Adsorption Configurations and Reactions of Nitric Acid on TiO(2) Rutile (110) and Anatase (101) surfaces

Abstract

The adsorption and reactions of the monomer and dimer of nitric acid on TiO(2) rutile (110) and anatase (101) surfaces have been studied by first-principles density functional theory with ultrasoft pseudopotential approximation. The most stable configuration of HNO(3) 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 NO(3)(a) + H(a). The rotation of NO(3) requires a barrier of 12.2 kcal/mol to form the didentate configuration, Ti(5c)-ON(O)-Ti(5c)H-O(2),(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 2HNO(3) molecules, the most stable configuration, 2(Ti(5c)-ON(O)OH...O(2c)(a)), has a structure similar to two single HNO(3) 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 HNO(3) molecule. The result suggests that the interaction of the two planar HNO(3) adsorbates is negligible. The dehydration from 2(Ti(5c)-ON(O)OH...O(2)c(a)) forming N(2)O(5)(a) + H(2)O(a) requires an energy barrier of 46.2 kcal/mol, indicating that the dimerization of the two HNO(3)(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 NO(3) radical, which may be employed as a linker between semiconductor quantum dots such as InN and the TiO(2) surface.