Hierarchical porous ZnWO₄ microspheres synthesized by ultrasonic spray pyrolysis: Characterization, mechanistic and photocatalytic NOₓ removal studies

Solar-light-driven photocatalysts with porous structure are preferred for gaseous pollutants removal at low concentration levels. In this study, hierarchical porous ZnWO₄ microspheres were synthesized by a facile ultrasonic spray pyrolysis method for the first time. The as-prepared ZnWO₄ samples were composed of microspheres with diameter ranging from 0.1 to 2 μm and it was revealed that these microspheres are formed by the self-assembly of nanoparticles. The photocatalytic performances of these microspheres were evaluated by the degradation of gaseous NOx under simulated solar light irradiation. It was found that the ZnWO₄ batch synthesized at 700 °C exhibited superior photocatalytic activity to those synthesized at 650 °C and 750 °C as well as Degussa TiO₂ P25. Both radical ·OH and O₂·⁻ radicals were found to be the major reactive species involved for NOₓ degradation as identified by electron spin resonance spectroscopy (ESR) method, which was consistent with the theoretical analysis. The excellent catalytic activity of ZWO-700 was attributed to its special hierarchical porous structure, which facilitated the separation/diffusion of the photogenerated charge carriers and the diffusion of intermediates and final products of NOₓ oxidation. The photocatalytic NOₓ removal mechanism over ZnWO₄ samples was also proposed. This study suggests that ultrasonic spray pyrolysis is a facile and scalable process to fabricate ZnWO₄ porous microspheres which are promising photocatalytic materials for gaseous pollutants purification. Copyright © 2016 Elsevier B.V.