Relationship between Pd oxidation states on TiO2 and the photocatalytic oxidation behaviors of nitric oxide

This study has been undertaken to investigate the relationship between Pd oxidation states on TiO2 photocatalysts and their photocatalytic oxidation behaviors of NO. Three types of Pd-modified TiO2 with different Pd oxidation states were prepared by wet impregnation method, neutralization method and photodeposition method, respectively. And these Pd-modified photocatalysts were characterized by X-ray diffraction analysis, X-ray photoelectron spectrum analysis (XPS), UV–Vis diffuse reflectance spectra and temperature programmed desorption (TPD). It was found from XPS results that the dominant oxidation states of Pd on these Pd-modified TiO2 catalysts were Pd2+, PdO, and Pd0, respectively. NO-TPD results showed that the NO adsorption capacity was improved greatly by the modification of Pd2+ ions. The activity tests showed that Pd-modified TiO2 by a wet impregnation method increased photocatalytic activity compared to pure TiO2 (Degussa P25). It was concluded that Pd2+ ions on as-prepared TiO2 catalysts provided key contributions to the improvement of photocatalytic activity. However, Pd0 and PdO deposits on TiO2 almost had no positive effect on NO oxidation. The mechanism of photocatalytic oxidation of NO in gas phase over Pd-modified TiO2 was also proposed.