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Low temperature selective catalytic reduction of NOx with NH3 over amorphous MnOx catalysts prepared by three methods
Catalysis Communications 8 (2007) 329–334 www.elsevier.com/locate/catcom
Low temperature selective catalytic reduction of NOx with NH3 over amorphous MnOx catalysts prepared by three methods Xiaolong Tang a
a,b
, Jiming Hao
a,*
, Wenguo Xu b, Junhua Li
a
Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China b The Institute for Chemical Physics, Beijing Institute of Technology, Beijing 100081, China Received 9 March 2006; received in revised form 9 June 2006; accepted 12 June 2006 Available online 4 July 2006
Abstract Unsupported manganese oxide catalysts with amorphous phase were prepared by three methods, and their activities for SCR of NOx with ammonia were investigated in the presence of O2. The results showed the catalysts have superior low temperature activity, and the NOx conversion is about 98% at 80 °C, and nearly 100% NOx conversion between 100 and 150 °C. Due to competing adsorption with the reactant, H2O has slight impact on the activity. The activity was suppressed with coexisting of SO2, however the deactivation of SO2 is reversible. The excellent low temperature catalytic activity of amorphous MnOx catalysts is mainly due to their amorphous phase and high specific areas. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Selective catalytic reduction; Low temperature; NO; Ammonia; MnOx
1. Introduction Selective catalytic reduction (SCR) of NOx with NH3 is the most common technique to remove NOx in the exhaust gas from stationary sources. In power plant, however, a drawback inherent to the use of SCR is the SO2 and dust present in the flue gas. A new efficient catalyst for low temperature SCR is greatly needed. This catalyst could be placed downstream of the dust catcher and desulfurizer without preheating the flue gas. In recent research, Mnbased catalysts show good activity during low temperature NO reduction with NH3. Examples of these catalysts are MnOx/Al2O3 [1,2], MnOx/TiO2 [3,4], MnOx/NaY [5], MnOx/AC [6–8], and MnOx/USY [9]. The SCR activity of different pure manganese oxides also was investigated [10]. Recently, some unsupported Mn-based catalysts with high activity at low temperature were reported such as MnOx–CeO2 [11] and Cu–Mn mixed oxides [12]. In this
*
Corresponding author. Tel.: +86 10 62782195; fax: +86 10 62773650. E-mail address: [email protected] (J. Hao).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.025
work, a series of unsupported amorphous MnOx catalysts were prepared by three methods. These catalysts present excellent catalytic activities in the temperature range of 80–150 °C. 2. Experimental 2.1. Catalysts preparation 2.1.1. Rheological phase reaction method (RP) Manganese acetate (MnAC) and salicylic acid (SA) were mixed in the desired proportion (mole ratio, MnAC:SA = 1:2.05). The mixture was thoroughly ground for 40 min, and then de-ionized water was stirred into the mixture until it turned into a rheologic phase [13]. This was placed into a stainless steel pressure reactor with a Teflon inner liner, sealed and put in a baking oven at 70 °C for 48 h. The product was washed 3–4 times with de-ionized water and 2–3 times with ethanol, then dried in vacuum drier at 60 °C, resulting in the catalyst precursor consisting of a white fine powder. The MnOx catalysts were obtained by calcining the precursor in the air at 350–650 °C for 6 h.
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X. Tang et al. / Catalysis Communications 8 (2007) 329–334
2.1.2. Low temperature solid phase reaction method (SP) The MnAC and potassium permanganate (KMnO4) were mixed in the desired proportion (mole ratio, MnAC: KMnO4 = 2:3), and ground fully for 40 min. The mixture was placed in the baking oven at 70 °C for 48 h. The product was washed 3–4 times with de-ionized water and 2–3 times with ethanol, then dried in vacuum drier at 60 °C. 2.1.3. Co-precipitation method (CP) Three solutions were prepared. In order to make sure that the solute was dissolved completely, the solutions could be prepared in a water bath at 40 °C and stirred well.
NOx conversion ¼
½NOx inlet ½NOx outlet 100%; ½NOx inlet where NOx represents NO þ NO2 :
All catalysts activity tests were carried out at low temperature (<150 °C). In order to avoid the impact of gas adsorption on the catalyst samples, we recorded the test data after the reactions had been kept in stable states for 1 h. Further, prior to the activity tests, the catalyst sample was pre-treated in a flow of 3% O2/N2 at 200 °C for 1 h and then cooled to room temperature. 3. Results and discussion
Solution I: 18.4 g MnAC, 500 mL H2O; Solution II: 2–3 g Polyethylene glycol (PEG400), 100 mL H2O; Solution III: 7.9 g KMnO4, 300 mL H2O. Solution II was added to solution I and mixed well, and then Solution III was gradually added with thorough stirring. The mixtures were stirred steadily for 6 h at room temperature and then filtered and washed with de-ionized water 3–4 times. The resulting solid product was dried in a vacuum drier at 100 °C for 6 h. In this work we adopted three methods to prepare unsupported amorphous MnOx catalysts, which were designated as MnOx (RP), MnOx (SP) and MnOx (CP), respectively. For comparison, MnOx (CA) was also prepared by the citric acid method [11]. Finally, all catalyst samples were crushed and sieved to a 40–60 mesh size. 2.2. Catalyst characterization The BET surface area, pore volume and pore size of catalyst samples were measured by N2 adsorption using a NOVA 4000 automated gas sorption system. The powder X-ray diffraction (XRD) measurement was carried out on a Rigaku D/max-IIIA system with Cu Ka radiation. The microscopic pattern of catalyst particles was observed with JEM 2010 High-resolution transmission electron microscopy and JSM 7401 F field emission scanning electron microscope. 2.3. Catalysts activity measurement Catalytic activity tests were performed in a quartz tube reactor of 9 mm internal diameter. NOx, O2 and SO2 concentration were simultaneously measured by a KM 9106 flue gas analyzer. At steady state, a gas N2 mixture containing 500 ppm NO, 500 ppm NH3, 3% O2, 100 ppm SO2 (when used) and 2.8–20% H2O (when used) was introduced into the reactor. The water vapor was adding by a WZ50C2 injection pump and an evaporator. In all the tests, the total flow rate was fixed at 300 cm3 min1, which corresponded to a GHSV (gas hourly space velocity) of 47,000 h1. NOx conversion was obtained by the following equation:
3.1. Characterizations of catalysts The BET surface area, pore volume and pore-size of different MnOx catalysts are summarized in Table 1. From Table 1 we can see that the surface area and the pore volume of MnOx (SP) are the greatest of all the samples. MnOx (CP) and MnOx (RP-350) have similar surface areas, but the pore volume and pore size of MnOx (CP) are larger than others. Furthermore, the surface area of MnOx (RP) was greatly affected by the calcination temperature, and decreased with the calcination temperature rising. The MnOx catalyst prepared by citric acid has the lowest surface area (19 m2 g1). The pore diameter of all catalysts were between 9.55 and 11.54 nm. 3.2. XRD results The calcination temperature has great effect on the characteristics of the catalysts (surface area, etc.), and also affects on the crystallinity and oxidation states. Fig. 1 shows the effect of the calcination temperature on the MnOx (RP) catalysts. The peaks of patterns from the catalysts calcined at low temperature become broad, and the intensity of the peaks are comparatively weak. This is due to the poor crystallinity of the catalysts, and the amorphous phase may formed in part of catalyst particles. According to the XRD patterns, it is clear that the crystallinity of the catalysts increased when the calcination temperature rose from 350 °C to 650 °C. Also we notice that the oxidation states of the catalysts were changed with Table 1 Characterization of the MnOx catalysts prepared by different methods Samplesa
BET surface area (m2 g1)
Pore volume (cm3 g1)
Average pore diameter (nm)
MnOx MnOx MnOx MnOx MnOx MnOx MnOx MnOx
99 73 46 28 150 96 19 27
0.212 0.186 0.128 0.074 0.365 0.278 0.072 0.071
9.55 9.61 10.74 11.26 9.69 11.54 10.63 10.45
a
(RP-350) (RP-450) (RP-550) (RP-650) (SP) (CP) (CA) (CA) [11]
The number in the bracket is the calcination temperature.
X. Tang et al. / Catalysis Communications 8 (2007) 329–334
ο
36.12 32.34 59.84
28.9 17.96
10
ο
550 C
Mn3O4 Mn2O3 Intensity (a.u.)
Intensity (a.u.)
650 C
20
44.44 50.74
30
331
40
64.66
50
60
70
80
10
20
30
40
2 ()
50
60
70
80
50
60
70
80
2 ()
ο
ο
350 C
Intensity (a.u.)
Intensity (a.u.)
450 C
58
10
20
30
40
50
60
70
80
10
20
30
40 2 ()
2 ()
Fig. 1. XRD patterns of MnOx (RP) catalysts calcined at different temperatures.
Mn2O3 a: MnOx(CA) b: MnOx(RP) c: MnOx(SP) d: MnOx(CP)
Intensity (a.u.)
the temperature increasing. Though the diffraction peaks of XRD profile of MnOx (RP-350) are weak, we can identify that most of them are attributed to Mn3O4 (PDF card 240734). When the calcination temperature rose up to 450 °C, the oxidation state of MnOx (RP-450) changed, most diffraction peaks are belong to Mn2O3 (PDF card 41-1442) except two small weak peaks, which could be attributed to Mn3O4. Then the MnOx (RP-450) could be considered as mixed oxides of Mn2O3 and Mn3O4, but main is Mn2O3. The diffraction peaks of the patterns of MnOx (RP-650) and MnOx (RP-550) are sharper than others of MnOx (RP-450) and MnOx (RP-350). As marked in the Fig. 1, the main oxidation state of MnOx (RP-650) and MnOx (RP-550) are Mn3O4 and Mn2O3 respectively. Then the catalysts oxidation states alternated as followed sequence with the calcination temperature increasing: Mn3O4 ! Mn2O3 ! Mn3O4. Fig. 2 shows the XRD patterns of different MnOx catalysts. The MnOx (RP) was calcined at 350 °C. As shown in curve a, which is the most different from the others, sharp peaks representing Mn2O3 were observed in the MnOx (CA) catalyst. The XRD peaks in curve b become broad, and the intensity of the peaks, which seem to be mixed oxides of Mn3O4 and Mn2O3, greatly decreased. The width and intensity of diffraction peaks of curve c are similar to b, whereas the locations of diffraction peaks are different, and three main peaks were listed in Table 2.
a b c d
20
40
60
80
2 Fig. 2. XRD patterns of MnOx catalysts prepared by different methods.
Table 2 The XRD data of MnOx (SP) catalyst Peak
2h (°)
PDF number
2h (°)
Crystal
I
18.024
44-0141 24-0734 39-1218 39-1218
18.122 18.014 17.937 18.126
MnO2 Mn3O4 Mn5O8 Mn5O8
II
37.512
44-0141
37.533
MnO2
III
59.763
24-0734
59.893
Mn3O4
332
X. Tang et al. / Catalysis Communications 8 (2007) 329–334
As shown in Fig. 2, the sample peak I and II match the characteristic diffraction peaks of MnO2 (PDF card 440141) well, so it means that the MnOx (SP) catalyst must has MnO2. Furthermore, the peak I is broadened greatly, so we also consider that it may be a superposition peak, which formed by several adjacent overlapped diffraction peaks. Besides the characteristic peak of MnO2 (18.122), it may also contain the characteristic peaks of Mn3O4 (18.014) and Mn5O8 (17.937, 18.126). Analogously, the sample peak III may contain the character peaks of Mn3O4 (59.893). Then, it is reasonable to believe that the MnOx (SP) catalyst may have other oxidation states, Mn3O4 and Mn5O8. The XRD pattern of MnOx (CP) (curve d) is quite different to the others. Its diffraction peaks become very broad, almost like a straight line. Therefore, it is difficult to identify the phase and oxidation state from this curve, but we are sure that its crystallinity must be very poor and its particles turn into amorphous phase. Compared with MnOx (CA), the diffraction peaks of XRD patterns of MnOx (RP), MnOx (SP) and MnOx (CP) are broad and weak obviously, which implies that
part of the last three catalysts (other than MnOx (CA)) turned into an amorphous phase. 3.3. Catalysts TEM photos In order to obtain more details about the microstructure of catalysts, we analyzed these samples with a transmission electron microscope. The TEM photos were shown in Fig. 3. The catalyst particles of MnOx (CA) have comparatively regular shape (photo A), and the particle sizes are mainly in the range of 40–60 nm. From photo B, we can see that the particles of MnOx (RP350) have no regular shape, and the particle size distribution is uneven. The particles of MnOx (SP) catalyst are shown in photo c, and it is clear that there are mainly two types of particles. One is the club-shaped particles (30–60 nm) with the better crystallinity, and the other is the floc, which is the aggregation of fine particles (<10 nm). In photo d, there is almost no regular shape to the MnOx (CP), but their sizes distribution is more even than those seen in the other photos, mainly in the range of 10–20 nm.
Fig. 3. The TEM photos of MnOx catalysts: (A) MnOx (CA); (B) MnOx (RP-350); (C) MnOx (SP) and (D) MnOx (CP).
X. Tang et al. / Catalysis Communications 8 (2007) 329–334
3.4. Activity test The de-NOx activities over the MnOx (RP) catalysts are listed in Table 3. It is clear that the MnOx (RP) calcined at low temperature can perform more efficiently. (We also tested the catalyst calcined at 250 °C, but its activity is poor, which may have been caused by the catalyst precursor decomposing incompletely.) We chose MnOx (RP-350) to compare with the other catalysts in the following tests. Kijlstra et al reported that the catalytic activity and product selectivity of manganese oxide catalysts are mainly affected by the oxidation state and crystallinity [1]. One noticeable thing is that the catalysts in his work were prepared with high calcination temperature, so the crystallinity of those catalysts must be well. However, in this work, it is difficult to consider the effect of oxidation states to activity. As listed in Table 3, the catalytic activity of MnOx (RP) is decreased as followed sequence: MnOx (RP-350) > MnOx (RP-450) > MnOx (RP-550) > MnOx (RP-650), while their main oxidation states turned as followed sequence: Mn3O4 ! Mn2O3 + Mn3O4 ! Mn2O3 ! Mn3O4. Therefore, we can not appraise the effect of oxidation states to the catalytic activity on the catalysts with poor crystallinity. But these data imply that there may be certain relation between the activity and the crystallinity of MnOx catalysts. Fig. 4 presents the NOx conversions over MnOx catalysts. It is obvious that the catalytic activity of amorphous catalysts, which include MnOx (SP), MnOx (RP-350) and MnOx (CP), are much better than the activity of MnOx Table 3 The de-NOx activity over MnOx (RP) catalysts calcined at different temperatures Samples
MnOx MnOx MnOx MnOx
(RP-350) (RP-450) (RP-550) (RP-650)
NOx conversion (%) 50 °C
80 °C
100 °C
120 °C
150 °C
59 26 20 7
98 76 54 38
100 95 77 62
100 98 91 87
100 100 99 97
(CA). More than 98% NOx conversion could be achieved at 80 °C over the amorphous catalysts, and almost all of the NOx can be converted in the range of 80–150 °C. In comparing the amorphous catalysts with the MnOx (CA), two obvious differences are the specific area and crystallinity, which may be the reasons why the amorphous catalysts have such superior low temperature catalytic activity. In the study of MnOx/NaY [5], the author believed that its substantial low temperature activity is due to the eggshell manganese oxides in which amorphous particles were found. Therefore, we speculate that the excellent low temperature activity of amorphous catalysts is mainly due to the high specific area and the amorphous phase. Coincidently, a recent study of MnOx catalyst prepared by a precipitation method reported similar conclusions to ours [14]. We notice that at the low temperature range of 50– 80 °C, the activity decrease order of the amorphous catalysts is: MnOx (CP) > MnOx (RP-350) > MnOx (SP), but their BET specific areas decrease according to the sequence: MnOx (SP) > MnOx (RP-350) MnOx (CP). Crystallinity diminishes in the following order: MnOx (SP) > MnOx (RP-350) > MnOx (CP). Among the three catalysts, although the BET specific area of MnOx (SP) is the highest, its low temperature activity is the lowest. Zhang et al. reported that the amorphous phase is favorable to the inserting and releasing of proton, so the chemical adsorption/desorption and redox reaction can be carried on rapidly in the body phase or surface of the catalyst particles [15], and which will promote the SCR reaction. The above discussions lead us to believe that the amorphous phase of the catalysts may have more impact on the low temperature activity than the specific area. Furthermore, we also analyzed the tail gas contents by Gas Chromatography (GC-17, Shimadzu), which equipped ˚ molecular sieve column for PQ column for N2O and 5 A N2. The results show that the product selectivity of N2 is more than 96% at 80 °C, and the concentration of N2O is less than 10 ppm.
100 80
N O X Conversion (%)
The MnOx (CP), MnOx (RP-350) and MnOx (SP) catalysts were also analyzed by electron diffraction, which can help us to confirm whether these catalysts have an amorphous phase. On the MnOx (RP-350) catalyst, the diffraction halation was observed on part of particles, which indicates that they are amorphous. Similar results were also observed on the floc particles of the MnOx (SP) catalyst. Especially to the MnOx (CP) catalyst, the diffraction halation was observed on almost all particles. The results of XRD and ED (Electron Diffraction) analysis permit us to confirm that the MnOx (SP) and MnOx (RP-350) catalysts turn into an amorphous phase, and this is especially true for the MnOx (CP) catalyst. In other words, the crystallinity of the catalysts decreased in the following sequence: MnOx (CA) > MnOx (SP) > MnOx (RP350) > and MnOx (CP).
333
60 MnOX(RP) MnOX(SP)
40
MnOX(CP) MnOX(CA)
20 0 40
60
80
100
120
140
160
Temperature / C Fig. 4. NOx conversion over four MnOx catalysts at different temperatures. Reaction conditions: 0.5 g samples, 500 ppm NO, 500 ppm NH3, 3% O2, N2 to balance, GHSV = 47,000 h1.
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X. Tang et al. / Catalysis Communications 8 (2007) 329–334
4. Conclusions
NO X Conversion (%)
100 90 Heating treatment
80 70 add
stop
MnOX (CP)
60
MnOX (SP)
50 0
1
2
3
4
5
6
7
8
9
10
Amorphous MnOx catalysts prepared by three methods showed excellent activity for the selective catalytic reduction of NOx with NH3 in the presence of excess O2 at low temperatures. Amorphous phase and a high specific area of manganese oxides are probably the main reasons for these catalysts with high de-NOx activities at low temperature. The amorphous phase is the more important. The inhibition of activity over the amorphous MnOx catalysts by SO2 and H2O is due to their competing adsorption, and the activity recovers to the initial level after removing H2O and SO2 gases.
Time / h Fig. 5. Effect of SO2 and H2O on NOx conversion over MnOx (CP) and MnOx (SP). Reaction conditions: 0.5 g sample, 80 °C, 500 ppm NO, 500 ppm NH3, 3% O2, 10% H2O (when used), 100 ppm SO2 (when used), N2 to balance, GHSV = 47,000 h1 (dotted line: only added 10% H2O; solid line: added 10% H2O + 100 ppm SO2).
Acknowledgement This work was supported by the Natural Science Foundation of China (20437010 and 20507012). References
3.5. Effect of SO2 and H2O The effects of SO2 and H2O on NOx conversion are noticeable in this study. As shown in Fig. 5, when only 10% H2O is added in the feed gas, the NOx conversion on the MnOx (CP) decreased to 90%; the activity recovered completely after removing H2O. In case of adding both SO2 (100 ppm) and H2O (10%) to the feed gas, the NOx conversion at 80 °C on MnOx (CP) changed from 98% to 73% in 3 h. After turning off the SO2 and H2O, the activity was quickly restored to 90%. When the catalyst was heated 1– 2 h in N2 at 280 °C, the activity could be restored to its initial level. Because the reaction temperature was only 80 °C, a small portion of adsorbed SO2 on the catalyst surface was not easily desorbed by blowing gas, so its activity could not recover completely when the supply of SO2 was ended. After a heating treatment, the activity totally recovered. Similar variation trends of activity were observed over MnOx (SP), and the little decrease of the activity may due to its higher specific area. The results indicate that the decrease in activity by H2O and SO2 are due to their competing adsorption with the reactant over the catalysts surface.
[1] W.S. Kijlstra, D.S. Brands, H.I. Smit, E.K. Poels, A. Bliek, J. Catal. 171 (1997) 219–230. [2] T.S. Park, S.K. Jeong, S.H. Hong, S.C. Hong, Ind. Eng. Chem. Res. 40 (2001) 4491–4495. [3] G. Qi, R.T. Yang, Appl. Catal. B 44 (2003) 217–225. [4] M. Kantcheva, J. Catal. 204 (2001) 479–494. [5] M. Richter, A. Trunschke, U. Bentrup, K.-W. Brzezinka, E. Schreier, M. Schneider, M.-M. Pohl, R. Fricke, J. Catal. 206 (2002) 98–113. [6] M. Yosikawa, A. Yastutake, I. Mochida, Appl. Catal. A 173 (1998) 239–245. [7] G. Marba´n, A.B. Fuertes, Appl. Catal. B 34 (2001) 43–53. [8] T. Valde´s-Sol’y´s, G. Marba´n, A.B. Fuertes, Appl. Catal. B 46 (2003) 261–371. [9] G. Qi, R.T. Yang, R. Chang, Catal. Lett. 87 (2003) 217. [10] F. Kapteijn, L. Singoredjo, A. Andreini, J.A. Moulijn, Appl. Catal. B 3 (1994) 173–189. [11] G. Qi, R.T. Yang, J. Catal. 217 (2003) 434–441. [12] M. Kang, E.D. Park, J.M. Kim, J.E. Yie, Catal. Today 111 (2006) 236–241. [13] J. Sun, L. Yuan, K. Zhang, Thermochim. Acta 333 (1999) 141– 145. [14] M. Kang, T.H. Yeon, E.D. Park, T.E. Yie, Catal. Lett. 106 (2006) 77– 80. [15] Z.A. Zhang, B.C. Yang, M.G. Deng, Y.D. Hu, B.H. Wang, Acta Chim. Sin. 62 (2004) 1617–1620.
Low temperature selective catalytic reduction of NOx with NH3 over amorphous MnOx catalysts prepared by three methods Xiaolong Tang a
a,b
, Jiming Hao
a,*
, Wenguo Xu b, Junhua Li
a
Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, China b The Institute for Chemical Physics, Beijing Institute of Technology, Beijing 100081, China Received 9 March 2006; received in revised form 9 June 2006; accepted 12 June 2006 Available online 4 July 2006
Abstract Unsupported manganese oxide catalysts with amorphous phase were prepared by three methods, and their activities for SCR of NOx with ammonia were investigated in the presence of O2. The results showed the catalysts have superior low temperature activity, and the NOx conversion is about 98% at 80 °C, and nearly 100% NOx conversion between 100 and 150 °C. Due to competing adsorption with the reactant, H2O has slight impact on the activity. The activity was suppressed with coexisting of SO2, however the deactivation of SO2 is reversible. The excellent low temperature catalytic activity of amorphous MnOx catalysts is mainly due to their amorphous phase and high specific areas. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Selective catalytic reduction; Low temperature; NO; Ammonia; MnOx
1. Introduction Selective catalytic reduction (SCR) of NOx with NH3 is the most common technique to remove NOx in the exhaust gas from stationary sources. In power plant, however, a drawback inherent to the use of SCR is the SO2 and dust present in the flue gas. A new efficient catalyst for low temperature SCR is greatly needed. This catalyst could be placed downstream of the dust catcher and desulfurizer without preheating the flue gas. In recent research, Mnbased catalysts show good activity during low temperature NO reduction with NH3. Examples of these catalysts are MnOx/Al2O3 [1,2], MnOx/TiO2 [3,4], MnOx/NaY [5], MnOx/AC [6–8], and MnOx/USY [9]. The SCR activity of different pure manganese oxides also was investigated [10]. Recently, some unsupported Mn-based catalysts with high activity at low temperature were reported such as MnOx–CeO2 [11] and Cu–Mn mixed oxides [12]. In this
*
Corresponding author. Tel.: +86 10 62782195; fax: +86 10 62773650. E-mail address: [email protected] (J. Hao).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.025
work, a series of unsupported amorphous MnOx catalysts were prepared by three methods. These catalysts present excellent catalytic activities in the temperature range of 80–150 °C. 2. Experimental 2.1. Catalysts preparation 2.1.1. Rheological phase reaction method (RP) Manganese acetate (MnAC) and salicylic acid (SA) were mixed in the desired proportion (mole ratio, MnAC:SA = 1:2.05). The mixture was thoroughly ground for 40 min, and then de-ionized water was stirred into the mixture until it turned into a rheologic phase [13]. This was placed into a stainless steel pressure reactor with a Teflon inner liner, sealed and put in a baking oven at 70 °C for 48 h. The product was washed 3–4 times with de-ionized water and 2–3 times with ethanol, then dried in vacuum drier at 60 °C, resulting in the catalyst precursor consisting of a white fine powder. The MnOx catalysts were obtained by calcining the precursor in the air at 350–650 °C for 6 h.
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X. Tang et al. / Catalysis Communications 8 (2007) 329–334
2.1.2. Low temperature solid phase reaction method (SP) The MnAC and potassium permanganate (KMnO4) were mixed in the desired proportion (mole ratio, MnAC: KMnO4 = 2:3), and ground fully for 40 min. The mixture was placed in the baking oven at 70 °C for 48 h. The product was washed 3–4 times with de-ionized water and 2–3 times with ethanol, then dried in vacuum drier at 60 °C. 2.1.3. Co-precipitation method (CP) Three solutions were prepared. In order to make sure that the solute was dissolved completely, the solutions could be prepared in a water bath at 40 °C and stirred well.
NOx conversion ¼
½NOx inlet ½NOx outlet 100%; ½NOx inlet where NOx represents NO þ NO2 :
All catalysts activity tests were carried out at low temperature (<150 °C). In order to avoid the impact of gas adsorption on the catalyst samples, we recorded the test data after the reactions had been kept in stable states for 1 h. Further, prior to the activity tests, the catalyst sample was pre-treated in a flow of 3% O2/N2 at 200 °C for 1 h and then cooled to room temperature. 3. Results and discussion
Solution I: 18.4 g MnAC, 500 mL H2O; Solution II: 2–3 g Polyethylene glycol (PEG400), 100 mL H2O; Solution III: 7.9 g KMnO4, 300 mL H2O. Solution II was added to solution I and mixed well, and then Solution III was gradually added with thorough stirring. The mixtures were stirred steadily for 6 h at room temperature and then filtered and washed with de-ionized water 3–4 times. The resulting solid product was dried in a vacuum drier at 100 °C for 6 h. In this work we adopted three methods to prepare unsupported amorphous MnOx catalysts, which were designated as MnOx (RP), MnOx (SP) and MnOx (CP), respectively. For comparison, MnOx (CA) was also prepared by the citric acid method [11]. Finally, all catalyst samples were crushed and sieved to a 40–60 mesh size. 2.2. Catalyst characterization The BET surface area, pore volume and pore size of catalyst samples were measured by N2 adsorption using a NOVA 4000 automated gas sorption system. The powder X-ray diffraction (XRD) measurement was carried out on a Rigaku D/max-IIIA system with Cu Ka radiation. The microscopic pattern of catalyst particles was observed with JEM 2010 High-resolution transmission electron microscopy and JSM 7401 F field emission scanning electron microscope. 2.3. Catalysts activity measurement Catalytic activity tests were performed in a quartz tube reactor of 9 mm internal diameter. NOx, O2 and SO2 concentration were simultaneously measured by a KM 9106 flue gas analyzer. At steady state, a gas N2 mixture containing 500 ppm NO, 500 ppm NH3, 3% O2, 100 ppm SO2 (when used) and 2.8–20% H2O (when used) was introduced into the reactor. The water vapor was adding by a WZ50C2 injection pump and an evaporator. In all the tests, the total flow rate was fixed at 300 cm3 min1, which corresponded to a GHSV (gas hourly space velocity) of 47,000 h1. NOx conversion was obtained by the following equation:
3.1. Characterizations of catalysts The BET surface area, pore volume and pore-size of different MnOx catalysts are summarized in Table 1. From Table 1 we can see that the surface area and the pore volume of MnOx (SP) are the greatest of all the samples. MnOx (CP) and MnOx (RP-350) have similar surface areas, but the pore volume and pore size of MnOx (CP) are larger than others. Furthermore, the surface area of MnOx (RP) was greatly affected by the calcination temperature, and decreased with the calcination temperature rising. The MnOx catalyst prepared by citric acid has the lowest surface area (19 m2 g1). The pore diameter of all catalysts were between 9.55 and 11.54 nm. 3.2. XRD results The calcination temperature has great effect on the characteristics of the catalysts (surface area, etc.), and also affects on the crystallinity and oxidation states. Fig. 1 shows the effect of the calcination temperature on the MnOx (RP) catalysts. The peaks of patterns from the catalysts calcined at low temperature become broad, and the intensity of the peaks are comparatively weak. This is due to the poor crystallinity of the catalysts, and the amorphous phase may formed in part of catalyst particles. According to the XRD patterns, it is clear that the crystallinity of the catalysts increased when the calcination temperature rose from 350 °C to 650 °C. Also we notice that the oxidation states of the catalysts were changed with Table 1 Characterization of the MnOx catalysts prepared by different methods Samplesa
BET surface area (m2 g1)
Pore volume (cm3 g1)
Average pore diameter (nm)
MnOx MnOx MnOx MnOx MnOx MnOx MnOx MnOx
99 73 46 28 150 96 19 27
0.212 0.186 0.128 0.074 0.365 0.278 0.072 0.071
9.55 9.61 10.74 11.26 9.69 11.54 10.63 10.45
a
(RP-350) (RP-450) (RP-550) (RP-650) (SP) (CP) (CA) (CA) [11]
The number in the bracket is the calcination temperature.
X. Tang et al. / Catalysis Communications 8 (2007) 329–334
ο
36.12 32.34 59.84
28.9 17.96
10
ο
550 C
Mn3O4 Mn2O3 Intensity (a.u.)
Intensity (a.u.)
650 C
20
44.44 50.74
30
331
40
64.66
50
60
70
80
10
20
30
40
2 ()
50
60
70
80
50
60
70
80
2 ()
ο
ο
350 C
Intensity (a.u.)
Intensity (a.u.)
450 C
58
10
20
30
40
50
60
70
80
10
20
30
40 2 ()
2 ()
Fig. 1. XRD patterns of MnOx (RP) catalysts calcined at different temperatures.
Mn2O3 a: MnOx(CA) b: MnOx(RP) c: MnOx(SP) d: MnOx(CP)
Intensity (a.u.)
the temperature increasing. Though the diffraction peaks of XRD profile of MnOx (RP-350) are weak, we can identify that most of them are attributed to Mn3O4 (PDF card 240734). When the calcination temperature rose up to 450 °C, the oxidation state of MnOx (RP-450) changed, most diffraction peaks are belong to Mn2O3 (PDF card 41-1442) except two small weak peaks, which could be attributed to Mn3O4. Then the MnOx (RP-450) could be considered as mixed oxides of Mn2O3 and Mn3O4, but main is Mn2O3. The diffraction peaks of the patterns of MnOx (RP-650) and MnOx (RP-550) are sharper than others of MnOx (RP-450) and MnOx (RP-350). As marked in the Fig. 1, the main oxidation state of MnOx (RP-650) and MnOx (RP-550) are Mn3O4 and Mn2O3 respectively. Then the catalysts oxidation states alternated as followed sequence with the calcination temperature increasing: Mn3O4 ! Mn2O3 ! Mn3O4. Fig. 2 shows the XRD patterns of different MnOx catalysts. The MnOx (RP) was calcined at 350 °C. As shown in curve a, which is the most different from the others, sharp peaks representing Mn2O3 were observed in the MnOx (CA) catalyst. The XRD peaks in curve b become broad, and the intensity of the peaks, which seem to be mixed oxides of Mn3O4 and Mn2O3, greatly decreased. The width and intensity of diffraction peaks of curve c are similar to b, whereas the locations of diffraction peaks are different, and three main peaks were listed in Table 2.
a b c d
20
40
60
80
2 Fig. 2. XRD patterns of MnOx catalysts prepared by different methods.
Table 2 The XRD data of MnOx (SP) catalyst Peak
2h (°)
PDF number
2h (°)
Crystal
I
18.024
44-0141 24-0734 39-1218 39-1218
18.122 18.014 17.937 18.126
MnO2 Mn3O4 Mn5O8 Mn5O8
II
37.512
44-0141
37.533
MnO2
III
59.763
24-0734
59.893
Mn3O4
332
X. Tang et al. / Catalysis Communications 8 (2007) 329–334
As shown in Fig. 2, the sample peak I and II match the characteristic diffraction peaks of MnO2 (PDF card 440141) well, so it means that the MnOx (SP) catalyst must has MnO2. Furthermore, the peak I is broadened greatly, so we also consider that it may be a superposition peak, which formed by several adjacent overlapped diffraction peaks. Besides the characteristic peak of MnO2 (18.122), it may also contain the characteristic peaks of Mn3O4 (18.014) and Mn5O8 (17.937, 18.126). Analogously, the sample peak III may contain the character peaks of Mn3O4 (59.893). Then, it is reasonable to believe that the MnOx (SP) catalyst may have other oxidation states, Mn3O4 and Mn5O8. The XRD pattern of MnOx (CP) (curve d) is quite different to the others. Its diffraction peaks become very broad, almost like a straight line. Therefore, it is difficult to identify the phase and oxidation state from this curve, but we are sure that its crystallinity must be very poor and its particles turn into amorphous phase. Compared with MnOx (CA), the diffraction peaks of XRD patterns of MnOx (RP), MnOx (SP) and MnOx (CP) are broad and weak obviously, which implies that
part of the last three catalysts (other than MnOx (CA)) turned into an amorphous phase. 3.3. Catalysts TEM photos In order to obtain more details about the microstructure of catalysts, we analyzed these samples with a transmission electron microscope. The TEM photos were shown in Fig. 3. The catalyst particles of MnOx (CA) have comparatively regular shape (photo A), and the particle sizes are mainly in the range of 40–60 nm. From photo B, we can see that the particles of MnOx (RP350) have no regular shape, and the particle size distribution is uneven. The particles of MnOx (SP) catalyst are shown in photo c, and it is clear that there are mainly two types of particles. One is the club-shaped particles (30–60 nm) with the better crystallinity, and the other is the floc, which is the aggregation of fine particles (<10 nm). In photo d, there is almost no regular shape to the MnOx (CP), but their sizes distribution is more even than those seen in the other photos, mainly in the range of 10–20 nm.
Fig. 3. The TEM photos of MnOx catalysts: (A) MnOx (CA); (B) MnOx (RP-350); (C) MnOx (SP) and (D) MnOx (CP).
X. Tang et al. / Catalysis Communications 8 (2007) 329–334
3.4. Activity test The de-NOx activities over the MnOx (RP) catalysts are listed in Table 3. It is clear that the MnOx (RP) calcined at low temperature can perform more efficiently. (We also tested the catalyst calcined at 250 °C, but its activity is poor, which may have been caused by the catalyst precursor decomposing incompletely.) We chose MnOx (RP-350) to compare with the other catalysts in the following tests. Kijlstra et al reported that the catalytic activity and product selectivity of manganese oxide catalysts are mainly affected by the oxidation state and crystallinity [1]. One noticeable thing is that the catalysts in his work were prepared with high calcination temperature, so the crystallinity of those catalysts must be well. However, in this work, it is difficult to consider the effect of oxidation states to activity. As listed in Table 3, the catalytic activity of MnOx (RP) is decreased as followed sequence: MnOx (RP-350) > MnOx (RP-450) > MnOx (RP-550) > MnOx (RP-650), while their main oxidation states turned as followed sequence: Mn3O4 ! Mn2O3 + Mn3O4 ! Mn2O3 ! Mn3O4. Therefore, we can not appraise the effect of oxidation states to the catalytic activity on the catalysts with poor crystallinity. But these data imply that there may be certain relation between the activity and the crystallinity of MnOx catalysts. Fig. 4 presents the NOx conversions over MnOx catalysts. It is obvious that the catalytic activity of amorphous catalysts, which include MnOx (SP), MnOx (RP-350) and MnOx (CP), are much better than the activity of MnOx Table 3 The de-NOx activity over MnOx (RP) catalysts calcined at different temperatures Samples
MnOx MnOx MnOx MnOx
(RP-350) (RP-450) (RP-550) (RP-650)
NOx conversion (%) 50 °C
80 °C
100 °C
120 °C
150 °C
59 26 20 7
98 76 54 38
100 95 77 62
100 98 91 87
100 100 99 97
(CA). More than 98% NOx conversion could be achieved at 80 °C over the amorphous catalysts, and almost all of the NOx can be converted in the range of 80–150 °C. In comparing the amorphous catalysts with the MnOx (CA), two obvious differences are the specific area and crystallinity, which may be the reasons why the amorphous catalysts have such superior low temperature catalytic activity. In the study of MnOx/NaY [5], the author believed that its substantial low temperature activity is due to the eggshell manganese oxides in which amorphous particles were found. Therefore, we speculate that the excellent low temperature activity of amorphous catalysts is mainly due to the high specific area and the amorphous phase. Coincidently, a recent study of MnOx catalyst prepared by a precipitation method reported similar conclusions to ours [14]. We notice that at the low temperature range of 50– 80 °C, the activity decrease order of the amorphous catalysts is: MnOx (CP) > MnOx (RP-350) > MnOx (SP), but their BET specific areas decrease according to the sequence: MnOx (SP) > MnOx (RP-350) MnOx (CP). Crystallinity diminishes in the following order: MnOx (SP) > MnOx (RP-350) > MnOx (CP). Among the three catalysts, although the BET specific area of MnOx (SP) is the highest, its low temperature activity is the lowest. Zhang et al. reported that the amorphous phase is favorable to the inserting and releasing of proton, so the chemical adsorption/desorption and redox reaction can be carried on rapidly in the body phase or surface of the catalyst particles [15], and which will promote the SCR reaction. The above discussions lead us to believe that the amorphous phase of the catalysts may have more impact on the low temperature activity than the specific area. Furthermore, we also analyzed the tail gas contents by Gas Chromatography (GC-17, Shimadzu), which equipped ˚ molecular sieve column for PQ column for N2O and 5 A N2. The results show that the product selectivity of N2 is more than 96% at 80 °C, and the concentration of N2O is less than 10 ppm.
100 80
N O X Conversion (%)
The MnOx (CP), MnOx (RP-350) and MnOx (SP) catalysts were also analyzed by electron diffraction, which can help us to confirm whether these catalysts have an amorphous phase. On the MnOx (RP-350) catalyst, the diffraction halation was observed on part of particles, which indicates that they are amorphous. Similar results were also observed on the floc particles of the MnOx (SP) catalyst. Especially to the MnOx (CP) catalyst, the diffraction halation was observed on almost all particles. The results of XRD and ED (Electron Diffraction) analysis permit us to confirm that the MnOx (SP) and MnOx (RP-350) catalysts turn into an amorphous phase, and this is especially true for the MnOx (CP) catalyst. In other words, the crystallinity of the catalysts decreased in the following sequence: MnOx (CA) > MnOx (SP) > MnOx (RP350) > and MnOx (CP).
333
60 MnOX(RP) MnOX(SP)
40
MnOX(CP) MnOX(CA)
20 0 40
60
80
100
120
140
160
Temperature / C Fig. 4. NOx conversion over four MnOx catalysts at different temperatures. Reaction conditions: 0.5 g samples, 500 ppm NO, 500 ppm NH3, 3% O2, N2 to balance, GHSV = 47,000 h1.
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X. Tang et al. / Catalysis Communications 8 (2007) 329–334
4. Conclusions
NO X Conversion (%)
100 90 Heating treatment
80 70 add
stop
MnOX (CP)
60
MnOX (SP)
50 0
1
2
3
4
5
6
7
8
9
10
Amorphous MnOx catalysts prepared by three methods showed excellent activity for the selective catalytic reduction of NOx with NH3 in the presence of excess O2 at low temperatures. Amorphous phase and a high specific area of manganese oxides are probably the main reasons for these catalysts with high de-NOx activities at low temperature. The amorphous phase is the more important. The inhibition of activity over the amorphous MnOx catalysts by SO2 and H2O is due to their competing adsorption, and the activity recovers to the initial level after removing H2O and SO2 gases.
Time / h Fig. 5. Effect of SO2 and H2O on NOx conversion over MnOx (CP) and MnOx (SP). Reaction conditions: 0.5 g sample, 80 °C, 500 ppm NO, 500 ppm NH3, 3% O2, 10% H2O (when used), 100 ppm SO2 (when used), N2 to balance, GHSV = 47,000 h1 (dotted line: only added 10% H2O; solid line: added 10% H2O + 100 ppm SO2).
Acknowledgement This work was supported by the Natural Science Foundation of China (20437010 and 20507012). References
3.5. Effect of SO2 and H2O The effects of SO2 and H2O on NOx conversion are noticeable in this study. As shown in Fig. 5, when only 10% H2O is added in the feed gas, the NOx conversion on the MnOx (CP) decreased to 90%; the activity recovered completely after removing H2O. In case of adding both SO2 (100 ppm) and H2O (10%) to the feed gas, the NOx conversion at 80 °C on MnOx (CP) changed from 98% to 73% in 3 h. After turning off the SO2 and H2O, the activity was quickly restored to 90%. When the catalyst was heated 1– 2 h in N2 at 280 °C, the activity could be restored to its initial level. Because the reaction temperature was only 80 °C, a small portion of adsorbed SO2 on the catalyst surface was not easily desorbed by blowing gas, so its activity could not recover completely when the supply of SO2 was ended. After a heating treatment, the activity totally recovered. Similar variation trends of activity were observed over MnOx (SP), and the little decrease of the activity may due to its higher specific area. The results indicate that the decrease in activity by H2O and SO2 are due to their competing adsorption with the reactant over the catalysts surface.
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