Optimization of Raman Spectroscopy Parameters for Characterizing Soot from Different Diesel Fuels

Raman analysis of carbonaceous materials requires selecting a number of parameters, some of which alter the sample and affect the resulting spectra. Much literature has discussed about the most appropriate values of these parameters for analyzing graphite, coals, or more disordered materials, but no published studies have proposed appropriate values for comparing soot derived from diesel engines fueled with different fuels. This comparison is essential for the design of the regenerative diesel particulate traps that all new automotive diesel engines must incorporate. The effect of three Raman parameters was studied over the same spot to eliminate dispersions associated with the heterogeneity of the sample: laser wavelength, intensity of the incident laser, and exposure time. Among the tested laser wavelengths, 488 nm was preferred for comparing soot samples because it maximized the difference between their spectra due to better signal-to-noise ratio. The selection of the incident laser power is limited by altering/burning the focused area of the sample; this limit depends on the laser wavelength and the type of fuel used. Varying the exposure time was useful to show faster burning for the biodiesel soot sample than for the diesel soot one. The obtained results qualitatively indicate higher reactivity for biodiesel soot, enabling faster and less energy-consuming particulate trap regeneration.