Acid gases (e.g., NO x and SO x ), commonly found in complex chemical and petrochemical streams, require material development for their selective adsorption and removal. Here, we report the NO x adsorption properties in a family of rare earth (RE) metal–organic frameworks (MOFs) materials. Fundamental understanding of the structure–property relationship of NO x adsorption in the RE-DOBDC materials platform was sought via a combined experimental and molecular modeling study. No structural change was noted following humid NO x exposure. Density functional theory (DFT) simulations indicated that H2O has a stronger affinity to bind with the metal center than NO2, while NO2 preferentially binds with the DOBDC ligands. Further modeling results indicate no change in binding energy across the RE elements investigated. Also, stabilization of the NO2 and H2O molecules following adsorption was noted, predicted to be due to hydrogen bonding between the framework ligands and the molecules and nanoconfinement within the MOF structure. This interaction also caused distinct changes in emission spectra, identified experimentally. Calculations indicated that this is due to the adsorption of NO2 molecules onto the DOBDC ligand altering the electronic transitions and the resulting photoluminescent properties, a feature that has potential applications in future sensing technologies.

Detailed understanding of the reactions and processes which govern silicate–water interactions is critical to geological, materials, and environmental sciences. Interactions between water and nanoporous silica were studied using classical molecular dynamics with a Reactive Force Field (ReaxFF), and the results were compared with density functional theory (DFT) based ab initio molecular dynamics (AIMD) simulations. Two versions of ReaxFF Si/O/H parametrizations (Yeon et al. J. Phys. Chem. C2016120305 and Fogarty et al. J. Chem. Phys.2010132174704) were compared with AIMD results to identify differences in local structures, water dissociation mechanisms, energy barriers, and diffusion behaviors. Results identified reaction mechanisms consisting of two different intermediate structures involved in the removal of high energy two-membered ring (2-Ring) defects on complex nanoporous silica surfaces. Intermediate defects lifetimes affect hydroxylation and 2-Ring defect removal. Additionally, the limited internal volume of the nanoporous silica results in decreased water diffusion related to the development of nanoconfined water. Hydrogen atoms in the water diffused 10–30% faster than the oxygen atoms, suggesting that increased hydrogen diffusion through hydrogen hopping mechanisms may be enhanced in nanoconfined conditions. Comparison of the two different ReaxFF parametrizations with AIMD data indicated that the Yeon et al. parameters resulted in reaction mechanisms, hydroxylation rates, defect concentrations, and activation energies more consistent with the AIMD simulations. Therefore, this ReaxFF parametrization is recommended for future studies of water–silica systems with high concentrations of surface defects and highly strained siloxane bonds such as in complex silica nanostructures.

We present the development of a ReaxFF reactive force field for Na/Si/O/H interactions, which enables reactive molecular dynamics simulation of the sodium silicate–water interfaces. The force field parameters were fitted against various quantum mechanical calculations, including equations of state of different NaSiO x crystalline phases, energy barriers of a sodium cation’s transport within the sodium silicate crystal structure, interactions between the hydroxylated silica surface and sodium cation–water systems, and dissociation energies of [NaOH–n(H2O)] (n = 1–6) clusters. After the optimization process, we validated the force field capability through calculating the structures of sodium silicate crystals and glasses and transport properties of sodium ions and protons within the amorphous structures. The force field was also applied to validate the dissociation behavior of sodium hydroxides within the bulk water. Our results with the developed force field are relevant to detailed chemical dissolution mechanisms, which involve (a) the interdiffusion process of sodium ions from glasses and protons from water, (b) subsequent ionic self-diffusion of sodium ions from the subsurface region to vacancy sites at the glass–water interface, and (c) sodium ions interaction with water after leaching from the amorphous sodium silicate system.

Surface energies of silicates influence crack propagation during brittle fracture and decrease with surface relaxation caused by annealing and hydroxylation. Molecular-level simulations are particularly suited for the investigation of surface processes. In this work, classical MD simulations of silica surfaces are performed with two force fields (ClayFF and ReaxFF) to investigate the effect of force field reactivity on surface structure and energy as a function of surface hydroxylation. An unhydroxylated fracture surface energy of 5.1 J/m is calculated with the ClayFF force field, and 2.0 J/m is calculated for the ReaxFF force field. The ClayFF surface energies are consistent with the experimental results from double cantilever beam fracture tests (4.5 J/m), whereas ReaxFF underestimated these surface energies. Surface relaxation via annealing and hydroxylation was performed by creating a low-energy equilibrium surface. Annealing condensed neighboring siloxane bonds increased the surface connectivity, and decreased the surface energies by 0.2 J/m for ClayFF and 0.8 J/m for ReaxFF. Posthydroxylation surface energies decreased further to 4.6 J/m with the ClayFF force field and to 0.2 J/m with the ReaxFF force field. Experimental equilibrium surface energies are ∼0.35 J/m, consistent with the ReaxFF force field. Although neither force field was capable of replicating both the fracture and equilibrium surface energies reported from experiment, each was consistent with one of these conditions. Therefore, future computational investigations that rely on accurate surface energy values should consider the surface state of the system and select the appropriate force field.

Mechanistic insight into the process of crack growth can be obtained through molecular dynamics (MD) simulations. In this investigation of fracture propagation, a slit crack was introduced into an atomistic amorphous silica model and mode I stress was applied through far-field loading until the crack propagates.Atomic displacements and forces and an Irving-Kirkwood method with a Lagrangian kernel estimator were used to calculate the J-integral of classical fracture mechanics around the crack tip. The resulting fracture toughness (K IC ), 0.76 AE 0.16 MPa√m, agrees with experimental values. In addition, the stress fields and dissipation energies around the slit crack indicate the development of an inelastic region~30 A in diameter. This is one of the first reports of K IC values obtained from up-scaled atomic-level energies and stresses through the J-integral.The application of the ReaxFF classical MD force field in this study provides the basis for future research into crack growth in multicomponent oxides in a variety of environmental conditions. K E Y W O R D S atomistic modeling, bulk amorphous materials, fracture toughness, generalized J-integral