Influence of solution pH on forming silver molybdates obtained by sonochemical method and its application for methylene blue degradation

Abstract

Catalytic processes such as heterogenous photocatalysis or adsorptions have been extensively studied due to the difficulty in treating organic effluents, mainly textiles. The catalysts stability is essential to enable their ap- plicability, so that the catalytic converter does not turn into secondary residues after the catalytic cycle, enabling its re-employment in the catalytic process. In this work was studied the photocatalytic activity of different phases of silver molybdates obtained by the sonochemical method at different pH in the medium reaction. The powders were characterized by X-ray diffraction (XRD) field emission scanning electron microscopy (FE-SEM), and diffuse reflectance spectroscopy was used to estimate the Egap of the powders. The photocatalytic activity of the powders was measured by varying the degradation of the methylene blue dye (MB). According to the XRD patterns, the crystalline structure of the obtained silver molybdates changes from Ag2Mo3O10. 2H2O at very acidic pH 1 and 2, to pure Ag2Mo2O7 with the values of pH 3 and 4 it is then converted into Ag2Mo2O7 and Ag2MoO4 phases in pH 5, and shifts to pure Ag2MoO4 phase when the pH is increased up to 6. The results indicate that the microstructures of Ag2Mo3O10. 2H2O and Ag2Mo2O7 phases have high adsorbent properties, with the powders obtained at pH 1 and 2 being able to completely remove the MB from the solution in 5 min and the powders of Ag2Mo2O7 phases removing up to 90% of MB after 5 min of exposure without UV irradiation. In contrast, the powders with Ag2MoO4 phase showed high photocatalytic properties, being able to removing up to 90% of MB in less than 20 min and 40 min of ultraviolet exposure to powders obtained at pH 5 and 6, respectively. All presented catalysts maintained their stability and performance after 3 test cycles