Spectroscopic Investigation of Fe(III) in FeMCM-41

Abstract

A series of Fe(III) containing mesoporous MCM-41 molecular sieves (FeMCM-41)have been synthesized hydrothermally. The samples were characterized systematically by various spectroscopic and analytical techniques in order to study the location, coordination, stability and oxidation state of iron within the matix. Here we report on the spectroscopic information, which is particularly being inferred from EPR, 29Si MAS NMR, Mössbauer and Magnetic susceptibility studies. Analysis of EPR signal indicated that Fe(III) is present in two different tetrahedral sites along with octahedral coordination. During template removal, Fe(III) is partly dislodged from the framework and migrates to non-framework and/or extra-framework sites, irrespective of calcination temperature. 29Si MAS NMR studies revealed that paramagnetic Fe(III) is present in the direct vicinity of [SiO4] tetrahedral and therefore results in broadening of spectral lines. Absences of side spinning bands are attributed to the uniform dispersion of FexOy within the matrix. The observed isomer shift value of <0.3mm/sec in the Mössbauer spectrum supported the tetrahedral coordination of Fe(III). Further, the appearance of sexset pattern is attributed to the superparamagnetic FexOy nanoparticles. The coexistence of paramagnetic and superparamagnetic Fe(III) in FeMCM-41 is also inferred from magnetic susceptibility studies.