Desorption energy, heterogeneous surface spectra

Next, the qualitative and quantitative compositions of the carrier gas, especially as to the most chemically active components, have not been the same in different laboratories. As a result it is not known whether the surfaces were modified to a comparable degree in different studies. An even more important question is, how complete was the coverage of the surface by the grafted molecular fragments These factors critically affect the resulting surface heterogeneity and the spectrum of the desorption energies. 

In the Monte Carlo simulations of the chromatographic and related processes we should also account for the new knowledge about the deep heterogeneity of surfaces, about the role of localized adsorption and about the occurrence of surface diffusion. Evidently, now the molecular desorption energy accepted for a concrete simulation is to be understood as the effective value over the spectrum of possible energies in the sense of Eq. 5.71. As such, it is related to a concrete form of the p(E ). The... 

Unimolecular (desorption of intact molecules =MOH- OH2 =MOH -I- H2O) or associative (=M(OH)—0(H)— =M-0-M= + H2O or =M(OH)—O—(HO)M= -> =M(Oo)M= + H2O) desorption of water molecules can be described by the rate with Equation (37.12) of first or second order, respectively. The relationship between the ion current measured and the reaction rate constants was described elsewhere [29]. The fumed oxide surfaces are heterogeneous and every type of surface sites can influence the corresponding desorption peak and the corresponding center (E ) in a desorption energy distribution for this peak. If the TPD spectrum is convoluted into several peaks without any restriction that the activation energy calculated over the total temperature ranges for each of the peaks can be underestimated due to the overestimation of the peak width. Therefore, we used some modification of the calculations described in detail elsewhere [25,29]. 

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