Adsorption and Diffusion of Hydrogen on C60-Supported Ptn Clusters

Article pubs.acs.org/JPCC

Adsorption and Diffusion of Hydrogen on C60-Supported Ptn Clusters R. Méndez-Camacho and R. A. Guirado-López* Instituto de Fı ́sica “Manuel Sandoval Vallarta”, Universidad Autónoma de San Luis Potosı ́, Alvaro Obregón 64, 78000 San Luis Potos, México ABSTRACT: We present extensive pseudopotential density functional theory calculations dedicated to analyze the adsorption properties and migration behavior of hydrogen on C60-supported Ptn (n = 1, 2, 5, 13) clusters. When adsorbing Pt species on C60, we find that the systems gain energy when the platinum atoms aggregate on the fullerene surface, forming clusters of different sizes and symmetries. Notable structural variations around the adsorption sites are obtained, consisting in expansions and contractions of the C−C, Pt−Pt, and Pt−C bond lengths as large as 7%. The adsorption energies vary in the range of 1.5−3.1 eV, and there is a notable Pt → C charge transfer (∼0.15e) that leads to the formation of robust Pt−C bonds. When the C60Ptn compounds are exposed to molecular hydrogen, the Pt-rich regions of the surface are the ones favorable for the dissociative chemisorption of H2. The density of states around the Fermi level is very sensitive to the presence and location of the hydrogen species in our C60Ptn structures, a result that could have strong effects on the transport properties of our fullerene compounds and can be used as a fingerprint to identify precise structural features in these kind of complexes. Using the nudged-elastic-band method, we obtain that atomic hydrogen diffuses very easily on the surface of both free-standing and C60-supported Ptn clusters. However, H-atom migration on the carbon surface is very unlikely, since barriers of the order of 1.5 eV need to be overcome. Hydrogen transfer events between platinum and carbon regions on our here-considered C60Ptn structures, so-called spillover processes, are highly dependent on the local atomic environment. When going from the single Pt atom to the small cluster regime, the spillover energy barriers vary between 0.7−1.6 eV, a result that is important to consider in order to more clearly understand recent experimental studies addressing hydrogen storage in carbon nanostructures via chemical adsorption.

I. INTRODUCTION Hydrogen storage in carbon nanostructures has been the subject of a lot of research in the past decade. The adsorption properties of various forms of carbon nanotubes,1 fullerenes,2 graphite nanofibers,3 and graphene4 have been extensively analyzed. However, despite the previous intense activity, the above-mentioned carbon materials have been found to be unable to store hydrogen at high densities (to be useful in practical applications), since their external and internal surfaces are chemically too inert. As has been clearly established, the H2 molecules are weakly bonded via van der Waals interaction, desorbing, thus, very easily at low temperatures. As a recent approach to increase their chemical reactivity, carbon-based nanostructures functionalized by light-, alkaline-, and transition-metal atoms have been the subject of active study. Previous works have revealed that the dopant atoms bind strongly to the carbon surfaces and that the as-formed singleatom doped carbon nanostructures serve as ideal molecular hydrogen attractors. Actually, upon single-atom attachment, there is a notable local charge density redistribution that transforms the chemisorbed atoms into cationic species due to the large electronegativity of the C 60 fullerene. As a consequence, H2 molecules located in close proximity can be trapped by the positively charged chemisorbed atom through a charge polarization mechanism. As representative examples, we mention the work of Yildirim et al.,5 where it has been found that the C60Ti structure can bind, around the Ti species, up to four hydrogen molecules with an average binding energy of 0.3−0.5 eV/H2. Furthermore, Ataca et al.6 have obtained that © 2013 American Chemical Society

Ca atoms adsorbed on graphene can also serve as a highcapacity hydrogen storage medium. In this work, each Ca can absorb up to five H2 molecules. Similar results were obtained for Ca-coated C60 fullerenes.7 Graphene doped with Li atoms can also store hydrogen, with each Li atom being able to hold up to four H2 species.8 Finally, single-wall carbon nanotubes (SWNTs) functionalized with Pt species can adsorb large amounts of hydrogen.9 This last theoretical work is important to underline, since the authors have found the existence of both dissociated and molecular hydrogen species on the carbon surface, with the former configurations being energetically preferred. The use of Pt as a dopant atom in carbon materials is very interesting, since platinum nanostructures are widely used in various types of processes, such as reduction reactions, electrolysis, and fuel cells. In addition, the chemisorption of isolated Pt species on the surface of SWNTs reported in ref 9 is in line with recent efforts related to the implementation of single-atom strategies to achieve selective heterogeneous hydrogenations. For example, several studies have shown that atomically dispersed Pt atoms supported on oxides or metals can efficiently catalyze a number of important reactions.10 This finding is highly relevant not only from the basic science point of view but also from practical applications, due to the small Received: November 15, 2012 Revised: April 16, 2013 Published: April 17, 2013 10059

dx.doi.org/10.1021/jp3113123 | J. Phys. Chem. C 2013, 117, 10059−10069

The Journal of Physical Chemistry C

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synthesized by means of different reduction procedures.17 However, it is well-known that prolongated hydrogenation of C60 molecules at elevated temperature and pressure can also induce the fragmentation and collapse of the fullerene cage. Still, spheroidal carbon fragments can be preserved by immediate termination of dangling bonds by hydrogen, leading to the formation of C58H2 and C59H species which have been detected by both time-of-flight and high-resolution Fourier transform mass spectrometry.18 In this work we have thus decided to perform systematic pseudopotential density functional theory (DFT) calculations dedicated to analyzing the adsorption properties and migration behavior of hydrogen on C60-supported Ptn (n = 1, 2, 5, 13) clusters. We choose the C60Ptn structures as working systems, since, on the one hand, cluster models of complex nanostructures can provide useful insight into the catalytic reactions occurring at their surfaces. On the other hand, fullerenes containing adsorbed Pt species are interesting systems on their own due to the high-stability and -potential applications already exhibited by metal-coated C60 fullerenes. In addition, they might offer a more accessible and less aggressive hydrogenation procedure of the C60 surface. We will show that low-energy atomic configurations are obtained when the platinum atoms aggregate on the fullerene surface (in line with experimental findings on SWNTs), forming clusters of different sizes and symmetries. When exposed to molecular hydrogen, we observe that the Pt-rich regions of the fullerene surface are the ones favorable for the dissociative chemisorption of H2. Using the NEB method,19 we reveal that H atoms diffuse very easily on the surface of both free-standing as well as C60supported Ptn clusters and that hydrogen migration on C60 is unlikely, since energy barriers of the order of 1.5 eV need to be overcome. However, spillover processes between platinum and carbon regions on our C60Ptn structures are highly dependent on the local atomic environment. When going from the single Pt atom to the small cluster regime, we notice the existence of migration pathways involving high-energy barriers and notable atomic displacements in neighboring atoms. We obtain that none of our here-considered diffusion paths, based on a single (and isolated) H-atom transfer event, defined viable ways to form stable C−H bonds on C60. However, we show that the presence of chemisorbed H on the fullerene surface, near the Pt-rich regions, reduces the energy barriers opposing hydrogen transfer events by 50%. We speculate about the importance of (i) interfacial effects, (ii) the presence of coadsorbed species on the Pt regions, as well as (iii) the existence of more complex diffusion pathways. We believe that our data is important to take into account in order to more clearly understand the conditions that need to be fulfilled for the spillover process to be realized. The rest of the paper is organized as follows. In section II we briefly describe the theoretical models used for the calculations. In section III we present the discussion of our results, and finally, in section IV the summary and conclusions are given.

amounts of the precious metal required to produce the socalled single-atom alloys. It is important to comment that Dag and co-workers9 also reported that, with increasing Pt coverage, the platinum species do not uniformly cover the SWNTs but instead prefer to aggregate on the nanotube surface, forming small clusters of different sizes. Most important, the authors speculated that the H2 uptake capacity can be reduced as a consequence of the clustering of Pt atoms (due to the increase in the Pt−Pt coupling and/or the weakening of Pt−SWNTs bonding). Actually, the previously predicted platinum aggregation has been experimentally observed by Kongkanand et al.,11 who have synthesized SWNTs containing well dispersed Pt nanoparticles of sizes ranging from 2 to 3 nm. Similarly, Bhowmick and co-workers12 have also recently fabricated SWNTs decorated with Pt nanoparticles of 2.05 ± 0.36 nm in size. Interestingly, the authors of ref 12 concluded that SWNTs having adsorbed Pt nanoparticles can still be used as hydrogen storage materials. When exposed to molecular hydrogen, they found (through XPS measurements) H atoms chemically attached on the carbon network and speculate that the previous hydrogen incorporation has been realized via a so-called spillover process. This process involves first the dissociative adsorption of H2 molecules on the deposited Pt nanoparticles, followed by an atomic hydrogen transfer event in which the individual hydrogen species end up chemically adsorbed on the surface of SWNTs. The presence of C−H bonds on the surface of Ptn−SWNTs structures has been corroborated through electrical conductivity measurements, being characterized by a notable increase in the resistance of the samples. We must point out that the spillover process is currently a subject of controversy. However, it has been proposed by several groups to explain the hydrogen storage in nanostructured carbons. For example Lachawiec et al.13 analyzed the hydrogen storage capacity of SWNTs decorated with Pd nanoparticles and concluded that hydrogen spillover can be increased by building carbon bridges that serve to improve the contact between the source (Pd nanoparticles) and the receptor (SWNTs). Chen and co-workers14 studied the hydrogen spillover from a fully hydrogenated Pt6H24 cluster to a graphene sheet. They demonstrated that the migration of H atoms from the Pt cluster catalyst to the carbon substrate is facile at ambient conditions with a small energy barrier of ∼0.23 eV per H atom. Finally, Han et al.15 analyzed the H-atom transfer process from the BH4− molecular anion to a graphene layer. The authors found that the hydrogen exchange event involves only a small energy barrier of 0.2 eV. Clearly, we notice that the spillover mechanism is very complex and that, in the studies mentioned above, only a few steps of the hydrogen transfer events were analyzed. Consequently, an atomic level understanding of these elemental processes is a necessary step toward the design of more efficient hydrogen storage materials. We must remark that hydrogen storage in pure carbon nanostructures, through the formation of stable C−H bonds, has also been extensively analyzed in the literature by means of different experimental techniques. For example, Nikitin et al.16 concluded that, for nanotube diameters around 2 nm, carbon nanotube-hydrogen complexes with close to 100% hydrogenation exist and are stable at room temperature, and the chemisorbed hydrogen can be removed by heating to 200−300 °C. On the other hand, in the case of spheroidal fullerenes, various C60Hx hydrides (x = 36, 38, 40, 42, and 44) have been

II. METHOD OF CALCULATION The structural and electronic properties of our hydrogenated C60Ptn structures will be obtained within the DFT approach using the ultrasoft pseudopotential approximation for the electron−ion interaction and a plane wave basis set for the wave functions with the use of the PWscf code.20 For all our considered structures, the cutoff energy for the plane wave expansion is taken to be 476 eV. A cubic supercell with a side 10060



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considerably expanded C−C pair (7% of expansion) below the platinum species. We found a Pt → C60 charge transfer of ∼0.14e, together with a notable modification of the electronic structure around the Fermi level. In particular, upon a single Pt adsorption, the energy gap between the highest-occupied and lowest-unoccupied molecular orbitals (HOMO−LUMO gap) is notably reduced, when compared with bare C60, being now equal to 1.32 eV. Finally, the NEB method was used to analyze simple Pt atomic displacements on the fullerene surface. We calculate an energy barrier as the Pt species moves from the onbridge adsorption shown in the figure, to an adjacent one, to be of the order of 0.25 eV. In real systems, this small energy barrier might lead to uncontrolled diffusion on C60, caused by the available thermal energy, playing thus a crucial role in the initial stages of the growth-mode of the metallic coating. The case of two adsorbed Pts is also interesting to analyze. In Figure 1b−d we present the optimized configurations of three contrasting arrays. We consider clustered (Figure 1b and c) and isolated (Figure 1d) adsorbed configurations. The most stable phase corresponds to two Pt atoms forming a dimer with its Pt−Pt bond ”parallel” to the fullerene surface (Figure 1b) and having a considerably elongated interatomic distance RPt−Pt of 2.71 Å [a value which is notably expanded with respect to the one found in the isolated Pt2 dimer (2.36 Å)]. This configuration is more stable by 0.65 eV than the columnarlike adsorbed configuration shown in Figure 1c. Furthermore, it is also more stable by 0.7 eV than the adsorbed array in which the Pt species are isolated on the surface (Figure 1d). When compared to Figure 1a, we obtain an increased adsorption energy of ∼3.1 eV together with a Pt2 → C60 direction for the charge transfer of 0.38e. In addition, the HOMO−LUMO gap is reduced to 0.43 eV, a result that reveals the highly interacting nature of the C60/Pt2 system. Finally, as in the C60Pt case, we also consider the energetics of a single Pt atom displacement on the fullerene surface. We use the NEB methodology starting from the initial configuration shown in Figure 1b to a final state in which one of the Pt species performs a single lateral displacement, ending up finally separated from the other Pt atom by two C−C bonds and a Pt−Pt interatomic distance of 5 Å. Interestingly, we obtain that the platinum species needs to overcome now an energy barrier of 1.18 eV, a value which is considerably enhanced with respect to the isolated-atom displacement discussed above (0.25 eV). The previous data implies the formation of highly stable Pt pairs on the C60 surface and a notable reduction in the mobility of the chemisorbed atoms. In Figure 2 we show the optimized atomic configurations for our considered C60Pt5 systems. In this case several adsorbed configurations are possible; however, we do not attempt to perform an extensive exploration of all resulting C60Pt5 isomers but instead to try to reveal more general tendencies concerning the electronic structure, interatomic interactions, and stability of these compounds. On the basis of our previous results, we show first in Figure 2a the optimized atomic configuration for the adsorption of a compact Pt5 cluster (so-called square pyramid) on the fullerene surface. Second, in Figure 2b−e, we consider the attachment of smaller Pt4, Pt3, Pt2 clusters coadsorbed with isolated Pt species but always maintaining the initial five-atom Pt coverage on C60. Finally, we end up in Figure 2f with an adsorbed phase in which five isolated platinums are attached to the carbon surface. We obtain that the most stable atomic configuration corresponds to the one shown in Figure 2a, where a five-atom cluster is adsorbed on a

dimension as large as 35 Å was employed in the calculations and the Γ point for the Brillouin zone integration. In all cases, we use the Perdew−Burke−Ernzerhof (PBE) pseudopotential21 (which in the case of Pt contains nonlinear core corrections) and perform fully unconstrained structural optimizations for all our considered isomers, using the conjugate gradient method. The convergence in energy was set as 1 meV, and the structural optimization was performed until a value of less than 1 meV/Å was achieved for the remaining forces for each atom. To determine the minimum energy paths as well as the transition states, we applied the NEB methodology.19 The NEB calculation scheme is a chain-of-states method where a set of images between the initial and final states must be created to achieve a smooth curve. In our case, the relatively small size of our fullerene structures will ensure that the calculations for the reaction pathways will remain computationally tractable. We use from seven to nine images to determine the energy profiles, which have been found to be enough to reveal the different stages of the hydrogen atom migration and spillover events in our C60Ptn structures. The relevant energy barriers between well-defined initial and final atomic configurations are obtained by calculating the energy difference of the initial position and the saddle point of each one of the energy profiles.

III. RESULTS AND DISCUSSION A. Stability and Electronic Properties of C60Pt n Compounds. In Figure 1 we show some low-energy atomic

Figure 1. Calculated low-energy atomic configurations for (a) the C60Pt structure as well as for (b−d) various isomers of the C60Pt2 fullerene compound.

arrays for C60Pt (Figure 1a) and C60Pt2 (Figure 1b−d) compounds. In the case of the single adsorption, the most stable atomic configuration, shown in Figure 1a, corresponds to the Pt atom attached to C60 in an on-bridge array (over a C−C bond joining two hexagons) with an adsorption energy of 2.77 eV. We obtain a Pt−C bond length of 2.04 Å and a 10061



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Figure 2. Calculated low-energy atomic configurations for various isomers of the C60Pt5 fullerene compound.

Figure 3. Calculated low-energy atomic configurations for various isomers of the C60Pt13 fullerene compound.

hexagonal ring of the fullerene, through his square facet, being more stable than the isolated-atom adsorbed array shown in Figure 2f by 4.3 eV. Interestingly, if we remove a single Pt atom from the Pt5 cluster to obtain the adsorbed configurations shown in Figures 2b and c, we notice that the total energy of the systems increases by values as large as 0.7 eV. Furthermore, by comparing parts b and c of Figure 2, we observe that the most stable structure is defined by the Pt4 cluster being in less contact with the fullerene surface (Figure 2b). From Figure 2d−f we notice as a clear trend that reducing the clustering of the Pt atoms on C60 reduces the stability of the fullerene compounds. The previous tendency to Pt-aggregation is in line with the clustering of Pt species theoretically obtained in SWNTs in ref 9 as well as with the experimental findings of Kongkanand et al.11 and Bhowmick and co-workers,12 where it has been revealed that platinum atoms do not uniformly cover the SWNTs but instead they like to aggregate on the carbon surface, forming Pt nanoparticles of about 2 nm in size. It is important to comment on the structural changes induced on the Pt5 cluster when deposited on C60. Upon adsorption, the square pyramid is found to be notably distorted when compared to the free-standing case, having now a square facet (see Figure 2a) with expanded distances which vary from 2.56 to 2.65 Å, as well as more elongated Pt−Pt interatomic separations from the apex to the square atoms of 2.68 and 2.81 Å. The previous structural variations are important from the point of view of the chemical reactivity, since strained substrates (in both nanoparticle and extended-surface forms) have adsorption properties which differ from those observed on perfect lattices. In Figure 3 we present our results for the attachment of a single fcc Pt13 cluster on C60. We consider the Pt13 structure as being initially adsorbed on a hexagonal carbon ring through the square (Figure 3c) and triangular (Figure 3a) facets. In addition, and with the purpose of covering some other possible

binding environments, we also analyze the adsorption of Pt13 through one of its square facets but now being initially placed on top of a C−C bond of the fullerene (Figure 3b). Clearly, there are a lot of additional adsorbed configurations; however, we believe that our considered arrays will provide a representative set of adsorbed phases that would be very helpful to study the energetics of the systems, as well as the structural details and electronic properties of the Pt/C interface. As in previous cases, we present our optimized configurations from the most stable, shown in Figure 3a, to the less stable atomic array, specified in Figure 3c. The most important features to mention are that, upon Pt13 attachment, global structural transformations on the platinum cluster geometry are observed and that these highly deformed structures are energetically preferred (by values as large as 1.23 eV) when compared with the more symmetric Pt13 structure shown in Figure 3c. The configurations shown in Figure 3a and b are very interesting from the point of view of the chemical reactivity, due to the existence of low-coordinated Pt atoms having both positive as well as negative charges, and a notable reduction in the HOMO−LUMO gap to 0.17 eV, which are two facts that define our fullerene compounds as efficient molecular attractors. In all cases we obtain a Pt13 → C60 direction for the charge transfer (of ∼0.11e) and interfacial Pt−C bond lengths that vary in the range of 2.08−2.14 Å. It is important to note that the previous values are of the order of the ones experimentally found by Shi et al.22 when analyzing the electronic and structural properties of C60 molecules adsorbed on a Pt(111) surface, a result that implies the local character of the Pt−C bonding. Finally, we must comment that the adsorbed low-symmetry 13-atom platinum structures shown in Figure 3a and b are still stable even if we reoptimize them as free-standing clusters. 10062



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After the minimization procedure we find, in both cases, small reconstruction processes, but the general structure of the clusters remains almost the same. Interestingly, the C60Pt13 fullerene compound shown in Figure 3a is more stable than the one shown in Figure 3b by 0.3 eV. However, in the freestanding case, the 13-atom bilayer-like platinum structure shown in Figure 3b is more stable by 0.4 eV than the Pt13 cluster shown in Figure 3a, a result that clearly underlines the important role played by the C60 surface in stabilizing novel platinum atomic arrangements. B. Hydrogen Adsorption on C 60 -Supported Pt n Clusters. As is well-known, small free-standing Pt clusters have the ability to adsorb a large amount of hydrogen molecules in both molecular and dissociated forms. In particular, Szarek et al.23 have performed an interesting and systematic DFT study of the local reactivity of small Pt clusters. These authors were able to visualize the reactive regions of the platinum particles and concluded that antibond orbitals constitute the preferable binding sites for hydrogen molecules. On the one hand, it is of fundamental importance to analyze if the previous chemical reactivity is modified when Pt clusters are deposited on C60, and on the other hand, it is necessary to evaluate how the structural transformations and the interfacial Ptn → C60 charge transfer discussed in the previous section influence the adsorption properties. In Figure 4 we present first our resulting optimized atomic configurations when a single Pt atom attached to the C60 surface interacts with an hydrogen molecule. In the calculations, we initially place the H2 species on top of the platinum atom at an intermolecular distance of 3 Å and consider different relative orientations between the H−H bond and the chemisorbed Pt species. In all cases, the molecular hydrogen undergoes a

dissociative chemisorption process upon interacting with the deposited platinum atom. We found no energy barriers for this reaction, and in Figure 4, we show our two obtained adsorbed arrays, which only differ in the relative orientation of the broken H−H bond with respect to the fullerene surface. In the two calculations, we obtain hydrogen adsorption energies of ∼1.26 eV and observe negligible variations on the underlying Pt−C bond, changing by 0.06 Å. We find (i) Pt−H bond lengths of ∼1.7 Å, (ii) an interatomic separation between the chemisorbed hydrogens of 0.9 Å (both values being of the order of the ones reported in ref 9), and (iii) a HOMO−LUMO energy gap of 1.46 eV, which is slightly increased when compared to the one obtained from Figure 1a. We must comment that the H2 molecule can also be found in various physisorbed states on the pure carbon regions, or near the chemisorbed Pt species, with the previous configurations being less stable by ∼1.2 eV with respect to the structures shown in Figure 4. Interestingly, the atomic arrays shown in Figure 4 are separated by a rotational barrier of ∼0.14 eV, implying that, in real experiments, both of them could be simultaneously present in the samples. In Figure 5 we present our lowest energy atomic configuration for the C60Pt5+H2 interacting system. For the

Figure 5. Calculated low-energy atomic configuration for the C60Pt5H2 fullerene compound.

C60Pt5 compound we consider the more stable structure shown in Figure 2a, which we believe is appropriate, since it contains various types of contrasting local atomic environments, such as triangular facets, corner atoms, edges, as well as a more defined Pt/C interface, which are expected to have different local reactivities. As in Figure 4, and in order to map the intermolecular potential, we assume that the H2 approaches from different directions as well as relative orientations, and with this procedure, we found that the interaction between the H2 and the deposited Pt5 cluster is very anisotropic. Actually, our C60Pt5 structure functions as an efficient molecular attractor only when the hydrogen molecule is initially placed near the apex atom (at ∼3 Å) of the deposited cluster. In this case, H2 spontaneously adsorbs and dissociates, forming a C60Pt5H2 compound with a Pt−H bond length of 1.58 Å and a H−H interatomic distance of 1.8 Å. Interestingly, the energy separation between the HOMO and LUMO levels of the C60Pt5 fullerene compound is not modified by hydrogen adsorption, with the previous gap being equal to 0.63 eV (0.7 eV) after (before) H attachment. We note that, for our initially considered intermolecular separation, the triangular facets, edges, as well as Pt atoms defining the Pt/C interface do not function as chemisorption sites, but as we will see in the following, this situation will change when we analyze the effect

Figure 4. Two calculated low-energy atomic configurations for the C60PtH2 fullerene compound. 10063



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of the presence of coadsorbed hydrogens on the catalytic activity of our C60Pt5 structure. We must remark that in Figure 5 an hydrogen adsorption energy of 0.96 eV is obtained, a value which is considerably reduced when compared with the one found in the C60Pt+H2 compound (1.8 eV) shown in Figure 4. This reduction is important in order to have a system that contains H species that are bound weakly enough to be able to easily diffuse and possibly spill onto C60. In Figure 6 we show our data for the C60Pt13+H2 system. For the C60Pt13 compound we consider the more stable structure

Figure 7. Calculated low-energy atomic configuration for the C60Pt5 fullerene compound shown in Figure 2a interacting with twelve H2 molecules.

representative example, a low-energy atomic configuration for the C60Pt5 structure shown in Figure 2a but now interacting with twelve H2 molecules. The hydrogen species are initially located in a random configuration around the C60Pt5 fullerene compound at an average intermolecular distance from the surface of ∼3 Å. From the figure the most important features to note are that (i) the apex Pt site is still defined (as in Figure 5) as a strong chemisorption site, (ii) the H2 molecules located far away from the Pt5 cluster are found in a physisorbed state (at an intermolecular distance of ∼3.7 Å), and (in contrast to Figure 5) (iii) atomic hydrogens can now be chemisorbed both in edges as well as on platinum atoms defining the Pt/C interface of the system. Actually, from the four H2 molecules initially placed near the Pt5 cluster, three of them are found to be dissociatively chemisorbed: an atomic configuration defined by a small adsorption energy of 0.3 eV/H2. We must remark that the existence of the previous adsorbed phase strongly perturbs the electronic spectra of the fullerene compound shown in Figure 2a. In Figure 8 we present a comparison of the average density of states (ADOS) obtained for the bare (continuous line) and hydrogen-covered (dashed line) C60Pt5 structures shown in Figures 2a and 7, respectively. From the figure we see that the presence of the coadsorbed hydrogens introduces strong variations in the shape of the

Figure 6. Calculated low-energy atomic configuration for the C60Pt13H2 fullerene compound. In part a (b) we show a dissociated H2 molecule adsorbed near (far from ) the Pt/C interface.

shown in Figure 3a, which is characterized by having a highly distorted adsorbed Pt13 cluster. As in previous cases, we found the existence of both physisorbed and chemisorbed arrays, the latters being preferred by as much as 1.2 eV. In Figure 6 we show, as representative examples, two low-energy atomic configurations in which the H2 molecule is dissociatively chemisorbed on a pure Pt region (Figure 6b) and near the Pt/ C interface (Figure 6a). The energy difference between the two arrays is of 0.03 eV, with the latter being the (slightly) preferred adsorbed phase. We obtain hydrogen adsorption energies of ∼1.3 eV, and as in C60Pt5, the Pt−H bond lengths and H−H interatomic separations are of the order of 1.6 and 1.9 Å, respectively, implying the local character of the hydrogen bonding. With respect to the electronic structure of our C60Pt13H2 compounds, we found that hydrogen attachment does not modify the HOMO−LUMO energy separation of the complexes. In fact, the fullerene structures shown in Figures 3a and 6 are all characterized by having the same (and considerably reduced) HOMO−LUMO gap of ∼0.17 eV. Finally, we must comment that, under a typical catalytic condition, our C60Ptn fullerenes will be exposed to a large amount of hydrogen molecules, and it is thus of fundamental importance to analyze the role played by the presence of additional H2 species on the adsorption properties of our C60supported platinum clusters. In Figure 7 we show, as a

Figure 8. Calculated average density of states (ADOS) for the C60Pt5 (continuous line) and C60Pt5+(H2)12 (dashed line) fullerene compounds. The Fermi energy is marked as a vertical line. 10064



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Figure 9. Diffusion path connecting the stable atomic configuration for the C60PtH2 structure shown in Figure 4a (defined as Im1) with the one specified in the inset of the figure as Im13. The hydrogen atom involved in the spillover process is labeled as H. We also include the numerical values of some relevant energy barriers.

Figure 10. Same as in Figure 9 but for the C60Pt5H2 fullerene compound shown in Figure 5.

Initial and final states were selected on the basis of the adsorption calculations, and the number of images (as large as 9) was chosen to derive smooth energy curves. As has been established in section IIIB, our C60-supported Pt clusters provide a source of H atoms, generated via dissociative chemisorption of molecular hydrogen, that can diffuse on the pure Pt regions or even jump to the carbon surface. In particular, the previous hydrogen transfer event is very important and is known as a spillover process. As stated in the Introduction of our paper, the spillover process has been proposed by several experimental groups to explain the hydrogen storage in nanostructured carbons via chemical adsorption. However, the existence of this process has been the subject of controversy. In this section, we are interested in calculating the energy barriers opposing hydrogen diffusion on

ADOS in the energy range of −10 to 1 eV. In particular, notice the intense peak that appears at ∼9 eV as well as the lowering of the ADOS around the Fermi level. Both features are expected to influence the transport properties and optical behavior of the system, allowing, thus, for a possible identification of the precise details of the atomic structure in these kind of compounds. In fact, a similar behavior has been found in the work of Bhowmick et al.,12 where SWNTs decorated with Pt nanoparticles exhibit a notable increase in their resistance after hydrogen exposure. C. Diffusion Behavior of Hydrogen Species on C60Ptn Structures. In this section, we discuss the migration behavior of H atoms chemisorbed on our C60Ptn structures presented in the previous sections. The NEB method19 was used to determine the minimum energy paths for hydrogen diffusion. 10065



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Figure 11. Same as in Figure 9 but for the C60Pt13H2 fullerene compound shown in Figure 6a.

study, since we will be able to analyze atomic displacements on both pure platinum and carbon regions as well as the hydrogen spillover events. From the figure we see that the beginning of the path (Im1 to Im9) is characterized by H-atom displacements on the Pt5 cluster having small energy barriers of 0.12 and 0.38 eV. In addition, hydrogen diffusion is accompanied by notable atomic relaxations of the Pt atoms as well as by sizable changes in the Pt−H bond lengths and H−Pt−H angles. For example, at Im5, where one of the H species is already adsorbed in an on-bridge configuration, we obtain Pt−Pt bonds varying from 2.65 to 2.83 Å. We found also Pt−H interatomic separations of 1.57, 1.64, and 2.1 Å, with the largest one corresponding to the distance between the H atom and one of the Pt species located at the Pt/C interface. At Im7 we found the two H atoms attached in an on-top configuration with Pt− H bond lengths of 1.58 Å. The Pt−Pt bond just below the hydrogen species is notably expanded, being of the order of 2.8 Å. In addition, the Pt−C bonds forming the Pt/C interface are also elongated (when compared with the initial state), ranging from 2.1 to 2.4 Å. Finally, at Im9 (not shown), we obtain two well separated chemisorbed H atoms (by 4.3 Å) together with an interatomic distance of 3.3 Å between the spilling hydrogen and the nearest carbon atom, an adsorbed configuration that defines the beginning of the H-atom transfer event. From Im9 to Im15 of Figure 10, the spilling H species performs a gradual rotation accompanied by a systematic elongation of the Pt−H bond, going from 1.57 to 1.95 Å, defining the latter adsorbed configuration the transition state (Im13) of this section of the path (see the inset). From the figure we note that the energy barrier that needs to be overcome to achieve the spillover process is of ∼1.48 eV, which is slightly higher than the one found in the C60Pt structure (Figure 9). Finally, from Im15 to Im21, we see, as in Figure 9, that hydrogen diffusion on the C60 surface is very unlikely, a process being defined by an energy barrier of 1.47 eV. Finally, in Figure 11 we present, starting from the lowest energy atomic configuration shown in Figure 6a, the diffusion of an hydrogen atom in our C60Pt13 structure. As in Figure 10 we notice again that H-atom displacements on Pt regions (Im1 to Im7) are very easy to achieve. When going from Im1 to Im7, the moving hydrogen performs a gradual rotation to end up

C60Ptn structures, as well as in analyzing the conditions that need to be present for the development of low-barrier exit routes that can allow the hydrogen spillover and subsequent adsorption to the C60 surface. In Figure 9 we plot the energetics of a possible spillover event occurring in the C60PtH2 fullerene compound shown in Figure 4a. In our calculations we assume a diffusion path in which only one hydrogen atom is detached from the Pt species (labeled as H in the figure) and chemisorbed on C60 (Im1 to Im7). Once a C−H bond is formed, we further consider a single H-atom displacement on the fullerene structure (Im7 to Im13). From the figure we note that, before jumping to C60, the H atom involved in the spillover process performs a gradual rotation, with the H−Pt−H angle varying from 33° at the initial configuration (Im1) to 92° at the transition state (Im5). In this last case, we note also the existence of a more complex adsorbed configuration in which the Pt atom is asymmetrically bonded to C60 with Pt−C distances of 2.1 and 2.4 Å together, with a considerably elongated Pt−H bond of ∼2 Å for the moving hydrogen species. The previous transition state defines an energy barrier of ∼1.35 eV, after which the H atom is found to be attached to C60 with a C−H bond length of 1.1 Å (Im7). Our here-obtained energy barrier clearly implies that the spillover process assumed in Figure 9 will be very unlikely. Finally, in the same figure we include (Im7 to Im13) the energetics of a single H-atom displacement on C60. The transition state in this case (Im 10) is characterized by an atomic configuration in which the H atom is adsorbed in an onbridge array, with C−H distances of 1.3 Å, and it defines an energy barrier of 1.45 eV. The high value implies that H diffusion on C60 is very difficult to achieve. This result is consistent with the experimental work of Bhowmick et al.12 addressing the hydrogen capacity of carbon nanotubes decorated with Pt nanoparticles in which it is concluded that, after spilling on the nanotubes’ surface, the hydrogen atoms mainly accumulate in the vicinity of the catalyst particles due to the existence of high-energy barriers opposing H-atom migration. In Figure 10 we present the migration behavior for one of the H atoms (labeled as H in the figure) chemisorbed on the C60Pt5 structure shown in Figure 5. This case is interesting to 10066



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Figure 12. Diffusion path connecting the stable atomic configurations for the C60PtH2 structure defined as Im1 and Im9 (see the inset). The hydrogen atom involved in the spillover process is labeled as H, and the one chemisorbed on C60 is defined as Hchem. We also include the numerical value of the spillover energy barrier.

finally attached in an on-bridge configuration, as seen from Im7, with Pt−H distances of 1.62 and 2.12 Å. Also in agreement with previous data, we obtain that hydrogen diffusion on C60 (Im13 to Im19) is very unlikely, involving energy barriers of ∼1.4 eV. However, in the region of the path involving the spillover event (Im7 to Im13), we observe the formation of an energy barrier which is even higher than the one found for H migration on C60, being of ∼1.6 eV. In this last case, the transition state (Im11) is defined by an adsorbed configuration in which both H atoms are attached in an on-top array with Pt−H bond lengths of 1.57 and 1.9 Å, with the larger distance corresponding to the spilling atom. When comparing the spillover energy barriers obtained from Figures 9−11 we see that they become higher as we increase the size of our deposited Pt cluster. In all three cases, hydrogen diffusion involved notable displacements on neighboring atoms and we conclude that none of the here-proposed diffusion paths defined low-barrier exit routes to find a viable way to form C− H bonds on the fullerene surface. Even if, in agreement with experiment, we predict the existence of highly mobile H species on carbon-supported Pt clusters as well as high-energy barriers opposing hydrogen diffusion on the carbon surface, our DFT calculations could imply that the development of low-barrier pathways for hydrogen spillover might require (among others) the contribution of more complex interfacial effects. In order to explore our previous assumption, we recalculate in Figure 12 the hydrogen transfer event shown in Figure 10 (Im9 to Im15) but now in the presence of a chemisorbed hydrogen atom on the C60 surface, labeled as Hchem, located near the Pt5 cluster (see Im1). From Figure 12 we note that (i) the spilling H atom performs a systematic rotation gradually approaching to the fullerene surface (compare Im1, Im4, and Im6), (ii) the previous rotation is accompanied by a progressive elongation of the Pt−C bond, and (iii) the spilling H atom ends up chemically bonded to the C60 surface (see Im9), being very close to the already chemisorbed species, Hchem. The atomic configuration defined by Im9 (see the inset) is important to emphasize, since it is more stable than the initial atomic array of the path (Im1) by 0.3 eV. This is in contrast with the data in

Figure 10, where the spilling H atom prefers to be bonded to the Pt5 cluster rather than being attached to the carbon network (by 0.15 eV). Clearly, the presence of the Hchem species results in an efficient hydrogen attractor, lowering the spillover energy barrier by ∼50%. We believe that the presence of coadsorbed hydrogens on the Pt regions could also be relevant. As we have found in section IIIB (Figure 7), the existence of coadsorbed hydrogens in both chemisorption and physisorption states considerably reduces the adsorption energy of the system, increasing, thus, the mobility of the adsorbates. Finally, it will also be interesting to analyze more complex diffusion paths. As is well-known, platinum nanoparticles can absorb large amounts of hydrogen,24 and as a consequence, the migration of H atoms in regions below the surface (or in deeper volumes) might play an important role in the opening of low-energy migration channels for spillover events. Calculations along the previous lines of research are currently underway.

IV. SUMMARY AND CONCLUSIONS In this work we have presented extensive pseudopotential density functional theory calculations dedicated to analyze the adsorption properties and diffusion behavior of hydrogen on C60-supported Ptn clusters. We have found that adsorbed phases in which the platinum atoms aggregate on the fullerene surface are energetically preferred. Notable structural variations around the adsorption sites are found, and the adsorption energies vary in the range of 1.5−3.1 eV. When the C60Ptn compounds are exposed to H2, we notice that the Pt-rich regions of the surface are the ones favorable for the dissociative chemisorption of molecular hydrogen. Using the nudgedelastic-band method, we obtain that atomic hydrogen diffuses very easily on the surface of both free-standing and C60supported Ptn clusters. In contrast, hydrogen migration on the carbon surface is very unlikely to occur, since energy barriers of ∼1.5 eV need to be overcome. We reveal that hydrogen transfer events between platinum and carbon regions are highly dependent on the local atomic environment. When going from the single Pt impurity to the 13-atom Pt cluster case, the 10067



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Whangbo, M.-H.; Yang, J. ”Narrow” Graphene Nanoribbons Made Easier by Partial Hydrogenation. Nano Lett. 2009, 9, 4025−4030. ́ (5) Yildirim, T.; Iniguez, J.; Ciraci, S. Molecular and Dissociative Adsorption of Multiple Hydrogen Molecules on Transition Metal Decorated C60. Phys. Rev. B 2005, 72 (153403), 1−4. (6) Ataca, C.; Aktürk, E.; Ciraci, S. Hydrogen Storage of Calcium Atoms Adsorbed on Graphene: First-Principles Plane Wave Calculations. Phys. Rev. B 2009, 79 (041406), 1−4. (7) Yoon, M.; Yang, S.; Hicke, C.; Wang, E.; Geohegan, D.; Zhang, Z. Calcium as the Superior Coating Metal in Functionalization of Carbon Fullerenes for High-Capacity Hydrogen Storage. Phys. Rev. Lett. 2008, 100 (206806), 1−4. (8) Ataca, C.; Aktürk, E.; Ciraci, S.; Ustunel, H. High-Capacity Hydrogen Storage by Metallized Graphene. Appl. Phys. Lett. 2008, 93 (043123), 1−3. (9) Dag, S.; Ozturk, Y.; Ciraci, S.; Yildirim, T. Adsorption and Dissociation of Hydrogen Molecules on Bare and Functionalized Carbon Nanotubes. Phys. Rev. B 2005, 72 (155404), 1−8. (10) Kyriakou, G.; Boucher, M. B.; Jewell, A. D.; Lewis, E. A.; Lawton, T. J.; Baber, A. E.; Tierney, H. L.; Flytzani-Stephanopoulos, M.; Sykes, E. C. H. Isolated Metal Atom Geometries as a Strategy for Selective Heterogeneous Hydrogenations. Science 2012, 335, 1209− 1212. Fierro-González, J. C.; Bhirud, V. A.; Gates, B. C. A Highly Active Catalyst for CO Oxidation at 298 K: Mononuclear AuIII Complexes Anchored to La2O3 Nanoparticles. Chem. Commun. 2005, 42, 5275. Thomas, J. M.; Saghi, Z.; Gai, P. L. Can a Single Atom Serve as the Active Site in Some Heterogeneous Catalysts? Top. Catal. 2011, 54, 588−594. (11) Kongkanand, A.; Vinodgopal, K.; Kuwabata, S.; Kamat, P. V. Highly Dispersed Pt Catalysts on Single-Walled Carbon Nanotubes and Their Role in Methanol Oxidation. J. Phys. Chem. B 2006, 110, 16185−16188. (12) Bhowmick, R.; Rajasekaran, S.; Friebel, D.; Beasley, C.; Jiao, L.; Ogasawara, H.; Dai, H.; Clemens, B.; Nilsson, A. Hydrogen Spillover in Pt-Single-Walled Carbon Nanotube Composites: Formation of Stable C−H Bonds. J. Am. Chem. Soc. 2011, 133, 5580−5586. (13) Lachawiec, A. J.; Qi, G.; Yang, R. T. Hydrogen Storage in Nanostructured Carbons by Spillover: Bridge-Building Enhancement. Langmuir 2005, 21, 11418−11424. (14) Chen, L.; Cooper, A. C.; Pez, G. P.; Cheng, H. Mechanistic Study on Hydrogen Spillover onto Graphitic Carbon Materials. J. Phys. Chem. C 2007, 111, 18995−19000. (15) Han, S. S.; Jung, H.; Jung, D. H.; Choi, S.-H.; Park, N. Stability of Hydrogenation States of Graphene and Conditions for Hydrogen Spillover. Phys. Rev. B 2012, 85 (155408), 1−5. (16) Nikitin, A.; Li, X.; Zhang, Z.; Ogasawara, H.; Dai, H.; Nilsson, A. Hydrogen Storage in Carbon Nanotubes through the Formation of Stable C−H Bonds. Nano Lett. 2008, 8, 162−167. (17) Peera, A.; Saini, R. K.; Alemany, L. B.; Billups, W. E.; Saunders, M.; Khong, A.; Syamala, M. S.; Cross, R. J. Formation, Isolation, and Spectroscopic Properties of Some Isomers of C60H38, C60H40, C60H42, and C60H44Analysis of the Effect of the Different Shapes of Various Helium-Containing Hydrogenated Fullerenes on Their 3He Chemical Shifts. Eur. J. Org. Chem. 2003, 4140−4145. Nossal, J.; Saini, R. K.; Sadana, A. K.; Bettinger, H. F.; Alemany, L. B.; Scuseria, G. E.; Billups, W. E.; Saunders, M.; Khong, A.; Weisemann, R. Formation, Isolation, Spectroscopic Properties, and Calculated Properties of Some Isomers of C60H36. J. Am. Chem. Soc. 2001, 123, 8482−8495. (18) Talyzin, A. V.; Tsybin, Y. O.; Peera, A. A.; Schaub, T. M.; Marshall, A. G.; Sundqvist, B.; Mauron, P.; Züttel, A.; Billups, W. E. Synthesis of C59Hx and C58Hx Fullerenes Stabilized by Hydrogen. J. Phys. Chem. B 2005, 109, 5403−5405. (19) Henkelman, G.; Uberuaga, B. P.; Jonsson, H. A Climbing Image Nudged Elastic Band Method for Finding Saddle Points and Minimum Energy Paths. J. Chem. Phys. 2000, 113 (9901), 1−4. (20) Baroni, S.; Dal Corso, A.; de Gironcoli, S.; Giannozzi, P.; Cavazzoni, C.; Ballabio, G.; Scandolo, S.; Chiarotti, G.; Focher, P.; Pasquarello, A.; et al. http://www.pwscf.org/.

spillover energy barriers vary between 0.7 and 1.6 eV. The previous barriers clearly indicate that our here-considered diffusion paths, based on a single H-atom transfer event, are very unlikely to occur. However, we show that the presence of chemisorbed H on the fullerene surface, near the Pt-rich regions, reduces the energy barriers, opposing hydrogen transfer events by 50%. We speculate about (i) the role that could be played by the atomic and electronic structure of the Pt/C interface when considering larger adsorbed platinum clusters, (ii) the importance of the presence of coadsorbed hydrogens, as well as (iii) the existence of more complex hydrogen migration paths, possibly involving subsurface diffusion or atomic displacement within deeper regions of the particles.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +52 (444) 8 26 23 62. Fax: +52 (444) 8 13 38 74. Email: guirado@ifisica.uaslp.mx. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS R.M.-C. and R.A.G.-L. would like to acknowledge the financial support from CONACyT (México) through Grants 169345 and 372584, as well as from PROMEP (SEP-México). Computer resources from the Centro Nacional de Supercómputo (CNS) of the Instituto Potosino de Investigación Cientı ́fica y Tecnológica (IPICyT), SLP, México, are also acknowledged.

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REFERENCES

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Adsorption and Diffusion of Hydrogen on C60-Supported Ptn Clusters

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