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Low-Temperature Selective Catalytic Reduction of NOx with NH3 over FeMn Mixed-Oxide Catalysts Containing Fe3Mn3O8 Phase Zhihang Chen,†,‡ Furong Wang,† Hua Li,† Qing Yang,† Lefu Wang,† and Xuehui Li*,† †
School of Chemistry and Chemical Engineering, Pulp & Paper Engineering State Key Laboratory of China, South China University of Technology, Guangzhou 510640, People's Republic of China ‡ South China Institute of Environmental Science, Ministry of Environmental Protection, Guangzhou 510655, People's Republic of China ABSTRACT: Novel FeMn mixed-oxide catalysts were prepared for the low-temperature selective catalytic reduction (SCR) of NOx with ammonia in the presence of excess oxygen. It was found that Fe(0.4)MnOx catalyst showed the highest activity, yielding 98.8% NOx conversion and 100% selectivity of N2 at 120 °C at a space velocity of 30 000 h1. XRD results suggested that a new crystal phase of Fe3Mn3O8 was formed in the FeMnOx catalysts. TPR and Raman data showed that there was a strong interaction between the iron oxide and manganese oxide, which is responsible for the formation of the active center ;Fe3Mn3O8. Intensive analysis of fresh, used, and regenerated catalysts by XPS revealed that electron transfer between Fen+ and Mnn+ ions in Fe3Mn3O8 may account for the long lifetime of the Fe(0.4)MnOx catalyst. In addition, the SCR activity was suppressed a little in the presence of SO2 and H2O, but it was reversible after their removal.
GHSV = 30 000 h1), but the activity rapidly decreased to 60% in the presence of SO2 (100 ppm). They also found that mixed oxides with the spinel structure such as CuMn2O4 promoted the activity of low-temperature SCR. Qi et al.6,7 reported that the denitrification efficiency of a MnOxCeO2 catalyst prepared by both the citric acid method and a coprecipitation method was more than 95% below 150 °C (GHSV = 42 000 h1) and its activity changed little in the presence of water and SO2. However, for reported low-temperature SCR catalysts, the stability of catalytic activity, sulfur tolerance, and water resistance should be enhanced through formulation modification, structure adjustment, and the development of novel catalysts. At the same time, it is noteworthy that previous studies on the internal structure of the above catalysts and the mechanism of NO oxidationreduction processes within the crystalline phase are rarely considered. In order to explore the most active catalyst, our primary investigation has prepared 120 kinds of mixed-oxide catalysts by the solid phase method through the combination of 16 potential metals, viz., Mg, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Zr, Mo, Ba, W, or Bi in pairs, and their activities for lowtemperature SCR of NOx in the presence of O2 were evaluated. It was found that CrMnOx and FeMnOx mixed-oxide catalysts showed potential low temperature SCR activity. The CrMnOx catalyst with CrMn1.5O4 active phase has been established in our previous work,8 and the formation of Fe3Mn3O8 phase in FeMnOx catalyst was also discussed in a previous paper.9 In this work, a systematic investigation of the physicochemical properties of the FeMnOx catalyst was carried out and the inclusion of a mixed-oxide phase, Fe3Mn3O8, was found to play a key role for higher activity of low-temperature SCR for the first
1. INTRODUCTION Acid pollution in the atmosphere mainly originates from emissions of SO2 and NOx (nitrogen oxides). The major source of nitrogen oxides is the combustion of fossil fuels such as coal in electrical power plants or of petroleum in the engines of vehicles, accounting for 46 and 49% of the total emission quantity, respectively.1 Selective catalytic reduction (SCR) is the most widely used, mature denitration technology employed in power stations, where V2O5WO3(MO3)/TiO2 catalysts have been successfully employed and operate in the temperature range 300400 °C. However, there are still some disadvantages of traditional commercial catalysts.2,3 For example, the SCR device must be located upstream of units for particle removal and/or desulfurization to avoid reheating the flue gas, which makes the catalyst susceptible to deactivation or shortens the service life owing to the high concentrations of dust and SO2. To solve these problems, the SCR device must be placed downstream of the electrostatic precipitator and/or desulfurization units, but the activity of traditional SCR catalysts decreases at lower temperature. Hence, the synthesis of active catalysts for low-temperature SCR (700 °C, which can be interpreted as the reduction of Fe2O3 to Fe3O4 (362 °C), Fe3O4 to FeO (638 °C), and FeO to elemental Fe (above 700 °C).2629 MnOx shows two stages of reduction at 187 and 439 °C, representing the reduction of Mn2O3 to Mn3O4 and then to MnO, respectively.30 The TPR curve of a physical mixture of FeOx and MnOx shows peaks at 310, 368, and 416 °C, representing the reduction of Mn2O3 to Mn3O4 (310 °C),31 Fe2O3 to Fe3O4 (368 °C), and Mn3O4 to MnO (416 °C), respectively. The reduction processes of this mixture can be expressed as the reduction of Fe2O3 and Mn2O3 providing that it is mainly composed of Fe2O3 and Mn2O3 as detected by the XRD characterization (Figure 2c). Reduction peaks can be interpreted as the reduction of high valence oxidation states as M2O3 to M3O4 at 310 °C, M3O4 to MO at 368 and 416 °C (M = Mn or Fe), and the reduction of FeO (596 °C). The Fe3Mn3O8 generated in this catalyst makes its reduction behavior much more complex. Further investigation of the reduction of FeMnOx(CA) catalysts shows that they are quite different from those of pure FeOx and MnOx. Both Fe(0.4) MnOx(CA-500) and Fe(0.5)MnOx(CA-500) showed the same reduction behavior with a sharp reduction peak around 285 °C and a wide peak beginning around 320 °C and extending to around
Figure 8. Raman spectra of FeMnOx series catalysts: (a) FeOx; (b): MnOx; (c) Fe(0.1)MnOx(CA-500); (d) Fe(0.2)MnOx(CA-500); (e) Fe(0.3)MnOx(CA-500); (f) Fe(0.4)MnOx(CA-500); (g) Fe(0.5)MnOx(CA-500).
Table 2. Raman Shift Data of Oxide Catalysts entry
catalyst
Raman shift (cm1)
1
FeOx(CA-500)
216.5, 283.0, 396.7, 599.9
2 3
MnOx(CA-500) Fe(0.1)MnOx(CA-500)
642.7 652.6
4
Fe(0.2)MnOx(CA-500)
644.8
5
Fe(0.3)MnOx(CA-500)
645.8
6
Fe(0.4)MnOx(CA-500)
653.9
7
Fe(0.5)MnOx(CA-500)
658.6
3.4. Analysis of Raman Spectra. To further understand the structure of FeMn mixed oxides, Raman spectra were recorded to examine the surface characteristics of the FeMnOx series catalysts (Figure 8), and their detailed Raman shifts are listed in Table 2. The spectrum of FeOx shows four Raman bands with peaks at 216.5, 283.0, 396.7, and 599.9 cm1 (Figure 8a), attributed to α-Fe2O3.1619 For MnOx, the broad Raman shift peak at 642.7 cm1 (Figure 8b) is the characteristic Raman band of Mn3O4, which represents the single MnO bond vibration.2022 Although the characteristic Raman band of the MnO bond in MnO lies at 537 cm1,23 it may not be detected by this technique as MnO can be easily transformed to Mn3O4 under a laser beam.24 The results are consistent with similarly prepared pure iron oxide and manganese oxide detected by XRD characterization in Figure 2a and 2b, respectively. It can be seen from Figure 8 and Table 2 that the Raman peak around 650 cm1 increases in intensity with increasing Fe content. Hence, these bands can be assigned to the Raman shift of FeO and MnO bonds in the form of FeOMn as the Fe3Mn3O8 phase increases with the increase in Fe content 208
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Figure 9. Crystalline structures of Mn3O4, MnO, Fe2O3, Fe3Mn3O8, and its unit cell: (a) Mn3O4; (b) MnO; (c) Fe2O3; (d) Fe3Mn3O8; (e) unit cell of Fe3Mn3O8.
Figure 11. Oxidation activity of NO to NO2 on FeMnOx catalysts. Reaction conditions: [NO] = 1000 ppm, [O2] = 3%, N2 as balance, and GHSV = 30 000 h1.
catalysts doped with various ratios of Fe/(Fe + Mn) at temperatures from 80 to 160 °C are shown in Figure 11. With the increase of iron content in FeMnOx catalysts, the NO oxidation activity improved gradually; however, when the molar ratio of Fe to the total of Fe and Mn reached 0.5, it declined. The NO oxidation activity over Fe(0.4)MnOx(CA-500) and Fe(0.5)MnOx(CA-500) is relatively high, which may be attributed to the Fe3Mn3O8 phase. Also, the promotional effect of the Fe3Mn3O8 phase could be interpreted in terms of NO oxidation to NO2 occurring easily at low temperatures. It is known that ammonium nitrite decomposes quickly into N2 (majority) and NO (minority) below 100 °C. The formation of NH4NO2 from NO, O2, and NH3 requires oxidation of NO to NO2 by O2.13 Koebel et al.33 and Long and Yang34 reported that a highly active catalyst for the oxidation of NO to NO2 at low temperature also gave enhanced SCR activities. The addition of Fe and formation of Fe3Mn3O8 here are sure to increase the activity for NO oxidation to NO2 and thereby enhance the SCR activity. 3.7. XPS Analysis and a Possible Redox Reaction over Fe3Mn3O8. In order to find out the surface chemical states of the most active catalyst, the XPS spectra and XPS measurements of Mn 2p3/2, Fe 2p3/2, and O 1s in fresh, used (quenched from 500 h operation), and regenerated Fe(0.4)MnOx(CA-500) catalysts (operated for 500 h and then treated by plasma) were obtained, and these are shown in Figure 12. Two main peaks due to Mn 2p3/2 (at 636 eV) and Mn 2p1/2 (at 660 eV) were observed (Figure 12A). By performing a peak fitting deconvolution, the Mn 2p3/2 spectra can be separated into three characteristic peaks:
Figure 10. H2-TPR profiles of catalyst samples: (a) FeOx; (b) MnO;x (c) FeOxMnOx; (d) Fe(0.4)MnOx(CA-500); (e) Fe(0.5)MnOx(CA-500); (f) Fe(0.4)MnOx(CP-500).
520 °C without the presence of reduction of FeO at elevated temperature. This behavior is similar to the reduction of Fe2.40Mn0.53O4.32 The reduction peak at 280 °C is attributed to the reduction of the high oxidation states of Fe3+ and Mn4+/Mn3+ to M2+ and then the reduction of M2+ beginning around 320 °C (M = Mn or Fe). It was established in the literature that a shift in the peak position of the reduction temperature can be attributed to many factors, such as a change in particle size, lattice oxygen mobility, phase composition, and structural defects.31 Due to the difference in the particle size (Figure 5, Table 1), the reduction behavior of Fe(0.4)MnOx(CP-500) has some differences from Fe(0.4) MnOx(CA-500), with the higher and lagged reduction temperature also confirming its well-organized lattice structure as discussed above (Figure 5g). 3.6. Oxidation Activities of NO to NO2 over FeMnOx Mixed Oxides. From the above results, the most active catalyst for low-temperature NOx reduction with NH3 comprises the Fe3Mn3O8 phase with suitable surface properties. The in-depth investigation of the oxidation activities of NO to NO2 on 209
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Figure 12. XPS spectra for (A) Mn 2p, (B) Fe 2p, and (C) O 1s of Fe(0.4)MnOx(CA-500) catalyst. (a) Fresh catalyst; (b) used catalyst; (c) regenerated catalyst.
Table 3. Atom Percentage of Fe(0.4)MnOx(CA-500) Catalyst Determined by XPS catalyst
Fe (%)
Mn (%)
O (%)
Table 4. BE (eV) of Core Electrons of Fe(0.4)MnOx(CA-500) Catalystsa
Fe/(Fe + Mn)
BE (percent of valence state, %)
fresh catalyst
21.9
37.3
40.8
0.35
XPS
element
used catalyst
25.6
41.1
33.3
0.38
spectra
valence
regenerated catalyst
23.7
37.2
39.1
0.38
Fe 2p
640.4640.7, 641.8642.0, and 644.0644.3 eV, which is consistent with the characteristic peaks of Mn2+, Mn3+, and Mn4+, respectively.35 Binding energies of Mn 2p3/2 in the Fe(0.4)MnOx(CA-500) catalyst measured here (640645 eV) were slightly higher than those of MnO, Mn2O3, and MnO2 reported elsewhere,36 which shows a nice distinction in the change in chemical environment between the Fe3Mn3O8 phase and MnOx alone, and also fits well with the result discussed above. For Fe 2p XPS in the crystalline phase of Fe3Mn3O8 (Figure 12 B), the characteristic peak of Fe 2p3/2 is located about 710.4 eV and that of Fe 2p1/2 is located at 724.0 eV. The Fe 2p3/2 peak is separated into two peaks by the same peak fitting deconvolution technique. The peaks at 709.6710.1 and 710.8712.4 eV can be assigned to Fe2+ and Fe3+,37,38 respectively. As shown in Figure 12 C, the asymmetric peak is observed in the XPS spectrum of O 1s for fresh, used, and regenerated samples. The peak at 530.7531.2 eV corresponds to O 1s states assigned to surface-adsorbed oxygen such as O22 or O, in the form of hydroxyl OH and carbonate CO32.39,40 The distribution of the elements on the surface obtained by XPS characterization is shown in Table 3. After 500 h, the relative content of Fe and Mn increased, and the O content reduced accordingly on the surface of Fe(0.4)MnOx(CA-500), which may have resulted from the removal of adsorbed oxygen or partial consumption of lattice oxygen involved in the catalytic reaction. It should be noted that the value of Fe/(Fe + Mn) on the surface of both used and regenerated catalysts is larger than that for fresh catalyst, which implies that iron oxide tends to migrate to the surface. It is well-known that Mn is the original active center for this catalyst;
Mn 2p
O 1s a
regenerated fresh catalyst
used catalyst
catalyst
Fe2+
709.9 (40.6)
709.7 (50.7)
710.3 (39.6)
Fe3+
712.2 (59.4)
712.3 (49.3)
712.4 (60.4)
Mn2+
640.6 (39.2)
640.7 (38.6)
640.7 (38.7)
Mn3+
642.0 (33.0)
641.9 (27.9)
641.8 (35.3)
Mn4+
644.3 (27.8)
644.0 (33.5)
644.3 (26.0)
O2
529.5 (53.0)
529.7 (51.8)
529.6 (56.6)
OH/CO32
530.7 (47.0)
531.0 (48.2)
531.2 (43.4)
Surface concentrations of different Mn, Fe, and O states are in parentheses.
hence, the decrease in surface Mn content during the SCR process means a decrease in its SCR activity. However, the above experimental results show that the activity of the Fe(0.4)MnOx (CA-500) catalyst and the distribution of Fe, Mn, and O over the surface can be recovered almost to the original level of fresh catalyst after 6 h of normal temperature and normal pressure plasma oxidation reduction activation treatment, although the value of Fe/(Fe + Mn) on the surface cannot be recovered. Hence, the distribution of metal ions in Fe(0.4)MnOx(CA-500) (Table 4) is analyzed in detail. After 500 h of SCR reaction, the concentrations of Fe3+ and Mn3+ decreased by 10.1 and 5.1%, respectively. On the other hand, the concentrations of the higher valence manganese state (Mn4+) and the lower valence iron state (Fe2+) increased by 10.1% and 5.7%, while the lower valence manganese state (Mn2+) changed little. Comparing the change between the increase and the consumption of Fe and Mn between fresh and used catalysts, it is noted that the trend does not accord with the electronic balance for individual Fen+ and Mnn+ ions in the process of oxidation and reduction. For example 2Mn3þ T Mn4þ þ Mn2þ 210
ð4Þ
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obtained over the Fe(0.4)MnOx(CA-500) catalyst, prepared by the citric acid method at 120 °C, under flow conditions of GHSV = 30 000 h1. From the XRD patterns, the spinel Fe3Mn3O8 was found to be present in active FeMnOx mixed-oxide catalysts. The TPR profiles revealed for the first time the reduction process of Fe3Mn3O8, and the existence of the Fe3Mn3O8 phase clearly depressed the reduction temperature of manganese oxides. The Raman peak at around 650 cm1 can be assigned to the characteristic Raman bands of the Fe3Mn3O8 lattice phase, a new phase to the best of our knowledge. Better oxidation of NO could be ascribed to the formation of Fe3Mn3O8, with confirmation of electron transfer between iron and manganese by XPS, which is also the reason that Fe(0.4) MnOx(CA-500) continues to maintain excellent SCR activity at low temperature. The addition of SO2 and H2O in the feeder gas could exert an adverse effect on the NOx conversion, but the Fe(0.4)MnOx(CA-500) catalyst showed a certain capability of sulfur tolerance and water resistance as this effect can be eliminated quickly after their removal. The redox catalytic cycles of the NO oxidation reactions over the Fe3Mn3O8 lattice is also proposed, presumably due to the existence of high redox potential pairs of Fe and Mn in the structure. Intensive investigation about the adsorption behaviors of reactant molecules (NO, NH3, and O2) and the effects of SO2 and H2O is helpful for the understanding of real surface catalysis, and are all under study.
Scheme 1. Redox Catalytic Cycles of the NO Oxidation Reactions over Fe3Mn3O8 Lattice
Therefore, we believe that another redox reaction may take place between Fe and Mn ions leading to certain electronic transfer processes. Normal temperature and pressure plasma treatment of these catalysts has the advantage of not changing the structure and morphological properties;41 hence it is employed to activate the Fe(0.4)MnOx(CA-500) catalyst, and it is thus possible to have the surface concentrations of both Mn3+ and Fe3+ recovered, giving rise to the recovery of the activity of Fe(0.4)MnOx(CA-500). From the XPS analysis (Figure 12, Tables 3 and 4), the concentrations of various ions change after plasma oxidationreduction activation. The concentrations of Fe3+ and Mn3+ increase by 11.1 and 7.4%, while Fe2+ and Mn4+ concentrations decrease significantly and Mn2+ concentration remains unchanged compared to the used catalyst. The electronic transfer plays a quite important role in the oxidation of NO to NO2 during the selective catalytic reduction of NOx. Table 4 also shows the change of the percentage contents of Fen+ and Mnn+ ions with different valences in the fresh, used, and regenerated Fe(0.4)MnOx(CA-500) catalysts. The concentrations of Fe3+ and Mn3+ are relatively high over the fresh catalyst and decrease by 25.2% after SCR reaction. Meanwhile, the concentrations of Fe2+ and Mn4+ increase relatively. However, after 6 h of plasma oxidationreduction activation, as the concentrations of Fe3+ and Mn3+ increase significantly those of of Fe2+ and Mn4+ clearly decrease. Hence, it can be deduced that electronic transfer takes place between Fen+ and Mnn+ with different oxidation states as shown in eq 5 and then possibly follows the mechanism illustrated by eqs 68. The redox catalytic cycles of the NO oxidation reactions over Fe3Mn3O8 lattice are also proposed for the first time and are depicted in Scheme 1. Fe3þ þ Mn3þ T Fe2þ þ Mn4þ
ð5Þ
NO þ Mn4þ f NOads þ þ Mn3þ
ð6Þ
1 O2 þ Fe2þ f Fe3þ þ Oads 2
ð7Þ
NOads þ þ Oads f NO2
ð8Þ
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Tel.: 0086 20 8711 4707. Fax: 0086 20 8711 4707.
’ ACKNOWLEDGMENT The National Natural Science Foundation of China (20876063, 21176088), the Natural Science Foundation of Guangdong province (S2011020001472), and the Fundamental Research Funds for the Central Universities (SCUT) are gratefully acknowledged for the financial support of this work. ’ REFERENCES (1) Schneider, H.; Scharf, U.; Wokaun, A.; Baiker, A. Chromia on Titania: IV. Nature of Active Sites for Selective Catalytic Reduction of NO by NH3. J. Catal. 1994, 146, 545. (2) Heck, R. M. Catalytic Abatement of Nitrogen Oxides;Stationary Applications. Catal. Today 1999, 53, 519. (3) Nova, I.; Lietti, L.; Casagrande, L.; Dall’Acqua, L.; Giamello, E.; Forzatti, P. Characterization and Reactivity of TiO2-Supported MoO3 De-Nox SCR Catalysts. Appl. Catal., B 1998, 17, 245. (4) Szoczynski, J.; Janas, J.; Machej, T.; Rynkowski, J.; Stoch, J. Catalytic Activity of Chromium Spinels in SCR of NO with NH3. Appl. Catal., B 2000, 24, 45. (5) Kang, M.; Park, E.; Kim, J.; Yie, J. E. Cu-Mn Mixed Oxides for Low Temperature NO Reduction with NH3. Catal. Today 2006, 111, 236. (6) Qi, G.; Yang, R. T. Performance and Kinetics Study for LowTemperature SCR of NO with NH3 over MnOx-CeO2 Catalyst. J. Catal. 2003, 217, 434. (7) Qi, G.; Yang, R. T.; Chang, R. MnOx-CeO2 Mixed Oxides Prepared by Co-precipitation for Selective Catalytic Reduction of NO with NH3 at Low Temperatures. Appl. Catal., B 2004, 51, 93. (8) Chen, Z.; Qing, Y.; Li, H.; Li, X.; Wang, L.; Tsang, S. C. Cr-MnOx Mixed-Oxide Catalysts for Selective Catalytic Reduction of NOx with NH3 at Low Temperature. J. Catal. 2010, 276, 56.
4. CONCLUSION A series of FeMn mixed oxides with high activity for the lowtemperature SCR of NOx with ammonia in the presence of oxygen were developed. Nearly 100% of NOx conversion was 211
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