Proton adsorption enthalpies

Assuming the validity of the 1 pK model, proton adsorption enthalpies at the pHzpc can be used to predict the temperature variation of Kj (and therefore the pHzpc) with the van t Hoff relation (6),... 

Proton adsorption enthalpy data at the pHzpc and equation (6) permit estimation of expected changes in pHzpc with temperature. Ferric and... 

Table I. Changes in pHzpc with temperature obtained from the literature and corresponding proton adsorption enthalpy values calculated using equation (6)...

Table II. Proton adsorption enthalpies determined using...

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).

Adsorption and desorption reactions of protons on iron oxides have been measured by the pressure jump relaxation method using conductimetric titration and found to be fast (Tab. 10.3). The desorption rate constant appears to be related to the acidity of the surface hydroxyl groups (Astumian et al., 1981). Proton adsorption on iron oxides is exothermic potentiometric calorimetric titration measurements indicated that the enthalpy of proton adsorption is -25 to -38 kj mol (Tab. 10.3). For hematite, the enthalpy of proton adsorption is -36.6 kJ mol and the free energy of adsorption, -48.8 kJ mol (Lyklema, 1987). 

Tab. 10.3 Enthalpies of adsorption and kinetics of proton adsorption and desorption on iron oxides.

T. Akrmoto, S. (1984) Mossbauer study on the high pressure phase ofiron(III) oxide. Solid State Commun. 50 97—100 Szczypa, J. Matysiak, J. Kosmilski, M. (1994) Standard enthalpies of proton adsorption on hematite in various solvent systems. Abstracts of 8 Int. Conf. Colloid Surf. Chem. Adelaide... 

Reviews by Gorte and coworkers [35, 36] deal with the adsorption complexes formed by strong and weak bases with acid sites in zeolites. They examine the adsorption enthalpies of a series of strongly basic molecules such as alkylamines, pyridines and imines. These workers also performed studies of the adsorption properties of weak bases, including water, alcohols, thiols, olefins, aldehydes, ketones and nitriles. They report a poor correlation between the differential heats of adsorption on H-MFl zeolites and the enthalpies of protonation in aqueous solutions, but a much better correlation with gas-phase proton affinities [37]. 

FIG. 3.104 Enthalpy of proton adsorption on (hydr)oxides as a function of their PZC. 

With isoperibol solution calorimetry, proton adsorption and desorption enthalpies are obtained and these can be used to predict how the pHzpc varies with temperature. The 1 pK model is especially useful for this purpose. 

The adsorption of alkanes on Bronsted sites also gives 1 1 hydrogen-bonded species, with the interaction being between the protonic charge and the induced dipole on the alkane. The adsorption enthalpy of hydrocarbons increase monotonically with size, as the dispersion forces involved increase. The adsorption of alkenes is stronger, as some electron density is transferred from the double bond. At all but the lowest temperatures reaction will then occur for all alkenes except ethene. 

The adsorption enthalpies of Ar, N2, CO, H2O, CH3CN, and NH3 on H-MFl zeolites have been measured calorimetrically at 303 K in order to assess the energetic features of Lewis and Brbnsted acidic sites [175]. Both the uptakes and enthalpy changes correlated well with the proton affinities and polarizabilities of the probes. Whereas CO and N2 were foimd to single out contributions from Lewis and BrOnsted acidic sites, Ar was only sensitive to confinement effects. H2O, CH3CN, and NH3 were not preferentially adsorbed on Lewis sites, suggesting that the adsorption on Brbnsted sites is competitive with Lewis sites [176,177]. With the exception of H2O, the heats of adsorption at zero coverage of the different probes on the various systems studied were foimd to correlate well with the proton affinities (PA) of the molecular probes [176,177]. 

FIG. 11 Because the enthalpies of proton adsorption are large and vary with pH, the derivatives of dissolution rates (here tephroite Mn2 Si04 ) vary with hoth temperature and pH. (See Refs. 8, 52 and 53.)... 

Gorman-Lewis D, Fein JB, Jensen MP (2006) Enthalpies and entropies of proton and cadmium adsorption onto Bacillus subtilis bacterial cells from calorimetric measurements. Geochim Cosmochim Acta 70 4862-4873... 

Adsorption of nonionic compounds on subsurface solid phases is subject to a series of mechanisms such as protonation, water bridging, cation bridging, ligand exchange, hydrogen bonding, and van der Waals interactions. Hasset and Banwart (1989) consider that the sorption of nonpolar organics by soils is due to enthalpy-related and entropy-related adsorption forces. 

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... 

Immobilization of the aminosilane molecule changes its interaction characteristics. Because the surface silanols are more acidic than silane silanols, the interaction with the surface silanols is thermodynamically favoured over intramolecular interaction. Kelly and Leyden10 measured the enthalpy of adsorption of the aminosilane molecules. Their results indicate that interaction with the surface involves more proton transfer than in the closed form dissolved molecules. 

The influence of the pre-treatment temperature on the acidic properties is a very important factor. For Bronsted 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 towards an electron-deficient, coordinatively unsaturated site, and probably an energy term related to the relaxation of the strained surface [40]. 

The important implication is that microcalorlmetrically determined enthalpies of double layer formation essentially measure the chemical part of the adsorption or desorption of protons, hence their Independence of straight lines in fig. 3.60). The slope is steeper for the latter, in line with Its higher pH°, see [3.6.501. For y-AljOg this trend was confirmed at low pH by Machesky and Jacobs ). Above pH 6 chemical reactions involving various aluminium hydroxides may also have contributed to... 

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