Langmuir 1991, 7, 709-713
709
Effect of Cadmium Sulfide Characteristics on the Photocatalytic Oxidation of Thioacetamide Allen P. Davis* and C. P. Huang Department of Civil Engineering, University of Delaware, Newark, Delaware 19716 Received June 6, 1990. I n Final Form: August 29, 1990 The variation in photocatalytic oxidation rate of thioacetamide was examined by using cadmium sulfide obtained from five sources. These results as well as analogous data obtained by using thiourea, and the effect of pH and temperature suggest that the observed activity variations result from catalyst properties alone. On a mass basis, the most reactive photocatalysts are those with the largest specific surface area. However, on a surface area basis, the purity of the electronic grade compounds is essential for the most efficient reaction. Thermal treatment of the CdS increases photocatalytic activity as long as specific surface area is not detrimentally affected.
Introduction Photocatalytic semiconductor processes have received a significant amount of interest in recent years. Potential applications have been in the areas of energy production, organic synthesis, and water treatment. Only in recent investigations have the importance of catalyst surface properties been realized. Characteristics of the photocatalyst/water interface determine kinetic and thermodynamic parameters that markedly influence the photocatalytic process. One of the most important of these parameters is the adsorption of both the oxidant and reductant. This interaction may be controlled by the surface site density and possibly electrokinetics for ionic adsorbates. This paper examines the photocatalytic oxidation of thioacetamide using cadmium sulfide catalysts obtained from different manufacturers and having different physicochemical characteristics. Photocatalyticactivity has long been found to be variable between compounds synthesized by different methods, and even between batches using identical work-up.'V2 In an attempt to optimize semiconductor photocatalytic hydrogen production, the reaction rate increased after a photocatalyst heat re treatment.^ Similarly, the photocurrents from an cu-Fe203electrode were increased by heat treatinge4 A nondependence upon the surface area has been noted often. The effect of several physical and chemical catalyst treatments is also examined in order to maximize the desired reaction and to again elucidate solid-state and interfacial reaction mechanisms. The variation in reactivity of these compounds should provide insights into the reaction-limiting properties of the catalyst. Thioacetamide is used as the organic compound. The photocatalytic oxidation of this compound using CdS has been previously e ~ a m i n e d . ~ Methods and Materials Five types of cadmium sulfidewere used in this study. Samples from General Electric (GE) and Aldrich Chemical Co. (Aldrich 5-9s)were both ultrapure electronic grade materials. Reagent
* To whom correspondenceshould be addressed at the Department of Civil Engineering,University of Maryland,College Park,MD 20742. (1)Stephens, R. E.; Ke, B.; Trivich, D. J . Phys. Chem. 1955,59, 966. (2) Serpone,N.; Borgarello, E.; Barbeni,M.; Pelizzetti,E. Inorg. Chim. Acta 1984, 90,191. (3) Abrahams, J.; Davidson, R. S.; Morrison, C. L. J . Photochem. 1985, 29, 353. (4) Ahmed, S. M.; Leduc, J.; Haller, S. F. J . Phys. Chem. 1988, 92, 6655.
(5) Davis, A. P.; Huang, C. P. Submitted for publication in Water Res.
0743-746319112407-0709$02.50/0
grade CdS samples from Aldrich (Aldrich 98), Fisher Chemical Co. (Fisher),and Pfaltz and Bauer (P & B) were also used. Table I presents the properties of these materials. The color of the compounds ranged from a very bright yellow for the ultrapure solids to a near orange for the Fisher CdS. Thioacetamide was obtained from Fisher and thiourea from the J. T. Baker Chemical co. The organic concentrations were measured with a Hitachi Perkin-Elmer UV-vis spectrophotometer at a wavelength of 261 nm for thioacetamide and 236 nm for thiourea. BET surface areas were determined by Nz adsorption. Na2S04 was used as the inert electrolyte; pH was adjusted with HzS04or NaOH. Representative experimental conditions were loT3mo1.L-l thioacetamide, 5 gL-'CdS, pH = 7 , temperature = 25 "C, light intensity = 700 W.m-2,and constant 02 purge. The reaction vessels were identical with that in previous work5" and used a 300-W ELH projection lamp (General Electric) as the visible light source. These lamps produce a spectrum similar to sunlight. Temperature was regulated by thermostated flow through jacketed reaction vessels. The oxidation reaction resulted in acid production and base was added to maintain the solution pH. Organic photocatalytic oxidation is described by Langmuir-Hinshelwood kinetics. Therefore, comparisons between photocatalyst types are made by using initial rates. Adsorption studies were completed in Pyrex glass test tubes wrapped in aluminum foil, at a concentration of 5 gL-l CdS in a 10-mL solution. In some cases, the cadmium sulfide particles were subjected to a base or acid wash to remove any soluble impurities that might impede the photocatalytic reaction. A base wash was completed by using 1 M NaOH in plastic bottles, covered with aluminum foil, and shaken overnight. The solids were allowed to settle and then rinsed with distilled water twice. The CdS was filtered with a Whatman 1 filter and dried overnight at 35 "C. Samples for acid washing were treated by using the same procedure, except that 10-l M HN03 was used and they were shaken for only 1h due to the instability of CdS in acid solution.
Results and Discussion Photocatalytic Oxidation. The removal of thioacetamide from a M solution is shown in Figure 1. Negligible removal is evident in the absence of either light or CdS. It is obvious that the reactivity is catalyst type dependent. The Fisher CdS is the most active while the two electronic grade materials, GE and Aldrich 5-9s, produce the slowest reaction rate. Initial rates vary by a factor of 3 and are listed in Table 11. The acid production rate follows the oxidation rate. The fastest oxidation requires more base addition to maintain pH; however once the organic is depleted, the base requirement is complete. (6) Park, S. W.; Huang, C. P. J . Colloid Interface Sci. 1987,117,431. (7) Davis, A. P.; Huang, C. P. Water Res. 1990, 24, 543.
0 1991 American Chemical Society
Davis and Huang
710 Langmuir, Vol. 7, No. 4,1991
purity, CdS
'(
specific surface area, m2.g-1
crystal structurea hexagonal
Aldrich 5-9s
99.999
1.5
GEb
99.999
0 8c
Pfaltz & Bauer ? Aldrich 98 98+ Fisher' 99.8
,
-2.8
Table I. Physical a n d Chemical Properties of Cadmium Sulfides Used i n T h i s S t u d y
,
,
I
impurities
Zn 20 ppm Ca
