目录结构

详情信息展示

参考文献

1 Introduction▲
2 Experimental▲
2.1 Catalyst preparation
2.2 Catalyst characterization
2.3 Activity measurements
3 Results and discussion▲
3.1 Catalyst characterization
3.2 NOx storage capacity (NSC)
3.3 NOx conversion efficiency
4 Conclusion▲
图表▲

Rare Metals2019年第1期

Enhancing low-temperature NO_x storage and reduction performance of a Pt-based lean NO_x trap catalyst

Tong Wang Li-Wei Jia Xiu-Ting Wang Gang Wang Fu-Qiang Luo Jia-Ming Wang

School of Automotive and Traffic Engineering, Jiangsu University

Wuxi Weifu Environmental Catalysts Co. Ltd

作者简介：*Jia-Ming Wang e-mail:wxt5409@126.com;

收稿日期：1 July 2018

基金：financially supported by the National Key R&D Program of China (No. 2017YFC0211100);

Enhancing low-temperature NO_x storage and reduction performance of a Pt-based lean NO_x trap catalyst

Tong Wang Li-Wei Jia Xiu-Ting Wang Gang Wang Fu-Qiang Luo Jia-Ming Wang

School of Automotive and Traffic Engineering, Jiangsu University

Wuxi Weifu Environmental Catalysts Co. Ltd

Abstract：

Two lean NO_x trap(LNT) catalysts, Pt/BaO/CeO₂ + Al₂O₃ and Pt/BaO/CeO₂-Al₂O₃, were prepared and compared for low-temperature(< 250℃) NO_x storage and reduction performance. The influence of the form of ceria on low-temperature NO_x storage and reduction performance of LNT catalysts was investigated with the focus on NO_x storage capacity, NO_x reduction efficiency during lean/rich cycling, product selectivity and thermal stability.Inductively coupled plasma-atomic emission spectrometry(ICP-AES), Brunner-Emmet-T eller(BET), H₂-pulse chemisorption and X-ray diffraction(XRD) were conducted to characterize the physical properties of LNT catalysts. NO_x storage capacity and NO_x conversion efficiency were measured to evaluate NO_x storage and reduction performance of LNT catalysts. Pt/BaO/CeO₂-Al₂O₃ catalyst exhibits higher NO_x storage capacity than Pt/BaO/CeO₂ + Al₂O₃ catalyst in the temperature range of 150-250 ℃. Meanwhile, Pt/BaO/CeO₂-Al₂O₃ catalyst shows better NO_x conversion efficiency and N₂ selectivity. XRD results indicate that the thermal stability of CeO₂-Al₂O₃ complex oxide is superior to that of pure CeO₂. H₂-pulse chemisorption results show that Pt/BaO/CeO₂-Al₂O₃ catalyst has higher Pt dispersion than Pt/BaO/CeO₂ + Al₂O₃ catalyst over fresh and aged samples. The improved physical properties of Pt/BaO/CeO₂-Al₂O₃ catalyst are attributed to enhance the NOx storage and reduction performance over Pt/BaO/CeO₂ + Al₂O₃ catalyst.

Keyword：

NO_x storage and reduction; Low temperature; Ceria; Ceria-alumina; LNT;

Received： 1 July 2018

1 Introduction

To meet the stringent China Stage-VI light-duty diesel vehicles’NO_x emission regulations,a promising approach has been explored by a combination of lean NO_x trap(LNT) and selective catalytic reduction (SCR) catalysts [ 1, 2, 3] ,which can enhance NO_x reduction while avoiding the need for urea injection to the SCR catalyst [ 4] .However,additional improvements are required in LNT system,especially for low-temperature NO_x conversion [ 5, 6] .

LNT catalysts operate under cyclic lean/rich conditions [ 7, 8] .During lean conditions,NO_x is stored on the catalyst;while during rich conditions,stored NO_x is released and reduced into N₂ along with possible byproducts,such as NH₃ and N₂O [ 9, 10, 11] .A typical LNT catalyst contains basic components (alkali metal or alkaline earth metal compounds) for NO_x storage,noble metals (Pt,Pd,Rh),and support oxides [ 7] .In addition,ceria-based materials have been shown to be beneficial for LNT catalysts at low temperatures [ 12, 13, 14] .CeO₂ is a common rare earth metal oxide with special structure and properties [ 15, 16, 17] ,and many current commercial LNT catalysts have already incorporated ceria [ 5, 18] .

In the present work,the goal was to study the effects of the form of ceria on low-temperature NO_x storage and reduction performance through surface/bulk analysis and activity tests.The first catalyst of Pt/BaO/CeO₂+Al₂O₃used ceria as a support material as well as alumina,and the second catalyst of Pt/BaO/CeO₂-Al₂O₃ used complex ceria-alumina as a new support material.The low-temperature NO_x storage capacity (NSC),NO_x reduction efficiency,NH₃ production and N₂O emissions were measured.Thermal stability of LNT catalysts was also investigated after aging for 20 h at 800℃.

2 Experimental

2.1 Catalyst preparation

Catalysts were prepared by incipient wetness impregnation technique,using aqueous solutions of Ba(CH₃COO)₂ and Pt(NO₃)₂.In the preparation of Pt/BaO/CeO₂+Al₂O₃(1wt%Pt and 9 wt%BaO),CeO₂ (specific surface area152 m²·g^-1) andγ-Al₂O₃ (150 m²·g^-1) were mechanically mixed at the weight ratio of 3:7.The impregnation was carried out in a sequential manner:The well-mixed CeO₂+Al₂O₃ support was firstly impregnated with the Ba acetate solution followed by Pt nitrate solution.On the other hand,Pt/BaO/CeO₂-Al₂O₃ (1 wt%Pt and 9 wt%BaO) was prepared similarly,but using a CeO₂-Al₂O₃complex oxide (30 wt%CeO₂ and 70 wt%Al₂O₃,154 m²·g^-1) as the support.After each impregnation step,the catalyst was dried at 100℃in air for overnight and then calcined in air at 500℃for 5 h.The obtained fresh catalysts were denoted as F-1 and F-2,respectively.In order to compare their thermal stability,the catalysts were further calcined in air at 800℃for 20 h.The aged catalysts were denoted as A-1 and A-2.

2.2 Catalyst characterization

The actual elemental compositions of catalysts were analyzed by inductively coupled plasma-atomic emission spectrometry (ICP-AES,Optima 2100 DV,PerkinElmer).The Brunner-Emmet-Teller (BET) specific surface areas were measured by N₂ adsorption-desorption at-196℃on a Micromeritics ASAP 2000 analyzer.Prior to measurements,the samples were pre-treated at 150℃under vacuum for 3 h to eliminate the adsorbed species.The Pt dispersions were determined by H₂-pulse chemisorption at-80℃by a Micromeritics AutoChemⅡAnalyzer.The crystalline phases of the catalysts were characterized by X-ray diffractometer (XRD,XRD-600,Shimadzu) with a Cu Kαradiation (λ=0.154056 nm) at 36 kV with a graphite monochromator.

2.3 Activity measurements

In NO_x storage capacity (NSC) and NO_xconversion efficiency measurements,0.25 g Pt/BaO/Al₂O₃ catalyst was mixed with 0.75 g quartz sand.The total flow rate is 1L·min^-1,corresponding to a space velocity of 60,000 h^-1.Before each experiment,the sample was oxidized in 10%O₂/N₂ balance at 350℃for 30 min,and then reduced in5%H₂/N₂ balance at 450℃for 20 min.The reactor was then cooled in N₂ to the target test temperatures at 150,200and 250℃.The outlet gas of the reactor was maintained at140℃to avoid condensation and NH₃ hold-up.A MKS MultiGas 2030 FT-IR analyzer was used to monitor NO,NO₂,N₂O,NH₃,CO,C₃H₆,CO₂ and H₂O concentrations of the outlet gas.

NO_x storage capacity (NSC) tests were conducted by exposing the catalyst to the flowing gas containing250×10^-6 NO.8%O2.5%H₂O,5%CO₂ at 150,200 and250℃.The storage time was 10 min at each temperature during NSC measurement.NO_x conversion efficiency tests were also measured at 150,200 and 250℃,and LNT performance was evaluated in a lean/rich (100 s/17 s)cycle under the gas composition detailed in Table 1.The average value of the last 5 cycles was calculated after 15lean/rich cycles.

3 Results and discussion

3.1 Catalyst characterization

The contents of Pt,Ba,Ce and A1 in catalysts are summarized in Table 2.F-1 and F-2 samples have similar Pt and Ba contents.Ce content in F-1 is slightly lower than that in F-2 (26.24 wt%vs.27.54 wt%).

BET specific surface areas and Pt dispersions of catalysts are listed in Table 3.The specific surface areas of F-1and F-2 samples are 117 and 122 m².g^-1,respectively.Pt dispersion of F-1 is 60%,slightly lower than 62%of F-2.After thermal aging at 800℃for 20 h,their surface areas decrease to 99 and 103 m².g-1,respectively.The Pt dispersions reach significantly low values,only 12%for A-1and 19%for A-2,indicating that the sintering of Pt particles occurs during aging due to the mobility of Pt crystallites and eventually merged to form larger particles [ 19] .

Figure 1 shows XRD patterns of fresh and aged catalysts.The results show that Pt-related phases in all four samples were not detected because of either the low loading ratio or its small size.The main crystallite phases detected are CeO_2,BaCO₃ andγ-Al₂O₃ in all samples.The BaAl₂O₄ peak only appears after aging [ 20, 21] .Applying Scherrer equation,the average CeO₂ crystal size was calculated based on CeO₂ diffraction peak at 2θ=28.5^°.TheCeO₂ crystal size in F-1 is estimated to be 10 nm,while6 nm in F-2,16 nm in A-1,and 7 nm in A-2,respectively.These XRD results suggest that the thermal stability of CeO₂-Al₂O₃ complex oxide is superior to that of pure CeO₂+Al₂O₃ with regard to CeO₂ crystal size.

下载原图

Table 1 NO_x conversion efficiency measurement:gas composition

下载原图

Table 2 Elemental composition of fresh catalysts (wt%)

F-1—fresh Pt/BaO/CeO₂+Al₂O₃ catalyst;F-2—fresh Pt/BaO/CeO₂—Al₂O₃ catalyst

下载原图

Table 3 BET specific surface areas and Pt dispersions of catalysts

3.2 NOx storage capacity (NSC)

The results of NO_x storage capacity tests are reported in Table 4.All samples’NSCs increase substantially as the test temperature increases from 150 to 250℃due to enhancing NO oxidation activity to produce NO₂ which is known to be more effective than NO to be adsorbed on LNT catalysts [ 22] .F-2 sample displays higher NSC compared to F-1 sample at 150,200 and 250℃.NSCs of aged samples are lower than those of the fresh samples.A-2 sample still has higher NSC than A-1 sample.The NSC results indicate that CeO₂-Al₂O₃ complex oxide as the support loaded with Ba and Pt can trap NO_x amount to more extent in comparison with CeO₂+Al₂O₃.

下载原图

Table 4 NO_x storage capacity (250×10^-6 NO,8%O₂,5%H₂O,5%CO₂,N₂ bal.,GHSV=60,000 h^-1) of catalysts (μmol·g^-1)

3.3 NOx conversion efficiency

NO_x concentration profiles during the last 5 lean/rich cycles are shown in Fig.2.Average NO_x conversions during the last 5 cycles are presented in Table 5.For F-1sample,NO_x breakthrough is immediately observed after the feed gas switched into lean condition at 150℃,and the outlet NO_x concentration decreases with time and gradually approaches the inlet NO_x concentration.When the feed gas is subsequently switched to the rich condition,a sharp and intense NO_x release peak appears and then intensity quickly decreases.The similar profile was also observed in a previous study [ 23] .The average NO_x conversion at150℃is 13.8%.At 200℃,the outlet NO_x concentration reaches only about 90×10^-6 at the end of the lean duration.The amount of NO_x release during switching to rich condition becomes significantly lower compared to that at 150℃.Therefore,the average NO_x conversion increases to 72.0%.At 250℃,NO_x concentration profile during lean duration is almost coincided with that at200℃.However,there is no NO_x release detected during switching to rich condition.Average NO_x conversion reaches 75.2%.Raising reaction temperature increases NO_xstorage capacity,and on the other hand,high temperature is beneficial to promoting NO_xreduction ability.As a result,the average NOxconversion increases with reaction temperature increasing from 150 to 250℃.

Fig.1 XRD patterns of fresh and aged catalysts:a F-1 and F-2 catalysts and b A-1 and A-2 catalysts

Fig.2 NO_x concentration profiles during last 5 lean/rich cycles:a F-1 catalyst,b F-2 catalyst,c A-1 catalyst,and d A-2 catalyst

下载原图

Table 5 NO_x conversion and selectivity of N-containing products during last 5 lean/rich cycles

In the case of F-2 sample,NO_x evolutions at three temperatures are similar to those of F-1 sample,but F-2sample has slightly higher average NO_x conversions,14.6%at 150℃,73.4%at 200℃,and 76.1%at 250℃,in accordance with the results of NSC measurements.

After thermal aging,the amount of NO_xtrapped on the aged catalyst at lean phase at 150℃becomes smaller in comparison with fresh catalysts and the outlet NO_xconcentrations gradually reach a constant level (around240×10^-6) for both aged catalysts.Higher NO_xslip is observed during rich phase,and average NO_xxconversions of A-1 and A-2 samples are only 6.6%and 9.0%,respectively.At 200℃,NO_x-spill-out over A-1 is relatively larger in comparison with that over A-2 during switching to the rich condition,leading to a lower NO_x conversion for A-1 (49.0%vs.58.9%for A-2).NO_x release still appears over aged samples at 250℃,resulting in that NO_x conversion decreases to some extent.Average NO_x conversion sharply declines at 150 and 200℃.Pt is known to have impacts on NO oxidation under lean conditions,NO_xstorage under rich/lean conditions and NO_x reduction under rich conditions [ 7] .The difference in NO_x conversion may be correlated with different Pt dispersions of catalysts.Clayton et al. [ 24] also found that the differences in storage and reduction activity were the largest among three catalysts which have different Pt dispersions at low temperatures (<200℃).

Fig.3 Evolution of NH₃ and N₂O concentrations during lean/rich cycles at 200℃:a fresh samples and b aged samples

From the perspective of study on the law and production of NH₃ and N₂O,their concentrations evolution at 200℃during lean/rich cycles is taken as an example and shown in Fig.3.For fresh and aged samples,NH₃ and N₂O are both formed in lean and rich phases.During lean phase,NH₃and N₂O are subsequently formed,NH₃ intensity sharply decreases to below 10×10^-6 at the initial lean phase,while a small N₂O peak appears,and then the N₂O intensity gradually becomes stabilized.A sharp N₂O peak is immediately observed at the beginning of rich phase,and NH₃ peak delays about 2 s.In comparison with the results obtained over fresh samples,the amount of NH₃ and N₂O formation decreases over aged samples.In the lean and rich phase,NH₃ is mainly formed via the reduction in NO_x by surface hydrogen,although NH₃ is also a product of isocyanate hydrolysis reaction [ 25, 26] ,Dasari et al. [ 25] summarized the following reaction mechanism:

N₂O formation in lean phase is related to the reactions between surface-deposited reductants/intermediates (CO,HC,NH₃,isocyanate) and gaseous NO/O₂,and N₂O release peak during rich phase may be attributed to that NO_x partially reduced over platinum group metal (PGM)sites [ 26, 27, 28] .

Concerning the product selectivity,N₂O is the main product at 150℃.At 200℃,selectivity of NH₃,N₂O and N₂ is similar for fresh samples,but NH₃ and N₂O become the main products for aged samples.By increasing the temperature to 250℃,N₂ selectivity of all samples shows a substantial enhancement.Moreover,N₂O selectivity decreases as the temperature increases from 150 to 250℃.N₂ selectivity decreases in the following order:F-2>F-1>A-2>A-1.Thermal aging results are in an increase in NH₃ and N₂O selectivity,except NH₃ selectivity at 150℃.Compared to fresh samples,the higher NH₃ selectivity over aged LNT catalysts above 200℃was also found by Chatterjee et al. [ 29] and Easterling et al. [ 30] .Chatterjee et al. [ 29] suggested that the lower noble metal and oxygen storage activities correspond to the shifted light-off of the ammonia oxidation reactions to higher temperatures,which supports the faster NH₃ breakthrough.

4 Conclusion

The influence of the form of ceria on low-temperature NO_xstorage and reduction performance of LNT catalysts was investigated,using CeO₂-Al₂O₃₃ complex oxide or CeO₂+Al₂O₃ mixed oxide as the support for BaO and Pt.CeO₂-Al₂O₃ support exhibits the improvements on NO_xstorage capacity,NO_x conversion efficiency (especially for aged samples at 200℃) and N₂ selectivity in comparison with CeO₂+Al₂O₃ support in the temperature range of150-250℃.After aging for 20 h at 800℃,Pt/BaO/CeO₂-Al₂O₃ catalyst exhibits better thermal stability and chemical distribution than Pt/BaO/CeO₂+Al₂O₃ catalyst.Overall,CeO₂-Al₂O₃-based catalysts show superior NO_xstorage and regeneration performance over CeO₂+Al₂O₃-based catalyst at low temperatures.

Concerning NH₃ and N₂O selectivity of NO_x reduction,N₂O is the main product at 150℃,and its selectivity decreases as the temperature increases from 150 to 250℃.Thermal aging results are in an increase in the NH₃ and N₂O selectivity at 200 and 250℃.