Reactivity of bare and Fe-doped alumino-silicate nanotubes (imogolite) with H2O2 and the azo-dye Acid Orange 7

Abstract

The adsorption and catalytic properties are reported of aluminosilicate nanotubes (NTs) of the imogolite-type both bare (IMO, (OH)3Al2O3SiOH) and Fe-doped. Iron-doped NTs were obtained by either direct synthesis (Fe-x-IMO) or post synthesis loading (Fe-L-IMO), with 0.70wt.% Fe (Fe-0.70-IMO) and 1.4wt.% Fe content (both Fe-1.4-IMO and Fe-L-IMO). The samples were characterized by X-ray diffraction (XRD); N2 sorption isotherms at −196°C; diffuse reflectance (DR) UV–vis spectroscopy; IR spectroscopy, and electrophoretic mobility in water (ζ-potential).In water, the intrinsic acidity of Fe(OH)Al groups, formed by isomorphic substitution (IS) of Fe3+ for Al3+, is only marginally different from that of parent Al(OH)Al groups, as shown by ζ-potential measurements. Substitution of one octahedral Al3+ site, however, gives rise to the formation of three Fe–O–Al groups and three Fe(OH)Al groups: such process modifies the adsorption and catalytic properties of NTs outer surface, as shown by adsorption of the azo-dye Acid Orange 7 (NaAO7) from water solutions and by AO7− catalytic degradation by H2O2. Interaction with AO7− anions in water occurs in different ways: (i) with bare IMO, AO7− anions preferentially adsorb by electrostatic interaction with the positively charged surface, and by H-bonding; (ii) with Fe-1.4-IMO, IS Fe3+ cations act as coordination centres for N atoms in the AO7− moiety, through a ligand displacement mechanism; (iii) with Fe-L-IMO, AO7− adsorption is hindered by the presence of Fe2O3 clusters.Concerning AO7− degradation by H2O2, the outer surface of proper IMO is much more reactive than Fe-doped IMO towards hydrogen peroxide, by forming very reactive Al-OOH bridges. IS Fe3+ species are probably not accessible to H2O2, and (the expected) Fenton reaction between Fe3+ and H2O2 is not observed.