Synergetic Effects of Pd0 Metal Nanoparticles and Pd2+ Ions on Enhanced Photocatalytic Activity of ZnWO4 Nanorods for Nitric Oxide Removal

Doping and novel metallic nanoparticles loading on the photocatalyst are two effective means to enhance its photocatalytic activity. In our study, Pd0/Pd2+-co-modified ZnWO4 nanorods were fabricated by a two-step hydrothermal process and room-temperature reduction method. The performance of the as-prepared samples was evaluated through the photocatalytic nitric oxide (NOx) removal under simulated solar and visible-light irradiation. Pd0/Pd2+-co-modified ZnWO4 nanorods present a significantly enhanced photocatalytic activity for NOx removal compared with Pd0-loaded or Pd2+-doped ZnWO4 under simulated sunlight irradiation owing to a narrower band gap of Pd2+ doping compared with that of pure ZnWO4. The role of Pd0 nanoparticles is to act as an electron reservoir to restrain the recombination of e-/h+ pairs. According to the trapping measurements, the photoinduced holes and electrons play critical roles during the photocatalytic process. In addition, electron spin resonance (ESR) results further confirm that •O2- and •OH radicals are present and assist in the photocatalysis under simulated solar light irradiation. Stability test demonstrated that 1.5% Pd0/0.5% Pd2+-co-modified ZnWO4 nanorods as photocatalyst have high photocatalytic stability in NOx removal. This work proved that Pd0/Pd2+-co-modified ZnWO4 nanorods can be considered as an efficient photocatalyst for NOx removal.