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ReviewLecture5b_2014 [read]

ReviewLecture5b_2014 [read] - ECH 256 Chemical Kinetics and...

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Lecture 5b: Example of Surface Reaction Brian G. Higgins Department of Chemical Engineering and Materials Science University of California, Davis Email: [email protected] ECH 256: Chemical Kinetics and Reaction Engineering
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Problem Statement Consider the surface catalyzed hydrogena;on of ethylene, with the following adsorp;on, reac;on, and diffusion steps: Adsorp;on of C 2 H 4 : Dissocia;ve adsorp;on of H 2 : Hydrogena;on of C 2 H 4 : Hydrogena;on of C 2 H 5 : Desorp;on of C 2 H 6 : H 2 + 2 S k 2 k -2 2 H ad ( C 2 H 4 ) ad + H ad k 3 k -3 ( C 2 H 5 ) ad + S ( C 2 H 5 ) ad + H ad k 4 k -4 ( C 2 H 6 ) ad + S ( C 2 H 6 ) ad k 5 k -5 ( C 2 H 6 ) + S Where S is an empty surface site and represents an adsorbed surface species. Note that this mechanism assumes that does not desorb (only is included). ( C n H m ) ad ( C 2 H 5 ) ad ( C 2 H 5 ) ad C 2 H 4 + S k 1 k -1 ( C 2 H 4 ) ad (2000Exam1)
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Problem Statement (ii) Tasks: (I) Assuming that the adsorp;on and desorp;on steps obey the Langmuir isotherm (for compe;;ve adsorp;on on the same surface sites), and that the hydrogena;on of C 2 H 4 is irreversible and rate limi;ng, derive an expression for the overall reac)on rate in terms of the adsorp;on equilibrium constants and the rate constant for the surface reac;ons.
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