Titania-Based Catalysts for Low

Jul 18, 2007 - DRIFT Study of Manganese/. Titania-Based Catalysts for. Low-Temperature Selective Catalytic. Reduction of NO with NH3. ZHONGBIAO WU ...
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Environ. Sci. Technol. 2007, 41, 5812-5817

DRIFT Study of Manganese/ Titania-Based Catalysts for Low-Temperature Selective Catalytic Reduction of NO with NH3 ZHONGBIAO WU, BOQIONG JIANG, YUE LIU,* HAIQIANG WANG, AND RUIBEN JIN Department of Environmental Engineering, Zhejiang University, Hangzhou 310027, China

Manganese oxides and iron-manganese oxides supported on TiO2 were prepared by the sol-gel method and used for low-temperature selective catalytic reduction (SCR) of NO with NH3. Base on the previous study, Mn(0.4)/ TiO2 and Fe(0.1)-Mn(0.4)/TiO2 were then selected to carry out the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) investigation for revealing the reaction mechanism. The DRIFT spectroscopy for the adsorption of NH3 indicated the presence of coordinated NH3 and NH4+ on both of the two catalysts. When NO was introduced, the coordinated NH3 on the catalyst surface was consumed rapidly, indicating these species could react with NO effectively. When NH3 was introduced into the sample preadsorbed with NO + O2, SCR reaction would not proceed on Mn(0.4)/TiO2. However, for Fe(0.1)-Mn(0.4)/ TiO2 the bands due to coordinated NH3 on Fe2O3 were formed. Simultaneously, the bidentate nitrates were transformed to monodentate nitrates and NH4+ was detected. And NO2 from the oxidation of NO on catalyst could react with NH4+ leading to the reduction of NO. Therefore, it was suggested that the SCR reaction on Fe(0.1)-Mn(0.4)/TiO2 could also take place in a different way from the reactions on Mn(0.4)/TiO2 proposed by other researchers. Furthermore, the SCR reaction steps for these two kinds of catalysts were proposed.

the reaction could only initiate with the adsorption of NH3 on the catalyst surface. When the catalysts were preadsorbed with NO, the monodentate and bidentate nitrates formed were unlikely candidates to participate in the SCR reaction. However, there are few studies on the reaction mechanism of Mn/TiO2 based catalysts especially Fe-Mn/TiO2 catalysts (10). In this paper, the catalysts based on Mn/TiO2 were prepared by the sol-gel method. According to our previous study (11), Mn/TiO2 had high activity when Mn:Ti ) 0.4. Furthermore, the addition of transition metals would enhance the catalytic activity at low temperature. Among the catalysts, Fe-Mn/TiO2 showed the highest activity when Fe:Mn:Ti ) 0.1:0.4:1. Therefore, the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) for Mn/TiO2 and Fe-Mn/ TiO2 was used to study the reaction mechanism of the SCR reaction.

2. Experimental Section 2.1. Catalyst Preparation. The catalyst was prepared by solgel method with butyl titanate (0.1-0.3 mol), ethanol (0.8 mol), water (0.6 mol), acetic acid (0.3 mol), and manganese nitrate as we reported (11). The ferric nitrate was added during the mix process. The catalyst was denoted as Fe(y)-Mn(z)/ TiO2, where y and z represent the mole ratio of Fe and Mn to Ti, respectively, e.g., Fe(0.1)-Mn(0.4)/TiO2. 2.2. Catalytic Activity Measurement. The SCR activity measurement was carried out in a fixed-bed, stainless steel reactor. The typical reactant gas composition was NO of 1000 ppm, NH3 of 1000 ppm, O2 of 3%, and balanced N2. And the GHSV (gas hourly space velocity) was 30 000 h-1. The concentration of NO, NO2, and O2 was monitored by a flue gas analyzer (KM9106 Quintox Kane International Limited), which was standardized by chemical methods. 2.3. Transient DRIFT Experiments. FTIR spectra were acquired using in situ DRIFT cell equipped with gas flow system. The DRIFT measurements were performed with Nicolet 5700 FTIR spectrometers at 4 cm-1 resolution with 64 co-added sans. In the DRIFT cell, the catalyst was pretreated at 773 K in He environment for 2 h, then cooled to 423 K. The background spectrum was recorded with the flowing of He and was subtracted from the sample spectrum.

3. Results 1. Introduction In recent years, manganese oxides have attracted interest because of its high SCR activity at low temperature. The manganese oxides contain various types of labile oxygen, which is necessary to complete a catalytic cycle (1, 2). To improve the dispersion of the active components, there were some investigations (3-6) on the bimetallic and ternary mixed-oxides catalysts such as Fe-Mn (4) and Pt/Fe/γ-Al2O3 (5). In these studies, it was suggested that the sintering could be reduced in these catalysts, and the catalytic activity was enhanced. Furthermore, the mechanism of SCR reaction has been studied and several mechanisms have been proposed. Kapteijn et al. (7) suggested that the first step in SCR was an oxidative abstraction of hydrogen from adsorbed ammonia. This suggestion was approved by Kijlstra et al. (8) and Qi and Yang (9). In the study of Pena et al. (10), it was suggested that * Corresponding author phone: (86)571-87952459; fax: (86)5718793088; e-mail: [email protected]. 5812

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3.1. Catalytic Activity. Figure 1 gave the variation of NO conversion with temperature by using Mn(0.4)/TiO2 and Fe(0.1)-Mn(0.4)/TiO2. By using Mn(0.4)/TiO2, 90% of NO could be converted at about 417 K, and the temperature at 90% NO conversion decreased to about 360 K with Fe(0.1)-Mn(0.4)/ TiO2. At 380 K, the reaction rate of NO removal was 10.6 × 10-3 mmol/g‚min and 16.1 × 10-3 mmol/g‚min for Mn(0.4)/ TiO2 and Fe(0.1)-Mn(0.4)/TiO2, respectively. Therefore, the activity of the catalysts was greatly improved with the addition of Fe within the low-temperature range. Furthermore, it was found that more NO2 was formed on Fe(0.1)-Mn(0.4)/TiO2 compared to Mn(0.4)/TiO2. At 423 K, when NH3 was switched off in the inlet gas, 92 ppm NO2 was measured at the outlet, while on Mn(0.4)/TiO2, only 15 ppm NO was oxidized. 3.2. DRIFT Study. 3.2.1. Ammonia Adsorption. Figure 2 showed the DRIFT spectra of Mn(0.4)/TiO2 after it was exposed to NH3/He for different times. Several bands at 1598, 1443, 1210, 1170 cm-1 and a broad band in the range of 1850-1640 cm-1 were detected. The bands at 1598 and 1210 cm-1 could be attributed to coordinated NH3 on Lewis acid sites (12). The bands at 1443 cm-1 and in the range of 10.1021/es0700350 CCC: $37.00

 2007 American Chemical Society Published on Web 07/18/2007

FIGURE 1. The variation of the catalytic activity with temperature by using Mn(0.4)/TiO2 and Fe(0.1)-Mn(0.4)/TiO2.

FIGURE 2. DRIFT spectra of Mn(0.4)/TiO2 exposed to 1000 ppm NH3 for various times and after purging by He for 30 min at 423 K. 1850-1640 cm-1 could be attributed to NH4+ species on Brønsted acid sites (13). In the NH stretching region (14), bands were found at 3349, 3245, and 3149 cm-1. Some negative bands around 3700 cm-1 were also found, which could be assigned to the surface O-H stretching. When the catalyst was with NH3 for 2 min, bands at 966 and 1230 cm-1 were detected, and these bands disappeared after the sample was purged by He. Thus, it could be assigned to weakly adsorbed NH3 or gas-phase NH3. After the sample was purged by He, the intensity of the band at 1170 cm-1 decreased, indicating that the adsorbed NH3 was not very stable. Figure 3 showed the DRIFT spectra of Fe(0.1)-Mn(0.4)/ TiO2 after it was exposed to NH3/He for different times. The spectra were very similar to that of Mn(0.4)/TiO2. However, the band due to weakly adsorbed NH3 was not detected, and the band at 1170 cm-1 was much stronger than on Mn(0.4)/ TiO2. After the sample was purged with He, the reduction of the intensity of the band at 1170 cm-1 was much smaller than that on Mn(0.4)/TiO2, indicating a stronger adsorption of NH3 due to the addition of Fe. According to the results of XPS from our previous study (15), it was known that the oxidation degree of Fe was +3. According to previous studies (13, 16), the band at 1170 cm-1 could be assigned to the coordinated NH3 on Fe2O3. Furthermore, the negative bands around 3700 cm-1 due to the surface O-H stretching was much stronger than those on Mn(0.4)/TiO2, indicating that more NH3 could be adsorbed on acidic O-H group. 3.2.2. NO Adsorption. Figure 4 showed the DRIFT spectra of Mn(0.4)/TiO2 after it was exposed with NO/He at different times. After introducing NO into the DRIFT cell, three bands

FIGURE 3. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 exposed to 1000 ppm NH3 for various times and after purging by He for 30 min at 423 K.

FIGURE 4. DRIFT spectra of Mn(0.4)/TiO2 exposed to 1000 ppm NO for various times and after purging by He for 30 min at 423 K. at 1625, 1440, and 1275 cm-1 appeared. There was another small band at 2237 cm-1 that could be due to N2O, and the band at 1625 cm-1 was due to NO2 disproportionation of NO (17). The bands at 1440 and 1275 cm-1 could be attributed to M-NO2 nitro compounds (18) and monodentate nitrate (19) on manganese oxides, respectively. After 30 min of the adsorption of NO on the sample, a band at 1580 cm-1 assigned to bidentate nitrate (20) formed on manganese oxides was detected. Furthermore, there was a small band at 1660 cm-1 assigned to N2O4 (21). Oxygen was not introduced to the system and there are two possibilities for the nitrite and nitrate species observed under this condition: the variation of N2O4 and the mobile lattice oxygen on the surface of the mixed oxides (21). After the sample was purged with He, the band due to NO2 disappeared and other bands were still visible. Figure 5 showed the DRIFT spectra of NO adsorption on Fe(0.1)-Mn(0.4)/TiO2, which was much different from the spectra of Mn(0.4)/TiO2. In the first 2 min, only a band at 1210 cm-1 was detected, which could be assigned to the nitrites formed on Fe2O3 (22). With time increasing, a band for NO2 appeared. After the catalyst was with NO for 30 min, a band at 1440 cm-1 assigned to nitro compounds was detected, but the intensity of this band was much weaker than on Mn(0.4)/TiO2. At 3740 cm-1, there was a band due to hydroxyl produced by H2O formed during the disproportionation of NO and NO2 (4, 23). As for Mn(0.4)/TiO2, the band at 1660 cm-1 assigned to N2O4 could also be detected. After the sample was purged by He, the intensity of all the bands decreased, whereas a band at 1580 cm-1 appeared. This band was also due to bidentate nitrate formed because VOL. 41, NO. 16, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 exposed to 1000 ppm NO for various times and after purging by He for 30 min at 423 K.

FIGURE 6. DRIFT spectra of Mn(0.4)/TiO2 pretreated by exposure to 1000 ppm NH3 +3% O2 followed by exposure to NO +3% O2 at 423 K for various times.

FIGURE 7. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 pretreated by exposure to 1000 ppm NH3 + 3% O2 followed by exposure to NO +3% O2 at 423 K for various times. of the variation of N2O4. Furthermore, the band corresponding to NO2 was still visible after He purging. 3.2.3. Co-adsorption of NH3+O2. The catalysts were treated with 1000 ppm NH3 and 3% O2 for 30 min, followed by purging with He for 30 min. Then the spectra of Mn(0.4)/TiO2 (Figure 6 a) and Fe(0.1)-Mn(0.4)/TiO2 (Figure 7 a) were recorded. The spectra obtained were similar to the spectra of NH3 adsorption, and the intensity of the bands appeared 5814

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FIGURE 8. DRIFT spectra of Mn(0.4)/TiO2 pretreated by exposure to 1000 ppm NO +3% O2 followed by exposure to NH3 at 423 K for various times.

FIGURE 9. DRIFT spectra of Fe(0.1)-Mn(0.4)/TiO2 pretreated by exposure to 1000 ppm NO +3% O2 followed by exposure to NH3 at 423 K for various times. to increase compared to the adsorption of NH3. On Fe(0.1)Mn(0.4)/TiO2, the band at 1210 cm-1 shifted to lower wavenumber at about 1198 cm-1. Furthermore, the intensity of the band at 1170 cm-1, attributed to adsorption on Fe2O3 sites, increased. 3.2.4. Co-adsorption of NO + O2. In this investigation, 1000 ppm NO and 3% O2 were introduced to the DRIFT cell for 30 min, and then He was purged for 30 min. After that the spectra were recorded. On Mn(0.4)/TiO2 (Figure 8 a), bands at 1580, 1555, 1530, 1440, 1275 cm-1 were detected. According to the summarization of adsorbed NO complexes by Kijlstra et al. (2), the band at 1580 and 1555 cm-1 were assigned to bidentate nitrate on manganese oxides, and the band at 1530 and 1275 cm-1 were due to monodentate nitrate (24). The band at 1440 cm-1 was nitro compounds. The intensity of the bands mentioned above increased greatly compared to the spectra shown in Figure 4. The intensity of the band at 1625 cm-1 due to NO2 was very low because NO2 was desorbed when the sample was purged with He. Compared to Mn(0.4)/TiO2, when NO was introduced with O2, more nitrates were formed on Fe(0.1)-Mn(0.4)/TiO2 (Figure 9 a). The new bands appeared at 1603 and 1250 cm-1, and they could be assigned to bridged nitrate formed on Fe2O3 (25), and the band at 1603 cm-1 overlapped the band due to NO2. 3.2.5. Nitric Oxide Adsorption after NH3. The catalysts were first purged with NH3 + O2 for 30 min followed by He purging. When NO + O2 were introduced, the IR spectra were recorded

as a function of time. For Mn(0.4)/TiO2 (Figure 6), the IR bands due to NH3 species decreased quickly. The intensity of bands at 3349, 3245, and 3149 cm-1 in the NH stretching region was weakened. In the first 10 min, there was little variation of the bands assigned to ammonium, in the range of 1640-1850 cm-1. The intensity of the band 1443 cm-1 due to NH4+ could not be determined, for this band was overlapped by the band at 1440 cm-1 due to nitro compounds. During this process, the intensity of the bands ranged from 1540 to 1570 cm-1 due to the increase in amide species (9), and the band at 3380 cm-1 could be assigned to H2O. After the catalyst was purged with NO + O2 for 20 min, the band at 1580 cm-1, attributed to nitrate species, could be observed, and the intensity of the band still increased with time. As compared to Mn(0.4)/TiO2, the bands assigned to adsorbed NH3 decreased more rapidly on Fe(0.1)-Mn(0.4)/TiO2 (Figure 7). NH3 adsorbed on Fe2O3 had disappeared after 20 min, and the NH3 adsorbed on manganese oxides was consumed gradually. After the catalyst was purged with NO + O2 for 10 min, the bands at 1580 and 1275 cm-1 corresponded to nitrate had been formed. And then the band at 1619 cm-1 appeared after 20 min. According to Qi and Yang (9), the NO2 band would shift to lower wavenumbers because of the electron transformation to NH4+. Therefore, the band at 1619 cm-1 was due to NO2. During this process, the bands assigned to nitrates on Fe2O3 did not appear. 3.2.6. NH3 Adsorption after Nitric Oxide. The catalysts were first purged with NO + O2 for 30 min followed by He purging. When NH3 was introduced, the spectra were recorded as a function of time. In the NH stretching region, the bands at 3349, 3249, and 3149 cm-1 were formed on both of the samples. On Mn(0.4)/TiO2 (Figure 8), introduction of NH3 into the system caused nearly no intensity change of the bands in the region of 1000-2000 cm-1. Only the intensity of the band due to monodentate nitrate increased a little, the band assigned to H2O could not be observed. However, on Fe(0.1)-Mn(0.4)/TiO2 (Figure 9), the spectra varied significantly with time. After the catalyst was purged with NH3 for 5 min, the bands at 1605 and 1250 cm-1 due to bridged nitrate on Fe2O3 had disappeared, and the band at 1170 cm-1 attributed to the coordinated NH3 formed on Fe2O3 was observed first. At the same time, the intensity of the bands at 1580 cm-1 assigned to bidentate nitrate on manganese oxides decreased gradually. Simultaneously, the intensity of the band at 1443 cm-1 due to NH4+ and the band at 1290 cm-1 due to monodentate nitrate increased. Furthermore, bands ranging from 1540 to 1570 cm-1 due to amide species were formed. In this process, the band at 3380 cm-1 attributed to H2O could be observed. It was difficult to determine the band around 1600 cm-1 because the water generated from the SCR reaction and the coordinated NH3 appeared in the same region. In this process, the band at 1210 cm-1 assigned to coordinated NH3 formed on manganese oxides did not appear.

4. Discussion From the DRIFT study, it was known that NH3 could be strongly adsorbed on Mn(0.4)/TiO2, resulting in coordinated NH3 and NH4+ (Figure 2). After NO + O2 was passed over the sample, the bands corresponding to coordinated NH3 quickly vanished and NH4+ species were gradually consumed (Figure 6). The disappearance of the bands due to coordinated NH3 strongly suggested that NH3 in this form was the activated form of ammonia, and it was capable of participating in lowtemperature SCR. When NH3 was introduced to the catalyst preadsorbed with NO + O2 (Figure 8), there was little variation with the bands corresponded to bidentate nitrate, and we failed to observe water bands in the DRIFT spectra. Thus, with the formation of bidentate nitrates, coordinated NH3 was difficult to form on the Mn(0.4)/TiO2, and the SCR

reaction did not proceed. This conclusion was in good agreement with the study of Pena et al. (10). On Fe(0.1)-Mn(0.4)/TiO2, coordinated NH3 could be also adsorbed on Fe2O3 (Figure 3). After NO + O2 was passed over the sample (Figure 7), the bands referring to coordinated NH3 disappeared more quickly than on Mn(0.4)/TiO2, indicating that all the bands formed during the NH3 adsorption was active in the SCR reaction. Furthermore, it could be detected that NO2 had interactions with NH4+ in this process. When NH3 was introduced to Fe(0.1)-Mn(0.4)/TiO2 preadsorbed with NO + O2 (Figure 9), the band due to bridged nitrates on Fe2O3 quickly vanished, and the band due to coordinated NH3 on Fe2O3 was detected. Furthermore, with the decreasing of the intensity of the band corresponding to bidentate nitrate, the intensity of the band due to monodentate nitrate on manganese oxides greatly increased. Thus, it could be proposed that in this process, bidentate nitrates could be transformed to monodentate nitrates. Simultaneously, a large band due to NH4+ was detected. In the investigation of NH3 adsorption (Figure 3 and Figure 7a), the amount of NH4+ formed was less than that on Mn(0.4)/TiO2. Thus, the large amount of NH4+ formed on Fe(0.1)-Mn(0.4)/TiO2 in Figure 9 was due to the Brønsted acid sites brought by the NO complexes. Though it was difficult to determine the band around 1600 cm-1, the amide species which was considered the intermediate of the SCR reaction, appeared when NH3 was induced. Therefore, it could be concluded that SCR reaction could also take place on Fe(0.1)-Mn(0.4)/TiO2 in this process. The mechanism of the SCR reaction had been investigated in some previous works (7-10, 26), and several hypotheses had been proposed. The formation of NH2NO was the key step in most of the mechanism of NO reduction. In our investigation of NO adsorption on the catalyst preadsorbed with NH3, the coordinated NH3 was consumed rapidly after NO + O2 was introduced, and the bands assigned to water were detected. XRD and XPS results from our previous study (15) suggested that most probably Mn existed as Mn4+ in the catalysts. Therefore, the SCR reaction could take place between the coordinated NH3 and NO in the way as follows:

When NH3 was introduced to Mn(0.4)/TiO2 pretreated with MnOx

NH3(g) 98 NH3(a)

(2)

NH3(a) + O(a) f NH2(a) + OH(a)

(3)

NH2(a) + NO(g) f NH2NO(a)

(4)

NH2NO(a) f H2O + N2(g)

(5)

NO + O2, the SCR reaction would not proceed. On Fe(0.1)Mn(0.4)/TiO2, bidentate nitrates were consumed and abundant of NH4+ species were formed. It was suggested by Long and Yang (26) that NH4+ would react with nitrate and form N2O and water. If the reaction was carried out in this way, the nitrates would be consumed and the bands due to nitrates should be reduced. It can be seen in Figure 9 that the bidentate nitrates were transformed to monodentate nitrates on Fe(0.1)-Mn(0.4)/TiO2. However, the intensity of the band due to monodentate nitrates did not decrease. Furthermore, as discussed above, NH4+ was formed due to the Brønsted acid VOL. 41, NO. 16, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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sites brought by the variation of NO complexes. Therefore, this phenomenon might happen due to the reaction mechanism suggested by Hadjiivanov et al. (27): with ammonia and water molecules, part of the bidentate nitrates could be transformed to monodentate nitrates, and NH4+ was formed. Bridged nitrates formed on Fe2O3 were not stable under the condition investigated (28). When NH3 was induced to Fe(0.1)-Mn(0.4)/TiO2, bridged nitrates disappeared rapidly, and coordinated NH3 on Fe2O3 was formed. This kind of NH3 species could not be detected during the same process on Mn(0.4)/TiO2 (Figure 8). Therefore, it was assumed that bidentate nitrate reacted with coordinated NH3 on Fe2O3, leading to the reduction of bidentate nitrate (Figure 9). Furthermore, it could be found in Figure 5 that on Fe(0.1)-Mn(0.4)/TiO2, the band attributed to NO2 was still visible after the sample was purged with He, indicating that the adsorption of NO2 on this sample was stronger. And it was found NO2 had interaction with NH4+ on Fe(0.1)-Mn(0.4)/TiO2 (Figure 7). It was suggested in a previous study (29) that the NO2 could react with NH4+, and the formed NO2[NH4+]2 would react with NO, producing N2 and H2O. In conclusion, by using Fe(0.1)-Mn(0.4)/TiO2, NO could be removed in another way as shown: Fe2O3

NH3(g) 98 NH3(a)

(6)

(3)

(4) (5) (6) (7) (8)

(9) (10)

(11)

It was mentioned above in catalytic activity section that more (12) (13) (14) +

+

NO2(a) + 2NH4 f NO2[NH4 ]2 +

NO2[NH4 ]2 + NO f ‚‚‚ f 2N2 + 3H2O + 2H

(8) (15) +

(9)

NO2 was formed on Fe(0.1)-Mn(0.4)/TiO2 compared to Mn(0.4)/TiO2. It was also found that the intensity of the band corresponded to NO2 on Fe(0.1)-Mn(0.4)/TiO2 was much stronger than that on Mn(0.4)/TiO2 (see Figures 4 and 5). The same findings were also reported by Huang and Yang (4). Therefore, the Fe(0.1)-Mn(0.4)/TiO2 has better redox properties compared with Mn(0.4)/TiO2. This may be one of the reasons of its higher activity for NO reduction. On the other hand, By using Mn(0.4)/TiO2, the SCR reaction could only proceed in the first way (eqs 1-5). And on Fe(0.1)Mn(0.4)/TiO2, with the formation of coordinated NH3 on Fe2O3, the reaction could also take place in another way (eqs 6-9). The second reaction pathway may give some contribution to the improvement of catalytic activity.

Acknowledgments The project is financially supported by the National Natural Science Foundation of China (NSFC-20577040) and New Century Excellent Scholar Program of Ministry of Education of China (NCET-04-0549).

(16) (17)

(18)

(19) (20) (21) (22) (23)

Supporting Information Available Additional figures and tables. This material is available free of charge via the Internet at http://pubs.acs.org.

(24)

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Received for review January 5, 2007. Revised manuscript received May 20, 2007. Accepted June 14, 2007. ES0700350

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