A resonance Raman spectroscopic study of the protonation of 4-(phenylazo)-diphenylamine and 4-(4′-nitrophenylazo)-diphenylamine in solution and adsorbed on oxide surfaces

Abstract

The adsorption of 4-(phenylazo)-diphenylamine (PADA) and 4-(4′-nitrophenylazo)-diphenylamine (Disperse Orange 1, DO1) on oxide surfaces has been studied by resonance Raman (RR) spectroscopy. The neutral and protonated forms of these molecules were modelled by ab initio calculations at the B3-LYP/DZ level, enabling calculation of the vibrational spectra of these species, together with vibrational assignments. Adsorption of PADA on silica-alumina and both PADA and DO1 on H-mordenite results in protonation, indicating the presence of Brønsted acidic sites on these oxide surfaces with pK_a values ≤1.5. RR studies of the adsorption of PADA and DO1 can provide useful information regarding the nature of surface acidity on oxides which is complementary to that obtained from other probe molecules.