DFT Calculations on N2O Decomposition by Binuclear Fe Complexes

N2O decomposition catalyzed by oxidized Fe clusters localized in the micropores of Fe/ZSM-5 has been studied using the DFT approach and a binuclear cl...
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J. Phys. Chem. B 2001, 105, 12297-12302

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DFT Calculations on N2O Decomposition by Binuclear Fe Complexes in Fe/ZSM-5 Alexei L. Yakovlev,*,† Georgy M. Zhidomirov,†,‡,§ and Rutger A. van Santen†,| Schuit Institute of Catalysis, EindhoVen UniVersity of Technology, P.O. Box 513, 5600 MB EindhoVen, The Netherlands, and BoreskoV Institute of Catalysis, pr. LaVrentieVa 5, 630090 NoVosibirsk, Russia ReceiVed: March 6, 2001; In Final Form: June 27, 2001

N2O decomposition catalyzed by oxidized Fe clusters localized in the micropores of Fe/ZSM-5 has been studied using the DFT approach and a binuclear cluster model of the active site. Three different reaction routes were found, depending on temperature and water pressure. The results show that below 200 °C the binuclear cluster is hydroxylated and is probably inactive. Above this temperature and up to 500 °C the catalytic site has the [HO-Fe-O-Fe-OH]2+ structure, and above 500 °C the site is predominantly [FeO-Fe]2+. The reaction paths on the latter two forms of the site are similar. N2O dissociates on each of the Fe ions with subsequent oxygen recombination and desorption. Some of the side reactions including NO formation are also considered.

Introduction Among the oxide catalysts for nitrous oxide decomposition the Fe/MFI type catalysts have drawn much attention. This catalyst is active in the presence of oxygen, NO, and water.1,2 Interest in the system has also been stimulated by the observation of water-induced kinetic oscillations.3 Earlier, considerable attention to these catalysts has been related to their activity in selective hydrocarbon oxidation with N2O.4 These findings actualize the need to establish the molecular structure of the iron-containing active sites catalyzing the reaction. A number of studies have been devoted to this problem, which is complex because of the inhomogeneity of the iron oxide-hydroxide complex distribution present in the zeolite. The process of oxide-hydroxide clusters formation is very sensitive to the type of the zeolite, Si/Al ratio, and details of the synthesis and activation.3,5-9 Recently, Chen et al.10 attempted a classification of the possible forms of iron in different Fe/MFI catalysts. They found four types of iron structures in zeolites: (a) iron oxide particles on the external surface, (b) charged iron oxide nanoclusters, (c) isolated iron ions, and (d) oxygen-bridged binuclear iron clusters. For extra-lattice oxygen-containing both binuclear and nanoclusters, their formation is associated with a high Si/Al ratio of the parent zeolite and, therefore, with a low probability of finding Al ions in local zeolitic structures such as rings necessary for stabilization of single multivalent cations. Although a possible preference in Al distribution in zeolite framework may exist,11 the overall picture is close to a random Al distribution. On the other hand, even in the case of a random Al distribution in the framework (obeying the Lo¨wenstein rule) the probability of finding two Al ions in neighboring rings, which makes it possible to stabilize a binuclear oxygen-bridged structure, is close to unity for a Si/Al ratio ranging between 15 and 20.12,13 Oxygen-bridged binuclear iron structures have been proposed * Corresponding author. E-mail: [email protected]. † Eindhoven University of Technology. ‡ Boreskov Institute of Catalysis. § E-mail: [email protected]. | E-mail: [email protected].

as active sites for SCR of NOx in over-exchanged Fe/ZSM-5 prepared by FeCl3 sublimation.12,14,15 FTIR and XRD, as well as H2- and CO-TPR studies were used to draw the conclusions about the structure of the active sites. Definite conclusions about the formation of the oxygen-bridged binuclear structures in these systems have been made in a number of studies using EPR and XAFS,10,16 IR, 27Al MAS NMR, and EXAFS,7 XAFS.17 Marturano et al.7 and Battiston et al.17 determined geometric parameters of the binuclear sites. Lazar et al.18 proposed the binuclear structure to consist of lattice and extra-lattice iron ions [Felat-O-Feextra-lat] based on their redox behavior and Mo¨ssbauer spectroscopy results. Up to now, there are very few published results of calculations on oxygen-bridged binuclear metal structures in zeolites. However, significant attention has been given to binuclear copper clusters,19-22 Rice et al.23 studied the stability of the Pd-O-Pd structure in a 6T-ring of ZSM-5. Filatov et al.24 used a binuclear iron hydroxide structure (HO)2Fe(OH)2Fe(OH)2 as an example of extra-lattice iron species in zeolites. They calculated N2O decomposition on a partially dehydroxylated cluster resulting in terminal FedO groups. Arbuznikov et al.25 found that an isomer of this structure formed the bis-peroxo [Fe-O-O-Fe] group. It was shown that this site is very reactive toward CH4 oxidation to CH3OH. Siegbahn and Crabtree26 showed the importance of isomeric transitions between peroxo-structures in binuclear iron complexes as models of the methane monooxygenase enzyme (MMO). We earlier showed the possibility of such transitions for a model of a binuclear site where one of the iron ions belongs in the zeolite lattice and the other one is in the cationic position.27 Mononuclear models of active sites of N2O decomposition have also been discussed in the literature. Yoshizawa et al.28 discussed N2O decomposition considering an FeO+ ion in a zeolite cationic position as an active site. Recently Yakovlev et al.29 studied N2O decomposition catalyzed by mono-center sites containing Fe, Co, and Rh. Most studies use a quite simplified model of the active site. A more realistic model was proposed recently in ref 30, where a binuclear ZnOZn cluster supported on two cationic centers

10.1021/jp0108566 CCC: $20.00 © 2001 American Chemical Society Published on Web 11/15/2001

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Figure 1. Cluster model Z[HO-Fe-O-Fe-OH] and its position in ZSM-5 straight channel.

of ZSM-5 was used as a model for the site catalytically active for ethane dehydrogenation. In this work we present the results of a cluster quantumchemical model study of N2O decomposition catalyzed by a binuclear iron structure supported by ZSM-5. We investigate the effect of the presence of water on the structure of the active site and its reactivity. Besides that we studied a possible route for NO formation as a side reaction of N2O decomposition. Computational Details The density functional theory (DFT) calculations were performed using the ADF v2.3 package.31 TZ+VP basis sets were used throughout (type IV basis sets in ADF). Geometry optimization was carried in the local spin density approximation (LSDA) using Slater exchange32 and Vosko-Wilk-Nussair correlation33 functionals. At the LSDA geometry the energy was computed at the GGA level using the exchange-correlation functional of Perdew and Wang.34 The choice of a cluster model is not a simple one, especially for zeolites with a complex structure such as MFI. The unique properties of MFI-supported catalysts in N2O decomposition encouraged us to look for features inherent to the MFI structure. One of such features is the presence of pairs of 5T rings sharing an edge. There are such pairs present both in straight and zigzag channels. In fact, all 5T rings that are part of the channel walls in the MFI structure are paired. Rice et al.35 showed that the 5T rings are the preferable sites for Fe exchangeable cations in Fe/ZSM-5. It is then reasonable to suggest that two cations can be stabilized on a pair of 5T rings, bridged by extra-lattice oxygen if there is not enough framework Al for charge compensation. Thus the basic cluster model Z consists of two 5T rings representing part of the wall of the straight channel of the ZSM-5 structure. The position of the cluster in the ZSM-5 structure is presented in Figure 1. This model was previously used for calculations of the ethane dehydrogenation reaction on Zn/ZSM-5.29 The nonequivalent lattice positions of silicon and oxygen atoms in the ZSM-5 structure36 are shown as TN and ON, respectively. The cluster contains the T12 position, which is believed to be the preferred location for silicon substitution by aluminum. For all aluminum models Al ions are placed in different rings (Si9 and Si12 are substituted) and are separated by two silicon-oxygen tetrahedrons. To make a choice of the spin multiplicity we calculated the Z(HOFeOFeOH) model with the numbers of unpaired electrons between 0 and 10, which correspond to multiplicities between 1 and 11. The cluster with 8 unpaired electrons has the lowest energy. Thus, all iron-containing models with an even number of electrons were calculated with 8 unpaired electrons. This corresponds to

the spin multiplicity of 9. For the models with an odd number of electrons the one with lowest energy of the models with 7 and 9 unpaired electrons was chosen. Using DFT, one introduces an error computing lower spin states because it is a combination of terms with different multiplicity. The only spin state that is computed accurately is the one with highest spin possible at a given electronic configuration. In the case of Fe2+ it is S ) 2, for Fe3+ it is S ) 5/2. The number of unpaired electrons of 8 in all cluster models was chosen because it gives a maximum spin for a binuclear cluster with two Fe2+ ions in high spinstate (model I). We keep this number of unpaired electrons throughout all our calculations with an even number of electrons to avoid spin transitions along the reaction path. The geometry optimization procedure used is as follows. First, for the ZH2 model the Si-H distances of the terminal hydrogen atoms were optimized. Then, the positions of these atoms were kept frozen in all subsequent calculations. This approach simulates the limited rigidity of the zeolite framework and helps to maintain the shape of the substrate cluster during geometry optimizations. One can question the applicability of this approach, since the optimization of the cluster’s core with frozen positions of the boundary atoms creates some artificial forces on them. An argument in favor of the applied approach is that the chemical processes considered here take place on the exchanged cations far from the cluster’s boundaries, thus the errors in the energies introduced by forces on the frozen atoms are canceled to a significant extent. Results and Discussion Interatomic distances in the first coordination sphere of the iron ions are presented in Table 1. The calculated reaction routes with corresponding energies are shown in Figures 2 and 3. All the calculated models can be divided into three groups based on the reaction route they are involved in. The first group includes models I to IV belonging to cycle A, and model V, which is the hydration product of I. In model I both Fe ions are in the formal oxidation state II. This model represents the initial state of the complex after the catalyst preparation after autoreduction in an inert atmosphere or in a vacuum, or after reduction of an Fe3+/ZSM-5 sample. The second group includes models VI through X belonging to cycle B. The key structure here is model VI, which is discussed in more detail below. This form of the active site is suggested as the one obtained after Fe/ZSM-5 preparation by means of FeCl3 sublimation followed by washing and calcination in oxygen.15 The third group comprises models XIV through XVII, which belong in cycle C. Structures XIV, XV, and XVI may be obtained by hydration of VI, VII, and X, respectively. Model XVII differs from XVI

N2O Decomposition by Binuclear Fe Complexes

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TABLE 1: Distances to the Nearest Neighbors in the First Coordination Sphere of the Iron Ions, in Åf Fe(1) structure Cycle A

Cycle B

NO formation cycle Cycle C

I II III IV V VI VII VIII IX X XI XII XIII XIV XV XVI

Fe-Fe

O17

O23

O32

OEL1

3.17 3.34 3.31 3.35 3.48 3.39 3.48 3.39 3.67 3.48 3.47 3.78 3.73 3.71 3.83 3.68

2.26 2.29 2.48 2.42 2.28 2.19 2.63 2.87 2.38 2.24 2.36 2.22 2.39 2.94 2.86 2.64

2.05 1.99 2.03 2.06 2.11 2.16 2.09 2.19 1.92 2.00 2.05 2.01 2.01 2.08 1.96 2.03

2.09 2.02 2.04 2.02 2.10 2.03 2.02 2.07 1.96 1.95 2.11 2.05 2.08 2.08 2.02 2.11

1.82 1.84 1.79 1.80 2.05 1.84 1.87 1.76 1.80 1.71 1.93 2.08 2.10 2.08 1.83 1.94

Fe(2) OEL2 1.63a 1.62a 1.74c 1.82 1.82 1.76 1.77 1.76 1.78 1.83 1.82 1.82 1.84 1.80 1.83

OEL4

1.61a 1.61a 2.40 1.90d 2.19 2.18 1.80 1.81 1.81

O30

O31

O20

OEL1

2.50 2.40 2.35b 2.36b 2.41b 2.28 2.40 2.84b 2.30b 2.38b 2.62 2.89b 2.21 2.38 2.18 2.15

2.19 2.15 2.17 2.12 2.13 2.40 2.20 2.27 2.28 2.51 2.09 2.74 2.47 2.05 2.43 2.54

1.99 1.98 2.06 1.99 1.96 2.04 2.04 2.25 2.06 2.12 1.96 2.04 1.99 1.95 1.97 1.95

1.79 1.76 1.78 1.78 1.85 1.83 1.73 1.76 2.14 1.91 1.78 2.06 2.01 1.85 2.13 1.85

OEL3

OEL5

1.61a 1.80 1.78 1.76 1.77 1.74 1.74 1.79 1.79 1.74 1.76 1.75

1.61a 1.61a 1.79c 1.67e 1.63a 1.78c

a Distance to the oxygen atom resulted from N O decomposition. b Fe -O25 distance is shown in cases when it is shorter than Fe -O30. c Adsorbed 2 (2) (2) oxygen molecule. d Fe(1)-ONNO distance. e Fe(2)-N distance. f Normally, only distances shorter than 2.5 Å are shown. Here OEL1 denotes the extra-lattice bridging oxygen atom between Fe ions; OEL2 and OEL3 are first O atoms bound only to Fe(1) or Fe(2), respectively; OEL4 and OEL5 are corresponding second O atoms.

Figure 2. Calculated routes of N2O decomposition with reaction energies in kJ/mol.

by the position of one of the protons. These 3 cycles are discussed in more detail below. Side reactions may lead to NO formation, as shown in the Figure 3, and comprise, in addition

to VI and VII, also structures XI, XII, and XIII. The resulted NO can further be oxidized to give NO2 and NO3- to give surface nitrite and nitrate, but this goes beyond the scope of

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Figure 3. Calculated route of NO formation with reaction energies in kJ/mol.

the present paper. Although some of the reactions in Figures 2 and 3 are shown as irreversible, they in fact may be reversible. From the Fe-O distances shown in Table 1 one notes that the iron ions are normally coordinated to 2 or 3 framework oxygen ions depending on their extraframework ligands. The coordination number of an iron ion in our structures does not exceed six. In the structures where Fe is six-coordinated, its local geometry is close to octahedral. In cases of fivecoordinated iron, the geometry is that of a tetragonal pyramid, which resembles an octahedron with open coordination site. Four-coordinated iron is found only in structures I to V, its local geometry is neither tetrahedral nor planar but instead is close to octahedral with two vacancies. The geometry of the Z(HOFeOFeOH) cluster model is shown on the left side of Figure 1 (model VI, Figure 2). The average coordination number (C.N.) of the Fe ions is 5 with Fe-O distances below 2.5 Å and 1 with a distance around 2.7 Å. The Fe-Fe distance in this cluster is 3.35 Å. For the Z(FeOFe) site (model I, Figure 2) the Fe-O C.N. is 3.5 with distances between 1.79 and 2.26 Å and the Fe-Fe distance is 3.17. Experimental EXAFS data indicate the Fe-O C.N. is about 4 at distances around 1.85-2.15 Å and one Fe-Fe distance about 3.05 Å with C.N. ) 1.7,17 Surprisingly, the experimental data are closer to our results obtained for Z(FeOFe) than for Z(HOFeOFeOH) although the experimental procedures used by the authors17 should lead to the formation of FeIII containing species corresponding in our case to the Z(HOFeOFeOH) structure. The difference between calculated and experimental Fe-Fe distances may be attributed to the choice of the zeolite framework cluster. For example, using a cluster model that favors the formation of two oxygen bridges one can obtain a shorter Fe-Fe distance. Another interesting point to mention is that in the interpretation of EXAFS data for Fe/ZSM-5 the peak around 3.05 Å is usually assigned to the Fe-Fe shell because a similar Fe-Fe distance is found in the MMO active site. However, one should not exclude the possibility that this peak arises due to an FeOframework distance. Equilibrium between Different Sites in the Presence of Water. Fe/ZSM-5 may be prepared using different techniques such as precipitation, conventional or solid-state ion exchange, or FeCl3 sublimation. The final step in the Fe/ZSM-5 preparation is usually calcination in O2 flow at a high temperature. This step leads to oxidation of intra-zeolite iron species to +3 state, regardless of their history. Calcination in a vacuum usually leads to +2 oxidation state by autoreduction. We calculated three structures containing two Fe3+ ions and a different number of water molecules. These are II, VI, and XIV that contain 0, 1, and 2 water molecules, respectively. The site VI may be obtained via hydration of II or via oxidation of V by N2O, both reactions being exothermic. Thus, taking into

Figure 4. Comparison of energy profiles for different N2O decomposition paths, cycles A, B, and C.

account the T∆S term in the Gibbs free energy at water pressure of 0.1 atm, which is about +75 kJ/mol at 200 °C and +130 kJ/mol at 500 °C for steps IfV and IIfVI, and close to zero for IfII and VfVI, one can conclude that in the presence of a significant amount of water and at temperatures below 500 °C and above 200 °C the equilibrium will be shifted toward formation of the structure VI. At temperatures below 200 °C and above 500 °C the equilibrium is shifted toward formation of structures XIV and II, respectively. Catalytic Cycles of N2O Decomposition. Energy profiles of cycles A, B, and C are shown in Figure 4. Let us first discuss the cycle A. Formally, we start with the site I as a reference. Both iron ions are in the +2 oxidation state. Upon N2O decomposition the site is transformed to II, where the iron ions are in the oxidation state +3. This step is highly exothermic, ∆E ) -132 kJ/mol. Based on the bond lengths, the chemical structure of the site II is best represented by the formula [Fe(2)OEL1-Fe(1))OEL2]2+/Z. Surprisingly, the Fe-OEL1 distances in the structure II are not much different from those in I (see Table 1). Judging from the electron and spin density distribution of the structure II, OEL2 has O- character and Fe(1) is essentially Fe3+. The Fe(1)-OEL2 interatomic distance (1.63 Å) is more characteristic of a double bond and thus Fe(1) and OEL2 should have Fe4+ and O2- character, respectively. The same situation is observed in structure III, where both OEL2 and OEL3 have O- character and both Fe ions are Fe3+ based on electronic structure, whereas bond distances correspond to double Fe-O bond. Transition III f IV is endothermic (+74 kJ/mol) and is probably the rate-limiting step of the whole cycle. Molecular oxygen desorption (step IV f I) is also endothermic (+58 kJ/ mol). In summary, taking into account the high-temperature range (above 500 °C), in which the cycle works, the overall energy profile is not unreasonable. The catalytic cycle B resembles cycle A with a few important differences. The first difference is that the cycle involves Fe3+ a Fe4+ transitions, not Fe2+ a Fe3+. The second difference is that, in cycle B, we found a stable intermediate IX, which can

N2O Decomposition by Binuclear Fe Complexes be considered as a chemisorbed oxygen molecule in bridging position. The O2- molecule includes OEL1 and OEL4 with an OEL1-OEL4 distance of 1.33 Å and is strongly bound to Fe(1) and weakly coordinated to Fe(2) by OEL1 (Table 1). Although we have not found other stable intermediates, we do not exclude the possibility that there may exist other(s) than those found in this study. For example, we failed to find any peroxidic or superoxo structure. This may be due to the fact that the chosen substrate cluster favors Fe-Fe distances in a range between 3.3 and 3.5 Å, which is probably too short to accommodate a peroxide or superoxide moiety. Similar to the cycle A, the decomposition of the first N2O molecule is more exothermic than that of the second one (steps VIfVII and VIIfVIII, Figure 2). However unlike cycle A, formation of the oxygen molecule (VIIIfIXfX) is almost isothermal, although it goes through high-energy intermediate IX. Oxygen desorption is only a slightly endothermic process and if we take the T∆S contribution to the ∆G of the desorption into account, the equilibrium becomes shifted toward VI even at a slightly elevated temperature. As in cycle A, formation of molecular oxygen during cycle B is probably the rate-limiting step. The electronic structure of the clusters containing adsorbed atomic oxygen indicates Fe4+-O- bonding character. The atomic spin density of the O- ion is close to 1. Siegbahn and Crabtree26 drew a similar conclusion about the character of the terminal Fe-O bond in their model of the oxidized form [(HO)2Fe-O-Fe(OH)dO]+(HCOO)-(H2O)2 of the MMO active site. Quite interesting, however, is that as in the case of the structures II and III, terminal Fe-O distances are equal to 1.61 Å, close to the value of the double Fe-O bond. In cycle C, one of the two Fe ions is effectively shielded from adsorption and the other one acts as a single-ion site. The reaction for a single-ion site was discussed in detail elsewhere.29 Although the ∆E of reactions XIV + N2O f XV + N2 and XV + N2O f XVI + N2 (Figure 2) differ from those obtained in the previous study29 for the corresponding reactions X + N2O f XO + N2 and XO + N2O f XO2 + N2 (here X is a singleion catalytic site represented by the Fe(OH)3(H2O)2 cluster), the mechanism remains essentially the same. It has been shown earlier that the activation energies for these two reaction steps are 41 and 135 kJ/mol, respectively.29 One can expect that the corresponding activation barriers in the cycle C will be of a similar magnitude. In contrast to the single-ion site, molecular oxygen desorption from the cluster XVI is slightly exothermic. Assuming that the barrier of this step is relatively small, one can conclude that this is a fast step and there should be no oxygen inhibition in this case. Indeed, oxygen inhibition for Fe/ZSM-5 catalysts is often found to be very weak, if any.2 Comparison of the reaction energy profiles in Figure 4 indicates that the cycle C has the smoothest one. However, one should keep in mind that the profiles do not include transition states and, thus, we cannot make conclusions about reaction rates judging from them. The steps involving NO formation require special consideration. In our case NO can be formed from an adsorbed O atom and an N2O molecule (reaction VIIfXI, Figure 3). This endothermic reaction results in adsorbed ONNO molecule, which dissociates into two NO molecules (structure XII, Figure 3). The high energy of ONNO formation can be explained by the fact that the N-N bond in N2O is almost as strong as in N2. It has to be significantly weakened to create an N-O bond. Indeed, in N2O the N-N bond length is 1.13 Å, whereas in the adsorbed ONNO it is 1.25 Å, almost the same length as in the

J. Phys. Chem. B, Vol. 105, No. 49, 2001 12301 gas-phase (NO)2 dimer. Although judging from the reaction energies, this route to NO formation seems to be very unlikely, the species considered here can be formed from gas-phase NO and, thus, are worthwhile to discuss. It is interesting to note that in the structure XII one of the NO molecules reacts with the bridging oxygen atom to produce adsorbed NO2 (in the Table 1, OEL1 and OEL4 belong to the NO2 molecule). The N-OEL1 and N-OEL4 bond distances in the structure XII are 1.30 and 1.23 Å, respectively (compare with ca. 1.19 Å in gas-phase NO2). In the presence of oxygen these adsorbed species may lead to formation of surface nitrates and nitrites. The reaction route VfVIfXIIIfV can provide a base for the explanation of the promoting effect of NO on the N2O decomposition over Fe/ZSM-5 as found experimentally.2 Summary The structures of the binuclear Fe/ZSM-5 active sites that catalyze N2O decomposition has been determined as a function temperature and water content. At temperatures above 500 °C the iron-containing complexes are completely dehydroxylated and below 200 °C the sites are fully hydroxylated, therefore they are not available for reaction. Between these temperatures in the presence of water the binuclear sites have predominantly the [OH-Fe-O-Fe-OH]2+ structure. A different degree of hydroxylation results in different reaction mechanisms for N2O decomposition. The low-temperature hydroxylated form may act as a single-ion site since the second Fe ion has a completely saturated first coordination sphere. N2O decomposition on a single-ion site proceeds via the Eley-Rideal mechanism. However, given the activation energies that correspond to this mechanism,29 the mono-center Fe3+ site appears to be inactive at the temperatures of existence of this site, i.e., below 200 °C. The other two forms have related mechanisms of N2O decomposition: N2O dissociates on two separate Fe ions followed by oxygen recombination and desorption. The difference between the reaction pathways is that at high temperature the reaction may involve Fe2+a Fe3+ transitions, whereas at lower temperatures it is Fe3+ a Fe4+. The reaction energy profile for the [OH-Fe-O-Fe-OH]2+ site is smoother than that for [Fe-O-Fe]2+ suggesting that at the same temperature the former one should be more active. Acknowledgment. Financial support from Hydro AGRI, DSM AGRO and VROM/NOVEM (Dutch Authority for Environmental Affairs) is gratefully acknowledged. References and Notes (1) Pophal, C.; Yogo, T.; Yamada, K.; Segawa, K. Appl. Catal. B EnViron. 1998, 16, 177. (2) Kapteijn, F.; Rodriguez-Mirasol, J.; Moulijn, J. A. Appl. Catal. B EnViron. 1996, 9, 25. (3) El-Malki, E. M.; van Santen, R. A.; Sachtler, W. M. H. Microporous Mesoporous Mater. 2000, 35-6, 235. (4) (a) Panov, G. I.; Sobolev, V. I.; Kharitonov, A. S. J. Mol. Catal. 1990, 61, 85. (b) Panov, G. I.; Kharitonov, A. S.; Sobolev, V. I. Appl. Catal. A 1993, 98, 1. (5) Joyner, R.; Stockenhuber, M. J. Phys. Chem. B 1999, 103, 5963. (6) Lobree, L. J.; Hwang, I. C.; Reimer, J. A.; Bell, A. T. J. Catal. 1999, 186, 242. (7) Marturano, P.; Drozdova, L.; Kogelbauer, A.; Prins, R. J. Catal. 2000, 192, 236. (8) Kucherov, A. V.; Shelef, M. J. Catal. 2000, 195, 106. (9) Ribera, A.; Arends, I. W. C. E.; de Vues, S.; Perez-Ramirez, J.; Sheldon, R. A. J. Catal. 2000, 195, 287. (10) Chen, H. Y.; El-Malki, E. M.; Wang, X.; van Santen, R. A.; Sachtler, W. M. H. J. Mol. Catal. A Chem. 2000, 162, 159. (11) Rice, M. J.; Chakraborty, A. K.; Bell, A. T. J. Catal. 1999, 186, 222. (12) Feng, X. B.; Hall, W. K. J. Catal. 1997, 166, 368.

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