The effect of calcination temperature on structure and photocatalytic properties of Au/Pd nanoparticles supported on TiO2

TiO2 modified with Au/Pd nanoparticles have been prepared using a water-in-oil microemulsion system of water/AOT/cyclohexane followed by calcination from 350 to 700 degrees C. The photocatalysts were characterized by scanning transmission electron microscopy (STEM), X-ray powder diffraction analysis (XRD), UV-vis diffuse-reflectance spectroscopy (DRS), BET surface area measurements and X-ray photoelectron spectroscopy (XPS). In-situ EPR spectroscopy was used to examine the relevance of paramagnetic species formation at the surface of Au/Pd-TiO2 photocatalysts under visible (vis) and UV-vis light excitation. The results show that enhancement of calcination temperature from 350 to 700 degrees C resulted in a slight drop of Au/Pd-TiO2 photoactivity under UV and quite rapid drop under visible light. The Au/Pd-TiO2 samples calcinated at 350 and 400 degrees C possess the highest photocatalytic activity when degrading phenol under visible light, which is more than 4 times of that of calcinated at 450 degrees C. It was observed that increasing temperature from 350 to 700 degrees C during calcination step, caused to segregation of metals and gold-enrichment in the shell region of Au/Pd bimetallic nanoparticles formed at the TiO2 surface and finally resulted in photoactivity drop. The Pd to Au ratio in the surface layer of Au/Pd nanoparticles decreased from 5:1 to 1:4 with temperature enhancement from 350 to 700 degrees C, respectively. Based on EPR spectra, it seems that formation of O.- radicals is mainly responsible for phenol degradation under UV light for all Au/Pd-TiO2 samples, since organic radicals could be related to phenol degradation under visible light. (C) 2014 Elsevier B.V. All rights reserved.