Energy of adsorbate

Vibrational energy states are too well separated to contribute much to the entropy or the energy of small molecules at ordinary temperatures, but for higher temperatures this may not be so, and both internal entropy and energy changes may occur due to changes in vibrational levels on adsoiption. From a somewhat different point of view, it is clear that even in physical adsorption, adsorbate molecules should be polarized on the surface (see Section VI-8), and in chemisorption more drastic perturbations should occur. Thus internal bond energies of adsorbed molecules may be affected. 

According to Snyder [28], the solvent strength 8° is the standard free energy of adsorbed solvent molecules in a standard state, and it is given by Equation 4.11 ... 

The situation becomes even more complex in the case of electrochemical adsorption. Kolb et al. [17] first demonstrated clearly that the binding energy of adsorbed... 

Adsorption experiments are conducted at constant temperature, and an empirical or theoretical representation of the amount adsorbed as a function of the equilibrium gas pressure is called an adsorption isotherm. Adsorption isotherms are studied for a variety of reasons, some of which focus on the adsorbate while others are more concerned with the solid adsorbent. In Chapter 7 we saw that adsorbed molecules can be described as existing in an assortment of two-dimensional states. Although the discussion in that chapter was concerned with adsorption at liquid surfaces, there is no reason to doubt that similar two-dimensional states describe adsorption at solid surfaces also. Adsorption also provides some information about solid surfaces. The total area accessible to adsorption for a unit mass of solid —the specific area Asp — is the most widely encountered result determined from adsorption studies. The energy of adsorbate-adsorbent interaction is also of considerable interest, as we see below. 

In this chapter, we discuss TPR and reduction theory in some detail, and show how TPR provides insight into the mechanism of reduction processes. Next, we present examples of TPO, TP sulfidation (TPS) and TPRS applied on supported catalysts. In the final section we describe how thermal desorption spectroscopy reveals adsorption energies of adsorbates from well-defined surfaces in vacuum. A short treatment of the transition state theory of reaction rates is included to provide the reader with a feeling for what a pre-exponential factor of desorption tells about a desorption mechanism. The chapter is completed with an example of TPRS applied in ultra-high vacuum (UHV), in order to illustrate how this method assists in unraveling complex reaction mechanisms. 

The large binding energies of adsorbed sulfur on all metallic catalysts can explain the high toxicities of this compound for various reactions. 

On the other hand, by a ligand effect, the reactivity of sites located at varying distances from the sulfur-occupied site may be affected. As a proof of charge transfer, adsorbed sulfur is able to decrease the binding energy of adsorbed hydrogen when the free energy of adsorption ofolefinic compounds can be increased on partially sulfurized metallic catalysts. 

A way to stretch or compress metal surface atoms in a controlled way is to deposit them on top of a substrate with similar crystal symmetry, yet with different atomic diameter and lattice constant. Such a single monolayer of a metal supported on another is called an overlayer. Metal overlayers strive to approach the lattice constant of their substrate without fully attaining it hence, they are strained compared to their own bulk state [24, 25]. The choice of suitable metal substrates enables tuning of the strain in the overlayer and of the chemisorption energy of adsorbates. A Pt monolayer on a Cu substrate, for instance, was shown to bind adsorbates much weaker than bulk platinum due to compressive strain induced by the lattice mismatch between Pt and Cu, with Cu being smaller [26]. 

The activation energy is least when the CO bond is weakened because of its interaction with the metal surface. Electron transfer from the metal surface into antibonding molecular orbitals weakens the CO bond. This is a very general feature of dissociative adsorption. The dissociation energy of adsorbed molecules decreases when they are adsorbed with considerable electron transfer from the metal surface to the adsorbate. 

Although the absolute accuracy of these methods is not better than 10 kj/mol for the energies of adsorbed reaction intermediates and 20-30 kJ/mol for transition state energies, the differences between computed energies are usually large enough to justify conclusions as to which reaction pathways are relevant. 

AEa is the change in the interaction energy of adsorbed C atoms with respect to a reference reaction center, Ci is the rate of "Ci" hydrogenation at reference site, and C2 is the value of X at reference site. 

The normalized distribution function for para-Ho (eqm. — H2 at T = 0) is shown in Figure 8. The heat capacities, C s, used in the calculation were measured in the same calorimeter employed for the isosteric heat experiments (20). The distribution function. Figure 8, can now be transformed into the function /(Di) using the tables in References 15 and 21. These tables give the ground state energy of adsorbed para-H2 as a function of D for various values of y. 

From the data summarized in Fig. 14 and Table 5 it can be concluded that for nonporous carbosil particles there is no correlation between the thickness of hydrated layers and the hydrophilic properties of the adsorbents. Thus in the case of the sample whose surface contains 0.5 wt% carbon, very large values for the thickness of adsorbed water layers and for the free surface energy of adsorbents have been recorded, which seems to be related to the formation of surface regions having some electric charge [25 ]. 

Figure 18 shows the variation in the free energy of adsorbed water as a function of the concentration of nonfreezing water for all the adsorbents studied. As C is proportional to the average thickness of layers of nonfreezing water, the above-mentioned dependence reflects the shape of the radial function de.scribing... 

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