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gas molecules only interact with adjacent layers; and; the Langmuir theory can be applied to each layer. the enthalpy of adsorption for the first layer is constant and greater than the second (and higher). the enthalpy of adsorption for the second (and higher) layers is the same as the enthalpy of liquefaction. The resulting BET equation is
Chemisorption is a kind of adsorption which involves a chemical reaction between the surface and the adsorbate. New chemical bonds are generated at the adsorbent surface. Examples include macroscopic phenomena that can be very obvious, like corrosion [clarification needed], and subtler effects associated with heterogeneous catalysis, where the catalyst and reactants are in different pha
Thus, the use of alternative probe molecules can often result in different obtained numerical values for surface area, rendering comparison problematic. The model also ignores adsorbate–adsorbate interactions. Experimentally, there is clear evidence for adsorbate–adsorbate interactions in heat of adsorption data.
The key assumption used in deriving the BET equation that the successive heats of adsorption for all layers except the first are equal to the heat of condensation of the adsorbate. The Langmuir isotherm is usually better for chemisorption, and the BET isotherm works better for physisorption for non-microporous surfaces.
In chemisorption, molecules are adsorbed on the surface by valence bonds and only form monolayer adsorption. A direct transition from physisorption to chemisorption has been observed by attaching a CO molecule to the tip of an atomic force microscope and measuring its interaction with a single iron atom. [ 12 ]
Vapor-solid reactions: formation of an inactive surface layer and/or formation of a volatile compound that exits the reactor. [22] This results in a loss of surface area and/or catalyst material. Solid-state transformation : solid-state diffusion of catalyst support atoms to the surface followed by a reaction that forms an inactive phase.
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These equations are simple and can be easily understood but cannot explain experimental results. In 1958, P. Kisliuk [1] presented an equation for the sticking probability that can explain experimental results. In his theory, molecules are trapped in precursor states of physisorption before chemisorption. Then the molecules meet adsorption ...