Research Article | Open Access
The Cooperative Effect of In2O2 and In/HZSM-5 for Reduction of Nitric Oxide with Methane
Compared with In/HZSM-5 catalyst, In/HZSM-5/In2O3 catalyst that contained two different kinds of In induced by the impregnation and the physical mixing method, respectively, has shown remarkable activity for methane selectively catalytic reduction (CH4-SCR) of NOx. The addition of In2O3 to In/HZSM-5 could improve the NO conversion. When a little In2O3 was added to the In/HZSM-5, the active sites of InO+ which can adsorb NO2 were increased. Moreover, at the internal surface of HZSM-5, highly dispersed In2O3 species could promote oxidation of NO to NO2. The adsorption of NO2 is the key step for the whole reaction, which benefits the activation of methane and the reduction of NOx by methane. Thus the activity of In/HZSM-5/In2O3 for CH4-SCR of NOx was higher than that of In/HZSM-5.
Extensive and intensive researches have been devoted to the selective catalytic reduction (SCR) by hydrocarbons, mainly on the automobile and autocatalyst industries [1–5]. Methane, the cheapest and most abundant hydrocarbon, as reducing agent for SCR of NOx, has attracted extra attention [6–9]. Li and Armor  reviewed the active catalysts for the SCR of NOx with CH4 and concluded that Co, Cu, Ga, and In appeared to be more reactive than other transition metal ions. Kikuchi and Yogo  reported that Ga-H-ZSM-5 and In-H-ZSM-5 had high reactivity and selectivity for CH4-SCR NOx under a dry atmosphere over 500°C. They also found that In/HZSM-5 (ion exchange) and In2O3/HZSM-5 (physically mixing) were equally active in CH4-SCR of NO2 [9, 10]. They indicated that InO+ site was the important active site for In catalyst, which could chemisorb NO2 and reduce it to N2 with CH4 . They found that the addition of the novel metals especially Iridium could enhance NO conversion to N2 over In/HZSM-5 . They also thought that the addition of Ir could accelerate the oxidation of NO to NO2 and NO2 was reduced to N2 by CH4 on InO+ [11, 12]. Gutierrez et al.  reported that addition of Pd improved the catalytic performance of PdInHM with high In loadings for CH4-SCR NOx and the promoting effect of palladium on InHmordenite catalysts was very sensitive to the In/Al ratio. Richter et al.  also compared the activity of the In/H-mordenite catalysts prepared by ion exchange and impregnation, respectively, for reduction of NOx by methane. They found that the preparation method did not affect the activity and the important active site was In on ion change positions, which had strong Lewis-acid character. Requejo et al.  indicated that there existed an important fraction of the indium present in the active sample coordinated as in the In2O3 case for In-containing HZSM-5. In our previous work , we prepared the In/HZSM-5/In2O3 catalyst that contained two different kinds of In induced by impregnation and physical mixing, respectively, and investigated the activity and selectivity for the removal of NO with CH4 over this catalyst. Our results showed that the addition of In2O3 to the In/HZSM-5 catalyst could effectually improve the conversion of NO and heighten the selectivity of methane to NOx. In order to further elucidate the effect of the different In species located on the In-containing HZSM-5 catalyst, we performed more experiments and characterization tests.
2.1. Catalyst Preparation
In/HZSM-5/In2O3 catalysts with different ratios were prepared by the impregnation and physical mixing method. For the detailed descriptions about the preparation method, please see our previous paper .
2.2. Catalytic Activity Test
The reactions of the selective catalytic reduction of NOx were studied in a single-pass flow reactor (i.d., 8 mm) made of quartz. All of the catalysts were palletized, crushed, and sieved to 32–60 mesh before use. 0.8 mL catalyst was loaded for each test. A typical inlet gas composition was 2,500 ppm NO, 2,000 ppm CH4, and 2.0% O2, and helium was used as the balance gas. The flow rate was 48 mL·min−1, which equals a space velocity of 3600 h−1. All the results in this paper were obtained under similar conditions. The products were analyzed by a GC.
The NO conversion was calculated by the yield of N2, and the methane conversion was calculated by its consumption, which included the methane consumed by reacting with nitric oxide and oxygen. The extent of NOx conversion to N2 was used to evaluate the catalytic activity of a catalyst.
2.3. Catalyst Characterization
NO-TPD and NO/O2-TPD in pure He stream were measured on a flowing reaction system using a mass spectrometer (Omini-star, GSD-300) as the detector. Before adsorption the catalyst was first pretreated at 500°C in flowing He for 1 h. Then the catalyst was exposed to the gas stream of 0.2% NO or 0.2% NO–2% O2 in He at room temperature for 2 h. Then the catalyst was purged with pure He to remove the gas-phase NO and weakly adsorbed NO. Then the TPD program detected by the mass spectrometer was started from room temperature to 700°C at a ramping rate of 15°C·min−1.
Hydrogen temperature-programmed reduction (H2-TPR) in pure He stream was carried out using an Autochem 2910 (Micromeritics Co.). It was conducted according to the following steps. 100 mg fresh catalyst was loaded in a quartz reactor. Before the test, the catalyst was first pretreated at 300°C in flowing He for 1 h. When the temperature cooled close to room temperature, 10% H2/He with a total flow rate 50 mL·min−1 was used to change He. The temperature was increased linearly from room temperature to 900°C at a ramping rate of 10°C·min−1.
The temperature-programmed desorption of ammonia (NH3-TPD) was performed on the same equipment with H2-TPR. The typical experiment was performed as follows. First 150 mg catalyst was placed in the quartz reactor and pretreated at 600°C in flowing He for 1 h so as to get clean surface. Then the catalyst was cooled down to 100°C and absorbed NH3 using an injector until saturation. Then the flow of He (40 mL·min−1) passed through the sample and the temperature was increased to 700°C at a ramping rate of 20°C·min−1. The ammonia desorption curve was recorded.
3. Results and Discussion
3.1. The Effect of In2O3 on the Activity of In/HZSM-5
Figure 1 shows the conversions of NO to N2 as a function of temperature for In/HZSM-5/In2O3 catalysts with different In2O3 ratios. When a little In2O3 (In : HZSM-5 : In2O3 = 1 : 20 : 2.5) was added to In/HZSM-5 catalyst, the conversion of NO to N2 slightly changed. As the quantity of In2O3 increased to 1 : 20 : 5, the catalytic activity clearly improved compared to In/HZSM-5. When the quantity of In2O3 further increased to 1 : 20 : 10, the conversion of NO to N2 began to decrease. So here within the scope of our investigation, the optimum ratio for In/HZSM-5/In2O3 should be 1 : 20 : 5. It was considered that when increasing the content of In2O3, the surface of In/HZSM-5 was covered completely by In2O3 powders. Hence, the active site could not exhibit its catalytic activity, which resulted in the decrease of the conversion of NOx over the whole catalyst.
Figure 2 compares the activity of the catalysts before and after pretreatment with steam at 700°C for 15 hours. After presteaming, the NO conversion decreased significantly over In/HZSM-5 but had little effect on In/HZSM-5/In2O3. This indicated that the addition of In2O3 can protect the active sites and avoid the loss of activity of In/HZSM-5 under the condition of water.
3.2. Test of the Activity of the In/HZSM-5/In2O3
Figure 3 shows the changes of NO and CH4 conversions with respect to O2 concentration over In/HZSM-5 (1 : 20) and In/HZSM-5/In2O3 (1 : 20 : 5) catalyst at 400°C, respectively. The changes of the NO conversion over In/HZSM-5/In2O3 had the same trends with that over In/HZSM-5. Moreover, the difference in CH4 concentration between these two catalysts was remarkable; CH4 conversion over In/HZSM-5/In2O3 was higher than that over In/HZSM-5, although both conversions increased slightly with the increase of the O2 concentration. This implies that In2O3 addition is favorable for CH4 activation.
We also studied the effect of NO content on the conversion of CH4 over In/HZSM-5/In2O3 (1 : 20 : 5) catalyst (Figure 4). It is well known that there exist two competing reactions for the conversion of methane under our reaction conditions. They are From Figure 4 we can see that the CH4 conversion improved as the content of NO increased, which can correlate with that methane is consumed by the SCR reaction. Over In/HZSM-5/In2O3 catalyst, the enhancement of NO content can benefit the activation of methane. So for methane, it easily chooses NOx to react first when NO and O2 both exist.
Figure 5 investigates the conversion of the NO to NO2 and N2 without CH4 over In/HZSM-5/In2O3. From the results, we can find that NO easily reacts with O2 to form NO2 at low temperatures. As the temperature increased, the amount of NO2 reduced gradually, and N2 began to be produced. It means that the decomposition of NOx will happen when the temperature is up to 400°C. We also investigated the NO conversion to N2 and CH4 conversion to CO2 without O2 over In/HZSM-5/In2O3 (Figure 6). For this reaction system, CH4 combustion would be avoided. The conversion of methane is mainly denoted by the reduction of NO. The detailed reaction is shown as follows: However, from our results, the conversion of CH4 was very low, and the conversion of NO to N2 increased rapidly as the temperature increased. The production of N2 clearly does not totally come from the CH4-SCR NO. From Figure 5 we can see that direct NO decomposition partly leads to the production of N2, which became the main resource of the N2 as the temperature increased. Combined with the former results, it can be concluded that the existence of the oxygen is in favor of the activation of methane. And the reduction of NO with methane without oxygen is more difficult than that with oxygen.
3.3. NO-TPD and NO/O2-TPD
Figure 7 shows the TPD profiles in flowing He after NO adsorption over In/HZSM-5/In2O3 catalyst. One NO desorption peak appeared at low temperatures of 100°C, which could be ascribed to the weakly adsorbed NO species. The other two overlapped NO desorption peaks appeared at much higher temperatures of 330°C and 380°C, which were attributed to the strongly adsorbed NO species. Accompanying with desorption of NO, desorption of N2O and N2 was observed. Obviously, a surface decomposition reaction likely occurred over In/HZSM-5/In2O3. Desorption of oxygen and NO2 was not detected in this case. The TPD profiles after coadsorption of NO and O2 are shown in Figure 8. Similarly, the NO desorption peak at 100°C was ascribed to the weakly bounded NOx species. A continuous NO desorption started at 250°C, climbed up with temperature and gave the first maximum at 400°C, and then dropped slightly. After that, second desorption peak with a maximum at 430°C was observed. Concomitantly, O2 and NO2 desorption peaks were observed, respectively. Thus, it is suggested that NO2 is formed over the In/HZSM-5/In2O3 catalyst during NO/O2 coadsorption. Part of the NO2 is decomposed to NO and O2, corresponding to the desorption peaks of NO and O2 at 100°C, 400°C, and 430°C. From Kikuchi’s research , we know that NO2 is the key for the whole SCR reaction. Combined with the result of Figure 5, we can easily conclud that NO2 can be easily produced when NO and O2 both exist over In/HZSM-5/In2O3.
3.4. Characterization of the Catalysts
The H2-TPR results of the In/HZSM-5 and In/HZSM-5/In2O3 are shown in Figure 9. For both In-containing HZSM-5 catalysts, the reduction peak near 400°C suggests the presence of In+ at zeolite exchange positions [17, 18]. When In2O3 was added to In/HZSM-5, an obvious new peak appeared at about 600°C. Besides, there is another new peak at low temperature. To assign these peaks, we investigated the H2-TPR profile for In/HZSM-5/In2O3 with the different In2O3 addition quantities (Figure 10). When a little In2O3 is added to In/HZSM-5, the reduction peak at 600°C appears. As the quantity of the In2O3 increased to 1 : 20 : 5, the low temperature reduction peak appears. Then with the increasing of the In2O3 content, the intensity of the 600°C reduction peak increases, while the low temperature peak disappears. It has been proposed that the 400°C reduction peak is caused by the reductive exchange of In2O3 with the HZSM-5. If this sample is later oxidized, (InO)+ is produced and a subsequent TPR shows a signal shifted to lower temperature . So the low temperature peak is ascribed to easily reducible dispersed indium phases (InO)+, which is associated with the active species for the CH4-SCR of NOx [17–19]. The 600°C reduction peak can be attributed to the reduction of bulk In2O3 phase dispersed at the internal surface of HZSM-5. When the content of In2O3 exceeded to 10%, the dispersed bulk In2O3 phase began to assemble, which made zeolite pores blocked. This could explain that the intensity of low temperature peak and 400°C peak both decrease for 10% In2O3 content compared with the low loading samples. We also investigated the H2-TPR profile for the In-containing (20%: weight ratio) HZSM-5 catalysts prepared through the different methods (Figure 11). No matter which method, the reduction peak of In+ at zeolite exchanged positions and bulk In2O3 phase dispersed at the internal surface of HZSM-5 appear. But the reduction peak of (InO)+ species only appears on the H2-TPR figure of In/HZSM-5/In2O3 catalyst. In/HZSM-5/In2O3 showed the highest catalytic activity for removal of NO by methane among the three catalysts . That means (InO)+ species plays an important role for the reduction of NO by methane. A combination of the impregnation and the physical mixing preparation method can produce the optimal catalytic result.
Figure 12 shows the NH3-TPD results for different catalyst. For HZSM-5, there are two deadsorbed peaks at 200°C and 450°C, representing the two different strengths of acid center, Lewis acid site, and Brønst acid site, respectively [8, 20–22]. When 5% of In was loaded on the HZSM-5, the desorption peak at the Brønst acid site disappeared. This means that B-acid sites had been replaced by In3+ Proton for In/HZSM-5 . Requejo et al.  pointed out that In would transform the InO+ when it was added to HZSM-5, which would lead to the decrease of the Brønst acid at HZSM-5. The Kikuchi group [8, 9] indicated that the addition of In2O3 not only accelerated the formation of InO+ but also formed the oxidized In species dispersed at the internal surface of HZSM-5. Combined with the results of H2-TPR and NH3-TPD, we think that the addition of In2O3 to In/HZSM-5 catalyst will provide more InO+ sites and also contribute to the In2O3 high dispersion at the internal surface of HZSM-5.
NO2 has been proposed by other researchers to be an important intermediate in the reaction scheme of SCR of NOx to N2 [12, 24]. For In/HZSM-5 catalyst, the adsorption of NO2 on the InO+ sites was thought to be the key step for this reaction [10–12, 23, 25]. For NO and NO2, methane preferred to react with NO2 other than NO . This also can be explained by the results of Figures 5 and 9. Lónyi et al. [7, 26, 27] pointed out that the selective catalytic reduction of NO by methane required two catalytic functions: one for initiating the oxidation of NO to NO2 (NO-COX) and another for the N2-forming reaction (CH4/NO-SCR). The NO-COX reaction proceeded over the Brønst acid sites and, if present, also over metal-oxide clusters. So we consider that the addition of a little In2O3 to In/HZSM-5 can accelerate the dispersion of the oxide In species at the internal surface of HZSM-5 and raise the InO+ active sites. In2O3 highly dispersed at the internal surface of HZSM-5 would benefit the oxidation of NO to NO2. Results of Figures 5 and 9 show that NO2 can be easily produced when NO and O2 both exist over In/HZSM-5/In2O3. On the other hand, the increasing of InO+ active sites contributed to the adsorption of NO2 on the catalyst; the adsorption of NO2 further enhanced the activation of methane. So the possible pathway of NO reduction by methane over In/HZSM-5/In2O3 was suggested as Figure 13.
In conclusion, In/HZSM-5/In2O3 (1 : 20 : 5) catalyst that contained two different kinds of In induced by the impregnation and the physical mixing method, respectively, had shown high catalytic performance for the CH4-SCR of NOx compared with In/HZSM-5. The addition of In2O3 promoted the reaction of NO to NO2 and increased the formation of InO+ active sites of the In/HZSM-5. Thus, the adsorption of NO2 on the catalyst was accelerated. Adsorbed NO2 further enhanced the activation of methane. Consequently, the activity of the In/HZSM-5 for removal of NO with methane was increased.
Conflict of Interests
The author declares that there is no conflict of interests regarding the publication of this paper.
This work was financially supported by the National Natural Science Foundation of China (21206018) and the Foundation of Ministry of Education of China (11YJCZH139).
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