ReaxFF Reactive Molecular Dynamics Simulation of the Hydration of

Mar 3, 2015 - *E-mail: [email protected]. Phone: 001-814-863-6291. ... The hydrated Cu cations can diffuse through the Cu-SSZ-13 zeolite pore windo...
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ReaxFF Reactive Molecular Dynamics Simulation of the Hydration of Cu-SSZ-13 Zeolite and the Formation of Cu Dimers George M. Psofogiannakis,*,† John F. McCleerey,‡ Eugenio Jaramillo,‡ and Adri C. T. van Duin† †

Department of Mechanical and Nuclear Engineering, Pennsylvania State University, University Park, State College, Pennsylvania 16801, United States ‡ Division of Mathematics and Natural Sciences, Pennsylvania State University, Altoona, Pennsylvania 16601, United States S Supporting Information *

ABSTRACT: A new Cu/Si/Al/O/H reactive ReaxFF force field was developed and used in molecular dynamics simulations of the hydration of Cu-exchanged SSZ-13 catalyst. It was observed that at temperatures close to room temperarture, all Cu cations, including those at the faces of the double 6-member rings (6MRs), become fully hydrated and detach from the framework. The hydrated Cu cations can diffuse through the Cu-SSZ-13 zeolite pore windows. At higher temperatures, the formation of cationic OH-bridged Cu dimers Cu2OH and Cu2(OH)2 within the cages was observed. The simulations show that the temperature at which these species are formed is lower for zeolites that stabilize [CuOH]+ ions, as would be expected for relatively high-Cu- and low-Al-content zeolites. The stable dimers bind preferentially to 8MRs beside the large cages in a configuration that blocks the openings of the pores. The consequences of the present results for selective catalytic reduction and NO oxidation catalysis in Cu-SSZ-13 are discussed.

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INTRODUCTION Selective catalytic reduction (SCR) of NOx (NO and NO2) is currently considered the most efficient method for preventing the release of the harmful nitrogen oxides generated by diesel engines.1 In this catalytic process, ammonia (NH3) generated by the hydrolysis of injected urea acts as a selective reducing agent for NOx to produce N2 and H2O in an oxygen-rich diesel exhaust stream passing through a suitable catalyst. In the past few years, it was found2,3 that the most efficient and hydrothermally stable SCR catalyst is the Cu-exchanged chabazite Cu-SSZ-13, which has also become the prevalent commercial catalyst.1 The presence or absence of H2O inside the zeolite cavities is known to affect significantly the local coordination environment around the Cu ions, as well as their mobility. A recent study explored a combination of synchrotron-based (X-ray diffraction (XRD) and X-ray absorption near-edge spectroscopy (XANES)) and vibrational (diffuse reflectance Fourier transform spectroscopy (DRIFTS)) spectroscopies to study changes in the location and coordination of Cu ions during dehydration of Cu-SSZ-13.4 The authors concluded that under fully hydrating conditions, the Cu2+ ions are located in the cavities of the zeolite, removed from their cationic framework locations. Upon dehydration, the Cu ions bind strongly to various framework oxygen sites. In another work, the hydrated Cu ions under ambient conditions were assigned spectroscopically to [Cu(H2O)6]2+ species.5 A pertinent basic question in understanding catalysis in Cuexchanged zeolites is whether or not there exist Cu dimers, trimers, or larger clusters within the zeolite pores to a significant extent, and what is the composition, chemical state, location, and role of such species if they do exist. The © 2015 American Chemical Society

existence and catalytic role of Cu dimers, likely in the oxocation form [Cu−O−Cu]2+, has been shown for SCR in various other Cu-exchanged zeolites, such as Cu-FAU6 and Cu-ZSM5.7,8 The formation of oxygen-bridged Cu dimers in Cu-SSZ-13 was suggested in a kinetic and spectroscopic study of NO oxidation5 in Cu-SSZ-13, where the NO oxidation rate was found to correlate with the formation of such dimers and was negligible at low Cu exchange ratio, at which these species did not form. The authors also showed, via DFT, favorable catalytic pathways for NO oxidation on oxygen-bridged Cu dimers. Operando X-ray absorption spectroscopy (XAS) was also used9 to identify the formation of CuxOy species for Cu-SSZ-13 with Cu/Al ratios greater than 0.2. It was argued that these species do not contribute to SCR rates, although oxygen-bridged dimeric species are the most probable catalytic SCR species in other Cu-exchanged zeolites.10 Several studies claim that the catalytic species in SCR is likely an isolated Cu(II) ion adsorbed at the faces of the double 6MR (6-member ring) structures (d6r) of Cu-SSZ-13.9,11,12 Cu-ion adsorption at the 6MRs that contain two Al atoms have indeed been identified as the most stable sites for Cu binding on the framework both experimentally9,13 and computationally.9,14 As a result of its stability, the Cu2+ ion is expected to statistically populate preferentially the 6MR sites for zeolites with sufficient Al content such that a large fraction of 6MRs are substituted with at least two Al atoms.14 For zeolites with high Cu/Al ratio and low Al content, several other binding sites in the SSZ-13 structure become populated. Recent studies indicate that Received: January 22, 2015 Revised: February 26, 2015 Published: March 3, 2015 6678

DOI: 10.1021/acs.jpcc.5b00699 J. Phys. Chem. C 2015, 119, 6678−6686

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The Journal of Physical Chemistry C Table 1. ReaxFF and DFT-Calculated Reaction Energies for Select Reactions reaction 1. 2. 3. 4. 5. 6. 7. 8. 9.

Cu[Al(OH)4] + H2O → Al(OH)3(H2O) + Cu(OH) Cu[Al(OH)4]2 + 2H2O → 2Al(OH)3(H2O) + Cu(OH)2 Cu[Al(OH)4] [Si(OH)4] + H2O → Al(OH)3(H2O) + Si(OH)4 + Cu(OH) Cu[Al(OH)4] [Si(OH)4] → Cu[Al(OH)4] + Si(OH)4 [T6(Al)Cu]+1 + H2O → [T6(Al)Cu(H2O)]+1 [T6(Al)Cu]+1 + 2H2O → [T6(Al)Cu(H2O)2]+1 [Cu[Si(OSiH3)4]]+1 + H2O → [Cu(H2O)[Si(OSiH3)4]]+1 Cu[Al(OSiH3)4] + H2O → Cu(H2O)[Al(OSiH3)4] Cu(OH)(H2O) + [Cu(H2O)]+1 → [Cu2(OH)(H2O)2]+1

ΔE (ReaxFF) (kcal/mol)

ΔE (DFT) (kcal/mol)

40.5 64.2 59.6 19.1 −27.0 −43.3 −35.3 −27.9 −87.0

33.1 63.1 53.7 20.6 −22.2 −37.0 −32.3 −22.8 −79.1

application described here targets Cu-exchanged zeolites, the ReaxFF force fields aim to be transferrable to diverse chemical systems. Thus, the original Si/Al/O/H force field parameters had been parametrized against extensive density functional theory (DFT) and experimental data that included silicon and silicon oxide phases and silicon−water interactions,19,20 aluminum oxidation and hydrogenation,21,22 as well as equations of state for several aluminosilicate crystal phases.23 The underlying force field was later used in zeolite-specific simulations24 and was recently fine-tuned to predict water and proton adsorption and diffusion properties in acidic zeolites.25 The original Cu/O/H force field26 had been trained against an extensive set of copper metal, copper oxides, and hydroxides condensed-phase data and Cu ion interaction with water molecules. It includes data for both Cu(I) and Cu(II) oxidation states. The force field parameters pertaining to the water phase, and inherently included in the Si/Al/O/H and Cu/O/H force fields, has been described in detail elsewhere.27 The training set for the water force field includes quantum mechanical data on water molecule dissociation energy and energy versus O−H bond distance curves, water angle distortion energies, charge distributions, water dimer interaction energies for various dimer configurations, [H2O]n cluster cohesive energies for n between 2 and 34, formation energies and equations of state for different ice structures, [H3O]+ and [OH]− solvation energies, concerted H-transfer reactions, and proton migration energies and barriers. The ReaxFF water force field was validated by comparison with dynamic properties of the liquid water phase via MD simulations, and it was found to reproduce well the experimental cohesive energy and density of liquid water; water molecule and proton diffusion coefficients; and H/ H, O/O, and O/H radial distribution functions, highlighting the accuracy of treating hydrogen-bonding interactions of the water and ice phases via the polarizable and reactive ReaxFF potential. The current force field (appended as Supporting Information) was developed by merging these extensively tested force fields and thus entirely preserves the validity of previous predictions. Additionally, Cu−O−Si and Cu−O−Al angle parameters and Cu−Si and Cu−Al nonbonded interactions (Coulomb, van der Waals) were fitted against a new set of DFT reaction energy data. All parameters that pertain to the original Si/Al/O/H and Cu/O/H force fields remained unaltered in the new force field so that all previous ReaxFF predictions on these systems are not affected in any way. ReaxFF-calculated and DFT-calculated data for the set of reactions, relevant to the Cu/Si/Al/O/H chemical system, are shown in Table 1, and relevant species are shown in Figure 1, with structures being very similar at the DFT and ReaxFF levels. The DFT data were

industrially relevant Cu-SSZ-13 zeolites contain a major fraction of Cu in the form of [Cu(OH)]+ and located at 8MR sites.15,16 It can be inferred from the above studies that a variety of Cu species are likely to be present in Cu-SSZ-13, depending on the Cu content and Si/Al ratio, including Cu2+ isolated ions at the 6MRs and [Cu(OH)]+ ions at the 8MR structures. A recent temperature-programmed reduction/electron paramagnetic resonance (TPR/EPR) study of NO oxidation, NH3 oxidation and standard NH3-SCR catalysis in Cu-SSZ-13 zeolites arrived at a more complex picture of the state of the catalytic Cu species under reaction conditions.17 The results indicated that the SCR catalytic species are the isolated Cu2+ ions at the 6MRs only for very low-Cu-content zeolites and in the high-temperature regime (>350 K). At low temperature, transient Cu dimers are the active centers. For intermediate Cu loadings, [CuOH]+ species at the 8MRs become the catalytic sites, whereas for high-Cu-content zeolites, stable oxygenbridged Cu dimers form that block pore openings and decrease the catalyst efficiency. It was also supported that these Cu dimers catalyze effectively the NO oxidation and NH3 oxidation reactions. It can be inferred from all the above that despite extensive research, there is still much uncertainty regarding not only the mechanism of SCR in Cu-SSZ-13 but also the state of Cu catalytic species under reaction conditions. Computational work can be useful for elucidating some of these aspects. As a first step in the direction of understanding the nature of Cu catalytic species in Cu-SSZ-13 zeolites we study in this work the influence of the hydrated environment on the state of the Cu ions using reactive molecular dynamics (MD) simulations with the ReaxFF force field method.18 It has to be acknowledged that although the zeolite catalyst is typically calcined prior to reaction, there is always abundant water within the zeolite cages under reaction conditions for two reasons: (a) there is always a high water content in the feed to the zeolite catalyst from the exhaust engine stream and (b) water is generated by the SCR reactions within the zeolite pores. Thus, there is water present in the zeolite pores at all conditions, even at elevated reaction temperatures, where under nonreactive conditions all water would be evaporated. In the present work we follow the hydration, diffusion, and interaction of Cu2+ ions within Cu-SSZ-13 via the reactive MD method.

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COMPUTATIONAL METHODS A new ReaxFF18 force field that describes Cu/Si/Al/O/H systems has been developed and used in the present work. The force field was developed by merging and expanding previously tested and published ReaxFF force fields for (a) Si/Al/O/H systems19−25 and (b) Cu/O/H systems.26 Although the 6679

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Figure 2. Starting structures for the 3 sets of NVE simulations. Color code follows that of Figure 1.

structure in NVE simulations. The time step for integration was 0.25 fs. The total simulation time for each run was 500 ps (0.5 ns), or 2 million time steps. Because the reactive events in the simulations were only mildly exothermic, the temperatures throughout the simulations did not deviate much from their initial values (no more than 50 K). Thus, we can categorize the results according to the initial temperatures. The choice of running two different sets of simulations (sets 2 and 3) starting from different structures, is explained at this point. A difficult issue arises in consideration of the actual charge-balancing state of Cu in Cu-SSZ-13. In general, when divalent metal ions (M2+) act as the charge-balancing species, there are several possibilities:31 (i) M2+ can interact with two framework [AlO2]− units, when these are proximal; (ii) M2+ can interact with one [AlO2]− unit, the other residing far from the metal ion; (iii) the metal ion can charge-balance only one [AlO2]− unit by forming a formally Cu1+ ion via incorporation of extra-lattice oxygen, usually [M2+(OH)−]+, the other chargebalancing species being typically H+; and (iv) metal ions can assemble into binuclear metal cations, bridged by either O2− or (OH)− extra-framework oxygen. In Cu-SSZ-13, Cu2+ ions are believed to be the charge-balancing species for high-Al, low-Cu content zeolites,9 when the Cu ions are preferably adsorbed at the face of a 2-Al substituted 6MR. For other conditions, recent spectroscopic work15,16 shows that [Cu2+(OH)−]+ and protons account for a substantial fraction of the charge-balancing species. Under hydrating conditions, we expect an equilibrium between these species that will depend on the Al and Cu contents, but the complex interplay between electronic and electrostatic effects would possibly render the ReaxFF force field incapable of predicting this equilibrium. As a result, for the case of 5 Cu ions in the supercell, we chose to do simulations at a particular Si/Al ratio (about 6) and a specific Cu/Al ratio (0.5) and start with different concentrations of the two possible charge-balancing species (Cu2+ or [Cu(OH)]+/H+ pairs). We will further show that binuclear Cu ions can form via the action of water, irrespective of the initial state of the monomeric Cu ions. As shown in Figure 2, 30 water molecules were randomly dispersed in each supercell to hydrate the zeolite. The choice of the water content was such that in the simulations with 5 Cu ions, the water content would be sufficient to fully hydrate all of them in the absence of Cu−framework interactions, as it is known that the Cu2+ ion can accommodate up to six water molecules in the first hydration shell. Nevertheless, we cannot claim any connection of the water content to any experiment other than the fact that during SCR water is always present in

Figure 1. Selected species from Table 1 reactions.

obtained using Jaguar28 with the B3LYP functional29 and LACV3P++** basis sets.30 Minor corrections due to van der Waals interactions are not treated in the DFT calculations. Nevertheless, ReaxFF slightly overpredicts the adsorption energy of H2O on Cu-ion models, and van der Waals attractive interactions would be in the direction of offsetting part of this difference. The training set includes reaction energies for the formation and hydrolysis of species containing the Cu−O−Si and Cu−O−Al pattern in both Cu(I) and Cu(II) oxidation states (reactions 1−4), H2O adsorption energies in small aluminosilicate clusters (reactions 5−8), as well as a reaction for the formation of a hydroxyl-bound cationic Cu dimer (reaction 9).These comparisons, when combined with previous data on the zeolite−H2O and Cu−H2O interactions,23−26 indicate that the new force field can describe the chemistry of Cu-exchanged zeolite-H2O systems with sufficient accuracy to be used in large-scale reactive MD simulations. Additional ReaxFF-DFT comparisons specific to Cu ion hydration and desorption within Cu-SSZ-13 appear in the next section, in conjunction with observations from MD simulation results.

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RESULTS AND DISCUSSION Three sets of calculations were performed: (i) simulations with two Cu ions in the orthogonal SSZ-13 supercell, initially positioned on the faces of two 2Al-substituted 6MRs (set 1); (ii) simulations with 5 Cu ions in the supercell, originally present as one Cu2+ ion (on the face of a 6MR) and four [CuOH]+−H+ pairs, all ions originally positioned at various different locations and bound to framework oxygen atoms proximal to Al atoms (set 2); and (iii) calculations with 5 Cu ions in the supercell, all of which were originally present as framework-attached Cu2+ ions at various locations (set 3). The periodic supercell starting structures for the simulations obtained after a short optimization run of the hydrated zeolites are shown in Figure 2. In all cases the supercell had as many Al atoms as required to make the Cu/Al ratio equal to 0.5 (i.e., 4, 10, and 10 for simulation sets 1, 2, and 3, repsectively). All 3 sets of simulations were performed at six different starting temperatures: 100, 300, 500, 700, 900, and 1100 K. A short NPT run was performed first, at 1 atm pressure for all temperatures (progressively raising the temperature) to preoptimize the supercell sizes. The final structure from the NPT run for each temperature was then used as the starting 6680

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such that the Cu2+ ion balanced the charge of two Al atoms of a 6MR, whereas the [CuOH]+ ions were attached to various framework oxygen atoms at both 6MRs and 8MR sites. The protons were positioned at Brønsted acid sites bound to oxygen atoms beside Al atoms in the framework. Figure 4 shows details

the pores, as it is produced in the reaction, and also present in variable concentrations in the feed. Observations from the simulations are discussed next. For the simulations with two Cu2+ ions initially in the supercell (set 1), the original cell contained two Al atoms at each of two 6MRs of different cages, and on each of these one Cu ion was attached (Figure 2, left-hand structure). Figure 3 shows details

Figure 3. Details from snapshots of the structure after 0.5 ns of NVE simulation at different temperatures, for the case of two Cu2+ ions in the initial structure (simulation set 1).

Figure 4. Details from snapshots of the structure after 0.5 ns of NVE simulation at different temperatures, for the case of four [CuOH]+− H+ pairs and one Cu2+ ion in the initial structure (simulation set 2).

from snapshots at the end of the 0.5 ns simulations (zoomed in to aid in visualizing details around the Cu ions; edge effects with broken bonds create visual artifacts in some of them). At 100 K, Cu ions were immobile and remained attached to the 6MR oxygen atoms. Water molecules displayed very little mobility. We observed the adsorption of two water molecules on one of the Cu ions and the associated change in the location of the Cu ion, which was pulled outward and above the 6MR. No other events of interest were observed at this temperature. At 300 K, within the simulation time, we observed the hydration and detachment of one of the two Cu ions from the zeolite framework. The Cu ion was hydrated with 4−5 water molecules after detachment throughout the simulation and remained in the cage adjacent to the 6MR. At 500 K, we observed the hydration and detachment of the other Cu ion from the framework. The Cu ion was hydrated with 5−6 water molecules after detachment and remained in the cage of its original location. Clearly, it is only the duration of the simulation time that did not permit the observation of the full desorption of both ions. At 700 K, both Cu ions desorbed, became hydrated with 4−5 water molecules, and were located on the cages adjacent to their original locations. At 900 and 1100 K, both Cu ions desorbed and became hydrated with 3−4 water molecules, but now one of them escaped the original cage by crossing an 8MR and ended up in an adjacent cage. The fact that this diffusion event was observed once in the simulation is sufficient evidence for the expected extensive diffusion of hydrated Cu ions at lower temperatures at experimental time scales (∼11 orders of magnitude larger). For the simulation set 2, with five Cu ions (four [CuOH]+− + H pairs and one Cu2+), the initial distribution of the ions was

from snapshots at the end of the 0.5 ns simulations. At 100 K, during the 0.5 ns NVE simulation, the Cu ions were immobile and remained attached to the framework at the initial positions. However, two of the [CuOH]+−H+ pairs reacted to form Cu2+ and a water molecule. In one of the two cases, the proton transfer happened via the Grotthuss mechanism, with the proton diffusion mediated by two water molecules. The proton transfer from the framework to the [CuOH]+ ion illustrates the reversible nature of the [CuOH]+ + H+ ↔ [Cu(H2O)]2+ equilibrium, where all species refer to adsorbed species at different framework sites. At 300 K, one of the Cu ions was sufficiently hydrated to desorb from the framework and remained hydrated with 4−5 water molecules throughout the simulation. The other Cu ions remained attached at their original sites. At 500 K, all four Cu ions that were originally [CuOH]+ detached from the framework and appeared as hydrated Cu ions, most frequently possessing five water molecules in the hydration shell. One of them still possessed the OH group and appeared as [CuOH]+ hydrated with two water molecules. Only one Cu ion remained attached to the framework. Not surprisingly, this is the Cu ion attached to the two Al-substituted 6MR, the strongest binding site. At 700 K, we observed the formation of stable Cu dimers. A Cu cationic dimer in which two Cu cations were bridged by an OH group, having the composition [Cu2(OH)] and a net positive charge (ReaxFF charge = +1.5) was formed when a desorbed hydrated [Cu(OH)]+ ion approached the Cu ion of the two Alsubstituted ring and abstracted it. The two Cu ions forming the dimer were initially present in the same cage. Subsequently, at a later point in the simulation a [Cu2(OH)2] cationic dimer was 6681

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formation and destruction of transient Cu dimers. Two Cu ions finding themselves within the same cage would slightly attract each other when their hydration shells are not complete. Such a transient Cu dimer, with adsorbed water molecules, is visible at 700 K in Figure 5. Nevertheless, the interaction is very weak; thus, Cu dimers were unstable and continuously formed and divided into monomers. At 1100 K, however, Cu atoms were observed to form a stable Cu dimer and a Cu trimer. The Cu dimer was identified as a positively charged hydroxo-bridged hydrated [Cu2(OH)] cation. The Cu trimer was identified as hydrated [Cu3(OH)2] cation. The protons from deprotonated H2O molecules protonated framework O atoms adjacent to framework Al (Brønsted sites−proton acceptors), whereas the resulting OH groups (negatively charged) formed bridges within the Cu dimers. Because the force field overestimates significantly the binding strength of OH-bridged Cu trimers (tested via comparison with DFT), we attach much more emphasis to the formation of dimers without disregarding the possibility that larger clusters might be generated. The formation of the stable Cu dimer and simultaneous formation of the framework Brønsted acidic sites is in agreement with the previous set of calculations (set 2), but now, because the OH is not originally present, it requires higher temperature to be achieved. The simulations discussed above demonstrate the roomtemperature hydration and desorption of Cu ions from the framework via substitution of Cu-framework bonds with Cu− H2O bonds. To show that this is expected, we demonstrate the exothermicity of the desorption process via ReaxFF and DFT optimizations. Figure 6 (top) shows ReaxFF optimized structures for 1−4 adsorbed water molecules on the 2Alsubstituted 6MR face of the d6r with one Cu2+ ion. Each of the first three adsorbed water molecules is adsorbed exothermically and moves the Cu ion further out of the ring plane while simultaneously reducing interaction of the Cu ion with framework O atoms. The addition of the fourth water molecule takes the ion away from the ring and into the cage in an overall exothermic step. The Cu cation−framework electrostatic interaction is now screened by the polarizable water molecules. Additional DFT calculations for 1−3 water molecules (Figure 6, middle) were performed for a small 6T cluster in which the terminal OH groups were fixed to the bulk SSZ-13 framework positions. The ReaxFF−DFT comparison of the energy changes (Figure 6, bottom) demonstrates the predictive ability of the force field and the exothermicity of the hydration− detachment process. A DTF structure with four water molecules is not shown, as it cannot be simulated with a small cluster. Despite the exothermic nature of the Cu desorption step, it was not observed at 100 K because steric reasons will unavoidably require overcoming energy barriers, particularly for the binding of the third and fourth water molecules. The second important reaction identified in the simulations is the formation of hydroxo-bridged Cu2OH and Cu2(OH)2 cationic species in the SSZ-13 cages. Both of these species were observed and were transformed into each other frequently at high temperatures. It was also observed that these species were formed at elevated temperatures via combination and deprotonation of water-lean (not fully hydrated) Cu and Cu(OH) cations. As an example of the exothermic nature of this process, Figure 7 shows the ReaxFF-optimized structure of the initial and final states of the formation of a Cu2OH moiety in the zeolite cage, when a Cu(H2O)3 ion reacts with a

formed by deprotonation of a water molecule bound to the dimer, where the H+ ended up binding to a framework oxygen. In the dimer, the Cu ions were bridged by two OH groups. Eventually, one hydroxyl group regained the proton (now attached to the framework) to form a water molecule, thus reforming the [Cu2(OH)] cation (the [Cu2(OH)2] dimer is shown in the 900 and 1100 K snapshots).The dihydroxy and monohydroxy bridged forms of the dimer transformed into each other rather easily. Both forms of the dimer cations [Cu2(OH)2] and [Cu2(OH)] were also formed (and transformed into each other) at even higher temperature simulations (900 and 1100 K). The third set of simulations was initiated by a structure in which all five Cu ions were isolated Cu2+ species, each balancing the negative charge of two Al atoms in the framework, irrespective of the spatial distribution of the Al atoms, which was randomly chosen. Only one of the Cu atoms was bound to a 6MR possessing two Al atoms. Details from snapshots are shown in Figure 5. At 100 K, despite hydration of

Figure 5. Details from snapshots of the structure after 0.5 ns of NVE simulation at different temperatures, for the case of 5 Cu2+ ions in the initial structure (simulation set 3).

the Cu ions with 1−3 water molecules, all ions remained at the initial binding positions. At 300 K, it was observed that one of the Cu ions was hydrated and detached from the framework during the 0.5 ns simulation time. At 500 K, all Cu ions except the one bound to the 6MR with two Al, were detached from the framework and hydrated with 4−5 water molecules, in agreement with the results of the previous sets of simulations. At 700 and 900 K, all Cu ions were detached from the framework but continuously formed and broke bonds with the framework oxygen atoms to complete coordination, due to weaker interaction with water molecules at increased temperatures. Moreover, we also observed the diffusion of a Cu ion to an adjacent cage from the original location by crossing the 8MR. These results are in line with the results of the previous set. However, no formation of stable Cu dimers was observed at 700 and 900 K. In fact, for this set, this was observed only at the much higher temperature of 1100 K. This is understandable because in this case the hydroxyl group does not pre-exist and must be generated by deprotonation of a water molecule. On the other hand, at 700 and 900 K, we observed a continuous 6682

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Figure 6. Top: ReaxFF optimized structure for the adsorption of 1−4 H2O molecules on the Cu2+ ion of a 2Al-substituted ring in Cu-SSZ-13. The hydrated cation desorbs from the framework when the fourth water molecule adsorbs. Middle: DFT-optimized structures of model corresponding to adsorption of 1−3 water molecules. Bottom: total binding energy of 1−4 water molecules by ReaxFF and DFT (B3LYP/LACV3P**).

framework-bound Cu(H2O)2 ion to form a Cu2OH dimeric cation hydrated by four water molecules and a proton bound to an acidic framework site. The overall process is exothermic by 15.5 kcal/mol. The dimer formation process is favorable when [CuOH]+ monomers are initially present, as suggested by the significantly lower formation temperatures in the simulations. The most probable location of the Cu2OH cation was identified after running a series of ReaxFF optimizations starting from several initial structures. The minimum energy structure is shown in Figure 8. The dimer makes multiple bonds with the O atoms of an 8MR. The location of this species is ideal to act as a reactive center if it has catalytic properties, but it can also block the channels, resulting in diffusion limitations. These structures were dominant for both Cu2OH and Cu2(OH)2 species in the high-temperature NVE simulations (Figures 4 and 5). It is conceivable that the Cu2(OH)2 species can dehydrate under oxidizing conditions (not explored in the present work) to the [Cu−O−Cu]2+ cation, often hypothesized to be the more stable Cu cationic dimer in Cu-exchanged zeolites.6−8

We now proceed to summarize the observations from all simulations and to examine them critically. First, we can infer from the simulations that, even at 300 K, under experimental hydrating conditions all Cu ions should desorb from the framework. One Cu ion desorption was observed in all sets of simulations at 300 K. In the first set of simulations, one of the Cu ions detached from the 2 Al-substituted 6MR (the strongest binding site) at 300 K. Thus, given enough simulation time, and noting the exothermicity of the process, all Cu ions should detach from the framework even at 300 K. This is in agreement with experimental results that identify the presence of mobile hydrated Cu ions at ambient conditions in Cu-SSZ-13.4,5 The hydration shell of the detached Cu ions in these simulations at relatively low temperatures suggests an average coordination environment consisting of a number of water molecules somewhat smaller than (typically between 4 and 5) the expected hexa-aqua complex.4,5 This probably suggests a slightly underestimated electrostatic attraction between the Cu ions and water molecules that can be caused by a slight 6683

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to be prevalent for high-Cu-content zeolites for two reasons: (a) Statistically, more Cu ions find themselves in the same cage in high-Cu-content zeolites. (b) Above a critical Cu content, the ions begin to populate the 8MR sites in the [CuOH]+ form.15,16 The present simulations show that dimer formation is observed at significantly lower temperatures when the ions initially contain OH groups (∼700 K for set 2), because in this case water deprotonation upon formation is not required. Oxygen-bridged Cu dimers, with structure similar to that of the hydroxy-bridged dimers found here, have long been proposed to be the catalytic species for both SCR and NO oxidation reactions in several other zeolites.10 We did not observe these oxygen-bridged ions, such as [Cu−O−Cu]2+, as the simulation environment employed was not an oxidizing one, but it is evident how the OH-bridged and O-bridged dimeric species are structurally related and transformable into each other. In a recent study, Gao et al.17 used electron paramagnetic resonance and temperature-programmed reduction to study Cu-SSZ-13 as a catalyst for three different reactions: NO oxidation, NH3 oxidation, and standard SCR. The authors reached the following conclusions, which are stated below because they strengthen the findings of the present simulations. (1) In the low-temperature range (273−525 K) in hydrated Cu-SSZ-13, isolated and hydrated Cu2+ ions are highly mobile. Even at extremely low Cu loadings, EPR registers transient Cu−Cu interactions. Hydrated Cu ions can therefore migrate through the 8MR windows at low temperatures and find themselves occasionally in the same cage, forming transient Cu dimers. This conclusion is in line with the present MD simulations. We observed the hydration and detachment of Cu ions from the network at temperatures as low as 300 K. We observed the migration of hydrated Cu ions through the pore windows at quite higher temperatures (700 K) but argued that the reason we did not observe migration at lower temperatures is the unavoidably huge gap between experimental and simulation time scales. Also at these temperatures (700 K) in the MD simulations, we observed transient Cu−Cu interactions between hydrated Cu2+cations that were not reactive. (2) Isolated Cu2+ ions do not catalyze the NO oxidation reaction, at least below 675 K. Nitric oxide oxidation occurs only above critical Cu loadings where stable oxygen-bridged Cu dimers can form. Their conclusion strengthens existing DFT simulations that demonstrate how O-bridged Cu dimers allow for an easier catalytic path compared to isolated Cu ions, as well as experimental data that show no catalytic effect at lower-thancritical Cu loadings.5 This conclusion is in line with the present simulations. In our simulation, we show a plausible process for the formation of cationic OH-bridged Cu dimers (Cu2OH and Cu2(OH)2). The formation temperature in the simulations was close to 700 K, in the presence of [Cu(OH)]+ and H+, as charge-balancing cations of two Al substituents above a critical Cu content. These dimers could be the catalytic species for NO oxidation or could be transformed to O-bridged dimers in oxidizing conditions. We also discussed, as an inference from the simulations, why the requirement of high Cu loadings is essential for the formation of stable Cu dimers. (3) With respect to the SCR reaction, we present two conclusions. (a) At low Cu loadings and temperatures lower than ∼575 K, transient Cu dimers are the catalytic species for SCR. This conclusion is complemented by the results of the present simulations, in which we observed that under hydrating conditions, mobile Cu ions detached from the framework can find themselves in the same cage, even at low Cu loadings.

Figure 7. ReaxFF-optimized structure on the left has a Cu(H2O)2 cation and a Cu(H2O)3 cation. The Cu(H2O)2 is bound to a framework oxygen. The ReaxFF optimized structure on the right has a Cu2(OH)(H2O)4 species in the cage and a proton bound to framework oxygen. The dimer structure is lower in energy by 15.5 kcal/mol, suggesting that formation of the dimer is exothermic when the reacting Cu ions are not fully hydrated.

Figure 8. ReaxFF-optimized structure of Cu2OH cation after removal of all water molecules from the structure.

underestimation of the Cu charge within the hydrated zeolite. Nevertheless, the main effects are captured. The formation of stable OH-bridged Cu dimers was observed during the 700 K simulation, when starting from a structure that incorporated [CuOH]+, and was also observed during the very high-temperature simulation (1100 K) when starting from a structure that had Cu2+ only. However, as shown in the ReaxFF testing simulations, the formation of the Cu dimer is a mildly exothermic process. Dimer formation was not observed at lower temperatures in the simulations when all detached Cu ions were fully hydrated, thus indicating that dimer formation requires the reaction of Cu ions partially stripped off their hydrating water molecules. At temperatures lower than that required for Cu dimerization (∼1100 K for set 3), transient Cu dimers can still form and break continuously, as seen in the present simulations, but the hydrated cations are still identified as Cu2+ ions in the transient dimers and interact only very weakly. Under conditions of partial hydration of the cations, and when they are brought in close contact, the dimerization is feasible. Thus, the onset temperature of dimer formation should coincide with the temperature at which Cu ions are partly stripped of their hydration shell and yet are not in a framework-bound state. Dimer formation can be expected 6684

DOI: 10.1021/acs.jpcc.5b00699 J. Phys. Chem. C 2015, 119, 6678−6686

Article

The Journal of Physical Chemistry C

(iii) The present results simultaneously complement and are in agreement with a recent experimental spectroscopic study of NO oxidation, NH3 oxidation, and selective catalytic reduction of NO in terms of the catalytic species under different Cu loadings of Cu-SSZ-13 and different temperature ranges. (iv) Finally, a new ReaxFF force field was developed that can describe effectively the interaction of water molecules with Cuexchanged zeolites in reactive MD simulations. Extension of this force field to include the other SCR species is required as a next step in order to apply the force field to the actual SCR reaction dynamics.

These hydrated Cu ions interact without forming stable species at low temperatures. In the SCR reactions, the Cu ions, detached from the framework would bring the reactants (NO, NH3 and O2) in contact. (b) Above ∼575 K, the catalytic sites shift from the transient Cu dimers to isolated Cu2+ or [CuOH]+ monomers, or both, depending on the Cu content. At high Cu loadings, the formation of stable Cu dimers decreases the efficiency of the catalyst because these dimers do not have sufficient activity for SCR and further cause diffusional limitations by blocking the pore windows. This conclusion is in agreement with the present results that show a mechanism for the formation of stable OH-bridged dimers and further show that these dimers end up being located at the 8MR pore windows and would thus interfere with the diffusion of reactant and product gases in the zeolites. On the basis of the observations from these simulations, it was discussed why high Cu loadings would favor the formation of Cu dimers at the expense of monomeric Cu species.

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ASSOCIATED CONTENT

S Supporting Information *

The new Cu/Si/Al/O/H force field that was used in the present work within the ReaxFF code. This material is available free of charge via the Internet at http://pubs.acs.org.

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CONCLUSIONS The reactive MD simulations described in this work, after critical comparison with recent experimental work in Cu-SSZ13 catalysis for SCR, suggest the following. (i) In the presence of water, and from temperatures as low as 300 K, Cu ions within Cu-SSZ-13 detach from the framework as hydrated Cu2+ or hydrated [Cu(OH)]+ ions, depending on the initial state of the ions (framework-bound Cu2+ or [Cu(OH)]+ ions). Water molecules mediate the Cu cations’ interaction with the negatively charged framework. The mean number of water molecules in the coordination shell decreases at higher temperature. The cations are mobile within the framework and can diffuse through the 8MR windows within the structure. Thus, even when the Cu loading is small, Cu2+ ions can be present in the same cage, resulting in direct interaction and continuous formation and destruction of transient Cu dimers. As a result, it is reasonable to suggest that at low temperatures the close contact of the cations within the cages is responsible for SCR catalytic activity. At higher temperatures, interaction with water is weaker, resulting in Cu ion attachment to the framework, and isolated Cu ions would be responsible for the catalytic SCR activity. (ii) For temperatures higher than 700 K in the simulations, Cu2+ and [Cu(OH)]+ hydrated ions react to form Cu2(OH) and Cu2(OH)2 hydroxyl-bridged cationic stable Cu dimers. At even higher temperatures (1100 K in the simulations), the formation of these dimers is possible by direct reactions between hydrated Cu ions accompanied by deprotonation of water molecules and return of a proton (H+) to the framework to form Brønsted acid sites. Formation of Cu dimers is expected to be a strong function of Cu loading and temperature under reactive conditions. When the Cu loading is small, most Cu ions occupy the energetically favorable sites at the 6MRs, in the Cu2+ state. For higher Cu loading, Cu ions that are less strongly bound at more exposed locations, possibly in the form [Cu(OH)]+ at 8MR sites, can form dimers at lower temperatures. The hydroxyl-bridged Cu dimers are likely the precursors of oxo-bridged Cu dimers, often observed in zeolites under oxidizing atmosphere. Previous studies suggest that these species may be inactive for SCR and may also possibly cause diffusional limitations, but they are the active species in the NO oxidation reaction. The energetically favorable site predicted by ReaxFF for these dimers at the 8MR pore windows is in agreement with this observation.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 001-814-863-6291. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS G.M.P. and A.C.T.v.D acknowledge financial support from NETL through the RES Contract DE-FE00400. E.J. acknowledges start-up support from Pennsylvania State University Altoona. J.F.M. acknowledges support from an Undergraduate Research Assistant Award from Pennsylvania State University Altoona.

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REFERENCES

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