Hydrogen Arrangements on Defective Quasi-Molecular BN Fragments

Abstract

Considering the ever-increasing interest in metal-free materials, some potential chemical applications of quasi-molecular boron nitride (BN) derivatives were tested. Specifically, the behavior of BN fragments was analyzed when given defects, producing local electron density changes, were introduced by using topological engineering approaches. The inserted structural faults were Schottky-like divacancy (BN-d) defects, assembled in the fragment frame by the subtraction of one pair of B and N atoms or Stone–Wales (SW) defects. This study is aimed at highlighting the role of these important classes of defects in BN materials hypothesizing their future use in H2-based processes, related to either (i) H2 activation or (ii) H2 production, from preadsorbed hydrogenated molecular species on BN sites. Here, it has been observed that BN species, embodying SW defects, are characterized by endothermic H2 adsorption and fragmentation phenomena in order to guess their potential use in processes based on the transformation or production of hydrogen. On the contrary, in the presence of BN-d defects, and for reasons strictly related to local structural changes occurring along with the hydrogen rearrangements on the defective BN fragments, a possible use can be inferred. Precautions must be however taken to decrease the material rigidity that could actually decrease the ability of the BN fragment to flatten. This conversely seems to be a necessary requirement to have strong exothermic effects, following the rearrangements of the H2 molecules.

Adsorption of Polycyclic Aromatic Hydrocarbons and C60 onto Forsterite: C–H Bond Activation by the Schottky Vacancy

“…The latter show larger angle deviation from 120° with respect to the other PAHs because of the presence of a five-membered ring that causes major bending of its structure. The major flexibility due to the presence of a five-membered ring has been identified also in other materials with PAH-like structures . The and dihedral Θ angles reported in Table indicate that PAHs adsorbed on the surface have the 120° angles expected for planar PAHs, whereas the fullerene (C 60 ) deviates from this by about 6°, as expected for a buckyball-like structure (Figure a,b,c) .…”

“…The major flexibility due to the presence of a five-membered ring has been identified also in other materials with PAH-like structures. 39 The CCC and dihedral Θ angles reported in Table 2 indicate that PAHs adsorbed on the surface have the 120°angles expected for planar PAHs, whereas the fullerene (C 60 ) deviates from this by about 6°, as expected for a buckyballlike structure (Figure 5a,b,c). 61 Thus, we conclude that the adsorption process does not induce significant changes in the conformational structure of the PAHs relative to the gas phase, apart from a moderate loss in planarity.…”

“…The bottom layers (about 4 Å thickness) have been constrained during the whole optimization procedure, whereas the topmost layers were allowed to relaxed unconstrainedly. The presence of defects has proved to increase the reactivity and catalytic activity of several materials. − Therefore, we decided to model the defective surfaces of [010]-fo by replacing an undercoordinated Mg atom on the surface with only one transition metal (e.g., Fe and Ni for this work) and optimizing the resulting slab model. Furthermore, V MgO -[010]-fo is created by removing a MgO unit from the [010]-fo surface, again followed by a reoptimization of the slab.…”

Do defects in PAHs promote catalytic activity in space? Stone–Wales pyrene as a test case

“…boron nitrides. 14 The defect is produced upon a 901 rotation of a C-C bond located in a hexagonal carbon ring, creating two seven-membered and two five-membered rings. The generation of SW defects can cause a curvature of the material that hosts it, but in the case of graphene or other planar hexagonal materials this defect results in a planar structure.…”