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Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 Zhongxian Song a, Qiulin Zhang a,∗, Ping Ning a,∗, Jie Fan a, Yankang Duan a, Xin Liu a, Zhenzhen Huang b a b

Faculty of Environmental Science and Engineering, Kunming University of Science and Technology. Kunming 650500, PR China College of Environmental Science and Engineering, Hunan University, Changsha 410082, PR China

a r t i c l e

i n f o

Article history: Received 24 February 2016 Revised 8 April 2016 Accepted 14 April 2016 Available online xxx Keywords: CeO2 supports Selective catalytic reduction Oxygen vacancies Lewis acid sites

a b s t r a c t A series of CeO2 supports over phosphotungstic acid doped CeO2 (P-W/CeO2 ) prepared by hydrothermal (Cat-A), sol–gel (Cat-B) and precipitation (Cat-C) methods were investigated in selective catalytic reduction of NOx by NH3 . The catalytic activity of P-W/CeO2 was dramatically affected by the preparation methods of CeO2 supports. Results implied that the incorporation of W species could cause the decease of cell parameter and lattice contraction, resulting in inducing the formation of more Ce3+ and oxygen vacancies over the Cat-A catalyst, leading to the excellent oxidation ability and SCR activity. Furthermore, the Cat-A catalyst possessed the most amount of Lewis acid sites, resulting in the superior catalytic activity. Besides, the larger specific area and the excellent redox ability were responsible for the outstanding SCR performance. In brief, the favorable activity in NH3 oxidation, as well as NO oxidation, contributed to the excellent catalytic activity. Therefore, the Cat-A catalyst exhibited the best SCR activity and over 90% NOx conversion was obtained at 190–450 °C. © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction With the sharp increase of vehicles, the issue of vehicle exhaust pollution has attracted more and more public attention. NOx is one of main air pollutants emitted from vehicle exhaust [1]. The selective catalytic reduction (SCR) of nitrogen oxides (NOx ) with ammonia is one of the state-of-the-art technologies for controlling NOx from vehicle exhaust [2]. V2 O5 /TiO2 promoted by WO3 or MoO3 has been extensively used for several decades to removal of NOx . However, this V-based catalyst is only efficient at 30 0–40 0 °C, the formation of N2 O and loss of vanadium at high temperatures also restrain its further industry application [3–5]. Besides, the temperature of diesel exhaust gases is in a dynamic range of 180–440 °C [6]. Therefore, it is of great significance to develop environmentfriendly NH3 -SCR catalysts with the broad operating temperature window. Recently, CeO2 -based catalysts have received much attention due to the excellent redox property. Meanwhile, pure CeO2 shows poor SCR activity and stability. In order to promote the SCR activity and widen the operation temperature window, some other

∗

Corresponding authors. Tel.: +86 871 65170905. E-mail addresses: [email protected] 126.com (P. Ning).

(Q.

Zhang),

ningping_58@

transition metals such as Mn, Ti and W are often doped into CeO2 based catalyst [7–11]. Gao et al. [12] compared the SCR performance of CeO2 /TiO2 catalysts prepared by different methods, and the results indicated that the catalyst prepared by sol–gel method presented the best SCR activity due to better dispersion of active nano-crystalline ceria. Shan et al. [13] reported that a superior CeW-Ti catalyst prepared by a homogeneous precipitation method exhibited outstanding catalytic activity by the addition of WO3 and nearly 100% NOx conversion was obtained at 250–450 °C. Chen et al. [14] also found that introduction of WO3 could increase the activity of CeO2 /TiO2 catalyst prepared by co-impregnation method. Based on the above discussions, it is well established that the catalytic activity of catalyst is affected by the nature of support or the active components for the support catalysts. Therefore, various views concerning the relationship between different supports and catalytic activity have been reported [15–17]. It is usually accepted that, even for a same metal oxide-support catalyst, the supported catalyst can possess a variety of structures, which depends on the experimental conditions such as the different supports and the calcination temperatures. In our previous study [18], heteropoly acids modification on CeO2 was developed, and the results implied that the catalyst activity of pure CeO2 was remarkably improved by the introduction of solid acid for the improvement of surface acidity. However, the low-temperature SCR activity is still to improve in

http://dx.doi.org/10.1016/j.jtice.2016.04.034 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 1. The catalytic activity and N2 selectivity of P-W/CeO2 catalysts: (a): SCR activity; (b): N2 selectivity.

order to satisfy the need of industry application. Thereby, the effect of CeO2 supports on the SCR activity is investigated over the phosphotungstic acid doped CeO2 catalysts. In this work, the CeO2 supports were prepared by hydrothermal, precipitation and sol–gel methods and used for selective catalytic reduction of NO by NH3 , and the phosphotungstic acid doped CeO2 supports (P-W/CeO2 ) was prepared by impregnation. The effect of different CeO2 supports on the structure and catalytic activity of P-W/CeO2 was investigated. The structure and physicochemical properties of the samples were systematically characterized by XRD, N2 physisorption, Raman, HR-TEM, H2 -TPR, NH3 -TPD and Py-IR. 2. Experimental methods

(0.01 mol), glucose (0.05 mol) and acrylic acid (0.03 mol) were dissolved in deionized water. Secondly, the 25 mL ammonia solution (25 wt%) was gradually dripped with vigorous stirring at room temperature. The above mixture was aged for 5 h at 20 °C, and then the above mixture was poured into a stainless autoclave and kept at 160 °C for three days. The final powder was filtered, washed, and dried at 80 °C for 12 h, and then calcined at 550 °C for 5 h in air. This catalyst was signed as CeO2 -1. 2.1.2. Sol–gel method Ce(NO3 )3 ·6H2 O (0.01 mol) and citric acid (0.02 mol) were dissolved in deionized water. The mixture was heated at 80 °C under stirring until it became a transparent yellow sol and then dried at 140 °C until the sol transformed to gel. The resulting powder was calcined at 550 °C for 5 h in air. The catalyst was labeled as CeO2 -2.

2.1. Catalyst preparation 2.1.1. Hydrothermal method This CeO2 supports were synthesized by the hydrothermal method. All chemicals were of analytical grate. Ce(NO3 )3 ·6H2 O

2.1.3. Precipitation method Ce(NO3 )3 ·6H2 O was dissolved in distilled water. And then NH3 ·H2 O and (NH4 )2 CO3 solution were dropwise added with continuous stirring until pH reached 10. The resulting precipitate was

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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maintained in stirring for 4 h, aged 5 h, and then filtered, washed several times with distilled water and calcined at 550 °C for 5 h in air. The catalyst was noted as CeO2 -3.

2.1.4. Impregnating method The phosphotungstic acid doped CeO2 supports was prepared by impregnation method. 0.6 g phosphotungstic acid was dissolved in deionized water. And then 2.4 g of the CeO2 -1, CeO2 -2 and CeO2 -3 supports were immersed and kept for 2 h, respectively. The powder was heated at 60 °C for 5 h and dried overnight at 105 °C. The resulting catalyst was calcined at 300 °C for 2 h and 550 °C for 3 h in air. The obtained catalysts were correspondingly marked as Cat-A, Cat-B and Cat-C, respectively.

2.2. Catalyst characterization The powder X-ray diffraction (XRD) measurements were carried out on a Rigaku D/Max 2500 system with Cu radiation (λ = 0.15418 nm). The X-ray tube was carried out at 40 kV and 100 mA. The catalysts were studied in the 2θ range of 10–70° at a step of 6° min−1 for determining the crystal structure. The N2 adsorption-desorption isotherms were obtained at −196 °C using a TristarⅡ3020 automated gas sorption system. Before N2 adsorption, all catalysts were degassed at 300 °C for 4 h. The specific surface areas of the samples were evaluated from these isotherms by the Brunauer–Emmett–Teller (BET) model. Raman spectra were performed on a Renishaw-20 0 0 Raman spectrometer at a resolution of 2 cm−1 using the 514.5-nm line of an Ar ion laser as the excitation source. X-ray photoelectron spectroscopy (XPS) was operated on an ULVAC PHI 50 0 0 Versa Probe-Ⅱequipment operating at 10−9 Pa with an Al Ka radiation (1486.6 eV) to analyze the surface atomic concentration and study the chemical states of the samples. The observed spectra were corrected with the C 1 s binding energy value of 284.8 eV. The high-resolution transmission electron microscopy (HRTEM): A JEOL JEM-2100 analytical transmission electron microscope was used to get TEM pictures of the samples. Prior to the test, the sample was put into high-purity ethanol for 30 min, and the obtained sample was transited to the copper-grid-supported amorphous carbon films, and then the impregnated mesh was dried. In order to attain grain size distribution of particle sizes, about 100 particles were identified and calculated. The H2 -temperature programmed reduction (H2 -TPR) was operated by using 5% H2 /N2 . 50 mg of catalyst was employed. Prior to the H2 -TPR experiments, the samples were pre-treatment in pure N2 at 400 °C for 60 min and cooled to 100 °C. The H2 -TPR runs were carried out in a flow of 5 vol.% H2 /Ar (30 ml min−1 ) from 100 to 900 °C at a heating rate of 10 °C min−1 . Total acidity measurement was investigated by the temperature programmed desorption of ammonia (NH3 -TPD) with a thermal conductivity detector (TCD) setup (Fuli, GC-9750, China). Before the experiment, the catalysts were pretreated in pure He at 400 °C for 60 min. Then the samples were saturated with 5% NH3 /He at a flow rate of 30 mL/min for about 60 min. Desorption was carried out by heating from 50 to 450 °C with a heating rate of 10 °C/min. The infrared spectroscopy of adsorbed pyridine (Py-IR) was operated on a Bruker IF113 V IR spectrometer. The sample was activated under vacuum (1 × 10−2 Pa) at 400 °C for 1 h and then cooled to room temperature. 30 mL/min He flow saturated with pyridine was added into the IR cell at room temperature for 40 min, and then the sample was heated, and IR spectra were collected at 50, 150, 250 and 350 °C, respectively.

Fig. 2. XRD patterns of different catalysts.

2.3. Activity test The SCR performance measurements were operated in a fixedbed quartz reactor (9 mm i.d.) with 0.4 mL catalysts of 40–60 mesh. The gas flow was measured by mass flow meters. The concentration of simulated gases were as follow: 600 ppm NO, 600 ppm NH3 , 5% O2 , 10% water vapor (when used), 100 ppm SO2 (when used), balance N2 . The total flow rate was 400 mL/min. The concentration of NOx (NO and NO2 ) and NH3 were continuously detected by the ECOM·J2KN and GXH-1050E flue gas analyzer, respectively. The N2 O product was measured using gas chromatograph (Fuli, 9790) equipment with electron capture detector. The reaction system was kept for 30 min at each reaction temperature before analysis. The calculation equation of NOx conversion and N2 selectivity were as follows:

N Ox conversion =

N Ox (inlet ) − N Ox (out let ) × 100% N Ox (inlet )

N2 (select ivit y ) =

N Ox (inlet ) − N Ox (out let ) − 2N2 O(out ) × 100% N Ox (inlet ) − N Ox (out let )

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 3. HR-TEM images of P-W/CeO2 catalysts. a: Cat-A; b: Cat-B; c: Cat-C.

3. Result and discussion 3.1. The SCR activity and N2 selectivity of P-W/CeO2 catalysts Fig. 1a compared the SCR performance of the P-W/CeO2 catalysts. Obviously, the NOx conversion was influenced remarkably by the different CeO2 supports at 150–450 °C. Cat-C exhibited slight SCR activity at low temperature (below 275 °C) and over 80% NOx conversion was attained at 250–450 °C. For the Cat-B catalyst with the CeO2 support prepared by sol–gel method, the catalytic activity of Cat-B was noticeably promoted. Cat-B showed more than 80% NOx conversion in the temperature range of 208–450 °C. The 80% NOx conversion of Cat-A (the CeO2 support by hydrothermal

method) decreased to 177 °C and its working temperature window (NOx conversion was over 90%) was 190–450 °C. Compared to Cat-B and Cat-C, the low-temperature SCR activity of Cat-A was evidently improved, and the operation temperature window was widened effectively. In brief, the SCR activity of Cat-A was dramatically higher than that of Cat-B and Cat-C with the same content but different CeO2 supports. Therefore, the SCR performances of P-W/CeO2 catalysts were remarkably affected by the different CeO2 supports. The N2 selectivity and N2 O outlet concentration of the PW/CeO2 catalysts at 150–450 °C were exhibited in Fig. 1b. The P-W/CeO2 catalysts showed the excellent N2 selectivity and over 95% N2 selectivity was obtained at 150–450 °C. For N2 O outlet concentration, Cat-C possessed the largest N2 O concentration at

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 4. N2 adsorption–desorption results of the P-W/CeO2 catalysts: (a) the N2 adsorption–desorption isotherms; (b) the BJH pore size distributions.

high temperature, but its maximum N2 O concentration was merely 12 ppm at 450 °C. Consequently, the P-W/CeO2 catalyst was an environment-friendly NH3 -SCR catalyst with the broad operation temperature window and high N2 selectivity. 3.2. XRD analysis The X-ray powder diffraction patterns (XRD) of the P-W/CeO2 catalysts were shown in Fig. 2a. It could be seen that the cubic fluorite phase CeO2 (PDF: 65-5923) was the main phase over the three catalysts. From Fig. 2a, the small diffraction peaks of triclinic WO3 (PDF: 20-1323) was observed over the Cat-B and Cat-C catalysts. No WO3 phase was found over the Cat-A catalyst. These indicated that WO3 existed as a highly dispersed species on the CeO2 support. Furthermore, the intensity of CeO2 over Cat-A was obviously stronger than that of Cat-B and Cat-C, indicating that Cat-A possessed a higher CeO2 crystallinity than the other two samples.

Additionally, the diffraction peaks of Cat-B and Cat-C were sharper than that of the Cat-A catalyst. Thereby, the crystallite sizes were measured from the (111) crystal plane of CeO2 , and the results were presented in Table 1. It was obvious that Cat-B and Cat-C showed a crystallite size of 14.3 and 15.9 nm, respectively, which were smaller than that of Cat-A (10.8 nm), implying that the PW/CeO2 particles were prone to be aggregated on the surface of CeO2 supports prepared by sol–gel and precipitation methods. A peak shift of the XRD patterns in the 2θ range of 27–30° for P-W/CeO2 catalysts was listed in Fig. 2b and Table 1, it was apparent that the 2θ of Cat-A was shifted to higher value compared to the Cat-B and Cat-C catalysts, which proved that the cell parameter a of Cat-A was decreased. It was accepted that the radius of W6+ ˚ was smaller than that of Ce4+ (0.87 A), ˚ and it was much (0.65 A) easier for W6+ to enter into the CeO2 lattice, and then resulted in a decrease in the cell parameter. The cell volume Vcell of the P-W/CeO2 catalysts was measured based on the formula: Vcell = a3 ,

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 5. Raman spectra of the P-W/CeO2 catalysts. Table 1 Cell volume, cell parameter and crystallite size of the P-W/CeO2 catalysts. Sample

2θ /°

Cell parameter: a /A˚

Cell volume/A˚ 3

Crystallite size/nm

Cat-A Cat-B Cat-C

28.63 28.60 28.58

5.393 5.399 5.404

156.85 157.38 157.81

10.8 14.3 15.9

and the results were shown in Table 1. It was clear that the cell volume was 156.85 A˚ 3 for Cat-A, 157.38 A˚ 3 for Cat-B and 157.81 A˚ 3 for Cat-C, which were smaller compared with that of pure CeO2 (158.78 A˚ 3 ) [19]. The phenomena demonstrated that W6+ had entered into the CeO2 lattice. Furthermore, Cat-A possessed the smallest cell volume of 156.85 A˚ 3 , suggesting that more W6+ had entered into the CeO2 lattice, which might be one of factors that the P-W/CeO2 catalysts showed various SCR performances due to the different CeO2 supports prepared by different methods. 3.3. HR-TEM analysis In order to get more information about the particle sizes of P-W/CeO2 catalysts, the average particle sizes were calculated by HR-TEM and the results were shown in Fig. 3. The mean diameters were by statistic analysis of more than 100 particles in HRTEM images. It was obvious that the particle size was 12.3 nm for Cat-A, 22.45 nm for Cat-B and 50.2 nm for Cat-C, respectively. Furthermore, Cat-A and Cat-B exhibited a uniform distribution and small particle sizes compared with Cat-C. The phenomena implied that the particle sizes were associated with the different preparations of CeO2 supports; the Cat-A catalyst possessed the excellent dispersion and inhibited the particle aggregation. This meant that the silicotungstic acid doped CeO2 prepared by hydrothermal method was beneficial to promote the dispersion, which could enhance the catalytic performance. In general, the smaller the average particle size was, the larger the specific surface area was. Therefore, the smaller particle size could provide more surface areas and active sites, which was beneficial to the SCR activity. Cat-B and Cat-C showed the quite large particle sizes, which would lead to a decrease in specific surface areas and pore volume, resulting in poor SCR activity, which was in good accord with the SCR activity results. It was reported that the metal dispersion played an important role in determining the SCR activity [20,21]. Besides, the

smaller particle sizes contributed to the redox ability of SCR catalysts, which also contributed to the catalytic activity [22]. Combined with the SCR activity results, the well dispersion of silicotungstic acid and smaller particle sizes were two of the main factors for affecting the SCR activity over silicotungstic acid doped CeO2 support prepared by hydrothermal method. 3.4. N2 adsorption–desorption Fig. 4a exhibited the N2 adsorption–desorption isotherms of the P-W/CeO2 catalysts with different preparation of CeO2 . As shown in Fig. 4a, the isotherms of the P-W/CeO2 catalysts corresponded to type IV isotherm according to the BDDT classification, which was typically associated with mesoporous materials (2–50 nm) [23]. The closure points of the hysteresis loops over the Cat-A catalyst existed when the value of P/P0 reached 0.6, implying the abundance of mesopores. However, for the CeO2 support prepared by sol–gel and precipitation methods, the closure points of the hysteresis loops of Cat-B and Cat-C moved to the higher P/P0 value (0.85 and 0.9, respectively), implying that much more macropores were generated due to the different preparations of CeO2 supports, which could lead to the bigger particle sizes or the aggregation of small oxide particles in the two samples. Fig. 4b showed the BJH pore size distributions of the P-W/CeO2 catalysts. The Cat-A mainly possessed mesopores and the pore size distribution gathered in the range of 2–30 nm, and that the Cat-B and Cat-C showed distribution peaks near 3.4 nm. Besides, Cat-B also presented the pore size distribution of 8 and 24 nm. It was obvious that the CatA possessed more abundance of mesopores than Cat-B and Cat-C. Combined with the BET results, it was reasonable that the excellent dispersion and the smaller particle sizes of P-W/CeO2 catalyst were responsible for the favored pore structure. Noticeably, the N2 desorption differential pore volumes per weight (cm3 /g·nm) of Cat-B and Cat-C presented a remarkably decrease compared with

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 6. (a). Ce 3d XPS spectra of P-W/CeO2 catalysts. (b). O 1 s XPS spectra of P-W/CeO2 catalysts.

that of Cat-A. This proved that Cat-A possessed much more abundant mesopores, which could provide more active sites and inner surface area in the NH3 -SCR reaction. Meanwhile, the pore size distributions also demonstrated that the amount of mesopores in the P-W/CeO2 catalysts varied for the different preparation of CeO2 supports, and an appropriate amount of mesopores and macropores could contributed to the catalytic activity of the P-W/CeO2 catalysts. Table 2 showed the structural parameters such as the specific surface areas, the BJH desorption pore volume and the average pore diameter of the P-W/CeO2 catalysts from the N2 physisorption results. The specific surface areas, total pore volume and average pore diameter of the Cat-A catalyst were 58.7 m2 /g, 0.2079 cm3 /g and 11.3 nm, respectively. For Cat-B and Cat-C catalysts, the specific surface areas and total pore volume both dramatically decreased, suggesting that the mesopores of CeO2 support were blocked or

Table 2 BET measurements for the P-W/CeO2 catalysts by different methods.

Sample

BET (m2 /g)

Total pore volume (cm3 /g)

Average pore diameter (nm)

Cat-A Cat-B Cat-C

58.9 11.7 6.8

0.208 0.07478 0.0243

11.3 18.5 13.3

filled by the aggregated WO3 or larger particle sizes, because that the mesopores mainly were beneficial to large surface area and pore volume. This further confirmed that the Cat-A catalyst possessed more excellent dispersity than that of Cat-B and Cat-C. Consequently, Cat-A showed the excellent texture properties.

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 7. H2-TPR results of P-W/CeO2 catalysts.

Table 3 Surface atomic concentration (atomic %) from XPS. Sample

Ce3+ /(Ce3+ + Ce4+ )

Oα /(Oα + Oβ )

Cat-A Cat-B Cat-C

17.2 14.6 12.6

37.4 30 27.6

dramatically affected by the CeO2 supports prepared by different methods. 3.6. XPS analysis

Fig. 8. NH3-TPD results of P-W/CeO2 catalysts.

3.5. Raman analysis The Raman spectra of Cat-A, Cat-B and Cat-C catalysts were shown in Fig. 5. The most intense band was observed at 464 cm−1 over the three samples, which was assigned to F2g Raman active mode of the fluorite structure. For the Cat-B and Cat-C catalysts, the band at 252 cm−1 was ascribed to O-W-O bending modes of the bridging oxygen, and the bands at 713 and 805 cm−1 were the corresponding stretching modes [24]. However, no band of WO3 was observed over the Cat-A catalyst, which was in line with the XRD results. It was obvious that the Raman spectra of Cat-B and Cat-C were stronger and broader than that of Cat-A. The increase in its asymmetry and the broadening of the F2g band were ascribed to the reduction of phonon lifetime in the nanocrystalline regime [25]. Hence, the microstructures of P-W/CeO2 catalysts were

The Ce 3d XPS spectra of P-W/CeO2 catalysts were showed in Fig. 6a. The peaks signed as u, u  , u   , v, v  and v   were ascribed to the state of Ce4+ ions [26,27]. While the bands labeled u’ and v’ corresponded to the state of Ce3+ . It was reported [28] that the Ce3+ could induce the formation of more oxygen vacancies and unsaturated chemical bonds, which could contribute to the catalytic activity. Therefore, the surface atomic concentrations of Ce4+ and Ce3+ were calculated and the results were shown in Table 3. The surface Ce3+ /(Ce3+ + Ce4+ ) ratio was 17.2%, 14.6% and 12.6% over the Cat-A, Cat-B and Cat-C catalysts, respectively. Combined with the XRD results, the most amount of W6+ had incorporated into the CeO2 lattice over Cat-A for the smallest the cell parameter a. It was reported [29] that MnOx could interact with CeO2 , which could induce the formation of oxygen vacancy. In this case, it was inferred that the W ions incorporated into the lattice of the CeO2 support, which contributed to the presence of Ce3+ and oxygen vacancy, resulting in the improvement of SCR performance. Hence, the Cat-A possessed the highest surface Ce3+ /(Ce3+ + Ce4+ ) ratio. Liu et al. reported [30] that the higher surface Ce3+ concentrations could improve the adsorption capacity of NO and NH3 , which contributed to the improvement of SCR performance in NH3 -SCR activity. This might be another reason that the Cat-A catalyst exhibited the best catalytic activity at 150–450 °C. Besides, the redox abillity of SCR catalyst was remarkably affected by the ratio of Ce3+ /(Ce3+ + Ce4+ ), and the higher surface Ce3+ concentrations usually contributed to the excellent redox property via

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 9. Py-IR spectra of the P-W/CeO2 catalysts degassed at different temperatures: a: 50 °C; b: 150 °C; c: 250 °C; d: 350 °C.

the redox shift between Ce4+ and Ce3+ , which was beneficial to the improvement of SCR performance [28]. In brief, the surface Ce3+ /(Ce3+ + Ce4+ ) ratio was closely associated with the CeO2 supports prepared by the different methods, and Cat-A was beneficial to the formation of Ce3+ species. Fig. 6b displayed the peak separation of the region for the PW/CeO2 catalysts. The peaks of O 1 s could be fitted into two peaks: the lattice oxygen (529.2–530.3 eV, marked as Oβ ) and the chemisorbed oxygen (531.3–532.3 eV, as denoted Oα ) [31]. It was widely accepted that Oα played an important role in NH3 -SCR reaction [32]. Since the surface chemisorbed oxygen (Oα ) was more active than lattice oxygen (Oβ ) for its higher mobility, and then the high Oα /(Oα + Oβ ) ratio could contribute to the superior SCR activity. Thereby, the surface Oα and Oβ concentrations were calculated and the results were listed in Table 3. As shown in Fig. 3, The Oα /(Oα + Oβ ) ratio decreased according to the sequence: 27.6% for Cat-C < 30% for Cat-B < 37.4% for Cat-A, which was in good accord with the sequence of SCR activity. Hence, it was speculated that the chemisorbed oxygen was one of the decisive factors for the SCR performance. Besides, the Oα and Oβ binding energy for Cat-A shifted towards lower value compared with that of Cat-B and Cat-C, indicating that the Oα and Oβ of Cat-A possessed the most electron cloud density, which was beneficial to the generation of

the reactive electrophilic oxygen species, resulting in the excellent SCR activity of Cat-A [33]. 3.7. H2 -TPR In order to study the effect of different CeO2 supports on the reduction ability of P-W/CeO2 catalysts, The H2 -TPR analysis was characterized. As shown in Fig. 7, Cat-C exhibited three reduction peaks. The peak centered about 627 °C was ascribed to the reduction of surface oxygen and Ce4+ to Ce3+ ; the high-temperature peaks at 733 and 815 °C corresponded to the reduction of WO3 and bulk CeO2 , respectively [34,35]. For Cat-B, it was obvious that the reduction temperature of WO3 shifted to lower temperature compared with that of Cat-C. Furthermore, Cat-A showed the lowest reduction temperatures among the three catalysts. The reduction temperature of surface oxygen and Ce4+ to Ce3+ shifted to 523 °C and it was 624 °C for WO3 over Cat-A. These phenomena might be associated with the particle sizes of catalysts. It was reported [22] that the smaller particle sizes and excellent dispersity contributed to redox property. Besides, the larger specific surface area and higher pore volume were beneficial to the redox ability [36]. Consequently, the small particle sizes, excellent dispersity and favored pore structure were responsible for the outstanding redox

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3.8. NH3 -TPD NH3 -TPD was performed to investigate the influence of different CeO2 supports on the surface acidity of P-W/CeO2 catalysts. The results were shown in Fig. 8. The broad desorption peaks spanned at 50–450 °C were observed over Cat-A, which corresponded to the desorption of NH3 on the acid sites. For the Cat-B catalyst, the NH3 -desorption peaks appeared at 50–220 °C and 240–420 °C. Furthermore, the intensity of the NH3 -desorption peaks over Cat-B decreased dramatically compared with that of Cat-A, which implied that the Cat-A catalyst possessed more surface acid sites. It was obvious that a weak NH3 -desorption peak centered at 175 °C appeared on the Cat-C catalyst. The phenomenon indicated that Cat-C possessed some amount of weak acid sites. The results suggested that the amount of surface acidity over the P-W/CeO2 catalyst was remarkably influenced by the preparation methods of CeO2 supports. Combined with the SCR activity, it was reasonable that the enhanced surface acidity could contribute to NH3 adsorption, which was a crucial factor for the improvement of SCR activity of P-W/CeO2 .

Fig. 10. NH3 oxidation activity (a) and by-products over the P-W/CeO2 catalysts: N2O concentration (b); and NO concentration (c).

property, which could contribute to the superior catalytic activity. These were dependence on the CeO2 supports prepared by different methods.

3.9. Py-IR In order to investigate the effect of type of surface acid sites on the SCR activity over P-W/CeO2 catalysts, the Py-IR spectroscopy was employed and the results were shown in Fig. 9. Py-IR spectra of these samples were degassed at 50, 150, 250 and 350 °C, respectively. The band at 1450 cm−1 was assigned to Lewis (L) acid sites, the band at 1490 cm−1 corresponded to the Lewis or Brønsted acid sites, and the band at 1540 cm−1 was commonly attributed to Brønsted (B) acid sites [37,38]. As shown in Fig. 9, the Cat-A, Cat-B and Cat-C catalysts mainly possessed L acid sites, while Furthermore, the strength of peak at 1450 over Cat-A was remarkably higher than that of Cat-B and Cat-C, which indicated that more amount of L acid sites was existed over Cat-A. Besides, the intensity peaks of L acid sites over the P-W/CeO2 catalysts decreased with increasing evacuation temperatures from 50 to 350 °C. It was reported that weak acid sites corresponded to ones from which pyridine evacuated by evacuation at 50–150 °C; medium acid sites were assigned to the ones dropped off below 300 °C, and the strong acid sites kept adsorbing pyridine after evacuation at high temperature (above 350 °C) [39]. The phenomenon implied that Cat-A, Cat-B and Cat-C catalysts owned the weak, medium and strong L acid sites, but the weak L acid sites of the catalysts were dominant due to a weaker intensity of peak at high evacuation temperature, which could contribute to the low-temperature SCR activity. Furthermore, the B (1540 cm−1 ) and L (1455 cm−1 ) acid site of Cat-A shifted to lower wavenumber compared with that of Cat-B and Cat-C, which implied that the strength of B and L acid sites over Cat-A was weaker than that of the other two samples. This might be another reason that Cat-A showed the best catalytic activity among the samples. It was notice that the intensity of B and L acid sites should be gradually weakened with the increasing the degasification temperature from 50 to 350 °C. However, some new peaks (1500–1506, 1510–1517 and 1530–1533 cm−1 ) appeared in the range of 1500–1535 cm−1 and the intensity of them became stronger when the degasification temperatures increased. This phenomenon resulted from an oxidative breakdown of Py species or Py cracking into carboxylate, carbonaceous and nitrites species, which was in good accord with the previous reports [40–42]. It was reported [43] that the large amount of L acid sites contributed to adsorbing and activating NH3 , which was regarded as the significant factors for the reduction of NO by NH3 , and thus promoted the NH3 -SCR activity. It was also found that L acid sites were thought to the adsorption sites for nitrates as the crucial intermediates in NH3 -SCR reaction [37]. Thereby, the improvement of surface acidity could enhance the catalytic activity. Combined with the catalytic

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Fig. 11. NO oxidation activity over the P-W/CeO2 catalysts.

Fig. 12. Influence of H2O + SO2 on NOx conversion over Cat-A catalysts at 275 °C.

Table 4 Concentration and distribution of L acid sites on PW/CeO2 catalysts. Concentration of L acid sites (μmol/g) Samples

Cat-A Cat-B Cat-C

weak

medium

50 °C

150 °C

250 °C

350 °C

strong

47.6 21.6 12.2

30.2 15.2 7.8

17.0 8.8 6.9

10.3 8.0 6.1

activity results, the SCR performance was in line with the intensity of L acid sites. In order to quantitatively study the concentration of L acid sites, the strength of the peak at 1445 cm−1 for L acid sites was calculated and the results was shown in Table 4, while the peaks at 1540 cm−1 for B acid sites could be neglected due to

the low concentration. The relationship between the number of L acid sites and the SCR activity was established. It was obvious that Cat-A possessed the most amount of L acid sites. Furthermore, a plenty of weak L acid sites were observed over the Cat-A catalyst, which could contribute to the low-temperature catalytic activity. Besides, the concentration of L acid sites ranked in the sequence: Cat-A > Cat-B > Cat-C, which was consistent with the SCR activity results. In a word, a increase in the amount of L acid sites could enhance the reactivity of P-W/CeO2 catalysts for the adsorption of NH3 , which contributed to ammonia oxidation, and then resulted in the superior SCR performance. 3.10. NH3 oxidation The NH3 oxidation performances and by-products (N2 O, NO and NO2 ) of the P-W/CeO2 catalysts were shown in Fig. 10. It was apparent that the NH3 conversion of Cat-A was higher than that of

Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034

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Cat-B and Cat-C. The concentration of N2 O and NO decreased as follows: Cat-C > Cat-B > Cat-A. The concentration of NO2 was not detected in the whole temperature range over these three samples. Obviously, the by-products (N2 O and NO) decreased with an increase in the number of L acid sites. The phenomena indicated that the improvement of surface acidity could enhance the NH3 oxidation and promote the N2 selectivity, resulting in the superior SCR activity, which was dependence on the preparation method of CeO2 over the P-W/CeO2 catalysts. 3.11. NO oxidation The NO oxidation activities over the P-W/CeO2 catalysts were also investigated and the results were shown in Fig. 11. NO2 production of Cat-B was dramatically higher than that of Cat-C in the temperature range of 150–450 °C. However, Cat-A possessed the most concentration of NO2 . These implied that Cat-A showed the best oxidation ability, which was in good accord with the XPS and H2 -TPR results. It was usually accepted that NO2 production was a key step in the NH3 -SCR reaction. Therefore, Cat-A exhibited the best catalytic activity at 150–450 °C. 3.12. Effect of H2 O + SO2 on SCR activity The emission of SO2 + H2 O was unavoidable from diesel exhaust. Thereby, it was necessary to investigate the influence of SO2 + H2 O exposure on the catalytic activity. The influence of H2 O and SO2 + H2 O on catalytic activity over Cat-A was exhibited in Fig. 12. When 10% H2 O was introduced to the reaction gases, almost no decrease of De-NOx performance was observed, which implied that the inhibitory effect of H2 O was ignored. However, when the CatA catalyst was exposed in a feed stream for 8 h in the presence of 10 vol.% H2 O and 100 ppm SO2 at 275 °C. The SCR activity was inhibited and NOx conversion decreased from 100 to 89.3%. Noticeably, the NOx conversion nearly could recover the original level after removal off H2 O + SO2 , which suggested that the Cat-A catalyst possessed the excellent resistance to the H2 O + SO2 . 4. Conclusion The present work studied the influence of CeO2 supports prepared by different methods on the SCR activity of P-W/CeO2 catalysts. The results indicated that Cat-A exhibited the best catalytic activity and above 90% NOx conversion was attained at 190– 450 °C. The favorable activity in NH3 oxidation, as well as NO oxidation, contributed to the superior catalytic activity. The XRD, Raman and HR-TEM results revealed that Cat-A showed the smallest particle sizes and excellent dispersity; The BET results indicated that the Cat-A catalyst showed the largest specific surface area and the highest total pore volume. The XPS results implied that the Cat-A catalyst possessed higher surface Ce3+ /(Ce3+ + Ce4+ ) and Oα /(Oα + Oβ ) ratio. The H2 -TPR results suggested that the CatA catalyst showed the lowest reduction temperature and exhibited the outstanding redox ability. The NH3 -TPD and Py-IR results implied that Cat-A possessed the most amount of surface L acid sites. Furthermore, Cat-A possessed the superior N2 selectivity, as well as resistance to H2 O+SO2 . As a result, these factors were dramatically affected by the different CeO2 supports over the P-W/CeO2 catalysts. Acknowledgments This work is supported by the National Natural Science Foundation of China (no. 21307047) and Academic Newcomer Award of Yunnan Province.

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Please cite this article as: Z. Song et al., Effect of CeO2 support on the selective catalytic reduction of NO with NH3 over P-W/CeO2 , Journal of the Taiwan Institute of Chemical Engineers (2016), http://dx.doi.org/10.1016/j.jtice.2016.04.034