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Al2O3 catalysts for selective catalytic reduction of NO by NH3
Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
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Characterization of MoO3–V2O5/Al2O3 catalysts for selective catalytic reduction of NO by NH3 Hyoung-Lim Koh a,1, Hea-Kyung Park b,* a b
R&D Center for Chemical Technology, Hyosung Corporation, Anyang, Kyunggi-do 431-080, Republic of Korea Department of Chemical Engineering, Hanseo University, Seosan, Chungnam 356-706, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 April 2012 Accepted 7 July 2012 Available online 16 July 2012
MoO3–V2O5/Al2O3 catalysts were characterized by B.E.T., XRD, LRS, XPS and TPR and the effect of MoO3 addition to alumina supported vanadia catalysts on the catalytic activity for the selective catalytic reduction of NO by ammonia was investigated. Upon the addition of MoO3, catalytic activity was enhanced and the particle size of V2O5 which is shown by the results of B.E.T., XRD and Raman spectroscopy decreased. This was one reason for increased catalytic activity. The results obtained by XPS and TPR showed that MoO3 addition to alumina supported vanadia catalysts increased the reducibility of vanadia and this was the another reason for synergy effect between MoO3 and V2O5 in MoO3–V2O5/Al2O3 catalysts. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: V2O5/Al2O3 Selective catalytic reduction (SCR) Nitric oxide Reducibility MoO3 addition
1. Introduction The increasing problem of air pollution by nitrogen oxides has resulted in more stringent emission regulations in many countries and caused a great increase of interest in the development of effective de-NOx catalysts. One of the most available technologies is the so-called selective catalytic reduction (SCR) of NOx by ammonia process where the NOx is reduced by ammonia to form N2 and H2O. Many different types of catalysts have been developed for this process. Among various base metal oxides, vanadia is the most active and selective catalysts [1,2]. The deposition of vanadia on supports leads to an increase in the catalytic activity and V2O5/ TiO2 based catalysts are the most commonly used. Commercial SCR catalysts consist of tungsten oxide, vanadia and titania. Since vanadia is active not only in the reduction of NO but also in the undesired oxidation of SO2 to SO3, its content is generally kept low. WO3 is employed in larger amounts (nearby 10 wt%): it acts both as ‘chemical’ and ‘structural’ promoter by enlarging the temperature window of the SCR reaction and by improving the mechanical, structural and morphological properties of the catalysts [3–5]. However, recently the price of tungsten has risen about 30 times compared to ten years ago. SCR catalyst producer began to use MoO3 instead of WO3. In these catalysts, MoO3
typically replaces WO3 by a similar amount on a molar basis, so that molybdena loadings as high as 6 wt% are used. These catalysts have been reported to be less active than the analogous WO3– V2O5/TiO2 samples, but they are even more competitive both in tolerant to As [4–9] and in price. The surface reactivity of WO3– V2O5/TiO2 catalyst system was well characterized by various kinds of surface analyzers but MoO3–V2O5/Al2O3 catalyst system was relatively less analyzed. Alumina has a higher specific surface area, superior mechanical strength and is highly resistant to sintering, compared to titania. Therefore, many efforts have been made to apply alumina to the SCR of NO by ammonia [10–13]. When alumina is used as the supporter for the SCR reaction, SO2 in the flue gas, reacting with alumina or ammonia, deactivates the catalytic activity [11]. But results of other research show that activity of catalyst used alumina support could show the resistance to SO2 deactivation, depending on the pore size and specific surface area of alumina [12]. In order to enhance the activity and availability of aluminasupported vanadia catalysts for the SCR of NO by ammonia, we prepared MoO3–V2O5/Al2O3 catalysts. In this work, we tried to investigate the effect of MoO3 addition on the activity of V2O5/ Al2O3 catalysts for the SCR of NO by ammonia in detail. 2. Experimental
* Corresponding author. Tel.: +82 41 660 1424. E-mail address: [email protected] (H.-K. Park). 1 Department of Chemical Engineering, Hankyung National University, Ansung, Kyunggi, 456-749, Republic of Korea.
2.1. Preparation of catalysts A series of V2O5/Al2O3 catalysts with different vanadia loadings were prepared by the impregnation of gamma-Al2O3 (B.E.T. surface
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.07.003
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
area: 167.5 m2/g, extra pure, Aldrich Chemical Co., USA) with aqueous solutions of ammonium vanadate (NH4VO3, extra pure, Yukari Co., Japan), followed by drying in air at 120 8C for 24 h and calcination in air at 500 8C for 3 h. MoO3–V2O5/Al2O3 catalysts with different loadings of MoO3 were prepared by the impregnation of gamma-Al2O3 with aqueous solutions of ammonium vanadate, followed by drying in air at 120 8C for 24 h and impregnation of these samples with aqueous solutions of ammonium molybdate ((NH4)6Mo7O244H2O, extra pure Yukari Co. Japan), followed by drying in air at 120 8C for 24 h and calcination in air at 500 8C for 3 h. These catalysts will be quoted in this paper according to their V2O5, and MoO3 contents: e.g., Mo6V12 identifies a catalyst sample with MoO3 = 6 wt%, V2O5 = 12 wt%, supported by alumina.
100
V6 V9 V12 V16 V23
90
NO Conversion (%)
74
80 70 60 50 40 30 20 10
2.1.1. Characterization of catalysts X-ray diffraction was measured using Dmax II X-ray diffractometer manufactured by Rigaku Co., Japan. The XPS analysis was performed at room temperature with a VG ESCALAB 220i-XL spectrometer manufactured by Fisons, USA. A monochromated Al anode (energy of the AlKa line 1486.6 eV), powered at 10 keV and 20 mA, was used for X-ray production. The binding energies were calculated with respect to the C 1s peak at 284.6 eV. Decomposition of peaks was done with the best fitting routine of the XPSPEAK program. Raman spectra were obtained by the Raman spectrometer (Raman-11, Nanophoton Co., Japan). The excitation source was Ar+laser (514.532 nm, 100 mW) with the detector of optical multichannel photodiode. H2-TPR (temperature programmed reduction, ASAP 2000 of Micromeritics, USA) experiments were performed in a quartz microreactor inserted into an electric furnace driven by a PID temperature programmed controller. The catalyst temperature was measured by means of a small K-type thermocouple inserted below the catalyst bed in a quartz microreactor. In a typical experiment, 0.2 g of catalyst was oxidized in He + 20% O2 at 500 8C for 1 h, cooled to 25 8C and purged with He and then heated to 750 8C with He + 5% H2 (total flowrate = 50 cm3/min (STP)). A dry-ice cooled silica gel trap was used at the reactor outlet to remove H2O formed during catalyst reduction and H2 consumption was monitored by means of a thermal conductivity detector. 2.1.2. Catalytic activity measurement Catalytic activity for selective catalytic reduction (SCR) of NO by ammonia has been measured in a tubular, downflow, fixed-bed reactor operated under isothermal and at slightly above atmospheric pressure and inserted into an electric furnace. In a typical test of catalytic activity, 1 cm3 of catalyst (20–40 mesh, 0.38–0.52 g) was used and the typical reactant gases for activity test consisted of 500 ppm NO, 500 ppm NH3, 10% O2 and balance He. The space velocity which is based on the ratio of the total gas flowrate to the volume of catalyst bed was 100,000 h 1, otherwise specified. The nitrogen oxide concentration of the gas stream was measured before and after the catalyst bed by means of chemiluminescent NO/NOx analyzer (Teledyne Technologies Co., USA) and for the measurement of oxygen content in the gas stream after reaction, inlet and outlet gases were also analyzed by gas analyzer (Greenline-MK2, Eutron Co., UK). 3. Results and discussion 3.1. Activity of V2O5/Al2O3 and MoO3–V2O5/Al2O3 catalysts Catalytic activity of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts for SCR of NO by ammonia was tested in the downflow fixed-bed reactor system. The results of the catalytic activity tests performed in SCR of NO by ammonia over V2O5/Al2O3 catalysts with different vanadia loadings (V2O5 = 6, 9, 12, 16, 23 wt%) are shown in Fig. 1.
0 0
100
200
300
400
500
600
o
Temperature ( C) Fig. 1. Conversion of NO with different V2O5 loadings of the V2O5/Al2O3 catalysts (GHSV: 100,000/h, NO: 500 ppm, NH3: 500 ppm, O2: 10%, He: balance).
The catalytic activity for SCR of NO by ammonia was expressed as a percentaged NO conversion at the reaction condition of NO 500 ppm, NH3 500 ppm, O2 10%, He balance and reaction temperatures from room temperature to 500 8C. This figure showed that the activity of the catalysts increased on increasing the V2O5 contents: in the case of V23 (23 wt% V2O5/Al2O3), containing highest vanadia loadings, appreciable NO consumption was observed at temperatures as low as 170 8C. As the vanadia loading increased from 3 to 23 wt%, the temperature required to achieve 50% NO conversion was progressively lowered from 320 8C down to 225 8C. At the higher investigated temperatures, for all the catalysts the NO conversion decreased due to the occurrence of the ammonia oxidation producing NO [11], and higher NO concentration than feed (500 ppm NO) was observed above 500 8C. While catalytic activity increased with increasing vanadia content of the catalysts, activity difference between V16 and V23 was relatively small in comparison with that of other catalysts. Besides, at the high temperature NO conversion of V23 was rather lower than that of V16 due to higher ammonia oxidation activity of V23 or structural effect of such a high loading of vanadia, e.g., decrease of surface area or pore volume. Over 16 wt% of vanadia content, increased vanadia content rarely showed another good effect, such as temperature window and maximum activity on the SCR of NO by ammonia. In the view of practical application as vanadia loading increased, the oxidation SO2 to SO3, which was undesired reaction, increased. In this regard V16 catalyst (16 wt% V2O5/Al2O3) showed the acceptable catalytic activity. In this study, in order to improve the catalytic activity of V2O5/ Al2O3 catalysts for SCR of NO by ammonia, molybdenum oxide (MoO3) was chosen for promoter added to V2O5/Al2O3 catalysts. Fig. 2 shows the activity of 6 wt% MoO3 added to 9 wt% and 12 wt% V2O5/Al2O3 catalysts for SCR of NO by ammonia. Okazaki [14] reported that alumina-supported molybdenum oxide showed low activity compared to titania-supported vanadia catalyst for the SCR of NO by ammonia. As seen in Fig. 1, activity of V23 was affected by structural limitation. Molybdenum oxide was added to V9 and V12, of which vanadia loadings were out of this limitation range and these Mo6V9 and Mo6V12 were tested for SCR of NO by ammonia. Fig. 2 shows the NO conversion of Mo6V9 and Mo6V12 and also compared the activity of V9 and V12. It is observed in Fig. 2 that catalytic activities of molybdenum containing catalysts were significantly higher active than those of the corresponding vanadia only catalysts and the temperature
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
* * *
100
A
75 A
A
V16
Conversion of NO (%)
90 80
Mo6V12
70 60 50
V12
40 30
Mo6V9 V9 V12 Mo6V9 Mo6V12
20 10 0
0
100
200
300
400
V9
500
0
o
Temperature ( C)
20
30
40
50
60
70
80
90
2q
Fig. 2. Effect of MoO3 addition to V9 and V12 catalysts on the catalytic activity (GHSV: 100,000/h, NO: 500 ppm, NH3: 500 ppm, O2: 10%, He: balance).
window for the reaction was widened and shifted toward lower temperatures. This was a clear indication that the presence of molybdenum oxide increased the activity of V2O5/Al2O3 catalysts. Besides, the catalytic activity of Mo6V9 and Mo6V12 reached nearly 100% NO conversion which was the highest activity obtained in this study. The ternary MoO3–V2O5/Al2O3 catalysts investigated in this study exhibit a higher activity in the SCR reaction compared to binary V2O5/Al2O3 catalyst. This result may suggest that Mo is a good promoter for binary V2O5/Al2O3SCR catalyst and synergy effect operates between the V and Mo surface oxide species. 3.1.1. Effect of MoO3 addition on the vanadia dispersion The effect of the vanadia loading on the catalysts’ structural and morphological properties was investigated by B.E.T., XRD and Raman spectroscopy. Table 1 reported the B.E.T surface area, pore volume and the V2O5 and MoO3 surface coverage (uv and uMo, respectively) calculated from the nominal V2O5 and MoO3 loading and the specific surface area of the sample. In this calculation a monolayer capacity of 13 mmol V5+/m2 and of 0.12 wt% for MoO3 m 2 [16] has been assumed. The overall V + Mo surface coverage has been estimated by simply summing those of V and Mo. As reported in Table 1, the theoretical V + Mo monolayer coverage was mostly reached in the sample with 16 wt% vanadia loading. The morphological characteristics of the catalysts were slightly modified upon MoO3 addition. The B.E.T. surface area of the V2O5/ Al2O3 catalysts decreased progressively from 166 down to 145 m2/g. In spite of adding MoO3 to 12 wt% V2O5/Al2O3 catalyst, the surface area of 6 wt% MoO3–12 wt% V2O5/Al2O3 increased slightly. A comparison of the surface areas of Mo6V12 and V12, having the Table 1 Surface area and pore volume of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts.
*
10
Catalyst
Surface area (m2/g)
Pore volume (cm3/g)
uv a
uMo a
uV+Mo a
Al2O3 V9 Mo6V9 V12 Mo6V12 V16
167.5 166.7 155.5 152.2 155.6 145.7
38.4 38.2 35.7 34.9 23.3 33.4
– 0.50 0.50 0.67 0.67 0.89
– – 0.29 – 0.29 –
– 0.50 0.79 0.67 0.96 0.89
uV, uMo, uV+Mo represent the surface coverage of V2O5, MoO3 and V2O5 + MoO3, respectively and calculated from the assumption of the text.
Fig. 3. XRD patterns of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts (A: Al2O3, *: V2O5).
same vanadia loading, implied that the presence of molybdenum oxide on the catalyst surface helped the vanadia to be well dispersed on the catalyst surface. To obtain the more information concerning the structure and morphology of the catalyst, X-ray diffraction data of the catalysts were collected. XRD patterns of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts are shown in Fig. 3. Diffraction lines of 9 wt% V2O5/Al2O3 and 6 wt% MoO3–9 wt% V2O5/Al2O3 showed no peaks of crystalline vanadia. In these two samples, it could be not excluded that particle size of vanadium oxide and molybdenum oxide on alumina was so small that diffractometer could not detect these species. For 12 wt% V2O5/Al2O3, 16 wt% V2O5/Al2O3 and 6 wt% MoO3–12 wt% V2O5/Al2O3 catalysts, diffraction lines due to V2O5 crystalline are observed as well as peaks for gamma-Al2O3. With the adding of the molybdenum oxide to 12 wt% V2O5/Al2O3 catalyst, the intensity of peaks and particle size of vanadium oxide is distinctively decreased. In line with the observation of increase of B.E.T. surface area in 6 wt% MoO3–12 wt% V2O5/Al2O3 in comparison with 12 wt% V2O5/Al2O3catalsyt, it was inferred that presence of molybdenum oxide improved the vanadia dispersion in the catalyst, so that particle size of vanadia could be small. Laser Raman spectroscopy could detect the amorphous monomeric, polymeric vanadates below monolayer coverage [15,17,18]. Went et al. [15] used in situ laser Raman spectroscopy and temperature-programmed reduction and temperature-programmed oxidation to establish the presence of monomeric vanadyls, polymeric vanadates and crystallines of V2O5 in titania supported catalysts. The distribution of vanadia species as a function of V2O5 loading was determined. At low V2O5 content in the catalyst the peak at 1030 cm 1 broadens significantly and the intensity of the bands located in the region 700–1000 cm 1 assigned to polymeric vanadates increases. The band at 840 cm 1 is assigned to V–O–V bending vibrations. The average polymer size varies from two to three monomer units depending on the vanadia loading. For vanadia amounts of 6 wt% and higher, a narrow band at 960 cm 1 appears [15]. Lietti and Forzatti [16] have shown by means of transient techniques such as TPD, TPSR, TPR and SSR (steady-state reaction experiments) that isolated vanadyls and polymeric metavanadate species are present on the surface of vanadia on titania catalysts with V2O5 loadings of up to 3.56 wt%. Polyvanadate species are more reactive than isolated vanadyls due to the presence of more weakly bonded oxygen atoms. Fig. 4(A) shows the results of Raman spectroscopy for 12 wt% V2O5/Al2O3 and 6 wt% MoO3–12 wt% V2O5/Al2O3 catalyst. It was
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
76
A
*
(a) V12 (b) Mo6V12
B
*
(a)
(a) V9 (b) Mo6V9
(a)
(b)
(b)
800
900
1000
1100
800
1200
900
1000
1100
1200
-1
Raman shift (cm )
-1
Raman shift (cm )
Fig. 4. Raman spectra of V2O5/Al2O3 and MoO3–V2O5/Al2O3 catalysts (*: V2O5).
observed that in 6 wt% MoO3–12 wt% V2O5/Al2O3, intensity of crystalline V2O5 peak remarkably decreased in comparison with that of V2O5/Al2O3 catalyst, having the same vanadia loading, and the peak of polymeric vanadates appears in 1040 cm 1. This data clearly indicated that the addition of MoO3 to V2O5/Al2O3 catalyst prevented the agglomeration of vanadia species into crystalline and enhanced the dispersion of vanadia acting the active component for SCR of NO by ammonia. For 6 wt% MoO3–12 wt% V2O5/Al2O3 and 12 wt% V2O5/Al2O3, XRD data shown in Fig. 3, was in line with the observation of Raman spectra of these catalysts. While XRD results showed no useful peak for 9 wt% V2O5/Al2O3 and 6 wt% MoO3–9 wt% V2O5/Al2O3 catalyst in Fig. 3, Raman spectra of these catalysts shown in Fig. 4(B) exhibited the effect of MoO3 addition to V2O5/Al2O3. Peak of crystalline V2O5 (presented as ‘‘*’’ in Fig. 4) in 9 wt% V2O5/Al2O3, which was not detectable with XRD technique because of small particle size of crystalline V2O5, diminished in 6 wt% MoO3–9 wt% V2O5/Al2O3 catalyst, having the same vanadia loading. Dispersion of active component on the surface of catalysts could also be measured with X-ray photoelectron spectroscopy. Results of XPS analysis were reported in Fig. 5. Fig. 5(A) shows V 2p region of the XPS spectra in V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts. The spectra of catalysts exhibited one sharp peak at about 517 eV, due to V 2p3/2, and a broad one at about 524.5 eV, due to V 2p1/2, respectively. The area of these peaks ratio (V/Al) could represent the surface concentration of vanadium, which implied the dispersion of the vanadia, indirectly. For both samples with or without MoO3, the profiles of the V 2p1/2 and V 2p3/2 lines were slightly asymmetric suggesting the presence of two types of vanadium on the surface of the catalysts. The deconvolution of the V 2p XPS spectra based on a Gaussian signal for the two types of
vanadium led to a good fitting of the results (Fig. 5(C) and (D)). The binding energies (516.0 and 517.2 eV for the V 2p3/2 line) thus were found to correspond to the value of V4+ in V2O4 and V5+ in V2O5, respectively [19–21]. Murakami et al. [21] reported V4+ ion, which acted as a Bronsted acid site and also interacted strongly with the alumina support, was inactive. Fig. 5(C), compared to (D), showed that much more portion of V5+ ions existed in 12 wt% V2O5/Al2O3 catalyst than 6 wt% MoO3– 12 wt% V2O5/Al2O3 catalyst. This result was also confirmed from the observation of color of catalyst, i.e., MoO3 added catalysts were brown and vanadia only catalysts were yellow. It was reported that for some reaction catalyzed by vanadia, e.g., the selective oxidation of o-xylene and the SCR of NO by ammonia in the presence of SO2 [22], the relative distribution of V4+ and V5+ species influences the activity and/or the selectivity. That effect on the latter SCR reaction might be attributed to the Bronsted acid site which was produced by V4+, acting as the active site in the high temperature range. Hence, distribution of these V4+ and V5+ ions and the ratio of sum of intensity of these two ions in XPS spectrum to Al 1s peak area due to the alumina support, were calculated from the deconvoluted XPS peak area, and the result is reported in Table 2. Table 2 shows that the fraction of V4+ in the molybdenum oxide added vanadia catalysts increased, compared to vanadia catalysts having the same vanadia loading, and the total surface concentration of the vanadia increased as MoO3 was added to catalyst. Fig. 5(B) shows that any difference did not exist between XPS Mo3d region of MoO3/Al2O3 and of MoO3–V2O5/Al2O3, i.e., oxidation state of Mo was unchanged in the ternary MoO3– V2O5/Al2O3. Because of the limitation of XPS technique, however, it
Table 2 Fraction of the surface concentration of V4+and V5+, calculated by XPS peak area.
V9 Mo6V9 V12 Mo6V12
Al
V4+
V5+
V4+/(V4+ + V5+)
(V4+ + V5+)/Al
355.72 273.40 376.15 288.54
152.61 392.99 203.51 523.68
479.88 298.32 641.04 398.47
0.271 0.572 0.240 0.567
1.832 2.193 2.451 2.926
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
A
B
(a) Mo6V12
V 2P
77
Mo 3d5/2
V 2P
Mo 3d3/2
Intensity (a.u.)
Intensity (a.u.)
(b) Mo6V9
(c) V12
(d) V9
510
515
520
525
225
530
230
Binding Energy (eV)
235
240
245
Binding Energy (eV)
C
D
a) V
510
5+
(b) V
Intensity (a.u)
Intensity (a.u.)
b) V
4+
515
520
525
Binding energy (eV)
(a) V
510
5+
4+
515
520
525
530
Binding Energy (eV)
Fig. 5. (A) XPS V2p spectra of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts. (B) XPS spectra Mo3d of MoO3/Al2O3, MoO3–V2O5/Al2O3 catalysts. (C) XPS deconvolution of V2p region of 12 wt% V2O5/Al2O3 catalyst. (D) XPS deconvolution of V2p region of 6 wt% MoO3–12 wt% V2O5/Al2O3 catalyst.
was not excluded that the interaction of Mo and V could exist in the ternary MoO3–V2O5/Al2O3. Combined above B.E.T. surface area, XRD, Raman spectroscopy, and XPS analysis with the reactivity test, it could be concluded that adding molybdenum oxide to the V2O5/Al2O3 catalysts, while keeping the loading of vanadium constant, provoked a drastic decrease of the V2O5 crystallinity, so this well-dispersed active vanadium oxide in the catalyst enhanced the SCR reaction of NO by ammonia. 3.1.2. Effect of MoO3 addition on the reducibility of vanadia The effect of Mo addition to V2O5/Al2O3 was discussed in the view of improvement of vanadia dispersion which increased the reactivity of ternary MoO3–V2O5/Al2O3 catalyst for SCR of NO by ammonia. Besides these structural effects, the electronic one could be considered for the reason of the increase of reactivity. Lietti [4–7,9,16] suggested by the spectroscopic data such as
EPR, FT-IR, FT-Raman spectroscopy that in the MoO3–V2O5/TiO2 and WO3–V2O5/TiO2 catalyst the promoter changed the redox property of vanadium and this affected the reactivity for SCR of NO by ammonia over these catalysts. Fig. 6 shows the pretreatment effect by feed on the catalytic activity test. NO conversion of catalyst which has undergone SCR reaction under steady-state condition at 300 8C during 2 h and then at 500 8C for same condition was monitored at reaction of 300 8C again. Two samples presented in figure exhibited the same behavior in these experiment. NO conversion in second reaction test at 300 8C increased with respect to first test. This phenomenon implied the change of catalyst surface through the first reaction test. Fig. 7 shows the XPS analysis of catalyst before reaction and after reaction. It was found that the surface of catalysts was much reduced as the reaction temperature increased. This implied the state of catalyst in the reaction condition. In other word, vanadia might be changed to reduced state in reaction condition. Hence, It
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
78
100 o
o
o
300 C
500 C
300 C
80
(a)
(b)
(c)
Intensity (a.u.)
90
Conversion of NO (%)
70 60 50 40 30
o
after reaction, 300 C o after reaction, 500 C after calcination
20
V12 V9
10
510
515
520
525
Binding Energy (eV)
0 0
2
4
6
Fig. 7. XPS spectra of calcined and aged V9 catalysts.
Time (hr) Fig. 6. Effect of pretreatment by feed on the catalytic activity test.
seemed that the redox property of catalyst affected the reactivity of catalyst for SCR of NO by ammonia. Confirming this assumption more strictly, XPS experiment was carried out for the hydrogen-reduced catalysts. XPS spectra of the reduced catalysts in hydrogen flow at 300 8C for 3 h and XPS of unreduced, calcined catalyst is shown in Fig. 8. For the reduced catalyst, the region of lower binding energy of V 2p slightly increased, compared to the unreduced, calcined sample. In the case of MoO3 added to V2O5/Al2O3 catalysts in Fig. 8(B), this phenomenon was more distinctive, so that the lower binding region much more broadened. Although this experiment could not fully indicate the state of the catalyst surface in the reaction condition, it was certain that as the MoO3 was added to the V2O5/Al2O3 catalysts, the reduced surface of catalyst easily remained after reduction experiments and the reducibility of catalysts was improved. In real reaction conditions, the catalysts were considered to be reduced by the ammonia.
B
Intensity (a.u.)
Intensity (a.u.)
A
The results obtained during the hydrogen-TPR experiments over the various MoO3–V2O5/Al2O3 catalysts are presented in Fig. 9 and compared to MoO3/Al2O3 catalyst. In this experiment, no hydrogen consumption was observed in any case below 250 8C. The binary MoO3/Al2O3 sample is reduced above 400 8C; the hydrogen consumption showed a maximum near 450 8C and a pronounced tailing which extended up to 800 8C, where the hydrogen consumption increased again. The binary V2O5/Al2O3 catalyst, 12 wt% V2O5/Al2O3 and 9 wt% V2O5/Al2O3, exhibited a different behavior since in both case a single peak is evident with maximum near 530 8C and 500 8C, respectively. The reducibility order shown in these two figures indicated that the addition of MoO3 favors the catalyst reduction, and that the ternary catalysts are more easily reduced than the corresponding binary samples. Especially, Mo6V12 sample in Fig. 9(B) exhibited the maximum peak near 580 8C where was between the maximum of V12 and Mo6 and clearly indicated the MoO3 addition effect on the reducibility of V2O5/Al2O3 catalyst.
calcined Mo6V12 reduced Mo6V12
calcined V12 reduced V12 505
510
515
520
Binding Energy (eV)
525
530
505
510
515
520
525
530
Binding Energy (eV)
Fig. 8. Comparison of XPS spectra of calcined and reduced catalysts. (A) XPS spectra of calcined and reduced V12 catalysts. (B) XPS spectra of calcined and reduced Mo6V12 catalysts.
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
A
B
Mo6V12 V12 Mo6
Intensity (a.u.)
Intensity (a.u)
Mo6V9 V9 Mo6
79
0
200
400
600
800
o
Temperature ( C)
0
200
400
600
800
o
Temperature ( C)
Fig. 9. TPR spectra of various catalysts. (A) Mo6, V9, Mo6V9 catalysts. (B) Mo6, V12, Mo6V12 catalysts.
4. Conclusion The effect of MoO3 addition on the dispersion of vanadia over the alumina surface was studied by B.E.T., XRD and Raman spectroscopy. As the result of these analysis, it was found that the MoO3 addition to the catalysts inhibited the growth of vanadia particle size, thus well-dispersed vanadia catalysts were obtainable. This vanadia species were more active for SCR reaction and become one of the reasons for enhanced reactivity. By the results of XPS and TPR, it has been concluded that the simultaneous presence of V and Mo in MoO3–V2O5/Al2O3 catalysts enhanced the reducibility of vanadia and that the higher reactivity of the ternary MoO3–V2O5/Al2O3 catalysts was related to this superior redox properties. Acknowledgement We owe thanks to the Resources Recycling Technology Development Program and Core Material Technology Development Program funded by the Ministry of Knowledge and Economy whose support was essential in carrying out this study. References [1] M.A.L. Vargas, M. Casanova, A. Trovarelli, G. Busca, Applied Catalysis B: Environmental 75 (2007) 303.
[2] L. Lietti, I. Nova, G. Ramis, G. Busca, E. Giamello, P. Forzatti, F. Bregani, Journal of Catalysis 187 (1999) 419. [3] Z. Si, D. Weng, X. Wu, J. Li, G. Li, Journal of Catalysis 271 (2010) 43. [4] L. Lietti, P. Forzatii, F. Bregani, Industrial and Engineering Chemistry Research 35 (1996) 3884. [5] M. Kobayashi, K. Miyoshi, Applied Catalysis B: Environmental 72 (2007) 253. [6] L. Lietti, I. Nova, G. Ramis, L. Dall’Acqua, G. Busca, E. Giamell, P. Fozatti, F. Bregani, Journal of Catalysis 187 (1999) 419. [7] G.B. Busca, L. Lietti, G. Ramis, F. Berti, Applied Catalysis B: Environmental 18 (1998) 1. [8] L. Casagrande, L. Lietti, I. Nova, P. Forzatti, A. Baiker, Applied Catalysis B: Environmental 22 (1999) 63. [9] L. Lietti, G. Ramis, F. Berti, G. Toledo, D. Robba, G.B. usca, P. Fozatti, Catalysis Today 187 (1999) 419. [10] G.E. Marnellos, E.A. Efthimiadis, I.A. Vasalos, Applied Catalysis B: Environmental 48 (2004) 1. [11] I. Nam, J.W. Eldrige, J.R. Kittrel, Industrial & Engineering Chemistry Product Research and Development 25 (1986) 192. [12] H.K. Matralis, M. Ciardelli, M. Ruwet, P. Grange, Journal of Catalysis 157 (1995) 368. [13] V.M. Mastikhin, V.V. Terskikh, O.B. Lapina, M. Seidl, H. Knozinger, Journal of Catalysis 156 (1995) 1. [14] S. Okazaki, M. Kumasaka, J. Yoshida, K. Kosaka, Industrial & Engineering Chemistry Product Research and Development 20 (1981) 301. [15] G.T. Went, L. Leu, A.T. Bell, Journal of Catalysis 134 (1992) 479. [16] L. Lietti, P. Forzatti, Journal of Catalysis 147 (1994) 241. [17] T. Kim, A. Burrows, C.J. Kiely, I.E. Wachs, Journal of Catalysis 246 (2007) 370. [18] I.E. Wachs, Journal of Catalysis 124 (1990) 570. [19] G. Centi, D. Pinelli, F. Ghoussoub, M. Guelton, L. Gengembre, Journal of Catalysis 130 (1991) 238. [20] M.C. Paganini, L.D. Acqua, E.G.L. Lietti, P. Forzatti, G. Busca, Journal of Catalysis 166 (1997) 195. [21] M. Inomata, K. Mori, A. Miyamoto, Y. Murakami, Journal of Physical Chemistry 87 (1983) 761. [22] G. Clarebout, M. Ruwet, H. Matralis, P. Grange, Applied Catalysis 76 (1991) L9.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Characterization of MoO3–V2O5/Al2O3 catalysts for selective catalytic reduction of NO by NH3 Hyoung-Lim Koh a,1, Hea-Kyung Park b,* a b
R&D Center for Chemical Technology, Hyosung Corporation, Anyang, Kyunggi-do 431-080, Republic of Korea Department of Chemical Engineering, Hanseo University, Seosan, Chungnam 356-706, Republic of Korea
A R T I C L E I N F O
A B S T R A C T
Article history: Received 11 April 2012 Accepted 7 July 2012 Available online 16 July 2012
MoO3–V2O5/Al2O3 catalysts were characterized by B.E.T., XRD, LRS, XPS and TPR and the effect of MoO3 addition to alumina supported vanadia catalysts on the catalytic activity for the selective catalytic reduction of NO by ammonia was investigated. Upon the addition of MoO3, catalytic activity was enhanced and the particle size of V2O5 which is shown by the results of B.E.T., XRD and Raman spectroscopy decreased. This was one reason for increased catalytic activity. The results obtained by XPS and TPR showed that MoO3 addition to alumina supported vanadia catalysts increased the reducibility of vanadia and this was the another reason for synergy effect between MoO3 and V2O5 in MoO3–V2O5/Al2O3 catalysts. ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
Keywords: V2O5/Al2O3 Selective catalytic reduction (SCR) Nitric oxide Reducibility MoO3 addition
1. Introduction The increasing problem of air pollution by nitrogen oxides has resulted in more stringent emission regulations in many countries and caused a great increase of interest in the development of effective de-NOx catalysts. One of the most available technologies is the so-called selective catalytic reduction (SCR) of NOx by ammonia process where the NOx is reduced by ammonia to form N2 and H2O. Many different types of catalysts have been developed for this process. Among various base metal oxides, vanadia is the most active and selective catalysts [1,2]. The deposition of vanadia on supports leads to an increase in the catalytic activity and V2O5/ TiO2 based catalysts are the most commonly used. Commercial SCR catalysts consist of tungsten oxide, vanadia and titania. Since vanadia is active not only in the reduction of NO but also in the undesired oxidation of SO2 to SO3, its content is generally kept low. WO3 is employed in larger amounts (nearby 10 wt%): it acts both as ‘chemical’ and ‘structural’ promoter by enlarging the temperature window of the SCR reaction and by improving the mechanical, structural and morphological properties of the catalysts [3–5]. However, recently the price of tungsten has risen about 30 times compared to ten years ago. SCR catalyst producer began to use MoO3 instead of WO3. In these catalysts, MoO3
typically replaces WO3 by a similar amount on a molar basis, so that molybdena loadings as high as 6 wt% are used. These catalysts have been reported to be less active than the analogous WO3– V2O5/TiO2 samples, but they are even more competitive both in tolerant to As [4–9] and in price. The surface reactivity of WO3– V2O5/TiO2 catalyst system was well characterized by various kinds of surface analyzers but MoO3–V2O5/Al2O3 catalyst system was relatively less analyzed. Alumina has a higher specific surface area, superior mechanical strength and is highly resistant to sintering, compared to titania. Therefore, many efforts have been made to apply alumina to the SCR of NO by ammonia [10–13]. When alumina is used as the supporter for the SCR reaction, SO2 in the flue gas, reacting with alumina or ammonia, deactivates the catalytic activity [11]. But results of other research show that activity of catalyst used alumina support could show the resistance to SO2 deactivation, depending on the pore size and specific surface area of alumina [12]. In order to enhance the activity and availability of aluminasupported vanadia catalysts for the SCR of NO by ammonia, we prepared MoO3–V2O5/Al2O3 catalysts. In this work, we tried to investigate the effect of MoO3 addition on the activity of V2O5/ Al2O3 catalysts for the SCR of NO by ammonia in detail. 2. Experimental
* Corresponding author. Tel.: +82 41 660 1424. E-mail address: [email protected] (H.-K. Park). 1 Department of Chemical Engineering, Hankyung National University, Ansung, Kyunggi, 456-749, Republic of Korea.
2.1. Preparation of catalysts A series of V2O5/Al2O3 catalysts with different vanadia loadings were prepared by the impregnation of gamma-Al2O3 (B.E.T. surface
1226-086X/$ – see front matter ß 2012 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2012.07.003
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
area: 167.5 m2/g, extra pure, Aldrich Chemical Co., USA) with aqueous solutions of ammonium vanadate (NH4VO3, extra pure, Yukari Co., Japan), followed by drying in air at 120 8C for 24 h and calcination in air at 500 8C for 3 h. MoO3–V2O5/Al2O3 catalysts with different loadings of MoO3 were prepared by the impregnation of gamma-Al2O3 with aqueous solutions of ammonium vanadate, followed by drying in air at 120 8C for 24 h and impregnation of these samples with aqueous solutions of ammonium molybdate ((NH4)6Mo7O244H2O, extra pure Yukari Co. Japan), followed by drying in air at 120 8C for 24 h and calcination in air at 500 8C for 3 h. These catalysts will be quoted in this paper according to their V2O5, and MoO3 contents: e.g., Mo6V12 identifies a catalyst sample with MoO3 = 6 wt%, V2O5 = 12 wt%, supported by alumina.
100
V6 V9 V12 V16 V23
90
NO Conversion (%)
74
80 70 60 50 40 30 20 10
2.1.1. Characterization of catalysts X-ray diffraction was measured using Dmax II X-ray diffractometer manufactured by Rigaku Co., Japan. The XPS analysis was performed at room temperature with a VG ESCALAB 220i-XL spectrometer manufactured by Fisons, USA. A monochromated Al anode (energy of the AlKa line 1486.6 eV), powered at 10 keV and 20 mA, was used for X-ray production. The binding energies were calculated with respect to the C 1s peak at 284.6 eV. Decomposition of peaks was done with the best fitting routine of the XPSPEAK program. Raman spectra were obtained by the Raman spectrometer (Raman-11, Nanophoton Co., Japan). The excitation source was Ar+laser (514.532 nm, 100 mW) with the detector of optical multichannel photodiode. H2-TPR (temperature programmed reduction, ASAP 2000 of Micromeritics, USA) experiments were performed in a quartz microreactor inserted into an electric furnace driven by a PID temperature programmed controller. The catalyst temperature was measured by means of a small K-type thermocouple inserted below the catalyst bed in a quartz microreactor. In a typical experiment, 0.2 g of catalyst was oxidized in He + 20% O2 at 500 8C for 1 h, cooled to 25 8C and purged with He and then heated to 750 8C with He + 5% H2 (total flowrate = 50 cm3/min (STP)). A dry-ice cooled silica gel trap was used at the reactor outlet to remove H2O formed during catalyst reduction and H2 consumption was monitored by means of a thermal conductivity detector. 2.1.2. Catalytic activity measurement Catalytic activity for selective catalytic reduction (SCR) of NO by ammonia has been measured in a tubular, downflow, fixed-bed reactor operated under isothermal and at slightly above atmospheric pressure and inserted into an electric furnace. In a typical test of catalytic activity, 1 cm3 of catalyst (20–40 mesh, 0.38–0.52 g) was used and the typical reactant gases for activity test consisted of 500 ppm NO, 500 ppm NH3, 10% O2 and balance He. The space velocity which is based on the ratio of the total gas flowrate to the volume of catalyst bed was 100,000 h 1, otherwise specified. The nitrogen oxide concentration of the gas stream was measured before and after the catalyst bed by means of chemiluminescent NO/NOx analyzer (Teledyne Technologies Co., USA) and for the measurement of oxygen content in the gas stream after reaction, inlet and outlet gases were also analyzed by gas analyzer (Greenline-MK2, Eutron Co., UK). 3. Results and discussion 3.1. Activity of V2O5/Al2O3 and MoO3–V2O5/Al2O3 catalysts Catalytic activity of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts for SCR of NO by ammonia was tested in the downflow fixed-bed reactor system. The results of the catalytic activity tests performed in SCR of NO by ammonia over V2O5/Al2O3 catalysts with different vanadia loadings (V2O5 = 6, 9, 12, 16, 23 wt%) are shown in Fig. 1.
0 0
100
200
300
400
500
600
o
Temperature ( C) Fig. 1. Conversion of NO with different V2O5 loadings of the V2O5/Al2O3 catalysts (GHSV: 100,000/h, NO: 500 ppm, NH3: 500 ppm, O2: 10%, He: balance).
The catalytic activity for SCR of NO by ammonia was expressed as a percentaged NO conversion at the reaction condition of NO 500 ppm, NH3 500 ppm, O2 10%, He balance and reaction temperatures from room temperature to 500 8C. This figure showed that the activity of the catalysts increased on increasing the V2O5 contents: in the case of V23 (23 wt% V2O5/Al2O3), containing highest vanadia loadings, appreciable NO consumption was observed at temperatures as low as 170 8C. As the vanadia loading increased from 3 to 23 wt%, the temperature required to achieve 50% NO conversion was progressively lowered from 320 8C down to 225 8C. At the higher investigated temperatures, for all the catalysts the NO conversion decreased due to the occurrence of the ammonia oxidation producing NO [11], and higher NO concentration than feed (500 ppm NO) was observed above 500 8C. While catalytic activity increased with increasing vanadia content of the catalysts, activity difference between V16 and V23 was relatively small in comparison with that of other catalysts. Besides, at the high temperature NO conversion of V23 was rather lower than that of V16 due to higher ammonia oxidation activity of V23 or structural effect of such a high loading of vanadia, e.g., decrease of surface area or pore volume. Over 16 wt% of vanadia content, increased vanadia content rarely showed another good effect, such as temperature window and maximum activity on the SCR of NO by ammonia. In the view of practical application as vanadia loading increased, the oxidation SO2 to SO3, which was undesired reaction, increased. In this regard V16 catalyst (16 wt% V2O5/Al2O3) showed the acceptable catalytic activity. In this study, in order to improve the catalytic activity of V2O5/ Al2O3 catalysts for SCR of NO by ammonia, molybdenum oxide (MoO3) was chosen for promoter added to V2O5/Al2O3 catalysts. Fig. 2 shows the activity of 6 wt% MoO3 added to 9 wt% and 12 wt% V2O5/Al2O3 catalysts for SCR of NO by ammonia. Okazaki [14] reported that alumina-supported molybdenum oxide showed low activity compared to titania-supported vanadia catalyst for the SCR of NO by ammonia. As seen in Fig. 1, activity of V23 was affected by structural limitation. Molybdenum oxide was added to V9 and V12, of which vanadia loadings were out of this limitation range and these Mo6V9 and Mo6V12 were tested for SCR of NO by ammonia. Fig. 2 shows the NO conversion of Mo6V9 and Mo6V12 and also compared the activity of V9 and V12. It is observed in Fig. 2 that catalytic activities of molybdenum containing catalysts were significantly higher active than those of the corresponding vanadia only catalysts and the temperature
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
* * *
100
A
75 A
A
V16
Conversion of NO (%)
90 80
Mo6V12
70 60 50
V12
40 30
Mo6V9 V9 V12 Mo6V9 Mo6V12
20 10 0
0
100
200
300
400
V9
500
0
o
Temperature ( C)
20
30
40
50
60
70
80
90
2q
Fig. 2. Effect of MoO3 addition to V9 and V12 catalysts on the catalytic activity (GHSV: 100,000/h, NO: 500 ppm, NH3: 500 ppm, O2: 10%, He: balance).
window for the reaction was widened and shifted toward lower temperatures. This was a clear indication that the presence of molybdenum oxide increased the activity of V2O5/Al2O3 catalysts. Besides, the catalytic activity of Mo6V9 and Mo6V12 reached nearly 100% NO conversion which was the highest activity obtained in this study. The ternary MoO3–V2O5/Al2O3 catalysts investigated in this study exhibit a higher activity in the SCR reaction compared to binary V2O5/Al2O3 catalyst. This result may suggest that Mo is a good promoter for binary V2O5/Al2O3SCR catalyst and synergy effect operates between the V and Mo surface oxide species. 3.1.1. Effect of MoO3 addition on the vanadia dispersion The effect of the vanadia loading on the catalysts’ structural and morphological properties was investigated by B.E.T., XRD and Raman spectroscopy. Table 1 reported the B.E.T surface area, pore volume and the V2O5 and MoO3 surface coverage (uv and uMo, respectively) calculated from the nominal V2O5 and MoO3 loading and the specific surface area of the sample. In this calculation a monolayer capacity of 13 mmol V5+/m2 and of 0.12 wt% for MoO3 m 2 [16] has been assumed. The overall V + Mo surface coverage has been estimated by simply summing those of V and Mo. As reported in Table 1, the theoretical V + Mo monolayer coverage was mostly reached in the sample with 16 wt% vanadia loading. The morphological characteristics of the catalysts were slightly modified upon MoO3 addition. The B.E.T. surface area of the V2O5/ Al2O3 catalysts decreased progressively from 166 down to 145 m2/g. In spite of adding MoO3 to 12 wt% V2O5/Al2O3 catalyst, the surface area of 6 wt% MoO3–12 wt% V2O5/Al2O3 increased slightly. A comparison of the surface areas of Mo6V12 and V12, having the Table 1 Surface area and pore volume of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts.
*
10
Catalyst
Surface area (m2/g)
Pore volume (cm3/g)
uv a
uMo a
uV+Mo a
Al2O3 V9 Mo6V9 V12 Mo6V12 V16
167.5 166.7 155.5 152.2 155.6 145.7
38.4 38.2 35.7 34.9 23.3 33.4
– 0.50 0.50 0.67 0.67 0.89
– – 0.29 – 0.29 –
– 0.50 0.79 0.67 0.96 0.89
uV, uMo, uV+Mo represent the surface coverage of V2O5, MoO3 and V2O5 + MoO3, respectively and calculated from the assumption of the text.
Fig. 3. XRD patterns of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts (A: Al2O3, *: V2O5).
same vanadia loading, implied that the presence of molybdenum oxide on the catalyst surface helped the vanadia to be well dispersed on the catalyst surface. To obtain the more information concerning the structure and morphology of the catalyst, X-ray diffraction data of the catalysts were collected. XRD patterns of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts are shown in Fig. 3. Diffraction lines of 9 wt% V2O5/Al2O3 and 6 wt% MoO3–9 wt% V2O5/Al2O3 showed no peaks of crystalline vanadia. In these two samples, it could be not excluded that particle size of vanadium oxide and molybdenum oxide on alumina was so small that diffractometer could not detect these species. For 12 wt% V2O5/Al2O3, 16 wt% V2O5/Al2O3 and 6 wt% MoO3–12 wt% V2O5/Al2O3 catalysts, diffraction lines due to V2O5 crystalline are observed as well as peaks for gamma-Al2O3. With the adding of the molybdenum oxide to 12 wt% V2O5/Al2O3 catalyst, the intensity of peaks and particle size of vanadium oxide is distinctively decreased. In line with the observation of increase of B.E.T. surface area in 6 wt% MoO3–12 wt% V2O5/Al2O3 in comparison with 12 wt% V2O5/Al2O3catalsyt, it was inferred that presence of molybdenum oxide improved the vanadia dispersion in the catalyst, so that particle size of vanadia could be small. Laser Raman spectroscopy could detect the amorphous monomeric, polymeric vanadates below monolayer coverage [15,17,18]. Went et al. [15] used in situ laser Raman spectroscopy and temperature-programmed reduction and temperature-programmed oxidation to establish the presence of monomeric vanadyls, polymeric vanadates and crystallines of V2O5 in titania supported catalysts. The distribution of vanadia species as a function of V2O5 loading was determined. At low V2O5 content in the catalyst the peak at 1030 cm 1 broadens significantly and the intensity of the bands located in the region 700–1000 cm 1 assigned to polymeric vanadates increases. The band at 840 cm 1 is assigned to V–O–V bending vibrations. The average polymer size varies from two to three monomer units depending on the vanadia loading. For vanadia amounts of 6 wt% and higher, a narrow band at 960 cm 1 appears [15]. Lietti and Forzatti [16] have shown by means of transient techniques such as TPD, TPSR, TPR and SSR (steady-state reaction experiments) that isolated vanadyls and polymeric metavanadate species are present on the surface of vanadia on titania catalysts with V2O5 loadings of up to 3.56 wt%. Polyvanadate species are more reactive than isolated vanadyls due to the presence of more weakly bonded oxygen atoms. Fig. 4(A) shows the results of Raman spectroscopy for 12 wt% V2O5/Al2O3 and 6 wt% MoO3–12 wt% V2O5/Al2O3 catalyst. It was
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
76
A
*
(a) V12 (b) Mo6V12
B
*
(a)
(a) V9 (b) Mo6V9
(a)
(b)
(b)
800
900
1000
1100
800
1200
900
1000
1100
1200
-1
Raman shift (cm )
-1
Raman shift (cm )
Fig. 4. Raman spectra of V2O5/Al2O3 and MoO3–V2O5/Al2O3 catalysts (*: V2O5).
observed that in 6 wt% MoO3–12 wt% V2O5/Al2O3, intensity of crystalline V2O5 peak remarkably decreased in comparison with that of V2O5/Al2O3 catalyst, having the same vanadia loading, and the peak of polymeric vanadates appears in 1040 cm 1. This data clearly indicated that the addition of MoO3 to V2O5/Al2O3 catalyst prevented the agglomeration of vanadia species into crystalline and enhanced the dispersion of vanadia acting the active component for SCR of NO by ammonia. For 6 wt% MoO3–12 wt% V2O5/Al2O3 and 12 wt% V2O5/Al2O3, XRD data shown in Fig. 3, was in line with the observation of Raman spectra of these catalysts. While XRD results showed no useful peak for 9 wt% V2O5/Al2O3 and 6 wt% MoO3–9 wt% V2O5/Al2O3 catalyst in Fig. 3, Raman spectra of these catalysts shown in Fig. 4(B) exhibited the effect of MoO3 addition to V2O5/Al2O3. Peak of crystalline V2O5 (presented as ‘‘*’’ in Fig. 4) in 9 wt% V2O5/Al2O3, which was not detectable with XRD technique because of small particle size of crystalline V2O5, diminished in 6 wt% MoO3–9 wt% V2O5/Al2O3 catalyst, having the same vanadia loading. Dispersion of active component on the surface of catalysts could also be measured with X-ray photoelectron spectroscopy. Results of XPS analysis were reported in Fig. 5. Fig. 5(A) shows V 2p region of the XPS spectra in V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts. The spectra of catalysts exhibited one sharp peak at about 517 eV, due to V 2p3/2, and a broad one at about 524.5 eV, due to V 2p1/2, respectively. The area of these peaks ratio (V/Al) could represent the surface concentration of vanadium, which implied the dispersion of the vanadia, indirectly. For both samples with or without MoO3, the profiles of the V 2p1/2 and V 2p3/2 lines were slightly asymmetric suggesting the presence of two types of vanadium on the surface of the catalysts. The deconvolution of the V 2p XPS spectra based on a Gaussian signal for the two types of
vanadium led to a good fitting of the results (Fig. 5(C) and (D)). The binding energies (516.0 and 517.2 eV for the V 2p3/2 line) thus were found to correspond to the value of V4+ in V2O4 and V5+ in V2O5, respectively [19–21]. Murakami et al. [21] reported V4+ ion, which acted as a Bronsted acid site and also interacted strongly with the alumina support, was inactive. Fig. 5(C), compared to (D), showed that much more portion of V5+ ions existed in 12 wt% V2O5/Al2O3 catalyst than 6 wt% MoO3– 12 wt% V2O5/Al2O3 catalyst. This result was also confirmed from the observation of color of catalyst, i.e., MoO3 added catalysts were brown and vanadia only catalysts were yellow. It was reported that for some reaction catalyzed by vanadia, e.g., the selective oxidation of o-xylene and the SCR of NO by ammonia in the presence of SO2 [22], the relative distribution of V4+ and V5+ species influences the activity and/or the selectivity. That effect on the latter SCR reaction might be attributed to the Bronsted acid site which was produced by V4+, acting as the active site in the high temperature range. Hence, distribution of these V4+ and V5+ ions and the ratio of sum of intensity of these two ions in XPS spectrum to Al 1s peak area due to the alumina support, were calculated from the deconvoluted XPS peak area, and the result is reported in Table 2. Table 2 shows that the fraction of V4+ in the molybdenum oxide added vanadia catalysts increased, compared to vanadia catalysts having the same vanadia loading, and the total surface concentration of the vanadia increased as MoO3 was added to catalyst. Fig. 5(B) shows that any difference did not exist between XPS Mo3d region of MoO3/Al2O3 and of MoO3–V2O5/Al2O3, i.e., oxidation state of Mo was unchanged in the ternary MoO3– V2O5/Al2O3. Because of the limitation of XPS technique, however, it
Table 2 Fraction of the surface concentration of V4+and V5+, calculated by XPS peak area.
V9 Mo6V9 V12 Mo6V12
Al
V4+
V5+
V4+/(V4+ + V5+)
(V4+ + V5+)/Al
355.72 273.40 376.15 288.54
152.61 392.99 203.51 523.68
479.88 298.32 641.04 398.47
0.271 0.572 0.240 0.567
1.832 2.193 2.451 2.926
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
A
B
(a) Mo6V12
V 2P
77
Mo 3d5/2
V 2P
Mo 3d3/2
Intensity (a.u.)
Intensity (a.u.)
(b) Mo6V9
(c) V12
(d) V9
510
515
520
525
225
530
230
Binding Energy (eV)
235
240
245
Binding Energy (eV)
C
D
a) V
510
5+
(b) V
Intensity (a.u)
Intensity (a.u.)
b) V
4+
515
520
525
Binding energy (eV)
(a) V
510
5+
4+
515
520
525
530
Binding Energy (eV)
Fig. 5. (A) XPS V2p spectra of V2O5/Al2O3, MoO3–V2O5/Al2O3 catalysts. (B) XPS spectra Mo3d of MoO3/Al2O3, MoO3–V2O5/Al2O3 catalysts. (C) XPS deconvolution of V2p region of 12 wt% V2O5/Al2O3 catalyst. (D) XPS deconvolution of V2p region of 6 wt% MoO3–12 wt% V2O5/Al2O3 catalyst.
was not excluded that the interaction of Mo and V could exist in the ternary MoO3–V2O5/Al2O3. Combined above B.E.T. surface area, XRD, Raman spectroscopy, and XPS analysis with the reactivity test, it could be concluded that adding molybdenum oxide to the V2O5/Al2O3 catalysts, while keeping the loading of vanadium constant, provoked a drastic decrease of the V2O5 crystallinity, so this well-dispersed active vanadium oxide in the catalyst enhanced the SCR reaction of NO by ammonia. 3.1.2. Effect of MoO3 addition on the reducibility of vanadia The effect of Mo addition to V2O5/Al2O3 was discussed in the view of improvement of vanadia dispersion which increased the reactivity of ternary MoO3–V2O5/Al2O3 catalyst for SCR of NO by ammonia. Besides these structural effects, the electronic one could be considered for the reason of the increase of reactivity. Lietti [4–7,9,16] suggested by the spectroscopic data such as
EPR, FT-IR, FT-Raman spectroscopy that in the MoO3–V2O5/TiO2 and WO3–V2O5/TiO2 catalyst the promoter changed the redox property of vanadium and this affected the reactivity for SCR of NO by ammonia over these catalysts. Fig. 6 shows the pretreatment effect by feed on the catalytic activity test. NO conversion of catalyst which has undergone SCR reaction under steady-state condition at 300 8C during 2 h and then at 500 8C for same condition was monitored at reaction of 300 8C again. Two samples presented in figure exhibited the same behavior in these experiment. NO conversion in second reaction test at 300 8C increased with respect to first test. This phenomenon implied the change of catalyst surface through the first reaction test. Fig. 7 shows the XPS analysis of catalyst before reaction and after reaction. It was found that the surface of catalysts was much reduced as the reaction temperature increased. This implied the state of catalyst in the reaction condition. In other word, vanadia might be changed to reduced state in reaction condition. Hence, It
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
78
100 o
o
o
300 C
500 C
300 C
80
(a)
(b)
(c)
Intensity (a.u.)
90
Conversion of NO (%)
70 60 50 40 30
o
after reaction, 300 C o after reaction, 500 C after calcination
20
V12 V9
10
510
515
520
525
Binding Energy (eV)
0 0
2
4
6
Fig. 7. XPS spectra of calcined and aged V9 catalysts.
Time (hr) Fig. 6. Effect of pretreatment by feed on the catalytic activity test.
seemed that the redox property of catalyst affected the reactivity of catalyst for SCR of NO by ammonia. Confirming this assumption more strictly, XPS experiment was carried out for the hydrogen-reduced catalysts. XPS spectra of the reduced catalysts in hydrogen flow at 300 8C for 3 h and XPS of unreduced, calcined catalyst is shown in Fig. 8. For the reduced catalyst, the region of lower binding energy of V 2p slightly increased, compared to the unreduced, calcined sample. In the case of MoO3 added to V2O5/Al2O3 catalysts in Fig. 8(B), this phenomenon was more distinctive, so that the lower binding region much more broadened. Although this experiment could not fully indicate the state of the catalyst surface in the reaction condition, it was certain that as the MoO3 was added to the V2O5/Al2O3 catalysts, the reduced surface of catalyst easily remained after reduction experiments and the reducibility of catalysts was improved. In real reaction conditions, the catalysts were considered to be reduced by the ammonia.
B
Intensity (a.u.)
Intensity (a.u.)
A
The results obtained during the hydrogen-TPR experiments over the various MoO3–V2O5/Al2O3 catalysts are presented in Fig. 9 and compared to MoO3/Al2O3 catalyst. In this experiment, no hydrogen consumption was observed in any case below 250 8C. The binary MoO3/Al2O3 sample is reduced above 400 8C; the hydrogen consumption showed a maximum near 450 8C and a pronounced tailing which extended up to 800 8C, where the hydrogen consumption increased again. The binary V2O5/Al2O3 catalyst, 12 wt% V2O5/Al2O3 and 9 wt% V2O5/Al2O3, exhibited a different behavior since in both case a single peak is evident with maximum near 530 8C and 500 8C, respectively. The reducibility order shown in these two figures indicated that the addition of MoO3 favors the catalyst reduction, and that the ternary catalysts are more easily reduced than the corresponding binary samples. Especially, Mo6V12 sample in Fig. 9(B) exhibited the maximum peak near 580 8C where was between the maximum of V12 and Mo6 and clearly indicated the MoO3 addition effect on the reducibility of V2O5/Al2O3 catalyst.
calcined Mo6V12 reduced Mo6V12
calcined V12 reduced V12 505
510
515
520
Binding Energy (eV)
525
530
505
510
515
520
525
530
Binding Energy (eV)
Fig. 8. Comparison of XPS spectra of calcined and reduced catalysts. (A) XPS spectra of calcined and reduced V12 catalysts. (B) XPS spectra of calcined and reduced Mo6V12 catalysts.
H.-L. Koh, H.-K. Park / Journal of Industrial and Engineering Chemistry 19 (2013) 73–79
A
B
Mo6V12 V12 Mo6
Intensity (a.u.)
Intensity (a.u)
Mo6V9 V9 Mo6
79
0
200
400
600
800
o
Temperature ( C)
0
200
400
600
800
o
Temperature ( C)
Fig. 9. TPR spectra of various catalysts. (A) Mo6, V9, Mo6V9 catalysts. (B) Mo6, V12, Mo6V12 catalysts.
4. Conclusion The effect of MoO3 addition on the dispersion of vanadia over the alumina surface was studied by B.E.T., XRD and Raman spectroscopy. As the result of these analysis, it was found that the MoO3 addition to the catalysts inhibited the growth of vanadia particle size, thus well-dispersed vanadia catalysts were obtainable. This vanadia species were more active for SCR reaction and become one of the reasons for enhanced reactivity. By the results of XPS and TPR, it has been concluded that the simultaneous presence of V and Mo in MoO3–V2O5/Al2O3 catalysts enhanced the reducibility of vanadia and that the higher reactivity of the ternary MoO3–V2O5/Al2O3 catalysts was related to this superior redox properties. Acknowledgement We owe thanks to the Resources Recycling Technology Development Program and Core Material Technology Development Program funded by the Ministry of Knowledge and Economy whose support was essential in carrying out this study. References [1] M.A.L. Vargas, M. Casanova, A. Trovarelli, G. Busca, Applied Catalysis B: Environmental 75 (2007) 303.
[2] L. Lietti, I. Nova, G. Ramis, G. Busca, E. Giamello, P. Forzatti, F. Bregani, Journal of Catalysis 187 (1999) 419. [3] Z. Si, D. Weng, X. Wu, J. Li, G. Li, Journal of Catalysis 271 (2010) 43. [4] L. Lietti, P. Forzatii, F. Bregani, Industrial and Engineering Chemistry Research 35 (1996) 3884. [5] M. Kobayashi, K. Miyoshi, Applied Catalysis B: Environmental 72 (2007) 253. [6] L. Lietti, I. Nova, G. Ramis, L. Dall’Acqua, G. Busca, E. Giamell, P. Fozatti, F. Bregani, Journal of Catalysis 187 (1999) 419. [7] G.B. Busca, L. Lietti, G. Ramis, F. Berti, Applied Catalysis B: Environmental 18 (1998) 1. [8] L. Casagrande, L. Lietti, I. Nova, P. Forzatti, A. Baiker, Applied Catalysis B: Environmental 22 (1999) 63. [9] L. Lietti, G. Ramis, F. Berti, G. Toledo, D. Robba, G.B. usca, P. Fozatti, Catalysis Today 187 (1999) 419. [10] G.E. Marnellos, E.A. Efthimiadis, I.A. Vasalos, Applied Catalysis B: Environmental 48 (2004) 1. [11] I. Nam, J.W. Eldrige, J.R. Kittrel, Industrial & Engineering Chemistry Product Research and Development 25 (1986) 192. [12] H.K. Matralis, M. Ciardelli, M. Ruwet, P. Grange, Journal of Catalysis 157 (1995) 368. [13] V.M. Mastikhin, V.V. Terskikh, O.B. Lapina, M. Seidl, H. Knozinger, Journal of Catalysis 156 (1995) 1. [14] S. Okazaki, M. Kumasaka, J. Yoshida, K. Kosaka, Industrial & Engineering Chemistry Product Research and Development 20 (1981) 301. [15] G.T. Went, L. Leu, A.T. Bell, Journal of Catalysis 134 (1992) 479. [16] L. Lietti, P. Forzatti, Journal of Catalysis 147 (1994) 241. [17] T. Kim, A. Burrows, C.J. Kiely, I.E. Wachs, Journal of Catalysis 246 (2007) 370. [18] I.E. Wachs, Journal of Catalysis 124 (1990) 570. [19] G. Centi, D. Pinelli, F. Ghoussoub, M. Guelton, L. Gengembre, Journal of Catalysis 130 (1991) 238. [20] M.C. Paganini, L.D. Acqua, E.G.L. Lietti, P. Forzatti, G. Busca, Journal of Catalysis 166 (1997) 195. [21] M. Inomata, K. Mori, A. Miyamoto, Y. Murakami, Journal of Physical Chemistry 87 (1983) 761. [22] G. Clarebout, M. Ruwet, H. Matralis, P. Grange, Applied Catalysis 76 (1991) L9.