Enthalpy changes adsorption

As already shown in Figures 2.6, 2.15, 2.22 and 5.26 very often it is found that the binding energy, Ebj, or enthalpy of adsorption, AHj, is related linearly to the change in work function [Pg.300]

Poisoning is caused by chemisorption of compounds in the process stream these compounds block or modify active sites on the catalyst. The poison may cause changes in the surface morphology of the catalyst, either by surface reconstruction or surface relaxation, or may modify the bond between the metal catalyst and the support. The toxicity of a poison (P) depends upon the enthalpy of adsorption for the poison, and the free energy for the adsorption process, which controls the equilibrium constant for chemisorption of the poison (KP). The fraction of sites blocked by a reversibly adsorbed poison (0P) can be calculated using a Langmuir isotherm (equation 8.4-23a) ... 

Care note the double A symbol in A A H(ads), which represents the change in AH(adSj from its mean value rather than the enthalpy of adsorption itself. 

This expression is analogous to Henry s Law for gas-liquid systems even to the extent that the proportionality constant obeys the van t Hoff equation and Ka = K0e AH/RT where AH is the enthalpy change per mole of adsorbate as it transfers from gaseous to adsorbed phase. At constant temperature, equation 17.1 becomes the simplest form of adsorption isotherm. Unfortunately, few systems are so simple. 

The site-selectively derived thermodynamic parameters obtained by adaptation of Equation 1.17 (Table 1.8) clearly revealed that the heat of adsorptions are exothermic on both enantioselective and nonenantioselective sites, and the difference in the adsorption enthalpies on enantioselective and nonenantioselective sites is about 10 and 15 kJ mol for/ - and 5-enantiomers, respectively. The differential enthalpy change upon adsorption of R- and 5-enantiomers at the enantioselective site AAEIg... 

Clay Resident inorganic cation CEC (pEq g- ) Organic adsorption (pEq g- ) Diquat Paraquat Enthalpy change AT/ (KJ moE ) Diquat Paraquat ... 

Molar entropy of an adsorbed layer perturbed by the solid surface Total enthalpy change for the immersion of an evacuated solid in a solution at a concentration at which monolayer adsorption occurs Heat of dilution of a solute from a solution Enthalpy change for the formation of an interface between an adsorbed mono-layer and solution Integral heat of adsorption of a monolayer of adsorbate vapor onto the solid surface... 

An important hypothesis related to interface energetics is that of Nyilas, who said that the free energy of adsorption basically drives conformational change 132>. He probed this approach by measuring the enthalpy of adsorption using micro calorimetry and attempted to relate surfaces with low heats of protein adsorption with increased blood compatibility. 

Here A//adS is the enthalpy of adsorption, T is the temperature, and AAads is the entropy change associated with the adsorption of the protein onto the surface. Protein adsorption will take place if AGads < 0. Considering a complex system, where proteins are dissolved in an aqueous environment, and are brought into contact with an artificial interface, there are a vast number of parameters that impact AGads due to their small size (i.e., large diffusion coefficient), water molecules are the first to reach the surface when a solid substrate is placed in an aqueous biological environment. Hence, a hydrate layer is formed. With some delay, proteins diffuse to the interface and competition for a suitable spot for adsorption starts. This competition... 

Af/a(is enthalpy of adsorption, A ads entropy change associated with the adsorption of the protein onto the surface, AGads Gibbs free energy of adsorption... 

Immobilization of the aminosilane molecule changes its interaction characteristics. Because the surface silanols are more acidic than silane silanols, the interaction with the surface silanols is thermodynamically favoured over intramolecular interaction. Kelly and Leyden10 measured the enthalpy of adsorption of the aminosilane molecules. Their results indicate that interaction with the surface involves more proton transfer than in the closed form dissolved molecules. 

Differential enthalpy changes of hydrogen and isobutylene adsorption were measured, as described elsewhere (36, 42). Figure 3 shows differential heat versus adsorbate coverage for hydrogen and isobutylene adsorption on Pt/Sn/Si02. Values of the differential enthalpy changes of adsorption,... 

The enthalpy changes for adsorption of acetaldehyde (step 3), ethanol (step 5), hydrogen (step 6), water (step 8), and acetic acid to form adsorbed acetate (step 9) were adjusted in the reaction kinetics analysis. The initial estimates of the heats of adsorption of acetaldehyde, ethanol, and hydrogen were obtained from the DFT predictions for these species on Cu(211) (Table VIII). The heat of adsorption of water was constrained to be equal to the heat of adsorption of ethanol in these analyses. The steps involving adsorption of ethanol, acetaldehyde, water, and the step in which acetic acid forms the surface acetate species were all assumed to be nonactivated. 

The standard entropy and enthalpy changes for the dissociative adsorption of ethyl acetate (step 1), acetic acid (step 7), and the hydrogenation of adsorbed acetaldehyde to form ethoxy species (step 4) were constrained to maintain thermodynamic consistency for the appropriate overall gas-phase reactions. 

The activation energies for the dissociative adsorption of ethyl acetate and acetic acid were adjusted in the analysis. To be consistent with the DFT results shown in Table VIII, the activation energy for the dissociative adsorption of ethyl acetate was constrained to be less that the activation energy for the dissociative adsorption of acetic acid. The standard entropy changes for these steps were constrained such that the activated complexes were immobile species. The standard entropy and enthalpy changes to form the activated complex for step 2 were adjusted. 

We first consider the parameterization of the enthalpy changes of adsorption. It has been established that the heats of adsorption of various paraffins in zeolites vary linearly with carbon number (124-126). Therefore, we define the enthalpy of formation for a surface species as... 

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