INTRODUCTION TO CATALYSIS KINETICS OF CATALYTIC REACTIONS CH
360 1
Slide 2
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
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
Slide 9
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
Slide 10
10
Slide 11
Transport & Kinetic Processes in Catalytic Reactions
external diffusion of A bulk gas phase porous catalyst particle
hydrodynamic boundary layer 11
Slide 12
Transport & Kinetic Processes in Catalytic Reactions
internal diffusion of A bulk gas phase porous catalyst particle
hydrodynamic boundary layer external diffusion of A 12
Slide 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 A + S AS 13
Slide 14
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
Slide 15
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
Slide 16
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
Slide 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 A + S AS AS BS BS B + S porous catalyst particle
bulk gas phase hydrodynamic boundary layer 17
Slide 18
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
Slide 19
The Rate Limiting Step: Which Step Has the Largest Resistance?
Electrical analog to heterogeneous reactions 19
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
Slide 22
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
Slide 23
Langmuir Adsorption Isotherm Adsorption: Molecular Isotherms
portray the amount of a gas adsorbed on a solid at different
pressures, but at one temperature. 23
Slide 24
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 weve treated both adsorption and desorption
in this analysis 24
Slide 25
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
Slide 26
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
Slide 27
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 )
31 AS + BS CS + DS The Surface Reaction Step AS + BS CS +
S
Slide 32
32 AS + BS' CS' + DS The Surface Reaction Step
Slide 33
33 AS + B (g) CS + D (g) The Surface Reaction Step
Slide 34
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
Desorption from catalytic surface A A rate of desorption: rate
of adsorption: net rate of desorption: Remember the rate of
adsorption: 36
Slide 37
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