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June 2009

A Window on Hydrate Lattices

Approach seeks most stable molecular scaffolds to hold renewable energy

Model to identify the most stable water molecule lattices
Researchers use an innovative model to identify the most stable water molecule lattices for energy storage. Enlarge Image.

Results: There's a lot more to water than meets the eye.  Water molecules form three-dimensional empty cages organized in a lattice with the potential to store large amounts of natural gases.  Gases such as hydrogen, methane, and carbon dioxide can be fit as "guest" molecules inside these "host" cavities. But water molecules can be arranged in millions of ways in the basic unit cell, and scientists need to know those arrangements that are energetically more stable compared to others, because lower-energy, stable arrangements have the greatest potential for storing energy.

With millions of possible combinations, finding the ones that are lower in energy becomes a challenge of staggering proportions.  Now a computer model built by scientists at Pacific Northwest National Laboratory and the Russian Academy of Sciences allows scientists to rapidly pinpoint the most stable structures and use them to model those systems that hold promise for energy storage.

Why it matters:  At high pressures and low temperatures, like those found in permafrost or on the ocean's floor, water molecules organize into complex hydrate lattices that are held together by hydrogen bonds.  Because these lattices are so complex, even sophisticated computer models struggle to find the lattices of most interest to researchers.

Previous methods used to analyze lattices relied on a "top down" approach. Under those methods, researchers started with the known arrangement of the oxygen atoms and randomly selected the positions for the hydrogen atoms to build a network that obeyed certain rules.  That approach had several drawbacks.  It was based on statistical sampling, which required very large unit cells to be effective.  Then too, the resulting structures could be more theoretical than practical, meaning they might not exist in reality.  Even more problematic, the model couldn't guarantee that the most appropriate structures would even be found.

The new approach starts from the bottom, by looking at the building blocks of the hydrate lattices and the characteristics needed for stability.  Researchers then use these blocks to build the more complex lattices.  This approach ensures that the resulting structures meet all requirements for stability. 

Methods:  The computer model screens quickly through all possible configurations of the water cages that are used to build the hydrate lattice by calculating the energy necessary to hold each collection of water molecules together.  Some arrangements require very large amounts of energy to make. These are discarded.  Others need less energy to build. These are most likely the arrangements that would actually exist.

PNNL chemist Dr. Sotiris S. Xantheas and colleagues Dr. Soohaeng Yoo, also at PNNL, and Dr. Mikhail Kirov of the Russian Academy of Sciences had previously applied this approach on a smaller arrangement consisting of a hollow cage of 20 water molecules.  They narrowed an initial list of 30,026 possible arrangements down to 64, which were most likely to be the more stable ones.  The most recent calculations involved hollow cages of 24 water molecules. The model refined the list of over three million structures down to just 321. With the use of the supercomputer at EMSL, the Department of Energy's national scientific facility at PNNL, the team performed detailed electronic structure calculations to further refine those arrangements.  The researchers were then able to computationally build up the unit cells into a three-dimensional structure I (sI) hydrate lattice that is capable of holding the alternative energy source hydrogen as well as natural gases.

The work provides realistic models for the hydrate lattices and allows researchers to understand the underlying intermolecular interactions that are responsible for storing "guest" molecules in these "host" lattices.

What's next:  Water molecules form hydrate lattices in three types of structures, labeled sI, sII, and sH.  The current work modeled the sI hydrate lattice.  In the future, the researchers hope to model the sII hydrate lattice, for which the unit cell is made from 16 units of hollow cages with 20 water molecules each and 8 units of an even larger hollow cage of 28 water molecules that can have over 60 million possible arrangements. The (sII) hydrate has been reported to meet current Department of Energy's target densities for an on-board hydrogen storage system.

Acknowledgments: DOE's Office of Basic Energy Sciences funded the research of Dr. Xantheas and Dr. Yoo. The Russian Academy of Sciences funded the research of Dr. Kirov.  The research was done using computational resources at DOE's EMSL, a national scientific user facility at PNNL.

This work supports PNNL's mission to strengthen U.S. scientific foundations for innovation by developing tools and understanding required to control chemical and physical processes in complex multiphase environments.

Reference:  Yoo S, MV Kirov, and S Xantheas.  "Low-Energy Networks of the T-Cage (H2O)24 Cluster and Their Use in Constructing Periodic Unit Cells of the Structure I (sI) Hydrate Lattice." Communication to the Editor,  Journal of the American Chemical Society 131(22):7564-7566.  The paper was recently highlighted in the "Science & Technology Concentrates" section of the June 1, 2009, issue of Chemical and Engineering News.

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