Examples :

Physisorption : Adsorption of nitrogen on carbon or alumina.

Chemisorption : Adsorption of hydrogen on active platinum sites on any support.

Potential energy diagram of approaching molecule towards a solid surface

The potential energy variation of a molecular system as it approaches a solid surface can be depicted by the potential energy diagram, where the potential energy is plotted as a function of distance of the approaching molecule from adsorbent surface. When the molecule approaches the surface, at first it becomes attracted by a weak attractive force resulting in relatively flat potential minimum corresponding to non – dissociative physical adsorption. Then depending on extent of interaction it can be carried to non-dissociative chemisorbed state and finally to stable dissociated state. When the extent of interaction is less, the adsorbate molecules are only physically adsorb on the adsorbent surface or may occur in non-dissociative chemisorbed state. If the interaction is only van der Waals type then the adsorbates will be in physisorbed state. In case of stronger electronic interaction the process may be directly carried on to dissociative chemisorption. If the crossing points are below the line of zero potential energy as shown in Fig. 5, then the overall process is non-activated. If they are above, the overall process requires activation.

Fig. 5. Potential energy diagram for non activated dissociative chemisorption

 

Book References:

•  J.J. Carberry, Chemical and catalytic reaction Engineering, Dover Publications, 2001

•  J. M. Thomas & W. J. Thomas, Principles and Practice of Heterogeneous Catalysis, VCH, 1997

•  J. M. Smith, Chemical Engineering Kinetics, McGraw-Hill Book Company, 1981

•  R. J. Farrauto & C. H. Bartholomew, Fundamentals of Industrial catalytic Processes, Blackie Academic & Professional, 1997

•  D.M. Ruthven, Principle of adsorption & adsorption processes, John Wiley & sons, 1984.