Density Functional Study of Hydrogen Adsorption on Tetrairidium

Sep 12, 2007 - ... zeolite clusters (M = K, Na and Li): a density functional theory study. Mehmet Ferdi Fellah. Journal of Porous Materials 2014 21 (5...
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14484

J. Phys. Chem. C 2007, 111, 14484-14492

Density Functional Study of Hydrogen Adsorption on Tetrairidium Supported on Hydroxylated and Dehydroxylated Zeolite Surfaces Galina P. Petrova,† Georgi N. Vayssilov,*,† and Notker Ro1 sch*,‡ Faculty of Chemistry, UniVersity of Sofia, 1126 Sofia, Bulgaria, and Department Chemie, Technische UniVersita¨t Mu¨nchen, 85747 Garching, Germany ReceiVed: May 23, 2007; In Final Form: July 20, 2007

We explored computationally successive adsorption of hydrogen on Ir4 clusters both in the gas phase and on hydroxylated or dehydroxylated zeolite support, the latter described by cluster models. Free and supported Ir4 clusters studied were calculated to adsorb dissociatively large amounts of hydrogen, up to three hydrogen atoms per metal atom. In the range covered, the energy for dissociative adsorption of hydrogen on Ir4Hx clusters (x ) 0, 3, 6, 9, 12) in the gas phase is almost independent of x, ∼70 kJ/mol per adsorbed atom. The corresponding energy gain is smaller for zeolite-supported Ir4. The average Ir-Ir distance of free and supported clusters increases with hydrogen loading at very similar rates, by ∼1.9 pm per adsorbate. Charged Ir4Hx complexes adsorbed on a dehydroxylated zeolite surface are always more stable than complexes of the same chemical composition in gas phase or supported on a hydroxylated surface. Therefore, reverse hydrogen spillover from OH groups of the support onto the metal cluster is calculated to be energetically favorable not only for bare clusters but also after partial loading of hydrogen. From the calculated structural and energetic parameters we suggest that the tetrairidium species in faujasite-type zeolites, produced experimentally and investigated by extended X-ray absorption fine structure (EXAFS), correspond to hydrogenated moieties Ir4Hx (x ) 9-12) supported on a dehydroxylated framework.

1. Introduction The interaction of hydrogen with supported metal clusters is of crucial importance in various catalytic processes related to hydrogenation, dehydrogenation, or hydrogenolysis of petrochemicals and other organic compounds.1-3 This interaction is also crucial for processes in fuel cells based on such catalysts4 as well as for storage and release of hydrogen.5 In particular, knowledge of the energy for dissociative adsorption of hydrogen, the concomitant structural changes in the cluster, and the saturation coverage is very helpful for understanding and predicting the adsorption and catalytic behavior of such clusters. One expects that these properties are notably affected not only by the type of the support but also its state, hydroxylated or dehydroxylated, which affects the charge (and to some extend the chemical composition) of the supported metal species.3,6 Other important factors are the properties of metal clusters themselves, such as structure, oxidation state, and affinity toward hydrogen adsorption, and it is helpful to understand how they vary between clusters in the gas phase and on support. In our computational model study, we used zeolite-supported Ir4 clusters as examples. The corresponding experimental background was provided in series of experimental papers by Gates and co-workers1,7 on preparation, structure determination, and catalytic activity of iridium clusters of well-defined nuclearity, supported on zeolites or oxides. They reported on the hydrogenation of arenes and alkenes with supported Ir4 and Ir6 as catalysts1a-d and showed that, at exposure to H2, iridium hydride species are formed.8 Such hydride species on metal * Corresponding authors. E-mail: [email protected] (G.N.V.); [email protected] (N.R.). † University of Sofia. ‡ Technische Universita ¨ t Mu¨nchen.

clusters could have been produced by dissociative adsorption of hydrogen molecules from the gas phase but also by interaction of metal clusters with OH groups of the support, so-called “reverse spillover”. In previous computational studies, we showed that the latter process can take place for zeolitesupported Rh6 clusters;9,10 also, it was calculated to be energetically preferred in a study on supported hexaatomic clusters of the transition metals of the groups 8-10.11 Very recently, we calculated the energy of reverse hydrogen spillover to Ir4 at 140 kJ/mol per proton.12 Whereas the experimentally derived Ir-Ir distances of supported Ir4 clusters, 266-272 ((1%) pm,7 were found similar to the bulk nearestneighbor distance, the average interatomic distance of bare Ir4 on zeolite support was calculated much shorter, 247-249 pm,12,13 as is to be expected for small bare metal clusters.14 To rationalize this large discrepancy, the hypothesis was advanced that Ir4 clusters in zeolites, studied in the extended X-ray absorption fine structure (EXAFS) experiment, are not free of additional ligands.12,13 However, the model studies performed so far did not lead to a successful identification of these ligands. The interaction of hydrogen with iridium clusters containing few metal atoms is interesting not only with respect to the heterogeneous catalysis but also in connection with the possibility for hydrogen storage in complexes with molecular ligands. For example, up to eight hydride species were found to be coordinated to a tetrahedral Ir4 moiety in molecular complexes with phosphorus-containing ligands and CO in solution.5a For free Ir4 clusters, two types of hydride coordination have been suggested on the basis of density functional modeling: on-top positions at iridium atoms and bridge positions at Ir-Ir bonds.15 The present work reports density functional investigations of hydrogen adsorption (up to six H2 molecules) on tetrahedral

10.1021/jp074001q CCC: $37.00 © 2007 American Chemical Society Published on Web 09/12/2007

Hydrogen Adsorption on Ir4 Supported on Zeolites

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TABLE 1: Calculated Energy Characteristics (in kJ/mol) of Optimized Structures, Obtained by Hydrogen Adsorption on Ir4 Clusters in the Gas Phase and Supported on Zeolite: Ir4Hx and Ir4Hx/zeo(nH) (x ) 0, 3, 6, 9, 12; n ) 0 or 3) gas phase

hydroxylated support

dehydroxylated support

Ir4H3 Ir4H6 Ir4H9 Ir4H12 Ir4/zeo(3H) Ir4H3/zeo(3H) Ir4H6/zeo(3H) Ir4H9/zeo(3H) Ir4H12/zeo(3H) Ir4/zeo Ir4H3/zeo Ir4H6/zeo Ir4H9/zeo Ir4H12/zeo

Erela

EDAb

-182 -475 -627 -847 -231 -439 -586 -665 -863 -430 -646 -782 -899 -914

-61 -79 -70 -71 -69 -59 -48 -53 -72 -59 -52 -41

Eads[Ir4Hx]c

-231 -257 -111 -38 -16

Ehydd

66 69 65 78 51

ERSe

-138 -114 -104 -83

Nsf 3 0 1 0 0 3 0 1 0 3 0 3 0 1

a Relative energy of the structures (see text). b Energy for dissociative adsorption of hydrogen molecules from the gas phase (per adsorbed hydrogen atom). c Adsorption energy of the bare or hydrogen-covered metal cluster, Ir4Hx (x ) 0, 3, 6, 9, 12), on the hydroxylated zeolite fragment. d Energy of hydroxylation of the zeolite fragment (per OH group formed). e Energy of reverse hydrogen spillover (per transferred hydrogen). f Number of unpaired electrons in the complex.

Ir4 clusters, supported on hydroxylated and dehydroxylated zeolite. For comparison, we will also discuss hydrogen adsorption on gas-phase tetrairidium species. We will analyze various aspects of the reaction energetics of these systems and compare them for clusters supported on surfaces of hydroxylated or dehydroxylated substrates. Following the variation of the average metal-metal distance in the iridium species with hydrogen coverage, we also suggest a rationalization of the problem regarding the large difference between experimental and calculated Ir-Ir distances for Ir4/zeolite systems, tackled for the first time by Ferrari et al.13 2. Computational Details 2.1. Method. All calculations were carried out with the linear combination of Gaussian-type orbitals fitting functions density functional method (LCGTO-FF-DF)16,17 as implemented in the program PARAGAUSS.18,19 We employed the gradient-corrected exchange-correlation functional suggested by Becke (exchange) and Perdew (correlation) (BP).20 The calculations were performed with a scalar relativistic variant of the LCGTO-FF-DF method as the relativistic effects were described by explicitly treating all electrons with the Douglas-Kroll-Hess approach of second order.17,21,22 Where appropriate, unrestricted KohnSham calculations were carried out; see Table 1. The KohnSham orbitals were represented by Gaussian-type basis sets, contracted in generalized form: (6s1p) f [4s1p] for H,23a (9s5p1d) f [5s4p1d] for O,23a,b (9s5p1d) f [5s4p1d] for C,23a (12s9p1d) f [6s4p1d] for Al and Si.23b,c The original basis set for Ir24 was extended by two s (0.0117, 0.2222), three p (0.02938, 0.07345, 0.18363), two d (0.05639, 0.14097), and two f (0.16792, 0.67168) exponents, and the resulting basis set (21s17p12d7f) was contracted in generalized form to [9s8p6d4f] as described earlier.25 The auxiliary basis set, used in the LCGTO-FF-DF method to represent the Hartree part of the electron-electron interaction, was derived from the orbital basis set in a standard fashion.16 On each atom except hydrogen centers, five p- and five d-type polarization exponents were supplemented, constructed as geometric series with a factor 2.5, starting with 0.1 and 0.2 au for p- and d-exponents, respectively. Only the p-type series was added for hydrogen centers. The geometry of the model clusters was optimized,26 imposing C3 symmetry constraints. The only exception is the doublet Ir4H9 cluster in the gas phase which was at first optimized in C3 symmetry. However, the cluster has the electronic config-

uration e1, i.e., due to the applied symmetry restrictions the HOMO is partially occupied, and thus the structure will feature a Jahn-Teller distortion of first order. The degeneracy is removed when the symmetry constraints are completely released (C1). This reoptimized structure is quite similar to that obtained in C3 symmetry but 113 kJ/mol more stable. In the following, we will present and discuss only the C1 structure of Ir4H9. All open-shell systems were checked for spin contamination of the Kohn-Sham determinant; it never exceeded 4%. Reported charges were obtained both by a Mulliken analysis and by fitting the electrostatic potential27 (potential derived charges s PDC). Core level binding energy shifts for the subvalence 5s shell of Ir atoms were estimated as differences of calculated Kohn-Sham orbital energies with respect to the corresponding core level energy of free tetrahedral Ir4 cluster (C3 symmetry). A positive value of the shift corresponds to a stabilization of core levels relative to the isolated cluster. 2.2. Models. The zeolite support was represented by a sixring fragment of faujasite structure denoted as zeo(3H) (Figure 1). The six T-atoms of this cluster model are Al and Si centers in alternating sequence, according to the Lo¨wenstein rule.28 Similar models have been used in our previous studies on zeolites containing metal clusters, cations, or metal complexes.9-11,13,29 The free valences of Si and Al centers are saturated by OH groups on the side of the supported metal cluster and by hydrogen atoms on the opposite side. The structures of the supported clusters and the zeolite fragment were optimized while constraining the distances between the T-atoms to crystallographically determined values30 and fixing the positions of the saturating H and OH groups; for details, see ref 31. The excess of negative charge on the fragment due to the three Al atoms is compensated by protons which form three bridging OH groups of the hydroxylated support model, zeo(3H). The dehydroxylated form of the support, zeo, does not have bridging OH groups, and thus the zeolite fragment carries a negative charge of -3 e; consequently, all adsorbed species Ir4Hx on such a support are positively charged, +3 e. Initial structures of the complexes Ir4Hx, adsorbed on a dehydroxylated fragment, were constructed by removing 3 H from the complex Ir4H3/zeo or adding (x - 3) H to Ir4H3/zeo (x ) 6, 9, 12).12 We modeled only the tetrahedral structure of the Ir4 moiety12,13 as EXAFS yields 2.9-3.3 ((20%) closest Ir-Ir contacts.7 The alternative square-planar structure of Ir4, as nonet in the gas phase 49 kJ/mol more stable than the tetrahedral

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Petrova et al.

EDA ) Erel[Ir4Hx]/x, cluster in the gas phase EDA ) {Erel[Ir4Hx/zeo(3H)] - Erel[Ir4/zeo(3H)]}/x, hydroxylated support EDA ) {Erel[Ir4Hx/zeo] - Erel[Ir4/zeo]}/x, dehydroxylated support (ii) Adsorption energy of the metal cluster at the hydroxylated zeolite:

Eads[Ir4Hx] ) E[Ir4Hx/zeo(3H)] - E[zeo(3H)] - E[Ir4Hx] ) Erel[Ir4Hx/zeo(3H)] - Erel[Ir4Hx]; (where x ) 0-12) Figure 1. Designations of the atomic centers of the zeolite fragment and the metal cluster on the example of the complex Ir4/zeo(3H): OHs oxygen centers of bridging OH groups of the hydroxylated zeolite sixring; OMsoxygen centers of the zeolite six-ring that do not participate in bridging OH group of the hydroxylated zeolite fragment; Irzsmetal atom of the cluster Ir4 that interacts with oxygen centers of the zeolite support (for Ir4 in the gas phase, this label is used for the atoms at the base of the triangular pyramid); Irtstop atom of the tetrahedral metal cluster (for Ir4 in the gas phase this label is used for the metal atom at the apex of the triangular pyramid).

structure as singlet,15,32 is characterized by two Ir-Ir contacts. As check for the accuracy of our computational approach we optimized the geometry of the cluster Ir4(CO)12 in the gas phase; the structure of this complex is available both from EXAFS33 and X-ray diffraction (XRD).34 The bonding distances were calculated 1% longer than the XRD values (see Table S1 of the Supporting Information). To facilitate the subsequent discussion of structures, we will refer to the various centers of the zeolite fragment and of the iridium cluster with the labels shown in Figure 1. We estimated the relative stability of all complexes studied by calculating the relative energy Erel with respect to fragmentation into a bare tetrahedral Ir4 cluster in the gas phase, the adsorbate-free neutral zeolite fragment zeo(3H), and an appropriate number of H2 molecules in the gas phase:

Erel ) E[Ir4Hx/zeo(3H)] - E[zeo(3H)] - E[Ir4] (x/2)E[H2], hydroxylated support Erel ) E[Ir4Hx/zeo] - E[zeo(3H)] - E[Ir4] ((x - 3)/2)E[H2], dehydroxylated support Ir4Hx species in the gas phase can be included on the same scale if one considers them together with a separated neutral zeo(3H) fragment, at long, noninteracting distance from the metal cluster. Then, one has

Erel ) E[Ir4Hx] - E[Ir4] - (x/2)E[H2], cluster in the gas phase With the help of Erel (Table 1), one can directly identify the most stable structure of a given chemical composition, e.g., Ir4H6/zeo, Ir4H3/zeo(3H), and Ir4H3. Thus, in this comparison, one can also include the pertinent gas-phase species, keeping in mind that the remaining three H are in the OH groups of the remote zeo(3H) fragment. In addition, we will use the following energy characteristics (Table 1; see section 4.3): (i) Energy for dissociatiVe adsorption of hydrogen from the gas phase (per H atom):

(iii) Energy of hydroxylation of the zeolite support via direct adsorption of hydrogen from the gas phase (per OH group formed):

Ehyd ) {Erel[Ir4Hx/zeo(3H)] - Erel[Ir4Hx/zeo]}/3 (iv) Energy of reVerse hydrogen spilloVer from bridging OH groups onto the supported cluster (per transferred proton):12

ERS ) {Erel[Ir4Hx+3/zeo] - Erel[Ir4Hx/zeo(3H)]}/3 A negative value of any of these energy characteristics implies that the final state is more stable than the initial state. Finite models of zeolite support9-11 yield only approximate energetics, even though structures of adsorption complexes are satisfactorily described. Very recently, using a sophisticated description of an extended zeolite support, hydrogenated supported Rh6 and Ir6 clusters on dehydroxylated zeolite support were confirmed to be energetically favored over the corresponding bare clusters on a hydroxylated support; however, the energy preference was reduced compared to the results from isolated cluster models.35 3. Results In the following subsections we present the results on structure and stability of the clusters Ir4Hx in the gas phase and on zeolite support, for various hydrogen loading x ) 0-12. We first describe clusters in the gas phase, then clusters supported on a hydroxylated (neutral) zeolite surface, and on a dehydroxylated surface (which is negatively charged). 3.1. Clusters in the Gas Phase. Bare and hydrogenated Ir4 clusters in the gas phase were modeled as reference for evaluating various energy characteristics just introduced. The initial structures were taken from the corresponding optimized complexes Ir4Hx/zeo (x ) 3-12). Dissociative adsorption of up to six hydrogen molecules from the gas phase was calculated to be energetically favorable; the relative energy Erel (stabilization) of the hydrogenated complexes decreases to -847 kJ/mol for x ) 12 (Table 1). In the upper row of Figure 2 we present the corresponding optimized structures. As reported earlier,15a the Ir-Ir distances of bare tetrahedral (triangular-pyramidal) Ir4 are 248 pm. In the complex Ir4H3 the three hydrogen atoms are coordinated at IrzIrt edges of the pyramid in a distorted bridge mode, with Irz-H ) 170 pm and Irt-H ) 183 pm (Table 2). This type of bonding is similar to the coordination of one H atom at tetrahedral Ir4 and to some of the structures with two H at Ir4.15 In the complexes with increasing hydrogen loading, the next three adsorbates are coordinated on-top of Irt, to be followed by one, then two H atoms at each Irz atom (Figure 2a). Ir-H distances

Hydrogen Adsorption on Ir4 Supported on Zeolites

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Figure 2. Sketches of optimized structures of bare and hydrogenated Ir4 clusters (a) in the gas phase as well supported on (b) a hydroxylated or (c) a dehydroxylated zeolite fragment.

TABLE 2: Interatomic Distances (in pm) of Optimized Structures, Obtained by Hydrogen Adsorption on Ir4 Clusters in the Gas Phase and Supported on Zeolite gas phased

hydroxylated support

dehydroxylated support

Ir4H3 Ir4H6 Ir4H9 Ir4H12 Ir4/zeo(3H) Ir4H3/zeo(3H) Ir4H6/zeo(3H) Ir4H9/zeo(3H) Ir4H12/zeo(3H) Ir4/zeo Ir4H3/zeo Ir4H6/zeo Ir4H9/zeo Ir4H12/zeo

Irz-Irza

Irz-Irta

〈Ir-Ir〉b

245 245 255e 268 249 255 251 261 268 246 243 245 252 267

260 268 270e 269 247 253 271 274 271 247 254 269 274 278

253 257 263 269 248 254 261 268 270 247 249 257 263 273

Irz-OMa

Irz-OHa

Irz-Si

Irz-Al

Ir-Hc 170/183 168/190, 158t 178/182, 158t, 162z, 168z 165/190, 159t, 159z, 162z

218 208 229 223 240 211 207 212 212 212

329 338 325 329 352 228 245 225 226 334

271 267 278 273 298 266 271 267 266 322

318 286 338 327 340 338 332 339 336 305

168/185 168/189,159t 171/183, 159t, 153z 167/185, 160t, 154z, 159z 168/188 177/177, 160t 172/184, 159t, 153z 165/190, 158t, 159z, 160z

a For the labels of the various atomic centers, see Figure 1. b Average Ir-Ir distance. c Ir-H distances for H adsorption at a distorted bridge site are given as pairs of values Irz-H/Irt-H. Subscripts t and z denote distances of H atoms coordinated on-top at Irt and Irz atoms, respectively. d The Ir-Ir distance of a free tetrahedral Ir4 cluster was calculated at 248 pm, ref 15. e The reported distances are averaged over all atoms of the corresponding type as the cluster Ir4H9 was optimized without symmetry constraints (C1 symmetry).

vary in the range of 158-168 pm for on-top positions, 165178 for shorter, and 182-190 pm for longer bridge coordination. All these values fall into the ranges of Ir-H distances for coordination of one or two hydrogen atoms at the cluster Ir4 in the gas phase, as obtained in previous DF studies with the same exchange-correlation functional and the same basis set.15 Coordination of H atoms at Irz-Irt edges elongates the Irz-Irt distances with respect to the bare Ir4 cluster: by 12 pm in Ir4H3 and by 20-22 pm in the complexes with larger hydrogen content. Despite a slight contraction of the base of the triangular pyramid in the complexes Ir4H3 and Ir4H6, their average distances 〈Ir-Ir〉 are elongated by 4-8 pm. The 〈Ir-Ir〉 distances of these model clusters increase regularly with the hydrogen loading (Figure 3). In a molecular complex of Ir4 with four phosphoruscontaining and four CO ligands,5a two types of hydride coordination were found by refining X-ray data, on-top with Ir-H distances of about 155 ( 4 pm and bridge positions with Ir-H distances of about 175 ( 4 pm, very similar to our results. In addition, hydride species in bridge position also resulted in elongation of the corresponding Ir-Ir bonds by 20 pm.

3.2. Clusters on Hydroxylated Zeolite Fragment. This series of models (second row of Figure 2) also includes structures with up to 12 hydrogen atoms on the metal cluster. The first complex of the series was obtained via adsorption of bare Ir4 at the hydroxylated zeolite fragment. In the optimized structure each Irz atom is bound to one OM center; the other oxygen centers of the zeolite six-ring participate in bridging OH groups, as in all structures with a hydroxylated zeolite fragment. Adsorption is energetically favorable, Eads[Ir4] ) -231 kJ/mol (Table 1). Dissociative adsorption of 1.5H2 from the gas phase on the zeolite-supported bare Ir4 cluster results in the formation of the stable complex Ir4H3/zeo(3H) in a quartet state with Erel ) -439 kJ/mol. The complex adsorbs at the support with E[Ir4H3] ) -257 kJ/mol, i.e., more strongly than bare Ir4, likely due to stronger electrostatic interaction between the zeolite OM centers and the Irz atoms which are partially oxidized in the ligated cluster (Table S2 of the Supporting Information). In fact, the Irz-OM distances are 10 pm shorter than in the complex Ir4/zeo(3H). Subsequent increase of the hydrogen coverage up to 12 hydrogen atoms results in a continuous reduction of the adsorption interaction, to finally Eads[Ir4H12] ) -16 kJ/mol. The

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Figure 3. Average Ir-Ir distance in Ir4Hx complexes as a function of the number of hydrogen atoms coordinated to the metal moiety: clusters in the gas phase (triangles; y ) 247.8 + 1.71x, correlation coefficient ) 0.997); clusters on hydroxylated support (squares; y ) 248.6 + 1.93x, correlation coefficient ) 0.988); clusters on dehydroxylated support (circles; y ) 244.6 + 2.20x, correlation coefficient ) 0.981). Straight line from fitting simultaneously all data shown: y ) 247.1 + 1.93x, correlation coefficient ) 0.978 (not shown). The experimental EXAFS interval according to Gates and co-workers (ref 7) for Ir4 clusters in zeolites is also indicated. Filled triangles correspond to Ir4H and Ir4H2 species in the gas phase; from ref 15.

distance between the hydrogenated cluster and the support increases concomitantly. In the complex Ir4H12/zeo(3H), IrzOM ) 240 pm, i.e., more than 10 pm longer than the corresponding distances of the complexes with zero to nine ligands, 208-229 pm (Table 2). By construction, hydrogen atoms in the models of supported clusters Ir4Hx/zeo(3H), x ) 3-12, are similarly located as in the corresponding models of clusters in gas phase: three symmetry equivalent hydrogen atoms at distorted Irz-Irt bridge positions, three hydrogen atoms on-top at Irt, and the rest coordinated “on-top” at Irz atoms (Figure 2, Table 2). The only notable difference is the contraction of one of the Irz-H distances of supported Ir4H9, by ∼10 pm compared to the corresponding complex in the gas phase. As already noted for clusters in the gas phase, Ir-Ir bonds of Ir4Hx/zeo(3H) with bridge coordinated H atoms are elongated, by 6-17 pm compared to bare free or supported Ir4. In addition, due to the interaction with zeolite oxygen centers, Irz-Irz bonds are 5-9 pm longer than in Ir4Hx. Similarly to the gas-phase clusters, the average Ir-Ir distance increases with the hydrogen loading of the cluster. 3.3. Clusters on Dehydroxylated Zeolite Fragment. As the dehydroxylated zeolite fragment has no bridging OH groups, all oxygen centers of the six-ring are available for interaction with the metal atoms of a supported cluster. In addition, the formal charges of the Ir4Hx moiety and the zeolite fragment of the overall neutral complexes Ir4Hx/zeo are 3 e and -3 e, respectively. As a result, the metal cluster in most of the complexes (bottom row of Figure 2) is oriented in such a way that Irz atoms are able to interact with both types of oxygen centers of the six-ring, OM and OH, i.e., each Irz atom participates in two Ir-O bonds. The complex Ir4H12/zeo is an exception, because each of its Irz centers interacts with three hydrogen ligands; bonding competition disfavors interaction of Irz with a second zeolite oxygen. Irz-OM distances of complexes Ir4Hx/ zeo vary in a small range, 207-212 pm, whereas Irz-OH distances are 245 pm in Ir4H3/zeo and 225-228 pm in the other complexes with two types of Irz-O bonds. The initial complex of this series, Ir4/zeo (Figure 2, Tables 1 and 2), was optimized after removal of three hydrogen atoms from the complex Ir4H3/zeo.12 The latter complex was produced via reverse spillover of hydrogen from the zeolite OH groups of Ir4/zeo(3H).12 With Erel[Ir4/zeo] ) -430 kJ/mol and

Petrova et al. Erel[Ir4H3/zeo] ) -646 kJ/mol, desorption of the three hydrogen atoms from supported Ir4H3 is disfavored. Yet, removal of hydrogen from the OH groups of the complex Ir4/zeo(3H) is energetically favored, by 199 kJ/mol, because Erel[Ir4/zeo(3H)] ) -231 kJ/mol (Table 1). Adsorption of hydrogen from the gas phase on Ir4H3/zeo results in series of increasingly more stable complexes, up to 12 hydrogen atoms coordinated to the metal moiety, as the relative energy of the modeled structures increases to Erel[Ir4Hx/zeo] ) -782, -899, and -914 kJ/mol for x ) 6, 9, and 12, respectively. In all cluster models, the hydrogen ligands are coordinated in a similar way as in the corresponding clusters in the gas phase and on a hydroxylated surface, with Ir-H distances in the range of 153-160 pm for on-top positions, as well as 165-177 pm and 177-190 pm for shorter and longer bonds at bridge positions. Average Ir-Ir distances of Ir4Hx/zeo clusters are comparable to those obtained for the other two series of Ir4Hx clusters. The value 〈Ir-Ir〉 ) 273 pm of the complex Ir4H12/ zeo is the largest average calculated for all structures studied. 4. Discussion 4.1. Variation of the Distances in the Clusters and Comparison with EXAFS. We now will analyze how the interatomic distances 〈Ir-Ir〉 and Ir-O vary in the optimized structures with presence and type of the support as well as hydrogen loading. In the three series of model clusters Ir4Hx, in the gas phase and supported on hydroxylated and dehydroxylated zeolite fragments, the 〈Ir-Ir〉 distances depend linearly on the number of H atoms (Figure 3; correlation coefficients 0.981-0.997). The series of gas-phase cluster contains two further values from previous studies,15 which represent tetrahedral Ir4 with one and two adsorbed hydrogen atoms.36 Both values fit perfectly the straight line determined for the gas-phase clusters modeled in the present work (with the same correlation coefficient). Most importantly, the 〈Ir-Ir〉 values of all three series of complexes also can be fit very well by a single straight line (Figure 3; correlation coefficient 0.978). Thus, the elongation of 〈Ir-Ir〉 with the hydrogen loading of the metal moiety is hardly affected by the presence of the support and its nature, hydroxylated or dehydroxylated. The observation that deposition on oxide support does not cause substantial changes in the structure of metal clusters was already made in previous computational37 and experimental38 studies. Here we showed in addition that the state of the support and the formal charge of the adsorbed moieties have little effect on the average metalmetal distance of bare or hydrogenated clusters. From the joint fit of all data of the present work (Figure 3), one deduces that adsorption of one additional hydrogen atom on Ir4 cluster results in an elongation of 〈Ir-Ir〉 by 1.9 pm. As done previously,12,13 we compare optimized distances of the structures studied with values derived from EXAFS measurements on Ir4/NaY.7 The range of these experimental Ir-Ir distances, 266-272 ((1%), is also shown in Figure 3. Only complexes with 9 or 12 adsorbed hydrogen atoms of the present study, either in the gas phase or on support, feature average metal-metal distances in that interval. Also Ir-O contacts permit a comparison with experimental results7 where the short Ir-O distances were reported as 210220 ((10%) pm, associated with average coordination numbers of the corresponding O centers between 0.8 and 2.2 ((30%). Such extended range for Ir-O contacts prevents a clear answer whether the iridium atoms of the “bottom” triangle of the tetrahedron observed in the experiments interact with one or

Hydrogen Adsorption on Ir4 Supported on Zeolites

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two oxygen centers of the support. Our computational models cover both situations: s Irz atoms of all complexes on hydroxylated zeolite and of Ir4H12 on dehydroxylated zeolite form one bond to oxygen centers of the zeolite support (average Ir-O coordination number 0.75); s Irz atoms of all clusters on dehydroxylated zeolite, except Ir4H12, form two Irz-O bonds (average Ir-O coordination number 1.50). The shortest Irz-OM distances of complexes on a dehydroxylated surface were calculated at 207-212 pm, whereas in complexes on a hydroxylated surface these distances are longer, 208-240 pm. The latter value corresponds to the complex Ir4H12/zeo(3H) which features the lowest adsorption energy (Table 1). Average Ir-O distances of structures with two Ir-O contacts per Irz atom are 218-226 pm, which also fits the EXAFS results. Comparison between experimental structure parameters and optimized species suggests that the distance of 261-280 ((20%) pm from EXAFS assigned to an Ir-O distance of farther oxygen centers (Olong),7 likely corresponds to Ir-Si distances (Table 2), which in our calculations are estimated at 266-278 pm for the model complexes with up to nine hydride ligands, either on a hydroxylated or a dehydroxylated surface. According to our calculations, the distance between Irz atoms and farther oxygen centers (not considered in the first oxygen coordination sphere of Ir) is above 300 pm. In summary: 〈Ir-Ir〉 values of Ir4 models with 9 and 12 H ligands on hydroxylated and dehydroxylated support are compatible with the corresponding EXAFS derived distances. Thus, at variance with Rh6, where we had suggested9,10 three H ligands on the zeolite-supported clusters examined by EXAFS,39 we are lead to conclude that supported Ir4 clusters examined by EXAFS1,7 carry a much larger load of hydrogen.8 The short Ir-O distances and coordination numbers of all supported complexes, except Ir4H12/zeo(3H), fit the corresponding EXAFS results. 4.2. Oxidation State of the Metal Cluster. Next, we analyze chemical changes in the metal particle that result from dissociative adsorption of H2 or deposition of the cluster on a hydroxylated or a dehydroxylated surface. For this purpose, we will discuss Mulliken as well as potential derived atomic charges together with shifts ∆E(Ir5s) of Ir 5s core level binding energies (see Table S2 of the Supporting Information). Dissociative adsorption of H2 as well as hydrogen transfer from the zeolite support onto the metal cluster (reverse spillover) lead to oxidation of the metal centers because hydride species are formed. Adsorption of each hydrogen atom originating from gas-phase H2 increases the formal charge of the Ir4 moiety by 1 e. For clusters in the gas phase and on hydroxylated support one has

Ir4 + (x/2)H2 f Ir4x+(H-)x

(1)

whereas for clusters on dehydroxylated support the formal charge of the Ir4 moiety is +3 e larger:

Ir43+/zeo3- + (x/2)H2 f Ir43+x(H-)x/zeo3-

(2)

Indeed, for clusters in the gas phase, potential derived [and Mulliken] charges indicate a gradual oxidation of the metal moiety with increasing hydrogen loading; the total charges of the Ir4 unit increase from 1.29 to 2.05 e [0.07-0.39 e] (see Table S2 and Figure S1 of the Supporting Information).

Figure 4. Relative energy, Erel, of the systems studied as a function of the total number of hydrogen atoms in the system Ir4Hm/zeo(nH), x ) m + n ) 0-15. Clusters in the gas phase (triangles; Erel ) 201.1 70.25x, correlation coefficient ) 0.996); clusters on hydroxylated support (squares; Erel ) -109.8 - 49.67x, correlation coefficient ) 0.990); clusters on dehydroxylated support (circles; Erel ) -429.9 80.80x + 3.34x2, correlation coefficient ) 0.998). Filled triangles represent data for Ir4H and Ir4H2 in the gas phase; from ref 15.

Unfortunately, neither charge analysis provides a similarly clear trend for supported Ir4 clusters. However, as indicated by eqs 1 and 2, the charges of the Ir4 unit increase with the hydrogen loading, and they are larger for the cluster on a dehydroxylated support than for the corresponding cluster on a hydroxylated surface. Indeed, the total charge of the Ir4Hx complexes on a dehydroxylated support are 0.64 ( 0.09 [1.33 ( 0.12] e higher than the charge of the corresponding complexes on a hydroxylated surface and 1.24 ( 0.07 [1.61 ( 0.04] e larger than the Ir4Hx complexes in the gas phase (Table S2 of the Supporting Information). The shifts of the Ir 5s core levels also reflect these changes of the charge on the metal moiety; the core levels of the complexes Ir4Hx/zeo on average are stabilized 0.78-2.04 eV more than in corresponding clusters in the gas phase and 1.38-2.00 eV more than in corresponding clusters adsorbed on hydroxylated support (Table S2). Similarly to observations in recent studies of zeolite-supported M6 clusters,11,35 Irz atoms (in direct contact with the support) are most strongly oxidized according to calculated charges and estimated Ir 5s core level shifts. This holds in particular for clusters on dehydroxylated support. From detailed characterization and catalytic experiments on hydrocarbon conversion Sachtler and co-workers suggested that metal species supported in zeolites exist as metal-proton adducts, which leads to the formation of electron-deficient metal clusters.3 Our present results on partial oxidation of the metal moiety of Ir4Hx adducts support this earlier hypothesis. 4.3. Energetics. The relatiVe energies Erel of the clusters in the gas phase and on support (section 2.2) allow us to assess the relative stability of different forms of complexes with the same composition. Inspection of Table 1 shows that the energy of the complexes depends on the number of hydrogen atoms in the system, both coordinated at the metal cluster and involved in bridging OH groups of the zeolite fragment. Thus, for clusters in the gas phase or adsorbed on a hydroxylated surface one counts three protons of zeolite OH groups to total number of hydrogen atoms of the system, whereas for complexes on a dehydroxylated surface one counts only the hydrogen species on the metal cluster. Figure 4 shows how Erel of the various complexes depends on the total number of hydrogen atoms in the system (counted as explained above). For the clusters Ir4Hx in the gas phase, Erel correlates linearly with the total number of hydrogen atoms (Figure 4; correlation coefficient 0.996). As in the analysis of average Ir-Ir distances (Figure 3), the corresponding energies for tetrahedral Ir4 in the gas phase with one and two hydrogen

14490 J. Phys. Chem. C, Vol. 111, No. 39, 2007 impurities15,40 also reflect this linear correlation.41 The slope of the straight line provides an average value of the energy EDA for dissociatiVe adsorption of H2 on the cluster (per H atom). Thus, addition of one hydrogen atom from an H2 molecule in the gas phase stabilizes the system on average by -70 kJ/mol. Similarly, the relative energies of the complexes Ir4Hx supported on a hydroxylated surface also correlated in linear fashion with the total number of hydrogen atoms in the system (Figure 4; correlation coefficient 0.990). The average energy of dissociative adsorption of H2, EDA, now is smaller, only -50 kJ/mol; in other words, clusters on the hydroxylated support bind hydrogen somewhat less strongly than the corresponding clusters in the gas phase. The difference of Erel[Ir4Hx] between systems in the gas phase and on hydroxylated surface, i.e., the adsorption energy of clusters Ir4Hx on the zeo(3H) fragment, decreases steadily with hydrogen loading x, from -257 kJ/mol for x ) 3 to only -16 kJ/mol for x ) 12 (Table 1). The relative energies of clusters supported on a dehydroxylated fragment are better represented by a second-order polynomial in the total number of hydrogen atoms of the system (Figure 4; correlation coefficient 0.998). This shape of the curve reflects the decrease of the energy of dissociative adsorption of hydrogen with increasing hydrogen loading of the supported metal moiety. Initially, up to Ir4H6, the system is stabilized by 60-70 kJ/mol per hydrogen atom from H2 in the gas phase, similar to Ir4Hx clusters in the gas phase. However, for the complexes Ir4H9/zeo and Ir4H12/zeo, this energy gain saturates because “new” hydrogen ligands are added close to the zeolite fragment. (Steric constraints of clusters with high hydrogen loading are more pronounced on the dehydroxylated surface because in these complexes the metal-oxygen distances are by 11-28 pm shorter than in the corresponding complexes on the hydroxylated fragment.) In particular, for the complex Ir4H12/ zeo steric constraints result in rotation of the hydrogenated iridium moiety and Irz-OH bonds break, i.e., each Irz atom of this complex is bound only to one oxygen center of the zeolite fragment. Therefore, the complex Ir4H12/zeo is only -15 kJ/ mol more stable than the preceding complex of this series, Ir4H9/ zeo, where each Irz atom forms two Irz-O bonds. Further hydrogen loading of the cluster beyond 12 atoms is expected to destabilize the adsorption complexes. Comparing relative energies of species with the same total composition (Table 1, Figure 4), one notes that clusters in the gas phase are the least stable species (for x ) 3-12), whereas the complexes adsorbed on a dehydroxylated support are most stable. However, extrapolation of the fitted curves in Figure 4 suggests that all systems with formally 15 hydrogen atoms (not modeled here) should have similar stability: Ir4H12, Ir4H12/zeo(3H), and Ir4H15/zeo. For even higher hydrogen loading, one expects the cluster in the gas phase to be more stable than either supported complex. The resulting energies EDA for dissociative adsorption of hydrogen molecules on bare Ir4 clusters in the gas phase or on support, -41 to -79 kJ/mol per hydrogen atom, are somewhat smaller than the corresponding energies calculated for the gasphase clusters Ir4H and Ir4H2 of similar structure, -75 and -90 kJ/mol, respectively.15,40 For the surface Ir(111), EDA was calculated at -60 kJ/mol per H,42 and the corresponding experimental value (for low hydrogen coverage) is -53 kJ/mol per hydrogen atom.43 Both values are similar to our estimates for hydrogen adsorption on supported Ir4 clusters. The relative energies of the various species allow us also to trace (depending on H coverage) the energies ERS of reverse hydrogen spillover and the energies Ehyd of hydroxylation of

Petrova et al. the dehydroxylated zeolite fragment by H2 from the gas phase. The corresponding values are collected in Table 1. The reVerse spilloVer of hydrogen from the zeolite OH groups onto the metal cluster is favorable not only for bare supported Ir4, Ir4/zeo(3H) f Ir4H3/zeo, but also when the iridium cluster supported on the hydroxylated surface is already hydrogenated, e.g., Ir4H9/zeo(3H) f Ir4H12/zeo. However, not unexpectedly, the corresponding energies ERS decrease steadily with the number of hydrogen ligands on the supported iridium cluster, from -138 kJ/mol per transferred hydrogen for the process Ir4/ zeo(3H) f Ir4H3/zeo to -83 kJ/mol per transferred hydrogen for the process Ir4H9/zeo(3H) f Ir4H12/zeo (Table 1). Whereas dissociative adsorption of hydrogen from the gas phase on the metal cluster is favorable (EDA < 0), adsorption of H2 on the dehydroxylated zeolite fragment leading to the formation of hydroxylated surfaces is disfavored (Ehyd > 0) for all studied Ir4Hx/zeo complexes. The energy per hydrogen required to form a new zeolite OH group varies between 51 and 78 kJ/mol (Table 1). 5. Conclusion With density functional calculations, we quantified successive adsorption of hydrogen from the gas phase on Ir4 clusters as isolated species or supported either on a hydroxylated or a dehydroxylated zeolite surface, described by a six-ring. The model of the dehydroxylated surface was derived from that of the hydroxylated configuration by transfer of three protons from bridging OH groups of the zeolite ring onto the adsorbed Ir4 cluster. This process, also referred to as reverse hydrogen spillover, lead to a partial oxidation of the metal cluster and a negative charge on the zeolite fragment. In a preceding study,12 we had demonstrated this reverse hydrogen spillover onto bare supported Ir4 to be energetically favorable. In this work, we extended this exploration to show that this process is also favorable for supported iridium moieties Ir4Hx, which already carry adsorbed hydride species. However, the energy gained decreases with the hydrogen loading on the cluster, from -138 kJ/mol per transferred proton for x ) 0 to -83 kJ/mol for x ) 9. In a recent study, using embedded cluster models of the extended zeolite support,35 we showed that the energetics of reverse hydrogen spillover is somewhat modified when the whole zeolite framework is taken into account, but the overall conclusions from isolated cluster models of supported iridium clusters remained unchanged. These results of our computational study suggest that adsorption of hydrogen from the gas phase on the small cluster Ir4, either in the gas phase or supported, is a favorable process up to at least 12 hydrogen atoms at the metal cluster, corresponding to a ratio H/Ir ) 3. Comparing the stability of three model states, in the gas phase as well as on a hydroxylated and dehydroxylated zeolite surface, for different amount of hydrogen loading on the metal moiety, we found that the preferred state of supported Ir4 are hydrogenated clusters, adsorbed on a dehydroxylated surface. Correlation of relative energies of the species as function of the total number of hydrogen atoms in the system suggests that adsorption of hydrogen at clusters in the gas phase affords an energy gain of about 70 kJ/mol per adsorbed atom, whereas the average energy for dissociative H2 adsorption on supported clusters is 40-50 kJ/mol per adsorbed hydrogen atom. Adsorption of hydrogen from the gas phase on free or supported Ir4 clusters results in an oxidation of the metal atoms. The total positive charge of the metal moiety increases from clusters in the gas phase, to clusters on hydroxylated support, to clusters on dehydroxylated support. We rationalized the result

Hydrogen Adsorption on Ir4 Supported on Zeolites that oxidation is strongest in the latter types of systems with the higher formal charge of the Ir4Hx moiety in these systems, by +3 e, which is required to compensate the negative charge of the dehydroxylated zeolite fragment. These trends were corroborated by the larger stabilization of the core levels of the metal atoms in complexes on dehydroxylated zeolite, compared to the corresponding species in the gas phase or on hydroxylated support. The structural characteristics of the metal moiety in the three series of model complexes suggest that the average Ir-Ir distances are almost independent of the presence of a support or its type (hydroxylated or dehydroxylated) as well as of the formal charge of the adsorbed moieties. Rather, the average IrIr distance of the cluster models increases with the number of hydrogen atoms adsorbed on the metal moiety, by about 1.9 pm per adsorbed hydrogen atom. The calculated average Ir-Ir distances of the clusters coincide with the experimental values derived from EXAFS only for the complexes with a large amount of hydrogen atoms adsorbed on the metal moiety, at least nine hydride centers. Experimental and calculated values of other structural criteria, Ir-O distances and the corresponding coordination numbers, agree for all supported complexes, except Ir4H12 on a hydroxylated zeolite surface. On the basis of available structural information from EXAFS and our various computational studies, we suggest that the tetrairidium species in Y zeolite, produced experimentally and investigated by EXAFS,7 correspond to supported hydrogenated moieties Ir4Hx containing 9-12 hydrogen hydride centers. Our computational result agrees well with experimental findings44 on the high hydrogen loading of supported iridium clusters, up to an atomic ratio R(H/Ir) ) 2.7. This particularly large amount of adsorbed hydrogen was specific to iridium clusters, as supported clusters of other transition metals showed notably lower maximum loading, e.g., R(H/Rh) ) 2.0 and R(H/ Pt) ) 1.2.44 According to the analysis on the basis of relative stabilities of the model complexes, part of these hydrogen atoms are transferred to the metal cluster from bridging zeolite OH groups in vicinity of the cluster. Therefore, the surrounding zeolite framework is dehydroxylated. Further hydrogen loading has to come from the gas phase, e.g., during preparation of the samples.1,7,8 Acknowledgment. This work was supported by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie (Germany), and the National Science Fund (Bulgaria). Supporting Information Available: Comparison of the optimized and experimental (XRD and EXAFS) structure of the Ir4(CO)12 cluster in the gas phase, table with calculated electronic characteristics of the clusters, and a figure that shows how the calculated total charges of the Ir4 moiety of the Ir4Hx clusters in the gas phase depend on the hydrogen content x. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Argo, A. M.; Gates, B. C. Langmuir 2002, 18, 2152. (b) Argo, A. M.; Odzak, J. F.; Gates, B. C. J. Am. Chem. Soc. 2003, 125, 7107. (c) Alexeev, O. S.; Li., F.; Amiridis, M. D.; Gates, B. C. J. Phys. Chem. B 2005, 109, 2338. (d) Argo, A. M.; Odzak, J. F.; Goellner, J. F.; Lai, F. S.; Xiao, F.-S.; Gates, B. C. J. Phys. Chem. B 2006, 110, 1775. (e) Alexeev, O.; Gates, B. C. Top. Catal. 2000, 10, 273. (2) (a) Wei, J.; Iglesia, E. Angew. Chem., Int. Ed. 2004, 43, 3685. (b) Wei, J.; Iglesia, E. Phys. Chem. Chem. Phys. 2004, 6, 3754.

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Petrova et al. (39) Weber, W. A.; Gates, B. C. J. Phys. Chem. B 1997, 101, 10423. (40) The energy values reported in refs 15a,b were recalculated with respect to the energy of the tetrahedral structure of the bare Ir4 cluster, to be comparable with energies of the present work. (41) According to the way of calculation of the total number of H in the system with gas-phase clusters assuming separate hydroxylated zeo(3H) fragment, the two points correspond to systems with four and five hydrogen atoms. (42) Faglioni, F.; Goddard, W. A., III. J. Chem. Phys. 2005, 122, 014704. Those calculations were performed with the different gradient-corrected functional (Perdew-Wang), using pseudopotentials and a plane-wave basis set. The EDA value quoted here was calculated from the adsorption energy of H atoms on Ir(111) at a coverage of one-third monolayer. (43) Christmann, K. Surf. Sci. Rep. 1988, 9, 1. (44) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J. Catal. 1987, 105, 26.