Desorption kinetic parameters

How to determine desorption kinetic parameters By changing / . If readsorption does not occur and n = 1, according to equation (7.94), plotting 21gTm — lg/3 l/ m, then desorption activation energy Ed is obtained from the slope of the straight line. The value of pre-exponential factor v can be obtained by Ed and intercept of the straight line. 

The applications of this simple measure of surface adsorbate coverage have been quite widespread and diverse. It has been possible, for example, to measure adsorption isothemis in many systems. From these measurements, one may obtain important infomiation such as the adsorption free energy, A G° = -RTln(K ) [21]. One can also monitor tire kinetics of adsorption and desorption to obtain rates. In conjunction with temperature-dependent data, one may frirther infer activation energies and pre-exponential factors [73, 74]. Knowledge of such kinetic parameters is useful for teclmological applications, such as semiconductor growth and synthesis of chemical compounds [75]. Second-order nonlinear optics may also play a role in the investigation of physical kinetics, such as the rates and mechanisms of transport processes across interfaces [76]. 

TPD Temperature programmed desorption After pre-adsorption of gases on a surface, the desorption and/or reaction products are measured while the temperature Increases linearly with time. Coverages, kinetic parameters, reaction mechanism... 

V. Processing of the Experimental Data to Estimate the Kinetic Parameters of Desorption. 372... 

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. 

Most often, the primary experimental desorption data [[mainly the P(t) or P(T) function] represent, after due corrections, the temperature dependence of the desorption rate, —dnjdt = Nt vs T. The resulting curves exhibit peaks and their most reliable point is the maximum at the temperature Tm, corresponding to the maximum desorption rate Nm. Its location on the temperature scale under various conditions is essential for estimating the kinetic parameters of the desorption process. 

A volcano plot correlates a kinetic parameter, such as the activation energy, with a thermodynamic parameter, such as the adsorption energy. The maximum in the volcano plot corresponds to the Sabatier principle maximum, where the rate of activation of reactant molecules and the desorption of product molecules balance. 

Unraveling catalytic mechanisms in terms of elementary reactions and determining the kinetic parameters of such steps is at the heart of understanding catalytic reactions at the molecular level. As explained in Chapters 1 and 2, catalysis is a cyclic event that consists of elementary reaction steps. Hence, to determine the kinetics of a catalytic reaction mechanism, we need the kinetic parameters of these individual reaction steps. Unfortunately, these are rarely available. Here we discuss how sticking coefficients, activation energies and pre-exponential factors can be determined for elementary steps as adsorption, desorption, dissociation and recombination. 

How do we derive the activation energy of desorption from TPD Data Unfortunately, the differential equation in (12) can not be solved analytically. Hence, analyzing TPD curves can be a cumbersome task, in particular because the kinetic parameters usually depend on surface coverage. 

Such behavior is known as the compensation effecf . The important point is that if we ignore the additional term in Eq. (18), we essentially force the kinetic parameters to satisfy Eq. (19) resulting in a correlation between the prefactor and the desorption energy according to the compensation effect ... 

Finally, although both temperature-programmed desorption and reaction are indispensable techniques in catalysis and surface chemistry, they do have limitations. First, TPD experiments are not performed at equilibrium, since the temperature increases constantly. Secondly, the kinetic parameters change during TPD, due to changes in both temperature and coverage. Thirdly, temperature-dependent surface processes such as diffusion or surface reconstruction may accompany desorption and exert an influence. Hence, the technique should be used judiciously and the derived kinetic data should be treated with care ... 

Having estimated the sticking coefficient of nitrogen on the Fe(lll) surface above, we now consider the desorption of nitrogen, for which the kinetic parameters are readily derived from a TPD experiment. Combining adsorption and desorption enables us to calculate the equilibrium constant of dissociative nitrogen adsorption from... 

The SCR catalyst is considerably more complex than, for example, the metal catalysts we discussed earlier. Also, it is very difficult to perform surface science studies on these oxide surfaces. The nature of the active sites in the SCR catalyst has been probed by temperature-programmed desorption of NO and NH3 and by in situ infrared studies. This has led to a set of kinetic parameters (Tab. 10.7) that can describe NO conversion and NH3 slip (Fig. 10.16). The model gives a good fit to the experimental data over a wide range, is based on the physical reality of the SCR catalyst and its interactions with the reacting gases and is, therefore, preferable to a simple power rate law in which catalysis happens in a black box . Nevertheless, several questions remain unanswered, such as what are the elementary steps and what do the active site looks like on the atomic scale ... 

The variations in the kinetic parameters (E, m,n) with chlorine coverage shown in Fig. 5 are entirely consistent with our studies by thermal desorption spectroscopy, which show the effects of chlorine... 

These experiments also show the value of NEXAFS as a technique for following the kinetics of surface processes. We have shown that experiments can be tailored so a specific reaction can be studied, even if gas evolution is not involved. This represents an advantage over thermal desorption experiments, where several steps may be required in order to desorb the products to be detected. Another advantage of NEXAFS is that rates are measured isothermally, so the kinetic parameters can be determined with accuracy. Finally, NEXAFS is not a destructive technique, so we need not to worry about modifying the surface compounds while probing the system, as would be the case with other techniques such as Auger electron spectroscopy. 

Keywords. Organic pollutants, Aqueous-solid phase systems, Sorption, Desorption, Kinetics, Modeling, Transport parameters, Solid waste materials, Slow sorption/desorption, Highway materials, Remediation... 

Fig. 3.36 (a) Desorption curves for the (MgHj + Xwt%Al) composites with the content of A1 additive equivalent to the content of free A1 formed after decomposition of Awt%LiAlH in (MgH + Xwt%LiAlH ) composites (300°C 0.1 MPa Hj). The (MgH + Xwt%Al) composites were baU milled for 20 h. (b) Dependence of an effective kinetic parameter, k, in the JMAK equation on the equivalent content of A1 metal additive... 

The generic shape of the desorption peak for TPD, zeroth order kinetics. (Parameters A=10 s , E = 100 kJ/mol, initial coverage 0.95.)... 

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