What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning

Letter pubs.acs.org/JPCL

What Makes Copper-Exchanged SSZ-13 Zeolite Efficient at Cleaning Car Exhaust Gases? Florian Göltl,*,† Rosa E. Bulo,†,‡ Jürgen Hafner,¶ and Philippe Sautet† †

Ecole Normale Supérieure de Lyon, Laboratoire de Chimie, Université de Lyon, CNRS, 46 Allée d’Italie, F-69342 Lyon Cedex 07, France ‡ Department of Theoretical Chemistry, Vrije Universiteit Amsterdam, De Boelelaan 1083, 1081 HV Amsterdam, The Netherlands ¶ Faculty of Physics, Computational Materials Physics, University of Vienna, Sensengasse 8/12, 1090 Vienna, Austria S Supporting Information *

ABSTRACT: Recently, the outstanding properties of Cu-SSZ-13 (a zeolite in the chabazite structure) for the selective catalytic reduction of nitrous oxides were discovered. However, the true nature of the active site is still not answered satisfactorily. In this work, we identify the active site for the given reaction from first-principles simulations of the total energy of Cu(II) ions in various positions in combination with previously published catalytic activity as a function of the copper exchange level. This attribution is confirmed by the simulation of vibrational properties of CO adsorbed to the reduced Cu(I) species. The relation between energetic considerations, vibrational calculations, and experiment allows a clear statement about the distribution of active sites in the catalyst. We furthermore discuss the structural properties of the active site leading to the high stability under reaction conditions over a large temperature range. The insights from this work allow a more targeted catalyst design and represent a step toward an industrial application of copper-exchanged zeolites in cleaning car exhaust gases. SECTION: Molecular Structure, Quantum Chemistry, and General Theory

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and creating divalent cations) are adsorbed, compensating for the charge deficiency and acting as active centers in chemical reactions. In this work, we investigate Cu-exchanged SSZ-13 from density functional theory calculations. This zeolite is basically built by double six-membered rings (6R), which are connected by four-membered ring units, and also form two different types of eight-membered rings (8R).20 The Cu atom represents the active site and is encountered in the Cu(II) state in the conditions of catalysis, but for analytical purposes, Cu can be reduced to Cu(I). It is, however, not clear whether Cu(I) occupies the same sites as Cu(II). Kwak et al. studied the catalytic activity as a function of the Cu(II) exchange level (EL). They found that the first sites to be exchanged are the most active ones,7 but the nature of these preferred and active sites is still unknown. Clearly, the chemical activity of these sites is determined by their environment and therefore the exact position within the zeolite framework. One way to investigate this environment of the Cu cation is to measure the infrared spectrum of adsorbed probe molecules. One of those molecules is CO, which, however, at room

atalysts containing only one atom as the reactive center are involved in some of the most important chemical processes. For instance, in nature, enzymes act as single-site catalysts and allow specific biochemical reactions, which are the foundations of life. One way to transfer this concept to industrial applications is the design of well-defined catalytic materials, one example being the positioning of transition-metal cations in the framework of microporous crystalline solids, namely, zeolites. It has been known for several years that such materials are very active in the selective catalytic reduction of nitrous oxides,1,2 environmental pollutants that are created in combustion engines. After this discovery, research has focused on different zeolite structures,3,4 but only recently, the outstanding properties of Cu-SSZ-13 (a zeolite in the chabazite structure) for this reaction were discovered.5 Its high hydrothermal stability paired with a high selectivity in this reaction makes it a prime candidate for an application in the next generations of car catalysts. These prospects have led to a large number of studies investigating this material,6−22 but the true nature of the active site is still not answered satisfactorily. Zeolites are micro- and mesoporous silicas, which have a basic chemical composition of SiO2. When one Si atom is substituted by Al, it is impossible to saturate all bonds to the neighboring O atoms, and the framework is activated. Close to these sites, electron donors such as H, Na, NH4 (formally donating one electron and creating a monovalent cation), Mg, Ca, or transition-metal atoms (formally donating two electrons © 2013 American Chemical Society

Received: April 18, 2013 Accepted: June 24, 2013 Published: June 25, 2013 2244

dx.doi.org/10.1021/jz400817c | J. Phys. Chem. Lett. 2013, 4, 2244−2249

The Journal of Physical Chemistry Letters

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temperature only adsorbs on the reduced Cu(I) species8 (this spectroscopy can only be performed ex situ). We modeled the vibrational spectrum of this molecule in previous work and found very similar values for all investigated configurations.22 Comparison with experimental results implied a single-peak spectrum with its maximum at 2156 cm−1, which is encountered in many zeolite structures. However, experimental measurements by Kwak et al.,8 which were published one month after our study, indicate a two-peak structure in the IR spectrum with one main peak at 2154 cm−1 but also a second peak at 2135 cm−1 for the case of SSZ-13. They attributed the second peak to a Cu(I) site in the 8R, which is in clear contradiction to previous theoretical work. The experimental data hence raises several questions: What is the nature of the preferred and most active Cu(II) sites in SSZ-13? Which is the Cu(I) site leading to the 2135 cm−1 band found in experiment and how is it connected to catalytic activity? Clearly, our assumption that every Cu(I) site is associated with one Al atom in the framework was able to describe the main peak but not the full spectrum. To find this missing active site, we closely studied the catalyst preparation process and investigated how the Cu(II) and, after reduction, the Cu(I) cations are positioned in the zeolite framework. After the zeolite is synthesized,23 Cu is introduced into the system via CuSO49 or Cu(NO3)2. In this approach, it can be assumed that Cu2+ is exchanged into the framework (see Scheme 1) and forms a Cu(II) site. Because the zeolite is not charged in the ion exchange process, every Cu(II) site has to be counterbalanced by two framework Al atoms. Different routes for the reduction of Cu(II) cations in the zeolite structure exist, and in this work we assume the most simple way, namely, the reduction of Cu(II) to Cu(I) by the addition of hydrogen. (Most often, Cu(II) is reduced to Cu(I) by autoreduction, which probably involves traces of the structure-directing agent still present in the framework after synthesis,7 but also, the possibility of reduction using H2 or CO exists.24) This catalyst preparation route has two consequences. First of all, every Cu site is associated with two Al atoms, which are not necessarily located in the same unit cell. The comparatively high Si/Al ratio indicates that most often, only one Al atom will be found in one unit cell, while the other atom is in one of the adjacent cells. However, the distribution of Al in the framework is never completely uniform,26 and statistics indicate that in some unit cells, two Al atoms will be present. We did not investigate cationic sites with three or more Al atoms in one unit cell because such configurations seem very unlikely at the experimentally given Si/Al ratios. In the catalyst, each Cu(II) center is associated with two Al atoms, and upon reduction to Cu(I), a H atom will bond to one of the O atoms adjacent to the Al farther away from the cation. The first part of this work concerns the energy and structure of Cu(II) in the various potential sites offered by SSZ-13. The previous discussion leads to eight possible, different cation structures for the Cu(II)-exchanged zeolite, which are displayed in Figure 1. Five Cu(II) structures (a−e in Figure 1) contain two Al atoms, while three others (f−h in Figure 1) are created by the presence of only one Al atom in the unit cell.20−22 A second Al should be located in a neighboring cell to ensure the correct electron count. In our calculations, this is modeled by subtracting an electron in the cell containing the Cu atom and adding an electron in another cell without a countercation. The sites are constructed in order to represent different situations. Sites (a) and (b) describe the case where both Al

Scheme 1. Schematic Representation of the Catalyst Preparation Process of Cu(I) Sites in Zeolitesa

a (a) Divalent Cu cations (Cu2+ in the scheme) in aqueous solution are inserted into the zeolite. (b) To avoid a charging of the framework, two monovalent cations (X+) are removed from the framework, leaving two Al atoms associated with the Cu(II) site. (c) Cu(II) is reduced to Cu(I), here by the addition of hydrogen. (d) At a microscopic level, this corresponds to a reduction of Cu(II) to Cu(I) and the addition of one proton to an O atom adjacent to one Al atom.

atoms and the cation are located within the same six-membered ring (6R). Site (c) represents a situation where one Al atom and the cation are placed in the same 6R while the second Al is located in the adjacent 6R. Sites (d) and (e) model a situation where the Cu cation is in an 8R with two Al’s close to it. Sites (f−h) represent a situation with a single Al in the unit cell and Cu placed in a 6R (f) or 8R (g,h). We modeled all of these sites using the range-separated hybrid functional HSE0631 and found Cu cations forming strong bonds with the activated O atoms for all configurations (detailed structural and energetic data are provided in the Supporting Information). Sites (a) (ΔECu(II) = exc 0 kJ/mol) and (b) (ΔECu(II) = 19 kJ/mol) where two Al’s are exc placed in a 6R are clearly favored compared to sites (c−e). Among cells containing a single Al atom in the unit cell, site (f) (where Al is again in the 6R) is favored. In order to compare with experimental measurements, it is necessary to analyze the weight of the different configurations within the distribution of active sites. From the following 2245



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Figure 2. The distribution of active sites in Cu(II)-SSZ-13 at different ELs obtained from total energy calculations. At an EL of 0.2, only the most stable sites (a) (blue), (b) (yellow), and (f) (green) will be occupied. With increased EL, the occupation of site (f) increases, and only at the end site (c) will it be exchanged. Comparison with experiment7 indicates that the most stable sites (a) and/or (b) are the active centers in the self-catalytic reduction of NOx.

mol) will be exchanged. For the other sites, the exchange process is slightly more complicated. For all of them, except configuration (c), different configurations to saturate the Al distribution are possible. For these cases, we will assume that the most stable configuration (f) (ΔECu(II) = 39 kJ/mol), which exc is accessible from all of those sites and shows the most favorable exchange energy, will be occupied. Only in the end of the process will the remaining configuration (c) (ΔECu(II) = 70 exc kJ/mol) be substituted. From the previous discussion, it is now possible to construct a distribution of Cu sites at different ELs. (The EL is defined as the amount of all Cu(II) sites occupied. An EL of 1.0 corresponds to one Cu atom for two Al atoms in the catalyst.) For an EL of 0.2, the configurations (a), (b), and (f) will be occupied. While configurations (a) and (b) are already saturated at this low EL, the occupancy of configuration (f) will increase with increased EL. Only short before reaching full exchange will site (c) be occupied. A graphical display of the distribution of active sites at an EL of 0.2, 0.4, 0.6, 0.8, and 1.0 is given in Figure 2. Combining this distribution with experimental data by Kwak et al.7 will allow us later in this text to identify that configurations (a) and/or (b) are the active sites in the selective catalytic reduction of nitrous oxides. To confirm the validity of the assumptions concerning the distribution of active sites, we also investigate the vibrational spectrum of CO adsorbed to the Cu site after reduction. These sites correspond to those of Figure 1. For structures (a−e), one H atom is added directly in the unit cell, while for (f−h), the natural number of electrons for the modeled one-Al unit cell is restored. All data concerning these Cu(I) sites is given in the SI. The optimal structures for Cu(I) are only weakly modified. In general, the bonds between Cu and O atoms are weakened, which leads to a lower coordination number of Cu for configurations (e) and (h) as well as to changes for (b) and (c). The energetic ordering of the sites is globally similar, Cu(I) structures (a) (ΔECu(I) stab = 10 kJ/mol) and (b) (ΔEstab = 0 kJ/ mol) being still favored among the two-Al unit cells, while (f) (ΔECu(I) stab = 28 kJ/mol) is still the most stable among unit cells

Figure 1. Eight different cation positions for Cu(II) in SSZ-13. Each Cu cation, together with a proton, has to be associated with two Al atoms. For sites (a−e), both Al atoms are found in the same unit cell, while sites (f−h) correspond to the limit of large Al separation. In the atomistic pictures, Si is displayed in yellow, O in red, Al in silver, and cations in blue. In the lower right-hand corner, the relative energetic [kJ/mol]) of these sites is displayed. stability (ΔECu(II) exc

discussion, we will assume that ∼30% of the total possible exchange sites are represented by the two-Al configurations (a− e). For typical Si/Al ratios, this seems to be a reasonable assumption based upon the calculations of Goodman et al. on different zeolite structures.26 We will furthermore assume that the initial exchange sites are equally distributed, which leads to the presence of 6% of possible exchange positions for each configuration. All other exchange sites contain only one Al atom in the unit cell and will therefore only allow the creation of configurations (f) and (g). These sites represent about 70% of the possible Cu exchange positions. For the following discussion, we assume that the exchange process is guided by thermodynamics. The charge deficiencies of the Al atoms are compensated for by the presence of a countercation. Studies for Na indicate that Na sites show very similar stabilities in all exchange sites.25 Therefore, only the Cu exchange energy determines the preferred order of ion exchange (see Figure 2). Initially, the sites leading to the highest energetic gain, namely, configuration (a) (ΔECu(II) =0 exc kJ/mol), will be occupied. After all of these site are occupied, the next most stable site configuration (b) (ΔECu(II) = 19 kJ/ exc 2246



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containing a single Al. This clearly indicates that the position of the active Cu cations will not be modified upon reduction. CO adsorbs to the Cu(I) site by a donation from the nonbonding CO p−σ orbitals to the Cu s states and a weak back-donation from Cu d states to the antibonding π* states. The Pauli repulsion due to the resulting partial occupation of the Cu s states leads to a reduction in the O coordination number of the Cu(I) upon adsorption. In most cases this leads to a two-fold coordination of the cation to the activated O atoms22 (see (a) in Figure 3; for structural data, see the

Figure 4. Calculated spectrum for CO adsorbed on Cu(I)-SSZ-13 at different ELs. The distribution displayed in Figure 2 is used to determine the possible sites accessible for CO adsorption. We calculated the thermal broadening using MD simulations33,34 at a temperature of 295 K. Different lines correspond to different ELs. At an EL of 0.2, we find a clear two-peak structure. At higher exchange rates, the blue-shifted peak becomes more and more dominant. This spectrum is within the limitations of the level of theory applied, in excellent agreement with experimental results.8 This confirms the assumptions leading to the distribution in Figure 2. The exact position of the maximum is given in the graph.

Figure 3. Different cation coordinations upon CO adsorption. In all cases, the Cu(I) cation bonds to two O atoms. (a) In most cases, the Cu atom bonds to two adjacent O atoms closest to an Al atom (configurations (a) and (c−h) in Figure 1). (b) For one cationic position ((b) in Figure 1), we find Cu bonded to O atoms further separated. This second configuration leads to a different signal in IR spectroscopy, which agrees well with experimental results.8 C atoms are displayed in green, Si atoms in yellow, O atoms in red, Al atoms in silver, and Cu in blue.

and +11 cm−1. With a higher EL, the intensity of the blueshifted peak starts to increase, and it becomes fully dominant at full cation exchange. Comparing this spectrum with the experimental measurements by Kwak et. al 8 shows a qualitatively excellent agreement. They find features at −8 and +12 cm−1. Similar to our calculations, the intensity of the blue-shifted peak increases with increased EL. While the differences between the two peak positions are 20 cm−1 in experiment, theory only predicts a shift of 11 cm−1. We see this as a deficiency of the applied exchange−correlation functional to properly describe the energies of CO and Cu states. This directly influences π* back-donation and can explain the discrepancy between theory and experiment.27 Overall, the comparison with experiment indicates that the previous assumptions concerning the Al distribution within SSZ-13 are reasonable. Kwak et al. also investigated the catalytic activity of the material at different Cu ELs.7 The catalyst showed already very high activity at an EL of 0.2. It slightly increased at 0.4 and then remained fairly constant. This indicates that almost all active sites are already present at an EL of 0.2, while the remaining sites are occupied for an EL lower than 0.4. The distribution of active sites calculated above shows that structures (a), (b), and (f) are occupied at these ELs and are hence potential active sites. However, the occupancy of site (f) continues to rise at higher loading while the activity saturates,7 which disqualifies this site. (NO also shows clearly different binding energies to Cu(II) (calculated as energy differences at the adsorption geometry) for sites (a) (−64 kJ/mol) and (b) (−57 kJ/mol) compared to configuration (f) (−45 kJ/mol). This is a further theoretical indication for differences in reactivity.) Therefore, total energy calculations coupled to experimental reactivity data indicate that configurations (a) and (b) with two Al atoms in one 6R are the active sites for this reaction. Our simple model predicts an even earlier occupation of these active sites than experiment. This small discrepancy is not unexpected because we neglected thermal fluctuations, dynamic limitations, and

Supporting Information). For configuration (b), the cation is also two-fold coordinated, but it is only bound to one activated O atom. (The charge deficiency close to one Al atom is compensated for by the presence of a proton. We find this coordination also for site (a), but here, it is higher in energy than the standard coordination.) The second bond is formed with an O atom adjacent to the Al atom closest to the H atom. This difference is also reflected in the IR wavenumbers. For most sites located in 6R, except site (b), we find a shift in wave numbers of +9 to +11 cm−1 compared to gas-phase CO (see the Supporting Information). Only for site (b) is the signal not shifted compared to the gas phase. For sites (d), (e), (g), and (h), where the cation is located in an 8R, we find shifts between +3 and +14 cm−1. As shown before, these sites in the 8MR are not favored energetically. To find the underlying reason for the distinct behavior for site (b), we performed a charge analysis. The positive charge at the Cu−CO complex is constant, but for sites leading to lower wavenumbers, the electronic population on the CO molecule is slightly reduced. This is a clear indication that the different Cu site structures lead to a variation in the π* back-donation, a bonding interaction between the Cu d states and the otherwise unoccupied, antibonding π* orbital. The stronger this bond is, the lower the IR wavenumber will be. However, these subtle differences are not reflected in the adsorption energies. Clearly, the changes in the cation coordination dominate over the small differences in the CO bond strength due to π* back-donation. We can now simulate the vibrational CO spectrum as a function of Cu EL in the zeolite, following the previously proposed filling scheme. The result is displayed in Figure 4. At an EL of 0.2, the spectrum shows a two-peak structure with one feature at 0 cm−1 and the other slightly broadened between +9 2247



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of Cu-SSZ-13 at different ELs7 allows to assign the activity of the catalyst to Cu cations placed in 6R containing two Al atoms. A control of the Si/Al ratio paired with a specific structure-directing agent in synthesis could modify the distribution of Al in the framework and increase the activity of the catalyst. Additionally, we discuss that the clear separation of the 6Rs prevents copper cations from occupying less active sites and therefore leads to constant activity over a large temperature range. It is therefore the high density of active sites and structural separation of these 6Rs that make SSZ-13 an efficient catalyst in cleaning car exhaust gases.

details in the ion exchange and reduction process. (An exact treatment of these processes is highly complicated and requires large-scale ab initio molecular dynamics calculations.) As we showed above, the presence of the red-shifted peak is a function of the amount of site (b) within the catalyst. However, the assumption of a statistical distribution made earlier in the text indicates that the presence of site (a) is directly proportional to the presence of site (b). Therefore, the catalytic activity of the material is directly proportional to the intensity of the measured IR peak at 2135 cm−1. Hence, total energy and frequency calculations closely agree about the key role of two Al atoms placed in a single 6R. In combination with experiments, this allows an unambiguous assignment of such sites as the ones responsible for the catalytic activity for NOx reduction. Upon the basis of statistics, the presence of these sites is closely connected to the Si/Al ratio. If this ratio is too high, too few sites are created, while at too low ratios, more than two Al atoms might be present in one unit cell. Therefore, an exact control of the Si/Al ratio together with the use of a specially designed template directing the synthesis toward the desired sites might help to increase the efficiency of future catalysts. (In the ideal catalyst, only unit cells containing the active site exist. The Si/Al ratio would be 5, and it would be 10−15 times more efficient than today’s catalysts.) A distinct advantage of copper-exchanged SSZ-13 is its longterm stability under reaction conditions and over a wide temperature range.6,28 Clearly, the activity of the material is directly correlated to the presence of the most stable Cu(II) sites. At reaction temperature, a diffusion of the Cu atoms in the zeolite framework might be expected. Concerning the migration of these atoms, the chabazite structure shows very special properties. The 6R are not directly linked, and the lowest-energy path is a diffusion from the 6R to an 8R (see the Supporting Information). Our calculations indicate that these sites are over 100 kJ/mol higher in energy than configurations (a) or (b). These large energetic differences indicate that even at high temperatures, the majority of the active sites will remain occupied, which is a key factor to avoid catalyst deactivation. (In this work, we only consider the energy differences between the sites. However, these large differences also lead to large differences in the barriers for diffusion between different cation locations, which allows these conclusions. In Y-zeolite. these energy differences would be only ∼40 kJ/mol.) SSZ-13 has a very high density of those clearly separated 6Rs, but we expect that other zeolite structures with similarly separated 6R structures should show a comparable stability under reaction conditions, even though the density of these 6R structures and hence the total catalytic activity should be lower. In conclusion, we identify the nature of the Cu(II) active sites for selective catalytic reduction of NOx in Cu-exchanged SSZ-13 from total energy calculations and an association with published experimental data.7 A combination of energetic considerations and assumptions about the statistical distribution of Al atoms in the zeolite framework allow us to predict a distribution of active sites at different levels of ion exchange. We confirm these assumptions by modeling the IR spectrum of CO adsorbed to Cu sites after ex situ reduction. The modeled two-peak structure in the IR spectrum is in excellent agreement with experimental measurements.8 The red-shifted feature of the CO spectrum corresponds to a specific site, where two next-nearest-neighbor Al atoms are positioned in a 6R, which contradicts the initial experimental assignment. Furthermore, a comparison with experimental catalytic activity measurements

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METHODS All ab initio calculations were performed using the Vienna ab initio simulation package (VASP).29,30 We used density functional theory and the HSE06 hybrid functional31,32 to describe the exchange and correlation interactions between electrons. After structure optimization, we calculated infrared absorption spectra using the frozen phonon approach. To arrive at the thermal broadening, we performed ab initio molecular dynamics simulations. We then used PyMD,33 a program supplied with the ADF package,34 to calculate the thermal broadening from the distance autocorrelation function. A more detailed description is given in the Supporting Information.

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ASSOCIATED CONTENT

S Supporting Information *

A detailed description of computational methods, data for Cu(I) (before and after CO adsorption) and Cu(II) sites, as well as a graphical displays of the diffusion thermodynamics. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: fl[email protected]. Author Contributions

This work was planned and calculations were performed by F.G. R.E.B. was involved in calculating the thermal broadening of the spectrum. All authors were involved in the writing process, with main contributions of F.G. and P.S. The initial manuscript was provided by F.G. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS F.G. acknowledges discussion with Laurent Bonneviot and Belen Albela as well as financial support by the ANR within the project DYQUMA. R.E.B. acknowledges funding by the European Union under a Marie Curie EIF (Contract No. MEIF-CT-2004-011109).

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REFERENCES

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