TiO2

Article pubs.acs.org/JPCC

Enhancement of Catalytic Activity Over the Iron-Modified Ce/TiO2 Catalyst for Selective Catalytic Reduction of NOx with Ammonia Yun Shu,† Hong Sun,‡ Xie Quan,*,† and Shuo Chen† †

Key Laboratory of Industrial Ecology and Environmental Engineering (Ministry of Education, China), School of Environmental Science and Technology, Dalian University of Technology, Dalian 116024, China ‡ School of Environmental & Chemical Engineering, Dalian Jiaotong University, Dalian 116028, China S Supporting Information *

ABSTRACT: A series of iron-modified Ce/TiO2 catalysts with different Fe/Ti molar ratios were prepared by an impregnation method and used for selective catalytic reaction (SCR) of NOx with NH3. The Fe−Ce/ TiO2 catalyst with a Fe/Ti molar ratio of 0.2 had good low-temperature activity and sulfur-poisoning resistance compared with the Ce/TiO2 catalyst. The introduction of Fe could increase the amount of Ce3+ and chemisorbed oxygen species on the catalyst surface and thereafter generate more ionic NH4+ and in situ formed NO2, respectively. In addition, the dispersion of cerium oxide could be improved by the addition of iron, and no visible phase of iron oxide could be observed at low Fe/Ti molar ratios (≤0.2). All of these factors played significant roles in the enhanced catalytic activity, especially the low-temperature activity. Furthermore, mechanisms of the SCR reaction and the SO2 poisoning of the Fe(0.2)−Ce/TiO2 catalyst were studied using in situ diffuse reflectance infrared Fourier transform spectroscopy. Coordinated NH3 and ionic NH4+ species as well as adsorbed NO2 might be the key intermediates in the SCR reaction in the relatively low-temperature range. The formation of ammonium sulfate appeared to be the dominant cause for the catalyst deactivation in SO2-containing gases. that the accessible Fe3+ species were active sites in the SCR of NO with NH3. Qi and Yang9 observed that the addition of Fe could improve the low-temperature SCR activity of the Mn/ TiO2 catalyst through enhancing the oxidation of NO to NO2. Wu et al.10 investigated the effect of transition metal addition on the manganese-based catalyst for SCR of NOx and found that iron had the most favorable effect on the catalytic activity. They thought that the addition of iron significantly improved the dispersion of Mn and Ti and thus enhanced the lowtemperature activity of the Mn/Ti catalyst. In addition, Li et al.11 synthesized the Fe−Ti spinel catalyst that had excellent SCR activity, selectivity, and H2O/SO2 durability at 300−400 °C. In their work the increase of reducible Fe3+ species due to the incorporation of Ti into γ-Fe2O3 was found to be a key factor in promoting SCR reaction through the Eley−Rideal mechanism. Therefore, iron may be a good candidate to improve the low-temperature SCR activity and sulfur-poisoning resistance of Ce-based catalysts. TiO2 is commonly used as a support during the SCR of NOx with NH3 because of its excellent sulfur tolerance. In this work, a series of Fe-modified Ce/TiO2 catalysts were prepared for SCR of NOx to investigate the influence of Fe on catalytic

1. INTRODUCTION Selective catalytic reduction (SCR) with ammonia is the dominant technology to remove NOx in the exhaust gas from stationary sources. The commercial catalysts for this process are based on TiO2-supported V2O5−WO3 and/or V2O5−MoO3 oxides.1−3 However, this type of catalyst is efficient only within a narrow temperature window of 300−400 °C. Furthermore, the loss of vanadium during the catalyst preparation and operation is hazardous to the environment and human health. Therefore, it is of great significance and interest to develop novel catalysts to reduce the vanadium loadings or replace the vanadium with other metal elements. Nowadays, ceria (CeO2) with the advantages of nontoxic, unique oxygen storage capacity, and good redox properties has received much attention for SCR of NO. Recently, a few researchers have reported Ce-based catalysts for NH3 SCR, such as CeO2−V2O5−WO3/TiO2,4 Ce/TiO2,5 Ce−Al2O3,6 and CeO2−WO3,7 all of which have shown various SCR activities under different conditions. Nevertheless, these catalysts commonly suffer from insufficient activity at low temperatures and/or severe deactivation by SO2 poisoning. Iron oxide is widely used in catalytic applications either as a promoter or as an active component, and the most ironcontaining SCR catalysts are Fe2O3-loaded or Fe3+-exchanged zeolites types. Brückner et al.8 prepared Fe-ZSM-5 catalysts and studied the roles of Fe on catalytic performance. They thought © 2012 American Chemical Society

Received: July 16, 2012 Revised: October 15, 2012 Published: November 12, 2012 25319

dx.doi.org/10.1021/jp307038q | J. Phys. Chem. C 2012, 116, 25319−25327

The Journal of Physical Chemistry C

Article

2.3. SCR Activity Measurements. SCR activity measurements were carried out in a fixed-bed quartz flow reactor (i.d. 9 mm) containing 0.5 g of catalyst. The typical composition of the reactant gas was: 1000 ppm NO, 1000 ppm NH3, 3 vol % O2, 500 ppm SO2 (when needed), and N2 as the balance gas. The total flow rate was 500 mL/min (refers to 1 atm and 298 K) which corresponded to a gas hourly space velocity (GHSV) of 30 000 h−1. The NO, NO2, and O2 concentrations were measured online by a flue gas analyzer (ecom-J2KN, rbr Messtechnik GmbH Inc.). The N2O concentration was analyzed by a gas chromatograph (Shimadzu GC-14C) with a Porapak Q column. The reaction system was kept for 1 h at each reaction temperature to reach a steady state before the analysis of the product was performed. The NOx conversion and N2 selectivity were calculated as follows

activity and SO2-poisoning resistance. The effects of iron on surface properties of the catalyst were also studied using N2 physisorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction (H2TPR), NH3 temperature-programmed desorption (NH3-TPD), and in situ diffuse reflectance infrared Fourier transform spectroscopy (in situ DRIFTS) of NH3 and NOx adsorption. Finally, mechanisms of the SCR reaction and the SO2 poisoning of the Fe−Ce/TiO2 catalyst were investigated using in situ DRIFTS.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The Fe-modified Ce/TiO2 catalysts were prepared by an impregnation method. In a typical synthesis, the required amount of cerium nitrate and/or iron nitrate was added into a 250 mL beaker containing 1.0 g of TiO2 support (P25, Degussa) in 80 mL of deionized water. The mixture was then treated in ultrasonic for 12 h followed by drying at 80 °C overnight and then calcined in air at 500 °C for 3 h. The catalyst was denoted as Fe(x)−Ce/TiO2 (“x” represents the Fe/Ti molar ratio; x = 0.02, 0.1, 0.2, 0.7), and the Ce/Ti molar ratio of all catalysts was 0.2. For comparison, Ce/TiO2 and Fe/TiO2 (Fe/Ti = 0.2 in molar ratio) were prepared using the same method. A state-of-the-art SCR catalyst, 1.5 wt % V2O5−7 wt % WO3/TiO2, was also prepared, using the impregnation method, as a reference in the SCR activity measurements. NH4VO3 and (NH4)10W12O41 were used as the source of vanadium and tungsten, respectively. All the above catalysts were ground and sieved to 40−60 mesh for evaluation. 2.2. Catalyst Characterization. The specific surface areas of the catalysts were measured by nitrogen adsorption using the Brunauer−Emmett−Teller (BET) method (Quadrasorb SI). Powder X-ray diffraction (XRD) measurements were carried out on a Rigaku D/MAX-2400 X-ray diffractometer with Cu Kα radiation. X-ray photoelectron spectroscopy (XPS) was implemented on a surface analysis system (Thermal ESCALAB 250) using Al Kα radiation. The C 1s line at 284.6 eV was considered as a reference for the binding energy calibration. Temperature-programmed reduction (H2-TPR) and temperature-programmed desorption (NH3-TPD) were carried out on a ChemBET PULSAR TPR/TPD instrument using 50 mg of each catalyst. Prior to TPR experiments, the catalysts were pretreated at 400 °C for 2 h under helium flow. For NH3-TPD experiments, after pretreatment in a helium stream at 400 °C for 1 h, the catalysts were saturated with anhydrous NH3 (2% in He) at a flow rate of 40 mL/min for about 1 h. Desorption was carried out by heating the catalyst in He (30 mL/min) from 100 to 600 °C with a heating rate of 10 °C/min. The in situ DRIFTS experiments were performed on a Bruker Vector FTIR spectrometer with in situ diffuse reflectance pool and high sensitivity MCT detector cooled by liquid N2. An approximately 12 mg sample was finely ground and pressed into a self-supported wafer. Mass flow controllers and a sample temperature controller were used to simulate the real reaction conditions. Prior to each experiment, the wafer was heated to 350 °C in N2 (99.999%) for 1 h and then cooled to the desired reaction temperature. The background spectrum was collected in flowing N2 and automatically subtracted from the sample spectrum. The total flow rate of the feeding gas was kept at 100 mL/min, and the reaction conditions were controlled as follows: 500 ppm NO, 500 ppm NH3, 250 ppm SO2 (when used), 3 vol % O2 and N2 balance.

⎛ [NOx ]out ⎞ NOx conversion = ⎜1 − ⎟ × 100% [NOx ]in ⎠ ⎝ N2 selectivity [NOx ]in + [NH3]in − [NOx ]out − [NH3]out − 2[N2O]out = × 100% [NOx ]in + [NH3]in − [NOx ]out − [NH3]out

The catalytic oxidation activities for NO to NO2 were also measured. The reaction conditions were as follows: 1000 ppm NO, 3 vol % O2, balance N2, GHSV = 30 000 h−1. The conversion of NO to NO2 was obtained by this equation: conversion of NO to NO2 = 100% × ([NOx]in − [NO]out)/ [NOx]in, where NOx represents NO + NO2.

3. RESULTS AND DISCUSSION 3.1. XRD and BET Studies. The X-ray diffraction patterns of Fe(x)−Ce/TiO2 (x = 0, 0.02, 0.09, 0.2, 0.7) and Fe/TiO2 catalysts are shown in Figure 1. All the Fe-doped Ce/TiO2

Figure 1. XRD profiles of Fe(x)−Ce/TiO2 (x = 0, 0.02, 0.1, 0.2, 0.7) and Fe/TiO2 catalysts.

catalysts provided typical diffraction patterns for the cubic CeO2 structure (PDF#34-0394), anatase phase (PDF#211272), and a little rutile phase (PDF#21-1276). No visible phase of iron species could be observed at low Fe/Ti molar ratios (≤0.2), but the Fe2O3 phase could be detected on a single component of the Fe(0.2)/TiO 2 catalyst. This demonstrated that the iron oxides were in a highly dispersed state or the crystallites formed were less than 5 nm beyond detection limitation. Further increasing the Fe/Ti to 0.7, some 25320



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weak peaks appear at 2θ = 24.5°, 33.9°, 35.7°, 49.7°, and 64.2°, respectively, corresponding to typical diffraction of crystallite Fe2O3. Furthermore, the full width at half-maximum (fwhm) of the cubic CeO2 peaks for Fe(0.2)−Ce/TiO2 were lower than those for Ce/TiO2, indicating that the grain size of CeO2 on Fe(0.2)−Ce/TiO2 was smaller than that on Ce/TiO2. This meant that the existence of iron oxide could lower the crystallinity of CeO2 and thus enhance the dispersion of CeO2 on the catalyst surface. In addition, the BET areas and pore volumes of the above catalysts are listed in Table 1. Upon Table 1. Physical Properties of Various Catalysts catalyst

BET surface area (m2/g)

pore volume (cm3/g)

Ce/TiO2 Fe(0.02)−Ce/TiO2 Fe(0.1)−Ce/TiO2 Fe(0.2)−Ce/TiO2 Fe(0.7)−Ce/TiO2 Fe/TiO2

59 61 66 68 48 43

0.22 0.24 0.26 0.28 0.20 0.16

increasing the Fe/Ti molar ratio from 0 to 0.2, the BET surface areas and pore volumes increased a little. However, the BET surface area and pore volume dropped dramatically when the Fe/Ti molar ratio was 0.7. In combination with XRD results, the decreases of surface area and pore volume could be attributed to the formation of Fe2O3 crystallites. 3.2. XPS Analysis. To identify the effect of iron on the state of surface species, Fe(x)−Ce/TiO2 catalysts (x = 0, 0.02, 0.1, 0.2, 0.7) were measured by XPS. Surface atomic concentrations of Ce, O, Ti, and Fe are summarized in Table 2, and Figure 2. XPS spectra of (A) Ce 3d and (B) O 1s for different catalysts. (a) Ce/TiO2, (b) Fe(0.02)−Ce/TiO2, (c) Fe(0.1)−Ce/ TiO2, (d) Fe(0.2)−Ce/TiO2, (e) Fe(0.7)−Ce/TiO2.

Table 2. XPS Results of Various Catalysts surface atomic concentration (%) catalyst

O

Ce

Ti

Fe

Ce/Ti

Ce/TiO2 Fe(0.02)−Ce/TiO2 Fe(0.1)−Ce/TiO2 Fe(0.2)−Ce/TiO2 Fe(0.7)−Ce/TiO2

71.9 72.8 75.0 76.3 77.9

8.2 7.9 7.5 7.4 6.8

19.9 18.7 16.4 14.8 11.6

0.6 1.1 1.5 3.7

0.4(0.2)a 0.4(0.2) 0.4(0.2) 0.5(0.2) 0.5(0.2)

The O 1s peaks could be fitted into two peaks referred to as the lattice oxygen O2− at 529.5−529.8 eV (hereafter denoted as Oβ) and the chemisorbed oxygen at 531.0−531.4 eV (hereafter denoted as Oα), such as O22− and O− belonging to the defectoxide or hydroxyl-like group.14 As shown in Figure 2B, the relative ratio of Oα calculated by Oα/(Oα + Oβ) on te Fe(0.2)− Ce/TiO2 catalyst (47.2%) was much higher than that on the Ce/TiO2 catalyst (24.4%), suggesting that the chemisorbed oxygen content was greatly increased after the introduction of iron. The above-mentioned XPS results indicated that the presence of the Ce3+ species could create a charge imbalance, vacancies, and unsaturated chemical bonds on the catalyst surface,15 which would lead to the increase of oxide defects or hydroxyl-like groups. On one hand, the chemisorbed oxygen was more reactive than the lattice oxygen in the oxidation reactions due to its higher mobility.16 This meant that the Femodified Ce/TiO2 catalyst might have better activity for the NO oxidation to NO2 than the Ce/TiO2 catalyst. On the other hand, the NH3 adsorption could be enhanced due to the production of a larger amount of surface hydroxyl groups. The enhancement of NOx and NH3 adsorption over the Femodified Ce/TiO2 catalyst would be discussed later. In addition, the binding energies of Fe 2p3/2 and Fe 2p1/2 (Figure S1 in Supporting Information) observed at 710.8 and 724.5 eV corresponded well with the Fe3+ species,17,18 implying that the Fe species of the Fe-doped Ce/TiO2 catalysts were in the Fe3+ oxidation state. With increasing Fe content, the

a

The number in the brackets is denoted as the corresponding bulk ratio.

photoelectron spectra of Ce 3d and O 1s are displayed in Figure 2. As listed in Table 2, the ratio of Ce/Ti was slightly higher than the corresponding bulk ratio, indicating the richness of surface Ce atoms. Furthermore, with the increase of Fe, the concentration of Ti decreased obviously, while the concentration of Ce remained almost unchanged. It suggested that the dispersion of Ce species could be improved by Fe doping. From the XPS results of Ce 3d of the Fe-doped Ce/TiO2 catalyst in Figure 2A, two multiplets (u and v) could be found after fitting. The bands labeled u1 and v1 represent the 3d104f1 initial electronic state, corresponding to Ce3+, whereas the peaks labeled u, u2, u3, v, v2, and v3 represent the 3d104f0 state of Ce4+ ions.12,13 It could be clearly seen that the intensities of Ce3+ characteristic peaks increased with the addition of iron, accompanied with a decrease of Ce4+ peaks. It demonstrated that the introduction of Fe resulted in great conversion of Ce4+ to Ce3+ on the catalyst surface compared to that on pure cerium oxide supported on TiO2. 25321



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intensities of both peaks gradually increased as a result of an increase of Fe2O3, which was in good agreement with XRD results. 3.3. H2-TPR Analysis. H2-TPR experiments were conducted to study the influence of Fe content on the reduction profile of the Ce/TiO2 catalyst (as shown in Figure 3). The

Figure 3. H2-TPR profiles of different catalysts.

TPR profile of Ce/TiO2 showed two obvious reduction peaks. The first broad peak in the range 400−700 °C corresponded to the reduction of surface Ce4+ to Ce3+,19 and the intense peak at higher temperature (848 °C) could be attributed to the reduction of bulk CeO2.19,20 After the introduction of Fe, the onset temperature of the first broad peak of Ce/TiO2 shifted to the lower temperatures, implying the enhancement of redox activity at lower temperatures, and the Tmax of reduction of the both peaks shifted to the lower temperatures. These results suggested that the mobility of surface oxygen was enhanced after the addition of iron, and therefore there was more chemisorbed oxygen formed on the catalyst surface (as shown in Figure 2B), which was beneficial to the SCR reaction. However, further increasing the Fe/Ti molar ratio from 0.2 to 0.7, the two main reduction peaks shifted to the higher temperatures, implying lower reducibility of the cerium oxides with more Fe substitution amounts. Thus, it was deduced that the catalytic activity of Fe−Ce/TiO2 would decrease upon increasing the Fe/Ti molar ratio from 0.2 to 0.7. Moreover, the first broad peak became more intensive with increasing Fe content, for this peak was overlapped by the reduction of Fe3+ to Fe2+.21 When the Fe content reached a certain level (Fe/Ti = 0.7), the Fe2O3 reduction peaks appeared,21,22 which was in good accordance with the XRD results. It was also noteworthy that after the introduction of iron a new peak emerged at about 900 °C and became more intensive with increasing Fe amounts. The ferric oxide showed a large broad peak around 800 °C, which could be assigned to the partial reduction of Fe2+ to Fe0.21 Thus, this new peak on Fe-modified Ce/TiO2 catalysts might be assigned to the partial reduction of Fe2+ to Fe0. 3.4. Effect of Fe Doping Molar Ratio on SCR Performance. The NOx conversions, N2 selectivities, and N2O formations of the Fe−Ce/TiO2 catalysts with different Fe/Ti molar ratios are shown in Figure 4. It could be seen that the addition of Fe to Ce/TiO2 could significantly increase NOx conversion below 250 °C, although the Fe/TiO2 catalyst had a low activity at low temperatures (as shown in Figure 4A). At 150 °C, the NOx conversion was greatly improved from 25 to

Figure 4. (A) NOx conversions and (B) N2 selectivities and N2O formation of Fe(x)−Ce/TiO2 (x = 0, 0.02, 0.1, 0.2, 0.7) and Fe/TiO2 catalysts. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 3 vol %, N2 balance, and GHSV = 30 000 h−1.

72% with an increase of Fe/Ti molar ratio from 0 to 0.2. For the Fe(0.2)−Ce/TiO2 catalyst, the NOx conversion was approximately 80% at 160 °C, and little NO could be detected in outlet in the temperature range of 200−350 °C. However, further increasing the Fe/Ti molar ratio lowered the NOx conversion at both low and high temperatures. The N2 selectivity decreased with an increase of Fe/Ti molar ratio from 0.1 to 0.7 (as shown in Figure 4B). For the Fe(0.2)−Ce/ TiO2 catalyst, the amount of N2O formed was below 10 ppm even at 350 °C, indicating its promising N2 selectivity. For NOx conversion and N2 selectivity, the Fe(0.2)−Ce/TiO2 catalyst was chosen as a model catalyst in the following work. 3.5. Comparison with V2O5−WO3/TiO2 and Ce/TiO2 Catalysts and the Influence of SO2. The V2O5−WO3/TiO2 catalyst has been widely used for the NOx removal from stationary sources and introduced into the market for diesel vehicles. To better evaluate the NH3−SCR performance of the Fe(0.2)−Ce/TiO2 catalyst, comparative SCR activity measurements over V2O5−WO3/TiO2 and Ce/TiO2 catalysts were also carried out (as shown in Figure 5). As for Fe(0.2)−Ce/TiO2, the low-temperature SCR activity was much better than that of V2O5−WO3/TiO2, although its SCR activity above 300 °C showed a small decrease (as shown in Figure 5A). The amount of N2O formed on Fe(0.2)−Ce/TiO2 was less than that on V2O5−WO3/TiO2 during the whole testing process, indicating the good N2 selectivity of Fe(0.2)−Ce/TiO2 (as shown in Figure 5B). Compared with Ce/TiO2, Fe(0.2)−Ce/TiO2 revealed a superior low-temperature activity. In all, it would be possible to use the Fe(0.2)−Ce/TiO2 catalyst for the removal of NOx from actual flue gases with low exhaust temperatures. 25322



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Figure 6. NOx conversions of Fe(0.2)−Ce/TiO2 and Ce/TiO2 catalysts in the presence of SO2 at 250 °C at two different GHSVs (30 000 or 60 000 h−1). Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 3 vol %, [SO2] = 500 ppm.

test were investigated (Table S1 in the Supporting Information). It clearly showed that both Fe(0.2)−Ce/TiO2 and Ce/TiO2 exhibited a loss of surface area and pore volume after the durability test, and the decrease in surface area and pore volume of Fe(0.2)−Ce/TiO2 was lower than those of Ce/ TiO2, especially at the higher GHSV. Some researchers thought that the decrease of the BET and pore volume of catalyst after the SCR reaction in the presence of SO2 was ascribed to the formation of ammonium-sulfate salts such as NH4HSO4 or (NH4)2SO4, which blocked the pores of the catalyst.23,24 Thus, it was deduced that the deposition of ammonium-sulfate salts on the catalyst surface was the dominant reason for the catalyst deactivation in the SO2 durability test, and the Fe(0.2)−Ce/ TiO2 had larger capacity to resist the poisoning resulting from ammonium sulfate than Ce/TiO2 under our conditions. 3.7. Effect of Fe Doping on the NO Oxidation. A previous study showed that the existence of NO2 could promote the low-temperature activity over SCR catalysts through a fast reaction of 2NH3 + NO + NO2 → 2N2 + 3H2O.25,26 Therefore, the oxidation activities of NO to NO2 by O2 over Fe(0.2)−Ce/TiO2 and Ce/TiO2 catalysts were measured (as shown in Figure 7). It could be seen that the conversion of NO to NO2 over Fe(0.2)−Ce/TiO2 was greater

Figure 5. Comparison of (A) NOx conversion and (B) N2 selectivity and N2O formation of the Fe(0.2)−Ce/TiO2 catalyst (in the presence or absence of 500 ppm SO2) with those of V2O5(1.5)−WO3(7)/TiO2 and Ce/TiO2 catalysts. Reaction conditions: [NO] = [NH3] = 1000 ppm, [O2] = 3 vol %, [SO2] = 500 ppm (when needed), N2 balance, and GHSV = 30 000 h−1.

To evaluate the sulfur-poisoning resistance of the catalyst, the effect of SO2 on NOx conversions over Fe(0.2)−Ce/TiO2, V2O5−WO3/TiO2, and Ce/TiO2 catalysts was investigated (as shown in Figure 5). When 500 ppm SO2 was added into the reaction gas, the catalytic activities of all catalysts showed a decrease below 250 °C and some increase above 300 °C (as shown in Figure 5A). However, it could be seen that the highest NOx conversion of Fe(0.2)−Ce/TiO2 could be obtained at 250 °C, which was 50 °C lower than that of V2O5−WO3/TiO2 and Ce/TiO2. This meant that the Fe(0.2)− Ce/TiO2 catalyst had good sulfur-poisoning resistance for the NH3−SCR reaction in the low-temperature region. It was noteworthy that at relatively high temperatures the presence of SO2 increased the NOx conversion over Fe(0.2)−Ce/TiO2. This might be caused by the inhibitory effect of SO2 on the unselective oxidation of NH3. 3.6. SO2 Durability Test. For further study of the sulfurpoisoning resistance of the catalyst, the SO2 durability of Fe(0.2)−Ce/TiO2 and Ce/TiO2 at two different GHSVs was investigated (as shown in Figure 6). It could be seen that when 500 ppm SO2 was added to the reaction gas at 30 000 h−1 the NOx conversion of Fe(0.2)−Ce/TiO2 was greater than 94% after 11 h, whereas the NOx conversion of Ce/TiO2 gradually decreased with time to 55% after 11 h. With a GHSV of 60 000 h−1, both catalysts were expected to show a further decrease in NOx conversion, and the NOx conversion of Fe(0.2)−Ce/TiO2 decreased more slowly than that of Ce/TiO2. These results indicated that the Fe(0.2)−Ce/TiO2 exhibited excellent SO2 durability compared to Ce/TiO2. Furthermore, the physical properties of fresh and used catalysts used in the SO2 durability

Figure 7. Oxidation activity of NO to NO2 by O2 over Fe(0.2)−Ce/ TiO2 and Ce/TiO2 catalysts. Reaction conditions: [NO] = 1000 ppm, [O2] = 3 vol %, N2 balance, and GHSV = 30 000 h−1. 25323



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S2 in the Supporting Information). Some researchers28,29 have reported that the Brønsted acid sites were the important active sites for the SCR reaction at low temperatures, and the transformation of Ce4+ to Ce3+ could increase the amount of Brønsted acid sites. In this work, the Ce species of Ce/TiO2 were mainly in the Ce4+ oxidation state (as shown in Figure 2A), and adsorbed ammonia species on Ce/TiO2 were mainly coordinated NH3 linked to Lewis acid sites (as shown in Figure 8A), which was confirmed by the results of Qi et al.30 that there were no bands due to NH4+ species over pure CeO2. Hence, the greater conversion of Ce4+ to Ce3+ caused by iron modification resulted in more Brønsted acid sites on the catalyst surface, which might be favorable for the promotion of low-temperature activity. In addition, Wu et al.31 thought that the coordinated NH3 could be adsorbed on Fe2O3. Thus, it was thought that Fe3+ species could make up the loss of amount of Lewis-acid sites caused by the conversion of Ce4+ to Ce3+. As shown in Figure 8B, after NOx adsorption and N2 purge, the catalyst surface was mainly covered by NO2 (1605 cm−1), monodentate nitrate (1290 and 1530 cm−1), bridging nitrate (1240 cm−1), and bidentate nitrate (1020, 1550, and 1580 cm−1) species, and some negative bands around 3700 cm−1 due to the interaction between surface basic hydroxyls and NOx also showed up.32−34 After introduction of Fe, more surface nitrate species appeared on Fe(0.2)−Ce/TiO2 than that on Ce/TiO2, including NO2 and monodentate nitrate species. According to the XPS results, the more NO2 formed mainly resulted from the increase of chemisorbed oxygen by Fe modification. A previous study has reported that the gaseous NO was easy to be oxidized by O 2 over Fe 3+ ,27 which resulted in more monodentate nitrate species Fe−O−NO2. Hence, it could be concluded that the increase of monodentate nitrate and NO2 species on the catalyst surface was ascribed to the formation of Fe3+ and chemisorbed oxygen, respectively. Moreover, Qi et al.30 studied the NOx adsorption over MnOx−CeO2 mixed oxides and thought that Ce ions could provide a number of adsorption sites for NO because the moderate basicity allowed surrounding oxide ions to react readily with NO, thus producing much adsorbed NO2 species. Combined with the in situ DRIFTS results of NO + O2 adsorption, it was deduced that the NO2 species was mainly bound to cerium oxide, while the monodentate nitrate species was mainly bound to the Fe site (Fe−O−NO2). Furthermore, Li et al.29 thought that the reaction between NO2 and ammonia was likely to have occurred over the Ce/TiO2 catalyst at relatively low temperatures, whereas Liu et al.35 thought that monodentate nitrate species could directly react with adsorbed NH3 species to give out N2 when they studied the SCR mechanism of the iron titanate catalyst at lower temperatures. The real reactive NOx adsorbed species in NH3−SCR reaction over Fe(0.2)−Ce/ TiO2 catalyst would be discussed later. In addition, the NH3/ NO + O2 adsorption over Fe(0.2)−Ce/TiO2 at various temperatures (50−350 °C) was also performed (Figure S3 in Supporting Information). With an increase in temperature, all bands due to NH3 adspecies observed at 50 °C became weaker. The ionic NH4+ species almost vanished at 250 °C, whereas the coordinated NH3 and NH2 species still remained even at 350 °C. Similarly, all the NOx adspecies formed at 50 °C decreased with increasing temperature, but the NO2 species did not decrease until 250 °C and disappeared at 350 °C. 3.8.2. Possible Reaction Mechanism for “NO + NH3 + O2” over the Fe(0.2)−Ce/TiO2 Catalyst. To understand the mechanism of the Fe(0.2)−Ce/TiO2 catalyst for SCR of NOx

than that over Ce/TiO2, especially at the temperature range of 100−250 °C. It suggested that addition of Fe could strengthen the oxidation activity of NO to NO2, which was beneficial to the enhancement of low-temperature SCR activity. In addition, the NO oxidation activity should decrease above 350 °C due to the thermodynamic equilibrium between NO and NO2, which shifted to the NO side for higher temperatures. 3.8. In Situ DRIFTS Studies. 3.8.1. In Situ DRIFTS of NH3/ NO Adsorption over Ce/TiO2 and Fe(0.2)−Ce/TiO2 Catalysts. In the NH3−SCR reaction, especially in the low-temperature range, both NH3 and NOx can adsorb onto the catalyst surface and participate in the NOx reduction process. Therefore, DRIFTS spectra of NH3/NO adsorption over Fe(0.2)−Ce/ TiO2 and Ce/TiO2 at 50 °C were recorded (as shown in Figure 8). After NH3 adsorption and N2 purge (as shown in Figure

Figure 8. In situ DRIFTS spectra of Fe(0.2)−Ce/TiO2 and Ce/TiO2 catalysts during (A) NH3 adsorption and (B) NO + O2 adsorption at 50 °C.

8A), the catalyst surface was mainly covered by ionic NH4+ bound to Brønsted acid sites (1440 and 1680 cm−1) and coordinated NH3 bound to Lewis acid sites (1170, 1230/1205, and 1602 cm−1).27,28 At the same time, a small band at 1515 cm−1 due to the scissoring vibration mode of NH2 species, the bands at 3100−3400 cm−1 attributed to N−H stretching vibration modes, and some negative bands around 3650 cm−1 due to the surface O−H stretching also showed up. After introduction of Fe, the bands attributed to Lewis acid sites showed no obvious changes, whereas the band at 1440 cm−1 due to Brønsted acid sites was much stronger than that on Ce/ TiO2, which was consistent with the NH3−TPD results (Figure 25324



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including bidentate nitrate (1550 and 1580 cm−1), monodentate nitrate (1530 cm−1), bridging nitrate (1240 cm−1), nitro compounds (1430 cm−1), and NO2 (1605 cm−1). These results indicated that both ionic NH4+ and coordinate NH3 could play roles as reducing agents to reduce NOx in the SCR reaction. In this experiment, the reactants were introduced to the Fe(0.2)−Ce/TiO2 catalyst in the reversed order (as shown in Figure 9B). After NH3 passed over the NO + O2 pretreated catalyst, the intensity of the band at 1605 cm−1 due to NO2 decreased, and the band at 1240 cm−1 shifted to 1260 cm−1 which could be ascribed to the deformation nitrate species.28 Meanwhile, the IR bands attributed to coordinated NH3 (1170 and 1602 cm−1), ionic NH4+ (1680 cm−1), and amid (1515 cm−1) species appeared, and the bands in the region of 3400− 3100 cm−1 attributed to N−H stretching vibration in coordinated NH3 were also observed. After the catalyst was purged with NH3 for 40 min, both adsorbed NH3 and NOx adspecies could be observed on the catalyst surface. These results indicated that the reaction between ammonia and nitrate species was unlikely to have occurred, except for the reaction between NO2 and ammonia. Thus, the more NO2 species on Fe(0.2)−Ce/TiO2 compared to that on Ce/TiO2 was responsible for the enhancement of the SCR reaction. In addition, the coexistence of NH3 and NOx adspecies showed that NH3 and NOx could be adsorbed over different active sites of the catalyst surface. To indentify the species present on the catalysts under SCR reaction condition, the DRIFTS spectra of Fe(0.2)−Ce/TiO2 in a flow of NO + NH3 + O2 from 150 to 350 °C was recorded (as shown in Figure 9C). The bands due to different nitrate species were observed at 1020, 1240, and 1550 cm−1 at 150 °C. The band at 1603 cm−1 might be caused by overlapping of bands of NO2 and coordinated NH3. Meanwhile, the bands attributed to the adsorbed NH3 species could also be observed at 1170, 1515, 1603 (overlapping of bands of NO2 and coordinated NH3), and 1690 cm−1. Band at 1240 cm−1 might be caused by overlapping of coordinated NH3 and bridging nitrate. The appearance of these bands indicated that both adsorbed NH3 and NOx species might be involved in the SCR reaction at temperatures below 200 °C. With an increase in reaction temperature, the intensities of the bands belonging to the adsorbed NOx species decreased noticeably and almost disappeared at 200 °C, while the bands ascribed to adsorbed NH3 species still remained. This meant that the reaction between adsorbed NH3 species and adsorbed NO was fast on the catalyst surface at a reaction temperature above 200 °C, so that the steady-state concentration of NOx adspecies was below the detection limits of our IR spectroscopy. In the relatively low- and high-temperature range, the Langmuir−Hinshelwood (L−H) and Eley−Rideal (E−R) mechanisms are two of the most accepted mechanisms proposed in normal SCR systems.27−29 However, for various catalytic systems, different hypotheses have been proposed for the mechanism. In general, adsorption of NH3 has been considered as the initial step for the SCR reaction. In view of previous findings, a simplified reaction mechanism over Fe(0.2)−Ce/TiO2 was proposed and presented in eqs 1−8. When the SCR reaction happened below 200 °C, both adsorbed NH3 and NOx species were considered to be involved in the SCR reaction (as shown in Figure 9C). On the one hand, coordinated NH3 and ionic NH4+ could favor the reaction with nitrate species (as shown in Figure 9A). On the other hand, the

with NH3, the in situ DRIFTS experiment of reaction between NO + O2 and preadsorbed NH3 species at 200 °C was carried out, and the results are shown in Figure 9A. After NO + O2

Figure 9. In situ DRIFT spectra of the Fe(0.2)−Ce/TiO2 catalyst for the reaction between (A) NO + O2 and adsorbed NH3 species at 200 °C, (B) NH3 and adsorbed NOx species at 200 °C, and (C) 500 ppm NH3 + 500 ppm NO + 3 vol % O2 (SCR condition) in a steady state at various temperatures.

passed over the NH3-pretreated Fe(0.2)−Ce/TiO2 catalyst, the bands attributed to ionic NH4+ (1440 and 1680 cm−1) and coordinate NH3 (1170, 1230, and 1602 cm−1) showed an obvious decrease in intensity and totally disappeared after 2 min, and then the nitrate species began to form on the catalyst surface. After the catalyst was purged with NO + O2 for 40 min, the catalyst surface was mainly covered by NOx adspecies, 25325



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in situ formed NO2 was generally accepted to be an important intermediate species for the low-temperature SCR reaction,14,15 which was confirmed by the observation of NO2 in Figure 9B. Hence, in the relatively low-temperature range (200 °C), Lewis acid Fe3+ species acted as active sites, where the reaction between gas-phase or weakly adsorbed NO and NH2 species occurred following an E−R mechanism (eqs 7 and 8). O2 (g) + 2* → 2O − *

(* surface active sites)

Figure 10. In situ DRIFTS spectra of the Fe(0.2)−Ce/TiO2 catalyst in a flow of NH3 + NO + O2 before and after the addition of 250 ppm SO2 at 250 °C.

and the bands at 1440 and 1680 cm−1 were assigned to NH4+ species. The above results indicated that the coordinated NH3 was little influenced by SO2, and the SCR reaction could still occur via the E−R reaction pathway. So, a high NOx conversion could be obtained over Fe(0.2)−Ce/TiO2 at high temperatures (>200 °C) in SO2-contaning gases (as shown in Figure 5A). Moreover, the bands due to NH4+ (1440 cm−1) and the hydroxyl vibration were strengthened with the reaction time, suggesting the formation of NH4HSO4 and (NH4)2SO4 on the catalyst surface, which might be the dominant cause for the deactivation of the Fe(0.2)−Ce/TiO2 in SO2-containing gases (as shown in Figure 6).

(1)

Ce 4 +,Fe3 +

NH3(g) ⎯⎯⎯⎯⎯⎯⎯⎯⎯→ NH3(a) Ce

(2)

3+

NH3(g) ⎯⎯⎯⎯→ NH+4 (a)

(3)

NO(g) + O − * → NO2 (a)

(4)

2NH3(a) + NO2 (a) + NO → 2N2 + 3H 2O

(5)

4. CONCLUSION The addition of iron could enhance the low-temperature activity and SO2-poisoning resistance of the Ce/TiO2 catalyst. The Fe−Ce/TiO2 catalyst with a molar ratio of 0.2 could obtain nearly 80% NOx conversion at 160 °C. More than 94% NOx conversion could be provided by Fe-modified Ce/TiO2 catalyst in the presence of 500 ppm SO2 at 250 °C for 11 h. In the case of Fe(0.2)−Ce/TiO2, both cerium and iron oxides dispersed well on the catalyst surface. The characterization results revealed that the addition of iron resulted in more Ce3+ and chemisorbed oxygen on the catalyst surface, thus increasing the amount of Brønsted acid sites and in situ formed NO2, respectively. A possible reaction mechanism was proposed over the Fe(0.2)−Ce/TiO2 catalyst. In the relatively low-temperature range, coordinated NH3 and ionic NH4+ species as well as adsorbed NO2 were involved in the SCR reaction following an L−H mechanism. In the relatively high-temperature range, coordinated NH3 and gas-phase or weakly adsorbed NO predominated on the catalyst surface following an E−R mechanism. Finally, the effect of SO2 for the SCR reaction over Fe(0.2)−Ce/TiO2 was also studied. The results revealed that the E−R reaction pathway was little influenced by SO2, which made this catalyst resistant to SO2 poisoning at relatively high temperatures (>200 °C). The formation of ammonium sulfate appeared to be the dominant cause for the catalyst deactivation in SO2-containing gases.

2NH4 +(a) + NO2 (a) + NO → 2N2 + 3H 2O + 2H+ (6) Fe

3+

NH3(a) ⎯⎯⎯→ NH 2(a) + H+ + e−

(7)

NH 2(a) + NO(g) → N2 + H 2O

(8)

3.8.3. Effect of SO2 for “NO + NH3 + O2” over the Fe(0.2)− Ce/TiO2 Catalyst. To understand the deactivation mechanism of Fe(0.2)−Ce/TiO2 for the SCR reaction, the DRIFTS spectra of Fe(0.2)−Ce/TiO2 under SCR conditions before and after the addition of 250 ppm SO2 at 250 °C were recorded (as shown in Figure 10). When the catalyst was purged by 500 ppm NO + 500 ppm NH3 + 3 vol % O2 for 30 min, the catalyst surface was mainly covered by NH3 adspecies, including coordinated NH3 (1230 and 1603 cm−1) and amid species (1515 cm−1). Meanwhile, bands in the region of 3400−3100 cm−1 attributed to N−H stretching vibration in coordinated NH3 also appeared. These indicated that the E−R reaction pathway dominated over the Fe(0.2)−Ce/TiO2 catalyst at 250 °C as discussed above. After SO2 passed over NO + NH3 + O2 pretreated catalyst for 40 min, besides the coordinated NH3 species that occurred on the catalyst surface, some new bands at 1050, 1145, 1305, 1440, and 1680 cm−1 also appeared. According to previous studies,36 the bands at 1050, 1145, and 1305 cm−1 could be assigned to the ν3 band splitting of SO42−, 25326



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(24) Kijlstra, W. S.; Bliek, A. Appl. Catal., B 1998, 16, 327−337. (25) Granger, P.; Parvulescu, V. I. Chem. Rev. 2011, 111, 3155−3207. (26) Long, R. Q.; Yang, R. T. J. Catal. 2002, 207, 274−285. (27) Long, R. Q.; Yang, R. T. J. Catal. 2002, 207, 224−231. (28) Guan, B.; Lin, H.; Zhu, L. J. Phys. Chem. C 2011, 115, 12850− 12863. (29) Chen, L.; Li, J. H. Environ. Sci. Technol. 2010, 44, 9590−9596. (30) Qi, G. S.; Yang, R. T.; Chang, R. Appl. Catal., B 2004, 51, 93− 106. (31) Wu, Z. B.; Jiang, B. Q. Environ. Sci. Technol. 2007, 41, 5812− 5817. (32) Kijlstra, W. S.; Brands, D. S.; Poels, E. K.; Bliek, A. J. Catal. 1997, 171, 208−218. (33) Kijlstra, W. S.; Brands, D. S.; Smit, H. I.; Poels, E. K.; Bliek, A. J. Catal. 1997, 171, 219−230. (34) Underwood, G. M.; Miller, T. M.; Grassian, V. H. J. Phys. Chem. A 1999, 103, 6184−6190. (35) Liu, F.; He, H. Catal. Today 2010, 153, 70−78. (36) Fu, H.; Wang, X.; Chen, J. J. Phys. Chem. C 2007, 111, 6077− 6083.

ASSOCIATED CONTENT

S Supporting Information *

The textual characterizations of Fe(0.2)−Ce/TiO2 and Ce/ TiO2 catalysts before and after the SO2 durability test, XPS spectra of Fe 2p over Fe(x)−Ce/TiO2 (x = 0.02, 0.1, 0.2, 0.7) catalysts, NH3-TPD profiles of Fe(0.2)−Ce/TiO2 and Ce/ TiO2 catalysts, and in situ DRIFTS spectra of the Fe(0.2)−Ce/ TiO2 catalyst during (A) NH3 and (B) NO + O2 adsorption in a steady state at various temperatures are shown. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 411 84706140. Fax: +86 (411) 84706263. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program of China (973 Program) (No. 2011CB936002), Program for Changjiang Scholars and Innovative Research Team in University (IRT0813), and Zhejiang Provincial Engineering Research Center of Industrial Boiler & Furnace Flue Gas Pollution Control (2010A01).

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