Structure of Active Sites in Pd-Exchanged Mordenite: A Density

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J. Phys. Chem. C 2007, 111, 6454-6464

Structure of Active Sites in Pd-Exchanged Mordenite: A Density Functional Investigation R. Grybos,*,†,| J. Hafner,† L. Benco,†,‡ and H. Toulhoat§ Fakulta¨t fu¨r Physik and Center for Computational Materials Science, UniVersita¨t Wien, Sensengasse 8, A-1090 Wien, Austria, Institute of Inorganic Chemistry, SloVak Academy of Sciences, DubraVska cesta 9, SK-84236 BratislaVa, SloVak Republic, and Institute Franc¸ ais du Petrole, F-92852 Rueil-Malmaison Cedex, France ReceiVed: September 25, 2006; In Final Form: March 9, 2007

Pd-exchanged mordenite (Pd-MOR) is considered as a promising catalyst for the selective catalytic reduction of nitrogen oxides (NOx) by hydrocarbons. However, it is well known that the chemical reactivity of the Pd2+ cations depends strongly on their location in the zeolite framework and on the coordination of the cation by the charge-compensating Al ions replacing Si framework atoms. In the present paper, we present a comprehensive investigation of the structure of the active sites in Pd-MOR based on periodic density functional calculations. For the possible configurations of Al in mordenite at a Si/Al ratio of 11, we find a rather narrow distribution of energies spreading over an interval of only 0.6 eV per primitive unit cell. This suggests that aluminum will be distributed more or less randomly over the framework. The stability of Pd2+ cations depends on the number of bonds between palladium and oxygen atoms directly connected to Al substitution sites. The energy difference between the most stable Pd locations in the main channel and the side pocket is found to be small (ca. 0.5 eV per primitive unit cell). However, the energy of a Pd cation in the side pocket is very sensitive to the local arrangement of Al sites, while Pd in the main channel remains quite stable for a number of different Al distributions.

1. Introduction 1.1. Selective Catalytic Reduction (SCR) of NO in Transition-Metal Exchanged Zeolites. Zeolites cation-exchanged with transition and noble metals are efficient catalysts for many reactions in chemical industry and environmental catalysis. The dispersion of the metallic cation throughout the volume of the catalyst bed, the chemical reactivity of the cations, and their accessibility to the reactant molecules are key features determining the performance of the catalyst. The removal of nitric oxides from exhaust gases is now of primary importance because of the reduction of the NOx emission limits that will enter in force by the year 2008.1 In particular, much attention was given to mordenite exchanged with palladium as a promising catalyst for the SCR of NO by methane. Palladium-exchanged mordenite (Pd-MOR) proved to be one of the most stable zeolite-based catalysts, even under rather harsh, hydrothermal conditions found in the car exhaust systems. According to the proposed reaction mechanism,2 isolated Pd2+ cations are the active sites. There is, however, a limit to the amount of Pd that can be introduced into the zeolite. At higher concentrations, palladium starts to form clusters, consisting either of PdO or metallic particles, and the catalyst becomes less active and/or selective. In addition, increasing the amount of aluminum to increase the zeolite cation-exchange capacity results in a destabilization of the framework, which becomes increasingly susceptible to dealumination and dehydroxylation. To discuss catalyst stability and performance, more information is needed about its structure on the atomic level. This paper * Corresponding author. E-mail: [email protected]. † Universita ¨ t Wien. ‡ Slovak Academy of Sciences. § Institute Franc ¸ ais du Petrole. | On leave from the Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, Niezapominajek 8, PL-30239 Krako´w, Poland.

aims at describing, using density functional theory (DFT), the structure of the active sites in Pd-exchanged mordenite. 1.2. Al in Mordenite. Replacement of Si atoms in the zeolitic framework by Al results in a charge deficit that can be neutralized by protons bound to framework oxygen atoms (creating Brønsted acid sites) or extraframework cations. Extraframework cations of noble, transition, or polyvalent simple metals act as strong Lewis acid sites.3,4 Al cations can replace silicon in tetrahedral (T) sites or be accommodated as extraframework Al (EFAl). EFAl species can also exhibit substantial acidity or can enhance the acidity of other sites.5-7 The distribution of Al within the zeolite framework is still an open question. It seems that aluminum cations are not distributed randomly but prefer specific T sites. However, because X-ray diffraction is unable to unambiguously distinguish Si and Al, experimental information is not readily accessible. The Al distribution is determined only indirectly, usually from the location of Brønsted acid sites (as obtained from neutron diffraction and infrared spectroscopy). The amount of Al present is another question. It is known that thermal treatment (calcination) and chemical reactions may lead to dealumination,8 but under some conditions reinsertion of Al into tetrahedral sites (realumination) may take place.9-11 Sonnemans et al.12 showed that mordenite samples with Si/Al ) 9.4 undergo dealumination under calcination conditions, resulting in Si/Al ) 12.3, as measured by 29Si magic-angle spinning nuclear magnetic resonance. In the present study, we consider mordenite with a Si/Al ratio of 11. This corresponds to two Al centers per primitive monoclinic unit cell (4 Al per orthorhombic unit cell), which can compensate the charge of a single Pd2+ cation per primitive cell.

10.1021/jp0662606 CCC: $37.00 © 2007 American Chemical Society Published on Web 04/11/2007

Structure of Active Sites in Pd-Exchanged Mordenite

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6455

Figure 1. Structure of mordenite viewed along the c-axis. Two unit cells are shown: orthorhombic (solid line) and primitive monoclinic (dashed line). Different channels within the structure are also shown. Black circles are used to represent four types of framework tetrahedral sites (T1, T2, T3, T4) as well as extraframework positions classified by Mortier (A, B, C, D, E, and H).

1.3. Extraframework Pd in Mordenite. Molecularly dispersed PdO (or Pd2+) species, which can act as active sites, are stabilized by framework aluminum.13-21 In this paper, only isolated Pd2+ cations were considered in accordance with the SCR mechanism proposed by Shimizu et al.2 Water vapor facilitates the removal of metal from the zeolite13 thus decreasing the activity. Pd-exchanged zeolites (Pd-zeolites) are more stable under the presence of water vapor and show higher activity in SCR22,23 than Ga, In,24,25 or Co-loaded26 zeolites. Additionally, Pd-MOR is more stable than Pd-exchanged ZSM or MFI zeolites.13,14,27 Other factors leading to deactivation and loss of zeolite acidity under hydrothermal conditions include oxidation to PdO agglomeration particles or metal clusters.13 2. Model and Method 2.1. Zeolite Model. Mordenite is a natural zeolite of varying composition; therefore, it is hard to unambiguously define its crystallographic structure. Many X-ray diffraction experiments can be found in the literature,28-36 and it is widely accepted that a purely siliceous structure has Cmcm space group symmetry but extraframework ions lower the symmetry. As reported by Alberti et al.30 and most commonly used in theoretical studies, the orthorhombic cell of mordenite contains 144 atoms (48 Si and 96 O). It is possible, however, to construct a smaller monoclinic primitive cell of 72 atoms (24 Si and 48 O), which is used in the present work. After optimizing volume and shape, the parameters of the primitive cell are the following: a ) b ) 13.79 Å, c ) 7.59 Å, and γ ) 83°. Previous calculations (albeit with lower energy cutoff)37 gave very similar results (a ) b ) 13.80 Å, c ) 7.61 Å, γ ) 83°) and also showed that the introduction of counterions does not change the size of the cell significantly. Therefore, in our calculations the size and shape of the unit cell were kept fixed.

The structure of mordenite viewed along the c direction is shown in Figure 1. The solid box denotes the orthorhombic cell, while the dashed box represents the monoclinic primitive cell. Three distinct areas are also shown with varying shades of gray. The main channel composed of 12-member rings and running parallel to the c-axis is pictured in light gray. The darkest area is sometimes referred to as the “side channel” running parallel to the main channel, although this is not strictly correct. This channel is not straight, but consists of a series of cavities connected by narrow, elliptical openings through which only the smallest molecules can travel. More often the areas marked “link channel” and side channel are together referred as the “side pocket”, which is perpendicular to the main channel. However, it is sometimes convenient to separate those two areas. Possible locations of extraframework cations have been classified by Mortier;52 they are labeled in Figure 1 by capital letters A-H. Sites A-C are located in the side- and link channels (and hence accessible only to small reactant molecules), while sites D, E, and H are located close to the wall of the main channel. 2.2. Calculation Method. Our calculations were performed using the Vienna ab initio simulation package (VASP)38-41 within DFT with the gradient-corrected PW9142,43 exchangecorrelation functional. The projector-augmented wave method (PAW)44,45 was used to describe the electron-ion interaction, and for valence electrons a plane wave basis set was employed. The energy cutoff was set to 400 eV. A modest Gaussian smearing was applied to band occupations around the Fermi level (σ ) 0.1 eV) and the total energies were extrapolated to σ f 0. The electronic entropy correction was below 1 meV/ atom. Because the mordenite supercell used is rather large, the integration over the Brillouin zone was performed using the Γ point only. All calculations were spin-polarized, and the possible

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Figure 2. Schematic representation of all 29 possible nonequivalent Al-Al locations in the primitive mordenite cell. Almost all tetrahedral sites (except T3) are located in the main channel. Therefore, the zeolite framework can be represented as a tube (the wall of the main channel) which after cutting along the c-axis can be rolled out. The inset shows such a projection of the wall around the main channel and the locations of the four types of tetrahedral sites. The 4-MRs on which the T3 sites are located are shown in “pseudoperspective” view as small trapezoids attached to the large 8-MR’s. In the real structure, the 4-MRs are perpendicular to the 8-MRs, pointing outwards from the main channel. Aluminum locations are denoted as black circles at appropriate positions. Note that the T2 sites on left and right edges overlap when the flat structure is rolled back into a tube; therefore, Al on those sites are represented by half-circles (structures T1T2b, T1T2d, T2T2c, and T2T2d).

spin multiplicities taken into account are discussed below. Unless stated explicitly otherwise, all energies are total energies per primitive unit cell. 3. Optimization of the Structure of Pd-Exchanged Mordenite 3.1. Al Distribution in the Zeolite Framework. From simple combinatorics, there are 276 possible ways for substituting two Al ions for Si in 24 tetrahedral sites. Out of this number, 48 combinations are excluded due to Loewenstein’s rule, which forbids that two Al ions share a common O-atom,46 leaving 228 possibilities. In the primitive unit cell used, three symmetry operations are present that reduce the number of possible AlAl arrangements: a mirror plane parallel to c-axis (bisecting the side pocket), a translation along c-axis, and an inversion center (located in the center of the main channel). It turns out that only 38 configurations represent all nonequivalent combinations of two Al ions in T sites. Another nine structures were excluded on the basis of the five-member ring (5-MR) avoidance rule,47 leaving 29 structures. All structures are shown schematically in Figure 2.

The Loewenstein’s rule is considered a “hard” rule; it can be broken only under rather unusual synthesis conditions.48 This is confirmed by our calculations that show that configurations breaking Loewenstein’s rule have energies by ≈1 eV/unit cell higher than other geometries. The 5-MR ring avoidance rule is much “softer”. There is experimental evidence of this rule being broken.49-51 According to our calculations, structures that break this rule have energies comparable with the least stable structures in the selected set. They were excluded from the calculations to be consistent with experiment and to reduce the computational workload, but under certain conditions they may be found in the zeolite. Substitution of Al for framework Si causes a local chargedeficit (as Al has valence 3 compared to 4 of silicon). In H-mordenite, this excess charge is neutralized by the creation of an OH group (Brønsted acid site) at one of the oxygen ions directly bound to Al. However, as there are four O-ions around each of the two Al sites, there are about 16 times as many possible combinations of two Al-OH Brønsted sites and a corresponding multiplication of the computational effort. Therefore, the excess charge created by the Al/Si substitution was

Structure of Active Sites in Pd-Exchanged Mordenite compensated by adding a positive background charge (while leaving the number of electrons constant). This approach allows one to study the effect of Al substitutions on the framework stability and structure without the added complexity of counterions or OH groups. In a system isoelectronic with purely siliceous mordenite, all electrons are paired, and therefore the ground state is a spin-singlet state. For a few cases, test calculations with triplet state were performed that showed that the spin-triplet is indeed an excited state with an energy higher by about 5 eV. 3.2. Extraframework Sites for Counterions. For the initial location of the counterions in the zeolite structure, a set of specific positions proposed by Mortier52 has been used. It should be noted, however, that the Mortier positions are devised for zeolites containing alkali and alkali earth metals, which are anchored to the framework through nondirectional, electrostatic interactions. Palladium, on the other hand, establishes strong covalent bonds with the framework oxygen atoms (with Pd-O distances of about 2 Å), and therefore upon relaxation Pd usually moves away from the ideal Mortier position toward the channel walls. Still, it is convenient to use the Mortier symbols, while keeping in mind that they represent only the positions closest to the actual placement of the Pd cation and from which the system relaxations have been started. Palladium cations were inserted in two distinct areas: in the main channel (Mortier positions D, E, and H) and in the side pocket (Mortier positions A, B, and C) (see Figures 1 and 3). Position D is located in the 8-MR opening, the access from the main channel to the side pocket, but was classified as a main channel site, as it is easily accessible to molecules that fit into the main channel. The entire structure was allowed to fully relax. Not all combinations of the six Pd positions with the 29 AlAl arrangements were investigated. From a first series of calculations, it became evident that the stability of the Pd cation is related to the combined distance between Pd and both Al atoms, or to be more precise, to the oxygen atoms that are directly bound to Al centers (for details see Section 4.3). Therefore for each Al-Al arrangement, two or three favorable Pd locations were chosen for calculations, leading to a total of 112 structures. The reason behind this correlation is that palladium is stabilized by binding to the Si-O-Al groups and the compensation of the excess negative charge is introduced by the Al/Si substitution. Therefore, a Pd cation, which is close to both Al atoms, provides a more efficient local charge compensation. This is in agreement with the conclusions of Aylor et al.14 for Pd in ZSM-5. 4. Results and Discussion 4.1. Stability of Al/Si Substitutions. Figure 4 reports the total energy (relative to the most stable configuration) of all 29 Al-Al configurations against the distance between translationally nonequivalent Al sites. Structures with shorter Al-Al distances are less stable than arrangements in which the Al cations are further apart. This is largely due to the repulsive interaction between the excess negative charges induced by the Al/Si substitution. As shown below, framework distortions induced by the substitution are strongly localized and influence the Al-Al interactions only at the shortest distances. Our results agree with the experimental findings that parts of the framework at which two Al centers are separated by only a single Si ion are the most susceptible for dealumination under hydrothermal conditions.12 Note that when the Al-Al distance exceeds ca. 7.6 Å, the energy levels off. This is because of a rather flat unit cell (7.59 Å along c direction) that causes interaction of Al cations with their translational images in neighboring cells.

J. Phys. Chem. C, Vol. 111, No. 17, 2007 6457 In the most stable structures, Al cations occupy T1 and T2 sites. This coincides with the experimental findings of Satsuma et al.53 that framework aluminum is located mostly in the main channel, but contrasts the suggestions of Maurin et al.54 that T3 and T4 sites are preferentially occupied by Al. However, the energy difference between the most and least stable structures is only ≈0.6 eV (and even less than 0.4 eV, if the two least stable configurations are excluded), suggesting that in thermodynamic equilibrium Al should be distributed almost randomly within the framework. The slight preference for T1 and T2 sites can be explained by the flexibility of the zeolite framework around these sites, as seen from distortions of T-O-T angles discussed in more detail in the following section. An increased flexibility allows for a better geometry relaxation and thus increases the stability of Al in T1 and T2 sites. On the other hand, T3 and T4 sites are located in a tight 4-MR where relaxation is strongly restricted. A preferential occupation of T3 and T4 sites as reported by Maurin et al.54 could be caused by an exceptional stability of acidic OH groups or counterions placed in the vicinity. There is also a possibility that the kinetics of the zeolite synthesis process are responsible for a nonrandom Al distribution. Both topics, however, are beyond the scope of this work. Because of such a narrow distribution of the total energies, all 29 Al-Al configurations were considered in our further calculations. 4.2. Framework Distortions Induced by Al/Si Substitutions. Aluminum-oxygen bonds are weaker and hence longer than Si-O bonds. Therefore, substitution of Al for Si in tetrahedral sites must result in a framework deformation. For the purpose of discussion, oxygen centers bound directly to Al cations will be denoted O*. The most notable effect is the elongation of bonds around Al substituted sites. In an allsiliceous structure, Si-O bond lengths fall into the range of 1.61-1.63 Å. Upon substitution by Al, the bond lengths around the tetrahedral site increase to 1.73-1.76 Å (the bond length changes by 0.121-0.138 Å). At the same time, the Si-O* bonds are contracted to 1.58-1.59 Å (decrease by 0.040-0.052 Å). Next-nearest-neighbor Si sites, which have no direct contact with the substitution site, experience only slight changes of the Si-O bond lengths, not exceeding 0.027 Å in any case. This shows that the effect of Al substitution on T-O bonds decays rapidly with increasing distance from the substitution site. The O-T-O angles in the all-siliceous zeolite are almost equal to the ideal tetrahedral angle (109.5°), ranging between 107.7° and 111.3°. The shift of the oxygen center from Al toward Si imposes a strain on the intratetrahedral (O-T-O) angle at the Si site. However, the O-T-O angles do not change significantly. The largest observed changes occur at the Si sites adjacent to Al and are usually below (4°. In extreme cases, when a Si atom is located between two Al, effects from both sites add up and the changes in O-T-O angles can reach 6.8°. In fact, the intertetrahedral angles (T-O-T) accommodate most of the framework deformations, and changes can be as large as 23°. The largest changes are found for angles between two T2 tetrahedrons or between T1 and T2. There is no clear correlation with the position of Al centers. Evidently, the T2O-T2 (and, to some extent, also T1-O-T2) intertetrahedral angles are “softest” and able to accommodate quite large distortions without substantial energetic penalties. In an all-Si structure, because of symmetry (inversion center) there is a T-O-T angle of 180°. Calculations show that when Al distribution breaks the inversion symmetry, this angle is reduced


Grybos et al.

Figure 3. Schematic drawings of the Mortier positions of counterions in the mordenite framework. Mortier positions are denoted by black circles. Surrounding rings are shown as balls and sticks, while a wire-frame model is used for the rest of the framework. By necessity, the viewing angle is different for each case, but to improve the readability on each figure the largest 12-membered ring in the main channel is shown highlighted by a thicker line.

to 156.5-178.6°, depending on the structure. The impact of symmetry is best seen when comparing two T3T3 structures (T3T3a and T3T3b, see Figure 2) that differ only slightly (i.e., one of the Al cations is shifted to a neighboring position in the 4-MR). In one case, the inversion center is preserved and the 180° angles are present, while in the other structure in which the symmetry is broken, this angle is reduced to 173.8°.

In summary, changes of the T-O bond lengths are highest at the substitution site (ca. 0.125 Å) and decrease rapidly (down to 0.05 Å) for Si sites located farther away. Intratetrahedral (OT-O) angles remain almost intact, with the exception of Si sites neighboring two Al sites, at which the angle can change by 6.8°. Intertetrahedral (T-O-T) angles accommodate most of the framework deformations, and can change by as much as



Figure 4. All 29 possible nonequivalent locations of two Al cations in the primitive cell of mordenite. The relative total energy (relative to the most stable configuration T2T2d, see Figure 2) is plotted versus the distance between translationally nonequivalent Al cations. Because the length of the unit cell along the c direction is 7.59 Å (denoted by a vertical dashed line) all points to the right of this line represent structures in which Al centers interact mainly with their own translational images from neighboring cells. Letters next to symbols (combined with the TxTy symbols) can be used to identify the structures shown in Figure 2.

23°. The T2-O-T2 angles are especially prone to large changes, whatever the Al distribution. Previous studies37 have determined framework distortions in the presence of a Na counterion. Comparison with the present results (where no counterion is present) can give insight into the effect of the counterion on the zeolite structure. In both calculations, the changes in bond lengths (with respect to allSi structure) are very similar, following the same trend and with discrepancies not exceeding 0.04 Å. The changes in intratetrahedral angles, however, show large differences. Often the angular distortions follow opposite trends and the discrepancies in values can reach 20°. This shows that a counterion heavily affects the way in which zeolite framework reacts to the Al/Si substitution, mainly by changing the intratetrahedral (T-OT) angles. 4.3. Stability of Lewis Sites in Mordenite. The Pd2+ cation has 8 d-electrons and therefore can be found in singlet or triplet states. Because the most stable framework state is singlet, there are two possible spin states for a Pd2+ cation within the zeolite with multiplicities of 1 and 3. Both were calculated for every structure and the one with lower energy was chosen. In addition, for a few selected cases calculations with varying magnetic moment were performed. For the most stable structures, the system is always in a singlet state with zero magnetic moment. Triplet states are found for some “exotic” structures (for example, when the Pd cation is placed far away from both Al sites, or when the geometry forces the Pd-O* bonds to be unusually elongated), but their energies are at least 1.5 eV above the most stable singlet states. Figure 5 shows the dependence of the relative energy of all considered Pd2+/zeolite configurations on the sum of the distances between the Pd2+ cation and the two nearest O* atoms activated by the presence of a nearest-neighbor Al site. Although the energies scatter rather widely, there is a net tendency to destabilize the Pd2+ in the zeolite framework if the two compensating Al centers are located at large distances. The vicinity of the Pd2+ cation to one or two Al sites allows the formation of covalent bonds to activated O* atoms located next

to Al atom. The importance of the formation of Pd-O* bonds for the stabilization of a cation site is illustrated by insets in Figure 5 showing four different configurations with Al in T1 and T2 sites and Pd2+ in the center of a 6-MR (Mortier site E). In configuration T1T2a (cf. Figure 2), there are two Al atoms in the 6-MR, in configuration T1T2c there is only one Al atom, and in configuration T2T2e there is no Al at all in the ring next to the Pd2+ cation. Hence, in these three configurations two, one, and no strong Pd-O* bond can be formed, resulting in a difference in the total energies of these three configurations of up to ∼2 eV, although the stability of the Al/Al configurations is almost the same (see Figure 4). It is also important to realize that each Al/Si substitution creates four activated O* atoms, out of which two are capable of forming strong Pd-O* bonds. Configurations T1T2c and T1T3a shown in Figure 5 have both only one Al site in the six-membered ring occupied by the Pd2+ cation, but because of the more symmetric Pd-Al arrangement, two strong PdO* bonds are formed in configuration T1T3a and only one in T1T2c with evident consequences for the stability. The same trend emerges from a comparison of configurations T1T2a and T1T3a, the former with two, the latter with one Al in the ring occupied by the Pd2+ cation; in both cases two Pd-O* bonds are formed, resulting in a comparable stability, although the combined distances are much larger for configuration T1T3a. 4.4. Dependence of Pd2+ Stability on Cation Location (Mortier Site). A very important point is the stability of the Pd2+ cation in different Mortier positions, because the location determines accessibility of the active site. Table 1 summarizes our results for the relative stability of the Pd2+ cation in different Mortier positions. For each position, a range is given that represents the relative stabilities of structures with various AlAl arrangements. Each row represents a characteristic Al-Al arrangement, grouped according to the number of Al and O* (oxygen center directly bound to Al) in the vicinity of the Pd cation. Energies are calculated with respect to the energetically optimal geometry. The negative value shown in the table corresponds to a special case and will be discussed separately.


Grybos et al.

Figure 5. Dependence of total energy of Pd-mordenite on the sum of two Pd-O(Al) distances. It is clear that the palladium cation is stabilized by having Al centers in the vicinity.

TABLE 1: Relative Energies [eV] (with Respect to the Most Stable Structure) of Pd Placed in Different Mortier Positions and with Varying Al-Al Configuration around the Sitea Al

O*

A

B

C

O* Al〈O* O*〉 Pd 〈O*〉 Al

2

4

0.00-0.37

1.30-1.34

AlO*-Pd〈O* O*〉 Al

2

3

0.73

0.27

AlO*-Pd-O* Al

2

2

0.73

Pd 〈O* O*〉 Al

1

2

0.74

Pd-O* Al Pd

1 0

1 0

1.21-1.39 1.91-1.99

D

E

H

0.02 0.38

-0.29

1.37

1.09

0.43-0.44

0.96-1.32

1.26-1.74

1.02-1.36

0.47-0.72

1.00 2.03-2.36

2.09

1.66-2.09 2.17-2.70

1.28-1.78 2.56

1.53

a Some combinations of Pd-position/Al-arrangement could not be realized (blank) because they are forbidden according to the Loewenstein or the 5-MR avoidance rules.

The results show no clear preference for a specific Mortier site. For all sites, the energies of the most favorable configurations lie within an interval of e0.5 eV (see Figure 6), and the less favorable configurations have energies of up to ∼2eV in all cases. However, as will be shown below, Al-Al configurations that break the 5-MR ring avoidance rule offer strong stabilization (at least 0.29 eV) for Pd2+ in Mortier position E. Experimental results do not give a definitive answer on the location of the palladium cation. A number of authors55-57 report that divalent cations (like Ca2+, Cu2+, Co2+) are stabilized in the side pockets. On the other hand, Satsuma et al.53 claim that framework aluminum is located only in the main channel, and as shown above, Pd cations are more stable when placed close to the Al substituted T-sites; thus, palladium in the main channel should be more stable. Pd cations are also rather mobile and capable of moving along mordenite channels. This aspect will be a subject of a separate study.58 Wherever palladium is located, the stability depends heavily on the arrangement of Al centers in the vicinity. Pd is stabilized by direct bonding to “activated” oxygen centers (framework O, which are bound to Al centers). As discussed above, the number of Al sites in the vicinity plays only a secondary role. Six typical situations can be found, each corresponding to a row in Table 1:

1. With two Al and four O* around, Pd2+ can form four PdO* bonds with a square-planar coordination. This arrangement is energetically most favorable in combination with Pd in Mortier position A (side channel) and C (link channel) in which indeed four (position C) and three (position A) strong Pd-O* bonds may be formed (see Figure 6). In both cases, the coordination of Pd cations is close to square-planar. The surprisingly high energies for Pd in position B are the result of placing Pd in the center of the 8-MR where all four Pd-O bonds are unusually elongated, while for Pd in position A only one Pd-O* bond is long. 2. Two Al in the vicinity can give three O* to which Pd can bond. The most energetically favorable Pd2+ location is site B (see Figure 6). Palladium is again in a square-planar coordination with one coordination site empty (the one pointing toward the middle of 8-MR ring). The least stable configuration (Pd in position A) is ca. 0.7 eV above the optimum. 3. With two Al and two O* in the vicinity, Pd2+ cations are still quite strongly bound. For some particular arrangements (e.g., Pd in position E) the energy can be as low as 0.43 eV; however, most of the configurations are more than 1 eV above the optimum. 4. Bonding to two O* but in the vicinity of only one Al. The energy varies between 0.47 eV (Pd2+ in position E, configuration



Figure 6. Most stable palladium configuration for each of the Mortier positions and their total energies relative to the most favorable configuration with Pd2+ in site A. Large white circles denote Si ions, small white circles denote O ions, gray circles denote Al ions, and black circles denote Pd ions. Note that positions B and D in a 8-MR look very much alike; however, palladium in position D is open for attack from the main channel, while position B is located deeper in the side pocket. Refer to Figure 1 for clarification on the position of various Mortier sites within mordenite structure. The lengths of the strongest Pd-O* bond are indicated.

Figure 7. Al-Al arrangements that exceptionally stabilize the Pd cation in position E. Unfortunately, both arrangements break the fivemembered ring avoidance rule and are unlikely to be found in the zeolite structure. The configuration on the right also breaks Loewenstein’s rule because of a rather flat supercell.

T1T3a, see Figure 5) and 1.74 eV (Pd2+ in position C). Note that for Pd in the main channel (Mortier position E) the maximum penalty is ≈0.7 eV. Because it is much easier to find one framework Al in the vicinity than two (especially for dealuminated zeolites), this arrangement can be very common within the zeolite. Additionally, it may be an important stage in the process of Pd diffusion along the main channel. 5. Bonding to one O*. In some special cases (e.g., in Mortier position B), the energetic penalty may be reduced to 1.00 eV, but it can also reach more than 2 eV (for Pd2+ in position D). This geometry is not very stable, as it is usually easy for Pd cation to shift to a more favorable position. 6. No bonding to O*. This is a rather unstable situation, located more than 2 eV above the optimal energy for all possible Mortier sites. After considering the above rules for Pd stabilization, it became apparent that for Pd in position E, two other, yet unconsidered, Al-Al arrangements exist that should provide exceptional stabilization (see Figure 7). These two structures were not taken into consideration at first because they violate the 5-MR avoidance rule;47 the structure on the right additionally violates Loewenstein’s rule due to the size of the cell used. However, a test calculation showed that the first structure is

actually by 0.29 eV more stable than the “optimal” structure considered so far. Therefore, it was included in Table 1 as a negative value. Note that this calculation already includes the energetic penalty of putting two Al centers in a 5-MR ring. This geometry, however, is not expected to be significant in catalytic reactions. For one reason, the 5-MR avoidance rule limits the abundance of specific Al-Al distributions necessary for palladium adsorption. Second, increased stability of palladium makes it less reactive toward approaching reagents. The local coordination of Pd in different Mortier positions is also interesting. In energetically favorable configurations, palladium is bound to the framework by three or four strong, partially covalent Pd-O* bonds. The Pd-O* distances vary between 2.0 and 2.2 Å. This is in agreement with extended X-ray absorption fine structure measurements59 that show that Pd in mordenite coordinates to four oxygen centers with a Pd-O bond length of ∼2.03 Å. The most favorable coordination for palladium is squareplanar; only in positions C and H the palladium cation and the oxygen anions are not coplanar. In position E, Pd is only slightly shifted out of the plane of four oxygen ions. Despite a multitude of possible Pd locations in the zeolite framework, some distinct patterns emerge (refer to Figure 3 for drawings of different Mortier sites): Position A. Mortier position A corresponds to Pd placed at the deep end of the side channel. The position A, exactly in the middle of the side channel, is in most cases not favorable (1.99 eV above optimum). However, for one special Al-Al arrangement (T3T3a, see Figure 2) (i.e., two T3 sites on opposite sides of the side channel) the Pd2+ cation remains exactly in the center of the channel, and the energy is only 0.37 eV above optimum. In this arrangement, Pd is in distorted square-planar coordination with two shorter (2.20 Å) and two longer (2.34-2.41 Å) Pd-O bonds. The acute O-Pd-O angle is 67°. This arrangement is

6462 J. Phys. Chem. C, Vol. 111, No. 17, 2007 stabilized by local symmetry and therefore is unlikely to be found in a real zeolite. If aluminum is distributed differently around the A position, the Pd cation moves toward the channel wall and creates three shorter (

Structure of Active Sites in Pd-Exchanged Mordenite: A Density

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