Effect of ceria nanoparticles on soot inception and growth in toluene-oxygen-argon mixtures

Soot formation from the combustion of toluene (C6H5CH3) and of two concentrations of nano-sized-ceria-laden toluene was monitored using a shock tube to observe the effect of the organometallic additive on the formation of soot from its point of inception. Two concentrations of ceria, of chemical composition CeO1.63, were employed to examine the effect on soot production of toluene over the range of temperature 1588-2370 K using two levels of inert gas dilution in which reflected-shock pressure was maintained near 1.5 atm. The ceria nanoparticles were synthesized using a microemulsion technique which employs sodium dioctyl sulfosuccinate (AOT), a surfactant, to retard agglomeration. Introduction of the nanoparticles into the shock tube is achieved using a novel, two-stage injection procedure. Soot yield measurements reveal that the presence of ceria has no direct implications on peak soot concentration near 1950 K. A shift in the parabolic soot profile of toluene in the direction of increased temperature was observed for each concentration of ceria with a larger shift occurring for increased concentration of ceria, although the same effect was exhibited for the toluene-AOT mixtures in absence of ceria, supporting an inefficaciousness of ceria on soot suppression on kinetic timescales. It is evidenced in measured soot delay times that the presence of the surfactant in absence of ceria significantly slows the rate of soot growth for T < 2000 K, while the presence of ceria has a relatively negligible impact. Under conditions of higher fuel concentration, a remarkable decrease in soot accumulation on the shock tube walls was observed in experiments using the ceria-toluene mixtures over that yielded by pure toluene combustion. In the present paper, the authors report the first measurements of nanoparticle-influenced combustion of a hydrocarbon as performed in a shock tube.