Site energy distribution differential

It is not surprising, in view of the material of the preceding section, that the heat of chemisorption often varies from the degree of surface coverage. It is convenient to consider two types of explanation (actual systems involving some combination of the two). First, the surface may be heterogeneous, so that a site energy distribution is involved (Section XVII-14). As an example, the variation of the calorimetric differential heat of adsorption of H2 on ZnO is shown in Fig. 

FIGURE 13.5 Calorimetric and volumetric data obtained from adsorption calorimetry measurements Raw pressure and heat flow data obtained for each dose of probe molecule and Thermokinetic parameter (a), Volumetric isotherms (b), Calorimetric isotherms (c), Integral heats (d), Differential heats (e), Site Energy Distribution Spectrum (f). (From Damjanovic, Lj. and Auroux, A., Handbook of Thermal Analysis and Calorimetry, Further Advances, Techniques and Applications, Elsevier, Amsterdam, 387-438, 2007. With permission.)... 

The second approximation to the F vs. b plot is shown in Figure 2,a as well as F3 vs. 6, obtained by repeating the above procedure. In the present example F i vs. b was taken to be in the terminal approximation (also shown in Figure 1,6) and, by means of Equation 4, the integral site energy distribution F(Q) was obtained. The corresponding differential site energy distribution is shown in... 

The following notes and symbols will be used in the other tables as well T, adsorption temperature Si/Al, silicon to aluminum ratio q, differential heat of adsorption n, surface coverage < inai < location of the maximum distribution of sites in the site energy distribution plot, with letters indicating the relative number of sites under the peak L, large I, intermediate S, small. 

Topspe and co-workers (231) studied calorimetrically the adsorption of CO at 303 K on MgO-supported Fe and on two unsupported Fe ammonia synthesis catalysts. These catalysts displayed quite heterogeneous site energy distributions. For example, the differential heat of adsorption on the Fe/MgO catalyst decreased from about 110 kJ moP to a large plateau at 80 kJ moP before decreasing abruptly to near 40 kJ moP. It was found that the amount of weakly held CO increased with decreasing Fe particle size. The authors used IR spectroscopy to demonstrate that the differences in the site energy... 

Fubini et al. (123) studied the adsorption of CO on reduced and oxidized Cu/ZnO catalysts. The differential heat for the reduced catalyst decreased rapidly with coverage from 120 to 40 kj mol and remained constant at that value. The site energy distribution on the oxidized sample showed a maximum concentration at about 90 kJ moC and no sites with diflerential heats lower than 65 kJ mol. The authors proposed that adsorption microcalorimetry is a good probe for the detection of different adsorption sites, namely, Cu° and Cu. ... 

The surface acidity of three commercial mordenite (MOR) zeolites with Si/Al ratios of 10, 60, and 80 has been evaluated by adsorption micro calorimetry at 423 K, using pyridine as a probe molecule [210]. As could be expected from the Si/Al ratios, the total pyridine uptakes varied in the order MOR-10 >MOR-60 >MOR-80. The initial differential heats of adsorption were in the range 215-220 kJ mol After a sudden drop, Qdiff changed slightly and stepwise over a relatively wide range of pyridine uptake (at least for MOR-10) and then steeply decreased. The site-energy distributions and the thermokinetic parameters versus coverage were also determined [210]. 

The surface aridity of a series of Beta zeolites with different Si/Al ratios has been assessed by adsorption calorimetry at 423 K using pyridine as a probe molecule. The site energy distribution was discussed in relation with FTIR results [231]. Differential heats of adsorption of pyridine on Beta zeolites (Si/Al15) were also measiued at 473 K by Diaz-Mendoza et al. [232]. 

A site distribution assuming no lateral interaction was determined using Steele s method (4) for neon on titanium dioxide. The site energies and distribution are given in Table II (columns 3 and 5). With this site distribution, Equations 2.3 and 3.6 of Aston, Tykodi, and Steele (2) were used to calculate the isotherm and differential heats of adsorption for neon on titanium dioxide which already has been treated by Aston, Tykodi, and Steele. Table II (columns 2 and 4) also gives the site distribution of Aston, Tykodi, and Steele. In Figure 4,... 

Adsorption is an exothermic process and when a reactive molecule interacts with the surface of the solid heat is evolved. This heat is related to the energy of the bonds formed between the adsorbed species and the adsorbent and hence to the nature of the bonds and to the chemical reactivity of the surface. From a number of techniques used to study this interaction only a few provide information about the strength of chemisorption itself. The determination of the differential heats evolved, by a suitable microcalorimeter, when known amounts of gas probe molecules are adsorbed on catalytic surface is the most suitable and accurate method which allows the determination of the number, strength and energy distribution of the adsorption sites [4-12]. The effective use of this technique in heterogeneous catalysis depends on choice of probe molecule, since the nature of the probe determines which surface property is going to be investigated. 

In order that adsorption microcalorimetry can give an accurate representation of active site strength distribution the adsorbed probe molecule must be equilibrated among all sites on the catalyst surface within the time frame of the experiment. If external or internal mass-transfer limitation exists the adsorption on surface sites is controlled by kinetics instead of thermodynamics [5]. In this case the profile of differential heats as function of coverage would not accurately represent energy spectrum of the surface active sites and the adsorbent surface would appear to be more homogeneous. For these reasons, it is important to study the effect of the adsorption temperature and verify that molecules possess sufficient thermal energy to obtain the thermodynamically stable site occupation. 

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