Oxygen Adsorption on β-Cristobalite Polymorph: Ab Initio Modeling and Semiclassical Time-Dependent Dynamics

Abstract

The adsorption dynamics of atomic oxygen on a model beta-cristobalite silica surface has been studied by combining ab initio electronic structure calculations with a molecular dynamics semiclassical approach. We have evaluated the interaction potential of atomic and molecular oxygen interacting with an active Si site of a model beta-cristobalite surface by performing DFT electronic structure calculations. As expected, O is strongly chemisorbed, E(b) = 5.57 eV, whereas molecular oxygen can be weakly adsorbed with a high-energy barrier to the adsorption state of approximately 2 eV. The binding energies calculated for silica clusters of different sizes have revealed the local nature of the O,O(2)-silica interaction. Semiclassical collision dynamic calculations show that O is mainly adsorbed in single-bounce collisions, with a smaller probability for adsorption via a multicollision mechanism. The probability for adsorption/desorption (reflected) collisions at the three impact energies is small but not negligible at the higher energy considered in the trajectory calculations, about P(r) = 0.2 at E(kin) = 0.8 eV. The calculations give evidence of a complex multiphonon excitation-deexcitation mechanism underlying the dynamics of stable adsorption and inelastic reflection collisions.

Oxygen Adsorption on β-Quartz Model Surfaces: Some Insights from Density Functional Theory Calculations and Semiclassical Time-Dependent Dynamics

“…16 The same trend has also been observed when we analyzed the potential energy curves calculated by increasing the size of the surface models to Si 8 When the O 2 molecule is concerned, the potential energy minimum profile for the O 2 −Si 3 O 4 H 6 system is found to involve, at a distance between the nearest atom to the surface oxygen of the O 2 and the on top Si in the range 1.75−4.00 Å, two triplet states in which the incoming molecule can be found in its ground state, that is, X 3 Σ g − , and in interaction with the 1 [Si 3 O 4 H 6 ] cluster (profile termed as triplet a in Figure 3, lower panel), or in a 1 Σ g + state in combination with a 3 [Si 3 O 4 H 6 ] moiety (triplet c ). At shorter distances, the presence of a spin crossing lying at almost 1.74 Å enables a second spin variation toward a singlet spin multiplicity curve (singlet profile in Figure 3 7 This analysis clearly evidences that the two silica polymorphs (β-quartz and β-cristobalite) show essentially the same PESs with respect to the oxygen sticking trajectory (Table 1 and Figure 4). This is due to the fact that the chemical interaction appears to be mainly confined within the silicon atom on top and, to a lesser extent, to its nearest oxygen atoms.…”

“…In turn, a fully classical description of the collisional pathways without any quantummechanics refinement of the N, O, N 2 , and O 2 interactions with silica polymorphs should be considered, at least in principle, as affected by such a limitation during the attachment to the surface of the adsorbant. 6 In this contribution, inspired by the above computational evidence, 6,7 we have addressed the O and O 2 interaction potentials with an on top silicon atom and its nearest oxygen both localized over predefined β-quartz surface models cleaved from the unit cell as reported in ref 12 (Figure 1). To this end, and for a direct comparison with previous data obtained for β-cristobalite, we used the hybrid PBE0 functional 13 based on Perdew−Burke−Erzenrhof exchange−correlation functionals 14 in which a portion of Hartree−Fock exchange is added selfconsistently to the DFT (PBE) contribution.…”

Molecular Physics of Elementary Processes Relevant to Hypersonics: Atom-Molecule, Molecule-Molecule and Atoms-Surface Processes

“…As previously mentioned the most critical step in molecular dynamics studies is the determination of a sufficiently accurate PES able to describe correctly the process under investigation. For the case of H, H 2 interaction over β -cristobalite some of us have recently developed a multidimensional PES based on electronic structure DFT calculations [67] whereas for the case of the O, O 2 interaction over β -cristobalite two different PESs have been developed by the two groups: a first one based on DFT cluster calculations [68] and another one based on DFT periodic boundary conditions calculations [69].…”

Molecular Dynamics Studies on Fundamental Molecular Surface Processes

“…The calculations were performed within the size-scaled cluster approach [11,12]. Spin polarized wave functions were used as basis sets.…”

“…We notice that atomic oxygen and atomic nitrogen are chemisorbed on top of a Si active site with a binding energy close to 6.0eV and 2.7eV, respectively. On the other hand, molecular oxygen can be slightly chemisorbed on condition that an activation energy of about 2eV is gained by the molecule to surmount the energy barrier [11]. These results have some important consequences on the dynamics of the surface processes.…”