Enhancement of Activity and Sulfur Resistance of CeO2 Supported on

May 1, 2012 - After the addition of SiO2, the number of Lewis-acid sites was not ... commercial interest for stationary and mobile source applications...
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Enhancement of Activity and Sulfur Resistance of CeO2 Supported on TiO2−SiO2 for the Selective Catalytic Reduction of NO by NH3 Caixia Liu,† Liang Chen,†,‡ Junhua Li,†,* Lei Ma,† Hamidreza Arandiyan,† Yu Du,† Jiayu Xu,† and Jiming Hao† †

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China ‡ CPI YUANDA Environmental-Protection Engineering Co., Ltd., Chongqing, 401122, China S Supporting Information *

ABSTRACT: A series of novel metal-oxide-supported CeO2 catalysts were prepared via the wet impregnation method, and their NH3−SCR activities were investigated. The Ce/TiO2−SiO2 catalyst with a Ti/Si mass ratio of 3/1 exhibited superior NH3−SCR activity and high N2 selectivity in the temperature range of 250−450 °C. The characterization results revealed that the activity enhancement was correlated with the properties of the support material. Cerium was highly dispersed on the TiO2−SiO2 binary metal oxide support, and the interaction of Ti and Si resulted in greater conversion of Ce4+ to Ce3+ on the surface of the catalyst compared to that on the single metal oxide supports. As a result of in the increased number of acid sites on Ce/TiO2−SiO2 that resulted from the addition of SiO2, the NH3 adsorption capacity was significantly improved. All of these factors played significant roles in the high SCR activity. More importantly, Ce/TiO2−SiO2 exhibited strong resistance to SO2 and H2O poisoning. After the addition of SiO2, the number of Lewis-acid sites was not decreased, but the number of Brønsted-acid sites on the TiO2−SiO2 carrier was increased. The introduction of SiO2 further weakened the alkalinity over the surface of the Ce/TiO2−SiO2 catalyst, which resulted in sulfate not easily accumulating on the surface of the Ce/TiO2−SiO2 catalyst in comparison with Ce/TiO2. TiO2,14 CexTi1‑xO2,15 CeO2−WO3/TiO2,16,17 CeO2−WO3,18,19 and CeO2/Al2O3,20 all of which have shown excellent SCR activity but with poor resistance to sulfur and water poisoning. Some support material such as TiO2, ZrO2, Al2O3 and SiO2 can be used for the NH3−SCR reaction. Among these oxides, anatase TiO221,22 is the most favorable support material for NO abatement; TiO2 also shows good low-temperature catalytic activity and good resistance to SO2 poisoning, especially when it is loaded with V2O5.23 It is well-known that SiO2 exhibits a high bonding strength, a large surface area, good optical properties, and excellent mechanical properties, and its surface silicon hydroxyl groups exhibit high thermal stability. Consequently, SiO2 is widely used in composite materials and as a catalyst support.24,25 Al2O3 is a porous solid material with a high specific surface area. This compound exhibits a high adsorption capacity, a large number of acid sites on its surface and good heat stability because of its microporous structure; these properties are vital to the catalytic properties of Al2O3. ZrO2 exhibits special characteristics, such as a high thermal stability, extreme hardness, acidic/basic features and stability under reducing conditions.26 ZrO2 also inhibits the sintering of

1. INTRODUCTION Nitrogen oxides are major atmospheric pollutants. The selective catalytic reduction (SCR) of nitrogen oxides with ammonia is of increasing commercial interest for stationary and mobile source applications.1 Currently, V2O5−WO3/TiO2 and V2O5− MoO3/TiO2 are the most widely used catalysts in a relatively narrow temperature window of 300−400 °C. However, several shortcomings of these systems still need to be addressed, including the narrow temperature window, the low N 2 selectivity at high temperatures, and the toxicity of vanadium to the environment.2 Given these disadvantages, the substitution of other metals for vanadium and the modification of the support have been investigated over the past several decades. Nonetheless, in practical applications, sulfur poisoning remains a problem for novel NH3−SCR catalysts that do not include vanadium. In the past few years, a large number of catalysts have been reported to be active for SCR of nitrogen oxides with ammonia to N2, such as Fe oxides supported on ZrO2 or TiO2,3,4 Cu/ TiO2;5 CuO/Al2O3;6 Mn-base catalyst;7,8 iron oxide and chromia supported on TiO2-pilllared clay;9 and Fe-exchanged ZSM-5.10 Nowadays, Ceria (CeO2) has received considerable attention for the SCR of NO because of its good oxidation and reduction abilities and because it promotes NO removal.1,11 Recently, a few researchers have reported CeO2 mixed with other oxides for NH3−SCR, such as Mn−Ce/TiO2,12,13 CeO2− © 2012 American Chemical Society

Received: Revised: Accepted: Published: 6182

January 19, 2012 March 26, 2012 May 1, 2012 May 1, 2012 dx.doi.org/10.1021/es3001773 | Environ. Sci. Technol. 2012, 46, 6182−6189

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isotherm using the Barrett−Joyner−Halenda (BJH) method. Prior to the surface area and pore size distribution measurements, the samples were degassed in vacuum at 300 °C for 4 h. X-ray diffraction (XRD) measurements were performed on a D/MAX-RB system equipped with a Cu Kα radiation source. The diffraction patterns were recorded in the 2θ range of 10 to 90° with a step size of 0.018° and a count time of 1 s per step. X-ray photoelectron spectroscopy (XPS) data were obtained with an ESCALab 220i-XL electron spectrometer from VG Scientific using 300 W Mg Kα radiation. The base pressure was approximately 3 × 10−9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. Temperature-programmed desorption (TPD) experiments of NH3 and SO2 were performed in a fixed-bed quartz reactor. A typical sample mass of 0.5 g and a gas flow rate of 300 mL/min were used during the experiments. The experiment consisted of four stages: (1) degasification of the sample in N2 at 500 °C for 1 h, (2) adsorption of 500 ppm NH3 (200 ppm SO2 + 3% O2) at room temperature for 1 h, (3) isothermal desorption in N2 at the temperature of the final step-response experiment until no NH3 (SO2) was detected, and (4) temperature-programmed desorption under N2 (TPD stage) at 10 °C/min up to 500 °C. In situ DRIFTS spectra were recorded using a Nicolet NEXUS 870 FTIR spectrometer equipped with a smart collector and an MCT detector cooled by liquid N2. The diffuse-reflectance FTIR measurements were performed in situ in a high-temperature cell fitted with ZnSe windows. The catalyst was finely ground, placed in a ceramic crucible, and manually pressed. Mass flow controllers and a sampletemperature controller were used to simulate the real reaction conditions. Prior to each experiment, the catalyst was heated to 350 °C under 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.

supported oxides in the presence of water at high temperatures.27 Composite oxide carriers, such as TiO2−Al2O3, CeO2−ZrO2, and TiO2−ZrO2, possess higher surface areas, better redox abilities, and improved sulfur resistance and oxygen storage capacities relative to single-component oxide carriers.28−31 Over the past few years, the performance of the catalyst carrier has rarely been discussed with respect to composites, even though their role in the NH3−SCR reaction should not be ignored. The aim of the present work is to investigate the effects of CeO2 loading on various supports, including singlecomponent oxides (TiO2, ZrO2, Al2O3, and SiO2) and composite oxides (TiO2−SiO2, TiO2−ZrO2, and TiO2− Al2O3). The results presented in this report show that CeO2/ TiO2−SiO2 exhibits the best catalytic activity for the SCR of NO by ammonia. The promotional effect of Si on a CeO2/ TiO2−SiO2 catalyst was also investigated.

2. EXPERIMENTAL SECTION 2.1. Preparation of catalysts. In the present work, TiO2− SiO2, TiO2−ZrO2, and TiO2−Al2O3 binary oxide supports were prepared using a hydrolyzation method. Tetrabutyltitanate was placed into three beakers; the solutions were subsequently stirred and heated in a water bath at 50 °C, and 1 M HNO3 was added. Either tetraethoxysilane, zirconium n-propoxide, or aluminum isopropoxide was slowly added to the prepared tetrabutyltitanate solutions. The solutions were subsequently stirred for 24 h and filtered, dried at 110 °C overnight, and calcined at 500 °C for 4 h in static air to produce TiO2−SiO2, TiO2−ZrO2, or TiO2−Al2O3 supports, which are denoted as Ti−Si, Ti−Zr, and Ti−Al, respectively. To investigate the effects of supports for cerium-based catalysts, all of the catalysts were prepared using the wetimpregnation method. The support materials of TiO2, Al2O3, ZrO2, SiO2, Ti−Si, Ti−Zr, or Ti−Al were impregnated with cerium nitrate solution, stirred for 2 h at 40 °C, exposed to ultrasonic energy for 2 h, dried overnight at 110 °C and calcined at 500 °C for 4 h in static air. Finally, the CeO2/TiO2, CeO2/Al2O3, CeO2/ZrO2, CeO2/SiO2, CeO2/TiO2−SiO2(x) CeO2/TiO2−Al2O3, and CeO2/TiO2−ZrO2 catalysts were prepared and denoted as Ce/Ti, Ce/Al, Ce/Zr, Ce/Si, Ce/ Ti−Si(x), Ce/Ti−Al, and Ce/Ti−Zr, respectively, where (x) corresponds to the mass ratio between TiO2 and SiO2. The loading of CeO2 on all catalysts was 10 wt %. 2.2. Evaluation of the Activity of the Catalysts. The selective catalytic reduction activities were performed in a fixedbed quartz reactor (i.d.: 9 mm) located inside a vertical furnace that was fed from the top. The reaction conditions were as follows: 500 ppm NO, 500 ppm NH3, 3% O2, 200 ppm SO2 (when used), 10% H2O (when used), and N2 as the balance gas. In all tests, the total flow rate of the feed gas was 300 mL/ min, which corresponded to a space velocity of approximately 28 000 h−1. The concentration of NOx, N2O, and NH3 in the inlet and outlet gases was measured using a Gasmet Dx-4000 FTIR gas analyzer. The activity data were collected when the catalytic reaction had been substantially maintained for 30 min at each temperature except for 150 °C, which was held at 60 min. 2.3. Catalyst Characterization. The BET surface area, average pore size, and average pore volume of the catalysts were measured by N2 adsorption at 77 K using a Quantachrome Autosorb AS-1 system. The pore size distribution was calculated from the desorption branch of the N2 adsorption

3. RESULTS 3.1. Activity of Various Catalysts. The activity curves of the cerium-base catalysts are shown in Figure 1. The Ce/Ti catalyst exhibited the highest activity among the cerium

Figure 1. NOx conversion during the SCR reaction over Ce-base catalysts with different carriers. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3% O2, N2 as balance gas, and GHSV: 28 000 h−1. 6183

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catalysts loaded onto the single metal oxide supports, and its NOx conversion reached 90% in the temperature range of 300− 400 °C. Notably, the activities of CeO2 supported on composite metal oxides were all higher than those of CeO2 supported on single-metal oxides. The NOx conversions of the Ce/Ti−Si catalysts with different TiO2/SiO2 ratios are shown in Figure 2. With the

Figure 3. Effect of SO2 on the SCR of NOx with NH3 over ceriumbase catalysts. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3% O2, 200 ppm SO2, 10% H2O, N2 as balance gas, 300 °C, and GHSV: 28 000 h−1.

3.3. Analysis of Specific Surface Area (BET) and XRD Patterns. The XRD patterns of the Ce/Ti and Ce/Ti−Si catalysts are presented in Figure 4. The diffraction peaks of Ce/ Figure 2. NOx conversion during the SCR reaction over Ce/Ti−Si catalysts with different TiO2/SiO2 mass ratios. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3% O2, N2 as balance gas, and GHSV: 28 000 h−1.

addition of SiO2 into TiO2, the activity temperature range was broadened; remarkably, Ce/Ti−Si (3:1) showed the best activity, and its NOx conversion was greater than 90% in the temperature range of 250−450 °C. With the further addition of SiO2, the NOx conversion decreased by 23% at 500 °C. Notably, N2 selectivity of all the catalysts was almost nearly 100% (shown in Figure S1 of the Supporting Information), which indicated that the Ce/Ti−Si catalyst exhibited excellent N2 selectivity. 3.2. Effect of SO2. The emission of SO2 is unavoidable in exhaust and flue gases, and the investigation of the effect of SO2 on SCR activities of catalysts is therefore important. The effects of SO2 were consequently investigated over Ce/Ti, Ce/Ti−Al, Ce/Ti−Si, and Ce/Ti−Zr catalysts (shown in Figure 3). All of the experiments were performed at a fixed temperature (300 °C), which was held constant for 24 h. As evident from the results presented in Figure 3, in the presence of SO2, the NOx conversion over Ce/Ti gradually decreased with time to 70% after 24 h. This result is consistent with the results in a recent report.32 The NOx conversion of Ce/Ti−Al and Ce/Ti−Zr catalysts also declined obviously. However, the NOx conversion over the Ce/Ti−Si catalyst was greater than 99% in the presence of SO2 after 24 h, which indicated that the catalytic activity of Ce/Ti−Si was not significantly influenced by the addition of SO2. In our previous work, the activity of a Ce/Ti catalyst slightly decreased in the temperature range of 150−450 °C 16 when H2O was added to the reaction gas. In comparison, the activity of the Ce/Ti−Si catalyst remained at 99% after the introduction of 10% H2O (shown in Figure S2 of the Supporting Information). Therefore, the Ce/Ti−Si catalyst exhibits strong resistance to water vapor and SO2 poisoning.

Figure 4. XRD patterns of Ce/Ti and Ce/Ti−Si catalysts.

Ti showed typical anatase-phase TiO2 (PDF-ICDD 21−1272) in addition to small amounts of rutile-phase TiO2 (PDF-ICDD 21−1276) and cubic fluorite-phase CeO2 (PDF-ICDD 34− 0394). From the XRD figure, in the pattern of the Ce/Ti−Si catalyst, only the broadened diffraction peak of anatase TiO2 was observed. It can be seen that the height of halfpeak breadth of anatase TiO2 on Ce/Ti−Si catalyst is much lower than that of Ce/Ti, which indicated the crystal particle of anatase TiO2 on Ce/Ti−Si catalyst was smaller than that on Ce/Ti catalyst. Notably, in the pattern of the Ce/Ti−Si catalyst, the diffraction peaks of rutile TiO2 and cubic CeO2 were almost completely disappeared. According to the literature,33 we concluded that it was doping SiO2 that hindered the crystal phase formation of TiO2, which might be main reason for us not gain the intensive peak. 6184

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were observed in its XPS spectrum, which suggested that the chemical state of Ce was primarily Ce4+. For the Ce/Ti and Ce/Ti−Si catalysts, the v1 and u1 peaks appeared in their XPS spectra, and the intensities of the two peaks that correspond to Ce3+ over Ce/Ti−Si were more intense than those over Ce/Ti. The Ce3+ ratio, calculated by Ce3+/(Ce3+ + Ce4+), of Ce/Ti−Si (66.9%) was significantly higher than that of Ce/Ti (55.0%). Furthermore, the peak of Ce3+ was more intense than the other peaks originated from Ce4+. These results indicated that Ce was in a partially reduced state on the surfaces of the Ce/Ti−Si catalysts, which might be attributable to the interaction between Ti and Si. In addition, the presence of Ce3+ may result in a charge imbalance, which that would lead to oxygen vacancies and unsaturated chemical bonds. This situation could generate additional chemisorbed oxygen or weakly adsorbed oxygen species on the surface of the catalyst, which is favorable for the SCR reaction.35,36 3.5. TPD. 3.5.1. NH3-TPD. The NH3−TPD profiles of the Ce/Ti and Ce/Ti−Si catalysts are shown in Figure 6. As evident from the results in part a of Figure 6, the amount of

The Brunauer−Emmett−Teller (BET) surface area, pore volume, and pore size of the catalysts are summarized in Table 1. The surface area of SiO2 was significantly greater than that of Table 1. Physical Properties of Various Catalysts samples

BET surface area (m2 g−1)

pore volume (mL g−1)

average pore diameter (nm)

TiO2 TiO2−SiO2 Ce/Ti Ce/Ti−Si

50.5 144.7 54.5 107.8

0.341 0.024 0.354 0.026

3.38 3.41 2.63 3.40

TiO2. Furthermore, after SiO2 was doped into TiO2, the surface area was increased, but the pore volume and average pore size were decreased, which was so small that it was considered that their effects for surface area were very little. If the crystal particle decreased, the surface area might increase. According to XRD result, doping SiO2 possibly restrained the crystal formation of TiO2 and the crystal particle of Ce/Ti−Si become smaller than that of Ce/Ti, which maybe affect the surface aera. Rutile-phase TiO2 is well-known to lead to smaller surface areas and decreased reactivity in gas−solid reactions, whereas anatase-phase TiO2 is recognized as a highly active carrier that can both help vanadium highly disperse onto the catalyst to form single vanadium-active centers.2 On the basis of the XRD result, doping SiO2 restrained the crystalline phase formation of TiO2, which indicated that doping SiO2 into TiO2 might hinder the rutile phase growth of TiO2. In addition, the incremental surface area with doping SiO2 into TiO2 were particularly beneficial for the dispersion of Cerium oxides, which might be the main reason for disappeared peak of cubic phase CeO2. Therefore, according to the BET and XRD results, the addition of an appropriate amount of SiO2 was beneficial not only for the formation of the TiO2 crystal phase but also for the dispersion of active sites over the carrier. 3.4. XPS. The chemical states of Ce over different catalysts were investigated using XPS; the results are shown in Figure 5. The peaks labeled v correspond to Ce 3d5/2 contributions, and those labeled u represent the Ce 3d3/2 contributions. The bands v, v2, v3, u, u2 and u3 are attributed to Ce4+, whereas v1 and u1 are due to Ce3+.34 For the Ce/Si catalyst, no v1 or u1 peaks

Figure 6. NH3−TPD profiles of the Ce/Ti and Ce/Ti−Si catalysts (a); SO2 + O2−TPD profiles of the Ce/Ti and Ce/Ti−Si catalysts (b).

Figure 5. Ce 3d XPS spectra for the Ce/Si, Ce/Ti, and Ce/Ti−Si catalysts. 6185

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NH3 adsorbed on the surface of the Ce/Ti−Si catalyst is obviously greater than that on the Ce/Ti catalyst. This result implies that the SiO2 doping could increase the number of acidic sites and lead to a significant improvement in the NH3 adsorption capacity. Furthermore, NH3 adsorption has been demonstrated to be crucial for the NH3−SCR reaction.37 Therefore, the increased NH3 adsorption may represent one of the most important reasons for the higher catalytic activity of the Ti−Si catalyst than that of the Ce/Ti catalyst. 3.5.2. SO2 + O2−TPD. To achieve deeper insight into the state of SO2 on the surfaces of the Ce/Ti and Ce/Ti−Si catalysts, the catalysts were investigated using SO2+O2−TPD experiments; the profiles are shown in part b of Figure 6. The absorption peaks of these two catalysts appeared primarily in the temperature range of 100−300 °C and 600−800 °C, respectively. The absorption peaks that appeared in the lowtemperature range might represent the physisorbed or weakly adsorbed SO2, whereas the peaks that appeared at higher temperatures might be due to the decomposition of sulfate on the surfaces of the catalysts.5,38,39 The SO2 adsorption over the Ce/Ti−Si catalyst at high temperatures was significantly lower than that observed over the Ce/Ti catalyst. This result illustrated that the alkalinity over the surface of the Ce/Ti−Si catalyst was weaker than that over the Ce/Ti catalyst. This reduced the alkalinity resulted in sulfate not easily forming on the surface of the Ti−Si binary oxide support, which decreased the blocking of active sites. That may represent a significant factor for the high activity of the Ce/Ti−Si catalyst in the presence of SO2. 3.6. In Situ DRIFTS Studies. 3.6.1. In Situ DRIFTS of Adsorbents over SiO2, TiO2, and TiO2−SiO2. Ammonia has been reported to be adsorbed on Brønsted- or Lewis-acid sites to form NH4+ or coordinated NH3 in the SCR reaction; gaseous or adsorbed nitric oxides then react with NH4+ or coordinated NH3 to form N2 and H2O.40 Therefore, the surface acidity of a catalyst is critical for the SCR reaction of NOx by NH3. DRIFTS spectra of NH3 adsorption over different carriers at room temperature are shown in Figure 7. The bands at 1600 cm−1 and 1182 cm−1 were attributed to asymmetrical and symmetrical bending vibrations of the N−H bonds in NH3 coordinately linked to Lewis-acid sites, whereas the bands at 1400 cm−1 were assigned to symmetrical bending vibrations of NH4+ species on Brønsted-acid sites.5,41 On the basis of the results in part a of Figure 7, few absorption peaks were observed on the surface of SiO2, and only one absorption band was apparent at 1182 cm−1 on the surface of TiO2. However, strong absorption peaks were observed at 1600 cm−1 and 1440 cm−1 on the surface of TiO2−SiO2. In contrast, TiO2 primarily adsorbed NH3 species on Lewis-acid sites; after the addition of SiO2, the number of Brønsted-acid sites was increased on Ti−Si carrier, which is consistent with the results of previous reports.42−45 Some researches46,47 argued that the silica is a SiO4 tetrahedral network and, when doping TiO2, will produce some distortion of the network, therefore increasing the amount of network strain which may give rise to a high −OH content when the strain is relieved. Therefore, these incremental Brønsted-acid sites on Ti−Si carrier have been suggested to be mainly attributed to NH4+ species adsorbed by a high level of −OH over the surface of the catalyst which resulted from the interaction TiO2 and SiO2. In principle, NO can be adsorbed in molecular form and oxidized on oxide surfaces, thereby giving rise to surface species, such as NO+, NO2−, NO2, and NO3−, which have been

Figure 7. DRIFTS spectra of NH3 adsorption over different carriers at room temperature (a), DRIFTS spectra of NO + O2 adsorption over different carriers at room temperature (b).

identified as key intermediates involved in SCR reactions.48−50 The DRIFTS spectra of NO+O2 adsorption over different carriers at room temperature are shown in part b of Figure 7. No obvious peaks were observed for SiO2, and a series of strong absorption peaks were observed on the surfaces of TiO2 and TiO2−SiO2 including those at 1610, 1584, 1510, 1480, and 1300 cm−1. The band at 1610 cm−1 was assigned to the bridge nitrate, and the band 1584 cm−1 was attributed to a bidentate nitrate. The bands at 1510, 1480, and 1300 cm−1 were attributed to monodentate nitrate. With the addition of SiO2 into TiO2, the positions of peaks at 1610, 1584, and 1300 cm−1 were changed; furthermore, the absorption peak (1480 cm−1) shifted to a higher band (1510 cm−1), which might represent another type of monodentate nitrate. All of the nitrate species absorbed on TiO2−SiO2 were similar to those on TiO2, except that the intensity of absorbance decreased. 3.6.2. In Situ DRIFTS of Effects of SO2 for SCR Reaction. To understand the deactivation mechanism of Ce/Ti and Ce/Ti− Si by SO2 for the SCR of NO with NH3, the DRIFTS spectra of 6186

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Ce/Ti and Ce/Ti−Si in a flow of NO + NH3 + O2 before and after the addition of 200 ppm SO2 at 300 °C were recorded and are depicted in Figure 8. Part a of Figure 8 shows the DRIFTS

SO2 at 300 °C. The presence of NH4+ species (1680 and 1429 cm−1) was obvious before SO2 was flowed into the reactor. After the addition of SO2, no obvious change was observed for the first 60 min. After 60 min, only a weak absorption peak linked to SO42‑ was detected, and the peaks of NH4+ and N−H vibrations were still not altered. These results demonstrate that the effect of SO2 over Ce/Ti−Si was much smaller than that over Ce/Ti.

4. DISCUSSION In our research, we investigated the effects of CeO2 loading on various supports, including single-component oxides (TiO2, ZrO2, Al2O3, and SiO2) and composite oxides (TiO2−SiO2, TiO2−ZrO2, and TiO2−Al2O3). The results presented in this report show that CeO2/TiO2−SiO2 exhibits the best catalytic activity for the SCR of NO by ammonia and high resistance ability to SO2 poisoning. According to the results of characterization of the supports and Cerium-base catalysts, the promotional effect aspects of SiO2 on TiO2−SiO2 support and CeO2/TiO2−SiO2 catalyst were further discussed in the following. 4.1. Effects of Doping SiO2 for TiO2−SiO2 Support. Doping SiO2 into TiO2 modified the physical−chemical properties of TiO2−SiO2 support in several aspects. First, on the basis of the XRD result, doping SiO2 restrained the crystalline phase formation of TiO2, which indicated that doping SiO2 into TiO2 might hinder the rutile phase growth of TiO2. Rutile-phase TiO2 is well-known to lead to smaller surface areas and decreased reactivity in gas−solid reactions, whereas anatase-phase TiO2 is recognized as a highly active carrier2 that can both help active ingredient highly disperse onto the catalyst. Second, according to BET result, the surface area of TiO2−SiO2 support was greatly enlarged after doping SiO2 into TiO2. In addition, from the DRIFTS results of NH3 adsorption over SiO2, TiO2, and TiO2−SiO2, TiO2 primarily adsorbed NH3 species on Lewis-acid sites; after the addition of SiO2, the number of Brønsted-acid sites was sharply increased on TiO2−SiO2 support and the Lewis-acid sites was not reduced. That was mainly for the fact that the silica oxide is a SiO4 tetrahedral network. After doping it into TiO2, the amount of network strain would be decomposed, which might give rise to a high −OH content. Furthermore, the incremental −OH would significantly increase the Brønsted-acid sites. 4.2. Effects of Doping SiO2 for Ce/Ti−Si Catalyst. With the addition of SiO2 into TiO2, the activity temperature range was broadened; remarkably, Ce/Ti−Si (3:1) showed the best activity, and its NOx conversion was greater than 90% in the temperature range of 250−450 °C. Significantly, compared to Ce/Ti, the stability of SCR activity over Ce/Ti−Si (3:1) was greatly improved in the presence of SO2. From the XPS result, more Ce4+ over Ce/Ti−Si (3:1) transformed into Ce3+ than that over Ce/Ti. The presence of Ce3+ may result in a charge imbalance, which that would lead to oxygen vacancies and unsaturated chemical bonds. In addition, NH3 adsorption capacity was enormously enhanced after doping SiO2 but SO2 adsorption over the Ce/Ti−Si catalyst was decreased at high temperature. That might be because that there are many −OH around the TiO2−SiO2, which may greatly increase the Brønsted-acid sites and weaken the alkalinity over the surface of the Ce/Ti−Si catalyst. Some researchers16 have reported he number of Brønstedacid sites might be an important factor for the NH3−SCR reaction at high temperatures (>200 °C) and part of the

Figure 8. DRIFTS spectra of the Ce/Ti (a) and the Ce/Ti−Si (b) catalysts in a flow of NH3 + NO + O2 before and after the addition of 200 ppm SO2 at 300 °C.

spectra of Ce/Ti. When the SCR gas was directed steadily into the reactor for 30 min, absorption peaks (at 1451 and 1198 cm−1) of NH3 and NH4+ appeared on the surface of the Ce/Ti catalyst, and their absorbance was weak. After SO2 was flowed for 5 min, the absorption peaks at 1680, 1351, and 1305 cm−1 successively appeared. Among these peaks, the bands at 1351 and 1305 cm−1 were attributed to SO42‑,38−40 and the band at 1680 cm−1 was assigned to NH4+ species. Moreover, the absorption peak for NH4+ (1451 cm−1) and the hydroxyl vibration were strengthened with time. These results indicated that SO2 and NH4+ produced (NH4)2SO4 and NH4HSO4 species on the catalyst and that these species covered the active sites, which was in agreement with a previous report.32 Part b of Figure 8 shows DRIFTS spectra of Ce/Ti−Si in a flow of NH3 + NO +O2 before and after the addition of 200 ppm 6187

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increased Brønsted acid sites could be resulted by the transformation of Ce4+ to Ce3+. Combining with our results, the reaction over the Ce/Ti−Si catalyst was proposed as eqs 1 and 2. As shown in Figures S3 and S4 of the Supporting Information, the introduction of SO2 had minimal impact on NH3 adsorption and restrained NO3− adsorption over the Ce/ Ti−Si catalyst. The NH3−SCR reaction of Ce/Ti−Si might follow the E-R mechanism at high temperatures (>200 °C) as eqs 3−5. Consequently, SO2 had slightly influence on the NH3−SCR process of the Ce/Ti−Si catalyst. Most importantly, because of the special network structure of the Ti−Si, sulfate formation was suppressed on the surface of the Ti−Si binary oxide support, which might be the most significant reason that Ce/Ti−Si maintains a high activity in the presence of SO2. In general, Ce/Ti−Si is a promising SCR catalyst for NOx abatement from mobile and stationary sources. TiO2 −SiO2 − OH

Ce+4 − NH3(g ) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ Ce+3 − NH4 +(a) TiO2 − SiO2

−OH − NH3 ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ −O− − NH4 + TiO2 − SiO2 − OH

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(1) (2)

NH3(g ) ⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NH4 +(a)(Brønsted acid site)

(3)

NH4 +(a) → NH 2(a) + 2H+ + e−

(4)

NH 2(a) + NO(g ) → NH 2NO(a) → N2 + H 2O

(5)

ASSOCIATED CONTENT

S Supporting Information *

Information regarding N2 selectivity during the SCR reaction over Ce/Ti−Si catalysts with different TiO2/SiO2 mass ratios, effect of H2O on NOx conversion over the Ce/Ti−Si catalyst at 300 °C, DRIFTS spectra of the influence of SO2 on NH3 adsorption on the Ce/Ti−Si catalyst, and DRIFTS spectra of the influence of SO2 on NO + O2 adsorption. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-10-62771093, fax: +86 1062771093, e-mail: address:[email protected] (J. Li). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Fund of China (Grant No. 51078203) and the National High-Tech Research and Development (863) Program of China (Grant No. 2010AA065001 and 2010AA065002).

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REFERENCES

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