Probing the Fe(III) sites in mesoporous FeMCM-41

Abstract

Spectroscopic techniques such as 29Si MAS-NMR, Mössbauer, magnetic susceptibility, EPR and microwave variation EPR studies were used in order to study the location, coordination, oxidation state and stability of trivalent in FeMCM-41. 29Si MAS-NMR studies revealed that paramagnetic Fe(III) is present in the direct vicinity of [SiO 4] tetrahedral and therefore results in broadening of spectral lines. Absence of side spinning bands are attributed to the uniform dispersion of Fe x O y within the matrix. Observed isomer shift value of <0.3 mm/s and the appearance of sexset pattern in Mössbauer spectrum is attributed to the tetrahedral coordination of Fe(III) and superparamagnetic Fe x O y nanoparticles, respectively. The co-existence of paramagnetic and superparamagnetic Fe(III) is also inferred from magnetic susceptibility studies. Analysis of EPR spectrum indicated that Fe(III) is present in two different tetrahedral sites along with octahedral coordination [S.K. Badamali, A. Sakthivel, P. Selvam, Catal. Lett. 65 (2000) 153]. Room temperature microwave power variation electron paramagnetic resonance (EPR) (0.5–10 mW) studies are in good agreement with above interpretations. During template removal, Fe(III) is preferably dislodged from the distorted framework sites irrespective of calcination temperature. The population of unsymmetrical Fe(III) species is inferred to be relatively less as compared to the symmetrical sites, in the calcined form.