Abstract
Tetraethoxysilane (TEOS) is used as a precursor in the industrial production of silica nanoparticles using thermal decomposition methods such as flame spray pyrolysis (FSP). Despite the industrial importance of this process, the current kinetic model of high temperature decomposition of TEOS to produce intermediate silicon species and eventually form amorphous silica nanoparticles remains inadequate. This is partly due to the fact only a small proportion of the possible species are considered. This work presents the thermochemistry of practically all the species that can exist in the early stages of the reaction mechanism. In order to ensure that all possible species are considered the process is automated by considering all species that can be formed from the reactions that are deemed reasonable in the standard ethanol combustion model in the literature. Thermochemical data for approximately 180 species (over 160 of which have not appeared in the literature before) is calculated using density functional theory with two different hybrid functionals; B3LYP and B97-1. The standard enthalpy of formation values for these species are calculated using isodesmic reactions. It is observed that internal rotation may be important because the barriers to rotation are reasonably low. Comparisons are then made between the rigid rotor harmonic oscillator approximation (RRHO), and the RRHO with some of the vibrational modes treated as hindered rotors. It is found that full treatment of the hindered rotors makes a significant difference to the thermochemistry and thus has an impact on equilibrium concentrations and kinetics in this system. For this reason, all of the species are treated using the hindered rotor approximation where appropriate. Finally, equilibrium calculations are performed to identify the intermediates that are likely to be most prevalent in the high temperature industrial process. Particularly, Si(OH)₄, SiH(OH)₃, SiH₂(OH)₂, SiH₃(OH), Si(OH)₃(OCH₃), Si(OH)₂(OCH₃)₂, the silicon dimers (CH₃)₃SiOSi(CH₃)₃, SiH₃OSiH₃; and the smaller hydrocarbon species CH₄, CO₂, C₂H₄ and C₂H₆ are highlighted as the important species.