THE CONTRIBUTION OF SURFACE DIFFUSION TO TRANSPORT IN NANOPOROUS SOLIDS

摘要

Transport in nanoporous solids is conventionally viewed as diffusion through the voids within the pores as the rate is modified by the presence of the pore walls. However in many cases, diffusion of adsorbed species across the surface in two-dimensions contributes to the transport. This surface diffusion can often dominate the rate of transport and must be considered. Adsorbed species of varying sorption energies can move across the surface. Physically adsorbed species can move from one position to another as a two-dimensional gas. The rate is fast and increases with temperature as T~n, with 1 < n < 2. Chemically sorbed species can move to nearby sites in activated "jumps" that depend exponentially on the temperature. Transport across a surface can also be enhanced if the sorbed species are transformed (activated) to moieties which are then surface mobile, a process called spillover. A molecule can either diffuse through gas phase or along the surface in order to move down the length of the pore, i.e., in the axial direction. Movement constrained to the surface can dominate the flux in this direction as the molecule transport in the gas phase can involve more less productive movement, unaligned with the pore axis. The concern is the relative rates of movement (i.e., the velocities) in the gas phase or at the surface. The velocity in the gas phase is readily estimated from molecular theory while the velocity at the surface depends on the mechanism of transport and the energetics across the surface. Several of these processes are discussed below.