A Theoretical Investigation of Brønsted Acids Chemisorption on ZnO(0001)

Density functional theory coupled to the molecular cluster approach has been used to investigate the chemisorption of simple Brønsted acids (H2O, H2S and HCN) on the ZnO(0001) polar surface. Geometrical parameters and vibrational frequencies for species molecularly and dissociatively chemisorbed have been computed. The agreement with experimental measurements, when available, has been found to be very good. The nature of the bonding between the conjugate base of each examined Brønsted acid and the Lewis acid site on the investigated surface has been thoroughly discussed. In particular, it has been elucidated the role played by the interaction between completely occupied levels in determining the actual chemisorption geometry. Enthalpy variations associated to titration displacement reactions have been evaluated, confirming the leading role played by the bonding between the Brønsted conjugate base anions and the Lewis acid site available on the surface in determining the actual relative acidity scale. The strength of this interaction follows the order OH− ≈ CN−>SH−.