I have applied to my experimental data of kinetic adsorption of a fungicide onto activated carbons at different temperatures the linearized form of the Arrhenius equation in order to get the values of the activation energies and frequency factors.
I have observed that the activated carbons with higher adsorption capacities show also higher values for the frequency factor and activation energy.
I see logical the behaviour for the frequency factor taking into account that this factor is related with the number of active centers (see attached paper). But I don't understand why better activated carbons present higher activation energies.
Thank you for your time and your kind attention.
M.A. Martín-González.
http://pdm-mipa.ugm.ac.id/ojs/index.php/ijc/article/viewFile/115/165
Buenos dias. Difficult to generalize, because the affecting factors can be numerous and complex. Typically a different order of magnitude of the apparent activation energy is detected between PHYSICAL adsorption and CHEMIsorption. What you see is the linearised macroscopic effect of potentially many steps in series including mass transfer (e.g. Glueckauf and Coates mechanism) and chemical reactions (i.e. Hinshelwood-Languimur kinetics). In addition, for reactions like these, Hinshelwood-Hougen kinetics works much better than simple Arrhenius (see for instance the classic textbook by Satterfield). Roughly speaking, the evidence could be due to a bigger promotion of a chemisorption with a corresponding higher activation, just due to the higher efficiency of the more powerful carbons.
I agree with Renato Benintendi. The higher activation energy may be caused by the chemisorption. In addition, for a series of same types of reactions, the frequency factor is related with the number of active sites. However, for differents types of reactions, there is no relationship between them. Firstly, you should identify what is the reason for higher adsorption capacity of active carbon. Generally, higher adsorption capacity of active carbon is attributed to the incresing porosity , the change of structure and diameter of pore. The active carbon with higher absorption capacity has bigger porosity and smaller average pore diameter. For phyical absorption, the absorption occuring at the small pore also has bigger activation energy.
Thank you Renato and Huang for your clear explanations. Your help is greatly appreciated.
- Badges
- Science topic
- Similar topics
- Physical Sciences
- Thermodynamics