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

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 directsynthesis (Fe-x-IMO) or post synthesis loading (Fe-L-IMO), with 0.70 wt.% Fe (Fe-0.70-IMO) and 1.4 wt.%Fe content (both Fe-1.4-IMO and Fe-L-IMO). The samples were characterized by X-ray diffraction (XRD);N2sorption isotherms at −196◦C; diffuse reflectance (DR) UV–vis spectroscopy; IR spectroscopy, andelectrophoretic mobility in water (-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 -potential measurements.Substitution of one octahedral Al3+site, however, gives rise to the formation of three Fe–O–Al groupsand three Fe(OH)Al groups: such process modifies the adsorption and catalytic properties of NTs outersurface, as shown by adsorption of the azo-dye Acid Orange 7 (NaAO7) from water solutions and byAO7−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 chargedsurface, and by H-bonding; (ii) with Fe-1.4-IMO, IS Fe3+cations act as coordination centres for N atomsin the AO7−moiety, through a ligand displacement mechanism; (iii) with Fe-L-IMO, AO7−adsorption ishindered by the presence of Fe2O3clusters.Concerning AO7−degradation by H2O2, the outer surface of proper IMO is much more reactive thanFe-doped IMO towards hydrogen peroxide, by forming very reactive Al-OOH bridges. IS Fe3+speciesare probably not accessible to H2O2, and (the expected) Fenton reaction between Fe3+and H2O2is notobserved.