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Dissociative adsorption, entropy

Clearly, the sticking coefficient for the direct adsorption process is small since a considerable amount of entropy is lost when the molecule is frozen in on an adsorption site. In fact, adsorption of most molecules occurs via a mobile precursor state. Nevertheless, direct adsorption does occur, but it is usually coupled with the activated dissociation of a highly stable molecule. An example is the dissociative adsorption of CH4, with sticking coefScients of the order 10 -10 . In this case the sticking coefficient not only contains the partition functions but also an exponential... [Pg.120]

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. [Pg.229]

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. [Pg.229]

It is often observed that molecular adsorption prevails at lower temperatures and that dissociative adsorption occurs at higher temperature. This may be caused by kinetics the activation energy for dissociative adsorption is then too high for dissociation at lower temperature. It could also have a thermodynamic reason. If the number of surface sites where adsorption can take place is equal for molecular and dissociative adsorption, the surface can accommodate twice as many molecules in the molecular state than in the dissociative state. Hence, molecular adsorption will prevail if the heat of dissociative adsorption is not very much higher than the heat of molecular adsorption. The entropy change upon... [Pg.126]

The initial entropy of cumene adsorption on a commercial aluminosilicate was measured to be about —750JmoP K , whereas the gas-phase entropy of cumene at 298 K is 389 J moP K (709). This seemingly inconsistent result appears to be caused by the dissociative adsorption of cumene at low coverages on this catalyst. In this case, the measured heat corresponds to a combination of heats of adsorption and reaction. Higher coverages produced lower, nearly constant heats and entropies of adsorption. These entropies correspond to the loss of between two and three degrees of translational freedom. The adsorption of benzene on these samples did not show abnormally... [Pg.181]

Developing this assumption, B. Eichler et al. (1992) determined the enthalpy and entropy of the dissociative adsorption for the reactions... [Pg.2449]

These criteria are listed in Table 6.9, and they are comprised of three strong rules and two guidelines. First, adsorption is invariably exothermic, thus the enthalpy of adsorption is negative, i.e., -AH°j > 0(Qad > 0). Second, there must be a decrease in entropy after adsorption, including dissociative adsorption of a diatomic molecule [32], thus AS = — S < 0,... [Pg.135]

For a localized adsorption the concentration of the adsorption sites has to be taken into account [4]. In other cases a reversible change of the chemical state of the adsorbate in the chromatography process has to be considered, e.g., dissociative adsorption and substimtive adsorption as described in Part II of this chapter (Sect. 1.3, Eqs. 55, 56). The reaction enthalpy and entropy have to be introduced into the calculations [5-8] as well as the equilibrium constant for the chemical reaction including the standard states of the occurring chemical states as ... [Pg.378]

Gas, cells, 464, 477, 511 characteristic equation, 131, 239 constant, 133, 134 density, 133 entropy, 149 equilibrium, 324, 353, 355, 497 free energy, 151 ideal, 135, 139, 145 inert, 326 kinetic theory 515 mixtures, 263, 325 molecular weight, 157 potential, 151 temperature, 140 velocity of sound in, 146 Generalised co-ordinates, 107 Gibbs s adsorption formula, 436 criteria of equilibrium and stability, 93, 101 dissociation formula, 340, 499 Helmholtz equation, 456, 460, 476 Kono-walow rule, 384, 416 model, 240 paradox, 274 phase rule, 169, 388 theorem, 220. Graetz vapour-pressure equation, 191... [Pg.541]

An example illustrates the usefulness of Table II. Suppose a certain adsorption reaction is 0.5 order, and it is concluded that dissociation accompanies adsorption that is. Step 2 applies. Suppose also that L has been found by a nonkinetic method to be 10 sites cm, and that according to TST L is calculated to be 10 sites cm . To decrease the calculated value of L by a factor of 100 means that AS (a negative quantity) as calculated from the model is 18.4 e.u. (that is, 2 x 9.2 e.u.) too low. Thus, in this example the gas did not lose as much entropy upon adsorption as had been supposed. Such a result could indicate that the dissociated fragments are mobile, not limited to fixed sites. [Pg.120]

It is possible to calculate the entropies in cases where association or dissociation occurs at the same time as localized adsorption. Chang (17) gave an expression... [Pg.238]

The three cases of chemisorption examined all showed localized adsorption, but there may be other examples in which a considerable mobility is possessed by the adsorbed molecules or atoms. Again, this would be more likely to occur at high temperatures. Entropy values can give indications of dissociation or, more particularly, of association when localized adsorption takes place. This was clearly noticeable in the values for the entropy of water on mercury. Likewise a knowledge of the changes in entropy during the course of an isotherm may be of use... [Pg.249]

Sorption of Cu(tfac)2 on a column depends on the amount of the compound injected, the content of the liquid phase in the bed, the nature of the support and temperature. Substantial sorption of Cu(tfac)2 by glass tubing and glass-wool plugs was observed. It was also shown that sorption of the copper chelate by the bed is partialy reversible . The retention data for Cr(dik)3, Co(dik)3 and Al(dik)3 complexes were measured at various temperatures and various flow rates. The results enable one to select conditions for the GC separation of Cr, Al and Co S-diketonates. Retention of tfac and hfac of various metals on various supports were also studied and were widely used for the determination of the metals. Both adsorption and partition coefficients were found to be functions of the average thickness of the film of the stationary phase . Specific retention volumes, adsorption isotherms, molar heats and entropy of solution were determined from the GC data . The retention of metal chelates on various stationary phases is mainly due to adsorption at the gas-liquid interface. However, the classical equation which describes the retention when mixed mechanisms occur is inappropriate to represent the behavior of such systems. This failure occurs because both adsorption and partition coefficients are functions of the average thickness of the film of the stationary phase. It was pointed out that the main problem is lack of stability under GC conditions. Dissociation of the chelates results in a smaller peak and a build-up of reactive metal ions. An improvement of the method could be achieved by addition of tfaH to the carrier gas of the GC equipped with aTCD" orFID" . ... [Pg.701]

Chemisorption differs from physisorption in that it involves the formation of chemical bonds and electron transfer between adsorbate and adsorbent. It is of primary concern with regard to catalysis, and it is only useful as an extreme measure for separation purposes. Removal of chemical warfare agents from breathing air is a prime example. One way to discem between physisorption and chemisorption is by heat effects and the effect of temperature. For example, physisorption is always exothermic, i.e., A// < 0. Furthermore, the adsorbed state is more ordered than the fluid state, so AA < 0. To be thermodynamically feasible, < 0, so AH < TAS. In contrast, endothermic adsorption is possible for dissociative chemisorption, even if the entropy of the adsorbate decreases, because the entropy of the adsorbent may increase to more than offset that, e.g., by expanding. [Pg.1133]


See other pages where Dissociative adsorption, entropy is mentioned: [Pg.304]    [Pg.246]    [Pg.68]    [Pg.230]    [Pg.183]    [Pg.314]    [Pg.263]    [Pg.183]    [Pg.313]    [Pg.58]    [Pg.103]    [Pg.980]    [Pg.182]    [Pg.213]    [Pg.110]    [Pg.550]    [Pg.169]    [Pg.173]    [Pg.234]    [Pg.247]    [Pg.20]    [Pg.177]    [Pg.169]    [Pg.14]    [Pg.127]    [Pg.116]    [Pg.129]    [Pg.667]    [Pg.319]    [Pg.81]    [Pg.496]    [Pg.390]    [Pg.154]   
See also in sourсe #XX -- [ Pg.234 , Pg.246 , Pg.247 , Pg.249 ]




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