Reactivity of TiO$_{2}$ 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), and synchrotron Ultraviolet photoemission (UPS). 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-N bond even without the presence of Au. The same superstructures, p(1 $\times $ 2) on anatase and c(2 $\times $ 2) on rutile and the same electronic structures, 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 $\leftrightarrow $ azobenzene conversion is the activation of O$_{2}$/H$_{2}$ for de/hydrogenation reactions.