In-line and in-situ diagnostics of particle formation mechanism in flame reactor: combustion synthesis of iron oxide

In this work we have applied combination of molecular beam sampling and laser-based techniques to study iron oxide nanoparticle formation in flame reactor. Flames of CH4/O2, doped with iron pentacarbonyl were studied using several methods: (1) a combined quartz-crystal-microbalance-particle-mass-spectrometry (QCM-PMS); (2) laser-induced quartz-crystal microbalance (LID-QCM) detuning and (3) intracavity laser absorption spectroscopy (ICLAS) techniques. The application of ICLAS, which is insensitive to the broadband losses induced by light scattering on solid particulates we were able to map the distribution of gas-phase FeO in heavily particle laden environment, where the sensitivity of techniques traditionally applied for laser-based combustion diagnostics (e.g. LIF and CRDS) is hampered. The newly-developed LID-QCM technique paves the way towards in-line optical characterization of as-sampled flame synthesized nanoparticles. Two flame configurations were investigated: (1) Flat quazi - 1-D flame, which allows direct comparisons with the output of kinetic modelling and (2) Hybrid two-stage flame, enabling operation under extremely wide range of synthesis conditions. The study of particle mass concentration and size distribution in these flames with QCM-PMS clearly indicates that the formation of nanoparticles in iron pentacarbonyl-doped flames occurs very early, in close proximity to the burner surface, prior to the flame front. This early rise of nanoparticle mass concentration is followed by a sharp drop in the nanoparticle concentration at the high temperature flame front. The dependence of early particle formation on the iron pentacarbonyl precursor load was also studied and compared to the predictions of analytical 2-reaction model and to the output of numerical simulation using detailed kinetic mechanism. Both the early particle formation and its dependence on precursor load are consistent with kinetic models including iron cluster formation. These observations challenge the existing mechanisms of gas phase synthesis of iron oxide nanoparticles.