Rates and heats of oxygen chemisorption on Saran chars at 100°C

Abstract

Chemisorption of oxygen on samples of Saran char was followed at 100°C by differential scanning calorimetry (over a 28-min period) and thermal gravimetric analysis. The three samples used were: a Saran char prepared at 900°C; this char containing about 100 ppm of iron; and this char activated in air at 425°C to 63.8% carbon burn-off. A chemisorption temperature of 100°C was chosen since physical adsorption of O 2 and carbon gasification were negligible. Both the addition of iron to Saran char and its activation increase the rate and enthalpy ( ΔH) of oxygen chemisorption. The ΔH and/or the rate of chemisorption on the activated char is also enhanced by increasing the degassing (desorption) temperature from 500 to 700°C, increasing the rate of heating to a desorption temperature of 600°C, and decreasing outgassing time at 600°C. With three cycles of desorption-chemisorption of oxygen on the activated char, the rate of oxygen chemisorption decreases for outgassing temperatures of 500, 600, and 700°C, but ΔH only changes following outgassing the char at 600 and 700°C. In all of these studies ΔH varied from a low of −42 to a high of −81 kcal/mole O 2. For the original and activated Saran chars, the rate of heat produced as a result of oxygen chemisorption was first order in O 2 pressure at constant coverages.