19518
J. Phys. Chem. 1996, 100, 19518-19524
Copper Coordination in Zeolite-Supported Lean NOx Catalysts Richard J. Blint Physics and Physical Chemistry Department, Research and DeVelopment Center, General Motors Corporation, 30500 Mound Road, Box 9055, Warren, Michigan 48090-9055 ReceiVed: June 11, 1996; In Final Form: September 3, 1996X
Copper ions exchanged into the ZSM-5 zeolite are known to catalyze lean NOx reduction. The majority of these copper ions are shown here to be hydrated ions attached to acid sites in the zeolite. This conclusion is reached using computations of hydrated copper structures and available EXAFS and ESR experiments. An acid site occurs when an Al substitutes for a Si in the zeolite framework. Four oxygens are attached to this aluminum atom, each of which can exhibit some acidic character. An analysis of the rigid zeolite structure shows only a small number of the square-planar symmetry sites which a majority of the Cu(II) ions are predicted to occupy by the ESR experiments. The ESR experiments are reinforced by the EXAFS experiments which show that the Cu(II) ions have four nearest-neighbor oxygen atoms. The calculations here show a four-member, first hydration shell for Cu(II) which includes one acid oxygen from the zeolite framework. The remaining three oxygens arise from a hydroxyl ion and two water molecules. The predicted Cu-O distance for this first hydration shell is approximately 2.0 Å which is only slightly longer than the 1.96 Å measured experimentally. The water molecules in the copper hydration shells also hydrogen bond to each other and to parts of the zeolite framework. Similar calculations for the hydration of the Cu(I) ion show a first shell with two to three oxygens as nearest neighbors at a distance of 2.1 Å. This also agrees with experiment. An examination of the pore size in ZSM-5 indicates sufficient room for a first and second hydration shell for most of the possible acid sites. The capability of waters to form hydration shells in the zeolite is enhanced by the formation of hydrogen bonds with the zeolite framework. In the copper hydration shell, each water molecule has inequivalent O-H distances. This occurs to accommodate the hydrogen bonding. The conclusion that the copper ions are typically hydrated suggests that the catalytic mechanism may have much in common with homogeneous catalysis. This catalytic environment is often termed heterogenized homogeneous catalysis.
Introduction The mechanism for the selective reduction of NOx by hydrocarbons under lean exhaust conditions has been shown to occur on a catalyst which is copper supported on a zeolite.1-3 Evidence suggests that the position, coordination, and oxidation state of the copper are each determining factors in the mechanism of catalysis.4-7 This report identifies likely sites, coordination, and hydration for the copper in the zeolite using computational techniques. These zeolites have Brønsted acid sites which are created by the substitution of an aluminum for a silicon in the cage structure of the zeolite. Hydrogen or alkali ions are typically bound to an oxygen at one of these acid sites which connects the substituted aluminum to another silicon in the zeolite structure. Copper is usually introduced into the zeolite by ion exchange (often as Cu++ acetate salt). It is expected that the copper ions exchange with these protons and are associated with that exchange site. However, the copper ions will most probably not occupy exactly the same positions as the hydrogens in the zeolites. The experiments aimed at identifying the copper environment show that the copper environment depends on its chemical state. The copper in the zeolite can have three oxidation states (Cu++, Cu+, or Cu0) which may have different binding sites depending on the oxidation state of the copper. Even though the copper is initially deposited as Cu(II), the flow of exhaust gases which can be either oxidizing or reducing can cause the copper to exist in all three states. Consequently individual coppers may exist as: X
Abstract published in AdVance ACS Abstracts, November 1, 1996.
S0022-3654(96)01707-8 CCC: $12.00
(1) hydrated -Cu(II)OH molecules chemically bound to one or two zeolite exchange sites; (2) CuO molecules or clusters loosely associated with the zeolite framework which may also have a hydration shell; (3) [Cu++-O- --Cu++] complexes8-10; (4) hydrated -Cu(I) ions attached to an acid site in the zeolite framework; (5) copper atoms loosely bound to the zeolite framework; (6) clusters of copper atoms. Part of the catalysis mechanism will be the interaction of exhaust gas species with copper in one or more of these states. Experiments by Kucherov et al. identify an ESR peak which indicates that the Cu(II) ions are in square-pyramidal and squareplanar environments which are typically 4-5 coordinated.11 Planar ligand structures for Cu(II) ions are quite common especially for complexes with halogens.12 EXAFS experiments by Liu and Robota13 show that the cupric ion is surrounded by an average of 4.2 oxygen atoms in its first-neighbor shell which has a radius of 1.96 Å. This is consistent with the observation that the copper ions are in a square-planar or even squarepyramidal structure. The experiments by Liu and Robota also suggest that part of that water coordination is due to water, since the coordination number drops to 3.7 after calcination. Further XANES experiments by Liu and Robota indicate that Cu(I) ions are only coordinated by two oxygens. Given the size and complexity of the ZSM-5 structure, it is likely that part of the coordination of the copper ions in the zeolite is made up of water or hydroxyl radicals. The Cu-O separations for Cu(II) in these experiments are 1.95 Å before calcination and 1.93 Å after calcination. © 1996 American Chemical Society
Cu Coordination in Lean NOx Catalysts
J. Phys. Chem., Vol. 100, No. 50, 1996 19519
TABLE 1: Experimental Cu-O Distances in Å from Crystallographic Analysis14 (Number in Parentheses is the Number of Ligands at that Separation from the Copper) ion
species
RCu-O (Å) 2+
Cu(I)
Cu(H2O)6 Cu2O
2.09(2), 2.16(2), 2.28(2) 1.85(2)
Cu(II)
Cu(OH)64Cu(OH)2 CuO
1.97(4), 2.81(2) 1.94(4), 2.63(2) 1.96(4), 2.78(2)
In highly symmetric, crystalline environments the Cu-O distances for Cu(II) oxidation states are fairly similar at approximately 1.94-7 Å (Table 1). The first hydration shell in these cases consists of four hydroxyl groups. A similar analysis for Cu(I) shows only two water molecules in the first hydration shell. The average Cu-O distance for water molecules is longer than the average Cu-O distance for Cu(II). There are also experiments which determine the binding energies for the sequential addition of water to gas phase Cu(I) ions up through the binding of four water molecules to the ion.15-17 A complementary set of calculations show good agreement with the binding energies and predict bond distances of 2.04-2.22 Å.18-20 These computations show that two to three waters are typically closer to the Cu(I) ion. The following section discusses the computational techniques. The Results section focuses on the calculational results, and the relation of those calculations to the experiments are discussed in the Discussion section.
Figure 1. Ball and stick model of the C2 cluster. The geometry of this structure comes from the zeolite crystal structure. The bridge oxygen is flanked by the two equivalent T12 silicon positions where one is replaced by an aluminum. This cluster also has the minimized position of the copper ion. All atoms except the hydrogens are labeled.
Computational Approach To calculate the binding energies of exhaust gas species to the copper in the zeolite, fragments (clusters) of the zeolite crystal have been constructed to simulate the environment to which the typical copper would be exposed. Two different clusters were used in these calculations. The first is the same as the cluster size 2 as described by Brand et al.21 The specific geometry of the cluster is derived from the crystal structure of ZSM-5.22 The two silicon sites are the adjacent T12 sites. For this cluster one of the silicons is replaced by an aluminum to describe a Brønsted acid site. The T12 sites were selected because they have been predicted to be the most stable sites for aluminum substitution.23 This cluster will be referred to as the C2 cluster (Figure 1). This first cluster has a bridge oxygen at the center with a silicon atom on one side and an aluminum atom on the other. Each of these atoms has an OH molecule attached at each of the tetrahedral positions. Each of the hydroxyl fragments occupy their unique crystallagraphic positions, and the hydrogens are in the direction of the next silicon atom in the crystal. However, the OH distances have been shortened to 0.92 Å to keep the disruption to the electronic wave function to a minimum. This cluster is a Brønsted acid which lacks only a single electron. Consequently it can only directly provide a site for a single Cu+ ion. A second cluster was developed to investigate multicenter binding at a Brønsted acid site. This is the T12 site where each bond to the aluminum is a -O-Si-H group, where again each of the atomic postions is defined by the crystal structure. In the case of the terminating hydrogens, the bond distances have been shortened to 1.47 Å similar to those of silane to give a structure which is closer to equilibrium distances for each of the bonds. This cluster is referred to as the C3 cluster (Figure 2). This cluster has the aluminum at the center. In this geometric configuration the copper can find whichever combination of the oxygen atoms attached to the aluminum that is most energetically advantageous.
Figure 2. Ball and stick model of the C3 cluster. The geometry of this structure comes from the zeolite crystal structure. The center aluminum is substituted into a T12 position, and the geometries of each of attached atoms comes from the crystal structure. All atoms except the hydrogens are labeled.
Both of these clusters are quite small and do not include the longer range effects which occur from neighboring atoms in the zeolite structure. This limits our understanding of the effects of pore sizes and adjacent acid sites. In effect these clusters model a generic zeolite as opposed to an actual ZSM-5 zeolite. The program used in these molecular quantum mechanics calculations is Gaussian 92/G94.24 These calculations are primarily unrestricted Hartree-Fock calculations. Two basis sets are used in these calculations. The first basis set is smaller and makes the computations demand less computational resources. The second treats all the atoms equivalently with high accuracy and is used to confirm the results determined from the calculations using the first basis set. The first (B1) is a combination basis where the bridge oxygen has the D95v**
19520 J. Phys. Chem., Vol. 100, No. 50, 1996
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TABLE 2: Calculated Cu-O Distances in Å for the Binding of Single Oxygen Containing Species to Copper in the Zeolite Cluster Using the B1 Basis species
Cu-OB
Cu-OAl
(B2) O H2O H2O (B2) OH O2
2.14 2.10 2.02 2.39 2.31 2.03 2.17
2.06 2.12 2.04 1.98 2.04 2.07 2.04
TABLE 3: Mulliken Population Analysis for Copper Bound to C2 Clusters Using the B1 Basis species
X
Cu
OB
Al
Si
OAl
0.60 0.61 0.98 0.98 0.50 0.54 0.39 0.55 0.46 1.02
-0.65 -0.98 -1.06 -1.41 -1.42 -1.41 -1.41 -1.42 -1.42 -1.41 -1.41 -1.41 -1.43
1.76 1.97 1.76 1.98 2.20 1.99 1.98 1.97 1.96 2.01 2.18 2.22 2.02
2.36 2.65 2.37 2.38 2.68 2.40 2.40 2.38 2.41 2.40 2.67 2.65 2.37
-0.95 -1.01 -0.97 -0.92 -1.04 -0.94 -0.94 -0.92 -0.91 -0.92 -1.02 -1.03 -0.97
Cu-Ospecies
1.81 1.97 2.09 1.75 2.18
basis of Dunning and Hay.25 The copper, silicon, and aluminum are calculated using the effective core potential of Wadt and Hay and use the accompanying basis set.26,27 The remaining oxygen and hydrogens use the 3-21g basis.28-30 To further evaluate the effects of basis, the 6-31g** basis31-33 (B2) is applied to each of the oxygens and hydrogens in the system. The effective core potentials are again used for the copper, silicon, and aluminum atoms. It should be noted that the calculations on the copper hydration shells discussed below have been optimized. However, since these calculations have many degrees of freedom, it is possible that some of the minimized structures may describe local minima. Reasonable efforts have been made to identify and report lowest energy structures. The binding energies for Cu(H2O)+ have been calculated with the effective core potential for copper, and both the D95v** and 3-21g bases are used to describe the water. These calculations give a binding energy of 33 kcal/mol for the water described by the D95v** basis and 42 kcal/mol for water described by the 3-21g basis. Bauschlicher et al. calculate a value of 34 kcal/mol for a more extensive basis than the D95v** 20 which compares quite well with the experimental value of 35 kcal/mol.16 As might be expected, the smaller basis gives a greater error in the binding energy; however, it is encouraging that the comparison with experiment is good with the more extensive basis. The binding energy of Cu(I) to the C2 cluster has also been calculated using both the B1 and B2 bases. The B1 basis gives a binding energy (C2 cluster plus copper atom) of about 125 kcal/mol, and the B2 basis gives a value of about 121 kcal/mol. The copper-oxygen distances for the B2 basis are both very similar at 2.1 Å compared with 2.14 and 2.06 Å for B1 (see Table 2). The population analysis (see Results below) has only 0.03 electron more on the copper for B2 than B1. Again this comparison suggests that basis set effects are small. Also, a comparison of the binding energy of water to the C(I)-C2 cluster has been calculated using both the B1 and B2 bases. The binding of water to the C(I)-C2 cluster cluster using the B1 basis gives a binding energy of 28 kcal/mol (see Table 5). The comparable binding energy for the B2 basis is 30 kcal/mol. The optimized geometries for the B1 and B2 basis sets are also compared for both the Cu(H2O)5 and the CuOH(H2O)3 clusters. For Cu(H2O)5 the general structures of the clusters calculated with B1 and B2 are similar (Table 4). The MPA on the copper is also consistent with a value of 0.46. Also, for the CuOH(H2O)3 clusters the general structures calculated with the two basis sets are similar and are compared (Table 6). The differences between these calculations are differences in absolute values, not differences in concept. Simulations were done using Cerius.34 Results The calculated Cu-O distances (Table 2) show values in the range between 1.75 and 2.38 Å. This compares with the
(B2) H Cu Cu(B2) Cu-OH Cu-O Cu-O2 Cu-H2O Cu-(H2O)5 Cu-H2O (B2) Cu-(H2O)5 (B2) Cu-OH(H2O)4
0.47H -0.47O -0.49O -0.01O -0.70O -0.72O
experimental range of 1.85-2.09 Å (Table 1) for the first hydration shell distances. Since these calculations are for isolated clusters, there should not be a one-to-one comparison with the crystallographic values. However, the comparison with these distances shows values in a similar range and a similar ordering of the bond distances for [OH, O] < H2O. Inherent in the experimental analysis of these zeolite systems is the identification of the oxidation state of the copper. The oxidation state is a measure of the partial charge which resides on the copper. There are various measures of this partial charge. The simplest is the Mulliken population analysis (MPA) which measures the fraction of the charge which resides on the various atoms in the structure (Table 3). This analysis is sufficient to differentiate the charge states of the copper. Since there is significant covalent bonding in these systems, the values for the charge states will be much less than one or two electrons; however, the results do separate the oxidation states in an acceptable fashion. From the MPA one can readily assign a value of 0.5-0.6 to Cu(I) and a value of 0.98 or greater to Cu(II). It is also clear that the addition of copper to the cluster changes the partial charges on the zeolite portion of the cluster; however, it is also clear that the addition of species to the copper which even changes the oxidation state of the copper does not significantly change the partial charges on the zeolite cluster. The change in oxidation state for the copper atom is determined by what is bonded to it, not the zeolitic framework. It is also interesting that the hydrogen bond to the bridge oxygen does very little to partial charges of the zeolite beyond the bridge oxygen, which is quite different from the effect of the copper on the zeolite. Hydrogen Binding to the C2 Cluster. In the C2 cluster the hydrogen atom is bonded primarily to the bridge oxygen (OB). In a large part this localization of the bond is due to the very short OB-H bond distance which is 0.965 Å. This distance is consistent with the normal OH bond distance. In the rigid zeolite structure of ZSM-5, the distance of the nearest oxygen to the hydrogen and also attached to the aluminum is 2.5 Å. The hydrogen does tilt slightly toward the aluminum; however, it is bonded primarily to the bridge oxygen. The binding energy for this hydrogen atom is 107.4 kcal/mol. Brand et al. calculate proton affinities for similar size clusters which have been held rigid as is true here and also where the atoms in the zeolite have been allowed to relax from the crytallographic values.21 For a similar rigid system they predict a proton affinity of 320.4 kcal/mol. The value calculated here for the C2 cluster is 331.3 kcal/mol. Given the basis set differences, this is acceptable agreement. The MPA analysis shows that the primary charge transfer in this system is between the acid hydrogen and the bridge oxygen. The bridge oxygen gains approximately 0.47 of an electron from the hydrogen atom.
Cu Coordination in Lean NOx Catalysts Copper Binding to the C2 Cluster. In the C2 cluster the copper could be coordinated with a combination of two oxygen atoms. Optimizing the position of the copper within the cluster leads to a structure where the copper is shared between the bridge oxygen (OB) and the other oxygen bonded to the aluminum atom (OAl). The binding energy is 124.6 kcal/mol. The copper is positioned 2.14 Å from the bridge oxygen and 2.06 Å from OAl (Table 2). This indicates that the optimum binding for this cluster is two centered where the aluminum binds almost equally with the copper through both of its oxygen atoms. A calculation was done where the copper was constrained to remain equidistant between the OAl and the OSi. The binding energy drops to 96.3 kcal/mol, and the Cu-OB distance decreases to 1.96 Å. The benefit of sharing the binding with two of the oxygens attached to the same aluminum is approximately 28.3 kcal/mol. For this bidentate bond, the MPA on the copper is approximately 0.60, which corresponds to Cu(I) as discussed above. If the copper is forced to remain equidistant between the two oxygens (OAl and OSi, effectively a one-center bond) the MPA is calculated to be 0.80. The MPA on OB does not change in these two calculations. The major charge exchange in this cluster is between the copper and the bridge oxygen. The charge exchange between them is almost 0.8 electrons. The primary difference in the charge exchange between the bidentate bond and the one-center bond occurs in the MPA on the aluminum. The aluminum shows a charge increase of approximately 0.2 for the bidentate bond which accounts for the decrease in charge on the copper for the bidentate bond. Apparently the bidentate bond is most effective in transferring charge from the aluminum to the copper. Copper Binding in the C3 Cluster. The C3 cluster allows the copper to coordinate with up to three oxygens all attached to the same aluminum atom. The optimized position for the copper in this cluster turns out to be approximately equidistant between two of the oxygen atoms. The distances are 2.1 and 2.16 Å. These distance are not exactly the same because the cluster is not exactly symmetric because the atomic positions were derived from the crystal structure as discussed above. This confirms the result from the C2 cluster that Cu(I) prefers a bidentate bond. Hydration in the Cu(I)-ZSM-5 Cluster. Hydration of Cu(I) bound to the zeolite support is investigated by sequential addition of water molecules around the copper ion. The positions of all water molecules are energetically reoptimized with the addition of each water molecule so that each of the waters finds a place in the hydration shell where it is energetically stable. This involves balancing the energy from the primary electrostatic bond to the copper and additional hydrogen bonds with neighboring water molecules and the zeolite framework with the repulsions among the water molecules which are crowding around the copper atom. This involves a great number of degrees of freedom in the optimization. Consequently the optimized structures are minima but are not rigorously proven to be the lowest minima. When the first H2O is bound to Cu(I) in the C2 cluster, the optimized positions of the copper and H2O are constrained by the zeolite structure. As has been noted previously, a lone copper finds a position which is approximately equidistant to two of the oxygen atoms which are bound to the aluminum. The addition of the first water molecule to copper causes the copper to shift away from the zeolite with Cu-OB and CuOAl distances of 2.39 and 1.98, respectively (Table 4). The water molecule bridges the gap between the copper and an oxygen (OSi) bonded to the silicon in the other T12 position.
J. Phys. Chem., Vol. 100, No. 50, 1996 19521 TABLE 4: Calculated Cu-O Distances in Å for the Binding of Water Molecules to the Cu(I)-ZSM-5 Cluster species Cu-OB Cu-OAl
Cu-Ospecies
first shell
2.14 2.39 2.56 2.65 2.58 2.88 2.71
1.97 2.07, 2.08 2.01, 2.13, 3.33 2.01, 2.07, 3.47, 3.71 2.12, 2.22, 2.30, 2.96, 3.55 2.12, 2.13, 2.98, 2.98, 3.21, 3.87
1.975(2) 2.09(3) 2.08(3) 2.07(3) 2.09(2) 2.12(3)
H 2O 2H2O 3H2O 4H2O 5H2O 6H2O
2.06 1.98 2.13 2.08 2.13 2.06 2.03
TABLE 5: Calculated Energy To Add Each Successive Water Molecule to the Cu(I) Hydration Shell in the ZSM-5 Cluster (Energies in kcal/mol) number of water ligands
energy to bind next H2O
MPA of copper
1 2 3 4 5 6
28.2 23.6 17.8 11.0 15.8 15.4
0.54 0.46 0.46 0.46 0.38 0.42
Consequently one of the hydrogens on the water is less than 2 Å from the OSi. The sequential addition of water molecules to the hydration shell of the copper then causes a localization into only one bond between the copper and the zeolite. The CuOB distance increases substantially with the addition of each water molecule. Consequently in these calculations the copperzeolite bond is localized on the OAl in the cluster. As noted previously the bidentate binding was worth about 28 kcal/mol. Examining the incremental binding energy from the sequential addition of each water to the copper hydration shell (Table 5) shows that the energy gained from the hydration of just the first water molecule exceeds 28 kcal/mol. Consequently hydration is energetically preferred over bidentate binding. This results in the localization of the copper-zeolite bond into a single bond to the zeolite framework. The nonmonotonic behavior of the binding energies for the successive addition of water molecules most likely occurs because the hydration shell is artifically constructed by sequential addition of water molecules. The number and strength of the hydrogen bonds formed are dependent on the structure of the hydration shell. The sequential formation of the hydration shell can create situations where the next water added can form more and stronger hydrogen bonds than the previous water molecule could. Nevertheless there is a decreasing trend in the binding energies because each successive water molecule gets less of a share in the binding to the copper ion (Table 5). The Cu-O distances for the clusters are shown along with a prediction of the first-shell oxygen distances (Table 4). The Cu-O distances separate into groups which are interpreted as the hydration shells of the copper (see Discussion below). The Cu-O distances can be resolved into a primary shell and the beginnings of second and third hydration shells. There are insufficient water molecules to form a clear second or third hydration shell. An identification of the primary hydration shell is shown in Table 4. The charge (MPA) on the copper should be 0.6 or below since it is the Cu(I) state. The additional water molecules in the hydration shell tend reduce it as more electronic charge is made available by the water molecules. Indeed the MPA of the copper drops as additional water molecules are added (Table 5). Hydration of OH-Cu(II)-ZSM-5 Cluster. To put the copper in the +2 state, an OH ion is bound to it. This gives the copper ion a MPA of approximately 1.0 (Table 3). The
19522 J. Phys. Chem., Vol. 100, No. 50, 1996
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TABLE 6: Calculated Cu-O Distances in Å for the Hydration of OH-Cu(II)-ZSM-5 species OH OH + H2O OH + 2H2O OH + 3H2O OH + 4H2O OH + 5H2O
Cu-OB Cu-OAl 2.03 2.06 2.64 2.58 3.25 3.27
2.07 2.24 2.01 2.08 2.02 2.04
Cu-Ospecies 1.75 1.78OH 2.03H2O 1.84OH (2.00, 2.04)H2O 1.89OH (2.02, 2.05, 2.20)H2O 1.89OH (2.07, 2.11, 2.27, 2.31)H2O 1.91OH (2.01, 2.04, 2.29, 2.41, 3.56)H2O
first shell 1.95(3) 1.96(3) 2.01(4) 2.01(4) 2.03(4) 2.00(4)
TABLE 8: Square-Planar Sites Identified in Undeformed ZSM-5 (Atom Designations are Silicon 1-12 and Oxygens 13-38) site
O shell distance Å
oxygens in first shell
1 2 3 4 5
2.2 2.2+ 2.4+ 2.5+ 2.6+
O22, O22, O23, O23 O18, O23, O31, O32 O16, O26, O28, O29 O20, O23, O24, O26 O15, O16, O25, O26
Discussion
TABLE 7: Calculated Energy of Addition for Each Successive Water Molecule to the Cu-OH Species in the Zeolite Cluster (Energies in kcal/mol) number of water ligands
energy to bind next H2O
MPA
1 2 3 4 5
21.3 30.7 20.5 2.9 18.7
0.99 1.01 0.99 1.02 1.01
additional charge lost from the copper goes almost completely to the OH group. The previously observed bidentate character of the Cu-ZSM-5 bond is not lost with the addition of the OH ion. The binding energy for the OH to Cu-ZSM-5 is 57.6 kcal/ mol. Adding the first water molecule to hydrate the OH-Cu(II)ZSM-5 causes very little change in the Cu-OH bond (Table 6). The bound water molecules has a Cu-OH2O distance of 2.03 Å similar to the first hydration bond distance for the hydrated Cu(I)-ZSM-5 (Table 4). Similar to the hydration of Cu(I)-ZSM-5, a hydrogen bond is formed with the water and another of the oxygen atoms in the zeolite. The sequential addition of water molecules to OH-Cu(II)ZSM-5 also causes a localization into only one bond between the Cu(II) and the ZSM-5 cluster. The localization of the Cu(II)-ZSM-5 bond occurs with the addition of about two water molecules. Further addition of water molecules causes the CuOB to lengthen and the Cu-OAl to stabilize at about 2.0 Å. Analysis of all the Cu-O distances shows that these distances can be resolved into shells. The first shell includes the oxygen in the zeolite, the oxygen in the OH, and some of the oxygens in the hydration layer. By the addition of the third water of hydration there is a distinct separation into a first shell which has four oxygens and then a beginning of a second hydration shell. The distance of this shell from the copper is slightly shorter than that calculated for Cu(I). This would be expected since the copper is more highly charged. As water molecules are added to the hydration shell, the CuOH distance (to the hydroxyl radical) increases (Table 6). The bond distance increases from 1.75 to over 1.9 Å as the number of waters in the hydration shell increases. This is occuring because the water molecules also bind to the copper and compete for the available charge on the copper. Again the variation in the binding energies are attributed to structural adjustments to gain additional hydrogen bonding. The binding energies do tend to decrease as the additional hydration molecules compete for the available charge on the copper ion. The hydration energies are again sufficient to make up for the loss of the bidentate binding of the copper to the zeolite framework (Table 7). In this case there is little gain in charge for the Cu(II) from the added waters of hydration. The MPA for the copper remains at approximately 1.0. This indicates that charge from the hydration shell is not localizing on the copper.
The experimental evidence suggests that Cu(II) ions are typically 3-5 coordinated and reside in a square-planar or square-pyramidal environment. The evidence further suggests that Cu(I) ions are only 2-3 coordinated and does not necessarily suggest a symmetry for the environment. Consequently two issues need to be addressed. One is the number and identity of nearest neighbors. The second is the symmetry of the nearest neighbors. Kucherov suggests that the majority of the Cu(II) ions are in square-pyramidal or square planar environments. The rigidity of the ZSM-5 structure limits the number of square-planar sites which can arise solely from the zeolite. That is, the zeolite is not expected to deform significantly to provide optimum coordination sites for Cu(II). Consequently either one must find a signficant number of highly symmetric sites or the copper will occupy lower symmetry sites, and water or hydroxide molecules will fill out the coordination structure for the copper ions. Examining the structure of ZSM-5 shows a small number of sites which can approximate square-planar sites (Table 8). The binding of CO and NO to some of these sites has been investigated theoretically.35,36 The unit cell of ZSM-5 has 288 atoms with 12 inequivalent T (silicon) sites and 26 inequivalent oxygen sites. The five symmetric sites found are a small number of the possible acid sites. These first-shell distances are significantly longer than the EXAFS measurements and the crystalline Cu-O distances in more common copper oxygen systems (Table 1). Given the high densities of copper exchanged into these zeolites, it is probable that the majority of the copper ions occupy lower symmetry zeolite sites and water molecules or hydroxyl ions make up the remainder of the coordination shell. It is simpler to construct a square-planar site where only one or two of the oxygen atoms come from the zeolite crystal and the remainder comes from mobile species like water or OH. Cu(II) Coordination. As noted previously, to produce a Cu(II) oxidation state at the Brønsted acid site it is necessary to bond a hydroxide ion to the copper. This provides one charge from the Brønsted acid site and one from the hydroxide radical. The first hydration shell is the one which defines the symmetry and so is the source of both the ESR and the EXAFS experimental measurements. To understand the hydration structure, water molecules are added one by one. The bidentate bonding structure with the zeolite cluster is lost when the first water is added to the hydration shell. This occurs because the bond to the first hydration molecule has more energy than is obtained from forming the bidentate bond. Hydration shell structures are determined by both bonding to the central ion and hydrogen bonding between the water molecules. The CuOB distance is found to increase as water molecules are added to the hydration shell because the copper ion rotates away from the second zeolitic oxygen to accommodate the most stable structure for the hydration shell. The first hydration shell is the group of shortest Cu-O distances. With the addition of three water molecules to the
Cu Coordination in Lean NOx Catalysts OH-Cu(II)-ZSM-5 cluster, the structure of the hydration shell is a single shell which contains five Cu-O bonds (Table 6). The standard deviation of the closest four bond distances is 0.08 Å. The addition of the fourth water molecule causes a definitive separation into more than one hydration shell. The first hydration shell consists of four Cu-O bonds coming from the zeolite, the hydroxide, and two water molecules. The separation into two hydration shells is characterized by the formation of hydrogen bonds between water molecules within the hydration shell. A property of the binding in this primary solvation shell is the formation of hydrogen bonds with the zeolite cluster. In addition to forming hydrogen bonds to the primary hydration shell, the second-shell water molecules also form hydrogen bonds with the zeolite framework. The oxygen distances in the primary hydration shell average out to a separation of 2.03 Å with a standard deviation of 0.09 Å. This structure with four hydration water molecules is a classic Cu(II) square-bipyramidal hydration structure. Consequently the results of the experiments can be interpreted as Cu(II) hydrated with one bond to a single oxygen in the zeolite framework. The loss of the bidentate bonding structure is advantageous because the it reduces steric hindrances on the hydration shell. If there are no steric hindrances by the remainder of the zeolite cage structure, then the copper primary hydration shell can form the well-known square-bypyramidal structure subject to limitations caused by the pore size of the zeolite. Cu(I) Coordination. The experiments for Cu(I) are not as extensive. However the experiments do describe a primary hydration shell with only two to three oxygen bonds. The calculations give a consistent interpretation with these experiments. As was noted previously, the hydration of a Cu(I) ion attached to the zeolite framework also starts out with bidentate binding to the zeolite. However, the addition of the first water molecule destroys the bidentate character of the binding. Immediately there are two Cu-O bonds appreciably closer (2.00 Å) to the copper. The second Cu-O bond to the zeolite is appreciably longer which indicates that the copper binding to the zeolite no longer has bidentate character. As each subsequent water is added, a definitive separation into hydration spheres occurs. When there are three water molecules hydrating the Cu(I) ion, there are clearly three Cu-O bonds in the primary hydration shell which occurs at about 2.1 Å. The remaining Cu-O bonds are at a distance of 2.2 Å or greater. Increasing to four water molecules gives a similar result with the closest second-shell water molecule moving out to 2.3 Å. This result characterizes hydration by four and five water molecules. The binding of these water molecules is enhanced by the formation of hydrogen bonds. In the primary shell hydrogen bonds are formed to the zeolite cluster. In the secondary shell the calculations show hydrogen bonding also to water molecules in the primary hydration shell. Summary and Conclusions Copper ions appear to be active species for lean NOx catalysis. These copper ions are supported on a crystalline zeolite support (typically ZSM-5). The zeolites have aluminum substituted into the crystalline zeolite framework. These aluminum atoms form localized Brønsted acid sites in the zeolite. The copper ions are associated with these Brønsted acid sites. Since the oxidiation number of the copper varies during the catalytic process, it is necessary to understand the binding of both Cu(II) and Cu(I) at these Brønsted acid sites. It should also be noted that the clusters employed in this study are of quite short range; consequently it is not possible to predict whether CuOH+ exists as an entity or complexed with another
J. Phys. Chem., Vol. 100, No. 50, 1996 19523 exchange site or bonded to another copper ion. However, charge exchange through the lattice structure seems to occur readily, suggesting that a bond with an adjacent exchange site should be especially stable. ESR experiments suggest that Cu(II) ions typically reside in a high-symmetry environment. However, the majority of the acid sites in the zeolite are low-symmetry sites. These highsymmetry environments have the Cu(II) ions associated with 3.6-4.5 oxygen atoms and in square-planar symmetry. These calculations show that a hydration shell can provide the highsymmetry environment experimentally observed. The hydration shell thus removes the need for square-planar symmetry sites to exist in the zeolite crystalline structure. Calculations on the first hydration shell show the calculated number of oxygens in the first shell to be four including one oxygen from the zeolite framework and three from a hydroxyl ion and two hydration water molecules. The average Cu-O separation of 2.0 Å for these four oxygen atoms compares well with the measured values of 1.93-1.95 Å from EXAFS experiments. Again this corresponds well to the experimental measurements. An additional observation from the calculations is that there is hydrogen bond formation to the zeolite framework and with the hydration shell. The primary conclusion from these calculations is that the Cu(II) ions can reside at any low-symmetry acid site and will be hydrated. ESR experiments cannot detect Cu(I). The EXAFS experiments indicate that the copper has 2.1 nearest-neighbor oxygens at a separation of 1.94 Å. These calculations indicate a similar hydrated structure as was seen with the Cu(II) ions. The calculations find two to three nearest neighbors for the Cu(I) of which one is the oxygen in the zeolite cage and the remainder comes from a waters of hydration. Again this is consistent with the EXAFS experiments. The calculations also show the existence of a second hydration shell which is also consistent with the experiments. The structure of this shell is also strongly influenced by the formation of hydrogen bonds with the zeolite framework. The results of these calculations suggest that the catalytic mechanism is different from the typical heterogeneous catalysis mechanism. These copper ions exist in an aqueous hydration shell which is more typically found in homogeneous catalysis. However, the ions are bound to the acid site in the zeolite. This type of catalytic system has been observed previously in organic systems and is termed heterogenized homogeneous catalysis in contrast to heterogeneous catalysis occurring in three-way automotive catalysts. Catalysis Implications. These calculations suggest that these copper/zeolite-supported catalysts may behave much like homogeneous catalysts during reduction of NOx under lean exhaust conditions. The two characteristics of homogeneous catalysis are: (1) A fully hydrated copper ion is the active site. (2) Any exhaust gas species must be able to displace a water molecule in the hydration shell to gain acess to the active site. This type of catalytic structure has been previously observed in other systems. The advantage of this type of catalysis is the ability to phase separate homogeneous catalysis agents. This particular type of catalysis has been termed heterogenized, homogeneous catalysis.37,38 Another implication of this work is that the extent of hydration of the metal-zeolite catalyst may impact the effectiveness of the catalyst. NMR experiments on zeolites show that the chemical environment surrounding the aluminum changes dramatically with the level of hydration. Further, flows of water vapor containing air or exhaust gases at higher
19524 J. Phys. Chem., Vol. 100, No. 50, 1996 temperatures do not rehydrate the zeolite. Since it is common to pretreat the catalysts by calcining at high temperatures (e.g., 500 °C), it is possible that many of the catalysts are operating in a water deficient environment. It is then possible that a water scarce environment would make it easier for nitrogen-containing exhaust gases to gain access to the copper ion, possibly providing an enhancement to the catalytic activity. References and Notes (1) Iwamoto, M.; Furukawa, H.; Mine, Y.; Uemura, F.; Mikurija, S.; Kagawa, S. J. Chem. Soc., Chem. Commun. 1986, 1272. (2) Li, Y.; Hall, W. K. J. Phys. Chem. 1990, 94, 6145. (3) Held, W.; Konig, A.; Richter, T.; Puppe, L. Catalytic NOx-reduction in net oxidizing exhaust. Techical Report 900496; Society of Automotive Engineers: Warrendale, PA, February 1990. (4) Burch, R.; Millington, P. J. Appl. Catal. B 1993, 2, 101. (5) Lei, G. D.; Adelman, B. J.; Sa´rka´ny, J.; Sachtler, W. M. H. Appl. Catal. B 1995, 5, 245. (6) Kharas, H. C. C. Appl. Catal. B 1993, 2, 207. (7) Ansell, G. P.; Diwell, A. F.; Golunski, S. E.; Hayes, J. W.; Rajarum, R. R.; Truex, T. J.; Walker, A. P. Appl. Catal. B 1993, 2, 81. (8) Chao, C. C.; Lunsford, J. H. J. Chem. Phys. 1972, 57, 2890. (9) Jacobs, P. A; Beyer, H. K. J. Phys. Chem. 1979, 83, 1174. (10) Iwamoto, M.; Nakamura, M.; Nagano, H.; Kagawa, S.; Selyama, T. J. Phys. Chem. 1982, 86, 153. (11) Kucherov, A. V.; Kucherova, T. N.; Slinkin, A. A. Catal. Lett. 1991, 10, 289. (12) Cotton, F. A.; Wilkinson, G. AdVanced inorganic chemistry, 3rd ed.; Wiley Interscience Publishers: New York, 1972. (13) Liu, D.; Robota, H. J. X-ray absorption spectroscopic study of Cu in Cu-ZSM-5 during NO catalytic decomposition; In ACS Symposium Series 587; American Chemical Society, Washington, DC, 1995; pp 147-165. (14) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon Press: Oxford, 1984. (15) Holland, P. M.; Castleman, A. W., Jr. J. Chem. Phys. 1982, 76, 4195. (16) Magnera, T. F.; David, D. E.; Michl, J. J. Am. Chem..Soc. 1989, 111, 4100. (17) Dalleska, N. F.; Honma, K.; Sunderlin, L. S.; Armentrout, P. B.. J. Am. Chem. Soc. 1994, 116, 3519.
Blint (18) Bauschlicher, C. W. J. Chem. Phys. 1986, 84, 260. (19) Rosi, M.; Bauschlicher, C. W. J. Chem. Phys. 1990, 92, 1876. (20) Bauschlicher, C. W., Jr. Langhoff, S. R.; Partridge, H. J. Chem. Phys. 1991, 94, 2068. (21) Brand, H. V.; Curtiss, L. A.; Iton, L. E. J. Phys. Chem. 1992, 96, 7725. (22) van Koningsveld, H.; van Bekkum, H.; Jansen, J. C. Acta Crystallogr. 1987, B34, 127. (23) Alvarado-Swaisgood, A. E.; Barr, M. K.; Hay, P. J.; Redondo, A. J. Phys. Chem. 1991, 95, 10031. (24) Frisch, M. J.; Trucks, G. W.; Head-Gordon, M.; Gill, P. M. W.; Wong, M. W.; Foresman, J. B.; Johnson, B. G.; Schlegel, H. B.; Robb, M. A.; Replogle, E. S.; Gomperts, R.; Andres, J. L.; Raghavachari, K.; Binkley, J. S.; Gonzalez, C.; Martin, R. L.; Fox, D. J.; Defrees, D. J.; Stewart, J. J. P.; Pople, J. A. Gaussian 92, ReVision A; Gaussian, Inc.: Pittsburgh, PA, 1992. (25) Dunning, T. H.; Hay, P. J. Gaussian basis sets for molecular calculations. In Modern Theoretical Chemistry; Schaeffer, H. R., Ed.; Plenum: New York, 1977; Vol. 3, pp 1-28. (26) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (27) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (28) Binkley, J. S.; Pople, J. A.; Hehre, W. J. J. Am. Chem. Soc. 1980, 102, 939. (29) Gordon, M. S.; Binkley, J. S.; Pople, J. A.; Pietro, W. J.; Hehre, W. J. J. Am. Chem. Soc. 1982, 104, 2797. (30) Pietro, W. J.; Francl, M. M.; Hehre, W. J.; Defrees, D. J.; Binkley, J. S. J. Am. Chem. Soc. 1982, 104, 5039. (31) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (32) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (33) Gordon, M. S. Chem. Phys. Lett. 1980, 76, 163. (34) The results published were generated using the program Cerius2. This program was developed by Molecular Simulations Incorporated. (35) Schneider, W. F.; Hass, K. C.; Ramprasad, R.; Adams, J. B. J. Phys. Chem. 1996, 100, 6032. (36) Schneider, W. F.; Hass, K. C. J. Phys. Chem., to be published. (37) Oyama, S. T.; Somorjai, G. A. J. Chem. Educ. 1988, 65, 765. (38) Moulijn, J. A.; van Leeuwen, P.; Santen, R. A. V. Catalysis, an integrated approach to homogeneous, heterogeneous and industrial catalysis; In Studies in Surface Science and Catalysis 79; Elsevier Science Publishers: Amsterdam, The Netherlands, 1993; p 441.
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