Characterization of alumina surfaces by fluorescence spectroscopy. Part 1. Grafting a pyrene derivative on γ- and δ-alumina supports

With the aim of characterizing the proximity and the distribution of hydroxyl groups on alumina surfaces by fluorescence spectroscopy, a fluorophore containing a pyrene moiety linked to a triethoxysilane group (PPTEOS) was synthesized and grafted on the surface of two alumina supports (g-Al2O3 and d-Al2O3) at different surface concentrations. Elemental analysis showed that the grafting of PPTEOS was less efficient on the delta-Al2O3 as compared to the gamma-Al2O3 . This observation is in full agreement with IR spectroscopy measurements on the commercially available gamma-Al2O3 and delta-Al2O3 dehydrated at 823 K. Information on alumina sintering can thus be obtained. The ability of pyrene to form excimers was used to detect probes that were in close proximity to each other and thus gave indirect information of the distribution of hydroxyl groups on the surface. The fluorescence intensities and surface concentration data indicated that pyrene derivatives chemically bound to alumina were not homogeneously distributed but rather clustered in regions. A scheme of the distribution of hydroxyl groups on the alumina surface is proposed.