Session H36: Focus Session: Environment II: Green Processes

Using Molecular Simulation to Develop New Materials for Energy and Environmental Applications

We face two enormous challenges in the coming decades. First, we must find ways to provide clean, affordable energy to an ever-growing population. Second, as underdeveloped countries advance, we must find ways to provide access to advanced technologies that enhance human wellbeing. What makes these two challenges even more daunting is that they both must be met using sustainable technologies; simply relying on old dirty technologies is not an option. In this talk, I will show how approaches rooted in chemical physics are being used to develop new materials that can be used to meet these challenges. In particular, I will give examples where molecular-based simulations are being used in three areas: 1) the discovery of new ionic liquid solvents for capturing CO2 produced from fossil fuel combustion; 2) the development of new refrigerants that have better performance and significantly lower global warming potentials than existing refrigerants; and 3) fundamental investigations of actinide ions in solution, with the objective of developing new separation processes for nuclear waste remediation and fuel reprocessing technologies.   

Highly Selective CO$_{2}$/CH$_{4 }$Gas Uptake by a Halogen-Decorated Borazine-Linked Polymer

We report herein a synergistic approach that combines synthesis and characterization of a new borazine-linked polymer, BLP-10(Cl), and theoretical calculations based on density functional theory to investigate its performance in small gas storage and separation. We focus on the binding of H$_{2}$, CO$_{2}$, and CH$_{4}$. The choice of these gases is motivated by their impact on energy and the environment. Given the relatively small and similar kinetic diameter of the CH$_{4}$ and CO$_{2}$ molecules, their efficient separation remains a nontrivial task. In the case of a dihydrogen molecule interacting with the chlorinated borazine, we find the H$_{2}$ to be bound molecularly with a bond length of 0.75 {\AA} and at a distance of 2.76 {\AA} from the boron site. CO$_{2}$ and CH$_{4}$, on the other hand, interact with the central ring system of borazine at a distance of 3.12 {\AA} and 3.33 {\AA} respectively. The bond length between the carbon and oxygen atoms of CO$_{2}$ is 1.16 {\AA} while the distance between the carbon and hydrogen of CH$_{4}$ is 1.10 {\AA}. The binding affinities of all gases with the chlorinated borazine rings obtained from using the M06 exchange-correlation potential agree very well with experimental data collected form pure gas component isotherms. Theoretical investigations also indicate that all of the gas molecules preferentially interact with the borazine ring rather than the phenyl substituent of the nitrogen atoms which highlight the significance of including polarizable building blocks in adsorbent materials.   

Carbon dioxide intercalation in Na-fluorohectorite clay at near-ambient conditions

A molecular dynamics study by Cygan et al.[1] shows the possibility of intercalation and retention of CO$_{2}$ in smectite clays at 37 $^{o}$C and 200 bar, which suggests that clay minerals may prove suitable for carbon capture and carbon dioxide sequestration. In this work we show from x-ray diffraction measurements that gaseous CO$_{2}$ intercalates into the interlayer space of the synthetic smectite clay Na-fluorohectorite. The mean interlayer distance of the clay when CO$_{2}$ is intercalated is 12.5 {\AA} at {\-}20 \r{ }C and 15 bar. The magnitude of the expansion of the interlayer upon intercalation is indistinguishable from that of the dehydrated-monohydrated intercalation of H$_{2}$O, but this possibility is ruled out by careful repeating the measurements exposing the clay to nitrogen gas. The dynamics of the CO$_{2}$ intercalation process displays a higher intercalation rate at increased pressure, and the rate is several orders of magnitude slower than that of water or vapor at ambient pressure and temperature.\\[4pt] [1] Cygan, R. T.; Romanov, V. N.; Myshakin, E. M. \textit{Natural materials for carbon capture}; Techincal report SAND2010-7217; Sandia National Laboratories: Albuquerque, New Mexico, November, 2010.   

Monoethanolamine adsorption on TiO2 (110) for solid supported CO2 capture

Solid supported CO2 capture materials are drawing substantial attention as a promising, cost-effective and environmentally friendly alternative to aqueous amine based CO2 capture. Recently CO2 capture was observed from monoethanolamine (MEA) adsorbed TiO2 powders. In order to facilitate the rational design of future solid CO2 capture materials, it is very important to understand the interaction between MEA and the TiO2 surface at the atomic level and how it affects the CO2 capture capabilities. In this work, we report a combined experimental and theoretical study of MEA adsorption on rutile TiO2 (110). We found that 1 ML of MEAs can form a stable and ordered patten on TiO2 (110). However, the amine group in MEA (the CO2 capture site) binds preferably to the TiO2 surface in the gauche mode. The binding energy of the gauche mode is about 0.8 eV larger than the trans mode, where the amine group is free, causing the present MEA/TiO2 system unable to capture CO2. We found that this large binding energy difference is originated from a combination of surface donor-acceptor bonds, H-bonds, and dipole-induced dipole interaction. Our study suggests that these effects are key factors to design future amine-based solid supported CO2 capture materials.   

Investigation of nanoparticle transformations to guide the design of greener products and processes

Nanoscale particles and products containing nanoparticles hold promise as higher performance materials; however, there are concerns that the production and use of nanoparticles might negatively impact human health or the environment. Within the context of greener nanoscience we aim to maximize the benefits, while minimizing hazards, of nanoscale products. A significant gap in the knowledge needed to develop greener products and processes is our understanding of the formation and transformation of nanoparticles. Such studies of nanoparticle dynamics are technically challenging and few studies have been reported. In this presentation, I will describe convenient methods to monitor nanoparticle dynamics and show how knowledge of nanoparticle transformations can guide the design of greener products and processes. In one example, chemically-modified transmission electron microscopy (TEM) grids are used to directly visualize silver nanoparticle transformations on surfaces. By indexing the TEM grids, it was possible to examine the same nanoparticles repeatedly throughout exposure to different environments. These studies show that larger particles can act as a source of smaller nanoparticles and that much larger particles also produce nanoparticles. With this knowledge, an improved design of nanoparticle coatings for antimicrobial fabrics was developed. A second example involves the use of small angle x-ray scattering (SAXS) to monitor nanoparticle formation reactions in solution in real-time. A combination of beam-line and lab-scale SAXS measurements, combined with simultaneous optical studies, showed that particle growth and coalescence compete under typical synthesis conditions, leading to loss of structural definition of the product. This mechanistic insight, in turn, guided the design of efficient and greener syntheses of well-defined nanoparticles.   

Ceria Nanoparticles: Environmental Impacts on Particle Structure and Chemistry

Ceria nanoparticles are widely studied for catalytic, energy, environmental and bio-medical applications. The performance of ceria often depends on the ability of Cr to switch between +3 and +4 oxidation states. This paper summarizes observations of the impact that synthesis route, processing conditions, storage and environmental conditions have on the chemical and physical properties of ceria nanoparticles. Particles less than 10 nm in diameter are highly dynamic and change their oxidation state not just as a function of size, but also as a function of aging (time) and environmental conditions. During particle nucleation and growth, both particle size and oxidation state change with time. These observations suggest that interpretations of experimental results based primarily on particle size may be misleading. Raman and microXRD studies indicate that these changes can be more complex than anticipated. Because synthesis, analysis and relevant operational conditions often place particles in different environments, understanding how particles change with time in operational conditions is essential to predicting their properties. Time and environmentally induced changes may also play a significant role in the discrepancies reported in various studies.   

Solubility and transport of cationic and anionic patterned nanoparticles

Diffusion and transport of nanoparticles (NPs) though nanochannels is important for desalination, drug delivery, and biomedicine. Their surface composition dictate their efficiency separating them by reverse osmosis, delivering into into cells, as well as their toxicity. We analyze bulk diffusion and transport through nanochannels of NPs with different hydrophobic-hydrophilic patterns achieved by coating a fraction of the NP sites with positive or negative charges via explicit solvent molecular dynamics simulations. The cationic NPs are more affected by the patterns, less water soluble, and have higher diffusion constants and fluxes than their anionic NPs counterparts. The NP-water interaction dependence on surface pattern and field strength explains these observations. For equivalent patterns, anionic NPs solubilize more than cationic NPs since the Coulomb interaction of free anionic NPs, which are much stronger than hydrophobic NP-water interactions, are about twice that of cationic NPs.   

Hydrogen bond density and strength analysis on hydrated Rutile (110) and Cassiterite (110) surfaces

We study the dynamics of water on the surface of cassiterite (110) and rutile (110) using ab-initio molecular dynamics simulation. Water adsorbs and dissociates on these surfaces. This dynamic equilibrium is dominated by the hydrogen bond (h-bond) network at the surface. The h-bond density analysis shows that adsorbed water molecules form higher average number of h-bonds on rutile ($\sim $2.3) as compared to the cassiterite surface ($\sim $2.1). On the other hand, bridging oxygen atoms form higher average number of h-bonds on cassiterite ($\sim $1.4) than rutile surface ($\sim $1.2). Dissociated species are found to have same average number of hydrogen bonds on both surfaces. As a consequence, the rutile surface has higher density of h-bonds at the surface than cassiterite, however, their strength is lower [N. Kumar et al., J. Chem. Phys. 134, 044706 (2011)]. This delicate balance is responsible for the different dynamical properties of both surfaces.   

Microfluidics and Stimulus-Responsive Materials -- The Key to Next Generation Chemical Sensors for Widely Distributed Environmental Monitoring

The fields of chemical sensing and microfluidics have promised much, but in terms of functional devices, have delivered relatively little. Issues like biofouling and surface degradation mean that sensor characteristics change rapidly in real samples. Consequently, chemical sensors must be regularly recalibrated to ensure the information they send is reliable. This results in complex and very costly devices that must integrate fluidics, standards, and waste storage, as well as sampling and analytical procedures. The fundamental challenge for realizing sensors for widely distributed environmental monitoring is this - how can we produce low cost sensing platforms that can function reliably in an autonomous manner for periods up to years? The key to progress lies in new, and more sophisticated materials that can respond to external stimuli, and communicate with the external world. For example, materials that can be activated from a passive state, reversibly bind and release targeted guest molecules, and return to a passive form. Activation and deactivation happen as part of an external control system, which can be local (chemical in nature) or external (e.g. photonic), and the material reports its status (passive, activated-free, activated-occupied) optically materials can be incorporated into more sophisticated platforms, such as micelles, beads, or complete fluidic systems that are much more biomimetic in nature than current platforms. They include polymer actuators that expand and contract dramatically under an external stimulus (e.g. light), enabling valve and pumping functions to be fully integrated into the microfluidic device. This lecture, I will present some of the exciting possibilities for chemical sensing that are now beginning to emerge through breakthroughs in fundamental materials science.