Skip to content
BY 4.0 license Open Access Published by De Gruyter Open Access December 31, 2019

The Influence Of NO/O2 On The NOx Storage Properties Over A Pt-Ba-Ce/γ-Al2O3 Catalyst

  • Xuedong Feng EMAIL logo , Jing Yi and Peng Luo
From the journal Open Chemistry

Abstract

With the purpose of studying the influence of NO/O2 on the NOx storage activity, a Pt-Ba-Ce/γ-Al2O3 catalyst was synthesized by an acid-aided sol-gel method. The physical and chemical properties of the catalyst were characterized by X-ray diffraction (XRD) and Transmission Electron Microscope (TEM) methods. The results showed that the composition of the catalyst was well-crystallized and the crystalline size of CeO2 (111) was about 5.7 nm. The mechanism of NO and NO2 storage and NOx temperature programmed desorption (NO-TPD) experiments were investigated to evaluate the NOx storage capacity of the catalyst. Pt-Ba-Ce/γ-Al2O3 catalyst presented the supreme NOx storage performance at 350℃, and the maximum value reached to 668.8 μmol / gcat. Compared with O2-free condition, NO oxidation to NO2 by O2 had a beneficial effect on the storage performance of NOx. NO-TPD test results showed that the NOx species stored on the catalyst surface still kept relatively stable even below 350℃.

1 Introduction

Diesel and lean-burn engines are becoming increasingly popular, which offer important fuel savings as compared to conventional gasoline engines [1]. In contrast, the presence of excess O2 in the diesel exhausts renders the classical three-way catalysts, which are effective only under stoichiometric conditions, inefficient in reducing NOx emissions in the lean-burn engine exhaust [2]. NOx storage and reduction (NSR) technology has considered to be a particularly means to reduce NOx from diesel engines. The principle of NSR is that NOx stored on the catalyst as nitrates or nitrites when O2 is in excess. Under rich conditions, with the presence of excessive reductants, NOx reduction reactions take place [3,4].

Many studies have focused on the optimization of the NSR catalyst for NOx reduction [5,6]. These NSR catalysts mainly contain precious metals such as Pt/Rh/ Pd to facilitate oxidation and reduction reactions, and alkali metal or alkaline earth metals compounds as the NOx absorbed component on a high surface area support material [7,8,9]. Pt-Ba/γ-Al2O3 catalyst is considered to be the most commonly studied formulation of NSR catalyst, which exhibits high NOx storage capacities in the low-temperature region within 200-400℃ [10,11]. Besides, with the addition of ceria to Pt-Ba-Al catalysts is reported to improve NOx storage capacity at low temperatures under 300℃ [12]. Shi et al. compared the NSR behaviour of Pt-Ba-Al2O3, Pt-Ba-CeO2 and Pt-Ba-CeO2(25%)/Al2O3 catalysts , and found that ceria containing catalysts could improve NOx conversions at low temperatures (200 ℃), which indicated that CeO2 could be a helpful assistant material for NOx storage under low temperature [3].

Literature reports have proved that many factors in diesel exhausts had a great impact on the properties of NOx storage and reduction. In particular, NO oxidation to NO2 over the precious metal is an essential precursor for NOx storage [13,14,15] due to that NO2 is more easily to be adsorbed on the trapping material in the form of nitrites/ nitrates [16]. Ceria has the capacity to increase mobility of O2 on the surface of catalysts and sometimes affects the oxidation performance of other elements and the incorporation of ceria are beneficial to the NO oxidation to NO2 and NO2 storage [17,18]. 6Mn10Ce/γ-Al2O3 displayed a higher oxidation activity of NO into NO2 in the temperature range of 150℃- 450℃ [19]. Various factors have positive effect on the stability of nitrates and the NOx conversion performance. It has been suggested that the stability of the nitrates is increased by the presence of O2, which otherwise are decomposed resulting in the release of NOx [20].

With the aim to investigate the properties of NOx storage under different operating conditions that close to real diesel exhausts, such as temperature, H2, CO, O2 and NO, etc. Such complex mixture would influence the optimum operation of the NSR catalysts under various operating temperature. The present study aims to understand the behavior and the influence of O2 and NO on the NOx storage capacity of the Pt-Ba-Ce/γ-Al2O3 catalyst. In the present work, the NOx storage experiments with the combined feed of NO/O2/N2 were carried out to focus on the NOx storage capacity of Pt-Ba-Ce/γ-Al2O3 under different operating conditions.

2 Materials and methods

2.1 NSR Catalyst Preparation

Pt-Ba-Ce/γ-Al2O3 catalyst used in this study was prepared by an acid-aided sol-gel method. First, the required amount of Ba (CH3COO)2, Ce(NO3)3·6H2O, and γ-Al2O3 were dissolved in deionized water, and then stirred for 30 min till to form solution. C6H8O7·H2O was added into the solution, the amount of which was twice times of Ba2+and Ce3+ molar concentration. Then polyethylene glycol, the quality of which is 10% of C 6H8O7·H2O, was introduced into the solution at 80℃ in the vigorous magnet-stirring device. After continuously stirring, the obtained transparent gel subsequently was dried as contacted with a homogenous solution of H2PtCl6·H2O, as a precursor for Pt, dried at 110℃ for 24 h, calcining then took place at 500℃ for 5 h in air. Finally, the powders of 40–60 mesh were obtained by ball milling for further experiments.

2.2 Catalyst Characterization

The XRD diffraction patterns of the samples were obtained with a Bruker D8 ADVANCE X-ray diffractometer. The samples were subjected to Cu Kα (λ= 0.154068 nm) radiation at 40 kV and 40 mA. Powder XRD patterns were recorded at 0.02° per second sampling interval in continuous scan mode from 20 to 90° of 2θ with a scanning velocity of 7°/min. The crystalline size was calculated by Scherrer’s formula D=Kλ/(βcosθ). Transmission electron microscopy (TEM) images of the catalysts were obtained on a TEM (Philips Tecnai 12) microscope operated at 120 kV. The catalyst samples were ground and dispersed in anhydrous ethanol by ultra-sonication previously. Then, the resulting suspension solution was dropped on a 200 mesh carbon-coated copper wire and sufficiently dried for TEM test.

2.3 Catalytic Activity

Catalytic activities of Pt/Ba-Ce/γ-Al2O3 measurements for NOx storage and NO temperature programmed desorption were carried out on the lab apparatus shown in Fig.1. The catalyst sample of 0.3 mL was placed in a fixed-bed quarz reactor and packed between two quarz plugs to prevent the sample blown away. The gas concentrations were continuously detected by Thermo Scientific Model 42i-HL Analyzer. Before the measurement, the catalysts were pretreated at 450℃ by imposing a feed of 1% H2 in N2 for 1h, and in N2 for 30 min separately. After cooling down to the required measurement temperatures, the gas flow (500 ppm NO,10% O2 and N2 balance) was feed by calibrated electronic mass flow controllers with a total flow rate of 300ml/min, and a space velocity of 60000 h-1. All measurements have been repeated three times to ensure the experimental errors did not exceed 3%. The NOx storage efficiency (NSE) and NOx storage capacity (NSC) were calculated according to the following formula:

Figure 1 Schematic of experimental system.
Figure 1

Schematic of experimental system.

(1)NSE=(10tsφ(NOx)dtφ(NO)ts)×100%
(2)NSC=α0ts[φ(NO)φ(NOx)]dtgcat

Where φ(NOx) was the detected NOx concentration (ppm), φ(NO) was the inlet NO concentration (ppm), ts was the storage time (s), gcat was the mass of the catalyst (g). And α was a constant that converted NSC units from ppm to μmol.

Ethical approval: The conducted research is not related to either human or animal use.

3 Results and Discussion

3.1 Catalyst characterization

The X-ray diffraction pattern of Pt/Ba-Ce/γ-Al2O3 catalyst was shown in Figure 2. As shown in Figure 2, the main

Figure 2 XRD pattern of catalyst.
Figure 2

XRD pattern of catalyst.

phases present in the sample were BaCO3 (JCPDS No. 45-1471) and γ-Al2O3 (JCPDS No. 47-1770). The main diffraction peaks at 2θ = 28.59°, 33.03°, 47.31°, and 56.03° were attributed to the cubic fluorite CeO2 (JCPDS: PDF43-1002). According to Scherrer’s formula, the crystalline size of CeO2 (111) is 26 nm. The characteristic reflections of Pt were not observed due to the small size of Pt particles in the Pt-Ba-Ce/γ-Al2O3 catalyst.

The TEM image of the Pt-Ba-Ce/γ-Al2O3 catalyst was shown in Figure 3. It’s obvious to find that the main structure of the catalyst was strip-shaped with nanoparticles. The small near-spherical particles with the sizes varying in the range of 10-20 nm (marked by the black circles) homogeneously dispersed on the surface of the catalyst, mainly were BaCO3 or CeO2 nanoparticles, which were deposited on the surface of γ-Al2O3. Besides, there are many nanoparticles small particles in the size range of 1-5 nm (marked by the red circles) might be Pt or PtOx particles.

Figure 3 TEM pattern of catalyst.
Figure 3

TEM pattern of catalyst.

3.2 Effect of Temperature on NOx storage capacity

Results on NOx storage capacity as a function of temperature over the Pt- Ba-Ce/ γ-Al2O3 catalyst were shown in Figure 4. Table 1 showed the values of NOx storage capacity, NOx storage efficiency and penetration time of the catalyst at different temperatures. It was obviously found that no NOx slip was detected at the beginning of the storage period (defined as the penetration time Tc), indicating that the NOx was completely stored at the active site of the catalyst surface during this process. The NOx slip began to gradually increase as time increased. NOx storage capacity of the catalyst was close to saturation when the detected NOx concentration reached to 500

Figure 4 NO, NO2 and NOX(NO+NO2) breakthrough profile for 30 min at different temperature with lean feed consisting of 500 ppm NO+10% O2+balance N2, 60,000 h-1.
Figure 4

NO, NO2 and NOX(NO+NO2) breakthrough profile for 30 min at different temperature with lean feed consisting of 500 ppm NO+10% O2+balance N2, 60,000 h-1.

Table1

NOX storage capacity, NOX storage efficiency, penetration time at different temperatures.

Temperature / °CNSC/μmol/gcatNSE / %Tc / min
25015222.61
30038257.02
35066999.812
40037055.36
45013820.52

ppm. Tc increased from 1 min to 12 min as the temperature increased from 250℃ to 350℃, which was shown to well correspond to the NOx storage capacity of the catalyst increased from 151.7 μmol / gcat to 668.8 μmol / gcat. The results could be attributed to the fact that the catalyst activity increased with the temperature raised in the region of low temperature, promoted the NOx storage process. However, when the temperature varied from 350-450℃, Tc decreased with the temperature increased, from 12 min to 2 min, resulting in a decrease in the NOx storage capacity of the catalyst. NOx storage capacity decreased was mainly due to an imbalance between the rates of NOx storage and nitrate decomposition. The catalyst presented the excellent NOx storage performance in high temperature range. Furthermore, the high temperature would reduce the thermal stability of the NOx stored on the surface of the catalyst to form the nitrate or nitrite, and the increased decomposition increased would lead to the NOx storage amount reduced.

3.3 Effect of O2 on NOx storage capacity

From the above analysis, Pt-Ba-Ce/γ-Al2O3 showed the supreme NOx storage performance at 350℃. In order to investigate the NOx storage capacity as a function of O2 over the Pt-Ba-Ce/γ-Al2O3 catalyst, the inlet gas was switched in the range of 0%-10% O2 at 350℃. The storage patterns of the catalyst under the process of 0% O2 to 10% O2 atmosphere at 350℃ were shown in Figure 5. As shown in Figure 5, Tc was only 1 min under 0% O2 condition. The concentration of NOx slip increased rapidly with the time, and then reached to storage saturation after about 13 min. However, the NO2 was not detected during the storage process. The results indicated that the catalyst performed poor NOx storage capacity in the absence of O2. It should be noted that the NOx concentration decreased rapidly and the NOx slip was not detected for about 10 min after switching to 10% O2 conditions, which indicated that NOx was completely stored on the catalyst surface. After about 58 min, the catalyst reached to the storage saturation state again, and simultaneously NO2 was began to be detected, indicating that NO oxidation to NO2 played the positive role in promoting the NOx storage capacity over the catalyst.

Figure 5 Adsorption patterns of catalyst under the process of 0% O 2 and 10% O2 at 350℃.
Figure 5

Adsorption patterns of catalyst under the process of 0% O 2 and 10% O2 at 350℃.

In order to further study the effect of O2 on the NOx storage performance of the catalyst, the O2 concentration in the experiments was set to 0%, 5%, 10% and 15%, respectively, and the temperature was fixed at 350℃. NOx storage capacity as a function of O2 concentrations over the Pt-Ba-Ce/γ-Al2O3 catalyst were shown in Figure 6. As shown in Figure 6, the maximum NOx storage efficiency was only 31.8% under the condition of 0% O2. When O2 concentration was increased to 5%, the penetration time was increased and the NOx slip rate decreased, and the NOx saturated storage efficiency was increased to 37.6%. When O2 concentration was 10%, the NOx penetration time increases to 12 min, and the NOx saturated storage efficiency was greatly improved to 51.6%. When O2 concentration was further increased to 15%, the NOx saturated storage efficiency was slightly reduced to 40.3%.

Figure 6 NOx (NO/NO2) breakthrough profile for 70 min at different O2 concentration with lean feed consisting of 500 ppm NO+O2+balance N2, 60,000 h-1.
Figure 6

NOx (NO/NO2) breakthrough profile for 70 min at different O2 concentration with lean feed consisting of 500 ppm NO+O2+balance N2, 60,000 h-1.

The values of NOx storage efficiency, NOx storage capacity, NO2/NOx ratio, penetration time and storage saturation time of the catalyst at different O2 concentrations were shown in Table 2. As can be seen from the Table 2, the Pt/Ba-Ce/γ-Al2O3 catalyst had the highest NOx adsorption efficiency when the O2 concentration was 10% at 350℃.

Table 2

NOx adsorption efficiency, NOX adsorption amount⍰NO2/ NOx ratio, breakthrough time and adsorption saturation time under different oxygen concentration at 350℃

O2 concentration/%NSC /μmol/gcatNSE /%Tc/min
012931.81
530237.12
1064151.612
1558640.38

3.4 Effect of NO concentrations on NOx storage capacity

NOx storage capacity as a function of NO concentrations over the Pt- Ba-Ce/ γ-Al2O3 catalyst was shown in Figure 7. As shown in Figure 7, with the increase of NO concentration, the penetration time was gradually decreased to 12 min, 3 min and 1 min, respectively. In addition, the storage saturation time was decreased respectively with the increase of NO concentration to 52 min, 33 min and 26 min, which indicated that the NOx storage efficiency was decreased as the NO concentration increased. The main reason was that the active site of NOx adsorption was constant under the different NO concentration, and then the relative NO adsorption capacity was decreased with the high NO concentration.

Figure 7 NOx (NO/NO2) breakthrough profile for 65 min at different NO concentration with lean feed consisting of 500 ppm NO+balance N2, 60,000 h-1.
Figure 7

NOx (NO/NO2) breakthrough profile for 65 min at different NO concentration with lean feed consisting of 500 ppm NO+balance N2, 60,000 h-1.

3.5 Thermal stability of stored NOx

The NO-TPD curve of the catalyst was shown in Figure 8. As shown in Figure 8, when the temperature was below 350℃, NOx was not detected, which indicated that the NOx species stored on the catalyst surface was relatively stable under the range of low temperature. When the temperature was higher than 350℃, the NOx slip began to be detected. Furthermore, as the temperature increased, the NOx slip was increased rapidly. At 450℃, the desorption peak of NO2 appeared and the desorption peak of NO took place at 470℃. Further analysis results showed that desorption peak value of NO was only 25 ppm, while the NO2 was up to 764 ppm. In addition, when the temperature was higher than 350℃, the thermal stability of the NOx stored on the surface of the catalyst was reduced. The involved overall reactions were explained with reaction (1) to (4) ⍰

Figure 8 NO - TPO profile of Pt/ Ba-Ce/γ-Al2O3 catalyst.
Figure 8

NO - TPO profile of Pt/ Ba-Ce/γ-Al2O3 catalyst.

(3)2BaO+4NO+3O22Ba(NO3)2
(4)BaO+3NO2Ba(NO3)2+NO
(5)2Ba(NO3)22BaO+4NO2+O2
(6)2Ba(NO3)22BaO+4NO+3O2

4 Conclusions

In the present work, the Pt-Ba-Ce/γ-Al2O3 catalyst was synthesized by the acid-aided sol-gel method and found to be well crystallized in the size of 5.7 nm. The active components of the catalyst were uniformly dispersed without obvious agglomeration. The catalyst had the optimal NOx storage performance at 350℃, and the maximum of NOx storage capacity is 669 μmol / gcat. In addition, with the presence of O2, NO is oxidized to NO2, promoting the catalyst on the storage performance of NOx. Moreover, the relative NO storage capacity of the catalyst was decreased on the high concentration of NO, which resulted in the NOx storage efficiency reduce. NO-TPD test results showed that when the temperature below 350℃, the NOx species stored on the catalyst surface was relatively stable. The desorption peaks of NO2 and NO appear at 450℃ and 470℃, respectively.

Acknowledgements

The authors wish to acknowledge the financial support of this research by The National Natural Science Found (51676090), and Natural Science Foundation of Jiangsu Province (BK20150513).

  1. Conflict of interest: Authors declare no conflict of interest.

References

[1] Zhang Z.S., Chen B.B., Wang X.K., et al., NOx, storage and reduction properties of model manganese-based lean NOx, trap catalysts. Applied Catalysis B Environmental, 2015, 165(165), 232-244.10.1016/j.apcatb.2014.10.001Search in Google Scholar

[2] Beñat P.A., Unai D.L.T., M. Pilar G.M., et al., Influence of ceria loading on the NOx, storage and reduction performance of model Pt–Ba/Al2O3, NSR catalyst. Catalysis Today, 2015, 241, 133-142.10.1016/j.cattod.2014.03.044Search in Google Scholar

[3] Takahashi N., Shinjoh H., Iijima T., et al., The new concept 3-way catalyst for automotive lean-burn engine: NOx storage and reduction catalyst. Catal.Today, 1996, 27(1-2), 63-69.10.1016/0920-5861(95)00173-5Search in Google Scholar

[4] Marcano S.J., Bensaid S., Deorsola F.A., et al., Multifunctional catalyst based on BaO/Pt/CeO2, for NO2 -assisted soot abatement and NOx, storage. Fuel, 2014, 149, 78-84.10.1016/j.fuel.2014.09.063Search in Google Scholar

[5] Righini L., Feng G., Lietti L., et al., Performance and properties of K and TiO2, based LNT catalysts [J]. Applied Catalysis B Environmental, 2016, 181, 862-873.10.1016/j.apcatb.2015.07.008Search in Google Scholar

[6] Choi J.S., Partridge W.P., Lance M.J., et al., Nature and spatial distribution of sulfur species in a sulfated barium-based commercial lean NOx, trap catalyst. Catalysis Today, 2010, 151(3–4), 354-361.10.1016/j.cattod.2010.01.016Search in Google Scholar

[7] Zhang Z.S., Crocker M., Chen B.B., et al.,. Pt-free, non-thermal plasma-assisted NOx, storage and reduction over M/Ba/Al2O3, (M=Mn, Fe, Co, Ni, Cu) catalysts. Catalysis Today, 2015, 256, 115-123.10.1016/j.cattod.2015.03.012Search in Google Scholar

[8] Mráček D., Kočí P., Marek M., et al., Dynamics of N2, and N2O peaks during and after the regeneration of lean NOx, trap. Applied Catalysis B Environmental, 2015, 166-167, 509-517.10.1016/j.apcatb.2014.12.002Search in Google Scholar

[9] Dupré J., Bazin P., Marie O., et al., Effects of temperature and rich-phase composition on the performance of a commercial NOx-Storage-Reduction material. Applied Catalysis B Environmental, 2016, 181, 534-541.10.1016/j.apcatb.2015.08.033Search in Google Scholar

[10] Beñat P.A., Unai D.L.T., González-Marcos M.P., et al., Influence of ceria loading on the NOx, storage and reduction performance of model Pt–Ba/Al2O3, NSR catalyst. Catalysis Today, 2015, 241, 133-142.10.1016/j.cattod.2014.03.044Search in Google Scholar

[11] Guo R.T., Chen Q.L., Ding H.L., et al., Preparation and characterization of CeOx/MnOx core–shell structure catalyst for catalytic oxidation of NO. Catalysis Communications, 2015, 69, 165-169.10.1016/j.catcom.2015.06.013Search in Google Scholar

[12] Parks J.E., Less Costly Catalysts for Controlling Engine Emission. Science, 2010, 327(5973), 1584-1585.10.1126/science.1187154Search in Google Scholar PubMed

[13] Lim C.B., Kusaba H., Einaga H., et al., Catalytic performance of supported precious metal catalysts for the combustion of diesel particulate matter. Catalysis Today, 2011, 175, 106-111.10.1016/j.cattod.2011.03.062Search in Google Scholar

[14] Karásková K., Obalová L., Jirátová K., et al., Effect of promoters in Co-Mn-Al mixed oxide catalyst on N2O decomposition. Chemical Engineering Journal, 2010, 160(2), 480-487.10.1016/j.cej.2010.03.058Search in Google Scholar

[15] Ma X., Zhang F., Xu H.M., et al., Throtteless and EGR-controlled stoichiometric combustion in a diesel-gasoline dual-fuel compression ignition engine. Fuel, 2014, 115, 765-777.10.1016/j.fuel.2013.07.052Search in Google Scholar

[16] Lietti L., Forzatti P., Nova I., et al., NOx Storage Reduction over Pt-Ba/γ-Al2O3 Catalyst. Journal of Catalysis, 2001, 204(1), 175-191.10.1006/jcat.2001.3370Search in Google Scholar

[17] Wang Q.Y., King L.Y., Miguel A., et al., Operando Raman-online FTIR investigation of ceria, vanadia/ceria and gold/ceria catalysts for toluene elimination. Journal of Catalysis, 2018, 364, 80-84.10.1016/j.jcat.2018.05.001Search in Google Scholar

[18] Unai D.L.T., Beat P.A., Onrubia J.A., et al., Effect of the Presence of Ceria in the NSR Catalyst on the Hydrothermal Resistance and Global DeNOx Performance of Coupled LNT-SCR Systems. Topics in Catalysis, 2018.Search in Google Scholar

[19] Wang P., Luo P., Yin J.C., et al., Evaluation of NO oxidation properties over a Mn-Ce/γ-Al2O3 catalyst. Journal of Nanomaterials, 2016.10.1155/2016/2103647Search in Google Scholar

[20] Fridell E., Skoglundh M., Westerberg B., et al., NOx Storage in Barium-Containing Catalysts. Journal of Catalysis, 1999, 183(2), 196-209.10.1006/jcat.1999.2415Search in Google Scholar

Received: 2018-08-17
Accepted: 2019-03-12
Published Online: 2019-12-31

© 2019 Xuedong Feng, Jing Yi, Peng Luo, published by De Gruyter

This work is licensed under the Creative Commons Attribution 4.0 Public License.

Downloaded on 21.2.2024 from https://www.degruyter.com/document/doi/10.1515/chem-2019-0153/html
Scroll to top button