Electron Microscopy Reveals How Gases Behave When Hot

Researchers have obtained a better understanding of gas dynamics by measuring the loss of energy of electrons as they pass through gas samples.

AsianScientist (Dec. 22, 2017) – In a study published in Scientific Reports, a team of researchers has used an electron beam to study the behavior of simple gases at the atomic level.

Gases are used throughout industry. Natural gas, for example, is ‘cracked’ in refineries to make products like acetylene. The efficiency of gaseous reactions depends on the dynamics of the molecules—their rotation, vibration, and translation (directional movement). These motions provide the kinetic energy to drive reactions. By understanding gas dynamics, more efficient and environmentally friendly industrial systems can be designed.

Gas molecules can be studied using transmission electron microscopy or TEM. Unlike optical microscopy, TEM uses a beam of electrons instead of light, and has a much higher resolution, capable of visualizing single atoms. In the present study, a team of researchers at the University of Tokyo’s Institute of Industrial Science, in collaboration with Hitachi High-Technologies Corporation, used the energetic electron beam of the TEM to perform an energy-loss near-edge structure (ELNES) experiment. This involves measuring the energy lost when electrons in the TEM beam give up part of their kinetic energy as they pass through a gas sample.

In theory, ELNES can also measure the dynamics of gas molecules, not just their chemical bonding. However, researchers had never extracted dynamic information from ELNES before. The researchers chose four gases—oxygen, methane, nitrogen and carbon monoxide—whose bonding is well understood, and performed ELNES at room temperature and at 1,000°C. Crucially, they also performed computer simulations of these gases, to theoretically predict the effects of high temperature.

The researchers found that oxygen and methane underwent dynamical changes at high temperature, with significantly faster vibration. However, nitrogen and carbon monoxide did not seem to vibrate any differently at 1,000°C, despite their extra kinetic energy. Moreover, the computer simulated high-temperature vibration of methane matched the experiments very closely, although the vibration of hot oxygen was overestimated.

“Gas molecules in a heater can gain kinetic energy in three ways, namely, by bouncing into each other, by directly touching the heating element, or by indirectly absorbing heat through infrared rays,” said corresponding author Associate Professor Teruyasu Mizoguchi.

“The third mechanism is only possible for gases with polar chemical bonds, where one element pulls electrons away from the other. That applies to methane, but not oxygen, as oxygen is a pure element. Therefore, oxygen heated up slower than the simulations predicted.”

Meanwhile, the failure of nitrogen and carbon monoxide to undergo vibrational excitation was also a result of their bonds. However, in this case, the bonds of nitrogen and carbon monoxide were simply too rigid to vibrate much faster. These findings underscore the importance of taking chemical bonding into account, even for apparently simple processes like the vibration of a two-atom molecule.

Nonetheless, the team believes that rapid developments in ELNES will soon make the method sensitive enough to detect vibrational changes even in rigid molecules. This will pave the way for an improved understanding of gas reactions at the atomic level.


The article can be found at: Katsukura et al. (2017) Estimation of the Molecular Vibration of Gases Using Electron Microscopy.

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Source: University of Tokyo; Photo: Shutterstock.
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