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

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℃.


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 O 2 in the diesel exhausts renders the classical three-way catalysts, which are effective only under stoichiometric conditions, inefficient in reducing NO x emissions in the lean-burn engine exhaust [2]. NO x storage and reduction (NSR) technology has considered to be a particularly means to reduce NO x from diesel engines. The principle of NSR is that NO x stored on the catalyst as nitrates or nitrites when O 2 is in excess. Under rich conditions, with the presence of excessive reductants, NO x reduction reactions take place [3,4].
Many studies have focused on the optimization of the NSR catalyst for NO x 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 NO x absorbed component on a high surface area support material [7,8,9]. Pt-Ba/γ-Al 2 O 3 catalyst is considered to be the most commonly studied formulation of NSR catalyst, which exhibits high NO x storage capacities in the lowtemperature region within 200-400℃ [10,11]. Besides, with the addition of ceria to Pt-Ba-Al catalysts is reported to improve NO x storage capacity at low temperatures under 300℃ [12]. Shi et al. compared the NSR behaviour of Pt-Ba-Al 2 O 3 , Pt-Ba-CeO 2 and Pt-Ba-CeO 2 (25%)/Al 2 O 3 catalysts , and found that ceria containing catalysts could improve NO x conversions at low temperatures (200 ℃), which indicated that CeO 2 could be a helpful assistant material for NO x storage under low temperature [3].
Literature reports have proved that many factors in diesel exhausts had a great impact on the properties of NO x storage and reduction. In particular, NO oxidation to NO 2 over the precious metal is an essential precursor for NO x storage [13,14,15] due to that NO 2 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 O 2 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 NO 2 and NO 2 storage [17,18]. 6Mn10Ce/γ-Al 2 O 3 displayed a higher oxidation activity of NO into NO 2 in the temperature range of 150℃-450℃ [19]. Various factors have positive effect on the stability of nitrates and the NO x conversion performance. It has been suggested that the stability of the nitrates is increased by the presence of O 2 , which otherwise are decomposed resulting in the release of NO x [20].
With the aim to investigate the properties of NO x storage under different operating conditions that close to real diesel exhausts, such as temperature, H 2 , CO, O 2 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 O 2 and NO on the NO x storage capacity of the Pt-Ba-Ce/γ-Al 2 O 3 catalyst. In the present work, the NO x storage experiments with the combined feed of NO/O 2 /N 2 were carried out to focus on the NO x storage capacity of Pt-Ba-Ce/γ-Al 2 O 3 under different operating conditions.

NSR Catalyst Preparation
Pt-Ba-Ce/γ-Al 2 O 3 catalyst used in this study was prepared by an acid-aided sol-gel method. First, the required amount of Ba (CH 3 COO) 2 , Ce(NO 3 ) 3 ·6H 2 O, and γ-Al 2 O 3 were dissolved in deionized water, and then stirred for 30 min till to form solution. C 6 H 8 O 7 ·H 2 O 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 6 H 8 O 7 ·H 2 O, 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 H 2 PtCl 6 ·H 2 O, 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.

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.

Catalytic Activity
Catalytic activities of Pt/Ba-Ce/γ-Al 2 O 3 measurements for NO x 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% H 2 in N 2 for 1h, and in N 2 for 30 min separately. After cooling down to the required measurement temperatures, the gas flow (500 ppm NO,10% O 2 and N 2 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 NO x storage efficiency (NSE) and NO x storage capacity (NSC) were calculated according to the following formula: Where φ(NO x ) was the detected NO x concentration (ppm), φ(NO) was the inlet NO concentration (ppm), t s 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.

Catalyst characterization
The X-ray diffraction pattern of Pt/Ba-Ce/γ-Al 2 O 3 catalyst was shown in Figure 2. As shown in Figure 2 The TEM image of the Pt-Ba-Ce/γ-Al 2 O 3 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 BaCO 3 or CeO 2 nanoparticles, which were deposited on the surface of γ-Al 2 O 3 . Besides, there are many nanoparticles small particles in the size range of 1-5 nm (marked by the red circles) might be Pt or PtO x particles.

Effect of Temperature on NO x storage capacity
Results on NO x storage capacity as a function of temperature over the Pt-Ba-Ce/γ-Al 2 O 3 catalyst were shown in Figure 4. Table 1 showed the values of NO x storage capacity, NO x storage efficiency and penetration time of the catalyst at different temperatures. It was obviously found that no NO x slip was detected at the beginning of the storage period (defined as the penetration time Tc), indicating that the NO x was completely stored at the active site of the catalyst surface during this process. The NO x slip began to gradually increase as time increased. NO x storage capacity of the catalyst was close to saturation when the detected NO x concentration reached to 500  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 NO x 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 NO x 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 NO x storage capacity of the catalyst. NO x storage capacity decreased was mainly due to an imbalance between the rates of NO x storage and nitrate decomposition. The catalyst presented the excellent NO x storage performance in high temperature range. Furthermore, the high temperature would reduce the thermal stability of the NO x stored on the surface of the catalyst to form the nitrate or nitrite, and the increased decomposition increased would lead to the NO x storage amount reduced.

Effect of O 2 on NO x storage capacity
From the above analysis, Pt-Ba-Ce/γ-Al 2 O 3 showed the supreme NO x storage performance at 350℃. In order to investigate the NO x storage capacity as a function of O 2 over the Pt-Ba-Ce/γ-Al 2 O 3 catalyst, the inlet gas was switched in the range of 0%-10% O 2 at 350℃. The storage patterns of the catalyst under the process of 0% O 2 to 10% O 2 atmosphere at 350℃ were shown in Figure 5. As shown in Figure 5, Tc was only 1 min under 0% O 2 condition. The concentration of NO x slip increased rapidly with the time, and then reached to storage saturation after about 13 min. However, the NO 2 was not detected during the storage process. The results indicated that the catalyst performed poor NO x storage capacity in the absence of O 2 . It should be noted that the NO x concentration decreased rapidly and the NO x slip was not detected for about 10 min after switching to 10% O 2 conditions, which indicated that NO x was completely stored on the catalyst surface. After about 58 min, the catalyst reached to the storage saturation state again, and simultaneously NO 2 was began to be detected, indicating that NO oxidation to NO 2 played the positive role in promoting the NO x storage capacity over the catalyst.
In order to further study the effect of O 2 on the NO x storage performance of the catalyst, the O 2 concentration in the experiments was set to 0%, 5%, 10% and 15%,   respectively, and the temperature was fixed at 350℃. NO x storage capacity as a function of O 2 concentrations over the Pt-Ba-Ce/γ-Al 2 O 3 catalyst were shown in Figure 6. As shown in Figure 6, the maximum NO x storage efficiency was only 31.8% under the condition of 0% O 2 . When O 2 concentration was increased to 5%, the penetration time was increased and the NO x slip rate decreased, and the NO x saturated storage efficiency was increased to 37.6%. When O 2 concentration was 10%, the NO x penetration time increases to 12 min, and the NO x saturated storage efficiency was greatly improved to 51.6%. When O 2 concentration was further increased to 15%, the NO x saturated storage efficiency was slightly reduced to 40.3%.
The values of NO x storage efficiency, NO x storage capacity, NO 2 /NO x ratio, penetration time and storage saturation time of the catalyst at different O 2 concentrations were shown in Table 2. As can be seen from the Table 2, the Pt/Ba-Ce/γ-Al 2 O 3 catalyst had the highest NO x adsorption efficiency when the O 2 concentration was 10% at 350℃.

Effect of NO concentrations on NO x storage capacity
NO x storage capacity as a function of NO concentrations over the Pt-Ba-Ce/γ-Al 2 O 3 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 NO x storage efficiency was decreased as the NO concentration increased. The main

Thermal stability of stored NO x
The NO-TPD curve of the catalyst was shown in Figure 8. As shown in Figure 8, when the temperature was below 350℃, NO x was not detected, which indicated that the NO x species stored on the catalyst surface was relatively stable under the range of low temperature. When the temperature was higher than 350℃, the NO x slip began to be detected. Furthermore, as the temperature increased, the NO x slip was increased rapidly. At 450℃, the desorption peak of NO 2 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 NO 2 was up to 764 ppm. In addition, when the temperature was higher than 350℃, the thermal stability of the NO x stored on the surface of the catalyst was reduced. The involved overall reactions were explained with reaction (1)

Conclusions
In the present work, the Pt-Ba-Ce/γ-Al 2 O 3 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 NO x storage performance at 350℃, and the maximum of NO x storage capacity is 669 μmol / gcat. In addition, with the presence of O 2 , NO is oxidized to NO 2 , promoting the catalyst on the storage performance of NO x . Moreover, the relative NO storage capacity of the catalyst was decreased on the high concentration of NO, which resulted in the NO x storage efficiency reduce. NO-TPD test results showed that when the temperature below 350℃, the NO x species stored on the catalyst surface was relatively stable. The desorption peaks of NO 2 and NO appear at 450℃ and 470℃, respectively.