Heterogeneous Photochemistry An Easy Experiment Jose Peral, Maria Trillas, and Xavier ~ o m e n e c h ' Departament de Quirnica, Facultat de Ciencies, Universitat Autonoma de Barcelona, 08193-Eellaterra, Spain Some laboratory experiments have been described in
this Journal concerning photochemical transformations (Id).However, none of these experiments deals with heterogeneous photochemistry. I n this article, the photochemical reduction of methyl orange by Ti02powder is presented a s a simple experimental example, suitable for senior students of physical chemistry, illustrating the heterogeneous photochemistry fundamentals. Aromatic azo compounds are important commercial dyes, being Methyl Orange (MO-) one of the most used. This compound is stable to visible or near-UV irradiation, but in aqueous solution it could be reduced by many ordinary reductants, giving the colorless hydrazine derivative (H2MO-1: = NC6H4S03-+ 2Ht + 2e(CH3i2NC6H4N
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MO-absorbance versus irradiation time. {bj i
Nevertheless, the rate of this process is usually low, which confers stability to its aqueous solutions, even in presence of reductants. The rate of MO- reduction can be increased by adding a suitable catalyst, for example Ti02.This oxide is an n-type semiconductor with a band-gap of 3.23 eV. For aqueous suspensions of Ti02 under illumination with light of h < 400 nm. electron-hole airs are created inside the semiconductor particles that cfin migrate to the semiconductor surface and react with s ~ e c i e sin solution of suitable redox potential (6).~ecause'theredox potential of the electrons ~hotoeenerateda t the Ti07 (-0.04 V versus NHE a t pH = 3) is more negative than the redox potential of t h e - ~ o /H2MO-couple (0.067 V versus NHE a t pH = 31, the photoreduction of M O is thermodynamically possible. Simultaneously, t h e photogenerated holes, w i t h a redox potential of 3.1 V versus NHE a t pH = 3, can photooxidize water to give oxygen. I t must be said that, in general, the quantum yields of these photocatalytic processes are not very high due to electron-hole recombination. For this reason. in order to increase the efficiencv of the ~hotoreduction processes a n easy oxidizable substance (sacrificial aeent) is nsuallv added to the solution. This s ~ e c i e reacts s w&h the holes ;reventing them from recomb&ed with the electrons. Procedure Ti02Degussa P-25 (80% anatase, mean particle size = 27 nm and BET surface area = 59 m2 .g-') was used a s photocatalyst . All other chemicals were a t least of reagent grade. The experiments were performed in presence of ascorbic acid in solution (0.01 mol.dm3 a t pH = 3) which acted a s sacrificial agent. The M0:acid ascorbic aqueous solutions with 0.5 g.dm3 of Ti02 were placed in a 200-mL beaker and the semiconductor was maintained in suspen'Author to whom correspondence should be addressed.
~ o= j4.10.~ mol.dm3 'I = 20%;' (c) [MOJ = 4.10" mol.dm 3, 1 = 80%; (d)[MOJ = 4.10.~mo1.dm3, I = 100%. Ascorbic acid conc. = 0.01 mol.dm3; mass of Ti02 in suspension 0.05 g.dm3: pH = 3. sion by magnetic stirring. A common slide projector was used a s the light source. The light intensity could be changed by placing neutral density filters (metallic grids) between the lamp and the beaker or by changing the distance between them. Light intensity measurements, as percentage of full lamp power, were carried out by means of a silicon photocell. After each irradiation the suspension was filtered through a 0.45 pm hydrophilic nylon membrane in order to separate the solid, and the concentration of MO- was determined bv measuring the solution absorbance a t h = 463 nm. ~bsorbanceunits (an.) are used to report the experimental data. The figure shows the absorbance of MO- solutions in presence of Ti02a t different irradiation times. I t must be said t h a t in absence of Ti02, both in the dark and under illumination, no decrease of MO- absorbance in solution was detected. However, in the presence of light a linear decrease of the solution absorbance with time is observed, indicating t h a t the MO- photoreduction follows a zero-order kinetics with r e s ~ e c to t MO- concentration. This is confirmed by the similar slopes (0.0115 a.u. min-' and 0.120 a.u. min-I for M O initial concentrations of 5.10-' and 4.10-5 mol.dm3, respectively) obtained for MO- solutions of different initial concentration (curves a and c in the figure). On the other hand, the yield of M O ohotoreduction decreases with decreasing t h e intensity' of the incident light (see curves b, c, and d in the figure). The slopes of the straight lines b, c and d (0.0094 a.u. min-', 0.0120 a.u. m i d and 0.0190 a.u. min-', respectively) and the zero rate obtained in absence of light can be fitted to a straight line when Volume 72 Number 6 June 1995
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plottedasfunctionoflightintensity.Consequently, assuming that the light intensity is proportional to the concentration of the photogenerated electrons, it can be concluded that the rate of MO- photoreduction ( u ) follows a first-order kinetics:
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whereI is the incident light intensity.
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Journal of Chemical Education
6 , nodes, G . ; G T ~ ~ Z ~ I , M .JN them U 1984,8,509-~20.
