Adsorption of Carbon Dioxide on Mesoporous Zirconia: Microcalorimetric Measurements, Adsorption Isotherm Modeling, and Density Functional Theory Calculations

Mesoporous zirconia was prepared using the sol–gel process and the EISA method. It presents a specific surface area of 90 m 2 ·g –1 and an interparticular porosity associated with a pore diameter of around 4–5 nm, and it crystallizes in the tetragonal symmetry. The determination of the CO 2 adsorption properties (both the isotherm and adsorption enthalpies) coupled with isotherm modeling using a multi-Langmuir model and density functional theory (DFT) calculations on different representative clusters evidenced that the surface of zirconia is heterogeneous from an energetic point of view. It emerges from both the experimental and the theoretical results that (i) the adsorption sites associated with the lowest enthalpies of adsorption (between −24 and −34 kJ·mol –1 ) represent nearly 65–70% of the total number of the adsorption sites present on the zirconia surface. They correspond to the interactions (physisorption) between carbon dioxide and oxygen atoms or hydroxyl groups of the surface. (ii) The adsorption sites associated with higher enthalpies of adsorption (around −65 kJ·mol –1 ) correspond to the interactions between carbon dioxide and Zr atoms; they represent around 5% of the total amount of adsorption sites. (iii) The adsorption sites associated with high enthalpies of adsorption (below −70 kJ·mol –1 ) represent only a small fraction of the adsorption sites (around 10%) and correspond probably to the interaction of CO 2 with structural surface defects or charged sites.