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The Origin of an Unexpected Increase in NHSCR Activity of Aged Cu-LTA Catalysts Nak Ho Ahn, Taekyung Ryu, Yonjoo Kang, Hyojun Kim, Jiho Shin, In-Sik Nam, and Suk Bong Hong ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b02852 • Publication Date (Web): 05 Sep 2017 Downloaded from http://pubs.acs.org on September 5, 2017
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The Origin of an Unexpected Increase in NH3-SCR Activity of Aged Cu-LTA Catalysts Nak Ho Ahn†, Taekyung Ryu†, Yonjoo Kang, Hyojun Kim, Jiho Shin, In-Sik Nam, and Suk Bong Hong* Center for Ordered Nanoporous Materials Synthesis, Division of Environmental Science and Engineering, POSTECH, Pohang 37673, Korea ABSTRACT: We have recently reported an increase in low-temperature NH3-SCR activity of the copper-exchanged (Cu/Al = 0.50), high-silica (Si/Al = 23) LTA catalyst when hydrothermally aged at 1023-1173 K. Here we demonstrate that this unexpected phenomenon originates from the migration of Cu+ ions present inside the sod cages of Cu-LTA to the vacant single 6-rings which accompanies their oxidation to Cu2+ ions during (hydro)thermal aging. Hence, the sod cages, which are inaccessible to almost all reactant species so as to be regarded as useless for zeolite catalysis, were found to serve as a “catalyst reservoir” during the course of nitrogen oxides (NOx) reduction with NH3.
KEYWORDS: Cu-LTA, copper cation migration, heterogeneous catalysis, NH3-SCR, zeolites There are unmet social needs for cleaner air. While this has led the automotive industry to wonder how to solve severe air quality problems, the internal combustion engine is expected to dominate the global market for the next several decades.1 However, NOx reduction from the oxygen-rich diesel exhaust remains one of the major technical challenges for the more widespread penetration of fuel-efficient diesel engines. Although selective catalytic reduction with urea and thus with ammonia (NH3-SCR) is a commercially proven technology, in addition, keen interest has developed in enhancing the hydrothermal durability of SCR catalysts for future diesel aftertreatment systems.2-5 Recently, we have been successful in synthesizing the highsilica (Si/Al > 8) version of zeolite A (framework type LTA) using benzylimidazolium-based structure-directing agents in fluoride media, when necessary, together with the tetramethylammonium ion.6,7 More importantly, divalent copper ions when fully exchanged into LTA zeolites with Si/Al = 16-23 were found to show remarkable NO reduction activities even after hydrothermal aging at 1173 K, where the commercial Cu-SSZ-13 (CHA) catalyst cannot maintain its structural integrity at all. An unexpected result is the improvement of the low-temperature deNOx activity of Cu-LTA with Si/Al = 23 by hydrothermal aging at temperatures up to 1173 K.6,7 On the other hand, the copper ions exchanged into the silicoaluminophosphate version of SSZ-13, i.e., SAPO-34, have also been reported to show similar catalytic behavior.8,9 Although the reason for this has not yet been elucidated with certainty, the migration of copper species remained on the external surface of SAPO-34 crystals to the double 6-ring (d6r) unit, a wellestablished active Cu2+ site in this small-pore material for NH3-SCR,2-5 during hydrothermal aging has been suggested as a plausible hypothesis. Given notable differences in the Cu2+
cation sites between LTA and CHA framework structures,10 however, the origin of an increase in deNOx activity of hydrothermally aged Cu-LTA-23-0.50 catalysts, where the last two values represent the Si/Al and Cu/Al ratios, respectively, can be inherently different from that of Cu-SAPO-34. Herein, we present clear evidence that the migration of monovalent copper ions located within the 14-hedral ([4668]) sod cages of CuLTA to unoccupied single 6-rings (s6rs), while being oxidized to divalent cations, is responsible for this unusual activity increase. Figure 1a shows NO conversion as a function of temperature in NH3-SCR reaction over fresh and 1023 K-, 1123 K-, and 1173 K-aged Cu-LTA-23-0.50 catalysts, with or without 10% H2O present in the feed. It can be seen that the activity of Cu-LTA-23-0.50 at temperatures lower than 673 K increases with increasing aging temperature. It is worth noting that the extent of increase is higher under wet conditions than under dry conditions, whereas the structure of Cu-LTA-23-0.50 remains intact even after hydrothermal aging at 1173 K for 12 h (Figure S1). This finding is of practical importance in that the activities of almost all known NH3-SCR catalysts become lower when thermally aged under wet conditions.2-7 A further increase of aging temperature to 1223 K resulted in a significant loss of its deNOx activity (Figure S2). From the slope of its Arrhenius plot (Figure 1b), the apparent activation energy (Eapp) of fresh Cu-LTA-23-0.50 was calculated to be ca. 38 kJ mol-1. Since this value is similar to the Eapp values (40-43 kJ mol-1) of Cu-SSZ-13 catalysts with quite low Cu contents (Cu/Al < 0.02), where the cations are located only in the vicinity of d6r units in the CHA framework,11,12 it appears that the effects of intracrystalline NO diffusion on the NH3-SCR activity of Cu-LTA-23-0.50 may be negligible.13,14 We were also able to observe no noticeable differences in the
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Figure 1. (a) NO conversion as a function of temperature over fresh (□) and 1023 K- (●), 1123 K- (▲), and 1173 K-aged (♦) Cu-LTA-230.50 catalysts in the absence (left) and presence (right) of 10% water vapor. The last two values of the catalyst identification correspond to the Si/Al and Cu/Al ratios, respectively, and the feed contains 500 ppm NH3, 500 ppm NO, 5% O2, and 10% H2O balanced with N2 at 100 000 h-1 gas hourly space velocity (GHSV). Aging time was 24 h at 1023 and 1123 K and 12 h at 1173 K. (b) Turnover frequency as a function of temperature over fresh (□) and 1023 K- (●), 1123 K- (▲), and 1173 K-aged (♦) Cu-LTA-23-0.50 catalysts in the presence of 10% water vapor. The feed contains 500 ppm NH3, 500 ppm NO, 5% O2, 10% H2O balanced with N2 at 400 000 h-1 GHSV.
Eapp value of hydrothermally aged catalysts at different temperatures, suggesting that the nature of active sites in this catalyst is hardly altered during aging. However, the specific activity (i.e., turnover frequency (TOF)) for NO reduction per mole of copper ions increased from 6.5 to 9.1 × 10-4 s-1 with increasing hydrothermal aging temperature to 1173 K. As such, the Cu content of Cu-LTA-23-0.50 cannot increase by treatment at high temperatures. Then, its increase in TOF value, although not so large, can be rationalized only by considering that the amount of active sites in this catalyst has increased upon hydrothermal aging. A linear combination fitting of the Cu K-edge X-ray absorption near-edge structure spectra of a series of Cu-LTA-23-0.50 catalysts prepare here reveals that copper oxides are hardly detectable in the fresh catalyst and the increase its amount caused by hydrothermal treatment is also negligible (Figure S3 and S4 and Table S1). We were able to draw a quite similar conclusion from their TEM images in which no large particles are located on the external crystal surface (Figure S5). It is interesting to note here that hydrothermal aging even at 1173 K resulted in no noticeable formation of intraparticle mesopores in Cu-LTA-23-0.50 (Figure S6), revealing the excellent structural stability of this high-silica LTA zeolite. To understand the low-temperature activity improvement described above, we located the extraframework cations in
fresh Cu-LTA-23-0.50 and its hydrothermally aged forms at different temperatures using synchrotron powder X-ray diffraction (XRD) and Rietveld analyses (Table S2-S10 and Figures S7-S10). Two different Cu sites were determined to exist in fresh, dehydrated Cu-LTA-23-0.50: one at the s6r center and the other inside the sod cage that is adjacent to 4-rings, labeled as Cu(1) and Cu(2), respectively (Figure 2a). Like those in Cu-LTA-16-0.48 with a larger Al content (Si/Al = 16) but a similar Cu/Al ratio (0.48 vs 0.50),3 the cation at site Cu(1) is coordinated to three framework oxygen atoms at 2.26 Å along the 3-fold axis and are thus divalent. However, that at site Cu(2), which was not found in Cu-LTA-16-0.48,3 is coordinated to four oxygen atoms at 2.52 Å along the 4-fold axis (Table S7), far longer than the Cu(1)-O distance. While this did not allow us to assign its oxidation state as +2, in fact, we note that the Cu(2)-O distance is applicable to the distances (2.46-3.11 Å) between the Cu+ ion and framework oxygen atoms in copper-exchanged zeolite A.15 If the s6rs in Cu-LTA are occupied by divalent copper ions, then they should have two AlO4- tetrahedra for charge balance. Considering Loewenstein’s rule,16 the two Al atoms in s6rs can be arranged in either meta or para position (Figure S11). On the other hand, the existence of two copper cation sites, i.e., Cu(1) and Cu(2), in Cu-LTA-23-0.50 suggests that the Al distribution in this material is considerably different from that
Figure 2. (a) Structure of the fresh, dehydrated Cu-LTA-23-0.50 catalyst with two different Cu2+ sites labeled as Cu(1) and Cu(2). Color code: yellow, Si; red, O; blue, Cu. (b) Numbers of Cu2+ ions at s6rs (●) and Cu+ ions within sod-cages (■) per unit cell of Cu-LTA-23-0.50 vs hydrothermal aging temperature. The NO conversions at 593 K of fresh and aged catalysts are given as a bar diagram.
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Figure 3. Emission spectra of fresh Cu-LTA catalysts with similar Cu/Al ratios (0.48-0.50) but different Si/Al ratios (11-23): CuLTA-11-0.48, green; Cu-LTA-16-0.48, red; Cu-LTA-23-0.50, blue. The excitation wavelength used is 375 nm.
in Cu-LTA-16-0.48. This is particularly true when considering the absence of any extraframework cation at site Cu(2) in the latter material.3 It should be noted here that the relative ratio of Cu2+ ions at site Cu(1) to Cu+ ions at site Cu(2) in fresh CuLTA-23-0.50 determined using powder XRD and Rietveld analyses is approximately 40:60 (Figure 2b and Table S11). To check whether the Cu+ ion could be located at site Cu(1) with one AlO4- tetrahedron as well, we performed density functional theory calculations (Figure S12). This monovalent cation was calculated to be more thermodynamically stable by about 30 kJ mol-1 when centered at the s6r than when located within the sod cage. It thus appears that the intrazeolitic migration and reduction (see below) of Cu2+ ions exchanged into Cu-LTA may be kinetically, rather than energetically controlled. Of particular interest in Figure 2b is that the number of Cu2+ ions at site Cu(1) per unit cell of Cu-LTA-23-0.50 increases with increasing hydrothermal aging temperature to 1173 K, whereas the opposite holds for the Cu+ ion at site Cu(2). This clearly shows that a portion of Cu+ ions within the sod cages migrates to the center of s6rs, together with the oxidation to Cu2+ ions, during hydrothermal aging at elevated temperatures. There is no doubt that the Cu+ ions within the sod cages of CuLTA-23-0.50 cannot serve as an active center for NH3–SCR, because NO with a kinetic diameter of 3.17 Å17 cannot diffuse into the sod cage with a 6-ring window size of 2.2 Å. Therefore, the activity increase observed for aged Cu-LTA-23-0.50 catalysts (Figure 2) can be attributed to the migration and oxidation of Cu+ ions at site Cu(2) to site Cu(1) as their divalent cation. Figure 3 shows the emission spectra of three fresh Cu-LTA catalysts with very similar Cu/Al ratios (0.48-0.50) but different Si/Al ratios (11-23). The spectra of fresh Cu-LTA-11-0.49 and Cu-LTA-23-0.50 are characterized by one broad, asymmetric band centered at 470-480 nm. Given the lack of Cu+ ions at s6rs in the latter catalyst (Figure 2) and also of oddmembered rings in the LTA framework, this band can be assigned to the S0 ← T1 transition in isolated Cu+ ions coordinated to four oxygen atoms within 4-rings in the LTA framework.18-20 The fact that no noticeable bands are observed in the emission spectrum of fresh Cu-LTA-16-0.48 reveals the absence of Cu+ ions in this catalyst, in excellent agreement with
our recent study.3 We also measured the emission spectra of a series of hydrothermally aged Cu-LTA-23-0.50 catalysts at different temperatures. All the spectra (not shown) gave a broad emission band around 470 nm. Unfortunately, however, no linear relationship between the emission band intensity and the hydrothermal aging temperature was observed. Besides a strong signal around 57 ppm, typical of tetrahedral Al, Cu-LTA-23-0 (i.e., the parent zeolite support) shows a weak resonance around 0 ppm corresponding to octahedral Al in the 27Al MAS NMR spectrum (Figure 4a). This resonance is not detectable in the spectra of fresh Cu-LTA-23-0.50 and its hydrothermally aged forms at 1023-1173 K. Nevertheless, considering their strong interactions with paramagnetic Cu2+ ions, some amount of octahedral Al species may also exist in both fresh and aged catalysts. On the other hand, if all copper species in fresh Cu-LTA-23-0.50 with Cu/Al = 0.50 are divalent cations, the resulting IR spectrum should then show very weak or no Si-OH-Al (Brønsted acid sites) bands. As shown in Figure 4b, however, the overall intensity of two OH bands appearing around 3620 and 3550 cm-1 in the IR spectrum of fresh Cu-LTA-23-0.50, assignable to the Brønsted acid sites located within large lta and smaller sod cages, respectively,21 is still ca. 50% of that of the corresponding bands from the parent zeolite Cu-LTA-23-0. From the structural results in Figure 2 that ca. 60% of the copper ions in fresh Cu-LTA-230.50 is monovalent, in fact, the intensity of its Si-OH-Al bands could be estimated to be approximately 70% of that from CuLTA-23-0. Figure 4b also shows that both Brønsted OH bands around 3620 and 3550 cm-1 decrease significantly in intensity with increasing hydrothermal aging temperature from 1023 to 1173 K. Three lines of reasoning are given below to rule out the possibility that dealumination could be responsible for this behavior. First, in our recent work, we have shown that the Cu2+ ions located at s6rs, i.e., site Cu(1), act not only as an active center for NH3-SCR, but also as a dealumination suppressor.3 Their number per unit cell of Cu-LTA-23-0.50 becomes larger when hydrothermally aged at higher temperature (Figure 2b) so that the resistance against dealumination of the resulting catalyst should be greater. Second, as shown in Figure 1b, the TOF value of Cu-LTA-23-0.50 increases with increasing hydrothermal aging temperature. Third, there are no significant differences in the 27Al NMR line shape of fresh and hydrothermally aged catalysts at different temperatures (Figure 4a). Therefore, a notable intensity decrease of Brønsted OH bands observed from aged Cu-LTA-23-0.50 catalysts can be considered as indirect evidence that the Cu+ ions within the sod cages are oxidized to divalent copper upon migration to the center of s6rs. This may lead to the disappearance of one Brønsted acid site in Cu-LTA-23-0.50 per migration/oxidation of each Cu+ ion. Based on the results presented thus far, the overall reaction for the migration and redox reaction of copper cations in CuLTA can be written as [Cu(H2O)]2+(ZO-s6r)2 + ZOsodH
migration reduction oxidation
[Cu(OH)]+(ZO-sod) + 2ZOs6rH Cu+ (ZO-sod) + 1/2H2O + 1/2O2 + 2ZOs6rH (1)
where ZO-s6r and ZO-sod are s6rs and sod cages in the LTA framework and ZOs6rH and ZOsodH their proton form. Here we assume that the copper ions in as-ion-exchanged Cu-LTA exist as divalent exclusively at site Cu(1). We also assumed that the
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Figure 4. (a) 27Al MAS NMR spectra and (b) IR spectra in the OH region of (from bottom to top) Cu-LTA-23-0, fresh Cu-LTA23-0.50, and 1023 K-, 1123 K-, and 1173 K-aged Cu-LTA-230.50 catalysts in the presence of 10% water vapor. Prior to IR experiments, each catalyst was dehydrated at 823 K.
Cu2+ be coordinated to only one water molecule in or near the s6r, because the s6r window size (2.2 Å) is not wide enough to allow the free diffusion of its hydrated form into the sod cage. From a thermodynamic point of view, on the other hand, the location of divalent copper ions at site Cu(1) should be more favorable than that of monovalent cations at site Cu(2) possessing only one AlO4- tetrahedron, because of the stronger electrostatic interactions. It is also clear that the (hydro)thermal aging conditions employed in this work provides enough energy to overcome a barrier to the copper ion migration from the thermodynamically less stable site Cu(2) to the more stable site Cu(1). The mobility of copper ions is known to be higher in hydrated zeolites than in dehydrated ones.14,22,23 This may explain why the extent of deNOx activity increase of Cu-LTA-23-0.50 is higher in the presence of 10% water vapor (Figure 1a). It has long been recognized that the catalytically active metal components occluded in the sod cages, for example, of FAU-type zeolites, cannot contribute to the heterogeneously catalyzed reaction, due to the very narrow 6-ring windows where most, if not all, reactant and product molecules cannot freely diffuse.24 As described so far, however, the Cu+ ions located with the sod cages of Cu-LTA come in useful for NH3SCR after (hydro)thermal aging at high temperatures. Thus, if Cu-LTA is implemented as the standard catalyst in the mobile SCR technology, its sod cages could function as a catalyst reservoir to supply new divalent copper ions to vacant s6rs with increasing vehicle mileage. In this regard, fine-tuning of the copper ion distribution in Cu-LTA has great practical implications in the enhancement of the catalyst activity and durability. We also determined the structure of fresh Cu-LTA-11-0.48 with a higher Al content (Si/Al = 11) in order to better understand the effects of framework Al content on the distribution of copper ions in Cu-LTA (Tables S12-S14 and Figure S13). Interestingly, the extraframework cations in this catalyst was found to be located at three different sites: Cu(1), Cu(2), and another site, labeled as Cu(3), with a population ratio of ca. 65:25:10 (Figure S14). We note here that site Cu(3), which is in the plane of 8-rings but not at their centers, is similar to the copper ion site at 8-ring windows in Cu-SSZ-13.3 The distances between the copper ion at site Cu(3) and the nearest framework oxygens O(1) and O(2) are 2.27 and 2.19 Å, respective-
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ly, indicating that the cation is divalent. Therefore, it is clear that the location of copper ions in Cu-LTA can differ notably according to the framework Al content. In addition, the Cu2+ ions in 8-rings with a window size of 4.1 Å may be more easily exposed to O2 and water molecules than the corresponding cations at narrower s6rs, thus being prone to transform into copper oxides (CuOx) during hydrothermal aging, like the case of Cu-SSZ-13.25 This may explain why the structure of CuLTA-11-0.48 collapsed even after hydrothermal aging at 1023 K.3 In summary, we have shown that the migration and oxidation of Cu+ ions located within the sod cages to vacant s6r sites as their divalent cation during (hydro)thermal aging is responsible for the unexpected increase in deNOx activity of aged high-silica (Si/Al = 23) Cu-LTA catalysts. This led us to consider the sod cages in Cu-LTA as a catalyst reservoir which provides an active center for NH3–SCR in the long run. It was also found that the framework Al content of the LTA zeolite support could dominate the distribution of copper ions and thus the activity and durability of resulting Cu-LTA catalysts, which is also true after (hydro)thermal aging. Therefore, the development of NH3-SCR catalysts with better activity maintenance than Cu-LTA may be possible with the precise control of the intrazeolitic location and distribution of active metal cations, as well as of the framework topology and composition of the zeolite support.
AUTHOR INFORMATION Corresponding Author *E-mail:
[email protected].
Author Contributions †
N.H.A. and T.R. contributed equally to this work.
Notes The authors declare no competing financial interest.
ASSOCIATED CONTENT Supporting Information. Experimental section, characterization data, crystallographic information, and additional results. This material is available free of charge via the Internet at http://pubs.acs.org.
ACKNOWLEDGMENT We acknowledge financial support from the National Creative Research Initiative Program (2012R1A3A2048833) through the National Research Foundation of Korea. We also thank K.-S. Lee and Y. H. Jung (8C and 9B, PAL, respectively) for help in X-ray absorption and diffraction data and PAL for beam time. PAL is supported by MSIP and POSTECH.
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SYNOPSIS TOC The Origin of an Unexpected Increase in NH3-SCR Activity of Aged Cu-LTA Catalysts Nak Ho Ahn, Taekyung Ryu, Yonjoo Kang, Hyojun Kim, Jiho Shin, In-Sik Nam, and Suk Bong Hong*
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