Article pubs.acs.org/IECR
Enhanced Photocatalytic Degradation of Aqueous Nitrobenzene Using Graphitic Carbon−TiO2 Composites Wan-Kuen Jo,† Yangsoo Won,‡ Ingyu Hwang,‡ and Rajesh J. Tayade†,§,* †
Department of Environmental Engineering, Kyungpook National University, 80 Daehek-Ro, Bukgu, Daegu 702-701, Republic of Korea ‡ Department of Environmental Engineering, Yeungnam University, 280 Daehak-Ro, Gyeongsan, Gyeongbuk 712-749, Republic of Korea § Discipline of Inorganic Materials and Catalysis, Central Salt and Marine Chemicals Research Institute, Council of Scientific and Industrial Research (CSIR-CSMCRI), G. B. Marg, Bhavnagar, Gujarat 364002, India ABSTRACT: Graphitic carbon−TiO2 nanocomposites with different carbon loadings were synthesized by a one-pot hydrothermal method. The prepared catalysts were characterized by X-ray diffractometry (XRD), scanning electron microscopy, UV−vis diffuse reflectance spectrophotometry, and Brunauer−Emmett−Teller surface area analysis. The XRD results confirmed the presence of graphite in the composite without alteration of the TiO2 structure. The photocatalytic efficiencies of the synthesized composites were determined by the degradation of aqueous nitrobenzene (NB) under UV irradiation. Because of the presence of graphitic carbon in the composite, there was an increase in the adsorption of NB (24%) on the composite surface, which led to a higher photocatalytic yield (up to 96% in 4 h at a graphitic carbon content of 1%). NB degradation was corroborated by chemical oxygen demand determinations.
1. INTRODUCTION Nitrobenzene (NB) is an essential raw material widely used in the production of various chemicals, such as anilines, dyes, perfumes, synthetic resins, explosives, pesticides, and drugs;1,2 however, it is considered a priority pollutant because of its toxicity.3,4 Annually, the release of NB from various sources has been estimated at approximately 19 million pounds.5,6 The National Institute of Environmental Health Sciences, U.S. has designated NB as a carcinogen based on the conclusions of the International Agency for Research on Cancer because of its high toxicity and difficult degradation. The exposure of the skin or eyes to very small amounts of NB can cause mild irritation, vomiting, and headache; continuous exposure may cause liver damage. Repeated exposure to high concentrations of NB can reduce the ability of blood to carry oxygen. Above 2 ppm, the presence of NB in waste streams is considered hazardous. Reports have shown that NB concentrations exceeding 100 ppm can be observed in wastewater generated by the organic chemicals and plastics industries, and some industrial levels exceed this limit.6 Various conventional techniques, including physical, chemical, and biological methods have been applied for the decomposition of NB in aqueous media.7 Recently, NB was decomposed under aerobic conditions using Rhodotorula mucilaginosa immobilized on polyurethane foam.8 The process required 30 h for complete NB degradation (200 ppm) under optimized conditions, which was comparatively slower than other processes. Similar results also reported in the literature suggest that aerobic processes are very slow for NB removal from water.9−11 NB degradation has also been effected through ozonation in combination with each H2O2 and/or UV, UV/Fe(III) separately.12−14 These studies used different irradiation sources, © 2014 American Chemical Society
including a mercury vapor lamp, a xenon lamp, and solar light. The direct photolysis of aqueous NB under a 150-W mercury− xenon lamp was ineffectual.13 The degradation of NB in aqueous solution has also been pursued using heterogeneous catalytic O3/UV processes. Catalysts have included TiO2, manganese-loaded activated carbon (MnOx/GAC), ceramic honeycomb, and synthetic goethite.14,15 Recently, to enhance the photocatalytic degradation of NB, a combination of ozonation and ultrasound at three frequencies, with or without ceramic honeycomb catalysis, was studied by Zhao et al.16,17 In recent years, semiconductor photocatalysts have been extensively investigated for the photocatalytic degradation and mineralization of water pollutants; such an approach could offer a more economical purification of the wastewater discharged from industries and households as compared to other processes. Semiconductor-based photocatalysts offer great potential for the decomposition of organic pollutants because of their unique ability to convert light energy to chemical energy. A representative example is the TiO2-based photocatalytic degradation of pollutants from air and water.18 However, many challenges need to be overcome for developing an economically feasible photocatalytic process. These challenges include the synthesis of a visible light-activated photocatalyst to exploit the full spectrum of solar energy and ensure an appropriate electron−hole recombination rate in the semiconductor. To enhance the efficiency of TiO2, several methods have been developed, including increasing its surface area, dye Received: Revised: Accepted: Published: 3455
January 17, 2014 February 12, 2014 February 14, 2014 February 14, 2014 dx.doi.org/10.1021/ie500245d | Ind. Eng. Chem. Res. 2014, 53, 3455−3461
Industrial & Engineering Chemistry Research
Article
sensitization,19,20 and doping with nitrogen, carbon, and sulfur, as well as various metal ions.21−24 Recently, the enhanced photocatalytic efficiency of TiO2 was studied by the development of graphitic carbon and TiO2 composites by various methods.25−28 Carbon materials are considered more environmentally and biologically friendly as compared to inorganic materials, because carbon is one of the most common elements in our environment. Graphite, in particular, is found naturally and people have used it in their daily lives for hundreds of years without any critical toxicity problems.29 The preparation of graphite-coated TiO2 composites by different methods and their photocatalytic activities have recently been reported.30,31 Recently, considerable effort has been directed toward the preparation of carbon−TiO2 composites, mainly by supporting TiO2 on the surface of expanded graphene oxide. The graphene oxide-based TiO2 composites were synthesized after converting graphite to expanded graphene oxide by the Hummers method.32 Another approach used to prepare a hybrid of TiO2 with graphite modified the TiO2 surface by applying a layer of glucose and then converting it into graphitic carbon to enhance the photocatalytic efficiency of the resulting catalyst.33 However, hardly any attention has been paid to in situ hybridization of TiO2 with graphite and its photocatalytic activity. In the present study, we developed a one-step graphitic carbon−TiO2 composite synthesis by a hydrothermal method, incorporating different amounts of graphite. The effect of the increase in the graphitic carbon loading on TiO2 was studied. The prepared catalysts were characterized by various techniques, and their photocatalytic performance was studied in the degradation of the toxic pollutant NB in aqueous media.
from the integrated peak intensities at 2θ = 25.3° (101) for the anatase phase and 2θ = 27.4° (110) for the rutile phase. The percentage of anatase phase, A(%), was determined using eq 1.35 A(%) = 100/(1 + 1.265IR /IA )
(1)
where IR is the intensity of the rutile peak at 2θ = 27.4°, and IA is the intensity of the anatase peak at 2θ = 25.3°. Crystallite sizes of the synthesized composites were determined from the half-height widths of the characteristic anatase peak at 2θ = 25.3° (101), using the Scherrer equation with a shape factor (K) of 0.9.36 crystallite size = Kλ/W cos θ
(2)
where W = Wb − Ws (Wb is the broadened profile width of the composite sample, and Ws is the standard profile width of a reference silica sample), λ is the wavelength of X-ray irradiation (Cu Kα = 0.154 056 nm), and θ is the diffracting angle. The Brunauer−Emmett−Teller (BET) surface areas, pore sizes, and volume distributions of the synthesized composites were determined from nitrogen adsorption/desorption isotherms at 77 K (ASAP 2010, Micromeritics, U.S.). Surface areas and pore size distributions were determined using the BET equation and the Barrett−Joyner−Halenda (BJH) method, respectively.37 The samples were degassed under vacuum (10−3 Torr) at 623 K for 4 h prior to measurement. The band gap energies and absorption edges of the graphitic carbon−TiO2 nanocomposites were determined using UV−vis diffuse reflectance spectroscopy (UV-DRS) (Scinco Co., Ltd., S-3100) with BaSO4 as a reference.6,38 The spectra were recorded at room temperature in the wavelength range 250− 600 nm. The band gap energies of the synthesized composites were calculated according to eq 3:
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. Titanium tetraisopropoxide (TTIP), graphite ( PT > GT-3 > GT-5.
4. CONCLUSIONS Graphitic carbon−TiO2 composites with varying graphite contents were successfully synthesized by a simple one-pot hydrothermal method without alteration of the TiO2 structure. The incorporation of graphitic carbon was confirmed by X-ray diffractometry, UV−visible diffuse reflectance spectrophotometry, and surface area analysis. The results demonstrated that the presence of graphitic carbon enhanced the adsorption of nitrobenzene in the synthesized composites, and was highest in the composite containing the lowest amount of graphitic carbon. The graphitic carbon also facilitates the separation of electron−hole pairs, which reduces electron−hole recombination and consequently leads to higher photocatalytic activity under ultraviolet irradiation.
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AUTHOR INFORMATION
Corresponding Author
*Phone: +91 278 2567760 ext: 7180; fax: +91 278 2567562/ 2566970; e-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This study was conducted with the support of the MSIP (Ministry of Science, ICT & Future Planning, Project No. 132S-5-3-0610) and the National Research Foundation of Korea (NRF) funded by the Korean Government (MEST) (No. 2011-0027916). R.J.T. would like to thank the Director of the CSMCRI-CSIR for granting leave to avail the Brainpool fellowship and would also like to thank Dr. S. Shin, Mr. Joon Y. Lee, and Miss Kang for their kind support.
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REFERENCES
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