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Adsorption: Screencasts

Explains the concept of adsorption and derives the Langmuir isotherm.

We suggest you list the important points in this screencast as a way to increase retention.

An introduction to Langmuir Isotherms.

We suggest you list the important points in this screencast as a way to increase retention.

Optional screencast: Langmuir Isotherm: Dissociative Adsorption

Important Equations:

Langmuir isotherm for species A adsorbed on a surface:

\[\theta _A = \frac{K_AP_A}{1 + K_AP_A}\]

where \(K_A\) = adsorption equilibrium constant for species A,

\(P_A\) = pressure of species A, and

\(\theta _A\) = fraction of the surface covered by species A.

The Lanmuir isotherm for molecules A and B co-adsorbed has the form:

\[Molecules \,\, of \,\, A\,\, adsorbed/site\,\,= \frac{K_AP_A}{1+K_AP_A + K_BP_B}\]

\[Molecules \,\, of \,\, B\,\, adsorbed/site\,\,= \frac{K_BP_B}{\alpha (1+K_AP_A +K_BP_B)}\]

where \(K_B\) = adsorption equilibrium constant for species B,

\(P_B\) = pressure of species B, and 

\[\alpha = \frac{sites\,\,needed\,\,for\,\,B}{sites\,\,needed\,\,for\,\,A}\]

\[K_A = K_{A0}exp \left( \frac{-\Delta H}{RT} \right)\]

where \(\Delta H\) = enthalpy change on adsorption, which is negative because adsorption is exothermic.

\(K_{A0}\) is a temperature-independent term.

Lanmuir adsorption isotherm for dissociative adsorption of H2:

\[\theta _H = \frac{K^{0.5} _{\alpha} P^{0.5} _{H_2}}{1+K^{0.5} _{\alpha} P^{0.5} _{H_2}}\]

where \(\theta _H\) = fraction of sites covered by H atoms

\(K_{\alpha}\) = adsorption equilibrium constant, and

\(P_{H_2}\) = H2 pressure.