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/γAl2O3 catalyst, which caused the NOx conversion to obviously drop, but the rate of NOx absorption declined slightly.


Introduction
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 NO x emissions have become more stringent and the efforts to reduce the NO x emissions still remain a challenging topic.NO x storage and reduction (NSR) is regarded as one of the most practical technologies to remove NO x from diesel engines.Diesel NO x emissions typically contain more than 90% NO and less than 10% NO 2 .Compared with NO, NO 2 is more promptly to be absorbed on storage components contained in NSR catalyst [1,2].NO oxidation to NO 2 is a critical chemical reaction step and benefit for promoting the NO x storage and reduction over NSR catalyst.Diesel oxidation catalyst (DOC) exhibits good activity for NO oxidation to NO 2 .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 NO x emissions.
Pt-Ba [3] mixed oxides are the components mainly used for NO x storage and reduction, and the sequence capacity of NO x storage is BaO > Ba(OH) 2 > BaCO 3 .It was also reported that Mn-Ce mixed oxides catalysts showed a higher NO x storage activity when the content of BaO is within the range of 14∼23 (wt%) [4][5][6][7][8].CeO x based oxides have been reported to have high activity for NO x storage and regeneration of NSR catalyst at low temperature [9].Pt-Me/Al 2 O 3 (Me = Ba, Ce, Cu) oxides catalysts have high NO x storage and reduction activity due to the NO conversion promptly improved by CeO x oxides [10].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 [11].Furthermore, the contents of MnO x -CeO 2 as main components of catalyst also have strong effect on NO oxidation; for example, among all the catalysts, MnO x (0.4)-CeO 2 shows the highest NO oxidation activity and NO conversion reaches up to 60% at 250 ∘ C [12].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 NO x emissions.
The objective of this paper is to evaluate the effect of Mn-Ce/-Al 2 O 3 catalyst on storage and reduction of NO x over Pt-Ce-Ba/-Al 2 O 3 catalyst, which is the combination of a DOC catalyst and a NSR catalyst.A series of NSR catalysts capable of NO x 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 Al 2 O 3 .The activities  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 K radiation ( = 0.154068 nm), operating at 40 kV and 40 mA and at a scanning rate of 7 ∘ /min, in a 2 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.

Experimental Methods.
NO to NO 2 oxidation activity experiments were performed and NO x 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 NO x analyzer.The measurements were then carried out by exposing the catalyst to flowing gas containing 500-1000 ppm NO, 10% O 2 , and balance N 2 .The NO and NO 2 concentration were followed by the NO x analyzer.A typical NO x 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 NO x .Then a continuous lean flow was admitted to the reactor to evaluate the stored NO x amounts.Measurements were carried out from 150 to 400 ∘ C in 50 ∘ C steps.A flow of 1000 ppm NO, 10% O 2 , balanced by N 2 was fed during the lean phase, while a flow of 1% H 2 , balanced by N 2 was fed during the rich phase.All flow conditions were operated at a gas hourly space velocity (GHSV) of 50,000 h −1 .

Results and Discussion
X-ray diffractometer of Pt-Ce-Ba/-Al 2 O 3 catalysts is shown in Figure 2. The characteristic reflections of Pt and PtO x were not observed due to the low concentration of Pt in the catalysts and/or the small size of the Pt particles.Xray diffraction (XRD) analysis showed that the main phases present in Pt-Ce-Ba/-Al   [ [13][14][15][16].In addition, the BaCO 3 phases are also observed as shown in the mark ( * ) of Figure 2, indicating that Ba(NO 2 ) 2 converted to BaCO 3 during calcination, in accordance with the results of Kwak et al. [17], in which the crystalline form of BaCO 3 was present as witherite in the catalysts.Therefore, the 0.855Pt10Ce15Ba/-Al 2 O 3 (denote as Pt-Ce-Ba/-Al 2 O 3 ) catalyst is chosen as further measurement and evaluation in this work due to its lowest BaCO 3 content contained.
Figure 3 shows the SEM image and EDS spectrum of Pt-Ce-Ba/-Al 2 O 3 catalyst.As shown in Figure 3 Figure 4 shows the variation of NO conversion as a function of catalyst temperature over Mn-Ce/-Al 2 O 3 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 NO 2 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, NO 2 , and NO   seconds and remain nearly constant between 1600 and 3600 seconds over Pt-Ce-Ba/-Al 2 O 3 catalyst.This means that the process from adsorption starting to fully saturated adsorption lasted 1600 seconds.And in the period of rich condition, NO x concentration decreases remarkably by the reductant reaction with H 2 .In comparison to that with Mn-Ce/-Al 2 O 3 catalyst, NO, NO 2 , and NO x 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 NO 2 adsorbent to be completely saturated, which is caused by the activity of oxidation NO to NO 2 on Mn-Ce/-Al 2 O 3 catalyst.The present results prove that the oxidation activity of Mn-Ce/-Al 2 O 3 catalyst can effectively improve the NO x adsorption rate, but the value of desorption peaks is lower in comparison with the treatment of Pt-Ce-Ba/-Al 2 O 3 catalyst alone.Therefore, it is difficult to discuss the results only as caused by differences in NO oxidation to NO 2 with oxidation catalyst.Figure 6 shows NO x storage and reduction experiments carried out for Pt-Ce-Ba/-Al 2 O 3 catalyst with and without Mn-Ce/-Al 2 O 3 catalyst samples.Comparing the different experiments, it can be noticed that the rise rate and the peak value of NO x concentration in the process of desorption reaction were higher and both obviously improved with the Mn-Ce/-Al 2 O 3 catalyst, attributed by more heat release from the H 2 oxidation reaction partly consumed by Mn-Ce/-Al 2 O 3 catalyst.As can be observed in Figure 6(b), there are more increase of NO concentration and decrease of NO 2 concentration during the process of desorption.This may suggest that NH 3 was formed during the reaction between H 2 and Ce(NO 3 ) 4 contained in the Mn-Ce/-Al 2 O 3 catalyst as shown in reaction (1) [18][19][20], followed by NO formation where O 2 was available for reacting with NH 3 as shown in reaction (2).In addition, more heat released by the oxidation between H 2 and O 2 will result in the decomposition of nitrate and nitrite oxides contained in Pt-Ce-Ba/-Al 2 O 3 catalyst.And this decomposition may also contribute to remarkably improving the NO concentration in the NO x desorption stage.However, H 2 consumed by the oxidation over Mn-Ce/-Al 2 O 3 catalyst resulted in the decrease of NO 2 concentration.More importantly, this reduction was relevant with respect to the total NO 2 concentration needed for NO x desorption, which had negligible effect on the rate of decomposition reaction.
Lower NO x conversion and less NO x storage capacity with Mn-Ce/-Al 2 O 3 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 NO x conversion is much smaller and at the same time the NO x storage capacity experiences less decline.The main reason is that H 2 consumption by oxidation over Mn-Ce/-Al 2 O 3 catalyst will lead to the incomplete desorption of NO x ; then the NO x conversion will be reduced.It is worth noticing that the rate of NO x storage and reduction is improved by the heat released from the Mn-Ce/-Al 2 O 3 catalyst, which is also benefit for making up for NO x storage capacity.

Conclusion
A series of Pt-Ce-Ba/-Al 2 O 3 and Mn-Ce/-Al 2 O 3 catalysts were prepared and found to exhibit small strip microstructure morphology mainly composed of CeO x or BaO.The behavior of Pt-Ce-Ba/-Al 2 O 3 catalyst was investigated at different lean-rich cycles in NO x storage and reduction process with and without Mn-Ce/-Al 2 O 3 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/-Al 2 O 3 catalyst treatment alone, the rate of NO x storage and reduction both was remarkably increased.The results indicate that Mn-Ce/-Al 2 O 3 catalyst can effectively improve the activity of Pt-Ce-Ba/-Al 2 O 3 catalyst, especially in the low temperature.However, it is still probable that the reducing agent as H 2 will be consumed by Mn-Ce/-Al 2 O 3 catalyst, which will lead the NO x conversion to decrease to a certain extent.

Figure 4 :
Figure 4: Variation of NO conversion with temperature over Mn-Ce/-Al 2 O 3 catalyst.

3 Time
x concentration as a function of the time over Pt-Ce-Ba/-Al 2 O 3 catalyst with and without the Mn-Ce/-Al 2 O 3 catalyst.It can be clearly seen that the NO, NO 2 , and NO x concentration rapidly increase in the initial absorption stage before 1600 Pt-Ce-Ba/-Al 2 O Mn-Ce/-Al 2 O 3 + Pt-Ce-Ba/-Al 2 O 3

Figure 5 :
Figure 5: Storage and reduction process of NSR catalyst and DOC combined with NSR catalyst technology.

Figure 6 :
Figure 6: Storage-reduction process of (a) NSR catalysts and (b) DOC combined NSR technology.