Improved Activity and H2O Resistance of Cu-Modified MnO2 Catalysts

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Improved activity and HO resistance of Cumodified MnO catalysts for NO oxidation 2

Chuanning Shi, Huazhen Chang, Chizhong Wang, Tao Zhang, Yue Peng, Mingguan Li, Yuanyuan Wang, and Junhua Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b04504 • Publication Date (Web): 21 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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Industrial & Engineering Chemistry Research

Improved activity and H2O resistance of Cu-modified MnO2 catalysts for NO oxidation Chuanning Shi, † Huazhen Chang, *,† Chizhong Wang, ‡ Tao Zhang, † Yue Peng, ‡ Mingguan Li, † Yuanyuan Wang, † and Junhua Li‡ †

School of Environment and Natural Resources, Renmin University of China, Beijing 100872,

China ‡

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Environment, Tsinghua University, Beijing 100084, China

∗

Corresponding author. Tel.: +86-10-62512572

E-mail address: [email protected] (H. Chang) 1

ACS Paragon Plus Environment

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ABSTRACT The catalytic oxidation of NO plays an important role in the process of DeNOx. In this study, a series of Cu-Mn-Ox mixed metal oxides were prepared by a co-precipitation method. The NO oxidation performances indicated that the catalyst with a molar ratio of Cu:Mn = 1:1 prepared at pH=6 (denoted as CuMnpH6) exhibited the best activity. In the presence of H2O, the activity of CuMnpH6 catalyst was improved obviously compared to those of the other samples. The basicity of α-MnO2 was weakened after adding Cu. The basicity modification might inhibit the adsorption of H2O on surface oxygen atoms and likely promote the adsorption of NO over CuMnpH6 catalyst. The redox properties were enhanced, and there existed dual redox cycles during NO oxidation. The redox properties and surface basicity, which are related to NO adsorption capacity, were correlated to the NO oxidation and resistance to H2O over Cu-Mn-Ox catalysts.

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1. INTRODUCTION The emissions of nitrogen oxides (NOx) from fossil fuel combustion has caused serious environmental problems and are harmful to human health.1 More than 90 % of NOx is NO in flue gas and exhaust.2, 3 The selective catalytic reduction of NOx with NH3 (NH3-SCR) is the most effective technology for NOx elimination.4, 5 The oxidation of NO to NO2 is very important for NH3-SCR, especially for “fast SCR”. On the other hand, a mixture of NO2 and NO could be easily removed by liquid-phase absorption technology. Therefore, the oxidation of NO is of great importance for the removal of NOx.6 In general, the process of converting NO into NO2 is slow under standard condition. The catalytic oxidation of NO to NO2 by O2 has attracted increasing attention. Plenty of catalysts, including noble metals and metal oxides, have been investigated for the catalytic oxidation of NO. Pt-based catalysts have been applied in exhaust treatment, and they have showed high activity in NO oxidation.7, 8 The commercial application of noble metal catalysts is limited by their high cost and poor resistance in the presence of SO2 and H2O.

7, 9

Recently, it was found that several transition metal oxides showed high

performances in NO oxidation. Among these catalysts, Mn-based catalysts10 exhibited excellent activities in NO oxidation due to their multivalent chemical properties and porous structure. However, the catalytic activity of NO oxidation was still inhibited severely by H2O. Most researchers have reported that the effect of H2O is reversible in NO oxidation.7, 11 It was believed that the competitive adsorption between water molecules and NO is an 3



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important reason for the inhibition effect of H2O.12 Iglesia et al. 7 proposed that unreactive hydroxyls formed, which occupy vacant sites (*) essential for O2 activation, in NO oxidation. The vacancy concentrations decreased in the presence of H2O. Thus, the (OH*)/(O*) ratio should be valuable during NO oxidation catalysis on surfaces predominantly covered by O* and OH*. 7 In our previous study, 13 the modification of the surface basicity proved to be an effective way to enhance the H2O resistance in NO oxidation over Sn-Co-O catalysts. In this study, a series of Cu-Mn-Ox mixed metal oxides were prepared by a co-precipitation method. A novel and practical design strategy for H2O resistance in NO oxidation by optimizing the preparation pH value was proposed. The effects of the pH value on the NO oxidation performance and water resistance were investigated extensively by several characterization methods, including CO2-TPD, XPS, H2-TPR, and NO+O2-TPD.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The mixed metal oxides consisting of Cu and Mn were prepared by a co-precipitation method. Cu(NO3)2 and a 50 wt% manganese nitrate solution were dissolved in deionized water under ambient conditions. In order to control the pH value, an ammonium carbonate solution or ammonia water was added as a precipitating agents. The sediment was dried and calcined. In most of this work, the molar ratio of Cu/Mn was kept at 1:1 in the preparation process. The catalysts were denoted as CuMnpHx, where x 4


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represents the pH value of the prepared solution. A CuO catalyst was also prepared by a similar precipitation method. The α-MnO2 sample was prepared by a hydrothermal method. Details of the preparation process can be found in the Supporting Information. 2.2. Catalytic Activity Test. The catalytic activity was tested in a fixed-bed flow quartz reactor (outer diameter: 8 mm, inside diameter: 6 mm) system at atmospheric pressure. A 150 mg sample (sieve fraction of 40–60 mesh) was placed in the quartz reactor. Fourier transform infrared (FTIR) analysis (Gasmet, Dx-4000) was used as the online monitoring system. The inlet gas composition (volume fraction) in the experiment contained 500 ppm NO, 5 % O2, 0 or 5 % H2O and N2 balance. In addition, the total gas flow rate was 200 mL min-1, and the gas hourly space velocity (GHSV) was 85000 h-1. The NO oxidation conversion was calculated as follows: CNO %=

NOin - NOout ×100 % NOin

(1)

where [NO]in and [NO]out represent the inlet and outlet concentrations of NO under steady-state conditions, respectively. 2.3. Catalyst Characterization. To illustrate the properties of the catalysts, various characterization methods were used. H2-TPR and CO2-TPD experiments were conducted on a Micromeritics ChemSorb 2920 chemical adsorption instrument. In the H2-TPR test, 0.05 g of a sample was treated in N2 for 1 h at a temperature of 300 ºC. XPS was obtained on an X-ray photoelectron spectrometer (VG Scientific, ESCALab220i-XL), and the excitation source was Mg Kα 5



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with an X-ray power of 0.3 kW and vacuum of 3×10-9 mbar. The C 1s pollution peak of 284.8 eV was used to correct the electron binding energy. For the NO+O2-TPD experiments, after pretreatment in N2 at 400 ºC for 1 h and cooling to 30 ºC, the catalysts were saturated with 500 ppm NO and 5 % O2 for 1 h. After purging with N2 (300 mL min-1) for 1 h, the desorption progress was performed from 30 to 500 ºC with a heating rate of 10 ºC min-1. Furthermore, NO+O2+H2O-TPD was measured from 110 to 500 ºC with the similar pretreatments as the NO+O2-TPD, except the adsorption temperature was 110 °C, and 5 % H2O was added in the NO and O2 adsorption steps.

3. RESULTS 3.1. Catalytic Activity. The NO oxidation activities over the Cu-Mn-Ox mixed metal oxide catalysts with different Cu/Mn molar ratios were investigated, and the results are shown in Figure 1.α-MnO2 showed excellent NO oxidation performance in the test temperature range. The activity over the mixed metal oxide catalyst with a molar ratio of Cu:Mn=1:1 was the best among the series of Cu-Mn-Ox catalysts. Afterwards, the NO oxidation performances over CuMn11 prepared at different pH values were tested (see Figure 2). Compared to the other catalysts, the CuMnpH6 sample exhibited better activity with a wide reaction temperature window. The maximum efficiency of NO oxidation reached 77 % at 300 ºC. The NO oxidation efficiency decreased over all the samples at high temperature due to thermodynamic restrictions.14, 15 However, poor activity was obtained over the CuMnpH4 catalyst, which might be due to the incomplete precipitation of metal 6


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ions in the solution. With an increasing pH value (pH > 8), the ratio of copper oxide decreased in the catalyst. The active sites decreased on the surface of CuMnpH11 sample, the NO oxidation performance became worse than the other catalysts. The effect of H2O on the NO oxidation activity was investigated, and the results are displayed in Figure 3. It can be seen that the activity decreased over all the catalysts in the presence of H2O. However, the NO oxidation performance only slightly decreased over the CuMnpH6 and CuMnpH8 catalysts, where ca. 60 % NO oxidation efficiency was obtained at 350 ºC. For the CuO and α-MnO2 samples, the activity decreased sharply compared to the other catalysts. At 300 ºC, the NO conversion (27 %) was much lower than that in the absence of H2O (82 %) over the α-MnO2 catalyst. This indicated that the NO oxidation activity was inhibited considerably by H2O over the CuO and α-MnO2 catalysts. It can be seen from the DRIFTS spectra (see Figure S2), with the arising of reaction temperature from 100 to 250 ºC, the intensity of adsorption peaks decreased. Meanwhile, adsorption of H2O might be difficult at such high temperature. Thus, H2O has little effect on NO adsorption at 400 ºC. To investigate the effect of H2O on NO oxidation activity, CO2-TPD and XPS were utilized to characterize the surface basicity of these samples. 3.2. Surface Basicity. 3.2.1. CO2-TPD. The distribution of the surface basic sites was characterized by CO2-TPD. As shown in Figure 4, a series of bands appeared in the temperature range of 200 °C - 1000 °C over 7



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these catalysts. For the CuO and α-MnO2 samples, one band at high temperature (> 700 °C) appeared, and another sharp band at 400 °C - 700 °C was observed on the α-MnO2 catalyst. Similar bands appeared at high temperature on the CuMnpH6 and CuMnpH8 catalysts. However, they shifted to lower temperatures (at 781 °C and 813 °C) compared to those of the single metal oxides (at 868 °C and 967 °C), indicating that the basicity strength decreased on these two catalysts.16, 17 In addition, the intensity of the low-temperature band decreased obviously on the CuMnpH6 and CuMnpH8 catalysts. In our previous study, 13, 18 the surface basicity of the catalysts was related to the adsorption of H2O. Hence, the decreasing strength and quantity of the surface basic sites might benefit H2O resistance in NO oxidation over the CuMnpH6 and CuMnpH8 catalysts. In previous literature, 16, 18 the band at 400-650 °C has been assigned to strong basic sites (such as OH groups), and the band at > 650 °C has been assigned to very strong basic sites, i.e., negative oxygen ions (O2-) or metal-oxygen pairs. The existence of much stronger basic sites (O2-) on CuO and plenty of strong basic sites on α-MnO2 might facilitate the adsorption of H2O. The CO2-TPD results indicated that the surface basicity could be modified by regulating the pH value in the preparation of the Cu-Mn-Ox mixed oxide catalysts. 3.2.2 XPS. XPS was applied to analyze the concentrations and valence states of the surface atoms of these catalysts. 19 As shown in Figure 5, there are two peaks in the O 1s XPS spectra of these catalysts. The peaks at 531.1-531.4 eV are attributed to surface adsorbed 8


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oxygen belonging to defect oxide or hydroxyl-like species (denoted as Oα), while the peaks at 529.7-530.0 eV are ascribed to lattice oxygen (denoted as Oβ). 20, 21 For the CuMnpH6 and CuMnpH8 catalysts, the binding energies of Oβ were higher than those of CuO and a-MnO2 (see Table 1). We can get some information from the area integral calculation (see Figure 5). The proportions of Oα to (Oα + Oβ) were listed in the picture, while the more Oα existed, the reaction would take place more easily. Thus, there is no clear difference among the prepared catalysts. According to previous studies, 16, 20 the binding energy (BE) of Oβ in XPS can reflect the strength of the surface basicity. It was proposed that the basicity decreased as the BE increased, which is in accordance with the CO2-TPD results over the CuMnpH6 and CuMnpH8 catalysts. 3.3. Redox Properties. The Cu 2p XPS spectra of the CuO, CuMnpH6 and CuMnpH8 catalysts are shown in Figure 6a. The peaks at 931.2 eV and 934.1 eV correspond to Cu+ and Cu2+, respectively. Such the reoxidation phenomenon in the previous work might be attributed to the change of copper ion. There are several peaks (such as peaks at 941 eV and 943 eV) that contributed to satellites peaks on the curves.22, 23 The Mn 2p XPS spectra are shown in Figure 6b. The peaks at 644.4-645.9 eV and 642.4-642.7 eV are attributed to Mn4+ and Mn3+, respectively. In addition, the peaks at 641.4-641.6 eV are attributed to Mn2+ on CuMnpH6 and CuMnpH8.

19, 24

The ratio of Mn4+/ (Mn4++ Mn3+) increased to 49.2 %

and 49.6 % over CuMnpH6 and CuMnpH8, respectively, which are higher than that over α-MnO2 (47.0 %). The existence of Cu+ and Mn2+ might improve the redox properties of 9



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the solid catalysts, and finally enhance the performance of NO oxidation over the CuMnpH6 and CuMnpH8 catalysts. To further characterize the redox properties of the prepared catalysts, H2-TPR experiments were carried out and the results are displayed in Figure 7. For α-MnO2, two obvious reduction peaks appeared at 406 ºC and 550 ºC, which can be attributed to the reduction of MnO2.25, 26 The reduction products may include Mn2O3, Mn3O4, and finally MnO. 27 For CuO, there was an obvious reduction peak at 361 ºC and a shoulder peak at 445 ºC. Cu2+ might be reduced to Cu+ as the temperature increases. Due to the limits of thermodynamic balance, further reduction of Cu+ to Cu0 is not likely under the existing experimental conditions.

28

The reduction peaks shifted to lower temperature over the

CuMnpH6 and CuMnpH8 samples. These peaks can be attributed to the reduction of Mn2O3 to Mn3O4 and to MnO, as well as the reduction of Cu2+ to Cu+.

27, 29

In addition,

the temperatures of the reduction peaks were much lower than those of α-MnO2 and CuO, indicating the redox properties were apparently enhanced in the mixed oxides catalysts. This should be an important factor to obtain better activity for NO catalytic oxidation over the CuMnpH6 and CuMnpH8 samples. 3.4. Effect of H2O on NO Adsorption behavior. The BET surface areas of these catalysts are shown in Table 1. The surface area of α-MnO2 was 37.1 m2 g-1. The largest surface area was obtained for the CuMnpH6 catalyst (55.3 m2 g-1). This might contribute to its better catalytic activity for NO oxidation. However, the surface area of CuMnpH8 was similar to that of the α-MnO2 catalyst, 10


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indicating the BET surface area did not have a considerable influence in the present study. To evaluate the adsorption capacity of NO on the Cu-Mn-Ox catalysts, NO+O2-TPD measurements were performed over the four catalysts (see Figure 8). One broad desorption band spanned the temperature range of 100-400 ºC over the CuMnpH6 and CuMnpH8 samples. Both NO and NO2 desorption occurred, which can be attributed to the desorption of nitrogen-containing species from the active sites. 14 The NOx desorption peak of CuMnpH6 was much stronger than that of the CuMnpH8 sample. The adsorption capacity of NO was calculated and the results are shown in Table 1. The NO adsorption capacity of CuMnpH6 was also much higher than those of other samples. Moreover, the desorption peak of NO appeared at a lower temperature over CuMnpH6 than the other catalysts, indicating that the adsorbed species could be desorbed more easily. To investigate the effect of H2O on NO adsorption, NO+O2-TPD was also tested with the addition of 5 wt% H2O in the NO+O2 adsorption procedure. As exhibited in Figure 8 and Table 1, the NO adsorption capacity was considerably affected after introducing H2O, demonstrating that the adsorption of NO was inhibited severely by H2O. However, a larger adsorption capacity was obtained over CuMnpH6 than over the other catalysts under the same conditions (see Table 1). It can be concluded that more NO could be adsorbed on the CuMnpH6 catalyst, even in the presence of H2O, which will be beneficial for low-temperature NO catalytic oxidation in the presence of H2O.

11



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4. DISCUSSION 4.1. Dual Redox Cycles for NO Oxidation. The redox properties of a catalyst are very important for NO oxidation. According to the H2-TPR results (see Figure 7), the reduction peaks moved toward lower temperature on the CuMnpH6 and CuMnpH8 samples. The Mn and Cu species can be reduced much easier on these two catalysts than on the α-MnO2 and CuO catalysts. This indicates that the redox ability of the two catalysts are improved. In the XPS spectra (see Figure 6), only Cu2+ was found on the CuO sample; meanwhile, Mn3+ and Mn4+ were found on the α-MnO2 sample. Comparatively, Cu+, Cu2+, Mn2+, Mn3+, and Mn4+ appeared on the CuMnpH6 and CuMnpH8 catalysts. The variable valence states might contribute to the enhanced redox properties of the two samples. The existence of Cu+ and Mn2+ might improve the adsorption and activation of O2. It is supposed that dual redox cycles might exist in NO oxidation over the CuMnpH6 catalyst: Cu2+ is reduced to Cu+, while Mn4+ is first reduced to Mn3+, and ultimately to Mn2+. They could be oxidized easily by O2. Meanwhile, the ratios of Mn4+/(Mn4++ Mn3+) over CuMnpH6 and CuMnpH8 are higher than that over α-MnO2. This should be another important reason for the excellent oxidation performances over the Cu-Mn-Ox catalysts. The possible redox progress were listed as follows: Cu2+ + e-

↔

Cu+

Mn4+ + e-

↔

Mn3+ + e-

(2)

↔

Mn2+

(3)

The XRD spectra indicated that Cu1.5Mn1.5O4 was the main phase. Moreover, the 12


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intensities of the diffraction peaks were lower compared with those of the single metal oxides. This indicates that smaller crystalline particles formed in the CuMnpH6 and CuMnpH8 catalysts. The specific crystal structure might facilitate electron transfer. Due to the dual redox cycles, additional chemical valence state transformations would occur on the surfaces of the Cu-Mn-Ox catalysts. Obviously, the redox properties of α-MnO2 were improved by the addition of Cu. Concerning the different species formed on the surfaces of the catalysts, an in situ DRIFTS study was performed on the CuO and CuMnpH6 samples, and the spectra are exhibited in Figure S2. Compared with CuO, more adsorbed NO species appeared on the CuMnpH6 catalyst, indicating that more NO adsorption sites formed on this sample. This might contribute to the superior NO adsorption capacity of the CuMnpH6 catalyst. 4.2. Strategies for the Design of Water-Resistant Catalysts. The resistance to H2O is very important for a NO oxidation catalyst in flue gas. In this work, excellent H2O tolerance was obtained over the CuMnpH6 and CuMnpH8 catalysts. At 250 ºC, much higher normalized reaction rates (Rs) were achieved over the CuMnpH6 and CuMnpH8 catalysts (2.4×10-9 and 4.4×10-9 mol s-1 m-2, respectively) in the presence of H2O (see Table 1), while the Rs values were lower over the α-MnO2 and CuO samples. Since the BET specific surface area of CuMnpH6 is larger than those of the other samples, the NO oxidation reaction rate (with H2O) was considerably promoted over the CuMnpH6 catalyst. The CO2-TPD results indicated that the surface basicity was modified on the 13



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Cu-Mn-Ox mixed oxide catalysts (see Figure 4). Several types of basic sites existed on the surfaces of the different catalysts. The peaks on CuMnpH6 shifted to lower temperature compared to the other catalysts, implying that the basicity strength decreased on this catalyst.

13

Meanwhile, the peak intensities decreased apparently compared to

α-MnO2 and CuO, suggesting that the amount of basic sites decreased over the CuMnpH6 and CuMnpH8 catalysts. According to previous studies, 12 surface active oxygen atoms play an important role in H2O adsorption. It is likely that hydrogen bonds formed between the surface oxygen and H atoms. The basicity modification might inhibit the adsorption of H2O over the CuMnpH6 and CuMnpH8 catalysts. In contrast, the existence of stronger basic sites (i.e., O2-) might promote the adsorption of H2O over the α-MnO2 and CuO samples. Competitive adsorption might exist between NO and water vapor on the surfaces of the catalysts. It was proven that the adsorption of NOx was affected by H2O on the α-MnO2 catalyst (see Figure 8). Once the adsorption of H2O was inhibited, more NO could absorb on the catalyst and be oxidized by O2. The mechanism of NO oxidation on Cu-Mn-Ox and α-MnO2 could be described as follows: 30 NO (g) → NO (a)

(4)

2NO (a) + O2 (g) → 2NO2 (a)

(5)

NO2 (a) → NO2 (g)

(6)

where “g” represents gas state, and “a” represents adsorption state. A mechanism for NO oxidation over the Cu-Mn-Ox catalysts is proposed in Scheme 14


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1. It was suggested that NO adsorption and the redox properties of the catalyst played important roles in NO oxidation in the presence of H2O. For the α-MnO2 catalyst, the adsorption of H2O was promoted by the existence of abundant basic sites. NO adsorption was affected because of competitive adsorption between NO and H2O. For the Cu-Mn-Ox catalysts, the adsorption of H2O was weakened owing to the decreased basicity. NO could be easily absorbed on the surface and oxidized by O2. The existence of dual redox cycles promoted the redox properties of the Cu-Mn-Ox catalysts. This might exist during NO oxidation over the CuMnpH6 catalyst: Cu2+ is reduced to Cu+, while Mn4+ is first reduced to Mn3+, and ultimately to Mn2+. They could be oxidized easily by O2. Meanwhile, a higher ratio of Mn4+/ (Mn4++ Mn3+) was an important reason for the excellent oxidation performance over the Cu-Mn-Ox catalysts.

5. CONCLUSION In this research work, the effect of pH value on catalytic performance over Cu-Mn mixed metal oxides were investigated. It can be seen from various characterization method, that CuMnpH6 catalyst performed a better activity on NO oxidation than other samples, especially in the appearance of H2O. The redox ability was improved, while the BET surface area increasing. There existing dual redox cycles and on the surface of Cu-Mn-Ox catalysts, played an important role on competitive adsorption between NO and H2O. This provided a design strategy on metal catalyst design.

ASSOCIATED CONTENT s Supporting Information ○

15



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The Supporting Information is available free of charge on the ACS Publications website at DOI: Complete set of simulation parameters (PDF)

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Huazhen Chang: 0000-0002-9915-6042 Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the National Key R&D Program of China (No. 2016YFC0203900, 2016YFC0203901), the National Natural Science Foundation of China (Grant No. 21577173, 21307071) and the National Basic Research Program of China (973 Program, Grant No. 2013CB430005).

16


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oxides in the selective catalytic reduction of nitric-oxide with ammonia. Appl. Catal. B-Environ. 1994, 3, 173-189. (27) Liu, T.; Yao, Y.; Wei, L.; Shi, Z.; Han, L.; Yuan, H.; Li, B.; Dong, L.; Wang, F.; Sun, C., Preparation and Evaluation of Copper Manganese Oxide as a High-Efficiency Catalyst for CO Oxidation and NO Reduction by Co. J. Phys. Chem. C. 2017, 121, 12757-12770. (28) Delahay, G.; Coq, B.; Kieger, S.; Neveu, B., The origin of N2O formation in the selective catalytic reduction of NOx by NH3 in O2 rich atmosphere on Cu-faujasite catalysts. Catalysis Today, 1999, 54, 431-438. (29) Fang, D.; Xie, J. L.; Hu, H.; Yang, H.; He, F.; Fu, Z. B. Identification of MnOx species and Mn valence states in MnOx/TiO2 catalysts for low temperature SCR. Chem. Eng. J. 2015, 271, 23-30. (30) Hadjiivanov, K. Identification of neutral and charged NxOy surface species by IR spectroscopy. Catal. Rev. 2000, 42, 71-144.

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Table 1. Physical properties, the XPS information, yield and specific reaction rate (Rs) and adsorption capacity of different catalysts. Rsb Qadsc (µmol g-1) BET surface Oβ B.E. -9 Catalyst Ad (µmol m-2) (×10 mol a 2 -1 area (m g ) (eV) no H2O 5% H2O s-1 m-2) CuMnpH6

55.3

530.0

2.4

44.2

43.7

0.02

CuMnpH8

28.6

530.0

4.4

39.2

29.9

0.13

CuO

10.0

529.7

1.0

8.5

8.3

0.04

α-MnO2

37.1

529.9

1.6

22.2

9.8

0.23

a

B.E. represents binding energy. Rs represents the normalized reaction rate of NO in the presence of H2O at 250 ºC. c Qads represents the total amount of the adsorbed NO, calculated from NO+O2-TPD and NO+O2+H2O-TPD. d A represents CO2 desorption on the medium basic site (at around 400-650 ºC) of specific surface area. b

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Figure Captions Figure 1. Comparison of activity performance over CuO, α-MnO2 and Cu-Mn-Ox mixed oxides catalysts with different molar ratios. Reaction conditions: 0.15g samples, 500 ppm NO, 5% O2, N2 balance, with a total flow rate of 200mL min-1. Figure 2. Comparison of activity performance over Cu-Mn-Ox mixed oxides catalysts prepared at different pH values. Reaction conditions: 0.15g samples, 500 ppm NO, 5% O2, N2 balance, with a total flow rate of 200mL min-1. Figure 3. NO conversion efficiency over CuO, α-MnO2 and CuMn catalysts in the absence (dotted lines) and presence (solid lines) of 5% H2O. Figure 4. CO2-TPD profiles of different metal oxides catalysts. Figure 5. XPS spectra for O 1s of the prepared catalysts. Figure 6. XPS spectra for (a) Cu 2p (b) Mn 2p of the prepared catalysts. Figure 7. H2-TPR profiles of different metal oxides catalysts measured at range of 100-800 oC. Figure 8. NO+O2-TPD profiles of (a) CuMnpH6, (b) CuMnpH8, (c) α - MnO2, and NO+O2+H2O-TPD profiles of (d) CuMnpH6, (e) CuMnpH8, (f) α - MnO2. Scheme 1. Proposed NO oxidation reaction mechanism over Cu-Mn-Ox and α-MnO2 catalysts. For Table of Contents Only

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Figure 1. Comparison of activity performance over CuO, α-MnO2 and Cu-Mn-Ox mixed oxides catalysts with different molar ratios. Reaction conditions: 0.15g samples, 500 ppm NO, 5% O2, N2 balance, with a total flow rate of 200mL min-1.

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Figure 2. Comparison of activity performance over Cu-Mn-Ox mixed oxides catalysts prepared at different pH values. Reaction conditions: 0.15g samples, 500 ppm NO, 5% O2, N2 balance, with a total flow rate of 200mL min-1.

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Figure 3. NO conversion efficiency over CuO, α-MnO2 and CuMn catalysts in the absence (dotted lines) and presence (solid lines) of 5% H2O.

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Figure 4. CO2-TPD profiles of different metal oxides catalysts.

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Figure 5. XPS spectra for O 1s of the prepared catalysts.

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Figure 6. XPS spectra for (a) Cu 2p (b) Mn 2p of the prepared catalysts.

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Figure 7. H2-TPR profiles of different metal oxides catalysts measured at range of 100-800 oC.

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Figure 8. NO+O2-TPD profiles of (a) CuMnpH6, (b) CuMnpH8, (c) α - MnO2, and NO+O2+H2O-TPD profiles of (d) CuMnpH6, (e) CuMnpH8, (f) α - MnO2.

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Scheme 1. Proposed NO oxidation reaction mechanism over Cu-Mn-Ox and α-MnO2 catalysts.

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For Table of Contents Only

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Improved Activity and H2O Resistance of Cu-Modified MnO2 Catalysts

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