# Effect of Fuel Composition on Carbon Black Formation Pathways
## 1 Introduction  
Reactive dimerization by the formation of covalent-covalent bonds is becoming recognized as a viable pathway and likely a dominant route to nuclei formation [1, 2]. Notably, this pathway does not require exceptionally large Polycyclic Aromatic Hydrocarbons (PAHs) to realize sufficient van der Waals forces at flame temperatures [3, 4, 5, 6]. Underscoring this new path are the observations that soot particles appear sooner than even moderately sized PAHs (e.g., pyrene) along the centerline of a diffusion flame [2]. Additionally, atomistic simulations provide evidence for PAH growth involving radicals that bypass the traditional small molecule growth pathways [3]. Most importantly, this mechanism implicates fuel-sourced PAHs as a significant source of soot nuclei. Small PAHs prevalent in the fuel provide high initial concentrations to feed nuclei formation, thereby affecting early-stage particle growth dynamics. In parallel, the recent study by [PERSON] et al. showed that soot nucleation must involve strong chemical bond formation between dimers [4]. Initially, unstable dimers are formed through reactive bonding--covalent bonds are formedthrough a sequence of hydrogen abstraction and carbon-carbon bond formation, or dehydrogenation, which stabilizes the dimers. The evolution of soot concentration is predicted through comparison to measurements. Nucleation involving all PAHs rather than a single one has been found essential for predicting PAH and soot concentrations. Alternatively, very large PAHs can contribute to particle nucleation by van der Waals forces, but these are generally in low concentrations-requiring first their formation in the flame environment or reactor. Similarly, for feedstocks with lower aromatic concentrations (and correspondingly higher paraffinic and naphthenic content)--pyrolysis reactions followed by aromatic ring building reactions are needed to raise aromatic concentrations to sufficient levels for reactive condensation to be an effective path to nuclei formation [7; 8]. These degradative and growth reaction sequences impose a kinetic delay-governed by temperature [9]. In this study, two types of carbon blacks are compared for their physical and chemical attributes, including morphology, nanostructure, and composition. These carbon blacks are formed under similar reactor conditions, with feedstock composition being the only significant variable. One carbon black is formed using coal tar as the feedstock (referred to as CT-CB), while the other carbon black is formed using fluid catalytic cracking decant oil (referred to as FCC-CB). It is difficult to ascribe a specific aspect of nanostructure to a particular component class in the original fuel given several component classes and their chemical and physical transformations by concerted pyrolysis and growth reactions. Still, recognized differences between classes of compounds present within CT versus FCC feeds may now be understood in the context of newly recognized soot nucleation/formation pathways. Specifically, these advanced pathways begin with aromatics and the resonance stabilized radical reactions between them and other small hydrocarbons [5; 6]. Such low energy pathways negate the preconditions for high temperature, high H-atom concentrations in order to form nuclei [7] while also removing the energetic and kinetic restriction of only large PAHs serving as soot nuclei [8]. The two feedstocks lead to significant differences in primary particle nanostructure and sp\({}^{2}\)/sp\({}^{3}\) chemistry--as observed and characterized by electron-based microscopic and spectroscopic measurements. The influence of the source fuel extends to oxidative reactivity, demonstrated here by isothermal thermogravimetric analysis conducted at \(\sim\)500 \({}^{\circ}\)C. To further differentiate nanostructure and chemistry differences, pulsed laser annealing is applied. Pulsed laser annealing has been recently shown to derivatize combustion generated carbons [9].