Small-Molecule Activation Driven by Confinement Effects
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2014 American Chemical Society. Electron-rich environments may arise from interactions between nanostructures at subnanometer separations and have been shown able to facilitate molecular dissociation of simple diatomic molecules. Electrons in these regions have energies close to the Fermi energy of the structures contributing to the electron-rich environment. These energies can be tuned to achieve specific interactions with the LUMO of molecules of interest. In this work, we focus on electron-rich regions generated between platinum nanoclusters and carbon nanotubes in the confined space defined by a nanopillared graphene (NPG) structure decorated with Pt22 nanoparticles. First, the reactivity of these regions is evaluated using density functional theory by comparing the bond strengths of O2, CO, and N2 adsorbed on Pt22 with that of the molecules adsorbed on similar surfaces in absence of surrounding electron-rich regions. Results show larger charge transfer to the adsorbed molecules in the presence of the electron-rich environments. These excesses of charge are located in molecular antibonding LUMO orbitals, thus weakening their molecular bonds and facilitating their dissociation. Second, the dissociation of O2 and CH4 molecules is investigated in the proposed Pt22/NPG structure at different temperature and pressure conditions and compared with that taking place in a larger-pore-size Pt26/graphite system using ab initio molecular dynamics simulations. The presence of the electron-rich regions may affect O2 dissociation via two mechanisms: (a) generating wider spectra of charges available for transfer to adsorbed O2 molecules which results in bond-length oscillations that facilitate molecular dissociation; (b) increasing the amount of charge transferred to adsorbed molecules, debilitating their molecular bond. On the other hand, when gas phase methane is exposed to electron-rich environments, it is found that the larger barrier between the Fermi energy of the system and the LUMO orbital of CH4 impedes the interaction of the electron-rich regions with the molecular LUMO. However, enhanced reactivity driven by the confined nature of the electron-rich environment in Pt22/NPG could also arise in this case as a result of a geometric effect, inducing an increased frequency of interaction between catalysts and molecules. (Figure Presented).