Desorption calculations involving

Although the calculations involved in this TPD model are very tedious and time-consuming, the results can be considered as highly reliable, since they are based on the evaluation of desorption energies. 

By applying the machinery of statistical thermodynamics we have derived expressions for the adsorption, reaction, and desorption of molecules on and from a surface. The rate constants can in each case be described as a ratio between partition functions of the transition state and the reactants. Below, we summarize the most important results for elementary surface reactions. In principle, all the important constants involved (prefactors and activation energies) can be calculated from the partitions functions. These are, however, not easily obtainable and, where possible, experimentally determined values are used. 

The results of the calculation could not distinguish hydroxyl or propyl radicals as the desorbed species, since they produced very similar product distributions. This is because they were both involved in the chain propagation reactions. Since the desorption of a hydroxyl radical required cleavage of metal-oxygen bonds in the oxide lattice, it would be energetically less favorable. Thus it was concluded that propyl radicals were the most likely desorbed species. 

We observed that only a small fraction of the adsorbed mass would desorb even after a number of successive desorption steps. The investigation on laboratory contaminated soil showed a biphasic behavior, namely an easily desorbed fraction and a desorption resistant fraction. Both field contaminated and aged soils also showed the same behavior. The first stage involved a loosely bound fraction and the second stage involved a tightly bound fraction. The desorption constants calculated or estimated for the two fractions were em-... 

Heterogeneous photochemical processes are concerned with the effect of light on interacting molecules and solid surfaces. The concept of photoinduced surface chemistry is commonly used to integrate these processes. As cited earlier, they involve surface phenomena such as adsorption, diffusion, chemical reaction and desorption [3]. Experiments and theoretical calculations make clear that the photochemical behavior of an adsorbed molecule can be very different from that of a molecule in the gas or liquid phase [4]. Photochemical reactions of this type involve molecules and systems of quite different complexity, from species composed of a few atoms in the stratosphere to large chiral organic molecules that presumably were formed in prebiotic systems. 

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