Activation energies of desorption

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. 

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. 

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<-... 

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 > ... 

Figure 8,16. Activation energy of desorption, Ed, of 02 from Ru02 as a function of the applied potential21,25 as extracted from TPD and the modified Redhead equation of Falconer and Madix.25,26 Reprinted from ref. 21 with permission from Elsevier Science.

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. 

Give a brief overview of methods that can be used to derive the activation energy of desorption from TPD experiments. 

TDS also gives information on the strength of the bond between adsorbate and substrate. An important check is obtained from desorption of Ag from a thick layer Here the activation energy of desorption should be equal to the heat of vaporization of Ag, 254 kJ/mol. Of course, the more interesting information is in the adsorption energies of Ag on Ru. This requires quite a bit of effort as we shall see. 

Particularly popular among surface scientists is the Redhead method [26], in which the activation energy of desorption is given by ... 

Figure 2.14 Simulated TDS spectra and the results of a number of different analysis procedures for determining the activation energy of desorption. The solid line represents the input for the simulations. Note that only the complete analysis [16] and the leading edge procedure of Habenschaden and Kiippers [29] give reliable results. The Chan-Aris-Weinberg curves [28] extrapolate to the correct activation energies at zero coverage (from de Jong and Niemantsverdriet [31]).

There is no distinct threshold between physical adsorption, i.e., reversible adsorption with small activation energy of desorption, chemisorption with a significant activation energy of desorption, and formation of surface compounds with a high activation energy for... 

Table 2.7 Summary of apparent activation energies of desorption for unmiUed Tego Magnan and ABCR powders estimated from the Arrhenius plot...

Powder Apparent activation energy of desorption, (kJ/mol) Coefficient of fit in the Arrhenius equation Kinetic curves at temperatures taken for calculation (°C) Activation... 

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