Excellent Tungsten Trioxide Based Photocatalyst with Ammonium Paratungstate

Up to date, titanium dioxide (TiO2) is accepted as the most feasible material for photodegradation of organic and inorganic pollutants. However, the fast recombination of the photocatalytically generated charge carriers competes with the electron transfer of the reactants adsorbed on the catalyst surface, which limits its photocatalysis activity.

Combining TiO2 with a metal oxide or a metal that absorbs visible light and possessing a conduction band (CB) at a lower energy level, and CB and a valance band (VB) with higher or equal energy compared to TiO2 increases the separation of the charge carriers. Thus, an sol-gel method to produce an excellent tungsten trioxide (WO3) based photocatalyst using ammonium paratungstate (APT). Platinum (Pt) was loaded onto TiO2/WO3 composite material to boosted its properties.

The synthesis method of Pt/TiO2-WO3 composite material is as below:

Titanium butoxide (Ti(oBu)4, purum ≥ 97.0% gravimetric), ammonium paratungstate hydrate (99.99% trace metal basis, gravimetric), formic acid (reagent grade ≥ 95%) and Chloroplatinic acid hydrate, (≥ 99% trace metal basis) were supplied by Sigma Aldrich, Canada. Hydrochloric acid (certified ACS plus) was supplied by Fischer Scientific. Commercial Alcohols (Boucherville, Quebec, Canada) provided the anhydrous ethyl alcohol. All the chemicals were used as received without any further purification. Laboratory deionized water was used throughout this work.

Traditional sol-gel method was used for the synthesis of TiO2. Titanium butoxide, anhydrous ethyl alcohol, formic acid and water were mixed in the molar ratio of 60:500:27:1500 for one hour at room temperature. A pre-established amount of titanium precursor was added to anhydrous ethyl alcohol under vigorous stirring, followed by the drop-wise addition of water and formic acid. A milky sol of titanium hydroxide instantaneously formed sue to hydrolysis and condensation reactions in the reacting mixture.

We simultaneously prepared WO3 by a crash precipitation technique. The Ammonium paratungstate (APT), hydrochloric acid (HCl) and water were used in the molar ratio of 0.005:200:500, Pt precursor solution was also added to the mother liquor with a concentration calculated basing on a Pt weight percentage of 0, 0.2, 0.4, 0.6, 0.8, 1.0. Ammonium paratungstate dissolved in HCl under stirring followed by rapid addition of this solution to water, resulting in the crash precipitation of a yellow white precipitate of WO3. The precipitate was washed several times with deionized water (8 x 100 mL).

In the next steps, we added the Pt-WO3 suspension to the stable sol of titanium hydroxide (molar ratio of TiO2:WO3 is 1:1.) and stirred the hybrid suspension for 3 h before spray drying it. In the final step the dried powders were calcined at a temperature of 600 °C in a muffle furnace for 2 h to produce the crystalline Pt/TiO2-WO3 material.  The Pt-TiO2/WO3 powders include XRD, FTIR, electron microscope imaging, N2 physisorption, UV–vis-DRS, Raman, photoluminescence (PL) and XPS spectroscopy.

In summary, a sol-gel method to produce an excellent tungsten trioxide (WO3) based photocatalyst using ammonium paratungstate (APT). We varied the amount of Pt from 0.2 to 1 wt.%. The highest photoactivity can be reached at 0.8 wt.% of Pt. This powder had the highest surface area (283 m2g−1) and the highest % of macro-meso pores among the hybrid samples. The high activity of the samples ascribes to the synergistic effect of both WO3 and Pt as co-catalyst for TiO2. This synergistic effect includes the charge separation promoted by WO3 together with the charge transfer promoted by Pt.

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