The Effect of Mesoporous Carbon Nitride Modification by Titanium Oxide Nanoparticles on Photocatalytic Degradation of 1 , 3-Dinitrobenzene

In the present work, well ordered, mesoporous carbon nitride (MCN) sorbent with uniform mesoporous wall, high surface area and pore volume has been fabricated using the simple polymerization reaction between ethylene diamine and carbon tetrachloride in mesoporous silica media, and then modified by TiO2 nanoparticles (Ti-MCN). The structural order and textural properties of the nanoporous materials were studied by XRD, elemental analysis, and nitrogen adsorption–desorption experiments. Photodegradation experiments for 1,3-dinitrobenzene were conducted in batch mode, the Ti-MCN catalysts were found to be more active compared to the free TiO2 nanoparticles for 1,3-dinitrobenzene degradation.


Introduction
Soil polluted through poisonous or hazardous organic pollutants is an ecological concern.Hydrophobic organic compounds have attracted intensive attention because they are powerfully adsorbed into soil and known as toxic, persistent and carcinogenic pollutants 1,2 in ecosystem.These compounds can reach the environment as a result of fossil fuel burning, coke burning, metal processing facilities, hydrocarbon production, and so on. 3,4veral treatment methods for the removal of organic pollutants from aqueous solutions have been reported, mainly electrochemical treatments, evaporation, solvent extraction, reverse osmosis, chemical precipitation, membrane filtration and adsorption. 5Most of these methods involve high capital cost and are not suitable for small-scale industries.The removal of toxic pollutants from water is a problem, particularly when they are present in low concentrations.Several studies have focused on the fate and transport of these pollutants, and the application of remedial technologies to manage them. 6,7The most common adsorbent material are carbon-based adsorbents, which have a huge specific surface area, plentiful micro and macro pores, and a high adsorption capacity.These adsorbents are economically favourable because they can be made from various low-cost materials that have high carbonaceous content including wood, coal, petroleum coke, sawdust and coconut shell. 8On the other hand, photocatalysis on TiO 2 surfaces is of great importance today with the intention to serve as a solution for numerous environmental issues like wastewater treatment and remediation, hazard-ous waste control and air cleansing.Moreover, TiO 2 is low--cost whereat titanium is the world's seventh most plentiful metal and ninth most plentiful element. 9,10TiO 2 has found uses as white pigment and UV absorber in food and paint industries and cosmetics. 11Though, the major drawback in TiO 2 photocatalysis is low efficiency due to the high rate of recombination of electrons and holes upon photoactivation. 12Due to its wide bandgap energy, eg., of 3.0 -3.2 eV, it can only be activated by UV light, which coincidentally accounts only 3 -4 % of sunlight spectrum. 13To have an improved photocatalytic efficiency, composite photocatalysts were designed by loading TiO 2 on certain support substrates with large surface areas. 14cently, Vinu et al. 15 reported the synthesis of mesoporous CN materials based on pyridine-and benzene-ring building blocks by using mesoporous silica as a hard template and ethylenediamine (EDA) and carbon tetrachloride (CTC) as precursors.
Scientists recently used carbon-based materials as catalyst supports associated with photo-or electro-catalysis, the good electron conducting character of carbon supports may have some helpful influence in increasing catalytic activities.However, there are only limited publications concerning this effect.
In this work, the influence of the TiO 2 loading on mesoporous carbon nitride support were systematically studied in the photodegradation of 1,3-dinitrobenzene (NB).For this purpose, the MCN were modified by TiO 2 nanoparticle.The activity of this catalyst was compared with pure TiO 2 in the photodegradation of 1,3-dinitrobenzene in aqueous solution.The effects of contact time and initial solution concentration on 1,3-dinitrobenzene degradation has been investigated.

Synthesis of mesoporous silica (MCM-48)
MCM-48 was prepared according to the synthesis procedure described by Yaofeng Shao. 16In a representative synthesis, the MCM-48 molecular sieves were prepared as follows: 10 ml of TEOS was mixed with 50 ml of deionized water, and the mixture was vigorously stirred for 40 min at 308 K, then 0.9 g of NaOH was added into mixture, and at the same time 0.19 g of NaF was added into the mixture.After the NaF was added completely, the required content of sucrose, respectively, was added.After another 60 min of vigorous stirring, 10.61 g of CTAB was added to the mixture, and stirring continued for 60 min.The mixture was heated for 24 h at 393 K in an autoclave under static conditions, and the resulting product was filtered, washed with distilled water, and dried at 373 K.The as-synthesized samples were then calcined in air for 4 h at 823 K, increasing the temperature to 823 K at 1 °C min −1 of the heating rate.

Synthesis of mesoporous carbon nitride (MCN)
Mesoporous carbon nitride materials were prepared by using mesoporous silica MCM-48 as template.In a typical synthesis, 0.5 g of calcined MCM-48 was added to a mixture of EDA (1.35 g) and CTC (3 g).The resultant mixture was refluxed and stirred at 363 K for 6 h.Then, the obtained dark brown solid mixture was placed in a drying oven for 12 h, and ground into fine powder.The template-carbon nitride polymer composites were then heat-treated in a nitrogen flow of 50 ml per min at 873 K with a heating rate of 3.0 °C min −1 and kept under these conditions for 5 h to carbonize the polymer.The mesoporous carbon nitride was recovered after dissolution of the silica framework in 5 % hydrofluoric acid, by filtration, washed several times with ethanol, and dried at 373 K.

TiO 2 loaded mesoporous carbon nitride photocatalyst
In order to incorporate the TiO 2 , 0.1 g mesoporous carbon nitride was mixed with 20 ml isopropanol, stirred for several minutes.Then 0.105 ml Ti-isopropoxide was added.The mixture was stirred overnight.Isopropanol was removed by centrifugation, and the material was further washed with acetone.The sample was air-dried at 80 °C overnight.

Characterization
The X-ray powder diffraction patterns were recorded on a Philips 1830 diffractometer using Cu K α radiation.The diffractograms were recorded in the 2θ range of 0.8 -10 with a 2θ step size of 0.01° and a step time of 1 s.
The adsorption-desorption isotherm of the as-synthesized sample was measured at 77 K on micromeritics model ASAP 2010 sorptometer to determine an average pore diameter.Pore-size distribution was calculated by the Barrett-Joyner-Halenda (BJH) method while Brunauer-Emmett-Teller (BET) measured surface area of the sample.

Photocatalytic activity measurements
The photodegradation of 1,3-dinitrobenzene was done in order to calculate the photocatalytic activity of Ti-MCN photocatalyst.About 10 mg of catalyst was mixed with 50 ml of 1,3-dinitrobenzene aqueous solution (100 ppm) under magnetic stirring in dark for 30 minutes to reach the adsorption equilibrium of the 1,3-dinitrobenzene on the catalyst prior to irradiation.The photocatalytic activity of Ti-MCN was evaluated in the degradation of 1,3-dinitrobenzene aqueous solution under UV irradiation.The UV light source was a 125 W Hg lamp (λ = 365 nm).The average light intensity was 60 mW cm −2 .The distance between the light and the reaction tube was fixed at 12 cm.The lamp was cooled with flowing water in a quartz cylindrical jacket around the lamp, and ambient temperature was maintained during the photocatalytic reaction because of good ventilation.An air diffuser was placed at the bottom of the reactor to uniformly disperse air into the solution and stirred with air at a flow rate of 0.2 m 3 h −1 .After irradiation and separation of the catalyst by centrifugation, the concentration of 1,3-dinitrobenzene was determined with a Perkin-Elmer Lambda 35 UV-Vis spectrophotometer.The 1,3-dinitrobenzene has an absorption maximum at 254 nm.The concentration of 1,3-dinitrobenzene at diverse irradiation times was gained by converting absorbance of the solution to 1,3-dinitrobenzene concentration (Beer-Lambert's law, A = ε b c, where A is absorbance, ε molar absorption coefficient, b pathlength, and c concentration).A standard calibration curve (not shown here) was built by adjusting a different concentration of 1,3-dinitrobenzene solution and the absorption at 254 nm.This calibration curve refers to the situation of the absence of by-products co-adsorption.A blank test was done in the solution without catalyst, where the concentration of the 1,3-dinitrobenzene indicates constancy.

Photodegradation Kinetics
Photocatalytic degradation of 1,3-dinitrobenzene yields carbon dioxide, nitrate and water.The zeros order kinetics is as shown in Eq. ( 1) (1) The first order kinetics, is as shown in Eq. ( 2) where [NB] i indicate initial 1,3-dinitrobenzene concentration and [NB] t concentration at time t.If the reaction follows second order kinetics, it gives Eq. ( 3) which can also be rewritten in a non-linear form as shown in Eq. ( 4). ( (4)

Adsorption-desorption analysis
Besides powder XRD, nitrogen physisorption is the method of choice to gain knowledge about mesoporous materials.This method gives information on the specific surface area and the pore diameter.Calculating pore diameters of mesoporous materials using the BJH method is common. 17[20][21] relative pressure relativni tlak volume adsorbed (STP) ⁄ cm 3 Fig. 1 shows nitrogen adsorption and desorption isotherms measured at 77 K by using a Micromeritics ASAP 2010 automatic analyser.BET surface areas and the pore size determined by BJH method for the synthesized mesoporous sorbents (MCN and Ti-MCN) were determined by the adsorption branches of the isotherms.All mesoporous materials yield a type IV isotherm.The isotherm of MCN is reversible and in good agreement with that previously reported.Table 1 summarizes the important physical properties of mesoporous sorbents.The surface areas of MCN and Ti-MCN samples were 1321 and 1280 m 2 g −1 , and their average pore size were 3.5 and 3.2 nm, respectively.This result mainly depends on the pore size and shape for all nanostructured carbon materials.

XRD analysis
The XRD patterns of Ti-MCN and MCN showed (Fig. 2) three diffraction peaks that can be indexed to (110), (210), and (220) in the 2θ range from 0.8° to 10°, representing well-ordered cubic I4 1 32 structure.The observation data from the original samples of all nanostructured carbons are in good agreement with that previously reported. 15The wide-angle XRD patterns of titanium oxide modified mesoporous photocatalyst (Ti-MCN) in Fig. 3 as revealed by the peaks at 25.3°, 37.7°, 48.0°, 53.9°, 55.1°, 62.7°, 68.6°, 70.4° and 75.0° (JCPDS 00-004-0477) 2θ, are characteristic of the existence of crystalline TiO 2 anatase phase, but the XRD pattern of mesoporous carbon nitride photocatalyst shows no diffraction peak.These outcomes propose that the loading of the titanium oxide particles on the surface of the support and the better accessibility of the reactants might be more favourable for the photocatalytic reaction.The mesoporous carbon nitride had a larger specific surface area than TiO 2 , and TiO 2 modified mesoporous carbon nitride due to better porosity and more ordered structure but had low catalytic activity.
The higher photocatalytic activity of mesoporous carbon nitride and titanium oxide modified sample here observed may be attributed to several factors.1) The increasing adsorption capacity of the photocatalyst; NB molecules would transfer from the solution to the catalysts' surface and be adsorbed with offset face-to-face orientation via p-p conjugation between NB and aromatic regions of the carbonaceous photocatalyst.Therefore, the adsorption of 1,3-dinitrobenzene is increased compared to TiO 2 .2) Extending light absorption; the chemical bonds of Ti−O−C and good transparency of photocatalyst made a red shift in the photo responding range, and facilitated a more efficient use of light for the photocatalysis.3) Overpowering charge recombination; the carbon nitride surface can act as an acceptor of the photo-generated electrons for titanium dioxide particles and confirm a fast charge moving in view of its great conductivity, and therefore, an effective charge separation can be attained. 22,23The better charge transportation would provide more photo-induced carriers for the associated photocatalytic reactions, due to a higher photocatalytic activity.
To find the order of the reaction of photochemical degradation of NB by Ti-MCN photocatalyst, pseudo-first-order and pseudo-second-order models were used.For first order reaction, the semi-log of residual NB (ln [NB] t ⁄ [NB] i ) versus reaction time (t) was plotted.Similarly, the 1 ⁄ [NB] t versus t was plotted for second-order reaction.A comparison of two kinetic models indicates that the first-order kinetics fits better with the data (R 2 = 0.998) than the second-order kinetics (R 2 = 0.965).The corresponding rate coefficient (k) for the first-order reaction was found to be 4.64 • 10 −2 min −1 .
The effect of photocatalyst dosage on the photodegradation of NB was examined in UV irradiation by different dosage of Ti-MCN varying from 0.02 g l −1 to 0.2 g l −1 at a fixed 1,3-dinitrobenzene concentration of 100 ppm.The photodegradation rate for photocatalyst is presented in Fig. 5, which shows that initial slopes of the curves rise greatly by increasing catalyst amount, and after that the amount of degradation remained almost constant.
Hence, the maximum photodegradation is shown with 0.15 g l −1 dose of Ti-MCN.Similar research was carried out by Muruganandham et al. 24 The NB degradation improved meaningfully by increasing the dosage of photocatalyst.The increase in the amount of catalyst increases the number of active sites of TiO 2 that in turn increases the number of OH • and O 2 − radicals.The photocatalytic destruction of other organic pollutants has also shown a similar type of dependence on photocatalyst dosage. 25,26i-MCN dosage ⁄ g l −1 doziranje Ti-MCN ⁄ g l

Conclusions
In summary, a highly ordered three-dimensional mesoporous carbon nitride material (MCN) with high surface area and pore volume has been prepared using MCM-48, as a template through a simple polymerization reaction between ethylenediamine and carbon tetrachloride, after that modified with TiO 2 .The structural order and textural properties of the modified and unmodified mesoporous photocatalysts was studied by XRD and nitrogen adsorption analyses.The photocatalytic activity of the Ti-MCN composite was examined by the degradation of NB in aqueous solutions under UV light irradiation.The degradation of NB fits the pseudo-first-order kinetic model.The TiO 2 modified mesoporous carbons prepared in this work are suitable for the NB dye photocatalysts.

List of abbreviations and symbols
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