KineticsAtSurfaces - Kinetics of Surface Catalysed...

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Kinetics of Surface Catalysed Reactions A wide variety of rate laws and reaction mechanisms can apply to surface catalysed reactions. A few of these are considered below. UNIMOLECULAR DECOMPOSITION Examples of unimolecular decomposition reactions include: 1. Decomposition of NH 3 to N 2 and H 2 on metal surfaces 2. Decomposition of phosphine on glass 3. Decomposition of formic acid on glass, Pt, Ag, Au, or TiO 2 (in this case there are two competing reactions: HCOOH CO + H 2 O and HCOOH CO 2 + H 2 ) Unimolecular surface reactions can often be described by the mechanism A (g) k 1 k -1 A (ads) k 2 Products The rate of product formation is k 2 θ A (where θ A is the surface coverage of A), and provided k 2 << k -1 (i.e. the rate of reaction of adsorbed A molecules is much greater than the rate of desorption of unreacted A), the Langmuir adsorption isotherm can be used to determine θ A , giving rate = k 2 θ A = k 2 K A P A 1+K A P A (1) This type of reaction shows two limiting rate laws, corresponding to the two extreme behaviours of the Langmuir isotherm: a) At low pressure (P A 0), θ A is very small and proportional to the pressure. The rate becomes first order in A(g). rate (low P) = k 2 K A P A (2) b) At high pressures θ A is approximately equal to unity, and the reaction is zeroth order rate (high P) = k 2 (3) Inhibition A complication in surface reactions arises when a substance other than the reactant is adsorbed on the surface. This reduces the effective surface area, and therefore the rate. If the fraction of the surface covered by the reactant (A) is θ A , and that covered by the inhibitor is θ I , we have θ A K A P A 1+K A P A +K I P I (4) [Exercise: Derive the above equation by setting up Langmuir equations for θ A and θ I (i.e. k A P A N(1- θ I - θ A ) = k’ A θ A ; k I P I N(1- θ I - θ A ) = k’ I θ I , where k and k’ are adsorption and desorption rates and N is the number of available surface sites) and solving for θ A ]
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The reaction rate, k 2 θ A , is therefore rate = k 2 K A P A 1+K A P A +K I P I (5) If the pressure of the reactant is low compared to that of the inhibitor, so that the available surface is only sparsely covered by the reactant, then K A P A <<1+K I P I , and rate = k 2 K A P A 1+K I P I (6)
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This note was uploaded on 05/09/2010 for the course CHE 3051 taught by Professor Sitimachmud during the Spring '08 term at National Central University.

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KineticsAtSurfaces - Kinetics of Surface Catalysed...

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