Manganese Oxides Supported on TiO2–Graphene Nanocomposite

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Manganese Oxides Supported on TiO2−Graphene Nanocomposite Catalysts for Selective Catalytic Reduction of NOX with NH3 at Low Temperature Xining Lu,† Cunyi Song,† Chein-Chi Chang,‡,§ Yunxiao Teng,† Zhensong Tong,*,† and Xiaolong Tang† †

Department of Environmental Engineering, College of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, P. R. China ‡ Department of Engineering and Technical Service, D. C. Water and Sewer Authority, Washington, D.C. 20032, United States § Department of Chemical, Bioengineering, and Environmental Engineering, University of Maryland, Baltimore County, Baltimore, Maryland 21250, United States ABSTRACT: TiO2−graphene (TiO2−GE) nanocomposites were prepared by the sol−gel method with different mass ratios of graphene (0−2% wt %). With the MnOX active component loaded by means of ultrasonic impregnation, the catalysts exhibited excellent structure and electrical properties, which favored the catalytic reaction. All the catalysts were characterized by XRD, SEM, TEM, BET, FT-IR, XPS, and Raman spectroscopy. The results indicated that the reduction of graphene oxide and formation of graphene in TiO2−GE supports and TiO2−graphene were readily indexed to anatase TiO2 in all samples. Various valences of manganese species coexisted in MnOX/TiO2−GE catalysts. Especially, nonstoichiometric MnOX/Mn on the surface of composite catalyst was beneficial to the electron transfer; therefore, the redox performance of the catalyst was improved. The MnOX/TiO2−0.8%GE catalyst exhibited good resistance to simultaneous H2O and SO2, as well as to only H2O, with an optimum Mn mass ratio of 7 wt %. All the samples showed excellent N2 selectivity. experimental results at about 100 °C were absent. 8 Padmanabha and co-workers found that it is highly significant to fill the oxygen vacancies on the catalyst surface by gas-phase oxygen, which is extremely important for the low-temperature SCR reaction of NOX with NH3 over MnOX/TiO2 catalysts.9 Casapu et al. investigated the NH3−SCR performance of MnOX-CeO2/cordierite catalysts and found that the activity on MnOX−CeO2−Nb2O5/cordierite was slightly higher than that of MnOx−CeO2/cordierite below 200 °C. Kijlstra et al. considered that the main reason for the deactivation of MnOX/Al2O3 catalysts was the formation of MnSO4. Qi et al. prepared MnOX−CeO2 catalysts which showed excellent SCR performance with 100 ppm of SO2 and 2.5% H2O at 150 °C.10 High specific surface areas can be provided by carbon materials, such as activated carbon (AC), activated carbon fiber (ACF), and carbon nanotubes (CNTs), which can also provide high chemical stability.11−14 Therefore, carbon-based catalysts, such as MnO X /ACF, 11 MnO X /AC/ceramic, 12 and MnO X / MWCNTs,15 have been widely studied, and they can provide good performance in the SCR reaction, under conditions of excess oxygen and low temperature (≤250 °C). Graphene (GE) as a new carbon nanomaterial has many exceptional properties, such as high electron mobility, high transparency, flexible structure, and large theoretical specific surface area.16−18 Each carbon atom of graphene is in an sp2 hybrid orbital, which contributes to the formation of a π bond with the remaining

1. INTRODUCTION Nitrogen oxides (NOX, i.e., NO and NO2) emitted from the burning of fossil fuels and mobile and stationary sources is one of the main atmospheric pollutants that leads to a lot of environmental problems, including acid rain, photochemical smog, ozone depletion, and greenhouse effects.1,2 Selective catalytic reduction (SCR) of NOX by NH3 in the presence of oxygen is one of the most effective methods to decrease the NOX levels in gaseous emissions.3 A well-known commercialized catalyst for this process is V2O5−WO3/TiO2. However, there are still some inevitable problems with this catalyst system including the toxicity of vanadium species, the narrow temperature window of 300−400 °C, the high conversion of SO2 to SO3, and the low N2 selectivity at high temperatures.4 Flue gas temperature after the desulfurizer and electrostatic precipitator has been decreased to below 200 °C, so it is very necessary to develop a low-temperature SCR process and catalysts.5 Mn-based catalyst has been reported to have excellent activity in the low-temperature SCR reaction. In addition, active catalysts are usually supported by material such as TiO2, which provides good thermal stability, strong mechanical strength, and high sulfur resistance6 but smaller surface area that often restricts the catalytic performance of TiO2 to some extent. Thirupathi and co-workers prepared Mn−Ni/TiO2 catalyst using commercial TiO2 in the incipient wetness method, and the results showed high surface area of the catalyst and high NO conversion at low temperature but no sulfur resistance.7 Kim and co-workers prepared the molecular sieve catalysts using ZSM5 as the carrier, which showed a high SCR activity close to 100% above 200 °C. However, in their study, © 2014 American Chemical Society

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orbital electrons of P. The π electrons move freely, which results in the excellent conductivity of graphene. Graphene is thought to be the best conductive material at room temperature, possessing an electron mobility of 2 × 105 cm2/(V·s) (about 140 times that of silicon) and a conductivity of 106 S/ m.19,20 The high-efficiency process of the gain and loss of electrons has resulted in the improvement of the redox performance and the catalytic reduction ability of the catalyst. By introducing graphene into TiO2, loading of the active component (MnOX) was improved and adsorption of NH3 was increased, while the catalytic activity was enhanced. Therefore, TiO2 in combination with graphene becomes a composite support achieving complementary advantages and the ability to improve the SCR performance. Recently, Gui and co-workers21 prepared the TiO2−graphene composite by a sol−gel method. Zhang and co-workers prepared MnOX/TiO2 catalyst by an ultrasonic impregnation method. The catalyst has a high Mn atom concentration on the surface, and the NO conversion reached 90% at low temperature.22 However, no reports have focused on the application of graphene in the low-temperature SCR field. In this work, a series of MnOX/TiO2−graphene catalysts were prepared by the sol−gel method and ultrasonic impregnation methods. The SCR activity tests were carried out in a fixed-bed reactor between 80 °C and 180 °C. To fully examine the structure and possible metal−support interaction between the manganese oxide species and TiO2−graphene support, the catalysts were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), Brunauer−Emmett−Teller (BET) analysis, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The effects of H2O and SO2 in flue gases on the activity of the catalyst for low-temperature SCR of NOX with NH3 were also investigated.

volume of tetrabutyl titanate and ethanol. Solution B was a mixture of a certain amount of glacial acetic acid, GO solution with different mass ratios, and anhydrous ethanol. Solution A was stirred steadily, and solution B was added dropwise to it in the meantime. The mixture was stirred for 90 min, and a brown, uniform, transparent titanium graphene sol preparation was obtained. After standing at room temperature for 24 h, the preparation was dried at a constant temperature of 80 °C for 12 h to afford brown crystals. Finally, the crystals were ground and calcined in a tubular furnace under an atmosphere of nitrogen and then warmed at 450 °C for 6 h to get the TiO2−GO powder. By adding a certain amount of hydrazine hydrate, the reduction of GO to GE was achieved, as verified by the following experimental results. The resulting composite was recovered by filtration, rinsed with deionized water several times, and fully dried at 80 °C for 12 h to get the final TiO2− GE nanocomposites with different mass ratios of GE. The samples prepared by this procedure were denoted as TiO2−X% GE, X = 0.2, 0.4, 0.8, and 2, respectively. A series of MnOX/TiO2−GE catalysts were prepared successfully with different Mn loadings by the ultrasonic impregnation method and manganese acetate as precursor. The typical synthetic process is described as follows: aqueous solution of manganese acetate was added to the beaker containing TiO2−GE. This mixture was sonicated at 60 °C for 90 min for better dispersion of TiO2−GE in the solution. After impregnation, the samples were dried in air at 80 °C for 12 h and then calcined in a tubular furnace under an atmosphere of nitrogen at 450 °C for 6 h. The catalyst was denoted as Y wt % MnOX/TiO2−GE, where Y represented Mn loading. 2.2. Catalyst Characterization. The morphology of the samples was observed with a Hitachi S-4800 scanning electron microscope (SEM) operating at 15 kV. X-ray spectrometry (EDS) (Bruker X Flash Detector 5010) was used for microstructure observation of the interface and element distribution of catalyst. X-ray diffraction (D/max-TTRIII) measurements were taken on a Rigaku diffractometer with Cu Kα radiation (18 kW, 20−60 kV, 10−300 mA) in the 2θ range of 5−150°. The XRD phases were identified by comparison with the reference data from Joint Committee on Power Diffraction Standards (JCPDS) data files. The morphologies of the MnOX/TiO2−GE catalysts were investigated with TEM on a JEOL JEM-2100HR transmission electron microscope at an accelerating voltage of 200 kV. The specific surface area, pore volume, and pore size of the samples were determined on a Quadrasorb SI-MP surface area analyzer with a nitrogen adsorption−desorption method. NLDFT model was used to analyze the result. XPS (Axis Ultra DLD, Kratos, UK) was used to analyze the chemical state and surface composition of samples. All spectra were acquired at a basic pressure 25 × 10−9 Torr with Al Kα radiation (ℏν = 1486.6 eV) at 15 kV. The binding energy calibration was performed using the C 1s peak in the background as the reference energy (284.6 eV). The quality of the samples was studied with a Labram HR800 laser Raman spectrometer (HJY, France) at an excitation laser beam wavelength of 532 nm. The effects of H2O and SO2 were analyzed with a Fourier transform infrared spectrometer (FT-IR PROTÉGÉ 460, Thermo Nicolet). 2.3. Catalyst Activity Test. Steady-state SCR reaction experiments were performed in a quartz tube fixed-bed continuous flow reactor using 500 mg of catalyst of 60−100 mesh. The reactor was placed inside an electrically heated furnace with a programmable controller. The typical reactant

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Graphene oxide (GO) was prepared using natural graphite powder (99.9% purity from Qingdao) through a modified Hummers method.23,24 In a typical experiment, 1 g of graphite (3000 mesh), 0.5 g of sodium nitrate, and 23 mL of concentrated H2SO4 were put into a 1000 mL beaker in an ice bath. After the mixture was stirred continuously for 10 min, 3 g of KMnO4 was added into the above reaction mixture. The ice bath was removed, and the beaker was placed in a water bath at 35 ± 3 °C for 30 min. Then a certain amount of H2O2 (30 wt %) and 46 mL of deionized water were added to the above reaction mixture, the water temperature was increased to 90 °C for 15 min, and warm deionized water was added to a volume of 140 mL. Then the mixture was filtered through a 0.45 mm cellulose membrane film. The resultant yellow-brown graphite oxide was washed with 150 mL of diluted HCl (10 wt %) and 500 mL of deionized water three times. The obtained graphite oxide was dispersed into 500 mL of water by ultrasonication at room temperature for 1 h. Unexfoliated graphite oxide in the suspension was removed by subsequent centrifugation at 4000 rpm for 30 min. The graphene oxide was recovered by filtration and finally dried at 80 °C for 24 h in the vacuum oven. The TiO2−graphene (TiO2−GE) nanocomposite with different mass ratios of graphene (0.2, 0.4, 0.8, 2 wt %, respectively) was obtained via the sol−gel method. First, GO was dissolved in 50 mL of deionized water by ultrasonic treatment for 1 h. Solution A was the mixture of a certain 11602

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gas composition included 500 ppm of NO, 500 ppm of NH3, 7 vol % O2, 10 vol % H2O (when used), and 200 ppm of SO2 (when used) with a balance of Ar. The Ar flow gas was divided into two branches. One branch converged with NO, NH3, O2, and SO2 to form the main gas flow, while the other one passed through a heated gas-wash bottle containing deionized water (80 °C) to introduce water vapor into the system when required. The gas flow rates were controlled by mass flow controllers. The feed flow rate was fixed at 600 mL/min, which corresponded to a gas hourly space velocity (GHSV) of 67 000 h−1. With an isotherm step of 20 °C, the reaction temperature ranged from 80 °C to 180 °C. At each temperature point, the reaction came to steady state around 30 min, at which time experimental data were collected. NO and NO2 concentrations at the inlet and outlet were monitored with a NOX analyzer (42i-HL, Thermo Scientific Instruments), while N2 product was monitored with a gas chromatograph (GC-7890A, Agilent Technologies). NO and NH3 oxidation experiments were also carried out with this equipment, and the outlet concentration of NO2 was also monitored with the 42i-HL NOX analyzer (Thermo Scientific Instruments). The effluent gas concentration of N2O was monitored with a Fourier transform infrared spectrometer (FT-IR PROTÉGÉ 460, Thermo Nicolet). The NOX removal efficiency and the N2 selectivity were obtained by the following equations: NOX conversion (%) = 100 ×

Figure 1. Effect of different mass ratios of graphene of MnOX/TiO2− GE catalysts on catalytic activity. Reaction conditions: 500 ppm of NO, 500 ppm of NH3, 7 vol % O2, Ar to balance, GHSV = 67 000 h−1, 500 mg of catalyst.

in out C NO − C NO X X in C NO X

(1)

N2 selectivity (%) = 100 ×

in in out C NO + C NH − C NO − 2C N2O 3 2 in in C NO − C NH 3

(2) in CNO , X

in CNO ,

in CNH3

where and correspond to the inlet concentration of NOX, NO, and NH3, respectively. Cout NOX and out CNO2 correspond to the outlet concentration of NOX and NO2, respectively. CN2O is the outlet concentration of N2O.

3. RESULTS AND DISCUSSION 3.1. SCR Activity of MnOX/TiO2−GE Catalysts. 3.1.1. Effect of Different Mass Ratios of Graphene. Figure 1 shows the NH3-SCR activity of 5% manganese supported on TiO2−GE which possessed different mass ratios of graphene. SCR activity for the catalyst was enhanced with the increase of mass ratio of graphene, and 0.8 wt % graphene of TiO2−GE reached the maximum, activity up to 91% at 180 °C. Especially, the catalytic activity of 5%MnOX/TiO2−0.8%GE was obviously better than the other samples at low temperature, between 80 °C and 120 °C. However, the activity decreased when the mass ratio of graphene rose to 2%; this phenomenon can be attributed to the BET surface areas and pore size distributions. This indicated that the TiO2−GE samples had higher SCR activity compared to that of pure TiO2 and GE at reaction temperatures lower than 140 °C. In addition, similar NOX conversion curves of manganese supported on these samples were observed at reaction temperatures between 140 °C and 180 °C. 3.1.2. Effect of Mn Loading and N2 Selectivity. Figure 2a illustrates the effect of Mn loading of MnOX/TiO2−0.8%GE catalysts on catalytic activity. The addition of manganese on TiO2−0.8%GE caused enhancement of the catalytic activity.

Figure 2. (a) Effect of different mass ratios of graphene of MnOX/ TiO2−GE catalysts on catalytic activity. (b) N2 selectivity and N2O formation over the various MnOX/TiO2−GE catalysts. Reaction conditions: 500 ppm of NO, 500 ppm of NH3, 7 vol % O2, Ar to balance, GHSV = 67 000 h−1, 500 mg of catalyst.

Increasing of manganese loading increased NOX conversion until the manganese loading reached 7%, and the NOX conversion was 93% at 180 °C. After this level, a further increase of manganese loading led to a decline in NOX conversion due to the aggregation of MnOX particles on the surface of TiO2−0.8%GE. Nevertheless, the results were interpreted by the following TEM and BET analysis. The N2 product selectivity and N2O formation as a function of the temperature for these catalysts is shown in Figure 2b. All of the 11603

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Figure 3. SEM images of different supports: (a) GE; (b−d) TiO2−0.8%GE; (e−g) EDS mapping of TiO2−0.8%GE support.

0.8%GE were distributed on the surface of lamellar GE uniformly. The results are shown in Figure 3e−g, which represented the distribution of C, Ti, and O elements on the support surface. The three elements were evenly distributed on the support surface, as shown in Figure 3, in a dense and orderly arrangement. Then it was further confirmed that each component was distributed uniformly on the catalyst carrier. 3.2.2. XRD of the Supports. The XRD patterns of different supports are shown in Figure 4. It was obvious that TiO2, TiO2−GO, and TiO2−GE nanocomposites with different mass ratios of graphene exhibited similar XRD patterns and that all diffraction peaks could be readily indexed to anatase TiO2 (JCPDS 21-1272). Anatase TiO2 had abundant active sites, which enhanced the SCR activity of the catalyst, and the peaks at 2θ values of 25.3°, 37.8°, 48.1°, 53.9°, 55.0°, 62.7°, 68.7°, 70.3° and 75.1° were indexed to (101), (004), (200), (105), (211), (204), (116), (220), and (215) crystal planes of anatase TiO2 in all samples, respectively. GO of TiO2−GO showed a characteristic peak at a 2θ value of 9° in Figure 4 and then disappeared after reduction by hydrazine. The results proved

catalysts exhibited high N2 selectivity (>96%) and a trace amount of N2O (less than 5 ppm) across all temperature ranges. Increasing the loading and reaction temperature decreased N2 selectivity. These results were due to the formation of undesired N2O at high loadings and the partial oxidation of NH3 at elevated temperatures. Singoredjo et al.25 reported similar results: with the increase of manganese loading and reaction temperature over MnOX/Al2O3, the amount of N2O increased; therefore, N2 selectivity decreased. 3.2. Characterization of the Catalysts. 3.2.1. SEM and EDS Mapping Analysis of the Supports. The SEM images of GE and TiO2−0.8%GE and EDS surface scanning spectra of TiO2−0.8%GE supports are shown in Figure 3. Figure 3a shows the GE single sheet morphology, and Figure 3b,c shows TiO2−0.8%GE lamellar morphology and topography of its cross-section. Tiny white particles of TiO2 were distributed on the surface of lamellar GE uniformly, and a graph of the crosssection showed that the structure of GE with TiO2−0.8%GE was formed as multigraphene layers stacked together. Further, EDS mapping analysis proved that the TiO2 particles in TiO2− 11604

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on the the in-plane vibration of sp2 bonded carbon atoms.30,31 The D band suggested the presence of sp3 defects.32 The second-order Raman feature, namely the 2D band (secondorder of the D band) at about 2700 cm−1, was very sensitive to the stacking order of the graphene sheets along the c-axis, as well as to the number of layers, and showed greater structure (often a doublet) with an increasing number of graphene layers. The stacking structure and agglomerated morphology of the GE sheets were therefore consistent with previous reports.33,34 Further observation indicated that three characteristic peaks at about 1350 cm−1 (D band), 1590 cm−1 (G band), and 2700 cm−1 (2D band) for the graphitized structures were observed in the Raman spectrum of TiO2−0.2%GE, TiO2−0.4%GE, TiO2− 0.8%GE, and TiO2−2%GE (inset in Figure 5), which correspond to the well-documented D, G, and 2D bands, suggesting that the structure of graphene was maintained in all composites. 3.2.4. TEM of the MnOX/TiO2−GE. As shown in Figure 6, TEM images revealed that particles were dispersed well on the

Figure 4. XRD patterns of TiO2−graphene composites (TiO2, TiO2− 0.8%GO, TiO2−0.2%GE, TiO2−0.4%GE, TiO2−0.8%GE, and TiO2− 2%GE).

that the GO was restored successfully. No characteristic diffraction peaks for carbon species were observed in the TiO2−GE composites because of the low amount and relatively low diffraction intensity of graphene. The main characteristic peak of graphene was at 24.5°,26 so the peaks for graphene might be shielded by the strong peak of anatase TiO2 at 25.3°. Nevertheless, the existence of graphene was determined by following the Raman spectra and XPS analysis. Further observation indicated that the full width at half-maximum (fwhm) of the anatase peaks for TiO2−GE composites were slightly broadened, implying a slight decrease in the anatase crystallite size.27 The incorporation of graphene into TiO2 particles did not have an effect on the anatase structure. Similar results have been reported by the other researchers.28,29 3.2.3. Raman Spectra of the Supports. Raman spectroscopy is a powerful, nondestructive tool for distinguishing ordered and disordered crystal structures of carbon. Figure 5 shows a

Figure 6. TEM images of various catalysts: (a: 3%MnOX/TiO2−0.8% GE; b: 5%MnOX/TiO2−0.8%GE; c: 7%MnOX/TiO2−0.8%GE; d: 9% MnOX/TiO2−0.8%GE; e: 11%MnOX/TiO2−0.8%GE).

lamellar GE. As shown in Figure 6a−c, manganese particles were uniformly dispersed on the surface of GE and their particle diameters were between 10 and 40 nm when the catalysts had lower Mn loadings (7 wt %) led to the aggregation of MnOX particles and then decreased the NOX conversion. All samples showed excellent N2 selectivity up to more than 95%. MnOX was considered as MnO, MnO2, Mn2O3, and MnOX/Mn (nonstoichiometric) in the sample, and redox reaction was apt to occur in the presence of manganese oxides with multiple valence states. Especially, nonstoichiometric MnOX/Mn on the surface of the composite catalyst favored electron transfer; therefore, the redox performance of the catalyst was improved. The results indicated that TiO2− graphene was able to exhibit high surface area, high activity and N2 selectivity, good resistance to H2O and SO2 at low temperatures, and excellent redox performance, which all favored the catalytic reaction. This work is expected to facilitate TiO2−C composites investigation and to promote practical application in the low-temperature SCR field.

be formed by the reaction between SO2 and the reactants (NH3 and O2), and the formed metal sulfates and ammonium sulfates can occupy active sites on the surface of catalyst, gradually deactivating the catalyst over the course of the reaction. The resistance of 7%MnOX/TiO2−0.8%GE to H2O and SO2 in the SCR reaction system was also studied. As shown in Figure 12, 7%MnOX/TiO2−0.8%GE stabilized at 60 min in simulated flue gas without H2O and SO2. After the addition of 10 vol % H2O and 200 ppm of SO2, the NOX conversion obviously decreased but still kept a relatively high level of activity. In addition, the NOX conversion by 7%MnOX/TiO2− 0.8%GE was restored to 79% when H2O and SO2 was removed, as indicated by the FT-IR spectra of fresh and poisoned 7% MnOX/TiO2−0.8%GE catalyst (Figure 13). Compared to the

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AUTHOR INFORMATION

Corresponding Author

*Tel: +86-10-62333434. Fax: +86-10-62333201. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by National Natural Science Foundation (21177051), the Fundamental Research Funds for the Central Universities (06101047), and Program for New Century Excellent Talents in University (NECT-13-0667).

Figure 13. FT-IR spectra of fresh and poisoned 7%MnOX/TiO2−0.8% GE catalyst.

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fresh catalysts in Figure 13, the band intensity at 1409 cm−1 of the poisoned catalyst was smaller, due to NH4+ species on Brønsted acid sites originated from ammonium sulfates. Absorption peaks of water ranged from 2600 to 3700 cm−1, and the intensities at 2925 cm−1 and 3425 cm−1 of both the fresh and poisoned catalysts remained almost unchanged. This result can be attributed to H2O adsorption that was hindered by the hydrophobic groups on the surface of graphene. Furthermore, the band at 1102 cm−1 of the poisoned sample was attributed to the characteristic band of SO42−, and it was also observed in the fresh one, because sulfuric acid was used in the preparation of GO, which played a role in the presulfurization process. The results indicated that the introduction of GE enhanced resistance to H2O and SO2, lowered the probability of surface active sites being taken by SO2, and decreased byproducts such as NH4SO3 and NH4HSO4. All these effects can convey good resistance to H2O and SO2 in the low-temperature SCR reaction.

REFERENCES

(1) Hao, J. M.; Wang, L. T.; Shen, M. J.; Li, L.; Hu, J. N. Air quality impacts of power plant emissions in Beijing. Environ. Pollut. 2007, 147, 401−408. (2) Busca, G.; Larrubia, M. A.; Arrighi, L.; Ramis, G. Catalytic abatement of NOX: Chemical and mechanistic aspects. Catal. Today 2005, 107−108, 139−148. (3) Ma, Z. X.; Yang, H. S.; Li, Q.; Zheng, J. W.; Zhang, X. B. Catalytic reduction of NO by NH3 over Fe-Cu-OX/CNTs-TiO2 composites at low temperature. Appl. Catal., A 2012, 427−428, 43−48. (4) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and mechanistic aspects of the selective catalytic reduction of NOX by ammonia over oxide catalysts: A review. Appl. Catal., B 1998, 18, 1− 36. (5) Yao, G.; Gui, K.; Wang, F. Low-temperature de-NOX by selective catalytic reduction based on iron-based catalysts. Chem. Eng. Technol. 2010, 33, 1093−1098. (6) Ettireddy, P. R.; Ettireddy, N.; Mamedov, S.; Boolchand, P.; Smirniotis, P. G. Surface characterization studies of TiO2 supported manganese oxide catalysts for low temperature SCR of NO with NH3. Appl. Catal., B 2007, 76, 123−134. (7) Thirupathi, B.; Smirniotis, P. G. Nickel-doped Mn/TiO2 as an efficient catalyst for the low-temperature SCR of NO with NH3: Catalytic evaluation and characterizations. J. Catal. 2012, 288, 74−83. (8) Kim, Y. J.; Kwon, H. J.; Heo, l.; Nam, I. S.; Cho, B. K.; Choung, J. W.; Cha, M. S.; Yeo, G. K. Mn-Fe/ZSM5 as a low-temperature SCR catalyst to remove NOx from diesel engine exhaust. Appl. Catal., B 2012, 126, 9−21. (9) Ettireddy, P. R.; Ettireddy, N.; Boningari, T.; Pardemann, R.; Smirniotis, P. G. Investigation of the selective catalytic reduction of nitric oxide with ammonia over Mn/TiO2 catalysts through transient isotopic labeling and in situ FT-IR studies. J. Catal. 2012, 292, 53−63.

4. CONCLUSION In summary, mesoporous TiO2−graphene nanocomposites with low loading (0−2% wt %) of graphene were successfully prepared by means of sol−gel and in situ reduction. Raman spectra and XPS analysis of TiO2−graphene composites confirmed the reduction of graphene oxide and formation of graphene. XRD patterns showed that all diffraction peaks of TiO2−graphene can be readily indexed to anatase TiO2, and the samples enhanced SCR activity at low temperatures. Among all supports prepared, TiO2−0.8%GE had the best SCR catalytic performance from 80 °C to 180 °C, especially less 11609

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(10) Li, J. H.; Chang, H. Z.; Ma, L.; Hao, J. M.; Yang, R. T. Lowtemperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts-A review. Catal. Today 2011, 175, 147−156. (11) Yoshikawa, M.; Yasutake, A.; Mochida, I. Low-temperature selective catalytic reduction of NOX by metal oxides supported on active carbon fibers. Appl. Catal., A 1998, 173, 239−245. (12) Tang, X. L.; Hao, J. M.; Yi, H. H.; Li, J. H. Low-temperature SCR of NO with NH3 over AC/C supported manganese-based monolithic catalysts. Catal. Today 2007, 126, 406−411. (13) Pradhan, B. K.; Sandle, N. K. Effect of different oxidizing agent treatments on the surface properties of activated carbons. Carbon 1999, 37, 1323−1332. (14) Solís, T. V.; Marbán, G.; Fuertes, A. B. Low-temperature SCR of NOX with NH3 over carbon-ceramic supported catalysts. Appl. Catal., B 2003, 46, 261−271. (15) Wang, L. S.; Huang, B. C.; Su, Y. X.; Zhou, G. Y.; Wang, K. L.; Luo, H. C.; Ye, D. Q. Manganese oxides supported on multi-walled carbon nanotubes for selective catalytic reduction of NO with NH3: Catalytic activity and characterization. Chem. Eng. J. 2012, 192, 232− 241. (16) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Grigorieva, I. V.; Firsov, A. A. Electric field effect in atomically thin carbon film. Science 2004, 306, 666−669. (17) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Fine Structure Constant Defines Visual Transparency of Graphene. Science 2008, 320, 1308. (18) Kim, K. S.; Zhao, Y.; Jang, H.; Lee, S. Y.; Kim, J. M.; Kim, K. S.; Ahn, J. H.; Kim, Philip.; Choi, J. Y.; Hong, B. H. Large-scale pattern growth of graphene film for stretchable transparent electrodes. Nature 2008, 457, 706−710. (19) Martin, I.; Blanter, Y. M.; Morpurgo, A. F. Topological confinement in bilayer graphene. Phys. Rev. Lett. 2008, 100, 036804. (20) Xavier, L. J. P.; Pereira, J. M.; Chaves, A.; Farias, G. A.; Peeters, F. M. Topological confinement in graphene bilayer quantum rings. Appl. Phys. Lett. 2010, 96, 212108. (21) Zhang, X. Y.; Li, H. P.; Cui, X. L.; Lin, Y. H. Graphene/TiO2 nanocomposites: Synthesis, characterization and application in hydrogen evolution from water photocatalytic splitting. J. Mater. Chem. 2010, 20, 2801−2806. (22) Zhang, Y. P.; Zhao, X. Y.; Xu, H. T.; Shen, K.; Zhou, C. C.; Jin, B. S.; Sun, K. Q. Novel ultrasonic-modified MnOx/TiO2 for lowtemperature selective catalytic reduction (SCR) of NO with ammonia. J. Colloid Interface Sci. 2011, 361, 212−218. (23) Hummers, W. S.; Offerman, R. E. Preparation of graphitic oxide. J. Am. Chem. Soc. 1958, 80, 1339. (24) Stankovich, S.; Dikin, D. A.; Dommett, G. H. B.; Kohlhaas, K. M. Graphene-based composite materials. Nature 2006, 442, 282−286. (25) Singoredjo, L.; Korver, R.; Kapteijn, F.; Moulijn, J. Alumina supported manganese oxides for the low-temperature selective catalytic reduction of nitric oxide with ammonia. Appl. Catal., B 1992, 1, 297−316. (26) Nethravathi, C.; Rajamathi, M. Chemically modified graphene sheets produced by the solvothermal reduction of colloidal dispersions of graphite oxide. Carbon 2008, 46, 1994−1998. (27) Zhang, Y. H.; Tang, Z. R.; Fu, X. Z.; Xu, Y. J. TiO2−graphene nanocomposites for gas-phase photocatalytic degradation of volatile aromatic pollutant: Is TiO2−graphene truly different from other TiO2−carbon composite materials? ACS Nano 2010, 4, 7303−7314. (28) Chen, C.; Cai, W. M.; Long, M. C.; Zhou, B. X.; Wu, Y. H.; Wu, D. Y. Synthesis of visible-light responsive graphene oxide/TiO2 composites with p/n heterojunction. ACS Nano 2010, 4, 6425−6432. (29) Wang, W. G.; Yu, J. G.; Xiang, Q. J.; Cheng, B. Enhanced photocatalytic activity of hierarchical macro/mesoporous TiO2− graphene composites for photodegradation of acetone in air. Appl. Catal., B 2012, 119−120, 109−116. (30) Dresselhaus, M. S.; Jorio, A.; Hofmann, M.; Dresselhaus, G.; Saito, R. Perspectives on carbon nanotubes and graphene Raman spectroscopy. Nano Lett. 2010, 10, 751−758.

(31) Zhang, W. X.; Cui, J. C.; Tao, C. A.; Wu, Y. G.; Li, Z. P.; Ma, L.; Wen, Y. Q.; Li, G. T. A strategy for producing pure single-layer graphene sheets based on a confined self-assembly approach. Angew. Chem., Int. Ed. 2009, 121, 5978−5982. (32) Lu, J.; Yang, J. X.; Wang, J. Z.; Lim, A.; Wang, S.; Loh, K. P. One-pot synthesis of fluorescent carbon nanoribbons, nanoparticles, and graphene by the exfoliation of graphite in ionic liquids. ACS Nano 2009, 3, 2367−2375. (33) Graf, D.; Molitor, F.; Ensslin, K.; Stampfer, C.; Jungen, A.; Hierold, C.; Wirtz, L. Spatially resolved Raman spectroscopy of single and few-layer graphene. Nano Lett. 2007, 7, 238−242. (34) Akhavan, O. Graphene nanomesh by ZnO nanorod photocatalysts. ACS Nano 2010, 4, 4174−4180. (35) Reinoso, F. R. The role of carbon materials in heterogeneous catalysis. Carbon 1998, 36, 159−175. (36) Yu, J. G.; Zhang, L. J.; Cheng, B.; Su, Y. R. Hydrothermal preparation and photocatalytic activity of hierarchically sponge-like macro-/mesoporous titania. J. Phys. Chem. C 2007, 111, 10582−10589. (37) Tang, X. L.; Hao, J. M.; Yi, H. H.; Ning, P. Low-temperature SCR of NO with NH3 on Mn-based catalysts modified with cerium. Catal. Today 2007, 25, 240−243. (38) Huang, H. C.; Ye, D. Q.; Huang, B. C.; Wei, Z. L. Vanadium supported on viscose-based activated carbon fibers modified by oxygen plasma for the SCR of NO. Catal. Today 2008, 139, 100−108. (39) Lu, P.; Li, C. T.; Zeng, G. M.; He, L. J.; Peng, D. L.; Cui, H. F.; Li, S. H.; Zhai, Y. B. Low temperature selective catalytic reduction of NO by activated carbon fiber loading lanthanum oxide and ceria. Appl. Catal., B 2010, 96, 157−161. (40) Planeix, J. M.; Coustel, N.; Coq, B.; Brotons, V.; Kumbhar, P. S.; Dutartre, R.; Geneste, P.; Bernier, P.; Ajayan, P. M. Application of carbon nanotubes as supports in heterogeneous catalysis. J. Am. Chem. Soc. 1994, 116, 7935−7936. (41) Gao, E.; Wang, W. Z.; Shang, M.; Xu, J. H. Synthesis and enhanced photocatalytic performance of graphene-Bi2WO6 composite. Phys. Chem. Chem. Phys. 2011, 13, 2887−2893. (42) Xiang, Q. J.; Yu, J. G.; Jaroniec, M. Enhanced photocatalytic H2production activity of graphene-modified titania nanosheets. Nanoscale 2011, 3, 3670−3678. (43) Akhavan, O.; Ghaderi, E. Photocatalytic Reduction of Graphene Oxide Nanosheets on TiO2 Thin Film for Photoinactivation of Bacteria in Solar Light Irradiation. J. Phys. Chem. C 2009, 113, 20214− 20220. (44) Lee, J. Y.; Hong, S. H.; Cho, S. P.; Hong, S. C. The study of deNOX catalyst in low temperature using nano-sized supports. Curr. Appl. Phys. 2006, 6, 996−1001. (45) Ponce, S.; Peña, M. A.; Fierro, J. L. G. Surface properties and catalytic performance in methane combustion of Sr-substituted lanthanum manganites. Appl. Catal., B 2000, 24, 193−205. (46) Zhao, Q. S.; Xiang, J.; Sun, L. S.; Su, S.; Hu, S. Adsorption and oxidation of NH3 and NO over sol-gel-derived CuO-CeO2-MnOx/γAl2O3 catalysts. Energy Fuel 2009, 23, 1539−1544. (47) Sjoerd, W. K.; Brands, D. S.; Smit, H. I.; Poels, E. K.; Bliek, A. Mechanism of the selective catalytic reduction of NO with NH3 over MnOx/Al2O3: II. Reactivity of adsorbed NH3 and NO complexes. J. Catal. 1997, 171, 219−230. (48) Zhu, L. L.; Huang, B. C.; Wang, W. H.; Wei, Z. L.; Ye, D. Q. Low-temperature SCR of NO with NH3 over CeO2 supported on modified activated carbon fibers. Catal. Commun. 2011, 12, 394−398. (49) Qi, G. S.; Yang, R. T.; Chang, R. MnOX-CeO2 mixed oxides prepared by co-precipitation for selective catalytic reduction of NO with NH3 at low temperatures. Appl. Catal., B 2004, 51, 93−106. (50) Tang, X. L.; Hao, J. M.; Xu, W. G.; Li, J. H. Low temperatures selective catalytic reduction of NOX with NH3 over amorphous MnOX catalysts prepared by three methods. Catal. Commun. 2007, 8, 329− 334.

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dx.doi.org/10.1021/ie5016969 | Ind. Eng. Chem. Res. 2014, 53, 11601−11610