Rates and Kinetic models of catalytic reactions

19
1 Chapter 3 Rates and kinetic models of catalytic reactions

description

Powerpoint

Transcript of Rates and Kinetic models of catalytic reactions

Page 1: Rates and Kinetic models of catalytic reactions

1

Chapter 3 Rates and kinetic models of catalytic reactions

Page 2: Rates and Kinetic models of catalytic reactions

2

A C

Page 3: Rates and Kinetic models of catalytic reactions

3

Page 4: Rates and Kinetic models of catalytic reactions

4

A + B → CθA θB

dPc/dt = Rate = k θA θB

PB: constant

max. rate: θA = θB

bAPA = bBPB

∴ bA/bB = PB/PA

θA = bAPA/(1+ bAPA+ bBPB)

θB = bBPB/(1+ bAPA+ bBPB)

Page 5: Rates and Kinetic models of catalytic reactions

5

Page 6: Rates and Kinetic models of catalytic reactions

6

↗A + B↘

C

DA + B → C → D

↗A + B↘

C

D↓

% Conversion: fraction of R converted into P% Selectivity of Pi: fraction of Pi/total products

% yield Pi = conversion x selectivity

Selective poison:

purification

Page 7: Rates and Kinetic models of catalytic reactions

7

Page 8: Rates and Kinetic models of catalytic reactions

8

KINETICS OF CATALYSED REACTIONS

θ ~ 00 < θ < 1

θ = 1

Eapp Etrue

b小, T大 b大, T小

k = A exp(-E/RT)

Page 9: Rates and Kinetic models of catalytic reactions

9

diffusion-limitedsurface rxn

Fick’s 1st law of diffusion -dnA/Adt = D‧dc/dx

Page 10: Rates and Kinetic models of catalytic reactions

10

log k

1/T

Rate = A exp(-E/RT)

Page 11: Rates and Kinetic models of catalytic reactions

11

Diffusion process – mass transport or mass transfer

Diffusion limited = mass-transport limited

1. Rxn rate ∝ (catalyst wt)n

or (conc. of active component)n n <1 [cf. n = 1]

2. Movement, agitation, → ↑ rate [ no effect]

3. Eapp = 10 – 15 kJ/mole [cf. Eapp ≧ 25 kJ/mole]

[truly chemical step]

Page 12: Rates and Kinetic models of catalytic reactions

12

Page 13: Rates and Kinetic models of catalytic reactions

13

Page 14: Rates and Kinetic models of catalytic reactions

14

KENETICS OF CATALYZED REACTIONS

k = A exp(-E/RT)ln A = m E + Cln A vs. E → linear

↑ ln A, k ↑↑ E, k ↓

compensation effect

C1

C2

n-C3

i-C3

n-C4

2o-C4

Page 15: Rates and Kinetic models of catalytic reactions

15

The compensation effectlog A = aE + b

An increase in lnA (which would increase the rate) is compensated by an increase in E (which would decrease the rate )

Page 16: Rates and Kinetic models of catalytic reactions

16

- facile or structure-insensitive rxns – one or two metal atoms- demanding or structure-sensitive rxns – specific aggregates of atoms

Active centers

Catalytically-inert elemnet

+ 3 H2

H CH

HC HH

H→ 2 CH4H2 +

Page 17: Rates and Kinetic models of catalytic reactions

17

Structure-insensitive rxn: catalytic reaction for which the reaction rate per unit surface area is practically independent of the size or shape of the supported metal crystallites, i.e., independent of the mode of preparation. Also known as a facile reaction.

Structure-sensitive rxn: catalytic reaction for which the activity of the catalyst depends on the atomic structure of the catalyst surface, i.e., the rate per unit surface area depends on the size or shape of the supported metal crystallites. Also known as a demanding reaction.

Page 18: Rates and Kinetic models of catalytic reactions

18

Langmuir-Hinshelwood Mechanism:The rate of a heterogeneous reaction is controlled by the reaction of the adsorbed molecules, and that all adsorption and desorptionpressure are in equilibrium.

Rideal-Eley mechanism:----between strongly adsorbed atoms (those chemisorbed) and molecules held to surface only be weak, van der Waals forces (those physically adsorbed).

Page 19: Rates and Kinetic models of catalytic reactions

19