SnxCe1-xO2 Catalysts for High-Temperature Lean NOx

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Kinetics, Catalysis, and Reaction Engineering

Efficient Pt/Ba/SnxCe1-xO2 Catalysts for High-Temperature Lean NOx Traps with High H2O and CO2 Tolerance Mengxin Yin, Dongyue Zhao, Yuexi Yang, Lingli Xing, Wei Ren, Zhongnan Gao, Qingpeng Cheng, Tong Ding, Ye Tian, and Xingang Li Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b01661 • Publication Date (Web): 30 Apr 2019 Downloaded from http://pubs.acs.org on April 30, 2019

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Efficient Pt/Ba/SnxCe1-xO2 Catalysts for High-Temperature Lean NOx Traps with High H2O and CO2 Tolerance

Mengxin Yin, † Dongyue Zhao, † Yuexi Yang, † Lingli Xing, † Wei Ren, † Zhongnan Gao, † Qingpeng Cheng, † Tong Ding, †Ye Tian, † and Xingang Li1*,† †Collaborative

Innovation Center of Chemical Science and Engineering (Tianjin),

Tianjin Key Laboratory of Applied Catalysis Science & Technology, School of Chemical Engineering & Technology, Tianjin University, Tianjin, 300072, People’s Republic of China

* Corresponding author: Prof. Xingang Li School of Chemical Engineering & Technology, Tianjin University, Tianjin, 300072, P. R. China EMAIL: [email protected]

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ABSTRACT: Herein, we investigate the De-NOx activity of Pt/Ba/SnxCe1-xO2 catalysts at high temperatures. Our results show that at high temperatures, their NOx storage capacity (NSC), linked to the quantity of highly dispersed BaCO3 species on the catalysts, determines the De-NOx activity instead of the NOx oxidation and reduction ability. Moreover, we identify that monodentate nitrate species is more stable than free ion nitrate species on the Ce-containing catalysts. Thus, the presence of monodentate nitrate species is beneficial to improve the H2O and CO2 tolerance by inhibiting the latter ones’ competitive adsorption on NOx storage sites. Compared with other catalysts, Pt/Ba/Sn0.8Ce0.2O2 possesses the larger amount of highly dispersed BaCO3 species and especially more monodentate nitrate species is formed on it during the NOx storage period. Thus, it exhibits the superior De-NOx performance with NOx removal percentage over 86 % in the presence of H2O and CO2 at 350~500 ℃. Key words: NOx storage reduction; High temperature; H2O and CO2 tolerance; Pt/Ba/SnxCe1-xO2; Highly dispersed BaCO3

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1. INTRODUCTION Nitrogen oxides (NOx, including NO and NO2) emitted from vehicle exhaust brings serious threats on both human health and ecological environment.1,

2

With

people’s environmental consciousness enhancing, increasingly stringent standards on NOx emission from vehicle exhaust are executed.3 Lean NOx trap technology, also known as NOx storage reduction (NSR), is effective in NOx elimination for lean-burn engines.4 In the NSR process, catalysts usually work in cyclic process of long lean-burn atmosphere and short fuel-rich atmosphere.4 During lean-burn phase with excessive oxygen, NOx is oxidized and then stored as nitrites or nitrates on the catalysts.3 In contrast, the stored NOx is released to gaseous phase and reduced to N2 during fuel-rich phase with excessive reductants.3 A typical NSR catalyst usually consists of three components: noble metal for NO oxidation and NOx reduction, alkali or alkaline earth metal compounds for NOx storage and supports for suitable surface areas.5 Up to now, many superior NSR catalysts have been proposed, for example, Pt/BaO/Al2O3 and La1-xSrxCoO3-based perovskites.6-10 NSR catalysts are generally studied at the medium temperature region from 250 to 400 ℃.11 Nevertheless, due to the widespread use of gasoline direct injection (GDI) engines, the temperature of exhaust gas is above 400 ℃.11 It puts forward more stringent requirements for the performance of the catalysts at high temperatures. Pt/K2Ti2O5 and MgAl2O4-based catalysts show high efficiency in NOx elimination at high temperatures.12, 13 However, few studies report the high-temperature (HT) NSR activity in the presence of H2O and CO2, which are inevitable in real vehicle exhaust. 3


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Some investigations have been reported about the effect of H2O and CO2 on the NSR catalysts at medium temperatures.14-18 Both H2O and CO2 are found to have negative impacts on NOx storage process, especially for CO2.3 H2O suppresses the NOx storage by inhibiting the oxidation of NO to NO2, while CO2 lowers NOx storage capacity by competitive adsorption with NOx on storage sites.16, 19, 20 In addition, high dispersion of Ba species on supports can improve H2O and CO2 tolerance of NSR catalysts.21 CeO2 is a support with higher stability for lean NOx trap than Al2O3.22 The NSR activity of CuO/CeO2 can be enhanced by doping with other cations with improved thermal stability and redox ability.23 Moreover, it is reported that the addition of SnO2 can increase the H2O tolerance of the catalysts in various reactions, for example, selective catalytic reduction of NO by NH3 and CO oxidation.24-26 In view of the characteristics of CeO2 and SnO2, Sn-Ce solid solution may act as an efficient support to improve the H2O and CO2 tolerance of the catalysts for HT NSR. According to the above analysis, we designed a series of Pt/Ba/SnxCe1-xO2 catalysts for NOx storage and reduction at high temperatures. Al2O3-based catalyst with the same Pt and Ba loading was used for comparison. The whole reaction process was divided into three parts: NO oxidation, NOx storage and NOx reduction. Corresponding experiments were designed to verify their relationship with the De-NOx performance of the catalysts. The catalytic performance in the presence of H2O and CO2 was also investigated to further understand the function of the catalyst properties on their tolerance.

2. EXPERIMENTAL SECTION 4



2.1. Preparation of the Catalysts. 2.1.1. Preparation of the Supports. The SnxCe1-xO2 (with x = 0, 0.2, 0.8, 1) supports were synthesized via the chemical co-precipitation method. Tin chloride and cerium nitrate with a certain molar ratio were dissolved in deionized water separately, and then mixed together with stirring. Thereafter, ammonium hydroxide (28 %) was slowly added to the mixed solution to adjust the alkalinity of the solution until a pH value of 9 was reached. The mixture kept stirring for another 0.5 h and aged for 2 h. The resulting product was then filtered and washed with deionized water for five times to ensure no Cl- existed in the solid. After drying at 60 ℃ for 24 h, the solid product was calcined at 650 ℃ for 6 h. Al2O3 was synthesized through the same process mentioned above using aluminum nitrate as raw material. 2.1.2. Preparation of the Catalysts. The catalysts were prepared by equivalent-volume impregnation with Pt and Ba loadings of 1.5 wt. % and 9 wt. %, respectively. The support of SnxCe1-xO2 or Al2O3 was impregnated with platinum nitrate solution. Thereafter, the support was dried at 60 ℃ for 12 h and calcined at 550 ℃ for 4 h. Barium acetate was added to the resulting powder by impregnation with the same steps as the addition of Pt. 2.2. Characterizations of the Catalysts. The X-ray diffraction (XRD) measurements were conducted on an X’pert Pro diffractometer from PANalytical company. The radiation source was Cu Kα (λ = 0.15418 nm) The specific surface areas (SBET) date was obtained through nitrogen adsorption 5


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experiments at -200 ℃ with a Quantachrome QuadraSorb SI instrument. The X-ray photoelectron spectroscopy (XPS) analysis was carried out on Thermo 250Xi facility. The radiation source was Al Kα (hν = 1486.6 eV). And the binding energies (BE) of the catalysts were calibrated with the C 1s peak at 284.6 eV. The X-ray fluorescence spectrometer (XRF) measurement was conducted with Bruker S4 Pioneer spectrometer to detect the contents of Pt and Ba on the catalysts. The CO2 temperature-programmed desorption (CO2-TPD) experiments were conducted on the TP-5079 instrument equipped with the mass spectrometry (MS) (Hiden HPR20). Typically, the catalysts of 100 mg were pre-treated in He flow at the temperature of 200 ℃ for 2 h, and then heated to 900 ℃ by a temperature program of 10 ℃ min-1. The outlet concentration of CO2 was monitored by MS. The in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements were carried out at 450 ℃ on a Nicolet Nexus spectrometer with a MCT detector. The background spectrum at 450 ℃ was collected before measurements. Thereafter, the reaction flow (400 ppm NO / 5 % O2 / N2, 100 mL min-1) was introduced to the sample chamber, and the spectrum was collected with a spectral resolution of 4 cm-1 and 32 scans. 2.3 Activity Measurements. NOx storage reduction activities of the catalysts were measured on a continuous-flow fixed-bed quartz reactor with inner diameter of 4 mm. Table 1 gives the gas compositions of lean-burn atmosphere and fuel-rich atmosphere. The reaction space velocity was as high as 120 000 mL g-1 h-1. At least 20 lean (50 s) - rich (10 s) 6



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cycles were performed. Compositions of the products were monitored with an online chemiluminescence NO-NO2-NOx analyzer (Model 42i-HL, Thermo Scientific). The NOx removal percentage (NRP) was calculated with the last six of the steady cycles using the following formula: NOx removal percentage (%) = (1 - ∫ [NOx]out dt / 2000) × 100 [NOx]out represents the outlet NOx concentration (ppm). Table 1. Gas Compositions in the Experiments gas compositions

lean-burn

fuel-rich

NO

400 ppm

0 ppm

O2

5%

0%

C3H6

0 ppm

1000 ppm

H2O

0 or 5 %

0 or 5 %

CO2

0 or 5 %

0 or 5 %

N2

balance

balance

NOx storage experiments were conducted in the same reactor as the cycling NOx storage reduction experiments. The reaction atmosphere was the same as lean-burn atmosphere, as listed in Table 1. The experiments were conducted at 500 ℃ for 10 min. NO oxidation tests were conducted in lean-burn atmosphere. The NO oxidation ability of the catalysts was calculated by the ratio of export NO2 and NOx after the catalysts adsorb NOx to reach saturation. NO to NO2 conversion (%) = [NO2]out / [NOx]out × 100 7


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[NO2]out represents the outlet NO2 concentration (ppm). In addition, the NOx reduction efficiency was calculated with the last steady cycle using the following formula: 60

50

NOx reduction efficiency (%) = [1 - ∫50[NOx]out dt / (50[NOx]in -∫0 [NOx]out dt)] × 100 [NOx]in represents the inlet NOx concentration (ppm). NOx-TPD experiments were performed with catalysts that had been adsorbing NOx to saturation in lean-burn atmosphere at 500 ℃. Then the catalysts were cooled down to 200 ℃, and were heated from 200 to 680 ℃ in N2 flow at the ramp of 3 ℃ min-1.

3. RESULTS AND DISCUSSIONS 3.1 Physical and Chemical Properties of the Catalysts.

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Figure 1. XRD patterns of the (a) supports, (b) enlarged view of the black rectangle area in (a), (c) enlarged view of the red rectangle area in (a) and (d) catalysts.

Figure 1a shows the XRD patterns of the supports. For the Ce-rich supports (CeO2 and Sn0.2Ce0.8O2), only the diffraction peaks of the CeO2 phase are detected (JCPDS No. 34-0394). According to the literature, we know that the radius of the Ce cations (Ce4+: 0.097 nm, Ce3+: 0.110 nm) is bigger than that of the Sn cations (Sn4+: 0.071 nm, Sn2+: 0.093 nm).27 The XRD patterns of CeO2 and Sn0.2Ce0.8O2 between 27 ° and 30 ° are enlarged and displayed in Figure 1b. A distinct shift of the CeO2-phase diffraction peak for Sn0.2Ce0.8O2 towards higher degree is observed, compared with pure CeO2. Besides, the CeO2 lattice constant of Sn0.2Ce0.8O2 is smaller than that of CeO2, as displayed in Table 2. These results demonstrate that Sn cations are incorporated into the CeO2 lattice to form solid solution.27,

28

For the

Sn-rich supports (SnO2 and Sn0.8Ce0.2O2), all the diffraction peaks are attributed to the SnO2 phase (JCPDS No. 41-1445) and no CeO2 phase is detected. Furthermore, as shown in Figure 1c, the diffraction peak of Sn0.8Ce0.2O2 shifts to lower degree 9


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compared with pure SnO2, meanwhile the SnO2 lattice constant of Sn0.8Ce0.2O2 is bigger than that of SnO2, demonstrating the successful incorporation of Ce cations into the SnO2 lattice.27 Based on the above analysis, the SnxCe1-xO2 solid solution supports are successfully synthesized. Figure 1d presents the XRD patterns of the catalysts. Obvious diffraction peaks of BaCO3 are observed on the catalysts without the detection of BaO. It demonstrates that the thermal decomposition of the Ba(CH3COO)2 precursor in air will produce BaCO3.29 In addition, no diffraction peaks belonging to Pt or PtOx are detected owing to the low loading or the high dispersion of Pt species on the catalysts. Table 2. Physical and Chemical Properties of the Catalysts catalysts

lattice constanta (nm)

Pt/Ba/CeO2 Pt/Ba/Sn0.2Ce0.8O2 Pt/Ba/Sn0.8Ce0.2O2 Pt/Ba/SnO2 Pt/Ba/Al2O3

0.5404 (CeO2) 0.5382 (CeO2) 0.4743 (SnO2) 0.4730 (SnO2) /

aObtained

SBET of catalysts (m2 g-1) 29.6 30.7 46.2 17.3 141.7

Pt loadingb (wt. %)

Ba loadingb (wt. %)

1.5 1.5 1.6 1.6 1.6

8.8 8.6 8.9 7.7 8.9

by Figure 1a.

bDetermined

by XRF.

Table 2 gives the specific surface area of the catalysts. Pt/Ba/Sn0.2Ce0.8O2 and Pt/Ba/Sn0.8Ce0.2O2 own larger specific surface areas than pure Pt/Ba/SnO2 and Pt/Ba/CeO2. Combining with the specific surface areas of the supports in Table S1, the specific surface areas of the catalysts decrease after the loading of Ba and Pt species, because the pores in the supports are partially blocked by the loaded species. In addition, the Pt content on the catalysts is about 1.5 wt. %. Similarly, the contents 10



of Ba on the catalysts are close to the theoretical values.

Figure 2. XPS spectra of the catalysts in the region of (a) Ce 3d, (b) Sn 3d and (c) O 1s.

Figure 2a displays the Ce 3d XPS spectra of Pt/Ba/CeO2, Pt/Ba/Sn0.2Ce0.8O2 and Pt/Ba/Sn0.8Ce0.2O2, which are numerically fitted with eight distributions. The peaks belonging to Ce4+ (blue lines) and Ce3+ (pink lines) are observed, as reported in the literatures.24,

30

The total peak areas of Ce4+ are significantly larger compared with

that of Ce3+, indicating that Ce mainly exists as Ce4+ on the surface of the catalysts. Ce3+ is only 12.8 % of the total Ce on pure CeO2 surfaces. However, the fraction of 11


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Ce3+ gradually increases with the increase of the Sn content, and reaches 35.1 % on Pt/Ba/Sn0.8Ce0.2O2, showing the highest fraction of Ce3+ as displayed in Table 3. Figure 2b shows the Sn 3d XPS spectra of Pt/Ba/SnO2, Pt/Ba/Sn0.8Ce0.2O2 and Pt/Ba/Sn0.2Ce0.8O2. For Pt/Ba/SnO2, the binding energies of Sn 3d5/2 and Sn 3d3/2 are located at 486.4 and 494.8 eV, respectively.24, 28 However, with the addition of Ce, the peaks of Sn shift to lower BE compared with Pt/Ba/SnO2, indicating the presence of electrons enriched Sn cations on Pt/Ba/Sn0.8Ce0.2O2 and Pt/Ba/Sn0.2Ce0.8O2.28 Changes of Sn and Ce can be explained by the charge transfer between Sn and Ce through the equilibrium of Sn4+ + 2Ce3+ ↔ Sn2+ + 2Ce4+.27, 28 Figure 2c shows the O 1s XPS spectra of the catalysts, which are numerically fitted into three peaks. The peak at the lowest binding energies is assigned to the lattice oxygen species (Olatt), the peak at about 531 eV is attributed to the adsorbed oxygen species (Oads), while the peak at about 532 eV belongs to the surface oxygen species (Osurf), such as carbonate and surface hydroxyl.30-32 For the Pt/Ba/SnxCe1-xO2 catalysts, as the Sn content increases, the peaks of O 1s shift to higher binding energy, because of the improved ability to capture electrons of Sn than Ce.28 Moreover, Table 3 provides the ratio of Oads / Olatt of the catalysts, which varies in a volcanic shape with the increase of the Sn content, and Pt/Ba/Sn0.8Ce0.2O2 owns the maximum Oads / Olatt ratio. The high Oads / Olatt ratio is beneficial to NO oxidation.

12



Table 3. Surface Element Contents of the Catalysts Determined by XPS catalysts

Oads / Olatt

Pt/Ba/CeO2 Pt/Ba/Sn0.2Ce0.8O2 Pt/Ba/Sn0.8Ce0.2O2 Pt/Ba/SnO2 Pt/Ba/Al2O3

0.4 0.5 0.7 0.4 0.4

Ce3+/ (Ce3+ + Ce4+) (%) 12.8 19.3 35.1 / /

3.2 De-NOx Activity of the Catalysts in the Absence of H2O and CO2.

Figure 3. NRP of the catalysts in the absence of H2O and CO2 between 250 and 550 ℃. Lean-burn phase: 400 ppm NO, 5 vol. % O2 and balanced with N2; fuel-rich phase: 1000 ppm C3H6 and balanced with N2.

Figure 3 shows the NRP results of the catalysts in the absence of H2O and CO2 at the temperature range of 250 to 550 ℃. The De-NOx activity of all the catalysts shows volcanic trend with the increase of temperatures, except that the optimum activity temperature is slightly different. At 250 ℃, the NRP is rather low with a conversion of less than 40 %, which is partly attributed to the limited NO oxidation rate and the weak NOx adsorption capacity at low temperatures.4 At 300 ℃, the activity of all the catalysts increases dramatically with the NRP over 60 %. In the temperature region of 13


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400 to 500 ℃, all the catalysts show high De-NOx activity and the NRP of all the catalysts is over 85 %. The optimal activity of Pt/Ba/Al2O3 and Pt/Ba/SnO2 appears at 350 ℃, while the NRP reaches the highest at 400 ℃ for Pt/Ba/CeO2, all of which are in the medium temperature range. Notably, the optimum De-NOx activity of Pt/Ba/Sn0.2Ce0.8O2 and Pt/Ba/Sn0.8Ce0.2O2 appears at 450 ℃, which locates in the high temperature range. Thereafter, as the temperature increases to 550 ℃, the NRP of all the catalysts decreases with the different degree. Additionally, Pt/Ba/Sn0.2Ce0.8O2 and Pt/Ba/Sn0.8Ce0.2O2 achieve the higher De-NOx activity than Pt/Ba/SnO2 and Pt/Ba/CeO2 in the temperature range of 300 to 550 ℃, especially for Pt/Ba/Sn0.8Ce0.2O2, the NRP of which remains above 95 % between 350 and 500 ℃. In this article, our research mainly concentrates on the activity of the catalysts at high temperatures above 400℃. For the Pt/Ba/SnxCe1-xO2 catalysts, the activity changes in volcanic shape with the increase of the Sn content, and Pt/Ba/Sn0.8Ce0.2O2 and Pt/Ba/Sn0.2Ce0.8O2 show the better De-NOx performance. Both of them show the much higher activity, compared with Pt/Ba/Al2O3, at high temperatures. 3.3 Factors Affecting the De-NOx Activity of the Catalysts at High Temperatures. Table 4. NO to NO2 Conversion (%), NOx Reduction Efficiency (%) and NRP (%) in the Absence of H2O and CO2 at 500 ℃ catalysts Pt/Ba/CeO2 Pt/Ba/Sn0.2Ce0.8O2 Pt/Ba/Sn0.8Ce0.2O2 Pt/Ba/SnO2

NO to NO2 conversion (%) 35.1 35.9 36.0 35.0

NOx reduction efficiency (%) 95.1 94.8 97.0 92.4

NRP (%) 90.5 93.0 96.2 86.3

The whole NSR process can be divided into three parts: NO oxidation on Pt 14



sites, NOx storage on Ba sites and NOx reduction on Pt sites. Generally, NSR catalysts capture NO2 more effectively than NO. Therefore, NO oxidation to NO2 on Pt sites is a crucial step in the whole process. Table 4 displays the similar NO conversion of the catalysts in the absence of H2O and CO2 at 500 ℃, which does no follow the values of Oads / Olatt displayed in Table 3. Probably, NO oxidation is thermodynamically controlled at high temperatures. Besides, the NOx reduction efficiency of the catalysts in Table 4 exhibits the similar values. Therefore, we can conclude that the Pt species on the catalysts has little effect on NO oxidation and NOx reduction. In addition, the reducibility of the catalysts in Figure S1 has little influence on the De-NOx activity, as well.

Figure 4. NOx storage curves of the catalysts in the absence of H2O and CO2 at 500 ℃. Gas composition: 400 ppm NO, 5 vol. % O2 and balanced with N2.

Figure 4 exhibits the NOx storage curves of the catalysts in the absence of H2O and CO2 at 500 ℃. The NOx storage capacity follows the order of Pt/Ba/SnO2 < Pt/Ba/CeO2 < Pt/Ba/Sn0.2Ce0.8O2 < Pt/Ba/Sn0.8Ce0.2O2, and is in agreement with the 15


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NRP of the catalysts in Table 4. During NOx storage, initially, the concentration of NOx drops rapidly to near zero and remains for at least 30 seconds, and this stage is called “complete NOx trap”. Thereafter, it rises slowly to recover the inlet concentration. Apparently, Pt/Ba/Sn0.8Ce0.2O2 has the much larger NOx storage capacity than other catalysts, showing the longer “complete NOx trap” period and the slower rate of NOx recovery. Figure S2 presents the NOx storage curves of the spent and the fresh catalysts at 500 ℃. They are quite similar, indicating that the storage sites of the catalysts can be well regenerated. It is because the formed nitrate is unstable at 500 ℃ and can be easily decomposed and released from the NOx adsorption sites during the following fuel-rich period. Therefore, at high temperatures, the intrinsic NOx storage capacity of each catalyst determines their corresponding De-NOx activity.

Figure 5. NOx-TPD curves of the catalysts.

Figure 5 exhibits the NOx-TPD curves of the catalysts. Apparently, the area of the NOx desorption curve is related to the amount of NOx stored on the catalysts. The peak areas of Pt/Ba/Sn0.2Ce0.8O2 and Pt/Ba/Sn0.8Ce0.2O2 are obviously larger than that 16



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of Pt/Ba/CeO2 and Pt/Ba/SnO2. It means that more NOx species are adsorbed on the former two catalysts, which is consistent with the NOx storage results in Figure 4. It will induce the enlarged NOx storage capacity to improve the De-NOx performance during the alternative lean-burn and fuel-rich cycling at high reaction temperatures. Obviously, the initial desorption temperature follows the order of Pt/Ba/CeO2 ≈ Pt/Ba/Sn0.2Ce0.8O2 < Pt/Ba/SnO2 < Pt/Ba/Sn0.8Ce0.2O2 in the squared area in Figure 5. Higher

initial

desorption

temperature

of

the

NOx

species

trapped

on

Pt/Ba/Sn0.8Ce0.2O2 indicates that NOx can more thermal-stably exist on it.

Figure 6. CO2-TPD profiles of the catalysts.

Figure 6 shows the CO2-TPD profiles of the fresh catalysts from 200 to 900 ℃. The peak of CO2 desorption is divided into two parts based on the temperature. The peak in the relatively low temperature zone (200 - 500 ℃) is attributed to highly dispersed BaCO3 (LT-BaCO3) on the surface. The peaks located at the high temperature region (above 500 ℃) are related to the decomposition of bulk-like BaCO3 (HT-BaCO3). It is reported that highly dispersed BaCO3 plays a key role in 17


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NOx storage.33,

34

Apparently, Pt/Ba/Sn0.8Ce0.2O2 and Pt/Ba/Sn0.2Ce0.8O2 with more

highly dispersed BaCO3 exhibit the higher De-NOx activity in Figure 3. It coincides with the discussion in Figure 4 that the De-NOx performance at high temperatures is determined by the NOx storage capacity. 3.4 NOx Storage Mechanism.

Figure 7. In situ DRIFTS spectra of NO and O2 co-adsorption on the catalysts at 450 ℃: (a) Pt/Ba/CeO2, (b) Pt/Ba/Sn0.2Ce0.8O2, (c) Pt/Ba/Sn0.8Ce0.2O2 and (d) Pt/Ba/SnO2.

Figure 7 shows the in situ DRIFTS spectra of NO and O2 co-adsorption on the catalysts at 450 ℃. Over Pt/Ba/CeO2, bands appearing at 1284 and 1309 cm-1 are 18



attributed to monodentate nitrate species.33, 35, 36 And the band emerging at 1347 cm-1 belongs to free ionic nitrate species.37 In the initial 15 minutes, the main adsorbent is monodentate nitrate species. However, after 20 minutes, free ionic nitrate species become the main adsorbent. The adsorption behavior of NOx on Pt/Ba/Sn0.2Ce0.8O2 is very similar to that of NOx on Pt/Ba/CeO2. But the main difference between the two catalysts is that the bands over Pt/Ba/Sn0.2Ce0.8O2 are stronger than those over Pt/Ba/CeO2, suggesting more nitrates species formed on Pt/Ba/Sn0.2Ce0.8O2 in the initial period. Moreover, free ionic nitrate species become the main adsorbent on Pt/Ba/Sn0.2Ce0.8O2 after 15 minutes, earlier than that on Pt/Ba/CeO2. As for Pt/Ba/Sn0.8Ce0.2O2, monodentate nitrate species are the main adsorbent throughout the whole adsorption process with the main peak locating at 1290 cm-1.35 Moreover, there is a secondary peak lying at 1341cm-1 on Pt/Ba/Sn0.8Ce0.2O2, which demonstrates the co-existence of free ionic nitrate species.37 However, for Pt/Ba/SnO2, free ionic nitrate species are dominant throughout the measurement.37 In the initial few minutes, monodentate nitrate species always appear prior to free ionic nitrate species for the Ce-containing catalysts. It indicates that the former is more stable than the latter on these catalysts. In addition, among all the Ce-containing catalysts, the weak absorption bands between 1506 and 1559 cm-1 are detected, which are attributed to chelating bidentate nitrate species.36, 38 It is noteworthy that no peak of nitrite species is detected on all the catalysts, because they are unstable at high temperatures. 3.5 De-NOx Activity of the Catalysts in the Presence of H2O and / or CO2. 19


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Figure 8. NRP of the catalysts at 500 ℃: (a) in the presence of H2O, (b) in the presence of CO2 and (c) in the presence of H2O and CO2. Lean-burn phase: 400 ppm NO, 5 vol. % O2, 5 vol. % H2O and / or 5 vol. % CO2 and balanced with N2; fuel-rich phase: 1000 ppm C3H6, 5 vol. % H2O and / or 5 vol. % CO2 and balanced with N2.

The NRP of the catalysts in the presence of H2O alone at the temperature range of 350~550 ℃ is displayed in Table S2. Pt/Ba/Sn0.8Ce0.2O2 shows the superior De-NOx activity with NRP over 92 % between 350 and 500 ℃. Besides, the NRP of Pt/Ba/SnO2 between 350 and 450 ℃ is also above 87 %, which signifies the excellent De-NOx activity. Figure 8a shows the NRP of the catalysts in the presence of H2O alone at 500 ℃. 20



The NRP of each catalyst decreases slightly and varies in volcanic shape with the increase of the Sn content. Pt/Ba/Sn0.8Ce0.2O2 shows the highest De-NOx activity with the NRP over 92 %. It’s worth noting that all the Pt/Ba/SnxCe1-xO2 catalysts show the higher De-NOx activity than Pt/Ba/Al2O3. Combined with the NRP in the presence of CO2 alone at the temperature range of 350~550 ℃ displayed in Table S2, CO2 has more negative impact than H2O on the De-NOx activity of the catalysts. Obviously, the NRP of the Sn-rich catalysts (Pt/Ba/Sn0.8Ce0.2O2 and Pt/Ba/SnO2) is much higher than that of the Pt/Ba/Al2O3 and

the Ce-rich catalysts (Pt/Ba/CeO2 and Pt/Ba/Sn0.2Ce0.8O2) in the presence of CO2. Figure 8b shows the NRP of the catalysts in the presence of CO2 alone at 500 ℃. The De-NOx activity is much lower than that in the presence of H2O alone. Moreover, the De-NOx activity follows the order of Ce-rich catalysts < Pt/Ba/Al2O3 < Sn-rich catalysts. Pt/Ba/Sn0.8Ce0.2O2 presents the highest De-NOx activity with the NRP over 90%. The in situ DRIFTS results in Figure 7 show that there are two adsorption sites on the Ce-containing catalysts: the strong adsorption sites where monodentate nitrate species are formed, as well as the weak adsorption sites where free ionic nitrate are formed. Combining with the H2O and CO2 tolerance of the catalysts, we suppose that H2O and CO2 competitively adsorb with NOx at weak adsorption sites to lower the activity of the Ce-containing catalysts. Because monodentate nitrate species are dominate on Pt/Ba/Sn0.8Ce0.2O2, it presents the best H2O and CO2 tolerance among the Ce-containing catalysts. 21


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Obviously, the NRP drops more seriously when both H2O and CO2 exist in the reaction atmosphere, as displayed in Table S2. The NRP in the presence of both H2O and CO2 follows the order of Ce-rich catalysts < Pt/Ba/Al2O3 < Sn-rich catalysts over the entire temperature range. Figure 8c shows the NRP of the catalysts in the presence of both H2O and CO2 at 500 ℃. Similar to the results with only CO2 in the reaction atmosphere, the Sn-rich catalysts show the higher NRP than other catalysts. Pt/Ba/Sn0.8Ce0.2O2 is still the most active one in NOx removal, with the NRP above 85 %.

Figure 9. NOx storage curves of the catalysts in the presence of H2O and CO2 at 500 ℃. Gas composition: 400 ppm NO, 5 vol. % O2, 5 vol. % CO2, 5 vol. % H2O and balanced with N2.

Figure 9 exhibits the NOx storage curves of the catalysts in the presence of H2O and CO2 at 500 ℃. Similar to the NOx storage behavior in Figure 4, the concentration of NOx drops rapidly to a rather low level and then increases slowly until it reaches the inlet concentration. However, the presence of H2O and CO2 lowers the NOx 22



storage capacity of the catalysts obviously. It is due to the competitive adsorption of H2O and CO2 with NOx on the same storage sites. From the above analysis, due to the poor NOx storage performance of the catalysts in the presence of H2O and CO2, the De-NOx activity decreases. Compared with other catalysts, Pt/Ba/Sn0.8Ce0.2O2, which has apparently the higher NRP in the presence of H2O and CO2, possesses the maximum NOx storage capacity.

4. CONCLUSIONS In this work, we studied the De-NOx activity of the Pt/Ba/SnxCe1-xO2 catalysts in the absence and presence of H2O and CO2 at high temperatures. Our results show that the De-NOx activity is determined by the NSC of the catalysts instead of the NOx oxidation and reduction ability at high temperatures, because they are quite similar. And the NSC depends on the amount of highly dispersed BaCO3 species on the catalysts. The in situ DRIFTS results show that at high temperatures, monodentate nitrate species and free ion nitrate species co-exist on the catalysts. However, the former is more stable than the latter one, and thus presents the higher H2O and CO2 tolerance by inhibiting their competitive adsorption. Our results show that Pt/Ba/Sn0.8Ce0.2O2 has the larger amount of active BaCO3 and more monodentate nitrate storage sites. Thus, it presents the highest active in NOx removal at the temperature range of 350~500 ℃ with the NRP over 86 % in the presence of H2O and CO2.

 ASSOCIATED CONTENT Supporting Information 23


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Specific surface areas of the supports(Table S1); H2-TPR profiles (Figure S1); NOx storage curves of the spent and fresh catalysts in the lean-burn phase in the absence of H2O and CO2 at 500 ℃ (Figure S2); NRP (%) in the presence of H2O and/or CO2 at the temperature range of 350 to 550 ℃ (Table S2)

 ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (21878213) and the Program for Introducing Talents of Discipline to Universities of China (No. B06006).

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 for TOC only

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SnxCe1-xO2 Catalysts for High-Temperature Lean NOx

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