Recent advances towards the use of renewable sources of energy warrant further investigation of the effect of oxygenated hydrocarbons on soot formation and reactivity. The recently developed tool, the micropyrolysis index (MPI)¹ was used as a basis for these studies. Carbon was deposited on same spot silicon nitride grids by placing the grid in a MPI flow reactor at a 45° angle from the direction of flow and injecting 20 μL of specific fuels in a stream of He across the grids at a temperature of 1000°C. Pyrolized particles deposited directly on the grids were analyzed via TEM and Raman. Partial oxidation of the grids were then conducted in a temperature programmed oxidation (TPO) reactor, with resulting analysis of the same particles via same spot TEM and Raman. Compounds investigated include toluene and benzaldehyde, in order to study the effect of oxygen on the soot formation via pyrolysis and reactivity. Reactivity was studied with and without the presence of an oxidation catalyst (CeO₂), allowing one to study the influence of both starting molecular structure and catalytic influence.

Similar studies were conducted by replacing the TEM grid with a bed of α-Al₂O₃ (same method used for MPI measurements), and analyzing the deposited carbon via TPO and SEM of the particles. Through this method, several compounds were tested, including benzene, benzaldehyde, dodecane, and methyl-dodecanoate. TPO measurements indicate a drop in the amount of soot produced from oxygenates when compared with their corresponding hydrocarbons. Also, the appearance of a high temperature peak is observed to be much more significant for toluene than for benzaldehyde, indicating larger, more graphitic particles. By combining the two studies, findings indicate that the effect of oxygen on soot formation via pyrolysis is to lower the total amount of soot, and thus average particle size through radical scavenging as the unstable oxygen species produced via pyrolysis act as radical scavengers, reducing the average particle size and total amount of soot produced.

1. A Novel Micropyrolyis Index (MPI) to Estimate Sooting Tendency of Fuels, Steven P. Crossley, Walter E. Alvarez, Daniel E. Resasco. Accepted: Energy and Fuels.