Adsorption isochor

The strength of acid sites can be approximated by determining the heat of adsorption using the Van t Hoff isochore. In this method the pressures P required to give a fixed surface coverage at a series of temperatures T is determined. The derivative of In P with respect to T is related to the isosteric heat of adsorption. This technique is not used routinely. Nor is that of IR spectroscopy, where it is reported that Lewis and Bronsted sites can be differentiated by using substituted compounds. 

The effect of temperature on the rate constants and adsorption coefficients was assumed to be given by the Arrhenius equation and Van t Hoff isochore respectively, i.e.. 

In a recent paper [3] we described how adsorption equilibrium constants for a range of paraffinic and olefinic species could be estimated using the following integrated form of van t Hoffs isochore equation,... 

High precision is especially hard to obtain in measurements on vapours. Pressure ranges in expansions are restricted by proximity to saturation conditions, and high-accuracy measurements of low pressures are difficult. Adsorption also complicates studies on vapours. For example, in measurements by the Burnett method on pure Ar and Kr near saturation, Weir et al. determined the effect of adsorption in their copper apparatus by performing experiments in vessels of different surface-to-volume ratios. Corrections for adsorption first became significant at temperatures at which the saturation pressure was just in excess of atmospheric, and rose rapidly at lower temperatures. For Ar at its triple point, where the saturation pressure is 70 kPa and B = — 280 cm mol, the correction due to adsorption was 8 cm mol. Hall and Eubank recommend that Burnett measurements be combined with isochoric data to avoid systematic errors due to adsorption. 

As there is a difference between the heat of adsorption of this weakly bound adsorbate on the support and the more strongly bound irreversible adsorption on the metal, it is possible to determine monolayer capacity of the metal by noting the value of coverage at which the heat of adsorption abruptly decreases to give a constant value. Heats of adsorption may be obtained from isochore type measurements where variations in pressure are measured while varying the temperature after exposing the catalyst to different initial amounts of adsorbate. 

Xiao et al. ° studied the adsorption of chlorobenzene and 1,3-dichlorobenzene at 313 to 453 K on a wood-based activated carbon, which was prepared by the activation of a char in CO2 at 1073 K. The adsorption isotherms for both the halogenated hydrocarbons are Type 1 of the BET classihcation (Figure 7.6). The amount adsorbed decreased systematically with the increasing temperature. The total volume of the halogenated hydrocarbon adsorbed was equal to the total pore volume of the carbon obtained from nitrogen adsorption at 77 K. The isosteric heats of adsorption of chlorobenzene calculated using the van Hoff isochore was 37 KJ/mol at an adsorption of 1 mmol/g and compared well with the heat of vaporization 35.19 KJ/mol. The adsorption appeared to be physical in nature involving micropores. The kinetics... 

Liquid-vapor phase transitions of confined fluids were extensively studied both by experimental and computer simulation methods. In experiments, the phase transitions of confined fluids appear as a rapid change in the mass adsorbed along adsorption isotherms, isochores, and isobars or as heat capacity peaks, maxima in light scattering intensity, etc. (see Refs. [28, 278] for review). A sharp vapor-liquid phase transition was experimentally observed in various porous media ordered mesoporous sifica materials, which contain non-interconnected uniform cylindrical pores with radii Rp from 10 A to more than 110 A [279-287], porous glasses that contain interconnected cylindrical pores with pore radii of about 10 to 10 A [288-293], silica aerogels with disordered structure and wide distribution of pore sizes from 10 to 10" A [294-297], porous carbon [288], carbon nanotubes [298], etc. 

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