Chemisorption potential energy

The minimum of the chemisorption potential energy curve (C) corresponds to the sum of the Ni and H atomic radii (y), a result of the formation of the Ni-H bonds. Fig. 2.7 also shows the atomic arrangements at the various... 

When an atom or molecule approaches the surface it feels the potential energy set up by the metal atoms in the solid. The interaction is usually divided into two regimes, namely physisorption and chemisorption, which we discuss separately. 

The interaction of hydrogen (deuterium) molecules with a transition metal surface c an be conveniently described in terms of a Lennard--Jones potential energy diagram (Pig. 1). It cxxislsts of a shallcw molecular precursor well followed by a deep atomic chemisorption potential. Depending on their relative depths and positions the wells m or may not be separated by an activation energy barrier E as schematically Indicated by the dotted cur e in Fig. 1. 

Here we shall be concerned with the interaction of inacming diatomic molecules (H-/ 0.) with either types of potential energy wells The molecular InteractJjon (responsible for elastic and direct-inelastic scattering with extremely short residence times of the irpinglng molecules in the potential) and the chemisorptive interaction (leading to dissociative adsorption and associative desorption, reflectively, and associated with H (D) atoms trapped in the chemisorption potential for an appreci le time). 

Figure 3.2. Potential energy diagram of chemisorption for the adsorption of hydrogen on nickel (after Le Page, 1987).

The influence of the Ni atoms becomes clear from a comparison of the actual reaction path, which consists of physical adsorption and subsequent dissociative chemisorption, with the theoretical alternative reaction path, consisting of dissociation of H2 followed by the formation of two Ni-H bonds. H2 is a very stable molecule and, as a consequence, the potential energy of the dissociated H-atoms is very high. In moving to the adsorbed state, Ni-... 

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

Figure 19.2 Potential energy curves for physical (II) and chemisorption (I).

The transition from physical adsorption to chemisorption occurs at point A. The potential energy at A is in excess of that for the adsorbate and the adsorbent when separated and represents the activation energy required for chemisorption, A fl. If curve I resided more to the right or curve II more to the left, then the transition from physical to chemical adsorption would occur with no activation energy since the crossover point would reside beneath zero potential energy. 

Fig. 7. Potential energy diagram for van der Waals (aa) and chemisorbed hydrogen (66) [J. E. Lennard Jones, Trana. Faraday Soc. 28, 333 (1932)]. aQ and A5 represent heats of chemisorption and van der Waals adsorption. aF represents activation energy for chemisorption.

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... 

Figure 9.17 Potential energy profile versus distance for chemisorption and physisorption.

Only monomolecular chemisorbed layers are possible. Chemisorption is a specific process which may require an activation energy and may, therefore, be relatively slow and not readily reversible. The nature of physical adsorption and chemisorption is illustrated by the schematic potential energy curves shown in Figure 5.2 for the adsorption of a diatomic gas X2 on a metal M. 

The BOC-MP method provides reasonably accurate estimates of the heats of chemisorption Q and the dissociation and recombination barriers AE for various molecules and molecular fragments. Combined with the knowledge of the molecular total bond (gas-phase dissociation) energies, this allows one to construct potential energy profiles of surface reactions. 

Figure 14 Potential energy curves for the H-graphite interaction for three cases physisorption on a rigid lattice (dotted line, filled squares), chemisorption where the lattice is allowed to relax (solid line, open circles), and chemisorption where the bonding carbon is fixed in the puckered position (dashed line, filled circles). The symbols correspond to the DFT calculations, and the lines correspond to the model PES. Taken from Ref. [90],...

Figure 6 Schematic potential energy diagrams for the interaction between O2 and Ag(l 11). Four panels are shown. In (a), the three states into which O2 can adsorb at the surfaces are depicted as a function of a reaction coordinate. In (b), the two potentials leading to direct inelastic scattering are shown. In (c), a trajectory representing a one dimensional representation of transient trapping-desorption in the O2 state is shown. In (d), two path ways leading to dissociative chemisorption are shown. From Kleyn et al. [45],...

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