Accounting for hydrolysis in the modeling of titanium dioxide nanoparticle synthesis in laminar TiCl4-seeded flames

The description of TiO2 synthesis in TiCl4-seeded flames is most often based on phenomenological mod-els that qualitatively reproduce experimental trends in terms of final production but they do not provide chemical insights into the actual kinetic pathways. Alternatively, thermodynamically-consistent detailed TiCl4 oxidation kinetics are available. However, since they have been developed under dry conditions they still need to be challenged and validated when employed for flame synthesis where the presence of wa-ter molecules may potentially activate TiCl4 hydrolysis. To derive accurate chemical descriptions for TiO2 flame synthesis, it is essential to evaluate the possible contribution of TiCl4 hydrolysis. For this, numer-ical simulations of TiO2 flame synthesis in laminar flames are carried out in this article using different chemical descriptions for TiCl4 conversion into TiO2. Detailed oxidation kinetics neglecting hydrolysis are shown to predict an extremely slow formation of TiO2 particles in flames when the O 2 concentration is small. As a consequence, a significant underestimation of the conversion yield is observed compared to experimental evidences and to trends deduced from phenomenological models. To correct this behavior, a new scheme is proposed by combining a detailed oxidation kinetics with a five-reaction mechanism describing the first steps of TiCl4 hydrolysis. Conversion of TiCl4 is found to be faster and more efficient with this new combined scheme, leading to log-normal particle size distributions in agreement with the experimental data for nanoparticles flame synthesis.(c) 2022 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Automated summary

The synthesis of TiO2 in TiCl4-seeded flames is often described using phenomenological models, which qualitatively reproduce experimental trends but lack chemical insights into the kinetic pathways. Alternatively, thermodynamically-consistent oxidation kinetics of TiCl4 have been developed, but their applicability to flame synthesis needs to be assessed considering the presence of water molecules. This article presents numerical simulations of TiO2 flame synthesis, incorporating different chemical descriptions of TiCl4 conversion. Detailed oxidation kinetics without hydrolysis predict slow TiO2 formation at low O2 concentrations, leading to an underestimation of conversion yield. A new scheme combining detailed oxidation kinetics with a mechanism for TiCl4 hydrolysis is proposed, resulting in faster and more efficient TiCl4 conversion and log-normal particle size distributions that agree with experimental data for flame synthesis of nanoparticles.