Degradation of methylene blue and methyl orange by palladium-doped TiO₂ photocatalysis for water reuse: Efficiency and degradation pathways

This work prepared palladium-doped titanium dioxide (Pd-doped TiO₂) catalysts by a facile sol-gel method to enhance degradation of single and binary methylene blue and methyl orange by ultraviolet photocatalysis. The physicochemical properties of Pd-doped TiO₂ particles were analyzed by a X-ray powder diffractometer, X-ray photoelectron spectroscope, scanning electron microscope, transmission electron microscope, UV–visible diffuse reflectance spectroscope, sorptiometer, and transient photocurrent spectroscope. More efficient degradation was achieved in acidic and basic/neutral media for methyl orange and methylene blue, respectively. Also, methylene blue was always degraded faster than methyl orange. The highest degradation efficiency of both dyes in single and binary systems was obtained using 0.5 wt.% and 0.75 wt.% Pd–TiO₂, respectively. The degradation pathways of single methylene blue and methyl orange with Pd-doped TiO₂ and commercial P25–TiO₂ were proposed and compared by identifying reaction intermediates produced during photocatalysis. Recycling tests have confirmed excellent stability of Pd–TiO₂ materials. Also, the prepared materials were easier to separate from the treated liquid using a simple sedimentation or filtration method than P25–TiO₂. In particular, the use of sol-gel method could readily synthesize a large number of catalysts, which was beneficial for scale up consideration. All these factors highlighted their potential and prospect in practical applications for water reuse.