||The applications of azo dye are extremely versatile; therefore, searching for more efficient and environmentally friendly catalysts to synthesize azo dye molecule has become a very important issue. Aniline can be used to synthesize azobenzene in the presence of a copper catalyst. Titanium dioxide can also catalyze the conversion from aniline to azobenzene, and the reaction intermediate is believed to be surface-bound phenyl imide species. In this study, we selected aniline and phenyl isocyanate as precursors to investigate their thermal chemistry on copper and titanium dioxide single crystal surfaces. Experiments were carried out under ultrahigh vacuum conditions and various surface analytic techniques, including temperature programmed desorption (TPD), reflection absorption infrared spectroscopy (RAIR) and Auger electron spectroscopy (AES), were employed to characterize the reaction products and intermediates.|
The study of aniline on Cu(111) shows that multilayer and monolayer desorption occurred at 194K and 228K, respectively. H2 was released at 300 ~ 400K as a result of aniline dehydrogenation. The definitive identity of the surface intermediates remains unclear. The thermal chemistry of phenyl isocyanate on Cu(111) is complex. RAIR spectra showed that C6H5NCO underwent dimerization, trimerization , and condensation to form diphenyl carbodiimide (C6H5NCNC6H5) and CO2 at 150K. Dimer and trimer desorbed at 230K and 250K, respectively. Benzoyl nitrene (C6H5C(= O) N) was proposed as an intermediate produced at 230K, and remained stable until 350K. Benzoyl nitrene then decomposed into C6H5CN and desorbed at about 360K. Carbodiimide split into two parts , C6H5N(ad) and C6H5NC fragments, at 350K, reacting with H2O and O respectively to form aniline and phenyl isocyanate evolved at 390K and 360K. Some strongly bonded C6H5NC species eventually decomposed into nitrogen and carbon deposits on the surface at high temperature.
The study on TiO2 (101) single crystal is only preliminary. The cleaning of the TiO2 surface could not be achieved, so experiements were conducted under non-ideal conditions. The results of surface reactions for aniline and phenyl isocyanate are inconclusive.