表面化学Chemical Adsorption ( Chemisorption ).pptVIP

2.7 Adsorption and desorption Some Terminologies Substrate - frequently used to describe the solid surface onto which adsorption can occur; the substrate is also occasionally (although not here) referred to as the adsorbent. Adsorbate - the general term for the atomic or molecular species which are adsorbed (or are capable of being adsorbed) onto the substrate. Adsorption - the process in which a molecule becomes adsorbed onto a surface of another phase (note - to be distinguished from absorption which is used when describing uptake into the bulk of a solid or liquid phase) Coverage - a measure of the extent of adsorption of a species onto a surface (unfortunately this is defined in more than one way ! ). Usually denoted by the lower case Greek theta, q Exposure - a measure of the amount of gas which a surface has seen; more specifically, it is the product of the pressure and time of exposure (normal unit is the Langmuir, where 1?L = 10-6 Torr s ). Coverage The coverage, q, of adsorbate on the surface is defined as Physical Adsorption ( Physisorption ) Physical Adsorption : the only bonding is by weak Van der Waals - type forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface. Chemical Adsorption ( Chemisorption ) Chemisorption : a chemical bond, involving substantial rearrangement of electron density, is formed between the adsorbate and substrate. The nature of this bond may lie anywhere between the extremes of virtually complete ionic or complete covalent character. There are two kinds of chemisorption: Non-dissociative chemiorption (molecule chemisorption) and dissociative chemisorption. Physisorption + Molecular Chemisorption Physisorption + Dissociative Chemisorption Activated dissociative chemisorption Comparison of chemisorption and physisorption Adsorption Kinetics - The Rate of Adsorption The rate of adsorption, Rads, of a molecule onto a surface can be expressed in the same manne