MM Study of the Effect of Local Environment on Dissociative

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J. Phys. Chem. C 2009, 113, 15643–15651

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QM/MM Study of the Effect of Local Environment on Dissociative Adsorption in BaY Zeolites Chun-Yi Sung, Linda J. Broadbelt,* and Randall Q. Snurr* Institute for Catalysis in Energy Processes and Chemical and Biological Engineering Department, Northwestern UniVersity, 2145 Sheridan Road, EVanston, Illinois 60208 ReceiVed: March 8, 2009; ReVised Manuscript ReceiVed: July 11, 2009

Quantum chemical calculations were carried out to study the characteristics of dissociative adsorption on BaY zeolites, that is, adsorption of neutral HX versus H+X-. The calculations were performed using the QM/MM method ONIOM with the B3LYP functional, SDD basis set, and UFF force field. Key intermediates from the NOx reduction reaction were studied, including nitromethane, aci-nitromethane, acetic acid, CH2COOH, HNO3, HNO2, H2O, and O2NCH2COOH. Two factors were found to affect dissociative adsorption: the local environment in the zeolite and the acidity of the adsorbate molecule. The local environment can vary due to different aluminum arrangements around the active site. It was found that adsorbates in both neutral and dissociated forms are further stabilized as aluminum atoms are further apart because of the electrostatics. Generally, neutral intermediates adsorb with one oxygen atom coordinated to barium, whereas the dissociated intermediates adsorb with two oxygen atoms coordinated to barium in a bidentate configuration. For all of the intermediates studied except HNO3, the neutral form is favored over the dissociated form, and adsorbates with higher acidity have stronger tendencies to dissociate. 1. Introduction Zeolites have been used in the petrochemical industry as heterogeneous catalysts since the 1960s.1 For example, proton exchanged zeolites are widely used for catalytic cracking, isomerization, and alkylation reactions.1-3 These reactions are catalyzed by Brønsted acid sites in proton-exchanged zeolites and are well studied. On the other hand, if metals are introduced into the framework or extraframework sites, zeolites can serve as oxidation or reduction catalysts. Most catalytic oxidation and reduction processes make use of the propensity of transition metals to change their oxidation state rather easily. The underlying chemistry is a cascade of oxidation steps of an organic molecule in which a transition-metal ion oscillates between two oxidation states.4 In addition, framework oxygen atoms are known to exhibit basic properties.5 The cations serve as Lewis acids and create strong electric fields.6 The framework oxygen atoms as the basic site along with the electric field created by cations are thought to be responsible for the reactivity of alkali and alkaline-earth exchanged zeolites for oxidation and alkylation reactions.7-11 Since Iwamoto discovered that some zeolite-based catalysts, such as Cu/ZSM-5, are able to selectively catalyze the reduction of NOx with hydrocarbons in the early 1990s,12,13 numerous studies have been dedicated to the application of cationexchanged zeolites for NOx reduction in automotive exhaust gas.14-23 Although zeolites exchanged with transition metals have been found to be very effective for treating model exhaust streams, they all suffer from some problems: either the reaction rate at low temperature is considered too low for practical application, or the NOx reduction activity is reduced by water vapor.24 Recently, BaNaY zeolites have received attention since it has been reported that acetaldehyde, which is formed from hydro* Corresponding authors. E-mail: [email protected] (L.J.B.); [email protected] (R.Q.S.).

carbons in a nonthermal plasma, reduces NOx to N2 over BaNaY catalysts at a relatively low temperature in the presence of water vapor.24-27 A detailed reaction mechanism for the reduction of NOx over BaNaY with acetic acid or acetaldehyde as a reductant was proposed by Yeom et al. using FTIR spectroscopy and isotopic labeling.6 It is interesting that BaNaY zeolites can reduce NOx without a transition metal that can change oxidation state. However, it is not clear what role the barium atom plays in the proposed NOx reduction mechanism in the zeolitic environment. One of the reaction routes of the NOx reduction reaction over BaNaY zeolites is proposed to be ionic with ionic intermediates in equilibrium with the neutral parent molecules, assuming that the high electric field in BaNaY zeolites will facilitate the formation of charged intermediates.6 Yeom et al. suggested that the reaction of acetic acid (as a reductant) with NO2 to form nitromethane is the rate-determining part of the mechanism, with many elementary steps involved in this tranformation. The proposed intermediates among these elementary steps are acetate ion dissociated from acetic acid, CH2COO- formed from hydrogen abstraction of acetate ion by NO2, and O2NCH2COOresulting from the addition of NO2 to CH2COO-.28 Another important step is that NO reduces surface HNO3 to HNO2, which then reacts with NH3 to form NH4NO2 that decomposes into N2 and H2O. Moreover, the enhanced adsorption of the reductant and the formation of charged species are thought to be the possible reasons for the superior reactivity of zeolite-based deNOx catalysts.29 However, some of the proposed anionic intermediates were not seen experimentally via FTIR. This could be due to the fact that their lifetimes are short or due to the equilibrium with the neutral parent molecules.6 The equilibrium between the dissociated and neutral intermediates is difficult to quantify. The effect of the high electric field produced by cations on the dissociation of intermediates under the influence of the local zeolitic framework with the basic sites in the vicinity is also

10.1021/jp9020905 CCC: $40.75  2009 American Chemical Society Published on Web 08/06/2009

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difficult to probe experimentally. Therefore, quantum chemical calculations were carried out in this study to unravel the characteristics of dissociative adsorption over BaNaY zeolites. Key intermediates that were proposed to dissociate in the NOx reduction mechanism were studied around the active site with three different Al arrangements. These intermediates are nitromethane (CH3NO2), aci-nitromethane (CH2NOOH), acetic acid (CH3COOH), CH2COOH, HNO3, HNO2, H2O, and O2NCH2COOH. Ab initio calculations of small clusters representing active sites have been widely employed to study adsorption or reactions on zeolites.30-36 However, interactions between guest molecules and the extended zeolite framework are neglected in ab initio calculations when utilizing small clusters. Periodic calculations have also been used and can provide accurate results, but they are too computationally expensive for zeolites that have large unit cells. For example, Y zeolites have a unit cell of 576 atoms. QM/MM methods such as ONIOM37 that consist of two layers linked via mechanical embedding have been increasingly used for studying zeolite systems at a lower computational cost.38-42 Solans-Monfort et al.43 studied the adsorption of ammonia and water in acidic chabazite, forming ion-pair and hydrogen-bonded structures, respectively, with both QM/MM and periodic calculations. They showed that if the inner layer includes at least all of the oxygen atoms that might be involved in the local interaction between the guest molecule and the zeolite, the results obtained are essentially equal to those obtained with full periodic density functional theory (DFT) calculations. In this work, the ONIOM method was used for the purpose of reducing the computational cost for the zeolite framework while treating atoms closest to the active site with density functional theory.

Sung et al. 10T 96T 10T EONIOM ) EB3LYP + EUFF - EUFF

(1)

where the superscript “10T” denotes the capped inner layer, and the superscript “96T” designates the whole zeolitic framework utilized. Subscripts “B3LYP” and “UFF” denote energies obtained at the B3LYP level of theory and UFF force field, respectively. The UFF atom types for Si, O, and Al were specified as Si3, O_3_z, and Al3, respectively. Progress in obtaining information about Si and Al ordering in zeolites has been made using high-resolution 29Si and 27Al MAS NMR.51-53 Inferences about the Al locations have been made from 29Si NMR spectra along with calculations of electrostatic energies: in Y zeolite, the hexagonal face may contain two Al either in the para or in the meta location. None contains three Al, and some may include only one Al.51,52,54

2. Computational Methods Experimental studies44-46 have shown that the majority of Ba2+ are located at site II in faujasite zeolites, which is in the center of a single six ring (S6R) in the supercage. Therefore, site II was assumed to be the active site. An ONIOM cluster of 96 tetrahedral (96T) atoms including the supercage was extracted from the crystal structure47 of Y zeolites to represent the zeolite framework as shown in Figure 1. A 10T cluster including a S6R, four more tetrahedral atoms extending from the S6R, and hydrogen atoms to cap the dangling bonds of the framework atoms was treated as the capped inner layer. The inner layer was treated with the B3LYP density functional and the effective core potential basis set SDD. This combination of functional and basis set was chosen on the basis of calculations of bariumcontaining molecules summarized in our previous study.42 The outer layer, consisting of 86T atoms around the inner 10T atoms, was treated with the universal force field (UFF),48 which was reported by Kasuriya et al.49 and Namuangruk et al.50 to provide a good description of van der Waals interactions between guest molecules and zeolitic frameworks. The terminating Si and O atoms were saturated with hydrogen atoms directed along the bond vector of what would have been the next zeolite framework atom in the crystal structure. The Si-H and O-H distances were fixed at 1.49 and 0.96 Å, respectively. All framework atoms in the outer layer and the outermost framework atoms of the inner layer were fixed at their crystallographic positions, as were the hydrogen atoms capping the inner layer. All other atoms including barium, atoms comprising the adsorbates, and the other framework atoms in the inner layer were allowed to relax. The total energy of the whole system studied with the twolayer ONIOM approach is expressed as:

Figure 1. Top and side views of ONIOM cluster including a supercage of faujasite zeolite showing site II, where the Ba2+ ion is located. Two Al atoms are in the para location, denoted as (1,4). Atoms treated with DFT are shown in a ball and stick representation, and atoms treated with molecular mechanics are shown in a wireframe representation.

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Figure 2. Side views of BaY around site II with (a) (1,4), (b) (1,5), and (c) (1,6) Al arrangement. The structures are shown in the top row. The (1,4), (1,5), and (1,6) notations denote the relative positions of the two aluminum atoms near S6R. Only the QM portions are shown for simplicity. The electrostatic potentials mapped on the 0.002 e/au3 electron density surfaces are shown in the bottom row. The colors ranging from red to blue correspond to electrostatic potentials ranging from -0.167 to 0.167 hartree.

Therefore, with the assumption that site II is the active site, three different Al distributions for the two Al atoms located surrounding site II were employed to study their effect on adsorption as shown in Figure 2: (1) two Al atoms in the para location, denoted as (1,4) arrangement, (2) two Al atoms are one T site further apart from the (1,4) arrangement with one Al atom in the S6R, denoted as (1,5) arrangement, and (3) two Al atoms are one T site further apart from the (1,5) arrangement with one Al atom in the S6R, denoted as (1,6) arrangement. Another Al arrangement in which two Al atoms are in the meta location in the S6R, denoted as (1,3) arrangement, was not employed in this study because it was shown in our previous study42 that its electronic properties barely differ from those of the (1,4) cluster. Attempts were made to stabilize adsorbates in both the neutral and the dissociated forms by placing them with different initial configurations for optimization, especially the hydrogen atoms. The stabilities of adsorbates in neutral and dissociated forms were then evaluated through their Gibbs free energies at 473 K as follows:

H ) E0 + EZPE + Ev + Et + Er + RT

(2)

S ) Sv + St + Sr

(3)

G ) H - TS

(4)

where H, E, S, and G denote enthalpy, internal energy, entropy, and Gibbs free energy, respectively. Subscripts “0”, “ZPE”, “v”, “t”, and “r” denote contributions from electronic energy, zeropoint energy, vibration, translation, and rotation, respectively. R is the gas constant, and T is the temperature. Frequencies were calculated for each cluster studied. Imaginary vibrational modes due to the artificially fixed atoms of the clusters were removed by the level-shift technique55,56 as in our

TABLE 1: Relative Energies (kcal/mol), Binding Energies of Ba2+ (kcal/mol), Height of Ba2+ to O(2) Plane (Å), and Mean Ba2+-O(2) Distance (Å) for BaY Cluster Models with (1,4), (1,5), and (1,6) Al Arrangements Al positions

relative energies

binding energies of Ba2+

height of Ba2+ to O(2) plane

mean Ba2+-O(2) distance

(1,4) (1,5) (1,6)

0.0 29.09 49.54

-464.3 -435.0 -410.2

1.637 1.587 1.642

2.615 2.654 2.665

previous study.42 Minima were confirmed by no imaginary frequencies after the level-shift procedure. The level-shifted frequencies were then used to calculate Gibbs free energy values within the rigid rotor, harmonic oscillator approximation. No scale factor was available for the particular combination of functional and basis set that we employed, so the frequencies were used unscaled. Basis set superposition error (BSSE) corrections using the counterpoise method57 were taken into account. Natural bond orbital analysis (NBO)58,59 was carried out to obtain the charge and electronic configurations of atoms in clusters. All calculations were carried out using the Gaussian 03 software.60 3. Results and Discussion 3.1. BaY Clusters with Different Al Arrangements. As shown in Table 1, the relative energies of clusters with different Al arrangements show that the BaY cluster is most stable when two aluminum atoms are in the S6R, that is, (1,4) Al arrangement. The calculated electronic energies for binding of Ba show that Ba is most stabilized by the cluster with two aluminum atoms closest to each other. This is reasonable because framework oxygen atoms bonded to aluminum atoms are more basic than those bonded to silicon atoms, and thus Ba can be more stabilized when these basic framework oxygen atoms are closer. There are four types of oxygen atoms in faujasite zeolites, O(1), O(2), O(3), and

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TABLE 2: NBO Charges and Electronic Configurations of Ba for BaY Cluster Models with (1,4), (1,5), and (1,6) Al Arrangements cluster

NBO charge of Ba

electronic configuration of Ba

(1,4) (1,5) (1,6)

1.914 1.917 1.920

core[53.97]6s[0.05]5d[0.05]6p[0.01] core[53.97]6s[0.05]5d[0.05]6p[0.01] core[53.98]6s[0.04]5d[0.05]6p[0.01]

O(4), as shown in Figure 1. The heights of the Ba atom with respect to the plane formed from three O(2) in the S6R and the mean distances of the Ba atom from the three O(2) in the S6R were calculated and are listed in Table 1 for different aluminum arrangements. The aluminum arrangement only has a limited effect on the position of barium. The heights of the Ba atom with respect to the plane formed from the three O(2) in the S6R vary within 0.05 Å, and the mean distances of the Ba atom from the three O(2) in the S6R vary within 0.055 Å. The mean distances of the Ba atom from the three O(2) in the S6R range from 2.615 to 2.665 Å, which agree well with the sum of the ionic radii of Ba2+ and O2-, 1.34 + 1.32 ) 2.66 Å.61 In the optimized (1,5) cluster, the single six ring is slightly more expanded than those of the (1,4) and (1,6) clusters, which is consistent with the shortest height of Ba2+ to the O(2) plane for the (1,5) Al arrangement even though its mean Ba2+-O(2) distance is not the shortest. The NBO charge on Ba and its electronic configuration are shown for the three different aluminum arrangements in Table 2. The charges and electronic configurations of Ba are almost independent of the Al arrangements. Cartesian coordinates of the optimized inner regions of the ONIOM clusters with different Al arrangements are listed in the Supporting Information. 3.2. Adsorption of Key Intermediates. 3.2.1. Structures. Several different initial configurations were explored for each of the adsorbates on clusters with (1,4), (1,5), and (1,6) Al arrangements, and the optimized geometries that were the most stable are discussed in the following paragraphs and shown in Figures 3 and 4 and Figures S1-S4 in the Supporting Information. (1,4) Al Arrangement. As shown in Figure 3, the optimal configurations found for all of the adsorbates studied except O2N-CH2COOH involve one oxygen atom of the adsorbate coordinated to barium for the (1,4) Al arrangement. The distances range from 2.61 to 2.76 Å and are listed as Ba-Oa in Table S1. For adsorbed O2N-CH2COOH, two oxygen atoms are coordinated to the barium atom. One oxygen atom from the carboxyl group is coordinated at 2.70 Å, and one oxygen atom from the nitro group is coordinated at 2.91 Å. Note that for adsorbates with a hydroxyl group, the hydrogen atom in the hydroxyl group is coordinated to a framework oxygen atom next to an aluminum atom at distances ranging from 1.29 to 1.70 Å, listed as Oz-H in Table S1. The bond lengths of the hydroxyl groups range from 1.05 to 1.14 Å, which is slightly longer than the typical OH bond length, 0.98 Å. The hydrogen atom in the methyl group of adsorbed nitromethane is coordinated to a zeolite oxygen at 1.97 Å. More selected geometric parameters are listed in Table S1. Structures of the dissociated adsorbates are shown in Figure 4. Except for O2N-CH2COOH, the hydrogen atoms of the adsorbate molecules are bonded to a framework oxygen atom next to an aluminum atom at a typical OH bond distance, 0.99 Å. For O2N-CH2COOH, the adsorbate hydrogen atoms are bonded to the framework oxygen atom at a slightly longer distance, 1.03 Å, while also being attracted by the third oxygen

Sung et al. atom of the adsorbate moieties. Aci-anion nitromethane (CH3NOO-), acetate ion (CH3COO-), CH2COO-, NO2-, and O2N-CH2COO- adsorb with bidentate configurations through two oxygen atoms coordinated to the barium atom. The two O-C or O-N distances are essentially equivalent to each other as listed in Table S1. Note that attempts to stabilize dissociated HNO3 with the (1,4) cluster were made by initializing the geometry optimization with a dissociated configuration. However, H+ and NO3- moieties recombined during the course of the geometry optimization, and no dissociated HNO3 could be stabilized with this Al arrangement. (1,5) Al Arrangement. Structures of the neutral adsorbates are shown in Figure S1. Structural features of the neutral adsorbates are generally the same as those of the neutral adsorbates on the cluster with a (1,4) Al arrangement: one oxygen atom of the adsorbate is coordinated to barium, and the hydrogen atoms in the hydroxyl groups of the adsorbates are coordinated to framework oxygen atoms next to aluminum atoms. Both Ba-Oa and Oz-H coordinations are at shorter distances than for the (1,4) Al arrangement and range from 2.56 to 2.73 Å, and 1.21 to 1.66 Å, respectively, as listed in Table S2. Note that the hydrogen atom of adsorbed HNO3 is coordinated to an oxygen atom of the nitrate group and a framework oxygen atom at an equal distance, 1.21 Å, while the OH bond lengths of the other adsorbates, ranging from 1.02 to 1.16 Å, are shorter than the Oz-H distances, ranging from 1.28 to 1.66 Å. As shown in Figure S2, dissociated adsorbates on the (1,5) Al cluster have the same general structural features as those interacting with the (1,4) Al arrangement. Except for HNO3 and O2N-CH2COOH, the hydrogen atoms of the adsorbate molecules are bonded to a framework oxygen atom next to an aluminum atom at a typical OH bond distance, 0.98 Å. Acianion nitromethane (CH3NOO-), acetate ion (CH3COO-), CH2COO-, NO3-, NO2-, and O2N-CH2COO- adsorb with bidentate configurations through two oxygen atoms coordinated to the barium atom. Geometric parameters for each adsorbate are listed in Table S2. (1,6) Al Arrangement. Neutrally adsorbed aci-NM, acetic acid, and CH2COOH interact with the barium atom through one oxygen atom at slightly longer distances, 2.60-2.63 Å, than the corresponding distances on the cluster with the (1,5) Al arrangement. The hydrogen atoms of the hydroxyl groups in aci-nitromethane, acetic acid, and CH2COOH coordinate to a framework oxygen atom at slightly longer distances as well, 1.46-1.48 Å. The structures are shown in Figure S3, and geometric parameters are listed in Table S3. Note that neutrally adsorbed HNO3 and HNO2 could not be stabilized even when the geometry optimization was initialized with their neutral form. As shown in Figure S4, dissociated adsorbates on the (1,6) Al cluster have the same general structural features as those interacting with the (1,5) Al arrangement. These adsorbates except for OH- interact with the barium atom with a bidentate configuration through two oxygen atoms, and the hydrogen atoms of the adsorbate molecules are bonded to framework oxygen atoms next to an aluminum atom at 0.98 Å except for HNO3 and O2N-CH2COOH. Geometric parameters for each adsorbate, listed in Table S3, are mostly invariant from those of dissociated adsorbates on the (1,5) cluster. 3.2.2. Partial Charges of Hydrogen Atoms in Adsorbates from NBO Analysis. Adsorbates on clusters with (1,4), (1,5), and (1,6) Al arrangements, as shown in Figures 3, S1, and S3, respectively, are generally considered neutral if the OH

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Figure 3. Structure of adsorbed molecules on 96 T BaY clusters with (1,4) Al arrangement: (a) nitromethane, (b) aci-nitromethane, (c) acetic acid, (d) CH2COOH, (e) HNO3, (f) HNO2, (g) H2O, and (h) O2N-CH2COOH. Only the QM portions are shown for simplicity. Selected interatomic distances in angstroms are indicated. Additional values are given in Table S1 in the Supporting Information.

bond lengths are shorter than the Oz-H distances. However, the fact that the Oz-H distances are often comparable to the OH bond lengths brings up the question of how close these adsorbates are to a dissociated state. Partial charges of the hydrogen atoms of adsorbates in the neutral and dissociated forms on the clusters were obtained from NBO analysis and are listed in Table 3. Gas-phase charges of the hydrogen atoms of these adsorbate molecules are also listed for comparison. The partial charges of the hydrogen atoms of O2NCH2COOH do not differ significantly whether it adsorbs neutrally or dissociatively. For neutrally adsorbed acetic acid and CH2COOH on (1,5) and (1,6) clusters, and HNO3 on (1,6) cluster, the hydrogen partial charges are nearly the average value of the hydrogen partial charges for the gas-phase and dissociated forms, suggesting that they are actually midway between the “neutral” and “dissociated” states. For neutrally adsorbed aciNM and HNO2, the partial charges of the hydrogen atoms suggest that they are closer to the neutral state than to the dissociated state. The net charges of the anionic moieties of

adsorbates in the neutral forms on the clusters, as listed in Table S4, also show that the molecules are partially ionized to various extents. 3.2.3. Adsorption Energetics. The adsorption enthalpies at 473 K were calculated by: Z+X Z X ∆H ) HONIOM - HONIOM - HDFT

(5)

where the superscript “z” denotes the zeolite cluster, and the Z+X Z superscript “x” denotes the adsorbate. HONIOM and HONIOM represent the enthalpies of the zeolite cluster interacting with the adsorbate and the bare zeolite cluster, respectively, calculated X denotes the enthalpy of the by the ONIOM approach. HDFT adsorbate in the gas phase calculated purely by DFT. The enthalpies can be obtained via eq 2 at 473 K. Adsorption Gibbs free energies at 473 K were calculated in a similar way: Z+X Z X ∆G ) GONIOM - GONIOM - GDFT

(6)

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Figure 4. Structure of adsorbed molecules on 96 T BaY clusters with (1,4) Al arrangement: (a) aci-anion nitromethane, H+, (b) acetate ion, H+, (c) CH2COO-, H+, (d) NO2-, H+, (e) OH-, H+, and (f) O2N-CH2COO-, H+. Only the QM portions are shown for simplicity. Selected interatomic distances in angstroms are indicated. Additional values are given in Table S1 in the Supporting Information.

TABLE 3: Partial Charge of Hydrogen Atomsa in Adsorbates Al arrangement aci-NM aci-anion NM, H+ acetic acid acetate ion, H+ CH2COOH CH2COO-, H+ HNO3 NO3, H+ HNO2 NO2, H+ H2O OH-, H+ O2NCH2COOH O2NCH2COO-, H+

gas phase

(1,4)

(1,5)

(1,6)

0.501

0.537 0.625 0.551 0.627 0.551 0.626 0.542

0.535 0.606 0.551 0.608 0.552 0.607 0.545 0.575 0.523 0.608 0.552 0.603 0.564 0.569

0.532 0.608 0.556 0.609 0.552 0.609

0.504 0.506 0.507 0.491 0.481 0.520

0.521 0.628 0.549 0.621 0.552 0.588

0.591 0.613 0.552 0.606 0.567 0.574

a

These are the hydrogen atom of the methyl group in nitromethane and the hydrogen atom of the hydroxyl group in other adsorbates that are dissociated from the adsorbates and bonded to a framework oxygen atom.

where G denotes Gibbs free energies that can be obtained via eq 4 at 473 K. The adsorption Gibbs free energies of the adsorbates on the different clusters are shown in Figure 5. The calculated Gibbs free energies of adsorption show that the neutral form is more stable than the ionic form for all of these adsorbates except HNO3. However, adsorbates in their neutral form are actually partially ionized as mentioned in the previous section. In addition, molecules in both the neutral and

the dissociated forms are more stabilized with the two aluminum atoms further apart. Except for HNO3 and HNO2, differences in the adsorption free energies between the neutral and ionic forms are smaller for the (1,6) than for the (1,5) and (1,4) Al arrangements, indicating that the equilibrium shifts more toward the dissociated state for the (1,6) than for the (1,5) and the (1,4) Al arrangements. The enhanced adsorption and dissociation when the two Al atoms are further apart can be attributed to the electrostatic effect created by the clusters with different Al arrangements. Figure 2 shows the electrostatic potential mapped onto the electron density isosurface of 0.002 e/au3. Red indicates a negative electrostatic potential with a concentrated electron density, whereas blue indicates a positive electrostatic potential with low electron density that does not completely shield the nuclear charge. When the two Al atoms are further apart, the difference between the positive and negative regions is more apparent, which means that the cluster is more polarized and has a stronger electric field. Thus, the adsorption is enhanced, and the dissociation is facilitated by the negative region around the framework oxygen atoms next to the Al atom, which attracts hydrogen atoms more strongly. One could expect that other divalent cations might show similar behavior. The calculated adsorption enthalpies are shown in Figure S5. The same trends are found for adsorption enthalpies as are observed for the Gibbs free energies of adsorption. The contributions from the inner (QM) and outer (MM) regions can be decomposed as in our previous study.42 Table S5 in

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Figure 5. Adsorption Gibbs free energies of nitromethane (NM), aci-NM, acetic acid, CH2COOH, HNO3, HNO2, H2O, and O2N-CH2 COOH in neutral (solid line) and dissociated (dashed line) forms for different Al atom arrangements at 473 K.

Figure 6. Correlation between adsorption Gibbs free energies of dissociatively adsorbed nitromethane (NM), acetic acid, HNO2, O2N-CH2COOH, and HNO3 and their acidities in water.

the Supporting Information shows that the contribution from the QM region to the adsorption enthalpy is dominant for all of the adsorbates studied on BaY clusters with different Al arrangements. Unfortunately, there are, to our knowledge, no available experimental data for adsorption enthalpies of these adsorbates in BaY zeolite. These calculated adsorption enthalpies are thus provided for future comparison with experimental values. 3.2.4. Correlation between Adsorption Gibbs Free Energy and Acidity. We next explored whether a correlation exists between the properties of the dissociatively adsorbed molecules and their adsorption Gibbs free energies. The most relevant property of these adsorbates was postulated to be their acidity, which can be characterized by pKa values. The experimental pKa values found for NM, acetic acid, HNO2, O2N-CH2COOH, and HNO3 at 298 K in water62-65 can be used to obtain the

standard Gibbs free energy change upon dissociation via the equation:

∆G ) 2.303RTpKa

(7)

The adsorption Gibbs free energies of dissociated NM, acetic acid, HNO2, O2N-CH2COOH, and HNO3 on clusters with different Al arrangements at 298 and 473 K are plotted versus ∆G calculated from pKa values using eq 7 at 298 K in Figure 6. The adsorption Gibbs free energies at 473 K of these dissociated adsorbates are parallel with those at 298 K and therefore correlate with the standard Gibbs free energy change upon dissociation. This simple correlation suggests that the more acidic molecules have a stronger tendency to dissociate and have

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lower adsorption Gibbs free energy when adsorbed dissociatively on clusters with different Al arrangements. The combination of a strong acid in a more polarized environment is the possible reason why HNO3 prefers to be in the dissociated form on clusters with (1,5) and (1,6) Al arrangements. Therefore, besides the local environment in the zeolite, the stabilities of dissociated adsorbates depend on the acidities of the adsorbates as well. 4. Conclusion To elucidate the characteristics of dissociative adsorption of intermediates in the NOx reduction reaction over BaNaY zeolites, quantum chemical calculations using the ONIOM(B3LYP/ SDD:UFF) approach were carried out. Three different aluminum arrangements around the active site were employed to investigate the effect of local zeolite environment on the stability of intermediates adsorbed neutrally and dissociatively. Nitromethane, aci-nitromethane, acetic acid, CH2COOH, HNO3, HNO2, H2O, and O2N-CH2COOH were studied. It was found by our calculations that intermediates could be stabilized in both neutral and dissociated forms. When intermediates adsorb neutrally, they interact with the barium atom through one oxygen atom, and there is an interaction of an adsorbate hydrogen atom with a framework oxygen atom. When intermediates adsorb dissociatively, the anionic moieties interact with the barium atom through two oxygen atoms, and the proton is bonded to a framework oxygen atom next to an aluminum atom. Adsorption Gibbs free energies of all of these adsorbates were calculated at 473 K, revealing that adsorbates, except for HNO3, are more stable in their neutral form than their dissociated form, and that adsorbates are more stabilized when two aluminum atoms are further apart. However, even though most intermediates appear to favor neutral adsorption (lower Gibbs free energies), the partial charges, obtained from NBO analysis, of the hydrogen atoms coordinated to framework oxygen atoms show that the adsorbates are intermediate between the fully dissociated state and the neutral state. Therefore, we conclude that intermediates in the NOx reduction reaction are stabilized by BaNaY zeolites through both the Ba atom and the zeolite oxygen atoms. The intermediates are predominantly in forms in which hydrogen atoms are partially “ionized” by basic framework oxygen atoms upon adsorption. Therefore, it is possible that the NOx reduction reaction over BaNaY zeolites proceeds through a series of intermediates that are partially ionized but not fully anionic. This is consistent with the fact that some proposed anionic intermediates were not observed by experiments.6 Finally, a correlation was found between the acidities of these adsorbates and their adsorption Gibbs free energies for the dissociated form, which suggests that the acidity of the adsorbate is a governing factor for the dissociative adsorption in addition to the local environment in the zeolite. The results of this work support the idea that one of the catalytic roles of BaY in NOx reduction is to open up additional reaction channels involving ionic species formed upon dissociative adsorption. The reason why this particular cation and zeolite combination works so well—and whether it could be further optimized—is an interesting question for future research. Acknowledgment. This work was supported by the Chemical Sciences, Geosciences, and Biosciences Division, Office of Basic Energy Sciences, Office of Science, U.S. Department of Energy Grant No. DE-FG02-03ER15457. This research used resources of the National Energy Research Scientific Computing Center, which is supported by the Office of Science of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231.

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