Mean adsorption enthalpy

True activation energies are obtained when the reaction order is zero and probably also when the rate coefficient, k, and adsorption coefficient, Ka, have been separated by treatment of rate data by means of eqn. (3). In the case of the first-order rate equation, the apparent activation energy, calculated from k values [eqn. (5)] by means of the Arrhenius equation, is the difference between the true activation energy and the adsorption enthalpy of the reactant A... 

It is noticeable that both linear and non linear modeling approaches explain changes in adsorption enthalpy data with the same molecular properties. This means that the lack of agreement observed is not due to the way of combining the explicative variables in the relationship. A better representation would only be found provided that an other pertinent molecular property is included among the input variables set. 

The standard adsorption enthalpy can also be used to gain information on changes in the surface characteristics of the carbon materials. For this purpose, molecular probes with different sizes and shapes can be used n-hexane, benzene, cyclohexane and 2,2 DMB. All these molecules have six carbon atoms and are linear (n-hexane), cyclic (benzene and cyclohexane) and branched (2,2 DMB), and their mean molecular sizes range from 0.405nm (n-hexane) to 0.62nm (2,2 DMB). 

Ex situ investigations can be carried out with all the methods usually applied to gas-phase adsorption problems, provided they do not require flat surfaces. These methods are of limited value, as separation from the liquid phase will generally influence the composition of the surface unless the adsorption enthalpy is very great, which means that a stable surface compound may be formed. Under this condition all additional information may be useful. 

In most cases, the separation of enantiomers on CSPs is controlled by the difference of adsorption enthalpies and a increases with decreasing temperature. The effects and thus the separation factors are small (which means unfortunately that nonchiral retention is dominating). An a of 1.05 corresponds to a free energy difference of adsorption of 120 J mol and a minimum of 8000 theoretical plates is necessary to obtain good resolution (gas constant, R = 1 with ki = 10 the calculation is performed by means of the chromatographic resolution equation). 

The adsorption of dihydrogen onto the zeohtes Na-ZSM-5 and K-ZSM-5 rendered the fundamental H-H stretching mode as IR active [05A1]. The corresponding IR adsorption bands were fonnd at 4101 and 4112 cm for H2/Na-ZSM-5 and H2/K-ZSM-5, respectively. By means of variable-temperature IR spectroscopy, the standard adsorption enthalpy and entropy were determined. 

The TAP reactor allows the fast and simultaneous determination of adsorption and diffiision parameters in activated carbons for the low-pressure limit. The adsorption and diffiision parameters determined agree well with those values obtained by other authors for n-butane adsorption over different activated carbons. The determined adsorption enthalpies are very high and almost do not depend on carbons, which means that all the samples have micropores whose dimensions are close to the n-butane molecular size. A TAP pulse response experiment allows gas access into the micropores. The value of the diffiision coefficient decreeises linear with decreasing pore size. 

Thermal desorption requires a means to apply heat to the trap while carrier gas is flowing through and to do so in a pulse mode, so as to simulate a typical syringe injection into a GC system. Selection of thermal desorption parameters depends on the thermal stability of sorbent and sorbate, the energy required to desorb the latter (related to its adsorption enthalpy), and close control of the chosen desorption temperature (particularly the risetime to achieve a rapid pulse) [561]. 

If the van t Hoff and isosteric methods are simple ways for estimating the adsorption enthalpy of single component from isothermal adsorption data, they have the disadvantage to not take into account the temperature dependence on the enthalpy and entropy and to be not enough accurate. Moreover they are not adapted to the adsorption of gas mixtures. The best mean to determine the adsorption and coadsorption enthalpy is to measure them by using a differential calorimetry technique coupled with others techniques allowing the measure of adsorbed amount and composition as for example the manometry and the chromatography. 

In the case of multi-components adsorption, the partial molar differential adsorption enthalpies and entropies of each component i present in the gas mixture cannot be directly measured by experiments. However, it is possible to estimate them by mean of the tangent method based on the well-known Gibbs-Duhem relation. 

Although we do not wish to imply that equation (6.20) is a general fundamental equation, we are also not aware of any published exceptions to the physical meaning it conveys, i.e. that the enthalpy of adsorption and thus, according to any isotherm, the coverage of an electron acceptor/donor adsorbate decreases/ increases with increasing work function O and thus decreasing Fermi level EF. 

The cooling cycle starts when all parts of the refrigerator are at about 1.3K. At this temperature, the 3He is completely adsorbed by the pump. The pump temperature is now raised to about 25 K by means of an heater. At 25 K, the 3He is desorbed, and its pressure increases over the saturation pressure at 1.3 K. Consequently, 3He condenses in the part of the tube T internal to the copper support C and drops down into the evaporator E. In this phase, the latent heat of condensation and the enthalpy variation are delivered to the 4He bath. The cooling phase starts when all the 3He is condensed in E and the power on the pump heater is switched off. The pump starts cooling towards the bath temperature, reducing the pressure on liquid 3He in E. The adsorption heat of the 3He vapour is delivered to the 4He bath by L. 

Figure 10.6 The enthalpy of adsorption A//fads) is a function of 0. The position of the x-axis represents the mean enthalpy the magnitude of the vertical deviation from the v-axis is A A H(iicls)...

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. 

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]. 

For the physical adsorption of gases on solids the attraction between the molecules and the surface is almost the exclusive driving force. Thermodynamically this means that such gas adsorption is exothermic. Usually the enthalpy of adsorption per molecule depends on 0 because of heterogeneity (upon filling an adsorbent with adsorbate the "highest energetic" parts are covered first) and because, with increasing 0, lateral interaction also increases (this contribution may be attractive or repulsive). 

The energetic meaning of the C parameter is also confirmed when one observes that it is closely related to the differential enthalpies of adsorption. 

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