In the Laboratory
A Simple Parallel Photochemical Reactor for Photodecomposition Studies
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Xiaobo Chen, Sarah M. Halasz, Eric C. Giles, Jessica V. Mankus, Joseph C. Johnson, and Clemens Burda* Department of Chemistry, Case Western Reserve University, Cleveland, OH 44106; *
[email protected] In recent years, environmental cleanup has been one of the most active areas in heterogeneous photocatalysis. For example, TiO2-based photocatalysts are used to completely photodecompose organic compounds in polluted air and wastewaters (1–3). In this article, a simple apparatus is described for the study of photodecomposition processes in a parallel experiment. This apparatus can be easily set up and used in an educational lab for undergraduate experiments on the study of environmentally relevant photocatalysis. Theory The electronic structure of a semiconductor is composed of the valence band filled with electrons and the empty conduction band. The energy difference between the valence and conduction bands is called the band gap (2–4). After absorbing light with energy equal to or higher than the band gap, the electrons in the valence band can be promoted into the conduction band, thereby creating excited electrons in the conduction band and leaving corresponding excited holes in the valence band (2–4). These excited electrons and holes are created for the redox processes on the surface of semiconductors, which act as sensitizers for collecting light in a photocatalytic reaction using semiconductors as photocatalysts (1–6). The excited electrons in the conduction band can migrate to the semiconductor surface and transfer to the chemicals adsorbed onto the surface, thereby reducing them. The excited holes in the valence band can recombine with electrons from the molecules on the surface and neutralize them (1–6). A good photocatalyst should be highly photoactive. For the degradation of organic compounds, the redox potential of the H2O兾⭈OH couple (OH− → ⭈OH + e−, E0 = ᎑2.8 V) should lie within the band gap of the semiconductor (2–4). Among candidates for photocatalysts, such as WO3, ZnO, ZnS, SrTiO3, titania (TiO2) is the most promising material for real environmental and industrial applications (2–6).
Also, the power dependence of the photodecomposition can be easily monitored by removing one or two beamsplitters in this system. Such a reactor can be used for a multitude of parallel photochemical transformations beyond the single reaction described in the experimental example below. The main elements of this system (Figure 1) consist of a Xe lamp (150 W), three 50兾50 beamsplitters, two Ag 100% reflecting mirrors, four convex lenses with short focusing distance (f = 50 mm), one weak convex lens, two magnetic stirrers, four cuvettes, four mini magnetic stir bars, and one electric shutter. The Xe lamp (model 6137) and the lamp power supply (model 8500) are from Oriel Corp. All the op-
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The Parallel Photochemical Reactor We have designed a simple and useful parallel photochemical reactor intended to study the photodecomposition of dyes using semiconductor photocatalysts. The photochemical reactions are followed through time-dependent changes in the ground-state absorption spectra of the dyes. A distinctive characteristic of this reactor is that four reaction cells can be run under identical conditions and turned on or off simultaneously. Four different reaction conditions, such as four different concentrations of dye or catalysts (or four different catalysts or dyes) can be monitored over precisely the same time frame and analyzed after the same degree of photolysis. www.JCE.DivCHED.org
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Xe lamp (150 W) light tube convex lens concave lens electrical shutter 50/50 beamsplitter Ag mirror convex lens (f = 50 mm) cuvette magnetic stir bar magnetic stirrer
Figure 1. Illustration of the parallel photochemical reactor set-up. Electrical components not shown.
Vol. 83 No. 2 February 2006
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Journal of Chemical Education
265
In the Laboratory CH3 N
tion time, the absorption spectrum of methylene blue is recorded.
CH3 S
H3C
N
ⴙ
CH3
Clⴚ
N
Optimal eye UV–A protection is required. Methylene blue is classified as harmful (11). Avoid direct contact with skin and eye, inhalation, ingestion, and prolonged exposure. A lab coat and gloves must be worn.
Figure 2. Structure of methylene blue.
tical components can be obtained from ThorLab or Melles Griot. The three 50兾50 beamsplitters (Melles Griot, part no. 03BTF007, $52.00) and the two 100% reflecting mirrors (ThorLab, part no. BB1-E02, $83.00) are arranged at a 45⬚ angle with regard to the light path, which ensures that four light paths with equal light flux reaching the cuvettes containing the dye solution with catalyst. The four convex lenses focus the light on the cuvettes at the same distance. The electric shutter (Melles Griot, part no. 04IES001 and 04ISC001) controls the illumination time and can be run automatically or manually. The main part of this system is built on a 12 in. × 24 in. breadboard (ThorLab, part no. MB1224, $389.00) and confined in a breadboard enclosure system (ThorLab, part no. XE25C3: 9 in. × 21 in. × 12 in. H, $149.00). The top of this enclosure system can be easily removed when taking the samples out for spectroscopic measurement. The optical system including the electrical shutter costs approximately $2,500. After the cuvettes containing the dye (with or without catalyst) are illuminated for predetermined times, the cuvettes can be taken out and analyzed via UV–visible absorption spectra to determine the fractions of decomposed dye. The light flux reaching the reaction can be measured with a power meter or by using the actinometry method (1, 7). Experimentally, the light flux is controlled with the output knob on the power supply. Experiment Methylene blue, also called Swiss blue, C16H18ClN3S, has the structure as shown in Figure 2. It is used as a stain in bacteriology and as an oxidation–reduction indicator. Also, it is used as a model dye in the study of photocatalytic activity of semiconductor photocatalysts (9–10). Commercial titanium dioxide P25 powder from Degussa was chosen as the photocatalyst. For the measurement of photocatalytic decay with pH dependence, each cuvette (1-cm width) is filled with 2.000 mL of methylene blue water solution with an optical density equal to 1.0, then 0.100 mL of 0.40 g兾L Degussa P25 solution is added, and the pH is adjusted with NaOH or HCl. For the measurement of photocatalytic decay on catalyst concentration dependence, each cuvette is filled with 2.000 mL of methylene blue water solution with an optical density equal to 1.0, then 0.200, 0.400, 0.600, and 0.800 mL of 0.40 g兾L Degussa P25 solution is added, and the pH adjusted to 10.0. Before adding the P25, the initial absorption spectrum of methylene blue solution is measured on a UV–visible absorption spectrophotometer (Varian Cary 50). After the addition of P25 and the predetermined illumina-
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Journal of Chemical Education
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Results The photodegradation of methylene blue on TiO2 dispersions under UV irradiation follows the Langmuir– Hinshelwood equation (1, 10, 12,13), r =
dc kd K ad c = dt 1 + Kad c
(1)
where kd is the rate constant of the intrinsic reactivity of the photoactivated TiO2 surface, Kad is the adsorption equilibrium constant, and c is the dye concentration. At low concentrations, when Kadc
