Article pubs.acs.org/est
Reaction Pathway Investigation on the Selective Catalytic Reduction of NO with NH3 over Cu/SSZ-13 at Low Temperatures Wenkang Su,† Huazhen Chang,† Yue Peng,† Chaozhi Zhang,*,‡ and Junhua Li*,† †
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China ‡ School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, P. R. China S Supporting Information *
ABSTRACT: The mechanism of the selective catalytic reduction of NO with NH3 was studied using Cu/SSZ-13. The adspecies of NO and NH3 as well as the active intermediates were investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy and temperature-programmed surface reaction. The results revealed that three reactions were possible between adsorbed NH3 and NOx. NO2− could be generated by direct formation or NO3− reduction via NO. In a standard selective catalytic reduction (SCR) reaction, NO3− was hard to form, because NO2− was consumed by ammonia before it could be further oxidized to nitrates. Additionally, adsorbed NH3 on the Lewis acid site was more active than NH4+. Thus, SCR mainly followed the reaction between Lewis acid site-adsorbed NH3 and directly formed NO2−. Higher Cu loading could favor the formation of active Cu-NH3, Cu-NO2−, and Cu-NO3−, improving the SCR activity at low temperature.
1. INTRODUCTION The selective catalytic reduction (SCR) of nitrogen oxides with NH3 is considered an effective way to control NOx emission from mobile sources. Among available catalytic formulations, metal exchanged zeolites have shown superior SCR performance. In general, the overall “standard SCR” reaction between NO and NH3 can be expressed as follows: 4NH3 + 4NO + O2 → 4N2 + 6H 2O
that the proton-adsorbed NH3 was less active compared to CuNH3. These researches implied that the Cu-linked NOx and NH3 species were active intermediates. On the other hand, Di et al.13 has proposed that the NH4+ form at Brönsted acid sites or migrate from Lewis acid sites was the active intermediates. Regarding the active NOx adspecies, Xie22 and Kwak23 has proposed the Cu−NO3− and Cu−NO as the active NOx adspecies in the SCR reaction, respectively. While, Ruggeri et al.24 proposed the nitrite as the intermediate, and directly detected the formation and reaction of nitrite with ammonia, which improved our understanding of reaction mechanism significantly. In the present study, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and TPSR were used to investigate the possible SCR reactions and active intermediates on Cu/SSZ-13 with different copper concentrations. In other words, we intended to investigate the active adspecies of NH3 and NOx and their transformation. Furthermore, we intended to elaborate on all the possible reactions involved in standard SCR and their relative importance. The data indicated that three reactions were possible at lower temperatures and the one playing the major role was the reaction between Lewis acid site-adsorbed NH3 and NO2−.
(1)
Recently, Cu(II) exchanged zeolites, Cu/SSZ-13, with the chabazite (CHA) structure have been commercialized as NH3SCR catalysts in diesel engines for their broad activity window, stable performance, and their high selectivity to N2.2−6 Investigations have been performed on the active copper sites and reaction mechanism.7−12 Most scientists agree that the reaction follows the Langmuir−Hinshelwood (L-H) mechanism at lower temperatures, in which the adsorbed NOx reacts with the NH3 adspecies to form N2 and H2O. On the basis of different intermediates formed during the reaction, two types of pathways belonging to the L-H type mechanism have been proposed on SCR catalyst: (i) the adspecies of NH4+ and NO2− forms ammonia nitrite, NH4NO2, which decomposes into N2 and H2O at temperatures higher than 100 °C;13−15 and (ii) adsorbed NH3 and NO oxidation species form ammonia nitrate NH4NO3 on metal sites, and it is subsequently reduced by NO.1,16−18 There is still no clear consensus on the detailed reaction step network on Cu/SSZ-13, including the active intermediates and reactive sites. Recently, Gao11 believed the NO and NH3 was oxidized on Cu sites. It was19−21 proposed © XXXX American Chemical Society
Received: July 16, 2014 Revised: November 25, 2014 Accepted: December 8, 2014
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DOI: 10.1021/es503430w Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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2. MATERIALS AND METHODS Catalyst Synthesis. The H/SSZ-13 (Si/Al = 10) was added into 0.1 M NH4NO3 solution and stirred at 70 °C for 4 h. The solid was then separated from the liquid phase by centrifugation and then dried at 110 °C overnight. The process was repeated three times to obtain NH4/SSZ-13. The Cu/SSZ13 catalysts were prepared by aqueous ion exchange. A 5 g portion of NH4/SSZ-13 was poured into four types of Cu(CH3COO)2·H2O solution with different concentrations, and stirred at 80 °C for 24 h. Then, the samples were thoroughly washed with deionized water followed by drying and calcination at 550 °C for 6 h. The elementary composition of the zeolite framework and Cu content in the four samples were measured by ICP-OES analysis. The results showed that the Si/Al ratio was 10 and the four catalysts contained 0.6, 0.9, 1.4, and 1.7 wt % copper. The corresponding ion exchange (IE) level was 17.3%, 26%, 44.5%, and 51.2%, respectively. The BaO/Al2O3 was synthesized as referenced.24 The mass ratio of mixture of BaO/Al2O3 and Cu/SSZ-13 was 3. Catalyst Characterization. The infrared spectra were recorded on a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer equipped with an MCT detector. The in situ diffuse reflectance FTIR spectroscopy (DRIFTS) experiments were performed in a high temperature chamber with a ZnSe window. The powder sample was placed in the Harrick IR cell with a copper sieve support. The total flow was maintained at 100 mL/min. Before each experiment, the catalyst was pretreated at 500 °C for 30 min at a flow of 5% O2 in N2. Then, the sample was cooled to the desired temperature. At each temperature, the background spectrum was recorded at N2 flow and subtracted from the sample spectrum obtained at the same temperature. The spectra were collected in the range of 4000−800 cm−1 at a resolution of 2 cm−1 and 32 scans of accumulation. To diminish the influence of absorbance from different samples, the absorbance intensity was set to 2.5 for every sample at 500 °C. The key band was quantified according to area integral, and the change in peak area was shown in terms of percentage. All of the infrared spectra were collected in the absence of water because of its wide interference with IR bands. We assumed that the surface intermediates and reaction mechanism still applied to the feed in the presence of water.13 All the designed DRIFT experiments were carried out parallelly in a fixed bed reactor. The reaction gas was fed to 100 mg of catalyst at a flow rate of 200 mL/min. The products were measured by an online FTIR spectrometer to monitor the concentration of NO, NO2, N2O, and NH3, and mass spectrometer (MS) to monitor the H2O and N2.
Figure 1. DRIFTS spectra obtained during NH3 desorption after the sample had been exposed to 500 ppm of NH3/N2, followed by a purging with N2.
NH3-TPD on 1.7 wt % Cu/SSZ-13. After the adsorption at 50 °C, coordinated NH3 (1625/1200 cm−1) and NH4+ (1470 cm−1) ions13,18,19,25 were generated. The peaks in the range of 3700−3500 cm−1 represented the Si−OH and Si−OH−Al structure consumed by the adsorption of NH4+.19,25 At 200 °C, the peak at 3665 cm−1 assigned to the terminal hydroxyl Si− OH decreased significantly, and the 1470 cm−1 peak representing NH4+ decreased simultaneously. The terminal hydroxyl acid was weak, indicating that terminal hydroxyl was not involved in SCR. When the temperature reached 350 °C, the coordinated NH3 on Lewis acid sites (1625 cm−1) decreased significantly, while the NH4+ ions adsorbed at the bridging Si−OH−Al hydroxyl maintained about 70% of their intensity at 350 °C. Thus, the acid strength followed the sequence: bridging Si−OH−Al Brönsted acid > Lewis acid > terminal hydroxyl Si−OH Brönsted acid. The sequence showed that the NH3 adsorbed on the Lewis acid was unstable compared to the Si−OH−Al Brönsted acid, which meant that the former might be more easily removed from the catalyst surface and be more active in the SCR reaction at lower temperatures than NH4+ ions on the Si−OH−Al Brönsted acid. The NH3 desorption pattern on the H/SSZ-13 and Cu/SSZ13 catalyst with the copper concentration ranging from 0.6 wt % to 1.7 wt % was shown in Supporting Information, Figure S1. Three primary peaks were found at 150, 300, and 450 °C on Cu/SSZ-13 catalysts. The quantitative results of NH 3 desorption were summarized in Supporting Information, Table S2. On the basis of our infrared spectra in Figure 1, the three peaks could be assigned to the ammonia desorption mainly from the terminal hydroxyl Si−OH Brönsted acid sites, Lewis acid sites, and bridging Si−OH−Al Brönsted acid sites, respectively. With increasing copper concentration, both the weak and strong Brönsted acid sites (150/450 °C) decreased; in addition, the Lewis acid sites (300 °C) increased significantly from 1.5 to 8 mL/g, verifying the fact that the Brönsted acid protons were substituted by Cu2+ ions. The increased new Lewis acid formed by substituted Cu2+ ions might be the key intermediate according to the strength sequence. In terms of the NO + O2 TPD experiment, the adsorption gas was switched to 500 ppm of NO in a 5% O2 flow with N2 as the balance at 50 °C, and the desorption temperature was set at 150, 300, and 500 °C. Figure 2 shows the DRIFT spectra collected on both the H/ SSZ-13 and Cu/SSZ-13 zeolite after NO + O2 adsorption at 50 °C. The 1891 and 1938 cm−1 peaks could be assigned to NO
3. RESULTS AND DISCUSSION 3.1. Adspecies of NH3 and NO. The acidic property of the SCR catalyst was an important factor that influenced the SCR performance. To elucidate the acidic properties of the four catalysts, the DRIFT spectra of NH 3 adsorption and subsequent temperature-programmed desorption were collected, and the type, strength, and quantity of the acidic sites on Cu/SSZ-13 and H/SSZ-13 were examined. The catalysts were exposed to 500 ppm of NH3 in a N2 flow for 30 min at 50 °C, and then purged by N2 for another 30 min. The infrared spectrum was collected at 50, 200, 350, and 500 °C after reaching a steady state. Additionally, the product of the NH3TPD was monitored by online FTIR to obtain the quantitative results. Figure 1 shows the DRIFT spectra obtained during B
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The NO adsorption and evolution on the zeolite surface was summarized as follows: NO + Cu 2 +−OH ··· Cu+−HONO
(R3)
Cu+−HONO + H−OZ ··· NO+−OZ + H 2O + Cu+ (R2) +
2−
Cu −ONO + O
··· Cu −ONO2
Cu+ + O2 − ··· Cu 2 +
coordinately adsorbed at the Cu oligomer and the Cu ions, respectively.26,27 The band intensity of Cu2+−NO (1891 cm−1) over 1.7% Cu-SSZ-13 was higher than that over 0.6% Cu-SSZ13, while the intensity of the Cu2+−NO (1935 cm−1) was lower. This may imply Cu aggregation with increased Cu loading. Cu oligomer was more oxidative than isolated ions,10 meaning the higher Cu loaded catalyst tended to favor the NOx oxidation. The weak bans at 1675/1510 cm−1 were assigned to NO2−.28 These two bands were very unstable, and they could not detected at reaction temperature (150 °C), making it difficult to investigate their reactivity. The bands at 2197/2155 cm−1 were assigned to NO+ linked to both the Cu and zeolite framework. This formed either by direct adsorption on zeolite or nitrite readsorption,23,24,28,29 +
+
NOg + 2H − OZ ⇔ NO −OZ + HO−H −Z
(R4) (R5)
The integration results of NH3 and NOx desorption were shown in Supporting Information, Table S1. To further study the adsorption adspecies and capacity of NH3 and NO under standard SCR conditions, 0.1 g of catalyst was exposed to 500 ppm of NO, 500 ppm of NH3, and 5% O2 for an hour, followed by purge and the temperature rising from 150 to 600 °C. Figure 3 shows the desorption results. It could be clearly seen that, the
Figure 2. DRIFTS spectra taken after exposing the H/SSZ-13 and Cu/SSZ-13 sample to 500 ppm of NO + 5% O2/N2 for 30 min at 50 °C.
+
+
Figure 3. Outlet gas concentrations in temperature-programmed desorption period after preadsorbed NH3, NO, and O2 at 150 °C on 1.7% Cu/SSZ-13 catalysts.
(R1)
Cu+−HONO + H−OZ ··· NO+−OZ + H 2O + Cu+
composition of desorption gas was mainly NH3 and only a trace amount of N2O was detected. The production of N2O was thought to be the decomposition of NH4NO3.14,31 The amount of N2O (0.022 mL/g) was negligible compared to NH3 (12 mL/g). However, the adsorption amount of NH3 was about three times that of NO in the separated adsorption experiments as shown in Supporting Information, Table S1. The difference indicated that NO3− was hard to form on the catalyst surface in the presence of NH3. DRIFTS spectra of the coadsorption of NH3, NO, and O2 at 150 °C were also obtained as shown in Supporting Information, Figure S3. The feature of the nitrate species was not obvious. Zhu19 had also found that the adsorption of NH3 is much stronger than that of NO. Additionally, as the intermediate nitrite was captured by ammonia adspecies promptly on formation, it could hardly be further oxidized to form NO3−. 3.2. Reaction Pathways at Low Temperatures on Cu/ SSZ-13. Reactivity of Ammonia Adspecies. To investigate the active NH3 adspecies on the Cu/SSZ-13, the NH3 preadsorbed catalysts were obtained by treating the catalyst with 500 ppm of NH3 for 30 min and then purging with N2 gas. Subsequently, the N2 was switched to 500 ppm of NO in 5% O2 gas. The spectra were recorded during the reaction between NO and the presorbed NH3 at 150 °C. Figure 4 shows the IR spectra of the reaction between the NH3 adspecies and NO + O2 at 150 °C on the 1.7 wt % Cu/ SSZ-13 catalysts. And the normalized peak areas of key bands were calculated to more clearly elucidate the evolution of the
(R2)
The 1575 cm−1 band only existed on the Cu exchanged zeolite and increased significantly with Cu concentration, indicating that this kind of nitrate species was adsorbed on copper sites, while the 1629/1609 cm−1 feature existed on both H/SSZ-13 and Cu/SSZ-13 zeolites and they could be nitrated adsorbed on the zeolite framework. In association with other related work, the features in this range were attributed as follows: monodentate nitrate linked to Cu sites (1575 cm−1), monodentate nitrate linked to Al sites(1609 cm−1), and bridging nitrate based on Al sites(1629 cm−1), respectively.1,28−30 To confirm the formation of NO2− at 150 °C, the NO + O2TPD experiment was performed in the same way as in ref 24 on the mixture of Cu/SSZ-13 and BaO/Al2O3 at 150 °C. The formed NO2− on Cu/SSZ-13 would transfer to BaO/Al2O3 to avoid further oxidation. The result was shown in Supporting Information, Figure S2. The desorption ratio of NO/NO2 was close to 1 below 300 °C, and the NO2 desorption at higher temperature was much lower than the NO desorption. The result verified that the nitrite was formed on the catalyst surface, and the absence of nitrate adspecies indicated that the NO3− formation originated from the further transformation of NO2−. Partial Cu+−ONO would be stored on the zeolite surface in the form of NO+−OZ, if it would not be further oxidized to Cu+−ONO2 or captured by NH3. And the NO+− OZ was not reactive as no direct evidence was detected by IR. C
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quickly. According to the former NO + O2-TPD results, NO2− was formed on the surface. This indicated that the NH3 on the Lewis acid sites have reacted with NO2− nitrite before the CuNO3− sites formed on the catalyst surface. As NH4NO2 was very unstable, it would easily decompose to N 2 and H2O.17,24,32−34 Thus, it was not detected in IR. The corresponding reaction between NH3 and NO2− could be expressed as following: Cu−NO2− + NH3−Cu−H ··· NH4NO2 ····N2 + H 2O (eq 1) +
The intensity of the NH4 ions and corresponding negative hydroxyl band was almost unchanged in the first 15 min. In the subsequent 15 min, the band at 3606 and 1470 cm−1 decreased in unison, accompanied by the significant decrease of Cu− NO3− at the same rate, indicating that the NH4+ ions mainly formed on bridging Si−OH−Al hydroxyl were consumed by Cu linked nitrate. The increase in the intensity of band at 1470 cm−1 in the beginning might originate from the strengthening of NH4+ ions vibration with nitrate bonded to it. Furthermore, the 1625 cm−1 band decreased more quickly compared with the bands corresponding to NH4+ ions, indicating the Lewis acid bonded NH3 was more active, consistent with the NH3-TPD results where the Lewis acid sites bonded NH3 was unstable compared to NH3 on Brönsted acid sites. Figure 5 shows the outlet concentration in the reaction between NO and gas phase ammonia (A) and presorbed ammonia (B) on 0.6% Cu/SSZ-13 at 150 °C followed by temperature-programmed desorption from 150 to 550 °C. As Figure 5A shows, both NH3 adsorbed on Lewis acid and Brönsted acid sites desorbed in the temperature rising period after the SCR reaction gas in the stream was cut off, indicating that both the adsorbed NH3 on the Lewis acid site and the NH4+ ions existed on the catalyst surface in the SCR reaction. In Figure 5B, where the ammonia was preadsorbed on the surface, only NH3 adsorbed on the Brönsted acid sites desorbed in the temperature rise period, indicating that the NH3 adsorbed on the Lewis acid site was consumed completely by NO. The results in Figure 5 verified that both the adsorbed NH3 on the Lewis acid sites and the NH4+ ions on the Brönsted acid sites were involved in the reaction and further confirmed that the adsorbed NH3 on the Cu sites reacted faster than the NH4+ ions. Reactivity of NOx Adspecies. To investigate the active nitrate adspecies (centered at 1575 cm−1) on the Cu/SSZ-13, the infrared spectra were recorded during the reaction between NH 3 and the presorbed NO at 150 °C. Supporting
Figure 4. (A) DRIFTS spectra taken at 150 °C upon passing 500 ppm of NO + 5% O2/N2 over NH3-adsorbed 1.7 wt % Cu/SSZ-13 for 0− 30 min. (B) Normalized DRIFT peaks intensity.
intermediates. When the catalyst surface was saturated with NH3, both bands assigned to the Lewis acid sites (1625 and 1200 cm−1) and Brőnsted acid sites (1470 and 3606 cm−1) were detected. As the figures show, the NH3 on the Lewis acid sites began to decrease slowly upon exposure to NO and O2, and after 20 min, it was hardly detected. In 10 min, new nitrate peaks centered at 1575 cm−1, 1609 cm−1, 1629 cm−1 emerged. Accompanied by the accumulation of Cu linked nitrate, the intensity of band centered at 1625 cm−1 decreased more
Figure 5. Temperature-programmed surface reaction: NO reaction with gas phase ammonia (A) or presorbed ammonia (B) on 0.6% Cu/SSZ-13 at 150 °C followed by temperature-programmed desorption from 150 to 550 °C. D
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almost at the same rate. The 1200 cm−1 band decreased down to zero at 8 min, only 2 min faster than the 3606 and 1470 cm−1 band. Simultaneously, the peaks assigned to the NOx species became sharp and increased after the consumption of NH4+, which meant an accumulation of nitrate and NO+ on the catalyst surface. The results indicated that the reaction between NH4+ and NO3− could not occur without gas phase NO. It had been proposed that NO3− could be reduced by NO in the gas phase to form NO2−.22,29,32,35 As NH4NO2 was very unstable, it would easily decompose to N2 and H2O.17,32,34,36 Thus, the nitrite species was hard to detect in our study because of its rapid reaction with the NH3 adspecies on the surface or its overlapping regions with the zeolite framework vibration.13,29,37 The faster reaction rate originated from the existence of more nitrate species on the catalyst surface formed by the preadsorption of NO + O2. For the catalysts that showed more affinity to NH3 than NO and the nitrite quick reaction, it was difficult for the nitrate species to form in the presence of ammonia adspecies. The NO preadsorption treatment could enhance the coverage of nitrate; thus, the reaction rate of the ammonia adspecies was accelerated. The corresponding reaction between NH4+ and NO3− could be expressed as follows:
Information, Figure S4 shows the DRIFT spectra collected during the reaction. The NH4+ ions and Lewis acid siteadsorbed NH3 peaks were detected as time increased, while the peaks assigned to nitrate were unchanged, which meant that NH4+ ions did not react with nitrates. Compared to the experimental conditions in Figure 4, gas phase NO did not exist in this case, indicating that the NO may have played an important role in the reaction between the NH4+ ions and nitrate. Simultaneously, N2 and H2O were not detected in the outlet gas by MS, indicating the NO+ (formed by NO2− transformation) was not active, either. Further experiment was performed to elucidate the role of NO in the SCR reaction as shown in Figure 6. A flow of NO in
Cu−NO3− + 2Z−O−(NH4)···(NH4)2 NO3
(NH4)2 NO3 + NO ····(NH4)2 (NO2 )2 ··· 2N2 + 3H 2O + 2ZO−H
(eq 2)
In Figure 6, the consumption of adsorbed NH3 on Cu sites was also accelerated by the preformation of nitrate species, and disappeared in 8 min which was much shorter than that needed in Figure 4. The accelerated reactivity of the Lewis acid site adsorbed NH3 indicated that it could also react with NO2− species generated by NO3− reduction. The reaction between Lewis acid-site adsorbed NH3 and NO3− could be expressed as follows: Cu−NO3− + 2NH3−Cu−H ···(NH4)2 NO3
(NH4)2 NO3 + NO ····(NH4)2 (NO2 )2 ··· 2N2 + 3H 2O + 2ZO−H
(eq 3)
3.3. The Main Reaction for SCR at Low Temperatures. The main adspecies of NO on the Cu/SSZ-13 was NO+, NO2−, and NO3−. NO3− originated from NO2− further oxidation. If Cu- NO2− would not be consumed or further oxidized, it would adsorb on the zeolite surface and form Z-O-NO+. Nitrate could hardly form for the competitive adsorption of NH3 based on the coadsorption results. Additionally, the quick reaction between NO2− and NH3 would inhibit the further oxidation of NO2− to form NO3−. And the NO+ species was not detected to be involved in the reaction. Thus, NO2− was believed to be the main active NOx species. As the Lewis acid site adsorbed NH3 showed more reactivity than NH4+ ions. The reaction mainly followed the pathway involved the reaction between Lewis acid adsorbed NH3 and directly formed NO2−. While the formation of active Cu-NO3− on the catalyst surface could improve the SCR activity significantly as the results shown in Figure 6. Thus, the reaction involved Cu-NO3− could not be neglected. The discussed reactions at low temperatures were summarized in Figure 7.
Figure 6. (A) DRIFTS spectra taken at 150 °C upon passing 500 ppm of NO over NO + O2 and NH3-adsorbed 1.7 wt % Cu/SSZ-13 for 0− 15 min. (B) Normalized DRIFT peaks intensity.
N2 was passed over the catalyst with presorbed NO and NH3 on the catalysts one after another. After the preadsorption, with peaks attributed to NH3 and NH4+, three different types of nitrates and Cu−NO+ were detected. The normalized peak areas of key bands were calculated, and the results are shown in Figure 6B. The ammonia adspecies, both Lewis acid sites bonded NH3 and NH4+ ions on Brönsted acid sites, decreased significantly upon exposure to NO, a process which was much faster than the reaction in Figure 4. According to the quantitive results, the bands at 1200, 3606, and 1470 cm−1 decreased E
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AUTHOR INFORMATION
Corresponding Authors
*Phone: +86 10 62771093; e-mail: [email protected]. *Phone: +86-25-58731221; e-mail: [email protected]. cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Fund of China (Grant Nos. 21325731 and 51478241) and the National High-Tech Research and Development (863) Program of China (Grant No. 2013AA065304), and Jiangsu Specially-Appointed Professor Program (R2012T01). The authors would also like to acknowledge the financial support of the Ford China University Research Program from the Chemical Engineering & Fuel Cell department, Research & Innovation Center, Ford Motor Company.
Figure 7. Reaction scheme of SCR of NO with ammonia on Cu/SSZ13 at lower temperatures.
Supporting Information, Figure S5 shows the NO conversion as a function of temperature for the series of catalysts with Cu concentrations ranging from 0.6 to 1.7 wt %. The SCR activity was clearly increased for the higher Cu loaded catalyst. Figure 8
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Figure 8. Amount of adspecies of NH4+ (black), Cu-NH3 (red), and Cu-NO3− (green) formed on Cu/SSZ-13 zeolites with varying Cu loading and the corresponding reaction rate at 150 °C: (1) 0.6 wt % Cu/SSZ-13; (2) 0.9 wt % Cu/SSZ-13; (3) 1.4 wt % Cu/SSZ-13; (4) 1.7 wt % Cu/SSZ-13).
showed the relationship between the quantity of active intermediates and the corresponding reaction rate at 150 °C on Cu/SSZ-13 zeolites with varying Cu loading. As was shown, the reaction rate increased with copper loading while the NH4+ ions showed a decreasing trend. It conformed to our consumption that Lewis acid site-adsorbed NH3 was responsible for the SCR activity at low temperatures. Additionally, the active nitrate amount was significantly enhanced with increased Cu concentration. It implied that the formation of nitrite and nitrate was accelerated. In a word, a high Cu exchange level could favor the formation of active Cu−NH3, Cu−NO2− and Cu−NO3−, making the SCR activity improved at low temperatures eventually.
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REFERENCES
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ASSOCIATED CONTENT
S Supporting Information *
Temperature-programmed desorption of NH3 (NH3-TPD), DRIFT spectra of coadsorption and reaction, SCR activity test. This material is available free of charge via the Internet at http://pubs.acs.org. F
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catalysts for the selective catalytic reduction of NOx. ACS Catal. 2013, 3 (3), 413−427. (13) Wang, D.; Zhang, L.; Kamasamudram, K.; Epling, W. S. In situDRIFTS study of selective catalytic reduction of NOx by NH3 over Cu-exchanged SAPO-34. ACS Catal. 2013, 3 (5), 871−881. (14) Grossale, A.; Nova, I.; Tronconi, E.; Chatterjee, D.; Weibel, M. The chemistry of the NO/NO2−NH3 “fast” SCR reaction over FeZSM-5 investigated by transient reaction analysis. J. Catal. 2008, 256 (2), 312−322. (15) Iwasaki, M.; Shinjoh, H. A comparative study of “standard”, “fast” and “NO2” SCR reactions over Fe/zeolite catalyst. Appl. Catal. A: Gen. 2010, 390 (1−2), 71−77. (16) Gao, F.; Walter, E. D.; Washton, N. M.; Szanyi, J.; Peden, C. H. F. Synthesis and evaluation of Cu-SAPO-34 catalysts for ammonia selective catalytic reduction. 1. Aqueous solution ion exchange. ACS Catal. 2013, 2083−2093. (17) Yeom, Y. H.; Henao, J.; Li, M. J.; Sachtler, W. M. H.; Weitz, E. The role of NO in the mechanism of reduction with ammonia over a BaNa−Y catalyst. J. Catal. 2005, 231 (1), 181−193. (18) Elzey, S.; Mubayi, A.; Larsen, S. C.; Grassian, V. H. FTIR study of the selective catalytic reduction of NO2 with ammonia on nanocrystalline NaY and CuY. J. Mol. Catal. A: Chem. 2008, 285 (1−2), 48−57. (19) Zhu, H. Y.; Kwak, J. H.; Peden, C. H. F.; Szanyi, J. In situ DRIFTS-MS studies on the oxidation of adsorbed NH3 by NOx over a Cu-SSZ-13 zeolite. Catal. Today 2013, 205, 16−23. (20) Centi, G.; Perathoner, S.; Biglino, D.; Giamello, E. Adsorption and reactivity of NO on copper-on-alumina catalysts: I. Formation of nitrate species and their influence on reactivity in No and NH3 conversion. J. Catal. 1995, 152 (1), 75−92. (21) Centi, G.; Perathoner, S. Adsorption and reactivity of NO on copper-on-alumina catalysts: II. Adsorbed species and competitive pathways in the reaction of NO with NH3 and O2. J. Catal. 1995, 152 (1), 93−102. (22) Xie, L.; Liu, F.; Liu, K.; Shi, X.; He, H. Inhibitory effect of NO2 on the selective catalytic reduction of NOx with NH3 over one-potsynthesized Cu-SSZ-13 catalyst. Catal. Sci. Technol. 2014, 4 (4), 1104− 1110. (23) Kwak, J. H.; Lee, J. H.; Burton, S. D.; Lipton, A. S.; Peden, C. H. F.; Szanyi, J. A common intermediate for N2 formation in enzymes and zeolites: Side-on Cu-nitrosyl complexes. Angew. Chem., Int. Ed. 2013, 52 (38), 9985−9989. (24) Ruggeri, M. P.; Selleri, T.; Colombo, M.; Nova, I.; Tronconi, E. Identification of nitrites/HONO as primary products of NO oxidation over Fe-ZSM-5 and their role in the Standard SCR mechanism: A chemical trapping study. J. Catal. 2014, 311, 266−270. (25) Wang, L.; Li, W.; Qi, G.; Weng, D. Location and nature of Cu species in Cu/SAPO-34 for selective catalytic reduction of NO with NH3. J. Catal. 2012, 289 (0), 21−29. (26) Fu, Y.; Tian, Y.; Lin, P. A low-temperature IR spectroscopic study of selective adsorption of NO and CO on CuO/γ-Al2O3. J. Catal. 1991, 132 (1), 85−91. (27) Boccuzzi, F.; Guglielminotti, E.; Martra, G.; Cerrato, G. Nitric oxide reduction by CO on Cu/TiO2 Catalysts. J. Catal. 1994, 146 (2), 449−459. (28) Skarlis, S. A.; Berthout, D.; Nicolle, A.; Dujardin, C.; Granger, P. Combined IR spectroscopy and kinetic modeling of NOx storage and NO oxidation on Fe-BEA SCR catalysts. Appl. Catal. B: Environ. 2014, 148−149 (0), 446−465. (29) Ruggeri, M. P.; Grossale, A.; Nova, I.; Tronconi, E.; Jirglova, H.; Sobalik, Z. FTIR in situ mechanistic study of the NH3NO/NO2 “Fast SCR” reaction over a commercial Fe-ZSM-5 catalyst. Catal. Today 2012, 184 (1), 107−114. (30) Kijlstra, W. S.; Brands, D. S.; Poels, E. K.; Bliek, A. Mechanism of the selective catalytic reduction of NO by NH3 over MnOx/Al2O3. J. Catal. 1997, 171 (1), 208−218. (31) Colombo, M.; Nova, I.; Tronconi, E. Detailed kinetic modeling of the NH3−NO/NO2 SCR reactions over a commercial Cu-zeolite G
DOI: 10.1021/es503430w Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Reaction Pathway Investigation on the Selective Catalytic Reduction of NO with NH3 over Cu/SSZ-13 at Low Temperatures Wenkang Su,† Huazhen Chang,† Yue Peng,† Chaozhi Zhang,*,‡ and Junhua Li*,† †
State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, P. R. China ‡ School of Environmental Science and Engineering, Nanjing University of Information Science and Technology, Nanjing 210044, P. R. China S Supporting Information *
ABSTRACT: The mechanism of the selective catalytic reduction of NO with NH3 was studied using Cu/SSZ-13. The adspecies of NO and NH3 as well as the active intermediates were investigated using in situ diffuse reflectance infrared Fourier transform spectroscopy and temperature-programmed surface reaction. The results revealed that three reactions were possible between adsorbed NH3 and NOx. NO2− could be generated by direct formation or NO3− reduction via NO. In a standard selective catalytic reduction (SCR) reaction, NO3− was hard to form, because NO2− was consumed by ammonia before it could be further oxidized to nitrates. Additionally, adsorbed NH3 on the Lewis acid site was more active than NH4+. Thus, SCR mainly followed the reaction between Lewis acid site-adsorbed NH3 and directly formed NO2−. Higher Cu loading could favor the formation of active Cu-NH3, Cu-NO2−, and Cu-NO3−, improving the SCR activity at low temperature.
1. INTRODUCTION The selective catalytic reduction (SCR) of nitrogen oxides with NH3 is considered an effective way to control NOx emission from mobile sources. Among available catalytic formulations, metal exchanged zeolites have shown superior SCR performance. In general, the overall “standard SCR” reaction between NO and NH3 can be expressed as follows: 4NH3 + 4NO + O2 → 4N2 + 6H 2O
that the proton-adsorbed NH3 was less active compared to CuNH3. These researches implied that the Cu-linked NOx and NH3 species were active intermediates. On the other hand, Di et al.13 has proposed that the NH4+ form at Brönsted acid sites or migrate from Lewis acid sites was the active intermediates. Regarding the active NOx adspecies, Xie22 and Kwak23 has proposed the Cu−NO3− and Cu−NO as the active NOx adspecies in the SCR reaction, respectively. While, Ruggeri et al.24 proposed the nitrite as the intermediate, and directly detected the formation and reaction of nitrite with ammonia, which improved our understanding of reaction mechanism significantly. In the present study, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and TPSR were used to investigate the possible SCR reactions and active intermediates on Cu/SSZ-13 with different copper concentrations. In other words, we intended to investigate the active adspecies of NH3 and NOx and their transformation. Furthermore, we intended to elaborate on all the possible reactions involved in standard SCR and their relative importance. The data indicated that three reactions were possible at lower temperatures and the one playing the major role was the reaction between Lewis acid site-adsorbed NH3 and NO2−.
(1)
Recently, Cu(II) exchanged zeolites, Cu/SSZ-13, with the chabazite (CHA) structure have been commercialized as NH3SCR catalysts in diesel engines for their broad activity window, stable performance, and their high selectivity to N2.2−6 Investigations have been performed on the active copper sites and reaction mechanism.7−12 Most scientists agree that the reaction follows the Langmuir−Hinshelwood (L-H) mechanism at lower temperatures, in which the adsorbed NOx reacts with the NH3 adspecies to form N2 and H2O. On the basis of different intermediates formed during the reaction, two types of pathways belonging to the L-H type mechanism have been proposed on SCR catalyst: (i) the adspecies of NH4+ and NO2− forms ammonia nitrite, NH4NO2, which decomposes into N2 and H2O at temperatures higher than 100 °C;13−15 and (ii) adsorbed NH3 and NO oxidation species form ammonia nitrate NH4NO3 on metal sites, and it is subsequently reduced by NO.1,16−18 There is still no clear consensus on the detailed reaction step network on Cu/SSZ-13, including the active intermediates and reactive sites. Recently, Gao11 believed the NO and NH3 was oxidized on Cu sites. It was19−21 proposed © XXXX American Chemical Society
Received: July 16, 2014 Revised: November 25, 2014 Accepted: December 8, 2014
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2. MATERIALS AND METHODS Catalyst Synthesis. The H/SSZ-13 (Si/Al = 10) was added into 0.1 M NH4NO3 solution and stirred at 70 °C for 4 h. The solid was then separated from the liquid phase by centrifugation and then dried at 110 °C overnight. The process was repeated three times to obtain NH4/SSZ-13. The Cu/SSZ13 catalysts were prepared by aqueous ion exchange. A 5 g portion of NH4/SSZ-13 was poured into four types of Cu(CH3COO)2·H2O solution with different concentrations, and stirred at 80 °C for 24 h. Then, the samples were thoroughly washed with deionized water followed by drying and calcination at 550 °C for 6 h. The elementary composition of the zeolite framework and Cu content in the four samples were measured by ICP-OES analysis. The results showed that the Si/Al ratio was 10 and the four catalysts contained 0.6, 0.9, 1.4, and 1.7 wt % copper. The corresponding ion exchange (IE) level was 17.3%, 26%, 44.5%, and 51.2%, respectively. The BaO/Al2O3 was synthesized as referenced.24 The mass ratio of mixture of BaO/Al2O3 and Cu/SSZ-13 was 3. Catalyst Characterization. The infrared spectra were recorded on a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer equipped with an MCT detector. The in situ diffuse reflectance FTIR spectroscopy (DRIFTS) experiments were performed in a high temperature chamber with a ZnSe window. The powder sample was placed in the Harrick IR cell with a copper sieve support. The total flow was maintained at 100 mL/min. Before each experiment, the catalyst was pretreated at 500 °C for 30 min at a flow of 5% O2 in N2. Then, the sample was cooled to the desired temperature. At each temperature, the background spectrum was recorded at N2 flow and subtracted from the sample spectrum obtained at the same temperature. The spectra were collected in the range of 4000−800 cm−1 at a resolution of 2 cm−1 and 32 scans of accumulation. To diminish the influence of absorbance from different samples, the absorbance intensity was set to 2.5 for every sample at 500 °C. The key band was quantified according to area integral, and the change in peak area was shown in terms of percentage. All of the infrared spectra were collected in the absence of water because of its wide interference with IR bands. We assumed that the surface intermediates and reaction mechanism still applied to the feed in the presence of water.13 All the designed DRIFT experiments were carried out parallelly in a fixed bed reactor. The reaction gas was fed to 100 mg of catalyst at a flow rate of 200 mL/min. The products were measured by an online FTIR spectrometer to monitor the concentration of NO, NO2, N2O, and NH3, and mass spectrometer (MS) to monitor the H2O and N2.
Figure 1. DRIFTS spectra obtained during NH3 desorption after the sample had been exposed to 500 ppm of NH3/N2, followed by a purging with N2.
NH3-TPD on 1.7 wt % Cu/SSZ-13. After the adsorption at 50 °C, coordinated NH3 (1625/1200 cm−1) and NH4+ (1470 cm−1) ions13,18,19,25 were generated. The peaks in the range of 3700−3500 cm−1 represented the Si−OH and Si−OH−Al structure consumed by the adsorption of NH4+.19,25 At 200 °C, the peak at 3665 cm−1 assigned to the terminal hydroxyl Si− OH decreased significantly, and the 1470 cm−1 peak representing NH4+ decreased simultaneously. The terminal hydroxyl acid was weak, indicating that terminal hydroxyl was not involved in SCR. When the temperature reached 350 °C, the coordinated NH3 on Lewis acid sites (1625 cm−1) decreased significantly, while the NH4+ ions adsorbed at the bridging Si−OH−Al hydroxyl maintained about 70% of their intensity at 350 °C. Thus, the acid strength followed the sequence: bridging Si−OH−Al Brönsted acid > Lewis acid > terminal hydroxyl Si−OH Brönsted acid. The sequence showed that the NH3 adsorbed on the Lewis acid was unstable compared to the Si−OH−Al Brönsted acid, which meant that the former might be more easily removed from the catalyst surface and be more active in the SCR reaction at lower temperatures than NH4+ ions on the Si−OH−Al Brönsted acid. The NH3 desorption pattern on the H/SSZ-13 and Cu/SSZ13 catalyst with the copper concentration ranging from 0.6 wt % to 1.7 wt % was shown in Supporting Information, Figure S1. Three primary peaks were found at 150, 300, and 450 °C on Cu/SSZ-13 catalysts. The quantitative results of NH 3 desorption were summarized in Supporting Information, Table S2. On the basis of our infrared spectra in Figure 1, the three peaks could be assigned to the ammonia desorption mainly from the terminal hydroxyl Si−OH Brönsted acid sites, Lewis acid sites, and bridging Si−OH−Al Brönsted acid sites, respectively. With increasing copper concentration, both the weak and strong Brönsted acid sites (150/450 °C) decreased; in addition, the Lewis acid sites (300 °C) increased significantly from 1.5 to 8 mL/g, verifying the fact that the Brönsted acid protons were substituted by Cu2+ ions. The increased new Lewis acid formed by substituted Cu2+ ions might be the key intermediate according to the strength sequence. In terms of the NO + O2 TPD experiment, the adsorption gas was switched to 500 ppm of NO in a 5% O2 flow with N2 as the balance at 50 °C, and the desorption temperature was set at 150, 300, and 500 °C. Figure 2 shows the DRIFT spectra collected on both the H/ SSZ-13 and Cu/SSZ-13 zeolite after NO + O2 adsorption at 50 °C. The 1891 and 1938 cm−1 peaks could be assigned to NO
3. RESULTS AND DISCUSSION 3.1. Adspecies of NH3 and NO. The acidic property of the SCR catalyst was an important factor that influenced the SCR performance. To elucidate the acidic properties of the four catalysts, the DRIFT spectra of NH 3 adsorption and subsequent temperature-programmed desorption were collected, and the type, strength, and quantity of the acidic sites on Cu/SSZ-13 and H/SSZ-13 were examined. The catalysts were exposed to 500 ppm of NH3 in a N2 flow for 30 min at 50 °C, and then purged by N2 for another 30 min. The infrared spectrum was collected at 50, 200, 350, and 500 °C after reaching a steady state. Additionally, the product of the NH3TPD was monitored by online FTIR to obtain the quantitative results. Figure 1 shows the DRIFT spectra obtained during B
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The NO adsorption and evolution on the zeolite surface was summarized as follows: NO + Cu 2 +−OH ··· Cu+−HONO
(R3)
Cu+−HONO + H−OZ ··· NO+−OZ + H 2O + Cu+ (R2) +
2−
Cu −ONO + O
··· Cu −ONO2
Cu+ + O2 − ··· Cu 2 +
coordinately adsorbed at the Cu oligomer and the Cu ions, respectively.26,27 The band intensity of Cu2+−NO (1891 cm−1) over 1.7% Cu-SSZ-13 was higher than that over 0.6% Cu-SSZ13, while the intensity of the Cu2+−NO (1935 cm−1) was lower. This may imply Cu aggregation with increased Cu loading. Cu oligomer was more oxidative than isolated ions,10 meaning the higher Cu loaded catalyst tended to favor the NOx oxidation. The weak bans at 1675/1510 cm−1 were assigned to NO2−.28 These two bands were very unstable, and they could not detected at reaction temperature (150 °C), making it difficult to investigate their reactivity. The bands at 2197/2155 cm−1 were assigned to NO+ linked to both the Cu and zeolite framework. This formed either by direct adsorption on zeolite or nitrite readsorption,23,24,28,29 +
+
NOg + 2H − OZ ⇔ NO −OZ + HO−H −Z
(R4) (R5)
The integration results of NH3 and NOx desorption were shown in Supporting Information, Table S1. To further study the adsorption adspecies and capacity of NH3 and NO under standard SCR conditions, 0.1 g of catalyst was exposed to 500 ppm of NO, 500 ppm of NH3, and 5% O2 for an hour, followed by purge and the temperature rising from 150 to 600 °C. Figure 3 shows the desorption results. It could be clearly seen that, the
Figure 2. DRIFTS spectra taken after exposing the H/SSZ-13 and Cu/SSZ-13 sample to 500 ppm of NO + 5% O2/N2 for 30 min at 50 °C.
+
+
Figure 3. Outlet gas concentrations in temperature-programmed desorption period after preadsorbed NH3, NO, and O2 at 150 °C on 1.7% Cu/SSZ-13 catalysts.
(R1)
Cu+−HONO + H−OZ ··· NO+−OZ + H 2O + Cu+
composition of desorption gas was mainly NH3 and only a trace amount of N2O was detected. The production of N2O was thought to be the decomposition of NH4NO3.14,31 The amount of N2O (0.022 mL/g) was negligible compared to NH3 (12 mL/g). However, the adsorption amount of NH3 was about three times that of NO in the separated adsorption experiments as shown in Supporting Information, Table S1. The difference indicated that NO3− was hard to form on the catalyst surface in the presence of NH3. DRIFTS spectra of the coadsorption of NH3, NO, and O2 at 150 °C were also obtained as shown in Supporting Information, Figure S3. The feature of the nitrate species was not obvious. Zhu19 had also found that the adsorption of NH3 is much stronger than that of NO. Additionally, as the intermediate nitrite was captured by ammonia adspecies promptly on formation, it could hardly be further oxidized to form NO3−. 3.2. Reaction Pathways at Low Temperatures on Cu/ SSZ-13. Reactivity of Ammonia Adspecies. To investigate the active NH3 adspecies on the Cu/SSZ-13, the NH3 preadsorbed catalysts were obtained by treating the catalyst with 500 ppm of NH3 for 30 min and then purging with N2 gas. Subsequently, the N2 was switched to 500 ppm of NO in 5% O2 gas. The spectra were recorded during the reaction between NO and the presorbed NH3 at 150 °C. Figure 4 shows the IR spectra of the reaction between the NH3 adspecies and NO + O2 at 150 °C on the 1.7 wt % Cu/ SSZ-13 catalysts. And the normalized peak areas of key bands were calculated to more clearly elucidate the evolution of the
(R2)
The 1575 cm−1 band only existed on the Cu exchanged zeolite and increased significantly with Cu concentration, indicating that this kind of nitrate species was adsorbed on copper sites, while the 1629/1609 cm−1 feature existed on both H/SSZ-13 and Cu/SSZ-13 zeolites and they could be nitrated adsorbed on the zeolite framework. In association with other related work, the features in this range were attributed as follows: monodentate nitrate linked to Cu sites (1575 cm−1), monodentate nitrate linked to Al sites(1609 cm−1), and bridging nitrate based on Al sites(1629 cm−1), respectively.1,28−30 To confirm the formation of NO2− at 150 °C, the NO + O2TPD experiment was performed in the same way as in ref 24 on the mixture of Cu/SSZ-13 and BaO/Al2O3 at 150 °C. The formed NO2− on Cu/SSZ-13 would transfer to BaO/Al2O3 to avoid further oxidation. The result was shown in Supporting Information, Figure S2. The desorption ratio of NO/NO2 was close to 1 below 300 °C, and the NO2 desorption at higher temperature was much lower than the NO desorption. The result verified that the nitrite was formed on the catalyst surface, and the absence of nitrate adspecies indicated that the NO3− formation originated from the further transformation of NO2−. Partial Cu+−ONO would be stored on the zeolite surface in the form of NO+−OZ, if it would not be further oxidized to Cu+−ONO2 or captured by NH3. And the NO+− OZ was not reactive as no direct evidence was detected by IR. C
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quickly. According to the former NO + O2-TPD results, NO2− was formed on the surface. This indicated that the NH3 on the Lewis acid sites have reacted with NO2− nitrite before the CuNO3− sites formed on the catalyst surface. As NH4NO2 was very unstable, it would easily decompose to N 2 and H2O.17,24,32−34 Thus, it was not detected in IR. The corresponding reaction between NH3 and NO2− could be expressed as following: Cu−NO2− + NH3−Cu−H ··· NH4NO2 ····N2 + H 2O (eq 1) +
The intensity of the NH4 ions and corresponding negative hydroxyl band was almost unchanged in the first 15 min. In the subsequent 15 min, the band at 3606 and 1470 cm−1 decreased in unison, accompanied by the significant decrease of Cu− NO3− at the same rate, indicating that the NH4+ ions mainly formed on bridging Si−OH−Al hydroxyl were consumed by Cu linked nitrate. The increase in the intensity of band at 1470 cm−1 in the beginning might originate from the strengthening of NH4+ ions vibration with nitrate bonded to it. Furthermore, the 1625 cm−1 band decreased more quickly compared with the bands corresponding to NH4+ ions, indicating the Lewis acid bonded NH3 was more active, consistent with the NH3-TPD results where the Lewis acid sites bonded NH3 was unstable compared to NH3 on Brönsted acid sites. Figure 5 shows the outlet concentration in the reaction between NO and gas phase ammonia (A) and presorbed ammonia (B) on 0.6% Cu/SSZ-13 at 150 °C followed by temperature-programmed desorption from 150 to 550 °C. As Figure 5A shows, both NH3 adsorbed on Lewis acid and Brönsted acid sites desorbed in the temperature rising period after the SCR reaction gas in the stream was cut off, indicating that both the adsorbed NH3 on the Lewis acid site and the NH4+ ions existed on the catalyst surface in the SCR reaction. In Figure 5B, where the ammonia was preadsorbed on the surface, only NH3 adsorbed on the Brönsted acid sites desorbed in the temperature rise period, indicating that the NH3 adsorbed on the Lewis acid site was consumed completely by NO. The results in Figure 5 verified that both the adsorbed NH3 on the Lewis acid sites and the NH4+ ions on the Brönsted acid sites were involved in the reaction and further confirmed that the adsorbed NH3 on the Cu sites reacted faster than the NH4+ ions. Reactivity of NOx Adspecies. To investigate the active nitrate adspecies (centered at 1575 cm−1) on the Cu/SSZ-13, the infrared spectra were recorded during the reaction between NH 3 and the presorbed NO at 150 °C. Supporting
Figure 4. (A) DRIFTS spectra taken at 150 °C upon passing 500 ppm of NO + 5% O2/N2 over NH3-adsorbed 1.7 wt % Cu/SSZ-13 for 0− 30 min. (B) Normalized DRIFT peaks intensity.
intermediates. When the catalyst surface was saturated with NH3, both bands assigned to the Lewis acid sites (1625 and 1200 cm−1) and Brőnsted acid sites (1470 and 3606 cm−1) were detected. As the figures show, the NH3 on the Lewis acid sites began to decrease slowly upon exposure to NO and O2, and after 20 min, it was hardly detected. In 10 min, new nitrate peaks centered at 1575 cm−1, 1609 cm−1, 1629 cm−1 emerged. Accompanied by the accumulation of Cu linked nitrate, the intensity of band centered at 1625 cm−1 decreased more
Figure 5. Temperature-programmed surface reaction: NO reaction with gas phase ammonia (A) or presorbed ammonia (B) on 0.6% Cu/SSZ-13 at 150 °C followed by temperature-programmed desorption from 150 to 550 °C. D
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almost at the same rate. The 1200 cm−1 band decreased down to zero at 8 min, only 2 min faster than the 3606 and 1470 cm−1 band. Simultaneously, the peaks assigned to the NOx species became sharp and increased after the consumption of NH4+, which meant an accumulation of nitrate and NO+ on the catalyst surface. The results indicated that the reaction between NH4+ and NO3− could not occur without gas phase NO. It had been proposed that NO3− could be reduced by NO in the gas phase to form NO2−.22,29,32,35 As NH4NO2 was very unstable, it would easily decompose to N2 and H2O.17,32,34,36 Thus, the nitrite species was hard to detect in our study because of its rapid reaction with the NH3 adspecies on the surface or its overlapping regions with the zeolite framework vibration.13,29,37 The faster reaction rate originated from the existence of more nitrate species on the catalyst surface formed by the preadsorption of NO + O2. For the catalysts that showed more affinity to NH3 than NO and the nitrite quick reaction, it was difficult for the nitrate species to form in the presence of ammonia adspecies. The NO preadsorption treatment could enhance the coverage of nitrate; thus, the reaction rate of the ammonia adspecies was accelerated. The corresponding reaction between NH4+ and NO3− could be expressed as follows:
Information, Figure S4 shows the DRIFT spectra collected during the reaction. The NH4+ ions and Lewis acid siteadsorbed NH3 peaks were detected as time increased, while the peaks assigned to nitrate were unchanged, which meant that NH4+ ions did not react with nitrates. Compared to the experimental conditions in Figure 4, gas phase NO did not exist in this case, indicating that the NO may have played an important role in the reaction between the NH4+ ions and nitrate. Simultaneously, N2 and H2O were not detected in the outlet gas by MS, indicating the NO+ (formed by NO2− transformation) was not active, either. Further experiment was performed to elucidate the role of NO in the SCR reaction as shown in Figure 6. A flow of NO in
Cu−NO3− + 2Z−O−(NH4)···(NH4)2 NO3
(NH4)2 NO3 + NO ····(NH4)2 (NO2 )2 ··· 2N2 + 3H 2O + 2ZO−H
(eq 2)
In Figure 6, the consumption of adsorbed NH3 on Cu sites was also accelerated by the preformation of nitrate species, and disappeared in 8 min which was much shorter than that needed in Figure 4. The accelerated reactivity of the Lewis acid site adsorbed NH3 indicated that it could also react with NO2− species generated by NO3− reduction. The reaction between Lewis acid-site adsorbed NH3 and NO3− could be expressed as follows: Cu−NO3− + 2NH3−Cu−H ···(NH4)2 NO3
(NH4)2 NO3 + NO ····(NH4)2 (NO2 )2 ··· 2N2 + 3H 2O + 2ZO−H
(eq 3)
3.3. The Main Reaction for SCR at Low Temperatures. The main adspecies of NO on the Cu/SSZ-13 was NO+, NO2−, and NO3−. NO3− originated from NO2− further oxidation. If Cu- NO2− would not be consumed or further oxidized, it would adsorb on the zeolite surface and form Z-O-NO+. Nitrate could hardly form for the competitive adsorption of NH3 based on the coadsorption results. Additionally, the quick reaction between NO2− and NH3 would inhibit the further oxidation of NO2− to form NO3−. And the NO+ species was not detected to be involved in the reaction. Thus, NO2− was believed to be the main active NOx species. As the Lewis acid site adsorbed NH3 showed more reactivity than NH4+ ions. The reaction mainly followed the pathway involved the reaction between Lewis acid adsorbed NH3 and directly formed NO2−. While the formation of active Cu-NO3− on the catalyst surface could improve the SCR activity significantly as the results shown in Figure 6. Thus, the reaction involved Cu-NO3− could not be neglected. The discussed reactions at low temperatures were summarized in Figure 7.
Figure 6. (A) DRIFTS spectra taken at 150 °C upon passing 500 ppm of NO over NO + O2 and NH3-adsorbed 1.7 wt % Cu/SSZ-13 for 0− 15 min. (B) Normalized DRIFT peaks intensity.
N2 was passed over the catalyst with presorbed NO and NH3 on the catalysts one after another. After the preadsorption, with peaks attributed to NH3 and NH4+, three different types of nitrates and Cu−NO+ were detected. The normalized peak areas of key bands were calculated, and the results are shown in Figure 6B. The ammonia adspecies, both Lewis acid sites bonded NH3 and NH4+ ions on Brönsted acid sites, decreased significantly upon exposure to NO, a process which was much faster than the reaction in Figure 4. According to the quantitive results, the bands at 1200, 3606, and 1470 cm−1 decreased E
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AUTHOR INFORMATION
Corresponding Authors
*Phone: +86 10 62771093; e-mail: [email protected]. *Phone: +86-25-58731221; e-mail: [email protected]. cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Fund of China (Grant Nos. 21325731 and 51478241) and the National High-Tech Research and Development (863) Program of China (Grant No. 2013AA065304), and Jiangsu Specially-Appointed Professor Program (R2012T01). The authors would also like to acknowledge the financial support of the Ford China University Research Program from the Chemical Engineering & Fuel Cell department, Research & Innovation Center, Ford Motor Company.
Figure 7. Reaction scheme of SCR of NO with ammonia on Cu/SSZ13 at lower temperatures.
Supporting Information, Figure S5 shows the NO conversion as a function of temperature for the series of catalysts with Cu concentrations ranging from 0.6 to 1.7 wt %. The SCR activity was clearly increased for the higher Cu loaded catalyst. Figure 8
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Figure 8. Amount of adspecies of NH4+ (black), Cu-NH3 (red), and Cu-NO3− (green) formed on Cu/SSZ-13 zeolites with varying Cu loading and the corresponding reaction rate at 150 °C: (1) 0.6 wt % Cu/SSZ-13; (2) 0.9 wt % Cu/SSZ-13; (3) 1.4 wt % Cu/SSZ-13; (4) 1.7 wt % Cu/SSZ-13).
showed the relationship between the quantity of active intermediates and the corresponding reaction rate at 150 °C on Cu/SSZ-13 zeolites with varying Cu loading. As was shown, the reaction rate increased with copper loading while the NH4+ ions showed a decreasing trend. It conformed to our consumption that Lewis acid site-adsorbed NH3 was responsible for the SCR activity at low temperatures. Additionally, the active nitrate amount was significantly enhanced with increased Cu concentration. It implied that the formation of nitrite and nitrate was accelerated. In a word, a high Cu exchange level could favor the formation of active Cu−NH3, Cu−NO2− and Cu−NO3−, making the SCR activity improved at low temperatures eventually.
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ASSOCIATED CONTENT
S Supporting Information *
Temperature-programmed desorption of NH3 (NH3-TPD), DRIFT spectra of coadsorption and reaction, SCR activity test. This material is available free of charge via the Internet at http://pubs.acs.org. F
DOI: 10.1021/es503430w Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology
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