Diffusion of oxygen and reaction of residual carbon in sol-gel derived silica

Priscilla Ann Robinson, University of Pennsylvania


Sol-gel processing offers a moderate temperature solution route to the formation of ceramics and glasses. During heat treatment, the residual alkoxy and hydroxyl groups remaining from the solution precursors must be retained to the extent required by the final application. This research examines the rate limiting steps which control the removal of the residual organics during heat treatment. Silica xerogels were fabricated via two-step acid-acid or acid-base catalyzed hydrolysis. The effects of interparticle diffusion, pore diffusion, and external mass transport limitations on the reactions occurring within the xerogel were examined using weight loss measurements obtained by a thermal gravimetric analyzer. The effects of heat transfer limitations were evaluated using predictions of the theoretical temperature profiles within the monolith during heat treatment. These investigations indicated that the diffusion through the pores can significantly limit the reactions occurring within the xerogel. The relative rates of oxidation of residual organics and oxygen diffusion in the silica xerogels were examined in detail in the temperature range from 360 to 460$\sp\circ$C. The rate of oxygen penetration through sol-gel derived xerogels was determined using the removal of visible carbon as a marker of the depth of penetration. The visible carbon concentration profile was measured using an image analysis system in combination with an optical microscope. A model was formulated to describe the removal of visible carbon in terms of oxygen diffusion and reaction, and the equations were solved numerically. The effect of external mass transport limitations on the removal of residual organics was determined by manipulating the value of the mass transfer Biot number and then observing the attendant changes in the predicted profiles. Estimates of the effective diffusion coefficient and effective reaction rate constant were obtained by numerically fitting the experimental visible carbon profiles to those calculated by the model. The relative rates of oxidation and oxygen diffusion were determined as a function of time and position within the monolith. Between 360 and 460$\sp\circ$C, the overall rate of removal of visible carbon was found to be affected by both oxygen diffusion through the porous network and chemical oxidation. Assuming the kinetics to be first order in both oxygen concentration and visible carbon concentration, the effective diffusion coefficient for oxygen is (3.1 $\pm$ 1.8) $\times$ 10$\sp{-8}$ m$\sp2$/s and the effective reaction rate constant is 0.021 $\pm$ 0.012 m$\sp3$/mole s.

Subject Area

Chemical engineering|Materials science

Recommended Citation

Robinson, Priscilla Ann, "Diffusion of oxygen and reaction of residual carbon in sol-gel derived silica" (1993). Dissertations available from ProQuest. AAI9331833.