Effect of Mo-Incorporation in the TiO2 Lattice: A Mechanistic Basis for Photocatalytic Dye Degradation

Photocatalytic activity of TiO2 (anatase) is appreciably enhanced by substitutional doping of Mo in anatase lattice, in conjunction with the incorporation of nanostructured MoO3 within the parent anatase lattice. The photocatalyst material was characterized in detail using X-ray diffraction, Raman spectroscopy, diffuse reflectance (DR-UV-Vis spectroscopy), X-ray photoelectron spectroscopy, and electron microscopy. Photocatalysis experiments were conducted using a model rhodamine-B (Rh-B) dye reaction using both UV and visible irradiation sources. The observed trends in the case of visible irradiative source can be summarized as follows: Mo-1 < Mo-2 < Mo-5 >> Mo-10. Attempts were made to isolate the structural factors that control photochemical behavior of these Mo-TiO2 photocatalysts and to correlate photocatalytic activity with different structural aspects like oxidation state, band gap, surface species, etc. Mechanistic insights were acquired from ex situ H-1 NMR studies showing different intermediates and different probable routes for the Rh-B dye degradation with UV and visible radiations. The stable intermediates were formed by a direct oxidative fragmentation route, without any evidence of the initial deethylation route. The intermediates found were benzoic acid, different amines, diols, and certain acids (mostly formic and acetic acid). The adsorption of the Rh-B dye on the catalytic surface via the N-charge centers of the Rh-B was also observed.