Photocatalytic Activity of Tio 2 on Activated Carbon under Visible Light in the Photodegradation of Phenol

The photodegradation of phenol under visible light by using TiO 2 and activated carbon prepared by physical activation was performed. Activated carbon was prepared from Tabebuia Pentaphyla wood by physical activation under CO 2 flow at 800ºC. UV-Vis lamp with a high proportion of visible light was used. Kinetic results of phenol photodegradation indicated a beneficial effect between TiO 2 and AC. An increase of apparent first-order rate constant (k app) by a factor of 1.74 in presence of TiO 2-AC was obtained in comparison than TiO 2 alone. This cooperative effect between both solids was ascribed to the high surface area and basic surface pH of this type of AC.


INTRODUCTION
Heterogeneous photocatalysis represent an efficient method of treatment of water and air [1,2].This technique requires the presence of a semiconductor and up to now TiO 2 has been found as the best photocatalyst.Several attempts have been performed to increase its photocatalytic efficiency and one way to increase this photoactivity consists in adding an inert co-adsorbent as activated carbon (AC).This coadsorbent has shown a remarkable increase in TiO 2 photoactivity [3][4][5][6][7][8][9] in the photodegradation of phenol, halophenols and acid herbicides.In these works, a synergistic effect between both solids in the photomineralization of these organic molecules has been reported.The physicochemical properties of AC represent an aspect important in the photocatalytic reaction.For example, surface area of AC has an important role because it is involved in the creation of a common contact interface between both solids [3,4].In addition, surface pH defined by the point zero charge of carbon (pH PZC ) seems to be the more important role because it remarkably affects photoactivity [5,6,9].Recently, it has shown that surface functionalization of AC play an important function on TiO 2 photoactivity on phenol and 4-chlorophenol degradations [5,6].For example, the use of specific L-type activated carbons (prepared by chemical activation) generate a synergy effect which could be attributed not only to a proper surface area but also to the presence of low surface concentration of acidic oxygenated groups in solution are dissociated to produce surface anions able to drive out electron density from aromatic system to TiO 2 [6,8].For the case of TiO 2 in the presence of H-type activated carbons (prepared by physical activation), a beneficial effect on TiO 2 photoactivity has been attributed to the presence of an intimated contact interface spontaneously created between both solids that promotes a very well dispersed layer of TiO 2 *Address correspondence to this author at the Engineering of Materials and Nanotechnology Centre, Catalysis, Venezuelan Institute for Scientific Research, I.V.I.C., 20632, Caracas 1020-A, Venezuela; Tel: 0058-212-5041306; Fax: 0058-212-5041166; E-mail: agarciag@ivic.venanoparticles [5].Furthermore, authors have proposed that this type of activated carbons play the role of electron carrier that could inhibit the recombination of photoelectrons to improve the photoactivity of TiO 2 [5,8].In order to verify this, the main objective of this work is to study the photodegradation of phenol by using TiO 2 and a specific Htype AC.

EXPERIMENTAL METHODS
Photocatalyst was TiO 2 P25 (Degussa).Activated carbon (AC) was prepared from sawdust of Tabebuia Pentaphyla wood by physical activation [5] under CO 2 flow at 800ºC and sample was denoted as AC CO2-800 .Samples were characterized by BET surface area (S BET ), Infrared (IR), surface pH (pH PZC ), and X-ray diffraction (XRD).Photocatalytic tests were performed at 25ºC with 200mg TiO 2 and 40mg AC under stirring in 200mL of 40ppm initial concentration of phenol.Samples were maintained in the dark for 60min to complete adsorption at equilibrium prior UVirradiation and then the suspension was irradiated.After centrifugation aliquots were analyzed by UV-spectrophotometer Perkin Elmer, Lambda.UV-Vis lamp of Metal Halide was used for the photocatalytic tests.From 522.7W.m -2 of total radiation of this lamp, 70.2W.m - corresponds to UV radiation and about 452.5W.m -2 to the visible radiation.The total radiation flux was about 1.44x10 17 photons.cm - .s - .

Characterization of Solids
Results of texture and surface pH of the solids are compiled in Table 1.AC CO2-800 presented a high surface area and a basic pH PZC which suggest the presence of basic oxygenated functional groups on the surface of carbon.For the case of TiO 2 -AC CO2-800 , the surface area BET clearly decreases one order magnitude with respect to AC CO2-800 .This fact can be attributed to a strong interaction between both the solids in agreement with previous works [3][4][5][6][7].This inference can be verified by means of FTIR analysis is shown in Fig. (1), which shows the FTIR spectra of TiO 2 with a peak located at about 3200-3600cm -1 which is associated at stretching vibration of the OH groups.In addition, it observed a broad peak identified to bulk Titania framework in the region from 400 to 800 cm -1 [10].It is also important to remark that the TiO 2 -AC system shows a broadening in this bulk titania framework region in presence of AC CO2-800 and this behavior was also observed in a previous work [7].In concordance, the corresponding signal to stretching vibration of peaks attributed to cyclic ethers and quinone in the activated carbon, located at 1100-1400cm -1 and 1610-1580cm -1 , respectively [11], clearly decreased.These changes in FTIR have been associated with a strong surface interaction between both solids consequence of ether groups coordination from carbon to the metallic centre in TiO 2 [5,7].XRD pattern of Fig. (2) shows no changes in the corresponding X-ray diffraction patterns for the case of TiO 2 -AC in comparison than that obtained for TiO 2 alone.The only change detected in the XRD pattern of mixed system was a remarkable decrease in the main peaks of Anatase phase attributed to a dilution effect by means of the presence of AC.In other words in spite of surface oxygenated functional groups on AC are able to coordinate to Ti +4 in the semiconductor, this interaction was unable to change the crystalline phase that compose the P25 TiO 2 .

Kinetics of Phenol Adsorption in the Dark and Photodegradation
Fig. (3) shows the kinetics of adsorption in the dark of phenol followed by 120min under stirring.In all cases, adsorption occurred within 30min but to ensure the equilibrium of adsorption, a period of 60 min of adsorption in the dark was selected for the photodegradation experiments.The results in Fig. (3) indicated that there are no additive effects in the adsorption capacities of both solids after they are mixed.This difference was ascribed to a strong interaction between TiO 2 particles and AC, which creates an intimate interface between both solids according to reported in previous works [3][4][5][6][7][8][9].Kinetics of photocatalytic disappearance of phenol is presented in Fig. (4A).It can be observed that the direct photolysis without solids can be neglected, because it only shows about 5% conversion.A second point to be considered corresponds to the fact that in the presence of TiO 2 -AC system, the photodegradation of phenol was obtained in shorter reaction time than that required on TiO 2 alone.Assuming a first-order reaction rate, Fig. (4B) shows the linear transformation from the kinetic data of Fig. (4B).These regressions were employed to estimate the apparent rate constant of first-order (k app ).These values are shown in the Table 2.It can be seen that k app obtained on TiO 2 -AC is clearly higher than that obtained on TiO 2 alone.Therefore, it can be considered, by means of the interaction factor (I F ) estimated from the comparison between the apparent rate constant of first-order (k app-TiO2-AC )/(k app-TiO2 ) the presence of a synergetic effects between both solids.The results indicated that the combination of TiO 2 -AC creates a kinetic synergy effect in phenol disappearance with an increase of in the rate constant by a factor of 1.75.This enhancement in the photoactivity of TiO 2 under visible light can be due to the textural properties and the surface chemistry of an H-type AC with a high surface area and basic pH PZC in concordance with the presence of basic functional groups, as cyclic ethers and quinones detected in the FTIR spectra shown in Fig. (1).This AC can play the role of an electron carrier that could inhibit the recombination of photoelectrons to improve the photoactivity of TiO 2 which has been reported previously [5,8].

CONCLUSION
Results suggest that under visible light the combination TiO 2 -AC creates a kinetic synergy effect in phenol disappearance with an increase in the rate constant by a factor of 1.75.This beneficial effect has been attributed to the specific properties of the AC that can induce the formation of a contact interface between both solids, where the AC could act as electron carrier and can drive out electrons from the TiO 2 surface and avoid the recombination of photoelectrons.