N-doped graphene and TiO2 supported manganese and cerium oxides on low-temperature selective catalytic reduction of NOx with NH3

A series of N-doped graphene (NG) and TiO2 supported MnOx–CeO2 catalysts were prepared by a hydrothermal method. The catalysts with different molar ratios of Mn/Ce (6: 1, 10: 1, 15: 1) were investigated for the low-temperature selective catalytic reduction (SCR) of NOx with NH3. The synthesized catalysts were characterized by HRTEM, SEM, XRD, BET, XPS, and NH3-TPD technologies. The characterization results indicated that manganese and cerium oxide particles dispersed on the surface of the TiO2–NG support uniformly, and that manganese and cerium oxides existed in different valences on the surface of the TiO2–NG support. At Mn element loading of 8 wt%, MnOx–CeO2(10: 1)/TiO2–1%NG displayed superior activity and improved SO2 resistance. On the basis of the catalyst characterization, excellent catalytic performance and SO2 tolerance at low temperature were attributed to the high content of manganese with high oxidation valence, extensive oxidation of NO into NO2 by CeO2 and strong NO adsorption capacity, and electron transfer of N-doped graphene.


Introduction 
Nitrogen oxides (NO x ) are one of the main atmospheric pollutants, which have given rise to a variety of health-related and environmental issues [1][2][3]. The environmental effects of nitrogen oxides (NO x ) include formation of photochemical smog, acid precipitation, greenhouse effect, ozone depletion, and fine particles. * Corresponding author.
Selective catalytic reduction (SCR) is most widely used in flue gas denitrification technology. The reaction mechanism of SCR technique is that the reducing agent (usually NH 3 ) selectively reduces NO x to N 2 under the action of the catalyst in an oxygen-containing atmosphere. In recent years, low-temperature selective catalytic reduction (SCR) with NH 3 is a promising method to remove NO x in flue gas because the catalyst unit can be located downstream of the particulate control device and desulfurization system, where the temperature is declined to 120-180 ℃ [4][5][6]. Mn-based catalysts E-mail: zhaochunlin@cbmamail.com.cn exhibit excellent DeNO x activity at low temperature of 80-220 , such as MnO ℃ x -CeO 2 /TiO 2 , MnO x -FeO x / TiO 2 , and so on [7][8][9][10][11]. Because manganic oxides have a variety of different surface active oxygen, they can be used to complete the catalytic cycle. But SO 2 in the exhaust gas could easily lead to serious poisoning effect on SCR catalytic in the low-temperature range [12,13]. Ce-based NH 3 -SCR catalysts have also been widely studied due to the high oxygen storage capacity and excellent redox properties of CeO 2 [14][15][16]. Therefore, low-temperature SCR catalysts with high activity and good SO 2 resistance have obtained wide concern [17,18].
In the past years, graphene has drawn an amount of attention as a promising candidate for wide applications in catalysis due to unique two-dimensional monolayer structure, and physical and chemical properties [19][20][21][22]. It has a large specific surface area, high electron mobility, and high stability, and is widely used in the study of heterogeneous catalyst support [23][24][25][26][27][28]. The graphene-supported catalytic system exhibits many special catalytic activities. By introducing graphene into the catalyst, the loading of the active component (MnO x ) is improved, and hence the catalytic activity is enhanced [6]. Some researches indicated that incorporation of nitrogen into the carbon structures enhances the SCR activity [29,30]. N element can be doped through post treatment of graphene or GO, such as hydrazine reduction, and thermal annealing in ammonia method [31][32][33]. It has been well established that the incorporation of N atoms into the graphene matrix can lead to three main types of N formats, including graphitic N with direct substitution structure, and pyridinic N and pyrrolic N structures [34]. Through the surface functionalization to form a controllable chemical defect, the special physical and chemical properties of N-doped graphene play a role in increasing the active site of the supported catalyst. The pyridine-like N is absorbing nitric oxide (NO) more easily than the graphite-like N [35]. It has been discovered and reported that N-doped graphene can be used as a new catalyst for the oxygen reduction reaction [36], C-H bond activation reaction [37], reduction of nitro compounds [38], oxidation of benzylic alcohols [39], and electrochemical biosensing [40]. However, there is few report about N-doped graphene for SCR reactions.
We recently found that the hydrothermal synthesis method has the advantages of simplicity, high efficiency, high purity, and good homogeneity. The Mn-Ce-Ti mixed oxide catalyst prepared by the hydrothermal method exhibited excellent NH 3 -SCR activity and strong resistance against H 2 O and SO 2 with a broad operating temperature window [41]. The purpose of this work is to study the effect of NH 3 -SCR on the removal of NO x in flue gas at low temperature and to develop low-temperature SCR denitrification catalyst with high activity and high durability with MnO x , CeO x , N-doped grapheme, and TiO 2 as main components. In this work, a series of MnO x -CeO 2 /TiO 2 -1%NG catalysts were prepared by the hydrothermal method, which few researchers have concerned about. To fully examine the structure and catalytic mechanism, the catalysts were characterized by SEM, HRTEM, XRD, BET, NH 3 -TPD, and XPS.

Experimental
Expandable graphite (50 mesh 36.5%-38%), and titanium dioxide (TiO 2 , 98.5%) were purchased from Beijing Chemical Reagent Company. All of the chemical reagents were of analytical grade and used as received without further purification. All aqueous solutions were prepared using deionized water.

1 Preparation of graphene oxide (GO)
GO is obtained by chemical oxidation treatment, which is synthesized by a pressurized oxidation [42]. Graphite, KMnO 4 , sulphuric acid (98%), and a Teflon reactor were completely cooled in a refrigerator at 0-4 ℃ before use. The Teflon reactor was placed in a stainless steel autoclave. The cooled graphite (2 g) and KMnO 4 (8 g) were put into the reactor, and then, sulphuric acid (60 mL) was added. As soon as the sulphuric acid was added, the reactor and stainless steel autoclave were covered and fasten down. The autoclave was kept at 0-4 ℃ for 1.5 h and then heated at 100 ℃ in an oven for 1.5 h. The obtained mud was diluted with 1 L www.springer.com/journal/40145 water. With mechanical stirring, H 2 O 2 (30%) was dripped into the suspension until the slurry turned golden yellow. The suspension was washed with hot HCl and deionized water until the pH reached 7, and humid graphite oxide was obtained. After drying, 1 g of GO was added under stirring to 1 L of deionized water. The suspension was placed in an ultrasonic bath for 3 h and then centrifuged at 4000 rpm. The supernatant, consisting of a dispersion of GO with a concentration of about 1 mg/mL, was finally recovered and used for the N-doped graphene preparation.

2 Preparation of N-doped graphene (NG)
0.9 g of urea was added to 60 mL of GO dispersion under magnetic stirring for 30 min. The mass ratio of GO to urea was 1 : 30 [43]. The suspension was placed in autoclave at 160 ℃ for 3 h, and then washed several times. The precipitation was dried. Finally, the product was tagged as NG.

3 Synthesis of catalyst
The MnO x -CeO 2 /TiO 2 -1%NG catalysts were prepared with different molar ratios of Mn/Ce by the hydrothermal method. Mn element accounted for 8 wt% of catalyst quality. Appropriate amounts of Mn(NO 3 ) 2 , Ce(NO 3 ) 3 · 6H 2 O, NG, and TiO 2 were dissolved in deionized water at room temperature and stirred for 20 min, and then ammonia solution (25 wt%, 20 mL) was slowly added to the above solution under vigorous stirring until pH = 11 was achieved. After stirring for 30 min, the obtained suspension was transferred to a 250 mL Teflon-sealed autoclave and allowed to react at 130 ℃ for 12 h. The precipitate was separated by centrifugation and washed several times with deionized water and ethanol, respectively. The resulting powder was dried at 100 ℃ for 12 h, and then calcined in a tubular furnace in a nitrogen atmosphere at 450 for 3 h. For comparison, ℃ MnO x -CeO 2 /TiO 2 and MnO x /TiO 2 -1%NG were also prepared by the same preparation method as described above.

4 Catalyst characterization
The morphology of the samples was characterized by scanning electron microscopy (SEM; Quanta 250 FEG, FEI, USA), and high-resolution transmission electron microscopy (HRTEM; JEM-2100, JEOL, Japan). The structure of the samples was determined by X-ray diffraction (XRD) performed on a Bruker D8 Advance diffractometer, running at 60 kV and 30 mA. The specific surface areas were calculated from adsorbed nitrogen volume by an automatic volumetric apparatus following standard Brunauer-Emmett-Teller (BET) theory, with a Micromeritics ASSP2020 equipment by N 2 physisorption at 77 K. Temperature-programmed desorption of NH 3 (NH 3 -TPD) was conducted using a TP5080 auto-adsorption apparatus (XQ, Tianjin). The catalysts (150 mg) were pretreated at 300 in a flow ℃ of N 2 (30 mL/min) for 0.5 h and cooled to 100 ℃ under N 2 flow. Then the samples were exposed to a flow of NH 3 at 100 for 1 h, followed by N ℃ 2 purging for 0.5 h. Finally, the reactor temperature was raised to 600 ℃ under N 2 flow at a constant rate of 10 ℃/min. The X-ray photoelectron spectroscopy (XPS) was carried out to analyze surface chemical composition and the valence state of the metal species on the surface of the catalysts on an Escalab 250 xi spectrometer (Thermo, USA) with Al Kα radiation source, and the binding energy was corrected using the C 1s spectrum at 284.8 eV.

5 SCR performance test
SCR activity measurement was performed in a fixedbed reactor using the catalyst of 40-60 mesh at 100-200 ℃ by ZHKPR Instrument Co., Ltd. (Chengdu, China). The reactor was placed in an electrically heated furnace with the typical reaction gas consisted of 500 ppm NO, 500 ppm NH 3 , 6 vol% O 2 , 100 ppm SO 2 (when used), balance N 2 gas, and GHSV = 30000 h -1 . The inlet and outlet concentrations of NO x were continually measured by an analyzer (Testo 350, Germany). All the data were obtained after 20 min as the SCR reaction reached steady state. NO x conversion was calculated according to the following formula:

1 Morphology and texture
The TEM image in Fig. 1(a) shows that N-doped graphene is transparent with some clearly visible wrinkles, suggesting that NG is mainly composed of few layers. Because the sheets have a high specific fringes. Figure 1(c) shows the fringe spacing is 0.352 nm, corresponding to the (101) plane of anatase TiO 2 . The low-magnification SEM image in Fig. 1(d) indicates that MnO x -CeO 2 and TiO 2 nanoparticles are anchored onto the surface of NG sheets and some particles aggregate together. A large amount of the catalyst nanoparticles uniformly disperse on the surface of the N-doped graphene carrier and the surface area of the MnO x -CeO 2 /TiO 2 -1%NG catalyst is greatly increased.

2 GO and NG by XPS
In order to investigate the effect of urea on GO reduction and nitrogen doping in hydrothermal process, XPS was used to qualitatively and quantitatively analyze the samples. It can be seen from the XPS full spectra ( Fig. 2(a)) that the intensity of the O 1s (531.3 eV) peak is significantly reduced after the hydrothermal reaction, indicating that GO is reduced. N 1s (~399.3 eV) peak of NG is also observed indicating that nitrogen element is doped into the sample, and N element content is up to 6.33% as shown in Table 1. C=C (284.6 eV), C-O/C-O-C (286.9 eV), and C=O (288.2 eV) are found in the C 1s XPS spectra of GO ( Fig. 2(b)) and NG (Fig.  2(c)). After hydrothermal process, the content of C-O (286.9 eV) is significantly reduced in NG (Fig. 2(c)) surface area, in order to reduce the surface energy, there will be overlapping phenomenon. Figure 1(b) shows the TEM image of the MnO x -CeO 2 /TiO 2 -1%NG catalyst at a Mn/Ce molar ratio of 10:1. It is clear that a large number of TiO 2 nanoparticles ranging from 100 to 200 nm with an average particle size of ca. 150 nm, are anchored onto the stacked and wrinkled NG sheets. The corresponding HRTEM image reveals clear lattice compared to that in GO (Fig. 2(b)), which indicates GO is reduced with urea. A C-N peak (285.9 eV) is appeared (Fig. 2(c)). N atoms are divided into "pyridinic N" (398.2 eV), "pyrrolic N" (399.5 eV), and "graphitic N" (401.5 eV) as shown in Fig. 2(d), which replace the C atoms in the graphene lattice [43]. Nitrogen atoms of NG due to its basic nature should have affinity towards weakly acidic molecules like NO. The presence of nitrogen in the carbon matrix was reported to enhance adsorption of NO [29,35], which may cause an electron transfer from the support surface to the NO molecules. Table 1 lists the surface atomic concentrations of GO and NG. Consequently, GO is reduced with urea after the removal of a large number of oxygenated functional groups, and nitrogen element is doped into the graphene lattice.

4 BET surface area and pore size distribution
Detailed data of the specific surface area, pore volume, and pore size of MnO x -CeO 2 /TiO 2 and MnO x -CeO 2 / TiO 2 -1%NG are listed in Table 2. Through adding NG, MnO x -CeO 2 /TiO 2 -1%NG has a larger specific surface area than MnO x -CeO 2 /TiO 2 , which leads to the high dispersion in the metal oxide composite with the support. The nitrogen adsorption-desorption isotherms are displayed in Fig. 4. According to the Brunauer-Deming-Deming-Teller (BDDT) classification, the majority of physisorption isotherms could be classified into six types. As shown in Fig. 4, MnO x -CeO 2 /TiO 2 and MnO x -CeO 2 /TiO 2 -1%NG could both be classified into the representative type IV adsorption-desorption isotherm with an H3-type hysteresis loop [6]. The samples have mesoporous structure, which could be derived from the packing of the nanoparticles. MnO x -CeO 2 /TiO 2 -1%NG presents larger pore volume than MnO x -CeO 2 /TiO 2 . As illustrated in Table 2, the pore distribution of MnO x -CeO 2 /TiO 2 and MnO x -CeO 2 / TiO 2 -1%NG shows an average pore size of 13.20 and 13.03 nm, respectively. In general, the larger specific surface area is expected to be beneficial to offer more active sites and increase the adsorption of reactants in  the catalytic reaction, resulting in the excellent catalytic performance of MnO x -CeO 2 /TiO 2 -1%NG.

5 Chemical composition by XPS
To obtain the information on the atomic concentration and element chemical state of manganese or cerium species in the catalysts, the surface of samples was further investigated by XPS. Figure 5 illustrates the XPS spectra of Mn 2p, Ce 3d, and O 1s. The atomic surface compositions of MnO x -CeO 2 /TiO 2 , fresh and spent MnO x -CeO 2 /TiO 2 -1%NG have been summarized by XPS in Table 3. Results show that MnO 2 and MnO/ Mn contents of the spent catalyst increase relative to  The Ce 3d spectrum is presented in Fig. 5(b). The peaks are attributed to 3d 3/2 and 3d 5/2 spin-orbit states. The peaks at the binding energy of 882.4 (v), 898.9 (v'''), 900.5 (u), 907 (u''), 916.9 (u''') eV are assigned to Ce 4+ . The peaks at the binding energy of 885.4 (v') and 904.5 (u') are assigned to Ce 3+ species [6]. Results imply that Ce 4+ is the main valence state in MnO x -CeO 2 /TiO 2 -1%NG catalyst. No obvious difference is observed from the Ce 3d XPS spectra of MnO x -CeO 2 /TiO 2 and MnO x -CeO 2 /TiO 2 -1%NG samples.
The XPS patterns of O 1s (Fig. 5(c)) show the presence of two types of surface oxygen in the samples. The peak at 529.4-529.7 eV corresponds to lattice oxygen (O β ), while that at 531.6-532.0 eV is assigned to chemisorbed oxygen (O α , surface-adsorbed oxygen), such as O 2 2-or O -, in the form of OHand CO 3 2- [26]. According to the XPS analysis, the surface concentration of O α species on MnO x -CeO 2 /TiO 2 -1%NG is higher than that on MnO x -CeO 2 /TiO 2 . It has been demonstrated that O α species are more active than O β species, due to their higher mobility [46]. Hence, the higher concentration of O α species is beneficial to the NH 3 -SCR of NO, resulting in the promotion of the reduction of NO and the subsequent facilitation of the "fast SCR" reaction.

6 Acidic properties
The adsorption and activation of NH 3 at active sites of the catalysts play an important role in the NH 3 -SCR reaction. NH 3 -TPD was performed to investigate the surface acid amount and strength of the catalysts, and the corresponding results are shown in Fig. 6(b). The area and position of these desorption peaks directly relate to the amounts of acidic sites and their acidic strength, respectively. One broad peak spanning the temperature range of 100-250 ℃ is observed for both samples, attributed to physisorbed NH 3 and NH 3 at weak acid sites. The NH 3 -TPD physisorption is too weak to activate NH 3 molecules, while the adsorbed NH 3 species on strong acid sites are hardly to desorb, which make not much contribution to low-temperature NH 3 -SCR reaction. Therefore, we focus on the adsorption of NH 3 molecules on medium-strong acid sites. The desorption peak at 306 ℃ of the MnO x -CeO 2 /TiO 2 -1%NG is obviously higher than MnO x -CeO 2 /TiO 2 as shown in Fig. 6. It is considered that the peak area correlates with the acid amount. This indicates that MnO x -CeO 2 /TiO 2 -1%NG catalysts have more acid sites than MnO x -CeO 2 /TiO 2 , which may be due to the increase in specific surface area of MnO x -CeO 2 /TiO 2 -1%NG and improve the dispersion of the catalyst nanoparticles. Therefore, MnO x -CeO 2 /TiO 2 -1%NG has a stronger acid intensity due to the addition of NG. The difference in the strength and the number of acid sites on the two catalysts might lead to the distinction of their catalytic performances. In other word, MnO x -CeO 2 /TiO 2 -1%NG catalyst possesses the largest amount of NH 3 molecules, and further promotes the enhancement of catalytic performance for NH 3 -SCR reaction. Figure 7 shows the NH 3 -SCR activity of these prepared catalysts with the variation in temperature. It can be seen that the NO x conversion over all the catalysts increases with increasing temperature in 80-200 .

7 Catalytic activity
℃ The loading of manganese element is 8 wt%, together with different molar cerium supported on TiO 2 -1%NG. When the Mn/Ce molar ratio is 10 : 1, the NO x conversion is up to 99% at 160 . Further when the ℃ Mn/Ce molar ratio is 6 : 1, NO x conversion decreases evidently. MnO x -CeO 2 (10 : 1)/TiO 2 -1%NG samples have higher SCR activity compared to MnO x /TiO 2 -1%NG in the temperature region. It also can be seen that the MnO x -CeO 2 (10 : 1)/TiO 2 -1%NG samples have higher SCR activity compared to MnO x -CeO 2 (10 : 1)/TiO 2 in the whole temperature region. The reaction mechanism may be gaseous NH 3 molecules are adsorbed onto the acid sites to form NH 4 + ions, and then the formed molecules of NO 2 react with adjacent NH 4 + ions to produce N 2 and H 2 O [29]. Nitrogen atoms of NG as a basic center can adsorb acid gas NO and the increased adsorption may be associated with an electron transfer between the support surface to the NO molecule, which is oxidized NO 2 . The reaction is probably the rate-determining step for SCR reaction of catalysts. This demonstrates that NG improves interaction of the species, which possibly provides more effective contact with the reactants resulting in the process of NO adsorption oxidation.

8 Influence of SO 2
The SO 2 resistance effects of the catalysts on NO x conversion at 160 ℃ are investigated in Fig. 8. It is obvious that the NG in MnO x -CeO 2 /TiO 2 -1%NG plays a great role in the high catalytic activity. It shows that when 100 ppm SO 2 is added to the system, the NO x conversion of MnO x -CeO 2 (10 : 1)/TiO 2 -1%NG decreases from an initial value of 99% to 55% in 2 h. And when SO 2 is removed from the flue gas, the activity of MnO x -CeO 2 (10 : 1)/TiO 2 -1%NG reaches a stable level of about 49%. For MnO x -CeO 2 /TiO 2 resistance to SO 2 , SCR reaction system was also studied and a similar phenomenon is observed. The NO x conversion markedly decreases to 50% in 2 h and is finally restored to 41%. For MnO x /TiO 2 -1%NG resistance to SO 2 , the NO x conversion markedly decreases to 32% in 2 h and is finally restored to 26%. These indicate that the NO x conversion of MnO x -CeO 2 /TiO 2 -1%NG obviously decreases but a relatively higher activity is still maintained compared with MnO x -CeO 2 /TiO 2 and MnO x / TiO 2 -1%NG. The results indicate that introduction of NG and Ce enhances the resistance of Mn-based catalysts to SO 2 . A possible reason is that nitrogen functional group of NG due to its basic nature can absorb acid gas SO 2 . Consequently, NG can act as a SO 2 trap to limit the sulfation of the main active phase when exposed to SO 2 . Introduction of Ce inhibits the formation of manganese sulfates, lowers the probability of surface active site poisoning by SO 2 , and decreases by-products, such as NH 4 SO 3 and NH 4 HSO 4 , all of which could improve the resistance of the catalyst to SO 2 poisoning during low-temperature SCR.

Conclusions
In summary, a series of MnO x -CeO 2 /TiO 2 -1%NG catalysts were successfully prepared with different molar ratios of Mn/Ce by the hydrothermal method. The obtained results of HRTEM images and XRD patterns showed the anatase TiO 2 and several valences of amorphous manganese and cerium oxides were uniformly distributed on the surface of the catalysts. Among the catalysts prepared, MnO x -CeO 2 (10:1)/ TiO 2 -1%NG catalyst exhibited the highest SCR activity (up to 99% at 160 ). MnO ℃ x was observed as MnO, MnO 2 , Mn 2 O 3 , and non-stoichiometric MnO x /Mn in the samples by XPS. Redox reactions were likely to occur in the presence of manganese oxides with multiple valence states. The introduction of NG could be associated with the high specific surface areas, which provided more active sites to adsorb and activate reagents. In particular, active sites on the surface of NG adsorb NO molecular. Addition of Ce increased chemisorbed oxygen on the catalyst surface and promoted NO oxidation into NO 2 , thereby improving the redox performance of the catalyst. MnO x -CeO 2 (10:1)/TiO 2 -1%NG exhibited a large surface area, high activity, and improved resistance to SO 2 at low temperatures. This work enhanced the low-temperature NH 3 -SCR performance and SO 2 tolerance of catalysts by adding NG and promoted the practical application of these catalysts in low-temperature SCR.