DFT study of carboxylic acids modes of adsorption on rutile TiO2(0Â 1Â 1) surfaces

Plane wave density functional theory (PWDFT) has been employed to study the energy and structure of formic and acetic acids dissociative adsorption on rutile TiO2(0 1 1) surfaces. Both the bulk terminated (1 × 1) and reconstructed (2 × 1) surfaces were considered. On both surfaces the bridging (bidentate) configuration was found more stable than the monodentate configuration for formate species by 25-40 kJ mol−1. The difference in stability between the bridging and monodentate species is more pronounced in the case of acetates on the 2 × 1 reconstructed surface where the former adsorption was found equal to 162 kJ mol−1 while the latter dropped to 100 kJ mol−1. The decrease in stability of the monodentate configuration in the case of acetate species is attributed to the considerable repulsion between the methyl group and terminal TiO groups. Analyses of the density of states indicated that while the dissociated adsorption of formic acid on the 2 × 1 reconstructed surface appears complete with a clear formation of surface hydroxyl that on the 1 × 1 surface may still involve electron sharing between surface hydroxyls and the formate species. The chelating mode, whereby the two oxygen atoms of the carboxylate species are in close proximity with one surface Ti atoms was found unstable for both acids and on both surfaces. Comparison with experimental TPD results of carboxylic acids over the rutile TiO2 (0 1 1) surface is presented and discussed.