Activated Carbon Honeycomb Catalyst for Selective

Article pubs.acs.org/IECR

MnOx−CeO2/Activated Carbon Honeycomb Catalyst for Selective Catalytic Reduction of NO with NH3 at Low Temperatures Yanli Wang, ChuanZhang Ge, Liang Zhan,* Cui Li, Wenming Qiao, and Licheng Ling State Key Laboratory of Chemical Engineering, East China University of Science and Technology, Shanghai 200237, PR China ABSTRACT: Activated carbon honeycomb supported manganese and cerium oxides (MnOx−CeO2/ACH) catalysts were investigated for selective catalytic reduction (SCR) of NO at low temperatures of 80−200 °C. Compared with ACH supported manganese oxide catalyst (MnOx/ACH), MnOx−CeO2/ACH catalysts show much higher SCR activity and higher selectivity to N2. NO conversion can be improved by the addition of CeO2 from less than 50% to 100% at 80−160 °C. The N2 selectivity of higher than 99.8% is obtained over the Ce(1)Mn/ACH catalyst at 80−200 °C. Results indicate that the addition of CeO2 improves the distribution of MnOx and enhances the oxidation of NO to NO2, producing more absorbed NO3− on the catalyst surface, which is then reduced into N2 by NH3. These behaviors account for the promoting effect of CeO2 on the SCR activity.

1. INTRODUCTION Nitrogen oxides (NOx) in flue gas are major air pollutants that must be removed before emission. Among various controlling technologies, selective catalytic reduction (SCR) of NO with NH3 is the most effective for NOx removal.1 Compared with granular catalysts for NOx removal, honeycomb catalysts exhibit obvious advantages, such as low pressure drop, high geometric surface areas, low tendency to fly ash plugging, and easy retrofitting to existing boiler systems.2 Up to now, two types of honeycomb catalysts, honeycomb V2O5/TiO2 based catalysts and cordierite-based CuO/Al2O3 catalysts, have been developed. Among the catalysts, honeycomb V2O5/TiO2 based catalysts have been used in industry to remove NO from flue gas, but the reaction temperature must be higher than 350 °C to avoid catalysts deactivation by SO2, requiring a high energy consumption.3,4 In this regard, low-temperature SCR catalysts have been widely developed to decrease the energy consumption and operating cost. Currently, various honeycomb SCR catalysts show high SCR activities at low temperatures, such as carbon-coated cordierite monoliths supported V2O5 (or MnOx), activated carbon honeycomb monoliths (ACH) supported CuO (or V2O5).5−11 However, the V2O5-based catalysts are easily deactivated in the presence of SO2 and H2O, and the temperatures must be higher than 200 °C.8−10 MnOx/ carbon-ceramic monoliths catalyst generally yielded NO conversion of 60−70% at about 150 °C, and the reduction of NOx could be enhanced by increasing the reaction temperature, while the selectivity for N2 decreasesd.5,7 Additionally, the carbon content of the carbon-coated cordierite monolith catalysts is less than 20 wt %, because the carbon was derived from carbonaceous precursors by pyrolysis and deposition, such as phenolic novolac resin and furan resin,5,7,8 leading to the operating temperature being very narrow and SO2 has a strong poisoning effect on the SCR activity. Therefore, a high resistance to SO2 and/or H2O poisoning must be of concern for low-temperature honeycomb SCR catalysts during the NOx removal process. Due to the unique redox properties, CeO2 has been extensively applied in many catalytic reactions, such as CO © XXXX American Chemical Society

oxidation and CO/NO removal. For example, CeO2 is a wellknown additive in the three-way catalyst for automotive exhaust gas purification. The reason is that labile oxygen vacancies and bulk oxygen species with relatively high mobility are easily formed during the redox shift between Ce4+ and Ce3+ under oxidizing and reducing conditions, respectively.12 Qi et al.13 reported that MnOx−CeO2 mixed oxides catalysts prepared by coprecipitation method showed high NO conversion at low temperatures range of 100−200 °C, and found that addition of cerium into MnOx resulted in different oxidation states of manganese. They also proposed that these catalysts exhibited the redox property based on reversible adsorption/desorption cycles of lattice oxygen, which led to increase of the oxidation of NO to NO2 and thus the SCR activity was promoted. Carja et al.14 prepared a Mn−Ce/ZSM-5 catalyst that exhibited 75− 100% NO conversion within a broad temperature window (240−500 °C). Interestingly, SO2 had only slight effect on its activity. Wu et al.15 studied the effect of Ce on SO2 resistance of Mn/TiO2 catalysts in the SCR activity and found that the SO2 resistance could be greatly enhanced for Ce modified Mn/TiO2 catalysts because the addition of Ce would prevent the formation of Ti(SO4)2 and Mn(SO4)2 and the formation of ammonia sulfate on catalyst surface. It should be noted that the granular configuration of these catalysts will cause a high flue resistance and plugging problem in flue gases, which restricts their practical application to some extent. Compared to carbon-coated monoliths, ACH monoliths combine the advantages of both activated carbons and honeycomb structures, exhibiting high carbon content, high surface area, and perfect channels for flue gases. In our previous work,16 we found that MnOx/ACH had good SCR activity at low temperatures, and more than 85% NO conversion could be achieved over MnOx/ACH with 15 wt % Mn loading in the temperature range of 160−220 °C and at the space velocity of Received: February 29, 2012 Revised: July 17, 2012 Accepted: August 24, 2012

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dx.doi.org/10.1021/ie300555f | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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1910 h−1. However, it should be pointed out that the catalyst must be operated in a higher temperature to avoid catalyst deactivation by SO2. Based on the successful application of CeO2 as additive in the three-way catalysts and the high activity of MnOx/ACH catalyst for NO removal, one thus expects ACH supported MnOx−CeO2 (MnOx−CeO2/ACH) catalyst to be a competitive catalyst for NOx removal. In this paper, we study the activities of MnOx−CeO2/ACH catalysts for SCR reaction in a wide temperature range of 80−200 °C with the aid of X-ray powder diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). The results are of importance for deeply understanding low temperature SCR reaction on porous carbon materials and beneficial to promote the development of NO removal technology from flue gases.

gas hourly space velocity (GHSV) of 1910 h−1 (based on the overall volume of the monolith). The reaction temperature was regulated from 80 to 200 °C. The inlet and outlet concentrations of NO and O2 were continually monitored online by an MRU VARIO PLUS flue gas analyzer. The effluent gas concentrations of NO and N2O were monitored by Nicolet (FTIR, PROTÉGÉ460, America). The SCR behaviors of the catalyst are expressed by NO conversion and selectivity to N2, which were calculated as literature reported.17

3. RESULTS AND DISCUSSION 3.1. NO Removal Activities of the Catalysts. Figure 1 compares the NO removal activities vs reaction temperature

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. ACH with a cell density of 10 cells per square inch (cpsi) and a wall thickness of 1.0 mm was prepared by extrusion from activated carbon (AC) powder with phenolic resin as binder. The MnOx−CeO2/ACH catalyst was prepared by pore volume impregnation of the treated ACH using a mixed solution of manganese acetate and cerium nitrate. After impregnation, the samples were dried at 50 °C for 6 h and then at 110 °C for 6 h, followed by calcination at 400 °C for 3 h in N2 atmosphere. Before NO removal test, the catalysts were preoxidized in air at 160 °C for 2 h. The weight percentage of Mn of the catalysts is 3.0 wt %. The final catalysts are labeled as Ce(x)Mn/ACH, where x denotes the molar ratio of Ce to Mn. For example, Ce(1)Mn/ACH refers to a MnOx−CeO2 catalyst with a molar ratio of Ce/Mn = 1:1. For comparison, MnOx/ ACH and CeO2/ACH catalysts were also prepared using the same method. 2.2. Characterization. Nitrogen adsorption/desorption isotherms of the ACH support and the catalysts were measured at 77 K using a Micromeritics ASAP 2020 M analyzer to determine the pore texture properties. The morphology of the samples and EDS data were obtained using a JEOL JSM6360LV scanning electron microscope (SEM) equipped with electron diffraction spectra. Powder X-ray diffraction (XRD) patterns were recorded using a Riguku D/Max 2550 diffractometer using Cu Kα radiation with 2θ in the range from 10 to 80° in steps of 0.05°. XPS measurements were carried out at room temperature on a PHI 5000 VersaProbe system with Al KR radiation (1486.6 eV) at a pressure lower than 10−8 Torr. The DRIFTS (diffuse reflectance infrared Fourier transform spectroscopy) experiments were carried out on a Nicolet 6700 FTIR spectrometer with in situ diffuse reflectance pool and high-sensitivity MTC detector. Prior to experiment, the samples were purged in a flow of Ar at 200 °C for 2 h and then cooled to 30 °C under Ar atmosphere. The background spectrum was recorded with Ar and was subtracted from the sample spectrum. Then the Ar flow was switched to a stream containing 1000 ppm NO2/Ar or 1000 ppm NO/Ar + 8.0 vol % O2 to adsorb NO2 or NO for 60 min at 30 °C. The total flow rate in all the runs was 50 mL/min. 2.3. NO Activity Test. NO removal activities were measured in a fixed-bed glass reactor with an internal diameter of 20 mm. The reactants consisted of 500 ppm NO, 500 ppm NH3, 5.0 vol % O2, and balance N2. In all experiments, the total flow rate was maintained at 500 mL/min, corresponding to a

Figure 1. NO conversions at different reaction temperatures over various catalysts.

over the MnOx/ACH, CeO2/ACH, and Ce(1)Mn/ACH catalysts. The results show that NO conversion decreases in the following sequence: Ce(1)Mn/ACH > CeO2/ACH> MnOx/ACH. MnOx/ACH catalyst shows low NO removal activity with NO conversion of less than 50% below 160 °C. The NO conversions of CeO2/ACH catalyst are more than 83% in the temperature range of 80−200 °C. The Ce(1)Mn/ ACH catalyst shows the highest SCR activity. In detail, 100% of NO conversion can be obtained at temperatures of 80−160 °C. With increasing temperature, NO conversion slightly decreases and still exhibits higher than 98% at 180−200 °C, suggesting that the addition of CeO2 to MnOx/ACH catalyst has a significant effect on the NO conversion. 3.2. Effect of Ce/Mn Ratio on NO Removal Activity. Figure 2 compares NO conversions over various MnOx-CeO2/

Figure 2. NO conversions over various MnOx−CeO2/ACH catalysts with different Ce/Mn ratio.

ACH catalysts obtained with different Ce/Mn ratio. It can be seen that NO conversion gradually increases with increasing the Ce/Mn ratio. When the Ce/Mn ratio is 0.25, NO conversion increases from 82.4% at 80 °C to 97.6% at 160 °C. If further increasing the reaction temperature, the NO conversion has a slightly decrease, but it still exceeds 93.6% at 200 °C. Increasing B



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the Ce/Mn ratio to 1, the NO conversion is 100% at 80 °C, nearly 100% at 160 °C, and about 98.4% at 200 °C. When the ratio of Ce/Mn increased to 2, the catalyst maintained the high level of NO conversion, giving a high activity. Figure 3 shows

Table 1. Pore Structure Characterization of the Different Catalystsa sample

SBET (m2/g)

Smic (m2/g)

Vtotal (cm3/g)

Vmic (cm3/g)

ACH MnOx/ACH Ce(0.25)Mn/ACH Ce(0.5)Mn/ACH Ce(1)Mn/ACH Ce(2)Mn/ACH

718 718 652 651 602 497

502 501 463 462 431 383

0.39 0.39 0.35 0.35 0.32 0.25

0.26 0.26 0.24 0.24 0.23 0.20

a SBET, BET specific surface area; Smic, specific surface area of micropores; Vtotal, total pore volume; Vmic, pore volume of micropores.

Ce/Mn ratio from 0.25 to 2, while NO conversion increases, suggesting that the SBET and Vmic values of the used ACH are high enough for MnOx−CeO2/ACH catalysts and thus the NO removal activities of MnOx−CeO2/ACH catalysts have no direct relationships with their pore structures. The above results also confirm that the main reasons for the promotion of NO removal activity may result from the rate improvement of NO oxidation to NO2, which is known to be more active than NO toward SCR reaction.19 For the ACH used in this work, the optimum Ce/Mn ratio is considered to be 1. 3.4. XRD Characterization. To understand the changes of physical properties caused by CeO2 additive, XRD analyses were performed on various MnOx−CeO2/ACH catalysts. Figure 5 exhibits the XRD patterns of Ce(0.25)Mn/ACH,

Figure 3. N2 selectivity and N2O formation at different reaction temperatures over MnOx/ACH and Ce(1)Mn/ACH catalysts.

the N2 selectivity and N2O formation at different temperatures over MnOx/ACH and Ce(1)Mn/ACH catalysts. The selectivity to N2 on MnOx/ACH catalyst is increased by the addition of CeO2, and the Ce(1)Mn/ACH catalyst yields a N2 selectivity of higher than 99.8% at 80−200 °C. For Ce(1)Mn/ACH catalyst, no N2O is detected at reaction temperatures below 140 °C, and a trace amount of N2O (less than 2 ppm) is detected at 160− 200 °C. However, the N2O concentration over MnOx/ACH catalyst increases with an increase in the temperature of 100− 200 °C and N2O concentration of 25 ppm is achieved at 200 °C. Such results suggest that the addition of CeO2 additive to MnOx/ACH catalyst can restrict the formation of N2O, and then improve the selectivity of N2. These behaviors are similar to the reported MnOx−CeO2 mixed oxides, V2O5/AC,17 and Fe−Mn/TiO218 catalysts. 3.3. Pore Structure Analysis. To reveal the effect of Ce/ Mn ratio on NO removal activity, pore structure characterization of the MnOx−CeO2/ACH catalysts obtained with different Ce/Mn ratio wass analyzed by N2 adsorption/ desorption isotherms, as shown in Figure 4. The pore

Figure 5. XRD patterns of the different samples.

Ce(0.5)Mn/ACH, and Ce(1)Mn/ACH, along with those of MnOx/ACH, CeO2/ACH, and ACH support for comparison. It is clear that the typical diffraction peaks of amorphous carbon (2θ = 23° and 43°) can be observed on ACH support. With introduction of 3.0 wt % manganese onto ACH, new peaks appear at 18.1°, 28.9°, 32.4°, 36.1°, 44.3°, 58.5°, and 59.9°, indicating the existence of crystallized Mn3O4 in MnOx/ACH catalyst. But the diffraction peaks of Mn3O4 or other forms of manganese disappear when CeO2 is introduced into MnOx/ ACH catalyst, demonstrating that manganese species is well dispersed or in amorphous form. It is worth noting that the typical diffraction peaks corresponding to the cubic CeO2 (2θ = 28.4°, 33°, 47.2°, and 56.3°) cannot be observed on Ce(0.25)Mn/ACH catalyst due to its low loading. However, Ce(0.5)Mn/ACH and Ce(1)Mn/ACH catalysts show the diffraction peaks of cubic CeO2, indicating the presence of CeO2 crystal. Furthermore, the intensities of CeO2 diffraction peaks increase with increasing amount of CeO2. Huang et al.20 have revealed that the catalytic activity of manganese oxides catalyst was mainly affected by the oxidation state and crystallization and found that amorphous phase of manganese

Figure 4. Nitrogen adsorption−desorption isotherms of MnOx− CeO2/ACH catalysts obtained with different Ce/Mn ratios.

parameters of the samples are summarized in Table 1. ACH support gives a BET surface area (SBET) of 718 m2/g and a micropore volume (Vmic) of 0.26 cm3/g. When 3.0 wt % manganese is loaded, the pore structures of ACH stay the same, indicating that impregnation of a small amount of MnOx on ACH cannot cause pore plugging. Addition of CeO2 additive to the MnOx/ACH catalyst decreases its SBET and Vmic. The corresponding values for Ce(0.25)Mn/ACH, Ce(0.5)Mn/ ACH, Ce(1)Mn/ACH, and Ce(2)Mn/ACH are 652 m2/g and 0.24 cm3/g, 651 m2/g and 0.24 cm3/g, 602 m2/g and 0.23 cm3/g, 497 m2/g and 0.20 cm3/g, respectively. It is worth noting that the values of SBET and Vmic decrease with increasing C



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by XRD (Figure 5). These data suggests that Mn3+ and Mn2+ manganese oxide phases coexist on the surface of the samples. The O1s peaks are fitted into three peaks at 529.5−530.1, 531.0−531.7, and 532.7−533.5 eV (Figure 6b).23,24 The peak at 529.5−530.1 eV is assigned to the lattice oxygen (denoted as Oα) for MnOx or MnOx−CeO2, the peak at 531.0−531.7 eV is assigned to chemisorbed oxygen (denoted as Oβ) and the peak at 532.7−533.5 eV is assigned to C−O combined oxygen (denoted as Oγ). Figure 6b and Table 2 show that the Oβ concentration of Ce(1)Mn/ACH catalyst is higher than that of MnOx/ACH catalyst, indicating that the addition of CeO2 to MnOx/ACH catalyst increases the chemisorbed oxygen on the catalyst surface. And the surface chemisorbed oxygen has been reported to be the most active oxygen and plays a critical role in oxidation reaction.25 Wu et al.26 also reported that surface chemisorbed oxygen content on the catalyst was greatly increased by addition of CeO2 to MnOx/TiO2, resulting in a higher SCR activity. Thus, the SCR activity of Ce(1)Mn/ACH catalyst is much higher than that of MnOx/ACH catalyst, which may be ascribed to the enhancement of surface chemisorbed oxygen concentration. The Ce3d spectra are presented in Figure 6c. The peaks labeled as u″′, u″, u, v″′, v″, and v are assigned to Ce4+, and u′ and v′ are assigned to Ce3+.27−29 It can be seen that the u″′, u″, u, v″′, v″, and v peaks for Ce4+ are much stronger than the u′ and v′ peaks for Ce3+. This implies that Ce4+ is mainly valence state in Ce(1)Mn/ACH catalyst. Because electron is easy to transform from Ce3+ to oxygen or Mn species during the catalyst preparation, and the presence of Ce3+ could create a charge imbalance, the vacancies and unsaturated chemical bond on the catalyst surface,30 which will lead to the increase in chemisorbed oxygen on the surface. The above XPS results show that the addition of CeO2 does not alter the oxidation state of Mn, however, the existence of CeO2 increases the oxygen storage capacity and the percentage of Ce4+ on the surface of catalyst. According to the cycle of Mars−Krevelen redox mechanism, these factors are beneficial to the catalytic activity during the redox process. Considering the addition of CeO2 to MnOx/ACH catalyst increases the percentage of Ce4+, the Ce4+ content further increases with increasing cerium oxides loading. Thus, more amount of CeO2 on Ce(1)Mn/ACH and Ce(2)Mn/ACH catalysts increases the oxygen storage capacity and transfer ability, and then enhances the SCR activity. 3.6. SEM-EDX. To understand the distribution of MnOx and physical changes caused by CeO2, MnOx/ACH and Ce(1)Mn/ACH catalysts were characterized by SEM. Figure 7a and c show the SEM images of MnOx/ACH and Ce(1)Mn/ ACH catalysts. The corresponding EDX mappings are displayed in Figure 7b, d, and e. It can be seen that MnOx/ ACH catalyst shows some block species of different size (Figure 7a). Figure 7c shows that addition of CeO2 into MnOx/ACH results in decreasing the size of block-like species. As can be observed from Figure 7b, the distribution of manganese in the MnOx/ACH catalyst is poor, while manganese becomes more homogeneously distributed on the surface of Ce(1)Mn/ACH catalyst (Figure 7d). These confirm that addition of CeO2 improves the distribution of the manganese oxide, as evidenced by XRD result. Mapping of cerium shown in Figure 7e demonstrates that cerium is well dispersed on the ACH support. The coexistence of manganese and cerium oxides not only improves the distribution of metal catalysts, but also decreases the degree of crystallinity, suggesting there are strong

oxides resulted in enhancement of the SCR activity, but crystalline manganese oxides led to low activity. Additionally, CeO2 diffraction peaks on MnOx−CeO2/ACH catalyst are broader than those of individual CeO2/ACH, suggesting decreased size of crystal particle and degree of crystallinity. 3.5. XPS Characterization. To further determine the form of manganese or cerium specie, MnOx/ACH and Ce(1)Mn/ ACH catalysts were examined by XPS analysis. Surface atomic concentrations of Ce, Mn, C, O are summarized in Table 2 and Table 2. XPS Results of the Different Catalysts surface atomic concentrations (%) O sample

Ce

Mn

C

Oα

Oβ

Oγ

MnOx/ACH Ce(1)Mn/ACH

0 3.29

3.31 1.37

87.19 81.99

4.06 3.84

4.66 7.61

0.78 1.88

the XPS spectra of Mn2p, O1s, and Ce3d are shown in Figure 6. As seen from Figure 6a, the binding energy (BE) of Mn 2p3/2 for the two samples is 641.8 eV. This value is in the range of that reported in the literature, which is 641.7 eV for Mn3O4 and 641.8 eV for Mn2O3.21,22 Therefore, the precise information on Mn specie can not be directly obtained from XPS analysis. It is worth noting that the manganese specie is Mn3O4 determined

Figure 6. XPS spectra of MnOx/ACH and Ce(1)Mn/ACH catalyst. D



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Figure 7. SEM images of (a) MnOx/ACH, (c) Ce(1)Mn/ACH catalysts, and Mn mapping of (b) MnOx/ACH and (d) Ce(1)Mn/ACH catalysts, and (e) Ce mapping of Ce(1)Mn/ACH catalyst.

interactions between the two metal oxides.31 The improved manganese and cerium oxides distribution can undoubtedly improve the NO removal activity of Ce(1)Mn/ACH catalyst. However, it should be noted that the significantly increased SCR activity may be ascribed to the addition of CeO2 to MnOx/ACH rather than the improved dispersion of manganese and cerium oxides, which will be discussed below. 3.7. DRIFTS. Figure 8a shows the DRIFTS spectra of MnOx/ACH and Ce(1)Mn/ACH catalysts exposed to 1000 ppm NO2/Ar after 60 min. As can be seen, the spectra of two samples shows obvious absorption bands at 1628, 1597, 1577, and 1381 cm−1. The bands at 1628, 1597, and 1577 cm−1 are assigned to the adsorbed NO2. The band at 1381 cm−1 is in accordance with the literature32 and assigned to the adsorbed NO3−, which is produced by the disproportionation of chemisorbed NO2 on the catalyst surface. Figure 8b shows the DRIFTS spectra of MnOx/ACH and Ce(1)Mn/ACH catalysts exposed to a mixture of NO and O2. New bands at 1906 and 1845 cm−1 are observed, indicating the weak NO adsorption. Additionally, the bands at 1628, 1597, 1577, and 1381 cm−1 are also observed, suggesting the oxidation of NO to NO2 by gaseous O2 and the formed NO3− on the catalysts

surface. Such results reveal that the NOx species on the surface of MnOx/ACH and Ce(1)Mn/ACH catalysts are mainly absorbed NO2 and NO3−. Compared to MnOx/ACH catalyst, Ce(1)Mn/ACH shows a much stronger NO3− band (1381 cm−1), suggesting more absorbed NO3− present on the surface of Ce(1)Mn/ACH catalyst. According to the XPS results, the presence of Ce3+ on the surface of Ce(1)Mn/ACH catalyst could create a charge imbalance, the vacancies and unsaturated chemical bonds on the catalyst surface, and then more NO could be oxidized to form NO2, resulting in more absorbed NO3− produced on the catalyst surface. Recently, the fast SCR process (4NH3 + 2NO + 2NO2 → 4N2 + 6H2O) has been developed that has faster reaction rate (almost 10 times) and high NOx removal efficiency than the standard SCR (4NH3 + 4NO + O2 → 4N2 + 6H2O).33−37 This implies that converting NO to NO2 is very necessary to realize the fast SCR reaction. More NO2 and NO3− species on the Ce(1)Mn/ACH catalyst surface are reduced very rapidly by NH3 to produce N2, which explains the increased NO conversion of the catalyst containing CeO2. This is also confirmed by the activities of MnOx/ACH and Ce(1)Mn/ACH catalysts shown in Figures 1 and 2. Therefore, the main reason for increased SCR activity of E



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ACKNOWLEDGMENTS

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REFERENCES

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We thank the National Natural Science Foundation of China (51002051, 20806024, 50730003 and 50672025), the Natural Science Foundation of Shanghai City (12ZR1407200), and Fundamental Research Funds for the Central Universities (WA1014016) for financial support.

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Figure 8. In situ DRIFTS spectra of the catalysts exposed to NO2 (a) and NO + O2 (b) at 30 °C.

Ce(1)Mn/ACH catalyst may result from the enhancement of the oxidation of NO to NO2 and more absorbed NO3− produced on the catalyst surface.

4. CONCLUSIONS ACH supported manganese−cerium oxides (MnOx−CeO2/ ACH) catalysts show high activity for the low-temperature SCR of NO with NH3. Nearly 100% NO conversion in a wide temperature range of 80−180 °C and more than 98.4% NO conversion at 200 °C are obtained over the Ce(1)Mn/ACH catalyst. The selectivity to N2 on MnOx/ACH is increased by the addition of CeO2, and Ce(1)Mn/ACH catalyst yields a N2 selectivity of higher than 99.8% at 80−200 °C. The distribution of manganese and cerium oxide on ACH is improved by the addition of CeO2, because there are strong interactions between these two metal oxides. Furthermore, the addition of CeO2 promotes NO oxidation and provides more absorbed NO3− on the catalyst surface of MnOx−CeO2/ACH catalyst, resulting in an enhancement of SCR activity. More in-depth studies are underway to understand the effects of SO2 and/or H2O addition on SCR activity of MnOx−CeO2/ACH.

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