Effect of nonreacting gases on the desorption of reaction-created CO from graphite

Temperature-programmed desorption (TPD) was used to determine whether inert gases influence the desorption step in the oxidation of carbon by CO₂. Surface oxides were formed on a graphite by reacting it at 1200 K with CO₂, and then quenching the reaction. The oxides then were removed by TPD to 1373 K, using He, Ar and Kr as carrier gases in separate desorptions. The oxides appeared as CO₂ and CO in the desorption carrier gases; some CO₂ was observed between 800 and 1050 K, but above 1050 K the product was almost entirely CO. The CO₂ desorption showed no effect from changing the carrier gas, but temperature of CO desorption peak and amount of CO desorbed by 1373 K both depended on which gas was used as a carrier. This shows that nonreactive gases affect the desorption step of the C-CO₂ reaction. Involved in the desorption step is movement of some surface oxide species to desorption sites; the nonreactive-gas influence occurs because the nonreactive gases affect this surface transport. In the presence of Ar and Kr, transport rates of surface oxides to desorption sites were higher than they were in the presence of He. Under reaction conditions this can result in greater CO desorption rates and faster overall reaction rates in the presence of Ar and Kr than occur in the presence of He.