INTRODUCTION TO CATALYSIS –KINETICS OF CATALYTIC REACTIONS CH 360 1

Catalysts The photo above shows a variety of different solid catalysts used in industry. Those in the front row consist of porous substrate material coated or impregnated with catalyst; The two samples in the back row are fine powders with large surface areas 2

Silica-Alumina Cat-Cracking Catalyst (100X ) FRESH SPENT 3

Silica-Alumina Cat-Cracking Catalyst (400X) FRESH SPENT 4

Silica-Alumina Cat-Cracking Catalyst (800X) FRESH SPENT 5

Fresh Silica-Alumina Cat-Cracking Catalyst (1700 & 3000X) 6

Silica-Alumina Cat-Cracking Catalyst (5000X) FRESHSPENT 7

Steps in a catalytic reaction 1.Mass transfer of reactants (External diffusion) reactants diffuse from the bulk gas to the external surface of the catalyst pellet 2.Internal diffusion of reactants reactant diffuse from the catalyst surface (pore mouth) through the catalyst pores to the immediate vicinity of the internal catalytic surface 3.Adsorption reactant attaches onto the catalyst surface 4.Reaction a reaction takes place 5. Desorption product leaves from the catalyst surface 6. Internal diffusion of products products diffuse from the catalyst pores to the catalyst surface (pore mouth) 7. Mass transfer of products (External diffusion) products diffuse from the catalyst surface to the bulk gas 8

Reactions are not catalyzed over the entire surface but only at certain active sites or centers that result from unsaturated atoms in the surface. An active site is a point on the surface that can form strong chemical bonds with an adsorbed atom or molecule. Active site is usually denoted as S Active sites 9

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Transport & Kinetic Processes in Catalytic Reactions external diffusion of A bulk gas phase porous catalyst particle hydrodynamic boundary layer 11

Transport & Kinetic Processes in Catalytic Reactions internal diffusion of A bulk gas phase porous catalyst particle hydrodynamic boundary layer external diffusion of A 12

Transport & Kinetic Processes in Catalytic Reactions adsorption of A bulk gas phase porous catalyst particle hydrodynamic boundary layer external diffusion of A internal diffusion of A A + S  AS 13

Transport & Kinetic Processes in Catalytic Reactions adsorption of A bulk gas phase porous catalyst particle hydrodynamic boundary layer external diffusion of A internal diffusion of A AS  BS A + S  AS reaction of A to B 14

Transport & Kinetic Processes in Catalytic Reactions external diffusion of A internal diffusion of A adsorption of A desorption of B reaction of A to B A + S  AS AS  BS BS  B + S porous catalyst particle bulk gas phase hydrodynamic boundary layer 15

Transport & Kinetic Processes in Catalytic Reactions external diffusion of A internal diffusion of A adsorption of A internal diffusion of B desorption of B reaction of A to B A + S  AS AS  BS BS  B + S porous catalyst particle bulk gas phase hydrodynamic boundary layer 16

Transport & Kinetic Processes in Catalytic Reactions external diffusion of A internal diffusion of A adsorption of A external diffusion of B internal diffusion of B desorption of B reaction of A to B A + S  AS AS  BS BS  B + S porous catalyst particle bulk gas phase hydrodynamic boundary layer 17

Transport & Kinetic Processes in Catalytic Reactions external diffusion of A internal diffusion of A adsorption of A external diffusion of B internal diffusion of B desorption of B reaction of A to B One of these seven transport and kinetic processes occurs the slowest. We say that step is “rate-limiting” The overall rate of the catalytic reaction is equal to the rate of the rate-limiting step It is necessary to determine the “rate-limiting” step to analyze the kinetics. these stepsStart by restricting ourselves to these steps… A + S  AS AS  BS BS  B + S 18

The Rate Limiting Step: Which Step Has the Largest Resistance? Electrical analog to heterogeneous reactions 19

1. Adsorption: Molecular or dissociative 2. Surface reaction 3. Desorption Rate-limiting catalytic steps 20

Adsorption-molecular A S represents an active site-it is a vacant site, with no atom, molecule, or complex adsorbed on it When species A is adsorbed on the site S : Site balance equation : Total molar concentration of active sites per unit mass of catalyst Molar concentration of vacant sites per unit mass of catalyst S The whole molecule gets adsorbed (molecular adsorption) 21 A

Adsorption: Molecular adsorption of A A + S  AS Treat as an elementary reaction rate of adsorption: rate of desorption: site balance: net rate of adsorption: K A  equilibrium constant K A  (k A /k -A ) At equilibrium pressure is a measure of collision frequency (from the molecular theory of gases) We cannot experimentally measure C V but can measure C t Concentration of AS at the catalytic surface 22

Langmuir Adsorption Isotherm Adsorption: Molecular Isotherms portray the amount of a gas adsorbed on a solid at different pressures, but at one temperature. 23

Langmuir Adsorption Isotherm adsorption of A A + S  AS Langmuir Adsorption Isotherm Langmuir Isotherm describes the equilibrium partition of gas between sorbed and desorbed states. Assumes monolayer coverage of surface uniform surface Interaction between gas/site only note that we’ve treated both adsorption and desorption in this analysis 24

Adsorption-dissociative A Site balance equation : Total molar concentration of active sites per unit mass of catalyst Molar concentration of vacant sites per unit mass of catalyst S The molecule dissociate as it gets adsorbed (dissociative adsorption) A2 A 25

Adsorption: Dissociative Treat as an elementary reaction rate of adsorption: rate of desorption: site balance: net rate of sorption At equilibrium: K A  (k A /k -A ) We cannot experimentally measure C V but can measure C t Concentration of AS at the catalytic surface adsorption of A 2 A 2 + 2S  2AS 26

Multicomponent Adsorption adsorption of A and B A + S  AS rate of adsorption:rate of desorption: site balance: net rate of sorption (at equilibrium): B + S  BS 27 This equation is the same as for The adsorption of single component A (like no B is present) K A  (k A /k -A ) K B  (k B /k -B )

1. Adsorption: Molecular or dissociative 2. Surface reaction 3. Desorption Rate-limiting catalytic steps 28

The Surface Reaction Step 29 AS  BS

30 AS + S  BS + S The Surface Reaction Step

31 AS + BS  CS + DS The Surface Reaction Step AS + BS  CS + S

32 AS + BS'  CS' + DS The Surface Reaction Step

33 AS + B (g)  CS + D (g) The Surface Reaction Step

Surface Reaction single site mechanism AS  BS dual site mechanisms AS + S  BS + S AS + BS  CS + DS AS + BS'  CS' + DS gas phase interaction AS + B (g)  CS + D (g) Eley-Rideal mechanism Langmuir-Hinshelwood mechanism 34

1. Adsorption: Molecular or dissociative 2. Surface reaction 3. Desorption Rate-limiting catalytic steps 35

Desorption from catalytic surface A A rate of desorption: rate of adsorption: net rate of desorption: Remember the rate of adsorption: 36

Important: When studying the kinetics of catalytic reactions, the rate expression of the rate limiting step must take into account both forward and the reverse reaction However, all the other catalytic steps are assumed to be in equilibrium (the rate of forward is equal to the rate of the reverse reaction): 37