Accounting for hydrolysis in the modeling of titanium dioxide nanoparticle synthesis in laminar TiCl[formula omitted]-seeded flames

Abstract

The description of TiO2 synthesis in TiCl4-seeded flames is most often based on phenomenological models 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 water 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, numerical 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 O2 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.