Enhancing the Rate of Organic Material Decomposition Photo Catalyzed by High Performance Visible Light Activated Tungsten Oxide

Abstract

We report a novel approach for increasing the rate of organic material decomposition photo-catalyzed by visible light activated tungsten oxide (WO₃). It was confirmed that photo-catalysis by WO₃ nano-particles fabricated by gas phase method and annealed at 600 °C for 1 hour achieves 30 times higher decomposition rate compared to photo-catalysis by commonly reported nitrogen doped titanium dioxides (TiO₂). 1) The high decomposition rate in the presence of WO₃ nano-particles could be attributed to their high crystalline quality and large surface area. It was also found that the decomposition rate can further be doubled by addition of metal oxides such as zirconium oxide (ZrO₂) or TiO₂. Increased adsorbability of organic material by the WO₃ nano-particles as a result of ZrO₂ or TiO₂ addition could explain the observed enhancement in the decomposition rate. Furthermore, it is shown that immersing WO3 nano-particles mixed with ZrO₂ or TiO₂ in Pt colloid or Ru ionic solutions achieve complete decomposition of organic material to CO₂ and H₂O. This is made possible by the catalytic effect of Ru and Pt, which decreases the activation energy required for breaking down C-C bonds. These novel methods are already in the mass production stage.

Experiment and Result

It is well known that photo-catalyst activity improves with higher specific surface area and/or better crystallinity.In order to achieve higher surface area and improved crystallinity we fabricated nano-sized WO₃ particles from vapor phase, followed by annealing at atmospheric conditions. A TEM image of the WO₃ nano particles obtained from the ammonium paratungstate vapor is shown in Figure 1. The grain size was about 10 nm before annealing and about 20 nm after one hour 600°C annealing. The Raman spectra are shown in Figure 2. Before annealing the WO₃ peaks at 700 and 800 cm-1 are broad, and the W=O terminating group peak appears in 930 cm^-1. After annealing and at higher annealing temperatures the former two peaks become sharper, while the latter almost disappears. The peaks at 140, 270 and 320 cm^-1 belong to monoclinic WO₃ crystals and indicate improved cristallinity with monoclinic structure at higher annealing temperatures.2)3)  With these WO₃ nano particles organic material decomposed at a 30 times higher rate as compared to decomposition catalyzed by commonly reported nitrogen-doped titanium dioxides.(Fig. 3) Comparing the measured decrease of organic material concentration in the presence of WO₃ with the reaction rate equations for the processes involved indicates that the decomposition rate is controlled by the speed of organic adsorption at the photocatalyst surface. To increase the adsorbability of the organic materials, metallic oxides such as ZrO₂ and TiO₂were added. As a result, the decomposition rate can further be doubled by addition of metal oxides.

The remaining issue is the weaker oxidizing ability of WO₃ as compared to TiO₂. This diminishes WO₃’s capability for decomposition of some organic molecules, e.g. acetic acids. As a solution to this problem it is proposed to add ruthenium or platinum by immersing the WO₃ nano-particles mixed with ZrO₂ or TiO₂ in Pt colloid or Ru ionic solutions. These results suggest that complete decomposition of acetic acid with WO₃photocatalyst may be possible, too. (Fig. 4)

It is believed that the synthesized photocatalyst reported here, can be used as indoor deodorant.

1)  R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki, Y. taga, Science, 293, 269 (2001)

2) M. Boulova and G. Lucazeau, J. Solid State Chem. 167, 425(2002)

3) A. G. Souza-Filho, V. N. Freire, J. M. Sasaki, J. Mendes Filho, J. F. Juliao, U.U. Gomes, J. Raman Spectrosc. 31, 451(2000)