In the Laboratory
Photocatalysis, A Laboratory Experiment for an Integrated Physical Chemistry–Instrumental Analysis Course
W
Steven Gravelle* and Beth Langham Department of Chemistry, Saint Vincent College, Latrobe, PA 15650; *
[email protected] Brian Geisbrecht Department of Biophysics and Biophysical Chemistry, Johns Hopkins University School of Medicine, Baltimore, MD 21205
Photocatalysis has proven to be a useful technique for a variety of chemical reactions and an interesting technique to study from a mechanistic viewpoint (1–4). In photocatalysis, one typically suspends a semiconducting material (such as TiO2) in an aqueous solution containing an unwanted contaminant. The mixture is irradiated by UV light, which excites electrons across the band gap in the semiconducting material, thus creating electron–hole pairs. The resulting electrons can induce reduction while the holes can induce oxidation. This is depicted schematically in Figure 1. Numerous types of compounds can be decomposed photocatalytically (5–10). One example is the photocatalytic decomposition of benzoquinone, which has been shown to follow a reductive pathway (5), whereas 2-chlorophenol follows an oxidative pathway (6). Other researchers (8, 9) have monitored the products formed from the gas-phase photocatalytic decomposition of trichloroethylene. Ideally, complete photocatalytic decomposition should yield HCl and CO2, but they observed the formation of a variety of other compounds, including phosgene and dichloroacetyl chloride. The determination of the various products provides an interesting puzzle for our students. There have been a number of articles in this Journal illustrating the effectiveness of photocatalysis for the decomposition of wastewater contaminants (11–14). These laboratory experiments test the effectiveness of using simple laboratory equipment to completely decompose unwanted contaminants in water. In this article we describe a set of open-ended experiments that are conducted by upper-level students in our integrated Advanced Physical Methods (laboratory) course. Advanced Physical Methods is an integrated laboratory course that combines the techniques of instrumental analysis with the conceptual underpinnings of physical chemistry. The experiments use a number of instrumental techniques to investigate the kinetics of gas-phase and solution-phase photocatalysis and fit in nicely with the integrated nature of the laboratory course. The students pick the system they wish to study from the choices presented in the laboratory handout and choose how they investigate their system. In addition, the students need to look up the literature relevant to their experiment so that they may adequately interpret their results.
the Instructor’s Guide and the details of the procedure are in the Student Manual.W The students should read the appropriate sources listed at the beginning of the Student Manual before coming to lab. Based on their reading the students decide which catalyst(s) to use (TiO2 or ZnO), which contaminant to photocatalytically decompose, and whether to conduct the experiment in the gas phase or in solution. The solution-phase experiments are done by placing the photocatalyst powder and the reagent solution in a 10-cm UV–vis cell and irradiating with a 400- or 500-W UV lamp while rotating the cell (Figure 2). After irradiation, aliquots of the solution are drawn out, filtered, and placed in a conventional 1-cm cuvet for UV–vis spectroscopic analysis. The students can either survey the different decomposition rates among the various photocatalyst–reagent combinations or can focus on one photocatalyst–reagent combination and investigate the kinetics in detail.
Rⴚ
R
eⴚ
conduction band hν
Eg ⴙ
valence band
h
O
Oⴙ
Figure 1. Schematic representation of photocatalysis depicting the free electron in the conduction band and the hole in the valence band. R indicates a species that is reduced and O indicates a species that is oxidized.
reaction cell
filters Oriel 400-W Xe lamp
Experimental There are two systems for students to investigate: (1) the decomposition of benzoquinone or 2-chlorophenol in the aqueous phase or (2) the oxidative decomposition of trichloroethylene in the gas phase. The details of the setup are in
roller
power supply
Figure 2. Solution-phase irradiation setup.
JChemEd.chem.wisc.edu • Vol. 80 No. 8 August 2003 • Journal of Chemical Education
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In the Laboratory
In the gas-phase experiments, the photocatalyst powder is placed in an IR cell and evacuated on a vacuum line. The gaseous trichloroethylene is then transferred to the cell followed by the O2. The irradiation setup is similar to that used for the solution phase except that the cell is held stationary and not rotated. The students conducting these gas-phase studies use FTIR and GC–MS as complimentary techniques to determine their products. Two three-hour class periods are allotted to this experiment in our course; three or four periods would allow the students to become more comfortable with the experimental technique and to investigate the kinetics more thoroughly. Hazards The high intensity UV lamp and reaction cell should be covered by a box during irradiation to prevent accidental viewing of the UV radiation. Students who need to view the lamp during photolysis must wear appropriate UV eyewear (such as welding goggles.) The vacuum line should be located in a vented hood with the sash closed as much as possible to contain fumes or in case of breakage. Students should wear face shields when operating the vacuum line.
A 3.5
Absorbance
3.0
before irradiation after 12 min
2.5
All reagents and reaction cells should be handled in a hood; students should wear appropriate eyewear and gloves when preparing samples. Results The results from three groups of students who have recently completed the laboratory are reported. The first group studied the decomposition of two different compounds (2chlorophenol and benzoquinone) using two different catalysts (TiO2 and ZnO). The UV–vis absorption spectra of 2-chlorophenol taken before and after irradiation in the presence of TiO2 and ZnO are shown in Figures 3A and 3B, respectively. Clearly, the 2-chlorophenol decomposed to a greater extent in the presence of TiO2. Conversely, when benzoquinone was irradiated in the presence of either catalyst, the fraction that decomposed in the presence of ZnO was twice as much as in the presence of TiO2 (spectra not shown). The students can ponder why there could be such a great difference between the two catalysts, given that they have similar band gaps and similar oxidation and reduction potentials for the valence and conduction bands (5–7). The second group of students studied a similar process to that of the first group, except that the students focused on the kinetics of one photocatalyst–reagent system. The students used a lower concentration of catalyst than typically recommended in the laboratory procedure so that they could observe the “saturation” of the photocatalyst at high concentrations of the reagent. The students first determined the appropriate range of concentrations to study, and then estimated the rates of photocatalytic decomposition at each concentration. Finally the data were fit with a Langmuir–Hinshelwood rate equation (15)
2.0
R = 1.5
kK [ X ] 1 + K [X ]
as shown in Figure 4. Students who have studied the Michaelis –Menten treatment for enzyme kinetics should recognize the resemblance of the two rate laws.
1.0 0.5 0.0 200
250
300
350
400
Wavelength / nm
B
180
3.0
Absorbance
2.5
Initial Rate / (µM/min)
160
before irradiation after 12 min
2.0 1.5 1.0
140 120 100 80
raw data best fit (L-H)
60 40 20
0.5
0
0.0 200
0.0
250
300
350
400
Wavelength / nm Figure 3. The photocatalytic decomposition of 2-chlorophenol in solution with (A) TiO2 and (B) ZnO.
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0.5
1.0
1.5
Concentration / µM Figure 4. Langmuir–Hinshelwood fit of the student data for the photocatalytic decomposition of benzoquinone by TiO2. For this set of data, k = 250 and K = 1.4.
Journal of Chemical Education • Vol. 80 No. 8 August 2003 • JChemEd.chem.wisc.edu
In the Laboratory
Conclusion Two observations can be drawn from the variety of student results. First, the open-ended nature of the laboratory allows students to investigate different aspects of photocatalysis and to explore the system they selected thoroughly. The students explore a photocatalytic mechanism by reading the literature, collecting pertinent data, analyzing their results, and constructing conclusions. Every group can come up with a different set of conclusions when conducting the laboratory. The second observation is that students can formulate reasonable conclusions using minimal data. For example, in both of the solution-phase experiments, a great amount of information is gleaned from semiquantitative data. The students simply estimate rates by acquiring two UV spectra of the reagent, before and after a known irradiation period. These approximations are sufficient either for making comparisons of different photocatalytic systems or for fitting the Langmuir–Hinshelwood mechanism to a single system. Although the data are approximate, the students are still able to draw some important conclusions regarding the mechanism of photocatalytic decomposition.
2.0
Change in Absorbance
In the gas-phase experiment, the third group of students generated qualitative data in contrast to the solution-phase portion of the laboratory. Their goal was to determine the various products that can be formed from the gas-phase photocatalytic decomposition of trichloroethylene in the presence of TiO2 (6). To do this the students acquired single beam IR spectra of the gaseous contents of the reaction cell before and after UV irradiation. The students then computed a difference absorbance spectrum like the one shown in Figure 5. In this figure, peaks below the baseline indicate depleted trichloroethylene reagent while peaks above the baseline indicate products. Ideally, if the decomposition of trichloroethylene were complete, the only products should be HCl and CO2. Clearly there were other products formed as well, which the students need to identify. The students looked up spectra in the spectral library of the FTIR to identify possible products. The second step in the analysis is to acquire GC–MS data of the gaseous products. To do this the students vacuum transferred the gaseous contents of the IR cell to a bulb outfitted with a septum that can be used with a GC syringe. Since the products are gaseous, the gas chromatograph was left at a low temperature (∼40 ⬚C) and did not need to be programmed for a temperature ramp. The students analyzed the mass spectra collected at different retention times and attempted to identify the products. They should observe that the two analysis techniques, IR and GC–MS, are complementary, not redundant. From the product determination the students made a reasonable guess of the mechanism governing the decomposition process. One obvious conclusion was that the C⫽C double bond in the trichloroethylene was oxidized to form carbonyl containing compounds. The students constructed mechanisms that were quite imaginative, but the process was instructive and enjoyable for them.
A carbonyl-containing product to be identified by students
1.5
1.0
0.5
HCl
CO2
0.0
Negative peaks indicate depletion of trichloroethylene −0.5 3000
2500
2000
1500
1000
Wavenumber / cmⴚ1 Figure 5. The difference IR spectrum showing the photocatalytic depletion of trichloroethylene
Acknowledgments This project was supported by faculty development grants at Saint Vincent College. We thank the students in the Advanced Physical Methods course whose data were used for the figures. W
Supplemental Material
The Student Manual and the Instructor Guide are available in this issue of JCE Online. Literature Cited 1. Mills, A.; Le Hunte, S. J. Photochem. Photobiol. A: Chemistry 1997, 108, 1. 2. Ollis, D. F.; Pelizzetti, E.; Serpone, N. Environ. Sci. Technol. 1991, 25, 1523–1529. 3. Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341–357. 4. Rajeshwar, K.; Ibanez, J. G. J. Chem. Educ. 1995, 72, 1044. 5. Richard, C. New J. Chem. 1994, 18, 443. 6. D’Oliveira, J. C.; Al-Sayyad, G.; Pichat, P. Environ. Sci. Technol. 1990, 24, 990. 7. Matthews, R. W. J. Phys. Chem. 1987, 91, 3328–3333. 8. Jacoby, W. A.; Nimlos, M. R.; Blake, D. M. Environ. Sci. Technol. 1994, 28, 1661. 9. Nimlos, M. R.; Jacoby, W. A.; Blake, D. M.; Milne, T. A. Environ. Sci. Technol. 1993, 27, 732. 10. Lichtin, N. N.; Dong, J.; Vijayakumar, K. M. Water Poll. Res. J. Canada 1992, 27, 203. 11. Herrera-Melián, J. A.; Doña-Rodríguez, J. M.; Rendón, E. T.; Soler Vila, A.; Brunet Quetglas, M.; Azcárate, A. A.; Pascual Pariente, L. J. Chem. Educ. 2001, 78, 775. 12. Bumpus, J. A.; Tricker, J.; Andrzejewski, K.; Rhoads, H.; Tatarko, M. J. Chem. Educ. 1999, 76, 1680. 13. Giglio, K. D.; Green, D. B.; Hutchinson, B. J. Chem. Educ. 1995, 72, 352. 14. Yu, J. C.; Chan, L. Y. L. J. Chem. Educ. 1998, 75, 750. 15. Turchi, C. S.; Ollis, D. F. J. Catal. 1990, 122, 178.
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