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CeO2 catalysts by optimal solvent effect
Accepted Manuscript Title: Enhanced low-temperature NH3 -SCR performance of MnOx /CeO2 catalysts by optimal solvent effect Authors: Xiaojiang Yao, Tingting Kong, Li Chen, Shimin Ding, Fumo Yang, Lin Dong PII: DOI: Reference:
S0169-4332(17)31511-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.156 APSUSC 36091
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-3-2017 10-5-2017 18-5-2017
Please cite this article as: Xiaojiang Yao, Tingting Kong, Li Chen, Shimin Ding, Fumo Yang, Lin Dong, Enhanced low-temperature NH3-SCR performance of MnOx/CeO2 catalysts by optimal solvent effect, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.156 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript Submitted to Applied Surface Science
Enhanced low-temperature NH3-SCR performance of MnOx/CeO2 catalysts by optimal solvent effect Xiaojiang Yao a,b,c,*, Tingting Kong a, Li Chen c, Shimin Ding a, Fumo Yang a,b,c,*, Lin Dong d
a
Collaborative Innovation Center for Green Development in Wuling Mountain Areas, Yangtze
Normal University, Chongqing 408100, PR China b
Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment,
Chinese Academy of Sciences, Xiamen 361021, PR China c
Research Center for Atmospheric Environment, Chongqing Institute of Green and Intelligent
Technology, Chinese Academy of Sciences, Chongqing 400714, PR China d
Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing
University, Nanjing 210093, PR China
* Corresponding authors at: Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Fangzheng Avenue 266#, Chongqing 400714, PR China. Tel.: +86 23 65935909; fax: +86 23 65935924. E-mail addresses: [email protected] (X.J. Yao); [email protected] (F.M. Yang).
Graphical abstract
1
Highlights ►NH3-SCR performance of MnOx/CeO2 catalyst was enhanced by optimal solvent effect. ►Oxalic acid solution can promote the dispersion of MnOx on the surface of CeO2. ►Oxalic acid solution can enhance the electron interaction between MnOx and CeO2. ►MnOx/CeO2 catalyst with oxalic acid as a solvent shows the best catalytic performance.
Abstract A series of MnOx/CeO2 catalysts were prepared by modulating the solvents (deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and oxalic acid (OA) solution) with the purpose of improving the low-temperature NH3-SCR performance, broadening the operating temperature window, and enhancing the H2O+SO2 resistance. The synthesized catalysts were characterized by means of N2-physisorption, XRD, EDS mapping, Raman, XPS, H2-TPR, NH3-TPD, and in situ DRIFTS technologies. Furthermore, the catalytic performance and H2O+SO2 resistance were evaluated by NH3-SCR model reaction. The obtained results indicate that MnOx/CeO2 catalyst prepared with oxalic acid solution as a solvent exhibits the best catalytic performance among these catalysts, which shows above 80% NO conversion during a wide operating temperature range of 100-250 °C and good H2O+SO2 resistance for low-temperature NH3-SCR reaction. This is related to that oxalic acid solution can promote the dispersion of MnOx and enhance the electron interaction between MnOx and CeO2, which are beneficial to improving the physicochemical property of MnOx/CeO2 catalyst, and further lead to the enhancement of catalytic performance and good H2O+SO2 resistance.
Keywords: MnOx/CeO2 catalysts; Solvent effect; Low-temperature NH3-SCR; Dispersion; Electron interaction
2
1. Introduction Nitrogen oxides (NOx), which emitted from stationary and mobile sources, caused serious atmospheric pollution and destroyed the ecological environment [1-4]. Therefore, it is an urgent task to control the emission of NOx. In the past decades, many purification techniques have been developed to satisfy this purpose. In which, the selective catalytic reduction of NOx by NH3 (i.e., NH3-SCR) is considered as the most cost-effective method to remove NOx from the flue gas of thermal power plants [5-8]. It is well known that the commercial denitration catalysts are V2O5-WO3/TiO2 or V2O5-MoO3/TiO2, which exhibit excellent catalytic performance between 300 and 400 °C, but not suitable for low-load operating condition and low-temperature denitration (below 300 °C) [9-11]. However, in order to slow down the deactivation of denitration catalysts caused by dusts and SO2, the denitration unit is planned to be installed downstream of the precipitator and desulfurization device, where the concentration of dusts and SO2 has been decreased greatly [10]. Simultaneously, with regard to the transformation of the old thermal power plants, the denitration unit can only be added downstream of the precipitator and desulfurization device, because there is no enough space to install upstream of the precipitator and desulfurization device. However, the temperature of flue gas is remarkably lower than 300 °C after dusts removal and desulfurization. Therefore, the development of low-temperature denitration catalysts to satisfy this operating condition becomes a hot research topic. Recently, MnOx/CeO2 catalysts have been widely investigated in low-temperature NH3-SCR reaction due to their excellent catalytic performance [12-16]. However, the operating temperature window and water resistance need to be further improved to satisfy the practical denitration application. Therefore, many methods have been adopted to broaden the operating temperature window and enhance the water resistance, such as the introduction of modifiers, change of preparation methods, modulation of synthesis parameters, and so on, which can remarkably affect the physicochemical properties of MnOx/CeO2 catalysts [10,17-20]. Liu et al. [21] prepared a series of MnOx-CeO2 catalysts with the acetic-acid-chelated 3
titania support, and used for low-temperature NH3-SCR reaction. They found that their catalytic performance is obviously better than that of commercial TiO2-supported MnOx-CeO2 catalysts due to the high dispersion of active species. Furthermore, Ge et al. [22] reported that the dispersion of CeO2 on the surface of γ-Al2O3 support is highly dependent on the solvents (water and acetic acid), which leads to the different physicochemical properties and catalytic performance of CuO-CeO2/γ-Al2O3 catalysts for NO reduction by CO. However, as far as we know, the solvent effect on the dispersion of active species and the catalytic performance of MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction has not been reported. Therefore, in the present work, we prepared a series of MnOx/CeO2 catalysts by modulating the solvents (deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and oxalic acid (OA) solution) with the purpose of improving the low-temperature NH3-SCR performance, broadening the operating temperature window, and enhancing the H2O+SO2 resistance. The prepared catalysts were characterized by means of N2-physisorption, XRD, EDS mapping, Raman, XPS, H2-TPR, NH3-TPD, in situ DRIFTS, and NH3-SCR model reaction to explore the effect of solvents on the physicochemical properties, catalytic performance, and H2O+SO2 resistance of MnOx/CeO2 catalysts.
2. Experimental section 2.1. Catalysts preparation CeO2 was obtained by thermal decomposition of Ce(NO3)3·6H2O at 500 °C for 5 h in the flowing air, and then, used as a support to prepare supported MnOx/CeO2 catalysts. In detail, CeO2 was wet impregnated with required amount of Mn(NO3)2 solution in different solvents of deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and 0.1 M oxalic acid (OA) solution, respectively. The suspension was kept in magnetic stirring for 1 h and evaporated to remove the solvents at 110 °C during an oil bath. The obtained cakes were oven dried at 110 °C for 12 h, and finally calcined at 450 °C for 5 h in the flowing air. Furthermore, the loading amount of MnOx was fixed at 0.5 mmol MnOx/g-CeO2. The prepared catalysts were denoted as 4
Mn/Ce-DW, Mn/Ce-AE, Mn/Ce-AA, and Mn/Ce-OA, respectively. 2.2. Catalysts characterization Textural characteristics of these catalysts were obtained by N2-physisorption at 77 K on a Belsorp-max analyzer via the Brunauer-Emmet-Teller (BET) method. Before each analysis, the catalyst was degassed under vacuum at 300 °C for 4 h. X-ray diffraction (XRD) patterns of these catalysts were collected on a Philips X’Pert3 Powder diffractometer with Ni-filtered Cu Kα radiation (λ = 0.15418 nm). X-ray tube was operated at 40 kV and 40 mA. Energy dispersive spectrometer (EDS) mapping images of these catalysts were captured on a Philips XL30 scanning electron microscopy (SEM) equipped with an Oxford EDS analysis system operated at beam energy of 10 kV. Raman spectra of these catalysts were recorded on a Renishaw inVia Reflex Laser Raman spectrometer with Ar+ laser beam. The excitation wavelength and laser power are 532 nm and 5 mW, respectively. X-ray photoelectron spectra (XPS) of these catalysts were collected on a PHI 5000 VersaProbe system with monochromatic Al Kα radiation (1486.6 eV), operating at an accelerating power of 15 kW. Prior to the measurement, the catalyst was outgassed in a UHV chamber (< 5×10–7 Pa) at ambient temperature. The charging effect of the sample was compensated by calibrating the binding energy with C 1s peak at 284.6 eV. H2-temperature programmed reduction (H2-TPR) experiments were carried out on a dynamic sorption analyzer (TP-5076) with a thermal conductivity detector (TCD). The reductant is H2-Ar mixture (7% H2 by volume, 30 ml·min–1). Before the reduction, 50 mg catalyst was pretreated in high purified N2 (30 ml·min–1) at 300 °C for 1 h, and then cooled to ambient temperature. After that, the H2-TPR started from room temperature to 600 °C at a rate of 10 °C·min–1. NH3-temperature programmed desorption (NH3-TPD) experiments were also performed on a dynamic sorption analyzer (TP-5076). 200 mg catalyst was pretreated in high purified N2 (30 ml·min–1) at 300 °C for 1 h. And then, the catalyst adsorbed NH3-N2 mixture (1% NH3 by volume, 30 ml·min–1) at 100 °C for 1 h to be saturated, 5
and subsequently flushed with high purified N2 (30 ml·min–1) at the same temperature for 1 h to eliminate the gaseous NH3, and further cooled to ambient temperature. Finally, the desorption process was carried out from room temperature to 700 °C at a rate of 10 °C·min–1. In situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) of these catalysts by NH3 adsorption were collected on a Nicolet 5700 FT-IR spectrometer with a high-sensitive MCT detector. Prior to the test, the catalyst was pretreated in high purified N2 at 300 ºC for 1 h to remove the surface adsorbed water. The sample background of each target temperature was collected during the cooling process. At room temperature, the catalyst was exposed to NH3-N2 mixture (1% NH3 by volume, 50 ml·min–1) for 1 h to be saturated. And then, the gaseous NH3 was purged by high purified N2 (50 ml·min–1) for 1 h at room temperature. In situ DRIFTS were recorded at various target temperatures from room temperature to 300 ºC at a rate of 10 °C·min–1 in high purified N2 by subtraction of the corresponding background reference. 2.3. Catalytic performance measurement The catalytic performance of these catalysts for NH3-SCR model reaction was measured under steady state, involving a feed stream with a fixed composition of 500 ppm NO, 500 ppm NH3, 5% O2, 5% H2O (when used), 100 ppm SO2 (when used), and N2 in balance. 400 mg catalyst was fitted in a quartz tube and pretreated in high purified N2 at 300 °C for 1 h, and then cooled to room temperature. After that, the mixed reaction gases were switched on. The reaction was conducted at different temperatures with a space velocity of 60000 ml·g–1·h–1. NO concentration of inlet ([NO]in) and outlet ([NO]out) was detected by a chemiluminescence NO analyzer. The concentration of by-product N2O was measured by a N2O analyzer. All of the data were collected after waiting for 30 min until the gaseous composition reached steady state at each reaction temperature. The same experiment was repeated three times, and taken the average value to ensure the reproducibility of the data and reduce the corresponding experimental error. Finally, NO conversion was calculated from the following equation: 6
NO conversion (%)
[ NO]in [ NO]out 100% [ NO]in
(1)
In addition, steady state kinetic measurements were carried out on these catalysts. Prior to each test, the catalyst was diluted 5 times by quartz sand to decrease the influence of diffusion on NO conversion. The apparent activation energy (Ea) for NH3-SCR model reaction over these catalysts was obtained from Arrhenius formula:
Lnk LnA
Ea RT
(2)
In which, k is the reaction rate constant, A is the pre-exponential factor, R is the standard gas constant, and T is the reaction temperature.
3. Results and discussion 3.1. Catalytic performance and H2O+SO2 resistance (NH3-SCR model reaction) Catalytic performance of these MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction is exhibited in Fig. 1. It can be seen from Fig. 1(a) that all of these catalysts present the same change trend during the heating process: firstly, NO conversion increases rapidly with the elevation of reaction temperature; secondly, the growth of NO conversion begins to slow with further elevation of reaction temperature; finally, NO conversion declines gradually when the temperature is above 200 °C due to the oxidation of NH3 [9]. In addition, for Mn/Ce-DW catalyst, NO conversion is below 80% during the whole test temperature range of 50-300 °C. However, Mn/Ce-OA catalyst exhibits above 80% NO conversion during a wide operating temperature range of 100-250 °C. NO conversion of Mn/Ce-AE and Mn/Ce-AA catalysts is located between Mn/Ce-DW and Mn/Ce-OA catalysts. In other words, the catalytic activity of these MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction can be ranked by Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA. It is well known that the target products for NH3-SCR of NO are N2 and H2O. However, by-product N2O usually generates during the reaction process due to the non-selective oxidation of NH3 and partial reduction of NO, which is also an important air pollutant. Therefore, the generated N2O concentration was also detected
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in the whole reaction temperature range of 50-300 °C to further evaluate the catalytic performance of these MnOx/CeO2 catalysts, and the results are presented in Fig. 1(b). When the temperature is below 250 °C, N2O concentration of Mn/Ce-DW, Mn/Ce-AE, and Mn/Ce-OA catalysts is very similar, which is obviously lower than that of Mn/Ce-AA catalyst. Moreover, when the temperature is above 250 °C, Mn/Ce-OA catalyst exhibits the lowest N2O concentration. Combining with the results of NO conversion in Fig. 1(a), we can conclude that Mn/Ce-OA catalyst displays the best catalytic performance for NH3-SCR of NO to N2 among these MnOx/CeO2 catalysts. Furthermore, Fig. 1(c) shows that the measured apparent activation energy (Ea) of NO conversion over these MnOx/CeO2 catalysts is 56.24, 32.73, 21.23, and 13.79 kJ mol–1 for Mn/Ce-DW, Mn/Ce-AE, Mn/Ce-AA, and Mn/Ce-OA, respectively. The obtained results suggest that the removal of NO is the easiest on the surface of Mn/Ce-OA catalyst, while the most difficult over Mn/Ce-DW catalyst, which is consistent with the results of catalytic activity. It is well known that H2O+SO2 resistance of low-temperature denitration catalysts is another important indicator in the practical application. Therefore, we evaluated the H2O+SO2 resistance of these MnOx/CeO2 catalysts, and the corresponding results are shown in Fig. 2. It can be seen from this figure that NO conversion of these MnOx/CeO2 catalysts decreases to some extent when H2O and SO2 are introduced; once the H2O and SO2 are cut off, NO conversion recovers gradually, but not recovers completely due to the generation of surface hydroxyl groups and the deposition of sulfates [6, 23]. Interestingly, we can find that Mn/Ce-OA catalyst exhibits the highest NO conversion among these MnOx/CeO2 catalysts during the whole test process, which indicates that Mn/Ce-OA catalyst is the most promising for practical application. The obtained results indicate that the solvents can remarkably influence the catalytic performance of these MnOx/CeO2 catalysts in the absence and presence of H2O and SO2 for NH3-SCR reaction. In order to further clarify the reason, the physicochemical properties of these catalysts are characterized and discussed in the following sections. 3.2. Textural and structural characteristics (N2-physisorption, XRD, EDS mapping, 8
and Raman) The textural properties (such as BET specific surface area and pore structure) of denitration catalysts can remarkably affect their catalytic performance, but we find that BET specific surface area, total pore volume, and mean pore diameter of these MnOx/CeO2 catalysts with different solvents have no obvious difference in Table 1. It suggests that textural property is not the most important influence factor in our present work. Therefore, we also carried out structural characterization over these MnOx/CeO2 catalysts. Fig. 3 shows that only the diffraction peaks of cubic fluorite-type structure CeO2 can be detected in the XRD patterns of these MnOx/CeO2 catalysts, while the signals of MnOx are absent, which indicate that MnOx species are highly dispersed on the surface of CeO2. However, there is no more information about the solvent effect on the dispersion of MnOx species can be given from XRD results. Moreover, in order to figure out the solvent effect on the dispersion of MnOx species, EDS mapping experiment was carried out, and the corresponding results are displayed in Fig. 4. We can find that the dispersion of MnOx species over Mn/Ce-AE, Mn/Ce-AA, and Mn/Ce-OA catalysts is remarkably better than that of Mn/Ce-DW catalyst, which suggests that the solvents of anhydrous ethanol (AE), acetic acid (AA), and oxalic acid (OA) solution can efficiently promote the dispersion of MnOx species on the surface of CeO2 compared with deionized water (DW) as a solvent. Furthermore, Raman spectra of these MnOx/CeO2 catalysts exhibit a main vibration band at 462 cm–1 and a wide shoulder around 608 cm–1, which can be attributed to F2g vibration mode of cubic fluorite-type structure CeO2 and defect-induced mode (denoted as D) related to oxygen vacancy, respectively (Fig. 5) [24-26]. It is widely reported that oxygen vacancy can promote the dissociation of NOx species, which is beneficial to the enhancement of catalytic performance for denitration reaction [27]. Quantitative analysis of Raman spectra shows that the oxygen vacancy concentration (i.e., ID/IF2g) of these MnOx/CeO2 catalysts is in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, which is consistent with the results of catalytic performance. These phenomena indicate that the solvents can obviously influence the dispersion of MnOx species, and consequently lead to different electron interaction 9
between MnOx and CeO2, and finally affect the generation of oxygen vacancy and the catalytic performance for NH3-SCR reaction. 3.3. Surface analysis (XPS) In order to further investigate the solvent effect on the surface properties of these MnOx/CeO2 catalysts, XPS measurements were carried out, and the corresponding results are presented in Fig. 6. Ce 3d spectra (Fig. 6(a)) of these MnOx/CeO2 catalysts can be fitted with eight binding energy peaks, labeled as u‴, u″, u′, u0 (u), and v‴, v″, v′, v0 (v), respectively. In which, u′ and v′ are attributed to Ce3+, while the other six peaks are ascribed to Ce4+ [28-30]. The results of Mn 2p spectra (Fig. 6(b)) exhibit that Mn4+, Mn3+, and Mn2+ co-exist on the surface of these MnOx/CeO2 catalysts due to the interaction between MnOx and CeO2 through electron transfer [31]. It is widely reported that the appearance of Ce3+ is accompanied with the generation of oxygen vacancy, and further promotes the dissociation of NOx species; Mn4+ is beneficial to the oxidation of NO to NO2, and enhances the catalytic performance for NH3-SCR reaction through a “fast SCR” route [20,31]. Therefore, the contents of Ce3+ and Mn4+ on the surface of these MnOx/CeO2 catalysts are calculated from XPS, and listed in Table 2. We can find that both of Ce3+ and Mn4+ contents are in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, which is in agreement with the results of catalytic performance. In addition, it is well known that surface oxygen species of the denitration catalysts also benefits for the oxidation of NO to NO2, and enhances the catalytic performance for NH3-SCR reaction through a “fast SCR” route. So, O 1s spectra of these MnOx/CeO2 catalysts are analyzed carefully. Fig. 6(c) shows that O 1s spectra of these catalysts can be fitted with two components, the strong binding energy peak at 529.0 eV (O′) is assigned to the lattice oxygen, and the shoulder around 531.2 eV (O″) is attributed to the surface adsorbed oxygen species [32]. Interestingly, the content of O″ (i.e., Oʺ/(Oʹ+Oʺ)) is also in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA (Table 2), which is consistent with the results of catalytic performance. Moreover, the atomic ratio of Mn/Ce over these MnOx/CeO2 catalysts is also ranked by Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, which further indicates that the solvents can remarkably influence the 10
dispersion of MnOx species. Especially, XPS analysis suggests that the solvent of oxalic acid (OA) is more beneficial to the dispersion of MnOx species on the surface of CeO2 (i.e., Mn/Ce-OA catalyst), and consequently leads to largest amounts of Ce3+, Mn4+, and surface adsorbed oxygen species, and finally exhibits the most excellent catalytic performance for NH3-SCR reaction. 3.4. Reduction behavior and surface acidity (H2-TPR, NH3-TPD, and NH3 adsorption in situ DRIFTS) It is well known that the redox properties and surface acidities of denitration catalysts are the most important influence factors for NH3-SCR reaction. Therefore, we adopted H2-TPR and NH3-TPD techniques to investigate the redox properties and surface acidities of these MnOx/CeO2 catalysts, respectively, as shown in Fig. 7. H2-TPR profiles of these MnOx/CeO2 catalysts (Fig. 7(a)) exhibit three reduction peaks α, β, and γ, which can be ascribed to the stepwise reduction of MnOx, i.e., MnO2/Mn2O3 → Mn3O4 → MnO, and the reduction of surface CeO2, respectively [32,33]. Interestingly, we can find that H2 consumption of these MnOx/CeO2 catalysts is in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, while the reduction peak temperature is in the sequence of Mn/Ce-DW > Mn/Ce-AE > Mn/Ce-AA > Mn/Ce-OA (Table 3). It suggests that the redox properties of these MnOx/CeO2 catalysts are highly dependent on the solvents, which may be related to the dispersion of MnOx species on the surface of CeO2, and the electron interaction between MnOx and CeO2 (according to XPS results). Especially, Mn/Ce-OA catalyst exhibits the optimal redox property due to the high dispersion of MnOx species and the strongest electron interaction between MnOx and CeO2 resulted from the solvent effect of oxalic acid (OA). It can be seen from NH3-TPD results (Fig. 7(b)) that all of these MnOx/CeO2 catalysts present four desorption peaks labeled as I, II, III, and IV, which are attributed to the desorption of physisorbed NH3, and the desorption of NH3 from weak, medium-strong, and strong acid sites, respectively [34]. Some interesting results are obtained from the quantitative analysis of NH3-TPD (Table 4). Physisorption (peak I) is too weak to activate NH3 molecules, while the adsorbed NH3 species on strong acid 11
sites (peak IV) are hardly to desorb, which are not much contribution to low-temperature NH3-SCR reaction. Therefore, we focus on the adsorption of NH3 molecules on weak and medium-strong acid sites (peaks II and III). Table 4 shows that the acid amounts of peak II and peak III (i.e., SII+SIII) over these MnOx/CeO2 catalysts are ranked by Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA. In other word, Mn/Ce-OA catalyst possesses the largest amount of acid sites, which is beneficial to the adsorption and activation of NH3 molecules, and further promotes the enhancement of catalytic performance for NH3-SCR reaction. In situ DRIFTS of NH3 adsorption was carried out over the representative samples (Mn/Ce-DW and Mn/Ce-OA catalysts) to clarify the type of surface acid sites and the evolution of adsorbed NH3 species during the heating process, as exhibited in Fig. 8. For Mn/Ce-DW catalyst (Fig. 8(a)), when it is exposed to NH3-N2 mixed gas at room temperature, NH3 molecules are adsorbed on the surface of Mn/Ce-DW catalyst to generate several IR vibration bands between 1000 and 1700 cm–1. According to the literatures, the bands at 1070, 1114, 1168, 1305, and 1580 cm–1 are attributed to NH3 molecules coordinated on Lewis (L) acid sites; while the bands at 1391 and 1440 cm–1 are ascribed to NH4+ ions bonded on Brønsted (B) acid sites [10,31,35]. Some interesting results can be obtained during the heating process: firstly, the bands of L acid at 1114 and 1580 cm–1 disappear when the temperature increases to 150 °C; secondly, two new bands appear at 1263 and 1594 cm–1 at the same temperature of 150 °C, which are assigned to the deformation species of coordinated NH3 [10]; finally, all of these IR bands related to NH3 adsorption decrease gradually with the elevation of temperature to 300 °C. In situ DRIFTS of NH3 adsorption over Mn/Ce-OA catalyst shows that IR bands of L acid, deformation species of coordinated NH3, and B acid can be also detected at the corresponding positions (Fig. 8(b)). Interestingly, compared with Mn/Ce-DW catalyst, the bands related to deformation species of coordinated NH3 over Mn/Ce-OA catalyst appear at a lower temperature of 75 °C, which indicates that Mn/Ce-OA catalyst is more efficient to activate the adsorbed NH3 species. Furthermore, it is widely reported that the strength of L acid is obviously stronger than that of B acid [34,36]. Therefore, B acid may be more 12
important than L acid for low-temperature NH3-SCR reaction, while the situation of high-temperature NH3-SCR reaction is just the opposite. Consequently, we further investigate the revolution of integral peak area of B acid with the increase of temperature over Mn/Ce-DW and Mn/Ce-OA catalysts (Fig. 9). We can find that both of them exhibit a similar volcanic curve, while the peak area of B acid over Mn/Ce-OA catalyst is remarkably larger than that on Mn/Ce-DW catalyst during the heating process, which indicates that Mn/Ce-OA catalyst is more beneficial to the adsorption and activation of NH3 molecules. 3.5. Interaction with reactants (NO+NH3+O2 co-adsorption in situ DRIFTS) In order to investigate the interaction between MnOx/CeO2 catalysts and reactants to further understand the reaction mechanism of NH3-SCR, in situ DRIFTS of NO+NH3+O2 co-adsorption were performed. Firstly, the catalyst was placed in an in situ DRIFTS cell and pretreated with high purified N2 at 300 ºC for 1 h to eliminate the physisorbed water. Secondly, the sample background of each target temperature was collected in the cooling process. At room temperature, the catalyst was exposed to a NO-NH3-O2-N2 mixture (3000 ppm NO, 3000 ppm NH3, and 5% O2 by volume) at a rate of 50 ml·min–1 for 1 h to be saturated. Finally, in situ DRIFTS data were recorded at various target temperatures from room temperature to 300 ºC at a rate of 10 °C·min–1 by subtraction of the corresponding sample background. In situ DRIFTS of NO+NH3+O2 co-adsorption for the representative MnOx/CeO2 catalysts are displayed in Fig. 10. For Mn/Ce-DW catalyst (Fig. 10(a)), when it is exposed to the NO-NH3-O2-N2 mixed gases at room temperature, several IR vibration bands related to the adsorbed NOx and NH3 species are observed between 1000 and 1600 cm–1. According to some literatures [10,11,37-39], the bands at 1017 and 1579 cm–1 are attributed to bidentate nitrates; the band at 1239 cm–1 can be ascribed to bridging nitrates; while the bands at 1278 and 1540 cm–1 are assigned to monodentate nitrates. Furthermore, combining with the results of NH3 adsorption in situ DRIFTS, the bands at 1314 and 1432 cm–1 are related to NH3 molecules coordinated on L acid sites and NH4+ ions bonded on B acid sites, respectively. Some interesting phenomena can be observed in the heating process: firstly, the bands of L acid and B acid disappear at 13
175 ºC due to the desorption of NH3 as well as the reaction between the adsorbed NH3 species and NOx species; secondly, the band intensity of bidentate nitrates, bridging nitrates, and monodentate nitrates increases with the elevation of temperature, and tends to steady state from 175 to 225 ºC, while decreases with further increase of temperature to 300 ºC. However, IR signals of the main by-product N2O and reactants (NO and NH3) are not detected during the heating process, which is because that the signals of these gas-phase molecules are obviously weaker than those of the adsorbed species. With regard to Mn/Ce-OA catalyst (Fig. 10(b)), all the bands of bidentate nitrates, monodentate nitrates, L acid, and B acid can be detected at the corresponding positions. Moreover, a new band for L acid appears at 1178 cm–1 [10,31], which may be related to its larger amount of acid sites than Mn/Ce-DW catalyst. Interestingly, we can find that all of the L acid and B acid disappear at a lower temperature of 150 ºC, while two new bands of bridging nitrates (1211 cm–1) and monodentate nitrates (1493 cm–1) appear at 100 ºC [10,38], which suggest that Mn/Ce-OA catalyst is more beneficial to the activation and transformation of the adsorbed NH3 species and NOx species than Mn/Ce-DW catalyst. These phenomena can be explained from the following aspects: firstly, the results of NH3-TPD and NH3 adsorption in situ DRIFTS present that Mn/Ce-OA catalyst displays the largest amount of acid sites, which is beneficial to the adsorption and activation of NH3 molecules; secondly, the results of H2-TPR and XPS show that Mn/Ce-OA catalyst exhibits the most excellent redox property as well as the largest amounts of Mn4+ and surface adsorbed oxygen species, which can promote the oxidation of NO to NO2 and the further adsorption to form nitrates; finally, the results of Raman and XPS exhibit that Mn/Ce-OA catalyst possesses abundant of oxygen vacancy and Ce3+, which benefit for the dissociation and transformation of NOx species. All of these improved physicochemical properties result in the excellent catalytic performance and good H2O+SO2 resistance of Mn/Ce-OA catalyst for low-temperature NH3-SCR reaction.
4. Conclusions 14
In summary, we have carefully investigated the solvent effect on the physicochemical properties and catalytic performance of MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction. The obtained results indicate that MnOx/CeO2 catalyst with oxalic acid as a solvent (i.e., Mn/Ce-OA catalyst) exhibits the best catalytic performance and good H2O+SO2 resistance. The reason may be that oxalic acid is beneficial to the dispersion of MnOx and enhances the electron interaction between MnOx and CeO2, which lead to the most excellent redox property and the largest amounts of oxygen vacancy, Ce3+, Mn4+, surface adsorbed oxygen species, and acid sites. Large amounts of Mn4+ and surface adsorbed oxygen species, as well as excellent redox property can promote the oxidation of NO to NO2; abundant of oxygen vacancy and Ce3+ benefit for the dissociation and transformation of NOx species; large amount of acid sites is conducive to the adsorption and activation of NH3 molecules. All of these result in the best catalytic performance and good H2O+SO2 resistance. In the present work, we have successfully enhanced the low-temperature NH3-SCR performance of MnOx/CeO2 catalysts by the optimal solvent effect, which can provide a scientific reference for the development of practical denitration catalysts.
Acknowledgements The financial supports of the National Natural Science Foundation of China (21507130), the Chongqing Science & Technology Commission (cstc2016jcyjA0070, cstc2014pt-gc20002, cstc2014yykfC20003, cstckjcxljrc13), the Open Project Program of Chongqing Key Laboratory of Catalysis and Functional Organic Molecules from Chongqing Technology and Business University (1456029), and the Open Project Program of Jiangsu Key Laboratory of Vehicle Emissions Control (OVEC001, OVEC007) are gratefully acknowledged. Furthermore, we also thank Dr. Shuohan Yu in Nanjing University for XPS measurement. References [1] S.G. Wu, X.J. Yao, L. Zhang, Y. Cao, W.X. Zou, L.L. Li, K.L. Ma, C.J. Tang, F. 15
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Fig. 1. (a) NO conversion, (b) N2O concentration, and (c) Arrhenius plots of these MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction.
NO conversion (%)
(a) 100 80 60 Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
40 20 0 50
100
150
200
250 o
Temperature ( C)
18
300
250
N2O concentration (ppm)
(b)
Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
200 150 100 50 0 50
100
150
200
250
300
o
Temperature ( C)
1
1
lnk (mmol g s )
(c)
-1
Ea = 13.79 kJ mol 1
-2
Ea = 21.2 3k
-3
E =3 a 2.73 k
-4
Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
-5
J m ol 1
J m ol 1
E = a 56 .2 4 kJ m ol 1
-6 2.3
2.4
2.5
2.6
1
2.7
1000/T (K )
Fig. 2. H2O+SO2 resistance of these MnOx/CeO2 catalysts at 200 °C.
NO conversion (%)
100
5% H2O + 100ppm SO2 in
80 60 5% H2O + 100ppm SO2 off
40
Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
20 0 0
100
200
300
400
500
600
Reaction time (min) 19
700
Fig. 3. XRD patterns of these MnOx/CeO2 catalysts.
(111)
Intensity (a.u.)
(220)
(200)
Mn Mn Mn
OA
/Ce-
AA
(420)
(311)
(222) (400) (331)
60
70
/Ce-
Mn /C
10
/Ce-
CeO2
XRD
AE e -D W 20
30
40
50
80
2Theta (degree)
Fig. 4. EDS mapping images of these MnOx/CeO2 catalysts.
Mn/Ce-DW
Mn/Ce-AE
Mn/Ce-AA
20
Mn/Ce-OA
Fig. 5. Raman spectra of these MnOx/CeO2 catalysts.
Raman
Intensity (a.u.)
F2g
D Mn/Ce-OA
ID/IF2g = 0.1431
Mn/Ce-AA
ID/IF2g = 0.1047
Mn/Ce-AE
ID/IF2g = 0.1008
Mn/Ce-DW
ID/IF2g = 0.0797
200
300
400
500
600
700
1
800
Raman shift (cm )
Fig. 6. XPS spectra of these MnOx/CeO2 catalysts: (a) Ce 3d, (b) Mn 2p, and (c) O 1s.
(a)
(b)
Ce 3d
u'''
u ,u v''' Mn/Ce-OA u'' u'
v''
Mn/Ce-AE
Mn/Ce-DW
920
910
2+
v ,v v'
Mn/Ce-AA
900
890
Mn/Ce-OA
Mn 3+ Mn
4+
Mn
Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW Mn 2p1/2
Ce 3d5/2
Ce 3d3/2
Mn 2p
0
Intensity (a.u.)
Intensity (a.u.)
0
656
880
652
Mn 2p3/2
648
644
640
Binding energy (eV)
Binding energy (eV)
21
636
(c)
O 1s O'
Intensity (a.u.)
O'' Mn/Ce-OA Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW
536
534
532
530
528
526
524
Binding energy (eV)
Fig. 7. (a) H2-TPR profiles, and (b) NH3-TPD profiles of these MnOx/CeO2 catalysts.
10
H2 consumption (a.u.)
(a)
H2-TPR
Mn/Ce-OA Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW
100
200
300
400
500
o
Temperature ( C)
22
600
(b)
NH3-TPD
20 I
III
IV
Intensity (a.u.)
II
Mn/Ce-OA Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW
0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 8. NH3 adsorption in situ DRIFTS of (a) Mn/Ce-DW, and (b) Mn/Ce-OA catalysts.
Kubelka-Munk (a.u.)
0.04
o C o C o C o C o C o C o C o C o
300 275 250 225 200 175 150 125 100o C 75 C o 50 oC 25 C
2000
1800
(b)
Mn/Ce-DW
0.04
Kubelka-Munk (a.u.)
(a)
1263 1070 1594 1440 1391 1168
L
L 1580
1600
L B B 1305
1400
1114 L L
1200 1
o C o C o C o C o C o C o C o C o
300 275 250 225 200 175 150 125 100o C 75 oC 50 C o 25 C
2000
1000
1800
Mn/Ce-OA 1604
1430 1262 1077 1393 1175
L
L L 1578
1600
1125 B L B 1314 L
1400
1200 1
Wavenumber (cm )
Wavenumber (cm )
23
1000
Fig. 9. Quantitative analysis of B acid over Mn/Ce-DW and Mn/Ce-OA catalysts according to NH3 adsorption in situ DRIFTS.
Peak area of B acid (a.u.)
2.5
Mn/Ce-DW Mn/Ce-OA
2.0 1.5 1.0 0.5 0.0 0
50
100
150
200
250
300
o
Temperature ( C)
Fig. 10. NO+NH3+O2 co-adsorption in situ DRIFTS of (a) Mn/Ce-DW, and (b) Mn/Ce-OA catalysts.
(a)
1540
1239
o
300 oC 275 C o 250 oC 225 oC 200 oC 175 oC 150 oC 125 oC 100o C 75 C o 50 oC 25 C
2000
1800
1314 1579 1432 1278
1600
1400
Mn/Ce-OA
0.2
Kubelka-Munk (a.u.)
Kubelka-Munk (a.u.)
(b)
Mn/Ce-DW
0.2
1
1000
2000
Wavenumber (cm )
1493
o
300 oC 275 C o 250 oC 225 oC 200 oC 175 oC 150 oC 125 oC 100o C 75 C o 50 oC 25 C
1017
1200
1211
1540
1800
1178 1598 143213141278 1017
1600
1400
1200 1
Wavenumber (cm )
24
1000
Table 1 BET specific surface area, total pore volume, and mean pore diameter of these MnOx/CeO2 catalysts.
BET specific surface area
Total pore volume
Mean pore diameter
(m2·g–1)
(cm3·g–1)
(nm)
Mn/Ce-DW 61.2
0.2048
13.4
Mn/Ce-AE
57.3
0.2022
14.1
Mn/Ce-AA
60.2
0.2100
14.0
Mn/Ce-OA
60.7
0.2014
13.3
Catalysts
25
Table 2 The surface composition and atomic ratio of these MnOx/CeO2 catalysts.
Atomic concentration (at%)
Atomic ratio (%)
Ce
Mn
O
Mn/Ce
Ce3+/(Ce3++Ce4+)
Mn4+/(Mn2++Mn3++Mn4+)
Oʺ/(Oʹ+Oʺ)
Mn/Ce-DW
17.71
1.95
80.34
11.01
16.18
27.58
25.91
Mn/Ce-AE
17.84
3.83
78.33
21.47
16.78
30.36
27.23
Mn/Ce-AA
15.79
3.91
80.30
24.76
17.09
32.06
28.35
Mn/Ce-OA
16.34
4.96
78.70
30.35
18.13
35.47
33.43
Catalysts
26
Table 3 The information of peak temperature and H2 consumption of these MnOx/CeO2 catalysts obtained from H2-TPR.
Peak temperature (°C)
H2 consumption (μmol·g–1)
Tα
Tβ
Tγ
Sα
Sβ
Sγ
Sα+Sβ+Sγ
Mn/Ce-DW
235
314
382
547
332
275
1154
Mn/Ce-AE
228
309
374
574
384
277
1235
Mn/Ce-AA
217
309
375
469
492
309
1270
Mn/Ce-OA
209
283
352
631
393
372
1396
Catalysts
Table 4 Quantitative analysis of NH3-TPD over these MnOx/CeO2 catalysts.
Acid amount (a.u.)
Total acid amount (a.u.)
Catalysts SI
SII
SIII
SIV
SII+SIII
SI+SII+SIII+SIV
Mn/Ce-DW 1296 1232 1212 219
2444
3959
Mn/Ce-AE
908
1486 1341 269
2827
4004
Mn/Ce-AA
961
1424 1434 220
2858
4039
Mn/Ce-OA
1014 1460 1492 286
2952
4252
27
S0169-4332(17)31511-8 http://dx.doi.org/doi:10.1016/j.apsusc.2017.05.156 APSUSC 36091
To appear in:
APSUSC
Received date: Revised date: Accepted date:
28-3-2017 10-5-2017 18-5-2017
Please cite this article as: Xiaojiang Yao, Tingting Kong, Li Chen, Shimin Ding, Fumo Yang, Lin Dong, Enhanced low-temperature NH3-SCR performance of MnOx/CeO2 catalysts by optimal solvent effect, Applied Surface Sciencehttp://dx.doi.org/10.1016/j.apsusc.2017.05.156 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Revised Manuscript Submitted to Applied Surface Science
Enhanced low-temperature NH3-SCR performance of MnOx/CeO2 catalysts by optimal solvent effect Xiaojiang Yao a,b,c,*, Tingting Kong a, Li Chen c, Shimin Ding a, Fumo Yang a,b,c,*, Lin Dong d
a
Collaborative Innovation Center for Green Development in Wuling Mountain Areas, Yangtze
Normal University, Chongqing 408100, PR China b
Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment,
Chinese Academy of Sciences, Xiamen 361021, PR China c
Research Center for Atmospheric Environment, Chongqing Institute of Green and Intelligent
Technology, Chinese Academy of Sciences, Chongqing 400714, PR China d
Jiangsu Key Laboratory of Vehicle Emissions Control, Center of Modern Analysis, Nanjing
University, Nanjing 210093, PR China
* Corresponding authors at: Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Fangzheng Avenue 266#, Chongqing 400714, PR China. Tel.: +86 23 65935909; fax: +86 23 65935924. E-mail addresses: [email protected] (X.J. Yao); [email protected] (F.M. Yang).
Graphical abstract
1
Highlights ►NH3-SCR performance of MnOx/CeO2 catalyst was enhanced by optimal solvent effect. ►Oxalic acid solution can promote the dispersion of MnOx on the surface of CeO2. ►Oxalic acid solution can enhance the electron interaction between MnOx and CeO2. ►MnOx/CeO2 catalyst with oxalic acid as a solvent shows the best catalytic performance.
Abstract A series of MnOx/CeO2 catalysts were prepared by modulating the solvents (deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and oxalic acid (OA) solution) with the purpose of improving the low-temperature NH3-SCR performance, broadening the operating temperature window, and enhancing the H2O+SO2 resistance. The synthesized catalysts were characterized by means of N2-physisorption, XRD, EDS mapping, Raman, XPS, H2-TPR, NH3-TPD, and in situ DRIFTS technologies. Furthermore, the catalytic performance and H2O+SO2 resistance were evaluated by NH3-SCR model reaction. The obtained results indicate that MnOx/CeO2 catalyst prepared with oxalic acid solution as a solvent exhibits the best catalytic performance among these catalysts, which shows above 80% NO conversion during a wide operating temperature range of 100-250 °C and good H2O+SO2 resistance for low-temperature NH3-SCR reaction. This is related to that oxalic acid solution can promote the dispersion of MnOx and enhance the electron interaction between MnOx and CeO2, which are beneficial to improving the physicochemical property of MnOx/CeO2 catalyst, and further lead to the enhancement of catalytic performance and good H2O+SO2 resistance.
Keywords: MnOx/CeO2 catalysts; Solvent effect; Low-temperature NH3-SCR; Dispersion; Electron interaction
2
1. Introduction Nitrogen oxides (NOx), which emitted from stationary and mobile sources, caused serious atmospheric pollution and destroyed the ecological environment [1-4]. Therefore, it is an urgent task to control the emission of NOx. In the past decades, many purification techniques have been developed to satisfy this purpose. In which, the selective catalytic reduction of NOx by NH3 (i.e., NH3-SCR) is considered as the most cost-effective method to remove NOx from the flue gas of thermal power plants [5-8]. It is well known that the commercial denitration catalysts are V2O5-WO3/TiO2 or V2O5-MoO3/TiO2, which exhibit excellent catalytic performance between 300 and 400 °C, but not suitable for low-load operating condition and low-temperature denitration (below 300 °C) [9-11]. However, in order to slow down the deactivation of denitration catalysts caused by dusts and SO2, the denitration unit is planned to be installed downstream of the precipitator and desulfurization device, where the concentration of dusts and SO2 has been decreased greatly [10]. Simultaneously, with regard to the transformation of the old thermal power plants, the denitration unit can only be added downstream of the precipitator and desulfurization device, because there is no enough space to install upstream of the precipitator and desulfurization device. However, the temperature of flue gas is remarkably lower than 300 °C after dusts removal and desulfurization. Therefore, the development of low-temperature denitration catalysts to satisfy this operating condition becomes a hot research topic. Recently, MnOx/CeO2 catalysts have been widely investigated in low-temperature NH3-SCR reaction due to their excellent catalytic performance [12-16]. However, the operating temperature window and water resistance need to be further improved to satisfy the practical denitration application. Therefore, many methods have been adopted to broaden the operating temperature window and enhance the water resistance, such as the introduction of modifiers, change of preparation methods, modulation of synthesis parameters, and so on, which can remarkably affect the physicochemical properties of MnOx/CeO2 catalysts [10,17-20]. Liu et al. [21] prepared a series of MnOx-CeO2 catalysts with the acetic-acid-chelated 3
titania support, and used for low-temperature NH3-SCR reaction. They found that their catalytic performance is obviously better than that of commercial TiO2-supported MnOx-CeO2 catalysts due to the high dispersion of active species. Furthermore, Ge et al. [22] reported that the dispersion of CeO2 on the surface of γ-Al2O3 support is highly dependent on the solvents (water and acetic acid), which leads to the different physicochemical properties and catalytic performance of CuO-CeO2/γ-Al2O3 catalysts for NO reduction by CO. However, as far as we know, the solvent effect on the dispersion of active species and the catalytic performance of MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction has not been reported. Therefore, in the present work, we prepared a series of MnOx/CeO2 catalysts by modulating the solvents (deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and oxalic acid (OA) solution) with the purpose of improving the low-temperature NH3-SCR performance, broadening the operating temperature window, and enhancing the H2O+SO2 resistance. The prepared catalysts were characterized by means of N2-physisorption, XRD, EDS mapping, Raman, XPS, H2-TPR, NH3-TPD, in situ DRIFTS, and NH3-SCR model reaction to explore the effect of solvents on the physicochemical properties, catalytic performance, and H2O+SO2 resistance of MnOx/CeO2 catalysts.
2. Experimental section 2.1. Catalysts preparation CeO2 was obtained by thermal decomposition of Ce(NO3)3·6H2O at 500 °C for 5 h in the flowing air, and then, used as a support to prepare supported MnOx/CeO2 catalysts. In detail, CeO2 was wet impregnated with required amount of Mn(NO3)2 solution in different solvents of deionized water (DW), anhydrous ethanol (AE), acetic acid (AA), and 0.1 M oxalic acid (OA) solution, respectively. The suspension was kept in magnetic stirring for 1 h and evaporated to remove the solvents at 110 °C during an oil bath. The obtained cakes were oven dried at 110 °C for 12 h, and finally calcined at 450 °C for 5 h in the flowing air. Furthermore, the loading amount of MnOx was fixed at 0.5 mmol MnOx/g-CeO2. The prepared catalysts were denoted as 4
Mn/Ce-DW, Mn/Ce-AE, Mn/Ce-AA, and Mn/Ce-OA, respectively. 2.2. Catalysts characterization Textural characteristics of these catalysts were obtained by N2-physisorption at 77 K on a Belsorp-max analyzer via the Brunauer-Emmet-Teller (BET) method. Before each analysis, the catalyst was degassed under vacuum at 300 °C for 4 h. X-ray diffraction (XRD) patterns of these catalysts were collected on a Philips X’Pert3 Powder diffractometer with Ni-filtered Cu Kα radiation (λ = 0.15418 nm). X-ray tube was operated at 40 kV and 40 mA. Energy dispersive spectrometer (EDS) mapping images of these catalysts were captured on a Philips XL30 scanning electron microscopy (SEM) equipped with an Oxford EDS analysis system operated at beam energy of 10 kV. Raman spectra of these catalysts were recorded on a Renishaw inVia Reflex Laser Raman spectrometer with Ar+ laser beam. The excitation wavelength and laser power are 532 nm and 5 mW, respectively. X-ray photoelectron spectra (XPS) of these catalysts were collected on a PHI 5000 VersaProbe system with monochromatic Al Kα radiation (1486.6 eV), operating at an accelerating power of 15 kW. Prior to the measurement, the catalyst was outgassed in a UHV chamber (< 5×10–7 Pa) at ambient temperature. The charging effect of the sample was compensated by calibrating the binding energy with C 1s peak at 284.6 eV. H2-temperature programmed reduction (H2-TPR) experiments were carried out on a dynamic sorption analyzer (TP-5076) with a thermal conductivity detector (TCD). The reductant is H2-Ar mixture (7% H2 by volume, 30 ml·min–1). Before the reduction, 50 mg catalyst was pretreated in high purified N2 (30 ml·min–1) at 300 °C for 1 h, and then cooled to ambient temperature. After that, the H2-TPR started from room temperature to 600 °C at a rate of 10 °C·min–1. NH3-temperature programmed desorption (NH3-TPD) experiments were also performed on a dynamic sorption analyzer (TP-5076). 200 mg catalyst was pretreated in high purified N2 (30 ml·min–1) at 300 °C for 1 h. And then, the catalyst adsorbed NH3-N2 mixture (1% NH3 by volume, 30 ml·min–1) at 100 °C for 1 h to be saturated, 5
and subsequently flushed with high purified N2 (30 ml·min–1) at the same temperature for 1 h to eliminate the gaseous NH3, and further cooled to ambient temperature. Finally, the desorption process was carried out from room temperature to 700 °C at a rate of 10 °C·min–1. In situ diffuse reflectance infrared Fourier transform spectra (in situ DRIFTS) of these catalysts by NH3 adsorption were collected on a Nicolet 5700 FT-IR spectrometer with a high-sensitive MCT detector. Prior to the test, the catalyst was pretreated in high purified N2 at 300 ºC for 1 h to remove the surface adsorbed water. The sample background of each target temperature was collected during the cooling process. At room temperature, the catalyst was exposed to NH3-N2 mixture (1% NH3 by volume, 50 ml·min–1) for 1 h to be saturated. And then, the gaseous NH3 was purged by high purified N2 (50 ml·min–1) for 1 h at room temperature. In situ DRIFTS were recorded at various target temperatures from room temperature to 300 ºC at a rate of 10 °C·min–1 in high purified N2 by subtraction of the corresponding background reference. 2.3. Catalytic performance measurement The catalytic performance of these catalysts for NH3-SCR model reaction was measured under steady state, involving a feed stream with a fixed composition of 500 ppm NO, 500 ppm NH3, 5% O2, 5% H2O (when used), 100 ppm SO2 (when used), and N2 in balance. 400 mg catalyst was fitted in a quartz tube and pretreated in high purified N2 at 300 °C for 1 h, and then cooled to room temperature. After that, the mixed reaction gases were switched on. The reaction was conducted at different temperatures with a space velocity of 60000 ml·g–1·h–1. NO concentration of inlet ([NO]in) and outlet ([NO]out) was detected by a chemiluminescence NO analyzer. The concentration of by-product N2O was measured by a N2O analyzer. All of the data were collected after waiting for 30 min until the gaseous composition reached steady state at each reaction temperature. The same experiment was repeated three times, and taken the average value to ensure the reproducibility of the data and reduce the corresponding experimental error. Finally, NO conversion was calculated from the following equation: 6
NO conversion (%)
[ NO]in [ NO]out 100% [ NO]in
(1)
In addition, steady state kinetic measurements were carried out on these catalysts. Prior to each test, the catalyst was diluted 5 times by quartz sand to decrease the influence of diffusion on NO conversion. The apparent activation energy (Ea) for NH3-SCR model reaction over these catalysts was obtained from Arrhenius formula:
Lnk LnA
Ea RT
(2)
In which, k is the reaction rate constant, A is the pre-exponential factor, R is the standard gas constant, and T is the reaction temperature.
3. Results and discussion 3.1. Catalytic performance and H2O+SO2 resistance (NH3-SCR model reaction) Catalytic performance of these MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction is exhibited in Fig. 1. It can be seen from Fig. 1(a) that all of these catalysts present the same change trend during the heating process: firstly, NO conversion increases rapidly with the elevation of reaction temperature; secondly, the growth of NO conversion begins to slow with further elevation of reaction temperature; finally, NO conversion declines gradually when the temperature is above 200 °C due to the oxidation of NH3 [9]. In addition, for Mn/Ce-DW catalyst, NO conversion is below 80% during the whole test temperature range of 50-300 °C. However, Mn/Ce-OA catalyst exhibits above 80% NO conversion during a wide operating temperature range of 100-250 °C. NO conversion of Mn/Ce-AE and Mn/Ce-AA catalysts is located between Mn/Ce-DW and Mn/Ce-OA catalysts. In other words, the catalytic activity of these MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction can be ranked by Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA. It is well known that the target products for NH3-SCR of NO are N2 and H2O. However, by-product N2O usually generates during the reaction process due to the non-selective oxidation of NH3 and partial reduction of NO, which is also an important air pollutant. Therefore, the generated N2O concentration was also detected
7
in the whole reaction temperature range of 50-300 °C to further evaluate the catalytic performance of these MnOx/CeO2 catalysts, and the results are presented in Fig. 1(b). When the temperature is below 250 °C, N2O concentration of Mn/Ce-DW, Mn/Ce-AE, and Mn/Ce-OA catalysts is very similar, which is obviously lower than that of Mn/Ce-AA catalyst. Moreover, when the temperature is above 250 °C, Mn/Ce-OA catalyst exhibits the lowest N2O concentration. Combining with the results of NO conversion in Fig. 1(a), we can conclude that Mn/Ce-OA catalyst displays the best catalytic performance for NH3-SCR of NO to N2 among these MnOx/CeO2 catalysts. Furthermore, Fig. 1(c) shows that the measured apparent activation energy (Ea) of NO conversion over these MnOx/CeO2 catalysts is 56.24, 32.73, 21.23, and 13.79 kJ mol–1 for Mn/Ce-DW, Mn/Ce-AE, Mn/Ce-AA, and Mn/Ce-OA, respectively. The obtained results suggest that the removal of NO is the easiest on the surface of Mn/Ce-OA catalyst, while the most difficult over Mn/Ce-DW catalyst, which is consistent with the results of catalytic activity. It is well known that H2O+SO2 resistance of low-temperature denitration catalysts is another important indicator in the practical application. Therefore, we evaluated the H2O+SO2 resistance of these MnOx/CeO2 catalysts, and the corresponding results are shown in Fig. 2. It can be seen from this figure that NO conversion of these MnOx/CeO2 catalysts decreases to some extent when H2O and SO2 are introduced; once the H2O and SO2 are cut off, NO conversion recovers gradually, but not recovers completely due to the generation of surface hydroxyl groups and the deposition of sulfates [6, 23]. Interestingly, we can find that Mn/Ce-OA catalyst exhibits the highest NO conversion among these MnOx/CeO2 catalysts during the whole test process, which indicates that Mn/Ce-OA catalyst is the most promising for practical application. The obtained results indicate that the solvents can remarkably influence the catalytic performance of these MnOx/CeO2 catalysts in the absence and presence of H2O and SO2 for NH3-SCR reaction. In order to further clarify the reason, the physicochemical properties of these catalysts are characterized and discussed in the following sections. 3.2. Textural and structural characteristics (N2-physisorption, XRD, EDS mapping, 8
and Raman) The textural properties (such as BET specific surface area and pore structure) of denitration catalysts can remarkably affect their catalytic performance, but we find that BET specific surface area, total pore volume, and mean pore diameter of these MnOx/CeO2 catalysts with different solvents have no obvious difference in Table 1. It suggests that textural property is not the most important influence factor in our present work. Therefore, we also carried out structural characterization over these MnOx/CeO2 catalysts. Fig. 3 shows that only the diffraction peaks of cubic fluorite-type structure CeO2 can be detected in the XRD patterns of these MnOx/CeO2 catalysts, while the signals of MnOx are absent, which indicate that MnOx species are highly dispersed on the surface of CeO2. However, there is no more information about the solvent effect on the dispersion of MnOx species can be given from XRD results. Moreover, in order to figure out the solvent effect on the dispersion of MnOx species, EDS mapping experiment was carried out, and the corresponding results are displayed in Fig. 4. We can find that the dispersion of MnOx species over Mn/Ce-AE, Mn/Ce-AA, and Mn/Ce-OA catalysts is remarkably better than that of Mn/Ce-DW catalyst, which suggests that the solvents of anhydrous ethanol (AE), acetic acid (AA), and oxalic acid (OA) solution can efficiently promote the dispersion of MnOx species on the surface of CeO2 compared with deionized water (DW) as a solvent. Furthermore, Raman spectra of these MnOx/CeO2 catalysts exhibit a main vibration band at 462 cm–1 and a wide shoulder around 608 cm–1, which can be attributed to F2g vibration mode of cubic fluorite-type structure CeO2 and defect-induced mode (denoted as D) related to oxygen vacancy, respectively (Fig. 5) [24-26]. It is widely reported that oxygen vacancy can promote the dissociation of NOx species, which is beneficial to the enhancement of catalytic performance for denitration reaction [27]. Quantitative analysis of Raman spectra shows that the oxygen vacancy concentration (i.e., ID/IF2g) of these MnOx/CeO2 catalysts is in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, which is consistent with the results of catalytic performance. These phenomena indicate that the solvents can obviously influence the dispersion of MnOx species, and consequently lead to different electron interaction 9
between MnOx and CeO2, and finally affect the generation of oxygen vacancy and the catalytic performance for NH3-SCR reaction. 3.3. Surface analysis (XPS) In order to further investigate the solvent effect on the surface properties of these MnOx/CeO2 catalysts, XPS measurements were carried out, and the corresponding results are presented in Fig. 6. Ce 3d spectra (Fig. 6(a)) of these MnOx/CeO2 catalysts can be fitted with eight binding energy peaks, labeled as u‴, u″, u′, u0 (u), and v‴, v″, v′, v0 (v), respectively. In which, u′ and v′ are attributed to Ce3+, while the other six peaks are ascribed to Ce4+ [28-30]. The results of Mn 2p spectra (Fig. 6(b)) exhibit that Mn4+, Mn3+, and Mn2+ co-exist on the surface of these MnOx/CeO2 catalysts due to the interaction between MnOx and CeO2 through electron transfer [31]. It is widely reported that the appearance of Ce3+ is accompanied with the generation of oxygen vacancy, and further promotes the dissociation of NOx species; Mn4+ is beneficial to the oxidation of NO to NO2, and enhances the catalytic performance for NH3-SCR reaction through a “fast SCR” route [20,31]. Therefore, the contents of Ce3+ and Mn4+ on the surface of these MnOx/CeO2 catalysts are calculated from XPS, and listed in Table 2. We can find that both of Ce3+ and Mn4+ contents are in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, which is in agreement with the results of catalytic performance. In addition, it is well known that surface oxygen species of the denitration catalysts also benefits for the oxidation of NO to NO2, and enhances the catalytic performance for NH3-SCR reaction through a “fast SCR” route. So, O 1s spectra of these MnOx/CeO2 catalysts are analyzed carefully. Fig. 6(c) shows that O 1s spectra of these catalysts can be fitted with two components, the strong binding energy peak at 529.0 eV (O′) is assigned to the lattice oxygen, and the shoulder around 531.2 eV (O″) is attributed to the surface adsorbed oxygen species [32]. Interestingly, the content of O″ (i.e., Oʺ/(Oʹ+Oʺ)) is also in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA (Table 2), which is consistent with the results of catalytic performance. Moreover, the atomic ratio of Mn/Ce over these MnOx/CeO2 catalysts is also ranked by Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, which further indicates that the solvents can remarkably influence the 10
dispersion of MnOx species. Especially, XPS analysis suggests that the solvent of oxalic acid (OA) is more beneficial to the dispersion of MnOx species on the surface of CeO2 (i.e., Mn/Ce-OA catalyst), and consequently leads to largest amounts of Ce3+, Mn4+, and surface adsorbed oxygen species, and finally exhibits the most excellent catalytic performance for NH3-SCR reaction. 3.4. Reduction behavior and surface acidity (H2-TPR, NH3-TPD, and NH3 adsorption in situ DRIFTS) It is well known that the redox properties and surface acidities of denitration catalysts are the most important influence factors for NH3-SCR reaction. Therefore, we adopted H2-TPR and NH3-TPD techniques to investigate the redox properties and surface acidities of these MnOx/CeO2 catalysts, respectively, as shown in Fig. 7. H2-TPR profiles of these MnOx/CeO2 catalysts (Fig. 7(a)) exhibit three reduction peaks α, β, and γ, which can be ascribed to the stepwise reduction of MnOx, i.e., MnO2/Mn2O3 → Mn3O4 → MnO, and the reduction of surface CeO2, respectively [32,33]. Interestingly, we can find that H2 consumption of these MnOx/CeO2 catalysts is in the order of Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA, while the reduction peak temperature is in the sequence of Mn/Ce-DW > Mn/Ce-AE > Mn/Ce-AA > Mn/Ce-OA (Table 3). It suggests that the redox properties of these MnOx/CeO2 catalysts are highly dependent on the solvents, which may be related to the dispersion of MnOx species on the surface of CeO2, and the electron interaction between MnOx and CeO2 (according to XPS results). Especially, Mn/Ce-OA catalyst exhibits the optimal redox property due to the high dispersion of MnOx species and the strongest electron interaction between MnOx and CeO2 resulted from the solvent effect of oxalic acid (OA). It can be seen from NH3-TPD results (Fig. 7(b)) that all of these MnOx/CeO2 catalysts present four desorption peaks labeled as I, II, III, and IV, which are attributed to the desorption of physisorbed NH3, and the desorption of NH3 from weak, medium-strong, and strong acid sites, respectively [34]. Some interesting results are obtained from the quantitative analysis of NH3-TPD (Table 4). Physisorption (peak I) is too weak to activate NH3 molecules, while the adsorbed NH3 species on strong acid 11
sites (peak IV) are hardly to desorb, which are not much contribution to low-temperature NH3-SCR reaction. Therefore, we focus on the adsorption of NH3 molecules on weak and medium-strong acid sites (peaks II and III). Table 4 shows that the acid amounts of peak II and peak III (i.e., SII+SIII) over these MnOx/CeO2 catalysts are ranked by Mn/Ce-DW < Mn/Ce-AE < Mn/Ce-AA < Mn/Ce-OA. In other word, Mn/Ce-OA catalyst possesses the largest amount of acid sites, which is beneficial to the adsorption and activation of NH3 molecules, and further promotes the enhancement of catalytic performance for NH3-SCR reaction. In situ DRIFTS of NH3 adsorption was carried out over the representative samples (Mn/Ce-DW and Mn/Ce-OA catalysts) to clarify the type of surface acid sites and the evolution of adsorbed NH3 species during the heating process, as exhibited in Fig. 8. For Mn/Ce-DW catalyst (Fig. 8(a)), when it is exposed to NH3-N2 mixed gas at room temperature, NH3 molecules are adsorbed on the surface of Mn/Ce-DW catalyst to generate several IR vibration bands between 1000 and 1700 cm–1. According to the literatures, the bands at 1070, 1114, 1168, 1305, and 1580 cm–1 are attributed to NH3 molecules coordinated on Lewis (L) acid sites; while the bands at 1391 and 1440 cm–1 are ascribed to NH4+ ions bonded on Brønsted (B) acid sites [10,31,35]. Some interesting results can be obtained during the heating process: firstly, the bands of L acid at 1114 and 1580 cm–1 disappear when the temperature increases to 150 °C; secondly, two new bands appear at 1263 and 1594 cm–1 at the same temperature of 150 °C, which are assigned to the deformation species of coordinated NH3 [10]; finally, all of these IR bands related to NH3 adsorption decrease gradually with the elevation of temperature to 300 °C. In situ DRIFTS of NH3 adsorption over Mn/Ce-OA catalyst shows that IR bands of L acid, deformation species of coordinated NH3, and B acid can be also detected at the corresponding positions (Fig. 8(b)). Interestingly, compared with Mn/Ce-DW catalyst, the bands related to deformation species of coordinated NH3 over Mn/Ce-OA catalyst appear at a lower temperature of 75 °C, which indicates that Mn/Ce-OA catalyst is more efficient to activate the adsorbed NH3 species. Furthermore, it is widely reported that the strength of L acid is obviously stronger than that of B acid [34,36]. Therefore, B acid may be more 12
important than L acid for low-temperature NH3-SCR reaction, while the situation of high-temperature NH3-SCR reaction is just the opposite. Consequently, we further investigate the revolution of integral peak area of B acid with the increase of temperature over Mn/Ce-DW and Mn/Ce-OA catalysts (Fig. 9). We can find that both of them exhibit a similar volcanic curve, while the peak area of B acid over Mn/Ce-OA catalyst is remarkably larger than that on Mn/Ce-DW catalyst during the heating process, which indicates that Mn/Ce-OA catalyst is more beneficial to the adsorption and activation of NH3 molecules. 3.5. Interaction with reactants (NO+NH3+O2 co-adsorption in situ DRIFTS) In order to investigate the interaction between MnOx/CeO2 catalysts and reactants to further understand the reaction mechanism of NH3-SCR, in situ DRIFTS of NO+NH3+O2 co-adsorption were performed. Firstly, the catalyst was placed in an in situ DRIFTS cell and pretreated with high purified N2 at 300 ºC for 1 h to eliminate the physisorbed water. Secondly, the sample background of each target temperature was collected in the cooling process. At room temperature, the catalyst was exposed to a NO-NH3-O2-N2 mixture (3000 ppm NO, 3000 ppm NH3, and 5% O2 by volume) at a rate of 50 ml·min–1 for 1 h to be saturated. Finally, in situ DRIFTS data were recorded at various target temperatures from room temperature to 300 ºC at a rate of 10 °C·min–1 by subtraction of the corresponding sample background. In situ DRIFTS of NO+NH3+O2 co-adsorption for the representative MnOx/CeO2 catalysts are displayed in Fig. 10. For Mn/Ce-DW catalyst (Fig. 10(a)), when it is exposed to the NO-NH3-O2-N2 mixed gases at room temperature, several IR vibration bands related to the adsorbed NOx and NH3 species are observed between 1000 and 1600 cm–1. According to some literatures [10,11,37-39], the bands at 1017 and 1579 cm–1 are attributed to bidentate nitrates; the band at 1239 cm–1 can be ascribed to bridging nitrates; while the bands at 1278 and 1540 cm–1 are assigned to monodentate nitrates. Furthermore, combining with the results of NH3 adsorption in situ DRIFTS, the bands at 1314 and 1432 cm–1 are related to NH3 molecules coordinated on L acid sites and NH4+ ions bonded on B acid sites, respectively. Some interesting phenomena can be observed in the heating process: firstly, the bands of L acid and B acid disappear at 13
175 ºC due to the desorption of NH3 as well as the reaction between the adsorbed NH3 species and NOx species; secondly, the band intensity of bidentate nitrates, bridging nitrates, and monodentate nitrates increases with the elevation of temperature, and tends to steady state from 175 to 225 ºC, while decreases with further increase of temperature to 300 ºC. However, IR signals of the main by-product N2O and reactants (NO and NH3) are not detected during the heating process, which is because that the signals of these gas-phase molecules are obviously weaker than those of the adsorbed species. With regard to Mn/Ce-OA catalyst (Fig. 10(b)), all the bands of bidentate nitrates, monodentate nitrates, L acid, and B acid can be detected at the corresponding positions. Moreover, a new band for L acid appears at 1178 cm–1 [10,31], which may be related to its larger amount of acid sites than Mn/Ce-DW catalyst. Interestingly, we can find that all of the L acid and B acid disappear at a lower temperature of 150 ºC, while two new bands of bridging nitrates (1211 cm–1) and monodentate nitrates (1493 cm–1) appear at 100 ºC [10,38], which suggest that Mn/Ce-OA catalyst is more beneficial to the activation and transformation of the adsorbed NH3 species and NOx species than Mn/Ce-DW catalyst. These phenomena can be explained from the following aspects: firstly, the results of NH3-TPD and NH3 adsorption in situ DRIFTS present that Mn/Ce-OA catalyst displays the largest amount of acid sites, which is beneficial to the adsorption and activation of NH3 molecules; secondly, the results of H2-TPR and XPS show that Mn/Ce-OA catalyst exhibits the most excellent redox property as well as the largest amounts of Mn4+ and surface adsorbed oxygen species, which can promote the oxidation of NO to NO2 and the further adsorption to form nitrates; finally, the results of Raman and XPS exhibit that Mn/Ce-OA catalyst possesses abundant of oxygen vacancy and Ce3+, which benefit for the dissociation and transformation of NOx species. All of these improved physicochemical properties result in the excellent catalytic performance and good H2O+SO2 resistance of Mn/Ce-OA catalyst for low-temperature NH3-SCR reaction.
4. Conclusions 14
In summary, we have carefully investigated the solvent effect on the physicochemical properties and catalytic performance of MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction. The obtained results indicate that MnOx/CeO2 catalyst with oxalic acid as a solvent (i.e., Mn/Ce-OA catalyst) exhibits the best catalytic performance and good H2O+SO2 resistance. The reason may be that oxalic acid is beneficial to the dispersion of MnOx and enhances the electron interaction between MnOx and CeO2, which lead to the most excellent redox property and the largest amounts of oxygen vacancy, Ce3+, Mn4+, surface adsorbed oxygen species, and acid sites. Large amounts of Mn4+ and surface adsorbed oxygen species, as well as excellent redox property can promote the oxidation of NO to NO2; abundant of oxygen vacancy and Ce3+ benefit for the dissociation and transformation of NOx species; large amount of acid sites is conducive to the adsorption and activation of NH3 molecules. All of these result in the best catalytic performance and good H2O+SO2 resistance. In the present work, we have successfully enhanced the low-temperature NH3-SCR performance of MnOx/CeO2 catalysts by the optimal solvent effect, which can provide a scientific reference for the development of practical denitration catalysts.
Acknowledgements The financial supports of the National Natural Science Foundation of China (21507130), the Chongqing Science & Technology Commission (cstc2016jcyjA0070, cstc2014pt-gc20002, cstc2014yykfC20003, cstckjcxljrc13), the Open Project Program of Chongqing Key Laboratory of Catalysis and Functional Organic Molecules from Chongqing Technology and Business University (1456029), and the Open Project Program of Jiangsu Key Laboratory of Vehicle Emissions Control (OVEC001, OVEC007) are gratefully acknowledged. Furthermore, we also thank Dr. Shuohan Yu in Nanjing University for XPS measurement. References [1] S.G. Wu, X.J. Yao, L. Zhang, Y. Cao, W.X. Zou, L.L. Li, K.L. Ma, C.J. Tang, F. 15
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Fig. 1. (a) NO conversion, (b) N2O concentration, and (c) Arrhenius plots of these MnOx/CeO2 catalysts for low-temperature NH3-SCR reaction.
NO conversion (%)
(a) 100 80 60 Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
40 20 0 50
100
150
200
250 o
Temperature ( C)
18
300
250
N2O concentration (ppm)
(b)
Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
200 150 100 50 0 50
100
150
200
250
300
o
Temperature ( C)
1
1
lnk (mmol g s )
(c)
-1
Ea = 13.79 kJ mol 1
-2
Ea = 21.2 3k
-3
E =3 a 2.73 k
-4
Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
-5
J m ol 1
J m ol 1
E = a 56 .2 4 kJ m ol 1
-6 2.3
2.4
2.5
2.6
1
2.7
1000/T (K )
Fig. 2. H2O+SO2 resistance of these MnOx/CeO2 catalysts at 200 °C.
NO conversion (%)
100
5% H2O + 100ppm SO2 in
80 60 5% H2O + 100ppm SO2 off
40
Mn/Ce-DW Mn/Ce-AE Mn/Ce-AA Mn/Ce-OA
20 0 0
100
200
300
400
500
600
Reaction time (min) 19
700
Fig. 3. XRD patterns of these MnOx/CeO2 catalysts.
(111)
Intensity (a.u.)
(220)
(200)
Mn Mn Mn
OA
/Ce-
AA
(420)
(311)
(222) (400) (331)
60
70
/Ce-
Mn /C
10
/Ce-
CeO2
XRD
AE e -D W 20
30
40
50
80
2Theta (degree)
Fig. 4. EDS mapping images of these MnOx/CeO2 catalysts.
Mn/Ce-DW
Mn/Ce-AE
Mn/Ce-AA
20
Mn/Ce-OA
Fig. 5. Raman spectra of these MnOx/CeO2 catalysts.
Raman
Intensity (a.u.)
F2g
D Mn/Ce-OA
ID/IF2g = 0.1431
Mn/Ce-AA
ID/IF2g = 0.1047
Mn/Ce-AE
ID/IF2g = 0.1008
Mn/Ce-DW
ID/IF2g = 0.0797
200
300
400
500
600
700
1
800
Raman shift (cm )
Fig. 6. XPS spectra of these MnOx/CeO2 catalysts: (a) Ce 3d, (b) Mn 2p, and (c) O 1s.
(a)
(b)
Ce 3d
u'''
u ,u v''' Mn/Ce-OA u'' u'
v''
Mn/Ce-AE
Mn/Ce-DW
920
910
2+
v ,v v'
Mn/Ce-AA
900
890
Mn/Ce-OA
Mn 3+ Mn
4+
Mn
Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW Mn 2p1/2
Ce 3d5/2
Ce 3d3/2
Mn 2p
0
Intensity (a.u.)
Intensity (a.u.)
0
656
880
652
Mn 2p3/2
648
644
640
Binding energy (eV)
Binding energy (eV)
21
636
(c)
O 1s O'
Intensity (a.u.)
O'' Mn/Ce-OA Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW
536
534
532
530
528
526
524
Binding energy (eV)
Fig. 7. (a) H2-TPR profiles, and (b) NH3-TPD profiles of these MnOx/CeO2 catalysts.
10
H2 consumption (a.u.)
(a)
H2-TPR
Mn/Ce-OA Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW
100
200
300
400
500
o
Temperature ( C)
22
600
(b)
NH3-TPD
20 I
III
IV
Intensity (a.u.)
II
Mn/Ce-OA Mn/Ce-AA Mn/Ce-AE Mn/Ce-DW
0
100
200
300
400
500
600
700
o
Temperature ( C)
Fig. 8. NH3 adsorption in situ DRIFTS of (a) Mn/Ce-DW, and (b) Mn/Ce-OA catalysts.
Kubelka-Munk (a.u.)
0.04
o C o C o C o C o C o C o C o C o
300 275 250 225 200 175 150 125 100o C 75 C o 50 oC 25 C
2000
1800
(b)
Mn/Ce-DW
0.04
Kubelka-Munk (a.u.)
(a)
1263 1070 1594 1440 1391 1168
L
L 1580
1600
L B B 1305
1400
1114 L L
1200 1
o C o C o C o C o C o C o C o C o
300 275 250 225 200 175 150 125 100o C 75 oC 50 C o 25 C
2000
1000
1800
Mn/Ce-OA 1604
1430 1262 1077 1393 1175
L
L L 1578
1600
1125 B L B 1314 L
1400
1200 1
Wavenumber (cm )
Wavenumber (cm )
23
1000
Fig. 9. Quantitative analysis of B acid over Mn/Ce-DW and Mn/Ce-OA catalysts according to NH3 adsorption in situ DRIFTS.
Peak area of B acid (a.u.)
2.5
Mn/Ce-DW Mn/Ce-OA
2.0 1.5 1.0 0.5 0.0 0
50
100
150
200
250
300
o
Temperature ( C)
Fig. 10. NO+NH3+O2 co-adsorption in situ DRIFTS of (a) Mn/Ce-DW, and (b) Mn/Ce-OA catalysts.
(a)
1540
1239
o
300 oC 275 C o 250 oC 225 oC 200 oC 175 oC 150 oC 125 oC 100o C 75 C o 50 oC 25 C
2000
1800
1314 1579 1432 1278
1600
1400
Mn/Ce-OA
0.2
Kubelka-Munk (a.u.)
Kubelka-Munk (a.u.)
(b)
Mn/Ce-DW
0.2
1
1000
2000
Wavenumber (cm )
1493
o
300 oC 275 C o 250 oC 225 oC 200 oC 175 oC 150 oC 125 oC 100o C 75 C o 50 oC 25 C
1017
1200
1211
1540
1800
1178 1598 143213141278 1017
1600
1400
1200 1
Wavenumber (cm )
24
1000
Table 1 BET specific surface area, total pore volume, and mean pore diameter of these MnOx/CeO2 catalysts.
BET specific surface area
Total pore volume
Mean pore diameter
(m2·g–1)
(cm3·g–1)
(nm)
Mn/Ce-DW 61.2
0.2048
13.4
Mn/Ce-AE
57.3
0.2022
14.1
Mn/Ce-AA
60.2
0.2100
14.0
Mn/Ce-OA
60.7
0.2014
13.3
Catalysts
25
Table 2 The surface composition and atomic ratio of these MnOx/CeO2 catalysts.
Atomic concentration (at%)
Atomic ratio (%)
Ce
Mn
O
Mn/Ce
Ce3+/(Ce3++Ce4+)
Mn4+/(Mn2++Mn3++Mn4+)
Oʺ/(Oʹ+Oʺ)
Mn/Ce-DW
17.71
1.95
80.34
11.01
16.18
27.58
25.91
Mn/Ce-AE
17.84
3.83
78.33
21.47
16.78
30.36
27.23
Mn/Ce-AA
15.79
3.91
80.30
24.76
17.09
32.06
28.35
Mn/Ce-OA
16.34
4.96
78.70
30.35
18.13
35.47
33.43
Catalysts
26
Table 3 The information of peak temperature and H2 consumption of these MnOx/CeO2 catalysts obtained from H2-TPR.
Peak temperature (°C)
H2 consumption (μmol·g–1)
Tα
Tβ
Tγ
Sα
Sβ
Sγ
Sα+Sβ+Sγ
Mn/Ce-DW
235
314
382
547
332
275
1154
Mn/Ce-AE
228
309
374
574
384
277
1235
Mn/Ce-AA
217
309
375
469
492
309
1270
Mn/Ce-OA
209
283
352
631
393
372
1396
Catalysts
Table 4 Quantitative analysis of NH3-TPD over these MnOx/CeO2 catalysts.
Acid amount (a.u.)
Total acid amount (a.u.)
Catalysts SI
SII
SIII
SIV
SII+SIII
SI+SII+SIII+SIV
Mn/Ce-DW 1296 1232 1212 219
2444
3959
Mn/Ce-AE
908
1486 1341 269
2827
4004
Mn/Ce-AA
961
1424 1434 220
2858
4039
Mn/Ce-OA
1014 1460 1492 286
2952
4252
27