Polycyclic aromatic hydrocarbons--catalysts for molecular hydrogen formation
Faraday Discussions
|October 11, 2014
Summary
Polycyclic aromatic hydrocarbons (PAHs) catalyze hydrogen formation through atom addition and subsequent abstraction. Deuterium atoms fully hydrogenate coronene, showing addition cross-sections are constant regardless of hydrogenation level.
Area of Science:
- Astrochemistry
- Physical Chemistry
- Materials Science
Background:
- Polycyclic aromatic hydrocarbons (PAHs) are prevalent in interstellar environments and play a role in chemical reactions.
- PAHs are known to catalyze the formation of molecular hydrogen (H2).
- The mechanism involves sequential addition of hydrogen atoms to PAHs, forming super-hydrogenated species, followed by H2 abstraction.
Purpose of the Study:
- To investigate the kinetics of hydrogen addition to PAHs.
- To determine the cross-sections for addition and abstraction reactions in the super-hydrogenation of PAHs.
- To understand how the degree of hydrogenation affects these reaction cross-sections.
Main Methods:
- Experimental study using quadrupole mass spectrometry to monitor reactions.
- Kinetic simulations to model the reaction pathways and cross-sections.
- Deuterium atom bombardment of coronene molecules.
Main Results:
- Coronene (C24H24) was observed to become fully deuterated (C24D36) under sufficient deuterium atom fluence.
- The mass distribution peaked at 358 amu, close to the theoretical maximum of 360 amu.
- Kinetic models supported experimental data with an abstraction cross-section (σ_abs) of 0.01 Ų/atom.
- Addition cross-sections (σ_add) were found to be in the range of 0.55–2.0 Ų, independent of the degree of hydrogenation.
Conclusions:
- The cross-section for hydrogen/deuterium atom addition to PAHs remains constant across a wide range of super-hydrogenation.
- Hydrogenation does not depend on the number of available addition sites on the PAH molecule.
- This provides crucial insights into the H2 formation pathways in interstellar space and on surfaces.
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