Nanocages trap and separate elusive noble gases

(Nanowerk News) Researchers have discovered how two-dimensional cages trap some noble gases. These cages are only nanometers, or billionths of a meter, thick. They can trap atoms of argon, krypton, and xenon at above freezing temperatures. Noble gases are hard to trap using other methods because they condense at temperatures far below freezing.
The research (Advanced Functional Materials, "Ionization-Facilitated Formation of Two-Dimensional (Alumino)silicate-Noble Gas Clathrate Compounds") studied the nanocages using a computational quantum modeling method called density functional theory (DFT) and X-ray photoelectron spectroscopy (XPS).
Depiction of a 2D-array of hexagonal prism silicate nanocages trapping individual atoms of argon, krypton, and xenon.
Depiction of a 2D-array of hexagonal prism silicate nanocages trapping individual atoms of argon, krypton, and xenon.
Noble gases are the least reactive elements in the periodic table, and they condense out of the air at very low temperatures. They are therefore extremely difficult to trap at temperatures above freezing. This creates health and environmental challenges for industry and energy production.
For example, krypton and xenon are produced in nuclear reactions, so trapping these elements could simplify nuclear power production and waste remediation. Radon, another noble gas, is a radioactive element that is often found in basements, where it causes tens of thousands of deaths every year due to lung cancer.
Noble gases condense at extremely low temperatures, from 80 degrees below zero Fahrenheit (F) for radon to 452 degrees below zero F for xenon. Scientists, industry, energy companies, and others share a need to capture noble gases at above-freezing temperatures.
However, capturing atoms of these elements is extremely difficult due to the weak trapping forces generated by most nanoscale materials, materials only billionths of a meter thick. Researchers have now identified how two-dimensional nanoscale compounds form when cages of ultrathin silica and aluminosilicate crystalline trap argon, krypton, or xenon at 80 degrees F.
The research was conducted at two DOE Office of Science Basic Energy Sciences user facilities, the National Synchrotron Light Source II and the Center for Functional Nanomaterials. The researchers used XPS and DFT to show how these traps, called clathrate compounds, form due to a novel activated physical adsorption mechanism supported by chemical changes in the noble gas atoms.
Computations were performed at the Scientific Data and Computing Center and at the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility. Noble gas ions are neutralized when entering the cages, allowing them to be trapped at temperatures well above freezing: 167 degrees F for argon to 437 degrees F for krypton, and 752 degrees F for xenon. The researchers calculate that radon could be trapped at even higher temperatures. The resulting clathrates are the thinnest ever reported.
The findings pave the way for new research on individual atoms of noble gases. The research could also lead to new gas capture and separation methods for environmental and health applications.
Source: U.S. Department of Energy, Office of Science
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