Pure and Zn-doped Pt clusters go flat and upright on MgO(100)

Abstract

Pure and doped sub-nanoclusters can exhibit superb catalytic activity, which, however, strongly depends on their size, shape, composition, and the nature of the support. This work is about surface-deposited sub-nano Pt-based clusters, which are promising catalysts for the reactions of dehydrogenation. Using density functional theory and ab initio calculations, and an ab initio genetic algorithm for finding the global minima of clusters, we found a peculiar effect that Pt 5 and Pt 4 Zn clusters exhibit upon deposition on MgO(100). Both of them change shapes from the gas phase 3-D form to a planar form, and they stand upright on the support. Several reasons are responsible for this behaviour. In part, clusters go flat due to the electron transfer from the support. Indeed, the anionic Pt 5 À and Pt 4 Zn À species are flat also in the gas phase. Charging induces the second-order Jahn-Teller effect (or partial covalency) facilitated by the recruitment of the higher-energy 6p atomic orbitals on Pt into the valence manifold, and that is the reason for the planarization of the anions. Secondly, clusters maximize interactions with the surface O atoms (resulting in further favouring of 2-D structures over 3-D), and avoid contacts with surface Mg atoms (resulting in upright morphologies).

Alloying Pt Sub-nano-clusters with Boron: Sintering Preventative and Coke Antagonist?

“…Previously, the authors have shown that Pt 4 takes on a tetrahedral structure in the gas-phase, 30 while neutral Pt 5 is trigonal-bipyramidal. 11 As seen in Figure 1b the addition of B dramatically deforms the structures, reorganizing the bonding between the Pt atoms. Such distortion and planarization of metal clusters are typically attributed to the onset of partial covalency in bonding.…”

“…A similar effect was observed when considering the substitution of one of the Pt atoms with Zn. 11 The situation with B is more complex. The boron-doped cluster (Pt 4 B) undergoes morphological changes when deposited on MgO, relative to its gas-phase geometry, as shown in Figure 2.…”

“…Previously, the authors have shown that both Pt 4 and Pt 5 undergo morphological modification when attached to MgO. 11,12 For the case of Pt 4 the cluster broadens, with three Pt atoms coordinating to surface O atoms and one Pt atom on top, balancing between metal−metal and metal−oxide bonding. 12 The case of Pt 5 was more striking, with the cluster going planar and standing upright as a result of an induced partial covalency due to large charge-transfer from the support.…”

“…Such distortion and planarization of metal clusters are typically attributed to the onset of partial covalency in bonding. 11,51,52 As a comparison, the Pt cluster was also doped with C ( Figure 1a) to determine how the presence of a coke-initiator would affect gas-phase cluster morphology. In both cases, the introduction of maingroup chemistry tremendously distorts pure Pt clusters and is done so in a similar way.…”

“…For some elements, such as Au, where relativistic effects become significant or in some charged clusters, like Pt 5 − , the s-and pAOs get recruited into the valence shell, sometimes leading to the formation of planar clusters with a partially covalent character of chemical bonding. 11,21 On the other hand, when clusters of Al (globular when composed only of Al atoms) were doped with B, they were shown to undergo notable structural transformations. 19 The presence of just one or two B atoms was enough to make the clusters flat or nearly flat.…”