Potential-energy diagram, physisorption

Figure 6.34. Schematic, strongly simplified potential energy diagram along the reaction coordinate of a molecule X2 approaching a metal surface. First the molecule feels the weak Van derWaals interaction, leading to physisorption. The next stage is associative...

Figure 10.1. Potential-energy diagram for physisorption and chemisorption of an A-A molecule.

The difference between physisorption and chemisorption can be explained using a potential-energy diagram. The potential-energy diagram for physisorption and chemisorption of an A-A molecule (e.g., H2) is shown in Figure 10.1. Curve P in... 

The thermodynanucs of gas-phase species adsorbing on a surface can be separated into the classes of physical adsorption (physisorption) and chemical adsorption (chemisorption). The difference between the two classes of adsorption is related to the heat of adsorption, A Hads. In physisorption, weak attractive forces (van der Waals) such as dipole-dipole and induced dipole interactions drive the adsorption of species from the gas phase. For chemisorbed systems, chemical bonds associated with electron transfer between adsorbate and substrate are formed upon adsorption. Each of these interactions can be described by a two-dimensional potential-energy diagram as in Figures 10 and 11. 

Figure 10 Potential energy diagram for the case of physisorption where adsorbates experience weak attractive forces near the surface...

Figure 6.16. One-dimensional potential-energy diagrams showing the possible transition from molecular physisorption to dissociative chemisorption.

Two schematic combined potential energy wells for the interaction of a gaseous species with a surface are shown in Fig. 2, illustrating the importance of the crossover point of the chemisorption and physisorption wells for adsorption and desorption kinetics. In the first case, adsorption is activated in the second, it is non-activated. (There are, in fact, only a few well-documented cases of activated chemisorption.) Recently, Lundqvist et al. [43] have made detailed calculations of the potential interaction between H2 and a magnesium surface which substantiate the presence of two minima. Their work is reviewed elsewhere [44]. It must be borne in mind that diagrams such as Fig. 2 grossly oversimplify the... 

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