Preparation of Bi2WO6- and BiOI-allophane composites for efficient photodegradation of gaseous acetaldehyde under visible light

Clays have been used in combination with various photocatalysts to enhance the removal of organic pollutants. The aim of the present work was to study the impact of allophane as the support material on photocatalytic activity of Bi2WO6 and BiOI. In this work, we report on the preparation of highly adsorptive mechanically-mixed and as-synthesized Bi2WO6- and BiOI-allophane composites by mechanical mixing and hydrothermal synthesis, respectively. The prepared composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy (Raman), ultraviolet-visible spectroscopy (UV-vis), and BET method. The allophane powders showed higher specific surface area, SSA, (SBET = 263 m2/g) in comparison with the Bi2WO6 (SBET = 21.3 m2/g) and BiOI (SBET = 23.1 m2/g) powders. The mechanically-mixed composites possessed higher SSA compared to the as-synthesized composites probably due to the pore collapse or pore filling during the hydrothermal synthesis of the composites. The light absorption by both composites started from 700 nm; however, the Bi2WO6-allophane composites only absorbed the photons > 325 nm, whereas the BiOI-allophane composites absorbed the photons > 500 nm. The samples with higher SSA showed higher acetaldehyde adsorption. On the basis of the CO2 liberation estimated from the photodegradation experiments, the BiOI and BiOI-allophane composites decomposed acetaldehyde completely within 5-7 h, whereas the Bi2WO6, mechanically-mixed and as-synthesized Bi2WO6-allophane composites decomposed 75.5%, 100%, and 85.6% within 8 h, respectively. With its significant adsorption of acetaldehyde, allophane contributed to the efficient photodegradation of acetaldehyde by allophane-containing composites. The obtained results suggested that both mechanically-mixed and as-synthesized Bi2WO6- and BiOI-allophane composites can be useful material for efficient photodegradation of acetaldehyde under visible light.