The Chemisorbed State Energetic Aspects

Once again the detail needs to be prefaced by some general considerations. The process of chemisorption is in essence a chemical reaction unfortunately, like many chemical reactions, it is not a simple process, as it does not always lead to a well-defined product. The occurrence of the different structures of the hydrogen atom adlayer exemplifies this. Nevertheless the fact that chemisorption takes place means that the Gibbs free energy of the system must decrease, and that because of the loss of translational entropy there has to be a decrease in the system s heat content chemisorption is thus of necessity exothermic. All manner of thermodynamic parameters can therefore be ascribed to the process and to the resulting state.  

We must note however that this equation rests on simplifying assumptions, namely, that each site can acquire only one atom, and that all sites are energetically 

Values of heats of adsorption at the flat regions or the initial value where there is no plateau are given in Table 3.1. This shows that for nickel, palladium and iron there is no strong dependence on surface geometry and indeed mean values are all between ca. 95 and 100 kJ mol Analysis of the TPD spectra for deuterium on 

TABLE 3.1. Initial Heats of Hydrogen Chemisorption on Single Crystal Faces,  

Ni( 111) by the isostere method gave slightly lower values ( 90 kJ mol With cobalt, rhodium, platinum (which is also structure insensitive) and iridium, values are somewhat lower (75—90 kJ mol ) and for copper and manganese lower still. The heat of adsorption (—A//ads) is converted into an M—H bond strength Dmh by the relation 

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