Researchers Examine Dynamics of Hot Gas Molecules by Joining Electron Microscopy and Simulation

Gases have been used throughout industry. Natural gas, for instance, is "cracked" in refineries to make products like acetylene. The efficiency of gaseous reactions relies on the dynamics of the molecules - their vibration, rotation and translation (directional movement). These motions offer the kinetic energy to drive reactions. By comprehending gas dynamics, scientists can engineer more efficient (and environmentally friendly) industrial systems.

Gas molecules can be researched using transmission electron microscopy (TEM). In contrast to optical microscopy, TEM employs a beam of electrons rather than light, and has a much better resolution, capable of visualizing single atoms. A new study published in Scientific Reports, describes the work of a research team at The University of Tokyo's Institute of Industrial Science (IIS) partnering with Hitachi High-Technologies Corp. The researchers used an advanced version of TEM to investigate the dynamics of simple gases at high temperature.

In TEM, the energetic electron beam can be used to perform another experiment at the same time, known as energy-loss near-edge structure [ELNES]. The electrons in the beam give up part of their kinetic energy as they pass through the sample. Measuring this energy loss reveals which elements are present and how they are bonded to each other.

Hirotaka Katsukura explains, Study First Author

In theory, ELNES will be able to also measure the dynamics of gas molecules, not only their chemical bonding. However, researchers have not extracted dynamic information from ELNES before. The IIS team selected four gases - oxygen, nitrogen, methane, and carbon monoxide - whose bonding is properly understood, and performed ELNES at room temperature and 1,000 °C. Significantly, they also did computer simulations of these gases, using molecular dynamics code, to hypothetically predict the effects of high temperature.

Usually, when molecules are heated up, they vibrate faster and the bonds between their atoms become longer. In the IIS experiments, two gases - methane and oxygen - did certainly exhibit dynamical changes at high temperature, with considerably faster vibration. However, carbon monoxide and nitrogen did not seem to vibrate any differently at 1000 °C, in spite of their additional kinetic energy. Furthermore, the simulated high-temperature vibration of methane matched the experiments extremely closely, but the vibration of hot oxygen was overrated.

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. This last one is only possible for gases with polar chemical bonds, where one element pulls electrons away from the other. That applies to methane (CH₄), but not oxygen, a pure element. Therefore, oxygen heated up slower than the simulations predicted.

Teruyasu Mizoguchi says, Corresponding Author

In the meantime, the failure of carbon monoxide and nitrogen to endure vibrational excitation was also a result of their bonds - however, in this case, they were just too inflexible to vibrate much faster. These findings emphasize the significance of taking chemical bonding into consideration, even for seemingly simple processes like the vibration of a two-atom molecule.

However, the team believes that fast developments in ELNES will soon make the technique sufficiently sensitive to detect vibrational variations even in stiff molecules. This will pave the way toward a better understanding of gas reactions at the atomic level.

The research paper titled "Estimation of the molecular vibration of gases using electron microscopy" was published in Scientific Reports.