Molecular modeling of the adsorption and initial photocatalytic oxidation step for para-nitrophenol on nano-sized TiO2 surface

In the present work, the non-constrained optimization for three proposed models of PNP molecule onto TiO2 (100) surface has resulted in three molecular adsorption conformations; a vertical alignment via bridged NO2, a parallel alignment bridges via NO2 and OH groups, and a monodentate vertical alignment bridged via OH. The first configuration is the dominant adsorption structure. The adsorption energy for the bridging geometries is found to be much more stable than that of the monodentate conformer. The predicted VDOS spectra indicate that the main interaction between PNP molecule and the surface of TiO2 takes place through a linkage between the Lewis acid sites, fivefold Ti (V) and fourfold Ti (IV) coordinated titanium surface atoms, and the nitro or phenolic oxygen atoms. Frontier electron theory (FET) and Wheland localization approach (WLA) have been applied for the prediction of the primary •OH initiated oxidation intermediates. These approaches highlight similar expectations for the variation in the reactivity of the ring positions, towards the hydroxyl radical attack. By comparison with the available experimental data and other theoretical calculations, it is shown that MSINDO can reproduce literature data with acceptable accuracy.