Introduction

As discussed in Chapters 3 and 10, light with energy higher than that of the band gap in a semiconductor (e.g., TiOi) can excite an electron from its valence band to the conduction band. This process also creates a positive hole in the valence band. The electron and hole thus produced can reduce or oxidize different pollutants (e.g., organic compounds, inorganic species, metal ions, bacteria). A summary of the advantages, challenges, and proposed solutions in Ti02 photocatalysis is given in Chapter 10. Even though a reduction and an oxidation always occur, only in a few cases are both the anodic and cathodic reactions advantageously utilized (see Colon, 2001 and Layman, 1995).

In the present experiment we describe a mi-croscale experiment in which the simultaneous oxidation of an organic compound (ethanol or citric

Materials

Reagents

1 quartz UV- pencil lamp

1 100-mm long quartz tube

2 rubber bands

1 100-mL beaker 1 1-mL graduated pipet 1 2-mL graduated pipet

1 magnetic stirrer

2 Beral pipets 1 stirring bar

6 triangular bars 6 one-hole rubber stoppers parafilm 1 Pasteur pipet 1 spectrophotometer 1 spectrophotometer cell 1 cardboard box to cover the UV source 6 test tubes (5 cm long,

5 mm I.D.) 1 black plastic bag 1 glass capillary U-tube

0.04 M CuS04 Ti02 (preferably Degussa P25) D. I. water 96% ethanol 0.33 M citric acid Ba(OH)2

To power source

Optional C02 test

Figure 1. Experimental set-up. (Reproduced from the Journal of Chemical Education 2005, 82, 1549-1551, with permission).

Arrange a set-up consisting of a UV light source (e.g., a miniature UV-quartz pencil lamp, Spectro-line model 11SC-1, Spectronics Corp.). If a miniature light source is used, it can be introduced into a quartz test tube (e.g., Ace Glass 13 mm OD, 100 mm long quartz tube without lip). Surround this tube with six quartz tubes containing different suspensions as described below, cover each one with parafilm, and secure the bundle with two rubber bands. Place the bundle inside a 100-mL beaker on top of a magnetic stirrer (see Figure 1). Add a small stirring bar to each of the six tubes (triangular bars of the type used with conical-bottom vials work well). Depending on your objectives, place the different solutions inside the test tubes according to the qualitative or quantitative procedures described below (or do both).

In the qualitative option, the test tubes will contain a Cu(II) solution, a Ti02 suspension and some ethanol. One component will be missing from each test tube and the observed results interpreted accordingly. In the quantitative option, citric acid will be used instead of ethanol, and all the test tubes will have the same contents; irradiation time will be varied and the Cu(II) removed will be analyzed as a function of time.

Because the oxidation of the organic compound produces C02, you may desire to do a test for its production. To this end, instead of using parafilm, cap each quartz tube with a one-hole rubber stopper connected by a glass capillary U-tube to a small test tube (e.g., 5 cm long, 5 mm I.D.) half-filled with a saturated Ba(OH)2 solution as a C02 trap (see Figure 1). Before initiating the UV illumination make sure that all six stirring bars are mixing well each solution. Caution: UV light is dangerous. Avoid exposure of any part of the body (particularly the eyes), by placing a thick cardboard box or another suitable protective covering between the UV lamp and the experimenter. For added protection, cover the whole set-up with a black polyethylene bag. Copper (II) sulfate is a strong irritant to skin and mucous membranes. BaC03 and Ba(OH)2 are poisonous. Overexposure to Ti02 may cause slight lung fibrosis, and it is a potential occupational carcinogen.

Because ambient dioxygen is a good electron trap, it should be removed from the suspensions since it competes as an electron acceptor with Cu(II). To this end, bubble nitrogen gas for a couple of minutes from a nitrogen tank into each test tube (except one, as described below) containing the prepared solutions and then seal each tube with parafilm.

1 ) Qualitative procedure: Observation of the photocatalytic effect.

With distilled or deionized water, prepare the following:

a) An aqueous slurry containing 100 mg of Ti02

(e.g., Degussa P25) in 50 mL of water b) A 0.04 M CuS04 solution c) 96% ethanol

As instructed in Table 1, place the following components in the corresponding test tubes. Ti02: add 2 mL of its suspension. Cu(II): add 2 mL of its solution. Ethanol: add 0.5 mL. Then, bubble nitrogen into the tubes, cover them with parafilm and proceed as instructed below.

Table 1. Components added to each test tube.

Tube#

Ti02

Cu(II)

Ethanol

n2

hv

1

-

V

V

V

V

2

V

-

V

V

V

3

•J

■J

-

V

V

4

V

V

-

-J

5

V

V

V

-

6

V

•J

V

Note: To prevent light from reaching the tube #5, wrap it with A1 foil, and subject it to the same conditions as the others.

Note: To prevent light from reaching the tube #5, wrap it with A1 foil, and subject it to the same conditions as the others.

Once all the components are in their respective test tubes, and these are all bundled and placed on site, cover the entire experimental set up with the box and bag described above and turn the UV light on. Then, wait approximately 10 min (this period of time works well for the UV lamp described above). After this time, turn it off, carefully observe the appearance of the resulting products in each test tube, write down these observations, and interpret them.

Interestingly enough, the reduction of Cu(II) to Cu(I) yields a reversible, insoluble purple complex with Ti02 (see Foster, 1993). A very small amount of elemental copper is sometimes observed as a thin layer deposit on the walls of some quartz tubes (like a copper mirror) or even on the Ti02 surface, which is further proof of the Cu(II) reduction. In addition, the production of a white precipitate in the small (optional) collection tubes confirms the production of C02 as a result of the photocatalytic oxidation of the organic molecule.

2) Quantitative procedure: Observation ofCu(II) removal as a function of time.

With distilled or deionized water, prepare the following:

a) An aqueous slurry containing 100 mg of Ti02 (e.g., Degussa P25) in 50 mL of water b) A 0.33 M citric acid solution c) A 0.04 M CuS04 solution

Repeat the set-up preparation procedure exactly as described above, but this time add 2 mL of the Ti02 suspension to all six tubes, as well as approximately 0.8 mL of the citric acid solution, and 2 mL of the CuS04 solution. Once all the components are in their respective test tubes, the nitrogen has been bubbled, and the tubes are covered with parafilm, bundled, and placed on site, cover the entire experimental set up with the box and bag described above. Turn the UV light on and allow irradiation for approximately 10 min. Then turn it off and remove with a Pasteur pipet the contents of test tube 1. Cover the set-up again and turn the UV light on once more. For protection, move out of the irradiation area, quickly filter the contents of test tube 1 through a fine filter (for example, place the contents of the Pasteur pipet in a syringe equipped with a 0.45 |xm syringe filter and push the contents out with the plunger), and analyze for the removal of Cu(II) with the aid of a colorimeter or spectrophotometer set at 810 nm. [You may want to see the absorption spectrum of Cu(II) in the experiment on Metal Ion Recovery by Cementation in this book; the Cu(II) as well as the Cu(II) + citric acid absorbance plots have their absorption maxima essentially at this same wavelength.] Note that a calibration plot is not needed since only the normalized absorbances (i.e., At/Ao) are needed so as to observe the removal of Cu(II). (Here, At is the absorbance at time t, and Ao is the initial absorbance).

Repeat the same procedure with the remaining five test tubes, removing one at a time after every 10 min of irradiation. To show that light is required for this process to occur, a separate blank experiment may be done with a sample in a tube wrapped with A1 foil, subject to the same conditions as the others, and then analyzed after a given period of time so as to compare it with an irradiated one.

A plot of the Cu(II) normalized absorbance decrease as a function of time will reveal the extent of removal. This is the result of a photocatalytic-adsorption combined effect, which can be tested separately if desired as described in the literature (see Bumpus, 1999).

Once the qualitative and/or the quantitative experiments are finished, students can bubble dioxygen from a tank into the purple suspension [containing Cu(I)—Ti02] and observe within a few minutes the color change back to light-blue as a result of reoxidation and dissolution of the Cu Jds) that reverts to Cu2^. Air can be used instead of dioxygen, but with less spectacular results. If desired, the photocatalyst may then be recovered by filtration with a 0.45 |xm membrane filter. This photoredox cycling can be envisaged as an environmentally friendly metal removal/concentration cycle. This is why the filtration step described in the quantitative procedure must be done quickly so as to avoid metal re-dissolution, and this is an additional reason for removing air by bubbling nitrogen at the beginning of the experiment.

166 16. Photocatalytic Remediation of Pollutants Name_Section_Date_

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