I have been reading several documents in the thermodynamic characterization of an adsorption process. However, I have found some differences about this way of characterize the physisorption or chemisorption. On one hand some authors work with the D-R model and obtain different mean free energy values for every temperature. They say that a value between 8 and 16 KJ/mol can be characterized as physisorption. Higher values are chemisorption. On the other hand by the change of temperature of the process Van't Hoff equation can be used in order to determine the enthalpy, entropy and Gibbs free energy in the process. Others authors consider that a change of enthalpy
Dear Oscar Balladares :
The interaction energy during chemisorption of 40-400 kJ / mol. It is 1-2 orders of magnitude larger than this value for the physical adsorption (10-40 kJ / mol). Physical adsorption is reversible and low specificity. Chemisorption is characterized by the specificity of the interaction, which leads to the formation of surface chemical compounds. Chemisorption is often irreversible. Chemisorption is a typical example of a heterogeneous reaction.
Best regards.
Hello Professor Yuri,
Probably my question was misunderstood, I was referring to the differences that both model presents, because in one case the D-R model consider a different mean free energy for every temperature and that is the parameter that differences physisorption from chemisorption. On the other hand, through the linearization of Van't Hoff equation it is possible to obtain the change of enthalpy in the process, this entalphy is used to characterize chemi or physorption. I was wondering what is the difference between this mean free energy from D-R model and thermodynamic parameters (enthalpy, entropy and Gibbs free energy) from Van't Hoff eq. Thank you so much.
Oscar
Oscar:
Adsorption may be on homogeneous of surface with a uniform monolayer. Adsorption centers are energetically equivalent. The enthalpy of adsorption determined from the experimental dependence of the equilibrium constant of adsorption on temperature according to the equation Van't Hoff.
On inhomogeneous surface adsorption energy parameters depend on the degree of filling of the surface adsorbed substance. Each corresponds to a certain degree of filling of the differential heat of adsorption at A = const. It decreases with increasing degree of filling. To determine the heat of adsorption isotherm is first obtained at different temperatures. Determine the isosteric heat of adsorption at each degree of filling of the adsorbent. This is a simple example of a non-uniform surface. A model D-R is used for more complex non-uniform surface of microporous adsorbents.
Regards.
Entropy: A concept that is not a physical quantity
https://www.researchgate.net/publication/230554936_Entropy_A_concept_that_is_not_a_physical_quantity
Dear Professor Yuri Mirgorod
I got an E value is over 200 Kj/mol/ it was meant that Chemisorption occur in my experiment? because I could not found the supporting data that this over value are represented the chemisorption. Could you please to sent some suppuration data to confirm this information
Regard,
Phacharapol
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