Enthalpies adsorption, factors affecting

The pretreatment temperature is an important factor that influences the acidic/ basic properties of solids. For Brpnsted sites, the differential heat is the difference between the enthalpy of dissociation of the acidic hydroxyl and the enthalpy of protonation of the probe molecule. For Lewis sites, the differential heat of adsorption represents the energy associated with the transfer of electron density toward an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [147,182]. Increasing the pretreatment temperature modifies the surface acidity of the solids. The influence of the pretreatment temperature, between 300 and 800°C, on the surface acidity of a transition alumina has been studied by ammonia adsorption microcalorimetry [62]. The number and strength of the strong sites, which should be mainly Lewis sites, have been found to increase when the temperature increases. This behavior can be explained by the fact that the Lewis sites are not completely free and that their electron pair attracting capacity can be partially modified by different OH group environments. The different pretreatment temperatures used affected the whole spectrum of adsorption heats... 

There are a number of different factors which may affect the level of uptake and the energetics of adsorption from solution the chemistry and electrical properties of the solid surface and the molecular/micellar/polymeric structure of the solution must all be taken into account. Whenever possible, a study of both adsorption isotherms and enthalpies of displacement is worthwhile, but it is often necessary to complement these measurements with others including electrophoretic mobilities, FI7R spectra-and various types of microscopy. 

Enthalpies of wetting are sometimes used to obtain (integral) enthalpies of adsorption by subtracting the enthalpy of condensation. This procedure is not exact because it presupposes a model in which the interaction between the first and the second surface layer is Interpreted as purely identical to that in condensation (BET theory assumes the same). However, the heat of adsorption of the second layer Is not exactly Identical to the heat of liquefaction and the configuration of the first layer is affected by the presence of a second. In other words, entropic factors also have to be considered, and. in this connection, the packing in the first layer must be known to convert A H (in J m ) into A H (in J mol 2). Notwithstanding these reservations, a certain similarity may be expected. 

Factors (1) to (5) above are known in one way or another to affect the kinetics of an electrode reaction including, in most cases, the Tafel slope. Since the anion adsorption is normally temperature dependent owing to the usually finite enthalpy of adsorption (most chemisorptions are energy as well as entropy controlled in their thermodynamics), it follows that anion effects could give rise to unconventional temperature dependence of a. 

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