Oxygen vacancy formation and their role in the CO2 activation on Ca doped ZrO2 surface: An ab-initio DFT study

Highlights: • Important modifications in the properties of zirconia surface due to the creation of oxygen vacancies and Ca doping are revealed by DFT. • Despite the generation of vacancies is energetically difficult, it is facilitated by the presence of Ca. • Ca and oxygen vacancies activate CO₂ differently. The adsorption modes are characteristic of monoclinic surfaces. • The electrons left in the vacancy play a key role to the surface-CO₂ interaction, resulting large differences of adsorption energies. A detailed investigation regarding modifications that occur in surface properties of zirconium oxide, through the generation of oxygen vacancies and through the doping process with calcium, was carried out within the ab-initio (periodic) density functional theory framework. Vacancy formation energies, geometric parameters, vibrational frequencies, and electronic properties are presented for both pure and doped surfaces. It was demonstrated that oxygen vacancies may alter substantially the surface properties of zirconia. The theoretical calculations show that the energy needed to remove a surface oxygen atom, creating a vacancy, is high, but the presence of calcium greatly facilitates this process. The surface adsorption and activation properties were examined through the interaction between the (−1 1 1) face (pure and doped) and the chemically stable CO₂ molecule. It was shown that this surface is capable of activating CO₂. The adsorption process results in different geometries (CO₂^−δ and CO₃^−δ species), which present different interaction strengths depending on the presence of a vacancy and the alignment between the molecule with the electrons localized in the vacancy site. A thermodynamic analysis revealed that the presence of calcium provides an overall energetically more stable environment and therefore the doping process increases the surface capacity of adsorbing CO₂.