Adsorption dissociation energies

Computational chemistry has reached a level in which adsorption, dissociation and formation of new bonds can be described with reasonable accuracy. Consequently trends in reactivity patterns can be very well predicted nowadays. Such theoretical studies have had a strong impact in the field of heterogeneous catalysis, particularly because many experimental data are available for comparison from surface science studies (e.g. heats of adsorption, adsorption geometries, vibrational frequencies, activation energies of elementary reaction steps) to validate theoretical predictions. 

This section introduces the principal experimental methods used to study the dynamics of bond making/breaking at surfaces. The aim is to measure atomic/molecular adsorption, dissociation, scattering or desorption probabilities with as much experimental resolution as possible. For example, the most detailed description of dissociation of a diatomic molecule at a surface would involve measurements of the dependence of the dissociation probability (sticking coefficient) S on various experimentally controllable variables, e.g., S 0 , v, J, M, Ts). In a similar manner, detailed measurements of the associative desorption flux Df may yield Df (Ef, 6f, v, 7, M, Ts) where Ef is the produced molecular translational energy, 6f is the angle of desorption from the surface and v, J and M are the quantum numbers for the associatively desorbed molecule. Since dissociative adsorption and... 

Fig. 1. Energy scheme of chemisorption and physical adsorption of oxygen vs. distance from the surface according to Lennard-Jones. E tt is the electron affinity of atomic oxygen, Eo the dissociation energy of oxygen molecules, Ecu the chemisorption energy, and Exot the activation energy. Position A is that of physically adsorbed O2, and position B is that of chemisorbed O".

M—O bond, Eu 0, s simply related with the adsorption energy, ad, and the dissociation energy of 02, Ed = 119 kcal/mol, through... 

We may turn now to consider the results provided by the several sets of DFT calculations (see Appendix) that relate to massive gold surfaces.13-15,20 21,24 25 45 These treat in particular the likelihood of molecular adsorption, the activation energy for dissociative chemisorption and the strength of adsorption (binding energy) of oxygen atoms on Au(lll), Au(211) (a stepped surface), Au(221) (a kinked surface), as well... 

The reaction can take place at anode potentials largely lower than the thermodynamic potential of water oxidation to oxygen (1.23V/SHE under standard conditions). However, in order to achieve the dissociative activation of water (1.7), electrocatalytic electrodes are needed. In fact, the dissociative adsorption of water can be achieved only at electrodes (M) on which the bonding energy of M-OH and M-H exceeds the dissociation energy of water to H + HO. This is the case of Pt-Ru-based electrodes on which water activation can be achieved at 0.2-0.3 V/SHE. 

Fig. 4.40. Potential energy plots for dissociative adsorption of a molecule XX. Dxx is the dissociation energy of the free molecule, Ed is the activation energy for dissociative adsorption.

Before we present a survey on the adsorption of several simple gases some general aspects of the dissociative adsorption will be treated. Table 4.1 tabulates the dissociation energies of reactants in the gas phase. Table 4.2 shows some examples of the heats of formation of hydrides, nitrides, carbides and oxides. The following relevant conclusions emerge ... 

The activation energy is least when the CO bond is weakened because of its interaction with the metal surface. Electron transfer from the metal surface into antibonding molecular orbitals weakens the CO bond. This is a very general feature of dissociative adsorption. The dissociation energy of adsorbed molecules decreases when they are adsorbed with considerable electron transfer from the metal surface to the adsorbate. 

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