NH3adsorption on MCM-41 and Ti-grafted MCM-41. FTIR, DR UV–Vis–NIR and photoluminescence studies

Abstract

NH₃ was used as molecular probe to monitor the nature of both Brønsted and Lewis acid sites as well as co-ordination sphere of Ti(IV) centres in Ti-grafted MCM-41 by means of FTIR, DR UV-Vis-NIR and photoluminescence spectroscopies. A bare MCM-41 sample was also studied for comparison. Diffuse reflectance UV–Vis–NIR spectroscopy was used to investigate both electronic and vibrational properties of Ti-MCM-41. The UV–Vis region (55 000–14 300 cm⁻¹) gave information on the co-ordination of Ti(IV) ions and the NIR region (14 300–4000 cm⁻¹) on overtones and combinations of fundamental modes of surface OH and adsorbed NH₃ falling in the medium IR (MIR) region. Significant effect related to the anarmonicity of the oscillators were observed. MCM-41, after calcination at 550 °C in O₂, exhibits only isolated Si–OH groups (fundamental ν_OH stretching band at 3745 cm⁻¹, overtone at 7321 cm⁻¹ and combination of stretching and bending modes at 4530 cm⁻¹). Ti-containing MCM-41 shows, besides isolated Si–OH groups, OH groups likely close to Ti Lewis acid sites (fundamental at 3725 cm⁻¹, overtone at 7300 cm⁻¹ and combination of stretching and bending modes at 4530 cm⁻¹). NH₃ was used as molecular probe to study the acidity of such materials by analysing the vibrational features of adsorbed ammonia in the NIR and MIR region. FTIR spectroscopy also revealed that the Brønsted groups are sufficiently acidic to protonate some ammonia molecules, to produce ammonium ions. DR UV–Vis and photoluminescence studies of Ti-MCM-41 revealed the presence of Ti(IV) Lewis sites in tetrahedral co-ordination, which, by the insertion of NH₃ molecules, enlarge the co-ordination sphere from tetrahedral to octahedral.