Cation-doping strategies for tuning of zirconia acid-base properties.

The role of Y-, Ca- and Ce-doping of cubic zirconia (c-ZrO2) (111) surface on its acidity, basicity and the interplay between surface acid-base pairs is investigated by computational methods. The most stable surface structures for this investigation were initially determined based on previous studies of Y-doped c-ZrO2 (111) and by a detailed exploration of the most stable configuration for Ca-doped c-ZrO2 (111) and Ce-doped c-ZrO2 (111). Next, surface mapping by basic probe molecules (NH3 and pyridine) revealed a general reduction of the acidity of the surface sites, although a few exceptions were observed for zirconium ions at next nearest neighbour (NNN) positions to the oxygen vacancy and at the nearest neighbour (NN) position to the dopants. Adsorption of CO2 over basic sites revealed a cooperative interplay between acid-base groups. In this case, the overall effect observed was the decrease of the calculated adsorption energies when compared with the pristine surface. Moreover, spontaneous formation of η 3-CO2 systems from initial η 2-CO2 configurations indicates a decrease in the required energy for forming oxygen vacancies in the doped ZrO2 systems at NNN positions or further away from the existing vacancy site.