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CeO2 catalysts
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Effect of support preparation with different concentration precipitant on the NOx storage performance of Pt/BaO/CeO2 catalysts Yan Zhanga,b, Yunbo Yua,c,d, , Wenpo Shana,b, Zhihua Liana,b, Hong Hea,c,d, ⁎
⁎
a
Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, 361021, China Ningbo Urban Environment Observation and Research Station-NUEORS, Institute of Urban Environment, Chinese Academy of Sciences, Ningbo, 315800, China c State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China d University of Chinese Academy of Sciences, Beijing, 100049, China b
ARTICLE INFO
ABSTRACT
Keywords: NOx storage reduction Pt/BaO/CeO2 NaOH concentration Oxygen vacancies Rich duration
In this study, the CeO2 nanorods as the supports of Pt/BaO/CeO2 catalysts were synthesis by the alkali-assisted hydrothermal method in a solution containing different content of NaOH. Pt/BaO/CeO2 catalyst with 7.5M NaOH pre-added in the support preparation process showed higher NOx storage capacity and NOx removal efficiency than samples derived from other concentration NaOH. Moreover, even under shorter rich time (3s) and more lean-rich cycles (60), Pt/BaO/CeO2-7.5M still maintained above 80% NOx conversion from 250 to 450 °C, exhibiting good fuel economy and durability. A series of characterization techniques including XRD, N2adsorption, TEM, XPS, Raman, H2-TPR, and NO2-TPD were conducted to investigate the physical, chemical properties. Combining the activity with the structure of catalysts, a linear relationship between the NOx storage capacity (NSC) of NSR catalysts and the amount of oxygen vacancies was drawn. Meanwhile, the strong synergetic effect among Pt and CeO2 attributed to the role of oxygen vacancies in anchoring the active component Pt, promoting the mobility of surface oxygen species, was also related to the NSC value of NSR catalysts. These results indicated that the oxygen vacancies were responsible for the excellent catalytic activity.
1. Introduction The push for the better fuel economy and lower vehicular CO2 emissions has led to increased deployment of lean-burn engines. However, due to the lean exhaust environment, diesel NOx emission control is challenging, especially with recent, more stringent legislation such as limits and measurement methods for emissions from diesel fuelled heavy-duty vehicles (China VI) and European Real-Driving Emission (RDE) regulations. In order to satisfy these regulations, current diesel NOx aftertreatment technologies such as selective catalytic reduction of NOx using urea (NH3-SCR) and NOx storage reduction (NSR, otherwise known as lean NOx traps (LNT)) are expected to require modifications or improvements [1,2]. In general, SCR technology is well suited for heavy duty diesel applications, while NSR technology offers a viable solution for light duty applications [3]. NSR catalysts comprise Pt-group metal (PGM) elements like Pt, Pd, Rh, etc. and storage components like Ba, K, dispersed over a high surface area support [4–8]. NSR operation is periodically switched between lean-fuel phase and rich-fuel phase. During the former phase
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typically lasting 1–2 min, NOx is trapped in the catalyst in the form of nitrates or nitrites, which was followed by a few seconds rich phase to regenerate the NSR catalyst by reducing the stored NOx to N2 [4]. The durations of lean and rich phases are vitally important operating parameters that influence the activity of NSR catalysts. Some studies have investigated the effect of cycle timing on overall NOx conversion [9–12]. Li et al. [9] illustrated the effect of lean and rich duration, finding that the overall NOx conversion on Pt-Rh/BaO/Al2O3 catalysts decreased with increasing lean duration while the NOx conversion increased with increasing the rich duration at 300 °C. Experiments and modeling performed by Shakya et al. [11] showed that longer diluted rich phase duration for fixed lean time and shorter overall cycle were favorable to attain high NOx conversion. Whitacre et al. [13] evaluated the exhaust system containing NOx traps using the heavy-duty transient test. Providing the rich atmosphere, an overall NOx conversion of > 90% was achieved, but the fuel economy penalty was relatively high at 8.2%. In order to avoid excessive fuel consumption, minimizing the duration of rich phase is an effective strategy. Aiming at obtaining high performance NSR catalysts, many
Corresponding authors at: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China. E-mail addresses: [email protected] (Y. Yu), [email protected] (H. He).
https://doi.org/10.1016/j.cattod.2019.03.021 Received 30 September 2018; Received in revised form 8 February 2019; Accepted 9 March 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Yan Zhang, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.021
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Catalytic tests were performed in a transient flow reactor system and a series of characterization were conducted to investigate the physical, chemical, and structural properties. It was found that preparation of ceria with suitable concentration of NaOH has a great influence on the NSR performance of Pt/BaO/CeO2 catalyst, which mainly related to the relative amount of oxygen vacancies presented on the catalyst surface. 2. Experiment section 2.1. Catalyst preparation 2.1.1. Preparation of different shapes CeO2 nanomaterials The CeO2 nanorods were prepared by a hydrothermal method based on previous studies [23,26,27]. Specifically, 6.9 mmol Ce(NO3)3∙6H2O (AR grade, Tianjin Fuchen Chemical Reagent Factory, China) and specific amount (be equal to 5 M, 7.5 M, 10 M, 15 M in the obtained solution) of NaOH (AR grade, Sinopharm Chemical Reagent Beijing Co., Ltd, China) were dissolved in 30 ml and 50 ml of deionized water, respectively. Then the mixture was stirred for 60 min to get purple slurry and subsequently transferred into a Teflon-lined stainless autoclave at a temperature of 100 °C and held for 12 h for nanorods. The fresh precipitates were thoroughly washed with deionized water and anhydrous ethanol (AR grade, Sinopharm Chemical Reagent Beijing Co., Ltd, China) to remove any possible ionic remnants. The solid obtained was dried at 60 °C in air for 24 h and calcined at 550 °C for 4 h in air. 2.1.2. Catalysts preparation The NSR catalysts were synthesized by an impregnation method using nanosized CeO2 materials as supports. Firstly, the CeO2 nanorods were impregnated in a Ba(CH3COO)2 solution. After stirring for 60 min, the excess water was removed in a rotary evaporator at 333 K. The samples were then dried at 120 °C for overnight and calcined at 500 °C for 3 h in air. Subsequently, Pt was loaded on the surface of Ba/CeO2 by the same procedure using PtCl4 as a precursor. The catalysts prepared with CeO2 nanorods (5 M, 7.5 M, 10 M, and 15 M) were hereafter denoted as Pt/BaO/CeO2-5 M, Pt/BaO/CeO2-7.5 M, Pt/BaO/CeO2-10 M, and Pt/BaO/CeO2-15 M, respectively, with a nominal BaO loading of 8 wt % and a value of 1 wt % for Pt loading.
Fig. 1. XRD patterns of CeO2 (a) and Pt/BaO/CeO2 (b) catalysts.
2.2. Characterization
literature reports have dealt with the optimization of the catalyst formulation [14–17]. Providing an excellent oxygen storage capacity, enabling high novel metal dispersion, and affording the surface basicity [18–20], CeO2 was effective promoter leading to a significant improvement in the NOx storage capacity, and NOx removal efficiency of the NSR catalysts [14,21,22]. In our previous research, the morphology effect of CeO2 nanomaterials on the activity of Pt/BaO/CeO2 catalysts was investigated deeply [23]. It was found that the NOx storage-reduction performance ranked by the CeO2 support was nanorods > nanoparticles > nanocubes. Moreover, recent studies have shown that CeO2 nanorods with (100) and (110) planes were usually more active for oxidation processes than conventional ceria nanoparticles with the preferred exposure of (111) planes [24,25]. So far, there are plenty of reports on the fabrication of CeO2 nanorods by several efficient synthetic routes, such as hydrothermal method [26–32], precipitation [33], and electrochemical reduction of cerium salts in liquid phase [34], among which the alkali-assisted hydrothermal method seems to be the most effective route [35]. In the reported literatures, the CeO2 nanorods could be fabricated through adjusting the synthesis temperature, synthesis time, and alkalinity [26,31,36]. However, the use of high synthesis temperature and long synthesis time leads to a high energy consumption. Using Ce(NO3)3∙6H2O in solutions containing different concentrations of NaOH, in this study, CeO2 nanorods were synthesized by a hydrothermal method at low temperature and short synthesis time. Effect of NaOH content on morphologies of ceria and NSR performance was studied and discussed.
Powder X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert PRO X-ray diffractometer (Japan, Cu Kα as radiation resource, λ = 0.154 nm). The specific surface areas of the catalysts were obtained at −196 °C over the whole range of relative pressures, using a Quantachrome Quadrasorb SI-MP. Prior to the N2 physisorption, the catalysts were degassed at 300 °C for 5 h. Specific surface areas were calculated from these isotherms by applying the BET equation in 0.05 − 0.3 partial pressure range. Transmission electron microscopy (TEM) images with low magnification were obtained on a Hitachi H-7500 transmission electron microscope (Hitachi) with an acceleration voltage of 80 kV. High-resolution transmission electron microscopy (HR-TEM) images were obtained on JEOL JEM 2010 TEM with 200 kV acceleration voltage. X-ray photoelectron spectra (XPS) measurements were recorded in a scanning X-ray microprobe (PHI Quantera, ULVAC-PHI, Inc) using Al Kα radiation. Binding energies were calibrated using the C 1 s (BE = 284.8 eV) as standard. NOx temperature programmed desorption (NOx–TPD) experiments were performed in a Micromeritics AutoChem II 2920 apparatus, equipped with a computer-controlled CryoCooler, a thermal conductivity detector (TCD), and a quadrupole mass spectrometer (MKS Cirrus). The samples were first pre-reduced with 10% H2/Ar at 450 °C for 30 min and cooled down to the room temperature. By purging with He for 30 min, then the gas was switched to NO2 adsorption for 60 min. After He flows for 30 min, the temperature was increased to 900 °C at a 2
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Fig. 2. TEM and HR-TEM images of CeO2 nanorods (a), corresponding Pt/BaO/CeO2 catalysts (b), and EDS elemental mapping images of all NSR catalysts (c).
heating rate of 10 °C/min, and the signal of NO (m/z = 30) was recorded simultaneously. To avoid the influence of H2O, a cold trap was set before the MS detector.
H2 temperature programmed reduction (H2–TPR) was performed in the same instrument as the NOx–TPD. The samples with a weight of 100 mg were pretreated at 450 °C in a flow of air (50 mL/min) for 3
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Table 1 The surface areas of CeO2 and corresponding Pt/BaO/CeO2 catalysts. Catalyst
Surface area (m2/g)
Da (nm)
CeO2-5 M CeO2-7.5 M CeO2-10 M CeO2-15 M Pt/BaO/CeO2-5 M Pt/BaO/CeO2-7.5 M Pt/BaO/CeO2-10 M Pt/BaO/CeO2-15 M
121.0 103.6 100.6 89.5 91.8 84.2 83.4 76.0
(7.2 (8.1 (9.2 (9.9 (7.2 (8.1 (9.2 (9.9
a b
± ± ± ± ± ± ± ±
CeO2 sizeb (nm) 0.7) 1.4) 1.3) 2.5) 0.7) 1.4) 1.3) 2.5)
× × × × × × × ×
(40–80) (40–110) (50–160) (70–180) (40–80) (40–110) (50–160) (70–180)
11.2 12.3 14.3 15.5 11.7 12.2 12.3 14.0
Calculated for 200 CeO2 nanorods from the HRTEM images. Calculated based on the CeO2 diffraction peak at 2θ = 28.5° by applying the Scherrer equation.
Fig. 3. Outlet NOx concentration as a function of time and temperature under lean condition: (a) Pt/BaO/CeO2-5 M, (b) Pt/BaO/CeO2-7.5 M, (c) Pt/BaO/CeO2-10 M, and (d) Pt/BaO/CeO2-15 M. Lean gas composition: 500 ppm NO, 8% O2, N2 balance.
2.3. Catalytic activity measurements
Table 2 NOx storage capacities (NSC) tested at different temperatures. (Unit: μmol/g). Catalysts
150°C
200°C
250°C
300°C
350°C
400°C
450°C
Pt/BaO/CeO2-5 M Pt/BaO/CeO2-7.5 M Pt/BaO/CeO2-10 M Pt/BaO/CeO2-15 M
215.3 252.7 204.3 196.2
446.5 589.0 435.7 350.6
534.5 692.6 501.2 367.5
722.9 865.6 584.2 610.8
716.1 913.8 711.9 645.6
655.7 782.8 619.3 572.6
492.5 553.8 330.9 374.8
2.3.1. NOx storage capacity experiment The NOx uptake experiments were carried out under lean fuel conditions as a function of temperature. Before each experiment, the catalyst was pretreated in 3% H2/N2 for 1 h at 450 °C, and then cooled to the desired temperature. A mixture of 500 ppm NO and 8% O2 at a total flow rate of 300 mL/min was used to simulate the lean fuel exhaust, using N2 as a balance gas. The outlet NOx (NO + NO2) concentration was monitored by a chemiluminescence detector (ECO Physics CLD 62). The reactor was also equipped with a bypass line to ensure inlet gas concentration.
30 min and cooled down to the room temperature. Then reduction profiles were obtained by passing through the sample with 10% H2/Ar at a rate of 50 mL/min, during which the temperature was increased from 50 to 900 °C at a ramp of 10 °C/ min.
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maintained their original crystal shapes, while no structural features of Pt and Ba were observed. Furthermore, elemental mapping in Fig. 2c revealed that the Pt and Ba were highly dispersed on all the NSR catalysts. The BET surface areas of CeO2 nanorods and Pt/BaO/CeO2 catalysts calculated from the N2 adsorption isotherms were compiled in Table 1. With increasing the pre-adding NaOH concentration, the surface area of CeO2 nanorods decreased from 121.0 m2/g for CeO2-5 M to 89.5 m2/g for CeO2-15 M. After loading 1% Pt and 8% BaO (followed by calcination), the specific area decreased to 91.8, 84.2, 83.4, and 76.0 m2/g, respectively. Such decrease in BET surface area is due to the collapse or clogging of the pores during the impregnation and calcination process. 3.2. Catalytic performance 3.2.1. NOx storage performance Fig. 3 showed the NOx uptake profiles of a series Pt/BaO/CeO2 catalysts at different temperatures for 60 min lean periods where the inlet NOx was present as NO (500 ppm). The profile of Pt/BaO/CeO27.5 M catalyst (in Fig. 3b) presented the typical NOx adsorption characteristics. Just after exposure to the lean condition, the NOx level drops immediately to 0 ppm (except for 150 °C), indicating that complete uptake of NOx by the catalysts. Increasing operating temperature from 200 to 350 °C lengthened the duration for NOx complete capture (from 65 to 344 s). With further rising the temperature to 450 °C, while the complete adsorption time decreased. And then, the outlet NOx level gradually increased with time-on-stream, reaching the inlet value after 60 min. Furthermore, the rate of NOx escape, signified by the slope of the breakthrough curve, is smaller at 350 °C than at any of the other temperatures examined, exhibiting the maximum NOx uptake. Similar features for NOx uptake were also observed over Pt/BaO/CeO2-5 M, Pt/ BaO/CeO2-10 M, and Pt/BaO/CeO2-15 M catalysts, while given a shorter duration for complete NOx uptake if compared with Pt/BaO/ CeO2-7.5 M. Based on these experiments (Fig. 3), the NOx storage capacities (NSC) of series NSR catalysts were calculated and shown in Table 2. With increase of the lean operation temperature, an increased NSC was observed over all the samples, with the highest value being attained at 350 °C. Further increasing the reaction temperature resulted in a decrease in NOx storage capacity, due to the relatively low thermal stability of the nitrates at 400 and 450 °C. Among all the NSR catalysts, obviously, the Pt/BaO/CeO2-7.5 M sample possessed the largest NSC within the full operating temperature range (150–450 °C). The NOx storage capacity of NSR catalyst is related to the ability of NO oxidation [37]. Fig. 4 gave the proportion of NO2/(NO + NO2) over series Pt/BaO/CeO2 catalysts at the end of NOx uptake measurement. For a given NSR catalyst, the oxidation ability of NO to NO2 was weak at low temperatures (150–250 °C). At temperature above 250 °C, the NO2/(NO + NO2) value increased sharply, exhibiting the maximum at 400 °C. In the whole temperature range, there was no difference among the Pt/BaO/CeO2-5 M, Pt/BaO/CeO2-7.5 M, Pt/BaO/CeO2-10 M, Pt/ BaO/CeO2-15 M catalysts, indicating that NO oxidation was not a critical step in determining the NOx storage capacity.
Fig. 4. The NO2/(NO + NO2) ratio over series Pt/BaO/CeO2 catalysts when the NSR catalysts were saturated by NOx.
2.3.2. Cyclic NOx storage reduction tests The NSR cyclic measurements were conducted with 100 mg of catalysts using a fixed-bed quartz reactor. The reactor was connected to a pneumatically actuated four-way valve, which provides a quick switching between the lean and rich atmospheres. Constant flows (300 mL/min) of 500 ppm NO + 8% O2 and 500 ppm NO + 3% H2 were introduced alternately, during which a lean period of 90 s and a rich period of 6 s (or 5 s, 4 s, 3 s) were performed between 150 °C and 450 °C. The NOx conversion was averaged over 20 (or 60) lean/rich cycles to give a mean value according to the following formula:
NOx conversion (%) =
NOx,in-NOx,out ×100 % NOx,in
3. Results and discussion 3.1. Structural properties of CeO2 and Pt/BaO/CeO2 catalysts The X-ray diffraction (XRD) pattern of the CeO2 nanorods (Fig. 1a) indicated the existence of a distinct cubic fluorite phase oxide structure (JCPDS No.43–1002) consisting of diffraction peaks at 28.5, 33.0, 47.5, 56.3, 59.1, 69.4, 76.7, 79.1, and 88.4°, which belong to the face-centered cubic structure of (111), (200), (220), (311), (222), (400), (331), (420), and (422), respectively. Fig. 1b showed that the main phases present in Pt/BaO/CeO2 catalysts were CeO2 (JCPDS No.43–1002) and BaCO3 (JCPDS No.71–2394), thereby confirming the decomposition of Ba(O2CCH3)2 into crystalline BaCO3 during calcination process. The characteristic reflection of Pt was not observed due to its low concentration and/or the high dispersion in the Pt/BaO/CeO2 catalysts. Fig. 2a presented TEM micrographs of the as-synthesized CeO2, which revealed a rod-like morphology for all the samples. Calculating for 200 CeO2 nanorods from the HR-TEM images (shown in Table 1), it was clearly found that the length of CeO2 nanorods increased with increase of the pre-adding NaOH. The HR-TEM images further aimed at clarifying the lattice plane of the CeO2 nanorods. From the HR-TEM image of CeO2-7.5 M nanorods, the interplanar distances of the CeO2 lattice fringes were determined to be 0.31, 0.27, and 0.19 nm, respectively. According to the fast Fourier transform (FFT) analysis, three kinds of lattice fringe directions attributed to (111), (002), and (220) were observed for the CeO2-7.5 M nanorods. Therefore, The CeO2-7.5 M sample showed a 1D growth structure with a preferred growth direction along [110] and were enclosed by (110) and (100) planes. After loading Pt and Ba, it can be clearly seen in Fig. 2b that the CeO2 nanomaterials
3.2.2. Dynamic NOx storage and reduction performance The NOx removal efficiency is not only concerned with the NOx storage capacity in the lean phase, but also related to the reduction of trapped NOx during the rich phase. Minimizing the duration of the rich phase can improve the fuel efficiency, so a rich regeneration time of several seconds is typically used in real systems [23]. In this study, the NOx removal efficiency was investigated under harsh cycling conditions, with a lean period of 90 s and a rich period of 6 s at a quite high gas hourly space velocity (GHSV) of 360, 000 h−1. Fig. 5 showed the transient effluent NOx concentration over all NSR catalysts during 20 lean/rich cycles. Firstly, over the Pt/BaO/CeO2-5 M (in Fig. 5a), NOx breakthrough rapidly increased at a low temperature 5
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Fig. 5. Evolutions of NOx concentrations under lean-rich condition at different temperatures over Pt/BaO/CeO2-5 M (a), Pt/BaO/CeO2-7.5 M (b), Pt/BaO/CeO2-10 M (c), and Pt/BaO/CeO2-15 M (d).
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3.2.3. Effects of lean/rich timing on the performance of Pt/BaO/CeO27.5M Lean/rich cycle timing is an important parameter that was identified to maximize NOx conversion while minimizing fuel penalty [38]. Thus, the effect of lean/rich timing on cyclic behavior was studied over Pt/BaO/CeO2-7.5 M by varying the rich time (3–6 s) with a fixed inlet H2 concentration (3%) and fixed storage time (90 s). As shown in Fig. 7, along with decreasing the rich time, the spillover concentration of NOx increased at the low temperatures while the outlet concentration of NOx was basically unchanged at the high temperatures. It was clear that when the rich time is 6 s, the NOx conversion of Pt/BaO/CeO27.5 M was excellent in the whole temperature window, especially in the 200–400 °C range, in which the purification efficiency of NOx could reach up to nearly 100% (Table 3). The reduction of rich time resulted in the decrease of NOx conversion at low temperatures. However, note that even if the rich time is just only 3 s, the Pt/BaO/CeO2-7.5 M catalyst still maintained more than 80% NOx conversion in the 250–450 °C. It is worth mentioning, during the 60 cycles operation, the overflow concentration of NOx remained consistent except a gradually increased concentration in the first few cycles, indicative of a good durability.
Fig. 6. NOx conversion of Pt/BaO/CeO2-5 M (a), Pt/BaO/CeO2-7.5 M (b), Pt/ BaO/CeO2-10 M (c), and Pt/BaO/CeO2-15 M (d) over 20 lean-rich cycles at different temperatures.
3.3. Thermal stabilities of the stored NOx The NOx storage features and thermal stabilities of the NOx stored on the NSR catalysts were measured by NO2-TPD measurement. Fig. 8 showed the signal of m/e = 30 on Pt/BaO/CeO2-5 M, Pt/BaO/CeO27.5 M, Pt/BaO/CeO2-10 M, and Pt/BaO/CeO2-15 M. Generally, the nitrate decomposition follows the equation: Ba(NO3)2 → BaO + NO2 + 1/2O2 [39]. Since NO2 is easily dissociated to NO fragment in our MS conditions, the main signal is the m/e = 30 (NO) not m/e = 46 (NO2) [40]. As seen in Fig. 8, almost all of the stored NOx on the NSR catalysts was desorbed within the temperature range of 350–650 °C. Such desorption of nitrates could lead to a decrease in amounts of trapped NOx, resulting in a decrease of NSC at high temperatures (Table 2). By integrating the area of NOx desorption peak, it was found that the four NSR catalysts displayed different NOx releasing amount. Pt/BaO/CeO27.5 M showed the largest desorption amount of NOx, followed by Pt/ BaO/CeO2-5 M, then Pt/BaO/CeO2-10 M, and finally Pt/BaO/CeO215 M. This sequence was in line with the NOx storage capacity shown in Fig. 2.
of 150 °C, reaching ˜380 ppm at the end of the lean periods. Upon switching to rich phase, the stored NOx was reduced to N2 by reductant H2, during which the storage sites were regenerated. However, because of the low NOx storage capacity of the catalyst itself at 150 °C, there was still a large amount of NOx spillover in the lean-rich dynamic cycle experiment. With the temperature increasing to 250 °C, the release of NOx obviously decreased. Within the temperature range of 300–400 °C, nearly no NOx was detected during the whole lean and rich period. At 450 °C, the inlet NOx was completely stored at the lean phase. When the atmosphere was switched from lean to rich flow, the NOx was released suddenly with its concentration increasing to a considerable value, but it decreased rapidly and sharply because of its reduction by H2. Over the other three NSR catalysts, the trends of effluent NOx concentration with reaction temperature were similar to that of Pt/BaO/CeO2-5 M. Over the Pt/BaO/CeO2-7.5 M (Fig. 5b), NOx release was a little lower at temperatures below 300 °C than that of Pt/BaO/CeO2-5 M, while slightly higher at other temperatures. As for Pt/BaO/CeO2-10 M and Pt/ BaO/CeO2-15 M, however, the outlet NOx concentration was often higher than that of Pt/BaO/CeO2-5 M in the temperature range of 150–450 °C. Besides, NOx breakthrough over the NSR catalysts increased with time-on-stream. This will be explained later in combination with TPR data. Experimental condition: lean (90 s): 500 ppm NO, 8% O2; rich (6 s): 500 ppm NO, 3% H2, N2 balance; GHSV˜360,000 h−1. Based on the cycle operation, the averaged NOx conversion was calculated, with results shown in Fig. 6. For the Pt/BaO/CeO2-5 M catalyst, the cycle-averaged NOx removal efficiency increased significantly when the temperature increased to 200 °C and the highest conversion of 98.8% was obtained at 300 °C. Similar high NOx conversion was also obtained over Pt/BaO/CeO2-7.5 M within the temperature range of 200-400 °C if compared with Pt/BaO/CeO2-5 M, while at the temperatures of 150 and 450 °C, the former sample gave a little higher NOx removal efficiency. At temperatures above 200 °C, Pt/ BaO/CeO2-10 M and Pt/BaO/CeO2-15 M always displayed lower NOx removal efficiency than Pt/BaO/CeO2-7.5 M. Taking the NOx storage capacity into account, it can be inferred that the Pt/BaO/CeO2-7.5 M is the optimal catalyst.
3.4. Oxygen vacancy and redox properties 3.4.1. XPS results of NSR catalysts It has been proved in our previous study that oxygen vacancies in Pt/BaO/CeO2 govern the NOx storage capacity by creating efficient sites or channels for the formation of nitrates and their further transformation to storage sites [23]. Over the ceria based materials, oxygen vacancies can be produced by the transformation between Ce3+ and Ce4+, 4Ce4+ + O2− → 2Ce4+ + 2Ce3+ + □ + 0.5O2 (□ represents an empty position) [30]. The higher the relative concentration of Ce3+, the more oxygen vacancies that form. As a result, XPS is a well technique to study the oxygen vancies in CeO2 materials, by meauring the relative concentration of Ce3+. Fig. 9 showed the Ce 3d electron core level spectra of CeO2 supports, fresh NSR catalysts, and the reduced ones. The Ce4+ has been fitted with six peaks: v, v’’, v’’’, u, u’’, and u’’’, while Ce3+ has been fitted with two peaks: v’ and u’. The surface concentrations of Ce3+ to the total Ce on the all samples are summarized in Table 4. The concentrations of Ce3+ on the CeO2-5 M, CeO27.5 M, CeO2-10 M, and CeO2-15 M are 14.6%, 14.8%, 14.5%, and 13.6%, separately. After loading active components Pt and BaO, the
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Fig. 7. The outlet NOx concentration of Pt/BaO/CeO2-7.5 M over 60 lean (90 s) -rich cycles at rich time of 6 s (a), 5 s (b), 4 s (c), and 3 s (d).
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BaO/CeO2-5 M catalyst, the peak below 300 °C was assigned to the reduction of PtOx and the surface CeO2 in direct contact with Pt (thereafter named promoted surface CeO2). The peak at 554 °C was attributed to the reduction of surface CeO2 being distant from Pt. In addition, the reduction peaks at about 750 °C was responsible for the reduction of bulk CeO2 [44,45]. Moreover, upon a further increase of NaOH concentration from 7.5 M to 15 M employed in CeO2 preparation, the reduction peaks at intermediate and high temperatures were similar to the Pt/BaO/CeO2-5 M catalyst. However, the reduction peak of PtOx and promoted surface CeO2 on the Pt/BaO/CeO2-7.5 M catalyst moved to the lowest temperature, which indicated that the mobility of surface O was greatly improved because of the strong synergetic effect between Pt and CeO2. It is believed that the synergetic effect facilitated oxygen diffusion from the subsurface layers and might progressively proceed deeper into the bulk [46], which was beneficial for the NOx storage capacity. With the results of H2-TPR in mind, meanwhile, one can deduce that, under the rich phase of NSR process, the H2 consumption would include the reduction of trapped NOx, PtOx, and promoted surface CeO2. Considering that a short rich period (3–6 s) was employed in NSR operation, the partial oxidation of PtOx would occur at low temperatures, leaving larger amount of PtOx in the Pt/BaO/CeO2 with time-onstream. This change may lower the activity for NO oxidation under lean phase and for NOx reduction under rich condition [47]. As a result, it is reasonable that, the NOx breakthrough often appeared as an increase in cyclic operation, within the low temperature range of 200–400 °C. As time going on, a gradually increased PtOx in the Pt/BaO/CeO2 may consume a greater amount of H2 during the rich phase of NSR operation [23]. As a result, the amount of H2 being available for the reduction of released NOx decreased with time-on-stream, leading to the partial regeneration of the storage sites in the rich phase and a decrease in NOx storage under lean condition.
Table 3 NOx conversion of NSR catalysts over 60 lean-rich cycles at different rich times. Lean- Rich
150°C
200°C
250°C
300°C
350°C
400°C
450°C
90 s-6 s 90 s-5 s 90 s-4 s 90 s-3 s
59.8% 61.1% 60.6% 55.5%
97.6% 82.5% 81.1% 70.1%
97.5% 84.2% 85.1% 82.3%
94.4% 80.7% 79.4% 86.6%
95.4% 90.8% 86.4% 86.8%
99.4% 96.8% 95.5% 94.7%
72.6% 85.8% 96.4% 91.4%
Fig. 8. NO2–TPD profiles of the NSR catalysts.
Ce3+ concentrations remain unchanged. However, the H2 pretreatment improved the ratio of Ce3+ on the Pt/BaO/CeO2. Obviously, the oxygen vacancies of the samples that prepared by 7.5 M NaOH are more than those of samples prepared by 5 M, 10 M, and 15 M NaOH, in good agreement with the result of NSC and NSR performance. This further confirmed the importance of oxygen vacancies on the NOx adsorption and storage. R[a]: the NSR catalysts reduced by 3% H2.
3.5. Relationship among oxygen vacancies, redox properties and catalytic performance It was perfectly clear in Table 1 that the BET surface areas of Pt/ BaO/CeO2 catalysts decreased with increasing the pre-adding NaOH concentration in CeO2 support preparation process. Among all the NSR catalysts, however, the specific surface area of Pt/BaO/CeO2-7.5 M catalyst was not the largest, while giving the best catalytic performance (Table 2 and Fig. 6), indicating that the BET surface area was not the crucial factor influencing the catalytic performance. Besides, all the NSR catalysts had almost the same Pt and BaO loadings, and the active components were highly dispersed on the support CeO2 nanorods. Therefore, the catalytic performance of the NSR catalysts was dependent on the intrinsic properties related to CeO2 support. In previous study, we have identified the determinative role of oxygen vacancies in the NOx storage process [23]. Combining the results of XPS in Table 3 with the NOx storage capacity in Table 2, the Fig. 12a showed a linear relationship also existing between the Ce3+ concentration on the reduced NSR catalysts and their NSC in the temperature range of 150–350 °C. This result again manifests that oxygen vacancies could possess the NOx storage process. At temperatures above 350 °C, however, the similar linear correlation was not observed. This phenomenon could be attributed to the decomposition of the stored NOx began at 350 °C (shown in Fig. 8), which makes a negative contribution to the NOx storage capacity of NSR catalysts. Furthermore, there was a linear relationship between the reduction temperature of PtOx and promoted surface CeO2 and the NOx storage capacity of all the NSR catalysts at 150–350 °C (Fig. 12b), revealing that
3.4.2. Raman spectra Raman measurement has been successfully used to discriminate between different structures on oxide surfaces [41], so we recorded and analyzed the Raman spectra of the different shapes CeO2 nanostructures. As presented in Fig. 10, a prominent peak at around 462 cm−1 was observed over all the CeO2 nanomaterials corresponding to the distinct F2g symmetry mode of the CeO2 phase and being viewed as a symmetric breathing mode of the oxygen atoms around cerium ions [41]. Meanwhile, the weak band centered at about 600 cm−1 should be due to the defect-induced mode (D) [42,43]. Consequently, this band can be linked to the defects (oxygen vacancies, etc.) in the CeO2 nanomaterials. The I600/I462 values, which indicates the defect concentration, such as oxygen vacancies, reached a maximum for CeO27.5 M, and then decreased in the sequence of CeO2-5 M, CeO2-10 M, and CeO2-15 M. This demonstrated that the CeO2-7.5 M owned the highest defects among them, being consistent with the results of XPS. 3.4.3. Redox property revealed by H2-TPR H2-TPR experiments were performed to investigate the redox ability of all the NSR catalysts, and the results are shown in Fig. 11. For the Pt/
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Fig. 9. 1 Ce 3d XPS spectra of CeO2-5 M (a), CeO2-7.5 M (b), CeO2-10 M (c), and CeO2-15 M (d). 2 Ce 3d XPS spectra of Pt/BaO/CeO2-5 M (a), Pt/BaO/CeO2-7.5 M (b), Pt/BaO/CeO2-10 M (c), and Pt/BaO/CeO2-15 M (d). 3 Ce 3d XPS spectra of Pt/BaO/CeO2-5 M-R (a), Pt/BaO/CeO2-7.5 M-R (b), Pt/BaO/CeO2-10 M-R (c), and Pt/ BaO/CeO2-15 M-R (d).
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Table 4 The Ce3+ concentration of CeO2 nanorods and NSR catalysts. Sample
CeO2 Pt/BaO/CeO2 Pt/BaO/CeO2-R[a]
Pre-added NaOH concentration 5M
7.5 M
10 M
15 M
14.6% 14.6% 15.2%
14.8% 15.1% 16.8%
14.5% 14.2% 14.6%
13.6% 13.7% 13.9%
the synergetic effect between Pt and CeO2 plays a key role in the NOx storage process. As discussed in the part of “3.4.3 Redox property revealed by H2-TPR”, the synergetic effect facilitated the mobility of surface oxygen species at low temperatures, benefiting the formation of NO2 on the Pt sites and nitrate production, thus increasing the NOx storage capacity. This result, in turn, indicated that the strong synergetic effect was due to the oxygen vacancies on the CeO2 surface anchoring the active component Pt [48]. 4. Conclusion Fig. 11. H2-TPR profiles of the NSR catalysts.
The Pt/BaO/CeO2 catalyst with 7.5 M NaOH pre-added in the support preparation process exhibited higher NOx storage capacity and overall NOx conversion than catalysts with other NaOH concentration. Surprisingly, when the number of lean–rich cycles increased from 20 to 60 in periodically alternating lean–rich atmospheres (90 s vs. 6 s), the NOx conversion on Pt/BaO/CeO2-7.5 M displayed an exceeding 94% NOx conversion in the temperature range of 200–400 °C, indicating the good durability. Even under an extremely short rich duration (only 3 s), Pt/BaO/CeO2-7.5 M still presented a relatively high NOx conversion, ensuring the fuel economy. The results of H2-TPR revealed that the strong synergetic effect between Pt and CeO2 on the Pt/BaO/CeO2-7.5 M catalyst promoted the mobility of surface oxygen species. The Pt/BaO/CeO2-7.5 M implied the
higher amount of oxygen vacancies confirmed by the XPS and Raman measurements than other NSR catalysts. Over all the catalysts, the oxygen vacancies were linearly correlated with the NOx storage capacity. This relationship revealed that the outstanding catalytic performance of Pt/BaO/CeO2 was attributed to the oxygen vacancies. Acknowledgments This work was financially supported by the National Key R&D Program of China (2016YFC0204902), and the National Natural Science Foundation of China (21673277 and 21637005).
Fig. 10. Raman spectra of CeO2 nanostructures (a) and the corresponding peak intensity ratios of I600/I462 (b).
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Fig. 12. The relationship between Ce3+ concentration of reduced NSR catalysts and the NOx storage capacity (NSC) of all the NSR catalysts (a), the relationship between the reduction temperature of PtOx and promoted surface CeO2 and the NOx storage capacity (NSC) of all the NSR catalysts (b).
References
Soc. Rev. 43 (2014) 1543–1574. [21] S. Jones, Y.Y. Ji, M. Crocker, Ceria-based catalysts for low temperature NOx storage and release, Catal. Letters 146 (2016) 909–917. [22] Y. Ryou, J. Lee, H. Lee, C.H. Kim, D.H. Kim, Low temperature NO adsorption over hydrothermally aged Pd/CeO2 for cold start application, Catal. Today 307 (2018) 93–101. [23] Y. Zhang, Y. Yu, H. He, Oxygen vacancies on nanosized ceria govern the NOx storage capacity of NSR catalysts, Catal. Sci. Technol. 6 (2016) 3950–3962. [24] W.-W. Wang, W.-Z. Yu, P.-P. Du, H. Xu, Z. Jin, R. Si, C. Ma, S. Shi, C.-J. Jia, C.H. Yan, Crystal plane effect of Ceria on supported copper oxide cluster catalyst for CO oxidation: importance of metal–support interaction, ACS Catal. 7 (2017) 1313–1329. [25] M. Lykaki, E. Pachatouridou, S.A.C. Carabineiro, E. Iliopoulou, C. Andriopoulou, N. Kallithrakas-Kontos, S. Boghosian, M. Konsolakis, Ceria nanoparticles shape effects on the structural defects and surface chemistry: implications in CO oxidation by Cu/CeO2 catalysts, Appl. Catal. B Environ. 230 (2018) 18–28. [26] H.X. Mai, L.D. Sun, Y.W. Zhang, R. Si, W. Feng, H.P. Zhang, H.C. Liu, C.H. Yan, Shape-selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes, J. Phys. Chem. B 109 (2005) 24380–24385. [27] K. Zhou, X. Wang, X. Sun, Q. Peng, Y. Li, Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes, J. Catal. 229 (2005) 206–212. [28] W.I. Hsiao, Y.S. Lin, Y.C. Chen, C.S. Lee, The effect of the morphology of nanocrystalline CeO2 on ethanol reforming, Chem. Phys. Lett. 441 (2007) 294–299. [29] X.S. Huang, H. Sun, L.C. Wang, Y.M. Liu, K.N. Fan, Y. Cao, Morphology effects of nanoscale ceria on the activity of Au/CeO2 catalysts for low-temperature CO oxidation, Appl. Catal. B Environ. 90 (2009) 224–232. [30] X. Liu, K. Zhou, L. Wang, B. Wang, Y. Li, Oxygen vacancy clusters promoting reducibility and activity of Ceria nanorods, J. Am. Chem. Soc. 131 (2009) 3140–3141. [31] N. Ta, J. Liu, W. Shen, Tuning the shape of ceria nanomaterials for catalytic applications, Chin. J. Catal. 34 (2013) 838–850. [32] Nanostructured ceria-based materials: effect of the hydrothermal synthesis conditions on the structural properties and catalytic activity, Catalysts 7 (2017) 174. [33] N. Bugayeva, J. Robinson, Synthesis of hydrated CeO2 nanowires and nanoneedles, Mater. Sci. Technol. 23 (2013) 237–241. [34] L. Čerović, V. Lair, O. Lupan, M. Cassir, A. Ringuedé, Electrochemical synthesis and characterization of nanorods, nanocolumnar ceria – based thin films on different glass substrates, Chem. Phys. Lett. 494 (2010) 237–242. [35] Q. Yuan, H.H. Duan, L.L. Li, L.D. Sun, Y.W. Zhang, C.H. Yan, Controlled synthesis and assembly of ceria-based nanomaterials, J. Colloid Interface Sci. 335 (2009) 151–167. [36] R. Si, M. Flytzani-Stephanopoulos, Shape and crystal-plane effects of nanoscale Ceria on the activity of Au-CeO2 catalysts for the water–Gas shift reaction, Angew. Chemie 120 (2008) 2926–2929. [37] H.P. Louise Olsson, Erik Fridell, Magnus Skoglundh, Bengt Andersson, A Kinetic Study of NO Oxidation and NOx Storage on Pt/Al2O3 and Pt/BaO/Al2O3, J. Phys. Chem. B 105 (2001) 6895–6906. [38] A. Reihani, B. Patterson, J. Hoard, G.B. Fisher, J.R. Theis, C.K. Lambert, Rapidly pulsed reductants for diesel NOx reduction with lean NOx traps: comparison of Alkanes and alkenes as the reducing agent, J. Eng. Gas Turbine. Power 139 (2017) 102805. [39] G. Zhou, T. Luo, R.J. Gorte, An investigation of NOx storage on Pt-BaO-Al2O3, Appl.
[1] A. Fritz, V. Pitchon, The current state of research on automotive lean NOx catalysis, Appl. Catal. B: Environ. 13 (1997) 1–25. [2] Z.M. Liu, S.I. Woo, Recent advances in catalytic DeNOx science and technology, Catal. Rev. Sci. Eng. 48 (2006) 43–89. [3] T.V. Johnson, Diesel emission control: 2001 in review, SAE Trans. 111 (2002) 85–101. [4] W.S. Epling, L.E. Campbell, A. Yezerets, N.W. Currier, J.E. Parks, Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts, Catal. Rev. Sci. Eng. 46 (2004) 163–245. [5] H.E. Jun, C. Ying, L.I. Xuehui, M.A. Yugang, C. Xiaoping, W. Lefu, A review on development of De-NOx storage-reduction catalysts, Precious Metals 27 (2006) 65–70. [6] S. Roy, A. Baiker, NOxStorage-reduction catalysis: from mechanism and materials properties to storage-reduction performance, Chem. Rev. 109 (2009) 4054–4091. [7] H. Chen, Y. Zhang, Y. Xin, Q. Li, Z. Zhang, Z. Jiang, Y. Ma, H. Zhou, J. Zhang, Enhanced NOx conversion by coupling NOx storage-reduction with CO adsorptionoxidation over the combined Pd−K/MgAlO and Pd/MgAlO catalysts, Catal. Today 258 (2015) 416–423. [8] Y. Zhang, X. Wang, Z. Wang, Q. Li, Z. Zhang, L. Zhou, Direct spectroscopic evidence of CO spillover and subsequent reaction with preadsorbed NOx on Pd and K cosupported Mg-Al mixed oxides, Environ. Sci. Technol. 46 (2012) 9614–9619. [9] Y. Li, S. Roth, J. Dettling, T. Beutel, Effects of lean/rich timing and nature of reductant on the performance of a NOx trap catalyst, Top. Catal. 16/17 (2001) 139–144. [10] L. Liu, Z.-j. Li, H.-y. Zhang, Q. Chang, B.-x. Shen, Effect of rich air/fuel ratio and temperature on NOx desorption of lean NOx trap, J. Zhejiang Univ. Sci. A 14 (2013) 835–842. [11] B.M. Shakya, M.P. Harold, V. Balakotaiah, Effect of cycle time on NH3 generation on low Pt dispersion Pt/BaO/Al2O3 catalysts: experiments and crystallite-scale modeling, Chem. Eng. J. 230 (2013) 584–594. [12] M. Li, Y. Zheng, D. Luss, M.P. Harold, Impact of rapid cycling strategy on reductant effectiveness during NOx storage and reduction, Emiss. Control. Sci. Technol. (2017). [13] T.V. Johnson, Diesel emission control in review, SAE Trans. 116 (2007) 76–87. [14] S. Andonova, Z.A. Ok, N. Drenchev, E. Ozensoy, K. Hadjiivanov, Pt/CeOx/ZrOx/γAl2O3 ternary mixed oxide DeNOx catalyst: surface chemistry and NOx interactions, J. Phys. Chem. C 122 (2018) 12850–12863. [15] A. Bueno-Lopez, D. Lozano-Castello, J.A. Anderson, NOxstorage and reduction over copper-based catalysts. 2 supports, Appl. Catal. B Environ. 198 (2016) 189–199. [16] A. Bueno-Lopez, D. Lozano-Castello, J.A. Anderson, NOxstorage and reduction over copper-based catalysts. 0.8M0.2O delta supports (M = Zr, La, Ce, Pr or Nd), Appl. Catal. B Environ. 198 (2016) 234–242. [17] W. Xie, Y. Yu, H. He, Shape dependence of support for NOx storage and reduction catalysts, J. Environ. Sci. (2018). [18] A. Trovarelli, Catalytic properties of Ceria and CeO2-Containing materials, Catal. Rev. Sci. Eng. 38 (1996) 439–520. [19] Y.T. Kim, E.D. Park, H.C. Lee, D. Lee, K.H. Lee, Water-gas shift reaction over supported Pt-CeOx catalysts, Appl. Catal. B Environ. 90 (2009) 45–54. [20] Y. Li, W. Shen, Morphology-dependent nanocatalysts: rod-shaped oxides, Chem.
12
Catalysis Today xxx (xxxx) xxx–xxx
Y. Zhang, et al. Catal. B Environ. 64 (2006) 88–95. [40] B. Zhao, G.F. Li, C.H. Ge, Q.Y. Wang, R.X. Zhou, Preparation of Ce0.67Zr0.33O2 mixed oxides as supports of improved Pd-only three-way catalysts, Appl. Catal. B Environ. 96 (2010) 338–349. [41] B.M. Reddy, A. Khan, Y. Yamada, T. Kobayashi, S. Loridant, J.C. Volta, Surface characterization of CeO2/SiO2 and V2O5/CeO2/SiO2 catalysts by Raman, XPS, and other techniques, J. Phys. Chem. B 106 (2002) 10964–10972. [42] M.S. Brogan, T.J. Dines, Raman spectroscopic study of the Pt-CeO2 interaction in the Pt/Al2O3-CeO2 catalyst, J. Chem. Soc. Faraday Trans. 90 (1994) 1461–1466. [43] R. Gao, D. Zhang, P. Maitarad, L. Shi, T. Rungrotmongkol, H. Li, J. Zhang, W. Cao, Morphology-dependent properties of MnOx/ZrO2–CeO2 nanostructures for the selective catalytic reduction of NO with NH3, J. Phys. Chem. C 117 (2013) 10502–10511. [44] N. Acerbi, S. Golunski, S.C. Tsang, H. Daly, C. Hardacre, R. Smith, P. Collier, Promotion of Ceria catalysts by precious metals: changes in nature of the interaction
[45] [46] [47] [48]
13
under reducing and oxidizing conditions, J. Phys. Chem. C 116 (2012) 13569–13583. N. Acerbi, S.C.E. Tsang, G. Jones, S. Golunski, P. Collier, Rationalization of interactions in precious metal/ceria catalysts using the d-band center model, Angew. Chemie 52 (2013) 7737–7741. S. Ding, F. Liu, X. Shi, K. Liu, Z. Lian, L. Xie, H. He, Significant promotion effect of Mo additive on a novel Ce-Zr mixed oxide catalyst for the selective catalytic reduction of NOx with NH3, ACS Appl. Mater. Interfaces 7 (2015) 9497–9506. L. Olsson, E. Fridell, The influence of Pt oxide formation and Pt dispersion on the reactions NO2⇔NO+1/2 O2 over Pt/Al2O3 and Pt/BaO/Al2O3, J. Catal. 210 (2002) 340–353. L. Nie, D. Mei, H. Xiong, B. Peng, Z. Ren, X.I.P. Hernandez, A. DeLaRiva, M. Wang, M.H. Engelhard, L. Kovarik, A.K. Datye, Y. Wang, Activation of surface lattice oxygen in single-atom Pt/CeO2 for low-temperature CO oxidation, Science 358 (2017) 1419–1423.
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Effect of support preparation with different concentration precipitant on the NOx storage performance of Pt/BaO/CeO2 catalysts Yan Zhanga,b, Yunbo Yua,c,d, , Wenpo Shana,b, Zhihua Liana,b, Hong Hea,c,d, ⁎
⁎
a
Center for Excellence in Regional Atmospheric Environment, Institute of Urban Environment, Chinese Academy of Sciences, Xiamen, 361021, China Ningbo Urban Environment Observation and Research Station-NUEORS, Institute of Urban Environment, Chinese Academy of Sciences, Ningbo, 315800, China c State Key Joint Laboratory of Environment Simulation and Pollution Control, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China d University of Chinese Academy of Sciences, Beijing, 100049, China b
ARTICLE INFO
ABSTRACT
Keywords: NOx storage reduction Pt/BaO/CeO2 NaOH concentration Oxygen vacancies Rich duration
In this study, the CeO2 nanorods as the supports of Pt/BaO/CeO2 catalysts were synthesis by the alkali-assisted hydrothermal method in a solution containing different content of NaOH. Pt/BaO/CeO2 catalyst with 7.5M NaOH pre-added in the support preparation process showed higher NOx storage capacity and NOx removal efficiency than samples derived from other concentration NaOH. Moreover, even under shorter rich time (3s) and more lean-rich cycles (60), Pt/BaO/CeO2-7.5M still maintained above 80% NOx conversion from 250 to 450 °C, exhibiting good fuel economy and durability. A series of characterization techniques including XRD, N2adsorption, TEM, XPS, Raman, H2-TPR, and NO2-TPD were conducted to investigate the physical, chemical properties. Combining the activity with the structure of catalysts, a linear relationship between the NOx storage capacity (NSC) of NSR catalysts and the amount of oxygen vacancies was drawn. Meanwhile, the strong synergetic effect among Pt and CeO2 attributed to the role of oxygen vacancies in anchoring the active component Pt, promoting the mobility of surface oxygen species, was also related to the NSC value of NSR catalysts. These results indicated that the oxygen vacancies were responsible for the excellent catalytic activity.
1. Introduction The push for the better fuel economy and lower vehicular CO2 emissions has led to increased deployment of lean-burn engines. However, due to the lean exhaust environment, diesel NOx emission control is challenging, especially with recent, more stringent legislation such as limits and measurement methods for emissions from diesel fuelled heavy-duty vehicles (China VI) and European Real-Driving Emission (RDE) regulations. In order to satisfy these regulations, current diesel NOx aftertreatment technologies such as selective catalytic reduction of NOx using urea (NH3-SCR) and NOx storage reduction (NSR, otherwise known as lean NOx traps (LNT)) are expected to require modifications or improvements [1,2]. In general, SCR technology is well suited for heavy duty diesel applications, while NSR technology offers a viable solution for light duty applications [3]. NSR catalysts comprise Pt-group metal (PGM) elements like Pt, Pd, Rh, etc. and storage components like Ba, K, dispersed over a high surface area support [4–8]. NSR operation is periodically switched between lean-fuel phase and rich-fuel phase. During the former phase
⁎
typically lasting 1–2 min, NOx is trapped in the catalyst in the form of nitrates or nitrites, which was followed by a few seconds rich phase to regenerate the NSR catalyst by reducing the stored NOx to N2 [4]. The durations of lean and rich phases are vitally important operating parameters that influence the activity of NSR catalysts. Some studies have investigated the effect of cycle timing on overall NOx conversion [9–12]. Li et al. [9] illustrated the effect of lean and rich duration, finding that the overall NOx conversion on Pt-Rh/BaO/Al2O3 catalysts decreased with increasing lean duration while the NOx conversion increased with increasing the rich duration at 300 °C. Experiments and modeling performed by Shakya et al. [11] showed that longer diluted rich phase duration for fixed lean time and shorter overall cycle were favorable to attain high NOx conversion. Whitacre et al. [13] evaluated the exhaust system containing NOx traps using the heavy-duty transient test. Providing the rich atmosphere, an overall NOx conversion of > 90% was achieved, but the fuel economy penalty was relatively high at 8.2%. In order to avoid excessive fuel consumption, minimizing the duration of rich phase is an effective strategy. Aiming at obtaining high performance NSR catalysts, many
Corresponding authors at: Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing, 100085, China. E-mail addresses: [email protected] (Y. Yu), [email protected] (H. He).
https://doi.org/10.1016/j.cattod.2019.03.021 Received 30 September 2018; Received in revised form 8 February 2019; Accepted 9 March 2019 0920-5861/ © 2019 Published by Elsevier B.V.
Please cite this article as: Yan Zhang, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2019.03.021
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Catalytic tests were performed in a transient flow reactor system and a series of characterization were conducted to investigate the physical, chemical, and structural properties. It was found that preparation of ceria with suitable concentration of NaOH has a great influence on the NSR performance of Pt/BaO/CeO2 catalyst, which mainly related to the relative amount of oxygen vacancies presented on the catalyst surface. 2. Experiment section 2.1. Catalyst preparation 2.1.1. Preparation of different shapes CeO2 nanomaterials The CeO2 nanorods were prepared by a hydrothermal method based on previous studies [23,26,27]. Specifically, 6.9 mmol Ce(NO3)3∙6H2O (AR grade, Tianjin Fuchen Chemical Reagent Factory, China) and specific amount (be equal to 5 M, 7.5 M, 10 M, 15 M in the obtained solution) of NaOH (AR grade, Sinopharm Chemical Reagent Beijing Co., Ltd, China) were dissolved in 30 ml and 50 ml of deionized water, respectively. Then the mixture was stirred for 60 min to get purple slurry and subsequently transferred into a Teflon-lined stainless autoclave at a temperature of 100 °C and held for 12 h for nanorods. The fresh precipitates were thoroughly washed with deionized water and anhydrous ethanol (AR grade, Sinopharm Chemical Reagent Beijing Co., Ltd, China) to remove any possible ionic remnants. The solid obtained was dried at 60 °C in air for 24 h and calcined at 550 °C for 4 h in air. 2.1.2. Catalysts preparation The NSR catalysts were synthesized by an impregnation method using nanosized CeO2 materials as supports. Firstly, the CeO2 nanorods were impregnated in a Ba(CH3COO)2 solution. After stirring for 60 min, the excess water was removed in a rotary evaporator at 333 K. The samples were then dried at 120 °C for overnight and calcined at 500 °C for 3 h in air. Subsequently, Pt was loaded on the surface of Ba/CeO2 by the same procedure using PtCl4 as a precursor. The catalysts prepared with CeO2 nanorods (5 M, 7.5 M, 10 M, and 15 M) were hereafter denoted as Pt/BaO/CeO2-5 M, Pt/BaO/CeO2-7.5 M, Pt/BaO/CeO2-10 M, and Pt/BaO/CeO2-15 M, respectively, with a nominal BaO loading of 8 wt % and a value of 1 wt % for Pt loading.
Fig. 1. XRD patterns of CeO2 (a) and Pt/BaO/CeO2 (b) catalysts.
2.2. Characterization
literature reports have dealt with the optimization of the catalyst formulation [14–17]. Providing an excellent oxygen storage capacity, enabling high novel metal dispersion, and affording the surface basicity [18–20], CeO2 was effective promoter leading to a significant improvement in the NOx storage capacity, and NOx removal efficiency of the NSR catalysts [14,21,22]. In our previous research, the morphology effect of CeO2 nanomaterials on the activity of Pt/BaO/CeO2 catalysts was investigated deeply [23]. It was found that the NOx storage-reduction performance ranked by the CeO2 support was nanorods > nanoparticles > nanocubes. Moreover, recent studies have shown that CeO2 nanorods with (100) and (110) planes were usually more active for oxidation processes than conventional ceria nanoparticles with the preferred exposure of (111) planes [24,25]. So far, there are plenty of reports on the fabrication of CeO2 nanorods by several efficient synthetic routes, such as hydrothermal method [26–32], precipitation [33], and electrochemical reduction of cerium salts in liquid phase [34], among which the alkali-assisted hydrothermal method seems to be the most effective route [35]. In the reported literatures, the CeO2 nanorods could be fabricated through adjusting the synthesis temperature, synthesis time, and alkalinity [26,31,36]. However, the use of high synthesis temperature and long synthesis time leads to a high energy consumption. Using Ce(NO3)3∙6H2O in solutions containing different concentrations of NaOH, in this study, CeO2 nanorods were synthesized by a hydrothermal method at low temperature and short synthesis time. Effect of NaOH content on morphologies of ceria and NSR performance was studied and discussed.
Powder X-ray diffraction (XRD) patterns were measured on a PANalytical X’Pert PRO X-ray diffractometer (Japan, Cu Kα as radiation resource, λ = 0.154 nm). The specific surface areas of the catalysts were obtained at −196 °C over the whole range of relative pressures, using a Quantachrome Quadrasorb SI-MP. Prior to the N2 physisorption, the catalysts were degassed at 300 °C for 5 h. Specific surface areas were calculated from these isotherms by applying the BET equation in 0.05 − 0.3 partial pressure range. Transmission electron microscopy (TEM) images with low magnification were obtained on a Hitachi H-7500 transmission electron microscope (Hitachi) with an acceleration voltage of 80 kV. High-resolution transmission electron microscopy (HR-TEM) images were obtained on JEOL JEM 2010 TEM with 200 kV acceleration voltage. X-ray photoelectron spectra (XPS) measurements were recorded in a scanning X-ray microprobe (PHI Quantera, ULVAC-PHI, Inc) using Al Kα radiation. Binding energies were calibrated using the C 1 s (BE = 284.8 eV) as standard. NOx temperature programmed desorption (NOx–TPD) experiments were performed in a Micromeritics AutoChem II 2920 apparatus, equipped with a computer-controlled CryoCooler, a thermal conductivity detector (TCD), and a quadrupole mass spectrometer (MKS Cirrus). The samples were first pre-reduced with 10% H2/Ar at 450 °C for 30 min and cooled down to the room temperature. By purging with He for 30 min, then the gas was switched to NO2 adsorption for 60 min. After He flows for 30 min, the temperature was increased to 900 °C at a 2
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Fig. 2. TEM and HR-TEM images of CeO2 nanorods (a), corresponding Pt/BaO/CeO2 catalysts (b), and EDS elemental mapping images of all NSR catalysts (c).
heating rate of 10 °C/min, and the signal of NO (m/z = 30) was recorded simultaneously. To avoid the influence of H2O, a cold trap was set before the MS detector.
H2 temperature programmed reduction (H2–TPR) was performed in the same instrument as the NOx–TPD. The samples with a weight of 100 mg were pretreated at 450 °C in a flow of air (50 mL/min) for 3
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Table 1 The surface areas of CeO2 and corresponding Pt/BaO/CeO2 catalysts. Catalyst
Surface area (m2/g)
Da (nm)
CeO2-5 M CeO2-7.5 M CeO2-10 M CeO2-15 M Pt/BaO/CeO2-5 M Pt/BaO/CeO2-7.5 M Pt/BaO/CeO2-10 M Pt/BaO/CeO2-15 M
121.0 103.6 100.6 89.5 91.8 84.2 83.4 76.0
(7.2 (8.1 (9.2 (9.9 (7.2 (8.1 (9.2 (9.9
a b
± ± ± ± ± ± ± ±
CeO2 sizeb (nm) 0.7) 1.4) 1.3) 2.5) 0.7) 1.4) 1.3) 2.5)
× × × × × × × ×
(40–80) (40–110) (50–160) (70–180) (40–80) (40–110) (50–160) (70–180)
11.2 12.3 14.3 15.5 11.7 12.2 12.3 14.0
Calculated for 200 CeO2 nanorods from the HRTEM images. Calculated based on the CeO2 diffraction peak at 2θ = 28.5° by applying the Scherrer equation.
Fig. 3. Outlet NOx concentration as a function of time and temperature under lean condition: (a) Pt/BaO/CeO2-5 M, (b) Pt/BaO/CeO2-7.5 M, (c) Pt/BaO/CeO2-10 M, and (d) Pt/BaO/CeO2-15 M. Lean gas composition: 500 ppm NO, 8% O2, N2 balance.
2.3. Catalytic activity measurements
Table 2 NOx storage capacities (NSC) tested at different temperatures. (Unit: μmol/g). Catalysts
150°C
200°C
250°C
300°C
350°C
400°C
450°C
Pt/BaO/CeO2-5 M Pt/BaO/CeO2-7.5 M Pt/BaO/CeO2-10 M Pt/BaO/CeO2-15 M
215.3 252.7 204.3 196.2
446.5 589.0 435.7 350.6
534.5 692.6 501.2 367.5
722.9 865.6 584.2 610.8
716.1 913.8 711.9 645.6
655.7 782.8 619.3 572.6
492.5 553.8 330.9 374.8
2.3.1. NOx storage capacity experiment The NOx uptake experiments were carried out under lean fuel conditions as a function of temperature. Before each experiment, the catalyst was pretreated in 3% H2/N2 for 1 h at 450 °C, and then cooled to the desired temperature. A mixture of 500 ppm NO and 8% O2 at a total flow rate of 300 mL/min was used to simulate the lean fuel exhaust, using N2 as a balance gas. The outlet NOx (NO + NO2) concentration was monitored by a chemiluminescence detector (ECO Physics CLD 62). The reactor was also equipped with a bypass line to ensure inlet gas concentration.
30 min and cooled down to the room temperature. Then reduction profiles were obtained by passing through the sample with 10% H2/Ar at a rate of 50 mL/min, during which the temperature was increased from 50 to 900 °C at a ramp of 10 °C/ min.
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maintained their original crystal shapes, while no structural features of Pt and Ba were observed. Furthermore, elemental mapping in Fig. 2c revealed that the Pt and Ba were highly dispersed on all the NSR catalysts. The BET surface areas of CeO2 nanorods and Pt/BaO/CeO2 catalysts calculated from the N2 adsorption isotherms were compiled in Table 1. With increasing the pre-adding NaOH concentration, the surface area of CeO2 nanorods decreased from 121.0 m2/g for CeO2-5 M to 89.5 m2/g for CeO2-15 M. After loading 1% Pt and 8% BaO (followed by calcination), the specific area decreased to 91.8, 84.2, 83.4, and 76.0 m2/g, respectively. Such decrease in BET surface area is due to the collapse or clogging of the pores during the impregnation and calcination process. 3.2. Catalytic performance 3.2.1. NOx storage performance Fig. 3 showed the NOx uptake profiles of a series Pt/BaO/CeO2 catalysts at different temperatures for 60 min lean periods where the inlet NOx was present as NO (500 ppm). The profile of Pt/BaO/CeO27.5 M catalyst (in Fig. 3b) presented the typical NOx adsorption characteristics. Just after exposure to the lean condition, the NOx level drops immediately to 0 ppm (except for 150 °C), indicating that complete uptake of NOx by the catalysts. Increasing operating temperature from 200 to 350 °C lengthened the duration for NOx complete capture (from 65 to 344 s). With further rising the temperature to 450 °C, while the complete adsorption time decreased. And then, the outlet NOx level gradually increased with time-on-stream, reaching the inlet value after 60 min. Furthermore, the rate of NOx escape, signified by the slope of the breakthrough curve, is smaller at 350 °C than at any of the other temperatures examined, exhibiting the maximum NOx uptake. Similar features for NOx uptake were also observed over Pt/BaO/CeO2-5 M, Pt/ BaO/CeO2-10 M, and Pt/BaO/CeO2-15 M catalysts, while given a shorter duration for complete NOx uptake if compared with Pt/BaO/ CeO2-7.5 M. Based on these experiments (Fig. 3), the NOx storage capacities (NSC) of series NSR catalysts were calculated and shown in Table 2. With increase of the lean operation temperature, an increased NSC was observed over all the samples, with the highest value being attained at 350 °C. Further increasing the reaction temperature resulted in a decrease in NOx storage capacity, due to the relatively low thermal stability of the nitrates at 400 and 450 °C. Among all the NSR catalysts, obviously, the Pt/BaO/CeO2-7.5 M sample possessed the largest NSC within the full operating temperature range (150–450 °C). The NOx storage capacity of NSR catalyst is related to the ability of NO oxidation [37]. Fig. 4 gave the proportion of NO2/(NO + NO2) over series Pt/BaO/CeO2 catalysts at the end of NOx uptake measurement. For a given NSR catalyst, the oxidation ability of NO to NO2 was weak at low temperatures (150–250 °C). At temperature above 250 °C, the NO2/(NO + NO2) value increased sharply, exhibiting the maximum at 400 °C. In the whole temperature range, there was no difference among the Pt/BaO/CeO2-5 M, Pt/BaO/CeO2-7.5 M, Pt/BaO/CeO2-10 M, Pt/ BaO/CeO2-15 M catalysts, indicating that NO oxidation was not a critical step in determining the NOx storage capacity.
Fig. 4. The NO2/(NO + NO2) ratio over series Pt/BaO/CeO2 catalysts when the NSR catalysts were saturated by NOx.
2.3.2. Cyclic NOx storage reduction tests The NSR cyclic measurements were conducted with 100 mg of catalysts using a fixed-bed quartz reactor. The reactor was connected to a pneumatically actuated four-way valve, which provides a quick switching between the lean and rich atmospheres. Constant flows (300 mL/min) of 500 ppm NO + 8% O2 and 500 ppm NO + 3% H2 were introduced alternately, during which a lean period of 90 s and a rich period of 6 s (or 5 s, 4 s, 3 s) were performed between 150 °C and 450 °C. The NOx conversion was averaged over 20 (or 60) lean/rich cycles to give a mean value according to the following formula:
NOx conversion (%) =
NOx,in-NOx,out ×100 % NOx,in
3. Results and discussion 3.1. Structural properties of CeO2 and Pt/BaO/CeO2 catalysts The X-ray diffraction (XRD) pattern of the CeO2 nanorods (Fig. 1a) indicated the existence of a distinct cubic fluorite phase oxide structure (JCPDS No.43–1002) consisting of diffraction peaks at 28.5, 33.0, 47.5, 56.3, 59.1, 69.4, 76.7, 79.1, and 88.4°, which belong to the face-centered cubic structure of (111), (200), (220), (311), (222), (400), (331), (420), and (422), respectively. Fig. 1b showed that the main phases present in Pt/BaO/CeO2 catalysts were CeO2 (JCPDS No.43–1002) and BaCO3 (JCPDS No.71–2394), thereby confirming the decomposition of Ba(O2CCH3)2 into crystalline BaCO3 during calcination process. The characteristic reflection of Pt was not observed due to its low concentration and/or the high dispersion in the Pt/BaO/CeO2 catalysts. Fig. 2a presented TEM micrographs of the as-synthesized CeO2, which revealed a rod-like morphology for all the samples. Calculating for 200 CeO2 nanorods from the HR-TEM images (shown in Table 1), it was clearly found that the length of CeO2 nanorods increased with increase of the pre-adding NaOH. The HR-TEM images further aimed at clarifying the lattice plane of the CeO2 nanorods. From the HR-TEM image of CeO2-7.5 M nanorods, the interplanar distances of the CeO2 lattice fringes were determined to be 0.31, 0.27, and 0.19 nm, respectively. According to the fast Fourier transform (FFT) analysis, three kinds of lattice fringe directions attributed to (111), (002), and (220) were observed for the CeO2-7.5 M nanorods. Therefore, The CeO2-7.5 M sample showed a 1D growth structure with a preferred growth direction along [110] and were enclosed by (110) and (100) planes. After loading Pt and Ba, it can be clearly seen in Fig. 2b that the CeO2 nanomaterials
3.2.2. Dynamic NOx storage and reduction performance The NOx removal efficiency is not only concerned with the NOx storage capacity in the lean phase, but also related to the reduction of trapped NOx during the rich phase. Minimizing the duration of the rich phase can improve the fuel efficiency, so a rich regeneration time of several seconds is typically used in real systems [23]. In this study, the NOx removal efficiency was investigated under harsh cycling conditions, with a lean period of 90 s and a rich period of 6 s at a quite high gas hourly space velocity (GHSV) of 360, 000 h−1. Fig. 5 showed the transient effluent NOx concentration over all NSR catalysts during 20 lean/rich cycles. Firstly, over the Pt/BaO/CeO2-5 M (in Fig. 5a), NOx breakthrough rapidly increased at a low temperature 5
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Fig. 5. Evolutions of NOx concentrations under lean-rich condition at different temperatures over Pt/BaO/CeO2-5 M (a), Pt/BaO/CeO2-7.5 M (b), Pt/BaO/CeO2-10 M (c), and Pt/BaO/CeO2-15 M (d).
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3.2.3. Effects of lean/rich timing on the performance of Pt/BaO/CeO27.5M Lean/rich cycle timing is an important parameter that was identified to maximize NOx conversion while minimizing fuel penalty [38]. Thus, the effect of lean/rich timing on cyclic behavior was studied over Pt/BaO/CeO2-7.5 M by varying the rich time (3–6 s) with a fixed inlet H2 concentration (3%) and fixed storage time (90 s). As shown in Fig. 7, along with decreasing the rich time, the spillover concentration of NOx increased at the low temperatures while the outlet concentration of NOx was basically unchanged at the high temperatures. It was clear that when the rich time is 6 s, the NOx conversion of Pt/BaO/CeO27.5 M was excellent in the whole temperature window, especially in the 200–400 °C range, in which the purification efficiency of NOx could reach up to nearly 100% (Table 3). The reduction of rich time resulted in the decrease of NOx conversion at low temperatures. However, note that even if the rich time is just only 3 s, the Pt/BaO/CeO2-7.5 M catalyst still maintained more than 80% NOx conversion in the 250–450 °C. It is worth mentioning, during the 60 cycles operation, the overflow concentration of NOx remained consistent except a gradually increased concentration in the first few cycles, indicative of a good durability.
Fig. 6. NOx conversion of Pt/BaO/CeO2-5 M (a), Pt/BaO/CeO2-7.5 M (b), Pt/ BaO/CeO2-10 M (c), and Pt/BaO/CeO2-15 M (d) over 20 lean-rich cycles at different temperatures.
3.3. Thermal stabilities of the stored NOx The NOx storage features and thermal stabilities of the NOx stored on the NSR catalysts were measured by NO2-TPD measurement. Fig. 8 showed the signal of m/e = 30 on Pt/BaO/CeO2-5 M, Pt/BaO/CeO27.5 M, Pt/BaO/CeO2-10 M, and Pt/BaO/CeO2-15 M. Generally, the nitrate decomposition follows the equation: Ba(NO3)2 → BaO + NO2 + 1/2O2 [39]. Since NO2 is easily dissociated to NO fragment in our MS conditions, the main signal is the m/e = 30 (NO) not m/e = 46 (NO2) [40]. As seen in Fig. 8, almost all of the stored NOx on the NSR catalysts was desorbed within the temperature range of 350–650 °C. Such desorption of nitrates could lead to a decrease in amounts of trapped NOx, resulting in a decrease of NSC at high temperatures (Table 2). By integrating the area of NOx desorption peak, it was found that the four NSR catalysts displayed different NOx releasing amount. Pt/BaO/CeO27.5 M showed the largest desorption amount of NOx, followed by Pt/ BaO/CeO2-5 M, then Pt/BaO/CeO2-10 M, and finally Pt/BaO/CeO215 M. This sequence was in line with the NOx storage capacity shown in Fig. 2.
of 150 °C, reaching ˜380 ppm at the end of the lean periods. Upon switching to rich phase, the stored NOx was reduced to N2 by reductant H2, during which the storage sites were regenerated. However, because of the low NOx storage capacity of the catalyst itself at 150 °C, there was still a large amount of NOx spillover in the lean-rich dynamic cycle experiment. With the temperature increasing to 250 °C, the release of NOx obviously decreased. Within the temperature range of 300–400 °C, nearly no NOx was detected during the whole lean and rich period. At 450 °C, the inlet NOx was completely stored at the lean phase. When the atmosphere was switched from lean to rich flow, the NOx was released suddenly with its concentration increasing to a considerable value, but it decreased rapidly and sharply because of its reduction by H2. Over the other three NSR catalysts, the trends of effluent NOx concentration with reaction temperature were similar to that of Pt/BaO/CeO2-5 M. Over the Pt/BaO/CeO2-7.5 M (Fig. 5b), NOx release was a little lower at temperatures below 300 °C than that of Pt/BaO/CeO2-5 M, while slightly higher at other temperatures. As for Pt/BaO/CeO2-10 M and Pt/ BaO/CeO2-15 M, however, the outlet NOx concentration was often higher than that of Pt/BaO/CeO2-5 M in the temperature range of 150–450 °C. Besides, NOx breakthrough over the NSR catalysts increased with time-on-stream. This will be explained later in combination with TPR data. Experimental condition: lean (90 s): 500 ppm NO, 8% O2; rich (6 s): 500 ppm NO, 3% H2, N2 balance; GHSV˜360,000 h−1. Based on the cycle operation, the averaged NOx conversion was calculated, with results shown in Fig. 6. For the Pt/BaO/CeO2-5 M catalyst, the cycle-averaged NOx removal efficiency increased significantly when the temperature increased to 200 °C and the highest conversion of 98.8% was obtained at 300 °C. Similar high NOx conversion was also obtained over Pt/BaO/CeO2-7.5 M within the temperature range of 200-400 °C if compared with Pt/BaO/CeO2-5 M, while at the temperatures of 150 and 450 °C, the former sample gave a little higher NOx removal efficiency. At temperatures above 200 °C, Pt/ BaO/CeO2-10 M and Pt/BaO/CeO2-15 M always displayed lower NOx removal efficiency than Pt/BaO/CeO2-7.5 M. Taking the NOx storage capacity into account, it can be inferred that the Pt/BaO/CeO2-7.5 M is the optimal catalyst.
3.4. Oxygen vacancy and redox properties 3.4.1. XPS results of NSR catalysts It has been proved in our previous study that oxygen vacancies in Pt/BaO/CeO2 govern the NOx storage capacity by creating efficient sites or channels for the formation of nitrates and their further transformation to storage sites [23]. Over the ceria based materials, oxygen vacancies can be produced by the transformation between Ce3+ and Ce4+, 4Ce4+ + O2− → 2Ce4+ + 2Ce3+ + □ + 0.5O2 (□ represents an empty position) [30]. The higher the relative concentration of Ce3+, the more oxygen vacancies that form. As a result, XPS is a well technique to study the oxygen vancies in CeO2 materials, by meauring the relative concentration of Ce3+. Fig. 9 showed the Ce 3d electron core level spectra of CeO2 supports, fresh NSR catalysts, and the reduced ones. The Ce4+ has been fitted with six peaks: v, v’’, v’’’, u, u’’, and u’’’, while Ce3+ has been fitted with two peaks: v’ and u’. The surface concentrations of Ce3+ to the total Ce on the all samples are summarized in Table 4. The concentrations of Ce3+ on the CeO2-5 M, CeO27.5 M, CeO2-10 M, and CeO2-15 M are 14.6%, 14.8%, 14.5%, and 13.6%, separately. After loading active components Pt and BaO, the
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Fig. 7. The outlet NOx concentration of Pt/BaO/CeO2-7.5 M over 60 lean (90 s) -rich cycles at rich time of 6 s (a), 5 s (b), 4 s (c), and 3 s (d).
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BaO/CeO2-5 M catalyst, the peak below 300 °C was assigned to the reduction of PtOx and the surface CeO2 in direct contact with Pt (thereafter named promoted surface CeO2). The peak at 554 °C was attributed to the reduction of surface CeO2 being distant from Pt. In addition, the reduction peaks at about 750 °C was responsible for the reduction of bulk CeO2 [44,45]. Moreover, upon a further increase of NaOH concentration from 7.5 M to 15 M employed in CeO2 preparation, the reduction peaks at intermediate and high temperatures were similar to the Pt/BaO/CeO2-5 M catalyst. However, the reduction peak of PtOx and promoted surface CeO2 on the Pt/BaO/CeO2-7.5 M catalyst moved to the lowest temperature, which indicated that the mobility of surface O was greatly improved because of the strong synergetic effect between Pt and CeO2. It is believed that the synergetic effect facilitated oxygen diffusion from the subsurface layers and might progressively proceed deeper into the bulk [46], which was beneficial for the NOx storage capacity. With the results of H2-TPR in mind, meanwhile, one can deduce that, under the rich phase of NSR process, the H2 consumption would include the reduction of trapped NOx, PtOx, and promoted surface CeO2. Considering that a short rich period (3–6 s) was employed in NSR operation, the partial oxidation of PtOx would occur at low temperatures, leaving larger amount of PtOx in the Pt/BaO/CeO2 with time-onstream. This change may lower the activity for NO oxidation under lean phase and for NOx reduction under rich condition [47]. As a result, it is reasonable that, the NOx breakthrough often appeared as an increase in cyclic operation, within the low temperature range of 200–400 °C. As time going on, a gradually increased PtOx in the Pt/BaO/CeO2 may consume a greater amount of H2 during the rich phase of NSR operation [23]. As a result, the amount of H2 being available for the reduction of released NOx decreased with time-on-stream, leading to the partial regeneration of the storage sites in the rich phase and a decrease in NOx storage under lean condition.
Table 3 NOx conversion of NSR catalysts over 60 lean-rich cycles at different rich times. Lean- Rich
150°C
200°C
250°C
300°C
350°C
400°C
450°C
90 s-6 s 90 s-5 s 90 s-4 s 90 s-3 s
59.8% 61.1% 60.6% 55.5%
97.6% 82.5% 81.1% 70.1%
97.5% 84.2% 85.1% 82.3%
94.4% 80.7% 79.4% 86.6%
95.4% 90.8% 86.4% 86.8%
99.4% 96.8% 95.5% 94.7%
72.6% 85.8% 96.4% 91.4%
Fig. 8. NO2–TPD profiles of the NSR catalysts.
Ce3+ concentrations remain unchanged. However, the H2 pretreatment improved the ratio of Ce3+ on the Pt/BaO/CeO2. Obviously, the oxygen vacancies of the samples that prepared by 7.5 M NaOH are more than those of samples prepared by 5 M, 10 M, and 15 M NaOH, in good agreement with the result of NSC and NSR performance. This further confirmed the importance of oxygen vacancies on the NOx adsorption and storage. R[a]: the NSR catalysts reduced by 3% H2.
3.5. Relationship among oxygen vacancies, redox properties and catalytic performance It was perfectly clear in Table 1 that the BET surface areas of Pt/ BaO/CeO2 catalysts decreased with increasing the pre-adding NaOH concentration in CeO2 support preparation process. Among all the NSR catalysts, however, the specific surface area of Pt/BaO/CeO2-7.5 M catalyst was not the largest, while giving the best catalytic performance (Table 2 and Fig. 6), indicating that the BET surface area was not the crucial factor influencing the catalytic performance. Besides, all the NSR catalysts had almost the same Pt and BaO loadings, and the active components were highly dispersed on the support CeO2 nanorods. Therefore, the catalytic performance of the NSR catalysts was dependent on the intrinsic properties related to CeO2 support. In previous study, we have identified the determinative role of oxygen vacancies in the NOx storage process [23]. Combining the results of XPS in Table 3 with the NOx storage capacity in Table 2, the Fig. 12a showed a linear relationship also existing between the Ce3+ concentration on the reduced NSR catalysts and their NSC in the temperature range of 150–350 °C. This result again manifests that oxygen vacancies could possess the NOx storage process. At temperatures above 350 °C, however, the similar linear correlation was not observed. This phenomenon could be attributed to the decomposition of the stored NOx began at 350 °C (shown in Fig. 8), which makes a negative contribution to the NOx storage capacity of NSR catalysts. Furthermore, there was a linear relationship between the reduction temperature of PtOx and promoted surface CeO2 and the NOx storage capacity of all the NSR catalysts at 150–350 °C (Fig. 12b), revealing that
3.4.2. Raman spectra Raman measurement has been successfully used to discriminate between different structures on oxide surfaces [41], so we recorded and analyzed the Raman spectra of the different shapes CeO2 nanostructures. As presented in Fig. 10, a prominent peak at around 462 cm−1 was observed over all the CeO2 nanomaterials corresponding to the distinct F2g symmetry mode of the CeO2 phase and being viewed as a symmetric breathing mode of the oxygen atoms around cerium ions [41]. Meanwhile, the weak band centered at about 600 cm−1 should be due to the defect-induced mode (D) [42,43]. Consequently, this band can be linked to the defects (oxygen vacancies, etc.) in the CeO2 nanomaterials. The I600/I462 values, which indicates the defect concentration, such as oxygen vacancies, reached a maximum for CeO27.5 M, and then decreased in the sequence of CeO2-5 M, CeO2-10 M, and CeO2-15 M. This demonstrated that the CeO2-7.5 M owned the highest defects among them, being consistent with the results of XPS. 3.4.3. Redox property revealed by H2-TPR H2-TPR experiments were performed to investigate the redox ability of all the NSR catalysts, and the results are shown in Fig. 11. For the Pt/
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Fig. 9. 1 Ce 3d XPS spectra of CeO2-5 M (a), CeO2-7.5 M (b), CeO2-10 M (c), and CeO2-15 M (d). 2 Ce 3d XPS spectra of Pt/BaO/CeO2-5 M (a), Pt/BaO/CeO2-7.5 M (b), Pt/BaO/CeO2-10 M (c), and Pt/BaO/CeO2-15 M (d). 3 Ce 3d XPS spectra of Pt/BaO/CeO2-5 M-R (a), Pt/BaO/CeO2-7.5 M-R (b), Pt/BaO/CeO2-10 M-R (c), and Pt/ BaO/CeO2-15 M-R (d).
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Table 4 The Ce3+ concentration of CeO2 nanorods and NSR catalysts. Sample
CeO2 Pt/BaO/CeO2 Pt/BaO/CeO2-R[a]
Pre-added NaOH concentration 5M
7.5 M
10 M
15 M
14.6% 14.6% 15.2%
14.8% 15.1% 16.8%
14.5% 14.2% 14.6%
13.6% 13.7% 13.9%
the synergetic effect between Pt and CeO2 plays a key role in the NOx storage process. As discussed in the part of “3.4.3 Redox property revealed by H2-TPR”, the synergetic effect facilitated the mobility of surface oxygen species at low temperatures, benefiting the formation of NO2 on the Pt sites and nitrate production, thus increasing the NOx storage capacity. This result, in turn, indicated that the strong synergetic effect was due to the oxygen vacancies on the CeO2 surface anchoring the active component Pt [48]. 4. Conclusion Fig. 11. H2-TPR profiles of the NSR catalysts.
The Pt/BaO/CeO2 catalyst with 7.5 M NaOH pre-added in the support preparation process exhibited higher NOx storage capacity and overall NOx conversion than catalysts with other NaOH concentration. Surprisingly, when the number of lean–rich cycles increased from 20 to 60 in periodically alternating lean–rich atmospheres (90 s vs. 6 s), the NOx conversion on Pt/BaO/CeO2-7.5 M displayed an exceeding 94% NOx conversion in the temperature range of 200–400 °C, indicating the good durability. Even under an extremely short rich duration (only 3 s), Pt/BaO/CeO2-7.5 M still presented a relatively high NOx conversion, ensuring the fuel economy. The results of H2-TPR revealed that the strong synergetic effect between Pt and CeO2 on the Pt/BaO/CeO2-7.5 M catalyst promoted the mobility of surface oxygen species. The Pt/BaO/CeO2-7.5 M implied the
higher amount of oxygen vacancies confirmed by the XPS and Raman measurements than other NSR catalysts. Over all the catalysts, the oxygen vacancies were linearly correlated with the NOx storage capacity. This relationship revealed that the outstanding catalytic performance of Pt/BaO/CeO2 was attributed to the oxygen vacancies. Acknowledgments This work was financially supported by the National Key R&D Program of China (2016YFC0204902), and the National Natural Science Foundation of China (21673277 and 21637005).
Fig. 10. Raman spectra of CeO2 nanostructures (a) and the corresponding peak intensity ratios of I600/I462 (b).
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Fig. 12. The relationship between Ce3+ concentration of reduced NSR catalysts and the NOx storage capacity (NSC) of all the NSR catalysts (a), the relationship between the reduction temperature of PtOx and promoted surface CeO2 and the NOx storage capacity (NSC) of all the NSR catalysts (b).
References
Soc. Rev. 43 (2014) 1543–1574. [21] S. Jones, Y.Y. Ji, M. Crocker, Ceria-based catalysts for low temperature NOx storage and release, Catal. Letters 146 (2016) 909–917. [22] Y. Ryou, J. Lee, H. Lee, C.H. Kim, D.H. Kim, Low temperature NO adsorption over hydrothermally aged Pd/CeO2 for cold start application, Catal. Today 307 (2018) 93–101. [23] Y. Zhang, Y. Yu, H. He, Oxygen vacancies on nanosized ceria govern the NOx storage capacity of NSR catalysts, Catal. Sci. Technol. 6 (2016) 3950–3962. [24] W.-W. Wang, W.-Z. Yu, P.-P. Du, H. Xu, Z. Jin, R. Si, C. Ma, S. Shi, C.-J. Jia, C.H. Yan, Crystal plane effect of Ceria on supported copper oxide cluster catalyst for CO oxidation: importance of metal–support interaction, ACS Catal. 7 (2017) 1313–1329. [25] M. Lykaki, E. Pachatouridou, S.A.C. Carabineiro, E. Iliopoulou, C. Andriopoulou, N. Kallithrakas-Kontos, S. Boghosian, M. Konsolakis, Ceria nanoparticles shape effects on the structural defects and surface chemistry: implications in CO oxidation by Cu/CeO2 catalysts, Appl. Catal. B Environ. 230 (2018) 18–28. [26] H.X. Mai, L.D. Sun, Y.W. Zhang, R. Si, W. Feng, H.P. Zhang, H.C. Liu, C.H. Yan, Shape-selective synthesis and oxygen storage behavior of ceria nanopolyhedra, nanorods, and nanocubes, J. Phys. Chem. B 109 (2005) 24380–24385. [27] K. Zhou, X. Wang, X. Sun, Q. Peng, Y. Li, Enhanced catalytic activity of ceria nanorods from well-defined reactive crystal planes, J. Catal. 229 (2005) 206–212. [28] W.I. Hsiao, Y.S. Lin, Y.C. Chen, C.S. Lee, The effect of the morphology of nanocrystalline CeO2 on ethanol reforming, Chem. Phys. Lett. 441 (2007) 294–299. [29] X.S. Huang, H. Sun, L.C. Wang, Y.M. Liu, K.N. Fan, Y. Cao, Morphology effects of nanoscale ceria on the activity of Au/CeO2 catalysts for low-temperature CO oxidation, Appl. Catal. B Environ. 90 (2009) 224–232. [30] X. Liu, K. Zhou, L. Wang, B. Wang, Y. Li, Oxygen vacancy clusters promoting reducibility and activity of Ceria nanorods, J. Am. Chem. Soc. 131 (2009) 3140–3141. [31] N. Ta, J. Liu, W. Shen, Tuning the shape of ceria nanomaterials for catalytic applications, Chin. J. Catal. 34 (2013) 838–850. [32] Nanostructured ceria-based materials: effect of the hydrothermal synthesis conditions on the structural properties and catalytic activity, Catalysts 7 (2017) 174. [33] N. Bugayeva, J. Robinson, Synthesis of hydrated CeO2 nanowires and nanoneedles, Mater. Sci. Technol. 23 (2013) 237–241. [34] L. Čerović, V. Lair, O. Lupan, M. Cassir, A. Ringuedé, Electrochemical synthesis and characterization of nanorods, nanocolumnar ceria – based thin films on different glass substrates, Chem. Phys. Lett. 494 (2010) 237–242. [35] Q. Yuan, H.H. Duan, L.L. Li, L.D. Sun, Y.W. Zhang, C.H. Yan, Controlled synthesis and assembly of ceria-based nanomaterials, J. Colloid Interface Sci. 335 (2009) 151–167. [36] R. Si, M. Flytzani-Stephanopoulos, Shape and crystal-plane effects of nanoscale Ceria on the activity of Au-CeO2 catalysts for the water–Gas shift reaction, Angew. Chemie 120 (2008) 2926–2929. [37] H.P. Louise Olsson, Erik Fridell, Magnus Skoglundh, Bengt Andersson, A Kinetic Study of NO Oxidation and NOx Storage on Pt/Al2O3 and Pt/BaO/Al2O3, J. Phys. Chem. B 105 (2001) 6895–6906. [38] A. Reihani, B. Patterson, J. Hoard, G.B. Fisher, J.R. Theis, C.K. Lambert, Rapidly pulsed reductants for diesel NOx reduction with lean NOx traps: comparison of Alkanes and alkenes as the reducing agent, J. Eng. Gas Turbine. Power 139 (2017) 102805. [39] G. Zhou, T. Luo, R.J. Gorte, An investigation of NOx storage on Pt-BaO-Al2O3, Appl.
[1] A. Fritz, V. Pitchon, The current state of research on automotive lean NOx catalysis, Appl. Catal. B: Environ. 13 (1997) 1–25. [2] Z.M. Liu, S.I. Woo, Recent advances in catalytic DeNOx science and technology, Catal. Rev. Sci. Eng. 48 (2006) 43–89. [3] T.V. Johnson, Diesel emission control: 2001 in review, SAE Trans. 111 (2002) 85–101. [4] W.S. Epling, L.E. Campbell, A. Yezerets, N.W. Currier, J.E. Parks, Overview of the fundamental reactions and degradation mechanisms of NOx storage/reduction catalysts, Catal. Rev. Sci. Eng. 46 (2004) 163–245. [5] H.E. Jun, C. Ying, L.I. Xuehui, M.A. Yugang, C. Xiaoping, W. Lefu, A review on development of De-NOx storage-reduction catalysts, Precious Metals 27 (2006) 65–70. [6] S. Roy, A. Baiker, NOxStorage-reduction catalysis: from mechanism and materials properties to storage-reduction performance, Chem. Rev. 109 (2009) 4054–4091. [7] H. Chen, Y. Zhang, Y. Xin, Q. Li, Z. Zhang, Z. Jiang, Y. Ma, H. Zhou, J. Zhang, Enhanced NOx conversion by coupling NOx storage-reduction with CO adsorptionoxidation over the combined Pd−K/MgAlO and Pd/MgAlO catalysts, Catal. Today 258 (2015) 416–423. [8] Y. Zhang, X. Wang, Z. Wang, Q. Li, Z. Zhang, L. Zhou, Direct spectroscopic evidence of CO spillover and subsequent reaction with preadsorbed NOx on Pd and K cosupported Mg-Al mixed oxides, Environ. Sci. Technol. 46 (2012) 9614–9619. [9] Y. Li, S. Roth, J. Dettling, T. Beutel, Effects of lean/rich timing and nature of reductant on the performance of a NOx trap catalyst, Top. Catal. 16/17 (2001) 139–144. [10] L. Liu, Z.-j. Li, H.-y. Zhang, Q. Chang, B.-x. Shen, Effect of rich air/fuel ratio and temperature on NOx desorption of lean NOx trap, J. Zhejiang Univ. Sci. A 14 (2013) 835–842. [11] B.M. Shakya, M.P. Harold, V. Balakotaiah, Effect of cycle time on NH3 generation on low Pt dispersion Pt/BaO/Al2O3 catalysts: experiments and crystallite-scale modeling, Chem. Eng. J. 230 (2013) 584–594. [12] M. Li, Y. Zheng, D. Luss, M.P. Harold, Impact of rapid cycling strategy on reductant effectiveness during NOx storage and reduction, Emiss. Control. Sci. Technol. (2017). [13] T.V. Johnson, Diesel emission control in review, SAE Trans. 116 (2007) 76–87. [14] S. Andonova, Z.A. Ok, N. Drenchev, E. Ozensoy, K. Hadjiivanov, Pt/CeOx/ZrOx/γAl2O3 ternary mixed oxide DeNOx catalyst: surface chemistry and NOx interactions, J. Phys. Chem. C 122 (2018) 12850–12863. [15] A. Bueno-Lopez, D. Lozano-Castello, J.A. Anderson, NOxstorage and reduction over copper-based catalysts. 2 supports, Appl. Catal. B Environ. 198 (2016) 189–199. [16] A. Bueno-Lopez, D. Lozano-Castello, J.A. Anderson, NOxstorage and reduction over copper-based catalysts. 0.8M0.2O delta supports (M = Zr, La, Ce, Pr or Nd), Appl. Catal. B Environ. 198 (2016) 234–242. [17] W. Xie, Y. Yu, H. He, Shape dependence of support for NOx storage and reduction catalysts, J. Environ. Sci. (2018). [18] A. Trovarelli, Catalytic properties of Ceria and CeO2-Containing materials, Catal. Rev. Sci. Eng. 38 (1996) 439–520. [19] Y.T. Kim, E.D. Park, H.C. Lee, D. Lee, K.H. Lee, Water-gas shift reaction over supported Pt-CeOx catalysts, Appl. Catal. B Environ. 90 (2009) 45–54. [20] Y. Li, W. Shen, Morphology-dependent nanocatalysts: rod-shaped oxides, Chem.
12
Catalysis Today xxx (xxxx) xxx–xxx
Y. Zhang, et al. Catal. B Environ. 64 (2006) 88–95. [40] B. Zhao, G.F. Li, C.H. Ge, Q.Y. Wang, R.X. Zhou, Preparation of Ce0.67Zr0.33O2 mixed oxides as supports of improved Pd-only three-way catalysts, Appl. Catal. B Environ. 96 (2010) 338–349. [41] B.M. Reddy, A. Khan, Y. Yamada, T. Kobayashi, S. Loridant, J.C. Volta, Surface characterization of CeO2/SiO2 and V2O5/CeO2/SiO2 catalysts by Raman, XPS, and other techniques, J. Phys. Chem. B 106 (2002) 10964–10972. [42] M.S. Brogan, T.J. Dines, Raman spectroscopic study of the Pt-CeO2 interaction in the Pt/Al2O3-CeO2 catalyst, J. Chem. Soc. Faraday Trans. 90 (1994) 1461–1466. [43] R. Gao, D. Zhang, P. Maitarad, L. Shi, T. Rungrotmongkol, H. Li, J. Zhang, W. Cao, Morphology-dependent properties of MnOx/ZrO2–CeO2 nanostructures for the selective catalytic reduction of NO with NH3, J. Phys. Chem. C 117 (2013) 10502–10511. [44] N. Acerbi, S. Golunski, S.C. Tsang, H. Daly, C. Hardacre, R. Smith, P. Collier, Promotion of Ceria catalysts by precious metals: changes in nature of the interaction
[45] [46] [47] [48]
13
under reducing and oxidizing conditions, J. Phys. Chem. C 116 (2012) 13569–13583. N. Acerbi, S.C.E. Tsang, G. Jones, S. Golunski, P. Collier, Rationalization of interactions in precious metal/ceria catalysts using the d-band center model, Angew. Chemie 52 (2013) 7737–7741. S. Ding, F. Liu, X. Shi, K. Liu, Z. Lian, L. Xie, H. He, Significant promotion effect of Mo additive on a novel Ce-Zr mixed oxide catalyst for the selective catalytic reduction of NOx with NH3, ACS Appl. Mater. Interfaces 7 (2015) 9497–9506. L. Olsson, E. Fridell, The influence of Pt oxide formation and Pt dispersion on the reactions NO2⇔NO+1/2 O2 over Pt/Al2O3 and Pt/BaO/Al2O3, J. Catal. 210 (2002) 340–353. L. Nie, D. Mei, H. Xiong, B. Peng, Z. Ren, X.I.P. Hernandez, A. DeLaRiva, M. Wang, M.H. Engelhard, L. Kovarik, A.K. Datye, Y. Wang, Activation of surface lattice oxygen in single-atom Pt/CeO2 for low-temperature CO oxidation, Science 358 (2017) 1419–1423.