Effect of nanostructure, oxidative pressure and extent of oxidation on model carbon reactivity

Abstract

Oxidation kinetics and fringe analysis studies of three "model" carbons, ranging from fullerenic to onion-like nanostructures and a reference diesel soot were performed in a thermogravimetric analyzer. The samples were oxidized isothermally at temperatures ranging from 575 to 775 °C in air. Multiple tests were performed to obtain the most favorable operating conditions to minimize mass-transfer diffusion limitations in the experiments. First-order reaction kinetics and an Arrhenius-type equation were used to extract the rate constants at each temperature. The activation energies for the oxidation of the carbon samples ranges from 124 to 204 kJ/mol, and it was approximately 140 kJ/mol for the reference diesel soot sample. The onion-like structure exhibited a slower kinetic rate compared to the other carbons. Similar kinetic parameters were found for "intermediate structure" model carbon and reference diesel soot. Fringe analysis explained the differences in the kinetic parameters between carbon samples studied. The onion-like carbon "nascent" sample had a broader range of lamellae length with smaller tortuosity distribution, suggesting stacking; the "nascent" fullerenic carbon had much shorter mean lamella length distribution and broader tortuosity, suggesting more curvature. Nanostructure metrics of the reference diesel soot and intermediate model carbon were between the other two carbons. Results confirm a structure-property relationship between oxidative reactivity with carbon nanostructure.

Comparative nanostructure analysis of gasoline turbocharged direct injection and diesel soot-in-oil with carbon black

“…A study by Wu et al [59] on GDI soot showed that the fringe length also varied with the air-fuel-ratio of the engine, from 0.83 nm to 0.91 nm. For soot obtained from diesel engines, Jaramillo et al [41] and Sakai et al [42] reported similar fringe lengths of 0.98 nm and 1 nm respectively. However, in studies by Li et al [9] and Xu et al [60] the fringe length of diesel soot was observed to be affected by fuel injection splitting (1.16e1.2 nm) and injection timing (1.12e1.4 nm), respectively.…”

“…Thus, the soot-in-oil would be less oxidised than soot from the exhaust gas due to a shorter residence time in the combustion chamber. Jaramillo et al [41] found a linear decrease of the fringe length with increasing degree of oxidation of diesel soot. Similarly, Rohani and Bae [61] observed lower fringe length and higher tortuosity for soot obtained directly from the in-cylinder spray than for particles from the exhaust gas.…”

“…Regarding carbon black, Jaramillo et al [41] found the fringe length to be shorter than for diesel soot (0.8e0.9 nm). In contrast, Seong and Boehman [45] found that the crystallite width is higher for carbon black than for engine soots.…”

“…Wu et al [59] observed a tortuosity range of 1.44e1.49 by varying the air-to-fuel ratio of a GDI engine. For diesel engine soot Sakai et al [42] identified a fringe tortuosity of 1.23 and Jaramillo et al [41] of 1.17. In a study by Li et al [9] changes to the diesel fuel injection, i.e.…”

“…Following initial advances by Vander Wal et al [38] towards quantitative measurements, Yehliu et al [39,40] developed an algorithm comprising image processing and geometric analysis of the fringes with improved quality. Using this algorithm, Jaramillo et al [41] identified the fringe length and tortuosity for soot from a diesel powered industrial forklift to be 1 nm and 1.17 respectively, while fringe separation was not analysed. For soot sampled directly from a diesel spray flame, Sakai et al [42] obtained similar values of fringe length, separation and tortuosity as 0.98 nm, 0.42 nm and 1.23, respectively.…”