Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 191surface area A, the pressure in the system p, and the instantaneous surface concentrationN. These quantities can be measured in a static system or a flow system.(i) Static systemGas is introduced into a system of volume V, containing the filament and aprdssure gauge, at time to. The rate of change of pressure, or number of moleculesin the gas phase, is given bywhere SF is the pumping speed of the filament and FF the rate of evolution fromthe filament. The process is indicated in Fig. 3. On admission of the gas, thepressure drops due to adsorption and continues to do so until the rate of evaporationis equal to the rate of adsorption. SF is related to the sticking coefficients byClosed systemI I I4 6 8Time (sec)Fig. 3. Pressure cycle for adsorption experiments in a closed system on sample of unit area. to,start of adsorption; rl, start of heating; r,, end of adsorption. Amount adsorbed = V(No-Nl) =amount desorbed. From ref. 88.Refirenma pp. 270-278N1