Activation energy spectrum

Determining the activation energy spectrum from Rock-Eval pyrolysis data is an inverse task of mathematical statistics having multiple solutions. Some problems are related to the restoration of chemical-kinetic parameters of effective reactions for organic matter maturation in source rocks. For example, reactions with activation energies of less than 50 Kcal moD do not contribute to the Rock-Eval pyrolysis Sj curve because these reactions can occur during the burial stage and would not contribute to S. ... 

Fig. 6.14. Hydrocarbon (HC) yields solid line), rates of hydrocarbon generation (dashed line), and expulsion threshold in the geological history of the Silurian source shales of the Oued el-Mya Basin. Calculations used time-temperature history of the Silurian rocks and activation energy spectrum shown in Makhous et al. (1997). Two stages of hydrocarbon generation took place in the basin history in the pre-erosion Carboniferous and during the Greta ceous-Cenezoic...

Tanaka, Y., Yamamoto, T. Enthalpy relaxation of comb-like polymer analysed by combining activation energy spectrum and tmn models. J. Non-Cryst. Solids 358(14), 1687-1698 (2012)... 

The ESR spectrum of the pyridazine radical anion, generated by the action of sodium or potassium, has been reported, and oxidation of 6-hydroxypyridazin-3(2//)-one with cerium(IV) sulfate in sulfuric acid results in an intense ESR spectrum (79TL2821). The self-diffusion coefficient and activation energy, the half-wave potential (-2.16 eV) magnetic susceptibility and room temperature fluorescence in-solution (Amax = 23 800cm life time 2.6 X 10 s) are reported. 

In actual experiments we do not usually observe directly the desorbed amount, but rather the derived read-out quantities, as is the time dependence of the pressure in most cases. In a closed system, this pressure is obviously a monotonously increasing function of time. In a flow or pumped system, the pressure-time dependence can exert a maximum, which is a function of the maximum desorption rate, but need not necessarily occur at the same time due to the effect of the pumping speed S. If there are particles on the surface which require different activation energies Ed for their desorption, several maxima (peaks) appear on the time curve of the recorded quantity reflecting the desorption process (total or partial pressure, weight loss). Thereby, the so-called desorption spectrum arises. It is naturally advantageous to evaluate the required kinetic parameters of the desorption processes from the primarily registered read-out curves, particularly from their maxima which are the best defined points. 

Let us consider that particles are adsorbed on surface sites whose activation energies of desorption form a continuous spectrum between certain limits. The problem now consists of finding the distribution of initial surface populations ne0i according to the energies EA<-... 

Figure 5 presents data for the non-lnteractlng Rh/S102 catalyst at similar pressures and at 48, 158, and 333 °C. Even with the scatter In the 333°C data, there Is an obvious transition In the spectra as the temperature Is Increased. The predominant peak around 10 rad/sec diminishes and the one around. 5 rad/sec Increases to dominate the spectrum, a trend similar to that observed In the Rh/T102 spectra. Presumably, these trends are the result of differences In apparent activation energies for H2 adsorption and desorption on the various types of sites. 

The observation of negative apparent activation energy can most simply be interpreted in terms of the competition between the adsorption and desorption of methylacetylene on the surface. This qualitative explanation is illustrated in Figure 3, where the steady-state production of trimethylbenzene is compared with the TPD trace of methylacetylene. The fall off in steady state cyclotrimerization rate matches the tail of the desorption spectrum and illustrates the role of reactant desorption at higher temperatiu-es controlling the availability of alkyne monomers and thus the overall cyclotrimerization rate in this temperatime/pressure regime. 

Moreover, the use of heat-flow calorimetry in heterogeneous catalysis research is not limited to the measurement of differential heats of adsorption. Surface interactions between adsorbed species or between gases and adsorbed species, similar to the interactions which either constitute some of the steps of the reaction mechanisms or produce, during the catalytic reaction, the inhibition of the catalyst, may also be studied by this experimental technique. The calorimetric results, compared to thermodynamic data in thermochemical cycles, yield, in the favorable cases, useful information concerning the most probable reaction mechanisms or the fraction of the energy spectrum of surface sites which is really active during the catalytic reaction. Some of the conclusions of these investigations may be controlled directly by the calorimetric studies of the catalytic reaction itself. 

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