Research Projects

Soot formation is a ubiquitous problem in combustion processes; this happens due to incomplete combustion of fuel (pyrolysis), which in turn leads to the formation of Polycyclic Aromatic Hydrocarbons (PAHs) and Polycyclic Aromatic Compounds (PACs). The production of these molecules marks the onset of soot, otherwise known as ‘inception of soot’. This is a critical step that happens early on during combustion. We seek to gain a molecular understanding of the important intermediates along the pathway of soot inception and formation. ***** The key intermediates during soot formation are both radical and molecular in nature. Our focus is on studying the structural aspects of the molecular reaction intermediates, which are composed of flexible PAHs and PACs. The flexibility of these molecules adds to the conformational complexity, hence influencing the inception of soot. We propose to do this by using high-resolution Fourier-Transform Infrared (FTIR) spectroscopy (lower than 0.2 cm-1) across the Far-IR (FIR) and the Mid-IR (MIR) regions (50-7800 cm-1) originating from multiple conformations. ***** The MIR region (350-7800 cm-1) has been very successfully used to characterize functional groups. But these vibrational transitions experience rather small spectral shifts due to conformational and gross structural changes. On the other hand, the FIR region (50-350 cm-1) shows low frequency vibrations including skeletal vibrations, torsional modes, and other modes involving flexible bonds of the molecular intermediates predicted to lie along the pathway to soot formation. Conformational and gross structural changes show up as larger spectral shifts in this region. Since the molecules will be probed in the gas-phase, the FIR region is devoid of phonon modes (in solid particles) and effects due to the environment or solvent (in liquid phase). ***** The molecules of relevance to the inception of soot can be broadly classified as (i) PAH moieties linked via flexible bonds, (ii) PAH with aliphatic chain derivatives, (iii) heteroatom containing flexible aromatic molecules, and (iv) aliphatic ring moieties. ***** Increasing molecular size like larger PAHs introduces low frequency skeletal modes. Adding flexible bonds introduces low frequency torsional modes. Methodically increasing size and flexibility (as shown above) will let us study structural effects on the FIR spectrum in a systematic manner. Instead on solely using the ‘shifts’ of peaks in the MIR region, complementing the analysis with peaks in the FIR region makes our analysis and assignments more reliable and ‘confirmatory’, providing us with synergistic insight. ***** Since number density in the gas-phase is low, we plan to use a multi-pass gas-cell which will increase the signal by a factor of 100, thereby reducing acquisition time by a factor of 10,000! As a contingency plan, various instrumentation parameters will be varied to optimize the experimental conditions like temperature and pressure. This will help us identify hot bands and also help increase the number density of the molecules in the gas-phase, which in turn enhances the molecular signal. ***** Potential future directions involving direct application of the proposed study include studying reaction kinetics and dynamics initiated by laser (ablation cell already designed), pyrolysis, discharge, etc. Flexibility in PAHs and PACs leads to myriad different soot inception and nucleation pathways. Understanding the correlation between FIR spectral activity and the extent of soot formation can be applied by the energy R&D sector to study real-time combustion processes. ***** The molecules of interest to this proposal which are not available commercially will be synthesized by Professor Ganesh Venkataraman’s group which is an expert in synthetic organic chemistry. He is the collaborator for this work.