Saturation of Small Supported Metal Clusters by Adsorbed Hydrogen

Apr 16, 2010 - strongly affects the adsorption energy of hydrogen, and the optimum H/M ratio varies with the metal, being. ∼2 for Rh and ∼3 for Ir...
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J. Phys. Chem. C 2010, 114, 8500–8506

Saturation of Small Supported Metal Clusters by Adsorbed Hydrogen. A Computational Study on Tetrahedral Models of Rh4, Ir4, and Pt4 Petko St. Petkov,† Galina P. Petrova,† Georgi N. Vayssilov,*,† and Notker Ro¨sch*,‡ Faculty of Chemistry, UniVersity of Sofia, 1126 Sofia, Bulgaria, and Department Chemie & Catalysis Research Center, Technische UniVersita¨t Mu¨nchen, 85747 Garching, Germany ReceiVed: February 16, 2010

With density functional calculations, we explored the successive adsorption of hydrogen on tetrahedrally shaped zeolite-supported M4 clusters (M ) Rh, Ir, and Pt). Similarly to our earlier results for models of Ir4, hydrogen adsorption on Rh4 and Pt4 causes an increase of the metal-metal distances. The type of metal strongly affects the adsorption energy of hydrogen, and the optimum H/M ratio varies with the metal, being ∼2 for Rh and ∼3 for Ir and Pt. As judged by the core-level shifts and atomic charges, Rh and Pt clusters are oxidized through interactions with the support and the hydrogen ligands, similarly to our earlier findings for Ir4. 1. Introduction Rhodium, iridium, and platinum are among the most active catalysts in various hydrogenation and dehydrogenation reactions,1-4 as well as in methane conversion,5 CO oxidation,6 and other processes of industrial or ecological importance. To decrease the amount of metal required in catalyzed processes, catalysts based on thin supported layers of the metals or small metal clusters supported on various metal oxide supports have been developed in recent years. Such catalytic materials ensure a larger active surface, a better surface/bulk ratio, and additional active centers specifically at the surface. Knowledge of the detailed structural and electronic properties of such systems and better understanding of their catalytic activity and the mechanism of their action are crucial for improving existing catalysts or developing novel materials. Subnanosize clusters of rhodium, iridium, and platinum have been thoroughly studied by both experimental1-6 and theoretical7-10 approaches. To rationalize the catalytic role of such species in hydrogenation processes, it is important to understand the geometric and electronic structure of hydrogenated metal moieties and the influence of the support on H2 adsorption. In early studies of hydrogen adsorption on supported metal clusters of undefined nuclearity, Kip et al.11 found that particularly large amounts of hydrogen adsorbed on iridium clusters up to a ratio of H/Ir ) 2.68, whereas for supported clusters of rhodium and platinum, the maximum hydrogen uptake per metal atom was notably lower, 1.98 and 1.14, respectively. In a computational model study,12 we confirmed that small iridium clusters (Ir4) in a zeolite cage are able to adsorb up to three hydrogen atoms per metal atom. Here, we extend this computational study of hydrogen uptake to small platinum and rhodium clusters. We simulated the adsorption of up to six H2 molecules on tetrahedral M4 clusters, supported on a faujasite zeolite fragment with deprotonated bridging hydroxyl groups. For comparison, we also modeled hydrogen adsorption on the corresponding metal species in the * To whom correspondence should be addressed. E-mail: gnv@ chem.uni-sofia.bg (G.N.V.), [email protected] (N.R.). † University of Sofia. ‡ Technische Universita¨t Mu¨nchen.

gas phase. Our calculations on models suggest that the type of metal strongly influences the hydrogen loading of the cluster and the value of the corresponding adsorption energy. The results of this study are of interest in the context of industrial heterogeneous catalysis, as well as for hydrogen storage or the utilization of such transition-metal clusters as active components in fuel cells.13 2. Method All calculations were carried out with the linear combination of Gaussian-type orbitals fitting-functions density functional (LCGTO-FF-DF) method14,15 as implemented in the program PARAGAUSS.16,17 We employed the gradient-corrected exchangecorrelation functional suggested by Becke (exchange) and Perdew (correlation) (BP).18 The calculations were performed with a scalar relativistic variant of the LCGTO-FF-DF method; relativistic effects were described by explicitly treating all electrons with the Douglas-Kroll-Hess approach of second order.15,19,20 We carried out unrestricted Kohn-Sham (KS) calculations where appropriate. The KS orbitals were represented by Gaussian-type basis sets, contracted in generalized form: (6s1p) f [4s1p] for H,21a (9s5p1d) f [5s4p1d] for O,21a,b and (12s9p1d) f [6s4p1d] for Al and Si.21b,c The original basis sets22 for Rh and Pt atoms were extended and contracted as described earlier:23,24 (18s13p9d) f [7s6p4d] and (21s17p12d7f) f [9s8p6d4f], respectively. 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.14 On each atom except the hydrogen centers, five p- and five d-type polarization exponents were supplemented, constructed as geometric series with factors of 2.5, starting with 0.1 and 0.2 au for the p and d exponents, respectively. Only the p-type series was added on hydrogen centers. All open-shell systems were checked for spin contamination of the KS determinant; it never exceeded 4%. The structure of the model clusters was optimized,25 under the imposition of C3 symmetry constraints. With this symmetry restriction, some complexes have a partially occupied highest occupied molecular orbital (HOMO); therefore, their structures will undergo a Jahn-Teller distortion of first order. Such a degeneracy is removed when the symmetry constraints are

10.1021/jp1014274  2010 American Chemical Society Published on Web 04/16/2010

Hydrogen Saturation of Small Supported Metal Clusters completely released (C1 symmetry). Therefore, we reoptimized the structures of the highly hydrogenated zeolite-supported clusters Rh4H12/zeo and Pt4H12/zeo without symmetry constraints. In the following discussion, the energy of the hydrogenated species Pt4Hn, in the gas phase or supported on zeolite, will be estimated with respect to the C1 structure of Pt4. Structural characteristics of the complexes optimized without symmetry constraints and the corresponding energies are provided and briefly discussed in the Supporting Information (Table S1). Because the two zeolite-supported species Rh4H12/ zeo and Pt4H12/zeo exhibit open structures when optimized without symmetry constraints, we used the data for the tetrahedral C3 structures in the discussion below for consistency. Atomic charges (see Supporting Information) were obtained by fitting the electrostatic potential26 (potential-derived charges, PDCs). Shifts of core-level binding energies of the M 2p shells were estimated (with acceptable accuracy27) as changes in the KS orbital energies relative to the average of the KS energies of the corresponding free tetrahedral M4 cluster (C3 symmetry for Ir and Rh, C1 symmetry for Pt; see above). A positive value of the shifts corresponds to a stabilization of the core levels relative to the isolated reference clusters. We optimized the tetrahedral shapes of the zeolite-supported metal moieties, similarly to the Ir4 clusters for which this type of structure has been determined experimentally.3 In analogy to the faujasite six-ring models zeo(3H) used previously,12 the distances between zeolite T atoms were constrained to crystallographically obtained values;28 also, the relative positions of the preoptimized subunits >TH(OH) were kept fixed. The T atoms of the faujasite model were chosen as Al and Si centers in alternating sequence, according to the Lo¨wenstein rule.29 In the current model study, we employed a zeolite fragment, denoted as zeo, with deprotonated bridging hydroxyl groups, considered as the dehydroxylated form of the support; for a detailed description, see refs 12a and 12b. As in those previous works,12b,c we explored here model structures of the hydrogenated clusters with n ) 3, 6, 9, or 12 hydrogen atoms on the metal moiety. All model clusters and complexes M4Hn/zeo and M4Hn are neutral. We estimated the stability of all complexes by calculating the average energy, EDA, of dissociative adsorption of hydrogen (per adsorbed H) onto the corresponding bare cluster obtained from the total energies E of the bare and hydrogenated clusters

EDA(n) ) [E(M4Hn /zeo) - E(M4 /zeo) - n/2E(H2)]/n, supported cluster

EDA(n) ) [E(M4Hn) - E(M4) - n/2E(H2)]/n, cluster in the gas phase A negative value of this energy characteristic implies a favorable process in which the final state is more stable than the initial state. In the discussion, we also use the total energy for adsorption of all hydrogen species on the cluster, calculated as Eads ) nEDA. The designations of the two types of metal atoms (presented in Figure 1 for the example of the complex Rh4/zeo) are analogous to those applied earlier.12 Mz denotes a metal atom of the cluster M4 that interacts with oxygen centers of the zeolite support, and Mt refers to the atom at the apex of the triangular pyramid M4.

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Figure 1. Sketches of optimized structures of the zeolite-supported cluster complexes Rh4Hn/zeo and Pt4Hn/zeo.

3. Results and Discussion We present the results obtained for modeling the adsorption of hydrogen on zeolite-supported metal clusters in three parts: (1) the geometries of the hydrogenated metal clusters, (2) the stabilities of the optimized structures, and (3) alternations of the electronic structure of the metal moieties due to the coordination of hydrogen ligands on the metal moiety. Table 1 collects pertinent structural, energetic, and electronic characteristics. 3.1. Geometries of the Hydrogenated Complexes. In discussing the calculated structures of the hydrogenated clusters, we focus on two aspects: the locations of the hydride ligands and the influence of the hydrogen loading on the structure of the metal moiety. Figure 1 displays the optimized structures of the bare Rh4 and Pt4 clusters and their hydrogenated congeners supported on zeolite. As mentioned before, the subsequent adsorption of 3, 6, 9, and 12 H ligands on the metal clusters was modeled. The first three ligands were initially coordinated as bridging the three Mz-Mt bonds of the metal tetrahedron in a distorted fashion. The subsequent three H atoms were located at the top atom of the metal cluster, and the following two triads of ligands were positioned in terminal mode at the Mz atoms at the base of the cluster. These types of coordination of adsorbed hydrogen atoms were previously obtained for the energetically most stable structures of the corresponding supported clusters Ir4Hn.12b In general, the initial locations of the ligands were preserved during optimization. In only two cases were hydrogen atoms

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Petkov et al.

TABLE 1: Characteristics of Optimized Complexes from Density Functional Calculations on Hydrogen Adsorption at M4 Clusters in the Gas Phase and Supported on a Dehydroxylated Zeolite Fragment ∆EMe Rh4 Rh4H3 Rh4H6 Rh4H9 Rh4H12 Rh4/zeo Rh4H3/zeo Rh4H6/zeo Rh4H9/zeo Rh4H12/zeo Ir4g Ir4H3 Ir4H6 Ir4H9 Ir4H12 Ir4/zeo Ir4H3/zeo Ir4H6/zeo Ir4H9/zeo Ir4H12/zeo Pt4h Pt4H3 Pt4H6 Pt4H9 Pt4H12 Pt4/zeo Pt4H3/zeo Pt4H6/zeo Pt4H9/zeo Pt4H12/zeo

Nsa

EDAb

0 3 0 3 0 3 0 3 0 1 0 3 0 1 0 3 0 3 0 1 4 3 2 1 0 1 0 1 2 1

-34 -30 -38 -41 -72 -41 -38 -23 -61 -79 -70 -71 -72 -59 -52 -41 -57 -74 -52 -50 -62 -64 -52 -43

Edefc

5 12 53 125 17

2 14 18 188 25

17 0 24 52 197

〈M-M〉d 247 257 262 265 277 247 250 266 267 278 248 253 257 263 269 247 249 257 263 273 257 267 266 278 277 251 269 268 278 289

Mz-Od

M-Hd 164z/181t 166z/187t, 156t 175z/171t, 167z/179z, 157t 161z/184t, 158z/197t, 155t, 156z

212, 232 207, 256 210, 221 214, 228 214, 337

166z/180t 182z/182z/197t, 155t 162z/203t, 161z/204t, 155t 163z/187t, 157z/194z, 156t, 156z 170z/183t 168z/190t, 158t 178z/182t, 158t, 162z, 168z 165z/190t, 159t, 159z, 162z

211, 228 207, 245 212, 225 212, 226 212, 334

168z/188t 177z/177t, 160t 172z/184t, 159t, 153z 165z/190t, 158t, 159z, 160z 167z/181t 154z, 154t 157z/205t, 155t, 158z 176z/189z, 154t, 154z, 159z

211, 240 216, 229 221, 221 212, 271 215, 339

165z/172t 154t, 157z 162z/202t, 155t, 153z 163z/190t, 165z/199z, 156t, 161z

Mt

Mz

〈∆EM〉f

0.16 1.01 2.39 1.89 2.41 2.63 1.80 2.17 3.21 1.09 0.86 1.50 1.65 1.75 1.90 1.86 1.25 2.37 0.18 1.78 2.45 1.98 1.18 1.01 2.72 2.86 2.89

0.03 0.67 1.18 1.64 3.10 3.04 2.95 3.24 3.04 1.17 1.03 1.48 1.66 2.39 2.47 2.62 2.66 2.93 0.46 0.92 0.98 1.66 2.20 1.94 1.90 2.45 2.71

0.06 0.75 1.48 1.70 2.93 2.94 2.67 2.97 3.09 1.15 0.99 1.48 1.65 2.23 2.33 2.43 2.31 2.80 0.39 1.13 1.35 1.74 1.95 1.71 2.11 2.55 2.76

a Number of unpaired electrons in the complex. b Energy of dissociative adsorption of a hydrogen molecule (in kJ/mol per adsorbed hydrogen atom). c Deformation energy of the supported cluster calculated as the total energy difference between the optimized complex in the gas phase and the same complex with a geometry corresponding to that of the supported complex (in kJ/mol). d Interatomic distances in pm; for the notation of atomic centers, see Figure 1. 〈M-M〉 is the average M-M distance in the modeled complexes. M-H represents the distances of H adsorption at a distorted bridge site, given as pairs of values Mz-H/Mt-H or Mz-H/Mz-H. In the case of a 3-fold coordination of the ligands, the three distances are separated by slashes. Subscripts t and z denote distances of H atoms coordinated on-top at Mt and Mz atoms, respectively. e See text for the reference values used to estimate the 2p core-level shifts. f Average ∆EM values of core-level shifts over Mz and Mt atoms, in eV. g Data reported for iridium clusters correspond to the structures reported in ref 12b. h Reported distances are averaged over all atoms of the corresponding type as both Pt4 and Pt4H3 clusters were optimized without symmetry constraints (C1 symmetry).

shifted from distorted bridge positions toward 3-fold (Rh4H6/ zeo) or terminal (Pt4H6/zeo) coordination. Comparison of the calculated results obtained for hydrogenated rhodium, iridium,12 and platinum tetrahedral clusters shows that the M-H distances of the ligands in the same positions vary by only small amounts, up to about 10 pm, whether the support is present or not. In rhodium complexes with 6 or 9 ligands, some of the M-H distances differ by up to 30 pm because of alternative locations of some of the ligands in the gas-phase clusters and the supported clusters. Similarly to Ir4, the coordination of up to 12 H ligands to rhodium and platinum clusters results in an increase of the M-M distances, by up to 30 pm in some cases (Figure 2). The Mz-Mz and Mz-Mt distances do not change uniformly in the various clusters. Whereas in the iridium clusters, both in the gas phase and supported on a zeolite surface, the Mz-Mt and Mz-Mz distances increase roughly synchronously, the distances in the rhodium and platinum clusters do not vary as smoothly. Applying a linear fit, one finds that the average M-M distance of the supported moieties Rh4, Ir4, and Pt4 increases for each additional H ligand by 2.6, 2.2, and 2.9 pm, respectively. For comparison of the structures with experiment, we refer to extended X-ray absorption fine structure (EXAFS) data on small Rh clusters in the cages of faujasite.30 From the average

Rh-Rh coordination number of 2.6 ( 20%, these clusters were hypothesized to comprise at most five metal atoms. The experimental average Rh-Rh distance, 265 pm,30 is 18 pm longer than the calculated (average) distances of tetrahedral bare Rh4, on a zeolite support or in the gas phase. A similar discrepancy between experimental and theoretical values was previously observed for Rh6 species, and the increased M-M distances of hexanuclear zeolite-supported rhodium species under experimental conditions were rationalized as being the result of moderate reverse spillover of protons from zeolite bridging OH groups onto the metal cluster.31 The complex Rh4H3/zeo studied here might be considered as a result of a hypothetical reverse spillover of protons from the zeolite support to the metal moiety. However, even in this case, the M-M distances in the metal moiety remain 15 pm shorter than the experimental M-M result. Thus, in analogy to the Ir4/zeo species,12a,b one could rationalize the increase of the M-M distances by adsorption of further hydrogen ligands and the formation of hydrogenated species M4Hn with n ≈ 6-9. A direct comparison of the calculated results for platinum clusters with experimental data is difficult because the size, nuclearity, and interatomic distances of the species studied experimentally were shown to depend on many factors, such as the precursor, the synthetic procedure, and the treatment of

Hydrogen Saturation of Small Supported Metal Clusters

Figure 2. Variation of the average M-M distances in M4Hn complexes with the number n of hydrogen atoms coordinated to the metal moiety: (a) clusters in the gas phase and (b) clusters on a dehydroxylated zeolite support. Rh4, circles (magenta); Ir4, triangles (black); Pt4, squares (blue). The data for the systems Ir4Hn and Ir4Hn/zeo are taken from ref 12b. The vertical lines indicate the spread of the values in a given system.

the catalyst.32 Interatomic distances of tetrahedral platinum clusters, anchored on alumina, were estimated by EXAFS spectroscopy at 275 pm as hydrogen treatment resulted in the sintering of the Pt4 species.32 This experimental value is more than 15 pm longer than the average interatomic distances determined in the present model of bare Pt4 on a zeolite support, 251 pm. Also, experiments showed metal-metal distances of platinum clusters on different supports to increase with hydrogen loading. The average interatomic distances of larger platinum moieties (∼50 atoms in Y zeolite in a He atmosphere) increased from 265 to 274 pm under hydrogen adsorption.33 More recently, Feast et al.34 prepared and studied platinum species on various microporous materials; for platinum clusters of less than 10 metal atoms in KLTL zeolite at a H/Pt ratio of 1.7, they determined M-M interatomic distances of 275 pm. These results fall nicely between two sets of our calculated data: we calculated average Pt-Pt distances of 268 and 278 pm for models with ratios H/Pt ) 1.50 (Pt4H6/zeo) and H/Pt ) 2.25 (Pt4H9/zeo), respectively. A trend to longer metal-metal distances with increasing hydrogen loading was also observed for larger (∼1 nm) platinum clusters supported on alumina or silica, for which the Pt-Pt distances increased from 269 to 276 pm for hydrogen coverages estimated to change from 0 to 1.35 The adsorption of hydrogen from the gas phase onto zeolitesupported metal clusters is also expected to affect the interaction between the metal moiety and the zeolite support with deprotonated bridging OH groups. In the bare Ir4 cluster and in hydrogenated Ir4Hn species with n up to 9, each of the Irz atoms was calculated to interact with two oxygen centers of the zeolite fragment.12 The supported bare Rh4 and Pt4 clusters also have two M-O contacts per Mz atom, with distances estimated at 212/232 pm and 211/240 pm, respectively. Adsorption of three or six hydrogen atoms on the metal moieties results in a decrease

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Figure 3. Adsorption energy, Eads ) nEDA, of hydrogen in M4Hn complexes as a function of the H/M ratio for species (a) in the gas phase and (b) on a dehydroxylated zeolite support. Rh4, circles (magenta); Ir4, triangles (black); Pt4, squares (blue). The data for the systems Ir4Hn and Ir4Hn/zeo are taken from ref 12b.

of the second, longer distance between the hydrogenated metal moiety and the zeolite support, likely due to further oxidation of the metal atoms as a result of hydrogen adsorption (see section 3.3). When the hydrogen loading increases to n ) 12 (Rh) and n ) 9 (Pt), the model clusters rotate around an approximately “vertical” axis, and each metal atom at the base of the tetrahedron coordinates to only one oxygen atom of the zeolite. A similar effect was also determined for the model Ir4H12/zeo.12 3.2. Relative Stabilities of the Hydrogenated Complexes. The affinity of the metal clusters toward hydrogen was evaluated with the help of the energies EDA of dissociative adsorption per H atom; see section 2 and Table 1. Figure 3 shows how the total adsorption energies Eads ) nEDA vary with the hydrogen content n of the system. The results obtained previously for tetrahedral iridium clusters are also presented to broaden the comparison.12b The energies Eads for clusters in the gas phase vary in a roughly linear fashion with the hydrogen loading n (Figure 3a); see also Table S2 of the Supporting Information for the corresponding linear fits. From these linear relationships, one can derive average energies, EDA, per H atom, for the dissociative adsorption of H2 on the metal moiety: -38 kJ/mol for Rh, -71 kJ/mol for Ir, and -54 kJ/mol for Pt (see also Table 1). Recent density functional calculations7a showed that the maximum hydrogen loading of platinum clusters in the gas phase with up to 9 metal atoms is H/Pt ) 4. This result is consistent with the lack of saturation in the dependence of Eads on the hydrogen loading of the clusters shown in Figure 3a. For the supported clusters, the energies EDA of dissociative adsorption initially, for n ) 3, are even larger (in absolute terms) than the corresponding values for the gas-phase clusters: -72 vs -34 kJ/mol for Rh, -72 vs -61 kJ/mol for Ir, and -62 vs -57 kJ/mol for Pt (Table 1). This can be related to the positive

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charge of the bare supported metal clusters, 1.10 e for Rh and 1.30 e for Ir and Pt. However, with growing ratio H/M ratio, the average increase of the adsorption energy slows. Inspection of Figure 3b reveals that Eads for supported clusters overall exhibits an approximately quadratic trend in the hydrogen loading; see Table S2 of the Supporting Information for the corresponding quadratic fits. Thus, the total average adsorption energy increases up to some critical hydrogen loading n, which has a value of ∼9 for Rh, ∼12 for Ir, and ∼13 for Pt, corresponding to H/M ratios of 2.2, 3.0, and 3.3, respectively. Beyond the critical loading n, further dissociative hydrogen adsorption becomes endothermic on average; that is, one can refer to this critical value n of a (zeolite-) supported species as the optimum hydrogen loading. Therefore, when one determines the surface of small (or defect-rich) supported metal clusters using hydrogen adsorption,36 one has to take into account that the low-coordinated atoms of the cluster might be able to adsorb more than one hydrogen atom; see also refs 12b and 37. As mentioned in the Introduction, the maximum hydrogen loading reported by Kip et al.11 for Rh, Ir, and Pt clusters supported on alumina or silica, evaluated by the H/M ratio, was 1.98, 2.68, and 1.14, respectively. For supported rhodium and iridium clusters, these values are not far from the optimum hydrogen loadings obtained in our model study of supported metal tetramers, 2.2 (Rh) and 3.0 (Ir). For platinum, however, the difference is much larger, about 3 times with respect to the calculated optimum value of 3.3. This discrepancy is likely related to the size of the metal species studied experimentally. Rh and Ir particles behave similar to subnanosize clusters, whereas the low H/Pt ratio measured might be due to a fraction of much larger metal particles (with a considerable fraction of bulk atoms) present in the platinum samples. Apparently, the adsorption energies of hydrogen on the metal clusters change with the H/M ratio quite differently for clusters in the gas phase or on an oxide support (Figure 3). As an explanation for this finding, one might invoke bond-order conservation, where an increasing hydrogen loading eventually reduces the marginal stability gained. However, the adsorption energies of the clusters in the gas phase do not exhibit any saturation effect in the range of H/M values studied, in contrast to the supported clusters. In this context, recall the model character of the systems. In experiments, clusters in the gas phase with more hydrogen adsorbed might actually fragment into smaller metal hydride species;38 such a possibility is not explored here. The effect of a support on the adsorption energies of supported clusters could have several causes, such as (i) steric hindrance/repulsion between adsorbed hydrogen ligands and atoms of the support; (ii) deformation of the cluster from the optimum structure (in the gas phase) to the structure optimal on the support; and (iii) a reduction of the H binding capability of Mz atoms that participate in M-O bonds, as suggested by the concept of bond-order competition. The repulsion between some of the hydride ligands and the oxygen centers of the zeolite support can be estimated, to some extent, from the marked reduction of the shortest H-O distances in the zeolite-supported adsorption complexes. For the Rh4Hn/ zeo models with 6, 9, and 12 ligands, we determined these shortest H-O distances to be 307, 301, and 211 pm, respectively. The same trend was found for the supported iridium hydride clusters: 346, 268, and 206 pm, respectively. For the Pt4Hn/zeo model systems, these shortest contacts were found to be roughly comparable: 296, 282, and 182 pm. To appreciate these distances, one can compare them to the sums of the van der Waals radii of H and O, 120 and 152 pm,39 respectively;

Petkov et al. that is, a notable repulsion between hydride ligands and zeolite oxygen centers is expected to occur below distances of about 272 pm. The second effect can be estimated from the deformation energy, Edef, of the metal moiety (Table 1), i.e. from the difference in total energies of bare and hydrogenated clusters in the gas phase, using the geometries of the species optimized in the gas phase and on the surface. For clusters with up to three hydrogen ligands, the deformation energy is small, below 20 kJ/mol. For rhodium and iridium, the largest value of Edef is calculated for complexes with nine H ligands, 125 and 188 kJ/ mol, because in both adsorption complexes the metal moiety is still bound relatively strongly to the zeolite ring, with two M-O bonds per Mz atom. In the complexes with 12 hydrogen ligands, the metal cluster is farther from the support, with only one M-O bond per Mz atom. Therefore, the influence of the support on the cluster structure is reduced, resulting in smaller Edef values, 17-25 kJ/mol (Table 1). The deformation energy for Pt4H9, 52 kJ/mol, is notably smaller than those for its congeners of Rh and Ir because this Pt cluster is already farther from the support, with only one M-O bonds per Mz atom. However, for Pt4H12, Edef was calculated to be large, 197 kJ/mol, as one type of hydrogen ligand atoms, at Mz, is not able to occupy a favorable position in the adsorption complex because of the repulsive interaction with the zeolite. 3.3. Oxidation of the Metal Clusters. The oxidation state of the metal moieties varies as a result of the hydrogen adsorption from the gas phase and the metal-support interaction. To analyze these changes of the metal moiety, we traced key quantities that describe the electron density distribution and the electronic structure of the metal atoms. The energy shifts of the Rh 2p and Pt 2p levels are provided in Table 1 and compared in Figure 4. Note that the M 2p core levels can be related to the position of the white line in the X-ray absorption near-edge structure (XANES) spectrum of the supported metal clusters.40 Although we traced only the metal 2p levels, other inner core levels are expected to exhibit analogous trends. Our calculations show that, upon adsorption of hydrogen onto all of the clusters in the gas phase, the metal atoms are oxidized by hydrogen. The calculated charge of the M4 metal moieties of the hydrogenated rhodium, iridium, and platinum clusters increases with the amount of adsorbed hydrogen, from 1.1 to 2.5 e (Figure S2 of the Supporting Information). In agreement with the calculated charges, the adsorption of hydrogen on the metal species in the gas phase stabilizes the M 2p core levels; in the hydrogen-rich clusters M4H12, the average stabilization is similar for all three metals, namely, 1.7 eV with respect to the corresponding bare cluster in the gas phase. Several mechanisms have been discussed to rationalize core-level shifts:41 (i) charge transfer leading to a (partial) ionization of an atomic center; (ii) the electrostatic field of the surrounding species; and (iii) a change in the electronic configuration of an atom, for example, in the s + p and d occupation numbers of the metal centers. In the case of hydrogen adsorption on metal clusters, all mechanisms might affect the core-level energies of the metal centers. On one hand, adsorbed hydrogen can oxidize the metal cluster and thus generate a positive partial charge on the metal centers that stabilizes their core levels, hence resulting in positive core-level shifts. The ligands, on the other hand, carry negative partial charges, and thus, their electrostatic field destabilize the metal core levels. Upon hydrogen adsorption, the (average) d-orbital populations of the metal atoms decrease by up to 0.2, 0.3, and 0.4 e for M ) Rh, Ir, and Pt, respectively; concomitantly, the combined s

Hydrogen Saturation of Small Supported Metal Clusters

Figure 4. Shifts ∆E of the (a) average M 2p core levels of the metal atoms in the complexes M4Hn (dashed lines) and the zeolite-supported clusters M4Hn/zeo (solid lines) and (b) M 2p level of the basal Mz (solid lines) and apical Mt (dotted lines) atoms of the zeolite-supported clusters M4Hn/zeo. The shifts were calculated with respect to the corresponding bare cluster M4 in the gas phase as a function of the number of hydrogen atoms adsorbed on the metal moiety, n. Rh4, circles (magenta); Ir4, triangles (black); Pt4, squares (blue).

+ p populations increase slightly (Figure S3 of the Supporting Information). Thus, according to a previous theoretical analysis,41 the variation in the electronic configuration also contributes to a stabilization of the metal core levels, as concluded from the calculated shifts (Table 1). The zeolite-supported clusters exhibit similar trends. Here, the oxidation of the metal centers and the corresponding stabilization of the core levels (e.g., M 2p) is due to the combined effect of the interaction of the metal moiety with the zeolite fragment and the adsorption of hydrogen (Figure 4). The former effect can be deduced from the increase of the (vertical) ionization potentials of supported bare metal clusters compared to their congeners in the gas phase, by 0.9, 0.7, and 0.8 eV for Rh, Ir, and Pt, respectively, as calculated from the difference in total energy of the ionized and neutral clusters (∆SCF procedure). Depending on the hydrogen coverage, the total charge of the M4 moiety in the supported metal clusters varies from 1.10 to 1.97 e for Rh, from 1.30 to 2.18 e for Ir, and from 1.30 to 2.73 e for Pt. As a result of this charge-transferdominated interaction with the zeolite support, the charges of the Mz atoms are more positive than those of the Mt atoms, by 0.12 e (Rh), 0.19 e (Ir), and 0.21 e (Pt). These charge differences allow one to rationalize the calculated stabilization of the core levels of the Mz atoms of bare clusters (with respect to the corresponding bare clusters in the gas phase) by 3.1 eV (Rh), 2.2 eV (Ir), and 2.2 eV (Pt), whereas the stabilization of the Mt atoms is smaller, by 2.4 eV (Rh), 1.6 eV (Ir), and 1.2 eV (Pt). The electrostatic field of the negatively charged oxygen centers of the zeolite is expected to destabilize the metal core levels. However, the polarization of the electron density, of the metal moiety in our case, should also be taken into account.41 Thus, in the bare clusters, the negatively charged oxygen centers near

J. Phys. Chem. C, Vol. 114, No. 18, 2010 8505 the metal atoms Mz at the cluster base induce a larger positive charge on Mz centers and correspondingly have a larger stabilizing effect on the Mz core levels than on the core levels of the Mt center, located at the vertex of the cluster. This strong polarization of the electron density at the Mz centers renders them less sensitive to an additional oxidation by hydrogen ligands; hence, only small variations of the metal core levels result from the hydrogen loading: within 0.3 eV (Rh), 0.7 eV (Ir), and 0.8 eV (Pt) (see the solid lines in Figure 4b). On the other hand, as Mt atoms are less affected by the support, they experience a stronger influence of the adsorbed hydrogen ligands; thus, the core levels of the Mt centers vary by 1.4 eV (Rh), 1.2 eV (Ir), and 1.9 eV (Pt). If one compares the (s + p)and d-orbital populations of the supported clusters to those of the corresponding clusters in the gas phase (Figure S3 of the Supporting Information), one is led to conclude that the oxidation of the metal moiety due to the interaction with the support mainly concerns the d-orbital populations. In the supported bare clusters, the d populations decrease by 0.52 e (Rh), 0.63 e (Ir), and 0.46 e (Pt). For Rh, one also notes an increase of the s + p population by 0.24 e. Inspection of Figure 4 shows that the core-level shifts of gasphase clusters vary more, by about 1.7 eV, hence indicating a stronger oxidation of the metal moieties by hydrogen, than the core-level shifts of the supported clusters, which vary by 0.2-0.8 eV. On the other hand, the supported clusters are oxidized more strongly because of their interaction with the support, as can be concluded from the larger overall stabilization of the metal core levels, by up to 3.1 eV. (Note that the core levels of the bare supported clusters are stabilized by 2.0-2.9 eV with respect to their congeners in the gas phase.) However, the core-level shifts in gas-phase clusters and supported clusters do not always vary in a regular fashion with the hydrogen loading (Figure 4) as they are the result of various, in part counteracting, effects: the oxidation of the metal cluster, the polarization of its electron density, the electrostatic field due to the support and the ligands, and the electronic configuration of the metal atoms. 4. Conclusions With density functional model calculations, we quantified the successive dissociative adsorption of hydrogen from the gas phase onto tetrahedral metal clusters M4 (M ) Rh, Ir, Pt), supported on a zeolite surface, modeled as a faujasite six-ring, and we compared the trends obtained with those for the analogous metal hydride species in the gas phase. The subsequent dissociative adsorption of up to six H2 molecules on the metal clusters in the gas phase and on a zeolite support resulted in an increase of the average M-M distances. For gas-phase species, the average increases (per adsorbed H atom) are about 2.3 pm for Rh, 1.7 pm for Ir, and 1.7 pm for Pt species; the corresponding values for the zeolite-supported complexes are 2.6, 2.2, and 2.9 pm, respectively. Individual M-M distances of the Rh4Hn and Pt4Hn clusters deviate notably from these average values. In the range studied, the adsorption energy of hydrogen on the gas-phase complexes M4Hn decreases essentially linearly with increasing H loading. On average, the energies of dissociative adsorption of hydrogen, EDA, per H atom are -38 kJ/ mol for Rh, -71 kJ/mol for Ir, and -54 kJ/mol for Pt. The adsorption energy for the supported hydrogenated species also decreases with increasing hydrogen loading on the metal moiety. However, the gain in stability of the hydrogenated clusters reaches a maximum at a critical hydrogen loading, which varies

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for the metal clusters studied, being ∼9 for Rh, ∼12 for Ir, and ∼13 for Pt. In the zeolite-supported model clusters, the maximum stability thus is reached at H/M ratios of 2.2, 3.0, and 3.3, respectively. Just as was found for the corresponding iridium clusters,12b small rhodium and platinum clusters are oxidized through their interactions with the zeolite support, as well as with the hydrogen ligands. Acknowledgment. We thank Sven Kru¨ger for helpful discussions. This work was supported by the Bulgarian National Science Fund (Contract VUH-303/07), the Bulgarian National Center of Advanced Materials UNION (Contract DO-02-82/ 2008), Deutsche Forschungsgemeinschaft, and Fonds der Chemischen Industrie (Germany). Supporting Information Available: Table with characteristics of selected hydrogenated metal clusters M4Hn in the gas phase and supported on zeolite, optimized in C1 symmetry; table with results of least-squares fits for the adsorption energy of hydrogen as a function of hydrogen loading; figure with optimized structures of Rh4Hn and Pt4Hn clusters in the gas phase; figure showing how the total charge of the M4 moiety depends on the hydrogen loading; and figure showing averages of (s + p)- and d-orbital populations of the metal atoms. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) (a) Sachtler, W. M. H.; Zhang, Z. AdV. Catal. 1993, 39, 129. (b) McCarthy, T. J.; Lei, G.-D.; Sachtler, W. M. H. J. Catal. 1996, 159, 90. (2) Hayek, K.; Goller, H.; Penner, S.; Rupprechter, G.; Zimmermann, C. Catal. Lett. 2004, 92, 1. (3) 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, and references therein. (4) 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. (5) Wei, J.; Iglesia, E. Angew. Chem., Int. Ed. 2004, 43, 3685. (6) Balaj, O. P.; Balteanu, I.; Rossteuscher, T. T. J.; Beyer, M. K.; Bondybey, V. E. Angew. Chem., Int. Ed. 2004, 43, 6519. (7) (a) Zhou, C.; Wu, J.; Nie, A.; Forrey, R. C.; Tachibana, A.; Cheng, H. J. Phys. Chem. C 2007, 111, 12773. (b) Chen, L.; Chen, B.; Zhou, C.; Wu, J.; Forrey, R. C.; Cheng, H. J. Phys. Chem. C 2008, 112, 13937. (8) Huda, M. N.; Kleinman, L. Phys. ReV. B 2006, 74, 195407. (9) (a) Mikhailov, M. N.; Mishin, I. V.; Kustov, L. M.; Mordkovich, V. Z. Catal. Today 2009, 144, 273. (b) Mineva, T.; Alexiev, V.; LacazeDufaure, C.; Sicilia, E.; Mijoule, C.; Russo, N. J. Mol. Struct. (THEOCHEM) 2009, 903, 59. (10) Ferrari, A. M.; Neyman, K. M.; Belling, T.; Mayer, M.; Ro¨sch, N. J. Phys. Chem. B 1999, 103, 216. (11) Kip, B. J.; Duivenvoorden, F. B. M.; Koningsberger, D. C.; Prins, R. J. Catal. 1987, 105, 26. (12) (a) Petrova, G. P.; Vayssilov, G. N.; Ro¨sch, N. Chem. Phys. Lett. 2007, 444, 215. (b) Petrova, G. P.; Vayssilov, G. N.; Ro¨sch, N. J. Phys. Chem. C 2007, 111, 14484. (c) Petrova, G. P.; Vayssilov, G. N.; Ro¨sch, N. J. Phys. Chem. C 2008, 112, 18572. (13) (a) Pandelov, S.; Stimming, U. Electrochim. Acta 2007, 52, 5548. (b) Holby, E. F.; Sheng, W.; Shao-Horn, Y.; Morgan, D. Energy EnViron. Sci. 2009, 2, 865.

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