Adsorption of methyl orange onto 1- Acid activated clay and 2- Al13-Pillared clay were performed separately. The pillared clay showed higher adsorption capacity but negative activation energy, while the acid activated clay showed positive activation energy .
One can guess - physical adsorption? chemical adsorption? Better, give more details about the experiment and the mathematical description of its results.
Regards,
Adsorption process showing negative activation energy is exothermic and it enhances at lower temperatures. As the temperature increases, the adsorbate may desorb (leave the surface) and unfavorable intermolecular forces between adsorbate and solvent are much stronger than those between adsorbate and adsorbent. As a result, the solubility of adsorbate species increases which causing adsorbate to be more difficult to adsorb. .
https://www.researchgate.net/post/What_is_the_phenomenon_of_negative_activation_energy2
Activation energy is here associated with overcoming the energy barrier of chemical bond formation between the adsorbate molecules and the adsorbent surface (molesules on it). Then it has a positive value. Negative activation energy may occur in the case of physical adsorption. The question is, however, whether in such a case the introduction of the concept of activation energy is justified. In my opinion - it is not. A negative activation energy could also occur in the case of reversible adsorption, but then the adsorption and desorption process must be considered separately. Similarly, as in the case of reversible reaction, where there are actually two reactions "forward" and "backward".
Regards,
Physical adsorption is associated with negative activation energy which is reverse i.e., positive for physical desorption.
If we talk about activation energy, then the process has to be kinetic. If the adsorption kinetics enhances with increasing temperature then the activation energy is positive. If the kinetic processes is slowed down with increasing temperature the activation energy is negative. The reverse will be for desorption.
To quote Yong Li (No negative activation energy exist!). In chemistry, activation barrier is associated with a certain elementary step and is by definition positive. I was teached that "negative" activation barrier is interpreted as a direct indication on a complex process mechanism and cannot be assigned to a certain rate-limiting elementary step, and therefore, it becomes a complexed empirical parameter. There is no place to assign the meaning of "physical"/"chemical" interactions or sorption/desorption control to negative activation energy until the mechanism is not established, and, perhaps, multiple factors such as, in a specific system, change of pH with temperature, decomposition of clay, release of silica or alumina, and/or collapse of clay structure (and many others) are not verified, and, it is certain, not a barrier of a rate-limiting step, to my understanding, obviously. In a specific case, when it is about a dye adsorption, the questions arising are the probablity for clay aggregation in solution and the validity of the experimental protocol. Since, when working with dyes on clays, a "classic" approach is to use spectrophotometry, and here the identity of the solution matrix to that used in calibrations curves may become very important.