Adsorption enthalpy effect

Adsorption enthalpies and vibrational frequencies of small molecules adsorbed on cation sites in zeolites are often related to acidity (either Bronsted or Lewis acidity of H+ and alkali metal cations, respectively) of particular sites. It is now well accepted that the local environment of the cation (the way it is coordinated with the framework oxygen atoms) affects both, vibrational dynamics and adsorption enthalpies of adsorbed molecules. Only recently it has been demonstrated that in addition to the interaction of one end of the molecule with the cation (effect from the bottom) also the interaction of the other end of the molecule with a second cation or with the zeolite framework (effect from the top) has a substantial effect on vibrational frequencies of the adsorbed molecule [1,2]. The effect from bottom mainly reflects the coordination of the metal cation with the framework - the tighter is the cation-framework coordination the lower is the ability of that cation to bind molecules and the smaller is the effect on the vibrational frequencies of adsorbed molecules. This effect is most prominent for Li+ cations [3-6], In this contribution we focus on the discussion of the effect from top. The interaction of acetonitrile (AN) and carbon monoxide with sodium exchanged zeolites Na-A (Si/AM) andNa-FER (Si/Al= 8.5 and 27) is investigated. 

Fig. 9. Effect of the chain length of hydrocarbons on the adsorption enthalpy and rates of desorption. (A) Hydrocarbon in interaction with zeolite framework. Methyl groups interact with the framework oxygen protons exhibit an additional attractive force. (B) Heat of adsorption as a function of carbon number for zeolites MFI and FAU in the acidic and non-acidic form. (C) Relative desorption rates of a C12, Ci6, and C20 alkane compared to octane at 348 K. Values calculated from the linear extrapolation of the heat of adsorption values shown in (B).

There is a wide range of adsorption enthalpies AH(adsi, ranging from effectively zero to as much a 600 kJ per mole of adsorbate. The adsorptive interaction cannot truly be said to be a bond if the enthalpy is small the interaction will probably be more akin to van der Waals forces, or maybe hydrogen bonds if the substrate bears a surface layer of oxide. We call this type of adsorption physical adsorption, which is often abbreviated to physisorption. At the other extreme are adsorption processes for which A//(ads) is so large that real chemical bond(s) form between the substrate and adsorbate. We call this type of adsorption chemical adsorption, although we might abbreviate this to chemisorption. 

A sharp decrease in adsorption enthalpy between 10 and 30% surface coverage of SAL can also be seen in Figure 2. This decrease may indicate that only a small number of surface sites are favorably oriented for SAL-goethite bond formation, although possible SAL-SAL interactions on the surface may also have an effect. Separate measurements of SAL adsorption on goethite, gave relatively small adsorption maxima (when compared to the phosphate and fluoride adsorption maxima discussed above) of 22 and 11 pmol/g at pH 4.8 and 6.3, respectively, in either 0.001 M NaN0 or 0.001 M KC1 06). J... 

A similar technique has been used to determine the acidic character of niobium oxide and niobyl phosphate catalysts in different solvents (decane, cyclohexane, toluene, methanol and isopropanol) using aniline and 2-phenyl-ethylamine as probe molecules [27, 28]. The heat evolved from the adsorption reaction derives from two different contributions the exothermic enthalpy of adsorption and the endothermic enthalpy of displacement of the solvent, while the enthalpy effects describing dilution and mixing phenomena can be neglected owing to the differential design and pre-heating of the probe solution. 

The adsorption of the homologous alkane series on zeolites and other adsorbents has been extensively studied by many research groups. These studies demonstrate the existence of linear relationships between adsorption enthalpy and entropy, and the carbon number [1-4]. Contrarily, there are few studies dealing with the adsorption of alkanes in liquid phase. This can be explained by the lack of selectivity effects that occur in the adsorption of alkanes on adsorbents in liquid phase. Indeed, classical stationary phases for HPLC show no separation of alkane mixtures, as a result of the rather weak interactions between the molecules and the force field exerted by the surface of the amorphous material. 

Effect Change in catalytic rate r Change in work function <1) Change in adsorption enthalpies AHj ... 

This uneasy situation, with such important quantities, calls for a search of the physicochemical rationale of the regularities. The poor understanding of the absolute values of adsorption enthalpy gives little ground to any far-going conclusions from the experimental data. It concerns, for example, the manifestation of relativistic effects in chemical properties of the new elements, which is believed to be evidenced if a TAE compound is more volatile (less adsorbable) then that of a lighter homolog. 

Since both albedo and gas adsorption depend on SSA, the climate response of the concentration of species adsorbed within the snowpack will be similar to that of albedo increase in regions where warming is accompanied by a change from HGM to QIM, such as the southern taiga and the warmer Alpine areas in the fall and decrease in the other regions. These effects will be modulated by the temperature increase, that will decrease the concentration of adsorbed species. For example, a temperature rise from -15 to -10 °C will desorb 40% of adsorbed acetone molecules, that have an adsorption enthalpy of 57 kJ/mol,, at constant SSA. The combined effect of warming and SSA change will then probably lead to a decrease in the concentration of adsorbed species in most areas. 

Adopting the approach of increasing the adsorption enthalpy for graphites should have a similar effect for SWNTs, and also have the effect of increasing the surface area of SWNTs. Nanotubes typically form rope structures due to van der Waals interactions which promote rope formation, limiting the surface area to -300 m /gm (interior and exterior surface areas for a SWNT material should be above 2,600 m /gm). Potassium intercalation of SWNTs will separate the individual tubes. Computational work on SWNTs shows that under certain conditions, increasing the van der Waals gap will increase the amount of hydrogen that can be adsorbed [5]. 

Finally, since the absorption of hydrogen is thermodynamically unfavourable with respect to adsorption (enthalpy of adsorption at low coverage, — lOOkJmor enthalpy of absorption at infinite dilution — 20kJmor ), and yet absorption occurs under conditions where adsorption is inhibited, it must be concluded that adsorption is suppressed by a purely surface effect. 

Since two-point adsorption is no longer possible if the metal-metal distance is too large, the optimal catalyst for ethylene hydrogenation should have a certain medium interatomic distance. This is the case for rhodium with 0.375 nm, but since energetic aspects (adsorption enthalpies) must also be taken into accoimt, it can not be said that this is solely the result of steric effects. 

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