Tungsten Trioxide Intercalation

This procedure was modified by Tess Jacquez and George Lisensky from "Hydrogen Insertion into WO₃" by M. Stanley Whittingham in Teaching General Chemistry: A Materials Science Companion, ACS Books (1993).

Tungsten trioxide has an idealized rhenium oxide, ReO₃, structure consisting of WO₆ octahedra joined at their corners. It may also be considered as having the perovskite structure of CaTiO₃ with all of the calcium sites vacant. An atom inserted into the center of the WO₃ structure gives M_xWO₃, where M can be an H, Li, Na, K, Rb, Ca, Sr, Ba, Al, In, Tl, Sn, Pb, Cu, Ag, Cd, the rare earth elements, or NH₄⁺. In this experiment, you are going to make H_xWO₃.

Partial reduction of the WO₃ and charge balance through intercalation of hydrogen ions into the cavities of the structure adds electrons to the conduction band resulting in color and conductivity changes. These electrons make the material behave like a metal, with both its conductivity and color being derived from free electron behavior. Redox coloration reactions are now being used in electrochromic displays for sunglasses and rear-view mirrors in cars, and has been proposed for adjusting the transmission of light through the glass panes in large buildings.

Clicking a thumbnail on this web page (http://chemistry.beloit.edu/edetc/nanolab/WO3/index.html) shows a movie of that step.

Procedure

Wear eye protection

Place 0.5 g of WO₃ into a 150-mL beaker. CAUTION: WO₃ is an irritant. Avoid creating or breathing dust. Avoid eye and skin contact. Wash hands thoroughly after handling.

Add about 50 mL of 3 M hydrochloric acid onto the WO₃. What happens?

Add 0.6 g zinc filings to the acid and observe what takes place. CAUTION: Keep away from open flames.

Wait for the reaction to finish.

(During this time record the conductivity and the x-ray powder diffraction of the starting material. See below.)

After all reaction has ceased, suction-filter off the solid product.

Wash twice with water and air-dry for a few minutes. Record the weight of dry product obtained.

X-Ray Powder Diffraction

Tightly wrap a piece of tape with the sticky side out all the way around a microscope slide. Stick to an aluminum holder.

Fill the holder with powder and smooth to a uniform depth. Prepare both the product and the starting material.

For both the product and for the starting material place the holder in the diffractometer and scan from 20 to 35°. Have you obtained a different material? WO₃ is triclinic with three strong peaks having d spacing greater than 3.6 Å, each corresponding to half a unit cell dimension. H_xWO₃ has higher (tetragonal) symmetry.

Electrical Resistance

Use a plastic nut and two bolts. Screw a bolt all the way into one side.

Add sample. Screw another bolt into the opposite side. Tighten the thumbscrew by hand.

Is the pressed powder conductive? Compare resistance of the starting material with that of the product.

Reversibility

Place the sample in an oven at 100°C for 90 minutes or keep the sample at room temperature for three days. Change in coloration can show alteration on the molecular level, but conductivity and x-ray diffraction analysis can verify if the reaction is indeed reversible.

Questions

1. What is the color of the WO₃? of H_xWO₃?
2. Are there any visible changes when hydrochloric acid is added to the WO₃ in beaker?
3. Which is a better reducing agent, H₂ or Zn(s)?
4. What weight of starting material did you use? What weight of product did you get?
5. Have you synthesized a different material? How do you know?
6. Are the pressed powders conductive? Is there a conductivity difference between starting material and product?
7. Is the reaction reversible? What data supports your answer?
8. What are the balanced chemical equations for the reduction and re-oxidation in air of the WO₃?

Materials

Tungsten trioxide, WO₃, Aldrich 232785

Zn, granule 20 mesh, Aldrich 243469

3 M HCl

150 mL beaker, 50 mL graduated cylinder

Suction flask, Hirsch funnel and adapter, filter paper

Tape, glass microscope slide, aluminum XRD holder

Obtain a piece of polyethylene rod. Drill all the way through. Twist in the tap from each end to thread the drilled hole. The thumbscrews should not touch even if both are screwed in all the way.

Polyethylene Rod 1/2" Diameter x 9/16" Long, McMaster-Carr 8701K41.

Drill and Bit (7/64" or #36), McMaster-Carr 8947A114 or 8947A189

6-32 tap, McMaster-Carr 2523A446, and T-handle tap wrench, McMaster-Carr 25605A63

Thumbscrews 6-32 x 1/4" (brass), McMaster-Carr 92421A144

Multimeter

---