Low-temperature stabilization of electrosynthesized tetragonal zirconia, its photoactivity toward methylene blue decolorization

Abstract

Zirconia nanoparticles (ZrO2) were prepared by a simple electrochemical method and characterized using X-ray diffraction, transmission electron microscopy, Brunnauer–Emmett–Teller surface area analysis, and ultraviolet-visible diffuse reflectance spectroscopy. In this study, tetragonal ZrO2 (t-ZrO2) particles were stabilized at a lower calcination temperature without the addition of a dopant in the preparation step. It was suggested that the use of tetraethylammonium perchlorate as the supporting electrolyte was responsible for the stabilization of t-ZrO2 by preventing the agglomeration of metal clusters with undesired powders in the electrolysis system, resulting in a decrease in ZrO2 crystal size. The photoactivity of the catalyst was optimized using central composite design with response surface methodology and the optimum values of the parameters were a solution pH of 11, contact time of 4 h, catalyst dosage of 1.0 g L-1, and calcination temperature of 523 K. This resulted in 84.9% decolorization of methylene blue (MB) obtained from the predicted model, which fitted well with the laboratory results of 83.6%. The kinetics study demonstrates that the reaction followed pseudo first-order kinetics, and the rate constants KR = 0.09 mg L-1 h-1 and KLH = 0.43 L mg-1 were determined using the Langmuir–Hinshelwood model. The mineralization of MB was measured by chemical oxygen demand removal, BOD5/COD, and TOC/TOC0 ratio analyses with values of 78.5%, 1.48, and 0.21, respectively, after 4 h of contact time. The regeneration study shows that the catalyst could be maintained with a slight decrease in decolorization (<10%) after five cycling runs. Furthermore, t-ZrO2 facilitated good photoactivity towards MB decolorization under UV light in a batch reactor compared with commercial t-ZrO2 (58.7%) and Degussa P25 TiO2 (64.5%).