Comparative Density Functional Study of Methanol Decomposition on

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J. Phys. Chem. B 2010, 114, 14458–14466

Comparative Density Functional Study of Methanol Decomposition on Cu4 and Co4 Clusters† F. Mehmood,‡ J. Greeley,§ P. Zapol,‡ and L. A. Curtiss*,‡,§ Materials Science DiVision and Center for Nanoscale Materials, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: February 22, 2010; ReVised Manuscript ReceiVed: July 9, 2010

A density functional theory study of the decomposition of methanol on Cu4 and Co4 clusters is presented. The reaction intermediates and activation barriers have been determined for reaction steps to form H2 and CO. For both clusters, methanol decomposition initiated by C-H and O-H bond breaking was investigated. In the case of a Cu4 cluster, methanol dehydrogenation through hydroxymethyl (CH2OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For a Co4 cluster, the dehydrogenation pathway through methoxy (CH3O) and formaldehyde (CH2O) is slightly more favorable. Each of these pathways results in formation of CO and H2. The Co cluster pathway is very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H2 and CO at low temperatures. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. The results are compared to our previous study of methanol decomposition on Pd4 and Pd8 clusters. Finally, all reaction energy changes and transition state energies, including those for the Pd clusters, are related in a linear, Brønsted-Evans-Polanyi plot. 1. Introduction Methanol is considered one of the most important candidates for storage and production of hydrogen and is a promising component in the next generation of renewable green fuels.1 While palladium-based catalysts have been found to be effective for the decomposition of methanol,2,3 nonpalladium-based catalysts for methanol decomposition4-20 have been of interest for methanol decomposition because of the prohibitive cost and low natural abundance of Pd-based catalysts. Among non-Pdbased catalysts, Cu-based catalysts have been shown to be structure sensitive for methanol decomposition on surfaces including some evidence for increased reactivity of methanol on small Cu nanoparticles, compared to bulk single-crystal Cu surfaces.20 A theoretical investigation of methanol adsorption on Cun (n ) 2-9) clusters has shown stronger binding for methanol compared to single-crystal surfaces.21 These results along with findings that small metal clusters exhibit novel catalytic properties for reactions10 such as propane dehydrogenation and propylene epoxidation22-26 suggest that small transition metal clusters might potentially have useful catalytic properties for methanol decompositions. In a recent paper,27 we have reported a thorough density functional investigation of methanol decomposition reaction pathways on Pd4 and Pd8 gas-phase clusters. We found that decomposition pathways to H2 and CO involving initial C-H or O-H bond breaking steps were favorable. However, CO binding was also found to be very favorable and suggested that CO poisoning could occur at low temperatures. In a further experimental/theoretical paper,28 we reported calculations for †

Part of the “Michael R. Wasielewski Festschrift”. * Corresponding author. E-mail: [email protected]. ‡ Materials Science Division. § Center for Nanoscale Materials.

key methanol decomposition steps on Pd4 and Pd8 clusters on an alumina surface that explicitly accounted for the possibility of high-coverage CO adsorption. These supported clusters showed trends similar to the gas-phase clusters for the energetics of methanol decomposition and were consistent with the experimental studies in the paper that showed evidence of CO poisoning at low temperatures. Thus, it is of interest to investigate other clusters such as those involving Cu for methanol decomposition to determine how the choice of metals influences the reaction pathways. In the present paper, we extend our previous density functional studies on Pd clusters to two other gas-phase metal atom clusters, Cu4 and Co4, of interest for methanol decomposition, to investigate how Cu and Co clusters might impact the CO poisoning problems and other reactivity issues associated with the Pd clusters. Selected results are also reported for supported clusters. The Cu and Co clusters are representative of ones being investigated in experimental studies of clusters for methanol decomposition.28 We first determine adsorption energies, geometries, reaction pathways, and activation energy barriers for the O-H, C-H, and C-O bond activation pathways. Next, we present a detailed analysis of the corresponding transition states and energy barriers. Finally, we compare our results on the small transition metal clusters to our previous work on Pd4 clusters and to reported reaction energies on nanoparticles and on single-crystal surfaces. 2. Theoretical Methods The calculations performed in this study are based on density functional theory (DFT);29 the Kohn-Sham equations were solved with a plane-wave basis set using the Vienna ab initio simulation package (VASP).30,31 The electron-ion interactions for C, H, O, Cu, and Co were described using ultrasoft

10.1021/jp101594z  2010 American Chemical Society Published on Web 08/12/2010

Methanol Decomposition on Cu4 and Co4 Clusters pseudopotentials with a plane-wave energy cutoff of 400 eV. In all calculations reported in this article, the generalized gradient correction of Perdew and Wang32 (GGA-PW91) was used. To assess our results against other related density functionals and core-electron treatments, binding energies for methanol and its dissociative reaction intermediates were also calculated using the Perdew-Burke-Ernzerhof (PBE) functional33 with projector augmented wave (PAW) potentials34 for Cu4 clusters. We found differences in the adsorbate binding energies were less than 0.05 eV for all cases. A 15 × 15 × 15 Å3 cubic supercell was used to optimize the structures of gas-phase metal clusters, and it was found to be large enough to ensure that the periodically repeated cluster images do not interact with one another. To improve convergence, a modest Gaussian smearing (σ ) 0.01 eV) was used for all calculations on gas-phase clusters and 0.1 eV on oxide-supported clusters. The geometry of the clusters was determined by static relaxation using a conjugate gradient minimization and the exact Hellmann-Feynman forces. The calculations of the supported clusters were performed on a θ-Al2O3(010) surface, which was modeled in a slab geometry with a (2 × 2) lateral periodicity. Six Al2O3 layers were used to describe the surface, of which the three top layers were allowed to relax. The periodic slabs are separated by a vacuum distance of 16 Å, which was found to be sufficient to prevent interactions among repeated images of slabs with adsorbed clusters and reaction products. For the gas-phase cluster calculations, a single k-point was used, whereas for surface cells, the k-point sampling was performed with (3,3,1) Monkhorst-Pack grids.35 Many of the systems studied have a magnetic moment; therefore, spin-polarization was used in all calculations. The climbing-image nudged elastic band (CI-NEB) method36 was used to determine the minimum-energy paths for the elementary reaction steps and the corresponding activation energy barriers; between 5 and 7 intermediate images are used in each CI-NEB pathway. Vibrational frequencies (Vi) for the initial, transition, and final states of the reaction were calculated by numerical differentiation of the forces using a second-order finite difference approach with a step size of 0.005 Å. The massweighted Hessian matrix was then diagonalized to yield the frequencies and normal modes for each system. All calculated transition state structures were confirmed to have only one negative (imaginary) frequency. Interaction energies of coadsorbed products were not included in the reported reaction energy differences of the potential energy surfaces (PESs) in this study. Instead, these plots were obtained by considering product complexes adsorbed on separate clusters. The energy differences between these two approaches are modest; with two H atoms coadsorbed on the same Pd4 cluster (a metal to which we compare our Cu and Co results below), for example, we have found that such a procedure changes the energetics by less than 0.05 eV per H atom. In all cases, the elementary reaction energies on the PESs were determined by taking the best adsorption sites of the reactants and products of the corresponding elementary steps. The reported transition state energies on the PESs, however, were taken from the NEB pathway that gave the lowest such energy; this pathway might not, in all cases, correspond to the most thermodynamically stable states of the isolated reactants and products. The elementary barriers for both the pathways corresponding to the most stable isolated reactants/products and the pathways that give the lowest transition state energy are reported in this contribution. To determine the Gibbs free energies of elementary reactions, we approximated the enthalpy (∆H) at 0 K as the ∆Ee from the

J. Phys. Chem. B, Vol. 114, No. 45, 2010 14459 DFT calculations. At this temperature, the entropy contribution is also zero, and the Gibbs free energy is thus simply approximated as ∆Ee. At temperatures greater than 0 K, however, the entropy term will have rotational, vibrational, and translational contributions. We have calculated these contributions for gas-phase (nonadsorbed) species from classical approximations at 448 K and 1 bar and use them to obtain ∆G at nonzero temperatures. 3. Results and Discussion In this section, we first present the structures of the clean gas-phase and supported clusters. We then discuss the adsorption energies and structures of the adsorbates that are produced during the decomposition of methanol, and we compare these results to previously reported theoretical and experimental observations on single-crystal surfaces and nanoparticles. Next, we describe the transition states associated with the various elementary steps in the methanol decomposition reaction network, and we compare the overall potential energy surfaces for the Cu and Co clusters to earlier calculations on Pd clusters.27 We conclude with a discussion of the role of the oxide support on the decomposition pathways and of Brønsted-Evans-Polanyi relationships between transition state energies and thermodynamic reaction energies. 3.1. Cluster Structures. The lowest energy optimized gasphase Cu4 cluster has a planar rhombic structure, as shown in Figure 1, with a binding energy of 1.77 eV per Cu atom. A distorted tetrahedron is slightly less stable (by 0.25 eV) than the planar rhombic structure. Therefore, all the calculations for adsorption and reactions were performed using the planar rhombic structure. Previous DFT-GGA calculations21 for gasphase Cu4 clusters also reported the rhombus-shaped cluster to be the preferred structure with a binding energy per Cu atom of 1.66 eV. Our calculated Cu-Cu bond lengths for the optimized planar rhombic structure also agree quite well with earlier cluster calculations,21,37 and the bond lengths are smaller than bulk Cu and single crystal Cu surfaces.38 For most cases, we find that the Cu4 cluster retains its rhombic shape upon adsorption of a methanol fragment, except for the cases of C, CH, and COH, where the adsorption occurs through carbon with very strong binding. In these cases, the rhombic Cu4 cluster transforms to a slightly distorted planar square-shaped cluster with Cu-Cu bond lengths ranging from 2.39 to 2.45 Å. For Co4, we find a distorted tetrahedron to be less stable by 0.11 eV than a rhombic structure, which has a binding energy of 3.63 eV per Co atom. Both structures are illustrated in Figure 1. Geometry relaxation of the planar rhombic Co4 cluster with an adsorbed species leads to the cluster becoming a distorted tetrahedron. Therefore, in all calculations on Co4 with adsorbed species, a tetrahedral structure was used as the starting point, and binding energies were calculated relative to it. Most adsorbed species transform the distorted tetrahedron to a more skewed structure, and in some cases bond lengths change by as much as 0.5 Å. Earlier studies on Pd4 clusters did not exhibit such significant structural transformations.27 As we discuss below, the transformations are due to a delicate interplay between the cluster binding energy per atom and the strength of adsorbate binding. Previous studies have found a tetrahedral Co4 cluster structure to be the most stable clean structure with an average Co-Co bond distance of 2.34 Å,39 close to our result of 2.26 Å and 0.16 Å smaller than the Co bulk value.40 3.2. Adsorbate Structure and Energies. The adsorption energies for methanol and methanol fragments adsorbed on Cu4 and Co4 are summarized in Table 1, and adsorption sites are shown in Figure 2.

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Figure 1. Optimized structures of (a,b) gas-phase Cu4 cluster and (c,d) gas-phase Co4 cluster.

TABLE 1: Adsorption Energies (Eads) in Electronvolts of Methanol and Reaction Intermediates on Preferred Adsorption Sites, As Shown in Figure 2a Eads (eV) Cu4 CH3OH CH2OH CH3O CH2O CHOH CHO COH CH3 CH2 CO CH OH C O H

-0.78 -1.73 -2.43 -1.16 -3.03 -1.95 -2.79 -2.08 -3.85 -1.71 -5.39 -3.47 -6.14 -4.97 -2.62

Co4 Cu4 Co4 -1.11 -2.65 -3.80 -2.16 -3.78 -3.06 -4.48 -3.01 -4.43 -2.62 -6.87 -4.17 -7.69 -6.53 -2.99

bond lengths (Å) and configuration

µB 0 1 1 0 0 1 1 1 0 0 1 1 0 0 1

10 9 9 8 10 7 9 9 8 8 9 9 8 8 9

Cu4

Co4

dCu-O ) 1.99 on top dCo-O ) 2.02 on top dCu-O ) 1.99, dCu-C ) 1.95 on long bridge through C-O dCo-C ) 1.93 on top dCu-O ) 1.80 on top dCo-O ) 1.77 on top dCu-O ) 1.89, dCu-C ) 2.18 on long bridge through C-O dCo-O ) 1.82, dCo-C ) 1.99 on bridge through C-O dCu1-C ) 1.88, dCu2-C ) 2.10 on long bridge dCo-C ) 1.96 on bridge dCu-O ) 1.98, dCu-C ) 1.91 on long bridge through C-O dCo1-C ) 1.89, dCo2-C ) 2.01 on bridge dCu-C ) 1.98 on 4-fold hollow dCo-C ) 1.84 on bridge dCu1-C ) 2.10, dCu2-C ) 2.05 on long bridge dCo-C ) 1.94 on top dCu1-C ) 1.95, dCu2-C ) 1.87 on long bridge dCo-C ) 1.95 on bridge dCu-C ) 1.80 on top dCo-C ) 1.74 on top dCu-C ) 1.92 on 4-fold hollow dCo-C ) 1.88 on 3-fold hollow dCu1-O ) 1.96, dCu2-O ) 1.93 on long bridge dCo-O ) 1.78 on top dCu-C ) 1.91 on 4-fold hollow dCo-C ) 1.79 on 3-fold hollow dCu1-O ) 1.79 on long bridge dCo-O ) 1.79 on bridge dCo-H ) 1.68 on bridge dCu1-H ) 1.62 on long bridge

a The µB is the magnetic moment of the cluster with the specified intermediate. Adsorption energies are given with respect to the corresponding gas-phase species and clean metal clusters at infinite separation from one another. For Cu4, the cluster is planar rhombus (except for C, CH, and COH where it is square planar), and for Co4, the cluster is a distorted tetrahedron.

Methanol. A methanol molecule adsorbs on a top site of gasphase Cu4 and Co4 clusters through an oxygen atom, as shown in Figure 2. Methanol is bound to the Cu4 cluster with an interaction energy of -0.78 eV and a Cu-O bond length of 1.99 Å. Methanol binds more strongly (by 0.3-0.4 eV) to Cu4 compared to DFT calculations41,42 on single-crystal Cu surfaces, but the difference is less than 0.1 eV when compared to DFT37 calculations on Cu3 clusters or to experimental adsorption energy estimates from Temperature Programmed Desorption studies4 on Cu(110). Finally, the calculated Cu-O bond length of 1.99 Å is close to what has been reported from DFT calculations in the literature (2.02-2.09 Å).21,37,42 In comparison to Cu4, methanol binds to Co4 clusters more strongly (-1.11 eV) and with a slightly longer Co-O bond of 2.02 Å. This adsorption is much stronger than what has been calculated on the Co(0001) surface (-0.29 eV) or on a stepped Co surface (-0.44 eV).43

O-H Bond Scission Adsorbates (Methoxy, Formaldehyde, and Formyl). One of the three possible ways to initiate decomposition of methanol on transition metals is by removing one hydrogen atom by O-H bond activation to yield a methoxy radical (CH3O). Experimentally, methoxy has been detected as a stable intermediate on single-crystal Cu and Co surfaces.4,9,44 On both Cu4 and Co4 clusters, we find that CH3O binds most strongly to a top site through an oxygen atom with adsorption energies of -2.43 and -3.80 eV, respectively, Previous calculations on Cu(111) show adsorption energies of -2.08 eV on fcC-Hollow sites,41 ∼0.35 eV smaller than what was calculated on the Cu(100).38,45 Clearly, the adsorption energy on Cu4 is stronger than that reported on Cu(111), and the same is true for Co, where the CH3O adsorption energy on stepped Co surfaces was reported to be -3.03 eV and even smaller (-2.63 eV) on Co(0001).43 The bond lengths given in Table 1

Methanol Decomposition on Cu4 and Co4 Clusters

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Figure 2. Comparison of the most energetically favorable adsorption configurations on Co4 (tetrahedral structures) and Cu4 (planar structures) clusters for methanol and selected methanol decomposition intermediates.

show that the metal-O bond length of the methoxy adsorbate is much shorter than the corresponding metal-O bond in the methanol adsorbate. A formaldehyde (CH2O) intermediate can be obtained either by breaking a C-H bond in methoxy or by breaking an O-H bond in hydroxymethyl. Our calculations indicate that CH2O binds by -1.16 eV to Cu4 when C and O are bonded to two Cu atoms at a long bridge site, as shown in Figure 2. On Co4, CH2O binds in a similar way with an adsorption energy that is 1 eV greater than on Cu4. Most earlier calculations show that formaldehyde also prefers adsorption in a similar manner38,42,46-48 on single-crystal Cu surfaces of various kinds. The calculated adsorption energies of CH2O on these surfaces, however, have been reported to be somewhat smaller, ranging from -0.25 to -0.70 eV for Cu38,42,46-48 and from -0.86 to -1.23 eV for Co.43 A formyl intermediate (CHO) can result from dehydrogenation of either formaldehyde or hydroxymethylene. The optimized structure of formyl on Cu4 and Co4 is shown in Figure 2. We find a long bridge site to be the most favorable adsorption site on Cu4 with an adsorption energy of -1.95 eV. The C and O

of CHO are bonded to two different Cu atoms. The adsorbed formyl on Co4 also binds at a bridge site with a much stronger adsorption energy of -3.06 eV. Calculations on Cu(110) revealed that the bridge site is favorable for formyl adsorption with an adsorption energy of -1.45 eV,42 whereas formyl adsorption energies on Cu(111) and Cu(100) range between -1.15 and -1.50 eV.41,45,49 The adsorption of HCO on stepped Co surfaces was found to be similar to that found for Co4 with an adsorption energy of about -2.75 eV.43 C-H Bond Scission Intermediates (Hydroxymethyl, Hydroxymethylene, and Hydroxymethylidyne). Methanol decomposition through C-H bond activation on Cu and Co surfaces has been proposed as a competitive reaction pathway to decomposition through O-H scission in numerous studies.42,43,45 This step leads to the formation of a hydroxymethyl (CH2OH) intermediate. The hydroxymethyl intermediate prefers to adsorb on a bridge site on Cu4 through both C and O, while on Co4 it adsorbs at a top site through a carbon atom, as shown in Figure 2. The adsorption energy is -1.73 eV on the Cu4 and -2.65 eV on Co4 cluster. This is larger than what was reported on

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Cu(111) and Cu(100) (-0.80 and -1.13 eV, respectively45), but a value reported for Cu(110) is 0.44 eV larger than what we find on Cu4, even though the adsorption site is the same.42 On a Co(0001) surface, the CH2OH intermediate also binds through a C atom (to an hcp site), while on a stepped surface it binds through both C and O with somewhat less exothermic adsorption.43 Removal of a hydrogen atom from hydroxymethyl results in the formation of a hydroxymethylene (CHOH) intermediate. Hydroxymethylene preferentially adsorbs on a bridge site through a carbon atom on both Cu4 and Co4 with adsorption energies of -3.03 and -3.78 eV, respectively. Additional C-H bond cleavage leads to a hydroxymethylidyne (COH) intermediate. We find COH to preferentially adsorb on a hollow site through carbon on the Cu4 cluster; the adsorption energy is -2.79 eV. The strongly adsorbed COH causes structural transformation of the Cu4 cluster from a rhombus to a square, thus creating a 4-fold hollow site. On the other hand, COH adsorbs on a bridge site on the Co4 cluster with an adsorption energy of -4.48 eV. Adsorption sites for both cases are shown in Figure 2, and selected bond distances are given in Table 1. Our calculated adsorption energies are significantly stronger than those reported on single-crystal Cu ((100), (111), or (110))42,45 and Co(0001) surfaces.43 C-O Bond Scission Intermediates (Methyl, Methylene, and Methine). An important question in the decomposition of methanol on subnanometer clusters is to what extent these clusters might activate C-O bond cleavage in methanol, which is known to be among the least competitive reaction steps in the decomposition of methanol on many transition metal surfaces.42,43 Activating the C-O bond in methanol itself results in methyl and hydroxyl groups adsorbed on the metal cluster. We find the CH3 radical to bind to a bridge site (Figure 2) with an energy of -2.08 eV. This site is the same as is reported in earlier calculations on Cu(100)38 and Cu(111),50,51 where the authors found the hcp/fcc hollow site to be the most energetically favorable, with adsorption energies ranging from -1.57 to -1.87 eV. On the Co4 cluster, we find that CH3 adsorbs on a top site with an adsorption energy of -3.06 eV. Previous calculations on flat and stepped Co surface reveal the same adsorption site but much weaker adsorption energies (∼ -1 eV).43 Methylene (CH2) is produced in three possible ways: (1) C-H bond activation in the methyl radical, (2) C-O bond activation in formaldehyde, or (3) C-O bond activation in hydroxymethyl. Methylene adsorption favors a long bridge site on Cu4 and Co4 clusters, as shown in Figure 2, with relatively strong adsorption energies of -3.85 and -4.43 eV, respectively. The structures and adsorption energies are similar to what was reported for Cun (n ) 1-6) clusters,43 a Co(0001) surface, and a stepped Co surface.43 Finally, C-O bond activation in formyl or hydroxymethylene radicals and C-H bond activation in the methylene radical will result in methine (CH) formation. We find that CH binds very strongly at a hollow site on Cu4 with an adsorption energy of -5.39 eV, which causes a structural transformation of the cluster to a square structure. The adsorption is even stronger on Co4 (-7.69 eV); however, no structural transformation is observed in this case. Other Adsorbates (Hydroxyl, Carbon Monoxide, and Atomic Species). Hydroxyl radicals prefer adsorption at a bridge site on a Cu4 cluster with an adsorption energy of -3.47 eV, slightly larger than the adsorption energy of -3.13 eV found for an on-top site. This trend is in general agreement with calculations

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Figure 3. Comparison of calculated binding energies of reactants, products, and intermediates of a methanol decomposition reaction on Cu, Co, and Pd clusters. The binding energies plotted here are the absolute values of the adsorption energies in Table 1.

of OH on other Cu surfaces,45 although the adsorption energies are much stronger in the case of the clusters. The difference between the top and bridge site on a Co4 cluster is smaller (0.06 eV), with the top site being energetically more stable. The adsorption of carbon monoxide has been widely investigated on surfaces of single-crystal transition metals and on transition metal nanoparticles, using both experiment and theory.20,41-43,45,52,53 We find that CO has a clear preference for adsorption on a top site on both Cu and Co clusters, with adsorption energies of -1.71 and -2.62 eV, respectively. This is in contrast to Pd4 where a bridge site is preferred.27 The difference between the top and bridge sites on Co4 is only 0.33 eV, whereas on Cu4 the difference in energy is quite large (1.31 eV). We note that previous calculations on surfaces report both top and bridge adsorption with similar energy differences, consistent with this work.42,43,52 The adsorption of carbon, hydrogen, and oxygen atoms has also been investigated. The adsorption energies on Cu4 are: -6.14 eV for carbon (4-fold-hollow site with Cu4 undergoing a structural transformation to a square structure), -4.97 eV for atomic oxygen (bridge site), and -2.62 eV for atomic hydrogen (bridge site). For O, the difference in energy between bridge and 3-fold-hollow sites is small (0.14 eV). The site preferences for C, O, and H on a Co4 cluster are the same as on Cu4, but the adsorption energies are stronger (-7.69, -6.53, and -2.99 eV, respectively). The difference between these sites on Co4 and their closest competitive site is relatively small (less than 0.33 eV). Comparison to Results of Adsorbates on a Pd4 Cluster. A comparison of the adsorption energies of the adsorbates on the Cu4 and Co4 clusters with our previously reported results27 on Pd4 is shown in Figure 3. In general, all adsorbate species bind more strongly to Co4 than Pd4 and less strongly to Cu4 than Pd4. Exceptions to this general rule are adsorbates that bind to Cu4 through the O atom, such as methanol and the methoxy radical, which have a stronger adsorption energy than on Pd4. 3.3. Thermodynamics and Kinetics of the Methanol Decomposition Reaction Network. Having established the adsorption energies and structures of adsorbates on Cu4 and Co4 clusters, we turn to a discussion of the kinetics and the thermodynamics of the complex reaction networks associated with methanol decomposition on these clusters. We begin with a description of the most energetically favorable processes (methanol dehydrogenation through either C-H or O-H bond scission), and we follow this with a description of the less energetically favorable processes associated with C-O bond

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TABLE 2: Activation Energy Barriers (Ea in eV) for the Reaction Pathways Involving C-H, O-H, and C-O Bond Cleavage in Methanol and Its Decomposition Intermediates on Cu, Co, and Pd Clustersa C-H bond activation CH3OH CH2OH CHOH CH3O CH2O CHO COH CH3 CH2 CO CH OH

O-H bond activation

C-O bond activation

Cu4

Co4

Pd4

Cu4

Co4

Pd4

Cu4

Co4

Pd4

1.24 (+0.60) 0.92 (-0.13) 0.94 (+0.31) 0.87 (-0.12) 1.26 (+0.62) 0.53 (-1.16) 1.52 (+0.58) 1.20 (+0.63) 1.92 (+0.42) -

0.62 (-0.35) 0.21 (-0.10) 0.37 (-0.30) 0.75 (-0.11) 0.45 (-0.99) 0.17 (-0.20) 0.50 (-0.28) 0.42 (-0.69) 1.27 (-0.02) -

0.86 (+0.17) 0.61 (+0.34) 1.34 (-0.02) 0.43 (+0.23) 0.46 (+0.40) 0.59 (+0.23) 0.71 (+0.51) 0.96 (-0.04) 1.23 (+0.82) -

1.30 (+0.37) 1.91 (-0.76) 0.94 (-0.02) -

0.47 (-1.01) 1.04 (-0.77) 0.72 (-1.86) 1.24 (-1.74) 1.21 (-0.26)

0.85 (+0.17) 1.16 (-0.24) 0.31 (-0.66) 0.55 (-1.34) 1.45 (+0.92)

1.36 (-0.42) 1.20 (-0.87) 2.77 (-0.11) 2.12 (+0.17) 2.63 (+0.87) 2.22 (+0.88) 2.01 (-0.67) -

0.97 (-1.60) 0.38 (-0.77) 2.25 (-1.54) 1.66 (-0.85) 1.11 (+0.43) 2.13 (+0.06) 1.82 (-1.23) 2.22 (+0.25) -

1.32 (+0.40) 0.79 (+0.07) 1.05 (-0.02) 1.89 (+1.13) 2.29 (+1.66) 2.18 (+2.06) 0.69 (+0.56) 4.17 (+3.26) -

2.36 (+0.97)

-

a The reference point for each energy barrier reported is considered to be an adsorbed reactant which is referred to as the “actual” or “true” barrier in earlier sections. The values in parentheses are reaction energies in eV of co-adsorbed species resulting from the dissociation.

Figure 4. Free energy surface for methanol dehydrogenation steps through O-H bond activation on Cu4, Co4, and Pd4. (The Pd4 results are from ref 27.)

Figure 5. Free energy surface for methanol dehydrogenation steps through C-H bond activation on Cu4, Co4, and Pd4. (The Pd4 results are from ref 27.)

scission through the various decomposition intermediates. We conclude with a discussion of competing pathways to methanol decomposition on Cu, Co, and Pd clusters. Dehydrogenation of Methanol (O-H or C-H Bond ActiWation). Methanol can initially dehydrogenate through either C-H bond activation to produce hydroxymethyl or O-H bond activation to produce methoxy. The activation barriers of these reaction steps for the Cu4 and Co4 gas-phase clusters are summarized in Table 2, and the free energies of full dehydrogenation pathways with initial O-H and C-H bond activations are plotted in Figure 4 and Figure 5, respectively. Results from our previous work27 for methanol decomposition on a gas-phase Pd4 cluster are also shown in Figure 4 and Figure 5. Figure 4 shows that the free energy change associated with O-H activation (relative to adsorbed methanol) in methanol is substantially exothermic for Co4 and endothermic for Cu4. Note that the free energy of the CH3O intermediate is higher than the corresponding free energy of gaseous methanol and clean Cu4 clusters because of the sizable entropy loss associated with methanol adsorption from the gas phase at the temperature considered in this study. The activation energy barriers with respect to the adsorbed methanol were found to be 0.47 and 1.30 eV on Co4 and Cu4 clusters, respectively (see Table 2). A previous DFT study of O-H scission of methanol on a Cu3 cluster gave similar results with a slightly larger barrier of 1.49 eV and a reaction energy of -0.44 eV.37 On a Cu(110) surface, on the other hand, Mei et al.42 report a much smaller activation energy barrier for CH3O formation (0.63 eV) with an exothermic

reaction energy (-0.24 eV). This significant difference is likely due to geometric variations between the O-H scission transition states on the clusters and the stepped (110) surface. Although the O-H bond length at the transition state is only 0.10 Å different between the Cu4 cluster and what is reported for Cu(110),42 other geometric changes are considerably more significant. On Cu4, at the transition state both CH3O and H are bonded to the same Cu atom, whereas on Cu(110), the dissociating H is closer to a separate row of Cu atoms than to the row on which CH3O adsorbs. Similarly, we find that, in the final state of this elementary reaction on Cu4, CH3O is on top of a Cu atom with H between it and an adjacent Cu atom, whereas on Cu(110), both dissociated products are located on top of two different Cu atoms. For Co-based catalysts, a previous DFT calculation of O-H scission on a Co(0001) surface gave an activation barrier of 0.80 eV, which drops to 0.42 eV when considering a stepped Co surface.43 After overcoming the small barrier on Co4 needed to create methoxy, subsequent dehydrogenation proceeds through CH2O and ultimately CHO and CO as summarized here

CH3OH f CH3O, H f CH2O, H2 f CHO, H, H2 f CO, 2H2 (1) These steps have relatively low barriers, as shown in Table 2 and Figure 4. In contrast, the subsequent steps on the Cu4 cluster for reaction 1 are endothermic.

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In Figure 5, it is seen that the free energy change associated with C-H activation in adsorbed methanol is also slightly exothermic for Co4, whereas it is substantially endothermic for Cu4. Previous DFT calculations by Mei et al.42 on the Cu(110) surface for the C-H scission barrier and reaction energy are within 0.09 eV of what we find for the Cu cluster. Unlike O-H bond activation, the similarities in transition states for C-H bond activation on Cu4 and Cu(110) are significant. In both cases, CH2OH and H are bonded to a single Cu atom with the C-H bond length slightly larger on Cu(110). The geometries of the reaction products are quite similar, with CH2OH bonded to a Cu atom through C and with dissociated H on an adjacent bridging site. Cheng et al.43 find a difference in activation barrier of O-H and C-H bond scission step on a flat Co surface, with C-H activation favored by 0.32 eV. They find a smaller difference on a stepped surface (less than 0.15 eV), which is in line with our gas-phase Co cluster results. After the initial C-H bond scission event in methanol, further dehydrogenation can occur via C-H or O-H cleavage in hydroxymethyl (CH2OH). The most thermodynamically favorable of these processes on Cu4 is C-H bond scission in CH2OH to yield hydroxymethylene (CHOH) and, ultimately, CHO and CO.

CH3OH f CH2OH, H f CHOH, H2 f CHO, H, H2 f CO, 2H2 (2) An alternate pathway, the cleavage of the O-H bond to yield formaldehyde (CH2O), has a much higher barrier (1.91 eV) compared to C-H bond scission in CH2OH (0.92 eV). As in the case of reaction 1, the subsequent steps in reaction 2 on Cu4 are endothermic. We note, in passing, that subsequent dehydrogenation steps (for both C-H and O-H scission) are modeled by removing previously dissociated hydrogen atoms from the cluster. In test calculations, we find that this approach introduces only small differences in the calculated barriers and energies; for O-H bond activation in CH2OH, for example, a difference of less than 0.02 eV was found in the barrier with and without coadsorbed H atoms. In the case of Co4, relatively low barriers for these subsequent steps are found, as shown in Table 2, Figure 4, and Figure 5. On the Co4 cluster, both the C-H and O-H bond scission step in methanol have essentially the same barrier; however, for the overall pathway the O-H bond scission pathway is slightly more favorable (see Figure 4 and Figure 5). On the Cu4 cluster, both the C-H and O-H bond scission steps in methanol also have essentially the same barrier, and for the overall pathway the C-H bond scission in methanol is slightly more favorable (see Figure 4 and Figure 5). The desorption of CO is relatively favorable thermodynamically on the Cu4 clusters compared to the Co4 clusters. The results in Figure 4 and Figure 5 also include the reaction pathways for the previously studied Pd4 clusters.27 The Pd4 pathways are intermediate between those of Co4 and Cu4. The desorption of CO is relatively unfavorable thermodynamically on the Pd4 clusters, similar to the Co4 clusters. These results imply that CO may poison the catalytic reaction on the Pd and Co clusters at low temperatures. This result is consistent with experimental observations for Pd clusters.28 Methanol Decomposition through C-O Bond ActiWation. In Table 2, we summarize the activation barriers and reaction energies for C-O bond cleavage in methanol and in its dehydrogenation products on the Co and Cu clusters. For

Mehmood et al. TABLE 3: Activation Energy Barriers (Ea in eV) for the Methanol Decomposition through C-H, O-H, and C-O Bond Cleavage on Cu, Co, and Pd Clusters Supported on θ-Al2O3a C-H O-H C-O

Cu4

Co4

Pd4

1.93(1.24) 0.92(1.30) 1.53(1.36)

0.92(0.62) 0.65(0.47) 0.94(0.97)

0.80(0.86) 0.98(0.85) 1.42(1.32)

a Corresponding activation energy barriers on gas-phase clusters are provided in parentheses.

methanol itself on Cu4 and Co4, C-O bond activation is exothermic by -0.42 and -1.60 eV, respectively. The barrier for this step on Cu4, 1.36 eV, is relatively larger than the barriers for competing steps in the most favorable dehydrogenation pathways (Table 2) and for C-O activation on Pd and Co. On the Co4 cluster, the barrier for C-O scission is 0.97 eV, which is considerably larger than the barriers for competing steps in the most favorable dehydrogenation pathways (Table 2) on the Co cluster. The C-O bond breaking steps in other dehydrogenation intermediates, including CH2OH, CHOH, CH3O, CHO, and COH, are substantially exothermic on Co and moderately endothermic in CH2O and CO. Each of these elementary steps has activation barriers that are smaller than both on the Cu cluster. These results strongly imply that C-O cleavage is unlikely to be a major reaction pathway on these clusters, as we also found for Pd4 clusters.27 3.4. Methanol Decomposition on Supported Clusters. To probe support effects on the methanol decomposition chemistry, we have studied the thermodynamics and kinetics of C-O, C-H, and O-H bond scission steps on Co4 and Cu4 clusters supported on a θ-Al2O3(010) surface. We chose this surface because alumina has been used as a support in several studies of catalysis on subnanometer clusters.54-57 Using the results from the gas-phase clusters as a starting point, we have calculated adsorption geometries and energies for the first bonding breaking steps on supported Cu4 and Co4 clusters. We considered both planar and tetrahedral structures of the clusters on θ-Al2O3(010). The planar rhombus structure of Cu4 was found to be 0.34 eV more stable than the tetrahedral structure on θ-alumina, whereas the tetrahedral Co4 structure was more stable than the planar structure by 0.26 eV. In the case of methanol adsorbed on Cu4/ Al2O3, we find the adsorption energy to increase by 0.25 eV compared to the gas-phase clusters. For the case of the Co4 cluster, we find methanol to adsorb with an adsorption energy 0.37 eV smaller than what was calculated on the gas-phase cluster. A comparison of the activation energy barriers for the C-O, C-H, and O-H bond scission steps on gas-phase and on supported Co and Cu clusters is given in Table 3. The changes in the activation energy with inclusion of the support range from a decrease of 0.38 eV to an increase of 0.69 eV for Cu and range from an increase of 0.30 eV to a decrease of 0.03 eV for Co. The range for Cu, in particular, is considerably larger than what we found in our previous study of Pd4 clusters, where changes in activation energy due to support effects were generally 0.1 eV or less (see Table 3). Although no single explanatory paradigm is likely to be sufficient to describe all of these effects, some understanding can be gained by considering the structures of the adsorbed metal clusters on the θ-Al2O3 support, together with corresponding differences in the transition state geometries. In the case of Cu4, where the support effects are largest, we find that the planar structure can place significant constraints on the adsorbate geometries when the cluster is

Methanol Decomposition on Cu4 and Co4 Clusters

J. Phys. Chem. B, Vol. 114, No. 45, 2010 14465 value, close to unity, suggests that the transition states in the studied reaction networks are generally final state-like in character, where the final state is defined in the exothermic direction for each elementary step. Conclusions

Figure 6. Brønsted-Evans-Polanyi plotsthe calculated transition state energy (ETS) versus final state energy (EFS)sfor C-H and O-H bond scission steps on the Cu4, Co4, and Pd4 clusters. Both transition and final state energies for each elementary step are calculated with reference to reactants for that particular step, where the direction of each step is defined such that the products are lower in energy than the reactants.

adsorbed on a support, which in turn results in the large differences in activation energies between the gas-phase and supported clusters, as shown in Table 3. In fact, the largest such difference in the activation energy barrier was found for C-H bond activation in methanol on Cu4. Examining the transition state of this step on gas-phase and supported clusters reveals that, in the gas-phase cluster, both CH2OH and H are in the cluster plane, whereas, on the supported clusters, these species are forced out-of-plane, with consequent increase in the activation barrier. We note, however, that these effects are not universal for all reactions on Cu4; for O-H bond activation, in particular, the activation barrier on the supported cluster is 0.38 eV smaller than on the gas-phase cluster. In this case, the effect may also be related to geometric differences; CH3O and H are on top of the same Cu atom for the gas-phase cluster, whereas H is adsorbed on a bridge site between two Cu atoms on the supported cluster. For most other bond-breaking reactions on the various metal clusters, we find relatively small differences between the transition states of supported and gas-phase clusters, perhaps resulting in the calculated relatively small changes in transition state energies. Again, however, we emphasize that a generalized description of support effects on subnanometer cluster chemistry could involve a variety of other effects, as well. 3.5. Correlation of Activation Energies with Reaction Energies. Additional information on the energetics of methanol dissociation over subnanometer metal clusters can be obtained by plotting Brønsted-Evans-Polanyi (BEP)-type curves. These BEP relations provide quantitative relationships between the activation barriers for elementary reaction steps and the corresponding elementary reaction energy changes. The BEP relations are often implicitly assumed to hold for surface reactions, and it has been shown that this kind of relationship holds quite well for subnanometer Pd clusters.27 Figure 6 shows a BEP plot for elementary reaction steps associated with C-H and O-H bond activation studied for the Cu4, Co4, and Pd4 clusters. According to this BEP relationship, the thermodynamically most endothermic step should have the highest activation barrier and vice versa. The O-H and C-H scission in methanol have intermediate energetics, and C-H dissociation in methine is the most exothermic. The equation relating transition and final state energies is provided in Figure 6 and has a slope of 0.85; this

We have used density functional theory for a thorough investigation of the decomposition of methanol on subnanometer Cu4 and Co4 clusters. The reaction intermediates and activation barriers in reaction steps to form H2 and CO have been determined, and likely reaction pathways have been elucidated. Among the pathways, methanol decomposition initiated by C-H and O-H bond breaking was investigated for both clusters. In the case of the Cu4 cluster, methanol dehydrogenation through hydroxymethyl (CH2OH), hydroxymethylene (CHOH), formyl (CHO), and carbon monoxide (CO) is found to be slightly more favorable. For the Co4 cluster, the dehydrogenation pathway through methoxy (CH3O) and formaldehyde (CH2O) is slightly more favorable. Each of these pathways results in formation of CO and H2. Both Co cluster pathways based on C-H and O-H initial steps are very favorable thermodynamically and kinetically for dehydrogenation. However, since CO binds strongly, it is likely to poison methanol decomposition to H2 and CO at low temperatures on this metal. In contrast, for the Cu cluster, CO poisoning is not likely to be a problem since it does not bind strongly, but the dehydrogenation steps are not energetically favorable. Pathways involving C-O bond cleavage are even less energetically favorable. We previously found that CO poisoning was probable for methanol decomposition on Pd clusters,27 and this was confirmed experimentally.28 These results may indicate a need to find other clusters that combine both favorable aspects of the Co (or Pd) and Cu clusters, possibly binary alloy clusters, for methanol decomposition. Finally, all reaction energy changes and transition state energies were related in a linear, Brønsted-Evans-Polanyi plot. Acknowledgment. Work, including use of the Center for Nanoscale Materials, is supported by the U.S. Department of Energy under Contract DE-AC0206CH11357. We acknowledge grants of computer time from EMSL, a national scientific user facility located at Pacific Northwest National Laboratory, and the ANL Laboratory Computing Resource Center (LCRC). References and Notes (1) Parsons, R.; Vandernoot, T. J. Electroanal. Chem. 1988, 257, 9. (2) Agrell, J.; Germani, G.; Jaras, S. G.; Boutonnet, M. Appl. Catal. A: Gen. 2003, 242, 233. (3) Liu, S.; Takahashi, K.; Eguchi, H.; Uematsu, K. Catal. Today 2007, 129, 287. (4) Wachs, I. E.; Madix, R. J. J. Catal. 1978, 53, 208. (5) Paul, J. Surf. Sci. 1985, 160, 599. (6) Chesters, M. A.; McCash, E. M. Spectrochim. Acta, Part A 1987, 43, 1625. (7) Knickelbein, M. B.; Koretsky, G. M. J. Phys. Chem. A 1998, 102, 580. (8) Mustain, W. E.; Kepler, K.; Prakash, J. Electrochim. Acta 2007, 52, 2102. (9) Nowitzki, T.; Borchert, H.; Jurgens, B.; Risse, T.; Zielasek, V.; Baumer, M. Chemphyschem 2008, 9, 729. (10) Gates, B. C. Chem. ReV. 1995, 95, 511. (11) Webber, K. M.; Gates, B. C.; Drenth, W. J. Mol. Catal. 1977, 3, 1. (12) Bowker, M.; Madix, R. J. Surf. Sci. 1980, 95, 190. (13) Francis, S. M.; Leibsle, F. M.; Haq, S.; Xiang, N.; Bowker, M. Surf. Sci. 1994, 315, 284. (14) Russell, J. N.; Gates, S. M.; Yates, J. T. Surf. Sci. 1985, 163, 516. (15) Sexton, B. A.; Hughes, A. E.; Avery, N. R. Surf. Sci. 1985, 155, 366. (16) Nakatsuji, H.; Hu, Z. M. Int. J. Quantum Chem. 2000, 77, 341. (17) Witko, M.; Hermann, K. J. Chem. Phys. 1994, 101, 10173.

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(18) Choi, H. J.; Kang, M. Int. J. Hydrogen Energy 2007, 32, 3841. (19) Gadhe, J. B.; Gupta, R. B. Int. J. Hydrogen Energy 2007, 32, 2374. (20) Varazo, K.; Parsons, F. W.; Ma, S.; Chen, D. A. J. Phys. Chem. B 2004, 108, 18274. (21) Hsu, W. D.; Ichihashi, M.; Kondow, T.; Sinnott, S. B. J. Phys. Chem. A 2007, 111, 441. (22) Vajda, S.; Pellin, M. J.; Greeley, J. P.; Marshall, C. L.; Curtiss, L. A.; Ballentine, G. A.; Elam, J. W.; Catillon-Mucherie, S.; Redfern, P. C.; Mehmood, F.; Zapol, P. Nat. Mater. 2009, 8, 213. (23) Hakkinen, H.; Yoon, B.; Landman, U.; Li, X.; Zhai, H. J.; Wang, L. S. J. Phys. Chem. A 2003, 107, 6168. (24) Lee, S.; Molina, L. M.; Lo´pez, M. J.; Alonso, J. A.; Hammer, B.; Lee, B.; Seifert, S.; Winans, R. E.; Elam, J. W.; Pellin, M. J.; Vajda, S. Angew. Chem., Int. Ed. 2009, 48, 1467. (25) Sakurai, H.; Tsubota, S.; Haruta, M. Sci. Technol. Catal. 1995, 92, 111. (26) Haruta, M. Catal. Today 1997, 36, 153. (27) Mehmood, F.; Greeley, J.; Curtiss, L. A. J. Phys. Chem. C 2009, 113, 21789. (28) Lee, S.; Lee, B.; Mehmood, F.; Seifert, S.; Libera, J. A.; Elam, J. W.; Greeley, J.; Zapol, P.; Winans, R. E.; Curtiss, L. C.; Stair, P. C.; Vajda, S. J. Phys. Chem. C 2010, 114, 10342. (29) Hohenberg, P.; Kohn, W. Phys. ReV. 1964, 136, B864. (30) Kresse, G.; Furthmuller, J. Phys. ReV. B 1996, 54, 11169. (31) Kresse, G.; Hafner, J. Phys. ReV. B 1993, 47, 558. (32) Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D. J.; Fiolhais, C. Phys. ReV. B 1992, 46, 6671. (33) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. ReV. Lett. 1996, 77, 3865. (34) Blochl, P. E. Phys. ReV. B 1994, 50, 17953. (35) Monkhorst, H. J.; Pack, J. D. Phys. ReV. B 1976, 13, 5188. (36) Henkelman, G.; Jonsson, H. J. Chem. Phys. 2000, 113, 9978. (37) Yamin, L. J.; Gomez, M. F.; Arrua, L. A. J. Mol. Struct.-Theochem 2004, 684, 159.

Mehmood et al. (38) Foo, A. S. Y.; Lim, K. H. Catal. Lett. 2009, 127, 113. (39) Rodrı´guez-Lo´pez, J. L.; Aguilera-Granja, F.; Michaelian, K.; Vega, A. J. Alloys Compd. 2004, 369, 93. (40) Bezemer, G. L.; Bitter, J. H.; Kuipers, H.; Oosterbeek, H.; Holewijn, J. E.; Xu, X. D.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956. (41) Greeley, J.; Mavrikakis, M. J. Catal. 2002, 208, 291. (42) Mei, D. H.; Xu, L. J.; Henkelman, G. J. Phys. Chem. C 2009, 113, 4522. (43) Cheng, J.; Gong, X. Q.; Hu, P.; Lok, C. M.; Ellis, P.; French, S. J. Catal. 2008, 254, 285. (44) Habermehl-Cwirzen, K.; Lahtinen, J.; Hautojarvi, P. Surf. Sci. 2005, 598, 128. (45) Ferrin, P.; Mavrikakis, M. J. Am. Chem. Soc. 2009, 131, 14381. (46) Gomes, J. R. B.; Gomes, J.; Illas, F. J. Mol. Catal. A-Chem. 2001, 170, 187. (47) Sakong, S.; Gross, A. J. Phys. Chem. A 2007, 111, 8814. (48) Sakong, S.; Groβ, A. J. Catal. 2005, 231, 420. (49) Gomes, J. R. B.; Gomes, J. J. Electroanal. Chem. 2000, 483, 180. (50) Robinson, J.; Woodruff, D. P. Surf. Sci. 2002, 498, 203. (51) Michaelides, A.; Hu, P. J. Chem. Phys. 2001, 114, 2523. (52) Mehmood, F.; Kara, A.; Rahman, T. S.; Bohnen, K. P. Phys. ReV. B 2006, 74, 155439. (53) Dai, X. P.; Yu, C. C.; Shen, S. K. Chin. J. Catal. 2001, 22, 104. (54) Bezemer, G. L.; Bitter, J. H.; Kuipers, H. P. C. E.; Oosterbeek, H.; Holewijn, J. E.; Xu, X.; Kapteijn, F.; van Dillen, A. J.; de Jong, K. P. J. Am. Chem. Soc. 2006, 128, 3956. (55) Vajda, S.; Winans, R. E.; Elam, J. W.; Lee, B.; Pellin, M. J.; Seifert, S.; Tikhonov, G. Y.; Tomczyk, N. A. Top. Catal. 2006, 39, 161. (56) Hellman, A.; Gronbeck, H. J. Phys. Chem. C 2009, 113, 3674. (57) Imamura, S.; Yamane, H.; Kanai, H.; Saito, Y.; Utani, K. J. Jpn. Pet. Inst. 2002, 45, 222.

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