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

doi:10.1016/j.chemosphere.2009.07.060

Chemosphere

Volume 77, Issue 2, September 2009, Pages 264-268

https://doi.org/10.1016/j.chemosphere.2009.07.060 Get rights and content

Abstract

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.

Introduction

Nitrogen oxides (NO_x) exhausted into an atmosphere from stationary sources and mobile sources have caused various environmental problems, such as ozone depletion, photochemical smog and the acid deposition (Castro et al., 2001, Farrell, 2001). Wet scrubbing method promises to an efficient and economical way to remove soluble pollutants (Jeong et al., 2006). In order to remove NO_x by wet scrubbing method, it is necessary to oxidize nitric oxide (NO) to more soluble nitrogen dioxide (NO₂) or dinitrogen pentoxide (N₂O₅) in either gas or liquid phase (Mok, 2006).

Recently, it has been reported in our study that photocatalytic oxidation of NO over Pd-modified TiO₂ showed very different behaviors from that on Degussa P25 (Sheng et al., 2008). In that study, it was observed that Pd dopant existed as combined states of Pd⁰ metal, Pd²⁺ ions and PdO particles on as-prepared photocatalysts prepared by a thermal impregnation method. Therefore, the inter-relationship between Pd oxidation states on TiO₂ and the photocatalytic oxidation behaviors of NO was worthy of investigation to reveal NO photocatalytic oxidation mechanism over Pd-modified TiO₂.

Until now, only little information about the effect of Pd oxidation states on the photocatalytic behaviors of TiO₂ has been reported (Papp et al., 1993, Belver et al., 2003, Iliev et al., 2004). Iliev et al. (2004) reported the enhanced photocatalytic degradation of xylenol orange in aqueous solution over Pd photodeposited on Degussa P25 TiO₂. It was found that the surface of loaded Pd was reduced from Pd²⁺ (PdO) to metallic Pd⁰ during the degradation, which suggested that the Pd oxidation states could also be subjected to dynamic changes under different reaction conditions. In the study of the photo-oxidation of toluene, it was observed by Belver et al. (2003) that the surface of metallic Pd⁰ particles on TiO₂ were partly oxidized to Pd²⁺ ions during photoreaction, which might be adsorption centers of CO.

In the present study, three types of Pd-modified TiO₂ with different Pd oxidation states were prepared by wet impregnation method, neutralization method and photodeposition method, respectively. And the catalysts were characterized by X-ray diffraction analysis (XRD), X-ray photoelectron spectrum analysis (XPS), UV–Vis diffuse reflectance spectra (UV–Vis DRS) and temperature programmed desorption (TPD). Finally, the relationship between the Pd oxidation states and NO photocatalytic oxidation activity of the catalysts was discussed.

Section snippets

Catalysts preparation

Commercial TiO₂ Degussa P25 was used as the precursor. Wet impregnation, neutralization and photodeposition methods were used to prepare Pd-modified TiO₂ photocatalysts with different Pd oxidation states.

Wet impregnation method (TiO₂ (W)): The samples were prepared by dispersal of 8.0 g TiO₂ powder into 100 mL Pd²⁺/HCl solution (40 mg 100 m L⁻¹) to obtain Pd loading of 0.5 wt.%. The slurry was then stirred for 48 h. After that, it was evaporated at 60 °C for 12 h in air.

Neutralization method (TiO₂ (N))

Characterization of the Pd-modified TiO₂

The XRD patterns of P25 and three types of Pd-modified photocatalysts with 0.5 wt.% Pd doping content are shown in Fig. 1. Peaks marked “A” and “R” correspond to anatase and rutile phases, respectively. From Fig. 1, all photocatalysts displayed a mixture of anatase and rutile, and no Pd containing species were observed. The deposition of Pd slightly modified the morphological properties of the TiO₂, as listed in Table 1. It was shown that the size of the crystallite of anatase and rutile

Conclusions

In this study, three types of Pd-modified TiO₂ with different Pd oxidation states were prepared by wet impregnation method, neutralization method and photodeposition method, respectively. XPS results showed that the dominant oxidation states of Pd on TiO₂ (W), TiO₂ (N) and TiO₂ (P) were Pd²⁺, PdO, and Pd⁰. Pd-modified TiO₂ prepared by a wet impregnation method showed a superior photocatalytic activity among these photocatalysts. It was concluded that Pd²⁺ deposited on TiO₂ gave the main

Acknowledgements

This work was financially supported by the National High-Tech Research and Development Program (863) of China (2007AA061701), the Excellent Young Teacher Support Program of Zhejiang University (2007) and the New Century Excellent Scholar Program of Ministry of Education of China (NCET-04-0549).

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

Abstract

Introduction

Section snippets

Catalysts preparation

Characterization of the Pd-modified TiO2

Conclusions

Acknowledgements

Appl. Catal., B

Atmos. Environ.

J. Photochem. Photobiol., A

Energy Policy

J. Catal.

J. Solid State Chem.

Catal. Commun.

Appl. Catal., B

J. Mol. Catal. A – Chem.

Chem. Eng. J.

Catal. Today

Chem. Eng. J.

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

Characterization of the Pd-modified TiO₂