) calculations. We performed X-ray diffraction (XRD) and temperature-programmed desorption (TPD) of CO2 on amorphous zirconia (am-ZrO2) and crystalline (tetragonal and monoclinic) zirconia. Using molecular dynamics simulations, the bulk structures of am-ZrO2 and am-zirconium(IV) hydroxide (am-Zr(OH)4) were obtained and the reproducibility of the experimental structure was confirmed by comparing the radial distribution functions. In addition, the hydroxyl density on the hydrogenated ZrO2 surfaces was found to be consistent with the experimental results. Both experiments and simulations indicate that the adsorption of CO2 on an am-ZrO2 surface is more heterogeneous and weaker than that on a crystalline zirconia surface. Because the charge environment and band structures of crystalline zirconia are approximately the same as those of am-ZrO2, the weak adsorption on the am-ZrO2 surface arises from the fewer and stronger Zr-O bonds on the surface. These findings provide molecular-level insight not only for the adsorption of CO2 but also into the molecular adsorption on ZrO2-based catalysts.