Hierarchical porous ZnWO₄ microspheres synthesized by ultrasonic spray pyrolysis: Characterization, mechanistic and photocatalytic NO_x 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 NO_x 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 OH and O₂^- radicals were found to be the major reactive species involved for NO_x 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 NO_x oxidation. The photocatalytic NO_x 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.