Reduction of fluorescence from high-area oxides of the silica, γ-alumina, silica-alumina, and Y-zeolite types and Raman spectra for a series of molecules adsorbed on these surfaces

Abstract

It is shown that strong fluorescence that occurs with untreated high-area samples of porous silica-glass, silica gel, Cab-O-Sil, γ-aluminas, a silica-alumina and sodium Y zeolite is in all cases eliminated or greatly reduced by heating in oxygen for several hours at 500 °C. This fluorescence is attributed to traces of hydrocarbons decomposed on the acidic oxide surfaces that burn away in oxygen. Some degree of fluorescence returns when alumina-containing oxides are heated at elevated temperatures in a normal high vacuum; this is attributed to slow migration of hydrocarbon impurities. Certain organic molecules, such as furan and acetone, also give rise to new fluorescence, through decomposition, on contact with porous glass surfaces or while a Raman spectrum is being obtained. After heating in oxygen at 500 °C the aluminas or alumina-containing oxides exhibit a residual weaker fluorescence between 13,000 and 14,600 cm −, which may be attributed to traces of Fe 3+ impurities. Raman scattering has been observed for all of the main bands active in pyridine, and variations in position, half-height width and relative intensity were observed. The bands in the 3000 and 1000 cm −1 regions from pyridine on the several forms of silica, when considered in relation to parallel infra-red studies, can be interpreted in terms of an initially adsorbed species involving a strong hydrogen bond between surface OH groups and the pyridine nitrogen lone-pair. Physically adsorbed pyridine, with a spectrum similar to that of the pure liquid, occurs at higher coverage. Three species are observed from pyridine on BDH γ-alumina, two of them similar to those found on silica, and the third probably held to the surface by a coordinate linkage involving the nitrogen atom. Pyridine adsorbed on NaY zeolite appears to be held to the cationic sites. Raman spectra have been obtained by adsorption of a number of other molecules on porous glass, including benzene, methyl iodide, and several substituted benzenes. In the early stages of adsorption benzene and aniline gave spectra notably different from those of the pure liquids, and possibly attributable to species held to the surface through hydrogen-bonding involving surface OH groups.