The physisorption/chemisorption of atomic hydrogen on a slab model of the Mg2SiO4 forsterite (010) surface mimicking the interstellar dust particle surface has been modeled using a quantum mechanical approach based on periodic B3LYP-D2* density functional calculations (DFT) combined with flexible polarized Gaussian type basis sets, which allows a balanced description of the hydrogen/surface interactions for both minima and attachment of H at the surface at 100 K, but prevents the same process to occur at 10 K.From this H-chemisorbed state, second hydrogen chemisorption mainly occurs on the neighboring Mg ion, thus forming a Mg-H surface group, giving rise to a surface species stabilized by favorable electrostatic interactions between the OH + /H -Mg pair. The formation of molecular hydrogen at the (010) forsterite surface adopting a LangmuirHinshelwood mechanism takes place either starting from two physisorbed H atoms with an almost negligible kinetic barrier through a spin-spin coupling driven reaction or from two chemisorbed H atoms with a barrier surmountable already at T higher than 10 K. We also suggest that a nanosized model of the interstellar dust built from a replica of the forsterite unit cell is able to adsorb half the energy released by the H2 formation by increasing its temperature by about 50 K which could then radiates in about 0.02 s.

B3LYP periodic simulations have been carried out to study some physico-chemical properties of the bulk structures and the corresponding non-polar (010) surfaces of Mgpure and Fe-containing olivine systems; i.e, Mg2SiO4 (Fo) and Mg1.5Fe0.5SiO4 (Fo75). A detailed structural analysis of the (010) In contrast, the IR spectra of the surfaces are shifted to upper values and exhibit more bands compared to the corresponding bulk systems due to the shorter metal-O distances given in the coordinatively unsaturated metals and to symmetry reduction which brings non-equivalent motions between the outermost and the internal modes, respectively.

The adsorption of H atoms and their recombination to form an H 2 molecule on slab models of the crystalline Mg 2 SiO 4 forsterite (001) and (110) surfaces was studied by means of quantum mechanical calculations based on periodic density functional theory (DFT). Present results are compared with those previously reported for the most stable (010) surface, showing the relevance of the surface morphology and their stability on the H 2 formation. Different H chemisorption states were identified, mostly on the outermost O atoms of the surfaces. In agreement with the higher instability of the (001) and (110) surfaces, the calculated adsorption energies are larger than those for the (010) surface. Computed energy barriers for the H hopping on these surfaces are exceedingly high to occur at the very low temperatures of deep space. For the adsorption of two H atoms, the most stable complexes are those in which the H atoms form Mg-H and SiOH surface groups. From these complexes, we did not identify energetically feasible paths for H 2 formation through a Langmuir-Hinshelwood mechanism on the (001) surface because the initial states are more stable than the final products. However, on the (110) surface one path was found to be exoergic with very low energy barriers. This differs to that observed for the (010) surface, for which two feasible Langmuir-Hinshelwoodbased channels were identified. H 2 formation through the Eley-Rideal mechanism was also simulated, in which an incoming H atom impinges on a pre-adsorbed H atom at the (001) and (110) surfaces in a barrierless way.

Using periodic DFT-D2 methods, atomistic simulations of interstellar H adsorption and H2 formation on a (010) Fe-containing olivine surface are presented. At variance with the (010) Mg2SiO4 surface and key to these processes are the large Fe/H interaction energies, suggesting that olivine surfaces are good reservoirs of H atoms for subsequent recombination to form H2.

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A crucial step in the preparation of supported metal catalysts is related to the choice of support, since it regulates the anchoring of the metal species in an environment of strong interactions. In this sense, the binding on the support, which may present defects, as well as induced modifications of the electronic structure, helps to determine efficient metal/support combinations for catalysis applications. In this work, first-principles studies have been carried out to model and describe the geometric and electronic properties of O-functionalized graphene as a model carbon support, in addition to a single Pd atom and Pd 13 nanocluster (experimentally observed on various carbon supports) as supported metal catalysts. The carbon-based nanomaterial includes experimentally probed abundant oxygen functional groups and point defects, demonstrating high thermal stability at room temperature. The characterization confirms the structural motifs presented in the model, distinguishing the organic functionalities and various defects using spectroscopy techniques. Interestingly, the oxygenated carbon-based support presents a metallic character when the formal charges in the embedded Pd atom and nanocluster have been determined to be [Pd] + and [Pd 13 ] 3+ . The hydrogenation process of the adsorbed cluster is also presented, with the determination of a ratio between the adsorbed hydrides and the surface metal atoms close to unity at room temperature, thanks to ab initio molecular dynamics.

Silicates are among the most abundant and important inorganic materials, not only in the Earth’s crust, but also in the interstellar medium in the form of micro/nanoparticles or embedded in the matrices of comets, meteorites, and other asteroidal bodies. Although the crystalline phases of silicates are indeed present in nature, amorphous forms are also highly abundant. Here, we report a theoretical investigation of the structural, dielectric, and vibrational properties of the amorphous bulk for forsterite (Mg2SiO4) as a silicate test case by a combined approach of classical molecular dynamics (MD) simulations for structure evolution and periodic quantum mechanical Density Functional Theory (DFT) calculations for electronic structure analysis. Using classical MD based on an empirical partial charge rigid ionic model within a melt-quenching scheme at different temperatures performed with the GULP 4.0 code, amorphous bulk structures for Mg2SiO4 were generated using the crystalline phase as the initial guess. This has been done for bulk structures with three different unit cell sizes, adopting a super-cell approach; that is, 1 × 1 × 2, 2 × 1 × 2, and 2 × 2 × 2. The radial distribution functions indicated a good degree of amorphization of the structures. Periodic B3LYP-geometry optimizations performed with the CRYSTAL14 code on the generated amorphous systems were used to analyze their structure; to calculate their high-frequency dielectric constants (ε∞); and to simulate their IR, Raman, and reflectance spectra, which were compared with the experimental and theoretical crystalline Mg2SiO4. The most significant changes of the physicochemical properties of the amorphous systems compared to the crystalline ones are presented and discussed (e.g., larger deviations in the bond distances and angles, broadening of the IR bands, etc.), which are consistent with their disordered nature. It is also shown that by increasing the unit cell size, the bulk structures present a larger degree of amorphization.

The addition of unsaturated molecules on H-terminated silicon surfaces is a way to functionalize them. Usual procedures are initiated by an H–Si bond cleavage, yielding an Si dangling bond, which in turn reacts with unsaturated molecules to form a carbon-centered radical intermediate (process called hydrosilylation). This intermediate can evolve according to two different channels: (i) via H-abstraction of a neighboring Si–H surface group, thus forming a stable adsorbed species and leading to a new Si dangling bond, which is able to newly undergo the same processes with other incoming unsaturated molecules; or (ii) by reacting with a second unsaturated molecule hence elongating the organic chain. In this work, these processes (i.e., hydrosilylation, surface H-abstraction, and alkyne addition) have been studied by means of hybrid B3LYP-D* and BHLYP-D* periodic calculations considering the reaction of acetylene with the (111) and (100) H-terminated silicon surfaces. The computed potential energy surfaces indicate that both processes are fairly competitive, as the energy barriers are similar and the reaction energies are large and negative. However, the computed free energy profiles indicate that polymerization has a significantly higher energy barrier than the H-abstraction reaction due to disfavored entropic effects, in agreement to what is observed to occur experimentally. Moreover, significant differences in the computed energy profiles between the two surfaces are observed, the kinetics and thermodynamics of these reactions being more favorable on H–Si(111).