Characterization adsorption, enthalpies

The observed frequency and the low adsorption enthalpy are similar to those found for CO on the prismatic faces of chromia and ferric oxide. This comparison suggests that the electric fields at Al3+ centers sensed by CO are very weak. The reason for the predominance of faces characterized by highly shielded cations may be related to the high temperature of sintering needed to form the a phase, a parameter which favors the preferential formation of homopolar faces (500, 538, 543). 

Elements 108 - 116 are homologues of Os through Po and are expected to be partially very noble metals. Thus it is obvious that their electrochemical deposition could be an attractive method for their separation from aqueous solutions. It is known that the potential associated with the electrochemical deposition of radionuclides in metallic form from solutions of extremely small concentration is strongly influenced by the electrode material. This is reproduced in a macroscopic model [70], in which the interaction between the microcomponent A and the electrode material B is described by the partial molar adsorption enthalpy and adsorption entropy. By combination with the thermodynamic description of the electrode process, a potential is calculated that characterizes the process at 50% deposition ... 

It is assumed that the binding energy of an adsorbed single molecule to the surface approximately equals its partial molar adsorption enthalpy at zero surface coverage. In the adsorbed state at zero surface coverage the individual variations of the entropy are partly but not completely suppressed. Hence, it is expected that this adsorption enthalpy is proportional to the standard sublimation enthalpy, which characterizes the volatility properties of pure solid phases as an integral value, ... 

The adsorption enthalpy equals the sum of desublimation enthalpy and net adsorption enthalpy. The net adsorption enthalpy is the enthalpy difference between a pure solid compound and its adsorbed state on a surface at zero surface coverage. Hence, the net adsorption enthalpy characterizes the interaction, which depends on the nature of both metals. On the other hand, the desublimation enthalpy is an exclusive property of the adsorbate. 

Alternative methods of characterizing adsorption thermodynamically involve the analogue of the isosteric enthalpy used in discussing gas/solid systems. However, as discussed in an earlier Report this method of analysis, which involves calculating the temperature coefficient of the equilibrium value of X2 corresponding to a constant surface mole fraction x", can be carried out only within the framework of an adsorbed phase model. It is sometimes supposed that the restriction of the partial differentiation to constant xj can be replaced by specifying constant nj. However, this implies that both the surface area of the adsorbent, and the thickness of the adsorbed layer are independent of temperature. In many instances this is an unjustified assumption. 

This is an important quantity likewise for single and for multi-component adsorbates which characterizes the energetic state of the adsorbed molecules. It normally is determined by measuring the difference (AH) between the enthalpies of the mass (m ) in the gaseous state (tf) and the adsorbed state (H ), i.e. the so-called adsorption enthalpy... 

Further information is obtained if the amount of liquid adsorbed on the surface of the particle is also determined, permitting the combination of the data on heat of immersion with those on the amount of adsorbed liquid. Thus, molar adsorption enthalpies can be given for the characterization of the stabilizing adsorption layer [12-16]. A further benefit of adsorption excess isotherms is that it is possible to calculate from them the free enthalpy of adsorption as a function of composition. When these data are combined with the results of calorimetric measurements, the entropy change associated with adsorption can also be calculated on the basis of the second law of thermodynamics. Thus, the combination of these two techniques makes possible the calculation of the thermodynamic potential functions describing adsorption [14,17-19]. 

The interaction of adsorbents with various surface energies with the liquid components studied are adequately characterized by the differences in molar adsorption enthalpies between components 1 and 2, h - (l/r )ft, listed in Table 1. In the case of the adsorption of the methanol-benzene liquid pair, these enthalpy differences in the adsorption layer are decreased by the effect of hydropho-bization. 

Hence, for small or equal net adsorption enthalpies it can be expected that the adsorption enthalpy is proportional to the standard sublimation enthalpy, which characterizes the volatility properties of pure solid phases as an integral value ... 

This model assumes that the adsorption enthalpy can be divided into two independent energetic parts—the desublimation enthalpy and the net adsorption enthalpy. Hence, the net adsorption enthalpy characterizes the interaction, which... 

The lower pressure sub-region is characterized by a considerable enhancement of the interaction potential (Chapter 1) and therefore of the enthalpy of adsorption consequently the pore becomes completely full at very low relative pressure (sometimes 0 01 or less), so that the isotherm rises steeply from the origin. This behaviour is observed with molecular sieve zeolites, the enhancement of the adsorption energy and the steepness of the isotherm being dependent on the nature of the adsorbent-adsorbate interaction and the polarizability of the adsorbate. -... 

The shift in the C=C frequency, vi, for adsorbed ethylene relative to that in the gas phase is 23 cm-1. This is much greater than the 2 cm-1 shift that is observed on liquefaction (42) but is less than that found for complexes of silver salts (44) (about 40 cm-1) or platinum complexes (48) (105 cm-1). Often there is a correlation of the enthalpy of formation of complexes of ethylene to this frequency shift (44, 45). If we use the curve showing this correlation for heat of adsorption of ethylene on various molecular sieves (45), we find that a shift of 23 cm-1 should correspond to a heat of adsorption of 13.8 kcal. This value is in excellent agreement with the value of 14 kcal obtained for isosteric heats at low coverage. Thus, this comparison reinforces the conclusion that ethylene adsorbed on zinc oxide is best characterized as an olefin w-bonded to the surface, i.e., a surface w-complex. 

The adsorption bond can be quite strong, and is characterized by an energy known as the enthalpy of adsorption A//(ac. sj- Since a hot iron is needed to remove the creases in the cotton, we realize that the iron must have supplied sufficient heat to the adsorbed water to overcome and hence break the adsorptive bond joining it to the underlying cotton substrate. The steam we experience is adsorbed water released in this way. 

Beside O P D it is well known that metal deposition can also take place at potentials positive of 0. For this reason called underpotential deposition (UPD) it is characterized by formation of just one or two layer(s) of metal. This happens when the free enthalpy of adsorption of a metal on a foreign substrate is larger than on a surface of the same metal [ 186]. This effect has been observed for a number of metals including Cu and Ag deposited on gold ]187]. Maintaining the formalism of the Nernst equation, deposition in the UPD range means an activity of the deposited metal monolayer smaller than one ]183]. 

Each patch is characterized by a different energy of adsorption, i.e., different heat (enthalpy) of adsorption Q. Thus, p will be also different for each patch because... 

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