Adsorption Configurations and Reactions of Nitric Acid on TiO₂ Rutile (110) and Anatase (101) surfaces

Abstract

The adsorption and reactions of the monomer and dimer of nitric acid on TiO₂ rutile (110) and anatase (101) surfaces have been studied by first-principles density functional theory with ultrasoft pseudopotential approximation. The most stable configuration of HNO₃ on the rutile surface is a molecular monodentate adsorbed on the 5-fold coordinated Ti atom with the hydrogen bonded to a neighboring surface bridging oxygen with the adsorption energy of 6.7 kcal/mol. It can dissociate its H atom to a nearest bridged oxygen with almost no barrier to produce NO₃(a) + H(a). The rotation of NO₃ requires a barrier of 12.2 kcal/mol to form the didentate configuration, Ti_5c−ON(O)−Ti_5cH−O_2c(a), which adsorbs on two 5-fold coordinated Ti atoms with the adsorption energy of 16.5 kcal/mol. In the case of the adsorption of 2HNO₃ molecules, the most stable configuration, 2(Ti_5c−ON(O)OH...O_2c(a)), has a structure similar to two single HNO₃ adsorbates on two 5-fold coordinated Ti atoms with the adsorption energy of 12.8 kcal/mol, which is about twice that of the single HNO₃ molecule. The result suggests that the interaction of the two planar HNO₃ adsorbates is negligible. The dehydration from 2(Ti_5c−ON(O)OH...O_2c(a)) forming N₂O₅(a) + H₂O(a) requires an energy barrier of 46.2 kcal/mol, indicating that the dimerization of the two HNO₃(a) is difficult. Similar adsorption phenomena appear on the anatase (101) surface. In addition, we find that the coadsorption of hydrogen plays a significant role in the adsorption energies of adsorbates, especially for the NO₃ radical, which may be employed as a linker between semiconductor quantum dots such as InN and the TiO₂ surface.