Desorption, activation energy

Polymorph Surface area (mVg) Particle diameter (nm) Activation energy desorption (kJ/ mol) Total enthalpy of formation, AH, (kJ/molAlH,)... 

Fig. XVIII-13. Activation energies of adsorption and desorption and heat of chemisorption for nitrogen on a single promoted, intensively reduced iron catalyst Q is calculated from Q = Edes - ads- (From Ref. 130.)...

This means that desorption activation energies can be much larger than those for adsorption and very dependent on 6 since the variation of Q with 6 now contributes directly. The rate of desorption may be written, following the kinetic treatment of the Langmuir model. 

In the case of nitrogen on iron, the experimental desorption activation energies are also shown in Fig. XVIII-13 the desorption rate was given by the empirical expression... 

Temperature progranuned desorption (TPD), also called thenual desorption spectroscopy (TDS), provides infonuation about the surface chemistry such as surface coverage and the activation energy for desorption [49]. TPD is discussed in detail in section B 1.25. In TPD, a clean surface is first exposed to a gaseous... 

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]. 

With the aid of (B1.25.4), it is possible to detennine the activation energy of desorption (usually equal to the adsorption energy) and the preexponential factor of desorption [21, 24]. Attractive or repulsive interactions between the adsorbate molecules make the desorption parameters and v dependent on coverage [22]- hr the case of TPRS one obtains infonnation on surface reactions if the latter is rate detennming for the desorption. 

Step 4 of the thermal treatment process (see Fig. 2) involves desorption, pyrolysis, and char formation. Much Hterature exists on the pyrolysis of coal (qv) and on different pyrolysis models for coal. These models are useful starting points for describing pyrolysis in kilns. For example, the devolatilization of coal is frequently modeled as competing chemical reactions (24). Another approach for modeling devolatilization uses a set of independent, first-order parallel reactions represented by a Gaussian distribution of activation energies (25). 

When the temperature of the analyzed sample is increased continuously and in a known way, the experimental data on desorption can serve to estimate the apparent values of parameters characteristic for the desorption process. To this end, the most simple Arrhenius model for activated processes is usually used, with obvious modifications due to the planar nature of the desorption process. Sometimes, more refined models accounting for the surface mobility of adsorbed species or other specific points are applied. The Arrhenius model is to a large extent merely formal and involves three effective (apparent) parameters the activation energy of desorption, the preexponential factor, and the order of the rate-determining step in desorption. As will be dealt with in Section II. B, the experimental arrangement is usually such that the primary records reproduce essentially either the desorbed amount or the actual rate of desorption. After due correction, the output readings are converted into a desorption curve which may represent either the dependence of the desorbed amount on the temperature or, preferably, the dependence of the desorption rate on the temperature. In principle, there are two approaches to the treatment of the desorption curves. 

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. 

IV. Fundamental Relationships for the Determination of the Activation Energy of Desorption, of the Order of Desorption and of the Preexponential Factor... 

Let us consider a surface on which particles are adsorbed on sites with different activation energy of desorption, and the distribution of these energies over the surface is discrete so that ni0 particles are initially in a state with an activation energy of desorption Edt, n particles with an energy Ed/, etc. Such a model corresponds to a concept of adsorption on different crystal planes each of which is homogeneous, or to a concept of different adsorption states of the particles adsorbed on a single crystal (26, 88). 

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<-... 

The paper by Dawson and Peng (98) can be quoted as an example of applying Eq. (58) to a kinetic analysis of both the first-order and second-order desorptions with an activation energy varying linearly with the surface coverage. 

Ei( activation energy of desorption for the ith desorption process (kcal mole-1)... 

Regardless of the exact extent (shorter or longer range) of the interaction of each alkali adatom on a metal surface, there is one important feature of Fig 2.6 which has not attracted attention in the past. This feature is depicted in Fig. 2.6c, obtained by crossploting the data in ref. 26 which shows that the activation energy of desorption, Ed, of the alkali atoms decreases linearly with decreasing work function . For non-activated adsorption this implies a linear decrease in the heat of chemisorption of the alkali atoms AHad (=Ed) with decreasing > ... 

On K modified Ni(100) and Ni(lll)62,63 and Pt(lll)64 the dissociative adsorption of hydrogen is almost completely inhibited for potassium coverages above 0.1. This would imply that H behaves as an electron donor. On the other hand the peaks of the hydrogen TPD spectra shift to higher temperatures with increasing alkali coverage, as shown in Fig. 2.22a for K/Ni(lll), which would imply an electron acceptor behaviour for the chemisorbed H. Furthermore, as deduced from analysis of the TPD spectra, both the pre-exponential factor and the activation energy for desorption... 

The effect of electronegative modifiers on the activation energy of CO desorption, Ed, and on the corresponding pre-exponential factor, vd, can be quantified by analysis of the TPD spectra at very low CO coverages. The... 

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