Stability of Ni Clusters and the Adsorption of CH₄: First-Principles Calculations

Abstract

Structural, magnetic and adsorption properties of Ni n (n = 2−16, 21, 55) clusters have been investigated based on density funciontal theory (DFT) with the spin polarized generalized gradient approximation, using the Perdew−Burke−Ernzerhof functional. The most stable isomers have been selected to study the adsorption of methane CH 4 and methyl CH 3 . It is found that the CH 4 molecule adsorbs on the top site for all clusters considered. The most selective Ni n clusters are the tetrahedron (n = 4) and icosahedral clusters due to highcoordinated edge atoms (n = 13, 21, and 55). For CH 3 , stronger adsorption tendencies were found with similar patterns. Our results show that clusters with n = 6, 10, and 13 with complete atomic shells are relatively more stable. Besides, they perform the lowest adsorption for CH 3 , indicating that they possess such a desirable property of a higher carbon poisoning resistance, than for the rest of the clusters. This result can be understood in terms of the electronic stability and localization of the frontier molecular orbitals. ■ INTRODUCTIONSmall clusters and nanoparticles have been widely studied in the past few years due to their high surface to volume ratio and enhanced reactivity properties compared with their bulk counterparts. 1 For example, gold nanoparticles are active catalyts although gold in the bulk is practically inert. 2,3 In catalytic reactions, they usually increase the selectivity and bond dissociation steps since they have more edge sites and lowcoordinated atoms. 4 Although in some reactions the presence of edge sites on the surface of nanoparticles can reduce the specific catalytic activity, 5 they are useful for different reactions, such as decomposition of hydrocarbons and alcohols, which can serve as important hydrogen sources. 6,7 Recently, much attention has been paid to the hydrogen production relating to the fuel cell technology. 8 In practice, hydrogen is produced from natural gas via steam reforming of methane, a process highly endothermic and very expensive, due to the high heat demand. 9 Among a wide range of heterogeneous catalyst, noble metal catalysts show higher activity and stability for this reaction, but the prohibitive cost and scarce resources makes their use very limited. On the other hand, Ni-based are promising catalysts in terms of cost and outstanding activity compared with noble-metals. However, the major problem of Ni-based systems is the strongly bonded carbon deposition on the catalyst surface which can deactivate the catalyst reducing its stability. 10 In a previous work, 11 we have shown that small Rh clusters possess an excellent selectivity to catalytically adsorb and dissociate the acid rain precursor N 2 O molecule. Furthermore, in the limit of going to arbitrary large clusters, it was interesting to compare this effect with that of a flat (111) slab of same metal, which happened to be provided with a much more smaller selectivity as the corresponding to small clusters. The problem, however, appears when we begin to consider cl...

Oxidation of Ni13 clusters

“…An interesting feature is that the oxygen chemisorption leads to a stabilization of an icosahedral Ni core in all configurations observed. The same is seen in two studies which found the capped bipyramid cluster to be the ground-state geometry: with CH 4 adsorption [48] or with one atom substituted by Fe, Co, or Ru, [46] the cluster core is stabilized in the icosahedral configuration.…”

“…[39,40] It is expected that the icosahedral cluster is the ground-state structure, [20,[40][41][42][43] but previous theoretical studies found that this is not the case. Instead, it was proposed that distorted [44,45] or the capped bipyramid (hexagonal close-packed) clusters [46][47][48] be the ground-state geometry. To elucidate this situation, we reproduced these results.…”

Graphene and graphene nanomesh supported nickel clusters: electronic, magnetic, and hydrogen storage properties

“…1a,b) show average Ni-Ni bond lengths of 2.31Å and 2.37Å, respectively, and magnetic moments per Ni atom of 1.4 µ B and 0.9 µ B , in agreement with previous calculations. 31,32 The density of states (DOS) ( Fig. 1c,d) demonstrates that the We calculate the storage capacities of the Ni x clusters (x = 6, 13) by adding H molecules, one at a time,allowing the system to structurally relax after each addition.…”

Theoretical insights into C–H bond activation of methane by transition metal clusters: the role of anharmonic effects

“…at a finite temperature (T ) and pressure (p). To better understand the situation theoretically, we consider a prototypical model system of nickel (Ni 4 , which is already experimentally synthesized and is having high selectivity [45][46][47] ) in a reactive atmosphere of O 2 and CH 4 gas molecules under realistic conditions. Note that Ni-based catalysts, owing to their low cost, high selectivity and high activity, have been extensively employed experimentally in catalysis over the past.…”