TiO2 for selective catalytic reduction of NOx with ammonia

TiO2 for selective catalytic reduction of NOx with ammonia

Applied Surface Science 356 (2015) 181–190 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 356 (2015) 181–190

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Promotional effect of tungsten-doped CeO2 /TiO2 for selective catalytic reduction of NOx with ammonia Dong Wook Kwon, Sung Chang Hong ∗ Department of Environmental Energy Engineering, Graduate School of Kyonggi University, 94-6 San, Iui-dong, Youngtong-ku, Suwon-si, Gyeonggi-do 443-760, Republic of Korea

a r t i c l e

i n f o

Article history: Received 16 May 2015 Received in revised form 29 July 2015 Accepted 9 August 2015 Available online 11 August 2015 Keywords: Cerium oxide SO2 resistance NH3 –SCR Ce/W/Ti Tungsten oxide

a b s t r a c t We examined the effects that the physicochemical properties of Ce/Me/Ti catalysts had on the selective catalytic reduction (SCR) activity after various metals (W, Mo, and La) were added to non-vanadiumbased catalysts in order to improve NH3 –SCR activity. We studied the properties of the catalysts through the use of physiochemical techniques, including Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2 -TPR), X-ray photoelectron spectroscopy (XPS) transmission infrared spectroscopy (IR), and inductively coupled plasma optic emission spectroscopy (ICP). The catalytic activity tests of the Ce/Ti catalysts with various ceria loadings revealed that the Ce/Ti with 10 wt.% ceria (10Ce/Ti) exhibited excellent activity. Thus, various metals were added to the 10Ce/Ti. The tungsten-doped 10Ce/Ti catalyst exhibited the highest activity (10Ce/W/Ti: Ce was deposited after tungsten had been deposited on TiO2 ). We investigated the correlation between the catalyst’s Ce valence state and its activity. Different Ce3+ ratios were observed when various metals were added to Ce/Ti. The highest Ce3+ ratio was observed in 10Ce/W/Ti at 0.3027, and the catalyst efficiency had a positive correlation with higher Ce3+ ratios. The SCR activity was found to increase as the Ce3+ ratio increased when tungsten was added to 10Ce/W/Ti. Furthermore, in the case of 10Ce/W/Ti, it seemed that the Brønsted acid sites were more abundant relative to those on 10Ce/Ti. Upon the injection of SO2 in the SCR reaction, 10Ce/Ti was rapidly deactivated. However, the 10Ce/W/Ti catalyst exhibited an excellent resistance to SO2 -induced deactivation relative to 10Ce/Ti. Thus, the addition of tungsten to Ce/Ti resulted in excellent NOx conversion and SO2 resistance. © 2015 Elsevier B.V. All rights reserved.

1. Introduction The NOx that is emitted during the combustion processes of fossil fuel causes photochemical smog, acid rain, the greenhouse effect, and the depletion of the ozone layer [1,2]. Thus, the selective catalytic reduction (SCR) of NOx to N2 , with NH3 as a reductant on VOx/TiO2 , has been known to be the best available control technology (BACT) to remove NOx from the atmosphere. It has been widely used in a large number of commercial combustion processes as result of its economic feasibility and efficiency [3]. The SCR catalysts that are most commonly used are those in which WO3 is added to VOx/TiO2 (anatase). These vanadium-based catalysts exhibit an excellent efficiency in a temperature range from 300 to 400 ◦ C [4]. However, these vanadium-based catalysts also suffer from several drawbacks, including the harmful effects of vanadium on the environment, the high likelihood of SO2 oxidizing

∗ Corresponding author. E-mail address: [email protected] (S.C. Hong). http://dx.doi.org/10.1016/j.apsusc.2015.08.073 0169-4332/© 2015 Elsevier B.V. All rights reserved.

to o SO3 , a narrow operation temperature window (300–400 ◦ C), and the formation of N2 O at high temperatures [5,6]. Therefore, recent efforts have been made to develop an environmentally friendly SCR catalyst to replace V-based catalysts. In particular, ceria is a potential substitute for vanadium due to its oxygen storage capacity and high redox ability when shifting between Ce4+ and Ce3+ [7,8]. Surface acid sites are well known to promote absorption and to increase NH3 –SCR activity [9], so various studies have been conducted to develop SCR catalysts using ceria, and several catalyst systems have been developed, including CeO2 /TiO2 [10,11], CeO2 /Al2 O3 [12], CeO2 /WO3 [13,14], MnOx CeO2 [15], CeO2 /ZrO2 [16], CeO2 /TiO2 SiO2 [17], Ge(or Mn)/CeO2 WO3 [18], and Fe Ce Mn/ZSM-5 catalysts [19]. Gao et al. [20,21] demonstrated that the Ce Cu Ti mixed oxide catalyst exhibited excellent SCR activity at a low temperature. According to Shan et al. [10], the CeO2 /TiO2 catalyst exhibited a higher catalytic activity than did V2 O5 WO3 /TiO2 and Fe-ZSM-5 catalysts. They claimed that the high NH3 –SCR activity was mainly a result of the synergistic effects between CeO2 and TiO2 . In addition, they examined the promotional effects of a tungsten-doped Ce/TiO2

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catalyst. The excellent catalytic performance of the Ce W Ti catalysts was associated with the highly dispersed ceria and primitive W species on TiO2 [22]. Liu et al. [23] reported that the addition of molybdenum to a Ce/Ti catalyst improved the NH3 –SCR activity. Thus, the above studies have revealed that the addition of a promoter to a Ce/Ti catalyst could improve the NH3 –SCR catalytic activity. Although the catalyst modified by the addition of a promoter shows excellent SCR activity, further studies are required to investigate the correlation between the activity and the catalytic characteristics of the metal added to the Ce/Ti system. Accordingly, we prepared catalysts in this study by adding various metals to Ce/Ti to improve the NH3 –SCR activity of non-vanadium-based catalysts. We examined the effects of the physicochemical properties of the metal-doped Ce/Me/TiO2 on the SCR activity, and we investigated the correlation between the catalyst’s Ce valence state and its activity. We also studied the properties of the catalyst through various physiochemical analyses, which included Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), H2 temperature-programmed reduction (H2 -TPR), X-ray photoelectron spectroscopy (XPS) transmission infrared spectroscopy (IR), and inductively coupled plasma optic emission spectroscopy (ICP). 2. Experimental 2.1. Catalyst preparation The Ce/Ti system was prepared via wet impregnation using cerium(III) nitrate [10 wt.% Ce; Ce(NO3 )3 ·6H2 O, Aldrich Chemical Co.] by using ceria (5, 10, 20, and 50 wt.% Ce) and anatase-type TiO2 powder (DT51, Cristal Global Co.). A calculated amount of TiO2 was gradually added to the ceria solution while stirring. The mixture was agitated in a slurry state for more than 1 h; then the moisture was removed at 70 ◦ C using a rotary vacuum evaporator (Eyela Co., N–N series); and the mixture was dried overnight at 110 ◦ C. Finally, the mixture was calcined in air for 5 h at 500 ◦ C. The Ce/Me/Ti catalyst was prepared via wet impregnation using TiO2 , cerium(III) nitrate, ammonium metatungstate hydrate [(NH4 )6 H2 W12 O40 , Aldrich Co.], ammonium molybdate [(NH4 )6 Mo7 O24 ·4H2 O, Aldrich Co.], and lanthanum(III) nitrate hexahydrate (LaN3 O9 ·6H2 O, Aldrich Co.). The calculated amount of the various metals (10 wt.% WO3 , 10 wt.% MoO3 , and 10 wt.% La2 O3 ) was immersed in distilled H2 O. After mixing the metal solution with TiO2 , the moisture was removed at 70 ◦ C using a rotary vacuum evaporator, and the mixture was then dried overnight at 110 ◦ C. The supports were calcined in air for 8 h at 600 ◦ C, and calculated amounts of cerium(III) nitrate (10 wt.% Ce) were dissolved in distilled water. After impregnation of the ceria solution into the W/Ti, Mo/Ti, or La/Ti supports, the moisture was removed at 70 ◦ C using a rotary vacuum evaporator, and the mixture was dried overnight at 110 ◦ C. The catalysts were then calcined in air for 5 h at 500 ◦ C. The 10Ce W/Ti catalyst was prepared according to the following process. Cerium nitrate (10 wt.% Ce) and ammonium paratungstate (10 wt.% WO3 ) were mixed in distilled H2 O. The calculated amount of the TiO2 support was impregnated in this solution by stirring for 1 h, and then the moisture was removed at 70 ◦ C using a rotary vacuum evaporator. The mixture was then dried overnight at 110 ◦ C, and was subsequently calcined in air for 5 h at 500 ◦ C. W/10Ce/Ti was prepared through impregnation of 10Ce/Ti and ammonium paratungstate using the method described above. 2.2. Catalytic activity tests The SCR activity tests were carried out in a fixed-bed quartz reactor with an inner diameter of 6 mm. The reaction conditions were

set to 750 ppm NO, 48 ppm NO2 , 800 ppm NH3 , 3 vol.% O2 , 6 vol.% H2 O, and 200 ppm SO2 (when used) in Ar. The reactant gases were fed to the reactor using a mass flow controller (MKS Co.). Approximately 0.15 g of the catalyst (40–50 mesh, 0.3 cc) were used for each test. Under ambient conditions, the total flow rate was 600 cc/min, and the gas hourly space velocity (GHSV) was 120,000 h−1 . The composition of the feed gases and the effluent streams were continuously monitored on-line using non-dispersive infrared gas analyzers: ZKJ-2 (Fuji Electric Co.) for NO/NO2 ; Ultramat 6 (Siemens Co.) for N2 O; and a pulsed fluorescence gas analyzer 43 C-High Level (Thermo Co.) for SO2 . Before the gas flowed into the analyzers, the moisture was removed with a moisture trap inside the chiller. The NOx conversion and the rate constant were calculated from the gas concentrations at a steady state according to the following equations: NOxconversion(%) = (([NO + NO2 ]in − [NO + NO2 ]out )/([NO + NO2 ]in )) × 100 (1)

NH3 conversion(%) = (([NH3 ]in − [NH3 ]out )/([NH3 ]in )) × 100

(2)

2.3. Catalyst characterization The BET surface areas of the catalysts were measured using ASAP 2010C (Micromeritics Co.). The specific surface area was estimated using the BET method, and the pore size of the distribution was calculated using the Barrett–Joyner–Halenda method that calculates the pore size based on the adsorption layer thickness in relation to the pressure and the average radius of the meniscus, as obtained using the Kelvin method. Each sample was then analyzed after degassing in a vacuum at 110 ◦ C for 3–5 h. The XRD measurements ˚ radiation, and the catwere carried out using Cu K␣ ( = 1.5056 A) alysts were measured at the 2 range from 10◦ to 80◦ with a step size of 0.1◦ and a time step of 1.0 s with a PANalytical X’Pert Pro MRD. The TPR of H2 was measured using 10% H2 /Ar and 0.3 g of the catalyst, at a total flow rate of 50 cc/min. Prior to the H2 -TPR measurements, the catalyst was pretreated under a flow of 5% O2 /Ar at 400 ◦ C for 0.5 h, followed by cooling to 50 ◦ C. The catalyst was then placed in hydrogen, and the hydrogen consumption was monitored with an Autochem 2920 (Micromeritics) instrument while the temperature increased to 800 ◦ C at a rate of 10 ◦ C/min. An ESCALAB 210 (VG Scientific) was used to conduct the XPS analysis with monochromatic Al K␣ (1486.6 eV) as the excitation source. After the complete removal of moisture from the catalysts by drying at 100 ◦ C for 24 h, the analysis was carried out without surface sputtering and etching so that the degree of vacuum in the XPS equipment could be maintained at 10−6 Pa. The spectra were analyzed using the XPS PEAK software (version 4.1), and the intensities were estimated from the integration of each peak, subtraction of the Shirley background, and fitting of the experimental curve to a combination of Lorentzian and Gaussian curves of various proportions. All of the binding energies were referenced to the C 1s line at 284.6 eV, and the binding energy values were measured with a precision of ±0.3 eV. This study used an in situ DRIFTs analysis performed with an FT-IR (Nicolet iS10, Thermo Co.). A DR (diffuse reflectance) 400 accessory was used for the analysis of the solid reflectance. The CaF2 window was used as a plate for the DR measurement, and spectra were collected using a MCT (mercury cadmium telluride) detector. All of the catalysts used for this analysis were ground using a rod in the sample pan of the in situ chamber with an installed temperature controller. To prevent the influence of moisture and impurities, the sample was preprocessed with Ar at a rate of 100 cc/min at 400 ◦ C for 1 h. Then, the vacuum state was maintained using a vacuum pump. The spectra of the catalyst was collected by

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measuring the single-beam spectrum of the preprocessed sample as background, and all analyses were performed by auto scanning at a resolution of 4 cm−1 . The elemental (sulfur) analyses were performed via inductively coupled plasma optic emission spectroscopy (ICP). The transmission IR spectra were recorded with a Nicolet Nexus spectrometer (Model Magna IR 550 II) equipped with a liquid nitrogen cooled MCT detector that collected 100 scans with a spectral resolution of 4 cm−1 . The samples were then mixed with KBr and were pressed into pellets to conduct the measurements.

3. Results and discussion 3.1. Catalytic activity and characteristics for NO reduction by NH3 over Ce/Ti catalysts The experiments were performed using Ce/Ti catalysts to examine the influence of the ceria loading on the SCR activity. As shown in Fig. 1a, the NOx conversion activities were determined on 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti at 250–400 ◦ C with a GHSV of 120,000 h−1 . The 10Ce/Ti catalyst exhibited an activity of 91% or higher at 300 ◦ C and showed excellent activity over the entire temperature range. However, if 10 wt.% ceria or more was deposited, the activity decreased. The 50Ce/Ti catalyst exhibited very low activity at 300 ◦ C or higher, and N2 O was not found in the SCR reactions of all catalysts. As shown in Fig. 1b, the concentration of NO2 emitted from the SCR reactions increased as the ceria loading increased, and Fig. 1c shows the NH3 conversions for the SCR reactions. These conversions are proportional to the NOx conversions because the SCR reactions are held at a molar ratio of 1.0 NH3 /NOx. Therefore, the NH3 conversion curves are related to the temperature range in which the NOx conversion decreases. The surface area for the 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti samples are collated in Table 1 with values of 86.6, 87.9, 55.5, and 44.8 m2 /g, respectively. 10Ce/Ti had the highest surface are. However, the specific surface area, total pore volume, and average pore diameter decreased with tungsten loadings that exceeded 10 wt.%. The variation in the catalyst structure with different ceria loadings was analyzed through the use of XRD. Fig. 2 shows the XRD patterns of the 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti catalysts. For cubic CeO2 and anatase TiO2 , main peaks were observed at 28.6◦ and 25.3◦ , respectively, and the intensity of the anatase TiO2 peak decreased as the ceria loading increased. For the 5Ce/Ti, 10Ce/Ti, and 20Ce/Ti catalysts, cubic CeO2 peaks were not observed in the XRD results, which indicates that the ceria oxides were well dispersed on the TiO2 support structure. When the 50Ce/Ti catalyst was analyzed, cubic CeO2 peaks were observed at 28.6, 33.1, and 47.5. When ceria was not evenly distributed on the support, crystalline ceria formed, and the catalytic activity decreased [24,25]. According to Gao et al. [7], when ceria exists in an amorphous state in Ce/Ti, the amount of chemisorbed oxygen and oxygen species that are bound to the catalyst increase, and excellent catalytic activity can be observed. Therefore, when 20 wt.% ceria or more was deposited, Ce did not exist in an amorphous state. Thus, crystalline Ce could be said to have formed, and the formation of crystalline ceria on the surface of the catalyst was thought to decrease the SCR activity. An H2 -TPR analysis was carried out to investigate the presence of reducible species in TiO2 , 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti. The TiO2 and ceria peaks in Ce/Ti catalysts were distinguished by performing H2 -TPR analyses of TiO2 and Ce/Ti. Fig. 3 shows the results of the H2 -TPR analysis of TiO2 , 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti. All samples exhibited a major reduction in the peaks in the 200 to 600 ◦ C temperature range. For the TiO2 support, the maximum temperature of the hydrogen consumption was 490 ◦ C. When Ce was impregnated into the TiO2 support, reduction peaks emerged

Fig. 1. Effect of reaction temperature on (a) NOx conversion, (b) NH3 conversion, and (c) outlet NO2 concentration of 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti catalysts (NO: 750 ppm, NO2 : 48 ppm, NH3 /NOx: 1.0, O2 : 8 vol.%, H2 O: 6 vol.%, GHSV: 120,000 h−1 ).

for the Ce/Ti catalysts at 410, 480, and 515 ◦ C. The first peak at 410 ◦ C could be attributed to the reduction in the surface oxygen of stoichiometric ceria (Ce4+ O Ce4+ ) [26], and the second peak at 480 ◦ C was attributed to the reduction of non-stoichiometric ceria (Ce3+ O Ce4+ ) [27]. In addition, the peak observed at 700 ◦ C in the 50Ce/Ti catalyst corresponded to that of bulk CeO2 [28]. For mixed oxide Ce/Ti catalysts, the reduction peak areas of the

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Table 1 Surface area, total pore volume, average pore diameter, and H2 consumption for Ce/Ti catalysts. Catalyst

SBET (m2 /g)

Total pore volume (p/p0 = 0.304) (cm3 /g)

Average pore diameter (nm)

H2 consumption (␮mol/g)

5Ce/Ti 10Ce/Ti 20Ce/Ti 50Ce/Ti

86.6 87.9 55.5 44.8

0.043 0.044 0.026 0.019

1.983 1.992 1.847 1.429

493 725 1042 1443

Ce/Ti catalysts increased as the ceria loading increased. As shown in Table 1, the H2 consumption values of 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti were 493, 725, 1042, and 1443 ␮mol/g, respectively. The H2 consumption increased as the ceria loading in the Ce/Ti catalysts increased, and as a result, the removal observed for oxygen from the 50Ce/Ti catalyst was the largest. The interaction between Ce and TiO2 significantly influenced the reducibility of the catalyst and is largely dependent on the ceria loading. However, when compared with the catalytic activity, the reducibility of Ce/Ti catalysts did not exhibit a direct correlation.

Fig. 2. XRD pattern of 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti catalysts.

Fig. 4. Effect of reaction temperature on NOx conversion of 10Ce/Ti, 10Ce/W/Ti, 10Ce/Mo/Ti, and 10Ce/La/Ti catalysts (NO: 750 ppm, NO2 : 48 ppm, NH3 /NOx: 1.0, O2 : 8 vol.%, H2 O: 6 vol.%, GHSV: 120,000 h−1 ).

Fig. 5. Effect of reaction temperature on NOx conversion of 10Ce/W/Ti, 10Ce W/Ti, and W/10Ce/Ti catalysts (NO: 750 ppm, NO2 : 48 ppm, NH3 /NOx: 1.0, O2 : 8 vol.%, H2 O: 6 vol.%, GHSV: 120,000 h−1 ).

3.2. Catalytic activity of catalysts doped with different transition metals

Fig. 3. H2 -TPR profiles of TiO2 , 5Ce/Ti, 10Ce/Ti, 20Ce/Ti, and 50Ce/Ti catalysts (experimental conditions: 30 mg catalyst, pretreatment at 400 ◦ C for 30 min with 5% O2 /Ar at 50 cm3 /min, 10% H2 /Ar reduction with a heating rate of 10 ◦ C/min, and a total flow rate of 50 cm3 /min).

The influence that the metal added to the catalyst had on the NOx conversion was evaluated, and the results are presented in Fig. 4, which the NOx conversion from 250 to 400 ◦ C with a GHSV of 120,000 h−1 using 10Ce/Ti, 10Ce/W/Ti, 10Ce/Mo/Ti, and 10Ce/La/Ti catalysts. The highest activity was achieved when 10 wt.% WO3 was added to 10Ce/Ti. In addition, we observed the catalytic activity of the 10Ce/W/Ti, 10Ce W/Ti, and W/10Ce/Ti catalysts to examine the effect of the different methods to add tungsten. As shown in Fig. 5, 10Ce/W/Ti had the highest activity over all temperature ranges. The surface areas of the 10Ce/Ti, 10Ce/W/Ti, 10Ce/Mo/Ti, 10Ce/La/Ti, 10Ce W/Ti, and W/10Ce/Ti catalysts were 87.9, 80.1, 79.6, 89.9, 76.5, and 74.7 m2 /g, respectively (Table 2), and different surface areas were obtained as metals were added to 10Ce/Ti. The 10Ce/La/Ti catalyst had the largest surface area at 89.9 m2 /g. As the metals were added to 10Ce/Ti, the surface areas of all catalysts decreased, except for that of 10Ce/La/Ti. An XPS analysis offers an effective method to analyze the surface of the catalysts. Fig. 6 shows the deconvoluted O 1s

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Table 2 Surface area, O␣ ratio, and Ce3+ ratio of various catalysts. Catalyst

SBET (m2 /g)

O␣ ratioa

Ce3+ ratiob

10Ce/Ti 10Ce/W/Ti 10Ce/Mo/Ti 10Ce/La/Ti 10Ce W/Ti W/10Ce/Ti

87.9 80.1 79.6 89.9 76.5 74.7

0.2604 0.4204 0.3032 0.2450 0.3723 0.3541

0.2678 0.3027 0.2762 0.2524 0.2725 0.2514

a b

Surface-adsorbed oxygen ratio calculated from XPS data. Ce3+ ratio calculated from XPS data.

Fig. 7. Correlation between O␣ ratio and NOx conversion over various catalysts.

Fig. 6. O 1s spectra of (a) 10Ce/Ti, (b) 10Ce/W/Ti, (c) 10Ce/Mo/Ti, (d) 10Ce/La/Ti, (e) 10Ce W/Ti, and (f) W/10Ce/Ti catalysts from XPS analysis.

spectra of the 10Ce/Ti, 10Ce/W/Ti, 10Ce/Mo/Ti, 10Ce/La/Ti, 10CeW/Ti, and W/10Ce/Ti catalysts. The O 1s peak was fitted into sub-bands by searching for an ideal combination of Gaussian bands. The two bands at 531.0–531.6 and 532.8–533.0 eV are respectively assigned to surface-adsorbed oxygen (O␣ ), such as O2 2− and O− , which are related to chemisorbed water (expressed as O ␣ ) [29]. The sub-bands from 529.5 to 530.0 eV are attributable to the lattice O2− oxygen (expressed as O␤ ) [27]. The O␣ ratio, which is calculated as O␣ /(O␣ + O ␣ + O␤ ), varied as metals were added to 10Ce/Ti (Table 2). The O␣ ratios in Ce/Me/Ti decreased in the order of 10Ce/W/Ti (0.4204) > 10Ce W/Ti (0.3723) > W/10Ce/Ti (0.3541) > 10Ce/Mo/Ti (0.3032) > 10Ce/Ti (0.2604) > 10Ce/La/Ti (0.2450). Wu et al. [30] mentioned that surface-chemisorbed oxygen (O␣ ) is the most active oxygen species, and that it is highly active in an oxidation reaction due to its high mobility. The surface-adsorbed oxygen is considered to be more reactive in oxidation reactions since its mobility is higher than that of lattice oxygen, and several researchers have insisted that a high O␣ ratio is beneficial for the

oxidation of NO to NO2 in an SCR reaction, resulting in an improvement to a “fast SCR” reaction [30,31]. Fig. 7 shows the correlation between the O␣ ratio and the catalytic activity, and the 10Ce/W/Ti catalyst with the highest O␣ ratio exhibited excellent catalytic activity. However, the increase in the O␣ ratio was not associated with the catalytic activity for all catalysts. The valence state of the active metals that are involved is an important factor in the NH3 –SCR reaction, which for Ce was of either Ce3+ or Ce4+ . Thus, we examined the influence of the addition of various metals on the Ce valence state in Mn/Ce/W/Ti. Fig. 8 shows the Ce 3d peaks in the XPS profiles of the 10Ce/Ti, 10Ce/W/Ti, 10Ce/Mo/Ti, 10Ce/La/Ti, 10Ce W/Ti, and W/10Ce/Ti catalysts. The Ce 3d peak was fitted into sub-bands by searching for the ideal combination of Gaussian bands, and the sub-bands marked u1 and v1 represent the 3d10 4f1 initial electronic state corresponding to Ce3+ while the sub-bands marked u, u2 , u3 , v, v2 , and v3 represent the 3d10 4f0 state of Ce4+ [22]. The Ce4+ ratios of all samples were calculated as Ce4+ /(Ce3+ + Ce4+ ) and are shown in Table 2. The Ce3+ ratios could be ranked as follows: 10Ce/W/Ti (0.3027) > 10Ce/Mo/Ti (0.2762) > 10Ce W/Ti (0.2725) > 10Ce/Ti (0.2678) > 10Ce/La/Ti (0.2524) > W/10Ce/Ti (0.2514), and the highest Ce3+ ratio of 0.3027 was observed for 10Ce/W/Ti. Fig. 9 shows the correlation between the Ce3+ ratio and the catalyst activity. An excellent SCR activity was achieved at higher Ce3+ ratios. According to Chen et al. [8], Ce3+ may improve the SCR reaction by promoting the oxidation of NO to NO2 , and NO oxidation experiments were carried out to investigate the influence that the tungsten in 10Ce/Ti had on the catalyst activity (Fig. 10). With the addition of tungsten to 10Ce/Ti, the conversion of NO to NO2 increased in a reaction from 250 to 400 ◦ C. Particularly, the NO2 production increased when 10Ce/W/Ti was used at 300 ◦ C or less. For this reason, SCR catalysts with the capacity for oxidation of NO to NO2 exhibit an excellent activity at low temperatures due to the influence of “fast SCR” [32]. Shan et al. [22,33] claimed that the introduction of W species could increase the Ce3+ ratio, number of acid sites, and number of oxygen vacancies of the catalyst, which are all features that improve the low-temperature activity of the catalyst. Thus, the SCR activity was observed to increase as the Ce3+ ratio increased when tungsten was added to 10Ce/W/Ti. The NH3 adsorption capacity of the 10Ce/Ti and 10Ce/W/Ti catalysts was compared in order to investigate the cause for which the activity improved after the addition of tungsten. The adsorption characteristics of the catalysts were measured by conducting a DRIFTs analysis, and the acid site of the catalysts in the SCR reaction was examined by performing an NH3 adsorption

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analysis using a DRIFT spectrometer at 250 ◦ C (Fig. 11a). The 1000 ppm NH3 was injected for 30 min, and the adsorbed species were examined. In these experiments, peaks corresponding to –OH, caused by the adsorption of NH4 + onto –OH, were observed at 3674 cm−1 . The adsorption of NH3 on Lewis acid sites was observed at 1605, 3170, 3256, and 3364 cm−1 [34], and the 10Ce/Ti and 10Ce/W/Ti exhibited peaks corresponding to Lewis acid sites and to –OH. In the case of the 10Ce/W/Ti impregnated with tungsten, a negative peak was observed at 2020 cm−1 , which corresponds to W O [35]. In addition, the adsorption of the NH4 + ion on the Brønsted acid sites was simultaneously observed at 1430 and

1670 cm−1 [34]. For 10Ce/Ti, the Brønsted acid sites showed a very small peak, and for 10Ce/W/Ti, it seemed that the Brønsted acid sites were more abundant relative to those for 10Ce/Ti. The largest negative peak of the adsorption of NH4 + onto –OH was observed at 3674 cm−1 for 10Ce/W/Ti, and furthermore, bands that did not belong to Lewis or Brønsted acid sites also appeared at 1560 cm−1 . Ramis et al. [36] proposed that bands at 1550 and 1570 cm−1 may be related to the intermediate of the oxidation of ammonia. Accordingly, bands at 1560 cm−1 might be attributed to amid (–NH2 ) species that can be assigned as an intermediate of the oxidation of ammonia. The 10Ce/Ti catalyst was purged with NH3 for 0.5 h, and NO + O2 /N2 was then introduced into the IR at 200 ◦ C, and the spectra were recorded as a function of time (Fig. 11b). After NO + O2 had been injected, all of the ammonia species decreased. In the case of 10Ce/Ti, the peaks of the ammonia species decreased after 15 min. At the same time, new bands were detected at 1622 and 1580 cm−1 that could be attributed to the NOx species [37]. When compared to 10Ce/Ti, the peaks assigned to the adsorbed ammonia species decreased more quickly on for the 10Ce/W/Ti catalyst (Fig. 11c). When the catalyst was purged with NO + O2 for only 5 min, all of the peaks resulting from ammonia species decreased. At the same time, new bands were detected at 1610 and 1580 cm−1 that could be attributed to NOx species [38]. On the basis of Fig. 11b and c, it could be concluded that NOx readily reacted with the adsorbed ammonia species, especially for the 10Ce/W/Ti catalyst. According to Chen et al. [39], the Ce/Ti and Ce/W/Ti catalysts represents the E–L mechanism. The adsorbed ammonia was first transformed to amide species, then formed nitrosamine (NH2 NO) after reacting with gaseous NO, and then decomposed to nitrogen. Since Ce4+ NH3 is converted to Ce3+ NH4 + by WO3 , the Brønsted acid sites of Ce/W/Ti increased more than for Ce/Ti [8]. The addition of tungsten in the 10Ce/Ti caused the peak corresponding to NH4 + adsorbed on the Brønsted acid sites to increase, which suggests that NH4 + adsorbs on the Brønsted acid sites, and this observation is strongly correlated to the catalytic activity that was also shown in previous studies [40]. An H2 -TPR analysis was performed to investigate the presence of the reducible species in the addition of tungsten in the 10Ce/Ti. The H2 -TPR analyses of the 10Ce/Ti and 10Ce/W/Ti were performed to distinguish the peaks of ceria and tungsten in 10Ce/W/Ti. Fig. 12 shows the results of the H2 -TPR analysis of 10Ce/Ti and 10Ce/W/Ti. When tungsten was impregnated into 10Ce/Ti, the reduction peaks for 10Ce/W/Ti emerged at approximately 410, 480,

Fig. 8. Ce 3d spectra of (a) 10Ce/Ti, (b) 10Ce/W/Ti, (c) 10Ce/Mo/Ti, (d) 10Ce/La/Ti, (e) 10Ce W/Ti, and (f) W/10Ce/Ti catalysts from XPS analysis.

Fig. 9. Correlation between Ce3+ ratio and NOx conversion over various catalysts.

Fig. 10. Effect of reaction temperature on NO to NO2 conversion of 10Ce/Ti and 10Ce/W/Ti catalysts (NO: 750 ppm, NO2 : 48 ppm, O2 : 8 vol.%, H2 O: 6 vol.%, GHSV: 120,000 h−1 ).

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Fig. 12. H2 -TPR profiles of 10Ce/Ti and 10Ce/W/Ti catalysts (experimental conditions: 30 mg catalyst, pretreatment at 400 ◦ C for 30 min with 5% O2 /Ar at 50 cm3 /min, 10% H2 /Ar reduction with a heating rate of 10 ◦ C/min, and a total flow rate of 50 cm3 /min).

515, 550–650, and 750–800 ◦ C. The peak at 410, 480, 515 ◦ C was attributed to the reduction in the surface oxygen of stoichiometric ceria (Ce4+ O Ce4+ ) and non-stoichiometric ceria (Ce3+ O Ce4+ ) [26,27]. The area of the peak that was attributed to WOx increased at ∼575 and from 757 to 790 ◦ C [41,42]. With the addition of tungsten to 10Ce/Ti, the area of the peak at 410 ◦ C corresponding to stoichiometric ceria (Ce4+ O Ce4+ ) decreased. In contrast, the area of the peak at 480 and 515 ◦ C corresponding to the reduction in the non-stoichiometric ceria (Ce3+ O Ce4+ ) increased. This means that the Ce3+ species of the 10Ce/W/Ti catalyst was present in large amounts. Fig. 13 shows the catalytic performance of the 10Ce/Ti and 10Ce/W/Ti catalysts in NH3 –SCR with and without H2 O. Under dry conditions, the catalytic activity of 10Ce/Ti and 10Ce/W/Ti decreased at 350 ◦ C or more while it shows a tendency to increase at 300 ◦ C or less. When moisture is present at a low temperature, the SCR activity is known to decrease. The ammonia that reacts with NO due to competitive adsorption of water and ammonia in the active site is reduced on the surface of the catalyst [43]. In contrast, the catalytic activity and the N2 selectivity increase since the oxidation of ammonia is inhibited [44]. In addition, the outlet NO2 concentration of 10Ce/Ti under dry conditions increases relative to the condition where water is present (Fig. 13a). However, the outlet NO2 concentration of the 10Ce/W/Ti catalyst does not exhibit a significant difference depending on the presence or absence of water (Fig. 13b). When tungsten is added to 10Ce/Ti, it is expected to inhibit the oxidation of the ammonia at high temperatures. 3.3. Effect of GHSV and O2 concentration on NO reduction by NH3 over Ce/W/Ti catalyst

Fig. 11. DRIFT spectra of (a) 10Ce/Ti and 10Ce/W/Ti exposed to 1,000 ppm NH3 for 30 min at 250 ◦ C, (b) taken at 250 ◦ C upon passing 1,000 ppm NO + 3% O2 over the NH3 presorbed on 10Ce/Ti for 0, 1, 2, 5, 10, 15, and 20 min, (c) taken at 250 ◦ C upon passing 1,000 ppm NO + 3% O2 over the NH3 presorbed on 10Ce/W/Ti for 0, 1, 2, 5, 10, 15, and 20 min.

The space velocity is a very important parameter for useful applications, and the influence of the GHSV on the catalytic activity was studied by varying the GHSV over the 10Ce/W/Ti catalyst. The activity of the 10Ce/W/Ti catalyst was measured over a wide GHSV range of 60,000–240,000 h−1 , and the results are shown in Fig. 14. The GHSV of 60,000 h−1 exhibited excellent catalytic NOx conversion activity of 74% or higher at 270–400 ◦ C. For a GHSV of 120,000 h−1 , 98% NO conversion was obtained at 350 ◦ C. Even with a GHSV as high as 240,000 h−1 , the maximum NO conversion was of 90% at 350 ◦ C. These results show that the 10Ce/W/Ti catalyst is highly effective for NOx removal within a wide GHSV range.

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Fig. 15. Effect of O2 concentration on NOx conversion of 10Ce/W/Ti catalyst (NO: 750 ppm, NO2 : 48 ppm, NH3 /NOx: 1.0, O2 : 8 vol.%, H2 O: 6 vol.%, GHSV: 120,000 h−1 ).

Fig. 13. Effect of reaction temperature on NOx conversion according to the with and without H2 O of (a) 10Ce/Ti and (b) 10Ce/W/Ti catalysts (NO: 750 ppm, NO2 : 48 ppm, O2 : 8 vol.%, H2 O: 6 vol.%, GHSV: 120,000 h−1 ). Fig. 16. Relative SCR activity in the presence of SO2 of NO by NH3 over 10Ce/Ti and 10Ce/W/Ti catalysts at 270 ◦ C (NO: 750 ppm, NO2 : 48 ppm, NH3 /NOx: 1.0, O2 : 8 vol.%, H2 O: 6 vol.%, SO2 : 200 ppm, GHSV: 120,000 h−1 ).

Oxygen plays an important role in the SCR reaction. In order to investigate the effect of the gaseous oxygen concentration on the catalytic activity in the SCR reaction of the 10Ce/W/Ti catalyst, an experiment was performed with an O2 concentration of 3–15%, and the results are shown in Fig. 15. The increase in the O2 concentration increases the catalytic activity at 300 ◦ C or less. The partial oxygen pressure was related to the diffusion of oxygen, and when you consider that the lattice oxygen is consumed in the SCR reaction and that gaseous O2 fills the lattice oxygen, the redox capacity increases since the transfer of oxygen is facilitated [45]. 3.4. SO2 resistance study

Fig. 14. Effect of space velocity on NOx conversion of 10Ce/W/Ti catalyst (NO: 750 ppm, NO2 : 48 ppm, NH3 /NOx: 1.0, O2 : 8 vol.%, H2 O: 6 vol.%, GHSV: 60,000–240,000 h−1 ).

The influence that the addition of SO2 had on the activity of Ce/Ti and 10Ce/W/Ti was monitored as a function of the time on the stream during SCR of NO with NH3 . The reaction conditions were set to 750 ppm NO, 48 ppm NO2 , NH3 /NOx = 1.0, 3 vol.% O2 , 6 vol.% H2 O, 200 ppm SO2 , GHSV = 120,000 h−1 , and a total flow rate of 600 cc/min at 270 ◦ C. Fig. 16 shows the NOx conversion when SO2 was added as a function of the reaction time. The 10Ce/Ti catalyst showed more signs of deactivation when SO2 was added than did the 10Ce/W/Ti catalyst. The activity of 10Ce/Ti rapidly decreased

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to the SCR activity of the catalyst. Due to the addition of tungsten in the 10Ce/Ti, the peak that corresponded to NH4 + adsorbed on the Brønsted acid sites tended to increase. After NO + O2 had been injected, as compared to 10Ce/Ti, the peaks assigned to adsorbed ammonia species decreased more quickly on the 10Ce/W/Ti catalyst. An excellent activity could be achieved by increasing the Ce3+ ratios. The 10Ce/W/Ti catalyst exhibited an excellent SO2 resistance relative to 10Ce/Ti, and thus, the addition of tungsten to 10Ce/Ti effectively increased the Ce3+ ratios and could be associated with excellent NOx conversion and SO2 resistance. References

Fig. 17. Transmission IR spectra of (a) 10Ce/Ti and (b) 10Ce/W/Ti samples after 5 h deactivation with SO2 .

immediately after the addition of SO2 , and in contrast, the activity of 10Ce/W/Ti decreased gradually. The activity of 10Ce/Ti was less than 38%, and the activity of 10Ce/W/Ti was 66% at a reaction time of 5 h. The decline in the catalyst activity upon the addition of SO2 was a result of the blocking of the active sites, which is caused by the formation of metal sulfates and ammonium sulfates [46]. The sulfated species that formed on the surface of the Ce-based composite oxide catalysts are stable and resulted in irreversible damage. The transmission IR spectra of the 10Ce/Ti and 10Ce/W/Ti samples after 5 h of deactivation with SO2 were determined (Fig. 17). For samples exposed to SO2 , several bands were detected at 982, 1128, 1196, 1414, and 1631 cm−1 , and at a wide band from 3400 to 3500 cm−1 . According to the literature [47,48], strong bands at 1196 and 1128 cm−1 are caused by bulk sulfate on the CeO2 catalyst, and bands at 982 and 1414 cm−1 are due to surface sulfate species with only one S O double bond and three S O bonds with O atoms linked to the surface. The broad bands between 3400 and 3500 cm−1 and the band at 1631 cm−1 correspond to H2 O [49]. It can thus be concluded that both the surface and bulk sulfate species formed on the samples exposed to SO2 , and in particular, the peak of the 10Ce/W/Ti catalyst exposed to SO2 was smaller than the 10Ce/Ti catalyst exposed to SO2 . In addition, the amount of sulfur of the 10Ce/Ti and 10Ce/W/Ti after 5 h deactivation with SO2 was measured via inductively coupled plasma optic emission spectroscopy (ICP). The sulfur for the 10Ce/Ti and 10Ce/W/Ti was 1.9 and 1.4 wt.%, respectively. The results of the IR pattern and ICP analysis of the catalysts exposed to SO2 for the same time revealed fewer sulfur from 10Ce/W/Ti catalyst than from the 10Ce/Ti catalyst. The 10Ce/W/Ti catalyst showed a stronger resistance to SO2 deactivation than the 10Ce/Ti catalyst, and thus, it was determined that the addition of tungsten to 10Ce/W/Ti inhibited the formation of sulfate species. 4. Conclusions We examined the effects that the physicochemical properties of the Ce/Me/Ti catalysts had on the SCR activity after the addition of various metals (W, Mo, and La) to the catalysts in order to promote the NH3 –SCR activity of non-vanadium-based catalysts. The activity in the SCR reaction significantly increased for 10Ce/W/Ti. Different surface areas and O␣ ratios could be observed as metals were added to Ce/Ti, and this did not have a direct correlation with the catalytic activity. The Ce3+ ratio of 10Ce/Me/Ti was proportional

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