PHOTOCATALYTIC OXIDATION OF CERTAIN ANILINES MEDIATED BY AQUEOUS Ti02 SUPENSIONS

Abstract

Both developed and developing nations are faced alike with enormous environmental problems related to discharge of hazardous effluents and emission of toxic air pollutants from various industries. It has posed a great challenge for the scientists and technologists to develop environmental friendly and economically viable systems so as to eliminate these toxic pollutants prior to their discharge from industries. Last 15-20 years have witnessed a tremendous upsurge of interest and activity in semiconductor mediated photocatalysis. The use of semiconductor suspensions in heterogeneous photocatalysis offer distinct advantages because of their (i) high surface area, (ii) varied redox potentials of conduction band electrons and valence band holes, (iii) controllable charge carrier dynamics, (iv) redox selectivity etc. Among various semiconductors, attention has mainly been focused on Ti02 because of it being relatively inexpensive, biologically and chemically inert, stable with respect to photocorrosion and demonstrates redox selectivity. Moreover, in this system the photogenerated holes have strong oxidizing power (E° =+2.53 Vvs NHE) and the photogenerated electrons are reducing enough (E° = -0.52 Vvs NHE) for performing photochemical transformation, and degradation / mineralization of a wide variety of inorganic and organic compounds at its interface. However, the photocatalytic efficiency of the processes initiated by pure Ti02 suspensions is generally observed to be poor because ofthe rapid recombination ofphotogenerated electrons and holes, which, in the absence of suitable electron acceptors, is extremely efficient. In the recent past a number ofstrategies have been employed to improve the separation ofcharge in these systems by intercepting e' and h+ prior to their recombination. These objectives have been achieved by modifying the surface properties of the semiconductor through derivatization/sensitization of surface, by doping of transition metal ions, alkali metal ions, alkaline earth metal ions, rare earth metal ions and various inorganic anions. In the present work, the surface modification of Ti02 suspensions has been carried out by the addition of Ag+ and C032' ions. Interestingly, both systems demonstrate enhanced charge separation in Ti02, which results in increased photocatalytic activity for the oxidation of certain anilines. The present thesis comprises ofsix chapters. The first chapter presents an overview of the work being carried out in some of the areas of homogeneous / heterogeneous photocatalysis during the last three decades. The mechanistic aspects of photocatalytic reactions initiated mainly by suspensions of semiconductors have been described with major emphasis on Ti02 based systems. It also specifies the objectives ofthe present investigation. The second chapter incorporates the experimental details about the used materials and equipment. It describes the preparation of different samples of photocatalyst(s) and methods used for their characterization. Abrief account of methods employed for the kinetic study and analysis of the reaction samples has also been included. The third chapter deals with the photocatalytic oxidation of aniline at pH 9.2 using Ag+ containing Ti02 suspensions as sensitizer. The characterization of the catalyst by XRD reveals the presence of metallic Ag at the surface of Ti02 particles. The nature of surface interaction of aniline with the photocatalyst(s) has been studied spectrophotometrically. In both cases, aniline gets adsorbed on the surface of Ti02 and the physisorption depicts Type II adsorption isotherm. At low concentrations of aniline, these systems follow Langmuir equation, from which the values of intensity of adsorption of in Ill ^ 1 1 aniline has been estimated to be 3.1x10 mol' dm and 3.6x10 mol" dm for bare Ti02and Ag+ containing Ti02 suspensions, respectively. The amounts of aniline, Ag+ and Ti02 have been optimized by following the initial rate of decomposition of aniline. Under the optimized reaction conditions, the oxygenated reaction mixture containing bare Ti02 - aniline, and Ag+ containing Ti02 - aniline were illuminated by light of Xex > 320 nm where aniline does not have any absorption. In both cases, azobenzene, p-benzoquinone and NH3 were detected to be the products of photooxidation of aniline but their amounts were relatively higher in the presence of Ag+. The photocatalytic decomposition of aniline was monitored kinetically both in the absence and presence of Ag+. The initial consumption of aniline follows pseudo-first-order kinetics. The loading of 0.2 wt % of Ag+ almost doubled this rate compared to that of the unmodified particles. The kinetic analysis showed the reaction to follow Langmuir-Hinshelwood model. Under the used experimental conditions, aniline could be completely decomposed in about 10 h, whereas in its absence during the sameperiod only80%decomposition takes place. Thefourth chapter includes the study of photocatalytic oxidation of aniline at pH 10.8 using bare Ti02 suspensions and Ti02 suspensions containing Na2C03 as photocatalysts. The characterization of the catalyst has been carried out by using IR spectroscopy and XRD technique. The addition of carbonate to Degussa Ti02 P-25 increased the number of active adsorption sites at its surface. The values of the intensity of adsorption of aniline were determined to be 5.5 x102 mor'dm3 and 6.9 x102 moP'dm3 for bare Ti02 and Ti02 suspensions containing Na2C03, respectively and this in turn enhanced the rate of photodecomposition of aniline from 2.7 x 10 mol dm" s' to 6.50 x 106 mol dm'3 s"\ in two cases. The maximum efficiency of the photocatalyst has been

obtained upon the addition of 0.11 mol dm'3 of Na2C03 at pH 10.8. In both cases, azobenzene and NH3 were detected as products of the reaction, while nitrobenzene was additionally produced only in case ofTi02 containing Na2C03 - aniline system. The time course of photodecomposition of aniline and formation of intermediates and products demonstrate that in the presence ofNa2C03, the rate of photodegradation of aniline along with the rates of formation of products has increased significantly. The kinetic analysis showed the reaction to follow Langmuir-Hinshelwood model. In the presence of Na2C03, 3 x 10'3 mol dm'3 of aniline could be completely photodegraded in about 6h, whereas in its absence, for the same duration only 80% aniline undergoes degradation. In efforts to further enhance the photoactivity ofTi02 suspensions, the combined effect ofaddition of C032' to Ag+ containing Ti02 suspensions at pH 9.2 has also been examined. It was found to enhance the rate of photodecomposition of aniline compared to bare Ti02 suspensions at pH 9.2, however, this rate was smaller to that observed in the presence of Na2C03 alone at pH 10.80. The fifth chapter presents a study on the photooxidation of xylidines, viz. 2,3- DMA, 2,6-DMA and 3,4-DMA using Ti02 suspensions containing Na2C03 as photocatalyst under optimized reaction conditions, as arrived in the previous chapter. These investigations were carried out in order to verify the findings recorded with the Ti02 - aniline system in the presence of Na2C03. The interaction of the studied dimethyl anilines with the photocatalyst was similar to that of Na2C03 containing Ti02 - aniline system except that the intensity ofadsorption was at variance. Interestingly, the products of photooxidation of 2,3-DMA and 2,6-DMA were found to be the corresponding derivatives of p-benzoquinone, azobenzene, nitrobenzene, besides NH3. However, there was no formation of dimethyl p-benzoquinone as a product in case of 3,4-DMA. The kinetic behavior of the xylidines in general was also found to be very similar to that of aniline, except that the initial rates of their photodecomposition were slightly less, which though enhanced subsequently on longer time scale. 2,3-DMA and 2,6-DMA photodegraded completely in about 4.5 and 4 h, respectively. The sixth chapter presents a brief account of the mechanistic behaviour of Ti02 mediated photocatalysis. It furnishes a summary and detailed discussion on the results of photooxidation (i) of aniline in the absence and presence of AgN03 at pH 9.2, (ii) of aniline, 2,3-DMA, 2,6-DMA and 3,4-DMA in the absence and presence of Na2C03 at pH 10.8 using Ti02 suspensions as photocatalyst. On the basis of these results, the nature of interaction of the substrates with the photocatalyst(s) and the mechanisms of their photocatalytic decomposition has been suggested.