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

Abstract

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.