Electron Spin Resonance Study of Manganese Ion Species Incorporated into Novel Aluminosilicate Nanospheres with Solid Core/Mesoporous Shell Structure

Abstract

An ion-exchanged reaction of MnCl2 with Al-incorporated solid core/mesoporous shell silica (AlSCMS) followed by calcinations generated manganese species, where average oxidation state of manganese ion is 3+, in the mesoporous materials. Dehydration results in the formation of Mn2+ ion species, which can be characterized by electron spin resonance (ESR). The chemical environments of the manganese centers in Mn-AlSCMS were investigated by diffuse reflectance, UV-VIS and ESR spectroscopic methods. Upon drying at 323 K, part of manganese is oxidized to higher oxidation state (Mn3+ and Mn4+) and further increase in (average) oxidation state takes place upon calcinations at 823 K. It was found that the manganese species on the wall of the Mn-AlSCMS were transformed to tetrahedral Mn3+ or Mn4+ and further changed to square pyramid by additional coordination to water molecules upon hydration. The oxidized Mn3+ or Mn4+ species on the surfaces were reversibly reduced to Mn2+ or Mn3+ species or lower valances by thermal process. Mn(II) species I with a well resolved sextet was observed in calcined, hydrated Mn-AlSCMS, while Mn (II) species II with g = 5.1 and 3.2 observed in dehydrated Mn-AlSCMS. Both species I and II are considered to be non-framework Mn(II).