Relationship between Pd oxidation states on TiO(2) and the photocatalytic oxidation behaviors of nitric oxide.

This study has been undertaken to investigate the relationship between Pd oxidation states on TiO(2) photocatalysts and their photocatalytic oxidation behaviors of NO. Three types of Pd-modified TiO(2) 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 TiO(2) catalysts were Pd(2+), PdO, and Pd(0), respectively. NO-TPD results showed that the NO adsorption capacity was improved greatly by the modification of Pd(2+) ions. The activity tests showed that Pd-modified TiO(2) by a wet impregnation method increased photocatalytic activity compared to pure TiO(2) (Degussa P25). It was concluded that Pd(2+) ions on as-prepared TiO(2) catalysts provided key contributions to the improvement of photocatalytic activity. However, Pd(0) and PdO deposits on TiO(2) almost had no positive effect on NO oxidation. The mechanism of photocatalytic oxidation of NO in gas phase over Pd-modified TiO(2) was also proposed.