Scientists Trap Noble Gas in a 2D Porous Structure at Room Temperature

Recently, scientists at the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory completed an experiment with a two-dimensional (2D) structure synthesized by them for catalysis research when, to their surprise, they studied that atoms of argon gas were trapped within the structure’s nanosized pores.

Argon and other noble gases have earlier been trapped in three-dimensional (3D) porous materials, however immobilizing them on surfaces had only been attained by either accelerating gas ions to implant them directly into materials, or by either cooling the gases to extremely low temperatures in order to condense them.

“We are the first team to trap a noble gas in a 2D porous structure at room temperature.

Anibal Boscoboinik, Materials Scientist, Lab Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory

This accomplishment, featured in a paper published recently in Nature Communications, will allow scientists to use conventional surface-science tools, such as infrared reflection absorption spectroscopy and -ray photoelectron, in order to carry out in depth studies of single gas atoms in confinement. The knowledge obtained from this research will help in updating the selection, design, and improvement of adsorbent materials and membranes for capturing gases such as xenon and radioactive krypton produced by nuclear power plants.

The team of scientists from Brookhaven Lab, Stony Brook University, and the National University of San Luis in Argentina synthesized 2D aluminosilicate (made up of oxygen, silicon, and aluminum) films on top of a ruthenium metal surface. This 2D model catalyst material was developed in order to study the chemical processes occurring in the industrially used 3D catalyst, known as zeolite, whose cage-like structure opens pores and channels the size of tiny molecules. It is difficult to probe with standard surface-science tools since the catalytically active surface is enclosed within these cavities. The 2D analogue material is made up of the same chemical composition and active site as the 3D porous zeolite, however its active site is displayed on a flat surface, which can be effortlessly accessed with such tools.

The scientists confirmed that the argon atoms were trapped in these “nanocages” by exposing the 2D material to argon gas and then measuring the kinetic energy and number of electrons released from the surface after striking it with an x-ray beam. These studies were carried out at the former National Synchrotron Light Source I (NSLS-I) and its successor facility, NSLS-II (both DOE Office of Science User Facilities at Brookhaven), with an instrument manufactured and operated by the CFN. The binding energies of core electrons are considered to be unique to every single chemical element, and because of this, the resulting spectra disclose the presence and concentration of elements on the surface. The scientists grazed a beam of infrared light over the surface while introducing argon gas in a separate experiment performed at the CFN. Atoms go through changes in their vibrational motions that are precise to that element’s molecular chemical bonds and structure when they absorb light of a particular wavelength.

The scientists improved their understanding of how the framework itself contributes to caging by investigating the trapping mechanism with silicate films, which are considered to be similar in structure to the aluminosilicates but no not contain aluminum. In this case, it was studied that not all of the argon gets trapped in the cages — a minimal amount passes to the interface between the ruthenium surface and the framework. This interface is extremely compressed in the aluminosilicate films for argon to squeeze in.

The scientists, after studying adsorption, analyzed the reverse process of desorption by incrementally increasing the temperature till the argon atoms were totally discharged from the surface at 350 ^oF. They confirmed their experimental spectra with theoretical calculations of the amount of energy linked with argon entering and leaving the cages.

In another infrared spectroscopy experiment performed in Brookhaven’s Chemistry Division, they investigated how the existence of argon in the cages affects the passage of carbon monoxide molecules via the framework. They discovered that argon limits the number of molecules that adsorb onto the ruthenium surface.

“In addition to trapping small atoms, the cages could be used as molecular sieves for filtering carbon monoxide and other small molecules, such as hydrogen and oxygen,” said first author Jian-Qiang Zhong, a CFN research associate.

Going forward, the scientists plan to mainly focus on continuing their investigation into zeolite catalytic processes on the 2D material and are also keen in studying the impact of varied pore sizes on the materials’ potential to trap and filter gas molecules.

As we seek to better understand the material, interesting and unexpected findings keep coming up. The ability to use surface-science methods to understand how a single atom of gas behaves when it is confined in a very small space opens up lots of interesting questions for researchers to answer.

Anibal Boscoboinik, Materials Scientist, Lab Center for Functional Nanomaterials (CFN), Brookhaven National Laboratory

This research used resources of the National Energy Research Scientific Computing Center, a DOE Office of Science User Facility at Lawrence Berkeley National Laboratory. Brookhaven’s Laboratory Directed Research and Development program and the National Scientific and Technical Research Council (CONICET) of Argentina supported the research.