Investigation of chromium oxide clusters grafted on SBA-15 using Cr-polycation sol

Abstract

A comparative study of chromium oxide clusters grafted on mesoporous silica SBA-15 was carried out using samples synthesized by one-pot, impregnation and Cr-polycation sol grafting methods. The nature of CrOx species incorporated into SBA-15 by direct hydrothermal one-pot method as well as impregnation of Cr(NO3)3·9H2O and Cr-polycation precursors was characterized by XRD, BET isotherms, UV–Vis DRS, FTIR, TGA, O1s XPS, 29Si-MAS NMR, H2-TPR, NH3-TPD, SEM and TEM. Powder XRD did not show the presence of Cr2O3 in the calcined samples obtained by one-pot method. It, however, shows the rhombohedral clusters of α-Cr2O3 dispersed over SBA-15 for CrOx/SBA-15 samples prepared by impregnation and polycation sol grafting methods. The absorption band at 296 nm, observed for Cr3+ in solution, is absent for the aqueous Cr-polycation sol. There is evidence that the presence of chromium precursor in the reaction medium can influence the morphology of SBA-15 as seen in the SEM micrographs. Charge transfer transitions demonstrate the insertion of CrOx species on SBA-15 matrix synthesized by one-pot method. Cr-polycation grafted SBA-15 sample shows unique vibrational features at 573 and 624 cm−1 attributed to extra-framework CrOx species. The ratio of Cr6+/Cr3+ species present in CrOx/SBA-15 samples depends on the Cr-precursor employed for grafting on SBA-15. One-pot synthesized samples predominantly contain coordinated water (δH–O–H at 1,635 cm−1) on SBA-15 while impregnated samples show water molecules associated with CrOx species (δH–O–H at 1,594 cm−1).