Determination of Intermediates and Mechanism for Soot Combustion

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Environ. Sci. Technol. 2010, 44, 8254–8258

Determination of Intermediates and Mechanism for Soot Combustion with NOx/O2 on Potassium-Supported Mg-Al Hydrotalcite Mixed Oxides by In Situ FTIR Z H A O L I A N G Z H A N G , * ,† Y E X I N Z H A N G , †,‡ QINGYUN SU,‡ ZHONGPENG WANG,† QIAN LI,† AND XIYAN GAO‡ College of Chemistry and Chemical Engineering, University of Jinan, 106 Jiwei Rd., Jinan 250022, P. R. China, and Liaoning key laboratory of Internal Combustion Engines, Institute of Internal Combustion Engine, Dalian University of Technology, 2 Linggong Rd., Dalian, 116024, P. R. China

Received July 13, 2010. Revised manuscript received September 4, 2010. Accepted September 23, 2010.

The soot combustion with NOx and/or O2 on potassiumsupported Mg-Al hydrotalcite mixed oxides under tight contact condition was studied using temperature-programmed oxidation (TPO), isothermal reaction and in situ FTIR techniques. The presence of NOx in O2 favors the soot combustion at lower temperatures (300 °C), which was accompaniedbyasubstantialNOx reduction.Theketene(CdCdO) and isocyanate (NCO-) species were determined as the reaction intermediates. In NOx + O2, NO2 directly interacts with the free carbon sites (CdC*) through two parallel reactions: (1) NO2 + CdC* f CdCdO + NO; (2) NO2 + CdC* f NCO- + CO2. The two reactions can proceed easily, which accounts for the promotion effect of NOx on soot combustion at lower temperatures. The further oxidation of NCO- by NO2 or O2 is responsible for the simultaneous reduction of NOx. However, the reactions between NO2 and CdC* are limited by the amount of free carbon sites, which can be provided by the oxidation of soot by O2 at higher temperatures. The interaction of NOx and catalyst results in the formation of nitrates and nitrites, which poisoned the active K sites.

1. Introduction The two prevalent pollutants, soot and NOx, from diesel engines have caused severe environmental and health problems, meaning that their emissions must be controlled. Soot can be eliminated by catalytic combustion with NOx and/or O2 using diesel particulate filter (DPF), during which NOx can also be reduced by soot forming N2 and CO2 simultaneously (1). The beneficial effect of NOx on soot combustion was well-known in the literature (2). However, little or deleterious effect of NOx on the soot combustion under tight contact condition was also reported (3). This uncertainty of the effects of NOx can be ascribed to the different reaction mechanism among the soot, O2 and NOx * Corresponding author phone: + 86 531 89736032; fax: + 86 531 89736032; e-mail: [email protected]. † University of Jinan. ‡ Dalian University of Technology. 8254

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on different catalysts, in which the clarification of the reaction intermediates is crucial. Generally, a carbon-oxygen complex (CO*) was reported to be the intermediates for soot combustion in O2 (4-7). While in NOx + O2, the intermediate for the reduction of NOx often involves a C[N, O] species, which further reacts with the adsorbed or gaseous NOx to produce N2 (6, 7). In both cases, the free carbon sites (CdC*) play an important role in the formation of the intermediate (5-7). The reaction intermediates and mechanism can be evidenced using in situ FTIR spectroscopy. Nevertheless, in situ FTIR spectra of soot combustion are thought to be difficult to be obtained due to the opacity of soot (8, 9). Thus, some diluents, such as KBr (10), were used. But the interaction between soot, catalysts, and diluents cannot be excluded, especially when analysis is performed at elevated temperatures (9, 11). In the previous work (11, 12), C60 was used as the soot substitute and the in situ FTIR spectra of soot combustion were obtained. This can be served as an efficient way to study the intermediates due to the FTIR absorption of C60 only at few well-defined frequencies. Sa´nchez Escribano et al. (13) performed in situ FTIR spectra of soot combustion using pressed disks of pure powders of catalyst impregnated with the soot and found the carboxylic species to be the intermediate. Li et al. (14) obtained the in situ diffuse reflectance infrared Fourier transform (DRIFT) spectra of soot combustion by reducing the amount of soot to trace, but the signal of the CO* is too weak. Among various catalysts for soot combustion, potassiumcontaining materials received increasing attention because of their considerable activity (15). In our previous work (16, 17), we studied soot combustion with O2 on potassiumsupported Mg-Al hydrotalcite mixed oxides. A carbonoxygen complex, ketene group was identified as the intermediate by ex situ FTIR spectroscopy, and thus an oxygen spillover mechanism was proposed (17). In the present work, soot combustion with NOx and/or O2 on the same catalysts has been examined using temperature-programmed oxidation (TPO) and isothermal reactions. Despite the difficulty as mentioned above, we obtained the in situ FTIR spectra of catalytic soot combustion without diluents since the opaque samples become transparent with soot depletion. The reaction intermediates, ketene and isocyanate species were determined, and thus the reaction mechanism has been proposed.

2. Experimental Section The potassium-supported Mg-Al hydrotalcite mixed oxides (K/MgAlO) with 0, 2, 5, and 8 wt.% of K were prepared as described in ref (17). Hereafter, they were denoted as MgAlO, 2K/MgAlO, 5K/MgAlO, and 8K/MgAlO. The detailed characterizations, for instance, X-ray powder diffraction (XRD) and Brunauer-Emmett-Teller (BET) surface areas can be obtained in ref 17. The TPO reactions were conducted in a fixed bed micro reactor consisting of a quartz tube (6 mm i.d.). Printex-U from Degussa is used as model soot. The soot was mixed with the catalyst in a weight ratio of 1:9 in an agate mortar for 30 min, which results in a tight contact between soot and catalyst. A 50 mg sample of the soot/catalyst mixture was pretreated in a flow of He (50 mL/min) at 200 °C for 1 h to remove surface-adsorbed species. After cooling down to room temperature, a gas flow with 5 vol.% oxygen in He or 1000 ppm NO + 5 vol.% O2 in He (100 mL/min) was introduced and then TPO was started at a heating rate of 5 °C/min until 800 °C. NOx (NO and NO2) and COx (CO and CO2) in the effluent were online analyzed by a chemiluminiscence NOx 10.1021/es102363f

 2010 American Chemical Society

Published on Web 10/05/2010

FIGURE 1. TPO patterns of CO2 and/or NOx for soot combustion on catalysts under tight contact condition with O2 and NO + O2.

TABLE 1. Reaction Rates (µmol/g · s) and NOx Conversions (%) of Isothermal Soot Combustion at 260 and 310 °C in NO + O2 and O2 Flows 260 °C sample 8K/MgAlO 5K/MgAlO 2K/MgAlO MgAlO a

NO + O2 0.35 0.23 0.16 0.01

(4.58)a (1.37)a (0.94)a (0.73)a

310 °C O2 0.13 0.08 0.02 0.00

NO + O2 0.66 0.46 0.13 0.03

(6.80)a (3.16)a (1.67)a (1.04)a

O2 0.81 0.53 0.13 0.00

NOx conversions.

analyzer (42i-HL, Thermo Environmental) and a gas chromatograph (GC) (SP-6890, Shandong Lunan Ruihong Chemical Instrument Corporation, China), respectively. The characteristic temperatures from the TPO profiles, T5 and T50 are defined as the temperatures at which 5% and 50% of the soot are converted, respectively. The selectivity to CO2 formation (SCO2) is defined as the percentage CO2 outlet concentration divided by the sum of the CO2 and CO outlet concentrations. NO conversion is evaluated by maximum NOx conversion (Cmax). The isothermal reactions for soot combustion at 260 and 310 °C were also conducted, respectively, at which a stable and small conversion of soot (1-5%) was achieved in an approximate kinetic regime. Thus, the reaction rates at 260 and 310 °C can be obtained. More details were given elsewhere (18). The in situ FTIR spectra were recorded on a Bruker Tensor 27 spectrometer over 400-4000 cm-1 after 32 scans at a resolution of 4 cm-1. The mixture of soot and catalyst in tight contact was pressed into a thin self-supporting wafer with a thickness of 7.5 mg/cm2. The wafer was loaded into an in situ infrared transmission cell which is capable of operating up to 500 °C and equipped with gas flow system. All experiments were performed in the flow of 100 mL/min with the heating rate of 5 °C/min. Background spectra were obtained without samples before each experiment in a He flow at room temperature.

3. Results and Discussion 3.1. Catalytic Reaction. Figure 1 shows the CO2 and NOx outlet concentrations during TPO for all samples in the flow of O2 and NO + O2. The outlet concentrations of NO2 and NO were given in Supporting Information (SI) Figure S1. T5, T50, SCO2, and Cmax are summarized in SI Table S1. The reaction rates and NOx conversions obtained from the isothermal reactions are listed in Table 1. Compared with the results in O2, the soot combustion in NO + O2 can be outlined as:

FIGURE 2. In situ FITR spectra for soot combustion on 5K/ MgAlO in the flows of 5 vol.% O2 + He (A) and 1000 ppm NO + 5 vol.% O2 + He (B). (1) Similar to the results obtained in O2, the TPO patterns in NO + O2 shift to lower temperatures after K addition. Furthermore, T5 and T50 decreased, whereas SCO2 slightly increased monotonously with the increase in K amount. This indicates that no matter what flows (O2 or NO + O2) are used, the presence of K improves the activity and the selectivity to CO2. (2) At lower temperatures (300 °C), the major CO2 peaks were observed in TPO patterns. No much difference in the major CO2 peaks was shown and T50 was comparable between in O2 and in NO + O2. For 5 K/MgAlO and 8K/MgAlO, nevertheless, the isothermal reaction rate at 310 °C in NO + O2 is lower than that in O2. This suggests that, at higher temperatures, the presence of NOx suppressed the soot combustion on K/MgAlO with high K amount (g5 wt.%). The large NOx reduction was observed simultaneously with the presence of the major CO2 peak, and Cmax increased with the increase in K amount. (4) In contrast with a weak CO2 desorption in O2 after soot combustion completion (inset in Figure 1), a strong NOx desorption peak was found in NO + O2 for 8K/MgAlO. 3.2. In Situ FTIR Study. Figure 2 shows the in situ FTIR spectra of soot combustion on 5K/MgAlO (the spectra on 8K/MgAlO and MgAlO were given in SI Figures S2 and S3). The assignments of the FTIR bands are summarized in Table 2. Before further discussions, three points must be pointed out: (1) The FTIR signal was very weak initially due to the opacity of soot but increased with soot depletion; (2) The reaction temperatures judged from the in situ FTIR spectra are higher than those in TPO tests due to the diffusion limitation of self-supporting wafer; and (3) No bands assigned to N2O were detected during TPO and isothermal reactions, indicating that the main product of NOx reduction is N2. VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. FTIR Bands and Assignments

SCHEME 1. Mechanism Illustration for Soot Oxidation With NO + O2 on K/MgAlO

3.2.1. Reaction with O2. Figure 2A shows the in situ FTIR spectra of soot combustion with O2 on 5K/MgAlO. The band at 2162 cm-1 can be attributed to ketene group (17), which reaches maximum at ∼460 °C and progressively decreases as the combustion goes further. This is also the case for 8K/ MgAlO (SI Figure S2A) but not for MgAlO (SI Figure S3A), confirming our findings that ketene group was the intermediate and soot combustion proceeds via an oxygen spillover mechanism (Scheme 1) (17): O2 + K+ f K+ - O*

(1)

K+ - O* + CdC* f CdCdO + K+-

(2)

CdCdO + K+ - O* f CO2 + K+-

(3)

K+-O* represents the surface-activated oxygen on K sites, while CdCdO is the ketene group. Besides the ketene group, the chelating bidentate carbonate (1544 and 1354 cm-1) and ionic carbonate (1402 cm-1) were formed on K/MgAlO. These carbonates originate from the adsorption of the produced CO2 on K sites, corresponding to the weak CO2 peak at higher temperatures (700-750 °C) in TPO (17). 3.2.2. Reaction with NO + O2. Figure 2B shows the soot combustion with NO + O2 on 5K/MgAlO. The ketene group was also found at 2162 cm-1. Differently, an isocyanate ion was detected at 2196 cm-1 during soot combustion. The same band was found for 8K/MgAlO (SI Figure S2B) but not for MgAlO (SI Figure S3B). The isocyanate ion has been con8256

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FIGURE 3. In situ FITR spectra for soot combustion on 5K/ MgAlO in the flows of 2000 ppm NO2 + He. sidered as the intermediate in the selective catalytic reduction of NO with hydrocarbons (HC-SCR) (20, 25-27). It is deduced that the intermediate in the NOx reduction by soot is also the isocyanate ion. In addition to ketene group and isocyanate ion, the chelating bidentate nitrate (1575 cm-1), ionic nitrate (1370 cm-1), and nitrite (1234 cm-1) were formed. This suggests the substitution of carbonates (in O2) with nitrates (in NO + O2), which coincides with the fact that only NOx desorption was observed in Figure 1 at higher temperatures (600-750 °C) after soot combustion completion. 3.2.3. Reaction with NO2. The promotion effect of NO to soot combustion is often attributed to the coexistent NO2 which is a much stronger oxidant than O2 and NO. Figure 3 shows the in situ FTIR spectra of soot combustion with NO2 on 5K/MgAlO. The band of isocyanate ion was observed at 2196 cm-1, which suggests that the isocyanate ion originates from the reaction of soot with NO2. At higher temperatures, the band of ketene group (2162 cm-1) was present, namely, the isocyanate ion appeared earlier than the ketene group. This rules out the possibility that both derive from an identical reaction. Another possibility is that the ketene group is the product of the reaction of isocyanate ion with soot. However, the NCO- with N in a -3 reduction state and O in a -2 reduction state cannot oxidize soot producing the ketene group. Moreover, the chelating bidentate nitrate (1575 cm-1), the monodentate nitrate (1475 and 1039 cm-1), the ionic nitrate (1370 cm-1) and the ionic nitrite (1234 cm-1) were observed. Compared with the in situ FTIR spectra of NOx (+ O2) adsorption on 5K/MgAlO (SI Figure S4), the monodentate nitrate and the ionic nitrate were formed by NO2 adsorption, while the ionic nitrite is attributed to NO adsorption. Thus, the appearance of the ionic nitrite in the flow of NO2 suggests that gaseous NO was produced. In Figure 3, it is noteworthy that the presence of the ionic nitrite was in concomitant with the formation of the ketene group. Therefore, the ketene group and NO should come from an identical reaction between soot and NO2. Some work (14, 22, 28) suggested that the reaction of soot with NO2 proceeds via surface nitrates. To check its validity, the surface nitrates on 5K/MgAlO were obtained in advanced through NO2 adsorption at 400 °C for 1 h (SI Figure S4), and then carbothermic reduction was performed for the mixture of the obtained sample and soot in He under tight contact condition (SI Figure S5). The TPO reaction between soot and NO2 took place at much lower temperature than that for soot

FIGURE 4. In situ FITR spectra of the mixture of 5K/MgAlO and soot at 200 °C in 2000 ppm NO2 + He after pretreatment in 5 vol.% O2 + He at 400 °C and then cooling to 200 °C in He. and nitrates. Furthermore, the temperature of CO2 peak for soot combustion in NO2 is nearly in agreement with that of the maximum CO2 peak at lower temperatures in NO + O2 (Figure 1). Based on above discussion, the intermediates, ketene group and isocyanate ion should come from two parallel direct reactions between NO2 and soot, which can be ascribed as (Scheme 1): NO2 + CdC* f CdCdO + NO

(4)

NO2 + CdC* f NCO- + CO2

(5)

Furthermore, the highly stable nitrates after soot combustion completion were observed in Figure 1, Figure 2B, SI Figure S2B and Figure 3, which suggests that the nitrates as intermediates might be excluded in NO + O2. 3.2.4. Reactivity of the Free Carbon Sites. As shown in Figure 2, 3 and SI Figure S2B, the isocyanate ion and the ketene group were observed at higher temperatures. However, it is unclear whether the intermediates were produced at lower temperatures because of the weak FTIR signal and the shortage of free carbon sites during initial soot conversion. To solve the problem, we designed the following in situ FTIR experiments, as shown in Figure 4. The mixture of 5K/MgAlO and soot was heated to 450 °C in O2 + He followed by purging with He, which results in a clear FTIR signal and a lot of free carbon sites. After cooling to 200 °C in He, the FTIR spectra show the presence of the ketene group (2162 cm-1), the chelating bidentate carbonate (1544 and 1354 cm-1) and the ionic carbonate (1402 cm-1). Then, different reaction gases, such as NO2, NO, and O2, were introduced, respectively. After the introduction of NO2, the bands of isocyanate ion (2196 cm-1), ketene group (2162 cm-1), ionic nitrite (1234 cm-1) and nitrate (1370 cm-1) were observed simultaneously. With the increase in time, the bands of isocyanate ion and ketene group decreased in intensity. While the chelating bidentate carbonate was gradually substituted by the chelating bidentate nitrate (1575-1590 cm-1), which is also present in Figure 3. This suggests that the reaction between NO2 and soot is rather vigorous at 200 °C, which can be attributed to the abundance of the free carbon sites. Reichert et al. (7) confirmed that the oxidation of soot by O2 can produce free carbon sites. However, after the introduction of NO or O2, neither isocyanate ion nor ketene group emerged (no reaction occurred, SI Figure S6), suggesting that the free carbon sites react exclusively with NO2 at lower temperatures.

FIGURE 5. In situ FITR spectra of the isocyanate ion in the flows of 1000 ppm NO + He (A), 5 vol.% O2 + He (B) and 2000 ppm NO2 + He at 450 °C (C). The isocyanate ion was obtained by heating the mixture of 5K/MgAlO and soot to 450 °C in the flow of 2000 ppm NO2 + He. 3.2.5. Reactivity of the Isocyanate Ion. In order to understand the fate of the isocyanate ion, Figure 5 shows the in situ FTIR spectra of the isocyanate ion which is formed in advance and further reacts with NO, O2 or NO2 at 450 °C. In the reaction with NO (Figure 5A), no marked change was observed for the band of isocyanate ion at 2196 cm-1. In the reaction with O2 (Figure 5B), the band of ketene group at 2162 cm-1 appeared and then disappeared together with the isocyanate ion after 20 min, which can be attributed to the reactions of (1-3) and (6): O2 + NCO- f N2 + CO2

(6)

NO2 also results in the completion of soot combustion after 20 min (Figure 5C). The isocyanate ion may be oxidized by NO2 via the reaction: NO2 + NCO- f N2 + CO2

(7)

This suggests that the isocyanate species is highly reactive toward O2 and NO2 (NO + O2), while less reactive to NO (29, 30). 3.3. Reaction Mechanism. All above results allow us to propose a mechanism for soot combustion with O2 or NO + O2 on K/MgAlO. As shown by the dash lines in Scheme 1, O2 first interacts with surface K species. This results in the formation of the surface-activated oxygen on K sites (1), which can spill over to the free carbon sites on soot to form the intermediate of ketene group (2). Then, the ketene group combined with another active oxygen species to give out CO2 (3). However, as shown by the solid lines in Scheme 1, NO2 directly reacts with the free carbon sites by two parallel reactions: producing NO and the ketene group (4); producing isocyanate ion (5) which is further oxidized into N2 by O2 (6) or NO2 (7), responsible for the reduction of NOx. The activity of soot combustion on K/MgAlO in NO + O2 can be well explained by the proposed mechanism. The reactions of NO2 and the free carbon sites can produce ketene and isocyanate species easily at lower temperatures, as confirmed in SI Figure S1. However, the further reactions of the ketene species to give out CO2 finally need the surfaceactivated oxygen on K sites. Therefore, the activity is promoted by the presence of K. The more K, the more active oxygen and the more evident the promotion is (Figure 1, SI Table VOL. 44, NO. 21, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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S1 and Table 1). On the other hand, the reactions of NO2 and soot are limited by the amount of the free carbon sites. At higher temperatures, soot oxidation with O2 would prevail, many free carbon sites are thus accessible to NO2, which results in the marked NOx reduction via the isocyanate intermediate. Furthermore, the interaction of NOx and catalyst results in the formation of stable nitrates and nitrites, which poisoned the active K sites. Therefore, the soot combustion at higher temperatures was suppressed by the presence of NOx. This is more serious to the samples with higher K amount.

Acknowledgments This work was supported by the 863 program of the Ministry of Science and Technology of the People’s Republic of China (No. 2008AA06Z320), the National Natural Science Foundation of China (No. 20777028 and 21077043), the Natural Science Foundation of Shandong Province (No. Y2007B36) and the Program of the Development of Science and Technology of Shandong Province (No. 2008GG10003026).

Supporting Information Available Table S1 and Figure S1-S6. This material is available free of charge via the Internet at http://pubs.acs.org.

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