In the subsurface, MgO engineered barriers are employed at the Waste Isolation Pilot Plant (WIPP), a transuranic waste repository near Carlsbad, NM. During service, the MgO will be exposed to high concentration brine environments and may form stable intermediate phases that can alter the barriers effectiveness. Here, MgO was aged in water and three different brine solutions. X-ray diffraction (XRD) and 1H nuclear magnetic resonance (NMR) analysis were performed to identify the formation of secondary phases. After aging, ~4% of the MgO was hydrated and fine-grained powders resulted in greater loss of crystallinity than hard granular grains. 1H magic angle spinning (MAS) NMR spectra resolved minor phases not visible in XRD, indicating that diverse 1H environments are present along with Mg(OH)2. Density functional theory (DFT) simulations for several proposed Mg-O-H, Mg-CI-O-H, and Na-O-H containing phases were performed to index peaks in the experimental 1H MAS NMR spectra. While proposed intermediate crystal structures exhibited overlapping 1H NMR peaks, Mg-O-H intermediates were attributed to the growth of the 1.0-0.0ppm peak while the Mg-CI-O-H structures contributed to the 2.5- 5.0ppm peak in the chloride containing brines. Overall, NMR analysis of aged MgO indicates the formation of a range of possible intermediate structures that cannot be resolved with XRD analysis alone.
Infrastructure resiliency depends on the ability of infrastructure systems to withstand, adapt, and recover from chronic and extreme stresses. In this white paper, we address the resiliency of infrastructure assets and discuss improving infrastructure stability through development of our understanding of cement and concrete degradation. The resiliency of infrastructure during extreme events relies on the condition, adaptability, and recoverability of built infrastructure (roads, bridges, dams), which serves as the backbone of existing infrastructure systems. Much of the built infrastructure in the US has consistently been rated D+ by the American Society of Civil Engineers (ASCE). Aged infrastructure introduces risk to the system, since unreliable infrastructure increases the likelihood of failures under chronic and extreme stress and are particularly concerning when extreme events occur. To understand and account for this added risk from poor infrastructure quality, more research is needed on (i) how the changing environment alters the aging of new and existing built infrastructure and (ii) how degradation causes unique failure mechanisms. The aging of built infrastructure is based on degradation of the structural materials, such as concrete and steel supports, which causes failure. Current work in cement/concrete degradation is based on (i) the development of high strength and degradation resistance concrete mixtures, (ii) methods of assessing the age and reliability of existing structures, and (3) modeling of structural stability and the microstructural evolution of concrete/cement from degradation mechanisms (sulfide attack, carbonation, decalcification). Sandia National Laboratories (SNL) has made several investments in studying the durability and degradation of cement based materials, including using SNL-developed codes and methodologies (peridynamics, PFLOTRAN) to focus on chemo-mechanical fracture of cement for energy applications. Additionally, a recent collaboration with the University of Colorado Boulder has included fracture of concrete gravity dams, scaling the existing work to applications in full sized infrastructure problems. Ultimately, SNL has the experience in degradation of cementitious materials to extend the current research portfolio and answer concerns about the resilience of aging built infrastructure.
The coordination behavior of the tridentate alkoxy ligand 6,6'-(((2-hydroxyethyl)azanediyl)bis(methylene)) bis(2,4-di-tert-butylphenol) (termed H3-AM-DBP2) with group 4 metal alkoxides ([M(OR)4]) in a 1:1 ratio was previously found to generate [(ONep)Ti(κ 4 (O,O’,O”,N)-AM-DBP2)] and [(OR)Zr(κ 4 (μ-O,O’,O”,N)-AM-DBP2)]2 (M = Zr, Hf). Additional studies revealed that increasing the stoichiometric ratio to 1:2 H3-AM-DBP2:[M(OR)4] led to the isolation of [(ONep)Ti(κ 4 (μ-O,O’,O”,N)-AM-DBP2)(μ-ONep)Ti(ONep)3] (1)•tol, [(OBu t)Zr(κ 4 (μ-O,O’,O”,N)-AM-DBP2)(μ-OBu t)Zr(OBu t)3] (2) and [(OBu t)Hf(κ 4 (μ-O,O’,O”,N)-AM-DBP2)(μ-OBu t)Hf(OBu t)3] (3). The asymmetric dinuclear complexes of 1-3 resemble the chelation of a [M(OR)4] moiety to a “(OR)M(κ 4 (O,O’,O”,N)-AM-DBP2)” fragment. The metal complexed by the AM-DBP2 ligand has a pseudo octahedral geometry while the other metal adopts an intermediate trigonal bipyramidal (TBP-5)/square base pyramidal (SBP-5) geometry for 1 but a distorted SBP-5 for both 2 and 3. The structure and properties of 1-3 were analyzed by computational modeling and fully characterized by standard analytical methods. (Figure presented.).
Permeability of salt formations is controlled by the equilibrium between the salt-brine and salt-salt interfaces described by the dihedral angle, which can change with the composition of the intergranular brine. Here, classical molecular dynamics (MD) simulations were used to investigate the structure and properties of the salt-brine interface to provide insight into the stability of salt systems. Mixed NaCl-KCl brines were investigated to explore differences in ion size on the surface energy and interface structure. Nonlinearity was noted in the salt-brine surface energy with increasing KCl concentration, and the addition of 10% KCl increased surface energies by 2-3 times (5.0 M systems). Size differences in Na+ and K+ ions altered the packing of dissolved ions and water molecules at the interface, impacting the surface energy. Additionally, ions at the interface had lower numbers of coordinating water molecules than those in the bulk and increased hydration for ions in systems with 100% NaCl or 100% KCl brines. Ultimately, small changes in brine composition away from pure NaCl altered the structure of the salt-brine interface, impacting the dihedral angle and the predicted equilibrium permeability of salt formations.
Magnesium oxide (MgO) can convert to different magnesium-containing compounds depending on exposure and environmental conditions. Many MgO-based phases contain hydrated species allowing 1H-nuclear magnetic resonance (NMR) spectroscopy to be used in the characterization and quantification of proton-containing phases; however, surprisingly limited examples have been reported. Here, 1H-magic angle spinning (MAS) NMR spectra of select Mg-based minerals are presented and assigned. These experimental results are combined with computational NMR density functional theory (DFT) periodic calculations to calibrate the predicted chemical shielding results. This correlation is then used to predict the NMR shielding for a series of different MgO hydroxide, magnesium chloride hydrate, magnesium perchlorate, and magnesium cement compounds to aid in the future assignment of 1H-NMR spectra for complex Mg phases.
Zeolite-supported Ag0 clusters have broad applications from catalysis to medicine, necessitating a mechanistic understanding of the formation of Ag0 clusters in situ. Density functional theory (DFT) simulations have been performed on silver, water, and silver–water clusters in silica mordenite (Si-MOR), to identify the role of the confinement on the structure and energetics of Ag0 cluster formation. The most favorable binding energy in the 12-membered ring (MR) pore of the Si-MOR is a 10–15-atom Ag0 cluster. Computational pair distribution function (PDF) data indicates that the Ag0 and Ag0–H2O clusters formed in vacuum versus in Si-MOR exhibit structural differences. Additionally, when the Ag0 cluster is confined, the density decreases and the surface area increases, hypothesized to be due to the limiting geometry of the 12-MR main channel. An energetic drive toward formation of larger Ag0 clusters was also identified, with hydrated silver atoms generating higher energy structures. In conclusion, this work identifies mechanistic and structural insight into the role of nanoconfinement on formation of Ag0 clusters in mordenite.
Organic linkers in metal-organic framework (MOF) materials exhibit differences in hydrogen bonding (H-bonding), which can alter the geometric, electronic, and optical properties of the MOF. Density functional theory (DFT) simulations were performed on a photoluminescent Y-2,5-dihydroxyterephthalic acid (DOBDC) MOF with H-bonding concentrations between 0 and 100%; the H-bonds were located on both bidentate-and monodentate-bound DOBDC linkers. At 0% H-bond concentration in the framework, the lattice parameters contracted, the density increased, and simulated X-ray diffraction patterns shifted. Comparison with published experimental data identified that Y-DOBDC MOF structures must have a degree of H-bond concentration. The concentration of H-bonds in the system shifted the calculated band gap energy from 2.25 eV at 100% to 3.00 eV at 0%. The band gap energies also indicate a distinction of H-bonds formed on bidentate-coordinated linkers compared to those on monodentate linkers. Additionally, when the calculated optical spectra are compared with experimental data, the ligand-to-ligand charge-transfer luminescence in Y-DOBDC MOFs is expected to result from an average of 20-40% H-bonding with at least 50% of the bidentate linkers containing H-bonding. Therefore, the type of H-bonding within the DOBDC linker determines the electronic structure and the optical absorption of the MOF framework structure. Tuning of the H-bonding in rare-earth MOFs provides an opportunity to control the specific optical and adsorption properties of the MOF framework on the basis of reactions between the linker and the environment.