Dependence of O2 diffusion dynamics on pressure and temperature in silica nanoparticles

An experimental study of the molecular O2 diffusion process in high purity non-porous silica nanoparticles having 50 m2/g BET specific surface and 20 nm average radius was carried out in the temperature range from 127 to 177 C at O2 pressure in the range from 0.2 to 66 bar. The study was performed bymeasuring the volume average interstitial O2 concentration by a Raman and photoluminescence technique using a 1,064 nmexcitation laser to detect the singlet to triplet emission at 1,272 nm of the molecular oxygen in silica. A dependence of the diffusion kinetics on the O2 absolute pressure, in addition to temperature dependence, was found. The kinetics can be fit by the solution of Fick’s diffusion equation using an effective diffusion coefficient related to temperature and O2 external pressure. The fit results have evidenced that the temperature and pressure dependencies can be disentangled and that the pressure effects are more pronounced at lower temperatures. An Arrhenius temperature law is determined for the effective diffusion coefficient and the activation energy and pre-exponential factor are found in the explored experimental range. The reported findings have not been evidenced previously in the studies in bulk silica and could probably be originated by the reduced spatial extension of the considered system.