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

Nanoporous silica systems with porosity between 30% and 70% were developed using two Molecular Dynamics (MD) simulation protocols to obtain structures with dissimilar pore morphologies. Short-and medium-range structural characteristics including bond angle distributions and pair distribution functions were analyzed and found to be consistent with experimental results. Surface area to volume ratio and pore microstructures were characterized and compared with experimental observations. Mechanical properties including elastic, shear, and bulk moduli of these nanoporous silica systems were calculated and their change as a function of porosity was compared with experimental data and theoretical models. It was found that the elastic modulus of porous silica with 50% porosity is 5-14 GPa which is consistent with experimental results. The elastic moduli-porosity relationship was fitted by exponential and power functions, and analysis of coefficients was performed to obtain microstructure characteristics of the simulated nanoporous silica structures. This works confirms that two distinct nanoporous silica microstructures are generated with MD simulations which result in variations in mechanical properties and highlight the importance of selecting a nanoporous silica simulation method which approximates experimental systems.

Computer simulations at the atomistic scale play an increasing important role in understanding the structure features, and the structure-property relationships of glass and amorphous materials. In this paper, we reviewed atomistic simulation methods ranging from first principles calculations and ab initio molecular dynamics (AIMD) simulations, to classical molecular dynamics (MD), and meso-scale kinetic Monte Carlo (KMC) simulations and their applications to study the reactions and interactions of inorganic glasses with water and the dissolution behaviors of inorganic glasses. Particularly, the use of these simulation methods in understanding the reaction mechanisms of water with oxide glasses, water-glass interfaces, hydrated porous silica gels formation, the structure and properties of multicomponent glasses, and microstructure evolution are reviewed. The advantages and disadvantageous of these simulation methods are discussed and the current challenges and future direction of atomistic simulations in glass dissolution presented.npj Materials Degradation (2017) 1:16 ; doi:10.1038/s41529-017-0017-yThe corrosion or degradation of glasses in aqueous solutions are critical in a number of engineering and technological processes ranging from microelectronic packaging, glass reaction chambers, and the immobilization of nuclear waste materials, as well as in healthcare and biomedical fields such as dissolution of inhaled glass fibers and bioactive glasses for biomedical applications. In particular, immobilizing radioactive waste in borosilicate glasses is widely accepted as a preferred method to treat nuclear waste materials generated from civilian and military sources. This process, also known as vitrification, is a critical component of the cycle of nuclear energy to combat global environmental and energy challenges. Researchers from around the world have extensively invested in understanding glass corrosion in an effort to predict the long-term stability and release rate radionuclides to the environment during nuclear waste storage. 1,2Various mechanisms for glass corrosion have been proposed and despite intensive experimental investigations with advanced characterization techniques results are unclear. It is generally accepted that the corrosion of glass consists of a set of complex processes including hydration, hydrolysis, and ion-exchange that are coupled during glass dissolution. The initial stage is interdiffusion of proton or hydronium ions from the solution with sodium or other alkali ions in the glass.3 This is followed by the hydrophilic attack of water on the Si-O-Si or Si-O-Al linkages that lead to hydroxylation of the silicate glass network. The remaining hydrolyzed glass skeleton then undergoes condensation and repolymerization to form the hydrated nanoporous silica rich gel layer which can be protective, decreasing dissolution to a residual rate. [4][5][6] The morphology of the gel layer, such as thickness, pore structure, and chemical composition, depends on the original glass composition and the pH...

The magnetic susceptibility of NO x -loaded RE-DOBDC (rare earth (RE): Y, Eu, Tb, Yb; DOBDC: 2,5-dihydroxy­terephthalic acid) metal–organic frameworks (MOFs) is unique to the MOF metal center. RE-DOBDC samples were synthesized, activated, and subsequently exposed to humid NO x . Each NO x -loaded MOF was characterized by powder X-ray diffraction, and the magnetic characteristics were probed by using a VersaLab vibrating sample magnetometer (VSM). Lanthanide-containing RE-DOBDC (Eu, Tb, Yb) are paramagnetic with a reduction in paramagnetism upon adsorption of NO x . Y-DOBDC has a diamagnetic moment with a slight reduction upon adsorption of NO x . The magnetic susceptibility of the MOF is determined by the magnetism imparted by the framework metal center. The electronic population of orbitals contributes to determining the extent of magnetism and change with NO x (electron acceptor) adsorption. Eu-DOBDC results in the largest mass magnetization change upon adsorption of NO x due to more available unpaired f electrons. Experimental changes in magnetic moment were supported by density functional theory (DFT) simulations of NO x adsorbed in lanthanide Eu-DOBDC and transition metal Y-DOBDC MOFs.