Preparation, characterization, and photocatalytic activity of three-dimensionally ordered macroporous hybrid monosubstituted polyoxometalate K5[Co(H2O)PW11O39] amine functionalized titanium catalysts

Abstract

A novel and efficient, amine-functionalized three-dimensionally ordered macroporous (3DOM) titanium (Ti) catalyst modified using transition metal-monosubstituted polyoxometalate (3DOM K5[Co(H2O)PW11O39]–(EtO)3SiCH2CH2CH2NH2–TiO2 or 3DOM PW11Co–APS–TiO2) was fabricated by self-assembly method. First, methanol bath was used to soak polystyrene spheres to obtain large size 3DOM TiO2, which was further functionalized using 3-aminopropyltriethoxysilane (APS). Amine-functionalized 3DOM TiO2 materials impregnated with monosubstituted polyoxometalate (POM, K5[Co(H2O)PW11O39]) were prepared by coordination of cobalt (Co) present in the units of POM, with the amine groups present on the surface of 3DOM TiO2. The materials so obtained were characterized by Fourier transform infrared spectroscopy, X-ray diffraction, Inductively coupled plasma atomic emission spectrometry, Ultraviolet–visible (UV–vis) diffuse reflectance spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy, Scanning electron microscopy, Transmission electron microscopy, and nitrogen adsorption–desorption tests. The results showed that the primary Keggin structures of the composites remained intact, simultaneously, 3DOM PW11Co–APS–TiO2 had anatase crystalline structure, and PW11Co was successfully bonded to 3DOM TiO2 by CoN bond with APS as the coupling agent. Broadening of absorption band was observed for the product compared to the monomers, 3DOM TiO2 and PW11Co. However, the 3DOM structure was retained in the product; therefore, the accessibility of the diffusing species was significantly improved because of the permeability offered by the macroporous structure. The UV photocatalytic degradation of organic pollutants revealed better activities of 3DOM PW11Co–APS–TiO2 compared to PW11Co, 3DOM TiO2, and PW11Co–APS–TiO2. Moreover, 3DOM PW11Co–APS–TiO2 exhibited superior photocatalytic degradation of Rhodamine B (RB), Salicylic acid (SA), Congo red (CR), and Malachite green oxalate (MG) under microwave-assisted irradiation. Leakage of PW11Co from the hybrid photocatalyst was barely observed during the photocatalytic tests, attributed to strong coordination interactions between the Keggin units and the amine-functionalized Ti surface. The capture experimental results of photocatalysis revealed that the dominant active species were OH• and O2−•. Moreover, in this study, the procedure and possible mechanism of the UV photodegradation of organic pollutants in the aqueous system was proposed and discussed.