Fourier Transform Raman Studies of Methyl Red Adsorbed on γ-Alumina and Silica-Alumina

Abstract

Fourier transform Raman spectra of methyl red adsorbed on untreated and pretreated <TEX>${\gamma}$</TEX>-alumina and silicaalumina calcined at 900 <TEX>$^{\circ}C$</TEX> under 1 atm steam flowing were recorded. Spectral analysis shows that the active species adsorbed on <TEX>${\gamma}$</TEX>-alumina was to be deprotonated methyl red, and on silica-alumina to be di-protonated. This indicates that <TEX>${\gamma}$</TEX>-alumina adapted in this work holds Bronsted basicity, and silica-alumina Bronsted acidity. Raman intensities of methyl red on pretreated <TEX>${\gamma}$</TEX>-alumina are about three times stronger than on untreated <TEX>${\gamma}$</TEX>-alumina, while spectral features are unchanged. For silica-alumina, spectral features show modified vibrational characteristics upon surface hydroxylations generated from pretreatment. Consequently, the acidity loss for silica-alumina and the basicity gain for <TEX>${\gamma}$</TEX>-alumina were observed by increasing the surface hydroxyl groups on the catalysts through pretreatment of the steam calcination.