Titiania Catalyst for Selective

Nov 18, 2010 - Alkali Metal Poisoning of a CeO2–WO3 Catalyst Used in the Selective Catalytic Reduction of NOx with NH3: an Experimental and Theoreti...
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Environ. Sci. Technol. 2010, 44, 9590–9596

DRIFT Study on Cerium-Tungsten/ Titiania Catalyst for Selective Catalytic Reduction of NOx with NH3 L I A N G C H E N , †,‡ J U N H U A L I , * ,†,§ A N D M A O F A G E * ,‡ Department of Environmental Science and Engineering, Tsinghua University, Beijing 100084, P.R. China, and State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Beijing National Laboratory for Molecular Sciences (BNLMS), Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China, and State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University, Beijing 100084, China

Received August 7, 2010. Revised manuscript received September 30, 2010. Accepted November 11, 2010.

CeO2/TiO2 and CeO2-WO3/TiO2 catalysts prepared by impregnation method assisted with ultrasonic energy were investigated on the selective catalytic reduction (SCR) of NOx (NO and NO2) by NH3. The catalytic activity of 10% CeO2/TiO2 (CeTi) was greatly enhanced by the addition of 6% WO3 in the broad temperature range of 200-500 °C, the promotion mechanism was proposed on basis of the results of in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH3 was introduced into both catalysts preadsorbed with NO + O2, SCR would not proceed except for the reaction between NO2 and ammonia. For CeO2/TiO2 catalysts, coordinated NH3 linked to Lewis acid sites were the main adsorbed ammonia species. When NO + O2 was introduced, all the ammonia species consumed rapidly, indicating that these species could react with NOx effectively. Two different reaction routes, L-H mechanism at low temperature (200 °C), were presented for SCR reaction over CeO2/TiO2 catalyst. For CeO2-WO3/TiO2 catalysts, the Lewis acid sites on Ce4+ state could be converted to Brønsted acid sites due to the unsaturated coordination of Cen+ and Wn+ ions. When NO + O2 was introduced, the reaction proceeded more quickly than that on CeO2/TiO2. The reaction route mainly followed E-R mechanism in the temperature range investigated (150-350 °C) over CeO2-WO3/TiO2 catalysts. Tungstation was beneficial for the formation of Ce3+, which would influence the active sites of the catalyst and further change the mechanisms of SCR reaction. In this way, the cooperation of tungstation and the presence of Ce3+ state resulted in the better activity of CeO2-WO3/TiO2 compared to that of CeO2/TiO2.

1. Introduction Nitrogen oxides (NOx), which results from automobile exhaust gas and industrial combustion of fossil fuels, is a major source * Corresponding author. † Department of Environmental Science and Engineering, Tsinghua University. ‡ Chinese Academy of Sciences. § State Key Joint Laboratory of Environment Simulation and Pollution Control, Tsinghua University. 9590

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of air pollution and can cause acid rain and photochemical smog (1). The selective catalytic reduction (SCR) of nitrogen oxides (NOx) with ammonia remains among the state-ofthe-art technologies for abating NOx in the flue gas from stationary sources (2). V2O5-WO3/TiO2 has been widely used for several decades to reduce NOx emitted from stationary power plants (3, 4). However, this type of catalyst is efficient only within a narrow temperature window of 300-400 °C, the loss of vanadium and formation of N2O at high temperatures (5, 6) are hazardous to the environment and to human health, and the high activity for oxidation of SO2 to SO3, which will be further reacted with H2O and NH3 to NH4HSO4, (NH4)2S2O7 and H2SO4 (7), could cause corrosion of downstream equipments and pore plugging of catalysts. Considering these disadvantages, many researchers continue to modify current catalysts and develop novel catalysts to reduce the vanadium loadings, or replace the vanadium with other metal elements. In recent years, cerium oxides have attracted much attention because of its oxygen and redox properties in the three way catalyst (TWC) used for gasoline engine emission control, and many researchers developed some cerium based catalysts to make it applied to NH3-SCR, such as CeO2/TiO2 (8), CeO2/Al2O3 (9), CeO2-WO3/TiO2 (10) and so on. All the catalysts showed high activity in the temperature range of 200-500 °C. Furthermore, mechanism involved in NH3-SCR has been studied by some researchers and several mechanisms have been proposed. Topsøe et al (11-14) proposed a “BrønstedNH4+” mechanism over V2O5-based catalyst which has gained the major support in the literature. However, The Lewis acid mechanism was first reported by Ramis et al. (15) in 1990 and supported later by Lietti et al. (16). They thought that in this catalytic sequence, the NH3 adsorbed on the Lewis acid sites is activated through H-abstraction (partial oxidation) to form NH2 (amide), which then reacts with the gas phase NO to form a nitrosamide (NH2NO) species, an intermediate that decomposes into N2 and H2O. Kijlstra et al (17, 18) suggested that over MnOx/Al2O3 catalyst, the initial step was the adsorption of NH3, and the NH2 species from the abstraction of hydrogen could react with both gaseous NO and adsorbed NOx species. In the studies of Marban et al (19, 20), it was suggested that surface adsorbed NH3 species mainly reacted with gaseous NO and NO2, which followed E-R mechanism over MnOx/Ac catalysts. However, there were few reports on the mechanism of CeO2/TiO2 based catalyst especially on this novel CeO2-WO3/TiO2 catalyst. In this work, both CeO2/TiO2 and CeO2-WO3/TiO2 catalysts were prepared by impregnation method. According to our previous study (10), the catalytic activity of 10% CeO2/ TiO2 (CeTi) was greatly enhanced by the addition of 6% WO3 in the broad temperature range of 200-500 °C. Therefore, by means of the in situ diffuse reflectance infrared transform spectroscopy (DRIFT), the modification of tungsten on the surface properties of catalysts and mechanisms of NH3-SCR reaction of CeO2/TiO2 and CeO2-WO3/TiO2 were explored.

2. Experimental Section 2.1. Catalyst Preparation. Both catalysts were prepared by the wet impregnation method with cerium nitrate, ammonium paratungstate, oxalic acid and Degussa AEROSIL TiO2 P25 support as we had reported (10).The catalysts were denoted as CeTi and CeWxTi (x represented the weight percentage of WO3, e.g., CeW6Ti.), and the cerium oxide contents in both catalysts were 10% by weight. 10.1021/es102692b

 2010 American Chemical Society

Published on Web 11/18/2010

FIGURE 1. NH3-SCR performances of CeTi and CeW6Ti catalysts as function of temperatures. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3% O2, N2 as balance gas, GHSV: 28 000 h-1. 2.2. Catalytic Performance Measurement. The SCR activity measurement was carried out in a fixed-bed quartz reactor (inner diameter 9 mm) using 0.5 g catalyst with 40-60 mesh. The typical reactant gas composition was as follows: 500 ppm NO, 500 ppm NH3, 3% O2 and N2 as the balance gas. The total flow rate was 300 mL/min, corresponding to a space velocity of about 28 000 h-1. The concentration of NOx, N2O, and NH3 in the inlet and outlet gas was measured by FT-IR gas analyzer Gasmet Dx-4000. The activity data were collected when the catalytic reaction substantially reached a steadystate condition for half an hour at each temperature. 2.3. In Situ DRIFTS Studies. In situ DRIFTS spectra were recorded by a Nicolet NEXUS 870-FTIR spectrometer equipped with a smart collector and an MCT detector cooled by liquid N2. The diffuse reflectance FT-IR measurements were carried out in situ in a high-temperature cell, fitted with ZnSe windows. The catalyst was finely ground and placed in a ceramic crucible and manually pressed. Mass flow controllers and a sample temperature controller were used to simulate the real reaction conditions. Prior to each experiment, the catalyst was heated to 350 °C in N2 with a total flow rate of 100 mL/min for 60 min. The IR spectra were recorded by accumulating 100 scans at a resolution of 4 cm-1.

3. Results 3.1. Catalytic Activity. NH3-SCR performances of CeTi and CeW6Ti catalysts were measured as a function of temperature, and the results are shown in Figure 1. For CeTi catalyst, the NOx conversion started at 200 °C and reached 90% at 300-400 °C. The onset temperature of NH3-SCR reaction decreased to 150 °C for CeW6Ti catalyst, and the NOx conversion was greater than 90% in a wider temperature range (200-400 °C). Therefore, the tungsten addition could remarkably enhance the NH3-SCR activity of CeTi catalyst and broaden the temperature window for NOx conversion. Furthermore, few N2O appeared over both catalysts, which indicated that Ce-based catalysts showed excellent N2 selectivity (10). 3.2. DRIFT Studies. 3.2.1. Co-Adsorption of NO and O2. NOx adsorbed species on CeTi at various temperatures were investigated by FTIR spectroscopy. Prior to NOx adsorption, the sample were treated at 350 °C in N2 for 1 h to remove any adsorbed species. After the sample was cooled to room temperature, 500 ppm NO and 3% O2 were introduced to the IR cell and IR spectra were recorded with increasing temperature. The DRIFT spectra of NO + O2 on CeTi at different temperatures are shown in Figure 2 (a). Several bands at 863, 1023, 1240, 1290, 1490, 1580, and 1610 cm-1 were observed. The bands at 1023 and 1580 cm-1 could be

FIGURE 2. DRIFT spectra of (a) CeTi and (b) CeW6Ti treated in flowing 500 ppm NO + 3% O2 at room temperature for 1 h and then purged by N2 at 50, 150, 200, 250, and 300 °C. attributed to bidentate nitrate (21).The bands at 1290 and 1490 cm-1 might be the monodentate nitrate due to their disappearance above 150 °C (22). The band at 1240 cm-1 might be assigned to the bridged nitrate (23). The band at 1610 cm-1 was very close to the asymmetric frequency of gaseous NO2 molecules (1617 cm-1) (24). But some researchers proposed different points. Si et al (25) assigned the band at 1622 cm-1 in the spectrum of W10ZrO2 to υ3 models of bridged nitrates; And Liu et al (26) also thought that the bands 1612 and 1244 cm-1 could be attributed to bridged nitrate species over FeMnTiOx catalysts. Here in our work, we could find that the band at 1240 cm-1 became visible at 200 °C, while the band at 1290 cm-1 attributed to monodentate nitrate almost vanished. It indicated the variation from monodentate into bridged nitrate happened with increasing temperature, which had a thermal stability different from the species due to the band at 1610 cm-1. Hence, we assigned the band at 1610 cm-1 to NO2. In the higher frequency, two weak bands at 1934 and 1996 cm-1 appeared which were due to weakly adsorbed NO (27). Figure 2 (b) showed the DRIFT spectra of NOx adsorption on CeW6Ti, which were very similar to that of CeTi. However, with the tungsten addition, the bands at 1290 and 1490 cm-1 due to unstable monodentate nitrate vanished below 150 °C, and all the bands showed less thermal stability than CeTi. These results strongly suggested that different nitrate species, gas phase or weekly adsorbed NO and NO2 would appear over cerium/titania-based catalysts, and unstable monoVOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 3. DRIFT spectra of (a) CeTi and (b) CeW6Ti treated in flowing 500 ppm NH3 at room temperature for 1 h and then purged by N2 at 50, 150, 200, 250, 300, and 350 °C.

FIGURE 4. DRIFT spectra taken at 200 °C upon passing 500 ppm NO + 3% O2 over the NH3 presorbed on (a) CeTi and (b) CeW6Ti for 0, 2, 5, 10, 20, and 30 min.

dentate nitrate could convert to more stable bridged nitrate at high temperatures. The tungsten addition worked upon not the forms but the thermal stability of the adsorbed nitrate species. 3.2.2. NH3 Adsorption. Figure 3 (a) showed the DRIFT spectra of NH3 adsorption on CeTi catalyst under the applied reaction temperature. Several bands in the range of 1100-1700 and 3100-3400 cm-1 were detected. The bands at 1600 and 1176, 1224 cm-1 can be assigned to asymmetric and symmetric bending vibrations of the NsH bonds in NH3 coordinately linked to Lewis acid sites (28), while the bands at 1440 cm-1 and in the range of 1850-1640 cm-1 could be attributed to asymmetric and symmetric bending vibrations of NH4+ species on Brønsted acid sites (29). In the NH stretching region, bands were found at 3360, 3260, and 3160 cm-1. Some negative bands around 3700 cm-1 were also seen, which could be assigned to the surface OsH stretching. Furthermore, two different bands also appeared at 1520 and 1560 cm-1 which did not belonged to Lewis or Brønsted acid sites. Kijlstra et al. (18) suggested that the band at 1510 cm-1 was due to an amide species. Ramis et al. (30-32) proposed that the bands at 1550 and 1570 cm-1 may be related to the intermediate of oxidation of ammonia. Accordingly, the bands at 1520 cm-1 might be attributed to amid (-NH2) species and 1560 cm-1 could be assigned to the intermediate of oxidation of ammonia. With the increase of temperature, all bands became weaker. The intensity of 1440 and 1684 cm-1 bands due to Brønsted acid sites decreased noticeably at higher temperature, while the bands at 1176 and 1224 cm-1

due to Lewis acid sites still remained. This result indicated that ammonia bonded to Lewis acid sites were more stable. Figure 3 (b) showed the DRIFT spectra of NH3 adsorption on CeW6Ti, which was much different from the spectra of CeTi. The band at 1600 cm-1 attributed to Lewis acid sites was not detected, whereas the band at 1440 cm-1 due to Brønsted acid sites appearing at room temperature was much stronger than that on CeTi. A broad band in the range of 1100-1260 cm-1 could be assigned to Lewis acid sites. Following the preceding discussion, the stronger band at 1560 cm-1 with increasing temperature could also be assigned to the intermediate of oxidation of ammonia, and band at 1510 cm-1 could be attributed to amid (-NH2) species. According to the results of XPS from our previous study (10), it has been known that most Ce4+ species could be transformed to Ce3+ species due to tungsten modification, which might be the key factor to affect the surface adsorbed NH3 species. Further detailed discussion would be expounded later. 3.2.3. Reaction between Nitrogen Oxides and Ammonia Adspecies. The catalysts were first purged with NH3 for 1 h followed by N2 purging. NO + O2/N2 was then introduced into the IR cell at 200 °C, and spectra were recorded as a function of time. As noted already, coordinated NH3 species (1176, 1224, and 1600 cm-1), amide species (1520 cm-1) and few NH4+ species (1684 cm-1) formed on CeTi catalyst upon treatment with NH3 (Figure 4 (a)). After NO + O2 was introduced into the cell, all ammonia adspecies decreased quickly. However, bands at 1560 cm-1 increased a bit, which

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FIGURE 5. DRIFT spectra taken at 200 °C upon passing 500 ppm NH3 over the NO + O2 presorbed on (a) CeTi and (b) CeW6Ti for 0, 5, 10, 20, and 30 min.

FIGURE 6. DRIFT spectra of (a) CeTi and (b) CeW6Ti in a flow of 500 ppm NO + 500 ppm NH3 + 3% O2 at 150, 200, 250, 300, and 350 °C.

could also prove that this band was the intermediate of oxidation of ammonia such as nitrate or nitrite, as report by Ramis et al (30).After 5 min, the bands due to NH3/NH2 diminished. At the same time, many new bands at 1610, 1580, 1375, and 1240 cm-1 were detected which were attributed to NOx species. As compared to CeTi, the bands assigned to adsorbed NH3 species decreased more quickly on CeW6Ti catalyst (Figure 4 (b)). When the catalyst was purged by NO + O2 for only 2 min, all the bands due to ammonia adspecies diminished. And then the bands due to NOx species appeared after 5 min. On the basis of Figure 4 (a) and (b), it could be concluded that NOx readily reacted with the adsorbed ammonia species, especially for CeW6Ti catalyst. 3.2.4. Reaction between Ammonia and Adsorbed Nitrogen Oxides Species. In this experiment, the reactants were introduced to both catalysts in the reversed order. The samples were first treated with NO + O2 /N2 for 1 h and then purged with N2 for 30 min. When NH3 was introduced at 200 °C, the spectra were recorded as a function of time. Both catalysts showed nearly the same variation (Figure 5 (a) and (b)). In the NH stretching region, the bands at 3360, 3260, and 3160 cm-1 were formed. The band at 1610 cm-1 due to NO2 decreased, and the peak at 1580 cm-1 shifted to 1560 cm-1. The spectra obtained from the reaction between ammonia and adsorbed NOx species at different times were relatively similar. After NH3 was purged over CeTi (Figure 5 (a)), the band at 1176 cm-1 attributed to the coordinated NH3 was observed, and the band at 1240 cm-1 due to bridged

nitrate transformed to two bands at 1290 and 1260 cm-1. Both bands might be also due to the nitrate species formed by oxidation of ammonia or the interaction between ammonia and adsorbed NOx species, which were different in the forms from the NOx species shown in Figure 2 (a). On CeW6Ti (Figure 5 (b)), after NH3 was introduced into the system, the intensity of the band at 1240 cm-1 due to bridged nitrate increased obviously, and its position moved to high wavenumber at 1260 cm-1. According to the analysis mentioned on CeTi catalyst, we could also assign the band at 1260 cm-1 to the deformation nitrate species. Simultaneously, the band at 1440 cm-1 attributed to NH4+ appeared. All the results indicated that the reaction between ammonia and nitrate species was unlikely to have occurred, except for the reaction between NO2 and ammonia. And the coexistence of ammonia and nitrate adspecies showed that NH3 and NOx could be adsorbed over different active sites of the catalyst surface. 3.2.5. DRIFT Spectra in a Flow of NO+NH3+O2. To indentify the species present on the catalysts under reaction condition, DRIFT spectra were recorded when samples were heated from room 150 to 300 °C in a flow of NO + NH3+O2. On CeTi (Figure 6 (a)), bands due to various nitrate species were observed at 1580, 1540, 1290, and 1260 cm-1 at 150 °C. Band at 1610 cm-1 might be caused by overlapping of bands of NO2 and coordinated NH3 on Lewis acid sides. In the regions of 3000-4000 cm-1, there was no NH stretching bands. Consequently, NOx could be adsorbed strongly on CeTi catalysts below 150 °C. Raising the temperature resulted VOL. 44, NO. 24, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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in a decrease in the band intensity of 1610 cm-1 (overlapping of bands of NO2 and coordinated NH3) and 1540, 1290 cm-1 (monodentate nitrate), whereas bands due to NH3 adspecies appeared (3360, 3260, 3160, and 1176 cm-1). A new band appeared at 1340 cm-1, which was quite different from adsorbed NOx and NH3 species mentioned above. And here we assigned this band to the intermediate species from the combination of surface adsorbed NH3 and NOx species. With an increase in temperature, the nitrate species diminished quickly, whereas the bands at 1176 and 1350 cm-1, which were attributed to coordinated NH3 and new intermediate species, still kept stable. Figure 6 (b) showed the DRIFT spectra in a flow of NO + NH3+O2 on CeW6Ti, which was much different from the spectra of CeTi. There was only one peak at 1260 cm-1 due to bridged nitrate in the whole region at 150 °C, which might be caused by the competitive adsorption and followed reactions among NH3, NO and O2. In the NH stretching region, obvious bands were found at 3360, 3260, and 3160 cm-1. Strong bands appearing at 1660 and 1440 cm-1could indicate the enhanced Brønsted acidic surface of CeW6Ti compared with the spectra obtained in NH3 adsorption (Figure 3 (b)), and it might result from the contact of water (generated in NH3-SCR process) and W ) O on surface of catalysts (33). Adsorption of NH3 on Lewis acid sites at 1600 and 1201 cm-1 were weak. It was worth noting that bidentate nitrate species (1580 cm-1), whose bands might be covered by the strong bands of ammonia species, failed to be detected in this process.

4. Discussion 4.1. Mechanism on CeTi Catalyst. From the DRIFT study, NO could be adsorbed and then oxidized to NO, NO3-, and NO2 on CeTi (Figure 2 (a)). The intensity of monodentate nitrate species decreased with increasing temperature. Meanwhile, larger amount of bridged nitrate species formed. These results strongly suggested that unstable monodentate nitrate could convert to more stable bridged nitrate at higher temperatures. With the temperature continuous increase, the adsorbed NOx species ceased changing forms, but decreased intensity. A large amount of nitrate species still existed steadily at a high temperature of 300 °C. After NH3 was passed over the NOx-adsorbed CeTi catalyst at 200 °C (Figure 5 (a)), NO2 decreased while bidentate nitrate species kept stable; and the bridged nitrate transformed to other forms of nitrate, indicating that NO2 could be reduced by ammonia, whereas bidentate nitrate could firmly occupy the active sites and hardly react with ammonia. The coexistence of ammonia and nitrate adspecies showed that NH3 and NOx could be adsorbed over different active sites of the catalyst surface. Ammonia could be adsorbed on both Lewis and Brønsted acid sites, resulting in coordinated NH3, NH4+, NH2, and other intermediates of ammonia oxidation. The number of coordinated NH3 was much higher than NH4+ ions. After NO + O2 was passed over the NH3-adsorbed sample, all adsorbed ammonia quickly vanished in 5 min. Hence, the reaction between NOx and coordinated NH3 predominated. The mechanism of surface reaction of NOx on mixedoxide catalysts has been studied extensively (34). Different hypotheses have been proposed for the mechanism (35, 36) including the reaction between the ammonium ion and adsorbed NO2, the reaction between an amide and gaseous NO, and the reaction between coordinated ammonia and a species generated by the spillover of NO on the support. Yang et al (37) proposed in a theoretical FTIR study that the SCR reaction of NO with NH3 on the MnOx(0.3)-CeO2(923) catalyst most probably took place according to the so-called amide-nitrosamide mechanism. The initial step was the 9594

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adsorption of NH3 on the Lewis acid sites of the catalyst, followed by reaction with nitrite species to produce N2 and H2O. In view of previous findings, we tried to propose a simplified reaction mechanism on CeTi catalyst, which was presented in eqs 1-6. Lewis acid sites were considered as the main active sites. Due to the impossibility of NO adsorbing directly on catalyst surface to form nitrate species, the oxidation of NO to NO2 is the only way to form nitrate species. The oxidation of NO to NO2 is now generally accepted to be an important reaction step to improve NOx reduction by NH3 (38). It was confirmed by the observation of NO2 in the stable and transient experiment described above. At low temperature below 200 °C, active sites for NH3 activation were covered by nitrate species (Figure 6 (a)), which might impede the NH3-SCR reaction (Figure 1). Hence, the reduction of NO to N2 by NH3 was suggested as followed L-H mechanism at temperature lower than 200 °C. The SCR reaction could take place between the coordinated NH3 and adsorbed NO3- species. When the SCR reaction happened above 200 °C, adsorption of NH3 became more active as Figure 6 (a) showed. Band at 1176 cm-1 due to Lewis acid appeared and kept stable with increasing temperature. Thus, E-R mechanism might become another important NH3-SCR reaction pathway at high temperature. Coordinated NH3 and amide species became main surface intermediates (Figure 3 (a)). Adsorbed ammonia was first transformed to amide species, and then reacted with gaseous NO to form nitrite and nitrate which then decomposed to nitrogen. The reaction of NH2 and NO and then the formation of nitrosamine (NH2NO) is a typical SCR mechanism reported for V2O5/TiO2 catalysts (39). O2(g) + 2* f 2O - *(*:surfaceactivesites)

(1)

NO(g) + O - * f NO2(a)

(2)

Ce4+

NH3(g) 98 NH3(a)(Lewis acid site)

(3)

2NH3(a) + NO2(a) + NO f 2N2 + 3H2O(low temperature: < 200oC) NH3(a) f NH2(a) + H+ + e-

(4) (5)

NH2(a) + NO(g) f NH2NO(a) f N2 + H2O(high temperature: > 200oC)

(6)

4.2. Mechanism on CeW6Ti Catalyst. From the XPS results (10), cerium species with lowered valence (Ce3+) were generated on catalyst due to tungsten modification, and this transformation might be an important factor to affect the mechanism of this Ce-based catalyst. The NOx adsorption on CeW6Ti (Figure 2 (b)) was similar to that on CeTi (Figure 2 (a)), except its stability. However, remarkable variation of NH3 adsorption happened for tungsten modification as Figure 3 (b) showed. Strong Brønsted acid sites were detected on CeWTi, which might rise from the unsaturated coordination of Ce3+ and W6+ ions. First, many reports (40) have proposed that WO3 on V2O5/TiO2 could increase the amount and the strength of Brønsted acid sites on the catalyst surface. Naturally, part of the enhanced Brønsted acid sites would be supplied by WO3. Second, adsorbed ammonia species on CeTi catalyst were mainly coordinated NH3 linked to Lewis acid sites (Figure 3 (a)). In Qi’s work (27), there were no bands due to NH4+ species over pure CeO2. Hence, with our XPS analysis, Ce4+ was the main state which would provide abundant Lewis acid sites. Tungsten modification caused the transformation from Ce4+ to Ce3+, accompanied by the

reduction of surface coordinated NH3. Therefore, part of the increased Brønsted acid sites could be resulted by the valence change of Ce state, and the reaction might be proposed as follows: WO3

Ce4+ - NH3 98 Ce3+ - NH+ 4

(7)

After NO + O2 was passed over NH3 adsorbed sample (Figure 4 (b)), the bands referring to surface ammonia species disappeared more quickly than that on CeTi, indicating that all the bands formed during the NH3 adsorption was active in SCR reaction. When NH3 was introduced to CeW6Ti preadsorbed with NO + O2 (Figure 5 (b)), stable bidnetate nitrate species formed, which is unfamiliar with ammonia, and the SCR reaction would not proceed in that way. This conclusion was also in good agreement with the above study of CeTi. According to DRIFT spectra of SCR activities, most active sites were covered by adsorbed ammonia species in the whole temperature range of 150-350 °C, and Brønsted acid sites served as important active sites. Hence, many nitrogen oxide species such as unfriendly bidentate nitrate could not be adsorbed over catalyst surface in the competition of NH3 and NOx, except bridged species. In this way, NH3-SCR process of CeW6Ti might mainly follow E-R mechanism in eqs 8-10. Ce3+, W6+

NH3(g) 98 NH+ 4 (a)(Brønsted acid site)

(8)

+ NH+ 4 (a) f NH2(a) + 2H + e

(9)

NH2(a) + NO(g) f NH2NO(a) f N2 + H2O

(10)

4.3. Effect of Tungstation. Tungsten affected the properties of catalyst in several aspects. First, tungstation was beneficial for the formation of Ce3+ state, while the presence of the Ce3+ species could create a charge imbalance, the vacancies and unsaturated chemical bonds on the catalysts surface (41), which will lead to the increase of chemisorbed oxygen on the surface. Second, tungstation could work upon not the forms but the thermal stability of the adsorbed nitrate species. However, WO3 could bring in many Brønsted acid sites, and the valence lower from Ce4+ to Ce3+ due to tungstation changed many Lewis acid sites into Brønsted acid sites. Third, tungstation could accelerate the NH3 activation, which made NH3 could be easily adsorbed at low temperature in the competition of NH3 and NOx. In a word, tungstation was beneficial for the formation of Ce3+, which would influence the active sites of the catalyst and further change the mechanisms of SCR reaction. In this way, the cooperation of tungstation and the presence of Ce3+ state resulted in the better activity of CeWTi in comparison to CeTi.

Acknowledgments This work was financially supported by the National Natural Science Fund of China (Grant No. 51078203), the National High-Tech Research and Development (863) Program of China (Grant Nos. 2006AA0618023 and 2007AA0618023).

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