HRTEM AND EELS INVESTIGATION OF FLAME-FORMED SOOT NANOSTRUCTURE

The formation mechanism and sources of soot, along with properties relevant for health as the reactivity and toxicity, can be inferred through the detailed characterization of soot nanostructure. In the present work HRTEM coupled with EELS were applied to soot probed along a premixed methane flame from the inception phase up to the burnout region in order to follow the transformations of soot nanostructure. HRTEM allows a direct molecular imaging of carbon structures at nanoscale level and the application of image analysis procedures makes possible to extract quantitative structural information describing the shape, size and orientation of sp2-bonded (aromatic) layers and their distribution inside the carbon particle. Moreover, EELS was used to provide quantitative information about the relative concentrations of sp3 and sp2 hybridized carbon. The main parameters extracted from HRTEM and related to the geometry of the individual fringes were the length and tortuosity. A structural improvement in terms of a better stacking of longer and more planar layers was observed along the methane flame. In particular, the nascent methane soot presented a higher contribution of amorphous-like components. It could be noticed that the nanostructural ordering observed for methane was found to occur to a lower level in respect to ethylene flames operating at a similar temperature reported in previous work, even though downstream of the soot formation region mature soot presented rather similar HRTEM features. EELS measures indicated higher sp3 content for the nascent methane soot suggesting that the chemical environment, rich of unburned methane fuel, hydrogen and light aliphatic hydrocarbon products, in which methane soot inception occurs, causes the highly disordered structure in the first nuclei of methane soot. Thereafter, young soot as well as intermediate and mature soot suddenly showed a largely prevalent sp2 content without undergoing a significant nanostructural restructuring, in terms of fringe length and sp2 character, up to the end of soot formation.