Global Optimization of FeNi Clusters by Basin Hopping: Insight into Awaruite Cluster Structures with High Hydrogenation Capacity

Abstract:

Naturally occurring Iron-nickel alloys form during water-rock interaction in the Earth’s crust (serpentinization), and stand out as a somewhat rare but highly catalytic phase. Interestingly, these “new” alloys are now receiving huge attention, primarily because of their catalytic capacity to accelerate bond rupture and formation processes in small organic molecules relevant to the origin of life, but also because they are viewed as nature’s stockpile of alloy surfaces that would assist in the hydrogenation of greenhouse gases (reductive sequestration).

Two forms of ‘Ni-Fe’ metal alloys have been examined in detail here, namely awaruite (Ni₈₁Fe₁₉) and tetrataenite (Ni₅₀Fe₅₀); however, intermediate, i.e. solid solutions do exist along the Fe-Ni binary, so that a large group of Ni-Fe alloy phases would probably form during serpentinization-type processes. In this study, the atomic structure of Ni-Fe alloy will be investigated containing 75 nickel atoms and 25 iron atoms (Ni₇₅Fe₂₅), a composition closely related to awaruite. We will also present results in which we have examined the occurrence of so-called magic number awaruite clusters at the positions n=13,55,147, all of which exhibit a high stability relative to their atom number precursors and successors. Awaruite clusters with n=13,55,147 stand out in terms of the total atom binding energy and maintain highly organized structures; in the case of n=13 the cluster is comprised of central metal atoms sandwiched between two pentameric ring moieties. These clusters may react with molecular hydrogen in preparation for various hydrogenation reactions that ultimately give rise to small but important molecules such as ammonia, methane, ethane etc. Here we will present results of a series of molecular simulations that we examine specific sites on the “awaruite” cluster surface and the affinity of H₂ to attach and dissociate on various surface sites of awaruite clusters. These simulations build on a basin-hopping algorithm and density functional calculations, in which stable structures of, i.e. Fe_n·H, Ni_n·H, Fe_13-n-Ni_n·H and related clusters are predicted. A detailed discussion of these results, particularly their relevance for hydrogenation processes on early Earth and its impact on Earth’s inventory of prebiotic molecules will be presented.