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CRE II Heterogeneous Catalysis

L3

Prof. K.K.Pant

Department of Chemical EngineeringIIT Delhi.

kkpant@chemical.iitd.ac.in

mailto:kkpant@chemical.iitd.ac.inmailto:kkpant@chemical.iitd.ac.in

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The fundamental equations of

thermodynamics

Study of the rates of chemical reactions and the factors

that influence the rates.

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Three key aspects of catalyst

actiontaking part in the reaction

it will change itself during the process by

interacting with other reactant/productmolecules.

altering the rates of reactions

in most cases the rates of reactions are

increased by the action of catalysts;

however, in some situations the rates of

undesired reactions are selectively

suppressed.

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A catalyst accelerates a chemical reaction

without affecting the position of the equilibrium.

Ostwald recognized catalysis as a ubiquitousphenomenon that was to be explained in terms

of the laws of physical chemistry.

Returning to its original form

After reaction cycles a catalyst with

exactly the same nature is reborn.

In practice a catalyst has its lifespan

it deactivates gradually during use.

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Schematic representation of the energetic in a catalytic cycle. The

uncatalyzed reaction(a)has a higher Gibbs energy of activationG than any

step in the catalyzed reaction (b).

The gibbs energy of reactionrG for the overall reaction is unchanged

from (a) to (b).

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The reaction activation energy is altered

The intermediates formed are different

from those formed in non-catalytic

reaction

The rates of reactions are altered (both

desired and undesired ones)

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Action of Catalysts

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-Reactions proceed under less demanding

conditions

Allow reactions occur under a milder

conditions, e.g. at lower temperatures for

those heat sensitive materials

It is important to remember that the useof catalyst DOES NOT vary DG& Keq

values of the reaction concerned, it

merely change the PACE of the process.

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The suitability of a catalyst for an industrialprocess depends mainly on the following

three properties:

ActivitySelectivity

Stability (deactivation behavior)

The question which of these functions is the

most important is generally difficult toanswer because the demands made on the

catalyst are different for each process.

Mode of Action of Catalysts

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Catalysts are investigated in continuously

operated test reactors, in which theconversions attained at constant space

velocity/space time are compared .

Space Velocity: is the volume flow rate V0,

relative to the catalyst mass mcat :

(V0/mcat)

(made under constant conditions of startingmaterial ratio, temperature, and pressure).

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Comparison of catalyst activities

Space time (mcat/V0)

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Chemical Kinetics

Collision Theory

Collisions between reacting molecules are

necessary before a reaction can occur.

Only those collisions having sufficient energy

are effective in bringing about a reaction

activation energy.

Colliding molecules must be properly oriented

with respect to one another for the reaction to

take place.

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Every catalytic reaction is a sequence of

elementary steps, in which reactant moleculesbind to the catalyst, where they react, after which

the product detaches from the catalyst, liberating

the latter for the next cycle.

Steps of Catalytic Reactions