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

Fourier transform Raman spectra of methyl red adsorbed on untreated and pretreated γ -alumina and silicaalumina calcined at 900 ∘C under 1 atm steam flowing were recorded. Spectral analysis shows that the active species adsorbed on γ -alumina was to be deprotonated methyl red, and on silica-alumina to be di-protonated. This indicates that γ -alumina adapted in this work holds Bronsted basicity, and silica-alumina Bronsted acidity. Raman intensities of methyl red on pretreated γ -alumina are about three times stronger than on untreated γ -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 γ -alumina were observed by increasing the surface hydroxyl groups on the catalysts through pretreatment of the steam calcination.