Research Article | Open Access
Lili Lei, Chuan Sun, Bo Zhang, Wenye Gu, "A Simulation Study of Effect of Mn-Ce/γ-Al2O3 on NOx Storage and Reduction over Pt-Ce-Ba/γ-Al2O3 Catalysts", Advances in Materials Science and Engineering, vol. 2016, Article ID 8594049, 6 pages, 2016. https://doi.org/10.1155/2016/8594049
A Simulation Study of Effect of Mn-Ce/γ-Al2O3 on NOx Storage and Reduction over Pt-Ce-Ba/γ-Al2O3 Catalysts
A series of Pt-Ce-Ba/γ-Al2O3 and Mn-Ce/γ-Al2O3 catalysts were synthesized by a sol-gel method and the samples were characterized by XRD, SEM, and EDS. The effect of Mn-Ce/γ-Al2O3 on the storage and reduction of NOx over Pt-Ce-Ba/γ-Al2O3 catalysts was studied in a fix-bed reactor with simulation gases NO, O2, and N2. The results indicated that NO oxidation to NO2 was reduced with the increase of inlet NO concentration, which was up to 83% when the concentration of NO was 500 ppm but reduced to 76% with the concentration of NO increasing to 1000 ppm. Comparing with the Pt-Ce-Ba/γ-Al2O3 catalysts, the rate of NOx storage and reduction was remarkably increased over Pt-Ce-Ba/γ-Al2O3 combined with Mn-Ce/γ-Al2O3 catalysts. However, the reductant used for NOx reduction reaction over Pt-Ce-Ba/γ-Al2O3 catalysts was consumed under the treatment of Mn-Ce/γ-Al2O3 catalyst, which caused the NOx conversion to obviously drop, but the rate of NOx absorption declined slightly.
Nitrogen oxides emissions as the main pollutants from diesel engine have attracted much attention due to their harmful effect on human health and the environment. The regulations on diesel NOx emissions have become more stringent and the efforts to reduce the NOx emissions still remain a challenging topic. NOx storage and reduction (NSR) is regarded as one of the most practical technologies to remove NOx from diesel engines. Diesel NOx emissions typically contain more than 90% NO and less than 10% NO2. Compared with NO, NO2 is more promptly to be absorbed on storage components contained in NSR catalyst [1, 2]. NO oxidation to NO2 is a critical chemical reaction step and benefit for promoting the NOx storage and reduction over NSR catalyst. Diesel oxidation catalyst (DOC) exhibits good activity for NO oxidation to NO2. According the oxide catalysts reported, Mn-Ce mixed oxides have evidenced high activity for NO oxidation, which has remarkable effects on the storage and reduction of NOx emissions.
Pt-Ba  mixed oxides are the components mainly used for NOx storage and reduction, and the sequence capacity of NOx storage is BaO > Ba(OH)2 > BaCO3. It was also reported that Mn-Ce mixed oxides catalysts showed a higher NOx storage activity when the content of BaO is within the range of 14~23 (wt%) [4–8]. CeOx based oxides have been reported to have high activity for NOx storage and regeneration of NSR catalyst at low temperature . Pt-Me/Al2O3 (Me = Ba, Ce, Cu) oxides catalysts have high NOx storage and reduction activity due to the NO conversion promptly improved by CeOx oxides . The Mn-Ce based oxides usually have much higher NO oxidation because of the improving of oxidation activity due to the addition of Ce element . Furthermore, the contents of MnOx-CeO2 as main components of catalyst also have strong effect on NO oxidation; for example, among all the catalysts, MnOx (0.4)-CeO2 shows the highest NO oxidation activity and NO conversion reaches up to 60% at 250°C . For these reasons, it is necessary to develop additives that has the better oxidation activity for NO oxidation combined with NSR catalyst to store and reduce NOx emissions.
The objective of this paper is to evaluate the effect of Mn-Ce/γ-Al2O3 catalyst on storage and reduction of NOx over Pt-Ce-Ba/γ-Al2O3 catalyst, which is the combination of a DOC catalyst and a NSR catalyst. A series of NSR catalysts capable of NOx storage and reduction combined with DOC catalysts were prepared by a sol-gel method that involves the addition of Pt, Ce, Mn, and Ba to Al2O3. The activities for NO oxidation to NO2 were measured by temperature programmed reactions. And the simulation experiments were carried out to investigate the storage and reduction of NOx with and without Mn-Ce/γ-Al2O3 catalyst combined with Pt-Ce-Ba/γ-Al2O3 catalyst.
2. Experimental Section
2.1. Catalyst Formulation and Characterization
The Mn-Ce/γ-Al2O3 and Pt-Ce-Ba/γ-Al2O3 catalysts used in this work have been prepared by the sol-gel method. The required amounts of Ce(NO3)3·6H2O, C4H6MnO4·4H2O, and γ-Al2O3 were dissolved in the solution containing deionized water, citric acid, and polyethylene glycol with a mass ratio Mn/Ce 6 : 10, forming homogenous and transparent solution. Stirred at 80°C until transparent gel was obtained, then dried at 110°C for 24 h, decomposed at 300°C for 1 h, and calcined at 500°C for 5 h in air. The catalyst prepared by loading only Pt, Ce, and Ba on γ-Al2O3 support is referred to as ()Pt()Ce() Ba/γ-Al2O3, where , , and () mean the weight percent (wt%) of each component that was loaded in the catalyst. The Mn/Ce/Ba mixed oxides reference catalyst was introduced in detail as previously reported. Catalyst powders were characterized by means of an X-ray diffractometer (Bruker D8 Advance) using Cu Kα radiation (λ = 0.154068 nm), operating at 40 kV and 40 mA and at a scanning rate of 7°/min, in a 2θ ranging from 20° to 80° in order to evaluate the achievement of the desired oxides. Field emission scanning electron microscopy (FESEM, JEOL JSM-7001F) equipped with an Oxford Instruments’ INCA system was utilized to investigate the morphology and the chemical components of the catalyst samples.
2.2. Experimental Methods
NO to NO2 oxidation activity experiments were performed and NOx storage and reduction capacity was evaluated in a fix-bed reactor as shown in Figure 1, composed of simulation gas and control unit, reactor, and NOx analyzer.
The measurements were then carried out by exposing the catalyst to flowing gas containing 500–1000 ppm NO, 10% O2, and balance N2. The NO and NO2 concentration were followed by the NOx analyzer. A typical NOx storage and reduction capacity test was carried out by performing several lean-rich cycles to obtain a steady state working condition for the catalyst, when the catalyst was completely saturated with NOx. Then a continuous lean flow was admitted to the reactor to evaluate the stored NOx amounts. Measurements were carried out from 150 to 400°C in 50°C steps. A flow of 1000 ppm NO, 10% O2, balanced by N2 was fed during the lean phase, while a flow of 1% H2, balanced by N2 was fed during the rich phase. All flow conditions were operated at a gas hourly space velocity (GHSV) of 50,000 h−1.
3. Results and Discussion
X-ray diffractometer of Pt-Ce-Ba/γ-Al2O3 catalysts is shown in Figure 2. The characteristic reflections of Pt and PtOx were not observed due to the low concentration of Pt in the catalysts and/or the small size of the Pt particles. X-ray diffraction (XRD) analysis showed that the main phases present in Pt-Ce-Ba/γ-Al2O3 were r-Al2O3 (PDF No. 48-0366), CeO2 (PDF No. 001-0800) located at 28.68° and 47.02°, and BaO (PDF No. 26-0178) located at 28.68° and 66.80°, thereby confirming the decomposition of Ce(NO3)3 and Ba(NO2)2 into crystalline CeO2 during catalyst calcination [13–16]. In addition, the BaCO3 phases are also observed as shown in the mark () of Figure 2, indicating that Ba(NO2)2 converted to BaCO3 during calcination, in accordance with the results of Kwak et al. , in which the crystalline form of BaCO3 was present as witherite in the catalysts. Therefore, the 0.855Pt10Ce15Ba/γ-Al2O3 (denote as Pt-Ce-Ba/γ-Al2O3) catalyst is chosen as further measurement and evaluation in this work due to its lowest BaCO3 content contained.
Figure 3 shows the SEM image and EDS spectrum of Pt-Ce-Ba/γ-Al2O3 catalyst. As shown in Figure 3(a), the catalyst presents the small strip microstructure morphology formed by Pt and/or PtOx and other irregular agglomerates mainly composed of CeOx or BaO obtained through the reaction between Pt and Ce oxides, while some larger particles could be observed over the catalyst sample, probably caused by accumulation of γ-Al2O3 with CeOx or BaO. EDS spectra show the presence of various spectra of the catalyst. Each element is identified by the presence of peaks at the characteristic spectrum. As seen in Figure 3(b), strong signals for C and Al elements were detected. Weak signals for Ba, Ce, and P have also been detected. C element was mainly coming from the residual carbon-containing reagents.
Figure 4 shows the variation of NO conversion as a function of catalyst temperature over Mn-Ce/γ-Al2O3 catalyst under 500 ppm, 750 ppm, and 1000 ppm NO, respectively. It can be seen from the figure that NO conversion increases gradually with the rise of temperature in the range of 150–300°C and then decreases after 300°C under each different inlet NO concentration, which can be explained by the fact that partial NO2 will decompose into NO at high temperature. Additionally, NO conversion decreases with the increase of inlet NO concentration. The total NO conversion is higher under the inlet NO concentration 500 ppm as compared to that of 750 and 1000 ppm NO, which increases from 33% at 150°C to maximum 82% at 300°C. It should be noticed that the difference between NO conversion under different inlet NO concentration experiences little change in the range of 350°C to 450°C, which indicates that the temperature may be responsible for NO oxidation when the temperature is high but not the inlet NO concentration.
Figure 5 shows the variation in NO, NO2, and NOx concentration as a function of the time over Pt-Ce-Ba/γ-Al2O3 catalyst with and without the Mn-Ce/γ-Al2O3 catalyst. It can be clearly seen that the NO, NO2, and NOx concentration rapidly increase in the initial absorption stage before 1600 seconds and remain nearly constant between 1600 and 3600 seconds over Pt-Ce-Ba/γ-Al2O3 catalyst. This means that the process from adsorption starting to fully saturated adsorption lasted 1600 seconds. And in the period of rich condition, NOx concentration decreases remarkably by the reductant reaction with H2. In comparison to that with Mn-Ce/γ-Al2O3 catalyst, NO, NO2, and NOx concentration increase occurs somewhat slowly and then becomes very fast between 1600 and 2000 seconds and after that remains the same general trend as the previous analysis. This means it would take a shorter time up to around 2000 seconds for NO and NO2 adsorbent to be completely saturated, which is caused by the activity of oxidation NO to NO2 on Mn-Ce/γ-Al2O3 catalyst. The present results prove that the oxidation activity of Mn-Ce/γ-Al2O3 catalyst can effectively improve the NOx adsorption rate, but the value of desorption peaks is lower in comparison with the treatment of Pt-Ce-Ba/γ-Al2O3 catalyst alone. Therefore, it is difficult to discuss the results only as caused by differences in NO oxidation to NO2 with oxidation catalyst.
(b) Mn-Ce/γ-Al2O3 + Pt-Ce-Ba/γ-Al2O3
Figure 6 shows NOx storage and reduction experiments carried out for Pt-Ce-Ba/γ-Al2O3 catalyst with and without Mn-Ce/γ-Al2O3 catalyst samples. Comparing the different experiments, it can be noticed that the rise rate and the peak value of NOx concentration in the process of desorption reaction were higher and both obviously improved with the Mn-Ce/γ-Al2O3 catalyst, attributed by more heat release from the H2 oxidation reaction partly consumed by Mn-Ce/γ-Al2O3 catalyst. As can be observed in Figure 6(b), there are more increase of NO concentration and decrease of NO2 concentration during the process of desorption. This may suggest that NH3 was formed during the reaction between H2 and Ce(NO3)4 contained in the Mn-Ce/γ-Al2O3 catalyst as shown in reaction (1) [18–20], followed by NO formation where O2 was available for reacting with NH3 as shown in reaction (2). In addition, more heat released by the oxidation between H2 and O2 will result in the decomposition of nitrate and nitrite oxides contained in Pt-Ce-Ba/γ-Al2O3 catalyst. And this decomposition may also contribute to remarkably improving the NO concentration in the NOx desorption stage. However, H2 consumed by the oxidation over Mn-Ce/γ-Al2O3 catalyst resulted in the decrease of NO2 concentration. More importantly, this reduction was relevant with respect to the total NO2 concentration needed for NOx desorption, which had negligible effect on the rate of decomposition reaction.
(b) Mn-Ce/γ-Al2O3 + Pt-Ce-Ba/γ-Al2O3
Lower NOx conversion and less NOx storage capacity with Mn-Ce/γ-Al2O3 catalyst can be clearly seen in Figure 6(b) compared to Figure 6(a). As clearly seen in Figure 6(b) compared to Figure 6(a), the NOx conversion is much smaller and at the same time the NOx storage capacity experiences less decline. The main reason is that H2 consumption by oxidation over Mn-Ce/γ-Al2O3 catalyst will lead to the incomplete desorption of NOx; then the NOx conversion will be reduced. It is worth noticing that the rate of NOx storage and reduction is improved by the heat released from the Mn-Ce/γ-Al2O3 catalyst, which is also benefit for making up for NOx storage capacity.
A series of Pt-Ce-Ba/γ-Al2O3 and Mn-Ce/γ-Al2O3 catalysts were prepared and found to exhibit small strip microstructure morphology mainly composed of CeOx or BaO. The behavior of Pt-Ce-Ba/γ-Al2O3 catalyst was investigated at different lean-rich cycles in NOx storage and reduction process with and without Mn-Ce/γ-Al2O3 catalyst through simulation experiments. The inlet NO concentration increase was not benefit for NO conversion in the range of low temperature, which will has no effect on NO conversion above high temperature. Compared with Pt-Ce-Ba/γ-Al2O3 catalyst treatment alone, the rate of NOx storage and reduction both was remarkably increased. The results indicate that Mn-Ce/γ-Al2O3 catalyst can effectively improve the activity of Pt-Ce-Ba/γ-Al2O3 catalyst, especially in the low temperature. However, it is still probable that the reducing agent as H2 will be consumed by Mn-Ce/γ-Al2O3 catalyst, which will lead the NOx conversion to decrease to a certain extent.
The authors declare that there is no conflict of interests regarding the publication of this paper.
Financial support of this research was provided by The National Natural Science Foundation (51206068), Natural Science Foundation of Jiangsu Province (BK2015040369), and State Key Laboratory of Engines, Tianjin University (K15-007).
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