21252
J. Phys. Chem. C 2010, 114, 21252–21261
Hydrogen Adsorption on Mixed Platinum and Nickel Nanoclusters: The Influence of Cluster Composition and Graphene Support Jiang Wu,† Sheau Wei Ong,† Hway Chuan Kang,*,† and Eng Soon Tok‡ Department of Chemistry and Department of Physics, National UniVersity of Singapore, Singapore 119260, Singapore ReceiVed: September 7, 2010; ReVised Manuscript ReceiVed: October 14, 2010
The physical and chemical properties of transition metal nanoclusters have been extensively investigated. In particular, we study the energetics of the mixed clusters Pt4-nNin, focusing on the binding energy of the clusters Ebind to a graphene support, and the hydrogenation energy Eads in both the gas-phase and the graphenesupported clusters. For each cluster composition, the cluster can bind to graphene in either a face-on or an edge-on configuration, and in each of these orientations, binding can occur through different atoms; we explore these binding configurations comprehensively. We discuss the variation of Ebind and Eads with respect to the composition of the cluster and the binding configuration of the cluster to the graphene support. Our results show that hydrogen is generally chemisorbed at a Pt site and physisorbed at a Ni site, with a dependence of the adsorption energy upon the composition and the adsorption configuration. Compared with the gas-phase cluster, the chemisorption energies are generally reduced, whereas the physisorption energies are generally increased when the cluster is supported on graphene. We show that the reduction in chemisorption energies can be understood in terms of the reduction in the intracluster bond strength and the binding energy to graphene, whereas the increase in physisorption energies can be understood in terms of an increase in the charge transferred to the adsorbed hydrogen. We also show that the binding energy to graphene depends upon composition, both through the elemental identity of the atoms binding to graphene and also through the strength of the intracluster bonding. In general, Ebind is reduced upon hydrogen adsorption on the cluster. In some cases, this changes the relative binding energies of different binding configurations, thus leading to a change in the most stable cluster orientation when hydrogen adsorption occurs. Our results show that Eads varies through a significant range with cluster composition and that this variation can be effectively understood through a consideration of the changes in localized charges on the hydrogen, the cluster atoms, and the graphene upon hydrogen adsorption. This dependence should be of broader relevance to other mixed transition metal clusters. Introduction The physical and chemical properties of transition metal nanoclusters is of great current interest because of their potential applications as novel materials and also because of longstanding fundamental interest in understanding the relationship of cluster properties to bulk or atomic scale properties. These nanomaterials by virtue of their reactivity and large surface area to volume ratios are of broad interest in catalysis,1-3 and thus, extensive work has been done by many groups on characterizing their reactivity. In particular, the electrocatalytic activity of alloys of Pt with other transition metals (Ni, Co, Fe, Ti, V) has been the focus of much work.4,5 Recently, it has been shown that a volcano-shaped relationship between the experimentally measured catalytic activity and the d-band center exists, reflecting the balance between the adsorption energies and the coverage of intermediate species that block reactive sites on the surface.6,7 Both pure Pt clusters and mixed clusters of Pt and other transition metals such as Fe, Co, Ni, Cr, and Au have also been extensively studied. This is because alloys of Pt with these metals have been found to be at least as effective as pure Pt in catalyzing, for example, the oxygen reduction reaction.7-9 The * To whom correspondence should be addressed. E-mail: chmkhc@ nus.edu.sg. † Department of Chemistry. ‡ Department of Physics.
reactivity of Pt alloyed with Ni has been studied extensively by Balbuena, et al.10-17 and Stamenkovic, et al.6 The adsorption and reaction on transition metal clusters of various species such as O2, H2O, OH, H3O+, and H2O2 have been experimentally probed and theoretically calculated using density functional theory.10-17 A number of particularly interesting alloys have been studied in detail. For example, trends in the electrocatalytic activity of Pt3M (with M ) Fe, Co, Ni, Ti, V) systems with respect to the electronic structure of the alloys have been examined.6 Pt-Co alloys have also been extensively investigated with a focus on the electronic structure, magnetic moments, and the relationship between the composition of the alloy surface and reactivity toward NO, O2. A number of groups18-23 have also investigated Pt-Au materials, especially characterizing the hydrogen adsorption rate as a function of the composition. This has been investigated by calculating the hydrogen adsorption energetics for AuPt2 and AuPt3. The latter cluster has been shown to have hydrogen dissociation paths with lower activation barriers than Pt424 and is thus of interest in redox catalysis. The reactivities of the Pt4 and Pt3Co clusters toward O2, CO and H2 have also been compared theoretically.25 Particularly relevant to our interest here, it was shown that hydrogen chemisorption is energetically more favorable on Pt3Co than on Pt4 because of the increased density of states near the Fermi level due to electron transfer from Co. Structural distortion of
10.1021/jp108517k 2010 American Chemical Society Published on Web 11/16/2010
Hydrogen Adsorption on Mixed Pt, Ni Nanoclusters
J. Phys. Chem. C, Vol. 114, No. 49, 2010 21253
Figure 1. Top view (top panels) and side view (bottom panels) of the face-on (left panels) and edge-on (right panels) binding configurations to graphene.
the clusters occurs due to adsorption of H2, O2, and CO, to the extent that with CO adsorption, the Pt3Co cluster becomes planar. For these alloyed clusters, the reactivity generally depends upon the elemental identity of the adsorption site. For Pt3Co, the binding of H2 to Co is typically physisorption, whereas the binding of H2 to Pt is typically chemisorption. In addition to gas-phase clusters, the effect of supports/ matrixes such as activated carbon,14,26-28 amorphous carbon,7,29,30 silica, and zeolite31-34 are of interest. Carbon-supported Pt-Co catalyst nanoparticles have been examined experimentally and found to have improved catalytic activity compared with carbonsupported Pt.35-37 Although much work has been done, the complexity of the problem is challenging, and the range of questions pertaining to the reactivity of transition metal clusters is rather large. It is thus particularly interesting to look for organizing principles, such as the relationship between the catalytic activity and the metal d-band center, as discussed by Stamenkovic, et al.6 In this article, we report results of density functional theory calculations to investigate hydrogen adsorption energetics for Pt4-nNin supported on graphene. We consider the nature of the adsorption at both Pt and Ni sites, whether physisorption or chemisorption, and characterize theoretically the energetics of hydrogen adsorption and the binding of the cluster to the graphene support. The dependence of these upon the cluster composition and the configuration in which the cluster is bound to the graphene is calculated. We relate the energetics to the charge transfers, within the cluster itself, from the cluster to the adsorbing hydrogen molecule, and from the cluster to the graphene support. Our results provide some general insights into how adsorption energies can be understood in supported mixed clusters. Computational Method All the calculations were performed with the pseudopotential planewave density functional theory method.38 All atoms (Pt, Ni, C, and H) were modeled with the Rappe-RabeKaxiras-Joannopoulos39 ultrasoft pseudopotential with the Perdew-Burke-Ernzerhof40 generalized-gradient correction (GGA) exchange-correlation functional. To allow faster convergence, a cold smearing with a Gaussian width of 0.001 Ry or 0.014 eV was used. As calibrated in earlier work,41 the energy cutoffs for the wave function and the electron density are set at 40 and 240 Ry, respectively, giving an error in the total energy of
