Fig. 4.
Dependence of the total diffusion coefficient DT (Eq. 9) of CO on the pore diameter dp when diffusing in a porous material at 1 bar and 25 °C. For large pore diameters (see left panel), DT approaches DM, i.e., the molecular diffusion coefficient (20.4⋅10–6 m2/s, green line), determined by intermolecular collisions. For pore diameters being significantly smaller than the mean free pathway in the bulk gas phase (see right panel), DT can be up to an order of magnitude smaller and is mostly determined by Knudsen diffusion. For 1 bar and 25 °C, this situation is given in case of the pore diameters typically occurring for as-prepared npAu samples