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Chapter 3 Rates and kinetic models of catalytic reactions

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A C

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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)

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↗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

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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)

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diffusion-limitedsurface rxn

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

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log k

1/T

Rate = A exp(-E/RT)

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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]

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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

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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 )

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- 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 +

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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.

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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).

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