Preparation and heterogeneous photocatalytic behaviors of the surface-modified porous silica materials impregnated with monosubstituted keggin units

Amine-funetionalized mesoporous and macroporous silica materials impregnated with transition-metal-monosubstituted polyoxometalate K-6[Ni(H2O)SiW11O39] (SiW11M) clusters were prepared by coordination of nickel centers in the cluster with the amine surface groups in silica supports. The structures and compositions of the materials were characterized by UV-vis diffuse reflectance spectra (DR-UV-vis), infrared (IR) spectra, and Si-29 MAS NMR, indicating that the SiW11Ni clusters were chemically attached to the silica supports, and the primary Keggin structure remained intact in the as-synthesized composites. The topologies and porosities of the materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and nitrogen adsorption determination. The heterogeneous photocatalytic behaviors of the materials were tested via degradation of dye rhodamine B (RB), and a RB degradation mechanism was proposed based on the detected intermediates and final products. That is, photocatalytic degradation of RB on the supported SiW11Ni underwent the successive steps of deethylation, deamination, decarboxylation, and cleavage of the chromophore ring structure; resulting in the final products of CO2, NO3-, and Cl- ions. A Drop of SiW11Ni clusters from the amine-modified silica matrix into the reaction system was hardly observed during the photocatalytic tests, attributed to the strong coordination interaction between the SiW11Ni molecule and the amine-modified silica support. (C) 2003 Elsevier Science (USA). All rights reserved.