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

The role of Y-, Ca- and Ce-doping of cubic zirconia (c-ZrO 2 ) (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-ZrO 2 (111) and by a detailed exploration of the most stable configuration for Ca-doped c-ZrO 2 (111) and Ce-doped c-ZrO 2 (111). Next, surface mapping by basic probe molecules (NH 3 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 CO 2 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 -CO 2 systems from initial η 2 -CO 2 configurations indicates a decrease in the required energy for forming oxygen vacancies in the doped ZrO 2 systems at NNN positions or further away from the existing vacancy site.