Ammonium Paratungstate as Precursor Material for Photocatalytic Gold-WO3 Materials
- Details
- Category: Tungsten Information
- Published on Monday, 01 February 2021 00:52
Due to widely use of fossil fuels, the environmental issues and climate change has been critical global topic. hydrogen fuel cells are clean energy considered to be possible substitutes of fossil fuel. It is even used as powerful rocket fuel for NASA’s space shuttles.
Recently a deposition of Au on WO3 by the cationic adsorption method has been reported as a potential photocatalytic material hydrogen production. Ammonium paratungstate (APT) has been used a starting material. The synthesis process can be divided into 3 major steps:
1. Preparation of the WO3 support
WO3 was prepared as described by Sánchez-Martínez et al. In a typical synthesis step, 5.478 g of ammonium paratungstate (APT, H42N10O42W12∙H2O) were dissolved in 50 mL of 30% v/v HNO3 at 70 °C. Then, 0.516 g of citric acid (C6H8O7) were added to the solution under vigorous stirring. The solution was stirred for 30 min, while the formation of a yellowish suspension occurred. The solid was separated by centrifugation and washed several times with 100 mL of distilled water per gram of material. After the washing step, the solid was dried under vacuum at 80 °C for 2 h. Lastly, the solid was annealed at 700 °C in a furnace for 3 h, using a temperature ramp of 10 °C min−1.
2. Preparation of the [Au(en)2]Cl3 complex
The [Au(en)2]Cl3 complex was synthesized following the procedure proposed by Block and Bailar [37]. In brief, 1 mL of ethylenediamine was mixed with 5 mL of diethyl ether. The solution was added to a second solution containing 10 mL of diethyl ether and 1 g of HAuCl4∙3H2O (violent reaction); a gummy yellowish precipitate was formed. The solid was dissolved in 1 mL of water and consecutively re-precipitated with 10 mL of ethyl alcohol. The process was repeated three times until a white solid was obtained.
3. Preparation of Au/WO3 by cationic adsorption
The preparation of the Au/WO3 material was accomplished via the cationic adsorption of the positively charged Au-ethylenediamine complex on the negatively charged WO3 surface. For this, 1 g of the support was dispersed in 100 mL of a preheated (45 °C) aqueous solution containing the [Au(en)2]Cl3 complex. The initial pH of the suspension was 4. The mass added of the Au-ethylenediamine complex was set to achieve nominal Au loadings of 0.1, 0.5, 1.0 and 3.0 wt.%. Calculations were done considering that 46% of the complex mass corresponded to Au, according to analysis by ICP-OES. The suspension was thoroughly stirred to promote the adsorption of the positively charged [Au(en)2]Cl3 complex on the negatively charged WO3 particles. The optimization of the cationic adsorption procedure was carried out by modifying either the pH of the suspension or the reaction time. For the former, four pH values were tested, namely 1, 4 (natural pH), 7 and 10; which were achieved by adding dropwise either 1 M ethylenediamine or 0.1 M hydrochloric acid. The reaction time was varied by testing 2, 12, and 16 h. All the reactions were performed under dark conditions and 45 °C in order to avoid the reduction of gold and the decomposition of the [Au(en)2]3+ complex, respectively [29]. Upon the Au adsorption, the solid was separated by centrifugation at 10,500 rpm for 8 min. Then, the sample was washed with 100 mL of distilled water at 40 °C, in order to remove the Cl− ions and other residues. The washing step was repeated four times; then, the solid was dried for 12 h at room temperature under vacuum conditions. To degrade the organic complex and to nucleate the gold nanoparticles, a thermal treatment from room temperature to 350 °C at a heating rate of 2 °C min−1, followed by a 3-h isothermal step at 350 °C was performed.
Air stream was used for activation of Au nanoparticles at a flow rate of 1 mL min−1 per milligram of material. After this step, electrostatic interactions of the Au complex were transformed to solid-solid interactions of the metallic nanoparticles with the WO3 support. All the samples were stored at room temperature under dark and vacuum conditions in order to prevent further growth of the Au nanoparticles.
In conclusion, the particle size of the as-prepared Au/WO3 material was 2–20 nm. Au/WO3 materials demonstrated to be more efficient than unmodified WO3 and Degussa P25 for the photocatalytic production of hydrogen. The 0.5 wt.% Au/WO3 material was 1.2 times more effective in the hydrogen production than unmodified WO3 and 2.57 times than Degussa P25.
- APT Manufacturer & Supplier, Chinatungsten Online: ammonium-paratungstate.com
- Tungsten News & Prices of China Tungsten Industry Association: www.ctia.com.cn
- Molybdenum News & Price: news.molybdenum.com.cn
- Tel.: 86 592 5129696; Fax: 86 592 5129797; Email: sales@chinatungsten.com