The oxidized soot surface: theoretical study of desorption mechanisms involving oxygenated functionalities and...

Area of Science:

• Surface Science
• Computational Chemistry
• Physical Chemistry

Background:

• Soot oxidation is crucial for combustion processes and atmospheric chemistry.
• Understanding the desorption mechanisms of oxygenated functionalities on soot is key to characterizing its surface.
• Previous studies have used temperature programmed desorption-mass spectrometry (TPD-MS) but lacked detailed mechanistic insights.

Purpose of the Study:

• To investigate the desorption mechanisms of oxygenated functionalities on soot using quantum mechanical calculations.
• To compare computational results with experimental TPD-MS data for accurate assignment of desorption maxima.
• To elucidate the role of different functional groups and their local environment in soot oxidation.

Main Methods:

• Quantum mechanical calculations on functionalized polycyclic aromatic hydrocarbon (PAH) models.
• Modeling of unimolecular and bimolecular fragmentation processes.
• Investigation of cooperative effects between adjacent functional groups.
• Comparison of computed desorption barriers with experimental TPD spectra.

Main Results:

• Calculated desorption barriers were compared with TPD maxima to assign specific functional groups to observed desorption events.
• The desorption behavior of carboxylic and lactone groups is highly dependent on the local chemical and geometric environment.
• Evidence suggests that not all carboxylic groups desorb at low temperatures as commonly assumed.
• Lactone groups were found to contribute to both CO2 and CO desorption.

Conclusions:

• The study provides a mechanistic basis for interpreting TPD-MS data of oxidized soot.
• Local environment significantly influences the desorption pathways of functional groups on soot surfaces.
• The findings challenge the general assumption about the low-temperature desorption of all carboxylic groups.
• Lactone groups are identified as potential sources of both CO and CO2 during soot oxidation.