Because of their significant impact on climate, environment and health, reducing the emission of soot from combustion processes remains a problem that requires detailed understanding of its formation as well as of the principles that govern how the result in terms of size, morphology, and chemical reactivity of soot particles depends upon the formation process. Especially very small, below ten nanometer-sized, particles in the early stages of the soot nucleation process are interesting targets for more detailed inspection, to reveal useful insight and to guide further model development. In this study, Helium-ion microscopy (HIM) is applied as an imaging technique new to combustion studies to analyze the morphology of soot particles >2 nm and to determine their geometrical characteristics. For this analysis, a series of premixed ethylene flames are investigated. Mobility size measurements from an earlier investigation have been compared with the particle sizes determined by HIM. Observed particle shapes and geometrical statistics suggest that in all flames under investigation, nascent soot possesses no well-defined morphologies. Additionally, investigations have been made using X-ray photoelectron spectroscopy (XPS) to obtain more information on the chemical characteristics of these particles.

For the first time, nascent soot particles are probed by using helium-ion microscopy (HIM). HIM is a technique that is similar to scanning electron microscopy (SEM) but it can achieve higher contrast and improved surface sensitivity, especially for carbonaceous materials. The HIM microscope yields images with a high contrast, which allows for the unambiguous recognition of smaller nascent soot particles than those observed in previous transmission electron microscopy studies. The results indicate that HIM is ideal for rapid and reliable probing of the morphology of nascent soot, with surface details visible down to approximately 5 nm, and particles as small as 2 nm are detectable. The results also show that nascent soot is structurally and chemically inhomogeneous, and even the smallest particles can have shapes that deviate from a perfect sphere.

The kinetics of catalytic oxidation of methane (1-3% in air) over a palladium oxide (PdO) surface was investigated by wire microcalorimetry at atmospheric pressure and over the temperature range from 560 to 800 K. Wire surface structures and compositions were characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and atom force microscopy. It was found that a porous PdO layer with a constant thickness of 1-2 mu m was formed on the Pd wire after it was heat treated in nitrogen followed by air at elevated temperatures. Under the condition of the experiment, the reaction was found to be in the pseudo-first-order regime with respect to the methane concentration. The apparent rate constant of methane oxidation on PdO was determined to be k(app)(cm/s) = (3.2 +/- 0.8) x 10(4)e(-(62.8 +/- 1.6)(kJ/mol)/RT) for 600 < T < 740 K. Experimental data were analyzed using a gas-surface reaction model proposed previously. Analysis shows that the overall catalytic oxidation rate is governed by equilibrium adsorption/desorption of molecular oxygen, which determines the density of surface palladium sites and dissociative adsorption of methane on these sites. The equilibrium constant of O-2 adsorption and desorption was estimated from literature values of desorption energy and molecular parameters of adsorbed oxygen atoms. The rate coefficient of methane dissociative adsorption was estimated to be k(16)(cm/s) = (7.7 +/- 1.6) x 10(4)e(-(59.9 +/- 1.2)(kJ/mol)/RT), derived from the equilibrium constant of oxygen adsorption over the same temperature range.

The impact of fuel bound oxygen on the sooting behavior of butanol fuels was examined by following the evolution of the particle size distribution function (PSDF) of nascent soot produced in atmospheric pressure burner stabilized stagnation (BSS) flames of n-butanol and i-butanol. Similar experiments were carried out for i-butane and n-butane flames to better understand the influence of fuel structure and the presence of the alcohol group on detailed processes of soot nucleation and growth. In terms of fuel structure, the branched chain functionality has the most observable effect on soot formation. The onset of soot nucleation is faster in the branched fuels in comparison to the straight-chain counterparts. Under the same C/O ratio, however, the butanol flames were found to nucleate soot earlier and have higher soot volume fraction than the butane flames. A combustion reaction model for i-butanol and n-butanol was used to explore the precursor chemistry. Similar to the measured PSDF, benzene is computed to rise earlier in flames of the branched fuels than the straight-chain fuels.