The speciation and morphology of chromium oxide nanoparticles in a diffusion flame

The impact of combustion conditions on the formation of chromium oxide aerosol has been studied in a hydrogen diffusion flame seeded with Cr(CO) ₆ vapor. Specifically, the effects of the precursor concentration, the flame temperature and the flame residence time on properties of the postflame chromium oxide aerosol were investigated. Each of these flame parameters was found to affect the particle size distribution and/or particle morphology. These effects were partially associated with the fact that the chromium speciation in the final flame products was sensitive to the precursor concentration and the flame temperature. The flame residence time was found to affect the crystallinity of the Cr(III) particles in the postflame chromium oxide aerosol. Conversion of Cr(III) particles to Cr(VI) in the high temperature zone of the flame was proposed to explain changes in the particle size distribution, composition, and morphology. The proposed mechanism was able to explain the experimental findings satisfactorily.