The CeO_X and MnO_X Nanocrystals Supported on TiO₂–Graphene Oxide Catalysts and Their Selective Catalytic Reduction Properties at Low Temperature

Abstract

A series of 9%CeO_x–MnO_x/TiO₂–GO nanocomposites with different molar ratios of Ce/Mn were synthesized by the sol-gel and ultrasonic impregnation methods and characterized by field emission scanning electron microscope (FESEM), high resolution transmission electron microscopy (HRTEM), N₂ adsorption (BET) analysis, X-ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT–IR). The results showed that various valences of Ce and Mn oxides were uniformly distributed on the surface of TiO₂–GO multilayered supports. The coexistence of various valences of Ce and Mn oxides can improve the redox performance of the catalyst. With the introduction of Ce, the amount of MnO₂ and non-stoichiometric MnO_x/Mn, the total oxygen and chemisorbed oxygen content, and the electron transfer ability of the catalyst increased significantly. When the molar ratio of Ce/Mn was 0.3, the catalysts exhibited high selective catalytic reduction activity (more than 99% at 180 °C) and N₂ selectivity. The presence of hydrophilic groups on the surface of the GO was considered as the critical factor influencing the H₂O resistance of the catalyst. Due to the pre-sulfuring process of GO, serious sulfation of the active component can be prevented, and the catalyst exhibited excellent SO₂ resistance.

1. Introduction

Removal of the nitrogen oxides (NO_x) emitted from the combustion of fossil fuels has attracted much more attention worldwide because NO_x can cause acid rain, photochemical smog, greenhouse effects, and damage to human health [1,2]. Currently, low-temperature selective catalytic reduction (SCR) has been considered as a reasonable and effective strategy to decrease the NO_x levels in gaseous emissions [3,4,5,6].

For their relatively high activities for low-temperature NH₃-SCR, Mn-based catalysts have attracted increasing attention. It has been shown that the reaction activity of manganese oxide is affected by several factors, such as Mn oxidation state, crystallinity, specific surface area, and morphology [7,8,9,10]. However, the selective catalytic reduction (SCR) activity of MnO_x is not as high as expected, and the resistance of MnO_x to SO₂ and/or H₂O is relatively poor, so some other metal oxides are often added, such as CeO_x [11], FeO_x [12], NbO_x [13], SnO_x [14], and ZrO_x [15]. Compared with other metal oxides, CeO_x is considered more effective. The redox shift between Ce⁴⁺ and Ce³⁺ will result in the increase in the oxygen storage capability of MnO_x and the oxygen migration speed, which are important for the reaction activity of the catalyst [16,17]. Additionally, it has also been shown that when CeO_x is added into MnO_x, the Mn ions will enter the CeO₂ lattice to form a nanoscale solid solution [18]. On the other hand, the addition of CeO₂ can improve the distribution status of MnO_x on the surface of the support and enhance the oxidation of NO to NO₂, producing more absorbed NO₃⁻ on the catalyst surface, which is then reduced into N₂ by NH₃. These behaviors account for the promoting effect of CeO₂ on the SCR activity [19]. Qi et al. prepared a CeO₂–MnO_x catalyst and showed that the addition of Mn into the Ce lattice significantly improves NO conversion; however, increasing the Mn content beyond a certain point degrades this conversion [20,21]. It has also been proven that the introduction of CeO_x is beneficial to the SO₂ resistance of the catalysts. Jin et al. reported that the introduction of Ce can not only lessen the sulfation of MnO_x, but also reduce thermal stabilities of the sulfate species covered on the catalyst surface [22]. Wu et al. prepared Mn/TiO₂ and ceria-modified Mn/TiO₂ catalysts by the sol-gel method, and the results showed that the doping of ceria can prevent the formation of Ti(SO₄)₂ and Mn(SO₄)_x, and the depositions of (NH₄)₂SO₄ and NH₄HSO₄ can also be significantly inhibited [23].

Carbon materials, such as activated carbon (AC), AC fiber (ACF), carbon nanotubes (CNTs), ordered mesoporous carbons, graphene, and graphene oxide (GO), are considered as ideal supports of catalysts for their large specific surface areas and relatively high chemical stabilities [24,25,26,27,28,29,30,31,32]. GO, as the derivative of graphene, consists of a hexagonal ring-based carbon network with both sp² and sp³ hybridized carbon atoms [33], both sides are accessible [34], and GO is a sheet-shaped material with large specific surface area up to 400–1500 m²/g [35,36]. It has also been reported that the existence of extensive reactive oxygen functional groups [37,38,39], holes [40], carbon vacancies, and defects on the surface of GO can introduce chemically-active sites during catalysis [41]. It has been suggested that GO could be an ideal support for the growth of functional nanoparticles and would render them electrically conductive, highly dispersive, and catalytically active [35].

We recently found that MnO_x/TiO₂–GO catalysts exhibited high NH₃–SCR activity and N₂ selectivity at low temperature. The results showed that the 9%MnO_x/TiO₂–0.8%GO catalyst had the highest activity at low temperature [42]. In order to improve the SCR activity further, a certain amount of CeO_x was added to the MnO_x/TiO₂–GO catalyst in this work, which was represented as 9%Ce–Mn/TiO₂–0.8%GO. The composite catalysts were characterized by field emission scanning electron microscope (FESEM), high resolution transmission electron microscopy (HRTEM), N₂ adsorption (BET) analysis, X–ray photoelectron spectroscopy (XPS), and Fourier transform infrared spectroscopy (FT–IR), temperature-programmed desorption of NH₃(NH₃–TPD), and temperature-programmed reduction of H₂(H₂–TPR). The focus of the work was to identify the effect of the introduction of Ce and the synergistic effect of Ce, Mn, and GO on the catalytic performance of the nanocomposites. The influences of the catalyst SCR activity by H₂O and SO₂ were also determined.

2. Results and Discussion

2.1. SCR Activity of Different Molar Ratios of Ce/Mn

The SCR activities of 9%Ce–Mn/TiO₂–0.8%GO with different molar ratios of Ce/Mn at low temperature are shown in Figure 1.

From Figure 1, it can be found that the NO_x conversion efficiencies of all catalysts increased with the reaction temperature and, with the addition of Ce, the 9%Ce–Mn/TiO₂–0.8%GO catalysts exhibited higher SCR activity than that of 9%Mn/TiO₂–0.8%GO. When the Ce/Mn ratio was below 0.4, the NO_x conversion of 9%Ce–Mn/TiO₂–0.8%GO catalysts increased with the Ce/Mn ratio. When the ratio was 0.3, the catalytic activity had the highest value of 99% at 180 °C. That meant that the introduction of a certain amount of Ce was beneficial to the increase of reaction activity of the catalyst, which will be explained by the following HRTEM and XPS analyses. It can also be observed that when the Ce/Mn ratio was as high as 0.4, the efficiency decreased. Similar results have been obtained in our previous work [43]. This may be attributed to the aggregation of active components on the surface of the support, which will be interpreted by the following BET analysis.

N₂ selectivity and N₂O formation of the catalysts with the reaction temperature are shown in Figure 2. It can be found that high N₂ selectivity (>97%) and a small amount of N₂O formation (less than 10 ppm) exists over all of the catalysts. However, the addition of Ce and the increase of temperature had negative effect on the N₂ selectivity and N₂O formation of the catalysts. With the increase of the reaction temperature, much more NH₃, which acted as a reducing agent in the SCR reaction, will be oxidized to generate N₂O. As a result, the N₂ selectivity of the catalyst decreased with temperature. It has also been proven by Liu et al. that with the increase the ratio of Ce/Mn, much more N₂O will be generated. According to the definition of N₂ selectivity in Equation (2) mentioned below, the N₂ selectivity was decreased [16]. For practical application, the catalyst with reasonable molar ratios of Ce/Mn (0.3) can be chosen as a candidate for its high SCR activity, N₂ selectivity, and trace amount of N₂O formation at low temperature.

2.2. Characterization of the Catalysts

2.2.1. FESEM of the Supports

FESEM images of GO and TiO₂–0.8%GO supports are shown in Figure 3a,b, respectively. From Figure 3a, the multilayer morphology of GO sheets can be observed clearly, which was considered as an ideal structure to provide a high specific surface area and prevent the aggregation of metal oxides on its surface. In Figure 3b, some white anatase TiO₂ particles could be observed distributed on the GO surface, and there was an accumulation of them in some areas of the GO, which will be proven by the following HRTEM analysis. It has been identified that the TiO₂–0.8%GO is an ideal support for catalysts [42].

2.2.2. HRTEM of the Catalyst

The microstructure of 9%Ce(0.3)–Mn/TiO₂–0.8%GO was characterized by TEM and HRTEM, as shown in Figure 4b, respectively. In Figure 4a, the sheet-like morphology of GO can be observed, and the TiO₂, MnO_x, and CeO_x nanoparticles were dispersed randomly on the surface of the GO sheets. Though there was a little aggregation in some area of GO, it was not serious, which may be attributed to the introduction of GO. The corresponding HRTEM image of the catalyst was shown in Figure 4b. The observed spacing between the lattice planes of the catalyst was around 0.218, 0.352, 0.384, 0.420, 0.436, and 0.610 nm, corresponding to the (510) crystallographic planes of Ce₂O₃, the (101) crystallographic planes of anatase TiO₂, the (211) crystallographic planes of Mn₂O₃, the (110) crystallographic planes of CeO₂, the (021) crystallographic planes of Mn₃O₄, and the (001) crystallographic planes of GO, respectively. The well-distributed status of active components on the surface of the support [44] and the coexistence of multivalent metal oxides [43] can improve the selective catalytic reduction abilities of the composite catalysts at low temperature.

2.2.3. BET Surface Areas and Pore Size Distributions

The specific surface area, pore volume, and pore size of the CeO_x−MnO_x/TiO₂–GO and MnO_x/TiO₂–GO are summarized in

Table 1. Structural parameters of CeO_x−MnO_x/TiO₂–0.8%GO catalysts.

Catalyst	SBET	Pore Volume (cm3/g)	Average Pore Size (nm)
9%Mn/TiO2–0.8%GO	149	30.48	9.0
9%Ce(0.1)–Mn/TiO2–0.8%GO	144	29.98	9.1
9%Ce(0.2)–Mn/TiO2–0.8%GO	138	29.12	9.2
9%Ce(0.3)–Mn/TiO2–0.8%GO	136	29.13	9.1
9%Ce(0.4)–Mn/TiO2–0.8%GO	127	25.56	11.5

. It can be found that the addition of CeO_x to the 9%Mn/TiO₂–0.8%GO had a negative effect on the specific surface area and the porous structure parameters, which has also been proven by some other analogous research [19,45,46]. When the ratio was no more than 0.3, this tendency was not very obvious, but when the Ce/Mn molar ratio reached 0.4, the specific surface area and total pore volume of the catalysts decreased significantly. For the SCR activity of 9%Ce(0.4)–Mn/TiO₂–0.8%, GO was lower than that of 9%Ce(0.3)–Mn/TiO₂–0.8%GO, and it can be concluded that the specific surface area and total pore volume of the catalyst had an effect on the reaction activity of the catalyst. On the other hand, though the 9%Ce(0.3)–Mn/TiO₂–0.8%GO had the highest activity among the catalysts, its specific area and pore volume did not have the highest value. This meant that the activity of the catalyst was affected by not only the specific surface area and total pore volume of the catalyst, but also some other important factors, which will be revealed by the following character results.

2.2.4. XPS Characterization

The atomic concentration and element chemical state on the surface of the 9%Mn/TiO₂0.8%GO and 9%Ce(0.3)Mn/TiO₂–0.8%GO catalysts were further investigated by XPS. Figure 5a–c illustrate the obtained XPS spectra for Mn2p, Ce3d, and O1s, respectively, and the corresponding surface atomic concentrations and relative percentages of various oxidation states are summarized in

Table 2. Structural parameters of CeO_x−MnO_x/TiO₂–0.8%GO catalysts.

Catalyst	Atomic Composition (%)
	C	Ce	Mn	Ti	O	O		Mn
						Oα	Oβ	MnO	MnO2	Mn2O3	MnOx/Mn
9%Mn/TiO2–0.8%GO	19.45	-	5.21	21.36	53.98	17.93	36.05	1.58	1.29	1.24	1.10
9%Ce(0.3)–Mn/TiO2–0.8%GO	16.91	3.85	4.61	17.59	57.04	15.99	41.05	1.03	1.62	0.65	1.31

.

From Figure 5a, it can be found that the Mn 2p3/2 spectra for the 9%Mn/TiO₂–0.8%GO and 9%Ce(0.3)–Mn/TiO₂–0.8%GO catalysts can both be divided into three characteristic peaks attributed to MnO (640.9 eV), Mn₂O₃ (642.2 eV), and MnO₂ (644.6 eV). However, the bonding energy of the Mn 2p3/2 spectra in 9%Ce(0.3)–Mn/TiO₂–0.8%GO showed a slight offset toward the high value, which indicated that the electron transferability could be improved by addition of Ce [43]. The Mn 2p1/2 spectra of 9%Ce(0.3)–Mn/TiO₂–0.8%GO catalyst exhibited two characteristic peaks at 653.0 eV (MnO_x/Mn) and 654.1 eV (MnO₂), but for the Mn 2p1/2 spectra of the 9%Mn/TiO₂–0.8%GO catalyst, there was only one peak corresponding to the existence of non-stoichiometric MnO_x/Mn. The contents of Mn with different vacancies in Table 2 were achieved by calculating the percentage of the corresponding peak area. According to Table 2, the content of MnO₂ for 9%Ce(0.3)–Mn/TiO₂–0.8%GO was higher than that of 9%Mn/TiO₂–0.8%GO. This meant the introduction of Ce can increase the content of MnO₂ in the catalyst. It has been reported that MnO₂ had the highest low-temperature NH₃–SCR activity among various manganese oxides [47]. In addition, from Table 2, it is also shown that, compared with 9%Mn/TiO₂–0.8%GO catalyst, 9%Ce(0.3)–Mn/TiO₂–0.8%GO nanocomposite had a higher content of the non-stoichiometric MnO_x/Mn, which has been demonstrated to be beneficial to the redox reaction of the catalyst and further increase the removal rate of NO_x in the SCR reaction [48]. Thus, with the addition of Ce, the content of MnO₂ and non-stoichiometric MnO_x/Mn will be increased, which results in the improvement of the catalytic activity of the nanocomposite catalyst [49,50,51].

The Ce3d spectrum of the 9%Ce(0.3)–Mn/TiO₂–0.8%GO catalyst, which is composed of the Ce 3d3/2 and Ce 3d5/2 spectra, are presented in Figure 5b. The Ce 3dp5/2 spectra can be divided into four characteristic peaks at 882.5 eV (Ce⁴⁺), 885.8 eV (Ce³⁺), 888.8 eV (Ce³⁺), and 898.3 eV (Ce²⁺), respectively, while the Ce3d3/2 spectra can also be divided into four characteristic peaks at 901.4 eV (Ce⁴⁺), 903.8 eV (Ce³⁺), 907.3 eV (Ce⁴⁺), and 916.8 eV (Ce⁴⁺), respectively. The coexistence of Ce²⁺, Ce³⁺, and Ce⁴⁺ can create a charge imbalance, vacancies, and unsaturated chemical bonds on the catalyst’s surface [52] and, as a result, can increase the SCR activity of the catalyst [53]. Among these cerium oxides, CeO₂ has been proven as the most effective oxide type of cerium for SCR activity, which was the main valence state in the 9%Ce(0.3)–Mn/TiO₂–0.8%GO catalyst.

From Figure 5c, it can be observed that there are two types of surface oxygen in the XPS patterns of O1s for 9%Mn/TiO₂–0.8%GO and 9%Ce(0.3)–Mn/TiO₂–0.8%GO catalysts, lattice oxygen (O_β) at 529.4–530.0 eV and chemisorbed oxygen (O_α) at 531.3–531.7 eV, respectively. The chemisorbed oxygen, with higher mobility than lattice oxygen (O_β), was mainly shown as O₂²⁻ or O⁻, in the form of hydroxyl, OH⁻, carbonate, and CO₃²⁻ [54]. In Table 2, from the comparison between 9%Mn/TiO₂–0.8%GO and 9% Ce(0.3)–Mn/TiO₂–0.8%GO catalysts, it was very clear that, with the loading of Ce, the total oxygen and chemisorbed oxygen content of the catalyst increased, which implied that the catalyst would have high SCR activity at low temperature.

2.3. Resistance to H₂O and SO₂

Figure 6a–c show the resistance property of the nanocomposite catalyst to H₂O, SO₂, and H₂O+SO₂ at 180 °C. When only 10 vol % H₂O was added to the reaction system, the denitrification efficiency decreased from about 99% to 90% with the reaction time. The water in the flue gas may cause serious deactivation of the catalyst because of the competitive adsorption with NH₃ on the active sites over the catalyst surface [55]. However, when the H₂O feed was stopped, the NO_x conversion of the catalyst nearly completely recovered to the original levels. Due to the presence of hydrophilic groups on the surface of the GO sheets, the water in the flue gas can be absorbed easily, which resulted in the competitive adsorption with NH₃. Consequently, the SCR activity of the catalyst decreased obviously with the feeding of H₂O.

When only 200 ppm SO₂ was fed into the reaction system, there was also a decline in the catalytic activity from 99% to a relatively high value of about 82% with time, as shown in Figure 6b. The deactivation of the catalyst caused by SO₂ is mainly attributed to the reaction between SO₂ and NH₃, active components of the catalyst. When the SO₂ feeding ceased, the conversion was restored to around 87%. As the addition of Ce can prevent the formation of manganese sulfate on the catalyst surface [22], the 9% Ce(0.3)–Mn/TiO₂–0.8%GO catalyst exhibited high resistance to the SO₂ pollutants.

When 10 vol % H₂O and 200 ppm SO₂ were added together into the SCR system, a much more serious decline in the catalytic activity from 99% to about 70% would occur. This meant there was a synergistic effect of the H₂O and SO₂ on the activation of the catalyst. When H₂O and SO₂ were removed from the system, the efficiency recovered to around 78%. In order to identify the mechanism of the deactivation of the catalyst caused by H₂O and SO₂, the FT–IR spectra of GO and TiO₂–0.8%GO support, and the fresh and poisoned 9%Ce(0.3)–Mn/TiO₂–0.8%GO catalyst were studied, as shown in Figure 7a,b, respectively.

In Figure 7a, for GO and TiO₂–0.8%GO support, the peak at 3425 cm⁻¹ corresponded to the −OH vibrations, and the vibration at 2925 cm⁻¹ was ascribed to the aliphatic stretching vibration of molecule residues of the synthesis. The peaks at 1726 cm⁻¹ and 1644 cm⁻¹ were attributed to the stretching vibration of C=O bond and the C=C stretching of the Csp² network of GO, while the signals at 1226 cm⁻¹ could be also ascribed to C–N groups originating from the catalyst synthesis [56,57,58]. For the TiO₂–0.8%GO support, there was a new peak at 470 cm⁻¹, which was attributed to the stretching vibration of Ti–O.

Compared the FT-IR spectra of the poisoned 9% Ce(0.3)–Mn/TiO₂−0.8%GO catalyst with that of fresh catalyst and TiO₂–0.8%GO support, it can be found that there was no obvious difference of the appearance of the character peaks. However, it can be found that the band intensity at 3425 cm⁻¹ of the poisoned catalyst was larger than that of fresh catalyst, which indicated the H₂O in the flue gas poisoned the catalyst during the SCR reaction process because of the hydrophilic character of GO. It can also be observed that, for the poisoned catalyst, the band intensity of the peak at 1407 cm⁻¹ was higher than that of fresh catalyst and the support. This difference implied the generation of (NH₄)₂SO₄ on the Lewis acid sites. This can also be demonstrated by the analysis of the peak at 1102 cm⁻¹. The peak at 1102 cm⁻¹ corresponded to the existence of SO₄²⁻, which can be observed only in the poisoned catalyst. For a large amount of concentrated sulfuric acid used in the synthesis process of GO, some SO₄²⁻ would be loaded on the surface of GO at the pre-sulfuring step. However, the band intensity at 1102 cm⁻¹ for the poisoned catalyst was only slightly larger than that of fresh catalyst, which indicated the slight poisoning of SO₂ to the catalyst. Thus, it can be concluded that for the pre-sulfuring process of GO, serious sulfation of the active component can be prevented and the catalyst exhibited excellent SO₂ resistance.

2.4. NH₃-TPD Analysis

The surface acidity is an important aspect of the NH₃–SCR reaction. By using NH₃–TPD analysis, the surface acidity of 9% Mn/TiO₂–0.8%GO and 9% Ce(0.3)–Mn/TiO₂–0.8%GO catalysts were determined, and the results are presented in Figure 8.

For the NH₃–TPD pattern of 9% Mn/TiO₂–0.8%GO and 9% Ce(0.3)–Mn/TiO₂–0.8%GO, there was a strong peak at about 144 °C. For the thermal stability, the NH₃ molecules that coordinated to the Lewis acid sites was higher than that of the NH₄⁺ ions bound to the Brønsted acid sites, and the desorption peak at low temperature (below 200 °C) was assigned to NH₄⁺ ions bound to Brønsted acid sites [59]. It was clear that the peak area of 9% Ce(0.3)–Mn/TiO₂–0.8%GO was much larger than that of 9% Mn/TiO₂–0.8%GO, which indicated that the 9% Ce(0.3)–Mn/TiO₂–0.8%GO catalyst provided more Brønsted acidic sites than that of the 9% Mn/TiO₂–0.8%GO. Thus, the introduction of CeO_x had a significant influence on promoting NH₃ adsorption at Brønsted acidic sites. For the 9% Mn/TiO₂–0.8%GO catalyst, there was a broad peak in the temperature range of about 450–700 °C, and for 9% Ce(0.3)–Mn/TiO₂–0.8%GO there was a peak at about 693 °C. The peaks at high temperature ranging from 400–550 °C were associated with coordinated NH₃ molecules originating from the Lewis acid sites, which may originate from the decomposition of nitrite-nitrate species which are formed from the oxidation of ammonia by MnO₂ [60]. The desorption peak above 600 °C may be related to the hydroxyl groups on the surface [61]. It has been proven that the Brønsted acid sites also have an important role in the SCR reaction [62], which may be one of the reasons that the SCR reaction activity of 9% Ce(0.3)–Mn/TiO₂–0.8%GO was higher than that of 9% Mn/TiO₂–0.8%GO.

2.5. H₂-TPR Analysis

H₂-TPR is a valid method to determine the redox performance of the catalyst. Figure 9 shows the TPR curves of the catalysts. It can be found that the H₂-TPR pattern of 9% Mn/TiO₂–0.8%GO and 9% Ce(0.3)–Mn/TiO₂–0.8%GO was similar, where there was a strong peak at about 446 °C. This peak was attributed to reduction from MnO₂ to Mn₂O₃ [63]. However, it was obvious that the peak area of 9% Ce(0.3)–Mn/TiO₂–0.8%GO was much larger than that of 9% Mn/TiO₂–0.8%GO, which meant that 9% Ce(0.3)–Mn/TiO₂–0.8%GO had larger H₂ consumption than that of 9% Mn/TiO₂–0.8%GO. Thus, it was demonstrated that 9% Ce(0.3)–Mn/TiO₂–0.8%GO had better redox performance than 9% Mn/TiO₂–0.8%GO, which was very important for the SCR reaction.

2.6. Stability Test of the Catalyst

Stability of the 9% Ce(0.3)–Mn/TiO₂–0.8%GO catalyst at 180 °C is shown in Figure 10. The NO_x conversion slightly decreased during the first 30 h and then reached a stable level of about 94%. The NO_x conversion of 94% could last for more than 40 h. Therefore, the 9% Ce(0.3)–Mn/TiO₂–0.8%GO catalyst presented good stability and high SCR activity.

3. Materials and Methods

3.1. Synthesis of the Catalyst

Graphene oxide (GO) was synthesized by the modified Hummers method [64]. Firstly, 1 g graphite powder (3000 mesh) and 0.5 g NaNO₃ were added into 23 mL of concentrated H₂SO₄ slowly in a 1000 mL beaker in an ice bath with stirring. After being stirred for 10 min, 3 g KMnO₄ was added to the mixture, slowly, and the temperature was kept at 35 ± 3 °C for 30 min. Then a certain amount of H₂O₂ (30 wt %) and 46 mL of deionized water were added, and the temperature was maintained at 90 °C for 15 min, and warm deionized water was added to a volume of 140 mL. Then the mixture was filtered and washed with 150 mL diluted HCl (10 wt %) and 500 mL of deionized water three times. Then the obtained graphite oxide was dispersed into 500 mL 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 finally dried at 80 °C for 24 h in a vacuum oven.

TiO₂–0.8 wt %GO nanocomposites were prepared by the sol-gel method. First, GO was dissolved in a certain amount of deionized water by ultrasonic treatment for 1 h. A certain volume of tetrabutyl titanate and ethanol were stirred to obtain solution A. Glacial acetic acid, GO solution, and anhydrous ethanol were dissolved to obtain solution B. Solution B was slowly dropped into solution A, under vigorous stirring, for 90 min to achieve a uniform brown transparent titanium–GO sol. Brown crystals were obtained after aging of this sol at room temperature for 24 h and then drying at a constant temperature of 80 °C for 12 h to remove the organic solution. Finally, the support was ground, calcined in a tubular furnace in a nitrogen atmosphere, and kept warm at 450 °C for 6 h to obtain the TiO₂–0.8%GO powder.

CeO_x–MnO_x/TiO₂–GO catalysts were synthesized with different Mn and Ce loadings totaling 9 wt % by the ultrasonic impregnation method. Manganese acetate and cerium nitrate were chosen as the precursors of the active components. According to the molar ratio of Ce/Mn, certain amount of manganese acetate and cerium nitrate were dissolved in deionized water and added into a breaker containing TiO₂–GO. The mixture was sonicated at 60 °C for 90 min to ensure the dispersion of TiO₂–GO and full contact of the active components with the support. The samples were air-dried at 80 °C for 12 h and then calcined in a tubular furnace in an atmosphere of nitrogen at 450 °C for 6 h. The fin catalysts were denoted as 9 wt % CeO_x(Y)–MnO_x/TiO₂–GO, where Y represents the molar ratio of Ce/Mn.

3.2. Catalyst Characterization

The morphology of the support of the catalysts was observed by FESEM (Hitachi S–4800, Tokyo, Japan). The HRTEM (FEI Tecnai G2 F20, Hillsboro, OR, USA) was used to observe the morphologies and interplanar crystal spacing of the CeO_x–MnO_x/TiO₂–GO catalysts. The specific surface area, pore volume, and pore size of the samples were determined on a Quadrasorb SI-MP surface area analyzer (Quantachrome Instrument, Boynton Beach, FL, USA) with a nitrogen adsorption-desorption method. XPS (Axis Ultra DLD, Kratos Analytical Ltd., Manchester, UK) was used to analyze the chemical state and surface composition of samples. H₂ temperature-programmed reduction (H₂–TPR) and NH₃ temperature-programmed desorption (NH₃–TPD) were performed on a Builder PCA–1200 auto-adsorption apparatus (Builder, Beijing, China). Prior to the H₂–TPR experiment, 100 mg of catalysts were pretreated with N₂ with a total flow rate of 30 mL·min^-1 at 300 °C for 0.5 h, then cooled to room temperature in an N₂ atmosphere. Finally, the temperature was raised to 600 °C with a constant heating rate of 10 °C·min⁻¹ in a flow of H₂ (5 vol %)/N₂ (30 mL·min⁻¹). Prior to the NH₃–TPD experiment, the catalysts (50 mg) were pretreated at 300 °C in a flow of N₂ (30 mL·min⁻¹) for 0.5 h and cooled to 100 °C under N₂ flow. Then the samples were exposed to a flow of NH₃ at 100 °C for 1 h, followed by N₂ purging for 0.5 h. Finally, the reactor temperature was raised to 800 °C under N₂ flow at a constant rate of 10 °C·min⁻¹. The effects of H₂O and SO₂ were analyzed by a Fourier transform infrared spectrometer (FT–IR PROTÉGÉ 460, Thermo Nicolet, Madison, WI, USA).

3.3. Catalyst Activity Test

Steady-state SCR reaction experiments were performed in a quartz tube fixed-bed continuous flow reactor using 500 mg catalyst of 60–100 mesh. The reactor was placed in an electrically-heated furnace with a programmable controller. The typical reactant gas composition included 500 ppm NO, 500 ppm NH₃, 7 vol % O₂, 10 vol % H₂O (when used), 200 ppm SO₂ (when used), with the balance being 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 feed flow rate was fixed at 600 mL/min, which was controlled by a mass flow controller and corresponded to a gas hourly space velocity (GHSV) of 67,000 h⁻¹, and the reaction temperature ranged from 80 to 180 °C. At each temperature point, the reaction came to a steady state around 30 min, at which time the experimental data were collected.

NO and NO₂ concentrations at the inlet and outlet were monitored by an NO_x analyzer (42i–HL, Thermo Scientific Ins., Waltham, MA, USA), while the N₂ product was monitored by a gas chromatograph (GC-7890A, Agilent Technologies, Santa Clara, CA, USA). The effluent gas concentrations of N₂O was monitored by a Fourier transform infrared spectrometer (FT–IR PROTÉGÉ 460, Thermo Nicolet, Madison, WI, USA).

The NO_x removal efficiency and the N₂ selectivity were obtained by the following equations:

$${\mathrm{NO}}_{\mathrm{x}} \mathrm{conversion}(\%)=\frac{{\mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}}^{\mathrm{in}}-{\mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}}^{\mathrm{out}}}{{\mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}}^{\mathrm{in}}}\times 100$$

(1)

$${\mathrm{N}}_2 \mathrm{selectivity}(\%)=\frac{{\mathrm{C}}_{\mathrm{NO}}^{\mathrm{in}}+{\mathrm{C}}_{{\mathrm{NH}}_3}^{\mathrm{in}}-{\mathrm{C}}_{{\mathrm{NO}}_2}^{\mathrm{out}}-2{\mathrm{C}}_{{\mathrm{N}}_2\mathrm{O}}}{{\mathrm{C}}_{\mathrm{NO}}^{\mathrm{in}}-{\mathrm{C}}_{{\mathrm{NH}}_3}^{\mathrm{in}}}\times 100$$

(2)

where ${\mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}}^{\mathrm{in}}$, ${\mathrm{C}}_{\mathrm{NO}}^{\mathrm{in}}$, and ${\mathrm{C}}_{{\mathrm{NH}}_3}^{\mathrm{in}}$ correspond to the inlet concentration of NO_x, NO, and NH₃, respectively. ${\mathrm{C}}_{{\mathrm{NO}}_{\mathrm{x}}}^{\mathrm{out}}$ and ${\mathrm{C}}_{{\mathrm{NO}}_2}^{\mathrm{out}}$ correspond to the outlet concentration of NO_x and NO₂, respectively. ${\mathrm{C}}_{{\mathrm{N}}_2\mathrm{O}}$ is the outlet concentration of N₂O.

4. Conclusions

In this paper, a series of 9%CeO_x–MnO_x/TiO₂–GO nanocomposites with different molar ratios of Ce/Mn were synthesized by the sol-gel and ultrasonic impregnation methods. The results showed that the TiO₂–0.8%GO was an ideal support for the catalyst and several valences of manganese and cerium oxides were uniformly distributed on its surface. MnO_x was characterized as MnO, MnO₂, Mn₂O₃, and non-stoichiometric MnO_x/Mn, while CeO_x was characterized as Ce⁴⁺, Ce³⁺, and Ce²⁺ in the samples. The coexistence of various valences of cerium and manganese oxide can improve the redox performance of the catalyst. With the addition of Ce, the amount of MnO₂ and non-stoichiometric MnO_x/Mn, the total oxygen and chemisorbed oxygen content, and the electron transfer ability of the catalyst increased significantly. As a result, the catalyst, with an introduction of Ce in certain amounts, exhibited high catalytic activity and N₂ selectivity at low temperature. When the molar of Ce/Mn was kept at 0.3, the 9%Ce(0.3)–Mn/TiO2-0.8%GO catalyst had the highest SCR activity (more than 99% at 180 °C) and reasonable N₂ selectivity. The catalyst exhibited good resistance to H₂O and SO₂. For the presence of hydrophilic groups on the surface of the GO sheets, the competitive adsorption between H₂O and NH₃ occurred easily, which was considered as the critical factor influencing the H₂O resistance of the catalyst. Due to the pre-sulfuring process of GO, serious sulfation of the active component can be prevented and the catalyst exhibited excellent SO₂ resistance.