Adsorption of H, H2, and H2O inside and outside of (M@Si16F)6 tubelike aggregates and wires (M = V, Ta). A first principles study

We have studied theoretically the adsorption of atomic and molecular hydrogen, as well as H2O molecules, on porous materials assembled from M@Si16F molecular units (M = V, Ta). Here we consider the (M@Si16F)6 star-like aggregate formed by two (M@Si16F)3 triangular supermolecules stacked along the vertical axis and twisted 60° each other, as well as the infinite wire which unit cell is that (M@Si16F)6 aggregate. That aggregate forms an internal barrel-shaped porous which size is ideal to encapsulate small molecules like H2 and H2O. Similarly, the external space between the arms of these star-like systems is also ideal to adsorb small molecules. Our calculations were performed using density functional theory (DFT) within the generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) for the exchange and correlation functional. From these calculations we have shown that adsorption of molecular H2 occurs preferably outside of (M@Si16F)6 tube-like structures and when the M dopant is Vanadium rather than Tantalum. The binding energy is higher for the infinite wire than for the finite aggregate. In order to estimate the effect of dispersion interactions, we studied the adsorption of H2 on a single M@Si16F molecular unit using the non local correlation van der Waals (vdW) functional of Klimes, Bouwler, and Michaelides (KBM). The corresponding H2 binding energy is about a factor 3 larger than that from the GGA–PBE approach for M = V dopant. From Mulliken population analysis it is shown that GGA–PBE describes the bond between H2 and M@Si16F as chemisorption whereas vdW–KBM approach leads to the physisorption picture. Considering all the finite ring and infinite tube-like systems studied in this work, the binding energy per H atom is larger than half the dissociation energy of gas phase H2 only for the V@Si16F finite aggregate, that is, an exothermic dissociative H2 adsorption is possible only in that case. Finally, we have shown that the encapsulation of a single H2O molecule inside of these M-doped silicon structures is favored (exothermic) when the H2O axis is not parallel to the aggregate or wire axis.