Pt/TiO2 Coupled with Water-Splitting Catalyst for Organic Pollutant Photodegradation: Insight into the Primary Reaction Mechanism

Hindawi Publishing Corporation Research Letters in Physical Chemistry Volume 2008, Article ID 810457, 5 pages doi:10.1155/2008/810457 Research Letter Pt/TiO2 Coupled with Water-Splitting Catalyst for Organic Pollutant Photodegradation: Insight into the Primary Reaction Mechanism Zizhong Zhang, Xuxu Wang, Jinlin Long, Xianliang Fu, Zhengxin Ding, Zhaohui Li, Ling Wu, and Xianzhi Fu State Key Laboratory Breeding Base of Photocatalysis, Research Institute of Photocatalysis, Fuzhou University, Fuzhou 350002, China Correspondence should be addressed to Xianzhi Fu, Received 23 December 2007; Accepted 28 February 2008 Recommended by T. An A composited system was fabricated by coupling Pt/TiO2 with water-splitting catalyst for photooxidation of organic pollutants in aqueous solutions. The new composited system exhibits more efficient photocatalytic activity than pure Pt/TiO2 does under UV light irradiation. The promoting effect is dependent on the photo-produced H2 over the composited system. The active oxygen species, hydroxyl radical (·OH) and hydrogen peroxide (H2 O2 ), are measured by fluorescence spectroscopy and photometric method, respectively. The results reveal that the produced H2 by photocatalytic water splitting over NiO/NaTaO3 :La transfers to Pt particle of TiO2 surface, then reacts with introducing O2 to generate in situ intermediate H2 O2 , and finally translates into ·OH radical to accelerate the photooxidation of organic pollutants. Copyright © 2008 Zizhong Zhang et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. 1. INTRODUCTION Photoinduced charge transfer occurring on semiconductor materials can achieve direct conversion of photo energy to chemical energy, and thus it can be used for elimination of organic pollutants and splitting water into hydrogen. However, the utility of semiconductor-based photocatalytic process is controlled to a large extent by the separation efficiency of the initially formed excited states (h+vb and e−cb ) [1]. A variety of approaches was made to enhance electron-accepting or electron-donating ability of the material surface to favor the interfacial charge separation and consequently increase the photocatalytic efficiency. One approach involves addition of surface adsorbed redox species capable of scavenging selectively either of the excited states to the photoreaction system [2, 3]. Another promising approach concerns modification of TiO2 with noble metals, other semiconductors, and coloring matters to improve the separation of the excited states [4–6]. Deposition of platinum on TiO2 has been reported to enhance extremely the photocatalytic efficiency for organic pollutant elimination due to its high electron-trapping effect [7], although an excessive number of platinum particles per grain of TiO2 can be detrimental to the performance of the reaction system [8]. We have recently demonstrated that trace amount of H2 can efficiently improve the activity of benzene photooxidation over Pt/TiO2 [9, 10]. However, the mechanisms have not been fully understood, and a practical approach for the environmental application has not yet to be achieved, due to the difficulties in realizing the integration of H2 gas and photocatalysis into a practical system. Herein, an alternative system was fabricated by coupling Pt/TiO2 with water-splitting catalyst NiO/NaTaO3 :La to supply the in situ H2 to enhance photocatalytic oxidation organic pollutants in an aqueous solution, where the obtained composited system is quite different from the classic coupled semiconductor system. The data show that the high photocatalytic efficiency of the composited system is attributed to the formation of more ·OH which is dependent on the generation of in situ H2 O2 from the combination between the photo-produced H2 by the NiO/NaTaO3 :La and bubbled O2 on Pt/TiO2 surface. 2 Research Letters in Physical Chemistry Table 1: Rate constants for SA photodegradation with different composited catalysts. Catalyst: 0.0500 g, the rate of NiO/NaTaO3 :La to Pt/TiO2 (or TiO2 ) is 25 wt%. reactant solution: 120 mL SA (5 × 10−4 mol L−1 ), with two 254 nm UV lamps irradiation. Photocatalyst Pt/TiO2 NiO/NaTaO3 :La-Pt/TiO2 TiO2 NiO/NaTaO3 :La-TiO2 NiO/Ta2 O5 -Pt/TiO2 NiO/Sr2 Ta2 O7 -Pt/TiO2 k (min−1 ) 0.00308 0.00429 0.00289 0.00286 0.00267 0.00415 collected with a Ba(OH)2 solution and then determined by a titrate with an oxalic acid (H2 C 2 O4 ) solution (0.02 mol L−1 ). The evolved H2 during the reaction was monitored by a hydrogen sensor (Dräger Pac III). Hydroxyl radical ·OH was captured by terephthalic acid to form fluorescent 2-hydroxyterephthalic acid [12] and then determined with fluorescence spectroscopy (FS/FL920, excitation wavelength: 312 nm, and fluorescence peak: 426 nm). Hydrogen peroxide was analyzed photometrically by the POD (horseradish peroxidase) catalyzed oxidation product of DPD (N,N-diethyl-p-phenylenediamine) at 551 nm [13]. 3. 2. EXPERIMENTAL 2.1. Sample preparation Titanium dioxide (TiO2 ) particles were prepared by a sol-gel technique. Titanium isopropoxide (0.1 mol) was first added dropwise to 100 mL of nitric acid aqueous solution. The suspension was stirred to clear and then dialyzed to pH of ca. 4 to obtain the TiO2 sol. The sol was dried at 333 K in an oven for 3 days. The resulting solid powders were ground to fine powders and finally calcined at 623 K for 3 hours. NaTaO3 :La was prepared by the solid state reaction according to the literature [11]. In typical, 0.02 mol Ta2 O5 , 0.0206 mol Na2 CO3 , and 0.0004 mol La2 O3 were mixed and then calcined in air at 1173 K for 1 hour and 1423 K for 10 hour. Platinum supported catalyst was prepared by the incipient wetness impregnation method. The calcined TiO2 was impregnated with a 5.22 × 10−2 M aqueous solution of H2 PtCl6 . The impregnated sample was dried at 393 K for 6 hours and subsequently reduced with an NaBH4 solution (0.1 M). After reduction, the solid sample was washed with deionized water to remove residual ion, and finally dried in air at 333 K (denoted as Pt/TiO2 ). The initial ratio of Pt to TiO2 was fixed at 1 wt%. NiO loaded catalysts were prepared by an impregnation method from a 2.36 × 10−2 M aqueous solution of Ni(NO3 )2 and then dried at 383 K for 2–5 hours. The sample thus obtained was subsequently calcined at 543 K for 1 hour in air using a muffle furnace. The initial ratio of NiO to NaTaO3 :La was fixed at 0.2 wt%. 2.2. Photocatalytic reactions and methods The photocatalytic reaction was performed at room temperature in a quartz tubal reactor surrounded with 254 nm UV lamps (Philips TUV, 4 W, Holland). The photocatalystpowders were dispersed in the salicylic acid (SA) solution bubbled with oxygen (10 mL min−1 ). The concentration of SA was analyzed by a high-performance liquid chromatograph (HPLC Waters) equipped with a re (...truncated)