Photoelectrocatalytic Degradation of Sulfosalicylic Acid and Its Electrochemical Impedance Spectroscopy Investigation Hong Liu,* ,² Shaoan Cheng, ‡ Ming Wu, ² Hejin Wu, ² Jianqing Zhang, ‡ Wenzhao Li, ² and Chunan Cao ‡ State Key Laboratory of Catalysis, Dalian Institute of Physical Chemistry, Chinese Academy of Sciences, Dalian 116023, China, and Department of Chemistry, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed: January 12, 2000; In Final Form: March 27, 2000 A three-electrode system composed of TiO 2 /Ni as the working electrode, porous nickel as the counter electrode, and saturated calomel electrode (SCE) as the reference electrode was used for the photoelectrocatalytic degradation of organic compounds. The photoelectrocatalytic degradation of sulfosalicylic acid (SSal) under anodic bias potential was investigated. It is shown that SSal can be degraded effectively as the external potential is increased up to 700 mV (vs SCE). The characteristics by electrochemical impedance spectroscopy (EIS) of the photoelectrocatalytic degradation of sulfosalicylic acid (SSal) was also investigated. It is shown from the EIS that the photoelectrocatalytic degradation appears to be a simple reaction on the electrode surface, suggesting that only one step of charge transfer is involved in the electrode process. The value of the resistance of charge transfer for the photoelectrocatalytic reaction of SSal manifests itself not only in the reaction rate, but also in the separation efficiency of the photogenerated electron-hole pairs. The separation efficiency of the electron-hole pairs under N 2 atmosphere is higher than that under O 2 atmosphere. Introduction Photocatalysis of the TiO 2 semiconductor has received increasing attention for the application of degrading a great variety of organic contaminants in water. 1-4 The interest lies in the simplicity and low cost of the photocatalytic system which is mainly composed of an ultraviolet or a visible light source, the TiO 2 , and atmospheric O 2 . Accordingly, commercial interest is increasing. However, two problems must be overcome before the TiO 2 photocatalytic system can be put into practical use. One is to reuse the photocatalyst, and the other is to decrease the simple recombination efficiency of the photogenerated electrons and holes. Some kinds of fixation techniques 5 have been adopted to prevent the separation process from reusing the photocatalyst. As far as the second problem is concerned, an externally applied anodic bias is proposed to improve the charge separation by driving the photogenerated electrons via the external circuit to the counter electrode, and the degradation of several organic compounds has been reported to be effective. 6-10 In fact, the pioneering work of photocatalysis was conducted in an electrode system by Fujishima and Honda. 11 Electrochemical methods have also been employed in direct wastewater treatments, for example, for the decoloration and degradation of recalcitrant pollutants such as cyanide, EDTA, and aniline. 13 Rajeshwar has made an interesting review about electrochemistry and the environment. 12 On the other hand, electrochemical measurements are found to be useful in the direct observation of TiO 2 surface reactions on the electrode. Some researchers have confirmed the synergic effect between the electrochemical and photoelectrochemical processes used to degrade reactive dyes by voltammetric results. 14 Kesselman et al. stated that no individual process, i.e., electron transfer, hole transfer or the recombination of electrons and holes, was “rate-determining” by flux-matching conditions applied to TiO 2 photoelecrodes. 15 As a widely used electrochemical method, electrochemical impedance spectroscopy (EIS) is very effective for studying the mechanism and kinetics of complicated electrode reaction. Gomes and Vanmaekelbergh reviewed the impedance spectros- copy at semiconductor electrodes. 16 Hens highlighted the electrochemical impedance of the electrode process involving charge transfer. 17 Actually, the photoelectrochemical or elec- trochemically assisted photocatalytic degradation reactions of organic compounds 6-10,14 can be considered as electrode reac- tions taking place on the electrode/liquid interface. The function of an anodic bias is to increase the separation efficiency of photogenerated electron-hole (e-h) pairs and to act as the electrode potential in the kinetic process. Therefore, the EIS method might provide some useful information if it is employed to investigate photoelectrochemical degradation reactions. In this paper, the photoelectrocatalytic degradation of sulfosalicylic acid (SSal) and its EIS investigation is reported. Experimental Section TiO 2 (anatase) was laboratory reagent grade, and was obtained from Shanghai Chemical Factory. The average radius of the particle is 458 nm. Other chemicals were analytical reagent grade, and SSal was purified by recrystallization before it was used for the EIS measurement. Nitrogen was highly pure and oxygen was generally pure. The solution pH values were adjusted by 0.01 mol L -1 HClO 4 and 0.01 mol L -1 NaOH solutions before the reactions. Distilled water was used through- out this work. Porous nickel (porosity > 95%) was washed with a dilute alkaline solution (NaOH:Na 2 CO 3 ) 1:1) prior to use. The TiO 2 powder was packed into the porous nickel substrate using 3 wt % poly(vinyl alcohol) (PVA) as the binder. The thickness of the immobilized TiO 2 was 0.6 mm. Figure 1 gives * Author to whom correspondence should be addressed. Fax: 86-411- 4691570. E-mail: hongliu203@hotmail.com. ² Dalian Institute of Physical Chemistry, Chinese Academy of Sciences. ‡ Department of Chemistry, Zhejiang University. 7016 J. Phys. Chem. A 2000, 104, 7016-7020 10.1021/jp000171q CCC: $19.00 © 2000 American Chemical Society Published on Web 07/11/2000