Reactivity of TiO₂ Rutile and Anatase Surfaces toward Nitroaromatics

The Au-TiO(2) system is a promising catalyst for the synthesis of nitro-aromatic compounds. The adsorption of azobenzene (C(6)H(5)N=NH(5)C(6)) and aniline (C(6)H(5)NH(2)) on two single-crystalline TiO(2) surfaces, anatase (101) and rutile (110), has been investigated with scanning tunneling microscopy (STM), low energy electron diffraction (LEED), and X-ray photoemission spectroscopy (XPS). While azobenzene adsorbs as an intact molecule at low coverages, ordered overlayers of phenyl imide (C(6)H(5)N) form at saturation coverage, indicating that TiO(2) surfaces cleave the N horizontal lineN bond even without the presence of Au. The same superstructures, p(1 x 2) on anatase and c(2 x 2) on rutile, form upon adsorption of aniline, suggesting the formation of the same, or a very similar, reaction intermediate. These results suggest that the main role of the supported Au in catalytic aniline <--> azobenzene conversion is the activation of O(2)/H(2) for de/hydrogenation reactions.